Scientific Output

Over 10.000 scientific papers have been published by members of the Materials Chain since the foundation of the University Alliance Ruhr in 2010. This tremendous output is proof of the excellent environment the Ruhr Area provides for research in the field of materials science and technology.

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  • 2020 • 586 Preparation of antibiofouling nanocomposite PVDF/Ag-SiO2 membrane and long-term performance evaluation in the MBR system fed by real pharmaceutical wastewater
    Ahsani, M. and Hazrati, H. and Javadi, M. and Ulbricht, M. and Yegani, R.
    Separation and Purification Technology 249 (2020)
    In this work, the Ag-SiO2 nanoparticles were successfully synthesized and their antibacterial property was confirmed using the plate colony counting method. The SiO2 and Ag-SiO2 nanoparticles were used to prepare the PVDF/SiO2 and PVDF/Ag-SiO2 nanocomposite membranes, respectively. Pure water flux, contact angle and mechanical strength measurement analyses were conducted to characterize and compare the performance of the neat and nanocomposite membranes. Moreover, in order to investigate the structure of the prepared membranes scanning electron microscope (SEM) was used to obtain surface and cross-section images. A long-term filtration test was carried out in a bench scale submerged membrane bioreactor (MBR) system, fed by real pharmaceutical wastewater, to evaluate the antibiofouling performance of the prepared neat and nanocomposite membranes. In comparison to the neat PVDF, the pure water flux of the nanocomposite PVDF membrane (PVDF/Ag-SiO2; 0.6 wt%) increased about 60% and the water contact angle decreased from about 99° to 89°. The obtained results showed that the nanocomposite PVDF/Ag-SiO2 membrane exhibits considerable antibiofouling properties such that the accumulated dried biofilm as well as the extra cellular polymeric substances (EPSs) collected from the cake layer decreased considerably for the nanocomposite membrane. Moreover, the flux recovery ratio increased from 58% for the neat PVDF membrane to 76% for the nanocomposite PVDF/Ag-SiO2 membrane. The excitation and emission matrix (EEM) fluorescence spectroscopy analysis revealed that the accumulated protein on the surface of nanocomposite membranes decreased considerably in a way that the peak corresponding to tryptophan protein-like substances diminished completely, indicating the high antibiofouling potential of nanocomposite membranes. The chemical oxygen demand (COD) and ammonium removal efficiencies of the neat and nanocomposite membranes were higher than 90% and 95%, respectively, indicating negligible impact of the membrane modification on the effluents’ quality. © 2020
    view abstractdoi: 10.1016/j.seppur.2020.116938
  • 2020 • 585 Gas atomization and laser additive manufacturing of nitrogen-alloyed martensitic stainless steel
    Boes, J. and Röttger, A. and Theisen, W. and Cui, C. and Uhlenwinkel, V. and Schulz, A. and Zoch, H.-W. and Stern, F. and Tenkamp, J. and Walther, F.
    Additive Manufacturing 34 (2020)
    Nitrogen as an alloying element can improve the corrosion resistance and the mechanical properties of stainless steels. Therefore, nitrogen-alloyed martensitic stainless steels, such as X30CrMoN151, have been developed in recent decades and conventional processing of this steel by casting or powder metallurgy is well understood. However, only very few attempts to process nitrogen-alloyed martensitically hardenable stainless steels containing more than 0.2 mass-% of carbon by laser powder bed fusion (L-PBF) have been reported so far. In this study, X30CrMoN15-1 steel powder has been produced from quasi nitrogen-free X30CrMo15-1 steel by gas atomization using N2 as the process gas to introduce nitrogen into the steel. The gas-atomized powder was characterized in terms of nitrogen content, particle size distribution, particle morphology, and flow properties. The powder was then processed by L-PBF under an N2 gas atmosphere, and microstructural investigations were performed on the L-PBF-built samples using scanning electron microscopy and X-ray computed tomography. Additionally, a first impression of the mechanical properties of the L-PBF-built steel in the as-built and quenched and tempered condition was obtained by means of fatigue tests. It was shown that a nitrogen content of 0.16 mass-% could be introduced into the steel during gas atomization. The resulting powder was successfully processed by means of L-PBF, and specimens with a high density were produced. During fatigue testing, a large amount of retained austenite in the as-built condition resulted in a greater damage tolerance of the specimens compared to the heat-treated condition. © 2020 Elsevier B.V.
    view abstractdoi: 10.1016/j.addma.2020.101379
  • 2020 • 584 Processing of X65MoCrWV3-2 Cold Work Tool Steel by Laser Powder Bed Fusion
    Boes, J. and Röttger, A. and Theisen, W.
    Steel Research International 91 (2020)
    Laser powder bed fusion (L-PBF) of forming tools has become of major interest in the tooling industry because of the high geometrical flexibility of this process. During L-PBF, a metallic powder bed is melted selectively by a laser beam, enabling the layer-wise manufacturing of parts from 3D computer-aided design data. The process is characterized by a locally and temporally unsteady heat flow in the solidified part and in the melt pool, causing nonequilibrium solidification and phase transformations. In addition, rapid heating and cooling occur, promoting the formation of microstructural defects, cold cracks, and distortion. Because of the high tendency to form cold cracks, processing of martensitic tool steels is still a challenging task. Tool steel X65MoCrWV3-2 is processed by L-PBF and the resulting microstructure and the associated local properties are investigated by microhardness measurements, nanoindentation, and scanning electron microscopy. It is gathered from the investigations that regions of different microstructures and mechanical properties on both micro- and macroscale are present in the L-PBF-densified steel. The different microstructures and properties are the result of the alternating heat insert at different temperature regimes, forming heat-affected zones in which the tempering processes are triggered and strongly varying properties are generated. © 2019 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/srin.201900445
  • 2020 • 583 On the influence of crystallography on creep of circular notched single crystal superalloy specimens
    Cao, L. and Thome, P. and Agudo Jácome, L. and Somsen, C. and Cailletaud, G. and Eggeler, G.
    Materials Science and Engineering A 782 (2020)
    The present work contributes to a better understanding of the effect of stress multiaxiality on the creep behavior of single crystal Ni-base superalloys. For this purpose we studied the creep deformation and rupture behavior of double notched miniature creep tensile specimens loaded in three crystallographic directions [100], [110] and [111] (creep conditions: 950 °C and 400 MPa net section stress). Crystal plasticity finite element method (CPFEM) was used to analyze the creep stress and strain distributions during creep. Double notched specimens have the advantage that when one notch fails, the other is still intact and allows to study a material state which is close to rupture. No notch root cracking was observed, while microstructural damage (pores and micro cracks) were frequently observed in the center of the notch root region. This is in agreement with the FEM results (high axial stress and high hydrostatic stress in the center of the notched specimen). Twinning was observed in the notch regions of [110] and [111] specimens, and <112> {111} twins were detected and analyzed using orientation imaging scanning electron microscopy. The present work shows that high lattice rotations can be detected in SXs after creep fracture, but they are associated with the high strains accumulated in the final rupture event. © 2020 The Authors
    view abstractdoi: 10.1016/j.msea.2020.139255
  • 2020 • 582 Effect of Spray Parameters in a Spray Flame Reactor During FexOy Nanoparticles Synthesis
    Carvajal, L. and Buitrago-Sierra, R. and Santamaría, A. and Angel, S. and Wiggers, H. and Gallego, J.
    Journal of Thermal Spray Technology 29 368-383 (2020)
    Abstract: Synthesis and characterization of FexOy nanoparticles were carried out in order to study reaction parameters influence in a spray flame reactor. FexOy powders were prepared with three different precursors aiming to understand how the reactor conditions, dispersion gas flow, and precursor solution flow affect morphology, shape, particle size distribution, crystalline phases, and residue content of the obtained materials. Thermogravimetric analysis, scanning electron microscopy, transmission electron microscopy (TEM), x-ray diffraction (XRD), and Raman spectroscopy were employed to characterize the materials. In addition, magnetic behavior of the obtained samples was evaluated. It was found that the evaluated parameters influenced the residue contents obtaining weight changes from 10 to 35%. Particle size distribution centers also showed differences between 17 and 24 nm. By XRD, Raman, and TEM, the presence of hematite (a-Fe2O3), maghemite (γ-Fe2O3), and magnetite (Fe3O4) was evidenced and explained based on the gas and liquid content in the flame. Additionally, the saturation magnetization was measured for selected samples, obtaining values between 26 and 32 emu g−1. These magnetic measurements were correlated with the crystalline phase composition and particle size distributions. Graphic Abstract: [Figure not available: see fulltext.] © 2020, ASM International.
    view abstractdoi: 10.1007/s11666-020-00991-1
  • 2020 • 581 Could face-centered cubic titanium in cold-rolled commercially-pure titanium only be a Ti-hydride?
    Chang, Y. and Zhang, S. and Liebscher, C.H. and Dye, D. and Ponge, D. and Scheu, C. and Dehm, G. and Raabe, D. and Gault, B. and Lu, W.
    Scripta Materialia 178 39-43 (2020)
    A face-centered cubic (FCC) phase in electro-polished specimens for transmission electron microscopy of commercially pure titanium has sometimes been reported. Here, a combination of atom-probe tomography, scanning transmission electron microscopy and low-loss electron energy loss spectroscopy is employed to study both the crystal structural and chemical composition of this FCC phase. Our results prove that the FCC phase is actually a TiHx (x ≥ 1) hydride, and not a new allotrope of Ti, in agreement with previous reports. The formation of the hydride is discussed. © 2019 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.scriptamat.2019.11.010
  • 2020 • 580 Sintering and biocompatibility of blended elemental Ti-xNb alloys
    Chen, Y. and Han, P. and Dehghan-Manshadi, A. and Kent, D. and Ehtemam-Haghighi, S. and Jowers, C. and Bermingham, M. and Li, T. and Cooper-White, J. and Dargusch, M.S.
    Journal of the Mechanical Behavior of Biomedical Materials 104 (2020)
    Titanium-niobium (Ti–Nb) alloys have great potential for biomedical applications due to their superior biocompatibility and mechanical properties that match closely to human bone. Powder metallurgy is an ideal technology for efficient manufacture of titanium alloys to generate net-shape, intricately featured and porous components. This work reports on the effects of Nb concentrations on sintered Ti-xNb alloys with the aim to establish an optimal composition in respect to mechanical and biological performances. Ti-xNb alloys with 33, 40, 56 and 66 wt% Nb were fabricated from elemental powders and the sintering response, mechanical properties, microstructures and biocompatibility assessed and compared to conventional commercial purity titanium (CPTi). The sintered densities for all Ti-xNb compositions were around 95%, reducing slightly with increasing Nb due to increasing open porosity. Higher Nb levels retarded sintering leading to more inhomogeneous phase and pore distributions. The compressive strength decreased with increasing Nb, while all Ti-xNb alloys displayed higher strengths than CPTi except the Ti–66Nb alloy. The Young's moduli of the Ti-xNb alloys with ≥40 wt% Nb were substantially lower (30–50%) than CPTi. In-vitro cell culture testing revealed excellent biocompatibility for all Ti-xNb alloys comparable or better than tissue culture plate and CPTi controls, with the Ti–40Nb alloy exhibiting superior cell-material interactions. In view of its mechanical and biological performance, the Ti–40Nb composition is most promising for hard tissue engineering applications. © 2020
    view abstractdoi: 10.1016/j.jmbbm.2020.103691
  • 2020 • 579 Tailored CNTs buckypaper membranes for the removal of humic acid and separation of oil-in- water emulsions
    Elnabawy, E. and Elsherbiny, I.M.A. and Abdelsamad, A.M.A. and Anis, B. and Hassan, A. and Ulbricht, M. and Khalil, A.S.G.
    Membranes 10 (2020)
    Carbon nanotubes (CNTs) are a robust material and proven as a promising candidate for a wide range of electronic, optoelectronic and environmental applications. In this work, two different methods were utilized for the preparation of CNTs exhibiting different aspect ratios via chemical vapor deposition (CVD). The as-prepared CNTs were analyzed using scanning electron microscopy (SEM), transmission electron microscopy (TEM), N2 adsorption isotherms, thermogravimetric analysis and Raman spectroscopy in order to investigate their morphological and structural properties. Free-standing CNTs “buckypaper” membranes were fabricated, characterized and tailored to meet the requirements of two applications, i.e., (1) the removal of humic acid (HA) from water and (2) separation of oil-in-water emulsions. It was revealed that the hydrophobic buckypapers showed high separation performance for Shell oil-in-water emulsions filtration, with up to 98% through the accumulation of oil droplets onto the membrane surface. The absorption capacity of buckypaper membranes for various organic liquids (oil, chloroform and toluene) was evaluated over 10 absorption cycles to investigate their recyclability and robustness. Moreover, surface modification was introduced to the pristine CNTs to increase their surface hydrophilicity and improve the pure water permeability of buckypapers. These modified buckypapers showed high flux for HA solutions and excellent HA rejection efficiency up to 95%via size exclusion and electrostatic repulsion mechanisms. © 2020 by the authors. Licensee MDPI, Basel, Switzerland.
    view abstractdoi: 10.3390/membranes10050097
  • 2020 • 578 Image-based size analysis of agglomerated and partially sintered particles via convolutional neural networks
    Frei, M. and Kruis, F.E.
    Powder Technology 360 324-336 (2020)
    There is a high demand for fully automated methods for the analysis of primary particle size distributions of agglomerated, sintered or occluded primary particles, due to their impact on material properties. Therefore, a novel, deep learning-based, method for the detection of such primary particles was proposed and tested, which renders a manual tuning of analysis parameters unnecessary. As a specialty, the training of the utilized convolutional neural networks was carried out using only synthetic images, thereby avoiding the laborious task of manual annotation and increasing the ground truth quality. Nevertheless, the proposed method performs excellent on real world samples of sintered silica nanoparticles with various sintering degrees and varying image conditions. In a direct comparison, the proposed method clearly outperforms two state-of-the-art methods for automated image-based particle size analysis (Hough transformation and the ImageJ ParticleSizer plug-in), thereby attaining human-like performance. © 2019 Elsevier B.V.
    view abstractdoi: 10.1016/j.powtec.2019.10.020
  • 2020 • 577 Bulk nanostructured AlCoCrFeMnNi chemically complex alloy synthesized by laser-powder bed fusion
    Jung, H.Y. and Peter, N.J. and Gärtner, E. and Dehm, G. and Uhlenwinkel, V. and Jägle, E.A.
    Additive Manufacturing 35 (2020)
    We report the synthesis of a bulk nanostructured alloy using laser-powder bed fusion. The equiatomic AlCoCrFeMnNi chemically complex alloy forms a nanoscale modulated structure, which is homogeneously distributed in the as-built condition. The nanostructure consists of Al & Ni-rich ordered and Cr & Fe-rich disordered BCC phases. The two phases form an interconnected phase network with coherent interface boundaries. Atom probe tomography and aberration-corrected scanning transmission electron microscopy analysis of the spatial distribution of the modulated structure suggests the occurrence of nano-scale spinodal decomposition. These results introduce a direct synthesis of bulk nanostructured alloys with promising geometric flexibility. © 2020 Elsevier B.V.
    view abstractdoi: 10.1016/j.addma.2020.101337
  • 2020 • 576 The impact of post manufacturing treatment of functionally graded Ti6Al4V scaffolds on their surface morphology and mechanical strength
    Khrapov, D. and Koptyug, A. and Manabaev, K. and Léonard, F. and Mishurova, T. and Bruno, G. and Cheneler, D. and Loza, K. and Epple, M. and Surmenev, R. and Surmeneva, M.
    Journal of Materials Research and Technology 9 1866-1881 (2020)
    An ultrasonic vibration post-treatment procedure was suggested for additively manufactured lattices. The aim of the present research was to investigate mechanical properties and the differences in mechanical behavior and fracture modes of Ti6Al4V scaffolds treated with traditional powder recovery system (PRS) and ultrasound vibration (USV). Scanning electron microscopy (SEM) was used to investigate the strut surface and the fracture surface morphology. X-ray computed tomography (CT) was employed to evaluate the inner structure, strut dimensions, pore size, as well as the surface morphology of additively manufactured porous scaffolds. Uniaxial compression tests were conducted to obtain elastic modulus, compressive ultimate strength and yield stress. Finite element analysis was performed for a body-centered cubic (BCC) element-based model and for CT-based reconstruction data, as well as for a two-zone scaffold model to evaluate stress distribution during elastic deformation. The scaffold with PRS post treatment displayed ductile behavior, while USV treated scaffold displayed fragile behavior. Double barrel formation of PRS treated scaffold was observed during deformation. Finite element analysis for the CT-based reconstruction revealed the strong impact of surface morphology on the stress distribution in comparison with BCC cell model because of partially molten metal particles on the surface of struts, which usually remain unstressed. © 2019 The Authors.
    view abstractdoi: 10.1016/j.jmrt.2019.12.019
  • 2020 • 575 In vivo biodistribution of calcium phosphate nanoparticles after intravascular, intramuscular, intratumoral, and soft tissue administration in mice investigated by small animal PET/CT
    Kollenda, S.A. and Klose, J. and Knuschke, T. and Sokolova, V. and Schmitz, J. and Staniszewska, M. and Costa, P.F. and Herrmann, K. and Westendorf, A.M. and Fendler, W.P. and Epple, M.
    Acta Biomaterialia 109 244-253 (2020)
    Calcium phosphate nanoparticles were covalently surface-functionalized with the ligand DOTA and loaded with the radioisotope 68Ga. The biodistribution of such 68Ga-labelled nanoparticles was followed in vivo in mice by positron emission tomography in combination with computer tomography (PET-CT). The biodistribution of 68Ga-labelled nanoparticles was compared for different application routes: intravenous, intramuscular, intratumoral, and into soft tissue. The particle distribution was measured in vivo by PET-CT after 5 min, 15 min, 30 min, 1 h, 2 h, and 4 h, and ex vivo after 5 h. After intravenous injection (tail vein), the nanoparticles rapidly entered the lungs with later redistribution into liver and spleen. The nanoparticles remained mostly at the injection site following intramuscular, intratumoral, or soft tissue application, with less than 10 percent being mobilized into the blood stream. Statement of Significance: The in vivo biodistribution of DOTA-terminated calcium phosphate nanoparticles was followed by PET/CT. To our knowledge, this is the first study of this kind. Four different application routes of clinical relevance were pursued: Intravascular, intramuscular, intratumoral, and into soft tissue. Given the high importance of calcium phosphate as biomaterial and for nanoparticular drug delivery and immunization, this is most important to assess the biofate of calcium phosphate nanoparticles for therapeutic application and also judge biodistribution of nanoscopic calcium phosphate ceramics, including debris from endoprostheses and related implants. © 2020
    view abstractdoi: 10.1016/j.actbio.2020.03.031
  • 2020 • 574 Irreversible Structural Changes of Copper Hexacyanoferrate Used as a Cathode in Zn-Ion Batteries
    Lim, J. and Kasiri, G. and Sahu, R. and Schweinar, K. and Hengge, K. and Raabe, D. and La Mantia, F. and Scheu, C.
    Chemistry - A European Journal 26 4917-4922 (2020)
    The structural changes of copper hexacyanoferrate (CuHCF), a Prussian blue analogue, which occur when used as a cathode in an aqueous Zn-ion battery, are investigated using electron microscopy techniques. The evolution of ZnxCu1−xHCF phases possessing wire and cubic morphologies from initial CuHCF nanoparticles are monitored after hundreds of cycles. Irreversible introduction of Zn ions to CuHCF is revealed locally using scanning transmission electron microscopy. A substitution mechanism is proposed to explain the increasing Zn content within the cathode material while simultaneously the Cu content is lowered during Zn-ion battery cycling. The present study demonstrates that the irreversible introduction of Zn ions is responsible for the decreasing Zn ion capacity of the CuHCF cathode in high electrolyte concentration. © 2019 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.
    view abstractdoi: 10.1002/chem.201905384
  • 2020 • 573 Coulometric Titration of Active Sites at Mesostructured Cobalt Oxide Spinel by Surface Interrogation Mode of Scanning Electrochemical Microscopy
    Lorenz, J. and Yu, M. and Tuÿsüz, H. and Harms, C. and Dyck, A. and Wittstock, G.
    Journal of Physical Chemistry C 124 7737-7748 (2020)
    Cobalt-based transition-metal oxides are promising candidates for the oxygen evolution reaction (OER). However, a complex interplay between the catalyst crystal structures and material morphologies as well as the surface reactions hampers a comprehensive understanding of the electrocatalytic OER at those materials. Here, we investigate the amount and reactivity of specific surface sites of a mesostructured cobalt oxide spinel powder by surface interrogation mode of scanning electrochemical microscopy (SI-SECM). The powder material was supplied in cavity microelectrodes and efficiently titrated with an Fe(II)-triethanolamine redox mediator generated at a gold microelectrode in an alkaline electrolyte. Thus, quantification of different surface sites was achieved, and their reactivity showed dependence on the cobalt oxidation state. Titration experiments after adjustable time delays with respect to the generation of the different surface sites indicated that these surface sites are active for the OER. Kinetic analysis revealed two pseudo-first-order decay constants that were related to fast and slow surface sites for the OER. Rate constants were determined for potentials where predominantly a mixed-valence CoIII/IV state might be present as the most active species. These results expand the great potential of the surface interrogation mode on studying the reaction kinetics of distinct surface sites for practically relevant powdered, nonprecious metal catalysts to address a highly relevant challenge in electrocatalysis. Copyright © 2020 American Chemical Society.
    view abstractdoi: 10.1021/acs.jpcc.9b11114
  • 2020 • 572 Tantalum and zirconium induced structural transitions at complex [111] tilt grain boundaries in copper
    Meiners, T. and Duarte, J.M. and Richter, G. and Dehm, G. and Liebscher, C.H.
    Acta Materialia 190 93-104 (2020)
    Alloying nanocrystalline copper (Cu) with immiscible elements, such as tantalum (Ta) and zirconium (Zr), is a promising technique to manipulate grain boundary properties and by this suppress grain growth at elevated temperatures. However, insights on the atomistic origins on the influence of impurity elements on grain boundaries are lacking. In this study, the atomistic effects of Ta and Zr on [111] tilt grain boundaries in Cu are investigated by high resolution scanning transmission electron microscopy techniques. In case of Ta, the formation of spherical, nano-scale precipitates in close vicinity to the grain boundaries is observed, but no sign of segregation. The particles induce a repelling force to migrating boundaries and act as local pinning points. The segregation of Zr is observed to occur either at confined grain boundary steps or homogeneously along the boundaries without steps. In both cases a strong disordering of the defect or grain boundary structure is revealed. Furthermore, at low Zr concentrations it induces structural grain boundary transitions and partial atomic reordering of the grain boundary structural units. © 2020 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2020.02.064
  • 2020 • 571 Forming amorphous calcium carbonate within hydrogels by enzyme-induced mineralization in the presence of N-(phosphonomethyl)glycine
    Milovanovic, M. and Unruh, M.T. and Brandt, V. and Tiller, J.C.
    Journal of Colloid and Interface Science 579 357-368 (2020)
    Amorphous inorganic materials have a great potential in material science. Amorphous calcium carbonate (ACC) is a widely useable system, however, its stabilization often turns out to be difficult and the synthesis is mostly limited to precipitation in solution as nanoparticles. Stable ACC in bulk phases would create new composite materials. Previous work described the enzyme-induced mineralization of hydrogels with crystalline calcium carbonate by entrapping urease into a hydrogel and treating this with an aqueous mineralization solution containing urea und calcium chloride. Here, this method was modified using a variety of crystallization inhibitors attached to the hydrogel matrix or added to the surrounding mineralization solution. It was found that only N-(phosphonomethyl)glycine (PMGly) in solution completely inhibits the crystallization of ACC in the hydrogel matrix. The stability of the homogeneously precipitated ACC could be accounted to the combination of stabilizing effects of the additive and stabilization through confinement. The crystallization could be accelerated at higher temperatures up to 60 °C. Here, a combination of Mg ions and PMGly was required to stabilize ACC in the hydrogel. Variation of these two compounds can be used to control a number of different calcium carbonate morphologies within the hydrogel. While the ACC nanoparticles within the hydrogel are stable over weeks even in water, a calcite layer grows on the surface of the hydrogel, which might be used as self-hardening mechanism of a surface. © 2020 Elsevier Inc.
    view abstractdoi: 10.1016/j.jcis.2020.06.047
  • 2020 • 570 Early stage phase separation of AlCoCr0.75Cu0.5FeNi high-entropy powder at the nanoscale
    Peter, N.J. and Duarte, M.J. and Liebscher, C.H. and Srivastava, V.C. and Uhlenwinkel, V. and Jägle, E.A. and Dehm, G.
    Journal of Alloys and Compounds 820 (2020)
    High entropy alloys are generally considered to be single phase material. This state is, however, typically a non-equilibrium state after fabrication at high cooling rates. Phase constitution after fabrication or heat treatment is mostly known for isothermal annealing only and for casts as well as rapidly quenched alloys. Knowledge on early phase separation stages of high entropy alloys and their mechanisms are missing so far. Here, we present results on phase separation at intermediate cooling rates, by characterization of gas atomized powder of the AlCoCr0.75Cu0.5FeNi alloy. Although investigation by X-ray diffraction and Electron Backscatter Diffraction indicates a single-phase nature of the powder particles, aberration-corrected scanning transmission electron microscopy and atom probe tomography reveal a nanoscale phase separation into Ni–Al-rich B2 and Fe–Cr-rich A2 regions as well as a high number density of 3.1 × 1024 Cu-rich clusters per m3 in the B2 matrix. The observed phase separation and cluster formation are linked to spinodal decomposition and nucleation processes, respectively. The study highlights that adequate characterization techniques need to be chosen when making statements about phase stability and structural evolution in compositionally complex alloys. © 2019 The Authors
    view abstractdoi: 10.1016/j.jallcom.2019.153149
  • 2020 • 569 One-step preparation of antifouling polysulfone ultrafiltration membranes via modification by a cationic polyelectrolyte based on polyacrylamide
    Plisko, T.V. and Bildyukevich, A.V. and Burts, K.S. and Ermakov, S.S. and Penkova, A.V. and Kuzminova, A.I. and Dmitrenko, M.E. and Hliavitskaya, T.A. and Ulbricht, M.
    Polymers 12 (2020)
    A novel method for one-step preparation of antifouling ultrafiltration membranes via a non-solvent induced phase separation (NIPS) technique is proposed. It involves using aqueous 0.05-0.3 wt. % solutions of cationic polyelectrolyte based on a copolymer of acrylamide and 2-acryloxyethyltrimethylammonium chloride (Praestol 859) as a coagulant in NIPS. Asystematic study of the effect of the cationic polyelectrolyte addition to the coagulant on the structure, performance and antifouling stability of polysulfone membranes was carried out. The methods for membrane characterization involved scanning electron microscopy (SEM), atomic force microscopy (AFM), Fourier transform infrared spectroscopy (FTIR), contact angle and zeta-potential measurements and evaluation of the permeability, rejection and antifouling performance in human serum albumin solution and surface water ultrafiltration. It was revealed that in the presence of cationic polyelectrolyte in the coagulation bath, its concentration has a major influence on the rate of "solvent-non-solvent" exchange and thus also on the rate of phase separation which significantly affects membrane structure. The immobilization of cationic polyelectrolyte macromolecules into the selective layer was confirmed by FTIR spectroscopy. It was revealed that polyelectrolyte macromolecules predominately immobilize on the surface of the selective layer and not on the bottom layer. Membrane modification was found to improve the hydrophilicity of the selective layer, to increase surface roughness and to change zeta-potential which yields the substantial improvement of membrane antifouling stability toward natural organic matter and human serum albumin. © 2020 by the authors. Licensee MDPI, Basel, Switzerland.
    view abstractdoi: 10.3390/POLYM12051017
  • 2020 • 568 Orientation-dependent deformation behavior of 316L steel manufactured by laser metal deposition and casting under local scratch and indentation load
    Pöhl, F. and Hardes, C. and Scholz, F. and Frenzel, J.
    Materials 13 (2020)
    This study analyzes the local deformation behavior of austenitic stainless steel 316L, manufactured conventionally by casting and additively by laser metal deposition (LMD). We produced directionally solidified 316L specimens with most grains showing (001) orientations parallel to the longitudinal specimen axis. We conducted nanoindentation and scratch experiments for local mechanical characterization and topography measurements (atomic force microscopy and confocal laser scanning microscopy) of indentation imprints and residual scratch grooves for the analysis of the deformation behavior and, in particular, of the pile-up behavior. The local mechanical properties and deformation behavior were correlated to the local microstructure investigated by scanning electron microscopy with energy dispersive X-ray spectroscopy and electron backscatter diffraction analysis. The results show that the local mechanical properties, deformation behavior, and scratch resistance strongly depend on the crystallographic orientation. Nearly (001)-oriented grains parallel to the surface show the lowest hardness, followed by an increasing hardness of nearly (101)-and (111)-oriented grains. Consequently, scratch depth is the greatest for nearly (001)-oriented grains followed by (101) and (111) orientations. This tendency is seen independently of the analyzed manufacturing route, namely Bridgman solidification and laser metal deposition. In general, the laser metal deposition process leads to a higher strength and hardness, which is mainly attributed to a higher dislocation density. Under the investigated loading conditions, the cellular segregation substructure is not found to significantly and directly change the local deformation behavior during indentation and scratch testing. © 2020 by the authors.
    view abstractdoi: 10.3390/MA13071765
  • 2020 • 567 Temperature-Induced Stress Relaxation in Alloyed Silver-Gold Nanoparticles (7-8 nm) by in Situ X-ray Powder Diffraction
    Prymak, O. and Grasmik, V. and Loza, K. and Heggen, M. and Epple, M.
    Crystal Growth and Design 20 107-115 (2020)
    Alloyed silver-gold nanoparticles (spherical, 8 nm) were wet-chemically prepared by reduction with sodium citrate/tannic acid and colloidally stabilized by poly(N-vinylpyrrolidone) (PVP), in steps of 10 atom %, including pure silver nanoparticles (35 nm) and pure gold nanoparticles (7 nm). The nanoparticles were subjected to in situ X-ray powder diffraction up to 850 °C to induce internal stress relaxation and recrystallization. The stress-induced negative deviation from Vegard's rule that was present in the original alloyed nanoparticles vanished between 150 and 250 °C, indicating the internal healing of defects. Simultaneously, a discontinuous increase in the crystallite size and a drop in the microstrain were observed. After heating to 850 °C, the original gradient structure (silver-rich shell, gold-rich core) had changed to a homogeneous elemental distribution as shown by high-angle annular dark-field scanning transmission electron microscopy/energy-dispersive X-ray spectroscopy (STEM/EDX). Thus, there is a considerable mobility of the metal atoms inside the nanoparticles that starts as low as 150 °C. © 2019 American Chemical Society.
    view abstractdoi: 10.1021/acs.cgd.9b00728
  • 2020 • 566 Microstructure and mechanical properties of 316L austenitic stainless steel processed by different SLM devices
    Röttger, A. and Boes, J. and Theisen, W. and Thiele, M. and Esen, C. and Edelmann, A. and Hellmann, R.
    International Journal of Advanced Manufacturing Technology 108 769-783 (2020)
    In this work, we examined the influence of different types of selective laser melting (SLM) devices on the microstructure and the associated material properties of austenitic 316L stainless steel. Specimens were built using powder from the same powder batch on four different SLM machines. For the specimen build-up, optimized parameter sets were used, as provided by the manufacturers for each individual SLM machine. The resulting microstructure was investigated by means of scanning electron microscopy, which revealed that the different samples possess similar microstructures. Differences between the microstructures were found in terms of porosity, which significantly influences the material properties. Additionally, the build-up direction of the specimens was found to have a strong influence on the mechanical properties. Thus, the defect density defines the material’s properties so that the ascertained characteristic values were used to determine a Weibull modulus for the corresponding values in dependence on the build-up direction. Based on these findings, characteristic averages of the mechanical properties were determined for the SLM-manufactured samples, which can subsequently be used as reference parameters for designing industrially manufactured components. © 2020, The Author(s).
    view abstractdoi: 10.1007/s00170-020-05371-1
  • 2020 • 565 Synthesis of plasmonic Fe/Al nanoparticles in ionic liquids
    Schmitz, A. and Meyer, H. and Meischein, M. and Garzón Manjón, A. and Schmolke, L. and Giesen, B. and Schlüsener, C. and Simon, P. and Grin, Y. and Fischer, R.A. and Scheu, C. and Ludwig, Al. and Janiak, C.
    RSC Advances 10 12891-12899 (2020)
    Bottom-up and top-down approaches are described for the challenging synthesis of Fe/Al nanoparticles (NPs) in ionic liquids (ILs) under mild conditions. The crystalline phase and morphology of the metal nanoparticles synthesized in three different ionic liquids were identified by powder X-ray diffractometry (PXRD), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), selected-area electron diffraction (SAED) and fast Fourier transform (FFT) of high-resolution TEM images. Characterization was completed by scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDX) for the analysis of the element composition of the whole sample consisting of the NPs and the amorphous background. The bottom-up approaches resulted in crystalline FeAl NPs on an amorphous background. The top-down approach revealed small NPs and could be identified as Fe4Al13 NPs which in the IL [OPy][NTf2] yield two absorption bands in the green-blue to green spectral region at 475 and 520 nm which give rise to a complementary red color, akin to appropriate Au NPs. © 2020 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/d0ra01111h
  • 2020 • 564 Analysis of strengthening due to grain boundaries and annealing twin boundaries in the CrCoNi medium-entropy alloy
    Schneider, M. and George, E.P. and Manescau, T.J. and Záležák, T. and Hunfeld, J. and Dlouhý, A. and Eggeler, G. and Laplanche, G.
    International Journal of Plasticity 124 155-169 (2020)
    CrCoNi exhibits the best combination of strength and ductility among all the equiatomic single-phase FCC subsets of the CrMnFeCoNi high-entropy alloy. Here, its yield strength was determined in compression as a function of grain size and temperature. Yield strength was also plotted as a function of "crystallite" size, which takes into account both annealing twin boundaries and grain boundaries. The resulting Hall-Petch slopes were straight lines but with different slopes that depend on the number of twin boundaries per grain. Scanning transmission electron microscopy of deformed specimens revealed the formation of dislocation pile-ups at grain and annealing twin boundaries indicating that the latter also act as obstacles to slip and contribute to strength. Using a simple pile-up model, the strengths of the grain and twin boundaries were estimated to lie in the range 900-1250 »MPa. Assuming that they have the same strength, in the case of twin boundaries this strength corresponds roughly to the stress required to constrict Shockley partials, which suggests that dissociated dislocations have to become compact before they can cross the annealing twin boundaries. © 2019 The Authors.
    view abstractdoi: 10.1016/j.ijplas.2019.08.009
  • 2020 • 563 In situ characterization of damage development in cottonid due to quasi-static tensile loading
    Scholz, R. and Delp, A. and Walther, F.
    Materials 13 (2020)
    Cottonid is a layered material based 100% on cellulose that holds excellent material properties by being completely sustainable. The finite nature of petroleum-based resources nowadays makes these properties significant for technical applications again. To understand how Cottonid reacts to application-oriented mechanical loads and how it fails, development of microstructural damage on the surface and in the volume of Cottonid was studied using innovative in situ testing techniques for the first time. Quasi-static tensile tests were comparatively performed in a scanning electron microscope as well as a microfocus computer tomograph, and the development of defects present in the initial condition of the material was investigated. In the elastic region, no visible damage initiation on the surface and a decrease of overall void volume within the gauge length could be detected. When reaching the yield strength, crack initiation on the surface starts at critical areas, like pores and microcracks, which propagation and assembly could be visualized via scanning electron micrographs. In the plastic region, an increase in void volume could be shown in the gauge length until final failure of the specimen. Innovative material testing techniques presented in this study support lifetime estimation in technical applications and understanding of process-structure-property relations. Particularly, characterization of microstructural damage development due to a mechanical load, which leads to final failure of the specimen, is essential to be able to create material models for lifetime prediction in respect to variable manufacturing or application parameters. © 2020 by the authors.
    view abstractdoi: 10.3390/ma13092180
  • 2020 • 562 Probing catalytic surfaces by correlative scanning photoemission electron microscopy and atom probe tomography
    Schweinar, K. and Nicholls, R.L. and Rajamathi, C.R. and Zeller, P. and Amati, M. and Gregoratti, L. and Raabe, D. and Greiner, M. and Gault, B. and Kasian, O.
    Journal of Materials Chemistry A 8 388-400 (2020)
    The chemical composition and the electronic state of the surface of alloys or mixed oxides with enhanced electrocatalytic properties are usually heterogeneous at the nanoscale. The non-uniform distribution of the potential across their surface affects both activity and stability. Studying such heterogeneities at the relevant length scale is crucial for understanding the relationships between structure and catalytic behaviour. Here, we demonstrate an experimental approach combining scanning photoemission electron microscopy and atom probe tomography performed at identical locations to characterise the surface's structure and oxidation states, and the chemical composition of the surface and sub-surface regions. Showcased on an Ir-Ru thermally grown oxide, an efficient catalyst for the anodic oxygen evolution reaction, the complementary techniques yield consistent results in terms of the determined surface oxidation states and local oxide stoichiometry. Significant chemical heterogeneities in the sputter-deposited Ir-Ru alloy thin films govern the oxide's chemistry, observed after thermal oxidation both laterally and vertically. While the oxide grains have a composition of Ir0.94Ru0.06O2, the composition in the grain boundary region varies from Ir0.70Ru0.30O2 to Ir0.40Ru0.60O2 and eventually to Ir0.75Ru0.25O2 from the top surface into the depth. The influence of such compositional non-uniformities on the catalytic performance of the material is discussed, along with possible engineering levers for the synthesis of more stable and reactive mixed oxides. The proposed method provides a framework for investigating materials of interest in the field of electrocatalysis and beyond. This journal is © The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c9ta10818a
  • 2020 • 561 Thermal treatment of lignin, cellulose and hemicellulose in nitrogen and carbon dioxide
    Senneca, O. and Cerciello, F. and Russo, C. and Wütscher, A. and Muhler, M. and Apicella, B.
    Fuel 271 (2020)
    The paper explores the primary products from fast pyrolysis of biomass components: Lignin, Cellulose and Hemicellulose (Xylan). A heated strip reactor is employed at temperatures of 1573 K and 2073 K with N2 and CO2 atmospheres. Volatiles quench immediately after volatilization on a cold pyrex bridge, while char remains on the heated strip for 3 s. Tar, soot and char are collected and subject to chemical treatments and analyses, including gas chromatography-mass spectrometry and Size Exclusion Chromatography, Thermogravimetric analysis, Raman spectroscopy and Scanning Electron Microscopy. Fast pyrolysis of Lignin produces “Light tar” (soluble in acetone) and “Heavy tar” (soluble in NMP), char, a minor fraction of soot. The “Light tar” contains Vanillin, which can be considered the main primary depolymerization product, but also aliphatics and PAHs. Higher temperature enhances “Heavy tar” and graphitization of the char. Cellulose at 1573 K produces only “Light tar”, largely made of Levoglucosan, as the result of depolymerization. At higher temperature the tar becomes heavier. Hemicellulose has a peculiar behavior: it produces a “Light tar” which is chemically similar to that of Cellulose and, at high temperature also “Heavy tar”. Hemicellulose pyrolysis results also in the production of an atypical solid residue: swollen ad spongy at lower temperature, bright and glassy at higher temperature. CO2 affects the pyrolysis products, particularly those of Lignin, promoting tar cracking and oxygenation already at the stage of primary pyrolysis and hindering thermal annealing and structural ordering of the solid carbonaceous structure. © 2020 Elsevier Ltd
    view abstractdoi: 10.1016/j.fuel.2020.117656
  • 2020 • 560 A combined experimental and modelling approach for the Weimberg pathway optimisation
    Shen, L. and Kohlhaas, M. and Enoki, J. and Meier, R. and Schönenberger, B. and Wohlgemuth, R. and Kourist, R. and Niemeyer, F. and van Niekerk, D. and Bräsen, C. and Niemeyer, J. and Snoep, J. and Siebers, B.
    Nature Communications 11 (2020)
    The oxidative Weimberg pathway for the five-step pentose degradation to α-ketoglutarate is a key route for sustainable bioconversion of lignocellulosic biomass to added-value products and biofuels. The oxidative pathway from Caulobacter crescentus has been employed in in-vivo metabolic engineering with intact cells and in in-vitro enzyme cascades. The performance of such engineering approaches is often hampered by systems complexity, caused by non-linear kinetics and allosteric regulatory mechanisms. Here we report an iterative approach to construct and validate a quantitative model for the Weimberg pathway. Two sensitive points in pathway performance have been identified as follows: (1) product inhibition of the dehydrogenases (particularly in the absence of an efficient NAD+ recycling mechanism) and (2) balancing the activities of the dehydratases. The resulting model is utilized to design enzyme cascades for optimized conversion and to analyse pathway performance in C. cresensus cell-free extracts. © 2020, The Author(s).
    view abstractdoi: 10.1038/s41467-020-14830-y
  • 2020 • 559 Influence of the cathode microstructure on the stability of inverted planar perovskite solar cells
    Sirotinskaya, S. and Schmechel, R. and Benson, N.
    RSC Advances 10 23653-23661 (2020)
    One of the main challenges for perovskite solar cells (PSC) is their environmental stability, as oxygen and water induced aging may result in mobile decomposition compounds, which can enhance the recombination rate and react with charge carrier extraction layers or the contact metallization. In this contribution the importance of the microstructure of the contact metallization on the environmental cell stability is investigated. For this purpose, the storage stability of inverted planar methylammonium lead iodide (MAPI)-based perovskite solar cells without encapsulation is tested, using the metals aluminum (Al), silver (Ag), gold (Au) and nickel (Ni) as representative cathode materials. For this study, scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX) analysis of the different electrodes as well as the perovskite is correlated with PSC device current-voltage (J-V) and impedance measurements. Our findings substantiate that the metal microstructure has a significant influence on the PSC aging properties. While a strong perovskite decomposition and iodide diffusion to the contacts were detected for devices using Al, Ag or Au cathodes with a polycrystalline microstructure, these effects were strongly reduced when Ni metallization was employed, where a nanocrystalline microstructure was exhibited under the chosen process conditions. This journal is © The Royal Society of Chemistry.
    view abstractdoi: 10.1039/d0ra00195c
  • 2020 • 558 Atomic scale configuration of planar defects in the Nb-rich C14 Laves phase NbFe2
    Šlapáková, M. and Zendegani, A. and Liebscher, C.H. and Hickel, T. and Neugebauer, J. and Hammerschmidt, T. and Ormeci, A. and Grin, J. and Dehm, G. and Kumar, K.S. and Stein, F.
    Acta Materialia 183 362-376 (2020)
    Laves phases belong to the group of tetrahedrally close-packed intermetallic phases, and their crystal structure can be described by discrete layer arrangements. They often possess extended homogeneity ranges and the general notion is that deviations from stoichiometry are accommodated by anti-site atoms or vacancies. The present work shows that excess Nb atoms in a Nb-rich NbFe2 C14 Laves phase can also be incorporated in various types of planar defects. Aberration-corrected scanning transmission electron microscopy and density functional theory calculations are employed to characterize the atomic configuration of these defects and to establish stability criteria for them. The planar defects can be categorized as extended or confined ones. The extended defects lie parallel to the basal plane of the surrounding C14 Laves phase and are fully coherent. They contain the characteristic Zr4Al3-type (O) units found in the neighboring Nb6Fe7 µ phase. An analysis of the chemical bonding reveals that the local reduction of the charge transfer is a possible reason for the preference of this atomic arrangement. However, the overall layer stacking deviates from that of the perfect µ phase. The ab initio calculations establish why these exceptionally layered defects can be more stable configurations than coherent nano-precipitates of the perfect µ phase. The confined defects are observed with pyramidal and basal habit planes. The pyramidal defect is only ~1 nm thick and resembles the perfect µ phase. In contrast, the confined basal defect can be regarded as only one single O unit and it appears as if the stacking sequence is disrupted. This configuration is confirmed by ab initio calculations to be metastable. © 2019
    view abstractdoi: 10.1016/j.actamat.2019.11.004
  • 2020 • 557 Surface Properties of Battery Materials Elucidated Using Scanning Electrochemical Microscopy: The Case of Type I Silicon Clathrate
    Tarnev, T. and Wilde, P. and Dopilka, A. and Schuhmann, W. and Chan, C.K. and Ventosa, E.
    ChemElectroChem 7 665-671 (2020)
    Silicon clathrates have attracted interest as potential anodes for lithium-ion batteries with unique framework structures. However, very little is known about the surface reactivity and solid electrolyte interphase (SEI) properties of clathrates. In this study, operando scanning electrochemical microscopy (SECM) is used to investigate the effect of pre-treatment on the formation dynamics and intrinsic properties of the SEI in electrodes prepared from type I Ba8Al16Si30 silicon clathrates. Although X-ray photoelectron spectroscopy (XPS) analysis does not reveal large changes in SEI composition, it is found through SECM measurements that ball-milling combined with chemical acid/base etching of the clathrates lead to a more stable and rapidly formed SEI as compared to purely ball-milled samples, resulting in enhanced coulombic efficiency. © 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/celc.201901688
  • 2020 • 556 Characterization of damage evolution on hot flat rolled mild steel sheets by means of micromagnetic parameters and fatigue strength determination
    Teschke, M. and Vasquez, J.R. and Lücker, L. and Walther, F.
    Materials 13 (2020)
    In continuous casting processes, inevitable voids (damage) are generated inside the material. The subsequent forming process of hot flat rolling offers the potential of healing these defects by closing the voids and bonding the internal surfaces. In this paper, different forming conditions from hot flat rolling process were characterized with micromagnetic measurement techniques and the influence of the damage evolution on the fatigue behavior was investigated. To characterize the reduction of voids through hot flat rolling processes, nondestructive testing techniques are required. Therefore, micromagnetic measurements such as Barkhausen noise, incremental permeability, and harmonic analysis were carried out, correlated with the number of voids, and compared with each other. The influence of damage evolution of different forming conditions on the fatigue behavior was characterized based on instrumented constant amplitude and multiple amplitude (load increase) tests. A significant increase in fatigue strength due to the hot flat rolling process, which leads to a reduction in the number of voids, was observed. In addition, the fracture surfaces of the specimens were analyzed in the scanning electron microscope. © 2020 by the authors.
    view abstractdoi: 10.3390/ma13112486
  • 2020 • 555 The effect of metal-oxide incorporation on the morphology of carbon nanostructures
    Tigges, S. and Wöhrl, N. and Hagemann, U. and Ney, M. and Lorke, A.
    Journal of Physics D: Applied Physics 53 (2020)
    Metal-organic, single-source, low-temperature, morphology-controlled growth of carbon nanostructures is achieved, using an inductively coupled plasma-enhanced chemical vapor deposition system. Three distinctive morphologies, namely nanoflakes, nanowalls (CNWs) and nanorods (and intermediates between these morphologies), can be reproducibly deposited, depending on the process parameters. The synthesized structures can be described as hybrid materials consisting of metal oxide incorporated in a carbon matrix material. Since the incorporation of metal oxide into the carbon structure significantly influences their growth, the synthesis cannot be described solely with the existing models for the growth of CNWs. Optical emission spectroscopy is used to measure the relative number density of suspected growth and etching species in the plasma, while physical and chemical surface analysis techniques (scanning electron microscopy, Raman spectroscopy, scanning Auger microscopy and x-ray photoelectron spectroscopy) were employed to characterize the properties of the different nanostructures. Therefore, by using methods for both plasma and surface characterization, the growth process can be understood. The precursor dissociation in the plasma can be directly linked to the deposited morphology, as the incorporation of Al2O3 into the nanostructures is found to be a major cause for the transition between morphologies, by changing the dominant type of defect within the carbon structure. © 2020 The Author(s). Published by IOP Publishing Ltd.
    view abstractdoi: 10.1088/1361-6463/ab6946
  • 2020 • 554 Microstructural and Tribological Characteristics of Sn-Sb-Cu-Based Composite Coatings Deposited by Cold Spraying
    Tillmann, W. and Hagen, L. and Kensy, M.D. and Abdulgader, M. and Paulus, M.
    Journal of Thermal Spray Technology 29 1027-1039 (2020)
    Sn-based Babbitt coatings are widely used for sliding in hydrodynamic bearings. The Babbitting of bearing surfaces is among others accomplished by casting; however, this implies some disadvantages such as segregations, or susceptibility to shrinkage defects. Thermal spraying represents a promising method to overcome these challenges. To date, no studies on Babbitt coatings deposited by means of low-pressure cold spraying (LPCS) are available in the literature. In this study, a first attempt is made to produce a Sn-Sb-Cu-based composite coating reinforced with alumina particles by means of LPCS which enables the coating of internal diameters (IDs) of cylindrical components. A tailor-made feedstock was utilized which consists of a powder mixture of Sn, Sb, Cu and alumina. The composite coating is investigated with regard to its microstructural and tribological characteristics using scanning electron microscopy (SEM), energy-dispersive x-ray spectroscopy (EDS), x-ray diffraction (XRD), as well as dry sliding experiments. Metallographic investigations demonstrate the feasibility of depositing an alumina-reinforced Sn-Sb-Cu-based composite coating with a dense microstructure and low porosity. The composite coating mainly consists of Sn, SbSn, Cu and hexagonal CuSn. Despite a small fraction of alumina particles, the microhardness of the composite coating is primarily determined by the formation of SbSn intermetallic phases dispersed in the soft Sn-Sb-rich matrix. The composite coating possesses a coefficient of friction of 0.43 ± 0.01 and wear coefficient k of 17.27 ± 7.77 × 10−5 mm3 N m−1 sliding against a 100Cr6 counterbody. © 2020, ASM International.
    view abstractdoi: 10.1007/s11666-020-01054-1
  • 2020 • 553 How tensile tests allow a screening of the fracture toughness of hard coatings
    Völker, B. and Du, C. and Fager, H. and Rueß, H. and Soler, R. and Kirchlechner, C. and Dehm, G. and Schneider, J.M.
    Surface and Coatings Technology 390 (2020)
    In hard coating development there is a need for fast and efficient screening methods to assess the influence of changes in composition and structure evolution on the mechanical behavior. Nanoindentation constitutes a method that allows fast and efficient probing of mechanical properties. For another important mechanical characteristic of hard coatings, the fracture toughness, no quick benchmark test to compare coatings with various compositions and/or morphologies, exist. Therefore, the goal of this investigation was to determine if a tensile test setup allows for fast and efficient qualitative screening of fracture toughness trends in hard coatings compared to accurate but time consuming micro-bending beam experiments. TiAlN and VAlN, each deposited on ductile Cu-substrates, were chosen for this investigation. In situ scanning electron microscopy tensile tests were performed. Here, the strain at crack initiation of the coating was utilized as the experimental parameter, which is most representative for the fracture toughness. The experiments indicate that TiAlN exhibits a higher fracture toughness than VAlN. This was confirmed using time demanding in situ micro-bending beam fracture experiments. Hence, it is established that under the given conditions macroscopic in situ SEM tensile tests can be used as a fast and efficient screening method for fracture toughness trends of hard coatings. © 2020 Elsevier B.V.
    view abstractdoi: 10.1016/j.surfcoat.2020.125645
  • 2020 • 552 Torsion plastometer trials to investigate the effect of non-proportional loading paths in caliber rolling on damage and performance of metal parts
    Wang, S. and Dunlap, A. and Möhring, K. and Lohmar, J. and Schwedt, A. and Aretz, A. and Walther, F. and Hirt, G.
    Production Engineering 14 17-32 (2020)
    The non-proportional loading path describes a strain-dependent development of stress triaxiality and Lode parameter during metal forming processes. Existing studies suggest a strong dependence of damage evolution on the non-proportional loading path. This work focuses on investigating the influence of non-proportional loading paths observed in hot caliber rolling of the case-hardening steel 16MnCrS5 using laboratory scale experiments. The applied torsion plastometer is highly flexible as it can apply combined loading types (tension, compression and torsion) on notched round specimen and enables deformation at elevated temperature. In this study, two characteristic non-proportional loading paths in caliber rolling and the maximal achievable non-proportional loading path variation were recreated in the torsion plastometer based on both FE simulations and experiments. After deformation, the specimen were further analyzed using Scanning Electron Microscopy (SEM) to quantify the damage. The results indicate an influence of the non-proportional loading path on damage evolution. Furthermore, fatigue tests were employed to characterize the fatigue performance of the deformed specimens. In the torsion plastometer trials carried out no clear correlation of performance and damage was observed. This is most likely due to differences in residual dislocation density after static recrystallization and deviations in the microstructure after hot working. Thus, the superposition of microstructure evolution and damage needs to be considered carefully when testing at elevated temperature. © 2020, The Author(s).
    view abstractdoi: 10.1007/s11740-019-00949-5
  • 2020 • 551 Defect Creation in Surface-Mounted Metal-Organic Framework Thin Films
    Wang, Z. and Henke, S. and Paulus, M. and Welle, A. and Fan, Z. and Rodewald, K. and Rieger, B. and Fischer, R.A.
    ACS Applied Materials and Interfaces 12 2655-2661 (2020)
    Defect engineering is a strategy for tailoring the properties of metal-organic frameworks (MOFs). Plenty of efforts have been devoted to study the defect chemistry and structures of bulk MOFs; however, the reported example of a defect-engineered surface-mounted MOF (SURMOF) thin film is rare. In this work, defects were incorporated in SURMOF thin films by using defect-generating linkers and taking advantage of the liquid-phase stepwise epitaxial layer-by-layer growth (LBL). Two methods based on the LBL, named mixing method and alternating method, are proposed for incorporating defects in the prototypical SURMOF HKUST-1 by partially substituting the parent H3btc (benzene-1,3,5-tricarboxylic acid) linker with a set of defect-generating linkers H2ip (isophthalic acid), H2OH-ip (5-hydroxyisophthalic acid), and H2pydc (3,5-pyridinedicarboxylic acid). The crystallinity and phase purity of the obtained "defected" SURMOFs were confirmed by X-ray diffraction, infrared reflection absorption spectroscopy, and Raman spectroscopy. The incorporation of the defect-generating linkers and the types of induced defects were characterized by ultraviolet-visible spectroscopy, time-of-flight secondary ion mass spectrometry, methanol adsorption, scanning electron microscopy, and 1H nuclear magnetic resonance spectroscopy (after digestion of the samples). These two methods provide avenues for controlling the defect formation in MOF thin films. © 2019 American Chemical Society.
    view abstractdoi: 10.1021/acsami.9b18672
  • 2020 • 550 Acidity enhancement through synergy of penta- and tetra-coordinated aluminum species in amorphous silica networks
    Wang, Z. and Li, T. and Jiang, Y. and Lafon, O. and Liu, Z. and Trébosc, J. and Baiker, A. and Amoureux, J.-P. and Huang, J.
    Nature Communications 11 (2020)
    Amorphous silica-aluminas (ASAs) are widely used in acid-catalyzed C-H activation reactions and biomass conversions in large scale, which can be promoted by increasing the strength of surface Brønsted acid sites (BAS). Here, we demonstrate the first observation on a synergistic effect caused by two neighboring Al centers interacting with the same silanol group in flame-made ASAs with high Al content. The two close Al centers decrease the electron density on the silanol oxygen and thereby enhance its acidity, which is comparable to that of dealuminated zeolites, while ASAs with small or moderate Al contents provide mainly moderate acidity, much lower than that of zeolites. The ASAs with enhanced acidity exhibit outstanding performances in C–H bond activation of benzene and glucose dehydration to 5-hydroxymethylfurfural, simultaneously with an excellent calcination stability and resistance to leaching, and they offer an interesting potential for a wide range of acid and multifunctional catalysis. © 2020, The Author(s).
    view abstractdoi: 10.1038/s41467-019-13907-7
  • 2020 • 549 Influence of Hot Hardness and Microstructure of High-Alloyed Powder Metallurgical Tool Steels on Abrasive Wear Behavior at Elevated Temperatures
    Wulbieter, N. and Pöhl, F. and Theisen, W.
    Steel Research International 91 (2020)
    Herein, the abrasive wear behavior of different high-alloyed powder metallurgical (PM) tool steels is investigated at elevated temperatures (400–600 °C) in a dry-pot wear tester containing Al2O3 particles. To identify the influence of the microstructure, PM tool steels with different hot hardnesses, carbide types, and carbide volume contents are selected. Wear tracks are analyzed by scanning electron microscopy (SEM) to clarify wear mechanisms. The results show that there is no direct correlation between wear resistance and only one material property such as hot hardness, carbide content, or carbide type. More important seems to be the best possible compromise between a sufficient hot hardness of the metallic matrix and a high volume content of carbides that are harder than the attacking abrasive particles at the respective temperature. When the test temperatures surpass the tempering temperature of the investigated steels, there is a pronounced change in wear behavior due to the stronger embedding of abrasive particles into the wear surface. It is thus necessary to discuss the microstructural properties as a function of temperature, considering interactions with the abrasive particles. © 2019 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/srin.201900461
  • 2020 • 548 Nanocutting mechanism of 6H-SiC investigated by scanning electron microscope online observation and stress-assisted and ion implant-assisted approaches
    Xu, Z. and Liu, L. and He, Z. and Tian, D. and Hartmaier, A. and Zhang, J. and Luo, X. and Rommel, M. and Nordlund, K. and Zhang, G. and Fang, F.
    International Journal of Advanced Manufacturing Technology 106 3869-3880 (2020)
    Nanocutting mechanism of single crystal 6H-SiC is investigated through a novel scanning electron microscope setup in this paper. Various undeformed chip thicknesses on (0001) < 1–100 > orientation are adopted in the nanocutting experiments. Phase transformation and dislocation activities involved in the 6H-SiC nanocutting process are also characterized and analyzed. Two methods of stress-assisted and ion implant-assisted nanocutting are studied to improve 6H-SiC ductile machining ability. Results show that stress-assisted method can effectively decrease the hydrostatic stress and help to activate dislocation motion and ductile machining; ion implant-induced damages are helpful to improve the ductile machining ability from MD simulation and continuous nanocutting experiments under the online observation platform. © 2020, Springer-Verlag London Ltd., part of Springer Nature.
    view abstractdoi: 10.1007/s00170-019-04886-6
  • 2020 • 547 (Al, Zn)3Zr dispersoids assisted η′ precipitation in anAl-Zn-Mg-Cu-Zr alloy
    Zhao, H. and Chen, Y. and Gault, B. and Makineni, S.K. and Ponge, D. and Raabe, D.
    Materialia 10 (2020)
    The influence of (Al,Zn)3Zr dispersoids on the precipitation of the main strengthening (Mg,Zn)-rich phases was investigated during isothermal aging of a model Al-Zn-Mg-Cu-Zr alloy. Upon homogenization of the alloy, dispersoids of (Al,Zn)3Zr with a L12 structure are present. Isothermal aging at 120 °C for 0.5 h leads to the homogeneous formation of spherical GP zones in the α-Al matrix and heterogeneous nucleation on (Al,Zn)3Zr dispersoids. After 2 h of aging, GP zones remain present in the α-Al matrix while the accelerated transformation of GP zones to plate-shaped (Mg,Zn)-rich ηʹ precipitates is shown on the {111} planes at the interface of the L12 dispersoids. Even at grain boundaries, the similar composite structure comprising ηʹ precipitates on the coarser Zr-dispersoid is observed, along with 10-nm wide precipitate-free zones around them. The composition and structure of pre-existing dispersoids, their role in the formation of the composite structure are discussed. © 2020
    view abstractdoi: 10.1016/j.mtla.2020.100641
  • 2019 • 546 Corrosion and material alterations of a CuZn38Pb3 brass under acoustic cavitation
    Abedini, M. and Reuter, F. and Hanke, S.
    Ultrasonics Sonochemistry 58 (2019)
    An alloy that is exposed to cavitation may experience mechanical cavitation damages as well as accelerated corrosion. In the present paper, the evolution of corrosion erosion behavior of brass samples (CuZn38Pb3) during continuous exposure to ultrasonic cavitation in a salt solution (NaCl) was investigated. Various samples were sonicated for times between 0 min and 5 h. The average surface roughness and the effective surface area of the samples were measured by confocal microscopy, and the surfaces were inspected by scanning electron microscopy. Different erosion behavior of the phases present on the surface is discussed. Complementary to the surface inspection, the corrosion behavior of the samples before, during and after sonication was investigated through open circuit potential, potentiodynamic polarization and electrochemical impedance spectroscopy techniques. The results show that at the initial times of sonication preferably the lead islets were removed from the brass surface, resulting in a change in the open circuit potential. α and β′ phases showed ductile and brittle behavior under sonication, respectively. The corrosion rate of the alloy under cavitation increased as the sonication time increased, mainly related to the increase in effective surface area and the rise of plastic deformation of the surface material. © 2019 Elsevier B.V.
    view abstractdoi: 10.1016/j.ultsonch.2019.104628
  • 2019 • 545 Electron-Beam-Induced Current Measurements of Thin-Film Solar Cells
    Abou-Ras, D. and Kirchartz, T.
    ACS Applied Energy Materials 2 6127-6139 (2019)
    The present tutorial review provides a practical guide to the analysis of semiconductor devices using electron-beam-induced currents (EBICs). The authors focus on cross-sectional EBIC measurements that provide an experimental assay of the efficiency of charge carrier collection in a semiconductor diode. The tutorial covers the fundamental physics of the technique, specimen preparation, data acquisition, and numerical simulation and analysis of the experimental data. A key focus is put on application cases from the field of thin-film photovoltaics as well as specific pitfalls that may occur, such as effects occurring under high-level injection and at grain boundaries of polycrystalline materials. © 2019 American Chemical Society.
    view abstractdoi: 10.1021/acsaem.9b01172
  • 2019 • 544 Porous poly(vinylidene fluoride) membranes with tailored properties by fast and scalable non-solvent vapor induced phase separation
    Alexowsky, C. and Bojarska, M. and Ulbricht, M.
    Journal of Membrane Science 577 69-78 (2019)
    Hydrophobic and highly porous poly(vinylidene fluoride) (PVDF) membranes with isotropic cross section as well as tunable and narrow barrier pore size distribution in the range from ~0.1 to ~1 µm have been prepared using the non-solvent vapor induced phase separation (VIPS) technique. The process conditions have been tuned to suit an industrial scale up, using a short production time and dimethyl sulfoxide (DMSO) as solvent for PVDF, instead of commonly used hazardous chemicals. Factors like kind of solvent, the relative humidity of air, the exposure time to humid air and the mass fraction of PVDF in the casting solutions have been used to tune membrane characteristics. Interestingly, it was revealed that DMSO as a less common solvent for PVDF shows better qualities regarding upscaling than other more frequently used polar aprotic solvents (e.g. dimethylacetamide) when using VIPS under suited conditions. The phenomenon was explained through investigations of the membrane formation step, in particular by water uptake and cloud point measurements, as well as structure and performance analyses, e.g. by scanning electron microscopy, gas flow/liquid dewetting permpometry, gas and water vapor permeability analyses and liquid water entry pressure measurements. Lab-scale manufactured membranes showed pore characteristics and performance desired for membrane contactor applications. Furthermore, fabrication on a roll-to-roll machine using a nonwoven support and the established VIPS conditions was realized in a short manufacturing time; resulting membranes structure and characteristics were found to be similar to the ones for the lab-scale membranes. Overall, the combination of PVDF with DMSO gives promising opportunities for a more eco-friendly industrial fabrication of porous membranes with advanced properties via VIPS. © 2019 Elsevier B.V.
    view abstractdoi: 10.1016/j.memsci.2019.01.033
  • 2019 • 543 Electrical conductivity of silver nanoparticle doped carbon nanofibres measured by CS-AFM
    Ali, W. and Shabani, V. and Linke, M. and Sayin, S. and Gebert, B. and Altinpinar, S. and Hildebrandt, M. and Gutmann, J.S. and Mayer-Gall, T.
    RSC Advances 9 4553-4562 (2019)
    In this work, a pioneering study on the electrical properties of composite carbon nanofibres (CNFs) using current-sensitive atomic force microscopy (CS-AFM) has been demonstrated. CNFs are highly interesting materials which are usable in a wide array of applications e.g. electrode materials for biosensors, lithium ion batteries, fuel cells and supercapacitors. CNFs offer a high specific surface area and thus have a high contact area for charge transfer. CNFs can be produced using spinnable polyacrylonitrile (PAN) as a precursor for carbonisation. For the purpose of developing efficient CNFs with high conductivity and power density, silver nanoparticle (AgNPs)-containing PAN solutions were electrospun to form composite nanofibres which was followed by heat treatment. The applied voltage of the spinning setup and the content of both PAN and the silver nanoparticles in the spinning solution were varied in order to study their influence on the morphology and the electrical properties of the nanofibres. The resultant morphologies and fibre diameters were determined by scanning electron microscopy (SEM). The formation of silver nanoparticles was characterised in solution by UV-visible absorption spectroscopy and dynamic light scattering (DLS), while energy-dispersive X-ray spectroscopy (EDX) and transmission electron microscopy (TEM) were carried out to investigate the presence as well as the average diameter of the AgNPs. The electrical properties of the CNFs were investigated using CS-AFM. This technique gives us the possibility to explore the electrical properties of single fibers and hence derive relationships between the structural features and the electrical properties. Our results show that the composite CNFs have a higher electrical conductivity than the neat CNFs and both the average diameter of the fibers and the electrical conductivity increase with an increasing AgNP content. © The Royal Society of Chemistry.
    view abstractdoi: 10.1039/C8RA04594A
  • 2019 • 542 High temperature pyrolysis of lignite and synthetic carbons
    Apicella, B. and Russo, C. and Ciajolo, A. and Cortese, L. and Cerciello, F. and Stanzione, F. and Wuetscher, A. and Muhler, M. and Senneca, O.
    Fuel 264-272 (2019)
    The paper explores changes in reactivity and chemico-physical characteristics of char and tar produced by severe heat treatment of lignite in both inert atmospheres and CO2 rich atmospheres. The role of mineral matter, in particular metal oxides, in catalysing chemical and physical transformations is also addressed. A Rhenish Lignite from the Garzweiler mine was studied and compared with: a) mineral-free synthetic carbon (HTC), obtained from cellulose; b) a synthetic carbon doped with iron oxide (Fe2O3). A heated strip reactor (HSR) was employed at temperatures of 1300 and 1800 °C in N2 and CO2 atmospheres. Liquid and solid products (tar and char) were analysed and compared. Tar composition was evaluated by extraction and gas chromatography-mass spectrometry, whereas the solid carbonaceous material produced by pyrolysis, mainly composed of char, was characterized regarding its thermal behaviour by thermogravimetric analysis and its structure by Raman spectroscopy and scanning electron microscopy. Results show that iron oxide exerts a catalytic influence on both pyrolysis and char oxidation. Upon severe heat treatment, it reduces char reactivity promoting graphitization and structural ordering. The overall effect on char reactivity is therefore not easy to predict. © 2018 Elsevier Ltd
    view abstractdoi: 10.1016/j.fuel.2018.12.065
  • 2019 • 541 Dental pulp stem cells in chitosan/gelatin scaffolds for enhanced orofacial bone regeneration
    Bakopoulou, A. and Georgopoulou, Α. and Grivas, I. and Bekiari, C. and Prymak, O. and Loza, Κ. and Epple, M. and Papadopoulos, G.C. and Koidis, P. and Chatzinikolaidou, Μ.
    Dental Materials 35 310-327 (2019)
    Objective: Biomimetic chitosan/gelatin (CS/Gel) scaffolds have attracted great interest in tissue engineering of several tissues. However, limited information exists regarding the potential of combining CS/Gel scaffolds with oral cells, such as dental pulp stem cells (DPSCs), to produce customized constructs targeting alveolar/orofacial bone reconstruction, which has been the aim of the present study. Methods: Two scaffold types, designated as CS/Gel-0.1 and CS/Gel-1, were fabricated using 0.1 and 1% (v/v) respectively of the crosslinker glutaraldehyde (GTA). Scaffolds (n = 240) were seeded with DPSCs with/without pre-exposure to recombinant human BMP-2. In vitro assessment included DPSCs characterization (flow cytometry), evaluation of viability/proliferation (live/dead staining, metabolic-based tests), osteo/odontogenic gene expression analysis (qRT-PCR) and structural/chemical characterization (scanning electron microscopy, SEM; energy dispersive X-ray spectroscopy, EDX; X-ray powder diffraction, XRD; thermogravimetry, TG). In vivo assessment included implantation of DPSC-seeded scaffolds in immunocompromised mice, followed by histology and SEM-EDX. Statistical analysis employed one/two-way ANOVA and Tukey's post-hoc tests (significance for p < 0.05). Results: Both scaffolds supported cell viability/proliferation over 14 days in culture, showing extensive formation of a hydroxyapatite-rich nanocrystalline calcium phosphate phase. Differential expression patterns indicated GTA concentration to significantly affect the expression of osteo/odontogenic genes, with CS/Gel-0.1 scaffolds being more effective in upregulating DSPP, IBSP and Osterix. In vivo analysis demonstrated time-dependent production of a nanocrystalline, mineralized matrix at 6, 8 and 10 weeks, being more prominent in constructs bearing rhBMP-2 pre-treated cells. The latter showed higher amounts of osteoid and fully mineralized bone, as well as empty space reduction. Significance: These results reveal a promising strategy for orofacial bone tissue engineering. © 2018 The Academy of Dental Materials
    view abstractdoi: 10.1016/j.dental.2018.11.025
  • 2019 • 540 Cobalt metalloid and polybenzoxazine derived composites for bifunctional oxygen electrocatalysis
    Barwe, S. and Andronescu, C. and Engels, R. and Conzuelo, F. and Seisel, S. and Wilde, P. and Chen, Y.-T. and Masa, J. and Schuhmann, W.
    Electrochimica Acta 297 1042-1051 (2019)
    The development of bifunctional oxygen electrodes is a key factor for the envisaged application of rechargeable metal-air batteries. In this work, we present a simple procedure based on pyrolysis of polybenzoxazine/metal metalloid nanoparticles composites into efficient bifunctional oxygen reduction and oxygen evolution electrocatalysts. This procedure generates nitrogen-doped carbon with embedded metal metalloid nanoparticles exhibiting high activity towards both, oxygen reduction and oxygen evolution, in 0.1 M KOH with a roundtrip voltage of as low as 0.81 V. Koutecký-Levich analysis coupled with scanning electrochemical microscopy reveals that oxygen is preferentially reduced in a 4e− transfer pathway to hydroxide rather than to hydrogen peroxide. Furthermore, the polybenzoxazine derived carbon matrix allows for stable catalyst fixation on the electrode surface, resulting in unattenuated activity during continuous alternate polarisation between oxygen evolution at 10 mA cm−2 and oxygen reduction at −1.0 mA cm−2. © 2018 Elsevier Ltd
    view abstractdoi: 10.1016/j.electacta.2018.12.047
  • 2019 • 539 Improvements on the recovery of 3D particle size distributions from 2D sections
    Benito, S. and Cuervo, C. and Pöhl, F. and Theisen, W.
    Materials Characterization 156 (2019)
    The recovery of underlying 3D particle size distributions by analyzing only 2D sections (such as thin layers or scanning electron microscopy images) has been thoroughly investigated in the last forty years. However, with the advent of increasingly more powerful computers, flexible programming languages and readily available source code, very interesting studies have recently been published on this topic. In this paper, we implement and discuss some key improvements to one particularly promising approach, which is based on the linear representation of the effect of each particle on the smaller apparent sizes that are measurable in the 2D plane. Two main improvements are examined: (i) the inclusion of either a parametric or a nonparametric fit to the measured data and (ii) the utilization of optimization tools to solve the resulting linear system. We endeavor to prove that the new method produces reliable results both in simulations and in an experimental validation example, while also reducing the number of required measurements. © 2019 Elsevier Inc.
    view abstractdoi: 10.1016/j.matchar.2019.109872
  • 2019 • 538 Multi-scale characterization of austenite reversion and martensite recovery in a cold-rolled medium-Mn steel
    Benzing, J.T. and Kwiatkowski da Silva, A. and Morsdorf, L. and Bentley, J. and Ponge, D. and Dutta, A. and Han, J. and McBride, J.R. and Van Leer, B. and Gault, B. and Raabe, D. and Wittig, J.E.
    Acta Materialia 166 512-530 (2019)
    A medium-Mn steel (Fe-12Mn-3Al-0.05C wt%) was designed using Thermo-Calc ® simulations to balance the fraction and stacking fault energy of reverted austenite. Intercritical annealing for 0.5, 8 and 48 h was carried out at 585 °C to investigate the microstructural evolution. X-ray diffraction (XRD), electron backscatter diffraction (EBSD), 3-dimensional EBSD, energy-dispersive spectroscopy via scanning-transmission electron microscopy (STEM-EDS) and atom probe tomography (APT) enable characterization of phase fraction, grain area, grain morphology and alloy partitioning. An increase in annealing time from 0.5 h to 48 h increases the amount of ultrafine-grained (UFG) reverted austenite from 3 to 40 vol %. EBSD and TEM reveal multiple morphologies of UFG austenite (equiaxed, rod-like and plate-like). In addition, most of the remaining microstructure consists of recovered α′-martensite that resembles the cold-rolled state, as well as a relatively small fraction of UFG ferrite (i.e., only a small amount of martensite recrystallization occurs). Multi-scale characterization results show that the location within the cold-rolled microstructure has a strong influence on boundary mobility and grain morphology during austenite reversion. Results from APT reveal Mn-decoration of dislocation networks and low-angle lath boundaries in the recovered α′-martensite, but an absence of Mn-decoration of defects in the vicinity of austenite grains, thereby promoting recovery. STEM-EDS and APT reveal Mn depletion zones in the ferrite/recovered α′-martensite near austenite boundaries, whereas gradients of C and Mn co-partitioning are visible within some of the austenite grains after annealing for 0.5 h. Relatively flat C-enriched austenite boundaries are present even after 8 h of annealing and indicate certain boundaries possess low mobility. At later stages the growth of austenite followed the local equilibrium (LE) model such that the driving force between two equilibrium phases moves the mobile interface, as confirmed by DICTRA simulations (a Thermo-Calc ® diffusion module). The sequence of austenite reversion is: (i) formation of Mn- and C-enriched face-centered-cubic nuclei from decorated dislocations and/or particles; (ii) co-partitioning of Mn and C and (iii) growth of austenite controlled by the LE mode. © 2019 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2019.01.003
  • 2019 • 537 A Novel Nanoconjugate of Landomycin A with C60 Fullerene for Cancer Targeted Therapy: In Vitro Studies
    Bilobrov, V. and Sokolova, V. and Prylutska, S. and Panchuk, R. and Litsis, O. and Osetskyi, V. and Evstigneev, M. and Prylutskyy, Y. and Epple, M. and Ritter, U. and Rohr, J.
    Cellular and Molecular Bioengineering 12 41-51 (2019)
    Introduction: Landomycins are a subgroup of angucycline antibiotics that are produced by Streptomyces bacteria and possess strong antineoplastic potential. Literature data suggest that enhancement of the therapeutic activity of this drug may be achieved by means of creating specific drug delivery systems. Here we propose to adopt C60 fullerene as flexible and stable nanocarrier for landomycin delivery into tumor cells. Methods: The methods of molecular modelling, dynamic light scattering and Fourier transform infrared spectroscopy were used to study the assembly of C60 fullerene and the anticancer drug Landomycin A (LA) in aqueous solution. Cytotoxic activity of this nanocomplex was studied in vitro towards two cancer cell lines in comparison to human mesenchymal stem cells (hMSCs) using 3-(4,5-dimethyl-2-thiazolyl)-2,5-diphenyl-2-H-tetrazolium bromide (MTT) test and a live/dead assay. The morphology of the cells incubated with fullerene–drug nanoparticles and their uptake into target cells were studied by scanning electron microscopy and fluorescence light microscopy. Results: The viability of primary cells (hMSCs, as a model for healthy cells) and cancer cell lines (human osteosarcoma cells, MG-63, and mouse mammary cells, 4T1, as models for cancer cells) was studied after incubation with water-soluble C60 fullerenes, LA and the mixture C60 + LA. The C60 + LA nanocomplex in contrast to LA alone showed higher toxicity towards cancer cells and lower toxicity towards normal cells, whereas the water-soluble C60 fullerenes at the same concentration were not toxic for the cells. Conclusions: The obtained physico-chemical data indicate a complexation between the two compounds, leading to the formation of a C60 + LA nanocomposite. It was concluded that immobilization of LA on C60 fullerene enhances selectivity of action of this anticancer drug in vitro, indicating on possibility of further preclinical studies of novel C60 + LA nanocomposites on animal tumor models. © 2018, Biomedical Engineering Society.
    view abstractdoi: 10.1007/s12195-018-0548-5
  • 2019 • 536 A Novel Nanoconjugate of Landomycin A with C 60 Fullerene for Cancer Targeted Therapy: In Vitro Studies
    Bilobrov, V. and Sokolova, V. and Prylutska, S. and Panchuk, R. and Litsis, O. and Osetskyi, V. and Evstigneev, M. and Prylutskyy, Y. and Epple, M. and Ritter, U. and Rohr, J.
    Cellular and Molecular Bioengineering 12 41-51 (2019)
    Introduction: Landomycins are a subgroup of angucycline antibiotics that are produced by Streptomyces bacteria and possess strong antineoplastic potential. Literature data suggest that enhancement of the therapeutic activity of this drug may be achieved by means of creating specific drug delivery systems. Here we propose to adopt C 60 fullerene as flexible and stable nanocarrier for landomycin delivery into tumor cells. Methods: The methods of molecular modelling, dynamic light scattering and Fourier transform infrared spectroscopy were used to study the assembly of C 60 fullerene and the anticancer drug Landomycin A (LA) in aqueous solution. Cytotoxic activity of this nanocomplex was studied in vitro towards two cancer cell lines in comparison to human mesenchymal stem cells (hMSCs) using 3-(4,5-dimethyl-2-thiazolyl)-2,5-diphenyl-2-H-tetrazolium bromide (MTT) test and a live/dead assay. The morphology of the cells incubated with fullerene–drug nanoparticles and their uptake into target cells were studied by scanning electron microscopy and fluorescence light microscopy. Results: The viability of primary cells (hMSCs, as a model for healthy cells) and cancer cell lines (human osteosarcoma cells, MG-63, and mouse mammary cells, 4T1, as models for cancer cells) was studied after incubation with water-soluble C 60 fullerenes, LA and the mixture C 60 + LA. The C 60 + LA nanocomplex in contrast to LA alone showed higher toxicity towards cancer cells and lower toxicity towards normal cells, whereas the water-soluble C 60 fullerenes at the same concentration were not toxic for the cells. Conclusions: The obtained physico-chemical data indicate a complexation between the two compounds, leading to the formation of a C 60 + LA nanocomposite. It was concluded that immobilization of LA on C 60 fullerene enhances selectivity of action of this anticancer drug in vitro, indicating on possibility of further preclinical studies of novel C 60 + LA nanocomposites on animal tumor models. © 2018, Biomedical Engineering Society.
    view abstractdoi: 10.1007/s12195-018-0548-5
  • 2019 • 535 Processing of gas-nitrided AISI 316L steel powder by laser powder bed fusion – Microstructure and properties
    Boes, J. and Röttger, A. and Becker, L. and Theisen, W.
    Additive Manufacturing 30 (2019)
    This work investigated the processing of high nitrogen-alloyed austenitic stainless steels by laser powder bed fusion (L-PBF). Prior to L-PBF processing, the AISI 316 L steel powder was nitrided at a temperature of 675°C in a 3 bar nitrogen atmosphere, thus achieving a N content of 0.58 mass-%. By mixing nitrided 316 L powder with untreated 316 L powder, two different powder mixtures were obtained with 0.065 mass-% and 0.27 mass-% nitrogen, respectively. After nitriding and mixing, the powder was characterized in terms of its flow properties and chemical composition. The nitrided steel powder was then processed by L-PBF, and the microstructure as well as the chemical composition were investigated by means of scanning electron microscopy and carrier gas hot extraction. It was shown that nitriding of steel powders in an N2 atmosphere can be used to significantly increase the nitrogen content of the powder without impairing its flow properties. With increasing nitrogen content of the powder, the porosity within the L-PBF built specimens increased. However, both the yield strength and the tensile strength were greatly improved without a marked reduction in the elongation at fracture of the respective steels. This work shows that nitrogen-alloyed austenitic stainless steels can be processed by L-PBF and the mechanical properties can be improved. © 2019 Elsevier B.V.
    view abstractdoi: 10.1016/j.addma.2019.100836
  • 2019 • 534 Bimetallic silver-platinum nanoparticles with combined osteo-promotive and antimicrobial activity
    Breisch, M. and Grasmik, V. and Loza, K. and Pappert, K. and Rostek, A. and Ziegler, N. and Ludwig, Al. and Heggen, M. and Epple, M. and Tiller, J.C. and Schildhauer, T.A. and Köller, M. and Sengstock, C.
    Nanotechnology 30 (2019)
    Bimetallic alloyed silver-platinum nanoparticles (AgPt NP) with different metal composition from Ag10Pt90 to Ag90Pt10 in steps of 20 mol% were synthesized. The biological effects of AgPt NP, including cellular uptake, cell viability, osteogenic differentiation and osteoclastogenesis as well as the antimicrobial activity towards Staphylococcus aureus and Escherichia coli were analyzed in comparison to pure Ag NP and pure Pt NP. The uptake of NP into human mesenchymal stem cells was confirmed by cross-sectional focused-ion beam preparation and observation by scanning and transmission electron microscopy in combination with energy-dispersive x-ray analysis. Lower cytotoxicity and antimicrobial activity were observed for AgPt NP compared to pure Ag NP. Thus, an enhanced Ag ion release due to a possible sacrificial anode effect was not achieved. Nevertheless, a Ag content of at least 50 mol% was sufficient to induce bactericidal effects against both Staphylococcus aureus and Escherichia coli. In addition, a Pt-related (≥50 mol% Pt) osteo-promotive activity on human mesenchymal stem cells was observed by enhanced cell calcification and alkaline phosphatase activity. In contrast, the osteoclastogenesis of rat primary precursor osteoclasts was inhibited. In summary, these results demonstrate a combinatory osteo-promotive and antimicrobial activity of bimetallic Ag50Pt50 NP. © 2019 IOP Publishing Ltd.
    view abstractdoi: 10.1088/1361-6528/ab172b
  • 2019 • 533 Creep properties of single crystal Ni-base superalloys (SX): A comparison between conventionally cast and additive manufactured CMSX-4 materials
    Bürger, D. and Parsa, A.B. and Ramsperger, M. and Körner, C. and Eggeler, G.
    Materials Science and Engineering A 762 (2019)
    The present work compares the microstructures and the creep properties of two types of single crystal Ni-base superalloy CMSX-4 materials (SXs). One was produced by conventional directional solidification Bridgman processing. The other was manufactured by selective electron beam melting (SEBM). The microstructures of the two types of materials are compared with emphasis placed on the large (dendritic/interdendritic regions) and small scale (γ-matrix/γ′-precipitates) microstructural heterogeneities, which characterize SX microstructures and their evolution during processing, heat treatment and creep. It is shown that heat treated SEBM materials have creep properties, which match or even outperform those of conventionally processed SX materials. Creep properties were assessed using a miniature creep test technique where [001] miniature tensile creep specimens were tested in the high temperature/low stress (1050 °C, 160 MPa) and in the low temperature/high stress (850 °C, 600 MPa) creep regimes. The creep behavior is interpreted based on microstructural results, which were obtained using analytical scanning and transmission electron microscopy (SEM and TEM). © 2019 The Authors
    view abstractdoi: 10.1016/j.msea.2019.138098
  • 2019 • 532 Functionalization of titania nanotubes with electrophoretically deposited silver and calcium phosphate nanoparticles: Structure, composition and antibacterial assay
    Chernozem, R.V. and Surmeneva, M.A. and Krause, B. and Baumbach, T. and Ignatov, V.P. and Prymak, O. and Loza, K. and Epple, M. and Ennen-Roth, F. and Wittmar, A. and Ulbricht, M. and Chudinova, E.A. and Rijavec, T. and Lapanje, A...
    Materials Science and Engineering C 97 420-430 (2019)
    Herein TiO2 nanotubes (NTs) were fabricated via electrochemical anodization and coated with silver and calcium phosphate (CaP) nanoparticles (NPs) by electrophoretic deposition. Scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX) revealed that Ag and CaP NPs were successfully deposited onto the TiO2 NTs. Using X-ray diffraction, only anatase and Ti were observed after deposition of Ag and CaP NPs. However, X-ray photoelectron spectroscopy (XPS) analysis revealed that the binding energy (BE) of the Ag and CaP NP core levels corresponded to metallic Ag, hydroxyapatite and amorphous calcium phosphate, based on the knowledge that CaP NPs synthesized by precipitation have the nanocrystalline structure of hydroxyapatite. The application of Ag NPs allows for decreasing the water contact angle and thus increasing the surface free energy. It was concluded that the CaP NP surfaces are superhydrophilic. A significant antimicrobial effect was observed on the TiO2 NT surface after the application of Ag NPs and/or CaP NPs compared with that of the pure TiO2 NTs. Thus, fabrication of TiO2 NTs, Ag NPs and CaP NPs with PEI is promising for diverse biomedical applications, such as in constructing a biocompatible coating on the surface of Ti that includes an antimicrobial effect. © 2018 Elsevier B.V.
    view abstractdoi: 10.1016/j.msec.2018.12.045
  • 2019 • 531 Adhesion, proliferation, and osteogenic differentiation of human mesenchymal stem cells on additively manufactured Ti6Al4V alloy scaffolds modified with calcium phosphate nanoparticles
    Chudinova, E.A. and Surmeneva, M.A. and Timin, A.S. and Karpov, T.E. and Wittmar, A. and Ulbricht, M. and Ivanova, A. and Loza, K. and Prymak, O. and Koptyug, A. and Epple, M. and Surmenev, R.A.
    Colloids and Surfaces B: Biointerfaces 176 130-139 (2019)
    In the present study, biocomposites based on 3D porous additively manufactured Ti6Al4V (Ti64) scaffolds modified with biocompatible calcium phosphate nanoparticles (CaPNPs) were investigated. Ti64 scaffolds were manufactured via electron beam melting technology using an Arcam machine. Electrophoretic deposition was used to modify the scaffolds with CaPNPs, which were synthesized by precipitation in the presence of polyethyleneimine (PEI). Dynamic light scattering revealed that the CaP/PEI nanoparticles had an average size of 46 ± 18 nm and a zeta potential of +22 ± 9 mV. Scanning electron microscopy (SEM) revealed that the obtained spherical CaPNPs had an average diameter of approximately 90 nm. The titanium-based scaffolds coated with CaPNPs exhibited improved hydrophilic surface properties, with a water contact angle below 5°. Cultivation of human mesenchymal stem cells (hMSCs) on the CaPNPs-coated Ti64 scaffolds indicated that the improved hydrophilicity was beneficial for the attachment and growth of cells in vitro. The Ti6Al4V/CaPNPs scaffold supported an increase in the alkaline phosphatase (ALP) activity of cells. In addition to the favourable cell proliferation and differentiation, Ti6Al4V/CaPNPs scaffolds displayed increased mineralization compared to non-coated Ti6Al4V scaffolds. Thus, the developed composite 3D scaffolds of Ti6Al4V functionalized with CaPNPs are promising materials for different applications related to bone repair. © 2018 Elsevier B.V.
    view abstractdoi: 10.1016/j.colsurfb.2018.12.047
  • 2019 • 530 Electrostatic precipitation of submicron particles in a molten carrier
    Dobrowolski, A. and Pieloth, D. and Wiggers, H. and Thommes, M.
    Pharmaceutics 11 (2019)
    Recently, submicron particles have been discussed as a means to increase the bioavailability of poorly water-soluble drugs. Separation of these small particles is done with both fibre and membrane filters, as well as electrostatic precipitators. A major disadvantage of an electrostatic precipitator (ESP) is the agglomerate formation on the precipitation electrode. These agglomerates frequently show low bioavailability, due to the decreased specific surface area and poor wettability. In this work, a new melt electrostatic precipitator was developed and tested to convert submicron particles into a solid dispersion in order to increase the bioavailability of active pharmaceutical ingredients. The submicron particles were generated by spray drying and transferred to the ESP, where the collection electrode is covered with a melt, which served as matrix after solidification. The newly developed melt electrostatic precipitator was able to collect isolated naproxen particles in a molten carrier. A solid naproxen xylitol dispersion was prepared, which showed a reduction of the dissolution time by 82%, and a release of 80% of the total drug, compared to the physical mixture. © 2019 by the authors. Licensee MDPI, Basel, Switzerland.
    view abstractdoi: 10.3390/pharmaceutics11060276
  • 2019 • 529 Bisphosphonate ligand mediated ultrasensitive capacitive protein sensor: complementary match of supramolecular and dynamic chemistry
    Ertürk, G. and Akhoundian, M. and Lueg-Althoff, K. and Shinde, S. and Yeung, S.Y. and Hedström, M. and Schrader, T. and Mattiasson, B. and Sellergren, B.
    New Journal of Chemistry 43 847-852 (2019)
    Modern healthcare demands rapid and accurate detection of proteins/enzymes at the ultratrace level. Herein we present a molecularly imprinted capacitive sensor for trypsin, developed by microcontact imprinting. High affinity and selectivity was achieved by doping the prepolymerization mixture with a stoichiometric amount of methacrylamide-based bisphosphonate (BP) monomer. Taking advantage of the specific interaction between bisphosphonate binding monomers and lysine/arginine residues on the surface of trypsin, we have constructed a powerful polymeric sensor. The BP based sensor has the ability to recognize trypsin over other arginine-rich proteins, even in high ionic strength buffers with a sub-picomolar detection limit (pM). We believe that the combination of supramolecular chemistry, molecular imprinting and advanced instrumentation has a potential for future drug development and diagnostics that extends beyond biomolecular recognition. © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique.
    view abstractdoi: 10.1039/c8nj05238g
  • 2019 • 528 Development of Multilayer Sinter Cladding of Cold Work Tool Steel on Hadfield Steel Plates for Wear-Resistant Applications
    Farayibi, P.K. and Blüm, M. and Theisen, W. and Weber, S.
    Journal of Materials Engineering and Performance 28 1833-1847 (2019)
    Machinery components used for mining and mineral processing activities are often subjected to high impact loads and wear which have placed demands for the development of materials with high resistance to dynamic loads and aggressive wear conditions. In this study, a multilayered cladding of high alloyed cold work tool steel (X245VCrMo9-4), interlayered with Hadfield steel (X120Mn12) plates, which was also used as substrate using super-solidus liquid-phase sintering technique was investigated. A stack of the cold work tool steel powder was prepared with interlayered X120Mn12 steel plates in an alumina crucible at tap density with the substrate placed on it and was sintered in a vacuum furnace at 1250 °C at a heating rate of 10 K/min, held for 30 min under a nitrogen atmosphere at 0.08 MPa and furnace-cooled. Sample from the as-sintered cladding was subjected to austenization at 1000 °C, quenched in oil and tempered at 150 °C for 2 h. Samples were subjected to microstructural examination using optical and scanning electron microscopy. The microstructural investigations were supplemented by hardness and impact wear tests. Computational thermodynamics was used to support experimental findings. The results revealed that a near-net densification of the sintered X245 was achieved with 99.93 ± 0.01% density. The sintered X245 was characterized by a dispersion of vanadium carbonitride precipitates, especially at the grain boundaries. The heat-treated X245 sample had the highest hardness of 680 ± 7 HV30 due to the matrix of tempered martensitic microstructure when compared to as-sintered with hardness of 554 ± 2 HV30. The X245/X120 interface was characterized by diffusion of Cr, Mo, Mn and C, which resulted in metallurgical bonding between the cladded materials. The impact wear resistance of the sintered X245 was eight times that of the X120; hence, a tough and wear-resistant tool is anticipated when the X120 work hardened in service. © 2019, ASM International.
    view abstractdoi: 10.1007/s11665-019-03942-2
  • 2019 • 527 Influence of spore size distribution, gas mixture, and process time on the removal rate of B. subtilis spores in low-pressure plasmas
    Fiebrandt, M. and Roggendorf, J. and Moeller, R. and Awakowicz, P.
    Journal of Physics D: Applied Physics 52 (2019)
    The size reduction of B. subtilis spores due to removal of biological material in low-pressure plasmas was analyzed in a double inductively coupled plasma system. Argon, nitrogen, and oxygen at 5 Pa were used as feed gases to investigate the impact of different reactive species and high energy radiation on the process. The spore size was determined using scanning electron microscopy images and the length of thousands of spores were evaluated using an automated algorithm. By applying a statistical test the precision of the mean spore size determination was increased and the applicability of a normal distribution to describe the spore size distribution was demonstrated. The removal rate was found to vary depending on the process gas as well as on the process time and was found to be largest with a mixture of nitrogen and oxygen and lowest in pure argon. With increasing treatment time the removal rate decreases significantly and tends to stop in all gases and inhibits the complete removal of spores and potentially hazardous biological material. Possible explanations for this effect are the aggregation of non-volatile compounds or the formation of cross-linked layers which significantly reduce the etching efficiency. © 2019 IOP Publishing Ltd.
    view abstractdoi: 10.1088/1361-6463/aafdcf
  • 2019 • 526 Development of filaments for fused deposition modeling 3D printing with medical grade poly(lactic-co-glycolic acid) copolymers
    Feuerbach, T. and Callau-Mendoza, S. and Thommes, M.
    Pharmaceutical Development and Technology 24 487-493 (2019)
    The manufacturing of custom implants and patient-tailored drug dosage forms with fused deposition modeling (FDM) three-dimensional (3D) printing is currently considered to be very promising. Most FDM printers are designed as an open filament system, for which filaments with a defined size are required. In addition to this processing requirement, the filament material must be of medical or pharmaceutical quality, in order to be suitable in these applications. In this work, filaments with nominal diameters of 1.75 mm and diameter tolerances of ±0.05 mm or lower were developed in a continuous extrusion process. The filaments were made from different medical grade poly(lactic-co-glycolic acid) (PLGA) copolymers. Thermal characterization of the material with differential scanning calorimetry (DSC) showed increased material degradation with increasing hydrophilicity. Mechanical characterization of the filaments showed tensile strengths in the range of 41–48 MPa and Young’s moduli in the range of 2055–2099 MPa. Stress relaxation tests showed no irreversible change in filament diameter under processing conditions similar to the utilized 3D printer. Due to unexpected differences in processability in the 3D printer, the molecular weight of the materials was identified as an additional relevant parameter. © 2018, © 2018 Informa UK Limited, trading as Taylor & Francis Group.
    view abstractdoi: 10.1080/10837450.2018.1514522
  • 2019 • 525 Synthesis, microstructure, and hardness of rapidly solidified Cu-Cr alloys
    Garzón-Manjón, A. and Christiansen, L. and Kirchlechner, I. and Breitbach, B. and Liebscher, C.H. and Springer, H. and Dehm, G.
    Journal of Alloys and Compounds 794 203-209 (2019)
    Cu-Cr alloys with ∼32 at.% Cr were rapidly solidified by splat quenching or laser melting techniques with the intention to form a very fine grained, non-equilibrium nanostructure similar to those obtained by severe plastic deformation or thin film deposition. The rapidly solidified Cu-Cr alloys were analyzed by X-ray diffraction, scanning electron microscopy and transmission electron microscopy. Both synthesis techniques lead to a similar two-phase microstructure with a nearly pure fcc Cu matrix with μm grain sizes and bcc Cr particles highly supersaturated with Cu. Splat quenching provides finer bcc particles with dimensions of less than 50 nm compared to laser melting with particle sizes of 100–2000 nm. In case of laser melting, (14 ± 2) at.% Cu are contained in the bcc phase, while splat quenching freezes (20 ± 2) at.% Cu in the bcc particles. The microstructures are discussed and compared to the non-equilibrium microstructures reported in literature using severe plastic deformation and thin films deposition. © 2019 Elsevier B.V.
    view abstractdoi: 10.1016/j.jallcom.2019.04.209
  • 2019 • 524 Surface roughening of Al2O3/Al2O3-ceramic matrix composites by nanosecond laser ablation prior to thermal spraying
    Gatzen, C. and Mack, D.E. and Guillon, O. and Vaßen, R.
    Journal of Laser Applications 31 (2019)
    Al2O3/Al2O3 ceramic matrix composites are candidate materials for high-temperature applications such as gas turbines. As water vapor corrosion of Al2O3/Al2O3-CMC (ceramic matrix composite) is a major issue, the application of suitable environmental barrier coatings is inevitable. An important factor for coating adhesion, especially in thermal spraying, is mechanical interlocking. Therefore, a rough substrate surface is needed. Although it has been proven that laser ablation is a suitable method for surface preparation of metallic substrates, no studies on Al2O3/Al2O3-CMCs are available. Therefore, the suitability of surface preparation of an Al2O3/Al2O3-CMC by laser ablation for use prior to atmospheric plasma spraying was examined. The laser ablation threshold fluence for Al2O3/Al2O3-CMC was determined. The effects of different processing parameters on the surface were studied. Various surface morphologies were obtained, such as cauliflower and honeycomb structures. The samples were characterized by white light interferometry, laser microscopy, and scanning electron microscopy. The obtained surface structures were coated with Gd2Zr2O7. It was found that the adhesion strength of coatings on laser treated samples was drastically increased. © 2019 Laser Institute of America.
    view abstractdoi: 10.2351/1.5080546
  • 2019 • 523 On pinning-depinning and microkink-flow in solid state dewetting: Insights by in-situ ESEM on Al thin films
    Hieke, S.W. and Willinger, M.-G. and Wang, Z.-J. and Richter, G. and Chatain, D. and Dehm, G. and Scheu, C.
    Acta Materialia 165 153-163 (2019)
    The dynamics of solid state dewetting phenomena of a 50 nm thick, mazed bicrystalline Al film on single crystalline α-Al2O3 (sapphire) substrates was studied in-situ using an environmental scanning electron microscope (ESEM). The bicrystalline Al thin films served as a model system where the influence of grain boundaries and texture effects are well determined compared to polycrystalline films. The experiments were performed in controlled oxidizing and reducing atmospheres at 773 K and 823 K, respectively, to shed light on the differences in dewetting mechanisms and dynamics. While the reducing atmosphere led to spontaneous dewetting at 823 K after an incubation time of a few minutes, a hierarchical dewetting process was observed for the sluggish dewetting under oxidizing conditions. Voids initiated at (substrate or surface) defects and expanded trying to maintain a hexagonal shape. Pinning and depinning processes led to a discontinuous void growth and irregular void shapes including finger instabilities. As a consequence, the void growth followed a variety of power law exponents between 0.10 and 0.55. A new microkink-flow mechanism was discovered at the terminating Al planes at the void. © 2018
    view abstractdoi: 10.1016/j.actamat.2018.11.028
  • 2019 • 522 Nanoscale Physical and Chemical Structure of Iron Oxide Nanoparticles for Magnetic Particle Imaging
    Hufschmid, R. and Landers, J. and Shasha, C. and Salamon, S. and Wende, H. and Krishnan, K.M.
    Physica Status Solidi (A) Applications and Materials Science 216 (2019)
    In this work, the role of the nanoscale chemical and magnetic structure on relaxation dynamics of iron oxide nanoparticles in the context of magnetic particle imaging (MPI) is investigated with Mössbauer spectroscopy (MS) and electron energy loss spectroscopy (EELS). Two samples of 27 nm monodisperse iron oxide nanoparticles are compared, with and without an additional oxidation optimization step, with corresponding differences in structure and properties. Iron oxide nanoparticles synthesized in the presence of sufficient oxygen form single crystalline, inverse-spinel magnetite (Fe 3 O 4 ) and display magnetic properties suitable for MPI. A secondary wüstite (FeO) phase is observed in the diffraction pattern of unoptimized nanoparticles, which is antiferromagnetic and therefore unsuitable for MPI. Mössbauer spectra confirm the composition of the optimized nanoparticles to be ≈70% magnetite, with the remaining 30% oxidized to maghemite; in contrast, the as-synthesized particles (without the oxidation step) contained about 40% wüstite and 60% magnetite. The authors use scanning transmission electron microscopy (STEM) with electron energy loss spectroscopy (EELS) to probe iron 2p-3d electronic transitions and correlate their intensities with the oxidation state with sub-nanometer spatial resolution. The optimally oxidized nanoparticles are uniform in crystallography and phase, while the mixed phase nanoparticles are core-shell wüstite/magnetite. Further confirming the core-shell structure of the mixed phase nanoparticles, considerable spin canting in the in-field Mössbauer spectrum, likely caused by interface coupling, is observed. © 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/pssa.201800544
  • 2019 • 521 Improved in vitro test procedure for full assessment of the cytocompatibility of degradable magnesium based on ISO 10993-5/-12
    Jung, O. and Smeets, R. and Hartjen, P. and Schnettler, R. and Feyerabend, F. and Klein, M. and Wegner, N. and Walther, F. and Stangier, D. and Henningsen, A. and Rendenbach, C. and Heiland, M. and Barbeck, M. and Kopp, A.
    International Journal of Molecular Sciences 20 (2019)
    Magnesium (Mg)-based biomaterials are promising candidates for bone and tissue regeneration. Alloying and surface modifications provide effective strategies for optimizing and tailoring their degradation kinetics. Nevertheless, biocompatibility analyses of Mg-based materials are challenging due to its special degradation mechanism with continuous hydrogen release. In this context, the hydrogen release and the related (micro-) milieu conditions pretend to strictly follow in vitro standards based on ISO 10993-5/-12. Thus, special adaptions for the testing of Mg materials are necessary, which have been described in a previous study from our group. Based on these adaptions, further developments of a test procedure allowing rapid and effective in vitro cytocompatibility analyses of Mg-based materials based on ISO 10993-5/-12 are necessary. The following study introduces a new two-step test scheme for rapid and effective testing of Mg. Specimens with different surface characteristics were produced by means of plasma electrolytic oxidation (PEO) using silicate-based and phosphate-based electrolytes. The test samples were evaluated for corrosion behavior, cytocompatibility and their mechanical and osteogenic properties. Thereby, two PEO ceramics could be identified for further in vivo evaluations. © 2019 by the authors. Licensee MDPI, Basel, Switzerland.
    view abstractdoi: 10.3390/ijms20020255
  • 2019 • 520 In vivo simulation of magnesium degradability using a new fluid dynamic bench testing approach
    Jung, O. and Porchetta, D. and Schroeder, M.-L. and Klein, M. and Wegner, N. and Walther, F. and Feyerabend, F. and Barbeck, M. and Kopp, A.
    International Journal of Molecular Sciences 20 (2019)
    The degradation rate of magnesium (Mg) alloys is a key parameter to develop Mg-based biomaterials and ensure in vivo-mechanical stability as well as to minimize hydrogen gas production, which otherwise can lead to adverse effects in clinical applications. However, in vitro and in vivo results of the same material often differ largely. In the present study, a dynamic test bench with several single bioreactor cells was constructed to measure the volume of hydrogen gas which evolves during magnesium degradation to indicate the degradation rate in vivo. Degradation medium comparable with human blood plasma was used to simulate body fluids. The media was pumped through the different bioreactor cells under a constant flow rate and 37?C to simulate physiological conditions. A total of three different Mg groups were successively tested: Mg WE43, and two different WE43 plasma electrolytically oxidized (PEO) variants. The results were compared with other methods to detect magnesium degradation (pH, potentiodynamic polarization (PDP), cytocompatibility, SEM (scanning electron microscopy)). The non-ceramized specimens showed the highest degradation rates and vast standard deviations. In contrast, the two PEO samples demonstrated reduced degradation rates with diminished standard deviation. The pH values showed above-average constant levels between 7.4-7.7, likely due to the constant exchange of the fluids. SEM revealed severe cracks on the surface of WE43 after degradation, whereas the ceramized surfaces showed significantly decreased signs of corrosion. PDP results confirmed the improved corrosion resistance of both PEO samples. While WE43 showed slight toxicity in vitro, satisfactory cytocompatibility was achieved for the PEO test samples. In summary, the dynamic test bench constructed in this study enables reliable and simple measurement of Mg degradation to simulate the in vivo environment. Furthermore, PEO treatment of magnesium is a promising method to adjust magnesium degradation. © 2019 by the authors. Licensee MDPI, Basel, Switzerland.
    view abstractdoi: 10.3390/ijms20194859
  • 2019 • 519 Synthesis of Mixed AuZn Nanoparticles by Spark Discharge Technique
    Kala, S. and Kruis, F.E.
    MRS Advances 4 1621-1629 (2019)
    In this study, feasibility of spark discharge technique to generate mixed metal nanoparticles is demonstrated. Two immiscible metals Au and Zn are selected to prepare AuZn mixed nanoparticles. Ignition of spark between Au and Zn electrodes under normal pressure, in the presence of carrier gas, leads to formation of mixed nanoparticles by condensation and nucleation. Online particle size-distribution is monitored by a scanning mobility particle sizer on changing carrier gas flow rate and capacitor charging current during co-sparking between Au and Zn electrodes. The technique provides flexibility to generate binary mixture of AuZn nanoparticles in the size range of 10-80 nm. Distribution of Au and Zn in the prepared mixed nanoparticles is mapped by scanning electron microscopy and high resolution electron microscopy. © Materials Research Society 2019.
    view abstractdoi: 10.1557/adv.2019.221
  • 2019 • 518 Repair of Ni-based single-crystal superalloys using vacuum plasma spray
    Kalfhaus, T. and Schneider, M. and Ruttert, B. and Sebold, D. and Hammerschmidt, T. and Frenzel, J. and Drautz, R. and Theisen, W. and Eggeler, G. and Guillon, O. and Vassen, R.
    Materials and Design 168 (2019)
    Turbine blades in aviation engines and land based gas-turbines are exposed to extreme environments. They suffer damage accumulation associated with creep, oxidation and fatigue loading. Therefore, advanced repair methods are of special interest for the gas-turbine industry. In this study, CMSX-4 powder is sprayed by Vacuum Plasma Spray (VPS) on single-crystalline substrates with similar compositions. The influence of the substrate temperature is investigated altering the temperature of the heating stage between 850 °C to 1000 °C. Different spray parameters were explored to identify their influence on the microstructure. Hot isostatic pressing (HIP) featuring fast quenching rates was used to minimize porosity and to allow for well-defined heat-treatments of the coatings. The microstructure was analysed by orientation imaging scanning electron microscopy (SEM), using electron backscatter diffraction (EBSD). The effects of different processing parameters were analysed regarding their influence on porosity and grain size. The results show that optimized HIP heat-treatments can lead to dense coatings with optimum γ/γ′ microstructure. The interface between the coating and the substrate is oxide free and shows good mechanical integrity. The formation of fine crystalline regions as a result of fast cooling was observed at the single-crystal surface, which resulted in grain growth during heat-treatment in orientations determined by the crystallography of the substrate. © 2019
    view abstractdoi: 10.1016/j.matdes.2019.107656
  • 2019 • 517 Femtosecond laser patterning of graphene electrodes for thin-film transistors
    Kasischke, M. and Subaşı, E. and Bock, C. and Pham, D.-V. and Gurevich, E.L. and Kunze, U. and Ostendorf, A.
    Applied Surface Science 478 299-303 (2019)
    The aim of this study is to assess femtosecond laser patterning of graphene in air and in vacuum for the application as source and drain electrodes in thin-film transistors (TFTs). The analysis of the laser-patterned graphene with scanning electron microscopy, atomic force microscopy and Raman spectroscopy showed that processing in vacuum leads to less debris formation and thus re-deposited carbonaceous material on the sample compared to laser processing in air. It was found that the debris reduction due to patterning in vacuum improves the TFT characteristics significantly. Hysteresis disappears, the mobility is enhanced by an order of magnitude and the subthreshold swing is reduced from S sub = 2.5 V/dec to S sub = 1.5 V/dec. © 2019 Elsevier B.V.
    view abstractdoi: 10.1016/j.apsusc.2019.01.198
  • 2019 • 516 Dual-source evaporation of silver bismuth iodide films for planar junction solar cells
    Khazaee, M. and Sardashti, K. and Chung, C.-C. and Sun, J.-P. and Zhou, H. and Bergmann, E. and Dunlap-Shohl, W.A. and Han, Q. and Hill, I.G. and Jones, J.L. and Lupascu, D.C. and Mitzi, D.B.
    Journal of Materials Chemistry A 7 2095-2105 (2019)
    Non-toxic and air-stable silver bismuth iodide semiconductors are promising light absorber candidates for photovoltaic applications owing to a suitable band gap for multi- or single-junction solar cells. Recently, solution-based film fabrication approaches for several silver bismuth iodide stoichiometries have been investigated. The current work reports on a facile and reproducible two-step coevaporation/annealing approach to deposit compact and pinhole-free films of AgBi 2 I 7 , AgBiI 4 and Ag 2 BiI 5 . X-ray diffraction (XRD) in combination with scanning electron microscopy (SEM)/energy-dispersive X-ray spectroscopy (EDX) analysis reveals formation of pure cubic (Fd3m) phase AgBi 2 I 7 , cubic (Fd3m) or rhombohedra (R3m) phase AgBiI 4 , each with &gt;3 μm average grain size, or the rhombohedral phase (R3m) Ag 2 BiI 5 with &gt;200 nm average grain size. A phase transition from rhombohedral to cubic structure is investigated via temperature-dependent X-ray diffraction (TD-XRD). Planar-junction photovoltaic (PV) devices are prepared based on the coevaporated rhombohedral AgBiI 4 films, with titanium dioxide (TiO 2 ) and poly(3-hexylthiophene) (P3HT) as electron- and hole-transport layers, respectively. The best-performing device exhibited a power conversion efficiency (PCE) of as high as 0.9% with open-circuit voltage (V OC ) &gt; 0.8 V in the reverse scan direction (with significant hysteresis). © 2019 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c8ta08679f
  • 2019 • 515 Induction of herpes simplex virus type 1 cell-to-cell spread inhibiting antibodies by a calcium phosphate nanoparticle-based vaccine
    Kopp, M. and Aufderhorst, U.W. and Alt, M. and Dittmer, U. and Eis-Hübinger, A.-M. and Giebel, B. and Roggendorf, M. and Epple, M. and Krawczyk, A.
    Nanomedicine: Nanotechnology, Biology, and Medicine 16 138-148 (2019)
    Herpes simplex viruses 1 and 2 are among the most ubiquitous human infections and persist lifelong in their host. Upon primary infection or reactivation from ganglia, the viruses spread by direct cell–cell contacts (cell-to-cell spread) and thus escape from the host immune response. We have developed a monoclonal antibody (mAb 2c), which inhibits the HSV cell-to-cell spread, thereby protecting from lethal genital infection and blindness in animal models. In the present study we have designed a nanoparticle-based vaccine to induce protective antibody responses exceeding the cell-to-cell spread inhibiting properties of mAb 2c. We used biodegradable calcium phosphate (CaP) nanoparticles coated with a synthetic peptide that represents the conformational epitope on HSV-1 gB recognized by mAb 2c. The CaP nanoparticles additionally contained a TLR-ligand CpG m and were formulated with adjuvants to facilitate the humoral immune response. This vaccine effectively protected mice from lethal HSV-1 infection by inducing cell-to-cell spread inhibiting antibodies. © 2018 The Authors
    view abstractdoi: 10.1016/j.nano.2018.12.002
  • 2019 • 514 Perforating Freestanding Molybdenum Disulfide Monolayers with Highly Charged Ions
    Kozubek, R. and Tripathi, M. and Ghorbani-Asl, M. and Kretschmer, S. and Madauß, L. and Pollmann, E. and O'Brien, M. and McEvoy, N. and Ludacka, U. and Susi, T. and Duesberg, G.S. and Wilhelm, R.A. and Krasheninnikov, A.V. and Ko...
    Journal of Physical Chemistry Letters 10 904-910 (2019)
    Porous single-layer molybdenum disulfide (MoS 2 ) is a promising material for applications such as DNA sequencing and water desalination. In this work, we introduce irradiation with highly charged ions (HCIs) as a new technique to fabricate well-defined pores in MoS 2 . Surprisingly, we find a linear increase of the pore creation efficiency over a broad range of potential energies. Comparison to atomistic simulations reveals the critical role of energy deposition from the ion to the material through electronic excitation in the defect creation process and suggests an enrichment in molybdenum in the vicinity of the pore edges at least for ions with low potential energies. Analysis of the irradiated samples with atomic resolution scanning transmission electron microscopy reveals a clear dependence of the pore size on the potential energy of the projectiles, establishing irradiation with highly charged ions as an effective method to create pores with narrow size distributions and radii between ca. 0.3 and 3 nm. © 2019 American Chemical Society.
    view abstractdoi: 10.1021/acs.jpclett.8b03666
  • 2019 • 513 Thermo-mechanical properties of mullite ceramics: New data
    Krenzel, T.F. and Schreuer, J. and Laubner, D. and Cichocki, M. and Schneider, H.
    Journal of the American Ceramic Society 102 416-426 (2019)
    Coefficients of elastic stiffnesses and thermal expansion of hot isostatically pressed, reaction-sintered and technical fused-mullite ceramics were measured between 100 and 1673 K in comparison with single crystal mullite employing resonant ultrasound spectroscopy and dilatometry, respectively. Additionally, chemical and phase compositions and the microstructure of the ceramics were studied using X-ray diffraction techniques and scanning electron microscopy. Our studies revealed that despite polycrystallinity and slight porosity of up to 1.6%, the elastic behavior of the hot isostatically pressed ceramics is near to ideal aggregate elastic properties of mullite single crystal, for example, their bulk moduli fit within 0.7% to B = 170.0 GPa of single crystal mullite. On the other hand, with B = 155 GPa, the reaction-sintered mullite behaves significantly softer. The difference can be explained with more tight grain to grain contacts in hot isostatically pressed ceramics as compared to reaction-sintered materials. The thermal expansion of both types of ceramics almost coincides with the corresponding averaged behavior of single crystal mullite. For example, between 573 and 1273 K, the volume expansion coefficients of all these materials are (18.0 ± 0.3)·10−6 K−1. Obviously, the microstructural features are less important for the macroscopic thermal expansion. Due to heterogeneous microstructure and high α-alumina and zirconia contents, the corresponding properties of fused-mullite refractory deviate strongly from those of the other mullite materials. © 2018 The American Ceramic Society
    view abstractdoi: 10.1111/jace.15925
  • 2019 • 512 Time-resolved ATP measurements during vesicle respiration
    Lin, J. and Weixler, D. and Daboss, S. and Seibold, G.M. and Andronescu, C. and Schuhmann, W. and Kranz, C.
    Talanta 205 (2019)
    In vitro synthesis of ATP catalyzed by the ATP-synthase requires membrane vesicles, in which the ATP-synthase is present within the bilayer membrane. Inverted vesicle prepared from Gram negative cells (e.g., Escherichia coli or Pseudomonas putida) can be readily obtained and used for in vitro ATP-synthesis. Up to now, quantification of ATP synthesized by membrane vesicles has been mostly analyzed via bioluminescence-based assays. Alternatively, vesicle respiration and the associated ATP level can be determined using biosensors, which not only provide high selectivity, but allow ATP measurements without the sample being illuminated. Here, we present a microbiosensor for ATP in combination with scanning electrochemical microscopy (SECM) using an innovative two-compartment electrochemical cell for the determination of ATP levels at E.coli or P. putida inverted vesicles. For a protein concentration of 22 mg/ml, a total amount of 0.29 ± 0.03 μM/μl ATP per vesicle was determined in case of E.coli; in turn, P. putida derived vesicles yielded 0.48 ± 0.02 μM/μl ATP per vesicle at a total protein concentration of 25.2 mg/ml. Inhibition experiments with Venturicidin A clearly revealed that the respiratory chain enzyme complex responsible for ATP generation is effectively involved. © 2019 Elsevier B.V.
    view abstractdoi: 10.1016/j.talanta.2019.06.083
  • 2019 • 511 Microscopic Determination of Carrier Density and Mobility in Working Organic Electrochemical Transistors
    Mariani, F. and Conzuelo, F. and Cramer, T. and Gualandi, I. and Possanzini, L. and Tessarolo, M. and Fraboni, B. and Schuhmann, W. and Scavetta, E.
    Small 15 (2019)
    A comprehensive understanding of electrochemical and physical phenomena originating the response of electrolyte-gated transistors is crucial for improved handling and design of these devices. However, the lack of suitable tools for direct investigation of microscale effects has hindered the possibility to bridge the gap between experiments and theoretical models. In this contribution, a scanning probe setup is used to explore the operation mechanisms of organic electrochemical transistors by probing the local electrochemical potential of the organic film composing the device channel. Moreover, an interpretative model is developed in order to highlight the meaning of electrochemical doping and to show how the experimental data can give direct access to fundamental device parameters, such as local charge carrier concentration and mobility. This approach is versatile and provides insight into the organic semiconductor/electrolyte interface and useful information for materials characterization, device scaling, and sensing optimization. © 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/smll.201902534
  • 2019 • 510 Highly Compact TiO 2 Films by Spray Pyrolysis and Application in Perovskite Solar Cells
    Möllmann, A. and Gedamu, D. and Vivo, P. and Frohnhoven, R. and Stadler, D. and Fischer, T. and Ka, I. and Steinhorst, M. and Nechache, R. and Rosei, F. and Cloutier, S.G. and Kirchartz, T. and Mathur, S.
    Advanced Engineering Materials 21 (2019)
    Transparent and pinhole free hole-blocking layers such as TiO 2 grown at low temperatures and by scalable processes are necessary to reduce production costs and thus enabling commercialization of perovskite solar cells. Here, the authors compare the transport properties of TiO 2 compact layers grown by spray pyrolysis from commonly used titanium diisopropoxide bisacetylacetonate ([Ti(OPr i ) 2 (acac) 2 ]) precursor to films grown by spray pyrolysis of TiCl 4 . Spray pyrolysis provides insights into the interdependence of precursor chemistry and electron transport properties of TiO 2 films and their influence on the performance of the perovskite solar cells. X-ray diffraction and X-ray photoelectron spectroscopy data confirm the chemical and structural composition of the obtained films. Thin film deposition at lower temperature (150 °C) are conducted using TiCl 4 to evaluate the influence of crystal growth and topography by scanning electron microscopy and atomic force microscopy as well as thickness (profilometry) and transmittance (UV/Vis spectroscopy) on the power conversion efficiency of perovskite solar cells. TiO 2 compact layers grown from TiCl 4 enhance the power conversion efficiency by acting as superior electron transfer medium and by reducing hysteresis behavior, when compared to films grown using titanium diisopropoxide bisacetylacetonate. UV/Vis spectroscopy and external quantum efficiency studies reveal the correlation of transmittance on the power conversion efficiency. © 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/adem.201801196
  • 2019 • 509 Catalytic Reactivation of Industrial Oxygen Depolarized Cathodes by in situ Generation of Atomic Hydrogen
    Öhl, D. and Franzen, D. and Paulisch, M. and Dieckhöfer, S. and Barwe, S. and Andronescu, C. and Manke, I. and Turek, T. and Schuhmann, W.
    ChemSusChem 12 2732-2739 (2019)
    Electrocatalytically active materials on the industrial as well as on the laboratory scale may suffer from chemical instability during operation, air exposure, or storage in the electrolyte. A strategy to recover the loss of electrocatalytic activity is presented. Oxygen-depolarized cathodes (ODC), analogous to those that are utilized in industrial brine electrolysis, are analyzed: the catalytic activity of the electrodes upon storage (4 weeks) under industrial process conditions (30 wt % NaOH, without operation) diminishes. This phenomenon occurs as a consequence of surface oxidation and pore blockage, as revealed by scanning electron microscopy, focused ion beam milling, X-ray photoelectron spectroscopy, and Raman spectroscopy. Potentiodynamic cycling of the oxidized electrodes to highly reductive potentials and the formation of “nascent” hydrogen re-reduces the electrode material, ultimately recovering the former catalytic activity. © 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/cssc.201900628
  • 2019 • 508 Cellulose acetate/layered double hydroxide adsorptive membranes for efficient removal of pharmaceutical environmental contaminants
    Raicopol, M.D. and Andronescu, C. and Voicu, S.I. and Vasile, E. and Pandele, A.M.
    Carbohydrate Polymers 214 204-212 (2019)
    The increasing amount of residual pharmaceutical contaminants in wastewater has a negative impact on both the environment and human health. In the present study, we developed new cellulose acetate/Mg-Al layered double hydroxide (Mg-Al LDH) nanocomposite membranes as an efficient method to remove pharmaceutical substances from wastewater. The morphology, porosity, surface properties and thermal stability of nanocomposite membranes containing various amounts of nanofiller were evaluated by scanning electron microscopy (SEM), X-ray microtomography (μCT), contact angle measurements and thermogravimetric analysis (TGA). The Mg-Al LDH nanofiller showed a high degree of exfoliation in the polymer matrix, evidenced by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The hydrodynamic properties and adsorption capacity were evaluated with pure water and aqueous solutions of two common drugs, diclofenac sodium (DS) and tetracycline (TC), and the nanocomposite membranes showed an improved permeability compared with neat cellulose acetate. The membrane prepared with 4 wt.% Mg-Al LDH loading exhibited the highest water flux compared with the pure polymer one (529 vs 36 L·m −2 ·h -1 ) and a tenfold increase in adsorption capacity for DS. This enhancement is attributed to electrostatic interactions between the negatively charged drug molecule and positively charged Mg-Al LDH layers. Conversely, in the case of TC, the increase in adsorption capacity was smaller and was assigned to hydrogen bonding interactions between the drug molecule and the nanofiller. © 2019 Elsevier Ltd
    view abstractdoi: 10.1016/j.carbpol.2019.03.042
  • 2019 • 507 The Key Role of Water Activity for the Operating Behavior and Dynamics of Oxygen Depolarized Cathodes
    Röhe, M. and Botz, A. and Franzen, D. and Kubannek, F. and Ellendorff, B. and Öhl, D. and Schuhmann, W. and Turek, T. and Krewer, U.
    ChemElectroChem 6 5671-5681 (2019)
    Advanced chlor-alkali electrolysis with oxygen depolarized cathodes (ODC) requires 30 % less electrical energy than conventional hydrogen-evolution-based technology. Herein, we confirm that the activities of hydroxide and water govern the ODC performance and its dynamics. Experimental characterization of ODC under varying mass transfer conditions on the liquid side reveals large differences in the polarization curves as well as in potential step responses of the electrodes. Under convective transport in the liquid electrolyte, the ODC is not limited by mass transfer in its current density at j&gt;3.9 kA m−2, whereas transport limitations are already reached at j≈1.3 kA m−2 with a stagnant electrolyte. Since gas phase conditions do not differ significantly between the measurements, these results are in contrast the common assumption that oxygen supply determines ODC performance. A dynamic model reveals the strong influence of the electrolyte mass transfer conditions on oxygen availability and thus performance. Dynamic responses of the current density to step-wise potential changes are dominated by the mass transport of water and hydroxide ions, which is by orders of magnitude faster with convective electrolyte flow. Without convective liquid electrolyte transport, a high accumulation of hydroxide ions significantly lowers the oxygen solubility. Thus, a fast mass transport of water and hydroxide is essential for high ODC performance and needs to be ensured for technical applications. The predicted accumulation of ions is furthermore validated experimentally by means of scanning electrochemical microscopy. We also show how the outlined processes can explain the distinctively different potential step responses with and without electrolyte convection. © 2019 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.
    view abstractdoi: 10.1002/celc.201901224
  • 2019 • 506 Local quasi-static and cyclic deformation behaviour of brazed AISI 304L/BAu-4 joints characterised by digital image correlation
    Schmiedt, A. and Manka, M. and Tillmann, W. and Walther, F.
    Welding in the World 63 501-509 (2019)
    For a reliable design of brazed components, the degradation of mechanical properties due to the corrosive attack by aggressive operating environments has to be considered. In this study, the effect of a condensate corrosion, which is performed according to VDA test sheet 230-214 up to 6 weeks, on the mechanical behaviour of brazed AISI 304L/BAu-4 stainless steel joints is investigated. A time-dependent reduction of the tensile and fatigue strength values down to 42% of the as-received condition is determined. As standard strain measurements are not appropriate to characterise the local strain distributions of heterogeneous material systems, the optical digital image correlation technique is used to evaluate the local quasi-static and cyclic deformation behaviour of the 50 μm wide brazing seam. A novel triggered image acquisition enables measurements in fatigue tests at a frequency of 10 Hz. The reduction of the virtual gauge length from 12.5 down to 0.5 mm leads to an increase of the total strain and ratcheting strain values, which is more pronounced for higher stresses and enhanced for pre-corroded brazed joints. For a microstructure-related analysis of the damage processes, scanning electron microscopy was used. © 2019, International Institute of Welding.
    view abstractdoi: 10.1007/s40194-018-00693-x
  • 2019 • 505 Retrofitting metal-organic frameworks
    Schneider, C. and Bodesheim, D. and Keupp, J. and Schmid, R. and Kieslich, G.
    Nature Communications 10 (2019)
    The post-synthetic installation of linker molecules between open-metal sites (OMSs) and undercoordinated metal-nodes in a metal-organic framework (MOF) — retrofitting — has recently been discovered as a powerful tool to manipulate macroscopic properties such as the mechanical robustness and the thermal expansion behavior. So far, the choice of cross linkers (CLs) that are used in retrofitting experiments is based on qualitative considerations. Here, we present a low-cost computational framework that provides experimentalists with a tool for evaluating various CLs for retrofitting a given MOF system with OMSs. After applying our approach to the prototypical system CL@Cu3BTC2 (BTC = 1,3,5-benzentricarboxylate) the methodology was expanded to NOTT-100 and NOTT-101 MOFs, identifying several promising CLs for future CL@NOTT-100 and CL@NOTT-101 retrofitting experiments. The developed model is easily adaptable to other MOFs with OMSs and is set-up to be used by experimentalists, providing a guideline for the synthesis of new retrofitted MOFs with modified physicochemical properties. © 2019, The Author(s).
    view abstractdoi: 10.1038/s41467-019-12876-1
  • 2019 • 504 Tungsten carbide as a deoxidation agent for plasma-facing tungsten-based materials
    Šestan, A. and Zavašnik, J. and Kržmanc, M.M. and Kocen, M. and Jenuš, P. and Novak, S. and Čeh, M. and Dehm, G.
    Journal of Nuclear Materials 524 135-140 (2019)
    Tungsten (W) and various composites are being considered as the primary plasma-facing materials for fusion reactors. Like all engineering materials, they contain certain levels of impurities, which can have an important impact on mechanical properties. In the present work, oxygen was identified as a major impurity in our starting tungsten powder. At elevated temperatures, the presence of interstitial elements such as oxygen leads to the formation of an oxide-rich tungsten phase at the tungsten grain boundaries. In this study, we determined the capacity of tungsten carbide (WC) nanoparticles to remove the oxide impurities from a tungsten body. Tungsten composites with 0.05, 0.25 and 0.51 wt. % carbon (C) in the form of WC were sintered using a field-assisted sintering technique (FAST) at 1900 °C for 5 min. The sintered samples were characterized using field-emission scanning and transmission electron microscopy. Thermodynamic and kinetic considerations allowed us to determine the optimum theoretical amount of WC to prevent the in-situ formation of WO2. © 2019 Andreja Šestan, Janez Zavašnik, Marjeta Maček Kržmanc, Matej Kocen, Petra Jenuš, Saša Novak, Miran Čeh, Gerhard Dehm
    view abstractdoi: 10.1016/j.jnucmat.2019.06.030
  • 2019 • 503 Contaminant transport in soil: A comparison of the Theory of Porous Media approach with the microfluidic visualisation
    Seyedpour, S.M. and Janmaleki, M. and Henning, C. and Sanati-Nezhad, A. and Ricken, T.
    Science of the Total Environment 686 1272-1281 (2019)
    Visualisation of the groundwater flow and contaminant transport can play a significant role for a better understanding of contaminant fate, which helps decision-makers and contaminated site planners to choose and implement the best remediation strategies. In this paper, a microfluidic chip coated with nanoclay was developed to mimic soil behaviour. Scanning electron microscopy (SEM) images and Fourier-transform infrared spectroscopy (FTIR) analysis confirmed that all the features and surfaces are coated with nanoclay. The change of contact angle for the native polydimethylsiloxane (PDMS) from 151° ± 4° to 73° ± 6° for modified ones is indicative of a considerable shift to hydrophilic behaviour. Moreover, the transport process in the developed chip was simulated utilising the Theory of Porous Media (TPM) and computational fluid dynamic (CFD) approaches. Although the results of both numerical approaches are in good agreement with experiments, the Root Mean Square Error (RMSE) of the predicted contaminant concentration by TPM at two observation points is less than that of CFD. © 2019
    view abstractdoi: 10.1016/j.scitotenv.2019.05.095
  • 2019 • 502 Achieving ultra-high strength and ductility in equiatomic CrCoNi with partially recrystallized microstructures
    Slone, C.E. and Miao, J. and George, E.P. and Mills, M.J.
    Acta Materialia 165 496-507 (2019)
    Despite having otherwise outstanding mechanical properties, many single-phase medium and high entropy alloys are limited by modest yield strengths. Although grain refinement offers one opportunity for additional strengthening, it requires significant and undesirable compromises to ductility. This work therefore explores an alternative, simple processing route to achieve strength by cold-rolling and annealing an equiatomic CrCoNi alloy to produce heterogeneous, partially recrystallized microstructures. Tensile tests reveal that our approach dramatically increases the yield strength (to ∼1100 MPa) while retaining good ductility (total elongation ∼23%) in the single-phase CrCoNi alloy. Scanning and transmission electron microscopy indicate that the strengthening is due to the non-recrystallized grains retaining their deformation-induced twins and very high dislocation densities. Load-unload-reload tests and grain-scale digital image correlation are also used to study the accumulation of plastic deformation in our highly heterogeneous microstructures. © 2018 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2018.12.015
  • 2019 • 501 Sound-driven single-electron transfer in a circuit of coupled quantum rails
    Takada, S. and Edlbauer, H. and Lepage, H.V. and Wang, J. and Mortemousque, P.-A. and Georgiou, G. and Barnes, C.H.W. and Ford, C.J.B. and Yuan, M. and Santos, P.V. and Waintal, X. and Ludwig, Ar. and Wieck, A.D. and Urdampilleta,...
    Nature Communications 10 (2019)
    Surface acoustic waves (SAWs) strongly modulate the shallow electric potential in piezoelectric materials. In semiconductor heterostructures such as GaAs/AlGaAs, SAWs can thus be employed to transfer individual electrons between distant quantum dots. This transfer mechanism makes SAW technologies a promising candidate to convey quantum information through a circuit of quantum logic gates. Here we present two essential building blocks of such a SAW-driven quantum circuit. First, we implement a directional coupler allowing to partition a flying electron arbitrarily into two paths of transportation. Second, we demonstrate a triggered single-electron source enabling synchronisation of the SAW-driven sending process. Exceeding a single-shot transfer efficiency of 99%, we show that a SAW-driven integrated circuit is feasible with single electrons on a large scale. Our results pave the way to perform quantum logic operations with flying electron qubits. © 2019, The Author(s).
    view abstractdoi: 10.1038/s41467-019-12514-w
  • 2019 • 500 Abrasion resistance of textiles: Gaining insight into the damaging mechanisms of different test procedures
    Textor, T. and Derksen, L. and Bahners, T. and Gutmann, J.S. and Mayer-Gall, T.
    Journal of Engineered Fibers and Fabrics 14 (2019)
    Three established test methods employed for evaluating the abrasion or wear resistance of textile materials were compared to gain deeper insight into the specific damaging mechanisms to better understand a possible comparability of the results of the different tests. The knowledge of these mechanisms is necessary for a systematic development of finishing agents improving the wear resistance of textiles. Martindale, Schopper, and Einlehner tests were used to analyze two different fabrics made of natural (cotton) or synthetic (polyethylene terephthalate) fibers, respectively. Samples were investigated by digital microscopy and scanning electron microscopy to visualize the damage. Damage symptoms are compared and discussed with respect to differences in the damaging mechanisms. © The Author(s) 2019.
    view abstractdoi: 10.1177/1558925019829481
  • 2019 • 499 Ni-base superalloy single crystal (SX) mosaicity characterized by the Rotation Vector Base Line Electron Back Scatter Diffraction (RVB-EBSD) method
    Thome, P. and Medghalchi, S. and Frenzel, J. and Schreuer, J. and Eggeler, G.
    Ultramicroscopy 206 (2019)
    In the present work we present the Rotation Vector Base Line Electron Back Scatter Diffraction (RVB-EBSD) method, a new correlative orientation imaging method for scanning electron microscopy (OIM/SEM). The RVB-EBSD method was developed to study crystal mosaicity in as-cast Ni-base superalloy single crystals (SX). The technique allows to quantify small crystallographic deviation angles between individual dendrites and to interpret associated accommodation processes in terms of geometrically necessary dislocations (GNDs). The RVB-EBSD method was inspired by previous seminal approaches which use cross correlation EBSD procedures. It applies Gaussian band pass filtering to improve the quality of more than 500 000 experimental patterns. A rotation vector approximation and a correction procedure, which relies on a base line function, are used. The method moreover features a novel way of intuitive color coding which allows to easily appreciate essential features of crystal mosaicity. The present work describes the key elements of the method and shows examples which demonstrate its potential. © 2019 Elsevier B.V.
    view abstractdoi: 10.1016/j.ultramic.2019.112817
  • 2019 • 498 Biomimetic structural coloration with tunable degree of angle-independence generated by two-photon polymerization
    Zyla, G. and Kovalev, A. and Heisterkamp, S. and Esen, C. and Gurevich, E.L. and Gorb, S. and Ostendorf, A.
    Optical Materials Express 9 2630-2639 (2019)
    A successful realization of photonic systems with characteristics of the Morpho butterfly coloration is reported using two-photon polymerization. Submicron structure features have been fabricated through the interference of the incident beam and the reflected beam in a thin polymer film. Furthermore, the influence of the lateral microstructure organization on the color formation has been studied in detail. The design of the polymerized structures was validated by scanning electron microscopy. The optical properties were analyzed using an angleresolved spectrometer. Tunable angle-independence, based on reflection intensity modulation, has been investigated by using photonic structures with less degree of symmetry. Finally, these findings were used to demonstrate the high potential of two-photon polymerization in the field of biomimetic research and for technical application, e.g. for sensing and anti-counterfeiting. © 2019 Optical Society of America.
    view abstractdoi: 10.1364/OME.9.002630
  • 2018 • 497 Influence of controlled functionalization of mesoporous silica nanoparticles as tailored fillers for thin-film nanocomposite membranes on desalination performance
    Abdelsamad, A.M.A. and Khalil, A.S.G. and Ulbricht, M.
    Journal of Membrane Science 563 149-161 (2018)
    Thin film nanocomposite (TFN) membranes comprising of controlled functionalized mesoporous silica nanoparticles (MSN) blended in the polyamide (PA) barrier layer were prepared via interfacial polymerization of m-phenylenediamine and trimesoyl chloride on a porous polyethersulfone support membrane. MSN were synthesized by sol-gel process and then functionalized with octadecyltrichlorosilane (OTS) using post-grafting method. The nitrogen adsorption measurements demonstrated that the hydrophobic alkyl chains of OTS can be grafted onto the internal pores of MSN or just be located on the external particle surface, depending on the functionalization procedure and the OTS concentration. The functionalized nanoparticles with a diameter of about 80 nm were thereafter easily dispersed in the organic phase during the interfacial polymerization. Evaluation of membranes’ performance was based on water and ethanol permeability measurements, in addition to salt rejection from aqueous solutions. The results indicated that the functionalization of the external surface of MSN only, without extension to the interior pores surface, significantly increased both water and ethanol permeabilities. Contrarily, the surface modification of the MSN internal pores only increased the permeability of ethanol and reduced the water permeability, mainly due to the hydrophobicity of OTS. The influence of nanoparticles loading, as well as the concentration of OTS and thus the extent of MSN functionalization on the separation performance of TFN membranes were also investigated. TFN membranes prepared using the optimized MSN functionalization and loading yielded up to 63% higher water permeability compared to the reference thin film composite membrane without sacrificing the membrane selectivity. This work clearly emphasizes the direct relationship between the internal pores of MSN as functional nanofiller in the PA barrier layer and increasing or decreasing the water permeability of resulting TFN membranes. © 2018 Elsevier B.V.
    view abstractdoi: 10.1016/j.memsci.2018.05.043
  • 2018 • 496 Laser beam welding of dual-phase DP1000 steel
    Alves, P.H.O.M. and Lima, M.S.F. and Raabe, D. and Sandim, H.R.Z.
    Journal of Materials Processing Technology 252 498-510 (2018)
    Autogeneous laser beam welding is an efficient process to join ferritic-martensitic dual-phase steels without large dimensional distortions. Localized softening may occur in the heat affected zone, particularly in DP1000 steel with a high martensite volume fraction. DP1000 steel was welded in bead-on-plate configuration varying the welding power between 0.4 and 2.0 kW and the welding speed between 20 and 150 mm/s. Light optical microscopy, scanning electron microscopy, and X-ray diffraction were used to perform the microstructural characterization of the welded joints. High-quality laser beam welds of thin sheets of DP1000 steel can be produced using appropriate welding parameters. The optimal welding condition was a nominal laser power of 2.0 kW and a welding speed of 150 mm/s. This condition minimizes the amount of softening of prior martensite and yields a narrow heat affected zone and a small volume fraction of retained austenite in the weld. © 2017 Elsevier B.V.
    view abstractdoi: 10.1016/j.jmatprotec.2017.10.008
  • 2018 • 495 Qualification of selective laser-melted Al alloys against fatigue damage by means of measurement and modeling techniques
    Awd, M. and Johannsen, J. and Siddique, S. and Emmelmann, C. and Walther, F.
    MATEC Web of Conferences 165 (2018)
    Aluminum alloys processed through selective laser melting possess unique features of microstructure, defect morphology and mechanical properties. Constitution of fine cellular dendrites results from the high-cooling rate of the melt pool during the consolidation process. Investigation of the microstructure by scanning electron microscopy identifies supersaturation of Si particles as a secondary strengthening mechanism. On the contrary, platform heating that induces coarser microstructure leads to migration of Si particles from the Al matrix to the eutectic phase. As a result, tensile strength is reduced by ∼3%, while fracture strain is increased by ∼17%. Fine-grained structures exhibit a lower amount of plastic damage accumulation as well as delayed crack initiation as determined by the applied measurement techniques. Finite element models of the investigated configurations are obtained using scans of computed tomography under consideration of process-induced defects. Comparison of modeling and experimental results concluded that dominant fatigue damage mechanisms are related to the loading regime from low-cycle (LCF) to very-high-cycle fatigue (VHCF). Thus, process-inherent features of microstructure and porosity have different quantitative effects concerning the applied load. In VHCF, a material configuration with platform heating possesses an improved fatigue strength by ∼33% at 1E9 cycles, concerning the material configuration without platform heating. © The Authors, published by EDP Sciences, 2018.
    view abstractdoi: 10.1051/matecconf/201816502001
  • 2018 • 494 Low-temperature MOCVD deposition of Bi2Te3 thin films using Et2BiTeEt as single source precursor
    Bendt, G. and Gassa, S. and Rieger, F. and Jooss, C. and Schulz, S.
    Journal of Crystal Growth 490 77-83 (2018)
    Et2BiTeEt was used as single source precursor for the deposition of Bi2Te3 thin films on Si(1 0 0) substrates by metal organic chemical vapor deposition (MOCVD) at very low substrate temperatures. Stoichiometric and crystalline Bi2Te3 films were grown at 230 °C, which is approximately 100 °C lower compared to conventional MOCVD processes using one metal organic precursors for each element. The Bi2Te3 films were characterized using scanning electron microscopy, high-resolution transmission electron microscopy and X-ray diffraction. The elemental composition of the films, which was determined by energy-dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy, was found to be strongly dependent of the substrate temperature. © 2018 Elsevier B.V.
    view abstractdoi: 10.1016/j.jcrysgro.2018.03.021
  • 2018 • 493 Local Activities of Hydroxide and Water Determine the Operation of Silver-Based Oxygen Depolarized Cathodes
    Botz, A. and Clausmeyer, J. and Öhl, D. and Tarnev, T. and Franzen, D. and Turek, T. and Schuhmann, W.
    Angewandte Chemie - International Edition 57 12285-12289 (2018)
    Local ion activity changes in close proximity to the surface of an oxygen depolarized cathode (ODC) were measured by scanning electrochemical microscopy (SECM). While the operating ODC produces OH− ions and consumes O2 and H2O through the electrocatalytic oxygen reduction reaction (ORR), local changes in the activity of OH− ions and H2O are detected by means of a positioned Pt microelectrode serving as an SECM tip. Sensing at the Pt tip is based on the pH-dependent reduction of PtO and obviates the need for prior electrode modification steps. It can be used to evaluate the coordination numbers of OH− ions and H2O, and the method was exploited as a novel approach of catalyst activity assessment. We show that the electrochemical reaction on highly active catalysts can have a drastic influence on the reaction environment. © 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/anie.201807798
  • 2018 • 492 An N-Heterocyclic Carbene Based Silver Precursor for Plasma-Enhanced Spatial Atomic Layer Deposition of Silver Thin Films at Atmospheric Pressure
    Boysen, N. and Hasselmann, T. and Karle, S. and Rogalla, D. and Theirich, D. and Winter, M. and Riedl, T. and Devi, A.
    Angewandte Chemie - International Edition 57 16224-16227 (2018)
    A new N-heterocyclic carbene (NHC)-based silver amide compound, 1,3-di-tert-butyl-imidazolin-2-ylidene silver(I) 1,1,1-trimethyl-N-(trimethylsilyl)silanaminide [(NHC)Ag(hmds)] was synthesized and analyzed by single-crystal X-ray diffraction, 1H and 13C NMR spectroscopy, as well as EI mass spectrometry, and subsequently evaluated for its thermal characteristics. This new halogen- and phosphine-free Ag atomic layer deposition (ALD) precursor was tested successfully for silver thin film growth in atmospheric pressure plasma enhanced spatial (APP-ALD). High-purity conductive Ag thin films with a low sheet resistance of 0.9 Ω/sq (resistivity: 10−5 Ωcm) were deposited at 100 °C and characterized by X-ray photoelectron spectroscopy, scanning electron microscopy, optical transmittance, and Rutherford back-scattering techniques. The carbene-based Ag precursor and the new APP-ALD process are significant developments in the field of precursor chemistry as well as metal ALD processing. © 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/anie.201808586
  • 2018 • 491 Atomic scale analysis of grain boundary deuteride growth front in Zircaloy-4
    Breen, A.J. and Mouton, I. and Lu, W. and Wang, S. and Szczepaniak, A. and Kontis, P. and Stephenson, L.T. and Chang, Y. and da Silva, A.K. and Liebscher, C.H. and Raabe, D. and Britton, T.B. and Herbig, M. and Gault, B.
    Scripta Materialia 156 42-46 (2018)
    Zircaloy-4 (Zr-1.5%Sn-0.2%Fe-0.1%Cr wt%) was electrochemically charged with deuterium to create deuterides and subsequently analysed with atom probe tomography and scanning transmission electron microscopy to understand zirconium hydride formation and embrittlement. At the interface between the hexagonal close packed (HCP) α-Zr matrix and a face centred cubic (FCC) δ deuteride (ZrD1.5–1.65), a HCP ζ phase deuteride (ZrD0.25–0.5) has been observed. Furthermore, Sn is rejected from the deuterides and segregates to the deuteride/α-Zr reaction front. © 2018 Elsevier Ltd
    view abstractdoi: 10.1016/j.scriptamat.2018.06.044
  • 2018 • 490 How evolving multiaxial stress states affect the kinetics of rafting during creep of single crystal Ni-base superalloys
    Cao, L. and Wollgramm, P. and Bürger, D. and Kostka, A. and Cailletaud, G. and Eggeler, G.
    Acta Materialia 158 381-392 (2018)
    Miniature tensile creep specimens are used to investigate the effect of mild circular notches on microstructural evolution during [001] tensile creep of a Ni-base single crystal superalloy. Creep deformed material states from a uniaxial (950 °C, uniaxial stress: 300 MPa) and a circular notched creep specimen (950 °C, net section stress in notch root: 300 MPa) are compared. For both types of tests, creep experiments were interrupted after 81, 169 and 306 h. Quantitative scanning electron microscopy (SEM) is used to assess the evolution of the γ/γ′-microstructure from rafting to topological inversion. Scanning transmission electron microscopy (STEM) was applied to study the evolution of dislocation densities during creep. As a striking new result it is shown that in circular notched specimen, the microstructural evolution is well coupled to the kinetics of the stress redistribution during creep. Rafting, the directional coarsening of the γ′-phase, and the increase of γ-channel dislocation density, start in the notch root before the center of the specimen is affected. When stresses in the circular notched specimens are fully redistributed, the microstructural differences between the notch root and the center of the circular notched specimen disappear. The comparison of the mechanical data and the microstructural findings in uniaxial and circular notched specimens contribute to a better understanding of the role of mild notches, of stress multiaxiality and of strain accumulation in the microstructure evolution of single crystal Ni-base superalloys during creep. The results obtained in the present work are discussed in the light of previous work published in the literature. © 2018 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2018.07.061
  • 2018 • 489 Sodium enhances indium-gallium interdiffusion in copper indium gallium diselenide photovoltaic absorbers
    Colombara, D. and Werner, F. and Schwarz, T. and Cañero Infante, I. and Fleming, Y. and Valle, N. and Spindler, C. and Vacchieri, E. and Rey, G. and Guennou, M. and Bouttemy, M. and Manjón, A.G. and Peral Alonso, I. and Melchior...
    Nature Communications 9 (2018)
    Copper indium gallium diselenide-based technology provides the most efficient solar energy conversion among all thin-film photovoltaic devices. This is possible due to engineered gallium depth gradients and alkali extrinsic doping. Sodium is well known to impede interdiffusion of indium and gallium in polycrystalline Cu(In,Ga)Se2 films, thus influencing the gallium depth distribution. Here, however, sodium is shown to have the opposite effect in monocrystalline gallium-free CuInSe2 grown on GaAs substrates. Gallium in-diffusion from the substrates is enhanced when sodium is incorporated into the film, leading to Cu(In,Ga)Se2 and Cu(In,Ga)3Se5 phase formation. These results show that sodium does not decrease per se indium and gallium interdiffusion. Instead, it is suggested that sodium promotes indium and gallium intragrain diffusion, while it hinders intergrain diffusion by segregating at grain boundaries. The deeper understanding of dopant-mediated atomic diffusion mechanisms should lead to more effective chemical and electrical passivation strategies, and more efficient solar cells. © 2018 The Author(s).
    view abstractdoi: 10.1038/s41467-018-03115-0
  • 2018 • 488 Biological imaging with scanning electrochemical microscopy
    Conzuelo, F. and Schulte, A. and Schuhmann, W.
    Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences 474 (2018)
    Scanning electrochemical microscopy (SECM) is a powerful and versatile technique for visualizing the local electrochemical activity of a surface as an ultramicroelectrode tip is moved towards or over a sample of interest using precise positioning systems. In comparison with other scanning probe techniques, SECM not only enables topographical surface mapping but also gathers chemical information with high spatial resolution. Considerable progress has been made in the analysis of biological samples, including living cells and immobilized biomacromolecules such as enzymes, antibodies and DNA fragments. Moreover, combinations of SECM with complementary analytical tools broadened its applicability and facilitated multi-functional analysis with extended life science capabilities. The aim of this review is to present a brief topical overview on recent applications of biological SECM, with particular emphasis on important technical improvements of this surface imaging technique, recommended applications and future trends. © 2018 The Author(s) Published by the Royal Society. All rights reserved.
    view abstractdoi: 10.1098/rspa.2018.0409
  • 2018 • 487 Polyarylsulfone-based blend ultrafiltration membranes with combined size and charge selectivity for protein separation
    Emin, C. and Kurnia, E. and Katalia, I. and Ulbricht, M.
    Separation and Purification Technology 193 127-138 (2018)
    Current research is actively devoted to the development of high-performance ultrafiltration membranes with high permeability at maximum selectivity for dedicated separations. This work reports the preparation of polymer blend charged ultrafiltration membranes for the improvement of fractionation selectivity. Flat sheet membranes made from polysulfone and various types of sulfonated poly(arylsulfone)s were prepared via non-solvent induced phase separation. Membrane morphology, hydrophilicity and surface charge were assessed by scanning electron microscopy, contact angle and zeta potential measurements, respectively. The separation performances, i.e. selectivity factor, permeability, and permeability recovery, were evaluated by ultrafiltration of model proteins – bovine serum albumin (BSA) and hemoglobin (Hb). Results showed that the membrane charge can be tuned by adjusting the fraction of sulfonated groups via the type of sulfonated polymer in the blend used for membrane preparation. Hence, a broad range of ultrafiltration membranes with different barrier pore sizes, permeability properties and surface charge was established. Pronounced differences between the effects of different types of sulfonated poly(arylsulfone)s at the same degree of sulfonation and mass fraction in the membranes had been found and related to different barrier structures formed during phase separation. Successful separation of protein mixtures was achieved by additionally adjusting filtration parameters. Overall, this work provides guidelines for a versatile and easy method to tailor membrane properties for the selective separation of binary mixtures of biomacromolecules with nearly identical molecular weight and potentially also to solve other separation problems. © 2017 Elsevier B.V.
    view abstractdoi: 10.1016/j.seppur.2017.11.008
  • 2018 • 486 Peripheral Blood Plasma Clot as a Local Antimicrobial Drug Delivery Matrix
    Gessmann, J. and Seybold, D. and Ayami, F. and Peter, E. and Baecker, H. and Schildhauer, T.A. and Köller, M.
    Tissue Engineering - Part A 24 809-818 (2018)
    Platelet-free blood plasma clots were loaded either with antibiotics (vancomycin, gentamicin, or linezolid) at concentrations of 5-300 μg/mL or with silver ions (silver acetate) at concentrations of 3.3-129 μg/mL. The release of antibiotics or silver from the clot matrix was analyzed after repeated immersion of the plasma clots using reversed-phase high-performance liquid chromatography (RP-HPLC) or atomic absorption spectroscopy (AAS). The antimicrobial activity was tested against Staphylococcus aureus; tissue cell compatibility was analyzed using human mesenchymal stem cells (hMSC). Fibrin fiber thickness of the clots was analyzed by scanning electron microscopy. While addition of linezolid and vancomycin did not significantly change the fibrin fiber thickness, gentamicin and silver ions led to an increase in fiber thickness. All antibiotics showed a concentration-dependent burst-like release from the plasma clots within 1 h followed by a general decay in elution. The release of vancomycin and gentamicin, or silver lasted up to 7 days (depending on initial concentrations), but lasted only up to 4 h for linezolid. A correlation (p < 0.0001) was noted between the concentration of released antibiotics analyzed by HPLC and antimicrobial activity (agar diffusion test). A decrease in antibacterial activity of gentamicin- and vancomycin-containing clots occurred within 4 or 5 days. In contrast, the corresponding antibacterial activity of plasma clots containing linezolid was limited to 3 h. Antibacterial activity of plasma clots containing silver at the highest concentrations decreased after day 3, but clots with lower concentrations induced incomplete bacterial lysis or displayed no antibacterial activity. The antibiotic-containing clots did not induce cytotoxic effects on the embedded hMSC in contrast to all clots containing silver. Our results indicate that an autologous plasma clot can be used to deliver antibiotics such as vancomycin and gentamicin in combination with hMSC and the antibacterial effects persist for days without inducing cytotoxic effects on the embedded stem cells. © Copyright 2018, Mary Ann Liebert, Inc.
    view abstractdoi: 10.1089/ten.tea.2017.0319
  • 2018 • 485 Synthesis of carbon nanowalls from a single-source metal-organic precursor
    Giese, A. and Schipporeit, S. and Buck, V. and Wöhrl, N.
    Beilstein Journal of Nanotechnology 9 1895-1905 (2018)
    In this work, the deposition of carbon nanowalls (CNWs) by inductively coupled plasma enhanced chemical vapor deposition (ICPPECVD) is investigated. The CNWs are electrically conducting and show a large specific surface area, which is a key characteristic to make them interesting for sensors, catalytic applications or energy-storage systems. It was recently discovered that CNW films can be deposited by the use of the single-source metal-organic precursor aluminium acetylacetonate. This precursor is relatively unknown in combination with the ICP-PECVD deposition method in literature and, thus, based on our previous publication is further investigated in this work to better understand the influence of the various deposition parameters on the growth. Silicon, stainless steel, nickel and copper are used as substrate materials. The CNWs deposited are characterized by scanning electron microscopy (SEM), Raman spectroscopy and Auger electron spectroscopy (AES). The combination of bias voltage, the temperature of the substrate and the substrate material had a strong influence on the morphology of the graphitic carbon nanowall structures. With regard to these results, a first growth model for the deposition of CNWs by ICP-PECVD and aluminium acetylacetonate is proposed. This model explains the formation of four different morphologies (nanorods as well as thorny, straight and curled CNWs) by taking the surface diffusion into account. The surface diffusion depends on the particle energies and the substrate material and thus explains the influence of these parameters. © 2018 Giese et al.
    view abstractdoi: 10.3762/bjnano.9.181
  • 2018 • 484 Imprinting and column damage on CoCrMo head taper surfaces in total hip replacements
    Hall, D.J. and McCarthy, S.M. and Ehrich, J. and Urban, R.M. and Fischer, A. and Jacobs, J.J. and Lundberg, H.J. and Pourzal, R.
    ASTM Special Technical Publication STP 1606 131-155 (2018)
    Material degradation within taper junctions of modular total hip replacements remains of great concern. Imprinting and column damage are two damage modes that frequently occur on head taper surfaces. Both can cover large areas and therefore can be associated with significant material loss. It was the purpose of this study to determine the prevalence of imprinting and column damage on a group of retrievals collected at our medical center and to identify damage pathways on selected cases. We asked two research questions: (1) How do CoCrMo stems imprint into CoCrMo heads? (2) Does alloy microstructure influence the column damage pattern in CoCrMo heads? In order to answer these questions, we conducted a retrieval study on moderately to severely worn femoral head taper surfaces that were paired with stems of different materials. All components were viewed under a stereo-light microscope to determine the presence of imprinting and column damage. Selected cases were further studied by means of scanning electron microscope, interferometry, and metallography to determine damage mode and the potential role of alloy microstructure. Our results demonstrated that imprinting is independent of the stem material but highly dependent on its topography. The imprinting process is at least initially driven by fretting and the generation of oxide particles. Column damage on the other hand is highly dependent on the microstructure of wrought CoCrMo alloys, which can exhibit banding resulting from slight alloy segregations that were characterized by molybdenum depletion. Therefore, column damage may be prevented by avoiding banding of the alloy during the thermomechanical processing. This study demonstrates that it is important to consider differences among the occurring degradation mechanisms and to understand how they relate to material and design factors. Copyright © 2018 by ASTM International.
    view abstractdoi: 10.1520/STP160620I70121
  • 2018 • 483 A method for the in-situ study of solid-state joining techniques using synchrotron radiation - observation of phase transformations in Ti-6Al-4V after friction surfacing
    Hanke, S. and Staron, P. and Fischer, T. and Fitseva, V. and dos Santos, J.F.
    Surface and Coatings Technology 335 355-367 (2018)
    The solid-state deposition process Friction Surfacing (FS) was applied to Ti-6Al-4V alloy on portable welding equipment at a high-energy synchrotron beamline. The heat input and coating thickness were altered by varying the deposition speed. X-ray diffraction was carried out in-situ during the deposition process and the cooling of the coated samples. Phase transformations were evaluated and correlated with thermal cycles determined by thermocouples and an infrared camera. SEM investigation of the coating microstructure was conducted to examine the morphology of the α phase. During FS the coating material is severely deformed and dynamically recrystallized in the β phase state at temperatures &gt; 1300 °C. Small changes in the β grain size were observed within the first 2 s after deposition only. Depending on the cooling rate it transforms into different types of α phase during cooling. Phase transformation rates were found to correlate well with the differences in α morphology. The two faster translational speeds showed transformation rates &gt; 45 vol%/s and a partially martensitic microstructure. When a thick coating is deposited at low translational speed, α → β transformation continues for several seconds after deposition, followed by a slow cooling rate resulting in martensite free coatings containing α m from massive transformation. The potential gain and the deficiencies of this complex in-situ study of a technical process, instead of simplified model experiments, for the understanding of fundamental mechanisms involved in FS are discussed. © 2017
    view abstractdoi: 10.1016/j.surfcoat.2017.12.049
  • 2018 • 482 A new approach to coat PA12 powders with laser-generated nanoparticles for selective laser sintering
    Hupfeld, T. and Laumer, T. and Stichel, T. and Schuffenhauer, T. and Heberle, J. and Schmidt, M. and Barcikowski, S. and Gökce, B.
    Procedia CIRP 74 244-248 (2018)
    The modification of selective laser sintering (SLS) powder materials by nanoadditives offers the possibility to adapt the powder properties to the laser sintering process or the resulting part properties. To avoid agglomeration of the nanofiller, a new approach in which surfactant-free laser-generated colloidal nanoparticles are adsorbed onto the polymer surface directly in an aqueous solution is demonstrated. Based on this novel approach, polyamide 12 (PA12) powders are decorated with metal and oxide nanoparticles and processed via SLS. Electron microscopy and confocal laser scanning imaging are utilized to analyze the dispersion of the filler. © 2018 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license.
    view abstractdoi: 10.1016/j.procir.2018.08.103
  • 2018 • 481 Machining β-titanium alloy under carbon dioxide snow and micro-lubrication: a study on tool deflection, energy consumption, and tool damage
    Iqbal, A. and Biermann, D. and Abbas, H. and Al-Ghamdi, K.A. and Metzger, M.
    International Journal of Advanced Manufacturing Technology 1-14 (2018)
    The alloys of the beta allotropic form of titanium are among the most difficult-to-cut materials. An extremely poor machinability calls for special ways of performing machining with an emphasis on developing new methods of heat dissipation. The paper focuses on evaluating effectiveness of using CO2 snow as a coolant in continuous machining of a β-titanium alloy. It also explores the most appropriate location of its application in the cutting area and usefulness of its hybridization with minimum quantity of lubrication. The effectiveness of using the two cutting fluids is compared with an emulsion-based flood coolant. The effects of varying work material’s yield strength and cutting speed are also investigated. The measured responses include tool displacement area (a measure of tool deflection obtained from tool acceleration data), cutting energy consumed (obtained from acoustic emissions data), and tool wear. The results show that the usage of CO2 snow and its location of application possess a significant effect on the responses. The combination of CO2 snow and minimum quantity of lubrication is found to be the most effective way of heat dissipation and lubrication. With regard to tool damage, the scanning electron microscopy shows the presence of gradual wear and cutting speed-dependent adhesion but no evidence of chipping. The paper also presents a possibility of estimating tool damage condition through acoustic emission and tool deflection data. In this regard, a strong uphill relationship between tool wear and cutting energy is observed. © 2018 Springer-Verlag London Ltd., part of Springer Nature
    view abstractdoi: 10.1007/s00170-018-2267-4
  • 2018 • 480 Temperature-Dependent Ultrastructure Transformation of Au-Fe Nanoparticles Investigated by in Situ Scanning Transmission Electron Microscopy
    Kamp, M. and Tymoczko, A. and Schürmann, U. and Jakobi, J. and Rehbock, C. and Rätzke, K. and Barcikowski, S. and Kienle, L.
    Crystal Growth and Design 18 5434-5440 (2018)
    Three-dimensional morphology changes of bimetallic nanoparticles (NPs) with nominal composition Au50Fe50 and Au20Fe80, generated by pulsed laser ablation in liquid, are monitored in situ and ex situ via scanning transmission electron microscopy and electron tomography. The samples are made up of a chemically segregated core-shell (CS) NPs structure, with an Au-rich shell and Fe-rich core, and solid solution (SS) NPs in the pristine state. Further, the examinations reveal information about a sequence of characteristic changes from the pristine metastable and intermediate ultrastructures up to thermodynamically stable products. In the case of the Au20Fe80 sample, a metastable spherical CS morphology is transformed at equilibrium conditions into a cube-shaped Fe-rich core faceted by truncated Au-rich pyramids. For the Au50Fe50 sample, the Au-rich shell is solved into the Fe-rich core, and chemically homogeneous (SS) NPs are formed. Interestingly, this transformation was proven to occur via an intermediate ultrastructure with lamellar segregation, not previously reported as a transient state during in situ heating. On the basis of these observations, a correlation between the composition and the morphology at equilibrium is suggested, in accordance with the bulk phase diagram of Au-Fe. At the same time, our examinations directly prove that laser ablation synthesis creates nonequilibrium NP morphologies, frozen in metastable, spherical core-shell particles. Copyright © 2018 American Chemical Society.
    view abstractdoi: 10.1021/acs.cgd.8b00809
  • 2018 • 479 A Versatile Thin-Film Deposition Method for Multidimensional Semiconducting Bismuth Halides
    Khazaee, M. and Sardashti, K. and Sun, J.-P. and Zhou, H. and Clegg, C. and Hill, I.G. and Jones, J.L. and Lupascu, D.C. and Mitzi, D.B.
    Chemistry of Materials 30 3538-3544 (2018)
    Despite the significant progress in fabricating hybrid organic-inorganic lead halide perovskite solar cells, their toxicity and low stability remain as major drawbacks, thereby hindering large-scale commercialization. Given the isoelectronic nature of lead(II) and bismuth(III) ions, potentially stable and nontoxic alternatives for efficient light absorption in thin-film photovoltaic (PV) devices may be found among bismuth-based halide semiconductors. However, high-quality polycrystalline films of many of these systems have not been demonstrated. Here we present a versatile and facile two-step coevaporation approach to fabricate A3Bi2I9 (A = Cs, Rb) and AgBi2I7 polycrystalline films with smooth, pinhole-free morphology and average grain size of &gt;200 nm. The process involves an initial two-source evaporation step (involving CsI, RbI or AgI, and BiI3 sources), followed by an annealing step under BiI3 vapor. The structural, optical, and electrical characteristics of the resulting thin films are studied by X-ray diffraction, optical spectroscopy, X-ray/UV photoelectron spectroscopy, and scanning electron microscopy. Copyright © 2018 American Chemical Society.
    view abstractdoi: 10.1021/acs.chemmater.8b01341
  • 2018 • 478 Determination of pore size gradients of virus filtration membranes using gold nanoparticles and their relation to fouling with protein containing feed streams
    Kosiol, P. and Müller, M.T. and Schneider, B. and Hansmann, B. and Thom, V. and Ulbricht, M.
    Journal of Membrane Science 548 598-608 (2018)
    Virus filtration membranes contribute to the virus safety of biopharmaceutical drugs due to their capability to retain virus particles mainly based on size-exclusion mechanisms. Typical product molecules like monoclonal antibodies with 9–12 nm in hydrodynamic diameter have to be transmitted by >95% while small viruses, e.g. parvoviridae (B19, MVM, PPV) with a diameter of 18–26 nm, have to be retained by at least 99.99%. Therefore, membrane fouling caused by product aggregates, which are similar in size compared to the viruses that have to be retained, is a common observation. Minimal membrane fouling is a requirement for economical processes and is influenced by both the membrane surface chemistry and the membrane structure, particularly with regard to the pore size gradient (PSG). In this work, virus filtration membranes were challenged with gold nanoparticles (GNPs) in order to determine PSGs for a wide range of different commercial and non-commercial parvovirus retentive membranes differing in structure, material and surface chemistry. GNP adsorption to the membrane material was suppressed by the use of an anionic surfactant, allowing to gain insights into size-exclusion properties of the membranes. Membrane performance with regard to fouling was further investigated by determination of protein mass throughputs up-to a defined membrane flux decay using solutions containing intravenous immunoglobulin (IVIG) as model protein. Additionally, the fouling mechanism of IVIG was investigated and confirmed to be caused by trace amounts of species larger than IVIG monomers and dimers, which were already present in the feed. The fouling results are discussed in relationship to the determined PSGs, since the porous support structure of virus filtration membranes can act as a depth pre-filter protecting the separation-active layer from particulate foulants. © 2017 Elsevier B.V.
    view abstractdoi: 10.1016/j.memsci.2017.11.048
  • 2018 • 477 Hardness and modulus of Fe2B, Fe3(C,B), and Fe23(C,B)6 borides and carboborides in the Fe-C-B system
    Lentz, J. and Röttger, A. and Theisen, W.
    Materials Characterization 135 192-202 (2018)
    This work provides a comparative and comprehensive study of the indentation hardness and indentation modulus of iron-rich borides and carboborides of types Fe2B, Fe3(C,B), and Fe23(C,B)6. In addition, the hardness and elastic modulus of Cr-rich M7C are investigated for comparative purposes. We investigated the impact of increasing B content and indentation size effect (ISE). The phases of interest were stabilized in cast Fe-C-B alloys that varied with respect to the B / (B + C) ratio and heat treatment. The resulting microstructures were characterized by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), and wavelength X-ray spectroscopy (WDS). Dynamic in-situ nanoindentation experiments based on the method of continuous stiffness measurement (CSM) were coupled to SEM and EBSD investigations to determine the mechanical properties of the individual borides and carboborides as a function of the indentation depth. The results were compared to values obtained for the Cr-rich M7C3 carbide. It was found that the hardness of the B-rich Fe3(C,B) phase is considerably higher than pure Fe3C and increases with increasing B content. The ISE was present in all investigated phases, and the hardness decreased as a function of indentation depth. The hardness at infinite indentation depth H0 was estimated according to the model of Nix and Gao. The Fe2B phase was found to be the hardest phase (H0 = 19.04 GPa), followed by M7C3 (H0 = 16.43 GPa), Fe3(C,B) (H0 = 11.18 to 12.24 GPa), and Fe23(C,B)6 (H0 = 10.39 GPa). © 2017 Elsevier Inc.
    view abstractdoi: 10.1016/j.matchar.2017.11.012
  • 2018 • 476 Enhancement of hardness, modulus and fracture toughness of the tetragonal (Fe,Cr)2B and orthorhombic (Cr,Fe)2B phases with addition of Cr
    Lentz, J. and Röttger, A. and Großwendt, F. and Theisen, W.
    Materials and Design 156 113-124 (2018)
    This study analyzes the influence of Cr content on hardness H, elastic modulus E and fracture toughness KIC of the M2B boride by means of nanoindentation experiments. Additionally, properties of the Fe3(C,B) phase are determined. Samples of the M2B phase are casted and microstructurally characterized by means of scanning electron microscopy, energy dispersive spectroscopy and X-ray diffraction. At a Cr content higher than 14.7 atom% the M2B phase transforms from tetragonal into orthorhombic structure. The tetragonal M2B type possesses an optimum of H (21 ± 1 GPa), E (373 ± 6) GPa and KIC (3.5 ± 0.7 MPam) at 4–5 atom% Cr. The hardness, modulus and toughness of the orthorhombic M2B phase increase with Cr content and reach values of H = 27 ± 0.7 GPa, E = 473 ± 9 of and KIC = 3.26 ± 0.8 MPam at maximal investigated Cr content of 55 atom%. The hardness of the M2B phases decreases around 2.3–3.2 GPa as a function of indentation depth, which is known as the indentation size effect. Hardness and fracture toughness of M2B phase outperform conventionally used M7C3 carbides and are similar to MC-carbides. Findings can be used in novel alloying approaches in order to optimize the performance and reduce cost of tool steels. © 2018 Elsevier Ltd
    view abstractdoi: 10.1016/j.matdes.2018.06.040
  • 2018 • 475 Primary particle diameter differentiation and bimodality identification by five analytical methods using gold nanoparticle size distributions synthesized by pulsed laser ablation in liquids
    Letzel, A. and Gökce, B. and Menzel, A. and Plech, A. and Barcikowski, S.
    Applied Surface Science 435 743-751 (2018)
    For a known material, the size distribution of a nanoparticle colloid is a crucial parameter that defines its properties. However, measured size distributions are not easy to interpret as one has to consider weighting (e.g. by light absorption, scattering intensity, volume, surface, number) and the way size information was gained. The radius of a suspended nanoparticle can be given as e.g. sphere equivalent, hydrodynamic, Feret or radius of gyration. In this study, gold nanoparticles in water are synthesized by pulsed-laser ablation (LAL) and fragmentation (LFL) in liquids and characterized by various techniques (scanning transmission electron microscopy (STEM), small-angle X-ray scattering (SAXS), analytical disc centrifugation (ADC), dynamic light scattering (DLS) and UV–vis spectroscopy with Mie-Gans Theory) to study the comparability of different analytical techniques and determine the method that is preferable for a given task related to laser-generated nanoparticles. In particular, laser-generated colloids are known to be bimodal and/or polydisperse, but bimodality is sometimes not analytically resolved in literature. In addition, frequently reported small size shifts of the primary particle mode around 10 nm needs evaluation of its statistical significance related to the analytical method. Closely related to earlier studies on SAXS, different colloids in defined proportions are mixed and their size as a function of the nominal mixing ratio is analyzed. It is found that the derived particle size is independent of the nominal mixing ratio if the colloid size fractions do not overlap considerably. Conversely, the obtained size for colloids with overlapping size fractions strongly depends on the nominal mixing ratio since most methods cannot distinguish between such fractions. Overall, SAXS and ADC are very accurate methods for particle size analysis. Further, the ability of different methods to determine the nominal mixing ratio of sizes fractions is studied experimentally. © 2017 Elsevier B.V.
    view abstractdoi: 10.1016/j.apsusc.2017.11.130
  • 2018 • 474 Strain-Induced Asymmetric Line Segregation at Faceted Si Grain Boundaries
    Liebscher, C.H. and Stoffers, A. and Alam, M. and Lymperakis, L. and Cojocaru-Mirédin, O. and Gault, B. and Neugebauer, J. and Dehm, G. and Scheu, C. and Raabe, D.
    Physical Review Letters 121 (2018)
    The unique combination of atomic-scale composition measurements, employing atom probe tomography, atomic structure determination with picometer resolution by aberration-corrected scanning transmission electron microscopy, and atomistic simulations reveals site-specific linear segregation features at grain boundary facet junctions. More specific, an asymmetric line segregation along one particular type of facet junction core, instead of a homogeneous decoration of the facet planes, is observed. Molecular-statics calculations show that this segregation pattern is a consequence of the interplay between the asymmetric core structure and its corresponding local strain state. Our results contrast with the classical view of a homogeneous decoration of the facet planes and evidence a complex segregation patterning. © 2018 American Physical Society.
    view abstractdoi: 10.1103/PhysRevLett.121.015702
  • 2018 • 473 Bidirectional Transformation Enables Hierarchical Nanolaminate Dual-Phase High-Entropy Alloys
    Lu, W. and Liebscher, C.H. and Dehm, G. and Raabe, D. and Li, Z.
    Advanced Materials 30 (2018)
    Microstructural length-scale refinement is among the most efficient approaches to strengthen metallic materials. Conventional methods for refining microstructures generally involve grain size reduction via heavy cold working, compromising the material's ductility. Here, a fundamentally new approach that allows load-driven formation and permanent refinement of a hierarchical nanolaminate structure in a novel high-entropy alloy containing multiple principal elements is reported. This is achieved by triggering both, dynamic forward transformation from a faced-centered-cubic γ matrix into a hexagonal-close-packed ε nanolaminate structure and the dynamic reverse transformation from ε into γ. This new mechanism is referred to as the “bidirectional transformation induced plasticity” (B-TRIP) effect, which is enabled through a near-zero yet positive stacking fault energy of γ. Modulation of directionality in the transformation is triggered by local dissipative heating and local micromechanical fields. The simple thermodynamic and kinetic foundations for the B-TRIP effect render this approach generally suited for designing metastable strong and ductile bulk materials with hierarchical nanolaminate substructures. © 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/adma.201804727
  • 2018 • 472 Zinc stannate by reactive laser sintering
    Mackert, V. and Gebauer, J.S. and Notthoff, C. and Winterer, M.
    Applied Surface Science 457 1174-1180 (2018)
    A novel procedure for producing polycrystalline zinc stannate (Zn 2 SnO 4 , ZTO) films is presented in this paper. Nanocrystals of zinc oxide (ZnO) and tin dioxide (SnO 2 ) are prepared by chemical vapor synthesis (CVS) and processed into stable aqueous dispersions including mixed colloids. These colloids are transformed into nanostructured films via electrophoretic deposition where the mixed colloid forms a homogeneous, nanoscaled composite. Ultraviolet (UV) laser sintering of these codeposited ZnO-SnO 2 nanocrystals generates the inverse cubic spinel Zn 2 SnO 4 phase by chemical reaction on the area of interest. The effects of UV laser sintering at a wavelength of 325 nm on the nanoscaled microstructure of pure deposited films are investigated by variation of laser power and scanning speed. The microstructure of composite films is compared to a film obtained by classical reactive sintering in a furnace. High-resolution scanning electron microscopy and energy dispersive X-ray spectroscopy are used to investigate film morphology and chemical composition. Structural characterization is performed by X-ray diffraction. © 2018 Elsevier B.V.
    view abstractdoi: 10.1016/j.apsusc.2018.06.304
  • 2018 • 471 Correlative Microscopy—Novel Methods and Their Applications to Explore 3D Chemistry and Structure of Nanoscale Lattice Defects: A Case Study in Superalloys
    Makineni, S.K. and Lenz, M. and Kontis, P. and Li, Z. and Kumar, A. and Felfer, P.J. and Neumeier, S. and Herbig, M. and Spiecker, E. and Raabe, D. and Gault, B.
    JOM 1-8 (2018)
    Nanoscale solute segregation to or near lattice defects is a coupled diffusion and trapping phenomenon that occurs in superalloys at high temperatures during service. Understanding the mechanisms underpinning this crucial process will open pathways to tuning the alloy composition for improving the high-temperature performance and lifetime. Here, we introduce an approach combining atom probe tomography with high-end scanning electron microscopy techniques, in transmission and backscattering modes, to enable direct investigation of solute segregation to defects generated during high-temperature deformation such as dislocations in a heat-treated Ni-based superalloy and planar faults in a CoNi-based superalloy. Three protocols were elaborated to capture the complete structural and compositional nature of the targeted defect in the alloy. © 2018 The Author(s)
    view abstractdoi: 10.1007/s11837-018-2802-7
  • 2018 • 470 Metalorganic Vapor-Phase Epitaxy Growth Parameters for Two-Dimensional MoS2
    Marx, M. and Grundmann, A. and Lin, Y.-R. and Andrzejewski, D. and Kümmell, T. and Bacher, G. and Heuken, M. and Kalisch, H. and Vescan, A.
    Journal of Electronic Materials 47 910-916 (2018)
    The influence of the main growth parameters on the growth mechanism and film formation processes during metalorganic vapor-phase epitaxy (MOVPE) of two-dimensional MoS2 on sapphire (0001) have been investigated. Deposition was performed using molybdenum hexacarbonyl and di-tert-butyl sulfide as metalorganic precursors in a horizontal hot-wall MOVPE reactor from AIXTRON. The structural properties of the MoS2 films were analyzed by atomic force microscopy, scanning electron microscopy, and Raman spectroscopy. It was found that a substrate prebake step prior to growth reduced the nucleation density of the polycrystalline film. Simultaneously, the size of the MoS2 domains increased and the formation of parasitic carbonaceous film was suppressed. Additionally, the influence of growth parameters such as reactor pressure and surface temperature is discussed. An upper limit for these parameters was found, beyond which strong parasitic deposition or incorporation of carbon into MoS2 took place. This carbon contamination became significant at reactor pressure above 100 hPa and temperature above 900°C. © 2017, The Minerals, Metals & Materials Society.
    view abstractdoi: 10.1007/s11664-017-5937-3
  • 2018 • 469 Operando Raman spectroscopy on CO2 methanation over alumina-supported Ni, Ni3Fe and NiRh0.1 catalysts: Role of carbon formation as possible deactivation pathway
    Mutz, B. and Sprenger, P. and Wang, W. and Wang, D. and Kleist, W. and Grunwaldt, J.-D.
    Applied Catalysis A: General 556 160-171 (2018)
    The methanation of CO2, as a part of the power-to-gas concept, was studied under various industrially relevant feed compositions with a focus on the formation and influence of carbonaceous species. For this purpose, 5 wt.% Ni/Al2O3, 5 wt.% Ni3Fe/Al2O3 and 3.4 wt.% NiRh0.1/Al2O3 catalysts were prepared and characterized by X-ray diffraction (XRD), temperature-programmed reduction (TPR), scanning transmission electron microscopy (STEM) combined with energy-dispersive X-ray spectroscopy (EDX) and electron energy loss spectroscopy (EELS). During the methanation of CO2, the Ni3Fe catalyst emerged as the most active and selective catalyst in the mid-temperature regime (300–350 °C). At 400 °C, all three tested catalysts showed high conversion of CO2 (67–75%; Ni &gt; Ni3Fe &gt; NiRh0.1) and selectivity towards CH4 (95–98%). Operando Raman spectroscopy was applied to elucidate the possible influence of carbonaceous species on the performance of the catalysts. Notably, no carbon deposition was observed under various feed compositions, even in CO2 or CO2/CH4 mixtures, e.g. as provided by biogas plants. Only in pure CH4 atmosphere an intensive carbon deposition with graphitic structure occurred as uncovered by operando Raman spectroscopy. Experiments in the lab-scale reactor and a spectroscopic microreactor could be correlated and revealed a strong catalytic deactivation of the carbon covered catalysts including a pronounced shift of the selectivity towards CO. The initial activity could be recovered after reactivation in H2 at elevated temperatures, which led to a removal of the deposits especially from the metal particles. Raman spectroscopy, supported by the results from high-resolution transmission electron microscopy (HRTEM) and EELS, revealed that carbon remained on the support material. The latter did not have any significant influence on the catalytic activity and could be removed in an oxidizing atmosphere. © 2018
    view abstractdoi: 10.1016/j.apcata.2018.01.026
  • 2018 • 468 In Aqua Electrochemistry Probed by XPEEM: Experimental Setup, Examples, and Challenges
    Nemšák, S. and Strelcov, E. and Guo, H. and Hoskins, B.D. and Duchoň, T. and Mueller, D.N. and Yulaev, A. and Vlassiouk, I. and Tselev, A. and Schneider, C.M. and Kolmakov, A.
    Topics in Catalysis 61 2195-2206 (2018)
    Recent developments in environmental and liquid cells equipped with electron transparent graphene windows have enabled traditional surface science spectromicroscopy tools, such as scanning X-ray photoelectron microscopy, X-ray photoemission electron microscopy (XPEEM), and scanning electron microscopy to be applied for studying solid–liquid and liquid–gas interfaces. Here, we focus on the experimental implementation of XPEEM to probe electrified graphene–liquid interfaces using electrolyte-filled microchannel arrays as a new sample platform. We demonstrate the important methodological advantage of these multi-sample arrays: they combine the wide field of view hyperspectral imaging capabilities from XPEEM with the use of powerful data mining algorithms to reveal spectroscopic and temporal behaviors at the level of the individual microsample or the entire array ensemble. © 2018, This is a U.S. Government work and not under copyright protection in the US; foreign copyright protection may apply.
    view abstractdoi: 10.1007/s11244-018-1065-4
  • 2018 • 467 Molecular engineering of Ga-ketoiminates: Synthesis, structure and evaluation as precursors for the additive-free spin-coated deposition of gallium oxide thin films
    O'Donoghue, R. and Rahman, S. and Mallick, B. and Winter, M. and Rogalla, D. and Becker, H.-W. and Devi, A.
    New Journal of Chemistry 42 3196-3210 (2018)
    A series of new homo- and heteroleptic gallium ketoiminate compounds, namely, tris[4-[2-(ethoxyethyl)imino]-2-pentanone] gallium(iii) [Ga(eeki)3] [1], tris[4-[3-(methoxypropyl)imino]-2-pentanone] gallium(iii), [Ga(mpki)3] [2], tris[4-[3-(methoxyethyl)imino]-2-pentanone] gallium(iii), [Ga(meki)3] [3], dichloro[4-[(isopropyl)imino]-2-pentanone] gallium(iii) [Ga(ipki)Cl2] [4], bisdimethylamido[4-[(isopropyl)imino]-2-pentanone] gallium(iii) [Ga(ipki)(NMe2)2] [5] and chloro-(bis[4-[3-(ethoxypropyl)imino]-2-pentanone]) gallium(iii) [Ga(epki)2Cl] [6], was synthesised through molecular engineering. The literature known compound chloro-(bis[4-[(isopropyl)imino]-2-pentanone]) gallium(iii) [Ga(ipki)2Cl] [7] was synthesised for comparison. Confirmation of the successful formation and spectroscopic purity of the compounds was determined using nuclear magnetic resonance (NMR) spectroscopy, single crystal X-ray diffraction (XRD), electron ionisation mass spectrometry (EI-MS), and elemental analysis (EA). The thermal properties of the compounds were assessed with thermogravimetric (TG) analysis and revealed compound [4] was suitable for vapour phase deposition processes while the others displayed a decompositional behaviour favourable for solution based thin film deposition processes. The EI-MS fragmentation behaviour of compound [1], with its thermal properties, and excellent solubility in a wide variety of organic solvents, suggested that it was highly eligible to be applied for chemical solution deposition (CSD). Thus, compound [1] was applied for the spin-coating of Ga2O3 thin films without the need for additives or aging to stabilise the solution prior to processing. The as-deposited thin films were amorphous, while annealing under ambient conditions at higher temperatures (850-1000 °C) yielded β-gallium oxide as indicated by XRD. The morphology and composition were analysed by scanning electron microscopy (SEM) and Rutherford backscattering spectrometry (RBS) respectively, while the optical properties were determined using UV-vis spectroscopy and illustrated that films grown with a spin-cycle number &lt;5 were highly transparent (&gt;80%) in the visible range. © 2018 The Royal Society of Chemistry and the Centre National de la Recherche Scientifique.
    view abstractdoi: 10.1039/c7nj04334a
  • 2018 • 466 Optimized Ag Nanovoid Structures for Probing Electrocatalytic Carbon Dioxide Reduction Using Operando Surface-Enhanced Raman Spectroscopy
    Öhl, D. and Kayran, Y.U. and Junqueira, J.R.C. and Eßmann, V. and Bobrowski, T. and Schuhmann, W.
    Langmuir 34 12293-12301 (2018)
    Surface-enhanced Raman spectroscopy is a powerful analytical tool and a strongly surface structure-dependent process. Importantly, it can be coupled with electrochemistry to simultaneously record vibrational spectroscopic information during electrocatalytic reactions. Highest Raman enhancements are obtained using precisely tuned nanostructures. The fabrication and evaluation of a high number of different nanostructures with slightly different properties is time-consuming. We present a strategy to systematically determine optimal nanostructure properties of electrochemically generated Ag void structures in order to find the void size providing highest signal enhancement for Raman spectroscopy. Ag-coated Si wafers were decorated with a monolayer of differently sized polymer nanospheres using a Langmuir-Blodgett approach. Subsequently, bipolar electrochemistry was used to electrodeposit a gradient of differently sized void structures. The gradient structures were locally evaluated using Raman spectroscopy of a surface-adsorbed Raman probe, and the surface regions exhibiting the highest Raman enhancement were characterized by means of scanning electron microscopy. High-throughput scanning droplet cell experiments were utilized to determine suitable conditions for the electrodeposition of the found highly active structure in a three-electrode electrochemical cell. This structure was subsequently employed as the working electrode in operando surface-enhanced Raman measurements to verify its viability as the signal amplifier and to spectroscopically rationalize the complex electrochemical reduction of carbon dioxide. © 2018 American Chemical Society.
    view abstractdoi: 10.1021/acs.langmuir.8b02501
  • 2018 • 465 Microstructure and mechanical properties in the thin film system Cu-Zr
    Oellers, T. and Raghavan, R. and Chakraborty, J. and Kirchlechner, C. and Kostka, A. and Liebscher, C.H. and Dehm, G. and Ludwig, Al.
    Thin Solid Films 645 193-202 (2018)
    A composition-spread Cu-Zr thin film library with Zr contents from 2.5 up to 6.5 at.% was synthesized by magnetron sputtering on a thermally oxidized Si wafer. The compositional and microstructural variations of the Cu-Zr thin film across the composition gradient were examined using energy dispersive X-ray spectroscopy, X-ray diffraction, and high-resolution scanning and transmission electron microscopy of cross-sections fabricated by focused ion beam milling. Composition-dependent hardness and elastic modulus values were obtained by nanoindentation for measurement areas with discrete Zr contents along the composition gradient. Similarly, the electrical resistivity was investigated by 4-point resistivity measurements to study the influence of Zr composition and microstructural changes in the thin film. Both, the mechanical and electrical properties reveal a significant increase in hardness and resistivity with increasing Zr content. The trends of the mechanical and functional properties are discussed with respect to the local microstructure and composition of the thin film library. © 2017
    view abstractdoi: 10.1016/j.tsf.2017.10.030
  • 2018 • 464 Overcoming Ehrlich-Schwöbel barrier in (1 1 1)A GaAs molecular beam epitaxy
    Ritzmann, J. and Schott, R. and Gross, K. and Reuter, D. and Ludwig, Ar. and Wieck, A.D.
    Journal of Crystal Growth 481 7-10 (2018)
    In this work, we first study the effect of different growth parameters on the molecular beam epitaxy (MBE) growth of GaAs layers on (1 1 1)A oriented substrates. After that we present a method for the MBE growth of atomically smooth layers by sequences of growth and annealing phases. The samples exhibit low surface roughness and good electrical properties shown by atomic force microscopy (AFM), scanning electron microscopy (SEM) and van-der-Pauw Hall measurements. © 2017 Elsevier B.V.
    view abstractdoi: 10.1016/j.jcrysgro.2017.10.029
  • 2018 • 463 Covalent Surface Functionalization of Calcium Phosphate Nanoparticles with Fluorescent Dyes by Copper-Catalysed and by Strain-Promoted Azide-Alkyne Click Chemistry
    Rojas-Sánchez, L. and Sokolova, V. and Riebe, S. and Voskuhl, J. and Epple, M.
    ChemNanoMat (2018)
    Spherical calcium phosphate nanoparticles with a solid core diameter around 90 nm (from scanning electron microscopy, SEM) were coated with a silica shell and then covalently functionalized by azide groups. To these azide groups, all kinds of alkyne-carrying molecules can be covalently attached by copper-catalysed azide-alkyne cycloaddition (CuAAC) and by strain-promoted azide-alkyne cycloaddition (SPAAC) at a very high density. This was demonstrated for a number of dyes (FAM, TAMRA, Cy5, Alexa Fluor™ 488, and an aromatic thioether with aggregation-induced emission (AIE) properties). It was also possible to attach more than one molecule to the surface of one particle by two-step click reaction, permitting the synthesis of multimodal nanoparticles that are stable under biological conditions. The nanoparticles have a hydrodynamic diameter of around 200 nm (from dynamic light scattering, DLS), which makes them suitable for uptake by cells. The strongly fluorescing nanoparticles were easily taken up by cells as demonstrated by fluorescence microscopy, confocal laser scanning microscopy (CLSM), and structured illuminated microscopy (SIM). © 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/cnma.201800509
  • 2018 • 462 Tensile and fatigue assessments of brazed stainless steel joints using digital image correlation
    Schmiedt, A. and Jaquet, S. and Manka, M. and Tillmann, W. and Walther, F.
    MATEC Web of Conferences 165 (2018)
    For mechanical tests of brazed stainless steel joints, the local deformation behaviour within the small area of the brazing seam is a major concern, because local strains cannot be detected with standard mechanical extensometers. The current study allows a fundamental comprehension of the gauge length influence on the strain measurements of brazed joints with smooth and notch-containing surfaces, under quasi-static and cyclic loadings. Therefore, the optical measurement technique of digital image correlation (DIC) is used within tensile and fatigue tests of brazed AISI 304L/BAu-4 joints in an as-received and pre-corroded condition. A triggered image acquisition of the DIC system is successfully applied to evaluate the local ratcheting fatigue behaviour in the area of the brazing seam at a frequency of 10 Hz. The gauge length influence, analysed in the range of 0.5 to 12.5 mm, is more pronounced with increasing tensile and fatigue stresses and is significantly enhanced for notch-containing surfaces. Instrumented load increase tests with strain, electrical, magnetic and temperature measuring techniques have proven to be appropriate to estimate fatigue properties of the brazed joints with a deviation of 4%. Fatigue and corrosion fatigue damage mechanisms are evaluated by using scanning electron microscopy with secondary and back-scattered electron detectors. © The Authors, published by EDP Sciences, 2018.
    view abstractdoi: 10.1051/matecconf/201816506003
  • 2018 • 461 Germanium Template Assisted Integration of Gallium Arsenide Nanocrystals on Silicon: A Versatile Platform for Modern Optoelectronic Materials
    Schmitt, S.W. and Sarau, G. and Speich, C. and Döhler, G.H. and Liu, Z. and Hao, X. and Rechberger, S. and Dieker, C. and Spiecker, E. and Prost, W. and Tegude, F.J. and Conibeer, G. and Green, M.A. and Christiansen, S.H.
    Advanced Optical Materials (2018)
    Metal organic vapor phase epitaxy is used to grow gallium arsenide (GaAs) nanocrystals (NCs) on germanium (Ge) templates on nanoscopic silicon (Si) threads prepared by reactive ion etching. Scanning transmission electron microscopy with energy dispersive X-ray measurements shows an epitaxial growth of the GaAs on the Ge template that is supported by the Si thread, and that Ge doping is induced to the GaAs by the template. On Ge templates of about 60 nm diameter, as-grown GaAs NCs show a very regular rhombic-dodecahedral outer shape that can be explained by a preferential growth along the <110> plane. Photoluminescence measurements of the Ge/GaAs structures reveal radiative emission peaks on top of the GaAs band-to-band emission and at sub-band gap energies. While high energy peaks are originating from Ge acceptor levels in GaAs, sub-band gap peaks can be explained by radiation from Ge donor and acceptor bands that are amplified by photonic modes hosted in the rhombic-dodecahedral GaAs NCs. This study shows that a template-assisted crystal growth at the nanoscale opens up routes for a versatile integration of strongly emitting nanomaterials for a use in on-chip solid state lighting and photonics. © 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/adom.201701329
  • 2018 • 460 On the Ni-Ion release rate from surfaces of binary NiTi shape memory alloys
    Ševčíková, J. and Bártková, D. and Goldbergová, M. and Kuběnová, M. and Čermák, J. and Frenzel, J. and Weiser, A. and Dlouhý, A.
    Applied Surface Science 427 434-443 (2018)
    The study is focused on Ni-ion release rates from NiTi surfaces exposed in the cell culture media and human vascular endothelial cell (HUVEC) culture environments. The NiTi surface layers situated in the depth of 70 μm below a NiTi oxide scale are affected by interactions between the NiTi alloys and the bio-environments. The finding was proved with use of inductively coupled plasma mass spectrometry and electron microscopy experiments. As the exclusive factor controlling the Ni-ion release rates was not only thicknesses of the oxide scale, but also the passivation depth, which was two-fold larger. Our experimental data strongly suggested that some other factors, in addition to the Ni concentration in the oxide scale, admittedly hydrogen soaking deep below the oxide scale, must be taken into account in order to rationalize the concentrations of Ni-ions released into the bio-environments. The suggested role of hydrogen as the surface passivation agent is also in line with the fact that the Ni-ion release rates considerably decrease in NiTi samples that were annealed in controlled hydrogen atmospheres prior to bio-environmental exposures. © 2017 Elsevier B.V.
    view abstractdoi: 10.1016/j.apsusc.2017.08.235
  • 2018 • 459 Synthesis of rare-earth metal and rare-earth metal-fluoride nanoparticles in ionic liquids and propylene carbonate
    Siebels, M. and Mai, L. and Schmolke, L. and Schütte, K. and Barthel, J. and Yue, J. and Thomas, J. and Smarsly, B.M. and Devi, A. and Fischer, R.A. and Janiak, C.
    Beilstein Journal of Nanotechnology 9 1881-1894 (2018)
    Decomposition of rare-earth tris(N, N'-diisopropyl-2-methylamidinato)metal(III) complexes [RE(MeC(N(iPr)2))3] (RE(amd)3; RE = Pr(III), Gd(III), Er(III)) and tris(2,2,6,6-tetramethyl-3,5-heptanedionato)europium(III) (Eu(dpm)3) induced by microwave heating in the ionic liquids (ILs) 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIm][BF4]), 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([BMIm][NTf2]) and in propylene carbonate (PC) yield oxide-free rare-earth metal nanoparticles (RE-NPs) in [BMIm][NTf2] and PC for RE = Pr, Gd and Er or rare-earth metal-fluoride nanoparticles (REF3-NPs) in the fluoridedonating IL [BMIm][BF4] for RE = Pr, Eu, Gd and Er. The crystalline phases and the absence of significant oxide impurities in RE-NPs and REF3-NPs were verified by powder X-ray diffraction (PXRD), selected area electron diffraction (SAED) and highresolution X-ray photoelectron spectroscopy (XPS). The size distributions of the nanoparticles were determined by transmission electron microscopy (TEM) and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) to an average diameter of (11 ± 6) to (38 ± 17) nm for the REF3-NPs from [BMIm][BF4]. The RE-NPs from [BMIm][NTf2] or PC showed diameters of (1.5 ± 0.5) to (5 ± 1) nm. The characterization was completed by energy-dispersive X-ray spectroscopy (EDX). © 2018 Siebels et al.
    view abstractdoi: 10.3762/bjnano.9.180
  • 2018 • 458 A systematic electron microscopic study on the uptake of barium sulphate nano-, submicro-, microparticles by bone marrow-derived phagocytosing cells
    Sokolova, V. and Loza, K. and Knuschke, T. and Heinen-Weiler, J. and Jastrow, H. and Hasenberg, M. and Buer, J. and Westendorf, A.M. and Gunzer, M. and Epple, M.
    Acta Biomaterialia 80 352-363 (2018)
    Nanoparticles can act as transporters for synthetic molecules and biomolecules into cells, also in immunology. Antigen-presenting cells like dendritic cells are important targets for immunotherapy in nanomedicine. Therefore, we have used primary murine bone marrow-derived phagocytosing cells (bmPCs), i.e. dendritic cells and macrophages, to study their interaction with spherical barium sulphate particles of different size (40 nm, 420 nm, and 1 µm) and to follow their uptake pathway. Barium sulphate is chemically and biologically inert (no dissolution, no catalytic effects), i.e. we can separate the particle uptake effect from potential biological reactions. The colloidal stabilization of the nanoparticles was achieved by a layer of carboxymethylcellulose (CMC) which is biologically inert and gives the particles a negative zeta potential (i.e. charge). The particles were made fluorescent by conjugating 6-aminofluoresceine to CMC. Their uptake was visualized by flow cytometry, confocal laser scanning microscopy (CLSM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and correlative light and electron microscopy (CLEM). Barium sulphate particles of all sizes were readily taken up by dendritic cells and even more by macrophages, with the uptake increasing with time and particle concentration. They were mainly localized inside phagosomes, heterophagosomes, and in the case of nanoparticles also in the nearby cytosol. No particles were found in the nucleus. In nanomedicine, inorganic nanoparticles from the nanometer to the micrometer size are therefore well suited as transporters of biomolecules, including antigens, into dendritic cells and macrophages. The presented model system may also serve to describe the aseptic loosening of endoprostheses caused by abrasive wear of inert particles and the subsequent cell reaction, a question which relates to the field of nanotoxicology. Statement of Significance: The interaction of particles and cells is at the heart of nanomedicine and nanotoxicology, including abrasive wear from endoprostheses. It also comprises the immunological reaction to different kinds of nanomaterials, triggered by an immune response, e.g. by antigen-presenting cells. However, it is often difficult to separate the particle effect from a chemical or biochemical reaction to particles or their cargo. We show how chemically inert barium sulphate particles with three different sizes (nano, sub-micro, and micro) interact with relevant immune cells (primary dendritic cells and macrophages). Particles of all three sizes are readily taken up into both cell types by phagocytosis, but the uptake by macrophages is significantly more prominent than that by dendritic cells. The cells take up particles until they are virtually stuffed, but without direct adverse effect. The uptake increases with time and particle concentration. Thus, we have an ideal model system to follow particles into and inside cells without the side effect of a chemical particle effect, e.g. by degradation or ion release. © 2018 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actbio.2018.09.026
  • 2018 • 457 Deformation of Mesoporous Titania Nanostructures in Contact with D2O Vapor
    Song, L. and Rawolle, M. and Hohn, N. and Gutmann, J.S. and Frielinghaus, H. and Müller-Buschbaum, P.
    Small 14 (2018)
    For many applications, mesoporous titania nanostructures are exposed to water or need to be backfilled via infiltration with an aqueous solution, which can cause deformations of the nanostructure by capillary forces. In this work, the degree of deformation caused by water infiltration in two types of mesoporous, nanostructured titania films exposed to water vapor is compared. The different types of nanostructured titania films are prepared via a polymer template assisted sol–gel synthesis in conjunction with a polymer-template removal at high-temperatures under ambient conditions versus nitrogen atmosphere. Information about surface and inner morphology is extracted by scanning electron microscopy and grazing incidence small-angle neutron scattering (GISANS) measurements, respectively. Furthermore, complementary information on thin film composition and porosity are probed via X-ray reflectivity. The backfilling induced deformation of near surface structures and structures inside the mesoporous titania films is determined by GISANS before and after D2O infiltration. The respective atmosphere used for template removal influences the details of the titania nanostructure and strongly impacts the degree of water induced deformation. Drying of the films shows reversibility of the deformation. © 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/smll.201801461
  • 2018 • 456 Preparation of spray dried submicron particles: Part A – Particle generation by aerosol conditioning
    Strob, R. and Dobrowolski, A. and Schaldach, G. and Walzel, P. and Thommes, M.
    International Journal of Pharmaceutics 548 423-430 (2018)
    The preparation of submicron-sized particles is relevant in chemical, food and pharmaceutical applications. In pharmaceutics, spray dried submicron-sized particles (0.1–1 µm) can increase the dissolution rate as well as the solubility of poorly water-soluble drugs. Since the particle size during spray drying is mainly influenced by the droplet size, the preparation of uniform droplets smaller than 3 µm is of particular interest. In this work, a two-fluid nozzle was combined with a cyclone droplet separator. Droplets larger than the cut-off size were separated with a cyclone droplet separator and returned to the liquid feed. The aerosol at the outlet of the droplet separator was subsequently dried. The drop size of the conditioned aerosol was small, d50,3=2 µm, and independent of the liquid-to-gas mass flow ratio and the viscosity of the liquid feed. Thus it only depended on the characteristics of the separator. Finally, the dried particles were spherical in shape and in the submicron-sized range. © 2018 Elsevier B.V.
    view abstractdoi: 10.1016/j.ijpharm.2018.06.067
  • 2018 • 455 Effect of Hf on the microstructure, mechanical properties, and oxidation behavior of sputtered CrAlN films
    Tillmann, W. and Lopes Dias, N.F. and Stangier, D.
    Vacuum 154 208-213 (2018)
    Al-rich CrAlN films with a varying Hf content between 0 and 11.6 at.-% were synthetized by dc magnetron sputtering. The structural changes in the morphology and topography caused by Hf were evaluated by scanning electron microscopy. In addition, the phase composition of the Hf-alloyed CrAlN films was determined utilizing X-ray diffraction. The hardness and indentation modulus were determined using nanoindentation. In order to evaluate the effect of Hf on the oxidation behavior, the films were tempered in ambient air at 800 °C and subsequently analyzed. The incorporation of Hf leads to a morphological change from a fully-dense structure with small columns to a structure with more pronounced columns, which are visible as larger column tops on the surface. The Al-rich CrAlHfN films consist of the Wurtzite structure of AlN. A hardness decrease from 22.2 to 18.6 GPa is observed with an increasing Hf content and ascribed to the structural changes. When exposed to higher temperatures, the Hf-alloyed CrAlN films form an oxide layer, whose thickness is affected by the Hf content. An improved oxidation resistance is already achieved by a small amount of 2.0 at.-% of Hf. © 2018 Elsevier Ltd
    view abstractdoi: 10.1016/j.vacuum.2018.05.015
  • 2018 • 454 Imaging Inelastic Fracture Processes in Biomimetic Nanocomposites and Nacre by Laser Speckle for Better Toughness
    Verho, T. and Karppinen, P. and Gröschel, A.H. and Ikkala, O.
    Advanced Science 5 (2018)
    Mollusk nacre is a prototypical biological inorganic–organic composite that combines high toughness, stiffness, and strength by its brick-and-mortar microstructure, which has inspired several synthetic mimics. Its remarkable fracture toughness relies on inelastic deformations at the process zone at the crack tip that dissolve stress concentrations and stop cracks. The micrometer-scale structure allows resolving the size and shape of the process zone to understand the fracture processes. However, for better scalability, nacre-mimetic nanocomposites with aligned inorganic or graphene nanosheets are extensively pursued, to avoid the packing problems of mesoscale sheets like in nacre or slow in situ biomineralization. This calls for novel methods to explore the process zone of biomimetic nanocomposites. Here the fracture of nacre and nacre-inspired clay/polymer nanocomposite is explored using laser speckle imaging that reveals the process zone even in absence of changes in optical scattering. To demonstrate the diagnostic value, compared to nacre, the nacre-inspired nanocomposite develops a process zone more abruptly with macroscopic crack deflection shown by a flattened process zone. In situ scanning electron microscopy suggests similar toughening mechanisms in nanocomposite and nacre. These new insights guide the design of nacre-inspired nanocomposites toward better mechanical properties to reach the level of synergy of their biological model. © 2017 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/advs.201700635
  • 2018 • 453 In-situ SEM observation of phase transformation and twinning mechanisms in an interstitial high-entropy alloy
    Wang, M. and Li, Z. and Raabe, D.
    Acta Materialia 147 236-246 (2018)
    The recently developed interstitial high-entropy alloys (iHEAs) exhibit an enhanced combination of strength and ductility. These properties are attributed to dislocation hardening, deformation-driven athermal phase transformation from the face-centered cubic (FCC) γ matrix into the hexagonal close-packed (HCP) ε phase, stacking fault formation, mechanical twinning and precipitation hardening. For gaining a better understanding of these mechanisms as well as their interactions direct observation of the deformation process is required. For this purpose, an iHEA with nominal composition of Fe-30Mn-10Co-10Cr-0.5C (at. %) was produced and investigated via in-situ and interrupted in-situ tensile testing in a scanning electron microscope (SEM) combining electron channeling contrast imaging (ECCI) and electron backscatter diffraction (EBSD) techniques. The results reveal that the iHEA is deformed by formation and multiplication of stacking faults along {111} microbands. Sufficient overlap of stacking faults within microbands leads to intrinsic nucleation of HCP ε phase and incoherent annealing twin boundaries act as preferential extrinsic nucleation sites for HCP ε formation. With further straining HCP ε nuclei grow into the adjacent deformed FCC γ matrix. γ regions with smaller grain size have higher mechanical stability against phase transformation. Twinning in FCC γ grains with a size of ∼10 μm can be activated at room temperature at a stress below ∼736 MPa. With increasing deformation, new twin lamellae continuously nucleate. The twin lamellae grow in preferred directions driven by the motion of the mobile partial dislocations. Owing to the individual grain size dependence of the activation of the dislocation-mediated plasticity, of the athermal phase transformation and of mechanical twinning at the different deformation stages, desired strain hardening profiles can be tuned and adjusted over the entire deformation regime by adequate microstructure design, providing excellent combinations of strength and ductility. © 2018 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2018.01.036
  • 2018 • 452 The effect of mixing on silver particle morphology in flow synthesis
    Yang, T. and Segets, D. and Thajudeen, T. and Han, Y. and Peukert, W.
    Chemical Engineering Science 192 254-263 (2018)
    Silver particles, prepared in a T-mixer under different flow rates, were selected to study the influences of mixing on particle shape evolution. Mixing effects on particle growth were visualized by scanning electron microscopy (SEM) and analysed quantitatively by sedimentation coefficient distributions derived from analytical centrifugation (AC). The mixing time under different flow rates was determined by the Villermaux-Dushman method to quantify the mixing quality. Based on the finding of a mixing-induced shape transformation from plates to dendrites, an extended growth mechanism involving mixing effects was proposed. Slow mixing leads to a non-uniform distributed reactant mixture and low effective supersaturation. This causes preferential growth of high-energy facets resulting in plate-like particles with broad, multimodal sedimentation distributions. In contrast, fast mixing, corresponding to uniform reactant mixture and thus high effective supersaturation and nucleation rate, leads to dendritic products, and narrow but bimodal sedimentation distributions. © 2018
    view abstractdoi: 10.1016/j.ces.2018.07.043
  • 2018 • 451 Ag-Segregation to Dislocations in PbTe-Based Thermoelectric Materials
    Yu, Y. and Zhang, S. and Mio, A.M. and Gault, B. and Sheskin, A. and Scheu, C. and Raabe, D. and Zu, F. and Wuttig, M. and Amouyal, Y. and Cojocaru-Mirédin, O.
    ACS Applied Materials and Interfaces 10 3609-3615 (2018)
    Dislocations have been considered to be an efficient source for scattering midfrequency phonons, contributing to the enhancement of thermoelectric performance. The structure of dislocations can be resolved by electron microscopy whereas their chemical composition and decoration state are scarcely known. Here, we correlate transmission Kikuchi diffraction and (scanning) transmission electron microscopy in conjunction with atom probe tomography to investigate the local structure and chemical composition of dislocations in a thermoelectric Ag-doped PbTe compound. Our investigations indicate that Ag atoms segregate to dislocations with a 10-fold excess of Ag compared with its average concentration in the matrix. Yet the Ag concentration along the dislocation line is not constant but fluctuates from ∼0.8 to ∼10 atom % with a period of about 5 nm. Thermal conductivity is evaluated applying laser flash analysis, and is correlated with theoretical calculations based on the Debye-Callaway model, demonstrating that these Ag-decorated dislocations yield stronger phonon scatterings. These findings reduce the knowledge gap regarding the composition of dislocations needed for theoretical calculations of phonon scattering and pave the way for extending the concept of defect engineering to thermoelectric materials. © 2018 American Chemical Society.
    view abstractdoi: 10.1021/acsami.7b17142
  • 2018 • 450 Printing structural colors via direct laser writing
    Zyla, G. and Kovalev, A. and Gurevich, E.L. and Esen, C. and Ostendorf, A. and Gorb, S.
    Proceedings of SPIE - The International Society for Optical Engineering 10544 (2018)
    Most common colors in our world as we see them, for example, in crystals, pigments, metals and salt solutions are the result from light scattering properties of electrons in atoms and molecules. Nevertheless, colors can also result from light interference effects, which are of great importance in the life of organisms. The structural colors of living organisms, e.g., the wings of some birds, insects and butteries, are often more intense and almost angle-independent. Understanding this specific color formation is of great interest for biology and for engineered materials with a broad range of biomimetic real world applications due to forgoing of toxic dyes and pigments. Therefore, the generation of artificial color formation with lithographic methods offers many advantages not available in coated multilayer systems because it avoids multiple complex fabrication steps. In the present work, we report an effortless fabrication method to generate structural coloration based on microand nano-structures using 3d-laser writing technique. The uniform micro- A nd nano-structures were produced in a thin polymer film with an refractive index of 1.51. The single structures are aligned in an array to create a blue color field. The identification of the influence of the structures on the artificial color formation was performed using scanning electron microscopy. The optical properties of the blue color was analyzed via an angle-resolved spectrometer. © 2018 SPIE.
    view abstractdoi: 10.1117/12.2289820
  • 2017 • 449 Fabrication of modified polyethersulfone membranes for wastewater treatment by submerged membrane bioreactor
    Abdel-Karim, A. and Gad-Allah, T.A. and El-Kalliny, A.S. and Ahmed, S.I.A. and Souaya, E.R. and Badawy, M.I. and Ulbricht, M.
    Separation and Purification Technology 175 36-46 (2017)
    Polyethersulfone (PES) flat sheet membranes were fabricated using different additives, i.e. polyvinylpyrrolidone (PVP), linear Pluronic 31R1, and star-like Tetronic 904, with different contents, by nonsolvent-induced phase separation method at similar preparation conditions. Their effects on characteristics and performance of the flat sheet PES membranes were investigated in order to select the best membrane to be applied in submerged membrane bioreactor (SMBR). The characteristics of the fabricated membranes were investigated by using attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectroscopy, scanning electron microscopy (SEM), atomic force microscopy (AFM), mechanical stability measurements, contact angle (CA) measurement, shrinkage measurement, and membrane porosity analysis. The membranes performance was examined by investigation of pure water permeability (PWP) and bovine serum albumin (BSA) rejection analyses. The characteristics and the performance of the neat PES membrane have been improved by the action of these additives. For instance, there is an increase in the PWP from 2.2 L m−2 h−1 bar−1 for neat PES membrane to more than 40, 116, and 64 L m−2 h−1 bar−1 for PES-PVP (3 wt.%), PES-P31R1 (5 wt.%), and PES-T904 (5 wt.%) modified membranes, respectively. Also, these membranes showed 60–70% BSA rejection compared to ∼90% for neat PES membrane. To sum up, these mentioned three membranes are considered as promising membranes for SMBRs. © 2016 Elsevier B.V.
    view abstractdoi: 10.1016/j.seppur.2016.10.060
  • 2017 • 448 Transmission electron microscopy study of the microstructural evolution during higherature and low-stress (011) [11] shear creep deformation of the superalloy single crystal LEK 94
    Agudo Jácome, L. and Göbenli, G. and Eggeler, G.
    Journal of Materials Research 32 4491-4502 (2017)
    The present work describes the shear creep behavior of the superalloy LEK 94 at temperatures between 980 and 1050 °C and shear stresses between 50 and 140 MPa for loading on the macroscopic crystallographic shear system (MCSS) (011) . The strain rate versus strain curves show short primary and extended secondary creep regimes. We find an apparent activation energy for creep of Q app = 466 kJ/mol and a Norton-law stress exponent of n = 6. With scanning transmission electron microscopy, we characterize three material states that differ in temperature, applied stress, and accumulated strain/time. Rafting develops perpendicular to the maximum principal stress direction, γ channels fill with dislocations, superdislocations cut γ′ particles, and dislocation networks form at γ/γ′ interfaces. Our findings are in agreement with previous results for higherature and low-stress [001] and [110] tensile creep testing, and for shear creep testing of the superalloys CMSX-4 and CMSX-6 on the MCSSs (111) and (001)[100]. The parameters that characterize the evolving γ/γ′ microstructure and the evolving dislocation substructures depend on creep temperature, stress, strain, and time. © 2017 Materials Research Society.
    view abstractdoi: 10.1557/jmr.2017.336
  • 2017 • 447 Production of precursors for micro-concentrator solar cells by femtosecond laser-induced forward transfer
    Andree, S. and Heidmann, B. and Ringleb, F. and Eylers, K. and Bonse, J. and Boeck, T. and Schmid, M. and Krüger, J.
    Applied Physics A: Materials Science and Processing 123 (2017)
    Single-pulse femtosecond laser-induced forward transfer (LIFT, 30 fs, 790 nm) is used to deposit micron-sized dots of copper and/or indium onto a molybdenum layer on glass. Such systems can serve as precursors for the bottom–up manufacturing of micro-concentrator solar cells based on copper–indium–gallium–diselenide. The influence of the thickness of the copper, indium, and combined copper–indium donor layers on the quality of the transferred dots was qualified by scanning electron microscopy, energy-dispersive X-ray analysis, and optical microscopy. The potential for manufacturing of a spatial arrangement adapted to the geometry of micro-lens arrays needed for micro-concentrator solar cells is demonstrated. © 2017, Springer-Verlag GmbH Germany.
    view abstractdoi: 10.1007/s00339-017-1282-x
  • 2017 • 446 Comparison of microstructure and mechanical properties of Scalmalloy® produced by selective laser melting and laser metal deposition
    Awd, M. and Tenkamp, J. and Hirtler, M. and Siddique, S. and Bambach, M. and Walther, F.
    Materials 11 (2017)
    The second-generation aluminum-magnesium-scandium (Al-Mg-Sc) alloy, which is often referred to as Scalmalloy®, has been developed as a high-strength aluminum alloy for selective laser melting (SLM). The high-cooling rates of melt pools during SLM establishes the thermodynamic conditions for a fine-grained crack-free aluminum structure saturated with fine precipitates of the ceramic phase Al3-Sc. The precipitation allows tensile and fatigue strength of Scalmalloy® to exceed those of AlSi10Mg by ~70%. Knowledge about properties of other additive manufacturing processes with slower cooling rates is currently not available. In this study, two batches of Scalmalloy® processed by SLM and laser metal deposition (LMD) are compared regarding microstructure-induced properties. Microstructural strengthening mechanisms behind enhanced strength and ductility are investigated by scanning electron microscopy (SEM). Fatigue damage mechanisms in low-cycle (LCF) to high-cycle fatigue (HCF) are a subject of study in a combined strategy of experimental and statistical modeling for calculation of Woehler curves in the respective regimes. Modeling efforts are supported by non-destructive defect characterization in an X-ray computed tomography (μ-CT) platform. The investigations show that Scalmalloy® specimens produced by LMD are prone to extensive porosity, contrary to SLM specimens, which is translated to ~30% lower fatigue strength. © 2017 by the author.
    view abstractdoi: 10.3390/ma11010017
  • 2017 • 445 Investigations of aging behaviour for aluminium powders during an atmosphere simulation of the LBM process
    Bauer, D.M. and Schwarzenböck, E. and Ludwig, I. and Schupp, N. and Palm, F. and Witt, G.
    Powder Metallurgy 1-9 (2017)
    Additive layer manufacturing (ALM) offers for production of parts and components for aeronautical applications potential cost benefits over conventional manufacturing routes. In particular, powder bed processes offer a high degree of design flexibility while enabling weight reduction due to topological optimisation. The quality and properties of the parts are strongly dependent on the powder quality which, in turn, is influenced by handling and storage of the powder. For this reasons an undefined contamination of atomised powder materials by oxygen and hydrogen has to be avoided. Aluminium-silicon powder was aged under atmosphere of different moistures and temperatures for defined duration. The effect of these environments as well as the effect of vacuum drying on the flowability was investigated. The morphology was evaluated by scanning electron microscope. The chemistry including oxygen content of the powder was measured by inductively coupled plasma optical emission spectrometry and hot fusion analysis. © 2017 Institute of Materials, Minerals and Mining Published by Taylor & Francis on behalf of the Institute
    view abstractdoi: 10.1080/00325899.2017.1288841
  • 2017 • 444 Microstructural stability of a niobium single crystal deformed by equal channel angular pressing
    Bernardi, H.H. and Sandim, H.R.Z. and Zilnyk, K.D. and Verlinden, B. and Raabe, D.
    Materials Research 20 1238-1247 (2017)
    A [211]-oriented niobium single crystal was deformed by equal channel angular pressing (ECAP) at room temperature using the route Bc to a total strain of 9.2. A sharp cube texture develops after ECAP processing. The deformed samples were annealed in vacuum from 400ºC (673 K) to 900ºC (1173 K) for 1 h to evaluate their microstructural stability. Scanning electron microscopy (SEM) was used to image the microstructures of as-deformed and annealed specimens. Electron backscatter diffraction (EBSD) was employed to determine the respective microtextures before and after annealing. Coarsening of the microstructure occurs at a maximum rate at 550ºC (823 K) due to discontinuous recrystallization. Normal grain growth replaces discontinuous recrystallization as the main coarsening mechanism above 700ºC (973 K).
    view abstractdoi: 10.1590/1980-5373-MR-2017-0288
  • 2017 • 443 Fundamental study of an industrial reactive HPPMS (Cr,Al)N process
    Bobzin, K. and Brögelmann, T. and Kruppe, N.C. and Engels, M. and Von Keudell, A. and Hecimovic, A. and Ludwig, Al. and Grochla, D. and Banko, L.
    Journal of Applied Physics 122 (2017)
    In this work, a fundamental investigation of an industrial (Cr,Al)N reactive high power pulsed magnetron sputtering (HPPMS) process is presented. The results will be used to improve the coating development for the addressed application, which is the tool coating for plastics processing industry. Substrate-oriented plasma diagnostics and deposition of the (Cr,Al)N coatings were performed for a variation of the HPPMS pulse frequency with values from f = 300 Hz to f = 2000 Hz at constant average power P = 2.5 kW and pulse length ton = 40 μs. The plasma was investigated using an oscilloscope, an intensified charge coupled device camera, phase-resolved optical emission spectroscopy, and an energy-dispersive mass spectrometer. The coating properties were determined by means of scanning electron microscopy, glow discharge optical emission spectroscopy, cantilever stress sensors, nanoindentation, and synchrotron X-ray diffraction. Regarding the plasma properties, it was found that the average energy within the plasma is nearly constant for the frequency variation. In contrast, the metal to gas ion flux ratio is changed from JM/JG = 0.51 to JM/JG = 0.10 for increasing frequency. Regarding the coating properties, a structure refinement as well as lower residual stresses, higher universal hardness, and a changing crystal orientation from (111) to (200) were observed at higher frequencies. By correlating the plasma and coating properties, it can be concluded that the change in the gas ion to metal ion flux ratio results in a competitive crystal growth of the film, which results in changing coating properties. © 2017 Author(s).
    view abstractdoi: 10.1063/1.4990997
  • 2017 • 442 Experimental and Theoretical Understanding of Nitrogen-Doping-Induced Strong Metal-Support Interactions in Pd/TiO2 Catalysts for Nitrobenzene Hydrogenation
    Chen, P. and Khetan, A. and Yang, F. and Migunov, V. and Weide, P. and Stürmer, S.P. and Guo, P. and Kähler, K. and Xia, W. and Mayer, J. and Pitsch, H. and Simon, U. and Muhler, M.
    ACS Catalysis 7 1197-1206 (2017)
    By doping the TiO2 support with nitrogen, strong metal-support interactions (SMSI) in Pd/TiO2 catalysts can be tailored to obtain high-performance supported Pd nanoparticles (NPs) in nitrobenzene (NB) hydrogenation catalysis. According to the comparative studies by X-ray diffraction, X-ray photoelectron spectroscopy (XPS), and diffuse reflectance CO FTIR (CO-DRIFTS), N-doping induced a structural promoting effect, which is beneficial for the dispersion of Pd species on TiO2. High-angle annular dark-field scanning transmission electron microscopy study of Pd on N-doped TiO2 confirmed a predominant presence of sub-2 nm Pd NPs, which are stable under the applied hydrogenation conditions. XPS and CO-DRIFTS revealed the formation of strongly coupled Pd-N species in Pd/TiO2 with N-doped TiO2 as support. Density functional theory (DFT) calculations over model systems with Pdn (n = 1, 5, or 10) clusters deposited on TiO2(101) surface were performed to verify and supplement the experimental observations. In hydrogenation catalysis using NB as a model molecule, Pd NPs on N-doped TiO2 outperformed those on N-free TiO2 in terms of both catalytic activity and stability, which can be attributed to the presence of highly dispersed Pd NPs providing more active sites, and to the formation of Pd-N species favoring the dissociative adsorption of the reactant NB and the easier desorption of the product aniline. (Figure Presented). © 2016 American Chemical Society.
    view abstractdoi: 10.1021/acscatal.6b02963
  • 2017 • 441 Hybrid biocomposites based on titania nanotubes and a hydroxyapatite coating deposited by RF-magnetron sputtering: Surface topography, structure, and mechanical properties
    Chernozem, R.V. and Surmeneva, M.A. and Krause, B. and Baumbach, T. and Ignatov, V.P. and Tyurin, A.I. and Loza, K. and Epple, M. and Surmenev, R.A.
    Applied Surface Science 426 229-237 (2017)
    In this study, biocomposites based on porous titanium oxide structures and a calcium phosphate (CaP) or hydroxyapatite (HA) coating are described and prepared. Nanotubes (NTs) with different pore dimensions were processed using anodic oxidation of Ti substrates in a NH4F-containing electrolyte solution at anodization voltages of 30 and 60 V with a DC power supply. The external diameters of the nanotubes prepared at 30 V and 60 V were 53 ± 10 and 98 ± 16 nm, respectively. RF-magnetron sputtering of the HA target in a single deposition run was performed to prepare a coating on the surface of TiO2 NTs prepared at 30 and 60 V. The thickness of the CaP coating deposited on the mirror-polished Si substrate in the same deposition run with TiO2 NTs was determined by optical ellipsometry (SE) 95 ± 5 nm. Uncoated and CaP-coated NTs were annealed at 500 °C in air. Afterwards, the presence of TiO2 (anatase) was observed. The scanning electron microscopy (SEM), X-ray diffraction (XRD), photoelectron spectroscopy (XPS) and nanoindentation results revealed the influence that the NT dimensions had on the CaP coating deposition process. The tubular surfaces of the NTs were completely coated with the HA coating when prepared at 30 V, and no homogeneous CaP coating was observed when prepared at 60 V. The XRD patterns show peaks assigned to crystalline HA only for the coated TiO2 NTs prepared at 30 V. High-resolution XPS spectra show binding energies (BE) of Ca 2p, P 2p and O 1s core-levels corresponding to HA and amorphous calcium phosphate on TiO2 NTs prepared at 30 V and 60 V, respectively. Fabrication of TiO2 NTs results in a significant decrease to the elastic modulus and nanohardness compared to the Ti substrate. The porous structure of the NTs causes an increase in the elastic strain to failure of the coating (H/E) and the parameter used to describe the resistance of the material to plastic deformation (H3/E2) at the nanoscale level compared to the Ti substrate. Furthermore, only the HA coating on the NTs exhibits a significantly increased H/E ratio and H3/E2 factor compared to the NTs and Ti substrate. Increases in resistance to penetration for the indenter were also observed for HA-coated TiO2 NTs prepared at 30 V compared to uncoated and CaP-coated NTs prepared at 60 V. © 2017 Elsevier B.V.
    view abstractdoi: 10.1016/j.apsusc.2017.07.199
  • 2017 • 440 In Operando Investigation of Electrical Coupling of Photosystem 1 and Photosystem 2 by Means of Bipolar Electrochemistry
    Eßmann, V. and Zhao, F. and Hartmann, V. and Nowaczyk, M.M. and Schuhmann, W. and Conzuelo, F.
    Analytical Chemistry 89 7160-7165 (2017)
    Electrochemical communication between two photobioelectrochemical half-cells based on photosystem 1 and photosystem 2 is investigated in operando. The driving force for the electron-transfer reactions is applied in a wireless mode using bipolar electrochemistry with the actual electrode potentials being self-regulated by the redox processes. Four parameters are assessed to understand the overall performance and elucidate the limiting reactions of the photobioelectrochemical cell. In addition to the potential differences for oxidation and reduction reactions, the current flowing between the half-cells as well as in situ collection of locally evolved O2 by photosystem 2 using a positioned scanning electrochemical microscopy tip are evaluated. In this way, changes in the enzymatic performances as a result of inactivation of either of the protein complexes or variations in the external conditions are monitored. © 2017 American Chemical Society.
    view abstractdoi: 10.1021/acs.analchem.7b01222
  • 2017 • 439 Coherent long-distance displacement of individual electron spins
    Flentje, H. and Mortemousque, P.-A. and Thalineau, R. and Ludwig, Ar. and Wieck, A.D. and Bäuerle, C. and Meunier, T.
    Nature Communications 8 (2017)
    Controlling nanocircuits at the single electron spin level is a possible route for large-scale quantum information processing. In this context, individual electron spins have been identified as versatile quantum information carriers to interconnect different nodes of a spin-based semiconductor quantum circuit. Despite extensive experimental efforts to control the electron displacement over long distances, maintaining electron spin coherence after transfer remained elusive up to now. Here we demonstrate that individual electron spins can be displaced coherently over a distance of 5 μm. This displacement is realized on a closed path made of three tunnel-coupled lateral quantum dots at a speed approaching 100 ms-1. We find that the spin coherence length is eight times longer than expected from the electron spin coherence without displacement, pointing at a process similar to motional narrowing observed in nuclear magnetic resonance experiments. The demonstrated coherent displacement will open the route towards long-range interaction between distant spin qubits. © 2017 The Author(s).
    view abstractdoi: 10.1038/s41467-017-00534-3
  • 2017 • 438 Local delivery of siRNA-loaded calcium phosphate nanoparticles abates pulmonary inflammation
    Frede, A. and Neuhaus, B. and Knuschke, T. and Wadwa, M. and Kollenda, S. and Klopfleisch, R. and Hansen, W. and Buer, J. and Bruder, D. and Epple, M. and Westendorf, A.M.
    Nanomedicine: Nanotechnology, Biology, and Medicine 13 2395-2403 (2017)
    The local interference of cytokine signaling mediated by siRNA-loaded nanoparticles might be a promising new therapeutic approach to dampen inflammation during pulmonary diseases. For the local therapeutic treatment of pulmonary inflammation, we produced multi-shell nanoparticles consisting of a calcium phosphate core, coated with siRNAs directed against pro-inflammatory mediators, encapsulated into poly(lactic-co-glycolic acid), and coated with a final outer layer of polyethylenimine. Nasal instillation of nanoparticles loaded with a mixture of siRNAs directed against different cytokines to mice suffering from TH1 cell-mediated lung inflammation, or of siRNA directed against NS-1 in an influenza infection model led to a significant reduction of target gene expression which was accompanied by distinct amelioration of lung inflammation in both models. Thus, this study provides strong evidence that the specific and local modulation of the inflammatory response by CaP/PLGA nanoparticle-mediated siRNA delivery could be a promising approach for the treatment of inflammatory disorders of the lung. © 2017 Elsevier Inc.
    view abstractdoi: 10.1016/j.nano.2017.08.001
  • 2017 • 437 Nanoscale x-ray investigation of magnetic metallofullerene peapods
    Fritz, F. and Westerström, R. and Kostanyan, A. and Schlesier, C. and Dreiser, J. and Watts, B. and Houben, L. and Luysberg, M. and Avdoshenko, S.M. and Popov, A.A. and Schneider, C.M. and Meyer, C.
    Nanotechnology 28 (2017)
    Endohedral lanthanide ions packed inside carbon nanotubes (CNTs) in a one-dimensional assembly have been studied with a combination of high resolution transmission electron microscopy (HRTEM), scanning transmission x-ray microscopy (STXM), and x-ray magnetic circular dichroism (XMCD). By correlating HRTEM and STXM images we show that structures down to 30 nm are resolved with chemical contrast and record x-ray absorption spectra from endohedral lanthanide ions embedded in individual nanoscale CNT bundles. XMCD measurements of an Er3N@C80 bulk sample and a macroscopic assembly of filled CNTs indicate that the magnetic properties of the endohedral Er3+ ions are unchanged when encapsulated in CNTs. This study demonstrates the feasibility of local magnetic x-ray characterisation of low concentrations of lanthanide ions embedded in molecular nanostructures. © 2017 IOP Publishing Ltd.
    view abstractdoi: 10.1088/1361-6528/aa8b4c
  • 2017 • 436 Plasma-Activated Copper Nanocube Catalysts for Efficient Carbon Dioxide Electroreduction to Hydrocarbons and Alcohols
    Gao, D. and Zegkinoglou, I. and Divins, N.J. and Scholten, F. and Sinev, I. and Grosse, P. and Roldan Cuenya, B.
    ACS Nano 11 4825-4831 (2017)
    Carbon dioxide electroreduction to chemicals and fuels powered by renewable energy sources is considered a promising path to address climate change and energy storage needs. We have developed highly active and selective copper (Cu) nanocube catalysts with tunable Cu(100) facet and oxygen/chlorine ion content by low-pressure plasma pretreatments. These catalysts display lower overpotentials and higher ethylene, ethanol, and n-propanol selectivity, resulting in a maximum Faradaic efficiency (FE) of ∼73% for C2 and C3 products. Scanning electron microscopy and energy-dispersive X-ray spectroscopy in combination with quasi-in situ X-ray photoelectron spectroscopy revealed that the catalyst shape, ion content, and ion stability under electrochemical reaction conditions can be systematically tuned through plasma treatments. Our results demonstrate that the presence of oxygen species in surface and subsurface regions of the nanocube catalysts is key for achieving high activity and hydrocarbon/alcohol selectivity, even more important than the presence of Cu(100) facets. © 2017 American Chemical Society.
    view abstractdoi: 10.1021/acsnano.7b01257
  • 2017 • 435 Complete Prevention of Dendrite Formation in Zn Metal Anodes by Means of Pulsed Charging Protocols
    Garcia, G. and Ventosa, E. and Schuhmann, W.
    ACS Applied Materials and Interfaces 9 18691-18698 (2017)
    Zn metal as anode in rechargeable batteries, such as Zn/air or Zn/Ni, suffers from poor cyclability. The formation of Zn dendrites upon cycling is the key limiting step. We report a systematic study of the influence of pulsed electroplating protocols on the formation of Zn dendrites and in turn on strategies to completely prevent Zn dendrite formation. Because of the large number of variables in electroplating protocols, a scanning droplet cell technique was adapted as a high-throughput methodology in which a descriptor of the surface roughness can be in situ derived by means of electrochemical impedance spectroscopy. Upon optimizing the electroplating protocol by controlling nucleation, zincate ion depletion, and zincate ion diffusion, scanning electron microscopy and atomic force microscopy confirmed the growth of uniform and homogenous Zn deposits with a complete prevention of dendrite growth. The implementation of pulsed electroplating as the charging protocol for commercially available Ni-Zn batteries leads to substantially prolonged cyclability demonstrating the benefits of pulsed charging in Zn metal-based batteries. © 2017 American Chemical Society.
    view abstractdoi: 10.1021/acsami.7b01705
  • 2017 • 434 New amidinate complexes of indium(III): Promising CVD precursors for transparent and conductive In2O3 thin films
    Gebhard, M. and Hellwig, M. and Kroll, A. and Rogalla, D. and Winter, M. and Mallick, B. and Ludwig, Ar. and Wiesing, M. and Wieck, A.D. and Grundmeier, G. and Devi, A.
    Dalton Transactions 46 10220-10231 (2017)
    For the first time, synthesis of two new amidinate-ligand comprising heteroleptic indium complexes, namely [InCl(amd)2] (1) and [InMe(amd)2] (2), via salt-metathesis and their detailed characterization is reported. For comparison, the earlier reported homoleptic tris-amidinate [In(amd)3] (3) was also synthesized and analyzed in detail especially with respect to the thermal properties and molecular crystal structure analysis which are reported here for the first time. From nuclear magnetic resonance spectroscopy (NMR) and single-crystal X-ray diffraction (XRD), all three compounds were found to be monomeric with C2 (compound 1 and 2) and C3 symmetry (compound 3). Both halide-free compounds 2 and 3 were evaluated regarding their thermal properties using temperature-dependent 1H-NMR, thermogravimetric analysis (TGA) and iso-TGA, revealing suitable volatility and thermal stability for their application as potential precursors for chemical vapor phase thin film deposition methods. Indeed, metalorganic chemical vapor deposition (MOCVD) experiments over a broad temperature range (400 °C-700 °C) revealed the suitability of these two compounds to fabricate In2O3 thin films in the presence of oxygen on Si, thermally grown SiO2 and fused silica substrates. The as-deposited thin films were characterized in terms of their crystallinity via X-ray diffraction (XRD), morphology by scanning electron microscopy (SEM) and composition through complementary techniques such as Rutherford-backscattering spectrometry (RBS) in combination with nuclear reaction analysis (NRA) and X-ray photoelectron spectroscopy (XPS). From UV/Vis spectroscopy, the deposited In2O3 thin films on fused silica substrates were found to be highly transparent (T &gt; 95% at 560 nm, compound 3). In addition, Hall measurements revealed high charge carrier densities of 1.8 × 1020 cm-3 (2) and 6.5 × 1019 cm-3 (3) with a Hall-mobility of 48 cm2 V-1 s-1 (2) and 74 cm2 V-1 s-1 (3) for the respective thin films, rendering the obtained thin films applicable as a transparent conducting oxide that could be suitable for optoelectronic applications. © 2017 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c7dt01280b
  • 2017 • 433 Microstructural characterization and simulation of damage for geared sheet components
    Gerstein, G. and Isik, K. and Gutknecht, F. and Sieczkarek, P. and Ewert, J. and Tekkaya, A.E. and Clausmeyer, T. and Nürnberger, F.
    Journal of Physics: Conference Series 896 (2017)
    The evolution of damage in geared components manufactured from steel sheets was investigated, to analyse the influence of damage caused by the sheet-bulk-metal forming. Due to the inhomogeneous and multi-axial deformation in the investigated parts, different aspects such as the location-dependent shape and size of voids are analysed by means of various microscopic methods. In particular, a method to characterize the state of damage evolution, i. e. void nucleation, growth and coalescence using scanning electron microscopy (SEM) is applied. The investigations reveal a strong dependence of the void area fraction, shape of voids and thus damage evolution on the loading mode. The microstructural analysis is complemented with FEM simulations using material models which consider the characteristics of the void evolution. © Published under licence by IOP Publishing Ltd.
    view abstractdoi: 10.1088/1742-6596/896/1/012076
  • 2017 • 432 Trisubstituted Pyridinylimidazoles as Potent Inhibitors of the Clinically Resistant L858R/T790M/C797S EGFR Mutant: Targeting of Both Hydrophobic Regions and the Phosphate Binding Site
    Günther, M. and Lategahn, J. and Juchum, M. and Döring, E. and Keul, M. and Engel, J. and Tumbrink, H.L. and Rauh, D. and Laufer, S.
    Journal of Medicinal Chemistry 60 5613-5637 (2017)
    Inhibition of the epidermal growth factor receptor represents one of the most promising strategies in the treatment of lung cancer. Acquired resistance compromises the clinical efficacy of EGFR inhibitors during long-term treatment. The recently discovered EGFR-C797S mutation causes resistance against third-generation EGFR inhibitors. Here we present a rational approach based on extending the inhibition profile of a p38 MAP kinase inhibitor toward mutant EGFR inhibition. We used a privileged scaffold with proven cellular potency as well as in vivo efficacy and low toxicity. Guided by molecular modeling, we synthesized and studied the structure-activity relationship of 40 compounds against clinically relevant EGFR mutants. We successfully improved the cellular EGFR inhibition down to the low nanomolar range with covalently binding inhibitors against a gefitinib resistant T790M mutant cell line. We identified additional noncovalent interactions, which allowed us to develop metabolically stable inhibitors with high activities against the osimertinib resistant L858R/T790M/C797S mutant. © 2017 American Chemical Society.
    view abstractdoi: 10.1021/acs.jmedchem.7b00316
  • 2017 • 431 Local photocurrent mapping and cell performance behaviour on a nanometre scale for monolithically interconnected Cu(In,Ga)Se2 solar cells
    Haggui, M. and Reinhold, B. and Andrae, P. and Greiner, D. and Schmid, M. and Fumagalli, P.
    Journal of Microscopy 268 66-72 (2017)
    The local efficiency of lamellar shaped Cu(In,Ga)Se2 solar cells has been investigated using scanning near-field optical microscopy (SNOM). Topographic and photocurrent measurements have been performed simultaneously with a 100 nm tip aperture. The lamellar shaped solar cell with monolithic interconnects (P scribe) has been investigated on a nanometre scale for the first time at different regions using SNOM. It was found that, the cell region between P1 and P2 significantly contributes to the solar cells overall photocurrent generation. The photocurrent produced depends locally on the sample topography and it is concluded that it is mainly due to roughness changes of the ZnO:Al/i-ZnO top electrode. Regions lying under large grains of ZnO produce significantly less current than regions under small granules. The observed photocurrent features were allocated primarily to the ZnO:Al/i-ZnO top electrode. They were found to be independent of the wavelength of the light used (532 nm and 633 nm). © 2017 The Authors Journal of Microscopy © 2017 Royal Microscopical Society
    view abstractdoi: 10.1111/jmi.12587
  • 2017 • 430 Degradation mechanisms of pcBN tool material during Friction Stir Welding of Ni-base alloy 625
    Hanke, S. and Lemos, G.V.B. and Bergmann, L. and Martinazzi, D. and dos Santos, J.F. and Strohaecker, T.R.
    Wear 376-377 403-408 (2017)
    In Friction Stir Welding (FSW), interactions between the plasticized stirred material and the tool significantly affect resulting weld properties. When welding metals with high strength and melting point, the tribological load on the tool is severe, and poses the main limiting factor for the technology's industrial exploitation. Since tool materials are loaded to their limits, it is essential to understand the interactions of specific tool material and welded metal combinations. In the present study 3.2 mm alloy 625 sheets were joined using a pcBN tool with W-Re binder phase. Wear lead to a change in tool geometry followed by tool fracture. In SEM investigations the welds revealed typical banded structures, composed of small grains and non-metallic phases containing W from the tool material. The tool surface is extensively covered by adhering sheet metal. Further, BN grain pull-outs and appearances of diffusive wear are visible on the worn tool surface. Tool wear is mainly caused by detachment of BN grains due to thermal softening of the metallic binder phase and dissolution of BN in the hot material in the stirred zone. Using low rotational speeds resulting in lower process temperatures reduces tool wear and results in a homogeneous stirred zone. © 2017 Elsevier B.V.
    view abstractdoi: 10.1016/j.wear.2017.01.070
  • 2017 • 429 Micrometer-Precise Determination of the Thin Electrolyte Layer of a Spectroelectrochemical Cell by Microelectrode Approach Curves
    Hiltrop, D. and Masa, J. and Botz, A.J.R. and Lindner, A. and Schuhmann, W. and Muhler, M.
    Analytical Chemistry 89 4367-4372 (2017)
    A spectroelectrochemical cell is presented that allows investigations of electrochemical reactions by means of attenuated total reflection infrared (ATR-IR) spectroscopy. The electrode holder for the working (WE), counter and reference electrode as mounted in the IR spectrometer cause the formation of a thin electrolyte layer between the internal reflection element (IRE) and the surface of the WE. The thickness of this thin electrolyte layer (dTL) was estimated by performing a scanning electrochemical microscopy-(SECM) like approach of a Pt microelectrode (ME), which was leveled with the WE toward the IRE surface. The precise lowering of the ME/WE plane toward the IRE was enabled by a micrometer screw. The approach curve was recorded in negative feedback mode of SECM and revealed the contact point of the ME and WE on the IRE, which was used as reference point to perform the electro-oxidation of ethanol over a drop-casted Pd/NCNT catalyst on the WE at different thin-layer thicknesses by cyclic voltammetry. The reaction products were detected in the liquid electrolyte by IR spectroscopy, and the effect of variations in dTL on the current densities and IR spectra were analyzed and discussed. The obtained data identify dTL as an important variable in thin-layer experiments with electrochemical reactions and FTIR readout. © 2017 American Chemical Society.
    view abstractdoi: 10.1021/acs.analchem.6b03732
  • 2017 • 428 Self-Assembly of a Tripodal Triszwitterion Forms a pH-Switchable Hydrogel that Can Reversibly Encapsulate Hydrophobic Guests in Water
    Jana, P. and Schmuck, C.
    Chemistry - A European Journal 23 320-326 (2017)
    The development of supramolecular smart materials, which exhibit physicochemical structural changes in response to external stimuli is of current interest for various applications. Herein, we have developed the novel tripodal triszwitterion 1, derived from a C 3-symmetric benzene-1,3,5-tricarboxamide (BTA) core, which forms a thermo-reversible and pH-switchable transparent hydrogel through intermolecular self-complementary zwitterionic interactions at a neutral pH value. The hierarchical supramolecular self-aggregation was fully analyzed by microscopy (AFM, field emission scanning electron microscopy (FESEM)), viscosity, dynamic light scattering (DLS), and rheology studies. Moreover, compound 1 enables to encapsulate hydrophobic guests, such as the dye Nile red in aqueous medium at pH 6, which makes it an interesting candidate for drug delivery and controlled release. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/chem.201601122
  • 2017 • 427 Synthesis and evaluation of new copper ketoiminate precursors for a facile and additive-free solution-based approach to nanoscale copper oxide thin films
    Karle, Sarah and Rogalla, Detlef and Ludwig, Arne and Becker, Hans-Werner and Wieck, Andreas Dirk and Grafen, Markus and Ostendorf, Andreas and Devi, Anjana
    Dalton Transactions 46 2670--2679 (2017)
    Novel copper ketoiminate compounds were synthesized and for the first time applied for additive-free solution-based deposition of nanoscale copper oxide thin films. The two closely related compounds, namely the bis[4-(2-ethoxyethyl-imino)-3-pentanonato] copper, [Cu(EEKI)(2)], and bis[4-(3-methoxypropylimino)- 3-pentanonato] copper, [Cu(MPKI)(2)], were characterized by means of elemental and thermogravimetric analysis (TGA), as well as electron impact mass spectrometry (EI-MS). The advantages of these compounds are that they are liquid and possess excellent solubility in common organic solvents in addition to an optimum reactivity towards ambient moisture that enables a facile solution-based approach to nanoscale copper oxide thin films. Moreover, no additives or aging is needed to stabilize the solution processing of the copper oxide layers. [Cu(MPKI)(2)] was tested in detail for the deposition of copper oxide thin films by spin coating. Upon one-step annealing, high-quality, uniform, crystalline copper oxide thin films were deposited on Si, SiO2, as well as on quartz substrates. Structural, morphological and compositional characteristics of the copper oxide nanostructures were investigated in detail by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and a combined analysis using Rutherford backscattering spectroscopy (RBS) and nuclear reaction analysis (NRA). It was possible to control the copper oxide phases (CuO and Cu2O) by systematic tuning of the post-deposition annealing conditions. The functional properties in terms of optical band gap were investigated using UV/Vis spectroscopy, while the transport properties, such as resistivity, mobility and carrier concentration were analyzed employing Hall measurements, which confirmed the p-type conductivity of the copper oxide layers.
    view abstractdoi: 10.1039/c6dt04399b
  • 2017 • 426 Polyvinylamine-coated polyester fibers as a carrier matrix for the immobilization of peroxidases
    Kiehl, K. and Opwis, K. and Gutmann, J.S.
    Engineering in Life Sciences 17 645-652 (2017)
    Biocatalytic transformations that employ immobilized enzymes become increasingly important for industrial applications. Synthetic or natural textile fiber materials such as polyester, polyamide or viscose are support materials that are comparatively inexpensive. Contrary to traditional support materials, their flexibility enables their use in reactors of any geometry and a fast and residue-free removal from batch reactors. In this study a permanently immobilized peroxidase (Baylase®) has been investigated on polyester felt as a solid support as a new heterogeneous catalyst system. The polyester felt was functionalized by coating with polyvinylamine and subsequent activation with glutaraldehyde as a crosslinking agent. The enzyme load on the textile surface, the activity of the immobilized protein after repeated use as well as the storage stability was evaluated. Scanning electron micrographs and UV Vis spectroscopy made it possible to verify the enzyme immobilization on the textile surface. Furthermore, the load of immobilized peroxidase was determined by ICP OE spectrometry to be 9-12 mg per gram of textile. The activity of immobilized Baylase® remained high over 35 reaction cycles and a storage period of 8 weeks. © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/elsc.201600170
  • 2017 • 425 Low-Temperature Atomic Layer Deposition of Cobalt Oxide as an Effective Catalyst for Photoelectrochemical Water-Splitting Devices
    Kim, J. and Iivonen, T. and Hämäläinen, J. and Kemell, M. and Meinander, K. and Mizohata, K. and Wang, L. and Räisänen, J. and Beranek, R. and Leskelä, M. and Devi, A.
    Chemistry of Materials 29 5796-5805 (2017)
    We have developed a low-temperature atomic layer deposition (ALD) process for depositing crystalline and phase pure spinel cobalt oxide (Co3O4) films at 120 °C using [Co(tBu2DAD)2] and ozone as coreagent. X-ray diffraction, UV-vis spectroscopy, atomic force microscopy, field emission scanning electron microscopy, X-ray photoelectron spectroscopy, and time-of-flight elastic recoil detection analysis were performed to characterize the structure and properties of the films. The as-deposited Co3O4 films are crystalline with a low amount of impurities (&lt;2% C and &lt;5% H) despite low deposition temperatures. Deposition of Co3O4 onto thin TiO2 photoanodes (100 nm) for water oxidation resulted in 30% improvement of photocurrent (after 10 ALD cycles yielding small Co3O4 particles) as compared to pristine TiO2 films), and exhibited no detrimental effects on photocurrent response up to 300 deposition cycles (approximately 35 nm thick films), demonstrating the applicability of the developed ALD process for deposition of effective catalyst particles and layers in photoelectrochemical water-splitting devices. © 2017 American Chemical Society.
    view abstractdoi: 10.1021/acs.chemmater.6b05346
  • 2017 • 424 Microstructure-based assessment of the corrosion fatigue behavior of the creep-resistant DieMag422 and AE42 magnesium alloys
    Klein, M. and Buhr, P. and Walther, F.
    Solid State Phenomena 258 SSP 530-533 (2017)
    Magnesium alloys offer high potential for lightweight constructions, e.g. in automotive applications. However, their application range is limited due to their low corrosion resistance. In the present study, the influence of corrosion on the microstructure and the depending mechanical properties under cyclic loading were characterized for the creep-resistant DieMag422 (Mg-4Al-2Ba-2Ca) and AE42 magnesium alloys. In this context, fatigue properties in distilled water and sodium chloride solutions were assessed in constant amplitude tests. The results were correlated with corrosion properties of the alloys, which were evaluated by immersion tests. Corrosion- and deformation-induced microstructural changes were observed by light and scanning electron microscopy (SEM), yielding a structure-property-relationship for a comprehensive understanding of mechanical and corrosive deterioration mechanisms. © 2017 Trans Tech Publications, Switzerland.
    view abstractdoi: 10.4028/www.scientific.net/SSP.258.530
  • 2017 • 423 Antibacterial activity of microstructured sacrificial anode thin films by combination of silver with platinum group elements (platinum, palladium, iridium)
    Köller, M. and Bellova, P. and Javid, S.M. and Motemani, Y. and Khare, C. and Sengstock, C. and Tschulik, K. and Schildhauer, T.A. and Ludwig, Al.
    Materials Science and Engineering C 74 536-541 (2017)
    Five different Ag dots arrays (16 to 400dots/mm2) were fabricated on a continuous platinum, palladium, or iridium thin film and for comparison also on titanium film by sputter deposition and photolithographic patterning. To analyze the antibacterial activity of these microstructured films Staphylococcus aureus (S. aureus) were placed onto the array surfaces and cultivated overnight. To analyze the viability of planktonic as well as surface adherent bacteria, the applied bacterial fluid was subsequently aspirated, plated on blood agar plates and adherent bacteria were detected by fluorescence microscopy. A particular antibacterial effect towards . S. aureus was induced by Ag dot arrays on each of the platinum group thin film (sacrificial anode system for Ag) in contrast to Ag dot arrays fabricated on the Ti thin films (non-sacrificial anode system for Ag). Among platinum group elements the Ir-Ag system exerted the highest antibacterial activity which was accompanied by most advanced dissolution of the Ag dots and Ag ion release compared to Ag dots on Pt or Pd. © 2016 Elsevier B.V.
    view abstractdoi: 10.1016/j.msec.2016.12.075
  • 2017 • 422 Size- and density-controlled deposition of Ag nanoparticle films by a novel low-temperature spray chemical vapour deposition method—research into mechanism, particle growth and optical simulation
    Liu, Y. and Plate, P. and Hinrichs, V. and Köhler, T. and Song, M. and Manley, P. and Schmid, M. and Bartsch, P. and Fiechter, S. and Lux-Steiner, M.C. and Fischer, C.-H.
    Journal of Nanoparticle Research 19 (2017)
    Ag nanoparticles have attracted interest for plasmonic absorption enhancement of solar cells. For this purpose, well-defined particle sizes and densities as well as very low deposition temperatures are required. Thus, we report here a new spray chemical vapour deposition method for producing Ag NP films with independent size and density control at substrate temperatures even below 100 °C, which is much lower than for many other techniques. This method can be used on different substrates to deposit Ag NP films. It is a reproducible, low-cost process which uses trimethylphosphine (hexafluoroacetylacetonato) silver as a precursor in alcoholic solution. By systematic variation of deposition parameters and classic experiments, mechanisms of particle growth and of deposition processes as well as the low decomposition temperature of the precursor could be explained. Using the 3D finite element method, absorption spectra of selected samples were simulated, which fitted well with the measured results. Hence, further applications of such Ag NP films for generating plasmonic near field can be predicted by the simulation. © 2017, Springer Science+Business Media Dordrecht.
    view abstractdoi: 10.1007/s11051-017-3834-6
  • 2017 • 421 Functional performance of Gd2Zr2O7/YSZ multi-layered thermal barrier coatings deposited by suspension plasma spray
    Mahade, S. and Curry, N. and Björklund, S. and Markocsan, N. and Nylén, P. and Vaßen, R.
    Surface and Coatings Technology 318 208-216 (2017)
    7-8. wt.% yttria stabilized zirconia (YSZ) is the standard ceramic top coat material used in gas turbines to insulate the underlying metallic substrate. However, at higher temperatures (>. 1200. C), phase stability and sintering becomes an issue for YSZ. At these temperatures, YSZ is also susceptible to CMAS (calcium magnesium alumino silicates) infiltration. New ceramic materials such as pyrochlores have thus been proposed due to their excellent properties such as lower thermal conductivity and better CMAS attack resistance compared to YSZ. However, pyrochlores have inferior thermo mechanical properties compared to YSZ. Therefore, double-layered TBCs with YSZ as the intermediate layer and pyrochlore as the top ceramic layer have been proposed. In this study, double layer TBC comprising gadolinium zirconate (GZ)/YSZ and triple layer TBC (GZdense/GZ/YSZ) comprising relatively denser GZ top layer on GZ/YSZ were deposited by suspension plasma spray. Also, single layer 8YSZ TBC was suspension plasma sprayed to compare its functional performance with the multi-layered TBCs. Cross sections and top surface morphology of as sprayed TBCs were analyzed by scanning electron microscopy (SEM). XRD analysis was done to identify phases formed in the top surface of as sprayed TBCs. Porosity measurements were made using water intrusion and image analysis methods. Thermal diffusivity of the as sprayed TBCs was measured using laser flash analysis and thermal conductivity of the TBCs was calculated. The multi-layered GZ/YSZ TBCs were shown to have lower thermal conductivity than the single layer YSZ. The as sprayed TBCs were also subjected to thermal cyclic testing at 1300. C. The double and triple layer TBCs had a longer thermal cyclic life compared to YSZ. The thermo cycled samples were analyzed by SEM. © 2016 Elsevier B.V.
    view abstractdoi: 10.1016/j.surfcoat.2016.12.062
  • 2017 • 420 Erosion Performance of Gadolinium Zirconate-Based Thermal Barrier Coatings Processed by Suspension Plasma Spray
    Mahade, S. and Curry, N. and Björklund, S. and Markocsan, N. and Nylén, P. and Vaßen, R.
    Journal of Thermal Spray Technology 26 108-115 (2017)
    7-8 wt.% Yttria-stabilized zirconia (YSZ) is the standard thermal barrier coating (TBC) material used by the gas turbines industry due to its excellent thermal and thermo-mechanical properties up to 1200 °C. The need for improvement in gas turbine efficiency has led to an increase in the turbine inlet gas temperature. However, above 1200 °C, YSZ has issues such as poor sintering resistance, poor phase stability and susceptibility to calcium magnesium alumino silicates (CMAS) degradation. Gadolinium zirconate (GZ) is considered as one of the promising top coat candidates for TBC applications at high temperatures (>1200 °C) due to its low thermal conductivity, good sintering resistance and CMAS attack resistance. Single-layer 8YSZ, double-layer GZ/YSZ and triple-layer GZdense/GZ/YSZ TBCs were deposited by suspension plasma spray (SPS) process. Microstructural analysis was carried out by scanning electron microscopy (SEM). A columnar microstructure was observed in the single-, double- and triple-layer TBCs. Phase analysis of the as-sprayed TBCs was carried out using XRD (x-ray diffraction) where a tetragonal prime phase of zirconia in the single-layer YSZ TBC and a cubic defect fluorite phase of GZ in the double and triple-layer TBCs was observed. Porosity measurements of the as-sprayed TBCs were made by water intrusion method and image analysis method. The as-sprayed GZ-based multi-layered TBCs were subjected to erosion test at room temperature, and their erosion resistance was compared with single-layer 8YSZ. It was shown that the erosion resistance of 8YSZ single-layer TBC was higher than GZ-based multi-layered TBCs. Among the multi-layered TBCs, triple-layer TBC was slightly better than double layer in terms of erosion resistance. The eroded TBCs were cold-mounted and analyzed by SEM. © 2016, ASM International.
    view abstractdoi: 10.1007/s11666-016-0479-4
  • 2017 • 419 Dislocation-twin boundary interaction in small scale Cu bi-crystals loaded in different crystallographic directions
    Malyar, N.V. and Micha, J.-S. and Dehm, G. and Kirchlechner, C.
    Acta Materialia 129 91-97 (2017)
    The mechanical behavior of several 5×5 micron sized Cu bi-crystals with a single coherent Σ3{111} twin boundary (TB) is studied by in situ Laue microdiffraction (μLaue) compression with the aim to unravel the slip transfer mechanisms through TBs. Single crystalline pillars (SCP) are additionally tested and used as reference samples. Engineering stress-strain curves and post mortem scanning electron microscopy (SEM) images were correlated to the local evolution of the TB angle, the storage of geometrically necessary dislocations and crystal orientations investigated by in situ X-ray Laue microdiffraction (μLaue). Both μLaue and post mortem SEM demonstrate multiple transmission events through the TB without significant storage of geometrically necessary dislocations in the crystals or at the boundary, independent on the compression direction. Nevertheless, at engineering strains larger than 5% a small dislocation pile-up was once observed temporarily at the boundary. Upper and lower bounds for the transmission stress are discussed based on the current experimental results. © 2017 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2017.02.067
  • 2017 • 418 Size effect in bi-crystalline micropillars with a penetrable high angle grain boundary
    Malyar, N.V. and Micha, J.S. and Dehm, G. and Kirchlechner, C.
    Acta Materialia 129 312-320 (2017)
    The implications of various size effects on the deformation behavior of and near grain boundaries is not yet fully understood. In this manuscript, slip transfer mechanisms through a general high angle grain boundary (HAGB) allowing for easy transfer are investigated in order to understand the size dependence of the dislocation-grain-boundary interaction. Complementary in situ micro compression tests on copper single and bi-crystals in the scanning electron microscope and with x-ray Laue microdiffraction were used to correlate the mechanical response with the evolving microstructure. It is shown that no dislocation pile-up is formed at the boundary. The lack of pile-up stresses results in a deformation process which is dominated by the initial dislocation source statistics. This is evidenced by similar size scaling of the single and bi-crystalline samples with the grain size being the characteristic length scale. © 2017 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2017.03.003
  • 2017 • 417 In-situ tensile testing of notched poly-and oligocrystalline 316L wires
    Mitevski, B. and Weis, S. and Fischer, A.
    Materialpruefung/Materials Testing 59 130-135 (2017)
    In-situ testing inside a scanning electron microscope is a helpful tool for detailed analyses of small sized specimens with respect to their mechanical properties and the correlated microstructural alterations. Thus, this test method is used to analyze the tensional properties of thin 316L (1.4441) wires used for microscale components, e. g., like coronary artery stents. Tensile tests were conducted on unnotched and circularly notched 316L wires (∅ 0.95 mm) with a special focus on the number of grains within the cross section as well as the notch geometry. Four combinations of notch width (2 and 4 mm) and notch depth (diameter at notch root: 0.5 and 0.75 mm) were chosen. Notch depth and notch shape were adjusted by means of electrochemical polishing. Previous investigations showed, that oligocrystalline structures exhibit a different mechanical behavior compared to polycrystalline ones or single crystals. There are only a few data available on mechanical testing of oligocrystalline structures with respect to varying notch geometries. Depending on the notch geometry, grain size and, therefore, the number of grains within the notch cross section widely scattering yield-and tensile strength as well as failure elongation values were measured. However, the transition criterion between poly-and oligocrystalline behavior could be quantified to be 6 to 7 grains within the cross section. © Carl Hanser Verlag GmbH &Co. KG.
    view abstractdoi: 10.3139/120.110973
  • 2017 • 416 In-situ tensile testing of notched poly- and oligocrystalline 316L wires
    Mitevski, Bojan and Weiss, Sabine and Fischer, Alfons
    Materials Testing 59 130--135 (2017)
    In-situ testing inside a scanning electron microscope is a helpful tool for detailed analyses of small sized specimens with respect to their mechanical properties and the correlated microstructural alterations. Thus, this test method is used to analyze the tensional properties of thin 316L (1.4441) wires used for microscale components, e. g., like coronary artery stents. Tensile tests were conducted on unnotched and circularly notched 316L wires (phi 0.95 mm) with a special focus on the number of grains within the cross section as well as the notch geometry. Four combinations of notch width (2 and 4 mm) and notch depth (diameter at notch root: 0.5 and 0.75 mm) were chosen. Notch depth and notch shape were adjusted by means of electrochemical polishing. Previous investigations showed, that oligocrystalline structures exhibit a different mechanical behavior compared to polycrystalline ones or single crystals. There are only a few data available on mechanical testing of oligocrystalline structures with respect to varying notch geometries. Depending on the notch geometry, grain size and, therefore, the number of grains within the notch cross section widely scattering yield-and tensile strength as well as failure elongation values were measured. However, the transition criterion between poly-and oligocrystalline behavior could be quantified to be 6 to 7 grains within the cross section.
    view abstractdoi: 10.3139/120.110973
  • 2017 • 415 Adaptation of TiC hard particles properties and morphology in metal matrix composites by refractory elements
    Mohr, A. and Röttger, A. and Theisen, W.
    Key Engineering Materials 742 KEM 99-105 (2017)
    High mechanical loads, corrosion, and abrasion decrease the lifetime of tools. One way to increase the wear resistance of tool materials can be achieved by adding hard particles to the metal matrix such as titanium carbide, which protect the softer metal matrix against abrasive particles. This material concept is designated as metal matrix composite (MMC). Ferro-Titanit® is such MMC material, possessing high wear and a simultaneously high corrosion resistance, for which reason this material is used in the polymers industry. The material concept is based on a corrosion-resistant Fe-base matrix with up to 45 vol% titanium carbide (TiC) as a hard particle addition to improve the wear resistance against abrasion. These TiC hard particles must be adapted to the present tribological system in terms of hardness, size and morphology. This study shows how the size and morphology of TiC hard particles can be influenced by the refractory element niobium (Nb). Therefore, the element Nb was added with 2 and 4 mass% to the soft-martensitic Ferro-Titanit® Grade Nikro128. The investigated materials were compacted by sintering, and the densified microstructure was further characterized by scanning electron microscopy (SEM), energy dispersive spectrometry (EDX), and optical image analyses. Furthermore, microstructure and properties of the compacted Nb-alloyed samples were compared to the reference material Nikro128. The results show that the addition of Nb influences the morphology, size and chemical composition of the TiC hard particle. These changes in the hard phase characteristics also influence the materials properties. It was shown that the phase niobium carbide (NbC) is formed around the TiC during the densification process, leading to a change in morphology and size of the TiC. © 2017 Trans Tech Publications, Switzerland.
    view abstractdoi: 10.4028/www.scientific.net/KEM.742.99
  • 2017 • 414 Potential of an alumina-supported Ni3Fe catalyst in the methanation of CO2: Impact of alloy formation on activity and stability
    Mutz, B. and Belimov, M. and Wang, W. and Sprenger, P. and Serrer, M.A. and Wang, D. and Pfeifer, P. and Kleist, W. and Grunwaldt, J.-D.
    ACS Catalysis 7 6802-6814 (2017)
    A promising bimetallic 17 wt % Ni3Fe catalyst supported on γ-Al2O3 was prepared via homogeneous deposition-precipitation for the application in the methanation of CO2 to gather more detailed insight into the structure and performance of the catalyst compared to state-of-the-art methanation systems. X-ray diffraction (XRD) analysis, detailed investigations using scanning transmission electron microscopy (STEM) combined with energy dispersive X-ray spectroscopy analysis (EDX) of single particles as well as larger areas, high-resolution transmission electron microscopy (HRTEM) imaging, temperature-programmed reduction (H2-TPR), and in-depth interpretation of Raman bands led to the conclusion that a high fraction of the Ni and Fe formed the desired Ni3Fe alloy resulting in small and well-defined nanoparticles with 4 nm in size and a dispersion of 24%. For comparison, a monometallic catalyst with similar dispersion using the same preparation method and analysis was prepared. Using a fixed-bed reactor, the Ni3Fe catalyst showed better low-temperature performance compared to a monometallic Ni reference catalyst, especially at elevated pressures. Longterm experiments in a microchannel packed bed reactor under industrially relevant reaction conditions in competition with a commercial Ni-based methanation catalyst revealed an improved performance of the Ni3Fe system at 358°C and 6 bar involving enhanced conversion of CO2 to 71%, selectivity to CH4 &gt; 98%, and most notably a high stability. Deactivation occurred only at lower temperatures, which was related to carbon deposition due to an increased CO production. Kinetic measurements were compared with literature models derived for Ni/Al2O3 catalysts, which fit well but underestimate the performance of the Ni3Fe system, emphasizing the synergetic effect of Ni and Fe. © 2017 American Chemical Society.
    view abstractdoi: 10.1021/acscatal.7b01896
  • 2017 • 413 Identification of a ternary μ-phase in the Co-Ti-W system – An advanced correlative thin-film and bulk combinatorial materials investigation
    Naujoks, D. and Eggeler, Y.M. and Hallensleben, P. and Frenzel, J. and Fries, S.G. and Palumbo, M. and Koßmann, J. and Hammerschmidt, T. and Pfetzing-Micklich, J. and Eggeler, G. and Spiecker, E. and Drautz, R. and Ludwig, Al.
    Acta Materialia 138 100-110 (2017)
    The formation of a ternary μ-phase is documented for the system Co-Ti-W. The relevant compositional stability range is identified by high-throughput energy dispersive X-ray spectroscopy, electrical resistance and X-ray diffraction maps from a thin-film materials library (1 μm thickness). Bulk samples of the identified compositions were fabricated to allow for correlative film and bulk studies. Using analytical scanning and transmission electron microscopy, we demonstrate that in both, thin film and bulk samples, the D85 phase (μ-phase) coexists with the C36-phase and the A2-phase at comparable average chemical compositions. Young's moduli and hardness values of the μ-phase and the C36-phase were determined by nanoindentation. The trends of experimentally obtained elastic moduli are consistent with density functional theory (DFT) calculations. DFT analysis also supports the experimental findings, that the μ-phase can solve up to 18 at.% Ti. Based on the experimental and DFT results it is shown that CALPHAD modeling can be modified to account for the new findings. © 2017 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2017.07.037
  • 2017 • 412 High-throughput study of binary thin film tungsten alloys
    Nikolić, V. and Wurster, S. and Savan, A. and Ludwig, Al. and Pippan, R.
    International Journal of Refractory Metals and Hard Materials 69 40-48 (2017)
    Combinatorial magnetron co-sputtering from elemental sources was applied to produce W-alloy thin film composition spread materials libraries with well-defined, continuous composition gradients (film thicknesses between 1 and 2.5 μm). Three systems were studied: W-Fe (0–7 at.%), W-Ti (0–15 at.%) and W-Ir (0–12 at.%). High-throughput characterization of the materials libraries comprised of chemical, morphological and microstructural analyses. Scanning electron microscope investigations revealed that the films have a columnar structure of inverted cone-like units separated by voided boundaries, with a strong correlation to the alloying element content. Significant morphological changes occurred with an increase in the amount of the added element; W films with lower at.% of the alloying element had higher density and tighter grain boundaries, altering towards an increased amount of voids as the concentration of the alloying element increased. Electron backscatter diffraction scanning was used to determine microstructural components (grain size, grain shape, texture evolution), in dependence on the concentration of the alloying element. © 2017 Elsevier Ltd
    view abstractdoi: 10.1016/j.ijrmhm.2017.07.017
  • 2017 • 411 Mechanical size effects in a single crystalline equiatomic FeCrCoMnNi high entropy alloy
    Raghavan, R. and Kirchlechner, C. and Jaya, B.N. and Feuerbacher, M. and Dehm, G.
    Scripta Materialia 129 52-55 (2017)
    The size dependence of the mechanical behavior of a single crystalline equiatomic FeCrCoMnNi single phase high entropy alloy was studied using in situ SEM microcompression. Electron back-scattered diffraction was used in conjunction with high-resolution scanning electron microscopy to identify the dominant slip system activated for accommodating plastic flow. The scaling of the yield strength with the size of the micropillar is discussed in comparison with the size dependence observed in face-centered and body-centered cubic single crystalline metals. © 2016 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.scriptamat.2016.10.026
  • 2017 • 410 Chemical and structural analysis of gallstones from the Indian subcontinent
    Ramana Ramya, J. and Thanigai Arul, K. and Epple, M. and Giebel, U. and Guendel-Graber, J. and Jayanthi, V. and Sharma, M. and Rela, M. and Narayana Kalkura, S.
    Materials Science and Engineering C 78 878-885 (2017)
    Representative gallstones from north and southern parts of India were analyzed by a combination of physicochemical methods: X-ray diffraction (XRD), infrared spectroscopy (IR), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), CHNS analysis, thermal analysis and Nuclear Magnetic Resonance (NMR) spectroscopy (1H and 13C). The stones from north Indian were predominantly consisting of cholesterol monohydrate and anhydrous cholesterol which was confirmed by XRD analysis. FTIR spectroscopy confirmed the presence of cholesterol and calcium bilirubinate in the south Indian gallstones. EDX spectroscopy revealed the presence of carbon, nitrogen, oxygen, calcium, sulfur, sodium and magnesium and chloride in both south Indian and north Indian gallstones. FTIR and NMR spectroscopy confirmed the occurrence of cholesterol in north Indian gallstones. The respective colour of the north Indian and south Indian gallstones was yellowish and black. The morphology of the constituent crystals of the north Indian and south Indian gallstones were platy and globular respectively. The appreciable variation in colour, morphology and composition of south and north Indian gallstones may be due to different food habit and habitat. © 2017 Elsevier B.V.
    view abstractdoi: 10.1016/j.msec.2017.04.004
  • 2017 • 409 Investigation of the resistance of open-column-structured PS-PVD TBCs to erosive and high-temperature corrosive attack
    Rezanka, S. and Mack, D.E. and Mauer, G. and Sebold, D. and Guillon, O. and Vaßen, R.
    Surface and Coatings Technology 324 222-235 (2017)
    In modern gas turbines, highly loaded components are internally cooled and furthermore covered with thermal barrier coatings (TBCs) to withstand the harsh operating conditions with temperatures exceeding the application limit of such coatings. Under realistic operating conditions, siliceous minerals, of a calcium-magnesium-aluminum-silicate (CMAS) composition, are ingested into the turbine and deposited on the TBCs. Besides erosion, this also leads to degradation by chemical interaction. The plasma spray-physical vapor deposition (PS-PVD) process is an advanced method for manufacturing TBCs, which fills the gap between traditional thermal spray processes and electron beam physical vapor deposition (EB-PVD). Due to the unique plasma conditions, coatings with columnar microstructures exhibiting high strain tolerance can be created. However, because of the high amount of open porosity the resistance of such structures to CMAS and erosion attack was expected to be low. In the present work, PS-PVD TBCs were investigated in a burner rig facility under thermal gradient cycling conditions and simultaneous CMAS attack. The interactions of the PS-PVD-deposited YSZ and the CMAS melt were studied by means of scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDS) and compared to EB-PVD coatings. Additionally, the resistance of PS-PVD TBCs to erosion is compared to APS TBCs by means of room temperature tests according to ASTM G76-13. © 2017
    view abstractdoi: 10.1016/j.surfcoat.2017.05.003
  • 2017 • 408 New approach on solid state joining of stainless steel tube to tube sheet joints
    Roos, A. and Winkler, M. and Wimmer, G. and Dos Santos, J.F. and Hanke, S.
    American Society of Mechanical Engineers, Pressure Vessels and Piping Division (Publication) PVP 6A-2017 (2017)
    Hybrid friction diffusion bonding (HFDB) is a solid-state bonding process first introduced by Helmholtz-Centre Geesthacht, Germany, to join aluminium tube-to-tube sheet joints of coil-wound heat exchangers (CWHE) for liquefaction of natural gas (LNG). This study describes how HFDB was in a first step successfully transferred to create austenitic S32100 single hole tube-to-tube sheet joints. Process parameters are presented and results from subsequent non-destructive bubble leak testing and destructive tensile pull-out testing are discussed. After pull out testing the bonded areas were further investigated using optical microscopy as well as scanning electron microscopy. Leak tight joints were generated due to the formation of a metallic bond close to the planar friction area of the employed tools, and failure in pull-out tests occurred by ductile fracture. The results show that the HFDB approach developed for Alalloys may well be transferred to steel, and in the future possibly to other high-temperature alloys. It thereby offers an alternative route for joining tube to tube-sheet connections in solid state, with the corresponding advantages, such as no open flames or arc, no spatter and no need for filler material. © Copyright 2017 ASME.
    view abstractdoi: 10.1115/PVP2017-65376
  • 2017 • 407 Uptake of the proteins HTRA1 and HTRA2 by cells mediated by calcium phosphate nanoparticles
    Rotan, O. and Severin, K.N. and Pöpsel, S. and Peetsch, A. and Merdanovic, M. and Ehrmann, M. and Epple, M.
    Beilstein Journal of Nanotechnology 8 381-393 (2017)
    The efficient intracellular delivery of (bio)molecules into living cells remains a challenge in biomedicine. Many biomolecules and synthetic drugs are not able to cross the cell membrane, which is a problem if an intracellular mode of action is desired, for example, with a nuclear receptor. Calcium phosphate nanoparticles can serve as carriers for small and large biomolecules as well as for synthetic compounds. The nanoparticles were prepared and colloidally stabilized with either polyethyleneimine (PEI; cationic nanoparticles) or carboxymethyl cellulose (CMC; anionic nanoparticles) and loaded with defined amounts of the fluorescently labelled proteins HTRA1, HTRA2, and BSA. The nanoparticles were purified by ultracentrifugation and characterized by dynamic light scattering and scanning electron microscopy. Various cell types (HeLa, MG-63, THP-1, and hMSC) were incubated with fluorescently labelled proteins alone or with protein-loaded cationic and anionic nanoparticles. The cellular uptake was followed by light and fluorescence microscopy, confocal laser scanning microscopy (CLSM), and flow cytometry. All proteins were readily transported into the cells by cationic calcium phosphate nanoparticles. Notably, only HTRA1 was able to penetrate the cell membrane of MG-63 cells in dissolved form. However, the application of endocytosis inhibitors revealed that the uptake pathway was different for dissolved HTRA1 and HTRA1-loaded nanoparticles. © 2017 Rotan et al.
    view abstractdoi: 10.3762/bjnano.8.40
  • 2017 • 406 Laser-induced surface activation of biocomposites for electroless metallization
    Rytlewski, P. and Bahners, T. and Polewski, F. and Gebert, B. and Gutmann, J.S. and Hartmann, N. and Hagemann, U. and Moraczewski, K.
    Surface and Coatings Technology 311 104-112 (2017)
    In this work biocomposites containing polylactide (PLA), polycaprolactone (PCL), copper(II) oxide and copper acetylacetonate were manufactured by an extrusion process. The extruded composites differed with respect to the PLA/PCL ratio whereas the content of mixed copper(II) oxide and copper acetylacetonate powders was held constant at 20 wt%. The main aims for the addition of PCL was to increase impact strength resistance, improve surface catalytic properties and reduce the temperature of extrusion, thus limiting degradation effects initiated by copper acetylacetonate. The composite samples were irradiated with an ArF excimer laser varying the number of laser pulses and then metalized by electroless plating. Based on optical microscopy, scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX) and X-ray photoelectron spectroscopy (XPS) measurements, it was found that (i) PCL was dispersed in the form of droplets in all volume of PLA, (ii) the copper compounds were preferably located in the dispersed PCL phase, and (iii) composites with higher PCL content were more effectively metalized. © 2016 Elsevier B.V.
    view abstractdoi: 10.1016/j.surfcoat.2016.12.048
  • 2017 • 405 Improving the zT value of thermoelectrics by nanostructuring: tuning the nanoparticle morphology of Sb2Te3 by using ionic liquids
    Schaumann, J. and Loor, M. and Ünal, D. and Mudring, A. and Heimann, S. and Hagemann, U. and Schulz, S. and Maculewicz, F. and Schierning, G.
    Dalton Transactions 46 656-668 (2017)
    A systematic study on the microwave-assisted thermolysis of the single source precursor (Et2Sb)2Te (1) in different asymmetric 1-alkyl-3-methylimidazolium- and symmetric 1,3-dialkylimidazolium-based ionic liquids (ILs) reveals the distinctive role of both the anion and the cation in tuning the morphology and microstructure of the resulting Sb2Te3 nanoparticles as evidenced by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDX), and X-ray photoelectron spectroscopy (XPS). A comparison of the electrical and thermal conductivities as well as the Seebeck coefficient of the Sb2Te3 nanoparticles obtained from different ILs reveals the strong influence of the specific IL, from which C4mimI was identified as the best solvent, on the thermoelectric properties of as-prepared nanosized Sb2Te3. This work provides design guidelines for ILs, which allow the synthesis of nanostructured thermoelectrics with improved performances. © The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c6dt04323b
  • 2017 • 404 Production-and microstructurebased fatigue assessment of metallic AISI 304/430 multilayer materials produced by hot pack rolling
    Schmiedt, A. and Luecker, L. and Kolesnikov, A.G. and Plokhikh, A. and Walther, F.
    Materialpruefung/Materials Testing 59 123-129 (2017)
    Metallic multilayer materials consisting of hundreds or thousands of layers offer a high potential for broad applications in modern technology. A uniform and gradual thinning of layers can be realized by using alloys with different crystal structures in combination with the efficient and high-performance technology of hot pack rolling. However, investigations on fatigue properties, especially to evaluate the influence of the number of layers, are still missing. In the present study, the fatigue behavior of metallic multilayer materials consisting of austenitic and ferritic stainless steels AISI 304 and AISI 430 with 100 and 1400 layers are characterized by applying a time-efficient load increase procedure. Therefore, instrumented stepwise load increase tests were performed to define suitable loading parameters for a convenient comparison of fatigue properties in constant amplitude tests. A benefit of the complex production process leading to 1400 layers was verified concerning the investigated load level in the range of low cycle fatigue with a significant improvement by the factor of 3.5. The alternating current potential drop method for measurements of change in voltage was determined to be most suitable to detect microstructural changes at an early state of fatigue damage for multilayer materials. Microstructures as well as fractured surfaces were investigated using light and scanning electron microscopy to evaluate the results of the two technological manufacturing routes as well as the crack and failure behavior. © Carl Hanser Verlag GmbH &Co. KG.
    view abstractdoi: 10.3139/120.110976
  • 2017 • 403 RF magnetron sputtering of a hydroxyapatite target: A comparison study on polytetrafluorethylene and titanium substrates
    Surmenev, R.A. and Surmeneva, M.A. and Grubova, I.Y. and Chernozem, R.V. and Krause, B. and Baumbach, T. and Loza, K. and Epple, M.
    Applied Surface Science 414 335-344 (2017)
    A pure hydroxyapatite (HA) target was used to prepare the biocompatible coating of HA on the surface of a polytetrafluorethylene (PTFE) substrate, which was placed on the same substrate holder with technically pure titanium (Ti) in the single deposition runs by radio-frequency (RF) magnetron sputtering. The XPS, XRD and FTIR analyses of the obtained surfaces showed that for all substrates, instead of the HA coating deposition, the coating of a mixture of calcium carbonate and calcium fluoride was grown. According to SEM investigations, the surface of PTFE was etched, and the surface topography of uncoated Ti was preserved after the depositions. The FTIR results reveal no phosphate bonds; only calcium tracks were observed in the EDX-spectra on the surface of the coated PTFE substrates. Phosphate oxide (V), which originated from the target, could be removed using a vacuum pump system, or no phosphate-containing bonds could be formed on the substrate surface because of the severe substrate bombardment process, which prevented the HA coating deposition. The observed results may be connected with the surface re-sputtering effect of the growing film by high-energy negatively charged ions (most probably oxygen or fluorine), which are accelerated in the cathode dark sheath. © 2017 Elsevier B.V.
    view abstractdoi: 10.1016/j.apsusc.2017.04.090
  • 2017 • 402 Incorporation of silver nanoparticles into magnetron-sputtered calcium phosphate layers on titanium as an antibacterial coating
    Surmeneva, M.A. and Sharonova, A.A. and Chernousova, S. and Prymak, O. and Loza, K. and Tkachev, M.S. and Shulepov, I.A. and Epple, M. and Surmenev, R.A.
    Colloids and Surfaces B: Biointerfaces 156 104-113 (2017)
    A three-layer system of nanocrystalline hydroxyapatite (first layer; 1000 nm thick), silver nanoparticles (second layer; 1.5 μg Ag cm−2) and calcium phosphate (third layer, either 150 or 1000 nm thick) on titanium was prepared by a combination of electrophoretic deposition of silver nanoparticles and the deposition of calcium phosphate by radio frequency magnetron sputtering. Scanning electron microscopy showed that the silver nanoparticles were evenly distributed over the surface. The adhesion of multilayered coating on the substrate was evaluated using the scratch test method. The resistance to cracking and delamination indicated that the multilayered coating has good resistance to contact damage. The release of silver ions from the hydroxyapatite/silver nanoparticle/calcium phosphate system into the phosphate-buffered saline (PBS) solution was measured by atomic absorption spectroscopy (AAS). Approximately one-third of the incorporated silver was released after 3 days immersion into PBS, indicating a total release time of the order of weeks. There were no signs of cracks on the surface of the coating after immersion after various periods, indicating the excellent mechanical stability of the multilayered coating in the physiological environment. An antimicrobial effect against Escherichia coli was found for a 150 nm thick outer layer of the calcium phosphate using a semi-quantitative turbidity test. © 2017 Elsevier B.V.
    view abstractdoi: 10.1016/j.colsurfb.2017.05.016
  • 2017 • 401 Tunable magneto-responsive mesoporous block copolymer membranes
    Tang, Y. and Lin, X. and Ito, K. and Hong, L. and Ishizone, T. and Yokoyama, H. and Ulbricht, M.
    Journal of Membrane Science 544 406-415 (2017)
    Mesoporous magneto-responsive membranes with remote-controllable step-wise tunable changes of sieving barrier pore size were successfully prepared. The membranes were composed of a thin mesoporous size-selective layer containing iron oxide nanoparticles (IONPs) embedded in poly(oligo(ethylene glycol) methyl ether methacrylate)-block-polystyrene-block-poly(oligo(ethylene glycol) methyl ether methacrylate) (PMEOnMA-b-PS-b-PMEOnMA), on top of a polyvinylidene fluoride (PVDF) macroporous support membrane. IONPs and PMEOnMA-b-PS-b-PMEOnMA mixture solutions were spun-cast on PVDF membranes, and then mesopores were introduced into the hydrophilic PMEOnMA domains by controlled selective swelling with a mixture of methanol and supercritical CO2. The loaded IONPs aggregated into small clusters (0.1–1 µm) and were finely dispersed in the PMEOnMA-b-PS-b-PMEOnMA mesoporous top layer. No defect was found in the membranes with IONPs based on water and dextran ultrafiltration performances, and scanning electron microscopy (SEM) images showed that their barrier pore sizes were almost identical to those of no IONPs containing membranes. Upon stimulation with alternating magnetic field with different input energy, the water permeability as well as the dextran rejection were tuned into different levels, indicating that the sieving barrier pore size distribution of these thin-film mixed matrix nanocomposite membranes was step-wise adjustable. © 2017 Elsevier B.V.
    view abstractdoi: 10.1016/j.memsci.2017.08.069
  • 2017 • 400 Self-sharpening-effect of nickel-diamond coatings sprayed by HVOF
    Tillmann, W. and Brinkhoff, A. and Schaak, C. and Zajaczkowski, J.
    IOP Conference Series: Materials Science and Engineering 181 (2017)
    The durability of stone working and drilling tools is an increasingly significant requirement in industrial applications. These tools are mainly produced by brazing diamond metal matrix composites inserts to the tool body. These inserts are produced by sintering diamonds and metal powder (e.g. nickel). If the wear is too high, the diamonds will break out of the metal matrix and other diamonds will be uncovered. This effect is called self-sharpening. But diamonds are difficult to handle because of their thermal sensitivity. Due to their high thermal influence, manufacturing costs, and complicate route of manufacturing (first sintering, then brazing), there is a great need for alternative production methods for such tools. One alternative to produce wear-resistant and self-sharpening coatings are thermal spray processes as examined in this paper. An advantage of thermal spray processes is their smaller thermal influence on the diamond, due to the short dwelling time in the flame. To reduce the thermal influence during spraying, nickel coated diamonds were used in the HVOF-process (high velocity oxygen fuel process). The wear resistance was subsequently investigated by means of a standardized ball-on-disc test. Furthermore, a SEM (scanning electron microscope) was used to gain information about the wear-mechanism and the self-sharpening effect of the coating. © Published under licence by IOP Publishing Ltd.
    view abstractdoi: 10.1088/1757-899X/181/1/012016
  • 2017 • 399 Influence of Si content on mechanical and tribological properties of TiAlSiN PVD coatings at elevated temperatures
    Tillmann, W. and Dildrop, M.
    Surface and Coatings Technology 321 448-454 (2017)
    TiN- and CrN-based binary or ternary coatings have been used for many years in order to extend the service life of machining tools. The increasing demands in the metalworking industry require more efficient coating systems. According to recent studies, silicon offers promising opportunities to positively influence the characteristics of thin titanium or chromium-based coatings. The nanocomposite TiAlSiN presents a high hardness and a fine grain structure. Furthermore, by adding silicon, the oxidation resistance as well as the tribological properties can be increased and improved. In this study, TiAlSiN coatings with different Si contents (0–10.9 at%) were produced by means of magnetron sputtering. In order to test the possibility to sputter pure, low conductive silicon targets, different sputter and bias modes were tested. The ratios of the other coating elements were kept constant while varying the silicon content inside the PVD coatings. Nitrided steel samples (AISI H11) were used as substrate materials. The influence of the Si content on the tribomechanical properties of TiAlSiN were analyzed. The analyses focused on the coatings with a silicon content of 5–10 at% due to the change of the coating morphology within this range. The coating morphology and different chemical compositions of the silicon-doped coatings were investigated by means of scanning electron microscopy and EDX analyses. Phase analyses were conducted and residual stresses were measured by means of X-ray diffraction. The hardness and Young's modulus of the PVD coatings were investigated using nanoindentation. Furthermore, scratch tests were performed in order to characterize the adhesion between the substrate and the coating. Finally, high temperature tribometer tests were executed to determine the wear resistance of the TiAlSiN coatings at room temperature as well as at elevated temperatures (500 °C, 800 °C). © 2017 Elsevier B.V.
    view abstractdoi: 10.1016/j.surfcoat.2017.05.014
  • 2017 • 398 Influence of bias voltage and sputter mode on the coating properties of TiAlSiN: Einfluss der Biasspannung und des Sputtermodus auf die Schichteigenschaften von TiAlSiN
    Tillmann, W. and Dildrop, M.
    Materialwissenschaft und Werkstofftechnik 48 855-861 (2017)
    Silicon offers promising opportunities to improve the characteristics of thin coatings. By adding silicon to TiAlN, the oxidation resistance as well as the tribological properties can be increased and improved. To analyze the influence of the silicon content on the coating properties of TiAlSiN, it is necessary to keep the ratio of the other coating elements constant by using the right target configuration. Within this study, TiAlSiN coatings were deposited on hot work steel AISI H11 by using magnetron sputtering (Cemecon CC800/9 sinox ML). This steel was previously plasma nitrided to increase the hardness and hence the carrying load of the substrate, avoiding shell egg effect during the analysis. Different sputter modes were used to analyze the possibility to produce TiAlSiN by utilizing a pure low conductive silicon target. The bias voltages were systematically varied to see their influence on the structure and chemical compositions of the coating which were investigated by means of scanning electron microscopy and energy dispersive X-ray spectroscopy (EDX). Furthermore, the roughness of the surface of the coatings was measured by an optical three-dimensional surface analyzer. The results of this study serve as a basis for further investigations regarding the variation of the silicon content of TiAlSiN coatings. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/mawe.201600731
  • 2017 • 397 Influence of PVD-duplex-treated, Bionic Surface Structures on the Wetting Behavior for Sheet-Bulk Metal Forming Tools
    Tillmann, W. and Stangier, D. and Lopes Dias, N.F.
    Journal of Bionic Engineering 14 520-531 (2017)
    Bionic surface structures, inspired by the flora, were developed for Sheet-Bulk Metal Forming (SBMF) in order to locally control the friction condition by adjusting the wetting behavior. Five bionic structures were micromilled on ASP®2023 in annealed as well as hardened and tempered conditions. Subsequently, the structured surfaces were plasma-nitrided and coated with a CrAlN thin film. The influence of the treatment method on the structural geometry was investigated with the aid of a scanning electron microscope and 3D-profilometer. The wetting behaviors of water and deep drawing oil (Berufluid ST6007) on bionic surfaces were evaluated using contact angle measurements. The resulting micro-milled structures exhibit an almost identical shape as their bionic models. However, the roughness of the structured surfaces is influenced by the microstructure. The combination of plasma-nitriding and Physical Vapor Deposition (PVD) leads to an increase in roughness. All bionic structures possess higher contact angles than that of the unstructured surfaces when wetted by water. This can be explained by the fact that the structural elevations block the spreading. When the bionic surfaces are wetted by deep drawing oil, the lubricant spreads in the structural cavities, leading to smaller contact angles. Furthermore, the anisotropy of the structure has an influence on the wetting behavior. © 2017 Jilin University
    view abstractdoi: 10.1016/S1672-6529(16)60418-3
  • 2017 • 396 Adhesion properties of a three-layer system based on RF-magnetron sputter deposited calcium-phosphate coating and silver nanoparticles
    Tkachev, M.S. and Melnikov, E.S. and Surmeneva, M.A. and Sharonova, A.A. and Surmenev, R.A. and Korneva, O.S. and Shulepov, I.A. and Loza, K. and Epple, M.
    Proceedings of the 11th International Forum on Strategic Technology, IFOST 2016 88-90 (2017)
    A three-layer system of hydroxyapatite (HA) coating - Ag nanoparticles - HA coating with an overall thickness of 1.2 μm was prepared. The radio-frequency (RF) magnetron sputtering was used to prepare the first layer of hydroxyapatite coating on titanium. Then electrophoretic deposition of silver nanoparticles on the prepared HA layer was done followed by deposition of the second layer of HA by RFmagnetron sputtering. The adhesion strength was investigated by the scratch test method. Scanning electron microscopy and optical microscopy allowed to qualitatively estimate the deformation mechanisms of the biocomposites after the scratch test. © 2016 IEEE.
    view abstractdoi: 10.1109/IFOST.2016.7884197
  • 2017 • 395 Multiphase Characterization of Cu-In-Sn Alloys with 17 at.% Cu and Comparison with Calculated Phase Equilibria
    Tumminello, S. and Del Negro, N. and Carrascal, C. and Fries, S.G. and Alonso, P.R. and Sommadossi, S.
    Journal of Phase Equilibria and Diffusion 38 276-287 (2017)
    Cu-In-Sn alloys are among the suggested materials to replace Pb-Sn alloys traditionally used in joining processes by the electronic industry. Thorough thermodynamic understanding is required for the selection/design of adequate and efficient alloys for specific applications. Understanding the effects that high cost elements such as In have on microstructure and phase stability is imperative for industrial use. In this work ternary alloys were prepared by melting high purity elements (5N) for selected compositions of the 17 at.% Cu isopleth, and cooling down to reproduce process conditions. Chemical composition was determined using scanning electron microscopy equipped with electron probe microanalysis. Measurements of transition temperatures were done by heat-flux differential scanning calorimetry. We present a comprehensive comparison between our experimental results and phase diagram calculations using Liu et al. (J Electron Mater 30:1093, 2001) thermodynamic description based in the CALPHAD method, available in the literature. © 2017 ASM International
    view abstractdoi: 10.1007/s11669-017-0538-7
  • 2017 • 394 The Production of Cu Nanoparticles on Large Area Graphene by Sputtering and in-Flight Sintering
    Ünlü, C.G. and Acet, M. and Tekgül, A. and Farle, M. and Atakan, Ş. and Lindner, J.
    Crystal Research and Technology 52 (2017)
    We have developed a simple method to synthesize Cu nanoparticles on graphene, which is a composite that is currently investigated for use as biosensors. Firstly, large area graphene (2 × 2 cm2) was prepared by chemical vapor deposition on Cu foils and then transferred onto SiO2 substrates by a transfer process. The Cu nanoparticles were collected on graphene/SiO2 by magnetron sputtering. The presence of graphene was verified by optical microscopy and Raman spectroscopy. The structure of graphene decorated with Cu nanoparticles was determined by scanning and transmission electron microscopy. The results show that the Cu nanoparticles acquire a cubic structure on graphene. © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/crat.201700149
  • 2017 • 393 Solid electrolyte interphase (SEI) at TiO2 electrodes in li-ion batteries: Defining apparent and effective SEI based on evidence from X-ay photoemission spectroscopy and scanning electrochemical microscopy
    Ventosa, E. and Madej, E. and Zampardi, G. and Mei, B. and Weide, P. and Antoni, H. and La Mantia, F. and Muhler, M. and Schuhmann, W.
    ACS Applied Materials and Interfaces 9 3123-3130 (2017)
    The high (de)lithiation potential of TiO2 (ca. 1.7 V vs Li/ Li+ in 1 M Li+) decreases the voltage and, thus, the energy density of a corresponding Li-ion battery. On the other hand, it offers several advantages such as the (de)lithiation potential far from lithium deposition or absence of a solid electrolyte interphase (SEI). The latter is currently under controversial debate as several studies reported the presence of a SEI when operating TiO2 electrodes at potentials above 1.0 V vs Li/Li+. We investigate the formation of a SEI at anatase TiO2 electrodes by means of X-ray photoemission spectroscopy (XPS) and scanning electrochemical microscopy (SECM). The investigations were performed in different potential ranges, namely, during storage (without external polarization), between 3.0-2.0 V and 3.0-1.0 V vs Li/Li+, respectively. No SEI is formed when a completely dried and residues-free TiO2 electrode is cycled between 3.0 and 2.0 V vs Li/Li+. A SEI is detected by XPS in the case of samples stored for 6 weeks or cycled between 3.0 and 1.0 V vs Li/Li+. With use of SECM, it is verified that this SEI does not possess the electrically insulating character as expected for a "classic" SEI. Therefore, we propose the term apparent SEI for TiO2 electrodes to differentiate it from the protecting and ef fective SEI formed at graphite electrodes. © 2016 American Chemical Society.
    view abstractdoi: 10.1021/acsami.6b13306
  • 2017 • 392 Partial recrystallization of gum metal to achieve enhanced strength and ductility
    Zhang, J.-L. and Tasan, C.C. and Lai, M.J. and Yan, D. and Raabe, D.
    Acta Materialia 135 400-410 (2017)
    Here we present a microstructure design approach which leads to partial recrystallization and nano-precipitation within the same single-step heat treatment. This produces a dual-constituent microstructure in Ti-Nb based gum metal, which consists of nano-ω-particle-rich ultrafine recrystallized grain chains embedded in ω-lean subgrain-containing recovered zones. This partially recrystallized microstructure exhibits an improved strength-ductility combination that surpasses the inverse strength-ductility relationship exhibited by materials with similar composition. The strengthening effects due to precipitates and grain refinement were studied by nanoindentation. The deformation mechanisms of the partially recrystallized material were investigated by in-situ scanning electron microscope tensile tests, micro-strain mapping and post-mortem microstructure characterization. The improved mechanical properties are attributed to the high yield strength of the recrystallized grains and the sequential activation of dislocation slip and dislocation channeling. © 2017 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2017.06.051
  • 2017 • 391 Nano-sized metal organic framework to improve the structural properties and desalination performance of thin film composite forward osmosis membrane
    Zirehpour, A. and Rahimpour, A. and Ulbricht, M.
    Journal of Membrane Science 531 59-67 (2017)
    In the present study, nano-sized metal-organic framework (MOF) particles consisting of silver (I) and 1,3,5-benzene tricarboxylic acid were synthesized and applied to improve the structural properties as well as desalination performance of thin-film composite (TFC) forward osmosis (FO) membranes. The MOF nanocrystals were incorporated into the polyamide layer of membranes through interfacial polymerization. Characterizations by Field emission scanning electron microscopy and X-ray photoelectron spectroscopy enabled the detection of MOF nanocrystals within the selective layer of the resultant membranes. The MOF incorporation led to changes of the membrane active layer in terms of hydrophilicity and transport properties, without detrimental effects on the layer selectivity. These features enhanced pure water permeability of the membranes to 129%, which was provided through 0.04% MOF loading of the organic phase during interfacial polymerization. As a result, the modified membrane exhibited an enhanced FO seawater desalination performance in comparison with the control membrane. The performance stability of TFC membrane was also improved by presence of MOF in active layer (as seen by a water flux decline of about 7% for modified membrane against about 18% for unmodified membrane when tested with real seawater). This study demonstrates the potential of MOF particles to enhance desalination performance of TFC membranes in FO systems. © 2017 Elsevier B.V.
    view abstractdoi: 10.1016/j.memsci.2017.02.049
  • 2017 • 390 Peculiarities in thermal evolution of precipitated amorphous calcium phosphates with an initial Ca/P ratio of 1:1
    Zyman, Z. and Epple, M. and Goncharenko, A. and Rokhmistrov, D. and Prymak, O. and Loza, K.
    Journal of Materials Science: Materials in Medicine 28 (2017)
    Thermal evolution of amorphous calcium phosphate (ACP) powder from a fast nitrate synthesis with a Ca/P ratio of 1:1 were studied in the range of 20–980 °C. The powder consisted of amorphous dicalcium phosphate anhydrate (CaHPO4) after heating to 200 °C. CaHPO4 gradually condensed to amorphous calcium pyrophosphate Ca2P2O7 (CPP) between 200 to 620 °C. Amorphous CPP crystallized at 620–740 °C to a metastable polymorph α′-CPP of the high-temperature phase α-CPP and β-CPP. The α′-CPP/ β-CPP phase ratio reached a maximum at 800 °C (60 wt% α′-CPP/40 wt% β-CPP), and α′-CPP gradually transformed to β-CPP at a higher temperature. Some β-TCP occurred at 900 °C, so that a three-phasic mixture was obtained in the powder heated to 980 °C. The occurrence of metastable α′-CPP is attributed to Ostwald’s step rule, and a mechanism for β-TCP formation is proposed. The advantages of prospective biomaterials from these powders are discussed. © 2017, Springer Science+Business Media New York.
    view abstractdoi: 10.1007/s10856-016-5820-4
  • 2016 • 389 Reverse water-gas shift reaction at the Cu/ZnO interface: Influence of the Cu/Zn ratio on structure-activity correlations
    Álvarez Galván, C. and Schumann, J. and Behrens, M. and Fierro, J.L.G. and Schlögl, R. and Frei, E.
    Applied Catalysis B: Environmental 195 104-111 (2016)
    The physicochemical properties of hydroxycarbonate-based precipitates [zincian malachite (ZM) and aurichalcite precursors], calcined CuO/ZnO precatalysts and finally reduced Cu/ZnO catalysts, with several Cu-Zn ratios, have been investigated by different characterization techniques. Results from isothermal physisorption of N2 (BET), X-ray Diffraction (XRD), Temperature Programmed Reduction (TPR), N2O Reactive Frontal Chromatography (N2O-RFC), X-ray Photoelectron Spectroscopy (XPS) and Scanning Electron Microscopy (SEM) have been correlated with the catalytic activity for the reverse water-gas shift (rWGS) reaction in order to provide insight into the controversial nature of active species in carbon dioxide activation, respectively the role of Cu and ZnO. Average crystalline domain size of CuO and ZnO show a relationship with the amount of each phase in the calcined sample. This is in agreement with the TPR profiles, which indicate a better dispersion of Cu for the ZnO-rich samples and a shift for the first reduction step to higher temperatures (Tonset for CuII to CuI). XPS measurements point out the surface enrichment of ZnO is less pronounced with higher ZnO/(ZnO + Cu) ratios. Activity results show that catalysts derived from high surface area ex-aurichalcite (Zn content, 50-70% atom) catalysts are more active in rWGS with lower apparent activation energies than ex-ZM catalysts (Zn content, 15-30% atom) with comparable apparent Cu surface area/N2O capacity. Thus, the CO formation rate as function of the apparent Cu surface area indicates that the reaction rate is not dependent on the exposed apparent Cu surface, but from an adjusted interface composition predetermined by the precursor structure and its thermal post-treatment. © 2016 Elsevier B.V.
    view abstractdoi: 10.1016/j.apcatb.2016.05.007
  • 2016 • 388 Nanoscale electron transport at the surface of a topological insulator
    Bauer, S. and Bobisch, C.A.
    Nature Communications 7 (2016)
    The use of three-dimensional topological insulators for disruptive technologies critically depends on the dissipationless transport of electrons at the surface, because of the suppression of backscattering at defects. However, in real devices, defects are unavoidable and scattering at angles other than 180° is allowed for such materials. Until now, this has been studied indirectly by bulk measurements and by the analysis of the local density of states in close vicinity to defect sites. Here, we directly measure the nanoscale voltage drop caused by the scattering at step edges, which occurs if a lateral current flows along a three-dimensional topological insulator. The experiments were performed using scanning tunnelling potentiometry for thin Bi2Se3 films. So far, the observed voltage drops are small because of large contributions of the bulk to the electronic transport. However, for the use of ideal topological insulating thin films in devices, these contributions would play a significant role.
    view abstractdoi: 10.1038/ncomms11381
  • 2016 • 387 Fast spin information transfer between distant quantum dots using individual electrons
    Bertrand, B. and Hermelin, S. and Takada, S. and Yamamoto, M. and Tarucha, S. and Ludwig, Ar. and Wieck, A.D. and Bäuerle, C. and Meunier, T.
    Nature Nanotechnology 11 672-676 (2016)
    Transporting ensembles of electrons over long distances without losing their spin polarization is an important benchmark for spintronic devices. It usually requires injecting and probing spin-polarized electrons in conduction channels using ferromagnetic contacts or optical excitation. In parallel with this development, important efforts have been dedicated to achieving control of nanocircuits at the single-electron level. The detection and coherent manipulation of the spin of a single electron trapped in a quantum dot are now well established. Combined with the recently demonstrated control of the displacement of individual electrons between two distant quantum dots, these achievements allow the possibility of realizing spintronic protocols at the single-electron level. Here, we demonstrate that spin information carried by one or two electrons can be transferred between two quantum dots separated by a distance of 4μm with a classical fidelity of 65%. We show that at present it is limited by spin flips occurring during the transfer procedure before and after electron displacement. Being able to encode and control information in the spin degree of freedom of a single electron while it is being transferred over distances of a few micrometres on nanosecond timescales will pave the way towards ' quantum spintronics' devices, which could be used to implement large-scale spin-based quantum information processing. © 2016 Macmillan Publishers Limited, All rights reserved.
    view abstractdoi: 10.1038/nnano.2016.82
  • 2016 • 386 Modelling of electron beam induced nanowire attraction
    Bitzer, L.A. and Speich, C. and Schäfer, D. and Erni, D. and Prost, W. and Tegude, F.J. and Benson, N. and Schmechel, R.
    Journal of Applied Physics 119 (2016)
    Scanning electron microscope (SEM) induced nanowire (NW) attraction or bundling is a well known effect, which is mainly ascribed to structural or material dependent properties. However, there have also been recent reports of electron beam induced nanowire bending by SEM imaging, which is not fully explained by the current models, especially when considering the electro-dynamic interaction between NWs. In this article, we contribute to the understanding of this phenomenon, by introducing an electro-dynamic model based on capacitor and Lorentz force interaction, where the active NW bending is stimulated by an electromagnetic force between individual wires. The model includes geometrical, electrical, and mechanical NW parameters, as well as the influence of the electron beam source parameters and is validated using in-situ observations of electron beam induced GaAs nanowire (NW) bending by SEM imaging. © 2016 AIP Publishing LLC.
    view abstractdoi: 10.1063/1.4945674
  • 2016 • 385 Characterisation of bifunctional electrocatalysts for oxygen reduction and evolution by means of SECM
    Chen, X. and Botz, A.J.R. and Masa, J. and Schuhmann, W.
    Journal of Solid State Electrochemistry 20 1019-1027 (2016)
    Electrocatalysts that can reversibly reduce oxygen and oxidise water are of prime importance for the advancement of new emerging electrochemical energy storage and conversion systems. We present in this work the application of scanning electrochemical microscopy (SECM) for characterisation of bifunctional catalysts. By using model bifunctional catalysts based on oxides of cobalt (CoxOy) and nickel (NixOy) embedded in nitrogen-doped carbon (NC), we specifically show the unique ability of using SECM to determine a range of the important electrocatalytic parameters including the selectivity of the oxygen reduction reaction (ORR), the initial mechanistic steps during the oxygen evolution reaction (OER), and the onset potential for both ORR and OER in a single experiment. We were able to observe directly that prior to oxygen evolution, local depletion of oxygen occurs at the SECM tip during redox transition accompanying most likely metal oxyhydroxide formation thus enabling direct in situ observation of the initial mechanistic steps of the OER. © 2015, Springer-Verlag Berlin Heidelberg.
    view abstractdoi: 10.1007/s10008-015-3028-z
  • 2016 • 384 Pseudomorphic Transformation of Organometal Halide Perovskite Using the Gaseous Hydrogen Halide Reaction
    Chen, K. and Deng, X. and Goddard, R. and Tüysüz, H.
    Chemistry of Materials 28 5530-5537 (2016)
    Halide exchange is a facile method of adjusting the band gap and optimizing the performance of organometal halide perovskite. During the halide exchange processes, preserving the crystallinity and morphology of highly crystalline materials will be desirable for preparing novel materials with outstanding performance. In this study, the gasous hydrogen halides were used as reactants for halide exchange processes. The mutual conversions among three halides for condense films were realized. Moreover, perovskite inverse opals and perovskite single crystals were also adopted as substrates to illustrate the morphology preservation and crystallinity preservation, respectively. Powder X-ray diffraction and UV-vis diffuse reflectance spectra demonstrated the segregation when smaller ions were substituted by larger ions. Scanning electron microscopy displayed the direct evidence for morphology preservation during the transformation. For the first time, single crystal X-ray diffraction confirmed the single-crystal-to-single-crystal transformation from bromide to chloride analogy, which demonstrated that the presented method can preserve the crystalline framework of large-sized perovskite during the halide exchange. © 2016 American Chemical Society.
    view abstractdoi: 10.1021/acs.chemmater.6b02233
  • 2016 • 383 Probing the Dynamic Structure and Chemical State of Au Nanocatalysts during the Electrochemical Oxidation of 2-Propanol
    Choi, Y. and Sinev, I. and Mistry, H. and Zegkinoglou, I. and Roldan Cuenya, B.
    ACS Catalysis 6 3396-3403 (2016)
    A size-dependent trend was observed for the electrochemical total oxidation of 2-propanol to CO2 over Au nanoparticles (NPs), with increasing activity (increased current density and lower overpotential) for decreasing NP size. Furthermore, an enhanced stability against poisoning by the unreacted acetone intermediate was also obtained for NPs smaller than ∼2 nm. Operando X-ray absorption fine structure (XAFS) measurements provided insight into the dynamic evolution of the NP structure and chemical state under reaction conditions, shedding light on the nature of the most catalytically active species and catalyst deactivation phenomena via chemically driven sintering. © 2016 American Chemical Society.
    view abstractdoi: 10.1021/acscatal.6b00057
  • 2016 • 382 Nanoelectrodes: Applications in electrocatalysis, single-cell analysis and high-resolution electrochemical imaging
    Clausmeyer, J. and Schuhmann, W.
    TrAC - Trends in Analytical Chemistry 79 46-59 (2016)
    High sensitivity and high spatial resolution in localized electrochemical measurements are the key advantages of electroanalysis using nanometer-sized electrodes. Due to recent progress in nanoelectrode fabrication and electrochemical instrument development, nanoelectrochemical methods are becoming more widespread. We summarize different protocols for the fabrication of needle-type nanoelectrodes and discuss their properties with regard to various applications. We discuss the limits of conventional theory to describe electrochemistry at the nanoscale and point out technical aspects for characterization and handling of nanometric electrodes. Different applications are highlighted: i) Nanoelectrodes are powerful tools for non-ensemble studies of electrocatalysis at single nanoparticles at high mass transport rates. ii) Electrochemical nanosensors are employed for highly localized non-invasive analysis of single living cells and intracellular detection of neurotransmitters and metabolites. iii) Used in scanning electrochemical probe techniques, nanoprobes afford topographical and truly chemical imaging of samples with high spatial resolution. © 2016 Published by Elsevier B.V.
    view abstractdoi: 10.1016/j.trac.2016.01.018
  • 2016 • 381 Dual-Templated Cobalt Oxide for Photochemical Water Oxidation
    Deng, X. and Bongard, H.-J. and Chan, C.K. and Tüysüz, H.
    ChemSusChem 9 409-415 (2016)
    Mesoporous Co3O4 was prepared using a dual templating approach whereby mesopores inside SiO2 nanospheres, as well as the void spaces between the nanospheres, were used as templates. The effect of calcination temperature on the crystallinity, morphology, and textural parameters of the Co3O4 replica was investigated. The catalytic activity of Co3O4 for photochemical water oxidation in a [Ru(bpy)3]2+[S2O8]2- system was evaluated. The Co3O4 replica calcined at the lowest temperature (150°C) exhibited the best performance as a result of the unique nanostructure and high surface area arising from the dual templating. The performance of Co3O4 with highest surface area was further examined in electrochemical water oxidation. Superior activity over high temperature counterpart and decent stability was observed. Furthermore, CoO with identical morphology was prepared from Co3O4 using an ethanol reduction method and a higher turnover-frequency number for photochemical water oxidation was obtained. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/cssc.201500872
  • 2016 • 380 Are Mo2BC nanocrystalline coatings damage resistant? Insights from comparative tension experiments
    Djaziri, S. and Gleich, S. and Bolvardi, H. and Kirchlechner, C. and Hans, M. and Scheu, C. and Schneider, J.M. and Dehm, G.
    Surface and Coatings Technology 289 213-218 (2016)
    Mo2BC nanocrystalline coatings were deposited on Cu substrates to compare their mechanical performance with bench-mark TiAlN, and pure Mo, Al and Al2O3 reference coatings. The Mo2BC coatings were characterized by X-ray diffraction and transmission electron microscopy to analyze the microstructure. In order to study the damage behavior, the coatings were subjected to uniaxial tensile loading and the crack spacing with increasing strain was monitored using optical and scanning electron microscopy. Based on crack density measurements, the Mo2BC coatings were found to be significantly less prone to cracking than the bench-mark TiAlN coatings. The higher resistance to cracking arises from the electronic structure of the Mo2BC nanolaminates, which imparts moderate ductility to the deformation behavior. © 2016 Elsevier B.V.
    view abstractdoi: 10.1016/j.surfcoat.2016.02.010
  • 2016 • 379 Direct metal deposition of refractory high entropy alloy MoNbTaW
    Dobbelstein, H. and Thiele, M. and Gurevich, E.L. and George, E.P. and Ostendorf, A.
    Physics Procedia 83 624-633 (2016)
    Alloying of refractory high entropy alloys (HEAs) such as MoNbTaW is usually done by vacuum arc melting (VAM) or powder metallurgy (PM) due to the high melting points of the elements. Machining to produce the final shape of parts is often needed after the PM process. Casting processes, which are often used for aerospace components (turbine blades, vanes), are not possible. Direct metal deposition (DMD) is an additive manufacturing technique used for the refurbishment of superalloy components, but generating these components from the bottom up is also of current research interest. MoNbTaW possesses high yield strength at high temperatures and could be an alternative to state-of-the-art materials. In this study, DMD of an equimolar mixture of elemental powders was performed with a pulsed Nd:YAG laser. Single wall structures were built, deposition strategies developed and the microstructure of MoNbTaW was analyzed by back scattered electrons (BSE) and energy dispersive X-ray (EDX) spectroscopy in a scanning electron microscope. DMD enables the generation of composition gradients by using dynamic powder mixing instead of pre-alloyed powders. However, the simultaneous handling of several elemental or pre-alloyed powders brings new challenges to the deposition process. The influence of thermal properties, melting point and vapor pressure on the deposition process and chemical composition will be discussed. © 2016 The Authors.
    view abstractdoi: 10.1016/j.phpro.2016.08.065
  • 2016 • 378 Synthesis and structure of strontium ferrite nanowires and nanotubes of high aspect ratio
    Ebrahimi, F. and Ashrafizadeh, F. and Bakhshi, S.R. and Farle, M.
    Journal of Sol-Gel Science and Technology 77 708-717 (2016)
    Abstract: Strontium hexaferrite nanowires and nanotubes were synthesized in porous anodic aluminum oxide templates. Different solution-based synthesis techniques (spin coating, vacuum suction, and dip coating) were investigated. Strontium ferrite nanopowders were also synthesized by a similar sol–gel process. The morphology, structure, and composition of the embedded hexaferrite nanostructures were examined by field emission scanning electron microscope, X-ray diffraction, and transmission electron microscopy. Strontium ferrite wires with Fe/Sr ratios from 10 to 12 under different annealing temperatures of 500–700 °C were studied. The results showed that dip coating could produce fine and uniform strontium ferrite nanowires. The ratio of Fe/Sr of 11 and a calcination temperature of 650 °C were found to be optimum conditions. The produced material may be of importance for novel microwave-frequency nanoscale devices. Graphical Abstract: [Figure not available: see fulltext.] © 2015, Springer Science+Business Media New York.
    view abstractdoi: 10.1007/s10971-015-3902-2
  • 2016 • 377 Fatigue life assessment of friction spot welded 7050-T76 aluminium alloy using Weibull distribution
    Effertz, P.S. and Infante, V. and Quintino, L. and Suhuddin, U. and Hanke, S. and Dos Santos, J.F.
    International Journal of Fatigue 87 381-390 (2016)
    Friction spot welding is a solid state welding process suitable to obtain spot like-joints in overlap configuration. The process is particularly useful to weld lightweight materials in similar and dissimilar combinations, and therefore an interesting alternative to other joining techniques (rivets, resistance welding, etc.). Optimum process parameters have been defined using the Taguchi method by maximizing the response variable (the lap shear strength). A study of the fatigue life was carried out on specimens welded with the above mentioned optimized process parameters. Fatigue tests were performed using a stress ratio of R = 0.10. Two-parameter Weibull distribution was used to analyze statistically the fatigue life for the joined overlapped sheets. Subsequently, the Weibull plots were drawn, as well as S-N curves considering different reliability levels. The results show that for a relatively low load, corresponding to 10% of the maximum supported by the joint, the number of cycles surpasses 1 × 106, hence infinite life of the service component can be attributed. Fatigue fracture surfaces were investigated for the highest and lowest loads tested using scanning electron microscope (SEM). © 2016 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.ijfatigue.2016.02.030
  • 2016 • 376 Differentiation between Single- and Double-Stranded DNA through Local Capacitance Measurements
    Estrada-Vargas, A. and Jambrec, D. and Kayran, Y.U. and Kuznetsov, V. and Schuhmann, W.
    ChemElectroChem 3 855-857 (2016)
    Local differentiation of single-stranded (ss) and double-stranded (ds) DNA in microarrays by using electrochemical techniques has gained increasing interest. We propose a method for distinguishing areas on gold electrodes modified with ssDNA and dsDNA based on the difference in their local capacitance. The local capacitance is visualized by means of scanning electrochemical impedance microscopy and alternating-current scanning electrochemical microscopy. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/celc.201600075
  • 2016 • 375 In Situ Characterization of Ultrathin Films by Scanning Electrochemical Impedance Microscopy
    Estrada-Vargas, A. and Bandarenka, A. and Kuznetsov, V. and Schuhmann, W.
    Analytical Chemistry 88 3354-3362 (2016)
    Control over the properties of ultrathin films plays a crucial role in many fields of science and technology. Although nondestructive optical and electrical methods have multiple advantages for local surface characterization, their applicability is very limited if the surface is in contact with an electrolyte solution. Local electrochemical methods, e.g., scanning electrochemical microscopy (SECM), cannot be used as a robust alternative yet because their methodological aspects are not sufficiently developed with respect to these systems. The recently proposed scanning electrochemical impedance microscopy (SEIM) can efficiently elucidate many key properties of the solid/liquid interface such as charge transfer resistance or interfacial capacitance. However, many fundamental aspects related to SEIM application still remain unclear. In this work, a methodology for the interpretation of SEIM data of "charge blocking systems" has been elaborated with the help of finite element simulations in combination with experimental results. As a proof of concept, the local film thickness has been visualized using model systems at various tip-to-sample separations. Namely, anodized aluminum oxide (Al2O3, 2-20 nm) and self-assembled monolayers based on 11-mercapto-1-undecanol and 16-mercapto-1-hexadecanethiol (2.1 and 2.9 nm, respectively) were used as model systems. (Figure Presented). © 2016 American Chemical Society.
    view abstractdoi: 10.1021/acs.analchem.6b00011
  • 2016 • 374 Exceptional damage-tolerance of a medium-entropy alloy CrCoNi at cryogenic temperatures
    Gludovatz, B. and Hohenwarter, A. and Thurston, K.V.S. and Bei, H. and Wu, Z. and George, E.P. and Ritchie, R.O.
    Nature Communications 7 (2016)
    High-entropy alloys are an intriguing new class of metallic materials that derive their properties from being multi-element systems that can crystallize as a single phase, despite containing high concentrations of five or more elements with different crystal structures. Here we examine an equiatomic medium-entropy alloy containing only three elements, CrCoNi, as a single-phase face-centred cubic solid solution, which displays strength-toughness properties that exceed those of all high-entropy alloys and most multi-phase alloys. At room temperature, the alloy shows tensile strengths of almost 1 GPa, failure strains of ∼70% and KJIc fracture-toughness values above 200 MPa m1/2; at cryogenic temperatures strength, ductility and toughness of the CrCoNi alloy improve to strength levels above 1.3 GPa, failure strains up to 90% and KJIc values of 275 MPa m1/2. Such properties appear to result from continuous steady strain hardening, which acts to suppress plastic instability, resulting from pronounced dislocation activity and deformation-induced nano-twinning. © 2016, Nature Publishing Group. All rights reserved.
    view abstractdoi: 10.1038/ncomms10602
  • 2016 • 373 Osteogenic differentiation and proliferation of bone marrow-derived mesenchymal stromal cells on PDLLA + BMP-2-coated titanium alloy surfaces
    Haversath, M. and Hülsen, T. and Böge, C. and Tassemeier, T. and Landgraeber, S. and Herten, M. and Warwas, S. and Krauspe, R. and Jäger, M.
    Journal of Biomedical Materials Research - Part A 104 145-154 (2016)
    RhBMP-2 is clinically applied to enhance bone healing and used in combination with titanium fixation implants. The purpose of this in vitro study was to compare the osteogenic differentiation and proliferation of hMSC on native polished versus sandblasted titanium surfaces (TS) and to test their behavior on pure poly-D,L-lactide (PDLLA) coated as well as PDLLA + rhBMP-2 coated TS. Furthermore, the release kinetics of PDLLA + rhBMP-2-coated TS was investigated. Human bone marrow cells were obtained from three different donors (A: male, 16 yrs; B: male, 27 yrs, C: male, 49 yrs) followed by density gradient centrifugation and flow cytometry with defined antigens. The cells were seeded on native polished and sandblasted TS, PDLLA-coated TS and PDLLA + rhBMP-2-coated TS. Osteogenic differentiation (ALP specific activity via ALP and BCA assay) and proliferation (LDH cytotoxicity assay) was examined on day 7 and 14 and release kinetics of rhBMP-2 was investigated on day 3, 7, 10, and 14. We found significant higher ALP specific activity and LDH activity on native polished compared to native sandblasted surfaces. PDLLA led to decreased ALP specific and LDH activity on both surface finishes. Additional rhBMP-2 slightly diminished this effect. RhBMP-2-release from coated TS decreased nearly exponentially with highest concentrations at the beginning of the cultivation period. The results of this in vitro study suggest that native TS stimulate hMSC significantly stronger toward osteogenic differentiation and proliferation than rhBMP-2 + PDLLA-layered TS in the first 14 days of cultivation. The PDLLA-layer seems to inhibit local hMSC differentiation and proliferation. © 2015 Wiley Periodicals, Inc.
    view abstractdoi: 10.1002/jbm.a.35550
  • 2016 • 372 On the Crystallography of Silver Nanoparticles with Different Shapes
    Helmlinger, J. and Prymak, O. and Loza, K. and Gocyla, M. and Heggen, M. and Epple, M.
    Crystal Growth and Design 16 3677-3687 (2016)
    The crystallographic properties of silver nanoparticles with different morphologies (two different kinds of spheres, cubes, platelets, and rods) were derived from X-ray powder diffraction and electron microscopy. The size of the metallic particle core was determined by scanning electron microscopy, and the colloidal stability and the hydrodynamic particle diameter were analyzed by dynamic light scattering. The preferred crystallographic orientation (texture) as obtained by X-ray powder diffraction, including pole figure analysis, and high resolution transmission electron microscopy showed the crystallographic nature of the spheres (almost no texture), the cubes (terminated by {100} faces), the platelets (terminated by {111} faces), and the rods (grown from pentagonal twins along [110] and terminated by {100} faces). The crystallite size was determined by Rietveld refinement of X-ray powder diffraction data and agreed well with the transmission electron microscopic data. © 2016 American Chemical Society.
    view abstractdoi: 10.1021/acs.cgd.6b00178
  • 2016 • 371 Silver nanoparticles with different size and shape: Equal cytotoxicity, but different antibacterial effects
    Helmlinger, J. and Sengstock, C. and Groß-Heitfeld, C. and Mayer, C. and Schildhauer, T.A. and Köller, M. and Epple, M.
    RSC Advances 6 18490-18501 (2016)
    The influence of silver nanoparticle morphology on the dissolution kinetics in ultrapure water as well as the biological effect on eukaryotic and prokaryotic cells was examined. Silver nanoparticles with different shapes but comparable size and identical surface functionalisation were prepared, i.e. spheres (diameter 40-80 and 120-180 nm; two different samples), platelets (20-60 nm), cubes (140-180 nm), and rods (diameter 80-120 nm, length &gt; 1000 nm). All particles were purified by ultracentrifugation and colloidally stabilized with poly(N-vinyl pyrrolidone) (PVP). Their colloidal dispersion in ultrapure water and cell culture medium was demonstrated by dynamic light scattering. Size, shape, and colloidal stability were analysed by scanning electron microscopy, atomic force microscopy, dynamic light scattering, and differential centrifugal sedimentation. The dissolution in ultrapure water was proportional to the specific surface area of the silver nanoparticles. The averaged release rate for all particle morphologies was 30 ± 13 ng s-1 m-2 in ultrapure water (T = 25 ± 1°C; pH 4.8; oxygen saturation 93%), i.e. about 10-20 times larger than the release of silver from a macroscopic silver bar (1 oz), possibly due to the presence of surface defects in the nanoparticulate state. All particles were taken up by human mesenchymal stem cells and were cytotoxic in concentrations of &gt;12.5 μg mL-1, but there was no significant influence of the particle shape on the cytotoxicity towards the cells. Contrary to that, the toxicity towards bacteria increased with a higher dissolution rate, suggesting that the toxic species against bacteria are dissolved silver ions. © The Royal Society of Chemistry 2016.
    view abstractdoi: 10.1039/c5ra27836h
  • 2016 • 370 Comparison of fatigue life assessment by analytical, experimental and damage accumulation modelling approach for steel SAE 1045
    Imran, M. and Siddique, S. and Guchinsky, R. and Petinov, S. and Walther, F.
    Fatigue and Fracture of Engineering Materials and Structures 39 1138-1149 (2016)
    Fatigue life assessment for two-phase steel SAE 1045 has been carried out by experimental and simulation techniques. Analytical approach, termed as fatigue lifetime calculation, was employed making use of a load increase testing procedure and constant amplitude tests equipped with measurement techniques – plastic strain amplitude, change in temperature and change in electrical potential difference. The predicted fatigue life has been validated by constant amplitude tests and compared with fatigue life estimation by microstructure-based simulation. Simulation has been carried out over the complete cross section of the specimen. The simulation uses damage accumulation in the gage section of the specimen culminating in the macro-crack propagation, taking into account the inhomogeneous fatigue resistance of the material element. The results show that at the initial intervals of high cycle fatigue range at relatively higher stress amplitudes, the experimental and simulation results are in agreement; whereas in the (high cycle fatigue) region at relatively low stress amplitudes, the simulation results were found more optimistic and the corresponding fatigue scatter is also increased. Each scatter is attributed to the relatively small number of analysed models of the material structure. Scanning electron microscope was used to determine volume fraction of the microstructure for simulation. Fatigue fracture surface analysis shows that crack initiated from internal defect of material and crack propagation is driven by silicon oxide inclusion. © 2016 Wiley Publishing Ltd.
    view abstractdoi: 10.1111/ffe.12426
  • 2016 • 369 Evaluation of Void Nucleation and Development during Plastic Deformation of Dual-Phase Steel DP600
    Isik, K. and Gerstein, G. and Clausmeyer, T. and Nürnberger, F. and Tekkaya, A.E. and Maier, H.J.
    Steel Research International 87 1583-1591 (2016)
    This paper presents investigations on the characterization of ductile damage and identification of the porosity-related material model parameters in a dual-phase steel DP600. As a modeling reference for the damage evolution, a variant from the Gurson model family is taken. The micromechanical investigations related to nucleation and growth of voids have been carried out. In order to show the void-volume evolution during the deformation, post-mortem scanning electron microscope (SEM) analysis of a notched tensile test is used. Using the ion beam slope cutting methodology to prepare the specimens for SEM analysis, the microstructure can be observed in 2D including the voids. In this way, for the dual-phase steel, characteristic damage behavior upon deformation due to interaction of martensite and ferrite can be investigated. The minimum void size (areal) that can be measured is 0.05 µm2. This resolution of the measurements provides the detection of the newly nucleated voids. For the related material parameters, void-size relevant criterion is applied to determine the newly nucleated voids at a certain plastic strain. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/srin.201500483
  • 2016 • 368 Investigations of ductile damage during the process chains of toothed functional components manufactured by sheet-bulk metal forming
    Isik, K. and Gerstein, G. and Schneider, T. and Schulte, R. and Rosenbusch, D. and Clausmeyer, T. and Nürnberger, F. and Vucetic, M. and Koch, S. and Hübner, S. and Behrens, B.-A. and Tekkaya, A.E. and Merklein, M.
    Production Engineering 10 5-15 (2016)
    Sheet-bulk metal forming processes combine conventional sheet forming processes with bulk forming of sheet semi-finished parts. In these processes the sheets undergo complex forming histories. Due to in- and out-of-plane material flow and large accumulated plastic strains, the conventional failure prediction methods for sheet metal forming such as forming limit curve fall short. As a remedy, damage models can be applied to model damage evolution during those processes. In this study, damage evolution during the production of two different toothed components from DC04 steel is investigated. In both setups, a deep drawn cup is upset to form a circumferential gearing. However, the two final products have different dimensions and forming histories. Due to combined deep drawing and upsetting processes, the material flow on the cup walls is three-dimensional and non-proportional. In this study, the numerical and experimental investigations for those parts are presented and compared. Damage evolution in the process chains is simulated with a Lemaitre damage criterion. Microstructural analysis by scanning electron microscopy is performed in the regions with high mechanical loading. It is observed that the evolution of voids in terms of void volume fraction is strongly dependent on the deformation path. The comparison of simulation results with microstructural data shows that the void volume fraction decreases in the upsetting stage after an initial increase in the drawing stage. Moreover, the concurrent numerical and microstructural analysis provides evidence that the void volume fraction decreases during compression in sheet-bulk metal forming. © 2016, German Academic Society for Production Engineering (WGP).
    view abstractdoi: 10.1007/s11740-016-0656-9
  • 2016 • 367 Precipitation Reactions in Age-Hardenable Alloys During Laser Additive Manufacturing
    Jägle, E.A. and Sheng, Z. and Wu, L. and Lu, L. and Risse, J. and Weisheit, A. and Raabe, D.
    JOM 68 943-949 (2016)
    We describe and study the thermal profiles experienced by various age-hardenable alloys during laser additive manufacturing (LAM), employing two different manufacturing techniques: selective laser melting and laser metal deposition. Using scanning electron microscopy and atom probe tomography, we reveal at which stages during the manufacturing process desired and undesired precipitation reactions can occur in age-hardenable alloys. Using examples from a maraging steel, a nickel-base superalloy and a scandium-containing aluminium alloy, we demonstrate that precipitation can already occur during the production of the powders used as starting material, during the deposition of material (i.e. during solidification and subsequent cooling), during the intrinsic heat treatment effected by LAM (i.e. in the heat affected zones) and, naturally, during an ageing post-heat treatment. These examples demonstrate the importance of understanding and controlling the thermal profile during the entire additive manufacturing cycle of age-hardenable materials including powder synthesis. © 2016, The Author(s).
    view abstractdoi: 10.1007/s11837-015-1764-2
  • 2016 • 366 Microscale Fracture Behavior of Single Crystal Silicon Beams at Elevated Temperatures
    Jaya, B.N. and Wheeler, J.M. and Wehrs, J. and Best, J.P. and Soler, R. and Michler, J. and Kirchlechner, C. and Dehm, G.
    Nano Letters 16 7597-7603 (2016)
    The micromechanical fracture behavior of Si [100] was investigated as a function of temperature in the scanning electron microscope with a nanoindenter. A gradual increase in KC was observed with temperature, in contrast to sharp transitions reported earlier for macro-Si. A transition in cracking mechanism via crack branching occurs at ∼300 °C accompanied by multiple load drops. This reveals that onset of small-scale plasticity plays an important role in the brittle-to-ductile transition of miniaturized Si. © 2016 American Chemical Society.
    view abstractdoi: 10.1021/acs.nanolett.6b03461
  • 2016 • 365 Coccospheres confer mechanical protection: New evidence for an old hypothesis
    Jaya, B.N. and Hoffmann, R. and Kirchlechner, C. and Dehm, G. and Scheu, C. and Langer, G.
    Acta Biomaterialia 42 258-264 (2016)
    Emiliania huxleyi has evolved an extremely intricate coccosphere architecture. The coccosphere is comprised of interlocking coccoliths embedded in a polysaccharide matrix. In this work, we performed in-situ scanning electron microscopy based compression tests and conclude that coccospheres have a mechanical protection function. The coccosphere exhibits exceptional damage tolerance in terms of inelastic deformation, recovery and stable crack growth before catastrophic fracture, a feature, which is not found in monolithic ceramic structures. Some of the mechanical features of the coccospheres are due to their architecture, especially polysaccharide matrix that acts as a kind of bio-adhesive. Our data provide strong evidence for the mechanical protection-hypothesis of coccolithophore calcification, without excluding other functions of calcification such as various biochemical roles discussed in the literature. Statement of Significance Although bio-mechanics of shell structures like nacre have been studied over the past decade, coccospheres present an architecture that is quite distinct and complex. It is a porous cell structure evolved to protect the living algae cell inside it in the oceans, subjected to significant hydrostatic pressure. Despite being made of extremely brittle constituents like calcium carbonate, our study finds that coccospheres possess significant damage tolerance especially due to their interlocking coccolith architecture. This will have consequences in bio-mimetic design, especially relating to high pressure applications. © 2016 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actbio.2016.07.036
  • 2016 • 364 Mesoporous nitrogen containing carbon materials for the simultaneous detection of ascorbic acid, dopamine and uric acid
    Joshi, A. and Schuhmann, W. and Nagaiah, T.C.
    Sensors and Actuators, B: Chemical 230 544-555 (2016)
    Mesoporous nitrogen rich carbonaceous (MNC) materials have been synthesized by pyrolyzing the polymerized ethylenediamine nanocasted into a SBA-15 hard template at 600 and 800 °C and explored for simultaneous determination of ascorbic acid (AA), dopamine (DA) and uric acid (UA). The electrocatalytic activity of these materials for the oxidation of analyte molecules was examined by means of redox-competition mode of scanning electrochemical microscopy (SECM), voltammetric, chronoamperometric and rotating disc electrode (RDE) measurements. MNC material exhibits a superior sensitivity towards the oxidation of AA, DA, and UA with a lowest detection limit of 0.01 μM, 0.001 μM and 0.01 μM respectively without any substantial interferences including glucose at physiologically relevant concentrations. © 2016 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.snb.2016.02.050
  • 2016 • 363 Flame retardants based on amino silanes and phenylphosphonic acid
    Kappes, R.S. and Urbainczyk, T. and Artz, U. and Textor, T. and Gutmann, J.S.
    Polymer Degradation and Stability 129 168-179 (2016)
    The sol-gel approach offers a new class of flame retardants with a high potential for textile applications. Pure inorganic sol-gel systems do, however, typically not provide an effect sufficient for a self-extinguishing behavior on its own. We therefore employed compounds with nitrogen and phosphorous containing groups. Especially the combination of compounds with both elements, using the synergism, is promising for the aim to find well-applicable, environmental friendly, halogen-free flame retardants. In our approach, the sol-gel network ensured on the one hand the link to the textile as non-flammable binder. On the other hand, the sol-gel-based networks modified with functional groups containing nitrogen groups provided flame retardancy. In this way, a flame retardant finishing for textiles could be obtained by simple finishing techniques as, e.g., padding. Besides a characterization with various flame tests (e.g., according to EN ISO 15025 - protective clothing), we used a combination of cone calorimetry, thermogravimetry coupled with infrared spectroscopy analysis and scanning electron microscopy to analyze the mechanism of flame retardancy. Thus, we could show that the main mechanism is based on the formation of a protection layer. This work provides a model system for sol-gel-based flame retardants and has the potential to show the principle feasibility of the sol-gel approach in flame retardancy of textiles. It therefore lays the groundwork for tailoring sol-gel layers from newly synthesized sol-gel precursors containing nitrogen and phosphorous groups. © 2016 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.polymdegradstab.2016.04.012
  • 2016 • 362 Preparing hydroxyapatite-silicon composite suspensions with homogeneous distribution of multi-walled carbon nano-tubes for electrophoretic coating of NiTi bone implant and their effect on the surface morphology
    Khalili, V. and Khalil-Allafi, J. and Xia, W. and Parsa, A.B. and Frenzel, J. and Somsen, C. and Eggeler, G.
    Applied Surface Science 366 158-165 (2016)
    Preparing a stable suspension is a main step towards the electrophoretically depositing of homogeneous and dense composite coatings on NiTi for its biomedical application. In the present study, different composite suspensions of hydroxyapatite, silicon and multi-walled carbon nano-tubes were prepared using n-butanol and triethanolamine as media and dispersing agent, respectively. Multi-walled carbon nanotubes were first functionalized in the nitric acid vapor for 15 h at 175 °C, and then mixed into suspensions. Thermal desorption spectroscopy profiles indicate the formation of functional groups on multi-walled carbon nano-tubes. An excellent suspension stability can be achieved for different amounts of triethanolamine. The amount of triethanolamine can be increased by adding a second component to a stable hydroxyapatite suspension due to an electrostatic interaction between components in suspension. The stability of composite suspension is less than that of the hydroxyapatite suspension, due to density differences, which under the gravitational force promote the demixing. The scanning electron microscopy images of the coatings surface show that more dense coatings are developed on NiTi substrate using electrophoretic deposition and sintering at 850 °C in the simultaneous presence of silicon and multi-walled carbon nanotubes in the hydroxyapatite coatings. The atomic force microscopy results of the coatings surface represent that composite coatings of hydroxyapatite-20 wt.% silicon and hydroxyapatite-20 wt.% silicon-1 wt.% multi-walled carbon nano-tubes with low zeta potential have rougher surfaces. © 2016 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.apsusc.2016.01.053
  • 2016 • 361 The role of metastable LPSO building block clusters in phase transformations of an Mg-Y-Zn alloy
    Kim, J.-K. and Ko, W.-S. and Sandlöbes, S. and Heidelmann, M. and Grabowski, B. and Raabe, D.
    Acta Materialia 112 171-183 (2016)
    We present a systematic atomic scale analysis of the structural evolution of long-period-stacking-ordered (LPSO) structures in the (i) α-Mg matrix and in the (ii) interdendritic LPSO phase of an Mg97Y2Zn1 (at. %) alloy annealed at 500°C, using high resolution high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). Various types of metastable LPSO building block clusters have been observed in both regions. The thermodynamic phase stabilities computed by density-functional-theory calculations explain the diversity of the LPSO structures which are distinguished by their different arrangements of the Y/Zn enriched LPSO building blocks that have a local fcc stacking sequence on the close packed planes. A direct evidence of the transformation from 18R to 14H is presented. This finding suggests that LPSO structures can change their separation distance - quantified by the number of α-Mg layers between them - at a low energy penalty by generating the necessary Shockley partial dislocation on a specific glide plane. Based on our results the most probable transformation sequence of LPSO precipitate plates in the α-Mg matrix is: single building block → various metastable LPSO building block clusters → 14H, and the most probable transformation sequence in the interdendritic LPSO phase is: 18R→ various metastable LPSO building block clusters → 14H. The thermodynamically most stable structures in both the α-Mg matrix and the interdendritic LPSO phase are a mixture of 14H and α-Mg. © 2016 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2016.04.016
  • 2016 • 360 The effect of silicon-substrate orientation on the local piezoelectric characteristics of LiNbO3 films
    Kiselev, D.A. and Zhukov, R.N. and Ksenich, S.V. and Kubasov, I.V. and Temirov, A.A. and Timushkin, N.G. and Bykov, A.S. and Malinkovich, M.D. and Shvartsman, V.V. and Lupascu, D.C. and Parkhomenko, Y.N.
    Journal of Surface Investigation 10 742-747 (2016)
    The domain structure of lithium-niobate thin films grown on Si(111) and Si(100) substrates coated with a native oxide layer with a thickness of no less than 2 nm is investigated by X-ray diffraction, scanning electron microscopy and piezoresponse force microscopy. The films are synthesized by the rf magnetron sputtering of a single-crystal lithium-niobate target. A high degree of grain orientation in the polycrystalline films is demonstrated. The piezoelectric coefficients dzz of the lithium-niobate films on Si(111) and Si(100) substrates are calculated from the measured dependences of the amplitude of the piezoresponse signal on the ac voltage applied between the cantilever tip and the substrate. Piezoelectric hysteresis loops are obtained in the remanent piezoelectric response regime © 2016, Pleiades Publishing, Ltd.
    view abstractdoi: 10.1134/S1027451016040091
  • 2016 • 359 Electrochemical-controlled Characterization of the Corrosion Fatigue Behavior of Creep-resistant Magnesium Alloys DieMag422 and AE42
    Klein, M. and Walther, F.
    Procedia Engineering 160 158-166 (2016)
    Magnesium alloys offer a high potential for lightweight construction, however their application range is limited due to their low corrosion resistance. In the present study the corrosion fatigue behaviors of the creep-resistant alloys DieMag422 and AE42 were characterized and compared. In this context, fatigue properties of specimens in sodium chloride solutions as well as under simultaneous galvanostatic anodic polarization were assessed in constant amplitude tests. The results were correlated with the corrosion behavior of the alloys, which was investigated in instrumented immersion tests. Corrosion- and deformation-induced changes in microstructure were observed by light and scanning electron microscopy, yielding a structure-property relationship for a comprehensive understanding of corrosion fatigue processes. The reduction of the corrosion fatigue strength with increasing corrosion impact could be quantitatively correlated with the adjusted corrosion rates. However, different corrosion morphologies between both materials were found, leading to varying influence on the corrosion fatigue behavior. © 2016 The Author(s).
    view abstractdoi: 10.1016/j.proeng.2016.08.876
  • 2016 • 358 Scanning electrochemical microscopy: Visualization of local electrocatalytic activity of transition metals hexacyanoferrates
    Komkova, M.A. and Maljusch, A. and Sliozberg, K. and Schuhmann, W. and Karyakin, A.A.
    Russian Journal of Electrochemistry 52 1159-1165 (2016)
    The redox competition mode of scanning electrochemical microscopy (SECM) was used to visualize differences in local electrocatalytic activity of Fe and Ni hexacyanoferrates (HCFs) in hydrogen peroxide reduction. The uniform round-shaped spots of electrocatalysts for the SECM measurements were electrochemically deposited using a scanning droplet cell. A negligible activity of NiHCF towards H2O2 reduction compared to Prussian Blue (PB) was observed. The dependence of local Prussian Blue activity on the applied potential was investigated. The proposed strategy explores the potential application of SECM as a rapid screening tool for HCF film activity within a single experiment. © 2016, Pleiades Publishing, Ltd.
    view abstractdoi: 10.1134/S1023193516120065
  • 2016 • 357 Preparation and characterization of low fouling novel hybrid ultrafiltration membranes based on the blends of GO-TiO2 nanocomposite and polysulfone for humic acid removal
    Kumar, M. and Gholamvand, Z. and Morrissey, A. and Nolan, K. and Ulbricht, M. and Lawler, J.
    Journal of Membrane Science 506 38-49 (2016)
    In this work, graphene oxide (GO)-TiO2 nanocomposite was synthesized by in situ sol-gel reaction at pH=2 using GO nanosheets suspension and titanium isopropoxide precursor. The synthesized GO-TiO2 nanocomposite was explored as a filler to fabricate improved antifouling novel hybrid ultrafiltration membranes for removal of humic acid from aqueous solution. Membranes were fabricated from polymer blend solutions containing polysulfone and GO-TiO2 with varied loading amount (0-5 wt%) by the non-solvent induced phase separation (NIPS) method. Contact angle, atomic force microscopy, scanning electron microscopy, Fourier transform infrared spectroscopy and outer surface zeta potential studies were conducted in order to characterise the membranes in terms of roughness, structure, surface properties and charge. The porous hydrophilic hybrid membranes were shown to have an asymmetric structure with improved surface roughness. The water permeability and antifouling capacity of hybrid membranes with 10 ppm HA solution were dependent on the loading amount of GO-TiO2. Incorporation of GO-TiO2 nanocomposite was found to improve the antifouling characteristics of the membranes when challenged with HA solutions. Irreversible HA fouling was substantially reduced with increased loading of GO-TiO2 nanocomposite (wt%). The lowest irreversible fouling ratio (3.2%) was obtained for the membrane containing 5 wt% nanocomposite (to total wt% of PSf, MG-5). Ultrafiltration of HA solutions of varied concentrations using hybrid membranes was studied at pH=7 and 1 bar feed pressure. The removal efficiency of hybrid membranes for HA was controlled by the membrane surface charge concentration, porosity and HA exclusion. The membrane MG-5 had the highest HA removal efficiency for 10 ppm HA solution at pH=7. © 2016 Published by Elsevier B.V.
    view abstractdoi: 10.1016/j.memsci.2016.02.005
  • 2016 • 356 On the mechanism of {332} twinning in metastable β titanium alloys
    Lai, M.J. and Tasan, C.C. and Raabe, D.
    Acta Materialia 111 173-186 (2016)
    {332} twinning, an unusual twinning mode in other body-centered cubic (bcc) metals and alloys, has been demonstrated to be a fundamental deformation mode in bcc metastable β titanium alloys. Recent studies suggest that this twinning mode plays an important role in enhancing the work hardening and thus improving the mechanical properties. Here, we studied the mechanism of this twinning mode in a metastable β Ti-36Nb-2Ta-3Zr (wt.%) alloy. Tensile tests were performed to induce the formation of {332} twins. By using electron backscatter diffraction, transmission electron microscopy and in situ scanning electron microscopy, the surface-to-bulk microstructures and the initiation and propagation of {332} twins were investigated. In addition to the previously reported high densities of straight dislocations within the twin, we have observed that an α″ martensite band is present near the surface adjacent to the twin. During annealing at 900°C, the α″ martensite band transforms into the adjacent twin rather than into the matrix, indicating that {332} twin nucleates within α″ martensite. Further evidence for this is the constitution of the twin in the initial stage of its formation, where the first portion formed consists of α″ martensite. During propagation, the twins propagating to the opposite directions can merge together when their lateral boundaries impinge on each other. Based on the experimental observations, an α″-assisted twinning mechanism is proposed and the origin of the dislocations within {332} twin is discussed accordingly. © 2016 Published by Elsevier Ltd on behalf of Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2016.03.040
  • 2016 • 355 Mechanism of the Fe3(B,C) and Fe23(C,B)6 solid-state transformation in the hypoeutectic region of the Fe-C-B system
    Lentz, J. and Röttger, A. and Theisen, W.
    Acta Materialia 119 80-91 (2016)
    This study investigates the microstructural mechanisms involved in the solid-state transformation of the Fe3(B,C) → Fe23(C,B)6 phases in the hypoeutectic region of the iron-carbon-boron (Fe-C-B) system. We analyzed the influence of different initial microstructural characteristics on the Fe3(B,C) → Fe23(C,B)6 transformation with regards to the matrix phase, matrix C content, B/(C + B) ratio, and agglomeration of the parental Fe3(B,C) phase. We performed thermodynamic calculations using the CALPHAD method, validated by laboratory melts with varying B/(B + C) ratios. These laboratory melts were then microstructurally characterized by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), and wavelength-dispersive X-ray spectroscopy (WDS). We particularly focused on solid-state transformation of borides and carboborides of type M3(C,B) and M23(C,B)6 in the hypoeutectic region of the ternary system Fe-C-B, investigated via both in situ and ex situ XRD measurements. It was found that the solid-state transformations are influenced by enriched B inside the eutectic structure, a result of solidification. This increased B content is not reduced in solid state due to the kinetic limitations of B and C inside the hard-phase structure. Thus phase stability is subject to local equilibria depending on the local C and B concentration of the hard-phase structure. In this process the Fe23(C,B)6 phase also forms a shell-like structure surrounding the Fe3(B,C) and Fe2B phases. © 2016 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2016.08.009
  • 2016 • 354 Copper(II), zinc(II) and copper(II)/zinc(II)-containing carbonate-substituted hydroxyapatite: Synthesis, characterization and thermal behaviour
    Livitska, O. and Strutynska, N. and Zatovsky, I. and Nikolenko, I. and Slobodyanik, N. and Prylutskyy, Y. and Epple, M. and Prymak, O. and Byeda, A.
    Materialwissenschaft und Werkstofftechnik 47 85-91 (2016)
    A new approach for the preparation of nanoscale copper- and zinc-containing sodium- and carbonate-substituted apatites is presented. The thermal transformations of the samples in the temperature range 80-1000 °C were determined by temperature-programmed desorption mass spectroscopy and thermogravimetry. The chemical and phase compositions of the copper- and zinc-containing sodium- and carbonate-substituted apatites were studied by atomic absorption spectroscopy and X-ray diffraction, respectively. The degree and nature of the carbonate substitution were determined by elemental analysis (C, H, N) and infrared spectroscopy, respectively. In addition, scanning electron microscopy (SEM) showed nanoparticles (about 10-20 nm in diameter) with a stability to aggregation under processes by microwave radiation. Samples annealed at 700 °C were crystalline and had an apatite structure. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/mawe.201600460
  • 2016 • 353 Setting mechanical properties of high strength steels for rapid hot forming processes
    Löbbe, C. and Hering, O. and Hiegemann, L. and Tekkaya, A.E.
    Materials 9 (2016)
    Hot stamping of sheet metal is an established method for the manufacturing of light weight products with tailored properties. However, the generally-applied continuous roller furnace manifests two crucial disadvantages: The overall process time is long and a local setting of mechanical properties is only feasible through special cooling techniques. Hot forming with rapid heating directly before shaping is a new approach, which not only reduces the thermal intervention in the zones of critical formability and requested properties, but also allows the processing of an advantageous microstructure characterized by less grain growth, additional fractions (e.g., retained austenite), and undissolved carbides. Since the austenitization and homogenization process is strongly dependent on the microstructure constitution, the general applicability for the process relevant parameters is unknown. Thus, different austenitization parameters are analyzed for the conventional high strength steels 22MnB5, Docol 1400M, and DP1000 in respect of the mechanical properties. In order to characterize the resulting microstructure, the light optical and scanning electron microscopy, micro and macro hardness measurements, and the X-ray diffraction are conducted subsequent to tensile tests. The investigation proves not only the feasibility to adjust the strength and ductility flexibly, unique microstructures are also observed and the governing mechanisms are clarified. © 2016 by the authors.
    view abstractdoi: 10.3390/ma9040229
  • 2016 • 352 Barium sulfate micro- and nanoparticles as bioinert reference material in particle toxicology
    Loza, K. and Föhring, I. and Bünger, J. and Westphal, G.A. and Köller, M. and Epple, M. and Sengstock, C.
    Nanotoxicology 10 1492-1502 (2016)
    The inhalation of particles and their exposure to the bronchi and alveoli constitute a major public health risk. Chemical as well as particle-related properties are important factors for the biological response but are difficult to separate from each other. Barium sulfate is a completely inert chemical compound, therefore it is ideally suited to separate these two factors. The biological response of rat alveolar macrophages (NR8383) was analyzed after exposure to barium sulfate particles with three different diameters (40 nm, 270 nm, and 1.3 μm, respectively) for 24 h in vitro (particle concentrations from 12.5 to 200 μg mL− 1). The particles were colloidally stabilized as well as fluorescently-labeled by carboxymethylcellulose, conjugated with 6-aminofluorescein. All kinds of barium sulfate particles were efficiently taken up by NR8383 cells and found inside endo-lysosomes, but never in the cell nucleus. Neither an inflammatory nor a cytotoxic response was detected by the ability of dHL-60 and NR8383 cells to migrate towards a chemotactic gradient (conditioned media of NR8383 cells) and by the release of inflammatory mediators (CCL2, TNF-α, IL-6). The particles neither caused apoptosis (up to 200 μg mL− 1) nor necrosis (up to 100 μg mL− 1). As only adverse reaction, necrosis was found at a concentration of 200 μg mL− 1 of the largest barium sulfate particles (1.3 μm). Barium sulfate particles are ideally suited as bioinert control to study size-dependent effects such as uptake mechanisms of intracellular distributions of pure particles, especially in nanotoxicology. © 2016 Informa UK Limited, trading as Taylor & Francis Group.
    view abstractdoi: 10.1080/17435390.2016.1235740
  • 2016 • 351 Investigation on femto-second laser irradiation assisted shock peening of medium carbon (0.4% C) steel
    Majumdar, J.D. and Gurevich, E.L. and Kumari, R. and Ostendorf, A.
    Applied Surface Science 364 133-140 (2016)
    In the present study, the effect of femtosecond laser irradiation on the peening behavior of 0.4% C steel has been evaluated. Laser irradiation has been conducted with a 100 μJ and 300 fs laser with multiple pulses under varied energy. Followed by laser irradiation, a detailed characterization of the processed zone was undertaken by scanning electron microscopy, and X-ray diffraction technique. Finally, the residual stress distribution, microhardness and wear resistance properties of the processed zone were also evaluated. Laser processing leads to shock peening associated with plasma formation and its expansion, formation of martensite and ferrito-pearlitic phase in the microstructure. Due to laser processing, there is introduction of residual stress on the surface which varies from high tensile (140 MPa) to compressive (-335 MPa) as compared to 152 MPa of the substrate. There is a significant increase in microhardness to 350-500 VHN as compared to 250 VHN of substrate. The fretting wear behavior against hardened steel ball shows a significant reduction in wear depth due to laser processing. Finally, a conclusion of the mechanism of wear has been established. © 2015 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.apsusc.2015.12.058
  • 2016 • 350 Investigation of the self-healing sliding wear characteristics of NiTi-based PVD coatings on tool steel
    Momeni, S. and Tillmann, W.
    Wear 368-369 53-59 (2016)
    Excellent damping capacity and superelasticity of the bulk NiTi shape memory alloy (SMA) makes it a suitable material of choice for tools in machining process as well as tribological systems. Although thin film of NiTi SMA has a same damping capacity as NiTi bulk alloys, it has a poor mechanical properties and undesirable tribological performance. This study aims at eliminating these application limitations for NiTi thin films. In order to achieve this goal, NiTi thin films were magnetron sputtered as an interlayer between reactively sputtered hard TiCN coatings and hot work tool steel substrates. The microstructure, composition, crystallographic phases, mechanical and tribological properties of the deposited thin films were analyzed by using field emission scanning electron microscopy (FESEM), X-ray diffraction (XRD), nanoindentation, ball-on-disc, scratch test, and three dimensional (3D) optical microscopy. It was found that under a specific coating architecture, the superelasticity of NiTi inter-layer can be combined with high hardness and wear resistance of TiCN protective layers. The obtained results revealed that the thickness of NiTi interlayers is an important factor controlling mechanical and tribological performance of bilayer composite coating systems. © 2016 Elsevier B.V.
    view abstractdoi: 10.1016/j.wear.2016.08.004
  • 2016 • 349 Composite cavitation resistant PVD coatings based on NiTi thin films
    Momeni, S. and Tillmann, W. and Pohl, M.
    Materials and Design 110 830-838 (2016)
    As a protective coating, TiCN hard PVD coating was deposited on magnetron sputtered NiTi thin films under various coating architectures. The microstructure, composition, mechanical properties, tribological performance as well as the cavitation resistance of deposited coatings were analyzed by using X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), nanoindentation, ball–on-disc, scratch test, three dimensional (3D) optical microscopy, and the standard ultrasonic cavitation test (ASTM G 32). The obtained results revealed that under a specific coating architecture, the superelasticity of NiTi can be combined with high hardness and wear resistance of TiCN coatings. As a consequence of the combination of these properties, the composite NiTi based coatings are capable of presenting remarkable cavitation resistance and tribological performance. © 2016 Elsevier Ltd
    view abstractdoi: 10.1016/j.matdes.2016.08.054
  • 2016 • 348 Influence of crystallographic orientation on cavitation erosion resistance of high interstitial CrMnCN austenitic stainless steels
    Niederhofer, P. and Pöhl, F. and Geenen, K. and Huth, S. and Theisen, W.
    Tribology International 95 66-75 (2016)
    The excellent resistance of high interstitial CrMnCN austenitic stainless steels to cavitation erosion was ascribed to strengthening by C+N and the low stacking fault energy causing improved resistance to fatigue and superior mechanical properties. Previous investigations revealed correlation of crystallographic orientation and cavitation erosion damage. In this study, different CrMnCN steels were investigated by ultrasonic cavitation testing. Results were correlated with indentation-derived properties. Alteration of the surface during cavitation was examined by scanning electron microscopy and electron back scatter diffraction. The investigations show influences of crystallographic orientation on the cavitation resistance of individual grains. A relationship between cavitation resistance and hardness and elastic indentation energy was derived in earlier research work. This study shows similar relationship for individual grains. © 2015 Published by Elsevier Ltd.
    view abstractdoi: 10.1016/j.triboint.2015.11.002
  • 2016 • 347 Decomposition of the single-phase high-entropy alloy CrMnFeCoNi after prolonged anneals at intermediate temperatures
    Otto, F. and Dlouhý, A. and Pradeep, K.G. and Kuběnová, M. and Raabe, D. and Eggeler, G. and George, E.P.
    Acta Materialia 112 40-52 (2016)
    Among the vast number of multi-principal-element alloys that are referred to as high-entropy alloys (HEAs) in the literature, only a limited number solidify as single-phase solid solutions. The equiatomic HEA, CrMnFeCoNi, is a face-centered cubic (FCC) prototype of this class and has attracted much attention recently because of its interesting mechanical properties. Here we evaluate its phase stability after very long anneals of 500 days at 500-900 °C during which it is reasonable to expect thermodynamic equilibrium to have been established. Microstructural analyses were performed using complementary analysis techniques including scanning and transmission electron microscopy (SEM/TEM/STEM), energy dispersive X-ray (EDX) spectroscopy, selected area electron diffraction (SAD), and atom probe tomography (APT). We show that the alloy is a single-phase solid solution after homogenization for 2 days at 1200 °C and remains in this state after a subsequent anneal at 900 °C for 500 days. However, it is unstable and forms second-phase precipitates at 700 and 500 °C. A Cr-rich σ phase forms at 700 °C, whereas three different phases (L10-NiMn, B2-FeCo and a Cr-rich body-centered cubic, BCC, phase) precipitate at 500 °C. These precipitates are located mostly at grain boundaries, but also form at intragranular inclusions/pores, indicative of heterogeneous nucleation. Since there is limited entropic stabilization of the solid solution state even in the extensively investigated CrMnFeCoNi alloy, the stability of other HEAs currently thought to be solid solutions should be carefully evaluated, especially if they are being considered for applications in vulnerable temperature ranges. © 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2016.04.005
  • 2016 • 346 Beam-induced atomic migration at Ag-containing nanofacets at an asymmetric Cu grain boundary
    Peter, N.J. and Liebscher, C.H. and Kirchlechner, C. and Dehm, G.
    Journal of Materials Research 32 968-982 (2016)
    Besides the high spatial resolution achieved in aberration-corrected scanning transmission microscopy, beam-induced dynamic effects have to be considered for quantitative chemical characterization on the level of single atomic columns. The present study investigates the influence of imaging conditions in an aberration-corrected scanning transmission electron microscope on the beam-induced atomic migration at a complex Ag-segregated, nanofaceted Cu grain boundary. Three distinct imaging conditions including static single image and serial image acquisition have been utilized. Chemical information on the Ag column occupation of single atomic columns at the grain boundary was extracted by the evolution of peak intensity ratios and compared to idealized scanning transmission electron microscopy image simulations. The atomic column occupation is underestimated when using conventional single frame acquisition due to an averaging of Ag atomic migration events during acquisition. Possible migration paths for the beam-induced atomic motion at a complex Cu grain boundary are presented. Copyright © Materials Research Society 2016
    view abstractdoi: 10.1557/jmr.2016.398
  • 2016 • 345 Improvement of biological indicators by uniformly distributing Bacillus subtilis spores in monolayers to evaluate enhanced spore decontamination technologies
    Raguse, M. and Fiebrandt, M. and Stapelmann, K. and Madela, K. and Laue, M. and Lackmann, J.-W. and Thwaite, J.E. and Setlow, P. and Awakowicz, P. and Moeller, R.
    Applied and Environmental Microbiology 82 2031-2038 (2016)
    Novel decontamination technologies, including cold low-pressure plasma and blue light (400 nm), are promising alternatives to conventional surface decontamination methods. However, the standardization of the assessment of such sterilization processes remains to be accomplished. Bacterial endospores of the genera Bacillus and Geobacillus are frequently used as biological indicators (BIs) of sterility. Ensuring standardized and reproducible BIs for reliable testing procedures is a significant problem in industrial settings. In this study, an electrically driven spray deposition device was developed, allowing fast, reproducible, and homogeneous preparation of Bacillus subtilis 168 spore monolayers on glass surfaces. A detailed description of the structural design as well as the operating principle of the spraying device is given. The reproducible formation of spore monolayers of up to 5 x 10^7 spores per sample was verified by scanning electron microscopy. Surface inactivation studies revealed that monolayered spores were inactivated by UV-C (254 nm), low-pressure argon plasma (500 W, 10 Pa, 100 standard cubic cm per min), and blue light (400 nm) significantly faster than multilayered spores were. We have thus succeeded in the uniform preparation of reproducible, highly concentrated spore monolayers with the potential to generate BIs for a variety of nonpenetrating surface decontamination techniques. © 2016, American Society for Microbiology. All Rights Reserved.
    view abstractdoi: 10.1128/AEM.03934-15
  • 2016 • 344 Resonant laser processing of nanoparticulate Au/TiO2 films on glass supports: Photothermal modification of a photocatalytic nanomaterial
    Schade, L. and Franzka, S. and Thomas, M. and Hagemann, U. and Hartmann, N.
    Surface Science 650 57-63 (2016)
    Resonant laser processing at λ = 532 nm is used to modify thin Au/TiO2 nanoparticle films on soda lime glass plates. A microfocused continuous-wave laser is employed for local patterning at distinct laser powers. In conjunction with microscopic techniques this approach allows for reproducible high-throughput screening of laser-induced material modifications. Optical microscopy and microspectroscopy reveal laser darkening, i.e. a significantly increased optical absorbance. Scanning electron microscopy and X-ray photoelectron spectroscopy show laser-induced film growth and roughening along with the integration of SiO2 from the glass supports. Raman spectroscopy displays a phase transition from anatase to rutile. Au evaporation and/or integration only takes place at high laser powers. All these modifications provide promising perspectives in view of photocatalytic applications. Data from complementary laser experiments with unblended pure TiO2 coatings at λ = 532 nm and λ = 355 nm point to a photothermal process, in which the optical energy is selectively deposited in the Au nanoparticles and transformed into heat. As a result, thermally activated modifications take place. General prospects of laser processing in targeted modification of nanomaterials for photocatalysis are emphasized. © 2016 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.susc.2016.01.006
  • 2016 • 343 Surface-enhanced Raman spectroscopy on laser-engineered ruthenium dye-functionalized nanoporous gold
    Schade, L. and Franzka, S. and Biener, M. and Biener, J. and Hartmann, N.
    Applied Surface Science 374 19-22 (2016)
    Photothermal processing of nanoporous gold with a microfocused continuous-wave laser at λ = 532 nm provides a facile means in order engineer the pore and ligament size of nanoporous gold. In this report we take advantage of this approach in order to investigate the size-dependence of enhancement effects in surface-enhanced Raman spectroscopy (SERS). Surface structures with laterally varying pore sizes from 25 nm to ≥200 nm are characterized using scanning electron microscopy and then functionalized with N719, a commercial ruthenium complex, which is widely used in dye-sensitized solar cells. Raman spectroscopy reveals the characteristic spectral features of N719. Peak intensities strongly depend on the pore size. Highest intensities are observed on the native support, i.e. on nanoporous gold with pore sizes around 25 nm. These results demonstrate the particular perspectives of laser-fabricated nanoporous gold structures in fundamental SERS studies. In particular, it is emphasized that laser-engineered porous gold substrates represent a very well defined platform in order to study size-dependent effects with high reproducibility and precision and resolve conflicting results in previous studies. ©2015 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.apsusc.2015.08.168
  • 2016 • 342 Squamous cell carcinoma in association with a red tattoo
    Schmitz, I. and Prymak, O. and Epple, M. and Ernert, C. and Tannapfel, A.
    JDDG - Journal of the German Society of Dermatology 14 604-610 (2016)
    Background and objectives Although tattoos have become exceedingly popular in recent years, only few cases of severe reactions leading to malignant transformation have been reported in the literature. This stands in contrast to the virtually innumerable number of tattoos worldwide. The composition of tattoo dyes is highly variable, and even the same colors may contain different compounds. The objective of our study was to investigate in what way tattoo dyes may potentially trigger skin cancer. Patient and methods We report the rare case of a 24-year-old woman who - seven months after getting a tattoo on the back of her foot - developed a squamous cell carcinoma in close proximity to the red dye used. Complications started in the form of nonspecific swelling. The lesion was histologically examined. The composition of the incorporated dye was analyzed using scanning electron microscopy in combination with energy dispersive element analysis. Thermogravimetry and powder diffraction were used for further characterization. Results and conclusions While the tattoo dye primarily consisted of barium sulfate, traces of Al, S, Ti, P, Mg, and Cl were also detected. The analysis showed pigment granules of varying sizes. In rare cases, tattoo inks may have carcinogenic effects, which appear to be multifactorial. © 2016 Deutsche Dermatologische Gesellschaft (DDG). Published by John Wiley & Sons Ltd.
    view abstractdoi: 10.1111/ddg.12730
  • 2016 • 341 Spray Deposition of Titania Films with Incorporated Crystalline Nanoparticles for All-Solid-State Dye-Sensitized Solar Cells Using P3HT
    Song, L. and Wang, W. and Körstgens, V. and Moseguí González, D. and Yao, Y. and Minar, N.K. and Feckl, J.M. and Peters, K. and Bein, T. and Fattakhova-Rohlfing, D. and Santoro, G. and Roth, S.V. and Müller-Buschbaum, P.
    Advanced Functional Materials 26 1498-1506 (2016)
    Spray coating, a simple and low-cost technique for large-scale film deposition, is employed to fabricate mesoporous titania films, which are electron-transporting layers in all-solid-state dye-sensitized solar cells (DSSCs). To optimize solar cell performance, presynthesized crystalline titania nanoparticles are introduced into the mesoporous titania films. The composite film morphology is examined with scanning electron microscopy, grazing incidence small-angle X-ray scattering, and nitrogen adsorption-desorption isotherms. The crystal phase and crystallite sizes are verified by X-ray diffraction measurements. The photovoltaic performance of all-solid-state DSSCs is investigated. The findings reveal that an optimal active layer of the all-solid-state DSSC is obtained by including 50 wt% titania nanoparticles, showing a foam-like morphology with an average pore size of 20 nm, featuring an anatase phase, and presenting a surface area of 225.2 m2 g-1. The optimized morphology obtained by adding 50 wt% presynthesized crystalline titania nanoparticles yields, correspondingly, the best solar cell efficiency of 2.7 ± 0.1%. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/adfm.201504498
  • 2016 • 340 Local Platinum Environments in a Solid Analogue of the Molecular Periana Catalyst
    Soorholtz, M. and Jones, L.C. and Samuelis, D. and Weidenthaler, C. and White, R.J. and Titirici, M.-M. and Cullen, D.A. and Zimmermann, T. and Antonietti, M. and Maier, J. and Palkovits, R. and Chmelka, B.F. and Schüth, F.
    ACS Catalysis 6 2332-2340 (2016)
    Combining advantages of homogeneous and heterogeneous catalysis by incorporating active species on a solid support is often an effective strategy for improving overall catalyst performance, although the influences of the support are generally challenging to establish, especially at a molecular level. Here, we report the local compositions, and structures of platinum species incorporated into covalent triazine framework (Pt-CTF) materials, a solid analogue of the molecular Periana catalyst, Pt(bpym)Cl2, both of which are active for the selective oxidation of methane in the presence of concentrated sulfuric acid. By using a combination of solid-state 195Pt nuclear magnetic resonance (NMR) spectroscopy, aberration-corrected scanning transmission electron microscopy (AC-STEM), X-ray photoelectron spectroscopy (XPS), and X-ray absorption spectroscopy (XAS), important similarities and differences are observed between the Pt-CTF and Periana catalysts, which are likely related to their respective macroscopic reaction properties. In particular, wide-line solid-state 195Pt NMR spectra enable direct measurement, identification, and quantification of distinct platinum species in as-synthesized and used Pt-CTF catalysts. The results indicate that locally ordered and disordered Pt sites are present in as-synthesized Pt-CTF, with the former being similar to one of the two crystallographically distinct Pt sites in crystalline Pt(bpym)Cl2. A distribution of relatively disordered Pt moieties is also present in the used catalyst, among which are the principal active sites. Similarly XAS shows good agreement between the measured data of Pt-CTF and a theoretical model based on Pt(bpym)Cl2. Analyses of the absorption spectra of Pt-CTF used for methane oxidation suggests ligand exchange, as predicted for the molecular catalyst. XPS analyses of Pt(bpym)Cl2, Pt-CTF, as well as the unmodified ligands, further corroborate platinum coordination by pyridinic N atoms. Aberration-corrected high-angle annular dark-field STEM proves that Pt atoms are distributed within Pt-CTF before and after catalysis. The overall results establish the close similarities of Pt-CTF and the molecular Periana catalyst Pt(bpym)Cl2, along with differences that account for their respective properties. (Figure Presented). © 2016 American Chemical Society.
    view abstractdoi: 10.1021/acscatal.5b02305
  • 2016 • 339 Gene transfection of human mesenchymal stem cells with a nano-hydroxyapatite–collagen scaffold containing DNA-functionalized calcium phosphate nanoparticles
    Tenkumo, T. and Vanegas Sáenz, J.R. and Takada, Y. and Takahashi, M. and Rotan, O. and Sokolova, V. and Epple, M. and Sasaki, K.
    Genes to Cells 21 682-695 (2016)
    This study aimed to fabricate a growth factor-releasing biodegradable scaffold for tissue regeneration. We prepared multishell calcium phosphate (CaP) nanoparticles functionalized with DNA, polyethyleneimine (PEI), protamine and octa-arginine (R8) and compared their respective transfection activity and cell viability measures using human mesenchymal stem cells. DNA–protamine complexes improved the transfection efficiency of CaP nanoparticles with the exception of those functionalized with R8. These complexes also greatly reduced the cytotoxicity of PEI. In addition, we also fabricated DNA–protamine-functionalized CaP nanoparticle-loaded nano-hydroxyapatite–collagen scaffolds and investigated their gene transfection efficiencies. These experiments showed that the scaffolds were associated with moderate hMSC cell viability and were capable of releasing the BMP-2 protein into hMSCs following gene transfection. In particular, the scaffold loaded with protamine-containing CaP nanoparticles showed the highest cell viability and transfection efficiency in hMSCs; thus, it might be suitable to serve as an efficient growth factor-releasing scaffold. © 2016 Molecular Biology Society of Japan and John Wiley & Sons Australia, Ltd
    view abstractdoi: 10.1111/gtc.12374
  • 2016 • 338 Tribological development of TiCN coatings by adjusting the flowing rate of reactive gases
    Tillmann, W. and Momeni, S.
    Journal of Physics and Chemistry of Solids 90 45-53 (2016)
    TiCN coatings were deposited by means of direct current magnetron sputtering of Ti targets in presence of N2 and C2H2 reactive gases. The microstructure, composition, mechanical and tribological properties of the deposited thin films were analyzed by using X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), nanoindentation, ball-on-disc, scratch test, and three dimensional (3D) optical microscopy. The obtained results presents a reproducible processing route for tailoring microstructure, mechanical and tribological behavior of TiCN coatings by controlling flowing rate of the reactive gases. © 2015 Elsevier Ltd.
    view abstractdoi: 10.1016/j.jpcs.2015.11.009
  • 2016 • 337 Influence of PVD-coating technology and pretreatments on residual stresses for sheet-bulk metal forming tools
    Tillmann, W. and Stangier, D. and Denkena, B. and Grove, T. and Lucas, H.
    Production Engineering 10 17-24 (2016)
    Residual stresses in the substrate material are significantly influencing the performance of PVD-coated parts and tools which are exposed to high forces. Especially for forming operations, such as sheet-bulk metal forming, during which normal contact pressures of 1.4 GPa can occur, the reduction of friction and, at the same time, the wear protection by means of thin Cr-based coatings are essential. To ensure a long service life of forming tool and tool coating, each step of the substrate pre-treatment, as well as the magnetron-sputtering process, has to be coordinated and compatible. Therefore, polished as well as nitrided samples consisting of high-speed steel (AISI M3:2) are exposed to a sequence of plasma-based pre-treatments prior to depositing a CrAlN coating. Hardness and Young’s modulus of the substrate and the coating are analysed by means of nanoindentation. To determine the adhesion between coating and substrate, scratch tests are conducted and analysed using a scanning electron microscope. For each step, the residual stresses are determined using sin2ψ measurements, which are correlated to the mechanical properties. A plasma-nitriding process before the CrAlN coating induces high compressive residual stresses into the sample subsurface and at the same time increases the hardness of the surface. This results in higher critical loads during the scratch tests and therefore a better adhesion of the coating on the substrate. © 2015, German Academic Society for Production Engineering (WGP).
    view abstractdoi: 10.1007/s11740-015-0653-4
  • 2016 • 336 Investigation of the brazing characteristics of a new iron-based brazing filler metal
    Tillmann, W. and Wojarski, L. and Manka, M. and Trelenberg, A.
    Welding in the World 60 869-875 (2016)
    High temperature applications of new class of iron-based filler metals provide brazements with high corrosion resistance and mechanical properties. These brazements are cost-effective alternative to those made of the conventional brazing alloys. However, a wiser usage demands a deeper understanding of the wetting as well as gap filling behavior in conjunction with the resulting microstructure, which is mainly influenced by the applied brazing cycles. Therefore, this paper presents results of the investigation of specific brazing fundamentals for the new iron-based brazing alloy Fe-24Cr-20Ni-10Cu-7P-5Mn-5Si. Followed by DTA/DSC measurements, the spreading and gap filling behavior were examined by using stainless steel AISI 304 as base material. In wetting tests and wedge-gap experiments, the influence of the applied brazing temperature and the dwell time were investigated for vacuum brazing processes. The resulting microstructure was evaluated using a scanning electron microscope (SEM), equipped with an energy dispersive X-ray spectroscopy (EDS). Finally, strength tests were conducted in order to determine the influence of the brazing parameters on the mechanical properties of the joint. © 2016, International Institute of Welding.
    view abstractdoi: 10.1007/s40194-016-0346-4
  • 2016 • 335 Study of biocompatibility effect of nanocarbon particles on various cell types in vitro
    Tolkachov, M. and Sokolova, V. and Loza, K. and Korolovych, V. and Prylutskyy, Y. and Epple, M. and Ritter, U. and Scharff, P.
    Materialwissenschaft und Werkstofftechnik 47 216-221 (2016)
    The viability of primary cells (human mesenchymal stem cells, hMSC, as a model for healthy cells) and a cancer cell line (human transformed cervix epithelial cells, HeLa, as a model for cancer cells) was studied with the MTT assay after the incubation with water-soluble C60 fullerenes and multi-walled carbon nanotubes filled by iron, respectively. The size of the particles was determined by dynamic light scattering. The morphology of the cells incubated with nanocarbon particles was studied by scanning electron microscopy. The effect of C60 fullerenes and Fe-multi-walled carbon nanotubes on the cells is depending on the concentration of applied nanoparticles. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/mawe.201600486
  • 2016 • 334 Gas-phase synthesis of Fe-Bi metastable and dumbbell particles
    Ünlü, C.G. and Li, Z.-A. and Acet, M. and Farle, M.
    Crystal Research and Technology 51 333-336 (2016)
    Fe-Bi nanoparticles were prepared in the gas-phase by DC magnetron sputtering and in-fight annealing. The morphological, structural and compositional properties were investigated by High-resolution transmission electron microscopy, energy dispersive X-ray spectroscopy and scanning transmission electron microscopy. High-resolution microscopy studies show that primary particles produced without in-flight annealing are spherical with a diameter of about 50 nm. Particles sintered at 773 K acquire a dumbbell structure with Fe-FeO and Bi sections. © 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/crat.201500329
  • 2016 • 333 Assessment of strain hardening in copper single crystals using in situ SEM microshear experiments
    Wieczorek, N. and Laplanche, G. and Heyer, J.-K. and Parsa, A.B. and Pfetzing-Micklich, J. and Eggeler, G.
    Acta Materialia 113 320-334 (2016)
    The effect of a pre-strain on the plasticity of copper single crystals subjected to in situ microshear deformation in a scanning electron microscope (SEM) is investigated. Pre-strains of 6.5 and 20% are imposed using [1 0 0] tensile testing. During tensile pre-deformation, several slip systems are activated and irregularly spaced slip bands form. A trace analysis revealed the presence of several slip bands on the tensile specimen near the grips while one family of slip bands parallel to the (1 1 1) crystallographic plane were detected in the middle of the tensile specimen. From the middle of the pre-deformed tensile specimens double microshear samples were prepared using focused ion beam (FIB) machining such that the [0 -1 -1] (1 -1 1) slip system could be directly activated. The results show how microshear behavior reacts to different levels of tensile pre-deformation. Sudden deformation events (SDEs) are observed during microshear testing. The critical stress associated with the first SDE is shown to increase with increasing pre-deformation as a result of an increasing number of slip bands introduced during pre-deformation per shear zone. The results allow also to obtain information on the interaction between dislocations activated during microshearing ([0 -1 -1] (1 -1 1)) and those which were introduced during tensile pre-deformation ([1 0 -1] (1 1 1) and [1 -1 0] (1 1 1)). When these slip systems interact glissile junctions and Lomer-Cottrell locks are likely to form. In the light of this analysis, we rationalize the occurrence of sudden deformation events based on piled up dislocation assemblies which overcome Lomer-Cottrell lock barriers. © 2016 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2016.04.055
  • 2016 • 332 Corrosion fatigue assessment of creep-resistant magnesium alloy Mg-4Al-2Ba-2Ca in aqueous sodium chloride solution
    Wittke, P. and Klein, M. and Dieringa, H. and Walther, F.
    International Journal of Fatigue 83 59-65 (2016)
    Low corrosion resistance of magnesium alloys strongly limits their application range. This study aims at the investigation of corrosion influence on microstructure and depending mechanical properties of newly developed magnesium alloy Mg-4Al-2Ba-2Ca. The fatigue properties of this creep-resistant magnesium alloy were investigated under three corrosive environments: double distilled water, 0.01 and 0.1 mol L-1 NaCl solutions. Potentiodynamic polarization measurements and immersion tests were performed to estimate the corrosion behaviour. Specimen surfaces were observed using light and scanning electron microscopy for microstructure-related assessment of corrosion mechanisms. The corrosion fatigue behaviour was characterized in continuous load increase tests using plastic strain and electrochemical measurements. Continuous load increase tests allow estimating the fatigue limit and determining the failure stress amplitude with one single specimen. Fatigue results showed a significant decrease in the estimated fatigue limit and determined failure stress amplitude with increasing corrosion impact of the environments. This corrosion-structure-property relation was quantitatively described by means of model-based correlation approaches and failure hypotheses. Plastic strain amplitude and deformation-induced changes in electrochemical measurands can be equivalently applied for precise corrosion fatigue assessment. © 2015 Elsevier Ltd.
    view abstractdoi: 10.1016/j.ijfatigue.2015.04.001
  • 2016 • 331 Determination of the young modulus of Ti-TiAl3 metallic intermetallic laminate composites by nano-indentation
    Yener, T. and Güler, S. and Siddique, S. and Walther, F. and Zeytin, S.
    Acta Physica Polonica A 129 604-606 (2016)
    Nano-indentation is an important technique to determine the Young modulus of multiphase materials where normal tensile tests are not appropriate. In this work, Ti-TiAl3 metallic-intermetallic laminate composites have been fabricated successfully in open atmosphere using commercial purity Al and Ti foils with 250 μm and 500 μm initial thicknesses, respectively. Sintering process was performed at 700 °C under 2 MPa pressure for 7.5 h. Mechanical properties including the Young modulus were determined after manufacturing. The Young moduli of metallic and intermetallic phases were determined as 89 GPa and 140 GPa, respectively. Microstructure analyses showed that aluminum foil was almost consumed by forming a titanium aluminide intermetallic compound. Titanium aluminides grow up through spherical shaped islands and metallic-intermetallic interface is a wavy form in Ti-Al system. Thus, the final microstructure consists of alternating layers of intermetallic compound and unreacted Ti metal. Microstructure and phase characterizations were performed by scanning electron microscopy, energy dispersive spectroscopy, and X-ray diffraction. Hardness of test samples was determined as 600 HV for intermetallic zone and 130 HV for metallic zone by the Vickers indentation method.
    view abstractdoi: 10.12693/APhysPolA.129.604
  • 2016 • 330 Autonomous Filling of Grain-Boundary Cavities during Creep Loading in Fe-Mo Alloys
    Zhang, S. and Fang, H. and Gramsma, M.E. and Kwakernaak, C. and Sloof, W.G. and Tichelaar, F.D. and Kuzmina, M. and Herbig, M. and Raabe, D. and Brück, E. and van der Zwaag, S. and van Dijk, N.H.
    Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science 47 4831-4844 (2016)
    We have investigated the autonomous repair of creep damage by site-selective precipitation in a binary Fe-Mo alloy (6.2 wt pct Mo) during constant-stress creep tests at temperatures of 813 K, 823 K, and 838 K (540 °C, 550 °C, and 565 °C). Scanning electron microscopy studies on the morphology of the creep-failed samples reveal irregularly formed deposits that show a close spatial correlation with the creep cavities, indicating the filling of creep cavities at grain boundaries by precipitation of the Fe2Mo Laves phase. Complementary transmission electron microscopy and atom probe tomography have been used to characterize the precipitation mechanism and the segregation at grain boundaries in detail. © 2016, The Author(s).
    view abstractdoi: 10.1007/s11661-016-3642-0
  • 2016 • 329 Magnetic surface domain imaging of uncapped epitaxial FeRh(001) thin films across the temperature-induced metamagnetic transition
    Zhou, X. and Matthes, F. and Bürgler, D.E. and Schneider, C.M.
    AIP Advances 6 (2016)
    The surface magnetic domain structure of uncapped epitaxial FeRh/MgO(001) thin films was imaged by in-situ scanning electron microscopy with polarization analysis (SEMPA) at various temperatures between 122 and 450 K. This temperature range covers the temperature-driven antiferromagnetic-to-ferromagnetic phase transition in the body of the films that was observed in-situ by means of the more depth-sensitive magneto-optical Kerr effect. The SEMPA images confirm that the interfacial ferromagnetism coexisting with the antiferromagnetic phase inside the film is an intrinsic property of the FeRh(001) surface. Furthermore, the SEMPA data display a reduction of the in-plane magnetization occuring well above the phase transition temperature which, thus, is not related to the volume expansion at the phase transition. This observation is interpreted as a spin reorientation of the surface magnetization for which we propose a possible mechanism based on temperature-dependent tetragonal distortion due to different thermal expansion coefficients of MgO and FeRh. © 2016 Author(s).
    view abstractdoi: 10.1063/1.4940758
  • 2015 • 328 The evolution of microstructure and mechanical properties of Ti-5Al-5Mo-5V-2Cr-1Fe during ageing
    Ahmed, M. and Li, T. and Casillas, G. and Cairney, J.M. and Wexler, D. and Pereloma, E.V.
    Journal of Alloys and Compounds 629 260-273 (2015)
    The phase transformations and compositional changes occurring during thermo-mechanical processing and subsequent high temperature ageing of Ti-5Al-5Mo-5V-2Cr-1Fe (wt.%) were investigated using scanning transmission electron microscopy (STEM) and atom probe tomography (APT). High resolution STEM revealed nano-sized α (< 10 nm) and athermal ω (∼1-3 nm) formed during accelerated cooling from 800°C and slow heating to an ageing temperature of 650°C. Nuclei of α were found to form heterogeneously in the β matrix as well as at the ω phase. APT revealed pronounced Mo compositional fluctuations in the β matrix. No direct connection was established between Mo-rich or Mo-lean regions and α or ω nuclei. APT also failed to detect the ω phase, which supports theories that it forms by a shuffle mechanism, without any compositional difference from the β phase. Very small α particles, after initial ageing, showed only a minute change in composition with respect to the β matrix, indicative of a displacive-diffusional transformation. With further ageing, growth of the α lamellae was accompanied by compositional changes according to the diffusion rates of β-stabilising elements. Pile-up of the slowest diffusing solutes (Mo, V) at the α/β interface were pronounced in the initial stages of ageing. The best combination of mechanical properties (1200 MPa ultimate tensile strength with 15% total elongation) was recorded after 3.6 ks of ageing. © 2015 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.jallcom.2015.01.005
  • 2015 • 327 Stability of Dealloyed Porous Pt/Ni Nanoparticles
    Baldizzone, C. and Gan, L. and Hodnik, N. and Keeley, G.P. and Kostka, A. and Heggen, M. and Strasser, P. and Mayrhofer, K.J.J.
    ACS Catalysis 5 5000-5007 (2015)
    We provide a comprehensive durability assessment dedicated to a promising class of electrocatalysts for the oxygen reduction reaction (i.e., porous platinum nanoparticles). The stability of these nanoengineered open structures is tested under two accelerated degradation test conditions (ADT), particularly selected to mimic the potential regimes experienced by the catalyst during the operative life of a fuel cell (i.e., load cycles (up to 1.0 V<inf>RHE</inf>) and start-up cycles (up to 1.4 V<inf>RHE</inf>)). To understand the evolution of the electrochemical performance, the catalyst properties are investigated by means of fundamental rotating disc electrode studies, identical location-transmission electron microscopy (IL-TEM) coupled with electron energy loss spectroscopy chemical mapping (IL-EELS), and post-use chemical analysis and online highly sensitive potential resolved dissolution concentration monitoring by scanning flow cell inductively coupled plasma-mass spectrometry (SFC-ICP-MS). The experimental results on the nanoporous Pt revealed distinctive degradation mechanisms that could potentially affect a wide range of other nanoengineered open structures. The study concludes that, although providing promising activity performance, under the relevant operational conditions of fuel cells, the nanoporosity is only metastable and subjected to a progressive reorganization toward the minimization of the nanoscale curvature. The rate and pathways of this specific degradation mechanism together with other well-known degradation mechanisms like carbon corrosion and platinum dissolution are strongly dependent on the selected upper limit potential, leading to distinctly different durability performance. © 2015 American Chemical Society.
    view abstractdoi: 10.1021/acscatal.5b01151
  • 2015 • 326 Van der Waals epitaxial MOCVD-growth of (BixSb1-x)2Te3 (0 < x < 1) films
    Bendt, G. and Sonntag, J. and Lorke, A. and Assenmacher, W. and Hagemann, U. and Schulz, S.
    Semiconductor Science and Technology 30 (2015)
    Epitaxial (Bi<inf>x</inf>Sb<inf>1-x</inf>)<inf>2</inf>Te<inf>3</inf> films with (0 < x < 1) were grown by the metal-organic chemical vapour deposition (MOCVD) process at 400 °C using the tailor-made precursors Et<inf>2</inf>Te<inf>2</inf>, i-Pr<inf>3</inf>Sb and Et<inf>3</inf>Bi. The films grown on Al<inf>2</inf>O<inf>3</inf>(0001) substrates show a very smooth surface morphology as shown by a scanning electron microscope (SEM), atomic force microscopy (AFM) and transmission electron microscopy (TEM), while those grown on Si(100) are rather polycrystalline. The chemical composition of the crystalline films (x-ray powder diffraction (XRD)) was investigated by energy-dispersive x-ray (EDX) and x-ray photoelectron spectroscopy (XPS), and the in-plane transport properties were measured, and a strong dependency from the bismuth content was found, which allows the tuning of the carrier concentration and mobility in a wide range. © 2015 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0268-1242/30/8/085021
  • 2015 • 325 Onset potential determination at gas-evolving catalysts by means of constant-distance mode positioning of nanoelectrodes
    Botz, A.J.R. and Nebel, M. and Rincón, R.A. and Ventosa, E. and Schuhmann, W.
    Electrochimica Acta 179 38-44 (2015)
    The onset potential of an electrocatalytic reaction is frequently used as an indicator to compare the catalytic performance of electrocatalysts. However, in addition to the fact that the onset potential is an undefined physico-chemical value which is dependent on the sensitivity of the used potentiostat its determination using voltammetry at the catalyst-modified electrode surface may be superimposed by additional Faradaic reactions e.g. from redox conversions of the catalyst material or corrosion processes. Gas-evolving electrodes suffer additionally from the dynamics of gas bubble formation and departure leading to inherent limitations of voltammetric studies directly performed at the catalyst-modified electrode. Nanometer-sized electrodes accurately positioned by means of shearforce-based constant-distance mode SECM are proposed for the highly sensitive determination of the onset potential of microcavity electrodes filled with different perovskites as oxygen evolution catalysts. Double barrel microcavity electrodes are additionally suggested for the simultaneous investigation of two catalysts. They enable direct referencing of a catalyst with a benchmark catalyst material in a single experiment. © 2015 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.electacta.2015.04.145
  • 2015 • 324 A quantitative metallographic assessment of the evolution of porosity during processing and creep in single crystal Ni-base super alloys
    Buck, H. and Wollgramm, P. and Parsa, A.B. and Eggeler, G.
    Materialwissenschaft und Werkstofftechnik 46 577-590 (2015)
    The present work reviews previous research on the evolution of porosity. It presents new results from a detailed study on the evolution of porosity during casting, heat treatment and creep of a single crystal Ni-base superalloy subjected to uniaxial tensile creep at 1050 °C and 160 MPa in [001] and [110] directions. A quantitative metallographic study was performed on carefully polished metallographic cross sections, monitoring sampling fields of 4500 × 1000 μm2 using the back scatter contrast of an analytical scanning electron microscope; evolutions of pore sizes and pore form factors were analyzed and all important details which were previously revealed in a synchrotron study could be reproduced. In addition, it was observed that micro cracks form at larger cast pores. They interlink and thus initiate final rupture. The [110] tensile creep tests showed lower rupture strains than the [001] experiments. In agreement with earlier work, this can be rationalized on the basis of aligned porosity along primary dendrites. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/mawe.201500379
  • 2015 • 323 Three-Dimensional, Fibrous Lithium Iron Phosphate Structures Deposited by Magnetron Sputtering
    Bünting, A. and Uhlenbruck, S. and Sebold, D. and Buchkremer, H.P. and Vaßen, R.
    ACS Applied Materials and Interfaces 7 22594-22600 (2015)
    Crystalline, three-dimensional (3D) structured lithium iron phosphate (LiFePO4) thin films with additional carbon are fabricated by a radio frequency (RF) magnetron-sputtering process in a single step. The 3D structured thin films are obtained at deposition temperatures of 600 °C and deposition times longer than 60 min by using a conventional sputtering setup. In contrast to glancing angle deposition (GLAD) techniques, no tilting of the substrate is required. Thin films are characterized by X-ray diffraction (XRD), Raman spectrospcopy, scanning electron microscopy (SEM), cyclic voltammetry (CV), and galvanostatic charging and discharging. The structured LiFePO4 + C thin films consist of fibers that grow perpendicular to the substrate surface. The fibers have diameters up to 500 nm and crystallize in the desired olivine structure. The 3D structured thin films have superior electrochemical properties compared with dense two-dimensional (2D) LiFePO4 thin films and are, hence, very promising for application in 3D microbatteries. © 2015 American Chemical Society.
    view abstractdoi: 10.1021/acsami.5b07090
  • 2015 • 322 NOx conversion properties of a novel material: Iron nanoparticles stabilized in carbon
    Busch, M. and Kompch, A. and Suleiman, S. and Notthoff, C. and Bergmann, U. and Theissmann, R. and Atakan, B. and Winterer, M.
    Applied Catalysis B: Environmental 166-167 211-216 (2015)
    Nitrogen oxides (NOx) belong to the most common pollutants from combustion processes and are a major threat to human health. Carbon-based catalysts exhibit strong advantages for NOx removal like low-toxic application and easy handling. However, gasification of the carbon matrix at elevated temperatures is still one of the greatest concerns. Hence, we have directed our focus on especially low temperature NOx-removal using a novel material, iron nanoparticles stabilized in a carbon matrix (nano-Fe/C). The investigations included NO2 uptake properties and catalytic conversion of NO2 in recycle flow at 425K and 328K, scanning transmission electron microscopy and 77K-N2-adsorption. Nano-Fe/C exhibits superior NOx-removal properties compared with untreated or iron-infiltrated activated carbon or magnetite reference catalysts. No severe catalyst deactivation or catalyst aging at 425K is observed. Even at 328K nano-Fe/C still exhibits NO2-conversion, although without converting the product NO. NO2 adsorption at 297K is suggested to occur in three stages with different kinetics: (1) NO2 adsorption and reduction to NO, (2) physisorption on the oxidized catalyst surface and (3) saturation of the catalyst and diffusion into the substrate matrix. At 425K, NO2 is quickly reduced to NO and the resulting NO is further converted to N2O. After complete consumption of NO, the residual NO2 is also converted to N2O. A possible reaction mechanism is suggested based on the conversion kinetics. © 2014 Published by Elsevier B.V.
    view abstractdoi: 10.1016/j.apcatb.2014.11.013
  • 2015 • 321 Synthesis and characterization of antimicrobial textile finishing based on Ag:ZnO nanoparticles/chitosan biocomposites
    Buşilə, M. and Muşat, V. and Textor, T. and Mahltig, B.
    RSC Advances 5 21562-21571 (2015)
    ZnO and Ag:ZnO nanoparticles were prepared by hydrolysis of zinc acetate in the presence of lithium hydroxide (LiOH). In combination with binders based on hybrid polymer sols, these metal oxide materials were applied for textile treatment. Hybrid coatings based on ZnO, Ag:ZnO/CS, chitosan (CS), 3-glycidyloxypropyltrimethoxysilane (GPTMS) and tetraethoxysilane (TEOS) prepared by sol-gel method were applied on cotton 100% and cotton/polyester (50/50%) textiles using "pad-dry-cure" technique. The obtained nanoparticles incorporated within chitosan matrix were characterised by X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, thermogravimetric analysis (TGA), UV-Vis spectroscopy and field emission scanning electron microscopy (FE-SEM). The antimicrobial activity of Ag/CS, ZnO/CS and Ag:ZnO/CS composite coatings was investigated in comparison to that of the pure chitosan using the paper disc method on Mueller-Hinton agar, against the Gram-negative E. coli and the Gram-positive S. aureus bacteria. For the same composite coatings applied on textile, the antimicrobial activity was investigated by UV-Vis absorption spectroscopy using TTC method, against the bacteria E. coli and M. luteus. The investigated nanocomposite materials showed good antimicrobial activity and are promising materials for use as medical applications. This journal is © The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c4ra13918f
  • 2015 • 320 Temperature-Induced Modulation of the Sample Position in Scanning Electrochemical Microscopy
    Clausmeyer, J. and Schäfer, D. and Nebel, M. and Schuhmann, W.
    ChemElectroChem 2 946-948 (2015)
    Feedback-mode scanning electrochemical microscopy (SECM) was conducted with insitu control of the sample temperature using a Peltier element. The intrinsic heat pulsing of the temperature control feedback loop causes the sample position to oscillate periodically. By synchronizing tip current acquisition with the oscillatory temperature modulation of the sample, tip current values at different tip-to-sample separations are recorded and used to estimate the slope of the distance-current relationship (z-approach curve). In-phase amplification of only the distance-dependent current signal is achieved using the approximated slope and hence the relative imaging contrast is enhanced. Moreover, sample-position modulated SECM allows distinguishing between electrochemically active and inactive domains on the sample in a single area scan without an apriori knowledge of the sample topography. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/celc.201500087
  • 2015 • 319 From layered zeolite precursors to zeolites with a three-dimensional porosity: Textural and structural modifications through alkaline treatment
    De Baerdemaeker, T. and Feyen, M. and Vanbergen, T. and Müller, U. and Yilmaz, B. and Xiao, F.-S. and Zhang, W. and Yokoi, T. and Bao, X. and De Vos, D.E. and Gies, H.
    Chemistry of Materials 27 316-326 (2015)
    The layered zeolite precursor RUB-36, consisting of ferrierite-type layers, can be transformed into a three-dimensional framework through various methods such as topotactic condensation into the CDO topology, or interlayer expansion either in the presence or absence of a silylating agent. However, the plate-like morphology of the micrometer sized crystals hampers the accessibility of the 2D micropore system, in which the channels run parallel to the plates. With the aim of introducing mesoporosity, alkaline treatments were performed on different RUB-36 derived expanded materials, and on RUB-36 itself. The effect on the physicochemical properties was examined using N2 physisorption, powder X-ray diffraction, scanning electron microscopy and 27Al MAS NMR whereas the influence on the catalytic activity was evaluated using esterification and alkylation reactions. After calcination, the purely inorganic, interlayer expanded material could be transformed into a mesopore containing FER-type material by selective removal of the interlayer T atom followed by the recombination of the layers. In the precalcination state, organic moieties, originating from the silylating agent or from the organic structure directing agent (OSDA), increase the hydrophobicity of the interlayer expanded structure and its stability against the alkaline treatment. In RUB-36, the high OSDA content limited the amount of mesopore formation through alkaline treatment. However, using the appropriate conditions, the subsequent interlayer expansion of alkaline treated RUB-36 also resulted in a mesopore containing interlayer expanded structure. © 2014 American Chemical Society.
    view abstractdoi: 10.1021/cm504014d
  • 2015 • 318 Pseudomorphic Generation of Supported Catalysts for Glycerol Oxidation
    Deng, X. and Dodekatos, G. and Pupovac, K. and Weidenthaler, C. and Schmidt, W. and Schüth, F. and Tüysüz, H.
    ChemCatChem 7 3832-3837 (2015)
    A catalyst consisting of copper nanoparticles (15-20 nm in size) supported on ordered mesoporous cobalt monoxide was synthesized by the one-step reduction of ethanol from nanocast copper cobalt spinel oxides. The small-angle X-ray scattering patterns showed that the ordered mesostructure was maintained after post-treatment, and the cross-section scanning electron microscopy images showed that the Cu nanoparticles were distributed homogeneously throughout the mesoporous CoO framework. The materials were tested as noble-metal-free catalysts for the oxidation of glycerol under alkaline conditions. The catalytic data showed that the presence of Cu nanoparticles greatly enhanced the catalytic performance. Nothing noble: A catalyst consisting of copper nanoparticles (NPs, 15-20 nm in size) supported on ordered mesoporous cobalt monoxide is synthesized by the one-step reduction with ethanol from nanocast copper cobalt spinel oxides. As a noble-metal-free catalyst for the oxidation of glycerol, the presence of Cu NPs greatly enhances the catalytic performance. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/cctc.201500703
  • 2015 • 317 Silicon/carbon nano-composite based anodes for advanced lithium-ion batteries
    Dobrowolny, S. and Mahlendorf, F. and Heinzel, A.
    ECS Transactions 66 29-36 (2015)
    In this study, the investigation of high capacity and high efficiency graphene coated silicon composite (Si/C composite) based electrodes prepared by using a wet chemical manufacturing process is presented. The active material provides a capacity of >2000 mAh g-1 with a coulombic efficiency >99% for more than 500 cycles. The focus is set to the investigation of the electrode structure during cycling progression by using galvanostatic cycling, electrochemical impedance spectroscopy, scanning electron microscopy, confocal microscopy and the measurement of the coating adhesion strength. Results show the applicability of improved Si/C composite electrodes for future lithium-ion batteries, both in half cells as well as in full cells in combination with a commercially available cathode material. © The Electrochemical Society.
    view abstractdoi: 10.1149/06609.0029ecst
  • 2015 • 316 Isotropic macroporous polyethersulfone membranes as competitive supports for high performance polyamide desalination membranes
    ElSherbiny, I.M.A. and Ghannam, R. and Khalil, A.S.G. and Ulbricht, M.
    Journal of Membrane Science 493 782-793 (2015)
    Novel macroporous isotropic polyethersulfone (PES) base membranes were developed using combined processes of vapor- and non-solvent-induced phase separation. Optimization of different preparation parameters was carried out. The newly prepared PES membranes exhibited well-defined isotropic porous structures with optimized average barrier pore diameter of 100. nm as well as hydrophilic surface and high water permeability. These isotropic membranes together with two more supports (i.e. commercial PES microfiltration and anisotropic hydrophobic polysulfone membranes), were utilized for the fabrication of polyamide (PA) thin-film composite (TFC) desalination membranes. The resulted PA TFC membranes showed significantly different film morphologies, surface characteristics as well as separation performance. The PA TFC membranes based on the hydrophilic PES supports with isotropic and optimized pore size, developed in this study, showed superior water permeability compared to the composite membranes based on the other supports, without compromising the salt rejection and providing high stability for the PA selective layer. © 2015 Published by Elsevier B.V.
    view abstractdoi: 10.1016/j.memsci.2015.05.064
  • 2015 • 315 Novel functionalization of porous polypropylene microfiltration membranes: Via grafted poly(aminoethyl methacrylate) anchored Schiff bases toward membrane adsorbers for metal ions
    Farjadian, F. and Schwark, S. and Ulbricht, M.
    Polymer Chemistry 6 1584-1593 (2015)
    In this work we introduce novel synthetic methods for the modification of a macroporous polypropylene (PP) membrane with poly(2-aminoethyl methacrylate) (polyAEMA) and subsequently anchoring a Schiff base with the aim of adsorbing specific metal ions from aqueous solution. The Schiff base synthesis on the surface of the PP membrane was done by a sequence of reactions. First the hydrophobic character of a commercial PP membrane (pore diameter 0.4 μm) was modified via UV irradiation-induced "grafting-from" of poly(2-hydroxyethyl methacrylate) (polyHEMA). The hydroxyl groups of polyHEMA were then reacted with the pre-synthesized photoinitiator 4-ethoxy-5-oxo-4,5-diphenylpentanoyl bromide. UV-irradiation was thereafter used for the "grafting-from" of polyAEMA. The free amino groups of this grafted comb-like brush layer on the surface were reacted with salicylaldehyde to form a Schiff base. ATR-FTIR spectroscopy was carried out to determine the functional groups' introduction and conversion. Scanning electron microscopy images showed the changes between unmodified and modified membranes. The specific surface area was determined by nitrogen adsorption and BET analysis, and the water permeability was also measured. The efficiency of membrane adsorbers with a Schiff base in the grafted layer in binding to Cu(ii) ions was determined by atomic absorption spectroscopy. Overall, the established functionalization sequences and the obtained functionality have potential for the development of efficient adsorber materials. This journal is © The Royal Society of Chemistry 2015.
    view abstractdoi: 10.1039/c4py01521e
  • 2015 • 314 Multifunctional calcium phosphate nanoparticles for combining near-infrared fluorescence imaging and photodynamic therapy
    Haedicke, K. and Kozlova, D. and Gräfe, S. and Teichgräber, U. and Epple, M. and Hilger, I.
    Acta Biomaterialia 14 197-207 (2015)
    Photodynamic therapy (PDT) of tumors causes skin photosensitivity as a result of unspecific accumulation behavior of the photosensitizers. PDT of tumors was improved by calcium phosphate nanoparticles conjugated with (i) Temoporfin as a photosensitizer, (ii) the RGDfK peptide for favored tumor targeting and (iii) the fluorescent dye molecule DY682-NHS for enabling near-infrared fluorescence (NIRF) optical imaging in vivo. The nanoparticles were characterized with regard to size, spectroscopic properties and uptake into CAL-27 cells. The nanoparticles had a hydrodynamic diameter of approximately 200 nm and a zeta potential of around +22 mV. Their biodistribution at 24 h after injection was investigated via NIRF optical imaging. After treating tumor-bearing CAL-27 mice with nanoparticle-PDT, the therapeutic efficacy was assessed by a fluorescent DY-734-annexin V probe at 2 days and 2 weeks after treatment to detect apoptosis. Additionally, the contrast agent IRDye® 800CW RGD was used to assess tumor vascularization (up to 4 weeks after PDT). After nanoparticle-PDT in mice, apoptosis in the tumor was detected after 2 days. Decreases in tumor vascularization and tumor volume were detected in the next few days. Calcium phosphate nanoparticles can be used as multifunctional tools for NIRF optical imaging, PDT and tumor targeting as they exhibited a high therapeutic efficacy, being capable of inducing apoptosis and destroying tumor vascularization. © 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actbio.2014.12.009
  • 2015 • 313 How electrophoretic deposition with ligand-free platinum nanoparticles affects contact angle
    Heinemann, A. and Koenen, S. and Schwabe, K. and Rehbock, C. and Barcikowski, S.
    Key Engineering Materials 654 218-223 (2015)
    Electrophoretic deposition of ligand-free platinum nanoparticles has been studied to elucidate how wettability, indicated by contact angle measurements, is linked to vital parameters of the electrophoretic deposition process. These parameters, namely the colloid concentration, electric field strength and deposition time, have been systematically varied in order to determine their influence on the contact angle. Additionally, scanning electron microscopy has been used to confirm the homogeneity of the achieved coatings. © (2015) Trans Tech Publications, Switzerland.
    view abstractdoi: 10.4028/www.scientific.net/KEM.654.218
  • 2015 • 312 Catalytic hydrodeoxygenation of guaiacol over platinum supported on metal oxides and zeolites
    Hellinger, M. and Carvalho, H.W.P. and Baier, S. and Wang, D. and Kleist, W. and Grunwaldt, J.-D.
    Applied Catalysis A: General 490 181-192 (2015)
    Hydrodeoxygenation of guaiacol over Pt-based catalysts was studied as a representative for phenolic compounds in pyrolysis oil. Screening of various Pt-based catalysts supported on different oxides and using different preparation methods resulted in 1%Pt/SiO2 and platinum supported on zeolites, such as 1% Pt/H-MFI-90, as the most promising catalysts in a temperature range up to 200 ° C. Thereby conversions of 86% and 100% were received, respectively. Particularly, selectivities to cyclohexane above 90% were achieved for 1% Pt/H-MFI-90. X-ray absorption near edge structure (XANES) uncovered that mild reduction temperatures were sufficient for the reduction of 1%Pt/SiO2 (up to 150°C) and 1%Pt/H-MFI-90 (up to 40°C) while 1%Pt/Al2O3 required a higher temperature of at least 320 °C. The average particle size obtained for Pt/SiO2 was 2-3 nm as unraveled by scanning transmission electron microscopy (STEM) and extended X-ray absorption fine structure (EXAFS). The deoxygenation ability of the catalysts was improved if the Pt particles were deposited on an acidic H-MFI zeolite (&gt;130 μmol acid sites per gram) as support. 1%Pt/SiO2 showed the highest selectivity towards deoxygenation at 50 °C, whereas for 1% Pt/H-MFI-90 temperatures of about 150 °C were required to achieve a high selectivity to cyclohexane. For the latter catalyst a longer reaction time was beneficial to maximize the selectivity towards cyclohexane. The hydrogen pressure did not have significant influence on the reaction rate. The results are in agreement with a hydrodeoxygenation mechanism over Pt/zeolite catalysts at temperatures up to 200 °C that comprises hydrogenation in the first step and acid catalyzed dehydration combined with hydrogenation in the second step. © 2014 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.apcata.2014.10.043
  • 2015 • 311 Combining structural and chemical information at the nanometer scale by correlative transmission electron microscopy and atom probe tomography
    Herbig, M. and Choi, P. and Raabe, D.
    Ultramicroscopy 153 32-39 (2015)
    In many cases, the three-dimensional reconstructions from atom probe tomography (APT) are not sufficiently accurate to resolve crystallographic features such as lattice planes, shear bands, stacking faults, dislocations or grain boundaries. Hence, correlative crystallographic characterization is required in addition to APT at the exact same location of the specimen. Also, for the site-specific preparation of APT tips containing regions of interest (e.g. grain boundaries) correlative electron microscopy is often inevitable. Here we present a versatile experimental setup that enables performing correlative focused ion beam milling, transmission electron microscopy (TEM), and APT under optimized characterization conditions. The setup was designed for high throughput, robustness and practicability. We demonstrate that atom probe tips can be characterized by TEM in the same way as a standard TEM sample. In particular, the use of scanning nanobeam diffraction provides valuable complementary crystallographic information when being performed on atom probe tips. This technique enables the measurement of orientation and phase maps as known from electron backscattering diffraction with a spatial resolution down to one nanometer. © 2015 Elsevier B.V.
    view abstractdoi: 10.1016/j.ultramic.2015.02.003
  • 2015 • 310 In Situ TEM Microcompression of Single and Bicrystalline Samples: Insights and Limitations
    Imrich, P.J. and Kirchlechner, C. and Kiener, D. and Dehm, G.
    JOM 67 1704-1712 (2015)
    In situ micromechanical compression experiments in a transmission electron microscope enable the study and analysis of small-scale deformation behavior. The implementation of instrumented indenter systems allows measuring the force and displacement, providing additionally insights on sample strength and flow behavior. Using focused ion beam sample preparation, single- and bicrystalline specimens can be fabricated to study the influence of individual grain boundaries on the mechanical behavior. Taperless single crystalline and bicrystalline Cu compression pillars including a coherent twin boundary were deformed in scanning and conventional transmission electron microscopy mode to study the applicability of both techniques for examining dislocation dynamics and interaction with the boundary. Based on experimental results, possibilities and limitations of such experiments are critically discussed, including sample preparation, in situ annealing to remove ion beam-induced defects, imaging of dislocations, and acquisition of stress–strain data. Finally, an outlook is given on the potential of micromechanical in situ transmission electron microscopic experiments for analyzing the influence of grain boundaries on mechanical behavior. © 2015, The Minerals, Metals & Materials Society.
    view abstractdoi: 10.1007/s11837-015-1440-6
  • 2015 • 309 Internal and external stresses: In situ TEM compression of Cu bicrystals containing a twin boundary
    Imrich, P.J. and Kirchlechner, C. and Kiener, D. and Dehm, G.
    Scripta Materialia 100 94-97 (2015)
    Uniaxial compression experiments on single- and twinned bicrystalline Cu samples using conventional and scanning in situ transmission electron microscopy reveal no increase in flow stress for the bicrystals. Dislocation curvature and dislocation source size analysis combined with indenter force measurements show agreement between local internal stresses acting on the dislocations and external stresses imposed by the indenter, indicating no stress concentrations due to the twin boundary. Furthermore, the dislocation density evolution shows stochastic variations but never a complete dislocation starvation. © 2015 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.scriptamat.2014.12.023
  • 2015 • 308 Importance of dislocation pile-ups on the mechanical properties and the Bauschinger effect in microcantilevers
    Kapp, M.W. and Kirchlechner, C. and Pippan, R. and Dehm, G.
    Journal of Materials Research 30 791-797 (2015)
    Copper microcantilevers were produced by focused ion beam milling and tested in situ using a scanning electron microscope. To provide different interfaces for piling up dislocations, cantilevers were fabricated to be single crystalline, bicrystalline, or single crystalline with a slit in the region of the neutral axis. The aim of the experiment was to study the influence of dislocation pile-ups on (i) strength and (ii) Bauschinger effects in micrometer-sized, focused ion beam milled bending cantilevers. The samples were loaded monotonically for several times under displacement control. Even though the cantilevers exhibited the same nominal strain gradient the strength varied by 34% within the three cantilever geometries. The Bauschinger effect can be promoted and prohibited by the insertion of different interfaces. © 2015 Materials Research Society.
    view abstractdoi: 10.1557/jmr.2015.49
  • 2015 • 307 MOCVD of TiO2 thin films from a modified titanium alkoxide precursor
    Kim, S.J. and Dang, V.-S. and Xu, K. and Barreca, D. and Maccato, C. and Carraro, G. and Bhakta, R.K. and Winter, M. and Becker, H.-W. and Rogalla, D. and Sada, C. and Fischer, R.A. and Devi, A.
    Physica Status Solidi (A) Applications and Materials Science 212 1563-1570 (2015)
    A new titanium precursor, [Ti(OPri)<inf>2</inf>(deacam)<inf>2</inf>] (deacam = N,N-diethylacetoacetamide), was developed by the reaction of the parent Ti alkoxide with the β-ketoamide. The compound, obtained as a monomeric six-coordinated complex, was used in metal organic chemical vapor deposition (MOCVD) of TiO<inf>2</inf> both as a single source precursor (SSP) and in the presence of oxygen. The high thermal stability of [Ti(OPri)<inf>2</inf>(deacam)<inf>2</inf>] enabled the fabrication of TiO<inf>2</inf> films over a wide temperature range, with steady growth rates between 500 and 800 °C. The microstructure of the obtained systems was analyzed by X-ray diffraction (XRD) and Raman spectroscopy, whereas atomic force microscopy (AFM) and field emission-scanning electron microscopy (FE-SEM) measurements were performed to investigate the surface morphology and nanoorganization. Film composition was investigated by complementary techniques like Rutherford backscattering spectrometry (RBS), nuclear reaction analysis (NRA), X-ray photoelectron spectroscopy (XPS), and secondary ion mass spectrometry (SIMS). The electrical properties of the layers were investigated by performing capacitance voltage (C-V) and leakage current measurements. © 2015 Wiley-VCH Verlag GmbH & Co. KGaA.
    view abstractdoi: 10.1002/pssa.201532271
  • 2015 • 306 Homogeneity and composition of AlInGaN: A multiprobe nanostructure study
    Krause, F.F. and Ahl, J.-P. and Tytko, D. and Choi, P.-P. and Egoavil, R. and Schowalter, M. and Mehrtens, T. and Müller-Caspary, K. and Verbeeck, J. and Raabe, D. and Hertkorn, J. and Engl, K. and Rosenauer, A.
    Ultramicroscopy 156 29-36 (2015)
    The electronic properties of quaternary AlInGaN devices significantly depend on the homogeneity of the alloy. The identification of compositional fluctuations or verification of random-alloy distribution is hence of grave importance. Here, a comprehensive multiprobe study of composition and compositional homogeneity is presented, investigating AlInGaN layers with indium concentrations ranging from 0 to 17. at% and aluminium concentrations between 0 and 39 at% employing high-angle annular dark field scanning electron microscopy (HAADF STEM), energy dispersive X-ray spectroscopy (EDX) and atom probe tomography (APT). EDX mappings reveal distributions of local concentrations which are in good agreement with random alloy atomic distributions. This was hence investigated with HAADF STEM by comparison with theoretical random alloy expectations using statistical tests. To validate the performance of these tests, HAADF STEM image simulations were carried out for the case of a random-alloy distribution of atoms and for the case of In-rich clusters with nanometer dimensions. The investigated samples, which were grown by metal-organic vapor phase epitaxy (MOVPE), were thereby found to be homogeneous on this nanometer scale. Analysis of reconstructions obtained from APT measurements yielded matching results. Though HAADF STEM only allows for the reduction of possible combinations of indium and aluminium concentrations to the proximity of isolines in the two-dimensional composition space. The observed ranges of composition are in good agreement with the EDX and APT results within the respective precisions. © 2015 Elsevier B.V.
    view abstractdoi: 10.1016/j.ultramic.2015.04.012
  • 2015 • 305 Quaternized polysulfone and graphene oxide nanosheet derived low fouling novel positively charged hybrid ultrafiltration membranes for protein separation
    Kumar, M. and McGlade, D. and Ulbricht, M. and Lawler, J.
    RSC Advances 5 51208-51219 (2015)
    Low fouling novel positively charged hybrid ultrafiltration membranes with adjustable charge density were fabricated from blends of polysulfone (PSf) and quaternized polysulfone (QPSf) in combination with varied fractions of graphene oxide (GO) nanosheets by a non-solvent induced phase separation method. Fourier transform infrared spectroscopy in the attenuated total reflection mode, scanning electron microscopy, outer surface zeta potential and contact angle studies were conducted to characterize the morphologies of hybrid membranes, structures, charge and surface properties. Results confirmed the fabrication of porous, hydrophilic and positively charged membranes. The water permeabilities (flux) and antifouling ability of membranes with protein solution were dependent on the fraction of quaternary ammonium groups and GO nanosheets in the membranes matrix. Antifouling ability of membranes was improved after the incorporation of GO nanosheets. In addition, irreversible protein fouling of membranes was substantially decreased with increasing fraction of GO nanosheets (%). The transmission of protein as a function of solution pH and the fraction of GO nanosheets (%) in the membranes was studied for two model proteins (bovine serum albumin; BSA or lysozyme; LYZ). The transmission of BSA or LYZ was controlled by size exclusion and the fraction of GO nanosheets in the membrane matrix. The highest transmission of proteins at their isoelectric points was obtained for membrane containing 2 wt% of GO nanosheets to total weight of polymers. © The Royal Society of Chemistry 2015.
    view abstractdoi: 10.1039/c5ra06893b
  • 2015 • 304 Characterization of dislocation structures and deformation mechanisms in as-grown and deformed directionally solidified NiAl-Mo composites
    Kwon, J. and Bowers, M.L. and Brandes, M.C. and McCreary, V. and Robertson, I.M. and Phani, P.S. and Bei, H. and Gao, Y.F. and Pharr, G.M. and George, E.P. and Mills, M.J.
    Acta Materialia 89 315-326 (2015)
    Directionally solidified (DS) NiAl-Mo eutectic composites were strained to plastic strain values ranging from 0% to 12% to investigate the origin of the previously observed stochastic versus deterministic mechanical behaviors of Mo-alloy micropillars in terms of the development of dislocation structures at different pre-strain levels. The DS composites consist of long, [1 0 0] single-crystal Mo-alloy fibers with approximately square cross-sections embedded in a [1 0 0] single-crystal NiAl matrix. Scanning transmission electron microscopy (STEM) and computational stress state analysis were conducted for the current study. STEM of the as-grown samples (without pre-straining) reveal no dislocations in the investigated Mo-alloy fibers. In the NiAl matrix, on the other hand, a〈1 0 0〉-type dislocations exist in two orthogonal orientations: along the [1 0 0] Mo fiber axis, and wrapped around the fiber axis. They presumably form to accommodate the different thermal contractions of the two phases during cool down after eutectic solidification. At intermediate pre-strain levels (4-8%), a/2〈1 1 1〉-type dislocations are present in the Mo-alloy fibers and the pre-existing dislocations in the NiAl matrix seem to be swept toward the interphase boundary. Some of the dislocations in the Mo-alloy fibers appear to be transformed from a〈1 0 0〉-type dislocations present in the NiAl matrix. Subsequently, the transformed dislocations in the fibers propagate through the NiAl matrix as a〈1 1 1〉 dislocations and aid in initiating additional slip bands in adjacent fibers. Thereafter, co-deformation presumably occurs by 〈1 1 1〉 slip in both phases. With a further increase in the pre-strain level (>10%), multiple a/2〈1 1 1〉-type dislocations are observed in many locations in the Mo-alloy fibers. Interactions between these systems upon subsequent deformation could lead to stable junctions and persistent dislocation sources. The transition from stochastic to deterministic, bulk-like behavior in sub-micron Mo-alloy pillars may therefore be related to an increasing number of multiple a〈1 1 1〉 dislocation systems within the Mo fibers with increasing pre-strain, considering that the bulk-like behavior is governed by the forest hardening of these junctions. © 2015 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2015.01.059
  • 2015 • 303 Microstructural evolution of a CoCrFeMnNi high-entropy alloy after swaging and annealing
    Laplanche, G. and Horst, O. and Otto, F. and Eggeler, G. and George, E.P.
    Journal of Alloys and Compounds 647 548-557 (2015)
    Abstract The processing parameters which govern the evolution of microstructure and texture during rotary swaging and subsequent heat treatments were studied in an equiatomic single-phase CoCrFeMnNi high-entropy alloy. After vacuum induction melting and casting, the diameter of the 40 mm cast ingot was reduced at room temperature to a final diameter of 16.5 mm by rotary swaging (diameter reduction of 60%/area reduction of 80%) and the alloy was then annealed at different temperatures for 1 h. The resulting microstructures were analyzed using scanning electron microscopy, energy-dispersive X-ray spectroscopy, electron backscatter diffraction and correlated with results of microhardness measurements. It was found that the microhardness first increases slightly upon annealing below the recrystallization temperature but then drops steeply at higher annealing temperatures due to the onset of recrystallization. Special emphasis was placed on how the microstructure evolves with respect to the radial and longitudinal position in the rod. Finally, a combination of swaging and heat treatment parameters were identified that can produce CoCrFeMnNi high-entropy alloys with a homogeneous composition and grain size and almost no texture. © 2015 Published by Elsevier B.V.
    view abstractdoi: 10.1016/j.jallcom.2015.05.129
  • 2015 • 302 Mechanisms of subgrain coarsening and its effect on the mechanical properties of carbon-supersaturated nanocrystalline hypereutectoid steel
    Li, Y.J. and Kostka, A. and Choi, P. and Goto, S. and Ponge, D. and Kirchheim, R. and Raabe, D.
    Acta Materialia 84 110-123 (2015)
    Carbon-supersaturated nanocrystalline hypereutectoid steels with a tensile strength of 6.35 GPa were produced from severely cold-drawn pearlite. The nanocrystalline material undergoes softening upon annealing at temperatures between 200 and 450°C. The ductility in terms of elongation to failure exhibits a non-monotonic dependence on temperature. Here, the microstructural mechanisms responsible for changes in the mechanical properties were studied using transmission electron microscopy (TEM), TEM-based automated scanning nanobeam diffraction and atom probe tomography (APT). TEM and APT investigations of the nanocrystalline hypereutectoid steel show subgrain coarsening upon annealing, which leads to strength reduction following a Hall-Petch law. APT analyzes of the Mn distribution near subgrain boundaries and in the cementite give strong evidence of capillary-driven subgrain coarsening occurring through subgrain boundary migration. The pronounced deterioration of ductility after annealing at temperatures above 350°C is attributed to the formation of cementite at subgrain boundaries. The overall segregation of carbon atoms at ferrite subgrain boundaries gives the nanocrystalline material excellent thermal stability upon annealing. © 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2014.10.027
  • 2015 • 301 Assembling Paramagnetic Ceruloplasmin at Electrode Surfaces Covered with Ferromagnetic Nanoparticles. Scanning Electrochemical Microscopy in the Presence of a Magnetic Field
    Matysiak, E. and Botz, A.J.R. and Clausmeyer, J. and Wagner, B. and Schuhmann, W. and Stojek, Z. and Nowicka, A.M.
    Langmuir 31 8176-8183 (2015)
    Adsorption of ceruloplasmin (Cp) at a gold electrode modified with ferromagnetic iron nanoparticles encapsulated in carbon (Fe@C Nps) leads to a successful immobilization of the enzyme in its electroactive form. The proper placement of Cp at the electrode surface on top of the nanocapsules containing an iron core allowed a preorientation of the enzyme, hence allowing direct electron transfer between the electrode and the enzyme. Laser ablation coupled with inductively coupled plasma mass spectrometry indicated that Cp was predominantly located at the paramagnetic nanoparticles. Scanning electrochemical microscopy measurements in the sample-generation/tip-collection mode proved that Cp was ferrooxidative inactive if it was immobilized on the bare gold surface and reached the highest activity if it was adsorbed on Fe@C Nps in the presence of a magnetic field. © 2015 American Chemical Society.
    view abstractdoi: 10.1021/acs.langmuir.5b01155
  • 2015 • 300 Swift Heavy Ion Induced Optical and Electronic Modifications of Graphene-TiO2 Nanocomposites
    Mishra, M. and Meinerzhagen, F. and Schleberger, M. and Kanjilal, D. and Mohanty, T.
    Journal of Physical Chemistry C 119 21270-21277 (2015)
    The effect of swift heavy ions irradiation on optical and electronic properties of chemically synthesized graphene-TiO<inf>2</inf> nanocomposites is presented. Modification of surface properties of these nanocomposites by irradiation with three different ions and with varying fluence was analyzed by Raman spectroscopy, transmission electron microscopy, and scanning Kelvin probe microscopy techniques. Raman spectra of irradiated samples exhibit systematic changes in the characteristic peaks of both graphene and TiO<inf>2</inf>. The nanocrystallite dimension calculated from Raman peak intensity decreases with fluence, indicating the occurrence of peripheral fragmentation. Furthermore, measurement of the surface contact potential difference using scanning Kelvin probe reveals that the work function of graphene-titanium dioxide nanocomposites can be effectively increased by more than 1 eV. © 2015 American Chemical Society.
    view abstractdoi: 10.1021/acs.jpcc.5b07297
  • 2015 • 299 Advancing from Rules of Thumb: Quantifying the Effects of Small Density Changes in Mass Transport to Electrodes. Understanding Natural Convection
    Ngamchuea, K. and Eloul, S. and Tschulik, K. and Compton, R.G.
    Analytical Chemistry 87 7226-7234 (2015)
    Understanding mass transport is prerequisite to all quantitative analysis of electrochemical experiments. While the contribution of diffusion is well understood, the influence of density gradient-driven natural convection on the mass transport in electrochemical systems is not. To date, it has been assumed to be relevant only for high concentrations of redox-active species and at long experimental time scales. If unjustified, this assumption risks misinterpretation of analytical data obtained from scanning electrochemical microscopy (SECM) and generator-collector experiments, as well as analytical sensors utilizing macroelectrodes/microelectrode arrays. It also affects the results expected from electrodeposition. On the basis of numerical simulation, herein it is demonstrated that even at less than 10 mM concentrations and short experimental times of tens of seconds, density gradient-driven natural convection significantly affects mass transport. This is evident from in-depth numerical simulation for the oxidation of hexacyanoferrate (II) at various electrode sizes and electrode orientations. In each case, the induced convection and its influence on the diffusion layer established near the electrode are illustrated by maps of the velocity fields and concentration distributions evolving with time. The effects of natural convection on mass transport and chronoamperometric currents are thus quantified and discussed for the different cases studied. © 2015 American Chemical Society.
    view abstractdoi: 10.1021/acs.analchem.5b01293
  • 2015 • 298 Structure of Biocompatible Coatings Produced from Hydroxyapatite Nanoparticles by Detonation Spraying
    Nosenko, V. and Strutynska, N. and Vorona, I. and Zatovsky, I. and Dzhagan, V. and Lemishko, S. and Epple, M. and Prymak, O. and Baran, N. and Ishchenko, S. and Slobodyanik, N. and Prylutskyy, Y. and Klyui, N. and Temchenko, V.
    Nanoscale Research Letters 10 1-7 (2015)
    Detonation-produced hydroxyapatite coatings were studied by scanning electron microscopy (SEM), X-ray powder diffraction (XRD), Raman spectroscopy, and electron paramagnetic resonance (EPR) spectroscopy. The source material for detonation spraying was a B-type carbonated hydroxyapatite powder. The coatings consisted of tetracalcium phosphate and apatite. The ratio depended slightly on the degree of crystallinity of the initial powder and processing parameters of the coating preparation. The tetracalcium phosphate phase was homogeneous; the apatite phase contained defects localized on the sixfold axis and consisted of hydroxyapatite and oxyapatite. Technological factors contributing to the transformation of hydroxyapatite powder structure during coating formation by detonation spraying are discussed. © 2015, Nosenko et al.
    view abstractdoi: 10.1186/s11671-015-1160-4
  • 2015 • 297 Diamond single micro-crystals and graphitic micro-balls' formation in plasmoids under atmospheric pressure
    Pothiraja, R. and Kartaschew, K. and Bibinov, N. and Havenith, M. and Awakowicz, P.
    Journal of Physics D: Applied Physics 48 (2015)
    Plasmoids are produced in the argon filamentary discharge. By going through hydrocarbon gas, the plasmoids collect carbon material. These plasmoids produce diamond single micro-crystals upon contact on the inner surface of cavity in air atmosphere. When the plasmoid's contact point on the substrate is in inert atmosphere, they deposit their material as micro-balls with a graphite core. The dimension and nature of the micro-materials deposited by the plasmoids are analysed using scanning electron microscopy and Raman microspectroscopy. The compressive residual stress in the deposited micro-diamonds varies in the range-7 to-21 GPa. © 2015 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0022-3727/48/11/115201
  • 2015 • 296 Bridging the gap between insightful simplicity and successful complexity: From fundamental studies on model systems to technical catalysts
    Prieto, G. and Schüth, F.
    Journal of Catalysis 328 59-71 (2015)
    When Haldor Topsøe founded his company in 1940, the application of solid catalysts in industrial chemical processes was still in its early phase. At that time, catalyst development and optimization strongly relied on phenomenological approaches and experimental know-how, whereas little knowledge existed on the nature of the catalytically active species and how to tune their structure and concentration. For more than 70 years, Topsøe has advocated the need of "bringing more scientific understanding to the field of catalysis," becoming a prominent figure in the transition of catalyst preparation - a word with an alchemical connotation - to catalyst synthesis, based on scientific principles. Numerous fundamental studies of his team and collaborators on simplified model catalysts have added substantially to the current understanding of a significant number of industrially relevant systems in particular, and the principles of action of solid catalysts in general. This article reviews some key advancements that the Topsøe team has contributed to the field of catalyst development, rooted in fundamental studies with either 2D or 3D model materials. Examples are provided of how the acquired scientific knowledge was successfully translated into innovations in the manufacture of technical catalysts. Next to the work of the Topsøe group, a broader and updated perspective of the use of model systems to investigate fundamental aspects of catalyst development is presented. A number of selected case studies are reviewed, which we find illustrative of recent findings with implications for the design and synthesis of solid catalysts. © 2015 Elsevier Inc. All rights reserved.
    view abstractdoi: 10.1016/j.jcat.2014.12.009
  • 2015 • 295 A continuous method to prepare poorly crystalline silver-doped calcium phosphate ceramics with antibacterial properties
    Range, S. and Hagmeyer, D. and Rotan, O. and Sokolova, V. and Verheyen, J. and Siebers, B. and Epple, M.
    RSC Advances 5 43172-43177 (2015)
    Silver-doped calcium phosphate ceramics were prepared in discontinuous and continuous processes with different amounts of incorporated silver (up to 1.8 wt% silver). In particular, the effects of pH, reaction time and light exposure on the incorporation of silver into the calcium phosphate ceramic were investigated. In the dark, silver can be incorporated as colourless silver ions (Ag+) into the apatite lattice, but the integration occurs slowly. Under ambient light, a rapid photoreduction to elemental silver (Ag0) occurs which permits a continuous process to prepare silver-doped calcium phosphate ceramics. The silver-doped calcium phosphate ceramics were characterized by scanning electron microscopy, X-ray powder diffraction, infrared spectroscopy, thermogravimetry, and elemental analysis (Ca, Ag, phosphate). The silver release from the silver-doped calcium phosphate ceramics was measured by a combination of dialysis and atomic absorption spectroscopy. The antimicrobial effect was tested on bacteria (Escherichia coli), and the cytotoxic effect was tested on HeLa cells (human epithelial cervical cancer cells). For comparison, stoichiometric silver phosphate, Ag<inf>3</inf>PO<inf>4</inf>, was prepared. The release of silver from silver phosphate is much faster, leading to pronounced antibacterial but also cytotoxic effects. © The Royal Society of Chemistry 2015.
    view abstractdoi: 10.1039/c5ra00401b
  • 2015 • 294 Post-polymerization of urease-induced calcified, polymer hydrogels
    Rauner, N. and Buenger, L. and Schuller, S. and Tiller, J.C.
    Macromolecular Rapid Communications 36 224-230 (2015)
    Urease-induced calcification is an innovative method to artificially produce highly filled CaCO3-based composite materials by intrinsic mineralization of hydrogels. The mechanical properties of these hybrid materials based on poly(2-hydroxyethylacrylate) cross-linked by triethylene glycol dimethacrylate are poor. Increasing the degree of calcification to up to 94 wt% improves the Young's moduli (YM) of the materials from some 40 MPa to more than 300 MPa. The introduction of calcium carbonate affine groups to the hydrogel matrix by copolymerizing acrylic acid and [2-(methacryloyloxy) ethyl]trimethylammonium chloride, respectively, does not increase the stiffness of the composites. A Young's modulus of more than 1 GPa is achieved by post-polymerization (PP) of the calcified hydrogels, which proves that the size of the contact area between the matrix and calcium carbonate crystals is the most crucial parameter for controlling the stiffness of hybrid materials. Switching from low Tg to high Tg hydrogel matrices (based on poly(N,N-dimethyl acrylamide)) results in a YM of up to 3.5 GPa after PP. (Chemical Equation Presented). © 2014 Wiley-VCH Verlag GmbH & Co. KGaA.
    view abstractdoi: 10.1002/marc.201400426
  • 2015 • 293 Nanostructure of wet-chemically prepared, polymer-stabilized silver-gold nanoalloys (6 nm) over the entire composition range
    Ristig, S. and Prymak, O. and Loza, K. and Gocyla, M. and Meyer-Zaika, W. and Heggen, M. and Raabe, D. and Epple, M.
    Journal of Materials Chemistry B 3 4654-4662 (2015)
    Bimetallic silver-gold nanoparticles were prepared by co-reduction using citrate and tannic acid in aqueous solution and colloidally stabilized with poly(N-vinylpyrrolidone) (PVP). The full composition range of silver:gold from 0:100 to 100:0 (n:n) was prepared with steps of 10 mol%. The nanoparticles were spherical, monodispersed, and had a diameter of ∼6 nm, except for Ag:Au 90:10 nanoparticles and pure Ag nanoparticles which were slightly larger. The size of the nanoalloys was determined by differential centrifugal sedimentation (DCS) and transmission electron microscopy (TEM). By means of X-ray powder diffraction (XRD) together with Rietveld refinement, precise lattice parameters, crystallite size and microstrain were determined. Scanning transmission electron microscopy (STEM) combined with energy-dispersive X-ray spectroscopy (EDX) and electron energy loss spectroscopy (EELS) showed that the particles consisted of a gold-rich core and a silver-rich shell. XRD and DCS indicated that the nanoparticles were not twinned, except for pure Ag and Ag:Au 90:10, although different domains were visible in the TEM. A remarkable negative deviation from Vegard's linear rule of alloy mixtures was observed (isotropic contraction of the cubic unit cell with a minimum at a 50:50 composition). This effect was also found for Ag:Au bulk alloys, but it was much more pronounced for the nanoalloys. Notably, it was much less pronounced for pure silver and gold nanoparticles. The microstrain was increased along with the contraction of the unit cell with a broad maximum at a 50:50 composition. The synthesis is based on aqueous solvents and can be easily scaled up to a yield of several mg of a well dispersed nanoalloy with application potential due to its tuneable antibacterial action (silver) and its optical properties for bioimaging. © The Royal Society of Chemistry 2015.
    view abstractdoi: 10.1039/c5tb00644a
  • 2015 • 292 Microstructural evolution in a Ti-Ta hightemperature shape memory alloy during creep
    Rynko, R. and Marquardt, A. and Paulsen, A. and Frenzel, J. and Somsen, C. and Eggeler, G.
    International Journal of Materials Research 106 331-341 (2015)
    Alloys based on the titanium-tantalum system are considered for application as high-temperature shape memory alloys due to their martensite start temperatures, which can surpass 200 °C. In the present work we study the evolution of microstructure and the influence of creep on the phase transformation behavior of a Ti70Ta30 (at.%) high-temperature shape memory alloy. Creep tests were performed in a temperature range from 470 to 530 °C at stresses between 90 and 150 MPa. The activation energy for creep was found to be 307 kJ mol-1 and the stress exponent n was determined as 3.7. Scanning and transmission electron microscopy investigations were carried out to characterize the microstructure before and after creep. It was found that the microstructural evolution during creep suppresses subsequent martensitic phase transformations. © Carl Hanser Verlag GmbH & Co. KG.
    view abstractdoi: 10.3139/146.111189
  • 2015 • 291 Resonant photothermal laser processing of hybrid gold/titania nanoparticle films
    Schade, L. and Franzka, S. and Dzialkowski, K. and Hardt, S. and Wiggers, H. and Reichenberger, S. and Wagener, P. and Hartmann, N.
    Applied Surface Science 336 48-52 (2015)
    Photothermal processing of thin anatase TiO2 and hybrid Au/anatase TiO2 nanoparticle films on glass supports is investigated using continuous-wave microfocused lasers at λ = 355 nm and λ = 532 nm. UV/Vis spectroscopy, Raman spectroscopy, optical microscopy, atomic force microscopy and scanning electron microscopy are used for characterization. Processing of TiO2 nanoparticle films is feasible at λ = 355 nm only. In contrast, the addition of Au nanoparticles enhances the overall absorbance of the material in the visible range and enables processing at both wavelengths, i.e. at λ = 355 nm and λ = 532 nm. Generally, laser heating induces a transition from anatase to rutile. The modification degree increases with increasing laser power and laser irradiation time. Resonant laser processing of hybrid Au/TiO2-mesoporous films provide promising perspectives in various applications, e.g. in photovoltaics, where embedded nanoparticulate Au could be exploited to enhance light trapping. © 2014 Published by Elsevier B.V.
    view abstractdoi: 10.1016/j.apsusc.2014.09.118
  • 2015 • 290 Systematic study on the influence of the morphology of α-MoO3 in the selective oxidation of propylene
    Schuh, K. and Kleist, W. and Høj, M. and Jensen, A.D. and Beato, P. and Patzke, G.R. and Grunwaldt, J.-D.
    Journal of Solid State Chemistry 228 42-52 (2015)
    Abstract A variety of morphologically different α-MoO<inf>3</inf> samples were prepared by hydrothermal synthesis and applied in the selective oxidation of propylene. Their catalytic performance was compared to α-MoO<inf>3</inf> prepared by flame spray pyrolysis (FSP) and a classical synthesis route. Hydrothermal synthesis from ammonium heptamolybdate (AHM) and nitric acid at pH 1-2 led to ammonium containing molybdenum oxide phases that were completely transformed into α-MoO<inf>3</inf> after calcination at 550 °C. A one-step synthesis of α-MoO<inf>3</inf> rods was possible starting from MoO<inf>3</inf>·2H<inf>2</inf>O with acetic acid or nitric acid and from AHM with nitric acid at 180°C. Particularly, if nitric acid was used during synthesis, the rod-like morphology of the samples could be stabilized during calcination at 550°C and the following catalytic activity tests, which was beneficial for the catalytic performance in propylene oxidation. Characterization studies using X-ray diffraction (XRD), scanning electron microscopy (SEM) and Raman spectroscopy showed that those samples, which retained their rod-like morphology during the activity tests, yielded the highest propylene conversion. © 2015 Elsevier Inc. All rights reserved.
    view abstractdoi: 10.1016/j.jssc.2015.04.011
  • 2015 • 289 Effects of retained austenite volume fraction, morphology, and carbon content on strength and ductility of nanostructured TRIP-assisted steels
    Shen, Y.F. and Qiu, L.N. and Sun, X. and Zuo, L. and Liaw, P.K. and Raabe, D.
    Materials Science and Engineering A 636 551-564 (2015)
    With a suite of multi-modal and multi-scale characterization techniques, the present study unambiguously proves that a substantially-improved combination of ultrahigh strength and good ductility can be achieved by tailoring the volume fraction, morphology, and carbon content of the retained austenite (RA) in a transformation-induced-plasticity (TRIP) steel with the nominal chemical composition of 0.19C-0.30Si-1.76Mn-1.52Al (weight percent, wt%). After intercritical annealing and bainitic holding, a combination of ultimate tensile strength (UTS) of 1100. MPa and true strain of 50% has been obtained, as a result of the ultrafine RA lamellae, which are alternately arranged in the bainitic ferrite around junction regions of ferrite grains. For reference, specimens with a blocky RA, prepared without the bainitic holding, yield a low ductility (35%) and a low UTS (800. MPa). The volume fraction, morphology, and carbon content of RA have been characterized using various techniques, including the magnetic probing, scanning electron microscopy (SEM), electron-backscatter-diffraction (EBSD), and transmission electron microscopy (TEM). Interrupted tensile tests, mapped using EBSD in conjunction with the kernel average misorientation (KAM) analysis, reveal that the lamellar RA is the governing microstructure component responsible for the higher mechanical stability, compared to the blocky one. By coupling these various techniques, we quantitatively demonstrate that in addition to the RA volume fraction, its morphology and carbon content are equally important in optimizing the strength and ductility of TRIP-assisted steels. © 2015 Elsevier B.V.
    view abstractdoi: 10.1016/j.msea.2015.04.030
  • 2015 • 288 Influence of process-induced microstructure and imperfections on mechanical properties of AlSi12 processed by selective laser melting
    Siddique, S. and Imran, M. and Wycisk, E. and Emmelmann, C. and Walther, F.
    Journal of Materials Processing Technology 221 205-213 (2015)
    Selective laser melting (SLM) offers high potential for manufacturing complex geometries and custom-made parts due to its unique layer-wise production process. A series of samples of AlSi12 have been manufactured by SLM process to study the effect of process parameters and post-build heat treatment on the microstructure and the corresponding mechanical properties. Optical microscope, scanning electron microscope, quasistatic tests, continuous load increase fatigue tests and constant amplitude fatigue tests have been employed for characterization. A remarkable eutectic microstructure, with dendritic width changing with SLM process parameters, has been observed. Relationship between SLM process parameters, resulting microstructure and the consequent changes in mechanical properties has been discussed. Base plate heating has been found critical in controlling the in-process microstructure. Mechanical properties of SLM parts outperform those of conventionally manufactured alloy, and can be varied as per requirement, by altering the build rate, keeping the process costs in control. Fatigue scatter can also be controlled by heating the base plate during the process. © 2015 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.jmatprotec.2015.02.023
  • 2015 • 287 Doping of inorganic materials in microreactors-preparation of Zn doped Fe3O4 nanoparticles
    Simmons, M.D. and Jones, N. and Evans, D.J. and Wiles, C. and Watts, P. and Salamon, S. and Escobar Castillo, M. and Wende, H. and Lupascu, D.C. and Francesconi, M.G.
    Lab on a Chip - Miniaturisation for Chemistry and Biology 15 3154-3162 (2015)
    Microreactor systems are now used more and more for the continuous production of metal nanoparticles and metal oxide nanoparticles owing to the controllability of the particle size, an important property in many applications. Here, for the first time, we used microreactors to prepare metal oxide nanoparticles with controlled and varying metal stoichiometry. We prepared and characterised Zn-substituted Fe3O4 nanoparticles with linear increase of Zn content (ZnxFe3-xO4 with 0 ≤ x ≤ 0.48), which causes linear increases in properties such as the saturation magnetization, relative to pure Fe3O4. The methodology is simple and low cost and has great potential to be adapted to the targeted doping of a vast array of other inorganic materials, allowing greater control on the chemical stoichiometry for nanoparticles prepared in microreactors. © 2015 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c5lc00287g
  • 2015 • 286 Conductivity Mechanisms in Sb-Doped SnO2 Nanoparticle Assemblies: DC and Terahertz Regime
    Skoromets, V. and Němec, H. and Kopeček, J. and Kužel, P. and Peters, K. and Fattakhova-Rohlfing, D. and Vetushka, A. and Müller, M. and Ganzerová, K. and Fejfar, A.
    Journal of Physical Chemistry C 119 19485-19495 (2015)
    Assemblies of undoped and antimony-doped tin oxide nanoparticles synthesized via a nonaqueous sol-gel procedure, pressed into pellets, and annealed under various conditions were investigated using time-domain terahertz spectroscopy, scanning electron microscopy, atomic force microscopy, and dc conductivity measurements. Combination of these methods made it possible to resolve the conductivity limitations imposed by intrinsic properties of the material and by the morphology of the samples. Percolation of the nanoparticles was confirmed in all samples. The undoped samples exhibit a weak hopping conductivity, whereas bandlike conduction of charges partially confined in the nanoparticles dominates in the doped samples. The conductivity of nanoparticles and their connectivity can be greatly controlled during the sample preparation, namely by the calcination temperature and by the order of technological steps. A substantial increase of the conductivity inside nanoparticles and of the charge transport between them is achieved upon calcination at 500 °C. © 2015 American Chemical Society.
    view abstractdoi: 10.1021/acs.jpcc.5b05091
  • 2015 • 285 Reversible or Not? Distinguishing Agglomeration and Aggregation at the Nanoscale
    Sokolov, S.V. and Tschulik, K. and Batchelor-McAuley, C. and Jurkschat, K. and Compton, R.G.
    Analytical Chemistry 87 10033-10039 (2015)
    Nanoparticles are prone to clustering either via aggregation (irreversible) or agglomeration (reversible) processes. It is exceedingly difficult to distinguish the two via conventional techniques such as dynamic light scattering (DLS), nanoparticle tracking analysis (NTA), or electron microscopy imaging (scanning electron microscopy (SEM), transmission electron microscopy (TEM)) as such techniques only generally confirm the presence of large particle clusters. Herein we develop a joint approach to tackle the issue of distinguishing between nanoparticle aggregation vs agglomeration by characterizing a colloidal system of Ag NPs using DLS, NTA, SEM imaging and the electrochemical nanoimpacts technique. In contrast to the conventional techniques which all reveal the presence of large clusters of particles, electrochemical nanoimpacts provide information regarding individual nanoparticles in the solution phase and reveal the presence of small nanoparticles (<30 nm) even in high ionic strength (above 0.5 M KCl) and allow a more complete analysis. The detection of small nanoparticles in high ionic strength media evidence the clustering to be a reversible process. As a result it is concluded that agglomeration rather than irreversible aggregation takes place. This observation is of general importance for all colloids as it provides a feasible analysis technique for a wide range of systems with an ability to distinguish subtly different processes. © 2015 American Chemical Society.
    view abstractdoi: 10.1021/acs.analchem.5b02639
  • 2015 • 284 Reciprocating sliding wear of case-hardened spheroidal cast iron against 100Cr6 under boundary lubrication
    Stickel, D. and Goeke, S. and Geenen, K. and Huth, S. and Theisen, W. and Biermann, D. and Fischer, A.
    Proceedings of the Institution of Mechanical Engineers, Part J: Journal of Engineering Tribology 229 1214-1226 (2015)
    Today cast iron with spheroidal graphite is used in a wide range of applications with a high production capacity per year. Due to optimized and well-controlled casting technology, the production of ductile cast iron became economic in such way that ductile cast iron replaced cast or wrought steel in many machinery components like crankshafts, piston rods, and engine mounts. These examples represent technical tribosystems of the automobile industry. Here, current political, economic, and ecological guidelines also demand downsizing combined with high power densities in order to minimize internal friction and reduce fuel consumption and satisfying CO2-emission limits. These guidelines can change the tribological loads and, therefore, result in more severe conditions. One example is the shift of the lubrication regime from hydrodynamic to mixed or boundary lubrication for larger periods of time. In these regimes, the applied load is partially or fully carried by the asperities. Still the need for maintaining as low as possible wear towards the ultra-mild sliding wear regime an integral approach is needed, which has to regard contact conditions, surface topography, interface chemistry, and sub-surface properties. One way to low wear can aim at lowering the run-in phase by e.g. optimizing the topography by means of adjusted machining processes. For this study, reciprocating sliding wear tests were conducted with grinded, milled, polished, and finished samples of case-hardened spheroidal cast iron slid against a 100Cr6 ball of a 5mm radius. The boundary lubrication was provided by a commercial combustion engine lubricant at 80°C. After predefined test cycles, 3D surface topographies were measured by means of confocal white-light microscopy within each wear test in order to analyse the development of the contact conditions over time. In combination with the measured forces and displacements, the tribological loads are calculated by means of a 3D elastic-ideal plastic contact model. Additionally the wear mechanism was analyzed by means of scanning electron microscopy. The overall wear rates and the coefficients of friction depend strongly on the initial surface topography and, therefore, on the machining process. This is also true for the development of a reaction layer (tribomaterial) allowing for ultra-mild siding wear even under boundary lubrication. © IMechE 2015.
    view abstractdoi: 10.1177/1350650115576245
  • 2015 • 283 Complex Nanotwin Substructure of an Asymmetric Σ9 Tilt Grain Boundary in a Silicon Polycrystal
    Stoffers, A. and Ziebarth, B. and Barthel, J. and Cojocaru-Mirédin, O. and Elsässer, C. and Raabe, D.
    Physical Review Letters 115 (2015)
    Grain boundaries in materials have substantial influences on device properties, for instance on mechanical stability or electronic minority carrier lifetime in multicrystalline silicon solar cells. This applies especially to asymmetric, less ordered or faceted interface portions. Here, we present the complex atomic interface structure of an asymmetric Σ9 tilt grain boundary in silicon, observed by high resolution scanning transmission electron microscopy (HR-STEM) and explained by atomistic modeling and computer simulation. Structural optimization of interface models for the asymmetric Σ9 and related symmetrical Σ9 and Σ3 tilt grain boundaries, by means of molecular-statics simulations with empirical silicon potentials in combination with first-principles calculations, results in a faceted asymmetric interface structure, whose grain-boundary energy is so low that it is likely to exist. The simulated local atomic structures match the observed HR-STEM images very well. © 2015 American Physical Society.
    view abstractdoi: 10.1103/PhysRevLett.115.235502
  • 2015 • 282 Microstructure design and mechanical properties in a near-α Ti-4Mo alloy
    Tarzimoghadam, Z. and Sandlöbes, S. and Pradeep, K.G. and Raabe, D.
    Acta Materialia 97 291-304 (2015)
    Abstract We study the effects of different heat treatment routes on microstructure engineering and the resulting mechanical response in a plain binary Ti-4Mo (wt%) model alloy. We observe a broad variety of microstructure formation mechanisms including diffusion driven allotropic phase transformations as well as shear and/or diffusion dominated modes of martensitic transformations, enabling a wealth of effective microstructure design options even in such a simple binary Ti alloy. This wide variety of microstructures allows tailoring the mechanical properties ranging from low yield strength (350 MPa) and high ductility (30-35% tensile elongation) to very high yield strength (1100 MPa) and medium ductility (10-15% tensile elongation) as well as a variety of intermediate states. Mechanical testing and microstructure characterization using optical microscopy, scanning electron microscopy based techniques, transmission electron microscopy and atom probe tomography were performed revealing that minor variations in the heat treatment cause significant changes in the resulting microstructures (e.g. structural refinement, transition between diffusive and martensitic transformations). The experimental results on microstructure evolution during the applied different heat treatment routes are discussed with respect to the mechanical properties. © 2015 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2015.06.043
  • 2015 • 281 Tribological performance of near equiatomic and Ti-rich NiTi shape memory alloy thin films
    Tillmann, W. and Momeni, S.
    Acta Materialia 92 189-196 (2015)
    Near equiatomic and Ti-rich NiTi shape memory alloy thin films were magnetron sputtered with the same processing parameters and thickness of 3 μm. The microstructure, composition, shape memory behavior, mechanical and tribological properties of the deposited thin films were analyzed by using X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), differential scanning calorimetry (DSC), nanoindentation, ball-on-disc, scratch test, and three dimensional (3D) optical microscopy. The obtained results clearly show how the crystallization evolution and precipitation formation of these two sets of thin films can drastically influence their mechanical and tribological performances. © 2015 Acta Materialia Inc. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2015.04.006
  • 2015 • 280 Comparison of NiTi thin films sputtered from separate elemental targets and Ti-rich alloy targets
    Tillmann, W. and Momeni, S.
    Journal of Materials Processing Technology 220 184-190 (2015)
    The kind of sputtering targets can adversely affect the microstructure, phase transformation behavior, mechanical and tribological properties of near equi-atomic NiTi thin films. This new finding was systematically investigated by employing comprehensive characterization and analysis techniques, including X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), differential scanning calorimetry (DSC), nanoindentation, ball-on-disc, and three dimensional (3D) optical microscopy. © 2015 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.jmatprotec.2015.01.014
  • 2015 • 279 In-situ annealing of NiTi thin films at different temperatures
    Tillmann, W. and Momeni, S.
    Sensors and Actuators, A: Physical 221 9-14 (2015)
    Magnetron sputtered NiTi thin films are usually sputtered at ambient temperature and need a post-annealing treatment to promote crystallization and obtain shape memory effect. However, this treatment could adversely affect the microstructure as well as the morphology of the film. Within this study, NiTi thin films were generated by annealing during the sputtering process. The effect of the sputtering temperature on the morphology of the film, the composition, and shape memory behavior was studied using X-ray diffraction (XRD), energy-dispersive X-ray spectroscopy (EDX), field emission scanning electron microscopy (FESEM), and differential scanning calorimetry (DSC). © 2014 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.sna.2014.10.034
  • 2015 • 278 Influence of in-situ and postannealing technique on tribological performance of NiTi SMA thin films
    Tillmann, W. and Momeni, S.
    Surface and Coatings Technology 276 286-295 (2015)
    Magnetron sputtered NiTi thin films were crystallized through two convenient techniques: (i) postannealing and (ii) in-situ annealing during the deposition. The annealing parameters (temperature and time) were kept constant by employing each technique. The microstructure, morphology, phase transformation behavior, mechanical and tribological properties of these thin films were investigated using X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), 4-point probe resistivity measurement, nanoindentation test, pin-on-disc, scratch test and three dimensional (3D) optical microscopy. The results show how postannealing and in-situ annealing techniques can differently affect properties of NiTi thin films in spite of employing similar annealing temperature and time. © 2015 Elsevier B.V..
    view abstractdoi: 10.1016/j.surfcoat.2015.07.012
  • 2015 • 277 Mechanical and microstructural analysis of ultrasonically assisted induction-brazed TiAl6V4 joints
    Tillmann, W. and Zimpel, M. and Dias, N.F.L. and Pfeiffer, J. and Wojarski, L. and Xu, Z.
    Welding in the World 59 901-909 (2015)
    This paper focuses on the process of ultrasonically assisted induction brazing with regard to titanium brazing. The titanium alloy TiAl6V4 was brazed using an aluminum-based filler alloy (AlMg2.5Cr0.3). It was apparent that the layer thickness of the brazing foil as well as the brazing temperature and the intensity of the ultrasound are significant influencing factors of the combined brazing process and microstructure. It is the aim of this paper to draw conclusions from the microstructural and mechanical investigations of the brazed joint about the process parameters, which are crucial for the properties and quality of the joint. The evaluation of the microstructure of the joint was conducted by means of metallographic investigations and results obtained by means of scanning electron microscopy. Besides mechanical microhardness measurements, strength investigations were conducted in order to evaluate the quality of the joint. Furthermore, the results of conventional vacuum brazing processes were correlated in order to be able to better facilitate and understand the adapted induction brazing process. © 2015, International Institute of Welding.
    view abstractdoi: 10.1007/s40194-015-0260-1
  • 2015 • 276 Charging effect reduction in electron beam lithography and observation of single nanopillars on highly insulating substrates
    Tirpanci, Ş. and Bürgler, D.E. and Schneider, C.M. and Rameev, B. and Aktaş, B.
    Microelectronic Engineering 140 33-37 (2015)
    Electron beam writing and imaging of nanoscale structures on highly insulating substrates severely suffer from charging effects, which cause reduction in pattern resolution, positioning precision, and imaging quality. Conductive layers deposited above or below the resist layer can effectively reduce charge accumulation, but often give rise to contamination impairing the physical and chemical properties of functional nanostructures. Here we deal with top and bottom contacted, sub-micron-sized nanopillars made from multilayer stacks comprising ferromagnetic and non-magnetic materials for the study of current-induced magnetization dynamics. We show how the charging effects in a previously established fabrication process for single-crystalline nanopillars by H. Dassow et al. (2006) [1] can be significantly reduced by using the bottom electrode layer as charge dissipater and only isolating and disconnecting the bottom electrodes from ground after the fabrication of the delicate nanopillar structure by electron beam lithography. The modified process is successfully applied to Co<inf>2</inf>MnSi/Ag/Co<inf>2</inf>MnSi(001) multilayer stacks grown on highly insulating MgO substrates. Ellipsoidal nanopillars with a cross-section of 75 × 120 nm2 reveal 2% giant magnetoresistance and angular dependent magnetization behavior due to the magnetic anisotropy of the elliptical nanomagnets. © 2015 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.mee.2015.05.007
  • 2015 • 275 Following crack path selection in multifilm structures with weak and strong interfaces by in situ 4-point-bending
    Völker, B. and Venkatesan, S. and Heinz, W. and Matoy, K. and Roth, R. and Batke, J.-M. and Cordill, M.J. and Dehm, G.
    Journal of Materials Research 30 1090-1097 (2015)
    In this study, the interfacial adhesion of Cu and TiN on an annealed borophosphosilicate glass (BPSG) in a multilayer material stack was investigated. The two material systems, Cu/BPSG and TiN/BPSG, are representatives for weak and strong interfaces, respectively. A weak and a strong interface was chosen to identify possible differences in the fracture path selection for the multilayer material systems. To investigate this, in situ 4-point-bending experiments were performed under an optical microscope and in a scanning electron microscope. Complementary ex situ 4-point-bending experiments were carried out on the identical material systems. These tests revealed that for the two analyzed systems there is a large discrepancy in the success rate of failure along the interface of interest, which is a prerequisite for determining the corresponding interface energy release rate. This phenomenon can be understood by using theoretical findings of earlier studies reported in the literature, which are in agreement with the experimental outcome of the in situ 4-point-bending measurements presented here. © 2015 Materials Research Society.
    view abstractdoi: 10.1557/jmr.2015.88
  • 2015 • 274 Nanolaminate transformation-induced plasticity-twinning-induced plasticity steel with dynamic strain partitioning and enhanced damage resistance
    Wang, M.-M. and Tasan, C.C. and Ponge, D. and Dippel, A.-Ch. and Raabe, D.
    Acta Materialia 85 216-228 (2015)
    Conventional martensitic steels have limited ductility due to insufficient microstructural strain-hardening and damage resistance mechanisms. It was recently demonstrated that the ductility and toughness of martensitic steels can be improved without sacrificing the strength, via partial reversion of the martensite back to austenite. These improvements were attributed to the presence of the transformation-induced plasticity (TRIP) effect of the austenite phase, and the precipitation hardening (maraging) effect in the martensitic matrix. However, a full micromechanical understanding of this ductilizing effect requires a systematic investigation of the interplay between the two phases, with regards to the underlying deformation and damage micromechanisms. For this purpose, in this work, a Fe-9Mn-3Ni-1.4Al-0.01C (mass%) medium-Mn TRIP maraging steel is produced and heat-treated under different reversion conditions to introduce well-controlled variations in the austenite-martensite nanolaminate microstructure. Uniaxial tension and impact tests are carried out and the microstructure is characterized using scanning and transmission electron microscopy based techniques and post mortem synchrotron X-ray diffraction analysis. The results reveal that (i) the strain partitioning between austenite and martensite is governed by a highly dynamical interplay of dislocation slip, deformation-induced phase transformation (i.e. causing the TRIP effect) and mechanical twinning (i.e. causing the twinning-induced plasticity effect); and (ii) the nanolaminate microstructure morphology leads to enhanced damage resistance. The presence of both effects results in enhanced strain-hardening capacity and damage resistance, and hence the enhanced ductility. © 2014 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2014.11.010
  • 2015 • 273 Chemical vapor deposition of Si/SiC nano-multilayer thin films
    Weber, A. and Remfort, R. and Wöhrl, N. and Assenmacher, W. and Schulz, S.
    Thin Solid Films 593 44-52 (2015)
    Stoichiometric SiC films were deposited with the commercially available single source precursor Et3SiH by classical thermal chemical vapor deposition (CVD) as well as plasma-enhanced CVD at low temperatures in the absence of any other reactive gases. Temperature-variable deposition studies revealed that polycrystalline films containing different SiC polytypes with a Si to carbon ratio of close to 1:1 are formed at 1000°C in thermal CVD process and below 100°C in the plasma-enhanced CVD process. The plasma enhanced CVD process enables the reduction of residual stress in the deposited films and offers the deposition on temperature sensitive substrates in the future. In both deposition processes the film thickness can be controlled by variation of the process parameters such as the substrate temperature and the deposition time. The resulting material films were characterized with respect to their chemical composition and their crystallinity using scanning electron microscope, energy dispersive X-ray spectroscopy (XRD), atomic force microscopy, X-ray diffraction, grazing incidence X-ray diffraction, secondary ion mass spectrometry and Raman spectroscopy. Finally, Si/SiC multilayers of up to 10 individual layers of equal thickness (about 450 nm) were deposited at 1000°C using Et3SiH and SiH4. The resulting multilayers features amorphous SiC films alternating with Si films, which feature larger crystals up to 300 nm size as measured by transmission electron microscopy as well as by XRD. XRD features three distinct peaks for Si(111), Si(220) and Si(311). © 2015 Published by Elsevier B.V.
    view abstractdoi: 10.1016/j.tsf.2015.08.042
  • 2015 • 272 Comprehensive investigation of phase transformation during diffusion alloying of Nb-rich powder metallurgical tool steels
    Weddeling, A. and Krell, J. and Huth, S. and Theisen, W.
    Powder Metallurgy 58 142-151 (2015)
    Stainless tool steels highly alloyed in niobium can be produced by powder metallurgy using diffusion alloying. Steel powder atomised without carbon is subsequently mixed with graphite and hot isostatically pressed. The atomised powder contains the intermetallic Laves phase NbFe<inf>2</inf>that transforms into MC-type carbides during HIP when graphite has been added. The obtained structure features a fine distribution of carbides to increase wear resistance and chromium fully dissolved in the matrix to provide corrosion resistance. X-ray diffraction (XRD) measurements and reflection position analysis with additional scanning electron microscopy (SEM) have been conducted to study the phase transition of NbFe<inf>2</inf>Laves phase into NbC carbides in two high Nb alloyed stainless tool steels. The results show that carburisation starts at 1000-1050°C and also confirm the correlation between oxide reduction and carburisation. The formed carbides are distinctly understoichiometric, which leads to an overestimation of the suitable quantitiy of added carbon in the thermodynamic calculations. © 2015 Institute of Materials, Minerals and Mining Published by Maney on behalf of the Institute.
    view abstractdoi: 10.1179/1743290115Y.0000000001
  • 2015 • 271 Nitrogen uptake of nickel free austenitic stainless steel powder during heat treatment-an XPS study
    Weddeling, A. and Lefor, K. and Hryha, E. and Huth, S. and Nyborg, L. and Weber, S. and Theisen, W.
    Surface and Interface Analysis 47 413-422 (2015)
    In austenitic stainless steel nitrogen stabilizes the austenitic phase improves the mechanical properties and increases the corrosion resistance. Nitrogen alloying enables to produce austenitic steels without the element nickel which is high priced and classified as allergy inducing. A novel production route is nitrogen alloying of CrMn-prealloyed steel powder via the gas phase. This is beneficial as the nitrogen content can be adjusted above the amount that is reached during conventional casting. A problem which has to be overcome is the oxide layer present on the powder surface which impedes both the sintering process and the uptake of nitrogen. This study focuses on whether heat treatment under pure nitrogen is an appropriate procedure to enable sintering and nitrogen uptake by reduction of surface oxides. X-ray photoelectron spectroscopy (XPS) in combination with scanning electron microscopy (SEM) and energy dispersive X-ray spectrometry (EDS) are used to investigate the surface of powdered FeMn19Cr17C0.4N heat treated under nitrogen atmosphere. The analyses showed reduction of iron oxides already at 500 °C leading to oxide-free metallic surface zones. Mn and Cr oxides are reduced at higher temperatures. Distinct nitrogen uptake was registered, and successful subsequent sintering was reached. Copyright © 2014 John Wiley & Sons, Ltd.
    view abstractdoi: 10.1002/sia.5730
  • 2015 • 270 Particle-induced cell migration assay (PICMA): A new in vitro assay for inflammatory particle effects based on permanent cell lines
    Westphal, G.A. and Schremmer, I. and Rostek, A. and Loza, K. and Rosenkranz, N. and Brüning, T. and Epple, M. and Bünger, J.
    Toxicology in Vitro 29 997-1005 (2015)
    Inflammation is a decisive pathophysiologic mechanism of particle toxicity and accumulation of neutrophils in the lung is believed to be a crucial step in this process. This study describes an in vitro model for investigations of the chemotactic attraction of neutrophils in response to particles using permanent cell lines. We challenged NR8383 rat macrophages with particles that were characterized concerning chemical nature, crystallinity, and size distribution in the dry state and in the culture medium. The cell supernatants were used to investigate migration of differentiated human leukemia cells (dHL-60 cells). The dose range for the tests was determined using an impedance-based Real-Time Cell Analyzer. The challenge of NR8383 cells with 32-96μgcm-2 coarse and nanosized particles resulted in cell supernatants which induced strong and dose-dependent migration of dHL-60 cells. Quartz caused the strongest effects - exceeding the positive control "fetal calf serum" (FCS) several-fold, followed by silica, rutile, carbon black, and anatase. BaSO<inf>4</inf> served as inert control and induced no cell migration. Particles caused NR8383 cells to secrete chemotactic compounds. The assay clearly distinguished between the particles of different inflammatory potential in a highly reproducible way. Specificity of the test is suggested by negative results with BaSO<inf>4</inf>. © 2015 The Authors.
    view abstractdoi: 10.1016/j.tiv.2015.04.005
  • 2015 • 269 Cyclic bending experiments on free-standing Cu micron lines observed by electron backscatter diffraction
    Wimmer, A. and Heinz, W. and Detzel, T. and Robl, W. and Nellessen, M. and Kirchlechner, C. and Dehm, G.
    Acta Materialia 83 460-469 (2015)
    Polycrystalline Cu samples 20 × 20 μm2 in size were cyclically bent inside a scanning electron microscope until fracture occurred. The microstructural changes were investigated by secondary electron imaging and electron backscatter diffraction. The in situ experiments revealed that, for the coarse-grained samples, it is not the external stress that dominates the cyclic deformation, but the local internal strength. This is in strong contrast to macroscopic bending samples, where deformation always happens near the fixed end of the bending beam and decreases constantly with increasing distance from the fixation. For micron-sized polycrystalline samples, the grain dimensions, dislocation density evolution and grain orientation (Taylor factor) can define the location of failure if the grain size and sample diameter become similar in size. A comparison with cyclic in situ tension-tension experiments (ratio of minimum stress to maximum stress R ≈ 0) reveals that cyclic bending experiments (R ≈ -1) undergo bulk-like fatigue deformation with extrusions/intrusions, in contrast to the experiments with R ≈ 0. Both the cyclic tension-tension and bending experiments can be described by a Basquin equation, although different mechanisms lead to failure of the samples. © 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2014.10.012
  • 2015 • 268 The effect of contact load on CoCrMo wear and the formation and retention of tribofilms
    Wimmer, M.A. and Laurent, M.P. and Mathew, M.T. and Nagelli, C. and Liao, Y. and Marks, L.D. and Jacobs, J.J. and Fischer, A.
    Wear 332-333 643-649 (2015)
    Tribochemical reactions in a protein lubricated metal-on-metal (MoM) sliding contact may play a significant role for its wear performance. Such reactions lead to the formation of a carbonaceous 'tribofilm', which can act as a protective layer against corrosion and wear. The purpose of this study was to determine the effect of contact load on wear and the formation and retention of tribofilms. Wear tests were performed in a custom-made ball-on-flat testing apparatus that incorporated an electrochemical cell. A ceramic ball was used to articulate against low-carbon wrought CoCrMo alloy pins in bovine serum. Using a range of contact loads at a single potentiostatic condition (close to free potential), weight loss and changes in surface properties were evaluated. We determined that wear was influenced by the loading condition. As expected, wear increased with load, but the association between applied load and measured weight loss was not linear. In the intermediate load region, in the range of 32-48 N (~58-80 MPa), there was more than an order of magnitude drop in the wear per unit load, and the wear versus load data suggested an inflexion point at 49N. Regression analyses yielded a cubic model (R2=0.991; p=0.0002), where the cubic term, which represents the inflexion, was highly significant (p=0.0021). This model is supported by the observations that the minimum in the friction versus load curve is at 52 N and the highest relative increase in polarization resistance occurred at 49 N. Scanning electron microscopy and Raman spectroscopy indicated the absence of a tribofilm for the low and within the contact area of the high load cases. Synergistic interactions of wear and corrosion seem to play an important role. © 2015 Elsevier B.V.
    view abstractdoi: 10.1016/j.wear.2015.02.013
  • 2015 • 267 Effect of the addition of low rare earth elements (lanthanum, neodymium, cerium) on the biodegradation and biocompatibility of magnesium
    Willbold, E. and Gu, X. and Albert, D. and Kalla, K. and Bobe, K. and Brauneis, M. and Janning, C. and Nellesen, J. and Czayka, W. and Tillmann, W. and Zheng, Y. and Witte, F.
    Acta Biomaterialia 11 554-562 (2015)
    Rare earth elements are promising alloying element candidates for magnesium alloys used as biodegradable devices in biomedical applications. Rare earth elements have significant effects on the high temperature strength as well as the creep resistance of alloys and they improve magnesium corrosion resistance. We focused on lanthanum, neodymium and cerium to produce magnesium alloys with commonly used rare earth element concentrations. We showed that low concentrations of rare earth elements do not promote bone growth inside a 750 μm broad area around the implant. However, increased bone growth was observed at a greater distance from the degrading alloys. Clinically and histologically, the alloys and their corrosion products caused no systematic or local cytotoxicological effects. Using microtomography and in vitro experiments, we could show that the magnesium-rare earth element alloys showed low corrosion rates, both in in vitro and in vivo. The lanthanum- and cerium-containing alloys degraded at comparable rates, whereas the neodymium-containing alloy showed the lowest corrosion rates. © 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actbio.2014.09.041
  • 2015 • 266 Routes towards catalytically active TiO2 doped porous cellulose
    Wittmar, A. and Thierfeld, H. and Köcher, S. and Ulbricht, M.
    RSC Advances 5 35866-35873 (2015)
    Cellulose-TiO<inf>2</inf> nanocomposites have been successfully prepared by non-solvent induced phase separation, either from cellulose solutions in ionic liquids or from cellulose acetate solutions in classical organic solvents followed by deacetylation ("regeneration"). Commercially available titania nanoparticles from gas phase synthesis processes have been used and processed as dispersions in the respective polymer solution. The used TiO<inf>2</inf> nanoparticles have been characterized by means of transmission electron microscopy (TEM) and X-ray diffraction (XRD), and their dispersions in ionic liquids and organic solvents have been evaluated by dynamic light scattering (DLS) and advanced rheology. The intermediate polymer solutions used in the phase separation process have been studied by advanced rheology. The resulting nanocomposites have been characterized by means of scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FT-IR). Special attention has been given to the complex relationship between the characteristics of the phase separation process and the porous structure of the formed nanocomposites. Two catalytic tests, based on the photocatalytic degradation of model organic dyes under UV irradiation, have been used for the characterization of the TiO<inf>2</inf> doped nanocomposites. The proof-of-concept experiments demonstrated the feasibility of photocatalyst immobilization in porous cellulose via phase separation of nanoparticle dispersions in polymer solutions, as indicated by UV-activated dye degradation in aqueous solution. © The Royal Society of Chemistry.2015.
    view abstractdoi: 10.1039/c5ra03707g
  • 2015 • 265 Two step and one step preparation of porous nanocomposite cellulose membranes doped with TiO2
    Wittmar, A. and Vorat, D. and Ulbricht, M.
    RSC Advances 5 88070-88078 (2015)
    Cellulose-TiO2 nanocomposites have been successfully prepared by non-solvent induced phase separation from cellulose acetate solutions in classical organic solvents followed by deacetylation ("regeneration"). The cellulose deacetylation has been performed either sequentially, i.e. after the completion of the phase separation process, or simultaneously, i.e. during the phase separation process. Commercially available titania nanoparticles from gas phase synthesis processes have been used and processed as a dispersion in the respective polymer solutions. The resulting nanocomposites have been characterized by means of scanning electron microscopy (SEM) and Fourier transform infrared (FT-IR) spectroscopy. Special attention has been given to the complex relation between the conditions of the deacetylation process, the structure of the resulting TiO2 doped cellulose membranes and their corresponding catalytic activities. Two catalytic activity tests, based on the photocatalytic degradation of model organic dyes under UV irradiation, have been used for the functional characterization of the TiO2 doped nanocomposites. The performed experiments demonstrated the successful photocatalyst immobilization in porous cellulose acetate together with good catalytic activity of this nanocomposite intermediate. By simply varying the conditions of the cellulose deacetylation, nanocomposite cellulose membranes with different structures and properties have been obtained. However after the regeneration of cellulose a partial decrease of the catalytic activity was observed. © 2015 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c5ra16337d
  • 2015 • 264 Atomic layer-by-layer construction of Pd on nanoporous gold via underpotential deposition and displacement reaction
    Yan, X. and Xiong, H. and Bai, Q. and Frenzel, J. and Si, C. and Chen, X. and Eggeler, G. and Zhang, Z.
    RSC Advances 5 19409-19417 (2015)
    Atomic layer-by-layer construction of Pd on nanoporous gold (NPG) has been investigated through the combination of underpotential deposition (UPD) with displacement reaction. It has been found that the UPD of Cu on NPG is sensitive to the applied potential and the deposition time. The optimum deposition potential and time were determined through potential- and time-sensitive stripping experiments. The NPG-Pd electrode shows a different voltammetric behavior in comparison to the bare NPG electrode, and the deposition potential was determined through the integrated charge control for the monolayer UPD of Cu on the NPG-Pd electrode. Five layers of Pd were constructed on NPG through the layer-by-layer deposition. In addition, the microstructure of the NPG-Pdx (x = 1, 2, 3, 4 and 5) films was probed by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM). The microstructural observation demonstrates that the atomic layers of Pd form on the ligament surface of NPG through epitaxial growth, and have no effect on the nanoporous structure of NPG. In addition, the hydrogen storage properties of the NPG-Pdx electrodes have also been addressed. This journal is © The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c4ra17014h
  • 2015 • 263 High resolution in situ mapping of microstrain and microstructure evolution reveals damage resistance criteria in dual phase steels
    Yan, D. and Tasan, C.C. and Raabe, D.
    Acta Materialia 96 399-409 (2015)
    Microstructures of multi-phase alloys undergo morphological and crystallographic changes upon deformation, corresponding to the associated microstructural strain fields. The multiple length and time scales involved therein create immense complexity, especially when microstructural damage mechanisms are also activated. An understanding of the relationship between microstructure and damage initiation can often not be achieved by post-mortem microstructural characterization alone. Here, we present a novel multi-probe analysis approach. It couples various scanning electron microscopy methods to microscopic-digital image correlation (μ-DIC), to overcome various challenges associated with concurrent mapping of the deforming microstructure along with the associated microstrain fields. For this purpose a contrast- and resolution-optimized μ-DIC patterning method and a selective pattern/microstructure imaging strategy were developed. They jointly enable imaging of (i) microstructure-independent pattern maps and (ii) pattern-independent microstructure maps. We apply this approach here to the study of damage nucleation in ferrite/martensite dual-phase (DP) steel. The analyses provide four specific design guidelines for developing damage-resistant DP steels. © 2015 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2015.05.038
  • 2015 • 262 Combined AFM/SECM Investigation of the Solid Electrolyte Interphase in Li-Ion Batteries
    Zampardi, G. and Klink, S. and Kuznetsov, V. and Erichsen, T. and Maljusch, A. and LaMantia, F. and Schuhmann, W. and Ventosa, E.
    ChemElectroChem 2 1607-1611 (2015)
    The solid electrolyte interphase (SEI) is an electronically insulating film formed from the decomposition of the organic electrolyte at the surface of the negative electrodes in Li-ion batteries (LIBs). This film is of vital importance in the performance and safety of LIBs. Atomic force microscopy (AFM) and scanning electrochemical microscopy (SECM) are combined in one platform for the consecutive insitu investigation of surface reactions in LIBs inside an Ar-filled glovebox. As proof of concept, the formation and the electrochemical properties of the SEI formed on glassy carbon electrodes are investigated. Changes in topography during film formation of the SEI are studied via AFM. The AFM tip is then used to partially remove a small area (50×50μm2) of the SEI, which is subsequently probed using SECM in feedback mode. The AFM-scratched spot is clearly visualized in the SECM image, demonstrating the strength of the AFM/SECM combination for the investigation in the field of LIBs. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/celc.201500085
  • 2015 • 261 In-operando evaluation of the effect of vinylene carbonate on the insulating character of the solid electrolyte interphase
    Zampardi, G. and La Mantia, F. and Schuhmann, W.
    Electrochemistry Communications 58 1--5 (2015)
    The solid electrolyte interphase (SEI) is an electronic insulating layer which highly affects the performance of lithium-ion batteries, especially when electrodes with low (de-)intercalation potentials such as graphite are employed. The formation of the SEI was investigated in-operando on graphite when vinylene carbonate (VC) was present as an additive in solution using feedback-mode SECM. The potential at which the surface started to become insulating was at 0.8 V vs. Li/Li+ in VC-free electrolytes, while it was at 1.3 V in VC-containing electrolytes. Nevertheless, potentials more cathodic than 0.8 V have to be reached to form a homogeneous SEI. No influence in the electronic properties of the formed SEI with different concentrations of VC was observed. (C) 2015 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.elecom.2015.05.013
  • 2015 • 260 Determination of the formation and range of stability of the SEI on glassy carbon by local electrochemistry
    Zampardi, G. and La Mantia, F. and Schuhmann, W.
    RSC Advances 5 31166-31171 (2015)
    The solid electrolyte interphase (SEI) is an electronic insulating and ionic conducting layer that is of main importance in lithium-ions batteries, since it critically affects the final performance of the battery system. The formation of this electronic insulating layer was determined in operando on a glassy carbon electrode by means of a microelectrode positioned in close proximity to its surface using scanning electrochemical microscopy (SECM). Glassy carbon was chosen as an ideal model system for carbonaceous materials, since it forms a SEI similar in composition to the one on graphite but concomitantly shows negligible intercalation of lithium ions. Moreover, the stability of the SEI was analysed depending on different potential ranges and the role of the cations on the insulating character of the SEI was investigated. © 2015 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c5ra02940f
  • 2015 • 259 Damage resistance in gum metal through cold work-induced microstructural heterogeneity
    Zhang, J.-L. and Tasan, C.C. and Lai, M.L. and Zhang, J. and Raabe, D.
    Journal of Materials Science 50 (2015)
    Cold-worked alloys exhibit high strength, but suffer from limited ductility. In contrast, Ti-based gum metal was reported to exhibit high strength combined with good ductility upon severe pre-straining. Motivated by this anomaly, we systematically studied the evolution of gum metal microstructure during severe cold working (swaging and rolling) and the resulting deformation and damage micro-mechanical mechanisms during follow-up tensile deformation. To this end, various experimental in situ and post-mortem methodologies are employed, including scanning electron microscopy imaging, high-resolution electron backscatter diffraction mapping and transmission electron microscopy. These observations reveal that intense grain refinement takes place through dislocation plasticity-dominated deformation banding upon cold working. The observed enhancement in crack blunting and failure resistance which prolongs the post-necking ductility of gum metal during follow-up tensile straining can be attributed to the deformation-induced development of local heterogeneities in texture and grain size. © 2015 Springer Science+Business Media New York
    view abstractdoi: 10.1007/s10853-015-9105-y
  • 2015 • 258 Light Induced H2 Evolution from a Biophotocathode Based on Photosystem 1 - Pt Nanoparticles Complexes Integrated in Solvated Redox Polymers Films
    Zhao, F. and Conzuelo, F. and Hartmann, V. and Li, H. and Nowaczyk, M.M. and Plumeré, N. and Rögner, M. and Schuhmann, W.
    Journal of Physical Chemistry B 119 13726-13731 (2015)
    We report on a biophotocathode based on photosystem 1 (PS1)-Pt nanoparticle complexes integrated in a redox hydrogel for photoelectrocatalytic H2 evolution at low overpotential. A poly(vinyl)imidazole Os(bispyridine)2Cl polymer serves as conducting matrix to shuttle the electrons from the electrode to the PS1-Pt complexes embedded within the hydrogel. Light induced charge separation at the PS1-Pt complexes results in the generation of photocurrents (4.8 ± 0.4 μA cm-2) when the biophotocathodes are exposed to anaerobic buffer solutions. Under these conditions, the protons are the sole possible electron acceptors, suggesting that the photocurrent generation is associated with H2 evolution. Direct evidence for the latter process is provided by monitoring the H2 production with a Pt microelectrode in scanning electrochemical microscopy configuration over the redox hydrogel film containing the PS1-Pt complexes under illumination. © 2015 American Chemical Society.
    view abstractdoi: 10.1021/acs.jpcb.5b03511
  • 2014 • 257 Electrochemical nanoprobes for single-cell analysis
    Actis, P. and Tokar, S. and Clausmeyer, J. and Babakinejad, B. and Mikhaleva, S. and Cornut, R. and Takahashi, Y. and López Córdoba, A. and Novak, P. and Shevchuck, A.I. and Dougan, J.A. and Kazarian, S.G. and Gorelkin, P.V. and...
    ACS Nano 8 875-884 (2014)
    The measurement of key molecules in individual cells with minimal disruption to the biological milieu is the next frontier in single-cell analyses. Nanoscale devices are ideal analytical tools because of their small size and their potential for high spatial and temporal resolution recordings. Here, we report the fabrication of disk-shaped carbon nanoelectrodes whose radius can be precisely tuned within the range 5-200 nm. The functionalization of the nanoelectrode with platinum allowed the monitoring of oxygen consumption outside and inside a brain slice. Furthermore, we show that nanoelectrodes of this type can be used to impale individual cells to perform electrochemical measurements within the cell with minimal disruption to cell function. These nanoelectrodes can be fabricated combined with scanning ion conductance microscopy probes, which should allow high resolution electrochemical mapping of species on or in living cells. © 2013 American Chemical Society.
    view abstractdoi: 10.1021/nn405612q
  • 2014 • 256 Tribological and mechanical properties of Ti/TiAlN/TiAlCN nanoscale multilayer PVD coatings deposited on AISI H11 hot work tool steel
    Al-Bukhaiti, M.A. and Al-Hatab, K.A. and Tillmann, W. and Hoffmann, F. and Sprute, T.
    Applied Surface Science 318 180-190 (2014)
    A new [Ti/TiAlN/TiAlCN]5 multilayer coatings were deposited onto polished substrate AISI H11 (DIN 1.2343) steel by an industrial magnetron sputtering device. The tribological performance of the coated system was investigated by a ball-on-disk tribometer against 100Cr6 steel and Al2O3 balls. The friction coefficients and specific wear rates were measured at various normal loads (2, 5, 8, and 10 N) and sliding velocities (0.2, 0.4, and 0.8 m/s) in ambient air and dry conditions. The phase structure, composition, wear tracks morphologies, hardness, and film/substrate adhesion of the coatings were characterized by light-microscopy, scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD), 3D-surface analyzer, nanoindentation, and scratch tests. Results showed that the deposited coatings showed low wear rates in the scale of 10-15 m3/N m, low friction coefficients against 100Cr6 and Al2O3 balls in the range of 0.25-0.37, and good hardness in the range of 17-20 GPa. Results also revealed that the friction coefficients and disc wear rates decrease and increase, respectively with the increase in normal load and sliding velocity for both coating/Al2O3 and coating/100Cr6 sliding system. Compared with the uncoated-H11 substrate, the deposited coating exhibited superior tribological and mechanical properties. The dominant wear mechanism was abrasive wear for coating/Al2O3 pair, while for coating/100Cr6 pair, a combination of mild adhesive wear, severe adhesive wear, and abrasive wear (extensive plowing) were the dominant wear mechanisms at different applied normal loads. © 2014 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.apsusc.2014.03.026
  • 2014 • 255 CrN/AlN nanolaminate coatings deposited via high power pulsed and middle frequency pulsed magnetron sputtering
    Bagcivan, N. and Bobzin, K. and Ludwig, Al. and Grochla, D. and Brugnara, R.H.
    Thin Solid Films 572 153-160 (2014)
    Nanolaminate coatings based on transition metal nitrides such as CrN, AlN and TiN deposited via physical vapor deposition (PVD) have shown great advantage as protective coatings on tools and components subject to high loads in tribological applications. By varying the individual layer materials and their thicknesses it is possible to optimize the coating properties, e.g. hardness, Young's modulus and thermal stability. One way for further improvement of coating properties is the use of advanced PVD technologies. High power pulsed magnetron sputtering (HPPMS) is an advancement of pulsed magnetron sputtering (MS). The use of HPPMS allows a better control of the energetic bombardment of the substrate due to the higher ionization degree of metallic species. It provides an opportunity to influence chemical and mechanical properties by varying the process parameters. The present work deals with the development of CrN/AlN nanolaminate coatings in an industrial scale unit by using two different PVD technologies. Therefore, HPPMS and mfMS (middle frequency magnetron sputtering) technologies were used. The bilayer period Λ, i.e. the thickness of a CrN/AlN double layer, was varied between 6.2nm and 47.8 nm by varying the rotational speed of the substrate holders. In a second step the highest rotational speed was chosen and further HPPMS CrN/AlN coatings were deposited applying different HPPMS pulse lengths (40, 80, 200 μs) at the same mean cathode power and frequency. Thickness, morphology, roughness and phase composition of the coatings were analyzed by means of scanning electron microscopy (SEM), confocal laser microscopy, and X-ray diffraction (XRD), respectively. The chemical composition was determined using glow discharge optical emission spectroscopy (GDOES). Detailed characterization of the nanolaminate was conducted by transmission electron microscopy (TEM). The hardness and the Young's modulus were analyzed by nanoindentation measurements. The residual stress was determined via Si microcantilever curvature measurements. The phase analysis revealed the formation of h-Cr2N, c-CrN and c-AlN mixed phases for the mfMS CrN/AlN coatings, whereas the HPPMS coatings exhibited only cubic phases (c-CrN, c-AlN). A hardness of 31.0 GPa was measured for the HPPMS coating with a bilayer period of 6.2 nm. The decrease of the HPPMS pulse length at constant mean power leads to a considerable increase of the cathode current on the Cr and Al target associated with an increased ion flux towards the substrate. Furthermore, it was observed that the deposition rate of HPPMS CrN/AlN decreases with shorter pulse lengths, so that a CrN/AlN coating with a bilayer period of 2.9 nm, a high hardness of 40.8 GPa and a high compressive stress (- 4.37 GPa) was achieved using a short pulse length of 40 μs. © 2014 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.tsf.2014.06.058
  • 2014 • 254 Localized impedance measurements for electrochemical surface science
    Bandarenka, A.S. and Maljusch, A. and Kuznetsov, V. and Eckhard, K. and Schuhmann, W.
    Journal of Physical Chemistry C 118 8952-8959 (2014)
    An approach for in-depth characterization of complex electrode/electrolyte interfaces based on localized impedance measurements is described in detail. The local ac probing of the interface is performed at different frequencies by means of scanning electrochemical microscopy (SECM) using ultramicroelectrodes (SECM tips) which enables visualization of dependences of the localized impedance spectra as a function of spatial coordinates. Subsequent fitting of these spectra to physical models visualize the local distribution of parameters describing the electrochemical interface, such as the electric double layer capacitance and the charge transfer resistance. Three model examples are analyzed dealing with typical situations, when the measurements are either affected or not by specific adsorption of anions at the SECM-tips. It is demonstrated that the approach holds promise for electrochemical surface science, particularly for better understanding of corrosion processes taking place at metal surfaces in aggressive, particularly aqueous electrolytes. © 2014 American Chemical Society.
    view abstractdoi: 10.1021/jp412505p
  • 2014 • 253 Composition-structure-function diagrams of Ti-Ni-Au thin film shape memory alloys
    Buenconsejo, P.J.S. and Ludwig, Al.
    ACS Combinatorial Science 16 678-685 (2014)
    Ti-Ni-Au thin film materials libraries were prepared from multilayer precursors by combinatorial sputtering. The materials libraries were annealed at 500, 600, and 700 °C for 1 h and then characterized by high-throughput methods to investigate the relations between composition, structure and functional properties. The identified relations were visualized in functional phase diagrams. The goal is to identify composition regions that are suitable as high temperature shape memory alloys. Phase transforming compositions were identified by electrical resistance measured during thermal cycles in the range of -20 and 250 °C. Three phase transformation paths were confirmed: (1) B2-R, (2) B2-R-B19', and (3) B2-B19. For the materials library annealed at 500 °C only the B2-R transformation was observed. For the materials libraries annealed at 600 and 700 °C, all transformation paths were observed. High transformation temperatures (Ms ≈100 °C) were only obtained by annealing at 600 or 700 °C, and with compositions of Ti ≈ 50 at. % and Au &gt; 20 at. %. This is the composition range that undergoes B2-B19 transformation. The phase transformation behaviors were explained according to the compositional and annealing temperature dependence of phase/structure formation, as revealed by X-ray diffraction analysis of the materials libraries. © 2014 American Chemical Society.
    view abstractdoi: 10.1021/co5000745
  • 2014 • 252 Electrochemical patterning as a tool for fabricating biomolecule microarrays
    Clausmeyer, J. and Schuhmann, W. and Plumeré, N.
    TrAC - Trends in Analytical Chemistry 58 23-30 (2014)
    High-density biomolecule arrays are powerful tools for the screening of pharmaceuticals, investigation of biomolecule interactions and patient diagnostics. Surfaces modified with electrochemically addressable films combined with electrochemical surface patterning techniques allow local triggering of DNA and protein immobilization. After a brief overview of classical patterning methods, such as printing, dip-pen nanolithography (DPN) and photolithography, we critically assess electrochemical strategies for local surface modification, such as the use of electrode arrays, electro-DPN and scanning electrochemical microscopy regarding their potential for fabrication and read-out of bioarrays. Capillary-based scanning probe methods are especially promising tools for truly chemoselective microarray and nanoarray generation due to their high patterning resolution and the possibility for directly probing the surface chemistry. © 2014 Elsevier Ltd.
    view abstractdoi: 10.1016/j.trac.2014.03.004
  • 2014 • 251 Structural and functional characterization of enamel pigmentation in shrews
    Dumont, M. and Tütken, T. and Kostka, A. and Duarte, M.J. and Borodin, S.
    Journal of Structural Biology 186 38-48 (2014)
    Pigmented tooth enamel occurs in several vertebrate clades, ranging from mammals to fish. Although an iron compound is associated with this orange to red colored pigmentation, its chemical and structural organization within the enamel is unknown. To determine the nature of the iron compound, we investigated heavily pigmented teeth of the northern short-tailed shrew Blarina brevicauda using combined characterization techniques such as scanning and transmission electron microscopy and synchrotron X-ray diffraction. We found that the pigmentation of the enamel with an iron content of around 8. wt% results from a close to amorphous magnetite phase deposited around the nm-sized enamel crystals. Furthermore, the influence of the pigmentation on the enamel hardness was determined by nanoindentation measurements. Finally, the biomechanical function and biological context are discussed in light of the obtained results. © 2014 Elsevier Inc.
    view abstractdoi: 10.1016/j.jsb.2014.02.006
  • 2014 • 250 Ultrastructural organization and micromechanical properties of shark tooth enameloid
    Enax, J. and Janus, A.M. and Raabe, D. and Epple, M. and Fabritius, H.-O.
    Acta Biomaterialia 10 3959-3968 (2014)
    The outer part of shark teeth is formed by the hard and mineral-rich enameloid that has excellent mechanical properties, which makes it a very interesting model system for the development of new bio-inspired dental materials. We characterized the microstructure, chemical composition and resulting local mechanical properties of the enameloid from teeth of Isurus oxyrinchus (shortfin mako shark) by performing an in-depth analysis using various high-resolution analytical techniques, including scanning electron microscopy, qualitative energy-dispersive X-ray spectroscopy and nanoindentation. Shark tooth enameloid reveals an intricate hierarchical arrangement of thin (50-80 nm) and long (>1 μm) crystallites of fluoroapatite with a high degree of structural anisotropy, which leads to exceptional mechanical properties. Both stiffness and hardness are surprisingly homogeneous in the shiny layer as well as in the enameloid: although both tooth phases differ in structure and composition, they show almost no orientation dependence with respect to the loading direction of the enameloid crystallites. The results were used to determine the structural hierarchy of shark teeth, which can be used as a base for establishing design criteria for synthetic bio-inspired and biomimetic dental composites. © 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actbio.2014.04.028
  • 2014 • 249 Comparison of different characterization methods for nanoparticle dispersions before and after aerosolization
    Fissan, H. and Ristig, S. and Kaminski, H. and Asbach, C. and Epple, M.
    Analytical Methods 6 7324-7334 (2014)
    A well-known and accepted aerosol measurement technique, the scanning mobility particle sizer (SMPS), is applied to characterize colloidally dispersed nanoparticles. To achieve a transfer from dispersed particles to aerosolized particles, a newly developed nebulizer (N) is used that, unlike commonly used atomizers, produces significantly smaller droplets and therefore reduces the problem of the formation of residual particles. The capabilities of this new instrument combination (N + SMPS) for the analysis of dispersions were investigated, using three different dispersions, i.e. gold-PVP nanoparticles (∼20 nm), silver-PVP nanoparticles (∼70 nm) and their 1:1 (m:m) mixture. The results are compared to scanning electron microscopy (SEM) measurements and two frequently applied techniques for characterizing colloidal systems: Dynamic light scattering (DLS) and analytical disc centrifugation (ADC). The differences, advantages and disadvantages of each method are discussed, especially with respect to the size resolution of the techniques and their ability to distinguish the particle sizes of the mixed dispersion. While DLS is, as expected, unable to resolve the binary dispersion, SEM, ADC and SMPS are able to give quantitative information on the two particle sizes. However, while the high-resolving ADC is limited due to the dependency on a predefined density of the investigated system, the transfer of dispersed particles into an aerosol and subsequent analysis with SMPS are an adequate way to characterize binary systems, independent of the density of concerned particles, but matching the high resolution of the ADC. We show that it is possible to use the well-established aerosol measurement technique (N + SMPS) in colloid science with all its advantages concerning size resolution and accuracy. © the Partner Organisations 2014.
    view abstractdoi: 10.1039/c4ay01203h
  • 2014 • 248 Effect of retained beta layer on slip transmission in Ti-6Al-2Zr-1Mo-1V near alpha titanium alloy during tensile deformation at room temperature
    He, D. and Zhu, J. and Zaefferer, S. and Raabe, D.
    Materials and Design 56 937-942 (2014)
    Slip is the main plastic deformation mechanism in titanium alloys at room temperature. This is especially so for near alpha titanium alloy like Ti-6Al-2Zr-1Mo-1V, which contains low beta stabilizing and high aluminum (alpha stabilizing) element additions. The effects of retained beta layers on slip transmission across α/β interfaces in Ti-6Al-2Zr-1Mo-1V during tensile deformation have been studied in the current work. High resolution scanning electron microscopy (HR-SEM) and electron backscatter diffraction (EBSD) techniques were used to study the deformation microstructure. The results indicate that the full Burgers crystal orientation relationship (OR) between the α and the thin retained β phase layers facilitates slip transition but is not the necessary requirement/restriction. Some novel slip trace morphologies that are characterized by deflection and bifurcation (fork-like morphology) are revealed in the retained β layers between two abutting α grains. The possible reasons for these different slip transmission patterns are analyzed by EBSD results and a schematic model is proposed. © 2013 Elsevier Ltd.
    view abstractdoi: 10.1016/j.matdes.2013.12.018
  • 2014 • 247 Microshear deformation of gold single crystals
    Heyer, J.-K. and Brinckmann, S. and Pfetzing-Micklich, J. and Eggeler, G.
    Acta Materialia 62 225-238 (2014)
    We perform microshear experiments on Au single crystals, directly imposing shear loading on the microscopic crystallographic h1-10i {111} slip system. We use a focused ion beam machined micro-double shear specimen which we load with a flat punch indenter inside a scanning electron microscope. Our method yields reproducible mechanical data (e.g. critical shear stresses of 63.5 ± 2.5 MPa). We study small-scale plasticity up to high strains (>50%) at constant slip geometry and document localized plastic deformation and sudden plastic deformation events. Strain bursts are observed, which can be related to the formation of new shear bands. Alternatively, they can result from sudden shear strain accumulation events in existing shear bands. Due to the stochastic nature of plastic deformation, the nature and the number of strain bursts can vary. We show and discuss how our in situ test technique captures these effects and how this affects the corresponding load-displacement curves. We discuss the advantages and inconveniences of our microshear test technique compared to other small-scale testing methods and relate our mechanical results to previous results reported for the micromechanical behavior of Au. © 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2013.10.002
  • 2014 • 246 Electronic and structural differences between wurtzite and zinc blende inas nanowire surfaces: Experiment and theory
    Hjort, M. and Lehmann, S. and Knutsson, J. and Zakharov, A.A. and Du, Y.A. and Sakong, S. and Timm, R. and Nylund, G. and Lundgren, E. and Kratzer, P. and Dick, K.A. and Mikkelsen, A.
    ACS Nano 8 12346-12355 (2014)
    We determine the detailed differences in geometry and band structure between wurtzite (Wz) and zinc blende (Zb) InAs nanowire (NW) surfaces using scanning tunneling microscopy/spectroscopy and photoemission electron microscopy. By establishing unreconstructed and defect-free surface facets for both Wz and Zb, we can reliably measure differences between valence and conduction band edges, the local vacuum levels, and geometric relaxations to the few-millielectronvolt and few-picometer levels, respectively. Surface and bulk density functional theory calculations agree well with the experimental findings and are used to interpret the results, allowing us to obtain information on both surface and bulk electronic structure. We can thus exclude several previously proposed explanations for the observed differences in conductivity of Wz-Zb NW devices. Instead, fundamental structural differences at the atomic scale and nanoscale that we observed between NW surface facets can explain the device behavior. © 2014 American Chemical Society.
    view abstractdoi: 10.1021/nn504795v
  • 2014 • 245 Tailoring the morphology of mesoporous titania thin films through biotemplating with nanocrystalline cellulose
    Ivanova, A. and Fattakhova-Rohlfing, D. and Kayaalp, B.E. and Rathouský, J. and Bein, T.
    Journal of the American Chemical Society 136 5930-5937 (2014)
    The tunable porosity of titania thin films is a key factor for successful applications in photovoltaics, sensing, and photocatalysis. Here, we report on nanocrystalline cellulose (NCC) as a novel shape-persistent templating agent enabling the straightforward synthesis of mesoporous titania thin films. The obtained structures are highly porous anatase morphologies having well-defined, narrow pore size distributions. By varying the titania-to-template ratio, it is possible to tune the surface area, pore size, pore anisotropy, and dimensions of titania crystallites in the films. Moreover, a post-treatment at high humidity and subsequent slow template removal can be used to achieve pore widening; this treatment is also beneficial for the multilayer deposition of thick films. The resulting homogeneous transparent films can be directly spin- or dip- coated on glass, silicon, and transparent conducting oxide (TCO) substrates. The mesoporous titania films show very high activity in the photocatalytic NO conversion and in the degradation of 4-chlorophenol. Furthermore, the films can be successfully applied as anodes in dye-sensitized solar cells. © 2014 American Chemical Society.
    view abstractdoi: 10.1021/ja411292u
  • 2014 • 244 Photoemission electron microscopy and scanning electron microscopy of magnetospirillum magnetotacticum 's magnetosome chains
    Keutner, C. and Von Bohlen, A. and Berges, U. and Espeter, P. and Schneider, C.M. and Westphal, C.
    Analytical Chemistry 86 9590-9594 (2014)
    Magnetotactic bacteria are of great interdisciplinary interest, since a vast field of applications from magnetic recording media to medical nanorobots is conceivable. A key feature for a further understanding is the detailed knowledge about the magnetosome chain within the bacteria. We report on two preparation procedures suitable for UHV experiments in reflective geometry. Further, we present the results of scanning electron microscopy, as well as the first photoemission electron microscopy experiments, both accessing the magnetosomes within intact magnetotactic bacteria and compare these to scanning electron microscopy data from the literature. From the images, we can clearly identify individual magnetosomes within their chains. © 2014 American Chemical Society.
    view abstractdoi: 10.1021/ac502050j
  • 2014 • 243 Vacancy-induced toughening in hard single-crystal V0.5Mo 0.5Nx/MgO(0 0 1) thin films
    Kindlund, H. and Sangiovanni, D.G. and Lu, J. and Jensen, J. and Chirita, V. and Birch, J. and Petrov, I. and Greene, J.E. and Hultman, L.
    Acta Materialia 77 394-400 (2014)
    Using a combination of experiments and density functional theory (DFT), we demonstrate the first example of vacancy-induced toughening, in this case for epitaxial pseudobinary NaCl-structure substoichiometric V0.5Mo 0.5Nx alloys, with N concentrations 0.55 ≤ x ≤ 1.03, grown by reactive magnetron sputter deposition. The nanoindentation hardness H(x) increases with increasing vacancy concentration from 17 GPa with x = 1.03 to 26 GPa with x = 0.55, while the elastic modulus E(x) remains essentially constant at 370 GPa. Scanning electron micrographs of indented regions show ductile plastic flow giving rise to material pile-up, rather than cracks as commonly observed for hard, but brittle, transition-metal nitrides. The increase in alloy hardness with an elastic modulus that remains constant with decreasing x, combined with the observed material pile-up around nanoindents, DFT-calculated decrease in shear to bulk moduli ratios, and increased Cauchy pressures (C12-C44), reveals a trend toward vacancy-induced toughening. Moreover, DFT crystal orbital overlap population analyses are consistent with the above results. © 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2014.06.025
  • 2014 • 242 First approach for thermodynamic modelling of the high temperature oxidation behaviour of ternary γ'-strengthened Co-Al-W superalloys
    Klein, L. and Zendegani, A. and Palumbo, M. and Fries, S.G. and Virtanen, S.
    Corrosion Science 89 1-5 (2014)
    In the present work, thermodynamic modelling of the high temperature oxidation behaviour of a γ'-strengthened Co-base superalloy is presented. The ternary Co-9Al-9W alloy (values in at%) was isothermally oxidised for 500h at 800 and 900°C in air. Results reveal that the calculated oxide layer sequence (Thermo-Calc, TCNI6) is in good agreement with the formed oxide scales on the alloy surface. Furthermore, prediction of the influence of oxygen partial pressure on Al2O3 formation is presented. The modelling results indicate pathways for alloy development or possible pre-oxidation surface treatments for improved oxidation resistance of the material. © 2014 Elsevier Ltd.
    view abstractdoi: 10.1016/j.corsci.2014.08.016
  • 2014 • 241 Investigation of ternary subsystems of superalloys by thin-film combinatorial synthesis and high-throughput analysis
    König, D. and Pfetzing-Micklich, J. and Frenzel, J. and Ludwig, Al.
    MATEC Web of Conferences 14 (2014)
    A Co-Ti-W thin film materials library was fabricated by magnetron sputtering. By using automated high-throughput measurement techniques (resistance mapping, automated XRD measurements) and cluster analysis a yet unknown phase region was revealed. The existence region of the new ternary phase is close to the composition Co60Ti15W25. In order to transfer the results from thin film to bulk material, a bulk sample was prepared by arc melting and subsequent heat treatment. Scanning electron microscopy and chemical micro-analysis data support that a yet unknown ternary phase exists in the system Co-Ti-W. © 2014 Owned by the authors, published by EDP Sciences.
    view abstractdoi: 10.1051/matecconf/20141418002
  • 2014 • 240 Large-scale synthesis and catalytic activity of nanoporous Cu-O system towards CO oxidation
    Kou, T. and Si, C. and Gao, Y. and Frenzel, J. and Wang, H. and Yan, X. and Bai, Q. and Eggeler, G. and Zhang, Z.
    RSC Advances 4 65004-65011 (2014)
    Nanoporous Cu-O system catalysts with different oxidation states of Cu have been fabricated through a combination of dealloying as-milled Al66.7Cu33.3 alloy powders and subsequent thermal annealing. X-ray diffraction (XRD), field-emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) have been used to characterize the microstructure and surface chemical states of Cu-O catalysts. The peculiar nanoporous structure can be retained in Cu-O catalysts after thermal treatment. Catalytic experiments reveal that all the Cu-O samples exhibit complete CO conversion below 170 °C. The optimal catalytic performance could be achieved through the combination of annealing in air with hydrogen treatment for the Cu-O catalyst, which shows a near complete conversion temperature (T90%) of 132 °C and an activation energy of 91.3 KJ mol-1. In addition, the present strategy (ball milling, dealloying and subsequent thermal treatment) could be scaled up to fabricate high-performance Cu-O catalysts towards CO oxidation. This journal is © The Royal Society of Chemistry 2014.
    view abstractdoi: 10.1039/c4ra12227e
  • 2014 • 239 Hydrogen-assisted decohesion and localized plasticity in dual-phase steel
    Koyama, M. and Tasan, C.C. and Akiyama, E. and Tsuzaki, K. and Raabe, D.
    Acta Materialia 70 174-187 (2014)
    Hydrogen embrittlement affects high-strength ferrite/martensite dual-phase (DP) steels. The associated micromechanisms which lead to failure have not been fully clarified yet. Here we present a quantitative micromechanical analysis of the microstructural damage phenomena in a model DP steel in the presence of hydrogen. A high-resolution scanning electron microscopy-based damage quantification technique has been employed to identify strain regimes where damage nucleation and damage growth take place, both with and without hydrogen precharging. The mechanisms corresponding to these regimes have been investigated by employing post-mortem electron channeling contrast imaging and electron backscatter diffraction analyses, as well as additional in situ deformation experiments. The results reveal that damage nucleation mechanism (i.e. martensite decohesion) and the damage growth mechanisms (e.g. interface decohesion) are both promoted by hydrogen, while the crack-arresting capability of the ferrite is significantly reduced. The observations are discussed on the basis of the hydrogen-enhanced decohesion and hydrogen-enhanced localized plasticity mechanisms. We discuss corresponding microstructure design strategies for better hydrogen-related damage tolerance of DP steels. © 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2014.01.048
  • 2014 • 238 Probing the electronic transport on the reconstructed Au/Ge(001) surface
    Krok, F. and Kaspers, M.R. and Bernhart, A.M. and Nikiel, M. and Jany, B.R. and Indyka, P. and Wojtaszek, M. and Möller, R. and Bobisch, C.A.
    Beilstein Journal of Nanotechnology 5 1463-1471 (2014)
    By using scanning tunnelling potentiometry we characterized the lateral variation of the electrochemical potential μec on the goldinduced Ge(001)-c(8 × 2)-Au surface reconstruction while a lateral current flows through the sample. On the reconstruction and across domain boundaries we find that μec shows a constant gradient as a function of the position between the contacts. In addition, nanoscale Au clusters on the surface do not show an electronic coupling to the gold-induced surface reconstruction. In combination with high resolution scanning electron microscopy and transmission electron microscopy, we conclude that an additional transport channel buried about 2 nm underneath the surface represents a major transport channel for electrons. © 2014 Krok et al.
    view abstractdoi: 10.3762/bjnano.5.159
  • 2014 • 237 Low fouling negatively charged hybrid ultrafiltration membranes for protein separation from sulfonated poly(arylene ether sulfone) block copolymer and functionalized multiwalled carbon nanotubes
    Kumar, M. and Ulbricht, M.
    Separation and Purification Technology 127 181-191 (2014)
    Low fouling negatively charged hybrid ultrafiltration membranes with adjustable charge density were fabricated from blends of poly(arylene ether sulfone) (PAES) block copolymer and the sulfonated copolymer (S-PAES) in combination with different fractions of sulfonic acid functionalized multiwalled carbon nanotubes (MWCNT-SO3H) by non-solvent induced phase separation method. Porous hybrid membrane morphologies, structure and surface properties were characterized comprehensively using scanning electron microscopy, Fourier transform infrared spectroscopy in the attenuated total reflection mode, as well as contact angle and zeta potential measurements. Results confirmed that the fabricated membranes were hydrophilic and negatively charged in the studied pH range 3-10. The water permeabilities and increased protein fouling resistances of the membranes were dependent on the fraction of MWCNT-SO3H in the membranes. The protein transmission as function of pH value and fraction of MWCNT-SO3H was studied for two model proteins (bovine serum albumin and myoglobin) and found to be controlled by size exclusion and the content of MWCNT-SO3H. The highest transmission of proteins at their isoelectric points was obtained for the membrane containing 2 wt% of MWCNT-SO3H relative to total membrane polymer. The selectivity of the hybrid membranes for the separation of the binary protein mixture could be systematically increased by increasing surface charge density by increasing fraction of MWCNT-SO3H. Consequently, the trade-off relationship between permeability and selectivity for conventional ultrafiltration membranes where separation is based on size exclusion solely could be overcome and performance be tuned by a small fraction of a functional additive. © 2014 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.seppur.2014.03.003
  • 2014 • 236 Novel ultrafiltration membranes with adjustable charge density based on sulfonated poly(arylene ether sulfone) block copolymers and their tunable protein separation performance
    Kumar, M. and Ulbricht, M.
    Polymer (United Kingdom) 55 354-365 (2014)
    A novel poly(arylene ether sulfone) (PAES) block copolymer was prepared from previously synthesized fluoride terminated oligomer (A16) and hydroxyl terminated oligomer (B12) by aromatic nucleophilic substitution polycondensation reaction. PAES was subsequently sulfonated under controlled conditions to yield a copolymer (S-PAES) with sulfonic acid groups selectively in the B12 segments and without chain degradation. Non-solvent induced phase separation method was used to prepare a series of ultrafiltration membranes from blends of S-PAES and PAES with varied ratios and, hence, sulfonic acid group densities. Porous membrane morphologies, structure and surface properties were characterized comprehensively using scanning electron microscopy, Fourier transform infrared spectroscopy in the attenuated total reflection mode, as well as contact angle and zeta potential measurements. Studies of membrane performance revealed systematically increasing water permeabilities and reduced protein fouling tendencies with increasing fraction of S-PAES in the membrane. The protein transmission as function of pH value (and hence protein charge) was studied for two model proteins (bovine serum albumin and lysozyme) and found to be controlled by combined size exclusion and charge effects. The selectivity for the separation of the binary protein mixture could be systematically increased with increasing membrane charge density (by increasing S-PAES fraction). Consequently, the trade-off relationship between permeability and selectivity for conventional ultrafiltration membranes where separation is based on size exclusion solely could be overcome. Due to their high stability and tunable functionality, the PAES block copolymers have also large potential as membrane material for other applications. © 2013 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.polymer.2013.09.003
  • 2014 • 235 Sudden stress-induced transformation events during nanoindentation of NiTi shape memory alloys
    Laplanche, G. and Pfetzing-Micklich, J. and Eggeler, G.
    Acta Materialia 78 144-160 (2014)
    This study investigates the stress-induced formation of martensite during nanoindentation of an austenitic NiTi shape memory alloy, where stress-induced martensite is stable at room temperature. An individual grain with a [1 1 1] surface normal was selected for spherical ex situ and in situ nanoindentation in a scanning electron microscope. The in situ load-displacement curves show several pop-ins which occur concomitantly with the formation of traces around the contact zone between the indenter tip and the sample. These traces exhibit a threefold symmetry around the remnant indent. A detailed study of the indentation-induced surface relief by atomic force microscopy before and after shape recovery allows to identify the formation of six twinned martensite plates. Post-mortem microstructural characterization shows that these twinned martensite plates are growing as the applied load is increasing. The activation of the experimentally observed twinned martensite plates is rationalized by analytical calculations of resolved shear stress and mechanical interaction energy density. Finally, the in situ nanoindentation results in combination with the post-mortem microstructural characterization show that the most likely deformation mechanism responsible for pop-in events corresponds to sudden increases of the thicknesses of twinned martensite plates. © 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2014.05.061
  • 2014 • 234 Yield strength dependence on strain rate of molybdenum- Alloy nanofibers
    Loya, P.E. and Xia, Y.Z. and Peng, C. and Bei, H. and Zhang, P. and Zhang, J. and George, E.P. and Gao, Y.F. and Lou, J.
    Applied Physics Letters 104 (2014)
    The yield strength dependence on strain rate was studied for molybdenum- Alloy nanofibers with varying initial dislocation density at three different pre-strain levels. In-situ tensile experiments at three displacement rates were carried out in a scanning electron microscope. Yield strength and its scatter decreased as a function of the pre-strain level for different displacement rates. A statistical model was used to analyze the results, and a negative strain rate dependence was inferred from the yield experiments. This finding suggests the need for theoretical investigations since classical models such as dynamic strain aging may have limitations at such nanoscales. © 2014 AIP Publishing LLC.
    view abstractdoi: 10.1063/1.4885377
  • 2014 • 233 In Situ Wear Test on Thermal Spray Coatings in a Large Chamber Scanning Electron Microscope
    Luo, W. and Tillmann, W. and Selvadurai, U.
    Journal of Thermal Spray Technology 24 263-270 (2014)
    Currently, the determination of the mass loss is usually used for a quantitative evaluation of wear tests, while the analysis of wear tracks is utilized for a qualitative evaluation of wear. Both evaluation methods can only be used after the wear testing process and their results only present the final outcome of the wear test. However, the changes during the wear test and the time-dependent wear mechanisms are of great interest as well. A running wear test in a large chamber scanning electron microscope (SEM) offers the first opportunity to observe the wear process in situ. Different wear mechanisms, such as the adhesive, abrasive wear, surface fatigue and tribochemical reaction, can be recorded with high magnification. Within this research, a special pin-on-disk testing device is designed for a vacuum environment. Using this device, arc-sprayed NiCrBSi coatings and high-velocity-oxygen-fuel-sprayed WC-12Co coatings were tested in a large chamber SEM with Al2O3 ceramic balls as wear counterparts. During the wear testing, different wear mechanisms were determined and the processes were recorded in short video streams. © 2014, ASM International.
    view abstractdoi: 10.1007/s11666-014-0163-5
  • 2014 • 232 Characterisation of non-uniform functional surfaces: Towards linking basic surface properties with electrocatalytic activity
    Maljusch, A. and Henry, J.B. and Tymoczko, J. and Bandarenka, A.S. and Schuhmann, W.
    RSC Advances 4 1532-1537 (2014)
    Functional materials, particularly heterogeneous catalysts, are often non-uniform at a microscopic level making their detailed characterisation extremely complex. This complexity inhibits the design and implementation of novel functional materials as such characterisation is a key to understanding interfaces for heterogeneous catalysis. We demonstrate that a combination of Scanning Kelvin Probe (SKP) and Scanning Electrochemical Microscopy (SECM) experiments made over the same sample surface using an integrated SKP-SECM system provides a powerful and robust tool to link basic surface properties with the observed electrocatalytic activity. As the SKP-response can be accurately assessed using modern quantum chemical approaches to benchmark analytical signals for different surface structures with varying compositions, application of an integrated SKP-SECM system can offer valuable insight into the origin of the observed electrocatalytic activity. As model objects, we used Pt(111)-like thin films modified with sub-monolayer and monolayer amounts of Cu atoms located at the electrode surface and in the sub-surface region. The exact position of the Cu atoms relative to the topmost Pt layer greatly affects basic surface properties and governs the electrocatalytic activity of the surface towards various reactions, i.e. the oxygen reduction reaction. SKP-SECM appeared to be a very sensitive tool to monitor those changes as a function of the spatial coordinates. © 2014 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c3ra45845h
  • 2014 • 231 Revealing onset potentials using electrochemical microscopy to assess the catalytic activity of gas-evolving electrodes
    Maljusch, A. and Ventosa, E. and Rincón, R.A. and Bandarenka, A.S. and Schuhmann, W.
    Electrochemistry Communications 38 142-145 (2014)
    Determination of the so-called onset potentials, i.e. the lowest (for the anodic reactions) or the highest (for the cathodic reactions) potentials at which a reaction product is formed at a given electrode and at defined conditions, is very important for the evaluation of the catalytic activity and even more for the comparison of different catalysts. We present an approach for the determination of the onset potentials based on scanning electrochemical microscopy (SECM) using the "substrate generation-tip collection" mode. In the proposed method, the potential applied to the catalyst sample is changed stepwise. A micro-electrode serving as SECM tip is positioned in known close proximity to the catalyst surface and is used to detect the onset of the formation of the product of the catalytic reaction, specifically gas generation at the sample surface. The oxygen evolution reaction (OER) at model RuO 2 and perovskite catalyst surfaces is used to evaluate the approach. The suggested method is supposed to provide a clearer and sensitive means for the detection of the onset potentials of electrolytic gas evolution reactions as compared to conventional procedures which mainly use cyclic voltammetry on stationary or rotating (ring) disk electrodes. Moreover, the detection of the reaction product at the SECM tip allows distinguishing between parasitic reactions at the catalyst surface and the true formation of the anticipated reaction product. © 2013 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.elecom.2013.11.024
  • 2014 • 230 Tribolayer formation in a metal-on-metal (MoM) hip joint: An electrochemical investigation
    Mathew, M.T. and Nagelli, C. and Pourzal, R. and Fischer, A. and Laurent, M.P. and Jacobs, J.J. and Wimmer, M.A.
    Journal of the Mechanical Behavior of Biomedical Materials 29 199-212 (2014)
    The demand for total hip replacement (THR) surgery is increasing in the younger population due to faster rehabilitation and more complete restoration of function. Up to 2009, metal-on-metal (MoM) hip joint bearings were a popular choice due to their design flexibility, post-operative stability and relatively low wear rates. The main wear mechanisms that occur along the bearing surface of MoM joints are tribochemical reactions that deposit a mixture of wear debris, metal ions and organic matrix of decomposed proteins known as a tribolayer. No in-depth electrochemical studies have been reported on the structure and characteristics of this tribolayer or about the parameters involved in its formation.In this study, we conducted an electrochemical investigation of different surfaces (bulk-like: control, nano-crystalline: new implant and tribolayer surface: retrieved implant) made out of two commonly used hip CoCrMo alloys (high-carbon and low-carbon). As per ASTM standard, cyclic polarization tests and electrochemical impedance spectroscopy tests were conducted. The results obtained from electrochemical parameters for different surfaces clearly indicated a reduction in corrosion for the tribolayer surface (Icorr: 0.76μA/cm2). Further, polarization resistance (Rp:2.39±0.60MΩ/cm2) and capacitance (Cdl:15.20±0.75μF/cm2) indicated variation in corrosion kinetics for the tribolayer surface, that attributed to its structure and stability in a simulated body environment. © 2013 Elsevier Ltd.
    view abstractdoi: 10.1016/j.jmbbm.2013.08.018
  • 2014 • 229 Designing Heusler nanoprecipitates by elastic misfit stabilization in Fe-Mn maraging steels
    Millán, J. and Sandlöbes, S. and Al-Zubi, A. and Hickel, T. and Choi, P. and Neugebauer, J. and Ponge, D. and Raabe, D.
    Acta Materialia 76 94-105 (2014)
    B2 NiMn and Ni2MnAl Heusler nanoprecipitates are designed via elastic misfit stabilization in Fe-Mn maraging steels by combining transmission electron microscopy (TEM) correlated atom probe tomography (APT) with ab initio simulations. Guided by these predictions, the Al content of the alloys is systematically varied, and the influence of the Al concentration on structure stability, size and distribution of precipitates formed during ageing at 450 °C is studied using scanning electron microscopy-electron backscatter diffraction, TEM and APT. Specifically, the Ni2MnAl Heusler nanoprecipitates exhibit the finest sizes and highest dispersion and hence lead to significant strengthening. The formation of the different types of precipitates and their structure, size, dispersion and effect on the mechanical properties of the alloys are discussed. © 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2014.05.016
  • 2014 • 228 Recycling of metallic chips by electro-discharge sintering
    Mohr, A. and Röttger, A. and Windmann, M. and Theisen, W.
    Materialwissenschaft und Werkstofftechnik 45 552-560 (2014)
    Composite material Ferro-Titanit® is produced powder-metallurgical by Deutsche Edelstahlwerke GmbH (DEW) and is commonly used for wear and corrosion resistant component parts. Materials properties can be attributed to the microstructure which consists of a corrosion resistant metallic matrix and a huge amount of approx. 50 vol.% of hard Ti-monocarbides. Although Ferro-Titanit® possesses a high amount of hard particles, the material can be machined by turning and drilling in solution annealed condition. Due to the alloying content (Mo, Cr, TiC) of Ferro-Titanit®, there is a high motivation to recover those elements by a recycling process of the chips, thus expensive and limited resources can be saved. On idea of a recycling process can be found in the redensification of those chips by electro discharge sintering (EDS). In this work, chips of the material Ferro-Titanit® were densified by EDS technique and the resulting microstructure was investigated by optical and scanning electron microscopy. Furthermore, microstructure and hardness of the EDS densified specimens was discussed with regard to the microstructure of conventionally sintered Ferro-Titanit®-samples in laboratory conditions. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/mawe.201400266
  • 2014 • 227 Adherence of human mesenchymal stem cells on Ti and TiO2 nano-columnar surfaces fabricated by glancing angle sputter deposition
    Motemani, Y. and Greulich, C. and Khare, C. and Lopian, M. and Buenconsejo, P.J.S. and Schildhauer, T.A. and Ludwig, Al. and Köller, M.
    Applied Surface Science 292 626-631 (2014)
    The interaction of human mesenchymal stem cells (hMSCs) with Ti and TiO2 nano-columnar surfaces fabricated using glancing angle sputter deposition was investigated. The adherence and proliferation of hMSCs on different nano-columnar surfaces, including vertical columns, slanted columns and chevrons, were examined with calcein-acetoxymethyl ester fluorescence staining and scanning electron microscopy. For comparison, adherence of hMSCs on compact, dense films was also studied. After 24 h and 7 days, adherent and viable cells were observed on both, Ti nano-columns as well as dense Ti films, which confirms the biocompatibility of these nanostructures. Very small pseudopodia with width of approximately 20-35 nm and length varying from 20 to 200 nm were observed between the nano-columns, independent of the type of the nano-columnar morphology. Large inter-column spacing and effectively increased surface area make these nanostructures promising candidates for bio-functionalization or drug loading on the surface of Ti-based implants. © 2013 Elsevier B.V.
    view abstractdoi: 10.1016/j.apsusc.2013.12.022
  • 2014 • 226 3D µCT and SEM Analysis of Resolidified Tips of Cored Wires Used in Twin-Wire Arc Spraying
    Nellesen, J. and Abdulgader, M. and Tillmann, W. and Beckmann, F.
    Journal of Thermal Spray Technology 24 55-62 (2014)
    In twin-wire arc spraying (TWAS), the in-flight particles are atomized from a melting bath which generates an inhomogeneous spraying plume. This inhomogeneity is due to the fact that these particles are generated by the impingement of fast continuous flowing air upon the melting tips of electrically conductive wires. This work aims to contribute to the understanding of the initiation of such particles in the TWAS process. For this purpose, cored wires filled with W-rich particles were sprayed. After interrupting the TWAS process, the tips of these cored wires were imaged by 3D µCT and scanning electron microscopy in order to analyze how the filling powder interacts with the melted part of the sheath. The analysis of the 3D tomograms shows that the resolidified melting bath of the cored wires is interspersed with both spherical and irregular-shaped W-rich particles. This irregular shape suggests a partial melting of the W-rich particles. © 2014, ASM International.
    view abstractdoi: 10.1007/s11666-014-0169-z
  • 2014 • 225 Microstructural evolution after thermomechanical processing in an equiatomic, single-phase CoCrFeMnNi high-entropy alloy with special focus on twin boundaries
    Otto, F. and Hanold, N.L. and George, E.P.
    Intermetallics 54 39-48 (2014)
    The FCC-structured equiatomic CoCrFeMnNi high-entropy alloy was produced by arc melting and drop casting. After homogenization, the drop-cast ingots were cold rolled to sheets with six different final thicknesses (thickness reductions of 21, 41, 61, 84, 92 and 96%). Samples were cut from the rolled sheets and annealed for 1 h at temperatures between 400 and 1000 °C. The recrystallization temperature was then determined as a function of cold work by means of scanning electron microscopy and electron backscatter diffraction measurements. Additionally, Vickers indentation was performed on these samples. It was found that the microhardness first tends to increase slightly upon annealing below the recrystallization temperature but then drops steeply for higher annealing temperatures due to the onset of recrystallization. To study grain growth kinetics, samples that underwent 96% cold rolling were first recrystallized for 1 h at 800 °C, which is the lowest temperature at which complete recrystallization occurs, and then annealed at temperatures between 800 and 1150 °C for various times. The grain growth exponent was determined to be approximately n = 3, and the activation energy Q = 325 kJ/mol, both of which agree well with published values for this alloy. EBSD measurements were made in the as-recrystallized and grain growth samples to analyze the annealing twins. The density of annealing twins in the grain growth samples was found to depend only on grain size, i.e., it was independent of annealing temperature and time. No such correlation could be found for the as-recrystallized samples. These observations are discussed in the framework of existing theories for the formation of annealing twins. © 2014 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.intermet.2014.05.014
  • 2014 • 224 Urease-induced calcification of segmented polymer hydrogels - A step towards artificial biomineralization
    Rauner, N. and Meuris, M. and Dech, S. and Godde, J. and Tiller, J.C.
    Acta Biomaterialia 10 3942-3951 (2014)
    Natural organic/inorganic composites, such as nacre, bones and teeth, are perfectly designed materials with exceptional mechanical properties. Numerous approaches have been taken to synthetically prepare such composites. The presented work describes a new way of mineralizing bulk materials on a large scale following the approach of bioinduced mineralization. To this end, a series of polymer conetworks with entrapped urease were prepared. After polymerization, the entrapped urease shows high enzymatic activity. The bioactive polymer conetworks were then treated with an aqueous mixture of urea and CaCl2. The urease-induced calcification indeed allows formation of carbonate crystals exclusively within the hydrogel even at room temperature. The influence of network composition, degree of cross-linking, immobilized urease concentration and temperature of calcification were investigated. By varying these parameters, spherical, monolithic clusters, as well as bar-like nanocrystals with different aspect ratios in spherical or dendritic arrays, are formed. The grown nanocrystals improve the stiffness of the starting material by up to 700-fold, provided that the microstructure shows a dense construction without pores and strong interaction between crystals and network. The process has the potential to generate a new class of hybrid materials that would be available on the macroscopic scale for use in lightweight design and medicine. © 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actbio.2014.05.021
  • 2014 • 223 Activation of oxygen evolving perovskites for oxygen reduction by functionalization with Fe-Nx/C groups
    Rincón, R.A. and Masa, J. and Mehrpour, S. and Tietz, F. and Schuhmann, W.
    Chemical Communications 50 14760-14762 (2014)
    The incorporation of Fe-Nx/C moieties into perovskites remarkably activates them for the oxygen reduction reaction (ORR) and also leads to notable improvement of their activity towards the oxygen evolution reaction (OER) thus presenting a new route for realizing high performance, low cost bifunctional catalysts for reversible oxygen electrodes. This journal is © the Partner Organisations 2014.
    view abstractdoi: 10.1039/c4cc06446a
  • 2014 • 222 New nanofibrous scaffold for corneal tissue engineering
    Salehi, S. and Grünert, A.K. and Bahners, T. and Gutmann, J.S. and Steuhl, K.P. and Czugala, M. and Singer, B.B. and Fuchsluger, T.A.
    Klinische Monatsblatter fur Augenheilkunde 231 626-630 (2014)
    Background: An estimated 10 million people suffer worldwide from vision loss caused by corneal damage. For the worst cases, the only available treatment is transplantation with human donor corneal tissue. However, in numerous countries there is a considerable shortage of corneal tissue of good quality, leading to various efforts to develop tissue substitutes. The present study aims to introduce a nanofibrous scaffold of poly(glycerol sebacate) PGS as a biodegradable implant, for the corneal tissue engineering. Materials and Methods: Nanofibrous scaffolds were produced from PGS and poly(ε- caprolactone) (PCL) by a modified electro-spinning process. The biocompatibility of the material was tested in vitro by colorimetric MTT assay on days 3, 5, and 7 to test the cell viability of human corneal endothelium cells (HCEC). To examine a potential immunological reaction of the scaffolds, samples were exposed to mononuclear cells derived from peripheral blood (PBMCs). After an incubation period of 3 days, supernatants were assayed for apoptotic assessment and immunogenic potentials by annexin V FITC//propidium iodide and flow-cytometric analysis. Results: We could successfully demonstrate that cultivation of HCECs on PGS/PCL scaffolds was possible. Compared to day 3, cell density determined by microplate absorbance was significantly higher after 7 days of cultivation (p 0.0001). According to the MTT data, none of the samples showed toxicity. Apoptotic assessments by FACS analysis showed that no composition stimulated apoptosis or activated PBMCs occurred. All the compositions were inert for native as well as activated T/B/NK cells and monocytes. It can be concluded that leukocytes and their activity was not affected by the scaffolds. Conclusion: A tissue-like scaffold mimicking the human stroma could be developed. The results indicate that PGS/PCL scaffolds could be considered as ideal candidates for corneal tissue engineering as they are biocompatible in contact to corneal endothelial cells and blood cells. © 2014 Georg Thieme Verlag KG Stuttgart New York.
    view abstractdoi: 10.1055/s-0034-1368533
  • 2014 • 221 Photothermal laser microsintering of nanoporous gold
    Schade, L. and Franzka, S. and Mathieu, M. and Biener, M.M. and Biener, J. and Hartmann, N.
    Langmuir 30 7190-7197 (2014)
    Photothermal processing of nanoporous gold using a microfocused continuous-wave laser at a wavelength of 532 nm and a 1/e2 spot diameter of 2.9 μm has been studied. In addition, complementary experiments have been carried out via conventional annealing. Scanning electron microscopy has been used for characterization. Local laser irradiation at distinct laser powers and pulse lengths results in coarsening of the porous gold structures. During laser processing the pore size of the native nanoporous gold increases to maximum values in the range of 0.25-3 μm. The affected areas exhibit lateral dimensions in the range of 2-10 μm. Overall two regions are distinguished. An inner region, where large pores and ligaments are formed and an outer region, where the pore size and ligament size gradually change and approach the feature sizes of the native material. A qualitative thermokinetic model allows one to reproduce the experimentally observed dependence of the laser-induced morphologies on the laser parameters. On the basis of this model the underlying processes are attributed to sintering and melting of the gold structures. The presented results demonstrate the prospects of photothermal laser processing in engineering porous gold with spatially varying porosities on micrometer to nanometer length scales. © 2014 American Chemical Society.
    view abstractdoi: 10.1021/la5011192
  • 2014 • 220 Photothermal laser fabrication of micro- and nanostructured chemical templates for directed protein immobilization
    Schröter, A. and Franzka, S. and Hartmann, N.
    Langmuir 30 14841-14848 (2014)
    Photothermal patterning of poly(ethylene glycol) terminated organic monolayers on surface-oxidized silicon substrates is carried out using a microfocused beam of a CW laser operated at a wavelength of 532 nm. Trichlorosilane and trimethoxysilane precursors are used for coating. Monolayers from trimethoxysilane precursors show negligible unspecific protein adsorption in the background, i.e., provide platforms of superior protein repellency. Laser patterning results in decomposition of the monolayers and yields chemical templates for directed immobilization of proteins at predefined positions. Characterization is carried out via complementary analytical methods including fluorescence microscopy, atomic force microscopy, and scanning electron microscopy. Appropriate labeling techniques (fluorescent markers and gold clusters) and substrates (native and thermally oxidized silicon substrates) are chosen in order to facilitate identification of protein adsorption and ensure high sensitivity and selectivity. Variation of the laser parameters at a 1/e2 spot diameter of 2.8 μm allows for fabrication of protein binding domains with diameters on the micrometer and nanometer length scale. Minimum domain sizes are about 300 nm. In addition to unspecific protein adsorption on as-patterned monolayers, biotin-streptavidin coupling chemistry is exploited for specific protein binding. This approach represents a novel facile laser-based means for fabrication of protein micro- and nanopatterns. The routine is readily applicable to femtosecond laser processing of glass substrates for the fabrication of transparent templates. (Graph Presented). © 2014 American Chemical Society.
    view abstractdoi: 10.1021/la503814n
  • 2014 • 219 Structure-related antibacterial activity of a titanium nanostructured surface fabricated by glancing angle sputter deposition
    Sengstock, C. and Lopian, M. and Motemani, Y. and Borgmann, A. and Khare, C. and Buenconsejo, P.J.S. and Schildhauer, T.A. and Ludwig, Al. and Köller, M.
    Nanotechnology 25 (2014)
    The aim of this study was to reproduce the physico-mechanical antibacterial effect of the nanocolumnar cicada wing surface for metallic biomaterials by fabrication of titanium (Ti) nanocolumnar surfaces using glancing angle sputter deposition (GLAD). Nanocolumnar Ti thin films were fabricated by GLAD on silicon substrates. S. aureus as well as E. coli were incubated with nanostructured or reference dense Ti thin film test samples for one or three hours at 37 °C. Bacterial adherence, morphology, and viability were analyzed by fluorescence staining and scanning electron microscopy and compared to human mesenchymal stem cells (hMSCs). Bacterial adherence was not significantly different after short (1 h) incubation on the dense or the nanostructured Ti surface. In contrast to S. aureus the viability of E. coli was significantly decreased after 3 h on the nanostructured film compared to the dense film and was accompanied by an irregular morphology and a cell wall deformation. Cell adherence, spreading and viability of hMSCs were not altered on the nanostructured surface. The results show that the selective antibacterial effect of the cicada wing could be transferred to a nanostructured metallic biomaterial by mimicking the natural nanocolumnar topography. © 2014 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0957-4484/25/19/195101
  • 2014 • 218 On the spheroidized carbide dissolution and elemental partitioning in high carbon bearing steel 100Cr6
    Song, W. and Choi, P.-P. and Inden, G. and Prahl, U. and Raabe, D. and Bleck, W.
    Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science 45 595-606 (2014)
    We report on the characterization of high carbon bearing steel 100Cr6 using electron microscopy and atom probe tomography in combination with multi-component diffusion simulations. Scanning electron micrographs show that around 14 vol pct spheroidized carbides are formed during soft annealing and only 3 vol pct remain after dissolution into the austenitic matrix through austenitization at 1123 K (850 °C) for 300 seconds. The spheroidized particles are identified as (Fe, Cr)3C by transmission electron microscopy. Atom probe analysis reveals the redistribution and partitioning of the elements involved, i.e., C, Si, Mn, Cr, Fe, in both, the spheroidized carbides and the bainitic matrix in the sample isothermally heat-treated at 773 K (500 °C) after austenitization. Homogeneous distribution of C and a Cr gradient were detected within the spheroidized carbides. Due to its limited diffusivity in (Fe, Cr) 3C, Cr exhibits a maximum concentration at the surface of spheroidized carbides (16 at. pct) and decreases gradually from the surface towards the core down to about 2 at. pct. The atom probe results also indicate that the partially dissolved spheroidized carbides during austenitization may serve as nucleation sites for intermediate temperature cementite within bainite, which results in a relatively softer surface and harder core in spheroidized particles. This microstructure may contribute to the good wear resistance and fatigue properties of the steel. Good agreement between DICTRA simulations and experimental composition profiles is obtained by an increase of mobility of the substitutional elements in cementite by a factor of five, compared to the mobility in the database MOBFE2. © The Minerals, Metals & Materials Society and ASM International 2013.
    view abstractdoi: 10.1007/s11661-013-2048-5
  • 2014 • 217 Fabrication of a Ni-Cu thin film material library using pulsed electrodeposition
    Srinivas, P. and Hamann, S. and Wambach, M. and Ludwig, Al. and Dey, S.R.
    Journal of the Electrochemical Society 161 D504-D509 (2014)
    A thin film composition gradient library of the Ni-Cu alloy system is generated through an electrodeposition technique using a complexing citrate electrolyte bath in a modified Hull cell. Energy dispersive X-ray spectroscopy, scanning electron microscopy and automated X-ray diffraction are performed to assess composition, surface morphology, and crystallographic structure of the deposited film as a function of the lateral position on the materials library. The results confirmed deposition of single phase polycrystalline f.c.c. Ni-Cu alloy system with varied lateral composition and lattice parameter, afcc as well. © 2014 The Electrochemical Society. All rights reserved.
    view abstractdoi: 10.1149/2.0451410jes
  • 2014 • 216 Recent progress in scanning electron microscopy for the characterization of fine structural details of nano materials
    Suga, M. and Asahina, S. and Sakuda, Y. and Kazumori, H. and Nishiyama, H. and Nokuo, T. and Alfredsson, V. and Kjellman, T. and Stevens, S.M. and Cho, H.S. and Cho, M. and Han, L. and Che, S. and Anderson, M.W. and Schüth, F. an...
    Progress in Solid State Chemistry 42 1-21 (2014)
    Research concerning nano-materials (metal-organic frameworks (MOFs), zeolites, mesoporous silicas, etc.) and the nano-scale, including potential barriers for the particulates to diffusion to/from is of increasing importance to the understanding of the catalytic utility of porous materials when combined with any potential super structures (such as hierarchically porous materials). However, it is difficult to characterize the structure of for example MOFs via X-ray powder diffraction because of the serious overlapping of reflections caused by their large unit cells, and it is also difficult to directly observe the opening of surface pores using ordinary methods. Electron-microscopic methods including high-resolution scanning electron microscopy (HRSEM) have therefore become imperative for the above challenges. Here, we present the theory and practical application of recent advances such as through-the-lens detection systems, which permit a reduced landing energy and the selection of high-resolution, topographically specific emitted electrons, even from electrically insulating nano-materials. © 2014 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.progsolidstchem.2014.02.001
  • 2014 • 215 A carbon-coated TiO2(B) nanosheet composite for lithium ion batteries
    Sun, Z. and Huang, X. and Muhler, M. and Schuhmann, W. and Ventosa, E.
    Chemical Communications 50 5506-5509 (2014)
    The carbon-coated TiO2(B) nanosheet composite synthesized by one-step hydrolysis of TiCl3 followed by vacuum annealing and air annealing delivers outstanding electrochemical performance as a negative electrode for Li-ion batteries, i.e. reversible capacity above 150 mA h g -1 at 30 C (10 A g-1). This journal is © the Partner Organisations 2014.
    view abstractdoi: 10.1039/c4cc01888e
  • 2014 • 214 Targeting and activation of antigen-specific B-cells by calcium phosphate nanoparticles loaded with protein antigen
    Temchura, V.V. and Kozlova, D. and Sokolova, V. and Überla, K. and Epple, M.
    Biomaterials 35 6098-6105 (2014)
    Cross-linking of the B-cell receptors of an antigen-specific B-cell is the initial signal for B-cell activation, proliferation, and differentiation into antibody secreting plasma cells. Since multivalent particulate structures are efficient activators of antigen-specific B-cells, we developed biodegradable calcium phosphate nanoparticles displaying protein antigens on their surface and explored the efficacy of the B-cell activation after exposure to these nanoparticles. The calcium phosphate nanoparticles were functionalized with the model antigen Hen Egg Lysozyme (HEL) to take advantage of a HEL-specific B-cell receptor transgenic mouse model. The nanoparticles were characterized by scanning electron microscopy and dynamic light scattering. The functionalized calcium phosphate nanoparticles were preferentially bound and internalized by HEL-specific B-cells. Co-cultivation of HEL-specific B-cells with the functionalized nanoparticles also increased surface expression of B-cell activation markers. Functionalized nanoparticles were able to effectively cross-link B-cell receptors at the surface of antigen-matched B-cells and were 100-fold more efficient in the activation of B-cells than soluble HEL. Thus, calcium phosphate nanoparticles coated with protein antigens are promising vaccine candidates for induction humoral immunity. © 2014 Elsevier Ltd.
    view abstractdoi: 10.1016/j.biomaterials.2014.04.010
  • 2014 • 213 Rational design of gold nanoparticle toxicology assays: A question of exposure scenario, dose and experimental setup
    Taylor, U. and Rehbock, C. and Streich, C. and Rath, D. and Barcikowski, S.
    Nanomedicine 9 1971-1989 (2014)
    Many studies have evaluated the toxicity of gold nanoparticles, although reliable predictions based on these results are rare. In order to overcome this problem, this article highlights strategies to improve comparability and standardization of nanotoxicological studies. To this end, it is proposed that we should adapt the nanomaterial to the addressed exposure scenario, using ligand-free nanoparticle references in order to differentiate ligand effects from size effects. Furthermore, surface-weighted particle dosing referenced to the biologically relevant parameter (e.g., cell number or organ mass) is proposed as the gold standard. In addition, it is recommended that we should shift the focus of toxicological experiments from 'live-dead' assays to the assessment of cell function, as this strategy allows observation of bioresponses at lower doses that are more relevant for in vivo scenarios. © 2014 Future Medicine Ltd.
    view abstractdoi: 10.2217/nnm.14.139
  • 2014 • 212 Rapid identification of areas of interest in thin film materials libraries by combining electrical, optical, X-ray diffraction, and mechanical high-throughput measurements: A case study for the system ni-al
    Thienhaus, S. and Naujoks, D. and Pfetzing-Micklich, J. and König, D. and Ludwig, Al.
    ACS Combinatorial Science 16 686-694 (2014)
    The efficient identification of compositional areas of interest in thin film materials systems fabricated by combinatorial deposition methods is essential in combinatorial materials science. We use a combination of compositional screening by EDX together with high-throughput measurements of electrical and optical properties of thin film libraries to determine efficiently the areas of interest in a materials system. Areas of interest are compositions which show distinctive properties. The crystallinity of the thus determined areas is identified by X-ray diffraction. Additionally, by using automated nanoindentation across the materials library, mechanical data of the thin films can be obtained which complements the identification of areas of interest. The feasibility of this approach is demonstrated by using a Ni-Al thin film library as a reference system. The obtained results promise that this approach can be used for the case of ternary and higher order systems. © 2014 American Chemical Society.
    view abstractdoi: 10.1021/co5000757
  • 2014 • 211 Wear analysis of thermal spray coatings on 3D surfaces
    Tillmann, W. and Luo, W. and Selvadurai, U.
    Journal of Thermal Spray Technology 23 245-251 (2014)
    Even though the application of thermal spray coatings on complex geometries gained a greater interest in the last decade, the effect of different geometrical features on the wear behavior is still ill-defined. In this study, the wear resistance of FTC-FeCSiMn coated 3D surfaces was investigated. The wear test was carried out by means of two innovative testing procedures. The first test is a Pin-on-Tubes test where the rotating motion is realized by a lathe chuck. The specimens in the second test were fixed on the table and a robot arm operated the pin. This wear test was applied on specimens with concave or convex surfaces. The residual stresses, which were determined by means of an incremental hole-drilling method, show a dependency on the substrate geometry. The obtained stresses were put in relation to the different radii. After the wear test, a 3D-profilometer determined the wear volume and the sections of the coatings were characterized by a scanning electron microscope. The results indicate that the wear resistance is strongly influenced by the geometry of the substrate. © 2013 ASM International.
    view abstractdoi: 10.1007/s11666-013-9983-y
  • 2014 • 210 Deposition of superelastic composite NiTi based films
    Tillmann, W. and Momeni, S.
    Vacuum 104 41-46 (2014)
    In recent years, NiTi shape memory alloys (SMA) thin films have been widely used as promising high-performance materials in the field of biomedical and microelectromechanical (MEMS) systems. However, there are still important problems such as their unsatisfactory mechanical and tribological properties including a limited hardness and wear resistance. This study aimed at deposition of layered composite thin films made of NiTi and TiCN thin films on Si (100) substrate by means of DC magnetron sputtering. Subsequently, microstructures, mechanical properties and shape memory behavior of these bilayers were investigated using Nanoindentation, X-ray diffraction (XRD), scanning electron microscopy (SEM) and differential scanning calorimetry (DSC). The results of this study confirmed that the presence of TiCN layer on NiTi thin film modifies its mechanical properties while maintaining the shape memory effects. The initial findings of this research work are suggestive of the potential for fabrication of self-healed composite NiTi based films. © 2014 Published by Elsevier Ltd.
    view abstractdoi: 10.1016/j.vacuum.2013.12.010
  • 2014 • 209 Apoptotic, inflammatory, and fibrogenic effects of two different types of multi-walled carbon nanotubes in mouse lung
    Van Berlo, D. and Wilhelmi, V. and Boots, A.W. and Hullmann, M. and Kuhlbusch, T.A.J. and Bast, A. and Schins, R.P.F. and Albrecht, C.
    Archives of Toxicology 88 1725-1737 (2014)
    There is increasing concern about the toxicity of inhaled multi-walled carbon nanotubes (MWCNTs). Pulmonary macrophages represent the primary cell type involved in the clearance of inhaled particulate materials, and induction of apoptosis in these cells has been considered to contribute to the development of lung fibrosis. We have investigated the apoptotic, inflammogenic, and fibrogenic potential of two types of MWCNTs, characterised by a contrasting average tube length and entanglement/agglomeration. Both nanotube types triggered H2O2 formation by RAW 264.7 macrophages, but in vitro toxicity was exclusively seen with the longer MWCNT. Both types of nanotubes caused granuloma in the mouse lungs. However, the long MWCNT induced a more pronounced pro-fibrotic (mRNA expression of matrix metalloproteinase-8 and tissue inhibitor of metalloproteinase-1) and inflammatory (serum level of monocyte chemotactic protein-1) response. Masson trichrome staining also revealed epithelial cell hyperplasia for this type of MWCNT. Enhanced apoptosis was detected by cleaved caspase 3 immunohistochemistry in lungs of mice treated with the long and rigid MWCNT and, to a lesser extent, with the shorter, highly agglomerated MWCNT. However, staining was merely localised to granulomatous foci, and neither of the MWCNTs induced apoptosis in vitro, evaluated by caspase 3/7 activity in RAW 264.7 cells. In addition, our study reveals that the inflammatory and pro-fibrotic effects of MWCNTs in the mouse lung can vary considerably depending on their composition. The in vitro analysis of macrophage apoptosis appears to be a poor predictor of their pulmonary hazard. © 2014 Springer-Verlag.
    view abstractdoi: 10.1007/s00204-014-1220-z
  • 2014 • 208 The effect of notches on the fatigue behavior in NiTi shape memory alloys
    Wang, X.M. and Cao, W. and Deng, C.H. and Liu, H. and Pfetzing-Micklich, J. and Yue, Z.F.
    Materials Science and Engineering A 610 188-196 (2014)
    Fatigue experiments have been performed on superelastic NiTi double-notched plate form specimens with different notch types and sizes. The fatigue loading was controlled by the average stress at the minimum cross-section. The fatigue life as well as the fatigue fracture surface has been investigated in detail. The fatigue life decreases from specimens without notches, to specimens with semi-circular notches followed by v-type notched specimens. Specimens with crack notches show the shortest fatigue lives. The fatigue life decreases with increasing the notch acuity. Not only notch type but also notch size affects the fatigue life under the same macro-average stress. The fatigue fracture surfaces are quite similar to those of the conventional ductile materials with crack initiation, propagation and final fracture area. Typical characteristics of low cycle fatigue surfaces such as multi-fatigue crack initiation sites, weak striations and tyre-like patterns were found. Finite element (FE) simulations were performed to investigate the stress and phase transformation distributions in order to explain and understand the experimental results. © 2014 Elsevier B.V.
    view abstractdoi: 10.1016/j.msea.2014.04.109
  • 2014 • 207 Smaller is less stable: Size effects on twinning vs. transformation of reverted austenite in TRIP-maraging steels
    Wang, M.-M. and Tasan, C.C. and Ponge, D. and Kostka, A. and Raabe, D.
    Acta Materialia 79 268-281 (2014)
    Steels containing reverted nanoscale austenite (γRN) islands or films dispersed in a martensitic matrix show excellent strength, ductility and toughness. The underlying microstructural mechanisms responsible for these improvements are not yet understood, but are observed to be strongly connected to the γRN island or film size. Two main micromechanical effects are conceivable in this context, namely: (i) interaction of γRN with microcracks from the matrix (crack blunting or arresting); and (ii) deformation-induced phase transformation of γRN to martensite (TRIP effect). The focus here is on the latter phenomenon. To investigate size effects on γRN transformation independent of other factors that can influence austenite stability (composition, crystallographic orientation, defect density, surrounding phase, etc.), a model (TRIP-maraging steel) microstructure is designed with support from diffusion simulations (using DICTRA software) to have the same, homogeneous chemical composition in all γRN grains. Characterization is conducted by in-situ tension and bending experiments in conjunction with high-resolution electron backscatter diffraction mapping and scanning electron microscopy imaging, as well as post-mortem transmission electron microscopy and synchrotron X-ray diffraction analysis. Results reveal an unexpected "smaller is less stable" effect due to the size-dependent competition between mechanical twinning and deformation-induced phase transformation. © 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2014.07.020
  • 2014 • 206 A facile solution-doping method to improve a low-temperature zinc oxide precursor: Towards low-cost electronics on plastic foil
    Weber, D. and Botnaraş, S. and Pham, D.V. and Merkulov, A. and Steiger, J. and Schmechel, R. and De Cola, L.
    Advanced Functional Materials 24 2537-2543 (2014)
    Optimization of thin-film transistors performance is usually accompanied by an increase of the process temperature. This work presents a method to raise the field effect mobility by a factor of 3 without a change of the process parameters. The modification involves a solution doping process where an ammine zinc complex is formed in the presence of metal ions of the 13th group, namely gallium and indium. Morphological studies, including scanning electron microscopy and atomic force microscopy, reveal the difference among the resulting films. Moreover, X-ray diffraction results show that the doping affects the preferred orientation of the zinc oxide crystals in the resulting film. The electrical properties vary distinctly and are best for a solution doped with both gallium and indium. With a double-layer system the performance of this new precursor exceeds field effect mobility values of 1 cm2 V-1 s-1 after a maximum process temperature of 160 °C. The performance of ZnO-based field-effect transistors is improved by a simple solution-doping procedure using ions of the 13th group. The method has a strong influence on the film morphology and orientation of the crystallites. This leads to field effect mobility values comparable to amorphous silicon. The low conversion temperature allows the fabrication on flexible substrates. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/adfm.201303461
  • 2014 • 205 Corrosion fatigue behaviour of creep-resistant magnesium alloy mg-4al-2ba-2ca
    Wittke, P. and Klein, M. and Walther, F.
    Procedia Engineering 74 78-83 (2014)
    Low corrosion resistance of magnesium alloys strongly limits their application range. This study aims at the investigation of corrosion influence on microstructure and depending mechanical properties of newly developed magnesium alloy Mg-4Al-2Ba- 2Ca. The fatigue properties of this creep-resistant magnesium alloy were investigated under three corrosive environments: double distilled water, 0.01 and 0.1 mol L-1 NaCl solutions. Potentiodynamic polarization measurements and immersion tests were performed to estimate the corrosion behaviour. Specimen surfaces were observed using light and scanning electron microscopy for microstructure-related assessment of corrosion mechanisms. The corrosion fatigue behaviour was characterized in continuous load increase tests using plastic strain and electrochemical measurements. Continuous load increase tests allow estimating the fatigue limit and determining the failure stress amplitude with a single specimen. Fatigue results showed a significant decrease in the estimated fatigue limit and determined failure stress amplitude with increasing corrosion impact of the environments. This corrosion-structure-property relation was quantitatively described by means of model-based correlation approaches and failure hypotheses. Plastic strain amplitude and deformation-induced changes in electrochemical measurands can be equivalently applied for precise corrosion fatigue assessment. © 2014 The Authors. Published by Elsevier Ltd.
    view abstractdoi: 10.1016/j.proeng.2014.06.228
  • 2014 • 204 Recovery, recrystallization, grain growth and phase stability of a family of FCC-structured multi-component equiatomic solid solution alloys
    Wu, Z. and Bei, H. and Otto, F. and Pharr, G.M. and George, E.P.
    Intermetallics 46 131-140 (2014)
    The equiatomic high-entropy alloy FeNiCoCrMn is known to crystallize as a single phase with the face-centered cubic (FCC) crystal structure. To better understand this quinary solid solution alloy, we investigate various binary, ternary and quaternary alloys made from its constituent elements. Our goals are twofold: (i) to investigate which of these lower order systems also form solid solution alloys consisting of a single FCC phase, and (ii) to characterize their phase stability and recovery, recrystallization, and grain growth behaviors. X-ray diffraction (XRD) and scanning electron microscopy with backscattered electron images showed that three of the five possible quaternaries (FeNiCoCr, FeNiCoMn and NiCoCrMn), five of the ten possible ternaries (FeNiCo, FeNiCr, FeNiMn, NiCoCr, and NiCoMn), and two of the ten possible binaries (FeNi and NiCo) were single-phase FCC solid solutions in the cast and homogenized condition, whereas the others either had different crystal structures or were multi-phase. The single-phase FCC quaternary, FeNiCoCr, along with its equiatomic ternary and binary subsidiaries, were selected for further investigations of phase stability and the thermomechanical processing needed to obtain equiaxed grain structures. Only four of these subsidiary alloys - two binaries (FeNi and NiCo) and two ternaries (FeNiCo and NiCoCr) - were found to be single-phase FCC after rolling at room temperature followed by annealing for 1 h at temperatures of 300-1100 C. Pure Ni, which is FCC and one of the constituents of the quinary high-entropy alloy (FeNiCoCrMn), was also investigated for comparison with the higher order alloys. Among the materials investigated after thermomechanical processing (FeNiCoCr, FeNiCo, NiCoCr, FeNi, NiCo, and Ni), FeNiCo and Ni showed abnormal grain growth at relatively low annealing temperatures, while the other four showed normal grain growth behavior. The grain growth exponents for all five of the equiatomic alloys were found to be ∼0.25 (compared to ∼0.5 for unalloyed Ni), suggesting that solute drag may control grain growth in the alloys. For all five alloys, as well as for pure Ni, microhardness increases as the grain size decreases in a Hall-Petch type way. The ternary alloy NiCoCr was the hardest of the alloys investigated in this study, even when compared to the quaternary FeNiCoCr alloy. This suggests that solute hardening in equiatomic alloys depends not just on the number of alloying elements but also their type. © 2013 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.intermet.2013.10.024
  • 2014 • 203 Nanostructured Er2O3 thin films grown by metalorganic chemical vapour deposition
    Xu, K. and Dang, V.-S. and Ney, A. and De Los Arcos, T. and Devi, A.
    Journal of Nanoscience and Nanotechnology 14 5095-5102 (2014)
    Metalorganic chemical vapor deposition (MOCVD) of nanostructured Er 2O3 thin films was performed using the Er-tris-guanidinate precursor [Er(DPDMG)3] (DPDMG = diisopropyl-2- dimethylamidoguanidinato) as the Er source and oxygen. Film deposition was carried out on Si(100) and quartz glass substrates and the process parameters namely temperature, pressure and oxygen flow rate were varied. The resulting thin films were characterised by X-ray diffraction (XRD), scanning electron microscopy (SEM) for investigating the crystallinity and morphology, respectively. The chemical composition of the film was investigated by X-ray photoelectron spectroscopy (XPS) measurements. Transmittance and absorption spectra of the 600 °C film grown on glass substrates were performed by UV-vis measurements revealing more than 80% transmittance. The potential of Er2O3 thin films as gate dielectrics was verified by carrying out capacitance-voltage (C-V ) and current-voltage (I-V ) measurements. Dielectric constants estimated from the accumulation capacitance were found to be in the range of 10-12 in AC frequencies of 1 MHz down to 10 kHz and the leakage current of the order of 2×10-8 A/cm2 at the applied field of 1 MV cm-1 was measured for films deposited under optimised process conditions. The low leakage current and high dielectric constant implies good quality of the Er2O3 layers relevant for high-k applications. These layers were found to be paramagnetic with a slightly reduced magnetic moment of the Er3+ ions. Copyright © 2014 American Scientific Publishers All rights reserved.
    view abstractdoi: 10.1166/jnn.2014.8848
  • 2014 • 202 Designing quadplex (four-phase) microstructures in an ultrahigh carbon steel
    Zhang, H. and Ponge, D. and Raabe, D.
    Materials Science and Engineering A 612 46-53 (2014)
    Here we present an approach to design a ferrite-based quadplex microstructure (ferrite/martensite/carbide/austenite) using a lean alloyed Mn-Si-Cr-Al ultrahigh carbon steel. The material has 1500MPa tensile strength and 11% elongation. The thermomechanical processing includes two main steps, namely, first, the formation of a ferrite plus carbide duplex microstructure by warm rolling below Ae1; and second, annealing just above Ae1 for a short time (~20min). The quadplex microstructure consists of 57vol% ultrafine ferrite (mean grain size ~1.5μm), 29vol% martensite, 12vol% spherical carbide and 2vol% austenite. Fracture analysis after tensile deformation reveals a mixed ductile and brittle failure mode without necking. Scanning electron microscopy (SEM), electron backscatter diffraction (EBSD) and dilatometry tests were conducted to map the microstructure characteristics and the contribution of each phase to the overall deformation. © 2014 Elsevier B.V.
    view abstractdoi: 10.1016/j.msea.2014.06.023
  • 2014 • 201 Enhanced superplasticity in an Al-alloyed multicomponent Mn-Si-Cr-C steel
    Zhang, H. and Pradeep, K.G. and Mandal, S. and Ponge, D. and Choi, P. and Tasan, C.C. and Raabe, D.
    Acta Materialia 63 232-244 (2014)
    Excellent superplasticity (elongation ∼720%) is observed in a novel multi-component (Mn-S-Cr-Al alloyed) ultrahigh carbon steel during tensile testing at a strain rate of 2 × 10-3 s-1 and a temperature of 1053 K (just above the equilibrium austenite-pearlite transformation temperature). In order to understand superplasticity in this material and its strong Al dependence, the deformation-induced microstructure evolution is characterized at various length scales down to atomic resolution, using X-ray diffraction, scanning electron microscopy, electron backscatter diffraction, energy-dispersive X-ray spectroscopy and atom probe tomography. The results reveal that 1 wt.% Al addition influences various microprocesses during deformation, e.g. it impedes Ostwald ripening of carbides, carbide dissolution, austenite nucleation and growth and void growth. As a result, the size of the austenite grains and voids remains relatively fine (< 10 μm) during superplastic deformation, and fine-grained superplasticity is enabled without premature failure. © 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2013.10.034
  • 2014 • 200 New insights into the austenitization process of low-alloyed hypereutectoid steels: Nucleation analysis of strain-induced austenite formation
    Zhang, H. and Pradeep, K.G. and Mandal, S. and Ponge, D. and Raabe, D.
    Acta Materialia 80 296-308 (2014)
    Austenite formation, which originated from a fined-grained ferrite plus carbide microstructure, was observed during tensile testing at 973 K (60 K below Ae1, the equilibrium austenite-pearlite transformation temperature). Scanning electron microscopy, electron backscatter diffraction and atom probe tomography results reveal the mechanism of austenitic transformation below Ae1. The initial fine-grained microstructure, in combination with the warm deformation process, determines the occurrence of strain-induced austenite formation below Ae1. The initial fine-grained microstructure essentially contains a higher dislocation density to facilitate the formation of Cottrell atmospheres and a larger area fraction of ferrite/carbide interfaces which serve as austenite nucleation sites. The warm deformation promotes the Ostwald ripening process and the increase in dislocation density, and hence promotes the accumulation of local high carbon concentrations in the form of Cottrell atmospheres to reach a sufficiently high thermodynamic driving force for austenite nucleation. The critical carbon concentration required for the nucleation of austenite was calculated using classical nucleation theory, which correlated well with the experimental observations. © 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2014.07.073
  • 2013 • 199 Tuning the nanofiltration performance of thin film strong polyelectrolyte hydrogel composite membranes by photo-grafting conditions
    Bernstein, R. and Antón, E. and Ulbricht, M.
    Journal of Membrane Science 427 129-138 (2013)
    Polyethersulfone ultrafiltration membranes were converted into charged nanofiltration membranes having a strong polyelectrolyte hydrogel as selective barrier layer through the UV-photo initiated graft polymerization technique. This was accomplished by using vinyl sulfonic acid as the functional monomer and N,N'-methylenbisacrylamide as a cross linker monomer (Bernstein et al., ACS Applied Materials & Interfaces, 4 (2012) 3438-3446). In this research the resulting composite membranes were further characterized using different methods (ATR-FTIR spectroscopy, zeta potential, contact angle, scanning electron microscopy). ATR-FTIR data were used to quantify the degree of grafting. The composite membranes' zeta potential was negative throughout the pH range and as high as -70. mV. The hydrogel composite membranes were also very hydrophilic with a contact angle of 11°. The membrane performance-salt rejection and water permeability-obtained at varied functionalization conditions-molecular weight cut-off of the base membrane, monomer concentration, cross linker fraction, UV irradiation intensity and time-was systematically investigated and the results were correlated to the membrane characterization data. Separation performance was also tested using mixed salt solutions. Larger composite membrane samples were prepared and long-term stability of nanofiltration (NF) performance was evaluated in cross-flow experiments. The performance of the best of the newly fabricated composite membranes was comparable to other polyelectrolyte-based NF membranes as well as to some commercial NF membranes presented in the literature. © 2012 Elsevier B.V.
    view abstractdoi: 10.1016/j.memsci.2012.09.034
  • 2013 • 198 Synthesis and characterization of Cu-doped ZnO one-dimensional structures for miniaturized sensor applications with faster response
    Chow, L. and Lupan, O. and Chai, G. and Khallaf, H. and Ono, L.K. and Roldan Cuenya, B. and Tiginyanu, I.M. and Ursaki, V.V. and Sontea, V. and Schulte, A.
    Sensors and Actuators, A: Physical 189 399-408 (2013)
    Detection of chemicals and biological species is an important issue to human health and safety. In this paper, we report the hydrothermal synthesis at 95 °C of Cu-doped ZnO low-dimensional rods for room-temperature (RT) sensing applications and enhanced sensor performances. X-ray diffraction, scanning electron microscopy, X-ray photoelectron spectroscopy, Raman and photoluminescence are used to characterize the material properties. To demonstrate the suitability of the Cu-doped ZnO rods for gas sensor applications and for comparison with pure ZnO, we fabricated a double rod device using Focused Ion Beam. The responses of pure-ZnO and Cu-doped ZnO rods studied in exactly the same condition are reported. We found that Cu-ZnO sensors have enhanced RT sensitivity, faster response time, and good selectivity. Miniaturized Cu-ZnO rod-based sensors can serve as a good candidate for effective H2 detectors with low power consumption. © 2012 Elsevier B.V.
    view abstractdoi: 10.1016/j.sna.2012.09.006
  • 2013 • 197 Atom Probe Tomography Studies on the Cu(In,Ga)Se-2 Grain Boundaries
    Cojocaru-Miredin, O. and Schwarz, T. and Choi, P. P. and Herbig, M. and Wuerz, R. and Raabe, D.
    Jove-journal of Visualized Experiments UNSP e50376 (2013)
    Compared with the existent techniques, atom probe tomography is a unique technique able to chemically characterize the internal interfaces at the nanoscale and in three dimensions. Indeed, APT possesses high sensitivity (in the order of ppm) and high spatial resolution (sub nm). Considerable efforts were done here to prepare an APT tip which contains the desired grain boundary with a known structure. Indeed, sitespecific sample preparation using combined focused-ion-beam, electron backscatter diffraction, and transmission electron microscopy is presented in this work. This method allows selected grain boundaries with a known structure and location in Cu(In, Ga)Se-2 thin-films to be studied by atom probe tomography. Finally, we discuss the advantages and drawbacks of using the atom probe tomography technique to study the grain boundaries in Cu(In,Ga)Se-2 thin-film solar cells.
    view abstractdoi: 10.3791/50376
  • 2013 • 196 Iron oxide film growth under ultrathin polysiloxane networks
    Degen, P. and Wieland, D.C.F. and Paulus, M. and Schroer, M.A. and Tolan, M. and Rehage, H.
    Colloid and Polymer Science 291 653-659 (2013)
    This study focuses on the preparation and characterization of magnetic switchable thin iron oxide-polymer films. In a series of experiments, the formation and growth of iron oxide under ultrathin polysiloxane layers was controlled by changing the concentration of iron ions in the aqueous subphase or by varying the residence time of ammonia in the gas phase above the liquid sample. The growth of the combined film structures is studied in situ by interfacial rheology, optical microscopy, and x-ray scattering experiments and ex situ by scanning electron microscopy. Different stages of iron oxide aggregation, from a very thin layer of amorphous iron oxide with thickness of a few nanometers up to micrometer thick coatings of crystalline maghemite (γ-Fe2O3) were investigated. The specific interactions between the inorganic iron oxide and the polymer membranes cause the creation of new composite materials which are sensitive to magnetic forces. © 2012 Springer-Verlag.
    view abstractdoi: 10.1007/s00396-012-2767-y
  • 2013 • 195 Application of SECM in tracing of hydrogen peroxide at multicomponent non-noble electrocatalyst films for the oxygen reduction reaction
    Dobrzeniecka, A. and Zeradjanin, A. and Masa, J. and Puschhof, A. and Stroka, J. and Kulesza, P.J. and Schuhmann, W.
    Catalysis Today 202 55-62 (2013)
    The redox competition mode of scanning electrochemical microscopy (RC-SECM) was used to study the electrocatalytic activity of three different non-noble metal O2 reduction catalysts at a pH value of 7.4, namely; multi-walled carbon nanotubes (MWCNTs), cobalt protoporphyrin (CoP) and a composite of MWCNTs/CoP. The collection efficiency of a scanning electrochemical microscopy (SECM) tip for the H2O2 generated by the reduction of O2 at the catalyst layer was almost 100%. Consequently, SECM experiments in a combined redox competition and generator/collector mode could be applied for the determination of the number of electrons exchanged during O2 reduction, leading to improved understanding of the intrinsic features of catalyst activity. This approach avoids the typical limitations encountered with rotating ring disk electrode (RRDE) voltammetry, notably, the variation of the quantity of H2O2 in the proximity of the electrode with the speed of electrode rotation or the chemical decomposition of reaction intermediates on the Pt ring, which often introduce inconsistencies and errors in the measured values of the number of exchanged electrons. It is commonly assumed that the O2 reduction reaction on most non-noble metal catalysts proceeds via formation of H2O 2 as an intermediate. The follow-up reaction of H2O 2, typically chemical decomposition or electrochemical reduction, influences the overall number of electrons exchanged during O2 reduction. In this study, we have confirmed by comparing the rate of electrochemical reduction of H2O2 using rotating disk electrode (RDE) measurements with its rate of chemical decomposition studied using a positioned SECM tip, that for the MWCNTs/CoP catalyst, chemical decomposition is predominantly determining the overall number of exchanged electrons per O2 molecule. © 2012 Elsevier B.V.
    view abstractdoi: 10.1016/j.cattod.2012.03.060
  • 2013 • 194 Design of next generation thermal barrier coatings - Experiments and modelling
    Gupta, M. and Curry, N. and Nylén, P. and Markocsan, N. and Vaßen, R.
    Surface and Coatings Technology 220 20-26 (2013)
    Thermal barrier coating (TBC) systems have been used in the gas turbine industry since the 1980s. The future needs both the air and land based turbine industry involve higher operating temperatures with longer lifetime on the component so as to increase power and efficiency of gas turbines. The aim of this study was to meet these future needs by further development of zirconia coatings. The intention was to design a coating system which could be implemented in industry within the next 3. years. Different morphologies of ceramic topcoat were evaluated; using dual layer systems and polymers to generate porosity. Dysprosia stabilised zirconia was also included in this study as a topcoat material along with the state-of-the-art yttria stabilised zirconia (YSZ). High purity powders were selected in this work. Microstructure was assessed with scanning electron microscope and an in-house developed image analysis routine was used to characterise porosity content. Evaluations were carried out using the laser flash technique to measure thermal conductivity. Lifetime was assessed using thermo-cyclic fatigue testing. Finite element analysis was utilised to evaluate thermal-mechanical material behaviour and to design the morphology of the coating with the help of an artificial coating morphology generator through establishment of relationships between microstructure, thermal conductivity and stiffness. It was shown that the combined empirical and numerical approach is an effective tool for developing high performance coatings. The results show that large globular pores and connected cracks inherited within the coating microstructure result in a coating with best performance. A low thermal conductivity coating with twice the lifetime compared to the industrial standard today was fabricated in this work. © 2012 Elsevier B.V.
    view abstractdoi: 10.1016/j.surfcoat.2012.09.015
  • 2013 • 193 The biocompatibility of dense and porous Nickel-Titanium produced by selective laser melting
    Habijan, T. and Haberland, C. and Meier, H. and Frenzel, J. and Wittsiepe, J. and Wuwer, C. and Greulich, C. and Schildhauer, T.A. and Köller, M.
    Materials Science and Engineering C 33 419-426 (2013)
    Nickel-Titanium shape memory alloys (NiTi-SMA) are of biomedical interest due to their unusual range of pure elastic deformability and their elastic modulus, which is closer to that of bone than any other metallic or ceramic material. Newly developed porous NiTi, produced by Selective Laser Melting (SLM), is currently under investigation as a potential carrier material for human mesenchymal stem cells (hMSC). SLM enables the production of highly complex and tailor-made implants for patients on the basis of CT data. Such implants could be used for the reconstruction of the skull, face, or pelvis. hMSC are a promising cell type for regenerative medicine and tissue engineering due to their ability to support the regeneration of critical size bone defects. Loading porous SLM-NiTi implants with autologous hMSC may enhance bone growth and healing for critical bone defects. The purpose of this study was to assess whether porous SLM-NiTi is a suitable carrier for hMSC. Specimens of varying porosity and surface structure were fabricated via SLM. hMSC were cultured for 8 days on NiTi specimens, and cell viability was analyzed using two-color fluorescence staining. Viable cells were detected on all specimens after 8 days of cell culture. Cell morphology and surface topography were analyzed by scanning electron microscopy (SEM). Cell morphology and surface topology were dependent on the orientation of the specimens during SLM production. The Nickel ion release can be reduced significantly by aligned laser processing conditions. The presented results clearly attest that both dense SLM-NiTi and porous SLM-NiTi are suitable carriers for hMSC. Nevertheless, before carrying out in vivo studies, some work on optimization of the manufacturing process and post-processing is required. © 2012 Elsevier B.V.
    view abstractdoi: 10.1016/j.msec.2012.09.008
  • 2013 • 192 Influence of the substrate bias on the stoichiometry and structure of RF-magnetron sputter-deposited silver-containing calcium phosphate coatings
    Ivanova, A.A. and Surmeneva, M.A. and Grubova, I.Y. and Sharonova, A.A. and Pichugin, V.F. and Chaikina, M.V. and Buck, V. and Prymak, O. and Epple, M. and Surmenev, R.A.
    Materialwissenschaft und Werkstofftechnik 44 218-225 (2013)
    A coating on the basis of silver-containing hydroxyapatite (silver-hydroxyapatite) was deposited by radio frequency (RF) magnetron sputtering. X-ray diffractometry, scanning electron microscopy, energy-dispersive X-ray spectroscopy, and ellipsometry were used to analyse the change in structure and stoichiometry of the coatings upon the change of the negative electrical bias (-50 V and -100 V) on the substrate. The chemical composition of the sputter-deposited coating was identical to the target. However, an increase in the negative electrical bias on the substrate led to a decrease of the coating thickness. In addition, the average size of the grains decreased from 55 ± 15 nm (grounded substrate) up to 30 ± 10 nm when an electrical bias of -100 V was applied. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/mawe.201300101
  • 2013 • 191 Osteoblastic potency of bone marrow cells cultivated on functionalized biometals with cyclic RGD-peptide
    Jäger, M. and Böge, C. and Janissen, R. and Rohrbeck, D. and Hülsen, T. and Lensing-Höhn, S. and Krauspe, R. and Herten, M.
    Journal of Biomedical Materials Research - Part A 101 2905-2914 (2013)
    The fixation of cementless endoprostheses requires excellent fixation at the bone implant interface. Although the surface structures of these implants are designed to promote osteoblastic differentiation, poor bone quality may prevent or delay osseointegration. There is evidence that RGD peptides known as recognition motifs for various integrins, promote cellular adhesion, influence cellular proliferation, and differentiation of local cells. In this study, five different metal surfaces were analyzed: Sandblasted (TiSa) and polished (TiPol) Ti6Al4V, porocoated (CCPor) and polished (CCPol) cobalt chrome and polished stainless steel (SS) were coated by ethanol amine and poly(ethylene glycol) to attach covalently RGD peptides. Human mesenchymal stromal cells of healthy donors were cultivated onto prior functionalized metal surfaces for 14 days without osteogenic stimulation. Cell proliferation and differentiation were quantitatively evaluated for native (I), NaOH pre-activated (II), NaOH pre-activated, and PEG-coated (III) as well as for RGD (IV) coated surfaces. The RGD immobilization efficiency was analyzed by epi-fluorescence spectroscopy, cell morphology was documented by light and scanning electron microscopy. The RGD-binding efficiency was TiSa > TiPol > SS > CCPor > CCPol. RGD coated surfaces showed the highest average cell proliferation on CCPol > SS > CCPor > TiSa ≥ TiPol, whereas cellular differentiation mostly correlated with the observed proliferation results, such as CCPol > TiSa > SS > CCPor > TiPol. Considering statistical analyses (significance level of α = 0.05), the RGD-coating of all biometals in comparison and in respect of their particular controls showed no significant improvement in cellular proliferation and osteoblastic differentiation. © 2013 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 101A: 2905-2914, 2013. Copyright © 2013 Wiley Periodicals, Inc., a Wiley Company.
    view abstractdoi: 10.1002/jbm.a.34590
  • 2013 • 190 Influence of the precipitation method on acid-base-catalyzed reactions over Mg-Zr mixed oxides
    Kozlowski, J.T. and Behrens, M. and Schlögl, R. and Davis, R.J.
    ChemCatChem 5 1989-1997 (2013)
    To examine the promotional effect that zirconia has on magnesia in catalysis, mixed oxides were prepared by coprecipitation under controlled-pH conditions or rising-pH conditions. The resulting mixed oxides were characterized by using NH3 and CO2 adsorption microcalorimetry, X-ray diffraction, and scanning electron microscopy. The samples were also tested as catalysts for transesterification of tributyrin with methanol, coupling of acetone, and conversion of ethanol to ethene, ethanal, and butanol. Zirconia promoted the activity of MgO for both transesterification and acetone coupling reactions, presumably by exposing new acid-base pairs at the surface. During ethanol conversion, however, zirconia promoted the dehydration reactions. Characterization and reactivity results suggest that a Mg-Zr sample prepared by controlled-pH precipitation exposes more ZrO2 than a sample prepared by the rising-pH method. © 2013 WILEY-VCH Verlag GmbH & Co.
    view abstractdoi: 10.1002/cctc.201200833
  • 2013 • 189 Novel antifouling positively charged hybrid ultrafiltration membranes for protein separation based on blends of carboxylated carbon nanotubes and aminated poly(arylene ether sulfone)
    Kumar, M. and Ulbricht, M.
    Journal of Membrane Science 448 62-73 (2013)
    In this study, an aminated hydrophilic poly(arylene ether sulfone) (APAES) multiblock copolymer was prepared from previously synthesized bromomethylated poly(arylene ether sulfone) (PAES-CH2Br) block copolymer via in situ amination with triethanolamine. Novel positively charged hybrid ultrafiltration membranes were fabricated by film casting with non-solvent induced phase separation of blends of PAES and APAES block copolymers with carboxylated multiwalled carbon nanotubes (MWCNT-COOH). Fourier transform infrared spectroscopy in the attenuated total reflection mode, scanning electron microscopy as well as contact angle and outer surface zeta potential studies were performed to characterize the membranes in detail. The results confirmed that the fabricated membranes were porous, hydrophilic, positively charged, and had improved antifouling capacity. The hybrid membranes were used in ultrafiltration of ovalbumin and lysozyme model solutions (individually) at varied pH values. Membrane performance depended on the contents of MWCNT-COOH, which could be adjusted by varying its fraction in the membrane casting solutions. Due to the combined effects of size exclusion and charge repulsion, the permeability, antifouling properties and separation selectivity of the hybrid ultrafiltration membranes could be improved simultaneously by increasing of charge density and fraction of MWCNT-COOH. © 2013.
    view abstractdoi: 10.1016/j.memsci.2013.07.055
  • 2013 • 188 Advanced ultrafiltration membranes based on functionalized poly(arylene ether sulfone) block copolymers
    Kumar, M. and Ulbricht, M.
    RSC Advances 3 12190-12203 (2013)
    In this study, poly(arylene ether sulfone) (PAES) block copolymer was synthesized from fluoride-terminated oligomer (A<inf>16</inf>) and hydroxyl terminated oligomer (B<inf>12</inf>) by aromatic nucleophilic substitution (S<inf>N</inf>Ar) polycondensation reaction. Then allylic free radical substitution reaction at methyl side group using N-bromosuccinimide and benzoyl peroxide was performed to synthesize PAES-CH<inf>2</inf>Br block copolymer. The chemical structure of PAES and PAES-CH<inf>2</inf>Br block copolymers was confirmed by 1H-NMR spectroscopy. Gel permeation chromatography was applied to determine the molecular weight and the degree of polymerization of A<inf>16</inf> and B<inf>12</inf> oligomers, PAES and PAES-CH<inf>2</inf>Br block copolymers. The synthesized block copolymer had high molecular weight and multiblock structure. Non-solvent induced phase separation of blends of PAES-CH<inf>2</inf>Br and PAES block copolymers followed by heterogeneous quaternization using trimethyl amine led to hydrophilic positively charged ultrafiltration membranes. Fourier transform infrared spectroscopy in the attenuated total reflection mode, scanning electron microscopy, contact angle, and outer surface zeta potential studies were performed to characterize the resulting membranes. The results confirmed that the prepared membranes were porous, hydrophilic and positively charged. Membranes were also characterized with respect to permeability and then used in ultrafiltration of bovine serum albumin and lysozyme model solutions at varied pH values. Membrane performance depended on the positive charge density which could be adjusted by varying the fraction of PAES-CH<inf>2</inf>Br in the membrane. Due to the combined effects of size exclusion and charge repulsion, permeability, antifouling properties and protein separation selectivity of membranes could be increased simultaneously with increasing charge density. © The Royal Society of Chemistry 2013.
    view abstractdoi: 10.1039/c3ra41483c
  • 2013 • 187 Characterization of deformation anisotropies in an α-Ti alloy by nanoindentation and electron microscopy
    Kwon, J. and Brandes, M.C. and Sudharshan Phani, P. and Pilchak, A.P. and Gao, Y.F. and George, E.P. and Pharr, G.M. and Mills, M.J.
    Acta Materialia 61 4743-4756 (2013)
    The crystallographic dependence of the mechanical responses of an α-Ti-7 wt.% Al alloy was measured by nanoindentation using spherical and Berkovich indenters. Both elastic moduli and hardness responses of indents on the (0 0 0 1), (1̄100) and (1̄21̄0) planes were quantified. The dislocation structures resulting from indentation were characterized by electron microscopy. While scanning electron microscopy techniques were used for the observation of surface slip structures, site-specific focused-ion-beam thin foil preparation and scanning transmission electron microscopy techniques were employed for the imaging of sub-surface dislocation structures. Elastic modulus, hardness and load at pop-in were found to vary with crystallographic orientation. Indentation-induced plasticity was found to occur by multiple slip/twin mechanisms and to be dependent on crystal orientation, although 〈a〉 slip on (0 0 0 1) planes was found to be common to all orientations. The observed dislocation structures are rationalized on the basis of theoretical predictions based on the anisotropic elastic contact analysis and resolved shear stress calculations. © 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2013.05.005
  • 2013 • 186 Online monitoring of the passivation breakthrough during deep reactive ion etching of silicon using optical plasma emission spectroscopy
    Leopold, S. and Mueller, L. and Kremin, C. and Hoffmann, M.
    Journal of Micromechanics and Microengineering 23 (2013)
    We present optical emission spectroscopy (OES) as a technique for process optimization of the etch step during deep reactive ion etching of silicon. For specific process steps, the spectrum of optical plasma emission is investigated. Two specific wavelengths are identified (fluorine at 703.8 nm and CS compounds at 257.6 nm), which significantly change intensity during the etch step. Their intensity drop is used for the recognition of the passivation layer breakthrough. Thus, the net silicon etch time can be measured. This time can be used for process optimization. A structural analysis of the passivation layer shows its fragmentation during its breakthrough. The plasma-surface interaction and their correlation with the plasma emission are described. Within an application example, the passivation breakthrough is investigated in detail. For different process regimes, the residues of the fragmented passivation layer are analyzed by scanning electron microscopy. Residue densities of 14-38 μm -2 are fabricated. For silicon grass generation, the OES technique offers a versatile tool for the process optimization of the mask generating process within the first cycles. © 2013 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0960-1317/23/7/074001
  • 2013 • 185 Iron metal-organic frameworks MIL-88B and NH2-MIL-88B for the loading and delivery of the gasotransmitter carbon monoxide
    Ma, M. and Noei, H. and Mienert, B. and Niesel, J. and Bill, E. and Muhler, M. and Fischer, R.A. and Wang, Y. and Schatzschneider, U. and Metzler-Nolte, N.
    Chemistry - A European Journal 19 6785-6790 (2013)
    Crystals of MIL-88B-Fe and NH2-MIL-88B-Fe were prepared by a new rapid microwave-assisted solvothermal method. High-purity, spindle-shaped crystals of MIL-88B-Fe with a length of about 2 μm and a diameter of 1 μm and needle-shaped crystals of NH2-MIL-88B-Fe with a length of about 1.5 μm and a diameter of 300 nm were produced with uniform size and excellent crystallinity. The possibility to reduce the as-prepared frameworks and the chemical capture of carbon monoxide in these materials was studied by in situ ultrahigh vacuum Fourier-transform infrared (UHV-FTIR) spectroscopy and Mössbauer spectroscopy. CO binding occurs to unsaturated coordination sites (CUS). The release of CO from the as-prepared materials was studied by a myoglobin assay in physiological buffer. The release of CO from crystals of MIL-88B-Fe with t1/2=38 min and from crystals of NH 2-MIL-88B-Fe with t1/2=76 min were found to be controlled by the degradation of the MIL materials under physiological conditions. These MIL-88B-Fe and NH2-MIL-88B-Fe materials show good biocompatibility and have the potential to be used in pharmacological and therapeutic applications as carriers and delivery vehicles for the gasotransmitter carbon monoxide. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/chem.201201743
  • 2013 • 184 Cyclic deformation and lifetime of Alloy 617B during thermo-mechanical fatigue
    Maier, G. and Riedel, H. and Nieweg, B. and Somsen, C. and Eggeler, G. and Klöwer, J. and Mohrmann, R.
    Materials at High Temperatures 30 27-35 (2013)
    Different heats of the nickel-base Alloy 617B are tested under in-phase and out-of-phase thermo-mechanical fatigue (TMF) conditions at temperatures between 50 and 900 °C. During one of the TMF tests the growth of microcracks is observed using the replica technique. After the tests, some of the specimens are inspected by scanning electron microscopy in order to analyse the prevailing damage mechanisms compared with those observed in isothermal low-cycle fatigue (LCF) tests. In addition, a Chaboche-type model and a fracture-mechanics-based lifetime model are employed to describe the cyclic viscoplastic deformation and fatigue lifetime. The Chaboche model adjusted to isothermal data is found to reasonably predict the cyclic viscoplastic behaviour of thermo-mechanically loaded specimens. Lifetime data of TMF tests fall into a common scatter band with LCF tests at temperatures above 400 °C if the test results are analysed based on the introduced lifetime model.
    view abstractdoi: 10.3184/096034013X13636905345685
  • 2013 • 183 Cyclic deformation and lifetime of Alloy 617B during isothermal low cycle fatigue
    Maier, G. and Riedel, H. and Somsen, C.
    International Journal of Fatigue 55 126-135 (2013)
    Isothermal low cycle fatigue tests are carried out on the nickel-base Alloy 617B in the solution-annealed, stabilized and long-term aged conditions at temperatures between room temperature and 900 C. In addition, fatigue microcrack growth is measured using the replica technique. Transmission electron microscopy studies suggest that the observed differences in cyclic hardening between the different heat treatments result from the precipitation of fine carbides. Scanning electron microscope observations indicate a change in fracture mode for the solution-annealed and long-term aged material with temperature. The Chaboche model is able to describe the time and temperature dependent cyclic plasticity of the three material conditions. The measured lifetimes and crack growth rates can be described using a fracture mechanics based lifetime model. However, the data for room temperature and for temperatures above 400 C fall into two different scatter bands due to differences in crack growth rates. © 2013 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.ijfatigue.2013.06.001
  • 2013 • 182 Excimer laser doping using highly doped silicon nanoparticles
    Meseth, M. and Kunert, B.C. and Bitzer, L. and Kunze, F. and Meyer, S. and Kiefer, F. and Dehnen, M. and Orthner, H. and Petermann, N. and Kummer, M. and Wiggers, H. and Harder, N.-P. and Benson, N. and Schmechel, R.
    Physica Status Solidi (A) Applications and Materials Science 210 2456-2462 (2013)
    Laser doping of crystalline Si (c-Si) using highly doped Si nanoparticles (NPs) as the dopant source is investigated. For this purpose Si NPs are deposited onto c-Si substrates from dispersion using a spin coater and subsequently laser annealed by scanning over the sample with a 248 nm line profile excimer laser. Scanning electron microscope (SEM) investigations demonstrate that the laser intensity as well as the oxide concentration in the NP thin film strongly influence the film forming properties of the annealed NPs. Substrate doping is substantiated using electrochemical capacitance voltage (ECV) measurements on realized pn-junctions. In dependence of the laser fluencies ranging from 0.81 to 2.54 J cm-2, the effective doping depth is determined to be in the range of 50 to 250 nm. The rectifying behaviour of the pn- or np-junctions is verified by current voltage measurements. A homogeneous in-plane doping distribution realized by the laser doping process is demonstrated on the μm scale by light beam induced current measurements. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/pssa.201329012
  • 2013 • 181 Electrochemically deposited Pd-Pt and Pd-Au codeposits on graphite electrodes for electrocatalytic H2O2 reduction
    Nagaiah, T.C. and Schäfer, D. and Schuhmann, W. and Dimcheva, N.
    Analytical Chemistry 85 7897-7903 (2013)
    Improved electrocatalytic activity and selectivity for the reduction of H2O2 were obtained by electrodepositing Pd-Pt and Pd-Au on spectrographic graphite from solutions containing salts of the two metals at varying ratio. The electrocatalytic activity of the resulting binary codeposits for H2O2 reduction was evaluated by means of the redox-competition mode of scanning electrochemical microscopy (SECM) and voltammetric methods. In a potential range from 0 to-600 mV (vs. Ag/AgCl/3 M KCl) at pH 7.0 in 0.1 M phosphate citrate buffer, the electrocatalytic activity of both Pd-Pt and Pd-Au codeposits was substantially improved as compared with the identically deposited single metals suggesting an electrocatalytic synergy of the codeposits. Pd-Pt and Pd-Au codeposits were characterized by X-ray diffraction (XRD) analysis and scanning electron microscopy (SEM). Codepositing with Au caused a change of hedgehog-like shaped Pd nanoparticles into cauliflower-like nanoparticles with the particle size decreasing with increasing Au concentration. Codepositing Pd with Pt caused the formation of oblong structures with the size initially increasing with increasing Pt content. However, the particle size decreases with further increase in Pt concentration. The improved electrocatalytic capability for H2O2 reduction of the Pd-Pt electrodeposits on graphite was further demonstrated by immobilizing glucose oxidase as a basis for the development of an interference-free amperometric glucose biosensor. © 2013 American Chemical Society.
    view abstractdoi: 10.1021/ac401317y
  • 2013 • 180 Visualization of oxygen consumption of single living cells by scanning electrochemical microscopy: The influence of the faradaic tip reaction
    Nebel, M. and Grützke, S. and Diab, N. and Schulte, A. and Schuhmann, W.
    Angewandte Chemie - International Edition 52 6335-6338 (2013)
    The influence of the reaction rate at the SECM tip on the overall imaging result is often neglected during respiration studies performed by SECM. The effect of the driving force of the tip reaction is elucidated using a potential pulse profile implemented into a constant-distance mode. Time-dependent data acquisition allows visualization of the transition between a tip behaving as a passive observer and a tip actively inducing transmembrane diffusion of oxygen. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/anie.201301098
  • 2013 • 179 Ring-disk microelectrodes for simultaneous constant-distance and constant-current mode scanning electrochemical microscopy
    Nebel, M. and Neugebauer, S. and Eckhard, K. and Schuhmann, W.
    Electrochemistry Communications 27 160-163 (2013)
    Ring-disk microelectrodes are proposed to be applied in a double constant-current mode using simultaneously an irreversible and a reversible reaction at the SECM tip. This allows an independent determination of the tip-to-sample distance concomitantly with the visualization of the lateral electrochemical reactivity of the investigated sample surface. The principle feasibility is demonstrated using a large and topographically as well as electrochemically challenging model sample. © 2012 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.elecom.2012.11.028
  • 2013 • 178 Design and fabrication of a bending rotation fatigue test rig for in situ electrochemical analysis during fatigue testing of NiTi shape memory alloy wires
    Neelakantan, L. and Zglinski, J.K. and Frotscher, M. and Eggeler, G.
    Review of Scientific Instruments 84 (2013)
    The current investigation proposes a novel method for simultaneous assessment of the electrochemical and structural fatigue properties of nickel-titanium shape memory alloy (NiTi SMA) wires. The design and layout of an in situ electrochemical cell in a custom-made bending rotation fatigue (BRF) test rig is presented. This newly designed test rig allows performing a wide spectrum of experiments for studying the influence of fatigue on corrosion and vice versa. This can be achieved by performing ex situ andor in situ measurements. The versatility of the combined electrochemicalmechanical test rig is demonstrated by studying the electrochemical behavior of NiTi SMA wires in 0.9 NaCl electrolyte under load. The ex situ measurements allow addressing various issues, for example, the influence of pre-fatigue on the localized corrosion resistance, or the influence of hydrogen on fatigue life. Ex situ experiments showed that a pre-fatigued wire is more susceptible to localized corrosion. The synergetic effect can be concluded from the polarization studies and specifically from an in situ study of the open circuit potential (OCP) transients, which sensitively react to the elementary repassivation events related to the local failure of the oxide layer. It can also be used as an indicator for identifying the onset of the fatigue failure. © 2013 American Institute of Physics.
    view abstractdoi: 10.1063/1.4793488
  • 2013 • 177 Spatial high resolution energy dispersive X-ray spectroscopy on thin lamellas
    Notthoff, C. and Winterer, M. and Beckel, A. and Geller, M. and Heindl, J.
    Ultramicroscopy 129 30-35 (2013)
    For conventional samples and measurement geometries the spatial resolution of energy dispersive X-ray spectroscopy is limited by a tear drop shaped emission volume to about 1 μm. This restriction can be substantially improved using thin samples and high acceleration voltage. In this contribution the spatial resolution of energy dispersive X-ray spectroscopy in a scanning electron microscope using thin lamella samples is investigated. At an acceleration voltage of 30kV, an edge resolution down to Δd edge = 40 ± 10 nm is observed performing linescans across an interface, using an 80nm thin sample prepared from a GaAs/AlAs-heterostructure. Furthermore, Monte-Carlo simulations of pure elements ranging from sodium to mercury are performed for different sample thicknesses. From the simulations we can derive a simple empirical formula to predict the spatial resolution as a function of sample thickness. © 2013 Elsevier B.V.
    view abstractdoi: 10.1016/j.ultramic.2013.02.008
  • 2013 • 176 Abnormal grain growth in ferritic-martensitic Eurofer-97 steel
    de Oliveira, V.B. and Padilha, A.F. and Möslang, A. and Raabe, D. and Zschommler Sandim, H.R.
    Materials Science Forum 753 333-336 (2013)
    Ferritic-martensitic steels like Eurofer-97 are candidate structural materials for future fusion reactors. In the tempered state, this steel contains fine particles dispersed in the ferritic matrix. The aim of this work is to investigate abnormal grain growth in Eurofer-97 steel. The microstructural evolution was followed by isothermal annealing between 200 and 800°C (ferritic phase field) after cold rolling to 70, 80, and 90% reductions. Representative samples were characterized by scanning electron microscopy in the backscattered electron mode. Microtexture was evaluated by electron backscattered diffraction. We propose a mechanism based on the size advantage acquired by nuclei with misorientation angles above 45° relative to their nearest neighbors to explain abnormal grain growth. Abnormal grain growth textures have components belonging to the α- and γ-fibers with predominance of {111}<110>, {111}<112>, and {100}<110>. © (2013) Trans Tech Publications, Switzerland.
    view abstractdoi: 10.4028/www.scientific.net/MSF.753.333
  • 2013 • 175 Thermal analysis on conversion of MoO3 to MoO2 and its silicothermic reduction
    Paul, B. and Kishor, J. and Chakraborty, S.P. and Suri, A.K.
    Journal of Thermal Analysis and Calorimetry 112 31-36 (2013)
    The present paper deals with differential thermal analysis studies conducted to find out the onset temperature for silicothermic reduction of MoO2 to Mo. The reaction kinetics of Si-MoO2 system has been analyzed by a model-free Kissinger method. X-ray diffraction analysis has confirmed the formation of Mo metal and SiO2 as the slag phase after silicothermic reduction of MoO2. The activation energy for silicothermic reduction of MoO2 to Mo was evaluated to be 309 kJ mol-1. © 2012 Akadémiai Kiadó, Budapest, Hungary.
    view abstractdoi: 10.1007/s10973-012-2828-y
  • 2013 • 174 Formation of Silicide Based Oxidation Resistant Coating Over Mo-30 wt% W Alloy
    Paul, B. and Chakraborty, S.P. and Suri, A.K.
    Transactions of the Indian Ceramic Society 72 39-42 (2013)
    Studies were carried out to develop silicide based oxidation resistant coatings over Mo-30W alloy substrate employing halide activated pack cementation coating process. Effect of activator content and temperature on coating was studied. Coated samples were characterized for phase and microstructure evaluation by SEM and EDS. Cyclic oxidation tests on coated alloy were performed at 1000°C up to 50 h. The coating provided enough protection from oxidation. © 2013 Copyright The Indian Ceramic Society.
    view abstractdoi: 10.1080/0371750X.2013.793993
  • 2013 • 173 On the crystallographic anisotropy of nanoindentation in pseudoelastic NiTi
    Pfetzing-Micklich, J. and Somsen, C. and Dlouhy, A. and Begau, C. and Hartmaier, A. and Wagner, M.F.-X. and Eggeler, G.
    Acta Materialia 61 602-616 (2013)
    We use a nanoindenter with a Berkovich tip to study local mechanical properties of two polycrystalline intermetallics with a B2 crystal structure, NiAl and NiTi. We use orientation imaging scanning electron microscopy to select a relevant number of grains with appropriate sizes and surface normals parallel to 〈0 0 1〉, 〈1 0 1〉 and 〈1 1 1〉. As a striking new result, we find a strong crystallographic orientation dependence for NiTi. This anisotropy is less pronounced in the case of NiAl. For NiTi, the indentation force required to impose a specific indentation depth is highest for indentation experiments performed in the 〈0 0 1〉 direction and lowest along the 〈1 1 1〉 direction. We consider transmission electron microscopy results from cross-sections below the indents and use molecular dynamics simulations and resolved shear stress calculations to discuss how this difference can be accounted for in terms of elementary deformation and transformation processes, related to dislocation plasticity (NiAl and NiTi), and in terms of the stress-induced formation and growth of martensite (NiTi). Our results show that the crystallographic anisotropy during nanoindentation of NiTi is governed by the orientation dependence of the martensitic transformation; dislocation plasticity appears to be less important. © 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2012.09.081
  • 2013 • 172 Impurity levels and fatigue lives of pseudoelastic NiTi shape memory alloys
    Rahim, M. and Frenzel, J. and Frotscher, M. and Pfetzing-Micklich, J. and Steegmüller, R. and Wohlschlögel, M. and Mughrabi, H. and Eggeler, G.
    Acta Materialia 61 3667-3686 (2013)
    In the present work we show how different oxygen (O) and carbon (C) levels affect fatigue lives of pseudoelastic NiTi shape memory alloys. We compare three alloys, one with an ultrahigh purity and two which contain the maximum accepted levels of C and O. We use bending rotation fatigue (up to cycle numbers &gt;108) and scanning electron microscopy (for investigating microstructural details of crack initiation and growth) to study fatigue behavior. High cycle fatigue (HCF) life is governed by the number of cycles required for crack initiation. In the low cycle fatigue (LCF) regime, the high-purity alloy outperforms the materials with higher number densities of carbides and oxides. In the HCF regime, on the other hand, the high-purity and C-containing alloys show higher fatigue lives than the alloy with oxide particles. There is high experimental scatter in the HCF regime where fatigue cracks preferentially nucleate at particle/void assemblies (PVAs) which form during processing. Cyclic crack growth follows the Paris law and does not depend on impurity levels. The results presented in the present work contribute to a better understanding of structural fatigue of pseudoelastic NiTi shape memory alloys. © 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2013.02.054
  • 2013 • 171 Infiltration of polymer hole-conductor into mesoporous titania structures for solid-state dye-sensitized solar cells
    Rawolle, M. and Sarkar, K. and Niedermeier, M.A. and Schindler, M. and Lellig, P. and Gutmann, J.S. and Moulin, J.-F. and Haese-Seiller, M. and Wochnik, A.S. and Scheu, C. and Müller-Buschbaum, P.
    ACS Applied Materials and Interfaces 5 719-729 (2013)
    The degree of filling of titania nanostructures with a solid hole-conducting material is important for the performance of solid-state dye-sensitized solar cells (ssDSSCs). Different ways to infiltrate the hole-conducting polymer poly(3-hexylthiophene) (P3HT) into titania structures, both granular structures as they are already applied commercially and tailored sponge nanostructures, are investigated. The solar cell performance is compared to the morphology determined with scanning electron microscopy (SEM) and time-of-flight grazing incidence small-angle neutron scattering (TOF-GISANS). The granular titania structure, commonly used for ssDSSCs, shows a large distribution of particle and pore sizes, with porosities in the range from 41 to 67%, including even dense parts without pores. In contrast, the tailored sponge nanostructure has well-defined pore sizes of 25 nm with an all-over porosity of 54%. Filling of the titania structures with P3HT by solution casting results in a mesoscopic P3HT overlayer and consequently a bad solar cell performance, even though a filling ratio of 67% is observed. For the infiltration by repeated spin coating, only 57% pore filling is achieved, whereas filling by soaking in the solvent with subsequent spin coating yields filling as high as 84% in the case of the tailored titania sponge structures. The granular titania structure is filled less completely than the well-defined porous structures. The solar cell performance is increased with an increasing filling ratio for these two ways of infiltration. Therefore, filling by soaking in the solvent with subsequent spin coating is proposed. © 2012 American Chemical Society.
    view abstractdoi: 10.1021/am302255c
  • 2013 • 170 Lignocellulosic jute fiber as a bioadsorbent for the removal of azo dye from its aqueous solution: Batch and column studies
    Roy, A. and Chakraborty, S. and Kundu, S.P. and Adhikari, B. and Majumder, S.B.
    Journal of Applied Polymer Science 129 15-27 (2013)
    The feasibility of the use of jute fiber for the adsorption of azo dye from an aqueous solution was evaluated with batch and fixed-bed column studies. The batch studies illustrated that dye uptake was highly dependent on different process variables, namely, the pH, initial dye concentration of the solution, adsorbent dosage, contact time, ionic strength, and temperature. The exothermic and spontaneous nature of adsorption was revealed from thermodynamic study. The equilibrium adsorption data were highly consistent with the Langmuir isotherm and yielded an R2 value of 0.999. Kinetic studies divulged that the adsorption followed a pseudo-second-order model with regard to the intraparticle diffusion. In the column studies, the total amount of adsorbed dye and the adsorption capacity decreased with increasing flow rate and increased with increasing bed height and initial dye concentration. Also, the breakthrough time and exhaustion time increased with increasing bed depth but decreased with increasing flow rate and influent dye concentration. The column performances were predicted by the application of the bed-depth service time model and Thomas model to the experimental data. The virgin and dye-adsorbed jute fiber was characterized by Fourier transform infrared spectroscopy and scanning electron microscopy analyses. The investigation suggested that jute fiber could be applied as a promising low-cost adsorbent for dye removal. Copyright © 2012 Wiley Periodicals, Inc.
    view abstractdoi: 10.1002/app.38222
  • 2013 • 169 Sintering of thin titanium dioxide nanoparticle films via photothermal processing with ultraviolet continuous-wave lasers
    Schade, L. and Franzka, S. and Hardt, S. and Wiggers, H. and Hartmann, N.
    Applied Surface Science 278 336-340 (2013)
    Photothermal laser processing of thin films of anatase titania nanoparticles (TiO2 NPs, diameter: 8-10 nm) with a thickness of about 500 nm is addressed. Laser processing in ambient air is carried out using a microfocused continuous-wave laser setup operating at a wavelength of 355 nm and a 1/e laser spot size of 1.6 μm. In conjunction with scanning electron microscopy, this approach provides a highly reproducible and convenient means in order to modify the local film structure and study the dependence of the resulting film morphology on the laser parameters. Generally, sintering of the nanoparticles is observed. At high laser power densities and/or long irradiation times the average particle/grain size increases reaching values of 200 nm and more. This opens up an opportunity to introduce scattering centers and optimize light trapping within the film, e.g., targeting photovoltaic or photocatalytic applications. © 2012 Elsevier B.V.
    view abstractdoi: 10.1016/j.apsusc.2012.11.077
  • 2013 • 168 Electric in-plane polarization in multiferroic CoFe2O 4/BaTiO3 nanocomposite tuned by magnetic fields
    Schmitz-Antoniak, C. and Schmitz, D. and Borisov, P. and De Groot, F.M.F. and Stienen, S. and Warland, A. and Krumme, B. and Feyerherm, R. and Dudzik, E. and Kleemann, W. and Wende, H.
    Nature Communications 4 (2013)
    Ferrimagnetic CoFe2O4 nanopillars embedded in a ferroelectric BaTiO3 matrix are an example for a two-phase magnetoelectrically coupled system. They operate at room temperature and are free of any resource-critical rare-earth element, which makes them interesting for potential applications. Prior studies succeeded in showing strain-mediated coupling between the two subsystems. In particular, the electric properties can be tuned by magnetic fields and the magnetic properties by electric fields. Here we take the analysis of the coupling to a new level utilizing soft X-ray absorption spectroscopy and its associated linear dichroism. We demonstrate that an in-plane magnetic field breaks the tetragonal symmetry of the (1,3)-type CoFe2O4/BaTiO3 structures and discuss it in terms of off-diagonal magnetostrictive-piezoelectric coupling. This coupling creates staggered in-plane components of the electric polarization, which are stable even at magnetic remanence due to hysteretic behaviour of structural changes in the BaTiO3 matrix. The competing mechanisms of clamping and relaxation effects are discussed in detail. © 2013 Macmillan Publishers Limited. All rights reserved.
    view abstractdoi: 10.1038/ncomms3051
  • 2013 • 167 Fabrication of chemical templates via selective laser-induced desorption of hexadecanethiol self-assembled monolayers
    Schröter, A. and Mathieu, M. and Franzka, S. and Feydt, J. and Irsen, S. and Hartmann, N.
    Applied Surface Science 278 57-61 (2013)
    A nonlinear photothermal laser patterning technique for rapid fabrication of chemical templates is demonstrated. Hexadecanethiol monolayers on Au-coated Si substrates are processed at λ = 532 nm, a 1/e2 spot diameter of d1/e2=2.8Tμm and ambient conditions. Local laser irradiation at high laser powers and short irradiation times in the micro-/millisecond range induces desorption of thiol molecules. The laser-depleted areas are backfilled with mercaptohexadecanoic acid in order to build up chemical templates. Atomic force microscopy, scanning electron microscopy and scanning Auger electron spectroscopy are used for characterization of these templates. In agreement with a selective laser process, the results indicate the formation of flat chemical patterns with well-defined boundaries. Complementary condensation experiments demonstrate the functionality of the patterns as hydrophilic/hydrophobic templates. In particular, upon decreasing the temperature below the dew point, selective formation of water droplets on the backfilled areas is observed. © 2013 Elsevier B.V.
    view abstractdoi: 10.1016/j.apsusc.2012.12.135
  • 2013 • 166 Lattice dependent motion of voids during electromigration
    Sindermann, S.P. and Latz, A. and Dumpich, G. and Wolf, D.E. and Meyer zu Heringdorf, F.-J.
    Journal of Applied Physics 113 (2013)
    The influence of the crystal lattice configuration to electromigration processes, e.g., void formation and propagation, is investigated in suitable test structures. They are fabricated out of self-assembled, bi-crystalline Ag islands, grown epitaxially on a clean Si(111) surface. The μm-wide and approximately 100 nm thick Ag islands are a composition of a Ag(001) and a Ag(111) part. By focused ion beam milling, they are structured into wires with a single grain boundary, the orientation of which can be chosen arbitrarily. In-situ scanning electron microscopy (SEM) allows to capture an image sequence during electrical stressing and monitors the development of voids and hillocks in time. To visualize the position and motion of voids, we calculate void maps using a threshold algorithm. Most of the information from the SEM image sequence is compressed into one single image. Our present electromigration studies are based on in-situ SEM investigations for three different lattice configurations: Ag(001) (with electron current flow in [110] direction), Ag(111) (with electron current flow in [112] direction), and additionally 90°rotated Ag(111) (with electron current flow in [110] direction). Our experimental results show that not only the formation and shape but also the motion direction of voids strongly depends on the crystal orientation. © 2013 American Institute of Physics.
    view abstractdoi: 10.1063/1.4798367
  • 2013 • 165 Utilization of low-pressure plasma to inactivate bacterial spores on stainless steel screws
    Stapelmann, K. and Fiebrandt, M. and Raguse, M. and Awakowicz, P. and Reitz, G. and Moeller, R.
    Astrobiology 13 597-606 (2013)
    A special focus area of planetary protection is the monitoring, control, and reduction of microbial contaminations that are detected on spacecraft components and hardware during and after assembly. In this study, wild-type spores of Bacillus pumilus SAFR-032 (a persistent spacecraft assembly facility isolate) and the laboratory model organism B. subtilis 168 were used to study the effects of low-pressure plasma, with hydrogen alone and in combination with oxygen and evaporated hydrogen peroxide as a process gas, on spore survival, which was determined by a colony formation assay. Spores of B. pumilus SAFR-032 and B. subtilis 168 were deposited with an aseptic technique onto the surface of stainless steel screws to simulate a spore-contaminated spacecraft hardware component, and were subsequently exposed to different plasmas and hydrogen peroxide conditions in a very high frequency capacitively coupled plasma reactor (VHF-CCP) to reduce the spore burden. Spores of the spacecraft isolate B. pumilus SAFR-032 were significantly more resistant to plasma treatment than spores of B. subtilis 168. The use of low-pressure plasma with an additional treatment of evaporated hydrogen peroxide also led to an enhanced spore inactivation that surpassed either single treatment when applied alone, which indicates the potential application of this method as a fast and suitable way to reduce spore-contaminated spacecraft hardware components for planetary protection purposes. © 2013 Mary Ann Liebert, Inc.
    view abstractdoi: 10.1089/ast.2012.0949
  • 2013 • 164 Effect of carrier gas composition on transferred arc metal nanoparticle synthesis
    Stein, M. and Kiesler, D. and Kruis, F.E.
    Journal of Nanoparticle Research 15 (2013)
    Metal nanoparticles are used in a great number of applications; an effective and economical production scaling-up is hence desirable. A simple and cost-effective transferred arc process is developed, which produces pure metal (Zn, Cu, and Ag) nanoparticles with high production rates, while allowing fast optimization based on energy efficiency. Different carrier gas compositions, as well as the electrode arrangements and the power input are investigated to improve the production and its efficiency and to understand the arc production behavior. The production rates are determined by a novel process monitoring method, which combines an online microbalance method with a scanning mobility particle sizer for fast production rate and size distribution measurement. Particle characterization is performed via scanning electron microscopy, energy-dispersive X-ray spectroscopy, and X-ray diffraction measurements. It is found that the carrier gas composition has the largest impact on the particle production rate and can increase it with orders of magnitude. This appears to be not only a result of the increased heat flux and melt temperature but also of the formation of tiny nitrogen (hydrogen) bubbles in the molten feedstock, which impacts feedstock evaporation significantly in bi-atomic gases. A production rate of sub 200 nm particles from 20 up to 2,500 mg/h has been realized for the different metals. In this production range, specific power consumptions as low as 0.08 kWh/g have been reached. © 2013 Springer Science+Business Media Dordrecht.
    view abstractdoi: 10.1007/s11051-012-1400-9
  • 2013 • 163 Adhesion, Vitality and Osteogenic Differentiation Capacity of Adipose Derived Stem Cells Seeded on Nitinol Nanoparticle Coatings
    Strauß, S. and Neumeister, A. and Barcikowski, S. and Kracht, D. and Kuhbier, J.W. and Radtke, C. and Reimers, K. and Vogt, P.M.
    PLoS ONE 8 (2013)
    Autologous cells can be used for a bioactivation of osteoimplants to enhance osseointegration. In this regard, adipose derived stem cells (ASCs) offer interesting perspectives in implantology because they are fast and easy to isolate. However, not all materials licensed for bone implants are equally suited for cell adhesion. Surface modifications are under investigation to promote cytocompatibility and cell growth. The presented study focused on influences of a Nitinol-nanoparticle coating on ASCs. Possible toxic effects as well as influences on the osteogenic differentiation potential of ASCs were evaluated by viability assays, scanning electron microscopy, immunofluorescence and alizarin red staining. It was previously shown that Nitinol-nanoparticles exert no cell toxic effects to ASCs either in soluble form or as surface coating. Here we could demonstrate that a Nitinol-nanoparticle surface coating enhances cell adherence and growth on Nitinol-surfaces. No negative influence on the osteogenic differentiation was observed. Nitinol-nanoparticle coatings offer new possibilities in implantology research regarding bioactivation by autologous ASCs, respectively enhancement of surface attraction to cells. © 2013 Strauß et al.
    view abstractdoi: 10.1371/journal.pone.0053309
  • 2013 • 162 Preparation of a silicate-containing hydroxyapatite-based coating by magnetron sputtering: Structure and osteoblast-like MG63 cells in vitro study
    Surmeneva, M.A. and Kovtun, A. and Peetsch, A. and Goroja, S.N. and Sharonova, A.A. and Pichugin, V.F. and Grubova, I.Y. and Ivanova, A.A. and Teresov, A.D. and Koval, N.N. and Buck, V. and Wittmar, A. and Ulbricht, M. and Prymak,...
    RSC Advances 3 11240-11246 (2013)
    Silicate-containing hydroxyapatite-based coatings with different structure and calcium/phosphate ratios were prepared by radio-frequency magnetron sputtering on silicon and titanium substrates, respectively. Scanning electron microscopy, X-ray diffraction and IR spectroscopy were used to investigate the effect of the substrate bias on the properties of the silicate-containing hydroxyapatite-based coatings. The deposition rate, composition, and microstructure of the deposited coatings were all controlled by changing the bias voltage from grounded (0 V) to -50 and -100 V. The biocompatibility was assessed by cell culture with human osteoblast-like cells (MG-63 cell line), showing a good biocompatibility and cell growth on the substrates. © 2013 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c3ra40446c
  • 2013 • 161 Phase and elemental composition of silicon-containing hydroxyapatite-based coatings fabricated by RF-magnetron sputtering for medical implants
    Surmeneva, M.A. and Surmenev, R.A. and Chaikina, M.V. and Kachaev, A.A. and Pichugin, V.F. and Epple, M.
    Inorganic Materials: Applied Research 4 227-235 (2013)
    We studied by X-ray diffraction analysis, IR spectroscopy, and scanning electron microscopy (SEM) the phase composition and the structure of coatings based on silicon-containing hydroxyapatite (Si-HA) deposited by RF-magnetron sputtering. The sputtering target contained two phases (apatite and tricalcium phosphate) and was produced by the ceramic technology from a single-phase mechanically activated powder precursor. The structure of the coating deposited by sputtering from the two-phase target was single phase (hydroxyapatite) and textured in the (002) direction. During deposition, silicate anions partially replaced phosphate ions in the apatite lattice. © Pleiades Publishing, Ltd., 2013.
    view abstractdoi: 10.1134/S2075113313030131
  • 2013 • 160 Carbon reactivity of binder metals in diamond-metal composites - Characterization by scanning electron microscopy and X-ray diffraction
    Tillmann, W. and Ferreira, M. and Steffen, A. and Rüster, K. and Möller, J. and Bieder, S. and Paulus, M. and Tolan, M.
    Diamond and Related Materials 38 118-123 (2013)
    Diamond tooling is a successfully used technique in machining of very hard materials such as minerals and concrete. The type and strength of bonding between the diamond grains, that are mainly responsible for the machining process (e.g. cutting or grinding), and the metallic binder phase is directly linked to the tools quality. Therefore it is of interest to investigate the carbon reactivity of commonly used binder materials. This paper reports about the investigation of the interfacial area between diamonds and one-component metallic binder matrices. As matrix material pure chromium, cobalt, copper, iron, and nickel was used. After the sintering process the diamonds were extracted from the metallic matrix and analyzed by scanning electron microscopy and X-ray diffraction. The morphology of the diamond surface was investigated and a phase analysis was done. These experimental studies support the hypothesis that the carbon reactivity of transition metals is linked to their d-orbital electron configuration. © 2013 Elsevier B.V.
    view abstractdoi: 10.1016/j.diamond.2013.07.002
  • 2013 • 159 Tribological comparison of different surface topographies coated with chromium aluminum nitride
    Tillmann, W. and Herper, J. and Laemmerhirt, I.-A.
    Materialwissenschaft und Werkstofftechnik 44 730-735 (2013)
    The reduction of friction and wear is one important goal for the extension of the tool life in many industrial applications. The forming and cutting industries in particular, are very interested in new techniques and surface coating characteristics that will improve the tribological behavior of the tools. Biomimetics is a very promising approach using biological surfaces or phenomena to optimize the properties of engineering components. The "Lotus Effect" is the most famous example, whereby surfaces are made water- and dirt-repellent. Taking a closer look at nature, it can be noticed that many different natural surfaces have perfectly adapted to their environment in order to meet the respective requirements. Extending the use of natural and biological effects to improve the material performance, it was discovered that the skin of many insects has an excellent frictional behavior and thus the potential to be transferred onto technical surfaces. In this paper, the surface structure of a dung beetle (db) was investigated. The main objective was to combine nature-adapted surface patterns with wear-resistant near-netshape PVD-coatings (PVD=physical vapour deposition), in order to improve the tribological properties of a tool surface. The shell of the beetle served as a pattern for the structure of the surfaces. A substrate, composed of high speed steel material, was structured by means of milling prior to the deposition of a chromium aluminum nitride multilayer coating system, using a magnetron sputtering process. The mechanical and tribological properties of the structured and coated surfaces were compared by means of nanoindentation, ball-on-disc-testing, and scanning electron microscopy. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/mawe.201300183
  • 2013 • 158 Measurement of the Young's modulus of thermal spray coatings by means of several methods
    Tillmann, W. and Selvadurai, U. and Luo, W.
    Journal of Thermal Spray Technology 22 290-298 (2013)
    Thermally sprayed coatings are usually defined by their hardness, porosity, roughness, and wear resistance. Even though the Young's modulus is an essential property, which describes the mechanical behavior of the coated components during their use, only few efforts have been made in the past to determine this property. The most common measurement methods of the Young's modulus of thermally sprayed coatings are tensile tests, bending tests, and nanoindentations. During the tensile and bending tests a sliding of the splats can occur due to the laminar structure of the thermally sprayed coatings, influencing the measurement value. When using the nanoindentation test, only the elastic behavior of some splats can be determined because of a minimal measuring volume. However, the Young's modulus of thermally sprayed coatings can also be determined by means of a resonant method, called impulse excitation technique. In this paper, the values of the Young's moduli of thermally sprayed coatings, measured by several methods, are compared with each other and correlated to the microstructure of the coatings, investigated by means of scanning electron microscopy. © 2012 ASM International.
    view abstractdoi: 10.1007/s11666-012-9855-x
  • 2013 • 157 Functionally graded vacuum plasma sprayed and magnetron sputtered tungsten/EUROFER97 interlayers for joints in helium-cooled divertor components
    Weber, T. and Stüber, M. and Ulrich, S. and Vaßen, R. and Basuki, W.W. and Lohmiller, J. and Sittel, W. and Aktaa, J.
    Journal of Nuclear Materials 436 29-39 (2013)
    Two coating technologies, magnetron sputtering and vacuum plasma spraying, have been investigated for their capability in producing functionally graded tungsten/EUROFER97 layers. In a first step, non-graded layers with different mixing ratios were deposited on tungsten substrates and characterized by nanoindentation, macroindentation, X-ray diffraction, transmission, Auger and scanning electron microscopy. The thermal stability of the sprayed layers against heat treatments at 800-1100 °C for 60 min was further analyzed. In a second step, the produced functionally graded layers deposited on tungsten substrates were joined to EUROFER97 bulk-material by diffusion bonding. The bonding and the graded joints were microscopically characterized and exposed to thermal cycles between 20 °C and 650 °C. Results from this study show that both coating technologies are ideal for the synthesis of functionally graded tungsten/EUROFER97 coatings. This is important in providing insights for fture development of joints with functionally graded interlayers. © 2013 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.jnucmat.2013.01.286
  • 2013 • 156 Hot embossed microtopographic gradients reveal morphological cues that guide the settlement of zoospores
    Xiao, L. and Thompson, S.E.M. and Röhrig, M. and Callow, M.E. and Callow, J.A. and Grunze, M. and Rosenhahn, A.
    Langmuir 29 1093-1099 (2013)
    Among different surface cues, the settlement of cells and larvae of marine macrofouling organisms has been found to be strongly influenced by surface microtopographies. In this article, the settlement of zoospores of the green alga Ulva linza on a surface topographic gradient has been investigated. "Honeycomb" gradient structures with feature sizes ranging from 1 to 10 μm were prepared by hot embossing, and the effect on the density of spores that attached in settlement assays was quantified. The highest density of spores was found when the size of the microstructures was similar to or larger than the size of the spores. With decreasing size of the structures, spore settlement density decreased. Interestingly, spore settlement density correlated with the Wenzel roughness of the surfaces. "Kink sites" on the surface played an important role and resembled preferred attachment positions. Furthermore, the gradients allowed the minimum pit size that the spores were able to squeeze into to be determined. © 2012 American Chemical Society.
    view abstractdoi: 10.1021/la303832u
  • 2013 • 155 Designing magnetic field responsive nanofiltration membranes
    Yang, Q. and Himstedt, H.H. and Ulbricht, M. and Qian, X. and Ranil Wickramasinghe, S.
    Journal of Membrane Science 430 70-78 (2013)
    Base, thin film composite polyamide, nanofiltration membranes have been modified using surface initiated atom transfer radical polymerization to graft poly(2-hydroxyethyl methacrylate) (polyHEMA) chains from the surface of the membrane. A modified Gabriel synthesis procedure was used to attach superparamagnetic (Fe3O4) nanoparticles to the chain ends. Chain density and chain length were independently varied by adjusting the initiator density and polymerization time. Membranes were characterized using scanning electron microscopy, X-ray photoelectron spectroscopy and contact angle measurements. The performance of modified membranes was investigated by determining deionized water fluxes as well as permeate fluxes and salt rejection for aqueous feed streams containing 500ppm CaCl2 and 2000ppm MgSO4. All experiments were conducted in dead end mode. Modified membranes display a reduced permeate flux and increased salt rejection compared to unmodified membranes in the absence of a magnetic field. Since both grafted chain density and chain length are expected to affect membrane performance differently, the decrease in permeate flux and increase in salt rejection is not directly proportional to the increase in grafted polymer weight. Modified membranes display both increased permeate fluxes and increased salt rejection in the presence of an oscillating magnetic field compared to their performance in the absence of an oscillating magnetic field. Magnetically responsive membranes could represent a new class of fouling resistant membranes. © 2012 Elsevier B.V.
    view abstractdoi: 10.1016/j.memsci.2012.11.068
  • 2013 • 154 The effect of surface roughness on the determination of optical constants of CuInSe2 and CuGaSe2 thin films
    Yin, G. and Merschjann, C. and Schmid, M.
    Journal of Applied Physics 113 (2013)
    To investigate the effect of surface roughness on the calculation of optical constants, e.g., the complex refractive index n + i k or (n, k) of CuIn1-xGaxSe2 (CIGSe) thin films, we took CuInSe2 (CISe) and CuGaSe2 (CGSe) as examples and applied the "Modified Transfer-Matrix (MTM)" method to calculate optical constants with considering the effect of scattering due to surface roughness. Compared to the Transfer-Matrix (TM) method without considering surface roughness, it was revealed that the MTM method could improve the accuracy of calculation. The calculated refractive index values from the MTM method increase by 6.89% for CISe and 2.59% for CGSe in contrast to those from the TM method. In addition, bromine solution was confirmed via Scanning Electron Microscopy and Atomic Force Microscopy to be able to reduce the surface roughness. Calculated results from smoothened samples showed that the accuracy of calculated optical constants was further improved. Finally, optical constants calculated by the MTM method were compared to those from smoothened samples, validating that the MTM method could eliminate the influence of surface roughness on the calculation of optical constants more effectively for CGSe with low surface roughness than for CISe with high surface roughness. © 2013 AIP Publishing LLC.
    view abstractdoi: 10.1063/1.4809550
  • 2013 • 153 Insights into the topotactic conversion process from layered silicate RUB-36 to FER-type zeolite by layer reassembly
    Zhao, Z. and Zhang, W. and Ren, P. and Han, X. and Müller, U. and Yilmaz, B. and Feyen, M. and Gies, H. and Xiao, F.-S. and De Vos, D. and Tatsumi, T. and Bao, X.
    Chemistry of Materials 25 840-847 (2013)
    Layered RUB-36 and PREFER (lamellar precursor of ferrierite) are the precursors of CDO and FER-type zeolites, respectively. Both are composed of the same ferrierite (FER) layer building blocks. Topotactic conversion from RUB-36 to pure silica zeolite ZSM-35 has been demonstrated in the presence of a surfactant cetyltrimethylammonium hydroxide (CTAOH). The transformation mechanism of this process was revealed, for the first time, by the detailed investigations of powder X-ray diffraction (XRD), scanning electron microscopy (SEM), thermal analysis, and one-and two-dimensional (2-D) solid-state magic-angle spinning nuclear magnetic resonance (MAS NMR) as well as theoretical simulations. During swelling at room temperature, cetyltrimethylammonium cations (CTA+) replacing the original template were intercalated into FER layers to expand the interlayer distance remarkably and consequently to destroy the strong hydrogen-bonding interactions between the layers. 2-D 1H-29Si heteronuclear correlation (HETCOR) NMR indicates that the surfactant polar heads approximate the FER layers in swollen RUB-36. After deswelling, only a small amount of CTA+ cations with long tails lay in the void space between the FER layers. The Monte Carlo simulations on the deswollen RUB-36 further elucidate the occlusion of CTA+ cations in the pre-10 member ring of the layered ferrierite precursor, which may act as the structure-directing agent for the formation of FER-structured zeolite. The FER layer reassembly from the alteration of CTA+ conformation at the interlayers is of key importance to the topotactic transformation of RUB-36 to FER-type zeolite by the dehydration-condensation reaction. This may open up more applications in the lamellar zeolite system by the layer restacking approach. © 2013 American Chemical Society.
    view abstractdoi: 10.1021/cm303131c
  • 2013 • 152 Experimental and theoretical investigation of molybdenum carbide and nitride as catalysts for ammonia decomposition
    Zheng, W. and Cotter, T.P. and Kaghazchi, P. and Jacob, T. and Frank, B. and Schlichte, K. and Zhang, W. and Su, D.S. and Schüth, F. and Schlögl, R.
    Journal of the American Chemical Society 135 3458-3464 (2013)
    Constant COx-free H2 production from the catalytic decomposition of ammonia could be achieved over a high-surface-area molybdenum carbide catalyst prepared by a temperature-programmed reduction-carburization method. The fresh and used catalyst was characterized by N2 adsorption/desorption, powder X-ray diffraction, scanning and transmission electron microscopy, and electron energy-loss spectroscopy at different stages. Observed deactivation (in the first 15 h) of the high-surface-area carbide during the reaction was ascribed to considerable reduction of the specific surface area due to nitridation of the carbide under the reaction conditions. Theoretical calculations confirm that the N atoms tend to occupy subsurface sites, leading to the formation of nitride under an NH3 atmosphere. The relatively high rate of reaction (30 mmol/((g of cat.) min)) observed for the catalytic decomposition of NH3 is ascribed to highly energetic sites (twin boundaries, stacking faults, steps, and defects) which are observed in both the molybdenum carbide and nitride samples. The prevalence of such sites in the as-synthesized material results in a much higher H2 production rate in comparison with that for previously reported Mo-based catalysts. © 2013 American Chemical Society.
    view abstractdoi: 10.1021/ja309734u
  • 2013 • 151 La-Sr-Fe-Co oxygen transport membranes on metal supports deposited by low pressure plasma spraying-physical vapour deposition
    Zotov, N. and Baumann, S. and Meulenberg, W.A. and Vaßen, R.
    Journal of Membrane Science 442 119-123 (2013)
    Dense La0.6Sr0.4Fe0.8Co0.2O3-δ (LSFC) membranes were successfully prepared on NiCoCrAlY metal supports by low pressure plasma spraying-physical vapour deposition. He leakage rate tests and microstructural analysis by SEM of the as-deposited films showed that a gas-tight membrane layer is achieved. Membrane thicknesses are between 40 and 65μm. The oxygen permeation flux was determined at atmospheric pressure in an air-Ar gradient at different temperatures between 700 and 950°C and compared with 1mm thick LSFC pellet. Although leakage was detected, noteworthy oxygen permeation rate of the supported membranes (~0.3mlcm-2min-1 at 885°C) is reported. © 2013 Elsevier B.V.
    view abstractdoi: 10.1016/j.memsci.2013.04.016
  • 2012 • 150 Optical emission spectroscopy as a tool for characterization of technical plasmas in medical applications
    Awakowicz, P. and Baldus, S. and Stapelmann, K. and Engelhardt, M. and Bibinov, N. and Denis, B.
    Plasma Medicine 2 151-168 (2012)
    Understanding the interactions of technical plasma discharges with biological systems is a key aspect to developing and optimizing plasma devices for use in medical practice. In this article, the characterization of 3 different plasma devices with absolutely and relatively calibrated optical emission spectroscopy is presented. Two low-pressure setups are described: a double inductively coupled plasma that serves as laboratory setup for basic research of sterilization of spores and germs and a very high frequency capacitively coupled plasma designed to meet commercial needs. An atmospheric pressure dielectric barrier discharge is designed for wound and skin treatment. Sterilization tests for each setup demonstrate the capability to inactivate bacteria and bacterial spores efficiently. In case of the double inductively coupled plasma, wavelength-dependent photo sterilization efficiency is investigated. As a result, Aspergillus brasiliensis spores are efficiently inactivated by irradiation below 235 nm, whereas Bacillus atrophaeus spores are sensitive to irradiation between 235 and 300 nm. The very high frequency capacitively coupled plasma demonstrates a reduction greater than log 6 of B. Atrophaeus endospores in a process challenge device, a metal box with 3 small slits (3 mm). With direct DBD treatment, a full inactivation of Escherichia coli is achieved within 10 seconds of treatment time. From measurements, data can be extracted only at certain positions. Simulations deliver spatially resolved data from whole-discharge volume. © 2012 by Begell House, Inc.
    view abstractdoi: 10.1615/PlasmaMed.2013006849
  • 2012 • 149 Characterization of the mechanical properties of technical fibers at extreme strain rates
    Bahners, T. and Schloßer, U. and Gutmann, J.
    Macromolecular Materials and Engineering 297 550-558 (2012)
    Technical textiles can be subject to mechanical stress with strain rates far from the quasi-static conditions of common standardized tests. The objective of the presented work was therefore to study the mechanical properties of technical yarns made of PA 6.6, PA 4.6, and PET under strain rates up to 200 s -1, making use of a "falling weight" apparatus. It can be summarized that the moduli at specific points of the stress-strain-curve increase with the strain rate to values up to 50% higher than the data determined under quasi-static strain. Saturation is observed for strain rates larger than 50 s -1. The analysis of failure morphology by scanning electron microscopy revealed molten material at the ends of broken fibers. This indicates thermal failure of the fibers due to local concentration of energy loading. Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/mame.201100312
  • 2012 • 148 Synthesis of high surface area ZnO powder by continuous precipitation
    Boz, I. and Kaluza, S. and Boroǧlu, M.S. and Muhler, M.
    Materials Research Bulletin 47 1185-1190 (2012)
    Synthesis of high surface area ZnO powder was achieved by continuous precipitation using zinc ions and urea at low temperature of 90 °C. The powder precipitated resulted in high-purity single-phase ZnO powder when calcined at 280 °C for 3 h in air. The solution pH and the precipitation duration strongly affected the surface area of the calcined ZnO powder. Detailed structural characterizations demonstrated that the synthesized ZnO powder were single crystalline with wurtzite hexagonal phase. The powdered samples precipitated by homogeneous precipitation crystallized directly to hydrozincite without any intermediate phase formation. The phase structures, morphologies and properties of the final ZnO powders were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), dynamic light scattering particle size analysis (DLS), and nitrogen physisorption in order to determine the specific surface area (BET) and the pore size distribution (BJH). © 2012 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.materresbull.2012.02.005
  • 2012 • 147 On the effect of manganese on grain size stability and hardenability in ultrafine-grained ferrite/martensite dual-phase steels
    Calcagnotto, M. and Ponge, D. and Raabe, D.
    Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science 43 37-46 (2012)
    Two plain carbon steels with varying manganese content (0.87 wt pct and 1.63 wt pct) were refined to approximately 1 μm by large strain warm deformation and subsequently subjected to intercritical annealing to produce an ultrafine grained ferrite/martensite dual-phase steel. The influence of the Mn content on microstructure evolution is studied by scanning electron microscopy (SEM). The Mn distribution in ferrite and martensite is analyzed by high-resolution electron backscatter diffraction (EBSD) combined with energy dispersive X-ray spectroscopy (EDX). The experimental findings are supported by the calculated phase diagrams, equilibrium phase compositions, and the estimated diffusion distances using Thermo-Calc (Thermo-Calc Software, McMurray, PA) and Dictra (Thermo-Calc Software). Mn substantially enhances the grain size stability during intercritical annealing and the ability of austenite to undergo martensitic phase transformation. The first observation is explained in terms of the alteration of the phase transformation temperatures and the grain boundary mobility, while the second is a result of the Mn enrichment in cementite during large strain warm deformation, which is inherited by the newly formed austenite and increases its hardenability. The latter is the main reason why the ultrafine-grained material exhibits a hardenability that is comparable with the hardenability of the coarse-grained reference material. © 2011 The Minerals, Metals & Materials Society and ASM International.
    view abstractdoi: 10.1007/s11661-011-0828-3
  • 2012 • 146 Polycrystal model of the mechanical behavior of a Mo-TiC 30 vol.% metal-ceramic composite using a three-dimensional microstructure map obtained by dual beam focused ion beam scanning electron microscopy
    Cédat, D. and Fandeur, O. and Rey, C. and Raabe, D.
    Acta Materialia 60 1623-1632 (2012)
    The mechanical behavior of a Mo-TiC 30 vol.% ceramic-metal composite was investigated over a wide temperature range (25-700 °C). High-energy X-ray tomography was used to reveal percolation of the hard titanium carbide phase through the composite. Using a polycrystal approach for a two-phase material, finite-element simulations were performed on a real three-dimensional (3-D) aggregate of the material. The 3-D microstructure, used as the starting configuration for the predictions, was obtained by serial sectioning in a dual beam focused ion beam scanning electron microscope coupled to an electron backscattered diffraction system. The 3-D aggregate consists of a molybdenum matrix and a percolating TiC skeleton. As for most body-centered cubic (bcc) metals, the molybdenum matrix phase is characterized by a change in plasticity mechanism with temperature. We used a polycrystal model for bcc materials which was extended to two phases (TiC and Mo). The model parameters of the matrix were determined from experiments on pure molydenum. For all temperatures investigated the TiC particles were considered to be brittle. Gradual damage to the TiC particles was treated, based on an accumulative failure law that is approximated by evolution of the apparent particle elastic stiffness. The model enabled us to determine the evolution of the local mechanical fields with deformation and temperature. We showed that a 3-D aggregate representing the actual microstructure of the composite is required to understand the local and global mechanical properties of the composite studied. © 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2011.11.055
  • 2012 • 145 A facile route to reassemble titania nanoparticles into ordered chain-like networks on substrate
    Cheng, Y.-J. and Wolkenhauer, M. and Bumbu, G.-G. and Gutmann, J.S.
    Macromolecular Rapid Communications 33 218-224 (2012)
    A facile route to reassemble titania nanoparticles within the titania-block copolymer composite films has been developed. The titania nanoparticles templated by the amphiphilic block copolymer of poly(styrene)-block-poly (ethylene oxide) (PS-b-PEO) were frozen in the continuous PS matrix. Upon UV exposure, the PS matrix was partially degraded, allowing the titania nanoparticles to rearrange into chain-like networks exhibiting a closer packing. The local structures of the Titania chain-like networks were investigated by both AFM and SEM; the lateral structures and vertical structures of the films were studied by GISAXS and X-ray reflectivity respectively. Both the image analysis and X-ray scattering characterization prove the reassembly of the titania nanoparticles after UV exposure. The mechanism of the nanoparticle assembly is discussed. Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/marc.201100638
  • 2012 • 144 A novel fully implantable wireless sensor system for monitoring hypertension patients
    Cleven, N.J. and Müntjes, J.A. and Fassbender, H. and Urban, U. and Görtz, M. and Vogt, H. and Gräfe, M. and Göttsche, T. and Penzkofer, T. and Schmitz-Rode, T. and Mokwa, W.
    IEEE Transactions on Biomedical Engineering 59 3124-3130 (2012)
    This paper presents a novel fully implantable wireless sensor system intended for long-term monitoring of hypertension patients, designed for implantation into the femoral artery with computed tomography angiography. It consists of a pressure sensor and a telemetric unit, which is wirelessly connected to an extracorporeal readout station for energy supply and data recording. The system measures intraarterial pressure at a sampling rate of 30Hz and an accuracy of ±1.0mmHg over a range of 30-300mmHg, while consuming up to 300μW. A special peel-away sheath introducer set was developed to support the implantation procedure. The system delivered stable measurements in initial animal trials in sheep, with results being in good agreement with reference sensor systems. © 2012 IEEE.
    view abstractdoi: 10.1109/TBME.2012.2216262
  • 2012 • 143 Nanoscale size effect on surface spin canting in iron oxide nanoparticles synthesized by the microemulsion method
    Darbandi, M. and Stromberg, F. and Landers, J. and Reckers, N. and Sanyal, B. and Keune, W. and Wende, H.
    Journal of Physics D: Applied Physics 45 (2012)
    Uniformly sized and crystalline iron oxide nanoparticles (IONPs) with spinel structure and mean diameters of about 3, 6 and 9nm were synthesized in high yield using the microemulsion route at room temperature. The nanoparticles (NPs) were stabilized in situ by organic surfactant molecules which acted both as a stabilizer of the microemulsion system and as a capping layer of the NP surface. NP size control was attained by careful adjustment of the preparation conditions. The structure, morphology and NP size distribution were investigated by x-ray diffraction, transmission electron microscopy and scanning electron microscopy. A particular effort was devoted in this work to study the effect of size and capping of these NPs on their magnetic structure by in-field Mössbauer spectroscopy at 4.2K. The mean canting angle (relative to the applied field direction) of the Fe spins was observed to increase with decreasing NP size due to the enhanced surface-to-volume ratio. Comparing bare and capped NPs of the same diameter, we verified that the spin canting was not affected by the organic capping. This implied almost identical magnetic orientations of bare and capped NPs. Simultaneously, the capping material was capable of preventing agglomeration effects which can occur in case of direct particle contact. Using a core/shell model, we showed that spin canting originated from the surface shell of the NPs. Furthermore, the Mössbauer spectral parameters provided evidence for the existence of a high fraction of Fe 3O 4 (magnetite) in the IONP. © 2012 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0022-3727/45/19/195001
  • 2012 • 142 Influence of process parameters on the crystallinity, morphology and composition of tungsten oxide-based thin films grown by metalorganic chemical vapor deposition
    De Los Arcos, T. and Cwik, S. and Milanov, A.P. and Gwildies, V. and Parala, H. and Wagner, T. and Birkner, A. and Rogalla, D. and Becker, H.-W. and Winter, J. and Ludwig, Al. and Fischer, R.A. and Devi, A.
    Thin Solid Films 522 11-16 (2012)
    The growth of tungsten oxide (WO 3) based thin films was achieved via metalorganic chemical vapor deposition using an all-nitrogen coordinated tungsten precursor in combination with oxygen. Film growth was performed on Si(100) substrates in the temperature range of 400-800 °C. Employing multi-technique approaches like X-ray diffraction, scanning electron microscopy, atomic force microscopy, Rutherford back scattering, nuclear reaction analysis and X-ray photoelectron spectroscopy, the variation of the growth characteristics and film properties with deposition temperature were studied in terms of crystallinity, structure, surface roughness and composition. Special attention was devoted to the investigation of variations in the film composition for the as-deposited and annealed films. © 2011 Elsevier B.V.
    view abstractdoi: 10.1016/j.tsf.2011.12.007
  • 2012 • 141 Magnetic switchable alginate beads
    Degen, P. and Leick, S. and Siedenbiedel, F. and Rehage, H.
    Colloid and Polymer Science 290 97-106 (2012)
    Calcium alginate beads are enclosed in a wide range of products including food, pharmaceuticals, and cosmetic formulations. The biopolymer matrix is often used to stabilize active ingredients and to provide a controlled release under well-defined conditions. In this context, it is of high interest to study the magnetic-induced attraction, elongation, and rupture of capsules or beads. In this work, we synthesized new types of magnetic switchable alginate beads. The magnetic sensitivity was achieved by incorporation of magnetic nanoparticles (MNPs) within the alginate gel. We measured the mechanical properties of single alginate beads in squeezing experiments, the evaporation of water and the magnetic sensitivity by stimulation of these beads in external fields. In all these measurements, the alginate and the nanoparticle concentration were systematically varied. We could show that the incorporation of MNPs generates a magnetic response of the beads and reduces the evaporation of water but has no influence on the mechanical stability of the beads during compression. Calculations of the shear modulus by means of the squeezing data result in good agreement in comparison to the shear moduli measured by rheological frequency sweep tests. With scanning electron microscopy, we could analyze the molecular structure of such composite systems, and we observed a homogeneous distribution of the MNPs within the gel matrix. © Springer-Verlag 2011.
    view abstractdoi: 10.1007/s00396-011-2524-7
  • 2012 • 140 Rare-earth substituted HfO2 thin films grown by metalorganic chemical vapor deposition
    Devi, A. and Cwik, S. and Xu, K. and Milanov, A.P. and Noei, H. and Wang, Y. and Barreca, D. and Meijer, J. and Rogalla, D. and Kahn, D. and Cross, R. and Parala, H. and Paul, S.
    Thin Solid Films 520 4512-4517 (2012)
    Thin films of HfGdOx and HfDyOx were deposited by metalorganic chemical vapor deposition (MOCVD) utilizing guanidinate precursors for Hf, Gd and Dy. The close match in the thermal properties of the precursors enabled the MOCVD of rare-earth (RE) substituted HfO2 over a wide temperature window. Film deposition was carried out in the temperature range 300-700 °C in the presence of oxygen on Si(100) substrates. HfGdO x films were analyzed in detail for their structure, composition and morphology using X-ray diffraction, Rutherford backscattering spectrometry, proton induced X-ray emission, X-ray photoelectron spectroscopy and scanning electron microscopy. The electrical properties of HfGdOx in terms of capacitance-voltage and current-voltage characteristics of metal-insulator- semiconductor device structures were evaluated. © 2011 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.tsf.2011.10.141
  • 2012 • 139 Structure, composition, and mechanical properties of shark teeth
    Enax, J. and Prymak, O. and Raabe, D. and Epple, M.
    Journal of Structural Biology 178 290-299 (2012)
    The teeth of two different shark species (Isurus oxyrinchus and Galeocerdo cuvier) and a geological fluoroapatite single crystal were structurally and chemically characterized. In contrast to dentin, enameloid showed sharp diffraction peaks which indicated a high crystallinity of the enameloid. The lattice parameters of enameloid were close to those of the geological fluoroapatite single crystal. The inorganic part of shark teeth consisted of fluoroapatite with a fluoride content in the enameloid of 3.1 wt.%, i.e., close to the fluoride content of the geological fluoroapatite single crystal (3.64 wt.%). Scanning electron micrographs showed that the crystals in enameloid were highly ordered with a special topological orientation (perpendicular towards the outside surface and parallel towards the center). By thermogravimetry, water, organic matrix, and biomineral in dentin and enameloid of both shark species were determined. Dentin had a higher content of water, organic matrix, and carbonate than enameloid but contained less fluoride. Nanoindentation and Vicker's microhardness tests showed that the enameloid of the shark teeth was approximately six times harder than the dentin. The hardness of shark teeth and human teeth was comparable, both for dentin and enamel/enameloid. In contrast, the geological fluoroapatite single crystal was much harder than both kinds of teeth due to the absence of an organic matrix. In summary, the different biological functions of the shark teeth (" tearing" for Isurus and "cutting" for Galeocerdo) are controlled by the different geometry and not by the chemical or crystallographic composition. © 2012 Elsevier Inc.
    view abstractdoi: 10.1016/j.jsb.2012.03.012
  • 2012 • 138 Low-temperature synthesis of crystalline BaTiO 3 nanoparticles by one-step "organosol"-precipitation
    Gao, Y. and Shvartsman, V.V. and Elsukova, A. and Lupascu, D.C.
    Journal of Materials Chemistry 22 17573-17583 (2012)
    The "organosol" precipitation method is proposed to produce nanosized particles of barium titanate (BaTiO 3) at temperatures as low as room temperature. The advantages of this method are a high yield, a simple but precise control of the size of the particles, low process temperature, short reaction time, as well as low cost of reagents. The particles were systematically characterized by powder X-ray diffraction (XRD), Raman scattering, scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), thermogravimetric thermal analysis (TGA/DSC), infrared spectroscopy (IR), and impedance analysis. The as-prepared BaTiO 3 nanocrystals exhibit a granular shape of around 15 nm in diameter. Oleic acid retards crystallization and thus allows generation of a uniformly small grain size and excellent dispersibility in organic solvents. The surface energy of the particles is modified and crystallization in cubes also arises. The mechanism of powder formation is discussed. The method offers an alternate low-cost route to perovskite nanopowders easily dispersed in organic media. © 2012 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c2jm33373b
  • 2012 • 137 Grain size effect on strain hardening in twinning-induced plasticity steels
    Gutierrez-Urrutia, I. and Raabe, D.
    Scripta Materialia 66 992-996 (2012)
    We investigate the influence of grain size on the strain hardening of two Fe-22Mn-0.6C (wt.%) twinning-induced plasticity steels with average grain sizes of 3 and 50 μm, respectively. The grain size has a significant influence on the strain hardening through the underlying microstructure. The dislocation substructure formed in the early deformation stages determines the density of nucleation sites for twins per unit grain boundary area which controls the developing twin substructure. © 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.scriptamat.2012.01.037
  • 2012 • 136 Dislocation density measurement by electron channeling contrast imaging in a scanning electron microscope
    Gutierrez-Urrutia, I. and Raabe, D.
    Scripta Materialia 66 343-346 (2012)
    We have measured the average dislocation density by electron channeling contrast imaging (ECCI) in a scanning electron microscope under controlled diffraction conditions in a Fe-3 wt.% Si alloy tensile deformed to a macroscopic stress of 500 MPa. Under optimal diffraction conditions, ECCI provides an average dislocation density close to that obtained by bright-field transmission electron microscopy. This result confirms that ECCI is a powerful technique for determining dislocation densities in deformed bulk metals. © 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.scriptamat.2011.11.027
  • 2012 • 135 In-situ tensile testing of single-crystal molybdenum-alloy fibers with various dislocation densities in a scanning electron microscope
    Johanns, K.E. and Sedlmayr, A. and Sudharshan Phani, P. and Mönig, R. and Kraft, O. and George, E.P. and Pharr, G.M.
    Journal of Materials Research 27 508-520 (2012)
    In-situ tensile tests have been performed in a dual beam focused ion beam and scanning electron microscope on as-grown and prestrained single-crystal molybdenum-alloy (Mo-alloy) fibers. The fibers had approximately square cross sections with submicron edge lengths and gauge lengths in the range of 9-41 μm. In contrast to previously observed yield strengths near the theoretical strength of 10 GPa in compression tests of ∼1-3-μm long pillars made from similar Mo-alloy single crystals, a wide scatter of yield strengths between 1 and 10 GPa was observed in the as-grown fibers tested in tension. Deformation was dominated by inhomogeneous plastic events, sometimes including the formation of Lüders bands. In contrast, highly prestrained fibers exhibited stable plastic flow, significantly lower yield strengths of ∼1 GPa, and stress-strain behavior very similar to that in compression. A simple, statistical model incorporating the measured dislocation densities is developed to explain why the tension and compression results for the as-grown fibers are different. © 2012 Materials Research Society.
    view abstractdoi: 10.1557/jmr.2011.298
  • 2012 • 134 Protective ability of hybrid nano-composite coatings with cerium sulphate as inhibitor against corrosion of AA2024 aluminium alloy
    Kozhukharov, S. and Kozhukharov, V. and Schem, M. and Aslan, M. and Wittmar, M. and Wittmar, A. and Veith, M.
    Progress in Organic Coatings 73 95-103 (2012)
    The corrosion protective ability of hybrid oxy silane nano-composite coatings deposited on AA2024 by sol-gel technique was studied. The coatings are developed as an environmentally friendly alternative of the toxic chromium containing coatings on aluminium. A cerium salt, Ce2(SO 4)3, was used as inhibitor of the corrosion process. Two methods were applied to introduce the salt in the hybrid matrix: directly in the matrix, or by porous Al2O3 nano-particles preliminary loaded by the salt. Atomic force microscopy (AFM) was used to evaluate the superficial morphology of the coatings, while their layer structure was studied by means of scanning electron microscopy (SEM). Linear voltammetry (LVA) and electrochemical impedance spectroscopy (EIS) were used for assessment of the barrier ability. The hybrid matrix was found to possess remarkable barrier ability which was preserved even after prolonged exposure of the coatings to a model corrosive medium of 0.05 M NaCl. In all cases, the cerium salt involved either directly or by Al2O3 nano-particles proved to deteriorate the protective properties of the coatings and to accelerate pitting nucleation. The experimental results have shown that cerium sulphate, introduced in the by the both manners in the hybrid matrix did not efficiently inhibit the corrosion of AA2024, unlike the reported inhibiting properties of other cerium salts. © 2011 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.porgcoat.2011.09.005
  • 2012 • 133 Well-defined amphiphilic poly(2-oxazoline) ABA-triblock copolymers and their aggregation behavior in aqueous solution
    Krumm, C. and Fik, C.P. and Meuris, M. and Dropalla, G.J. and Geltenpoth, H. and Sickmann, A. and Tiller, J.C.
    Macromolecular Rapid Communications 33 1677-1682 (2012)
    Self-organization of block copolymers in solution is a way to obtain advanced functional superstructures. The synthesis of well-defined polymethyloxazoline-block-polyphenyloxazoline-block-polymethyloxazoline triblock copolymers is described and proven by 1H NMR spectroscopy, SEC, and ESI-MS. The surprisingly water- soluble block copolymers do self-organize in aqueous solutions uniquely forming three coexisting well-defined structures: unimolecular micelles, micellar aggregates, and very form-stable polymersomes. This is the first example of a polymersome forming ABA-triblock copolymer with a glassy middle block. The spherical vesicles are analysed by scanning electron microscopy and transmission electron microscopy. It could be shown that these vesicles are indeed hollow spheres. © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/marc.201200192
  • 2012 • 132 Synthesis of an improved hierarchical carbon-fiber composite as a catalyst support for platinum and its application in electrocatalysis
    Kundu, S. and Nagaiah, T.C. and Chen, X. and Xia, W. and Bron, M. and Schuhmann, W. and Muhler, M.
    Carbon 50 4534-4542 (2012)
    A hierarchical carbon-fiber composite was synthesized based on carbon cloth (CC) modified with primary carbon microfibers (CMF) and subsequently secondary carbon nanotubes (CNT), thus forming a three-dimensional hierarchical structure with high BET surface area. The primary CMFs and the secondary CNTs are grown with electrodeposited iron nanoparticles as catalysts from methane and ethylene, respectively. After deposition of Pt nanoparticles by chemical vapor deposition from (trimethyl)cyclopentadienylplatinum, the resulting hierarchical composite was used as catalyst in the electrocatalytic oxygen reduction (oxygen reduction reaction, ORR) as specific test reaction. The modification of the CC with CMFs and CNTs improved the electrochemical properties of the carbon composite as revealed by electrochemical impedance measurements evidencing a low charge transfer resistance for redox mediators at the modified CC. X-ray photoelectron spectroscopy measurements were carried out to identify the chemical state and the surface atomic concentration of the Pt catalysts deposited on the hierarchical carbon composites. The ORR activity of Pt supported on different composites was investigated using rotating disk electrode measurements and scanning electrochemical microscopy. These electrochemical studies revealed that the obtained structured catalyst support is very promising for electrochemical applications, e.g. fuel cells. © 2012 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.carbon.2012.05.037
  • 2012 • 131 Synthesis and characterization of α-, β-, and γ-Ga 2o 3 prepared from aqueous solutions by controlled precipitation
    Li, L. and Wei, W. and Behrens, M.
    Solid State Sciences 14 971-981 (2012)
    α, β and γ-Ga 2O 3 have been successfully obtained in an easily scalable synthesis using aqueous solution of gallium nitrate and sodium carbonate as starting materials without any surfactant and additive. α and β-Ga 2O 3 were obtained by calcination at 350 and 700 °, respectively, of α-GaOOH, prepared by controlled precipitation at constant pH 6 and T = 55 °, with 24 h of aging. Aging was necessary to fully convert the initially preciptated gel into a well-crystalline and phase-pure material. γ-Ga 2O 3 was obtained after calcination at 500 ° of gallia gel, synthesized at constant pH 4 and T = 25 °, without aging. These three polymorphs have a for gallia relatively high surface area: 55 m 2/g (α-Ga 2O 3), 23 m 2/g (β-Ga 2O 3) and 116 m 2/g (γ-Ga 2O 3). The combination of X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDX), nitrogen physisorption and thermogravimetry (TG) was employed to characterize the samples and their formation. © 2012 Elsevier Masson SAS. All rights reserved.
    view abstractdoi: 10.1016/j.solidstatesciences.2012.04.037
  • 2012 • 130 Silver, gold, and alloyed silver-gold nanoparticles: Characterization and comparative cell-biologic action
    Mahl, D. and Diendorf, J. and Ristig, S. and Greulich, C. and Li, Z.A. and Farle, M. and Köller, M. and Epple, M.
    Journal of Nanoparticle Research 14 (2012)
    Silver, gold, and silver-gold-alloy nanoparticles were prepared by citrate reduction modified by the addition of tannin during the synthesis, leading to a reduction in particle size by a factor of three. Nanoparticles can be prepared by this easy waterbased synthesis and subsequently functionalized by the addition of either tris(3-sulfonatophenyl)phosphine or poly(N-vinylpyrrolidone). The resulting nanoparticles of silver (diameter 15-25 nm), gold (5-6 nm), and silver-gold (50:50; 10-12 nm) were easily dispersable in water and also in cell culture media (RPMI + 10 % fetal calf serum), as shown by nanoparticle tracking analysis and differential centrifugal sedimentation. High-resolution transmission electron microscopy showed a polycrystalline nature of all nanoparticles. EDX on single silver-gold nanoparticles indicated that the concentration of gold is higher inside a nanoparticle. The biologic action of the nanoparticles toward human mesenchymal stem cells (hMSC) was different: Silver nanoparticles showed a significant concentration-dependent influence on the viability of hMSC. Gold nanoparticles showed only a small effect on the viability of hMSC after 7 days. Surprisingly, silver-gold nanoparticles had no significant influence on the viability of hMSC despite the silver content. Silver nanoparticles and silver-gold nanoparticles in the concentration range of 5-20 μg mL -1 induced the activation of hMSC as indicated by the release of IL-8. In contrast, gold nanoparticles led to a reduction of the release of IL-6 and IL-8. © Springer Science+Business Media B.V. 2012.
    view abstractdoi: 10.1007/s11051-012-1153-5
  • 2012 • 129 Combined high resolution Scanning Kelvin probe - Scanning electrochemical microscopy investigations for the visualization of local corrosion processes
    Maljusch, A. and Senöz, C. and Rohwerder, M. and Schuhmann, W.
    Electrochimica Acta 82 339-348 (2012)
    An integrated SKP-SECM system was successfully optimised with respect to improved lateral resolution. An aluminum alloy was synthesised by solidification of a liquid melt of pure Al, Cu and Mg metal powders in order to visualize single S-phase intermetallic particles (IMPs) using a newly proposed "glass free" SKP-SECM tip. The obtained IMPs were randomly distributed in the solid solution matrix of the alloy and their average chemical composition was in agreement with that of S-phase IMPs in commercially available AA2024-T351 alloys. The S-phase IMPs were localized in the SKP mode of the SKP-SECM system. The increased electrochemical activity of the S-phase IMPs was visualized using the feedback mode of SECM and the in situ consumption of O 2 on the surface of a single S-phase IMP was visualized in the redox-competition mode of the SECM using the same tip. Thus, the local Volta potential difference obtained in the SKP mode could be overlaid with the local electrochemical activity for O 2 reduction. © 2012 Elsevier Ltd.
    view abstractdoi: 10.1016/j.electacta.2012.05.134
  • 2012 • 128 Modification of pineapple leaf fibers and graft copolymerization of acrylonitrile onto modified fibers
    Maniruzzaman, M. and Rahman, M.A. and Gafur, M.A. and Fabritius, H. and Raabe, D.
    Journal of Composite Materials 46 79-90 (2012)
    Raw pineapple leaf fibers (PALFs) were chemically modified by scouring, NaOH treatment, and bleaching (NaClO2). The graft copolymerization of synthetic acrylonitrile monomer onto bleached PALFs was carried out in aqueous medium using potassium persulfate (K2S2O8/FeSO4) as a redox initiator. The maximum grafting level at optimum conditions, namely, monomer concentration, initiator concentration, catalyst concentration, reaction time, and temperature have been determined. The main objective of this study is to decrease the amorphous region of lignocellulose in PALFs and improve its hydrophobic nature by incorporation of synthetic polymer of polyacrylonitrile and mechanical properties. The modified and grafted fibers were characterized by Fourier transform infrared spectroscopy, scanning electron microscope, thermogravimetric analysis, and X-ray diffraction study techniques. The moisture content and tensile strength properties were also evaluated for their environmental and mechanical performances. © The Author(s) 2011.
    view abstractdoi: 10.1177/0021998311410486
  • 2012 • 127 Enhanced electrocatalytic stability of platinum nanoparticles supported on a nitrogen-doped composite of carbon nanotubes and mesoporous titania under oxygen reduction conditions
    Masa, J. and Bordoloi, A. and Muhler, M. and Schuhmann, W. and Xia, W.
    ChemSusChem 5 523-525 (2012)
    Cheers for titania: An N-doped composite of carbon nanotubes (CNTs) and mesoporous TiO 2 is used as support for Pt nanoparticles applied in the oxygen reduction reaction. The composite Pt/N-TiO 2-CNT shows a higher stability than Pt particles on carbon black or N-doped CNTs, as indicated by accelerated stress tests of up to 2000 cycles. The enhanced stability is attributed to strong interactions between TiO 2 and Pt and a higher corrosion resistance of TiO 2 as well as CNTs. © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/cssc.201100643
  • 2012 • 126 Observation and implications of magnetic domains in lateral spin valves
    Mennig, J. and Matthes, F. and Bürgler, D.E. and Schneider, C.M.
    Journal of Applied Physics 111 (2012)
    Co/Cu/Co lateral spin valves (LSV), with Co being the topmost layer, are in situ prepared and measured under ultrahigh vacuum conditions. The clean process yields a non-local spin signal of 0.9 mω. Scanning electron microscopy with polarization analysis (SEMPA) reveals domain structures in both magnetic electrodes that depend on the LSV dimensions. The spin signal correlates to SEMPA images as well as the anisotropic magnetoresistance of both Co magnets, revealing a strong impact of multi-domain states on the spin signal. © 2012 American Institute of Physics.
    view abstractdoi: 10.1063/1.3671415
  • 2012 • 125 Surface modification of poly(ethylene terephthalate) fabric via photo-chemical reaction of dimethylaminopropyl methacrylamide
    Mohamed, N.H. and Bahners, T. and Wego, A. and Gutmann, J.S. and Ulbricht, M.
    Applied Surface Science 259 261-269 (2012)
    Photo-chemical reactions and surface modifications of poly(ethylene terephthalate) (PET) fabrics with the monomer dimethylaminopropyl methacrylamide (DMAPMA) and benzophenone (BP) as photo-initiator using a broad-band UV lamp source were investigated. The tertiary amino groups of the grafted poly(DMAPMA) chains were subsequently quaternized with alkyl bromides of different chain lengths to establish antibacterial activity. The surface composition, structure and morphology of modified PET fabrics were characterized by Fourier transform infrared spectroscopy (FTIR/ATR), X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). To evaluate the amount of quaternary and tertiary ammonium groups on the modified surface, PET was dyed with an acid dye which binds to the ammonium groups. Therefore, the color depth is a direct indicator of the amount of ammonium groups. The resulting antibacterial activity of the modified PET fabrics was tested with Escherichia coli. The results of all experiments show that a photochemical modification of PET is possible using DMAPMA, benzophenone and UV light. Also, the quaternization of tertiary amino groups as well as the increase in antibacterial activity of the modified PET by the established quaternary ammonium groups were successful. © 2012 Elsevier B.V.
    view abstractdoi: 10.1016/j.apsusc.2012.07.029
  • 2012 • 124 Self-assembled iron oxide nanoparticle multilayer: X-ray and polarized neutron reflectivity
    Mishra, D. and Benitez, M.J. and Petracic, O. and Badini Confalonieri, G.A. and Szary, P. and Brüssing, F. and Theis-Bröhl, K. and Devishvili, A. and Vorobiev, A. and Konovalov, O. and Paulus, M. and Sternemann, C. and Toperverg...
    Nanotechnology 23 (2012)
    We have investigated the structure and magnetism of self-assembled, 20nm diameter iron oxide nanoparticles covered by an oleic acid shell for scrutinizing their structural and magnetic correlations. The nanoparticles were spin-coated on an Si substrate as a single monolayer and as a stack of 5ML forming a multilayer. X-ray scattering (reflectivity and grazing incidence small-angle scattering) confirms high in-plane hexagonal correlation and a good layering property of the nanoparticles. Using polarized neutron reflectivity we have also determined the long range magnetic correlations parallel and perpendicular to the layers in addition to the structural ones. In a field of 5kOe we determine a magnetization value of about 80% of the saturation value. At remanence the global magnetization is close to zero. However, polarized neutron reflectivity reveals the existence of regions in which magnetic moments of nanoparticles are well aligned, while losing order over longer distances. These findings confirm that in the nanoparticle assembly the magnetic dipoledipole interaction is rather strong, dominating the collective magnetic properties at room temperature. © 2012 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0957-4484/23/5/055707
  • 2012 • 123 Factors affecting the sieving behavior of anti-fouling thin-layer cross-linked hydrogel polyethersulfone composite ultrafiltration membranes
    Peeva, P.D. and Million, N. and Ulbricht, M.
    Journal of Membrane Science 390-391 99-112 (2012)
    Anti-fouling thin-layer hydrogel composite membranes based on polyethersulfone (PES) ultrafiltration (UF) membranes were synthesized by addition of the crosslinking agents N,N'-methylene bisacrylamide (MBAA) and pentaerythritol triallyl ether (PETAE) to the modifier solution of poly(ethylene glycol)400 methacrylate (PEGMA) in water under variation of the UV irradiation intensity and dose. The resulting membrane properties were characterized by means of water permeability, degree of grafting (DG), modification depth and skin-layer cross-section structure (scanning electron microscopy; SEM) as well as sieving/rejection behavior via filtration of polyethylene glycol (PEG) and dextran mixtures with wide molecular weight distribution (MWD) in water, the latter as additional study to already performed experiments on protein rejection [1]. For better understanding of the obtained results, bulk hydrogels were also prepared and the degree of swelling (DS) was determined. The performed experiments showed that the used crosslinkers changed the hydrogel layer structure and properties, which significantly affected the sieving/rejection behavior of the composite membranes. Using test solutions with low fouling tendency facilitated the validation of the obtained data in comparison to the previously reported protein rejection results. It was shown that polyPEGMA/MBAA composite membranes exhibited higher solute rejection, while polyPEGMA/PETAE showed lower solute rejection compared to uncrosslinked polyPEGMA composite membranes prepared with the same UV irradiation dose. Increasing the amount of crosslinkers amplified these effects. Furthermore, a trade-off analysis between water permeability and molecular weight cut-off (MWCO) for virgin and composite membranes was done in order to evaluate the membrane performance improvement by the composite membranes. The obtained results showed that the prepared composite membranes exhibited better fluxes and selectivity compared to commercial membranes with analogous characteristics. © 2011 Elsevier B.V.
    view abstractdoi: 10.1016/j.memsci.2011.11.025
  • 2012 • 122 Layer-by-layer fabrication of an anatase titania multilayer with gradual sponge-like morphology
    Perlich, J. and Memesa, M. and Diethert, A. and Metwalli, E. and Wang, W. and Roth, S.V. and Gutmann, J.S. and Müller-Buschbaum, P.
    Colloid and Polymer Science 290 119-126 (2012)
    The fabrication of a functional multilayer system with a gradually hierarchical order formed by individual titania thin films of different porosity is investigated. The porous or sponge-like nanostructures are fabricated using a diblock copolymer assisted sol-gel process. The successive spin-coating of the sol-gel solution onto the silicon substrate deposits a thin polymer nanocomposite film which is transformed to purely anatase titania nanostructures via calcination. In total, this procedure is repeated layer by layer for three times. This layer-by-layer approach is monitored with grazing incidence small-angle X-ray scattering (GISAXS) after each fabrication step. The GISAXS investigation is complemented in real space with a scanning electron microscopy characterization of the respective preparation stages. From the characterization, a porous titania multilayer system with gradually structured levels is clearly identified. © Springer-Verlag 2011.
    view abstractdoi: 10.1007/s00396-011-2529-2
  • 2012 • 121 Investigation of early degradation processes at coated metals by AC-scanning electrochemical microscopy
    Santana, J.J. and Souto, R.M. and González, S. and Pähler, M. and Schuhmann, W.
    ECS Transactions 41 29-38 (2012)
    Alternating-current scanning electrochemical microscopy (AC-SECM) is employed to characterize the early stages of the degradation reactions occurring in metal-coating systems upon exposure to an aqueous environment. The spatial resolution of the technique results from the measurement of changes in the resistance of the thin electrolyte layer comprised between the tip and the surface of the coating due to variations in the tip-substrate distance. Resistance measurements are conducted at various frequencies of the AC perturbation signal effectively allowing topographic changes to be monitored as a function of time. Furthermore, AC-SECM can be used to determine the tip-substrate distance without the addition of redox mediators to the electrolyte, which might affect the chemical properties of the system. In this way, the effect of chloride ions from the aqueous phase to induce either the heterogeneous absorption of water by the coating, or its accumulation at the metal-substrate interface, has been imaged. ©The Electrochemical Society.
    view abstractdoi: 10.1149/1.3696868
  • 2012 • 120 Direct evidence of early blister formation in polymer-coated metals from exposure to chloride-containing electrolytes by alternating-current scanning electrochemical microscopy
    Santana, J.J. and Pähler, M. and Souto, R.M. and Schuhmann, W.
    Electrochimica Acta 77 60-64 (2012)
    An early specific effect of chloride ions on the heterogeneous swelling at the metal-polymer interface in non-defective coated metals is elucidated using frequency-dependent alternating-current scanning electrochemical microscopy (AC-SECM). Spatially-resolved differences in the topography of the coated sample are visualized upon prolonged incubation with chloride-containing electrolyte, thus allowing to assume a direct evidence of chloride ion permeation through the polymer matrix simultaneous to water uptake. The implications of this finding are particularly relevant towards the development of a new mechanistic model for blister initiation in coated metals. © 2012 Elsevier Ltd All rights reserved.
    view abstractdoi: 10.1016/j.electacta.2012.05.062
  • 2012 • 119 Ag/ZnO nanomaterials as high performance sensors for flammable and toxic gases
    Simon, Q. and Barreca, D. and Gasparotto, A. and MacCato, C. and Tondello, E. and Sada, C. and Comini, E. and Devi, A. and Fischer, R.A.
    Nanotechnology 23 (2012)
    Ag/ZnO nanocomposites supported on polycrystalline Al 2O 3 were synthesized by an unprecedented approach combining plasma enhanced chemical vapor deposition (PE-CVD) of ZnO matrices and the subsequent deposition of Ag nanoparticles (NPs) by radio frequency (RF) sputtering. The system structure, composition and morphology were investigated by glancing incidence x-ray diffraction (GIXRD), secondary ion mass spectrometry (SIMS), field emission scanning electron microscopy (FE-SEM) and energy dispersive x-ray spectroscopy (EDXS). A tailored dispersion and distribution of silver particles could be obtained under mild conditions by the sole variation of the sputtering time. Gas sensing properties toward flammable and toxic gases, both reducing (CH 3CH 2OH, CH 3COCH 3) and oxidizing (O 3), were investigated in the temperature range 100400°C. Beside the high sensitivity, the developed sensors exhibited a response proportional to Ag content, thanks to catalytic and electronic effects promoted by silver NPs. In addition, discrimination between oxidizing and reducing analytes was enabled by a suitable choice of the adopted working temperature.
    view abstractdoi: 10.1088/0957-4484/23/2/025502
  • 2012 • 118 Novel temperature dependent tensile test of freestanding copper thin film structures
    Smolka, M. and Motz, C. and Detzel, T. and Robl, W. and Griesser, T. and Wimmer, A. and Dehm, G.
    Review of Scientific Instruments 83 (2012)
    The temperature dependent mechanical properties of the metallization of electronic power devices are studied in tensile tests on micron-sized freestanding copper beams at temperatures up to 400 °C. The experiments are performed in situ in a scanning electron microscope. This allows studying the micromechanical processes during the deformation and failure of the sample at different temperatures. © 2012 American Institute of Physics.
    view abstractdoi: 10.1063/1.4725529
  • 2012 • 117 Calcium phosphate nanoparticles as versatile carrier for small and large molecules across cell membranes
    Sokolova, V. and Rotan, O. and Klesing, J. and Nalbant, P. and Buer, J. and Knuschke, T. and Westendorf, A.M. and Epple, M.
    Journal of Nanoparticle Research 14 (2012)
    The successful transport of molecules across the cell membrane is a key point in biology and medicine. In most cases, molecules alone cannot penetrate the cell membrane, therefore an efficient carrier is needed. Calcium phosphate nanoparticles (diameter: 100-250 nm, depending on the functionalization) were loaded with fluorescent oligonucleotides, peptide, proteins, antibodies, polymers or porphyrins and characterized by dynamic light scattering, nanoparticle tracking analysis and scanning electron microscopy. Any excess of molecules was removed by ultracentrifugation, and the dissolved molecules at the same concentration were used as control. The uptake of such fluorescence-labeled nanoparticles into HeLa cells was monitored by fluorescence microscopy and confocal laser scanning microscopy. Calcium phosphate nanoparticles were able to transport all molecules across the cell membrane, whereas the dissolved molecules alone were taken up only to a very small extent or even not at all. © 2012 Springer Science+Business Media B.V.
    view abstractdoi: 10.1007/s11051-012-0910-9
  • 2012 • 116 A combined experimental-numerical investigation of ductile fracture in bending of a class of ferritic-martensitic steel
    Soyarslan, C. and Malekipour Gharbi, M. and Tekkaya, A.E.
    International Journal of Solids and Structures 49 1608-1626 (2012)
    We present a combined experimental-numerical study on fracture initiation at the convex surface and its propagation during bending of a class of ferritic-martensitic steel. On the experimental side, so-called free bending experiments are conducted on DP1000 steel sheets until fracture, realizing optical and scanning electron microscopy analyses on the post mortem specimens for fracture characterization. A blended Mode I - Mode II fracture pattern, which is driven by cavitation at non-metallic inclusions as well as martensitic islands and resultant softening-based intense strain localization, is observed. Phenomena like crack zig-zagging and crack alternation at the bend apex along the bending axis are introduced and discussed. On the numerical side, based on this physical motivation, the process is simulated in 2D plane strain and 3D, using Gurson's dilatant plasticity model with a recent shear modification, strain-based void nucleation, and coalescence effects. The effect of certain material parameters (initial porosity, damage at coalescence and failure, shear modification term, etc.), plane strain constraint and mesh size on the localization and the fracture behavior are investigated in detail. © 2012 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.ijsolstr.2012.03.009
  • 2012 • 115 Effect of membrane hydrophilization on ultrafiltration performance for biomolecules separation
    Susanto, H. and Roihatin, A. and Aryanti, N. and Anggoro, D.D. and Ulbricht, M.
    Materials Science and Engineering C 32 1759-1766 (2012)
    This paper compares the performance of different hydrophilization methods to prepare low fouling ultrafiltration (UF) membranes. The methods include post-modification with hydrophilic polymer and blending of hydrophilic agent during either conventional or reactive phase separation (PS). The post-modification was done by photograft copolymerization of water-soluble monomer, poly(ethylene glycol) methacrylate (PEGMA), onto a commercial polyethersulfone (PES) UF membrane. Hydrophilization via blend polymer membrane with hydrophilic additive was performed using non-solvent induced phase separation (NIPS). In reactive PS method, the cast membrane was UV-irradiated before coagulation. The resulting membrane characteristic, the performance and hydrophilization stability were systematically compared. The investigated membrane characteristics include surface hydrophilicity (by contact angle /CA/), surface chemistry (by FTIR spectroscopy), and surface morphology (by scanning electron microscopy). The membrane performance was examined by investigation of adsorptive fouling and ultrafiltration using solution of protein or polysaccharide or humic acid. The results suggest that all methods could increase the hydrophilicity of the membrane yielding less fouling. Post-modification decreased CA from 44.8 ± 4.2 o to 37.8 ± 4.2 o to 42.5 ± 4.3 o depending on the degree of grafting (DG). The hydrophilization via polymer blend decreased CA from from 65 o to 54 o for PEG concentration of 5%. Nevertheless, decreasing hydraulic permeability was observed after post-modification as well as during polymer blend modification. Stability examination showed that there was leaching out of modifier agent from the membrane matrix prepared via conventional PS after 10 days soaking in both water and NaOH. Reactive PS could increase the stability of the modifier agent in membrane matrix. © 2012 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.msec.2012.04.036
  • 2012 • 114 Wear analysis of rolled and ground iron-based-alloy-coatings with and without hard material reinforcement
    Tillmann, W. and Luo, W. and Tekkaya, A.E. and Franzen, V.
    Materialwissenschaft und Werkstofftechnik 43 485-494 (2012)
    In this research work the wear behavior of thermal sprayed wear resistant coatings, which are finished by incremental roller burnishing and by grinding in order to smooth the surface, are analyzed by means of the Pin-on-Disc test. Two different arc sprayed coatings WSC-FeCSiMn and FeCrBSiMn are compared to each other. At first the microstructure of the smoothed coatings were characterized by investigation of the topography and morphology. After that the wear behavior was analyzed with two different counterparts made of stainless steel and ceramic. In order to determine the different wear mechanisms the wear traces have been investigated by scanning electron microscope. Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/mawe.201200852
  • 2012 • 113 Atomic layer deposition of HfO 2 thin films employing a heteroleptic hafnium precursor
    Xu, K. and Milanov, A.P. and Parala, H. and Wenger, C. and Baristiran-Kaynak, C. and Lakribssi, K. and Toader, T. and Bock, C. and Rogalla, D. and Becker, H.-W. and Kunze, U. and Devi, A.
    Chemical Vapor Deposition 18 27-35 (2012)
    The application of a heteroleptic hafnium amide-guanidinate precursor for the deposition of HfO 2 thin films via a water-assisted atomic layer deposition (ALD) process is demonstrated for the first time. HfO 2 films are grown in the temperature range 100-300 °C using the compound [Hf(NMe 2) 2(NMe 2-Guan) 2] (1). This compound shows self-limiting ALD-type growth characteristics with growth rates of the order of 1.0-1.2 Å per cycle in the temperature range 100-225 °C. The saturation behavior and a linear dependence on film thickness as a function of number of cycles are verified at various temperatures within the ALD window. The as-deposited HfO 2 films are characterized by atomic force microscopy (AFM), scanning electron microscopy (SEM), Rutherford backscattering spectroscopy (RBS), X-ray photoelectron spectroscopy (XPS), and electrical measurements. For a direct comparison of the precursor performance with that of the parent alkyl amide [Hf(NMe 2) 4] (2), ALD experiments are also performed employing compound 2 under similar process conditions, and in this case no typical ALD characteristics are observed. The application of a heteroleptic hafnium amide-guanidinate precursor [Hf(NMe 2) 2(NMe 2-Guan) 2] for the deposition of HfO 2 thin films via a water assisted ALD process has been demonstrated for the first time. This compound showed self-limiting ALD type growth characteristics with the growth rates as high as 1.0-1.2 Å per cycle in the temperature range 100-225 °C. Typical ALD characteristics such as saturation behavior and linear dependence on the film thickness as a function of number of cycles were verified at different temperatures within the ALD window. The as-deposited HfO 2 films were characterized by AFM, SEM, RBS, XPS and electrical measurements. Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/cvde.201106934
  • 2012 • 112 Nanoindentation of pseudoelastic NiTi containing Ni4Ti 3 precipitates
    Young, M.L. and Frotscher, M. and Bei, H. and Simon, T. and George, E.P. and Eggeler, G.
    International Journal of Materials Research 103 1434-1439 (2012)
    Depending on the processing method, pseudoelastic NiTi alloys can have small, lenticular Ni4Ti3 precipitates; however, the mechanical properties of these precipitates are not well understood. By performing nanoindentation with a spherical indenter, Ni4Ti 3 precipitates within a pseudoelastic NiTi alloy were examined. Scanning electron microscopy was used to examine the indents after nanoindentation. After unloading, the hardness and remnant depth ratios of the indents in the Ni4Ti3 precipitates, the NiTi matrix, and the "average" NiTi alloy were compared. To decouple the effects of elasticity from those of pseudoelasticity, similar nanoindentation experiments were performed on an NiAl sample and compared with results from the NiTi sample. © 2012 Carl Hanser Verlag GmbH & Co. KG.
    view abstractdoi: 10.3139/146.110792
  • 2012 • 111 Evaluation of the catalytic performance of gas-evolving electrodes using local electrochemical noise measurements
    Zeradjanin, A.R. and Ventosa, E. and Bondarenko, A.S. and Schuhmann, W.
    ChemSusChem 5 1905-1911 (2012)
    Characterization of gas evolution reactions at the electrode/electrolyte boundary is often difficult due to the dynamic behavior of interfacial processes. Electrochemical noise measurements determined by scanning electrochemical microscopy were used to characterize Cl 2 evolution at gas-evolving electrodes (GEEs). Analysis of the electrochemical noise is a powerful method to evaluate the efficiency of the catalyst layer at a GEE. The high sensitivity of the developed measurement system enabled accurate monitoring of the current fluctuations caused by gas-bubble detachment from the electrode surface. Fourier transform analysis of the obtained current responses allows extraction of the characteristic frequency, which is the main parameter of the macrokinetics of GEEs. The characteristic frequency was used as part of a methodology to evaluate the catalyst performance and, in particular, to estimate the fraction of the catalyst layer that is active during the gas evolution reaction. Tip of the iceberg: Positioned scanning electrochemical microscopy tips are used to determine the characteristic frequency of gas-bubble detachment from ruthenium-based dimensionally stable anodes at different applied potentials (see picture). Geometrical factors and optimized microstructures of the electrode surface are essential for improving the overall catalytic activity for industrial applications. © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/cssc.201200262
  • 2012 • 110 Characterization of the surface of Fe-19Mn-18Cr-C-N during heat treatment in a high vacuum - An XPS study
    Zumsande, K. and Weddeling, A. and Hryha, E. and Huth, S. and Nyborg, L. and Weber, S. and Krasokha, N. and Theisen, W.
    Materials Characterization 71 66-76 (2012)
    Nitrogen-containing CrMn austenitic stainless steels offer evident benefits compared to CrNi-based grades. The production of high-quality parts by means of powder metallurgy could be an appropriate alternative to the standard molding process leading to improved properties. The powder metallurgical production of CrMn austenitic steel is challenging on account of the high oxygen affinity of Mn and Cr. Oxides hinder the densification processes and may lower the performance of the sintered part if they remain in the steel after sintering. Thus, in evaluating the sinterability of the steel Fe-19Mn-18Cr-C-N, characterization of the surface is of great interest. In this study, comprehensive investigations by means of X-ray photoelectron spectroscopy and scanning electron microscopy combined with energy dispersive X-ray spectroscopy were performed to characterize the surface during heat treatment in a high vacuum. The results show a shift of oxidation up to 600 °C, meaning transfer of oxygen from the iron oxide layer to Mn-based particulate oxides, followed by progressive reduction and transformation of the Mn oxides into stable Si-containing oxides at elevated temperatures. Mass loss caused by Mn evaporation was observed accompanied by Mn oxide decomposition starting at 700 °C. © 2012 Elsevier Inc. All rights reserved.
    view abstractdoi: 10.1016/j.matchar.2012.06.002
  • 2011 • 109 Gold catalyst initiated growth of GaN nanowires by MOCVD
    Ahl, J.-P. and Behmenburg, H. and Giesen, C. and Regolin, I. and Prost, W. and Tegude, F.J. and Radnoczi, G.Z. and Pécz, B. and Kalisch, H. and Jansen, R.H. and Heuken, M.
    Physica Status Solidi (C) Current Topics in Solid State Physics 8 2315-2317 (2011)
    Our study shows the impact of the process parameters V/III ratio, pressure and temperature on growth and morphology of GaN nanowires (NWs) synthesized by an Au-initiated vapour-liquid-solid mechanism on a sapphire substrate. We confined a temperature window for successful GaN NW growth and show how the variation of reactor pressure changes the NW morphology. Using a very low V/III ratio, NW tapering, which was observed for higher V/III ratios, could be avoided. The optimization of these process parameters led to non-tapered GaN NWs, aligned perpendicular to the substrate. Further evaluation by scanning electron microscopy showed a high density (~3·109/cm2) of hexagonal c-plane GaN NWs having diameters of 60 ± 9 nm. Transmission electron microscopy revealed single-crystalline NWs without threading dislocations but some stacking faults. The use of a very low V/III ratio was found to be important for the successful selective growth and, most interestingly, led to a difference in NW and gold catalyst droplet diameter. For chemical analysis of the NW and its catalyst droplet, electron energy loss spectroscopy was employed confirming gold as the catalyst material. © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/pssc.201000992
  • 2011 • 108 Plasma enhanced-CVD of undoped and fluorine-doped Co3O 4 nanosystems for novel gas sensors
    Barreca, D. and Bekermann, D. and Comini, E. and Devi, A. and Fischer, R.A. and Gasparotto, A. and Gavagnin, M. and MacCato, C. and Sada, C. and Sberveglieri, G. and Tondello, E.
    Sensors and Actuators, B: Chemical 160 79-86 (2011)
    Co3O4-based nanosystems were prepared on polycrystalline Al2O3 by plasma enhanced-chemical vapor deposition (PE-CVD), at temperatures ranging between 200 and 400 °C. The use of two different precursors, Co(dpm)2 (dpm = 2,2,6,6-tetramethyl-3, 5-heptanedionate) and Co(hfa)2·TMEDA (hfa = 1,1,1,5,5,5-hexafluoro-2,4-pentanedionate; TMEDA = N,N,N′,N′- tetramethylethylenediamine) enabled the synthesis of undoped and fluorine-doped Co3O4 specimens, respectively. A thorough characterization of their properties was performed by glancing incidence X-ray diffraction (GIXRD), atomic force microscopy (AFM), field emission-scanning electron microscopy (FE-SEM), secondary ion mass spectrometry (SIMS) and X-ray photoelectron spectroscopy (XPS). For the first time, the gas sensing properties of such PE-CVD nanosystems were investigated in the detection of ethanol and acetone. The results show an appreciable response improvement upon doping and functional performances directly dependent on the fluorine content in the Co3O4 system. © 2011 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.snb.2011.07.016
  • 2011 • 107 MOCVD of ZnO films from bis(ketoiminato)Zn(II) precursors: Structure, morphology and optical properties
    Bekermann, D. and Ludwig, Ar. and Toader, T. and MacCato, C. and Barreca, D. and Gasparotto, A. and Bock, C. and Wieck, A.D. and Kunze, U. and Tondello, E. and Fischer, R.A. and Devi, A.
    Chemical Vapor Deposition 17 155-161 (2011)
    Two closely related bis(ketoiminato) zinc precursors, which are air stable and possess favorable properties for metal-organic (MO)CVD, are successfully employed for the growth of ZnO films on silicon and borosilicate glass substrates at temperatures between 400 and 700 °C. The as-deposited films are investigated by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), nuclear reaction analysis (NRA), as well as by UV-vis absorption spectroscopy and photoluminescence (PL) measurements. The structure, morphology, and composition of the as-grown films show a strong dependence on the substrate temperature. The formation of pure and (001)-oriented wurtzite-type stoichiometric ZnO is observed. PL measurements are performed both at room temperature and 77 K, revealing a defect-free emission of ZnO films. Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/cvde.201006898
  • 2011 • 106 p-Co3O4/n-ZnO, Obtained by PECVD, Analyzed by X-ray Photoelectron Spectroscopy
    Bekermann, D. and Gasparotto, A. and Barreca, D. and Devi, A. and Fischer, R.A.
    Surface Science Spectra 18 36-45 (2011)
    The present work focuses on the X-ray Photoelectron Spectroscopy (XPS) and X-ray Excited Auger Electron Spectroscopy (XE-AES) of a Co3O4/ZnO nanosystem. The composite material was obtained via a two-step Plasma Enhanced-Chemical Vapor Deposition (PECVD) process in Ar/O2 mixtures, consisting in the initial deposition of ZnO and the subsequent growth of Co3O4 onto the pristine matrices. Zn(ketoimi)2 (ketoimi = [CH3O(CH2)3NC(CH3)=C(H)C(CH3)=O]) and Co(dpm)2 (dpm = 2,2,6,6-tetramethyl-3,5-heptanedionate) were used as zinc and cobalt precursors, respectively. In particular, strongly 〈001〉 oriented ZnO was grown at 300 °C, followed by the deposition of Co3O4 at 200 °C, applying a radio-frequency (RF) power of 20 W. Structural, morphological and compositional investigations were performed by Glancing Incidence X-ray Diffraction (GIXRD), Field Emission-Scanning Electron Microscopy (FE-SEM) and Energy Dispersive X-ray Spectroscopy (EDXS). Surface XPS and XE-AES analyses were carried out to study in detail the system O 1s, Zn 2p3/2, Zn 3p and Co 2p core levels, as well as the Zn and Co Auger peaks. The obtained results evidenced the formation of a composite material, in which ZnO and Co3O4 preserved their chemical identity. © 2011 American Vacuum Society.
    view abstractdoi: 10.1116/11.20111003
  • 2011 • 105 Structural and magnetic characterization of self-assembled iron oxide nanoparticle arrays
    Benitez, M.J. and Mishra, D. and Szary, P. and Badini Confalonieri, G.A. and Feyen, M. and Lu, A.H. and Agudo, L. and Eggeler, G. and Petracic, O. and Zabel, H.
    Journal of Physics Condensed Matter 23 (2011)
    We report about a combined structural and magnetometric characterization of self-assembled magnetic nanoparticle arrays. Monodisperse iron oxide nanoparticles with a diameter of 20nm were synthesized by thermal decomposition. The nanoparticle suspension was spin-coated on Si substrates to achieve self-organized arrays of particles and subsequently annealed at various conditions. The samples were characterized by x-ray diffraction, and bright and dark field high resolution transmission electron microscopy. The structural analysis is compared to magnetization measurements obtained by superconducting quantum interference device magnetometry. We can identify either multi-phase FexO/γ-Fe2O3 or multi-phase Fe xO/Fe3O4 nanoparticles. The Fe xO/γ-Fe2O3 system shows a pronounced exchange bias effect which explains the peculiar magnetization data found for this system. © 2011 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0953-8984/23/12/126003
  • 2011 • 104 Wear protection by Fe-B-C hard phases
    Berns, H. and Saltykova, A. and Röttger, A. and Heger, D.
    Steel Research International 82 786-794 (2011)
    Four as-cast iron alloys with (mass%) 1B + 3C, 2B + 2C, 3B + 1C and 4B + 1C were investigated in respect to their microstructure by optical and scanning electron microscopy with EDX and ESBD and by microprobe analysis. The microhardness of eutectic Fe3(C,B) increased with the B/C ratio and raised the resistance to scratching by Flint particles. The low melting range of the castings was used for the powder metallurgical production of a metal matrix composite by liquid phase sintering of admixed hard particles in an Fe-B-C base material. Abrasive wear tests showed that the eutectic carborides in the base material raised the wear resistance even more than the admixed particles. Copyright © 2011 WILEY-VCH Verlag GmbH &amp; Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/srin.201000255
  • 2011 • 103 Prevention of catalyst deactivation in the hydrogenolysis of glycerol by Ga 2O 3-modified copper/zinc oxide catalysts
    Bienholz, A. and Blume, R. and Knop-Gericke, A. and Girgsdies, F. and Behrens, M. and Claus, P.
    Journal of Physical Chemistry C 115 999-1005 (2011)
    Copper/zinc oxide catalysts prepared by coprecipitation were proved to be highly active and selective in the hydrogenolysis of glycerol. However, they suffer from strong deactivation in the course of reaction. Modifying the CuO/ZnO catalyst with Ga 2O 3 extremely enhances the stability of the catalyst as even after four consecutive experiments over a Cu/ZnO/Ga 2O 3 catalyst no deactivation is observed. The catalysts were characterized by temperature-programmed reduction, powder X-ray diffraction, X-ray photoelectron spectroscopy, scanning electron microscopy coupled with energy-dispersive X-ray analysis, and inductively coupled plasma optical emission spectrometry. As the Cu/ZnO/Ga 2O 3 catalyst is stable even under harsh reaction conditions of 220 °C and in the presence of water, a space-time-yield as high as 22.1 g propylene glycol/(g Cu h) can be obtained. © 2010 American Chemical Society.
    view abstractdoi: 10.1021/jp104925k
  • 2011 • 102 Synthesis of active carbon-based catalysts by chemical vapor infiltration for nitrogen oxide conversion
    Busch, M. and Bergmann, U. and Sager, U. and Schmidt, W. and Schmidt, F. and Notthoff, C. and Atakan, B. and Winterer, M.
    Journal of Nanoscience and Nanotechnology 11 7956-7961 (2011)
    Direct reduction of nitrogen oxides is still a challenge. Strong efforts have been made in developing noble and transition metal catalysts on microporous support materials such as active carbons or zeolites. However, the required activation energy and low conversion rates still limit its break-through. Furthermore, infiltration of such microporous matrix materials is commonly performed by wet chemistry routes. Deep infiltration and homogeneous precursor distribution are often challenging due to precursor viscosity or electrostatic shielding and may be inhibited by pore clogging. Gas phase infiltration, as an alternative, can resolve viscosity issues and may contribute to homogeneous infiltration of precursors. In the present work new catalysts based on active carbon substrates were synthesized via chemical vapor infiltration. Iron oxide nano clusters were deposited in the microporous matrix material. Detailed investigation of produced catalysts included nitrogen oxide adsorption, X-ray diffraction, scanning electron microscopy and energy-dispersive X-ray spectroscopy. Catalytic activity was studied in a recycle flow reactor by time-resolved mass spectrometry at a temperature of 423 K. The infiltrated active carbons showed very homogeneous deposition of iron oxide nano clusters in the range of below 12 to 19 nm, depending on the amount of infiltrated precursor. The specific surface area was not excessively reduced, nor was the pore size distribution changed compared to the original substrate. Catalytic nitrogen oxides conversion was detected at temperatures as low as 423 K. Copyright © 2011 American Scientific Publishers. All rights reserved.
    view abstractdoi: 10.1166/jnn.2011.5074
  • 2011 • 101 Deformation and fracture mechanisms in fine- and ultrafine-grained ferrite/martensite dual-phase steels and the effect of aging
    Calcagnotto, M. and Adachi, Y. and Ponge, D. and Raabe, D.
    Acta Materialia 59 658-670 (2011)
    Three ferrite/martensite dual-phase steels varying in the ferrite grain size (12.4, 2.4 and 1.2 μm) but with the same martensite content (∼30 vol.%) were produced by large-strain warm deformation at different deformation temperatures, followed by intercritical annealing. Their mechanical properties were compared, and the response of the ultrafine-grained steel (1.2 μm) to aging at 170 °C was investigated. The deformation and fracture mechanisms were studied based on microstructure observations using scanning electron microscopy and electron backscatter diffraction. Grain refinement leads to an increase in both yield strength and tensile strength, whereas uniform elongation and total elongation are less affected. This can be partly explained by the increase in the initial strain-hardening rate. Moreover, the stress/strain partitioning characteristics between ferrite and martensite change due to grain refinement, leading to enhanced martensite plasticity and better interface cohesion. Grain refinement further promotes ductile fracture mechanisms, which is a result of the improved fracture toughness of martensite. The aging treatment leads to a strong increase in yield strength and improves the uniform and total elongation. These effects are attributed to dislocation locking due to the formation of Cottrell atmospheres and relaxation of internal stresses, as well as to the reduction in the interstitial carbon content in ferrite and tempering effects in martensite. © 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2010.10.002
  • 2011 • 100 Microstructure evolution and mechanical properties of an intermetallic Ti-43.5Al-4Nb-1Mo-0.1B alloy after ageing below the eutectoid temperature
    Cha, L. and Clemens, H. and Dehm, G.
    International Journal of Materials Research 102 703-708 (2011)
    Intermetallic γ-TiAl based alloys with a chemical composition of Ti-(42-45)Al-(3-5)Nb-(0.1-2)Mo-(0.1-0.2)B (in atom percent) are termed TNM ™ alloys. They exhibit several distinct characteristics, including excellent hot-workability and balanced mechanical properties. In this study, the relationship between microstructure and mechanical behavior in a Ti-43.5Al-4Nb-1Mo-0.1B alloy after two different heat treatments was investigated. One of the analyzed microstructures consisted of lamellar γ-TiAl/α2-Ti3Al colonies with a small volume fraction of globular γ-TiAl and β0-TiAl grains at their grain boundaries, whereas the second microstructure basically exhibited the same arrangement of the microstructural constituents, but a fraction of the lamellar colonies was altered by a cellular reaction. The prevailing microstructures have been analyzed by means of scanning electron microscopy and transmission electron microscopy. Macro-and micro-hardness measurements as well as room temperature tensile tests have revealed that the sample with both cellular and lamellar features show lower yield stress and hardness than the ones exhibiting undisturbed lamellar microstructures. The strength and hardness properties are primarily connected to the lamellar spacing within the colonies, where strength increases with decreasing lamellar spacing. The appearance of a cellular reaction leads to a refinement of the lamellar colonies which in turn influences positively the plastic fracture strain at room temperature. © Hanser Verlag GmbH & Co. KG.
    view abstractdoi: 10.3139/146.110526
  • 2011 • 99 Shape transformation mechanism of silver nanorods in aqueous solution
    Damm, C. and Segets, D. and Yang, G. and Vieweg, B.F. and Spiecker, E. and Peukert, W.
    Small 7 147-156 (2011)
    The spontaneous shape transformation of silver nanorods with an initial length of several hundred nanometers towards spherical particle shapes in aqueous solution is investigated by means of scanning electron microscopy, UV-vis absorption spectroscopy, anodic stripping voltammetry, and high-resolution transmission electron microscopy (HRTEM). The consolidation of the results reveals an increase in the particle number density with time. Moreover, HRTEM image analysis along the cross section of the rods evidences the presence of fivefold twinning defects which extend along the whole rod length. According to the analytical model of Monk et al. this kind of rod structure is only thermodynamically stable if the rod length is below a critical value at a given diameter. The rods investigated in the present work do not fulfill the stability criterion as they exceed the critical length. Thus, the rods decay into smaller "nanobuns" and defective as well as defect-free spheres. A mechanism based on findings from the literature, HRTEM image analysis of former rods, transition states, and the final particle structures is proposed. The defects along the surface are seen as starting points for the dissolution of material, which is reintegrated into the solid phase by homogeneous as well as heterogeneous nucleation and growth. The decay process of silver nanorods in aqueous suspension is investigated. During ageing the aspect ratio decreases with time whereas the absolute particle number increases. Defects play a decisive role in rod decay and underline how crystal structure influences particle shape. Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/smll.201001600
  • 2011 • 98 Scanning electrochemical microscopy for investigation of multicomponent bioelectrocatalytic films
    Dobrzeniecka, A. and Zeradjanin, A. and Masa, J. and Stroka, J. and Goral, M. and Schuhmann, W. and Kulesza, P.J.
    ECS Transactions 35 33-44 (2011)
    Scanning electrochemical microscopy in the redox competition mode (RC-SECM) is proposed as an useful technique for local investigation of the electrocatalytic activity of different catalysts towards the oxygen reduction reaction (ORR) with simultaneous detection of the produced undesirable intermediate hydrogen peroxide. We have used cobalt porphyrin dispersed in a multiwalled carbon nanotubes (MWCNTs) matrix as a model electrocatalyst for the predominant two-electron reduction of oxygen to hydrogen peroxide. Furthermore, Prussian Blue or horseradish peroxidase were used as catalysts for the further reduction of hydrogen peroxide to water. The properties of each component of the film were examined along with their potential interactions with the other components. As a result an efficient electrocatalyst for oxygen reduction at physiological pH could be obtained. ©The Electrochemical Society.
    view abstractdoi: 10.1149/1.3646486
  • 2011 • 97 Synthesis and magnetic properties of cobalt ferrite nanoparticles
    Etier, M.F. and Shvartsman, V.V. and Stromberg, F. and Landers, J. and Wende, H. and Lupascu, D.C.
    Materials Research Society Symposium Proceedings 1398 12-19 (2011)
    Nanopowders of cobalt iron oxide (CoFe2O4) were successfully fabricated by the co-precipitation method followed by a