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.

Below, you can either scroll through the complete list of our annually published material, or search for a specific author or term via the free text search to get to know our research strengths. You can also review the publication record of every Materials Chain member via his or her personal member’s page.

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  • 2024 • 422 Effect of stacking fault energy on the thickness and density of annealing twins in recrystallized FCC medium and high-entropy alloys
    Schneider, Mike and Couzinié, Jean-Philippe and Shalabi, Amin and Ibrahimkhel, Farhad and Ferrari, Alberto and Körmann, Fritz and Laplanche, Guillaume
    Scripta Materialia 240 (2024)
    This work aims to predict the microstructure of recrystallized medium and high-entropy alloys (MEAs and HEAs) with a face-centered cubic structure, in particular the density of annealing twins and their thickness. Eight MEAs and five HEAs from the Cr-Mn-Fe-Co-Ni system are considered, which have been cast, homogenized, cold-worked and recrystallized to obtain different grain sizes. This work thus provides a database that could be used for data mining to take twin boundary engineering for alloy development to the next level. Since the stacking fault energy is known to strongly affect recrystallized microstructures, the latter was determined at 293 K using the weak beam dark-field technique and compared with ab initio simulations, which additionally allowed to calculate its temperature dependence. Finally, we show that all these data can be rationalized based on theories and empirical relationships that were proposed for pure metals and binary Cu-based alloys. © 2023
    view abstractdoi: 10.1016/j.scriptamat.2023.115844
  • 2023 • 421 Cooperative deformation mechanisms in a fatigued CoCrNi multi-principal element alloy: A case of low stacking fault energy
    Lu, Kaiju and Chauhan, Ankur and Litvinov, Dimitri and Schneider, Mike and Laplanche, Guillaume and Aktaa, Jarir
    Journal of the Mechanics and Physics of Solids 180 (2023)
    Multi-principal element alloys (MPEAs) of low stacking fault energies (SFEs) often exhibit outstanding damage tolerance. In contrast to the well-understood monotonic deformation behaviors, fatigue deformation mechanisms of the low-SFE MPEAs remain fundamental questions. Using TEM, this work demonstrated two main dislocation configurations cooperating in an exemplary low-SFE CoCrNi MPEA, namely slip bands and veins/walls/cells, which are usually formed in materials with low- and high-SFEs, respectively. Under low strain amplitude of 0.3%, the deformation features are found to include primary and double slip bands (dominating in ≈71% and 29% grains, respectively). Upon increasing strain amplitude, apart from the slip bands, dislocation substructures formed (i.e., veins/walls/cells present in ≈ 33% and 60% grains under strain amplitude of 0.5% and 0.7%, respectively) due to cross slip, despite the alloy's low-SFE. Moreover, the slip bands and walls/cells are found to serve more likely as geometrically necessary dislocations and statistically stored dislocations, respectively. Besides, the constraints from neighboring grains and Copley-Kear effect (rather than grain orientation) were suggested to play more important role in determining these dislocation patterns formation. Lastly, this work evidenced unexpected partial dislocations and stacking faults (in addition to usually observed screw dislocations) shuttling between the walls/cells by planar slip. This predominant planar slip behavior explains well the exceptional damage tolerance of low-SFE MPEAs. These insights can also advance the understanding of fatigue deformation of conventional alloys with low-SFEs. © 2023 Elsevier Ltd
    view abstractdoi: 10.1016/j.jmps.2023.105419
  • 2023 • 420 Effect of grain size on critical twinning stress and work hardening behavior in the equiatomic CrMnFeCoNi high-entropy alloy
    Wagner, Christian and Laplanche, Guillaume
    International Journal of Plasticity 166 (2023)
    While the impact of grain boundary strengthening on dislocation slip is particularly effective in the equiatomic CrMnFeCoNi high-entropy alloy (HEA), its effect on deformation twinning remains unclear. To better understand how a grain size reduction affects the onset of deformation twinning and the work hardening behavior of the CrMnFeCoNi HEA, chemically homogeneous, nearly untextured, and single-phase face-centered cubic alloys with different grain sizes were investigated. Tensile tests were performed at 293 and 77 K and interrupted at different strains followed by systematic transmission electron microscopy observations. In all cases, deformation twinning occurs above a critical stress that is independent of temperature. This uniaxial twinning stress decreases from ∼785 to ∼615 MPa when the grain size increases from 6 to 242 µm, respectively, following the Hall-Petch equation. The resistance of the grain boundaries against slip and twinning is found to be nearly identical (Hall-Petch slope: ∼500 MPa·µm1/2) but the twinning stress extrapolated to infinite grain size (592 ± 30 MPa) is larger than the uniaxial friction stress against dislocation glide at 293 and 77 K (130 and 320 MPa, respectively). Deformation twinning at 77 K is found to sustain a high work hardening rate when it is triggered in a plastic regime dominated by planar glide of dislocations. In contrast, it does not significantly contribute to the work hardening rate at 293 K when dislocation cells have already formed and the dislocation mean free path is smaller than the mean twin spacing. © 2023 The Author(s)
    view abstractdoi: 10.1016/j.ijplas.2023.103651
  • 2022 • 419 Determining the sintering kinetics of Fe and FexOy-Nanoparticles in a well-defined model flow reactor
    Rosenberger, T. and Skenderović, I. and Sellmann, J. and Wollny, P. and Levish, A. and Wlokas, I. and Kempf, A. and Winterer, M. and Kruis, F.E.
    Aerosol Science and Technology 56 833-846 (2022)
    A model flow reactor provides a narrow particle temperature-residence time distribution with well-defined conditions and is mandatory to measure changes of the particle structure precisely. The experimental data of iron and iron oxide agglomerates are used to determine the sintering kinetics considering the temperature-time history of the particles. Thousand particle trajectories are tracked in a validated CFD model at three different furnace temperatures each. Strongly agglomerated particles with a small primary particle size (∼4 nm) are synthesized by spark discharge and are size-selected (25–250 nm) before sintering. The structure development is measured simultaneously with different online instrumentations and the structure calculated by means of structure models. A simple sintering model, based on the reduction of surface energy, is numerically quantified with the experimental results. The surface of the particles is strongly dependent on the primary particle size and the agglomerate structure. The chemical phase is analyzed using the offline techniques XANES, XRD, and EELS. It is observed that the addition of hydrogen led to a reduction of iron oxide to iron nanoparticles and to changes of the sintering kinetics. The sintering exponent (Formula presented.) = 1 was found to be optimal. For Fe, an activation energy (Formula presented.) of 59.15 kJ/mol and a pre-exponential factor (Formula presented.) of 1.57 104 s/m were found, for Fe3O4 an activation energy (Formula presented.) of 55.22 kJ/mol and a pre-exponential factor (Formula presented.) of 2.54 104 s/m. Copyright © 2022 American Association for Aerosol Research. © 2022 American Association for Aerosol Research.
    view abstractdoi: 10.1080/02786826.2022.2089011
  • 2022 • 418 In Situ Monitoring of Palladium-Catalyzed Chemical Reactions by Nanogap-Enhanced Raman Scattering using Single Pd Cube Dimers
    Wang, D. and Shi, F. and Jose, J. and Hu, Y. and Zhang, C. and Zhu, A. and Grzeschik, R. and Schlücker, S. and Xie, W.
    Journal of the American Chemical Society 144 5003-5009 (2022)
    The central dilemma in label-free in situ surface-enhanced Raman scattering (SERS) for monitoring of heterogeneously catalyzed reactions is the need of plasmonically active nanostructures for signal enhancement. Here, we show that the assembly of catalytically active transition-metal nanoparticles into dimers boosts their intrinsically insufficient plasmonic activity at the monomer level by several orders of magnitude, thereby enabling the in situ SERS monitoring of various important heterogeneously catalyzed reactions at the single-dimer level. Specifically, we demonstrate that Pd nanocubes (NCs), which alone are not sufficiently plasmonically active as monomers, can act as a monometallic yet bifunctional platform with both catalytic and satisfactory plasmonic activity via controlled assembly into single dimers with an ∼1 nm gap. Computer simulations reveal that the highest enhancement factors (EFs) occur at the corners of the gap, which has important implications for the SERS-based detection of catalytic conversions: it is sufficient for molecules to come in contact with the "hot spot corners", and it is not required that they diffuse deeply into the gap. For the widely employed Pd-catalyzed Suzuki-Miyaura cross-coupling reaction, we demonstrate that such Pd NC dimers can be employed for in situ kinetic SERS monitoring, using a whole series of aryl halides as educts. Our generic approach based on the controlled assembly into dimers can easily be extended to other transition-metal nanostructures. © 2022 American Chemical Society.
    view abstractdoi: 10.1021/jacs.1c13240
  • 2021 • 417 Biodegradable supramolecular micellesviahost-guest interaction of cyclodextrin-terminated polypeptides and adamantane-terminated polycaprolactones
    Pottanam Chali, S. and Azhdari, S. and Galstyan, A. and Gröschel, A.H. and Ravoo, B.J.
    Chemical Communications 57 9446-9449 (2021)
    Biodegradable supramolecular micelles were prepared exploiting the host-guest interaction of cyclodextrin and adamantane. Cyclodextrin-initiated polypeptides acted as the hydrophilic corona, whereas adamantane-terminated polycaprolactones served as the hydrophobic core. © The Royal Society of Chemistry 2021.
    view abstractdoi: 10.1039/d1cc03372g
  • 2021 • 416 In situ correlation between metastable phase-transformation mechanism and kinetics in a metallic glass
    Orava, J. and Balachandran, S. and Han, X. and Shuleshova, O. and Nurouzi, E. and Soldatov, I. and Oswald, S. and Gutowski, O. and Ivashko, O. and Dippel, A.-C. and Zimmermann, M. and Ivanov, Y.P. and Greer, A.L. and Raabe, D. and...
    Nature Communications 12 (2021)
    A combination of complementary high-energy X-ray diffraction, containerless solidification during electromagnetic levitation and transmission electron microscopy is used to map in situ the phase evolution in a prototype Cu-Zr-Al glass during flash-annealing imposed at a rate ranging from 102 to 103 K s−1 and during cooling from the liquid state. Such a combination of experimental techniques provides hitherto inaccessible insight into the phase-transformation mechanism and its kinetics with high temporal resolution over the entire temperature range of the existence of the supercooled liquid. On flash-annealing, most of the formed phases represent transient (metastable) states – they crystallographically conform to their equilibrium phases but the compositions, revealed by atom probe tomography, are different. It is only the B2 CuZr phase which is represented by its equilibrium composition, and its growth is facilitated by a kinetic mechanism of Al partitioning; Al-rich precipitates of less than 10 nm in a diameter are revealed. In this work, the kinetic and chemical conditions of the high propensity of the glass for the B2 phase formation are formulated, and the multi-technique approach can be applied to map phase transformations in other metallic-glass-forming systems. © 2021, The Author(s).
    view abstractdoi: 10.1038/s41467-021-23028-9
  • 2021 • 415 Phenotypical and myopathological consequences of compound heterozygous missense and nonsense variants in slc18a3
    Marina, A.D. and Arlt, A. and Schara-Schmidt, U. and Depienne, C. and Gangfuß, A. and Kölbel, H. and Sickmann, A. and Freier, E. and Kohlschmidt, N. and Hentschel, A. and Weis, J. and Czech, A. and Grüneboom, A. and Roos, A.
    Cells 10 (2021)
    Background: Presynaptic forms of congenital myasthenic syndromes (CMS) due to pathogenic variants in SLC18A3 impairing the synthesis and recycling of acetylcholine (ACh) have recently been described. SLC18A3 encodes the vesicular ACh transporter (VAChT), modulating the active transport of ACh at the neuromuscular junction, and homozygous loss of VAChT leads to lethality. Methods: Exome sequencing (ES) was carried out to identify the molecular genetic cause of the disease in a 5-year-old male patient and histological, immunofluorescence as well as electron-and CARS-microscopic studies were performed to delineate the muscle pathology, which has so far only been studied in VAChT-deficient animal models. Results: ES unraveled compound heterozygous missense and nonsense variants (c.315G>A, p.Trp105* and c.1192G>C, p.Asp398His) in SLC18A3. Comparison with already-published cases suggests a more severe phenotype including impaired motor and cognitive development, possibly related to a more severe effect of the nonsense variant. Therapy with pyridostigmine was only partially effective while 3,4 diaminopyridine showed no effect. Microscopic investigation of the muscle biopsy revealed reduced fibre size and a significant accumulation of lipid droplets. Conclusions: We suggest that nonsense variants have a more detrimental impact on the clinical manifestation of SLC18A3-associated CMS. The impact of pathogenic SLC18A3 variants on muscle fibre integrity beyond the effect of denervation is suggested by the build-up of lipid aggregates. This in turn implicates the importance of proper VAChT-mediated synthesis and recycling of ACh for lipid homeostasis in muscle cells. This hypothesis is further supported by the pathological observations obtained in previously published VAChT-animal models. © 2021 by the authors. Licensee MDPI, Basel, Switzerland.
    view abstractdoi: 10.3390/cells10123481
  • 2021 • 414 The molecular tweezer clr01 inhibits antibody-resistant cell-to-cell spread of human cytomegalovirus
    Brenner, S. and Braun, B. and Read, C. and Weil, T. and Walther, P. and Schrader, T. and Münch, J. and von Einem, J.
    Viruses 13 (2021)
    Human cytomegalovirus (HCMV) uses two major ways for virus dissemination: infection by cell-free virus and direct cell-to-cell spread. Neutralizing antibodies can efficiently inhibit infection by cell-free virus but mostly fail to prevent cell-to-cell transmission. Here, we show that the ‘molecular tweezer’ CLR01, a broad-spectrum antiviral agent, is not only highly active against infection with cell-free virus but most remarkably inhibits antibody-resistant direct cell-to-cell spread of HCMV. The inhibition of cell-to-cell spread by CLR01 was not limited to HCMV but was also shown for the alphaherpesviruses herpes simplex viruses 1 and 2 (HSV-1,-2). CLR01 is a rapid acting small molecule that inhibits HCMV entry at the attachment and penetration steps. Electron microscopy of extracellular virus particles indicated damage of the viral envelope by CLR01, which likely impairs the infectivity of virus particles. The rapid inactivation of viral particles by CLR01, the viral envelope as the main target, and the inhibition of virus entry at different stages are presumably the key to inhibition of cell-free virus infection and cell-to-cell spread by CLR01. Importance: While cell-free spread enables the human cytomegalovirus (HCMV) and other herpesviruses to transmit between hosts, direct cell-to-cell spread is thought to be more relevant for in vivo dissemination within infected tissues. Cell-to-cell spread is resistant to neutralizing antibodies, thus contributing to the maintenance of virus infection and virus dissemination in the presence of an intact immune system. Therefore, it would be therapeutically interesting to target this mode of spread in order to treat severe HCMV infections and to prevent dissemination of virus within the infected host. The molecular tweezer CLR01 exhibits broad-spectrum antiviral activity against a number of enveloped viruses and efficiently blocks antibody-resistant cell-to-cell spread of HCMV, thus representing a novel class of small molecules with promising antiviral activity. © 2021 by the authors. Licensee MDPI, Basel, Switzerland.
    view abstractdoi: 10.3390/v13091685
  • 2021 • 413 Ultra-Shallow All-Epitaxial Aluminum Gate GaAs/AlxGa1−xAs Transistors with High Electron Mobility
    Ashlea Alava, Y. and Wang, D.Q. and Chen, C. and Ritchie, D.A. and Ludwig, A. and Ritzmann, J. and Wieck, A.D. and Klochan, O. and Hamilton, A.R.
    Advanced Functional Materials (2021)
    The electron mobility in shallow GaAs/AlxGa1−xAs heterostructures is strongly suppressed by charge wafer surface, which arises from native surface oxide layers formed when the wafer is removed from the crystal growth system. Here an in situ epitaxial aluminum gate, grown as part of the wafer, is used to eliminate surface charge scattering. Transmission electron microscope characterization shows that the in situ epitaxial aluminum is crystalline, and the wafer surface is free of native oxide. The influence of Al thickness and the use of different semiconductor wetting layers at the semiconductor-aluminum interface are examined and correlated with electron mobility. The electron mobility is found to strongly depend on aluminum thickness. For 8 nm thick aluminum, the electron mobility is also influenced by the wetting layer, with aluminum grown on GaAs producing higher mobility compared to AlAs or Al0.33Ga0.67As wetting layers. The suppression of surface charge scattering in these all-epitaxial devices allows for high mobilities across a wide density range despite the shallow conduction channel (35 nm below the gate). These measurements also provide a uniquely sensitive method of determining the electrical quality of the semiconductor–metal interface, relevant to the formation of hybrid semiconductor–superconductor devices. © 2021 Wiley-VCH GmbH
    view abstractdoi: 10.1002/adfm.202104213
  • 2021 • 412 Xenotropic and polytropic retrovirus receptor 1 regulates procoagulant platelet polyphosphate
    Mailer, R.K. and Allende, M. and Heestermans, M. and Schweizer, M. and Deppermann, C. and Frye, M. and Pula, G. and Odeberg, J. and Gelderblom, M. and Rose-John, S. and Sickmann, A. and Blankenberg, S. and Huber, T.B. and Kubisch,...
    Blood 137 1392-1405 (2021)
    Polyphosphate is a procoagulant inorganic polymer of linear-linked orthophosphate residues. Multiple investigations have established the importance of platelet polyphosphate in blood coagulation; however, the mechanistic details of polyphosphate homeostasis in mammalian species remain largely undefined. In this study, xenotropic and polytropic retrovirus receptor 1 (XPR1) regulated polyphosphate in platelets and was implicated in thrombosis in vivo. We used bioinformatic analyses of omics data to identify XPR1 as a major phosphate transporter in platelets. XPR1 messenger RNA and protein expression inversely correlated with intracellular polyphosphate content and release. Pharmacological interference with XPR1 activity increased polyphosphate stores, led to enhanced platelet-driven coagulation, and amplified thrombus formation under flow via the polyphosphate/factor XII pathway. Conditional gene deletion of Xpr1 in platelets resulted in polyphosphate accumulation, accelerated arterial thrombosis, and augmented activated platelet-driven pulmonary embolism without increasing bleeding in mice. These data identify platelet XPR1 as an integral regulator of platelet polyphosphate metabolism and reveal a fundamental role for phosphate homeostasis in thrombosis. Key Points: • Xenotropic and polytropic retrovirus receptor 1 (XPR1) is a major phosphate exporter in platelets. • Inhibiting XPR1 in platelets increases procoagulant polyphosphate levels and augments arterial and venous thrombosis in mice. © 2021 American Society of Hematology
    view abstractdoi: 10.1182/blood.2019004617
  • 2020 • 411 Coactosin-like 1 integrates signaling critical for shear-dependent thrombus formation in mouse platelets
    Scheller, I. and Stritt, S. and Beck, S. and Peng, B. and Pleines, I. and Heinze, K.G. and Braun, A. and Otto, O. and Ahrends, R. and Sickmann, A. and Bender, M. and Nieswandt, B.
    Haematologica 105 1667-1676 (2020)
    Platelet aggregate formation is a multistep process involving receptormediated, as well as biomechanical, signaling cascades, which are highly dependent on actin dynamics. We have previously shown that actin depolymerizing factor (ADF)/n-cofilin and Twinfilin 2a, members of the ADF homology (ADF-H) protein family, have distinct roles in platelet formation and function. Coactosin-like 1 (Cotl1) is another ADF-H protein that binds actin and was also shown to enhance biosynthesis of pro-inflammatory leukotrienes (LT) in granulocytes. Here, we generated mice lacking Cotl1 in the megakaryocyte lineage (Cotl1-/-) to investigate its role in platelet production and function. Absence of Cotl1 had no impact on platelet counts, platelet activation or cytoskeletal reorganization under static conditions in vitro. In contrast, Cotl1 deficiency markedly affected platelet aggregate formation on collagen and adhesion to immobilized von Willebrand factor at high shear rates in vitro, pointing to an impaired function of the platelet mechanoreceptor glycoprotein (GP) Ib. Furthermore, Cotl1-/- platelets exhibited increased deformability at high shear rates, indicating that the GPIb defect may be linked to altered biomechanical properties of the deficient cells. In addition, we found that Cotl1 deficiency markedly affected platelet LT biosynthesis. Strikingly, exogenous LT addition restored defective aggregate formation of Cotl1-/- platelets at high shear in vitro, indicating a critical role of platelet-derived LT in thrombus formation. In vivo, Cotl1 deficiency translated into prolonged tail bleeding times and protection from occlusive arterial thrombus formation. Together, our results show that Cotl1 in platelets is an integrator of biomechanical and LT signaling in hemostasis and thrombosis. © 2020 Ferrata Storti Foundation.
    view abstractdoi: 10.3324/haematol.2019.225516
  • 2020 • 410 Correlative chemical and structural investigations of accelerated phase evolution in a nanocrystalline high entropy alloy
    Li, Y.J. and Kostka, A. and Savan, A. and Ludwig, Al.
    Scripta Materialia 183 122-126 (2020)
    Based on our recently-developed combinatorial processing platforms for accelerated investigations of phase evolution in multinary alloys, a novel correlative atom probe tomography and transmission electron microscopy approach is proposed to study phase stability in a nanocrystalline CrMnFeCoNi alloy. We observed that the material can decompose at 250 °C for 5 h or 300 °C for 1 h, having the same decomposed products as in its coarse-grained counterpart after annealing at 500 °C for 500 days. A low apparent activation energy for the diffusion of Ni in the nanocrystalline alloy is derived and explains the fast kinetics of phase decomposition in nanocrystalline alloys. © 2020 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.scriptamat.2020.03.016
  • 2020 • 409 Efficient OER Catalyst with Low Ir Volume Density Obtained by Homogeneous Deposition of Iridium Oxide Nanoparticles on Macroporous Antimony-Doped Tin Oxide Support
    Böhm, D. and Beetz, M. and Schuster, M. and Peters, K. and Hufnagel, A.G. and Döblinger, M. and Böller, B. and Bein, T. and Fattakhova-Rohlfing, D.
    Advanced Functional Materials 30 (2020)
    A multistep synthesis procedure for the homogeneous coating of a complex porous conductive oxide with small Ir nanoparticles is introduced to obtain a highly active electrocatalyst for water oxidation. At first, inverse opal macroporous Sb doped SnO2 (ATO) microparticles with defined pore size, composition, and open-porous morphology are synthesized that reach a conductivity of ≈3.6 S cm−1 and are further used as catalyst support. ATO-supported iridium catalysts with a controlled amount of active material are prepared by solvothermal reduction of an IrOx colloid in the presence of the porous ATO particles, whereby homogeneous coating of the complete outer and inner surface of the particles with nanodispersed metallic Ir is achieved. Thermal oxidation leads to the formation of ATO-supported IrO2 nanoparticles with a void volume fraction of ≈89% calculated for catalyst thin films based on scanning transmission electron microscope tomography data and microparticle size distribution. A remarkably low Ir bulk density of ≈0.08 g cm−3 for this supported oxide catalyst architecture with 25 wt% Ir is determined. This highly efficient oxygen evolution reaction catalyst reaches a current density of 63 A gIr −1 at an overpotential of 300 mV versus reversible hydrogen electrode, significantly exceeding a commercial TiO2-supported IrO2 reference catalyst under the same measurement conditions. © 2019 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/adfm.201906670
  • 2020 • 408 Identifying the nature of the active sites in methanol synthesis over Cu/ZnO/Al2O3 catalysts
    Laudenschleger, D. and Ruland, H. and Muhler, M.
    Nature Communications 11 (2020)
    The heterogeneously catalysed reaction of hydrogen with carbon monoxide and carbon dioxide (syngas) to methanol is nearly 100 years old, and the standard methanol catalyst Cu/ZnO/Al2O3 has been applied for more than 50 years. Still, the nature of the Zn species on the metallic Cu0 particles (interface sites) is heavily debated. Here, we show that these Zn species are not metallic, but have a positively charged nature under industrial methanol synthesis conditions. Our kinetic results are based on a self-built high-pressure pulse unit, which allows us to inject selective reversible poisons into the syngas feed passing through a fixed-bed reactor containing an industrial Cu/ZnO/Al2O3 catalyst under high-pressure conditions. This method allows us to perform surface-sensitive operando investigations as a function of the reaction conditions, demonstrating that the rate of methanol formation is only decreased in CO2-containing syngas mixtures when pulsing NH3 or methylamines as basic probe molecules. © 2020, The Author(s).
    view abstractdoi: 10.1038/s41467-020-17631-5
  • 2020 • 407 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 • 406 Synthesis of graphene-related carbon nanoparticles from a liquid isopropanol precursor by a one-step atmospheric plasma process
    Bodnar, W. and Schiorlin, M. and Frank, A. and Schulz, T. and Wöhrl, N. and Miron, C. and Scheu, C. and Kolb, J.F. and Kruth, A.
    Applied Surface Science 514 (2020)
    This study presents a cost-efficient single-step-method to synthesize nanographite from isopropanol by bipolar pulsed electric discharges. The influence of pulse width within the nanosecond range, repetition frequency within the kilohertz range and processing time on the product was systematically investigated by Raman spectroscopy, high-resolution transmission electron microscopy and gas chromatography - mass spectrometry. It was found that long pulses in the microsecond range promote the creation of amorphous and oxidic carbon structures. Although, hydrocarbon cracking and subsequent graphitization do occur, these process conditions are not suitable to drive intermediate reduction processes. In contrast, applying short pulses in the nanosecond regime ensures fast reduction processes and formation of graphene-related nanostructures. The number of observed nanographite layers lies in the range of 3–13 with an average interlayer spacing of 3.4(0.3) Å and an average distance between defects of 11.5(6.0) nm meaning that the produced nanographite is in the area of small defect density. Furthermore, no significant influence of process times on the product properties over a period up to 15 min was observed, indicating good process homogeneity. © 2020 Elsevier B.V.
    view abstractdoi: 10.1016/j.apsusc.2020.145926
  • 2020 • 405 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 • 404 Temperature-controlled magnetic nanoparticles hyperthermia inhibits primary tumor growth and metastases dissemination
    Garanina, A.S. and Naumenko, V.A. and Nikitin, A.A. and Myrovali, E. and Petukhova, A.Y. and Klimyuk, S.V. and Nalench, Y.A. and Ilyasov, A.R. and Vodopyanov, S.S. and Erofeev, A.S. and Gorelkin, P.V. and Angelakeris, M. and Savch...
    Nanomedicine: Nanotechnology, Biology, and Medicine 25 (2020)
    Magnetic hyperthermia (MHT) is a promising approach for cancer therapy. However, a systematic MHT characterization as function of temperature on the therapeutic efficiency is barely analyzed. Here, we first perform comparative temperature-dependent analysis of the cobalt ferrite nanoparticles-mediated MHT effectiveness in two murine tumors models – breast (4T1) and colon (CT26) cancer in vitro and in vivo. The overall MHT killing capacity in vitro increased with the temperature and CT26 cells were more sensitive than 4T1 when heated to 43 °C. Well in line with the in vitro data, such heating cured non-metastatic CT26 tumors in vivo, while only inhibiting metastatic 4T1 tumor growth without improving the overall survival. High-temperature MHT (>47 °C) resulted in complete 4T1 primary tumor clearance, 25–40% long-term survival rates, and, importantly, more effective prevention of metastasis comparing to surgical extraction. Thus, the specific MHT temperature must be defined for each tumor individually to ensure a successful antitumor therapy. © 2020 Elsevier Inc.
    view abstractdoi: 10.1016/j.nano.2020.102171
  • 2020 • 403 Time-dependent plasticity in silicon microbeams mediated by dislocation nucleation
    Elhebeary, M. and Harzer, T. and Dehm, G. and Saif, M.T.A.
    Proceedings of the National Academy of Sciences of the United States of America 117 16864-16871 (2020)
    Understanding deformation mechanisms in silicon is critical for reliable design of miniaturized devices operating at high temperatures. Bulk silicon is brittle, but it becomes ductile at about 540 °C. It creeps (deforms plastically with time) at high temperatures (∼800 °C). However, the effect of small size on ductility and creep of silicon remains elusive. Here, we report that silicon at small scales may deform plastically with time at lower temperatures (400 °C) above a threshold stress. We achieve this stress by bending single-crystal silicon microbeams using an in situ thermomechanical testing stage. Small size, together with bending, localize high stress near the surface of the beam close to the anchor. This localization offers flaw tolerance, allowing ductility to win over fracture. Our combined scanning, transmission electron microscopy, and atomic force microscopy analysis reveals that as the threshold stress is approached, multiple dislocation nucleation sites appear simultaneously from the high-stressed surface of the beam with a uniform spacing of about 200 nm between them. Dislocations then emanate from these sites with time, lowering the stress while bending the beam plastically. This process continues until the effective shear stress drops and dislocation activities stop. A simple mechanistic model is presented to relate dislocation nucleation with plasticity in silicon. © 2020 National Academy of Sciences. All rights reserved.
    view abstractdoi: 10.1073/pnas.2002681117
  • 2020 • 402 Ultrasmall gold nanoparticles (2 nm) can penetrate and enter cell nuclei in an in vitro 3D brain spheroid model
    Sokolova, V. and Nzou, G. and van der Meer, S.B. and Ruks, T. and Heggen, M. and Loza, K. and Hagemann, N. and Murke, F. and Giebel, B. and Hermann, D.M. and Atala, A.J. and Epple, M.
    Acta Biomaterialia 111 349-362 (2020)
    The neurovascular unit (NVU) is a complex functional and anatomical structure composed of endothelial cells and their blood-brain barrier (BBB) forming tight junctions. It represents an efficient barrier for molecules and drugs. However, it also prevents a targeted transport for the treatment of cerebral diseases. The uptake of ultrasmall nanoparticles as potential drug delivery agents was studied in a three-dimensional co-culture cell model (3D spheroid) composed of primary human cells (astrocytes, pericytes, endothelial cells). Multicellular 3D spheroids show reproducible NVU features and functions. The spheroid core is composed mainly of astrocytes, covered with pericytes, while brain endothelial cells form the surface layer, establishing the NVU that regulates the transport of molecules. After 120 h cultivation, the cells self-assemble into a 350 µm spheroid as shown by confocal laser scanning microscopy. The passage of different types of fluorescent ultrasmall gold nanoparticles (core diameter 2 nm) both into the spheroid and into three constituting cell types was studied by confocal laser scanning microscopy. Three kinds of covalently fluorophore-conjugated gold nanoparticles were used: One with fluorescein (FAM), one with Cy3, and one with the peptide CGGpTPAAK-5,6-FAM-NH2. In 2D cell co-culture experiments, it was found that all three kinds of nanoparticles readily entered all three cell types. FAM- and Cy3-labelled nanoparticles were able to enter the cell nucleus as well. The three dissolved dyes alone were not taken up by any cell type. A similar situation evolved with 3D spheroids: The three kinds of nanoparticles entered the spheroid, but the dissolved dyes did not. The presence of a functional blood-brain barrier was demonstrated by adding histamine to the spheroids. In that case, the blood-brain barrier opened, and dissolved dyes like a FITC-labelled antibody and FITC alone entered the spheroid. In summary, our results qualify ultrasmall gold nanoparticles as suitable carriers for imaging or drug delivery into brain cells (sometimes including the nucleus), brain cell spheroids, and probably also into the brain. Statement of significance: 3D brain spheroid model and its permeability by ultrasmall gold nanoparticles. We demonstrate that ultrasmall gold nanoparticles can easily penetrate the constituting cells and sometimes even enter the cell nucleus. They can also enter the interior of the blood-brain barrier model. In contrast, small molecules like fluorescing dyes are not able to do that. Thus, ultrasmall gold nanoparticles can serve as carriers of drugs or for imaging inside the brain. © 2020
    view abstractdoi: 10.1016/j.actbio.2020.04.023
  • 2019 • 401 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 • 400 Advances in in situ nanomechanical testing
    Minor, A.M. and Dehm, G.
    MRS Bulletin 44 438-442 (2019)
    In situ nanomechanical testing provides critical insight into the fundamental processes that lead to deformation phenomena in materials. Often, in situ tests are performed in relevant conditions such as high or low temperatures, tribological contact, gas environments, or under radiation exposure. Modern diffraction and imaging methods of materials under load provide high spatial resolution and enable extraction of quantitative mechanical data from local microstructure components or nano-sized objects. The articles in this issue cover recent advances in different types of in situ nanomechanical testing methods, spanning from dedicated nanomechanical testing platforms and microelectromechanical systems devices to deformation analyses via in situ diffraction and imaging methods. This includes scanning electron microscopy, advanced scanning transmission electron microscopy, electron diffraction, x-ray diffraction, and synchrotron techniques. Emerging areas such as in situ tribology enable novel insights into the origin of deformation mechanisms, while the evolution of microelectromechanical systems for controlled in situ testing provide opportunities for advanced control and loading strategies. Discussion on the current state of the art for in situ nanomechanical testing and future opportunities in imaging, strain sensing, and testing environments are also addressed. Copyright © Materials Research Society 2019.
    view abstractdoi: 10.1557/mrs.2019.127
  • 2019 • 399 Alkylated Aromatic Thioethers with Aggregation-Induced Emission Properties—Assembly and Photophysics
    Riebe, S. and Saccone, M. and Stelzer, J. and Sowa, A. and Wölper, C. and Soloviova, K. and Strassert, C.A. and Giese, M. and Voskuhl, J.
    Chemistry - An Asian Journal 14 814-820 (2019)
    In this contribution, we present the synthesis and self-assembly of alkylated thioethers with interesting photophysical properties. To this end, the emission, absorption and excitation spectra in organic solvents and as aggregates in water were measured as well as the corresponding photoluminescence quantum yields and lifetimes. The aggregates in aqueous media were visualized and measured using transmission electron microscopy. Besides that, crystal structures of selected compounds allowed a detailed discussion of the structure–property relationship. Furthermore, the mesomorphic behavior was investigated using polarized optical microscopy (POM) as well as differential scanning calorimetry (DSC). © 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/asia.201801564
  • 2019 • 398 BioSAxS measurements reveal that two antimicrobial peptides induce similar molecular changes in Gram-negative and Gram-positive bacteria
    Von Gundlach, A. and Ashby, M.P. and Gani, J. and Lopez-Perez, P.M. and Cookson, A.R. and Huws, S.A. and Rumancev, C. and Garamus, V.M. and Mikut, R. and Rosenhahn, A. and Hilpert, K.
    Frontiers in Pharmacology 10 (2019)
    Two highly active short broad-spectrum AMPs (14D and 69D) with unknown mode of action have been investigated in regards to their effect against the Gram-negative bacteria Escherichia coli and the Gram-positive bacteria methicillin-resistant Staphylococcus aureus (MRSA). Minimal inhibitory concentration (MIC) measurements using a cell density of 108 cfu/ml resulted in values between 16 and 32 µg/ml. Time-kill experiments using 108 cfu/ml revealed complete killing, except for 69D in combination with MRSA, where bacterial load was reduced a million times. Small-angle X-ray scattering of biological samples (BioSAXS) at 108 cfu/ml was applied to investigate the ultrastructural changes in E. coli and MRSA in response to these two broad-spectrum AMPs. In addition, electron microscopy (EM) was performed to visualize the treated and non-treated bacteria. As expected, the scattering curves generated using BioSAXS show the ultrastructure of the Gram-positive and Gram-negative bacteria to be very different (BioSAXS is not susceptible to the outer shape). After treatment with either peptide, the scattering curves of E. coli and MRSA cells are much more alike. Whereas in EM, it is notoriously difficult to observe changes for spherical Gram-positives; the BioSAXS results are superior and reveal strongly similar effects for both peptides induced in Gram-positive as well as Gram-negative bacteria. Given the high-throughput possibility and robust statistics, BioSAXS can support and speed up mode of action research in AMPs and other antimicrobial compounds, making a contribution toward the development of urgently needed drugs against resistant bacteria. Copyright © 2019 von Gundlach, Ashby, Gani, Lopez-Perez, Cookson, Ann Huws, Rumancev, Garamus, Mikut, Rosenhahn and Hilpert. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.
    view abstractdoi: 10.3389/fphar.2019.01127
  • 2019 • 397 Cascade Reactions in Nanozymes: Spatially Separated Active Sites inside Ag-Core-Porous-Cu-Shell Nanoparticles for Multistep Carbon Dioxide Reduction to Higher Organic Molecules
    O'Mara, P.B. and Wilde, P. and Benedetti, T.M. and Andronescu, C. and Cheong, S. and Gooding, J.J. and Tilley, R.D. and Schuhmann, W.
    Journal of the American Chemical Society 141 14093-14097 (2019)
    Enzymes can perform complex multistep cascade reactions by linking multiple distinct catalytic sites via substrate channeling. We mimic this feature in a generalized approach with an electrocatalytic nanoparticle for the carbon dioxide reduction reaction comprising a Ag core surrounded by a porous Cu shell, providing different active sites in nanoconfined volumes. The architecture of the nanozyme provides the basis for a cascade reaction, which promotes C-C coupling reactions. The first step occurs on the Ag core, and the subsequent steps on the porous copper shell, where a sufficiently high CO concentration due to the nanoconfinement facilitates C-C bond formation. The architecture yields the formation of n-propanol and propionaldehyde at potentials as low as-0.6 V vs RHE. Copyright © 2019 American Chemical Society.
    view abstractdoi: 10.1021/jacs.9b07310
  • 2019 • 396 Development of a high flow rate aerodynamic lens system for inclusion of nanoparticles into growing PVD films to form nanocomposite thin films
    Kiesler, D. and Bastuck, T. and Kennedy, M.K. and Kruis, F.E.
    Aerosol Science and Technology 53 630-646 (2019)
    Hard coatings for wear protection of tools, bearings, and sliding parts play an important role in industrial manufacturing. Nanocomposite coatings are being used in this context to improve the mechanical properties. The technology applied therefore is often based on physical vapor deposition (PVD), in which the different materials are co-deposited. In these processes it is not possible to control the properties of the disperse phase and continuous phase independently. Here, we present a technology which combines aerosol technology with thin film technology to produce nanocomposite coatings directly, which gives us full control over both phases. It is based on an upscaled three-stage aerodynamic lens, which allows to bring nanoparticles from an atmospheric-pressure aerosol reactor into a PVD vacuum chamber operating at low pressure (2 Pa). This requires the use of a higher mass flow rate than conventionally used in aerodynamic lenses, so that a rational upscaling strategy for designing an aerodynamic lens for larger mass flow rates is proposed. Here, an array consisting of eight parallel three-stage aerodynamic lenses having each a mass flow rate of 0.6 slm using argon and 0.71 slm using nitrogen is built and optimized, assisted by CFD and numerical trajectory analysis. The transfer efficiency has been investigated numerically and experimentally. It is possible to transfer 80% of the particles with only 1.3% of the gas into the deposition chamber. A number of coatings consisting of titanium carbonitride nanoparticles embedded in a PVD chromium oxynitride film with varying nanoparticle content were produced. Electron microscopy shows the successful incorporation of the nanoparticles in the thin film. A reduction in film crystallite size with increasing nanoparticle content was found. A reverse Hall–Petch behavior was observed. Copyright © 2019 American Association for Aerosol Research. © 2019, © 2019 American Association for Aerosol Research.
    view abstractdoi: 10.1080/02786826.2019.1587149
  • 2019 • 395 Elemental re-distribution inside shear bands revealed by correlative atom-probe tomography and electron microscopy in a deformed metallic glass
    Balachandran, S. and Orava, J. and Köhler, M. and Breen, A.J. and Kaban, I. and Raabe, D. and Herbig, M.
    Scripta Materialia 168 14-18 (2019)
    A density variation in shear bands visible by electron microscopy is correlated with compositionally altered locations measured by atom-probe tomography in plastically-deformed Al 85.6 Y 7.5 Fe 5.8 metallic-glass ribbons. Two compositionally distinct regions are identified along shear bands, one is Al-rich (~92 at.%), the other is Al-depleted (~82.5 at.%) and both regions show marginal concentration fluctuations of Y and Fe. The elemental re-distribution is observed within shear bands only, and no chemical exchange with the surrounding glassy matrix is observed. © 2019
    view abstractdoi: 10.1016/j.scriptamat.2019.04.014
  • 2019 • 394 Engineering atomic-level complexity in high-entropy and complex concentrated alloys
    Oh, H.S. and Kim, S.J. and Odbadrakh, K. and Ryu, W.H. and Yoon, K.N. and Mu, S. and Körmann, F. and Ikeda, Y. and Tasan, C.C. and Raabe, D. and Egami, T. and Park, E.S.
    Nature Communications 10 (2019)
    Quantitative and well-targeted design of modern alloys is extremely challenging due to their immense compositional space. When considering only 50 elements for compositional blending the number of possible alloys is practically infinite, as is the associated unexplored property realm. In this paper, we present a simple property-targeted quantitative design approach for atomic-level complexity in complex concentrated and high-entropy alloys, based on quantum-mechanically derived atomic-level pressure approximation. It allows identification of the best suited element mix for high solid-solution strengthening using the simple electronegativity difference among the constituent elements. This approach can be used for designing alloys with customized properties, such as a simple binary NiV solid solution whose yield strength exceeds that of the Cantor high-entropy alloy by nearly a factor of two. This study provides general design rules that enable effective utilization of atomic level information to reduce the immense degrees of freedom in compositional space without sacrificing physics-related plausibility. © 2019, The Author(s).
    view abstractdoi: 10.1038/s41467-019-10012-7
  • 2019 • 393 Hydroxyapatite nanowires rich in [Ca-O-P] sites for ethanol direct coupling showing high C6-12 alcohol yield
    Wang, Q.-N. and Zhou, B.-C. and Weng, X.-F. and Lv, S.-P. and Schüth, F. and Lu, A.-H.
    Chemical Communications 55 10420-10423 (2019)
    Herein, we have shown that the [Ca-O-P] sites exposed on hydroxyapatite are clearly responsible for C-C formation in ethanol direct-coupling, and their high density accelerates the C-C coupling rate and boosts C6-12 alcohol production. Notably, nanowire-like hydroxyapatite exhibited 30.4% selectivity to n-butanol and 63.9% selectivity to C6-12OH at a conversion of 45.7% at 325 °C, and thereby close to 30% yield of C6-12OH, which is greatly higher than that using the state-of-the-art catalysts (6%). © 2019 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c9cc05454e
  • 2019 • 392 Influence of composition and precipitation evolution on damage at grain boundaries in a crept polycrystalline Ni-based superalloy
    Kontis, P. and Kostka, A. and Raabe, D. and Gault, B.
    Acta Materialia 166 158-167 (2019)
    The microstructural and compositional evolution of intergranular carbides and borides prior to and after creep deformation at 850 °C in a polycrystalline nickel-based superalloy was studied. Primary MC carbides, enveloped within intergranular γ′ layers, decomposed resulting in the formation of layers of the undesirable η phase. These layers have a composition corresponding to Ni3Ta as measured by atom probe tomography and their structure is consistent with the D024 hexagonal structure as revealed by transmission electron microscopy. Electron backscattered diffraction reveals that they assume various misorientations with regard to the adjacent grains. As a consequence, these layers act as brittle recrystallized zones and crack initiation sites. The composition of the MC carbides after creep was altered substantially, with the Ta content decreasing and the Hf and Zr contents increasing, suggesting a beneficial effect of Hf and Zr additions on the stability of MC carbides. By contrast, M5B3 borides were found to be microstructurally stable after creep and without substantial compositional changes. Borides at 850 °C were found to coarsen, resulting in some cases into γ′- depleted zones, where, however, no cracks were observed. The major consequences of secondary phases on the microstructural stability of superalloys during the design of new polycrystalline superalloys are discussed. © 2018 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2018.12.039
  • 2019 • 391 Major Differences between the Self-Assembly and Seeding Behavior of Heparin-Induced and in Vitro Phosphorylated Tau and Their Modulation by Potential Inhibitors
    Despres, C. and Di, J. and Cantrelle, F.-X. and Li, Z. and Huvent, I. and Chambraud, B. and Zhao, J. and Chen, J. and Chen, S. and Lippens, G. and Zhang, F. and Linhardt, R. and Wang, C. and Klärner, F.-G. and Schrader, T. and La...
    ACS Chemical Biology 14 1363-1379 (2019)
    Self-assembly of the microtubule-associated protein tau into neurotoxic oligomers, fibrils, and paired helical filaments, and cell-to-cell spreading of these pathological tau species are critical processes underlying the pathogenesis of Alzheimer's disease and other tauopathies. Modulating the self-assembly process and inhibiting formation and spreading of such toxic species are promising strategies for therapy development. A challenge in investigating tau self-assembly in vitro is that, unlike most amyloidogenic proteins, tau does not aggregate in the absence of posttranslational modifications (PTM), aggregation inducers, or preformed seeds. The most common induction method is addition of polyanions, such as heparin; yet, this artificial system may not represent adequately tau self-assembly in vivo, which is driven by aberrant phosphorylation and other PTMs, potentially leading to in vitro data that do not reflect the behavior of tau and its interaction with modulators in vivo. To tackle these challenges, methods for in vitro phosphorylation of tau to produce aggregation-competent forms recently have been introduced (Despres et al. (2017) Proc. Natl. Acad. Sci. U.S.A., 114, 9080-9085). However, the oligomerization, seeding, and interaction with assembly modulators of the different forms of tau have not been studied to date. To address these knowledge gaps, we compared here side-by-side the self-assembly and seeding activity of heparin-induced tau with two forms of in vitro phosphorylated tau and tested how the molecular tweezer CLR01, a negatively charged compound, affected these processes. Tau was phosphorylated by incubation either with activated extracellular signal-regulated kinase 2 or with a whole rat brain extract. Seeding activity was measured using a fluorescence-resonance energy transfer-based biosensor-cell method. We also used solution-state NMR to investigate the binding sites of CLR01 on tau and how they were impacted by phosphorylation. Our systematic structure-activity relationship study demonstrates that heparin-induced tau behaves differently from in vitro phosphorylated tau. The aggregation rates of the different forms are distinct as is the intracellular localization of the induced aggregates, which resemble brain-derived tau strains suggesting that heparin-induced tau and in vitro phosphorylated tau have different conformations, properties, and activities. CLR01 inhibits aggregation and seeding of both heparin-induced and in vitro phosphorylated tau dose-dependently, although heparin induction interferes with the interaction between CLR01 and tau. © 2019 American Chemical Society.
    view abstractdoi: 10.1021/acschembio.9b00325
  • 2019 • 390 On the evolution of dislocation cell structures in two Al-alloys (Al-5Mg and Al-11Zn) during reciprocal sliding wear at high homologous temperatures
    Parsa, A.B. and Walter, M. and Theisen, W. and Bürger, D. and Eggeler, G.
    Wear 1-12 (2019)
    The formation of dislocation substructures in up to 10 µm deep subsurface regions of two aluminium alloys, Al-5Mg and Al-11Zn, was investigated under conditions of high homologous temperature reciprocal sliding wear (HT-RSW). Under creep conditions, Al-5Mg shows a solid solution type of inverse primary creep. In contrast, Al-11Zn creeps obstacle controlled and exhibits normal primary creep. These two materials were subjected to reciprocal sliding wear at 200 and 300 °C for 100 and 1000 cycles. Flat polished disks were exposed to the 1 mm reciprocal movements of a spherical aluminium oxide counterbody under normal forces of 5 and 10 N at an oscillation frequency of 1 Hz. Using focused ion beam (FIB) micromachining thin electron transparent foils were prepared from the surface regions of the as received and worn material states. Transmission electron microscopy (TEM) was used to study the evolution of nano and micro grain sizes in the surface regions. Despite the different creep behavior, the two materials behave similar under conditions of reciprocal sliding wear. The results obtained in the present work show that subgrain sizes decrease with increasing numbers of wear cycles and increasing normal forces. Subgrain sizes also increase with increasing temperature. At 300 °C, dynamic recrystallization was observed in both Al-alloys. The results of the present work are discussed in the light of previous results reported in the literature. Areas in need of further work are highlighted. © 2018 Elsevier B.V.
    view abstractdoi: 10.1016/j.wear.2018.10.018
  • 2019 • 389 Sensitive and selective detection of Cu2+ ions based on fluorescent Ag nanoparticles synthesized by R-phycoerythrin from marine algae Porphyra yezoensis
    Xu, Y. and Hou, Y. and Wang, Y. and Wang, Y. and Li, T. and Song, C. and Wei, N. and Wang, Q.
    Ecotoxicology and Environmental Safety 168 356-362 (2019)
    In this study, using a natural and green protein R-phycoerythrin (R-PE) extracted from marine Porphyra yezoensis as the stabilizer and reducer, silver nanoparticles (AgNPs) were synthesized. Based on this, a highly sensitive and selective method for the detection of Cu2+ ions was developed using R-PE-AgNPs as fluorescent probe. The interactions between R-PE-AgNPs and Cu2+ ions were systematically characterized by fluorescence spectroscopy, transmission electron microscopy (TEM), elemental mapping and Fourier transform infrared (FTIR). It was found that Cu2+ ions could cause aggregation of the R-PE-AgNPs, accompanied by the greatly increased particle size. Importantly, the method offered a wide linear detection range from 0 μM to 100.0 μM with a detection limit of 0.0190 μM. Moreover, the proposed method was successfully applied to analyze Cu2+ ions in tap water and lake water samples, acquiring satisfactory recovery between 91.6% and 102.2%. Such a green, fast and cost-effective fluorimetric method of the R-PE-AgNPs probe has great potential for tracing Cu2+ ions in diverse aqueous media. © 2018 Elsevier Inc.
    view abstractdoi: 10.1016/j.ecoenv.2018.10.102
  • 2019 • 388 Shape-preserving machining produces gradient nanolaminate medium entropy alloys with high strain hardening capability
    Guo, W. and Pei, Z. and Sang, X. and Poplawsky, J.D. and Bruschi, S. and Qu, J. and Raabe, D. and Bei, H.
    Acta Materialia 170 176-186 (2019)
    A high density of grain boundaries can potentially increase structural materials' strength, but at the expense of losing the materials' strain hardening ability at high flow stress levels. However, endowing materials with grain size gradients and a high density of internal interfaces can simultaneously increase the strength and strain hardening ability. This applies particularly for through-thickness gradients of nanoscale interface structures. Here we apply a machining method that produces metals with nanoscale interface gradients. Conventional bulk plastic deformation such as rolling, a process applied annually to about 2 billion tons of material, aims to reduce the metal thickness. We have modified this process by introducing severe strain path changes, realized by leading the sheet through a U-turn while preserving its shape, an approach known as ‘hard turning’. We applied this process at both room temperature and 77 K to a NiCrCo medium entropy alloy. Micropillar compression was conducted to evaluate the mechanical response. After hard turning at room temperature, the surface microstructure obtained a ∼50% increase in yield stress (0.9 GPa) over the original state with homogeneous grain size (0.4 GPa), but the initial strain hardening rate did not show significant improvement. However, after hard turning at 77 k, the gradient nanolaminate structure tripled in yield stress and more than doubled its initial strain hardening rate. The improvements were achieved by introducing a specific microstructure that consists of gradient nanolaminates in the form of nanospaced twins and martensite in the face center cubic (fcc) phase. This microstructure was formed only at cryogenic temperature. It was found after turning at room temperature that only nanospaced twins were present in the fcc phase inside nanolaminates that had formed at the surface. The origin of the enhanced strain hardening mechanism was studied. Joint density functional theory (DFT) and axial next nearest neighbor Ising (ANNNI) models were used to explain the temperature-dependent phase formation of the NiCrCo nanolaminate at the surface of the hard-turned material. © 2019 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2019.03.024
  • 2019 • 387 Single-pass STEM-EMCD on a zone axis using a patterned aperture: progress in experimental and data treatment methods
    Thersleff, T. and Schönström, L. and Tai, C.-W. and Adam, R. and Bürgler, D.E. and Schneider, C.M. and Muto, S. and Rusz, J.
    Scientific Reports 9 (2019)
    Measuring magnetic moments in ferromagnetic materials at atomic resolution is theoretically possible using the electron magnetic circular dichroism (EMCD) technique in a (scanning) transmission electron microscope ((S)TEM). However, experimental and data processing hurdles currently hamper the realization of this goal. Experimentally, the sample must be tilted to a zone-axis orientation, yielding a complex distribution of magnetic scattering intensity, and the same sample region must be scanned multiple times with sub-atomic spatial registration necessary at each pass. Furthermore, the weak nature of the EMCD signal requires advanced data processing techniques to reliably detect and quantify the result. In this manuscript, we detail our experimental and data processing progress towards achieving single-pass zone-axis EMCD using a patterned aperture. First, we provide a comprehensive data acquisition and analysis strategy for this and other EMCD experiments that should scale down to atomic resolution experiments. Second, we demonstrate that, at low spatial resolution, promising EMCD candidate signals can be extracted, and that these are sensitive to both crystallographic orientation and momentum transfer. © 2019, The Author(s).
    view abstractdoi: 10.1038/s41598-019-53373-1
  • 2019 • 386 Ti and its alloys as examples of cryogenic focused ion beam milling of environmentally-sensitive materials
    Chang, Y. and Lu, W. and Guénolé, J. and Stephenson, L.T. and Szczpaniak, A. and Kontis, P. and Ackerman, A.K. and Dear, F.F. and Mouton, I. and Zhong, X. and Zhang, S. and Dye, D. and Liebscher, C.H. and Ponge, D. and Korte-Ker...
    Nature Communications 10 (2019)
    Hydrogen pick-up leading to hydride formation is often observed in commercially pure Ti (CP-Ti) and Ti-based alloys prepared for microscopic observation by conventional methods, such as electro-polishing and room temperature focused ion beam (FIB) milling. Here, we demonstrate that cryogenic FIB milling can effectively prevent undesired hydrogen pick-up. Specimens of CP-Ti and a Ti dual-phase alloy (Ti-6Al-2Sn-4Zr-6Mo, Ti6246, in wt.%) were prepared using a xenon-plasma FIB microscope equipped with a cryogenic stage reaching −135 °C. Transmission electron microscopy (TEM), selected area electron diffraction, and scanning TEM indicated no hydride formation in cryo-milled CP-Ti lamellae. Atom probe tomography further demonstrated that cryo-FIB significantly reduces hydrogen levels within the Ti6246 matrix compared with conventional methods. Supported by molecular dynamics simulations, we show that significantly lowering the thermal activation for H diffusion inhibits undesired environmental hydrogen pick-up during preparation and prevents pre-charged hydrogen from diffusing out of the sample, allowing for hydrogen embrittlement mechanisms of Ti-based alloys to be investigated at the nanoscale. © 2019, The Author(s).
    view abstractdoi: 10.1038/s41467-019-08752-7
  • 2018 • 385 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 • 384 A TEM Investigation of Columnar-Structured Thermal Barrier Coatings Deposited by Plasma Spray-Physical Vapor Deposition (PS-PVD)
    Rezanka, S. and Somsen, C. and Eggeler, G. and Mauer, G. and Vaßen, R. and Guillon, O.
    Plasma Chemistry and Plasma Processing 38 791-802 (2018)
    The plasma spray-physical vapor deposition technique (PS-PVD) is used to deposit various types of ceramic coatings. Due to the low operating pressure and high enthalpy transfer to the feedstock, deposition from the vapor phase is very effective. The particular process conditions allow for the deposition of columnar microstructures when applying thermal barrier coatings (TBCs). These coatings show a high strain tolerance similar to those obtained by electron beam-physical vapor deposition (EB-PVD). But compared to EB-PVD, PS-PVD allows significantly reducing process time and costs. The application-related properties of PS-PVD TBCs have been investigated in earlier work, where the high potential of the process was described and where the good resistance to thermo-mechanical loading conditions was reported. But until now, the elementary mechanisms which govern the material deposition have not been fully understood and it is not clear, how the columnar structure is built up. Shadowing effects and diffusion processes are assumed to contribute to the formation of columnar microstructures in classical PVD processing routes. For such structures, crystallographic textures are characteristic. For PS-PVD, however, no crystallographic textures could initially be found using X-ray diffraction. In this work a more detailed TEM investigations and further XRD measurements of the columnar PS-PVD microstructure were performed. The smallest build units of the columnar TBC structure are referred to as sub-columns. The observed semi-single crystal structure of individual sub-columns was analyzed by means of diffraction experiments. The absence of texture in PS-PVD coatings is confirmed and elementary nucleation and growth mechanisms are discussed. © 2018, Springer Science+Business Media, LLC, part of Springer Nature.
    view abstractdoi: 10.1007/s11090-018-9898-y
  • 2018 • 383 Atomic Layer Deposition of Nickel on ZnO Nanowire Arrays for High-Performance Supercapacitors
    Ren, Q.-H. and Zhang, Y. and Lu, H.-L. and Wang, Y.-P. and Liu, W.-J. and Ji, X.-M. and Devi, A. and Jiang, A.-Q. and Zhang, D.W.
    ACS Applied Materials and Interfaces 10 468-476 (2018)
    A novel hybrid core-shell structure of ZnO nanowires (NWs)/Ni as a pseudocapacitor electrode was successfully fabricated by atomic layer deposition of a nickel shell, and its capacitive performance was systemically investigated. Transmission electron microscopy and X-ray photoelectron spectroscopy results indicated that the NiO was formed at the interface between ZnO and Ni where the Ni was oxidized by ZnO during the ALD of the Ni layer. Electrochemical measurement results revealed that the Ti/ZnO NWs/Ni (1500 cycles) electrode with a 30 nm thick Ni-NiO shell layer had the best supercapacitor properties including ultrahigh specific capacitance (∼2440 F g-1), good rate capability (80.5%) under high current charge-discharge conditions, and a relatively better cycling stability (86.7% of the initial value remained after 750 cycles at 10 A g-1). These attractive capacitive behaviors are mainly attributed to the unique core-shell structure and the combined effect of ZnO NW arrays as short charge transfer pathways for ion diffusion and electron transfer as well as conductive Ni serving as channel for the fast electron transport to Ti substrate. This high-performance Ti/ZnO NWs/Ni hybrid structure is expected to be one of a promising electrodes for high-performance supercapacitor applications. © 2017 American Chemical Society.
    view abstractdoi: 10.1021/acsami.7b13392
  • 2018 • 382 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 • 381 Crystallographic characterization of laser-generated, polymer-stabilized 4 nm silver-gold alloyed nanoparticles
    Prymak, O. and Jakobi, J. and Rehbock, C. and Epple, M. and Barcikowski, S.
    Materials Chemistry and Physics 207 442-450 (2018)
    Monometallic silver and gold nanoparticles and bimetallic silver-gold (AgAu) nanoparticles were prepared by laser ablation in liquids in the atomic composition range of Ag:Au from 0:100 to 100:0 with steps of 10 at% and colloidally stabilized with poly(N-vinylpyrrolidone) (PVP). As metallic bulk targets for laser ablation, pure silver, pure gold, and alloyed AgAu foils with the desired composition were used. Size separation by centrifugation and freeze-drying gave monodisperse spherical nanoparticles with a diameter of 4 nm as determined by differential centrifugal sedimentation (DCS) and transmission electron microscopy (TEM). A crystallographic characterization of the nanoparticles was carried out by X-ray powder diffraction (XRD) and Rietveld refinement, leading to highly precise cubic lattice parameters (fcc crystal system) and crystallite sizes. For comparison, the same analysis including the determination of the microstrain was carried out for the bulk target materials (AgAu alloys in the full concentration range). Both nanoparticles and bulk target materials obeyed Vegard's rule, with only slight deviations. The fact that the crystallite size as determined by XRD was identical to the hydrodynamic diameter by DCS and the Feret diameter by TEM indicates that the particles consist of only one domain, i.e. they are single crystals. The combination of UV-vis spectroscopy with energy-dispersive X-ray spectroscopy (EDX) as line scan along the nanoparticle showed a homogenous distribution of the gold and silver inside the nanoparticles, indicating solid solution alloys, in contrast to what was observed earlier for chemically prepared AgAu nanoparticles by reduction of metal ions in water. © 2018 Elsevier B.V.
    view abstractdoi: 10.1016/j.matchemphys.2017.12.080
  • 2018 • 380 Deformation mechanisms, activated slip systems and critical resolved shear stresses in an Mg-LPSO alloy studied by micro-pillar compression
    Chen, R. and Sandlöbes, S. and Zehnder, C. and Zeng, X. and Korte-Kerzel, S. and Raabe, D.
    Materials and Design 154 203-216 (2018)
    We study the micro-mechanical behaviour of single-crystalline long-period-stacking ordered (LPSO) structures, α-Mg and bi-crystalline Mg/LPSO micro-pillars, all cut from the same Mg97Y2Zn1 (at.%) alloy. To investigate the deformation and co-deformation mechanisms of Mg-LPSO alloys we performed micro-pillar compression experiments with micro-pillars of an orientation inclined by 7°, 46° and 90° to (0001) orientation, respectively. Electron backscatter diffraction-assisted slip trace analysis and post-mortem transmission electron microscopy analysis showed predominant deformation by basal 〈a〉 dislocation slip in 46°(0001) and 7°(0001) oriented micro-pillars. In 7°(0001) oriented micro-pillars additional non-basal dislocation slip and the formation of micro shear bands along pyramidal planes were activated in the α-Mg and the LPSO structure, respectively. In 90°(0001) oriented micro-pillars 11¯001¯1¯20 prismatic slip was predominantly activated during the early deformation stages. The relative magnitude of the critical stresses depends on the crystal phase as well as the crystallographic orientation, i.e. the activated slip system. Specifically, basal 〈a〉 slip has the lowest critical resolved shear stress in both, α-Mg and the LPSO structure, while the CRSS of prismatic 〈a〉 slip is about 5 times higher than basal 〈a〉 slip in α-Mg and about 15 times higher than basal 〈a〉 slip in LPSO. © 2018 Elsevier Ltd
    view abstractdoi: 10.1016/j.matdes.2018.05.037
  • 2018 • 379 Deposition of polycrystalline zinc substituted hydroxyapatite coatings with a columnar structure by RF magnetron sputtering: Role of in-situ substrate heating
    Prosolov, K.A. and Belyavskaya, O.A. and Rau, J.V. and Prymak, O. and Epple, M. and Sharkeev, Y.P.
    Journal of Physics: Conference Series 1115 (2018)
    Zn incorporation into hydroxyapatite structure leads to enhanced osteointegration and antibacterial activity of deposited coatings. Radiofrequency magnetron sputtering is a physical vapor deposition technique which can be used to create thin coatings with a controlled level of crystallinity. The material state is a crucial parameter for biocoatings as it governs cell response. Bioactive Zn substituted hydroxyapatite coatings were deposited onto Ti by radiofrequency magnetron sputtering at increased substrate temperatures (100, 200, 300 and 400°C). XRD showed crystallization of the coatings at elevated substrate temperatures starting from 300°C. Cross-section transmission electron microscopy showed a polycrystalline columnar grain structure of Zn substituted coatings deposited at 400°C substrate temperature. An amorphous TiO 2 sublayer of several monolayers thickness was detected in the interface between the polycrystalline coating and the Ti substrate. In-column energy dispersive X-ray analysis revealed coatings to be substoichiometric with the average Ca/P ratio being 1.5. It is established that it is possible to deposit Zn substituted hydroxyapatite in a form of a well-crystalline coating when the substrate temperature is exceeding 400°C. © Published under licence by IOP Publishing Ltd.
    view abstractdoi: 10.1088/1742-6596/1115/3/032077
  • 2018 • 378 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 • 377 Effects of strain rate on mechanical properties and deformation behavior of an austenitic Fe-25Mn-3Al-3Si TWIP-TRIP steel
    Benzing, J.T. and Poling, W.A. and Pierce, D.T. and Bentley, J. and Findley, K.O. and Raabe, D. and Wittig, J.E.
    Materials Science and Engineering A 711 78-92 (2018)
    The effects of quasi-static and low-dynamic strain rate (ε̇ = 10−4 /s to ε̇ = 102 /s) on tensile properties and deformation mechanisms were studied in a Fe-25Mn-3Al-3Si (wt%) twinning and transformation-induced plasticity [TWIP-TRIP] steel. The fully austenitic microstructure deforms primarily by dislocation glide but due to the room temperature stacking fault energy [SFE] of 21 ± 3 mJ/m2 for this alloy, secondary deformation mechanisms such as mechanical twinning (TWIP) and epsilon martensite formation (TRIP) also play an important role in the deformation behavior. The mechanical twins and epsilon-martensite platelets act as planar obstacles to subsequent dislocation motion on non-coplanar glide planes and reduce the dislocation mean free path. A high-speed thermal camera was used to measure the increase in specimen temperature as a function of strain, which enabled the use of a thermodynamic model to predict the increase in SFE. The influence of strain rate and strain on microstructural parameters such as the thickness and spacing of mechanical twins and epsilon-martensite laths was quantified using dark field transmission electron microscopy, electron channeling contrast imaging, and electron backscattered diffraction. The effect of sheet thickness on mechanical properties was also investigated. Increasing the tensile specimen thickness increased the product of ultimate tensile strength and total elongation, but had no significant effect on uniform elongation or yield strength. The yield strength exhibited a significant increase with increasing strain rate, indicating that dislocation glide becomes more difficult with increasing strain rate due to thermally-activated short-range barriers. A modest increase in ultimate tensile strength and minimal decrease in uniform elongation were noted at higher strain rates, suggesting adiabatic heating, slight changes in strain-hardening rate and observed strain localizations as root causes, rather than a significant change in the underlying TWIP-TRIP mechanisms at low values of strain. © 2017 Elsevier B.V.
    view abstractdoi: 10.1016/j.msea.2017.11.017
  • 2018 • 376 Electrostatic Self-Assembly Enabling Integrated Bulk and Interfacial Sodium Storage in 3D Titania-Graphene Hybrid
    Xu, G.-L. and Xiao, L. and Sheng, T. and Liu, J. and Hu, Y.-X. and Ma, T. and Amine, R. and Xie, Y. and Zhang, X. and Liu, Y. and Ren, Y. and Sun, C.-J. and Heald, S.M. and Kovacevic, J. and Sehlleier, Y.H. and Schulz, C. and Matt...
    Nano Letters 18 336-346 (2018)
    Room-temperature sodium-ion batteries have attracted increased attention for energy storage due to the natural abundance of sodium. However, it remains a huge challenge to develop versatile electrode materials with favorable properties, which requires smart structure design and good mechanistic understanding. Herein, we reported a general and scalable approach to synthesize three-dimensional (3D) titania-graphene hybrid via electrostatic-interaction-induced self-assembly. Synchrotron X-ray probe, transmission electron microscopy, and computational modeling revealed that the strong interaction between titania and graphene through comparably strong van der Waals forces not only facilitates bulk Na+ intercalation but also enhances the interfacial sodium storage. As a result, the titania-graphene hybrid exhibits exceptional long-term cycle stability up to 5000 cycles, and ultrahigh rate capability up to 20 C for sodium storage. Furthermore, density function theory calculation indicated that the interfacial Li+, K+, Mg2+, and Al3+ storage can be enhanced as well. The proposed general strategy opens up new avenues to create versatile materials for advanced battery systems. © 2017 American Chemical Society.
    view abstractdoi: 10.1021/acs.nanolett.7b04193
  • 2018 • 375 From Quasicrystals to Crystals with Interpenetrating Icosahedra in Ca-Au-Al: In Situ Variable-Temperature Transformation
    Pham, J. and Meng, F. and Lynn, M.J. and Ma, T. and Kreyssig, A. and Kramer, M.J. and Goldman, A.I. and Miller, G.J.
    Journal of the American Chemical Society 140 1337-1347 (2018)
    The irreversible transformation from an icosahedral quasicrystal (i-QC) CaAu4.39Al1.61 to its cubic 2/1 crystalline approximant (CA) Ca13Au56.31(3)Al21.69 (CaAu4.33(1)Al1.67, Pa3 (No. 205); Pearson symbol: cP728; a = 23.8934(4)), starting at ∼570 °C and complete by ∼650 °C, is discovered from in situ, high-energy, variable-temperature powder X-ray diffraction (PXRD), thereby providing direct experimental evidence for the relationship between QCs and their associated CAs. The new cubic phase crystallizes in a Tsai-type approximant structure under the broader classification of polar intermetallic compounds, in which atoms of different electronegativities, viz., electronegative Au + Al vs electropositive Ca, are arranged in concentric shells. From a structural chemical perspective, the outermost shell of this cubic approximant may be described as interpenetrating and edge-sharing icosahedra, a perspective that is obtained by splitting the traditional structural description of this shell as a 92-atom rhombic triacontahedron into an 80-vertex cage of primarily Au [Au59.86(2)Al17.14□3.00] and an icosahedral shell of only Al [Al10.5□1.5]. Following the proposal that the cubic 2/1 CA approximates the structure of the i-QC and on the basis of the observed transformation, an atomic site analysis of the 2/1 CA, which shows a preference to maximize the number of heteroatomic Au-Al nearest neighbor contacts over homoatomic Al-Al contacts, implies a similar outcome for the i-QC structure. Analysis of the most intense reflections in the diffraction pattern of the cubic 2/1 CA that changed during the phase transformation shows correlations with icosahedral symmetry, and the stability of this cubic phase is assessed using valence electron counts. According to electronic structure calculations, a cubic 1/1 CA, "Ca24Au88Al64" (CaAu3.67Al2.67) is proposed. © 2017 American Chemical Society.
    view abstractdoi: 10.1021/jacs.7b10358
  • 2018 • 374 Fully automated primary particle size analysis of agglomerates on transmission electron microscopy images via artificial neural networks
    Frei, M. and Kruis, F.E.
    Powder Technology 332 120-130 (2018)
    There is a high demand for fully automated methods for the analysis of primary particle size distributions of agglomerates on transmission electron microscopy images. Therefore, a novel method, based on the utilization of artificial neural networks, was proposed, implemented and validated. The training of the artificial neural networks requires large quantities (up to several hundreds of thousands) of transmission electron microscopy images of agglomerates consisting of primary particles with known sizes. Since the manual evaluation of such large amounts of transmission electron microscopy images is not feasible, a synthesis of lifelike transmission electron microscopy images as training data was implemented. The proposed method can compete with state-of-the-art automated imaging particle size methods like the Hough transformation, ultimate erosion and watershed transformation and is in some cases even able to outperform these methods. It is however still outperformed by the manual analysis. © 2018 Elsevier B.V.
    view abstractdoi: 10.1016/j.powtec.2018.03.032
  • 2018 • 373 In situ atomic-scale observation of oxidation and decomposition processes in nanocrystalline alloys
    Guo, J. and Haberfehlner, G. and Rosalie, J. and Li, L. and Duarte, M.J. and Kothleitner, G. and Dehm, G. and He, Y. and Pippan, R. and Zhang, Z.
    Nature Communications 9 (2018)
    Oxygen contamination is a problem which inevitably occurs during severe plastic deformation of metallic powders by exposure to air. Although this contamination can change the morphology and properties of the consolidated materials, there is a lack of detailed information about the behavior of oxygen in nanocrystalline alloys. In this study, aberration-corrected high-resolution transmission electron microscopy and associated techniques are used to investigate the behavior of oxygen during in situ heating of highly strained Cu-Fe alloys. Contrary to expectations, oxide formation occurs prior to the decomposition of the metastable Cu-Fe solid solution. This oxide formation commences at relatively low temperatures, generating nanosized clusters of firstly CuO and later Fe2O3. The orientation relationship between these clusters and the matrix differs from that observed in conventional steels. These findings provide a direct observation of oxide formation in single-phase Cu-Fe composites and offer a pathway for the design of nanocrystalline materials strengthened by oxide dispersions. © 2018 The Author(s).
    view abstractdoi: 10.1038/s41467-018-03288-8
  • 2018 • 372 In situ TEM observation of rebonding on fractured silicon carbide
    Zhang, Z. and Cui, J. and Wang, B. and Jiang, H. and Chen, G. and Yu, J. and Lin, C. and Tang, C. and Hartmaier, A. and Zhang, J. and Luo, J. and Rosenkranz, A. and Jiang, N. and Guo, D.
    Nanoscale 10 6261-6269 (2018)
    Silicon carbide (SiC) is widely used in harsh environments and under extreme conditions, including at high-power, higherature, high-current, high-voltage and high-frequency. The rebonding and self-matching of stack faults (SFs) is highly desirable to avoid catastrophic failure for SiC devices, especially for specific applications in the aerospace and nuclear power industries. In this study, a novel approach was developed using an eyebrow hair to pick up and transfer nanowires (NWs), in order to obtain in situ transmission electron microscope (TEM) images of the rebonding and self-matching of SFs at atomic resolution. During rebonding and healing, the electron beam was shut off. Rebonding on the fractured surfaces of monocrystalline and amorphous SiC NWs was observed by in situ TEM at room temperature. The fracture strength was 1.7 GPa after crack-healing, restoring 12.9% of that of a single crystal NW. Partial recrystallization along the <111> orientation and the self-matching of SFs are responsible for the rebonding of the monocrystalline NW. In comparison, the fracture strengths were 6.7 and 5.5 GPa for the first and second rebonding, respectively recovering 67% and 55% of that of an amorphous NW. Atomic diffusion contributed enormously to the rebonding on fractured surfaces of an amorphous NW, resulting in a healed surface consisting of an amorphous phase and crystallites. This rebonding function provides new insight into the fabrication of high-performance SiC devices for the aerospace, optoelectronic and semiconductor industries. © 2018 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c8nr00341f
  • 2018 • 371 Kinetic study of gold nanoparticles synthesized in the presence of chitosan and citric acid
    Simeonova, S. and Georgiev, P. and Exner, K.S. and Mihaylov, L. and Nihtianova, D. and Koynov, K. and Balashev, K.
    Colloids and Surfaces A: Physicochemical and Engineering Aspects 557 106-115 (2018)
    In this work colloidal gold nanoparticles (GNPs) are prepared using a citrate-reduction route, in which citric acid serves as reductive agent for the gold precursor HAuCl4. We demonstrate that a temperature variation on the one hand enables to tune the reaction rate of GNP formation and on the other hand allows modifying the morphology of the resulting metal nanoparticles. The use of chitosan, a biocompatible and biodegradable polymer with a multitude of functional amino and hydroxyl groups, facilitates the simultaneous synthesis and surface modification of GNPs in one pot. The resulting GNPs, which are stabilized by a network of chitosan and ß-ketoglutaric acid units, are characterized by UV–vis spectroscopy, atomic force microscopy (AFM), transmission electron microscopy (TEM) as well as fluorescence correlation spectroscopy (FCS) and reveal an average diameter of about 10 nm at the end of the synthesis. The kinetics of GNP formation is studied by calculating activation parameters based on UV–vis and AFM data such as the apparent activation energy, entropy and free energy applying the concept of the Finke-Watzky model and harmonic transition state theory. © 2018 Elsevier B.V.
    view abstractdoi: 10.1016/j.colsurfa.2018.02.045
  • 2018 • 370 Mechanochemical synthesis of porous carbon at room temperature with a highly ordered sp2 microstructure
    Casco, M.E. and Badaczewski, F. and Grätz, S. and Tolosa, A. and Presser, V. and Smarsly, B.M. and Borchardt, L.
    Carbon 139 325-333 (2018)
    Carbon nanostructures with a well-developed turbostratic sp2 structure and high porosity are synthesized at room temperature inside a planetary ball mill. The obtained carbons were analyzed in-depth by means of gas adsorption, wide-angle X-ray scattering (WAXS), Raman spectroscopy, and transmission electron microscopy (TEM). Our approach involves the solvent-free reaction between calcium carbide (CaC2) and hexachlorobenzene (C6Cl6) conducted under mechanochemical conditions. After certain mechanical activation time, the exothermic nature of the reaction (−492 kcal) provokes a combustion-like event that results in innocuous salt (CaCl2) and a carbonaceous material. Carbon with a high degree of structural order in the constituting graphene and the graphene stacks, possessing almost no internal surface, can be obtained after 5 min of milling time with a mass ratio CaC2/C6Cl6 of 0.9, while carbon exhibiting a surface area as high as 915 m2/g can be obtained after 2 h of milling time with a mass ratio CaC2/C6Cl6 of 5.1. WAXS results and TEM observations reveal a mixture of amorphous carbon and non-graphitic phases. Among the last one, spherical-shaped carbons and curved nanosized strips can be easily distinguished. © 2018 Elsevier Ltd
    view abstractdoi: 10.1016/j.carbon.2018.06.068
  • 2018 • 369 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 • 368 Microstructure and mechanical properties of Al0.7CoCrFeNi high-entropy-alloy prepared by directional solidification
    Liu, G. and Liu, L. and Liu, X. and Wang, Z. and Han, Z. and Zhang, G. and Kostka, A.
    Intermetallics 93 93-100 (2018)
    The high-entropy-alloy Al0.7CoCrFeNi (molar ratio) was prepared by vacuum arc melting followed by directional solidification (DS) with &lt;001&gt; oriented seed. The unique lamellar-dendrite microstructure was obtained over a wide cooling rate range. During solidification, Fe and Co are prone to segregate to the dendrite, while Cr and Al segregate to interdendrite. The solute pile-up of Cr and Al at the solid/liquid interface leads to the dendritic solidification. During the following cooling process, the BCC phase precipitates from the FCC dendrite to form the lamellar structure, while the ordered B2 phase precipitates from the interdendrite. Moreover, the lamellar spacing is significantly refined with increasing cooling rate, resulting in the higher hardness and compressive yield strength. Directional solidification is proved to be an efficient way to improve the mechanical properties of multi-phases high-entropy alloys. © 2017 Elsevier Ltd
    view abstractdoi: 10.1016/j.intermet.2017.11.019
  • 2018 • 367 Multiscale Characterization of Microstructure in Near-Surface Regions of a 16MnCr5 Gear Wheel After Cyclic Loading
    Medghalchi, S. and Jamebozorgi, V. and Bala Krishnan, A. and Vincent, S. and Salomon, S. and Basir Parsa, A. and Pfetzing, J. and Kostka, A. and Li, Y. and Eggeler, G. and Li, T.
    JOM 1-7 (2018)
    The dependence of the microstructure on the degree of deformation in near-surface regions of a 16MnCr5 gear wheel after 2.1 × 106 loading cycles has been investigated by x-ray diffraction analysis, transmission electron microscopy, and atom probe tomography. Retained austenite and large martensite plates, along with elongated lamella-like cementite, were present in a less deformed region. Comparatively, the heavily deformed region consisted of a nanocrystalline structure with carbon segregation up to 2 at.% at grain boundaries. Spheroid-shaped cementite, formed at the grain boundaries and triple junctions of the nanosized grains, was enriched with Cr and Mn but depleted with Si. Such partitioning of Cr, Mn, and Si was not observed in the elongated cementite formed in the less deformed zone. This implies that rolling contact loading induced severe plastic deformation as well as a pronounced annealing effect in the active contact region of the toothed gear during cyclic loading. © 2018 The Minerals, Metals & Materials Society
    view abstractdoi: 10.1007/s11837-018-2931-z
  • 2018 • 366 On the origin of the improvement of shape memory effect by precipitating VC in Fe–Mn–Si-based shape memory alloys
    Lai, M.J. and Li, Y.J. and Lillpopp, L. and Ponge, D. and Will, S. and Raabe, D.
    Acta Materialia 155 222-235 (2018)
    We studied the role of VC precipitation in improving the shape memory effect (SME) of the as-solution treated Fe–Mn–Si-based shape memory alloys by examining the microstructures developed during aging and deformation using transmission electron microscopy and electron channeling contrast imaging. Our results suggest that VC particles are not the only product of aging. Upon aging at 650 °C, the precipitation of VC particles is accompanied by the formation of profuse dislocations (2.26 ± 0.098 × 1013 m−2). In this case, the SME is not improved compared to the as-solution treated reference state. Upon aging at high temperatures (700–900 °C), a number of stacking faults are formed accompanying the VC precipitation and the SME is significantly improved, where the recovery ratios reach almost twice that of the as-solution treated state (&lt;50%). For these high-temperature aged states, in situ straining experiments reveal that the stacking faults rather than the VC particles play an important role in the stress-induced martensitic transformation, leading to the formation of very thin (&lt;3 nm) martensite plates with a single crystallographic variant within each grain. These martensite plates are in contrast to the very thick (from tens to hundreds of nanometers) and multi-variant martensite plates that prevail in the as-solution treated state. By comparing the characteristics of the martensite plates between the as-solution treated and the high-temperature aged states, the reasons for the improvement of SME by precipitating VC were discussed. © 2018 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2018.06.008
  • 2018 • 365 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 • 364 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 • 363 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 • 362 Thermal stability of nanocomposite Mo2BC hard coatings deposited by magnetron sputtering
    Gleich, S. and Breitbach, B. and Peter, N.J. and Soler, R. and Bolvardi, H. and Schneider, J.M. and Dehm, G. and Scheu, C.
    Surface and Coatings Technology 349 378-383 (2018)
    The investigation of hard coatings under thermal load is crucial in order to obtain information on the thermal stability and possible changes of microstructure and mechanical properties. In addition, advanced heating studies may also provide feedback for the grain growth mechanism occurring during annealing and thus, help to predict optimum post-growth annealing conditions for producing high-performance hard coatings. Here, we investigate the thermal response of Mo2BC, deposited by bipolar pulsed direct current magnetron sputtering in an industrial chamber on a silicon substrate at a substrate temperature of 380 °C. Ex-situ and in-situ X-ray diffraction and transmission electron microscopy studies are performed at elevated temperatures to track changes in the structure. Whereas the as-deposited nanocomposite coating exhibits small spherical nanocrystals (1.2 nm in diameter) embedded in an amorphous matrix, a fully crystalline structure, mainly consisting of elongated and interconnected crystals with lengths of up to 1 μm, is obtained at elevated annealing temperatures. Hardness and Young's modulus increase by ~8% and ~47%, respectively, compared to the as-deposited coating. Delamination from the silicon substrate only occurs at temperatures above 840 °C. Thus, our detailed study of the micro- and nanostructure evolution upon thermal annealing suggests that heat treatments below 840 °C are a suitable method to improve the crystallinity and mechanical properties of nanocomposite Mo2BC coatings. © 2018
    view abstractdoi: 10.1016/j.surfcoat.2018.06.006
  • 2017 • 361 Coarsening of Y-rich oxide particles in 9%Cr-ODS Eurofer steel annealed at 1350 °C
    Sandim, M.J.R. and Souza Filho, I.R. and Bredda, E.H. and Kostka, A. and Raabe, D. and Sandim, H.R.Z.
    Journal of Nuclear Materials 484 283-287 (2017)
    Oxide-dispersion strengthened (ODS) Eurofer steel is targeted for structural applications in future fusion nuclear reactors. Samples were cold rolled down to 80% reduction in thickness and annealed at 1350 °C up to 8 h. The microstructural characterization was performed using Vickers microhardness testing, electron backscatter diffraction, scanning and scanning transmission electron microscopies. Experimental results provide evidence of coarsening of the Y-rich oxide particles in ODS-Eurofer steel annealed at 1350 °C within delta ferrite phase field. © 2016 Elsevier B.V.
    view abstractdoi: 10.1016/j.jnucmat.2016.12.025
  • 2017 • 360 Complexion-mediated martensitic phase transformation in Titanium
    Zhang, J. and Tasan, C.C. and Lai, M.J. and Dippel, A.-C. and Raabe, D.
    Nature Communications 8 (2017)
    The most efficient way to tune microstructures and mechanical properties of metallic alloys lies in designing and using athermal phase transformations. Examples are shape memory alloys and high strength steels, which together stand for 1,500 million tons annual production. In these materials, martensite formation and mechanical twinning are tuned via composition adjustment for realizing complex microstructures and beneficial mechanical properties. Here we report a new phase transformation that has the potential to widen the application window of Ti alloys, the most important structural material in aerospace design, by nanostructuring them via complexion-mediated transformation. This is a reversible martensitic transformation mechanism that leads to a final nanolaminate structure of α″ (orthorhombic) martensite bounded with planar complexions of athermal ω (a-ω hexagonal). Both phases are crystallographically related to the parent β (BCC) matrix. As expected from a planar complexion, the a-ω is stable only at the hetero-interface. © The Author(s) 2017.
    view abstractdoi: 10.1038/ncomms14210
  • 2017 • 359 Confined chemical and structural states at dislocations in Fe-9wt%Mn steels: A correlative TEM-atom probe study combined with multiscale modelling
    Kwiatkowski da Silva, A. and Leyson, G. and Kuzmina, M. and Ponge, D. and Herbig, M. and Sandlöbes, S. and Gault, B. and Neugebauer, J. and Raabe, D.
    Acta Materialia 124 305-315 (2017)
    We investigated a high-purity cold-rolled martensitic Fe-9wt%Mn alloy. Tensile tests performed at room temperature after tempering for 6 h at 450 °C showed discontinuous yielding. Such static strain ageing phenomena in Fe are usually associated with the segregation of interstitial elements such as C or N to dislocations. Here we show by correlative transmission electron microscopy (TEM)/atom probe tomography (APT) experiments that in this case Mn segregation to edge dislocations associated with the formation of confined austenitic states causes similar effects. The local chemical composition at the dislocation cores was investigated for different tempering temperatures by APT relative to the adjacent bcc matrix. In all cases the Mn partitioning to the dislocation core regions matches to the one between ferrite and austenite in thermodynamic equilibrium at the corresponding tempering temperature. Although a stable structural and chemical confined austenitic state has formed at the dislocation cores these regions do not grow further even upon prolonged tempering. Simulation reveals that the high Mn enrichment along the edge dislocation lines (25 at.%Mn at 450 °C) cannot be described merely as a Cottrell atmosphere formed by segregation driven by size interaction. Thermodynamic calculations based on a multiscale model indicate that these austenite states at the dislocation cores are subcritical and defect-stabilized by the compression stress field of the edge dislocations. Phenomenologically, these states are the 1D equivalent to the so-called complexions which have been extensively reported to be present at 2D defects, hence have been named linear complexions. © 2016 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2016.11.013
  • 2017 • 358 Dislocation activities at the martensite phase transformation interface in metastable austenitic stainless steel: An in-situ TEM study
    Liu, J. and Chen, C. and Feng, Q. and Fang, X. and Wang, H. and Liu, F. and Lu, J. and Raabe, D.
    Materials Science and Engineering A 703 236-243 (2017)
    Understanding the mechanism of martensitic transformation is of great importance in developing advanced high strength steels, especially TRansformation-Induced Plasticity (TRIP) steels. The TRIP effect leads to enhanced work-hardening rate, postponed onset of necking and excellent formability. In-situ transmission electron microscopy has been performed to systematically investigate the dynamic interactions between dislocations and α′ martensite at microscale. Local stress concentrations, e.g. from notches or dislocation pile-ups, render free edges and grain boundaries favorable nucleation sites for α′ martensite. Its growth leads to partial dislocation emission on two independent slip planes from the hetero-interface when the austenite matrix is initially free of dislocations. The kinematic analysis reveals that activating slip systems on two independent {111} planes of austenite are necessary in accommodating the interfacial mismatch strain. Full dislocation emission is generally observed inside of austenite regions that contain high density of dislocations. In both situations, phase boundary propagation generates large amounts of dislocations entering into the matrix, which renders the total deformation compatible and provide substantial strain hardening of the host phase. These moving dislocation sources enable plastic relaxation and prevent local damage accumulation by intense slipping on the softer side of the interfacial region. Thus, finely dispersed martensite distribution renders plastic deformation more uniform throughout the austenitic matrix, which explains the exceptional combination of strength and ductility of TRIP steels. © 2017 Elsevier B.V.
    view abstractdoi: 10.1016/j.msea.2017.06.107
  • 2017 • 357 Dislocation interaction and twinning-induced plasticity in face-centered cubic Fe-Mn-C micro-pillars
    Choi, W.S. and Sandlöbes, S. and Malyar, N.V. and Kirchlechner, C. and Korte-Kerzel, S. and Dehm, G. and De Cooman, B.C. and Raabe, D.
    Acta Materialia 132 162-173 (2017)
    Deformation twinning contributes to a high work-hardening rate through modification of the dislocation structure and a dynamic Hall-Petch effect in polycrystalline steel. Due to the well-defined compression axis and limited deformation volume of micro-pillars, micro-compression testing is a suitable method to investigate the mechanisms of deformation twinning and the interactions of dislocations with twin boundaries. The material investigated is an austenitic Fe-22 wt%Mn-0.6 wt%C twining-induced plasticity steel. Micro-pillars oriented preferentially for deformation twinning and dislocation glide are compressed and the activated deformation systems are characterized. We observe that deformation twinning induces higher flow stresses and a more unstable work-hardening behavior than dislocation glide, while dislocation glide dominated deformation results in a stable work-hardening behavior. The higher flow stresses and unstable work-hardening behavior in micro-pillars oriented for deformation twinning are assumed to be caused by the activation of secondary slip systems and accumulated plastic deformation. © 2017 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2017.04.043
  • 2017 • 356 Dislocation mechanisms and 3D twin architectures generate exceptional strength-ductility-toughness combination in CrCoNi medium-entropy alloy
    Zhang, Z. and Sheng, H. and Wang, Z. and Gludovatz, B. and Zhang, Z. and George, E.P. and Yu, Q. and Mao, S.X. and Ritchie, R.O.
    Nature Communications 8 (2017)
    Combinations of high strength and ductility are hard to attain in metals. Exceptions include materials exhibiting twinning-induced plasticity. To understand how the strength-ductility trade-off can be defeated, we apply in situ, and aberration-corrected scanning, transmission electron microscopy to examine deformation mechanisms in the medium-entropy alloy CrCoNi that exhibits one of the highest combinations of strength, ductility and toughness on record. Ab initio modelling suggests that it has negative stacking-fault energy at 0K and high propensity for twinning. With deformation we find that a three-dimensional (3D) hierarchical twin network forms from the activation of three twinning systems. This serves a dual function: conventional twin-boundary (TB) strengthening from blockage of dislocations impinging on TBs, coupled with the 3D twin network which offers pathways for dislocation glide along, and cross-slip between, intersecting TB-matrix interfaces. The stable twin architecture is not disrupted by interfacial dislocation glide, serving as a continuous source of strength, ductility and toughness. © 2017 The Author(s).
    view abstractdoi: 10.1038/ncomms14390
  • 2017 • 355 Duplex stainless steel fabricated by selective laser melting - Microstructural and mechanical properties
    Hengsbach, F. and Koppa, P. and Duschik, K. and Holzweissig, M.J. and Burns, M. and Nellesen, J. and Tillmann, W. and Tröster, T. and Hoyer, K.-P. and Schaper, M.
    Materials and Design 133 136-142 (2017)
    In the scope of the present study, microstructural and mechanical characterizations of duplex stainless steel UNS S31803 processed by selective laser melting (SLM) are conducted. The findings shed light on the phase arrangement evolving in the as-built condition and in several heat-treated conditions. In the as-built condition, austenite formation is almost suppressed due to process-related high cooling rates. Therefore, several heat treatments ranging from 900 °C to 1200 °C for 5 min each were performed in order to adjust to the desired austenitic-ferritic microstructure. Results generated by transmission electron microscopy (TEM) reveal a high dislocation density induced during SLM fabrication, such that a recrystallized microstructure prevails after the heat treatment. Tensile tests display the severe impact of the heat treatment on the resulting mechanical response. The nearly complete ferritic as-built specimens obtain a higher ultimate tensile strength and a reduced elongation at fracture compared to the heat-treated specimens. © 2017 Elsevier Ltd
    view abstractdoi: 10.1016/j.matdes.2017.07.046
  • 2017 • 354 Electronic structure of metastable bcc Cu-Cr alloy thin films: Comparison of electron energy-loss spectroscopy and first-principles calculations
    Liebscher, C.H. and Freysoldt, C. and Dennenwaldt, T. and Harzer, T.P. and Dehm, G.
    Ultramicroscopy 178 96-104 (2017)
    Metastable Cu-Cr alloy thin films with nominal thickness of 300nm and composition of Cu67Cr33 (at%) are obtained by co-evaporation using molecular beam epitaxy. The microstructure, chemical phase separation and electronic structure are investigated by transmission electron microscopy (TEM). The thin film adopts the body-centered cubic crystal structure and consists of columnar grains with ~50nm diameter. Aberration-corrected scanning TEM in combination with energy dispersive X-ray spectroscopy confirms compositional fluctuations within the grains. Cu- and Cr-rich domains with composition of Cu85Cr15 (at%) and Cu42Cr58 (at%) and domain size of 1-5nm are observed. The alignment of the interface between the Cu- and Cr-rich domains shows a preference for (110)-type habit plane. The electronic structure of the Cu-Cr thin films is investigated by electron energy loss spectroscopy (EELS) and is contrasted to an fcc-Cu reference sample. The experimental EEL spectra are compared to spectra computed by density functional theory. The main differences between bcc-and fcc-Cu are related to differences in van Hove singularities in the electron density of states. In Cu-Cr solid solutions with bcc crystal structure a single peak after the L3-edge, corresponding to a van Hove singularity at the N-point of the first Brillouin zone is observed. Spectra computed for pure bcc-Cu and random Cu-Cr solid solutions with 10at% Cr confirm the experimental observations. The calculated spectrum for a perfect Cu50Cr50 (at%) random structure shows a shift in the van Hove singularity towards higher energy by developing a Cu-Cr d-band that lies between the delocalized d-bands of Cu and Cr. © 2016 Elsevier B.V.
    view abstractdoi: 10.1016/j.ultramic.2016.07.011
  • 2017 • 353 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 • 352 Formation Mechanism of Laser-Synthesized Iron-Manganese Alloy Nanoparticles, Manganese Oxide Nanosheets and Nanofibers
    Zhang, D. and Ma, Z. and Spasova, M. and Yelsukova, A.E. and Lu, S. and Farle, M. and Wiedwald, U. and Gökce, B.
    Particle and Particle Systems Characterization 34 1600225 (2017)
    Laser ablation in liquids (LAL) has emerged as a versatile approach for the synthesis of alloy particles and oxide nanomaterials. However, complex chemical reactions often take place during synthesis due to inevitable atomization and ionization of the target materials and decomposition/hydrolysis of solvent/solution molecules, making it difficult to understand the particle formation mechanisms. In this paper, a possible route for the formation of FeMn alloy nanoparticles as well as MnOx nanoparticles, -sheets, and -fibers by LAL is presented. The observed structural, compositional, and morphological variations are clarified by transmission electron microscopy (TEM). The studies suggest that a reaction between Mn atoms and Fe ions followed by surface oxidation result in nonstoichiometric synthesis of Fe-rich FeMn@FeMn2O4 core-shell alloy particles. Interestingly, a phase transformation from Mn3O4 to Mn2O3 and finally to Ramsdellite γ-MnO2 is accompanied by a morphology change from nanosheets to nanofibers in gradually increasing oxidizing environments. High-resolution TEM images reveal that the particle-attachment mechanism dominates the growth of different manganese oxides. © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/ppsc.201600225
  • 2017 • 351 Formation of nanometer-sized Cu-Sn-Se particles in Cu2ZnSnSe4 thin-films and their effect on solar cell efficiency
    Schwarz, T. and Cojocaru-Mirédin, O. and Mousel, M. and Redinger, A. and Raabe, D. and Choi, P.-P.
    Acta Materialia 132 276-284 (2017)
    Atom probe tomography and transmission electron microscopy are used to study the formation of nano-sized Cu-Sn-Se particles in Cu2ZnSnSe4 thin-films. For a Cu-rich precursor, which was deposited at 320 °C under Cu- and Zn-rich growth conditions, Cu2-xSe grains at the surface are detected. During annealing the precursor at 500 °C in a SnSe + Se atmosphere most of the Cu2-xSe is transformed to Cu2ZnSnSe4 via the consumption of excessive ZnSe and incorporation of Sn. However, atom probe tomography studies also reveal the formation of various nanometer-sized Cu-Sn-Se particles close to the CdS/Cu2ZnSnSe4 interface. One of those particles has a composition close to the Cu2SnSe3 compound. This phase has a smaller band gap than Cu2ZnSnSe4 and is proposed to lead to a significant drop in the open-circuit voltage and could be the main cause for a detrimental p-n junction and the zero efficiency of the final device. Possible effects of the other phases on solar cell performance and formation mechanisms are discussed as well. © 2017 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2017.04.056
  • 2017 • 350 Functional NiTi grids for in situ straining in the TEM
    Schürmann, U. and Chluba, C. and Wolff, N. and Smazna, D. and Lima de Miranda, R. and Junker, P. and Adelung, R. and Quandt, E. and Kienle, L.
    Ultramicroscopy 182 10-16 (2017)
    In situ measurements are a pivotal extension of conventional transmission electron microscopy (TEM). By means of the shape memory alloy NiTi thin film Functional Grids were produced for in situ straining as alternative or at least complement of expensive commercial holders. Due to the martensite-austenite transition temperature straining effects can be observed by use of customary heating holders in the range of 50 to 100  °C. The grids can be produced in diversified designs to fit for different strain situations. Micro tensile tests were performed and compared with finite element simulations to estimate the applied forces on the sample and to predict the functionality of different grid designs. As a first example of this Functional Grid technology, we demonstrate the impact of applying a strain to a network of ZnO tetrapods. © 2017 Elsevier B.V.
    view abstractdoi: 10.1016/j.ultramic.2017.06.003
  • 2017 • 349 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 • 348 In-situ tracking the structural and chemical evolution of nanostructured CuCr alloys
    Zhang, Z. and Guo, J. and Dehm, G. and Pippan, R.
    Acta Materialia 138 42-51 (2017)
    We report the thermal stability of supersaturated CuCr nanocrystallines alloys at the atomic resolution using modern spherical aberration-corrected transmission electron microscopy (TEM) via performing in-situ structural and spectroscopy experiments. It is found that CuCr nanocrystallines are not only subjected to a structural change but also undergo a chemical evolution upon annealing. Chemical destabilization of supersaturated CuCr nanocrystallines occurs at a quite low temperature. Heating triggers a rapid separation of Cu and Cr grains at the forced intermixing zone, accompanied by an obvious decrease of average interface width whereas the grain growth is not significant. Elemental profiles and images recorded in real time reveal that the local compositions vary with heating, which in turn permits to derive the concentration of excess vacancy generated by deformation and observe its evolution with temperature, further to analyze the dynamic behavior in nanocrystalline materials. Electronic structure changes at the interface forced intermixing zone are revealed by the fine structure analysis. The study uncovers the interplay between the thermal stability and chemical decomposition process of bulk nanostructured materials in real-time. © 2017
    view abstractdoi: 10.1016/j.actamat.2017.07.039
  • 2017 • 347 Influence of rhenium on γ′-strengthened cobalt-base superalloys
    Kolb, M. and Zenk, C.H. and Kirzinger, A. and Povstugar, I. and Raabe, D. and Neumeier, S. and Göken, M.
    Journal of Materials Research 32 2551-2559 (2017)
    The element Re is known to be a very potent strengthener concerning the creep properties of Ni-base superalloys. In this paper the influence of Re on the properties of new γ′-strengthened Co-base superalloys is addressed. Atom probe tomography reveals that Re partitions preferentially to the γ phase, but not as pronounced as in ni-base superalloys. Nanoindentation and micro-pillar compression tests of the γ′ phase indicate an increase of the hardness and the critical resolved shear stress caused by a considerable concentration of Re in the γ′ phase. Creep investigations show that the positive effect of Re is by far not as pronounced as in Ni-base superalloys. Several effects, which can contribute to this behavior, such as the lower Re concentration in γ and hence a slightly reduced effective diffusion coefficient, a smaller diffusion barrier of Re in Co compared to Ni, a slightly lower lattice misfit and γ′ volume fraction of the Re-containing alloy, are discussed. © Materials Research Society 2017.
    view abstractdoi: 10.1557/jmr.2017.242
  • 2017 • 346 In–situ TEM study of diffusion kinetics and electron irradiation effects on the Cr phase separation of a nanocrystalline Cu–4 at.% Cr thin film alloy
    Harzer, T.P. and Duarte, M.J. and Dehm, G.
    Journal of Alloys and Compounds 695 1583-1590 (2017)
    The Cr phase separation process and kinetics of a metastable Cu96Cr4 alloy film were investigated by isothermal annealing at different temperatures of up to 500 °C using transmission electron microscopy. It is shown that the Cr phase separation proceeds predominantly via enrichment of Cr at grain boundaries and grain boundary diffusion. Temperature dependent diffusion coefficients and the activation energy for grain boundary diffusion of Cr in face–centered cubic Cu are determined from analytical in–situ transmission electron microscopy experiments. In addition, the influence of electron beam irradiation on the diffusion kinetics is considered. © 2016 Elsevier B.V.
    view abstractdoi: 10.1016/j.jallcom.2016.10.302
  • 2017 • 345 Kinetics and crystallization path of a Fe-based metallic glass alloy
    Duarte, M.J. and Kostka, A. and Crespo, D. and Jimenez, J.A. and Dippel, A.-C. and Renner, F.U. and Dehm, G.
    Acta Materialia 127 341-350 (2017)
    The thermal stability and the quantification of the different transformation processes involved in the overall crystallization of the Fe50Cr15Mo14C15B6 amorphous alloy were investigated by several characterization techniques. Formation of various metastable and stable phases during the devitrification process in the sequence α-Fe, χ-Cr6Fe18Mo5, M23(C,B)6, M7C3, η-Fe3Mo3C and FeMo2B2 (with M = Fe, Cr, Mo), was observed by in-situ synchrotron high energy X-ray diffraction and in-situ transmission electron microscopy. By combining these techniques with differential scanning calorimetry data, the crystallization states and their temperature range of stability under continuous heating were related with the evolution of the crystallized fraction and the phase sequence as a function of temperature, revealing structural and chemical details of the different transformation mechanisms. © 2017 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2017.01.031
  • 2017 • 344 Laser-induced growth of YVO4:Eu3+ nanoparticles from sequential flowing aqueous suspension
    Wang, H. and Lau, M. and Sannomiya, T. and Gökce, B. and Barcikowski, S. and Odawara, O. and Wada, H.
    RSC Advances 7 9002-9008 (2017)
    Ligand-free lanthanide ion-doped oxide nanoparticles have critical biological applications. An environmentally friendly and chemically green synthesis of YVO4:Eu3+ nanoparticles with high crystallinity is achieved using a physical method, laser irradiation from sequential flowing aqueous suspension in a free liquid reactor. The fabricated nanoparticles have an ovoid or spindle shape depending on the number of laser irradiation cycles. A transmission electron microscopy study showed that spindle-like particles are single-crystalline with high crystallinity, which is beneficial for high luminescence efficiency. Strong light emission even from a single particle was confirmed by cathodoluminescence mapping. A possible mechanism of nanoparticle formation was proposed as follows. Primary nanocrystals were produced from the plasma plume and self-assembled into ovoid-like nanoparticles via oriented attachment. After several cycles of laser irradiation, we observed spindle-like nanoparticles that were much longer than the ovoid-like particles. The spindle-like nanoparticles grew as a result of the diffusion and coalescence of the ovoid-like nanoparticles during repetitive laser irradiation. These findings provide useful information for the formation of ligand-free luminescent nanoparticles with different sizes based on YVO4. © The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c6ra28118d
  • 2017 • 343 Linear and nonlinear behaviour of near-IR intersubband transitions of cubic GaN/AlN multi quantum well structures
    Wecker, T. and Jostmeier, T. and Rieger, T. and Neumann, E. and Pawlis, A. and Betz, M. and Reuter, D. and As, D.J.
    Journal of Crystal Growth 477 149-153 (2017)
    The linear and nonlinear behaviour of intersubband transitions of cubic GaN/AlN multi quantum well (QW) structures in the IR spectral region is investigated. In this study photoluminescence, IR absorption as well as pump-probe measurements are done. Two cubic GaN/AlN multi quantum wells with Si content of NSi ~1019 cm-3 in the cubic GaN quantum wells were grown on 3C-SiC (001) substrate by radio-frequency plasma-assisted molecular beam epitaxy. A broad IR absorption with a FWHM of 370meV was found with a maximum at 0.7eV, corresponding to the intersubband transition of the multi quantum wells. The nonlinear optical measurement reveals a clear change of transmission for a pump pulse with an angle of incidence of 65°. Furthermore, transmission electron microscopy measurements are used to determine the real layer thicknesses. These thickness values are exploited in the calculation with the Schrödinger-Poisson solver nextnano3. The simulated transition energies agree very well with the experimental data for the photoluminescence and the absorption measurement. © 2017 Elsevier B.V.
    view abstractdoi: 10.1016/j.jcrysgro.2017.01.022
  • 2017 • 342 Maintaining strength in supersaturated copper–chromium thin films annealed at 0.5 of the melting temperature of Cu
    Raghavan, R. and Harzer, T.P. and Djaziri, S. and Hieke, S.W. and Kirchlechner, C. and Dehm, G.
    Journal of Materials Science 52 913-920 (2017)
    The thermal stability of evaporated copper–chromium alloy films was studied by correlating hardness trends from nanoindentation to nanostructural–compositional changes from transmission electron microscopy. In particular, the hardness evolution with ageing time at ambient and elevated temperatures of two compositions, dilute (Cu96Cr4) and chromium-rich (Cu67Cr33) solutions, was studied. Due to the negligible mutual miscibility of copper and chromium, the chosen solid solutions are trapped in metastable states as supersaturated solid solutions with face-centred cubic and body-centred cubic phases. Nano-mechanical probing of the nanostructural evolution as a function of temperature provided interesting insights into the phase separation of these systems. © 2016, Springer Science+Business Media New York.
    view abstractdoi: 10.1007/s10853-016-0386-6
  • 2017 • 341 On the threshold for ion track formation in CaF2
    Karlušić, M. and Ghica, C. and Negrea, R.F. and Siketić, Z. and Jakšić, M. and Schleberger, M. and Fazinić, S.
    New Journal of Physics 19 (2017)
    There is an ongoing debate regarding the mechanism of swift heavy ion (SHI) track formation in CaF2. The objective of this study is to shed light on this important topic using a range of complementary experimental techniques. Evidence of the threshold for ion track formation being below 3 keV nm-1 is provided by both transmission electron microscopy (TEM) and Rutherford backscattering spectroscopy in the channelling mode, which has direct consequences for the validity of models describing the response of CaF2 to SHI irradiation. Furthermore, information about the elemental composition within the ion tracks is obtained using scanning TEM, electron energy loss spectroscopy, and with respect to the stoichiometry of the materials surface by in situ time of flight elastic recoil detection analysis. Advances in the analyses of the experimental data presented here pave the way for a better understanding of the ion track formation. © 2017 IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.
    view abstractdoi: 10.1088/1367-2630/aa5914
  • 2017 • 340 Photocatalytic Polymerization of 3,4-Ethylenedioxythiophene over Cesium Lead Iodide Perovskite Quantum Dots
    Chen, K. and Deng, X. and Dodekatos, G. and Tüysüz, H.
    Journal of the American Chemical Society 139 12267-12273 (2017)
    The outstanding performance of halide perovskites in optoelectronic applications can be partly attributed to their high absorption coefficient and long carrier lifetime, which are also desirable for photocatalysts. Herein, we report that cesium lead iodide perovskite quantum dots (CsPbI3 QDs) can be used as catalysts to promote the polymerization of 2,2′,5′,2″-ter-3,4-ethylenedioxythiophene under visible light illumination while preserving the quantum dot in the desirable cubic crystal phase. Simultaneously, the generated conducting poly(3,4-ethylenedioxythiophene), PEDOT, encapsulates and stabilizes the morphology of the CsPbI3 QDs. The photocatalytic polymerization clearly depends on the concentration of the CsPbI3 QDs, and the CsPbI3 QDs maintain the desirable perovskite phase when the concentration of the QD increases. Molecular oxygen and 1,4-benzoquinone can serve as electron acceptors during the photocatalytic polymerization reaction. When molecular oxygen is used, the structure of the CsPbI3 QD transforms from cubic to orthorhombic, while usage of 1,4-benzoquinone preserves the cubic phase of CsPbI3 QD. This novel approach enables the one-step formation of CsPbI3/PEDOT composite, which could be promising for the preparation of novel optoelectronic materials and high performance devices. © 2017 American Chemical Society.
    view abstractdoi: 10.1021/jacs.7b06413
  • 2017 • 339 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 • 338 Reasons for the superior mechanical properties of medium-entropy CrCoNi compared to high-entropy CrMnFeCoNi
    Laplanche, G. and Kostka, A. and Reinhart, C. and Hunfeld, J. and Eggeler, G. and George, E.P.
    Acta Materialia 128 292-303 (2017)
    The tensile properties of CrCoNi, a medium-entropy alloy, have been shown to be significantly better than those of CrMnFeCoNi, a high-entropy alloy. To understand the deformation mechanisms responsible for its superiority, tensile tests were performed on CrCoNi at liquid nitrogen temperature (77 K) and room temperature (293 K) and interrupted at different strains. Microstructural analyses by transmission electron microscopy showed that, during the early stage of plasticity, deformation occurs by the glide of 1/2&lt;110&gt; dislocations dissociated into 1/6&lt;112&gt; Shockley partials on {111} planes, similar to the behavior of CrMnFeCoNi. Measurements of the partial separations yielded a stacking fault energy of 22 ± 4 mJ m−2, which is ∼25% lower than that of CrMnFeCoNi. With increasing strain, nanotwinning appears as an additional deformation mechanism in CrCoNi. The critical resolved shear stress for twinning in CrCoNi with 16 μm grain size is 260 ± 30 MPa, roughly independent of temperature, and comparable to that of CrMnFeCoNi having similar grain size. However, the yield strength and work hardening rate of CrCoNi are higher than those of CrMnFeCoNi. Consequently, the twinning stress is reached earlier (at lower strains) in CrCoNi. This in turn results in an extended strain range where nanotwinning can provide high, steady work hardening, leading to the superior mechanical properties (ultimate strength, ductility, and toughness) of medium-entropy CrCoNi compared to high-entropy CrMnFeCoNi. © 2017 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2017.02.036
  • 2017 • 337 Role of Nanostructuring and Microstructuring in Silver Antimony Telluride Compounds for Thermoelectric Applications
    Cojocaru-Mirédin, O. and Abdellaoui, L. and Nagli, M. and Zhang, S. and Yu, Y. and Scheu, C. and Raabe, D. and Wuttig, M. and Amouyal, Y.
    ACS Applied Materials and Interfaces 9 14779-14790 (2017)
    Thermoelectric (TE) materials are of utmost significance for conversion of heat flux into electrical power in the low-power regime. Their conversion efficiency depends strongly on the microstructure. AgSbTe2-based compounds are high-efficiency TE materials suitable for the mid-temperature range. Herein, we explore an Ag16.7Sb30Te53.3 alloy (at %) subjected to heat treatments at 380 °C for different durations aimed at nucleation and coarsening of Sb2Te3-precipitates. To characterize the Sb2Te3-precipitation, we use a set of methods combining thermal and electrical measurements in concert with transmission electron microscopy and atom probe tomography. We find correlations between the measured TE transport coefficients and the applied heat treatments. Specifically, the lowest electrical and thermal conductivity values are obtained for the as-quenched state, whereas the highest values are observed for alloys aged for 8 h. In turn, long-term heat treatments result in intermediate values of transport coefficients. We explain these findings in terms of interplay between precipitate formation and variations in the matrix composition, highlighting the importance of thermal stability of the material under service conditions. © 2017 American Chemical Society.
    view abstractdoi: 10.1021/acsami.7b00689
  • 2017 • 336 Room temperature deformation of LPSO structures by non-basal slip
    Chen, R. and Sandlöbes, S. and Zeng, X. and Li, D. and Korte-Kerzel, S. and Raabe, D.
    Materials Science and Engineering A 682 354-358 (2017)
    We investigated the deformation mechanisms of long period stacking ordered (LPSO) structures in an extruded Mg97Y2Zn1 (at%) alloy. Tensile deformation was performed in such a way that basal slip and kink band formation were inhibited. Slip trace analysis and transmission electron microscopy reveal a predominant activity of non-basal < a&gt; slip. © 2016 Elsevier B.V.
    view abstractdoi: 10.1016/j.msea.2016.11.056
  • 2017 • 335 Stability, phase separation and oxidation of a supersaturated nanocrystalline Cu-33 at.% Cr thin film alloy
    Harzer, T.P. and Dehm, G.
    Thin Solid Films 623 48-58 (2017)
    A binary nanocrystalline Cu67Cr33 thin film alloy consisting of columnar grains was synthesized via co-evaporation of the constituent elements under non-equilibrium ultra-high vacuum conditions using molecular beam epitaxy. In the as-deposited state, the alloy film is a supersaturated solid solution with a single-phase body-centered cubic structure. In order to study the thermal stability of the microstructure and phase separation behavior towards the two phase equilibrium structure, isothermal annealing experiments in a temperature range of 150 °C – 500 °C were conducted inside a transmission electron microscope and compared to data obtained by X-ray diffraction under protective N2 atmosphere. It is shown that the single-phase nature of the alloy film is maintained for annealing temperatures of ≤ 300 °C, whereas heat treatment at temperatures of ≥ 400 °C results in the formation of a second phase, i.e. the equilibrium face-centered cubic phase of Cu. Phase separation proceeds predominantly by a spinodal-type decomposition process but a simultaneous diffusion of Cr along the columnar grain boundaries to the surface of the alloy film is observed as well. Temperature dependent diffusion coefficients for volume and grain boundary diffusion along with the activation energy for volume diffusion of Cr within the crystal lattice of the alloy film in a temperature range between 400 °C – 500 °C are determined from analytical in situ transmission electron microscopy experiments. Moreover, grain boundary diffusion of Cr leads to the growth of an external Cr-rich oxide scale. It is found that the growth kinetics of this oxide scale exhibits a transition from a linear to a nearly parabolic growth rate. © 2016 Elsevier B.V.
    view abstractdoi: 10.1016/j.tsf.2016.12.048
  • 2017 • 334 Surface optical phonon propagation in defect modulated nanowires
    Venkatesan, S. and Mancabelli, T. and Krogstrup, P. and Hartschuh, A. and Dehm, G. and Scheu, C.
    Journal of Applied Physics 121 (2017)
    Planar defects, such as stacking faults and twins, are the most common defects in III-V semiconductor nanowires. Here we report on the effect of surface perturbation caused by twin planes on surface optical (SO) phonon modes. Self-catalyzed GaAs nanowires with varying planar defect density were grown by molecular beam epitaxy and investigated by Raman spectroscopy and transmission electron microscopy (TEM). SO phonon peaks have been detected, and the corresponding spatial period along the nanowire axis were measured to be 1.47 μm (±0.47 μm) and 446 nm (±35 nm) for wires with twin densities of about 0.6 (±0.2) and 2.2 (±0.18) per micron. For the wires with extremely high density of twins, no SO phonon peaks were detected. TEM analysis of the wires reveal that the average distance between the defects are in good agreement with the SO phonon spatial period determined by Raman spectroscopy. © 2017 Author(s).
    view abstractdoi: 10.1063/1.4976564
  • 2017 • 333 Tailoring microstructure, mechanical and tribological properties of NiTi thin films by controlling in-situ annealing temperature
    Momeni, S. and Biskupek, J. and Tillmann, W.
    Thin Solid Films 628 13-21 (2017)
    Magnetron sputtered near equiatomic NiTi thin films were deposited on Si (100) and hot work tool steel substrates. The deposited thin films were in-situ annealed at four different temperatures viz., 80 °C, 305 °C, 425 °C, and 525 °C. The effect of the in-situ annealing temperature on the microstructure of the film, the morphology, as well as mechanical and tribological properties was studied using X-ray diffraction, synchrotron diffraction, transmission electron microscopy, energy dispersive X-ray spectroscopy, ball-on-disc, scratch test, and three dimensional optical microscopy. The obtained results revealed how the variation of in-situ annealing temperature affects the crystallization, microstructure evolution, as well as mechanical and tribological properties of NiTi thin films. © 2017
    view abstractdoi: 10.1016/j.tsf.2017.02.052
  • 2016 • 332 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 • 331 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 • 330 Barrierless growth of precursor-free, ultrafast laser-fragmented noble metal nanoparticles by colloidal atom clusters - A kinetic in situ study
    Jendrzej, S. and Gökce, B. and Amendola, V. and Barcikowski, S.
    Journal of Colloid and Interface Science 463 299-307 (2016)
    Unintended post-synthesis growth of noble metal colloids caused by excess amounts of reactants or highly reactive atom clusters represents a fundamental problem in colloidal chemistry, affecting product stability or purity. Hence, quantified kinetics could allow defining nanoparticle size determination in dependence of the time. Here, we investigate in situ the growth kinetics of ps pulsed laser-fragmented platinum nanoparticles in presence of naked atom clusters in water without any influence of reducing agents or surfactants. The nanoparticle growth is investigated for platinum covering a time scale of minutes to 50 days after nanoparticle generation, it is also supplemented by results obtained from gold and palladium. Since a minimum atom cluster concentration is exceeded, a significant growth is determined by time resolved UV/Vis spectroscopy, analytical disc centrifugation, zeta potential measurement and transmission electron microscopy. We suggest a decrease of atom cluster concentration over time, since nanoparticles grow at the expense of atom clusters. The growth mechanism during early phase (<1. day) of laser-synthesized colloid is kinetically modeled by rapid barrierless coalescence. The prolonged slow nanoparticle growth is kinetically modeled by a combination of coalescence and Lifshitz-Slyozov-Wagner kinetic for Ostwald ripening, validated experimentally by the temperature dependence of Pt nanoparticle size and growth quenching by Iodide anions. © 2015.
    view abstractdoi: 10.1016/j.jcis.2015.10.032
  • 2016 • 329 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 • 328 Characterization of the oleic acid/iron oxide nanoparticle interface by magnetic resonance
    Masur, S. and Zingsem, B. and Marzi, T. and Meckenstock, R. and Farle, M.
    Journal of Magnetism and Magnetic Materials 415 8-12 (2016)
    The synthesis of colloidal nanoparticles involves surfactant molecules, which bind to the particle surface and stabilize nanoparticles against aggregation. In many cases these protecting shells also can be used for further functionalization. In this study, we investigated monodisperse single crystalline iron oxide core/shell nanoparticles (FexOy-NPs) in situ covered with an oleic acid layer which showed two electron spin resonance (ESR) signals. The nanoparticles with the ligands attached were characterized by transmission electron microscopy (TEM) and ferro- and paramagnetic resonance (FMR, EPR). Infrared spectroscopy confirmed the presence of the functional groups and revealed that the oleic acid (OA) is chemisorbed as a carboxylate on the iron oxide and is coordinated symmetrically to the oxide atoms. We show that the EPR signal of the OA ligand molecule can be used as a local probe to determine the temperature changes at the surface of the nanoparticle. © 2016 Published by Elsevier B.V.
    view abstractdoi: 10.1016/j.jmmm.2016.03.045
  • 2016 • 327 Continuous synthesis of nanostructured silica based materials in a gas-liquid segmented flow tubular reactor
    Knossalla, J. and Mezzavilla, S. and Schüth, F.
    New Journal of Chemistry 40 4361-4366 (2016)
    A continuous synthesis of several spherical silica structures-by means of a gas-liquid segmented flow tubular reactor-is reported. Specifically, as proof of concept, we showed that 300-400 nm mesoporous core-shell spheres (SiO2@mSiO2), mesoporous spheres (mSiO2) as well gold-encapsulated spheres (Au@SiO2) can be effectively produced in a continuous manner in a tubular reactor. Thus, the successful conversion of classical batch methods to continuous processes opens new possibilities for the up-scaled synthesis of advanced nanostructured materials. © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2016.
    view abstractdoi: 10.1039/c5nj03033a
  • 2016 • 326 Cr2O3 Nanoparticles on Ba5Ta4O15 as a Noble-Metal-Free Oxygen Evolution Co-Catalyst for Photocatalytic Overall Water Splitting
    Soldat, J. and Busser, G.W. and Muhler, M. and Wark, M.
    ChemCatChem 8 153-156 (2016)
    The (1 1 1)-layered perovskite material Ba5Ta4O15 represents a suitable photoabsorber with remarkable photocatalytic activity in overall water splitting. We are the first to demonstrate overall water splitting without the presence of a noble-metal-based co-catalyst over this catalyst. The photocatalytic activity of Ba5Ta4O15 was investigated by overall water splitting after reductive photodeposition of amorphous Cr2O3. The formation of Cr2O3 nanoparticles for water splitting was evidenced by X-ray photoelectron spectroscopy and transmission electron microscopy. The reductive photodeposition of very low amounts of Cr2O3 on Ba5Ta4O15 induces stable rates in overall water splitting up to 465 μmol h-1 H2 and 228 μmol h-1 O2. © 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/cctc.201500977
  • 2016 • 325 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 • 324 Double minimum creep of single crystal Ni-base superalloys
    Wu, X. and Wollgramm, P. and Somsen, C. and Dlouhy, A. and Kostka, A. and Eggeler, G.
    Acta Materialia 112 242-260 (2016)
    Low temperature (750°C) and high stress (800 MPa) creep curves of single crystal superalloy ERBO/1 tensile specimens loaded in the (001) direction show two creep rate minima. Strain rates decrease towards a first sharp local creep rate minimum at 0.1% strain (reached after 30 min). Then deformation rates increase and reach an intermediate maximum at 1% (reached after 1.5 h). Subsequently, strain rates decrease towards a global minimum at 5% (260 h), before tertiary creep (not considered in the present work) leads to final rupture. We combine high resolution miniature creep testing with diffraction contrast transmission electron microscopy and identify elementary processes which govern this double-minimum type of creep behavior. We provide new quantitative information on the evolution of microstructure during low temperature and high stress creep, focusing on γ-channel dislocation activity and stacking fault shear of the γ′-phase. We discuss our results in the light of previous work published in the literature and highlight areas in need of further work. © 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2016.04.012
  • 2016 • 323 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 • 322 Efficient synthesis of polyoxazoline-silica hybrid nanoparticles by using the "grafting-onto" approach
    Bissadi, G. and Weberskirch, R.
    Polymer Chemistry 7 1271-1280 (2016)
    Well-defined silica poly(2-methyl-2-oxazoline) nanoparticles were prepared via the "grafting to" method employing either click chemistry or silane coupling using different reaction conditions. In the first approach, alkyne-functionalized poly(2-methyl-2-oxazoline), P1, was prepared by ring opening cationic polymerization and clicked on azide-functionalized silica nanoparticles (SNPs), which led to the fabrication of hybrid nanoparticles. In the second approach, trimethoxysilane-functionalized poly(2-methyl-2-oxazoline), P2, was prepared similar to P1 and grafted on the surface of SNPs using coupling reactions between trimethoxysilane and hydroxyl groups of the silica nanoparticle. Hybrid particles were characterized by Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA), transmission electron microscopy (TEM), and elemental analysis (EA). The grafting density ranged from 0.183 chains per nm2 for the click chemistry approach up to 0.45 chains per nm2 when using trimethoxysilane-functionalized P2 in acetonitrile at 80 °C. The water-in-oil microemulsion approach resulted still in a relatively high grafting density of 0.353 chains per nm2 and has the advantage of a one-step process and mild reaction conditions. © The Royal Society of Chemistry 2016.
    view abstractdoi: 10.1039/c5py01775k
  • 2016 • 321 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 • 320 Formation of polyoxazoline-silica nanoparticles: Via the surface-initiated cationic polymerization of 2-methyl-2-oxazoline
    Bissadi, G. and Weberskirch, R.
    Polymer Chemistry 7 5157-5168 (2016)
    Well-defined polyoxazoline-silica hybrid nanoparticles were prepared by coating silica nanoparticles (SNPs) with poly(2-methyl-2-oxazoline) using a surface-initiated cationic ring-opening polymerization process. First, reverse microemulsion was used to synthesize monodisperse SNPs followed by immobilizing (chloromethyl)phenylethyl)trimethoxysilane on the surface of the nanoparticles acting as an initiator. The grafting density of the polymeric shell was controlled by varying the polymerization time, PSNPs-A, and the monomer/initiator ratio concentration, PSNPs-B. Hybrid nanoparticles were characterized by Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA), and transmission electron microscopy (TEM). The molecular weight and polydispersity indices of the polymer chains were determined by size exclusion chromatography (SEC) after etching the silica core. The hybrid nanoparticles were further functionalized with fluorescein isothiocyanate (FITC) and folic acid (FA) as a fluorescence imaging molecule and a cancer-targeting ligand, respectively. Moreover, hybrid nanoparticles with Rubpy as a fluorophore encapsulated in the silica core and the poly(2-methyl-2-oxazoline) shell were prepared. © 2016 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c6py01034b
  • 2016 • 319 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 • 318 Growth of bainitic ferrite and carbon partitioning during the early stages of bainite transformation in a 2 mass% silicon steel studied by in situ neutron diffraction, TEM and APT
    Timokhina, I.B. and Liss, K.D. and Raabe, D. and Rakha, K. and Beladi, H. and Xiong, X.Y. and Hodgson, P.D.
    Journal of Applied Crystallography 49 399-414 (2016)
    In situ neutron diffraction, transmission electron microscopy (TEM) and atom probe tomography (APT) have been used to study the early stages of bainite transformation in a 2 mass% Si nano-bainitic steel. It was observed that carbon redistribution between the bainitic ferrite and retained austenite at the early stages of the bainite transformation at low isothermal holding occurred in the following sequence: (i) formation of bainitic ferrite nuclei within carbon-depleted regions immediately after the beginning of isothermal treatment; (ii) carbon partitioning immediately after the formation of bainitic ferrite nuclei but substantial carbon diffusion only after 33 min of bainite isothermal holding; (iii) formation of the carbon-enriched remaining austenite in the vicinity of bainitic laths at the beginning of the transformation; (iv) segregation of carbon to the dislocations near the austenite/ferrite interface; and (v) homogeneous redistribution of carbon within the remaining austenite with the progress of the transformation and with the formation of bainitic ferrite colonies. Bainitic ferrite nucleated at internal defects or bainite/austenite interfaces as well as at the prior austenite grain boundary. Bainitic ferrite has been observed in the form of an individual layer, a colony of layers and a layer with sideplates at the early stages of transformation. © 2016 International Union of Crystallography.
    view abstractdoi: 10.1107/S1600576716000418
  • 2016 • 317 Heat-Induced Phase Transformation of Three-Dimensional Nb3O7(OH) Superstructures: Effect of Atmosphere and Electron Beam
    Betzler, S.B. and Harzer, T. and Ciston, J. and Dahmen, U. and Dehm, G. and Scheu, C.
    Crystal Growth and Design 16 4309-4317 (2016)
    Nanostructured niobium oxides and hydroxides are potential candidates for photochemical applications due to their excellent optical and electronic properties. In the present work the thermal stability of Nb3O7(OH) superstructures prepared by a simple hydrothermal approach is investigated at the atomic scale. Transmission electron microscopy and electron energy-loss spectroscopy provide insights into the phase transformation occurring at elevated temperatures and probe the effect of the atmospheric conditions. In the presence of oxygen, H2O is released from the crystal at temperatures above 500 °C, and the crystallographic structure changes to H-Nb2O5. In addition to the high thermal stability of Nb3O7(OH), the morphology was found to be stable, and first changes in the form of a merging of nanowires are not observed until 850 °C. Under reducing conditions in a transmission electron microscope and during electron beam bombardment, an oxygen-deficient phase is formed at temperatures above 750 °C. This transformation starts with the formation of defects in the crystal lattice at 450 °C and goes along with the formation of pores in the nanowires which accommodate the volume differences of the two crystal phases. © 2016 American Chemical Society.
    view abstractdoi: 10.1021/acs.cgd.6b00386
  • 2016 • 316 High-Temperature Stable Ni Nanoparticles for the Dry Reforming of Methane
    Mette, K. and Kühl, S. and Tarasov, A. and Willinger, M.G. and Kröhnert, J. and Wrabetz, S. and Trunschke, A. and Scherzer, M. and Girgsdies, F. and Düdder, H. and Kähler, K. and Ortega, K.F. and Muhler, M. and Schlögl, R. an...
    ACS Catalysis 6 7238-7248 (2016)
    Dry reforming of methane (DRM) has been studied for many years as an attractive option to produce synthesis gas. However, catalyst deactivation by coking over nonprecious-metal catalysts still remains unresolved. Here, we study the influence of structural and compositional properties of nickel catalysts on the catalytic performance and coking propensity in the DRM. A series of bulk catalysts with different Ni contents was synthesized by calcination of hydrotalcite-like precursors NixMg0.67-xAl0.33(OH)2(CO3)0.17·mH2O prepared by constant-pH coprecipitation. The obtained Ni/MgAl oxide catalysts contain Ni nanoparticles with diameters between 7 and 20 nm. High-resolution transmission electron microscopy (HR-TEM) revealed a nickel aluminate overgrowth on the Ni particles, which could be confirmed by Fourier transform infrared (FTIR) spectroscopy. In particular, catalysts with low Ni contents (5 mol %) exhibit predominantly oxidic surfaces dominated by Ni2+ and additionally some isolated Ni0 sites. These properties, which are determined by the overgrowth, effectively diminish the formation of coke during the DRM, while the activity is preserved. A large (TEM) and dynamic (microcalorimetry) metallic Ni surface at high Ni contents (50 mol %) causes significant coke formation during the DRM. © 2016 American Chemical Society.
    view abstractdoi: 10.1021/acscatal.6b01683
  • 2016 • 315 High-yield and scalable synthesis of a Silicon/Aminosilane-functionalized Carbon NanoTubes/Carbon (Si/A-CNT/C) composite as a high-capacity anode for lithium-ion batteries
    Sehlleier, Y.H. and Dobrowolny, S. and Plümel, I. and Xiao, L. and Mahlendorf, F. and Heinzel, A. and Schulz, C. and Wiggers, H.
    Journal of Applied Electrochemistry 46 229-239 (2016)
    In this study, we present a novel anode architecture for high-performance lithium-ion batteries based on a Silicon/3-aminosilane-functionalized CNT/Carbon (Si/A-CNT/C) composite. A high-yield, low-cost approach has been developed to stabilize and support silicon as an active anode material. Silicon (Si) nanoparticles synthesized in a hot-wall reactor and aminosilane-functionalized carbon nanotubes (A-CNT) were dispersed in styrene and divinylbenzene (DVB) and subsequently polymerized forming a porous Si/A-CNT/C composite. Transmission electron microscopy showed that this method enables the interconnection and a uniform encapsulation of Si nanoparticles within a porous carbon matrix especially using aminosilane-functionalized CNT (A-CNT). Electrochemical characterization shows that this material can deliver a delithiation capacity of 2293 mAh g−1 with a capacity retention of more than 90 % after 200 cycles at lithiation and delithiation rate of 0.5 C. We conclude that the porous Si/A-CNT/C composite material can accommodate sufficient space for Si volume expansion and extraction and improve the electronic and ionic conduction. Excellent electrochemical performance during repeated cycling can thus be achieved. © 2015, Springer Science+Business Media Dordrecht.
    view abstractdoi: 10.1007/s10800-015-0897-x
  • 2016 • 314 Hollow Zn/Co Zeolitic Imidazolate Framework (ZIF) and Yolk-Shell Metal@Zn/Co ZIF Nanostructures
    Rösler, C. and Aijaz, A. and Turner, S. and Filippousi, M. and Shahabi, A. and Xia, W. and Van Tendeloo, G. and Muhler, M. and Fischer, R.A.
    Chemistry - A European Journal 22 3304-3311 (2016)
    Metal-organic frameworks (MOFs) feature a great possibility for a broad spectrum of applications. Hollow MOF structures with tunable porosity and multifunctionality at the nanoscale with beneficial properties are desired as hosts for catalytically active species. Herein, we demonstrate the formation of well-defined hollow Zn/Co-based zeolitic imidazolate frameworks (ZIFs) by use of epitaxial growth of Zn-MOF (ZIF-8) on preformed Co-MOF (ZIF-67) nanocrystals that involve in situ self-sacrifice/excavation of the Co-MOF. Moreover, any type of metal nanoparticles can be accommodated in Zn/Co-ZIF shells to generate yolk-shell metal@ZIF structures. Transmission electron microscopy and tomography studies revealed the inclusion of these nanoparticles within hollow Zn/Co-ZIF with dominance of the Zn-MOF as shell. Our findings lead to a generalization of such hollow systems that are working effectively to other types of ZIFs. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/chem.201503619
  • 2016 • 313 Incorporation of a Non-Natural Arginine Analogue into a Cyclic Peptide Leads to Formation of Positively Charged Nanofibers Capable of Gene Transfection
    Li, M. and Ehlers, M. and Schlesiger, S. and Zellermann, E. and Knauer, S.K. and Schmuck, C.
    Angewandte Chemie - International Edition 55 598-601 (2016)
    Functionalization of the tetracationic cyclic peptide (Ka)4 with a single guanidiniocarbonyl pyrrole (GCP) moiety, a weakly basic but highly efficient arginine analogue, completely alters the self-assembly properties of the peptide. In contrast to the nonfunctionalized peptide 2, which does not self-assemble, GCP-containing peptide 1 forms cationic nanofibers of micrometer length. These aggregates are efficient gene transfection vectors. DNA binds to their cationic surface and is efficiently delivered into cells. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/anie.201508714
  • 2016 • 312 Ionically Cross-Linked Shape Memory Polypropylene
    Raidt, T. and Hoeher, R. and Meuris, M. and Katzenberg, F. and Tiller, J.C.
    Macromolecules 49 6918-6927 (2016)
    An ionically cross-linked syndiotactic polypropylene (ix-sPP) was synthesized by subsequent grafting of maleic anhydride (MA) to the polymer followed by compounding with ZnO. The polymer network was investigated by X-ray scattering, transmission electron microscopy, and various thermal and mechanical analyses. The optimized polymer network with 1 wt % MA grafting and 20 wt % ZnO exhibits a crystal melting temperature of 125 °C and rubber elastic behavior up to 203 °C and becomes a viscous polymer melt at higher temperatures. This process is fully reversible. Further, ix-sPP is an exceptionally stable ionic polymer network that matches the stability of the respective covalently cross-linked polymer in terms of shape memory properties. Additionally, the ionic cross-linking affords thermoplastic processability and shape memory assisted self-healing. © 2016 American Chemical Society.
    view abstractdoi: 10.1021/acs.macromol.6b01387
  • 2016 • 311 Laser-induced incandescence from laser-heated silicon nanoparticles
    Menser, J. and Daun, K. and Dreier, T. and Schulz, C.
    Applied Physics B: Lasers and Optics 122 (2016)
    This work describes the application of temporally and spectrally resolved laser-induced incandescence to silicon nanoparticles synthesized in a microwave plasma reactor. Optical properties for bulk silicon presented in the literature were extended for nanostructured particles analyzed in this paper. Uncertainties of parameters in the evaporation submodel, as well as measurement noise, are incorporated into the inference process by Bayesian statistics. The inferred nanoparticle sizes agree with results from transmission electron microscopy, and the determined accommodation coefficient matches the values of the preceding study. © 2016, Springer-Verlag Berlin Heidelberg.
    view abstractdoi: 10.1007/s00340-016-6551-4
  • 2016 • 310 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 • 309 Magnetic microstructure in a stress-annealed Fe73.5Si15.5B7Nb3Cu1 soft magnetic alloy observed using off-axis electron holography and Lorentz microscopy
    Kovács, A. and Pradeep, K.G. and Herzer, G. and Raabe, D. and Dunin-Borkowski, R.E.
    AIP Advances 6 (2016)
    Fe-Si-B-Nb-Cu alloys are attractive for high frequency applications due to their low coercivity and high saturation magnetization. Here, we study the effect of stress annealing on magnetic microstructure in Fe73.5Si15.5B7Nb3Cu1 using off-axis electron holography and the Fresnel mode of Lorentz transmission electron microscopy. A stress of 50 MPa was applied to selected samples during rapid annealing for 4 s, resulting in uniaxial anisotropy perpendicular to the stress direction. The examination of focused ion beam milled lamellae prepared from each sample revealed a random magnetic domain pattern in the sample that had been rapidly annealed in the absence of stress, whereas a highly regular domain pattern was observed in the stress-annealed sample. We also measured a decrease in domain wall width from ∼ 94 nm in the sample annealed without stress to ∼ 80 nm in the stress-annealed sample. © 2016 Author(s).
    view abstractdoi: 10.1063/1.4942954
  • 2016 • 308 Microstructure evolution and critical stress for twinning in the CrMnFeCoNi high-entropy alloy
    Laplanche, G. and Kostka, A. and Horst, O.M. and Eggeler, G. and George, E.P.
    Acta Materialia 118 152-163 (2016)
    At low homologous temperatures (down to cryogenic temperatures), the CrMnFeCoNi high-entropy alloy possesses good combination of strength, work hardening rate (WHR), ductility, and fracture toughness. To improve understanding of the deformation mechanisms responsible for its mechanical properties, tensile tests were performed at liquid nitrogen and room temperature (77 K and 293 K) and interrupted at different strains to quantify the evolution of microstructure by transmission electron microscopy. Dislocation densities, and twin widths, their spacings, and volume fractions were determined. Nanotwins were first observed after true strains of ∼7.4% at 77 K and ∼25% at 293 K; at lower strains, deformation occurs by dislocation plasticity. The tensile stress at which twinning occurs is 720 ± 30 MPa, roughly independent of temperature, from which we deduce a critical resolved shear stress for twinning of 235 ± 10 MPa. In the regime where deformation occurs by dislocation plasticity, the shear modulus normalized WHR decreases with increasing strain at both 77 K and 293 K. Beyond ∼7.4% true strain, the WHR at 77 K remains constant at a high value of G/30 because twinning is activated, which progressively introduces new interfaces in the microstructure. In contrast, the WHR at room temperature continues to decrease with increasing strain because twinning is not activated until much later (close to fracture). Thus, the enhanced strength-ductility combination at 77 K compared to 293 K is primarily due to twinning starting earlier in the deformation process and providing additional work hardening. Consistent with this, when tensile specimens were pre-strained at 77 K to introduce nanotwins, and subsequently tested at 293 K, flow stress and ductility both increased compared to specimens that were not pre-strained. © 2016 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2016.07.038
  • 2016 • 307 Multiple mechanisms of lath martensite plasticity
    Morsdorf, L. and Jeannin, O. and Barbier, D. and Mitsuhara, M. and Raabe, D. and Tasan, C.C.
    Acta Materialia 121 202-214 (2016)
    The multi-scale complexity of lath martensitic microstructures requires scale-bridging analyses to better understand the deformation mechanisms activated therein. In this study, plasticity in lath martensite is investigated by multi-field mapping of deformation-induced microstructure, topography, and strain evolution at different spatial resolution vs. field-of-view combinations. These investigations reveal site-specific initiation of dislocation activity within laths, as well as significant plastic accommodation in the vicinity of high angle block and packet boundaries. The observation of interface plasticity raises several questions regarding the role of thin inter-lath austenite films. Thus, accompanying transmission electron microscopy and synchrotron x-ray diffraction experiments are carried out to investigate the stability of these films to mechanical loading, and to discuss alternative boundary sliding mechanisms to explain the observed interface strain localization. © 2016 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2016.09.006
  • 2016 • 306 Nanostructured Ternary FeCrAl Oxide Photocathodes for Water Photoelectrolysis
    Kondofersky, I. and Müller, A. and Dunn, H.K. and Ivanova, A. and Štefanić, G. and Ehrensperger, M. and Scheu, C. and Parkinson, B.A. and Fattakhova-Rohlfing, D. and Bein, T.
    Journal of the American Chemical Society 138 1860-1867 (2016)
    A sol-gel method for the synthesis of semiconducting FeCrAl oxide photocathodes for solar-driven hydrogen production was developed and applied for the production of meso- and macroporous layers with the overall stoichiometry Fe0.84Cr1.0Al0.16O3. Using transmission electron microscopy and energy-dispersive X-ray spectroscopy, phase separation into Fe- and Cr-rich phases was observed for both morphologies. Compared to prior work and to the mesoporous layer, the macroporous FeCrAl oxide photocathode had a significantly enhanced photoelectrolysis performance, even at a very early onset potential of 1.1 V vs RHE. By optimizing the macroporous electrodes, the device reached current densities of up to 0.68 mA cm-2 at 0.5 V vs RHE under AM 1.5 with an incident photon-to-current efficiency (IPCE) of 28% at 400 nm without the use of catalysts. Based on transient measurements, this performance increase could be attributed to an improved collection efficiency. At a potential of 0.75 V vs RHE, an electron transfer efficiency of 48.5% was determined. © 2016 American Chemical Society.
    view abstractdoi: 10.1021/jacs.5b08040
  • 2016 • 305 Neurotoxicity of the parkinson disease-associated pesticide ziram is synuclein-dependent in zebrafish embryos
    Lulla, A. and Barnhill, L. and Bitan, G. and Ivanova, M.I. and Nguyen, B. and O’Donnell, K. and Stahl, M.C. and Yamashiro, C. and Klärner, F.-G. and Schrader, T. and Sagasti, A. and Bronstein, J.M.
    Environmental Health Perspectives 124 1766-1775 (2016)
    Background: Exposure to the commonly used dithiocarbamate (DTC) pesticides is associated with an increased risk of developing Parkinson disease (PD), although the mechanisms by which they exert their toxicity are not completely understood. Objective: We studied the mechanisms of ziram’s (a DTC fungicide) neurotoxicity in vivo. Methods: Zebrafish (ZF) embryos were utilized to determine ziram’s effects on behavior, neuronal toxicity, and the role of synuclein in its toxicity. Results: Nanomolar-range concentrations of ziram caused selective loss of dopaminergic (DA) neurons and impaired swimming behavior. Because ziram increases α-synuclein (α-syn) concentrations in rat primary neuronal cultures, we investigated the effect of ziram on ZF γ-synuclein 1 (γ1). ZF express 3 synuclein isoforms, and ZF γ1 appears to be the closest functional homologue to α-syn. We found that recombinant ZF γ1 formed fibrils in vitro, and overexpression of ZF γ1 in ZF embryos led to the formation of neuronal aggregates and neurotoxicity in a manner similar to that of α-syn. Importantly, knockdown of ZF γ1 with morpholinos and disruption of oligomers with the molecular tweezer CLR01 prevented ziram’s DA toxicity. Conclusions: These data show that ziram is selectively toxic to DA neurons in vivo, and this toxicity is synuclein-dependent. These findings have important implications for understanding the mechanisms by which pesticides may cause PD. © 2016, Public Health Services, US Dept of Health and Human Services. All rights reserved.
    view abstractdoi: 10.1289/EHP141
  • 2016 • 304 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 • 303 Optical and electron microscopy study of laser-based intracellular molecule delivery using peptide-conjugated photodispersible gold nanoparticle agglomerates
    Krawinkel, J. and Richter, U. and Torres-Mapa, M.L. and Westermann, M. and Gamrad, L. and Rehbock, C. and Barcikowski, S. and Heisterkamp, A.
    Journal of Nanobiotechnology 14 (2016)
    Background: Cell-penetrating peptides (CPPs) can act as carriers for therapeutic molecules such as drugs and genetic constructs for medical applications. The triggered release of the molecule into the cytoplasm can be crucial to its effective delivery. Hence, we implemented and characterized laser interaction with defined gold nanoparticle agglomerates conjugated to CPPs which enables efficient endosomal rupture and intracellular release of molecules transported. Results: Gold nanoparticles generated by pulsed laser ablation in liquid were conjugated with CPPs forming agglomerates and the intracellular release of molecules was triggered via pulsed laser irradiation (λ = 532 nm, τpulse = 1 ns). The CPPs enhance the uptake of the agglomerates along with the cargo which can be co-incubated with the agglomerates. The interaction of incident laser light with gold nanoparticle agglomerates leads to heat deposition and field enhancement in the vicinity of the particles. This highly precise effect deagglomerates the nanoparticles and disrupts the enclosing endosomal membrane. Transmission electron microscopy images confirmed this rupture for radiant exposures of 25 mJ/cm2 and above. Successful intracellular release was shown using the fluorescent dye calcein. For a radiant exposure of 35 mJ/cm2 we found calcein delivery in 81 % of the treated cells while maintaining a high percentage of cell viability. Furthermore, cell proliferation and metabolic activity were not reduced 72 h after the treatment. Conclusion: CPPs trigger the uptake of the gold nanoparticle agglomerates via endocytosis and co-resident molecules in the endosomes are released by applying laser irradiation, preventing their intraendosomal degradation. Due to the highly localized effect, the cell membrane integrity is not affected. Therefore, this technique can be an efficient tool for spatially and temporally confined intracellular release. The utilization of specifically designed photodispersible gold nanoparticle agglomerates (65 nm) can open novel avenues in imaging and molecule delivery. Due to the induced deagglomeration the primary, small particles (~5 nm) are more likely to be removed from the body. © 2016 Krawinkel et al.
    view abstractdoi: 10.1186/s12951-015-0155-8
  • 2016 • 302 Phase selection and nanocrystallization in Cu-free soft magnetic FeSiNbB amorphous alloy upon rapid annealing
    Morsdorf, L. and Pradeep, K.G. and Herzer, G. and Kovács, A. and Dunin-Borkowski, R.E. and Povstugar, I. and Konygin, G. and Choi, P. and Raabe, D.
    Journal of Applied Physics 119 (2016)
    Nucleation of soft magnetic Fe3Si nanocrystals in Cu-free Fe74.5Si15.5Nb3B7 alloy, upon rapid (10 s) and conventional (30 min) annealing, was investigated using x-ray diffraction, transmission electron microscopy, Mössbauer spectroscopy, and atom probe tomography. By employing rapid annealing, preferential nucleation of Fe3Si nanocrystals was achieved, whereas otherwise there is simultaneous nucleation of both Fe3Si and undesired Fe-B compound phases. Analysis revealed that the enhanced Nb diffusivity, achieved during rapid annealing, facilitates homogeneous nucleation of Fe3Si nanocrystals while shifting the secondary Fe-B crystallization to higher temperatures resulting in pure soft magnetic nanocrystallization with very low coercivities of ∼10 A/m. © 2016 AIP Publishing LLC.
    view abstractdoi: 10.1063/1.4944595
  • 2016 • 301 Plasmonic Au/TiO2 nanostructures for glycerol oxidation
    Dodekatos, G. and Tüysüz, H.
    Catalysis Science and Technology 6 7307-7315 (2016)
    Au nanoparticles supported on P25 TiO2 (Au/TiO2) were prepared by a facile deposition-precipitation method with urea and investigated for surface plasmon-assisted glycerol oxidation under base-free conditions. Au/TiO2 samples were characterized in detail by X-ray diffraction, UV-vis spectroscopy, transmission electron microscopy and energy-dispersive X-ray spectroscopy. The adopted synthetic methodology permits deposition of Au nanoparticles with similar mean particle sizes up to 12.5 wt% loading that allows for the evaluation of the influence of the Au amount (without changing the particle size) on its photocatalytic performance for glycerol oxidation. The reaction conditions were optimized by carrying out a systematic study with different Au loadings on TiO2, reaction times, temperatures, catalyst amounts, O2 pressures and Au particle sizes for photocatalytic reactions as well as traditional heterogeneous catalysis. It has been shown that visible light irradiation during the reaction has a beneficial effect on the conversion of glycerol where the best catalytic results were observed for 7.5 wt% Au loading with an average particle size of around 3 nm. The main product observed, with selectivities up to 63%, was high-value dihydroxyacetone that has important industrial applications, particularly in the cosmetic industry. © 2016 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c6cy01192f
  • 2016 • 300 Rational design of ABC triblock terpolymer solution nanostructures with controlled patch morphology
    Löbling, T.I. and Borisov, O. and Haataja, J.S. and Ikkala, O. and Gröschel, A.H. and Müller, A.H.E.
    Nature Communications 7 (2016)
    Block copolymers self-assemble into a variety of nanostructures that are relevant for science and technology. While the assembly of diblock copolymers is largely understood, predicting the solution assembly of triblock terpolymers remains challenging due to complex interplay of block/block and block/solvent interactions. Here we provide guidelines for the self-assembly of linear ABC triblock terpolymers into a large variety of multicompartment nanostructures with C corona and A/B cores. The ratio of block lengths NC/NA thereby controls micelle geometry to spheres, cylinders, bilayer sheets and vesicles. The insoluble blocks then microphase separate to core A and surface patch B, where N B controls the patch morphology to spherical, cylindrical, bicontinuous and lamellar. The independent control over both parameters allows constructing combinatorial libraries of unprecedented solution nanostructures, including spheres-on-cylinders/sheets/vesicles, cylinders-on-sheets/vesicles, and sheets/vesicles with bicontinuous or lamellar membrane morphology (patchy polymersomes). The derived parameters provide a logical toolbox towards complex self-assemblies for soft matter nanotechnologies.
    view abstractdoi: 10.1038/ncomms12097
  • 2016 • 299 Small angle X-ray scattering as a high-throughput method to classify antimicrobial modes of action
    Von Gundlach, A.R. and Garamus, V.M. and Gorniak, T. and Davies, H.A. and Reischl, M. and Mikut, R. and Hilpert, K. and Rosenhahn, A.
    Biochimica et Biophysica Acta - Biomembranes 1858 918-925 (2016)
    Multi-drug resistant bacteria are currently undermining our health care system worldwide. While novel antimicrobial drugs, such as antimicrobial peptides, are urgently needed, identification of new modes of action is money and time consuming, and in addition current approaches are not available in a high throughput manner. Here we explore how small angle X-ray scattering (SAXS) as high throughput method can contribute to classify the mode of action for novel antimicrobials and therefore supports fast decision making in drug development. Using data bases for natural occurring antimicrobial peptides or predicting novel artificial peptides, many candidates can be discovered that will kill a selected target bacterium. However, in order to narrow down the selection it is important to know if these peptides follow all the same mode of action. In addition, the mode of action should be different from conventional antibiotics, in consequence peptide candidates can be developed further into drugs against multi-drug resistant bacteria. Here we used one short antimicrobial peptide with unknown mode of action and compared the ultrastructural changes of Escherichia coli cells after treatment with the peptide to cells treated with classic antibiotics. The key finding is that SAXS as a structure sensitive tool provides a rapid feedback on drug induced ultrastructural alterations in whole E. coli cells. We could demonstrate that ultrastructural changes depend on the used antibiotics and their specific mode of action. This is demonstrated using several well characterized antimicrobial compounds and the analysis of resulting SAXS curves by principal component analysis. To understand the result of the PCA analysis, the data is correlated with TEM images. In contrast to real space imaging techniques, SAXS allows to obtain nanoscale information averaged over approximately one million cells. The measurement takes only seconds, while conventional tests to identify a mode of action require days or weeks per single substance. The antimicrobial peptide showed a different mode of action as all tested antibiotics including polymyxin B and is therefore a good candidate for further drug development. We envision SAXS to become a useful tool within the high-throughput screening pipeline of modern drug discovery. This article is part of a Special Issue entitled: Antimicrobial peptides edited by Karl Lohner and Kai Hilpert. © 2015 Elsevier B.V.
    view abstractdoi: 10.1016/j.bbamem.2015.12.022
  • 2016 • 298 Strain hardening by dynamic slip band refinement in a high-Mn lightweight steel
    Welsch, E. and Ponge, D. and Hafez Haghighat, S.M. and Sandlöbes, S. and Choi, P. and Herbig, M. and Zaefferer, S. and Raabe, D.
    Acta Materialia 116 188-199 (2016)
    The strain hardening mechanism of a high-Mn lightweight steel (Fe-30.4Mn-8Al-1.2C (wt%)) is investigated by electron channeling contrast imaging (ECCI) and transmission electron microscopy (TEM). The alloy is characterized by a constant high strain hardening rate accompanied by high strength and high ductility (ultimate tensile strength: 900 MPa, elongation to fracture: 68%). Deformation microstructures at different strain levels are studied in order to reveal and quantify the governing structural parameters at micro- and nanometer scales. As the material deforms mainly by planar dislocation slip causing the formation of slip bands, we quantitatively study the evolution of the slip band spacing during straining. The flow stress is calculated from the slip band spacing on the basis of the passing stress. The good agreement between the calculated values and the tensile test data shows dynamic slip band refinement as the main strain hardening mechanism, enabling the excellent mechanical properties. This novel strain hardening mechanism is based on the passing stress acting between co-planar slip bands in contrast to earlier attempts to explain the strain hardening in high-Mn lightweight steels that are based on grain subdivision by microbands. We discuss in detail the formation of the finely distributed slip bands and the gradual reduction of the spacing between them, leading to constantly high strain hardening. TEM investigations of the precipitation state in the as-quenched state show finely dispersed atomically ordered clusters (size < 2 nm). The influence of these zones on planar slip is discussed. © 2016 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2016.06.037
  • 2016 • 297 Structural and optical properties of (1122) InGaN quantum wells compared to (0001) and (1120)
    Pristovsek, M. and Han, Y. and Zhu, T. and Oehler, F. and Tang, F. and Oliver, R.A. and Humphreys, C.J. and Tytko, D. and Choi, P.-P. and Raabe, D. and Brunner, F. and Weyers, M.
    Semiconductor Science and Technology 31 (2016)
    We benchmarked growth, microstructure and photo luminescence (PL) of (112-2) InGaN quantum wells (QWs) against (0001) and (112-0). In incorporation, growth rate and the critical thickness of (112-2) QWs are slightly lower than (0001) QWs, while the In incorporation on (112-0) is reduced by a factor of three. A small step-bunching causes slight fluctuations of the emission wavelength. Transmission electron microscopy as well as atom probe tomography (APT) found very flat interfaces with little In segregation even for 20% In content. APT frequency distribution analysis revealed some deviation from a random InGaN alloy, but not as severe as for (112-0). The slight deviation of (112-2) QWs from an ideal random alloy did not broaden the 300 K PL, the line widths were similar for (112-2) and (0001) while (112-0) QWs were broader. Despite the high structural quality and narrow PL, the integrated PL signal at 300 K was about 4 lower on (112-2) and more than 10 lower on (112-0). © 2016 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0268-1242/31/8/085007
  • 2016 • 296 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 • 295 The effect of stress, temperature and loading direction on the creep behaviour of Ni-base single crystal superalloy miniature tensile specimens
    Wollgramm, P. and Bürger, D. and Parsa, A.B. and Neuking, K. and Eggeler, G.
    Materials at High Temperatures 33 346-360 (2016)
    In the present work, we use a miniature test procedure to investigate the tensile creep behaviour of the single crystal superalloy ERBO1. We test precisely oriented [0 0 1], [1 1 0] and [1 1 1] creep specimens and determine the stress and the temperature dependence of characteristic creep rates in limited stress and temperature regimes, where the stress and temperature dependence of characteristic creep rates can be well described by power law and Arrhenius type of relations, with stress exponents n and apparent activation energies Qapp. n-values increase with stress and decrease with temperature. Qapp-values, on the other hand, increase with increasing temperature and decrease with increasing stress. Creep curve shapes gradually evolve from the high temperature low stress to the low temperature high stress (LTHS) regime. This implies that there is a gradual change in elementary deformation and softening mechanisms, which is qualitatively confirmed using transmission electron microscopy. While at high temperatures different loading directions only have a moderate influence on creep, there is a very strong effect of loading direction at low temperatures. The [1 1 0] tests show the fastest deformation rates and the shortest rupture times. In the LTHS creep regime, we confirm the double minimum (DM) type of creep behaviour, which was previously reported but never explained. Further work is required to rationalise DM-creep. The implications of this type of creep behaviour on scatter and on extrapolation of creep data is discussed in the light of previous results published in the literature. © 2016 Informa UK Limited, trading as Taylor & Francis Group.
    view abstractdoi: 10.1080/09603409.2016.1186414
  • 2016 • 294 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 • 293 The role of ω in the precipitation of α in near-β Ti alloys
    Li, T. and Kent, D. and Sha, G. and Cairney, J.M. and Dargusch, M.S.
    Scripta Materialia 117 92-95 (2016)
    To identify the conditions under which ω assists α formation in a near-β Ti alloy, we employed transmission electron microscopy and atom probe tomography to study α precipitation in alloys designed to contain two different types of ω. Coherent incommensurate embryonic ω formed upon isothermal ageing, does not directly assist α precipitation. When this incommensurate embryonic ω grows to a critical size, it transforms into commensurate isothermal ω, during which stress is thought to be the dominant factor. Regions of O enrichment at the semi-coherent isothermal ω/β interfaces are observed, which is thought to promote α formation. © 2016 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.scriptamat.2016.02.026
  • 2016 • 292 Transmission electron microscopy of a CMSX-4 Ni-base superalloy produced by selective electron beam melting
    Parsa, A.B. and Ramsperger, M. and Kostka, A. and Somsen, C. and Körner, C. and Eggeler, G.
    Metals 6 (2016)
    In this work, the microstructures of superalloy specimens produced using selective electron beam melting additive manufacturing were characterized. The materials were produced using a CMSX-4 powder. Two selective electron beam melting processing strategies, which result in higher and lower effective cooling rates, are described. Orientation imaging microscopy, scanning transmission electron microscopy and conventional high resolution transmission electron microscopy are used to investigate the microstructures. Our results suggest that selective electron beam melting processing results in near equilibrium microstructures, as far as γ′ volume fractions, the formation of small amounts of TCP phases and the partitioning behavior of the alloy elements are concerned. As expected, higher cooling rates result in smaller dendrite spacings, which are two orders of magnitude smaller than observed during conventional single crystal casting. During processing, columnar grains grow in <100> directions, which are rotated with respect to each other. There are coarse γ/γ′ microstructures in high angle boundary regions. Dislocation networks form low angle boundaries. A striking feature of the as processed selective electron beam melting specimens is their high dislocation density. From a fundamental point of view, this opens new possibilities for the investigation of elementary dislocation processes which accompany solidification. © 2016 by the authors; licensee MDPI, Basel, Switzerland.
    view abstractdoi: 10.3390/met6110258
  • 2016 • 291 Zeolite Beta Formation from Clear Sols: Silicate Speciation, Particle Formation and Crystallization Monitored by Complementary Analysis Methods
    Castro, M. and Haouas, M. and Lim, I. and Bongard, H.J. and Schüth, F. and Taulelle, F. and Karlsson, G. and Alfredsson, V. and Breyneart, E. and Kirschhock, C.E.A. and Schmidt, W.
    Chemistry - A European Journal 22 15307-15319 (2016)
    The formation of silicate nanoaggregates (NAs) at the very early stages of precursor sols and zeolite beta crystallization from silicate nanoparticles (NPs) are investigated in detail using a combination of different analysis methods, including liquid-state29Si,27Al,14N, and1H NMR spectroscopy, mass spectrometry (MS), small-angle X-ray scattering (SAXS), X-ray diffraction (XRD), and transmission electron microscopy at cryogenic temperatures (cryo-TEM). Prior to hydrothermal treatment, silicate NAs are observed if the Si/OH ratio in the reaction mixture is greater than 1. Condensation of oligomers within the NAs then generates NPs. Aluminum doped into the synthesis mixtures is located exclusively in the NPs, and is found exclusively in a state that is fourfold connected to silicate, favoring their condensation and aggregation. These results are in agreement with general trends observed for other systems. Silicate NAs are essential intermediates for zeolite formation and are generated by the aggregation of hydrated oligomers, aluminate, and templating cations. Subsequent further intra-nanoaggregate silicate condensation results in the formation of NPs.1H and14N liquid NMR as well as diffusion ordered spectroscopy (DOSY) experiments provide evidence for weakly restricted rotational and translational mobility of the organic template within NAs as a consequence of specific silicate–template interactions. NAs thus appear as key species in clear sols, and their presence in the precursor sol favors silicate condensation and further crystallization, promoted either by increasing the Si/OH ratio or by heating. © 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/chem.201600511
  • 2015 • 290 3D structural and atomic-scale analysis of lath martensite: Effect of the transformation sequence
    Morsdorf, L. and Tasan, C.C. and Ponge, D. and Raabe, D.
    Acta Materialia 95 366-377 (2015)
    To improve the fundamental understanding of the multi-scale characteristics of martensitic microstructures and their micro-mechanical properties, a multi-probe methodology is developed and applied to low-carbon lath martensitic model alloys. The approach is based on the joint employment of electron channeling contrast imaging (ECCI), electron backscatter diffraction (EBSD), transmission electron microscopy (TEM), atom probe tomography (APT) and nanoindentation, in conjunction with high precision and large field-of-view 3D serial sectioning. This methodology enabled us to resolve (i) size variations of martensite sub-units, (ii) associated dislocation sub-structures, (iii) chemical heterogeneities, and (iv) the resulting local mechanical properties. The identified interrelated microstructure heterogeneity is discussed and related to the martensitic transformation sequence, which is proposed to intrinsically lead to formation of a nano-composite structure in low-carbon martensitic steels. © 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2015.05.023
  • 2015 • 289 A molecular tweezer antagonizes seminal amyloids and HIV infection
    Lump, E. and Castellano, L.M. and Meier, C. and Seeliger, J. and Erwin, N. and Sperlich, B. and Stürzel, C.M. and Usmani, S. and Hammond, R.M. and Von Einem, J. and Gerold, G. and Kreppel, F. and Bravo-Rodriguez, K. and Pietschma...
    eLife 4 (2015)
    Semen is the main vector for HIV transmission and contains amyloid fibrils that enhance viral infection. Available microbicides that target viral components have proven largely ineffective in preventing sexual virus transmission. In this study, we establish that CLR01, a ‘molecular tweezer’ specific for lysine and arginine residues, inhibits the formation of infectivity-enhancing seminal amyloids and remodels preformed fibrils. Moreover, CLR01 abrogates semen-mediated enhancement of viral infection by preventing the formation of virion–amyloid complexes and by directly disrupting the membrane integrity of HIV and other enveloped viruses.We establish that CLR01 acts by binding to the target lysine and arginine residues rather than by a non-specific, colloidal mechanism. CLR01 counteracts both host factors that may be important for HIV transmission and the pathogen itself. These combined anti-amyloid and antiviral activities make CLR01 a promising topical microbicide for blocking infection by HIV and other sexually transmitted viruses. © 2015, Lump et al.
    view abstractdoi: 10.7554/eLife.05397
  • 2015 • 288 Advanced scale bridging microstructure analysis of single crystal Ni-base superalloys
    Parsa, A.B. and Wollgramm, P. and Buck, H. and Somsen, C. and Kostka, A. and Povstugar, I. and Choi, P.-P. and Raabe, D. and Dlouhy, A. and Müller, J. and Spiecker, E. and Demtroder, K. and Schreuer, J. and Neuking, K. and Eggeler, G.
    Advanced Engineering Materials 17 216-230 (2015)
    In the present work, we show how conventional and advanced mechanical, chemical, and microstructural methods can be used to characterize cast single crystal Ni-base superalloy (SX) plates across multiple length scales. Two types of microstructural heterogeneities are important, associated with the castmicrostructure (dendrites (D) and interdendritic (ID) regions - large scale heterogeneity) and with the well-known γ/γ′ microstructure (small scale heterogeneity). Using electron probe microanalysis (EPMA), we can showthat elements such as Re, Co, andCr partition to the dendrites while ID regions contain more Al, Ta, and Ti. Analytical transmission electron microscopy and atom probe tomography (APT) show that Al, Ta, and Ti partition to the γ′ cubes while g channels show higher concentrations of Co, Cr, Re, andW.We can combine large scale (EPMA) and small-scale analytical methods (APT) to obtain reasonable estimates for γ′ volume fractions in the dendrites and in the ID regions. The chemical and mechanical properties of the SX plates studied in the present work are homogeneous, when they are determined from volumes with dimensions, which are significantly larger than the dendrite spacing. For the SX plates (140mm x 100mm x 20mm) studied in the present work this holds for the average chemical composition as well as for elastic behavior and local creep properties. We highlight the potential of HRTEM and APT to contribute to a better understanding of the role of dislocations during coarsening of the γ′ phase and the effect of cooling rates after high temperature exposure on the microstructure. © 2014 Wiley-VCH Verlag GmbH & Co. KGaA.
    view abstractdoi: 10.1002/adem.201400136
  • 2015 • 287 Amorphous and Crystalline Sodium Tantalate Composites for Photocatalytic Water Splitting
    Grewe, T. and Tüysüz, H.
    ACS Applied Materials and Interfaces 7 23153-23162 (2015)
    A facile hydrothermal synthesis protocol for the fabrication of sodium tantalates for photocatalytic water splitting is presented. Mixtures of tantalum and sodium ethoxide precursors were dispersed in ethanol, and ammonium hydroxide solution was used as mineralizer. By adjusting the amount of mineralizer, a variety of sodium tantalates with various morphologies, textural parameters, band gaps, crystal phases, and degrees of crystallinity were fabricated. The reaction was carefully monitored with a pressure sensor inside the autoclave reactor, and the obtained samples were characterized using X-ray diffraction, transmission electron microscopy, N2-physisorption, and ultraviolet-visible light spectroscopy. Among the series, the amorphous sample and the composite sample that consists of amorphous and crystalline phases showed superior activity toward photocatalytic hydrogen production than highly crystalline samples. Particularly, an amorphous sodium tantalate with a small fraction of crystalline nanoparticles with perovskite structure was found to be the most active sample, reaching a hydrogen rate of 3.6 mmol h-1 from water/methanol without the use of any cocatalyst. Despite its amorphous nature, this photocatalyst gave an apparent photocatalyst activity of 1200 μmol g-1 L-1 h-1 W1-, which is 4.5-fold higher than highly crystalline NaTaO3. In addition, the most active sample gave promising activity for overall water splitting with a hydrogen production rate of 94 μmol h-1, which is superior to highly crystalline NaTaO3 prepared by conventional solid-solid state route. © 2015 American Chemical Society.
    view abstractdoi: 10.1021/acsami.5b06965
  • 2015 • 286 Assessment of soot particle-size imaging with LII at Diesel engine conditions
    Cenker, E. and Kondo, K. and Bruneaux, G. and Dreier, T. and Aizawa, T. and Schulz, C.
    Applied Physics B: Lasers and Optics 119 765-776 (2015)
    Two-time-step laser-induced incandescence (LII) imaging was performed in Diesel engine-relevant combustion to investigate its applicability for spatially resolved measurements of soot primary particle sizes. The method is based on evaluating gated LII signals acquired with two cameras consecutively after the laser pulse and using LII modeling to deduce the particle size from the ratio of local signals. Based on a theoretical analysis, optimized detection times and durations were chosen to minimize measurement uncertainties. Experiments were conducted in a high-temperature high-pressure constant-volume pre-combustion vessel under the Engine Combustion Network’s “Spray A” conditions at 61–68 bar with additional parametric variations in injection pressure, gas temperature, and composition. The LII measurements were supported by pyrometric imaging measurements of particle heat-up temperatures. The results were compared to particle-size and size-dispersion measurements from transmission electron microscopy of soot thermophoretically sampled at multiple axial distances from the injector. The discrepancies between the two measurement techniques are discussed to analyze uncertainties and related error sources of the two diagnostics. It is found that in such environment where particles are small and pressure is high, LII signal decay times are such that LII with standard nanosecond laser and detector equipment suffers from a strong bias toward large particles. © 2015, Springer-Verlag Berlin Heidelberg.
    view abstractdoi: 10.1007/s00340-015-6106-0
  • 2015 • 285 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 • 284 Atomic scale study of CU clustering and pseudo-homogeneous Fe-Si nanocrystallization in soft magnetic FeSiNbB(CU) alloys
    Pradeep, K.G. and Herzer, G. and Raabe, D.
    Ultramicroscopy 159 285-291 (2015)
    A local electrode atom probe has been employed to trace the onset of Cu clustering followed by their coarsening and subsequent growth upon rapid (10s) annealing of an amorphous Fe73.5Si15.5Cu1Nb3B7 alloy. It has been found that the clustering of Cu atoms introduces heterogeneities in the amorphous matrix, leading to the formation of Fe rich regions which crystallizes pseudo-homogeneously into Fe-Si nanocrystals upon annealing. In this paper, we present the data treatment method that allows for the visualization of these different phases and to understand their morphology while still quantifying them in terms of their size, number density and volume fraction. The crystallite size of Fe-Si nanocrystals as estimated from the atom probe data are found to be in good agreement with other complementary techniques like XRD and TEM, emphasizing the importance of this approach towards accurate structural analysis. In addition, a composition driven data segmentation approach has been attempted to determine and distinguish nanocrystalline regions from the remaining amorphous matrix. Such an analysis introduces the possibility of retrieving crystallographic information from extremely fine (2-4nm sized) nanocrystalline regions of very low volume fraction (< 5Vol%) thereby providing crucial in-sights into the chemical heterogeneity induced crystallization process of amorphous materials. © 2015 Elsevier B.V.
    view abstractdoi: 10.1016/j.ultramic.2015.04.006
  • 2015 • 283 Bulk morphologies of polystyrene-block-polybutadiene-block-poly(tert-butyl methacrylate) triblock terpolymers
    Löbling, T.I. and Hiekkataipale, P. and Hanisch, A. and Bennet, F. and Schmalz, H. and Ikkala, O. and Gröschel, A.H. and Müller, A.H.E.
    Polymer (United Kingdom) 72 479-489 (2015)
    The self-assembly of block copolymers in the bulk phase enables the formation of complex nanostructures with sub 100 nm periodicities and long-range order, both relevant for nanotechnology applications. Here, we map the bulk phase behavior of polystyrene-block-polybutadiene-block-poly(tert-butyl methacrylate) (SBT) triblock terpolymers on a series of narrowly distributed polymers with widely different block volume fractions, PdblS, PdblB and PdblT. In dependence of Pdbl, we find the lamella-lamella, core-shell cylinder, cylinder-in-lamella and core-shell gyroid morphology, but also a rarely observed cylinder-in-lamella phase. The bulk morphologies are thoroughly characterized by transmission electron microscopy (TEM) and small angle X-ray scattering (SAXS) and display unusually broad stability regions, i.e. morphologies are observed over a broad range of compositions. We attribute this phase behavior to the asymmetric distribution of block-block incompatibilities, along the SBT block sequence, which are relatively large for S/B and S/T interfaces, but small for B/T. The higher enthalpic penalties at the S/B and S/T interface cause B to preferentially spread on the T microdomain thereby adopting its geometry. The morphological behavior of SBT is thus dominated by the volume ratio of the end blocks, PdblS and PdblT, which reduces the number of potential morphologies to only a few, mostly the core-shell analogue of diblock copolymer morphologies. In general, a simplified terpolymer bulk behavior with large stability regions for morphologies offers straightforward synthetic targeting of specific morphologies that usually only appear in a small parameter space as demonstrated here on the core-shell gyroid. © 2015 Elsevier Ltd.
    view abstractdoi: 10.1016/j.polymer.2015.02.025
  • 2015 • 282 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 • 281 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 • 280 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 • 279 CO oxidation as a test reaction for strong metal-support interaction in nanostructured Pd/FeOx powder catalysts
    Kast, P. and Friedrich, M. and Teschner, D. and Girgsdies, F. and Lunkenbein, T. and D'Alnoncourt, R.N. and Behrens, M. and Schlögl, R.
    Applied Catalysis A: General 502 8-17 (2015)
    A series of differently loaded palladium-iron catalysts was prepared by a controlled co-precipitation method of the nitrate precursors, in order to ensure homogeneous Pd particle size-distribution. After characterization of the pre-catalysts by various techniques, different controlled reduction conditions were applied to investigate the interactions within the Pd-iron system, containing reversible and irreversible processes like phase transformations, SMSI, sintering and alloying. Strong indications for the reversible surface decoration of the Pd nanoparticles with iron oxide species via strong metal-support interaction were found by the combined results of DRIFTS, CO-chemisorption, TEM and XPS measurements. This SMSI state was found to be unstable. It was observed independent of bulk phase or palladium particle size. Catalytic CO-oxidation was found to be a suitable test reaction for the study of the phenomenon: higher activity as well as oxidative deactivation of the SMSI state was observed by investigating the light-off behavior in repeated, temperature-programmed cycles as well as by isothermal measurements. The instability was found to be higher in case of higher Pd dispersion. In addition, bulk properties of the Pd-Fe system, like alloying, were investigated by detailed XRD measurements. © 2015 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.apcata.2015.04.010
  • 2015 • 278 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 • 277 Comparing small scale plasticity of copper-chromium nanolayered and alloyed thin films at elevated temperatures
    Raghavan, R. and Harzer, T.P. and Chawla, V. and Djaziri, S. and Phillipi, B. and Wehrs, J. and Wheeler, J.M. and Michler, J. and Dehm, G.
    Acta Materialia 93 175-186 (2015)
    Abstract The yield strengths and deformation mechanisms of Cu-Cr nanolayered and alloyed thin films were studied by microcompression testing at elevated temperatures. The mechanical response of the films with alternating layers of Cu and Cr with sub-100 nm interlayer thicknesses and alloyed films of the same average composition was compared to determine the role of the interfaces on deformation. Higher resistance to plastic flow at elevated temperatures was exhibited by the nanolayered films with smaller interlayer thickness among the layered films, while the alloyed film revealed an anomalous increase in strength with temperature exhibiting a deformation mechanism similar to the pure Cr film. © 2015 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2015.04.008
  • 2015 • 276 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 • 275 Cysteine-containing oligopeptide β-sheets as redispersants for agglomerated metal nanoparticles
    Mizutaru, T. and Sakuraba, T. and Nakayama, T. and Marzun, G. and Wagener, P. and Rehbock, C. and Barcikowski, S. and Murakami, K. and Fujita, J. and Ishii, N. and Yamamoto, Y.
    Journal of Materials Chemistry A 3 17612-17619 (2015)
    Oligopeptide β-sheets comprising a fluorenyl methoxy carbonyl (Fmoc) group on its N-terminus and five amino acid residues of cysteine, lysine and valine displays redispersive properties with respect to agglomerated metal nanoparticles (MNPs, M = Au, Cu, Pt and Pd). The ligand-free MNPs prepared by a laser ablation technique in liquid maintain a high dispersion state due to the inherent surface charges delivered by anionic species present in solution, but may agglomerate after the preparation depending on concentration or salinity. We show how the agglomerated MNPs can be returned to the dispersed state by adding the Fmoc-oligopeptide β-sheets in methanol, as characterized by photoabsorption spectroscopy and transmission electron microscopy. Systematic studies in which we vary the concentration, the amino acid sequences and the secondary structures of a series of the oligopeptides clarify that the β-sheet structure is essential for the redispersion of the MNPs, where metal-binding thiol groups are integrated on one side and positively charged amino groups are located on the other side of the β-sheet. A possible mechanism for the redispersion may be that the agglomerated MNPs are subsequently enwrapped by the flexible β-sheets and gradually separated due to the reconstruction of peptide β-sheets under the assembly/disassembly equilibrium. © The Royal Society of Chemistry 2015.
    view abstractdoi: 10.1039/c5ta02098k
  • 2015 • 274 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 • 273 Deformation induced alloying in crystalline - metallic glass nano-composites
    Guo, W. and Yao, J. and Jägle, E.A. and Choi, P.-P. and Herbig, M. and Schneider, J.M. and Raabe, D.
    Materials Science and Engineering A 628 269-280 (2015)
    We study the mechanisms of deformation driven chemical mixing in a metallic nanocomposite model system. More specific, we investigate shear banding at the atomic scale in an amorphous CuZr/ crystalline Cu nanolaminate, deformed by microindentation. Three CuZr/Cu multilayer systems (100 nm Cu/100 nm CuZr, 50 nm Cu/100 nm CuZr, and 10 nm Cu/100 nm CuZr) are fabricated to study the effect of layer thickness on shear band formation and deformation induced alloying. The chemical and structural evolution at different strain levels are traced by atom probe tomography and transmission electron microscopy combined with nano-beam diffraction mapping. The initially pure crystalline Cu and amorphous CuZr layers chemically mix by cross-phase shear banding after reaching a critical layer thickness. The Cu inside the shear bands develops a high dislocation density and can locally undergo transition to an amorphous state when sheared and mixed. We conclude that the severe deformation in the shear bands in the amorphous layer squeeze Zr atoms into the Cu dislocation cores in the Cu layers (thickness <5 nm), resulting in local chemical mixing. © 2015 Elsevier B.V.
    view abstractdoi: 10.1016/j.msea.2015.01.062
  • 2015 • 272 Deformation mechanism of ω-enriched Ti-Nb-based gum metal: Dislocation channeling and deformation induced ω-β transformation
    Lai, M.J. and Tasan, C.C. and Raabe, D.
    Acta Materialia 100 290-300 (2015)
    Gum metal, a class of multifunctional β titanium alloys, has attracted much attention in the past decade due to its initially-proposed dislocation-free deformation mechanism based on giant faults, i.e., macroscopic planar defects carrying significant plastic strain. Special deformation features were observed in these alloys, such as plastic flow localization, pronounced surface steps, low work hardening, and large elongation. These were all proposed to arise from the special giant fault mechanism activated in the β-Ti matrix, while the initial presence or mechanically-induced formation of other phases was debated in several follow-up studies. Here, we set off with Ti-Nb-based gum metal samples with confirmed presence of large amounts of nanometer-sized hexagonal ω particles. Deformation experiments demonstrate all the features observed in the original reports, mentioned above. However, careful characterization reveals that the deformation bands (similar to giant faults) where plastic flow localized are "dislocation channels" that are depleted of ω phase. These channels are proposed to form by a {1 1 2}<1 1 1> dislocation dissociation mechanism, promoting reverse transformation of the ω phase into the β phase. The deformation induced ω-β transformation and the associated dislocation channeling process can explain the presence of the aforementioned special deformation features in the current gum metal. © 2015 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2015.08.047
  • 2015 • 271 Determination of small soot particles in the presence of large ones from time‑resolved laser‑induced incandescence
    Cenker, E. and Bruneaux, G. and Dreier, T. and Schulz, C.
    Applied Physics B: Lasers and Optics 118 169-183 (2015)
    A novel strategy for the analysis of time-resolved laser-induced incandescence (TiRe-LII), called two-exponential reverse ftting (TERF), is introduced. The method is based on combined monoexponential fts to the LII signal decay at various delay times and approximates the particle-size distribution as a weighted combination of one large and one small monodisperse equivalent mean particle size without requiring assumption on the particle-size distribution. The effects of particle size, heat-up temperature, aggregate size, and pressure on the uncertainty of this method are evaluated using numerical experiments for lognormal and bimodal size distributions. TERF is applied to TiRe-LII measured in an atmospheric pressure laminar non-premixed ethylene/air fame at various heights above burner. The results are compared to transmission electron microscopy (TEM) measurements of thermophoretically sampled soot. The particle size of the large particle-size class agreed well for both methods. The size of the small particle-size class and the relative contribution did not agree which is attributed to missing information in the TEM results for very small particles. These limitations of TEM measurements are discussed and the effect of the exposure time of the sampling grid is evaluated. © Springer-Verlag Berlin Heidelberg 2014.
    view abstractdoi: 10.1007/s00340-014-5966-z
  • 2015 • 270 Detoxification of hexavalent chromium in wastewater containing organic substances using simonkolleite-TiO2 photocatalyst
    Abdel Moniem, S.M. and Ali, M.E.M. and Gad-Allah, T.A. and Khalil, A.S.G. and Ulbricht, M. and El-Shahat, M.F. and Ashmawy, A.M. and Ibrahim, H.S.
    Process Safety and Environmental Protection 95 247-254 (2015)
    Innovative simple method for the preparation of simonkolleite-TiO<inf>2</inf> photocatalyst with different Zn contents was achieved. The prepared photocatalysts were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), FT-IR, Raman and diffuse reflectance spectroscopy techniques. The photocatalytic activities of the materials were evaluated for the simultaneous detoxification of hexavalent chromium (Cr(VI)) and oxidation of organic compounds commonly present in wastewater under simulated solar light. The best photoreduction efficiency of Cr(VI) has been achieved at 1000 ppm simonkolleite-TiO<inf>2</inf> photocatalyst of 5% Zn/TiO<inf>2</inf> weight ratio, and pH value of 2.5 to enhance the adsorption onto catalyst surface. Photoreduction was significantly improved by using formic acid as holes scavenger owing to its chemical adsorption on the catalyst surface. Finally, 100% photoreduction of Cr(VI) could be achieved using formic/simonkolleite-TiO<inf>2</inf> systems under sunlight.
    view abstractdoi: 10.1016/j.psep.2015.03.010
  • 2015 • 269 Downscaling metal-dielectric interface fracture experiments to sub-micron dimensions: A feasibility study using TEM
    Völker, B. and Heinz, W. and Roth, R. and Batke, J.M. and Cordill, M.J. and Dehm, G.
    Surface and Coatings Technology 270 1-7 (2015)
    In this study a bending beam approach is miniaturized to the sub-micrometer size regime for in-situ transmission electron microscopy (TEM) investigations. The purpose of this study was to evaluate the feasibility of this method for determining interface fracture energies for metal-dielectric systems, which are comparable to other methods. The interfaces under investigation are Cu/borophosphosilicate glass (BPSG) and W(20. at.% Ti)/BPSG as examples for a weak and a strong interface, respectively. It was possible to fracture the Cu/BPSG interface during in-situ TEM experiments and to obtain estimates for the interface fracture energy comparable to literature values. However, the TEM approach imposes challenges which can complicate the determination of an interface energy release rate; these influences are discussed. The miniaturized TEM approach failed for W(20. at.% Ti)/BPSG because no interface fracture occurred in this case. Thus it is concluded, that the experimental setup of the in-situ TEM bending beam method utilized in this study is useful for weak interfaces but for strongly adhering interfaces it is inappropriate. © 2015 Elsevier B.V.
    view abstractdoi: 10.1016/j.surfcoat.2015.03.027
  • 2015 • 268 Dynamic strain aging studied at the atomic scale
    Aboulfadl, H. and Deges, J. and Choi, P. and Raabe, D.
    Acta Materialia 86 34-42 (2015)
    Dynamic strain aging arises from the interaction between solute atoms and matrix dislocations in strained metallic alloy. It initiates jerky dislocation motion and abrupt softening, causing negative strain rate sensitivity. This effect leads to instable flow phenomena at the macroscopic scale, appearing as a serrated stress-strain response and deformation banding. These macroscopic features are referred to as the Portevin-Le Chatelier effect (PLC). Here we study the atomistic origin of dynamic strain aging in an Al-4.8 at.% Mg alloy using atom probe tomography (APT) and transmission electron microscopy (TEM). Samples were prepared from as-cold rolled (90% thickness reduction), stabilized (120 °C, 20 h) and recrystallized sheets (400°C, 10 min), respectively. In the stabilized state, Mg was found to decorate <1 1 0> aligned dislocations with up to ∼12.5 at.%. Tensile tests in combination with thermographic and laser speckle observations were used to map the deformation bands for the site-specific extraction of APT samples from regions inside the PLC bands. We observed an asymmetrical Mg distribution along some of the dislocations, matching model predictions for high dislocation speeds at peak drag stress by Zhang and Curtin. In this case, the Mg distribution is characterized by depletion in the compressive regime above the dislocation slip plane and enrichment in the dilatation region below the slip plane. Mg also depletes in a tail-like form behind fast-moving dislocations, further promoting slip localization. © 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2014.12.028
  • 2015 • 267 Effect of process parameters on the formation of laser-induced nanoparticles during material processing with continuous solid-state lasers
    Scholz, T. and Dickmann, K. and Ostendorf, A. and Uphoff, H. and Michalewicz, M.
    Journal of Laser Applications 27 (2015)
    During laser material processing with high laser beam intensities, a laser-induced vapor formation can occur. Due to the shockwave behavior of the vapor plume and the associated rapid cooling, a significant particle formation can be initiated by nucleation. The laser radiation interacts with the particles which can result in a dynamic change of the intensity distribution on the surface. Especially in the field of laser remote processing, the attenuation of laser radiation by nanoparticles can influence the process stability and reduce the processing quality. The presented work is focused on the particle formation at a height of 10 mm above the material surface during the laser welding of stainless steel with a fiber laser. The laser beam intensity on the surface was varied between 1.3 and 5.1 MW/cm2. Transmission electron microscopy images of the nanoparticles and high speed images of the vapor propagation in the ambient atmosphere were analyzed. The attenuation of a probe beam in the vapor plume was evaluated in dependence on the wavelength. The results indicate a linear connection between the laser beam power and the particle formation rate. © 2015 Laser Institute of America.
    view abstractdoi: 10.2351/1.4916081
  • 2015 • 266 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 • 265 Effects of strain amplitude, cycle number and orientation on low cycle fatigue microstructures in austenitic stainless steel studied by electron channelling contrast imaging
    Nellessen, J. and Sandlöbes, S. and Raabe, D.
    Acta Materialia 87 86-99 (2015)
    Substructure analysis on cyclically deformed metals is typically performed by time-consuming transmission electron microscopy probing, thus limiting such studies often to a single parameter. Here, we present a novel approach which consists in combining electron backscatter diffraction (EBSD), digital image correlation and electron channelling contrast imaging (ECCI), enabling us to systematically probe a large matrix of different parameters with the aim of correlating and comparing their interdependence. The main focus here is to identify the influence of cycle number, initial grain orientation and local strain amplitude on the evolving dislocation patterns. Therefore, experiments up to 100 cycles were performed on a polycrystalline austenitic stainless steel with local strain amplitudes between 0.35% and 0.95%. EBSD and ECCI maps reveal the individual influence of each parameter while the others remained constant. We find that the dislocation structures strongly depend on grain orientation. Dislocation structures in grains with double-slip (〈1 1 2〉 // LD, 〈1 2 2〉 // LD and 〈0 1 2〉 // LD) and multiple-slip (〈1 1 1〉 // LD, M 〈0 1 1〉 // LD and 〈0 0 1〉 // LD) orientations with respect to the loading direction (LD) are characterized under the variation of strain amplitude and cycle number. © 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2014.12.024
  • 2015 • 264 Formation and Movement of Cationic Defects during Forming and Resistive Switching in SrTiO3 Thin Film Devices
    Lenser, C. and Koehl, A. and Slipukhina, I. and Du, H. and Patt, M. and Feyer, V. and Schneider, C.M. and Lezaic, M. and Waser, R. and Dittmann, R.
    Advanced Functional Materials 25 6360-6368 (2015)
    The resistance switching phenomenon in many transition metal oxides is described by ion motion leading to the formation of oxygen-deficient, highly electron-doped filaments. In this paper, the interface and subinterface region of electroformed and switched metal-insulator-metal structures fabricated from a thin Fe-doped SrTiO3 (STO) film on n-conducting Nb-doped SrTiO3 crystals are investigated by photoemission electron microscopy, transmission electron microscopy, and hard X-ray photoelectron spectroscopy in order to gain a deeper understanding of cation movement in this specific system. During electroforming, the segregation of Sr to the top interface and the generation of defect-rich cones in the film are observed, apparently growing from the anode toward the cathode during electroforming. An unusual binding energy component of the Sr 3d emission line is observed which can be assigned to Sr Ti-VO∗ defect complexes by performing ab initio calculations. Since this Sr component can be reversibly affected by an external electrical bias, the movement of both oxygen and Sr point defects and the formation of defect complexes Sr Ti-VO∗ during resistive switching are suggested. These findings are discussed with regard to the point defect structure of the film and the local oxidation of the donor-doped substrate. In particular, the apparent dichotomy between the observation of acceptor-type defects and increased electronic conductivity in STO is addressed. A low binding energy component of the Sr 3d photoemission line is observed in Fe-doped SrTiO3 memristive devices and assigned to Sr′Ti-V∗O defect complexes by ab initio calculations. Since this Sr component can be reversibly affected by an electrical bias, the movement of both oxygen and Sr vacancies and the formation of Sr′Ti-V∗O defect complexes during resistive switching are suggested. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/adfm.201500851
  • 2015 • 263 Formation of a ZnO Overlayer in Industrial Cu/ZnO/Al2O3 Catalysts Induced by Strong Metal-Support Interactions
    Lunkenbein, T. and Schumann, J. and Behrens, M. and Schlögl, R. and Willinger, M.G.
    Angewandte Chemie - International Edition 54 4544-4548 (2015)
    In industrially relevant Cu/ZnO/Al<inf>2</inf>O<inf>3</inf> catalysts for methanol synthesis, the strong metal support interaction between Cu and ZnO is known to play a key role. Here we report a detailed chemical transmission electron microscopy study on the nanostructural consequences of the strong metal support interaction in an activated high-performance catalyst. For the first time, clear evidence for the formation of metastable "graphite-like" ZnO layers during reductive activation is provided. The description of this metastable layer might contribute to the understanding of synergistic effects between the components of the Cu/ZnO/Al<inf>2</inf>O<inf>3</inf> catalysts. © 2015 Wiley-VCH Verlag GmbH & Co. KGaA.
    view abstractdoi: 10.1002/anie.201411581
  • 2015 • 262 Impact of ambient pressure on titania nanoparticle formation during spray-flame synthesis
    Hardt, S. and Wlokas, I. and Schulz, C. and Wiggers, H.
    Journal of Nanoscience and Nanotechnology 15 9449-9456 (2015)
    Nanocrystalline titania was synthesized via liquid-fed spray-flame synthesis in a hermetically closed system at various pressures. Titanium tetraisopropoxide dissolved in isopropanol was used as precursor. The size, crystal structure, degree of agglomeration, morphology and the band gap of the as-prepared particles were investigated ex situ by nitrogen adsorption, transmission electron microscopy, X-ray diffraction, and UV-VIS absorption spectroscopy. In comparison to synthesis at atmospheric pressure it was found that decreasing pressure has a significant influence on the particle size distribution leading to smaller particles with reduced geometric standard deviation while particle morphology and crystal structure are not affected. Computational fluid dynamics simulations support the experimental findings also indicating a significant decrease in particle size at reduced pressure. Although it is well known that decreasing pressure leads to smaller particle sizes, it is (to our knowledge) the first time that this relation was investigated for spray-flame synthesis. Copyright © 2015 American Scientific Publishers All rights reserved.
    view abstractdoi: 10.1166/jnn.2015.10607
  • 2015 • 261 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 • 260 Interface engineering and characterization at the atomic-scale of pure and mixed ion layer gas reaction buffer layers in chalcopyrite thin-film solar cells
    Cojocaru-Mirédin, O. and Fu, Y. and Kostka, A. and Sáez-Araoz, R. and Beyer, A. and Knaub, N. and Volz, K. and Fischer, C.-H. and Raabe, D.
    Progress in Photovoltaics: Research and Applications 23 705-716 (2015)
    In this work, we investigate the p-n junction region for two different buffer/Cu(In,Ga)(Se,S)<inf>2</inf> (CIGSSe) samples having different conversion efficiencies (the cell with pure In<inf>2</inf>S<inf>3</inf> buffer shows a lower efficiency than the nano-ZnS/In<inf>2</inf>S<inf>3</inf> buffered one). To explain the better efficiency of the sample with nano-ZnS/In<inf>2</inf>S<inf>3</inf> buffer layer, combined transmission electron microscopy, atom probe tomography, and X-ray photoelectron spectroscopy studies were performed. In the pure In<inf>2</inf>S<inf>3</inf> buffered sample, a CuIn<inf>3</inf>Se<inf>5</inf> ordered-defect compound is observed at the CIGSSe surface, whereas in the nano-ZnS/In<inf>2</inf>S<inf>3</inf> buffered sample no such compound is detected. The absence of an ordered-defect compound in the latter sample is explained either by the presence of the ZnS nanodots, which may act as a barrier layer against Cu diffusion in CIGSSe hindering the formation of CuIn<inf>3</inf>Se<inf>5</inf>, or by the presence of Zn at the CIGSSe surface, which may disturb the formation of this ordered-defect compound. In the nano-ZnS/In<inf>2</inf>S<inf>3</inf> sample, Zn was found in the first monolayers of the absorber layer, which may lead to a downward band bending at the surface. This configuration is very stable (Fermi level pinning at the conduction band, as observed for Cd in Cu(In,Ga)Se<inf>2</inf>) and reduces the recombination rate at the interface. This effect may explain why the sample with ZnS nanodots possesses a higher efficiency. This work demonstrates the capability of correlative transmission electron microscopy, atom probe tomography, and X-ray photoelectron spectroscopy studies in investigating buried interfaces. The study provides essential information for understanding and modeling the p-n junction at the nanoscale in CIGSSe solar cells. Copyright © 2014 John Wiley & Sons, Ltd.
    view abstractdoi: 10.1002/pip.2484
  • 2015 • 259 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 • 258 Interplay of strain and interdiffusion in Heusler alloy bilayers
    Dutta, B. and Hickel, T. and Neugebauer, J. and Behler, C. and Fähler, S. and Behler, A. and Waske, A. and Teichert, N. and Schmalhorst, J.-M. and Hütten, A.
    Physica Status Solidi - Rapid Research Letters 9 321-325 (2015)
    Combining conventional and inverse magnetocaloric materials promises to enhance solid state refrigeration. As a first step here we present epitaxial Ni-Mn-Ga/Ni-Mn-Sn bilayer films. We examine the dependence of the lateral and normal lattice constants on the deposition sequence by combining experimental and ab initio techniques. Structural properties are determined with X-ray diffraction as well as highresolution transmission electron microscopy, while ab initio calculations explain the interplay of strain, local relaxations and the interdiffusion of atoms. The latter is confirmed by Auger electron spectroscopy and is expected to have a noticeable impact on the functional properties of the Heusler materials. ( © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/pssr.201510070
  • 2015 • 257 Ledges and grooves at γ/γ′ interfaces of single crystal superalloys
    Parsa, A.B. and Wollgramm, P. and Buck, H. and Kostka, A. and Somsen, C. and Dlouhy, A. and Eggeler, G.
    Acta Materialia 90 105-117 (2015)
    In the present work we study the formation of grooves and ledges (typical size: <100 nm) at γ/γ′ interfaces of single crystal Ni-base superalloys. We highlight previous work which documents the presence of such interface irregularities and shows that their number and size increases during high temperature exposure and creep. We use diffraction contrast stereo transmission electron microscopy (TEM) to provide new evidence for the presence of ledges and grooves near dislocations at γ/γ′ interfaces after heat treatment and creep. We present a 2D model of the interfacial region which shows how dislocation stress fields alter local chemical potentials and drive diffusional fluxes which result in the formation of a groove. The results of the numerical study yield realistic groove sizes in relevant time scales. The results obtained in the present study suggest that the formation of grooves and ledges represents an elementary process which needs to be considered when rationalizing the kinetics of rafting, the directional coarsening of the γ′ phase. © 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2015.02.005
  • 2015 • 256 Linear complexions: Confined chemical and structural states at dislocations
    Kuzmina, M. and Herbig, M. and Ponge, D. and Sandlöbes, S. and Raabe, D.
    Science 349 1080-1083 (2015)
    For 5000 years, metals have been mankind's most essential materials owing to their ductility and strength. Linear defects called dislocations carry atomic shear steps, enabling their formability. We report chemical and structural states confined at dislocations. In a body-centered cubic Fe-9 atomic percent Mn alloy, we found Mn segregation at dislocation cores during heating, followed by formation of face-centered cubic regions but no further growth. The regions are in equilibrium with the matrix and remain confined to the dislocation cores with coherent interfaces. The phenomenon resembles interface-stabilized structural states called complexions. A cubic meter of strained alloy contains up to a light year of dislocation length, suggesting that linear complexions could provide opportunities to nanostructure alloys via segregation and confined structural states.
    view abstractdoi: 10.1126/science.aab2633
  • 2015 • 255 Mechanical and chemical investigation of the interface between tungsten-based metallizations and annealed borophosphosilicate glass
    Völker, B. and Heinz, W. and Matoy, K. and Roth, R. and Batke, J.M. and Schöberl, T. and Cordill, M.J. and Dehm, G.
    Thin Solid Films 583 170-176 (2015)
    The focus of this study was on the interface between W-based metallizations and an annealed borophosphosilicate glass (BPSG) dielectric. W-based metallizations are often used in semiconductor devices because of their favourable properties as a diffusion barrier. The interface was characterized mechanically and chemically. For the determination of the interface energy release rate the 4-point-bending method was used. The fracture surfaces resulting from the 4-point-bending experiments were examined to determine the failing interface and the topography of the fracture surfaces. Chemical characterizations of intact interfaces were performed using an electron dispersive X-ray approach in a scanning transmission electron microscope to provide information why Ti incorporated in a W-layer improves the adhesion on annealed BPSG significantly compared to a pureW-layer. © 2015 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.tsf.2015.03.047
  • 2015 • 254 Mechanical properties, microstructure and thermal stability of a nanocrystalline CoCrFeMnNi high-entropy alloy after severe plastic deformation
    Schuh, B. and Mendez-Martin, F. and Völker, B. and George, E.P. and Clemens, H. and Pippan, R. and Hohenwarter, A.
    Acta Materialia 96 258-268 (2015)
    An equiatomic CoCrFeMnNi high-entropy alloy (HEA), produced by arc melting and drop casting, was subjected to severe plastic deformation (SPD) using high-pressure torsion. This process induced substantial grain refinement in the coarse-grained casting leading to a grain size of approximately 50 nm. As a result, strength increased significantly to 1950 MPa, and hardness to ∼520 HV. Analyses using transmission electron microscopy (TEM) and 3-dimensional atom probe tomography (3D-APT) showed that, after SPD, the alloy remained a true single-phase solid solution down to the atomic scale. Subsequent investigations characterized the evolution of mechanical properties and microstructure of this nanocrystalline HEA upon annealing. Isochronal (for 1 h) and isothermal heat treatments were performed followed by microhardness and tensile tests. The isochronal anneals led to a marked hardness increase with a maximum hardness of ∼630 HV at about 450 °C before softening set in at higher temperatures. The isothermal anneals, performed at this peak hardness temperature, revealed an additional hardness rise to a maximum of about 910 HV after 100 h. To clarify this unexpected annealing response, comprehensive microstructural analyses were performed using TEM and 3D-APT. New nano-scale phases were observed to form in the originally single-phase HEA. After times as short as 5 min at 450 °C, a NiMn phase and Cr-rich phase formed. With increasing annealing time, their volume fractions increased and a third phase, FeCo, also formed. It appears that the surfeit of grain boundaries in the nanocrystalline HEA offer many fast diffusion pathways and nucleation sites to facilitate this phase decomposition. The hardness increase, especially for the longer annealing times, can be attributed to these nano-scaled phases embedded in the HEA matrix. The present results give new valuable insights into the phase stability of single-phase high-entropy alloys as well as the mechanisms controlling the mechanical properties of nanostructured multiphase composites. © 2015 Acta Materialia Inc. Published by Elsevier Ltd.
    view abstractdoi: 10.1016/j.actamat.2015.06.025
  • 2015 • 253 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 • 252 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 • 251 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 • 250 Model-independent measurement of the charge density distribution along an Fe atom probe needle using off-axis electron holography without mean inner potential effects
    Migunov, V. and London, A. and Farle, M. and Dunin-Borkowski, R.E.
    Journal of Applied Physics 117 134301 (2015)
    The one-dimensional charge density distribution along an electrically biased Fe atom probe needle is measured using a model-independent approach based on off-axis electron holography in the transmission electron microscope. Both the mean inner potential and the magnetic contribution to the phase shift are subtracted by taking differences between electron-optical phase images recorded with different voltages applied to the needle. The measured one-dimensional charge density distribution along the needle is compared with a similar result obtained using model-based fitting of the phase shift surrounding the needle. On the assumption of cylindrical symmetry, it is then used to infer the three-dimensional electric field and electrostatic potential around the needle with ∼10 nm spatial resolution, without needing to consider either the influence of the perturbed reference wave or the extension of the projected potential outside the field of view of the electron hologram. The present study illustrates how a model-independent approach can be used to measure local variations in charge density in a material using electron holography in the presence of additional contributions to the phase, such as those arising from changes in mean inner potential and specimen thickness. © 2015 AIP Publishing LLC.
    view abstractdoi: 10.1063/1.4916609
  • 2015 • 249 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 • 248 Nanoscale origins of the damage tolerance of the high-entropy alloy CrMnFeCoNi
    Zhang, Z. and Mao, M.M. and Wang, J. and Gludovatz, B. and Zhang, Z. and Mao, S.X. and George, E.P. and Yu, Q. and Ritchie, R.O.
    Nature Communications 6 (2015)
    Damage tolerance can be an elusive characteristic of structural materials requiring both high strength and ductility, properties that are often mutually exclusive. High-entropy alloys are of interest in this regard. Specifically, the single-phase CrMnFeCoNi alloy displays tensile strength levels of ∼1 GPa, excellent ductility (∼60-70%) and exceptional fracture toughness (KJIc &gt;200 MPa √m). Here through the use of in situ straining in an aberration-corrected transmission electron microscope, we report on the salient atomistic to micro-scale mechanisms underlying the origin of these properties. We identify a synergy of multiple deformation mechanisms, rarely achieved in metallic alloys, which generates high strength, work hardening and ductility, including the easy motion of Shockley partials, their interactions to form stacking-fault parallelepipeds, and arrest at planar slip bands of undissociated dislocations. We further show that crack propagation is impeded by twinned, nanoscale bridges that form between the near-tip crack faces and delay fracture by shielding the crack tip.
    view abstractdoi: 10.1038/ncomms10143
  • 2015 • 247 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 • 246 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 • 245 On the identification of superdislocations in the γ′-phase of single-crystal Ni-base superalloys - An application of the LACBED method to complex microstructures
    Müller, J. and Eggeler, G. and Spiecker, E.
    Acta Materialia 87 34-44 (2015)
    Ni-base superalloys are used for turbine blades, which operate in the creep range at temperatures above 1000 °C. One of the objectives of modern materials science is to analyze the combination of elementary deformation and microstructural coarsening processes and to identify physically based micromechanical models which allow one to predict the mechanical behavior on the macroscale. High-temperature creep of single-crystal Ni-base superalloys is governed by dislocation plasticity in the well-known γ/γ′-microstructure. For a comprehensive description of plasticity, it is important to understand the nucleation, glide and climb of superdislocations in the γ′-phase. The rate-controlling dislocation processes have to be identified and therefore a reliable Burgers vector analysis of superdislocations is essential. Superdislocations exhibit complex dislocation cores, typically comprising superpartial dislocations and planar defects. Therefore, conventional Burgers vector analysis based on the invisibility criterion often fails, due to the presence of pronounced residual contrast. In the present work, large-angle convergent-beam electron diffraction (LACBED) is employed for Burgers vector determination of two characteristic superdislocations, of the standard <1 1 0> and the more complex <1 0 0> type. LACBED results are compared with results obtained using the conventional invisibility analysis. While both techniques work for the standard superdislocation, the conventional analysis fails to analyze the <1 0 0> superdislocation, which shows pronounced residual contrast even under conditions of g · b = 0 and g · b × u = 0. In contrast, the LACBED technique allows for an unambiguous determination of the Burgers vector, including its magnitude and absolute sense. In the present study, the use of LACBED to identify dislocations in the complex microstructure of an Ni-base superalloy is outlined and the better performance of LACBED as compared to the conventional gb-analysis is discussed. © 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2014.12.029
  • 2015 • 244 On the room temperature deformation mechanisms of a Mg-Y-Zn alloy with long-period-stacking-ordered structures
    Kim, J.-K. and Sandlöbes, S. and Raabe, D.
    Acta Materialia 82 414-423 (2015)
    We present a transmission electron microscopy study on the room temperature deformation mechanisms in a Mg97Y2Zn1 (at.%) alloy with long-period-stacking-order (LPSO) phase. The alloy consists of α-Mg matrix with platelet-shaped LPSO precipitates 3-5 nm thick and interdendritic LPSO (18R structures) phase grains. The interdendritic LPSO phase was found to deform either by kink-banding in conjunction with basal < a&gt; slip or by basal < a&gt; slip and the formation of dislocation walls. No orientation dependence of these different deformation modes was observed. The α-Mg matrix deforms by basal < a&gt; slip and pyramidal < c + a&gt; slip. No twinning was observed in the α-Mg matrix during room temperature deformation. The elastic modulus mismatch between α-Mg matrix and LPSO plates is suggested to be the main source for activating non-basal dislocations. The combination of the soft α-Mg matrix strengthened by LPSO precipitates and harder "bulk" interdendritic LPSO grains is suggested to contribute to the well-known good mechanical properties of Mg-LPSO alloys at room temperature. © 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2014.09.036
  • 2015 • 243 Origin of shear induced β to ω transition in Ti-Nb-based alloys
    Lai, M.J. and Tasan, C.C. and Zhang, J. and Grabowski, B. and Huang, L.F. and Raabe, D.
    Acta Materialia 92 55-63 (2015)
    Ti-Nb-based alloys are essential materials for biomedical implant and aerospace applications. They reveal complex phase transformation behavior. Here, a {2 1 1}<inf>β</inf>〈1 1 1〉<inf>β</inf> twinning induced β (body-centered cubic phase) to ω (hexagonal phase) transition in Ti-Nb-based alloys is demonstrated by transmission electron microscopy and analyzed employing ab initio calculations and the linear elastic inclusion theory. Our theoretical results reveal a distinct energy barrier for the β to ω transition, where the contribution from lattice rearrangement, rather than the elastic contribution associated with lattice parameter mismatch, plays the major role. It is shown that this energy barrier can be overcome by {2 1 1}<inf>β</inf>〈1 1 1〉<inf>β</inf> shear, explaining why {2 1 1}<inf>β</inf>〈1 1 1〉<inf>β</inf> twinning or, alternatively, the β to α″ (orthorhombic phase) transition promotes local formation of the ω phase. © 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2015.03.040
  • 2015 • 242 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 • 241 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 • 240 Size and orientation effects in partial dislocation-mediated deformation of twinning-induced plasticity steel micro-pillars
    Choi, W.S. and De Cooman, B.C. and Sandlöbes, S. and Raabe, D.
    Acta Materialia 98 391-404 (2015)
    Abstract Bulk and micro-pillar single crystals were used to investigate the twinning-induced plasticity mechanism in austenitic Fe-22 wt%Mn-0.6 wt%C TWIP steel. Compression of micro-pillars oriented either for deformation-induced twinning or for perfect dislocation glide was carried out for pillars with diameters in the range of 600 nm to 4 μm. The same size dependence of the critical resolved shear stress was observed for both orientations. The critical micro-pillar diameter for size-independent plasticity was approximately 7.6 μm. Partial dislocation-mediated formation of twins and ε-martensite was observed in micro-pillars oriented for twinning by transmission electron microscopy. The elastic-plastic transition in micro-pillars oriented for deformation twinning did not involve twinning, and dislocation-dislocation interactions were a necessary precondition for twin formation. © 2015 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2015.06.065
  • 2015 • 239 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 • 238 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 • 237 Synthesis, characterization and in vitro effects of 7 nm alloyed silver-gold nanoparticles
    Ristig, S. and Chernousova, S. and Meyer-Zaika, W. and Epple, M.
    Beilstein Journal of Nanotechnology 6 1212-1220 (2015)
    Alloyed silver-gold nanoparticles were prepared in nine different metal compositions with silver/gold molar ratios of ranging from 90:10 to 10:90. The one-pot synthesis in aqueous medium can easily be modified to gain control over the final particle diameter and the stabilizing agents. The purification of the particles to remove synthesis by-products (which is an important factor for subsequent in vitro experiments) was carried out by multiple ultracentrifugation steps. Characterization by transmission electron microscopy (TEM), differential centrifugal sedimentation (DCS), dynamic light scattering (DLS), UV-vis spectroscopy and atomic absorption spectroscopy (AAS) showed spherical, monodisperse, colloidally stable silver-gold nanoparticles of ≈7 nm diameter with measured molar metal compositions very close to the theoretical values. The examination of the nanoparticle cytotoxicity towards HeLa cells and human mesenchymal stem cells (hMSCs) showed that the toxicity is not proportional to the silver content. Nanoparticles with a silver/gold molar composition of 80:20 showed the highest toxicity. © 2015 Ristig et al.
    view abstractdoi: 10.3762/bjnano.6.124
  • 2015 • 236 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 • 235 The influence of partitioning on the growth of intragranular α in near-β Ti alloys
    Li, T. and Ahmed, M. and Sha, G. and Shi, R. and Casillas, G. and Yen, H.-W. and Wang, Y. and Pereloma, E.V. and Cairney, J.M.
    Journal of Alloys and Compounds 643 212-222 (2015)
    Abstract We report on partitioning of alloying elements during the formation of fine intragranular α plates in a Ti-55521 alloy after thermo-mechanical processing (TMP) and isothermal ageing at 923 K. The microstructures were characterised using atom probe tomography and high-resolution transmission electron microscopy. The partitioning of Mo, V and Al are strongly affected by their diffusivities and their mutual interaction. This leads to a deviation of the measured contents of alloying elements in the two phases from the predicted equilibrium values. The alloying elements at the broad faces and tips of α plates were found to exhibit different pile-up and segregation behaviours, which is thought to affect the lengthening and thickening kinetics of the α plates. As a result, the aspect ratio of α plates decreased rapidly with increasing ageing time. This study suggests that careful selection of alloying elements could be an effective way in controlling the growth anisotropy of α plates and thus α + β microstructures in near-β Ti alloys. © 2015 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.jallcom.2015.04.143
  • 2015 • 234 The Lys-Specific Molecular Tweezer, CLR01, Modulates Aggregation of the Mutant p53 DNA Binding Domain and Inhibits Its Toxicity
    Herzog, G. and Shmueli, M.D. and Levy, L. and Engel, L. and Gazit, E. and Klärner, F.-G. and Schrader, T. and Bitan, G. and Segal, D.
    Biochemistry 54 3729-3738 (2015)
    The tumor suppressor p53 plays a unique role as a central hub of numerous cell proliferation and apoptotic pathways, and its malfunction due to mutations is a major cause of various malignancies. Therefore, it serves as an attractive target for developing novel anticancer therapeutics. Because of its intrinsically unstable DNA binding domain, p53 unfolds rapidly at physiological temperature. Certain mutants shift the equilibrium toward the unfolded state and yield high-molecular weight, nonfunctional, and cytotoxic β-sheet-rich aggregates that share tinctorial and conformational similarities with amyloid deposits found in various protein misfolding diseases. Here, we examined the effect of a novel protein assembly modulator, the lysine (Lys)-specific molecular tweezer, CLR01, on different aggregation stages of misfolded mutant p53 in vitro and on the cytotoxicity of the resulting p53 aggregates in cell culture. We found that CLR01 induced rapid formation of β-sheet-rich, intermediate-size p53 aggregates yet inhibited further p53 aggregation and reduced the cytotoxicity of the resulting aggregates. Our data suggest that aggregation modulators, such as CLR01, could prevent the formation of toxic p53 aggregates. © 2015 American Chemical Society.
    view abstractdoi: 10.1021/bi501092p
  • 2015 • 233 The nucleation of Mo-rich Laves phase particles adjacent to M23C6 micrograin boundary carbides in 12% Cr tempered martensite ferritic steels
    Isik, M.I. and Kostka, A. and Yardley, V.A. and Pradeep, K.G. and Duarte, M.J. and Choi, P.P. and Raabe, D. and Eggeler, G.
    Acta Materialia 90 94-104 (2015)
    We study the nucleation of Mo-rich Laves phase particles during aging and creep of 12 wt.% Cr tempered martensite ferritic steels (TMFS). Recently, in Isik et al. (2014) we reported that Laves phase particles tend to form at micrograin boundaries of TMFSs after Mo and Si had segregated from the ferritic matrix to these internal interfaces. In the present work, we employ transmission electron microscopy (TEM) and atom probe tomography (APT) to study the formation of Laves phase particles. We investigate the preference of Laves phase particles to nucleate next to M23C6 micrograin boundary carbides. Our results suggest that this joint precipitation effect is due to the combined segregation of Mo and Si from the matrix to the micrograin boundaries and Si and P enrichment around the growing carbides.
    view abstractdoi: 10.1016/j.actamat.2015.01.027
  • 2015 • 232 Thermal dissolution mechanisms of AlN/CrN hard coating superlattices studied by atom probe tomography and transmission electron microscopy
    Tytko, D. and Choi, P.-P. and Raabe, D.
    Acta Materialia 85 32-41 (2015)
    AlN/CrN superlattices with a B1 cubic crystal structure and a bilayer period of 4 nm were deposited by reactive radiofrequency magnetron sputtering. The coatings were investigated with respect to their thermal stability and changes in microstructure and chemical composition at 900 °C. The AlN layers show high chemical stability but undergo dissolution by pinching off at grain boundaries. A transformation from cubic to hexagonal AlN with subsequent coarsening at grain boundary triple junctions is observed. In contrast to AlN, the CrN layers show poor chemical stability and their compositions are shifted towards Cr2N upon annealing in a protective argon atmosphere due to nitrogen loss. However, even after establishing Cr2N stoichiometry the crystal structure of the layers remains cubic. © 2014 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2014.11.004
  • 2015 • 231 Toxicity Inhibitors Protect Lipid Membranes from Disruption by Aβ42
    Malishev, R. and Nandi, S. and Kolusheva, S. and Levi-Kalisman, Y. and Klärner, F.-G. and Schrader, T. and Bitan, G. and Jelinek, R.
    ACS Chemical Neuroscience 6 1860-1869 (2015)
    Although the precise molecular factors linking amyloid β-protein (Aβ) to Alzheimer's disease (AD) have not been deciphered, interaction of Aβ with cellular membranes has an important role in the disease. However, most therapeutic strategies targeting Aβ have focused on interfering with Aβ self-assembly rather than with its membrane interactions. Here, we studied the impact of three toxicity inhibitors on membrane interactions of Aβ42, the longer form of Aβ, which is associated most strongly with AD. The inhibitors included the four-residue C-terminal fragment Aβ(39-42), the polyphenol (-)-epigallocatechin-3-gallate (EGCG), and the lysine-specific molecular tweezer, CLR01, all of which previously were shown to disrupt different steps in Aβ42 self-assembly. Biophysical experiments revealed that incubation of Aβ42 with each of the three modulators affected membrane interactions in a distinct manner. Interestingly, EGCG and CLR01 were found to have significant interaction with membranes themselves. However, membrane bilayer disruption was reduced when the compounds were preincubated with Aβ42, suggesting that binding of the assembly modulators to the peptide attenuated their membrane interactions. Importantly, our study reveals that even though the three tested compounds affect Aβ42 assembly differently, membrane interactions were significantly inhibited upon incubation of each compound with Aβ42, suggesting that preventing the interaction of Aβ42 with the membrane contributes substantially to inhibition of its toxicity by each compound. The data suggest that interference with membrane interactions is an important factor for Aβ42 toxicity inhibitors and should be taken into account in potential therapeutic strategies, in addition to disruption or remodeling of amyloid assembly. © 2015 American Chemical Society.
    view abstractdoi: 10.1021/acschemneuro.5b00200
  • 2015 • 230 Two-component self-assembly of a tetra-guanidiniocarbonyl pyrrole cation and Na4EDTA: Formation of pH switchable supramolecular networks
    Samanta, K. and Schmuck, C.
    Chemical Communications 51 16065-16067 (2015)
    A guanidiniocarbonyl pyrrole (GCP) cation forms stable H-bond assisted ion pairs with carboxylates even in aqueous solutions. A tetra GCP cation 1 undergoes efficient two-component self-assembly with Na4EDTA, a tetra-carboxylate, leading to 3D supramolecular networks. These networks show dual pH responsiveness and reversibly dissociate back into monomers upon addition of either acid or base. © The Royal Society of Chemistry 2015.
    view abstractdoi: 10.1039/c5cc06392b
  • 2015 • 229 Uniform 2 nm gold nanoparticles supported on iron oxides as active catalysts for CO oxidation reaction: Structure-activity relationship
    Guo, Y. and Gu, D. and Jin, Z. and Du, P.-P. and Si, R. and Tao, J. and Xu, W.-Q. and Huang, Y.-Y. and Senanayake, S. and Song, Q.-S. and Jia, C.-J. and Schüth, F.
    Nanoscale 7 4920-4928 (2015)
    Uniform Au nanoparticles (∼2 nm) with narrow size-distribution (standard deviation: 0.5-0.6 nm) supported on both hydroxylated (Fe-OH) and dehydrated iron oxide (Fe-O) have been prepared by either deposition-precipitation (DP) or colloidal-deposition (CD) methods. Different structural and textural characterizations were applied to the dried, calcined and used gold-iron oxide samples. Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) showed high homogeneity in the supported Au nanoparticles. The ex situ and in situ X-ray absorption fine structure (XAFS) characterization monitored the electronic and short-range local structure of active gold species. The synchrotron-based in situ X-ray diffraction (XRD), together with the corresponding temperature-programmed reduction by hydrogen (H<inf>2</inf>-TPR), indicated a structural evolution of the iron-oxide supports, correlating to their reducibility. An inverse order of catalytic activity between DP (Au/Fe-OH < Au/Fe-O) and CD (Au/Fe-OH > Au/Fe-O) was observed. Effective gold-support interaction results in a high activity for gold nanoparticles, locally generated by the sintering of dispersed Au atoms on the oxide support in the DP synthesis, while a hydroxylated surface favors the reactivity of externally introduced Au nanoparticles on Fe-OH support for the CD approach. This work reveals why differences in the synthetic protocol translate to differences in the catalytic performance of Au/FeO<inf>x</inf> catalysts with very similar structural characteristics in CO oxidation. © The Royal Society of Chemistry 2015.
    view abstractdoi: 10.1039/c4nr06967f
  • 2015 • 228 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
  • 2014 • 227 Amine-based solvents for exfoliating graphite to graphene outperform the dispersing capacity of N-methyl-pyrrolidone and surfactants
    Sun, Z. and Huang, X. and Liu, F. and Yang, X. and Rösler, C. and Fischer, R.A. and Muhler, M. and Schuhmann, W.
    Chemical Communications 50 10382-10385 (2014)
    Four organic amine-based solvents were discovered which enable direct exfoliation of graphite to produce high-quality and oxygen-free graphene nanosheets. These solvents outperform previously used solvents and additives such as N-methyl-pyrrolidone and surfactants in terms of their dispersing capacity. The resulting dispersions allow the facile fabrication of zeolitic imidazolate framework (ZIF)-graphene nanocomposites with remarkable CO 2 storage capability. This journal is © the Partner Organisations 2014.
    view abstractdoi: 10.1039/c4cc03923h
  • 2014 • 226 Atomic-scale analysis of carbon partitioning between martensite and austenite by atom probe tomography and correlative transmission electron microscopy
    Toji, Y. and Matsuda, H. and Herbig, M. and Choi, P.-P. and Raabe, D.
    Acta Materialia 65 215-228 (2014)
    Carbon partitioning between ferritic and austenitic phases is essential for austenite stabilization in the most advanced steels such as those produced by the quenching and partitioning (Q&P) process. The atomistic analysis of the carbon partitioning in Q&P alloys is, however, difficult owing to the simultaneous occurrence of bainite transformation, which can also contribute to carbon enrichment into remaining austenite and hence overlap with the carbon partitioning from martensite into austenite. Therefore, we provide here a direct atomic-scale evidence of carbon partitioning from martensite into austenite without the presence of bainite transformation. Carbon partitioning is investigated by means of atom probe tomography and correlative transmission electron microscopy. A model steel (Fe-0.59 wt.% C (2.7 at.% C)-2.0 wt.% Si-2.9 wt.% Mn) with martensite finish temperature below room temperature was designed and used in order to clearly separate the carbon partitioning between martensite and austenite from the bainite transformation. The steel was austenitized at 900°C, then water-quenched and tempered at 400°C. Approximately 8 vol.% retained austenite existed in the asquenched state. We confirmed by X-ray diffraction and dilatometry that austenite decomposition via bainite transformation did not occur during tempering. No carbon enrichment in austenite was observed in the as-quenched specimen. On the other hand, clear carbon enrichment in austenite was observed in the 400°C tempered specimens with a carbon concentration inside the austenite of 5-8 at.%. The results hence quantitatively revealed carbon partitioning from martensite to austenite, excluding bainite transformation during the Q&P heat treatment. © 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2013.10.064
  • 2014 • 225 Atomic-scale quantification of grain boundary segregation in nanocrystalline material
    Herbig, M. and Raabe, D. and Li, Y.J. and Choi, P. and Zaefferer, S. and Goto, S.
    Physical Review Letters 112 (2014)
    Grain boundary segregation leads to nanoscale chemical variations that can alter a material's performance by orders of magnitude (e.g., embrittlement). To understand this phenomenon, a large number of grain boundaries must be characterized in terms of both their five crystallographic interface parameters and their atomic-scale chemical composition. We demonstrate how this can be achieved using an approach that combines the accuracy of structural characterization in transmission electron microscopy with the 3D chemical sensitivity of atom probe tomography. We find a linear trend between carbon segregation and the misorientation angle ω for low-angle grain boundaries in ferrite, which indicates that ω is the most influential crystallographic parameter in this regime. However, there are significant deviations from this linear trend indicating an additional strong influence of other crystallographic parameters (grain boundary plane, rotation axis). For high-angle grain boundaries, no general trend between carbon excess and ω is observed; i.e., the grain boundary plane and rotation axis have an even higher influence on the segregation behavior in this regime. Slight deviations from special grain boundary configurations are shown to lead to unexpectedly high levels of segregation. © 2014 American Physical Society.
    view abstractdoi: 10.1103/PhysRevLett.112.126103
  • 2014 • 224 Bulk mixed ion electron conduction in amorphous gallium oxide causes memristive behaviour
    Aoki, Y. and Wiemann, C. and Feyer, V. and Kim, H.-S. and Schneider, C.M. and Ill-Yoo, H. and Martin, M.
    Nature Communications 5 (2014)
    In thin films of mixed ionic electronic conductors sandwiched by two ion-blocking electrodes, the homogeneous migration of ions and their polarization will modify the electronic carrier distribution across the conductor, thereby enabling homogeneous resistive switching. Here we report non-filamentary memristive switching based on the bulk oxide ion conductivity of amorphous GaOx (x~1.1) thin films. We directly observe reversible enrichment and depletion of oxygen ions at the blocking electrodes responding to the bias polarity by using photoemission and transmission electron microscopies, thus proving that oxygen ion mobility at room temperature causes memristive behaviour. The shape of the hysteresis I-V curves is tunable by the bias history, ranging from narrow counter figure-eight loops to wide hysteresis, triangle loops as found in the mathematically derived memristor model. This dynamical behaviour can be attributed to the coupled ion drift and diffusion motion and the oxygen concentration profile acting as a state function of the memristor. © 2014 Macmillan Publishers Limited. All rights reserved.
    view abstractdoi: 10.1038/ncomms4473
  • 2014 • 223 Carbide precipitation during tempering of a tool steel subjected to deep cryogenic treatment
    Gavriljuk, V.G. and Sirosh, V.A. and Petrov, Y.N. and Tyshchenko, A.I. and Theisen, W. and Kortmann, A.
    Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science 45 2453-2465 (2014)
    Using transmission electron microscopy, Mössbauer spectroscopy, and measurements of hardness, the carbide precipitation during tempering of steel X153CrMoV12 containing (mass pct) 1.55C, 11.90Cr, 0.70V, and 0.86Mo is studied after three treatments: quenching at RT and deep cryogenic treatment, DCT, at 77 K or 123 K (-196 °C or -150 °C). In contrast to some previous studies, no fine carbide precipitation after long-time holding at cryogenic temperatures is detected. After quenching at room temperature, RT, the transient ε(ε) carbide is precipitated between 373 K and 473 K (100 °C and 200 °C) and transformed to cementite starting from 573 K (300 °C). In case of DCT at 123 K (-150 °C), only fine cementite particles are detected after tempering at 373 K (200 °C) with their delayed coarsening at higher temperatures. Dissolution of cementite and precipitation of alloying element carbides proceed at 773 K (500 °C) after quenching at RT, although some undissolved cementite plates can also be observed. After DCT at 123 K (-150 °C), the transient ε(ε) carbide is not precipitated during tempering, which is attributed to the intensive isothermal martensitic transformation accompanied by plastic deformation. In this case, cementite is the only carbide phase precipitated in the temperature range of 573 K to 773 K (300 °C to 500 °C). If DCT is carried out at 77 K (-196 °C), the ε(ε) carbide is found after tempering at 373 K to 473 K (100 °C to 200 °C). Coarse cementite particles and the absence of alloying element carbides constitute a feature of steel subjected to DCT and tempering at 773 K (500 °C). As a result, a decreased secondary hardness is obtained in comparison with the steel quenched at RT. According to Mössbauer studies, the structure after DCT and tempering at 773 K (500 °C) is characterized by the decreased fraction of the retained austenite and clustering of alloying elements in the α solid solution. It is suggested that a competition between the strain-induced transformation of the retained austenite and carbide precipitation during the wear can control the life of steel tools. © 2014 The Minerals, Metals & Materials Society and ASM International.
    view abstractdoi: 10.1007/s11661-014-2202-8
  • 2014 • 222 Comparison of silver nanoparticles stored under air or argon with respect to the induction of intracellular free radicals and toxic effects toward keratinocytes
    Ahlberg, S. and Meinke, M.C. and Werner, L. and Epple, M. and Diendorf, J. and Blume-Peytavi, U. and Lademann, J. and Vogt, A. and Rancan, F.
    European Journal of Pharmaceutics and Biopharmaceutics 88 651-657 (2014)
    Bacterial infections decreased considerably after the discovery of antibiotics. Nevertheless, because of the rising rate of infections caused by antibiotic-resistant bacteria strains, the search for new bactericidal agents has again become a crucial topic in clinical medicine. Silver nanoparticles (AgNP) have a huge potential in dermatology and wound care management because of their ability to release silver ions (Ag+ ions) in a prolonged and sustained way. However, negative effects of silver on the patient's cells should not be underestimated. Furthermore, it has been controversially discussed whether AgNP are responsible for nanoparticle-specific outcomes or not. In this study, we investigated the effects of AgNP on human skin keratinocytes (HaCaT) in order to better understand the mechanisms of cytotoxicity and to improve the use of this highly reactive biocide in wound healing. We found that most of the cells with internalized AgNP displayed the typical morphological signs of apoptosis. The cell viability assay (XTT) showed concentration-dependent toxic effects of the AgNP toward HaCaT cells. The generation of reactive oxygen species (ROS) induced by AgNP was investigated in cell suspensions by means of electron paramagnetic resonance (EPR) spectroscopy. In order to distinguish between the effects of Ag+ ions released during AgNP storage and those of Ag+ ions released after nanoparticle application, we compared AgNP stored under air (O2) with AgNP stored under argon (Ar). Dispersions of AgNP stored under Ar have a low content of Ag+ ions because of the absence of oxygen which is needed for oxidative dissolution. The results show that Ag+ ions released during particle storage are responsible for most of the ROS produced during 1 h incubation with the cells. AgNP (Ar) also induced intracellular ROS but to a much smaller extent compared to AgNP (O2). These findings highlight the complexity of experiments to assess the toxicity of AgNP and suggest the possibility of reducing AgNP toxic effects by storing AgNP formulations and even silver-containing wound dressing under an inert gas atmosphere. © 2014 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.ejpb.2014.07.012
  • 2014 • 221 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 • 220 Crystallization, phase evolution and corrosion of Fe-based metallic glasses: An atomic-scale structural and chemical characterization study
    Duarte, M.J. and Kostka, A. and Jimenez, J.A. and Choi, P. and Klemm, J. and Crespo, D. and Raabe, D. and Renner, F.U.
    Acta Materialia 71 20-30 (2014)
    Understanding phase changes, including their formation and evolution, is critical for the performance of functional as well as structural materials. We analyze in detail microstructural and chemical transformations of the amorphous steel Fe50Cr15Mo14C15B6 during isothermal treatments at temperatures ranging from 550 to 800 °C. By combining high-resolution transmission electron microscopy and Rietveld analyses of X-ray diffraction patterns together with the local chemical data obtained by atom probe tomography, this research provides relevant information at the atomic scale about the mechanisms of crystallization and the subsequent phases evolution. During the initial stages of crystallization a stable (Fe,Cr) 23(C,B)6 precipitates as well as two metastable intermediates of M3(C,B) and the intermetallic χ-phase. When full crystallization is reached, only a percolated nano-scale Cr-rich (Fe,Cr) 23(C,B)6 and Mo-rich η-Fe3Mo3C structure is detected, with no evidence to suggest that other phases appear at any subsequent time. Finally, the corrosion behavior of the developed phases is discussed from considerations of the obtained atomic information. © 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2014.02.027
  • 2014 • 219 Cyclic degradation mechanisms in aged FeNiCoAlTa shape memory single crystals
    Krooß, P. and Somsen, C. and Niendorf, T. and Schaper, M. and Karaman, I. and Chumlyakov, Y. and Eggeler, G. and Maier, H.J.
    Acta Materialia 79 126-137 (2014)
    This study focuses on the functional stability of [0 0 1]-oriented Fe 41Ni28Co17Al11.5Ta2.5 (at.%) single crystals. It is shown that functional degradation of aged FeNiCoAlTa, containing fine dispersed γ′-particles ∼5-8 nm in diameter is caused by the interaction of different martensite variants under cyclic loading in tension. Superelastic cycling experiments up to 4.5% total strain resulted in the accumulation of permanent strain mainly caused by the formation of retained martensite. In situ observations were conducted in order to evaluate the local strain evolution and martensite variant interactions on the meso- and microscale. Optical microscopy and transmission electron microscopy observations revealed various differently oriented martensite variants which were retained upon 100 superelastic cycles. In addition, fine martensitic structures remaining in the vicinity of the γ′ precipitates were found after mechanical cycling, which are shown to be important for cyclic degradation in Fe-based shape memory alloys. © 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2014.06.019
  • 2014 • 218 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 • 217 Direct microstructural evidence for the stress induced formation of martensite during nanonindentation of NiTi
    Pfetzing-Micklich, J. and Wieczorek, N. and Simon, T. and Maaß, B. and Eggeler, G.
    Materials Science and Engineering A 591 33-37 (2014)
    When a pseudoelastic NiTi alloy is loaded and subsequently unloaded, stress induced martensite forms and disappears. It is challenging to directly observe and characterize the local martensitic features in such alloys. In the present study we use a specific NiTi alloy where stress induced martensite is thermally stable during unloading to prove the formation of martensite during nanoindentation. TEM investigations provide direct microstructural evidence for the martensitic phase transformation during nanoindentation. Subsequent in-situ heating in TEM shows how the stress induced martensite transforms back to austenite. © 2013 Elsevier B.V.
    view abstractdoi: 10.1016/j.msea.2013.10.035
  • 2014 • 216 Fabrication of Ag/TiO2 photocatalyst for the treatment of simulated hospital wastewater under sunlight
    Badawy, M.I. and Souaya, E.M.R. and Gad-Allah, T.A. and Abdel-Wahed, M.S. and Ulbricht, M.
    Environmental Progress and Sustainable Energy 33 886-894 (2014)
    In this study, photocatalytic activity of titanium dioxide was modified by doping with silver metal. This was done by simple preparation procedure at room temperature. Different preparation conditions were studied and their effects on photocatalytic activity were investigated. The obtained nanopowders were characterized using X-ray diffraction (XRD), X-ray fluorescence (XRF), specific surface area measurement, UV-Visible absorption, and transmission electron microscope (TEM). Photocatalytic activities of the prepared samples under simulated sunlight were evaluated with respect to their efficiencies for the degradation of five pharmaceutical compounds commonly present in hospital wastewater. Prepared samples showed very high efficiency for photodegradation of the studied pharmaceutical compounds. Maximum photodegradation rate of the simulated hospital wastewater was obtained using 1000 ppm of the material with 0.1% Ag/TiO2 (weight ratio) calcined at 300°C and pH 5. However, the best pH of the degradation of each pharmaceutical compound varies according to the pKa. © 2013 American Institute of Chemical Engineers.
    view abstractdoi: 10.1002/ep.11869
  • 2014 • 215 Fetuin-A and albumin alter cytotoxic effects of calcium phosphate nanoparticles on human vascular smooth muscle cells
    Dautova, Y. and Kozlova, D. and Skepper, J.N. and Epple, M. and Bootman, M.D. and Proudfoot, D.
    PLoS ONE 9 (2014)
    Calcification is a detrimental process in vascular ageing and in diseases such as atherosclerosis and arthritis. In particular, small calcium phosphate (CaP) crystal deposits are associated with inflammation and atherosclerotic plaque de-stabilisation. We previously reported that CaP particles caused human vascular smooth muscle cell (VSMC) death and that serum reduced the toxic effects of the particles. Here, we found that the serum proteins fetuin-A and albumin (≥1 μM) reduced intracellular Ca2+ elevations and cell death in VSMCs in response to CaP particles. In addition, CaP particles functionalised with fetuin-A, but not albumin, were less toxic than naked CaP particles. Electron microscopic studies revealed that CaP particles were internalised in different ways; via macropinocytosis, membrane invagination or plasma membrane damage, which occurred within 10 minutes of exposure to particles. However, cell death did not occur until approximately 30 minutes, suggesting that plasma membrane repair and survival mechanisms were activated. In the presence of fetuin-A, CaP particle-induced damage was inhibited and CaP/plasma membrane interactions and particle uptake were delayed. Fetuin-A also reduced dissolution of CaP particles under acidic conditions, which may contribute to its cytoprotective effects after CaP particle exposure to VSMCs. These studies are particularly relevant to the calcification observed in blood vessels in patients with kidney disease, where circulating levels of fetuin-A and albumin are low, and in pathological situations where CaP crystal formation outweighs calcification-inhibitory mechanisms. © 2014 Dautova et al.
    view abstractdoi: 10.1371/journal.pone.0097565
  • 2014 • 214 Generation of NiTi nanoparticles by femtosecond laser ablation in liquid
    Chakif, M. and Essaidi, A. and Gurevich, E. and Ostendorf, A. and Prymak, O. and Epple, M.
    Journal of Materials Engineering and Performance 23 2482-2486 (2014)
    NiTi was investigated as a model system for a binary alloy where the properties strongly depend on the relative proportion of the two elements and on the grain size. The NiTi nanoparticles were generated by laser ablation in water. For the analysis of the particle size distribution, we used transmission electron microscopy and dynamic light scattering. Here, we found a broad particle size distribution (10-200 nm). Furthermore, the temperature-resolved x-ray powder diffraction and differential scanning calorimetry (DSC) were used to evaluate the phase transition behavior of the generated NiTi nanoparticles. Here, we found an interesting effect. During the heating by DSC, an austenite phase transition and a weak martensite phase transition in the NiTi nanoparticles appeared. Moreover, the phase transformation temperature was about 40 K lower than that of the bulk target. © 2014 ASM International.
    view abstractdoi: 10.1007/s11665-014-1007-7
  • 2014 • 213 Gold nanoparticles interfere with sperm functionality by membrane adsorption without penetration
    Taylor, U. and Barchanski, A. and Petersen, S. and Kues, W.A. and Baulain, U. and Gamrad, L. and Sajti, L. and Barcikowski, S. and Rath, D.
    Nanotoxicology 8 118-127 (2014)
    To examine gold nanoparticle reprotoxicity, bovine spermatozoa were challenged with ligand-free or oligonucleotide-conjugated gold nanoparticles synthesized purely without any surfactants by laser ablation. Sperm motility declined at nanoparticle mass dose of 10 μg/ml (corresponding to ∼14 000 nanoparticles per sperm cell) regardless of surface modification. Sperm morphology and viability remained unimpaired at all concentrations. Transmission electron microscopy showed an modification dependant attachment of nanoparticles to the cell membrane of spermatozoa, but provided no evidence for nanoparticle entrance into sperm cells. A molecular examination revealed a reduction of free thiol residues on the cell membrane after nanoparticle exposure, which could explain the decrease in sperm motility. Sperm fertilising ability decreased after exposure to 10 μg/ml of ligand-free nanoparticles indicating that agglomerated ligand-free nanoparticles interfere with membrane properties necessary for fertilisation. In conclusion, nanoparticles may impair key sperm functions solely by interacting with the sperm surface membrane. © 2014 Informa UK Ltd. All rights reserved.
    view abstractdoi: 10.3109/17435390.2013.859321
  • 2014 • 212 Growth mechanism of Al2Cu precipitates during in situ TEM heating of a HPT deformed Al-3wt.%Cu alloy
    Rashkova, B. and Faller, M. and Pippan, R. and Dehm, G.
    Journal of Alloys and Compounds 600 43-50 (2014)
    The microstructural evolution of Al2Cu precipitates in an ultrafine-grained Al-3wt.% Cu model alloy produced by high-pressure torsion (HPT) was studied by in situ transmission electron microscopy (TEM). The precipitation growth was systematically investigated by isothermal heating experiments in the temperature range of 120 C to 170 C. The experimental data is analysed with respect of the diffusion kinetics and activation energy to determine the most prominent diffusion path: lattice or grain boundary diffusion. The results imply that grain boundary diffusion is the relevant mechanism for Al2Cu growth in the HPT deformed material. © 2014 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.jallcom.2014.02.090
  • 2014 • 211 Hidden structural features of multicompartment micelles revealed by cryogenic transmission electron tomography
    Löbling, T.I. and Haataja, J.S. and Synatschke, C.V. and Schacher, F.H. and Müller, M. and Hanisch, A. and Gröschel, A.H. and Müller, A.H.E.
    ACS Nano 8 11330-11340 (2014)
    The demand for ever more complex nanostructures in materials and soft matter nanoscience also requires sophisticated characterization tools for reliable visualization and interpretation of internal morphological features. Here, we address both aspects and present synthetic concepts for the compartmentalization of nanoparticle peripheries as well as their in situ tomographic characterization. We first form negatively charged spherical multicompartment micelles from ampholytic triblock terpolymers in aqueous media, followed by interpolyelectrolyte complex (IPEC) formation of the anionic corona with bis-hydrophilic cationic/neutral diblock copolymers. At a 1:1 stoichiometric ratio of anionic and cationic charges, the so-formed IPECs are charge neutral and thus phase separate from solution (water). The high chain density of the ionic grafts provides steric stabilization through the neutral PEO corona of the grafted diblock copolymer and suppresses collapse of the IPEC; instead, the dense grafting results in defined nanodomains oriented perpendicular to the micellar core. We analyze the 3D arrangements of the complex and purely organic compartments, in situ, by means of cryogenic transmission electron microscopy (cryo-TEM) and tomography (cryo-ET). We study the effect of block lengths of the cationic and nonionic block on IPEC morphology, and while 2D cryo-TEM projections suggest similar morphologies, cryo-ET and computational 3D reconstruction reveal otherwise hidden structural features, e.g., planar IPEC brushes emanating from the micellar core. (Figure Presented). © 2014 American Chemical Society.
    view abstractdoi: 10.1021/nn504197y
  • 2014 • 210 High-concentration graphene dispersions with minimal stabilizer: a scaffold for enzyme immobilization for glucose oxidation
    Sun, Z. and Vivekananthan, J. and Guschin, D.A. and Huang, X. and Kuznetsov, V. and Ebbinghaus, P. and Sarfraz, A. and Muhler, M. and Schuhmann, W.
    Chemistry (Weinheim an der Bergstrasse, Germany) 20 5752-5761 (2014)
    Modified acrylate polymers are able to effectively exfoliate and stabilize pristine graphene nanosheets in aqueous media. Starting with pre-exfoliated graphite greatly promotes the exfoliation level. The graphene concentration is significantly increased up to 11 mg mL(-1) by vacuum evaporation of the solvent from the dispersions under ambient temperature. TEM shows that 75 % of the flakes have fewer than five layers with about 18 % of the flakes consisting of monolayers. Importantly, a successive centrifugation and redispersion strategy is developed to enable the formation of dispersions with exceptionally high graphene-to-stabilizer ratio. Characterization by high-resolution transmission electron microscopy, X-ray photoelectron spectroscopy, X-ray diffraction, and Raman spectroscopy shows the flakes to be of high quality with very low levels of defects. These dispersions can act as a scaffold for the immobilization of enzymes applied, for example, in glucose oxidation. The electrochemical current density was significantly enhanced to be approximately six times higher than an electrode in the absence of graphene, thus showing potential applications in enzymatic biofuel cells. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/chem.201400098
  • 2014 • 209 High-throughput fabrication of Au-Cu nanoparticle libraries by combinatorial sputtering in ionic liquids
    König, D. and Richter, K. and Siegel, A. and Mudring, A.-V. and Ludwig, Al.
    Advanced Functional Materials 24 2049-2056 (2014)
    Materials libraries of binary alloy nanoparticles (NPs) are synthesized by combinatorial co-sputter deposition of Cu and Au into the ionic liquid (IL) 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([C 1C4im][Tf2N]), which is contained in a micromachined cavity array substrate. The resulting NPs and NP-suspensions are investigated by transmission electron microscopy (TEM), X-ray diffraction (XRD), UV-Vis measurements (UV-Vis), and attenuated total reflection Fourier transformed infrared (ATR-FTIR) spectroscopy. Whereas the NPs can be directly observed in the IL using TEM, for XRD measurements the NP concentration is too low to lead to satisfactory results. Thus, a new NP isolation process involving capping agents is developed which enables separation of NPs from the IL without changing their size, morphology, composition, and state of aggregation. The results of the NP characterization show that next to the unary Cu and Au NPs, both stoichiometric and non-stoichiometric Cu-Au NPs smaller than 7 nm can be readily obtained. Whereas the size and shape of the alloy NPs change with alloy composition, for a fixed composition the NPs have a small size distribution. The measured lattice constants of all capped NPs show unexpected increased values, which could be related to the NP/surfactant interactions. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/adfm.201303140
  • 2014 • 208 Impact of nanodiffusion on the stacking fault energy in high-strength steels
    Hickel, T. and Sandlöbes, S. and Marceau, R.K.W. and Dick, A. and Bleskov, I. and Neugebauer, J. and Raabe, D.
    Acta Materialia 75 147-155 (2014)
    A key requirement of modern steels - the combination of high strength and high deformability - can best be achieved by enabling a local adaptation of the microstructure during deformation. A local hardening is most efficiently obtained by a modification of the stacking sequence of atomic layers, resulting in the formation of twins or martensite. Combining ab initio calculations with in situ transmission electron microscopy, we show that the ability of a material to incorporate such stacking faults depends on its overall chemical composition and, importantly, the local composition near the defect, which is controlled by nanodiffusion. Specifically, the role of carbon for the stacking fault energy in high-Mn steels is investigated. Consequences for the long-term mechanical properties and the characterisation of these materials are discussed. © 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2014.04.062
  • 2014 • 207 Impact of solvent mixture on iron nanoparticles generated by laser ablation
    Chakif, M. and Prymak, O. and Slota, M. and Heintze, E. and Gurevich, E.L. and Esen, C. and Bogani, L. and Epple, M. and Ostendorf, A.
    Progress in Biomedical Optics and Imaging - Proceedings of SPIE 8955 (2014)
    The present work reveals the structural and magnetic properties of iron oxide (FexOy) nanoparticles (NPs) prepared by femtosecond laser ablation. The FexOy-NPs were produced in solutions consisting of different ratios of water and acetone. Laser ablation in water yields agglomerates and that in acetone yields chain structures whereas that in water/acetone show a mixture of both. We observe significant fabrication dependent properties such as different crystallinities and magnetic behaviors. The structural characterization shows a change from iron (Fe) to a Fe xOy state of the NPs which depends on the solution composition. Furthermore, transmission electron microscopy measurements exhibit a broad particle size distribution in all samples but with significant differences in the mean sizes. Using magnetic measurements we show that nanoparticles fabricated in pure acetone have lower coercive fields which come along with a smaller mean particle size and therefore increasing superparamagnetic behavior. © 2014 SPIE.
    view abstractdoi: 10.1117/12.2037682
  • 2014 • 206 Impacts of geometry, symmetry, and morphology of nanocast Co3O4 on its catalytic activity for water oxidation
    Deng, X. and Schmidt, W. and Tüysüz, H.
    Chemistry of Materials 26 6127-6134 (2014)
    Herein, we report a systematic study on the synthesis of ordered mesoporous Co3O4 nanocast from cubically (KIT-6) and hexagonally (SBA-15) ordered mesoporous silica hard templates. By increasing the number of impregnation cycles, the effect of loading amount on the replica symmetry as well as on its microstructure and textural parameters was investigated in detail by transmission electron microscopy (TEM), small-angle X-ray scattering (SAXS), and N2 sorption. By changing the loading amount of the metal precursor, we could modify the symmetry, pore systems, and morphologies of the replicas. Low loading favors formation of different symmetry in case of replication of cubically ordered mesoporous Co3O4. Increasing the loading amount results in a perfect negative replica of the KIT-6 silica template. Using the 2D ordered SBA-15, the symmetry of the Co3O4 replicas followed that of the template, regardless of its loading amount. However, the degree of the interconnectivity and the length of the nanowires increased. From the cubically ordered Co3O4 replicas the one with lowest symmetry and open pore system performed best as catalyst for water oxidation whereas for hexagonally ordered Co3O4 replicas highest activity was observed with nanowires that have higher degree of the ordering and interconnectivity. The electrocatalytic results for water oxidation showed that hexagonally ordered Co3O4 shows superior activity to the cubically ordered one. © 2014 American Chemical Society.
    view abstractdoi: 10.1021/cm5023163
  • 2014 • 205 In Situ Particle Size Measurements of Gas-borne Silicon Nanoparticles by Time-resolved Laser-induced Incandescence
    Sipkens, T. A. and Petermann, N. and Daun, K. J. and Titantah, J. and Karttunen, M. and Wiggers, H. and Dreier, T. and Schulz, C.
    Proceedings of the Asme Summer Heat Transfer Conference - 2013, Vol 1 V001T03A001 (2014)
    The functionality of silicon nanoparticles is strongly size-dependent, so there is a pressing need for laser diagnostics that can characterize aerosolized silicon nanoparticles. The present work is the first attempt to extend time-resolved laser-induced incandescence (TiRe-LII), a combustion diagnostic used for sizing soot, to size silicon nanoparticles. TiRe-LII measurements are made on silicon nanoparticles synthesized in a low-pressure plasma reactor containing argon. Molecular dynamics (MD) is used to predict the accommodation coefficient between silicon nanoparticles and argon and helium, which is needed to interpret the TiRe-LII data. The MD-derived thermal accommodation coefficients will be validated by comparing them to experimentally-derived values found using transmission electron microscopy (TEM) and Brunauer-Emmett-Teller (BET) analysis.
    view abstractdoi: 10.1115/HT2013-17246
  • 2014 • 204 Interface reactions of Ag@TiO2 nanocomposite films
    Zuo, J. and Rao, J. and Eggeler, G.
    Materials Chemistry and Physics 145 90-98 (2014)
    TiO2 films were sputtered on 100-nm-thick Ag layers at various O2 partial pressures to study forming processes at the interface. The interfacial reactions during the deposition process were investigated by means of transmission electron microscopy, X-ray photoelectron spectroscopy, X-ray diffraction, atomic force microscopy and UV-vis reflection spectra. The sputtering process led to formation of Ag nanoparticles surrounded by Ag 2O and TiO2 in the TiO2 film matrix as well as on the surface. The presence of oxygen in the plasma resulted in enrichment of silver oxides on the surface and an intermixing of Ag in the TiO2 matrix. The film structures could be explained based on the interplay among the formation of silver oxide, the nucleation and growth of TiO2, as well as the mobility of silver and silver oxides within the growing TiO2 films. © 2014 Elsevier B.V.
    view abstractdoi: 10.1016/j.matchemphys.2014.01.041
  • 2014 • 203 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 • 202 Ligand-free gold atom clusters adsorbed on graphene nano sheets generated by oxidative laser fragmentation in water
    Lau, M. and Haxhiaj, I. and Wagener, P. and Intartaglia, R. and Brandi, F. and Nakamura, J. and Barcikowski, S.
    Chemical Physics Letters 610-611 256-260 (2014)
    Over three decades after the first synthesis of stabilized Au 55-clusters many scientific questions about gold cluster properties are still unsolved and ligand-free colloidal clusters are difficult to fabricate. Here we present a novel route to produce ultra-small gold particles by using a green technique, the laser ablation and fragmentation in water, without using reductive or stabilizing agents at any step of the synthesis. For fabrication only a pulsed laser, a gold-target, pure water, sodium hydroxide and hydrogen peroxide are deployed. The particles are exemplarily hybridized to graphene supports showing that these carbon-free colloidal clusters might serve as versatile building blocks. © 2014 Published by Elsevier B.V.
    view abstractdoi: 10.1016/j.cplett.2014.07.047
  • 2014 • 201 Microstructural and defect analysis of metal nanoparticles in functional catalysts by diffraction and electron microscopy: The Cu/ZnO catalyst for methanol synthesis
    Kandemir, T. and Kasatkin, I. and Girgsdies, F. and Zander, S. and Kühl, S. and Tovar, M. and Schlögl, R. and Behrens, M.
    Topics in Catalysis 57 188-206 (2014)
    The application of different methods for a microstructural analysis of functional catalysts is reported for the example of different Cu/ZnO-based methanol synthesis catalysts. Transmission electron microscopy and diffraction were used as complementary techniques to extract information on the size and the defect concentration of the Cu nano-crystallites. The results, strengths and limitations of the two techniques and of different evaluation methods for line profile analysis of diffraction data including Rietveld-refinement, Scherrer- and (modified) Williamson-Hall-analyses, single peak deconvolution and whole powder pattern modeling are compared and critically discussed. It was found that in comparison with a macrocrystalline pure Cu sample, the catalysts were not only characterized by a smaller crystallite size, but also by a high concentration of lattice defects, in particular stacking faults. Neutron diffraction was introduced as a valuable tool for such analysis, because of the larger number of higher-order diffraction peaks that can be detected with this method. An attempt is reported to quantify the different types of defects for a selected catalyst. © 2013 Springer Science+Business Media New York.
    view abstractdoi: 10.1007/s11244-013-0175-2
  • 2014 • 200 MSC-derived exosomes: A novel tool to treat therapy-refractory graft-versus-host disease
    Kordelas, L. and Rebmann, V. and Ludwig, A.-K. and Radtke, S. and Ruesing, J. and Doeppner, T.R. and Epple, M. and Horn, P.A. and Beelen, D.W. and Giebel, B.
    Leukemia 28 970-973 (2014)
    doi: 10.1038/leu.2014.41
  • 2014 • 199 Nanoscale understanding of bond formation during cold welding of aluminum and steel
    Altin, A. and Wohletz, S. and Krieger, W. and Kostka, A. and Groche, P. and Erbe, A.
    Advanced Materials Research 966-967 445-452 (2014)
    Cold welding, e.g. by cold forging, is a smart manufacturing technology, enabling novel multi material designs. A material combination, which is particularly attractive for manufacturing, though challenging to handle in a cold welding process, is steel and aluminum. We investigate the bond formation between cold forged C 15 (mainly primary heat treated) and AW 6082. Analysis starts with numerical simulations using the finite element analysis (FEA) to identify optimum conditions for bond formation. The bond strength was determined by tensile tests from samples eroded from the cold-welded specimen. Best performing samples showed a maximum tensile strength of ~200 MPa with ductile failure in the AW 6082. Transmission electron microscopy (TEM) inspection of the bonded area between aluminum and steel show a reaction layer consisting of iron and aluminum of few nm thickness throughout the sample. The formation of such a reaction layer is hypothesized to be crucial for bond formation. © (2014) Trans Tech Publications, Switzerland.
    view abstractdoi: 10.4028/
  • 2014 • 198 Near-field-enhanced, off-resonant laser sintering of semiconductor particles for additive manufacturing of dispersed Au-ZnO-micro/nano hybrid structures
    Lau, M. and Niemann, R.G. and Bartsch, M. and O'Neill, W. and Barcikowski, S.
    Applied Physics A: Materials Science and Processing 114 1023-1030 (2014)
    Off-resonant near-field enhancement by gold nanoparticles adsorbed on crystalline zinc oxide significantly increases the energy efficiency of infrared laser sintering. In detail, ten different gold mass loads on zinc oxide were exposed to 1,064 nm cw-laser radiation. Variation of scan speed, laser power, and spot size showed that the energy threshold required for sintering decreases and sintering process window widens compared to laser sintering of pure zinc oxide powder. Transmission electron microscope analysis after focused ion beam cross sectioning of the sintered particles reveals that supported gold nanoparticles homogenously resolidify in the sintered semiconductor matrix. The enhanced sintering process with ligand-free gold nanoparticles gives access to metal-semiconductor hybrid materials with potential application in light harvesting or energy conversion. © 2014 Springer-Verlag Berlin Heidelberg.
    view abstractdoi: 10.1007/s00339-014-8270-1
  • 2014 • 197 On the nucleation of Laves phase particles during high-temperature exposure and creep of tempered martensite ferritic steels
    Isik, M.I. and Kostka, A. and Eggeler, G.
    Acta Materialia 81 230-240 (2014)
    This paper reports on the formation of an Mo-rich Laves phase during high-temperature exposure and creep of a tempered martensite ferritic steel with 12 wt.% Cr and 1 wt.% Mo. The material was exposed to 550 °C for time intervals between 864 and 81,984 h. For comparison, a few creep tests were carried out at 550 °C and 120 MPa (duration between 864 and 12,456 h). All tests were interrupted after specific time periods and microstructures were investigated using transmission electron microscopy and atom probe tomography. Laves phase formation occurs during both heat treatment and creep. Creep stress and strain have no significant effect on the early stages of Laves phase formation. In the present work we show that prior to Laves phase nucleation Si and Mo segregate to micrograin boundaries, where subsequently Laves phase particles appear next to M23C6carbides. © 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2014.08.008
  • 2014 • 196 On the physical nature of tribolayers and wear debris after sliding wear in a superalloy/steel tribosystem at 25 and 300°C
    Rynio, C. and Hattendorf, H. and Klöwer, J. and Eggeler, G.
    Wear 317 26-38 (2014)
    Dry sliding wear of metals is strongly affected by the formation of oxide particles and their incorporation into compacted oxide layers, so-called glaze layers. A high-temperature reciprocating pin-on-disc tribometer was used to study the tribological response of a Ni-based Alloy 80A pin on a cast iron disc at ambient temperature and at 300. °C. Alloy 80A is used for valves and specific cast irons are used for valve seat-inserts in automotive diesel engines, where wear limits the service life of the valve/seat-insert tribosystems. Measurements of the friction coefficient, the total linear wear and the electrical contact resistance were used to monitor the formation of oxide layers during the experiments. Electron dispersive X-ray (EDX) element mappings from the surface regions with wear scars provide clear evidence for the formation of glaze layers and material transfer between pin and disc. Focused ion beam (FIB) micromachining was used to cut out thin lamellae from specific surface regions of glaze layers and from metallic wear particles. These lamellae were investigated in a transmission electron microscope (TEM). It was shown that the glaze layers generated at 25 and 300. °C exhibit distinct differences, which led to a reduction in wear rate by a factor of five at the higher temperature. We also report on the mechanical mixing of oxide particles and metal matrix that results in a metal/oxide nanocomposite directly below the sliding surfaces. Such composite structures were also observed inside of metallic wear debris. © 2014 Elsevier B.V.
    view abstractdoi: 10.1016/j.wear.2014.04.022
  • 2014 • 195 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 • 194 One-step synthesis of bismuth molybdate catalysts via flame spray pyrolysis for the selective oxidation of propylene to acrolein
    Schuh, K. and Kleist, W. and Høj, M. and Trouillet, V. and Jensen, A.D. and Grunwaldt, J.-D.
    Chemical Communications 50 15404-15406 (2014)
    Flame spray pyrolysis (FSP) of Bi(iii)- and Mo(vi)-2-ethylhexanoate dissolved in xylene resulted in various nanocrystalline bismuth molybdate phases depending on the Bi/Mo ratio. Besides α-Bi2Mo3O12 and γ-Bi2MoO6, FSP gave direct access to the metastable β-Bi2Mo2O9 phase with high surface area (19 m2 g-1). This phase is normally only obtained at high calcination temperatures (&gt;560 °C) resulting in lower surface areas. The β-phase was stable up to 400 °C and showed superior catalytic performance compared to α- and γ-phases in selective oxidation of propylene to acrolein at temperatures relevant for industrial applications (360 °C). This journal is © The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c4cc07527g
  • 2014 • 193 Platinum-cobalt bimetallic nanoparticles in hollow carbon nanospheres for hydrogenolysis of 5-hydroxymethylfurfural
    Wang, G.-H. and Hilgert, J. and Richter, F.H. and Wang, F. and Bongard, H.-J. and Spliethoff, B. and Weidenthaler, C. and Schüth, F.
    Nature Materials 13 293-300 (2014)
    The synthesis of 2,5-dimethylfuran (DMF) from 5-hydroxymethylfurfural (HMF) is a highly attractive route to a renewable fuel. However, achieving high yields in this reaction is a substantial challenge. Here it is described how PtCo bimetallic nanoparticles with diameters of 3.6 ± 0.7 nm can solve this problem. Over PtCo catalysts the conversion of HMF was 100% within 10 min and the yield to DMF reached 98% after 2 h, which substantially exceeds the best results reported in the literature. Moreover, the synthetic method can be generalized to other bimetallic nanoparticles encapsulated in hollow carbon spheres. © 2014 Macmillan Publishers Limited.
    view abstractdoi: 10.1038/nmat3872
  • 2014 • 192 Polydisperse NiTi nanoparticles investigated by X-ray standing waves and electron microscopy - A comparative study
    Brücher, M. and Chakif, M. and Gurevich, E.L. and Hergenröder, R.
    Spectrochimica Acta - Part B Atomic Spectroscopy 98 60-64 (2014)
    A polydisperse mixture of nickel-titanium nanoparticles generated by femtosecond-laser ablation was investigated by the application of different analytical methods in order to characterize the distribution of particle sizes. Images obtained with scanning and transmission electron microscopy and intensity curves of fluorescence excited by X-ray standing waves (XSW) were evaluated and the resulting distributions were plotted in several histograms. Based on the differences found in the results, the possibilities, limitations and appropriate criteria of application of the respective technique are discussed. The principles of the XSW technique and the evaluation procedure are explained in more detail. The respective analytical methods were compared in terms of spatial resolution, information content, the risk of artifacts, the statistics of the evaluation and the availability of experimental facilities. © 2014 Elsevier B.V.
    view abstractdoi: 10.1016/j.sab.2014.05.004
  • 2014 • 191 Precipitation of the α-phase in an ultrafine grained beta-titanium alloy processed by severe plastic deformation
    Li, T. and Kent, D. and Sha, G. and Dargusch, M.S. and Cairney, J.M.
    Materials Science and Engineering A 605 144-150 (2014)
    A fine and uniform distribution of α phase at grain boundaries is expected to improve the mechanical properties and thermal stability of beta Ti alloys. To design high strength alloys, a key factor is the volume fraction of α, which is related to the concentration of the α phase. In this study, α-phase precipitates were characterized in an ultrafine-grained Ti-15Nb-2Mo-2Zr-1Sn (at%) alloy processed by severe plastic deformation in two different ways (hot drawing and cold rolling in conjunction with annealing). A combination of transmission Kikuchi diffraction, transmission electron microscopy and atom-probe tomography revealed that ultra-fine α precipitates precipitate at grain boundaries in hot-drawn material or at sub-grain boundaries in the cold-rolled samples. The Nb concentrations of α phases formed were not those expected for an equilibrium state, which highlights the importance of understanding the chemistry of the α precipitates for engineering microstructures in advanced Ti alloys. © 2014 Elsevier B.V.
    view abstractdoi: 10.1016/j.msea.2014.03.044
  • 2014 • 190 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 • 189 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 • 188 Role and evolution of nanoparticle structure and chemical state during the oxidation of NO over size- and shape-controlled Pt/γ-Al2O 3 catalysts under operando conditions
    Lira, E. and Merte, L.R. and Behafarid, F. and Ono, L.K. and Zhang, L. and Roldan Cuenya, B.
    ACS Catalysis 4 1875-1884 (2014)
    The structure and chemical state of size-selected Pt nanoparticles (NPs) supported on γ-Al2O3 were studied during the oxidation of NO using X-ray absorption near-edge spectroscopy and extended X-ray absorption fine-structure spectroscopy measurements under operando conditions. The data revealed the formation of PtOx species in the course of the reaction that remained present at the maximum temperature studied, 350 °C. The PtOx species were found in all samples, but those with the smallest NPs showed the highest degree of oxidation. Moreover, NO-induced nanoparticle redispersion was observed at temperatures below 150 °C for all catalysts studied. Catalytic tests showed activity toward the oxidation of NO for all samples. Nevertheless, the catalyst with the smallest NPs was found to be the least active, which is explained by a more extensive formation of PtOx species in this catalyst and their detrimental contribution to the oxidation of NO. © 2014 American Chemical Society.
    view abstractdoi: 10.1021/cs500137r
  • 2014 • 187 Shape-dependent catalytic oxidation of 2-butanol over Pt nanoparticles supported on γ-Al2O3
    Mistry, H. and Behafarid, F. and Zhou, E. and Ono, L.K. and Zhang, L. and Roldan Cuenya, B.
    ACS Catalysis 4 109-115 (2014)
    This study illustrates the effect of nanoparticle (NP) shape on the reactivity of size-selected Pt/γ-Al2O3 nanocatalysts for 2-butanol oxidation. Nanoparticles similar in size [transmission electron microscopy (TEM) diameter of ∼1 nm] but with different shapes were prepared via encapsulation in inverse micelles. The NP shape was resolved by combining information extracted from extended X-ray absorption fine structure spectroscopy (EXAFS) data, TEM, and modeling. A correlation was observed between the average first nearest neighbor coordination number of atoms at the NP surface and their catalytic activity. In particular, the NPs with the largest number of weakly coordinated surface atoms (i.e., edges and corners) were found to be the least active for the total oxidation of 2-butanol. This result highlights that not only size but also shape control must be achieved to tailor the catalytic properties of nanoscale materials. © 2013 American Chemical Society.
    view abstractdoi: 10.1021/cs400888n
  • 2014 • 186 Shear-induced mixing governs codeformation of crystalline-amorphous nanolaminates
    Guo, W. and Jägle, E.A. and Choi, P.-P. and Yao, J. and Kostka, A. and Schneider, J.M. and Raabe, D.
    Physical Review Letters 113 (2014)
    Deformation of ductile crystalline-amorphous nanolaminates is not well understood due to the complex interplay of interface mechanics, shear banding, and deformation-driven chemical mixing. Here we present indentation experiments on 10 nm nanocrystalline Cu-100 nm amorphous CuZr model multilayers to study these mechanisms down to the atomic scale. By using correlative atom probe tomography and transmission electron microscopy we find that crystallographic slip bands in the Cu layers coincide with noncrystallographic shear bands in the amorphous CuZr layers. Dislocations from the crystalline layers drag Cu atoms across the interface into the CuZr layers. Also, crystalline Cu blocks are sheared into the CuZr layers. In these sheared and thus Cu enriched zones the initially amorphous CuZr layer is rendered into an amorphous plus crystalline nanocomposite. © 2014 American Physical Society.
    view abstractdoi: 10.1103/PhysRevLett.113.035501
  • 2014 • 185 Site occupation of Nb atoms in ternary Ni-Ti-Nb shape memory alloys
    Shi, H. and Frenzel, J. and Martinez, G.T. and Van Rompaey, S. and Bakulin, A. and Kulkova, S. and Van Aert, S. and Schryvers, D.
    Acta Materialia 74 85-95 (2014)
    Nb occupancy in the austenite B2-NiTi matrix and Ti2Ni phase in Ni-Ti-Nb shape memory alloys was investigated by aberration-corrected scanning transmission electron microscopy and precession electron diffraction. In both cases, Nb atoms were found to prefer to occupy the Ti rather than Ni sites. A projector augmented wave method within density functional theory was used to calculate the atomic and electronic structures of the austenitic B2-NiTi matrix phase and the Ti2Ni precipitates both with and without addition of Nb. The obtained formation energies and analysis of structural and electronic characteristics explain the preference for Ti sites for Nb over Ni sites. © 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2014.03.062
  • 2014 • 184 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 • 183 Spectroelectrochemical and morphological studies of the ageing of silver nanoparticles embedded in ultra-thin perfluorinated sputter deposited films
    Ebbert, C. and Alissawi, N. and Somsen, C. and Eggeler, G. and Strunskus, T. and Faupel, F. and Grundmeier, G.
    Thin Solid Films 571 161-167 (2014)
    This paper focuses on the investigation of the ageing behaviour of silver nanoparticle containing polytetrafluoroethylene thin films during exposure to phosphate buffer solution (pH = 7.5). In order to investigate the effect of the electrical connection between the silver nanoparticles via a conductive substrate, two kinds of composite films were compared. One model where the nanoparticles are directly deposited on an inert conducting substrate and then covered by an ultra-thin polytetrafluoroethylene like film. In the second case a polytetrafluoroethylene/silver nanoparticle/polytetrafluoroethylene sandwich film was prepared on the same substrate to prevent electrical connection of the silver nanoparticles. Degradation was followed in-situ by means of the combination of ultraviolet-visible spectroscopy and electrochemical impedance spectroscopy. In the case of electrically connected nanoparticles electrochemical Ostwald ripening took place, while this process was not observed for the insulated nanoparticles. The electrochemical impedance spectroscopy studies allowed for the parallel study of the correlated loss of barrier properties. Transmission electron microscopy images of both composite films confirmed the results obtained by means of the in situ electrochemical ultraviolet-visible studies. © 2014 Elsevier B.V.
    view abstractdoi: 10.1016/j.tsf.2014.10.054
  • 2014 • 182 Spinel Mn-Co oxide in N-doped carbon nanotubes as a bifunctional electrocatalyst synthesized by oxidative cutting
    Zhao, A. and Masa, J. and Xia, W. and Maljusch, A. and Willinger, M.-G. and Clavel, G. and Xie, K. and Schlögl, R. and Schuhmann, W. and Muhler, M.
    Journal of the American Chemical Society 136 7551-7554 (2014)
    The notorious instability of non-precious-metal catalysts for oxygen reduction and evolution is by far the single unresolved impediment for their practical applications. We have designed highly stable and active bifunctional catalysts for reversible oxygen electrodes by oxidative thermal scission, where we concurrently rupture nitrogen-doped carbon nanotubes and oxidize Co and Mn nanoparticles buried inside them to form spinel Mn-Co oxide nanoparticles partially embedded in the nanotubes. Impressively high dual activity for oxygen reduction and evolution is achieved using these catalysts, surpassing those of Pt/C, RuO2, and IrO2 and thus raising the prospect of functional low-cost, non-precious-metal bifunctional catalysts in metal-air batteries and reversible fuel cells, among others, for a sustainable and green energy future. © 2014 American Chemical Society.
    view abstractdoi: 10.1021/ja502532y
  • 2014 • 181 Stable zinc oxide nanoparticle dispersions in ionic liquids
    Wittmar, A. and Gautam, D. and Schilling, C. and Dörfler, U. and Mayer-Zaika, W. and Winterer, M. and Ulbricht, M.
    Journal of Nanoparticle Research 16 (2014)
    The influence of the hydrophilicity and length of the cation alkyl chain in imidazolium-based ionic liquids on the dispersability of ZnO nanoparticles by ultrasound treatment was studied by dynamic light scattering and advanced rheology. ZnO nano-powder synthesized by chemical vapor synthesis was used in parallel with one commercially available material. Before preparation of the dispersion, the nanoparticles characteristics were determined by transmission electron microscopy, X-ray diffraction, nitrogen adsorption with BET analysis, and FT-IR spectroscopy. Hydrophilic ionic liquids dispersed all studied nanopowders better and in the series of hydrophilic ionic liquids, an improvement of the dispersion quality with increasing length of the alkyl chain of the cation was observed. Especially, for ionic liquids with short alkyl chain, additional factors like nanoparticle concentration in the dispersion and the period of the ultrasonic treatment had significant influence on the dispersion quality. Additionally, nanopowder characteristics (crystallite shape and size as well as the agglomeration level) influenced the dispersion quality. The results indicate that the studied ionic liquids are promising candidates for absorber media at the end of the gas phase synthesis reactor allowing the direct preparation of non-agglomerated nanoparticle dispersions without supplementary addition of dispersants and stabilizers. © Springer Science+Business Media 2014.
    view abstractdoi: 10.1007/s11051-014-2341-2
  • 2014 • 180 Structural and electronic properties of micellar Au nanoparticles: Size and ligand effects
    Behafarid, F. and Matos, J. and Hong, S. and Zhang, L. and Rahman, T.S. and Roldan Cuenya, B.
    ACS Nano 8 6671-6681 (2014)
    Gaining experimental insight into the intrinsic properties of nanoparticles (NPs) represents a scientific challenge due to the difficulty of deconvoluting these properties from various environmental effects such as the presence of adsorbates or a support. A synergistic combination of experimental and theoretical tools, including X-ray absorption fine-structure spectroscopy, scanning transmission electron microscopy, atomic force microscopy, and density functional theory was used in this study to investigate the structure and electronic properties of small (∼1-4 nm) Au NPs synthesized by an inverse micelle encapsulation method. Metallic Au NPs encapsulated by polystyrene 2-vinylpiridine (PS-P2VP) were studied in the solution phase (dispersed in toluene) as well as after deposition on γ-Al2O3. Our experimental data revealed a size-dependent contraction of the interatomic distances of the ligand-protected NPs with decreasing NP size. These findings are in good agreement with the results from DFT calculations of unsupported Au NPs surrounded by P2VP, as well as those obtained for pure (ligand-free) Au clusters of analogous sizes. A comparison of the experimental and theoretical results supports the conclusion that the P2VP ligands employed to stabilize the gold NPs do not lead to strong distortions in the average interatomic spacing. The changes in the electronic structure of the Au-P2VP NPs were found to originate mainly from finite size effects and not from charge transfer between the NPs and their environment (e.g., Au-ligand interactions). In addition, the isolated ligand-protected experimental NPs only display a weak interaction with the support, making them an ideal model system for the investigation of size-dependent physical and chemical properties of structurally well-defined nanomaterials. © 2014 American Chemical Society.
    view abstractdoi: 10.1021/nn406568b
  • 2014 • 179 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 • 178 Structural evolution of silver nanoparticles during wet-chemical synthesis
    Banerjee, S. and Loza, K. and Meyer-Zaika, W. and Prymak, O. and Epple, M.
    Chemistry of Materials 26 951-957 (2014)
    The formation of silver nanoparticles during the reduction with glucose in the presence of poly(N-vinyl pyrrolidone) as capping agent was followed for more than 3000 min. First, spherical silver nanoparticles are formed, but in later stages, an increasing fraction of nanotriangles and also a few nanorods develop. Both spherical and trigonal nanoparticles grow with time, indicating separate nucleation pathways. The domain size in the spherical nanoparticles increases proportionally to the particle diameter and is always about 1/ 4 of the diameter, indicating that twinned seeds are formed very early in the process and then simply grow by extending their domains. The lattice constant of the nanoparticles is systematically increased in comparison to microcrystalline silver (4.0877 vs 4.08635 Å) but did not change as a function of particle diameter. A thorough analysis of the texture coefficient, supported by transmission electron microscopy data, showed that the apparently spherical particles are in fact flattened pentagonal prisms, which typically lie on their flat pentagonal face. Neither the presence of oxygen nor the presence of ambient light had any influence on the particle properties. © 2014 American Chemical Society.
    view abstractdoi: 10.1021/cm4025342
  • 2014 • 177 The evolution of tribolayers during high temperature sliding wear
    Rynio, C. and Hattendorf, H. and Klöwer, J. and Eggeler, G.
    Wear 315 1-10 (2014)
    High temperature reciprocating sliding wear experiments of a Ni-based superalloy pin against a cast iron disc were performed at 600 and 800. °C (load: 20. N, frequency: 20. Hz, stroke: 1. mm). The evolution of tribolayers was investigated using scanning and transmission electron microscopy (SEM and TEM) and energy dispersive X-ray spectroscopy (EDX). Four distinct subsurface zones are identified and discussed in terms of plastic strain accumulation and microstructure evolution. The development of protective nanocrystalline oxide-layers (glaze-layers) on top of the wear surfaces leads to very low wear rates due to a suppression of the direct metal-metal contact between the pin and the disc. The nanohardness, microstructure and chemical composition of the glaze-layers are reported. © 2014 Elsevier B.V.
    view abstractdoi: 10.1016/j.wear.2014.03.007
  • 2014 • 176 The role of carbonaceous deposits in the activity and stability of Ni-based catalysts applied in the dry reforming of methane
    Düdder, H. and Kähler, K. and Krause, B. and Mette, K. and Kühl, S. and Behrens, M. and Scherer, V. and Muhler, M.
    Catalysis Science and Technology 4 3317-3328 (2014)
    Highly stable Ni catalysts with varying Ni contents up to 50 mol% originating from hydrotalcite-like precursors were applied in the dry reforming of methane at 800 and 900 °C. The integral specific rate of methane conversion determined after 10 h on stream was 3.8 mmol s-1 g cat -1 at 900 °C. Due to the outstanding high activity, a catalyst mass of just 10 mg had to be used to avoid operating the reaction in thermodynamic equilibrium. The resulting WHSV was as high as 1.44 × 106 ml gcat -1 h-1. The observed axial temperature distribution with a pronounced cold spot was analyzed by computational fluid dynamics simulations to verify the strong influence of this highly endothermic reaction. Transmission electron microscopy and temperature-programmed oxidation experiments were used to probe the formation of different carbon species, which was found to depend on the catalyst composition and the reaction temperature. Among the formed carbon species, multi-walled carbon nanofibers were detrimental to the long-term stability at 800 °C, whereas their formation was suppressed at 900 °C. The formation of graphitic carbon at 900 °C originating from methane pyrolysis played a minor role. The methane conversion after 100 h of dry reforming at 900 °C compared to the initial one amounted to 98% for the 25 mol% Ni catalyst. The oxidative regeneration of the catalyst was achieved in the isothermal mode using only carbon dioxide in the feed. © the Partner Organisations 2014.
    view abstractdoi: 10.1039/c4cy00409d
  • 2014 • 175 [FeFe]-hydrogenase models assembled into vesicular structures
    Menzel, K. and Apfel, U.-P. and Wolter, N. and Rüger, R. and Alpermann, T. and Steiniger, F. and Gabel, D. and Förster, S. and Weigand, W. and Fahr, A.
    Journal of Liposome Research 24 59-68 (2014)
    Compartmentalization is a major prerequisite for the origin of life on earth according to Wächtershäuser "Iron-Sulfur-World". The hypothesis is mainly based on an autocatalytic inorganic energy reproducing redox system consisting of iron and sulfur as requirement for the subsequent synthesis of complex organic structures. Here, we modified [FeFe]-hydrogenase models by means of covalent coupling to either oleic acid or the amphiphilic block copolymer polybutadiene-polyethyleneoxide (PB-PEO) and incorporated those into the membranes of vesicles composed of phospholipids (liposomes) or the unmodified amphiphilic polymer (polymersomes). We employed a [2Fe-2S] cluster as a hydrogenase model, since these structures are known to be suitable catalysts for the generation of H2 in the presence of weak acids. Successful incorporation was confirmed by spectrophotometric iron quantification and the vesicles formed were characterized by size determination (photon correlation spectroscopy (PCS)), and zeta potential as well as by cryo-transmission electron microscopy (Cryo-TEM). The modified models could be incorporated into liposomes or polymersomes up to molar proportions of 3.15% and 28%, respectively. Due to the immobilization in vesicular bilayers the [FeFe]-hydrogenase models can even exhibit catalytic action under the particular conditions of the intravesicular microenvironment. Our results suggest that the vesicular systems described may be applied as a nanoreactor for the reduction of encapsulated substances by generating hydrogen and thus as a minimal cell model. © 2014 Informa Healthcare USA, Inc.
    view abstractdoi: 10.3109/08982104.2013.833225
  • 2013 • 174 A simple stochastic model for yielding in specimens with limited number of dislocations
    Sudharshan Phani, P. and Johanns, K.E. and George, E.P. and Pharr, G.M.
    Acta Materialia 61 2489-2499 (2013)
    A simple statistical model is developed based on a random distribution and orientation of dislocations in order to explain recent experimental observations of the strength of small specimens containing a limited number of dislocations. Two different types of randomness are introduced, namely, randomness in the spatial location of the dislocations and randomness in the stress needed to activate them. For convenience, the randomness in the activation stress is modeled by assigning a random Schmid factor to the dislocations. In contrast to previous stochastic models, the current model predicts the yield strength not only in the presence of dislocations but also in their absence. Furthermore, the model predicts the scatter in the yield strength in addition to the mean. The model is found to quantitatively explain the yield strength and scatter in micro-compression/tension tests of Mo-alloy fibers using dislocation densities and arrangements measured by transmission electron microscopy. The results of Brenner's classic tensile tests on metallic whiskers are qualitatively reconciled. The model adds credence to the notion that "smaller is stronger" from a purely statistical point of view.
    view abstractdoi: 10.1016/j.actamat.2013.01.023
  • 2013 • 173 An elevated temperature study of a Ti adhesion layer on polyimide
    Taylor, A.A. and Cordill, M.J. and Bowles, L. and Schalko, J. and Dehm, G.
    Thin Solid Films 531 354-361 (2013)
    Titanium layers are used to promote adhesion between polymer substrates for flexible electronics and the Cu or Au conducting lines. Good adhesion of conducting lines in flexible circuits is critical in improving circuit performance and increasingcircuit lifetime. Nominally 50 nm thick Ti films on polyimide (PI) are investigated by fragmentation testing under uniaxial tensile load in the as-deposited state, at 350 C, and after annealing. The cracking and buckling of the films show clear differences between the as-deposited and the thermally treated samples, cracks are much straighter and buckles are smaller following heat treatment. These changes are correlated to a drop in adhesion of the samples following heat treatment. Adhesion values are determined from the buckle dimensions using a total energy approach as described in the work of Cordill et al. (Acta Mater. 2010). Cross-sectional transmission electron microscopy of the Ti/PI interface found evidence of a ~ 5 nm thick interlayer between the largely columnar Ti and the amorphous PI. This interlayer is amorphous in the as-deposited state but nano-crystalline in those coatings tested at elevated temperature or annealed. It is put forward that this alteration of the interfacial structure causes the reduced adhesion. © 2013 Elsevier B.V.
    view abstractdoi: 10.1016/j.tsf.2013.01.016
  • 2013 • 172 Atomic-scale compositional characterization of a nanocrystalline AlCrCuFeNiZn high-entropy alloy using atom probe tomography
    Pradeep, K.G. and Wanderka, N. and Choi, P. and Banhart, J. and Murty, B.S. and Raabe, D.
    Acta Materialia 61 4696-4706 (2013)
    We have studied a nanocrystalline AlCrCuFeNiZn high-entropy alloy synthesized by ball milling followed by hot compaction at 600 C for 15 min at 650 MPa. X-ray diffraction reveals that the mechanically alloyed powder consists of a solid-solution body-centered cubic (bcc) matrix containing 12 vol.% face-centered cubic (fcc) phase. After hot compaction, it consists of 60 vol.% bcc and 40 vol.% fcc. Composition analysis by atom probe tomography shows that the material is not a homogeneous fcc-bcc solid solution but instead a composite of bcc structured Ni-Al-, Cr-Fe- and Fe-Cr-based regions and of fcc Cu-Zn-based regions. The Cu-Zn-rich phase has 30 at.% Zn α-brass composition. It segregates predominantly along grain boundaries thereby stabilizing the nanocrystalline microstructure and preventing grain growth. The Cr- and Fe-rich bcc regions were presumably formed by spinodal decomposition of a Cr-Fe phase that was inherited from the hot compacted state. The Ni-Al phase remains stable even after hot compaction and forms the dominant bcc matrix phase. The crystallite sizes are in the range of 20-30 nm as determined by transmission electron microscopy. The hot compacted alloy exhibited very high hardness of 870 ± 10 HV. The results reveal that phase decomposition rather than homogeneous mixing is prevalent in this alloy. Hence, our current observations fail to justify the present high-entropy alloy design concept. Therefore, a strategy guided more by structure and thermodynamics for designing high-entropy alloys is encouraged as a pathway towards exploiting the solid-solution and stability idea inherent in this concept. © 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2013.04.059
  • 2013 • 171 Atomic-scale investigation of ε and θ precipitates in bainite in 100Cr6 bearing steel by atom probe tomography and ab initio calculations
    Song, W. and Von Appen, J. and Choi, P. and Dronskowski, R. and Raabe, D. and Bleck, W.
    Acta Materialia 61 7582-7590 (2013)
    Carbide precipitation during upper and lower bainite formation in high-carbon bearing steel 100Cr6 is characterized using transmission electron microscopy and atom probe tomography. The results reveal that both ε and θ carbides precipitate in lower bainite isothermally held at 260 C and only θ precipitates form in upper bainite isothermally held at 500 C. ε and θ precipitate under paraequilibrium condition at 260 C in lower bainite and θ precipitates under negligible partitioning local equilibrium condition in upper bainite at 500 C. In order to theoretically study ε and θ precipitation and the ε → θ transition in bainite, thermodynamic calculations have been carried out using ab initio techniques. We find that ε and θ carbides in ferrite have almost identical thermodynamic stability, and hence have similar formation probability. In austenite, however, cementite formation is clearly preferred: it is favored by 5 kJ mol-1 at room temperature and still by 4 kJ mol-1 at 500 C. Hence, the thermodynamic predictions agree well with the atom probe tomography results. © 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2013.08.051
  • 2013 • 170 Basal and non-basal dislocation slip in Mg–Y
    Sandlöbes, S. and Friák, M. and Neugebauer, J. and Raabe, D.
    Materials Science and Engineering A 576 61-68 (2013)
    The activation of non-basal slip systems is of high importance for the ductility in hcp Mg and its alloys. In particular, for Mg–Y alloys where a higher activation of pyramidal dislocation slip causes an increased ductility detailed characterization of the activated slip systems is essential to understand and describe plasticity in these alloys. In this study a detailed analysis of the activated dislocations and slip systems via post-mortem TEM and SEM-EBSD based slip band analysis in 3% deformed Mg–3 wt% Y is presented. The analysis reveals a substantial activity of pyramidal <c+a> dislocations with different Burgers vectors. The obtained dislocation densities and active slip systems are discussed with respect to atomistic simulations of non-basal dislocations in hcp Mg. © 2013 Elsevier B.V.
    view abstractdoi: 10.1016/j.msea.2013.03.006
  • 2013 • 169 Blocked-micropores, surface functionalized, bio-compatible and silica-coated iron oxide nanocomposites as advanced MRI contrast agent
    Darbandi, M. and Laurent, S. and Busch, M. and Li, Z.-A. and Yuan, Y. and Krüger, M. and Farle, M. and Winterer, M. and Vander Elst, L. and Muller, R.N. and Wende, H.
    Journal of Nanoparticle Research 15 (2013)
    Biocompatible magnetic nanoparticles have been found promising in several biomedical applications for tagging, imaging, sensing and separation in recent years. In this article, a systematic study of the design and development of surface-modification schemes for silica-coated iron oxide nanoparticles (IONP) via a one-pot, in situ method at room temperature is presented. Silica-coated IONP were prepared in a water-in-oil microemulsion, and subsequently the surface was modified via addition of organosilane reagents to the microemulsion system. The structure and the morphology of the as synthesized nanoparticles have been investigated by means of transmission electron microscopy (TEM) and measurement of N2 adsorption-desorption. Electron diffraction and high-resolution transmission electron microscopic (TEM) images of the nanoparticles showed the highly crystalline nature of the IONP structures. Nitrogen adsorption indicates microporous and blocked-microporous structures for the silica-coated and amine functionalized silica-coated IONP, respectively which could prove less cytotoxicity of the functionalized final product. Besides, the colloidal stability of the final product and the presence of the modified functional groups on top of surface layer have been proven by zeta-potential measurements. Owing to the benefit from the inner IONP core and the hydrophilic silica shell, the as-synthesized nanocomposites were exploited as an MRI contrast enhancement agent. Relaxometric results prove that the surface functionalized IONP have also signal enhancement properties. These surface functionalized nanocomposites are not only potential candidates for highly efficient contrast agents for MRI, but could also be used as ultrasensitive biological-magnetic labels, because they are in nanoscale size, having magnetic properties, blocked-microporous and are well dispersible in biological environment. © 2013 Springer Science+Business Media Dordrecht.
    view abstractdoi: 10.1007/s11051-013-1664-8
  • 2013 • 168 Characteristics of the stress-induced formation of R-phase in ultrafine-grained NiTi shape memory wire
    Olbricht, J. and Yawny, A. and Pelegrina, J.L. and Eggeler, G. and Yardley, V.A.
    Journal of Alloys and Compounds 579 249-252 (2013)
    The transformation between the cubic B2 and monoclinic B19' phases in ultrafine-grained pseudoelastic NiTi can occur as a two-step process involving the intermediate rhombohedral R-phase. Experimental work using differential scanning calorimetry, electrical resistance measurements and transmission electron microscopy has demonstrated the formation of this intermediate phase during thermal cycling and during mechanical loading. In the present paper, complementary mechanical and thermographic results are presented which allow to further assess the character of the stress-induced R-phase formation. The transformation from B2 to R-phase is demonstrated to occur homogeneously within the gauge length rather than via advancing Lüders-type transition regions as it is the case in the localized transformation from B2 or R-phase to B19'. © 2013 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.jallcom.2013.06.056
  • 2013 • 167 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 • 166 Cooperative self-assembly of discoid dimers: Hierarchical formation of nanostructures with a pH switch
    Fenske, M.T. and Meyer-Zaika, W. and Korth, H.-G. and Vieker, H. and Turchanin, A. and Schmuck, C.
    Journal of the American Chemical Society 135 8342-8349 (2013)
    Derivatives of the self-complementary 2-guanidiniocarbonyl pyrrole 5-carboxylate zwitterion (1) (previously reported by us to dimerize to 1•1 with an aggregation constant of ca. &gt;1010 M-l in DMSO) aggregate in a diverse manner depending on, e.g., variation of concentration or its protonation state. The mode of aggregation was analyzed by spectroscopic (NMR, UV) and microscopic (AFM, SEM, HIM, and TEM) methods. Aggregation of dimers of these zwitterions to higher supramolecular structures was achieved by introduction of sec-amide substituents at the 3-position, i.e., at the rearward periphery of the parent binding motif. A butyl amide substituent as in 2b enables the discoid dimers to further aggregate into one-dimensional (rod-like) stacks. Quantitative UV dilution studies showed that this aggregation is strongly cooperative following a nucleation elongation mechanism. The amide hydrogen seems to be essential for this rod-like aggregation, as neither 1 nor a corresponding tert-amide congener 2a form comparable structures. Therefore, a hydrogen bond-assisted π-π-interaction of the dimeric zwitterions is suggested to promote this aggregation mode, which is further affected by the nature of the amide substituent (e.g., steric demand), enabling the formation of bundles of strands or even two-dimensional sheets. By exploiting the zwitterionic nature of the aggregating discoid dimers, a reversible pH switch was realized: dimerization of all compounds is suppressed by protonation of the carboxylate moiety, converting the zwitterions into typical cationic amphiphiles. Accordingly, typical nanostructures like vesicles, tubes, and flat sheets are formed reversibly under acidic conditions, which reassemble into the original rod-like aggregates upon readjustment to neutral pH. © 2013 American Chemical Society.
    view abstractdoi: 10.1021/ja4025148
  • 2013 • 165 Coupling of electron channeling with EBSD: Toward the quantitative characterization of deformation structures in the sem
    Gutierrez-Urrutia, I. and Zaefferer, S. and Raabe, D.
    JOM 65 1229-1236 (2013)
    The coupling of electron channeling contrast imaging (ECCI) with electron backscatter diffraction (EBSD) provides an efficient and fast approach to perform ECCI of crystal defects, such as dislocations, cells, and stacking faults, under controlled diffraction conditions with enhanced contrast. From a technical point of view, the ECCI technique complements two of the main electron microscopy techniques, namely, EBSD and conventional diffraction-based transmission electron microscopy. In this review, we provide several application examples of the EBSD-based ECCI approach on microstructure characterization, namely, characterization of single dislocations, measurement of dislocation densities, and characterization of dislocation substructures in deformed bulk materials. We make use of a two-beam Bloch wave approach to interpret the channeling contrast associated with crystal defects. The approach captures the main features observed in the experimental contrast associated with stacking faults and dislocations. © 2013 TMS.
    view abstractdoi: 10.1007/s11837-013-0678-0
  • 2013 • 164 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 • 163 Diffusive and massive phase transformations in Ti-Al-Nb alloys-Modelling and experiments
    Gamsjäger, E. and Liu, Y. and Rester, M. and Puschnig, P. and Draxl, C. and Clemens, H. and Dehm, G. and Fischer, F.D.
    Intermetallics 38 126-138 (2013)
    The thermodynamic properties of the Ti-Al-Nb system are obtained from recently published thermodynamic assessments. Based on these data the phase boundaries of the (α-Ti + γ-TiAl) two phase region are calculated by utilizing the CALPHAD approach and are compared to those, obtained by ab-initio calculations. It is found that the ab-initio phase boundaries deviate significantly from those based on the CALPHAD fit to experimental data which can be rationalized by the lack of vibrational entropy contributions in the present approach. Consequently a thermodynamic description based on the CALPHAD approach is used to further investigate the kinetics of the massive α → γm phase transformation in the Ti-Al-Nb system by means of a recently developed thick-interface model. Simulation of the transformation kinetics results in a massive transformation in the single-phase region only. However, very thin mole fraction spikes are obtained due to comparatively high interface velocities. It is likely that these spikes cannot be fully developed in experiments meaning that diffusion processes are partly suppressed (quasi-diffusionless transformation). A massive transformation in the two-phase region would then be possible. The theoretical predictions are compared to experimental studies performed on a Ti-45Al-5Nb alloy (composition in atomic percent). The alloy is heat treated slightly above the α-transus temperature and subsequently oil quenched to room temperature to generate γm-α2 interfaces. Energy-dispersive X-ray spectroscopy measurements were performed across γm- α2 interfaces in a scanning transmission electron microscope to search for chemical spikes. © 2013 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.intermet.2013.03.001
  • 2013 • 162 Direct evidence for the formation of ordered carbides in a ferrite-based low-density Fe-Mn-Al-C alloy studied by transmission electron microscopy and atom probe tomography
    Seol, J.-B. and Raabe, D. and Choi, P. and Park, H.-S. and Kwak, J.-H. and Park, C.-G.
    Scripta Materialia 68 348-353 (2013)
    We study the structure and chemical composition of the κ-carbide formed as a result of isothermal transformation in an Fe-3.0Mn-5.5Al-0.3C alloy using transmission electron microscopy and atom probe tomography. Both methods reveal the evolution of κ-particle morphology as well as the partitioning of solutes. We propose that the κ-phase is formed by a eutectoid reaction associated with nucleation growth. The nucleation of κ-carbide is controlled by both the ordering of Al partitioned to austenite and the carbon diffusion at elevated temperatures.© 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.scriptamat.2012.08.013
  • 2013 • 161 Direct gas-phase synthesis of single-phase β-FeSi2 nanoparticles
    Bywalez, R. and Orthner, H. and Mehmedovic, E. and Imlau, R. and Kovacs, A. and Luysberg, M. and Wiggers, H.
    Journal of Nanoparticle Research 15 (2013)
    For the first time, phase-pure β-FeSi2 nanoparticles were successfully produced by gas-phase synthesis. We present a method to fabricate larger quantities of semiconducting β-FeSi2 nanoparticles, with crystallite sizes between 10 and 30 nm, for solar and thermoelectric applications utilizing a hot-wall reactor. A general outline for the production of those particles by thermal decomposition of silane and iron pentacarbonyl is provided based on kinetic data. The synthesized particles are investigated by X-ray diffraction and transmission electron microscopy, providing evidence that the as-prepared materials are indeed β-FeSi2, while revealing morphological characteristics inherent to the nanoparticles created. © 2013 Springer Science+Business Media.
    view abstractdoi: 10.1007/s11051-013-1878-9
  • 2013 • 160 Em characterization of precipitates in as-cast and annealed Ni45.5Ti45.5Nb9 shape memory alloys
    Shi, H. and Frenzel, J. and Schryvers, D.
    Materials Science Forum 738-739 113-117 (2013)
    Nb-rich precipitates in the matrix of as-cast and annealed Ni45.5Ti45.5Nb9alloys are investigated by scanning and scanning transmission electron microscopy, including slice-and-view and geometric phase analysis (GPA). The Nb-rich bcc nano-precipitates in the as-cast alloy have a 10% lattice parameter difference with the B2 matrix and reveal compensating interface dislocations. The 3D reconstruction of the configuration of small Nb-rich precipitates in the annealed alloy reveals a wall-like distribution of precipitates, which may increase the thermal hysteresis of the material. © (2013) Trans Tech Publications, Switzerland.
    view abstractdoi: 10.4028/
  • 2013 • 159 Eu-doped ZnO nanowire arrays grown by electrodeposition
    Lupan, O. and Pauporté, T. and Viana, B. and Aschehoug, P. and Ahmadi, M. and Cuenya, B.R. and Rudzevich, Y. and Lin, Y. and Chow, L.
    Applied Surface Science 282 782-788 (2013)
    The preparation of efficient light emitting diodes requires active optical layers working at low voltage for light emission. Trivalent lanthanide doped wide-bandgap semiconducting oxide nanostructures are promising active materials in opto-electronic devices. In this work we report on the electrochemical deposition (ECD) of Eu-doped ZnO (ZnO:Eu) nanowire arrays on glass substrates coated with F-doped polycrystalline SnO2. The structural, chemical and optical properties of ZnO:Eu nanowires have been systematically characterized by X-ray diffraction, transmission electron microscopy, Raman spectroscopy, X-ray photoelectron spectroscopy, secondary ion mass spectrometry, and photoluminescence. XRD results suggest the substitution of Zn2+ by Eu ions in the crystalline lattice. High-resolution TEM and associated electron diffraction studies indicate an interplanar spacing of 0.52 nm which corresponds to the (0 0 0 1) crystal plane of the hexagonal ZnO, and a growth along the c-direction. The ZnO:Eu nanowires have a single crystal structure, without noticeable defects. According to EDX, SIMS and XPS studies, cationic Eu species are detected in these samples showing the incorporation of Eu into the ZnO matrix. The oxidation states of europium ions in the nanowires are determined as +3 (74%) and +2 (26%). Photoluminescence studies demonstrated red emission from the Eu-doped ZnO nanowire arrays. When Eu was incorporated during the nanowire growth, the sharp 5D0-7F 2 transition of the Eu3+ ion at around 612 nm was observed. These results suggest that Eu doped ZnO nanowires could pave the way for efficient, multispectral LEDs and optical devices. © 2013 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.apsusc.2013.06.053
  • 2013 • 158 Evidence for metal-support interactions in Au modified TiO x/SBA-15 materials prepared by photodeposition
    Mei, B. and Wiktor, C. and Turner, S. and Pougin, A. and Van Tendeloo, G. and Fischer, R.A. and Muhler, M. and Strunk, J.
    ACS Catalysis 3 3041-3049 (2013)
    Gold nanoparticles have been efficiently photodeposited onto titanate-loaded SBA-15 (Ti(x)/SBA-15) with different titania coordination. Transmission electron microscopy shows that relatively large Au nanoparticles are photodeposited on the outer surface of the Ti(x)/SBA-15 materials and that TiOx tends to form agglomerates in close proximity to the Au nanoparticles, often forming core-shell Au/TiOx structures. This behavior resembles typical processes observed due to strong-metal support interactions. In the presence of gold, the formation of hydrogen on Ti(x)/SBA-15 during the photodeposition process and the performance in the hydroxylation of terephthalic acid is greatly enhanced. The activity of the Au/Ti(x)/SBA-15 materials is found to depend on the TiOx loading, increasing with a larger amount of initially isolated TiO4 tetrahedra. Samples with initially clustered TiOx species show lower photocatalytic activities. When isolated zinc oxide (ZnOx) species are present on Ti(x)/SBA-15, gold nanoparticles are smaller and well dispersed within the pores. Agglomeration of TiOx species and the formation of Au/TiO x structures is negligible. The dispersion of gold and the formation of Au/TiOx in the SBA-15 matrix seem to depend on the mobility of the TiOx species. The mobility is determined by the initial degree of agglomeration of TiOx. Effective hydrogen evolution requires Au/TiOx core-shell composites as in Au/Ti(x)/SBA-15, whereas hydroxylation of terephthalic acid can also be performed with Au/ZnO x/TiOx/SBA-15 materials. However, isolated TiOx species have to be grafted onto the support prior to the zinc oxide species, providing strong evidence for the necessity of Ti-O-Si bridges for high photocatalytic activity in terephthalic acid hydroxylation. © 2013 American Chemical Society.
    view abstractdoi: 10.1021/cs400964k
  • 2013 • 157 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 • 156 Facile synthesis of polymer core@silver shell hybrid nanoparticles with super surface enhanced Raman scattering capability
    Huo, D. and He, J. and Yang, S. and Zhou, Z. and Hu, Y. and Epple, M.
    Journal of Colloid and Interface Science 393 119-125 (2013)
    Silver nano-shells (SNSs) were synthesized via a two-step seeds-mediated method. Polymer cores were composed of ultrafine gold nanoparticles (NPs) modified chitosan-poly(acrylic acid) nanoparticles (CS-PAA NPs). Then, deposition of silver upon gold nucleus leads to the seed enlargement and finally forms silver shell on the surface of CS-PAA NPs to get SNSs. Transmission electron microscope (TEM) showed SNSs had a discrete silver shell plus some pores and gaps, which could acted as "hot spots" and provided the great potential of these SNSs to be used as SERS substrates with wavelength ranging from visible to infrared region (700-1000. nm) by tuning shell coverage of silver. SERS experiments with dibenzyl disulphide (DBDS) as the indicator showed that the resulting SNSs allowed the production of highly consistent enhancement of the Raman signals down to nM concentrations of DBDS. Considering the excellent biocompatibility of polymer core and their small size, these SNSs are highly desirable candidates as the enhancers for high performance SERS analysis and as SERS optical labels in biomedical imaging. © 2012 Elsevier Inc.
    view abstractdoi: 10.1016/j.jcis.2012.11.003
  • 2013 • 155 Friction and wear mechanisms of tungsten-carbon systems: A comparison of dry and lubricated conditions
    Stoyanov, P. and Stemmer, P. and Järvi, T.T. and Merz, R. and Romero, P.A. and Scherge, M. and Kopnarski, M. and Moseler, M. and Fischer, A. and Dienwiebel, M.
    ACS Applied Materials and Interfaces 5 6123-6135 (2013)
    The unfolding of a sheared mechanically mixed third-body (TB) in tungsten/tungsten carbide sliding systems is studied using a combination of experiments and simulations. Experimentally, the topographical evolution and the friction response, for both dry and lubricated sliding, are investigated using an online tribometer. Ex situ X-ray photoelectron spectroscopy, transmission electron microscopy, and cross-sectional focused ion beam analysis of the structural and chemical changes near the surfaces show that dry sliding of tungsten against tungsten carbide results in plastic deformation of the tungsten surface, leading to grain refinement, and the formation of a mechanically mixed layer on the WC counterface. Sliding with hexadecane as a lubricant results in a less pronounced third-body formation due to much lower dissipated frictional power. Molecular dynamics simulations of the sliding couples predict chemical changes near the surface in agreement with the interfacial processes observed experimentally. Finally, online topography measurements demonstrate an excellent correlation between the evolution of the roughness and the frictional resistance during sliding. © 2013 American Chemical Society.
    view abstractdoi: 10.1021/am4010094
  • 2013 • 154 Grain boundary segregation in a bronze-route Nb3Sn superconducting wire studied by atom probe tomography
    Sandim, M.J.R. and Tytko, D. and Kostka, A. and Choi, P. and Awaji, S. and Watanabe, K. and Raabe, D.
    Superconductor Science and Technology 26 (2013)
    Atom probe tomography was used to characterize the A15 phase in a bronze-route Nb3Sn superconducting wire with a bronze matrix composition of Cu-8Sn-0.3Ti (in at.%). We observed depletion of niobium and segregation of Cu and Ti atoms at Nb3Sn grain boundaries. While the Nb depletion is about 15% relative to the grain interior, the average ratio between Cu and Ti excess values is 9 to 2. Segregation extends to a distance d ∼ 9 Å from the point of maximum Cu and Ti concentrations. Such local variation in the stoichiometry at the grain boundary region can be an additional source of flux-pinning in the Nb3Sn phase. Other microstructural parameters, such as the grain size and chemical composition of the Nb 3Sn layer, were investigated by electron backscatter diffraction and transmission electron microscopy. © 2013 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0953-2048/26/5/055008
  • 2013 • 153 Hierarchical self-assembly of miktoarm star polymers containing a polycationic segment: A general concept
    Hanisch, A. and Gröschel, A.H. and Förtsch, M. and Löbling, T.I. and Schacher, F.H. and Müller, A.H.E.
    Polymer (United Kingdom) 54 4528-4537 (2013)
    We recently introduced a concept for the counterion-mediated hierarchical self-assembly of an amphiphilic ABC miktoarm star terpolymer in aqueous media into micrometer-sized compartmentalized particles with a highly periodic lamellar fine structure ("woodlice"). Herein, we extend this concept to different miktoarm star polymer systems containing a polycationic segment. The presence of a poly(N-methyl-2-vinylpyridinium) (P2VPq) block and its interaction with iodide/triiodide counterions is crucial. In analogy to linear diblock copolymer systems the hydrophilic/hydrophobic balance of polybutadiene-arm-poly(N-methyl-2-vinylpyridinium iodide)-arm-polystyrene miktoarm star terpolymers determines the morphology of the primary building blocks (spherical micelles and cylindrical micelles/vesicles) and the obtained superstructures (stacked lamellar structures and multilamellar vesicles) during this hierarchical process. When an ABA' miktoarm star copolymer (polystyrene-arm-poly(N-methyl-2-vinylpyridinium iodide)-arm-polystyrene) without a dynamic core-forming block was investigated, a different mechanism into "woodlouse" structured aggregates via aggregation and deformation of intermediate vesicles was found. The individual steps of the different self-assembly processes were investigated by transmission electron microscopy and additionally supported by dynamic light scattering, differential scanning calorimetry, and small-angle X-ray scattering. © 2013 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.polymer.2013.05.071
  • 2013 • 152 High temperature wear testing of a Ni-based superalloy pin on a cast iron disc
    Rynio, C. and Hattendorf, H. and Klöwer, J. and Lüdecke, H.-G. and Eggeler, G.
    Materialwissenschaft und Werkstofftechnik 44 825-831 (2013)
    A pin-on-disc wear test rig is described, which allows to extract reproducible mechanical and microstructural wear data from very small sample volumes at temperatures up to 900°C. The friction and wear behavior of Alloy 80A against a cast iron is evaluated at temperatures from ambient to 800°C. The wear rate of Alloy 80A decreased with increasing temperature. This was attributed to the development of protective tribolayers, which prevented a direct contact between the two sliding partners. Energy Dispersive X-Ray (EDX) mapping of surface wear products and Transmission Electron Microscopy (TEM) results for the evolution of subsurface microstructures are exemplarily presented for wear experiments performed for 10 min at 300°C (frequency: 20 Hz, load: 20 N, stroke: 1 mm). EDX investigations provide a good insight into material transfers and oxide layer generations during sliding wear. TEM-micrographs revealed cell structure formation and very small nanograins directly beneath the surface. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/mawe.201300165
  • 2013 • 151 Incipient plasticity and deformation mechanisms in single-crystal Mg during spherical nanoindentation
    Catoor, D. and Gao, Y.F. and Geng, J. and Prasad, M.J.N.V. and Herbert, E.G. and Kumar, K.S. and Pharr, G.M. and George, E.P.
    Acta Materialia 61 2953-2965 (2013)
    Incipient plasticity in Mg single crystals was investigated using the pop-ins generated during spherical nanoindentation on (0 0 0 1), (1 0 -1 2) and (1 0 -1 0) surfaces. Representative deformed regions extracted from underneath indents by means of focused ion beam machining were examined by transmission electron microscopy (TEM) to identify the deformation mechanisms. Anisotropic elastic Hertzian contact theory was used to calculate indentation Schmid factors and the relevant resolved shear stresses at pop-in from the load-displacement curves. The pop-in statistics in conjunction with the TEM analysis showed that the most likely deformation mechanism responsible for pop-in is slip via 〈a〉 dislocations even in the case of indentation along the c-axis. © 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2013.01.055
  • 2013 • 150 Influence of the cation alkyl chain length of imidazolium-based room temperature ionic liquids on the dispersibility of TiO2 nanopowders
    Wittmar, A. and Gajda, M. and Gautam, D. and Dörfler, U. and Winterer, M. and Ulbricht, M.
    Journal of Nanoparticle Research 15 (2013)
    The influence of the length of the cation alkyl chain on the dispersibility by ultrasonic treatment of TiO2 nanopowders in hydrophilic imidazolium-based room temperature ionic liquids was studied for the first time by dynamic light scattering and advanced rheology. TiO2 nanopowders had been synthesized by chemical vapor synthesis (CVS) under varied conditions leading to two different materials. A commercial nanopowder had been used for comparison. Characterizations had been done using transmission electron microscopy, X-ray diffraction, nitrogen adsorption with BET analysis, and FT-IR spectroscopy. Primary particle sizes were about 6 and 8 nm for the CVS-based and 26 nm for the commercial materials. The particle size distribution in the dispersion was strongly influenced by the length of the cation alkyl chain for all the investigated powders with different structural characteristics and concentrations in the dispersion. It was found that an increase of the alkyl chain length was beneficial, leading to a narrowing of the particle size distribution and a decrease of the agglomerate size in dispersion. The smallest average nanoparticle sizes in dispersion were around 30 nm. Additionally, the surface functionality of the nanoparticles, the concentration of the solid material in the liquid, and the period of ultrasonic treatment control the dispersion quality, especially in the case of the ionic liquids with the shorter alkyl chain. The influence of the nanopowders characteristics on their dispersibility decreases considerably with increasing cation alkyl chain length. The results indicate that ionic liquids with adapted structure are candidates as absorber media for nanoparticles synthesized in gas phase processes to obtain liquid dispersions directly without redispergation. © 2013 Springer Science+Business Media Dordrecht.
    view abstractdoi: 10.1007/s11051-013-1463-2
  • 2013 • 149 Interface-directed spinodal decomposition in TiAlN/CrN multilayer hard coatings studied by atom probe tomography
    Povstugar, I. and Choi, P.-P. and Tytko, D. and Ahn, J.-P. and Raabe, D.
    Acta Materialia 61 7534-7542 (2013)
    Microstructural and compositional changes in TiAlN/CrN multilayered films occurring at temperatures up to 1000 C were studied at different length scales by a combination of atom probe tomography, transmission electron microscopy and X-ray diffraction. We observe the onset of decomposition of the multilayer structure at 700 C via the mechanism of interface-directed spinodal decomposition of TiAlN layers, where Al atoms preferentially move toward the nearest interface and segregate there. The interface-directed mechanism later transforms into isotropic spinodal decomposition and is accompanied by intense interdiffusion between the constituting layers. Distinct compositional gradients across columnar grain boundaries (extending perpendicular to the multilayers) are detected at this stage of decomposition. Drastic differences in decomposition behavior across the film depth were observed at elevated temperatures (800-1000 C): the layered structure completely dissolves in the near-surface part but persists in the regions distant from the surface. The influence of residual stresses caused by the sputter deposition process on the thermally induced evolution of the multilayer thin films is discussed. © 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2013.08.028
  • 2013 • 148 Interfacial structure and chemistry of GaN on Ge(111)
    Zhang, S. and Zhang, Y. and Cui, Y. and Freysoldt, C. and Neugebauer, J. and Lieten, R.R. and Barnard, J.S. and Humphreys, C.J.
    Physical Review Letters 111 (2013)
    The interface of GaN grown on Ge(111) by plasma-assisted molecular beam epitaxy is resolved by aberration corrected scanning transmission electron microscopy. A novel interfacial structure with a 5:4 closely spaced atomic bilayer is observed that explains why the interface is flat, crystalline, and free of GeNx. Density functional theory based total energy calculations show that the interface bilayer contains Ge and Ga atoms, with no N atoms. The 5:4 bilayer at the interface has a lower energy than a direct stacking of GaN on Ge(111) and enables the 5:4 lattice-matching growth of GaN. © 2013 American Physical Society.
    view abstractdoi: 10.1103/PhysRevLett.111.256101
  • 2013 • 147 Low-temperature martensitic transformation in tool steels in relation to their deep cryogenic treatment
    Gavriljuk, V.G. and Theisen, W. and Sirosh, V.V. and Polshin, E.V. and Kortmann, A. and Mogilny, G.S. and Petrov, Yu.N. and Tarusin, Ye.V.
    Acta Materialia 61 1705-1715 (2013)
    The low-temperature martensitic transformation in steel X153CrMoV12 containing (mass%) 1.55C, 11.90Cr, 0.70V, 0.86Mo is studied using dilatometry, Mössbauer spectroscopy, X-ray diffraction, mechanical spectroscopy and transmission electron microscopy. Some additional measurements were carried out on steel X220CrMoV13-4. It is shown that, in contrast to the widely known absence of martensitic transformation during deep cryogenic treatment, this transformation occurs with isothermal kinetics within the temperature range of -100 down to -170 °C with its largest intensity near -150 °C. No transformation is observed at -196 °C. The remarkable features of the isothermal martensitic transformation are: (i) the plastic deformation, which is explained by the absence of ageing of martensite at low temperatures; and (ii) the abnormally low tetragonality of martensite. In contrast to existing interpretations, the abnormally low c/a ratio is interpreted in terms of the capture of immobile carbon atoms by gliding dislocations during plastic deformation at low temperatures. A recommendation is proposed for optimizing the deep cryogenic treatment of tool steels. © 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2012.11.045
  • 2013 • 146 Measurement of the indium concentration in high indium content InGaN layers by scanning transmission electron microscopy and atom probe tomography
    Mehrtens, T. and Schowalter, M. and Tytko, D. and Choi, P. and Raabe, D. and Hoffmann, L. and Jönen, H. and Rossow, U. and Hangleiter, A. and Rosenauer, A.
    Applied Physics Letters 102 (2013)
    A method for determining concentrations from high-angle annular dark field-scanning transmission electron microscopy images is presented. The method is applied to an InGaN/GaN multi-quantum well structure with high In content, as used for the fabrication of light emitting diodes and laser diodes emitting in the green spectral range. Information on specimen thickness and In concentration is extracted by comparison with multislice calculations. Resulting concentration profiles are in good agreement with a comparative atom probe tomography analysis. Indium concentrations in the quantum wells ranging from 26 at. to 33 at. are measured in both cases. © 2013 American Institute of Physics.
    view abstractdoi: 10.1063/1.4799382
  • 2013 • 145 Measuring composition in InGaN from HAADF-STEM images and studying the temperature dependence of Z-contrast
    Mehrtens, T. and Schowalter, M. and Tytko, D. and Choi, P. and Raabe, D. and Hoffmann, L. and Jönen, H. and Rossow, U. and Hangleiter, A. and Rosenauer, A.
    Journal of Physics: Conference Series 471 (2013)
    In this contribution, the indium concentration profile of an In xGa1-xN/GaN five-fold multi quantum well structure is measured from high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) images. The results are compared with an atom probe tomography study. Indium concentrations in the range of 26 at.% to 33 at.% are measured in the centre of the quantum wells. An additional indium layer of 14 at.% has been found on top of the quantum wells. In the second part, the temperature dependence of measured intensities in GaN is investigated. Here, multislice calculations in the frozen lattice approximation are carried out in dependence of specimen thickness and compared to experimental data. An increase of intensity with specimen temperature is found.
    view abstractdoi: 10.1088/1742-6596/471/1/012009
  • 2013 • 144 Nanocrystalline Fe-C alloys produced by ball milling of iron and graphite
    Chen, Y.Z. and Herz, A. and Li, Y.J. and Borchers, C. and Choi, P. and Raabe, D. and Kirchheim, R.
    Acta Materialia 61 3172-3185 (2013)
    A series of nanocrystalline Fe-C alloys with different carbon concentrations (xtot) up to 19.4 at.% (4.90 wt.%) are prepared by ball milling. The microstructures of these alloys are characterized by transmission electron microscopy and X-ray diffraction, and partitioning of carbon between grain boundaries and grain interiors is determined by atom probe tomography. It is found that the segregation of carbon to grain boundaries of α-ferrite can significantly reduce its grain size to a few nanometers. When the grain boundaries of ferrite are saturated with carbon, a metastable thermodynamic equilibrium between the matrix and the grain boundaries is approached, inducing a decreasing grain size with increasing xtot. Eventually the size reaches a lower limit of about 6 nm in alloys with x tot &gt; 6.19 at.% (1.40 wt.%); a further increase in xtot leads to the precipitation of carbon as Fe3C. The observed presence of an amorphous structure in 19.4 at.% C (4.90 wt.%) alloy is ascribed to a deformation-driven amorphization of Fe3C by severe plastic deformation. By measuring the temperature dependence of the grain size for an alloy with 1.77 at.% C additional evidence is provided for a metastable equilibrium reached in the nanocrystalline alloy. © 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2013.02.006
  • 2013 • 143 On the annealing mechanism of AuGe/Ni/Au ohmic contacts to a two-dimensional electron gas in GaAs/AlxGa1-xAs heterostructures
    Koop, E.J. and Iqbal, M.J. and Limbach, F. and Boute, M. and Van Wees, B.J. and Reuter, D. and Wieck, A.D. and Kooi, B.J. and Van Der Wal, C.H.
    Semiconductor Science and Technology 28 (2013)
    Ohmic contacts to a two-dimensional electron gas (2DEG) in GaAs/Al xGa1 -xAs heterostructures are often realized by annealing of AuGe/Ni/Au that is deposited on its surface. We studied how the quality of this type of ohmic contact depends on the annealing time and temperature, and how optimal parameters depend on the depth of the 2DEG below the surface. Combined with transmission electron microscopy and energy-dispersive x-ray spectrometry studies of the annealed contacts, our results allow for identifying the annealing mechanism. We use this for proposing a model that can predict the optimal annealing time when our commonly applied recipe is used for a certain heterostructure at a certain temperature. © 2013 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0268-1242/28/2/025006
  • 2013 • 142 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 • 141 Polyoxometalate-stabilized, water dispersible Fe2Pt magnetic nanoparticles
    Seemann, K.M. and Bauer, A. and Kindervater, J. and Meyer, M. and Besson, C. and Luysberg, M. and Durkin, P. and Pyckhout-Hintzen, W. and Budisa, N. and Georgii, R. and Schneider, C.M. and Kögerler, P.
    Nanoscale 5 2511-2519 (2013)
    Magnetic Fe2Pt core-shell nanoparticles with 2 nm cores were synthesized with a monolayer coating of silicotungstate Keggin clusters. The core-shell composition is substantiated by structural analysis performed using high-resolution scanning transmission electron microscopy (HR-STEM) and small angle X-ray scattering (SAXS) in a liquid suspension. The molecular metal oxide cluster shell introduces an enhanced dispersibility of the magnetic Fe-Pt core-shell nanoparticles in aqueous media and thereby opens up new routes to nanoparticle bio-functionalization, for example, using pre-functionalized polyoxometalates. © 2013 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c3nr33374d
  • 2013 • 140 Preparation of amorphous and nanocrystalline sodium tantalum oxide photocatalysts with porous matrix structure for overall water splitting
    Tüysüz, H. and Chan, C.K.
    Nano Energy 2 116-123 (2013)
    Herein, we report the preparation of a series of surfactant-free nanostructured sodium tantalum oxide using NaTa(OC3H7)6 as a single precursor. The reaction conditions for the novel synthetic method were optimized and the morphology and crystal structure of the prepared materials were investigated by transmission electron microscopy (TEM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). Condensation and polymerization of NaTa(OC3H7)6 under atmospheric pressure gave a porous amorphous structure that could be converted to crystalline NaTaO3 while crystalline Na2Ta2O6 nanocrystals with a 25nm average particle size could be obtained from a hydrothermal method using NH3 as a base catalyst. In addition, the photocatalytic behaviors of the prepared materials were investigated for overall water splitting into hydrogen and oxygen. Unexpectedly, porous amorphous sodium tantalum oxide showed much better catalytic activity over the crystalline one. The synthesized Na2Ta2O6 nanocrystals also indicated promising activity for overall water splitting without any co-catalyst in comparison to bulk NaTaO3. © 2012 Elsevier Ltd.
    view abstractdoi: 10.1016/j.nanoen.2012.08.003
  • 2013 • 139 Preparation of cubic ordered mesoporous silicon carbide monoliths by pressure assisted preceramic polymer nanocasting
    Wang, J. and Oschatz, M. and Biemelt, T. and Lohe, M.R. and Borchardt, L. and Kaskel, S.
    Microporous and Mesoporous Materials 168 142-147 (2013)
    Ordered mesoporous silicon carbide monoliths (OMSCMs) with three-dimensional (3D) bi-continuous cubic structure (Ia3d) have been successfully prepared using KIT-6 silica as the hard template and the commercial polycarbosilane (PCS-800) as the preceramic precursor. Tablet-like SiC/KIT-6 composite monoliths were formed via nanocasting of PCS-800 into the mesopores of KIT-6 silica by the wet impregnation, followed by pressing the PCS-800/KIT-6 composite powder with the addition of triblock copolymer P123 as a binder, and subsequent pyrolysis at 1073, 1273, or 1473 K in argon. The KIT-6 silica template was then dissolved in hydrogen fluoride (HF) solution to generate the silicon carbide (SiC) replicated monoliths with cubic ordered mesoporous structure. The OMSCMs demonstrated good macroscopic tablet-like appearances and no any cracks could be found in spite of the evident shrinkage. They were characterized by small-angle and wide-angle X-ray diffraction (XRD), nitrogen adsorption, Fourier-transform infrared (FT-IR), elemental analysis, transmission electron microscopy (TEM), and scanning electron microscopy (SEM). Nitrogen adsorption and small-angle XRD measurements showed that the OMSCMs had very high stability even after re-treatment at 1673 K under argon. And the transformation of amorphous into nano-crystalline state for SiC framework in the OMSCMs proceeded with the retention of the tablet-like morphology. © 2012 Elsevier Inc. All rights reserved.
    view abstractdoi: 10.1016/j.micromeso.2012.09.037
  • 2013 • 138 Purified oxygen- and nitrogen-modified multi-walled carbon nanotubes as metal-free catalysts for selective olefin hydrogenation
    Chen, P. and Chew, L.M. and Kostka, A. and Xie, K. and Muhler, M. and Xia, W.
    Journal of Energy Chemistry 22 312-320 (2013)
    Oxygen- and nitrogen-functionalized carbon nanotubes (OCNTs and NCNTs) were applied as metal-free catalysts in selective olefin hydrogenation. A series of NCNTs was synthesized by NH3 post-treatment of OCNTs. Temperature-programmed desorption, N2 physisorption, Raman spectroscopy, high-resolution transmission electron microscopy and X-ray photoelectron spectroscopy were employed to characterize the surface properties of OCNTs and NCNTs, aiming at a detailed analysis of the type and amount of oxygen- and nitrogen-containing groups as well as surface defects. The gas-phase treatments applied for oxygen and nitrogen functionalization at elevated temperatures up to 600 °C led to the increase of surface defects, but did not cause structural damages in the bulk. NCNTs showed a clearly higher activity than the pristine CNTs and OCNTs in the hydrogenation of 1,5-cyclooctadiene, and also the selectivity to cyclooctene was higher. The favorable catalytic properties are ascribed to the nitrogen-containing surface functional groups as well as surface defects related to nitrogen species. In contrast, oxygen-containing surface groups and the surface defects caused by oxygen species did not show clear contribution to the hydrogenation catalysis. Copyright © 2013, Dalian Institute of Chemical Physics, Chinese Academy of Sciences.
    view abstractdoi: 10.1016/S2095-4956(13)60038-8
  • 2013 • 137 Segregation engineering enables nanoscale martensite to austenite phase transformation at grain boundaries: A pathway to ductile martensite
    Raabe, D. and Sandlöbes, S. and Millán, J. and Ponge, D. and Assadi, H. and Herbig, M. and Choi, P.-P.
    Acta Materialia 61 6132-6152 (2013)
    In an Fe-9 at.% Mn maraging alloy annealed at 450 C reversed allotriomorphic austenite nanolayers appear on former Mn decorated lath martensite boundaries. The austenite films are 5-15 nm thick and form soft layers among the hard martensite crystals. We document the nanoscale segregation and associated martensite to austenite transformation mechanism using transmission electron microscopy and atom probe tomography. The phenomena are discussed in terms of the adsorption isotherm (interface segregation) in conjunction with classical heterogeneous nucleation theory (phase transformation) and a phase field model that predicts the kinetics of phase transformation at segregation decorated grain boundaries. The analysis shows that strong interface segregation of austenite stabilizing elements (here Mn) and the release of elastic stresses from the host martensite can generally promote phase transformation at martensite grain boundaries. The phenomenon enables the design of ductile and tough martensite. © 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2013.06.055
  • 2013 • 136 Simple synthesis of superparamagnetic magnetite nanoparticles as highly efficient contrast agent
    Jha, D.K. and Shameem, M. and Patel, A.B. and Kostka, A. and Schneider, P. and Erbe, A. and Deb, P.
    Materials Letters 95 186-189 (2013)
    Magnetite nanoparticles have been prepared by one-pot thermal decomposition process using iron (III) acetylacetonate in stearic acid in ambient environment. In this process, stearic acid acts as solvent as well as capping agent for the particles. These as-prepared hydrophobic magnetite nanoparticles have been converted into a hydrophilic form using tetramethylammonium hydroxide. This controlled surface functionalization approach limits microstructural and phase alteration due to the ligand exchange. A detailed investigation was carried out on the microstructural characteristics of these nanoparticles with the aid of X-ray diffraction, infrared spectroscopy, XPS and transmission electron microscopy. The hydrophilic superparamagnetic magnetite particles posses extraordinary transverse relaxivity and contrast property, making them potential T2 contrast agent in clinical magnetic resonance imaging. © 2013 Elsevier B.V.
    view abstractdoi: 10.1016/j.matlet.2012.12.096
  • 2013 • 135 Star-shaped poly(styrene)-block-Poly(4-vinyl-N-methylpyridiniumiodide) for semipermanent antimicrobial coatings
    Siedenbiedel, F. and Fuchs, A. and Moll, T. and Weide, M. and Breves, R. and Tiller, J.C.
    Macromolecular Bioscience 13 1447-1455 (2013)
    Goal of the present work is to develop an antimicrobial coating that can be applied from an aqueous solution and resists short washing cycles, but can be rinsed off by thorough washing. To this end, a series of star-shaped polystyrene-block-poly(4-vinyl-N-methylpyridinium iodide) polymers are synthesized by anionic polymerization using a core-first approach. The optimal resulting polymers are applied as coatings on glass slides, showing high antimicrobial efficiency against Staphylococcus aureus as well as Escherichia coli. The coatings, characterized by atomic force microscopy and transmission electron microscopy, stay at the surface even after at least 20 flush-like washings with water, and retain their antimicrobial activity. Semipermanent antimicrobial coatings can be achieved by synthesizing 3-arm star-blockcopolymer with an inner poly(styrene)-block and an outer poly(4-vinyl-N-methylpyridiniumiodide)-block. Solubility, antimicrobial activity, and coating stability strongly depend on the block ratios. The coatings can be applied from aqueous solution, withstand flush-like washings and prevent growth of S. aureus and E. coli. Nonetheless, the coatings are removable of by extended rinsing. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/mabi.201300219
  • 2013 • 134 Synthesis of small carbon nanoparticles in a microwave plasma flow reactor
    Gurentsov, E. and Priemchenko, K. and Grimm, H. and Orthner, H. and Wiggers, H. and Borchers, C. and Jander, H. and Eremin, A. and Schulz, C.
    Zeitschrift fur Physikalische Chemie 227 357-370 (2013)
    Unusually small carbon nanoparticles were synthesized in a microwave plasma flow-reactor by pyrolysis of 0.3-1.2% CH4, C2H 4, and C2H2 with 0.3-3.6% addition of molecular hydrogen in argon. Final particle sizes were analyzed by in-line particle-mass spectrometry (PMS) and by transmission electron microscopy (TEM). TEM measurements of primary particle sizes were found to be in a good agreement with PMS data. The carbon particles formed in the plasma generated by a 2.45 GHz magnetron with an applied power of 180 W and a total pressure of 13 mbar have diameters of 4-6 nm. The type of hydrocarbon precursor and 0.3-3.6% of hydrogen addition did not noticeably influence the final particle sizes. The formation of such small particles is attributed to the low pressure and the comparably low operation power. This method of small carbon nanoparticles synthesis could be useful for the production of carbon black material, where large surface area is important. © by Oldenbourg Wissenschaftsverlag.
    view abstractdoi: 10.1524/zpch.2013.0369
  • 2013 • 133 The structural and electronic promoting effect of nitrogen-doped carbon nanotubes on supported Pd nanoparticles for selective olefin hydrogenation
    Chen, P. and Chew, L.M. and Kostka, A. and Muhler, M. and Xia, W.
    Catalysis Science and Technology 3 1964-1971 (2013)
    A high-performance Pd catalyst for selective olefin hydrogenation was synthesized by supporting Pd nanoparticles on nitrogen-doped carbon nanotubes (NCNTs). X-ray diffraction, hydrogen chemisorption, transmission electron microscopy and X-ray photoelectron spectroscopy (XPS) were used to characterize Pd supported on NCNTs and nitrogen-free oxygen-functionalized CNTs (OCNTs). The Pd nanoparticles were stabilized on NCNTs with narrower size distribution compared with OCNTs. The XPS analysis revealed that the nitrogen functional groups favor the reduction of Pd on CNTs suggesting an electronic promoter effect. The Pd/NCNT catalyst showed extraordinary catalytic performance in terms of activity, selectivity and stability in the selective hydrogenation of cyclooctadiene, which is related to the structural and electronic promoting effect of the NCNT support. © 2013 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c3cy00097d
  • 2013 • 132 The transition from spark to arc discharge and its implications with respect to nanoparticle production
    Hontañón, E. and Palomares, J.M. and Stein, M. and Guo, X. and Engeln, R. and Nirschl, H. and Kruis, F.E.
    Journal of Nanoparticle Research 15 (2013)
    The synthesis of nanoparticles by means of electrical discharges between two electrodes in an inert gas at atmospheric pressure, as driven by a constant current ranging from a few milliamps to tens of amps, is investigated in this work. An extensive series of experiments are conducted with copper as a consumable electrode and pure nitrogen as the inert gas. Three different DC power supplies are used to drive electrical discharges for the entire operating current range. Then, three electrical discharge regimes (spark, glow, and arc) with distinct voltage-current characteristics and plasma emission spectra are recognized. For the first time, nanoparticles are synthesized by evaporation of an electrode by atmospheric pressure inert gas DC glow discharge of a few millimeters in size. The discharge regimes are characterized in terms of the mass output rate and the particle size distribution of the copper aerosols by means of online (tapered element oscillating microbalance, TEOM; and scanning mobility particle sizer, SPMS) and offline (gravimetric analysis; small and wide angle X-ray scattering, SWAXS; and transmission electron microscopy, TEM) techniques. The electrical power delivered to the electrode gap and the gas flow rate are two major parameters determining the aerosol mass output rate and the aerosol particle size distribution. The mass output rate of copper aerosols raises from 2 mg h-1 to 2 g h-1 when increasing the electrical power from 9 to 900 W. The particle mean size (SMPS dg) varies between 20 and 100 nm depending upon the electrical power and the gas flow rate, whereas the particle size dispersion (SMPS σg) ranges from 1.4 to 1.7 and is only weakly dependent on the gas flow rate. © 2013 Springer Science+Business Media.
    view abstractdoi: 10.1007/s11051-013-1957-y
  • 2013 • 131 Time- and space-resolved high-throughput characterization of stresses during sputtering and thermal processing of Al-Cr-N thin films
    Grochla, D. and Siegel, A. and Hamann, S. and Buenconsejo, P.J.S. and Kieschnick, M. and Brunken, H. and König, D. and Ludwig, Al.
    Journal of Physics D: Applied Physics 46 (2013)
    (Al100-xCrx)N thin-film materials libraries (x = 31-79 at%) were fabricated on micro-machined cantilever arrays, in order to simultaneously investigate the evolution of stresses during film growth as well as during thermal processing by analysing the changes in cantilever curvature. The issue of the dependence of stress in the growing films on composition, at comparable film thicknesses, was investigated. Among the various experimental parameters studied, it was found that the applied substrate bias has the strongest influence on stress evolution and microstructure formation. The compositions of the films, as well as the applied substrate bias, have a pronounced effect on the lattice parameter and the coherence length. For example, applying a substrate bias in general leads to compressive residual stress, increases the lattice parameter and decreases the coherence length. Moreover, bias can change the film texture from [1 1 1] orientation to [2 0 0]. Further detailed analysis using x-ray diffraction and transmission electron microscopy clearly revealed the presence of a [1 1 1] highly textured face centred cubic (B1 type) Al-Cr-N phase in the as-deposited state as well as the coexistence of the hexagonal [1 1 0] textured Cr2N phase, which forms in the Cr-rich region. These results show that the combinatorial approach provides insight into how stresses and compositions are related to phases and microstructures of different Al-Cr-N compositions fabricated in the form of materials libraries. © 2013 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0022-3727/46/8/084011
  • 2013 • 130 Towards the understanding of sintering phenomena at the nanoscale: Geometric and environmental effects
    Behafarid, F. and Roldan Cuenya, B.
    Topics in Catalysis 56 1542-1559 (2013)
    One of the technologically most important requirements for the application of supported metal nanoparticles (NPs) to the field of heterogeneous catalysis is the achievement of thermally and chemically stable systems under reaction conditions. For this purpose, a thorough understanding of the different pathways underlying coarsening phenomena is needed. In particular, in depth knowledge must be achieved on the role of the NP synthesis method, geometrical features of the NPs (size and shape), initial NP dispersion on the support (interparticle distance), support pre-treatment (affecting its morphology and chemical state), and reaction environment (gaseous or liquid medium, pressure, temperature). This study provides examples of the stability and sintering behavior of nanoscale systems monitored ex situ, in situ, and under operando conditions via transmission electron microscopy, atomic force microscopy, scanning tunneling microscopy, and X-ray absorption fine-structure spectroscopy. Experimental data corresponding to physical-vapor-deposited and micelle-synthesized metal (Pt, Au) NPs supported on TiO<inf>2</inf>, SiO<inf>2</inf> and Al<inf>2</inf>O <inf>3</inf> will be used to illustrate Ostwald-ripening and diffusion coalescence processes. In addition, the role of the annealing environment (H<inf>2</inf>, O<inf>2</inf>, water vapor) on the stability of NPs will be discussed. © 2013 Springer Science+Business Media New York.
    view abstractdoi: 10.1007/s11244-013-0149-4
  • 2013 • 129 Transmission electron microscopy characterization of CrN films on MgO(001)
    Harzer, T.P. and Daniel, R. and Mitterer, C. and Dehm, G. and Zhang, Z.L.
    Thin Solid Films 545 154-160 (2013)
    Two CrN(001) films with different thickness were grown on MgO(001) substrates using unbalanced d.c.magnetron sputtering.The morphology and interfacial structure of the films are characterized by using conventional transmission electron microscopy, weak-beam dark-field microscopy and spherical aberration (CS)-corrected high-resolution transmission electron microscopy.The microscopy studies revealed the well-known cube-on-cube orientation relationship.While an interface dislocation network with b→=1/2aCrN< 100&gt; edge dislocations was identified, only part of the lattice mismatch is relaxed.The misfit dislocation structure and growth defects are analyzed and discussed based on the weak-beam dark-field and high-resolution transmission electron microscopy results.© 2013 Elsevier B.V.All rights reserved.
    view abstractdoi: 10.1016/j.tsf.2013.07.064
  • 2012 • 128 Advanced nanomechanics in the TEM: Effects of thermal annealing on FIB prepared Cu samples
    Kiener, D. and Zhang, Z. and Šturm, S. and Cazottes, S. and Imrich, P.J. and Kirchlechner, C. and Dehm, G.
    Philosophical Magazine 92 3269-3289 (2012)
    The effect of focused ion beam (FIB) fabrication on the mechanical properties of miniaturized mechanical tests has recently been realized, but is not well documented. In this study, the effect of post thermal annealing on the plastic properties of FIB fabricated micro- and nanometer-sized Cu samples was studied by means of advanced analytic and in situ transmission electron microscopy. In situ heating experiments on thin films and pillars revealed a reduction of the initially high dislocation density, but never a recovery of the bulk dislocation density. Aberration-corrected atomic imaging documented the recovery of a pristine crystalline surface structure upon annealing, while electron energy-loss spectroscopy showed that the remaining contamination layer consisted of amorphous carbon. These structural observations were combined with the mechanical data from in situ tests of annealed micro- and nanometer-sized tensile and compression samples. The thermal annealing in the micron regime mainly influences the initial yield point, as it reduces the number of suited dislocation sources, while the flow behavior is mostly unaffected. For the submicron samples, the annealed material sustains significantly higher stresses throughout the deformation. This is explained by the high stresses required for surface-mediated dislocation nucleation of the annealed material at the nanoscale. In the present case, the FIB affected the surface near defects and facilitated dislocation nucleation, thereby lowering the material strength. © 2012 Taylor & Francis.
    view abstractdoi: 10.1080/14786435.2012.685966
  • 2012 • 127 Advanced scanning transmission stereo electron microscopy of structural and functional engineering materials
    Agudo Jácome, L. and Eggeler, G. and Dlouhý, A.
    Ultramicroscopy 122 48-59 (2012)
    Stereo transmission electron microscopy (TEM) provides a 3D impression of the microstructure in a thin TEM foil. It allows to perform depth and TEM foil thickness measurements and to decide whether a microstructural feature lies inside of a thin foil or on its surface. It allows appreciating the true three-dimensional nature of dislocation configurations. In the present study we first review some basic elements of classical stereo TEM. We then show how the method can be extended by working in the scanning transmission electron microscope (STEM) mode of a modern analytical 200. kV TEM equipped with a field emission gun (FEG TEM) and a high angle annular dark field (HAADF) detector. We combine two micrographs of a stereo pair into one anaglyph. When viewed with special colored glasses the anaglyph provides a direct and realistic 3D impression of the microstructure. Three examples are provided which demonstrate the potential of this extended stereo TEM technique: a single crystal Ni-base superalloy, a 9% Chromium tempered martensite ferritic steel and a NiTi shape memory alloy. We consider the effect of camera length, show how foil thicknesses can be measured, and discuss the depth of focus and surface effects. © 2012 Elsevier B.V.
    view abstractdoi: 10.1016/j.ultramic.2012.06.017
  • 2012 • 126 Atomic layer deposition of Gd 2O 3 and Dy 2O 3: A study of the ALD characteristics and structural and electrical properties
    Xu, K. and Ranjith, R. and Laha, A. and Parala, H. and Milanov, A.P. and Fischer, R.A. and Bugiel, E. and Feydt, J. and Irsen, S. and Toader, T. and Bock, C. and Rogalla, D. and Osten, H.-J. and Kunze, U. and Devi, A.
    Chemistry of Materials 24 651-658 (2012)
    Gd 2O 3 and Dy 2O 3 thin films were grown by atomic layer deposition (ALD) on Si(100) substrates using the homoleptic rare earth guanidinate based precursors, namely, tris(N,N′- diisopropyl-2-dimethylamido-guanidinato)gadolinium(III) [Gd(DPDMG) 3] (1) and tris(N,N′-diisopropyl-2-dimethylamido-guanidinato)dysprosium(III) [Dy(DPDMG) 3] (2), respectively. Both complexes are volatile and exhibit high reactivity and good thermal stability, which are ideal characteristics of a good ALD precursor. Thin Gd 2O 3 and Dy 2O 3 layers were grown by ALD, where the precursors were used in combination with water as a reactant at reduced pressure at the substrate temperature ranging from 150 °C to 350 °C. A constant growth per cycle (GPC) of 1.1 Å was obtained at deposition temperatures between 175 and 275 °C for Gd 2O 3, and in the case of Dy 2O 3, a GPC of 1.0 Å was obtained at 200-275 °C. The self-limiting ALD growth characteristics and the saturation behavior of the precursors were confirmed at substrate temperatures of 225 and 250 °C within the ALD window for both Gd 2O 3 and Dy 2O 3. Thin films were structurally characterized by grazing incidence X-ray diffraction (GI-XRD), atomic force microscopy (AFM), and transmission electron microscopy (TEM) analyses for crystallinity and morphology. The chemical composition of the layer was examined by Rutherford backscattering (RBS) analysis and Auger electron spectroscopy (AES) depth profile measurements. The electrical properties of the ALD grown layers were analyzed by capacitance-voltage (C-V) and current-voltage (I-V) measurements. Upon subjection to a forming gas treatment, the ALD grown layers show promising dielectric behavior, with no hysteresis and reduced interface trap densities, thus revealing the potential of these layers as high-k oxide for application in complementary metal oxide semiconductor based devices. © 2012 American Chemical Society.
    view abstractdoi: 10.1021/cm2020862
  • 2012 • 125 Degradation mechanisms of Pt/C fuel cell catalysts under simulated start-stop conditions
    Meier, J.C. and Galeano, C. and Katsounaros, I. and Topalov, A.A. and Kostka, A. and Schüth, F. and Mayrhofer, K.J.J.
    ACS Catalysis 2 832-843 (2012)
    This manuscript investigates the degradation of a Pt/Vulcan fuel cell catalyst under simulated start-stop conditions in an electrochemical half-cell. Identical location transmission electron microscopy (IL-TEM) is used to visualize the several different degradation pathways occurring on the same catalyst material under potential cycling conditions. The complexity of degradation on the nanoscale leading to macroscopic active surface area lossis demonstrated and discussed. Namely, four different degradation pathways at one single Pt/Vulcan aggregate are clearly observed. Furthermore, inhomogeneous degradation behavior for different catalyst locations is shown, and trends in degradation mechanisms related to the platinum particle size are discussed in brief. Attention is drawn to the vast field of parameters influencing catalyst stability. We also present the development of a new technique to study changes of the catalyst not only with 2D projections of standard TEM images but also in 3D. For this purpose, identical location tomography (IL-tomography) is introduced, which visualizes the 3D structure of an identical catalyst location before and after degradation. © 2012 American Chemical Society.
    view abstractdoi: 10.1021/cs300024h
  • 2012 • 124 Determination of the quantum dot band gap dependence on particle size from optical absorbance and transmission electron microscopy measurements
    Segets, D. and Lucas, J.M. and Klupp Taylor, R.N. and Scheele, M. and Zheng, H. and Alivisatos, A.P. and Peukert, W.
    ACS Nano 6 9021-9032 (2012)
    This work addresses the determination of arbitrarily shaped particle size distributions (PSDs) from PbS and PbSe quantum dot (QD) optical absorbance spectra in order to arrive at a relationship between band gap energy and particle size over a large size range. Using a modified algorithm which was previously developed for ZnO, we take only bulk absorption data from the literature and match the PSDs derived from QD absorbance spectra with those from transmission electron microscopical (TEM) image analysis in order to arrive at the functional dependence of the band gap on particle size. Additional samples sized solely from their absorbance spectra with our algorithm show excellent agreement with TEM results. We investigate the influence of parameters of the TEM image analysis such as threshold value on the final result. The band gap versus size relationship developed from analysis of just two samples lies well within the bounds of a number of published data sets. We believe that our methodology provides an attractive shortcut for the study of various novel quantum-confined direct band gap semiconductor systems as it permits the band gap energies of a broad size range of QDs to be probed with relatively few synthetic experiments and without quantum mechanical simulations. © 2012 American Chemical Society.
    view abstractdoi: 10.1021/nn303130d
  • 2012 • 123 Discovery and structure activity relationship of small molecule inhibitors of toxic β-amyloid-42 fibril formation
    Kroth, H. and Ansaloni, A. and Varisco, Y. and Jan, A. and Sreenivasachary, N. and Rezaei-Ghaleh, N. and Giriens, V. and Lohmann, S. and López-Deber, M.P. and Adolfsson, O. and Pihlgren, M. and Paganetti, P. and Froestl, W. and N...
    Journal of Biological Chemistry 287 34786-34800 (2012)
    Increasing evidence implicates Aβ peptides self-assembly and fibril formation as crucial events in the pathogenesis of Alzheimer disease. Thus, inhibiting Aβ aggregation, among others, has emerged as a potential therapeutic intervention for this disorder. Herein, we employed 3-aminopyrazole as a key fragment in our design of non-dye compounds capable of interacting with Aβ42 via a donor-acceptor-donor hydrogen bond pattern complementary to that of the β-sheet conformation of Aβ42. The initial design of the compounds was based on connecting two 3-aminopyrazole moieties via a linker to identify suitable scaffold molecules. Additional aryl substitutions on the two 3-aminopyrazole moieties were also explored to enhance π-π stacking/hydrophobic interactions with amino acids of Aβ42. The efficacy of these compounds on inhibiting Aβ fibril formation and toxicity in vitro was assessed using a combination of biophysical techniques and viability assays. Using structure activity relationship data from the in vitro assays, we identified compounds capable of preventing pathological self-assembly of Aβ42 leading to decreased cell toxicity. © 2012 by The American Society for Biochemistry and Molecular Biology, Inc.
    view abstractdoi: 10.1074/jbc.M112.357665
  • 2012 • 122 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 • 121 Dislocation starvation and exhaustion hardening in Mo alloy nanofibers
    Chisholm, C. and Bei, H. and Lowry, M.B. and Oh, J. and Syed Asif, S.A. and Warren, O.L. and Shan, Z.W. and George, E.P. and Minor, A.M.
    Acta Materialia 60 2258-2264 (2012)
    The evolution of defects in Mo alloy nanofibers with initial dislocation densities ranging from 0 to ∼1.6 × 10 14 m -2 were studied using an in situ "push-to-pull" device in conjunction with a nanoindenter in a transmission electron microscope. Digital image correlation was used to determine stress and strain in local areas of deformation. When they had no initial dislocations the Mo alloy nanofibers suffered sudden catastrophic elongation following elastic deformation to ultrahigh stresses. At the other extreme fibers with a high dislocation density underwent sustained homogeneous deformation after yielding at much lower stresses. Between these two extremes nanofibers with intermediate dislocation densities demonstrated a clear exhaustion hardening behavior, where the progressive exhaustion of dislocations and dislocation sources increases the stress required to drive plasticity. This is consistent with the idea that mechanical size effects ("smaller is stronger") are due to the fact that nanostructures usually have fewer defects that can operate at lower stresses. By monitoring the evolution of stress locally we find that exhaustion hardening causes the stress in the nanofibers to surpass the critical stress predicted for self-multiplication, supporting a plasticity mechanism that has been hypothesized to account for the rapid strain softening observed in nanoscale bcc materials at high stresses. © 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2011.12.027
  • 2012 • 120 Dispersions of various titania nanoparticles in two different ionic liquids
    Wittmar, A. and Ulbricht, M.
    Industrial and Engineering Chemistry Research 51 8425-8433 (2012)
    The dispersibility of different lab-made and commercial TiO 2 nanoparticles prepared by gas-phase processes in room temperature ionic liquids was for the first time studied by dynamic light scattering and advanced rheology. The characterization of the nanopowders has been done with transmission electron microscopy, X-ray diffraction analysis, nitrogen adsorption, and Brunauer-Emmett-Teller (BET) analysis and FT-IR spectroscopy. The colloidal stabilities of the resulting dispersions were strongly influenced by particle characteristics such as aggregation level, mean particle size, and surface functionality. The period of the ultrasound treatment, the powder concentration in the dispersion, and the hydrophilicity of the ionic liquid were also important influences. It was found that most types of powders disperse better in the hydrophilic ionic liquid because of the hydroxyl groups and adsorbed water present on the powders' surfaces. The best dispersions over a broader concentration range were obtained for a lab-made powder produced by chemical vapor synthesis (aerosol method) which had the smallest nonaggregated particles. © 2012 American Chemical Society.
    view abstractdoi: 10.1021/ie203010x
  • 2012 • 119 Electron transport in partially filled iron carbon nanotubes
    Migunov, V. and Li, Z.-A. and Spasova, M. and Farle, M.
    Solid State Phenomena 190 498-501 (2012)
    We report electron transport properties of iron filled multiwalled carbon nanotubes (MWCNT) with outer diameters of 30 to 80 nm and lengths of 1 to 10 μm. Our study is combined with a structural investigation of the iron core using transmission electron microscopy (TEM) and electron energy loss spectroscopy (EELS). It was found that high current densities of 1.8×107A/cm2 increase the conductivity of the MWCNT by a factor of two at 300 K, while the Fe core disappears probably forming defect states in the carbon shells. The enhanced diffusion of iron is most probably the result of local heating of the iron followed by implantation of iron atoms in the nanotube layers. © (2012) Trans Tech Publications.
    view abstractdoi: 10.4028/
  • 2012 • 118 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 • 117 Evolution of strength and microstructure during annealing of heavily cold-drawn 6.3 GPa hypereutectoid pearlitic steel wire
    Li, Y.J. and Choi, P. and Goto, S. and Borchers, C. and Raabe, D. and Kirchheim, R.
    Acta Materialia 60 4005-4016 (2012)
    Hypereutectoid steel wires with 6.35 GPa tensile strength after a cold-drawing true strain of 6.02 were annealed between 300 and 723 K. The ultrahigh strength remained upon annealing for 30 min up to a temperature of 423 K but dramatically decreased with further increasing temperature. The reduction of tensile strength mainly occurred within the first 2-3 min of annealing. Atom probe tomography and transmission electron microscopy reveal that the lamellar structure remains up to 523 K. After annealing at 673 K for 30 min, coarse hexagonal ferrite (sub)grains with spheroidized cementite, preferentially located at triple junctions, were observed in transverse cross-sections. C and Si segregated at the (sub)grain boundaries, while Mn and Cr enriched at the ferrite/cementite phase boundaries due to their low mobility in cementite. No evidence of recrystallization was found even after annealing at 723 K for 30 min. The stability of the tensile strength for low-temperature annealing (<473 K) and its dramatic drop upon high-temperature annealing (>473 K) are discussed based on the nanostructural observations. © 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2012.03.006
  • 2012 • 116 Formation of highly ordered alloy nanoparticles based on precursor-filled latex spheres
    Manzke, A. and Plettl, A. and Wiedwald, U. and Han, L. and Ziemann, P. and Schreiber, E. and Ziener, U. and Vogel, N. and Weiss, C.K. and Landfester, K. and Fauth, K. and Biskupek, J. and Kaiser, U.
    Chemistry of Materials 24 1048-1054 (2012)
    An experimental approach is presented, allowing the preparation of substrate supported, hexagonally arranged metallic alloy nanoparticles with narrow size distributions, well-defined interparticle distances, and controlled chemical composition. The method is based on miniemulsion polymerization and isotropic plasma etching. Polystyrene (PS) and poly(methyl methacrylate) (PMMA) colloids-in the present study containing Fe- and Pt-precursor complexes in a predefined ratio-are deposited onto hydrophilic Si/SiO 2 substrates by dip-coating, forming a highly ordered monolayer. Contrary to colloidal lithography, here, precursor-filled polystyrene colloids serve as carriers for the alloy forming elements. After reactive ion etching and annealing, hexagonally ordered arrays of crystalline FePt nanoparticles are formed exhibiting the desired 1:1 Fe-Pt ratio, as revealed by detailed analysis after each preparation step. Formation of stoichiometric binary alloy FePt nanoparticles is confirmed by determining magnetic hysteresis loops, as well as applying aberration-corrected high-resolution transmission electron microscopy. © 2012 American Chemical Society.
    view abstractdoi: 10.1021/cm203241p
  • 2012 • 115 Gas phase oxidation as a tool to introduce oxygen containing groups on metal-loaded carbon nanofibers
    Gosselink, R.W. and Van Den Berg, R. and Xia, W. and Muhler, M. and De Jong, K.P. and Bitter, J.H.
    Carbon 50 4424-4431 (2012)
    Oxygen containing groups were introduced, onto carbon nanofibers (CNFs) that were previously loaded with palladium, using HNO 3 vapor. Using traditional liquid-phase oxidations this is not possible due to severe metal leaching. For the samples oxidized using HNO 3 vapor temperature programmed desorption and X-ray photoelectron spectroscopy revealed the presence of two major classes of oxygen containing groups, i.e. carboxylic acid groups which are thermally stable up to 300 °C and less acidic (e.g. phenol) and basic groups which were stable up to 700 °C. The amount of acidic oxygen containing groups introduced by this gas-phase treatment ranged from 0.1 to 0.3 mmol/g, as determined by titration. The latter amount is comparable to that introduced by traditional liquid-phase treatment in 65% HNO 3 on bare CNFs. Transmission electron microscopy and H 2-chemisorption measurements show a gradual increase of the average metal particle size from 2.1 nm for the starting Pd/CNF to 4.5 nm for Pd/CNF treated for 75 h in HNO 3 vapor indicating that the extent of sintering with gas-phase treatment is limited. Elemental analysis showed that no leaching occurred upon gas-phase oxidation, whereas 90% of the metal was lost with a liquid-phase reflux HNO 3 treatment. © 2012 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.carbon.2012.05.020
  • 2012 • 114 Growth optimization and characterization of lattice-matched Al 0.82In 0.18N optical confinement layer for edge emitting nitride laser diodes
    Kim-Chauveau, H. and Frayssinet, E. and Damilano, B. and De Mierry, P. and Bodiou, L. and Nguyen, L. and Vennéguès, P. and Chauveau, J.-M. and Cordier, Y. and Duboz, J.Y. and Charash, R. and Vajpeyi, A. and Lamy, J.-M. and Akhte...
    Journal of Crystal Growth 338 20-29 (2012)
    We present the growth optimization and the doping by the metal organic chemical vapor deposition of lattice-matched Al 0.82In 0.18N bottom optical confinement layers for edge emitting laser diodes. Due to the increasing size and density of V-shaped defects in Al 1-xIn xN with increasing thickness, we have designed an Al 1-xIn xN/GaN multilayer structure by optimizing the growth and thickness of the GaN interlayer. The Al 1-xIn xN and GaN interlayers in the multilayer structure were both doped using the same SiH 4 flow, while the Si levels in both layers were found to be significantly different by SIMS. The optimized 8×(Al 0.82In 0.18N/GaN=54/6 nm) multilayer structures grown on free-standing GaN substrates were characterized by high resolution X-ray diffraction, atomic force microscopy and transmission electron microscopy, along with the in-situ measurements of stress evolution during growth. Finally, lasing was obtained from the UV (394 nm) to blue (436 nm) wavelengths, in electrically injected, edge-emitting, cleaved-facet laser diodes with 480 nm thick Si-doped Al 1-xIn xN/GaN multilayers as bottom waveguide claddings. © 2011 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.jcrysgro.2011.10.016
  • 2012 • 113 In situ study of γ-TiAl lamellae formation in supersaturated α 2-Ti 3Al grains
    Cha, L. and Schmoelzer, T. and Zhang, Z. and Mayer, S. and Clemens, H. and Staron, P. and Dehm, G.
    Advanced Engineering Materials 14 299-303 (2012)
    In situ heating transmission electron microscopy (TEM) was used to investigate the initial stage of γ-TiAl lamellae formation in an intermetallic Ti-45Al-7.5Nb alloy (in at.%). The material was heat treated and quenched in a non-equilibrium state to consist mainly of supersaturated, ordered α 2-Ti 3Al grains. Subsequently, specimens were annealed inside a TEM up to 750 °C. The in situ TEM study revealed that ultra-fine γ-TiAl laths precipitate in the α 2-matrix at ≈730 °C which exhibit the classical Blackburn orientation relationship, i.e. (0001)α 2//(111)γ and [$112̄0] α 2//< 110]γ. The microstructural development observed in the in situ TEM experiment is compared to results from conventional ex situ TEM studies. In order to investigate the precipitation behavior of the γ-phase with a complementary method, in situ high energy X-ray diffraction experiments were performed which confirmed the finding that γ-laths start to precipitate at ≈730 °C from the supersaturated α 2- matrix. © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/adem.201100272
  • 2012 • 112 Influence of impurity elements on the nucleation and growth of Si in high purity melt-spun Al-Si-based alloys
    Li, J.H. and Zarif, M.Z. and Dehm, G. and Schumacher, P.
    Philosophical Magazine 92 3789-3805 (2012)
    The nucleation and growth of Si has been investigated by TEM in a series of high purity melt spun Al-5Si (wt%)-based alloys with a trace addition of Fe and Sr. In the as-melt-spun condition, some twinned Si particles were found to form directly from the liquid along the grain boundary. The addition of Sr into Al-5Si-based alloys promotes the twinning of Si particles on the grain boundary and the formation of Si precipitates in the α-Al matrix. The majority of plate-shaped and truncated pyramid-shaped Si precipitates were also found to nucleate and grow along {111}-Al planes from supersaturated solid solution in the α-Al matrix. In contrast, controlled slow cooling decreased the amount of Si precipitates, while the size of the Si precipitates increased. The orientation relationship between these Si precipitates and the α-Al matrix still remained cube to cube. The β-Al5 FeSi intermetallic was also observed, depending on subsequent controlled cooling. © 2012 Copyright Taylor and Francis Group, LLC.
    view abstractdoi: 10.1080/14786435.2012.687840
  • 2012 • 111 Interface of nanoparticle-coated electropolished stents
    Neumeister, A. and Bartke, D. and Bärsch, N. and Weingärtner, T. and Guetaz, L. and Montani, A. and Compagnini, G. and Barcikowski, S.
    Langmuir 28 12060-12066 (2012)
    Nanostructures entail a high potential for improving implant surfaces, for instance, in stent applications. The electrophoretic deposition of laser-generated colloidal nanoparticles is an appropriate tool for creating large-area nanostructures on surfaces. Until now, the bonding and characteristics of the interface between deposited nanoparticles and the substrate surface has not been known. It is investigated using X-ray photoelectron spectroscopy, Auger electron spectroscopy, and transmission electron microscopy to characterize an electropolished NiTi stent surface coated by laser-generated Au and Ti nanoparticles. The deposition of elemental Au and Ti nanoparticles is observed on the total 3D surface. Ti-coated samples are composed of Ti oxide and Ti carbide because of nanoparticle fabrication and the coating process carried out in 2-propanol. The interface between nanoparticles and the electropolished surface consists of a smooth, monotone elemental depth profile. The interface depth is higher for the Ti nanoparticle coating than for the Au nanoparticle coating. This smooth depth gradient of Ti across the coating-substrate intersection and the thicker interface layer indicate the hard bonding of Ti-based nanoparticles on the surface. Accordingly, electron microscopy reveals nanoparticles adsorbed on the surface without any sorption-blocking intermediate layer. The physicomechanical stability of the bond may benefit from such smooth depth gradients and direct, ligand-free contact. This would potentially increase the coating stability during stent application. © 2012 American Chemical Society.
    view abstractdoi: 10.1021/la300308w
  • 2012 • 110 Interfacial interaction driven CO oxidation: Nanostructured Ce 1-xLa xO 2-δ/TiO 2 solid solutions
    Katta, L. and Reddy, B.M. and Muhler, M. and Grünert, W.
    Catalysis Science and Technology 2 745-753 (2012)
    Titania supported ceria-lanthana solid solutions (Ce xLa 1-xO 2-δ/TiO 2; CLT) have been synthesized by a facile and economical route. Existence of synergism between ceria-lanthana (CL) solid solutions and titania-anatase phase, which leads to decrease in the crystallite size, retarded titania phase transformation, and improved redox properties, has been thoroughly investigated by various techniques, namely, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), UV-visible diffuse reflectance spectroscopy (UV-vis DRS), Raman spectroscopy (UV-RS and Vis-RS), BET surface area analysis, and temperature programmed reduction (TPR). Two key observations made from the whole exercise were (i) mutual interaction of Ce and Ti ions could impose typical Ce-O-Ti modes at the interfacial region and (ii) the La 3+ ion as a dopant provokes a large number of oxygen vacancies via a charge compensation mechanism. The promising role of these factors in the CO oxidation (one of the most formidable challenges) has been comprehensively described. The observed enhanced activity for the CLT sample is primarily attributed to an apparent specific orientation of the active component over the support, which is endorsed by the interfacial interaction. This specific mode could facilitate the CO adsorption with simultaneous bulk oxygen diffusion for more consumption and in turn better activity. © 2012 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c2cy00449f
  • 2012 • 109 Leaf-like dislocation substructures and the decrease of martensitic start temperatures: A new explanation for functional fatigue during thermally induced martensitic transformations in coarse-grained Ni-rich Ti-Ni shape memory alloys
    Zhang, J. and Somsen, C. and Simon, T. and Ding, X. and Hou, S. and Ren, S. and Ren, X. and Eggeler, G. and Otsuka, K. and Sun, J.
    Acta Materialia 60 1999-2006 (2012)
    During repeatedly imposed thermally induced martensitic transformations in Ti-Ni shape memory alloys, the martensite start temperature M s decreases. This has been rationalized on the basis of a scenario where an increasing dislocation density makes it more and more difficult for martensite to form. However, it is not clear why dislocations which form because they accommodate the growth of martensite during the first cooling cycle should act as obstacles during subsequent transformation cycles. In the present work we use diffraction contrast transmission electron microscopy to monitor the formation of unique leaf-like dislocation substructures which form as the martensite start temperature decreases during thermal cycling. We interpret our microstructural results on the basis of a microstructural scenario where dislocations play different roles with respect to the propagation of a big martensite needle in one transformation cycle and the nucleation and growth of new martensite needles in the following cycles. As a consequence, chestnut-leaf-like dislocation arrays spread out in different crystallographic directions. © 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2011.12.014
  • 2012 • 108 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 • 107 Monitoring structural influences on quantum transport in InAs nanowires
    Frielinghaus, R. and Flöhr, K. and Sladek, K. and Weirich, T.E. and Trellenkamp, S. and Hardtdegen, H. and Schäpers, T. and Schneider, C.M. and Meyer, C.
    Applied Physics Letters 101 (2012)
    A sample design that allows for quantum transport and transmission electron microscopy (TEM) on individual suspended nanostructures is used to investigate moderately n-type doped InAs nanowires (NWs). The nanowires were grown by metal organic vapor phase epitaxy. Universal conductance fluctuations in the nanowires are investigated at temperatures down to 0.35 K. These fluctuations show two different temperature dependences. The very same nanowire segments investigated in transport are subsequently analyzed by TEM revealing crystal phase mixing. However, we find no correspondence between the atomic structure of the wires and the temperature dependences of the conductance fluctuations. © 2012 American Institute of Physics.
    view abstractdoi: 10.1063/1.4742326
  • 2012 • 106 Multistage strain hardening through dislocation substructure and twinning in a high strength and ductile weight-reduced Fe-Mn-Al-C steel
    Gutierrez-Urrutia, I. and Raabe, D.
    Acta Materialia 60 5791-5802 (2012)
    We investigate the kinetics of the deformation structure evolution and its contribution to the strain hardening of a Fe-30.5Mn-2.1Al-1.2C (wt.%) steel during tensile deformation by means of transmission electron microscopy and electron channeling contrast imaging combined with electron backscatter diffraction. The alloy exhibits a superior combination of strength and ductility (ultimate tensile strength of 1.6 GPa and elongation to failure of 55%) due to the multiple-stage strain hardening. We explain this behavior in terms of dislocation substructure refinement and subsequent activation of deformation twinning. The early hardening stage is fully determined by the size of the dislocation substructure, namely, Taylor lattices, cell blocks and dislocation cells. The high carbon content in solid solution has a pronounced effect on the evolving dislocation substructure. We attribute this effect to the reduction of the dislocation cross-slip frequency by solute carbon. With increasing applied stress, the cross-slip frequency increases. This results in a gradual transition from planar (Taylor lattices) to wavy (cells, cell blocks) dislocation configurations. The size of such dislocation substructures scales inversely with the applied resolved stress. We do not observe the so-called microband-induced plasticity effect. In the present case, due to texture effects, microbanding is not favored during tensile deformation and, hence, has no effect on strain hardening. © 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2012.07.018
  • 2012 • 105 Nanoscale austenite reversion through partitioning, segregation and kinetic freezing: Example of a ductile 2 GPa Fe-Cr-C steel
    Yuan, L. and Ponge, D. and Wittig, J. and Choi, P. and Jiménez, J.A. and Raabe, D.
    Acta Materialia 60 2790-2804 (2012)
    Austenite reversion during tempering of a Fe-13.6 Cr-0.44 C (wt.%) martensite results in an ultra-high-strength ferritic stainless steel with excellent ductility. The austenite reversion mechanism is coupled to the kinetic freezing of carbon during low-temperature partitioning at the interfaces between martensite and retained austenite and to carbon segregation at martensite-martensite grain boundaries. An advantage of austenite reversion is its scalability, i.e. changing tempering time and temperature tailors the desired strength-ductility profiles (e.g. tempering at 400 °C for 1 min produces a 2 GPa ultimate tensile strength (UTS) and 14% elongation while 30 min at 400 °C results in a UTS of ∼1.75 GPa with an elongation of 23%). The austenite reversion process, carbide precipitation and carbon segregation have been characterized by X-ray diffraction, electron back-scatter diffraction, transmission electron microscopy and atom probe tomography in order to develop the structure-property relationships that control the material's strength and ductility. © 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2012.01.045
  • 2012 • 104 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 • 103 Nanostructured supported palladium catalysts - Non-oxidative methane coupling
    Moya, S.F. and Martins, R.L. and Ota, A. and Kunkes, E.L. and Behrens, M. and Schmal, M.
    Applied Catalysis A: General 411-412 105-113 (2012)
    The Pd on α-Al2O3 catalysts with Pd particles in the low nanometer range have been prepared by a sonochemical reduction and a colloidal method, respectively. The two catalysts differ in their particle size, the widths of their particle size distributions and the amount of carbon incorporation in the Pd lattice. The adsorptive properties of the Pd/Al 2O3 samples are different as a result of the different preparation methods. The methane adsorption capacity of that sample with smaller particles is lower than that of the catalyst with larger particles and the energy of activation is nearly doubled. DRIFTS and TPD results of CO adsorption supported by transmission electron microscopy data indicate that the PdSON catalyst with smaller and more homogeneous particles than PdCOL is highly dispersed which influences the coupling-hydrogenolysis process. The catalytic activity evidenced the formation of different adspecies during methane coupling and chemisorption on both catalysts. During the hydrogenation the carbon adspecies formed mainly methane at low adsorption temperatures. The significant amount of adsorbed methane at 773 K is governed by the highly active coordination unsaturated sites at the surface. © 2011 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.apcata.2011.10.025
  • 2012 • 102 New insights into hard phases of CoCrMo metal-on-metal hip replacements
    Liao, Y. and Pourzal, R. and Stemmer, P. and Wimmer, M.A. and Jacobs, J.J. and Fischer, A. and Marks, L.D.
    Journal of the Mechanical Behavior of Biomedical Materials 12 39-49 (2012)
    The microstructural and mechanical properties of the hard phases in CoCrMo prosthetic alloys in both cast and wrought conditions were examined using transmission electron microscopy and nanoindentation. Besides the known carbides of M23C6-type (M=Cr, Mo, Co) and M6C-type which are formed by either eutectic solidification or precipitation, a new mixed-phase hard constituent has been found in the cast alloys, which is composed of ~100nm fine grains. The nanosized grains were identified to be mostly of M23C6 type using nano-beam precession electron diffraction, and the chemical composition varied from grain to grain being either Cr- or Co-rich. In contrast, the carbides within the wrought alloy having the same M23C6 structure were homogeneous, which can be attributed to the repeated heating and deformation steps. Nanoindentation measurements showed that the hardness of the hard phase mixture in the cast specimen was ~15.7GPa, while the M23C6 carbides in the wrought alloy were twice as hard (~30.7GPa). The origin of the nanostructured hard phase mixture was found to be related to slow cooling during casting. Mixed hard phases were produced at a cooling rate of 0.2°C/s, whereas single phase carbides were formed at a cooling rate of 50°C/s. This is consistent with sluggish kinetics and rationalizes different and partly conflicting microstructural results in the literature, and could be a source of variations in the performance of prosthetic devices in-vivo. © 2012 Elsevier Ltd.
    view abstractdoi: 10.1016/j.jmbbm.2012.03.013
  • 2012 • 101 On the presence of work-hardened zones around fibers in a short-fiber-reinforced Al metal matrix composite
    Kurumlu, D. and Payton, E.J. and Somsen, C. and Dlouhy, A. and Eggeler, G.
    Acta Materialia 60 6051-6064 (2012)
    Dislocation densities are investigated in a short-fiber-reinforced Al-11 wt.% Zn-0.2 wt.% Mg metal matrix composite (MMC) with a special focus on regions near the fiber-matrix interfaces. Clear microstructural evidence is provided for the formation of work-hardened zones (WHZs) around fibers during creep using transmission electron microscopy (TEM). The dislocation densities in the WHZs are higher after creep than after squeeze casting, where the plastic strains associated with the thermal stresses that build up during solidification also result in an increased dislocation density close to fibers. The effect of heating and cooling on the dislocation substructure is also considered. The results are discussed in light of previous findings and provide microstructural evidence for the presence of WHZs as predicted by the Dlouhy model of MMC creep. © 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2012.07.042
  • 2012 • 100 Ordered mesoporous materials with MFI structured microporous walls - Synthesis and proof of wall microporosity
    Reichinger, M. and Schmidt, W. and Narkhede, V.V. and Zhang, W. and Gies, H. and Grünert, W.
    Microporous and Mesoporous Materials 164 21-31 (2012)
    Ordered mesoporous materials (OMMs) of 1-dimensional hexagonal and 3-dimensional cubic symmetry of the pore systems were synthesized via well-established soft templating routes starting from precursor solutions of MFI-type zeolites (Silicalite-1, TS-1). The products were characterized by XRD, nitrogen and argon physisorption, DTG/DTA, IR, UV-vis spectroscopy, XANES, TEM, 119Xe NMR, and determination of the pair distribution function (PDF) in order to elucidate their structure, in particular to prove the presence of microporosity in arrays smaller than the coherence lengths of XRD, i.e. in the pore walls. The mesoporosity of the OMMs was well supported by physisorption studies and by TEM while the regularity of the structure was documented by XRD, which also served to exclude the presence of microporous crystalline grains. Instead, microporosity was detected by adsorption/desorption of water on tetrahedrally coordinated Ti-sites (XANES), by 119Xe NMR, by the comparison of the PDF with those of amorphous and of MFI-type solids, and by sequential decomposition of the structure directing agents for meso and micropore systems. From comparison of XRD and physisorption data and from the TEM micrographs, the thickness of the microporous mesopore walls was concluded to be ≈1.5 nm. Therefore, the failure of Ar physisorption to detect pores of sizes typical of MFI structures was attributed to the small micropore volume and the very short pore extension. The structural integrity of OMMs with 1-dimensional hexagonal pore system could be improved by a hydrothermal post-treatment despite the microporous nature of their pore walls, which resulted in more narrow mesopore size distributions peaking at somewhat larger pore sizes. © 2012 Elsevier Inc. All rights reserved.
    view abstractdoi: 10.1016/j.micromeso.2012.07.025
  • 2012 • 99 Pseudoelastic deformation and size effects during in situ transmission electron microscopy tensile testing of NiTi
    Manchuraju, S. and Kroeger, A. and Somsen, C. and Dlouhy, A. and Eggeler, G. and Sarosi, P.M. and Anderson, P.M. and Mills, M.J.
    Acta Materialia 60 2770-2777 (2012)
    The stress-induced B2-B19′ transformation in aged 51 at.% NiTi was investigated using in situ straining transmission electron microscopy (TEM). Increased applied strain along [1 1 0] B2 transforms B2 into plates containing B19′ variants that are related by a (1 1 0) B2 compound twin plane. This atypical twin plane is explained by relaxing the invariant plane constraint in the crystallographic theory of martensite (CTM) to an invariant line constraint. The relaxation is rationalized from the thin foil geometry. The relaxed CTM approach, coupled with conditions to maximize transformation strain along the loading axis and minimize elastic energy, predicts the observed twin interface, diffraction patterns, and interface with the B2 austenite. These results demonstrate subtleties in interpreting thin foil TEM results regarding martensitic transformations, and translating those results to bulk response. © 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2012.01.043
  • 2012 • 98 Rapid and surfactant-free synthesis of bimetallic Pt-Cu nanoparticles simply via ultrasound-assisted redox replacement
    Sun, Z. and Masa, J. and Xia, W. and König, D. and Ludwig, Al. and Li, Z.-A. and Farle, M. and Schuhmann, W. and Muhler, M.
    ACS Catalysis 2 1647-1653 (2012)
    The synthesis of bimetallic nanoparticles (NPs) with well-defined morphology and a size of <5 nm remains an ongoing challenge. Here, we developed a facile and efficient approach to the design of bimetallic nanostructures by the galvanic replacement reaction facilitated by high-intensity ultrasound (100 W, 20 kHz) at low temperatures. As a model system, Pt-Cu NPs deposited on nitrogen-doped carbon nanotubes (NCNTs) were synthesized and characterized by spectroscopic and microscopic techniques. Transmission electron microscopy (TEM) inspection shows that the mean diameter of Pt-Cu NPs can be as low as ≈2.8 nm, regardless of the much larger initial Cu particle size, and that a significant increase in particle number density by a factor of 35 had occurred during the replacement process. The concentration of the Pt precursor solution as well as of the size of the seed particles were found to control the size of the bimetallic NPs. Energy dispersive X-ray spectroscopy performed in the scanning TEM mode confirmed the alloyed nature of the Pt-Cu NPs. Electrochemical oxygen reduction measurements demonstrated that the resulting Pt-Cu/NCNT catalysts exhibit an approximately 2-fold enhancement in both mass- and area-related activities compared with a commercial Pt/C catalyst. © 2012 American Chemical Society.
    view abstractdoi: 10.1021/cs300187z
  • 2012 • 97 Scaling potential of local redox processes in memristive SrTiO 3 thin-film devices
    Dittmann, R. and Muenstermann, R. and Krug, I. and Park, D. and Menke, T. and Mayer, J. and Besmehn, A. and Kronast, F. and Schneider, C. M. and Waser, R.
    Proceedings of the IEEE 100 1979-1990 (2012)
    In this work, we address the following question: Where do the resistive switching processes take place in memristive thin-film devices of the single crystalline model material Fe-doped SrTiO 3? We compare resistive switching induced by the tip of the atomic force microscope on the bare thin-film surface with the switching properties observed in memristive devices with Pt top electrode. In order to close the gap between these two approaches, we combine conductive-tip atomic force microscopy with a delamination technique to remove the top electrode of Fe-doped SrTiO 3 metal-insulator-metal (MIM) structures to gain insight into the active switching interface with nanoscale lateral resolution. This enables us to prove the coexistence of a filamentary and area-dependent switching process with opposite switching polarities in the same sample. The spatially resolved analysis by transmission electron microscopy and photoelectron spectromicroscopy gives us some hints that the two switching types take place in device regions with different defect density and significant stoichiometry difference. © 1963-2012 IEEE.
    view abstractdoi: 10.1109/JPROC.2012.2188771
  • 2012 • 96 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 • 95 Solution behavior of double-hydrophilic block copolymers in dilute aqueous solution
    Casse, O. and Shkilnyy, A. and Linders, J. and Mayer, C. and Häussinger, D. and Völkel, A. and Thünemann, A.F. and Dimova, R. and Cölfen, H. and Meier, W. and Schlaad, H. and Taubert, A.
    Macromolecules 45 4772-4777 (2012)
    The self-assembly of double-hydrophilic poly(ethylene oxide)-poly(2-methyl- 2-oxazoline) diblock copolymers in water has been studied. Isothermal titration calorimetry, small-angle X-ray scattering, and analytical ultracentrifugation suggest that only single polymer chains are present in solution. In contrast, light scattering and transmission electron microscopy detect aggregates with radii of ca. 100 nm. Pulsed field gradient NMR spectroscopy confirms the presence of aggregates, although only 2% of the polymer chains undergo aggregation. Water uptake experiments indicate differences in the hydrophilicity of the two blocks, which is believed to be the origin of the unexpected aggregation behavior (in accordance with an earlier study by Ke et al. [Macromolecules2009, 42, 5339-5344]). The data therefore suggest that even in double-hydrophilic block copolymers, differences in hydrophilicity are sufficient to drive polymer aggregation, a phenomenon that has largely been overlooked or ignored so far. © 2012 American Chemical Society.
    view abstractdoi: 10.1021/ma300621g
  • 2012 • 94 Stability investigations of electrocatalysts on the nanoscale
    Meier, J.C. and Katsounaros, I. and Galeano, C. and Bongard, H.J. and Topalov, A.A. and Kostka, A. and Karschin, A. and Schüth, F. and Mayrhofer, K.J.J.
    Energy and Environmental Science 5 9319-9330 (2012)
    The search for more stable electrocatalyst materials for electrochemical energy conversion requires a fundamental understanding of the underlying degradation processes. Advanced characterization techniques like identical location transmission electron microscopy (IL-TEM) can provide invaluable insight into the stability of electrode materials on the nanoscale. In this review, the basic principles and the methodology of IL-TEM are described, and its capabilities are revealed by demonstrating the recent progress that has been achieved in research on the stability of fuel cell catalysts. Moreover, we provide future perspectives of the identical location approach towards implementing other electron microscopic and tomographic applications, which will help us to gain an even broader view of the degradation of electrocatalysts. © 2012 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c2ee22550f
  • 2012 • 93 Structural properties of amorphous metal carbides: Theory and experiment
    Kádas, K. and Andersson, M. and Holmström, E. and Wende, H. and Karis, O. and Urbonaite, S. and Butorin, S.M. and Nikitenko, S. and Kvashnina, K.O. and Jansson, U. and Eriksson, O.
    Acta Materialia 60 4720-4728 (2012)
    By means of theoretical modeling and experimental synthesis and characterization, we investigate the structural properties of amorphous Zr-Si-C. Two chemical compositions are selected: Zr0.31Si0.29 C0.40 and Zr0.60Si0.33C0.07. Amorphous structures are generated in the theoretical part of our work by the stochastic quenching (SQ) method, and detailed comparison is made regarding the structure and density of the experimentally synthesized films. These films are analyzed experimentally using X-ray absorption spectroscopy, transmission electron microscopy and X-ray diffraction. Our results demonstrate a remarkable agreement between theory and experiment concerning bond distances and atomic coordination of this complex amorphous metal carbide. The demonstrated power of the SQ method opens up avenues for theoretical predictions of amorphous materials in general. © 2012 Acta Materialia Inc. Published by Elsevier Ltd.
    view abstractdoi: 10.1016/j.actamat.2012.04.044
  • 2012 • 92 Surfactant-induced nonhydrolytic synthesis of phase-pure ZrO2 nanoparticles from metal-organic and oxocluster precursors
    Sliem, M.A. and Schmidt, D.A. and Bétard, A. and Kalidindi, S.B. and Gross, S. and Havenith, M. and Devi, A. and Fischer, R.A.
    Chemistry of Materials 24 4274-4282 (2012)
    Nonhydrolytic/non-sol-gel pyrolytic synthesis technique, as a convenient method, was applied to synthesize zirconium oxide nanoparticles (ZrO2 NPs). Pyrolysis of either the mononuclear keto ester/alkoxide complex zirconium bis(isopropoxide)bis(tert-butylacetoacetate) [Zr(OiPr) 2(tbaoac)2] (I) or the oligonuclear oxocluster compound [Zr6(OH)4O4(OMc)12] (II, Mc = methacrylate) generated ZrO2 NPs at moderate conditions of 300-400 °C. Trioctylamine, stearic acid, and/or oleic acid, which act as both solvents and stabilizing agents, were used. Under the adopted process conditions, the stabilizing agent oleic acid plays a vital role in determining the phase of as-synthesized colloidal ZrO2 nanoparticles, which yield the high-temperature tetragonal phase at moderate conditions of 335 °C. Those as-synthesized samples that contained both monoclinic and tetragonal ZrO2 phases (depending on the choice of the surfactant) were transformed into pure tetragonal phase at 1000 °C. An unambiguous phase determination of ZrO2 nanoparticles was carried out by the combination of powder X-ray diffraction (XRD) and Raman spectroscopy. Furthermore, the samples were analyzed by transmission electron microscopy (TEM), ultraviolet-visible (UV-vis) and photoluminescence (PL) spectroscopy, dynamic light scattering (DLS), and Fourier transform infrared (FT-IR) spectroscopy to elucidate the structure, chemical composition, and morphology of the obtained nanoparticles. Also, the phase transformations of the as-synthesized ZrO2 nanoparticles upon annealing were followed via Raman spectroscopy. © 2012 American Chemical Society.
    view abstractdoi: 10.1021/cm301128a
  • 2012 • 91 Synthesis of hybrid microgels by coupling of laser ablation and polymerization in aqueous medium
    Nachev, P. and Van 'T Zand, D.D. and Coger, V. and Wagener, P. and Reimers, K. and Vogt, P.M. and Barcikowski, S. and Pich, A.
    Journal of Laser Applications 24 (2012)
    Loading microgels with bioactive nanoparticles (NPs) often requires multiple synthesis and purification steps, and organic solvents or precursors that are difficult to remove from the gel. Hence, a fast and aqueous synthesis procedure would facilitate the synthesis of inorganic-organic hybrid microgels. Two microgel compounds were hybridized with laser-generated zinc oxide (ZnO) NPs prepared in a single-step procedure. ZnO NPs were formed by laser ablation in liquid, while the polymer microgels were synthesized in-situ inside the ablation chamber. Further, the authors report the preparation of two different microgel systems. The first one was produced without the use of chemical initiator forming hydrogels with ZnO NPs and diffuse morpholgy. Typical microgel colloids were also synthesized via a conventional chemical method in a preheated reaction chamber. The existence of microgel colloids partially loaded with ZnO NPs was confirmed in a transmission electron microscopy investigation. Fourier transform infrared spectroscopic measurements and dynamic light scattering verify the formation of polymer colloids. These initial results indicate the application potential of laser ablation in microgel precursor solution for the fabrication of polymeric carriers for inorganic nanoparticles. Preliminary biological tests using zinc chloride demonstrated negative dose effects on primary cell culture with zinc concentrations above 200 μM but no noticeable influence at 100 μM. © 2012 Laser Institute of America.
    view abstractdoi: 10.2351/1.4730803
  • 2012 • 90 Synthesis of titanium carbonitride coating layers with star-shaped crystallite morphology
    Garcia, J. and Pitonak, R. and Agudo, L. and Kostka, A.
    Materials Letters 68 71-74 (2012)
    Titanium carbonitride coating layers with star-shaped crystallite morphology were produced by chemical vapor deposition. Crystallites presenting a pentagonal symmetry nucleate at 880 °C and grow perpendicular to the surface by controlling the heating rate of the deposition process. Detailed transmission electron microscopy analyses of star-shaped crystallites along the [110] zone axis showed that each crystallite consists of five tetrahedra separated by (111) twins. A small-angle boundary consisting of edge dislocations forms as a result of elastic stress relaxation in the crystallites. The coatings presented a preferential texture in the direction (110) and an overall composition of Ti(C 0.15 N 0.85). © 2011 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.matlet.2011.10.008
  • 2012 • 89 Template-directed mild synthesis of anatase hybrid nanotubes within cylindrical core-shell-corona polymer brushes
    Müllner, M. and Lunkenbein, T. and Schieder, M. and Gröschel, A.H. and Miyajima, N. and Förtsch, M. and Breu, J. and Caruso, F. and Müller, A.H.E.
    Macromolecules 45 6981-6988 (2012)
    We demonstrate the synthesis of uniform one-dimensional (1D) titania hybrid nanotubes using core-shell-corona cylindrical polymer brushes (CPBs) as soft templates. The CPBs consist of a polymethacrylate backbone with densely grafted poly(ε-caprolactone) (PCL) as the core, poly(2-(dimethlamino)ethyl methacrylate) (PDMAEMA) as the cationic shell, and poly(oligo(ethylene glycol) methyl ether methacrylate) (POEGMA) as the corona. The weak polyelectrolyte shell complexed an oppositely charged titania precursor, namely titanium(IV) bis(ammonium lactate) dihydroxide (TALH), and then acted as a nanoreactor for the hydrolysis and condensation of TALH, resulting in crystalline TiO 2. The POEGMA shell provides solubility in aqueous and organic solvents. The hybrid titania nanotubes containing anatase nanoparticles were characterized by atomic force microscopy (AFM), transmission electron microscopy (TEM), and scanning electrion microscopy (SEM). The phase purity of the crystalline nanostructures was verified by powder X-ray diffractometry (PXRD). © 2012 American Chemical Society.
    view abstractdoi: 10.1021/ma301232m
  • 2012 • 88 The relation between ductility and stacking fault energies in Mg and Mg-Y alloys
    Sandlöbes, S. and Friák, M. and Zaefferer, S. and Dick, A. and Yi, S. and Letzig, D. and Pei, Z. and Zhu, L.-F. and Neugebauer, J. and Raabe, D.
    Acta Materialia 60 3011-3021 (2012)
    The underlying mechanisms that are responsible for the improved room-temperature ductility in Mg-Y alloys compared to pure Mg are investigated by transmission electron microscopy and density functional theory. Both methods show a significant decrease in the intrinsic stacking fault I 1 energy (I 1 SFE) with the addition of Y. The influence of the SFE on the relative activation of different competing deformation mechanisms (basal, prismatic, pyramidal slip) is discussed. From this analysis we suggest a key mechanism which explains the transition from primary basal slip in hexagonal close-packed Mg to basal plus pyramidal slip in solid solution Mg-Y alloys. This mechanism is characterized by enhanced nucleation of 〈c + a〉 dislocations where the intrinsic stacking fault I 1 (ISF 1) acts as heterogeneous source for 〈c + a〉 dislocations. Possible electronic and geometric reasons for the modification of the SFE by substitutional Y atoms are identified and discussed. © 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2012.02.006
  • 2012 • 87 The Role of Oxygen- and Nitrogen-containing Surface Groups on the Sintering of Iron Nanoparticles on Carbon Nanotubes in Different Atmospheres
    Sánchez, M.D. and Chen, P. and Reinecke, T. and Muhler, M. and Xia, W.
    ChemCatChem 4 1997-2004 (2012)
    The sintering of iron nanoparticles on carbon nanotubes (CNTs) under different atmospheres was investigated. CNTs were first treated with HNO3 vapor at 200°C to obtain O-functionalized CNTs (OCNTs). The OCNTs were treated in ammonia at 400°C to obtain N-doped CNTs (NCNTs). Highly dispersed FeOx nanoparticles were subsequently deposited by chemical vapor deposition from ferrocene under oxidizing conditions. The obtained FeOx/OCNT and FeOx/NCNT samples were allowed to sinter at 500°C under flowing helium, hydrogen, or ammonia. The samples were studied by X-ray diffraction, transmission electron microscopy, and X-ray photoelectron spectroscopy. A significant increase in particle size and a decrease in Fe surface atomic concentration were observed in all the sintered samples. The sintering on OCNTs was more severe than on NCNTs, which can be attributed to stronger metal-substrate interactions and a higher amount of surface defects on NCNTs. The applied gas atmosphere had a substantial influence on the sintering behavior of the nanoparticles: treatment in helium led to the growth of particles and a significant widening of particle size distributions, whereas treatment in hydrogen or ammonia resulted in the growth of particles, but not in the widening of particle size distributions. © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/cctc.201200286
  • 2012 • 86 Toward highly stable electrocatalysts via nanoparticle pore confinement
    Galeano, C. and Meier, J.C. and Peinecke, V. and Bongard, H. and Katsounaros, I. and Topalov, A.A. and Lu, A. and Mayrhofer, K.J.J. and Schüth, F.
    Journal of the American Chemical Society 134 20457-20465 (2012)
    The durability of electrode materials is a limiting parameter for many electrochemical energy conversion systems. In particular, electrocatalysts for the essential oxygen reduction reaction (ORR) present some of the most challenging instability issues shortening their practical lifetime. Here, we report a mesostructured graphitic carbon support, Hollow Graphitic Spheres (HGS) with a specific surface area exceeding 1000 m2 g-1 and precisely controlled pore structure, that was specifically developed to overcome the long-term catalyst degradation, while still sustaining high activity. The synthetic pathway leads to platinum nanoparticles of approximately 3 to 4 nm size encapsulated in the HGS pore structure that are stable at 850 C and, more importantly, during simulated accelerated electrochemical aging. Moreover, the high stability of the cathode electrocatalyst is also retained in a fully assembled polymer electrolyte membrane fuel cell (PEMFC). Identical location scanning and scanning transmission electron microscopy (IL-SEM and IL-STEM) conclusively proved that during electrochemical cycling the encapsulation significantly suppresses detachment and agglomeration of Pt nanoparticles, two of the major degradation mechanisms in fuel cell catalysts of this particle size. Thus, beyond providing an improved electrocatalyst, this study describes the blueprint for targeted improvement of fuel cell catalysts by design of the carbon support. © 2012 American Chemical Society.
    view abstractdoi: 10.1021/ja308570c
  • 2012 • 85 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
  • 2011 • 84 3D assembly of semiconductor and metal nanocrystals: Hybrid CdTe/Au structures with controlled content
    Lesnyak, V. and Wolf, A. and Dubavik, A. and Borchardt, L. and Voitekhovich, S.V. and Gaponik, N. and Kaskel, S. and Eychmüller, A.
    Journal of the American Chemical Society 133 13413-13420 (2011)
    A 3D metal ion assisted assembly of nanoparticles has been developed. The approach relies on the efficient complexation of cadmium ions and 5-mercaptomethyltetrazole employed as the stabilizer of both colloidal CdTe and Au nanoparticles. It enables in a facile way the formation of hybrid metal-semiconductor 3D structures with controllable and tunable composition in aqueous media. By means of critical point drying, these assemblies form highly porous aerogels. The hybrid architectures obtained are characterized by electron microscopy, nitrogen adsorption, and optical spectroscopy methods. © 2011 American Chemical Society.
    view abstractdoi: 10.1021/ja202068s
  • 2011 • 83 A (S)TEM and atom probe tomography study of InGaN
    Mehrtens, T. and Bley, S. and Schowalter, M. and Sebald, K. and Seyfried, M. and Gutowski, J. and Gerstl, S.S.A. and Choi, P.-P. and Raabe, D. and Rosenauer, A.
    Journal of Physics: Conference Series 326 (2011)
    In this work we show how the indium concentration in high indium content InxGa1-xN quantum wells, as they are commonly used in blue and green light emitting diodes, can be deduced from high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images. This method bases on introducing normalized intensities which can be compared with multislice simulations to determine the specimen thickness or the indium concentration. The evaluated concentrations are compared with atom probe tomography measurements. It is also demonstrated how the quality of focused ion beam prepared TEM-lamellas can be improved by an additional etching with low energy ions.
    view abstractdoi: 10.1088/1742-6596/326/1/012029
  • 2011 • 82 A facile synthesis of shape- and size-controlled α-Fe 2O3 nanoparticles through hydrothermal method
    Wang, G.-H. and Li, W.-C. and Jia, K.-M. and Lu, A.-H. and Feyen, M. and Spliethoff, B. and SchÜth, F.
    Nano 6 469-479 (2011)
    α-Fe2O3 nanoparticles have wide-ranging applications such as in catalysis, sensoring, painting, etc. This is the reason to study their controlled synthesis. Here we have investigated the synthesis of uniform α-Fe2O3 nanoparticles using amino acids as morphology control agents. The products were characterized by transmission electron microscopy (TEM), X-ray diffraction (XRD), thermogravimetry (TG) and differential thermal analysis (DTA). It was found that the type and the amount of amino acids as well as the reaction temperatures have significant influence on the shape and size of the obtained α-Fe2O3 nanoparticles. The use of acidic amino acids (always contain C=O in the side chain) typically leads to the formation of α-Fe2O3 nanoparticles with spindle shape. However, rhombohedrally shaped α-Fe 2O3 nanoparticles were formed in presence of basic amino acids (always contain -NH2 in the side chain). Increasing the amount of amino acid generally results in α-Fe2O3 nanoparticles with decreasing particle sizes. © 2011 World Scientific Publishing Company.
    view abstractdoi: 10.1142/S1793292011002846
  • 2011 • 81 A journey through 12 years of interacting molecules: From artificial amino acid receptors to the recognition of biomolecules and switchable nanomaterials
    Schmuck, C.
    Synlett 1798-1815 (2011)
    Research in supramolecular chemistry has been carried out in my laboratory for the past 12 years. Intrigued by the fascinating power of supramolecular chemistry, as seen in biomolecular recognition events in nature, we started out by trying to mimic the basic principles of such recognition events in small artificial model systems. Our first targets were amino acids and oligopeptides. We then moved on to proteins and nucleic acids, and we started to develop supramolecular systems that, besides simple binding, also featured functions, e.g. shutting down enzymes or allowing gene delivery into cells. In recent years and in a completely different yet related field, we have begun to develop self-assembling nanomaterials. The basic idea for this derived from an accidental discovery of the interesting self-assembling properties of a simple zwitterion, which was a synthetic intermediate on the route to our amino acid receptors. This personal account summarizes this journey and is intended not only to present the most-important findings from our laboratory so far, but also to shed light on how all these projects developed over the years and how our journey took us to where we are now. 1 Introduction 2 How It All Began 3 The Beginning: The Design of a New Oxoanion Binding Site 4 Tailor-Made Receptors for Small Peptides 5 Combinatorial Development of Peptide Receptors 6 From Small Peptides to Proteins 7 Nucleic Acids as Targets 8 Self-Assembling Zwitterions 9 pH-Switchable Nanostructures 10 Conclusion. © Georg Thieme Verlag Stuttgart . New York.
    view abstractdoi: 10.1055/s-0030-1260946
  • 2011 • 80 Amphiphilic gold nanoparticles: Synthesis, characterization and adsorption to PEGylated polymer surfaces
    Tarnawski, R. and Ulbricht, M.
    Colloids and Surfaces A: Physicochemical and Engineering Aspects 374 13-21 (2011)
    The direct synthesis of water-soluble gold nanoparticles with a mixed shell of two different thiols, 1-mercaptoundec-11-yl-hexa(ethylene glycol) (EG6) and dodecanethiol (C12), and their characterization are reported. Data from IR spectroscopy and contact angle (CA) measurements as well as the solubility of the nanoparticles in water support that the composition of the shell is in the range of the thiol ratio used for synthesis (EG6:C12 = 72:28). Results of transmission electron microscopy and atomic force microscopy (AFM) for deposited particles as well as the UV-vis spectrum in solution are in line with a size of ≤10. nm. Self-assembled monolayers (SAMs) as model surfaces were prepared from mixtures of EG6 and C12 on planar gold films. Polystyrene (PSt) spin-coated films on silicon wafers and on gold-coated surface plasmon resonance (SPR) sensor disks were used as substrates for surface functionalization via adsorption/self-assembly of a polystyrene poly(ethylene glycol) diblock copolymer (PSt- b-PEG) from aqueous solutions. CA and AFM results revealed pronounced differences of the hydrophilicity/hydrophobicity and topography of the surface as a function of PSt- b-PEG concentration used for the modification. The adsorption of myoglobin and the novel gold nanoparticles to the PSt- b-PEGylated surfaces was analyzed by SPR. A control of adsorbed amounts by the degree of surface PEGylation, i.e. a reduction by up to 55% for the highest degree of modification, could be confirmed for both kinds of colloids. Adsorption of the novel gold nanoparticles to the mixed SAM surfaces as analyzed by SPR showed an even stronger dependency of surface composition. All experiments demonstrate that amphiphilic, water-soluble gold-based nanoparticles can be used as model colloids for the investigation of interactions with polymer surfaces of varied structure and architecture, and that they could be further developed for analytical or biological applications. © 2010 Elsevier B.V.
    view abstractdoi: 10.1016/j.colsurfa.2010.10.027
  • 2011 • 79 Atomic-scale distribution of impurities in cuinse2-based thin-film solar cells
    Cojocaru-Miredin, O. and Choi, P. and Wuerz, R. and Raabe, D.
    Ultramicroscopy 111 552-556 (2011)
    Atom Probe Tomography was employed to investigate the distribution of impurities, in particular sodium and oxygen, in a cuinse2-based thin-film solar cell. It could be shown that sodium, oxygen, and silicon diffuse from the soda lime glass substrate into the cuinse2 film and accumulate at the grain boundaries. Highly dilute concentrations of sodium and oxygen were measured in the bulk. Selenium was found to be depleted at the grain boundaries. These observations could be confirmed by complementary energy dispersive X-ray spectroscopy studies. Our results support the model proposed by Kronik et al. (1998) [1], which explains the enhanced photovoltaic efficiency of sodium containing cuinse2 solar cells by the passivation of selenium vacancies at grain boundaries. © 2011 Elsevier B.V.
    view abstractdoi: 10.1016/j.ultramic.2010.12.034
  • 2011 • 78 Atomic-scale mechanisms of deformation-induced cementite decomposition in pearlite
    Li, Y.J. and Choi, P. and Borchers, C. and Westerkamp, S. and Goto, S. and Raabe, D. and Kirchheim, R.
    Acta Materialia 59 3965-3977 (2011)
    Pearlitic steel can exhibit tensile strengths higher than 5 GPa after severe plastic deformation, where the deformation promotes a refinement of the lamellar structure and cementite decomposition. However, a convincing correlation between deformation and cementite decomposition in pearlite is still absent. In the present work, a local electrode atom probe was used to characterize the microstructural evolution of pearlitic steel, cold-drawn with progressive strains up to 5.4. Transmission electron microscopy was also employed to perform complementary analyses of the microstructure. Both methods yielded consistent results. The overall carbon content in the detected volumes as well as the carbon concentrations in ferrite and cementite were measured by atom probe. In addition, the thickness of the cementite filaments was determined. In ferrite, we found a correlation of carbon concentration with the strain, and in cementite, we found a correlation of carbon concentration with the lamella thickness. Direct evidence for the formation of cell/subgrain boundaries in ferrite and segregation of carbon atoms at these defects was found. Based on these findings, the mechanisms of cementite decomposition are discussed in terms of carbon-dislocation interaction. © 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2011.03.022
  • 2011 • 77 Characterization of grain boundaries in Cu(In,Ga)Se 2 films using atom-probe tomography
    Cojocaru-Mirédin, O. and Choi, P.-P. and Abou-Ras, D. and Schmidt, S.S. and Caballero, R. and Raabe, D.
    IEEE Journal of Photovoltaics 1 207-212 (2011)
    This paper discusses the advantages of pulsed laser atom-probe tomography (APT) to analyze Cu(In,Ga)Se 2-based solar cells. Electron backscatter diffraction (EBSD) was exploited for site-specific preparation of APT samples at selected Cu(In,Ga)Se 2 grain boundaries. This approach is very helpful not only to determine the location of grain boundaries but also to classify them as well. We demonstrate that correlative transmission electron microscopy (TEM) analyses on atom-probe specimens enable the atom-probe datasets to be reconstructed with high accuracy. Moreover, EBSD and TEM can be very useful to obtain complementary information about the crystal structure in addition to the compositional analyses. The local chemical compositions at grain boundaries of a solar grade Cu(In,Ga)Se 2 film are presented here. Na, K, and O impurities are found to be segregated at grain boundaries. These impurities most likely diffuse from the soda lime glass substrate into the absorber layer during cell fabrication and processing. Based on the experimental results, we propose that Na, K, and O play an important role in the electrical properties of grain boundaries in Cu(In,Ga)Se 2 thin films for solar cells. © 2011 IEEE.
    view abstractdoi: 10.1109/JPHOTOV.2011.2170447
  • 2011 • 76 Characterization of wear particles generated from CoCrMo alloy under sliding wear conditions
    Pourzal, R. and Catelas, I. and Theissmann, R. and Kaddick, C. and Fischer, A.
    Wear 271 1658-1666 (2011)
    Biological effects of wear products (particles and metal ions) generated by metal-on-metal (MoM) hip replacements made of CoCrMo alloy remain a major cause of concern. Periprosthetic osteolysis, potential hypersensitivity response and pseudotumour formation are possible reactions that can lead to early revisions. To accurately analyse the biological response to wear particles from MoM implants, the exact nature of these particles needs to be characterized. Most previous studies used energy-dispersive X-ray spectroscopy (EDS) analysis for characterization. The present study used energy filtered transmission electron microscopy (TEM) and electron diffraction pattern analysis to allow for a more precise determination of the chemical composition and to gain knowledge of the crystalline structure of the wear particles.Particles were retrieved from two different test rigs: a reciprocating sliding wear tribometer (CoCrMo cylinder vs. bar) and a hip simulator according to ISO 14242-1 (CoCrMo head vs. CoCrMo cup). All tests were conducted in new born calf serum (30 g/l protein content). Particles were retrieved from the test medium using a previously published enzymatic digestion protocol.Particles isolated from tribometer samples had a size of 100-500nm. Diffraction pattern analysis clearly revealed the lattice structure of strain induced hcp e{open}-martensite. Hip simulator samples revealed numerous particles of 15-30nm and 30-80nm size. Most of the larger particles appeared to be only partially oxidized and exhibited cobalt locally. The smallest particles were Cr2O3 with no trace of cobalt. It optically appeared that these Cr2O3 particles were flaking off the surface of larger particles that depicted a very high intensity of oxygen, as well as chromium, and only background noise of cobalt. The particle size difference between the two test rigs is likely related to the conditions of the two tribosystems, in particular the difference in the sample geometry and in the type of sliding (reciprocating vs. multidirectional).Results suggest that there may be a critical particle size at which chromium oxidation and cobalt ionization are accelerated. Since earlier studies have shown that wear particles are covered by organic residue which may act as a passive layer inhibiting further oxidation, it would suggest that this organic layer may be removed during the particle isolation process, resulting in a change of the particle chemical composition due to their pyrophoric properties. However, prior to being isolated from the serum lubricant, particles remain within the contact area of head and cup as a third-body. It is therefore possible that during that time, particles may undergo significant transformation and changes in chemical composition in the contact area of the head and cup within the tribological interface due to mechanical interaction with surface asperities. © 2011 Elsevier B.V.
    view abstractdoi: 10.1016/j.wear.2010.12.045
  • 2011 • 75 Creep in directionally solidified NiAl-Mo eutectics
    Dudová, M. and Kuchařová, K. and Barták, T. and Bei, H. and George, E.P. and Somsen, C. and Dlouhý, A.
    Scripta Materialia 65 699-702 (2011)
    A directionally solidified NiAl-Mo eutectic and an NiAl intermetallic, having respective nominal compositions Ni-45.5Al-9Mo and Ni-45.2Al (at.%), were loaded in compression at 1073 and 1173 K. Formidable strengthening by regularly distributed Mo fibres (average diameter 600 nm, volume fraction 14%) was observed. The fibres can support compression stresses transferred from the plastically deforming matrix up to a critical stress of the order of 2.5 GPa, at which point they yield. Microstructural evidence is provided for the dislocation-mediated stress transfer from the NiAl to the Mo phase. © 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.scriptamat.2011.07.019
  • 2011 • 74 Deformation mechanisms in micron-sized PST TiAl compression samples: Experiment and model
    Rester, M. and Fischer, F.D. and Kirchlechner, C. and Schmoelzer, T. and Clemens, H. and Dehm, G.
    Acta Materialia 59 3410-3421 (2011)
    Titanium aluminides are the most promising intermetallics for use in aerospace and automotive applications. Consequently, it is of fundamental interest to explore the deformation mechanisms occurring in this class of materials. One model material which is extensively used for such studies are polysynthetically twinned (PST) TiAl crystals, which consist predominantly of parallel γ-TiAl and, fewer, α2-Ti3Al lamellae. In the present study, PST TiAl crystals with a nominal composition of Ti-50 at.% Al were machined by means of the focused ion beam (FIB) technique into miniaturized compression samples with a square cross-section of approximately 9 μm × 9 μm. Compression tests on the miniaturized samples were performed in situ inside a scanning electron microscope using a microindenter equipped with a diamond flat punch. After deformation, thin foils were cut from the micro-compression samples and thinned to electron transparency using a FIB machine in order to study the deformation structure by transmission electron microscopy (TEM). The TEM studies reveal mechanical twinning as the main deformation mechanism at strains of 5.4%, while at strains of 8.3% dislocation glide becomes increasingly important. The experimentally observed twins scale in size with the width of the γ-TiAl lamella. A kinematic and thermodynamic model is developed to describe the twin-related length change of the micro-compression sample at small strains as well as the relationship of an increase of twin width with increasing γ-TiAl lamella thickness. The developed twin model predicts a width of the twins in the range of a few nanometers, which is in agreement with experimental findings. © 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2011.02.016
  • 2011 • 73 Dislocation plasticity of Al film on polyimide investigated by cross-sectional in situ transmission electron microscopy straining
    Oh, S.H. and Rentenberger, C. and Im, J. and Motz, C. and Kiener, D. and Karnthaler, H.-P. and Dehm, G.
    Scripta Materialia 65 456-459 (2011)
    Tensile straining of a cross-sectional Al/polyimide was performed in a transmission electron microscope. The tensile deformation of Al was accomplished mainly by dislocations emitted from the film surface, which glide and impinge on the Al/polyimide interface. During further straining the interfacial dislocations disappeared, indicating dislocation core spreading, whereas threading dislocations moved towards the film surface. While the Al/polyimide interface remains flat and becomes depleted of dislocations, the Al surface becomes increasingly rough accompanied by a noticeable increase in dislocation density. © 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.scriptamat.2011.06.001
  • 2011 • 72 Effect of heat-treatment on grain growth of nanocrystalline tricalcium phosphate powder synthesized via the precipitation method
    Mobasherpour, I. and Salahi, E. and Manafi, S.A. and Kamachali, R.D.
    Materials Science- Poland 29 203-208 (2011)
    Nanocrystalline tricalcium phosphate powder was synthesized via the solution- precipitation method followed by heat treatment in order to achieve phase evolution, which was then studied by XRD and TEM techniques. The crystallites sizes were estimated by the Scherrer method and results were confirmed by TEM micrographs. The experimental observations showed that nanocrystalline tricalcium phosphate can be successfully prepared from raw materials by the precipitation technique. This technique is a competitive method for nanocrystalline tricalcium phosphate synthesis compared to other techniques. Moreover, a simple kinetic growth investigation was performed on the nanocrystalline growth process during heat treatment. Results have shown growth rate to increase exponentially with temperature and the growth rate constants to increase with time. The average activation energies of tricalcium phosphate grain growth obtained by this method were 84.78 and 134.38 KJ/mol. © Wroclaw University of Technology.
    view abstractdoi: 10.2478/s13536-011-0032-6
  • 2011 • 71 Elementary deformation and damage mechanisms during fatigue of pseudoelastic NiTi microstents
    Frotscher, M. and Wu, S. and Simon, T. and Somsen, C. and Dlouhy, A. and Eggeler, G.
    Advanced Engineering Materials 13 B181-B186 (2011)
    In the present study, we investigate the fatigue behavior of Nickel Titanium (NiTi) microstents at 22°C (room temperature) and 37°C up to 30×10 6 load cycles. We briefly describe our test procedure, which applies displacement-controlled pull-pull fatigue cycling between displacements corresponding to apparent strains of 5 and 7.5%. The response of the microstents to mechanical loading indicates cyclic softening during 30×10 4 cycles. Subsequently, the maximum load remains constant throughout the remainder of the test. We use transmission electron microscopy (TEM) to clarify the microstructural reasons for cyclic softening. A focused ion beam (FIB) technique is used to take out thin foil specimens from critical microstent locations. Our TEM results show that the dislocation density increases during cycling. We also find that microstructural regions with stabilized stress-induced B19 martensite can be detected. © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/adem.201180001
  • 2011 • 70 Evolution of the structure and chemical state of Pd nanoparticles during the in situ catalytic reduction of NO with H2
    Paredis, K. and Ono, L.K. and Behafarid, F. and Zhang, Z. and Yang, J.C. and Frenkel, A.I. and Cuenya, B.R.
    Journal of the American Chemical Society 133 13455-13464 (2011)
    An in-depth understanding of the fundamental structure of catalysts during operation is indispensable for tailoring future efficient and selective catalysts. We report the evolution of the structure and oxidation state of ZrO2-supported Pd nanocatalysts (∼5 nm) during the in situ reduction of NO with H2 using X-ray absorption fine-structure spectroscopy and X-ray photoelectron spectroscopy. Prior to the onset of the reaction (≤120 °C), a NO-induced redispersion of our initial metallic Pd nanoparticles over the ZrO2 support was observed, and Pd δ+ species were detected. This process parallels the high production of N2O observed at the onset of the reaction (&gt;120 °C), while at higher temperatures (≥150 °C) the selectivity shifts mainly toward N2 (∼80%). Concomitant with the onset of N 2 production, the Pd atoms aggregate again into large (6.5 nm) metallic Pd nanoparticles, which were found to constitute the active phase for the H2-reduction of NO. Throughout the entire reaction cycle, the formation and stabilization of PdOx was not detected. Our results highlight the importance of in situ reactivity studies to unravel the microscopic processes governing catalytic reactivity. © 2011 American Chemical Society.
    view abstractdoi: 10.1021/ja203709t
  • 2011 • 69 F-doped Co3O4 photocatalysts for sustainable H 2 generation from water/ethanol
    Gasparotto, A. and Barreca, D. and Bekermann, D. and Devi, A. and Fischer, R.A. and Fornasiero, P. and Gombac, V. and Lebedev, O.I. and MacCato, C. and Montini, T. and Van Tendeloo, G. and Tondello, E.
    Journal of the American Chemical Society 133 19362-19365 (2011)
    p-Type Co3O4 nanostructured films are synthesized by a plasma-assisted process and tested in the photocatalytic production of H 2 from water/ethanol solutions under both near-UV and solar irradiation. It is demonstrated that the introduction of fluorine into p-type Co3O4 results in a remarkable performance improvement with respect to the corresponding undoped oxide, highlighting F-doped Co 3O4 films as highly promising systems for hydrogen generation. Notably, the obtained yields were among the best ever reported for similar semiconductor-based photocatalytic processes. © 2011 American Chemical Society.
    view abstractdoi: 10.1021/ja210078d
  • 2011 • 68 Fast and cost-effective purification of gold nanoparticles in the 20-250 nm size range by continuous density gradient centrifugation
    Steinigeweg, D. and Schütz, M. and Salehi, M. and Schlücker, S.
    Small 7 2443-2448 (2011)
    A multilayer quasi-continuous density gradient centrifugation method for separating 20-250 nm metal colloids with high size resolution while maintaining particle stability is presented. Colloidal mixtures containing monodisperse gold nanospheres and clusters thereof, in particular, gold dimers, are purified with yields up to 94%. The rapid method uses standard laboratory equipment. Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/smll.201100663
  • 2011 • 67 Formation of interpenetrating hierarchical titania structures by confined synthesis in inverse opal
    Mandlmeier, B. and Szeifert, J.M. and Fattakhova-Rohlfing, D. and Amenitsch, H. and Bein, T.
    Journal of the American Chemical Society 133 17274-17282 (2011)
    Hierarchical periodic titania nanostructures composed of a macroporous crystalline scaffold and mesoporous titania were prepared by confined synthesis. The strategy for the generation of these hierarchical structures involves preparation of inverse opal titania layers and subsequent filling of the interstitial macroporous voids with surfactant-containing titania precursors to obtain a mesostructured titania phase using the surfactant Pluronic P123. The formation of mesostructure in the confined space of the macroporous scaffold upon thermal treatment was investigated with in situ grazing incidence small-angle X-ray scattering (GISAXS). The macroporous scaffold strongly influences the mesostructure assembly and leads to much larger structural parameters of the formed mesostructure, this effect becoming more pronounced with decreasing pore size of the macroporous host. Furthermore, the inverse opal scaffold acts as a stabilizing matrix, limiting the shrinkage of the mesopores upon heating. This effect is coupled with an enhanced crystallization of the mesophase, which is attributed to the crystalline walls of the macroporous host. Sorption measurements of the final hierarchical titania structure of 5 μm thickness show that the porous system is fully accessible, has a high total surface area of 154 m2/g, and has an average mesopore size of 6.1 nm, which is about 20% larger than the pore size of 5.1 nm for the reference mesoporous film obtained on a flat substrate. These hierarchical structures were implemented as anodes in dye-sensitized solar cells (DSCs), showing a conversion efficiency of 4% under one sun illumination, whereas the calcined macroporous scaffold alone shows an efficiency of only 0.4%. © 2011 American Chemical Society.
    view abstractdoi: 10.1021/ja204667e
  • 2011 • 66 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 • 65 Highly dispersed MoO 3/Al 2O 3 shell-core composites synthesized by CVD of Mo(CO) 6 under atmospheric pressure
    Shi, G. and Franzke, T. and Xia, W. and Sanchez, M.D. and Muhler, M.
    Chemical Vapor Deposition 17 162-169 (2011)
    MoO 3/γ-Al 2O 3 composites are synthesized by CVD under atmospheric pressure using Mo(CO) 6 as the precursor and porous γ-Al 2O 3 particles in a horizontal, rotating, hot-wall reactor, which is also used for calcination in air. The composites are characterized by N 2 physisorption, atomic absorption spectroscopy, X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), transmission electron microscopy (TEM), and laser Raman spectroscopy (LRS). The synthesized samples exhibit excellent porosity, even at high Mo loadings. A much higher Mo yield is achieved when applying sublimation-adsorption in static air instead of using flowing N 2. A high degree of Mo dispersion on alumina is confirmed by XRD, LRS, and TEM; with a Mo surface density as high as 5.2 atoms nm -2, the sample is X-ray amorphous, there are no polymeric molybdate species detectable by LRS, and the island size of the molybdate species is about 1 nm according to TEM. The XPS analysis shows that exclusively Mo VI species are present on all synthesized samples. Thus, the applied rotating, hot-wall reactor achieves efficient mixing and homogeneous deposition. Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/cvde.201106909
  • 2011 • 64 Hydrothermally highly stable acidic mesoporous aluminosilicate spheres with radial channels
    Gu, X. and Jiang, T. and Tao, H. and Zhou, S. and Liu, X. and Ren, J. and Wang, Y. and Lu, G. and Schmidt, W.
    Journal of Materials Chemistry 21 880-886 (2011)
    Hydrothermally highly stable mesoporous aluminosilicate spheres with radial channels were synthesized in the CTAB-NaF-TPAOH system through a one-step procedure at high aging temperature. The characterization by transmission electron microscopy (TEM), X-ray diffraction (XRD), nitrogen adsorption/desorption analysis, 27Al MAS solid state NMR spectroscopy, pyridine adsorption FT-IR combined with the typical hydrothermal treatments showed that this kind of material exhibited large surface area, specific pore arrangement, strong acidity and high hydrothermal stability. Detailed studies suggest that F- ions direct the perpendicular arrangement of aluminosilicate clusters during the hydrothermal treatment at 160°C, while TPA+ stabilized the structure. Both F- and TPA+ ions are considered to improve the acidity and hydrothermal stability of this material through coordination of framework atoms, thus, enhancing the condensation of Si-O-Si bonds in the amorphous pore walls. Due to the accessible radial pore arrangement and high acidity, the catalytic activity for Friedel-Crafts alkylation of toluene with benzyl alcohol was excellent with 100% conversion of benzyl alcohol.
    view abstractdoi: 10.1039/c0jm01973a
  • 2011 • 63 In-situ TEM heating study of the γ lamellae formation inside the α2 matrix of a Ti-45Al-7.5Nb alloy
    Cha, L. and Clemens, H. and Dehm, G. and Zhang, Z.
    Advanced Materials Research 146-147 1365-1368 (2011)
    In-situ heating transmission electron microscopy (TEM) was employed to investigate the initial stage of lamellae formation in a high Nb containing γ-TiAl based alloy. A Ti-45Al-7.5Nb alloy (at %), which was heat treated and quenched in a non-equilibrium state such that the matrix consists of ordered α2 grains, was annealed inside a TEM up to 750 °C. The in-situ TEM study reveals that γ laths precipitate in the α2 matrix at ∼750 °C possessing the classical Blackburn orientation relationship, i.e. (0001)α2 // (111)γ and [11-20]α2 // < 110]γ. The microstructure of the in-situ TEM experiment is compared to results from ex-situ heating and subsequent TEM studies.
    view abstractdoi: 10.4028/
  • 2011 • 62 Metal ion release kinetics from nanoparticle silicone composites
    Hahn, A. and Brandes, G. and Wagener, P. and Barcikowski, S.
    Journal of Controlled Release 154 164-170 (2011)
    Metal ion release kinetics from silver and copper nanoparticle silicone composites generated by laser ablation in liquids are investigated. The metal ion transport mechanism is studied by using different model equations and their fit to experimental data. Results indicate that during the first 30 days of immersion, Fickian diffusion is the dominant transport mechanism. After this time period, the oxidation and dissolution of nanoparticles from the bulk determine the ion release. This second mechanism is very slow since the dissolution of the nanoparticle is found to be anisotropic. Silver ion release profile is best described by pseudo-first order exponential equation. Copper ion release profile is best described by a second order exponential equation. For practical purposes, the in vitro release characteristics of the bioactive metal ions are evaluated as a function of nanoparticle loading density, the chemistry and the texture of the silicone. Based on the proposed two-step release model, a prediction of the release characteristics over a time course of 84 days is possible and a long-term ion release could be demonstrated. © 2011 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.jconrel.2011.05.023
  • 2011 • 61 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 • 60 Nanostructured Ti-catalyzed MgH2 for hydrogen storage
    Shao, H. and Felderhoff, M. and Schüth, F. and Weidenthaler, C.
    Nanotechnology 22 (2011)
    Nanocrystalline Ti-catalyzed MgH2 can be prepared by a homogeneously catalyzed synthesis method. Comprehensive characterization of this sample and measurements of hydrogen storage properties are discussed and compared to a commercial MgH2 sample. The catalyzed MgH2 nanocrystalline sample consists of two MgH2 phases-a tetrahedral β-MgH2 phase and an orthorhombic high-pressure modification γ-MgH2. Transmission electron microscopy was used for the observation of the morphology of the samples and to confirm the nanostructure. N2 adsorption measurement shows a BET surface area of 108m 2g-1 of the nanostructured material. This sample exhibits a hydrogen desorption temperature more than 130 °C lower compared to commercial MgH2. After desorption, the catalyzed nanocrystalline sample absorbs hydrogen 40 times faster than commercial MgH2 at 300 °C. Both the Ti catalyst and the nanocrystalline structure with correspondingly high surface area are thought to play important roles in the improvement of hydrogen storage properties. The desorption enthalpy and entropy values of the catalyzed MgH2 nanocrystalline sample are 77.7kJmol-1H2 and 138.3JK-1mol -1H2, respectively. Thermodynamic properties do not change with the nanostructure. © 2011 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0957-4484/22/23/235401
  • 2011 • 59 On the stress-induced formation of R-phase in ultra-fine-grained Ni-rich NiTi shape memory alloys
    Olbricht, J. and Yawny, A. and Pelegrina, J.L. and Dlouhy, A. and Eggeler, G.
    Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science 42 2556-2574 (2011)
    Phase transformations in binary ultra-fine-grained (UFG) pseudoelastic NiTi wires were studied in a wide temperature range using mechanical loading/unloading experiments, resistance measurements, differential scanning calorimetry (DSC), thermal infrared imaging, and transmission electron microscopy (TEM). The formation of R-phase can be detected in the mechanical experiments. It is shown that the stress-strain response of the R-phase can be isolated from the overall stress-strain data. The R-phase always forms prior to B19' when good pseudoelastic properties are observed. The stress-induced B2 to R-phase transition occurs in a homogeneous manner, contrary to the localized character of the B2/R to B19' transformations. The temperature dependence of the critical stress values for the formation of the martensitic phases shows a Clausius Clapeyron type of behavior with constants close to 6 MPa/K (B19') and 18 MPa/K (R-phase). A stress-temperature map is suggested that summarizes the experimentally observed sequences of elementary transformation/deformation processes. © The Minerals, Metals & Materials Society and ASM International 2011.
    view abstractdoi: 10.1007/s11661-011-0679-y
  • 2011 • 58 Oxygen chemisorption, formation, and thermal stability of Pt oxides on Pt nanoparticles supported on SiO2/Si(001): Size effects
    Ono, L.K. and Croy, J.R. and Heinrich, H. and Roldan Cuenya, B.
    Journal of Physical Chemistry C 115 16856-16866 (2011)
    The changes induced in the structure and chemical state of size-selected Pt nanoparticles (NPs) supported on ultrathin SiO2 films upon exposure to oxygen have been investigated by atomic force microscopy (AFM), transmission electron microscopy (TEM), in situ X-ray photoelectron spectroscopy (XPS), and temperature-programmed desorption (TPD). For low atomic oxygen exposures, chemisorbed oxygen species were detected on all samples. Exposure to higher atomic oxygen coverages at room temperature leads to the formation and stabilization of PtOx species (PtO2 and PtO). On all samples, a two-step thermal decomposition process was observed upon annealing in ultrahigh vacuum: PtO2 → PtO → Pt. For NPs in the 2-6 nm range, the NP size was found to affect the strength of the O binding. Contrary to the case of Pt(111), where no oxides were detected above 700 K, 10-20% PtO was detected on the NP samples via XPS at the same temperature, suggesting the presence of strongly bound oxygen species. In addition, for identical atomic oxygen exposures, decreasing the NP size was found to favor their ability to form oxides. Interestingly, regardless of whether the desorption of chemisorbed oxygen species or that of oxygen in PtOx species was considered, our TPD data revealed higher O2-desorption temperatures for the Pt NPs as compared with the Pt(111) surface. Furthermore, a clear size-dependent trend was observed, with an increase in the strength of the oxygen bonding with decreasing NP size. © 2011 American Chemical Society.
    view abstractdoi: 10.1021/jp204743q
  • 2011 • 57 Penetratin-conjugated gold nanoparticles - Design of cell-penetrating nanomarkers by femtosecond laser ablation
    Petersen, S. and Barchanski, A. and Taylor, U. and Klein, S. and Rath, D. and Barcikowski, S.
    Journal of Physical Chemistry C 115 5152-5159 (2011)
    Gold nanoparticles (AuNPs) are promising imaging agents for the long-term visualization and tracing of intracellular functions because they bear outstanding optical properties and are fairly easily bioconjugated. However, the design of such multifunctional nanosystems might be limited by their bioavailability. Cell-penetrating peptides (CPPs) have been shown to be efficient molecular transporters with very few indices of cytotoxicity also in conjunction with nanoparticles. In this context, the current work aims to explore the approach of in situ conjugation during laser ablation in liquids for the design of CPP-NP conjugates at the example of penetratin-conjugated AuNPs. Because penetratin is positively charged at neutral pH, the conjugation process most likely differs from the previously reported coupling of oligonucleotides with their negatively charged phosphate backbone. Results reveal that penetratin is more efficiently bound to AuNPs, increasing the pH value of the ablation media, whereas the size and morphology of the bioconjugates function in terms of the penetratin concentration during the laser process. Probable underlying processes such as size quenching, aggregation, and laser-induced partial melting are assessed by the means of transmission electron microscopy and UV-vis spectroscopy. In a preliminary biological study, laser scanning confocal and transmission electron microscopy revealed a successful uptake of penetratin-conjugated AuNPs for the first time in up to 100% of coincubated cells within 2 h. © 2010 American Chemical Society.
    view abstractdoi: 10.1021/jp1093614
  • 2011 • 56 Phase transformations and functional properties of NiTi alloy with ultrafine-grained structure
    Prokofiev, E. and Burow, J. and Frenzel, J. and Gunderov, D. and Eggeler, G. and Valiev, R.
    Materials Science Forum 667-669 1059-1064 (2011)
    Severe plastic deformation (SPD) processes, are successfully employed to produce ultra fine grain (UFG) and nanocrystalline (NC) microstructures in Ni50.7Ti49.3 shape memory alloy. The effect of grain size on phase transformations during annealing is investigated by differential scanning calorimetry (DSC) and transmission electron microscopy (TEM). The results of comparative studies of phase transformations in coarse-grained, UFG and NC alloys after SPD and subsequent long-term (up to 100 hours) annealing at 400°C is presented. The functional properties and the innovation potential of UFG NiTi alloys is considered and discussed.
    view abstractdoi: 10.4028/
  • 2011 • 55 Photomodulation of the magnetisation of Co nanocrystals decorated with Rhodamine B
    Comesaña-Hermo, M. and Estivill, R. and Ciuculescu, D. and Amiens, C. and Farle, M. and Batat, P. and Jonusauskas, G. and McClenaghan, N.D. and Lecante, P. and Tardin, C. and Mazeres, S.
    ChemPhysChem 12 2915-2919 (2011)
    How exciting! Upon excitation of Rhodamine B with visible light in magnetic Co nanocrystal-Rhodamine B nanocomposites, electron transfer from the nanocrystal to the dye is evidenced as well as an increase in magnetisation (see picture), affording a new access to photomodulation of the magnetic properties of nanocrystal assemblies. Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/cphc.201100616
  • 2011 • 54 Planar-defect characteristics and cross-sections of 〈001〉, 〈111〉, and 〈112〉 InAs nanowires
    Li, Z.-A. and Mller, C. and Migunov, V. and Spasova, M. and Farle, M. and Lysov, A. and Gutsche, C. and Regolin, I. and Prost, W. and Tegude, F.-J. and Ercius, P.
    Journal of Applied Physics 109 (2011)
    We report on detailed structural and morphological characterizations of InAs nanowires of 〈001〉, 〈111〉, and 〈112〉 crystallographic directions grown on (001)B InAs wafer substrates using high-resolution transmission electron microscopy. We find that 〈001〉 -oriented InAs nanowires are cubic zincblende-type structure and free of planar defects. The 〈111〉- and 〈112〉-oriented InAs nanowires both have densely twinned (111) planar defects that are perpendicular and parallel to the growth direction, respectively. The cross sections of all three types of InAs nanowires are obtained from 3D reconstructions using electron tomography. The characteristics of the planar defects and the 3D wire shape should provide better estimations of microstructure-relevant physical properties, such as conductivity and Young's modulus of InAs nanowires. © 2011 American Institute of Physics.
    view abstractdoi: 10.1063/1.3592186
  • 2011 • 53 Possibilities and limitations of different analytical methods for the size determination of a bimodal dispersion of metallic nanoparticles
    Mahl, D. and Diendorf, J. and Meyer-Zaika, W. and Epple, M.
    Colloids and Surfaces A: Physicochemical and Engineering Aspects 377 386-392 (2011)
    Silver nanoparticles (about 70. nm) and gold nanoparticles (about 15. nm) were prepared and colloidally stabilized with poly(vinylpyrrolidone) (PVP). The pure nanoparticles as well as a 1:1 mixture (w:w) were analysed with a variety of methods which probe the size distribution: Scanning electron microscopy, transmission electron microscopy, dynamic light scattering, analytical disc centrifugation, and Brownian motion analysis (nanoparticle tracking analysis). The differences between the methods are highlighted and their ability to distinguish between silver and gold nanoparticles in the mixture is demonstrated. The size distribution data from the different methods were clearly different, therefore it is recommended to apply more than one method to characterize the nanoparticle dispersion. In particular, the smaller particles were undetectable by dynamic light scattering and nanoparticle tracking analysis in the presence of the large particles. For the 1:1 mixture, only electron microscopy and analytical disc centrifugation were able to give quantitative data on the size distribution. On the other hand, it is not possible to make statements about an agglomeration in dispersion with electron microscopy because an agglomeration may also have occurred during the drying process. © 2011 Elsevier B.V.
    view abstractdoi: 10.1016/j.colsurfa.2011.01.031
  • 2011 • 52 Pulsed-laser atom probe studies of a precipitation hardened maraging TRIP steel
    Dmitrieva, O. and Choi, P. and Gerstl, S.S.A. and Ponge, D. and Raabe, D.
    Ultramicroscopy 111 623-627 (2011)
    A precipitation hardened maraging TRIP steel was analyzed using a pulsed laser atom probe. The laser pulse energy was varied from 0.3 to 1.9 nJ to study its effect on the measured chemical compositions and spatial resolution. Compositional analyses using proximity histograms did not show any significant variations in the average matrix and precipitate compositions. The only remarkable change in the atom probe data was a decrease in the++/+ charge state ratios of the elements. The values of the evaporation field used for the reconstructions exhibit a linear dependence on the laser pulse energy. The adjustment of the evaporation fields used in the reconstructions for different laser pulse energies was based on the correlation of the obtained cluster shapes to the TEM observations. No influence of laser pulse energy on chemical composition of the precipitates and on the chemical sharpness of their interfaces was detected. © 2010 Elsevier B.V.
    view abstractdoi: 10.1016/j.ultramic.2010.12.007
  • 2011 • 51 Rational design of β-sheet ligands against Aβ42- induced toxicity
    Hochdörffer, K. and März-Berberich, J. and Nagel-Steger, L. and Epple, M. and Meyer-Zaika, W. and Horn, A.H.C. and Sticht, H. and Sinha, S. and Bitan, G. and Schrader, T.
    Journal of the American Chemical Society 133 4348-4358 (2011)
    A β-sheet-binding scaffold was equipped with long-range chemical groups for tertiary contacts toward specific regions of the Alzheimer's Aβ fibril. The new constructs contain a trimeric aminopyrazole carboxylic acid, elongated with a C-terminal binding site, whose influence on the aggregation behavior of the Aβ42 peptide was studied. MD simulations after trimer docking to the anchor point (F19/F20) suggest distinct groups of complex structures each of which featured additional specific interactions with characteristic Aβ regions. Members of each group also displayed a characteristic pattern in their antiaggregational behavior toward Aβ. Specifically, remote lipophilic moieties such as a dodecyl, cyclohexyl, or LPFFD fragment can form dispersive interactions with the nonpolar cluster of amino acids between I31 and V36. They were shown to strongly reduce Thioflavine T (ThT) fluorescence and protect cells from Aβ lesions (MTT viability assays). Surprisingly, very thick fibrils and a high β-sheet content were detected in transmission electron microscopy (TEM) and CD spectroscopic experiments. On the other hand, distant single or multiple lysines which interact with the ladder of stacked E22 residues found in Aβ fibrils completely dissolve existing β-sheets (ThT, CD) and lead to unstructured, nontoxic material (TEM, MTT). Finally, the triethyleneglycol spacer between heterocyclic β-sheet ligand and appendix was found to play an active role in destabilizing the turn of the U-shaped protofilament. Fluorescence correlation spectroscopy (FCS) and sedimentation velocity analysis (SVA) provided experimental evidence for some smaller benign aggregates of very thin, delicate structure (TEM, MTT). A detailed investigation by dynamic light scattering (DLS) and other methods proved that none of the new ligands acts as a colloid. The evolving picture for the disaggregation mechanism by these new hybrid ligands implies transformation of well-ordered fibrils into less structured aggregates with a high molecular weight. In the few cases where fibrillar components remain, these display a significantly altered morphology and have lost their acute cellular toxicity. © 2011 American Chemical Society.
    view abstractdoi: 10.1021/ja107675n
  • 2011 • 50 Recrystallization and grain growth in ultrafine-grained materials produced by high pressure torsion
    Khorashadizadeh, A. and Raabe, D. and Winning, M. and Pippan, R.
    Advanced Engineering Materials 13 245-250 (2011)
    Ultrafine-grained (UFG) materials processed by severe plastic deformation are known to exhibit good mechanical properties. Much about the annealing behavior of such materials is still unknown, and this work aims to provide a better understanding of the thermal properties of UFG materials. For this purpose a Cu-0.17 wt%Zr alloy was subjected to high pressure torsion (HPT) with a maximal pressure of 4.8GPa at room temperature. The microstructures of the specimens were characterized using electron back scatter (EBSD) measurements, transmission electron microscopy (TEM), and hardness measurements. During annealing of the samples, dispersoids were formed which improved the thermal stability of the alloy. At higher strain levels the fraction of high angle grain boundaries (HAGBs) increased above 70% of the total grain boundaries. Ultrafine-grained materials processed by severe plastic deformation are known to exhibit good mechanical properties. Much about the annealing behavior of such materials is still unknown, and this work aims to provide a better understanding of the thermal properties of such materials. For this purpose a Cu-0.17 wt%Zr alloy was subjected to high pressure torsion. The microstructures of the specimens were characterized in the deformed state as well as after annealing using EBSD and hardness measurements. Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA.
    view abstractdoi: 10.1002/adem.201000253
  • 2011 • 49 Role of oxygen on microstructure and thermoelectric properties of silicon nanocomposites
    Schierning, G. and Theissmann, R. and Stein, N. and Petermann, N. and Becker, A. and Engenhorst, M. and Kessler, V. and Geller, M. and Beckel, A. and Wiggers, H. and Schmechel, R.
    Journal of Applied Physics 110 (2011)
    Phosphorus-doped silicon nanopowder from a gas phase process was compacted by DC-current sintering in order to obtain thermoelectrically active, nanocrystalline bulk silicon. A density between 95 and 96 compared to the density of single crystalline silicon was achieved, while preserving the nanocrystalline character with an average crystallite size of best 25 nm. As a native surface oxidation of the nanopowder usually occurs during nanopowder handling, a focus of this work is on the role of oxygen on microstructure and transport properties of the nanocomposite. A characterization with transmission electron microscopy (TEM) showed that the original core/shell structure of the nanoparticles was not found within the sintered nanocomposites. Two different types of oxide precipitates could be identified by energy filtered imaging technique. For a detailed analysis, 3-dimensional tomography with reconstruction was done using a needle-shaped sample prepared by focused ion beam (FIB). The 3-dimensional distribution of silicon dioxide precipitates confirmed that the initial core/shell structure breaks down and precipitates are formed. It is further found that residual pores are exclusively located within oxide precipitates. Thermoelectric characterization was done on silicon nanocomposites sintered between 960 C and 1060 C with varying oxygen content between room temperature and 950 C. The higher sintering temperature led to a better electrical activation of the phosphorus dopant. The oxidic precipitates support densification and seem to be able to reduce the thermal conductivity therefore enhancing thermoelectric properties. A peak figure of merit, zT, of 0.5 at 950 C was measured for a sample sintered at 1060 C with a mean crystallite size of 46 nm. © 2011 American Institute of Physics.
    view abstractdoi: 10.1063/1.3658021
  • 2011 • 48 Scanning transmission electron microscope observations of defects in as-grown and pre-strained Mo alloy fibers
    Phani, P.S. and Johanns, K.E. and Duscher, G. and Gali, A. and George, E.P. and Pharr, G.M.
    Acta Materialia 59 2172-2179 (2011)
    Compression testing of micro-pillars has recently been of great interest to the small-scale mechanics community. Previous compression tests on single crystal Mo alloy micro-pillars produced by directional solidification of eutectic alloys showed that as-grown pillars yield at strengths close to the theoretical strength while pre-strained pillars yield at considerably lower stresses. In addition, the flow behavior changes from stochastic to deterministic with increasing pre-strain. In order to gain a microstructural insight into this behavior, an aberration corrected scanning transmission electron microscope was used to study the defect structures in as-grown and pre-strained single crystal Mo alloy fibers. The as-grown fibers were found to be defect free over large lengths while the highly pre-strained (16%) fibers had high defect densities that were uniform throughout. Interestingly, the fibers with intermediate pre-strain (4%) exhibited an inhomogeneous defect distribution. The observed defect structures and their distributions are correlated with the previously reported stress-strain behavior. Some of the mechanistic interpretations of Bei et al. are examined in the light of new microstructural observations. © 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2010.12.018
  • 2011 • 47 Self-assembly of calcium phosphate nanoparticles into hollow spheres induced by dissolved amino acids
    Hagmeyer, D. and Ganesan, K. and Ruesing, J. and Schunk, D. and Mayer, C. and Dey, A. and Sommerdijk, N.A.J.M. and Epple, M.
    Journal of Materials Chemistry 21 9219-9223 (2011)
    Nanoparticles of calcium phosphate assemble spontaneously within a few seconds into hollow spheres with a diameter around 200-300 nm in the presence of dissolved amino acids and dipeptides. The process of formation was followed by cryo-transmission electron microscopy (cryoTEM), proving their hollow nature which was also confirmed by nano-indentation by atomic force microscopy (AFM). The hollow spheres were analyzed by scanning electron microscopy (SEM), dynamic light scattering (DLS), nanoparticle tracking analysis (NTA) and elemental analysis. The hollow spheres were moderately stable against heating and ultrasonication. A self-assembly of the primarily formed calcium phosphate nanoparticles around amino acid-rich domains in water is proposed. As this process was observed with different amino acids, it appears to be a more general phenomenon. © 2011 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c1jm11316j
  • 2011 • 46 Size and size distribution of apatite crystals in sauropod fossil bones
    Dumont, M. and Kostka, A. and Sander, P.M. and Borbely, A. and Kaysser-Pyzalla, A.
    Palaeogeography, Palaeoclimatology, Palaeoecology 310 108-116 (2011)
    Two complementary techniques, X-ray diffraction and transmission electron microscopy, were used to analyze the size and size distribution of apatite crystallites in subfossil mammal and fossil sauropod dinosaur bones. The transmission electron microscope (TEM) samples were prepared by Focused Ion Beam (FIB). X-ray diffraction indicates that crystals in sauropod bones are larger than in subfossil mammal bones. For sauropods the average crystallite size (both in length and width) does not seem to increase with bone length. A similar result was obtained from TEM investigations regarding crystallites aspect ratio (length/width). The distribution of crystallite sizes in a juvenile sauropod was found to be much wider compared to an adult bone originating from the same bed and locality, consistent with the hypothesis that young animal bone contains a mixture of crystallite sizes, with a beneficial effect on mechanical properties. Diagenesis effects on crystal size dimensions have been considered. © 2011 Elsevier B.V.
    view abstractdoi: 10.1016/j.palaeo.2011.06.021
  • 2011 • 45 Small-scale deposition of thin films and nanoparticles by microevaporation sources
    Meyer, R. and Hamann, S. and Ehmann, M. and König, D. and Thienhaus, S. and Savan, A. and Ludwig, Al.
    Journal of Microelectromechanical Systems 20 21-27 (2011)
    This paper reports on a novel miniaturized deposition technique based on micro-hotplates which are used as microevaporation sources (MES) for a localized deposition of thin films and nanoparticles. The feasibility of this small-scale deposition technique and its general properties are shown for depositions of Ag on unpatterned and microstructured substrates. The deposited films are rotationally symmetric and show a distinct lateral thickness change. We take advantage of this latter effect, as, e.g., all stages of film condensation can be observed within one experiment on one sample, in a size suitable for transmission electron microscopy investigations. For realizing the most laterally confined depositions, a micro-Knudsen cell was used. It is shown that the use of MES is also very suitable for the fabrication and deposition of nanoparticles. © 2011 IEEE.
    view abstractdoi: 10.1109/JMEMS.2010.2090506
  • 2011 • 44 Stoichiometry of alloy nanoparticles from laser ablation of PtIr in acetone and their electrophoretic deposition on PtIr electrodes
    Jakobi, J. and Menéndez-Manjón, A. and Chakravadhanula, V.S.K. and Kienle, L. and Wagener, P. and Barcikowski, S.
    Nanotechnology 22 (2011)
    Charged Pt-Ir alloy nanoparticles are generated through femtosecond laser ablation of a Pt9Ir target in acetone without using chemical precursors or stabilizing agents. Preservation of the target's stoichiometry in the colloidal nanoparticles is confirmed by transmission electron microscopy (TEM)-energy-dispersive x-ray spectroscopy (EDX), high angle annular dark field (HAADF) scanning transmission electron microscopy (STEM)-EDX elemental maps, high resolution TEM and selected area electron diffraction (SAED) measurements. Results are discussed with reference to thermophysical properties and the phase diagram. The nanoparticles show a lognormal size distribution with a mean Feret particle size of 26nm. The zeta potential of - 45mV indicates high stability of the colloid with a hydrodynamic diameter of 63nm. The charge of the particles enables electrophoretic deposition of nanoparticles, creating nanoscale roughness on three-dimensional PtIr neural electrodes within a minute. In contrast to coating with Pt or Ir oxides, this method allows modification of the surface roughness without changing the chemical composition of PtIr. © 2011 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0957-4484/22/14/145601
  • 2011 • 43 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 • 42 Structural characteristics and catalytic performance of alumina-supported nanosized ceria-lanthana solid solutions
    Katta, L. and Thrimurthulu, G. and Reddy, B.M. and Muhler, M. and Grünert, W.
    Catalysis Science and Technology 1 1645-1652 (2011)
    Alumina-supported nanosized ceria-lanthana solid solutions (CeO <inf>2</inf>-La<inf>2</inf>O<inf>3</inf>/Al<inf>2</inf>O<inf>3</inf> (CLA) = 80:20:100 mol% based on oxides) were synthesized by a modified deposition coprecipitation method from ultra-high dilute aqueous solutions. The synthesized materials were subjected to various calcination temperatures from 773 to 1073 K to understand the surface structure and the thermal stability. Structural and redox properties were deeply investigated by different characterization techniques, namely, X-ray diffraction (XRD), Raman spectroscopy (RS), transmission electron microscopy (TEM), UV-visible diffuse reflectance spectroscopy (UV-vis DRS), X-ray photoelectron spectroscopy (XPS), temperature programmed reduction (H<inf>2</inf>-TPR), and Brunauer-Emmett-Teller (BET) surface area. The catalytic efficiency was evaluated for CO oxidation at normal atmospheric pressure. BET surface area measurements revealed that synthesized samples exhibit reasonably high specific surface area. As revealed by XRD measurements, samples maintain structural integrity up to 1073 K without any disproportionation of phases. XPS results suggested that there is no significant change in the Ce3+ amount during thermal treatments due to the absence of undesirable cerium aluminate formation. A significant number of oxygen vacancies were confirmed from Raman and UV-vis DRS measurements. The CLA 773 sample exhibited superior CO oxidation activity. The better activity of the catalyst was proved to be due to a high dispersion in the form of nanosized ceria-lanthana solid solutions over the alumina support, facile reduction, and a high oxygen storage capacity. © The Royal Society of Chemistry 2011.
    view abstractdoi: 10.1039/c1cy00312g
  • 2011 • 41 Structure, morphology, and aging of Ag-Fe dumbbell nanoparticles
    Elsukova, A. and Li, Z.-A. and Möller, C. and Spasova, M. and Acet, M. and Farle, M. and Kawasaki, M. and Ercius, P. and Duden, T.
    Physica Status Solidi (A) Applications and Materials Science 208 2437-2442 (2011)
    Dumbbell-shaped or Janus-type nanocomposites provide multifunctional properties with various diagnostic and therapeutic applications in biomedicine. We have prepared dumbbell Ag-Fe nanoparticles by magnetron sputtering with subsequent in-flight annealing. Structural properties and chemical compositions of freshly prepared and 5-month aged particles were examined by means of transmission electron microscopy including high-resolution imaging, energy dispersive X-ray spectroscopy, and 3D electron tomography. Fresh particles consist of a faceted Ag part on a Fe-Fe 3O 4 composite particle of more spherical shape. Aging changes the crystallinity and morphology of the particles. The aged nanocomposite consists of a silver spherical particle that is attached to a hollow iron oxide sphere containing one or several silver clusters inside. TEM images of the fresh (a) and aged (b) Ag-Fe nanoparticles. (c) 3D reconstructed image of an aged Ag-Fe particle with color segmentation. © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/pssa.201127104
  • 2011 • 40 Synthesis of tailored WO3 and WOx (2.9 \< x \< 3) nanoparticles by adjusting the combustion conditions in a H2/O 2/Ar premixed flame reactor
    Gupta, A. and Ifeacho, P. and Schulz, C. and Wiggers, H.
    Proceedings of the Combustion Institute 33 1883-1890 (2011)
    Flame synthesis of WO3 and WOx (2.9 < x < 3) nanoparticles is carried out by adding a dilute concentration of WF6 as precursor in a low-pressure H2/O2/Ar premixed flame reactor. The reactor is equipped with molecular-beam sampling and particle mass spectroscopy (PMS) to determine particle composition and sizes as a function of height above burner. Varying the H2/O2 ratio allowed us to tune the stoichiometry of the product. With a H2/O2 ratio of 0.67 white colored stoichiometric WO3 is formed, whereas the H2/O2 ratio &gt;0.8 yields blue colored non-stoichiometric WOx (2.9 < x < 3) nanoparticles. The size of nanoparticles can be controlled by varying the residence time in the high-temperature zone of the reactor as observed by molecular-beam sampling with subsequent analysis using PMS. Transmission electron microscopy (TEM) images of as-synthesized nanoparticles show that particles are non-agglomerated and have an almost spherical morphology. The X-ray diffraction (XRD) pattern of the as-synthesized material indicates that the powders exhibit poor crystallinity, however, subsequent thermal annealing of the sample in air changes its structure from amorphous to crystalline phase. It is observed that particles with sub-stoichiometric composition (WOx) show higher conductivity compared to the stoichiometric WO3 sample. © 2010 The Combustion Institute. Published by Elsevier Inc. All rights reserved.
    view abstractdoi: 10.1016/j.proci.2010.06.162
  • 2011 • 39 The impact of microstructural alterations at spray coated cylinder running surfaces of diesel engines - Findings from motor and laboratory benchmark tests
    Hahn, M. and Bauer, C. and Theissmann, R. and Gleising, B. and Dudzinski, W. and Fischer, A.
    Wear 271 2599-2609 (2011)
    Future motor engine design and technology demand new materials that meet the requirements in terms of friction and wear properties. In order to reduce weight of the engine block and further more reduce the emissions thermal spray coatings are options to manufacture wear resistant and low-friction cylinder running surfaces. A motor cylinder with a thermally sprayed mild steel coating after a road test is prepared for transmission electron microscopy (TEM) investigations in the positions top dead center (TDC) and bottom dead center (BDC). In the dead centers of engines mixed/boundary lubrication conditions prevail. Within this study a new nanocrystalline thermal spray coating is investigated using two different laboratory tests where mixed/boundary lubrication regime is presumed. Conventional mild steel coatings and the new high alloyed Fe-base coatings are tested against two different piston rings in a reciprocating sliding wear test and an oscillating sliding wear test, known as SRV© test. TEM studies of the surface near microstructure demonstrate the difference in the microstructural alterations and wear mechanisms.Findings from the motor cylinder in surface and subsurface modifications are considered when discussing the results from laboratory tests in terms of initial grain size, influence of lattice defects and chemical influences due to oil additives. © 2011 Elsevier B.V.
    view abstractdoi: 10.1016/j.wear.2011.01.090
  • 2011 • 38 The influence of the inhibitor particle sizes to the corrosion properties of hybrid sol-gel coatings
    Wittmar, A. and Wittmar, M. and Caparrotti, H. and Veith, M.
    Journal of Sol-Gel Science and Technology 59 621-628 (2011)
    The influence of the inhibitor particle size (nano and micro cerium dioxide) embedded in several hybrid sol-gel coating systems for the corrosion protection of aluminium AA2024 alloy was studied, as well as the influence of other parameters like the inhibitor loading level and the method of reticulation. The properties of the obtained coatings were evaluated by means of transmission electron microscopy, accelerated salt spray test and electrochemical impedance spectroscopy. All the varied parameters proved to have an important influence on the corrosion mechanism, and an improvement induced by the use of nanometric inhibitors in comparison with the micrometric ones was demonstrated. In the case of the strong reticulated matrix, an inhomogeneous dispersion of the inhibiting species (micro inhibitor) favours the tension formation, making the coating more vulnerable to the corrosion attack. For the same doping level it was observed that the reticulation with 1-methylimidazole (MI) leads to a slightly better corrosion protection. Concomitantly, it was shown that high loadings with inhibitors have an adverse effect on the corrosion protection. © Springer Science+Business Media, LLC 2011.
    view abstractdoi: 10.1007/s10971-011-2536-2
  • 2011 • 37 The relation between shear banding, microstructure and mechanical properties in Mg and Mg-Y alloys
    Sandlöbes, S. and Schestakow, I. and Yi, S. and Zaefferer, S. and Chen, J. and Friák, M. and Neugebauer, J. and Raabe, D.
    Materials Science Forum 690 202-205 (2011)
    The formation of deformation-induced shear bands plays an important role for the room temperature deformation of both, Mg and Mg-Y alloys, but the formation and structure of shear bands is distinctively different in the two materials. Due to limited deformation modes in pure Mg, the strain is localized in few shear bands leading to an early failure of the material during cold deformation. Contrarily, Mg-RE (RE: rare earth) alloys exhibit a high density of homogeneously distributed local shear bands during deformation at room temperature. A study of the microstructure of the shear bands by electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM) at different strains was performed. These investigations give insight into the formation of shear bands and their effects on the mechanical behaviour of pure Mg and Mg-3Y. Since in pure Mg mainly extension twinning and basal <a> dislocation slip are active, high stress fields at grain resp. twin boundaries in shear bands effect fast growth of the shear bands. In Mg-RE alloys additionally contraction and secondary twinning and pyramidal <c+a> dislocation slip are active leading to the formation of microscopic shear bands which are limited to the boundary between two grains. The effects of shear bands on the mechanical behaviour of pure Mg and Mg-RE alloys are discussed with respect to their formation and growth. © (2011) Trans Tech Publications.
    view abstractdoi: 10.4028/
  • 2011 • 36 Thermal stability of TiAIN/CrN multilayer coatings studied by atom probe tomography
    Choi, P.-P. and Povstugar, I. and Ahn, J.-P. and Kostka, A. and Raabe, D.
    Ultramicroscopy 111 518-523 (2011)
    This study is about the microstructural evolution of TiAlN/CrN multilayers (with a Ti:Al ratio of 0.75:0.25 and average bilayer period of 9 nm) upon thermal treatment. Pulsed laser atom probe analyses were performed in conjunction with transmission electron microscopy and X-ray diffraction. The layers are found to be thermally stable up to 600 °C. At 700 °C TiAlN layers begin to decompose into Ti- and Al-rich nitride layers in the out-of-plane direction. Further increase in temperature to 1000 °C leads to a strong decomposition of the multilayer structure as well as grain coarsening. Layer dissolution and grain coarsening appear to begin at the surface. Domains of AlN and TiCrN larger than 100 nm are found, together with smaller nano-sized AlN precipitates within the TiCrN matrix. Fe and V impurities are detected in the multilayers as well, which diffuse from the steel substrate into the coating along columnar grain boundaries. © 2010 Elsevier B.V.
    view abstractdoi: 10.1016/j.ultramic.2010.11.012
  • 2011 • 35 TiO 2 coating of high surface area silica gel by chemical vapor deposition of TiCl 4 in a fluidized-bed reactor
    Xia, W. and Mei, B. and Sánchez, M.D. and Strunk, J. and Muhler, M.
    Journal of Nanoscience and Nanotechnology 11 8152-8157 (2011)
    TiO 2 was deposited on high surface area porous silica gel (400 m 2g -1) in a fluidized bed reactor. Chemical vapor deposition was employed for the coating under vacuum conditions with TiCl 4 as precursor. Nitrogen physisorption, X-ray diffraction, transmission electron microscopy, X-ray photoelectron spectroscopy and UV-vis spectroscopy were applied to characterize the obtained TiO 2-SiO 2 composites with different Ti loadings up to 5 wt%. Only a slight decrease in the specific surface area was detected at low Ti loadings. At a Ti loading of 2 wt%, TiO 2 was found to be highly dispersed on the SiO 2 surface likely in form of a thin film. At higher Ti loadings, two weak reflections corresponding to anatase TiO 2 were observed in the diffraction patterns indicating the presence of crystalline bulk TiO 2. High resolution XPS clearly distinguished two types of Ti species, i.e., Ti-O-Si at the interface and Ti-O-Ti in bulk TiO 2. The presence of polymeric TiOx species at low Ti loadings was confirmed by a blue shift in the UV-vis spectra as compared to bulk TiO 2. All these results point to a strong interaction between the TiO 2 deposit and the porous SiO 2 substrate especially at low Ti loadings. Copyright © 2011 American Scientific Publishers All rights reserved.
    view abstractdoi: 10.1166/jnn.2011.5107
  • 2011 • 34 Visualization and functions of surface defects on carbon nanotubes created by catalytic etching
    Xia, W. and Yin, X. and Kundu, S. and Sánchez, M. and Birkner, A. and Wöll, C. and Muhler, M.
    Carbon 49 299-305 (2011)
    Surface defects were created on carbon nanotubes (CNTs) by catalytic steam gasification or catalytic etching with iron as catalysts. The structure and morphology of the etched CNTs were studied by transmission electron microscopy (TEM) and scanning tunneling microscopy (STM). The electronic structure of the etched CNTs was investigated by ultraviolet photoelectron spectroscopy (UPS). The etched CNTs were treated by nitric acid to obtain oxygen-containing functional groups. The amount and the thermal stability of these groups were studied by temperature-resolved X-ray photoelectron spectroscopy (XPS). Temperature-programmed desorption with ammonia as a probe molecule (NH 3-TPD) was employed to investigate the interaction of the surface defects with foreign molecules in gas phase. TEM and STM studies disclosed the presence of surface defects especially edge planes on the etched CNTs. Etching of CNTs led to a less pronounced p-π band than the as-is CNTs, as evidenced by UPS studies. The XPS and NH 3-TPD studies demonstrated that the defects on the CNTs enhanced the reactivity of the exposed surfaces allowing obtaining a higher degree of oxygen functionalization and more active adsorption sites. © 2010 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.carbon.2010.09.025
  • 2010 • 33 A new family of nonionic dendritic amphiphiles displaying unexpected packing parameters in micellar assemblies
    Trappmann, B. and Ludwig, K. and Radowski, M.R. and Shukla, A. and Mohr, A. and Rehage, H. and Böttcher, C. and Haag, R.
    Journal of the American Chemical Society 132 11119-11124 (2010)
    In this paper we report on the synthesis of a new family of nonionic dendritic amphiphiles that self-assemble into defined supramolecular aggregates. Our approach is based on a modular architecture consisting of different generations of hydrophilic polyglycerol dendrons [G1-G3] connected to hydrophobic C11 or C16 alkyl chains via mono- or biaromatic spacers, respectively. All amphiphiles complex hydrophobic compounds as demonstrated by solubilization of Nile Red or pyrene. The structure of the supramolecular assemblies as well as the aggregation numbers are strongly influenced by the type of the dendritic headgroup. While the [G1] amphiphiles form different structures such as ringlike and fiberlike micelles, the [G2] and [G3] derivatives aggregate toward spherical micelles of low polydispersity clearly proven by transmission electron microscopy (TEM) measurements. In the case of the biaromatic [G2] derivative, the structural persistence of the micelles allowed a three-dimensional structure determination from the TEM data and confirmed the aggregation number obtained by static light scattering (SLS) measurements. On the basis of these data, molecular packing geometries indicate a drastic mass deficit of alkyl chains in the hydrophobic core volume of spherical micelles. It is noteworthy that these highly defined micelles contain as little as 15 molecules and possess up to 74% empty space. This behavior is unexpected as it is very different from classical detergent micelles such as sodium dodecyl sulfate (SDS), where the hydrophobic core volume is completely filled by alkyl chains. © 2010 American Chemical Society.
    view abstractdoi: 10.1021/ja101523v
  • 2010 • 32 A review of crystallographic textures in chemical vapor-deposited diamond films
    Liu, T. and Raabe, D. and Mao, W.-M.
    Signal, Image and Video Processing 4 1-16 (2010)
    Diamond is one of the most important functional materials for film applications due to its extreme physical and mechanical properties, many of which depend on the crystallographic texture. The influence of various deposition parameters matters to the texture formation and evolution during chemical vapor deposition (CVD) of diamond films. In this overview, the texture evolutions are presented in terms of both simulations and experimental observations. The crystallographic textures in diamond are simulated based on the van der Drift growth selection mechanism. The film morphology and textures associated with the growth parameters α (proportional to the ratio of the growth rate along the 〈100〉 direction to that along the 〈111〉 direction) are presented and determined by applying the fastest growth directions. Thick films with variations in substrate temperature, methane concentration, film thickness, and nitrogen addition were analyzed using high-resolution electron back-scattering diffraction (HR-EBSD) as well as X-ray diffraction (XRD), and the fraction variations of fiber textures with these deposition parameters were explained. In conjunction with the focused ion beam (FIB) technique for specimen preparation, the grain orientations in the beginning nucleation zones were studied using HR-EBSD (50 nm step size) in another two sets of thin films deposited with variations in methane concentration and substrate material. The microstructures, textures, and grain boundary character were characterized. Based on the combination of an FIB unit for serial sectioning and HR-EBSD, diamond growth dynamics was observed using a 3D EBSD technique, with which individual diamond grains were investigated in 3D. Microscopic defects were observed in the vicinity of the high-angle grain boundaries by using the transmission electron microscopy (TEM) technique, and the advances of TEM orientation microscopy make it possible to identify the grain orientations in nano-crystalline diamond. © 2010 Higher Education Press and Springer Berlin Heidelberg.
    view abstractdoi: 10.1007/s11760-008-0099-7
  • 2010 • 31 Analysis of local microstructure after shear creep deformation of a fine-grained duplex γ-TiAl alloy
    Peter, D. and Viswanathan, G.B. and Dlouhy, A. and Eggeler, G.
    Acta Materialia 58 6431-6443 (2010)
    The present work characterizes the microstructure of a hot-extruded Ti-45Al-5Nb-0.2B-0.2C (at.%) alloy with a fine-grained duplex microstructure after shear creep deformation (temperature 1023 K; shear stress 175 MPa; shear deformation 20%). Diffraction contrast transmission electron microscopy (TEM) was performed to identify ordinary dislocations, superdislocations and twins. The microstructure observed in TEM is interpreted taking into account the contribution of the applied stress and coherency stresses to the overall local stress state. Two specific locations in the lamellar part of the microstructure were analyzed, where either twins or superdislocations provided c-component deformation in the L10 lattice of the γ phase. Lamellar γ grains can be in soft and hard orientations with respect to the resolved shear stress provided by the external load. The presence of twins can be rationalized by the superposition of the applied stress and local coherency stresses. The presence of superdislocations in hard γ grains represents indirect evidence for additional contributions to the local stress state associated with stress redistribution during creep. © 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2010.08.005
  • 2010 • 30 Artificial single variant martensite in freestanding Fe70Pd 30 films obtained by coherent epitaxial growth
    Bechtold, C. and Buschbeck, J. and Lotnyk, A. and Erkartal, B. and Hamann, S. and Zamponi, C. and Schultz, L. and Ludwig, Al. and Kienle, L. and Fähler, S. and Quandt, E.
    Advanced Materials 22 2668-2671 (2010)
    (Figure Presented) The mechanically soft behavior of the magnetic shape-memory material Fe70Pd30 allows huge tetragonal distortions to be stabilized in sputtered thin films by coherent epitaxial growth on various metallic buffers. Furthermore, it is demonstrated that epitaxial films more than 1 μm thick can be grown, which makes possible freestanding films in an artificial single variant state suitable for microactuators and sensors. © 2010 WILEY-VCH Verlag GmbH & Co. KGaA.
    view abstractdoi: 10.1002/adma.201000599
  • 2010 • 29 Creep properties beyond 1100°C and microstructure of Co-Re-Cr alloys
    Brunner, M. and Hüttner, R. and Bölitz, M.-C. and Völkl, R. and Mukherji, D. and Rösler, J. and Depka, T. and Somsen, C. and Eggeler, G. and Glatzel, U.
    Materials Science and Engineering A 528 650-656 (2010)
    The melting point of a novel Co-17Re-23Cr alloy (numbers given in at.%) could be increased by 250 °C as compared to established Ni-based superalloys, by optimising the content of Re. Samples were produced by vacuum arc-melting in order to evaluate the creep behaviour at temperatures beyond 1100 °C and for microstructural analysis. Three alloys (the Co-17Re-23Cr-based material, and the carbide strengthened alloys Co-17Re-23Cr-2.6C and Co-17Re-23Cr-2.6C-1.2Ta) were investigated. Creep properties, especially the minimum creep rate and the Larson-Miller plots, were compared. The Co-17Re-23Cr-2.6C-1.2Ta alloy has a higher minimum creep rate than Co-17Re-23Cr at 1200 °C but it has a lower minimum creep rate than Co-17Re-23Cr at 1100 °C. TaC coarsening, detected via transmission electron microscope (TEM) measurements may explain this effect. The overall creep behaviour of Co-17Re-23Cr-2.6C at 1200 °C is better than that of Co-17Re-23Cr-2.6C-1.2Ta, but worse than that of Co-17Re-23Cr.Microstructural investigations by scanning electron microscopy and TEM reveal a hexagonal closed-packed (hcp) matrix and σ-phases. The microhardness of the σ-phase was about 1570. HV (load: 1. g) and around 800. HV for the matrix. Pores and cracks occur along the brittle σ-phases and grain boundaries in the Co-Re-Cr alloys. A Norton exponent n in between 1.4 and 3.0 points to grain boundary dominated creep mechanisms. © 2010 Elsevier B.V.
    view abstractdoi: 10.1016/j.msea.2010.09.035
  • 2010 • 28 Effect of low-temperature precipitation on the transformation characteristics of Ni-rich NiTi shape memory alloys during thermal cycling
    Wagner, M.F.-X. and Dey, S.R. and Gugel, H. and Frenzel, J. and Somsen, C. and Eggeler, G.
    Intermetallics 18 1172-1179 (2010)
    Thermal cycling of NiTi shape memory alloys is associated with functional fatigue: the characteristic phase transformation temperatures decrease with increasing number of cycles, and the transformation behavior changes from a single- to a two-stage martensitic transformation involving the intermediate R-phase. These effects are usually attributed to a gradual increase of dislocation density associated with micro-plasticity during repeated cycling through the transformation range. Here, these changes are shown to increase at a higher maximum temperature (in the fully austenitic state) during differential scanning calorimetric cycling of a Ni-rich alloy. Additional thermal cycling experiments without repeated phase transformations, and post-mortem microstructural observations by transmission electron microscopy, demonstrate that a relevant portion of functional fatigue is due to the formation of nano-scale Ni-rich precipitates of type Ni4Ti3 even at temperatures relatively close to the austenite finish temperature. These results show that both dislocation generation during the diffusion-less phase transformation, and diffusion-controlled nucleation and growth of Ni4Ti3 precipitates, can interact and contribute to the evolution of functional properties during thermal cycling of Ni-rich NiTi. © 2010 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.intermet.2010.02.048
  • 2010 • 27 Energy threshold for the creation of nanodots on SrTiO3 by swift heavy ions
    Karlušić, M. and Akcöltekin, S. and Osmani, O. and Monnet, I. and Lebius, H. and Jakšić, M. and Schleberger, M.
    New Journal of Physics 12 (2010)
    We present experimental and theoretical data on the threshold behaviour of nanodot creation with swift heavy ions. A model calculation based on a two-temperature model that takes into account the spatially resolved electron density gives a threshold of 12keVnm-1 below which the energy density at the end of the track is no longer high enough to melt the material. In the corresponding experiments, we irradiated SrTiO3 surfaces under grazing incidence with swift heavy ions. The resulting chains of nanodots were analysed by atomic force microscopy (AFM). In addition, some samples of SrTiO3 irradiated under normal incidence were analysed by transmission electron microscopy (TEM). Both experiments showed two thresholds, which were connected with the appearance of tracks and the creation of fully developed tracks. The threshold values were similar for surface and bulk tracks, suggesting that the same processes occur at both glancing and normal incidence. © IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.
    view abstractdoi: 10.1088/1367-2630/12/4/043009
  • 2010 • 26 Focused ion beam/scanning electron microscopy tomography and conventional transmission electron microscopy assessment of Ni4Ti3 morphology in compression-aged Ni-rich Ni-Ti single crystals
    Cao, S. and Somsen, C. and Croitoru, M. and Schryvers, D. and Eggeler, G.
    Scripta Materialia 62 399-402 (2010)
    The size, morphology and configuration of Ni4Ti3 precipitates in a single-crystal Ni-Ti alloy have been investigated by two-dimensional transmission electron microscopy-based image analysis and three-dimensional reconstruction from slice-and-view images obtained in a focused ion beam/scanning electron microscopy (FIB/SEM) dual-beam system. Average distances between the precipitates measured along the compression direction correlate well between both techniques, while particle shape and configuration data is best obtained from FIB/SEM. Precipitates form pockets of B2 of 0.54 μm in the compression direction and 1 μm perpendicular to the compression direction. © 2009 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.scriptamat.2009.11.040
  • 2010 • 25 Formation and thermal stability of platinum oxides on size-selected platinum nanoparticles: Support effects
    Ono, L.K. and Yuan, B. and Heinrich, H. and Roldan Cuenya, B.
    Journal of Physical Chemistry C 114 22119-22133 (2010)
    This article presents a systematic study of the formation and thermal stability of Pt oxide species on sizeselected Pt nanoparticles (NPs) supported on SiO2, ZrO2, and TiO2 thin films. The studies were carried out in ultrahigh vacuum (UHV) by temperature-dependent X-ray photoelectron spectroscopy (XPS) measurements and ex situ transmission electron microscopy and atomic force microscopy. The NPs were synthesized by inverse micelle encapsulation and oxidized in UHV at room temperature by an oxygen plasma treatment. For a given particle size distribution, the role played by the NP support on the stability of Pt oxides was analyzed. PtO2 species are formed on all supports investigated after O2-plasma exposure. A two-step thermal decomposition (PtO2 → PtO → Pt) is observed from 300 to 600 K upon annealing in UHV. The stability of oxidized Pt species was found to be enhanced on ZrO2 under annealing treatments in O2. Strong NP/support interactions and the formation of Pt-Ti-O alloys are detected for Pt/TiO2 upon annealing in UHV above 550 K but not under an identical treatment in O2. Furthermore, thermal treatments in both environments above 700 K lead to the encapsulation of Pt by TiOx. The final shape of the micellar Pt NPs is influenced by the type of underlying support as well as by the post-deposition treatment. Spherical Pt NPs are stable on SiO2, ZrO2, and TiO 2 after in situ ligand removal with atomic oxygen at RT. However, annealing in UHV at 1000 K leads to NP flattening on ZrO2 and to the diffusion of Pt NPs into TiO2. The stronger the nature of the NP/support interaction, the more dramatic is the change in the NP shape (TiO2 &gt; ZrO2 &gt; SiO2). © 2010 American Chemical Society.
    view abstractdoi: 10.1021/jp1086703
  • 2010 • 24 Formation, stability and crystal structure of the r phase in Mo-Re-Si alloys
    Bei, H. and Yang, Y. and Viswanathan, G.B. and Rawn, C.J. and George, E.P. and Tiley, J. and Chang, Y.A.
    Acta Materialia 58 6027-6034 (2010)
    The formation, stability and crystal structure of the σ phase in Mo-Re-Si alloys were investigated. Guided by thermodynamic calculations, six critically selected alloys were arc melted and annealed at 1600 °C for 150 h. Their as-cast and annealed microstructures, including phase fractions and distributions, the compositions of the constituent phases and the crystal structure of the r phase were analyzed by thermodynamic modeling coupled with experimental characterization by scanning electron microscopy, electron probe microanalysis, X-ray diffraction and transmission electron microscopy. Two key findings resulted from this work. One is the large homogeneity range of the r phase region, extending from binary Mo-Re to ternary Mo-Re-Si. The other is the formation of a r phase in Mo-rich alloys either through the peritectic reaction of liquid + Moss → σ or primary solidification. These findings are important in understanding the effects of Re on the microstructure and providing guidance on the design of Mo-Re-Si alloys. © 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2010.07.020
  • 2010 • 23 Highly active iron oxide supported gold catalysts for CO oxidation: How small must the gold nanoparticles be?
    Liu, Y. and Jia, C.-N. and Yamasaki, J. and Terasaki, O. and Schüth, F.
    Angewandte Chemie - International Edition 49 5771-5775 (2010)
    (Figure Presented) The shape of gold: The title catalyst has been prepared through a colloidal deposition method. Scanning transmission electron microscopy studies confirmed that for the catalyst, gold clusters with a bilayer structure and a diameter of about 0.5 nm are not mandatory to achieve the high activity (see image). © 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/anie.201000452
  • 2010 • 22 How dislocation substructures evolve during long-term creep of a 12% Cr tempered martensitic ferritic steel
    Pešička, J. and Aghajani, A. and Somsen, C. and Hartmaier, A. and Eggeler, G.
    Scripta Materialia 62 353-356 (2010)
    We document the evolution of dislocation densities in tempered martensite ferritic steels during long-term aging and creep. Scanning transmission electron microscopy in combination with a high-angle annular dark-field detector is used to study dislocations in a 12% Cr steel. During aging, the dislocation density quickly decreases by a factor 2 and then remains constant. Long-term creep results in an initial decrease by a factor 10, and after this sharp drop, the dislocation density continues to decrease. © 2009 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.scriptamat.2009.10.037
  • 2010 • 21 Identification of magnetic properties of few nm sized FePt crystalline particles by characterizing the intrinsic atom order using aberration corrected S/TEM
    Biskupek, J. and Jinschek, J.R. and Wiedwald, U. and Bendele, M. and Han, L. and Ziemann, P. and Kaiser, U.
    Ultramicroscopy 110 820-825 (2010)
    Hard-magnetic nanomaterials like nanoparticles of FePt are of great interest because of their promising potential for data storage applications. The magnetic properties of FePt structures strongly differ whether the crystal phases are face centered cubic (fcc) or face centered tetragonal (fct). We evaluated aberration corrected HRTEM, electron diffraction and aberration corrected HAADF-STEM as methods to measure the chemical degree of order S that describes the ordering of Pt and Fe atoms within the crystals unit cells. S/TEM experiments are accompanied by image calculations. The findings are compared with results obtained from X-ray diffraction on a FePt film. Our results show that STEM is a reasonable fast approach over HRTEM and electron diffraction to locally determine the chemical degree of order S. © 2010 Elsevier B.V.
    view abstractdoi: 10.1016/j.ultramic.2010.02.043
  • 2010 • 20 In situ TEM study of microplasticity and Bauschinger effect in nanocrystalline metals
    Rajagopalan, J. and Rentenberger, C. and Peter Karnthaler, H. and Dehm, G. and Saif, M.T.A.
    Acta Materialia 58 4772-4782 (2010)
    In situ transmission electron microscopy straining experiments with concurrent macroscopic stress-strain measurements were performed to study the effect of microstructural heterogeneity on the deformation behavior of nanocrystalline metal films. In microstructurally heterogeneous gold films (mean grain size dm = 70 nm) comprising randomly oriented grains, dislocation activity is confined to relatively larger grains, with smaller grains deforming elastically, even at applied strains approaching 1.2%. This extended microplasticity leads to build-up of internal stresses, inducing a large Bauschinger effect during unloading. Microstructurally heterogeneous aluminum films (dm = 140 nm) also show similar behavior. In contrast, microstructurally homogeneous aluminum films comprising mainly two grain families, both favorably oriented for dislocation glide, show limited microplastic deformation and minimal Bauschinger effect despite having a comparable mean grain size (dm = 120 nm). A simple model is proposed to describe these observations. Overall, our results emphasize the need to consider both microstructural size and heterogeneity in modeling the mechanical behavior of nanocrystalline metals. © 2010 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2010.05.013
  • 2010 • 19 Investigation of strain-induced martensitic transformation in metastable austenite using nanoindentation
    Ahn, T.-H. and Oh, C.-S. and Kim, D.H. and Oh, K.H. and Bei, H. and George, E.P. and Han, H.N.
    Scripta Materialia 63 540-543 (2010)
    Strain-induced martensitic transformation of metastable austenite was investigated by nanoindentation of individual austenite grains in multi-phase steel. A cross-section prepared through one of these indented regions using focused ion beam milling was examined by transmission electron microscopy. The presence of martensite underneath the indent indicates that the pop-ins observed on the load-displacement curve during nanoindentation correspond to the onset of strain-induced martensitic transformation. The pop-ins can be understood as resulting from the selection of a favorable martensite variant during nanoindentation. © 2010 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.scriptamat.2010.05.024
  • 2010 • 18 Low-temperature martensitic transformation and deep cryogenic treatment of a tool steel
    Tyshchenko, A.I. and Theisen, W. and Oppenkowski, A. and Siebert, S. and Razumov, O.N. and Skoblik, A.P. and Sirosh, V.A. and Petrov, Y. and Gavriljuk, V.G.
    Materials Science and Engineering A 527 7027-7039 (2010)
    The tool steel X220CrVMo 13-4 (DIN 1.2380) containing (mass%) 2.2C, 13Cr, 4V, 1Mo and the binary alloy Fe-2.03. mass% C were studied using transmission electron microscopy, Mössbauer spectroscopy, X-ray diffraction and internal friction with the aim of shedding light on processes occurring during deep cryogenic treatment. It is shown that the carbon atoms are essentially immobile at temperatures below -50 °C, whereas carbon clustering in the virgin martensite occurs during heating above this temperature. An increase in the density of dislocations, the capture of immobile carbon atoms by moving dislocations, the strain-induced partial dissolution of the carbide phase, and the abnormally low tetragonality of the virgin martensite are found and interpreted in terms of plastic deformation that occurs during martensitic transformation at low temperatures where the virgin martensite is sufficiently ductile. © 2010 Elsevier B.V.
    view abstractdoi: 10.1016/j.msea.2010.07.056
  • 2010 • 17 Magnetic coupling mechanisms in particle/thin film composite systems
    Confalonieri, G.A.B. and Szary, P. and Mishra, D. and Benitez, M.J. and Feyen, M. and Lu, A.H. and Agudo, L. and Eggeler, G. and Petracic, O. and Zabel, H.
    Beilstein Journal of Nanotechnology 1 101-107 (2010)
    Magnetic Γ-Fe 2O 3 nanoparticles with a mean diameter of 20 nm and size distribution of 7% were chemically synthesized and spin-coated on top of a Si-substrate. As a result, the particles self-assembled into a mono layer with hexagonal close-packed order. Subsequently, the nanoparticle array was coated with a Co layer of 20 nm thickness. The magnetic properties of this composite nanopar-ticle/thin film system were investigated by magnetometry and related to high-resolution transmission electron microscopy studies. Herein three systems were compared: i.e. a reference sample with only the particle monolayer, a composite system where the particle array was ion-milled prior to the deposition of a thin Co film on top, and a similar composite system but without ion-milling. The nanoparticle array showed a collective super-spin behavior due to dipolar interparticle coupling. In the composite system, we observed a decoupling into two nanoparticle subsystems. In the ion-milled system, the nanoparticle layer served as a magnetic flux guide as observed by magnetic force microscopy. Moreover, an exchange bias effect was found, which is likely to be due to oxygen exchange between the iron oxide and the Co layer, and thus forming of an antiferromagnetic CoO layer at the Γ-Fe 2O 3/Co interface. © 2010 Confalonieri et al.
    view abstractdoi: 10.3762/bjnano.1.12
  • 2010 • 16 Metal-free and electrocatalytically active nitrogen-doped carbon nanotubes synthesized by coating with polyaniline
    Jin, C. and Nagaiah, T.C. and Xia, W. and Spliethoff, B. and Wang, S. and Bron, M. and Schuhmann, W. and Muhler, M.
    Nanoscale 2 981-987 (2010)
    Nitrogen doping of multi-walled carbon nanotubes (CNTs) was achieved by the carbonization of a polyaniline (PANI) coating. First, the CNTs were partially oxidized with KMnO4 to obtain oxygen-containing functional groups. Depending on the KMnO4 loading, thin layers of birnessite-type MnO2 (10 wt% and 30 wt%) were obtained by subsequent thermal decomposition. CNT-supported MnO2 was then used for the oxidative polymerization of aniline in acidic solution, and the resulting PANI-coated CNTs were finally heated at 550 °C and 850 °C in inert gas. The samples were characterized by transmission electron microscopy and X-ray photoelectron spectroscopy. A thin layer of carbonized PANI was observed on the CNT surface, and the surface nitrogen concentration of samples prepared from 30% MnO 2 was found to amount to 7.6 at% and 3.8 at% after carbonization at 550 °C and 850 °C, respectively. These CNTs with nitrogen-containing shell were further studied by electrochemical impedance spectroscopy and used as catalysts for the oxygen reduction reaction. The sample synthesized from 30 wt% MnO2 followed by carbonization at 850 °C showed the best electrochemical performance indicating efficient nitrogen doping. © 2010 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/b9nr00405j
  • 2010 • 15 Microplasticity phenomena in thermomechanically strained nickel thin films
    Taylor, A.A. and Oh, S.H. and Dehm, G.
    Journal of Materials Science 45 3874-3881 (2010)
    Magnetron sputtered Ni thin films on both oxidised Si (100) and α-Al2O3 (0001) substrates of thickness 150-1000 nm were tested thermomechanically with a wafer curvature system, as well as in situ in a transmission electron microscope. The films on oxidised Si have a {111}-textured columnar microstructure with a mean grain size similar to the film thickness. On (0001) α-Al2O3 a near single crystal epitaxy with two growth variants is achieved leading to a significantly larger grain size. The thermomechanical testing was analysed in terms of the room temperature/high temperature flow stresses in the films and the observed thermoelastic slopes. It was found that the room temperature flow stresses increased with decreasing film thickness until a plateau of ∼1100 MPa was reached for films thinner than 400 nm. This plateau is attributed to the present experiments exerting insufficient thermal strain to induce yielding in these thinner films. At 500 °C the compressive flow stresses of the films show a competition between dislocation and diffusion mediated plasticity. A size effect is also observed in the thermoelastic slopes of the films, with thinner films coming closer to the slope predicted by mismatch in thermal expansion coefficients. It is put forward here that this is due to a highly inhomogeneous stress distribution in the films arising from the grain size distribution. © 2010 Springer Science+Business Media, LLC.
    view abstractdoi: 10.1007/s10853-010-4445-0
  • 2010 • 14 Microstructure and magnetic properties of FeCo/Ti thin film multilayers annealed in nitrogen
    Brunken, H. and Somsen, C. and Savan, A. and Ludwig, Al.
    Thin Solid Films 519 770-774 (2010)
    Multifunctional nanocomposites consisting of at least one ferromagnetic phase (e.g. FeCo) and one protective, wear resistant phase (e.g. TiN) are of interest for applications as sensors or actuators in harsh environments. This paper reports on the fabrication and characterization of nanocomposite thin films, prepared from FeCo/Ti metallic precursor multilayer composition spreads using a combinatorial sputter-deposition system. After deposition, the composition spread was annealed in nitrogen (5 × 10 5 Pa pressure) at 850 °C for 1.5 h, leading to preferential nitriding of Ti to TiN, thus forming the protective phase. Automated energy dispersive X-ray analysis, Auger electron spectroscopy, X-ray diffraction measurements, transmission electron microscopy (TEM) and vibrating sample magnetometry were used for the characterization of the as deposited and nitrided composition spreads. As an unexpected result, the appearance of a Heusler phase (Co 2FeSi) in the nanocomposite was observed by TEM. After N 2 annealing, the nanocomposites show reduced saturation magnetization values μ 0M S between 0.5 and 0.95 T and improved coercive field values μ 0H c between 4 and 13.8 mT, dependent on the TiN content. © 2010 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.tsf.2010.09.008
  • 2010 • 13 Modulation of aggregate size- and shape-distributions of the amyloid-β peptide by a designed β-sheet breaker
    Nagel-Steger, L. and Demeler, B. and Meyer-Zaika, W. and Hochdörffer, K. and Schrader, T. and Willbold, D.
    European Biophysics Journal 39 415-422 (2010)
    A peptide with 42 amino acid residues (Aβ42) plays a key role in the pathogenesis of the Alzheimer's disease. It is highly prone to self aggregation leading to the formation of fibrils which are deposited in amyloid plaques in the brain of diseased individuals. In our study we established a method to analyze the aggregation behavior of the Aβ peptide with a combination of sedimentation velocity centrifugation and enhanced data evaluation software as implemented in the software package UltraScan. Important information which becomes accessible by this methodology is the s-value distribution and concomitantly also the shape-distribution of the Aβ peptide aggregates generated by self-association. With this method we characterized the aggregation modifying effect of a designed bsheet breaker molecule. This compound is built from three head-to-tail connected aminopyrazole moieties and represents a derivative of the already described Tripyrazole. By addition of this compound to a solution of the Aβ42 peptide the maximum of the s-value distribution was clearly shifted to smaller s-values as compared to solutions where only the vehicle DMSO was added. This shift to smaller s-values was stable for at least 7 days. The information about size- and shape-distributions present in aggregated Aβ42 solutions was confirmed by transmission electron microscopy and by measurement of amyloid formation by thioflavin T fluorescence. © European Biophysical Societies' Association 2009.
    view abstractdoi: 10.1007/s00249-009-0416-2
  • 2010 • 12 Molecular recognition of vesicles: Host-guest interactions combined with specific dimerization of zwitterions
    Voskuhl, J. and Fenske, T. and Stuart, M.C.A. and Wibbeling, B. and Schmuck, C. and Ravoo, B.J.
    Chemistry - A European Journal 16 8300-8306 (2010)
    The aggregation of β-cyclodextrin vesicles can be induced by an adamantyl-substituted zwitterionic guanidiniocarbonylpyrrole carboxylate guest molecule (1). Upon addition of 1 to the cyclodextrin vesicles at neutral pH, the vesicles aggregate (but do not fuse), as shown by using UV/Vis and fluorescence spectroscopy, dynamic light scattering, ζ-potential measurements, cryogenic transmission electron microscopy, and atomic force microscopy. Aggregation of the vesicles is in-duced by a twofold supramolecular interaction. First, the adamantyl group of 1 forms an inclusion complex with ßcyclodextrin. Second, at neutral pH the guanidiniocarbonylpyrrole carboxylate zwitterion dimerizes through the formation of hydrogen-bonded ion pairs. Because the dimerization of 1 depends on the zwitterionic protonation state of 1, the aggregation of the cyclodextrin vesicles is also pH dependent; the cyclodextrin vesicles do not interact at pH 5 or 9, at which 1 is either cationic or anionic and, therefore, not self-complementary. These observations are consistent with molecular recognition of the vesicles through a combination of two different supramolecular interactions, that is, host-guest inclusion and dimerization of zwitterions, at the bilayer membrane surface. © 2010 Wiley-VCH Verlag GmbH& Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/chem.201000623
  • 2010 • 11 On the multiplication of dislocations during martensitic transformations in NiTi shape memory alloys
    Simon, T. and Kröger, A. and Somsen, C. and Dlouhy, A. and Eggeler, G.
    Acta Materialia 58 1850-1860 (2010)
    In situ and post-mortem diffraction contrast transmission electron microscopy (TEM) was used to study the multiplication of dislocations during a thermal martensitic forward and reverse transformation in a NiTi shape memory alloy single crystal. An analysis of the elongated dislocation loops which formed during the transformation was performed. It is proposed that the stress field of an approaching martensite needle activates an in-grown dislocation segment and generates characteristic narrow and elongated dislocation loops which expand on {1 1 0}B2 planes parallel to {0 0 1}B19′ compound twin planes. The findings are compared with TEM results reported in the literature for NiTi and other shape memory alloys. It is suggested that the type of dislocation multiplication mechanism documented in the present study is generic and that it can account for the increase in dislocation densities during thermal and stress-induced martensitic transformations in other shape memory alloys. © 2010.
    view abstractdoi: 10.1016/j.actamat.2009.11.028
  • 2010 • 10 Plastic anisotropy of γ-TiAl revealed by axisymmetric indentation
    Zambaldi, C. and Raabe, D.
    Acta Materialia 58 3516-3530 (2010)
    Single crystals of γ-TiAl cannot be grown in the near-stoichiometric compositions that are present inside two-phase γ / α2-microstructures with attractive mechanical properties. Therefore, the single-crystal constitutive behavior of γ-TiAl was studied by nanoindentation experiments in single-phase regions of these γ / α2-microstructures. The experiments were characterized by orientation microscopy and atomic force microscopy to quantify the orientation-dependent mechanical response during nanoindentation. Further, they were analyzed by a three-dimensional crystal plasticity finite element model that incorporated the deformation behavior of γ-TiAl. The spatially resolved activation of competing deformation mechanisms during indentation was used to assess their relative strengths. A convention was defined to unambiguously relate any indentation axis to a crystallographic orientation. Experiments and simulations were combined to study the orientation-dependent surface pile-up. The characteristic pile-up topographies were simulated throughout the unit triangle of γ-TiAl and represented graphically in the newly introduced inverse pole figure of pile-up patterns. Through this approach, easy activation of ordinary dislocation glide in stoichiometric γ-TiAl was confirmed independently from dislocation observation by transmission electron microscopy. © 2010 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2010.02.025
  • 2010 • 9 Rh-RhSx nanoparticles grafted on functionalized carbon nanotubes as catalyst for the oxygen reduction reaction
    Jin, C. and Xia, W. and Nagaiah, T.C. and Guo, J. and Chen, X. and Li, N. and Bron, M. and Schuhmann, W. and Muhler, M.
    Journal of Materials Chemistry 20 736-742 (2010)
    Rhodium-rhodium sulfide nanoparticles supported on multi-walled carbon nanotubes (CNTs) were synthesized via a multi-step colloid route. The CNTs were first exposed to nitric acid to generate oxygen-containing functional groups, and then treated with thionyl chloride to generate acyl chloride groups. The grafting of thiol groups was subsequently carried out by reaction with 4-aminothiophenol. Colloidal rhodium nanoparticles were synthesized using rhodium chloride as metal source, sodium citrate as stabilizer, and sodium borohydride as reducing agent. The immobilization of the generated colloidal rhodium nanoparticles was achieved by adding the thiolated CNTs to the colloidal suspension. All these steps were monitored by X-ray photoelectron spectroscopy, which disclosed the presence of rhodium sulfide, whereas metallic rhodium was detected by X-ray diffraction, suggesting that the nanoparticles probably consist of a metallic Rh core covered by a sulfide layer. Scanning and transmission electron microscopy studies showed that the diameter of the catalyst particles was about 7 nm even at high Rh loadings. Rotating disc electrode measurements and cyclic voltammetry were employed to test the electrocatalytic activity in the oxygen reduction reaction in hydrochloric acid. Among all the synthesized catalysts with different rhodium loadings (4.3-21.9%), the 16.1% rhodium catalyst was found to be the most active catalyst. In comparison to the commercial E-TEK Pt/C catalyst, the 16.1% catalyst displayed a higher electrochemical stability in the highly corrosive electrolyte, as determined by stability tests with frequent current interruptions. © 2010 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/b916192a
  • 2010 • 8 Spatially and size selective synthesis of Fe-based nanoparticles on ordered mesoporous supports as highly active and stable catalysts for ammonia decomposition
    Lu, A.-H. and Nitz, J.-J. and Comotti, M. and Weidenthaler, C. and Schlichte, K. and Lehmann, C.W. and Terasaki, O. and Schüth, F.
    Journal of the American Chemical Society 132 14152-14162 (2010)
    Uniform and highly dispersed γ-Fe 2O 3 nanoparticles with a diameter of ∼6 nm supported on CMK-5 carbons and C/SBA-15 composites were prepared via simple impregnation and thermal treatment. The nanostructures of these materials were characterized by XRD, Mössbauer spectroscopy, XPS, SEM, TEM, and nitrogen sorption. Due to the confinement effect of the mesoporous ordered matrices, γ-Fe 2O 3 nanoparticles were fully immobilized within the channels of the supports. Even at high Fe-loadings (up to about 12 wt %) on CMK-5 carbon no iron species were detected on the external surface of the carbon support by XPS analysis and electron microscopy. Fe 2O 3/CMK-5 showed the highest ammonia decomposition activity of all previously described Fe-based catalysts in this reaction. Complete ammonia decomposition was achieved at 700 °C and space velocities as high as 60 000 cm 3 g cat -1 h -1. At a space velocity of 7500 cm 3 g cat -1 h -1, complete ammonia conversion was maintained at 600 °C for 20 h. After the reaction, the immobilized γ-Fe 2O 3 nanoparticles were found to be converted to much smaller nanoparticles (γ-Fe 2O 3 and a small fraction of nitride), which were still embedded within the carbon matrix. The Fe 2O 3/CMK-5 catalyst is much more active than the benchmark NiO/Al 2O 3 catalyst at high space velocity, due to its highly developed mesoporosity. γ-Fe 2O 3 nanoparticles supported on carbon-silica composites are structurally much more stable over extended periods of time but less active than those supported on carbon. TEM observation reveals that iron-based nanoparticles penetrate through the carbon layer and then are anchored on the silica walls, thus preventing them from moving and sintering. In this way, the stability of the carbon-silica catalyst is improved. Comparison with the silica supported iron oxide catalyst reveals that the presence of a thin layer of carbon is essential for increased catalytic activity. © 2010 American Chemical Society.
    view abstractdoi: 10.1021/ja105308e
  • 2010 • 7 Sphere-on-pillar optical nano-antennas
    Cui, X. and Fan, Z. and Tao, X. and Zhang, W. and Erni, D. and Fan, X. and Zhang, X. and Dong, L.
    2010 IEEE Nanotechnology Materials and Devices Conference, NMDC2010 171-176 (2010)
    We propose an optical nano-antenna consisting of a pair of sphere-on-pillar structures. Experiments show that the controlled fabrication of metallic nanospheres on the tip of carbon nanotubes (CNTs) is effective, and numerical investigation revealed that a pair of such structures are capable to convert free space radiation into an intense near-field; hence can function as an optical antenna. The fabrication process, electron-beam-induced bubbling (EBIB) and electromigration-based bubbling (EMBB), are based on nanofluidic mass delivery at the attogram scale using metal-filled CNTs. Under the irradiation of a high energy electron beam of a transmission electron microscope (TEM), the encapsulated metal is melted and extruded out from the tip of the nanotube; generating a metallic sphere. In the case that the encapsulated materials inside the CNT have a higher melting point than that of the beam energy can reach, electromigration-based mass delivery is an optional process to apply. Under a low bias (2-2.5V), spherical nanoparticles are formed on the tips of nanotubes. The optical properties of the nano-antenna are analyzed numerically using the finite element method. Our investigations have revealed that the field enhancement, the resonances, and the radiation patterns can be easily tuned since all these quantities strongly depend on the size of the nanotubes and the metallic spheres, as well as on their material properties. Sphere-on-pillar optical antennas carry a great potential for bio-sensing, tip-enhanced spectroscopy applications, and interfacing integrated nanophotonic circuits. © 2010 IEEE.
    view abstractdoi: 10.1109/NMDC.2010.5652241
  • 2010 • 6 Suppression of Ni4Ti3 precipitation by grain size refinement in Ni-rich NiTi shape memory alloys
    Prokofiev, E.A. and Burow, J.A. and Payton, E.J. and Zarnetta, R. and Frenzel, J. and Gunderov, D.V. and Valiev, R.Z. and Eggeler, G.
    Advanced Engineering Materials 12 747-753 (2010)
    Severe plastic deformation (SPD) processes, such as equal channel angular pressing (ECAP) and high pressure torsion (HPT), are successfully employed to produce ultra fine grain (UFG) and nanocrystalline (NC) microstructures in a Ti-50.7 at% Ni shape memory alloy. The effect of grain size on subsequent Ni-rich particle precipitation during annealing is investigated by transmission electron microscopy (TEM), selected area electron diffraction (SAD, SAED), and X-ray diffraction (XRD). It is observed that Ni4Ti3 precipitation is suppressed in grains of cross-sectional equivalent diameter below approximately 150 nm, and that particle coarsening is inhibited by very fine grain sizes. The results suggest that fine grain sizes impede precipitation processes by disrupting the formation of selfaccommodating particle arrays and that the arrays locally compensate for coherency strains during nucleation and growth. © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/adem.201000101
  • 2010 • 5 Synthesis and characterization of ZnO nanowires for nanosensor applications
    Lupan, O. and Emelchenko, G.A. and Ursaki, V.V. and Chai, G. and Redkin, A.N. and Gruzintsev, A.N. and Tiginyanu, I.M. and Chow, L. and Ono, L.K. and Roldan Cuenya, B. and Heinrich, H. and Yakimov, E.E.
    Materials Research Bulletin 45 1026-1032 (2010)
    In this paper we report the synthesis of ZnO nanowires via chemical vapor deposition (CVD) at 650 °C. It will be shown that these nanowires are suitable for sensing applications. ZnO nanowires were grown with diameters ranging from 50 to 200 nm depending on the substrate position in a CVD synthesis reactor and the growth regimes. X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), photoluminescence (PL), and Raman spectroscopy (RS) have been used to characterize the ZnO nanowires. To investigate the suitability of the CVD synthesized ZnO nanowires for gas sensing applications, a single ZnO nanowire device (50 nm in diameter) was fabricated using a focused ion beam (FIB). The response to H2 of a gas nanosensor based on an individual ZnO nanowire is also reported. © 2010 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.materresbull.2010.03.027
  • 2010 • 4 The ferromagnetic shape memory system Fe-Pd-Cu
    Hamann, S. and Gruner, M.E. and Irsen, S. and Buschbeck, J. and Bechtold, C. and Kock, I. and Mayr, S.G. and Savan, A. and Thienhaus, S. and Quandt, E. and Fähler, S. and Entel, P. and Ludwig, Al.
    Acta Materialia 58 5949-5961 (2010)
    A new ferromagnetic shape memory thin film system, Fe-Pd-Cu, was developed using ab initio calculations, combinatorial fabrication and high-throughput experimentation methods. Reversible martensitic transformations are found in extended compositional regions, which have increased fcc-fct transformation temperatures in comparison to previously published results. High resolution transmission electron microscopy verified the existence of a homogeneous ternary phase without precipitates. Curie temperature, saturation polarization and orbital magnetism are only moderately decreased by alloying with nonmagnetic Cu. Compared to the binary system; enhanced Invar-type thermal expansion anomalies in terms of an increased volume magnetostriction are predicted. Complementary experiments on splat-fabricated bulk Fe-Pd-Cu samples showed an enhanced stability of the disordered transforming Fe70Pd30 phase against decomposition. From the comparison of bulk and thin film results, it can be inferred that, for ternary systems, the Fe content, rather than the valence electron concentration, should be regarded as the decisive factor determining the fcc-fct transformation temperature. © 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2010.07.011
  • 2010 • 3 Therapeutic window for bioactive nanocomposites fabricated by laser ablation in polymer-doped organic liquids
    Hahn, A. and Stöver, T. and Paasche, G. and Löbler, M. and Sternberg, K. and Rohm, H. and Barcikowski, S.
    Advanced Engineering Materials 12 B156-B162 (2010)
    Polymeric nanomaterials are gaining increased interest in medical applications due to the sustained release ofbioactive agents. Within this study nanomaterials are fabricated using laser ablation of silver and copper in polymer-doped organic liquids thus allowing to produce customized drug release systems. A strategy is shown to determine the therapeutic window for cells relevant for cochlear implant electrodes, defined by the viability of L929 fibroblasts, PC12 neuronal cells, and spiral ganglion cells on different concentrations ofsilver and copper ions. The distribution ofnanoparticles within the silicone polymer matrix is determined using transmission electron microscopy. Hexane doped with 1% silicone resin is found to be an appropriate liquid matrix to fabricate a nanocomposite with a constant ion release rate. Silver ions of 10 μmol L-1 or copper ions of 100 μmol L -1 cause a suppression of tissue growth without inhibiting neuronal cell growth. The copper nanoparticle content of 0.1 wt% of the silicone composite releases ion concentrations which fit the therapeutic window. © 2010 Wiley-VCH Verlag GmbH & Co. KGaA.
    view abstractdoi: 10.1002/adem.200980071
  • 2010 • 2 Very low temperature CO oxidation over colloidally deposited gold nanoparticles on Mg(OH)2 and MgO
    Jia, C.-N. and Liu, Y. and Bongard, H. and Schüth, F.
    Journal of the American Chemical Society 132 1520-1522 (2010)
    (Figure Presented) The colloidal deposition method was used to prepare Au/Mg(OH)2 (0.7 wt % gold) catalysts with gold particle sizes between 1.5 to 5 nm which exhibited very high activity for CO oxidation with specific rates higher than 3.7 molCO·h-1·g Au-1 even at temperatures as low as -89° C. © 2010 American Chemical Society.
    view abstractdoi: 10.1021/ja909351e
  • 2010 • 1 Wear mechanisms in metal-on-metal bearings: The importance of tribochemical reaction layers
    Wimmer, M.A. and Fischer, A. and Büscher, R. and Pourzal, R. and Sprecher, C. and Hauert, R. and Jacobs, J.J.
    Journal of Orthopaedic Research 28 436-443 (2010)
    Metal-on-metal (MoM) bearings are at the forefront in hip resurfacing arthroplasty. Because of their good wear characteristics and design flexibility, MoM bearings are gaining wider acceptance with market share reaching nearly 10% worldwide. However, concerns remain regarding potential detrimental effects of metal particulates and ion release. Growing evidence is emerging that the local cell response is related to the amount of debris generated by these bearing couples. Thus, an urgent clinical need exists to delineate the mechanisms of debris generation to further reduce wear and its adverse effects. In this study, we investigated the microstructural and chemical composition of the tribochemical reaction layers forming at the contacting surfaces of metallic bearings during sliding motion. Using X-ray photoelectron spectroscopy and transmission electron microscopy with coupled energy dispersive X-ray and electron energy loss spectroscopy, we found that the tribolayers are nanocrystalline in structure, and that they incorporate organic material stemming from the synovial fluid. This process, which has been termed "mechanical mixing," changes the bearing surface of the uppermost 50 to 200 nm from pure metallic to an organic composite material. It hinders direct metal contact (thus preventing adhesion) and limits wear. This novel finding of a mechanically mixed zone of nanocrystalline metal and organic constituents provides the basis for understanding particle release and may help in identifying new strategies to reduce MoM wear. © 2009 Orthopaedic Research Society. Published by Wiley Periodicals, Inc.
    view abstractdoi: 10.1002/jor.21020