Scientific Output

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

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  • 2021 • 230 Surface metal matrix nano-composite of magnesium/hydroxyapatite produced by stir-centrifugal casting
    Khalili, V. and Moslemi, S. and Ruttert, B. and Frenzel, J. and Theisen, W. and Eggeler, G.
    Surface and Coatings Technology 406 (2021)
    The present study aims to investigate a liquid state method of stir and centrifugal casting as an in-situ and cost-attractive processing technology for the production of magnesium/hydroxyapatite surface metal matrix nano-composites (Mg/HA surface metal matrix nano-composite). The main contribution of this study is to design the best condition for achieving a uniform Mg/HA surface nano-composite as a potential bone implant. It was shown how casting parameters and the distribution of hydroxyapatite affect mechanical properties of nano-composites measured using nano-indentation, nano-scratch, and compression tests. Response surface method in Design Expert software was used to predict the best model and the optimum condition of casting based on the experimentally measured data. The surface metal matrix nano-composites, consisting of a magnesium matrix with different amounts of nano-sized hydroxyapatite and silicon-doped hydroxyapatite (0.75-3 wt%) particles, were prepared. Hot isostatic pressing was used to homogenize the nano-composites in terms of particle distribution and to reduce porosity. It was shown that the weight percent of hydroxyapatite reinforcement is the parameter which is best suited to tailor targeted strength values. The target values of maximum compression strength (187 MPa) and elastic modulus (33 GPa) were achieved with a combination of the following parameters: 1.83 wt% hydroxyapatite, 800 rpm mold rotational speed, and a propeller rotational time of 6.3 min. A specimen prepared under these conditions had a homogeneous distribution of nano-hydroxyapatite in magnesium metal matrix after hot isostatic pressing at 450 °C and 100 MPa for a holding time of 120 min. It indicated the best mechanical resistance in terms of hardness and material loss during the nano-scratch testing. Moreover, the XRD results show that there is no considerable chemical reaction between the reinforcement particles of n-HA and Mg metallic matrix during casting at 700 °C and thermo-mechanical treatment of HIP at 450 °C. © 2020
    view abstractdoi: 10.1016/j.surfcoat.2020.126654
  • 2021 • 229 Identifying the Bottleneck for Heat Transport in Metal–Organic Frameworks
    Wieser, S. and Kamencek, T. and Dürholt, J.P. and Schmid, R. and Bedoya-Martínez, N. and Zojer, E.
    Advanced Theory and Simulations 4 (2021)
    Controlling the transport of thermal energy is key to most applications of metal–organic frameworks (MOFs). Analyzing the evolution of the effective local temperature, the interfaces between the metal nodes and the organic linkers are identified as the primary bottlenecks for heat conduction. Consequently, changing the bonding strength at that node–linker interface and the mass of the metal atoms can be exploited to tune the thermal conductivity. This insight is generated employing molecular dynamics simulations in conjunction with advanced, ab initio parameterized force fields. The focus of the present study is on MOF-5 as a prototypical example of an isoreticular MOF. However, the key findings prevail for different node structures and node–linker bonding chemistries. The presented results lay the foundation for developing detailed structure-to-property relationships for thermal transport in MOFs with the goal of devising strategies for the application-specific optimization of heat conduction. © 2020 The Authors. Advanced Theory and Simulations published by Wiley-VCH GmbH
    view abstractdoi: 10.1002/adts.202000211
  • 2020 • 228 Atomic Scale Origin of Metal Ion Release from Hip Implant Taper Junctions
    Balachandran, S. and Zachariah, Z. and Fischer, A. and Mayweg, D. and Wimmer, M.A. and Raabe, D. and Herbig, M.
    Advanced Science 7 (2020)
    Millions worldwide suffer from arthritis of the hips, and total hip replacement is a clinically successful treatment for end-stage arthritis patients. Typical hip implants incorporate a cobalt alloy (Co–Cr–Mo) femoral head fixed on a titanium alloy (Ti-6Al-4V) femoral stem via a Morse taper junction. However, fretting and corrosion at this junction can cause release of wear particles and metal ions from the metallic implant, leading to local and systemic toxicity in patients. This study is a multiscale structural-chemical investigation, ranging from the micrometer down to the atomic scale, of the underlying mechanisms leading to metal ion release from such taper junctions. Correlative transmission electron microscopy and atom probe tomography reveals microstructural and compositional alterations in the subsurface of the titanium alloy subjected to in vitro gross-slip fretting against the cobalt alloy. Even though the cobalt alloy is comparatively more wear-resistant, changes in the titanium alloy promote tribocorrosion and subsequent degradation of the cobalt alloy. These observations regarding the concurrent occurrence of electrochemical and tribological phenomena are vital to further improve the design and performance of taper junctions in similar environments. © 2020 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/advs.201903008
  • 2020 • 227 Static and oscillation superimposed ring compression tests with structured and coated tools for Sheet-Bulk Metal Forming
    Behrens, B.-A. and Meijer, A. and Stangier, D. and Hübner, S. and Biermann, D. and Tillmann, W. and Rosenbusch, D. and Müller, P.
    Journal of Manufacturing Processes 55 78-86 (2020)
    Forming tools with tailored surfaces or functional surface modifications offer great potential for the adaption and optimization of forming processes. However, the interaction of the resulting tribological conditions with additional process oscillations and lubricants has not yet been sufficiently investigated. In the field of sheet-bulk metal forming the superimposition of oscillations is a new, highly promising approach for optimizing the forming of metallic materials. The aim of this study is therefore to investigate the forming behavior of metallic materials with an oscillation superimposition in combination with structured tool surfaces. In order to examine to what extend the friction factor and the forming force can be influenced by structured surfaces and PVD-coatings ring compression tests were conducted to re-create the real process conditions. The experiments were carried out statically and with an oscillation superimposition in the main force flow of the machine under lubrication and dry conditions. Occurring interactions between surface applications, lubrication and oscillation superimposition are identified and analyzed within the context of this work. Thereby, three different deterministic surface structures of the ring compression tools were considered. A radial and tangential arrangement of a wave-like structure as well as an isotropic honeycomb structure were applied on the forming tools by means of micromilling. For a lubricated and vibration superimposed process, especially the isotropic honeycomb structure caused a significant decrease in friction. This was attributed to the formation of lubrication pockets. Despite the reduced friction, in some cases no reduction of the forming force occurred by using an oscillation superimposed forming process. This behavior was attributed to damping effects caused by the lubricating pockets. © 2020 The Society of Manufacturing Engineers
    view abstractdoi: 10.1016/j.jmapro.2020.04.007
  • 2020 • 226 Polymer nanocomposite ultrafiltration membranes: The influence of polymeric additive, dispersion quality and particle modification on the integration of zinc oxide nanoparticles into polyvinylidene difluoride membranes
    Berg, T.D. and Ulbricht, M.
    Membranes 10 1-19 (2020)
    This study aims to improve the understanding of the influence of metal oxide nanofillers on polyvinylidene difluoride (PVDF) ultrafiltration membranes. Zinc oxide nanoparticles were chosen as the model filler material. The membranes were prepared by non-solvent induced phase separation from PVDF solutions in N-methylpyrrolidone. The influences of the addition of polyvinylpyrrolidone (PVP), the nanoparticle dispersion quality, and a surface modification of the ZnO particles with PVP on the nanofiller integration into the polymer matrix and the resulting membrane separation performance, were evaluated. Unmodified and PVP-modified nanoparticles were characterized by evaluation of their Hansen solubility parameters. The membranes were characterized by ultrafiltration experiments, scanning electron microscopy (SEM) and with respect to mechanical properties, while the dope solutions were analyzed by rheology in order to judge about dispersion quality. Pure water permeability and solute rejection data revealed that the dominant effect of the addition of pristine ZnO nanoparticles was a major decrease in permeability caused by pore blocking. In SEM analyses, it was seen that the plain nanofiller did not integrate well into the polymer matrix. Importantly, it was found that the surface modification of the nanofiller, as well as a high dispersion quality, can be strategically used to enhance the integration of the nanofiller and thus suppress pore blocking, leading to membranes with high ultrafiltration rejection and permeability simultaneously. Overall, the study provides relevant insights into a new approach to integrating nanofillers into polymer nanocomposite membranes for improving their properties and performance. © 2020 by the authors. Licensee MDPI, Basel, Switzerland.
    view abstractdoi: 10.3390/membranes10090197
  • 2020 • 225 The introduction of functional side groups and the application of the mixed-linker concept in divalent MIL-53(Ni) materials
    Bitzer, J. and Titze-Alonso, A. and Roshdy, A. and Kleist, W.
    Dalton Transactions 49 9148-9154 (2020)
    Metal-organic framework materials with functional side groups are commonly used for various purposes like post-synthetic modification reactions or the tuning of pore geometries. Additionally, the mixed-linker concept, in which different linkers are statistically distributed over equivalent lattice positions, is a versatile approach to adjust the number of functional groups within the framework. However, neither functional side groups nor the mixed-linker approach have been used in MIL-53 materials with divalent metal ions yet. In the present work, we report on the synthesis of MIL-53(Ni)-Br(100), which contains only 2-bromoterephthalate as a linker molecule, and mixed-linker MIL-53(Ni)-Br(50) containing both terephthalate and 2-bromoterephthalate. These two materials represent the first functionalized and mixed-linker MIL-53 materials in combination with a divalent metal. Consequently, the possibilities to tailor the properties of the still rarely used divalent MIL-53 materials were expanded. © 2020 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/d0dt01222j
  • 2020 • 224 Evaluation of molecular orbital symmetry via oxygen-induced charge transfer quenching at a metal-organic interface
    Cojocariu, I. and Sturmeit, H.M. and Zamborlini, G. and Cossaro, A. and Verdini, A. and Floreano, L. and D'Incecco, E. and Stredansky, M. and Vesselli, E. and Jugovac, M. and Cinchetti, M. and Feyer, V. and Schneider, C.M.
    Applied Surface Science 504 (2020)
    Thin molecular films under model conditions are often exploited as benchmarks and case studies to investigate the electronic and structural changes occurring on the surface of metallic electrodes. Here we show that the modification of a metallic surface induced by oxygen adsorption allows the preservation of the geometry of a molecular adlayer, giving access to the determination of molecular orbital symmetries by means of near-edge X-ray absorption fine structure spectroscopy, NEXAFS. As a prototypical example, we exploited nickel tetraphenylporphyrin molecules deposited on a bare and on an oxygen pre-covered Cu(1 0 0) surface. We find that adsorbed atomic oxygen quenches the charge transfer at the metal-organic interface but, in contrast to a thin film sample, maintains the ordered adsorption geometry of the organic molecules. In this way, it is possible to disentangle π* and σ* symmetry orbitals, hence estimate the relative oscillator strength of core level transitions directly from the experimental data, as well as to evaluate and localize the degree of charge transfer in a coupled system. In particular, we neatly single out the σ* contribution associated with the N 1s transition to the mixed N 2px,y-Ni 3dx 2 -y 2 orbital, which falls close to the leading π*-symmetry LUMO resonance. © 2019
    view abstractdoi: 10.1016/j.apsusc.2019.144343
  • 2020 • 223 Fabrication of a novel and highly selective ion-imprinted PES-based porous adsorber membrane for the removal of mercury(II) from water
    Esmali, F. and Mansourpanah, Y. and Farhadi, K. and Amani, S. and Rasoulifard, A. and Ulbricht, M.
    Separation and Purification Technology 250 (2020)
    Herein, poly(ether sulfone) based ion imprinted membranes (IIM) were prepared through phase inversion, using ion imprinted polymer (IIP) particles obtained by radical copolymerization of acrylamide, acrylonitrile and ethyleneglycoldimethacrylate along with a template of Hg(II) complexed with bathophenanthroline (BPh). Optimization of the ability for Hg(II) removal from water and pure water flux of the IIM were investigated through Central Composite Design (CCD) combined with Response Surface Methodology (RSM). Accordingly, the optimized factors were obtained as IIP percentage of 2.5 wt% used in membrane preparation, as well as trans-membrane pressure of 0.19 bar, pH 7.95 and Hg(II) concentration of 4 ppm during filtration through the membrane. Using the optimum parameters, the removal percentage and flux of IIM were about 98.1% and 37.5 kg/m2 h, respectively. The maximum adsorption capacity of IIM was 432 mg/m2 (or 21.6 mg/g), almost four times higher than that of non-imprinted membrane (NIM; 105 mg/m2) (or 5.25 mg/g) which was prepared using copolymer particles prepared without the Hg(II) template. The IIM showed a high selectivity toward Hg(II) ions compared to other metal ions and could be effectively recycled for at least 6 times without any major loss of adsorption capacity. The synthesized imprinted membranes have demonstrated considerable potentials to selectively separate mercury(II) from simulated industrial wastewater. © 2020 Elsevier B.V.
    view abstractdoi: 10.1016/j.seppur.2020.117183
  • 2020 • 222 Electronic structure based design of thin film metallic glasses with superior fracture toughness
    Evertz, S. and Kirchlechner, I. and Soler, R. and Kirchlechner, C. and Kontis, P. and Bednarcik, J. and Gault, B. and Dehm, G. and Raabe, D. and Schneider, J.M.
    Materials and Design 186 (2020)
    High fracture toughness is crucial for the application of metallic glasses as structural materials to avoid catastrophic failure of the material in a brittle manner. One fingerprint for fracture toughness in metallic glasses is the fraction of hybridized bonds, which is affected by alloying Pd57.4Al23.5Y7.8M11.3 with M = Fe, Ni, Co, Cu, Os, Ir, Pt, and Au. It is shown that experimental fracture toughness data is correlated to the fraction of hybridized bonds which scale with the localized bonds at the Fermi level. Thus, the localized bonds at the Fermi level are utilized quantitatively as a measure for fracture toughness. Based on ab initio calculations, the minimum fraction of hybridized bonds was identified for Pd57.4Al23.5Y7.8Ni11.3. According to the ansatz that the crystal orbital overlap population at the Fermi level scales with fracture toughness, for Pd57.4Al23.5Y7.8Ni11.3 a value of around 95 ± 20 MPa·m0.5 is predicted quantitatively for the first time. Consistent with this prediction, in micro-mechanical beam bending experiments Pd57.4Al23.5Y7.8Ni11.3 thin films show pronounced plasticity and absence of crack growth. © 2018 The Authors
    view abstractdoi: 10.1016/j.matdes.2019.108327
  • 2020 • 221 Multi-Stimuli-Responsive Supramolecular Polymers Based on Noncovalent and Dynamic Covalent Bonds
    Hatai, J. and Hirschhäuser, C. and Niemeyer, J. and Schmuck, C.
    ACS Applied Materials and Interfaces 12 2107-2115 (2020)
    Several modes of supramolecular assembly relying on noncovalent as well as dynamic covalent interactions were combined in a single molecule. The supramolecular self-assembly of 1 can be controlled by three stimuli, namely light, pH, and addition of metal ions, in both organic and aqueous media. The multi-stimuli-responsive nature of 1 was used successfully for the controlled encapsulation and on-demand release of hydrophobic molecules, such as dyes and drugs. © 2019 American Chemical Society.
    view abstractdoi: 10.1021/acsami.9b19279
  • 2020 • 220 Structural and photoelectrochemical properties in the thin film system Cu-Fe-V-O and its ternary subsystems Fe-V-O and Cu-V-O
    Kumari, S. and Junqueira, J.R.C. and Sarker, S. and Mehta, A. and Schuhmann, W. and Ludwig, Al.
    Journal of Chemical Physics 153 (2020)
    Thin-film material libraries in the ternary and quaternary metal oxide systems Fe-V-O, Cu-V-O, and Cu-Fe-V-O were synthesized using combinatorial reactive co-sputtering with subsequent annealing in air. Their compositional, structural, and functional properties were assessed using high-throughput characterization methods. Prior to the investigation of the quaternary system Cu-Fe-V-O, the compositions (Fe61V39)Ox and (Cu52V48)Ox with promising photoactivity were identified from their ternary subsystems Fe-V-O and Cu-V-O, respectively. Two Cu-Fe-V-O material libraries with (Cu29-72Fe4-27V22-57)Ox and (Cu11-55Fe27-73V12-34)Ox composition spread were investigated. Seven mixed ternary and quaternary phase regions were identified: I (α-Cu3FeV6O26/FeVO4), II (Cu5V2O10/FeVO4/α-Cu3Fe4V6O26), III (Cu5V2O10), IV (Cu5V2O10/FeVO4, V (FeVO4/γ-Cu2V2O7/α-Cu3Fe4V6O26), VI (β-Cu2V2O7/α-Cu3Fe4V6O26/FeVO4), and VII (β-Cu3Fe4V6O26/FeVO4). In the investigated composition range, two photoactive regions, (Cu53Fe7V40)Ox and (Cu45Fe21V34)Ox, were identified, exhibiting 103 μA/cm2 and 108 μA/cm2 photocurrent density for the oxygen evolution reaction at 1.63 V vs reversible hydrogen electrode, respectively. The highest photoactive region (Cu45Fe21V34)Ox comprises the dominant α-Cu3Fe4V6O24 phase and minor FeVO4 phase. This photoactive region corresponds to having an indirect bandgap of 1.87 eV and a direct bandgap of 2.58 eV with an incident photon-to-current efficiency of 30% at a wavelength of 310 nm. © 2020 Author(s).
    view abstractdoi: 10.1063/5.0009512
  • 2020 • 219 Detection Method for Liquid Metal Embrittlement Cracks Inside the Intermediate Sheet Zone of Dissimilar Resistance Spot Welds
    Lindner, S. and Deike, R.
    Steel Research International 91 (2020)
    Liquid metal embrittlement (LME) is a phenomenon where a liquid metal damages a solid bulk metal. Notwithstanding previous investigations, LME remains an actual as well as a still not fully understood topic. A so far not yet investigated area can be defined with ultra-high strength austenitic stainless steels for passenger cars. The most commonly used joining procedure in car body engineering is still resistance spot welding. During this vehicle assembly step, the uncoated surface of an austenitic stainless steel is because of the process-related lap joint configuration in direct contact with zinc-coated surface-finished steels as a dissimilar material combination. During welding, liquid zinc could penetrate inside the intermediate sheet zone in the grain boundaries of the austenitic steel and is therefore able to initiate cracks in the heat-affected welding zone. Herein, the radiographic inspection is introduced, which is a process-reliable, nondestructive detection method for the inaccessible intermediate sheet zone area, which is usable in automotive big-industrial scale. With the implemented detection method, liquid metal-induced cracks can be detected in the intermediate sheet zone down to a crack length of 50 μm. Subsequently, the radiographic inspection helps to analyze the crack characteristic depending on surrounding conditions and welding parameters. © 2020 Wiley-VCH GmbH
    view abstractdoi: 10.1002/srin.202000044
  • 2020 • 218 Electron Irradiation of Metal Contacts in Monolayer MoS2Field-Effect Transistors
    Pelella, A. and Kharsah, O. and Grillo, A. and Urban, F. and Passacantando, M. and Giubileo, F. and Iemmo, L. and Sleziona, S. and Pollmann, E. and Madauß, L. and Schleberger, M. and Di Bartolomeo, A.
    ACS Applied Materials and Interfaces 12 40532-40540 (2020)
    Metal contacts play a fundamental role in nanoscale devices. In this work, Schottky metal contacts in monolayer molybdenum disulfide (MoS2) field-effect transistors are investigated under electron beam irradiation. It is shown that the exposure of Ti/Au source/drain electrodes to an electron beam reduces the contact resistance and improves the transistor performance. The electron beam conditioning of contacts is permanent, while the irradiation of the channel can produce transient effects. It is demonstrated that irradiation lowers the Schottky barrier at the contacts because of thermally induced atom diffusion and interfacial reactions. The simulation of electron paths in the device reveals that most of the beam energy is absorbed in the metal contacts. The study demonstrates that electron beam irradiation can be effectively used for contact improvement through local annealing. Copyright © 2020 American Chemical Society.
    view abstractdoi: 10.1021/acsami.0c11933
  • 2020 • 217 Orientation-dependent deformation behavior of 316L steel manufactured by laser metal deposition and casting under local scratch and indentation load
    Pöhl, F. and Hardes, C. and Scholz, F. and Frenzel, J.
    Materials 13 (2020)
    This study analyzes the local deformation behavior of austenitic stainless steel 316L, manufactured conventionally by casting and additively by laser metal deposition (LMD). We produced directionally solidified 316L specimens with most grains showing (001) orientations parallel to the longitudinal specimen axis. We conducted nanoindentation and scratch experiments for local mechanical characterization and topography measurements (atomic force microscopy and confocal laser scanning microscopy) of indentation imprints and residual scratch grooves for the analysis of the deformation behavior and, in particular, of the pile-up behavior. The local mechanical properties and deformation behavior were correlated to the local microstructure investigated by scanning electron microscopy with energy dispersive X-ray spectroscopy and electron backscatter diffraction analysis. The results show that the local mechanical properties, deformation behavior, and scratch resistance strongly depend on the crystallographic orientation. Nearly (001)-oriented grains parallel to the surface show the lowest hardness, followed by an increasing hardness of nearly (101)-and (111)-oriented grains. Consequently, scratch depth is the greatest for nearly (001)-oriented grains followed by (101) and (111) orientations. This tendency is seen independently of the analyzed manufacturing route, namely Bridgman solidification and laser metal deposition. In general, the laser metal deposition process leads to a higher strength and hardness, which is mainly attributed to a higher dislocation density. Under the investigated loading conditions, the cellular segregation substructure is not found to significantly and directly change the local deformation behavior during indentation and scratch testing. © 2020 by the authors.
    view abstractdoi: 10.3390/MA13071765
  • 2020 • 216 A compact and powerful EMAT design for contactless detection of inhomogeneities inside the liquid volume of metallic tanks Ein einfaches und leistungsstarkes EMAT-Design für die kontaktlose Detektion von Inhomogenitäten in metallischen Flüssigkeitsbehältern
    Rieger, K. and Erni, D. and Rueter, D.
    Technisches Messen 87 349-359 (2020)
    A simple and powerful design of an electromagnetic acoustic transducer (EMAT) without bulky permanent magnets is presented. The EMAT is operated in a pulse echo modality and generates longitudinal ultrasound at about 1 MHz. Unlike shear waves, these longitudinal ultrasound pulses can propagate in liquids. The generally addressed application scenario is the examination of a liquid volume inside a metallic container or tank, e. g., the detection of inhomogeneities within the liquid. The herein proposed EMAT operates for virtually all metallic containers, i. e., it succeeds for container walls made of aluminum or ferromagnetic steel, and even for non-ferromagnetic (stainless) steel. Moreover, unlike piezo transducers, EMAT techniques allow for a noncontacting ultrasound transduction: the air gap between the EMAT sensor coil and the tank s metallic surface extends up to 2 mm. Even with this relatively large air gap, the biasing magnetic field approaches a flux density of 3.2 T at the surface, more than what is possible to achieve with the permanent magnets of conventional and bulkier EMATs. Strong fields improve the coupling efficiency of the principally low-efficiency EMAT mechanism, which is important for both ultrasound transmission and reception. For that superior field intensity, a unipolar current pulse of up to 3.6 kA is applied through the thin windings (0.5 mm) of the EMAT coil. This paper presents a novel solid-state EMAT circuitry for such strong currents and MHz pulsed voltages >1 kV. As a particularly delicate task, the powerful circuitry must also detect the rather weak echo signals in the V range. A very short recovery time is required after such a strong emission burst. The discussed circuitry consists of three unipolar high-current modules, which can each be independently launched. This allows for received echo signals that can be timed independently, e. g., objects deep inside the liquid tank can be specifically addressed. In general, this work concentrates on the novel circuitry in parallel connection, the general pulse-echo functionality and the magnetic fields. A detailed analysis and shaping of the ultrasonic fields through different EMAT coil geometries would exceed the scope of this contribution and is to be reported separately. © 2020 De Gruyter Oldenbourg. All rights reserved.
    view abstractdoi: 10.1515/teme-2019-0124
  • 2020 • 215 Influence of the cathode microstructure on the stability of inverted planar perovskite solar cells
    Sirotinskaya, S. and Schmechel, R. and Benson, N.
    RSC Advances 10 23653-23661 (2020)
    One of the main challenges for perovskite solar cells (PSC) is their environmental stability, as oxygen and water induced aging may result in mobile decomposition compounds, which can enhance the recombination rate and react with charge carrier extraction layers or the contact metallization. In this contribution the importance of the microstructure of the contact metallization on the environmental cell stability is investigated. For this purpose, the storage stability of inverted planar methylammonium lead iodide (MAPI)-based perovskite solar cells without encapsulation is tested, using the metals aluminum (Al), silver (Ag), gold (Au) and nickel (Ni) as representative cathode materials. For this study, scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX) analysis of the different electrodes as well as the perovskite is correlated with PSC device current-voltage (J-V) and impedance measurements. Our findings substantiate that the metal microstructure has a significant influence on the PSC aging properties. While a strong perovskite decomposition and iodide diffusion to the contacts were detected for devices using Al, Ag or Au cathodes with a polycrystalline microstructure, these effects were strongly reduced when Ni metallization was employed, where a nanocrystalline microstructure was exhibited under the chosen process conditions. This journal is © The Royal Society of Chemistry.
    view abstractdoi: 10.1039/d0ra00195c
  • 2020 • 214 Atomistic description of self-diffusion in molybdenum: A comparative theoretical study of non-Arrhenius behavior
    Smirnova, D. and Starikov, S. and Leines, G.D. and Liang, Y. and Wang, N. and Popov, M.N. and Abrikosov, I.A. and Sangiovanni, D.G. and Drautz, R. and Mrovec, M.
    Physical Review Materials 4 (2020)
    According to experimental observations, the temperature dependence of self-diffusion coefficient in most body-centered cubic metals (bcc) exhibits non-Arrhenius behavior. The origin of this behavior is likely related to anharmonic vibrational effects at elevated temperatures. However, it is still debated whether anharmonicity affects more the formation or migration of monovacancies, which are known to govern the self-diffusion. In this extensive atomistic simulation study we investigated thermodynamic properties of monovacancies in bcc molybdenum, here taken as a representative model system, from zero temperature to the melting point. We combined first-principles calculations and classical simulations based on three widely used interatomic potentials for Mo. In our analysis we employ static and dynamic atomistic calculations as well as statistical sampling techniques and thermodynamic integration to achieve thorough information about temperature variations of vacancy formation and migration free energies and diffusivities. In addition, we carry out large-scale molecular dynamics simulations that enable direct observation of high-temperature self-diffusion at the atomic scale. By scrutinizing the results obtained by different models and methods, we conclude that the peculiar self-diffusion behavior is likely caused by strong temperature dependence of the vacancy formation energy. © 2020 American Physical Society.
    view abstractdoi: 10.1103/PhysRevMaterials.4.013605
  • 2020 • 213 Machine learning for metallurgy II. A neural-network potential for magnesium
    Stricker, M. and Yin, B. and Mak, E. and Curtin, W.A.
    Physical Review Materials 4 (2020)
    Interatomic potentials are essential for studying fundamental mechanisms of deformation and failure in metals and alloys because the relevant defects (dislocations, cracks, etc.) are far above the scales accessible to first-principles studies. Existing potentials for non-fcc metals and nearly all alloys are, however, not sufficiently quantitative for many crucial phenomena. Here machine learning in the Behler-Parrinello neural-network framework is used to create a broadly applicable potential for pure hcp magnesium (Mg). Lightweight Mg and its alloys are technologically important while presenting a diverse range of slip systems and crystal surfaces relevant to both plasticity and fracture that present a significant challenge for any potential. The machine learning potential is trained on first-principles density-functional theory (DFT) computable metallurgically relevant properties and is then shown to well predict metallurgically crucial dislocation and crack structures and competing phenomena. Extensive comparisons to an existing very good modified embedded atom method potential are made. These results demonstrate that a single machine learning potential can represent the wide scope of phenomena required for metallurgical studies. The DFT database is openly available for use in any other machine learning method. The method is naturally extendable to alloys, which are necessary for engineering applications but where ductility and fracture are controlled by complex atomic-scale mechanisms that are not well predicted by existing potentials. © 2020 American Physical Society.
    view abstractdoi: 10.1103/PhysRevMaterials.4.103602
  • 2020 • 212 Distortion and Dilution Behavior for Laser Metal Deposition onto Thin Sheet Metals
    Tebaay, L.M. and Hahn, M. and Tekkaya, A.E.
    International Journal of Precision Engineering and Manufacturing - Green Technology 7 625-634 (2020)
    The combination of additive manufacturing and incremental sheet forming offers great flexibility in the manufacture of function-integrated parts. In this study, both processes were carried out by the same CNC machine. This offers the possibility to manufacture large-scale lightweight parts with smaller additive parts on it in one machine and clamping device. Additionally, the process combination can lead to a reduced energy and material consumption for small batch sizes. DC01 sheets are used as a substrate with two different initial conditions. The first condition is as delivered steel sheet and the second is an incrementally formed with a thickness of 0.5 mm. The additive manufacturing was conducted by laser metal deposition (LMD). The powder material is a stainless steel 316 L. A segmentation of the cladding surface was applied and the path strategy of the laser movement was varied simultaneously to analyse the warpage of the thin substrate. It is shown that there is a dependency between the build-up strategies and the melt pool temperature, the thermal distortion, the dilution and the size of the cladding area. A segmentation of the working surface causes a lower melt pool temperature and thermal distortion. The lower melt pool temperature also generates a reduced dilution zone. © 2020, Korean Society for Precision Engineering.
    view abstractdoi: 10.1007/s40684-020-00203-9
  • 2020 • 211 The effect of metal-oxide incorporation on the morphology of carbon nanostructures
    Tigges, S. and Wöhrl, N. and Hagemann, U. and Ney, M. and Lorke, A.
    Journal of Physics D: Applied Physics 53 (2020)
    Metal-organic, single-source, low-temperature, morphology-controlled growth of carbon nanostructures is achieved, using an inductively coupled plasma-enhanced chemical vapor deposition system. Three distinctive morphologies, namely nanoflakes, nanowalls (CNWs) and nanorods (and intermediates between these morphologies), can be reproducibly deposited, depending on the process parameters. The synthesized structures can be described as hybrid materials consisting of metal oxide incorporated in a carbon matrix material. Since the incorporation of metal oxide into the carbon structure significantly influences their growth, the synthesis cannot be described solely with the existing models for the growth of CNWs. Optical emission spectroscopy is used to measure the relative number density of suspected growth and etching species in the plasma, while physical and chemical surface analysis techniques (scanning electron microscopy, Raman spectroscopy, scanning Auger microscopy and x-ray photoelectron spectroscopy) were employed to characterize the properties of the different nanostructures. Therefore, by using methods for both plasma and surface characterization, the growth process can be understood. The precursor dissociation in the plasma can be directly linked to the deposited morphology, as the incorporation of Al2O3 into the nanostructures is found to be a major cause for the transition between morphologies, by changing the dominant type of defect within the carbon structure. © 2020 The Author(s). Published by IOP Publishing Ltd.
    view abstractdoi: 10.1088/1361-6463/ab6946
  • 2020 • 210 LPBF-M manufactured Zr-based bulk metallic glasses coated with magnetron sputtered ZrN films
    Tillmann, W. and Fehr, A. and Wegner, J. and Stangier, D. and Kleszczynski, S. and Witt, G.
    Surface and Coatings Technology 386 (2020)
    The fabrication of Zr-based bulk metallic glasses (BMGs) by means of laser powder bed fusion of metals (LPBF-M) is recently emerging. This production route allows to widely overcome current geometrical restrictions of casting routes while maintaining the amorphous character, which is decisive for the unique mechanical properties, for instance. However, the roughness of the LPBF-M fabricated BMGs is still a challenging property, impeding the application of near-net shaped thin films that modify BMG surfaces, e.g. with respect to wear resistance. Zr59.3Cu28.8Al10.4Nb1.5 (at.%) substrates were manufactured by means of LPBFM, applying various exposure strategies, including laser remelting of the last solidified layer to influence the surface topography. Furthermore, BMG substrates were post-treated by grinding and polishing. Thus, varying degrees of crystallinity as well as surface roughness states were generated to analyze the effect of these characteristics on the microstructural properties of additionally applied magnetron sputtered ZrN films. Substrates that were fabricated with higher energy densities during LPBF-M exhibited (101)-Zr as well as (013)- and (110)-CuZr2 phases, which were accompanied by a decreased surface roughness. It was shown that all films had a crystalline structure on amorphous and partly crystalline BMG surfaces. A decreased surface roughness of the BMG substrates could be directly correlated with a higher hardness and a better adhesion of the ZrN film. © 2020 Elsevier B.V.
    view abstractdoi: 10.1016/j.surfcoat.2020.125463
  • 2020 • 209 Diamond-impregnated 316L metal matrix composites fabricated by powder bed fusion with laser beam - Influences of the energy input on the microstructural properties
    Wegner, J. and Fehr, A. and Platt, S. and Kleszczynski, S. and Witt, G. and Tillmann, W.
    Diamond and Related Materials 109 (2020)
    Fabricating diamond metal matrix composites (DMMCs) by means of powder bed fusion of metals using a laser beam (PBF-LB/M) is a new approach to extensively expand the spectrum of geometrical freedom for diamond tools. However, it must be borne in mind that the temperature input has a significant influence on the diamond condition since graphitizations are likely to occur. Therefore, it was analyzed how varying volume energy densities and substrate heating affect the microstructure and the densification of a 316 L stainless steel matrix, which was impregnated with 5 vol.-% Ni-coated diamonds. With regard to the densification, it was shown that an elevated substrate temperature (473 K) allowed to apply reduced volume energy densities and reduce stress-induced cracking. Thus, a relative density of 99.5% could be achieved. Furthermore, decreasing the volume energy density avoided graphitizations of the diamonds. Cr and Fe contents of the matrix material dissolved at the Ni-coated diamond surface revealing a diamond-metal interaction. A longer heat flux generally supported these diffusion processes at the interfaces. Finally, it became obvious that increased laser powers resulted in a higher densification, while low scan speeds and laser powers are desirable to foster diffusion and to avoid graphitizations. © 2020 Elsevier B.V.
    view abstractdoi: 10.1016/j.diamond.2020.108040
  • 2020 • 208 Ultrasmall gold and silver/gold nanoparticles (2 nm) as autofluorescent labels for poly(D,L-lactide-co-glycolide) nanoparticles (140 nm)
    Wey, K. and Epple, M.
    Journal of Materials Science: Materials in Medicine 31 (2020)
    Ultrasmall metallic nanoparticles show an efficient autofluorescence after excitation in the UV region, combined with a low degree of fluorescent bleaching. Thus, they can be used as fluorescent labels for polymer nanoparticles which are frequently used for drug delivery. A versatile water-in-oil-in-water emulsion-evaporation method was developed to load poly(D,L-lactide-co-glycolide) (PLGA) nanoparticles with autofluorescent ultrasmall gold and silver/gold nanoparticles (diameter 2 nm). The metallic nanoparticles were prepared by reduction of tetrachloroauric acid with sodium borohydride and colloidally stabilised with 11-mercaptoundecanoic acid. They were characterised by UV–Vis and fluorescence spectroscopy, showing a large Stokes shift of about 370 nm with excitation maxima at 250/270 nm and emission maxima at 620/640 nm for gold and silver/gold nanoparticles, respectively. The labelled PLGA nanoparticles (140 nm) were characterised by dynamic light scattering (DLS), scanning electron microscopy (SEM), and UV–Vis and fluorescence spectroscopy. Their uptake by HeLa cells was followed by confocal laser scanning microscopy. The metallic nanoparticles remained inside the PLGA particle after cellular uptake, demonstrating the efficient encapsulation and the applicability to label the polymer nanoparticle. In terms of fluorescence, the metallic nanoparticles were comparable to fluorescein isothiocyanate (FITC). [Figure not available: see fulltext.]. © 2020, The Author(s).
    view abstractdoi: 10.1007/s10856-020-06449-8
  • 2020 • 207 Influence of Hot Hardness and Microstructure of High-Alloyed Powder Metallurgical Tool Steels on Abrasive Wear Behavior at Elevated Temperatures
    Wulbieter, N. and Pöhl, F. and Theisen, W.
    Steel Research International 91 (2020)
    Herein, the abrasive wear behavior of different high-alloyed powder metallurgical (PM) tool steels is investigated at elevated temperatures (400–600 °C) in a dry-pot wear tester containing Al2O3 particles. To identify the influence of the microstructure, PM tool steels with different hot hardnesses, carbide types, and carbide volume contents are selected. Wear tracks are analyzed by scanning electron microscopy (SEM) to clarify wear mechanisms. The results show that there is no direct correlation between wear resistance and only one material property such as hot hardness, carbide content, or carbide type. More important seems to be the best possible compromise between a sufficient hot hardness of the metallic matrix and a high volume content of carbides that are harder than the attacking abrasive particles at the respective temperature. When the test temperatures surpass the tempering temperature of the investigated steels, there is a pronounced change in wear behavior due to the stronger embedding of abrasive particles into the wear surface. It is thus necessary to discuss the microstructural properties as a function of temperature, considering interactions with the abrasive particles. © 2019 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/srin.201900461
  • 2020 • 206 Post-synthetic Modification of DUT-5-based Metal Organic Frameworks for the Generation of Single-site Catalysts and their Application in Selective Epoxidation Reactions
    Yildiz, C. and Kutonova, K. and Oßwald, S. and Titze-Alonso, A. and Bitzer, J. and Bräse, S. and Kleist, W.
    ChemCatChem 12 1134-1142 (2020)
    New single-site catalysts based on mixed-linker metal-organic frameworks with DUT-5 structure, which contain immobilized Co2+, Mn2+ and Mn3+ complexes, have successfully been synthesized via post-synthetic modification. 2,2’-Bipyridine-5,5’-dicarboxylate linkers were directly metalated, while 2-amino-4,4’-biphenyldicarboxylate linkers were post-synthetically modified by their conversion to Schiff-base ligands and a subsequent immobilization of the metal complexes. The resulting materials were used as catalysts in the selective epoxidation of trans-stilbene and the activities and selectivities of the different catalysts were compared. The influence of various reaction parameters on conversion, yield and selectivity were investigated. Very low catalyst amounts of 0.02 mol % were sufficient to obtain a high conversion of trans-stilbene using molecular oxygen from air as the oxidant. For cobalt-containing MOF catalysts, conversions up to 90 % were observed and, thus, they were more active than their manganese-containing counterparts. Recycling experiments and hot filtration tests proved that the reactions were mainly catalyzed via heterogeneous pathways. © 2019 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.
    view abstractdoi: 10.1002/cctc.201901434
  • 2020 • 205 Nanocellulose-Mediated Transition of Lithium-Rich Pseudo-Quaternary Metal Oxide Nanoparticles into Lithium Nickel Cobalt Manganese Oxide (NCM) Nanostructures
    Zehetmaier, P.M. and Zoller, F. and Beetz, M. and Plaß, M.A. and Häringer, S. and Böller, B. and Döblinger, M. and Bein, T. and Fattakhova-Rohlfing, D.
    ChemNanoMat 6 618-628 (2020)
    We report the syntheses of various compounds within the pseudo-quaternary system of the type LiwNixCoyMnzOδ (δ≤1) (pre-NCMs). Four different compositions of this compound were realized as ultrasmall crystalline nanoparticles of 1–4 nm diameter using low-temperature solvothermal reaction conditions in tert-butanol at only 170 °C. All of the pre-NCMs crystallize in the rock-salt structure and their lithium content is between 20% and 30% with respect to the complete metal content. By adjusting the lithium content to 105% stoichiometry in the solvothermal reaction, the pre-NCMs can easily react to the respective Li(NixCoyMnz)O2 (NCM) nanoparticles. Furthermore, nanosized desert-rose structured NCMs were obtained after addition of nanocellulose during the synthesis. By using the mixed metal monoxides as precursor for the NCMs, cation mixing between lithium and nickel is favored and gets more pronounced with increasing nickel content. The cation mixing effect compromises good electrochemical capacity retention, but the desert-rose structure nevertheless enables enhanced stability at high power conditions, especially for NCM333. © 2020 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.
    view abstractdoi: 10.1002/cnma.201900748
  • 2019 • 204 Non-destructive testing derived parameters for microstructure-based residual service life assessment of aging metallic materials in nuclear engineering
    Acosta, R. and Boller, C. and Starke, P. and Jamrozy, M. and Knyazeva, M. and Walther, F. and Heckmann, K. and Sievers, J. and Schopf, T. and Weihe, S.
    Materialpruefung/Materials Testing 61 1029-1038 (2019)
    Metallic components in nuclear engineering are exposed to extensive loads such as pressurization and temperature changes which can affect the properties of the material significantly depending on the load spectrum applied. In view of developing a procedure to evaluate the residual service life of metallic components in nuclear power plants aged during service, metastable austenitic steel AISI 347 (German designation: X6CrNiNb18-10) has been considered as an example. To this purpose, total strain-controlled fatigue tests were carried out under different environmental conditions and monitored by continuously measuring thermometric, resistometric, electromagnetic and electrochemical parameters. These parameters provide an information gain in terms of material characterization when compared to conventional strain measurements. Based on these parameters, the short time evaluation procedure StrainLife has been developed, which allows the determination of local S-N curves with a significantly reduced effort as compared with traditional procedures. This method has been implemented into the structural simulation program PROST for the integrity assessment of the components while considering local fatigue properties. This very effective method allows for the determination of local fatigue properties including the strain-specific local scatter of the metallic microstructure properties of the material which has not been possible by traditional means. © Carl Hanser Verlag GmbH & Co. KG
    view abstractdoi: 10.3139/120.111417
  • 2019 • 203 Oxygen Reduction Reaction Activity of Mesostructured Cobalt-Based Metal Oxides Studied with the Cavity-Microelectrode Technique
    Behnken, J. and Yu, M. and Deng, X. and Tüysüz, H. and Harms, C. and Dyck, A. and Wittstock, G.
    ChemElectroChem 6 3460-3467 (2019)
    Cobalt oxides are known as abundant and stable catalysts for the oxygen reduction reaction (ORR) in an alkaline environment. Here, the ORR activity of Co3O4 and mixed metal oxides NiCo2O4 and CuCo2O4 was studied. Synthesis by using the nanocasting procedure resulted in a mesostructured spinel phase with uniform morphology and high surface area. However, the evaluation of the specific activity of this material class is often hampered by limitations in determining the real surface area. The cavity-microelectrode technique did not require the addition of any additives to the catalytic material. Thus, measuring the double layer capacitance was used to assess the surface area. This approach showed comparable and reliable values for all samples and different cavity depths. Furthermore, the in situ derived surface area enabled the determination of the specific ORR activity, which is more accurate than utilizing the geometric and nitrogen absorption derived surface area. Although the activity of Co3O4 was rather low, the presence of Ni2+ and Cu2+ in the mixed metal oxides led to a substantial activity enhancement, possibly by providing additional active sites. © 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/celc.201900722
  • 2019 • 202 CT extended hounsfield unit range in radiotherapy treatment planning for patients with implantable medical devices
    Ese, Z. and Qamhiyeh, S. and Kreutner, J. and Schaefers, G. and Erni, D. and Zylka, W.
    IFMBE Proceedings 68 599-603 (2019)
    Radiotherapy (RT) treatment planning is based on computed tomography (CT) images and traditionally uses the conventional Hounsfield unit (CHU) range. This HU range is suited for human tissue but inappropriate for metallic materials. To guarantee safety of patient carrying implants precise HU quantification is beneficial for accurate dose calculations in planning software. Some modern CT systems offer an extended HU range (EHU). This study focuses the suitability of these two HU ranges for the quantification of metallic components of active implantable medical devices (AIMD). CT acquisitions of various metallic and non-metallic materials aligned in a water phantom were investigated. From our acquisitions we calculated that materials with mass-density ρ, >, 3.0, g/cm3 cannot be represented in the CHU range. For these materials the EHU range could be used for accurate HU quantification. Since the EHU range does not effect the HU values for materials ρ, <, 3.0, g/cm3, it can be used as a standard for RT treatment planning for patient with and without implants. © Springer Nature Singapore Pte Ltd. 2019.
    view abstractdoi: 10.1007/978-981-10-9023-3_111
  • 2019 • 201 Regulating the size and spatial distribution of Pd nanoparticles supported by the defect engineered metal-organic framework HKUST-1 and applied in the aerobic oxidation of cinnamyl alcohol
    Guo, P. and Fu, Q. and Yildiz, C. and Chen, Y.-T. and Ollegott, K. and Froese, C. and Kleist, W. and Fischer, R.A. and Wang, Y. and Muhler, M. and Peng, B.
    Catalysis Science and Technology 9 3703-3710 (2019)
    The functional composite of metal nanoparticles (NPs) and defect-engineered metal-organic frameworks (DE-MOFs), NPs@DE-MOFs, is an emerging field of MOF materials chemistry. Herein, we report on a series of novel Pd-NPs@DE-HKUST-1(Cu/Pd) catalysts containing both micro- and mesopores through the incorporation of the defect-generating linker 2,6-pyridyldicarboxylate (pydc). The Pd NPs are formed by partial reduction of the Pd2+ sites of the pristine mixed-metal DE-HKUST-1(Cu/Pd) with methanol. The size regime and the spatial distribution of the Pd NPs can be controlled by the amount of framework-incorporated pydc. The samples exhibit superior catalytic activity in the aerobic oxidation of cinnamyl alcohol as compared to the parent HKUST-1. © 2019 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c9cy00560a
  • 2019 • 200 Molecule-Metal Bond of Alternant versus Nonalternant Aromatic Systems on Coinage Metal Surfaces: Naphthalene versus Azulene on Ag(111) and Cu(111)
    Klein, B.P. and Morbec, J.M. and Franke, M. and Greulich, K.K. and Sachs, M. and Parhizkar, S. and Bocquet, F.C. and Schmid, M. and Hall, S.J. and Maurer, R.J. and Meyer, B. and Tonner, R. and Kumpf, C. and Kratzer, P. and Gottfried, J.M.
    Journal of Physical Chemistry C 123 29219-29230 (2019)
    Interfaces between polycyclic π-electron systems and metals play prominent roles in organic or graphene-based (opto)electronic devices, in which performance-related parameters depend critically on the properties of metal/semiconductor contacts. Here, we explore how the topology of the π-electron system influences the bonding and the electronic properties of the interface. We use azulene as a model for nonalternant pentagon-heptagon (5-7) ring pairs and compare it to its isomer naphthalene, which represents the alternant 6-6 ring pair. Their coverage-dependent interaction with Ag(111) and Cu(111) surfaces was studied with the normal-incidence X-ray standing wave (NIXSW) technique, near-edge X-ray absorption fine structure (NEXAFS) spectroscopy, UV and X-ray photoelectron spectroscopies (UPS and XPS), and density functional theory (DFT). Coverage-dependent adsorption heights and spectroscopic data reveal that azulene forms shorter interfacial bonds than naphthalene and engages in stronger electronic interactions with both surfaces. These differences are more pronounced on Cu. Increasing coverages lead to larger adsorption heights, indicating bond weakening by intermolecular repulsion. The extensive DFT calculations include dispersive interactions using (1) the DFT-D3 scheme, (2) the vdWsurf correction based on DFT-TS, (3) a many-body dispersion (MBD) correction scheme, and (4) the D3surf scheme. All methods predict the adsorption heights reasonably well with an average error below 0.1 »Å. The stronger bond of azulene is attributed to its nonalternant topology, which results in a reduced highest occupied molecular orbital (HOMO)-lowest occupied molecular orbital (LUMO) gap and brings the LUMO energetically close to the Fermi energy of the metal, causing stronger hybridization with electronic states of the metal surfaces. © 2019 American Chemical Society.
    view abstractdoi: 10.1021/acs.jpcc.9b08824
  • 2019 • 199 Reconfigurable lateral anionic heterostructures in oxide thin films via lithographically defined topochemistry
    Lefler, B.M. and Duchoň, T. and Karapetrov, G. and Wang, J. and Schneider, C.M. and May, S.J.
    Physical Review Materials 3 (2019)
    Laterally structured materials can exhibit properties uniquely suited for applications in electronics, magnetoelectric memory, photonics, and nanoionics. Here, a patterning approach is presented that combines the precise geometric control enabled by lithography with topochemical anionic manipulation of complex oxide films. Utilizing oxidation and fluorination reactions, striped patterns of SrFeO2.5/SrFeO3,SrFeO2.5/SrFeO2F, and SrFeO3/SrFeO2F have been prepared with lateral periodicities of 200, 20, and 4 μm. Coexistence of the distinct chemical phases is confirmed through x-ray diffraction, optical and photoemission microscopies, and optical spectroscopy. The lateral heterostructures exhibit highly anisotropic electronic transport and also enable transience and regeneration of patterns through reversible redox reactions. This approach can be broadly applied to a variety of metal-oxide systems, enabling chemically reconfigurable lateral heterostructures tailored for specific electronic, optical, ionic, thermal, or magnetic functionalities. © 2019 American Physical Society.
    view abstractdoi: 10.1103/PhysRevMaterials.3.073802
  • 2019 • 198 Future perspectives of thermal energy storage with metal hydrides
    Manickam, K. and Mistry, P. and Walker, G. and Grant, D. and Buckley, C.E. and Humphries, T.D. and Paskevicius, M. and Jensen, T. and Albert, R. and Peinecke, K. and Felderhoff, M.
    International Journal of Hydrogen Energy 44 7738-7745 (2019)
    Thermochemical energy storage materials have advantage of much higher energy densities compared to latent or sensible heat storage materials. Metal hydrides show good reversibility and cycling stability combined with high enthalpies. They can be used for short and long-term heat storage applications and can increase the overall flexibility and efficiency of solar thermal energy production. Metal hydrides with working temperatures less than 500 °C were in the focus of research and development over the last years. For the new generation of solar thermal energy plants new hydrides materials with working temperatures above 600 °C must be developed and characterized. In addition to thorough research on new metal hydrides, the construction and engineering of heat storage systems at these high temperatures are challenging. Corrosion problems, hydrogen embrittlement and selection of heat transfer fluids are significant topics for future research activities. © 2018 Hydrogen Energy Publications LLC
    view abstractdoi: 10.1016/j.ijhydene.2018.12.011
  • 2019 • 197 Ni-Metalloid (B, Si, P, As, and Te) Alloys as Water Oxidation Electrocatalysts
    Masa, J. and Piontek, S. and Wilde, P. and Antoni, H. and Eckhard, T. and Chen, Y.-T. and Muhler, M. and Apfel, U.-P. and Schuhmann, W.
    Advanced Energy Materials 9 (2019)
    Breakthroughs toward effective water-splitting electrocatalysts for mass hydrogen production will necessitate material design strategies based on unexplored material chemistries. Herein, Ni-metalloid (B, Si, P, As, Te) alloys are reported as an emergent class of highly promising electrocatalysts for the oxygen evolution reaction (OER) and insight is offered into the origin of activity enhancement on the premise of the surface electronic structure, the OER activation energy, influence of the guest metalloid elements on the lattice structure of the host metal (Ni), and surface-oxidized metalloid oxoanions. The metalloids modify the lattice structure of Ni, causing changes in the nearest Ni–Ni interatomic distance (dNi–Ni). The activation energy Ea scales with dNi–Ni indicating an apparent dependence of the OER activity on lattice properties. During the OER, surface Ni atoms are oxidized to nickel oxyhydroxide, which is the active state of the catalyst, meanwhile, the surface metalloids are oxidized to the corresponding oxoanions that affect the interfacial electrode/electrolyte properties and hence the adsorption/desorption interaction energies of the reacting species. © 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/aenm.201900796
  • 2019 • 196 High-temperature 2D Fermi surface of SrTiO3 studied by energy-filtered PEEM
    Mathieu, C. and Gonzalez, S. and Lubin, C. and Copie, O. and Feyer, V. and Schneider, C.M. and Barrett, N.
    Surface and Interface Analysis 51 7-11 (2019)
    Functional oxides displaying phenomena such as 2D electron gas (2DEG) at oxide interfaces represent potential technological breakthroughs for post-CMOS (Complementary Metal Oxide Semiconductor) electronics. Noninvasive techniques are required to study the surface chemistry and electronic structure underlying their often unique electrical properties. The sensitivity of photoemission electron microscopy (PEEM) to local potential, chemistry, and electronic structure makes it an invaluable tool for probing the near surface region of microscopic regions and domains of functional materials. In particular, PEEM allows single shot acquisition of the 2D Fermi surface and full angular probing of the symmetry-induced intensity modulations. We present results demonstrating a 2DEG at the surface of SrTiO3(001) at 140 K. The 2DEG is created by soft X-ray irradiation and can be reversibly controlled by a combination of soft X-rays and oxygen partial pressure. © 2018 John Wiley & Sons, Ltd.
    view abstractdoi: 10.1002/sia.6533
  • 2019 • 195 High-temperature 2D Fermi surface of SrTiO 3 studied by energy-filtered PEEM
    Mathieu, C. and Gonzalez, S. and Lubin, C. and Copie, O. and Feyer, V. and Schneider, C.M. and Barrett, N.
    Surface and Interface Analysis 51 7-11 (2019)
    Functional oxides displaying phenomena such as 2D electron gas (2DEG) at oxide interfaces represent potential technological breakthroughs for post-CMOS (Complementary Metal Oxide Semiconductor) electronics. Noninvasive techniques are required to study the surface chemistry and electronic structure underlying their often unique electrical properties. The sensitivity of photoemission electron microscopy (PEEM) to local potential, chemistry, and electronic structure makes it an invaluable tool for probing the near surface region of microscopic regions and domains of functional materials. In particular, PEEM allows single shot acquisition of the 2D Fermi surface and full angular probing of the symmetry-induced intensity modulations. We present results demonstrating a 2DEG at the surface of SrTiO 3 (001) at 140 K. The 2DEG is created by soft X-ray irradiation and can be reversibly controlled by a combination of soft X-rays and oxygen partial pressure. © 2018 John Wiley & Sons, Ltd.
    view abstractdoi: 10.1002/sia.6533
  • 2019 • 194 On the effects of microstructure on the mechanical properties of open-pore Al–11Zn foams
    Matz, A.M. and Matz, B.S. and Parsa, A.B. and Jost, N. and Eggeler, G.
    Materials Science and Engineering A 759 552-564 (2019)
    The mechanical properties of investment casted open-pore metal foams have been investigated on the example of the binary alloy Al–11Zn. The samples were subjected to different cooling conditions subsequent to casting and to different homogenization and ageing treatments. Variation in cooling was done either by quenching the mold in water or slowly cooling it in air. Homogenization and ageing varied in terms of temperature and time. The effects of the different treatments were investigated through microstructural and mechanical characterization methods. Using TEM, we found that the presence of GP zones and their morphological arrangement are the main factors dominating the mechanical performance. Micro- and nanoindentation testing of single foam struts reveal maximum hardness H when room temperature ageing was applied. Ageing at a temperature of 150 °C results in the lowest H in the present study; that is approximately 2/3 of the hardness achieved when ageing at room temperature. This can also be confirmed by the strength of non-porous bulk material obtained by tensile tests, which further show an increase in ductility up to a factor of 5 due to ageing at elevated temperatures. By compression testing of open-pore Al–11Zn foams, we notice that the presence of the microstructural effects varies in extent as a function of the strain ε. At low strains, we observe differences in mechanical performance to a high extent, becoming less with increasing compaction of the samples until they behave as non-porous bulk material. Based on these findings, we deduce a strong interaction of the structural morphology of the foam and its microstructure that determines the mechanical properties dominated by strength and ductility of the base material. © 2019 Elsevier B.V.
    view abstractdoi: 10.1016/j.msea.2019.05.087
  • 2019 • 193 Milling Down to Nanometers: A General Process for the Direct Dry Synthesis of Supported Metal Catalysts
    Schreyer, H. and Eckert, R. and Immohr, S. and de Bellis, J. and Felderhoff, M. and Schüth, F.
    Angewandte Chemie - International Edition 58 11262-11265 (2019)
    Supported catalysts are among the most important classes of catalysts. They are typically prepared by wet-chemical methods, such as impregnation or co-precipitation. Here we disclose that dry ball milling of macroscopic metal powder in the presence of a support oxide leads in many cases to supported catalysts with particles in the nanometer size range. Various supports, including TiO2, Al2O3, Fe2O3, and Co3O4, and different metals, such as Au, Pt, Ag, Cu, and Ni, were studied, and for each of the supports and the metals, highly dispersed nanoparticles on supports could be prepared. The supported catalysts were tested in CO oxidation, where they showed activities in the same range as conventionally prepared catalysts. The method thus provides a simple and cost-effective alternative to the conventionally used impregnation methods. © 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/anie.201903545
  • 2019 • 192 Effects of selenization time and temperature on the growth of Cu2ZnSnSe4 thin films on a metal substrate for flexible solar cells
    Stanchik, A.V. and Gremenok, V.F. and Juskenas, R. and Tyukhov, I.I. and Tivanov, M.S. and Fettkenhauer, C. and Shvartsman, V.V. and Giraitis, R. and Hagemann, U. and Lupascu, D.C.
    Solar Energy 142-149 (2019)
    Thin film Cu2ZnSnSe4 (CZTSe) solar cells can be grown on flexible and lightweight metal substrates allowing their direct integration on bendable surfaces and where the weight of solar cell is an important criterion. Flexible substrates make it possible to use the roll-to-roll technology of solar cells, which leads to an additional reduction in the cost of production and final cost of solar cells. The CZTSe thin films were fabricated by selenization of electrodeposited metallic precursors onto tantalum (Ta) flexible substrates at different temperature and time. The results of the effect of selenization temperature and time on the morphology, structural, and optical property of the CZTSe films are presented in this work. It was found that the morphology of the CZTSe thin films depend on their elemental composition and time of selenization. Experimental data indicate that composition of the CZTSe films selenized within 10 and 20 min at 560 °C have the CZTSe basic phase and secondary phases (CuSe, SnSe and ZnSe). In contrast, the increase in selenization temperature and/or time leads to disappearing of the secondary phases (CuSe, SnSe) and better crystallization of the CZTSe films. It was found that films selenized at 560 and 580 °C within the same time have similar characteristics. Depending on selenization time and temperature of the CZTSe, thin films exhibited a shift in band gap from 1.16 to 1.19 and to 1.22 eV, respectively. The change of band gap of the CZTSe thin films is associated with changes of elemental and phase compositions, and thickness of the film. These results showed that the received CZTSe films on Ta foil can be used for fabrication of thin film solar cells. © 2018
    view abstractdoi: 10.1016/j.solener.2018.12.025
  • 2019 • 191 Development and construction of AISI H11/ZrO2 joints for injection molding tools
    Tillmann, W. and Anar, N.B. and Manka, M. and Wojarski, L. and Lehmert, B.
    Welding in the World 63 1861-1871 (2019)
    Increasing demands in industrial applications and simultaneous efforts to provide long-lasting and cost-efficient tools in the injection molding industry lead to the use of metal–ceramic joints with the aim to combine the specific properties of both materials. Due to its high CTE, zirconium oxide (ZrO2) is used for the ceramic part and is joined with the tool steel AISI H11 (1.2343). In this work, suitable joining techniques with a low heat input and therefore a low thermal load are applied and characterized for the production of metal–ceramic composites. The selection of joining techniques is based on the boundary conditions during the injection molding process, in which the composites have to resist the temperature, pressure, as well as shear and tensile loads. Therefore, besides brazing, other joining processes such as gluing, screwing, shrinking, and clamping were analyzed as possible low temperature joining techniques for ceramic-metal-compounds. The best results for the tensile strengths with 90 MPa were achieved by a brazing process, carried out in vacuum with approximately 10−5 mbar, at a temperature of 920 °C for 5 min, using the commercially available brazing filler alloy CB4. © 2019, International Institute of Welding.
    view abstractdoi: 10.1007/s40194-019-00800-6
  • 2019 • 190 Influence of the etching processes on the adhesion of TiAlN coatings deposited by DCMS, HiPIMS and hybrid techniques on heat treated AISI H11
    Tillmann, W. and Grisales, D. and Stangier, D. and Ben Jebara, I. and Kang, H.
    Surface and Coatings Technology 378 (2019)
    Ensuring a good adhesion of the coatings to the substrate is one of the key points during the manufacturing of machining and forming tools. The nature of the substrate material and the way it is pre-treated and cleaned before the deposition plays an important role in the adhesion of the coatings. The in-situ cleaning processes as plasma etching and metal ion etching have demonstrated to have an influence on the adhesion of different coating/substrate systems. The introduction of HiPIMS technologies for the in-situ cleaning and the deposition of PVD coatings throughout this technique has opened varied opportunities to improve the performance of the coated parts. Systematic and comparative analysis of the influence of plasma etching itself (PE), plasma etching and metal ion etching (PE + MIE), and plasma etching and HiPIMS etching (PE + HiPE) etching processes on the adhesion of the TiAlN to the hot work tool steel AISI H11 has been performed. Additionally, subsequent to the etching processes, TiAlN coatings have been deposited using DCMS, HiPIMS and hybrid (DCMS/HiPIMS) technologies. Residual stresses of the heat treated AISI H11 were evaluated before and after the different etching process as well as after coating's deposition. It has been shown that the etching process affects the growth direction and microstructure of the TiAlN coatings, especially of those deposited by DCMS and hybrid. For instance, DCMS and hybrid TiAlN coatings deposited after PE have the presence of the TiAlN (200) reflection, not evidenced on the coatings deposited after PE + MIE and PE + HiPE. Moreover, hybrid coatings on PE + HiPE have a preferential (220) growth orientation and a (111) orientation when deposited on PE and PE + MIE cleaned substrate. Finally, in order to evaluate the adhesion of the coatings to the substrate, both scratch test and Rockwell C indentation test were used. The coatings deposited on the mere plasma etched (PE) substrate turn out to be the coatings with the highest critical load Lc3 and the lowest HF standards (HF1–HF3). This behaviour is attributed to the conservation of the substrate's surface integrity and the no promotion of surface tensions that can act in detriment of the adhesion of the coating. © 2019 Elsevier B.V.
    view abstractdoi: 10.1016/j.surfcoat.2019.125075
  • 2019 • 189 Influence of etching-pretreatment on nano-grained WC-Co surfaces and properties of PVD/HVOF duplex coatings
    Tillmann, W. and Hagen, L. and Stangier, D. and Krabiell, M. and Schröder, P. and Tiller, J. and Krumm, C. and Sternemann, C. and Paulus, M. and Elbers, M.
    Surface and Coatings Technology 374 32-43 (2019)
    The deposition of coatings by means of Physical Vapor Deposition (PVD)is an established process to enhance the lifetime and performance of carbide bulk tools. Although the effect of surface pretreatments on conventional WC-Co surfaces is well known, this investigation examines for the first time, how different surface pretreatments affect the surface integrity of thermally sprayed WC-Co substrates prior a subsequent PVD layer deposition and its resulting properties. Therefore, a WC-12Co feedstock with an average WC particle size of 100 nm was thermally sprayed on AISI M3 steel substrates using High Velocity Oxy-Fuel (HVOF)technique. Hereinafter, the HVOF sprayed WC-Co coatings were grounded and polished, thus serving as substrates for further surface pretreatments and the deposition of a CrAlN PVD hard coating by means of magnetron sputtering. To evaluate the influence of various surface pretreatments on the HVOF sprayed WC-Co coatings, several sequences such as heating, inert gas ion etching, metal ion etching, and High Power Impulse Magnetron Sputtering (HIPIMS)-etching were carried out. With respect to the subsequent PVD layer deposition, the results show that the pretreatment does neither affect the hardness nor Young's modulus of the CrAlN top layer. Yet, different effects on the WC-Co surface and PVD coating adhesion are observed. Inert gas ion etching leads to a faster removal of the carbides than of the Co-binder matrix. In contrast, metal ion etching provides a “micro-blasting” effect and removes the binder matrix as verified by Atomic Force Microscope (AFM)measurements. As a result, a decrease of the compressive residual stress state and an increase of the surface free energy are observed. With respect to HIPIMS-etching, a Cr-nanolayer was applied onto the WC-Co surface, which enhances the adhesion of the CrAlN top layer. Nevertheless, HRC Rockwell adhesion and scratch tests reveal a superior adhesion for samples pretreated with the metal ion etching. © 2019 Elsevier B.V.
    view abstractdoi: 10.1016/j.surfcoat.2019.05.054
  • 2019 • 188 Photocatalytic and magnetic porous cellulose macrospheres for water purification
    Wittmar, A.S.M. and Fu, Q. and Ulbricht, M.
    Cellulose (2019)
    Abstract: In this work, we report the preparation of photocatalytically active and easy to recycle porous cellulose-based spheres from polymer solutions in ionic liquid/dimethylsulfoxide mixtures by using the dropping cum phase separation technique. The factors affecting the sphere structure formation in relation to their efficiency as photocatalysts have been studied in detail. It was found that the increase of the nanoparticulate dopant fraction (TiO 2 and/or Fe 3 O 4 ) in the casting solution led to the formation of nanocomposites with a higher specific surface area as well as with enhanced photocatalytic activity. The embedment of the TiO 2 nanoparticles in the polymeric matrix did not change the bandgap of the photocatalyst. Furthermore, the co-doping with Fe 3 O 4 had no negative impact on the photocatalytic activity of the TiO 2 doped porous cellulose spheres. The addition of a moderate amount of dimethylsulfoxide led to an improvement of the photocatalytic activity of the formed nanocomposites, due to an increase of the matrix porosity without an agglomeration of the active nanoparticles. However, higher fractions of dimethylsulfoxide led to the agglomeration of the photocatalytic nanoparticles and therefore a decrease of the photocatalytic activity of the hybrid materials. The obtained porous spheres could be successfully recycled and reused in at least five consecutive cycles for the photocatalytic degradation of the model organic pollutant Rhodamine B in aqueous solution. Additionally, the prepared porous spheres also exhibited good adsorber properties toward Cu 2+ ions which were used in this study as model metal ion pollutant in water. Graphical abstract: [Figure not available: see fulltext.]. © 2019, Springer Nature B.V.
    view abstractdoi: 10.1007/s10570-019-02401-4
  • 2019 • 187 Correlation between sputter deposition parameters and I-V characteristics in double-barrier memristive devices
    Zahari, F. and Schlichting, F. and Strobel, J. and Dirkmann, S. and Cipo, J. and Gauter, S. and Trieschmann, J. and Marquardt, R. and Haberfehlner, G. and Kothleitner, G. and Kienle, L. and Mussenbrock, T. and Ziegler, M. and Kers...
    Journal of Vacuum Science and Technology B: Nanotechnology and Microelectronics 37 (2019)
    Sputter deposition is one of the most important techniques for the fabrication of memristive devices. It allows us to adjust the concentration of defects within the fabricated metal-oxide thin film layers. The defect concentration is important for those memristive devices whose resistance changes during device operation due to the drift of ions within the active layer while an electric field is applied. Reversible change of the resistance is an important property for devices used in neuromorphic circuits to emulate synaptic behavior. These novel bioinspired hardware architectures are ascertained in terms of advantageous features such as lower power dissipation and improved cognitive capabilities compared to state-of-the-art digital electronics. Thus, memristive devices are intensively studied with regard to neuromorphic analog systems. Double-barrier memristive devices with the layer sequence Nb/Al/Al2O3/NbOx/Au are promising candidates to emulate analog synaptic behavior in hardware. Here, the niobium oxide acts as the active layer, in which charged defects can drift due to an applied electric field causing analog resistive switching. In this publication, crucial parameters of the process plasma for thin film deposition, such as floating potential, electron temperature, and the energy flux to the substrate, are correlated with the I-V characteristics of the individual memristive devices. The results from plasma diagnostics are combined with microscopic and simulation methods. Strong differences in the oxidation state of the niobium oxide layers were found by transmission electron microscopy. Furthermore, kinetic Monte Carlo simulations indicate the impact of the defect concentration within the NbOx layer on the I-V hysteresis. The findings may enable a new pathway for the development of plasma-engineered memristive devices tailored for specific application. © 2019 Author(s).
    view abstractdoi: 10.1116/1.5119984
  • 2018 • 186 UV-light assisted patterned metallization of textile fabrics
    Bahners, T. and Gebert, B. and Prager, A. and Hartmann, N. and Hagemann, U. and Gutmann, J.S.
    Applied Surface Science 436 1093-1103 (2018)
    A UV-assisted process allows full-faced or local deposition of silver domains on textiles made of natural as well as synthetic fibers, which act as nuclei for subsequent galvanic metallization. SEM and XPS analyses indicate that the process generates particulate depositions – particles, aggregates – of elementary silver. Masking the UV irradiation confines silver deposition strictly to the exposed areas thus allowing patterning. Adhesion of the deposited silver is high on the studied natural fiber cotton and polyamide fibers. Adhesion on smooth and chemically inert synthethic fibers such as, e.g., poly(ethylene terephthalate) or para- and meta-aramids could be enhanced by finishing with poly(vinylamine) thus providing complex-forming amino groups. Although the process does not deposit a closed, electrically conducting layer, all studied samples could be metallized by galvanization. The resulting metal coatings exhibit high conductivity and wash stability. Following a patterned silver deposition, the subsequent galvanic metallization produced conductive patterns of identical geometry thus opening an avenue towards printed circuits on textile fabrics. © 2017 Elsevier B.V.
    view abstractdoi: 10.1016/j.apsusc.2017.12.119
  • 2018 • 185 Predictive quality control of hybrid metal-CFRP components using information fusion
    Berger, D. and Zaiß, M. and Lanza, G. and Summa, J. and Schwarz, M. and Herrmann, H.-G. and Pohl, M. and Günther, F. and Stommel, M.
    Production Engineering 12 161-172 (2018)
    The paper presents an approach to determine the durability of hybrid metal-CFRP components combining the results of non-destructive testing (ndt) and finite element simulation The advantage of hybrid metal-CFRP components lies in the use of the properties of the materials used. CFRP parts with higher specific stiffness and strength are combined with metallic joining points, so that established joining processes for metal components can be applied to these lightweight components. In order to further promote the use of these hybrids in industry, it is necessary to guarantee a high level of component reliability through 100% quality control in order to avoid production-related defects. These defects such as delamination or fibre disorientation however vary in shape, size and position and lead to different effects on the part performance and reliability. Therefore the presented approach includes the application of non-destructive testing methods that are applied as in-line quality control measures in order to determine defect characteristics of the inspected parts. Due to the novelty of the component under test it is necessary to evaluate the individual criticality of detected defects and how they affect part performance during the testing procedure. Therefore the acquired ndt-data is used in finite element simulations where defect characteristics are added to the component model and whose effects on part reliability are evaluated. The generation of additional information combining non-destructive testing and simulation is referred to as data fusion. In order to evaluate the validity of the presented approach the determined part performances are compared to experimental mechanic tests. © 2018, German Academic Society for Production Engineering (WGP).
    view abstractdoi: 10.1007/s11740-018-0816-1
  • 2018 • 184 Influence of spokes on the ionized metal flux fraction in chromium high power impulse magnetron sputtering
    Biskup, B. and Maszl, C. and Breilmann, W. and Held, J. and Böke, M. and Benedikt, J. and Von Keudell, A.
    Journal of Physics D: Applied Physics 51 (2018)
    High power impulse magnetron sputtering (HiPIMS) discharges are an excellent tool for deposition of thin films with superior properties. By adjusting the plasma parameters, an energetic metal and reactive species growth flux can be controlled. This control requires, however, a quantitative knowledge of the ion-to-neutral ratio in the growth flux and of the ion energy distribution function to optimize the deposited energy per incorporated atom in the film. This quantification is performed by combining two diagnostics, a quartz crystal microbalance (QCM) combined with an ion-repelling grid system (IReGS) to discriminate ions versus neutrals and a HIDEN EQP plasma monitor to measure the ion energy distribution function (IEDF). This approach yields the ionized metal flux fraction (IMFF) as the ionization degree in the growth flux. This is correlated to the plasma performance recorded by time resolved ICCD camera measurements, which allow to identify the formation of pronounced ionization zones, so called spokes, in the HiPIMS plasma. Thereby an automatic technique was developed to identify the spoke mode number. The data indicates two distinct regimes with respect to spoke formation that occur with increasing peak power, a stochastic regime with no spokes at low peak powers followed by a regime with distinct spokes at varying mode numbers at higher peak powers. The IMFF increases with increasing peak power reaching values of almost 80% at very high peak powers. The transition in between the two regimes coincides with a pronounced change in the IMFF. This change indicates that the formation of spokes apparently counteracts the return effect in HiPIMS. Based on the IMFF and the mean energy of the ions, the energy per deposited atom together with the overall energy flux onto the substrate is calculated. This allows us to determine an optimum for the peak power density around 0.5 kW cm-2 for chromium HiPIMS. © 2018 IOP Publishing Ltd.
    view abstractdoi: 10.1088/1361-6463/aaac15
  • 2018 • 183 Influence of surface roughness on the shear strength of direct injection molded plastic-aluminum hybrid-parts
    Bonpain, B. and Stommel, M.
    International Journal of Adhesion and Adhesives 82 290-298 (2018)
    The strength of hybrid metal and plastic joints is strongly influenced by the surface roughness of metal. Although many investigations on the change in shear strength of adhesively bonded joints due to roughening have been published, it is not completely understood how different mean roughness indexes Ra of directly joined plastic-metal-parts correlate to the resulting shear strength. This paper describes a schematic roughness - shear strength curve for adhesively bonded specimens and an experimental one for direct injection molded hybrid specimens which is reconciled with the state of the art. Roughening the surface of the metal is realized by grit blasting using fused alumina. After that, the metal is coated by direct injection molded PA 6.6 with 30% short glass fiber and shear strength tests are carried out. It can be concluded that with increasing Ra the shear strength of adhesively bonded specimens increases to a first maximum, then decreases and finally increases again. Direct injection molded samples exhibit a similar trend. The difference is that the second increase is significantly more pronounced, leading to a second maximum which is nearly twice as high as the first one. This difference is explained by the lower strength of the adhesive compared to the plastic and the different pronounced so called surface area effects, notch effects and thickness effects. By increasing Ra they promote cohesive fracture which is detrimental for adhesively bonded but more beneficial for direct injection molded samples. To further explain the experimental results, a FEM model to predict the probability of failure, the expected stress and the failure mode for direct injection molded samples is developed. The FE-analyses confirm the explanatory approach on the joint strength in dependence of the Ra value. © 2018 Elsevier Ltd
    view abstractdoi: 10.1016/j.ijadhadh.2018.02.003
  • 2018 • 182 The Role of Composition of Uniform and Highly Dispersed Cobalt Vanadium Iron Spinel Nanocrystals for Oxygen Electrocatalysis
    Chakrapani, K. and Bendt, G. and Hajiyani, H. and Lunkenbein, T. and Greiner, M.T. and Masliuk, L. and Salamon, S. and Landers, J. and Schlögl, R. and Wende, H. and Pentcheva, R. and Schulz, S. and Behrens, M.
    ACS Catalysis 8 1259-1267 (2018)
    Cation substitution in transition-metal oxides is an important approach to improve electrocatalysts by the optimization of their composition. Herein, we report on phase-pure spinel-type CoV2-xFexO4 nanoparticles with 0 ≤ x ≤ 2 as a new class of bifunctional catalysts for the oxygen evolution (OER) and oxygen reduction reactions (ORR). The mixed-metal oxide catalysts exhibit high catalytic activity for both OER and ORR that strongly depends on the V and Fe content. CoV2O4 is known to exhibit a high conductivity, while in CoFe2O4 the cobalt cation distribution is expected to change due to the inversion of the spinel structure. The optimized catalyst, CoV1.5Fe0.5O4, shows an overpotential for the OER of â300 mV for 10 mA cm-2 with a Tafel slope of 38 mV dec-1 in alkaline electrolyte. DFT+U+SOC calculations on cation ordering confirm the tendency toward the inverse spinel structure with increasing Fe concentration in CoV2-xFexO4 that starts to dominate already at low Fe contents. The theoretical results also show that the variations of oxidation states are related to the surface region, where the redox activity was found experimentally to be manifested in the transformation of V3+ ↠V2+. The high catalytic activity, facile synthesis, and low cost of the CoV2-xFexO4 nanoparticles render them very promising for application in bifunctional electrocatalysis. © 2017 American Chemical Society.
    view abstractdoi: 10.1021/acscatal.7b03529
  • 2018 • 181 Composition-Dependent Effect of the Calcination of Cobalt-, Nickel-, and Gallium-Based Layered Double Hydroxides to Mixed Metal Oxides in the Oxygen Evolution Reaction
    Chakrapani, K. and Özcan, F. and Ortega, K.F. and Machowski, T. and Behrens, M.
    ChemElectroChem 5 93-100 (2018)
    Mixed cobalt and nickel based layered double hydroxides (LDHs) with Ga as the third cation and the mixed metal oxides (MMOs) resulting from their thermal decomposition were synthesized in various compositions through constant pH co-precipitation and calcination. The structural and textural properties of the catalysts with variable Co/Ni ratios were assessed by N2 physisorption, powder X-ray diffraction, and electron microscopy. The obtained materials exhibit electrocatalytic activity for the oxygen evolution reaction in alkaline solution. The highest activity was found for catalysts containing both transition-metal cations, Co and Ni. However, comparison of the LDH precursors and the calcined MMOs revealed a composition-dependent effect of calcination. Co-rich LDH tends to lose activity when calcined, whereas Ni-rich LDH gains activity. The optimal cation composition of the LDH was Co1.5Ni0.5Ga with an overpotential of 382 mV. The highest performance among the MMOs, on the other hand, has been encountered for the Co0.5Ni1.5Ga composition, reaching a similar overpotential. © 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/celc.201700936
  • 2018 • 180 Oxide dispersion-strengthened alloys generated by laser metal deposition of laser-generated nanoparticle-metal powder composites
    Doñate-Buendía, C. and Frömel, F. and Wilms, M.B. and Streubel, R. and Tenkamp, J. and Hupfeld, T. and Nachev, M. and Gökce, E. and Weisheit, A. and Barcikowski, S. and Walther, F. and Schleifenbaum, J.H. and Gökce, B.
    Materials and Design 154 360-369 (2018)
    A new method is proposed for producing nanoparticle-metal composite powders for laser additive manufacturing of oxide-dispersion strengthened (ODS) alloys. Different composite powders containing laser-generated Y2O3 and yttrium iron garnet (YIG) nanoparticles were produced and consolidated by Laser Metal Deposition (LMD). The structural properties of the manufactured ODS alloys were analyzed, and their hardness, remnant porosity, and temperature-dependent compression behavior were characterized to study the effect of the composition and size of the nanoparticles on the structural and mechanical properties. While the structural analyses did not show significant differences between the processed samples within the limits of the characterization methods that were used, the temperature-dependent compression behavior showed an increase of up to 22 ± 11% in the high-temperature strength of the specimens that contained only 0.08 wt% of laser-generated nanoparticles. This increase is attributed to the dispersed and deagglomerated nature of the nanoparticles that were used during the powder-preparation step. © 2018 Elsevier Ltd
    view abstractdoi: 10.1016/j.matdes.2018.05.044
  • 2018 • 179 Bifunctional Oxygen Reduction/Oxygen Evolution Activity of Mixed Fe/Co Oxide Nanoparticles with Variable Fe/Co Ratios Supported on Multiwalled Carbon Nanotubes
    Elumeeva, K. and Kazakova, M.A. and Morales, D.M. and Medina, D. and Selyutin, A. and Golubtsov, G. and Ivanov, Y. and Kuznetzov, V. and Chuvilin, A. and Antoni, H. and Muhler, M. and Schuhmann, W. and Masa, J.
    ChemSusChem 11 1204-1214 (2018)
    A facile strategy is reported for the synthesis of Fe/Co mixed metal oxide nanoparticles supported on, and embedded inside, high purity oxidized multiwalled carbon nanotubes (MWCNTs) of narrow diameter distribution as effective bifunctional catalysts able to reversibly drive the oxygen evolution reaction (OER) and the oxygen reduction reaction (ORR) in alkaline solutions. Variation of the Fe/Co ratio resulted in a pronounced trend in the bifunctional ORR/OER activity. Controlled synthesis and in-depth characterization enabled the identification of an optimal Fe/Co composition, which afforded a low OER/OER reversible overvoltage of only 0.831 V, taking the OER at 10 mA cm−2 and the ORR at −1 mA cm−2. Importantly, the optimal catalyst with a Fe/Co ratio of 2:3 exhibited very promising long-term stability with no evident change in the potential for both the ORR and the OER after 400 charge/discharge (OER/ORR) cycles at 15 mA cm−2 in 6 m KOH. Moreover, detailed investigation of the structure, size, and phase composition of the mixed Fe/Co oxide nanoparticles, as well as their localization (inside of or on the surface of the MWCNTs) revealed insight of the possible contribution of the individual catalyst components and their synergistic interaction in the catalysis. © 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/cssc.201702381
  • 2018 • 178 Exceeding 6500 cycles for LiFePO4/Li metal batteries through understanding pulsed charging protocols
    García, G. and Dieckhöfer, S. and Schuhmann, W. and Ventosa, E.
    Journal of Materials Chemistry A 6 4746-4751 (2018)
    Improving the performance of Li metal anodes is of key importance for the next generation high energy-density batteries. Here, we study an easily implementable strategy for prolonging the cycle stability of Li metal anodes that is based on the application of pulsed charging protocols. Introducing short periods of relaxation without current flow allows the concentration of Li+ ions to be replenished in front of the electrode surface promoting a uniform and efficient plating of Li metal. We demonstrate that the cycle life of LiFePO4/Li metal batteries is prolonged from 700 to more than 6500 cycles at high charge-rates. In contrast to the assumed failure due to Li dendrite formation, we show that the proposed potential pulse protocols mitigate the growth of a porous film within the Li metal electrode which appears to be responsible for the battery failure. © 2018 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c8ta00962g
  • 2018 • 177 Tailoring metal oxide nanoparticle dispersions for inkjet printing
    Gebauer, J.S. and Mackert, V. and Ognjanović, S. and Winterer, M.
    Journal of Colloid and Interface Science 526 400-409 (2018)
    There is a growing interest in science and industry for printed electronics. Printed electronics enable the production of large quantities of electronic components at low cost. Even though organic semiconductors are already widely used for printed components, inorganic materials may be advantageous due to their higher durability and superior device performance. Nevertheless, inorganic materials still remain difficult to print making the development of printable and functional inks a necessity. In this work we present the formulation, inkjet printing and processing of newly developed inks based on ethylene glycol as dispersion medium. Different metal oxide nanoparticles (ZnO, TiO2, CuO, SnO2 and In2O3) with high crystallinity and narrow size distribution were produced by chemical vapor synthesis. The particles were stabilized and the colloidal stability was evaluated by a combination of DLVO simulations and dynamic light scattering measurements. Measurements of rheological and interfacial properties, like viscosity and surface tension, are used to determine the printability on the basis of the inverse Ohnesorge number. Inks, developed in this work, have adjustable rheological properties as well as long-term stabilities without particle sedimentation over a period of several months. They are suitable for printing on different substrate materials like silicon and flexible polymeric substrates. © 2018 Elsevier Inc.
    view abstractdoi: 10.1016/j.jcis.2018.05.006
  • 2018 • 176 Highly Efficient Photocatalytic Degradation of Dyes by a Copper–Triazolate Metal–Organic Framework
    Liu, C.-X. and Zhang, W.-H. and Wang, N. and Guo, P. and Muhler, M. and Wang, Y. and Lin, S. and Chen, Z. and Yang, G.
    Chemistry - A European Journal 24 16804-16813 (2018)
    A copper(I) 3,5-diphenyltriazolate metal–organic framework (CuTz-1) was synthesized and extensively characterized by using a multi-technique approach. The combined results provided solid evidence that CuTz-1 features an unprecedented Cu5tz6 cluster as the secondary building unit (SBU) with channels approximately 8.3 Å in diameter. This metal–organic framework (MOF) material, which is both thermally and chemically (basic and acidic) stable, exhibited semiconductivity and high photocatalytic activity towards the degradation of dyes in the presence of H2O2. Its catalytic performance was superior to that of reported MOFs and comparable to some composites, which has been attributed to its high efficiency in generating .OH, the most active species for the degradation of dyes. It is suggested that the photogenerated holes are trapped by CuI, which yields CuII, the latter of which behaves as a catalyst for a Fenton-like reaction to produce an excess amount of .OH in addition to that formed through the scavenging of photogenerated electrons by H2O2. Furthermore, it was shown that a dye mixture (methyl orange, methyl blue, methylene blue, and rhodamine B) could be totally decolorized by using CuTz-1 as a photocatalyst in the presence of H2O2 under the irradiation of a Xe lamp or natural sunlight. © 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/chem.201803306
  • 2018 • 175 On the accumulation of irreversible plastic strain during compression loading of open-pore metallic foams
    Matz, A.M. and Matz, B.S. and Jost, N. and Eggeler, G.
    Materials Science and Engineering A 728 40-44 (2018)
    The accumulation of plastic strain as an essential element of the compression behavior of metal foams is investigated by analyzing effective stress-strain curves which were recorded during testing. By applying loading/unloading cycles within the low-strain region until reaching the stress plateau, it is studied how reversible elastic deformation is gradually transformed into irreversible plastic deformation and it is shown that both, elastic and plastic strains, contribute to the total strain ε. This behavior is found to be independent on the investigated mesostructural foam morphologies. Furthermore, a method is derived which can be used to determine a proof stress σϕPl=0.5 at which yielding dominates the deformation of a metal foam. © 2018 Elsevier B.V.
    view abstractdoi: 10.1016/j.msea.2018.05.012
  • 2018 • 174 Metal-insulator-metal sensors monitoring charge flow during thermal desorption
    Meyburg, J.P. and Diesing, D. and Hasselbrink, E.
    Surface Science 678 91-98 (2018)
    Metal-insulator-metal sensors with a new design have been developed which allow us to perform thermal desorption spectroscopy of weakly bound adsorbates. The sensor can be temperature ramped utilising an ITO layer which is electrically isolated from the two metal layers such that a sensitive measurement of the device current is not hampered. An accurate reading of the temperature of the metal surface is derived from the I-V curve of the sensor which is found to be temperature sensitive. Concurrently with the thermal desorption spectrum the device current is recorded allowing us to correlate the latter with distinct desorption processes. For the molecules, H2O, small alcohols, NH3 and HCOOH we obtain thermal desorption spectra. Concurrently with recoding the spectra we detect characteristic device currents. These are found to solely result from interactions between the substrate and the first monolayer of molecules. We suggest that these are due to shifts of the Fermi level of the top metal film in the MIM device induced by the charge transfer processes connected with molecular adsorption and desorption. © 2018 Elsevier B.V.
    view abstractdoi: 10.1016/j.susc.2018.04.012
  • 2018 • 173 Poly(ethylene oxide)-block-poly(methyl methacrylate) diblock copolymers as functional additive for poly(vinylidene fluoride) ultrafiltration membranes with tailored separation performance
    Meyer, J. and Ulbricht, M.
    Journal of Membrane Science 545 301-311 (2018)
    In this work, poly(ethylene oxide)-block-poly(methyl methacrylate) (PEO-b-PMMA) diblock copolymers were established as functional additive for polyvinylidene fluoride (PVDF) ultrafiltration (UF) membranes, originally with the intention to increase their hydrophilicity and thereby decrease fouling. Additionally, however, it was found that copolymer micelles can be induced by complexing the PEO block of PEO-b-PMMA with specific metal salts. The formation of micelles as function of specific solution compositions was observed in dynamic light scattering and rheology experiments; the formation of PEO-metal ion complexes was shown via proton nuclear magnetic resonance (1H NMR) spectroscopy. Integration of micelle-forming compositions into typical PVDF-based casting solutions for UF membranes could lead to a higher surface porosity and a more regular barrier pore structure through microphase separation during the nonsolvent induced phase separation process used for membrane preparation. It was found that membranes containing small amounts of PEO-b-PMMA show a significantly higher permeance than membranes made from an otherwise equal casting solution without the copolymer, while maintaining the solute rejection properties. By using different types and amounts of metal salts to complex the PEO block it was possible to tailor the molecular weight cut-off of the membranes between 30 kDa and 110 kDa. Fouling studies in lab-scale cross-flow filtration cells showed an increased relative flux recovery compared to membranes without the functional copolymer additive. The results of this study are relevant because small fractions of a tailored diblock copolymer and metal salt as additives allow tailoring the barrier and separation properties at significantly higher overall performance within an otherwise unchanged membrane manufacturing process. © 2017 Elsevier B.V.
    view abstractdoi: 10.1016/j.memsci.2017.09.034
  • 2018 • 172 Polymer brush guided templating on well-defined rod-like cellulose nanocrystals
    Morits, M. and Hynninen, V. and Nonappa and Niederberger, A. and Ikkala, O. and Gröschel, A.H. and Müllner, M.
    Polymer Chemistry 9 1650-1657 (2018)
    Cellulose is a natural biomaterial harvested from regrowing resources and it is one of the most attractive components for the construction of functional materials with low adverse ecological impact. Among various nanocelluloses, cellulose nanocrystals (CNC) are rod-like nanoparticles whose high crystallinity and stiffness make them viable candidates for templating materials. We report here on CNC-based polymer brushes used as templates for the synthesis of porous inorganic nanorods with tunable diameters and aspect ratios. The CNC were modified with initiation sites for surface-initiated polymerisation (SI-ATRP) to act as a backbone for the grafting of poly(2-(dimethyl amino)ethyl methacrylate) (PDMAEMA) brushes. Controlled polymerisation conditions allowed for adjusting the brush length and consequently the morphology of the hybrid nanomaterials. The PDMAEMA brush served as coordination and nucleation sites for the mineralisation of tetramethyl orthosilicate (TMOS) into SiO2@CNC-g-PDMAEMA hybrids. After calcination, microscopy and N2-sorption measurements revealed hollow silica nanorods with accessible micro-and meso-pores. We foresee that this strategy can be adapted to other nanocelluloses to create high-Aspect ratio porous silica nanotubes, or to achieve uniform depositions in the mineralisation of other inorganic metals or metal oxide compounds. © The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c7py01814b
  • 2018 • 171 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 • 170 Comparative biological effects of spherical noble metal nanoparticles (Rh, Pd, Ag, Pt, Au) with 4-8 nm diameter
    Rostek, A. and Breisch, M. and Pappert, K. and Loza, K. and Heggen, M. and Köller, M. and Sengstock, C. and Epple, M.
    Beilstein Journal of Nanotechnology 9 2763-2774 (2018)
    For a comparative cytotoxicity study, nanoparticles of the noble metals Rh, Pd, Ag, Pt, and Au (spherical, average diameter 4 to 8 nm) were prepared by reduction in water and colloidally stabilized with poly(N-vinyl pyrrolidone) (PVP). Thus, their shape, size, and surface functionalization were all the same. Size and morphology of the nanoparticles were determined by dynamic light scattering (DLS), analytical disc centrifugation (differential centrifugal sedimentation, DCS), and high-resolution transmission electron microscopy (HRTEM). Cell-biological experiments were performed to determine the effect of particle exposure on the viability of human mesenchymal stem cells (hMSCs). Except for silver, no adverse effect of any of the metal nanoparticles was observed for concentrations up to 50 ppm (50 mg L-1) incubated for 24 h, indicating that noble metal nanoparticles (rhodium, palladium, platinum, gold) that do not release ions are not cytotoxic under these conditions. © 2018 Rostek et al.
    view abstractdoi: 10.3762/bjnano.9.258
  • 2018 • 169 Different Breathing Mechanisms in Flexible Pillared-Layered Metal-Organic Frameworks: Impact of the Metal Center
    Schneemann, A. and Vervoorts, P. and Hante, I. and Tu, M. and Wannapaiboon, S. and Sternemann, C. and Paulus, M. and Wieland, D.C.F. and Henke, S. and Fischer, R.A.
    Chemistry of Materials 30 1667-1676 (2018)
    The pillared-layered metal-organic framework compounds M2(BME-bdc)2(dabco) (M2+ = Zn2+, Co2+, Ni2+, Cu2+; BME-bdc2- = 2,5-bis(2-methoxyethoxy)-1,4-benzenedicarboxylate; dabco = diazabicyclo[2.2.2]octane) exhibit structural flexibility and undergo guest and temperature-induced reversible phase transitions between a narrow pore (np) and a large pore (lp) form. These transitions were analyzed in detail by powder X-ray diffraction ex and in situ, isothermal gas adsorption measurements and differential scanning calorimetry. The threshold parameters (gas pressure or temperature), the magnitude of the phase transitions (volume change) as well as their transition enthalpies are strikingly dependent on the chosen metal cation M2+. This observation is assigned to the different electronic structures and ligand field effects on the coordination bonds. Accordingly, in situ powder X-ray diffraction measurements as a function of CO2 pressure reveal different mechanisms for the np to lp phase transition during CO2 adsorption. © 2018 American Chemical Society.
    view abstractdoi: 10.1021/acs.chemmater.7b05052
  • 2018 • 168 Scaling-up metal nanoparticle production by transferred arc discharge
    Stein, M. and Kruis, F.E.
    Advanced Powder Technology 29 3138-3144 (2018)
    The number of applications and products containing metal nanomaterials has significantly increased over the past years. In order to address the upcoming demand for metal nanoparticles, new scale-up strategies are required. The scale-up of nanoparticle synthesis, especially for metals, is however very challenging. This study reports about a production facility with a new scale-up approach for pure metal nanoparticles. The scale-up approach is the parallelization of multiple transferred arcs in one reactor, which were previously individually optimized. Furthermore, a novel filtration and bagging system is introduced, which is designed to handle pyrophoric metal nanoparticles. It is shown that the production rate of the process scales linearly with the number of transferred arcs, while the particle size stays on the nanoscale. © 2018 The Society of Powder Technology Japan
    view abstractdoi: 10.1016/j.apt.2018.08.016
  • 2018 • 167 Simultaneous Rayleigh/Mie and Raman/Fluorescence Characterization of Molecularly Functionalized Colloids by Correlative Single-Particle Real-Time Imaging in Suspension
    Wissler, J. and Wehmeyer, M. and Bäcker, S. and Knauer, S. and Schlücker, S.
    Analytical Chemistry 90 723-728 (2018)
    Many applications of nano- and microparticles require molecular functionalization. Assessing the heterogeneity of a colloidal sample in terms of its molecular functionalization is highly desirable but not accessible by conventional ensemble experiments. Retrieving this information necessitates single-particle experiments which simultaneously detect both functionalized and nonfunctionalized particles via two separate imaging channels. In this contribution, we present an optical setup for performing correlative single-particle imaging using laser light-sheet illumination: the first detection channel records elastic light scattering (Rayleigh/Mie), while the second channel detects inelastic light scattering (Raman) or fluorescence. The instrument is tested with Raman reporter-functionalized SERS-active metal nanoparticles (core/satellite silver nanoparticles, dimers and monomers of gold nanoparticles) and fluorophore-functionalized colloids (fluorescent polymer microparticles, dye-labeled protein on gold nanoparticles). © 2017 American Chemical Society.
    view abstractdoi: 10.1021/acs.analchem.7b02528
  • 2018 • 166 Coffee-Waste Templating of Metal Ion-Substituted Cobalt Oxides for the Oxygen Evolution Reaction
    Yu, M. and Chan, C.K. and Tüysüz, H.
    ChemSusChem 11 605-611 (2018)
    A facile and scalable method using coffee waste grounds as a hard template has been developed to fabricate nanostructured Co3O4 for the oxygen evolution reaction (OER). Co3O4 incorporating metals with different valences (M/Co=1:4; M=Cu, Ni, Fe, Cr, and W) were also prepared with similar sheet-like structures comprising nanosized crystallites. After detailed characterization by X-ray diffraction, electron microscopy, and nitrogen sorption, the oxides were employed as OER electrocatalysts. Substitution of octahedral and tetrahedral sites of the spinel structure with divalent and trivalent transition metals (Cu, Ni, Fe, and Cr) increased the activity of Co3O4 for the OER, whereas incorporation of hexavalent W led to formation of a second crystal phase and significantly higher electrocatalytic performance. Furthermore, this method is easily scaled up for mass production of Co3O4 with the same nanostructure, which is highly desirable for large-scale application. © 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/cssc.201701877
  • 2017 • 165 Fundamental study of an industrial reactive HPPMS (Cr,Al)N process
    Bobzin, K. and Brögelmann, T. and Kruppe, N.C. and Engels, M. and Von Keudell, A. and Hecimovic, A. and Ludwig, Al. and Grochla, D. and Banko, L.
    Journal of Applied Physics 122 (2017)
    In this work, a fundamental investigation of an industrial (Cr,Al)N reactive high power pulsed magnetron sputtering (HPPMS) process is presented. The results will be used to improve the coating development for the addressed application, which is the tool coating for plastics processing industry. Substrate-oriented plasma diagnostics and deposition of the (Cr,Al)N coatings were performed for a variation of the HPPMS pulse frequency with values from f = 300 Hz to f = 2000 Hz at constant average power P = 2.5 kW and pulse length ton = 40 μs. The plasma was investigated using an oscilloscope, an intensified charge coupled device camera, phase-resolved optical emission spectroscopy, and an energy-dispersive mass spectrometer. The coating properties were determined by means of scanning electron microscopy, glow discharge optical emission spectroscopy, cantilever stress sensors, nanoindentation, and synchrotron X-ray diffraction. Regarding the plasma properties, it was found that the average energy within the plasma is nearly constant for the frequency variation. In contrast, the metal to gas ion flux ratio is changed from JM/JG = 0.51 to JM/JG = 0.10 for increasing frequency. Regarding the coating properties, a structure refinement as well as lower residual stresses, higher universal hardness, and a changing crystal orientation from (111) to (200) were observed at higher frequencies. By correlating the plasma and coating properties, it can be concluded that the change in the gas ion to metal ion flux ratio results in a competitive crystal growth of the film, which results in changing coating properties. © 2017 Author(s).
    view abstractdoi: 10.1063/1.4990997
  • 2017 • 164 Corrosion resistance of high-alloyed stainless steel membrane supports under flue gas conditions of a lignite-fueled power plant
    Bram, M. and Seifert, M. and Kot, A. and Wilkner, K. and Wulbieter, N. and Theisen, W.
    Materials and Corrosion (2017)
    Metal-supported silica membranes are attractive candidates for CO2 capture from the exhaust of coal-fueled power plants. Compared to their full ceramic counterparts, the introduction of the metal support facilitates sealing of the membrane by established technologies, such as welding, and enhances the robustness of the membrane in the harsh environment of the power plant. As well-known from other steel components in flue gas desulfurization units, long-term corrosion resistance of the metal support is mandatory for the success of this new membrane concept. In the present work, a research concept is introduced enabling a systematic benchmark of stainless steels regarding their suitability to be used for the metal support of the CO2 selective silica membranes. The study combines field tests of porous samples in direct contact with the exhaust gas of a lignite-fueled power plant and standardized corrosion tests of dense and porous samples in the laboratory according to DIN 50918 using exhaust gas condensate as the corrosive medium. Preliminary results are achieved on austenitic steel (AISI 316L) as well as on two ferritic steels (Crofer22APU, Plansee ITM). Ferritic steels are chosen due to their availability as substrates with well-defined porosity and with adapted thermal expansion coefficient enabling successful coating of the CO2 selective silica membrane. © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/maco.201709456
  • 2017 • 163 Influence of nitrogen admixture to argon on the ion energy distribution in reactive high power pulsed magnetron sputtering of chromium
    Breilmann, W. and Maszl, C. and Hecimovic, A. and Von Keudell, A.
    Journal of Physics D: Applied Physics 50 (2017)
    Reactive high power impulse magnetron sputtering (HiPIMS) of metals is of paramount importance for the deposition of various oxides, nitrides and carbides. The addition of a reactive gas such as nitrogen to an argon HiPIMS plasma with a metal target allows the formation of the corresponding metal nitride on the substrate. The addition of a reactive gas introduces new dynamics into the plasma process, such as hysteresis, target poisoning and the rarefaction of two different plasma gases. We investigate the dynamics for the deposition of chromium nitride by a reactive HiPIMS plasma using energy- and time-resolved ion mass spectrometry, fast camera measurements and temporal and spatially resolved optical emission spectroscopy. It is shown that the addition of nitrogen to the argon plasma gas significantly changes the appearance of the localized ionization zones, the so-called spokes, in HiPIMS plasmas. In addition, a very strong modulation of the metal ion flux within each HiPIMS pulse is observed, with the metal ion flux being strongly suppressed and the nitrogen molecular ion flux being strongly enhanced in the high current phase of the pulse. This behavior is explained by a stronger return effect of the sputtered metal ions in the dense plasma above the racetrack. This is best observed in a pure nitrogen plasma, because the ionization zones are mostly confined, implying a very high local plasma density and consequently also an efficient scattering process. © 2017 IOP Publishing Ltd.
    view abstractdoi: 10.1088/1361-6463/aa5bfc
  • 2017 • 162 Manufacturing of hard composite materials on Fe-base with oxide particles
    Brust, S. and Röttger, A. and Kimm, J. and Usta, E. and Theisen, W.
    Key Engineering Materials 742 KEM 106-112 (2017)
    Metal matrix composites (MMC) are often applied to tool surfaces to increase resistance to wear and tear. However, some matrix and particle materials such as Ni, Co, WC or TiC are expensive and partly classified as critical elements. With respect to tribo-mechanical properties, Fe-alloys reinforced with oxide particles are promising compound materials to produce wear-resistant MMC with low-cost and readily available materials. However, thus far the technical application of such MMCs is limited due to poor wettability of the oxides by Fe-base melts and an associated weak bonding between the oxide particles and the metal matrix phases. In this work two novel production techniques (namely pre-metallization and active sintering) are introduced, which improve the wettability and interfacial reactions between both materials and therefore enable supersolidus liquid-phase sintering (SLPS) of the MMC. For the first technique the oxide particles are pre-metallized by depositing a thin film of TiN on the surfaces. The second technique is called active sintering. For this technique the alloy design is adapted from active brazing, so that wettability of the oxide particles by the alloy-melt is increased. The resulting effects of these techniques are investigated using wetting and sintering experiments, and are analyzed with respect to the developed microstructures and interfacial reactions between the oxide particles and the metallic phases. © 2017 Trans Tech Publications, Switzerland.
    view abstractdoi: 10.4028/www.scientific.net/KEM.742.106
  • 2017 • 161 Protocol for the Nanocasting Method: Preparation of Ordered Mesoporous Metal Oxides
    Deng, X. and Chen, K. and Tüysüz, H.
    Chemistry of Materials 29 40-52 (2017)
    Ordered mesoporous transition metal oxides have attracted considerable research attention due to their unique properties and wide applications. The preparation of these materials has been reported in the literature using soft and hard templating pathways. Compared with soft templating, hard templating, namely, nanocasting, is advantageous for synthesizing rigid mesostructures with high crystallinity and has already been applied to numerous transition metal oxides such as Co3O4, NiO, Fe2O3, and Mn3O4. However, nanocasting is often complicated by the multiple steps involved: first, the preparation of ordered mesoporous silica as the hard template, then infiltration of the metal precursor into the pores, and finally, formation of the metal oxide and removal of the hard template. In this paper, we provide a complete protocol that covers the preparation of most widely used ordered mesoporous silica templates (MCM-41, KIT-6, SBA-15) and the nanocasting process for obtaining ordered mesoporous metal oxides, with emphasizing cobalt oxide as an example. Characterization of the products is presented, and the factors that can potentially affect the process are discussed. © 2016 American Chemical Society.
    view abstractdoi: 10.1021/acs.chemmater.6b02645
  • 2017 • 160 Modifications of aluminum film caused by micro-plasmoids and plasma spots in the effluent of an argon non-equilibrium plasma jet
    Engelhardt, M. and Ries, S. and Hermanns, P. and Bibinov, N. and Awakowicz, P.
    Journal of Physics D: Applied Physics 50 (2017)
    A smooth layer of hard aluminium film is deposited onto a glass substrate with a multi-frequency CCP discharge and then treated in the effluent of a non-equilibrium atmospheric pressure plasma jet (N-APPJ) operated with Ar flow. A thin filament is formed in the argon N-APPJ through contraction of a diffuse feather-like discharge. The aluminium surface treated in the effluents of the N-APPJ is significantly modified. Erosion tracks of different forms and micro-balls composed of aluminium are observed on the treated surface. Based on CCD images of active plasma discharge channels, SEM images of the treated surface and current-voltage characteristics, these surface modifications are interpreted as traces of plasma spots and plasmoids. Plasma spots are focused plasma channels, which are characterized by an intense emission in CCD images at the contact point of a plasma channel with the treated metal surface and by deep short tracks on the aluminium surface, observed in SEM images. Plasmoids are plasma objects without contact to any power supply which can produce long, thin and shallow traces, as can be observed on the treated surface using electron microscopy. Based on observed traces and numerous transformations of plasma spots to plasmoids and vice versa, it is supposed that both types of plasma objects are formed by an extremely high axial magnetic field and differ from each other due to the existence or absence of contact to a power supply and the consequential transport of electric current. The reason for the magnetic field at the axis of these plasma objects is possibly a circular current of electron pairs in vortices, which are formed in plasma by the interaction of ionization waves with the substrate surface. The extremely high magnetic field of plasma spots and plasmoids leads to a local destruction of the metal film and top layer of the glass substrate and to an attraction of paramagnetic materials, namely aluminium and oxygen. The magnetic attraction of aluminium is a reason for the extraction of some pieces of metal and the formation of erosion tracks and holes in the metal film. In the absence of metal atomization, the extracted aluminium forms spherical micro-particles, which are distributed over the surface of the treated metal film by the gas flow. A thin (100 nm) gold (diamagnetic) layer on top of the aluminium film surface reduces the erosion rate of plasma spots and plasmoids drastically (more than three orders of magnitude). © 2017 IOP Publishing Ltd.
    view abstractdoi: 10.1088/1361-6463/aa802f
  • 2017 • 159 Thin SnOx films for surface plasmon resonance enhanced ellipsometric gas sensing (SPREE)
    Fischer, D. and Hertwig, A. and Beck, U. and Lohse, V. and Negendank, D. and Kormunda, M. and Esser, N.
    Beilstein Journal of Nanotechnology 8 522-529 (2017)
    Background: Gas sensors are very important in several fields like gas monitoring, safety and environmental applications. In this approach, a new gas sensing concept is investigated which combines the powerful adsorption probability of metal oxide conductive sensors (MOS) with an optical ellipsometric readout. This concept shows promising results to solve the problems of cross sensitivity of the MOS concept.Results: Undoped tin oxide (SnOx) and iron doped tin oxide (Fe:SnOx) thin add-on films were prepared by magnetron sputtering on the top of the actual surface plasmon resonance (SPR) sensing gold layer. The films were tested for their sensitivity to several gas species in the surface plasmon resonance enhanced (SPREE) gas measurement. It was found that the undoped tin oxide (SnOx) shows higher sensitivities to propane (C3H8) then to carbon monoxide (CO). By using Fe:SnOx, this relation is inverted. This behavior was explained by a change of the amount of binding sites for CO in the layer due to this iron doping. For hydrogen (H2) no such relation was found but the sensing ability was identical for both layer materials. This observation was related to a different sensing mechanism for H2 which is driven by the diffusion into the layer instead of adsorption on the surface. Conclusion: The gas sensing selectivity can be enhanced by tuning the properties of the thin film overcoating. A relation of the binding sites in the doped and undoped SnOx films and the gas sensing abilities for CO and C3H8 was found. This could open the path for optimized gas sensing devices with different coated SPREE sensors. © 2017 Fischer et al.; licensee Beilstein-Institut.
    view abstractdoi: 10.3762/bjnano.8.56
  • 2017 • 158 Microstructural characterization and simulation of damage for geared sheet components
    Gerstein, G. and Isik, K. and Gutknecht, F. and Sieczkarek, P. and Ewert, J. and Tekkaya, A.E. and Clausmeyer, T. and Nürnberger, F.
    Journal of Physics: Conference Series 896 (2017)
    The evolution of damage in geared components manufactured from steel sheets was investigated, to analyse the influence of damage caused by the sheet-bulk-metal forming. Due to the inhomogeneous and multi-axial deformation in the investigated parts, different aspects such as the location-dependent shape and size of voids are analysed by means of various microscopic methods. In particular, a method to characterize the state of damage evolution, i. e. void nucleation, growth and coalescence using scanning electron microscopy (SEM) is applied. The investigations reveal a strong dependence of the void area fraction, shape of voids and thus damage evolution on the loading mode. The microstructural analysis is complemented with FEM simulations using material models which consider the characteristics of the void evolution. © Published under licence by IOP Publishing Ltd.
    view abstractdoi: 10.1088/1742-6596/896/1/012076
  • 2017 • 157 Experimental and analytical investigation of the force requirements in shear cutting of metal-polymer-metal composites
    Groche, P. and Übelacker, D. and Stein, P. and Steinbach, F. and Erman Tekkaya, A.
    International Journal of Material Forming 1-12 (2017)
    The increasing demand for lightweight design requires the use of multi materials such as metal-polymer-metal composites. These so-called sandwich panels offer a good stiffness-to-weight ratio. Production technologies like shear cutting have to be adapted for these materials. For a targeted adaption, a comprehensive knowledge about the cutting phases of the shear cutting process of sandwich panels is essential. Therefore, within this paper, the shear cutting process of sandwich panels is studied in detail. The conducted experimental studies indicate that the shear cutting process can be divided into five stages. Based on these findings, a new analytic model is introduced to predict the force displacement curves of sandwich panels. The quality of the new model is proven by comparison with existing analytic models for monolithic materials as well as with the experimental data. © 2017 Springer-Verlag France
    view abstractdoi: 10.1007/s12289-017-1343-x
  • 2017 • 156 Degradation mechanisms of pcBN tool material during Friction Stir Welding of Ni-base alloy 625
    Hanke, S. and Lemos, G.V.B. and Bergmann, L. and Martinazzi, D. and dos Santos, J.F. and Strohaecker, T.R.
    Wear 376-377 403-408 (2017)
    In Friction Stir Welding (FSW), interactions between the plasticized stirred material and the tool significantly affect resulting weld properties. When welding metals with high strength and melting point, the tribological load on the tool is severe, and poses the main limiting factor for the technology's industrial exploitation. Since tool materials are loaded to their limits, it is essential to understand the interactions of specific tool material and welded metal combinations. In the present study 3.2 mm alloy 625 sheets were joined using a pcBN tool with W-Re binder phase. Wear lead to a change in tool geometry followed by tool fracture. In SEM investigations the welds revealed typical banded structures, composed of small grains and non-metallic phases containing W from the tool material. The tool surface is extensively covered by adhering sheet metal. Further, BN grain pull-outs and appearances of diffusive wear are visible on the worn tool surface. Tool wear is mainly caused by detachment of BN grains due to thermal softening of the metallic binder phase and dissolution of BN in the hot material in the stirred zone. Using low rotational speeds resulting in lower process temperatures reduces tool wear and results in a homogeneous stirred zone. © 2017 Elsevier B.V.
    view abstractdoi: 10.1016/j.wear.2017.01.070
  • 2017 • 155 Performance study of a 248 GHz voltage controlled hetero-integrated source in InP-on-BiCMOS technology
    Hossain, M. and Ostermay, I. and Weimann, N.G. and Schmueckle, F.J. and Borngraeber, J. and Meliani, C. and Lisker, M. and Tillack, B. and Krueger, O. and Krozer, V. and Heinrich, W.
    International Journal of Microwave and Wireless Technologies 9 259-268 (2017)
    This paper presents the performance study of a 248 GHz voltage-controlled hetero-integrated signal source using indium phosphide (InP)-on-bipolar complementary metal-oxide-semiconductor (BiCMOS) technology. The source consists of a voltage controlled oscillator (VCO) in 0.25 μm BiCMOS technology and a frequency multiplier in 0.8 μm transferred-substrate InP-heterojunction bipolar transistor technology, which is integrated on top of the BiCMOS monolithic microwave integrated circuit in a wafer-level based benzocyclobutene bonding process. The vertical transitions from BiCMOS to InP in this process exhibit broadband properties with insertion losses below 0.5 dB up to 325 GHz. The VCO operates at 82.7 GHz with an output power of 6 dBm and the combined circuit delivers -9 dBm at 248 GHz with 1.22% tuning range. The phase noise of the combined circuit is -85 dBc/Hz at 1 MHz offset. The measured output return loss of the hetero-integrated source is >10 dB within a broad frequency range. This result shows the potential of the hetero integrated process for THz frequencies. © 2015 Cambridge University Press and the European Microwave Association.
    view abstractdoi: 10.1017/S1759078715001634
  • 2017 • 154 Nanoporous Nitrogen-Doped Graphene Oxide/Nickel Sulfide Composite Sheets Derived from a Metal-Organic Framework as an Efficient Electrocatalyst for Hydrogen and Oxygen Evolution
    Jayaramulu, K. and Masa, J. and Tomanec, O. and Peeters, D. and Ranc, V. and Schneemann, A. and Zboril, R. and Schuhmann, W. and Fischer, R.A.
    Advanced Functional Materials (2017)
    Engineering of controlled hybrid nanocomposites creates one of the most exciting applications in the fields of energy materials and environmental science. The rational design and in situ synthesis of hierarchical porous nanocomposite sheets of nitrogen-doped graphene oxide (NGO) and nickel sulfide (Ni7S6) derived from a hybrid of a well-known nickel-based metal-organic framework (NiMOF-74) using thiourea as a sulfur source are reported here. The nanoporous NGO/MOF composite is prepared through a solvothermal process in which Ni(II) metal centers of the MOF structure are chelated with nitrogen and oxygen functional groups of NGO. NGO/Ni7S6 exhibits bifunctional activity, capable of catalyzing both the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) with excellent stability in alkaline electrolytes, due to its high surface area, high pore volume, and tailored reaction interface enabling the availability of active nickel sites, mass transport, and gas release. Depending on the nitrogen doping level, the properties of graphene oxide can be tuned toward, e.g., enhanced stability of the composite compared to commonly used RuO2 under OER conditions. Hence, this work opens the door for the development of effective OER/HER electrocatalysts based on hierarchical porous graphene oxide composites with metal chalcogenides, which may replace expensive commercial catalysts such as RuO2 and IrO2. © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/adfm.201700451
  • 2017 • 153 Metallic NiPS3@NiOOH Core-Shell Heterostructures as Highly Efficient and Stable Electrocatalyst for the Oxygen Evolution Reaction
    Konkena, B. and Masa, J. and Botz, A. J. R. and Sinev, I. and Xia, W. and Kossmann, J. and Drautz, R. and Muhler, M. and Schuhmann, W.
    ACS Catalysis 7 229--237 (2017)
    We report metallic NiPS3@NiOOH core shell heterostructures as an efficient and durable electrocatalyst for the oxygen evolution reaction, exhibiting a low onset potential of 1.48 V (vs RHE) and stable performance for over 160 h. The atomically thin NiPS3 nanosheets are obtained by exfoliation of bulk NiPS3 in the presence of an ionic surfactant. The OER mechanism was studied by a combination of SECM, in situ Raman spectroscopy, SEM, and XPS measurements, which enabled direct observation of the formation of a NiPS3@NiOOH core shell heterostructure at the electrode interface. Hence, the active form of the catalyst is represented as NiPS3@NiOOH core shell structure. Moreover, DFT calculations indicate an intrinsic metallic character of the NiPS3 nanosheets with densities of states (DOS) similar to the bulk material. The high OER activity of the NiPS3 nanosheets is attributed to a high density of accessible active metallic-edge and defect sites due to structural disorder, a unique NiPS3@NiOOH core shell heterostructure, where the presence of P and S modulates the rface electronic structure of Ni in NiPS3, thus providing excellent conductive pathway for efficient electron-transport to the NiOOH shell. These findings suggest that good size control during liquid exfoliation may be advantageously used for the formation of electrically conductive NiPS3@ NiOOH core shell electrode materials for the electrochemical water oxidation.
    view abstractdoi: 10.1021/acscatal.6b02203
  • 2017 • 152 Detection of adsorbed transition-metal porphyrins by spin-dependent conductance of graphene nanoribbon
    Kratzer, P. and Tawfik, S.A. and Cui, X.Y. and Stampfl, C.
    RSC Advances 7 29112-29121 (2017)
    Electronic transport in a zig-zag-edge graphene nanoribbon (GNR) and its modification by adsorbed transition metal porphyrins is studied by means of density functional theory calculations. The detachment reaction of the metal centre of the porphyrin is investigated both in the gas phase and for molecules adsorbed on the GNR. As most metal porphyrins are very stable against this reaction, it is found that these molecules bind only weakly to a perfect nanoribbon. However, interaction with a single-atom vacancy in the GNR results in chemical bonding by the transition metal centre being shared between nitrogen atoms in the porphyrin ring and the carbon atoms next to the vacancy in the GNR. For both the physisorbed and the chemisorbed geometry, the inclusion of van der Waals interaction results in a significant enlargement of the binding energy and reduction of the adsorption height. Electronic transport calculations using non-equilibrium Greens functions show that the conductivity of the GNR is altered by the chemisorbed porphyrin molecules. Since the metal centers of porphyrins carry an element-specific magnetic moment, not only the net conductance, but also the spin-dependent conductance of the GNR is affected. In particular, the adsorption of Ru-porphyrin on the single-atom vacancy results in a very large spin polarization of the current of 88% at small applied source-drain voltages. Based on our results, we suggest that a spin valve constructed from a GNR with ferromagnetic contacts could be used as a sensitive detector that could discriminate between various metal porphyrins. © 2017 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c7ra04594h
  • 2017 • 151 Effect of titania surface modification of mesoporous silica SBA-15 supported Au catalysts: Activity and stability in the CO oxidation reaction
    Kučerová, G. and Strunk, J. and Muhler, M. and Behm, R.J.
    Journal of Catalysis 356 214-228 (2017)
    As part of an ongoing effort to understand the deactivation and improve the stability of metal oxide-supported Au catalysts in the low-temperature CO oxidation reaction while maintaining their high activity, we have investigated the influence of a mesoporous silica SBA-15 substrate on the activity and stability of Au/TiO2 catalysts, which consist of a SBA-15 support surface modified by a monolayer of TiOx with Au nanoparticles on top. The extent of the TiOx surface modification was systematically increased, while the Au loading and the Au particle sizes were largely kept constant. Employing kinetic measurements at three different temperatures (30 °C, 80 °C, 180 °C) and a number of ex situ methods as well as in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) for catalyst characterization, we found that the activity of these catalysts increases significantly with the Ti concentration and with reaction temperature. The tendency for deactivation remains essentially unchanged. Detailed in situ DRIFTS measurements reveal that the Au nanoparticles are largely formed on the TiOx surface-modified areas of the SBA-15 support and that the tendency for surface carbonate formation is very low. The observed deactivation may at least partly be related to the accumulation of molecularly adsorbed H2O species, in particular at low temperatures (30 °C). These are likely to be formed from surface hydroxyl groups, they may affect the reaction either by blocking of active sites or by blocking the adsorption of reactants on the substrate. Other effects, such as reaction induced changes in the titania layer, must however, play a role as well, both at 80 °C and in particular at 180 °C, where accumulation of adsorbed species is negligible. The mechanistic ideas are supported by reactivation tests subsequent to calcination at 400 °C, which were found to fully restore the initial activity. © 2017 Elsevier Inc.
    view abstractdoi: 10.1016/j.jcat.2017.09.017
  • 2017 • 150 Modeling of ductile fracture from shear to balanced biaxial tension for sheet metals
    Lou, Y. and Chen, L. and Clausmeyer, T. and Tekkaya, A.E. and Yoon, J.W.
    International Journal of Solids and Structures 112 169-184 (2017)
    A ductile fracture model is proposed to describe shear fracture of sheet metals from shear to balanced biaxial tension via uniaxial and plane strain tension. The fracture criterion models plastic damage as strain-induced void nucleation, triaxiality-governed void enlargement, Lode-controlled void torsion, and shear-restrained coalescence of voids. Its flexibility is investigated by a parameter study of the ductile fracture model proposed. The fracture model is employed to describe ductile fracture behavior of an aluminum alloy AA6082 T6 (thickness: 1.0. mm). Dogbone specimens are strained to characterize the strain hardening properties, while another four different specimens are tested to characterize fracture behavior in shear, uniaxial tension, plane strain tension and balanced biaxial tension. The loading processes are analyzed numerically with the stress invariant-based Drucker yield function which is for the first time specified for body-centered cubic and face-centered cubic metals. Fracture strains in various loading conditions are measured with a hybrid experimental-numerical approach. The measured fracture strains are then used to calibrate the ductile fracture model proposed. The ductile fracture model calibrated above is employed to predict the onset of ductile fracture for these four specimens. For the purpose of comparison, the predicted fracture strokes of these four loading conditions are compared with those predicted by the modified Mohr-Coulomb model (), and two micromechanism-inspired criteria proposed recently (). The comparison reveals that the proposed model predicts the fracture behavior in much better agreement compared with experimental results from shear to the balanced biaxial tension. Accordingly, the proposed ductile fracture criterion is recommended for the prediction of ductile fracture in sheet metal forming processes, optimization of forming parameters and design of tools for both solid elements and shell elements. Besides, the ductile fracture model proposed can also be applied in various bulk metal forming processes in case that the model is calibrated by proper sets of experiments. © 2017.
    view abstractdoi: 10.1016/j.ijsolstr.2016.11.034
  • 2017 • 149 Laser synthesis, structure and chemical properties of colloidal nickel-molybdenum nanoparticles for the substitution of noble metals in heterogeneous catalysis
    Marzun, G. and Levish, A. and Mackert, V. and Kallio, T. and Barcikowski, S. and Wagener, P.
    Journal of Colloid and Interface Science 489 57-67 (2017)
    Platinum and iridium are rare and expensive noble metals that are used as catalysts for different sectors including in heterogeneous chemical automotive emission catalysis and electrochemical energy conversion. Nickel and its alloys are promising materials to substitute noble metals. Nickel based materials are cost-effective with good availability and show comparable catalytic performances. The nickel-molybdenum system is a very interesting alternative to platinum in water electrolysis. We produced ligand-free nickel-molybdenum nanoparticles by laser ablation in water and acetone. Our results show that segregated particles were formed in water due to the oxidation of the metals. X-ray diffraction shows a significant change in the lattice parameter due to a diffusion of molybdenum atoms into the nickel lattice with increasing activity in the electrochemical oxygen evolution reaction. Even though the solubility of molecular oxygen in acetone is higher than in water, there were no oxides and a more homogeneous metal distribution in the particles in acetone as seen by TEM-EDX. This showed that dissolved molecular oxygen does not control oxide formation. Overall, the laser ablation of pressed micro particulate mixtures in liquids offers a combinational synthesis approach that allows the screening of alloy nanoparticles for catalytic testing and can convert micro-mixtures into nano-alloys. © 2016 Elsevier Inc.
    view abstractdoi: 10.1016/j.jcis.2016.09.014
  • 2017 • 148 Adaptation of TiC hard particles properties and morphology in metal matrix composites by refractory elements
    Mohr, A. and Röttger, A. and Theisen, W.
    Key Engineering Materials 742 KEM 99-105 (2017)
    High mechanical loads, corrosion, and abrasion decrease the lifetime of tools. One way to increase the wear resistance of tool materials can be achieved by adding hard particles to the metal matrix such as titanium carbide, which protect the softer metal matrix against abrasive particles. This material concept is designated as metal matrix composite (MMC). Ferro-Titanit® is such MMC material, possessing high wear and a simultaneously high corrosion resistance, for which reason this material is used in the polymers industry. The material concept is based on a corrosion-resistant Fe-base matrix with up to 45 vol% titanium carbide (TiC) as a hard particle addition to improve the wear resistance against abrasion. These TiC hard particles must be adapted to the present tribological system in terms of hardness, size and morphology. This study shows how the size and morphology of TiC hard particles can be influenced by the refractory element niobium (Nb). Therefore, the element Nb was added with 2 and 4 mass% to the soft-martensitic Ferro-Titanit® Grade Nikro128. The investigated materials were compacted by sintering, and the densified microstructure was further characterized by scanning electron microscopy (SEM), energy dispersive spectrometry (EDX), and optical image analyses. Furthermore, microstructure and properties of the compacted Nb-alloyed samples were compared to the reference material Nikro128. The results show that the addition of Nb influences the morphology, size and chemical composition of the TiC hard particle. These changes in the hard phase characteristics also influence the materials properties. It was shown that the phase niobium carbide (NbC) is formed around the TiC during the densification process, leading to a change in morphology and size of the TiC. © 2017 Trans Tech Publications, Switzerland.
    view abstractdoi: 10.4028/www.scientific.net/KEM.742.99
  • 2017 • 147 Toughness and Fracture Properties in Nacre-Mimetic Clay/Polymer Nanocomposites
    Morits, M. and Verho, T. and Sorvari, J. and Liljeström, V. and Kostiainen, M.A. and Gröschel, A.H. and Ikkala, O.
    Advanced Functional Materials 27 1605378 (2017)
    Nacre inspires researchers by combining stiffness with toughness by its unique microstructure of aligned aragonite platelets. This brick-and-mortar structure of reinforcing platelets separated with thin organic matrix has been replicated in numerous mimics that can be divided into two categories: microcomposites with aligned metal oxide microplatelets in polymer matrix, and nanocomposites with self-assembled nanoplatelets-usually clay or graphene oxide-and polymer. While microcomposites have shown exceptional fracture toughness, current fabrication methods have limited nacre-mimetic nanocomposites to thin films where fracture properties remained unexplored. Yet, fracture resistance is the defining property of nacre, therefore centrally important in any mimic. Furthermore, to make use of these properties in applications, bulk materials are required. Here, up to centimeter-thick nacre-mimetic clay/polymer nanocomposites are produced by the lamination of self-assembled films. The aligned clay nanoplatelets are separated by poly(vinyl alcohol) matrix, with 106-107 nanoplatelets on top of each other in the bulk plates. Fracture testing shows crack deflection and a fracture toughness of 3.4 MPa m1/2, not far from nacre. Flexural tests show high stiffness (25 GPa) and strength (220 MPa) that, despite the hydrophilic constituents, are not substantially affected by exposure to humidity. © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/adfm.201605378
  • 2017 • 146 Rigidly Tethered Bis-phosphoric Acids: Generation of Tunable Chiral Fluorescent Frameworks and Unexpected Selectivity for the Detection of Ferric Ions
    Octa-Smolin, F. and Mitra, R. and Thiele, M. and Daniliuc, C.G. and Stegemann, L. and Strassert, C. and Niemeyer, J.
    Chemistry - A European Journal 23 10058-10067 (2017)
    We describe the straightforward synthesis of a series of bis-phosphoric acids (R,R)-1 a–d, featuring two chiral 1,1′-binaphthyl-phosphoric acid units that are tethered by rigid, π-conjugated linkers. The nature of the linker has a profound influence on the properties of the bis-phosphoric acids, such as their self-association behavior and their interaction with metal ions. This led to the identification of one preferred bis-phosphoric acid (R,R)-1 d, which shows selective fluorescence quenching in the presence of ferric ions (Fe3+). Thus, (R,R)-1 d could be applied for the detection of Fe3+, even in the presence of a variety of other metal ions. The chiral nature of the bis-phosphoric acid enables the interaction with Fe3+ to be followed by CD spectroscopy, providing a complementary detection mode with the same probe. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/chem.201700954
  • 2017 • 145 Organometallic Fe–Fe Interactions: Beyond Common Metal–Metal Bonds and Inverse Mixed-Valent Charge Transfer
    Ringenberg, M.R. and Schwilk, M. and Wittkamp, F. and Apfel, U.-P. and Kaim, W.
    Chemistry - A European Journal 23 1770-1774 (2017)
    The compounds [Fe(CO)3(dRpf)]n+, n=0, 1, 2 and dRpf=1,1′-bis(dicyclohexylphosphino)ferrocene ([1]n+) or 1,1′-bis(diisopropylphosphino)ferrocene ([2]n+), were obtained as two-step reversible redox systems by photolytic and redox reactions. The iron–iron distance decreases from about 4 Å to about 3 Å on oxidation, which takes place primarily at the tricarbonyliron moiety. Whereas ferrocene oxidation is calculated to occur only in excited states, the near infrared absorptions of the mixed-valent monocations are due to an unprecedented “inverse” inter-valence charge transfer from the electron-rich iron(II) in the ferrocene backbone to the electron-deficient tricarbonyliron(I). Protonation of complex 1 results in the formation of the structurally characterized hydride [1H]BF4, which reacts with acetone to form the dication, 12+, and isopropanol. While the hydride [2H]BF4was found to be unstable, protonation of 2 in acetone resulted in the clean formation of 22+,formally a hydrogen transfer. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/chem.201605527
  • 2017 • 144 Encapsulation of Bimetallic Metal Nanoparticles into Robust Zirconium-Based Metal-Organic Frameworks: Evaluation of the Catalytic Potential for Size-Selective Hydrogenation
    Rösler, C. and Dissegna, S. and Rechac, V.L. and Kauer, M. and Guo, P. and Turner, S. and Ollegott, K. and Kobayashi, H. and Yamamoto, T. and Peeters, D. and Wang, Y. and Matsumura, S. and Van Tendeloo, G. and Kitagawa, H. and Mu...
    Chemistry - A European Journal 23 3583-3594 (2017)
    The realization of metal nanoparticles (NPs) with bimetallic character and distinct composition for specific catalytic applications is an intensively studied field. Due to the synergy between metals, most bimetallic particles exhibit unique properties that are hardly provided by the individual monometallic counterparts. However, as small-sized NPs possess high surface energy, agglomeration during catalytic reactions is favored. Sufficient stabilization can be achieved by confinement of NPs in porous support materials. In this sense, metal-organic frameworks (MOFs) in particular have gained a lot of attention during the last years; however, encapsulation of bimetallic species remains challenging. Herein, the exclusive embedding of preformed core-shell PdPt and RuPt NPs into chemically robust Zr-based MOFs is presented. Microstructural characterization manifests partial retention of the core-shell systems after successful encapsulation without harming the crystallinity of the microporous support. The resulting chemically robust NP@UiO-66 materials exhibit enhanced catalytic activity towards the liquid-phase hydrogenation of nitrobenzene, competitive with commercially used Pt on activated carbon, but with superior size-selectivity for sterically varied substrates. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/chem.201603984
  • 2017 • 143 Laser-induced surface activation of biocomposites for electroless metallization
    Rytlewski, P. and Bahners, T. and Polewski, F. and Gebert, B. and Gutmann, J.S. and Hartmann, N. and Hagemann, U. and Moraczewski, K.
    Surface and Coatings Technology 311 104-112 (2017)
    In this work biocomposites containing polylactide (PLA), polycaprolactone (PCL), copper(II) oxide and copper acetylacetonate were manufactured by an extrusion process. The extruded composites differed with respect to the PLA/PCL ratio whereas the content of mixed copper(II) oxide and copper acetylacetonate powders was held constant at 20 wt%. The main aims for the addition of PCL was to increase impact strength resistance, improve surface catalytic properties and reduce the temperature of extrusion, thus limiting degradation effects initiated by copper acetylacetonate. The composite samples were irradiated with an ArF excimer laser varying the number of laser pulses and then metalized by electroless plating. Based on optical microscopy, scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX) and X-ray photoelectron spectroscopy (XPS) measurements, it was found that (i) PCL was dispersed in the form of droplets in all volume of PLA, (ii) the copper compounds were preferably located in the dispersed PCL phase, and (iii) composites with higher PCL content were more effectively metalized. © 2016 Elsevier B.V.
    view abstractdoi: 10.1016/j.surfcoat.2016.12.048
  • 2017 • 142 Ultra-stiff metallic glasses through bond energy density design
    Schnabel, V. and Köhler, M. and Music, D. and Bednarcik, J. and Clegg, W.J. and Raabe, D. and Schneider, J.M.
    Journal of Physics Condensed Matter 29 (2017)
    The elastic properties of crystalline metals scale with their valence electron density. Similar observations have been made for metallic glasses. However, for metallic glasses where covalent bonding predominates, such as metalloid metallic glasses, this relationship appears to break down. At present, the reasons for this are not understood. Using high energy x-ray diffraction analysis of melt spun and thin film metallic glasses combined with density functional theory based molecular dynamics simulations, we show that the physical origin of the ultrahigh stiffness in both metalloid and non-metalloid metallic glasses is best understood in terms of the bond energy density. Using the bond energy density as novel materials design criterion for ultra-stiff metallic glasses, we are able to predict a Co33.0Ta3.5B63.5 short range ordered material by density functional theory based molecular dynamics simulations with a high bond energy density of 0.94 eV Å-3 and a bulk modulus of 263 GPa, which is 17% greater than the stiffest Co-B based metallic glasses reported in literature. © 2017 IOP Publishing Ltd.
    view abstractdoi: 10.1088/1361-648X/aa72cb
  • 2017 • 141 Oscillatory combustion of propene during in situ mechanical activation of solid catalysts
    Schreyer, H. and Immohr, S. and Schüth, F.
    Journal of Materials Science 52 12021-12030 (2017)
    Mechanochemical activation of solids can lead to a strong increase in their activity as catalysts in heterogeneously catalyzed reactions. In the following, we report on the effects of solid catalyst activation during ball milling that lead to oscillatory behavior in CO and CO2 formation during propene oxidation. The oscillations arise under in situ ball milling conditions over chromium(III) oxide (Cr2O3) and cerium(IV) oxide (CeO2), respectively. The experiments were conducted under continuous gas flow at ambient pressure and temperature, using both a modified steel and a tungsten carbide milling vessel. Abrasion of particles from the steel milling vessel could be eliminated as the sole cause for the oscillations through substitution by a tungsten carbide milling vessel. The intensity and frequency of oscillations are shown to be dependent on the propene-to-oxygen ratio, the milling frequency, milling ball size and metal oxide used. Overall, Cr2O3 shows higher activity for oscillatory propene combustion under in situ mechanical activation than CeO2. © 2017, The Author(s).
    view abstractdoi: 10.1007/s10853-017-1153-z
  • 2017 • 140 Adaptive Behavior of Dynamic Orthoester Cryptands
    Shyshov, O. and Brachvogel, R.-C. and Bachmann, T. and Srikantharajah, R. and Segets, D. and Hampel, F. and Puchta, R. and von Delius, M.
    Angewandte Chemie - International Edition 56 776-781 (2017)
    The integration of dynamic covalent bonds into macrocycles has been a tremendously successful strategy for investigating noncovalent interactions and identifying effective host–guest pairs. While numerous studies have focused on the dynamic responses of macrocycles and larger cages to various guests, the corresponding constitutionally dynamic chemistry of cryptands remains unexplored. Reported here is that cryptands based on orthoester bridgeheads offer an elegant entry to experiments in which a metal ion selects its preferred host from a dynamic mixture of competing subcomponents. In such dynamic mixtures, the alkali metal ions Li+, Na+, K+, Rb+, and Cs+exhibit pronounced preferences for the formation of cryptands of certain sizes and donor numbers, and the selection is rationalized by DFT calculations. Reported is also the first self-assembly of a chiral orthoester cryptate and a preliminary study on the use of stereoisomers as subcomponents. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/anie.201609855
  • 2017 • 139 Standardized Benchmarking of Water Splitting Catalysts in a Combined Electrochemical Flow Cell/Inductively Coupled Plasma-Optical Emission Spectrometry (ICP-OES) Setup
    Spanos, I. and Auer, A.A. and Neugebauer, S. and Deng, X. and Tüysüz, H. and Schlögl, R.
    ACS Catalysis 7 3768-3778 (2017)
    The oxygen evolution reaction (OER) is the limiting step in splitting water into its constituents, hydrogen and oxygen. Hence, research on potential OER catalysts has become the focus of many studies. In this work, we investigate capable OER catalysts but focus on catalyst stability, which is, especially in this case, at least equally as important as catalyst activity. We propose a specialized setup for monitoring the corrosion profiles of metal oxide catalysts during a stability testing protocol, which is specifically designed to standardize the investigation of OER catalysts by means of differentiating between catalyst corrosion and deactivation, oxygen evolution efficiency, and catalyst activity. For this purpose, we combined an electrochemical flow cell (EFC) with an oxygen sensor and an inductively coupled plasma-optical emission spectrometry (ICP-OES) system for the simultaneous investigation of catalyst deactivation, activity, and faradaic efficiency of catalysts. We tested various catalysts, with IrO2 and NiCoO2 used as benchmark materials in acidic and alkaline environment, respectively. The scalability of our setup will allow the user to investigate catalytic materials with supports of higher surface area than those which are typical for microelectrochemical flow cells (thus, under conditions more similar to those of commercial electrolyzers). © 2017 American Chemical Society.
    view abstractdoi: 10.1021/acscatal.7b00632
  • 2017 • 138 Recent Advances in Selective Propylene Oxidation over Bismuth Molybdate Based Catalysts: Synthetic, Spectroscopic, and Theoretical Approaches
    Sprenger, P. and Kleist, W. and Grunwaldt, J.-D.
    ACS Catalysis 7 5628-5642 (2017)
    The selective oxidation of propylene to acrolein is an important reaction in the chemical industry which has been extensively studied over the last few decades. Today, spectroscopic, computational, and synthetic approaches allow a renewed view of this established and well-understood catalytic process at a fundamental level. Consequently, a revised mechanistic pathway for the selective propylene oxidation over bismuth molybdates has been suggested recently. Furthermore, studies concerning the local interaction of specific surface entities as well as concepts from semiconductor science have provided valuable information to describe the operation mode of oxidation catalysts. New synthetic methods can be used not only to tune the specific surface area and surface species of a catalyst but also to give direct access to distinct metal oxide phases or specific crystalline phases with a synergetic interplay on the nanoscale. Since complex multicomponent systems, which exhibit both higher selectivity and activity in comparison to pure bismuth molybdates, are used for industrial applications, it is important to transfer the research concepts from such model systems to those more complex systems. This also involves operando characterization techniques on multiple length scales. Recent research activities shine a renewed light on this well-studied reaction, which therefore may become one of the drivers in selective oxidation catalysis to apply and further establish new tools that have been developed in theory, modeling, synthesis, and operando spectroscopy. (Chemical Equation Presented). © 2017 American Chemical Society.
    view abstractdoi: 10.1021/acscatal.7b01149
  • 2017 • 137 Feasibility study of fluxless brazing cemented carbides to steel
    Tillmann, W. and Sievers, N.
    IOP Conference Series: Materials Science and Engineering 181 (2017)
    One of the most important brazing processes is the joints between cemented carbides and steel for the tool industry such as in rotary drill hammers or saw blades. Even though this technique has already been used for several decades, defects in the joint can still occur and lead to quality loss. Mostly, the joining process is facilitated by induction heating and the use of a flux to enhance the wetting of the filler alloy on the surface of the steel and cemented carbide in an ambient atmosphere. However, although the use of flux enables successful joining, it also generates voids within the joint, which reduces the strength of the connection while the chemicals within the flux are toxic and polluting. In this feasibility study, a fluxless brazing process is used to examine the joint between cemented carbides and steel for the first time. For this, ultrasound is applied during induction heating to enable the wetting between the liquid filler metal and the surfaces of the cemented carbide and steel. The ultrasound generates cavitations within the liquid filler metal, which remove the oxides from the surface. Several filler metals such as a silver based alloy Ag449, pure Zn, and an AlSi-alloy were used to reduce the brazing temperature and to lower the thermal residual stresses within the joint. As a result, every filler metal successfully wetted both materials and led to a dense connection. The ultrasound has to be applied carefully to prevent a damage of the cemented carbide. In this regard, it was observed that single grains of the cemented carbide broke out and remained in the joint. This positive result of brazing cemented carbides to steel without a flux but using ultrasound, allows future studies to focus on the shear strength of these joints as well as the behavior of the thermally induced residual stresses. © Published under licence by IOP Publishing Ltd.
    view abstractdoi: 10.1088/1757-899X/181/1/012007
  • 2017 • 136 High-Performance Energy Storage and Conversion Materials Derived from a Single Metal-Organic Framework/Graphene Aerogel Composite
    Xia, W. and Qu, C. and Liang, Z. and Zhao, B. and Dai, S. and Qiu, B. and Jiao, Y. and Zhang, Q. and Huang, X. and Guo, W. and Dang, D. and Zou, R. and Xia, D. and Xu, Q. and Liu, M.
    Nano Letters 17 2788-2795 (2017)
    Metal oxides and carbon-based materials are the most promising electrode materials for a wide range of low-cost and highly efficient energy storage and conversion devices. Creating unique nanostructures of metal oxides and carbon materials is imperative to the development of a new generation of electrodes with high energy and power density. Here we report our findings in the development of a novel graphene aerogel assisted method for preparation of metal oxide nanoparticles (NPs) derived from bulk MOFs (Co-based MOF, Co(mIM)2 (mIM = 2-methylimidazole). The presence of cobalt oxide (CoOx) hollow NPs with a uniform size of 35 nm monodispersed in N-doped graphene aerogels (NG-A) was confirmed by microscopic analyses. The evolved structure (denoted as CoOx/NG-A) served as a robust Pt-free electrocatalyst with excellent activity for the oxygen reduction reaction (ORR) in an alkaline electrolyte solution. In addition, when Co was removed, the resulting nitrogen-rich porous carbon-graphene composite electrode (denoted as C/NG-A) displayed exceptional capacitance and rate capability in a supercapacitor. Further, this method is readily applicable to creation of functional metal oxide hollow nanoparticles on the surface of other carbon materials such as graphene and carbon nanotubes, providing a good opportunity to tune their physical or chemical activities. © 2017 American Chemical Society.
    view abstractdoi: 10.1021/acs.nanolett.6b05004
  • 2017 • 135 Nanoparticulate versus ionic silver: Behavior in the tank water, bioaccumulation, elimination and subcellular distribution in the freshwater mussel Dreissena polymorpha
    Zimmermann, S. and Ruchter, N. and Loza, K. and Epple, M. and Sures, B.
    Environmental Pollution 222 251-260 (2017)
    Zebra mussels (Dreissena polymorpha) were exposed to polyvinylpyrrolidone (PVP)-coated silver nanoparticles (AgNP; hydrodynamic diameter 80 nm; solid diameter 50 nm) to investigate the behavior of Ag in the tank water with respect to its uptake, bioaccumulation, elimination and subcellular distribution in the mussel soft tissue. Parallel experiments were performed with ionic Ag (AgNO3) to unravel possible differences between the metal forms. The recovery of the applied Ag concentration (500 μg/L) in the tank water was clearly affected by the metal source (AgNP < AgNO3) and water type (reconstituted water < tap water). Filtration (< 0.45 μm) of water samples showed different effects on the quantified metal concentration depending on the water type and Ag form. Ag accumulation in the mussel soft tissue was neither influenced by the metal source nor by the water type. Ag concentrations in the mussel soft tissue did not decrease during 14 days of depuration. For both metal forms the Ag distribution within different subcellular fractions, i.e. metal-rich granules (MRG), cellular debris, organelles, heat-sensitive proteins (HSP) and metallothionein-like proteins (MTLP), revealed time-dependent changes which can be referred to intracellular Ag translocation processes. The results provide clear evidence for the uptake of Ag by the mussel soft tissue in nanoparticulate as well as in ionic form. Thus, zebra mussels could be used as effective accumulation indicators for environmental monitoring of both Ag forms. © 2016 Elsevier Ltd.
    view abstractdoi: 10.1016/j.envpol.2016.12.048
  • 2016 • 134 Co@Co3O4 Encapsulated in Carbon Nanotube-Grafted Nitrogen-Doped Carbon Polyhedra as an Advanced Bifunctional Oxygen Electrode
    Aijaz, A. and Masa, J. and Rösler, C. and Xia, W. and Weide, P. and Botz, A.J.R. and Fischer, R.A. and Schuhmann, W. and Muhler, M.
    Angewandte Chemie - International Edition 55 4087-4091 (2016)
    Efficient reversible oxygen electrodes for both the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER) are vitally important for various energy conversion devices, such as regenerative fuel cells and metal-air batteries. However, realization of such electrodes is impeded by insufficient activity and instability of electrocatalysts for both water splitting and oxygen reduction. We report highly active bifunctional electrocatalysts for oxygen electrodes comprising core-shell Co@Co3O4 nanoparticles embedded in CNT-grafted N-doped carbon-polyhedra obtained by the pyrolysis of cobalt metal-organic framework (ZIF-67) in a reductive H2 atmosphere and subsequent controlled oxidative calcination. The catalysts afford 0.85 V reversible overvoltage in 0.1 m KOH, surpassing Pt/C, IrO2, and RuO2 and thus ranking them among one of the best non-precious-metal electrocatalysts for reversible oxygen electrodes. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/anie.201509382
  • 2016 • 133 Closed-loop control of product properties in metal forming
    Allwood, J.M. and Duncan, S.R. and Cao, J. and Groche, P. and Hirt, G. and Kinsey, B. and Kuboki, T. and Liewald, M. and Sterzing, A. and Tekkaya, A.E.
    CIRP Annals - Manufacturing Technology 65 573-596 (2016)
    Metal forming processes operate in conditions of uncertainty due to parameter variation and imperfect understanding. This uncertainty leads to a degradation of product properties from customer specifications, which can be reduced by the use of closed-loop control. A framework of analysis is presented for understanding closed-loop control in metal forming, allowing an assessment of current and future developments in actuators, sensors and models. This leads to a survey of current and emerging applications across a broad spectrum of metal forming processes, and a discussion of likely developments. © 2016 The Author(s)
    view abstractdoi: 10.1016/j.cirp.2016.06.002
  • 2016 • 132 The effect of sodium on the structure-activity relationships of cobalt-modified Cu/ZnO/Al2O3 catalysts applied in the hydrogenation of carbon monoxide to higher alcohols
    Anton, J. and Nebel, J. and Song, H. and Froese, C. and Weide, P. and Ruland, H. and Muhler, M. and Kaluza, S.
    Journal of Catalysis 335 175-186 (2016)
    A series of Co-modified Cu/ZnO/Al2O3 methanol synthesis catalysts with different Na loadings was prepared and applied in higher alcohol synthesis (HAS) at 280 °C, 60 bar and a ratio of H2/CO = 1. The bulk and surface properties of the catalysts were characterized after reduction and after 40 h time on stream (TOS) without exposing the catalysts to air during the transfer and the measurements. Increased presence of metallic Co0 after reduction at 350 °C was confirmed by X-ray photoelectron spectroscopy indicating metallic Cu0 to act as a reduction promoter. Catalysts with low Na loadings (≤0.6 wt%) showed strong initial deactivation presumably due to coking of isolated Co0 surface sites favoring hydrocarbon formation. The selectivity to higher alcohols gradually increased during the first 10 h TOS indicating enhanced Cu-Co surface alloy formation considered as active sites for HAS. In contrast, with high Na loadings (≥0.8 wt%) deactivation did not occur and stable performance with constant CO conversion and product distribution was observed indicating significantly altered structural properties. High Na loadings caused the stabilizing amorphous oxide matrix to collapse resulting in strong sintering of the metallic Cu particles, and an increased carbidization of metallic Co0 forming bulk Co2C was observed by X-ray diffraction. Close contact between metallic Co0 and Co2C, which is known to facilitate molecular CO adsorption, is assumed to generate additional active sites for HAS. © 2016 Elsevier Inc. All rights reserved.
    view abstractdoi: 10.1016/j.jcat.2015.12.016
  • 2016 • 131 In-situ metal matrix composite steels: Effect of alloying and annealing on morphology, structure and mechanical properties of TiB2 particle containing high modulus steels
    Aparicio-Fernández, R. and Springer, H. and Szczepaniak, A. and Zhang, H. and Raabe, D.
    Acta Materialia 107 38-48 (2016)
    We systematically study the morphology, size and dispersion of TiB2 particles formed in-situ from Fe-Ti-B based melts, as well as their chemical composition, crystal structure and mechanical properties. The effects of 5 wt.% additions of Cr, Ni, Co, Mo, W, Mn, Al, Si, V, Ta, Nb and Zr, respectively, as well as additional annealing treatments, were investigated in order to derive guidelines for the knowledge based alloy design of steels with an increased stiffness/density ratio and sufficiently high ductility. All alloying elements were found to increase the size of the coarse primary TiB2 particles, while Co led to the most homogeneous size distribution. The size of the eutectic TiB2 constituents was decreased by all alloying additions except Ni, while their aspect ratio was little affected. No clear relation between chemical composition, crystal structure and mechanical properties of the particles could be observed. Annealing of the as-cast alloys slightly increased the size of the primary particles, but at the same time strongly spheroidised the eutectics. Additions of Co and Cr appear thus as the best starting point for designing novel in-situ high modulus metal matrix composite steels, while using Mn in concert with thermo-mechanical processing is most suited to adapt the matrix' microstructure and optimise the particle/matrix co-deformation processes. © 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2016.01.048
  • 2016 • 130 Efficient liquid metallurgy synthesis of Fe-TiB2 high modulus steels via in-situ reduction of titanium oxides
    Baron, C. and Springer, H. and Raabe, D.
    Materials and Design 97 357-363 (2016)
    We studied the in-situ reduction of Ti oxides by Al as an alternative and cost effective route for the liquid metallurgical synthesis of low density, high stiffness steels (high modulus steels) containing about 10 vol.% TiB2. TiO2, TiO1.83 and TiO were inserted via iron tubes into Fe-B melts, with Al either premixed with the oxide powders or liquid in the melt. Depending on Ti oxide type and location of the redox partner Al, greatly differing reaction kinetics, slag formation and corresponding microstructures of the high modulus steels were observed. TiO1.83 and TiO premixed with Al showed the highest TiB2 yield in the cast steel and are thus favourable candidates for the cost effective production of high modulus steels. Based on our findings, a novel synthesis process is proposed, based on filling wire injection into a continuous casting process, allowing the utilisation of the additionally formed oxide particles for the further improvement of the property profile of high modulus steels. © 2016 Elsevier Ltd.
    view abstractdoi: 10.1016/j.matdes.2016.02.076
  • 2016 • 129 Super-resolution for scanning light stimulation systems
    Bitzer, L.A. and Neumann, K. and Benson, N. and Schmechel, R.
    Review of Scientific Instruments 87 (2016)
    Super-resolution (SR) is a technique used in digital image processing to overcome the resolution limitation of imaging systems. In this process, a single high resolution image is reconstructed from multiple low resolution images. SR is commonly used for CCD and CMOS (Complementary Metal-Oxide-Semiconductor) sensor images, as well as for medical applications, e.g., magnetic resonance imaging. Here, we demonstrate that super-resolution can be applied with scanning light stimulation (LS) systems, which are common to obtain space-resolved electro-optical parameters of a sample. For our purposes, the Projection Onto Convex Sets (POCS) was chosen and modified to suit the needs of LS systems. To demonstrate the SR adaption, an Optical Beam Induced Current (OBIC) LS system was used. The POCS algorithm was optimized by means of OBIC short circuit current measurements on a multicrystalline solar cell, resulting in a mean square error reduction of up to 61% and improved image quality. © 2016 Author(s).
    view abstractdoi: 10.1063/1.4961748
  • 2016 • 128 The second Sandia Fracture Challenge: predictions of ductile failure under quasi-static and moderate-rate dynamic loading
    Boyce, B.L. and Kramer, S.L.B. and Bosiljevac, T.R. and Corona, E. and Moore, J.A. and Elkhodary, K. and Simha, C.H.M. and Williams, B.W. and Cerrone, A.R. and Nonn, A. and Hochhalter, J.D. and Bomarito, G.F. and Warner, J.E. and ...
    International Journal of Fracture 198 5-100 (2016)
    Ductile failure of structural metals is relevant to a wide range of engineering scenarios. Computational methods are employed to anticipate the critical conditions of failure, yet they sometimes provide inaccurate and misleading predictions. Challenge scenarios, such as the one presented in the current work, provide an opportunity to assess the blind, quantitative predictive ability of simulation methods against a previously unseen failure problem. Rather than evaluate the predictions of a single simulation approach, the Sandia Fracture Challenge relies on numerous volunteer teams with expertise in computational mechanics to apply a broad range of computational methods, numerical algorithms, and constitutive models to the challenge. This exercise is intended to evaluate the state of health of technologies available for failure prediction. In the first Sandia Fracture Challenge, a wide range of issues were raised in ductile failure modeling, including a lack of consistency in failure models, the importance of shear calibration data, and difficulties in quantifying the uncertainty of prediction [see Boyce et al. (Int J Fract 186:5–68, 2014) for details of these observations]. This second Sandia Fracture Challenge investigated the ductile rupture of a Ti–6Al–4V sheet under both quasi-static and modest-rate dynamic loading (failure in (Formula presented.) 0.1 s). Like the previous challenge, the sheet had an unusual arrangement of notches and holes that added geometric complexity and fostered a competition between tensile- and shear-dominated failure modes. The teams were asked to predict the fracture path and quantitative far-field failure metrics such as the peak force and displacement to cause crack initiation. Fourteen teams contributed blind predictions, and the experimental outcomes were quantified in three independent test labs. Additional shortcomings were revealed in this second challenge such as inconsistency in the application of appropriate boundary conditions, need for a thermomechanical treatment of the heat generation in the dynamic loading condition, and further difficulties in model calibration based on limited real-world engineering data. As with the prior challenge, this work not only documents the ‘state-of-the-art’ in computational failure prediction of ductile tearing scenarios, but also provides a detailed dataset for non-blind assessment of alternative methods. © 2016, The Author(s).
    view abstractdoi: 10.1007/s10704-016-0089-7
  • 2016 • 127 An analytical model to predict the shock pressure amplitude in vaporizing foils
    Cai, S. and Weddeling, C. and Tekkaya, A.E.
    Journal of Materials Processing Technology 231 374-381 (2016)
    Metal wires or foils can be vaporized when high currents are applied. The generated metal gas or plasma (at higher temperature) will expand very rapidly with high pressure. A shock wave is induced thereafter and then transmits through a polyurethane plate and finally provides the pressure pulse to the sheet metal, which results in a deformation of the sheet. In this work, an analytical model is introduced which describes the shock pressure on the sheet metal. In order to verify the analytical model, measurements of shock pressure pulses with different charging energies and polyurethane plate thicknesses are conducted. It turns out that the results predicted by the analytical model are in good agreement with the experimental data. Hence, the analytical model is successfully established for analyzing the shock pressures induced by vaporizing foils. © 2016 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.jmatprotec.2016.01.013
  • 2016 • 126 Direct metal deposition of refractory high entropy alloy MoNbTaW
    Dobbelstein, H. and Thiele, M. and Gurevich, E.L. and George, E.P. and Ostendorf, A.
    Physics Procedia 83 624-633 (2016)
    Alloying of refractory high entropy alloys (HEAs) such as MoNbTaW is usually done by vacuum arc melting (VAM) or powder metallurgy (PM) due to the high melting points of the elements. Machining to produce the final shape of parts is often needed after the PM process. Casting processes, which are often used for aerospace components (turbine blades, vanes), are not possible. Direct metal deposition (DMD) is an additive manufacturing technique used for the refurbishment of superalloy components, but generating these components from the bottom up is also of current research interest. MoNbTaW possesses high yield strength at high temperatures and could be an alternative to state-of-the-art materials. In this study, DMD of an equimolar mixture of elemental powders was performed with a pulsed Nd:YAG laser. Single wall structures were built, deposition strategies developed and the microstructure of MoNbTaW was analyzed by back scattered electrons (BSE) and energy dispersive X-ray (EDX) spectroscopy in a scanning electron microscope. DMD enables the generation of composition gradients by using dynamic powder mixing instead of pre-alloyed powders. However, the simultaneous handling of several elemental or pre-alloyed powders brings new challenges to the deposition process. The influence of thermal properties, melting point and vapor pressure on the deposition process and chemical composition will be discussed. © 2016 The Authors.
    view abstractdoi: 10.1016/j.phpro.2016.08.065
  • 2016 • 125 Improving the Out-Coupling of a Metal-Metal Terahertz Frequency Quantum Cascade Laser Through Integration of a Hybrid Mode Section into the Waveguide
    Fobbe, T. and Nong, H. and Schott, R. and Pal, S. and Markmann, S. and Hekmat, N. and Zhu, J. and Han, Y. and Li, L. and Dean, P. and Linfield, E.H. and Davies, A.G. and Wieck, A.D. and Jukam, N.
    Journal of Infrared, Millimeter, and Terahertz Waves 37 426-434 (2016)
    A hybrid mode section is integrated into the end of the metal-metal waveguide of a terahertz (THz) frequency quantum cascade laser (QCL) by removing sub-wavelength portions of the top metal layer. This allows a hybrid mode to penetrate into the air, which reduces the effective index of the mode and improves the out-coupling performance at the facet. The transmission of the hybrid section is further increased by ensuring its length fulfills the criterion for constructive interference. These simple modifications to a 2.5-THz metal-metal QCL waveguide result in a significant increase in the output emission power. In addition, simulations show that further improvements in out-coupling efficiency can be achieved for lower frequencies with effective refractive indices close to the geometric mean of the indices of the metal-metal waveguide and air. © 2016, Springer Science+Business Media New York.
    view abstractdoi: 10.1007/s10762-015-0239-4
  • 2016 • 124 Interlayer expansion using metal-linker units: Crystalline microporous silicate zeolites with metal centers on specific framework sites
    Gies, H. and Feyen, M. and De Baerdemaeker, T. and De Vos, D.E. and Yilmaz, B. and Müller, U. and Meng, X. and Xiao, F.-S. and Zhang, W. and Yokoi, T. and Tatsumi, T. and Bao, X.
    Microporous and Mesoporous Materials 222 235-240 (2016)
    Interlayer expansion using silylating agents to connect layer silicates to 3D framework structures has shown to be a versatile synthesis route to new crystalline, microporous frameworks. We demonstrate here that also Me cations can be introduced on the linker sites applying the same synthesis procedure. An acidic aqueous Fe-chloride solution was used in a hydrothermal reaction to convert the layered hydrous silicate precursor RUB-36 into an interlayer expanded zeolite, containing Fe at the linker sites, Fe-IEZ-RUB-36, Si19.14Fe0.86O38(OH)4. Structure analysis from powder X-ray data using the Rietveld technique confirmed that the porous framework is stable upon calcination and contains Fe on T-sites at the linker position. SEM-EDX analysis is in agreement with the analysis of the electron density maps showing that almost every other linker T-position is occupied by Fe-ions. The material crystallizes in the monoclinic space group Pm with a = 12.200(9) Å, b = 13.981(8) Å, c = 7.369(2) Å, and β = 106.9(1)°. Applying a similar synthesis procedure, the Sn-analog, Sn-IEZ-RUB-36, Si38.6Sn1.4O76(OH)8, has been obtained and structurally characterized. Despite its limited crystallinity, Rietveld analysis of the PXRD data set confirmed the materials framework topology and chemical composition (a = 23.856(14) Å, b = 14.103(7) Å, c = 7.412(7) Å, in SG Pnm21). We conclude, that the synthesis procedure is flexible and, meanwhile, has been extended to other metal cations such as Ti, Zn, Eu and Al leading to microporous materials with potentially active metal cations on well defined sites of the silicate framework. © 2015 Published by Elsevier Inc.
    view abstractdoi: 10.1016/j.micromeso.2015.09.051
  • 2016 • 123 Alkali metals incorporated ordered mesoporous tantalum oxide with enhanced photocatalytic activity for water splitting
    Grewe, T. and Tüysüz, H.
    Journal of Materials Chemistry A 4 3007-3017 (2016)
    Herein, we report a novel synthetic approach for the preparation of alkali (Na, K) metal incorporated ordered mesoporous tantalate composites and their photocatalytic performance for water splitting. With the main focus on sodium based composite materials, a series of samples with ordered mesoporosity and high surface area (108-120 m2 g-1) was prepared by a variation of the Ta/Na ratios through a soft templating route. The structural parameters and properties of the samples were analyzed by low angle XRD, N2-physisorption, TEM and STEM analysis, EDX, XPS, Raman and diffuse reflectance UV-Vis spectroscopy. The incorporation of alkali metals resulted in ordered mesoporous tantalate composites. Furthermore, the addition of alkaline earth (Ca, Ba, Sr) metals to the precursor solution of ordered mesoporous tantalum oxide was attempted. However, alkaline earth metals gave unordered tantalate composites. Photocatalytic investigations for water splitting, by using methanol as a sacrificial agent, indicated that the incorporation of alkali metals enhanced the hydrogen production rate of the photocatalyst whereas addition of alkaline earth metals decreased the hydrogen production. Among the sodium based samples, a Ta/Na ratio of 9 showed the best performance. The efficiency of this sample was further improved through decorated NiOx as co-catalyst. A 2.5 wt% NiOx loading was found to be the optimal loading amount, generating 64 μmol h-1 H2 and 31 μmol h-1 O2 when tested for overall water splitting. © 2016 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c5ta07086d
  • 2016 • 122 Hyperbranched potassium lanthanum titanate perovskite photocatalysts for hydrogen generation
    Grewe, T. and Yang, T. and Tüysüz, H. and Chan, C.K.
    Journal of Materials Chemistry A 4 2837-2841 (2016)
    Semiconductors with hierarchical nanostructured morphologies may be promising as high surface area photocatalysts for producing hydrogen from water. However, there are few scalable synthesis methods that can achieve such morphologies in metal oxide semiconductors such as titanates. Here, hydrothermal methods were used to synthesize nanostructured potassium lanthanum titanate (KLTO) perovskite without using templates or structure-directing agents. The obtained materials were octahedral particles composed of orthogonal hyperbranched nanowires, a morphology that is usually obtained using catalyst-mediated vapor phase methods. Several fundamental materials properties of KLTO were determined for the first time, including the bandgap (3.3 eV), semiconductor type (n-type), flat band potential, and conduction band maximum (-0.265 V and-0.835 V vs. NHE, respectively). The KLTO hyperbranched structures were also investigated as UV-photocatalysts for H2 production and displayed higher activities than P25 TiO2 and KLTO nanoparticles. The H2 production rate for KLTO decorated with 1 wt% Pt using thermal decomposition of K2PtCl4 reached ca. 2.5 mmol h-1 and was stable for 20 h of irradiation. © 2016 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c5ta07424j
  • 2016 • 121 Manufacturing of functional elements by sheet-bulk metal forming processes
    Gröbel, D. and Schulte, R. and Hildenbrand, P. and Lechner, M. and Engel, U. and Sieczkarek, P. and Wernicke, S. and Gies, S. and Tekkaya, A.E. and Behrens, B.A. and Hübner, S. and Vucetic, M. and Koch, S. and Merklein, M.
    Production Engineering 10 63-80 (2016)
    Due to increasing economic and ecological restrictions, conventional sheet and bulk forming operations often reach their limits with regard to part weight and functional integration. One solution to meet those challenges is provided by sheet-bulk metal forming (SBMF) processes. SBMF is defined as the application of bulk forming operations on sheet metal. SBMF can be combined with conventional sheet forming operations and offers the opportunity to form highly functional integrated parts out of sheet metal. It contains the benefit of an optimization of the part weight and a shortening of the process chain. Recent research has found different solutions regarding the actual implementation of SBMF in several process variants. In this paper, a categorisation for functional elements on sheet metal parts is proposed. A selection of possible approaches for their manufacturing is presented. The process variants are compared by means of the main process characteristics. By these means, the choice of a suitable option shall be facilitated for practical manufacturing design and for a particular relevant product. © 2016, German Academic Society for Production Engineering (WGP).
    view abstractdoi: 10.1007/s11740-016-0662-y
  • 2016 • 120 From electronic structure to phase diagrams: A bottom-up approach to understand the stability of titanium–transition metal alloys
    Huang, L.-F. and Grabowski, B. and Zhang, J. and Lai, M.-J. and Tasan, C.C. and Sandlöbes, S. and Raabe, D. and Neugebauer, J.
    Acta Materialia 113 311-319 (2016)
    We have computed formation energies for all technologically relevant transition metal solutes in the α, β, and ω phases of Ti, employing ab initio simulations. We analyze and explain their periodic-table trends, and from their differences we derive stabilization energies which provide direct insight into phase stabilization effects of the various solutes with respect to α, β, and ω. This allows us to identify strong β stabilizers in the middle of each electronic d shell in consistency with experimental knowledge. Based on an extension of the stabilization energies to free energies we derive a wide range of Ti-transition metal phase diagrams. A detailed comparison to available experimental martensitic transformation temperatures and to measurements performed in this study shows that, despite some quantitative discrepancies, the qualitative trends can be expected to be correct. An important feature that is displayed by a limited range of the computed phase diagrams is a triple point at which the three phases, α, β, and ω, meet. This insight provides a plausible explanation for the complexity observed in gum metals, a class of Ti alloys with very special materials properties. © 2016 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2016.04.059
  • 2016 • 119 Investigations of ductile damage during the process chains of toothed functional components manufactured by sheet-bulk metal forming
    Isik, K. and Gerstein, G. and Schneider, T. and Schulte, R. and Rosenbusch, D. and Clausmeyer, T. and Nürnberger, F. and Vucetic, M. and Koch, S. and Hübner, S. and Behrens, B.-A. and Tekkaya, A.E. and Merklein, M.
    Production Engineering 10 5-15 (2016)
    Sheet-bulk metal forming processes combine conventional sheet forming processes with bulk forming of sheet semi-finished parts. In these processes the sheets undergo complex forming histories. Due to in- and out-of-plane material flow and large accumulated plastic strains, the conventional failure prediction methods for sheet metal forming such as forming limit curve fall short. As a remedy, damage models can be applied to model damage evolution during those processes. In this study, damage evolution during the production of two different toothed components from DC04 steel is investigated. In both setups, a deep drawn cup is upset to form a circumferential gearing. However, the two final products have different dimensions and forming histories. Due to combined deep drawing and upsetting processes, the material flow on the cup walls is three-dimensional and non-proportional. In this study, the numerical and experimental investigations for those parts are presented and compared. Damage evolution in the process chains is simulated with a Lemaitre damage criterion. Microstructural analysis by scanning electron microscopy is performed in the regions with high mechanical loading. It is observed that the evolution of voids in terms of void volume fraction is strongly dependent on the deformation path. The comparison of simulation results with microstructural data shows that the void volume fraction decreases in the upsetting stage after an initial increase in the drawing stage. Moreover, the concurrent numerical and microstructural analysis provides evidence that the void volume fraction decreases during compression in sheet-bulk metal forming. © 2016, German Academic Society for Production Engineering (WGP).
    view abstractdoi: 10.1007/s11740-016-0656-9
  • 2016 • 118 Failure by fracture in sheet-bulk metal forming
    Isik, K. and Wernicke, S. and Silva, M.B. and Martins, P.A.F. and Tekkaya, A.E.
    Journal of Strain Analysis for Engineering Design 51 387-394 (2016)
    This article investigates the possibility of failure by crack-opening mode III (out-of-plane shearing) in sheet-bulk metal forming processes. The investigation makes use of experimentally and theoretically determined fracture-forming limits of aluminium AA1050-H111 sheets with 1 mm thickness, experimental tests in incremental ploughing with a roll-tipped tool and numerical simulation using a commercial finite element programme. Results show that incremental ploughing of thin sheets with a roll-tipped tool under large indentation depths gives rise to transverse cracks that are triggered at the upper groove surface and propagate downward across thickness along an inclined direction to the sheet surface. In contrast to sheet-metal forming processes that only fail by fracture in crack-opening modes I and II, sheet-bulk metal forming processes present the unique ability of failing in all three possible crack-opening modes, namely, in mode III that is typical of bulk metal-forming processes. © Institution of Mechanical Engineers.
    view abstractdoi: 10.1177/0309324716639773
  • 2016 • 117 Experimental and numerical analysis of tribological effective surfaces for forming tools in Sheet-Bulk Metal Forming
    Kersting, P. and Gröbel, D. and Merklein, M. and Sieczkarek, P. and Wernicke, S. and Tekkaya, A.E. and Krebs, E. and Freiburg, D. and Biermann, D. and Weikert, T. and Tremmel, S. and Stangier, D. and Tillmann, W. and Matthias, S....
    Production Engineering 10 37-50 (2016)
    Sheet-Bulk Metal Forming (SBMF) allows the manufacture of complex parts with integrated functional form elements, such as teeth and thickened areas. Therefore, bulk forming operations are applied to sheets with initial thicknesses of 2 or 3 mm. The design and functionality of the tools are as important as the process itself. Therefore, the working group "Tools" of the Transregional Collaborative Research Centre on Sheet-Bulk Metal Forming (CRC/TR73) focuses on the optimization of the technical tool design. By varying topographies or applying tailored coatings, the friction behavior is changed to achieve a better form filling and to reduce process forces during the forming operations. In this paper, the potential of different tailored surfaces is validated by simulations and experimental studies. The tribological behavior of 14 surface microstructures is evaluated using a half-space model in order to select structures suitable for application. Those were characterized experimentally by ring-compression and pin-extrusion tests. The determined friction factors were used in a forming simulation to predict the form filling of small cavities in a flow forming operation. Furthermore, special attention is paid to the utilization of the anisotropic behavior of specific structures. The results were validated by an incremental gear forming process. © 2016, German Academic Society for Production Engineering (WGP).
    view abstractdoi: 10.1007/s11740-015-0651-6
  • 2016 • 116 Experimental and modelling characterisation of residual stresses in cylindrical samples of rapidly cooled bulk metallic glass
    Korsunsky, A.M. and Sui, T. and Salvati, E. and George, E.P. and Sebastiani, M.
    Materials and Design 104 235-241 (2016)
    Quench processing is widely used in industry to impart the desired structural and mechanical properties by controlling microstructure and compositional gradients, e.g. to obtain supersaturated solid solutions in aluminium alloys, or to achieve martensitic hardening in steels. Rapid cooling, also referred to as quenching or tempering, is also the principal production route for bulk metallic glasses that exhibit high hardness and strength due to their amorphous structure that precludes plastic deformation by easy crystal slip. Importantly, rapid cooling is accompanied by the creation of residual stresses that also have a strong effect on the deformation behaviour. The present study aims to obtain insight into the residual stresses in cylindrical samples of Zr-based bulk metallic glass (BMG) by combining analytical modelling of thermal and mechanical problems with experimental measurements using Focused Ion Beam–Digital Image Correlation (FIB-DIC) ring-core milling. The results show good agreement between the two approaches, providing improved confidence in the validity of the two approaches considered here. © 2016 Elsevier Ltd
    view abstractdoi: 10.1016/j.matdes.2016.05.017
  • 2016 • 115 High Thermopower with Metallic Conductivity in p-Type Li-Substituted PbPdO2
    Lamontagne, L.K. and Laurita, G. and Gaultois, M.W. and Knight, M. and Ghadbeigi, L. and Sparks, T.D. and Gruner, M.E. and Pentcheva, R. and Brown, C.M. and Seshadri, R.
    Chemistry of Materials 28 3367-3373 (2016)
    PbPdO2 is a band semiconductor with a band gap arising from the filled d8 nature of square-planar Pd2+. We establish that hole doping through Li substitution for Pd in PbPdO2 results in a p-type metallic oxide with a positive temperature coefficient of resistance for substitution amounts as small as 2 mol % Li for Pd. Furthermore, PbPd1-xLixO2 demonstrates a high Seebeck coefficient and is therefore an oxide thermoelectric material with high thermopower despite the metallic conductivity. Up to 4 mol % Li is found to substitute for Pd as verified by Rietveld refinement of neutron diffraction data. At this maximal Li substitution, the resistivity is driven below the Mott metallic maximum to 3.5 × 10-3 ω cm with a Seebeck coefficient of 115 μV/K at room temperature, which increases to 175 μV/K at 600 K. These electrical properties are almost identical to those of the well-known p-type oxide thermoelectric NaxCoO2. Nonmagnetic Li-substituted PbPdO2 does not possess a correlated, magnetic state with high-spin degeneracy as found in some complex cobalt oxides. This suggests that there are other avenues to achieving high Seebeck coefficients with metallic conductivities in oxide thermoelectrics. The electrical properties coupled with the moderately low lattice thermal conductivities allow for a zT of 0.12 at 600 K, the maximal temperature measured here. The trend suggests yet higher values at elevated temperatures. First-principles calculations of the electronic structure and electrical transport provide insight into the observed properties. © 2016 American Chemical Society.
    view abstractdoi: 10.1021/acs.chemmater.6b00447
  • 2016 • 114 Dy uniform film morphologies on graphene studied with SPA-LEED and STM
    McDougall, D. and Hattab, H. and Hershberger, M.T. and Hupalo, M. and Horn-von Hoegen, M. and Thiel, P.A. and Tringides, M.C.
    Carbon 108 283-290 (2016)
    The use of graphene for microelectronics and spintronic applications requires strategies for metals to wet graphene and to grow layer-by-layer. This is especially important when metals will be used as electrical contacts or as spin filters. Extensive work in the literature so far has shown that this is very challenging, since practically all metals grow 3D, with multi-height islands forming easily. Reasons for the 3D morphology are the much weaker metal carbon bond when compared to the metal cohesive energy and the role of Coulomb repulsion of the poorly screened charges at the metal graphene interface. We employed the complementary techniques of SPA-LEED and STM to study the growth of Dy on graphene. It was found that under kinetic limitations it is possible to fully cover graphene with a bilayer Dy film, by growing well below room temperature in stepwise deposition experiments. The Dy film, however, is amorphous but ways to crystallize it within the 2D morphology are possible, since long range order improves at higher growth temperature. © 2016
    view abstractdoi: 10.1016/j.carbon.2016.06.083
  • 2016 • 113 Dioxygen binding to Fe-MOF-74: Microscopic insights from periodic QM/MM calculations
    Moeljadi, A.M.P. and Schmid, R. and Hirao, H.
    Canadian Journal of Chemistry 94 1144-1150 (2016)
    Accurate MOF-FF parameter sets were determined for the ferrous and ferric forms of an iron-based metal-organic framework (MOF) called Fe-MOF-74. For this purpose, density functional theory (DFT) calculations were applied to truncated cluster models of Fe-MOF-74, and the DFT-calculated geometries and energy derivatives were used for the force-field parameterization. The resultant parameter sets performed remarkably well in reproducing the experimentally determined structure of the MOF. We also performed periodic quantum mechanics (QM) / molecular mechanics (MM) calculations employing a subtractive scheme called ONIOM, with the optimized MOF-FF parameters used for the MM calculations, in an attempt to evaluate the binding energies between O2 and several Fe-MOF-74 variants. The calculated binding energy for Fe-MOF-74 agreed very well with the experimental value, and QM/MM geometry optimization calculations confirmed that the O2-bound complex has a side-on geometry. Our calculations also predicted that, when the two neighboring iron ions around the O2-binding site are replaced with other metal ions (Mg2+, Ni2+, Zn2+, Co2+, or Mn2+), there are noticeable variations in the binding energy, indicating that these substituted metal ions affect the O2 binding indirectly. © 2016 Published by NRC Research Press.
    view abstractdoi: 10.1139/cjc-2016-0284
  • 2016 • 112 Few-layer graphene modified with nitrogen-rich metallo-macrocyclic complexes as precursor for bifunctional oxygen electrocatalysts
    Morales, D.M. and Masa, J. and Andronescu, C. and Kayran, Y.U. and Sun, Z. and Schuhmann, W.
    Electrochimica Acta 222 1191-1199 (2016)
    We propose a method for the formation of highly active bifunctional oxygen electrocatalysts, by exploiting the unique features of nitrogen-rich metallo-macrocyclic complexes and the structural and electronic properties of few-layer graphene. The precursors of the electrocatalysts were synthesized by sonication of graphite in DMF leading to exfoliation and the formation of few-layer graphene sheets in the presence of a suitable transition metal macrocyclic complex. After pyrolysis and subsequent mild calcination metal oxide nanoparticles as well as metal-nitrogen (MNx) moieties embedded within a N-doped graphitic carbon matrix are obtained. The formation, in-depth characterization and electrochemical performance of two different catalysts derived from Co and Ni containing precursor complexes are demonstrated. © 2016 Elsevier Ltd
    view abstractdoi: 10.1016/j.electacta.2016.11.092
  • 2016 • 111 Size and orientation dependent mechanical behavior of body-centered tetragonal Sn at 0.6 of the melting temperature
    Philippi, B. and Kirchlechner, C. and Micha, J.S. and Dehm, G.
    Acta Materialia 115 76-82 (2016)
    Although, tin is one of the most prominent metals in soldering, very little is known about its mechanical behavior. In addition, possible size-effects of tin can become restricting for the ongoing miniaturization of microelectronic devices. Due to the low melting temperature of 505.15 K and the body-centered tetragonal crystal structure, differences in the mechanical behavior compared to face-centered cubic and body-centered cubic metals can be expected. Since Tin is especially interesting because of its multiple different slip systems, post mortem slip step analysis allowed to determine the activated slip systems and thus, to calculate size dependent critical resolved shear stresses. The measured size scaling exponent (-1.07 ± 0.06) is close to model-predictions of -1, irrespective of the activated families of slip systems in different orientations. Furthermore, an exceptional low scatter of the flow stress in various samples and no apparent hardening is found. It is concluded, that the activation of dislocation sources instead of dislocation-dislocation interactions are responsible for the observed behavior. This is in line with complementary μLaue diffraction experiments which indicate an unresolvable low density of geometrical necessary dislocations. © 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2016.05.055
  • 2016 • 110 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 • 109 Influence of the degree of infiltration of modified activated carbons with CuO/ZnO on the separation of NO2 at ambient temperatures
    Sager, U. and Däuber, E. and Bathen, D. and Asbach, C. and Schmidt, F. and Tseng, J.-C. and Pommerin, A. and Weidenthaler, C. and Schmidt, W.
    Adsorption Science and Technology 34 307-319 (2016)
    The reduction of NO2 in air at ambient temperatures with activated carbons can be increased by the infiltration of metal oxide nanoparticles into the sorbents. The NO2 is first adsorbed to the activated carbon and subsequently catalytically reduced to physiologically neutral substances by the metal oxides. The catalytic reduction at ambient temperatures is rather slow. In a former study concerning the application in cabin air filters, it was shown that the modification of activated carbon with 5 wt% CuO/ZnO leads to reduced breakthrough of NO2 and that the adsorbent was able to regenerate between repeated NO2 adsorption cycles. Here we show that the efficiency of the sorbent can be more than doubled by increasing the metal oxide infiltration to 20 wt% whereas a further increase in loading yields no additional improvement, due to a partial transformation of the oxidic compounds. © 2016, © The Author(s) 2016.
    view abstractdoi: 10.1177/0263617416653120
  • 2016 • 108 Effects of phase stability, lattice ordering, and electron density on plastic deformation in cubic TiWN pseudobinary transition-metal nitride alloys
    Sangiovanni, D.G. and Hultman, L. and Chirita, V. and Petrov, I. and Greene, J.E.
    Acta Materialia 103 823-835 (2016)
    We carry out density functional theory calculations to compare the energetics of layer glide, as well as stress vs. strain curves, for cubic Ti0.5W0.5N pseudobinary alloys and reference B1-structure TiN. Irrespective of the degree of ordering on the metal sublattice, the hardness and stiffness of Ti0.5W0.5N, as estimated by stress-strain results and resistance to layer glide, are comparable to that of the parent binary TiN, while ductility is considerably enhanced. After an initial elastic response to an applied load, the pseudobinary alloy deforms plastically, thus releasing accumulated mechanical stress. In contrast, stress continues to increase linearly with strain in TiN. Layer glide in Ti0.5W0.5N is promoted by a high valence-electron concentration which enables the formation of strong metallic bonds within the slip direction upon deformation. [111]-oriented Ti0.5W0.5N layers characterized by high local metal-sublattice ordering exhibit low resistance to slip along &lt;110&gt; directions due to energetically favored formation of (111) hexagonal stacking faults. This is consistent with the positive formation energy of &lt;111&gt;-ordered Ti0.5W0.5N with respect to mixing of cubic-B1 TiN and hexagonal WC-structure WN. In the cubic pseudobinary alloy, slip occurs parallel, as well as orthogonal, to the resolved applied stress at the interface between layers with the lowest friction. We suggest that analogous structural metastability (mixing cubic and hexagonal TM nitride binary phases) and electronic (high valence electron concentration) effects are responsible for the enhanced toughness recently demonstrated experimentally for cubic single-crystal pseudobinary V0.5W0.5N and V0.5Mo0.5N epitaxial layers. © 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2015.10.039
  • 2016 • 107 Revealing the relationships between chemistry, topology and stiffness of ultrastrong Co-based metallic glass thin films: A combinatorial approach
    Schnabel, V. and Köhler, M. and Evertz, S. and Gamcova, J. and Bednarcik, J. and Music, D. and Raabe, D. and Schneider, J.M.
    Acta Materialia 107 213-219 (2016)
    An efficient way to study the relationship between chemical composition and mechanical properties of thin films is to utilize the combinatorial approach, where spatially resolved mechanical property measurements are conducted along a concentration gradient. However, for thin film glasses many properties including the mechanical response are affected by chemical topology. Here a novel method is introduced which enables spatially resolved short range order analysis along concentration gradients of combinatorially synthesized metallic glass thin films. For this purpose a CoZrTaB metallic glass film of 3 μm thickness is deposited on a polyimide foil, which is investigated by high energy X-ray diffraction in transmission mode. Through the correlative chemistry-topology-stiffness investigation, we observe that an increase in metalloid concentration from 26.4 to 32.7 at% and the associated formation of localized (hybridized) metal - metalloid bonds induce a 10% increase in stiffness. Concomitantly, along the same composition gradient, a metalloid-concentration-induced increase in first order metal - metal bond distances of 1% is observed, which infers itinerant (metallic) bond weakening. Hence, the metalloid concentration induced increase in hybridized bonding dominates the corresponding weakening of metallic bonds. © 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2016.01.060
  • 2016 • 106 Metal hydrides for concentrating solar thermal power energy storage
    Sheppard, D.A. and Paskevicius, M. and Humphries, T.D. and Felderhoff, M. and Capurso, G. and Bellosta von Colbe, J. and Dornheim, M. and Klassen, T. and Ward, P.A. and Teprovich, J.A., Jr. and Corgnale, C. and Zidan, R. and Grant...
    Applied Physics A: Materials Science and Processing 122 (2016)
    The development of alternative methods for thermal energy storage is important for improving the efficiency and decreasing the cost of concentrating solar thermal power. We focus on the underlying technology that allows metal hydrides to function as thermal energy storage (TES) systems and highlight the current state-of-the-art materials that can operate at temperatures as low as room temperature and as high as 1100 °C. The potential of metal hydrides for thermal storage is explored, while current knowledge gaps about hydride properties, such as hydride thermodynamics, intrinsic kinetics and cyclic stability, are identified. The engineering challenges associated with utilising metal hydrides for high-temperature TES are also addressed. © 2016, Springer-Verlag Berlin Heidelberg.
    view abstractdoi: 10.1007/s00339-016-9825-0
  • 2016 • 105 Efficient additive manufacturing production of oxide- and nitride-dispersion-strengthened materials through atmospheric reactions in liquid metal deposition
    Springer, H. and Baron, C. and Szczepaniak, A. and Jägle, E.A. and Wilms, M.B. and Weisheit, A. and Raabe, D.
    Materials and Design 111 60-69 (2016)
    Despite being extremely attractive compounds for strengthening, oxides and nitride particles have found only limited use in metallic materials design, as obtaining appropriate size and dispersion up to now necessitates production by time- and cost-intensive powder metallurgy processes. Here we present an alternative production method, based on the oxide and nitride formation during liquid-metal-deposition procedures in oxygen and/or nitrogen containing atmospheres. Rapid solidification of the small liquid zone suppresses floatation and agglomeration of particles, while subsequent thermo-mechanical treatments densify the material and aids particle dispersion. The in-situ particle formation coupled to the high deposition rates ensures a drastically shortened production chain. The feasibility of the method is exemplarily demonstrated on austenitic stainless steel and commercially available deposition techniques as used in additive manufacturing, performed without shielding gas but instead at air. Even without substantial optimisation of processes and material, &gt; 2 vol.% of hard and stable Cr2N particles with sizes down to 80 nm could be evenly dispersed, resulting in pronounced strengthening at both room temperature and 700 °C without significant loss in ductility. Future possibilities for creating novel generations of cost effective and lean high strength materials, especially for high temperature applications, are outlined and discussed. © 2016 Elsevier Ltd
    view abstractdoi: 10.1016/j.matdes.2016.08.084
  • 2016 • 104 Optimization of a transferred arc reactor for metal nanoparticle synthesis
    Stein, M. and Kruis, F.E.
    Journal of Nanoparticle Research 18 (2016)
    The demand for metal nanoparticles is increasing strongly. Transferred arc synthesis is a promising process in this respect, as it shows high production rates, good quality particles and the ability of up-scaling. The influence of several process parameters on the performance of the process in terms of production rate and particle size is investigated. These parameters are the electrode design and adjustment, the gas flow rate and power input. A novel feeding mechanism allows process operation over an extended time period. It is shown that the process is capable of producing pure metal nanoparticles with variable primary particle sizes and comparatively high production rates. Optimal process conditions for a single transferred arc electrode pair are found, which allow further scale-up by numbering up. © 2016, The Author(s).
    view abstractdoi: 10.1007/s11051-016-3559-y
  • 2016 • 103 X-ray diffraction (XRD)-studies on the temperature dependent interface reactions on hafnium, zirconium, and nickel coated monocrystalline diamonds used in grinding segments for stone and concrete machining
    Tillmann, W. and Tolan, M. and Pinho Ferreira, M. and Paulus, M. and Becke, M. and Stangier, D.
    Materialwissenschaft und Werkstofftechnik 47 1193-1201 (2016)
    Diamond impregnated metal matrix composites are the state of the art solution for the machining of mineral materials. The type of interface reactions between the metal matrix and diamond surface has an essential influence on the tool performance and durability. To improve the diamond retention, the diamonds can be coated by physical vapour deposition with metallic materials, which enforce interface reactions. Hence, this paper focuses on the investigation of the interfacial area on metal-coated monocrystalline diamonds. Hafnium and zirconium, both known as carbide forming elements, are used as coating materials. The third coating, which is used to determine its catalytic influences when applied as a physical vapour deposition (PVD)-layer, is nickel. Additionally, the coated diamond samples were heat-treated to investigate the starting point of the formation of new phases. X-ray diffraction-analyses revealed the assumed carbide formation on hafnium and zirconium coated samples. The formation temperature was identified between 800 °C and 1000 °C for hafnium and zirconium coatings. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/mawe.201600713
  • 2016 • 102 Demonstration of Mg2FeH6 as heat storage material at temperatures up to 550 °C
    Urbanczyk, R. and Meggouh, M. and Moury, R. and Peinecke, K. and Peil, S. and Felderhoff, M.
    Applied Physics A: Materials Science and Processing 122 (2016)
    The storage of heat at high temperatures, which can be used to generate electricity after sunset in concentrating solar power plants, is one of the most challenging technologies. The use of metal hydride could be one possibility to solve the problem. During the endothermic heat storage process, the metal hydride is decomposed releasing hydrogen, which then can be stored. During the exothermic reaction of the metal with the hydrogen gas, the stored heat is then released. Previous research had shown that Mg and Fe powders can be used at temperatures up to 550 °C for heat storage and shows excellent cycle stability over hundreds of cycles without any degradation. Here, we describe the results of testing of a tube storage tank that contained 211 g of Mg and Fe powders in 2:1 ratio. Twenty-three dehydrogenations (storage) and 23 hydrogenations (heat release) in the temperature range between of 395 and 515 °C and pressure range between 1.5 and 8.6 MPa were done. During the dehydrogenation, 0.41–0.42 kWhth kg−1 of heat based on material 2 Mg/Fe can be stored in the tank. After testing, mainly Mg2FeH6 was observed and small amounts of MgH2 and Fe metal can be detected in the hydride samples. This means that the heat storage capacity of the system could be further increased if only Mg2FeH6 is produced during subsequent cycles. © 2016, The Author(s).
    view abstractdoi: 10.1007/s00339-016-9811-6
  • 2016 • 101 In Situ EPR Study of the Redox Properties of CuO-CeO2 Catalysts for Preferential CO Oxidation (PROX)
    Wang, F. and Büchel, R. and Savitsky, A. and Zalibera, M. and Widmann, D. and Pratsinis, S.E. and Lubitz, W. and Schüth, F.
    ACS Catalysis 6 3520-3530 (2016)
    Understanding the redox properties of metal oxide based catalysts is a major task in catalysis research. In situ electron paramagnetic resonance (EPR) spectroscopy is capable of monitoring the change of metal ion valences and formation of active sites during redox reactions, allowing for the identification of ongoing redox pathways. Here in situ EPR spectroscopy combined with online gas analysis, supported by ex situ X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), X-ray absorption near edge structure (XANES), temporal analysis of product (TAP), and mass spectrometry (MS) studies, was utilized to study the redox behavior of CuO-CeO2 catalysts under PROX conditions (preferential oxidation of carbon monoxide in hydrogen). Two redox mechanisms are revealed: (i) a synergetic mechanism that involves the redox pair Ce4+/Ce3+ during oxidation of Cu0/Cu+ species to Cu2+ and (ii) a direct mechanism that bypasses the redox pair Ce4+/Ce3+. In addition, EPR experiments with isotopically enriched 17O2 established the synergetic mechanism as the major redox reaction pathway. The results emphasize the importance of the interactions between Cu and Ce atoms for catalyst performance. With the guidance of these results, an optimized CuO-CeO2 catalyst could be designed. A rather wide temperature operation window of 11 K (from 377 to 388 K), with 99% conversion efficiency and 99% selectivity, was achieved for the preferential oxidation of CO in a H2 feed. © 2016 American Chemical Society.
    view abstractdoi: 10.1021/acscatal.6b00589
  • 2016 • 100 SciFab -a wafer-level heterointegrated InP DHBT/SiGe BiCMOS foundry process for mm-wave applications
    Weimann, N.G. and Stoppel, D. and Schukfeh, M.I. and Hossain, M. and Al-Sawaf, T. and Janke, B. and Doerner, R. and Sinha, S. and Schmückle, F.-J. and Krüger, O. and Krozer, V. and Heinrich, W. and Lisker, M. and Krüger, A. and...
    Physica Status Solidi (A) Applications and Materials Science 213 909-916 (2016)
    We present a wafer-level heterointegrated indium phosphide double heterobipolar transistor on silicon germanium bipolar-complementary metal oxide semiconductor (InP DHBT on SiGe BiCMOS) process which relies on adhesive wafer bonding. Subcircuits are co-designed in both technologies, SiGe BiCMOS and InP DHBT, with more than 300 GHz bandwidth microstrip interconnects. The 250 nm SiGe HBTs offer cutoff frequencies around 200 GHz, the 800 nm InP DHBTs exceed 350 GHz. Heterointegrated signal sources are demonstrated including a 328 GHz quadrupling source with -12 dBm RF output power. A common design kit for full InP DHBT/SiGe BiCMOS co-design was set up. The technology is being opened to third-party customers through IHP's multi-purpose wafer foundry interface. Microphotograph of InP DHBT / SiGe BiCMOS wafer. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/pssa.201532549
  • 2016 • 99 Time-Resolved InSitu X-ray Diffraction Reveals Metal-Dependent Metal-Organic Framework Formation
    Wu, Y. and Henke, S. and Kieslich, G. and Schwedler, I. and Yang, M. S. and Fraser, D. A. X. and O'Hare, D.
    Angewandte Chemie-international Edition 55 14081--14084 (2016)
    Versatility in metal substitution is one of the key aspects of metal-organic framework (MOF) chemistry, allowing properties to be tuned in a rational way. As a result, it important to understand why MOF syntheses involving different metals arrive at or fail to produce the same topological outcome. Frequently, conditions are tuned by trial-and-error to make MOFs with different metal species. We ask: is it possible to adjust synthetic conditions in a systematic way in order to design routes to desired phases? We have used insitu X-ray powder diffraction to study the solvothermal formation of isostructural M-2(bdc)(2)dabco (M=Zn, Co, Ni) pillared-paddlewheel MOFs in real time. The metal ion strongly influences both kinetics and intermediates observed, leading in some cases to multiphase reaction profiles of unprecedented complexity. The standard models used for MOF crystallization break down in these cases; we show that a simple kinetic model describes the data and provides important chemical insights on phase selection.
    view abstractdoi: 10.1002/anie.201608463
  • 2016 • 98 Interactions between metal species and nitrogen-functionalized carbon nanotubes
    Xia, W.
    Catalysis Science and Technology 6 630-644 (2016)
    Nitrogen-functionalized carbon nanotubes are promising materials in catalysis due to their versatile surface properties involving nitrogen groups, oxygen groups, surface defects and metal impurities. These factors can be used to tune the dispersion, morphology, crystal structure, electronic structure, mobility/stability and finally the catalytic performance of supported metal nanoparticles. This review focuses on selected examples aiming at understanding the interactions between surface groups, defects, and metal species and their impact on the catalytic properties in electrocatalysis and gas-phase redox catalysis. © 2016 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c5cy01694k
  • 2016 • 97 Conductive single nanowires formed and analysed on microfluidic devices
    Xing, Y. and Esser, N. and Dittrich, P.S.
    Journal of Materials Chemistry C 4 9235-9244 (2016)
    In this work, we studied the formation of fibres and particles made of metal salts and derivatives of tetrathiafulvalene (TTF) on a microfluidic device and in a conventional reaction flask, and characterized their morphologies, optical properties and electrical conductivities. A series of uniform one-dimensional (1D) structures were successfully formed via charge transfer interactions at the interface of two laminar streams on the microdevice. In general, these structures were significantly thinner and longer than those obtained in the standard bulk approach. The reaction between metal ions and different TTF derivatives indicated that the high planarity and strong molecular interaction of TTF derivatives are beneficial for nanowire formation. The conductivities of these metal-tetrathiafulvalene (M-TTF) and metal-formyl-tetrathiafulvalene (M-FTTF) nanowires were in the range of 10-1 to 101 S cm-1 at room temperature, being one to two orders of magnitude greater than those of metal-bis(ethylenedithio)tetrathiafulvalene (M-BEDT-TTF) (4.8 × 10-3 to 1.2 × 10-2 S cm-1 at room temperature). However, not all combinations of metal salts and TTF derivatives yielded fibres. In many cases, we obtained particles or dendritic structures both in bulk reactions and on the microdevice. Hence, our study provides a comprehensive overview of the morphologies of the products obtained from reactions of metal salts and TTF including different commercially available derivatives. © 2016 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c6tc02791a
  • 2016 • 96 Determination of the young modulus of Ti-TiAl3 metallic intermetallic laminate composites by nano-indentation
    Yener, T. and Güler, S. and Siddique, S. and Walther, F. and Zeytin, S.
    Acta Physica Polonica A 129 604-606 (2016)
    Nano-indentation is an important technique to determine the Young modulus of multiphase materials where normal tensile tests are not appropriate. In this work, Ti-TiAl3 metallic-intermetallic laminate composites have been fabricated successfully in open atmosphere using commercial purity Al and Ti foils with 250 μm and 500 μm initial thicknesses, respectively. Sintering process was performed at 700 °C under 2 MPa pressure for 7.5 h. Mechanical properties including the Young modulus were determined after manufacturing. The Young moduli of metallic and intermetallic phases were determined as 89 GPa and 140 GPa, respectively. Microstructure analyses showed that aluminum foil was almost consumed by forming a titanium aluminide intermetallic compound. Titanium aluminides grow up through spherical shaped islands and metallic-intermetallic interface is a wavy form in Ti-Al system. Thus, the final microstructure consists of alternating layers of intermetallic compound and unreacted Ti metal. Microstructure and phase characterizations were performed by scanning electron microscopy, energy dispersive spectroscopy, and X-ray diffraction. Hardness of test samples was determined as 600 HV for intermetallic zone and 130 HV for metallic zone by the Vickers indentation method.
    view abstractdoi: 10.12693/APhysPolA.129.604
  • 2016 • 95 Strong competition between electromagnetic enhancement and surface-energy-transfer induced quenching in plasmonic dye-sensitized solar cells: A generic yet controllable effect
    Yip, C.T. and Liu, X. and Hou, Y. and Xie, W. and He, J. and Schlücker, S. and Lei, D.Y. and Huang, H.
    Nano Energy 26 297-304 (2016)
    Light harvesting strategy using plasmonic metal nanostructures as subwavelength light concentrators provides a highly attractive solution to enhancing the performance of dye-sensitized solar cells (DSSCs). Through comprehensive optical spectroscopy and electrical characterizations together with a theoretical analysis, we demonstrate a strong competition between the surface energy transfer induced non-radiative quenching and the plasmonic electromagnetic enhancement effect in metal-dielectric-semiconductor core-shell-shell nanoparticle doped DSSCs, a generic yet unavoidable phenomenon in all types of plasmonic solar cells. The competition of the two effects results in a non-monotonic relationship between the device efficiency and the thickness of the dielectric shell covering the metal nanoparticles, and leads to an optimal thickness for the highest power conversion efficiency. This observation is further corroborated by photoluminescence spectroscopic measurements. Our experimental results are in good agreement with the Persson model that predicts a strong energy quenching effect when the distance between the photogenerated charge carrier and the metal core is short enough. Both experiment and theory show that the localized surface plasmon resonance enhanced light harvesting efficiency is suppressed by the surface energy transfer to the metal cores for the dielectric shell thickness shorter than a characteristic value (~7 nm in our study). Our work sheds new insights into the fundamental understanding of the photophysics mechanisms of plasmonic DSSCs and could push forward the study of plasmonic solar cells in terms of device design and fabrication. © 2016 Elsevier Ltd.
    view abstractdoi: 10.1016/j.nanoen.2016.05.016
  • 2015 • 94 Structure-activity relationships of Co-modified Cu/ZnO/Al2O3 catalysts applied in the synthesis of higher alcohols from synthesis gas
    Anton, J. and Nebel, J. and Song, H. and Froese, C. and Weide, P. and Ruland, H. and Muhler, M. and Kaluza, S.
    Applied Catalysis A: General 505 326-333 (2015)
    Cu-Co-based catalysts were synthesized by co-precipitation using Cu, Co, Zn and Al nitrates and applied in higher alcohol synthesis (HAS) at 280 °C, 60 bar and a ratio of H<inf>2</inf>/CO = 1. The catalyst exhibiting a Cu/Co ratio of 2.5 was found to provide the best trade-off between product distribution and degree of CO conversion. After activation and 40 h time on stream reaching steady-state conditions the bulk and surface properties of the catalyst were thoroughly investigated without exposing it to air during the transfer and the measurements. The conditions during activation and HAS led to a significant enrichment of Zn in the surface composition of the catalysts. The XRD pattern of the catalyst after reaction compared with the reduced catalyst revealed further sintering of the metallic Cu nanoparticles and the growth of crystalline ZnO nanoparticles, but there were no indications for the presence of bulk metallic Co or for bulk alloying. With increasing time on stream the product distribution shifted favorably towards higher alcohols presumably due to an increased intimate interface contact between the large metallic Cu0 particles detected by XRD and the X-ray amorphous metallic Co surface species probed by XPS. © 2015 Elsevier B.V.
    view abstractdoi: 10.1016/j.apcata.2015.07.002
  • 2015 • 93 Coprecipitation: An excellent tool for the synthesis of supported metal catalysts - From the understanding of the well known recipes to new materials
    Behrens, M.
    Catalysis Today 246 46-54 (2015)
    Constant-pH co-precipitation is a standard synthesis technique for catalyst precursors. The general steps of this synthesis route are described in this work using the successfully applied industrial synthesis of the Cu/ZnO/(Al2O3) catalyst for methanol synthesis as an example. Therein, co-precipitation leads to well-defined and crystalline precursor compound with a mixed cationic lattice that contains all metal species of the final catalyst. The anions are thermally decomposed to give the mixed oxides and the noblest component, in this current case copper, finally segregates on a nano-metric level to yield supported and uniform metal nanoparticles. Recent examples of the application of this synthesis concept for supported catalysts are reported with an emphasis on the layered double hydroxide precursor (Cu,Zn,Al; Ni,Mg,Al; Pd,Mg,Al; Pd,Mg,Ga). This precursor material is very versatile and can lead to highly loaded base metal as well as to mono- and bi-metallic highly dispersed noble metal catalysts. © 2014 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.cattod.2014.07.050
  • 2015 • 92 Experimental and simulative investigations of tribology in sheet-bulk metal forming
    Beyer, F. and Blum, H. and Kumor, D. and Rademacher, A. and Willner, K. and Schneider, T.
    Key Engineering Materials 639 283-290 (2015)
    Friction has a considerable influence in metal forming both in economic and technical terms. This is especially true for sheet-bulk metal forming (SBMF). The contact pressure that occurs here can be low making Coulomb's friction law advisable, but also very high so that Tresca's friction law is preferable. By means of an elasto-plastic half-space model rough surfaces have been investigated, which are deformed in such contact states. The elasto-plastic half-space model has been verified and calibrated experimentally. The result is the development of a constitutive friction law, which can reproduce the frictional interactions for both low and high contact pressures. In addition, the law gives conclusion regarding plastic smoothening of rough surfaces. The law is implemented in the framework of the finite element method (FEM). However, compared to usual friction relations the tribological interplay presented here comes with the disadvantage of rising numerical effort. In order to minimise this drawback, a model adaptive finite element simulation is performed additionally. In this approach, contact regions are identified, where a conventional friction law is applicable, where the newly developed constitutive friction law should be used, or where frictional effects are negligible. The corresponding goal-oriented indicators are derived based on the "dual-weighted-residual" (DWR) method taking into account both the model and the discretisation error. This leads to an efficient simulation that applies the necessary friction law in dependence of contact complexity. © 2015 Trans Tech Publications, Switzerland.
    view abstractdoi: 10.4028/www.scientific.net/KEM.639.283
  • 2015 • 91 Influence of surface modifications on friction, using high-feed milling and wear resistant PVD-coating for sheet-metal forming tools
    Biermann, D. and Freiburg, D. and Hense, R. and Tillmann, W. and Stangier, D.
    Key Engineering Materials 639 275-282 (2015)
    Increasing technological requirements, as well as the demand for an efficient production demands high performance materials and enhanced manufacturing processes. The development of a new manufacturing process, sheet-bulk metal forming (SBMF), is one approach to produce lightweight forming parts with an increased number of functional properties while, at the same time, combining the advantages of sheet and bulk metal forming. For SBMF processes, the specific adjustment of the friction between tool and workpiece for a specifically designed material flow, which is called tailored friction, is of great importance. The reduction of friction is essential in order to ensure a homogeneous forming zone. However, a higher friction can be used to control the material flow to increase the local thickness of the work piece for additional functional integration. This paper shows the development of surface structures for SBMF tools by means of high-feed milling. Process parameters like the tilt angle or the feed are varied to influence the surface parameters of the structures, which results in different tribological properties of the forming tool. The structured surfaces are subsequently coated with a wear resistant CrAlN coating, processed by a magnetron-sputtering process (PVD) to enhance the lifetime and performance of the forming tool. Finally, a ring compressing test is used to investigate the tribological behavior of the coated structures. © 2015 Trans Tech Publications, Switzerland.
    view abstractdoi: 10.4028/www.scientific.net/KEM.639.275
  • 2015 • 90 Metal-support interactions in surface-modified Cu-Co catalysts applied in higher alcohol synthesis
    Bordoloi, A. and Anton, J. and Ruland, H. and Muhler, M. and Kaluza, S.
    Catalysis Science and Technology 5 3603-3612 (2015)
    Cu-Co-based model catalysts were prepared by a sophisticated alkali-free synthesis method and tested in the conversion of synthesis gas to higher alcohols. MoO<inf>3</inf>-coated alumina was used as the support, providing both high specific surface area and strongly interacting sites for the deposition of the active metals. A bulk Cu/Co ratio of ∼2 was found to be most suitable in terms of activity and product distribution. Surface enrichment of Mo for all samples was observed by XPS, which significantly influenced the performance of the catalysts. Mo was found to be both a structural and a chemical promoter. Strong metal-support interactions were further achieved by modification of alumina with magnesia. With 12 wt% Mg incorporated, the catalysts showed 40% total oxygenate selectivity including 11% selectivity to ethanol. © The Royal Society of Chemistry 2015.
    view abstractdoi: 10.1039/c5cy00421g
  • 2015 • 89 A quantitative metallographic assessment of the evolution of porosity during processing and creep in single crystal Ni-base super alloys
    Buck, H. and Wollgramm, P. and Parsa, A.B. and Eggeler, G.
    Materialwissenschaft und Werkstofftechnik 46 577-590 (2015)
    The present work reviews previous research on the evolution of porosity. It presents new results from a detailed study on the evolution of porosity during casting, heat treatment and creep of a single crystal Ni-base superalloy subjected to uniaxial tensile creep at 1050 °C and 160 MPa in [001] and [110] directions. A quantitative metallographic study was performed on carefully polished metallographic cross sections, monitoring sampling fields of 4500 × 1000 μm2 using the back scatter contrast of an analytical scanning electron microscope; evolutions of pore sizes and pore form factors were analyzed and all important details which were previously revealed in a synchrotron study could be reproduced. In addition, it was observed that micro cracks form at larger cast pores. They interlink and thus initiate final rupture. The [110] tensile creep tests showed lower rupture strains than the [001] experiments. In agreement with earlier work, this can be rationalized on the basis of aligned porosity along primary dendrites. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/mawe.201500379
  • 2015 • 88 Three-dimensional Cu foam-supported single crystalline mesoporous Cu2O nanothorn arrays for ultra-highly sensitive and efficient nonenzymatic detection of glucose
    Dong, C. and Zhong, H. and Kou, T. and Frenzel, J. and Eggeler, G. and Zhang, Z.
    ACS Applied Materials and Interfaces 7 20215-20223 (2015)
    Highly sensitive and efficient biosensors play a crucial role in clinical, environmental, industrial, and agricultural applications, and tremendous efforts have been dedicated to advanced electrode materials with superior electrochemical activities and low cost. Here, we report a three-dimensional binder-free Cu foam-supported Cu<inf>2</inf>O nanothorn array electrode developed via facile electrochemistry. The nanothorns growing in situ along the specific direction of <011> have single crystalline features and a mesoporous surface. When being used as a potential biosensor for nonenzyme glucose detection, the hybrid electrode exhibits multistage linear detection ranges with ultrahigh sensitivities (maximum of 97.9 mA mM-1 cm-2) and an ultralow detection limit of 5 nM. Furthermore, the electrode presents outstanding selectivity and stability toward glucose detection. The distinguished performances endow this novel electrode with powerful reliability for analyzing human serum samples. These unprecedented sensing characteristics could be ascribed to the synergistic action of superior electrochemical catalytic activity of nanothorn arrays with dramatically enhanced surface area and intimate contact between the active material (Cu<inf>2</inf>O) and current collector (Cu foam), concurrently supplying good conductivity for electron/ion transport during glucose biosensing. Significantly, our findings could guide the fabrication of new metal oxide nanostructures with well-organized morphologies and unique properties as well as low materials cost. © 2015 American Chemical Society.
    view abstractdoi: 10.1021/acsami.5b05738
  • 2015 • 87 Metal-metal terahertz quantum cascade laser with hybrid mode section
    Fobbe, T. and Nong, H. and Schott, R. and Pal, S. and Markmann, S. and Hekmat, N. and Zhu, J. and Han, Y. and Li, L. and Dean, P. and Linfield, E.H. and Davies, A.G. and Wieck, A.D. and Jukam, N.
    IRMMW-THz 2015 - 40th International Conference on Infrared, Millimeter, and Terahertz Waves (2015)
    A hybrid mode section is integrated into the end of the metal-metal (MM) waveguide of a terahertz (THz) frequency quantum cascade laser (QCL) by removing sub-wavelength portions of the top metal layer. This allows a hybrid mode to penetrate into the air, which reduces the effective index of the mode and improves the out-coupling performance at the facet. The transmission of the processed metal-metal hybrid section (MMHS) waveguide is further increased by ensuring its length fulfills the criterion for constructive interference. These simple modifications to a 2.5 THz MM QCL waveguide result in a significant increase in the output emission power. In addition, simulations show that further improvements in out-coupling efficiency can be achieved for lower frequencies with effective refractive indices close to the geometric mean of the indices of the MM waveguide and air. © 2015 IEEE.
    view abstractdoi: 10.1109/IRMMW-THz.2015.7327687
  • 2015 • 86 Synthesis and post-synthetic modification of amine-, alkyne-, azide- and nitro-functionalized metal-organic frameworks based on DUT-5
    Gotthardt, M.A. and Grosjean, S. and Brunner, T.S. and Kotzel, J. and Gänzler, A.M. and Wolf, S. and Bräse, S. and Kleist, W.
    Dalton Transactions 44 16802-16809 (2015)
    Functionalized 4,4′-biphenyldicarboxylic acid molecules with additional amine, alkyne, azide or nitro groups were prepared and applied in the synthesis of novel metal-organic frameworks and mixed-linker metal-organic frameworks isoreticular to DUT-5. The properties of the frameworks could be tuned by varying the number of functional groups in the materials and the amine groups were employed in post-synthetic modification reactions without changing the framework structure or significantly decreasing the porosity of the materials. © The Royal Society of Chemistry 2015.
    view abstractdoi: 10.1039/c5dt02276b
  • 2015 • 85 Highly Ordered Mesoporous Cobalt-Containing Oxides: Structure, Catalytic Properties, and Active Sites in Oxidation of Carbon Monoxide
    Gu, D. and Jia, C.-J. and Weidenthaler, C. and Bongard, H.-J. and Spliethoff, B. and Schmidt, W. and Schüth, F.
    Journal of the American Chemical Society 137 11407-11418 (2015)
    Co<inf>3</inf>O<inf>4</inf> with a spinel structure is a very active oxide catalyst for the oxidation of CO. In such catalysts, octahedrally coordinated Co3+ is considered to be the active site, while tetrahedrally coordinated Co2+ is assumed to be basically inactive. In this study, a highly ordered mesoporous CoO has been prepared by H<inf>2</inf> reduction of nanocast Co<inf>3</inf>O<inf>4</inf> at low temperature (250 °C). The as-prepared CoO material, which has a rock-salt structure with a single Co2+ octahedrally coordinated by lattice oxygen in Fm3¯m symmetry, exhibited unexpectedly high activity for CO oxidation. Careful investigation of the catalytic behavior of mesoporous CoO catalyst led to the conclusion that the oxidation of surface Co2+ to Co3+ causes the high activity. Other mesoporous spinels (CuCo<inf>2</inf>O<inf>4</inf>, CoCr<inf>2</inf>O<inf>4</inf>, and CoFe<inf>2</inf>O<inf>4</inf>) with different Co species substituted with non/low-active metal ions were also synthesized to investigate the catalytically active site of cobalt-based catalysts. The results show that not only is the octahedrally coordinated Co3+ highly active but also the octahedrally coordinated Co2+ species in CoFe<inf>2</inf>O<inf>4</inf> with an inverse spinel structure shows some activity. These results suggest that the octahedrally coordinated Co2+ species is easily oxidized and shows high catalytic activity for CO oxidation. © 2015 American Chemical Society.
    view abstractdoi: 10.1021/jacs.5b06336
  • 2015 • 84 Metal-to-Insulator Transition in Au Chains on Si(111)-5×2-Au by Band Filling: Infrared Plasmonic Signal and Ab Initio Band Structure Calculation
    Hötzel, F. and Seino, K. and Chandola, S. and Speiser, E. and Esser, N. and Bechstedt, F. and Pucci, A.
    Journal of Physical Chemistry Letters 6 3615-3620 (2015)
    The Si(111)-5×2-Au surface is increasingly of interest because it is one of the rare atomic chain systems with quasi-one-dimensional properties. For the deposition of 0.7 monolayers of Au, these chains are metallic. Upon the evaporation of an additional submonolayer amount of gold, the surface becomes insulating but keeps the 5×2 symmetry. This metal-to-insulator transition was in situ monitored based on the infrared plasmonic signal change with coverage. The phase transition is theoretically explained by total-energy and band-structure calculations. Accordingly, it can be understood in terms of the occupation of the originally half-filled one-dimensional band at the Fermi level. By annealing the system, the additional gold is removed from the surface and the plasmonic signal is recovered, which underlines the stability of the metallic structure. So, recent results on the infrared plasmonic signals of the Si(111)-5 × 2-Au surface are supported. The understanding of potential one-dimensional electrical interconnects is improved. © 2015 American Chemical Society.
    view abstractdoi: 10.1021/acs.jpclett.5b01530
  • 2015 • 83 High-Quality Solution-Processed Silicon Oxide Gate Dielectric Applied on Indium Oxide Based Thin-Film Transistors
    Jaehnike, F. and Pham, D.V. and Anselmann, R. and Bock, C. and Kunze, U.
    ACS Applied Materials and Interfaces 7 14011-14017 (2015)
    A silicon oxide gate dielectric was synthesized by a facile sol-gel reaction and applied to solution-processed indium oxide based thin-film transistors (TFTs). The SiO<inf>x</inf> sol-gel was spin-coated on highly doped silicon substrates and converted to a dense dielectric film with a smooth surface at a maximum processing temperature of T = 350 °C. The synthesis was systematically improved, so that the solution-processed silicon oxide finally achieved comparable break downfield strength (7 MV/cm) and leakage current densities (<10 nA/cm2 at 1 MV/cm) to thermally grown silicon dioxide (SiO<inf>2</inf>). The good quality of the dielectric layer was successfully proven in bottom-gate, bottom-contact metal oxide TFTs and compared to reference TFTs with thermally grown SiO<inf>2</inf>. Both transistor types have field-effect mobility values as high as 28 cm2/(Vs) with an on/off current ratio of 108, subthreshold swings of 0.30 and 0.37 V/dec, respectively, and a threshold voltage close to zero. The good device performance could be attributed to the smooth dielectric/semiconductor interface and low interface trap density. Thus, the sol-gel-derived SiO<inf>2</inf> is a promising candidate for a high-quality dielectric layer on many substrates and high-performance large-area applications. (Graph Presented). © 2015 American Chemical Society.
    view abstractdoi: 10.1021/acsami.5b03105
  • 2015 • 82 Metal-organic CVD of Y2O3 Thin Films using Yttrium tris-amidinates
    Karle, S. and Dang, V.-S. and Prenzel, M. and Rogalla, D. and Becker, H.-W. and Devi, A.
    Chemical Vapor Deposition 21 335-342 (2015)
    Thin films of Y2O3 are deposited on Si(100) and Al2O3 (0001) substrates via metal-organic (MO)CVD for the first time using two closely related yttrium tris-amidinate compounds as precursors in the presence of oxygen in the temperature range 400-700 °C. The structural, morphological, and compositional features of the films are investigated in detail. At deposition temperatures of 500 °C and higher both the precursors yield polycrystalline Y2O3 thin films in the cubic phase. The compositional analysis revealed the formation of nearly stoichiometric Y2O3. The optical band gaps are estimated using UV-Vis spectroscopy. Preliminary electrical measurements are performed in the form of a metal oxide semiconductor (MOS) structure of Al/Y2O3/p-Si/Ag. Leakage currents and dielectric constants are also determined. Y2O3 thin films are grown by MOCVD at 400-700 °C using yttrium amidinate presursors in the presence of oxygen. The films, which are polycrystalline in case of deposition temperatures &gt; 400 °C, are dense and exhibit good purity and homogeneity. © 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/cvde.201507189
  • 2015 • 81 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 • 80 Experimental verification of a benchmark forming simulation
    Landkammer, P. and Loderer, A. and Krebs, E. and Söhngen, B. and Steinmann, P. and Hausotte, T. and Kersting, P. and Biermann, D. and Willner, K.
    Key Engineering Materials 639 251-258 (2015)
    Forming of near-net-shaped and load-adapted functional components, as it is developed in the Transregional Collaborative Research Centre on Sheet-Bulk Metal Forming SFB/TR 73, causes different problems, which lead to non-optimal manufacturing results. For these high complex processes the prediction of forming effects can only be realized by simulations. A stamping process of pressing eight punches into a circular blank is chosen for the considered investigations. This reference process is designed to reflect the main aspects, which strongly affect the final outcome of forming processes. These are the orthotropic material behaviour, the optimal design of the initial blank and the influences of different contact and friction laws. The aim of this work is to verify the results of finite element computations for the proposed forming process by experiments. Evaluation methods are presented to detect the influence of the anisotropy and also to quantify the optimal blank design, which is determined by inverse form finding. The manufacturing accuracy of the die plate and the corresponding roughness data of the milled surface are analysed, whereas metrological investigations are required. This is accomplished by the help of advanced measurement techniques like a multi-sensor fringe projection system and a white light interferometer. Regarding the geometry of the punches, micromilling of the die plate is also a real challenge, especially due to the hardness of the high-speed steel ASP 2023 (approx. 63 HRC). The surface roughness of the workpiece before and after the forming process is evaluated to gain auxiliary data for enhancing the friction modelling and to characterise the contact behaviour. © 2015 Trans Tech Publications, Switzerland.
    view abstractdoi: 10.4028/www.scientific.net/KEM.639.251
  • 2015 • 79 On the Role of Metals in Nitrogen-Doped Carbon Electrocatalysts for Oxygen Reduction
    Masa, J. and Xia, W. and Muhler, M. and Schuhmann, W.
    Angewandte Chemie - International Edition 54 10102-10120 (2015)
    The notion of metal-free catalysts is used to refer to carbon materials modified with nonmetallic elements. However, some claimed metal-free catalysts are prepared using metal-containing precursors. It is highly contested that metal residues in nitrogen-doped carbon (NC) catalysts play a crucial role in the oxygen reduction reaction (ORR). In an attempt to reconcile divergent views, a definition for truly metal-free catalysts is proposed and the differences between NC and M-N<inf>x</inf>/C catalysts are discussed. Metal impurities at levels usually undetectable by techniques such as XPS, XRD, and EDX significantly promote the ORR. Poisoning tests to mask the metal ions reveal the involvement of metal residues as active sites or as modifiers of the electronic structure of the active sites in NC. The unique merits of both M-N<inf>x</inf>/C and NC catalysts are discussed to inspire the development of more advanced nonprecious-metal catalysts for the ORR. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/anie.201500569
  • 2015 • 78 Guided in situ polymerization of MEH-PPV in mesoporous titania photoanodes
    Minar, N.K. and Docampo, P. and Fattakhova-Rohlfing, D. and Bein, T.
    ACS Applied Materials and Interfaces 7 10356-10364 (2015)
    Incorporation of conjugated polymers into porous metal oxide networks is a challenging task, which is being pursued via many different approaches. We have developed the guided in situ polymerization of poly(2-methoxy-5-(2′-ethylhexyloxy)-p-phenylenevinylene) (MEH-PPV) in porous titania films by means of surface functionalization. The controlled polymerization via the Gilch route was induced by an alkoxide base and by increasing the temperature. The selected and specially designed surface-functionalizing linker molecules mimic the monomer or its activated form, respectively. In this way, we drastically enhanced the amount of MEH-PPV incorporated into the porous titania phase compared to nonfunctionalized samples by a factor of 6. Additionally, photovoltaic measurements were performed. The devices show shunting or series resistance limitations, depending on the surface functionalization prior to in situ polymerization of MEH-PPV. We suggest that the reason for this behavior can be found in the orientation of the grown polymer chains with respect to the titania surface. Therefore, the geometry of the anchoring via the linker molecules is relevant for exploiting the full electronic potential of the conjugated polymer in the resulting hybrid composite. This observation will help to design future synthesis methods for new hybrid materials from conjugated polymers and n-type semiconductors to take full advantage of favorable electronic interactions between the two phases. © 2015 American Chemical Society.
    view abstractdoi: 10.1021/acsami.5b01262
  • 2015 • 77 Avalanche-Discharge-Induced Electrical Forming in Tantalum Oxide-Based Metal-Insulator-Metal Structures
    Skaja, K. and Baumer, C. and Peters, O. and Menzel, S. and Moors, M. and Du, H. C. and Bornhofft, M. and Schmitz, C. and Feyer, V. and Jia, C. L. and Schneider, C. M. and Mayer, J. and Waser, R. and Dittmann, R.
    Advanced Functional Materials 25 7154--7162 (2015)
    Oxide-based metal-insulator-metal structures are of special interest for future resistive random-access memories. In such cells, redox processes on the nanoscale occur during resistive switching, which are initiated by the reversible movement of native donors, such as oxygen vacancies. The formation of these filaments is mainly attributed to an enhanced oxygen diffusion due to Joule heating in an electric field or due to electrical breakdown. Here, the development of a dendrite-like structure, which is induced by an avalanche discharge between the top electrode and the Ta2O5- x layer, is presented, which occurs instead of a local breakdown between top and bottom electrode. The dendrite-like structure evolves primarily at structures with a pronounced interface adsorbate layer. Furthermore, local conductive atomic force microscopy reveals that the entire dendrite region becomes conductive. Via spectromicroscopy it is demonstrated that the subsequent switching is caused by a valence change between Ta4+ and Ta5+, which takes place over the entire former Pt/Ta2O5- x interface of the dendrite-like structure.
    view abstractdoi: 10.1002/adfm.201502767
  • 2015 • 76 Microstructure refinement for high modulus in-situ metal matrix composite steels via controlled solidification of the system Fe-TiB2
    Springer, H. and Aparicio Fernandez, R. and Duarte, M.J. and Kostka, A. and Raabe, D.
    Acta Materialia 96 47-56 (2015)
    Microstructures of Fe-TiB<inf>2</inf> metal-matrix-composites formed in-situ from Fe-Ti-B melts were investigated for hypo- and hyper-eutectic concentrations down to atomic-scale resolution. Special emphasis is laid on the influence of the solidification rate on particle size, morphology and distribution as well as their relation to mechanical properties. Innovative routes for the cost-effective production of stiff and ductile high modulus steels for lightweight structural applications are discussed, focusing on hyper-eutectic compositions due to their high stiffness/density ratio: firstly, very slow cooling allows the primary particles floating to the top of the cast, from which they can either be easily removed for retaining bulk material containing only fine-dispersed eutectic particles, or be kept and utilised as a wear resistant surface. Secondly, annealing of amorphous matrix material obtained from very fast solidification leads to fine dispersed nano-scaled precipitation of TiB<inf>2</inf> particles. © 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2015.06.017
  • 2015 • 75 One-Pot Synthesis of Carbon-Coated Nanostructured Iron Oxide on Few-Layer Graphene for Lithium-Ion Batteries
    Sun, Z. and Madej, E. and Wiktor, C. and Sinev, I. and Fischer, R.A. and Van Tendeloo, G. and Muhler, M. and Schuhmann, W. and Ventosa, E.
    Chemistry - A European Journal 21 16154-16161 (2015)
    Nanostructure engineering has been demonstrated to improve the electrochemical performance of iron oxide based electrodes in Li-ion batteries (LIBs). However, the synthesis of advanced functional materials often requires multiple steps. Herein, we present a facile one-pot synthesis of carbon-coated nanostructured iron oxide on few-layer graphene through high-pressure pyrolysis of ferrocene in the presence of pristine graphene. The ferrocene precursor supplies both iron and carbon to form the carbon-coated iron oxide, while the graphene acts as a high-surface-area anchor to achieve small metal oxide nanoparticles. When evaluated as a negative-electrode material for LIBs, our composite showed improved electrochemical performance compared to commercial iron oxide nanopowders, especially at fast charge/discharge rates. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/chem.201501935
  • 2015 • 74 QuickFF: A program for a quick and easy derivation of force fields for metal-organic frameworks from ab initio input
    Vanduyfhuys, L. and Vandenbrande, S. and Verstraelen, T. and Schmid, R. and Waroquier, M. and Van Speybroeck, V.
    Journal of Computational Chemistry 36 1015-1027 (2015)
    QuickFF is a software package to derive accurate force fields for isolated and complex molecular systems in a quick and easy manner. Apart from its general applicability, the program has been designed to generate force fields for metal-organic frameworks in an automated fashion. The force field parameters for the covalent interaction are derived from ab initio data. The mathematical expression of the covalent energy is kept simple to ensure robustness and to avoid fitting deficiencies as much as possible. The user needs to produce an equilibrium structure and a Hessian matrix for one or more building units. Afterward, a force field is generated for the system using a three-step method implemented in QuickFF. The first two steps of the methodology are designed to minimize correlations among the force field parameters. In the last step, the parameters are refined by imposing the force field parameters to reproduce the ab initio Hessian matrix in Cartesian coordinate space as accurate as possible. The method is applied on a set of 1000 organic molecules to show the easiness of the software protocol. To illustrate its application to metal-organic frameworks (MOFs), QuickFF is used to determine force fields for MIL-53(Al) and MOF-5. For both materials, accurate force fields were already generated in literature but they requested a lot of manual interventions. QuickFF is a tool that can easily be used by anyone with a basic knowledge of performing ab initio calculations. As a result, accurate force fields are generated with minimal effort. © 2015 Wiley Periodicals, Inc.
    view abstractdoi: 10.1002/jcc.23877
  • 2015 • 73 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 • 72 Multiscale Simulation of Plasticity in bcc Metals
    Weygand, D. and Mrovec, M. and Hochrainer, T. and Gumbsch, P.
    Annual Review of Materials Research 45 369-390 (2015)
    Significant progress in our understanding of plasticity in body-centered cubic (bcc) metals during the last decade has enabled rigorous multiscale modeling based on quantitative physical principles. Significant advances have been made at the atomistic level in the understanding of dislocation core structures and energetics associated with dislocation glide by using high-fidelity models originating from quantum mechanical principles. These simulations revealed important details about the influence of non-Schmid (nonglide) stresses on the mobility of screw dislocations in bcc metals that could be implemented to mesoscopic discrete dislocation simulations with atomistically informed dislocation mobility laws. First applications of dislocation dynamics simulations to studies of plasticity in small-scale bcc single crystals have been performed. Dislocation dynamics simulations inspired the development of continuum models based on advanced 3D dislocation density measures with evolution equations that naturally track dislocation motion. These advances open new opportunities and perspectives for future quantitative and materials-specific multiscale simulation methods to describe plastic deformation in bcc metals and their alloys. Copyright © 2015 by Annual Reviews. All rights reserved.
    view abstractdoi: 10.1146/annurev-matsci-070214-020852
  • 2015 • 71 Thermal conductivity of advanced TiC reinforced metal matrix composites for polymer processing applications
    Wilzer, J. and Windmann, M. and Weber, S. and Hill, H. and Bennekom, A.V. and Theisen, W.
    Journal of Composite Materials 49 243-250 (2015)
    Tools used for fabricating polymers are often required to have low thermal conductivities, e.g. for pelletizing, because this lowers the risk of the polymer nozzle being obstructed by molten polymer solidifying as it exits. Latterly, advanced corrosion and wear resistant metal matrix composites (MMCs) are used for pelletizing tools. Therefore, with respect to polymer processing it is important to know how the thermal conductivity of MMC gets influenced by hard phase and metal matrix contribution. In this study, the thermal conductivity of a TiC reinforced corrosion and wear resistant MMC gets analyzed. Especially the influence of chemical interdiffusion between TiC and metal matrix on the resulting thermal conductivity gets analyzed. It is shown that changes in the chemical composition lead to distinct decrease in thermal conductivity of the TiC which has to be considered when MMC thermal conductivities have to be examined. © The Author(s) 2013 Reprints and permissions: sagepub.co.uk/journalsPermissions.nav.
    view abstractdoi: 10.1177/0021998313516143
  • 2015 • 70 The effect of contact load on CoCrMo wear and the formation and retention of tribofilms
    Wimmer, M.A. and Laurent, M.P. and Mathew, M.T. and Nagelli, C. and Liao, Y. and Marks, L.D. and Jacobs, J.J. and Fischer, A.
    Wear 332-333 643-649 (2015)
    Tribochemical reactions in a protein lubricated metal-on-metal (MoM) sliding contact may play a significant role for its wear performance. Such reactions lead to the formation of a carbonaceous 'tribofilm', which can act as a protective layer against corrosion and wear. The purpose of this study was to determine the effect of contact load on wear and the formation and retention of tribofilms. Wear tests were performed in a custom-made ball-on-flat testing apparatus that incorporated an electrochemical cell. A ceramic ball was used to articulate against low-carbon wrought CoCrMo alloy pins in bovine serum. Using a range of contact loads at a single potentiostatic condition (close to free potential), weight loss and changes in surface properties were evaluated. We determined that wear was influenced by the loading condition. As expected, wear increased with load, but the association between applied load and measured weight loss was not linear. In the intermediate load region, in the range of 32-48 N (~58-80 MPa), there was more than an order of magnitude drop in the wear per unit load, and the wear versus load data suggested an inflexion point at 49N. Regression analyses yielded a cubic model (R2=0.991; p=0.0002), where the cubic term, which represents the inflexion, was highly significant (p=0.0021). This model is supported by the observations that the minimum in the friction versus load curve is at 52 N and the highest relative increase in polarization resistance occurred at 49 N. Scanning electron microscopy and Raman spectroscopy indicated the absence of a tribofilm for the low and within the contact area of the high load cases. Synergistic interactions of wear and corrosion seem to play an important role. © 2015 Elsevier B.V.
    view abstractdoi: 10.1016/j.wear.2015.02.013
  • 2015 • 69 Route to the Smallest Doped Semiconductor: Mn2+-Doped (CdSe)13 Clusters
    Yang, J. and Fainblat, R. and Kwon, S.G. and Muckel, F. and Yu, J.H. and Terlinden, H. and Kim, B.H. and Iavarone, D. and Choi, M.K. and Kim, I.Y. and Park, I. and Hong, H.-K. and Lee, J. and Son, J.S. and Lee, Z. and Kang, K. and...
    Journal of the American Chemical Society 137 12776-12779 (2015)
    Doping semiconductor nanocrystals with magnetic transition-metal ions has attracted fundamental interest to obtain a nanoscale dilute magnetic semiconductor, which has unique spin exchange interaction between magnetic spin and exciton. So far, the study on the doped semiconductor NCs has usually been conducted with NCs with larger than 2 nm because of synthetic challenges. Herein, we report the synthesis and characterization of Mn2+-doped (CdSe)13 clusters, the smallest doped semiconductors. In this study, single-sized doped clusters are produced in large scale. Despite their small size, these clusters have semiconductor band structure instead of that of molecules. Surprisingly, the clusters show multiple excitonic transitions with different magneto-optical activities, which can be attributed to the fine structure splitting. Magneto-optically active states exhibit giant Zeeman splittings up to elevated temperatures (128 K) with large g-factors of 81(±8) at 4 K. Our results present a new synthetic method for doped clusters and facilitate the understanding of doped semiconductor at the boundary of molecules and quantum nanostructure. © 2015 American Chemical Society.
    view abstractdoi: 10.1021/jacs.5b07888
  • 2015 • 68 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 • 67 Peptide self-assembly triggered by metal ions
    Zou, R. and Wang, Q. and Wu, J. and Wu, J. and Schmuck, C. and Tian, H.
    Chemical Society Reviews 44 5200-5219 (2015)
    Through their unique and specific interactions with various metal ions, naturally occurring proteins control structures and functions of many biological processes and functions in organisms. Inspired by natural metallopeptides, chemists have developed artificial peptides which coordinate with metal ions through their functional groups either for introducing a special reactivity or for constructing nanostructures. However, the design of new coordination peptides requires a deep understanding of the structures, assembly properties, and dynamic behaviours of such peptides. This review briefly discusses strategies of peptide self-assembly induced by metal coordination to different natural and non-natural binding sites in the peptide. The structures and functions of the obtained aggregates are described as well. We also highlight some examples of a metal-induced peptide self-assembly with relevance to biotechnology applications. © The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c5cs00234f
  • 2014 • 66 Analytic bond-order potentials for the bcc refractory metals Nb, Ta, Mo and W
    Čák, M. and Hammerschmidt, T. and Rogal, J. and Vitek, V. and Drautz, R.
    Journal of Physics Condensed Matter 26 (2014)
    Bond-order potentials (BOPs) are based on the tight-binding approximation for determining the energy of a system of interacting atoms. The bond energy and forces are computed analytically within the formalism of the analytic BOPs. Here we present parametrizations of the analytic BOPs for the bcc refractory metals Nb, Ta, Mo and W. The parametrizations are optimized for the equilibrium bcc structure and tested for atomic environments far from equilibrium that had not been included in the fitting procedure. These tests include structural energy differences for competing crystal structures; tetragonal, trigonal, hexagonal and orthorhombic deformation paths; formation energies of point defects as well as phonon dispersion relations. Our tests show good agreement with available experimental and theoretical data. In practice, we obtain the energetic ordering of vacancy, [1 1 1], [1 1 0], and [1 0 0] self-interstitial atom in agreement with density functional theory calculations. © 2014 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0953-8984/26/19/195501
  • 2014 • 65 Characterization of Ta-Ti Thin Films by using a Scanning Droplet Cell in Combination with AC Linear Sweep Voltammetry
    Fan, M. and Sliozberg, K. and La Mantia, F. and Miyashita, N. and Hagymási, M. and Schnitter, C. and Ludwig, Al. and Schuhmann, W.
    ChemElectroChem 1 903-908 (2014)
    A binary Ta-Ti thin film composition-spread materials library is prepared through magnetron sputter co-deposition. An automated microelectrochemical investigation on selected surface areas, corresponding to a concentration gradient of Ti varying from 0.5 to 36at%, is achieved by using a scanning droplet cell. Simultaneously, during the anodic oxide growth, a small alternating current (AC) voltage is superimposed on the increasing direct current (DC) potential in order to record the capacitance of the mixed-metal oxide by using alternating current linear sweep voltammetry (AC-LSV). Valve metal behavior, with the current stabilizing after an initial rapid increase, is found for all investigated compositions. AC-LSV allows the ratio of the formation factor to the relative permittivity for different compositions to be calculated. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/celc.201300153
  • 2014 • 64 Structural complexity in metal-organic frameworks: Simultaneous modification of open metal sites and hierarchical porosity by systematic doping with defective linkers
    Fang, Z. and Dürholt, J.P. and Kauer, M. and Zhang, W. and Lochenie, C. and Jee, B. and Albada, B. and Metzler-Nolte, N. and Pöppl, A. and Weber, B. and Muhler, M. and Wang, Y. and Schmid, R. and Fischer, R.A.
    Journal of the American Chemical Society 136 9627-9636 (2014)
    A series of defect-engineered metal-organic frameworks (DEMOFs) derived from parent microporous MOFs was obtained by systematic doping with defective linkers during synthesis, leading to the simultaneous and controllable modification of coordinatively unsaturated metal sites (CUS) and introduction of functionalized mesopores. These materials were investigated via temperature-dependent adsorption/desorption of CO monitored by FTIR spectroscopy under ultra-high-vacuum conditions. Accurate structural models for the generated point defects at CUS were deduced by matching experimental data with theoretical simulation. The results reveal multivariate diversity of electronic and steric properties at CUS, demonstrating the MOF defect structure modulation at two length scales in a single step to overcome restricted active site specificity and confined coordination space at CUS. Moreover, the DEMOFs exhibit promising modified physical properties, including band gap, magnetism, and porosity, with hierarchical micro/mesopore structures correlated with the nature and the degree of defective linker incorporation into the framework. © 2014 American Chemical Society.
    view abstractdoi: 10.1021/ja503218j
  • 2014 • 63 Hydrogen evolution from metal-surface hydroxyl interaction
    Fujimori, Y. and Kaden, W.E. and Brown, M.A. and Roldan Cuenya, B. and Sterrer, M. and Freund, H.-J.
    Journal of Physical Chemistry C 118 17717-17723 (2014)
    The redox interaction between hydroxyl groups on oxide surfaces and metal atoms and clusters deposited thereon, according to which metals get oxidized and hydrogen released, is an effective route to tune both the morphological (particle size and shape) and electronic (oxidation state) properties of oxide-supported metals. While the oxidation state of the metals can straightforwardly be probed by X-ray based methods (e.g., XPS), hydrogen is much more difficult to capture, in particular in highly reactive systems where the redox interaction takes place directly during the nucleation of the metals at room temperature. In the present study, the interaction of Pd with a hydroxylated MgO(001) surface was studied using a combination of vibrational spectroscopy, electronic structure studies including Auger parameter analysis, and thermal desorption experiments. The results provide clear experimental evidence for the redox nature of the interaction by showing a direct correlation between metal oxidation and hydrogen evolution at slightly elevated temperature (390 K). Moreover, a second hydrogen evolution pathway opens up at 500 K, which involves hydroxyl groups on the MgO support and carbon monoxide adsorbed on the Pd particles (water-gas shift reaction). © 2014 American Chemical Society.
    view abstractdoi: 10.1021/jp504655e
  • 2014 • 62 Support and challenges to the melanosomal casing model based on nanoscale distribution of metals within iris melanosomes detected by X-ray fluorescence analysis
    Gorniak, T. and Haraszti, T. and Suhonen, H. and Yang, Y. and Hedberg-Buenz, A. and Koehn, D. and Heine, R. and Grunze, M. and Rosenhahn, A. and Anderson, M.G.
    Pigment Cell and Melanoma Research 27 831-834 (2014)
    Melanin within melanosomes exists as eumelanin or pheomelanin. Distributions of these melanins have been studied extensively within tissues, but less often within individual melanosomes. Here, we apply X-ray fluorescence analysis with synchrotron radiation to survey the nanoscale distribution of metals within purified melanosomes of mice. The study allows a discovery-based characterization of melanosomal metals, and, because Cu is specifically associated with eumelanin, a hypothesis-based test of the 'casing model' predicting that melanosomes contain a pheomelanin core surrounded by a eumelanin shell. Analysis of Cu, Ca, and Zn shows variable concentrations and distributions, with Ca/Zn highly correlated, and at least three discrete patterns for the distribution of Cu vs. Ca/Zn in different melanosomes - including one with a Cu-rich shell surrounding a Ca/Zn-rich core. Thus, the results support predictions of the casing model, but also suggest that in at least some tissues and genetic contexts, other arrangements of melanin may co-exist. © 2014 John Wiley & Sons A/S.
    view abstractdoi: 10.1111/pcmr.12278
  • 2014 • 61 Multifunctional, defect-engineered metal-organic frameworks with ruthenium centers: Sorption and catalytic properties
    Kozachuk, O. and Luz, I. and Llabrés I Xamena, F.X. and Noei, H. and Kauer, M. and Albada, H.B. and Bloch, E.D. and Marler, B. and Wang, Y. and Muhler, M. and Fischer, R.A.
    Angewandte Chemie - International Edition 53 7058-7062 (2014)
    A mixed-linker solid-solution approach was employed to modify the metal sites and introduce structural defects into the mixed-valence Ru II/III structural analogue of the well-known MOF family [M 3 II,II(btc)2] (M=Cu, Mo, Cr, Ni, Zn; btc=benzene-1,3,5-tricarboxylate), with partly missing carboxylate ligators at the Ru2 paddle-wheels. Incorporation of pyridine-3,5-dicarboxylate (pydc), which is the same size as btc but carries lower charge, as a second, defective linker has led to the mixed-linker isoreticular derivatives of Ru-MOF, which display characteristics unlike those of the defect-free framework. Along with the creation of additional coordinatively unsaturated sites, the incorporation of pydc induces the partial reduction of ruthenium. Accordingly, the modified Ru sites are responsible for the activity of the "defective" variants in the dissociative chemisorption of CO 2, the enhanced performance in CO sorption, the formation of hydride species, and the catalytic hydrogenation of olefins. The defect engineering in Ru-based metal-organic frameworks (MOFs) at coordinatively unsaturated metal centers (CUS) induces partial reduction of the metal nodes and leads to properties that are absent for the parent MOF, such as dissociative chemisorption of CO2 and enhanced sorption capacity of CO. The modified MOFs offer new perspectives as multifunctional materials whose performance is controlled by design of the defects. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/anie.201311128
  • 2014 • 60 Possibilities to improve the antioxidative capacity of beer by optimized hopping regimes
    Kunz, T. and Frenzel, J. and Wietstock, P.C. and Methner, F.-J.
    Journal of the Institute of Brewing 120 415-425 (2014)
    Different hopping regimes were evaluated to investigate the effect on the oxidative stability of wort and beer. Compared with a single hop dosage at the beginning of wort boil, it was possible to increase the concentration of α-acids in pitching wort and beer by applying incremental hop dosage, dry hopping or the use of a pre-isomerized hop product in combination with an α-acid extract, which concomitantly resulted in lower iron concentrations and an enhanced flavour stability as indicated by standard wort and beer analyses, atomic absorption spectroscopy, electron spin resonance spectroscopy and sensory analysis of fresh and force-aged beers. The functional principle of hop dosage variations is explained by saving of α-acids throughout the wort production process, which yields an increased formation and precipitation of pro-oxidative acting transition metal ions (e.g. Fe) in α-acid-complexes during the whirlpool rest and fermentation. Consequently, fewer reactive oxygen species are generated. Additional laboratory trials simulating wort cooling and beer storage in buffered model solutions proved that un-isomerized α-acids are strong iron chelators and confirmed the functional principle of the applied hopping regimes. Negative effects of higher α-acid contents on fermentation performance and depletion of the zinc concentration, which is an essential nutrient for yeast, could be excluded. © 2014 The Institute of Brewing & Distilling.
    view abstractdoi: 10.1002/jib.162
  • 2014 • 59 Reactivity of metal catalysts in glucose-fructose conversion
    Loerbroks, C. and vanRijn, J. and Ruby, M.-P. and Tong, Q. and Schüth, F. and Thiel, W.
    Chemistry - A European Journal 20 12298–12309 (2014)
    A joint experimental and computational study on the glucose-fructose conversion in water is reported. The reactivity of different metal catalysts (CrCl3, AlCl3, CuCl2, FeCl3, and MgCl2) was analyzed. Experimentally, CrCl3 and AlCl3 achieved the best glucose conversion rates, CuCl2 and FeCl3 were only mediocre catalysts, and MgCl2 was inactive. To explain these differences in reactivity, DFT calculations were performed for various metal complexes. The computed mechanism consists of two proton transfers and a hydrogen-atom transfer; the latter was the rate-determining step for all catalysts. The computational results were consistent with the experimental findings and rationalized the observed differences in the behavior of the metal catalysts. To be an efficient catalyst, a metal complex should satisfy the following criteria: moderate Brønsted and Lewis acidity (pKa=4-6), coordination with either water or weaker σ donors, energetically low-lying unoccupied orbitals, compact transition-state structures, and the ability for complexation of glucose. Thus, the reactivity of the metal catalysts in water is governed by many factors, not just the Lewis acidity. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/chem.201402437
  • 2014 • 58 Metal-free catalysts for oxygen reduction in alkaline electrolytes: Influence of the presence of Co, Fe, Mn and Ni inclusions
    Masa, J. and Zhao, A. and Wei, X. and Muhler, M. and Schuhmann, W.
    Electrochimica Acta 128 271-278 (2014)
    Metal-free nitrogen modified carbon catalysts (NC) are very closely related to MNC catalysts which contain a transition metal(s) (M), usually Fe or Co as an essential constituent. We investigated the influence of metal inclusions on the activity of nitrogen-doped carbon black in the electrocatalysis of the oxygen reduction reaction (ORR). A reference metal-free NC catalyst was prepared by pyrolysis of a polypyrrole/Vulcan XC72 composite at 800 °C for 2 h under helium. Controlled amounts of Co, Fe, Mn and Ni in low concentrations were then introduced into NC by impregnating it with the corresponding meso-tetra(4-pyridyl) porphyrin metal complex followed by further pyrolysis at 650 °C for 2 h under helium. The resulting catalysts were investigated for ORR using rotating disk electrode and rotating-ring disk electrode voltammetry in 0.1 M KOH. Additionally, the rate of decomposition of hydrogen peroxide by the different catalysts was determined in order to probe the influence of the metal inclusions on the mechanism and selectivity of the ORR. The results show that Fe, Co and Mn inclusions cause a substantial decrease of the overpotential of the reaction and enhance the catalytic current, whereas the presence of Ni has a poisoning effect on ORR. In the presence of Fe, the catalysts apparently reduce oxygen selectively to OH- in a direct four electron transfer process as opposed to the two-step, two electron pathway involving hydrogen peroxide as an intermediate for the case of the NC catalyst. © 2013 Elsevier Ltd.
    view abstractdoi: 10.1016/j.electacta.2013.11.026
  • 2014 • 57 Tribolayer formation in a metal-on-metal (MoM) hip joint: An electrochemical investigation
    Mathew, M.T. and Nagelli, C. and Pourzal, R. and Fischer, A. and Laurent, M.P. and Jacobs, J.J. and Wimmer, M.A.
    Journal of the Mechanical Behavior of Biomedical Materials 29 199-212 (2014)
    The demand for total hip replacement (THR) surgery is increasing in the younger population due to faster rehabilitation and more complete restoration of function. Up to 2009, metal-on-metal (MoM) hip joint bearings were a popular choice due to their design flexibility, post-operative stability and relatively low wear rates. The main wear mechanisms that occur along the bearing surface of MoM joints are tribochemical reactions that deposit a mixture of wear debris, metal ions and organic matrix of decomposed proteins known as a tribolayer. No in-depth electrochemical studies have been reported on the structure and characteristics of this tribolayer or about the parameters involved in its formation.In this study, we conducted an electrochemical investigation of different surfaces (bulk-like: control, nano-crystalline: new implant and tribolayer surface: retrieved implant) made out of two commonly used hip CoCrMo alloys (high-carbon and low-carbon). As per ASTM standard, cyclic polarization tests and electrochemical impedance spectroscopy tests were conducted. The results obtained from electrochemical parameters for different surfaces clearly indicated a reduction in corrosion for the tribolayer surface (Icorr: 0.76μA/cm2). Further, polarization resistance (Rp:2.39±0.60MΩ/cm2) and capacitance (Cdl:15.20±0.75μF/cm2) indicated variation in corrosion kinetics for the tribolayer surface, that attributed to its structure and stability in a simulated body environment. © 2013 Elsevier Ltd.
    view abstractdoi: 10.1016/j.jmbbm.2013.08.018
  • 2014 • 56 Mesostructural design and manufacturing of open-pore metal foams by investment casting
    Matz, A.M. and Mocker, B.S. and Müller, D.W. and Jost, N. and Eggeler, G.
    Advances in Materials Science and Engineering 2014 (2014)
    The present paper describes the manufacturing process of open-pore metal foams by investment casting and the mesostructural/morphological evolution resulting from a new technique of modifying the precursor. By this technique, the precursor is coated with a polymer layer whereby a thickening of the struts occurs. Relative densities in the range of 1.85≤ρrel≤25% of open-pore metal foams can be achieved with high accuracy. The samples investigated have pore densities of ρP=7 ppi, 10 ppi, and 13 ppi. The relevant processing parameters needed for a homogenous formation of the polymer layer are determined for two different coating materials and the resulting open-pore foam's mesostructure is characterized qualitatively and quantitatively. The alloy used for investment casting open-pore metal foamsis AlZn11. The microstructural evolution of these foams is evaluated as a function of the mesostructure. Differences in the microstructure are observed for foams with low and high relative densities and discussed in terms of cooling subsequent to investment casting. © 2014 Alexander Martin Matz et al.
    view abstractdoi: 10.1155/2014/421729
  • 2014 • 55 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 • 54 Targeted manipulation of metal-organic frameworks to direct sorption properties
    Schneemann, A. and Henke, S. and Schwedler, I. and Fischer, R.A.
    ChemPhysChem 15 823-839 (2014)
    Metal-organic frameworks are promising materials for manifold applications. This Minireview highlights approaches for the fine-tuning of specific sorption properties (e.g. capacity, selectivity, and breathing behavior) of this interesting class of materials. Central aspects covered are the control over the crystal morphology, the targeted tuning of sorption properties by judicious choice of metal centers and linkers, and the preparation of host-guest systems. We want to introduce the reader to these topics on the basis of the manipulation of a handful of outstanding prototypical metal-organic frameworks. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/cphc.201300976
  • 2014 • 53 Dry shearing of micro-alloyed steels
    Steinbach, F. and Güner, A. and Tekkaya, A.E.
    Key Engineering Materials 622-623 1058-1065 (2014)
    In the production process of sheet metal parts, oil is widely used as lubricant, not only in sheet metal forming but also in shearing and blanking. Due to environment, health and cost reasons, the absence of lubricants is an aim for future production as it has initiated for machining in the last years. For lubricant-free shearing, it has to be known if there is an influence on the process itself when using oil or not. To find this out, experiments are carried out with a small testing device installed in a tensile testing machine and a blanking tool installed in a servo press. With the small device it is possible to make a piercing process with a circle punch of 16 mm diameter. The blanking tool produces a larger cut part with different holes and open cuts. Without lubricant, there is no difference in the maximum shearing force for the small device while the stripping force is higher and the cut edge zones differs slightly. Using oil or not has a small effect on the force using the blanking tool. © (2014) Trans Tech Publications, Switzerland.
    view abstractdoi: 10.4028/www.scientific.net/KEM.622-623.1058
  • 2014 • 52 A proof-of-concept - Using pre-created nucleation centres to improve the limit of detection in anodic stripping voltammetry
    Toh, H.S. and Batchelor-Mcauley, C. and Tschulik, K. and Damm, C. and Compton, R.G.
    Sensors and Actuators, B: Chemical 193 315-319 (2014)
    Anodic stripping voltammetry is a much-utilised method for trace metal analysis. We provide a simple proof-of-concept technique to improve the sensitivity of the method, which is illustrated by the detection of silver cations. This approach requires an electrode pre-treatment, which involves drop casting a metal nanoparticle suspension and oxidising the nanoparticles, leaving small metal nuclei on the electrode surface. In turn, the small metal nuclei act as nucleation sites for subsequent metal deposition when used to interrogate target solutions. In particular, the pre-treatment increases the amount of deposited metal in a given amount of time. Silver nitrate concentrations from 30 nM to 1 μM were tested and at silver ion concentration of 300 nM, the pre-treated electrode gave a signal, which was 40 times larger than the untreated electrode. The larger signal leads to the enhancement of sensitivity and a lowering of the detection limit of anodic stripping voltammetry without introducing other organic molecules, metals or impurities. © 2013 Elsevier B.V.
    view abstractdoi: 10.1016/j.snb.2013.11.120
  • 2014 • 51 Atomic-Layer-Deposited Aluminum and Zirconium Oxides for Surface Passivation of TiO2 in High-Efficiency Organic Photovoltaics
    Vasilopoulou, M. and Georgiadou, D.G. and Soultati, A. and Boukos, N. and Gardelis, S. and Palilis, L.C. and Fakis, M. and Skoulatakis, G. and Kennou, S. and Botzakaki, M. and Georga, S. and Krontiras, C.A. and Auras, F. and Fatta...
    Advanced Energy Materials 4 (2014)
    The reduction in electronic recombination losses by the passivation of surfaces is a key factor enabling high-efficiency solar cells. Here a strategy to passivate surface trap states of TiO<inf>2</inf> films used as cathode interlayers in organic photovoltaics (OPVs) through applying alumina (Al<inf>2</inf>O<inf>3</inf>) or zirconia (ZrO<inf>2</inf>) insulating nanolayers by thermal atomic layer deposition (ALD) is investigated. The results suggest that the surface traps in TiO<inf>2</inf> are oxygen vacancies, which cause undesirable recombination and high electron extraction barrier, reducing the open-circuit voltage and the short-circuit current of the complete OPV device. It is found that the ALD metal oxides enable excellent passivation of the TiO<inf>2</inf> surface followed by a downward shift of the conduction band minimum. OPV devices based on different photoactive layers and using the passivated TiO<inf>2</inf> electron extraction layers exhibit a significant enhancement of more than 30% in their power conversion efficiencies compared to their reference devices without the insulating metal oxide nanolayers. This is a result of significant suppression of charge recombination and enhanced electron extraction rates at the TiO<inf>2</inf>/ALD metal oxide/organic interface. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/aenm.201400214
  • 2014 • 50 Analysis of the potential of incremental stress superposition on air bending
    Weinrich, A. and Becker, C. and Maevus, F. and Chatti, S. and Tekkaya, A.E.
    Key Engineering Materials 622-623 1173-1180 (2014)
    Springback and limited forming limits of modern high strength steels are a big challenge in manufacturing engineering. Both aspects are crucial in sheet metal bending processes. Different modifications of the air bending process have already been developed in order to reduce springback and also to increase the forming limits of materials. A new method (the incremental stress superposition on air bending) has been developed. Studies of this new process alternative show a positive effect on the springback behavior. In order to investigate the potential of this process a comparison with other already established bending processes have been carried out. A possible process control to extend the forming limits has also been investigated. © (2014) Trans Tech Publications, Switzerland.
    view abstractdoi: 10.4028/www.scientific.net/KEM.622-623.1173
  • 2013 • 49 Detection of nitric oxide and nitroxyl with benzoresorufin-based fluorescent sensors
    Apfel, U.-P. and Buccella, D. and Wilson, J.J. and Lippard, S.J.
    Inorganic Chemistry 52 3285-3294 (2013)
    A new family of benzoresorufin-based copper complexes for fluorescence detection of NO and HNO is reported. The copper complexes, CuBRNO1-3, elicit 1.5-4.8-fold emission enhancement in response to NO and HNO. The three sensors differ in the nature of the metal-binding site. The photophysical properties of these sensors are investigated with assistance from density functional theory calculations. The fluorescence turn-on observed upon reaction with HNO is an unexpected result that is discussed in detail. The utility of the new sensors for detecting HNO and NO in HeLa cells and RAW 264.7 macrophages is demonstrated. © 2013 American Chemical Society.
    view abstractdoi: 10.1021/ic302793w
  • 2013 • 48 Wear patterns of taper connections in retrieved large diameter metal-on-metal bearings
    Bishop, N. and Witt, F. and Pourzal, R. and Fischer, A. and Rütschi, M. and Michel, M. and Morlock, M.
    Journal of Orthopaedic Research 31 1116-1122 (2013)
    Wear of the modular taper between head and shaft has been related to clinical failure resulting from adverse reactions to metallic debris. The problem has become pronounced in large metal-on-metal bearings, but the mechanism has not yet been fully understood. We analyzed retrieved components from five patients revised with various diagnoses. Two distinct wear patterns were observed for the head tapers. Three samples demonstrated "asymmetric" wear towards the inner end of the head taper. The other two showed "axisymmetric" radial wear (up to 65 μm) presenting the largest wear volumes (up to 20 mm3). Stem tapers demonstrated relatively little wear, and the fine thread on the stem taper surface was observed to be imprinted on the taper inside of the head. Our findings demonstrate that the cobalt-chrome head wears preferentially to the titanium stem taper. "asymmetric" wear suggests toggling due to the offset of the joint force vector from the taper. In contrast, samples with "axisymmetric" radial wear and a threaded imprint suggested that corrosion led to head subsidence onto the stem taper with gradual rotation. © 2013 Orthopaedic Research Society.
    view abstractdoi: 10.1002/jor.22326
  • 2013 • 47 Silver as antibacterial agent: Ion, nanoparticle, and metal
    Chernousova, S. and Epple, M.
    Angewandte Chemie - International Edition 52 1636-1653 (2013)
    The antibacterial action of silver is utilized in numerous consumer products and medical devices. Metallic silver, silver salts, and also silver nanoparticles are used for this purpose. The state of research on the effect of silver on bacteria, cells, and higher organisms is summarized. It can be concluded that the therapeutic window for silver is narrower than often assumed. However, the risks for humans and the environment are probably limited. Silver shield: Silver is used in different forms as an antibacterial agent. Earlier, sparingly soluble silver salts were predominantly used, but today, silver nanoparticles (see picture for an SEM image of cubic silver nanoparticles) are gaining increasing importance. As silver is also toxic towards mammalian cells, there is the question of the therapeutic window in the cases of consumer products and medical devices. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/anie.201205923
  • 2013 • 46 Massive anisotropic thermal expansion and thermo-responsive breathing in metal-organic frameworks modulated by linker functionalization
    Henke, S. and Schneemann, A. and Fischer, R.A.
    Advanced Functional Materials 23 5990-5996 (2013)
    Functionalized metal-organic frameworks (fu-MOFs) of general formula [Zn2(fu-L)2dabco]n show unprecedentedly large uniaxial positive and negative thermal expansion (fu-L = alkoxy functionalized 1,4-benzenedicarboxylate, dabco = 1,4-diazabicyclo[2.2.2]octane). The magnitude of the volumetric thermal expansion is more comparable to property of liquid water rather than any crystalline solid-state material. The alkoxy side chains of fu-L are connected to the framework skeleton but nevertheless exhibit large conformational flexibility. Thermally induced motion of these side chains induces extremely large anisotropic framework expansion and eventually triggers reversible solid state phase transitions to drastically expanded structures. The thermo-responsive properties of these hybrid solid-liquid materials are precisely controlled by the choice and combination of fu-Ls and depend on functional moieties and chain lengths. In principle, this combinatorial approach allows for a targeted design of extreme thermo-mechanical properties of MOFs addressing the regime between crystalline solid matter and the liquid state. Extremely large thermal expansion is shown by pillared-layered metal-organic frameworks (MOFs) exhibiting alkoxy-functionalized 1,4-benzenedicarboxylate linkers. At a certain threshold temperature the materials reversibly switch from a narrow pore to large pore form. This unprecedented thermo-mechanical behavior is an intrinsic property of the materials and can be modulated substantially by mixing differently functionalized linkers to obtain mixed linker MOF solid solutions. Copyright © 2013 WILEY-VCH Verlag GmbH &amp; Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/adfm.201301256
  • 2013 • 45 Generation of AuGe nanocomposites by co-sparking technique and their photoluminescence properties
    Kala, S. and Theissmann, R. and Kruis, F.E.
    Journal of Nanoparticle Research 15 (2013)
    The feasibility of spark discharge technique for preparing metal-semiconductor nanocomposites is demonstrated. In the AuGe system, Au shows only 10-3 atomic percent solid solubility in Ge, whereas 3.1 at.% Ge is soluble in Au. During the co-sparking, Au is used as anode material; the cathode is composed of Ge. The relative atomic percent of Au and Ge in the initially generated mixture can be changed by changing the charging current to the capacitor used to trigger the sparking. Depending upon the atomic ratio of Au and Ge in the initial mixture, AuGe agglomerates form AuGe composite nanoparticles on subsequent sintering, in which AuGe alloy nanoparticles are found dispersed in a Ge matrix. The size of the dispersed AuGe alloy nanoparticles depend on the relative atomic concentration of Au and Ge in the initial mixture as well as on the sintering temperature. AuGe alloy nanoparticles dispersed in the Ge matrix are observed to exhibit an intense photoluminescence between 550 and 600 nm. © 2013 Springer Science+Business Media.
    view abstractdoi: 10.1007/s11051-013-1963-0
  • 2013 • 44 Trace metal residues promote the activity of supposedly metal-free nitrogen-modified carbon catalysts for the oxygen reduction reaction
    Masa, J. and Zhao, A. and Xia, W. and Sun, Z. and Mei, B. and Muhler, M. and Schuhmann, W.
    Electrochemistry Communications 34 113-116 (2013)
    We show in this study that the presence of trace metal residues in some supposedly metal-free catalysts for oxygen reduction, at concentrations which are difficult to detect using conventional methods such as XPS and EDX, can profoundly promote the ORR activity of the catalysts. © 2013 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.elecom.2013.05.032
  • 2013 • 43 On the significance of thermoelectric and thermionic emission currents induced by chemical reactions catalyzed on nanofilm metal-semiconductor heterostructures
    Nedrygailov, I.I. and Karpov, E.G. and Hasselbrink, E. and Diesing, D.
    Journal of Vacuum Science and Technology A: Vacuum, Surfaces and Films 31 (2013)
    The possible origins and the magnitude of an electric current arising in nanofilm metal-semiconductor heterostructures when the metal surface is used to catalyze an exothermic chemical reaction are discussed. Two key mechanisms are considered that are responsible for the current generation: electron motion due to a temperature drop across the metal-semiconductor interface (thermionic emission mechanism) and the Seebeck effect in the two layers (thermoelectric mechanism). It is predicted that (i) current up to 10-3 A·cm-2 can arise, (ii) thermoelectric mechanism due to the Seebeck effect in the semiconductor layer plays a dominant role for the current generated under stationary chemical reaction conditions, and (iii) thermoelectric current strongly depends on the temperature. The carrier transport through the metal-semiconductor interface is described by the thermionic emission theory. The obtained results are discussed in view of the experimental data reported earlier for the chemical reaction-induced currents in metal/n-Si structures. © 2013 American Vacuum Society.
    view abstractdoi: 10.1116/1.4774217
  • 2013 • 42 On dislocation involvement in Ti-Nb gum metal plasticity
    Plancher, E. and Tasan, C.C. and Sandloebes, S. and Raabe, D.
    Scripta Materialia 68 805-808 (2013)
    The excellent mechanical properties of the Ti-Nb-based gum metal were originally proposed to arise from a "dislocation-free" giant fault mechanism; however, the involvement of lattice dislocations in the process is still under debate. To address this issue, gum metal deformation mechanisms are investigated systematically on cast specimens, employing postmortem and in situ analysis techniques. The results demonstrate that a giant fault mechanism (which appears to be a phase-transformation-assisted nanotwinning mechanism) governs gum metal plasticity without direct assistance from dislocations during the process. © 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.scriptamat.2013.01.034
  • 2013 • 41 Surface anchored metal-organic frameworks as stimulus responsive antifouling coatings
    Sancet, M.P.A. and Hanke, M. and Wang, Z. and Bauer, S. and Azucena, C. and Arslan, H.K. and Heinle, M. and Gliemann, H. and Wöll, C. and Rosenhahn, A.
    Biointerphases 8 (2013)
    Surface-anchored, crystalline and oriented metal organic frameworks (SURMOFs) have huge potential for biological applications due to their well-defined and highly-porous structure. In this work we describe a MOF-based, fully autonomous system, which combines sensing, a specific response, and the release of an antimicrobial agent. The Cu-containing SURMOF, Cu-SURMOF 2, is stable in artificial seawater and shows stimulus-responsive anti-fouling properties against marine bacteria. When Cobetia marina adheres on the SURMOF, the framework's response is lethal to the adhering microorganism. A thorough analysis reveals that this response is induced by agents secreted from the microbes after adhesion to the substrate, and includes a release of Cu ions resulting from a degradation of the SURMOF. The stimulus-responsive antifouling effect of Cu-SURMOF 2 demonstrates the first application of Cu-SURMOF 2 as autonomous system with great potential for further microbiological and cell culture applications. © 2013 Arpa Sancet et al.
    view abstractdoi: 10.1186/1559-4106-8-29
  • 2013 • 40 Microstructure of retrievals made from standard cast HC-CoCrMo alloys
    Stemmer, P. and Pourzal, R. and Liao, Y. and Marks, L. and Morlock, M. and Jacobs, J.J. and A.wimmer, M. and Fischer, A.
    ASTM Special Technical Publication 1560 STP 251-267 (2013)
    During the past decade, self-mating metal bearings based on cobalt- chromium-molybdenum (CoCrMo) alloys have become very popular in total hip replacements and hip resurfacings. This led to a market share of more than 35 % for metal-on-metal (MoM) bearings in the United States before several cases of high wear with biologic consequences led to a sharp drop in popularity. In part, these failures are a result of a very shallow understanding of the wear mechanisms in MoM joints and their relation to the microstructure. In order to find such a relation, one has to keep in mind that the microstructures of metallic materials depend distinctly on the entire production sequence. In addition, they change markedly under tribological stresses. This paper does not discuss the wear of any specific retrieval or even try to relate that to the specific microstructure, because such a task would be impossible based on the unknown loading history of such retrievals. Thus, we depict only the possible range of microstructures from standardized high carbon (HC)-CoCrMo retrievals. These reveal different types of hard phases: carbides and/or intermetallic phases. Some are fine (<10μm) and homogeneously distributed, whereas others appear as thin (<1 μm) and brittle cord-shaped arrangements at grain or dendrite boundaries. Coarser (>30 μm) types of mixed hard phases, which consist of carbides and interme-tallic phases, often show microcracks already below the articulating surfaces. Such subsurface microcracks are known to destabilize the gradient below the surface and the balance between tribochemical reactions and surface fatigue. In this paper, the microstructures of retrievals manufactured from standard cast CoCrMo alloys are shown and evaluated. Copyright © 2013 by ASTM International.
    view abstractdoi: 10.1520/STP156020120033
  • 2013 • 39 Tribochemical reactions in metal-on-metal hip joints influence wear and corrosion
    Wimmer, M.A. and Mathew, M.T. and Laurent, M.P. and Nagelli, C. and Liao, Y. and Marks, L.D. and Pourzal, R. and Fischer, A. and Jacobs, J.J.
    ASTM Special Technical Publication 1560 STP 292-309 (2013)
    Recent findings indicate the presence of tribochemically gener-ated layers on metal-on-metal (MoM) bearing surfaces. These tribolayers are films of a few-hundred-nanometer thickness and are constituted of carbonaceous material mixed with metal and oxide particles. The purpose of the study was to characterize these tribofilms mechanically and electrochemically. Using a nanoindenter, the local mechanical properties of the tribolayer were measured. On average a hardness of ∼1.0 GPa was determined, which was softer than the underlying metal. The influence of tribomaterial on the electrochemistry of the cobalt-chromium-molybdenum alloy (CoCrMo) was investigated. Bovine calf serum mixture was used as the electrolyte. High- and low-carbon CoCrMo-samples with and without tribolayer were compared using potentiodynamic testing. This corrosive investigation was followed by tribocorrosive tests using a custom made apparatus, where a ceramic ball oscillated against a flat CoCrMo surface. Potential and coefficient of friction were monitored throughout this 100 K cycle test. Electrochemical impedance spectroscopy tests before and after testing were conducted. Weight loss was determined using planimetric analysis. It was found that the tribolayered surface had better corrosion resistance than the corresponding tribolayer-free (polished) surface. The tribolayered surface also exhibited a more noble potential during tribocorrosive testing and demonstrated less wear. High- carbon was the superior alloy compared with low carbon for all surface conditions; however, the differences seemed to equalize in the presence of a tribo- film. There were also differences in tribofilm generation, possibly related to the microstructure of the two alloys. Copyright © 2013 by ASTM International.
    view abstractdoi: 10.1520/STP156020120050
  • 2012 • 38 Aerobic epoxidation of olefins catalyzed by the cobalt-based metal-organic framework STA-12(Co)
    Beier, M.J. and Kleist, W. and Wharmby, M.T. and Kissner, R. and Kimmerle, B. and Wright, P.A. and Grunwaldt, J.-D. and Baiker, A.
    Chemistry - A European Journal 18 887-898 (2012)
    A Co-based metal-organic framework (MOF) was investigated as a catalytic material in the aerobic epoxidation of olefins in DMF and exhibited, based on catalyst mass, a remarkably high catalytic activity compared with the Co-doped zeolite catalysts that are typically used in this reaction. The structure of STA-12(Co) is similar to that of STA-12(Ni), as shown by XRD Rietveld refinement and is stable up to 270 °C. For the epoxidation reaction, significantly different selectivities were obtained depending on the substrate. Although styrene was epoxidized with low selectivity due to oligomerization, (E)-stilbene was converted with high selectivities between 80 and 90%. Leaching of Co was low and the reaction was found to proceed mainly heterogeneously. The catalyst was reusable with only a small loss of activity. The catalytic epoxidation of stilbene with the MOF featured an induction period, which was, interestingly, considerably reduced by styrene/stilbene co-epoxidation. This could be traced back to the formation of benzaldehyde promoting the reaction. Detailed parameter and catalytic studies, including in situ EPR and EXAFS spectroscopy, were performed to obtain an initial insight into the reaction mechanism. Aim for the STAs: The metal-organic framework catalyst STA-12(Co) showed remarkable activity in the aerobic epoxidation of E-stilbene in DMF (see figure) and could be efficiently recycled. High selectivity (80-90%) for stilbene oxide was observed. The induction period of the reaction could be reduced by styrene/stilbene co-epoxidation. Detailed parameter studies combined with spectroscopic investigations were performed to obtain an initial insight into the reaction mechanism. Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/chem.201101223
  • 2012 • 37 Polycrystal model of the mechanical behavior of a Mo-TiC 30 vol.% metal-ceramic composite using a three-dimensional microstructure map obtained by dual beam focused ion beam scanning electron microscopy
    Cédat, D. and Fandeur, O. and Rey, C. and Raabe, D.
    Acta Materialia 60 1623-1632 (2012)
    The mechanical behavior of a Mo-TiC 30 vol.% ceramic-metal composite was investigated over a wide temperature range (25-700 °C). High-energy X-ray tomography was used to reveal percolation of the hard titanium carbide phase through the composite. Using a polycrystal approach for a two-phase material, finite-element simulations were performed on a real three-dimensional (3-D) aggregate of the material. The 3-D microstructure, used as the starting configuration for the predictions, was obtained by serial sectioning in a dual beam focused ion beam scanning electron microscope coupled to an electron backscattered diffraction system. The 3-D aggregate consists of a molybdenum matrix and a percolating TiC skeleton. As for most body-centered cubic (bcc) metals, the molybdenum matrix phase is characterized by a change in plasticity mechanism with temperature. We used a polycrystal model for bcc materials which was extended to two phases (TiC and Mo). The model parameters of the matrix were determined from experiments on pure molydenum. For all temperatures investigated the TiC particles were considered to be brittle. Gradual damage to the TiC particles was treated, based on an accumulative failure law that is approximated by evolution of the apparent particle elastic stiffness. The model enabled us to determine the evolution of the local mechanical fields with deformation and temperature. We showed that a 3-D aggregate representing the actual microstructure of the composite is required to understand the local and global mechanical properties of the composite studied. © 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2011.11.055
  • 2012 • 36 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 • 35 Toughness enhancement in TiAlN-based quarternary alloys
    Sangiovanni, D.G. and Chirita, V. and Hultman, L.
    Thin Solid Films 520 4080-4088 (2012)
    Improved toughness in hard and superhard thin films is a primary requirement for present day ceramic hard coatings, known to be prone to brittle failure during in-use conditions. We use density functional theory calculations to investigate a number of (TiAl) 1 - xM xN thin films in the B1 structure, with 0.06 ≤ x ≤ 0.75, obtained by alloying TiAlN with M = V, Nb, Ta, Mo and W. Results show significant ductility enhancements, hence increased toughness, in these compounds. Importantly, these thin films are also predicted to be superhard, with similar or increased hardness values, compared to Ti 0.5Al 0.5 N. For (TiAl) 1 - xW xN the results are experimentally confirmed. The ductility increase originates in the enhanced occupancy of d-t 2g metallic states, induced by the valence electrons of substitutional elements (V, Nb, Ta, Mo, W). This effect is more pronounced with increasing valence electron concentration, and, upon shearing, leads to the formation of a layered electronic structure in the compound material, consisting of alternating layers of high and low charge density in the metallic sublattice, which in turn, allows a selective response to normal and shear stresses. © 2012 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.tsf.2012.01.030
  • 2011 • 34 Basic investigation of HfO2 based metal-insulator-metal diodes
    Dudek, P. and Schmidt, R. and Lukosius, M. and Lupina, G. and Wenger, C. and Abrutis, A. and Albert, M. and Xu, K. and Devi, A.
    Thin Solid Films 519 5796-5799 (2011)
    Very fast frequency response of metal-insulator-metal (MIM) diodes extends into the terahertz regime making them attractive as key elements as alternative to photovoltaic solar energy harvesting and ultrahigh speed wireless communication systems. The tunnelling phenomena, which is crucial for achieving high performance in these devices is extremely sensitive to the nanoscale structural and chemical quality of interface regions. Modern chemical deposition techniques like Pulsed Injected Metal-Organic Chemical Vapour Deposition (PICVD), Atomic Layer Deposition (ALD) and Atomic Vapour Deposition (AVD®) will be used for the extremely precise growth of thin HfO2 films on TiN bottom electrodes. However, different deposition techniques may give unpredictably different results in terms of film density, surface and interface property and consequently in physical properties of the device. In this work, the influence of deposition techniques on the charge transport characteristics of HfO2 MIM diodes was investigated by Conducting Atomic Force Microscopy (C-AFM) and X-ray Photoelectron Spectroscopy (XPS). © 2010 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.tsf.2010.12.195
  • 2011 • 33 Thermally sprayed coatings as effective tool surfaces in sheet metal forming applications
    Franzen, V. and Witulski, J. and Brosius, A. and Trompeter, M. and Tekkaya, A.E.
    Journal of Thermal Spray Technology 20 939-947 (2011)
    Two approaches to produce wear-resistant effective surfaces for deep drawing tools by thermal arc wire spraying of hard materials are presented. Arc wire spraying is a very economic coating technique due to a high deposition rate. The coated surface is very rough compared to that of conventional sheet metal forming tools. In the first approach, the coated surface is smoothed in a subsequent CNC-based incremental roller burnishing process. In this process, the surface asperities on the surface are flattened, and the roughness is significantly reduced. In the second approach, the hard material coatings are not sprayed directly on the tool but on a negative mould. Afterward, the rough "as-sprayed" side of the coating is backfilled with a polymer. The bonded hard metal shell is removed from the negative mould and acts as the surface of the hybrid sheet metal forming tool. Sheet metal forming experiments using tools based on these two approaches demonstrate that they are suitable to form high-strength steels. Owing to a conventional body of steel or cast iron, the first approach is suitable for large batch sizes. The application of the second approach lies within the range of small up to medium batch size productions. © 2011 ASM International.
    view abstractdoi: 10.1007/s11666-011-9627-z
  • 2011 • 32 Switchable supramolecular polymers from the self-assembly of a small monomer with two orthogonal binding interactions
    Gröger, G. and Meyer-Zaika, W. and Böttcher, C. and Gröhn, F. and Ruthard, C. and Schmuck, C.
    Journal of the American Chemical Society 133 8961-8971 (2011)
    The low molecular weight heteroditopic monomer 1 forms supramolecular polymers in polar solution as shown, for example, by infrared laser-based dynamic light scattering (DLS), small-angle neutron scattering (SANS), electron microscopy (TEM, cryo-TEM), and viscosity measurements. Self-assembly of 1 is based on two orthogonal binding interactions, the formation of a Fe(II)-terpyridine 1:2 metal-ligand complex and the dimerization of a self-complementary guanidiniocarbonyl pyrrole carboxylate zwitterion. Both binding interactions have a sufficient stability in polar (DMSO) and even aqueous solutions to ensure formation of linear polymers of considerable length (up to 100 nm). The supramolecular polymerization follows a ring-chain mechanism causing a significant increase in the viscosity of the solutions at millimolar concentrations and above. The linear polymers then further aggregate in solution into larger globular aggregates with a densely packed core and a loose shell. Both binding interactions can be furthermore switched on and off either by adding a competing ligand to remove the metal ion and subsequent readdition of Fe(II) or by reversible protonation and deprotonation of the zwitterion upon addition of acid or base. The self-assembly of 1 can therefore be switched back and forth between four different states, the monomer, a metal-complexed dimer or an ion paired dimer, and finally the polymer. © 2011 American Chemical Society.
    view abstractdoi: 10.1021/ja200941a
  • 2011 • 31 Inverse method for identification of initial yield locus of sheet metals utilizing inhomogeneous deformation fields
    Güner, A. and Yin, Q. and Soyarslan, C. and Brosius, A. and Tekkaya, A.E.
    International Journal of Material Forming 4 121-128 (2011)
    Accurate finite element simulation of sheet metal forming processes requires among others accurate description of plastic behaviour of materials. This is achieved by utilization of sophisticated yield criteria having several material parameters. This work proposes a procedure which makes use of the distribution of strains to identify the initial yield locus of sheet metals by the help of inverse analysis. For this purpose a flat specimen having a varying cross-section is introduced, which is capable of revealing different deformation states in one test. Numerical simulations are performed with 2 representative materials for steel and aluminium, using the material model Yld2000-2d. The results of these simulations are treated as experimentally obtained results and with the inverse methods it is tried to obtain the given yield locus. The relation between the supplied input and the outcome of the inverse algorithm is studied by examining different objective function definitions. The numerical studies show that inclusion of the strain distribution in the definition of objective function is a key issue in identification of the yield locus. The orientation of the specimen with respect to the rolling direction also determines the amount and quality of the information used for parameter identification. Consequently the circumstances, under which the inverse method can predict the initial yield locus, are defined. © 2010 Springer-Verlag France.
    view abstractdoi: 10.1007/s12289-010-1009-4
  • 2011 • 30 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 • 29 Electrochemistry-controlled metal ion release from silicone elastomer nanocomposites through combination of different metal nanoparticles
    Hahn, A. and Günther, S. and Wagener, P. and Barcikowski, S.
    Journal of Materials Chemistry 21 10287-10289 (2011)
    Electrochemistry-controlled metal ion release is achieved using nanoparticle mixtures embedded into a silicone matrix. Synergistic metal ion release from silicone matrix filled with silver and copper nanoparticles as well as silver and gold nanoparticles embedded into silicone is investigated in terms of qualitative and quantitative influences. Results are compared to nanoparticle composites with only one metal. The mechanism enhancing the release of the less noble metal nanoparticle is based on the ion-mediated electrochemistry rather than on contact corrosion of both elements. A retardation as well as an enhancement of metal ion release is observed allowing a time- and rate-controlled design of bioactive nanocomposites. © 2011 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c0jm04480f
  • 2011 • 28 Investigation of corrosion and wear properties of fe based metal matrix composites consolidated by sintering and hot isostatic pressing
    Hill, H. and Weber, S. and Siebert, S. and Theisen, W.
    Powder Metallurgy 54 455-462 (2011)
    Many industrial applications, e.g. processing of polymers, suffer from high costs caused by corrosion and wear. Particularly the combination of both increases the requirements for the materials used. Corrosion resistant cold work steels were developed to withstand the combined attack. Resistance is achieved by a sufficient content of chromium in the metal matrix and by carbides dispersed in a martensitic matrix. A further gain in wear resistance is possible by adding hard phases to the steel to produce a particulate reinforced metal matrix composite (MMC). The common consolidation process for such MMCs is hot isostatic pressing, but they can also be processed by solid state or liquid phase sintering. This work focuses on detailed investigations of the properties in dependence on the processing route. The results show that the resulting corrosion and wear resistance depend not only on the processing method, but also on the incorporated hard phases in combination with the manufacturing method. In addition, the unreinforced metal matrices were compared to the MMC. © 2011 Institute of Materials, Minerals and Mining.
    view abstractdoi: 10.1179/003258910X12678035166656
  • 2011 • 27 Dislocation interactions and low-angle grain boundary strengthening
    Liu, B. and Raabe, D. and Eisenlohr, P. and Roters, F. and Arsenlis, A. and Hommes, G.
    Acta Materialia 59 7125-7134 (2011)
    The transmission of an incoming dislocation through a symmetrical low-angle tilt grain boundary (GB) is studied for {1 1 0}〈1 1 1〉 slip systems in body-centered cubic metals using discrete dislocation dynamics (DD) simulations. The transmission resistance is quantified in terms of the different types of interactions between the incoming and GB dislocations. Five different dislocation interaction types are considered: collinear, mixed-symmetrical junction, mixed-asymmetrical junction, edge junction, and coplanar. Mixed-symmetrical junction formation events are found not only to cause a strong resistance against the incident dislocation penetration, but also to transform the symmetrical low-angle tilt GB into a hexagonal network (a general low-angle GB). The interactions between the incident dislocation and the GB dislocations can form an array of 〈1 0 0〉 dislocations (binary junctions) in non-coplanar interactions, or a single 〈1 0 0〉 dislocation in coplanar interaction. We study how the transmission resistance depends on the mobility of 〈1 0 0〉 dislocations. 〈1 0 0〉 dislocations have usually been treated as immobile in DD simulations. In this work, we discuss and implement the mobility law for 〈1 0 0〉 dislocations. As an example, we report how the mobility of 〈1 0 0〉 dislocations affects the equilibrium configuration of a ternary dislocation interaction. © 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2011.07.067
  • 2011 • 26 Ultrathin metal oxidation for vacuum monitoring device applications
    Mäder, S. and Haas, T. and Kunze, U. and Doll, T.
    Physica Status Solidi (A) Applications and Materials Science 208 1223-1228 (2011)
    The oxide growth on thin metal films at room temperature has been investigated in terms of resistance change during oxidation. These data have been interpreted using the extended Cabrera-Mott theory of oxidation by Boggio. The resulting oxide thickness as well as the oxidation kinetics was found to depend on pressure. According to this dependence, oxidation of ultrathin metal films can be applied for monitoring the vacuum quality inside an evacuated environment. The performance of aluminum and copper sensing layers are compared with respect to sensor lifetime and response. Furthermore, the theoretically evaluated and resistively measured oxide thicknesses are verified by TEM studies. © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/pssa.201000921
  • 2011 • 25 Machines and tools for sheet-bulk metal forming
    Merklein, M. and Tekkaya, A.E. and Brosius, A. and Opel, S. and Kwiatkowski, L. and Plugge, B. and Schunck, S.
    Key Engineering Materials 473 91-98 (2011)
    The demand on closely-tolerated and complex functional components in the automotive sector, like e.g. synchronizer rings, leads to the development of a new process-class named "sheet-bulk metal forming". Within this technology bulk metal forming operations are applied on sheet metals. In the following two novel approaches considering machines and tools for sheet-bulk metal forming are presented. The first approach aims on a technology based on rolling, which is suitable for mass production. The second one is an incremental forming solution for low batch production. Both machine concepts allow the application of different forming strategies to manufacture individual tailored semi-finished products in term of a pre-distribution of material. These products feature variable sheet thicknesses and mechanical properties, which can be adapted to their case of application. Depending on the individual batch size, the blanks can be finished to functional parts by subsequent forming processes like deep drawing and upsetting, extrusion or incremental forming. In this paper the case of an incremental tooth-forming is mainly considered. Forming sequences and resulting loads are modeled and calculated by finite elements simulations for all discussed processes to serve as a basis for the design and dimensioning of the machine components and forming tools. © (2011) Trans Tech Publications.
    view abstractdoi: 10.4028/www.scientific.net/KEM.473.91
  • 2011 • 24 Atomically smooth p-doped silicon nanowires catalyzed by aluminum at low temperature
    Moutanabbir, O. and Senz, S. and Scholz, R. and Alexe, M. and Kim, Y. and Pippel, E. and Wang, Y. and Wiethoff, C. and Nabbefeld, T. and Meyer zu Heringdorf, F.-J. and Horn-Von Hoegen, M.
    ACS Nano 5 1313-1320 (2011)
    Silicon nanowires (SiNWs) are powerful nanotechnological building blocks. To date, a variety of metals have been used to synthesize high-density epitaxial SiNWs through metal-catalyzed vapor phase epitaxy. Understanding the impact of the catalyst on the intrinsic properties of SiNWs is critical for precise manipulation of the emerging SiNW-based devices. Here we demonstrate that SiNWs synthesized at low-temperature by ultrahigh vacuum chemical vapor deposition using Al as a catalyst present distinct morphological properties. In particular, these nanowires are atomically smooth in contrast to rough {112}-type sidewalls characteristic of the intensively investigated Au-catalyzed SiNWs. We show that the stabilizing effect of Al plays the key role in the observed nanowire surface morphology. In fact, unlike Au which induces (111) and (113) facets on the nanowire sidewall surface, Al revokes the reconstruction along the [1̄1̄2] direction leading to equivalent adjacent step edges and flat surfaces. Our finding sets the lower limit of the Al surface density on the nanowire sidewalls at ∼2 atom/nm2. Additionally, despite using temperatures of ca. 110-170 K below the eutectic point, we found that the incorporation of Al into the growing nanowires is sufficient to induce an effective p-type doping of SiNWs. These results demonstrate that the catalyst plays a crucial role is shaping the structural and electrical properties of SiNWs. © 2011 American Chemical Society.
    view abstractdoi: 10.1021/nn1030274
  • 2011 • 23 Aminotroponiminatozinc(I) complexes: Syntheses and spectroscopic analyses
    Nayek, H.P. and Lühl, A. and Schulz, S. and Köppe, R. and Roesky, P.W.
    Chemistry - A European Journal 17 1773-1777 (2011)
    Aminotroponiminates-very effective ligands for the stabilization of low-valent ZnI-ZnI bonded compounds (see scheme). For the first time a stretching motion of the Zn-Zn bond, which is almost uncoupled to other vibrations, was found in the Raman spectrum. © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/chem.201002443
  • 2011 • 22 Application of AC-SECM in corrosion science: Local visualisation of inhibitor films on active metals for corrosion protection
    Pähler, M. and Santana, J.J. and Schuhmann, W. and Souto, R.M.
    Chemistry - A European Journal 17 905-911 (2011)
    The suitability of frequency-dependent alternating-current scanning electrochemical microscopy (4D AC-SECM) for investigation of thin passivating layers covering the surface of corrosion-inhibited metals has been demonstrated. Inhibition of copper corrosion by benzotriazole (BTAH) and methylbenzotriazole (MBTAH), which are effective inhibitors for this metal under many environmental conditions, was investigated. Strong dependencies were found for the AC z-approach curves with both the duration of the inhibitor treatment and the frequency of the AC excitation signal applied in AC-SECM. Both negative and positive feedback behaviours were observed in the AC approach curves for untreated copper and for Cu/BTAH and Cu/MBTAH samples. Negative feedback behaviour occurred in the low-frequency range, whereas a positive feedback effect was observed at higher frequencies. A threshold frequency related to the passage from negative to positive regimes could be determined in each case. The threshold frequency for inhibitor-modified samples was found always to be significantly higher than for the untreated metal, because the inhibitor film provides electrical insulation for the surface. Moreover, the threshold frequency increased with increasing surface coverage by the inhibitor. 4D AC-SECM was successfully applied to visualizing spatially resolved differences in local electrochemical activity between inhibitor-free and inhibitor-covered areas of the sample. Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA.
    view abstractdoi: 10.1002/chem.201000689
  • 2011 • 21 Modeling plasmonic scattering combined with thin-film optics
    Schmid, M. and Klenk, R. and Lux-Steiner, M.Ch. and Topič, M. and Krč, J.
    Nanotechnology 22 (2011)
    Plasmonic scattering from metal nanostructures presents a promising concept for improving the conversion efficiency of solar cells. The determination of optimal nanostructures and their position within the solar cell is crucial to boost the efficiency. Therefore we established a one-dimensional optical model combining plasmonic scattering and thin-film optics to simulate optical properties of thin-film solar cells including metal nanoparticles. Scattering models based on dipole oscillations and Mie theory are presented and their integration in thin-film semi-coherent optical descriptions is explained. A plasmonic layer is introduced in the thin-film structure to simulate scattering properties as well as parasitic absorption in the metal nanoparticles. A proof of modeling concept is given for the case of metal-island grown silver nanoparticles on glass and ZnO:Al/glass substrates. Using simulations a promising application of the nanoparticle integration is shown for the case of CuGaSe2 solar cells. © 2011 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0957-4484/22/2/025204
  • 2011 • 20 Solvation dynamics of surface-trapped electrons at NH3 and D2O crystallites adsorbed on metals: from femtosecond to minute timescales
    Stähler, J. and Meyer, M. and Bovensiepen, U. and Wolf, M.
    Chemical Science 2 907-916 (2011)
    The creation and stabilization of localized, low-energy electrons is investigated in polar molecular environments. We create such excess electrons in excited states in ice and ammonia crystallites adsorbed on metal surfaces and observe their relaxation in real time using time-resolved photoelectron spectroscopy. The observed dynamics proceed up to minute timescales and are therefore slowed down considerably compared to ultrafast excited state relaxation in front of metal surfaces, which proceeds typically on femto- or picosecond time scales. It is the highly efficient wave function constriction of the electrons from the metal that ultimately enables the investigation of the relaxation dynamics over a large range of timescales (up to 17 orders of magnitude). Therefore, it gives novel insight into the solvated electron ground state formation at interfaces. As these long-lived electrons are observed for both, D2O and NH3 crystallites, they appear to be of general character for polar molecule-metal interfaces. Their time- and temperature-dependent relaxation is analyzed for both, crystalline ice and ammonia, and compared using an empirical model that yields insight into the fundamental solvation processes of the respective solvent. © The Royal Society of Chemistry 2011.
    view abstractdoi: 10.1039/c0sc00644k
  • 2011 • 19 Investigation of the spin-dependent properties of electron doped cobalt-CuPc interfaces
    Steil, S. and Goedel, K. and Ruffing, A. and Sarkar, I. and Cinchetti, M. and Aeschlimann, M.
    161 570-574 (2011)
    We have grown metal-organic interfaces by in-situ deposition of ultrathin copper phthalocyanine (CuPc) films on a thin cobalt film on Cu(0 0 1). Evidence for layer-by-layer growth is found. The spin-dependent electronic properties of the Co-CuPc interface and their modification under caesium doping are investigated by spin-resolved photoemission spectroscopy. We observe a doping-induced shift of the highest occupied molecular orbital (HOMO) of CuPc away from the Fermi level (EF), accompanied by the formation of an unpolarised gap-state at 0.7 eV below EF in the high doping regime. Such features are reflected in the behaviour of the detected interfacial spin-polarisation. © 2010 Elsevier B.V.
    view abstractdoi: 10.1016/j.synthmet.2010.11.031
  • 2011 • 18 Emission data and costs for environmental measures during laser joining of metals
    Walter, J. and Hustedt, M. and Hennigs, C. and Stein, J. and Barcikowski, S.
    Journal of Laser Micro Nanoengineering 6 138-144 (2011)
    Laser welding and soldering are important joining processes in the automotive industry. Typical examples are the production of the car body of the VW Golf or the automatic gearbox of the Mercedes-Benz A-Class. Furthermore, there is a general trend to increase the use of lightweight materials (e.g. Mg, Al, alloys), and to combine different metallic materials to produce complex components (e.g. in tailored blanks). In order to ensure good practices with regard to occupational health and safety as well as environmental issues, laser joining processes have to be analyzed in detail. Avoiding and controlling emission products caused by laser processing of metals or metal composites is an important task in this context. Typically, costs for environmental measures represent a significant percentage of the total manufacturing costs related to a laser process. In this work, emission measurements of several laser welding and soldering processes for metal sheets from steel and brass are reported. Different steel grades and surface treatments of the metal sheets have been taken into account: pure, zinc-coated, PTFE-coated, oiled, cold cleaner residues. The hazardous potential of these processes has been assessed by means of analyzing the specific emissions with respect to the relevant threshold limit values (TLVs). Based on the experimental results, the processes have been classified according to measures which are required by environmental legislation. Finally, a cost calculation for measures related to emission capturing is presented. It has been shown that these environmental measures are manageable for all industrial laser processes regarded here, and the costs for these measures remain acceptable, i.e. in many cases below 8 % of the total processing costs. The results are made permanently available in an interactive internet database. Using this database, the planning of appropriate exhaust systems for laser welding and soldering is facilitated significantly.
    view abstractdoi: 10.2961/jlmn.2011.02.0008
  • 2011 • 17 Production of low-volume aviation components using disposable electromagnetic actuators
    Woodward, S. and Weddeling, C. and Daehn, G. and Psyk, V. and Carson, B. and Tekkaya, A.E.
    Journal of Materials Processing Technology 211 886-895 (2011)
    Electromagnetic forming is commonly used to produce high strain rates to improve the formability of sheet metal. The objective of this paper is to discuss the feasibility of the use of disposable actuators during electromagnetic forming of two aluminum components: a very simple part with a one-dimensional curve, and a realistic part whose main feature is a convex flange with two joggles. The main forming complications after the parts were formed using conventional methods were the presence of wrinkles and excessive springback. The goal of this work is to use large controlled electromagnetic impulses to minimize the springback of these components from a rough-formed shape. The optimum test protocols for electromagnetic calibration of the components were determined by optimizing parameters such as actuator design, tool material, press force, and capacitor discharge energy. The use of disposable actuators for electromagnetic calibration of the parts showed significant reductions in springback compared to the parts which were only preformed using conventional techniques (hydroforming and rubber pad forming). Springback was decreased in the curved component by up to 87%. The wrinkles were eliminated on the flanged component, the joggles were formed properly, and the average bending angle of the part was improved from 95.3° to 90.3°, very near the target bending angle of 90°. This study demonstrates that these techniques can be used to improve current sheet metal production processes. © 2010 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.jmatprotec.2010.07.020
  • 2011 • 16 Highly active metal-free nitrogen-containing carbon catalysts for oxygen reduction synthesized by thermal treatment of polypyridine-carbon black mixtures
    Xia, W. and Masa, J. and Bron, M. and Schuhmann, W. and Muhler, M.
    Electrochemistry Communications 13 593-596 (2011)
    A straight-forward method for the synthesis of metal-free catalysts for oxygen reduction by thermal treatment of a mixture of poly(3,5-pyridine) with carbon black in helium is reported. The catalyst was characterized by X-ray diffraction and photoelectron spectroscopy, cyclic voltammetry and rotating disk electrode measurements. The new catalyst exhibited remarkable activity similar to Pt-based catalysts in alkaline media. © 2011 Elsevier B.V. All Rights Reserved.
    view abstractdoi: 10.1016/j.elecom.2011.03.018
  • 2011 • 15 Surface chemistry of metal-organic frameworks at the liquid-solid interface
    Zacher, D. and Schmid, R. and Wöll, C. and Fischer, R.A.
    Angewandte Chemie - International Edition 50 176-199 (2011)
    MOFs on surfaces: Many parameters need to be considered in the formation of metal-organic frameworks (MOFs; see structures) at the liquid-solid interface. The methods and growth mechanisms for the layer-by-layer deposition of MOFs on functional materials, the homo- and heteroepitaxial deposition of MOF heterocrystals, and the coordination modulation of MOF surfaces are reviewed. Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/anie.201002451
  • 2010 • 14 Tailoring the spin functionality of a hybrid metal-organic interface by means of alkali-metal doping
    Cinchetti, M. and Neuschwander, S. and Fischer, A. and Ruffing, A. and Mathias, S. and Wüstenberg, J.-P. and Aeschlimann, M.
    104 (2010)
    We employ a recently developed purpose-made technique based on spin-resolved two-photon photoemission spectroscopy to study the influence of alkali-metal doping (Cs and Na) on the spin functionality of the interface between a cobalt thin film and the organic semiconductor copper phthalocyanine. We find two alkali-metal-induced effects. First, alkali-metal atoms act as impurities and increase the spin-flip probability for the electrons crossing the interface (detrimental effect). Second, they allow one to modify the interface energy level alignment and, consequently, to enhance the efficiency of spin injection at an arbitrary energy above the Fermi level of the cobalt (intrinsic effect). We show that the intrinsic effect dominates over the detrimental one, opening the possibility to actively tailor the spin functionality of the considered hybrid metal-organic interface by changing the doping concentration. © 2010 The American Physical Society.
    view abstractdoi: 10.1103/PhysRevLett.104.217602
  • 2010 • 13 Optical properties of a nanomatch-like plasmonic structure
    Cui, X. and Zhang, W. and Erni, D. and Dong, L.
    Journal of the Optical Society of America A: Optics and Image Science, and Vision 27 1783-1790 (2010)
    The optical properties of a match-like plasmonic nanostructure are numerically investigated using full-wave finite-difference time-domain analysis in conjunction with dispersive material models. This work is mainly motivated by the developed technique enabling reproducible fabrication of nanomatch structures as well as the growing applications that utilize the localized field enhancement in plasmonic nanostructures. Our research revealed that due to the pronounced field enhancement and larger resonance tunabilities, some nanomatch topologies show potentials for various applications in the field of, e.g., sensing as well as a novel scheme for highly reproducible tips in scanning near field optical microscopy, among others. Despite the additional degrees of freedom that are offered by the composite nature of the proposed nanomatch topology, the paper also reflects on a fundamental complication intrinsic to the material interfaces especially in the nanoscale: stoichiometric mixing. We conclude that the specificity in material modeling will become a significant issue in future research on functionalized composite nanostructures. © 2010 Optical Society of America.
    view abstractdoi: 10.1364/JOSAA.27.001783
  • 2010 • 12 Finishing of thermally sprayed tool coatings for sheet metal forming operations by roller burnishing
    Franzen, V. and Trompeter, M. and Brosius, A. and Tekkaya, A.E.
    International Journal of Material Forming 3 147-150 (2010)
    Processing high strength sheet metal materials causes high tribological loads to the surface of forming tools. To increase the wear resistance and therewith the service time of tools, surface coatings containing hard materials are applied by thermal spraying. In the initial "as sprayed" state, the coatings show a rough surface and porous structure, which is not suitable for sheet forming tools. This article describes the finishing of the coatings by subsequent roller burnishing. By the rolling process surface asperities of the coating are flattened and the contact area between the finished surface and the sheet is increased. Furthermore, residual compressive stresses in the surface zone are generated by the rolling process. The tribological properties of the coated and finished surfaces are analyzed in strip drawing tests with uncoated DP600 sheets varying the contact pressure and drawing velocity. It is shown that the process parameters of the roller burnishing process have a strong influence on the surface topology of the friction elements and their tribological properties. The coated and finished friction elements are compared to conventional grinded steel friction elements, made of C60. © 2010 Springer-Verlag France.
    view abstractdoi: 10.1007/s12289-010-0728-x
  • 2010 • 11 Novel synthetic pathway for new Zn-Zn-bonded compounds from dizincocene
    Gondzik, S. and Bläser, D. and Wölper, C. and Schulz, S.
    Chemistry - A European Journal 16 13599-13602 (2010)
    Making it new! A novel synthetic pathway for the synthesis of Zn-Zn-bonded complexes (see graphic) under mild reaction conditions is presented. Copyright © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/chem.201002482
  • 2010 • 10 Plasma mediated collagen-I-coating of metal implant materials to improve biocompatibility
    Hauser, J. and Koeller, M. and Bensch, S. and Halfmann, H. and Awakowicz, P. and Steinau, H.-U. and Esenwein, S.
    Journal of Biomedical Materials Research - Part A 94 19-26 (2010)
    This study describes the collagen-I coating of titanium and steel implants via cold low-pressure gas plasma treatment. To analyze the coatings in terms of biocompatibility osteoblast-like osteosarcoma cells and human leukocytes were cultivated on the metal surfaces. Two different implant materials were assessed (Ti6Al4V, X2CrNiMo18) and four different surface properties were evaluated: (a) plasma pretreated and collagen-I coated implant materials; (b) collagen-I dip-coated without plasma pretreatment; (c) plasma treated but not collagen-I coated; (d) standard implant materials served as control. The different coating characteristics were analyzed by scanning electron microscopy (SEM). For adhesion and viability tests calcein-AM staining of the cells and Alamar blue assays were performed. The quantitative analysis was conducted by computer assisted microfluorophotography and spectrometer measurements. SEM analysis revealed that stable collagen-I coatings could not be achieved on the dip-coated steel and titanium alloys. Only due to pretreatment with low-pressure gas plasma a robust deposition of collagen I on the surface could be achieved. The cell viability and cell attachment rate on the plasma pretreated, collagen coated surfaces was significantly (p < 0.017) increased compared to the non coated surfaces. Gas plasma treatment is a feasible method for the deposition of proteins on metal implant materials resulting in an improved biocompatibility in vitro. © 2010 Wiley Periodicals, Inc.
    view abstractdoi: 10.1002/jbm.a.32672
  • 2010 • 9 Flexibility and Sorption Selectivity in Rigid Metal-Organic Frameworks: The Impact of Ether-Functionalised Linkers
    Henke, S. and Schmid, R. and Grunwaldt, J. D. and Fischer, R. A.
    Chemistry-a European Journal 16 14296--14306 (2010)
    The functionalisation of well-known rigid metal organic frameworks (namely, [Zn4O(bdc)(3)](n), MOF-5, IRMOF-1 and [Zn-2(bdc)(2)(dabco)](n); bdc = 1,4-benzene dicarboxylate, dabco=diazabicyclo[2.2.2]octane) with additional alkyl ether groups of the type -O-(CH2)(n)-O-CH3 (n = 2-4) initiates unexpected structural flexibility, as well as high sorption selectivity towards CO2 over N-2 and CH, in the porous materials. These novel materials respond to the presence/absence of guest molecules with structural transformations. We found that the chain length of the alkyl ether groups and the substitution pattern of the bdc-type linker have a major impact on structural flexibility and sorption selectivity. Remarkably, our results show that a high crystalline order of the activated material is not a prerequisite to achieve significant porosity and high sorption selectivity.
    view abstractdoi: 10.1002/chem.201002341
  • 2010 • 8 MOF-5 based mixed-linker metal-organic frameworks: Synthesis, thermal stability and catalytic application
    Kleist, W. and Maciejewski, M. and Baiker, A.
    Thermochimica Acta 499 71-78 (2010)
    Based on the well-known metal-organic framework material MOF-5 we developed a new route for the synthesis of highly porous mixed-linker metal-organic frameworks (MIXMOFs) where 5% and 10% of the benzene-1,4-dicarboxylate linkers have been substituted by a functionalized linker, namely 2-aminobenzene-1,4-dicarboxylate. The thermal stability of the materials decreased with increasing degree of substitution. However, all materials showed thermal stability up to at least 350 °C in oxidizing atmosphere which renders the MIXMOFs promising for catalytic applications. Choosing the optimum ratio of the two linker molecules both the number of active sites and thermal stability of the resulting catalysts could be tuned. The amino group at the functionalized linker proved to be beneficial for the immobilization of Pd species. The Pd loading achieved by equilibrium adsorption could be controlled by the number of NH 2 groups in the material. Although the thermal stability of the organic framework was affected to some extent in the presence of Pd, the Pd/MIXMOF materials could successfully be applied as catalysts in the oxidation of CO at elevated temperatures which was chosen as a test reaction. © 2009 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.tca.2009.11.004
  • 2010 • 7 Experimental characterization and modeling of the hardening behavior of the sheet steel LH800
    Noman, M. and Clausmeyer, T. and Barthel, C. and Svendsen, B. and Huétink, J. and van Riel, M.
    Materials Science and Engineering A 527 2515-2526 (2010)
    In complex forming processes, sheet metal undergoes large plastic deformations involving significant induced flow anisotropy resulting from the development of persistent oriented (planar) dislocation structures. The aim of the present work is the formulation and identification of a phenomenological model which accounts for the effect of the evolution of this oriented dislocation microstructure on the anisotropic hardening behavior. The model accounts for changes in the size, center, and shape, of the yield surface associated with isotropic, kinematic, and cross hardening, respectively. Identification of the model for the ferritic sheet metal steel LH800 is carried out with the help of shear, reverse shear, and tension-shear tests. The identified model has been validated using it to predict the stress-strain behavior of the material along different tension-shear loading paths and comparison with analogous experimental results. The results and in particular the comparison of theoretical predictions with experimental results clearly demonstrate the need of including cross hardening effects in the modeling of sheet metals like LH800. © 2009 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.msea.2009.12.013
  • 2010 • 6 Micro-structural alterations in MoM hip implants
    Pourzal, R. and Theissmann, R. and Gleising, B. and Williams, S. and Fischer, A.
    Materials Science Forum 638-642 1872-1877 (2010)
    Since the introduction of CoCrMo alloy metal-on-metal hip replacements have shown a great clinical performance. Metal-on-metal couplings produce a much lower wear rate and volume than e.g. metal-on-polyethylene. However, the particle size is significantly smaller within a nm-range. To evaluate the formation of nano-size wear particles in metal-on-metal hip replacements it is essential to understand the micro-structural changes in the sub-surface region of the CoCrMo alloy. For this study a MoM hip implant was analyzed by means of TEM. The results revealed that the good wear performance of this CoCrMo alloy is linked to a strain induced fcc → hcp phase transformation and in-situ re-crystallization under high shear stresses. The result is a nanocrystalline surface zone of ∼200 to 400 nm thickness which undergoes an ongoing process of mechanical intermixing with componants of the interfacial fluid. The incorporation of organic carbon from proteins in between the nano-crystals could be visualised by EFTEM and EDS. This mechanically mixed nc-zone must be the origin of the wear particle detachment. An earlier study by Catelas et. al confirms the hypothesis of the location of wear particle detachment by analyzing the shape and chemical composition of emitted wear particles which exhibits the same size and shape of crystals observed in the nc-zone of the implant analyzed in this study. © (2010) Trans Tech Publications.
    view abstractdoi: 10.4028/www.scientific.net/MSF.638-642.1872
  • 2010 • 5 Development of cold work tool steel based-MMC coating using HVOF spraying and its HIP densification behaviour
    Rajasekaran, B. and Mauer, G. and Vaßen, R. and Röttger, A. and Weber, S. and Theisen, W.
    Surface and Coatings Technology 204 3858-3863 (2010)
    The aim of the present study is to develop a Fe-based metal matrix composite (MMC) coating using high velocity oxy-fuel spraying (HVOF) process. A ledeburitic high alloyed cold work tool steel (X220CrVMo13-4) and NbC with an average size of 2μm at different volume fractions have been considered as metal matrix and hard particles respectively. MMC coatings were deposited on austenitic stainless substrates and the coatings were subsequently densified by hot isostatic pressing (HIP) with and without encapsulation. Microstructural analysis of the as-sprayed and HIPed coatings were characterized by SEM and XRD methods. Results showed that the feedstock preparation involving fine NbC was an influencing factor on the coating deposition. A relatively homogeneous dispersion of fine NbC up to 30. vol.% in cold work tool steel matrix was possible using optimized HVOF spraying. Besides, HVOF spraying and its subsequent HIP treatment induced significant microstructural and phase changes in the MMC coatings. The study showed the potential of HVOF spraying for the development of steel based MMC coatings and its subsequent densification can be achieved by HIP process with and without encapsulation. © 2010 Elsevier B.V.
    view abstractdoi: 10.1016/j.surfcoat.2010.05.001
  • 2010 • 4 Thermal stability, vapor pressures, and diffusion coefficients of some metal 2,2,6,6-tetramethyl-3,5-heptandionate [M(tmhd)n] compounds
    Siddiqi, M.A. and Siddiqui, R.A. and Atakan, B.
    Journal of Chemical and Engineering Data 55 2149-2154 (2010)
    Many metal 2,2,6,6-tetramethyl-3,5-heptandionate [M(tmhd)n] compounds are volatile enough to be useful as precursors of the metals in vapor-phase deposition processes, for example, metal organic chemical vapor deposition (MOCVD). The thermal stability, vapor pressures, and gaseous diffusion coefficients of these compounds are, therefore, of fundamental importance for achieving reproducible and effective depositions. The present communication reports the thermal stability, vapor pressures, enthalpies of sublimation, and diffusion coefficients (in nitrogen and/or helium) for some metal 2,2,6,6-tetramethyl-3,5-heptandionate compounds [M(tmhd)n], namely, [Al(tmhd)3], [Cr(tmhd)3], [Cu(tmhd)2], [Fe(tmhd)3], [Mn(tmhd)3], and [Ni(tmhd)2] at temperatures between (341 and 412) K at ambient pressure. All of these are found to be stable under the investigated experimental conditions and thus are suitable precursors for CVD. © 2010 American Chemical Society.
    view abstractdoi: 10.1021/je9006822
  • 2010 • 3 Optical response of metal-insulator-metal heterostructures and their application for the detection of chemicurrents
    Thissen, P. and Schindler, B. and Diesing, D. and Hasselbrink, E.
    New Journal of Physics 12 (2010)
    The optical response of thin-film metal-insulator-metal (MIM) systems of tantalum-tantalum oxide-Au type is studied by recording the macroscopic current across the device resulting from the low-energy electron-hole pairs excited in the metals by red and near-infrared (NIR) light (hν < 2 eV). It is observed that current flows from the top Au to the back Ta electrode, although a larger number of photons is absorbed in the latter. This directional preference is attributed to the built-in electric field across the oxide layer. The yield per photon increases strongly as photon energy becomes comparable to the barrier height. Current exhibits a strong dependence on bias voltages applied across the oxide layer. Photoyields induced by NIR light (hν ∼ 1.5 eV) were found to be comparable to recently observed chemicurrents arising from exposure of a MIM sensor to atomic hydrogen, when compared on a current per photon to current per impinging hydrogen atom basis. © IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.
    view abstractdoi: 10.1088/1367-2630/12/11/113014
  • 2010 • 2 Toward process optimization in laser welding of metal to polymer
    Tillmann, W. and Elrefaey, A. and Wojarski, L.
    Materialwissenschaft und Werkstofftechnik 41 879-883 (2010)
    The joining technology of dissimilar lightweight materials between metals and polymer is essential for realizing cars with hybrid structures and for other engineering applications. These types of joints are still difficult to generate and their behaviour is not fully understood. Laser welding offers specific process advantages over conventional technologies, such as short process times, while providing optically and qualitatively valuable weld seams and imposing minimal thermal stress. Furthermore, the process is compatible with automation. This paper summarizes the efforts to attain suitable joint strengths with the stainless steel plate type S30400 and a Polyethylene Terephtalate Glycol (PETG) plastic sheet. The study considers the optimization of two important process parameters, namely laser power, and welding speed. Microstructure features, test of tensile shear strength, investigation of the fracture location, and morphology were used to evaluate the joint performance. The result indicates that there is an optimum value for laser power, which achieves a sufficient melting and heat transfer to the joint without decomposing the plastic sheet and hence, enables to obtain high joint strength. Moreover, a low welding speed is preferable in most combinations of welding parameters since it achieves an adequate melting and wetting of the polymer to the steel surface. Copyright 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/mawe.201000674
  • 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