Scientific Output

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

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

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  • 2020 • 55 Enhanced antibacterial performance of ultrathin silver/platinum nanopatches by a sacrificial anode mechanism
    Abuayyash, A. and Ziegler, N. and Meyer, H. and Meischein, M. and Sengstock, C. and Moellenhoff, J. and Rurainsky, C. and Heggen, M. and Garzón-Manjón, A. and Scheu, C. and Tschulik, K. and Ludwig, A. and Köller, M.
    Nanomedicine: Nanotechnology, Biology, and Medicine 24 (2020)
    The development of antibacterial implant surfaces is a challenging task in biomaterial research. We fabricated a highly antibacterial bimetallic platinum (Pt)/silver(Ag) nanopatch surface by short time sputtering of Pt and Ag on titanium. The sputter process led to a patch-like distribution with crystalline areas in the nanometer-size range (1.3–3.9 nm thickness, 3–60 nm extension). Structural analyses of Pt/Ag samples showed Ag- and Pt-rich areas containing nanoparticle-like Pt deposits of 1–2 nm. The adhesion and proliferation properties of S. aureus on the nanopatch samples were analyzed. Consecutively sputtered Ag/Pt nanopatches (Pt followed by Ag) induced enhanced antimicrobial activity compared to co-sputtered Pt/Ag samples or pure Ag patches of similar Ag amounts. The underlying sacrificial anode mechanism was proved by linear sweep voltammetry. The advantages of this nanopatch coating are the enhanced antimicrobial activity despite a reduced total amount of Ag/Pt and a self-limited effect due the rapid Ag dissolution. © 2019 Elsevier Inc.
    view abstractdoi: 10.1016/j.nano.2019.102126
  • 2020 • 54 Role of coherency loss on rafting behavior of Ni-based superalloys
    Ali, M.A. and Görler, J.V. and Steinbach, I.
    Computational Materials Science 171 (2020)
    The role of coherency loss on rafting of superalloys under high temperature low stress creep conditions is investigated by phase-field crystal plasticity simulations. It is demonstrated that coalescence, critically depending on the state of coherency between precipitate and matrix is crucial to understand the rafting behavior of superalloys. An explicit mechanisms is developed predicting coherency loss based on the plastic activity in the matrix. The simulations are verified using experimental creep test results. © 2019 Elsevier B.V.
    view abstractdoi: 10.1016/j.commatsci.2019.109279
  • 2020 • 53 Subharmonic Injection Locking for Phase and Frequency Control of RTD-Based THz Oscillator
    Arzi, K. and Suzuki, S. and Rennings, A. and Erni, D. and Weimann, N. and Asada, M. and Prost, W.
    IEEE Transactions on Terahertz Science and Technology 10 221-224 (2020)
    Phase and frequency control of resonant tunneling diode (RTD) based terahertz oscillators are major challenges in realizing coherent signal sources for arrayed applications, such as spatial power combining, beam steering, or multi-in multi-out systems. In this letter, we demonstrate frequency locking and control of an RTD oscillating at f0 ∼ 550 GHz, via radiative injection of a weak sinusoidal subharmonic signal at f0/2. Precise frequency control, within the locking range of around 2 GHz, is demonstrated. A peak output power enhancement of 14 dB in the whole locking range, compared to the free running oscillator, is achieved. Furthermore, occurrence of phase locking is identified by the spectral linewidth reduction, quantifiable in the full-width at half-maximum parameter. A signal linewidth of 490 Hz was achieved in locked operation. © 2011-2012 IEEE.
    view abstractdoi: 10.1109/TTHZ.2019.2959411
  • 2020 • 52 An energy-relaxation-based framework for the modelling of magnetic shape memory alloys—Simulation of key response features under homogeneous loading conditions
    Bartel, T. and Kiefer, B. and Buckmann, K. and Menzel, A.
    International Journal of Solids and Structures 182-183 162-178 (2020)
    In this contribution we present a constitutive modelling framework for magnetic shape memory alloys (MSMA) that builds on a global variational principle. The approach relies on concepts of energy relaxation and generalised notions of convexity to compute effective energy hulls to the non-convex energy landscape associated with the underlying multi-phase solid, from which the prediction of microstructure evolution results. In this sense it fundamentally distinguishes itself from MSMA models that essentially follow phenomenological concepts of classical plasticity (Kiefer and Lagoudas, 2005; 2009). The microstructure is not spatially resolved, but micro-scale quantities are taken into account in an effective sense by additional state variables—such as volume fractions or interface orientations—and appropriate mixture rules. The model allows all mechanisms central to MSMA behaviour—i.e. variant switching, magnetisation rotation away from easy axes, and magnetic domain evolution—to occur simultaneously. The authors have previously been able to demonstrate that such a modelling approach can quantitatively capture the key characteristics of single-crystalline MSMA response under standard loading scenarios (Kiefer et al., 2015). The modelling framework presented here is now further able to predict much more general response features, such as variant switching diagrams, magnetic field-biased pseudo-elasticity and the influence of specimen shape anisotropy. Moreover, the global variational framework is formulated in a manner that lends itself to finite element implementation. In this work, however, numerical examples are considered in which the nonlocal nature of the demagnetisation field is taken into account in an approximate sense through appropriate shape factors. © 2019 Elsevier Ltd
    view abstractdoi: 10.1016/j.ijsolstr.2019.07.016
  • 2020 • 51 Spatially controlled VLS epitaxy of gallium arsenide nanowires on gallium nitride layers
    Blumberg, C. and Liborius, L. and Ackermann, J. and Tegude, F.-J. and Poloczek, A. and Prost, W. and Weimann, N.
    CrystEngComm 22 1239-1250 (2020)
    We present Au catalyzed p-GaAs nanowire growth on n-GaN layers as a possible method to grow an arsenide on a nitride compound semiconductor by metal organic vapor phase epitaxy. The GaAs growth position, the nanowire density and the nanowire growth direction are controlled by a combination of vapor-liquid-solid growth and selective area epitaxy. Thus, a spatially controlled nanowire growth is attained, which is mandatory for device fabrication. The growth position is defined by lithographically positioned Au discs on n-GaN. By adapting the growth conditions (QTBAs, presaturation) the nanowire density is optimized. Lateral and vertical anisotropic nanowire growth is attained through VLS growth in structured SiOx openings. Critical technological parameters for successful control of the growth direction are the positioning of the Au catalyst in relation to the SiOx mask, the size of the eutectic in relation to the opening dimensions, and the SiOx thickness. These results lead to distinct pn-junction positions and adjustable nanowire growth dimensions and directions. This journal is © The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c9ce01926j
  • 2020 • 50 Chirality in the plane
    Böhmer, C.G. and Lee, Y. and Neff, P.
    Journal of the Mechanics and Physics of Solids 134 (2020)
    It is well-known that many three-dimensional chiral material models become non-chiral when reduced to two dimensions. Chiral properties of the two-dimensional model can then be restored by adding appropriate two-dimensional chiral terms. In this paper we show how to construct a three-dimensional chiral energy function which can achieve two-dimensional chirality induced already by a chiral three-dimensional model. The key ingredient to this approach is the consideration of a nonlinear chiral energy containing only rotational parts. After formulating an appropriate energy functional, we study the equations of motion and find explicit soliton solutions displaying two-dimensional chiral properties. © 2019
    view abstractdoi: 10.1016/j.jmps.2019.103753
  • 2020 • 49 Enhanced dissolution of silver nanoparticles in a physical mixture with platinum nanoparticles based on the sacrificial anode effect
    Breisch, M. and Loza, K. and Pappert, K. and Rostek, A. and Rurainsky, C. and Tschulik, K. and Heggen, M. and Epple, M. and Tiller, J.C. and Schildhauer, T.A. and Köller, M. and Sengstock, C.
    Nanotechnology 31 (2020)
    A strategy to reduce implant-related infections is the inhibition of the initial bacterial implant colonization by biomaterials containing silver (Ag). The antimicrobial efficacy of such biomaterials can be increased by surface enhancement (nanosilver) or by creating a sacrificial anode system for Ag. Such a system will lead to an electrochemically driven enhanced Ag ion release due to the presence of a more noble metal. Here we combined the enlarged surface of nanoparticles (NP) with a possible sacrificial anode effect for Ag induced by the presence of the electrochemically more noble platinum (Pt) in physical mixtures of Ag NP and Pt NP dispersions. These Ag NP/Pt NP mixtures were compared to the same amounts of pure Ag NP in terms of cell biological responses, i.e. the antimicrobial activity against Staphylococcus aureus and Escherichia coli as well as the viability of human mesenchymal stem cells (hMSC). In addition, Ag NP was analyzed by ultraviolet-visible (UV-vis) spectroscopy, cyclic voltammetry, and atomic absorption spectroscopy. It was found that the dissolution rate of Ag NP was enhanced in the presence of Pt NP within the physical mixture compared to a dispersion of pure Ag NP. Dissolution experiments revealed a fourfold increased Ag ion release from physical mixtures due to enhanced electrochemical activity, which resulted in a significantly increased toxicity towards both bacteria and hMSC. Thus, our results provide evidence for an underlying sacrificial anode mechanism induced by the presence of Pt NP within physical mixtures with Ag NP. Such physical mixtures have a high potential for various applications, for example as antimicrobial implant coatings in the biomedicine or as bactericidal systems for water and surface purification in the technical area. © 2019 IOP Publishing Ltd.
    view abstractdoi: 10.1088/1361-6528/ab4e48
  • 2020 • 48 Investigation and equalisation of the flow distribution in a fuel cell stack
    Bürkle, F. and Moyon, F. and Feierabend, L. and Wartmann, J. and Heinzel, A. and Czarske, J. and Büttner, L.
    Journal of Power Sources 448 (2020)
    The possibility to use fuel cells as an electrical power source makes them interesting for a wide range of applications. In this work, computational fluid dynamics (CFD) simulations and optical measurements are performed to predict the flow distribution in a flow setup resembling the parallel flow circuits in fuel cell stacks. For the first time it is shown that by an adaptation of the port sizes in the inlet manifold to the individual fuel cells, the average global deviation between the flow rates can be reduced from 10.1% to 4.0% by means of a model experiment. The measurements are performed with a high resolution laser Doppler velocity profile sensor (LD-PS) specifically developed for measurements in small-scale channels, in this work 4×1 mm2, allowing for a spatial resolution below 2 μm and relative velocity uncertainties below 0.1%, helping to resolve installation effects possibly occurring in fuel cells to improve their efficiency. The presented results can be used by manufacturers to increase the efficiency of their fuel cell stacks. © 2019 Elsevier B.V.
    view abstractdoi: 10.1016/j.jpowsour.2019.227546
  • 2020 • 47 A six-compound, high performance gasoline surrogate for internal combustion engines: Experimental and numerical study of autoignition using high-pressure shock tubes
    Cancino, L.R. and da Silva, A., Jr. and De Toni, A.R. and Fikri, M. and Oliveira, A.A.M. and Schulz, C. and Curran, H.J.
    Fuel 261 (2020)
    This paper presents experimental and modeling data for the autoignition of a novel, six-component, high performance gasoline surrogate fuel comprising ethanol, n-heptane, i-octane, 1-hexene, methylcyclohexane, and toluene (AL-P-I-O-N-A). Experimental tests are conducted in two high-pressure shock tubes to determine the ignition delay time as a function of pressure, temperature and equivalence ratio. Ignition delay times were measured at 10 and 30 bar in the temperature range from 749 to 1204 K and equivalence ratios ranging from 0.35 to 1.30. A modified Arrhenius equation is defined to mathematically describe the ignition delay time of the proposed surrogate. For experimental data with temperatures higher than 900 K, a multiple linear regression identified the pressure dependence exponent of 0.72 and stoichiometry dependence exponent of 0.62, as well as a global activation energy of ≈109 kJ/mol. A simplistic approach to mechanism reduction based on the elimination of reactions with no relevant rate of progress was used in order to reduce an extensive detailed kinetics model (hierarchically constructed with more than 17800 reactions). The reduced detailed kinetics model with 4885 elementary reactions among 326 chemical species was used for numerical simulations. Comparisons between the experimental and numerical data are favorable, with the predictions using the reduced kinetics model differing by less than 0.056% when compared to the complete mechanism. It was observed that for low temperatures the proposed reduced kinetics model agrees only qualitatively with the measurements. In order to understand the likely cause of this discrepancy a brute force sensitivity analysis on IDT was performed, elucidating the more influencing reactions on the ignition delay times. The experimental data obtained in this research was compared to available data in the literature in terms of anti-knock index (AKI) and for a scaled pressure of 30 bar (τ30) at a stoichiometric composition. A modified Arrhenius equation was then fitted and an AKI dependence exponent of -1.11 was obtained, inferring that the higher the AKI the higher the IDT, independent of fuel composition at temperatures lower than the NTC region. This trend should be confirmed by further studies. © 2019 Elsevier Ltd
    view abstractdoi: 10.1016/j.fuel.2019.116439
  • 2020 • 46 Could face-centered cubic titanium in cold-rolled commercially-pure titanium only be a Ti-hydride?
    Chang, Y. and Zhang, S. and Liebscher, C.H. and Dye, D. and Ponge, D. and Scheu, C. and Dehm, G. and Raabe, D. and Gault, B. and Lu, W.
    Scripta Materialia 178 39-43 (2020)
    A face-centered cubic (FCC) phase in electro-polished specimens for transmission electron microscopy of commercially pure titanium has sometimes been reported. Here, a combination of atom-probe tomography, scanning transmission electron microscopy and low-loss electron energy loss spectroscopy is employed to study both the crystal structural and chemical composition of this FCC phase. Our results prove that the FCC phase is actually a TiHx (x ≥ 1) hydride, and not a new allotrope of Ti, in agreement with previous reports. The formation of the hydride is discussed. © 2019 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.scriptamat.2019.11.010
  • 2020 • 45 Optimal control problems with control complementarity constraints: existence results, optimality conditions, and a penalty method
    Clason, C. and Deng, Y. and Mehlitz, P. and Prüfert, U.
    Optimization Methods and Software 35 142-170 (2020)
    A special class of optimal control problems with complementarity constraints on the control functions is studied. It is shown that such problems possess optimal solutions whenever the underlying control space is a first-order Sobolev space. After deriving necessary optimality conditions of strong stationarity-type, a penalty method based on the Fischer–Burmeister function is suggested and its theoretical properties are analyzed. Finally, the numerical treatment of the problem is discussed and results of computational experiments are presented. © 2019, © 2019 Informa UK Limited, trading as Taylor & Francis Group.
    view abstractdoi: 10.1080/10556788.2019.1604705
  • 2020 • 44 Optimal designs for estimating individual coefficients in polynomial regression with no intercept
    Dette, H. and Melas, V.B. and Shpilev, P.
    Statistics and Probability Letters 158 (2020)
    We identify optimal designs for estimating individual coefficients in a polynomial regression with no intercept. Here the regression functions do not form a Chebyshev system such that the seminal results of Studden (1968) characterizing c-optimal designs are not applicable. © 2019
    view abstractdoi: 10.1016/j.spl.2019.108636
  • 2020 • 43 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 • 42 Image-based size analysis of agglomerated and partially sintered particles via convolutional neural networks
    Frei, M. and Kruis, F.E.
    Powder Technology 360 324-336 (2020)
    There is a high demand for fully automated methods for the analysis of primary particle size distributions of agglomerated, sintered or occluded primary particles, due to their impact on material properties. Therefore, a novel, deep learning-based, method for the detection of such primary particles was proposed and tested, which renders a manual tuning of analysis parameters unnecessary. As a specialty, the training of the utilized convolutional neural networks was carried out using only synthetic images, thereby avoiding the laborious task of manual annotation and increasing the ground truth quality. Nevertheless, the proposed method performs excellent on real world samples of sintered silica nanoparticles with various sintering degrees and varying image conditions. In a direct comparison, the proposed method clearly outperforms two state-of-the-art methods for automated image-based particle size analysis (Hough transformation and the ImageJ ParticleSizer plug-in), thereby attaining human-like performance. © 2019 Elsevier B.V.
    view abstractdoi: 10.1016/j.powtec.2019.10.020
  • 2020 • 41 Conductive films prepared from inks based on copper nanoparticles synthesized by transferred arc discharge
    Fu, Q. and Stein, M. and Li, W. and Zheng, J. and Kruis, F.E.
    Nanotechnology 31 (2020)
    Copper nanoparticles (NPs) are considered as a promising alternative for silver and gold NPs in conductive inks for the application of printing electronics, since copper shows a high electrical conductivity but is significantly cheaper than silver and gold. In this study, copper NPs were synthesized in the gas phase by transferred arc discharge, which has demonstrated scale-up potential. Depending on the production parameters, copper NPs can be continuously synthesized at a production rate of 1.2-5.5 g h-1, while their Brunauer-Emmett-Teller sizes were maintained below 100 nm. To investigate the suitability in electronic printing, we use ball milling technique to produce copper conductive inks. The effect of ball milling parameters on ink stability was discussed. In addition, the electrical resistivity of copper films sintered at 300 °C in reducing atmosphere was measured to be 5.4 ± 0.6 μΩ cm which is about three times higher than that of bulk copper (1.7 μΩ cm). This indicates that conductive inks prepared from gas-phase synthesized copper NPs are competitive to the conductive inks prepared from chemically synthesized copper NPs. © 2019 IOP Publishing Ltd.
    view abstractdoi: 10.1088/1361-6528/ab4524
  • 2020 • 40 Are Onsager's reciprocal relations necessary to apply Thermodynamic Extremal Principles?
    Hackl, K. and Fischer, F.D. and Zickler, G.A. and Svoboda, J.
    Journal of the Mechanics and Physics of Solids 135 (2020)
    Onsager's Reciprocal Relations between thermodynamic forces and fluxes, for which Onsager was awarded the Nobel Prize, automatically follow from Thermodynamic Extremal Principles. Thus, the Principles are up to now non-applicable for the treatment of experimentally determined or theoretically modeled non-reciprocal systems as e.g. those in the magnetic field. However, we can demonstrate that adding of a certain barrier constraint as bilinear form of thermodynamic forces and fluxes accounted by the Thermodynamic Extremal Principles provides to non-reciprocal relations between the thermodynamic forces and fluxes. Such a novel idea may contribute to a better understanding of physics behind non-reciprocal systems. © 2019 Elsevier Ltd
    view abstractdoi: 10.1016/j.jmps.2019.103780
  • 2020 • 39 Using spectral-based representative volume element crystal plasticity simulations to predict yield surface evolution during large scale forming simulations
    Han, F. and Diehl, M. and Roters, F. and Raabe, D.
    Journal of Materials Processing Technology 277 (2020)
    We present a new approach to predict the evolution of anisotropic yield functions by coupling large scale forming simulations with crystal plasticity-spectral based virtual experiments, realizing a multi-scale model for metal forming. Employing a fast spectral method solver enables us to conduct on-the-fly full-field virtual experiments to evolve the yield surface at each integration point of the macroscopic finite element model. As illustrative example, two advanced anisotropic yield functions, namely Yld2000-2D and Yld2004-18p, are used in finite element simulations of deep drawing for a 2090-T3 aluminum alloy sheet. The simulated earing profiles are compared to the experimental ones as well as to simulations with non-evolving yield functions. It is found that the prediction of the earing is improved for the case of the evolving Yld2000-2D yield function. The evolution of the plastic anisotropy during cup drawing is systematically analyzed, showing that the evolution of anisotropy can have considerable effect on the prediction accuracy of the macroscopic simulations. © 2019 Elsevier B.V.
    view abstractdoi: 10.1016/j.jmatprotec.2019.116449
  • 2020 • 38 On the atomic solute diffusional mechanisms during compressive creep deformation of a Co-Al-W-Ta single crystal superalloy
    He, J. and Zenk, C.H. and Zhou, X. and Neumeier, S. and Raabe, D. and Gault, B. and Makineni, S.K.
    Acta Materialia 184 86-99 (2020)
    We investigated the solute diffusional behavior active during compressive creep deformation at 150 MPa / 975 °C of a Co-Al-W-Ta single crystal superalloy in the [001] orientation. We report the formation of shear-bands that involves re-orientation of γ/γʹ rafts to {111} from {001} planes, referring to as γ/γ′ raft-rotation. In the shear-band regions, we observed abundant micro-twins, stacking faults (SFs), disordered zones within the γʹ termed as ‘γ pockets’ and also few geometrically-close-packed (GCP) phases. We used a correlative approach blending electron microscopy and atom probe tomography to characterize the structure and composition of these features. The SFs were identified as intrinsic and exhibit a W enrichment up to 14.5 at.% and an Al deficiency down to 5.1 at.%, with respect to the surrounding γʹ phase. The micro-twin boundaries show a solute enrichment similar to the SFs with a distinct W compositional profile gradients perpendicular from the boundaries into the twin interior, indicating solute diffusion within the micro-twins. The γ-pockets have a composition close to that of γ but richer in W/Ta. Based on these observations, we propose (i) a solute diffusion mechanism taking place during micro-twinning, (ii) a mechanism for the γ/γʹ raft-rotation process and evaluate their influence on the overall creep deformation of the present Co-based superalloy. © 2019
    view abstractdoi: 10.1016/j.actamat.2019.11.035
  • 2020 • 37 Toward Mobile Integrated Electronic Systems at THz Frequencies
    Hillger, P. and van Delden, M. and Thanthrige, U.S.M. and Ahmed, A.M. and Wittemeier, J. and Arzi, K. and Andree, M. and Sievert, B. and Prost, W. and Rennings, A. and Erni, D. and Musch, T. and Weimann, N. and Sezgin, A. and Pohl...
    Journal of Infrared, Millimeter, and Terahertz Waves (2020)
    This paper discusses advances related to the integration of future mobile electronic THz systems. Without claiming to provide a comprehensive review of this surging research area, the authors gathered research on selected topics that are expected to be of relevance for the future exploration of components for practical mobile THz imaging and sensing applications. First, a brief technology review of integrated mobile THz components is given. Advances in III-V technology, silicon technology, and resonant-tunneling diodes (RTD) are discussed. Based on an RTD source and a SiGe-HBT direct detector, low-cost and compact computed tomography is presented for volumetric continuous-wave imaging at around 300 GHz. Moreover, aspects of system integration of mobile THz MIMO radars are discussed. Thereby, a novel phase-locked loop concept utilizing a high-stability yttrium-iron-garnet-tuned oscillator to synthesize ultra-stable reference mmWave signals is shown, and an adaptive self-interference cancellation algorithm for THz MIMO in the digital domain based on Kalman filter theory is proposed. © 2020, Springer Science+Business Media, LLC, part of Springer Nature.
    view abstractdoi: 10.1007/s10762-020-00699-x
  • 2020 • 36 Study on machinability of additively manufactured and conventional titanium alloys in micro-milling process
    Hojati, F. and Daneshi, A. and Soltani, B. and Azarhoushang, B. and Biermann, D.
    Precision Engineering 62 1-9 (2020)
    Capability of Additive Manufacturing (AM) technology in the production of complex parts with high flexibility has led to the growing interest in their application as an alternative for conventional manufacturing processes. Despite the outstanding benefits of the AM process, due to their poor surface quality, the precision parts produced by this method generally need to be machined, ground, or polished. This paper addresses the machinability of AM Ti6Al4V titanium alloy parts in the micro-milling process with a specific focus on cutting forces, specific cutting energy, burr formation, and surface quality. Additive parts were produced by Electron Beam Melting (EBM) technique and were compared with the extruded Ti6Al4V parts in the micro-milling process. No significant difference could be observed in the cutting forces of both materials at chip thicknesses between 7.4 and 37.3 μm, despite the higher hardness of the EBM Ti6Al4V compared to the extruded Ti6Al4V. However, micro-milling of the EBM parts produced finer surfaces. Cutting forces and specific cutting energies of EBM parts were less than those of extruded parts at minimal chip thicknesses (lower than 7.4 μm). Continuous wavy-type burrs were formed in micro-milling of the EBM Ti6Al4V and were larger than those of extruded Ti6Al4V. © 2019 Elsevier Inc.
    view abstractdoi: 10.1016/j.precisioneng.2019.11.002
  • 2020 • 35 Development of an energy-based approach for optimized frequency selection for fatigue testing on polymers – Exemplified on polyamide 6
    Hülsbusch, D. and Kohl, A. and Striemann, P. and Niedermeier, M. and Strauch, J. and Walther, F.
    Polymer Testing 81 (2020)
    Polymers and composite materials show temperature-dependent material properties. Therefore, the frequency resembles a critical part in fatigue testing, due to its influence on the self-heating of the polymeric material and thereby on the number of cycles to failure. The aim of this paper is the development of a testing method, which allows comparable results with varying frequencies. To minimize the self-heating effect on the fatigue behavior, a model was established for selecting optimized frequencies regarding the load-specific temperature increase of the specimen. A new energy-parameter, the induced energy-rate, was introduced and correlated to the load-specific increase in temperature in multiple and constant amplitude tests at ambient conditions. With this approach, it was possible to determine a threshold value for the newly defined induced energy-rate. A stress-specific model was developed and a limit frequency was calculated. The results were verified in multiple and constant amplitude tests and S/N-curves. © 2019
    view abstractdoi: 10.1016/j.polymertesting.2019.106260
  • 2020 • 34 Thermodynamic modelling of the Ni–Zr system
    Jana, A. and Sridar, S. and Fries, S.G. and Hammerschmidt, T. and Kumar, K.C.H.
    Intermetallics 116 (2020)
    In this work, we report the thermodynamic modelling of the Ni–Zr system using the Calphad method combined with ab initio calculations. Density functional theory (DFT) is employed to calculate the enthalpy of formation of the intermediate phases. The calculated enthalpies of formation are in close agreement with the experimental data. An approach based on special quasirandom structures (SQS) was used for calculating the enthalpy of mixing of the fcc solid solution. The vibrational contribution to the heat capacities of NiZr, NiZr2, Ni3Zr and Ni7Zr2 phases were calculated using the quasiharmonic approximation (QHA) and the corresponding electronic contribution was obtained using an approach based on Mermin statistics. The total heat capacities for these phases were fitted to appropriate expressions and integrated to obtain the Gibbs energy functions valid down to 0 K. The calculated thermochemical properties along with critically selected experimental constitutional and thermochemical data served as input for the thermodynamic optimisation of the system. The calculated phase equilibria and the thermodynamic properties using the optimised Gibbs energy functions are in good agreement with the input data. The calculated congruent melting points of NiZr and NiZr2 phases are close to the recent experimental data. The Ni10Z7 phase forms by a peritectic reaction, which is also in agreement with the experimental data. © 2019 Elsevier Ltd
    view abstractdoi: 10.1016/j.intermet.2019.106640
  • 2020 • 33 Element-specific displacements in defect-enriched TiO2: Indication of a flash sintering mechanism
    Jongmanns, M. and Wolf, D.E.
    Journal of the American Ceramic Society 103 589-596 (2020)
    Flash sintering experiments of ceramics indicate the formation of a state far from equilibrium. It is hypothesized that this state is enriched by Frenkel defects. The possibility is investigated that such lattice defects are being generated by a proliferation of lattice vibrations that lie close to the Brillouin zone edge. We show by means of Molecular Dynamics simulations of rutile TiO2 that this mechanism generates Frenkel defects in concentrations far beyond equilibrium. These defects deform the whole lattice in a way that the mean-square displacements of the vibration amplitudes of the Ti and O atoms are specifically enhanced. This finding compares well to atomic displacement data of flash sintered rutile TiO2 reported recently. © 2019 The Authors. Journal of the American Ceramic Society published by Wiley Periodicals, Inc. on behalf of American Ceramic Society (ACERS)
    view abstractdoi: 10.1111/jace.16696
  • 2020 • 32 Modeling the Noise of Transferred-Substrate InP DHBTs at Highest Frequencies
    Kaule, E. and Doerner, R. and Weimann, N. and Rudolph, M.
    GeMIC 2020 - Proceedings of the 2020 German Microwave Conference 52-55 (2020)
    This paper investigates noise modeling of transferred-substrate indium phosphide double heterobipolar transistors (InP DUBTs). It is shown that the shot noise of these devices exhibits a pronounced correlation which allows for a reliable extrapolation of the noise performance based on standard noise measurement at lower frequencies, or even on the knowledge of small-signal model parameters alone. © 2020 IMA-Institut fur Mikrowellen-und Antennentechnik e.V.
    view abstract
  • 2020 • 31 Computer modeling of semiconductor nanotubes for water splitting
    Kenmoe, S. and Spohr, E.
    Current Opinion in Electrochemistry 19 88-95 (2020)
    One-dimensional nanostructures such as nanorods and nanotubes (NTs) are regarded as promising materials for photoelectrochemical water splitting. Modeling the electronic properties of oxidic NTs with diameters in the range of 3–30 nm in contact with liquid water is challenging owing to the fact that the systems are too large for direct ab initio molecular dynamics simulations. Here we summarize recent efforts to develop strained two-dimensional model systems for pristine and doped titania NTs. We have studied their structural, optical and photoelectrochemical properties by a number of different techniques attributable to quantum mechanical density functional theory. © 2019 Elsevier B.V.
    view abstractdoi: 10.1016/j.coelec.2019.10.013
  • 2020 • 30 Size dependent strength, slip transfer and slip compatibility in nanotwinned silver
    Kini, M.K. and Dehm, G. and Kirchlechner, C.
    Acta Materialia 184 120-131 (2020)
    Perfect slip transfer through single coherent Σ3 twin boundaries is known to be a cross-slip-like mechanism occurring at low stresses, which is expected to strongly depend on material properties like stacking fault energy. In the present study, we extend the argument of perfect slip transfer to (i) multiple closely spaced coherent twin boundaries in a nanotwinned thin film and (ii) to materials with very low stacking fault energy. The slip transfer is indicated by the continuity of slip steps and observed across up to 100 coherent Σ3 boundaries. The study addresses size scaling due to multiple weak obstacles for dislocation motion and discusses the underlying deformation mechanisms. The importance of strain compatibility is further extended to incoherent twin boundaries. © 2019
    view abstractdoi: 10.1016/j.actamat.2019.11.042
  • 2020 • 29 Regularized, parameter free scale similarity type models for Large Eddy Simulation
    Klein, M. and Ketterl, S. and Engelmann, L. and Kempf, A. and Kobayashi, H.
    International Journal of Heat and Fluid Flow 81 (2020)
    The fidelity of Large Eddy Simulations (LES) depends strongly on the closures of the sub-grid scale (SGS) stress tensor. Although it is well known that the SGS stresses in LES are not aligned with the strain rate tensor, the most widely used models are still of eddy viscosity type, due to their robust behavior in LES and reasonable performance in a posteriori testing. The unstable behavior of more advanced anisotropic models, that is typically found in LES, has been attributed to either the fact that these models provide backscatter or to the fact that they do not provide a sufficient amount of dissipation. Based on recent advances in the field, an alternative modeling strategy is suggested, which can be used to regularize an arbitrary anisotropic (e.g. scale similarity type) model. The resulting model is easy to implement, can be written in compact form and is free of model parameters. The model has been tested a-posteriori and results are presented for a Taylor-Green-Vortex, a free plane jet and a turbulent channel flow of friction Reynolds numbers 395, 590 and 934. The results are compared to well-known eddy viscosity models and when applicable, to simulations without explicit LES model. The new model exhibits good performance for a variety of mesh resolutions and for all configurations. Furthermore, a-priori analysis results in the context of liquid atomization indicate that the model might be suitable as well in more complex physical scenarios. The a-priori analysis performance of the model is found to be nearly equivalent to the underlying structural anisotropic model in terms of its correlation coefficient, but the model is free of backscatter and provides good stability in LES. © 2019 Elsevier Inc.
    view abstractdoi: 10.1016/j.ijheatfluidflow.2019.108496
  • 2020 • 28 Real-Time Capable Calculation of Reaction Forces of Multibody Systems Using Optimized Bushings on the Example of a Vehicle Wheel Suspension
    Kracht, F.E. and Schramm, D.
    Computational Methods in Applied Sciences 53 409-416 (2020)
    This paper presents an object-oriented modeling method capable of simulating the dynamics including the reaction forces of multibody systems with kinematic loops in hard real-time, called RTOOM. The modeling method describes the system by explicit equations, which can be solved numerically stable with a standard explicit numerical integrator with fixed step size. By knowing the application and the desired accuracy, the model can be adapted to fit the problem. Algebraic loops are resolved with low-pass filters parameterized for the frequency range of the application. Bushings with optimized spring and damping constants are used to avoid iterative methods for solving kinematics loops. For the optimization, a high accurate, non-manipulated and non-real-time multibody model is used. The optimization targets are stability, computing time and accuracy. The double wishbone suspension of the Formula Student racing car A40-02 of the University of Duisburg-Essen is used as an example. It has been successfully proven that a simulation up to 30 Hz with a required step size of 1 ms can be achieved. The simulation results show a very good accuracy up to 15 Hz with a deviation of the force below 4% and the acceleration below 7%. If the parameterization of the bushings remains the same, the accuracy is still acceptable even at higher frequencies. © 2020, Springer Nature Switzerland AG.
    view abstractdoi: 10.1007/978-3-030-23132-3_49
  • 2020 • 27 Processing of a single-crystalline CrCoNi medium-entropy alloy and evolution of its thermal expansion and elastic stiffness coefficients with temperature
    Laplanche, G. and Schneider, M. and Scholz, F. and Frenzel, J. and Eggeler, G. and Schreuer, J.
    Scripta Materialia 177 44-48 (2020)
    The equiatomic CrCoNi alloy is regarded as a model single-phase face-centered cubic medium-entropy alloy. A CrCoNi single crystal was grown by a Bridgman technique using a Ni-base superalloy seed. The elastic stiffnesses and thermal expansion coefficient were determined between 100 K and 673 K employing resonant ultrasound spectroscopy and dilatometry, respectively. All data were found to be in excellent agreement with those reported for polycrystalline CrCoNi. A comparison of the normalized Cauchy pressure of CrCoNi with those of other alloys indicates that interatomic bonds become more directional with increasing Cr-concentration while Co and Ni promote a metallic character. © 2019 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.scriptamat.2019.09.020
  • 2020 • 26 Behavioral Analysis of Human-Machine Interaction in the Context of Demand Planning Decisions
    Lauer, T. and Welsch, R. and Ramlah Abbas, S. and Henke, M.
    Advances in Intelligent Systems and Computing 965 130-141 (2020)
    The trend of digitalization has led to disruptive changes in production and supply chain planning, where autonomous machines and artificial intelligence gain competitive advantages. Besides, the satisfaction of customers’ wishes has reached top priority for demand-driven companies. Consequently, companies implement digital applications, for instance neural networks for accurate demand forecasting and optimized decision-making tools, to cope with nervous operational planning activities. Since planning tasks require human-machine interaction to increase performance and efficiency of planning decisions, this analysis focuses on forms of interaction to determine the right level of collaboration. The paper outlines various levels of interaction and analyses the impact of human reactions in the context of an industrial demand planning algorithm use case at Infineon Technologies AG conducting a behavioral experiment. The results show that a variance in the levels of human-machine interaction has influence on human acceptance of algorithms, but further experiments need to be conducted to outline an overall framework. © 2020, Springer Nature Switzerland AG.
    view abstractdoi: 10.1007/978-3-030-20454-9_13
  • 2020 • 25 n-Doped InGaP Nanowire Shells in GaAs/InGaP Core–Shell p–n Junctions
    Liborius, L. and Bieniek, J. and Nägelein, A. and Tegude, F.-J. and Prost, W. and Hannappel, T. and Poloczek, A. and Weimann, N.
    Physica Status Solidi (B) Basic Research 257 (2020)
    Herein, the characterization of n-doped InGaP:Si shells in coaxial not-intentionally doped (nid)-GaAs/n-InGaP as well as n–p–n core–multishell nanowires grown by metalorganic vapor-phase epitaxy is reported. The multi-tip scanning tunneling microscopy technique is used for contact-independent resistance profiling along the tapered nid-GaAs/n-InGaP core–shell nanowires to estimate the established emitter shell doping concentration to ND ≈ 3 · 1018 cm−3. Contacts on these shells are demonstrated and exhibit ohmic current–voltage characteristics after annealing. Application potential is demonstrated by the growth and processing of coaxial p-GaAs/n-InGaP junctions in n–p–n core–multishell nanowires, with n-InGaP being the electron-supplying emitter material. Current–voltage characteristics and temperature-dependent electroluminescence measurements substantiate successful doping of the n-InGaP shell. A tunneling-assisted contribution to the leakage currents of the investigated p–n junctions is verified by the sub-bandgap luminescence at low temperatures and is attributed to radiative tunneling processes. © 2019 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/pssb.201900358
  • 2020 • 24 Continuum Damage Mechanics—Modelling and Simulation
    Menzel, A. and Sprave, L.
    Solid Mechanics and its Applications 262 231-256 (2020)
    Continuum damage mechanics elaborates the continuum mechanics-based modelling and simulation of mechanical degradation effects. The objective of this contribution is to briefly review different aspects of continuum damage mechanics of solid continua with a focus on general modelling concepts and application to isotropic as well as anisotropic damage approaches on the one hand, and to discuss possible solution strategies in the context of finite element simulations on the other. In particular, viscous regularisation and gradient-enhanced regularisation—as a reduced form of general non-local theories—are considered. Several numerical examples including ductile damage, i.e. the coupling of damage with plasticity related phenomena, are addressed which show the applicability of the particular modelling and simulation frameworks highlighted. © 2020, Springer Nature Switzerland AG.
    view abstractdoi: 10.1007/978-3-030-31547-4_8
  • 2020 • 23 Moving cracks form white etching areas during rolling contact fatigue in bearings
    Morsdorf, L. and Mayweg, D. and Li, Y. and Diederichs, A. and Raabe, D. and Herbig, M.
    Materials Science and Engineering A 771 (2020)
    White etching cracks (WECs) and the associated white etching areas (WEAs) are responsible for failure of widely spread engineering applications such as bearings and railways. Although the phenomenon is known for more than 100 years, the underlying mechanisms are still a matter of debate. In this work, we thoroughly investigate a 100Cr6 wind turbine gearbox bearing after failure in service operation. Based on our findings from detailed microstructure characterization on multiple length scales we formulate a new consistent explanation for the formation of WEAs during rolling contact fatigue. We propose a mechanism of moving WECs - not only in terms of conventional crack propagation but also as a movement of the crack normal to its plane. During cyclic loading the crack continuously changes its position and leaves behind a severely plastically deformed area consisting of ferritic nano-grains, i.e. the WEAs. The atomic-scale delocalization of the crack plane in a single loading cycle adds up to micron-sized WEAs during repetitive loading/unloading. After the initial formation of a fatigue crack around inclusions, crack face rubbing occurs during compressive loading cycles. This leads to the formation of WEA by local severe plastic deformation. It also leads to partial cohesion of the abutting crack faces and material transport between them. As a result, the WEC opens at a slightly shifted position with respect to its former location during unloading. © 2019 Elsevier B.V.
    view abstractdoi: 10.1016/j.msea.2019.138659
  • 2020 • 22 Confinement of a three-dimensional organic molecule to two dimensions on a surface
    Müller, M. and Henzl, J. and Morgenstern, K.
    Chemical Physics Letters 738 (2020)
    We investigate the adsorption geometry of a three-dimensional organic molecule, anilino-nitro azobenzene, within hydrogen-bonded supramolecular structures on Au(111) by low temperature scanning tunneling microscopy. Therein, three conformational isomers exist, completely planar trans and cis-isomers and a non-planar, but surface adapted cis-isomer. The anilino-end of the molecule is planar for all isomers. In contrast, the nitro-end of the cis-isomer is only planar, if the nitro-end of the molecule forms hydrogen bonds. Our study pinpoints the subtle balance between molecule-substrate and molecule-molecule interaction in adsorption-induced bond-angle distortion that drive partial or full planarization of the molecule. © 2019 Elsevier B.V.
    view abstractdoi: 10.1016/j.cplett.2019.136906
  • 2020 • 21 Influence of the Feed Rate in the Single-Lip Deep Hole Drilling Process on the Surface Integrity of Steel Components
    Nickel, J. and Baak, N. and Walther, F. and Biermann, D.
    Lecture Notes in Mechanical Engineering 198-212 (2020)
    High strength steels like AISI 4140 are commonly used in many technical areas in which the mechanical properties of materials have to meet special requirements, for example, in the case of dynamically loaded parts. In the automotive industry increasing requirements due to lightweight design or energy efficiency lead to increasing demands on the mechanical and dynamic material strength. In response to this development, optimized machining processes are capable of improving the mechanical properties like fatigue performance by influencing the surface integrity of the machined components. In this paper, the influence of the single-lip deep hole drilling process on the surface integrity of quenched and tempered AISI 4140 specimens is analyzed in detail. Under variation of one of the main process parameters, the feed rate, the process output parameters such as cutting forces and the resulting condition of the machined surface and subsurface are determined. In combination with the analysis of the resulting hardness, microstructure and surface conditions of the machined surface, a magnetic Barkhausen noise (MBN) analysis with a custom-built sensor is applied and further developed. With this non-destructive technique, the surface integrity of the bore wall and the fatigue damage over the lifecycle of the part can be analyzed. The correlation of the surface integrity produced by the single-lip deep hole drilling process with the results from the micro-magnetic measurements are used to improve the possibility of predicting a components fatigue performance. © 2020, Springer Nature Singapore Pte Ltd.
    view abstractdoi: 10.1007/978-981-15-0054-1_21
  • 2020 • 20 Random walk methods for Monte Carlo simulations of Brownian diffusion on a sphere
    Novikov, A. and Kuzmin, D. and Ahmadi, O.
    Applied Mathematics and Computation 364 (2020)
    This paper is focused on efficient Monte Carlo simulations of Brownian diffusion effects in particle-based numerical methods for solving transport equations on a sphere (or a circle). Using the heat equation as a model problem, random walks are designed to emulate the action of the Laplace–Beltrami operator without evolving or reconstructing the probability density function. The intensity of perturbations is fitted to the value of the rotary diffusion coefficient in the deterministic model. Simplified forms of Brownian motion generators are derived for rotated reference frames, and several practical approaches to generating random walks on a sphere are discussed. The alternatives considered in this work include projections of Cartesian random walks, as well as polar random walks on the tangential plane. In addition, we explore the possibility of using look-up tables for the exact cumulative probability of perturbations. Numerical studies are performed to assess the practical utility of the methods under investigation. © 2019 Elsevier Inc.
    view abstractdoi: 10.1016/j.amc.2019.124670
  • 2020 • 19 Poling and annealing of piezoelectric Poly(Vinylidene fluoride) micropillar arrays
    Pariy, I.O. and Ivanova, A.A. and Shvartsman, V.V. and Lupascu, D.C. and Sukhorukov, G.B. and Surmeneva, M.A. and Surmenev, R.A.
    Materials Chemistry and Physics 239 (2020)
    This work reports on the effect of calcination and poling processes on the crystalline phase and piezoresponse of poly(vinylidene fluoride) (PVDF) micropillar arrays. PVDF micropillars were prepared by the imprinting method, heated and treated with high-voltage poling. The effect of the treatment conditions on the crystallization behaviour and the piezoelectric properties of the patterned PVDF films was investigated by piezoresponse force microscopy (PFM), Fourier transform infrared spectroscopy (FTIR), and differential scanning calorimetry (DSC). DSC data show that poling of the PVDF micropillars increases the crystallinity of the polymer from 12% to 22.7%. FTIR measurements of PVDF films show that the calcination and poling processes affect the γ to β phase transformation. In the imprinted and annealed samples, the γ phase was predominant (58% and 46%, respectively). For the poled samples, up to 42% of the β phase was found. Piezoelectric measurements using PFM showed that the poled PVDF micropillars possess a much higher piezoelectric coefficient (29 pm/V) compared to the annealed sample (10 pm/V). The piezoresponse of the PVDF micropillar arrays is thus substantially enhanced by poling. © 2019 Elsevier B.V.
    view abstractdoi: 10.1016/j.matchemphys.2019.122035
  • 2020 • 18 An experimental and modeling study on the reactivity of extremely fuel-rich methane/dimethyl ether mixtures
    Porras, S. and Kaczmarek, D. and Herzler, J. and Drost, S. and Werler, M. and Kasper, T. and Fikri, M. and Schießl, R. and Atakan, B. and Schulz, C. and Maas, U.
    Combustion and Flame 212 107-122 (2020)
    Chemical reactions in stoichiometric to fuel-rich methane/dimethyl ether/air mixtures (fuel air equivalence ratio ϕ = 1–20) were investigated by experiment and simulation with the focus on the conversion of methane to chemically more valuable species through partial oxidation. Experimental data from different facilities were measured and collected to provide a large database for developing and validating a reaction mechanism for extended equivalence ratio ranges. Rapid Compression Machine ignition delay times and species profiles were collected in the temperature range between 660 and 1052 K at 10 bar and equivalence ratios of ϕ = 1–15. Ignition delay times and product compositions were measured in a shock tube at temperatures of 630–1500 K, pressures of 20–30 bar and equivalence ratios of ϕ = 2 and 10. Additionally, species concentration profiles were measured in a flow reactor at temperatures between 473 and 973 K, a pressure of 6 bar and equivalence ratios of ϕ = 2, 10, and 20. The extended equivalence ratio range towards extremely fuel-rich mixtures as well as the reaction-enhancing effect of dimethyl ether were studied because of their usefulness for the conversion of methane into chemically valuable species through partial oxidation at these conditions. Since existing reaction models focus only on equivalence ratios in the range of ϕ = 0.3–2.5, an extended chemical kinetics mechanism was developed that also covers extremely fuel-rich conditions of methane/dimethyl ether mixtures. The measured ignition delay times and species concentration profiles were compared with the predictions of the new mechanism, which is shown to predict well the ignition delay time and species concentration evolution measurements presented in this work. Sensitivity and reaction pathway analyses were used to identify the key reactions governing the ignition and oxidation kinetics at extremely fuel-rich conditions. © 2019 The Authors
    view abstractdoi: 10.1016/j.combustflame.2019.09.036
  • 2020 • 17 Ultra-fast measurement circuit for transient space charge limited current in organic semiconductor thin films
    Rojek, K. and Schmechel, R. and Benson, N.
    Measurement Science and Technology 31 (2020)
    The charge carrier mobility is a crucial parameter determining the device performance for numerous different semiconductor applications. Consequently, an accurate measurement of this quantity is crucial. For this purpose, the transient space charge limited current (SCLC) method is commonly applied and is preferable over, for example, Hall or field effect measurements, as the analyzed current direction is in line with typical device architectures. For the transient SCLC method, a voltage step is applied and the transit time of injected charge carriers is determined using displacement currents. Consequently, the difficulty of this method is the use of an adequate RC time constant for the sample charging, as it needs to be much shorter than the transit time. This parameter generally limits the application of transient SCLC strongly, in terms of obtainable charge carrier mobility or minimum required film thickness. Here, we demonstrate a measurement circuit with a low RC time constant, which works in a wide current range (1 ϵA-0.5 A) and thus allows for significant flexibility in terms of minimum film thickness or detectable charge carrier mobility. The circuit is fast enough to measure, for example, charge carrier mobilities of up to 10-4 cm2 Vs-1 for a 76 nm thick 4,4',4"-Tris[phenyl(m-tolyl)amino]triphenylamine (MTDATA) layer, without using limited bridge circuitry. For this purpose, a capacitor coupled fast transistor switch generates a voltage step to avoid voltage oscillations and a fast operational amplifier is used for amplification of the voltage over a variable measurement resistor. We demonstrate the circuit working principle by measuring benchmarked MTDATA diodes and discuss its range of application. © 2019 IOP Publishing Ltd.
    view abstractdoi: 10.1088/1361-6501/ab3b2d
  • 2020 • 16 Dynamics of a cavitation bubble near a solid surface and the induced damage
    Sagar, H.J. and el Moctar, O.
    Journal of Fluids and Structures 92 (2020)
    Numerical and experimental studies of the dynamics of a cavitating bubble near a resilient metal surface were performed. To augment the experimental flow visualizations of a collapsing bubble, numerical simulations were conducted to more thoroughly identify the collapse dynamics and analyze the flow. A bubble collapse was captured using a high-speed camera and back illumination. The metal sample was made of pure aluminum placed near a collapsing cavitation bubble at various distances from the metal surface. Width, depth, and volume of the induced material deformations were measured using an optical microscope and a three-dimensional profilometer and then compared against existing experimental data from the literature. The cavitating bubble's dynamics and the related flow were simulated numerically using the open source finite volume based flow solver CavitatingFOAM. This code solved the Navier–Stokes equations for compressible two-phase flows using an Euler–Euler approach, including the barotropic equations of state. Bubble shapes, collapse times, and obtained damage parameters were compared to experimental observations. Impact velocities, pressures, shear rates, and various flow phenomena were discussed, providing broad insight into bubble dynamics and the induced damage. © 2019 Elsevier Ltd
    view abstractdoi: 10.1016/j.jfluidstructs.2019.102799
  • 2020 • 15 Analysis of strengthening due to grain boundaries and annealing twin boundaries in the CrCoNi medium-entropy alloy
    Schneider, M. and George, E.P. and Manescau, T.J. and Záležák, T. and Hunfeld, J. and Dlouhý, A. and Eggeler, G. and Laplanche, G.
    International Journal of Plasticity 124 155-169 (2020)
    CrCoNi exhibits the best combination of strength and ductility among all the equiatomic single-phase FCC subsets of the CrMnFeCoNi high-entropy alloy. Here, its yield strength was determined in compression as a function of grain size and temperature. Yield strength was also plotted as a function of "crystallite" size, which takes into account both annealing twin boundaries and grain boundaries. The resulting Hall-Petch slopes were straight lines but with different slopes that depend on the number of twin boundaries per grain. Scanning transmission electron microscopy of deformed specimens revealed the formation of dislocation pile-ups at grain and annealing twin boundaries indicating that the latter also act as obstacles to slip and contribute to strength. Using a simple pile-up model, the strengths of the grain and twin boundaries were estimated to lie in the range 900-1250 »MPa. Assuming that they have the same strength, in the case of twin boundaries this strength corresponds roughly to the stress required to constrict Shockley partials, which suggests that dissociated dislocations have to become compact before they can cross the annealing twin boundaries. © 2019 The Authors.
    view abstractdoi: 10.1016/j.ijplas.2019.08.009
  • 2020 • 14 Data compilation on the effect of grain size, temperature, and texture on the strength of a single-phase FCC MnFeNi medium-entropy alloy
    Schneider, M. and Werner, F. and Langenkämper, D. and Reinhart, C. and Laplanche, G.
    Data in Brief 28 (2020)
    This data article presents a compilation of microstructural and mechanical data regarding the ternary single-phase FCC MnFeNi medium-entropy alloy (MEA). For the analysis, interpretation, and comparison of the data to literature values, the reader can refer to the original related research article entitled “Effect of Temperature and Texture on Hall-Petch Strengthening by Grain and Annealing Twin Boundaries in the MnFeNi Medium-Entropy Alloy”, see Schneider et al. (Metals 9, 2019, 84). The microstructural data reported here include: (i) raw backscatter electron (BSE) micrographs (tif-files) obtained using a scanning electron microscope (SEM) for nine different grain sizes with four images for each grain size and (ii) pdf reports and tables shown below presenting the distributions of the grain- (d, accounting for grain boundaries only) and crystallite- (c, which accounts for both grain and annealing twin boundaries) sizes and of the annealing twin thicknesses (t). These datasets may be useful to develop new algorithms for the automated evaluation of microstructural parameters in recrystallized alloys, i.e. with these benchmark data, an algorithm for image analysis could be trained to assess the above mentioned microstructural parameters. This would help to speed up the analysis of microstructures and improve its reliability. Additional tables describing the recrystallized microstructures and texture include the average number of annealing twin boundaries per grain (n), and the average Taylor factors (M). Raeisinia et al. (Model. Simul. Mater. Sc. 16, 2008, 025001) recently used a viscoplastic model to show that differences in the distribution of microstructural parameters affect the Hall-Petch parameters, but no attempt has been carried out so far to experimentally investigate this possibility since grain size distributions are rarely reported. Here, our benchmark data (e.g. distribution in grain/crystallite sizes, annealing twins per grain, distribution of annealing twin thicknesses) could be used to address these issues. The data describing the mechanical properties reported here are excel-sheets of raw stress-strain curves for temperatures ranging from 77 K to 873 K and different grain sizes. The yield stress (σ0.2%) and the normalized Hall-Petch parameters (σ0/G and ky/Gb2) are given for all temperatures. The normalized Hall-Petch parameters are reported here since they allow to better compare the strength and the magnitude of grain boundary strengthening of different alloys with the same crystallographic structure, see Cordero et al. (Int. Mater. Rev. 61, 2016, 495–512). Moreover, the Hall-Petch parameters as well as the mechanical data reported here could be used for data mining and implemented in programs used for alloy design. © 2019 The Author(s)
    view abstractdoi: 10.1016/j.dib.2019.104807
  • 2020 • 13 Exergoeconomic analysis of an HCCI engine polygeneration process
    Schröder, D. and Hegner, R. and Güngör, A. and Atakan, B.
    Energy Conversion and Management 203 (2020)
    A polygeneration system producing hydrogen, electricity, process steam, and heating water is modeled and studied by conducting an exergoeconomic analysis. This system includes a homogeneous charge compression ignition (HCCI) engine burning a rich methane-air mixture, a water-gas shift reactor (WGSR) and a palladium membrane for hydrogen separation. Different cost-apportioning methods were considered in the present work in order to assess their suitability for the studied system. Furthermore, a global sensitivity analysis was used to identify the relevant system parameters as well as to quantify the influence that the input data uncertainty causes on the costs of the system products. It is shown that these costs are sensitive to the investment costs of only few system components and that the highest exergy destruction rates and costs occur in the engine. With the predicted cost of hydrogen ranging from 3.23 to 3.99 €/kg at EGR ratios of up to 25%, the studied process is promising. © 2019 Elsevier Ltd
    view abstractdoi: 10.1016/j.enconman.2019.112085
  • 2020 • 12 A note on non-homogeneous deformations with homogeneous Cauchy stress for a strictly rank-one convex energy in isotropic hyperelasticity
    Schweickert, E. and Mihai, L.A. and Martin, R.J. and Neff, P.
    International Journal of Non-Linear Mechanics 119 (2020)
    It has recently been shown that for a Cauchy stress response induced by a strictly rank-one convex hyperelastic energy potential, a homogeneous Cauchy stress tensor field cannot correspond to a non-homogeneous deformation if the deformation gradient has discrete values, i.e. if the deformation is piecewise affine linear and satisfies the Hadamard jump condition. In this note, we expand upon these results and show that they do not hold for arbitrary deformations by explicitly giving an example of a strictly rank-one convex energy and a non-homogeneous deformation such that the induced Cauchy stress tensor is constant. In the planar case, our example is related to another previous result concerning criteria for generalized convexity properties of conformally invariant energy functions, which we extend to the case of strict rank-one convexity. © 2019 Elsevier Ltd
    view abstractdoi: 10.1016/j.ijnonlinmec.2019.103282
  • 2020 • 11 Atomic scale configuration of planar defects in the Nb-rich C14 Laves phase NbFe2
    Šlapáková, M. and Zendegani, A. and Liebscher, C.H. and Hickel, T. and Neugebauer, J. and Hammerschmidt, T. and Ormeci, A. and Grin, J. and Dehm, G. and Kumar, K.S. and Stein, F.
    Acta Materialia 183 362-376 (2020)
    Laves phases belong to the group of tetrahedrally close-packed intermetallic phases, and their crystal structure can be described by discrete layer arrangements. They often possess extended homogeneity ranges and the general notion is that deviations from stoichiometry are accommodated by anti-site atoms or vacancies. The present work shows that excess Nb atoms in a Nb-rich NbFe2 C14 Laves phase can also be incorporated in various types of planar defects. Aberration-corrected scanning transmission electron microscopy and density functional theory calculations are employed to characterize the atomic configuration of these defects and to establish stability criteria for them. The planar defects can be categorized as extended or confined ones. The extended defects lie parallel to the basal plane of the surrounding C14 Laves phase and are fully coherent. They contain the characteristic Zr4Al3-type (O) units found in the neighboring Nb6Fe7 µ phase. An analysis of the chemical bonding reveals that the local reduction of the charge transfer is a possible reason for the preference of this atomic arrangement. However, the overall layer stacking deviates from that of the perfect µ phase. The ab initio calculations establish why these exceptionally layered defects can be more stable configurations than coherent nano-precipitates of the perfect µ phase. The confined defects are observed with pyramidal and basal habit planes. The pyramidal defect is only ~1 nm thick and resembles the perfect µ phase. In contrast, the confined basal defect can be regarded as only one single O unit and it appears as if the stacking sequence is disrupted. This configuration is confirmed by ab initio calculations to be metastable. © 2019
    view abstractdoi: 10.1016/j.actamat.2019.11.004
  • 2020 • 10 Evaluation of spray impact on a sphere with a two-fluid nozzle
    Strob, R. and Babaria, T. and Rodeck, M. and Schaldach, G. and Walzel, P. and Thommes, M.
    Journal of Aerosol Science 140 (2020)
    The generation of a secondary aerosol after impact, consisting of smaller droplets at a given velocity and mass flow, is relevant for various applications. Thus far, the investigations and modelling approaches on spray impact are based on extrapolation of the single-droplet impingement or empirical correlations. The validity of the models presented is limited to the given experimental setup and conditions such as initial droplet size, velocity and the impact surface characteristics. The aim of this work was to empirically evaluate the spray impact of a two-fluid nozzle on a sphere. A small-scale nozzle was used, which produced a primary aerosol with a mass median diameter of about 12μm (liquid-to-gas mass flow ratio = 1, gas pressure: ΔpG = 5 bar). After impact on a sphere, a multimodal distribution was observed and a higher mass flowrate of droplets in the small micrometer range (2 and 3μm) was produced for a liquid mass flow rate in the range of 1.2–6 kg/h and an atomizing gas mass flow rate of 1–4 kg/h. For easier observation, a geometrically similar, larger nozzle was used, which produced an aerosol with a mass median diameter of about 80μm (liquid-to-gas mass flow ratio = 4, gas pressure: ΔpG = 1 bar). The measured droplet size after impact is smaller for a lower liquid-to-gas mass flow ratio and increased atomizing gas inlet pressure. Droplet formation mechanisms such as splashing, crown formation and spreading on the sphere surface were observed. A characteristic film with large variations in thickness was generated. © 2019 Elsevier Ltd
    view abstractdoi: 10.1016/j.jaerosci.2019.105483
  • 2020 • 9 Dependence of hydrogen embrittlement mechanisms on microstructure-driven hydrogen distribution in medium Mn steels
    Sun, B. and Krieger, W. and Rohwerder, M. and Ponge, D. and Raabe, D.
    Acta Materialia 183 313-328 (2020)
    The risk of hydrogen embrittlement (HE) is currently one important factor impeding the use of medium Mn steels. However, knowledge about HE in these materials is sparse. Their multiphase microstructure with highly variable phase conditions (e.g. fraction, percolation and dislocation density) and the feature of deformation-driven phase transformation render systematic studies of HE mechanisms challenging. Here we investigate two austenite-ferrite medium Mn steel samples with very different phase characteristics. The first one has a ferritic matrix (~74 vol.% ferrite) with embedded austenite and a high dislocation density (~1014 m−2) in ferrite. The second one has a well recrystallized microstructure consisting of an austenitic matrix (~59 vol.% austenite) and embedded ferrite. We observe that the two types of microstructures show very different response to HE, due to fundamental differences between the HE micromechanisms acting in them. The influence of H in the first type of microstructure is explained by the H-enhanced local plastic flow in ferrite and the resulting increased strain incompatibility between ferrite and the adjacent phase mixture of austenite and strain-induced α'-martensite. In the second type of microstructure, the dominant role of H lies in its decohesion effect on phase and grain boundaries, due to the initially trapped H at the interfaces and subsequent H migration driven by deformation-induced austenite-to-martensite transformation. The fundamental change in the prevalent HE mechanisms between these two microstructures is related to the spatial distribution of H within them. This observation provides significant insights for future microstructural design towards higher HE resistance of high-strength steels. © 2019
    view abstractdoi: 10.1016/j.actamat.2019.11.029
  • 2020 • 8 Effects of acetylene flow rate and bias voltage on the structural and tribo-mechanical properties of sputtered a-C:H films
    Tillmann, W. and Ulitzka, H. and Lopes Dias, N.F. and Stangier, D. and Thomann, C.A. and Moldenhauer, H. and Debus, J.
    Thin Solid Films 693 (2020)
    The properties of sputtered a-C:H films are significantly influenced by the C2H2 flow rate and bias voltage. A suitable Design of Experiments allows to consider their effects on the mechanical and tribological properties. The a-C:H films are deposited by varying the C2H2 flow rate from 5.9 to 34.1 sccm and the bias voltage from −83 to −197 V, following the Central Composite Design. In Raman scattering studies, the presence of C[sbnd]H bands with increasing C2H2 flow rate is identified. Additionally, a decrease of the I(D)/I(G) ratio is observed with increasing C2H2 flow rate. Both observations indicate the formation of sp³-hybridized C[sbnd]H bonds. In contrast, a low C2H2 flow rate and a high bias voltage result in a higher I(D)/I(G) ratio and a lower intensity of the C[sbnd]H stretching bands, indicating a lower amount of C[sbnd]H bonds. The mechanical properties are also considerably influenced by these parameters. A higher C2H2 proportion results in a lower hardness and elastic modulus, which are related to a higher H content. However, a higher bias voltage increases the hardness and elastic modulus due to densification mechanisms, which increase the degree of distortion of the a-C:H films. Consequently, a low C2H2 flow rate and a high bias voltage ensure a high hardness of up to ~24 GPa due to a lower amount of C[sbnd]H bonds and a higher degree of distortion. In tribometer tests, most a-C:H films exhibit a low coefficient of friction against steel, ranging from 0.23 to 0.25. All a-C:H films are marked by a deformative wear, indicating a high resistance against abrasive wear when sliding against steel. © 2019 Elsevier B.V.
    view abstractdoi: 10.1016/j.tsf.2019.137691
  • 2020 • 7 Performance of YSZ and Gd2Zr2O7/YSZ double layer thermal barrier coatings in burner rig tests
    Vaßen, R. and Bakan, E. and Mack, D. and Schwartz-Lückge, S. and Sebold, D. and Jung Sohn, Y. and Zhou, D. and Guillon, O.
    Journal of the European Ceramic Society 40 480-490 (2020)
    Double layer thermal barrier coatings (TBCs) consisting of a Gd2Zr2O7 (GZO) top and an ytrria stabilized zirconia (YSZ) interlayer have been tested in a burner rig facility and the results compared to the ones of conventional YSZ single layers. In order to gain insight in the high temperature capability of the alternative TBC material, high surface temperatures of up to 1550 °C have been chosen while keeping the bond coat temperature similar. It turned out that the performance of all systems is largely depending on the microstructure of the coatings especially reduced porosity levels of GZO being detrimental. In addition, it was more difficult in GZO than in YSZ coatings to obtain highly porous and still properly bonded microstructures. Another finding was the reduced lifetime with increasing surface temperatures, the amount of reduction is depending on the investigated system. The reasons for this behavior are analyzed and discussed in detail. © 2019 Elsevier Ltd
    view abstractdoi: 10.1016/j.jeurceramsoc.2019.10.021
  • 2020 • 6 Effects on tool performance of cutting edge prepared by pressurized air wet abrasive jet machining (PAWAJM)
    Wang, W. and Biermann, D. and Aßmuth, R. and Arif, A.F.M. and Veldhuis, S.C.
    Journal of Materials Processing Technology 277 (2020)
    Edge preparation techniques are used to shape a proper edge microgeometry for enhanced tool performance. However, depending on the edge preparation method, the edge properties are also altered. Most of the reported work is limited to the effect of edge micro-geometry. In this paper, cutting edges prepared by pressurized air wet abrasive jet machining (PAWAJM) were evaluated from several aspects including tool edge geometries, tool surface quality and topographies, edge hardness (H) and residual stresses. Furthermore, the influence of the prepared edge on the tool performance of uncoated tungsten carbide cutting inserts with different average cutting edge rounding (S¯) as well as different form factor (K) were experimentally investigated through the orthogonal turning of AISI 4140 alloy steel. Results show that the performance of the prepared cutting edge depends on the combined effect of the initial state of edge geometry and edge properties For symmetric edges (form factor K = 1), the optimum range for average cutting edge rounding (S¯) was found to be 20 μm to 30 μm when using cemented carbide tools for dry machining AISI 4140 steel at a feed rate (f) =0.1 mm/rev, width of cut = 3 mm, and cutting speed (vc) = 300 m/min. © 2019 Elsevier B.V.
    view abstractdoi: 10.1016/j.jmatprotec.2019.116456
  • 2020 • 5 Corrosion behavior of NiTi alloy subjected to femtosecond laser shock peening without protective coating in air environment
    Wang, H. and Jürgensen, J. and Decker, P. and Hu, Z. and Yan, K. and Gurevich, E.L. and Ostendorf, A.
    Applied Surface Science 501 (2020)
    Laser shock peening with femtosecond laser was used to improve the corrosion resistance of biomedical NiTi alloy without protective coating in the air environment. The energy dispersive X-ray analysis (EDX) and X-ray diffraction (XRD) based analysis showed that the laser ablation could produce titanium oxide layer and femtosecond laser shock peening (FsLSP) can generate residual stress in the surface layer of NiTi alloy. The FsLSP improved the corrosion resistance of NiTi in 3.5% NaCl solution and Hank's solution and also prevented the formation of corrosion cracks and pits during corrosion testing. The reasons for the improvement of corrosion behavior may be the generation of residual stress and titanium oxide film during the laser surface treatment. © 2019 Elsevier B.V.
    view abstractdoi: 10.1016/j.apsusc.2019.144338
  • 2020 • 4 Towards an understanding of grain boundary step in diamond cutting of polycrystalline copper
    Wang, Z. and Zhang, J. and Zhang, J. and Li, G. and Zhang, H. and ul Hassan, H. and Hartmaier, A. and Yan, Y. and Sun, T.
    Journal of Materials Processing Technology 276 (2020)
    Microstructural deformation at the grain level has an inherent impact on the achievable ultimate machining accuracy of polycrystalline materials. In the present work, numerical simulations and experiments of diamond cutting of polycrystalline copper are carried out to investigate the formation of surface step at grain boundaries on machined surface. Single crystal diamond cutting tool with straight cutting edge is chosen for experiments to mimic the tool geometry utilized in 2D crystal plasticity finite element simulations. Moreover, the same crystallography configuration of bi-crystal Cu is employed between experiments and simulations. Formation mechanisms of surface steps at grain boundaries are revealed by finite element simulations and corresponding experimental validation, as well as cross-sectional transmission electron microscope characterization. Finally, finite element simulations of orthogonal cutting of bi-crystal Cu are carried out to examine effects of both extrinsic cutting edge radius of diamond cutting tool and intrinsic misorientation angle of grain boundary on the propensity of grain boundary surface step formation. The present work provides theoretical guidelines on the strategy of suppressing grain boundary surface step formation for achieving superior surface finish of polycrystalline materials by diamond cutting. © 2019 Elsevier B.V.
    view abstractdoi: 10.1016/j.jmatprotec.2019.116400
  • 2020 • 3 Crystal structure evolution of complex metal aluminum hydrides upon hydrogen release
    Weidenthaler, C.
    Journal of Energy Chemistry 42 133-143 (2020)
    Complex aluminum hydrides have been widely studied as potential hydrogen storage materials but also, for some time now, for electrochemical applications. This review summarizes the crystal structures of alkali and alkaline earth aluminum hydrides and correlates structure properties with physical and chemical properties of the hydride compounds. The crystal structures of the alkali metal aluminum hydrides change significantly during the stepwise dehydrogenation. The general pathway follows a transformation of structures built of isolated [AlH4]− tetrahedra to structures built of isolated [AlH6]3− octahedra. The crystal structure relations in the group of alkaline earth metal aluminum hydrides are much more complicated than those of the alkali metal aluminum hydrides. The structures of the alkaline earth metal aluminum hydrides consist of isolated tetrahedra but the intermediate structures exhibit chains of corner-shared octahedra. The coordination numbers within the alkali metal group increase with cation sizes which goes along with an increase of the decomposition temperatures of the primary hydrides. Alkaline earth metal hydrides have higher coordination numbers but decompose at slightly lower temperatures than their alkali metal counterparts. The decomposition pathways of alkaline metal aluminum hydrides have not been studied in all cases and require future research. © 2019 Science Press and Dalian Institute of Chemical Physics, Chinese Academy of Sciences
    view abstractdoi: 10.1016/j.jechem.2019.05.026
  • 2020 • 2 Tribological studies on multi-coated forming tools
    Weikert, T. and Tremmel, S. and Stangier, D. and Tillmann, W. and Krebs, E. and Biermann, D.
    Journal of Manufacturing Processes 49 141-152 (2020)
    Processes of Sheet-Bulk Metal Forming combine operations of both sheet and bulk metal forming, enabling the production of closely-toleranced functional components. Locally differing friction conditions on a single tool surface are crucial to control the material flow and to reduce process forces and tool wear. In order to understand how different friction conditions on a multi-coated tool surface affect the material flow during forming operations, variants of amorphous carbon coatings (a-C:H:W) and Cr-based hard coatings (CrAlN, CrAlCN) were deposited in combinations side by side on punches of ring compression tests. Evaluating the rings’ shapes revealed the coatings’ effects on material flow in terms of ring inner contours with sections of varying curvatures. Characteristic friction mechanisms were observed, which allowed a local delimitation of friction conditions by selectively applied coatings. © 2019 The Society of Manufacturing Engineers
    view abstractdoi: 10.1016/j.jmapro.2019.11.021
  • 2020 • 1 Self-assembled nano-silicon/graphite hybrid embedded in a conductive polyaniline matrix for the performance enhancement of industrial applicable lithium-ion battery anodes
    Wiggers, H. and Sehlleier, Y.H. and Kunze, F. and Xiao, L. and Schnurre, S.M. and Schulz, C.
    Solid State Ionics 344 (2020)
    Nano-structured silicon-based composite materials have generated significant excitement for use as anode materials in high-performance Li-ion batteries. For making these materials commercially applicable, a high Coulombic efficiency at the first cycle must be achieved. Additionally, scalable synthesis routes need to be developed to provide access to practically-relevant material quantities. In this work, we propose a strategy for the production of Si/graphite/polyaniline (Si/graphite/PANI) composites that addresses both above mentioned challenges. Si nanoparticles were produced in a pilot-plant-scale microwave-plasma reactor using monosilane (SiH4) as precursor. This process enables the formation of high-purity Si nanoparticles with controllable crystal sizes at a production rate of 45 g/h. Si/graphite hybrids are fabricated through self-assembly by electrostatic attraction. The Si/graphite/PANI nanocomposite is then prepared by in situ polymerization of aniline monomer in the presence of the Si/graphite hybrid. With this approach, ~40 g of Si/graphite/PANI composite per batch can be produced at lab scale. The scalability of the underlying processes enables the use for commercial products. The nanocomposite shows favorable characteristics inherited from its three components: Si nanoparticles provide high capacity, graphite acts as an electrical conductor and gives a high Coulombic efficiency, and the polyaniline coating further enhances the electrical conductivity and protects the entire structure. A very good Coulombic efficiency of 86.2% at the initial cycle is recorded for this nanocomposite material. Galvanostatic charge/discharge tests demonstrate that this material can deliver a discharge capacity of 2000 mAh/g with a very good capacity retention of 76% after 500 cycles at a discharge rate of 0.5C (1.25 A/g). The capacity is 870 mAh/g measured at 5C (12.5 A/g). © 2019 Elsevier B.V.
    view abstractdoi: 10.1016/j.ssi.2019.115117