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 • 260 CHAPTER 6: Molecular Tweezers and Clips that Modify Protein Function
    Alavijeh, N.S. and Kirupakaran, A. and Klärner, F.-G. and Schrader, T.
    Monographs in Supramolecular Chemistry 2021-January 161-198 (2021)
    Protein recognition by designed protein ligands is highly challenging, but bears great opportunities. Supramolecular chemists have recently been able to synthesize tailored ligands with remarkable protein recognition properties which are absent in the natural binding partners, and which lead to synergistic effects, positive cooperativity and exquisite selectivity. Thus, the combination of powerful charged interactions with hydrophobic forces has recently led to new prototypes of protein surface binders. This review summarizes the development of molecular tweezers (part 1) and clips (part 2) as unique tools for protein recognition. The parts begin with molecular tweezers for basic amino acid inclusion and the discovery of diphosphate clips for efficient cofactor binding, respectively. Gratifyingly, both host molecules complement each other due to their different molecular shapes. Molecular tweezers will be presented first in their interaction with amino acids and small disordered peptides, where they generally complex each Lys and Arg; then the review will proceed to tweezer complexation with protein surfaces, elucidating the preference for well-accessible basic residues and various examples of protein targeting and interference with protein-protein interactions. Finally, we discuss the advantages of additional recognition elements on the tweezer skeleton, which opens the door to numerous advanced applications in chemical biology and drug discovery. For the clips, we describe in detail the inclusion of two important cationic cofactors, followed by applications on cofactor-mediated enzymatic processes. © The Royal Society of Chemistry 2021.
    view abstractdoi: 10.1039/9781788019798-00161
  • 2021 • 259 Femtosecond pulse generation from external cavity diode laser based on self-mode-locking
    Alloush, M.A. and Brenner, C. and Calò, C. and Hofmann, M.R.
    Optics letters 46 344-347 (2021)
    In this Letter, we report optical pulse generation from a single-section diode gain chip, employed in an external cavity geometry based on the self-mode-locking regime. The gain chip emits light at 1550 nm wavelength range. The external cavity is operated at various repetition rates, ranging from 1 to 2.5 GHz. An optical pulse width of approximately 650 fs is obtained by fitting a Lorentzian distribution. A low RF spectral width of 78.875 kHz is measured corresponding to a low pulse-to-pulse RMS timing jitter of 1.273 ps. This system paves the way towards ultra-compact, cost-effective, and chirp-compensated femtosecond laser pulse sources with adjustable repetition rates.
    view abstractdoi: 10.1364/OL.415336
  • 2021 • 258 Mechanochemical Synthesis of Catalytic Materials
    Amrute, A.P. and De Bellis, J. and Felderhoff, M. and Schüth, F.
    Chemistry - A European Journal (2021)
    The mechanochemical synthesis of nanomaterials for catalytic applications is a growing research field due to its simplicity, scalability, and eco-friendliness. Besides, it provides materials with distinct features, such as nanocrystallinity, high defect concentration, and close interaction of the components in a system, which are, in most cases, unattainable by conventional routes. Consequently, this research field has recently become highly popular, particularly for the preparation of catalytic materials for various applications, ranging from chemical production over energy conversion catalysis to environmental protection. In this Review, recent studies on mechanochemistry for the synthesis of catalytic materials are discussed. Emphasis is placed on the straightforwardness of the mechanochemical route—in contrast to more conventional synthesis—in fabricating the materials, which otherwise often require harsh conditions. Distinct material properties achieved by mechanochemistry are related to their improved catalytic performance. © 2021 The Authors. Chemistry - A European Journal published by Wiley-VCH GmbH
    view abstractdoi: 10.1002/chem.202004583
  • 2021 • 257 Electrocatalysis in confined space
    Andronescu, C. and Masa, J. and Tilley, R.D. and Gooding, J.J. and Schuhmann, W.
    Current Opinion in Electrochemistry 25 (2021)
    The complex interplay of restricted mass transport leading to local accumulation or depletion of educts, intermediates, products, counterions and co-ions influences the reactions at the active sites of electrocatalysts when electrodes are rough, three-dimensionally mesoporous or nanoporous. This influence is important with regard to activity, and even more to selectivity, of electrocatalytic reactions. The underlying principles are discussed based on the growing awareness of these considerations over recent years. © 2020 Elsevier B.V.
    view abstractdoi: 10.1016/j.coelec.2020.100644
  • 2021 • 256 Spray-Flame Synthesis of LaMnO3+δNanoparticles for Selective CO Oxidation (SELOX)
    Angel, S. and Tapia, J.D. and Gallego, J. and Hagemann, U. and Wiggers, H.
    Energy and Fuels (2021)
    LaMnO3+δ nanoperovskites were prepared via the continuous and scalable spray-flame synthesis (SFS) technique from metal nitrate-based solutions by using either ethanol (EtOH) as solvent or a mixture of ethanol (50 vol %) and 2-ethylhexanoic acid (50 vol %) (EtOH/2-EHA). Solutions based on pure EtOH generated a mixture of several phases and a broad and multimodal particle size distribution, which is attributed to a combination of gas-to-particle and droplet-to particle formation of particles. The product contained a bimodal distribution of the orthorhombic (Pnma II) LaMnO3 perovskite-like phase and additional, unwanted phases such as La2O3 and sub-20 nm Mn-rich amorphous/poorly crystalline particles. The incorporation of 2-EHA led to high surface area (>100 m2 g-1), small, and crystalline LaMnO3+δ nanoparticles with sizes ranging between 4 and 15 nm in the presence of few sub-200 nm particles (<10 wt %). This sample is mainly composed of the orthorhombic Mn4+ rich (Pnma I) LaMnO3+δ phase, and it counts with a very high specific surface area that makes it highly promising for catalytic applications. FTIR and UV-VIS spectroscopy of the precursor solutions revealed the oxidation of the Mn2+ precursor in advance of the particle formation process along with the esterification of the solvent mixture. It is assumed that the observed liquid-phase oxidation supports the formation of Mn4+-rich perovskites. According to O2-TPD and H2-TPR measurements, the EtOH/2-EHA sample presented a much higher formation of adsorbed active oxygen species and higher reducibility than the EtOH-made material, leading to a superior performance for both the catalytic oxidation of CO and the selective oxidation (SELOX) of CO. © 2021 American Chemical Society.
    view abstractdoi: 10.1021/acs.energyfuels.0c03659
  • 2021 • 255 Nucleation and growth of α phase in a metastable β-Titanium Ti-5Al-5Mo-5V-3Cr alloy: Influence from the nano-scale, ordered-orthorhombic O″ phase and α compositional evolution
    Antonov, S. and Shi, R. and Li, D. and Kloenne, Z. and Zheng, Y. and Fraser, H.L. and Raabe, D. and Gault, B.
    Scripta Materialia 194 (2021)
    We investigated the nucleation and growth of α precipitates in the presence of nano-scale, titanium-rich, ordered-orthorhombic O″ precipitates formed during heating at 5 °C/min to 400 °C in a metastable β-Ti alloy, Ti-5Al-5Mo-5V-3Cr. The Oʺ precipitates are found to assist α nucleation by serving as preferential nucleation sites. In the very initial stages of its precipitation, the α phase is found to have formed via a coupled diffusional-displacive mode as titanium-rich plates. At this stage, aluminum partitions equally between the β matrix and the growing α precipitates, but slowly diffuses up-hill into the α phase during isothermal aging at 400 °C for 15 h. The interplay between such pre-formed metastable phases and α can be exploited to tailor the microstructure, by refining the α distribution, and consequently improve the mechanical properties of β-Ti alloys. Our work paves the way for exploiting this cascade of metastable phases for further microstructural engineering of β-Ti alloys. © 2020
    view abstractdoi: 10.1016/j.scriptamat.2020.113672
  • 2021 • 254 Coherent Spin-Photon Interface with Waveguide Induced Cycling Transitions
    Appel, M.H. and Tiranov, A. and Javadi, A. and Löbl, M.C. and Wang, Y. and Scholz, S. and Wieck, A.D. and Ludwig, Ar. and Warburton, R.J. and Lodahl, P.
    Physical Review Letters 126 (2021)
    Solid-state quantum dots are promising candidates for efficient light-matter interfaces connecting internal spin degrees of freedom to the states of emitted photons. However, selection rules prevent the combination of efficient spin control and optical cyclicity in this platform. By utilizing a photonic crystal waveguide we here experimentally demonstrate optical cyclicity up to ≈15 through photonic state engineering while achieving high fidelity spin initialization and coherent optical spin control. These capabilities pave the way towards scalable multiphoton entanglement generation and on-chip spin-photon gates. © 2021 American Physical Society. All rights reserved.
    view abstractdoi: 10.1103/PhysRevLett.126.013602
  • 2021 • 253 Model order reduction for deformable porous materials in thin domains via asymptotic analysis
    Armiti-Juber, A. and Ricken, T.
    Archive of Applied Mechanics (2021)
    We study fluid-saturated porous materials that undergo poro-elastic deformations in thin domains. The mechanics in such materials are described using a biphasic model based on the theory of porous media (TPM) and consisting of a system of differential equations for material’s displacement and fluid’s pressure. These equations are in general strongly coupled and nonlinear, such that exact solutions are hard to obtain and numerical solutions are computationally expensive. This paper reduces the complexity of the biphasic model in thin domains with a scale separation between domain’s width and length. Based on standard asymptotic analysis, we derive a reduced model that combines two sub-models. Firstly, a limit model consists of averaged equations that describe the fluid pore pressure and displacement in the longitudinal direction of the domain. Secondly, a corrector model re-captures the mechanics in the transverse direction. The validity of the reduced model is finally tested using a set of numerical examples. These demonstrate the computational efficiency of the reduced model, while maintaining reliable solutions in comparison with original biphasic TPM model in thin domain. © 2021, The Author(s).
    view abstractdoi: 10.1007/s00419-021-01907-3
  • 2021 • 252 Water influence on the uniaxial tensile behavior of polytetrafluoroethylene-coated glass fiber fabric
    Asadi, H. and Uhlemann, J. and Stranghoener, N. and Ulbricht, M.
    Materials 14 1-23 (2021)
    Polytetrafluoroethylene (PTFE)-coated glass fiber fabrics are used for long-lasting membrane structures due to their outstanding mechanical properties, chemical stabilities, and satisfying service life. During their operation time, different environmental impacts might influence their per-formance, especially regarding the mechanical properties. In this contribution, the impact of water on the tensile strength deterioration was assessed experimentally, providing evidence of consider-able but partially reversible loss of strength by up to 20% among the various types of investigated industrially established fabrics. © 2021 by the authors. Licensee MDPI, Basel, Switzerland.
    view abstractdoi: 10.3390/ma14040846
  • 2021 • 251 Artificial weathering mechanisms of uncoated structural polyethylene terephthalate fabrics with focus on tensile strength degradation
    Asadi, H. and Uhlemann, J. and Stranghoener, N. and Ulbricht, M.
    Materials 14 1-24 (2021)
    In the past five decades, reinforced coated textile membranes have been used increasingly as building materials, which are environmentally exposed. Thus, their weathering degradation over the service life must be taken into account in design, fabrication, and construction. Regarding such structural membranes, PVC (polyvinylchloride)-coated PET (polyethylene terephthalate) fabric is one of the most common commercially available types. This paper focuses on the backbone of it, i.e., the woven PET fabric. Herein, weathering of uncoated PET, as the load-bearing component of the composite PET-PVC, was studied. This study assessed the uniaxial tensile strength degradation mechanisms of uncoated PET fabric during artificial accelerated weathering tests. For this purpose, exploratory data analysis was carried out to analyze the chemical and physical changes which were traced by Fourier transform infrared spectroscopy and molecular weight measurements. Finally, with the help of degradation mechanisms determined from the aforementioned evaluations, a degradation pathway network model was constructed. With that, the relationship between applied stress, mecha-nistic variables, structural changes, and performance level responses (tensile strength degradation) was assessed. © 2021 by the authors. Licensee MDPI, Basel, Switzerland.
    view abstractdoi: 10.3390/ma14030618
  • 2021 • 250 Laser- and Ion-Induced Defect Engineering in WS2 Monolayers
    Asaithambi, A. and Kozubek, R. and Prinz, G.M. and Reale, F. and Pollmann, E. and Ney, M. and Mattevi, C. and Schleberger, M. and Lorke, A.
    Physica Status Solidi - Rapid Research Letters 15 (2021)
    Tungsten disulfide is one of the prominent transition metal dichalcogenide materials, which shows a transition from an indirect to a direct bandgap as the layer thickness is reduced down to a monolayer. To use (Formula presented.) monolayers in devices, detailed knowledge about the luminescence properties regarding not only the excitonic but also the defect-induced contributions is needed. Herein, (Formula presented.) monolayers are irradiated with (Formula presented.) ions with different fluences to create different defect densities. Apart from the excitonic contributions, two additional emission bands are observed at low temperatures. These bands can be reduced or even suppressed, if the flakes are exposed to laser light with powers up to 1.5 mW. Increasing the temperature up to room temperature leads to recovery of this emission, so that the luminescence properties can be modified using laser excitation and temperature. The defect bands emerging after ion irradiation are attributed to vacancy defects together with physisorbed adsorbates at different defect sites. © 2020 The Authors. Physica Status Solidi (RRL) – Rapid Research Letters published by Wiley-VCH GmbH
    view abstractdoi: 10.1002/pssr.202000466
  • 2021 • 249 Synthesis, sintering, and effect of surface roughness on oxidation of submicron Ti2AlC ceramics
    Badie, S. and Dash, A. and Sohn, Y.J. and Vaßen, R. and Guillon, O. and Gonzalez-Julian, J.
    Journal of the American Ceramic Society 104 1669-1688 (2021)
    Submicron Ti2AlC MAX phase powder was synthesized by molten salt shielded synthesis (MS3) using a Ti:Al:C molar ratio of 2:1:0.9 at a process temperature of 1000°C for 5 hours. The synthesized powder presented a mean particle size of ~0.9 µm and a purity of 91 wt. % Ti2AlC, containing 6 wt. % Ti3AlC2. The Ti2AlC powder was sintered by pressureless sintering, achieving a maximal relative density of 90%, hence field-assisted sintering technology/spark plasma sintering was used to enhance densification. The fine-grained microstructure was preserved, and phase purity of Ti2AlC was unaltered in the latter case, with a relative density of 98.5%. Oxidation was performed at 1200°C for 50 hours in static air of dense monolithic Ti2AlC with different surface finish, (polished, ground and sandblasted) which resulted in the formation of an approx. 8 µm thin aluminum oxide (Al2O3) layer decorated with titanium dioxide (rutile, TiO2) colonies. Surface quality had no influence on Al2O3 scale thickness, but the amount and size of TiO2 crystals increased with surface roughness. A phenomenon of rumpling of the thermally grown oxide (TGO) was observed and a model to estimate the extent of deformation is proposed. © 2020 The Authors. Journal of the American Ceramic Society published by Wiley Periodicals LLC on behalf of American
    view abstractdoi: 10.1111/jace.17582
  • 2021 • 248 Application of artificial neural networks for active roll control based on actor-critic reinforcement learning
    Bahr, M. and Reicherts, S. and Sieberg, P. and Morss, L. and Schramm, D.
    Advances in Intelligent Systems and Computing 1260 AISC 61-82 (2021)
    This work shows the application of artificial neural networks for the control task of the roll angle in passenger cars. The training of the artificial neural network is based on the specific actor-critic reinforcement learning training algorithm. It is implemented and trained utilizing the Python API for TensorFlow and set up in a co-simulation with the vehicle simulation realized in IPG CarMaker via MATLAB/Simulink to enable online learning. Subsequently it is validated in different representative driving maneuvers. For showing the practicability of the resulting neural controller it is also validated for different vehicle classes with respect to their corresponding structure, geometries and components. An analytical approach to adjust the resulting controller to various vehicle bodies dependent on physical correlations is presented. © Springer Nature Switzerland AG 2021.
    view abstractdoi: 10.1007/978-3-030-55867-3_4
  • 2021 • 247 Visible light-induced controlled surface grafting polymerization of hydroxyethyl methacrylate from isopropylthioxanthone semipinacol-terminated organic monolayers
    Balasubramaniam, A. and Manderfeld, E. and Krause, L.M.K. and Wanka, R. and Schwarze, J. and Beyer, C.D. and Rosenhahn, A.
    Polymer Chemistry 12 618-628 (2021)
    A visible light-induced living polymerization of a hydrophilic model monomer was initiated on organic silane monolayers using isopropylthioxanthone (ITX). The type II photoinitiator ITX was covalently introduced to the octadecyltrichlorosilane monolayers by UV-induced (254 nm) hydrogen abstraction and a subsequent coupling step through recombination. The resulting dormant isopropylthioxanthone semi-pinacol (ITXSP) groups can be reactivated by irradiation with visible light to initiate a controlled surface grafting polymerization. Using this surface-initiated polymerization approach, hydroxyethyl methacrylate (HEMA) was polymerized under visible light irradiation (385 nm) at room temperature. The polymer layer thickness depends linearly on the irradiation time, which is in good agreement with previous reports on the living characteristics of the polymerization reactions. It is possible to accurately control the thickness of the grafted layer by simply altering the irradiation time. © The Royal Society of Chemistry 2021.
    view abstractdoi: 10.1039/d0py01410a
  • 2021 • 246 Evaluation of antithrombogenic pHPC on CoCr substrates for biomedical applications
    Bannewitz, C. and Lenz-Habijan, T. and Lentz, J. and Peters, M. and Trösken, V. and Siebert, S. and Weber, S. and Theisen, W. and Henkes, H. and Monstadt, H.
    Coatings 11 1-12 (2021)
    Bare metal endovascular implants pose a significant risk of causing thrombogenic complications. Antithrombogenic surface modifications, such as phenox’s “Hydrophilic Polymer Coating” (pHPC), which was originally developed for NiTi implants, decrease the thrombogenicity of metal surfaces. In this study, the transferability of pHPC onto biomedical CoCr-based alloys is examined. Coated surfaces were characterized via contact-angle measurement and atomic force microscopy. The equivalence of the antithrombogenic effect in contact with whole human blood was demonstrated in vitro for CoCr plates compared to NiTi plates on a platform shaker and for braided devices in a Chandler loop. Platelet adhesion was assessed via scanning electron microscopy and fluorescence microscopy. The coating efficiency of pHPC on CoCr plates was confirmed by a reduction of the contact angle from 84.4° ± 5.1° to 36.2° - 5.2°. The surface roughness was not affected by the application of pHPC. Platelet adhesion was significantly reduced on pHPC-coated specimens. The platelet covered area was reduced by 85% for coated CoCr plates compared to uncoated samples. Uncoated braided devices were completely covered by platelets, while on the pHPC-coated samples, very few platelets were visible. In conclusion, the antithrombogenic effect of pHPC coating can be successfully applied on CoCr plates as well as stent-like CoCr braids. © 2021 by the authors. Licensee MDPI, Basel, Switzerland.
    view abstractdoi: 10.3390/coatings11010093
  • 2021 • 245 An energy-relaxation-based framework for the modeling of magnetic shape memory alloys—Simulation of three-dimensional effects under homogeneous loading conditions
    Bartel, T. and Kiefer, B. and Menzel, A.
    International Journal of Solids and Structures 208-209 221-234 (2021)
    It is known from experimental findings that three-dimensional effects can have a strong influence on magnetic shape memory behavior. Such phenomena are, however, often neglected in MSMA constitutive models, as they only become meaningful under complex loading conditions. The extensions of our original modeling framework, cf. Bartel et al. (2020), to include 3D-effects is threefold: (i) vector-valued microstructural variables are now elements in R3, i.e. no longer parameterizable in polar coordinates, (ii) a third tetragonal martensite variant may form/vanish by switching from/back into both other variants, and (iii) a more general and robust algorithmic treatment is necessary. The latter includes the implementation of a staggered Augmented Lagrangian scheme to handle the now much larger and numerically more advanced sets of equality and inequality constraints. In this context, two extended model formulations are presented. The first considers a first-order, two-variant laminate approach (rank-one convexification), in which domain magnetizations, interface orientations etc. are now three-dimensional vectors. The second model is based on a convexification approach, for which the incorporation of the third martensitic variant is quite natural. Numerical examples are investigated to test the generalized modeling framework. Firstly, it is confirmed that both extended models recover the solution of the previously established two-dimensional model for a simple loading case. Secondly, response predictions for more complex loading scenarios (non-proportional bi-axial stresses, orthogonal magnetic field), motivated by experiments, are investigated. It is found that capturing the formation, elimination and mutual interaction of all martensitic variants as well as general three-dimensional magnetization vector orientations is of key importance under these conditions. The extended convexification model and modified algorithmic formulation are shown to reliably handle even such general cases. © 2020 Elsevier Ltd
    view abstractdoi: 10.1016/j.ijsolstr.2020.10.024
  • 2021 • 244 Complex-Solid-Solution Electrocatalyst Discovery by Computational Prediction and High-Throughput Experimentation**
    Batchelor, T.A.A. and Löffler, T. and Xiao, B. and Krysiak, O.A. and Strotkötter, V. and Pedersen, J.K. and Clausen, C.M. and Savan, A. and Li, Y. and Schuhmann, W. and Rossmeisl, J. and Ludwig, Al.
    Angewandte Chemie - International Edition (2021)
    Complex solid solutions (“high entropy alloys”), comprising five or more principal elements, promise a paradigm change in electrocatalysis due to the availability of millions of different active sites with unique arrangements of multiple elements directly neighbouring a binding site. Thus, strong electronic and geometric effects are induced, which are known as effective tools to tune activity. With the example of the oxygen reduction reaction, we show that by utilising a data-driven discovery cycle, the multidimensionality challenge raised by this catalyst class can be mastered. Iteratively refined computational models predict activity trends around which continuous composition-spread thin-film libraries are synthesised. High-throughput characterisation datasets are then used as input for refinement of the model. The refined model correctly predicts activity maxima of the exemplary model system Ag-Ir-Pd-Pt-Ru. The method can identify optimal complex-solid-solution materials for electrocatalytic reactions in an unprecedented manner. © 2021 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH
    view abstractdoi: 10.1002/anie.202014374
  • 2021 • 243 Process characteristics, particle behavior and coating properties during HVOF spraying of conventional, fine and nanostructured WC-12Co powders
    Baumann, I. and Hagen, L. and Tillmann, W. and Hollingsworth, P. and Stangier, D. and Schmidtmann, G. and Tolan, M. and Paulus, M. and Sternemann, C.
    Surface and Coatings Technology 405 (2021)
    In recent years, great effort has been taken in science and industry to find novel material-related solutions, which provide improved properties for future technological applications. One of these approaches is the use of fine structured and nanostructured materials. Within the field of wear protection, the use of fine or nanostructured WC-Co powder feedstock in the thermal spray process enables the application of highly wear resistant, thin near net-shape coatings on parts with complex geometries. In this study, the processing of WC-12Co powders by means of High Velocity Oxy-Fuel (HVOF) flame spraying is fundamentally investigated and the results are compared to those obtained with conventional powders. The influence of process parameter and scaling effects on the spray process and the thermo-kinetic particle behavior in the flame, the heating of the substrate as well as on the coating properties, the microstructure, the behavior of elements and phases and the residual stress is discussed comprehensively. The investigations of this work have shown that HVOF spraying of fine and nanostructured WC-12Co powders instead of conventional ones leads to a significant alteration of the thermo-kinetic spray conditions. Under optimized spray conditions, achieved by the use of special spray equipment and statistical design of experiments (DoE), improvements in terms of the economy of the spray process (higher deposition efficiencies) and the mechanical properties (higher microhardness and fracture toughness, lower porosity and roughness) can be achieved. © 2020
    view abstractdoi: 10.1016/j.surfcoat.2020.126716
  • 2021 • 242 Small-Angle X-Ray Scattering Measurements on Amphiphilic Polymer Conetworks Swollen in Orthogonal Solvents
    Benski, L. and Viran, I. and Katzenberg, F. and Tiller, J.C.
    Macromolecular Chemistry and Physics 222 (2021)
    Amphiphilic polymer conetworks (APCNs), which combine two different polymer nanophases, have a broad range of applications that involve their unique potential to separately swell one of these nanophases in a selective solvent. Little is known about the structural changes of such APCNs upon swelling in dependence on the topology. Here, conetworks composed of poly(2-ethylhexyl acrylate) crosslinked by poly(2-methyl-2-oxazoline) (PMOx) are investigated with small-angle X-ray scattering in dry and swollen state using the orthogonal solvents water and toluene. The data clearly show that the structural changes induced by swelling are strongly dependent on the topology of the APCNs. While water leads to fusion of PMOx phases resulting in larger structures than found in the dry APCN, toluene is only swelling the hydrophobic phases without structural changes. © 2020 The Authors. Macromolecular Chemistry and Physics published by Wiley-VCH GmbH
    view abstractdoi: 10.1002/macp.202000292
  • 2021 • 241 Intercritical annealing to achieve a positive strain-rate sensitivity of mechanical properties and suppression of macroscopic plastic instabilities in multi-phase medium-Mn steels
    Benzing, J.T. and Luecke, W.E. and Mates, S.P. and Ponge, D. and Raabe, D. and Wittig, J.E.
    Materials Science and Engineering A 803 (2021)
    This study investigates the high strain-rate tensile properties of a cold-rolled medium-Mn steel (Fe–12Mn–3Al-0.05C % in mass fraction) designed to have a multi-phase microstructure and positive strain-rate sensitivity. At the intercritical annealing temperature of 585 °C, increasing the annealing time from 0.5 h to 8 h increased the phase volume fraction of ultrafine-grained (UFG) austenite from 2% to 35% by reversion. The remainder of the microstructure was composed of UFG ferrite and recovered α′-martensite (the latter resembles the cold-rolled state). Servo hydraulic tension testing and Kolsky-bar tension testing were used to measure the tensile properties from quasi-static strain rates to dynamic strain rates (ε˙ = 10-4 s-1 to ε˙ = 103 s-1). The strain-rate sensitivities of the yield strength (YS) and ultimate tensile strength (UTS) were positive for both annealing times. Tensile properties and all non-contact imaging modalities (infrared imaging and digital image correlation) indicated an advantageous suppression of Lüders bands and Portevin Le Chatelier (PLC) bands (a critical challenge in multi-phase medium-Mn steel design) due to the unique combination of microstructural constituents and overall composition. Fracture surfaces of specimens annealed for 0.5 h showed some instances of localized cleavage fracture (approximately 30 μm wide areas and lath-like ridges). Specimens annealed for 8 h maintained a greater product of strength and elongation by at least 2.5 GPa % (on average for each strain rate). The relevant processing-structure-property relationships are discussed in the context of recommendations for design strategies concerning multi-phase steels such that homogeneous deformation behavior and positive strain-rate sensitivities can be achieved. © 2020
    view abstractdoi: 10.1016/j.msea.2020.140469
  • 2021 • 240 Weakly symmetric stress equilibration and a posteriori error estimation for linear elasticity
    Bertrand, F. and Kober, B. and Moldenhauer, M. and Starke, G.
    Numerical Methods for Partial Differential Equations (2021)
    This paper proposes and analyzes a posteriori error estimator based on stress equilibration for linear elasticity with emphasis on the behavior for (nearly) incompressible materials. It is based on an H(div)-conforming, weakly symmetric stress reconstruction from the displacement-pressure approximation computed with a stable finite element pair. Our focus is on the Taylor-Hood combination of continuous finite element spaces of polynomial degrees k + 1 and k for the displacement and the pressure, respectively. This weak symmetry allows us to prove that the resulting error estimator constitutes a guaranteed upper bound for the error with a constant that depends only on local constants associated with the patches and thus on the shape regularity of the triangulation. It does not involve global constants like those from Korn's in equality which may become very large depending on the location and type of the boundary conditions. Local efficiency, also uniformly in the incompressible limit, is deduced from the upper bound by the residual error estimator. Numerical results for the popular Cook's membrane test problem confirm the theoretical predictions. © 2021 The Authors. Numerical Methods for Partial Differential Equations published by Wiley Periodicals LLC.
    view abstractdoi: 10.1002/num.22741
  • 2021 • 239 Digital holography for spatially resolved analysis of the semiconductor optical response
    Besaga, V.R. and Gerhardt, N.C. and Hofmann, M.R.
    Applied Optics 60 A15-A20 (2021)
    We present spatially resolved measurements of the below-band-gap carrier-induced absorption and concurrent phase change in a semiconductor with the help of transmission digital holography. The application is demonstrated for a bulk GaAs sample, while the holograms are recorded with a conventional CMOS sensor. We show that the phase information enables spatially resolved monitoring of excess carrier distributions. Based on that, we discuss a phase-based approach for separation of carrier and heat related effects in the semiconductor optical response. © 2020 Optical Society of America.
    view abstractdoi: 10.1364/AO.402488
  • 2021 • 238 Novel approach to study diffusion of hydrogen bearing species in silicate glasses at low temperatures
    Bissbort, T. and Becker, H.-W. and Fanara, S. and Chakraborty, S.
    Chemical Geology 562 (2021)
    Diffusion of hydrogen bearing species in glasses plays a significant role in numerous applications in commercial as well as scientific domains. The investigation of diffusion of water in glasses at low temperatures led to experimental and analytical difficulties in the past. We present a new approach that lets us overcome these complications. Diffusion couples of An50Di50 glass (mol %, NBO/T = 0.67) were produced by coating anhydrous glass substrates with thin films of hydrated glass (~200 nm, ~2 wt% H2O) using pulsed laser deposition (PLD). Bonding the diffusant to the glass matrix of the thin film instead of using free water at the interface during experiments precludes other glass altering processes such as dissolution and precipitation. This allows us to confidently interpret the measured profiles to be a result of diffusion only. Nanoscale concentration profiles that result from diffusion at low temperatures on experimentally feasible time scales were measured with the Nuclear Resonance Reaction Analysis (NRRA, 1H(15N,αγ)12C). The non-destructive nature of NRRA enables us to observe and better understand the evolution of diffusion profiles with time within one sample. Evaluation of the sample quality by EPMA, SEM, optical microscopy, Rutherford backscattering spectroscopy (RBS), and NRRA was performed and confirmed the suitability of the samples for diffusion studies. Experiments at 1 atm in a box furnace and at 2 kbar in a CSPV (pressure medium = water) and an IHPV (pressure medium = Argon) prove that the diffusion couples can be used under various experimental conditions. We present diffusion profiles that were measured in experiments carried out in these devices and discuss the distinct features of each that result from different boundary conditions in the experiments. © 2020 Elsevier B.V.
    view abstractdoi: 10.1016/j.chemgeo.2020.120037
  • 2021 • 237 Increasing the Complexity in the MIL-53 Structure: The Combination of the Mixed-Metal and the Mixed-Linker Concepts
    Bitzer, J. and Teubnerová, M. and Kleist, W.
    Chemistry - A European Journal 27 1724-1735 (2021)
    The isoreticular mixed-component concept is a promising approach to tailor the material properties of metal–organic frameworks. While isoreticular mixed-metal or mixed-linker materials are commonly synthesized, the combination of both concepts for the development of isoreticular materials featuring both two metals and two linkers is still rarely investigated. Herein, we present the development of mixed-metal/mixed-linker MIL-53 materials that contain different metal combinations (Al/Sc, Al/V, Al/Cr, Al/Fe) and different linker ratios (terephthalate/2-aminoterephthalate). The possibility of changing the metal combination and the linker ratio independently from each other enables a large variety of modifications. A thorough characterization (PXRD, ATR-IR, TGA, 1H NMR, ICP-OES) confirmed that all components were incorporated into the framework structure with a statistical distribution. Nitrogen physisorption measurements showed that the breathing behavior can be tailored by adjusting the linker ratio for all metal combinations. All materials were successfully used for post-synthetic modification reactions with maleic anhydride. © 2020 The Authors. Published by Wiley-VCH GmbH
    view abstractdoi: 10.1002/chem.202003304
  • 2021 • 236 Subcritical crack growth in hard alloys under cyclic loading
    Brackmann, L. and Röttger, A. and Weber, S. and Theisen, W.
    Fatigue and Fracture of Engineering Materials and Structures 44 349-365 (2021)
    In this work, the subcritical crack growth in Fe-, Ni- and Co-base hard alloys was investigated. Specimens were cyclically loaded in the pressure threshold range until a ring crack resulted as a failure criterion. Crack propagation along with the individual microstructural constituents and the associated resistance of the individual materials to crack propagation was investigated by scanning electron microscopy and by the methods adapted to it. For the Ni-base alloys, the formation of a closed ring fracture occurred after the lowest load cycle number, followed by the Co- and Fe-base alloys. Almost no crack deflection by the hard phases was detected in the Ni-base alloys. The higher number of loading cycles to produce a closed crack ring in the Fe-base alloys is attributed to the pronounced crack deflection by the hard phases and to the higher matrix strength. Besides, phase transformations were registered in front of the crack tip of the Co- and the partially austenitic Fe-base alloy. This phase transformation counteracts crack formation in the case of the Fe-base matrix but promotes crack propagation in the Co-base alloy. © 2020 The Authors. Fatigue & Fracture of Engineering Materials & Structures published by John Wiley & Sons Ltd
    view abstractdoi: 10.1111/ffe.13363
  • 2021 • 235 Prediction of short fiber composite properties by an artificial neural network trained on an rve database
    Breuer, K. and Stommel, M.
    Fibers 9 1-14 (2021)
    In this study, an artificial neural network is designed and trained to predict the elastic properties of short fiber reinforced plastics. The results of finite element simulations of three-dimensional representative volume elements are used as a data basis for the neural network. The fiber volume fraction, fiber length, matrix-phase properties, and fiber orientation are varied so that the neural network can be used within a very wide range of parameters. A comparison of the predictions of the neural network with additional finite element simulations shows that the stiffnesses of short fiber reinforced plastics can be predicted very well by the neural network. The average prediction accuracy is equal or better than by a two-step homogenization using the classical method of Mori and Tanaka. Moreover, it is shown that the training of the neural network on an extended data set works well and that particularly calculation-intensive data points can be avoided without loss of prediction quality. © 2021 by the authors. Licensee MDPI, Basel, Switzerland.
    view abstractdoi: 10.3390/fib9020008
  • 2021 • 234 Predicting the API partitioning between lipid-based drug delivery systems and water
    Brinkmann, J. and Becker, I. and Kroll, P. and Luebbert, C. and Sadowski, G.
    International Journal of Pharmaceutics 595 (2021)
    Partitioning tests in water are early-stage standard experiments during the development of pharmaceutical formulations, e.g. of lipid-based drug delivery system (LBDDS). The partitioning behavior of the active pharmaceutical ingredient (API) between the fatty phase and the aqueous phase is a key property, which is supposed to be determined by those tests. In this work, we investigated the API partitioning between LBDDS and water by in-silico predictions applying the Perturbed-Chain Statistical Associating Fluid Theory (PC-SAFT) and validated these predictions experimentally. The API partitioning was investigated for LBDDS comprising up to four components (cinnarizine or ibuprofen with tricaprylin, caprylic acid, and ethanol). The influence of LBDDS/water mixing ratios from 1/1 up to 1/200 (w/w) as well as the influence of excipients on the API partitioning was studied. Moreover, possible API crystallization upon mixing the LBDDS with water was predicted. This work showed that PC-SAFT is a strong tool for predicting the API partitioning behavior during in-vitro tests. Thus, it allows rapidly assessing whether or not a specific LBDDS might be a promising candidate for further in-vitro tests and identifying the API load up to which API crystallization can be avoided. © 2021 Elsevier B.V.
    view abstractdoi: 10.1016/j.ijpharm.2021.120266
  • 2021 • 233 Studying the mechanism of phase separation in aqueous solutions of globular proteins via molecular dynamics computer simulations
    Brudar, S. and Gujt, J. and Spohr, E. and Hribar-Lee, B.
    Physical chemistry chemical physics : PCCP 23 415-424 (2021)
    Proteins are the most abundant biomacromolecules in living cells, where they perform vital roles in virtually every biological process. To maintain their function, proteins need to remain in a stable (native) state. Inter- and intramolecular interactions in aqueous protein solutions govern the fate of proteins, as they can provoke their unfolding or association into aggregates. The initial steps of protein aggregation are difficult to capture experimentally, therefore we used molecular dynamics simulations in this study. We investigated the initial phase of aggregation of two different lysozymes, hen egg-white (HEWL) and T4 WT* lysozyme and also human lens γ-D crystallin by using atomistic simulations. We monitored the phase stability of their aqueous solutions by calculating time-dependent density fluctuations. We found that all proteins remained in their compact form despite aggregation. With an extensive analysis of intermolecular residue-residue interactions we discovered that arginine is of paramount importance in the initial stage of aggregation of HEWL and γ-D crystallin, meanwhile lysine was found to be the most involved amino acid in forming initial contacts between T4 WT* molecules.
    view abstractdoi: 10.1039/d0cp05160h
  • 2021 • 232 Extremely Low Vapor-Pressure Data as Access to PC-SAFT Parameter Estimation for Ionic Liquids and Modeling of Precursor Solubility in Ionic Liquids
    Bülow, M. and Greive, M. and Zaitsau, D.H. and Verevkin, S.P. and Held, C.
    ChemistryOpen (2021)
    Precursor solubility is a crucial factor in industrial applications, dominating the outcome of reactions and purification steps. The outcome and success of thermodynamic modelling of this industrially important property with equations of states, such as Perturbed-Chain Statistical Associating Fluid Theory (PC-SAFT), vastly depends on the quality of the pure-component parameters. The pure-component parameters for low-volatile compounds such as ionic liquids (ILs) have been commonly estimated using mixture properties, e. g. the osmotic pressure of aqueous solutions. This leads to parameters that depend on the solvent, and transferability to other mixtures often causes poor modeling results. Mixture-independent experimental properties would be a more suitable basis for the parameter estimation offering a way to universal parameter sets. Model parameters for ILs are available in the literature [10.1016/j.fluid.2012.05.029], but they were estimated using pure-IL density data. The present work focuses on a step towards a more universal estimation strategy that includes new experimental vapor-pressure data of the pure IL. ILs exhibit an almost negligible vapor pressure in magnitude of usually 10−5 Pa even at elevated temperatures. In this work, such vapor-pressure data of a series of 1-ethyl-3-methyl-imidazolium-based [C2mim]-ILs with various IL-anions (e. g. tetrafluoroborate [BF4]−, hexafluorophosphate [PF6]−, bis(trifluoromethylsulfonyl)imide [NTf2]−) were experimentally determined and subsequently used for PC-SAFT parameter estimation. The so-determined parameters were used to predict experimental molecular precursor solubility in ILs and infinitely diluted activity coefficients of various solvents in ILs. The parameters were further compared to modeling results using classical parametrization methods (use of liquid-density data only for the molecular PC-SAFT and the ion-based electrolyte PC-SAFT). As a result, the modeled precursor solubilities using the new approach are much more precise than using the classical parametrization methods, and required binary parameters were found to be much smaller (if needed). In sum, including the pure-component vapor-pressure data of ILs opens the door towards parameter estimation that is not biased by mixture data. This procedure might be suitable also for polymers and for all kind of ionic species but needs extension to ion-specific parametrization in the long term. © 2021 The Authors. Published by Wiley-VCH GmbH
    view abstractdoi: 10.1002/open.202000258
  • 2021 • 231 An extended model of the ISO-2631 standard to objectify the ride comfort in autonomous driving
    Burkhard, G. and Berger, T. and Enders, E. and Schramm, D.
    Work 68 S37-S45 (2021)
    BACKGROUND: With the development of autonomous driving, the occupants' comfort perception and their activities during the drive are becoming increasingly the focus of research. Especially in one of the first applications, a drive on a motorway, vertical dynamics play a major role. OBJECTIVE: To be able to robustly objectify ride comfort, better models need to be developed. Initial studies have shown, that the current ISO-2631 standard creates good results in the objectification and can be regarded as benchmark. METHODS: To increase the accuracy in objectification, an extended model with the occupants' head as additional measuring point is introduced. Instead of the known frequency filters, weighting (k-factors) is used to differentiate possible excitations. For comparing the model with the ISO-2631, a simulator study with 5 excitations and 50 inattentive subjects is carried out. RESULTS: Evaluating the study with the ISO-2631, 3 out of 5 excitations indicate a significant difference between the occupant's impression and the calculated comfort value. In comparison the extended model has no significant difference. CONCLUSION: The results further show, that inattentive occupants move their heads significantly more. By measuring accelerations of the head, the extended model creates equivalent or more accurate comfort values than the ISO-2631. © 2021 - IOS Press. All rights reserved.
    view abstractdoi: 10.3233/WOR-208004
  • 2021 • 230 Non-ideal mixing behavior in dibutyl phosphate-propylamine binary liquids: Dielectric and nuclear magnetic resonance investigations
    Calandra, P. and Turco Liveri, V. and Proietti, N. and Capitani, D. and Lombardo, D. and Gainaru, C. and Böhmer, R. and Kozak, M. and Dobies, M. and Fojud, Z. and Pochylski, M.
    Journal of Molecular Liquids 323 (2021)
    Owing to the amphiphilic nature of their constituent molecules, binary mixtures of pure liquid surfactants are usually characterized by enhanced nano-segregation and thus can exhibit interesting transport properties and complex macroscopic behavior. In this ambit it was recently shown by Turco Liveri et al. (J. Mol. Liq. 263 (2018) 274–281) at room temperature that mixtures of short aliphatic chains compounds, such as dibutyl phosphate (DBP) and n-propylamine (PA) liquids, due to their ability to allow for phosphate-to-amine proton transfer, display ionic liquid–like behavior with composition-dependent enhanced conductivity, viscosity, and magnetically-induced birefringence. To understand the molecular mechanisms at the basis of this behavior, in the present study a combination of nuclear magnetic resonance (NMR) and dielectric spectroscopy investigations has been carried out for the same materials for different amine molar ratios. It was found that at certain compositions all studied dynamical processes (conformational changes, local hopping of “free” protons among neighboring polar headgroups, long-range charge migration) exhibit significant deviations from ideal mixing behavior. The microscopic origin of these deviations is discussed. © 2020
    view abstractdoi: 10.1016/j.molliq.2020.114963
  • 2021 • 229 Flexibilization of Biorefineries: Tuning Lignin Hydrogenation by Hydrogen Partial Pressure
    Cao, Z. and Xu, Y. and Lyu, P. and Dierks, M. and Morales-García, Á. and Schrader, W. and Nachtigall, P. and Schüth, F.
    ChemSusChem 14 373-378 (2021)
    The present study describes an interesting and practical catalytic system that allows flexible conversion of lignin into aromatic or aliphatic hydrocarbons, depending on the hydrogen partial pressure. A combination of experiment and theory shows that the product distribution between aromatics and aliphatics can be simply tuned by controlling the availability of hydrogen on the catalyst surface. Noticeably, these pathways lead to almost complete oxygen removal from lignin biomass, yielding high-quality hydrocarbons. Thus, hydrogen–lignin co-refining by using this catalytic system provides high flexibility in hydrogen storage/consumption towards meeting different regional and temporal demands. © 2020 The Authors. ChemSusChem published by Wiley-VCH GmbH
    view abstractdoi: 10.1002/cssc.202002248
  • 2021 • 228 Solvent effect on the kinetics of the hydrogenation of n-butyl levulinate to γ-valerolactone
    Capecci, S. and Wang, Y. and Casson Moreno, V. and Held, C. and Leveneur, S.
    Chemical Engineering Science 231 (2021)
    The use of lignocellulosic biomass in the chemical industry can significantly contribute to respect the various international agreements on climate change. One of the most promising platform molecules issued from the lignocellulosic biomass hydrolysis is γ-valerolactone (GVL). GVL can be upgraded to valuable chemicals and produced by the hydrogenation of alkyl levulinates. Although these reactions are widely studied, seldom research focused on the solvent effect. To fill this gap, the effect of three different reaction mixtures with an excess of butyl levulinate (BL), of butanol and GVL was studied on the kinetics of BL hydrogenation to GVL over Ru/C. PC-SAFT (Perturbed-Chain Statistical Associating Fluid Theory) shows that the solubility of hydrogen is not constant during the reaction progress, and it was taken into account. To allow a fair comparison, kinetic models were developed using Bayesian statistics for each reaction mixture. The best performances were obtained when the reaction mixture has an excess of GVL. © 2020 Elsevier Ltd
    view abstractdoi: 10.1016/j.ces.2020.116315
  • 2021 • 227 Comparison between optical coherence tomography imaging and histological sections of peripheral nerves
    Carolus, A.E. and Möller, J. and Hofmann, M.R. and van de Nes, J.A.P. and Welp, H. and Schmieder, K. and Brenke, C.
    Journal of Neurosurgery 134 270-277 (2021)
    OBJECTIVE Optical coherence tomography (OCT) is an imaging technique that uses the light-backscattering properties of different tissue types to generate an image. In an earlier feasibility study the authors showed that it can be applied to visualize human peripheral nerves. As a follow-up, this paper focuses on the interpretation of the images obtained. METHODS Ten different short peripheral nerve specimens were retained following surgery. In a first step they were examined by OCT during, or directly after, surgery. In a second step the nerve specimens were subjected to histological examination. Various steps of image processing were applied to the OCT raw data acquired. The improved OCT images were compared with the sections stained by H & E. The authors assigned the structures in the images to the various nerve components including perineurium, fascicles, and intrafascicular microstructures. RESULTS The results show that OCT is able to resolve the myelinated axons. A weighted averaging filter helps in identifying the borders of structural features and reduces artifacts at the same time. Tissue-remodeling processes due to injury (perineural fibrosis or neuroma) led to more homogeneous light backscattering. Anterograde axonal degeneration due to sharp injury led to a loss of visible axons and to an increase of light-backscattering tissue as well. However, the depth of light penetration is too small to allow generation of a complete picture of the nerve. CONCLUSIONS OCT is the first in vivo imaging technique that is able to resolve a nerve’s structures down to the level of myelinated axons. It can yield information about focal and segmental pathologies. © AANS 2021, except where prohibited by US copyright law
    view abstractdoi: 10.3171/2019.8.JNS191278
  • 2021 • 226 Oxygenated PAH Formation Chemistry Investigation in Anisole Jet Stirred Reactor Oxidation by a Thermodynamic Approach
    Chen, B. and Kruse, S. and Schmid, R. and Cai, L. and Hansen, N. and Pitsch, H.
    Energy and Fuels 35 1535-1545 (2021)
    Oxygenated poly aromatic hydrocarbons (OPAH) are widely produced in biomass combustion. Recent studies suggest significantly higher toxicity for OPAH in comparison to PAH and soot. However, the present understanding of OPAH formation chemistry is rudimentary. Hence, fundamental knowledge on the formation pathways of OPAH is urgently required to develop predictive models for adequate emission control strategies on OPAH emission in biomass combustion. In this work, the OPAH formation from oxidation of anisole, a representative biomass surrogate, was studied in a jet stirred reactor (JSR). The reaction products were in-situ sampled by molecular beam (MB) and analyzed by time-of-flight mass spectrometry (TOF-MS) using synchrotron radiation as a photon ionization source. The unique experimental setup allows direct detection and identification of large OPAH molecules. Over 40 sum formula of OPAH species were detected and identified by experiments, and a computational thermodynamic approach was applied to deduce possible isomers of OPAH species. The thermodynamic modeling approach assumes that isomers with relatively lower Gibbs free energies are more likely to be present due to possible lower activation energies in the formation pathways. Furthermore, the formation pathways of elucidated OPAH structures are proposed by analogy to the literature based on the intermediate information. The joint study of OPAH by experiments and quantum chemistry advances the understanding of OPAH formation chemistry. © 2020 American Chemical Society.
    view abstractdoi: 10.1021/acs.energyfuels.0c03829
  • 2021 • 225 Scalable and Recyclable All-Organic Colloidal Cascade Catalysts
    Chen, C. and Janoszka, N. and Wong, C.K. and Gramse, C. and Weberskirch, R. and Gröschel, A.H.
    Angewandte Chemie - International Edition 60 237-241 (2021)
    We report on the synthesis of core–shell microparticles (CSMs) with an acid catalyst in the core and a base catalyst in the shell by surfactant-free emulsion polymerization (SFEP). The organocatalytic monomers were separately copolymerized in three synthetic steps allowing the spatial separation of incompatible acid and base catalysts within the CSMs. Importantly, a protected and thermo-decomposable sulfonate monomer was used as acid source to circumvent the neutralization of the base catalyst during shell formation, which was key to obtain stable, catalytically active CSMs. The catalysts showed excellent performance in an established one-pot model cascade reaction in various solvents (including water), which involved an acid-catalyzed deacetalization followed by a base-catalyzed Knoevenagel condensation. The CSMs are easily recycled, modified, and their synthesis is scalable, making them promising candidates for organocatalytic applications. © 2020 The Authors. Published by Wiley-VCH GmbH
    view abstractdoi: 10.1002/anie.202008104
  • 2021 • 224 Lattice-shifted nematic quantum critical point in FeSe1−xSx
    Chibani, S. and Farina, D. and Massat, P. and Cazayous, M. and Sacuto, A. and Urata, T. and Tanabe, Y. and Tanigaki, K. and Böhmer, A.E. and Canfield, P.C. and Merz, M. and Karlsson, S. and Strobel, P. and Toulemonde, P. and Paul...
    npj Quantum Materials 6 (2021)
    We report the evolution of nematic fluctuations in FeSe1−xSx single crystals as a function of Sulfur content x across the nematic quantum critical point (QCP) xc ~ 0.17 via Raman scattering. The Raman spectra in the B1g nematic channel consist of two components, but only the low energy one displays clear fingerprints of critical behavior and is attributed to itinerant carriers. Curie–Weiss analysis of the associated nematic susceptibility indicates a substantial effect of nemato-elastic coupling, which shifts the location of the nematic QCP. We argue that this lattice-induced shift likely explains the absence of any enhancement of the superconducting transition temperature at the QCP. The presence of two components in the nematic fluctuations spectrum is attributed to the dual aspect of electronic degrees of freedom in Hund’s metals, with both itinerant carriers and local moments contributing to the nematic susceptibility. © 2021, The Author(s).
    view abstractdoi: 10.1038/s41535-021-00336-3
  • 2021 • 223 Neuro-fuzzy fatigue life assessment using the wavelet-based multifractality parameters
    Chin, C.H. and Abdullah, S. and Singh, S.S.K. and Ariffin, A.K. and Schramm, D.
    Journal of Mechanical Science and Technology 35 439-447 (2021)
    This study aims to establish a fatigue life predictive model based on multifractality of road excitations using neuro-fuzzy method to assess the durability of suspension spring. Traditional durability analysis in time domain is complicated and time-consuming due to the needs of large data amount. Thus, it is an idea to adopt an adaptive neuro-fuzzy inference system (ANFIS) for relating the performance of coil spring to the multifractal properties of road excitations, giving a meaningful fatigue life prediction. Different membership function numbers were tested to obtain the optimum membership function number. During the data training process, the checking data was used to test the trained model each Epoch of training for overfitting detection. As a result, the Morrow-based fatigue life prediction model was found to give the most suitable result with three membership functions. The SWT-based model needed five membership functions due to nonlinear properties in the SWT-based fatigue life data. Training process of Morrow-based-ANFIS was stopped at Epoch 8 given its lowest checking root-mean-square-error of 0.6953. SWT-based model recorded a higher error of 0.7940. The neuro-fuzzy models gave accurate fatigue life predictions with 96 % of the data distributed within the acceptance boundary, hence, contributing to an acceptable assessment of coil spring fatigue life based on load multifractality. This study had shown a nonlinear relationship between road multifractality and durability performance of coil spring. Multifractality had been proven an important feature to characterise various road excitations for durability prediction. © 2021, The Korean Society of Mechanical Engineers and Springer-Verlag GmbH Germany, part of Springer Nature.
    view abstractdoi: 10.1007/s12206-021-0102-6
  • 2021 • 222 Computing low-frequency vibration energy with Hölder singularities as durability predictive criterion of random road excitation
    Chin, C.H. and Abdullah, S. and Singh, S.S.K. and Ariffin, A.K. and Schramm, D.
    Soft Computing (2021)
    This study aims to compute low-frequency energy with Hölder singularities in vibration signals of suspension system to predict the durability of coil spring. High frequencies in vibrations often had minimal contribution towards fatigue damage due to low amplitude range and thus induce errors in energy analysis of vibration signals. Since traditional low-pass method had not only been ineffective in reducing high frequencies, it also resulted in the loss of signal information. This study had therefore proposed characterising low-frequency energy for road excitations using Hölder singularities and power spectral analyses. Singularities and low-frequency energy of road vibration signals would first be identified through Hölder local regularity analysis. This was then followed by fatigue life prediction using the strain-life approaches (i.e. Coffin-Manson, Morrow and Smith–Watson–Topper models). The energy-based fatigue life prediction models had not only shown good fit with R2 values higher than 0.8, but had also demonstrated an accurate prediction of fatigue life with more than 95% of the data being within the acceptance boundary. The Morrow-based model provided the highest accuracy in fatigue life prediction because of its highest R2 value of 0.8625 and 100% data survival in the fatigue life correlation study. This showed that energy-based fatigue life prediction models provide an accurate and effective prediction of the durability performance. This study proposed a more precise energy characterisation method for energy-based durability prediction of suspension coil spring under random loading conditions. © 2021, The Author(s), under exclusive licence to Springer-Verlag GmbH, DE part of Springer Nature.
    view abstractdoi: 10.1007/s00500-021-05640-5
  • 2021 • 221 Durability prediction of coil spring through multibody-dynamics-based strain generation
    Chin, C.H. and Abdullah, S. and Singh, S.S.K. and Schramm, D. and Ariffin, A.K.
    Mechanical Systems and Signal Processing 154 (2021)
    The purpose of this study is to develop an acceleration-strain conversion model that considers torsional strain and spring curvature effects in inducing strain for a suspension coil spring. Measurements of strain–time histories at the coil spring are limited by complex geometry and insufficient workspace. This condition increases the demand for strain signal generation through multibody dynamics (MBD) simulation, reducing the need for real strain measurement of coil spring. Road tests were conducted to obtain the unsprung mass acceleration and strain signals of a coil spring under four road conditions (rural, industrial, highway, and campus road). Quarter-car suspension MBD simulation was modelled to simulate the deflection of a spring excited under unsprung mass acceleration. By using this model, simulated strain data with similar properties as the experimental data were generated for fatigue life prediction. The predicted fatigue life from the generated strain indicated a good correlation with the experimental fatigue life within the boundary and showed very low normalised root-mean-square error (NRMSE) between 4 × 10−6 and 2 × 10−4. Finally, it is suggested that the acceleration-strain conversion model showed an enhanced performance for producing realistic strain signals in accurately predicting the durability of coil spring. This can, therefore, further reduce the need for real strain measurement at the coil spring that can result in an erroneous signal. © 2020
    view abstractdoi: 10.1016/j.ymssp.2020.107580
  • 2021 • 220 Orientation-dependent plastic deformation mechanisms and competition with stress-induced phase transformation in microscale NiTi
    Choi, W.S. and Pang, E.L. and Ko, W.-S. and Jun, H. and Bong, H.J. and Kirchlechner, C. and Raabe, D. and Choi, P.-P.
    Acta Materialia 208 (2021)
    Understanding the orientation-dependent deformation behavior of NiTi shape-memory alloys at small length scales is of importance for designing nano- and micro-electromechanical systems. However, a complete understanding of the orientation- and size-dependent competition between the various modes of slip, deformation twinning, and martensitic transformation in NiTi shape-memory alloys is still lacking, especially in micron-scale specimens. In the present study, we perform micro-compression tests on [001]- and [112]-oriented micro-pillars of a solutionized Ti-49.9at.% Ni alloy. Post-mortem TEM analysis of the deformed pillars reveal that the operating plastic deformation modes are {011}&lt;100&gt; slip and {114}&lt;221¯&gt; deformation twinning, which compete with the martensitic transformation, depending on the crystal orientation. Furthermore, in both experiments and molecular dynamics simulations, we consistently find residual B19′ martensite in a herringbone microstructure composed of finely spaced (001)B19′ compound twins instead of the generally assumed [011]B19′ type II twins common in bulk samples, suggesting that the operative martensitic transformation mode may be size-dependent. Schmid factors in compression are calculated for all commonly reported slip, deformation twinning, and martensitic transformation modes as a function of crystallographic orientation, which rationalize the orientation-dependent competition between these deformation modes. © 2021 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2021.116731
  • 2021 • 219 Biomineralization, dissolution and cellular studies of silicate bioceramics prepared from eggshell and rice husk
    Choudhary, R. and Venkatraman, S.K. and Bulygina, I. and Senatov, F. and Kaloshkin, S. and Anisimova, N. and Kiselevskiy, M. and Knyazeva, M. and Kukui, D. and Walther, F. and Swamiappan, S.
    Materials Science and Engineering C 118 (2021)
    The current investigation aims to replace the synthetic starting materials with biowaste to synthesize and explore three different silicate bioceramics. Pure silica from rice husk was extracted by decomposition of rice husk in muffle furnace followed by alkali treatment and acid precipitation. Raw eggshell and extracted silica were utilized for the preparation of wollastonite, diopside and forsterite by the solid-state method. The TG-DSC analysis shows that the crystallization temperature of wollastonite, diopside and forsterite was found to be 883 °C, 870 °C and 980 °C, respectively. The phase purity of wollastonite was attained at 1100 °C whereas diopside and forsterite were composed of secondary phases even after calcination at 1250 °C and 1300 °C respectively. All three materials behaved differently when exposed to the physiological environment, as wollastonite exhibited remarkable apatite deposition within 3 days whereas a distinct apatite phase was noticed on the surface of diopside after 2 weeks and forsterite shows the formation of apatite phase after five weeks of immersion. The rapid dissolution of Mg2+ ion from forsterite lowered the leaching of silicate ions into the simulated body fluid leading to poor apatite deposition over its surface. Chemical composition was found to plays a key role in the biomineralization ability of these bioceramics. Hemolysis and Lactate Dehydrogenase (LDH) release assays were performed to evaluate the hemocompatibility of silicate ceramics cultured at different concentrations (62.5, 125, and 250 μg/mL) with red blood cells and mononuclear leucocytes (MLs) of mice. The hemolytic activity of all the tested bioceramics was insignificant (less than 1%). The interaction between diopside and mouse multipotent mesenchymal stromal cells (MMSCs) caused a negligible increase in the number of apoptosis-associated Annexin V-binding cells whereas forsterite and wollastonite induced an increase in the number of the apoptotic cells only at the concentration of 250 μg/mL. The LDH assay did not show statistically significant changes in the proliferation of MMSCs after treatment with the bioceramics at the tested concentrations when compared to control (p &gt; 0.05). This finding showed that the death of a part of cells during the first 24 h of incubation did not prevent the proliferation of MMSCs incubated with diopside, forsterite and wollastonite for 72 h. © 2020 Elsevier B.V.
    view abstractdoi: 10.1016/j.msec.2020.111456
  • 2021 • 218 Ferrous to Ferric Transition in Fe-Phthalocyanine Driven by NO2 Exposure
    Cojocariu, I. and Carlotto, S. and Sturmeit, H.M. and Zamborlini, G. and Cinchetti, M. and Cossaro, A. and Verdini, A. and Floreano, L. and Jugovac, M. and Puschnig, P. and Piamonteze, C. and Casarin, M. and Feyer, V. and Schneider, C.M.
    Chemistry - A European Journal 27 3526-3535 (2021)
    Due to its unique magnetic properties offered by the open-shell electronic structure of the central metal ion, and for being an effective catalyst in a wide variety of reactions, iron phthalocyanine has drawn significant interest from the scientific community. Nevertheless, upon surface deposition, the magnetic properties of the molecular layer can be significantly affected by the coupling occurring at the interface, and the more reactive the surface, the stronger is the impact on the spin state. Here, we show that on Cu(100), indeed, the strong hybridization between the Fe d-states of FePc and the sp-band of the copper substrate modifies the charge distribution in the molecule, significantly influencing the magnetic properties of the iron ion. The FeII ion is stabilized in the low singlet spin state (S=0), leading to the complete quenching of the molecule magnetic moment. By exploiting the FePc/Cu(100) interface, we demonstrate that NO2 dissociation can be used to gradually change the magnetic properties of the iron ion, by trimming the gas dosage. For lower doses, the FePc film is decoupled from the copper substrate, restoring the gas phase triplet spin state (S=1). A higher dose induces the transition from ferrous to ferric phthalocyanine, in its intermediate spin state, with enhanced magnetic moment due to the interaction with the atomic ligands. Remarkably, in this way, three different spin configurations have been observed within the same metalorganic/metal interface by exposing it to different doses of NO2 at room temperature. © 2020 The Authors. Chemistry - A European Journal published by Wiley-VCH GmbH
    view abstractdoi: 10.1002/chem.202004932
  • 2021 • 217 On the simultaneous use of simple geometrically exact shear-rigid rod and shell finite elements
    Costa e Silva, C. and Maassen, S.F. and Pimenta, P.M. and Schröder, J.
    Computational Mechanics (2021)
    This work addresses simultaneous use of geometrically exact shear-rigid rod and shell finite elements and describes both models within the same framework. Parameterization of the rotation field is performed by Rodrigues rotation vector, which makes the incremental updating of the rotational variables remarkably simple. For the rod element, cubic Hermitian interpolation for the displacements together with quadratic Lagrange interpolation for the incremental torsion angle were employed, while, for the triangular shell element, a complete quadratic Lagrange interpolation was used. The internal incremental torsion angle resulting from the displacement field within the shell element is then made compatible with the boundary incremental torsion angle of the shell element by an internal Lagrange multiplier. The compatibility between contiguous shell elements as well rod elements is mastered in the standard way by simply connecting nodes. This technique is an important contribution of the work, whose performance is illustrated by several numerical examples. © 2021, The Author(s), under exclusive licence to Springer-Verlag GmbH, DE part of Springer Nature.
    view abstractdoi: 10.1007/s00466-020-01967-2
  • 2021 • 216 Immobilization of peroxidase on textile carrier materials and their application in the bleaching of colored whey
    Courth, K. and Binsch, M. and Ali, W. and Ingenbosch, K. and Zorn, H. and Hoffmann-Jacobsen, K. and Gutmann, J.S. and Opwis, K.
    Journal of Dairy Science 104 1548-1559 (2021)
    Textiles represent promising support materials for enzymes. The goal of the present work was to investigate the immobilization of commercial peroxidase on a polyester needle felt and the repeated use in the gentle degradation of norbixin in whey from dairy cheese as a practical application. High enzyme loads were obtained by a 2-step immobilization procedure. First, the number of functional groups on the textile surface was increased by a modification with amino-functional polyvinylamine. Second, the enzyme was immobilized by using 2 types of crosslinking agents. Due to the iron content of peroxidase, inductively coupled plasma–optical emission spectrometry was used for the quantitative determination of the enzyme load on the textile. The enzyme activity was evaluated using common 2,2'-azino-di-(3-ethylbenzthiazoline-6-sulfonic acid) assay for peroxidases. By the variation of enzyme input and crosslinker concentration, a maximal enzyme load of 80 mg/g of textile was achieved, and a maximum specific activity of 57 U/g of textile. For the visualization of the enzyme on the fiber surface, fluorescence microscopy as well as scanning probe microscopy were used. The immobilized peroxidase showed significant activity, even after 50 reuse cycles. In addition, the potential of the new support and enzyme combination in commercial whey bleaching was demonstrated successfully on a 10-L scale. © 2021 American Dairy Science Association
    view abstractdoi: 10.3168/jds.2019-17110
  • 2021 • 215 A similarity measure for second order properties of non-stationary functional time series with applications to clustering and testing
    van Delft, A. and Dette, H.
    Bernoulli 27 469-501 (2021)
    Due to the surge of data storage techniques, the need for the development of appropriate techniques to identify patterns and to extract knowledge from the resulting enormous data sets, which can be viewed as collections of dependent functional data, is of increasing interest in many scientific areas. We develop a similarity measure for spectral density operators of a collection of functional time series, which is based on the aggregation of Hilbert–Schmidt differences of the individual time-varying spectral density operators. Under fairly general conditions, the asymptotic properties of the corresponding estimator are derived and asymptotic normality is established. The introduced statistic lends itself naturally to quantify (dis)-similarity between functional time series, which we subsequently exploit in order to build a spectral clustering algorithm. Our algorithm is the first of its kind in the analysis of non-stationary (functional) time series and enables to discover particular patterns by grouping together ‘similar’ series into clusters, thereby reducing the complexity of the analysis considerably. The algorithm is simple to implement and computationally feasible. As a further application, we provide a simple test for the hypothesis that the second order properties of two non-stationary functional time series coincide. © 2021 ISI/BS.
    view abstractdoi: 10.3150/20-BEJ1246
  • 2021 • 214 Adaptation of cluster analysis methods to optimize a biomechanical motion model of humans in a nursing bed
    Demmer, J. and Kitzig, A. and Stockmanns, G. and Naroska, E. and Viga, R. and Grabmaier, A.
    European Signal Processing Conference 2021-January 1323-1327 (2021)
    The paper considers the optimization of a Hidden-Markov Model (HMM) based method for the generation of averaged motion sequences. To create averaged motion sequences, motion sequences of different test persons were originally recorded with a motion capture system (MoCap system) and then averaged using an HMM approach. The resulting averaged data sets, however, partly showed serious motion artifacts and uncoordinated intermediate movements, especially in the extremities. The aim of this work was to combine only movements with similar courses in the extremities by a suitable cluster analysis. For each test person, model body descriptions of 21 body elements are available, each of which is represented in three-dimensional time series. For optimization, the MoCap data are first compared using time warp edit distance (TWED) and clustered using an agglomerative hierarchical procedure. Finally, the data of the resulting clusters are used to generate new averaged motion sequences using the HMM approach. The resulting averaged data can be used, for example, in a simulation in a multilevel biomechanical model. © 2021 European Signal Processing Conference, EUSIPCO. All rights reserved.
    view abstractdoi: 10.23919/Eusipco47968.2020.9287503
  • 2021 • 213 Identifying shifts between two regression curves
    Dette, H. and Dhar, S.S. and Wu, W.
    Annals of the Institute of Statistical Mathematics (2021)
    This article studies the problem whether two convex (concave) regression functions modelling the relation between a response and covariate in two samples differ by a shift in the horizontal and/or vertical axis. We consider a nonparametric situation assuming only smoothness of the regression functions. A graphical tool based on the derivatives of the regression functions and their inverses is proposed to answer this question and studied in several examples. We also formalize this question in a corresponding hypothesis and develop a statistical test. The asymptotic properties of the corresponding test statistic are investigated under the null hypothesis and local alternatives. In contrast to most of the literature on comparing shape invariant models, which requires independent data the procedure is applicable for dependent and non-stationary data. We also illustrate the finite sample properties of the new test by means of a small simulation study and two real data examples. © 2021, The Institute of Statistical Mathematics, Tokyo.
    view abstractdoi: 10.1007/s10463-020-00771-2
  • 2021 • 212 A note on optimal designs for estimating the slope of a polynomial regression
    Dette, H. and Melas, V.B. and Shpilev, P.
    Statistics and Probability Letters 170 (2021)
    In this note we consider the optimal design problem for estimating the slope of a polynomial regression with no intercept at a given point, say z. In contrast to previous work, we investigate the model on the non-symmetric interval. © 2020 Elsevier B.V.
    view abstractdoi: 10.1016/j.spl.2020.108992
  • 2021 • 211 Solvent influence on the phase behavior and glass transition of Amorphous Solid Dispersions
    Dohrn, S. and Luebbert, C. and Lehmkemper, K. and Kyeremateng, S.O. and Degenhardt, M. and Sadowski, G.
    European Journal of Pharmaceutics and Biopharmaceutics 158 132-142 (2021)
    Understanding the long-term stability of amorphous solid dispersions (ASDs) is important for their successful approval for market. ASD stability does not only depend on the interplay between the active pharmaceutical ingredient (API) and the polymer in the final formulation but may already be disadvantageously influenced by process steps during the production (e.g. selection of inappropriate solvent for spray drying). Residual solvent can affect the API solubility in the polymer, molecular mobility (by influencing the glass-transition temperature) and induce liquid-liquid phase separation. Enhanced mobility in the ASD due to residual solvent can promote recrystallization in ASDs. The removal of residual solvent can be expensive, time-consuming, and usually requires secondary drying procedures to fulfil the regulatory requirements. The aim of this work is to predict the API solubility in polymer-solvent mixtures, solvent influence on the glass transition, and the occurrence of liquid-liquid phase separation of solvent-loaded ASDs using the thermodynamic model PC-SAFT and to experimentally validate these predictions. ASDs containing the APIs ritonavir or naproxen and the polymers poly (vinylpyrrolidone), poly (vinylpyrrolidone-co-vinyl acetate), or hydroxypropyl methylcellulose acetate succinate were spray-dried using the solvents acetone, ethanol, and dichloromethane. API solubility, sorption behavior, liquid-liquid phase separation and glass transition in the ternary API/polymer/solvent mixtures were predicted based on the binary phase behavior between API/solvent, API/polymer, and polymer/solvent and successfully validated experimentally using dynamic vapor sorption (DVS), and Raman spectroscopy. Thus, the presented methodology allows for an in-silico selection of appropriate solvent systems for solvent-based ASD preparation based on a limited amount of experimental data for binary systems only. © 2020 The Authors
    view abstractdoi: 10.1016/j.ejpb.2020.11.002
  • 2021 • 210 Microstructure formation and mechanical properties of ODS steels built by laser additive manufacturing of nanoparticle coated iron-chromium powders
    Doñate-Buendia, C. and Kürnsteiner, P. and Stern, F. and Wilms, M.B. and Streubel, R. and Kusoglu, I.M. and Tenkamp, J. and Bruder, E. and Pirch, N. and Barcikowski, S. and Durst, K. and Schleifenbaum, J.H. and Walther, F. and G...
    Acta Materialia 206 (2021)
    Oxide dispersion strengthened (ODS) steels are known for their enhanced mechanical performance at high temperatures or under radiation exposure. Their microstructure depends on the manufacturing process, from the nanoparticle addition to the base steel powder, to the processing of the nanoparticle enriched powder. The optimization and control of the processing steps still represent a challenge to establish a clear methodology for the additive manufacturing of ODS steels. Here, we evaluate the microstructure, nanoparticle evolution, and mechanical properties of ODS steels prepared by dielectrophoretic controlled adsorption of 0.08 wt% laser-synthesized yttrium oxide (Y2O3) on an iron-chromium ferritic steel powder (PM2000). The influence of the ODS steel fabrication technique is studied for two standard additive manufacturing techniques, directed energy deposition (DED) and laser powder bed fusion (LPBF). The compressive strength of the ODS steels at 600 °C is increased by 21% and 29% for the DED and LPBF samples, respectively, compared to the DED and LPBF steels manufactured without Y2O3 nanoparticle addition. The Martens hardness is enhanced by 9% for the LPBF ODS steel while no significant change is observed in the DED ODS steel. The microstructure and nanoparticle composition and distribution are evaluated by electron backscatter diffraction, scanning electron microscopy–energy-dispersive X-ray spectroscopy, and atom probe tomography, to compare the microstructural features of DED and LPBF manufactured parts. Smaller grain size and more homogeneous distribution with lower agglomeration of Y-O nanoparticles in the LPBF sample are found to be key factors for enhanced mechanical response at 600 °C. The enhanced mechanical properties of the LPBF-processed sample and the more homogeneous nanoparticle dispersion can be linked to results obtained by finite element methods simulations of the melt pool that show two orders of magnitude faster cooling rates for LPBF than for DED. Therefore, this work presents and validates a complete laser-based methodology for the preparation and processing of an ODS steel, proving the modification of the microstructure and enhancement of the high-temperature strength of the as-built parts. © 2020
    view abstractdoi: 10.1016/j.actamat.2020.116566
  • 2021 • 209 A mechanical analysis of chemically stimulated linear shape memory polymer actuation
    Dumlu, H. and Marquardt, A. and Zirdehi, E.M. and Varnik, F. and Shen, Y. and Neuking, K. and Eggeler, G.
    Materials 14 1-21 (2021)
    In the present work, we study the role of programming strain (50% and 100%), end loads (0, 0.5, 1.0, and 1.5 MPa), and chemical environments (acetone, ethanol, and water) on the exploitable stroke of linear shape memory polymer (SMP) actuators made from ESTANE ETE 75DT3 (SMP-E). Dynamic mechanical thermal analysis (DMTA) shows how the uptake of solvents results in a decrease in the glass temperature of the molecular switch component of SMP-E. A novel in situ technique allows chemically studying triggered shape recovery as a function of time. It is found that the velocity of actuation decreases in the order acetone > ethanol > water, while the exploitable strokes shows the inverse tendency and increases in the order water > ethanol > acetone. The results are interpreted on the basis of the underlying chemical (how solvents affect thermophysical properties) and micromechanical processes (the phenomenological spring dashpot model of Lethersich type rationalizes the behavior). The study provides initial data which can be used for micromechanical modeling of chemically triggered actuation of SMPs. The results are discussed in the light of underlying chemical and mechanical elementary processes, and areas in need of further work are highlighted. © 2021 by the authors. Licensee MDPI, Basel, Switzerland.
    view abstractdoi: 10.3390/ma14030481
  • 2021 • 208 Trust and stock market volatility during the COVID-19 crisis
    Engelhardt, N. and Krause, M. and Neukirchen, D. and Posch, P.N.
    Finance Research Letters 38 (2021)
    We investigate if trust affects global stock market volatility during the COVID-19 pandemic. Using a sample of 47 national stock markets, we find the stock markets’ volatility to be significantly lower in high-trust countries (in reaction to COVID-19 case announcements). Both trust in fellow citizens as well as in the countries’ governments are of significant importance. © 2020 Elsevier Inc.
    view abstractdoi: 10.1016/
  • 2021 • 207 Elucidation of mass transfer mechanisms in pellet formation by spheronization
    Evers, M. and Mattusch, A. and Weis, D. and Garcia, E. and Antonyuk, S. and Thommes, M.
    European Journal of Pharmaceutics and Biopharmaceutics 160 92-99 (2021)
    Previously published mechanisms of pellet formation during extrusion-spheronization include a transfer of material between different granules. This research aimed to specify the origin of this transfered mass, enabling further insight into the extrusion-spheronization process. Granules of various diameters were rounded simultaniously in a spheronizer to ascertain if mass transfer between smaller and larger granules is truly in balance, or if mass transfer from smaller to larger granules is preferred. Granules were also marked with a fluorescent tracer to enable quantification of mass transfer. By using differently sized and shaped granules as starting material, different modes of mass transfer were investigated. Samples were taken after various process durations to investigate the kinetics of the tranfer mechanism. It was found that both small and large granules dispense and receive mass during spheronization. In general, small granules increase their size, while large granules maintain their size or show a slight size decrease, resulting in the particularly narrow monomodal size distribution. © 2021 Elsevier B.V.
    view abstractdoi: 10.1016/j.ejpb.2021.01.013
  • 2021 • 206 Design and Characterization of a Screw Extrusion Hot-End for Fused Deposition Modeling
    Feuerbach, T. and Thommes, M.
    Molecules (Basel, Switzerland) 26 (2021)
    The filament is the most widespread feedstock material form used for fused deposition modeling printers. Filaments must be manufactured with tight dimensional tolerances, both to be processable in the hot-end and to obtain printed objects of high quality. The ability to successfully feed the filament into the printer is also related to the mechanical properties of the filament, which are often insufficient for pharmaceutically relevant excipients. In the scope of this work, an 8 mm single screw hot-end was designed and characterized, which allows direct printing of materials from their powder form and does not require an intermediate filament. The capability of the hot-end to increase the range of applicable excipients to fused deposition modeling was demonstrated by processing and printing several excipients that are not suitable for fused deposition modeling in their filament forms, such as ethylene vinyl acetate and poly(1-vinylpyrrolidone-co-vinyl acetate). The conveying characteristic of the screw was investigated experimentally with all materials and was in agreement with an established model from literature. The complete design information, such as the screw geometry and the hot-end dimensions, is provided in this work.
    view abstractdoi: 10.3390/molecules26030590
  • 2021 • 205 Impact of incorporated drugs on material properties of amorphous solid dispersions
    Flügel, K. and Schmidt, K. and Mareczek, L. and Gäbe, M. and Hennig, R. and Thommes, M.
    European Journal of Pharmaceutics and Biopharmaceutics 159 88-98 (2021)
    Formulation development of amorphous solid dispersions (ASD) still is challenging although several poorly water-soluble drugs have been marketed using this technique. During development of novel drugs, the selection of the preparation technique and polymer matrix is commonly performed for the certain drug via screening tools. However, if general trends regarding material properties are to be investigated, this approach is not beneficial, although often utilized in literature. The main component of the ASD usually is the polymer and thus it predominantly determines the material properties of the system. Therefore, to study the impact of different drugs and their drug loads on mechanical properties and wettability, three poorly soluble model drugs with drug loads ranging from 10% to 40% were incorporated into copovidone via hot-melt extrusion. The obtained extrudates were subsequently characterized regarding mechanical properties by applying diametral compression test and nanoindentation and the results were compared to the performance during tablet compression. Incorporation of all tested drugs resulted in a similar increase in brittleness of the ASDs, whereas the Young's modulus and hardness changed differently in dependence of the incorporated drug. These observations correlated well with the performance during tablet compression and it was concluded, that the brittleness seemed to be the predominant factor influencing the compression behavior of copovidone-based ASDs. Furthermore, the degree of water absorption and wettability was assessed by applying dynamic vapor sorption experiments and contact angle measurements. Here, the incorporated drugs impacted the contact angle to different degrees and a strong correlation between the contact angle and disintegration time was observable. These results highlight the importance of thorough characterization of the ASDs as it helps to predict their performance during tablet compression and thus facilitates the optimal selection of excipients. © 2020 Elsevier B.V.
    view abstractdoi: 10.1016/j.ejpb.2020.12.017
  • 2021 • 204 FibeR-CNN: Expanding Mask R-CNN to improve image-based fiber analysis
    Frei, M. and Kruis, F.E.
    Powder Technology 377 974-991 (2021)
    Fiber-shaped materials (e.g. carbon nano tubes) are of great relevance, due to their unique properties but also the health risk they can impose. Unfortunately, image-based analysis of fibers still involves manual annotation, which is a time-consuming and costly process. We therefore propose the use of region-based convolutional neural networks (R-CNNs) to automate this task. Mask R-CNN, the most widely used R-CNN for semantic segmentation tasks, is prone to errors when it comes to the analysis of fiber-shaped objects. Hence, a new architecture – FibeR-CNN – is introduced and validated. FibeR-CNN combines two established R-CNN architectures (Mask and Keypoint R-CNN) and adds additional network heads for the prediction of fiber widths and lengths. As a result, FibeR-CNN is able to surpass the mean average precision of Mask R-CNN by 33% (11 percentage points) on a novel test data set of fiber images. © 2020 Elsevier B.V.
    view abstractdoi: 10.1016/j.powtec.2020.08.034
  • 2021 • 203 Thermoplastic forming of additively manufactured Zr-based bulk metallic glass: A processing route for surface finishing of complex structures
    Frey, M. and Wegner, J. and Neuber, N. and Reiplinger, B. and Bochtler, B. and Adam, B. and Ruschel, L. and Riegler, S.S. and Jiang, H.-R. and Kleszczynski, S. and Witt, G. and Busch, R.
    Materials and Design 198 (2021)
    Additive manufacturing of bulk metallic glasses (BMGs) through laser powder bed fusion (LPBF) has drawn growing interest in the last years, especially concerning industry-relevant alloys based on iron or zirconium. The process-inherent high cooling rates and localized melting pools allow to overcome geometrical restrictions given for the production of BMGs by classical casting routes. Yet, the achievable surface qualities are still limited, making an adequate post-processing necessary. In this work, we report on applying thermoplastic forming on LPBF-formed parts for the first time to decrease surface roughness and imprint finely structured surface patterns without the need for complex abrasive machining. This BMG-specific post-processing approach allows to functionalize surface areas on highly complex LPBF-formed specimens, which could be of interest especially for medical or jewelry applications. © 2020 The Authors
    view abstractdoi: 10.1016/j.matdes.2020.109368
  • 2021 • 202 Characterization of a surface plasmon antenna fabricated on a gate-defined lateral quantum dot
    Fukai, R. and Sakai, Y. and Nakagawa, T. and Fujita, T. and Kiyama, H. and Ludwig, Ar. and Wieck, A.D. and Oiwa, A.
    Japanese Journal of Applied Physics 60 (2021)
    Quantum repeater composed of a quantum memory and an interface between photon qubits and memory qubits is indispensable for long-distance quantum communication. Gate-defined lateral quantum dots (QDs) can be a suitable platform for such quantum repeaters because of its aptitude for spin qubit and feasibility of quantum state transfer from photon polarization to electron spin. So far, the reported photoelectron excitation probabilities in such a QD are not high enough to implement practical repeater protocols. To improve the photoexcitation probability, we combine a surface plasmon antenna (SPA) with QDs. We fabricated a SPA designed to enhance the optical transmission to the QDs in a practical illumination setup in a refrigerator and characterized the fabricated antenna by measuring photocurrents at room temperature. © 2021 The Japan Society of Applied Physics
    view abstractdoi: 10.35848/1347-4065/abd533
  • 2021 • 201 Experimental multi-scale approach to determine the local mechanical properties of foam base material in polyisocyanurate metal panels
    Gahlen, P. and Fröbel, S. and Karbach, A. and Gabriel, D. and Stommel, M.
    Polymer Testing 93 (2021)
    Polyisocyanurate (PIR) foams were examined regarding their local chemical composition using ATR-IR spectroscopy. As a special parameter the PIR: Amide III intensity ratio is to be mentioned, which represents the quantity of the formed PIR groups. Based on the local PIR: Amide III intensity ratio, the mechanical properties (Young's modulus) of the foam base material were analyzed at defined positions by AFM and Nanoindentation. It turned out that the AFM method is only suitable for qualitative analysis, because the values differ strongly from macroscopic measurements. For the measurements using nanoindentation, a new embedding method was developed, which achieves significantly more realistic and reproducible results compared to the embedding method used in the literature and shows a very good agreement with the macroscopic values. In general, it has been shown that a higher PIR: Amide III intensity ratio tends to lead to a higher Young's modulus. Nevertheless, there are other, currently unknown characteristic values which also influence the Young's modulus. © 2020 The Author(s)
    view abstractdoi: 10.1016/j.polymertesting.2020.106965
  • 2021 • 200 Reducing cohesion of metal powders for additive manufacturing by nanoparticle dry-coating
    Gärtner, E. and Jung, H.Y. and Peter, N.J. and Dehm, G. and Jägle, E.A. and Uhlenwinkel, V. and Mädler, L.
    Powder Technology 379 585-595 (2021)
    Additive manufacturing processes, such as laser powder bed fusion, require steady powder processing but often exhibit poor flowability and low powder bed densities. Reducing the attractive Van-der-Waals force through nanoparticle coating can enhance initially poor flowability. We investigated the effect of dry-coating nanosized SiO2 on gas-atomized CoCrFeNi powders containing different amounts of particles &lt; 20 μm with respect to nanoparticle concentration and mixing time. The dynamic angle of repose of a 0–90 μm powder reduced 50% and bulk powder density increased 30% with nanoparticle concentrations up to 0.153 wt.-%. The granular Bond-number was correlated with the powder flowability and porosity. The effect of mixing time was investigated with mixing two fractions 20–90 μm and 0–90 μm at a constant nominal nanoparticle surface area coverage of 128% for 2 to 1440 min. Short mixing times improved the flowability, while extensive mixing resulted in nanoparticle reagglomeration and deteriorated flow. © 2020
    view abstractdoi: 10.1016/j.powtec.2020.10.065
  • 2021 • 199 Existence results for non-homogeneous boundary conditions in the relaxed micromorphic model
    Ghiba, I.-D. and Neff, P. and Owczarek, S.
    Mathematical Methods in the Applied Sciences 44 2040-2049 (2021)
    In this paper, we notice a property of the extension operator from the space of tangential traces of H(curl; Ω) in the context of the linear relaxed micromorphic model, a theory that is recently used to describe the behavior of some metamaterials showing unorthodox behaviors with respect to elastic wave propagation. We show that the new property is important for existence results of strong solution for non-homogeneous boundary condition in both the dynamic and the static case. © 2020 John Wiley & Sons, Ltd.
    view abstractdoi: 10.1002/mma.6913
  • 2021 • 198 Charge Regulation at a Nanoporous Two-Dimensional Interface
    Ghosh, M. and Junker, M.A. and Van Lent, R.T.M. and Madauß, L. and Schleberger, M. and Lebius, H. and Benyagoub, A. and Wood, J.A. and Lammertink, R.G.H.
    ACS Omega 6 2487-2493 (2021)
    In this work, we have studied the pH-dependent surface charge nature of nanoporous graphene. This has been investigated by membrane potential and by streaming current measurements, both with varying pH. We observed a lowering of the membrane potential with decreasing pH for a fixed concentration gradient of potassium chloride (KCl) in the Donnan dominated regime. Interestingly, the potential reverses its sign close to pH 4. The fitted value of effective fixed ion concentration (C¯ R) in the membrane also follows the same trend. The streaming current measurements show a similar trend with sign reversal around pH 4.2. The zeta potential data from the streaming current measurement is further analyzed using a 1-pK model. The model is used to determine a representative pK (acid-base equilibrium constant) of 4.2 for the surface of these perforated graphene membranes. In addition, we have also theoretically investigated the effect of the PET support in our membrane potential measurement using numerical simulations. Our results indicate that the concentration drop inside the PET support can be a major contributor (up to 85%) for a significant deviation of the membrane potential from the ideal Nernst potential. © 2021 The Authors. Published by American Chemical Society.
    view abstractdoi: 10.1021/acsomega.0c03958
  • 2021 • 197 Nickel nanoparticles supported on nitrogen–doped carbon nanotubes are a highly active, selective and stable CO2 methanation catalyst
    Gödde, J. and Merko, M. and Xia, W. and Muhler, M.
    Journal of Energy Chemistry 54 323-331 (2021)
    CO2 methanation using nickel-based catalysts has attracted large interest as a promising power-to-gas route. Ni nanoparticles supported on nitrogen-doped CNTs with Ni loadings in the range from 10 wt% to 50 wt% were synthesized by impregnation, calcination and reduction and characterized by elemental analysis, X-ray powder diffraction, H2 temperature-programmed reduction, CO pulse chemisorption and transmission electron microscopy. The Ni/NCNT catalysts were highly active in CO2 methanation at atmospheric pressure, reaching over 50% CO2 conversion and over 95% CH4 selectivity at 340 °C and a GHSV of 50,000 mL g−1 h−1 under kinetically controlled conditions. The small Ni particle sizes below 10 nm despite the high Ni loading is ascribed to the efficient anchoring on the N-doped CNTs. The optimum loading of 30 wt%–40 wt% Ni was found to result in the highest Ni surface area, the highest degree of conversion and the highest selectivity to methane. A constant TOF of 0.3 s−1 was obtained indicating similar catalytic properties of the Ni nanoparticles in the range from 10 wt% to 50 wt% Ni loading. Long-term experiments showed that the Ni/NCNT catalyst with 30 wt% Ni was highly stable for 100 h time on stream. © 2020 Science Press
    view abstractdoi: 10.1016/j.jechem.2020.06.007
  • 2021 • 196 A new approach for open-end sequential change point monitoring
    Gösmann, J. and Kley, T. and Dette, H.
    Journal of Time Series Analysis 42 63-84 (2021)
    We propose a new sequential monitoring scheme for changes in the parameters of a multivariate time series. In contrast to procedures proposed in the literature which compare an estimator from the training sample with an estimator calculated from the remaining data, we suggest to divide the sample at each time point after the training sample. Estimators from the sample before and after all separation points are then continuously compared calculating a maximum of norms of their differences. For open-end scenarios our approach yields an asymptotic level (Formula presented.) procedure, which is consistent under the alternative of a change in the parameter. By means of a simulation study it is demonstrated that the new method outperforms the commonly used procedures with respect to power and the feasibility of our approach is illustrated by analyzing two data examples. © 2020 The Authors. Journal of Time Series Analysis published by John Wiley & Sons Ltd.
    view abstractdoi: 10.1111/jtsa.12555
  • 2021 • 195 Prehistoric salt mining in Hallstatt, Austria. New chronologies out of small wooden fragments
    Grabner, M. and Wächter, E. and Nicolussi, K. and Bolka, M. and Sormaz, T. and Steier, P. and Wild, E.M. and Barth, F.E. and Kern, A. and Rudorfer, J. and Kowarik, K. and Stöllner, T. and Reschreiter, H.
    Dendrochronologia 66 (2021)
    The prehistoric salt mine of Hallstatt together with its burial ground is one of the most prominent archaeological sites in the world, and has given its name to the “Hallstatt period”, an epoch of European prehistory (800 to 400 BCE). Due to the perfect conservation in rock salt a high number of organic materials have been found, including mostly wooden artefacts and structural timber. More than 2000 samples were taken from various archaeological sites in the mines as well as at the surface. It was possible to date 763 samples by the means of dendrochronology and by 14C wiggle matching. The dendrochronological dating was possible due to crossdating with various available chronologies (like Villingen-Magdalenenberg or Dachstein/Schwarzer See). The fir (Abies alba Mill.) chronologies span the periods: -1232 to -1063; -819 to -425 and -371 to-129. The spruce (Picea abies L. Karst.) chronologies span the periods: -1228 to -1063; -813 to -669 and -342 to -123. The larch (Larix decidua Mill.) chronologies span the periods: -1393 ± 18 to-1286 ± 18 based on wiggle matching data and -252 to -164. A beech (Fagus sylvatica L.) chronology span the time -1182 to -1062. © 2021 The Author(s)
    view abstractdoi: 10.1016/j.dendro.2021.125814
  • 2021 • 194 Resistance to avapritinib in pdgfra-driven gist is caused by secondary mutations in the pdgfra kinase domain
    Grunewald, S. and Klug, L.R. and Mühlenberg, T. and Lategahn, J. and Falkenhorst, J. and Town, A. and Ehrt, C. and Wardelmann, E. and Hartmann, W. and Schildhaus, H.-U. and Treckmann, J. and Fletcher, J.A. and Jung, S. and Czodro...
    Cancer Discovery 11 108-125 (2021)
    Gastrointestinal stromal tumors (GIST) harboring activating mutations of PDGFRA respond to imatinib, with the notable exception of the most common mutation, D842V. Avapritinib is a novel, potent KIT/PDGFRA inhibitor with substantial clinical activity in patients with the D842V genotype. To date, only a minority of PDGFRA-mutant patients treated with avapri-tinib have developed secondary resistance. Tumor and plasma biopsies in 6 of 7 patients with PDGFRA primary mutations who progressed on avapritinib or imatinib had secondary resistance mutations within PDGFRA exons 13, 14, and 15 that interfere with avapritinib binding. Secondary PDGFRA mutations causing V658A, N659K, Y676C, and G680R substitutions were found in 2 or more patients each, representing recurrent mechanisms of PDGFRA GIST drug resistance. Notably, most PDGFRA-mutant GISTs refractory to avapritinib remain dependent on the PDGFRA oncogenic signal. Inhibitors that target PDGFRA protein stability or inhibition of PDGFRA-dependent signaling pathways may overcome avapritinib resistance. © 2020 American Association for Cancer Research.
    view abstractdoi: 10.1158/2159-8290.CD-20-0487
  • 2021 • 193 Large-scale synthesis of iron oxide/graphene hybrid materials as highly efficient photo-Fenton catalyst for water remediation
    Hammad, M. and Fortugno, P. and Hardt, S. and Kim, C. and Salamon, S. and Schmidt, T.C. and Wende, H. and Schulz, C. and Wiggers, H.
    Environmental Technology and Innovation 21 (2021)
    The Photo-Fenton reaction is an advanced oxidation process to break down organic pollutants in aqueous systems. Moreover, the scalable synthesis and engineering of stable catalysts with a high specific surface area is extremely important for the practical application of the Photo-Fenton process. In the current study, we developed a low-cost method for large-scale production of iron oxide/graphene nanostructures with a controllable graphene loading for the photo-Fenton reaction. Under optimal condition, high efficiencies of degradation (&gt;99%) of methylene blue, rhodamine B, acid orange 7, and phenol at a concentration (60 mg/mL) were reached in 60 min under UV-A irradiation (1.6 mW/cm2) with mineralization of 72, 77, 82, and 48%, respectively. More importantly, the iron oxide/graphene nanocomposites exhibited good stability over a wide range of pH (from 3 to 9) and can be magnetically separated from the solution and repeatedly used with consistent photocatalytic performance. This enhanced removal efficiency of the iron oxide/graphene nanostructure compared to iron oxide nanoparticles is attributed to the accelerated transfer of photo-generated electrons between iron oxide and graphene and its relatively large surface area. The results demonstrate that the iron oxide/graphene system could be potentially utilized for many environmental treatment processes. © 2020 Elsevier B.V.
    view abstractdoi: 10.1016/j.eti.2020.101239
  • 2021 • 192 Toward tailoring the degradation rate of magnesium-based biomaterials for various medical applications: Assessing corrosion, cytocompatibility and immunological effects
    Hartjen, P. and Wegner, N. and Ahmadi, P. and Matthies, L. and Nada, O. and Fuest, S. and Yan, M. and Knipfer, C. and Gosau, M. and Walther, F. and Smeets, R.
    International Journal of Molecular Sciences 22 1-13 (2021)
    Magnesium (Mg)-based biomaterials hold considerable promise for applications in regenerative medicine. However, the degradation of Mg needs to be reduced to control toxicity caused by its rapid natural corrosion. In the process of developing new Mg alloys with various surface modifications, an efficient assessment of the relevant properties is essential. In the present study, a WE43 Mg alloy with a plasma electrolytic oxidation (PEO)-generated surface was investigated. Surface microstructure, hydrogen gas evolution in immersion tests and cytocompatibility were assessed. In addition, a novel in vitro immunological test using primary human lymphocytes was introduced. On PEO-treated WE43, a larger number of pores and microcracks, as well as increased roughness, were observed compared to untreated WE43. Hydrogen gas evolution after two weeks was reduced by 40.7% through PEO treatment, indicating a significantly reduced corrosion rate. In contrast to untreated WE43, PEO-treated WE43 exhibited excellent cytocompatibility. After incubation for three days, untreated WE43 killed over 90% of lymphocytes while more than 80% of the cells were still vital after incubation with the PEO-treated WE43. PEO-treated WE43 slightly stimulated the activation, proliferation and toxin (perforin and granzyme B) expression of CD8+ T cells. This study demonstrates that the combined assessment of corrosion, cytocompatibility and immunological effects on primary human lymphocytes provide a comprehensive and effective procedure for characterizing Mg variants with tailorable degradation and other features. PEO-treated WE43 is a promising candidate for further development as a degradable biomaterial. © 2021 by the authors. Licensee MDPI, Basel, Switzerland.
    view abstractdoi: 10.3390/ijms22020971
  • 2021 • 191 Control of electron velocity distributions at the wafer by tailored voltage waveforms in capacitively coupled plasmas to compensate surface charging in high-aspect ratio etch features
    Hartmann, P. and Wang, L. and Nösges, K. and Berger, B. and Wilczek, S. and Brinkmann, R.P. and Mussenbrock, T. and Juhasz, Z. and Donkó, Z. and Derzsi, A. and Lee, E. and Schulze, J.
    Journal of Physics D: Applied Physics 54 (2021)
    Low pressure single- or dual-frequency capacitively coupled radio frequency (RF) plasmas are frequently used for high-aspect ratio (HAR) dielectric etching due to their capability to generate vertical ion bombardment of the wafer at high energies. Electrons typically reach the wafer at low energies and with a wide angular distribution during the local sheath collapse. Thus, in contrast to positive ions, electrons cannot propagate deeply into HAR etch features and the bottom as well as the sidewalls of such trenches can charge up positively, while the mask charges negatively. This causes etch stops and distortion of profile shapes. Here, we investigate low pressure, high voltage capacitively coupled RF argon gas discharges by Particle-In-Cell/Monte Carlo collisions simulations and demonstrate that this problem can be solved by Voltage Waveform Tailoring, i.e. the velocity and angular distribution of electrons impacting on the electrodes can be tuned towards high velocities and small angles to the surface-normal, while keeping the energies of the impacting ions high. The applied voltage waveforms consist of a base frequency of 400 kHz with 10 kV amplitude and a series of higher harmonics. A high frequency component at 40 or 60 MHz is used additionally. Square voltage waveforms with different rise-times are examined as well. We show that high fluxes of electrons towards the wafer at normal velocities of up to 2.2 × 107 m s-1 (corresponding to 1.4 keV energy) can be realized. © 2021 The Author(s). Published by IOP Publishing Ltd.
    view abstractdoi: 10.1088/1361-6463/abf229
  • 2021 • 190 Beam divergence reduction of vortex waves with a tailored lens and a tailored reflector
    Hassan, M.H. and Sievert, B. and Svejda, J.T. and Ahmed, A.M. and Barowski, J. and Rennings, A. and Rolfes, I. and Sezgin, A. and Erni, D.
    IEEE Access 9 9800-9811 (2021)
    Reducing the strong beam divergence inherent to Orbital Angular Momentum waves (also known as OAM waves or vortex waves), a tailored lens and a tailored reflector are presented in this study. The generation of the OAM waves is accomplished by a Uniform Circular Patch Antenna Array (UCA) operating at 10 GHz. Here, the tailored lens and reflector are set up by two correspondingly designed shape functions rotated around the antenna's center axis in broadside direction (i.e. body of revolution approach). Initially, the tailored lens is introduced to be compared to the UCA in the presence and absence of the conventional lens separately. Upon the usage of the tailored lens, a gain improvement of 5.8 dB has been obtained in the simulation compared to a gain of 4.8 dB in the measurement. On the other hand, the tailored reflector is set under the same procedure to be compared also to the UCA with and without a conventional reflector. Both of the reflectors are simulated under idealized conditions with the aid of an OAM impressed field source used as an emitter for a meaningful comparison. The simulated gain has shown a better performance accomplished by the tailored reflector as the height r0 reaches a level less than 1.5 λ as well as the opening angle θ is less than 38° (given an UCA with an element separation distance d=λ/2). Furthermore, three different ground plane shapes with realistic UCA are applied for the simulation procedure where each of them is perturbing the radiation of the reflector. All of the lenses and the reflectors are manufactured and later measured in an anechoic chamber to undergo a comparison with the simulated results. This article demonstrates that the vortex waves need a tailored lens or a tailored reflector to decrease the beam divergence effectively especially when the radius of the UCA becomes increasingly large. © 2013 IEEE.
    view abstractdoi: 10.1109/ACCESS.2021.3050043
  • 2021 • 189 Robust dynamic adaptation of the Smagorinsky model based on a sub-grid activity sensor
    Hasslberger, J. and Engelmann, L. and Kempf, A. and Klein, M.
    Physics of Fluids 33 (2021)
    The present study deals with the application of a sub-grid activity sensor to an eddy viscosity type base model in the context of Large Eddy Simulation (LES). The coherent structure function is used to build this sensor in combination with explicit test filtering. The proposed sensor features two main advantages: First, it attenuates the sub-grid scale dissipation of the base model for transitional flows. Depending on local conditions, the sensor is essentially blending the eddy viscosity between zero and the value resulting from the standard Smagorinsky model. Second, the sensor rectifies the incorrect near-wall scaling of the standard Smagorinsky model. Application of the sensor requires no averaging (in the homogeneous direction) and is easy to implement, and the additional computational cost is insignificant. In order to assess the model, three different configurations have been examined: laminar-to-turbulent transition in the Taylor-Green vortex, wall-dominated channel flows, and a free planar jet flow including passive scalar mixing. Based on a posteriori LES, it has been found that the new sensor-enhanced Smagorinsky model often outperforms established eddy viscosity models from the literature, such as the standard Smagorinsky model and the sigma model, as well as the LES without the explicit sub-grid model. © 2021 Author(s).
    view abstractdoi: 10.1063/5.0032117
  • 2021 • 188 Effect of interface dislocations on mass flow during high temperature and low stress creep of single crystal Ni-base superalloys
    He, J. and Cao, L. and Makineni, S.K. and Gault, B. and Eggeler, G.
    Scripta Materialia 191 23-28 (2021)
    In this work, the nanometer-scale mass flow coupled to dislocation processes near the γ/γ′-interface during high temperature and low stress creep of a model Ni-base single crystal superalloy is investigated. In the early creep stages, the dislocation networks in the γ-phase at γ/γ′-interfaces attract γ-stabilizing elements like Cr, Co and in particular Re, resulting in compositional gradients close to the interface. At larger strains, where dislocations frequently cut into the γ′-phase, this local interfacial enrichment in these elements is no longer observed. The cutting dislocations take part of the segregated atoms away, whilst the remaining atoms are released and diffuse back into the γ-channels. © 2020
    view abstractdoi: 10.1016/j.scriptamat.2020.09.016
  • 2021 • 187 Elemental segregation to lattice defects in the CrMnFeCoNi high-entropy alloy during high temperature exposures
    Heczko, M. and Mazánová, V. and Gröger, R. and Záležák, T. and Hooshmand, M.S. and George, E.P. and Mills, M.J. and Dlouhý, A.
    Acta Materialia 208 (2021)
    The influence of small plastic pre-strains on the elevated-temperature stability and microstructure of the equiatomic CrMnFeCoNi FCC solid solution is investigated. Particular attention is given to whether any of the alloy elements segregate to individual dislocations. To that end, CrMnFeCoNi samples were first deformed in tension at room temperature to plastic strains of 0.2 and 2.3%, and subsequently annealed at 973 K for 800 hours. The pre-strains activated planar slip of 1/2&lt;110&gt;-type dislocations on {111}-type glide planes. Interactions of this planar slip with special Σ3 grain boundaries formed a large number of dislocation segments with a &lt;110&gt;-type crystallographic orientation suitable for a credible end-on analysis of dislocation cores in HR-STEM. The cores of the 1/2&lt;110&gt; dislocations pushed up against the investigated grain boundaries were found to be close to the compact configuration. Within the sensitivity of the Super-X EDS mapping, no concentration gradient was detected near dislocations that would indicate enrichment at dislocation cores of any of the elemental constituents of the alloy after the pre-deformation and annealing. However, a Cr-rich tetragonal sigma phase nucleated and grew at grain boundary triple junctions during this anneal, processes that were not accelerated by the enhanced dislocation density present after pre-strain. A clear chromium gradient was observed in the Cr-depleted zones near grain boundaries suggesting that Cr transport occurred by relatively slow diffusion from the bulk to the grain boundaries and then by relatively fast diffusion along the grain boundaries to the precipitates. Accompanying the Cr depletion near grain boundaries is a simultaneous Ni and Mn enrichment, which promotes formation of the L10 NiMn phase that is observed on the grain boundaries after prolonged annealing. © 2021
    view abstractdoi: 10.1016/j.actamat.2021.116719
  • 2021 • 186 Dislocation networks in gamma/gamma’-microstructures formed during selective laser melting of a Ni-base superalloy
    Heep, L. and Schwalbe, C. and Heinze, C. and Dlouhy, A. and Rae, C.M.F. and Eggeler, G.
    Scripta Materialia 190 121-125 (2021)
    A dislocation network which formed during selective laser melting (SLM) of a Ni-base superalloy was analyzed using scanning transmission electron microscopy (STEM). This network traverses an ordered Gamma'-phase domain, in between two adjacent Gamma-solid solution regions. The Gamma’-phase region has formed when two Gamma’-phase particles have started to coalesce, trapping the dislocation network in this ordered region so that it formed two dislocation families with pairs of anti-phase boundary (APB) coupled super partial dislocations. The network features are presented and unusual features (twist character and low APB energies), not previously reported, are discussed. © 2020
    view abstractdoi: 10.1016/j.scriptamat.2020.08.019
  • 2021 • 185 Protein signature of human skin fibroblasts allows the study of the molecular etiology of rare neurological diseases
    Hentschel, A. and Czech, A. and Münchberg, U. and Freier, E. and Schara-Schmidt, U. and Sickmann, A. and Reimann, J. and Roos, A.
    Orphanet Journal of Rare Diseases 16 (2021)
    Background: The elucidation of pathomechanisms leading to the manifestation of rare (genetically caused) neurological diseases including neuromuscular diseases (NMD) represents an important step toward the understanding of the genesis of the respective disease and might help to define starting points for (new) therapeutic intervention concepts. However, these “discovery studies” are often limited by the availability of human biomaterial. Moreover, given that results of next-generation-sequencing approaches frequently result in the identification of ambiguous variants, testing of their pathogenicity is crucial but also depending on patient-derived material. Methods: Human skin fibroblasts were used to generate a spectral library using pH8-fractionation of followed by nano LC-MS/MS. Afterwards, Allgrove-patient derived fibroblasts were subjected to a data independent acquisition approach. In addition, proteomic signature of an enriched nuclear protein fraction was studied. Proteomic findings were confirmed by immunofluorescence in a muscle biopsy derived from the same patient and cellular lipid homeostasis in the cause of Allgrove syndrome was analysed by fluorescence (BODIPY-staining) and coherent anti-Stokes Raman scattering (CARS) microscopy. Results: To systematically address the question if human skin fibroblasts might serve as valuable biomaterial for (molecular) studies of NMD, we generated a protein library cataloguing 8280 proteins including a variety of such linked to genetic forms of motoneuron diseases, congenital myasthenic syndromes, neuropathies and muscle disorders. In silico-based pathway analyses revealed expression of a diversity of proteins involved in muscle contraction and such decisive for neuronal function and maintenance suggesting the suitability of human skin fibroblasts to study the etiology of NMD. Based on these findings, next we aimed to further demonstrate the suitability of this in vitro model to study NMD by a use case: the proteomic signature of fibroblasts derived from an Allgrove-patient was studied. Dysregulation of paradigmatic proteins could be confirmed in muscle biopsy of the patient and protein-functions could be linked to neurological symptoms known for this disease. Moreover, proteomic investigation of nuclear protein composition allowed the identification of protein-dysregulations according with structural perturbations observed in the muscle biopsy. BODIPY-staining on fibroblasts and CARS microscopy on muscle biopsy suggest altered lipid storage as part of the underlying disease etiology. Conclusions: Our combined data reveal that human fibroblasts may serve as an in vitro system to study the molecular etiology of rare neurological diseases exemplified on Allgrove syndrome in an unbiased fashion. © 2021, The Author(s).
    view abstractdoi: 10.1186/s13023-020-01669-1
  • 2021 • 184 Investigation of the frequency dependent spatio-temporal dynamics and controllability of microdischarges in unipolar pulsed plasma electrolytic oxidation
    Hermanns, P. and Boeddeker, S. and Bracht, V. and Bibinov, N. and Grundmeier, G. and Awakowicz, P.
    Journal of Physics D: Applied Physics 54 (2021)
    The unipolar pulsed-plasma electrolytic oxidation (PEO) of aluminum has been replaced by bipolar pulsed methods that use a so-called 'soft-sparking'mode. This method results in an effective reduction of intense microdischarges, which are detrimental to the oxide layer. In a previous publication, we developed an in-situ multivariable microdischarge control scheme using unipolar pulsing. Using this method, it is possible to restrict the mean microdischarge size to well-defined limits, while at the same time influencing the mean microdischarge energy, number density or spectral emission behaviour. This method operates well inside a frequency range of f = 1-20 kHz. Although this method shows highly desirable plasma control properties, the mechanisms defining this frequency-dependent controllability are unclear. The aim of this study is to visualize the spatio-temporal behavior of microdischarges in higher frequency ranges. First, a wavelet transform was performed to estimate the temporal evolution of microdischarge lifetimes. Ceramic coatings were then deposited on aluminum alloy substrates in an aqueous solution using unipolar pulsed galvanostatic PEO. The aluminum samples were coated for 30 min at frequencies of f1 = 50 Hz, f2 = 5 kHz and f3 = 100 kHz. High-speed imaging was carried out utilizing four synchronized intensified charge-coupled device (ICCD) cameras, each with a 500 ns exposure time. At f2 = 5 kHz, the microdischarges were still able to follow the electrical pulses. In this regime, the process can be divided into two stages, an initial charging of the substrate surface without plasma emission and a subsequent slower evolution of microdischarges. Equivalent circuit model descriptions are given for both processes. At f3 = 100 kHz, microdischarges were not able to follow the pulse frequency, as the lifetimes and risetimes of the microdischarge characteristics were longer than the pulse length. Reignition at the same spatial location, clustering and permanent ignition through pulse periods were observed. © 2020 IOP Publishing Ltd.
    view abstractdoi: 10.1088/1361-6463/abbde4
  • 2021 • 183 Threshold photoionization shows no sign of nitryl hydride in methane oxidation with nitric oxide
    Hoener, M. and Bodi, A. and Hemberger, P. and Endres, T. and Kasper, T.
    Physical Chemistry Chemical Physics 23 1265-1272 (2021)
    Methane was doped with nitric oxide and oxidized in a high-pressure flow reactor. The nitrogen chemistry during partial oxidation was studied using photoelectron photoion coincidence spectroscopy with vacuum ultraviolet synchrotron radiation. The adiabatic ionization energy of nitrous acid, HONO, has been determined as 10.95 ± 0.03 eV. The HONO breakdown diagram was plotted based solely on the measured parent signal and the computed Franck-Condon envelope of trans-HONO, confirming the trans-HONO dissociative photoionization threshold to NO+ + OH at 11.34 eV. The spectra show strong indication for the presence of cis-HONO. We expected the m/z 47 photoion mass selected threshold photoelectron signal to rebound near 12 eV, i.e., at the ionization energy of nitryl hydride, the third HNO2 isomer. Recent computational studies suggest nitryl hydride is formed at a rate similar to trans-HONO, is more thermally stable than nitrous acid, its cation is bound, and its photoelectron spectrum is predicted to exhibit a strong origin band near 12 eV. The absence of its mass selected threshold photoelectron signal shows that nitryl hydride is either not formed in measurable amounts or is consumed faster than nitrous acid, for instance by isomerization to trans-HONO. This journal is © the Owner Societies.
    view abstractdoi: 10.1039/d0cp04924g
  • 2021 • 182 Thermoelastic properties and γ’-solvus temperatures of single-crystal Ni-base superalloys
    Horst, O.M. and Schmitz, D. and Schreuer, J. and Git, P. and Wang, H. and Körner, C. and Eggeler, G.
    Journal of Materials Science 56 7637-7658 (2021)
    Abstract: The present work shows that thermal expansion experiments can be used to measure the γʼ-solvus temperatures of four Ni-base single-crystal superalloys (SX), one with Re and three Re-free variants. In the case of CMSX-4, experimental results are in good agreement with numerical thermodynamic results obtained using ThermoCalc. For three experimental Re-free alloys, the experimental and calculated results are close. Transmission electron microscopy shows that the chemical compositions of the γ- and the γʼ-phases can be reasonably well predicted. We also use resonant ultrasound spectroscopy (RUS) to show how elastic coefficients depend on chemical composition and temperature. The results are discussed in the light of previous results reported in the literature. Areas in need of further work are highlighted. Graphical abstract: [Figure not available: see fulltext.] © 2021, The Author(s).
    view abstractdoi: 10.1007/s10853-020-05628-w
  • 2021 • 181 A W-Band Transceiver Chip for Future 5G Communications in InP-DHBT Technology
    Hossain, M. and Shivan, T. and Hrobak, M. and Al-Sawaf, T. and Stoppel, D. and Yacoub, H. and Weimann, N. and Heinrich, W. and Krozer, V.
    EuMIC 2020 - 2020 15th European Microwave Integrated Circuits Conference 193-196 (2021)
    This paper presents a W-band transceiver chip using InP-DHBT technology for future 5G application. It consists of a transceiver switch, a medium power amplifier (MPA) and a low noise amplifier (LNA) in 0.8 µm InP-DHBT technology. The switch operates from 75 GHz to 110 GHz and simulation results show more than 20 dB isolation and 1 dB output power (P1dBout) of 15 dBm. The measured MPA exhibits 16 dBm saturated output power (Psat) with 18 % power added efficiency (PAE) at 90 GHz. The measured LNA small signal gain is higher than 30 dB from 75 to 110 GHz and the measured noise figure values are below 9 dB. After integrating individual components (switch, LNA and PA), the entire transceiver chip achieves a measured isolation of more than 15 dB. The entire circuit consumes total 280 mW DC power. The chip area is only 2.5x1.5 mm2, To the knowledge of the authors, this is the first monolithically integrated transceiver covering the W-band for future 5G communication reported so far. © 2021 EuMA.
    view abstract
  • 2021 • 180 Study on the Fuel Consumptions Using Traffic Simulation with Example of City Duisburg
    Hu, X. and Ma, X. and Schramm, D.
    Mechanisms and Machine Science 88 488-495 (2021)
    With the development of microscopic traffic simulation and computer performance, traffic simulation is no longer limited to solving problems in the field of transportation. Microscopic traffic simulation can provide massive details, which can provide more possibilities for many studies. For studying traffic emissions, fuel consumptions and how the vehicle dynamic characteristics will affect traffic flow, a series of driver model and vehicle model for traffic simulation were built in this work. A traffic scenario of part of City Duisburg was also built in Simulation of Urban MObility (SUMO), which took advantage of the road network from Open Street Map (OSM) and traffic counter data provided by the municipality of Duisburg. As the vehicle models in SUMO are quite abstract and simplified, more detailed driver model and vehicle models were developed and applied in the simulation. The driver model is a fuzzy logic model based on behavior data of human drivers collected from driving simulator. In the vehicle models the characteristics of powertrains, braking systems, and the changes of driving resistances are all considered. These models make the simulated vehicles more similar to human drivers and realistic vehicles. The average fuel consumptions in this scenario was studied and the results with different models were also compared. © 2020, The Editor(s) (if applicable) and The Author(s), under exclusive license to Springer Nature Switzerland AG.
    view abstractdoi: 10.1007/978-3-030-60076-1_44
  • 2021 • 179 Determination of residual dimethyl sulfoxide by high-resolution continuum source graphite furnace molecular absorption spectrometry
    Huang, M.D. and Esser, N. and Hinrichs, K. and Tan, Q. and Rappich, J. and Nickel, N.H. and Dittrich, T.
    Spectrochimica Acta - Part B Atomic Spectroscopy 177 (2021)
    High-resolution continuum source molecular absorption spectrometry (HR-CSMAS) has been extended to the determination of sulfur-containing organic molecule, on the example of dimethyl sulfoxide (DMSO) as a residual solvent in CH3NH3PbI3 perovskite thin films. For this purpose, DMSO molecules were converted into CS molecules by pyrolysis in a graphite furnace at low temperature and the sulfur content was determined by measuring molecular absorption of CS at 258.055 nm. An aqueous solution of DMSO was used for calibration. A characteristic mass of 17 ng was achieved for S after pyrolysis at 160 °C by using Pd as a chemical modifier. Furthermore, the content of DMSO was normalized to that of Pb whereas the content of Pb was determined by reducing the Pb in the perovskite to metallic Pb with Zn powder and measuring absorption of the weak Pb line at 261.418 nm by high-resolution continuum source atomic absorption spectrometry (HR-CSAAS). The Pb content remained constant whereas the S/Pb molar ratio decreased with increasing annealing time. Our results open new opportunities for the characterization of residual DMSO in wide classes of materials. © 2020 Elsevier B.V.
    view abstractdoi: 10.1016/j.sab.2020.106050
  • 2021 • 178 New signatures of the spin gap in quantum point contacts
    Hudson, K.L. and Srinivasan, A. and Goulko, O. and Adam, J. and Wang, Q. and Yeoh, L.A. and Klochan, O. and Farrer, I. and Ritchie, D.A. and Ludwig, Ar. and Wieck, A.D. and von Delft, J. and Hamilton, A.R.
    Nature Communications 12 (2021)
    One dimensional semiconductor systems with strong spin-orbit interaction are both of fundamental interest and have potential applications to topological quantum computing. Applying a magnetic field can open a spin gap, a pre-requisite for Majorana zero modes. The spin gap is predicted to manifest as a field dependent dip on the first 1D conductance plateau. However, disorder and interaction effects make identifying spin gap signatures challenging. Here we study experimentally and numerically the 1D channel in a series of low disorder p-type GaAs quantum point contacts, where spin-orbit and hole-hole interactions are strong. We demonstrate an alternative signature for probing spin gaps, which is insensitive to disorder, based on the linear and non-linear response to the orientation of the applied magnetic field, and extract a spin-orbit gap ΔE ≈ 500 μeV. This approach could enable one-dimensional hole systems to be developed as a scalable and reproducible platform for topological quantum applications. © 2021, The Author(s).
    view abstractdoi: 10.1038/s41467-020-19895-3
  • 2021 • 177 Solvent Selection in Homogeneous Catalysis - Optimization of Kinetics and Reaction Performance
    Huxoll, F. and Jameel, F. and Bianga, J. and Seidensticker, T. and Stein, M. and Sadowski, G. and Vogt, D.
    ACS Catalysis 11 590-594 (2021)
    Solvents have an enormous impact on yield and turnover of chemical reactions in complex media. There is, however, a lack of consistent model-based tools to a priori identify the appropriate solvent for homogeneously catalyzed reactions. Here, a thermodynamically consistent approach for a reductive amination reaction is presented. It combines solvent screening using a thermodynamic-activity model and quantum chemical calculations. The optimization of activity coefficient-based predicted kinetics gives a suitable list of candidate solvents. The results were confirmed by batch experiments in selected solvents. This approach allows reducing time and lab resources for solvent selection to a minimum. ©
    view abstractdoi: 10.1021/acscatal.0c04431
  • 2021 • 176 Melt milling as manufacturing method for solid crystalline suspensions
    da Igreja, P. and Erve, A. and Thommes, M.
    European journal of pharmaceutics and biopharmaceutics : official journal of Arbeitsgemeinschaft fur Pharmazeutische Verfahrenstechnik e.V 158 245-253 (2021)
    Production of submicron particles (0.1-1 μm) has been identified by the pharmaceutical industry as a key technology to enhance the bioavailability of poorly water-soluble drugs. However, nanosuspensions derived from commonly applied wet milling suffer from long-term stability issues, making further downstream processing necessary. In previous works, the formulation as a long-term stable solid crystalline suspension (SCS) was introduced, for which the crystalline drug is ground in a (molten) hydrophilic carrier matrix. The model formulation of the antimycotic Griseofulvin and the sugar alcohol Xylitol was reused for comparative purposes. Due to process limitations regarding the degree of comminution, the present work demonstrates the application of fine grinding in the framework of SCS manufacturing. A custom-built mill with annular gap geometry successfully yielded particles in the targeted submicron range. A process optimization study lead to improved energy utilization during grinding, which reduced the necessary grinding time and, thereby, the thermal exposition of the drug. Investigation of solid-state properties of the SCS, via differential scanning calorimetry and x-ray powder diffraction, showed no alteration even for extended grinding times. In dissolution experiments, the melt-milled SCS outperformed its predecessors, although mostly agglomerates were found by SEM imaging in the solidified product. In conclusion, melt milling is a valuable tool to overcome low aqueous solubility. Copyright © 2020. Published by Elsevier B.V.
    view abstractdoi: 10.1016/j.ejpb.2020.11.020
  • 2021 • 175 Low temperature sintering of fully inorganic all-solid-state batteries – Impact of interfaces on full cell performance
    Ihrig, M. and Finsterbusch, M. and Tsai, C.-L. and Laptev, A.M. and Tu, C.-H. and Bram, M. and Sohn, Y.J. and Ye, R. and Sevinc, S. and Lin, S.-K. and Fattakhova-Rohlfing, D. and Guillon, O.
    Journal of Power Sources 482 (2021)
    One of the necessary prerequisites to advance the electrochemical performance of Li7La3Zr2O12 (LLZ) based all-solid-state lithium batteries is the manufacturing of dense composite cathodes from cathode active material (CAM) and the LLZ ceramic solid electrolyte. However, free co-sintering of LLZ and CAM mixtures requires temperatures above 1000 °C which often leads to decomposition and secondary phase formation, especially for high energy CAMs. In our study we present a completely dry processing route which is fast, free of any sintering additives and coatings and suitable to fabricate dense mixed cathodes, pure LLZ separators and multilayers of the two. Through application of high mechanical pressure during Field-Assisted Sintering we were able to reduce the sintering temperature down to 675–750 °C with dwell times as low as 10 min, while still obtaining 95% theoretical density for LCO/LLZ mixtures. The low sintering temperature is suitable for high energy CAMs, but leads to a significant effect of surface impurities, especially from powder handling in air, and affects the crystallinity of the CAM/LLZ interface. In the present paper we investigate the impact of resulting interfaces on the ionic conductivity, the interfacial impedance and the cycling stability of produced cells and propose the optimization strategy. © 2020 Elsevier B.V.
    view abstractdoi: 10.1016/j.jpowsour.2020.228905
  • 2021 • 174 Detailed simulations of the DLR auto-igniting pulsed jet experiment
    Inanc, E. and Lipkowicz, J.T. and Kempf, A.M.
    Fuel 284 (2021)
    Numerical simulations of an auto-igniting pulsed jet in a vitiated co-flow experiment by DLR (German Aerospace Center) are conducted by highly-resolved large-eddy simulations using direct chemistry with an augmented reduced mechanism. The experiments consist of two operation modes: continuous injection used for code-verification and pulsed injection utilized for fundamental investigation of auto-ignition dynamics. Initially, reference one-dimensional self-igniting counter-flow flames are investigated. Then, a grid convergence study has been performed. It is shown that even a coarser grid would be sufficient to describe the ignition chemistry since the ignition kernel appears at low velocities and fuel-lean conditions in zones of low scalar dissipation rates. For the statistically steady jet, numerical predictions are in a very good agreement with the experiments, giving confidence in the applied models. For the pulsed jet, all of the predicted ignition delay times and locations are in the range of the experimental observations. Time-resolved statistics reveal that thermochemical properties of the gas in a pulsed jet achieve states that are impossible to reproduce in laminar conditions. For further analysis, hydroxyl and formaldehyde are chosen as a marker for the established flame and for the ignition, respectively. In laminar conditions, these two species are perfectly correlated. However, the unsteady dynamics of the pulsed jet invalidates the correlation between the minor species chemistry prior to ignition. This yields the discrepancy in the auto-ignition delay time and the location of the ignition kernel between different pulses, as the thermochemical state needed for the ignition occurs in a random manner. © 2020 Elsevier Ltd
    view abstractdoi: 10.1016/j.fuel.2020.118947
  • 2021 • 173 Implementation and operation of a fiber-coupled CMOS detector in a low energy electron Microscope
    Janoschka, D. and Dreher, P. and Rödl, A. and Franz, T. and Schaff, O. and Horn-von Hoegen, M. and Meyer zu Heringdorf, F.-J.
    Ultramicroscopy 221 (2021)
    The intrinsically weak signals in ultrafast electron microscopy experiments demand an improvement in the signal-to noise ratio of suitable electron detectors. We provide an experience report describing the installation and operation of a fiber-coupled CMOS based detector in a low energy electron microscope. We compare the detector performance to the traditional multi-channel-plate-based setup. The high dynamic range CMOS detector is capable of imaging spatially localized large intensity variations with low noise. The detector is blooming-free and overexposure appears uncritical. Overall, we find dramatic improvements in the imaging with the fiber-coupled CMOS detector compared to imaging with our previously used multi-channel-plate detector. © 2020 Elsevier B.V.
    view abstractdoi: 10.1016/j.ultramic.2020.113180
  • 2021 • 172 Aperture Synthesis Method to Investigate on the Reflection Properties of Typical Road Surfaces
    Jebramcik, J. and Rolfes, I. and Barowski, J.
    2020 50th European Microwave Conference, EuMC 2020 634-637 (2021)
    In this contribution, a millimeter wave FMCW (frequency modulated continuous wave) radar is utilized to investigate the surface reflection of an asphalt sample at 80 GHz. Since the reflection properties of such a rough surface usually strongly depend on the characteristics of the antenna (i.e. opening angle, spot on sample) that is used in the measurement, a more generalized approach is presented here. Based on the backprojection algorithm, the measurement data, obtained by a relatively undirected antenna, are used to infer the reflection properties for different antenna systems with other radiation patterns. Exemplary measurement results are presented in this paper, proving the suitability of the proposed method. © 2021 EuMA.
    view abstractdoi: 10.23919/EuMC48046.2021.9338033
  • 2021 • 171 Plug-flow reactor and shock-tube study of the oxidation of very fuel-rich natural gas/DME/O2 mixtures
    Kaczmarek, D. and Herzler, J. and Porras, S. and Shaqiri, S. and Fikri, M. and Schulz, C. and Atakan, B. and Maas, U. and Kasper, T.
    Combustion and Flame 225 86-103 (2021)
    A polygeneration process with the ability to provide work, heat, and useful chemicals according to the specific demand is a promising alternative to traditional energy conversion systems. By implementing such a process in an internal combustion engine, products like synthesis gas or unsaturated hydrocarbons and very high exergetic efficiencies can be obtained through partial oxidation of natural gas, in addition to the already high flexibility with respect to the required type of energy. To enable compression ignition with natural gas as input, additives such as dimethyl ether are needed to increase the reactivity at low temperatures. In this study, the reaction of fuel-rich natural gas/dimethyl ether (DME) mixtures is investigated to support the further development of reaction mechanisms for these little studied reaction conditions. Temperature-resolved species concentration profiles are obtained by mass spectrometry in a plug-flow reactor at equivalence ratios ϕ = 2, 10, and 20, at temperatures between 473 and 973 K and at a pressure of 6 bar. Ignition delay times and product-gas analyses are obtained from shock-tube experiments, for ϕ = 2 and 10, at 710 – 1639 K and 30 bar. The experimental results are compared to kinetic simulations using two literature reaction mechanisms. Good agreement is found for most species. Reaction pathways are analyzed to investigate the interaction of alkanes and DME. It is found that DME forms radicals at comparatively low temperatures and initiates the conversion of the alkanes. Additionally, according to the reaction pathways, the interaction of the alkanes and DME promotes the formation of useful products such as synthesis gas, unsaturated hydrocarbons and oxygenated species. © 2020
    view abstractdoi: 10.1016/j.combustflame.2020.10.004
  • 2021 • 170 Path to single-crystalline repair and manufacture of Ni-based superalloy using directional annealing
    Kalfhaus, T. and Schaar, H. and Thaler, F. and Ruttert, B. and Sebold, D. and Frenzel, J. and Steinbach, I. and Theisen, W. and Guillon, O. and Clyne, T.W. and Vassen, R.
    Surface and Coatings Technology 405 (2021)
    Advanced methods for the repair of single-crystalline (SX) Ni-based superalloys are of special interest for the gas turbine industry. Polycrystalline repair approaches show promising results, while the repair of SX materials is still challenging. Directional annealing experiments resulted in large columnar grains by imposing thermal gradients at the abnormal grain growth temperature of a specific Ni-based superalloy. A numerical model of the Bridgman process is applied to provide an insight into the temperature evolution during zone annealing of the Vacuum-Plasma-Spray (VPS) repair coatings with the aim of promoting grain growth from the SX substrate. The results presented here suggest that this is a promising approach to repair or manufacture SX turbine blades. © 2020 Elsevier B.V.
    view abstractdoi: 10.1016/j.surfcoat.2020.126494
  • 2021 • 169 Higgs-like pair amplitude dynamics in superconductor-quantum-dot hybrids
    Kamp, M. and Sothmann, B.
    Physical Review B 103 (2021)
    We consider a quantum dot weakly tunnel coupled to superconducting reservoirs. A finite superconducting pair amplitude can be induced on the dot via the proximity effect. We investigate the dynamics of the induced pair amplitude after a quench and under periodic driving of the system by means of a real-time diagrammatic approach. We find that the quench dynamics is dominated by an exponential decay towards equilibrium. In contrast, the periodically driven system can sustain coherent oscillations of both the amplitude and the phase of the induced pair amplitude in analogy to Higgs and Nambu-Goldstone modes in driven bulk superconductors. © 2021 American Physical Society.
    view abstractdoi: 10.1103/PhysRevB.103.045414
  • 2021 • 168 Investigation of the combustion of iron pentacarbonyl and the formation of key intermediates in iron oxide synthesis flames
    Karakaya, Y. and Kluge, S. and Wiggers, H. and Schulz, C. and Kasper, T.
    Chemical Engineering Science 230 (2021)
    The information of the gas phase kinetics are relevant for the development of detailed reaction mechanisms as well as for process design and control in flame synthesis. In this study, the decomposition of iron pentacarbonyl and the reaction pathways towards iron oxide clusters and particles in laminar H2/O2/Ar low-pressure synthesis flames are investigated. Gas-phase species are analyzed by photoionization and electron ionization mass spectrometry. The extraction of a representative sample from the particle-laden flow of a synthesis flame by an intrusive sampling technique for the analysis is challenging, because iron-intermediate species can condense easily. Cations can be extracted from the flame with a high efficient ion sampling technique that results in high sensitivity. Iron-containing cations provide evidence of the presence of key intermediates, e.g., Fe(OH)2, Fe(OH)3, Fe2O3, and larger Fe-O-clusters which are the dominant intermediates with respect to particle formation and need to be considered in future gas-phase reaction mechanisms. © 2020 Elsevier Ltd
    view abstractdoi: 10.1016/j.ces.2020.116169
  • 2021 • 167 Nanopatterning surfaces by grazing incidence swift heavy ion irradiation
    Karlušić, M. and Mičetić, M. and Kresić, M. and Jakšić, M. and Šantić, B. and Bogdanović-Radović, I. and Bernstorff, S. and Lebius, H. and Ban-d'Etat, B. and Žužek Rožman, K. and O'Connell, J.H. and Hagemann, U. and ...
    Applied Surface Science 541 (2021)
    Nanopatterned surfaces play a key role for many applications exploiting unique features such as an enhanced surface area, long- and short-ranged morphology modulations or a spatial variation of electronic and chemical properties. Ion beam irradiation has been frequently used for nanostructuring bulk materials because it is efficient, fast, and cost-effective. In this paper we show that ion irradiation under extremely grazing incidence in conjunction with other scalable processing methods such as wet etching and thermal annealing, is a perfect tool for nanopatterning of dielectric surfaces. We demonstrate that by tuning ion energy and fluence, one can select different surface nanopattern morphologies like individual chains of nanohillocks, nanostripes, or nanoscaled ripples. Furthermore, chemical etching of the irradiated surface can be used to create a negative replica of the nanopattern as only the material making up the surface track is susceptible to the etching process and is thus removed. Also, a removal of the surface track can be achieved by thermal annealing in vacuum. All these presented strategies open up new ways for achieving control over nanoscale surface modifications using swift heavy ion beams. © 2020 Elsevier B.V.
    view abstractdoi: 10.1016/j.apsusc.2020.148467
  • 2021 • 166 Plastic deformation of single crystals of the equiatomic Cr−Mn−Fe−Co−Ni high-entropy alloy in tension and compression from 10 K to 1273 K
    Kawamura, M. and Asakura, M. and Okamoto, N.L. and Kishida, K. and Inui, H. and George, E.P.
    Acta Materialia 203 (2021)
    The plastic deformation behavior of single crystals of the quinary, equiatomic Cr−Mn−Fe−Co−Ni high-entropy alloy (HEA) with the face-centered cubic structure has been investigated in tension and compression as a function of crystal orientation and temperature from 10 K to 1373 K. The critical resolved shear stress (CRSS) for {111}<110> slip at room temperature is 42−45 MPa. It does not depend much on crystal orientation (i.e., the Schmid law holds true) and the sense (tension vs. compression) of the applied load. The CRSS for {111}<110> slip increases with the decrease in temperature, without showing any significant inertia effects at cryogenic temperatures below 77 K. Extrapolation from the measured yield stresses down to 10 K yields a CRSS value at 0 K of 168 MPa. At cryogenic temperatures, the measured strain-rate sensitivity of flow stress is consistent with a very small activation volume. The concept of stress equivalence holds true both for the temperature dependence of CRSS and the stress dependence of activation volume, indicating that solid-solution hardening is the major strengthening mechanism. Deformation twinning occurs at 77 K but not at room temperature, resulting in higher tensile elongation to failure at 77 K than at room temperature. Deformation twinning at 77 K occurs at a shear stress of 378 MPa on conjugate (1¯1¯1) planes in the form of Lüders deformation after large plastic strain (about 85%) achieved by the stage I (easy glide) and stage II (linear work-hardening) deformation. © 2020
    view abstractdoi: 10.1016/j.actamat.2020.10.073
  • 2021 • 165 Enhancing Engineering Education by Virtual Laboratories: A Comparison Between Two Different Approaches
    Keddi, D. and Frerich, S.
    Advances in Intelligent Systems and Computing 1231 AISC 359-365 (2021)
    The aim of this contribution is to compare two different settings of two virtual laboratories. Both of them are situated in the context of chemical engineering. One of them is used as online preparation for international students, while the other is implemented in lectures and seminars as demonstrating unit of subjects related to porous materials. While the online preparation for international students has already been at use, the demonstration unit is still work in progress. The students benefit from this kind of digital preparation to a high degree. Theoretical knowledge is available on an individual level, and they can choose time and place when to attend the courses. Many students mastered their course, understood the underlying concepts, and also exceeded usual expectations with their final reports. Regarding their comments, the implemented visualizations were highly appreciated, and the students also rated the set-ups as affirmative. However, a reasonable amount of participants complained about the absence of a real person in charge throughout the experiment, as they have experienced it in hands-on laboratories on site. Although it was found that virtual laboratories are an appropriate way to explain scientific topics, it can be observed that the actual implementation is still facing some issues. This contribution gives an overview of experiences made and discusses the potential for future applications of virtual laboratories in engineering education. © 2021, Springer Nature Switzerland AG.
    view abstractdoi: 10.1007/978-3-030-52575-0_30
  • 2021 • 164 Intersection-distribution-based remapping between arbitrary meshes for staggered multi-material arbitrary Lagrangian-Eulerian hydrodynamics
    Kenamond, M. and Kuzmin, D. and Shashkov, M.
    Journal of Computational Physics 429 (2021)
    We present a new intersection-distribution-based remapping method between arbitrary polygonal meshes for indirect staggered multi-material arbitrary Lagrangian-Eulerian hydrodynamics. All cell-centered material quantities are conservatively remapped using intersections between the Lagrangian (old, source) mesh and the rezoned (new, target) mesh. The new nodal masses are obtained by conservative distribution of all material masses in each new cell to the cell's corners and then collecting those corner masses at new nodes. This distribution is done using a local constrained optimization approach for each cell in the new mesh. In order to remap nodal momentum we first define cell-centered momentum for each cell in the old mesh, conservatively remap this to the new mesh and then conservatively distribute the new zonal momentum to each cell's bounding nodes, again using local constrained optimization. Our method also conserves total energy by applying a new nodal kinetic energy correction that relies on a process similar to that used for remapping nodal mass and momentum. Cell-centered kinetic energy is computed, conservatively remapped and then distributed to nodes. The discrepancy between this conservatively remapped and actual nodal kinetic energy is then conservatively distributed to the internal energies of the materials in the cells surrounding each node. Unlike conventional cell-based corrections of this type, this new nodal kinetic energy correction has not been observed to drive material internal energy negative in any of our testing. Unlike flux based remapping, our new intersection-distribution method can be applied to remapping between source and target meshes that are arbitrarily different, which provides superior flexibility in the rezoning strategy. Our method is accurate, essentially conservative and essentially bounds preserving. © 2020 Elsevier Inc.
    view abstractdoi: 10.1016/
  • 2021 • 163 Quantum Sensor for Nanoscale Defect Characterization
    Kerski, J. and Lochner, P. and Ludwig, Ar. and Wieck, A.D. and Kurzmann, A. and Lorke, A. and Geller, M.
    Physical Review Applied 15 (2021)
    The optical and electronic properties of semiconductors are strongly affected by structural and stoichiometric defects. The precise incorporation of dopants and the control of impurities are essentially what makes semiconductors useful materials for a broad range of devices. The standard defect and impurity characterization methods are sensitive only on a macroscopic scale, like the most widely used method of deep-level transient spectroscopy (DLTS). We perform time-resolved measurements of the resonance fluorescence of a single self-assembled (In,Ga)As quantum dot (QD) at low temperatures (4.2K). By pulsing the applied gate voltage, we are able to selectively occupy and unoccupy individual defects in the vicinity of the dot. We address the exciton transition of the QD with a tunable diode laser. Our time-resolved measurements exhibit a shift of the resonance energy of the optical transition. We attribute this to a change of the electric field in the dot's vicinity, caused by electrons tunneling from a reservoir to the defect sites. Furthermore, we are able to characterize the defects concerning their position and activation energy by modeling our experimental data. Our results thus demonstrate how a quantum dot can be used as a quantum sensor to characterize the position and activation energy of individual shallow defects on the nanoscale. © 2021 authors. Published by the American Physical Society.
    view abstractdoi: 10.1103/PhysRevApplied.15.024029
  • 2021 • 162 Influence of flexible side-chains on the breathing phase transition of pillared layer MOFs: A force field investigation
    Keupp, J. and Dürholt, J.P. and Schmid, R.
    Faraday Discussions 225 324-340 (2021)
    The prototypical pillared layer MOFs, formed by a square lattice of paddle-wheel units and connected by dinitrogen pillars, can undergo a breathing phase transition by a "wine-rack"type motion of the square lattice. We studied this behavior, which is not yet fully understood, using an accurate first principles parameterized force field (MOF-FF) for larger nanocrystallites on the example of Zn2(bdc)2(dabco) [bdc: benzenedicarboxylate, dabco: (1,4-diazabicyclo[2.2.2]octane)], and found clear indications for an interface between a closed and an open pore phase traveling through the system during the phase transformation [J. Keupp and R. Schmid, Adv. Theory Simul., 2019, 2, 1900117]. In conventional simulations in small supercells this mechanism is prevented by periodic boundary conditions (PBCs), enforcing a synchronous transformation of the entire crystal. Here, we extend this investigation to pillared layer MOFs with flexible side-chains, attached to the linker. Such functionalized (fu-)MOFs are experimentally known to have different properties with the side-chains acting as fixed guest molecules. First, in order to extend the parameterization for such flexible groups, a new parameterization strategy for MOF-FF had to be developed, using a multi-structure force based fit method. The resulting parameterization for a library of fu-MOFs is then validated with respect to a set of reference systems and shows very good accuracy. In the second step, a series of fu-MOFs with increasing side-chain length is studied with respect to the influence of the side-chains on the breathing behavior. For small supercells in PBCs a systematic trend of the closed pore volume with the chain length is observed. However, for a nanocrystallite model a distinct interface between a closed and an open pore phase is visible only for the short chain length, whereas for longer chains the interface broadens and a nearly concerted transformation is observed. Only by molecular dynamics simulations using accurate force fields can such complex phenomena can be studied on a molecular level. © 2021 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/d0fd00017e
  • 2021 • 161 Degradation behavior of the MgO/HA surface ceramic nano-composites in the simulated body fluid and its use as a potential bone implant
    Khalili, V. and Frenzel, J. and Eggeler, G.
    Materials Chemistry and Physics 258 (2021)
    In this work, we studied the effects of hot isostatic pressing and surface anodizing on the behavior of an in-situ surface modified magnesium matrix nano-composite with different wt.% of hydroxyapatite by stir-centrifugal casting. The hot isostatic pressing and anodizing were conducted to reduce the defects and to replace the surface of Mg/HA with a ceramic matrix nano-composite layer of MgO/HA, respectively. The composition of the conversion layer of anodizing was evaluated using energy dispersive spectroscopy and X-ray diffraction. The electrochemical tests were conducted in the simulated body fluid. The results show that the dominant deposition is vertical Mg(OH)2 nano-rods on the hot isostatic pressed-anodized surface during immersion in the simulated body fluid. According to the electrochemical results, a homogeneous distribution of 1.8 wt% nano-hydroxyapatite in the magnesium oxide matrix with a well-arranged nanostructure on the surface, after hot isostatic pressing and anodizing, reduces the H2 release and corrosion rate. Also, the mentioned specimen demonstrates the lowest thermodynamic tendency for corrosion (−1.345 V) and the corrosion rate of 3.8388 mm × year−1 with the highest protection efficiency of 42.26% compared to the as-cast pure magnesium. Therefore, it can be considered as a promising material in designing biomedical bone implants. © 2020 Elsevier B.V.
    view abstractdoi: 10.1016/j.matchemphys.2020.123965
  • 2021 • 160 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 • 159 Variational interface element model for 2D and 3D hydraulic fracturing simulations
    Khisamitov, I. and Meschke, G.
    Computer Methods in Applied Mechanics and Engineering 373 (2021)
    This paper presents an extension of the variational interface fracture model proposed in Khisamitov and Meschke (2018) to model fluid driven fracture in porous materials. The fluid saturated material is described by the theory of poroelasticity and the effective stress concept, while the coupling between fluid flow and fracture initiation and propagation is accomplished via a variational interface element formulation, using a damage variable c as an additional degree of freedom in conjunction with linear momentum and mass balance equations along the interface zones. Introducing the damage variable c, fracture propagates according to the values of the minimizers of the total potential energy expressed in terms of effective stresses. Hence, neither local crack propagation criteria nor tracking algorithms are required. Taking into account that a fracture grows quasi-statically, the discretized system of PDEs is solved by the backward Euler-scheme, ignoring the contribution from inertia forces. In the Newton–Raphson iterative solution procedure, an operator splitting algorithm is employed to solve first the poroelastic equations, and updating subsequently the damage variable c. The proposed model for hydraulic fracturing is validated first by means of one-dimensional analytical solutions in toughness as well as in viscosity dominated regimes. The performance and the applicability of the model to simulate interactions between propagating and existing fractures is demonstrated by means of 2D and 3D numerical examples. © 2020 Elsevier B.V.
    view abstractdoi: 10.1016/j.cma.2020.113450
  • 2021 • 158 Nanocrystalline equiatomic CoCrFeNi alloy thin films: Are they single phase fcc?
    Kini, M.K. and Lee, S. and Savan, A. and Breitbach, B. and Addab, Y. and Lu, W. and Ghidelli, M. and Ludwig, Al. and Bozzolo, N. and Scheu, C. and Chatain, D. and Dehm, G.
    Surface and Coatings Technology 410 (2021)
    The bulk quaternary equiatomic CoCrFeNi alloy is studied extensively in literature. Under experimental conditions, it shows a single-phase fcc structure and its physical and mechanical properties are similar to those of the quinary equiatomic CoCrFeMnNi alloy. Many studies in literature have focused on the mechanical properties of bulk nanocrystalline high entropy alloys or compositionally complex alloys, and their microstructure evolution upon annealing. The thin film processing route offers an excellent alternative to form nanocrystalline alloys. Due to the high nucleation rate and high density of defects in thin films synthesized by sputtering, the kinetics of microstructure evolution is often accelerated compared to those taking place in the bulk. Here, thin films are used to study the phase evolution in nanocrystalline CoCrFeNi deposited on Si/SiO2 and c-sapphire substrates by magnetron co-sputtering from elemental sources. The phases and microstructure of the films are discussed in comparison to the bulk alloy. The main conclusion is that second phases can form even at room temperature provided there are sufficient nucleation sites. © 2021 Elsevier B.V.
    view abstractdoi: 10.1016/j.surfcoat.2021.126945
  • 2021 • 157 Mechanism-oriented characterization of the anisotropy of extruded profiles based on solid-state recycled EN AW-6060 aluminum chips
    Koch, A. and Henkel, T. and Walther, F.
    Engineering Failure Analysis 121 (2021)
    Because of the great potential to reduce the amount of energy, the direct recycling of scrap like aluminum chips by hot extrusion is a hopeful alternative to the usual remelting process. Previous investigations showed that the chips, which are encased by oxide layers, are elongated due to the extrusion process. Therefore, the aim of this study is to test to what extend anisotropic properties, in analogy to fiber-reinforced materials, can be determined. The mechanical properties of cast-based and chip-based specimens with orientations of 0°, 30° and 90° to extrusion direction were characterized by means of mechanical quasistatic and cyclic experiments. It could be shown that quasistatic properties of the 0° orientation are highest for chip-based specimens, whereby the differences to the other orientations are slight. On the other hand, large differences in cyclic creep behavior between the orientations as well as in damage behavior could be determined. © 2020 Elsevier Ltd
    view abstractdoi: 10.1016/j.engfailanal.2020.105099
  • 2021 • 156 Chern insulating phases and thermoelectric properties of EuO/MgO(001) superlattices
    Köksal, O. and Pentcheva, R.
    Physical Review B 103 (2021)
    The topological and thermoelectric properties of (EuO)n/(MgO)m(001) superlattices (SLs) are explored using density functional theory calculations including a Hubbard U term together with Boltzmann transport theory. In (EuO)1/(MgO)3(001) SL at the lattice constant of MgO a sizable band gap of 0.51 eV is opened by spin-orbit coupling (SOC) due to a band inversion between occupied localized Eu 4f and empty 5d conduction states. This inversion between bands of opposite parity is accompanied by a reorientation in the spin texture along the contour of band inversion surrounding the Γ point and leads to a Chern insulator with C = -1, also confirmed by the single edge state. Moreover, this Chern insulating phase shows promising thermoelectric properties, e.g., a Seebeck coefficient between 400 and 800μVK-1. A similar SOC-induced band inversion takes place also in the ferromagnetic semimetallic (EuO)2/(MgO)2(001) SL. Despite the vanishing band gap, it leads to a substantial anomalous Hall conductivity with values up to -1.04 e2/h and somewhat lower Seebeck coefficient. Both cases emphasize the relation between nontrivial topological bands and thermoelectricity also in systems with broken inversion symmetry. © 2021 American Physical Society.
    view abstractdoi: 10.1103/PhysRevB.103.045135
  • 2021 • 155 Some Martingales Associated With Multivariate Bessel Processes
    Kornyik, M. and Voit, M. and Woerner, J.
    Acta Mathematica Hungarica 163 194-212 (2021)
    We study Bessel processes on Weyl chambers of types A and Bon RN. Using elementary symmetric functions, we present several space-timeharmonicfunctions and thus martingales for these processes (Xt)t≥0which areindependent from one parameter of these processes. As a consequence, pt(y):=E(∏i=1N(y-Xti)) can be expressed via classical orthogonal polynomials. Suchformulas on characteristic polynomials admit interpretations in random matrixtheory where they are partially known by Diaconis, Forrester, and Gamburd. © 2020, Akadémiai Kiadó, Budapest, Hungary.
    view abstractdoi: 10.1007/s10474-020-01096-5
  • 2021 • 154 Atomic oxygen generation in atmospheric pressure RF plasma jets driven by tailored voltage waveforms in mixtures of He and O2
    Korolov, I. and Steuer, D. and Bischoff, L. and Hübner, G. and Liu, Y. and Schulz-Von der Gathen, V. and Böke, M. and Mussenbrock, T. and Schulze, J.
    Journal of Physics D: Applied Physics 54 (2021)
    Absolute atomic oxygen densities measured space resolved in the active plasma volume of a COST microplasma reference jet operated in He/O2 and driven by tailored voltage waveforms are presented. The measurements are performed for different shapes of the driving voltage waveform, oxygen admixture concentrations, and peak-to-peak voltages. Peaks- and valleys-waveforms constructed based on different numbers of consecutive harmonics, N, of the fundamental frequency f 0 =13.56 MHz, different relative phases and amplitudes are used. The results show that the density of atomic oxygen can be controlled and optimized by voltage waveform tailoring (VWT). It is significantly enhanced by increasing the number of consecutive driving harmonics at fixed peak-to-peak voltage. The shape of the measured density profiles in the direction perpendicular to the electrodes can be controlled by VWT as well. For N >1 and peaks-/valleys-waveforms, it exhibits a strong spatial asymmetry with a maximum at one of the electrodes due to the spatially asymmetric electron power absorption dynamics. Thus, the atomic oxygen flux can be directed primarily towards one of the electrodes. The generation of atomic oxygen can be further optimized by changing the reactive gas admixture and by tuning the peak-to-peak voltage amplitude. The obtained results are understood based on a detailed analysis of the spatio-temporal dynamics of energetic electrons revealed by phase resolved optical emission spectroscopy. © 2021 Institute of Physics Publishing. All rights reserved.
    view abstractdoi: 10.1088/1361-6463/abd20e
  • 2021 • 153 Beschreibung der Festigkeit von Fels unter echten triaxialen Bedingungen mit der Grenzbedingung nach Mogi-Coulomb
    Kosmann, B. and Perau, E.
    Geotechnik (2021)
    Describing rock strength under true triaxial conditions with the limit condition of Mogi-Coulomb. It is known that investigations of rock strength under true triaxial conditions can show the influence of the mean principal stress on failure. For numerical calculations of tunnels and boreholes, where the mean and minimum principal stresses are not identical, limit conditions that take into account the influence of the mean principal stress would be advantageous. With the limit condition according to Mogi-Coulomb, unlike with the well-known limit condition according to Mohr-Coulomb, the influence of the mean principal stress can be taken into account. At the same time, however, these two limit conditions are identical for triaxial extension and compression. In order to be able to map limit states even more appropriate, the modified limit condition according to Mogi-Coulomb is presented in this article. In this modified form, the limit conditions according to Drucker-Prager and von Mises are included as a special case. In order to understand the Mogi-Coulomb criterion and to show possible applications, the present article shows its form, the influence of the parameters, the convexity and parameter acquisition of laboratory tests are examined. , Ernst und Sohn. All rights reserved.
    view abstractdoi: 10.1002/gete.202000013
  • 2021 • 152 Failure mode map for E-PBF manufactured Ti6Al4V sandwich panels
    Kotzem, D. and Tazerout, D. and Arold, T. and Niendorf, T. and Walther, F.
    Engineering Failure Analysis 121 (2021)
    In the present work the mechanical properties and failure modes of Ti6Al4V sandwich panels made by electron beam powder bed fusion (E-PBF) were investigated. Analytical models were employed to predict the most probable failure mode of the specimen by adjusting the dimensions of the core and the face sheet. Thereby, different failure modes such as core failure, face wrinkling and face yielding were taken into account in the analytical model. Subsequently, a failure mode map was constructed. Sandwich panels for experimental validation were manufactured by E-PBF. The deformation response of the sandwich panels under three-point bending load was characterized until failure. A good agreement between analytical predictions and experimental data was found. © 2020 Elsevier Ltd
    view abstractdoi: 10.1016/j.engfailanal.2020.105159
  • 2021 • 151 From π-Bonded Gallapnictenes to Nucleophilic, Redox-Active Metal-Coordinated Pnictanides
    Krüger, J. and Wölper, C. and Schulz, S.
    Angewandte Chemie - International Edition 60 3572-3575 (2021)
    A comprehensive reactivity study of gallapnictenes LGaEGa(Cl)L (E=As, Sb; L=HC[C(Me)N(Ar)]2, Ar=Dip=2,6-i-Pr2C6H3) proved the nucleophilic character of the pnictogen and the electrophilic nature of the Ga atom. Reactions of LGaEGa(Cl)L with imidazolium chloride [IPrH][Cl] yielded {[LGa(Cl)]2E−}{IPrH+} (E=As 1, Sb 2), and those with HCl and MeI gave pnictanes [LGa(Cl)]2EH (E=As 5, Sb 6) and L(I)GaE(Me)Ga(Cl)L (E=As 7, Sb 8). Pnictanides 1 and 2 also react with [H(OEt2)2][BArF4] (BArF4=B(C6F5)4) to 5 and 6, while reactions with MeI yielded [LGa(Cl)]2EMe (E=As 9, Sb 10). Single electron oxidation reactions of pnictanides 1 and 2 gave the corresponding radicals [LGa(Cl)]2E. (E=As, Sb). © 2020 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH
    view abstractdoi: 10.1002/anie.202013618
  • 2021 • 150 Importance of catalyst–photoabsorber interface design configuration on the performance of Mo-doped BiVO4 water splitting photoanodes
    Krysiak, O.A. and Junqueira, J.R.C. and Conzuelo, F. and Bobrowski, T. and Masa, J. and Wysmolek, A. and Schuhmann, W.
    Journal of Solid State Electrochemistry 25 173-185 (2021)
    Photoelectrochemical water splitting is mostly impeded by the slow kinetics of the oxygen evolution reaction. The construction of photoanodes that appreciably enhance the efficiency of this process is of vital technological importance towards solar fuel synthesis. In this work, Mo-modified BiVO4 (Mo:BiVO4), a promising water splitting photoanode, was modified with various oxygen evolution catalysts in two distinct configurations, with the catalysts either deposited on the surface of Mo:BiVO4 or embedded inside a Mo:BiVO4 film. The investigated catalysts included monometallic, bimetallic, and trimetallic oxides with spinel and layered structures, and nickel boride (NixB). In order to follow the influence of the incorporated catalysts and their respective properties, as well as the photoanode architecture on photoelectrochemical water oxidation, the fabricated photoanodes were characterised for their optical, morphological, and structural properties, photoelectrocatalytic activity with respect to evolved oxygen, and recombination rates of the photogenerated charge carriers. The architecture of the catalyst-modified Mo:BiVO4 photoanode was found to play a more decisive role than the nature of the catalyst on the performance of the photoanode in photoelectrocatalytic water oxidation. Differences in the photoelectrocatalytic activity of the various catalyst-modified Mo:BiVO4 photoanodes are attributed to the electronic structure of the materials revealed through differences in the Fermi energy levels. This work thus expands on the current knowledge towards the design of future practical photoanodes for photoelectrocatalytic water oxidation. © 2020, The Author(s).
    view abstractdoi: 10.1007/s10008-020-04636-9
  • 2021 • 149 Use of optimal mixture-process designs and response-surface models to study properties of calcium silicate units
    Kuhnt, S. and Becker-Emden, E.-C. and Schade, T. and Eden, W. and Middendorf, B.
    Quality and Reliability Engineering International 37 391-408 (2021)
    Calcium silicate units are versatile and widely used construction materials for edifices. Their production process involves several factors that concern either the mixture of the raw materials or the curing process. The understanding of how raw materials and process variables interact in achieving the compressive strength of the final product enables a cost- and energy-efficient layout of the production process. In this paper, we use mixture-process experiments to derive a prediction model for compressive strength. We compare computer-generated D-optimal designs with different numbers of center points by various criteria and by their prediction variance throughout the design space. In contrast to traditional mixture designs, these designs take additional constraints on the mixture components into account and can include process variables. We review suitable response-surface models, which combine mixture and process variables. Based on results from 72 experimental runs, a model for the mean compressive strength is built, combining expert knowledge with statistical model-selection strategies. The resulting model covers not only linear effects of mixture components and process variables but also interactions and quadratic terms. For example, the influence of the lime share on compressive strength differs among the use of various sand mixtures. For desired values of predicted compressive strength, factor settings can thereby be found reducing costs and energy emission. © 2020 John Wiley & Sons Ltd.
    view abstractdoi: 10.1002/qre.2758
  • 2021 • 148 Ultraviolet resonance Raman spectroscopy with a continuously tunable picosecond laser: Application to the supramolecular ligand guanidiniocarbonyl pyrrole (GCP)
    Kumar, V. and Holtum, T. and Sebena, D. and Giese, M. and Voskuhl, J. and Schlücker, S.
    Spectrochimica Acta - Part A: Molecular and Biomolecular Spectroscopy 250 (2021)
    We present a UVRR spectroscopy setup which is equipped with a picosecond pulsed laser excitation source continuously tunable in the 210–2600 nm wavelength range. This laser source is based on a three-stage optical parametric amplifier (OPA) pumped by a bandwidth-compressed second harmonic output of an amplified Yb:KGW laser. It provides &lt;15 cm−1 linewidth pulses below 270 nm, which is sufficient for resolving Raman lines of samples in condensed phase studies. For demonstrating the capability of this tunable setup for UVRR spectroscopy we present its application to the artificial ligand guanidiniocarbonyl pyrrole (GCP), a carboxylate binder used in peptide and protein recognition. A UVRR excitation study in the range 244–310 nm was performed for identifying the optimum laser excitation wavelength for UVRR spectroscopy of this ligand (λmax = 298 nm) at submillimolar concentrations (400 µM) in aqueous solution. The optimum UVRR spectrum is observed for laser excitation with λexc = 266 nm. Only in the relatively narrow range of λexc = 266–275 nm UVRR spectra with a sufficiently high signal-to-noise ratio and without severe interference from autofluorescence (AF) were detectable. At longer excitation wavelengths the UVRR signal is masked by AF. At shorter excitation wavelengths the UVRR spectrum is sufficiently separated from the AF, but the resonance enhancement is not sufficient. The presented tunable UVRR setup provides the flexibility to also identify optimum conditions for other supramolecular ligands for peptide/protein recognition. © 2020
    view abstractdoi: 10.1016/j.saa.2020.119359
  • 2021 • 147 A new perspective on flux and slope limiting in discontinuous Galerkin methods for hyperbolic conservation laws
    Kuzmin, D.
    Computer Methods in Applied Mechanics and Engineering 373 (2021)
    In this work, we discuss and develop multidimensional limiting techniques for discontinuous Galerkin (DG) discretizations of scalar hyperbolic problems. To ensure that each cell average satisfies a local discrete maximum principle (DMP), we impose inequality constraints on the local Lax–Friedrichs fluxes of a piecewise-linear (P1) approximation. Since the piecewise-constant (P0) version corresponds to a property-preserving low-order finite volume method, the validity of DMP conditions can always be enforced using slope and/or flux limiters. We show that the (currently rather uncommon) use of direct flux limiting makes it possible to construct more accurate DMP-satisfying approximations in which a weak form of slope limiting is used to prevent unbounded growth of solution gradients. After presenting two flux limiters that ensure the validity of local DMPs for cell averages, we discuss the design of slope limiters based on different kinds of inequality constraints. In particular, we derive new limiting procedures based on flux constraints and constraints for directional derivatives. The latter approach makes it possible to preserve directional monotonicity in applications to problems that require different treatment of different space directions. At the flux limiting stage, the anisotropy of the problem at hand can be taken into account by using a customized definition of local bounds for the DMP constraints. At the slope limiting stage, we adjust the magnitude of individual directional derivatives using low-order reconstructions from cell averages to define the bounds. In this way, we avoid unnecessary limiting of well-resolved derivatives at smooth peaks and in internal/boundary layers. The properties of selected algorithms are explored in numerical studies for DG-P1 discretizations of two-dimensional test problems. In the context of hp-adaptive DG methods, the new limiting procedures can be used in P1 subcells of macroelements marked as ‘troubled’ by a smoothness indicator. © 2020 Elsevier B.V.
    view abstractdoi: 10.1016/j.cma.2020.113569
  • 2021 • 146 Functional Rotaxanes in Catalysis
    Kwamen, C. and Niemeyer, J.
    Chemistry - A European Journal 27 175-186 (2021)
    Mechanically interlocked molecules (MIMs) have gained attention in the field of catalysis due to their unique molecular properties. Central to MIMs, rotaxanes are highly promising and attractive supramolecular catalysts due to their unique three-dimensional structures and the flexibility of their subcomponents. This Minireview discusses the use of rotaxanes in organocatalysis and transition-metal catalysis. © 2020 The Authors. Published by Wiley-VCH GmbH
    view abstractdoi: 10.1002/chem.202002876
  • 2021 • 145 Revisiting ω phase embrittlement in metastable β titanium alloys: Role of elemental partitioning
    Lai, M.J. and Li, T. and Yan, F.K. and Li, J.S. and Raabe, D.
    Scripta Materialia 193 38-42 (2021)
    The role of elemental partitioning between β and ω phase in embrittling an originally ductile ω-containing Ti–12Mo (wt.%) model alloy was studied using transmission electron microscopy and atom probe tomography. It is revealed that the embrittlement of this alloy already occurs after aging at 400 °C for as short as 10 min, when the size, inter-particle spacing and volume fraction of the ω particles remain almost unchanged. The origin of the aging-induced embrittlement is attributed to the significant rejection of Mo (>5 at.%) from the ω particles during aging, which leads to remarkable increase in the shear modulus (>30 GPa) of the ω particles, promoting intense plastic flow localization and facilitating crack nucleation prior to macroscopic yielding. © 2020
    view abstractdoi: 10.1016/j.scriptamat.2020.10.031
  • 2021 • 144 Experimental Reconstruction of the Few-Photon Nonlinear Scattering Matrix from a Single Quantum Dot in a Nanophotonic Waveguide
    Le Jeannic, H. and Ramos, T. and Simonsen, S.F. and Pregnolato, T. and Liu, Z. and Schott, R. and Wieck, A.D. and Ludwig, Ar. and Rotenberg, N. and García-Ripoll, J.J. and Lodahl, P.
    Physical Review Letters 126 (2021)
    Coherent photon-emitter interfaces offer a way to mediate efficient nonlinear photon-photon interactions, much needed for quantum information processing. Here we experimentally study the case of a two-level emitter, a quantum dot, coupled to a single optical mode in a nanophotonic waveguide.We carry out few-photon transport experiments and record the statistics of the light to reconstructthe scattering matrix elements of one- and two-photon components. This provides direct insight to the complex nonlinear photon interaction that contains rich many-body physics. © 2021 American Physical Society.
    view abstractdoi: 10.1103/PhysRevLett.126.023603
  • 2021 • 143 Comparative study of hydrogen embrittlement resistance between additively and conventionally manufactured 304L austenitic stainless steels
    Lee, D.-H. and Sun, B. and Lee, S. and Ponge, D. and Jägle, E.A. and Raabe, D.
    Materials Science and Engineering A 803 (2021)
    Hydrogen embrittlement in 304L austenitic stainless steel fabricated by laser powder-bed-fusion (LPBF) was investigated and compared to conventionally produced 304L samples with two different processing histories; casting plus annealing (CA) and CA plus thermomechanical treatment (CA-TMT). Interestingly, no significant difference in the amount of deformation-induced α′ martensite between the LPBF and CA-TMT samples was observed, suggesting that the solidification substructure in the LPBF sample enhanced the strength without promoting the harmful hydrogen embrittlement effect. These results are discussed in terms of the chemical inhomogeneity, hydrogen-assisted cracking behavior, and hydrogen diffusion and trapping in the present 304L samples. © 2020
    view abstractdoi: 10.1016/j.msea.2020.140499
  • 2021 • 142 Synthesis and Reactivity of Heteroleptic Ga-P-C Allyl Cation Analogues
    Li, B. and Wölper, C. and Haberhauer, G. and Schulz, S.
    Angewandte Chemie - International Edition 60 1986-1991 (2021)
    Oxidative addition of cyclic alkyl(amino)carbene-coordinated phosphinidenes (MecAAC)PX to LGa affords gallium-coordinated phosphinidenes LGa(X)-P(MecAAC) (L=HC[C(Me)N(2,6-i-Pr2C6H3)]2; X=Cl 1, Br 2), which react with NaBArF4 and LiAl(ORF)4 to [LGaP(MecAAC)][An] (An=B(C6H3(CF3)2)4 3, B(C6F5)4 4, Al(OC(CF3)3)4 5). The cations in 3–5 show substantial Ga−P double bond character and represent heteronuclear analogues of allyl cations according to quantum chemical calculations. The reaction of 4 with 4-dimethylaminopyridine (dmap) to adduct 6 confirms the strong electrophilic nature of the gallium center, whereas 5 reacts with ethyl isocyanate with C−C bond formation to the γ-C atom of the β-diketiminate ligand and formation of compound 7. © 2020 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH
    view abstractdoi: 10.1002/anie.202012595
  • 2021 • 141 Sodium control in Ultrathin Cu(In,Ga)Se2 solar cells on transparent back contact for efficiencies beyond 12%
    Li, Y. and Yin, G. and Gao, Y. and Köhler, T. and Lucaßen, J. and Schmid, M.
    Solar Energy Materials and Solar Cells 223 (2021)
    Ultrathin Cu(In,Ga)Se2 (CIGSe) solar cells on transparent conductive oxide (TCO) back contacts combine advantages of ultrathin cells for reducing material consumption of rare indium and gallium and TCO-transparency benefited applications in tandems, bifacial configurations etc. However, their efficiencies are still limited and the back barrier potential is a primary reason from an electrical perspective. In this work, we explore the effects of Nadoping by post deposition treatment (PDT) on the performance of ultrathin CIGSe solar cells on ITO (Sn:In2O3)-coated Na-free glass substrates. Na doping enhances not only the open circuit-voltage (Voc) by increasing the doping level, but also the fill factor (FF) by switching the Schottky contact to an Ohmic contact at the CIGSe/ITO interface, which we propose is due to the increased recombination at the back interface. The optimum performance is achieved at a NaF dose of 2 mg with a top efficiency of 12.9%, which exhibits an enhancement by nearly 48% relative compared to the references without Na doping. To our best knowledge, this is the highest efficiency achieved for ultrathin cells (&lt;500 nm absorber thickness) on TCO without additional antireflection or back reflecting layer. Therefore, the results show that sodium control offers a solid basis for the development of ultrathin CIGSe cells on TCO in above-mentioned promising applications. © 2021 Elsevier B.V.
    view abstractdoi: 10.1016/j.solmat.2021.110969
  • 2021 • 140 Tunneling-Related Leakage Currents in Coaxial GaAs/InGaP Nanowire Heterojunction Bipolar Transistors
    Liborius, L. and Bieniek, J. and Possberg, A. and Tegude, F.-J. and Prost, W. and Poloczek, A. and Weimann, N.
    Physica Status Solidi (B) Basic Research 258 (2021)
    Herein, a detailed analysis of leakage mechanisms in epitaxially grown nanowire heterojunction bipolar transistors (NW-HBTs) is presented. Coaxial npn-GaAs/InGaP core–multishell nanowires are grown via gold-catalyzed metalorganic vapor phase epitaxy, processed to three terminal devices and electrically characterized. The key for successful NW-HBT device functionality is the identification of tunneling as the dominant leakage mechanism in highly doped nanowire pn-junctions. The suppression of forward tunneling currents by adjustment of the tunneling barrier width reduces the junction leakage current density into the nA cm−2 regime, which is further verified by tunneling-related electroluminescence measurements. In addition, the suppressed tunneling accordingly increases the number of electrons that are injected from the n-emitter into the p-base. The latter effect influences the performance of pn-junction based devices and is found to enable bipolar transistor functionality. Measured common emitter Gummel plots of the NW-HBT exhibit a current gain of up to 9 and the transistor function is additionally verified by current-controlled output characteristics. © 2020 The Authors. Published by Wiley-VCH GmbH
    view abstractdoi: 10.1002/pssb.202000395
  • 2021 • 139 Numerical Investigation of Remote Ignition in Shock Tubes
    Lipkowicz, J.T. and Nativel, D. and Cooper, S. and Wlokas, I. and Fikri, M. and Petersen, E. and Schulz, C. and Kempf, A.M.
    Flow, Turbulence and Combustion 106 471-498 (2021)
    Highly resolved two- and three-dimensional computational fluid dynamics (CFD) simulations are presented for shock-tube experiments containing hydrogen/oxygen (H2/O2) mixtures, to investigate mechanisms leading to remote ignition. The results of the reactive cases are compared against experimental results from Meyer and Oppenheim (Proc Combust Inst 13(1): 1153–1164, 1971. and Hanson et al. (Combust Flame 160(9): 1550–1558, 2013. The results of the non-reactive case are compared against shock tube experiments, recently carried out in Duisburg and Texas. The computational domain covers the end-wall region of the shock tube and applies high order numerics featuring an all-speed approximate Riemann scheme, combined with a 5th order interpolation scheme. Direct chemistry is employed using detailed reaction mechanisms with 11 species and up to 40 reactions, on a grid with up to 2.2 billion cells. Additional two-dimensional simulations are performed for non-reactive conditions to validate the treatment of boundary-layer effects at the inlet of the computational domain. The computational domain covers a region at the end part of the shock tube. The ignition process is analyzed by fields of localized, expected ignition times. Instantaneous fields of temperature, pressure, entropy, and dissipation rate are presented to explain the flow dynamics, specifically in the case of a bifurcated reflected shock. In all cases regions with locally increased temperatures were observed, reducing the local ignition-delay time in areas away from the end wall significantly, thus compensating for the late compression by the reflected shock and therefore leading for first ignition at a remote location, i.e., away from the end wall where the ignition would occur under ideal conditions. In cases without a bifurcated reflected shock, the temperature increase results from shock attenuation. In cases with a bifurcated reflected shock, the formation of a second normal shock and shear near the slip line is found to be crucial for the remote ignition to take place. Overall, the two- and three-dimensional simulations were found to qualitatively explain the occurrence of remote ignition and to be quantitatively correct, implying that they include the correct physics. © 2020, The Author(s).
    view abstractdoi: 10.1007/s10494-020-00219-w
  • 2021 • 138 Electron heating mode transitions in radio-frequency driven micro atmospheric pressure plasma jets in He/O2: A fluid dynamics approach
    Liu, Y. and Korolov, I. and Hemke, T. and Bischoff, L. and Hübner, G. and Schulze, J. and Mussenbrock, T.
    Journal of Physics D: Applied Physics 54 (2021)
    A two-dimensional fluid model is used to investigate the electron heating dynamics and the production of neutral species in a capacitively coupled radio-frequency micro atmospheric pressure helium plasma jet - specifically the COST jet - with a small oxygen admixture. Electron heating mode transitions are found to be induced by varying the driving voltage amplitude and the O2 concentration numerically and experimentally. The helium metastable density, and the charged species densities are highly relevant to the electron heating dynamics. By analyzing the creation and destruction mechanisms of the negative ions, we find that the generation of negative ions strongly depends on the O2 concentration. The increase of the electronegativity with the increasing O2 concentration leads to an enhancement of the bulk drift electric field. The distributions of the different neutral species densities along the direction of the gas flow inside the jet, as well as in the effluent differ a lot due to the relevant chemical reaction rates and the effect of the gas flow. The simulated results show that a fluid model can be an effective tool for qualitative investigations of micro atmospheric pressure plasma jets. © 2021 The Author(s). Published by IOP Publishing Ltd.
    view abstractdoi: 10.1088/1361-6463/abf370
  • 2021 • 137 Non-linear effects and electron heating dynamics in radio-frequency capacitively coupled plasmas with a non-uniform transverse magnetic field
    Liu, Y. and Trieschmann, J. and Berger, B. and Schulze, J. and Mussenbrock, T.
    Physics of Plasmas 28 (2021)
    A non-uniform transverse magnetic field is used to increase the plasma density and create an asymmetry in radio frequency capacitively coupled plasmas for plasma sputtering and plasma vapor deposition. Based on one-dimensional particle-in-cell/Monte Carlo collision simulations, the effect of the magnetic field magnitude on the non-linear behavior and the electron heating dynamics is studied for a pure helium plasma at a pressure of 30 mTorr. The results show that increasing the magnetic field magnitude can generate a more positive DC self-bias. As a result, non-linear oscillations of the electron current density and the electric field close to the grounded electrode are enhanced. An electric field reversal is induced when the powered electrode sheath collapses to balance electron and ion fluxes toward this boundary due to the strong confinement of electrons. Anomalous energetic electron beams are observed propagating from the collapsed sheath toward the plasma bulk. It is shown that such beams are reflections of the beams originating from the opposite expanding sheath based on the analysis of single particle motions. We show that energetic electron beams can be reflected by the transverse magnetic field. © 2021 Author(s).
    view abstractdoi: 10.1063/5.0045947
  • 2021 • 136 Comparing the Activity of Complex Solid Solution Electrocatalysts Using Inflection Points of Voltammetric Activity Curves as Activity Descriptors
    Löffler, T. and Waag, F. and Gökce, B. and Ludwig, Al. and Barcikowski, S. and Schuhmann, W.
    ACS Catalysis 11 1014-1023 (2021)
    Complex solid solution (CSS) (often denoted as high-entropy alloy) electrocatalysts enable access to unique possibilities for tailoring active sites while overcoming ever-existing limitations in electrocatalysis by unique interactions of various elements in direct neighborhood. The challenge lies in the development of strategies, which allow for systematic design of element combination and composition optimization in the multinary composition space. This challenge is accompanied by a lack of a suitable analysis method of experimental activity measurements, which can cope with the complex surface structure of this catalyst class. In this work, we propose the advantageous use of inflection points of voltammetric activity curves as activity descriptors enabling to correlate the potential of individual surface site groups to the respective peaks in the adsorption energy distribution pattern. This concept allows to methodologically gather information about the importance of each element in a CSS with respect to activity and stability of the relevant active sites and provides the basis for a guideline for systematic composition optimization. Further, the effect of phase stability on specific surface site groups as induced by degradation of the CSS phase or oxidation can be monitored. These concepts are experimentally evaluated using Cr-Mn-Fe-Co-Ni as a model system. Nanoparticles are synthesized with systematically varied compositions by means of scalable laser ablation synthesis using a multinary target. The composition is optimized with respect to the electrocatalytic activity for the oxygen reduction reaction (ORR) by varying its Mn content via laser ablation synthesis in ethanol. Subsequently, the concept is applied using rotating disk electrodes for ORR analysis in alkaline media. © 2021 American Chemical Society. All rights reserved.
    view abstractdoi: 10.1021/acscatal.0c03313
  • 2021 • 135 Superior low-cycle fatigue properties of CoCrNi compared to CoCrFeMnNi
    Lu, K. and Chauhan, A. and Walter, M. and Tirunilai, A.S. and Schneider, M. and Laplanche, G. and Freudenberger, J. and Kauffmann, A. and Heilmaier, M. and Aktaa, J.
    Scripta Materialia 194 (2021)
    We report on the low-cycle fatigue behavior of single-phase, face-centered cubic CoCrNi and CoCrFeMnNi at room temperature. Both alloys manifest cyclic hardening followed by softening and a near steady state until failure. CoCrNi exhibits higher strength, lower inelastic-strain, and longer lifetime than CoCrFeMnNi. For both alloys, microstructural investigations reveal no noticeable changes of texture, grain size and twin fraction. Nevertheless, CoCrNi exhibits planar dislocation structures, while CoCrFeMnNi shows well-defined wavy dislocation structures. This is due to CoCrNi lower stacking fault energy, which enhances planar slip and delays deformation localization leading to its superior fatigue resistance, compared to CoCrFeMnNi. © 2020
    view abstractdoi: 10.1016/j.scriptamat.2020.113667
  • 2021 • 134 Polymorphic arrangement of an organic molecule in its hydration environment
    Lucht, K. and Morgenstern, K.
    Journal of Chemical Physics 154 (2021)
    We investigate the polymorphism of complexes formed by the hydration of a functionalized azobenzene molecule by low-temperature scanning tunneling microscopy. Under conditions at which the water-less azobenzene molecules remain as monomers on Au(111), co-adsorption of water leads to water-azobenzene complexes. These complexes prefer to adopt linear arrangements of the azobenzene mediated by its functionalized end groups. Such structures may serve as model systems for investigating the influence of a solvent on a surface reaction. © 2021 Author(s).
    view abstractdoi: 10.1063/5.0033081
  • 2021 • 133 Powder properties and flowability measurements of tailored nanocomposites for powder bed fusion applications
    Lüddecke, A. and Pannitz, O. and Zetzener, H. and Sehrt, J.T. and Kwade, A.
    Materials and Design 202 (2021)
    The modification of metal alloy powders by coating with nanoparticles offers the possibility to improve additive manufacturing processes, in particular the powder bed fusion of metals with laser beams (PBF/LB-M), from the material side of view. Subsequently, component qualities including mechanical properties and microstructural characteristics could be improved. Furthermore, the modification enables improved energy utilization due to an increase in laser absorption. In this work three commercial additive manufacturing powders, namely stainless steel (1.4404), tool steel (1.2709), and aluminum alloy (3.2381) were coated with three different nanoparticles (Silicon carbide (SiC), few layer graphene (FLG), and iron oxide black (IOB) to increase the laser light absorption in the PBF/LB/M process, mechanical properties, and flowability of the powders. The coating was conducted within a fluidized bed system, resulting in homogeneous coatings. This study demonstrates, that well scalable processes i.e. stirred media milling and fluidized bed coating have the potential to improve the commercial AM powders regarding their bulk density, flowability, and energy absorption, which is a crucial step towards an improvement in the efficiency of the whole PBF process. Overall important information and relations were gathered to transfer them to the real powder deposition process in future work. © 2021
    view abstractdoi: 10.1016/j.matdes.2021.109536
  • 2021 • 132 Phase behavior of ASDs based on hydroxypropyl cellulose
    Luebbert, C. and Stoyanov, E. and Sadowski, G.
    International Journal of Pharmaceutics: X 3 (2021)
    Novel polymeric carriers for amorphous solid dispersions (ASDs) are highly demanded in pharmaceutical industry to improve the bioavailability of poorly-soluble drug candidates. Besides established polymer candidates, hydroxypropyl celluloses (HPC) comes more and more into the focus of ASD production since they have the availability to stabilize drug molecules in aqueous media against crystallization. The thermodynamic long-term stability of HPC ASDs with itraconazole and fenofibrate was predicted in this work with PC-SAFT and compared to three-months enduring long-term stability studies. The glass-transition temperature is a crucial attribute of a polymer, but in case of HPC hardly detectable by differential scanning calorimetry. By investigating the glass transition of HPC blends with a miscible polymer, we were for the first time able to estimate the HPC glass transition. Although both, fenofibrate and itraconazole reveal a very low crystalline solubility in HPC regardless of the HPC molecular weight, we observed that low-molecular weight HPC grades such as HPC-UL prevent fenofibrate crystallization for a longer period than the higher molecular weight HPC grades. As predicted, the ASDs with higher drug load underwent amorphous phase separation according to the differential scanning calorimetry thermograms. This work thus showed that it is possible to predict critical drug loads above which amorphous phase separation and/or crystallization occurs in HPC ASDs. © 2020 The Authors
    view abstractdoi: 10.1016/j.ijpx.2020.100070
  • 2021 • 131 Optical and magneto-optical properties of epitaxial Mn2GaC MAX phase thin film
    Lyaschenko, S. and Maximova, O. and Shevtsov, D. and Varnakov, S. and Tarasov, I. and Wiedwald, U. and Rosen, J. and Ovchinnikov, S. and Farle, M.
    Journal of Magnetism and Magnetic Materials 528 (2021)
    We report measurements of the dielectric permittivity, optical conductivity and magnetic circular dichroism (MCD) of the epitaxial Mn2GaC MAX-phase thin film in an external magnetic field of up to 200 mT, at temperatures of 296 and 140 K and 1.4 to 3.5 eV. The optical conductivity and MCD spectra show absorption peaks which are consistent with the interband electronic transitions for different positions of Mn, Ga, and C ions as confirmed by theoretical calculations of the spin-dependent density of electronic states. The well-known structural phase transition at 214 K is also seen in the changes of optical, magneto-optical and surface magnetic properties of Mn2GaC in our experiment. © 2021 Elsevier B.V.
    view abstractdoi: 10.1016/j.jmmm.2021.167803
  • 2021 • 130 A vehicle guidance model with a close-to-reality driver model and different levels of vehicle automation
    Ma, X. and Hu, X. and Schweig, S. and Pragalathan, J. and Schramm, D.
    Applied Sciences (Switzerland) 11 1-11 (2021)
    This paper presents a microscopic vehicle guidance model which adapts to different levels of vehicle automation. Independent of the vehicle, the driver model built is different from the common microscopic simulation models that regard the driver and the vehicle as a unit. The term “Vehicle Guidance Model” covers, here, both the human driver as well as a combination of human driver and driver assistance system up to fully autonomously operated vehicles without a (human) driver. Therefore, the vehicle guidance model can be combined with different kinds of vehicle models. As a result, the combination of different types of driver (human/machine) and different types of vehicle (internal combustion engine/electric) can be simulated. Mainly two parts constitute the vehicle guidance model in this paper: The first part is a traditional microscopic car-following model adjusted according to different degrees of automation level. The adjusted model represents the automation level for the present and the near and the more distant future. The second part is a fuzzy control model that describes how humans adjust the pedal position when they want to reach a target speed with their vehicle. An experiment with 34 subjects was carried out with a driving simulator based on the experimental data and the fuzzy control strategy was determined. Finally, when comparing the simulated model data and actual driving data, it is found that the fuzzy model for the human driver can reproduce the behavior of human participants almost accurately. © 2021 by the authors. Licensee MDPI, Basel, Switzerland.
    view abstractdoi: 10.3390/app11010380
  • 2021 • 129 Experiences with establishing a simulation scenario of the city of Duisburg with real traffic volume
    Ma, X. and Hu, X. and Weber, T. and Schramm, D.
    Applied Sciences (Switzerland) 11 1-11 (2021)
    This article presents the experience of building a simulation scenario of the whole city of Duisburg using real traffic data. The establishment of the simulation scenario is based on road network and traffic volume. In most cases, it is hard to collect all data sources with high precision. Moreover, it is time-consuming to set up a realistic traffic scenario. Even with available data, conversion, calibration, and validation all take a large effort. With the increase of the respective simulation area, the difficulty and workload rise. In this study, a simulation scenario of the whole city of Duisburg with the road network area of 232 km2 and Origin/Destination (OD) matrix area over 800 km2 was established in the software package SUMO. Four cases with different networks and traffic volumes were built and compared with real traffic data collected from induction loops. The percentage of simulated traffic volume in real traffic volume range can be up to 72.22%. © 2021 by the authors. Licensee MDPI, Basel, Switzerland.
    view abstractdoi: 10.3390/app11031193
  • 2021 • 128 Effects of Automated Vehicles on Traffic Flow with Different Levels of Automation
    Ma, X. and Hu, X. and Weber, T. and Schramm, D.
    IEEE Access 9 3630-3637 (2021)
    Highly automated vehicles are regarded as the next revolution of the transport system. Automated vehicles include a spectrum from vehicles with driver assistance systems through to highly automated vehicles. These vehicles will only gradually appear in the overall vehicle fleet. Their impact as part of future traffic is of reference value for transport decision-makers. The present paper starts from assumptions for the shares of vehicles with different levels of automation in 2030 and 2050 (representing the near and far-distant future) and compares the effects of these automated vehicles on traffic flow using microscopic traffic simulations. The simulated vehicles include non-assisted vehicles, semi-automated vehicles with driver assistance systems, and fully autonomous vehicles. To obtain a more realistic result, a traffic scenario of the city of Duisburg is used in this thesis. With the support of the city administration, existing data of the origin/target matrix, detector data including induction loops, and cameras were available. Thus, the data of the origin/target matrix are used to generate the real traffic scenario and the detector data to investigate the accuracy of the generated traffic. The result shows that automated vehicles would have a positive impact on traffic, a proportion of automated vehicles can reduce the average travel time. For areas with different traffic conditions, the degree of impact of automated vehicles can be very different. © 2013 IEEE.
    view abstractdoi: 10.1109/ACCESS.2020.3048289
  • 2021 • 127 Traffic simulation of future intelligent vehicles in duisburg city inner ring
    Ma, X. and Hu, X. and Weber, T. and Schramm, D.
    Applied Sciences (Switzerland) 11 1-12 (2021)
    Intelligent vehicles gradually enter the vehicular fleet with advanced driver-assistance technologies. Their impact on traffic should, therefore, be considered by transportation decision-makers. This paper examines the effect of vehicles with different levels of automation on traffic flow, such as non-assisted vehicles, vehicles with driver assistance systems, and fully autonomous vehicles. The accuracy of the examined traffic scenario is also an important factor in microscopic traffic simulation. In this paper, the central part of the city of Duisburg, Duisburg’s inner ring, is chosen for the traffic scenario. Through the cooperation with local government, official data of Origin/Destination matrices, induction loops, and traffic light plans are provided for this work. Thus, traffic demand from Origin/Destination matrices and induction loops are generated and compared, respectively. Finally, vehicles with different levels of automation are simulated in the Duisburg inner ring scenario. © 2020 by the authors. Li-censee MDPI, Basel, Switzerland.
    view abstractdoi: 10.3390/app11010029
  • 2021 • 126 Experimental and statistical analysis of the wear of diamond impregnated tools
    Malevich, N. and Müller, C.H. and Dreier, J. and Kansteiner, M. and Biermann, D. and De Pinho Ferreira, M. and Tillmann, W.
    Wear 468-469 (2021)
    Diamond impregnated tools are considered which are used to machine concrete. During their application, the bonding as well as the diamonds need to wear down in a certain way to gain a sharp tool. This required wear is called self-sharpening and means a continuous exposure of new diamonds. Within the development phase of diamond tools, time and cost intensive testing is necessary for the assessment of the tool performance. Hence, an extrapolation based on a minimal amount of testing is desirable to forecast the tool lifetime. A further reduction of the development and testing cost can be achieved by reducing the data needed to forecast the tool performance. Within this paper, the development of a statistical model is shown which was used to forecast the lifetime of the single diamonds on the tool. The statistical analysis is based on single segment tests which were carried out with different segment specification. During the tests, the exposed and broken out diamonds were counted to serve as the necessary input data for the statistical analysis. The counting of the diamonds on the segment was done in two different ways: based on the 2-dimensional microscopic pictures made after every minute of drilling and based on the 3-dimensional surface measurements made after every 5 min of drilling. It turns out that these two approaches of the wear analysis provide similar results. © 2020 Elsevier B.V.
    view abstractdoi: 10.1016/j.wear.2020.203574
  • 2021 • 125 High-Power Lensless THz Imaging of Hidden Objects
    Mansourzadeh, S. and Damyanov, D. and Vogel, T. and Wulf, F. and Kohlhaas, R.B. and Globisch, B. and Schultze, T. and Hoffmann, M. and Balzer, J.C. and Saraceno, C.J.
    IEEE Access 9 6268-6276 (2021)
    The potential of pulsed THz radiation for time-of-flight imaging applications is well recognized. However, advances in this field are currently severely limited by the low average power of ultrafast THz sources. Typically, this results in impractically long acquisition times and a loss in resolution and contrast. These limitations make imaging of the objects in real-life scenarios impossible. Here, conclusively, the potential of state-of-the-art high-average power THz time-domain spectrometer (TDS), driven by a 100-W class, one-box ultrafast oscillator for imaging applications is shown by demonstrating lensless THz imaging in reflection mode of a dielectric sample with low reflectivity. Images obtained with our home-built 20-mW average power THz-TDS system show a significant contrast enhancement compared to a state-of-the-art commercial THz-TDS with less than 200~mu text{W} of average power. Our unique setup even allows us to obtain images of such an object with high-contrast hidden inside a medium-density fiberboard (MDF) box. This opens the door to THz time-of-flight imaging of concealed objects of unknown shape and orientation in various real-life scenarios which were so far impossible to realize. © 2013 IEEE.
    view abstractdoi: 10.1109/ACCESS.2020.3048781
  • 2021 • 124 Object-oriented framework for 3D bending and free vibration analysis of multilayer plates: Application to cross-laminated timber and soft-core sandwich panels
    Marjanović, M. and Meschke, G. and Damnjanović, E.
    Composite Structures 255 (2021)
    In the paper, the main steps involved in the development of an object-oriented computational framework for the 3D bending and free vibration analysis of multilayer plates are presented. The mathematical formulation for layered finite elements is based on Reddy's plate theory for laminated composites. The analysis model has been implemented into Matlab, and the pre- and post-processing phases are performed using GiD. The proposed solver is characterized by a fast assembly procedure of sparse matrices using matrix vectorization, and a novel algorithm for the evaluation of interlaminar stresses satisfying continuity at layer interfaces. The performance, efficiency and accuracy of the computational framework are demonstrated through a number of validation examples by comparing the obtained results against the exact solution. Results from both static and dynamic analyses of multilayer panels are shown. © 2020 Elsevier Ltd
    view abstractdoi: 10.1016/j.compstruct.2020.112859
  • 2021 • 123 Effect of synthesis temperature on the phase formation of NiTiAlFeCr compositionally complex alloy thin films
    Marshal, A. and Singh, P. and Music, D. and Wolff-Goodrich, S. and Evertz, S. and Schökel, A. and Johnson, D.D. and Dehm, G. and Liebscher, C.H. and Schneider, J.M.
    Journal of Alloys and Compounds 854 (2021)
    The synthesis temperature dependent phase formation of Ni10Ti10Al25Fe35Cr20 thin films is compared to a bulk processed sample of identical composition. The as-cast alloy exhibits a dual-phase microstructure which is composed of a disordered BCC phase and AlNiTi-based B2- and/or L21-ordered phase(s). Formation of the BCC phase as well as an ordered AlNi-based B2 phase is observed for a thin film synthesised at 500 °C (ratio of synthesis temperature of thin film to melting temperature of bulk alloy: T/Tm = 0.49), which is attributed to both surface and bulk diffusion mediated growth. Post deposition annealing at 900 °C (T/Tm = 0.75) of a thin film deposited without intentional heating results in the formation of NiAlTi-based B2 and/or L21-phase(s) similar to the bulk sample, which is attributed to bulk diffusion. Depositions conducted at room temperature without intentional substrate heating (T/Tm = 0.20) resulted in the formation of an X-ray amorphous phase, while a substrate temperature increase to 175 °C (T/Tm = 0.28) causes the formation of a BCC phase. Atom probe tomography of the thin films deposited without intentional substrate heating and at 175 °C indicates the formation of ∼5 nm and ∼10 nm FeAl-rich domains, respectively. This can be rationalized based on the activation energy for surface diffusion, as Ti and Ni exhibt 2.5 to 4 times larger activation energy barriers than Al, Fe and Cr. It is evident from the homologous temperature that the phase formation observed at 500 °C (T/Tm = 0.49) is a result of both surface and bulk diffusion. As the temperature is reduced, the formation of FeAl-rich domains can be understood based on the differences in activation energy for surface diffusion and is consistent with kinetically limited thin film growth. © 2020 Elsevier B.V.
    view abstractdoi: 10.1016/j.jallcom.2020.155178
  • 2021 • 122 Structural forces in segmental linings: process-oriented tunnel advance simulations vs. conventional structural analysis
    Marwan, A. and Gall, V.E. and Alsahly, A. and Meschke, G.
    Tunnelling and Underground Space Technology 111 (2021)
    Tunnel linings are designed to permanently fulfill basic structural, serviceability and durability requirements throughout the lifetime of a tunnel. In order to ensure structural stability, it is important to correctly assess the response of tunnel linings with respect to the loading from the ground and process loads to which lining structures are subjected. For the design of segmental tunnel linings, precise structural models are needed, as the segmentation imbues the lining system with non-trivial kinematics. In this contribution, a technique for modeling the segment-wise installation of tunnel linings in the context of a 3D tunnel advance simulation is proposed in order to better predict the time dependent structural forces that develop in segmental lining systems during tunnel advance. The segments of the lining ring are explicitly modeled as separate bodies, and the interactions between segments at the longitudinal and ring joints are modeled by means of a surface-to-surface frictional contact algorithm. In order to examine the 3D stress distribution in the segmental concrete lining under realistic, time-dependent process loadings, the lining model is integrated into the process oriented finite element simulation model ekate. The influence of the joint arrangement and the segmentation is investigated through comparison with simulations in which a standard, continuous lining modeling technique is employed and with standard structural beam models used in engineering practice. It is shown that the magnitude of structural forces obtained by the explicit modelling of segmental lining joints and their time-dependent installation process within a 3D structural model diverges significantly from those obtained using standard methods, i.e. bedded beam models. © 2021 Elsevier Ltd
    view abstractdoi: 10.1016/j.tust.2021.103836
  • 2021 • 121 How Hydrogen Admixture Changes Plasma Jet Characteristics in Spray Processes at Low Pressure
    Mauer, G.
    Plasma Chemistry and Plasma Processing 41 109-132 (2021)
    In plasma spraying, hydrogen is widely used as a secondary working gas besides argon. In particular under low pressure, there are strong effects on the plasma jet characteristics even by small hydrogen percentages. Under such conditions, fundamental mechanisms like diffusion and recombination are affected while this is less relevant under atmospheric conditions. This was investigated for argon–hydrogen mixtures by optical emission spectroscopy (OES). The small electron densities under the investigated low pressure conditions implied specific difficulties in the application of several OES-based methods which are discussed in detail. Adding hydrogen to the plasma gas effected an increased plasma enthalpy. Moreover, the jet expanded radially as the reactive part of the thermal conductivity was enhanced by recombination of atomic hydrogen so that the shock waves were less reflected at the cold jet rims. In the jet cores, the lowest temperatures were found for the highest hydrogen admixture because the energy consumption due to the dissociation of molecular hydrogen outbalanced the increase of the plasma enthalpy. Variations in the radial temperature profiles were related to the jet structure and radial thermal conductivity. The local hydrogen–argon concentration ratios revealed an accumulation of hydrogen atoms at the jet rims. Clear indications were found, that higher hydrogen contents promoted the fast recombination of electrons and ions. However, it is assumed that the transport properties of the plasma were hardly affected by this, since the electron densities and thus the ionization degrees were generally small due to the low pressure conditions. © 2020, The Author(s).
    view abstractdoi: 10.1007/s11090-020-10143-6
  • 2021 • 120 Energetic characterization of faujasite zeolites using a sensor gas calorimeter
    Mauer, V. and Bläker, C. and Pasel, C. and Bathen, D.
    Catalysts 11 1-19 (2021)
    In addition to the adsorption mechanism, the heat released during exothermic adsorption influences the chemical reactions that follow during heterogeneous catalysis. Both steps depend on the structure and surface chemistry of the catalyst. An example of a typical catalyst is the faujasite zeolite. For faujasite zeolites, the influence of the Si/Al ratio and the number of Na+ and Ca2+ cations on the heat of adsorption was therefore investigated in a systematic study. A comparison between a NaX (Sodium type X faujasite) and a NaY (Sodium type Y faujasite) zeolite reveals that a higher Si/Al ratio and therefore a smaller number of the cations in faujasite zeolites leads to lower loadings and heats. The exchange of Na+ cations for Ca2+ cations also has an influence on the adsorption process. Loadings and heats first decrease slightly at a low degree of exchange and increase significantly with higher calcium contents. If stronger interactions are required for heterogeneous catalysis, then the CaNaX zeolites must have a degree of exchange above 53%. The energetic contributions show that the highest‐quality adsorption sites III and III’ make a contribution to the load‐dependent heat of adsorption, which is about 1.4 times (site III) and about 1.8 times (site III’) larger than that of adsorption site II. © 2021 by the authors. Licensee MDPI, Basel, Switzerland.
    view abstractdoi: 10.3390/catal11010098
  • 2021 • 119 Towards blood flow in the virtual human: Efficient self-coupling of HemeLB: Virtual Human Blood Flow with HemeLB
    McCullough, J.W.S. and Richardson, R.A. and Patronis, A. and Halver, R. and Marshall, R. and Ruefenacht, M. and Wylie, B.J.N. and Odaker, T. and Wiedemann, M. and Lloyd, B. and Neufeld, E. and Sutmann, G. and Skjellum, A. and Kran...
    Interface Focus 11 (2021)
    Many scientific and medical researchers are working towards the creation of a virtual human- A personalized digital copy of an individual-that will assist in a patient's diagnosis, treatment and recovery. The complex nature of living systems means that the development of this remains a major challenge. We describe progress in enabling the HemeLB lattice Boltzmann code to simulate 3D macroscopic blood flow on a full human scale. Significant developments in memory management and load balancing allow near linear scaling performance of the code on hundreds of thousands of computer cores. Integral to the construction of a virtual human, we also outline the implementation of a self-coupling strategy for HemeLB. This allows simultaneous simulation of arterial and venous vascular trees based on human-specific geometries. © 2020 The Authors.
    view abstractdoi: 10.1098/rsfs.2019.0119rsfs20190119
  • 2021 • 118 Recovering activity of anodically challenged oxygen reduction electrocatalysts by means of reductive potential pulses
    Medina, D. and Löffler, T. and Morales, D.M. and Masa, J. and Bobrowski, T. and Barwe, S. and Andronescu, C. and Schuhmann, W.
    Electrochemistry Communications 124 (2021)
    The stability of electrocatalysts is of great importance to ensure their applicability, but stability is generally only considered for catalysts polarised to a constant potential or current density. This excludes stability evaluation under start/stop conditions in a fuel cell or in reversible batteries in which the catalyst is alternately polarised to high opposite potentials. For example, the poor cyclability of metal-air batteries is mainly due to the decrease in the oxygen reduction activity of electrocatalysts during the high applied potentials for the oxygen evolution reaction during battery charging. To investigate and at least partially mitigate the loss of electrocatalytic activity for the oxygen reduction reaction, we employed reductive pulses with the aim of restoring the catalytic activity of the active sites for the oxygen reduction reaction. Optimisation of the reductive pulse parameters makes it possible to substantially prolong the oxygen reduction activity of a Fe-Nx-doped carbon-based oxygen reduction electrocatalyst. © 2021 The Author(s)
    view abstractdoi: 10.1016/j.elecom.2021.106960
  • 2021 • 117 New tools to probe the protein surface: Ultrasmall gold nanoparticles carry amino acid binders
    van der Meer, S.B. and Hadrovic, I. and Meiners, A. and Loza, K. and Heggen, M. and Knauer, S.K. and Bayer, P. and Schrader, T. and Beuck, C. and Epple, M.
    Journal of Physical Chemistry B (2021)
    A strategy toward epitope-selective functionalized nanoparticles is introduced in the following: ultrasmall gold nanoparticles (diameter of the metallic core about 2 nm) were functionalized with molecular tweezers that selectively attach lysine and arginine residues on protein surfaces. Between 11 and 30 tweezer molecules were covalently attached to the surface of each nanoparticle by copper-catalyzed azide alkyne cycloaddition (CuAAC), giving multiavid agents to target proteins. The nanoparticles were characterized by high-resolution transmission electron microscopy, differential centrifugal sedimentation, and 1H NMR spectroscopy (diffusion-ordered spectroscopy, DOSY, and surface composition). The interaction of these nanoparticles with the model proteins hPin1 (WW domain; hPin1-WW) and Survivin was probed by NMR titration and by isothermal titration calorimetry (ITC). The binding to the WW domain of hPin1 occurred with a KD of 41 ± 2 μM, as shown by ITC. The nanoparticle-conjugated tweezers targeted cationic amino acids on the surface of hPin1-WW in the following order: N-terminus (G) ≈ R17 &gt; R14 ≈ R21 &gt; K13 &gt; R36 &gt; K6, as shown by NMR spectroscopy. Nanoparticle recognition of the larger protein Survivin was even more efficient and occurred with a KD of 8 ± 1 μM, as shown by ITC. We conclude that ultrasmall nanoparticles can act as versatile carriers for artificial protein ligands and strengthen their interaction with the complementary patches on the protein surface. © XXXX American Chemical Society
    view abstractdoi: 10.1021/acs.jpcb.0c09846
  • 2021 • 116 Controlling the Surface Functionalization of Ultrasmall Gold Nanoparticles by Sequence-Defined Macromolecules
    van der Meer, S.B. and Seiler, T. and Buchmann, C. and Partalidou, G. and Boden, S. and Loza, K. and Heggen, M. and Linders, J. and Prymak, O. and Oliveira, C.L.P. and Hartmann, L. and Epple, M.
    Chemistry - A European Journal 27 1451-1464 (2021)
    Ultrasmall gold nanoparticles (diameter about 2 nm) were surface-functionalized with cysteine-carrying precision macromolecules. These consisted of sequence-defined oligo(amidoamine)s (OAAs) with either two or six cysteine molecules for binding to the gold surface and either with or without a PEG chain (3400 Da). They were characterized by 1H NMR spectroscopy, 1H NMR diffusion-ordered spectroscopy (DOSY), small-angle X-ray scattering (SAXS), and high-resolution transmission electron microscopy. The number of precision macromolecules per nanoparticle was determined after fluorescent labeling by UV spectroscopy and also by quantitative 1H NMR spectroscopy. Each nanoparticle carried between 40 and 100 OAA ligands, depending on the number of cysteine units per OAA. The footprint of each ligand was about 0.074 nm2 per cysteine molecule. OAAs are well suited to stabilize ultrasmall gold nanoparticles by selective surface conjugation and can be used to selectively cover their surface. The presence of the PEG chain considerably increased the hydrodynamic diameter of both dissolved macromolecules and macromolecule-conjugated gold nanoparticles. © 2020 The Authors. Published by Wiley-VCH GmbH
    view abstractdoi: 10.1002/chem.202003804
  • 2021 • 115 Augmented Reality Application for the Mobile Measurement of Strain Distributions
    Mogylenko, O. and Selvaggio, A. and Upadhya, S. and Grodotzki, J. and Tekkaya, A.E.
    Advances in Intelligent Systems and Computing 1231 AISC 235-245 (2021)
    In mechanical engineering studies hands-on laboratories are an integral part of the students’ education. In manufacturing related fields, material characterization labs are often used to enable students to foster their understanding of different materials. To enhance the laboratory experience and to educate about specific aspects of the uniaxial tensile test, an Augmented Reality (AR) application has been developed. With this applications, it is possible to visualize the inhomogeneous strain field that arises during the experiment on the surface of the specimen. The technical components and structure of the implementation are described in this paper. The usability of several algorithms, technical and software implementation is discussed and evaluated. © 2021, Springer Nature Switzerland AG.
    view abstractdoi: 10.1007/978-3-030-52575-0_19
  • 2021 • 114 Ti-bearing lightweight steel with large high temperature ductility via thermally stable multi-phase microstructure
    Moon, J. and Jo, H.-H. and Park, S.-J. and Kim, S.-D. and Lee, T.-H. and Lee, C.-H. and Lee, M.-G. and Hong, H.-U. and Suh, D.-W. and Raabe, D.
    Materials Science and Engineering A 808 (2021)
    The global demand for lightweight design is increasing to provide sustainable solutions to counteract climate change. We developed a novel Ti-bearing lightweight steel (8% lower mass density than general steels), which exhibits an excellent combination of strength (491 MPa ultimate tensile strength) and tensile ductility (31%) at elevated temperature (600 °C). The developed steel is suitable for parts subjected to high temperature at reduced dynamical load. The composition of the developed steel (Fe–20Mn–6Ti–3Al–0.06C–NbNi (wt%)) lends the alloy a multiphase structure with austenite matrix, partially ordered ferrite, Fe2Ti Laves phase, and fine MC carbides. At elevated temperature (600 °C), the ductility of the new material is at least 2.5 times higher than that of conventional lightweight steels based on the Fe–Mn–Al system, which become brittle at elevated temperatures due to the inter/intragranular precipitation of κ-carbides. This is achieved by the high thermal stability of its microstructure and the avoidance of brittle κ-carbides in this temperature range. © 2021 Elsevier B.V.
    view abstractdoi: 10.1016/j.msea.2021.140954
  • 2021 • 113 Multi-line SiO fluorescence imaging in the flame synthesis of silica nanoparticles from SiCl4
    Moussawi, A.E. and Endres, T. and Peukert, S. and Zabeti, S. and Dreier, T. and Fikri, M. and Schulz, C.
    Combustion and Flame 224 260-272 (2021)
    Flame synthesis is a powerful and scalable method for generating nanoparticles for a wide range of applications. The chemical interaction of the flame and the precursor combined with the spatial and temporal temperature distribution determine the product properties. For controlled nanoparticle synthesis that can also be scaled to industrial production rates, detailed knowledge of the underlying chemical kinetics and their interaction with the reactive flow is essential. Laser diagnostics has the capability to analyze the process by probing the concentration of important intermediates in shock tubes and reactive flows. The gas-phase synthesis of silica nanoparticles from SiCl4 in a premixed H2/O2 low-pressure flame reactor is studied by laser-induced fluorescence imaging of SiO mole fractions and temperature. The literature value-based spectroscopy model of SiO used for fitting the LIF spectra are validated based on absorption cross-sections measurements in a shock tube, where SiO is formed under precisely defined conditions (temperature, pressure, mole fraction) using a well-known kinetics mechanism for SiH4/CO2/Ar decomposition. Based on literature sources, a reaction mechanism is assembled to describe the oxidation of SiCl4 in the flame, which is then compared to the measured SiO mole fractions distribution to shed light on the current state of the understanding of SiCl4 combustion chemistry and to direct further refinements. © 2020 The Author(s)
    view abstractdoi: 10.1016/j.combustflame.2020.12.020
  • 2021 • 112 Atmospheric pressure metal-organic chemical vapor deposition (AP-MOCVD) growth of undoped and aluminium-doped ZnO thin film using hot wall reactor
    Nebatti Ech Chergui, A. and Pflitsch, C. and Atakan, B.
    Surfaces and Interfaces 22 (2021)
    In this contribution, a hot wall reactor via economic atmospheric pressure metal-organic chemical vapor deposition (AP-MOCVD) was adopted for Un-doped and Al-doped Zinc oxide films were deposited on borosilicate glass and silicon substrates. To avoid the use of an expensive vacuum system, all experiments were realized at atmospheric pressure. The chemical reagents used for this experiment are Zinc acetylacetonate (Zn(acac)2) and aluminium acetylacetonates (Al (acac)2) under atmospheric conditions. The obtained films are characterized by X-ray diffraction (XRD),scanning electron microscopy (SEM), energy dispersive X-ray (EDX) and Uv-–vis spectrometer, respectively. As results, it is found that The un-doped ZnO films are polycrystalline. However, a significant enhancement in the intensity of the relevant (100) reflection is observed when Zinc oxide films are doped with Al. It is also observed that the Al-doped Zinc oxide films present higher transparency in the visible region and resistivities of 2.55 ohm cm and 1.44 ohm cm for un-doped and Al doped films respectively © 2020
    view abstractdoi: 10.1016/j.surfin.2020.100883
  • 2021 • 111 Software-in-the-loop optimization of actuator and sensor placement for a smart piezoelectric funnel-shaped inlet of a magnetic resonance imaging tomograph
    Nestorović, T. and Hassw, K. and Oveisi, A.
    Mechanical Systems and Signal Processing 147 (2021)
    Performance of smart piezoelectric structures strongly depends on placement of integrated piezoelectric actuators and sensors, which may be implemented in the form of thin film layers on the structure surface or embedded within the structure. In both cases actuator and sensor placement plays an important role, since after applying they remain permanently integrated with structure. In this paper the optimization procedure for piezoelectric structures with curved surfaces is proposed based on the Software-in-the-Loop (SiL) methodology and balanced modal order reduction in combination with H2 and H∞ norms used in placement indices. The optimization procedure is a global one, since it seeks for optima across the entire domain of the structure. A special challenge is tackling the problem of curved surfaces. This problem is solved in this work for a funnel shaped structure – inlet of the magnetic resonance imaging tomopraph. A thorough mesh convergence study with respect to the eigenfrequency analysis is performed in order to obtain a reliable numeric finite element model for further optimization purposes. Material parameter optimization is performed as well. Based on placement indices optimal placement study is performed under consideration of several eigenmodes of interest. The optimization is performed for individual modes as well as for simultaneous consideration of multiple modes. The SiL approach with recurrent communication in each iteration of the optimization between the numerical simulation FE software and optimization tool designed in Python is implemented through the evaluation of the placement indices for candidate locations over the entire curved surface of the structure. Depending on support conditions, optimal locations of piezoelectric actuators and sensors are proposed. © 2020 Elsevier Ltd
    view abstractdoi: 10.1016/j.ymssp.2020.107097
  • 2021 • 110 From digital models to numerical analysis for mechanised tunnelling: A fully automated design-through-analysis workflow
    Ninic, J. and Alsahly, A. and Vonthron, A. and Bui, H.-G. and Koch, C. and König, M. and Meschke, G.
    Tunnelling and Underground Space Technology 107 (2021)
    Large infrastructure projects involving the construction of tunnels in urban areas constitute complex, integrated and multi-disciplinary systems, which require building and construction information modelling as well as computational design assessment tools for decision making during all project phases and during their complete life cycle. Even if the underlying information needed for computational analysis is stored in an information model, the translation to computational models is still cumbersome and requires significant manual work for model generation and set-up as well as excessive computing resources and time. To address these shortcomings, this paper presents a systematic summary of concepts for integrated information modelling, numerical analysis and visualisation for urban mechanized tunnelling. Our first approach “BIM-to-FEM” is characterised by a fully automated link for error-free data exchange between a standalone Tunnelling Information Model and the process-oriented simulation model for mechanized tunnelling “ekate”. In the second approach “SATBIM”, a fully automated data exchange workflow is established between a parametric multi-level information model for tunnelling and multi-level numerical models based on both Finite Element and Isogeometric Analysis, where meta models are employed for real-time design assessment. We discuss the different applications of these concepts, such as scenario-based exploration of design alternatives, real-time design assessment within a TIM based on meta-models, and the potentials of using these models for the process control during construction. Furthermore, we present two case studies where real project data has been used for the integration of information and numerical modelling. The examples in this paper indicate clear advantages of this approach compared to traditional approaches in terms of efficiency of modelling achieved by reduced user interactions and error-free information exchange, and show the benefits of multi-level model representation and real-time analysis tasks. © 2020 Elsevier Ltd
    view abstractdoi: 10.1016/j.tust.2020.103622
  • 2021 • 109 Modeling Fused Filament Fabrication using Artificial Neural Networks
    Oehlmann, P. and Osswald, P. and Blanco, J.C. and Friedrich, M. and Rietzel, D. and Witt, G.
    Production Engineering (2021)
    With industries pushing towards digitalized production, adaption to expectations and increasing requirements for modern applications, has brought additive manufacturing (AM) to the forefront of Industry 4.0. In fact, AM is a main accelerator for digital production with its possibilities in structural design, such as topology optimization, production flexibility, customization, product development, to name a few. Fused Filament Fabrication (FFF) is a widespread and practical tool for rapid prototyping that also demonstrates the importance of AM technologies through its accessibility to the general public by creating cost effective desktop solutions. An increasing integration of systems in an intelligent production environment also enables the generation of large-scale data to be used for process monitoring and process control. Deep learning as a form of artificial intelligence (AI) and more specifically, a method of machine learning (ML) is ideal for handling big data. This study uses a trained artificial neural network (ANN) model as a digital shadow to predict the force within the nozzle of an FFF printer using filament speed and nozzle temperatures as input data. After the ANN model was tested using data from a theoretical model it was implemented to predict the behavior using real-time printer data. For this purpose, an FFF printer was equipped with sensors that collect real time printer data during the printing process. The ANN model reflected the kinematics of melting and flow predicted by models currently available for various speeds of printing. The model allows for a deeper understanding of the influencing process parameters which ultimately results in the determination of the optimum combination of process speed and print quality. © 2021, The Author(s).
    view abstractdoi: 10.1007/s11740-021-01020-y
  • 2021 • 108 The effect of short silica fibers (0.3 μm 3.2 μm) on macrophages
    Olejnik, M. and Breisch, M. and Sokolova, V. and Loza, K. and Prymak, O. and Rosenkranz, N. and Westphal, G. and Bünger, J. and Köller, M. and Sengstock, C. and Epple, M.
    Science of the Total Environment 769 (2021)
    Silica fibers with a dimension of 0.3 μm ∙ 3.2 μm2 nm were prepared by a modified Stöber synthesis as model particles. The particles were characterized by scanning electron microscopy, elemental analysis, thermogravimetry and X-ray powder diffraction. Their uptake by macrophages (THP-1 cells and NR8383 cells) was studied by confocal laser scanning microscopy and scanning electron microscopy. The uptake by cells was very high, but the silica fibers were not harmful to NR8383 cells in concentrations up to 100 μg mL−1. Only above 100 μg mL−1, significant cell toxic effects were observed, probably induced by a high dose of particles that had sedimented on the cells and led to the adverse effects. The chemotactic response as assessed by the particle-induced migration assay (PICMA) was weak in comparison to a control of agglomerated silica particles. The as-prepared fibers were fully X-ray amorphous but crystallized to β-cristobalite after heating to 1000 °C and converted to α-cristobalite upon cooling to ambient temperature. The fibers had sintered to larger aggregates but retained their elongated primary shape. The particle cytotoxicity towards THP-1 cells was not significantly enhanced by the crystallization. © 2021 Elsevier B.V.
    view abstractdoi: 10.1016/j.scitotenv.2020.144575
  • 2021 • 107 In situ total scattering experiments of nucleation and crystallisation of tantalum-based oxides: From highly dilute solutions via cluster formation to nanoparticles
    Onur Şahin, E. and Tüysüz, H. and Chan, C.K. and Moon, G.-H. and Dai, Y. and Schmidt, W. and Lim, J. and Scheu, C. and Weidenthaler, C.
    Nanoscale 13 150-162 (2021)
    The exact formation mechanism of tantalum oxides (and in general, metal/mixed metal oxides) from alkoxide precursors is still not fully understood, particularly when forming cluster-like or amorphous materials. The structural evolution of Ta-based oxides was studied in detail using X-ray total scattering experiments along with subsequent pair distribution function (PDF) analyses. Starting from a tantalum alkoxide precursor (Ta2(OEt)10), the formation of hydrolysed TaxOyHz clusters in highly diluted aqueous solution was analysed. From the PDF data, the connectivity and arrangement of TaxOy octahedra in the cluster could be deduced as well as the approximate size of the clusters (<1 nm). Construction of cluster models allowed for identification of common structural motifs in the TaxOyHz clusters, ruling out the formation of chain- or ring-like clusters. More likely, bulky clusters with a high number of corner-sharing octahedra are formed. After separation of the amorphous solid from the liquid, temperature-induced crystallisation processes were monitored via in situ total scattering experiments. Between room temperature and 600 °C, only small rearrangements of the amorphous structure are observed. At about 610 °C, amorphous TaxOyHz transforms directly into crystalline orthorhombic L-Ta2O5 without formation of any crystalline intermediate structures. © 2021 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/d0nr07871a
  • 2021 • 106 Failure criterion for PA 12 multi-jet fusion additive manufactured parts
    Osswald, P.V. and Obst, P. and Mazzei Capote, G.A. and Friedrich, M. and Rietzel, D. and Witt, G.
    Additive Manufacturing 37 (2021)
    Offering the possibility of producing complex geometries in a compressed product development cycle, it comes as no surprise that additive manufacturing (AM) techniques have become attractive to multiple industries, including the automotive and aerospace segments. Unfortunately, the ubiquitous stratified build approach used by these technologies is responsible for the pain point that hinders their adoption in production of parts that will be subjected to complex loads: the junction of adjacent layers tends to have subpar mechanical properties when compared to those of the bulk material, and thus, assessing the structural integrity of an AM part becomes difficult. In the advent of the industrialization of series production of AM parts for the automotive industry, the necessity to understand and predict how and why AM parts fail under complex stress states becomes of paramount importance. This paper applies a failure criterion for materials with anisotropic properties with stress interactions, to predict failure of multi-jet fusion (MJF) parts manufactured using polyamide 12 powder. The results are compared to the failure surfaces of Selective Laser Sintering (SLS) components. Special test specimens were designed, produced, and tested to measure failure under tensile, compressive, shear, and combined loading scenarios. The results show that much like SLS, MJF parts have a notable difference in tensile and compressive strengths. Unlike SLS however, MJF parts do not exhibit a strong interaction between stresses when under combined loading. The experimental data shows an excellent fit with the failure criterion, precisely capturing the strength behavior of MJF printed parts under complex loading conditions. Of great interest in this study is that the stress interactions with MJF parts were determined to be negligible when compared to SLS specimens, which emphasizes the fact that when performing stress analyses, each one of these powder-based additive manufacturing techniques must be treated differently. © 2020 Elsevier B.V.
    view abstractdoi: 10.1016/j.addma.2020.101668
  • 2021 • 105 Supramolecular polymers with reversed viscosity/temperature profile for application in motor oils
    Ostwaldt, J.-E. and Hirschhäuser, C. and Maier, S.K. and Schmuck, C. and Niemeyer, J.
    Beilstein Journal of Organic Chemistry 17 105-114 (2021)
    We report novel supramolecular polymers, which possess a reversed viscosity/temperature profile. To this end, we developed a series of ditopic monomers featuring two self-complementary binding sites, either the guanidiniocarbonyl pyrrole carboxylic acid (GCP) or the aminopyridine carbonyl pyrrole carboxylic acid (ACP). At low temperatures, small cyclic structures are formed. However, at elevated temperatures, a ring–chain transformation leads to the formation of a supramolecular polymer. We demonstrate that this effect is dependent on the concentration of the solution and on the polarity of the solvent. This effect can counteract the loss of viscosity of the solvent at elevated temperatures, thus opening an application of our systems as viscosity index improvers (VIIs) in working fluids. This was tested for different motor oils and led to the identification of one compound as a promising VII. © 2021 Ostwaldt et al.
    view abstractdoi: 10.3762/BJOC.17.11
  • 2021 • 104 Electronic interactions between graphene and cobaltite thin film La0.7Sr0.3CoO3 and its magnetic consequences
    Othmen, Z. and Othmen, R. and Daoudi, K. and Boudard, M. and Cavanna, A. and Madouri, A. and Gemeiner, P. and Lupascu, D.C. and Oueslati, M. and Dkhil, B.
    Surfaces and Interfaces 23 (2021)
    We have successfully synthesized and transferred graphene (Gr) monolayers on top of epitaxial mixed valence La0.7Sr0.3CoO3 (LSCO) thin films. Raman spectroscopy reveals that Jahn-Teller (JT) modes associated with the oxygen octahedral distortions usually unobserved for bare LSCO are activated by the deposited graphene. The appearance of these JT modes in the Gr/LSCO heterostructure is attributed to the electronic interactions at the interface between the graphene and the LSCO thin film promoting intermediate spin states of the Co ions. As a result, the magnetic properties of LSCO are affected. Indeed, magnetization measurements show a phase transition at ~135 K which is due to the presence of the graphene while the ferromagnetic transition of bare LSCO films is observed at~200 K. This magnetic phase is confirmed by Raman spectroscopy measurements as a function of temperature revealing a vibrational transition around the same temperature. © 2021 Elsevier B.V.
    view abstractdoi: 10.1016/j.surfin.2020.100919
  • 2021 • 103 NIR-Sensitized Cationic and Hybrid Radical/Cationic Polymerization and Crosslinking
    Pang, Y. and Shiraishi, A. and Keil, D. and Popov, S. and Strehmel, V. and Jiao, H. and Gutmann, J.S. and Zou, Y. and Strehmel, B.
    Angewandte Chemie - International Edition 60 1465-1473 (2021)
    NIR-sensitized cationic polymerization proceeded with good efficiency, as was demonstrated with epoxides, vinyl ether, and oxetane. A heptacyanine functioned as sensitizer while iodonium salt served as coinitiator. The anion adopts a special function in a series selected from fluorinated phosphates (a: [PF6]−, b: [PF3(C2F5)3]−, c: [PF3(n-C4F9)3]−), aluminates (d: [Al(O-t-C4F9)4]−, e: [Al(O(C3F6)CH3)4]−), and methide [C(O-SO2CF3)3]− (f). Vinyl ether showed the best cationic polymerization efficiency followed by oxetanes and oxiranes. DFT calculations provided a rough pattern regarding the electrostatic potential of each anion where d showed a better reactivity than e and b. Formation of interpenetrating polymer networks (IPNs) using trimethylpropane triacrylate and epoxides proceeded in the case of NIR-sensitized polymerization where anion d served as counter ion in the initiator system. No IPN was formed by UV-LED initiation using the same monomers but thioxanthone/iodonium salt as photoinitiator. Exposure was carried out with new NIR-LED devices emitting at either 805 or 870 nm. © 2020 The Authors. Published by Wiley-VCH GmbH
    view abstractdoi: 10.1002/anie.202010746
  • 2021 • 102 Damage mechanisms in cavitation erosion of nitrogen-containing austenitic steels in 3.5% NaCl solution
    Paolantonio, M. and Hanke, S.
    Wear 464-465 (2021)
    Interruptions of the passive layer of stainless steels by cavitation erosion expose the bare metal surface to the environment and can lead to cavitation-erosion-corrosion damage and synergistic effects. However, the probability for pitting corrosion is decreased during cavitation exposure of stainless steels in chloride solutions because mechanical passive film removal shifts corrosion potentials to lower cathodic values. In this study, the impact of 3.5 wt% NaCl in water on mass loss and damage features of two austenitic stainless N-containing steels is investigated to amend the understanding of cavitation erosion of passivating steels. Ultrasonic cavitation tests were carried out on steels 316LVM and CNMo0.95 in distilled water and 3.5% NaCl solution. Exposed surfaces were characterized qualitatively by light- and electron-microscopy and quantitatively by confocal microscopy. Damage mechanisms vary between the two steels but not with NaCl content in the solution. 316LVM also displayed the same mass loss in both solutions. CNMo0.95 possesses twice the strength as 316LVM, resulting in lower intensities of ductile damage mechanisms and slower damage progression. Mass loss of CNMo0.95 was lower in 3.5% NaCl solution compared to distilled water, which was primarily assigned to the effect of the salt content in the water on cavitation bubble formation. © 2020 Elsevier B.V.
    view abstractdoi: 10.1016/j.wear.2020.203526
  • 2021 • 101 Control of residual oxygen of the process atmosphere during laser-powder bed fusion processing of Ti-6Al-4V
    Pauzon, C. and Dietrich, K. and Forêt, P. and Dubiez-Le Goff, S. and Hryha, E. and Witt, G.
    Additive Manufacturing 38 (2021)
    The effect of the residual oxygen concentration in the process atmosphere during laser-powder bed fusion (L-PBF) of Ti-6Al-4V was investigated, using an external oxygen monitoring system equipped with two types of oxygen sensors typically used in L-PBF hardware: a lambda probe and an electrochemical oxygen sensor. The recordings of the oxygen variations during L-PBF highlighted that the electrochemical sensor is more reliable than the lambda probe, whose signal showed a maximum deviation of about 700 ppm O2 after 7 h, attributed to its sensitivity to hydrogen present in the system. The study revealed that proper monitoring of the oxygen in the laboratory scale L-PBF system used is necessary to limit oxygen and nitrogen pick-ups by the built material. Concentrations as high as 2200 ppm O2 and 500 ppm N2 in the Ti-6Al-4V part built under standard conditions were measured, compared to maximum levels of 1800 ppm O2 and 250 ppm N2 with the external oxygen control. In addition, the findings underline the critical effect of the component design, such as the high aspect ratio columns or the lattice structures, on the heat accumulation in case of Ti-6Al-4V, leading to enhanced oxygen and nitrogen pick-up, as high as 600 ppm O2 and 150 ppm N2 difference between the bottom and top of the cylindrical samples of 70 mm height used in this study. The determination of tensile properties of samples built at different heights put in evidence the detrimental effect of the oxygen increase with build height on the ductility, which decreased from 12% to below 6% between the bottom and top positions. This work highlights that the possible presence of impurities in the L-PBF atmosphere can have harmful impact on the properties of Ti-6Al-4V components, which can be mitigated adjusting the oxygen control system. © 2020 The Authors
    view abstractdoi: 10.1016/j.addma.2020.101765
  • 2021 • 100 Mitigating oxygen pick-up during laser powder bed fusion of Ti-6Al-4V by limiting heat accumulation
    Pauzon, C. and Dietrich, K. and Forêt, P. and Hryha, E. and Witt, G.
    Materials Letters 288 (2021)
    The dissolution of oxygen in Ti-6Al-4V during laser powder bed fusion (L-PBF) is a limitation for the final ductility of the produced components and a challenge for the end-users. In the present work, the effect of the residual oxygen in the process atmosphere of a laboratory scale L-PBF machine, as well as the role of heat accumulation, are studied. It was shown that oxygen content in the as-built Ti-6Al-4V is determined by the size of the scanned area and build time. The heat accumulation aspect was investigated by adjusting the inter-layer time (ILT), by increasing the recoating time or the number of produced parts. The results showed that oxygen pick-up could be limited by reducing residual oxygen level in the atmosphere or heat accumulation. A 400 ppm O2 reduction measured at the top of a 70 mm column was achieved by increasing the ILT manually by 4.5 s, and a 1200 ppm O2 reduction by increasing the scanned area by 7 times. By doing so, the hardness at full height was reduced by approximately 30 HV10. It is shown that design features characterised by high aspect ratio can absorb significant amount of oxygen resulting in increased brittleness. © 2021 The Author(s)
    view abstractdoi: 10.1016/j.matlet.2021.129365
  • 2021 • 99 Tracking changes in adaptation to suspension growth for MDCK cells: cell growth correlates with levels of metabolites, enzymes and proteins
    Pech, S. and Rehberg, M. and Janke, R. and Benndorf, D. and Genzel, Y. and Muth, T. and Sickmann, A. and Rapp, E. and Reichl, U.
    Applied Microbiology and Biotechnology (2021)
    Abstract: Adaptations of animal cells to growth in suspension culture concern in particular viral vaccine production, where very specific aspects of virus-host cell interaction need to be taken into account to achieve high cell specific yields and overall process productivity. So far, the complexity of alterations on the metabolism, enzyme, and proteome level required for adaptation is only poorly understood. In this study, for the first time, we combined several complex analytical approaches with the aim to track cellular changes on different levels and to unravel interconnections and correlations. Therefore, a Madin-Darby canine kidney (MDCK) suspension cell line, adapted earlier to growth in suspension, was cultivated in a 1-L bioreactor. Cell concentrations and cell volumes, extracellular metabolite concentrations, and intracellular enzyme activities were determined. The experimental data set was used as the input for a segregated growth model that was already applied to describe the growth dynamics of the parental adherent cell line. In addition, the cellular proteome was analyzed by liquid chromatography coupled to tandem mass spectrometry using a label-free protein quantification method to unravel altered cellular processes for the suspension and the adherent cell line. Four regulatory mechanisms were identified as a response of the adaptation of adherent MDCK cells to growth in suspension. These regulatory mechanisms were linked to the proteins caveolin, cadherin-1, and pirin. Combining cell, metabolite, enzyme, and protein measurements with mathematical modeling generated a more holistic view on cellular processes involved in the adaptation of an adherent cell line to suspension growth. Key points: • Less and more efficient glucose utilization for suspension cell growth • Concerted alteration of metabolic enzyme activity and protein expression • Protein candidates to interfere glycolytic activity in MDCK cells © 2021, The Author(s).
    view abstractdoi: 10.1007/s00253-021-11150-z
  • 2021 • 98 Gate-Controlled Field Emission Current from MoS2 Nanosheets
    Pelella, A. and Grillo, A. and Urban, F. and Giubileo, F. and Passacantando, M. and Pollmann, E. and Sleziona, S. and Schleberger, M. and Di Bartolomeo, A.
    Advanced Electronic Materials 7 (2021)
    Monolayer molybdenum disulfide (MoS2) nanosheets, obtained via chemical vapor deposition onto SiO2/Si substrates, are exploited to fabricate field-effect transistors with n-type conduction, high on/off ratio, steep subthreshold slope, and good mobility. The transistor channel conductance increases with the reducing air pressure due to oxygen and water desorption. Local field emission measurements from the edges of the MoS2 nanosheets are performed in high vacuum using a tip-shaped anode. It is demonstrated that the voltage applied to the Si substrate back-gate modulates the field emission current. Such a finding, that it is attributed to gate-bias lowering of the MoS2 electron affinity, enables a new field-effect transistor based on field emission. © 2020 Wiley-VCH GmbH
    view abstractdoi: 10.1002/aelm.202000838
  • 2021 • 97 Catalyst-enhanced plasma oxidation of n-butane over α-MnO2 in a temperature-controlled twin surface dielectric barrier discharge reactor
    Peters, N. and Schücke, L. and Ollegott, K. and Oberste-Beulmann, C. and Awakowicz, P. and Muhler, M.
    Plasma Processes and Polymers (2021)
    A twin surface dielectric barrier discharge is used for the catalyst-enhanced plasma oxidation of 300 ppm n-butane in synthetic air. Plasma-only operation results in the conversion of n-butane into CO and CO2. Conversion is improved by increasing the temperature of the feed gas, but selectivity shifts to undesired CO. α-MnO2 is used as a catalyst deposited on the electrodes by spray coating with a distance of 1.5 mm between the uncoated grid lines and the square catalyst patches to prevent the inhibition of plasma ignition. The catalyst strongly influences selectivity, reaching 40% conversion and 73% selectivity to CO2 at a specific energy density of 390 J·L−1 and 140°C, which is far below the onset temperature of thermocatalytic n-butane conversion. © 2021 The Authors. Plasma Processes and Polymers published by Wiley-VCH GmbH.
    view abstractdoi: 10.1002/ppap.202000127
  • 2021 • 96 Bearable Local Stress of High-Strength SFRC
    Plückelmann, S. and Breitenbücher, R. and Smarslik, M. and Mark, P.
    RILEM Bookseries 30 176-188 (2021)
    In the case of partial-area loading, compressive forces are transmitted into concrete members only over a limited area. For plain concretes and conventionally reinforced concretes, numerous investigations have already been carried out analyzing the load-bearing behavior under partial-area loading. Due to the tendency towards higher concrete strengths and the increasingly widespread use of steel fibers in recent years, it becomes also necessary to investigate the performance of high-strength steel fiber reinforced concrete (SFRC) under partial-area loading. This paper describes experimental tests on high-strength steel fiber reinforced concrete under partial-area loading with spatial and plane load distribution. Different area ratios and concretes with different fiber types and contents as well as fiber cocktails were considered. On the basis of the test results, a calculation approach is proposed for the determination of the bearable ultimate local stress. It is shown that by referring to the flexural tensile strength, instead of the compressive strength, as in the case of common calculation approaches, a more precise approximation of the ultimate local stresses for high-strength steel fiber reinforced concrete is possible. © 2021, RILEM.
    view abstractdoi: 10.1007/978-3-030-58482-5_16
  • 2021 • 95 A Minimally Invasive Monitoring Concept for Plasma-Assisted Surface Treatments in PET Bottles
    Pohle, D. and Mitschker, F. and Jenderny, J. and Rudolph, M. and Schulz, C. and Awakowicz, P. and Rolfes, I.
    2020 50th European Microwave Conference, EuMC 2020 479-482 (2021)
    This paper presents a novel approach to plasma monitoring in the context of plasma-assisted surface treatments in PET bottles. In industrial state-of-the-art production of PET-based beverage bottles, a so-called Plasmaline antenna is inserted into the bottle which provides both process gases and microwave excitation to generate the plasma state required for coating or sterilization on its inside. The proposed concept based on the planar multipole resonance probe (pMRP) allows for a non-invasive supervision of the plasma from the outside of the bottle wall. Since plasma and probe head are only separated by dielectric materials in between, the sensor's electric field is able to interact with the plasma and the resonance behavior that occurs can be evaluated and tracked. The performance of the concept regarding changes of the plasma electron frequency and the electron collision frequency are investigated within 3D full-wave simulations in CST Microwave Studio. Measurements of an argon plasma are presented as a proof-of-concept, with the plasma being monitored from the plasma-remote side of a PET bottle section. © 2021 EuMA.
    view abstractdoi: 10.23919/EuMC48046.2021.9338200
  • 2021 • 94 Coin minting by additive manufacturing and forming
    Pragana, J.P.M. and Rosenthal, S. and Alexandrino, P. and Araújo, A. and Bragança, I.M.F. and Silva, C.M.A. and Leitão, P.J. and Tekkaya, A.E. and Martins, P.A.F.
    Proceedings of the Institution of Mechanical Engineers, Part B: Journal of Engineering Manufacture 235 819-828 (2021)
    Additive manufacturing is proposed as a novel alternative to coin blank’s production routes based on rolling, blanking and edge rimming. The presentation draws from laser powder bed fusion of cylinders, slicing into individual coin blanks by electro discharge machining and surface preparation by polishing, to coin minting in a laboratory press-tool system. Special emphasis is given to material deposition and coin minting due to the originality of producing coin blanks with complex intricate contoured holes and to the necessity of subjecting the additive manufactured coin blanks to extreme compressive stresses that are typical of coin minting. Numerical and experimental results confirm the excellent performance of the additive manufactured coin blanks. The new design layouts included in the additive manufactured coin blanks open the way to produce high value-added singular collector coins, which are disruptively different from those available in the market nowadays. © IMechE 2020.
    view abstractdoi: 10.1177/0954405420971128
  • 2021 • 93 Comparison of Hilbert Transform and Complex Demodulation for Defect Identification in Cutting Discs using Vibration-Based Feature Extraction
    Priebe, S. and Brackmann, L. and Alabd-Allah, A. and Butt, S. and Röttger, A. and Meschke, G. and Mueller, I.
    Lecture Notes in Civil Engineering 127 564-572 (2021)
    This paper presents a novel concept for vibration-based feature extraction to identify damages in cutting discs of Tunnel Boring Machines (TBM). Defect frequencies resulting from repeated interaction of rock and disc defects are analysed. The data set is represented by the normal force acting on the edge of a cutting disc and the rock. Two different methods, the Hilbert transform and the complex demodulation, are used to generate the envelope of the time series, which was used to analyse whether the signal shows a feature representing an existing defect in the frequency domain. For the first proof of concept two numerical models were used - a multi-body system and a peridynamics 3D model simulating time series of normal forces. With both models, the linear motion of the disc on a rock sample with constant velocity was simulated. An experimental setup, mechanically similar to the simulations, was used in two experiments for further comparison. All implemented defects could be detected using vibration data of forces and one of the proposed data analysis techniques. © 2021, Springer Nature Switzerland AG.
    view abstractdoi: 10.1007/978-3-030-64594-6_55
  • 2021 • 92 Synthetic ferripyrophyllite: Preparation, characterization and catalytic application
    Qiao, Y. and Theyssen, N. and Spliethoff, B. and Folke, J. and Weidenthaler, C. and Schmidt, W. and Prieto, G. and Ochoa-Hernández, C. and Bill, E. and Ye, S. and Ruland, H. and Schüth, F. and Leitner, W.
    Dalton Transactions 50 850-857 (2021)
    Sheet silicates, also known as phyllosilicates, contain parallel sheets of tetrahedral silicate built up by [Si2O5]2- entities connected through intermediate metal-oxygen octahedral layers. The well-known minerals talc and pyrophyllite are belonging to this group based on magnesium and aluminium, respectively. Surprisingly, the ferric analogue rarely occurs in nature and is found in mixtures and conglomerates with other materials only. While partial incorporation of iron into pyrophyllites has been achieved, no synthetic protocol for purely iron-based pyrophyllite has been published yet. Here we report about the first artificial synthesis of ferripyrophyllite under exceptional mild conditions. A similar ultrathin two-dimensional (2D) nanosheet morphology is obtained as in talc or pyrophyllite but with iron(iii) as a central metal. The high surface material exhibits a remarkably high thermostability. It shows some catalytic activity in ammonia synthesis and can serve as catalyst support material for noble metal nanoparticles. © The Royal Society of Chemistry.
    view abstractdoi: 10.1039/d0dt03125a
  • 2021 • 91 Single-Entity Electrocatalysis of Individual “Picked-and-Dropped” Co3O4 Nanoparticles on the Tip of a Carbon Nanoelectrode
    Quast, T. and Aiyappa, H.B. and Saddeler, S. and Wilde, P. and Chen, Y.-T. and Schulz, S. and Schuhmann, W.
    Angewandte Chemie - International Edition 60 3576-3580 (2021)
    Nano-electrochemical tools to assess individual catalyst entities are critical to comprehend single-entity measurements. The intrinsic electrocatalytic activity of an individual well-defined Co3O4 nanoparticle supported on a carbon-based nanoelectrode is determined by employing an efficient SEM-controlled robotic technique for picking and placing a single catalyst particle onto a modified carbon nanoelectrode surface. The stable nanoassembly is microscopically investigated and subsequently electrochemically characterized. The hexagonal-shaped Co3O4 nanoparticles demonstrate size-dependent electrochemical activity and exhibit very high catalytic activity with a current density of up to 11.5 A cm−2 at 1.92 V (vs. RHE), and a turnover frequency of 532±100 s−1 at 1.92 V (vs. RHE) towards catalyzing the oxygen evolution reaction. © 2020 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH
    view abstractdoi: 10.1002/anie.202014384
  • 2021 • 90 Optimisation model for multi-item multi-echelon supply chains with nested multi-level products
    Quetschlich, M. and Moetz, A. and Otto, B.
    European Journal of Operational Research 290 144-158 (2021)
    This study proposes a mathematical optimisation model for the horizontal integration of the basic processes of production, inventory, and transportation planning on a tactical level. The model enables planning for cutting, assembly, transportation, and inventory while considering process times. Several items (raw materials, intermediate goods, finished products, and transport vehicles) are routed through a network. The general structure is defined by the layout of the supply network and the configurable multi-level bills of materials of products and transportation equipment, which can be converging, diverging, or mixed. To stress the practical applications of this study, we demonstrate how this generic model can be used and customised to solve an actual planning task in a multi-stage automotive production network using real industry data. © 2020 The Authors
    view abstractdoi: 10.1016/j.ejor.2020.08.005
  • 2021 • 89 Insights into the mechanism of combustion synthesis of iron oxide nanoparticles gained by laser diagnostics, mass spectrometry, and numerical simulations: A mini-review
    Rahinov, I. and Sellmann, J. and Lalanne, M.R. and Nanjaiah, M. and Dreier, T. and Cheskis, S. and Wlokas, I.
    Energy and Fuels (2021)
    To fully master a scaled-up combustion synthesis of nanoparticles toward a wide library of materials with tailored functionalities, a detailed understanding of the underlying kinetic mechanism is required. In this respect, flame synthesis of iron oxide nanoparticles is a model case, being one of the better understood systems and guiding the way how other material synthesis systems could be advanced. In this mini-review, we highlight, on the example of an iron oxide system, an approach combining laser spectroscopy and mass spectrometry with detailed simulations. The experiments deliver information on time-temperature history and concentration field data for gas-phase species and condensable matter under well-defined conditions. The simulations, which can be considered as in silico experiments, combining detailed kinetic modeling with computational fluid dynamics, serve both for mechanism validation via comparison to experimental observables as well as for shedding light on quantities inaccessible by experiments. This approach shed light on precursor decomposition, initial stages of iron oxide particle formation, and precursor role in flame inhibition and provided insights into the effect of temperature-residence time history on nanoparticle formation, properties, and flame structure. © XXXX American Chemical Society.
    view abstractdoi: 10.1021/acs.energyfuels.0c03561
  • 2021 • 88 A Tandem Solar Biofuel Cell: Harnessing Energy from Light and Biofuels
    Riedel, M. and Höfs, S. and Ruff, A. and Schuhmann, W. and Lisdat, F.
    Angewandte Chemie - International Edition 60 2078-2083 (2021)
    We report on a photobioelectrochemical fuel cell consisting of a glucose-oxidase-modified BiFeO3 photobiocathode and a quantum-dot-sensitized inverse opal TiO2 photobioanode linked to FAD glucose dehydrogenase via a redox polymer. Both photobioelectrodes are driven by enzymatic glucose conversion. Whereas the photobioanode can collect electrons from sugar oxidation at rather low potential, the photobiocathode shows reduction currents at rather high potential. The electrodes can be arranged in a sandwich-like manner due to the semi-transparent nature of BiFeO3, which also guarantees a simultaneous excitation of the photobioanode when illuminated via the cathode side. This tandem cell can generate electricity under illumination and in the presence of glucose and provides an exceptionally high OCV of about 1 V. The developed semi-artificial system has significant implications for the integration of biocatalysts in photoactive entities for bioenergetic purposes, and it opens up a new path toward generation of electricity from sunlight and (bio)fuels. © 2020 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH
    view abstractdoi: 10.1002/anie.202012089
  • 2021 • 87 Analytical solutions of the simple shear problem for micromorphic models and other generalized continua
    Rizzi, G. and Hütter, G. and Madeo, A. and Neff, P.
    Archive of Applied Mechanics (2021)
    To draw conclusions as regards the stability and modelling limits of the investigated continuum, we consider a family of infinitesimal isotropic generalized continuum models (Mindlin–Eringen micromorphic, relaxed micromorphic continuum, Cosserat, micropolar, microstretch, microstrain, microvoid, indeterminate couple stress, second gradient elasticity, etc.) and solve analytically the simple shear problem of an infinite stripe. A qualitative measure characterizing the different generalized continuum moduli is given by the shear stiffness μ∗. This stiffness is in general length-scale dependent. Interesting limit cases are highlighted, which allow to interpret some of the appearing material parameter of the investigated continua. © 2021, The Author(s), under exclusive licence to Springer-Verlag GmbH, DE part of Springer Nature.
    view abstractdoi: 10.1007/s00419-021-01881-w
  • 2021 • 86 On the long-term aging of S-phase in aluminum alloy 2618A
    Rockenhäuser, C. and Rowolt, C. and Milkereit, B. and Darvishi Kamachali, R. and Kessler, O. and Skrotzki, B.
    Journal of Materials Science (2021)
    The aluminum alloy 2618A is applied for engine components such as radial compressor wheels which operate for long time at elevated temperatures. This results in coarsening of the hardening precipitates and degradation in mechanical properties during the long-term operation, which is not taken into account in the current lifetime prediction models due to the lack of quantitative microstructural and mechanical data. To address this issue, a quantitative investigation on the evolution of precipitates during long-term aging at 190 °C for up to 25,000 h was conducted. Detailed transmission electron microscopy (TEM) was combined with Brinell hardness measurements and thorough differential scanning calorimetry (DSC) experiments. The results show that GPB zones and S-phase Al2CuMg grow up to &lt; 1,000 h during which the GPB zones dissolve and S-phase precipitates form. For longer aging times, only S-phase precipitates coarsen, which can be well described using the Lifshitz–Slyozov–Wagner theory of ripening. A thorough understanding of the underlying microstructural processes is a prerequisite to enable the integration of aging behavior into the established lifetime models for components manufactured from alloy 2618A. © 2021, The Author(s).
    view abstractdoi: 10.1007/s10853-020-05740-x
  • 2021 • 85 Unravelling the nature of citric acid:l-arginine:water mixtures: The bifunctional role of water
    Roda, A. and Santos, F. and Chua, Y.Z. and Kumar, A. and Do, H.T. and Paiva, A. and Duarte, A.R.C. and Held, C.
    Physical Chemistry Chemical Physics 23 1706-1717 (2021)
    The use of water as a component of deep eutectic systems (DES) has raised some questions regarding its influence on the nature of the mixture. Does it form a DES or an aqueous solution and what is the role of water? In this work, the nature of citric acid:l-arginine:water mixtures was explored through phase equilibria studies and spectroscopic analysis. In a first step, PC-SAFT was validated as a predictive tool to model the water influence on the solid liquid equilibria (SLE) of the DES reline using the individual-component approach. Hence, activity coefficients in the ternary systems citric acid:l-arginine:water and respective binary combinations were studied and compared using ePC-SAFT. It was observed that the water-free mixtures citric acid:l-arginine showed positive deviation from Raoult's law, while upon addition of water strong negative deviation from Raoult's law was found, yielding melting depressions around 100 K. Besides these strong interactions, pH was found to become acidic (pH = 3.5) upon water addition, which yields the formation of charged species ([H2Cit]- and [l-arg]+). Thus, the increased interactions between the molecules upon water addition might be caused by several mechanisms such as hydrogen bonding or ionic forces, both being induced by water. For further investigation, the liquid mixtures citric acid:l-arginine:water were studied by FTIR and NMR spectroscopy. FTIR spectra disproved a possible solubility enhancement caused by salt formation between citric acid and l-arginine, while NMR spectra supported the formation of a hydrogen bonding network different from the binary systems citric acid:water and l-arginine:water. Either being a DES or other type of non-ideal solution, the liquefaction of the studied systems is certainly caused by a water-mediator effect based on the formation of charged species and cross interactions between the mixture constituents. This journal is © the Owner Societies.
    view abstractdoi: 10.1039/d0cp04992a
  • 2021 • 84 Identification of thermal material parameters for thermo-mechanically coupled material models: Verification and model dependency
    Rose, L. and Menzel, A.
    Meccanica (2021)
    The possibility of accurately identifying thermal material parameters on the basis of a simple tension test is presented, using a parameter identification framework for thermo-mechanically coupled material models on the basis of full field displacement and temperature field measurements. Main objective is to show the impact of the material model formulation on the results of such an identification with respect to accuracy and uniqueness of the result. To do so, and as a proof of concept, the data of two different experiments is used. One experiment including cooling of the specimen, due to ambient temperature, and one without specimen cooling. The main constitutive relations of two basic material models are summarised (associated and non-associated plasticity), whereas both models are extended so as to introduce an additional material parameter for the thermodynamically consistent scaling of dissipated energy. The chosen models are subjected to two parameter identifications each, using the data of either experiment and focusing on the determination of thermal material parameters. The influence of the predicted dissipated energy of the models on the identification process is investigated showing that a specific material model formulation must be chosen carefully. The material model with associated evolution equations used within this work does neither allow a unique identification result, nor is any of the solutions for the underlying material parameters close to literature values. In contrast to that, a stable, that is locally unique, re-identification of the literature values is possible for the boundary problem at hand if the model with non-associated evolution equation is used and if cooling is included in the experimental data. © 2021, The Author(s).
    view abstractdoi: 10.1007/s11012-020-01267-2
  • 2021 • 83 Investigation of natural gas/hydrogen mixtures for exergy storage in a piston engine
    Rudolph, C. and Atakan, B.
    Energy 218 (2021)
    The conversion of mechanical to chemical energy offers an option for long-term and versatile energy storage. It was already proven that piston engines can be used as flexible reactors for energy conversion. Here, a novel method for energy conversion in piston engines is investigated, the pyrolysis of natural gas/hydrogen mixtures for energy storage. The supplied energy is stored by chemical conversion into hydrogen and higher energy hydrocarbons. The storage efficiency and the product composition are addressed here. To reach sufficiently high temperatures after compression, a dilution with 85–99% argon is used. The main products are hydrogen, acetylene, ethylene and benzene but also soot precursors are formed. The piston engine is simulated as a time-dependent four-stroke single-zone model with detailed chemical kinetics. The intake pressure is kept constant at 2 bar, while intake temperature, intake argon mole fraction and the hydrogen/natural gas ratio is varied. The hydrogen addition allows a reduction of the intake temperature and argon dilution but also reduces the storage power and efficiency. Yields of acetylene or ethylene are increased and the formation of soot precursors is suppressed. A storage power of 1.59 kW is reached with an efficiency of 52%. © 2020 Elsevier Ltd
    view abstractdoi: 10.1016/
  • 2021 • 82 Micro x-ray fluorescence analysis of trace element distribution in frozen hydrated HeLa cells at the P06 beamline at Petra III
    Rumancev, C. and Vöpel, T. and Stuhr, S. and Von Gundlach, A. and Senkbeil, T. and Ebbinghaus, S. and Garrevoet, J. and Falkenberg, G. and De Samber, B. and Vincze, L. and Rosenhahn, A. and Schroeder, W.
    Biointerphases 16 (2021)
    X-ray fluorescence analysis enables the study of trace element distributions in biological specimens. When this analysis is done under cryogenic conditions, cells are cryofixed as closely as possible to their natural physiological state, and the corresponding intracellular elemental densities can be analyzed. Details about the experimental setup used for analysis at the P06 beamline at Petra III, DESY and the used cryo-transfer system are described in this work. The system was applied to analyze the elemental distribution in single HeLa cells, a cell line frequently used in a wide range of biological applications. Cells adhered to silicon nitride substrates were cryoprotected within an amorphous ice matrix. Using a continuous scanning scheme and a KB x-ray focus, the distribution of elements in the cells was studied. We were able to image the intracellular potassium and zinc levels in HeLa cells as two key elements relevant for the physiology of cells. © 2021 Author(s).
    view abstractdoi: 10.1116/6.0000593
  • 2021 • 81 Cutting path-dependent machinability of SiCp/Al composite under multi-step ultra-precision diamond cutting
    LU, S. and ZHANG, J. and LI, Z. and ZHANG, J. and WANG, X. and HARTMAIER, A. and XU, J. and YAN, Y. and SUN, T.
    Chinese Journal of Aeronautics 34 241-252 (2021)
    Particle-tool interactions, which govern the synergetic deformation of SiC particle reinforced Al matrix composites under mechanical machining, strongly depend on the geometry of particle position residing on cutting path. In the present work, we investigate the influence of cutting path on the machinability of a SiCp/Al composite in multi-step ultra-precision diamond cutting by combining finite element simulations with experimental observations and characterization. Be consistent with experimentally characterized microstructures, the simulated SiCp/Al composite is considered to be composed of randomly distributed polygonally-shaped SiC particles with a volume fraction of 25vol%. A multi-step cutting strategy with depths of cut ranging from 2 to 10 μm is adopted to achieve an ultimate depth of cut of 10 μm. Intrinsic material parameters and extrinsic cutting conditions utilized in finite element simulations of SiCp/Al cutting are consistent with those used in corresponding experiments. Simulation results reveal different particle-tool interactions and failure modes of SiC particles, as well as their correlations with machining force evolution, residual stress distribution and machined surface topography. A detailed comparison between numerical simulation results and experimental data of multi-step diamond cutting of SiCp/Al composite reveals a substantial impact of the number of cutting steps on particle-tool interactions and machined surface quality. These findings provide guidelines for achieving high surface finish of SiCp/Al composites by ultra-precision diamond cutting. © 2020 Chinese Society of Aeronautics and Astronautics
    view abstractdoi: 10.1016/j.cja.2020.07.039
  • 2021 • 80 Influence of Nanoparticle Processing on the Thermoelectric Properties of (BixSb1−X)2Te3 Ternary Alloys
    Salloum, S. and Bendt, G. and Heidelmann, M. and Loza, K. and Bayesteh, S. and Sepideh Izadi, M. and Kawulok, P. and He, R. and Schlörb, H. and Perez, N. and Reith, H. and Nielsch, K. and Schierning, G. and Schulz, S.
    ChemistryOpen (2021)
    The synthesis of phase-pure ternary solutions of tetradymite-type materials (BixSb1−x)2Te3 (x=0.25; 0.50; 0.75) in an ionic liquid approach has been carried out. The nanoparticles are characterized by means of energy-dispersive X-ray spectroscopy (EDX), powder X-ray diffraction (PXRD), scanning electron microscopy (SEM), and transmission electron microscopy. In addition, the role of different processing approaches on the thermoelectric properties - Seebeck coefficient as well as electrical and thermal conductivity - is demonstrated. © 2021 The Authors. Published by The Chemical Society of Japan & Wiley-VCH GmbH
    view abstractdoi: 10.1002/open.202000257
  • 2021 • 79 Assessment of a Dual Kalman Filter-Based Approach for Input/Output Estimation in an Aluminum Plate
    Sattarifar, A. and Nestorović, T.
    Lecture Notes in Civil Engineering 127 584-593 (2021)
    Vulnerability of structures to damage during their service time brings up the necessity of design and implementation of an intelligent procedure to assure the health of the structure. In the sight of this requisite, current work deals with extending the capability of a dual Kalman filter (DKF) state estimation scheme to assist vibration-based health monitoring methods. This is met by estimating the response of the structure for locations at which a sensor cannot be placed. The capability of the DKF method in the estimation of states of a linear system with an unknown input has been presented in various recent works. In this paper, a DKF approach incorporated with a reduced order structural model (in this case an aluminum plate) is utilized to obtain an estimation of applied force and the response of the structure in terms of acceleration, velocity, and displacement. These estimations are based on measured accelerations at a limited number of points on the aluminum plate as well as the state-space model of the dynamic system. Numerical simulations and experimental works are performed to obtain the mentioned datasets. To assess the robustness of the method concerning various conditions, the effect of the frequency, as well as type of the function of the input force on the validity of the method, is presented. Moreover, it is shown to what extent the number of selected modes in model reduction procedure can influence the accuracy of the DKF technique. © 2021, Springer Nature Switzerland AG.
    view abstractdoi: 10.1007/978-3-030-64594-6_57
  • 2021 • 78 Simulation of wear and effective friction properties of microstructured surfaces
    Schewe, M. and Wilbuer, H. and Menzel, A.
    Wear 464-465 (2021)
    Wear and friction characteristics are simulated on metal forming tools with tailored surfaces generated by micro-milling. Friction homogenisation is applied to study surface cut-outs on the meso-scale where the structures are resolved by means of finite element methods and where asperities are represented by a combined friction law appropriate for metal forming. Dissipation based and pressure based Archard wear relations are implemented in a postprocessor, and wear distributions as well as effective friction properties are investigated. Sinusoidal surface structures are able to provide anisotropic structural resistance throughout the progress of wear. A bionic surface structure shows quasi-isotropic structural resistance where sliding directions across the edge directions are benefitial with regard to the wear progress. Experimental measurements from a wear experiment give hints which support the dissipation based Archard relation while more experimental evidence is necessary. © 2020 The Authors
    view abstractdoi: 10.1016/j.wear.2020.203491
  • 2021 • 77 Ab initio based models for temperature-dependent magnetochemical interplay in bcc Fe-Mn alloys
    Schneider, A. and Fu, C.-C. and Waseda, O. and Barreteau, C. and Hickel, T.
    Physical Review B 103 (2021)
    Body-centered cubic (bcc) Fe-Mn systems are known to exhibit a complex and atypical magnetic behavior from both experiments and 0 K electronic-structure calculations, which is due to the half-filled 3d band of Mn. We propose effective interaction models for these alloys, which contain both atomic-spin and chemical variables. They were parameterized on a set of key density functional theory (DFT) data, with the inclusion of noncollinear magnetic configurations being indispensable. Two distinct approaches, namely a knowledge-driven and a machine-learning approach have been employed for the fitting. Employing these models in atomic Monte Carlo simulations enables the prediction of magnetic and thermodynamic properties of the Fe-Mn alloys, and their coupling, as functions of temperature. This includes the decrease of Curie temperature with increasing Mn concentration, the temperature evolution of the mixing enthalpy, and its correlation with the alloy magnetization. Also, going beyond the defect-free systems, we determined the binding free energy between a vacancy and a Mn atom, which is a key parameter controlling the atomic transport in Fe-Mn alloys. © 2021 American Physical Society.
    view abstractdoi: 10.1103/PhysRevB.103.024421
  • 2021 • 76 Data compilation regarding the effects of grain size and temperature on the strength of the single-phase FCC CrFeNi medium-entropy alloy
    Schneider, M. and Laplanche, G.
    Data in Brief 34 (2021)
    In the present article, we present a data compilation reflecting recrystallized microstructures and the corresponding mechanical properties of an equiatomic, single-phase face-centered cubic (FCC) CrFeNi medium-entropy alloy (MEA). For the analysis, interpretation, and discussion of the data, the reader can refer to the original research article entitled “Effects of temperature on mechanical properties and deformation mechanisms of the equiatomic CrFeNi medium-entropy alloy”, see Ref. (Schneider and Laplanche, Acta Mater. 204, 2020). The data related to recrystallized microstructures comprise raw backscatter electron (BSE) micrographs (tif-files) obtained using a scanning electron microscope (SEM) for six grain sizes in the range [10–160 µm], optical micrographs of the alloy with the largest grain size (d = 327 µm), pdf-reports and tables presenting the corresponding grain-size distributions (d, accounting for grain boundaries only) and crystallite-size distributions (c, which accounts for both grain and annealing twin boundaries), the annealing twin thicknesses (t), the average number of annealing twin boundaries per grain (n), and the average Taylor factor (M) of each recrystallized microstructure. These are benchmark datasets that may serve to develop new algorithms for the automated evaluation of microstructural parameters. Such algorithms would help to speed up the analyses of microstructures and improve their reliability. Furthermore, several groups pointed out that in addition to the mean grain size, other microstructural parameters such as the grain size distribution (Raeisinia et al., Model. Simul. Mater. Sc. 16, 2008) and the average number of twins per grain (Schneider et al., Int. J. Plasticity, 124, 2020) may affect some material properties (e.g. Hall-Petch strengthening). Therefore, an effort was made here to determine and report almost all the microstructural parameters describing recrystallized microstructures of FCC alloys. The mechanical-properties data are provided as excel-sheets in which the raw stress-strain curves can be found. Compression tests for alloys with different grain sizes were performed at room temperature. Additional compression tests and tensile tests for the grain size d = 160 µm were performed at temperatures between 77 K and 873 K. Characteristic mechanical properties, such as yield stresses at 0.2% plastic strain (σ0.2%) and Hall-Petch parameters (σ0 and ky) are given for all temperatures in the tables below. Moreover, the Hall-Petch parameters as well as the mechanical data reported in the present study could be used for data mining and implemented in programs used for alloy design. © 2021 The Author(s)
    view abstractdoi: 10.1016/j.dib.2020.106712
  • 2021 • 75 Effects of temperature on mechanical properties and deformation mechanisms of the equiatomic CrFeNi medium-entropy alloy
    Schneider, M. and Laplanche, G.
    Acta Materialia 204 (2021)
    An equiatomic CrFeNi medium-entropy alloy (MEA) that constitutes a cornerstone of austenitic stainless steels and Fe-based superalloys is investigated. Anneals at various temperatures revealed that CrFeNi forms a stable face-centered cubic (FCC) solid solution above ~1223 K. Based on this result, this alloy was cold-worked and recrystallized between 1273 K and 1473 K to produce different grain sizes. Compression tests were carried out at 293 K to investigate grain boundary strengthening (Hall-Petch slope: 966 MPa µm1/2) and this contribution was then subtracted from the overall strength to reveal the intrinsic uniaxial lattice strength (80 MPa). Additional compression and tensile tests were performed between 77 K and 873 K to study the effect of temperature on mechanical properties and deformation mechanisms. Ductility, yield and ultimate tensile strengths increased with decreasing temperature. To reveal the active deformation mechanisms in CrFeNi with the coarsest grain size (160 µm), tensile tests at 77 K and 293 K were interrupted at different strains followed by transmission electron microscopy analyses. In all cases, the deformation was accommodated by dislocation glide at low strains, while twinning additionally occurred above a critical resolved shear stress of 165 MPa, which was roughly temperature independent. This value compares well with predictions (180 MPa) based on the Kibey's model for twin nucleation. Moreover, the fact that this value is roughly temperature-independent is also consistent with the Kibey's model since the twin nucleation barrier (unstable twin stacking fault energy) of FCC metals and alloys does not vary significantly with temperature. © 2020 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2020.11.012
  • 2021 • 74 Confocal laser scanning holographic microscopy of buried structures
    Schnitzler, L. and Neutsch, K. and Schellenberg, F. and Hofmann, M.R. and Gerhardt, N.C.
    Applied Optics 60 A8-A14 (2021)
    In this paper, we present a confocal laser scanning holographic microscope for the investigation of buried structures. The multimodal system combines high diffraction limited resolution and high signal-to-noise-ratio with the ability of phase acquisition. The amplitude and phase imaging capabilities of the system are shown on a test target. For the investigation of buried integrated semiconductor structures, we expand our system with an optical beam induced current modality that provides additional structure-sensitive contrast. We demonstrate the performance of the multimodal system by imaging the buried structures of a microcontroller through the silicon backside of its housing in reflection geometry. © 2020 Optical Society of America
    view abstractdoi: 10.1364/AO.403687
  • 2021 • 73 Lensless digital holographic microscopy as an efficient method to monitor enzymatic plastic degradation
    Schnitzler, L. and Zarzycki, J. and Gerhard, M. and Konde, S. and Rexer, K.-H. and Erb, T.J. and Maier, U.G. and Koch, M. and Hofmann, M.R. and Moog, D.
    Marine Pollution Bulletin 163 (2021)
    A big challenge of the 21st century is to cope with the huge amounts of plastic waste on Earth. Especially the oceans are heavily polluted with plastics. To counteract this issue, biological (enzymatic) plastic decomposition is increasingly gaining attention. Recently it was shown that polyethylene terephthalate (PET) can be degraded in a saltwater-based environment using bacterial PETase produced by a marine diatom. At moderate temperatures, plastic biodegradation is slow and requires sensitive methods for detection, at least at initial stages. However, conventional methods for verifying the plastic degradation are either complex, expensive, time-consuming or they interfere with the degradation process. Here, we adapt lensless digital holographic microscopy (LDHM) as a new application for efficiently monitoring enzymatic degradation of a PET glycol copolymer (PETG). LDHM is a cost-effective, compact and sensitive optical method. We demonstrate enzymatic PETG degradation over a time course of 43 days employing numerical analysis of LDHM images. © 2020 Elsevier Ltd
    view abstractdoi: 10.1016/j.marpolbul.2020.111950
  • 2021 • 72 A Selection of Benchmark Problems in Solid Mechanics and Applied Mathematics
    Schröder, J. and Wick, T. and Reese, S. and Wriggers, P. and Müller, R. and Kollmannsberger, S. and Kästner, M. and Schwarz, A. and Igelbüscher, M. and Viebahn, N. and Bayat, H.R. and Wulfinghoff, S. and Mang, K. and Rank, E. ...
    Archives of Computational Methods in Engineering 28 713-751 (2021)
    In this contribution we provide benchmark problems in the field of computational solid mechanics. In detail, we address classical fields as elasticity, incompressibility, material interfaces, thin structures and plasticity at finite deformations. For this we describe explicit setups of the benchmarks and introduce the numerical schemes. For the computations the various participating groups use different (mixed) Galerkin finite element and isogeometric analysis formulations. Some programming codes are available open-source. The output is measured in terms of carefully designed quantities of interest that allow for a comparison of other models, discretizations, and implementations. Furthermore, computational robustness is shown in terms of mesh refinement studies. This paper presents benchmarks, which were developed within the Priority Programme of the German Research Foundation ‘SPP 1748 Reliable Simulation Techniques in Solid Mechanics—Development of Non-Standard Discretisation Methods, Mechanical and Mathematical Analysis’. © 2020, The Author(s).
    view abstractdoi: 10.1007/s11831-020-09477-3
  • 2021 • 71 Kinetics of the Thermal Decomposition of Ethylsilane: Shock-Tube and Modeling Study
    Sela, P. and Peukert, S. and Somnitz, H. and Janbazi, H. and Wlokas, I. and Herzler, J. and Fikri, M. and Schulz, C.
    Energy and Fuels (2021)
    The thermal decomposition of ethylsilane (H3SiC2H5, EtSiH3) is investigated behind reflected shock waves and the gas composition is analyzed by gas chromatography/mass spectrometry (GC/MS) and high-repetition-rate time-of-flight mass spectrometry (HRR-TOF-MS) in a temperature range of 990-1330 K and pressure range of 1-2.5 bar. The unimolecular decomposition of EtSiH3 is considered to be initiated via a molecular elimination of H2 (H3SiC2H5 → H2 + HSiC2H5) followed by reactions of cyclic silicon-containing species. The main observed stable products were ethylene (C2H4) and silane (SiH4). Measurements are performed with a large excess of a silylene scavenger (C2H2) to suppress bimolecular reactions caused by silylene (SiH2) and to extract unimolecular rate constants. A kinetics mechanism accounting for the gas-phase chemistry of EtSiH3 is developed, which consists of 24 Si-containing species, 31 reactions of Si-containing species, and a set of new thermochemical data. The derived unimolecular rate constant is represented by the Arrhenius expression kuni(T) = 1.96 × 1012 s-1 exp(-205 kJ mol-1/RT). The experimental data is reproduced very well by simulations based on the mechanism of this work and is in very good agreement with literature values. It is shown that EtSiH3 is a promising precursor for the synthesis of SiC nanoparticles. © 2021 American Chemical Society.
    view abstractdoi: 10.1021/acs.energyfuels.0c03425
  • 2021 • 70 Measurement and PC-SAFT Modeling of the Solubility of Gallic Acid in Aqueous Mixtures of Deep Eutectic Solvents
    Sepúlveda-Orellana, B. and Gajardo-Parra, N.F. and Do, H.T. and Pérez-Correa, J.R. and Held, C. and Sadowski, G. and Canales, R.I.
    Journal of Chemical and Engineering Data (2021)
    Deep eutectic solvents have appeared as potential solvents for improving the extraction of polyphenols from vegetable or fruit matrixes. Since gallic acid is abundant in these sources, it is considered as a typical standard for quantifying their total polyphenol content after extraction with solvents. However, there are no extensive studies on the solubility behavior of gallic acid in different solvents or deep eutectic solvents. Thus, in this work, the solubility of gallic acid is measured in pure water; aqueous solutions of different hydrogen bond donors such as ethylene glycol, levulinic acid, and glycerol; and aqueous mixtures of deep eutectic solvents using choline chloride as the hydrogen bond acceptor and ethylene glycol, levulinic acid, and glycerol as the hydrogen bond donors. All of the measurements were performed at 293.15, 303.15, and 313.15 K and at 101.3 kPa and were validated by comparing the solubility of gallic acid in water from the literature. Results suggest that a 50 wt % aqueous solution of deep eutectic solvent based on ethylene glycol or glycerol improves the gallic acid solubility compared with a 50 wt % aqueous solution of its corresponding hydrogen bond donor. The deep eutectic solvent containing levulinic acid acts as the best aqueous mixture for gallic acid dissolution. Nondissolved gallic acid was measured after equilibrium using powder X-ray diffraction, showing that its structure does not change upon mixing with all of the liquid mixtures. All of the solid-liquid equilibrium results were accurately modeled with perturbed-chain statistical associating fluid theory (PC-SAFT). © 2021 American Chemical Society.
    view abstractdoi: 10.1021/acs.jced.0c00784
  • 2021 • 69 Optimization of the groundwater remediation process using a coupled genetic algorithm-finite difference method
    Seyedpour, S.M. and Valizadeh, I. and Kirmizakis, P. and Doherty, R. and Ricken, T.
    Water (Switzerland) 13 (2021)
    In situ chemical oxidation using permanganate as an oxidant is a remediation technique often used to treat contaminated groundwater. In this paper, groundwater flow with a full hydraulic conductivity tensor and remediation process through in situ chemical oxidation are simulated. The numerical approach was verified with a physical sandbox experiment and analytical solution for 2D advection-diffusion with a first-order decay rate constant. The numerical results were in good agreement with the results of physical sandbox model and the analytical solution. The developed model was applied to two different studies, using multi-objective genetic algorithm to optimize remediation design. In order to reach the optimised design, three objectives considering three constraints were defined. The time to reach the desired concentration and remediation cost regarding the number of required oxidant sources in the optimised design was less than any arbitrary design. © 2021 by the authors.Licensee MDPI, Basel, Switzerland.
    view abstractdoi: 10.3390/w13030383
  • 2021 • 68 Carrier-phase DNS of detailed NOx formation in early-stage pulverized coal combustion with fuel-bound nitrogen
    Shamooni, A. and Debiagi, P. and Wang, B. and Luu, T.D. and Stein, O.T. and Kronenburg, A. and Bagheri, G. and Stagni, A. and Frassoldati, A. and Faravelli, T. and Kempf, A.M. and Wen, X. and Hasse, C.
    Fuel 291 (2021)
    Carrier-phase direct numerical simulation of detailed NOx formation in pulverized coal flames (PCC) with fuel-bound nitrogen is conducted in a 3D temporally evolving mixing layer setup where Lagrangian particles (Colombian bituminous coal) in an air stream (upper half of the domain) mix with the products of lean volatile/air combustion in the lower stream. The release of fuel-N is represented by ammonia, hydrogen cyanide, and a lumped nitrogenated tar (pyridine). Devolatilization is modeled by fitting a 2-step pyrolysis approach to the detailed heterogeneous PoliMi kinetics. A comprehensive homogeneous mechanism including all standard pathways of NOx and pyridine oxidation is adopted. Results show a partition of NO in two distinct branches of scatter plots of NO mass fraction vs. volatile mixture fraction after flame establishment, corresponding to NO in the lower stream flame region and hot spots near the upper stream. The contribution of NO2, prompt, and thermal mechanisms to total NOx is limited in the early stages of PCC. The main source of NO is fuel-N, with NH being the most important precursor. Pyridine plays an important role for NO production in the upper stream through CN formed from CHCHCN. CN and ammonia oxidation have the highest contribution to NH production. Regarding NO destruction, NO reactions with HCCO, CHi and C through the reburn process constitute the largest share. NO conversion to N2O by NH followed by conversion of N2O to N2 and NO+N→N2+O are the two most important pathways directly reducing NO to N2. © 2020 Elsevier Ltd
    view abstractdoi: 10.1016/j.fuel.2020.119998
  • 2021 • 67 Multi-atom quasiparticle scattering interference for superconductor energy-gap symmetry determination
    Sharma, R. and Kreisel, A. and Sulangi, M.A. and Böker, J. and Kostin, A. and Allan, M.P. and Eisaki, H. and Böhmer, A.E. and Canfield, P.C. and Eremin, I. and Séamus Davis, J.C. and Hirschfeld, P.J. and Sprau, P.O.
    npj Quantum Materials 6 (2021)
    Complete theoretical understanding of the most complex superconductors requires a detailed knowledge of the symmetry of the superconducting energy-gap Δkα, for all momenta k on the Fermi surface of every band α. While there are a variety of techniques for determining ∣Δkα∣, no general method existed to measure the signed values of Δkα. Recently, however, a technique based on phase-resolved visualization of superconducting quasiparticle interference (QPI) patterns, centered on a single non-magnetic impurity atom, was introduced. In principle, energy-resolved and phase-resolved Fourier analysis of these images identifies wavevectors connecting all k-space regions where Δkα has the same or opposite sign. But use of a single isolated impurity atom, from whose precise location the spatial phase of the scattering interference pattern must be measured, is technically difficult. Here we introduce a generalization of this approach for use with multiple impurity atoms, and demonstrate its validity by comparing the Δkα it generates to the Δkα determined from single-atom scattering in FeSe where s± energy-gap symmetry is established. Finally, to exemplify utility, we use the multi-atom technique on LiFeAs and find scattering interference between the hole-like and electron-like pockets as predicted for Δkα of opposite sign. © 2021, Crown.
    view abstractdoi: 10.1038/s41535-020-00303-4
  • 2021 • 66 Limited Elemental Mixing in Nanoparticles Generated by Ultrashort Pulse Laser Ablation of AgCu Bilayer Thin Films in a Liquid Environment: Atomistic Modeling and Experiments
    Shih, C.-Y. and Chen, C. and Rehbock, C. and Tymoczko, A. and Wiedwald, U. and Kamp, M. and Schuermann, U. and Kienle, L. and Barcikowski, S. and Zhigilei, L.V.
    Journal of Physical Chemistry C (2021)
    Pulsed laser ablation in liquids (PLAL) is a promising technique for the generation of colloidal alloy nanoparticles that are of high demand in a broad range of fields, including catalysis, additive manufacturing, and biomedicine. Many of the applications have stringent requirements on the nanoparticle composition and size distributions, which can only be met through innovations in the PLAL technique guided by a clear understanding of the nanoparticle formation mechanisms. In this work, we undertake a combined computational and experimental study of the nanoparticle formation mechanisms in ultrashort PLAL of Ag/Cu and Cu/Ag bilayer thin films. Experimental probing of the composition of individual nanoparticles and predictions from large-scale atomistic simulations provide consistent evidence of limited mixing between the two components from bilayer films by PLAL. The simulated and experimental distributions of nanoparticle compositions exhibit an enhanced abundance of Ag-rich and Cu-rich nanoparticles, as well as a strongly depressed population of well-mixed alloy nanoparticles. The surprising observation that the nanoscale phase separation of the two components in the bilayer films manifests itself in the sharp departure from the complete quantitative mixing in the colloidal nanoparticles is explained by the complex dynamic interaction between the ablation plume and liquid environment revealed in the simulations of the initial stage of the ablation process. The simulations predict that rapid deceleration of the ablation plume by the liquid environment results in the formation of a transient hot and dense metal region at the front of the plume, which hampers the mixing of the two components and, at the same time, contributes to the stratification of the plume in the emerging cavitation bubble. As a result, nanoparticles of different sizes and compositions are produced in different parts of the emerging cavitation bubble during the first nanoseconds of the ablation process. Notably, the diameters of the largest nanoparticles generated in the simulations of the initial stage of the ablation process are more than twice larger than the thickness of the original bilayer films. This observation provides a plausible scenario for the formation of large nanoparticles observed in the experiments. The conclusion on limited elemental mixing in the nanoparticles is validated in simulations of bilayers with different spatial order of Cu and Ag layers, even though the two systems exhibit some notable quantitative differences mainly related to the different strength of electron-phonon coupling in Cu and Ag. Overall, the results of this study provide new insights into the formation mechanism of bimetallic nanoparticles in ultrashort PLAL from thin bilayer targets and suggest that the formation of alloy nanoparticles from immiscible elements may be hampered for targets featuring distinctive elemental segregation. © 2021 American Chemical Society.
    view abstractdoi: 10.1021/acs.jpcc.0c09970
  • 2021 • 65 Hybrid State Estimation-A Contribution Towards Reliability Enhancement of Artificial Neural Network Estimators
    Sieberg, P.M. and Blume, S. and Reicherts, S. and Maas, N. and Schramm, D.
    IEEE Transactions on Intelligent Transportation Systems (2021)
    Data-driven models are obtained purely from data without complex theoretical modeling and without explicit model knowledge. This results in black box models whose traceability and reliability constitute a major challenge. This contribution addressed this issue and presents a novel hybrid estimation method, which enhances the reliability of artificial neural networks. Within this method, a simple model based on physical knowledge secures a model based on an artificial neural network. An unscented Kalman filter realizes the interaction of the two individual models. Thereby, a confidence level determines which model is trusted to a greater extent or even entirely. As part of the method for adjusting this confidence level, the input variables of the artificial neural network are related to the data used in training. The more often the artificial neural network has encountered a situation in the training process, the greater the confidence level will be. Finally, the confidence level is used to set the covariances of the unscented Kalman filter. In this contribution, the method is presented using the application of roll angle estimation for passenger cars. By using the hybrid method the reliability of the estimation is increased in comparison to the artificial neural network. For this purpose, sensor malfunctions as well as a sensor failure are simulated. These disturbances are compensated by the introduced method. In addition, the hybrid state estimator increases the estimation quality compared to the individual estimators. The proposed method can be applied to any problem, where knowledge-based models are available to secure data-driven models. IEEE
    view abstractdoi: 10.1109/TITS.2021.3055800
  • 2021 • 64 Journal of Chemical & Engineering Data: An Update from the Editorial Team
    Siepmann, J.I. and Gardas, R.L. and Kofke, D.A. and Pini, R. and Sadowski, G. and Schwarz, C.E. and Wu, J.
    Journal of Chemical and Engineering Data 66 1-2 (2021)
    doi: 10.1021/acs.jced.0c01080
  • 2021 • 63 Time-of-flight mass spectrometry of particle emission during irradiation with slow, highly charged ions
    Skopinski, L. and Ernst, P. and Herder, M. and Kozubek, R. and Madauß, L. and Sleziona, S. and Maas, A. and Königstein, N. and Lebius, H. and Wucher, A. and Schleberger, M.
    Review of Scientific Instruments 92 (2021)
    We describe a setup for the analysis of secondary ions and neutrals emitted from solid surfaces and two-dimensional materials during irradiation with highly charged ions. The ultrahigh vacuum setup consists of an electron beam ion source to produce bunches of ions with various charge states q (e.g., Xe1+-Xe46+) and thus potential energies, a deceleration/acceleration section to tune the kinetic energy of the ions in the range of 5 keV to 20 × q keV, a sample stage for laser-cleaning and positioning of freestanding as well as supported samples, a pulsed excimer laser for post-ionization of sputtered neutrals, and a reflectron type time-of-flight mass spectrometer, enabling us to analyze mass and velocity distributions of the emitted particles. With our setup, contributions from potential and kinetic energy deposition can be studied independently of each other. Charge dependent experiments conducted at a constant kinetic energy show a clear threshold for the emission of secondary ions from SrTiO3. Data taken with the same projectile charge state, but at a different kinetic energy, reveal a difference in the ratio of emitted particles from MoS2. In addition, first results are presented, demonstrating how velocity distributions can be measured with the new setup. © 2021 Author(s).
    view abstractdoi: 10.1063/5.0025812
  • 2021 • 62 Determination of inherent dissolution performance of drug substances
    Sleziona, D. and Mattusch, A. and Schaldach, G. and Ely, D.R. and Sadowski, G. and Thommes, M.
    Pharmaceutics 13 1-12 (2021)
    The dissolution behavior of novel active pharmaceutical ingredients (API) is a crucial parameter in drug formulation since it frequently affects the drug release. Generally, a distinction is made between surface-reaction-and diffusion-controlled drug release. Therefore, dissolution studies such as the intrinsic dissolution test defined in the pharmacopeia have been performed for many years. In order to overcome the disadvantages of the common intrinsic dissolution test, a new experimental setup was developed within this study. Specifically, a flow channel was designed and tested for measuring the mass transfer from a flat, solid surface dissolving into a fluid flowing over the surface with well-defined flow conditions. A mathematical model was developed that distinguishes between surface-reaction-and diffusion-limited drug release based on experimental data. Three different drugs—benzocaine, theophylline and griseofulvin—were used to investigate the mass flux during dissolution due to surface reaction, diffusion and convection kinetics. This new technique shows potential to be a valuable tool for the identification of formulation strategies. © 2021 by the authors. Licensee MDPI, Basel, Switzerland.
    view abstractdoi: 10.3390/pharmaceutics13020146
  • 2021 • 61 Impact of test temperature on functional degradation in Fe-Ni-Co-Al-Ta shape memory alloy single crystals
    Sobrero, C. and Lauhoff, C. and Langenkämper, D. and Somsen, C. and Eggeler, G. and Chumlyakov, Y.I. and Niendorf, T. and Krooß, P.
    Materials Letters 291 (2021)
    The present paper focuses on the analysis of functional fatigue properties in 〈001〉-oriented single crystalline Fe-Ni-Co-Al-Ta shape memory alloys. Superelastic cycling experiments up to 4.5% at different temperatures were conducted and revealed excellent cyclic stability at lower testing temperatures. Transmission electron microscopy observations shed light on the influence of precipitation and dislocation activity on functional stability. © 2021
    view abstractdoi: 10.1016/j.matlet.2021.129430
  • 2021 • 60 Methodology Based on the Theory of Information to Describe the Representation Ability of the DMC + Alkane Behavior
    Sosa, A. and Ortega, J. and Fernández, L. and Haarmann, N. and Sadowski, G.
    Industrial and Engineering Chemistry Research 60 1036-1054 (2021)
    An information theory-based methodology has been applied to the multiproperty modeling of solution properties. Under this framework, a practical application on a set of binary solutions formed by dimethyl carbonate and six even saturated hydrocarbons (from C6 to C16) is carried out. A dense experimental database is generated composed of volumetric and energetic properties (from mixing processes), and phase equilibria, in order to disambiguate some discrepancies showed by the literature data, mainly for the binary with dodecane. The experimental information is modeled with a semiempirical equation for gE, and with the PCP-SAFT equation of state, which presents a solid theoretical basis. The optimal parameterizations are sought using the precision-complexity binomial whose aim is to increase the validity range of the set of parameters obtained. The Akaike Information Criterion is used to search the best parameterizations, that is, the appropriate number of parameters (complexity) and their best values (precision). With regard to the suitability of the precision-complexity methodology on the models tested, the following is concluded: with PCP-SAFT, precise and reliable estimates are obtained; for the gE model, the proposed approach is essential to control the number of free parameters and to preserve the stable numerical behavior in a wide range of conditions. © 2021 American Chemical Society.
    view abstractdoi: 10.1021/acs.iecr.0c05301
  • 2021 • 59 Photonic NO2 Gas Sensing with Binaphthyl-Based Dopants
    Spengler, M. and Pschyklenk, L. and Niemeyer, J. and Kaul, P. and Giese, M.
    Advanced Optical Materials (2021)
    A series of reactive binaphthyl-diimine-based dopants is prepared and investigated with respect to their potential for the chiral induction of structural coloration in nematic liquid crystal mixture E7 and the selective photonic sensing of nitrogen dioxide (NO2). Studies of the helical twisting power (HTP) in 4-cyano-4′-pentylbiphenyl (5CB) reveal HTP values as high as 375 µm-1 and the tremendous impact of structural compatibility and changes of the dihedral binaphthyl angle on the efficiency of the chiral transfer. Detailed investigation of the sensing capabilities of the systems reveals an extraordinarily high selectivity for NO2 and a response to concentrations as low as 100 ppm. The systems show a direct response to the analyte gas leading to a concentration-dependent shift of the reflectance wavelength of up to several hundred nanometers. Incorporation of copper ions remarkably improves the sensor's properties in terms of sensitivity and selectivity, enabling the tailored tweaking of the system's properties. © 2021 The Authors. Advanced Optical Materials published by Wiley-VCH GmbH
    view abstractdoi: 10.1002/adom.202001828
  • 2021 • 58 High-Temperature Oxidation in Dry and Humid Atmospheres of the Equiatomic CrMnFeCoNi and CrCoNi High- and Medium-Entropy Alloys
    Stephan-Scherb, C. and Schulz, W. and Schneider, M. and Karafiludis, S. and Laplanche, G.
    Oxidation of Metals 95 105-133 (2021)
    Abstract: Surface degradation phenomena of two model equiatomic alloys from the CrMnFeCoNi alloy system were investigated in 2% O2 and 10% H2O (pO2 = 0.02 and 10−7 atm, respectively) at 800 °C for times up to 96 h. The crystallographic structures, morphologies, and chemical compositions of the corrosion layers developing on CrMnFeCoNi and CrCoNi were comparatively analyzed by mass gain analysis, X-ray diffraction, and scanning electron microscopy combined with energy-dispersive X-ray spectroscopy and electron backscatter diffraction. The oxidation resistance of CrMnFeCoNi is relatively poor due to the fast growth of porous Mn-oxide(s). CrCoNi forms an external chromia layer that is dense and continuous in a dry 2% O2 atmosphere. This layer buckles and spalls off after exposure to 10% H2O atmosphere. Beneath the chromia layer, a Cr-depleted zone forms in the CrCoNi alloy in both environments. As the oxide scale spalls off in the H2O-containing atmosphere, a secondary chromia layer was observed and correspondingly enlarges the Cr-depleted zone. In contrast, as the chromia layer remains without significant spallation when CrCoNi is exposed to a dry oxidizing atmosphere, the region depleted in Cr is narrower. Graphic Abstract: [Figure not available: see fulltext.]. © 2020, The Author(s).
    view abstractdoi: 10.1007/s11085-020-10014-7
  • 2021 • 57 Dedicated setup to isolate plasma catalysis mechanisms
    Stewig, C. and Urbanietz, T. and Chauvet, L. and Böke, M. and Von Keudell, A.
    Journal of Physics D: Applied Physics 54 (2021)
    Plasma catalysis, the combination of plasma and catalysis, is used to achieve efficient molecule conversion, supporting the flexibility of operating parameters and feed gases. By combining plasmas with conventional thermal catalysis, the temperature windows may be changed and the process may be made insensitive to catalyst poisoning. However, understanding plasma catalysis mechanisms is extremely difficult, due to the strong coupling between plasma, gas-phase chemistry and surface. A multitude of reaction pathways may be enhanced or reduced by the presence of a plasma that provides excited species as reaction partners. We developed a robust setup to analyse those processes, based on a parallel-plate atmospheric-pressure plasma jet that allows a plug flow design. The plasma chemistry is analysed by Fourier transform infrared absorption spectroscopy and mass spectrometry. The electrodes in contact with the plasma are temperature controlled and can easily be replaced to apply a catalyst on top of them. The basic characteristics of the setup are discussed and three examples for its application are given: (a) the analysis of methane oxidation using the plug flow scheme; (b) the plasma catalytic conversion of CO2, and (c) the plasma catalytic conversion of methane in methane–oxygen mixtures. © 2021 IOP Publishing Ltd.
    view abstractdoi: 10.1088/1361-6463/abd65b
  • 2021 • 56 A comparative study of micro-mechanical models for fiber pullout behavior of reinforced high performance concrete
    Storm, J. and Pise, M. and Brands, D. and Schröder, J. and Kaliske, M.
    Engineering Fracture Mechanics 243 (2021)
    The pseudo-ductile material behavior of fiber reinforced high performance concrete is mainly characterized by the fiber pullout process. Thereby, complex fiber–concrete interactions, i.e. interface debonding, concrete micro cracks, slippage, adhesion and further unknown processes, are commonly investigated in single-fiber pullout tests. The study in this contribution is based on the experimental results of Gebuhr etal., (2019) and compares three different numerical models applied to the fiber pullout test. An accurate and efficient model for fiber pullout behavior forms the basis for the prediction of the overall behavior by means of composite models or multi-scale approaches in subsequent studies. © 2020 Elsevier Ltd
    view abstractdoi: 10.1016/j.engfracmech.2020.107506
  • 2021 • 55 How to Design Digitalized Laboratories?: Lessons Learned from Implementing Virtual and Remote Labs
    Strenger, N. and Frerich, S.
    Advances in Intelligent Systems and Computing 1231 AISC 103-111 (2021)
    This contribution is showing ways to overcome issues on the way to digitalized laboratories in engineering education. The results presented in this paper were gained throughout a long-term study: 10 different laboratories were surveyed over a time period of 8 years. A non-standardized survey method was chosen for this evaluation, including a semi-structured guideline with open questions developed during several phases of pre-testing. The didactics and technical concepts of the laboratories are addressed, as well as challenges encountered during implementation and operation. Key findings of the whole study were identified by looking at didactical set-ups, technical aspects, and project managing topics. Although some aspects of hands-on experiments on-site were easily conveyed into virtual or remote laboratories, others needed to overcome severe impairments. However, personal commitment and financial support were identified as important success factors. By addressing original learning objectives as well as technical challenges that arose during set-up and digitalization of the laboratories, the results of this contribution clearly emphasize the connection between didactical purposes and technical realization. In some cases, both virtual and remote laboratories needed additional assistance in rephrasing learning objectives and adapting them throughout the process. © 2021, Springer Nature Switzerland AG.
    view abstractdoi: 10.1007/978-3-030-52575-0_8
  • 2021 • 54 Shear Characterization of Additively Manufactured Short Carbon Fiber-Reinforced Polymer
    Striemann, P. and Bulach, S. and Hülsbusch, D. and Niedermeier, M. and Walther, F.
    Macromolecular Symposia 395 (2021)
    The additive manufacturing (AM) techniques Fused Deposition Modeling (FDM™) or Fused Filament Fabrication (FFF) are all based on material extrusion and are one of the most widely used AM methods. Serial production with this technology requires a deep understanding for the process and the effects of the process parameters on the mechanical properties. In literature tensile, compression, or flexural properties are often considered. However, the basic structure of a FFF part is strongly anisotropic and therefore similar to a composite material. The characterization of a carbon fiber-reinforced polymer (CFRP) involves additional material properties. One of the most informative quality values for CFRP is the interlaminar shear strength (ILSS) in the midplane direction. The ILSS can be handled in an equivalent way for AM parts in form of the interlayer shear strength. The literature shows various approaches, but no existing standard to determine the interlayer shear strength for FFF parts. The aim of the study is a suitable experimental setup for identifying the interlayer shear strength of AM parts. The experimental setup and the specimen geometry in accordance with DIN 65148 are adapted to ensure an interlayer failure mode between two layers. The newly developed test approach is validated with a test series showing a decrease in ILSS of up to 40% by increasing the layer height. © 2021 Wiley-VCH GmbH
    view abstractdoi: 10.1002/masy.202000247
  • 2021 • 53 Influence of Nb as Microalloying Element on the Recovery and Recrystallization of Fe–25Mn–12Cr–C–N Twinning-Induced Plasticity Steels
    Suárez Sierra, A. and Rodríguez Baracaldo, R. and Mujica Roncery, L. and Egels, G. and Theisen, W.
    Steel Research International (2021)
    The influence of Nb on the microstructure during annealing at 950, 1000, and 1100 °C is analyzed in two types of twinning-induced plasticity (TWIP) steels, Fe–25Mn–12Cr–C–N (TWIP-0) and Fe–25Mn–12Cr–C–N–Nb (TWIP-Nb). The addition of Nb as a microalloying element affects various phenomena taking place during annealing, namely, recrystallization, grain coarsening, and recovery processes. Microstructural characterization is conducted via light microscopy, scanning electron microscopy, and electron back scattering diffraction (EBSD). Recovery takes place after annealing at 950 °C, where remaining deformation and grain nucleation can be seen. Microstructural analyses indicate that the location of the recrystallization nuclei in the recovered structure is associated with the local chemical segregation of Mn and Cr, which leads to differences in the driving force for the martensitic transformation at microscale, and therefore local deformation mechanisms. The presence of Nb as a microalloying element decelerates recovery and recrystallization kinetics. At 1100 °C/10 min, both steels exhibit complete recrystallization; moreover, abnormal grain growth starts. © 2021 Wiley-VCH GmbH
    view abstractdoi: 10.1002/srin.202000417
  • 2021 • 52 Reducing hot tearing by grain boundary segregation engineering in additive manufacturing: example of an AlxCoCrFeNi high-entropy alloy
    Sun, Z. and Tan, X. and Wang, C. and Descoins, M. and Mangelinck, D. and Tor, S.B. and Jägle, E.A. and Zaefferer, S. and Raabe, D.
    Acta Materialia 204 (2021)
    One major hindrance that alloy design for additive manufacturing (AM) faces nowadays is hot tearing. Contrary to the previous works which either try to reduce solidification range or introduce grain refinement, the current work presents a new approach of employing segregation engineering to alter the residual stress states at the interdendritic and grain boundary regions and consequently prevent hot tearing. Here, in situ Al alloying is introduced into an existing hot-cracking susceptible high-entropy alloy CoCrFeNi. It is found that within a certain range of compositions, such as Al0.5CoCrFeNi, the hot crack density was drastically decreased. During the solidification of this specific alloy composition, Al is firstly ejected from the primary dendritic face-centred cubic (FCC) phase and segregates into the interdendritic regions. Spinodal decomposition then occurs in these Al-enriched regions to form the ordered B2 NiAl and disordered body-centred cubic (BCC) Cr phases. Due to the higher molar volume and lower homologous temperatures of these B2/BCC phases, the inherent residual strain is accommodated and transformed from a maximum 0.006 tensile strain in CoCrFeNi to a compressive strain of ~0.001 in Al0.5CoCrFeNi. It is believed that this grain boundary segregation engineering method could provide a new pathway to systematically counteract the hot tearing problem in additive manufacturing of metals and alloys, using available thermodynamic and kinetic database information. © 2020
    view abstractdoi: 10.1016/j.actamat.2020.116505
  • 2021 • 51 Influence of strain rate on the activation of {110}, {112}, {123} slip in ferrite of DP800
    Tian, C. and Dehm, G. and Kirchlechner, C.
    Materialia 15 (2021)
    We have performed micro pillar compression to investigate the influence of strain rate on the activation of three slip plane families, namely {110}, {112} and {123}, in ferrite of a dual phase steel. The critical resolved shear stress of all three slip plane families rises with increased strain rate. The strain rate sensitivity drops with increasing strain. Increasing strain rate does not reduce the number of activated slip systems, instead resulting in slip plane activation outside of that predicted by Schmid´s law. The activation volume of 13b³ to 16b³ suggests that the Peierl's process is the rate controlling mechanism in ferrite of DP800. © 2020
    view abstractdoi: 10.1016/j.mtla.2020.100983
  • 2021 • 50 Residual stresses and tribomechanical behaviour of TiAlN and TiAlCN monolayer and multilayer coatings by DCMS and HiPIMS
    Tillmann, W. and Grisales, D. and Stangier, D. and Thomann, C.-A. and Debus, J. and Nienhaus, A. and Apel, D.
    Surface and Coatings Technology 406 (2021)
    The deposition of ternary nitrides with the incorporation of carbon atoms into its structure has demonstrated to be a promising approach in the pursuit of wear-resistant and self-lubricating coatings. Firstly, both TiAlN and TiAlCN monolayers were deposited using direct current magnetron sputtering (DCMS) and high-power impulse magnetron sputtering (HiPIMS) onto quenched and tempered AISI H11 tool steel to be used as references. Acetylene was used as a carbon precursor, producing DCMS and HiPIMS TiAlCN coatings with 9.0 and 21.7 at.% C, respectively. Subsequently, TiAlN/TiAlCN multilayers of various designs were also developed as follows: 5×[10/500], 5×[50/500] and 5×[100/500] nm. Residual stresses of the coating systems were determined by X-ray radiation utilising an ETA-diffractometer with a Cu-Kα radiation source applying the sin2ψ method. Additionally, residual stresses depth gradients of the substrate before and after the deposition of the coatings were determined in a LEDDI 8-circle diffractometer equipped with a W-X-ray tube and operated in the energy-dispersive mode of diffraction. Great reduction of the compressive residual stresses in the coatings was observed after the introduction of carbon into the TiAlN coating structure, shifting from −1047 ± 149 to −307 ± 211 MPa for the DCMS and from −7035 ± 1361 to +989 ± 187 MPa for the HiPIMS coatings. In the multilayer coatings, compressive residual stresses increase along with the increment of the TiAlN interlayer. Additionally, residual stresses of the substrate in the near-surface are dragged from low compressive stresses (−218 ± 61) to tensile stresses in the range of 1000 to 2000 MPa for all the DCMS/substrate systems, a behaviour only presented in HiPIMS by the TiAlN monolayer. Wear coefficients of all the evaluated HiPIMS systems are notoriously lower than their DCMS counterparts. Compared to TiAlN, TiAlCN HiPIMS presented a lower coefficient of friction but a higher wear coefficient, which in turn was not reduced by the introduction of the multilayer systems. Finally, Scratch test and Rockwell C adhesion tests have shown higher adhesion of DCMS coatings than HiPIMS coatings, and a detriment of the monolayers adhesion by the implementation of TiAlN/TiAlCN multilayer systems. The understanding of the residual stresses, both in the coating and in the substrate, and the way they affect the tribomechanical performance of the system coating/substrate continues to be of great importance, especially for coatings deposited by new technologies such as HiPIMS and self-lubricating coatings. © 2020 Elsevier B.V.
    view abstractdoi: 10.1016/j.surfcoat.2020.126664
  • 2021 • 49 Influence of spray gun parameters on inflight particle's characteristics, the splat-type distribution, and microstructure of plasma-sprayed YSZ coatings
    Tillmann, W. and Khalil, O. and Baumann, I.
    Surface and Coatings Technology 406 (2021)
    Ceramic coatings, fabricated with specific properties using the atmospheric plasma spray (APS) process, are widely used for many applications in which the porosity and splat interfaces are the main factors affecting the performance. Since the coating microstructure is composed of large numbers of molten and semi-molten particles impinged successfully at the substrate (known as splats), the produced coatings are characterized by the melting degree of these particles and their relative splat-type fractions. In the present work, the effect of process parameters settings has been studied systematically, relating the characteristics of impinging particles to splat formation and eventually to microstructure development and properties of the coating. Therefore, individual splats were collected on mirror-polished substrates and observed using image analysis (IA). These were evaluated and categorized into different splat types, based on their melting degree and morphology, under each combination of spray conditions. It was found that gun current and standoff distance have a profound effect on the characteristics of impinging particles. These, in turn, determine the relative fractions of splat types, layered structure, and final properties of the deposit. The effect of splat-type distribution on the bonding strength between layers, lamellar structure, and coating porosity was investigated. © 2020 Elsevier B.V.
    view abstractdoi: 10.1016/j.surfcoat.2020.126705
  • 2021 • 48 Influence of the PVD process conditions on the incorporation of TiN nanoparticles into magnetron sputtered CrN thin films
    Tillmann, W. and Kokalj, D. and Stangier, D. and Fu, Q. and Kruis, F.E.
    Surface and Coatings Technology 409 (2021)
    CrTiN thin films are known to form a solid solution independent from the Ti content. Using a novel spatially separated synthesis approach, consisting of magnetron sputtering and atmospheric-pressure arc evaporation, artificial CrTiN nanocomposites were deposited. For the nanocomposite formation, TiN nanoparticles were synthesized using a transferred arc reactor and directly injected into growing CrN thin films using an aerodynamic lens system. The CrN and CrTiN thin films were deposited using various deposition conditions, such as heating power, substrate rotation velocity, nanoparticle injection distance, and cathode setup. The deposited thin films were analyzed regarding their physical structure, microstructure and mechanical properties. Based on the investigations, between 0.02 and 0.11 at.-% of TiN nanoparticles are embedded in the CrN matrix dependent on the deposition parameters. 2D GI-XRD experiments using synchrotron radiation confirm the nanocomposite structure for the two thin films with the highest TiN nanoparticle content. The crystallite size of the CrN thin film decreases from 9.4 ± 2.3 nm to 5.3 ± 1.2 nm due to the embedding of the nanoparticles. Concerning the physical structure, the nanoparticle injection leads to a change of the texture, as shown by the Debbye-Scherrer rings. Based on TEM-investigations, TiN nanoparticle agglomerates lead to a coarser microstructure of the CrN matrix. The hardness of the thin films is not significantly affected by the nanoparticle embedment. The nanoparticle injection distance and cathode setup reveal the highest impact on the film properties. © 2021 Elsevier B.V.
    view abstractdoi: 10.1016/j.surfcoat.2021.126935
  • 2021 • 47 On the synthesis and structural evolution of artificial CrN/TiN nanocomposites
    Tillmann, W. and Kokalj, D. and Stangier, D. and Fu, Q. and Kruis, F.E. and Kesper, L. and Berges, U. and Westphal, C.
    Applied Surface Science 535 (2021)
    The synthesis of nanocomposites is limited to thermodynamically immiscible phases or to phase separation by exceeding the limits of solution. Hence, the formation of nanocomposites based on transition metals, revealing a nanocrystalline Metal-Nitride/nanocrystalline Metal-Nitride structure, is restricted. These restrictions can be overruled by a spatially separated synthesis of the two phases and a recombination during the deposition. With this approach, the limits of current systems can be expanded, enabling the synthesis of artificial nanocomposites based on a variety of materials. We demonstrate the synthesis of a composite of two nanocrystalline phases of the miscible transition metal-nitrides CrN and TiN. TiN nanoparticles were synthesized using an atmospheric-pressure arc reactor and in-situ injected into a growing CrN thin film. The thin films are analyzed regarding their physical- and microstructure using two-dimensional GIXRD, XPS based on synchrotron radiation and TEM. The CrTiN thin film reveals a two-phase structure consisting of nanocrystalline CrN and TiN phases with crystallite sizes of 9 nm and 4 nm according to GIXRD. XPS indicates bonding of Cr-N, Cr-Cr, and Ti-N. No hint for Cr-Ti bonding was found, excluding (Cr,Ti)N solid solution formation. Based on the TEM-investigations, TiN nanoparticles are embedded as agglomerates in the CrN matrix. © 2020 Elsevier B.V.
    view abstractdoi: 10.1016/j.apsusc.2020.147736
  • 2021 • 46 Design of Cu- and Ag-containing amorphous carbon multilayers with improved tribo-mechanical properties
    Tillmann, W. and Lopes Dias, N.F. and Stangier, D. and Matveev, S. and Arne Thomann, C. and Debus, J.
    Materials Letters 284 (2021)
    The modification of amorphous carbon (a–C) films by adding either Cu or Ag is a common approach to tailor the film properties. These films become less hard, while they demonstrate higher friction and lower wear resistance than a–C. To enhance tribologically relevant features, multilayers of alternating a–C and a–C:Cu or a–C:Ag layers are synthetized by magnetron sputtering. The a–C/a–C:Cu and a–C/a–C:Ag multilayers possess a bilayer period of ~200 nm, a layer ratio of 1, and a bilayer number of 5. These structures are characterized by higher hardness and lower friction and wear against 100Cr6 counterparts as compared to monolayered a–C:Cu and a–C:Ag. © 2020 Elsevier B.V.
    view abstractdoi: 10.1016/j.matlet.2020.128905
  • 2021 • 45 Temperature-dependent tribological behavior of MoSx thin films synthesized by HiPIMS
    Tillmann, W. and Wittig, A. and Stangier, D. and Moldenhauer, H. and Thomann, C.-A. and Debus, J. and Aurich, D. and Bruemmer, A.
    Tribology International 153 (2021)
    Understanding the interaction between the structure and the tribological properties of sputtered molybdenum disulfide films at elevated temperatures is essential for their use in industrial applications. Therefore, the friction and wear behavior up to of 400°C of one stoichiometric MoS2 and a sub-stoichiometric MoS1.6 film are investigated against 100Cr6 counterparts. With an increasing temperature up to 200°C, the friction decreases, which is attributed to a thermally activated water desorption and an increasing intensity of the (002) basal plane. Due to a passivation mechanism caused by the sulfur defect sites, the friction is lower for the sub-stoichiometric film. Above this temperature the friction increases for both films and failure occurs at 400°C. Therefore, the friction at elevated temperatures result from a complex interaction of re-orientation mechanisms, desorption and oxidation processes. © 2020 Elsevier Ltd
    view abstractdoi: 10.1016/j.triboint.2020.106655
  • 2021 • 44 Deformation measurement of a monopile subject to vortex- induced vibration using digital image correlation
    Tödter, S. and el Sheshtawy, H. and Neugebauer, J. and el Moctar, O. and Schellin, T.E.
    Ocean Engineering 221 (2021)
    Monopile towers used for offshore wind turbines are sensitive to Vortex-Induced Vibration (VIV). Here, their structural response to VIV was experimentally investigated with models of a partially and a fully immerged offshore monopile. The partially and fully immersed model cylinders had aspect ratios of 18.75 and 28.13, respectively. They were subject to reduced velocities of up to 9.78 and 5.45, respectively, corresponding to Reynolds numbers of up to 77120 and 63680, respectively. Normalised transverse motion amplitudes of up to 1.19 were measured. “Strouhal-like” numbers down to 0.13 were obtained, and they decreased with increasing flow velocity and corresponding motion amplitude. The 3D Digital Image Correlation (DIC) processing method was used to measure response without influencing model properties and the flow field. To assess the suitability of DIC for VIV investigations, results obtained from the conventional technique using triaxial accelerometers were compared. The influence of different processing methods, pattern designs and their assemblies, the repeatability of the model tests, and the influence of ventilation were investigated also. Additionally, forces and torques were measured directly. Results from these three measurement techniques were compared and discussed. © 2020
    view abstractdoi: 10.1016/j.oceaneng.2020.108548
  • 2021 • 43 A bright and fast source of coherent single photons
    Tomm, N. and Javadi, A. and Antoniadis, N.O. and Najer, D. and Löbl, M.C. and Korsch, A.R. and Schott, R. and Valentin, S.R. and Wieck, A.D. and Ludwig, Ar. and Warburton, R.J.
    Nature Nanotechnology (2021)
    A single-photon source is an enabling technology in device-independent quantum communication1, quantum simulation2,3, and linear optics-based4 and measurement-based quantum computing5. These applications employ many photons and place stringent requirements on the efficiency of single-photon creation. The scaling on efficiency is typically an exponential function of the number of photons. Schemes taking full advantage of quantum superpositions also depend sensitively on the coherence of the photons, that is, their indistinguishability6. Here, we report a single-photon source with a high end-to-end efficiency. We employ gated quantum dots in an open, tunable microcavity7. The gating provides control of the charge and electrical tuning of the emission frequency; the high-quality material ensures low noise; and the tunability of the microcavity compensates for the lack of control in quantum dot position and emission frequency. Transmission through the top mirror is the dominant escape route for photons from the microcavity, and this output is well matched to a single-mode fibre. With this design, we can create a single photon at the output of the final optical fibre on-demand with a probability of up to 57% and with an average two-photon interference visibility of 97.5%. Coherence persists in trains of thousands of photons with single-photon creation at a repetition rate of 1 GHz. © 2021, The Author(s), under exclusive licence to Springer Nature Limited.
    view abstractdoi: 10.1038/s41565-020-00831-x
  • 2021 • 42 Methods for measuring large shear strains in in-plane torsion tests
    Traphöner, H. and Clausmeyer, T. and Tekkaya, A.E.
    Journal of Materials Processing Technology 287 (2021)
    The in-plane torsion test achieves true strains far beyond 1.0 for sheet metals, especially using specimens with circular grooves. The accurate measurement of these high strains is a challenge for the conventional digital image correlation (DIC). Thus, the determination of flow curves is limited and fracture strains for very ductile materials cannot be measured. A new grooved specimen is introduced to avoid strain localization. Shear stress and shear strain along a defined area in the groove are constant so that strains can be measured independently of the DIC system setting without error due to strain localization. Furthermore, three methods for the measurement of very high shear strains in the in-plane torsion test are presented: Firstly, the limit of the optical strain measurement is extended by multiple renewal of the digital image correlation (DIC) pattern on the samples. Secondly, the shear strain for the planar specimen is calculated exactly from the rotation angle curve. Lastly, a new incremental method is presented. This method enables to determine shear strains for plane and grooved specimen exactly by only measuring the torque and the angle of rotation. All methods were applied for three steel sheet materials namely DP1000, CP1000 and DC04. The equivalent strain in a grooved in-plane torsion test of sheet steel DC04 was determined as 3.3 with the new incremental method. Such high strains far exceed conventional methods for determining the flow curve. © 2019 The Author(s)
    view abstractdoi: 10.1016/j.jmatprotec.2019.116516
  • 2021 • 41 Numerical simulation and benchmarking of drops and bubbles
    Turek, S. and Mierka, O.
    Handbook of Numerical Analysis 22 419-465 (2021)
    The numerical simulation of immiscible multiphase flow problems, particularly including drops and bubbles, is very important in many applications, and performing accurate, robust and efficient numerical computations has been the object of numerous research and simulation projects for many years. One of the main challenges for the underlying numerical methods – besides the fact that the computational simulation of the incompressible Navier–Stokes equations is challenging by itself – is that the position of the moving interface between two fluids is unknown and must be determined as part of the boundary value problem to be solved. In this contribution, we provide a compact description of state-of-the-art numerical solvers for such multiphase flow problems, namely interface tracking and interface capturing methods. It is demonstrated that corresponding discretization and solution approaches which are based on Finite Element and Discrete Projection methods for the Navier–Stokes equations, combined with corresponding numerical tools for both interface capturing, resp., tracking approaches, lead to robust, accurate, flexible, and efficient simulation tools. Moreover, we present several numerical test cases of benchmarking type which first of all shall help to evaluate the quality of the underlying flow solvers. In particular, we describe the settings for a quantitative 3D Rising Bubble benchmark which can be used for ‘simple’ validation and evaluation of multiphase CFD codes without the necessity of complex postprocessing operations. Finally, we also provide numerical reference values for a ‘Taylor bubble’ setting, and we show simulation results of a reactive Taylor bubble flow in the framework of estimating reaction parameters to match corresponding experimentally obtained results. All reference benchmark quantities can be downloaded from © 2021 Elsevier B.V.
    view abstractdoi: 10.1016/bs.hna.2019.09.001
  • 2021 • 40 Residual stresses in hot bulk formed parts: two-scale approach for austenite-to-martensite phase transformation
    Uebing, S. and Brands, D. and Scheunemann, L. and Schröder, J.
    Archive of Applied Mechanics (2021)
    In production engineering, current research focuses on the induction of targeted residual stress states in components in order to improve their properties. Therein, the combination of experiment and simulation plays an important role. In this contribution, a focus is laid on the investigation of hot forming processes with subsequent cooling. A numerical approach is presented to analyze the distribution of residual stresses resulting from cooling of a cylinder with an eccentric hole made of chromium-alloyed steel. The occurring phase transformation, which is evoked by cooling, is considered in order to compute residual stress distributions inside the material. © 2021, The Author(s).
    view abstractdoi: 10.1007/s00419-020-01836-7
  • 2021 • 39 Investigation of the stress corrosion cracking behavior on T24 material under the operational conditions in the water wall
    Ullrich, C. and Tillmann, W. and Rademacher, H.-G. and Zielke, R. and Körner, P.
    International Journal of Pressure Vessels and Piping 190 (2021)
    The requirements for materials and their strength significantly increased with the new generation of coal fired power plants operating at steam temperatures of up to 620 °C. Therefore, new materials were introduced to fulfill the defined needs. During the commissioning process of the first plant many cracks occurred in welds of T24 material. The cracks showed clear characteristics of stress corrosion cracking (SCC). Not knowing the exact parameters that lead to cracking, experiments in high temperature water were carried out. Slow tensile tests in a controlled environment are extremely well suited to generate information about material's SCC sensitivity. In the present paper, the influence of the temperature, the oxygen concentration of water, the pre-treatment of the specimen and the heat treatment to the SCC are investigated. Furthermore critical limits for the cracking are defined where possible. © 2021
    view abstractdoi: 10.1016/j.ijpvp.2021.104317
  • 2021 • 38 Remote Lab to Illustrate the Influence of Process Parameters on Product Properties in Additive Manufacturing
    Upadhya, S. and Grodotzki, J. and Selvaggio, A. and Mogylenko, O. and Tekkaya, A.E.
    Advances in Intelligent Systems and Computing 1231 AISC 456-464 (2021)
    Additive manufacturing, which enables the production of highly complex components that were previously next to impossible to manufacture, has evolved from a tool for rapid prototyping to an integral part of many production lines in the metal manufacturing industry. Therefore, it is critical that today’s students, the manufacturing engineers of tomorrow, get a fundamental understanding of the process and the influence of the various process parameters on the process and the final product properties. The developed remote lab offers the students an opportunity to vary different process parameters and characterize the performance of specimens manufactured under different conditions with help of the uniaxial tensile test and to quantitatively analyze the interplay of different process parameters on the final product. From the educator’s point of view, the remote lab will allow for a higher throughput of students in the field of additive manufacturing without compromising on the machine and user safety, as well as the effectiveness of the lab. © 2021, Springer Nature Switzerland AG.
    view abstractdoi: 10.1007/978-3-030-52575-0_37
  • 2021 • 37 Oxygen Removal from a Hydrocarbon Containing Gas Stream by Plasma Catalysis
    Urbanietz, T. and Stewig, C. and Böke, M. and von Keudell, A.
    Plasma Chemistry and Plasma Processing 41 619-642 (2021)
    Hydrocarbon exhaust gases containing residual amounts of oxygen may pose challenges for their conversion into value added chemicals downstream, because oxygen may affect the process. This could be avoided by plasma treating the exhaust to convert O 2 in presence of hydrocarbons into CO or CO 2 on demand. The underlying reaction mechanisms of plasma conversion of O 2 in the presence of hydrocarbons are analysed in a model experiment using a radio frequency atmospheric pressure helium plasma in a plug flow design with admixtures of O 2 and of CH 4. The plasma process is analysed with infrared absorption spectroscopy to monitor CH 4 as well as the reaction products CO, CO 2 and H 2O. It is shown that the plasma reaction for oxygen (or methane removal) is triggered by the formation of oxygen atoms from O 2 by electron. Oxygen atoms are efficiently converted into CO, CO 2 and H 2O with CO being an intermediate in that reaction sequence. However, at very high oxygen admixtures to the gas stream, the conversion efficiency saturates because electron induced O 2 dissociation in the plasma seems to be counterbalanced by a reduction of the efficiency of electron heating at high admixtures of O 2. The impact of a typical industrial manganese oxide catalyst is evaluated for methane conversion. It is shown that the conversion efficiency is enhanced by 15–20% already at temperatures of 430 K. © 2021, The Author(s).
    view abstractdoi: 10.1007/s11090-020-10151-6
  • 2021 • 36 Hybrid chitosan/gelatin/nanohydroxyapatite scaffolds promote odontogenic differentiation of dental pulp stem cells and in vitro biomineralization
    Vagropoulou, G. and Trentsiou, M. and Georgopoulou, A. and Papachristou, E. and Prymak, O. and Kritis, A. and Epple, M. and Chatzinikolaidou, M. and Bakopoulou, A. and Koidis, P.
    Dental Materials 37 e23-e36 (2021)
    Objective: Hybrid chitosan/gelatin/nanohydroxyapatite (CS/Gel/nHA) scaffolds have attracted considerable interest in tissue engineering (TE) of mineralized tissues. The present study aimed to investigate the potential of CS/Gel/nHA scaffolds loaded with dental pulp stem cells (DPSCs) to induce odontogenic differentiation and in vitro biomineralization. Methods: CS/Gel/nHA scaffolds were synthesized by freeze-drying, seeded with DPSCs, and characterized with flow cytometry. Scanning Electron Microscopy (SEM), live/dead staining, and MTT assays were used to evaluate cell morphology and viability; real-time PCR for odontogenesis-related gene expression analysis; SEM-EDS (Energy Dispersive X-ray spectroscopy), and X-ray Diffraction analysis (XRD) for structural and chemical characterization of the mineralized constructs, respectively. Results: CS/Gel/nHA scaffolds supported viability and proliferation of DPSCs over 14 days in culture. Gene expression patterns indicated pronounced odontogenic shift of DPSCs, evidenced by upregulation of DSPP, BMP-2, ALP, and the transcription factors RunX2 and Osterix. SEM-EDS showed the production of a nanocrystalline mineralized matrix inside the cell-based and - to a lesser extent - the cell-free constructs, with a time-dependent production of net-like nanocrystals (appr. 25−30 nm in diameter). XRD analysis gave the crystallite size (D = 50 nm) but could not distinguish between the initially incorporated and the biologically produced nHA. Significance: This is the first study validating the potential of CS/Gel/nHA scaffolds to support viability and proliferation of DPSCs, and to provide a biomimetic microenvironment favoring odontogenic differentiation and in vitro biomineralization without the addition of any inductive factors, including dexamethasone and/or growth/morphogenetic factors. These results reveal a promising strategy towards TE of mineralized dental tissues. © 2020 The Academy of Dental Materials
    view abstractdoi: 10.1016/
  • 2021 • 35 Unique performance of thermal barrier coatings made of yttria-stabilized zirconia at extreme temperatures (>1500°C)
    Vaßen, R. and Mack, D.E. and Tandler, M. and Sohn, Y.J. and Sebold, D. and Guillon, O.
    Journal of the American Ceramic Society 104 463-471 (2021)
    Yttria-stabilized zirconia (YSZ) has been for several decades the state of the art material for thermal barrier coating (TBC) applications in gas turbines. Although the material has unique properties, further efficiency improvement by increasing the temperature is limited due to its maximum temperature capability of about 1200°C. Above this temperature the deposited metastable tetragonal (t´) phase undergoes a detrimental phase transformation as well as enhanced sintering. Both processes promote the failure of the coatings at elevated temperatures and this early failure has been frequently observed in gradient tests. In this paper, we now experimentally shown for the first time that under typical cycling conditions not the time at elevated temperatures leads to the reduced lifetime but the transient cooling rates. If cooling rates were reduced to 10K/s, TBC systems could be operated in a burner rig at a surface temperature well above 1500°C without showing a lifetime reduction. The explanation of these astonishing findings is given by the evaluation of energy release rate peaks during fast transient cooling in combination with the phase evolution during cooling with the used cooling rates. © 2020 The Authors. Journal of the American Ceramic Society published by Wiley Periodicals LLC on behalf of American Ceramic Society (ACERS)
    view abstractdoi: 10.1111/jace.17452
  • 2021 • 34 Combining crystalline and polymeric excipients in API solid dispersions – Opportunity or risk?
    Veith, H. and Wiechert, F. and Luebbert, C. and Sadowski, G.
    European Journal of Pharmaceutics and Biopharmaceutics 158 323-335 (2021)
    Amorphous solid dispersions (ASDs) are often metastable against crystallization of the active pharmaceutical ingredient (API) and thus might undergo unwanted changes during storage. The crystallization tendency of ASDs is influenced by the API crystallization driving force (CDF) and the mobility of the molecules in the ASD. Low molecular weight-excipients are known to stabilize amorphous APIs in so-called co-amorphous formulations. Due to their success in stabilizing co-amorphous APIs, low-molecular weight excipients might also enhance the stability of polymeric ASDs. In this work, we investigated the potential of combined low-molecular weight excipient/polymer formulations with in-silico tools and validated the predictions with long-term stability tests of the most promising excipient/polymer combinations. The considered critical quality attributes for the ASDs were the occurrence of amorphous phase separation, API CDF, and molecular mobility in the ASD. As an example, carbamazepine/polyvinylpyrrolidone ASDs were investigated combined with the excipients fructose, lactose, sucrose, trehalose, saccharin, tryptophan, and urea. Although all excipients had a negative impact on the ASD stability, saccharin still turned out to be the most promising one. Long-term stability studies with ASDs containing either saccharin or tryptophan verified -in agreement to the predictions- that API crystallization occurred faster than in the reference ASDs without additional excipient. This work showed that the addition of crystalline excipients to polymeric ASDs might not only offer opportunities but might also bear risks for the long-term stability of the ASD, even though the crystalline excipient stabilizes the polymer-free API. Consequently, excipients should be evaluated based on the thermodynamic phase behavior of the individual mixture of API/polymer/excipient, rather than based on pure-component properties of the excipient only. In-silico predictions proposed in this work remarkably decrease the number of screening tests for identifying suitable formulation excipients. © 2020 Elsevier B.V.
    view abstractdoi: 10.1016/j.ejpb.2020.11.025
  • 2021 • 33 Configurational Entropy Driven High-Pressure Behaviour of a Flexible Metal–Organic Framework (MOF)
    Vervoorts, P. and Keupp, J. and Schneemann, A. and Hobday, C.L. and Daisenberger, D. and Fischer, R.A. and Schmid, R. and Kieslich, G.
    Angewandte Chemie - International Edition 60 787-793 (2021)
    Flexible metal–organic frameworks (MOFs) show large structural flexibility as a function of temperature or (gas)pressure variation, a fascinating property of high technological and scientific relevance. The targeted design of flexible MOFs demands control over the macroscopic thermodynamics as determined by microscopic chemical interactions and remains an open challenge. Herein we apply high-pressure powder X-ray diffraction and molecular dynamics simulations to gain insight into the microscopic chemical factors that determine the high-pressure macroscopic thermodynamics of two flexible pillared-layer MOFs. For the first time we identify configurational entropy that originates from side-chain modifications of the linker as the key factor determining the thermodynamics in a flexible MOF. The study shows that configurational entropy is an important yet largely overlooked parameter, providing an intriguing perspective of how to chemically access the underlying free energy landscape in MOFs. © 2020 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH
    view abstractdoi: 10.1002/anie.202011004
  • 2021 • 32 There is Plenty of Room for THz Tunneling Electron Devices beyond the Transit Time Limit
    Villani, M. and Clochiatti, S. and Prost, W. and Weimann, N. and Oriols, X.
    IEEE Electron Device Letters 42 224-227 (2021)
    The traditional transmission coefficient present in the original Landauer formulation, which is valid for quasi-static scenarios with working frequencies below the inverse of the electron transit time, is substituted by a novel time-dependent displacement current coefficient valid for frequencies above this limit. Our model captures in a simple way the displacement current component of the total current, which at frequencies larger than the inverse of the electron transit time can be more relevant than the particle component. The proposed model is applied to compute the response of a resonant tunneling diode from 10 GHz up to 5 THz. We show that tunneling electron devices are intrinsically nonlinear at such high frequencies, even under small-signal conditions, due to memory effects related to the displacement current. We show that these intrinsic nonlinearities (anharmonicities) represent an advantage, rather than a drawback, as they open the path for tunneling devices in many THz applications, and avoid further device downscaling. © 1980-2012 IEEE.
    view abstractdoi: 10.1109/LED.2021.3049229
  • 2021 • 31 Adaptive thermodynamic topology optimization
    Vogel, A. and Junker, P.
    Structural and Multidisciplinary Optimization 63 95-119 (2021)
    The benefit of adaptive meshing strategies for a recently introduced thermodynamic topology optimization is presented. Employing an elementwise gradient penalization, stability is obtained and checkerboarding prevented while very fine structures can be resolved sharply using adaptive meshing at material-void interfaces. The usage of coarse elements and thereby smaller design space does not restrict the obtainable structures if a proper adaptive remeshing is considered during the optimization. Qualitatively equal structures and quantitatively the same stiffness as for uniform meshing are obtained with less degrees of freedom, memory requirement and overall optimization runtime. In addition, the adaptivity can be used to zoom into coarse global structures to better resolve details of interesting spots such as truss nodes. © 2020, The Author(s).
    view abstractdoi: 10.1007/s00158-020-02667-4
  • 2021 • 30 Functional central limit theorems for multivariate Bessel processes in the freezing regime
    Voit, M. and Woerner, J.H.C.
    Stochastic Analysis and Applications 39 136-156 (2021)
    Multivariate Bessel processes (Formula presented.) describe interacting particle systems of Calogero-Moser-Sutherland type and are related with β-Hermite and β-Laguerre ensembles. They depend on a root system and a multiplicity k. Recently, several limit theorems were derived for (Formula presented.) with fixed starting point. Moreover, the SDEs of (Formula presented.) were used to derive strong laws of large numbers for (Formula presented.) with starting points of the form (Formula presented.) with x in the interior of the Weyl chambers. Here we provide associated almost sure functional central limit theorems which are locally uniform in t. The Gaussian limit processes admit explicit representations in terms of the solutions of associated deterministic ODEs. © 2020 Taylor & Francis Group, LLC.
    view abstractdoi: 10.1080/07362994.2020.1786402
  • 2021 • 29 Sharp Rank-One Convexity Conditions in Planar Isotropic Elasticity for the Additive Volumetric-Isochoric Split
    Voss, J. and Ghiba, I.-D. and Martin, R.J. and Neff, P.
    Journal of Elasticity (2021)
    We consider the volumetric-isochoric split in planar isotropic hyperelasticity and give a precise analysis of rank-one convexity criteria for this case, showing that the Legendre-Hadamard ellipticity condition separates and simplifies in a suitable sense. Starting from the classical two-dimensional criterion by Knowles and Sternberg, we can reduce the conditions for rank-one convexity to a family of one-dimensional coupled differential inequalities. In particular, this allows us to derive a simple rank-one convexity classification for generalized Hadamard energies of the type W(F)=μ2∥F∥2detF+f(detF); such an energy is rank-one convex if and only if the function f is convex. © 2021, The Author(s).
    view abstractdoi: 10.1007/s10659-021-09817-9
  • 2021 • 28 Influence of the Brazing Paste Composition on the Wetting Behavior of Reactive Air Brazed Metal–Ceramic Joints
    Waetzig, K. and Schilm, J. and Mosch, S. and Tillmann, W. and Eilers, A. and Wojarski, L.
    Advanced Engineering Materials 23 (2021)
    Reactive air brazing (RAB) is a cost-effective way to produce ceramic–ceramic or ceramic–metal brazed joints in air, without applying a protective gas atmosphere or a vacuum. In addition to conventional furnace technology, the brazing with induction heating can also be used effectively. Within the scope of this study the shrinkage and wetting behavior of self-developed brazing pastes with different CuO contents and two qualities of silver powders with coarse and fine particle size are investigated by optical dilatometry on alumina (Al2O3, 99.7% purity). Thereby, the fine silver powder quality reveals a significant swelling effect at high temperatures, leading to an expansion of densified powder compacts caused by evolving gases. Joining tests are performed on ceramic–steel brazed joints using a muffle furnace and induction heating for short brazing cycles. The brazing seams and interfaces of the joints are investigated using scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS). As a result, correlations between the brazing filler metal composition, the steel, and the brazing conditions are obtained. © 2020 The Authors. Published by Wiley-VCH GmbH
    view abstractdoi: 10.1002/adem.202000711
  • 2021 • 27 Investigation of Turbulent Pulverized Solid Fuel Combustion with Detailed Homogeneous and Heterogeneous Kinetics
    Wang, B. and Shamooni, A. and Stein, O.T. and Kronenburg, A. and Kempf, A.M. and Debiagi, P. and Hasse, C.
    Energy and Fuels (2021)
    A comprehensive Euler-Lagrange framework for pulverized coal combustion using detailed multi-step heterogeneous kinetics is presented. The heterogeneous kinetics employ the POLIMI model that involves 37 species (22 solid species and 15 gas species) and 49 reactions to describe detailed pyrolysis as well as char oxidation, gasification, and annealing for a wide range of coals. The porous structure of the coal particles is considered, and the heterogeneous reactions are assumed to occur throughout the entire particle in a volume-based approach. The ordinary differential equations of the heterogeneous kinetics are integrated on each Lagrangian coal particle and predict the conversion of the raw coal components to light volatile hydrocarbons, heavy tar species, and char off-gases. Hence, the composition of the solid fuel components and the released gas changes dynamically in space and time, providing high-fidelity predictions of solid fuel combustion. The chemical conversion of the released species in the gas phase is described by a homogeneous kinetic mechanism with 76 species and 973 reactions that was reduced from the comprehensive CRECK-G-1407 kinetic mechanism. The new modeling framework is employed within carrier-phase direct numerical simulations (CP-DNS) of pulverized coal combustion in a three-dimensional turbulent mixing layer. This configuration includes the additional physics of turbulence and particle group combustion by mixing solid fuel particles suspended in a primary oxidizer stream with the products from lean volatile combustion in a secondary stream. The CP-DNS results are analyzed with and without the available set of 14 char conversion reactions, and a low degree of char conversion indicated by an increased rate of CO production is captured for particles with temperatures higher than 1800 K. The CP-DNS results from the detailed POLIMI approach feature a distinct bimodal shape of the volatile release curve and multi-regime combustion. The POLIMI data are used to evaluate the predictive capability of simpler pyrolysis models. The original competing two-step model (C2SM) by Kobayashi is investigated and shown to predict heavily delayed ignition. A new competing two-step devolatilization approach is proposed as an alternative model reduction suitable for fitting bimodal volatile release rates, such as that predicted by POLIMI. The CP-DNS using the alternative pyrolysis model faithfully captures the onset of ignition and multi-regime flame branches. Differences arise in the local tar species compositions in the gas phase as a result of the time-varying (POLIMI) and fixed (new C2SM) volatile compositions for the respective models. The flame structure is further analyzed by chemical explosive mode analysis (CEMA), and the occurrence of premixed and non-premixed flames zones is confirmed, whereas a simpler flame index analysis fails to correctly indicate the multi-regime nature of the flame. This recognition of multi-regime combustion serves as a guidance for selecting suitable conditioning variables for flamelet and other combustion submodels in large eddy simulation. © 2021 American Chemical Society.
    view abstractdoi: 10.1021/acs.energyfuels.0c03479
  • 2021 • 26 Highly Ordered Mesoporous Co3O4 Electrocatalyst for Efficient, Selective, and Stable Oxidation of 5-Hydroxymethylfurfural to 2,5-Furandicarboxylic Acid
    Wang, C. and Bongard, H.-J. and Yu, M. and Schüth, F.
    ChemSusChem (2021)
    Electrochemical oxidation of biomass substrates to valuable bio-chemicals is highly attractive. However, the design of efficient, selective, stable, and inexpensive electrocatalysts remains challenging. Here it is reported how a 3D highly ordered mesoporous Co3O4/nickel foam (om-Co3O4/NF) electrode fulfils those criteria in the electrochemical oxidation of 5-hydroxymethylfurfural (HMF) to value-added 2,5-furandicarboxylic acid (FDCA). Full conversion of HMF and an FDCA yield of &gt;99.8 % are achieved with a faradaic efficiency close to 100 % at a potential of 1.457 V vs. reversible hydrogen electrode. Such activity and selectivity to FDCA are attributed to the fast electron transfer, high electrochemical surface area, and reduced charge transfer resistance. More impressively, remarkable catalyst stability under long-term testing is obtained with 17 catalytic cycles. This work highlights the rational design of metal oxides with ordered meso-structures for electrochemical biomass conversion. © 2021 The Authors. ChemSusChem published by Wiley-VCH GmbH
    view abstractdoi: 10.1002/cssc.202002762
  • 2021 • 25 The effect of laser nitriding on surface characteristics and wear resistance of niti alloy with low power fiber laser
    Wang, H. and Nett, R. and Gurevich, E.L. and Ostendorf, A.
    Applied Sciences (Switzerland) 11 1-10 (2021)
    The laser nitriding was performed in nitrogen gas at room temperature (20◦ C) and low temperature (−190◦ C) by a low power fiber laser to modify the wear and abrasion resistance of NiTi alloy. The surface roughness and element composition were analyzed by roughness device and energy-dispersive X-ray spectroscopy respectively. The results of roughness show that laser treatment can change the surface roughness due to the laser remelting. The effect of laser nitriding on the microhardness, friction coefficient, and worn scars of NiTi alloy was also studied, which shows that the microhardness of the NiTi alloy increases after laser nitriding. The optical microscope and scanning electron microscope were used to characterize the surface of NiTi alloy after wear testing to observe the microstructure of worn scars. The results show that the laser nitriding with different parameters can induce a nitride layer with different thicknesses and the higher energy deposition is the key factor for the formation of the nitride layer, which can decrease the friction coefficient and reduce wear loss during the application of NiTi alloy. The improvement of wear resistance can be attributed to the hard nitriding layer. © 2021 by the authors. Li-censee MDPI, Basel, Switzerland.
    view abstractdoi: 10.3390/app11020515
  • 2021 • 24 Closing the Gap for Electronic Short-Circuiting: Photosystem I Mixed Monolayers Enable Improved Anisotropic Electron Flow in Biophotovoltaic Devices
    Wang, P. and Frank, A. and Zhao, F. and Szczesny, J. and Junqueira, J.R.C. and Zacarias, S. and Ruff, A. and Nowaczyk, M.M. and Pereira, I.A.C. and Rögner, M. and Conzuelo, F. and Schuhmann, W.
    Angewandte Chemie - International Edition 60 2000-2006 (2021)
    Well-defined assemblies of photosynthetic protein complexes are required for an optimal performance of semi-artificial energy conversion devices, capable of providing unidirectional electron flow when light-harvesting proteins are interfaced with electrode surfaces. We present mixed photosystem I (PSI) monolayers constituted of native cyanobacterial PSI trimers in combination with isolated PSI monomers from the same organism. The resulting compact arrangement ensures a high density of photoactive protein complexes per unit area, providing the basis to effectively minimize short-circuiting processes that typically limit the performance of PSI-based bioelectrodes. The PSI film is further interfaced with redox polymers for optimal electron transfer, enabling highly efficient light-induced photocurrent generation. Coupling of the photocathode with a [NiFeSe]-hydrogenase confirms the possibility to realize light-induced H2 evolution. © 2020 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH
    view abstractdoi: 10.1002/anie.202008958
  • 2021 • 23 Influence of low Bi contents on phase transformation properties of VO2studied in a VO2:Bi thin film library
    Wang, X. and Rogalla, D. and Kostka, A. and Ludwig, Al.
    RSC Advances 11 7231-7237 (2021)
    A thin-film materials library in the system V-Bi-O was fabricated by reactive co-sputtering. The composition of Bi relative to V was determined by Rutherford backscattering spectroscopy, ranging from 0.06 to 0.84 at% along the library. The VO2phase M1 was detected by X-ray diffraction over the whole library, however a second phase was observed in the microstructure of films with Bi contents &gt; 0.29 at%. The second phase was determined by electron diffraction to be BiVO4, which suggests that the solubility limit of Bi in VO2is only ∼0.29 at%. For Bi contents from 0.08 to 0.29 at%, the phase transformation temperatures of VO2:Bi increase from 74.7 to 76.4 °C by 8 K per at% Bi. With X-ray photoemission spectroscopy, the oxidation state of Bi was determined to be 3+. The V5+/V4+ratio increases with increasing Bi content from 0.10 to 0.84 at%. The similarly increasing tendency of the V5+/V4+ratio andTcwith Bi content suggests that although the ionic radius of Bi3+is much larger than that of V4+, the charge doping effect and the resulting V5+are more prominent in regulating the phase transformation behavior of Bi-doped VO2 © The Royal Society of Chemistry 2021.
    view abstractdoi: 10.1039/d0ra09654g
  • 2021 • 22 Synthesis and Characterization of Dendritic and Linear Glycol Methacrylates and Their Performance as Marine Antifouling Coatings
    Wanka, R. and Koschitzki, F. and Puzovic, V. and Pahl, T. and Manderfeld, E. and Hunsucker, K.Z. and Swain, G.W. and Rosenhahn, A.
    ACS Applied Materials and Interfaces (2021)
    Dendritic polyglycerol (PG) was covalently coupled to 2-hydroxyethyl methacrylate (HEMA) by an anionically catalyzed ring-opening polymerization generating a dendritic PG-HEMA with four PG repetition units (PG4MA). Coatings of the methacrylate monomer were prepared by grafting-through and compared against commercially available hydrophilic monomers of HEMA, poly(ethylene) glycol methacrylate (PEGMA), and poly(propylene) glycol methacrylate (PPGMA). The obtained coatings were characterized by modern surface analytical techniques, including water contact angle goniometry (sessile and captive bubble), attenuated total internal reflection Fourier transform infrared spectroscopy, and atomic force microscopy. The antifouling (AF) and fouling-release (FR) properties of the coatings were tested against the model organisms Cobetia marina and Navicula perminuta in laboratory-scale dynamic accumulation assays as well as in a dynamic short-term field exposure (DSFE) in the marine environment. In addition, the hydration of the coatings and their susceptibility toward silt uptake were evaluated, revealing a strong correlation between water uptake, silt incorporation, and field assay performance. While all glycol derivatives showed good resistance in laboratory settlement experiments, PPGMA turned out to be less susceptible to silt incorporation and outperformed PEGMA and PG4MA in the DSFE assay. © 2021 American Chemical Society.
    view abstractdoi: 10.1021/acsami.0c21212
  • 2021 • 21 Machine-learning-enhanced time-of-flight mass spectrometry analysis
    Wei, Y. and Varanasi, R.S. and Schwarz, T. and Gomell, L. and Zhao, H. and Larson, D.J. and Sun, B. and Liu, G. and Chen, H. and Raabe, D. and Gault, B.
    Patterns 2 (2021)
    Mass spectrometry is a widespread approach used to work out what the constituents of a material are. Atoms and molecules are removed from the material and collected, and subsequently, a critical step is to infer their correct identities based on patterns formed in their mass-to-charge ratios and relative isotopic abundances. However, this identification step still mainly relies on individual users' expertise, making its standardization challenging, and hindering efficient data processing. Here, we introduce an approach that leverages modern machine learning technique to identify peak patterns in time-of-flight mass spectra within microseconds, outperforming human users without loss of accuracy. Our approach is cross-validated on mass spectra generated from different time-of-flight mass spectrometry (ToF-MS) techniques, offering the ToF-MS community an open-source, intelligent mass spectra analysis. Time-of-flight mass spectrometry (ToF-MS) is a mainstream analytical technique widely used in biology, chemistry, and materials science. ToF-MS provides quantitative compositional analysis with high sensitivity across a wide dynamic range of mass-to-charge ratios. A critical step in ToF-MS is to infer the identity of the detected ions. Here, we introduce a machine-learning-enhanced algorithm to provide a user-independent approach to performing this identification using patterns from the natural isotopic abundances of individual atomic and molecular ions, without human labeling or prior knowledge of composition. Results from several materials and techniques are compared with those obtained by field experts. Our open-source, easy-to-implement, reliable analytic method accelerates this identification process. A wide range of ToF-MS-based applications can benefit from our approach, e.g., hunting for patterns of biomarkers or for contamination on solid surfaces in high-throughput data. A machine-learning application for the accelerated data processing and interpretation of time-of-flight mass spectrometry is presented. The machine learns patterns in a human-label-free manner, making the process easy to implement and the result highly reproducible. © 2020 The Authors
    view abstractdoi: 10.1016/j.patter.2020.100192
  • 2021 • 20 Implementation of formation mechanisms in DEM simulation of the spheronization process of pharmaceutical pellets
    Weis, D. and Grohn, P. and Evers, M. and Thommes, M. and García, E. and Antonyuk, S.
    Powder Technology 378 667-679 (2021)
    In the production process of pharmaceutical pellets with a narrow size distribution and a high sphericity, a combined extrusion-spheronization technique is frequently used. The rounding of the wet cylindrical extrudates in the spheronizer after the extrusion step is influenced by various interfering mechanisms, in particular plastic deformation, breakage, attrition and coalescence. Due to the complexity of these mechanisms which depend on the particle dynamics, there is no sufficient description of the particle rounding process in the spheronizer. In this study, the Discrete Element Method (DEM) which runs on the micro scale is coupled with a Particle Shape Evolution (PSE) model on the macro scale to describe how the particle shape changes due to collisions. For the DEM simulation a new contact model was used which was developed to capture the cyclic, dominant visco plastic deformation behaviour. Based on the DEM collision data, the changing particle shape was described in the PSE model by applying the proposed submodels for the different formation mechanisms. The resulting particle shapes obtained with this simulation framework are in a good agreement with experimental data. © 2020
    view abstractdoi: 10.1016/j.powtec.2020.09.013
  • 2021 • 19 Additive manufacturing of PA12 carbon nanotube composites with a novel laser polymer deposition process
    Wencke, Y.L. and Kutlu, Y. and Seefeldt, M. and Esen, C. and Ostendorf, A. and Luinstra, G.A.
    Journal of Applied Polymer Science 138 (2021)
    The facile manufacture of PA12 MWCNT/silica (50/50 by weight) nanocomposite powders through a high energy mixing process is presented, which are useful to generate 3D objects by a novel Laser Polymer Deposition (LPD) process. The mixing as well as the LPD process led to no discernible changes in the material properties (DSC, SEM, LD) of the core-shell nanocomposites, enabling the recycling of unconverted powder. The built parts yield ultimate tensile stresses and Young's modulus at 10%–20% of the bulk material. Partially unmolten particles and voids were identified as the main mechanical failure mechanism in the built parts. The mechanical properties are better with low additive content (Young's modulus: 89.8 ± 5.4 MPa; UTS: 12.9 ± 5.3 MPa with 0.25 wt% additives). Electronic conductivity up to the region of moderate conductivity could be achieved by multiwalled carbon nanotube (MWCNT) network formation (8 × 10−4 S cm−1 at 1.25 wt% of additives). A variant of the processing strategy revealed that a higher mechanical strength can be achieved by a laser induced remelting of the traces following their initial construction. © 2020 The Authors. Journal of Applied Polymer Science published by Wiley Periodicals LLC.
    view abstractdoi: 10.1002/app.50395
  • 2021 • 18 Amphiphilic poly(arylene ether sulfone) multiblock copolymers with quaternary ammonium groups for novel thin-film composite nanofiltration membranes
    Wieczorek, J. and Ulbricht, M.
    Polymer 217 (2021)
    Amphiphilic poly(arylene ether sulfone) (PAES) multiblock copolymers with quaternary ammonium groups were evaluated as tunable, size-selective barrier material in thin-film composite (TFC) nanofiltration membranes. Using a two-step synthesis, well-defined PAES multiblock copolymers with molecular weight (Mn) of at least 50 kg/mol were obtained. Conversion to anion-exchange polymers was accomplished by block-selective bromination of methyl side groups at adjusted degree of functionalization and subsequent quantitative amination using triethanolamine. A library of copolymers with varied block length ratios and ion-exchange capacities (IEC; up to 2 mmol/g) was obtained. PAES multiblock copolymers with suited hydrophilic/hydrophobic balance to yield films that are stable in water were further evaluated. Film casting of solutions of anion-exchange copolymers on a porous polyacrylonitrile support and solvent evaporation yielded TFC membranes with barrier layer thickness in the range of 1.5–1.9 μm. Nanofiltration performance was measured with glycerine, glucose, sucrose, NaCl, MgCl2 and FeCl3 in water. While for a random copolymer with similar composition and same thickness, no water flux could be measured, the novel TFC membranes had permeances in the range of 1 L m−2 bar−1·h−1, at &gt;99.9% rejection for glucose. Permeance increased and rejection (for glycerine and salt) decreased systematically with increasing IEC; an additional influence of block length ratio was identified. A membrane made from a block copolymer with longer hydrophobic block and moderate IEC of 0.9 mmol/g showed the best “trade-off” between permeability and selectivity. Furthermore, the stability of the novel membranes under oxidative disinfection conditions was demonstrated. © 2021 Elsevier Ltd
    view abstractdoi: 10.1016/j.polymer.2021.123446
  • 2021 • 17 Numerical simulation of the viral entry into a cell driven by receptor diffusion
    Wiegold, T. and Klinge, S. and Gilbert, R.P. and Holzapfel, G.A.
    Computers and Mathematics with Applications 84 224-243 (2021)
    The present study focuses on the receptor driven endocytosis typical of viral entry into a cell. A locally increased density of receptors at the time of contact between the cell and the virus is necessary in this case. The virus is considered as a substrate with fixed receptors on its surface, whereas the receptors of the host cell are free to move over its membrane, allowing a local change in their concentration. In the contact zone the membrane inflects and forms an envelope around the virus. The created vesicle imports its cargo into the cell. This paper assumes the diffusion equation accompanied by boundary conditions requiring the conservation of binders to describe the process. Moreover, it introduces a condition defining the energy balance at the front of the adhesion zone. The latter yields the upper limit for the size of virus which can be engulfed by the cell membrane. The described moving boundary problem in terms of the binder density and the velocity of the adhesion front is well posed and numerically solved by using the finite difference method. The illustrative examples have been chosen to show the influence of the process parameters on the initiation and the duration of the process. © 2021 Elsevier Ltd
    view abstractdoi: 10.1016/j.camwa.2020.12.012
  • 2021 • 16 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
  • 2021 • 15 Determining interface fracture toughness in multi layered environmental barrier coatings with laser textured silicon bond coat
    Wolf, M. and Kakisawa, H. and Süß, F. and Mack, D.E. and Vaßen, R.
    Coatings 11 1-14 (2021)
    In the high temperature combustion atmosphere inside of aircraft turbines, the currently used ceramic matrix composites require a protective environmental barrier coating (EBC) to mitigate corrosion of the turbine parts. Besides thermomechanical and thermochemical properties like matching thermal expansion coefficient (CTE) and a high resistance against corrosive media, mechanical properties like a high adhesion strength are also necessary for a long lifetime of the EBC. In the present work, the adhesion between an air plasma sprayed silicon bond coat and a vacuum plasma sprayed ytterbium disilicate topcoat was aimed to be enhanced by a laser surface structuring of the Si bond coat. An increase in interface toughness was assumed, since the introduction of structures would lead to an increased mechanical interlocking at the rougher bond coat interface. The interface toughness was measured by a new testing method, which allows the testing of specific interfaces. The results demonstrate a clear increase of the toughness from an original bond coat/topcoat interface (8.6 J/m2) compared to a laser structured interface (14.7 J/m2). Observations in the crack propagation indicates that the laser structuring may have led to a strengthening of the upper bond coat area by sintering. Furthermore, in addition to cohesive failure components, adhesive components can also be observed, which could have influenced the determined toughness. © 2021 by the authors.
    view abstractdoi: 10.3390/coatings11010055
  • 2021 • 14 Combinatorial exploration of B2/L21 precipitation strengthened AlCrFeNiTi compositionally complex alloys
    Wolff-Goodrich, S. and Marshal, A. and Pradeep, K.G. and Dehm, G. and Schneider, J.M. and Liebscher, C.H.
    Journal of Alloys and Compounds 853 (2021)
    Using both novel high-throughput screening via combinatorial thin film deposition and conventional bulk alloy synthesis techniques, a large region of the AlCrFeNiTi composition space has been probed for alloys that could serve as low cost alternatives to nickel-base superalloys for medium-to-high temperature structural applications. Phase formation trends in this highly complex alloying system have been determined using characterisation techniques that span multiple length scales—from bulk X-ray diffraction and differential scanning calorimetry to atomically resolved scanning transmission electron microscopy and energy dispersive X-ray spectroscopy. A large region of stability for both disordered A2 and ordered B2/L21 type phases is observed, with several compositions exhibiting fine-scaled precipitation structures of these two phases. For alloys with ≥20 at.% Al, the precipitation structure was further refined to a nano-scale lamellar arrangement of A2 and B2/L21 phases. Formation of C14 Laves phase, especially for compositions with &gt;10 at.% Ti, has consistently been observed. We include a screening of the mechanical properties based on nanoindentation and macroscopic hardness test data correlated with scanning electron microscope (SEM) observations of the hardness indents. The phase formation trends observed by both combinatorial thin film deposition and bulk alloy synthesis are discussed in detail for samples in the as-deposited and as-cast conditions, respectively. © 2020 Elsevier Ltd
    view abstractdoi: 10.1016/j.jallcom.2020.156111
  • 2021 • 13 Solubility of DNP-amino acids and their partitioning in biodegradable ATPS: Experimental and ePC-SAFT modeling
    Wysoczanska, K. and Nierhauve, B. and Sadowski, G. and Macedo, E.A. and Held, C.
    Fluid Phase Equilibria 527 (2021)
    Predicting the behavior of dinitrophenylated amino acids (DNP-AA) in aqueous solutions requires an understanding and accurate description of interactions that can occur in such systems. In this work, some properties of DNP-AA (DNP-glycine, DNP-alanine, DNP-valine, DNP-leucine) have been determined experimentally. These were liquid densities obtained at T=298.15 – 318.15 K, p=1 bar, and pH-dependent solubility data measured at T=298.15 K, p=1 bar. It was observed that the solubility order for DNP-AA does not follow the same sequence as for aliphatic amino acids. The thermodynamic model ePC-SAFT has been applied to predict the properties density and solubility, and additionally to estimate partition coefficients of DNP-AA in PEG (PEG 4000, PEG 6000, PEG 8000) - organic salt (sodium citrate, potassium citrate, potassium sodium tartrate) aqueous two-phase systems (ATPS). ePC-SAFT pure-component and binary interaction parameters for neutral DNP-AA were acquired using the joinzt joint-parameter method, namely by combining the parameters for dinitrobenzene with parameters for amino acids (glycine, L-alanine, L-valine, L-leucine) from literature. The pure-component parameters for charged DNP-AA- were inherited from their parent neutral DNP-AA. This work shows that ePC-SAFT allows predicting liquid densities and solubilities of neutral DNP-AA with good agreement to experimental results. Moreover, adjusting in sum six binary parameters between charged DNP-AA- and phase-forming species allowed modeling partition coefficients of four DNP-AA in nine different PEG – organic salt ATPS, each at four different ATPS compositions. This can be considered an excellent modeling result and proofs the suitability of ePC-SAFT for such systems. © 2020 Elsevier B.V.
    view abstractdoi: 10.1016/j.fluid.2020.112830
  • 2021 • 12 Gene transfection achieved by utilizing antibacterial calcium phosphate nanoparticles for enhanced regenerative therapy
    Xiang, C. and Tenkumo, T. and Ogawa, T. and Kanda, Y. and Nakamura, K. and Shirato, M. and Sokolova, V. and Epple, M. and Kamano, Y. and Egusa, H. and Sasaki, K.
    Acta Biomaterialia 119 375-389 (2021)
    Protamine-coated multi-shell calcium phosphate (CaP) was developed as a non-viral vector for tissue regeneration therapy. CaP nanoparticles loaded with different amounts of plasmid DNA encoding bone morphogenetic protein 2 (BMP-2) and insulin-like growth factor 1 (IGF-1) were used to treat MC3T3E1 cells, and the yield of the released BMP-2 or IGF-1 was measured using ELISA 3 days later. Collagen scaffolds containing CaP nanoparticles were implanted into rat cranial bone defects, and BMP-2 and IGF-1 yields, bone formation, and bone mineral density enhancement were evaluated 28 days after gene transfer. The antibacterial effects of CaP nanoparticles against Streptococcus mutans and Aggregatibacter actinomycetemcomitans increased with an increase in the protamine dose, while they were lower for Staphylococcus aureus and Porphyromonas gingivalis. In the combination treatment with BMP-2 and IGF-1, the concentration ratio of BMP-2 and IGF-1 is an important factor affecting bone formation activity. The calcification activity and OCN mRNA of MC3T3E1 cells subjected to a BMP-2:IGF-1 concentration ratio of 1:4 was higher at 14 days. During gene transfection treatment, BMP-2 and IGF-1 were released simultaneously after gene transfer; the loaded dose of the plasmid DNA encoding IGF-1 did not impact the BMP-2 or IGF-1 yield or new bone formation ratio in vitro and in vivo. In conclusion, two growth factor-releasing systems were developed using an antibacterial gene transfer vector, and the relationship between the loaded plasmid DNA dose and resultant growth factor yield was determined in vitro and in vivo. © 2020
    view abstractdoi: 10.1016/j.actbio.2020.11.003
  • 2021 • 11 Development of Fluorescent Chemosensors for Amino-sugars
    Yadav, R. and Kwamen, C. and Niemeyer, J.
    Israel Journal of Chemistry (2021)
    In this account, we describe the application of a series of multidentate BINOL-based phosphoric acids for the fluorescence-based chemosensing of amino sugars. To this end, we developed a novel synthetic protocol for three isomerically pure phosphoric acid monoesters of the type ArOP(O)(OH)2. These were investigated with respect to their binding towards amino sugars (glucosamine, galactosamine and mannosamine) in comparison to the previously reported diesters of the type (ArO)2P(O)(OH). We could find that the diesters show no significant binding, while the monoesters can be used as hosts for amino sugars. Indeed, one host selectively binds to galactosamine, while a second host binds both glucosamine and mannosamine. This allows the fluorescence-based detection and discrimination of the amino sugars by a novel class of phosphate-based supramolecular hosts. © 2021 The Authors. Israel Journal of Chemistry published by Wiley-VCH GmbH
    view abstractdoi: 10.1002/ijch.202000104
  • 2021 • 10 Dynamical properties of a driven dissipative dimerized S= 12 chain
    Yarmohammadi, M. and Meyer, C. and Fauseweh, B. and Normand, B. and Uhrig, G.S.
    Physical Review B 103 (2021)
    We consider the dynamical properties of a gapped quantum spin system coupled to the electric field of a laser, which drives the resonant excitation of specific phonon modes that modulate the magnetic interactions. We deduce the quantum master equations governing the time-evolution of both the lattice and spin sectors, by developing a Lindblad formalism with bath operators providing an explicit description of their respective phonon-mediated damping terms. We investigate the nonequilibrium steady states (NESS) of the spin system established by a continuous driving, delineating parameter regimes in driving frequency, damping, and spin-phonon coupling for the establishment of physically meaningful NESS and their related nontrivial properties. Focusing on the regime of generic weak spin-phonon coupling, we characterize the NESS by their frequency and wave-vector content, explore their transient and relaxation behavior, and discuss the energy flow, the system temperature, and the critical role of the type of bath adopted. Our study lays a foundation for the quantitative modeling of experiments currently being designed to control coherent many-body spin states in quantum magnetic materials. © 2021 American Physical Society.
    view abstractdoi: 10.1103/PhysRevB.103.045132
  • 2021 • 9 Magnetoelectric Tuning of Pinning-Type Permanent Magnets through Atomic-Scale Engineering of Grain Boundaries
    Ye, X. and Yan, F. and Schäfer, L. and Wang, D. and Geßwein, H. and Wang, W. and Chellali, M.R. and Stephenson, L.T. and Skokov, K. and Gutfleisch, O. and Raabe, D. and Hahn, H. and Gault, B. and Kruk, R.
    Advanced Materials 33 (2021)
    Pinning-type magnets with high coercivity at high temperatures are at the core of thriving clean-energy technologies. Among these, Sm2Co17-based magnets are excellent candidates owing to their high-temperature stability. However, despite intensive efforts to optimize the intragranular microstructure, the coercivity currently only reaches 20–30% of the theoretical limits. Here, the roles of the grain-interior nanostructure and the grain boundaries in controlling coercivity are disentangled by an emerging magnetoelectric approach. Through hydrogen charging/discharging by applying voltages of only ≈1 V, the coercivity is reversibly tuned by an unprecedented value of ≈1.3 T. In situ magneto-structural characterization and atomic-scale tracking of hydrogen atoms reveal that the segregation of hydrogen atoms at the grain boundaries, rather than the change of the crystal structure, dominates the reversible and substantial change of coercivity. Hydrogen reduces the local magnetocrystalline anisotropy and facilitates the magnetization reversal starting from the grain boundaries. This study opens a way to achieve the giant magnetoelectric effect in permanent magnets by engineering grain boundaries with hydrogen atoms. Furthermore, it reveals the so far neglected critical role of grain boundaries in the conventional magnetization-switching paradigm of pinning-type magnets, suggesting a critical reconsideration of engineering strategies to overcome the coercivity limits. © 2020 The Authors. Advanced Materials published by Wiley-VCH GmbH
    view abstractdoi: 10.1002/adma.202006853
  • 2021 • 8 Tunable eg Orbital Occupancy in Heusler Compounds for Oxygen Evolution Reaction**
    Yu, M. and Li, G. and Fu, C. and Liu, E. and Manna, K. and Budiyanto, E. and Yang, Q. and Felser, C. and Tüysüz, H.
    Angewandte Chemie - International Edition 60 5800-5805 (2021)
    Heusler compounds have potential in electrocatalysis because of their mechanical robustness, metallic conductivity, and wide tunability in the electronic structure and element compositions. This study reports the first application of Co2YZ-type Heusler compounds as electrocatalysts for the oxygen evolution reaction (OER). A range of Co2YZ crystals was synthesized through the arc-melting method and the eg orbital filling of Co was precisely regulated by varying Y and Z sites of the compound. A correlation between the eg orbital filling of reactive Co sites and OER activity was found for Co2MnZ compounds (Z=Ti, Al, V, and Ga), whereby higher catalytic current was achieved for eg orbital filling approaching unity. A similar trend of eg orbital filling on the reactivity of cobalt sites was also observed for other Heusler compounds (Co2VZ, Z=Sn and Ga). This work demonstrates proof of concept in the application of Heusler compounds as a new class of OER electrocatalysts, and the influence of the manipulation of the spin orbitals on their catalytic performance. © 2020 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH
    view abstractdoi: 10.1002/anie.202013610
  • 2021 • 7 From volatility to solubility: Thermodynamics of imidazolium-based ionic liquids containing chloride and bromide anions
    Zaitsau, D.H. and Siewert, R. and Pimerzin, A.A. and Bülow, M. and Held, C. and Loor, M. and Schulz, S. and Verevkin, S.P.
    Journal of Molecular Liquids 323 (2021)
    Ionic liquids (ILs) are effectively used for tuning the composition and the morphology of nanoparticles or stabilizing agents for nanoparticles for catalytic dehydrogenation. Thermodynamic properties of ionic liquids, e.g. vapor pressures and vaporization enthalpies help optimise these processes. Vapor pressures and vaporization enthalpies of the series of 1-alkyl-3-methylimidazolium ionic liquids with chloride and bromide anions have been measured by using quartz-crystal microbalance (QCM). Possible thermal decomposition pathways of [C2C1Im][Br] during vaporization were analyzed by using high-level quantum-chemical methods. These theoretical results explained and supported the absence of decomposition in QCM experimental conditions. According to the measured vapor pressures the [CnC1Im][Cl] and [CnC1Im][Br] series are very suitable for catalytic applications, taking also into account their sufficient thermal stability at the level of 523–543 K. Solubility parameters of ILs and practically relevant solutes were assessed with help of experimental vaporization enthalpies. © 2020 Elsevier B.V.
    view abstractdoi: 10.1016/j.molliq.2020.114998
  • 2021 • 6 Tracer diffusion in the σ phase of the CoCrFeMnNi system
    Zhang, J. and Muralikrishna, G.M. and Asabre, A. and Kalchev, Y. and Müller, J. and Butz, B. and Hilke, S. and Rösner, H. and Laplanche, G. and Divinski, S.V. and Wilde, G.
    Acta Materialia 203 (2021)
    A single Cr-rich σ-phase alloy with a composition of Co17Cr46Fe16.3Mn15.2Ni5.5 (at.%) and a tetragonal lattice structure was produced. The tracer diffusion coefficients of Ni and Fe were measured by secondary electron mass spectroscopy using the highly enriched 64Ni and 58Fe natural isotopes. On the homologous temperature scale, Ni and Fe diffuse in the σ phase faster as compared to the corresponding diffusion rates in the equiatomic and face-centered cubic CoCrFeMnNi alloy. In contrast, on the absolute temperature scale, these elements diffuse roughly at the same rates in both materials. Factors influencing element diffusion and phase stability of the σ phase compared to the equiatomic alloy are discussed. © 2020 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2020.116498
  • 2021 • 5 Tensile creep properties of a CrMnFeCoNi high-entropy alloy
    Zhang, M. and George, E.P. and Gibeling, J.C.
    Scripta Materialia 194 (2021)
    Tensile creep tests were performed on a CrMnFeCoNi high-entropy alloy at temperatures from 1023 K to 1173 K. A uniform stress exponent 3.7 ± 0.1 was found across all temperatures. The apparent activation energies of creep under various applied stresses were determined to be around 230 kJ/mol and decrease with increasing stress, indicating a stress-assisted, thermally activated behavior. Steady-state creep microstructures feature no subgrain formation and high dislocation density within grains. Based on our results, the creep rate of CrMnFeCoNi is believed to be controlled by both dislocation-dislocation interactions and dislocation-lattice interactions. © 2020
    view abstractdoi: 10.1016/j.scriptamat.2020.113633
  • 2021 • 4 Depth-sensing ductile and brittle deformation in 3C-SiC under Berkovich nanoindentation
    Zhao, L. and Zhang, J. and Pfetzing, J. and Alam, M. and Hartmaier, A.
    Materials and Design 197 (2021)
    The interplay between ductile and brittle deformation modes in hard brittle materials exhibits a strong size effect. In the present work, indentation depth-dependent deformation mechanisms of single-crystal 3C-SiC under Berkovich nanoindentation are elucidated by finite element simulations and corresponding experiments. A novel finite element framework, that combines a crystal plasticity constitutive model for describing dislocation slip-based ductile deformation and a cohesive zone model for capturing crack initiation and propagation-induced brittle fracture, is established. The utilized parameters in the crystal plasticity model of 3C-SiC are calibrated according to the load-displacement curves obtained from corresponding Berkovich nanoindentation experiments. Subsequent finite element simulations and experiments of nanoindentation jointly reveal co-existing microscopic plastic deformation and brittle fracture of 3C-SiC at different indentation depths, which significantly affect the observed macroscopic mechanical response and surface pile-up topography. In particular, the predicted morphology of surface cracks at an indentation depth of 500 nm agrees well with experimental observation, and the correlation of crack initiation and propagation with surface pile-up topography is theoretically analyzed. © 2020 The Authors
    view abstractdoi: 10.1016/j.matdes.2020.109223
  • 2021 • 3 Atomistic investigation of machinability of monocrystalline 3C–SiC in elliptical vibration-assisted diamond cutting
    Zhao, L. and Zhang, J. and Zhang, J. and Hartmaier, A.
    Ceramics International 47 2358-2366 (2021)
    Deformation-induced characteristics of surface layer strongly rely on loading condition-related operating deformation modes. In the current study we reveal the mechanisms governing machined surface formation of hard brittle monocrystalline 3C–SiC in ultrasonic elliptical vibration-assisted diamond cutting by molecular dynamics simulations. Simulation results show different deformation modes including phase transformation, dislocation activity, and crack nucleation and propagation, as well as their correlations with surface integrity in terms of machined surface morphology and subsurface damage. In particular, molecular dynamics simulations of ordinary cutting are also carried out, which demonstrate the effectiveness of applying ultrasonic vibration of cutting tool in decreasing machining force and suppressing crack events, i.e., promoting ductile-mode cutting for achieving high surface integrity. The physical mechanism governing the machining differences between the two machining processes are also revealed. Furthermore, the effect of cutting depth on machined surface integrity under vibration-assisted cutting and ordinary cutting is addressed. © 2020 Elsevier Ltd and Techna Group S.r.l.
    view abstractdoi: 10.1016/j.ceramint.2020.09.078
  • 2021 • 2 A fully automated approach to calculate the melting temperature of elemental crystals
    Zhu, L.-F. and Janssen, J. and Ishibashi, S. and Körmann, F. and Grabowski, B. and Neugebauer, J.
    Computational Materials Science 187 (2021)
    The interface method is a well established approach for predicting melting points of materials using interatomic potentials. However, applying the interface method is tedious and involves significant human intervention. The whole procedure involves several successive tasks: estimate a rough melting point, set up the interface structure, run molecular dynamic calculations and analyze the data. Loop calculations are necessary if the predicted melting point is different from the estimated one by more than a certain convergence criterion, or if full melting/solidification occurs. In this case monitoring the solid–liquid phase transition in the interface structure becomes critical. As different initial random seeds for the molecular dynamic simulations within the interface method induce slightly different melting points, a few ten or hundred interface method calculations with different random seeds are necessary for performing a statistical analysis on these melting points. Considering all these technical details, the work load for manually executing and combining the various involved scripts and programs quickly becomes prohibitive. To simplify and automatize the whole procedure, we have implemented the interface method into pyiron ( Our fully automatized procedure allows to efficiently and precisely predict melting points of stable unaries represented by arbitrary potentials with only two user-specified parameters (interatomic potential file and element). For metastable or dynamically unstable unary phases, the crystal structure needs to be provided as an additional parameter. We have applied our automatized approach on fcc Al, Ni, dynamically unstable bcc Ti and hcp Mg and employed a large set of available interatomic potentials. Melting points for classical interatomic potentials of these metals have been obtained with a numerical precision well below 1 K. © 2020 The Authors
    view abstractdoi: 10.1016/j.commatsci.2020.110065
  • 2021 • 1 On the size effect of additives in amorphous shape memory polymers
    Zirdehi, E.M. and Dumlu, H. and Eggeler, G. and Varnik, F.
    Materials 14 1-16 (2021)
    Small additive molecules often enhance structural relaxation in polymers. We explore this effect in a thermoplastic shape memory polymer via molecular dynamics simulations. The additiveto-monomer size ratio is shown to play a key role here. While the effect of additive-concentration on the rate of shape recovery is found to be monotonic in the investigated range, a non-monotonic dependence on the size-ratio emerges at temperatures close to the glass transition. This work thus identifies the additives’ size to be a qualitatively novel parameter for controlling the recovery process in polymer-based shape memory materials. © 2021 by the authors. Licensee MDPI, Basel, Switzerland.
    view abstractdoi: 10.3390/ma14020327