Zeitschriftenaufsätze und Buchbeiträge

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Li, Feitao; Wang, Dong; Klingenhof, Malte; Flock, Dominik; Wang, Honglei; Strasser, Peter; Schaaf, Peter;
Controllable Si oxidation mediated by annealing temperature and atmosphere. - In: Journal of materials science, ISSN 1573-4803, Bd. 57 (2022), 24, S. 10943-10952

The morphology evolution by thermal annealing induced dewetting of gold (Au) thin films on silicon (Si) substrates with a native oxide layer and its dependences on annealing temperature and atmosphere are investigated. Both dewetting degree of thin film and Au/Si interdiffusion extent are enhanced with the annealing temperature. Au/Si interdiffusion can be observed beyond 800 ˚C and Au-Si droplets form in both argon and oxygen (Ar + O2) and argon and hydrogen (Ar + H2) environments. In Ar + O2 case, the passive oxidation (Si + O2 &flech; SiO2) of diffused Si happens and thick silicon oxide (SiOx) covering layers are formed. A high temperature of 1050 ˚C can even activate the outward growth of free-standing SiOx nanowires from droplets. Similarly, annealing at 800 ˚C under Ar + H2 situation also enables the slight Si passive oxidation, resulting in the formation of stripe-like SiOx areas. However, higher temperatures of 950-1050 ˚C in Ar + H2 environment initiate both the SiOx decomposition and the Si active oxidation (2Si + O2 &flech; 2SiO(g)), and the formation of solid SiOx is absent, leading to the only formation of isolated Au-Si droplets at elevated temperatures and droplets evolve to particles presenting two contrasts due to the Au/Si phase separation upon cooling.

Biele, Lukas; Schaaf, Peter; Schmid, Florian;
Influence of contact pressure on the specific electrical contact resistance of copper. - In: IEEE transactions on components, packaging and manufacturing technology, ISSN 2156-3985, Bd. 12 (2022), 6, S. 973-980

The specific electrical contact resistance (SECR) [ Ωm2 ] of Cu-ETP (CW004A) contacts is characterized in dependence of contact pressure up to high pressures close to the tensile strength of the base material. Two different material states are considered, R200 (soft) and R300. Static four-wire measurements are carried out on a special experimental setup, which favors homogeneous contact states during mechanical loading. A finite-element simulation is utilized for interpretation and evaluation of the measurements, including the deformation of the samples during the test. The results show that the SECR of the harder material state R300 is higher than that of the softer material R200. The developed data show a decrease of SECR with increasing contact pressure. For contact pressures up to 0.5 of the tensile strength σt of the base material, the SECR decreases according to a power law. For contact pressures above 0.6 σt , the decrease shifts toward a linear behavior. In addition, the measurements show that the SECR vanishes at contact pressures in region of the tensile strength of the base material.

Hartmann, Robert; Puch, Florian;
Numerical simulation of the deformation behavior of softwood tracheids for the calculation of the mechanical properties of wood-polymer composites. - In: Polymers, ISSN 2073-4360, Bd. 14 (2022), 13, 2574, S. 1-25

From a fiber composite point of view, an elongated softwood particle is a composite consisting of several thousand tracheids, which can be described as fiber wound hollow profiles. By knowing their deformation behavior, the deformation behavior of the wood particle can be described. Therefore, a numerical approach for RVE- and FEM-based modelling of the radial and tangential compression behavior of pine wood tracheids under room climate environment is presented and validated with optical and laser-optical image analysis as well as tensile and compression tests on pine sapwood veneer strips. According to the findings, at 23 ˚C and 12% moisture content, at least 10 MPa must be applied for maximum compaction of the earlywood tracheids. The latewood tracheids can withstand at least 100 MPa compression pressure and would deform elastically at this load by about 20%. The developed model can be adapted for other wood species and climatic conditions by adjusting the mechanical properties of the base materials of the cell wall single layers (cellulose, hemicellulose, lignin), the dimensions and the structure of the vessel elements, respectively.

Schulz, Thomas; Knauer, Andrea; Schaaf, Peter; Töpfer, Jörg;
Tuning of high-temperature dielectric properties in the system (Bi0.5Na0.5)TiO3-BaTiO3-CaZrO3. - In: Ceramics international, ISSN 0272-8842, Bd. 48 (2022), 15, S. 22228-22236

Solid solutions of the (1-x)(0.94Bi0.5Na0.5TiO3-0.06BaTiO3)-xCaZrO3 system are regarded as promising dielectrics for high-temperature capacitors as they exhibit a remarkable flat trend of the permittivity over a large temperature range coupled with comparable low dielectric losses. In this work, the composition 0.8(0.94Bi0.5Na0.5TiO3-0.06BaTiO3)-0.2CaZrO3 was chosen in an attempt to optimize especially the high-temperature dielectric properties above 200 &ring;C. In particular, the influence of excess bismuth to account for element losses caused by evaporation, and the effect of manganese as acceptor dopant are reported. Conventional solid-state reaction route was used to synthesize selected compositions. X-ray diffraction was used to confirm a pseudo-cubic perovskite main phase in all examined compositions, although small traces of a zirconia secondary phase were also detected. All samples exhibit an expected flat trend of the relative permittivity with a maximum deviation of the permittivity lower than 15% between -80 &ring;C and 300 &ring;C. The unmodified base composition shows small dielectric loss (<2%) between -55 &ring;C and 265 &ring;C. By using small quantities of manganese doping, the small-loss temperature range was extended (-70 &ring;C and 300 &ring;C). Excess bismuth also affects the temperature-dependent dielectric losses, resulting in a narrowed temperature range, eventually limiting the application possibilities.

Kurtash, Vladislav; Thiele, Sebastian; Mathew, Sobin; Jacobs, Heiko O.; Pezoldt, Jörg;
Designing MoS2 channel properties for analog memory in neuromorphic applications. - In: Journal of vacuum science & technology, ISSN 2166-2754, Bd. 40 (2022), 3, S. 030602-1-030602-5

In this paper, we introduce analog nonvolatile random access memory cells for neuromorphic computing. The analog memory cell MoS2 channel is designed based on the simulation model including Fowler-Nordheim tunneling through a charge-trapping stack, trapping process, and transfer characteristics to describe a full write/read circle. 2D channel materials provide scaling to higher densities as well as preeminent modulation of the conductance by the accumulated space charge from the oxide trapping layer. In this paper, the main parameters affecting the distribution of memory states and their total number are considered. The dependence of memory state distribution on channel doping concentration and the number of layers is given. In addition, how the nonlinearity of memory state distribution can be overcome by variation of operating conditions and by applying pulse width modulation to the bottom gate voltage is also shown.

Lambri, Osvaldo Agustin; Weidenfeller, Bernd; Bonifacich, Federico Guillermo; Mohr-Weidenfeller, Laura; Lambri, F. D.; Xu, Jiayi; Zelada, Griselda I.; Endres, Frank;
Study of the damping behaviour in samples consisting of iron electro-deposited on copper in an ionic liquid. - In: Journal of alloys and compounds, Bd. 918 (2022), 165462

Copper-iron alloys were produced at room temperature by means of electrodeposition of iron on a copper substrate in an ionic liquid (1-butyl-1-methylpyrrolidinium trifluoromethylsulfonate [Py1,4]TfO). Samples with different electrodeposition times were studied using mechanical spectroscopy, scanning electron microscopy, light microscopy and magnetic loops techniques. Independent of the electrodeposition time the electrodeposition process leads to the promotion of a thin layer of iron onto the copper surface without iron diffusion into the substrate. The damping spectra for electrosposited samples in the as-electrodeposited state show the characteristic low and intermediate grain boundary damping peaks from copper as well as the solvent grain boundary damping peak from the electrodeposited iron. Thermal annealing at temperatures near 973 K leads to the appearance of Fe particles at the interface between the copper and iron (Cu + α-Fe phase) leading to a new damping peak at around 680 K whose driving force is the diffusion of copper atoms around the second phase particles.

Zhang, Xuyan; Cheng, Pengfei; Song, Weiming; Rong, Shiya; Huang, Jieming; Wang, Dong; Schaaf, Peter; Zhou, Guofu; Zhang, Zhang; Liu, Junmin;
Photo-thermoelectric conversion and photo-induced thermal imaging using 2D/3D ReS2carbon framework with enhanced photon harvesting. - In: The chemical engineering journal, ISSN 1873-3212, Bd. 446 (2022), 137084

Solar energy is a promising renewable energy with the potential for the sustainable development of the world. Efficient photo-thermal conversion is essential for harvesting and conversion of solar energy, therefore, the main challenge is the development of efficient and low-cost photothermal conversion materials. Carbon framework can be considered as a candidate but somehow its application potential can be still constrained due to the limited absorption of near-infrared (NIR) light. Herein, we propose a general strategy for preparing two-dimensional (2D) transition metal dichalcogenides nanosheets and three-dimensional (3D) carbon framework composites (2D/3D ReS2C) as a photothermal material, which has an excellent broadband light absorption performance (in the wavelength range from 200 to 2500 nm). A small thermoelectric (TE) module with an area of 4 × 4 cm2 is integrated with annealed ReS2@C as a light absorber for the investigation of photo-thermoelectric conversion. The open-circuit voltage of the assembled device increases clearly under solar illumination and reaches the maximum value of 136.3 mV, which is ∼ five times larger than that without the absorber. In addition, 20 TE modules coated with ReS2@C absorber layers are connected in series, which can produce a maximum open-circuit voltage of 2.12 V (∼66.25 V/m2) to light up a red light-emitting diode (LED) under natural sunlight. Moreover, the annealed ReS2@C powder demonstrates a rapid and strong photothermal response under NIR light (wavelength >800 nm), which indicates a great application potential in photothermal imaging and photothermal cancer therapy.

Hofmann, Martin; Holz, Mathias; Plank, Harald; Strehle, Steffen;
Localized direct material removal and deposition by nanoscale field emission scanning probes. - In: Micro and nano engineering, ISSN 2590-0072, Bd. 16 (2022), 100146, S. 1-5

The manufactory of advanced micro- and nanoscale devices relies on capable patterning strategies. Focused electron beams, as for instance implemented since long in electron beam lithography and electron beam induced deposition, are in this regard key enabling tools especially at the early stages of device development and research. We show here that nanoscale field emission scanning probes can be potentially utilized as well for a prospective direct device fabrication by localized material deposition but notably, also by localized material removal. Field emission scanning probe processing was specifically realized on 10 nm chromium and 50 nm gold thin film stacks deposited on a (1 × 1) cm2 fused silica substrate. Localized material deposition and metal removal was studied in various atmospheres comprising high vacuum, nitrogen, ambient air, naphthalene and carbon-dioxide. Stable and reliable regimes were in particular obtained in a carbonaceous atmosphere. Hence, localized carbon deposits were obtained but also localized metal removal was realized. We demonstrate furthermore that the selected electron emission parameters (20 V - 80 V, 180 pA) and the overall operation environment are crucial aspects that determine the degree of material deposition and removal. Based on our findings, direct tip-based micro- to nanoscale material patterning appears possible. The applied energy regime is also enabling new insights into low energy (< 100 eV) electron interaction. However, the underlying mechanisms must be further elucidated.

Peipmann, Ralf; Bund, Andreas; Schmidt, Udo;
Simulation verschiedener Hull-Zellen-Geometrien, Teil 2 - Erweiterung der Simulationsmodelle mittels 3D. - In: Galvanotechnik, ISSN 0016-4232, Bd. 113 (2022), 5, S. 575-585

Kovácsovics, Iris; Patzelt, Thomas; Herrmann, Johannes; Bund, Andreas;
Reverse numerical simulation of kinetic parameters from acidic copper Hull cell deposition. - In: Journal of the Electrochemical Society, ISSN 1945-7111, Bd. 169 (2022), 5, 052501

Hull cell depositions are industrially used to monitor electrolytes and study the influence of additives. By combining the Hull cell deposition and a numerical simulation based on the boundary element method via a curve-fitting approach allows to obtain kinetic parameters (e.g. transfer coefficient, exchange current density) and assessing the effects of additives without losing the visual information and the opportunity to get the structural and physical properties of the metal deposition (reverse determination). In an acidic copper electrolyte, an additive based on polyethylene glycol decreases the effective exchange current density, by up to two orders of magnitude, while the transfer coefficient is hardly influenced. By adding another additive based on bis-(3-sulfopropyl)disulphide, the effect is counteracted and increases in dependence on the ratio of both additives. The combined approach enables obtaining more information about visual and structural effects and the deposition kinetics from one experimental analysis.