Publication list of the Group of Materials in Electrical Engineering

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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.
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.
Grieseler, Rolf; Gallino, Isabella; Duboiskaya, Natallia; Döll, Joachim; Shekhawat, Deepshikha; Reiprich, Johannes; Guerra, Jorge A.; Hopfeld, Marcus; Honig, Hauke L.; Schaaf, Peter; Pezoldt, Jörg;
Silicon carbide formation in reactive silicon-carbon multilayers. - In: Materials science forum, ISSN 1662-9752, Bd. 1062 (2022), S. 44-48

An alternative low thermal budget silicon carbide syntheses route is presented. The method is based on self-propagating high-temperature synthesis of binary silicon-carbon-based reactive mul­tilayers. With this technique, it is possible to obtain cubic polycrystalline silicon carbide at relatively low annealing temperatures by a solid state reaction. The reaction starts above 600 &ring;C. The transformation process proceeds in a four-step process. The reaction enthalpy was determined to be (-70 ± 4) kJ/mol.
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.
Cheng, Pengfei; Ziegler, Mario; Ripka, Valentin; Wang, Honglei; Pollok, Kilian; Langenhorst, Falko; Wang, Dong; Schaaf, Peter;
Black silver: three-dimensional Ag hybrid plasmonic nanostructures with strong photon coupling for scalable photothermoelectric power generation. - In: ACS applied materials & interfaces, ISSN 1944-8252, Bd. 14 (2022), 14, S. 16894-16900

The conversion of solar energy into electric power has been extensively studied, for example, by photovoltaics. However, photo-thermoelectric (P-TE) conversion as an effective solar-to-electricity conversion process is less studied. Here, we present an efficient full-solar-spectrum plasmonic absorber for scalable P-TE conversion based on a simple integration of light absorber and commercial thermoelectric modules. Our developed light absorber of silica-silver hybrid structures achieves an average absorption of 99.4% in the wavelength range from 200 to 2500 nm, which covers over 98% solar energy in this range. It thus appears fully matte black and is named black silver. The light absorber includes a hierarchical structure with Ag nanoparticles attached on three-dimensional SiO2 nanostructures, resulting in ultrahigh absorption. Strong localized surface plasmon resonance hybridization together with multiple scattering causes the perfect light absorption. Using the black silver as a light absorber for P-TE power generation, it can achieve a peak voltage density as high as 82.5 V m-2 under a solar intensity of 100 mW cm-2, which is large enough to power numerous electronic devices. By assembling 20 thermoelectric modules in series, we test their possibility of practical application, and they can also achieve an average voltage density of 70.66 V m-2. Our work opens up a promising technology that facilitates high-efficiency and scalable solar energy conversion via the P-TE effect.
Muñoz-Piña, Sandra; Alcaide, A. M.; Limones-Ahijón, Blanca; Oliva Ramírez, Manuel; Rico, Victor; Alcalá, German; González, Maria U.; García-Martín, Jose M.; Alvarez, Rafael; Wang, Dong; Schaaf, Peter; González-Elipe, Agustin R.; Palmero, Alberto;
Thin film nanostructuring at oblique angles by substrate patterning. - In: Surface and coatings technology, ISSN 0257-8972, Bd. 436 (2022), 128293, S. 1-12

It is demonstrated that, besides classical nanocolumnar arrays, the oblique angle geometry induces the growth of singular structures in the nanoscale when using wisely designed patterned substrates. Well-ordered array of crosses, cylindrical nanorods or hole structures arranged in square or hexagonal regular geometries are reported as examples, among others. The fundamental framework connecting substrate topography and film growth at oblique angles is presented, allowing the use of substrate patterning as a feasible thin film nanostructuring technique. A systematic analysis of the growth of TiO2 thin films on 4 different lithographic patterned substrates in 4 different scale lengths is also presented. A first conclusion is the existence of a height-based selective growth in the initial stages of the deposition, by which the film preferentially develops on top of the tallest substrate features. This behavior is maintained until the film reaches a critical thickness, the so-called Oblivion Thickness, above which the film topography becomes gradually independent of the substrate features. A general formula relating the spatial features of the pattern, the coarsening exponent and the Oblivion Thickness has been deduced.
Smyrnova, Kateryna; Sahul, Martin; Haršáni, Marián; Pogrebnjak, Aleksandr Dmitrievič; Ivashchenko, Volodymyr; Beresnev, Vyacheslav; Stolbovoy, Vyacheslav; Čaplovič, &hacek;Lubomír; Čaplovičová, Mária; Vančo, &hacek;Lubomír; Kusý, Martin; Kassymbaev, Alexey; Satrapinskyy, Leonid; Flock, Dominik;
Microstructure, mechanical and tribological properties of advanced layered WN/MeN (Me = Zr, Cr, Mo, Nb) nanocomposite coatings. - In: Nanomaterials, ISSN 2079-4991, Bd. 12 (2022), 3, 395, S. 1-23

Due to the increased demands for drilling and cutting tools working at extreme machining conditions, protective coatings are extensively utilized to prolong the tool life and eliminate the need for lubricants. The present work reports on the effect of a second MeN (Me = Zr, Cr, Mo, Nb) layer in WN-based nanocomposite multilayers on microstructure, phase composition, and mechanical and tribological properties. The WN/MoN multilayers have not been studied yet, and cathodic-arc physical vapor deposition (CA-PVD) has been used to fabricate studied coating systems for the first time. Moreover, first-principles calculations were performed to gain more insight into the properties of deposited multilayers. Two types of coating microstructure with different kinds of lattices were observed: (i) face-centered cubic (fcc) on fcc-W2N (WN/CrN and WN/ZrN) and (ii) a combination of hexagonal and fcc on fcc-W2N (WN/MoN and WN/NbN). Among the four studied systems, the WN/NbN had superior properties: the lowest specific wear rate (1.7 × 10^-6 mm^3/Nm) and high hardness (36 GPa) and plasticity index H/E (0.93). Low surface roughness, high elastic strain to failure, Nb2O5 and WO3 tribofilms forming during sliding, ductile behavior of NbN, and nanocomposite structure contributed to high tribological performance. The results indicated the suitability of WN/NbN as a protective coating operating in challenging conditions.
Li, Feitao; Oliva Ramírez, Manuel; Wang, Dong; Schaaf, Peter;
Effect of SiO2 interlayer thickness in Au/SiO2/Si multilayer systems on Si sources and the formation of Au-based nanostructures. - In: Advanced materials interfaces, ISSN 2196-7350, Bd. 9 (2022), 2, 2101493, insges. 9 S.

Si sources involved in the growth of Au-SiOx nanostructures are investigated through the rapid thermal annealing of gold thin films on SiO2/Si substrates with various SiO2 layer thicknesses (3, 25, 100, 500 nm) in a reducing atmosphere. This method reveals three Si sources whose involvement depends on the thickness of the SiO2 layers, i.e., Si diffusion from the substrate, and SiO from SiO2 decomposition and from Si active oxidation. Increasing thicknesses of the SiO2 layer hampers the Si diffusion and the decomposition of regions of the SiO2 layer, which decreases the concentrations of discovered regions weakening the Si active oxidation. These discovered regions appear in systems with a SiO2 layer of 25 or 100 nm, while they are absent for a 500 nm layer. Furthermore, Au-SiOx nanostructures of different shapes form in each system. Both behaviors indicate that the influence and transport mechanisms of the different Si sources are largely dependent on the thicknesses of the SiO2 layers and that they control the evolution of the Au-SiOx nanostructures. A clear understanding of the relationship between these thicknesses and the possible Si sources and their roles in the evolution of the nanostructures makes the tailored fabrication of nanostructures possible.
Bohm, Sebastian; Grunert, Malte; Honig, Hauke; Wang, Dong; Schaaf, Peter; Runge, Erich; Zhong, Jinhui; Lienau, Christoph;
Optical properties of nanoporous gold sponges using model structures obtained from three-dimensional phase-field Simulation. - In: IEEE Xplore digital library, ISSN 2473-2001, (2021), S. 517-523

Nanoporous sponge structures show fascinating optical properties related to a strong spatial localization of field modes and a resulting strong field enhancement. In this work, a novel efficient method for the generation of three-dimensional nanoporous sponge structures using time-resolved phase-field simulations is presented. The algorithm for creating the geometries and the underlying equations are discussed. Different sponge geometries are generated and compared with sponges that have been experimentally measured using FIB tomography. Meaningful parameters are defined for the comparison of the geometric properties of the random sponge structures. In addition, the optical properties of the simulated sponges are compared with the experimentally measured sponges. It is shown that a description using effective media does not provide a good agreement to the actual spectra. This shows that the optical properties are largely determined by the local structures. In contrast, the numerically obtained spectra of the phase-field sponge models accounting for the real-space structure show excellent agreement with the spectra of the experimentally measured sponges.