Dissertations from 2018 to 2021

Anzahl der Treffer: 622
Erstellt: Tue, 17 May 2022 23:03:26 +0200 in 0.0502 sec

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.

Wang, Qi; Cheng, Xing; Sun, Yukun; Sun, Zaicheng; Wang, Dong; Chen, Ge; Schaaf, Peter;
A synergetic effect between photogenerated carriers and photothermally enhanced electrochemical urea-assisted hydrogen generation on the Ni-NiO/Nickel Foam catalyst. - In: Materials advances, ISSN 2633-5409, Bd. 2 (2021), 6, S. 2104-2111

The urea-assisted water electrolysis reactions are of great significance for solving the increasingly serious energy crisis and environmental pollution. Recently, the photo-driven effect strategy has been demonstrated to be an efficient external driving force for improving electrocatalytic activities. Herein, we synthesized Ni-NiO heterostructured nanosheet arrays grown on Ni foam (denoted as Ni-NiO/NF) as a bifunctional electrocatalyst enhancing the urea oxidation reaction (UOR) and hydrogen evolution reaction (HER) activities simultaneously under light irradiation. Moreover, when the catalyst is used in a two-electrode system for the urea-assisted water electrolysis reaction, the cell potential could be reduced to 1.48 V to achieve the current density of 10 mA cm-2 after exposure to light irradiation, as well as remarkable stability. Our studies demonstrate that the enhancement of the HER & UOR activities is attributed to a synergetic effect between photogenerated carriers and photothermy.

Cheng, Pengfei; Kampmann, Ronald; Wang, Dong; Sinzinger, Stefan; Schaaf, Peter;
Tailoring patterned visible-light scattering by silicon photonic crystals. - In: ACS applied materials & interfaces, ISSN 1944-8252, Bd. 13 (2021), 50, S. 60319-60326

Searching for the relationship between the nanostructure and optical properties has always been exciting the researchers in the field of optics (linear optics as well as non-linear optics), energy harvesting (anti-reflective Si solar cells, perovskite solar cells, ..., etc.), and industry (anti-reflection coating on car windows, sunglasses, etc.). In this work, we present an approach for nanostructuring the silicon substrate to silicon photonic crystals. By precisely controlling the etching time and etching path after using nanoimprint lithography, ordered arrays of inverted Si nanopyramids and Si nanopillars with good homogeneity, uniform surface roughness, high reproducibility of pattern transfer, and a controllable aspect ratio are prepared. Experimental investigation of the optical properties indicates that the reflections of these Si nanostructures are mainly determined by the aspect ratio as well as the period of nanostructures. Furthermore, we have experimentally observed visible-light scattering (V-LS) patterns on inverted Si nanopyramids and Si nanopillars, and their corresponding patterns can be precisely controlled by the patterned nanostructures. The V-LS pattern, background, and "ghost peaks" on the angle-resolved scattering results are caused by constructive interference, destructive interference, and the interference situation between both. This controllable nanopatterning on crystalline Si substrates with precisely tunable optical properties shows great potential for applications in many fields, for example, optics, electronics, and energy.

Schulz, Alexander; Bartsch, Heike; Gutzeit, Nam; Matthes, Sebastian; Glaser, Marcus; Ruh, Andreas; Müller, Jens; Schaaf, Peter; Bergmann, Jean Pierre; Wiese, Steffen;
Characterization of reactive multilayer systems deposited on LTCC featuring different surface morphologies. - In: MikroSystemTechnik Kongress 2021, (2021), S. 506-510

Baloochi, Mostafa; Shekhawat, Deepshikha; Riegler, Sascha Sebastian; Matthes, Sebastian; Glaser, Marcus; Schaaf, Peter; Bergmann, Jean Pierre; Gallino, Isabella; Pezoldt, Jörg;
Influence of initial temperature and convective heat loss on the self-propagating reaction in Al/Ni multilayer foils. - In: Materials, ISSN 1996-1944, Bd. 14 (2021), 24, 7815, insges. 15 S.

A two-dimensional numerical model for self-propagating reactions in Al/Ni multilayer foils was developed. It was used to study thermal properties, convective heat loss, and the effect of initial temperature on the self-propagating reaction in Al/Ni multilayer foils. For model adjustments by experimental results, these Al/Ni multilayer foils were fabricated by the magnetron sputtering technique with a 1:1 atomic ratio. Heat of reaction of the fabricated foils was determined employing Differential Scanning Calorimetry (DSC). Self-propagating reaction was initiated by an electrical spark on the surface of the foils. The movement of the reaction front was recorded with a high-speed camera. Activation energy is fitted with these velocity data from the high-speed camera to adjust the numerical model. Calculated reaction front temperature of the self-propagating reaction was compared with the temperature obtained by time-resolved pyrometer measurements. X-ray diffraction results confirmed that all reactants reacted and formed a B2 NiAl phase. Finally, it is predicted that (1) increasing thermal conductivity of the final product increases the reaction front velocity; (2) effect of heat convection losses on reaction characteristics is insignificant, e.g., the foils can maintain their characteristics in water; and (3) with increasing initial temperature of the foils, the reaction front velocity and the reaction temperature increased.

Wu, Xuping; Chen, Honglei; Luo, Xuemei; Wang, Dong; Schaaf, Peter; Zhang, Guangping;
Ultrasensitive strain sensors based on Cu-Al alloy films with voided cluster boundaries. - In: Advanced materials technologies, ISSN 2365-709X, Bd. 6 (2021), 12, 2100524, S. 1-12