Werkstoffkunde

Anthropogenic climate change is one of the biggest problems facing society today, and counteracting it requires replacing the fossil fuel-based infrastructure with renewable energy alternatives, such as green hydrogen. Photoelectrochemical cells (PECs) offer a sustainable way to perform solar water splitting, and thus to convert energy provided by the sun into chemical form through the generation of hydrogen. A main component of PECs are photoelectrodes, which are manufactured with semiconducting materials in order to absorb light, with generation of charge carriers, which contribute to drive the photoelectrochemical reactions at the interface to the electrolyte. The structure of photoelectrodes at the nanoscale determines their performance for light absorption, carrier generation, and carrier transport and transfer. Nanofabrication methods are a group of technologies that enable the construction of interfaces and devices with nanometer precision. In this thesis, nanofabrication methods, in particular atomic layer deposition (ALD), are explored for the manufacture of individual components for photoelectrodes. The resulting photoelectrode components are investigated in terms of performance as well as stability, which are the main challenges towards large scale implementation of PEC technology. Stability has the most significant impact on the longevity and hence industrial applicability of photoelectrochemical cells. A significant part of this work focuses on the synthesis and investigation of thin coatings to protect photoelectrodes from corrosion. Thin films of TiO2 were deposited by atomic layer deposition, and the influence of deposition conditions on crystallinity, and on optoelectronic and photoelectrochemical properties were investigated. The effect of film structure on stability under different photoelectrochemical operating conditions was directly quantified by ex-situ spectroscopic ellipsometry. An additional significant component of modern photoelectrodes are layers for selective charge transport, usually transparent conducting oxides, which help extract the generated charge carriers from the photoabsorber. In this work, a series of transparent conductive oxides (ZnO:Al, ZnO:Hf, ZnO:Ti) with different doping atoms at various doping concentrations were obtained by atomic layer deposition using supercycles, and their optoelectronic properties were characterized, compared, and optimized with respect to resistivity. A further step towards large-scale deployment of PECs requires the implementation of materials capable of optimally using the solar spectrum. In this work, selected earth-abundant photoabsorbers (Fe2O3, CuO, Cu2O) with suitable band gaps for solar water splitting, were synthesized by conformal and uniform atomic layer deposition, and approaches were developed towards the possible first time synthesis of the promising photoabsorber CuFeO2 by the atomic layer deposition. Finally, using the nanofabrication methods of direct write lithography and reactive ion etching, different reproducible geometric structures with varying aspect ratios and high specific surface areas were created in square and hexagonal arrangements in silicon wafers, in order to improve the studied absorption properties and photocurrent densities of deposited photoabsorbers by orthogonalization of the light absorption and charge separation processes.

In this thesis, the development of sustainable materials for hydrogen storage is studied. Four model types of hydrogen storage materials are included in this work, which are the complex metal hydride NaAlH₄, solid solution room temperature FeTi alloys, compositionally complex alloys (CCAs), and the reactive hydrides composite (RHC) 2NaBH₄ + MgH₂. The first part of this thesis focuses on developing a complex metal hydride of NaAlH₄ by using waste Al alloy as raw material. The synthesised less pure NaAlH₄ exhibits good reversible hydrogen capacity, whereas the pure NaAlH₄ is not reversible. The second part of this thesis is devoted to developing a FeTi-based metal hydride by using waste steel and Ti alloy scraps as raw materials. Astonishingly, at 50 °C and 100 bar of H₂, the hydrogen storage capacities measured for the FeTi alloys synthesised from recycled scraps are extremely close to the value measured for the pure FeTi. In the third part of this thesis, the hydrogen storage properties of some CCAs are investigated, which further helps in screening the suitable types of waste metal alloys to be used as raw materials. With the synthesised hydrogen storage materials, the main issues are kinetic and thermodynamic tuning. The selected waste steels, Ti alloys, Mg alloys, and Al alloys, are just examples of the vast variety of scrap materials potentially useful for synthesising sustainable hydrogen storage materials. In addition, the influence of single impurities cannot be distinguished. Therefore, the RHC system of 2NaBH₄ + MgH₂ was chosen as a model system to study the effects of well-defined additives on hydrogen storage performances, as can be seen in the last part of this thesis. This work shows that by using waste metal scraps instead of high-purity elements as raw materials, the carbon footprint and costs are tremendously reduced for producing hydrogen storage materials without deteriorating the hydrogen storage properties. This work opens a new path to the development of environmentally sustainable alloys for hydrogen storage purposes.

In recent years, cold spraying (CS) has emerged as a promising technology for repair applications, particularly for oxidation-sensitive materials. In order to obtain an optimum repair result that fulfills the highest requirements regarding material properties, simple geometric shape restoration is not sufficient. Any additive manufacturing process results in particular features in microstructure, possible defects and respective – potentially even anisotropic – mechanical properties. To systematically tailor these microstructures and properties to the specific component and geometry requires complex routines. This work proposes the design of a knowledge-based cold spray repair system that facilitates a complete individual repair procedure for aircraft components. This system includes the elements of (i) reverse engineering to analyze, classify and generate digital data of the damaged component, (ii) pre-processing to obtain the ideal conditions for the CS process, (iii) toolpath planning to optimize robotics for the CS process, (iv) on-line monitoring to ensure process quality, (v) post-processing and (vi) performance testing of the material properties to meet the challenging requirements of the aerospace industry. By using an industrial robot and computer-aided planning of the trajectories, components are to be repaired under cold spray and geometrical conditions for ideal material deposition. The goal is to obtain repaired components that fulfill the required property profile equally well as respective new parts.

© 2021, The Author(s). Up to now, the role of particle sizes on the impact behavior of ceramic particles in aerosol deposition not yet fully understood. Hence, with the aim to supply a more general understanding, modeling series of low strain rate compression and high-speed impact were performed by molecular dynamics on single-crystalline particles in sizes of 10-300 nm that are tuned to match mechanical properties of TiO2-anatase. The modeling results reveal that particles with original diameter of 25-75 nm exhibit three different impact behaviors that could be distinguished as (i) rebounding, (ii) bonding and (iii) fragmentation, depending on their initial impact velocity. In contrast, particles larger than 75 nm do not exhibit the bonding behavior. Detailed stress and strain field distributions reveal that combination of “localized inelastic deformation” along the slip systems and “shear localization” cause bonding of the small and large particles to the substrate. The analyses of associated temperature rise by the inelastic deformation revealed that heat diffusion at these small scales depend on size. Whereas small particles could reach a rather homogeneous temperature distribution, the evolved heat in the larger ones keeps rather localized to areas of highest deformation and may support deformation and the formation of dense layers in aerosol deposition.

Cold spray (CS) technology has shown many advantages over the traditional thermal spray methods, such as lower deposition temperature, higher deposition efficiency and lower coating porosity. The inherent deficiencies such as the oxidation, decomposition, phase transformation and grain growth in the nano structured materials associated with fusion of materials can be thus avoided, which allow the deposition of oxidation sensitive materials, thermal sensitive materials and phase transformation sensitive materials. It is worth noting that as the development of cold spray systems it is used as one solid state rapid manufacturing method to realize the industrial applications of metal additive manufacturing and repairing the damaged aerospace component and parts. Due to the fact that cold spray has been increasingly attracted more and more attention from domestic researchers and industries, a detailed summary and discussion on the advancement of cold spray technology and its system based on the literature survey and a long-term tracking of domestic and foreign CS systems were presented. Firstly, the main components for a conventional cold spray system, the sprayed materials and the microstructures-properties of the corresponding deposits were briefly described. Then, the systems and applications of the traditional cold spray technologies, such as high pressure cold spray, low pressure cold spray and vacuum cold spray were discussed. In addition, the novel cold spray technologies developed based on the spray gun design or spray nozzle design (such as the inner diameter cold spray, the pulsed gas dynamic spray, the shock tunnel produced cold spray and the radial supersonic spray), and the hybrid processes of other techniques assisting cold spray (such as, the laser assisted cold spray, the electrostatic force assisted cold spray, the magnetic assisted cold spray and the friction stir processing assisted cold spray additive manufacturing) were analyzed. Finally, the existed key problems of cold spray technologies and its system were pointed out, and the prospect of its material research and industrial application was given.