Journal articles and book contributions

Hard nitride coatings are commonly employed to protect components subjected to friction, whereby such coatings should possess excellent tribomechanical properties in order to endure high stresses and temperatures. In this study, WN/NbN coatings are synthesized by using the cathodic-arc evaporation (CA-PVD) technique at various negative bias voltages in the 50-200 V range. The phase composition, microstructural features, and tribomechanical properties of the multilayers are comprehensively studied. Fabricated coatings have a complex structure of three nanocrystalline phases: β-W2N, δ-NbN, and ε-NbN. They demonstrate a tendency for (111)-oriented grains to overgrow (200)-oriented grains with increasing coating thickness. All of the data show that a decrease in the fraction of ε-NbN phase and formation of the (111)-textured grains positively impact mechanical properties and wear behavior. Investigation of the room-temperature tribological properties reveals that with an increase in bias voltage from −50 to −200 V, the wear mechanisms change as follows: oxidative &flech; fatigue and oxidative &flech; adhesive and oxidative. Furthermore, WN/NbN coatings demonstrate a high hardness of 33.6-36.6 GPa and a low specific wear rate of (1.9-4.1) × 10-6 mm3/Nm. These results indicate that synthesized multilayers hold promise for tribological applications as wear-resistant coatings.

In order to address energy and environmental challenges effectively, there is a need to promote renewable energy-driven electrochemical conversion technologies, particularly electrosynthesis. Electrosynthesis has the potential to convert abundant molecules into valuable chemicals and fuels. However, the widespread adoption of electrosynthesis is often hindered by the slow oxygen evolution reaction (OER). To overcome this limitation, we can employ the more efficient alcohol electrooxidation reaction (AOR), utilizing renewable biomass-derived alcohols as an alternative to OER for producing high-value chemicals. Consequently, the development of efficient AOR catalysts, in conjunction with cathodic reduction reactions (hydrogen evolution, oxygen, and nitrogen electroreduction, etc.), is crucial for sustainable and environmentally-friendly advancements. A thorough understanding of AOR mechanisms is essential for catalyst design and can be achieved through the utilization of in situ characterization techniques and density functional theory (DFT) calculations. This review summarizes recent progress in AOR catalysts, with a particular focus on the electrooxidation of monohydric alcohols, polyols, and associated studies on reaction mechanisms. Additionally, the review identifies key factors impeding AOR development and provides insights into future prospects.

The influence of intentionally externally induced cracks in reactive Al/Ni multilayer systems is investigated. These cracks affect the reaction dynamics and enable tailoring of the reaction path and the overall velocity of the reaction front. The influence of layer variations onto mechanical crack formation and resulting reaction behavior are investigated. High-speed camera imaging shows the meandering propagation of the reaction front along the crack paths. Therefore, the mechanical cracking process significantly changes the total velocity of the reaction front and thus offers a possibility to control the self-propagating high-temperature synthesis process. It is shown that the phase formation remains unaffected despite the applied strains and cracks. This favorable stability in phase formation ensures predictability and provides insight into the adaptation of RMS for precision applications in joints. The results expand the understanding of mechanical cracking as a tool to influence high-temperature synthesis in reactive multilayer coatings and provide an opportunity to expand the range of applications.

Progressive developments in the field of lightweight construction and engineering demand continuous substitution of metals with suitable polymers. However, the combination of dissimilar materials results in a multitude of challenges based on different chemical and physical material properties. Reactive multilayer systems offer a promising joining method for flexible and low-distortion joining of dissimilar joining partners with an energy source introduced directly into the joining zone. Within this publication, hybrid lap joints between semi-crystalline polyamide 6 and surface-structured austenitic steel X5CrNi18-10 (EN 1.4301) were joined using reactive Al/Ni multilayer foils of the type Indium-NanoFoil®. Main objective is to examine possibilities of influencing crack initiation in the foil plane by variation of joining pressure and different metal surface structures with regard to geometry, density and orientation. Thus, the position of foil cracks is superimposed onto the metal structure and associated filling with molten plastic is improved. Consequently, characterisation of occurring crack positions as function of joining pressure and metal structure, analysis of the composite in terms of structural filling and joint strength as well as possible causes of crack initiation are evaluated.

Artificial leaves could be the breakthrough technology to overcome the limitations of storage and mobility through the synthesis of chemical fuels from sunlight, which will be an essential component of a sustainable future energy system. However, the realization of efficient solar-driven artificial leaf structures requires integrated specialized materials such as semiconductor absorbers, catalysts, interfacial passivation, and contact layers. To date, no competitive system has emerged due to a lack of scientific understanding, knowledge-based design rules, and scalable engineering strategies. Here, we will discuss competitive artificial leaf devices for water splitting, focusing on multi-absorber structures to achieve solar-to-hydrogen conversion efficiencies exceeding 15%. A key challenge is integrating photovoltaic and electrochemical functionalities in a single device. Additionally, optimal electrocatalysts for intermittent operation at photocurrent densities of 10-20 mA cm^-2 must be immobilized on the absorbers with specifically designed interfacial passivation and contact layers, so-called buried junctions. This minimizes voltage and current losses and prevents corrosive side reactions. Key challenges include understanding elementary steps, identifying suitable materials, and developing synthesis and processing techniques for all integrated components. This is crucial for efficient, robust, and scalable devices. Here, we discuss and report on corresponding research efforts to produce green hydrogen with unassisted solar-driven (photo-)electrochemical devices. This article is protected by copyright. All rights reserved.