Werkstoffkunde

In this thesis, atomistic modeling and simulation methods are thoroughly employed to understand and explore the mechanism of oxidation and subsequent hydrogenation of the TiFe intermetallic compound. Parallel to the material investigations, the computational methodology has also advanced with the incorporation of machine-learning techniques. TiFe is a commercially utilized stationary hydrogen storage material with attractive properties for practical applications. The formation of oxide-films on the TiFe surface is a major concern limiting its wide application. The first chapter of this work constructs and characterizes the oxygen-exposed TiFe surface with the use of global-optimization techniques and ab-initio methods. The energetics and dynamics of early-oxidation of the material surface is studied at different temperatures and the activation barriers for hydrogen penetration through the oxide layers are evaluated with a focus on the activation mechanisms. Structure fingerprint tools employed are able to identify metal oxide structural motifs consistent with experimental findings. This work also produces valuable mechanistic insights for the activation of TiFe. Progressively, the influence of transition metal dopants, V, Cr, Mn and Ni, substituting Fe in TiFe is investigated with a focus on their relative preference to oxidize and their involvement in the surface oxidation. Their suitability to promote hydrogenation is evaluated and characterized by assessing the change in surface chemistry and through a comparison with the pristine TiFe surface. Accurate atomistic simulations are burdened by heavy computational cost and limited to small systems. Hence, in the last chapter of this thesis, an extensive machine-learning interatomic model with ab-initio accuracy is developed for Ti-Fe-H system utilizing the first-principles data generated throughout this work. This deep neural network model provides upto 1000 times faster computation and scales linearly with the size of the system bridging the gap between ab-initio and large-scale atomistic simulations.

Diese Arbeit beschäftigt sich mit der Herstellung von keramischen Schichten mittels kinetischer Spritzverfahren. Als Modellsystem wurden die MAX Phasen Ti₃SiC₂, Ti₂AlC und Cr₂AlC, die sowohl keramische als auch metallische Eigenschaften aufweisen, in Partikelgrößen im Bereich von einigen zehn Mikrometern mittels Kaltgasspritzen aufgetragen. Hierfür wurden die Einflüsse von primären Spritzparametern wie Gastemperatur und -druck, sowie von sekundären Spritzparametern wie Substrattemperatur, Traversengeschwindigkeit und Phasenreinheit des Spritzpulvers untersucht, um so Informationen über die Voraussetzungen für die Schichtbildung zu gewinnen. Die MAX Phasen Ti₃SiC₂ und Cr₂AlC wurden zusätzlich in Partikelgrößen von wenigen Mikrometern mittels Aerosolspritzen abgeschieden, um so generelle Einflüsse der Partikelgröße zu analysieren. Im Hinblick auf das rein keramische Verhalten und mögliche Anwendungspotenziale in der solaren Wasserstoffproduktion wurden des Weiteren die Halbleiter BiVO₄ und Fe₂O₃ mittels Aerosolspritzen als Schicht aufgetragen. Die Untersuchungen zur Ableitung von Randbedingungen für einen erfolgreichen Schichtaufbau umfassen dabei die Einflüsse der Partikelgröße, unterschiedlicher Gasarten, Spritzparameter und Substrattemperaturen.

Interstitial metal-hydrides can reversibly store sustainable energy in the form of hydrogen. Among these materials, the FeTi alloy has the advantage of operating under near-ambient temperature and pressure conditions, as well as exhibiting a generally lower cost of the raw materials compared with other intermetallics in the same class. These properties are associated with a high volumetric hydrogen storage capacity that surpasses even the storage of hydrogen in its molecular form. This translates into an economical advantage because the energy-intensive processes of pressurizing to extreme pressure levels or cooling to cryogenic temperatures are avoided. Although this material has been extensively studied with experimental methods, the characterized properties of alloying elements, structural transformations, chemical stability of the phases, mechanical properties, macroscopic thermodynamics, just to mention a few, have not yet been systematically described in a multiscale model capable of yielding these properties and their mechanisms in a comprehensive integrated manner. To establish a foundation for developing this digital twin, the hydrogenation process of the interstitial intermetallic FeTi metal-hydride is investigated computationally across various hierarchical levels. Three distinct theoretical approaches are utilized with the intention of integrating them into a comprehensive model that can quantitatively address questions in materials science based on precise properties across different material scales. At the atomistic level, the properties of the FeTi-H system are studied with quantum mechanics within a high-throughput approach to analyze the equilibrium and non-equilibrium structures and their thermochemical and micromechanical properties. Computational thermodynamics is subsequently employed to wrap up these properties and integrate the chemical bulk equilibrium of the material in a macroscopic dependence with the external temperature and pressure engineering conditions. The atomistic and thermodynamic models are finally integrated into a thermokinetic mesoscale model capable of simulating the evolution of the properties when the material is subject to many different manipulated engineering conditions. To guarantee successful integration of the different scale properties, the spatial microstructural evolution should yield good agreement with the lower-level properties. The last part of this thesis work thus demonstrate with simulations that these properties have been well integrated, utmostly providing a basis of a comprehensive multiscale computational model for FeTi hydrogenation. The computational modeling of hydrogen storage in FeTi hydrides allows us to anticipate the evolution of the materials properties during their application, helping to develop new processes. In this context, this thesis work established a quantitatively integrated multiphysical multiscale model for simulation of the evolving hydrogenation phenomenon of the FeTi alloy. Ultimately, paving the path to the expansion of the model into a FeTi-based multi-component and multiphase mesoscale model.

In this work, 1.25 t of AB₂-commercially available hydride-forming alloy is taken as a case study for material selection for large-scale systems. Systematic experimental characterizations, modeling, and life cycle-cost assessment at this industrial scale are performed. Based on the thermodynamic characterization, the equilibrium pressure is calculated by applying the most used Nishizaki and novel 3D representation with 2D-bilinear interpolation approaches, giving accurate values. The kinetic model is comprehensively and successfully developed in a wide range of temperatures and pressures by applying the separable variable method. Life cycle assessment shows that the CO₂ emissions of these kinds of systems can be minimized by increasing the share of recycled material and by using waste heat sources for dehydrogenation. The economic analysis clarifies the influence of the components on the economic viability of large hydride-based systems for emergency power supply. Finally, guidelines are proposed for the development of hydride-based integrated renewable energy systems.

In metal hydride beds (MHBs), reaction heat transfer often limits the dynamic performance. Heat transfer within the MHB usually involves solid and gas phases. To account for both, an effective thermal conductivity (ETC) is defined. Measuring and predicting the ETC of metal hydride beds is of primary importance when designing hydride-based systems for high dynamics. This review paper presents an integral overview of the experimental and modeling approaches to characterize the ETC in MHBs. The most relevant methods for measuring the ETC of metal hydride beds are described, and the results and scopes are shown. A comprehensive description of the models applied to calculate the ETC of the MHBs under different conditions is developed. Moreover, the effects of operation parameters such as P, T, and composition on the ETC of the presented models are analyzed. Finally, a summary and conclusions about experimental techniques, a historical overview with a classification of the ETC models, a discussion about the needed parameters, and a comparison between ETC experimental and calculated results are provided.

Das Kaltgasspritzen entwickelt sich zu einem Verfahren mit großem Potenzial für die Reparatur metallischer Bauteile, insbesondere für das Aufbringen von hitze- und oxidationsempfindlichen Materialien. In diesem Zusammenhang ermöglicht der Einsatz von Automatisierung und Robotik eine flexible Steuerung des Reparaturprozesses. Um einen optimalen Reparaturprozess zu gewährleisten, müssen die verschiedenen Anforderungen des robotergeführten Kaltgasspritzens bereits in der simulativen Planungsphase berücksichtigt werden. Herkömmliche Trajektorien zum Materialauftrag berücksichtigen jedoch oft nicht die bei Reparaturen zu beachtenden geometrischen Randbedingungen des Materialaufbaus, den effizienten Materialeinsatz und die zugrundeliegenden Einschränkungen der Roboterkinematik. In dieser Arbeit wird daher ein Konzept zur automatisierten Trajektorienplanung und anschließenden Trajektorienoptimierung zur Reparatur durch robotergeführtes Kaltgasspritzen beschrieben. Das Ziel ist es, eine optimierte Trajektorie zu erzeugen, die die Anforderungen des Kaltgasspritzens und der Roboterkinematik berücksichtigt, um eine qualitativ hochwertige Reparatur und einen effizienten Materialeinsatz zu gewährleisten. Dazu gehören die Minimierung des überschüssigen Materials und die Minimierung des Rucks bei der Roboterbewegung. Die Ergebnisse zeigen die erfolgreiche Anwendung der initialen Trajektorienplanung und der anschließenden Trajektorienoptimierung für die Bauteilreparatur durch Kaltgasspritzen.

With the integration of renewable energy production into grids, hydrogen storage is an effective solution for coping with the fluctuating nature of the resources and reliably providing energy demands. Metal hydride storage is seen as a key technology due to its low operating pressure and temperatures near ambient, while it has a significant volumetric capacity (for room temperature hydrides: 50-110 kg/m³) compared to pressurized (40 kg/m³ under 700 bar and room temperature) or even liquified hydrogen (70 kg/m³ at – 253 ºC and 1 bar). One potential application with metal hydride storage lies in the flexibilization of residential energy demand. Excess photovoltaic generation from a house can power an electrolyser to produce hydrogen, which is then stored in the metal hydride storage. When power and heat are needed in the building, the hydrogen is released into a fuel cell. This case study investigates the dispatch optimization of a metal hydride storage system within a residential household energy system. The interaction of the electrolyser, metal hydride storage, and fuel cell, all components of a container solution called Smart Energy Transform Unit, was studied during summer and winter. Results show that in an exemplary period in winter, from 21 December 2021 to 28 December 2021, the total electricity demand is 98% covered by supply from the grid due to the low photovoltaic generation, which also yields a low hydrogen production; the total heat demand is 90% covered by the heat pump and the thermal storage as a buffer. During an exemplary period in summer, from 20 June 2021 to 27 June 2021, the system is self-sufficient, as hydrogen was stored during the day due to the high yield of photovoltaic generation, and hydrogen is used in a fuel cell at night to provide energy demands. In addition, heat pump operation during summer is small due to the heat provided by the electrolyser, the fuel cell, and the thermal buffer storage. The PV system, together with the Smart Energy Transform Unit, covers 99% of the total electric demand during this period in summer, while for the total heat demand, a coverage of 85% is observed, and the heat pump covers 15%.

The Digi-HyPro (Digitalized Hydrogen Process Chain for the Energy Transition) project's conceptual development of the SET-Unit investigates and facilitates the connection between the electric, gas, and mobility grid. This application report describes the experimental design of the Smart Energy Transition unit (SET-Unit), contemplating the bottom-up and top-down approaches. For the bottom-up approach, the design of core devices such as metal hydride-based hydrogen storage (MHS) and compressor (MHC) systems are shown. The gas separation system (GSS) concept is based on a hybrid process composed of membrane and pressure swing adsorption (PSA) for the gas grid coupling. Commercial anion exchange membrane electrolyzer (AEM-EL) and polymer exchange membrane fuel cell (PEM-FC) are assembled for the power grid connection. For the top-down approach, the first experimental SET-Unit composed of AEM-EL–MHS–PEM-FC in the nominal power range between 5 and 10 kWel and its control strategy for the optimal hydrogen and heat coupling is presented. All experimental development is carried out in the facilities of the Helmholtz-Zentrum Hereon in the frame of a cooperation agreement with the Helmut Schmidt University/University of the Federal Armed Forces.

Effective hydrogen storage is vital for the widespread adoption of hydrogen in energy systems, as it enables flexibility across various sectors. However, assessing such energy storage systems' suitability in future energy system configurations presents several challenges. One such challenge is the identification of representative operational scenarios for experimental testing of storage systems. Against this background, this paper presents an approach to derive such operational scenarios with the help of energy system modelling and optimization. Using the open-source energy system model and data set of Europe, PyPSA-Eur, cost-optimal future energy system configurations are identified, allowing the derivation of operational scenarios for energy storage facilities from the operation of the overall energy system. For this purpose, the methodology provides a way to identify a representative storage system from the entirety of corresponding storages in the energy system. Further, it allows determining representative time series sections using a segment identification algorithm, providing a basis for experimental technology testing. For an exemplary application of this methodology, further post-processing is implemented to consider the feasibility limits of subsystem components. The results showcase the effectiveness of the approach, offering a transparent and reproducible framework for defining operational scenarios for storage testing aligned with future energy system requirements.

Material deposition in cold spraying occurs in solid state and thus avoids undesired effects of melting and solidification. However, residual stress conditions in cold sprayed coatings could limit possible part performance. The temperature distribution and thermal history of the cold sprayed components has significant influence on stress distribution and thus deposition and part quality. The present study investigates the effect of substrate material and nozzle traverse speed (as a secondary parameter) on effective temperatures and residual stress distributions of titanium-grade 1 deposits. The results demonstrate that substrate material properties and nozzle traverse speeds have significant influence on residual stresses of the cold spray deposit. It is understood that coefficient of thermal expansion (CTE) difference of the coating and substrate materials has significant effect on residual stress state. On the other hand, the residual stresses change from more compressive to more tensile state as the temperature of the components increases by decreasing the nozzle traverse speed. These findings indicate that thermal parameters affect residual stresses substantially. Thus, by adjusting the kinematic parameters and reducing maximum reached local temperatures within the part, more favorable stress states of the finished component can be obtained. The attained knowledge is essential for the development of high-quality deposits and the selection of the best strategies for repair and additive manufacturing applications.