Hydrogen is an efficient, carbon-free and safe energy carrier. However, its compact and weight-efficient storage is an ongoing subject for research and development. Among the intensively investigated hydrogen storage materials, lithium aluminum hydride (LiAlH4) is an attractive candidate because of its high theoretical hydrogen density (volumetric: 96.7g H2/l material; gravimetric: 10.6 wt.%-H2) in combination with rather low decomposition temperatures (onset temperature <100°C after doping). Although the reversible dehydrogenation of LiAlH4 must be carried out with the help of organic solvent, LiAlH4 can serve as single-use hydrogen storage material for various special applications, for example, hydrogen fuel cell systems. This thesis deals with transition metal (TM)-doped LiAlH4 aiming at tailored dehydrogenation properties. The crystal structure and morphology of TM-doped LiAlH4 is characterized by XRD and SEM respectively. The positive effects of four dopants (NiCl2, TiCl3, ZrCl4 and TiCl4) on promoting the dehydrogenation kinetics of LiAlH4 are systematically studied by thermal analysis. Based on the state of each TM chloride (solid or liquid), three low-energy-input doping methods (1. ball-milling at low rotation speed; 2. manual grinding or magnetic stirring; 3. magnetic stirring in ethyl ether) are compared in order to prepare LiAlH4 with the maximum amount of hydrogen release in combination with fast dehydrogenation kinetics. The dehydrogenation properties of the TM-doped LiAlH4 powders are measured under isothermal conditions at 80°C at a H2 pressure of 1 bar, which is within the operating temperature range of proton exchange membrane (PEM) fuel cells, aiming at applications where the exhaust heat of the fuel cell is used to trigger the dehydrogenation of the hydrogen storage material. Furthermore, the mid-term dehydrogenation behavior of TM-doped LiAlH4 was monitored up to a few months in order to test its mid-term storability. In addition, the pelletization of TM-doped LiAlH4 is investigated aiming at a higher volumetric hydrogen storage capacity. The effects of compaction pressure, temperature and the H2 back-pressure on the dehydrogenation properties of TM-doped LiAlH4 pellets are systematically studied. Moreover, the volume change through dehydrogenation and the short-term storage of the TM-doped LiAlH4 pellets are discussed in view of practical applications for PEM fuel cell systems. / Wasserstoff ist ein effizienter, kohlenstofffreier und sicherer Energieträger. Jedoch die kompakte und gewichtseffiziente Speicherung ist ein permanentes Forschungs- und Entwicklungsthema. Unter den intensiv untersuchten Materialien für die Wasserstoffspeicherung ist aufgrund der hohen theoretischen Speicherdichte (volumetrisch: 96,7 g H2/L, gravimetrisch: 10.6 Gew.%-H2) in Kombination mit sehr niedrigen Zersetzungstemperaturen (Anfangstemperatur < 100°C nach Dotierung) Lithium Aluminiumhydrid (LiAlH4) ein vielversprechender Kandidat. Obwohl die reversible Dehydrierung von LiAlH4 mit Hilfe von organischen Lösungsmitteln durchgeführt werden muss, kann LiAlH4-Pulver als Einweg-Speichermaterial für verschiedene Anwendungen dienen, beispielsweise für Wasserstoff/Brennstoffzellensysteme. Diese Doktorarbeit beschäftigt sich mit LiAlH4 dotiert mit Übergangsmetall, mit dem Ziel maßgeschneiderte Dehydrierungseigenschaften zu erreichen. Die Kristallstruktur und die Morphologie der mit Übergangsmetallen dotierten LiAlH4-Pulver wurden mit Röntgenbeugung (XRD) und Rasterelektronenmikroskopie (REM) charakterisiert. Weiterhin wurde der positive Effekt der Dotanden auf die reaktionsfördernde Dehydrierung von LiAlH4 systematisch mit Hilfe thermoanalytischer Methoden untersucht. Für jedes Übergangsmetall, welches in Form von Übergangsmetallchloriden vorlag, wurden drei Dotierungsmethoden mit niedrigem Energieeintrag (Kugelmahlen mit geringer Rotations-geschwindigkeit, manuelles Schleifen/Magnetrühren, Magnetrühren mit Ethylether) verglichen, um LiAlH4-Pulver mit einer maximalen Wasserstofffreisetzungsmenge in Kombination mit einer schnellen Dehydrierungskinetik zu erzielen. Die Dehydrierung des dotierten LiAlH4-Pulvers wurde unter isothermen Bedingungen bei 80°C und einem H2-Druck von 1 bar gemessen, was im Bereich der Betriebstemperatur von PEM-Brennstoffzellen (Proton Exchange Membran) liegt. Dadurch sollen Anwendungen anvisiert werden, bei denen die entstehende Abwärme der Brennstoffzelle genutzt wird, um die Dehydrierung des Wasserstoffspeichermaterials auszulösen. Zudem wurde das Dehydrierungsverhalten des dotierten LiAlH4 bis zu einigen Monaten kontrolliert, um die mittelfristige Haltbarkeit zu testen. Weiterhin wurde die Pelletierung des mit Übergangsmetallen dotierten LiAlH4 mit dem Ziel untersucht, eine hohe volumetrische Speicherkapazität zu erreichen. Der Einfluss des Pressdrucks, der Dehydrierungstemperatur und des H2-Gegendrucks auf die Dehydrierungseigenschaften der mit Übergangsmetallen dotierten LiAlH4-Presslinge wurde systematisch analysiert. Außerdem wird die Volumenveränderung durch die Dehydrierung und die Kurzzeitspeicherung der mit Übergangsmetallen dotierten LiAlH4-Presslinge im Hinblick auf praktische Anwendungen unter Nutzung der Brennstoffzelle diskutiert.

Additive manufacturing (AM) is a technology that inverts the procedure of traditional machining. Instead of starting with a billet of material and removing unwanted parts, the AM manufacturing process starts with an empty workspace and proceeds to fill this workspace with material where it is desired, often in a layer-by-layer fashion. Materials available for AM processing include polymers, concrete, metals, ceramics, paper, photopolymers, and resins. This thesis is concerned with electron beam melting (EBM), which is a powder bed fusion technology that uses an electron beam to selectively melt a feedstock of fine powder to form geometries based on a computer-aided design file input. There are significant differences between EBM and conventional machining. Apart from the process differences, the ability to manufacture extremely complex parts almost as easily as a square block of material gives engineers the freedom to disregard complexity as a cost-driving factor. The engineering benefits of AM also include manufacturing geometries which were previously almost impossible, such as curved internal channels and complex lattice structures. Lattices are lightweight structures comprising a network of thin beams built up by multiplication of a three-dimensional template cell, or unit cell. By altering the dimensions and type of the unit cell, one can tailor the properties of the lattice to give it the desired behavior. Lattices can be made stiff or elastic, brittle or ductile, and even anisotropic, with different properties in different directions. This thesis focuses on alleviating one of the problems with EBM and AM, namely the relatively few materials available for processing. The method is to take a closer look at the widely used stainless steel 316LN, and investigate the possibility of processing 316LN powder via the EBM process into both lattices and solid material. The results show that 316LN is suitable for EBM processing, and a processing window is presented. The results also show that some additional work is needed to optimize the process parameters for increased tensile strength if the EBM-processed material is to match the yield strength of additively laser-processed 316L material. / <p>Vid tidpunkten för framläggningen av avhandlingen var följande delarbete opublicerat: delarbete 3 (inskickat).</p><p>At the time of the defence the following paper was unpublished: paper 3 (submitted).</p>

Additive manufacturing (AM) is the name given to the technology of building 3D parts by adding layer-by-layer of materials, including metals, plastics, concrete, etc. Of the different types of AM techniques, electron beam melting (EBM), as a powder bed fusion technology, has been used in this study. EBM is used to build parts by melting metallic powders by using a highly intense electron beam as the energy source. Compared to a conventional process, EBM offers enhanced efficiency for the production of customized and specific parts in aerospace, space, and medical fields. In addition, the EBM process is used to produce complex parts for which other technologies would be either expensive or difficult to apply. This thesis has been divided into three sections, starting from a wider window and proceeding to a smaller one. The first section reveals how the position-related parameters (distance between samples, height from build plate, and sample location on build plate) can affect the microstructural characteristics. It has been found that the gap between the samples and the height from the build plate can have significant effects on the defect content and niobium-rich phase fraction. In the second section, through a deeper investigation, the behavior of Alloy 718 during the EBM process as a function of different geometry-related parameters is examined by building single tracks adjacent to each other (track-by-track) andsingle-wall samples (single tracks on top of each other). In this section, the main focus is to understand the effect of successive thermal cycling on microstructural evolution. In the final section, the correlations between the main machine-related parameters (scanning speed, beam current, and focus offset) and the geometrical (melt pool width, track height, re-melted depth, and contact angle) and microstructural (grain structure, niobium-rich phase fraction, and primary dendrite arm spacing) characteristics of a single track of Alloy 718 have been investigated. It has been found that the most influential machine-related parameters are scanning speed and beam current, which have significant effects on the geometry and the microstructure of the single-melted tracks.