Hydrogen gas is considered as the key "green" technology for its endless production-consumption procedure, known better as the "water-cycle". In this cycle, hydrogen is directly produced from water electrolysis and while forms water again, when combusted with oxygen, releasing energy. However, as the most lightweight gas, the volumetric density of hydrogen makes it inappropriate for use in most fuel applications. Hydrogen storage technologies that promote the compression of the gas, along with a solid state form of storage, based on the consumer needs, are a critical component to further development of a hydrogen fuel-based economy.
Among the breakthrough hydrogen technologies, metal hydrides have drawn special attention due to their multiple properties. Their ability to react with hydrogen at various pressure and temperature conditions opened a whole new universe for storing energy in the form of hydrogen. Moreover, their unique capability to operate as thermal machines in order to compress hydrogen is of major importance and it will probably affect future hydrogen technologies.
The research described in this thesis tries to reveal new metal hydrides with enhanced properties or to enrich knowledge on already investigated compounds. The research conducted was multidisciplinary in nature and all the involved detailed investigations tried to identify correlations between micro-structural and chemical properties with the thermodynamic properties of the metal hydrides. The research undertaken focused in three key research areas: (i) structural characterization by means of the X-Ray Diffraction (XRD) technique and Rietveld analysis, (ii) microstructural observation and microchemical characterization by SEM/EDX analysis and (iii) thermodynamic properties investigation by hydrogen absorption/desorption measurements. The interactions between these different properties can be complex, and they are not always resulting to data that can be easily exploited.
In a more extensive manner, several Ti- or Zr-based intermetallic alloys have been synthesized from pure elements either using the arc-melting or the induction-levitation melting method. Both techniques are characterized as "Rapid Solidification Processes" with crucial influence on the crystal structure of the investigated compounds. The word "crucial" has been used in the last phrase since the rapid solidification of the liquid compounds result in fine, well-structured microstructures that are, as well as with the chemical properties, the key factor of their thermodynamic properties.
All the compounds have been fully characterized by XRD and SEM/EDX, and they are mostly exhibiting the hexagonal C14 or the cubic C15 type Laves phases. More specifically, Zr-based compounds with Laves phase structures are considered advantageous hydrides for the large span of plateau pressures (0 - 1000 bar) they can exhibit when forming reversible metal hydrides.
Since these intermetallic compounds are considered as low temperature hydrides, all hydrogen Pressure-Composition Isotherms (PCI) have been conducted in the range of 0 to ~100 oC. It has been shown that small compositional changes can affect the atomic structure that has a direct effect, in respect, on the thermodynamic properties. Thus, compounds with really close composition can have a very large difference in the equilibrium pressures in some given temperatures.
Finally, the metal hydride hydrogen compression technology is presented in terms that metal hydrides with successive equilibrium pressures can be used in order to increase hydrogen pressure using only waste heat as driving force
