Preparation of Metallic Hydrides by High Pressure Gaseous Hydrogen
Several metallic components require a high thermodynamic activity of hydrogen for the formation of hydride phases. This can be achieved by several distinctly different methods. The most simple include cathodic procedures, electric discharges in low pressure gaseous hydrogen or implantation techniques. The disadvantage of all these procedures is the non-equilibrium character of the hydrogen supply which often limits the reproducibility of the results and does not provide any valuable thermodynamic information . Gaseous hydrogen under high pressure conditions, on the other hand, offers a unique and reproducible defined thermodynamic activity for hydrogen. With a knowledge of the virial coefficients for the equation of State of gaseous hydrogen  the fugacity of this component, corresponding to a given hydrostatic pressure, can be calculated. It is fortunate that the departure from ideality leads to a large increase in the fugacity as a function of the hydrostatic pressure; for example, at 25°C gaseous hydrogen at 12 kbar corresponds to a fugacity of 107 bar, about three Orders of magnitude higher.
KeywordsHydrostatic Pressure Formation Pressure Gaseous Hydrogen Virial Coefficient Hydride Phase
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