Energy conditions determining size range impacting formation of magnesium hydride

Hydrogen, one of the most abundant elements in nature, is potentially suitable to produce, store and consume clean energy, namely for various industrial uses. The safest technique for hydrogen storage is metal hydride, e.g. as MgH2, with safe facilities to store and transport. A critical indicator of a metal-hydrogen system is the kinetic of hydrogen sorption, which is known particularly low for magnesium. However, under given thermodynamic conditions, the kinetics of magnesium/hydrogen reaction is especially sensitive to the particle size i.e. specific surface, crystallite/grain size i.e. boundary extension and nature of potential additives as catalysts. The present calculations aimed at determining the occurrence of hydride nucleation are based on a new energy ratio, as proposed at first time, which takes into account both physics-chemical and mechanical factors. The calculations of the energy ratio are based on the minimum total energy of the system, which correctly reflects the processes occurring during hydride formation in magnesium. The critical size of a nucleus at the phase formation (hydride) in magnesium is controlled by the ratio of the volume and surface area of the emerging component, similarly to a crystallization process from a solution. The influence of the mechanical response of the system to the formation of hydride allows one to propose an interpretation of some phenomena regularly recorded during such experiments, i.e. the influence of special additives and mechanical texture, which lead to the acceleration of hydride formation. The results obtained suggest a mechanism favoring oriented nucleation of the hydride in a textured magnesium matrix thanks to the anisotropy of the elastic characteristics of the newly formed phase.