Influence of uniaxial pressing and nickel catalytic additive on activation of magnesium hydride thermal decomposition

Speed-up of thermal decomposition of magnesium hydride due to uniaxial pressing, including pressing in presence of nickel catalyst, was studied by barometry, SEM, DSC, XRD methods, and using mathematical modeling. Pressing even with no catalyst is shown to hasten the hydrogen desorption. The most probable reason for this is the formation of multiple defects in crystal lattice. They can serve as nucleation centers: metal nuclei significantly hasten hydrogen desorption. Besides, metal magnesium not converted to the hydride during the synthesis stage can possibly appear at surface. Adding nickel powder before pressing hastens dehydriding process even more. Comparing hydrogen evolution curves for samples with different amount of nickel allowed to propose the probable mechanism of hydrogen evolution. It is based on description of reactions of hydrogen desorption and hydride decomposition and of the metal-hydride phase morphology change due to these reactions. We develop a mathematical model in form of ordinary differential equations that fits the experimental data well. The model is based on conservation and symmetry assumptions. The fitting allowed to evaluate rate parameters of hydride phase decomposition.