Comparative study of the formation-decomposition mechanisms and kinetics in LaNi 5 and magnesium reversible hydrides

Abstract

Comparison of kinetics and formation decomposition mechanisms between Mg and LaNi 5 hydrides is carried out as a function of the number of cycles. Kinetic studies are performed, as a function of pressure and temperature domains either by high sensitivity thermogravimetry or by differential manometry. Mechanisms are confirmed by microscopic observations. Differences in mechanical properties and temperature-pressure domains and the absence of an insertion solid-solution of H 2 in Mg give quite different mechanisms: For LaNi 5, the β-hydride results from a precipitation of the new phase in the whole sample volume, the reaction speed is probably controlled by the stress-density. For Mg, MgH 2 formation appears by superficial nucleation and growth of a gas-proof hydride without fragmentation of the sample.