STRUCTURAL-CHANGES OF THE LAVES PHASE ERFE2 DURING STATIC AND CYCLIC HYDROGENATION TREATMENTS

Abstract

To investigate the structural changes of C15 Laves compound ErFe2 during hydrogenation, which has a good potential as a hydrogen storage material, static and cyclic hydriding treatments were performed. During the static isothermal hydrogenation, an amorphous hydride phase is observed at about 200-degrees-C without decomposition. From the variations in the charging conditions (temperature, time and pressure), it is found that amorphization is a thermally activated process and that some critical amounts of hydrogen atoms should be supplied to the matrix. Temperature-induced hydrogen absorption-desorption cycling results in a decrease in the reversible hydrogen content with increasing number of cycles. As a result of intrinsic degradation, amorphous or mixed ErH2+Fe phases are formed. The thermal energy released during the hydriding reaction and the repetitive strain induce a phase decomposition. However, in the case of a smaller heat effect, the crystal structure is transformed to an amorphous state by the cyclic strain accumulation. Therefore, from the observed results, it is suggested that the amorphous phase is an intermediate and metastable phase formed during phase decomposition and that the corresponding reaction mechanism is closely related to the motion of the iron atoms. Also, amorphous phase formation is one of the possible degradation mechanisms of the metal hydrides during cyclic operation.