Kinetics of hydride formation in massive LaAl_0.25Ni_4.75 samples

Hydrogen pressures, ranging from 80 to 1 atm, were utilized to study the hydriding kinetics of small, well-shaped LaAl_0.25Ni_4.75 samples at temperatures between 246 K and 281 K. The hydrogen absorption was monitored as a function of time at constant temperature and approximately constant pressure. Either a shrinking core (sc) or a low-dimensional nucleation and growth (ng) models fit the experimental data. In one case an interesting feature of thin cracked layer, consisting of bent flakes was revealed in a SEM micrograph of a partly hydrogenated sample. This observation is examined in view of the above models. The pressure dependence of the rate constants indicates an interface-controlled phase transition as the hydride formation rate-determining step. The activation energy for the hydriding process is estimated from Arrhenius plots of the reaction rates to be 0.25 eV/H atom. The results are compared with kinetic data of hydride formation in massive LaNi₅ and powdered LaAl_0.25Ni_4.75 samples.