KINETICS OF HYDRIDE FORMATION IN MASSIVE LAAL0.25NI4.75 SAMPLES

Hydrogen pressures, ranging from 80 to 1 atm, were utilized to study t he hydriding kinetics of small, well-shaped LaAl0.25Ni4.75 samples at temperatures between 246 K and 281 K. The hydrogen absorption was moni tored as a function of time at constant temperature and approximately constant pressure. Either a shrinking core (sc) or a low-dimensional n ucleation and growth (ng) models fit the experimental data. in one cas e an interesting feature of thin cracked layer, consisting of bent fla kes 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 ph ase transition as the hydride formation rate-determining step. The act ivation energy for the hydriding process is estimated from Arrhenius p lots of the reaction rates to be 0.25 eV/H atom. The results are compa red with kinetic data of hydride formation in massive LaNi5 and powder ed LaAl0.25Ni4.75 samples.