KINETICS OF HYDRIDE FORMATION IN MASSIVE LANI5 SAMPLES

The hydriding kinetics of parallelepiped, 50-200 mg LaNi5 samples were investigated as a function of temperature and pressure in the ranges 245-293 K and 5-70 atm H-2 respectively. Mathematical analyses of the time-dependent hydrogen absorption curves indicate agreement with both shrinking core and low-dimensional nucleation and growth-type models. Visual and metallographic examinations of partially hydrided samples do not provide any additional evidence in support of the two hydriding models. The temperature dependence of the hydrogenation rate constant s follows an Arrhenius-type behavior for a given driving force, determ ined by a constant ratio between the applied and equilibrium plateau h ydrogen pressures. The pressure dependence of the rate constants indic ates an interface-controlled phase transition as the rate-determining step of hydride formation. The activation energy for the hydriding pro cess is estimated as 0.37 eV/H atom. This value is independent of the applied model.