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 H₂ 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 either of the two hydriding models. The temperature dependence of the hydrogenation rate constants follows an Arrhenius-type behavior for a given driving force, determined by a constant ratio between the applied and equilibrium plateau hydrogen pressures. The pressure dependence of the rate constants indicates an interface-controlled phase transition as the rate-determining step of hydride formation. The activation energy for the hydriding process is estimated as 0.37 eV/H atom. This value is independent of the applied model.