Protium diffusion in La-Ni alloys

The effect of microstructure on the diffusion of protium (hydrogen) in La-Ni alloys was studied by the desorption method. Five alloys were prepared; one was the well known protium absorbing alloy LaNi₅ with the stoichiometric composition, and the others were the two phase alloys LaNi_x (x = 5.5-8.1) consisting of the LaNi₅ phase and the Ni phase. By the analysis of the desorption curves for the temperature range 353-573 K, the diffusion coefficient of protium in the alloys was determined under the assumption that the sample particles were spheres. The Arrhenius plots of the diffusion coefficients, D, for all the alloys show linear relationships. The value of D in LaNi_x (x = 6.0, 6.5, 8.1) increases with increasing the value of x, and they are larger than that in LaNi₅, although the value of D in LaNi_5.5 is the smallest in the temperature range examined. The literature value of D in LaNi₅ obtained by the quasielastic neutron scattering (QNS) method is in the middle of the coefficients of the five alloys. This suggests that the effective surface area of the LaNi₅ specimen for the desorption is reduced by the formation of oxide layer and that in the two phase alloys the protium diffusion is enhanced via a network of the Ni phase deformed by the expansion of the LaNi₅ phase due to the absorption of protium, although the Ni phase in LaNi_5.5 is insufficient to form the network.