Protium diffusion in La-Ni alloys

The effect of microstructure on the diffusion of protium (hydrogen) in La-N i alloys was studied by the desorption method. Five alloys were prepared; o ne was the well known protium absorbing alloy LaNi5 with the stoichiometric composition, and the others were the two phase alloys LaNix (x=5.5-8.1) co nsisting of the LaNi5 phase and the Ni phase. By the analysis of the desorp tion curves for the temperature range 353-573 K, the diffusion coefficient of protium in the alloys was determined under the assumption that the sampl e particles were spheres. The Arrhenius plots of the diffusion coefficients , D, for all the alloys show linear relationships. The value of D in LaNix (x=6.0, 6.5, 8.1) increases with increasing the value of x, and they are la rger than that in LaNi5, although the value of D in LaNi5.5 is the smallest in the temperature range examined. The literature value of D in LaNi5 obta ined by the quasielastic neutron scattering (QNS) method is in the middle o f the coefficients of the five alloys. This suggests that the effective sur face area of the LaNi5 specimen for the desorption is reduced by the format ion of oxide layer and that in the two phase alloys the protium diffusion i s enhanced via a net work 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. (C) 1999 Elsevier Science S.A. All rights reserved.