The kinetics of H absorption in thin (2 to 80nm) layers, that had been evaporated (under ultra-high vacuum) onto a previously hydrided Pd foil, were studied. The Pd foil acted as an essentially infinite reservoir of atomic H. In this way, it was possible to avoid the problem of a slow dissociation of gaseous H on the Mg surface. In order to study the diffusion of H through Mg, and the subsequent nucleation and growth of the hydride, X-ray photo-electron spectroscopy was used to measure the temporal evolution of the hydride concentration on the vacuum-exposed surface of the Mg film. The aim was to develop a simple diffusion model, which described the measured curves, in order to determine the overall diffusion coefficient. The model exploited the fact that the hydride formed preferentially at the PdHx/Mg interface and constituted a diffusion barrier to the subsequent diffusion of H particles. This was taken into account by using a time-dependent boundary condition for the H concentration at the PdHx/Mg interface. It was found that the model curves agreed well with the experimental results. An overall diffusion coefficient of 1.1 x 10-20m2/s was found at 305K.

P.Spatz, H.A.Aebischer, A.Krozer, L.Schlapbach: Zeitschrift für Physikalische Chemie, 1993, 181[1-2], 393-7