The diffusion coefficients of grain boundary diffusion and grain boundary assisted lattice diffusion of Pd in Mg in Pd/Mg/Si system was studied at 473K in vacuum. The grain boundary diffusivity was measured using the Whipple model and grain boundary assisted lattice diffusivity by the plateau rise method using Pd depth profiles constructed by Rutherford back-scattering spectrometry. It was established that the grain boundary diffusivities were about six orders of magnitude higher than lattice diffusivities. Fine-grained microstructure of the Pd film, a high abundance of defects in the Mg film and a higher stability associated with Pd–Mg intermetallics were responsible for the diffusion of Pd into grain boundaries and subsequently in the interior of Mg. Besides the in-diffusion of Pd, annealing also brought about out-diffusion of Mg into the Pd film. Examination by nuclear reaction analysis involving 24Mg(p,p′γ)24Mg resonance reaction showed the occurrence of Mg out-diffusion. Minimization of surface energy was presumed to be the driving force for the process. In addition to Pd/Mg interface, diffusion occurred across the Mg/Si (substrate) interface as well upon increasing the annealing temperature above 473K. The studies showed that dehydrogenation of films accomplished by vacuum annealing should be limited to temperatures less than 473K in order to minimize the loss of surface Pd and Mg by way of interfacial reactions.

Studies on Interdiffusion in Pd/Mg/Si Films: Towards Improved Cyclic Stability in Hydrogen Storage. Y.Sunitha, G.L.N.Reddy, S.Kumar, V.S.Raju: Applied Surface Science, 2009, 256[5], 1553-9