Studies on interdiffusion in Pd/Mg/Si films: Towards improved cyclic stability in hydrogen storage

The paper reports the diffusion coefficients of grain boundary diffusion and grain boundary assisted lattice diffusion of Pd in Mg in Pd/Mg/Si system, a useful material for hydrogen storage, at 473 K in vacuum. The grain boundary diffusivity is measured by Whipple model and grain boundary assisted lattice diffusivity by plateau rise method using Pd depth profiles constructed by Rutherford backscattering spectrometry. It is established that grain boundary diffusivities are about six orders of magnitude faster than lattice diffusivities. Fine grained microstructure of Pd film, high abundance of defects in Mg film and higher stability associated with Pd-Mg intermetallics are responsible for the diffusion of Pd into grain boundaries and subsequently in the interiors of Mg. Besides the indiffusion of Pd, annealing also brings about an outdiffusion of Mg into Pd film. Examination by nuclear reaction analysis involving ²⁴Mg(p,p′γ)²⁴Mg resonance reaction shows the occurrence of Mg outdiffusion. Minimization of surface energy is presumably the driving force of the process. In addition to Pd/Mg interface, diffusion occurs across Mg/Si (substrate) interface as well on increasing the annealing temperature above 473 K. These studies show that dehydrogenation of films accomplished by vacuum annealing should be limited to temperatures less than 473 K to minimize the loss of surface Pd, the catalyst of the hydrogen absorption-desorption process and Mg, the hydrogen storing element, by way of interfacial reactions.