Moving interface hydride formation in multilayered metal thin films

In order to improve the often slow reaction kinetics associated with many metal hydride hydrogen storage materials, we must first develop a detailed understanding of the limiting kinetic mechanisms associated with the materials in question. We used in situ x-ray diffraction to monitor hydride formation in highly controlled epitaxial magnesium thin films capped with a thin palladium layer to examine the kinetics of hydride formation. We compare the kinetic data with a transport-based hydride formation model and conclude that hydride formation in these films occurs via a growing hydride layer originating at the surface of the film. Diffusion of hydrogen through this growing hydride layer controls the growth rate for hydride layer thicknesses above similar to 600 angstrom for the conditions examined, while reactions at an interface or diffusion through the fixed palladium layer control the growth rate for smaller hydride layer thicknesses. (C) 2010 American Institute of Physics. [doi: 10.1063/1.3447868]