The hydrogen desorption kinetics of Pd-coated LaNi 5-type films

Abstract

Hydrogen desorption kinetics in over-stoichiometric LaNi 5+ x ( x∼1) films coated with palladium have been studied by thermal desorption spectroscopy (TDS). La–Ni films with a thickness of 1.3 μm were deposited on sapphire substrates by ion beam sputtering, and then covered ‘in situ’ with a 70 nm thick palladium coating. To get amorphous and crystalline La–Ni structures, the substrates were heated during sputtering either to 325 or to 600 K, respectively. Desorption experiments were carried out on these samples and on a 70 nm thick palladium film after hydrogenation at room temperature in the pressure range of 1–100 mbar. Pd controls hydrogen desorption from crystalline LaNi 5-type films at temperatures lower than 300 K, whereas it behaves as a free permeation membrane for the amorphous alloy. Such behavior is related to a faster long-range diffusion of hydrogen in the crystalline structure in comparison with the amorphous one. This paper reveals that the kinetics of hydrogen desorption in LaNi 5-type metal hydride systems can be changed to a considerable extent by both microstructure and surface characteristics of films.