The hydrogen desorption kinetics of Pd-coated LaNi5-type films

Hydrogen desorption kinetics in over-stoichiometric LaNi5+x (x similar to1) films coated with palladium have been studied by thermal desorption spectroscopy (TDS). La-Ni films with a thickness of 1.3 mum 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 LaNi5-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 LaNi5-type metal hydride systems can be changed to a considerable extent by both microstructure and surface characteristics of films. (C) 2000 Elsevier Science B.V. All rights reserved.