Etude de l'absorption et de la desorption de l'hydrogene par les films de magnesium (French text)

Metal hydrides are one of the most promising systems for safe hydrogen storage. Among these, magnesium hydride is particularly attractive because of its high hydrogen storage capacity and low cost.; Our aim was to understand the mechanism involved in the formation and the decomposition of magnesium hydride. Magnesium films were used throughout this work because they have a simple geometry and they offer a suitable way to study the mechanism, the interface and the surface properties of the hydriding reaction. Thus, we investigated the hydrogenation/dehydrogenation characteristics of 30 μm air-exposed magnesium film prepared by vacuum evaporation. Some of them were implanted with vanadium and palladium ion in order to improve their kinetics and lower the temperature of operation.; Magnesium films are prepared by vacuum evaporation. The structural and hydrogen sorption properties of pure magnesium and ion-implanted magnesium films are presented. Morphology and structural properties were analyzed by scanning electron microscopy (SEM), X-Ray Diffraction (XRD), Auger depth profile (AES Profile) and Rutherford Backscattering Spectrometry (RBS). Hydrogen sorption properties and pressure-composition isotherms were measured with an automatic Sievert apparatus. Pure magnesium films exposed in air were found to absorb hydrogen at 623 K under 10 bar of hydrogen pressure and to desorb it at the same temperature under a residual hydrogen pressure of 0.21 bar. Furthermore, macroscopic and microscopic observations made at different steps of the absorption/desorption process are presented. We have particularly studied the influence of the rate of evaporation and the film thickness on the absorption/desorption characteristics. Finally, the effects of vanadium and palladium implantation on hydrogen diffusion is also described and discussed. It was found that the hydrogenation kinetics depends much more on ion species and the dose implanted rather than on the implantation energy. Indeed, the vanadium-implanted magnesium films have faster kinetics than the palladium-implanted ones. The fastest kinetics were obtained with vanadium-implanted magnesium film at an ion energy of 42 keV and a dose of 1 × 1016 atoms/cm2: a concentration of 6 wt.% of hydrogen was reached in 270 minutes during the first hydrogenation and in 45 minutes during the second one, which is faster than 400 minutes and 220 minutes respectively, for pure magnesium film. (Abstract shortened by UMI.)...