Hydrogen uptake kinetics of Pd coated FeTi films

Abstract

The kinetics of hydrogen uptake of thin films of FeTi deposited on Si substrates and covered with 20nm Pd were studied. The films serve as a model system for powdered FeTi, with grains that are (partly) covered with Pd, which serves as a protection for severe oxidation. Two FeTi compositions near the 50/50 composition were studied. The hydrogen uptake kinetics as a function of temperature and pressure were measured by probing the differential pressure between a small hydriding reaction chamber and a reference chamber. Both compositions showed first order kinetics for the largest portion of the uptake. Using results of additional measurements in which the thickness of the layers was varied, a model is proposed in which the uptake proceeds via fast channels through the film, followed by slower diffusion into the bulk. Finally, the influence of oxidation was studied. An FeTi-oxide underneath the Pd layer is a barrier for H diffusion. It was found that by annealing the H uptake rate could be increased. This is probably due to the decomposition of the oxide. Samples partly covered with Pd and partly by FeTi-oxides, obtained by decomposing the fully covered structure by air annealing at 250°C, showed uptake throughout the entire FeTi film with an even faster rate than in a fully covered film. Some explanation with simple models of the observed phenomena is given.