Real time characterization of elastic variations in palladium films produced by hydrogen adsorption

The magnitude and time variations of the SAW propagation velocity Δv/v, of the mass and elastic loading, and of the fractional changes in the density Δρ/ρ and in the elastic modulii Δc_ij/c_ij of Pd films are experimentally evaluated at room temperature and atmospheric pressure for 0.1, 0.5, 1% H₂, 1% CO, 1% N₂O and 1% NO gas mixtures in pure nitrogen. Two different thermally evaporated Pd films have been tested: one as evaporated, the other one after annealing at 423°K and 10^-5 Pa for 15 hours. The annealing is shown to change all set of the parameters. For not annealed films, hydrogen gives rise to large variations in the elastic constants (≃5-40%) and to an increase in the phase velocity; while for annealed films, it produces small variations in the elastic constants (≃1-2%) and a decrease in the phase velocity. The difference is attributed to weakly and strongly coupled oxygen preadsorbed in the film. The results obtained can allow an improvement of Pd-based sensors