REAL TIME CHARACTERIZATION OF ELASTIC VARIATIONS IN PALLADIUM FILMS PRODUCED BY HYDROGEN ADSORPTION

The magnitude and time variations of the loading, and of the fractional changes in the density Aplp and in the elastic modulii A+, of Pd fdms are experimentally evaluated at room temperature and amospheric pressure for 0.1 ,0.5,1 % Hz, 1 % CO, 1 % N20 and 1 % NO gas mixtures in pure nitrogen. Two different thermally evaporated Pd fdms have been tested: one as evaporated, the other one after annealing at 423 'K and IO5 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 (G 5 - 40 %) and to an increase in the phase velocity; while for annealed films, it produces small variations in the elastic constants (S 1-2 is attributed to weakly and strongly coupled oxygen 96) and a decrease in the phase velocity. The d i f f e w preadsorbed in the film. The results obtained can allow an improvement of Pd-based sensors.