HYDROGEN DIFFUSION AND SOLUBILITY IN PALLADIUM THIN-FILMS

Palladium thin films (220, 460 and 1350 Angstrom thick) were fabricate d by physical vapor deposition and characterized electrochemically. Th e reversible hydrogen storage capacity of the films was determined by electrochemical cycling. The hydrogen solubility of the thin films was larger in the alpha-phase but smaller in the beta-phase than that of the bulk. The hydrogen diffusion coefficient, D-H, in the Pd films was measured by the electrochemical stripping method in the Pd-H solid so lution (a-phase) at the temperatures ranging from 280 to 330 K. A well -defined potential range was found for applying the electrochemical st ripping technique. The D-H decreases with decreasing film thickness an d is 2-3 orders of magnitude smaller than that in the bulk. The D-H vs 1/T plots show that all the thin films follow the Arrhenius behavior, D-H = D-0 exp(-E(a)/RT), with approximately the same activation energ y E(a) and a decreasing prefactor D-H as the film thickness decreases.