Hydrogen-diffusion in Pd1-xAgx (0<=x<=1)

By further improvement of the electrochemical pulse time-lag technique it has now become possible to measure the hydrogen diffusion coefficient on the silver rich side of the series of PdAg-alloys, despite the extremely low hydrogen solubility in pure silver and the silver rich alloys. The measured macroscopic diffusion coefficient of hydrogen in Ag reaches values comparable to the diffusion in pure Pd, but is, by orders of magnitude, higher than in the Pd0.5Ag0.5 alloy. In order to describe this the diffusion process has been regarded theoretically from a microscopical point of view. This process has been simulated by computer calculations assuming hydrogen as a random walker in a Pd matrix (with high hydrogen solubility) where silver is distributed statistically, blocking the hydrogen diffusion paths to a great extent due to the extremely small hydrogen solubility. The calculated mean square distance shows an analogous behavior with increasing silver content as the measured macroscopic diffusion coefficient. This means that hydrogen diffusion through PdAg alloys takes place preferably by a roundabout way via ''Pd-rich diffusion paths'' avoiding Ag-rich regions.