SELF STRAIN GRADIENT INDUCED DIFFUSION OF HYDROGEN IN PD-AG ALLOY MEMBRANES

Self strain gradient induced uphill diffusion effects of hydrogen in a series of Pd-Ag alloys with up to 39.9at% Ag have been investigated by an electrochemical permeation method. The uphill diffusion effects during hydrogen absorption into and desorption from the alloys have been observed over a range of initial hydrogen content from about H/M = 0.01 to H/M = 0.25. In higher Ag content alloys of Pd-30.0 and 39.9at% Ag, the effects have been observed even to occur at initial hydrogen content of more than H/M = 0.3. The uphill diffusion effects in relatively low Ag content alloys have been primarily associated with lattice strain gradients corresponding to the temporary formation of quite well differentiated lattice volume differences across (alpha + beta)/beta and (alpha + beta)/alpha interfaces during absorption and desorption processes, respectively. In higher Ag content alloys, the non-Fickian diffusion effects seem better to be associated with the more usual lattice strain gradients corresponding to sequences of more uniform concentration gradients of the permeating hydrogen. The magnitude of these uphill effects, compared at almost the same initial hydrogen content, have a tendency to increase with an increase of the Ag content of the alloys. This may be related to a decrease of hydrogen diffusivity in Pd-Ag alloys with increasing Ag content and also may be related to a decrease in the bulk modulus of the alloys with Ag content.