Hydrogen chemical potentials and diffusion coefficients in hydrogen diffusion membranes

Abstract

Palladium and palladium alloys are quite widely used as membranes for hydrogen permeation, because of their resistance to embrittlement and to irreversible deformations during cycles of absorption and desorption of hydrogen^1–3. The isothermal relationships between hydrogen diffusion coefficient, D_H, and hydrogen content, n (ratio of hydrogen/metal atoms) has both technological and academic interest. The most detailed form of relationships^2,3 between D_H and n have been those reported for deuterium in Pd₇₅Ag₂₅ by Hickman⁴ within the temperature range 300–500°C, and for protium in Pd₇Ag₂₃ within the range 30–75°C by Küssner⁵. For increasing values of n up to n ∼ 0.2, Küssner⁵ reported significant corresponding decreases of D_H (by a factor of ∼ 8 from n=0 up to n ∼ 0.16 at 30°C). This is the reverse of trends in the values of D_H with increasing n suggested by measurements of anelastic effects^3,6,7 and also by the permeation rate measurements of Hickman⁴ at 300 and 400°C. Here we report measurements of diffusion through Pd₈₁Pt₁₉ alloy, the results of which are consistent with Gorskii's hypothesis relating long-range anelastic diffusion of lattice interstitials to elastic strain gradients.