Permeation hysteresis in PdCu membranes.

H 2 permeation hysteresis has been observed during cycling of a 3 mum thick supported PdCu membrane with approximately 50 atom % Pd through the fcc/bcc (face-centered cubic/body-centered cubic) miscibility gap between 723 and 873 K. Structural investigations after annealing of membrane fragments under H 2 at 823 K reveal retardation of the fcc(H) --> bcc(H) transition, which is attributed to the occurrence of metastable hydrogenated fcc PdCu(H) phases. The H(2) flux at 0.1 MPa H(2) pressure difference in the well-annealed bcc single phase regime below 723 K can be described by J(H2) = (1.3 +/- 0.2) mol.m (-2).s (-1) exp[(-11.1 +/- 0.6) kJ.mol (-1)/( RT)] and that in the fcc single phase regime above 873 K by J(H2) = (7 +/- 2) mol.m (-2).s (-1) exp[(-30.3 +/- 2.5) kJ.mol (-1)/( RT)].