Thickness dependence of oxygen permeation through erbia-stabilized bismuth oxide-silver composites

Oxygen permeation measurements were performed on erbia-stabilized bismuth oxide-silver (40 v/o) composite membranes in the range of thickness of 1.60–0.23 mm and temperature of 850–650 °C. Air was fed at one side of the membranes while permeated oxygen on the other side was swept away with helium. An oxygen flux as high as 1.56 × 10⁻⁷ mol/cm²/s was observed for a 0.23 mm thick membrane at 750 °C and P_o2(1) = 0.056 arm. The examination of the dependence of permeance on the membrane thickness reveals that (a) the oxygen transport kinetics are controlled by the diffusion of oxygen ions in the bulk of membrane with thickness down to about 1.0 mm; (b) with further decrease in thickness, the oxygen permeation becomes partially limited by surface oxygen exchange. The surface exchange rate has been found to decrease less pronouncedly than the bulk process as temperature decreases. The mass transfer is suggested to be the rate-limiting step for the surface process.