THICKNESS DEPENDENCE OF OXYGEN PERMEATION THROUGH ERBIA-STABILIZED BISMUTH OXIDE SILVER COMPOSITES

Oxygen permeation measurements were performed on erbia-stabilized bism uth oxide-silver (40 v/o) composite membranes in the range of thicknes s of 1.60-0.23 mm and temperature of 850-650 degrees C. Air was fed at one side of the membranes while permeated oxygen on the other side wa s swept away with helium. An oxygen flux as high as 1.56 x 10(-7) mol/ cm(2)/s was observed for a 0.23 mm thick membrane at 750 degrees C and p(O2)(1)=0.056 atm. 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 membran e with thickness down to about 1.0 mm; (b) with further decrease in th ickness, the oxygen permeation becomes partially limited by surface ox ygen exchange. The surface exchange rate has been found to decrease le ss pronouncedly than the bulk process as temperature decreases. The ma ss transfer is suggested to be the rate-limiting step for the surface process.