Yttria-stabilized zirconia (YSZ) could be used as an O permeating membrane at high temperatures (>1400C) due to its chemical and mechanical stability. However, the permeation kinetics of zirconia had not been studied in detail, especially in low O partial-pressure (PO2). The O flux of YSZ was measured here as a function of temperature (1400–1600C) and PO2 (1 to 10−3, 2 x 10−3 to 2 x 10−7 and 3 x 10−12 to 2 x 10−8atm). In high and middle PO2 regions, the measured O flux matched well the value estimated from the reported electrical conductivity. This observation showed that the O permeation was mostly limited by the bulk diffusion as proved by the PO2¼ (high PO2)- or PO2-¼ (middle PO2)-dependence of O flux. However, in the low PO2 region, the measured O flux was about one order of magnitude smaller than the expected value from the electrical conductivity. In this region, the O permeation was mostly limited by the surface oxygen-exchange kinetics in spite of very high temperature. Therefore, the coating of YSZ surface with porous layer was considered as one method to increase the O flux. YSZ, GDC (Gd–doped Ceria), YSZ–GDC or LaCrO3 were coated on both sides of YSZ and the flux was measured and compared with that without coating layer. The O flux of GDC–coated YSZ drastically increased but the increase was not maintained for long time due to the reaction and the sintering of porous layer. LaCrO3 was found to be the best coating material which maintains the porous layer and thus flux for extended time.

The Effect of Surface Coating on the Oxygen Permeation Characteristics of Zirconia. H.J.Park, G.M.Choi: Journal of the European Ceramic Society, 2005, 25[12], 2577-81