The effect of surface coating on the oxygen permeation characteristics of zirconia

Yttria-stabilized-zirconia (YSZ) can be used as an oxygen permeating membrane at elevated temperature (>1400 °C) due to its chemical and mechanical stability. However, the permeation kinetics of zirconia has not been studied in detail, especially in low oxygen partial-pressure ( P O 2 ). In this study, the oxygen flux of YSZ was measured as a function of the temperature (1400 – 1600 °C) and P O 2 (1 to 1 × 10−3, 2 × 10−3 to 2 × 10−7 and 3 × 10−12 to 2 × 10−8 atm). In high and middle P O 2 regions, the measured oxygen flux matched well the value estimated from the reported electrical conductivity. This observation shows that the oxygen permeation was mostly limited by the bulk diffusion as proved by the P O 2 1 / 4 , (high P O 2 )- or P O 2 − 1 / 4 (middle P O 2 )-dependence of oxygen flux. However, in low P O 2 region, the measured oxygen flux was about one order of magnitude smaller than the expected value from the electrical conductivity. In this region, the oxygen 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 oxygen 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 oxygen 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.