The oxygen-permeation property of doped-zirconia membrane at high temperature and in reduced condition

Doped zirconia with fast oxygen ionic conduction can be applied to an oxygen ion transport membrane for the deoxidation from metal melts and the production of metals from molten metal oxides. For the application, an oxygen-permeation study is required in low-oxygen partial pressure and high temperature. In this study, the electrical conductivity of the zirconia at high temperature (above 1200 °C) was estimated with a 4-probe DC method in order to estimate the theoretical oxygen-permeation flux. The oxygen permeation was investigated with two types of apparatus: a permeation concentration cell and a common permeation structure. As a result, the measured oxygen flux was much lower than the value expected from the electrical conductivity, indicating that the oxygen permeation was mostly limited by the surface-exchange kinetics in spite of the very high temperature. The rate-determining elementary step of the surface-exchange kinetics was the surface ionic diffusion and the charge transfer of minor charge carriers. To enhance the oxygen permeation of the zirconia, a surface-coating strategy using a double layer of ceria and lanthanum-chromate was applied with various coating thickness. The oxygen-permeation flux was maximized by the surface modification with optimal coating thickness.