Oxygen Permeation Mechanism in Lanthanum Chromites, La1-xCaxCrO3-δ

The oxygen permeation mechanism through La1-xCaxCrO3-δ was investigated by two experimental methods, firstly by the determination of the oxygen tracer diffusion coefficient by 18O2 exchange in different oxygen partial pressures and SIMS analyses, and secondly by the direct measurement of the permeation flux in a steady state under a p(O2) gradient. The oxygen tracer diffusion coefficient increased with decreasing oxygen partial pressure in the range p(O2) = 10-9 to 1 Pa at T = 1273 K. On the contrary, the oxygen vacancy diffusion coefficient was found to be 1.7 × 10-9 m2 s-1, which was independent of oxygen partial pressure or oxygen vacancy content. The oxygen permeation flux as oxide ion current density (JO2-) through La1-xCaxCrO3-δ was determined by using an electrochemical method in the p(O2) range from 10-11 to 105 Pa at T = 1273 K. When p(O2) was lower than 10-5 Pa, JO2- was suppressed by the effect of surface reaction as the rate determining step. The contribution of grain boundary diffusion on the permeation flux was considered when p(O2) was larger than 1 Pa.