Intrinsic and extrinsic oxygen diffusion and surface exchange reaction in cerium oxide

Polycrystalline CeO2 with a relative density in excess of 97% was prepared. The specimens contained a lower concentration of impurities than those examined previously. Oxygen diffusion experiments were performed for the temperature range from 800 to 1300 C, in an oxygen partial pressure of 6.6 x 10 exp(3) Pa. The concentration profile of 18O in the specimens following diffusion annealing was measured by SIMS. In the high-temperature region (intrinsic region, above 1000 C), the oxygen self-diffusion coefficient obtained using SIMS showed reasonable agreement with that obtained by gas phase analysis in a previous study, but the activation energy was found to be slightly smaller. The present result, D = 3.16 x 10 exp(-4) exp(-226 kJ/mol RT) m2/s (T = 1100-1300 C), is thought to represent the intrinsic behaviour of undoped CeO2. In contrast, in the low temperature region (extrinsic region, less than 1000 C), the activation energy was smaller than that in the high temperature region. Comparison with literature data for CeO2 doped with Y and Gd suggests that the low-temperature oxygen diffusion region is controlled by a trivalent impurity. The surface exchange coefficients obtained from gas phase analysis and SIMS agreed very well with each other and were represented by k = 1.93 x 10 exp(-3) exp(-136 kJ/mol RT) m/s (T = 800-1300 C). The data were also in good agreement with the surface exchange coefficient in ThO2, suggesting that the oxygen surface exchange reaction is insensitive to cation species. 30 refs.