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 degrees C, in an oxygen partial pressure of 6.6 x 10(3) Pa. The concentration profile of O-18 in the specimens following diffusion annealing was measured by secondary ion mass spectroscopy (SIMS). In the high-temperature region (intrinsic region, above 1000 degrees C), the oxygen self-diffusion coefficient obtained using SIMS was observed to agree reasonably with that obtained by phase analysis in a previous study, but the activation energy was found to be slightly smaller. The present result, D = 3.16 x 10(-4) (T = 1100-1300 degrees C), is thought to represent the intrinsic behavior of undoped CeO2. In contrast, exp(-226 kJ mol(-1)/RT) m(2) s(-1) in the low temperature region (extrinsic region, less than 1000 degrees C), the activation energy was smaller than that in the high temperature region. Comparison wit with data reported in the literature 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(-3) exp(- 136 kJ mol(-1)/RT) m s (-1) (T = 800-1300 degrees C). The data were also in good agreement with the surface exchange coefficient in ThO2, suggesting that thr. oxygen surface exchange reaction is insensitive to cation species. (C) 2000 The Electrochemical Society.