Oxygen self-diffusion in oxides

Some topics are presented on the self-diffusion coefficient of oxygen ions which were determined for oxide systems by the gas-solid exchange method using oxygen-18 as a tracer. For a single crystal MgO.Al2O3 spinl, the self-diffusion coefficient of oxygen ions was about five orders of magnitude smaller as compared with that of magnesium ions in the temperature range from 13500 to 16000C. The determined self-diffusion coefficient of oxygen ions was close to the apparent self-diffusion coefficient calculated from the densification rate in sintering spinel. The self-diffusion coefficient of oxygen ions for MgO.2Al2O3 and MgO.3Al2O3 was larger than that for MgO.Al2O3, but the difference was relatively small. This is interpreted that the solid solution of Al2O3 in MgO.Al2O3 spinel increases the concentration of cationic vacancies, but oxygen vacancy is varied little. The self-diffusion of oxygen in UOsub(2+x)-MgO solid solution system was investigated at 14000C under the oxygen partial pressure of 5.5 mmHg. Increase of the mole fraction of MgO in the solid solution should decrease interstitial oxygen atoms, and lead to the decrease of the self-diffusion coefficient of oxygen. This was confirmed experimentally. Further the increase of MgO mole fraction in the solid solution should produce or increase vacancies at oxygen sites, but the self-diffusion coefficient of oxygen determined experimentally remains unchanged. (Kato, T.)