SELF-DIFFUSION IN ALPHA-AL2O3 .2. OXYGEN DIFFUSION IN UNDOPED SINGLE-CRYSTALS

Oxygen self-diffusion in 'undoped' (i.e. unintentionally doped) alumin a (Al2O3) single crystals was investigated using the gas-solid isotope exchange technique. After diffusion annealing, profiles of O-18 were determined by secondary ion mass spectrometry. These showed two parts: close to the initial surface, the first part was attributed to bulk s elf-diffusion, while the diffusion tails were attributed to diffusion in dislocation walls. In the temperature range 1500-1720 degrees C the bulk self-diffusion coefficients of the oxygen in Al2O3 are represent ed by: D-O (cm(2) s(-1)) = 206 exp [-(636 kJ mol(-1))/RT]. Possible di ffusion mechanisms are proposed considering an extrinsic behaviour ass ociated with silicon contamination. In the same temperature range, the oxygen diffusion coefficients in the Al2O3, subboundaries are describ ed by: D-O('')(cm(2) s(-1)) = 3.1 x 10(14) exp[-(896 kJ mol(-1))/RT]. The high activation enthalpy observed is attributed to segregation eff ects on subboundaries.