Self-diffusion in α‒Al₂O₃. II. Oxygen diffusion in ‘undoped’ single crystals

Abstract

Oxygen self-diffusion in ‘undoped’ (i.e. unintentionally doped) alumina (Al₂O₃) single crystals was investigated using the gas-solid isotope exchange technique. After diffusion annealing, profiles of ¹⁸O were determined by secondary ion mass spectrometry. These showed two parts: close to the initial surface, the first part was attributed to bulk self-diffusion, while the diffusion tails were attributed to diffusion in dislocation walls.

In the temperature range 1500–1720°C the bulk self-diffusion coefficients of the oxygen in Al₂O₃ are represented by: D_o(cm²s⁻¹) = 206 exp[-(636 kJ mol⁻¹)/RT]. Possible diffusion mechanisms are proposed considering an extrinsic behaviour associated with silicon contamination. In the same temperature range, the oxygen diffusion coefficients in the Al₂O₃ subboundaries are described by: D″_o(cm² s⁻¹) = 3.1 × 10¹⁴exp[-(896 kJ mol⁻¹)/RT]. The high activation enthalpy observed is attributed to segregation effects on subboundaries.