Lattice Diffusion Kinetics in Undoped and Impurity‐Doped Sapphire (α‐Al ₂ O ₃ ): A Dislocation Loop Annealing Study

Abstract

The shrinkage rates of small prismatic dislocation loops in thin foils of sapphire (α‐Al 2 O 3 ) have been determined using transmission electron microscopy; between observations the thin foils were annealed ex situ in the temperature range 1200° < T < 1500°C. The shrinkage rates of individual loops were used to determine the diffusivity of the rate‐controlling species, assumed to be oxygen. The loop annealing results agree well with an extrapolation of oxygen self‐diffusion data for undoped crystals obtained by conventional tracer techniques in the temperature range 1400° < T < 1900°C. The oxygen diffusion rate was slower in 600‐ppm‐Ti‐doped and faster in 250‐ppm‐Mg‐doped crystals compared to undoped sapphire, consistent at first sight with a “classical” picture of diffusion via oxygen vacancies in α‐Al 2 O 3 . However, consideration of the experimental activation energies in terms of the concentration of free point defects suggests that substantial modification of the classical picture may be necessary.