Diffusion and Reaction Studies in the System Al ₂ O ₃ ‐SiO ₂

Abstract

High‐temperature diffusion kinetics and phase relations between couples of fused SiO 2 or cristobalite and sapphire or mullite were investigated in air and in helium. Subsolidus liquid formation between sapphire and cristobalite indicates the existence of a metastable system without mullite. A liquid phase is considered to be essential for the nucleation of mullite. The growth rate of mullite exceeded its dissolution rate in semi‐infinite fused‐SiO 2 ‐sapphire couples above 1634°C. The inter‐facial liquid compositions provided data for a minor revision of the mullite liquidus curve. Diffusion coefficients calculated from the Al profiles vary greatly with concentration and temperature, resulting in a large range of values for apparent activation energy, which decreases with increasing Al 2 O 3 content (∼310 to ∼60 kcal/mol for ∼4 to ∼22 wt% Al 2 O 3 ). The diffusion process in the liquid is considered to be cooperative movement of oxygen‐containing Al and Si complexes whose nature changes with composition and temperature; this change in the diffusing species contributes to the range in the values of experimental apparent activation energies.