Diffusion and Reaction Studies in the System Al2O3‐SiO2

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Abstract
High‐temperature diffusion kinetics and phase relations between couples of fused SiO2or 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‐SiO2‐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 Al2O3content (∼310 to ∼60 kcal/mol for ∼4 to ∼22 wt% Al2O3). 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.