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Diffusion and Reaction Studies in the System Al 2 O 3 ‐SiO 2
Author(s) -
DAVIS ROBERT F.,
PASK JOSEPH A.
Publication year - 1972
Publication title -
journal of the american ceramic society
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.9
H-Index - 196
eISSN - 1551-2916
pISSN - 0002-7820
DOI - 10.1111/j.1151-2916.1972.tb13421.x
Subject(s) - mullite , cristobalite , activation energy , diffusion , atmospheric temperature range , nucleation , dissolution , metastability , analytical chemistry (journal) , phase (matter) , liquidus , materials science , chemistry , sapphire , mineralogy , thermodynamics , quartz , metallurgy , chromatography , ceramic , physics , organic chemistry , laser , optics
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.