Oxygen tracer diffusivities of low- and high-alumina mullite ceramics (72wt%Al2O3, 28wt%SiO2 and 78wt%Al2O3, 22wt%SiO2, respectively) were determined. Gas/solid exchange experiments were conducted in an atmosphere enriched in the rare stable isotope 18O, and the resulting 18O isotope depth distributions were analyzed by using SIMS depth profiling. The investigation showed that grain-boundary diffusivities for both mullite ceramics were several orders of magnitude higher than mullite volume diffusivity. Activation enthalpies of O diffusion were 363kJ/mol for the low-alumina and 548kJ/mol for the high-alumina materials. Because the glassy grain-boundary films were not identified, the differences between the low- and high-alumina materials might be explained by different impurity concentrations in the grain boundaries of the two materials.

Oxygen Grain-Boundary Diffusion in Polycrystalline Mullite Ceramics. P.Fielitz, G.Borchardt, M.Schmücker, H.Schneider, P.Willich: Journal of the American Ceramic Society, 2004, 87[12], 2232