Oxygen grain-boundary diffusion in polycrystalline mullite ceramics

Oxygen tracer diffusivities of low‐ and high‐alumina mullite ceramics (72 wt% Al₂O₃, 28 wt% SiO₂ and 78 wt% Al₂O₃, 22 wt% 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 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 oxygen diffusion were 363 ± 25 kJ/mol for the low‐alumina and 548 ± 46 kJ/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