Anisotropic grain growth in diphasic-gel-derived titania-doped mullite

Densification and anisotropic grain growth in diphasic-gel-derived, titania-doped mullite were studied. Titania enhanced initial and intermediate stage densification in diphasic mullite gels by reducing the glass viscosity. Rodlike anisotropic mullite grains started to grow in titania-doped diphasic mullite gels once a dense, equiaxed microstructure was achieved. The onset temperature for anisotropic grain growth decreased with increasing titania concentration because the sintering temperature for final-stage densification decreased. The lowest onset temperature for anisotropic grain growth was ∼1500°C in 5 wt% titania-doped mullite. The aspect ratio and area fraction of anisotropic mullite grains increased with higher titania concentration and were strongly dependent on the initial titania particle size. Kinetic studies demonstrated that anisotropic grain growth in titania-doped diphasic mullite gels followed the empirical equation Gⁿ - G₀ⁿ = Kt, with growth exponents of 3 and 6 for the length [001] and thickness [110] directions, respectively. The activation energies for grain growth were 690 kJ/mol for the length and 790 kJ/mol for the thickness directions.