ANISOTROPIC GRAIN-GROWTH IN DIPHASIC-GEL-DERIVED TITANIA-DOPED MULLITE

Densification and anisotropic grain growth in diphasic-gel-derived, ti tania-doped mullite were studied. Titania enhanced initial and interme diate stage densification in diphasic mullite gels by reducing the gla ss viscosity. Rodlike anisotropic mullite grains started to grow in ti tania-doped diphasic mullite gels once a dense, equiaxed microstructur e was achieved. The onset temperature for anisotropic grain growth dec reased with increasing titania concentration because the sintering tem perature for final-stage densification decreased. The lowest onset tem perature for anisotropic grain growth was similar to 1500 degrees C in 5 wt% titania-doped mullite. The aspect ratio and area fraction of an isotropic mullite grains increased with higher titania concentration a nd were strongly dependent on the initial titania particle size. Kinet ic studies demonstrated that anisotropic grain growth in titania-doped diphasic mullite gels followed the empirical equation G(n) - G(0)(n) = Kt, with growth exponents of 3 and 6 for the length [001] and thickn ess [110] directions, respectively. The activation energies for grain growth were 690 kJ/mol for the length and 790 kJ/mol for the thickness directions.