Correlation of the precursor type with densification behavior and microstructure of sintered mullite ceramics

The effect of alumina component in diphasic mullite precursors containing alkoxy-derived silica on the crystallization and sintering behavior of compacts was studied. The phenomena observed were characterized using differential thermal analysis (DTA), powder X-ray diffraction (XRD), dilatometry and transmission and scanning electron microscopy (TEM, SEM). In order to change the characteristics of as-prepared gels, the alumina source was varied while keeping the silica source constant. Al(NO3)3·9H2O, γ-Al2O3 and boehmite (γ-AlOOH) were used as the alumina source and TEOS as the silica source. Clear differences were found in the microstructure of sintered samples derived from the precursors with aluminum nitrate nonahydrate in comparison to the samples containing γ-Al2O3 or boehmite (γ-AlOOH). The former exhibited elongated mullite grains embedded into the “equiaxial mullite matrix”. This morphology is due to the overlapping of mullite crystallization and viscous flow sintering temperatures. Transient alumina, either added as γ-Al2O3 or formed in situ by decomposition of boehmite, shifts the mullite formation above the sintering temperature, and enables formation of equiaxial mullite. The smaller are the transient alumina particles, the smaller are mullite grains of sintered bodies.