Effect of composition on the sintering and microstructure of diphasic mullite gels

Diphasic gels with alumina/silica ratio of 68/32, 71.8/28.2, 74/26, and 76/24 were prepared and used to study the effect of the precursor composition on the mullite formation process, densification behavior, and microstructure development. Alumina contents of mullites in the samples sintered at high temperature (1650 °C) for 4 h or at low temperature (1550 °C) for longer time increased with the increase in alumina/silica ratio of the gels. A sharp decrease in densification rate was observed on transition from the mullite+silica-rich liquid phase field to the mullite solid solution. This decrease was associated with elimination of the viscous flow of the amorphous silica phase. However, the densification rate was higher for the samples with excess alumina than for the 74/26 sample. Both in the silica-rich sample (68/32) and the stoichiometric mullite sample (71.8/28.2), elongated mullite grains were observed. Such elongated grains grew at the expense of the surrounding smaller mullite grains. In the 74/26 and 76/24 samples, mullite grains were equiaxed. Furthermore, in the latter, elongated α-Al₂O₃ grains were observed.