Sintering and Microstructure of Mullite Aerogels

Mullite (3Al2O3·2SiO2) was prepared by a sol-gel process and dried by supercritical extraction with CO2. the aerogel (\u3c0.05 of the theoretical density of mullite) experienced a shrinkage of up to 0.6 and reached a density of only ∼0.5 of theoretical after 1 h at 1350°C Mechanically compacted aerogels, however, sintered to nearly theoretical density below 1200°C. This density is somewhat higher than those of gels prepared by conventional drying (i.e., exposure to the atmosphere) and is considerably higher than mullite prepared from mixed powders at higher temperatures. Although the X-ray diffraction pattern of the sintered gels was almost identical to that of mullite, transmission electron microscopy and energy-dispersive microanalysis showed two types of grain structure. Elongated grains with an Al/Si atomic ratio corresponding to that of stoichiometric mullite were surrounded by equiaxed grains with a lower Al/Si ratio