Densification behaviour and microstructure of gel-derived phase-pure mullite in the presence of sinter additives

Gels with high pure mullite composition have been synthesized using aluminum nitrate nanohydrate and ethyl silicate. Pure mullite has been sintered to 88.5% of the T.D., when sintered at 1600°C for 3 h. Suitable sintering aids such as SrO (0.5 wt.%) and clay (2 wt.%) have tremendous effect in promoting the densification behavior of mullite to attain density to 98 and 95% of T.D. respectively at relatively low temperatures by pressureless sintering. SrO promotes the densification of pure mullite via liquid phase sintering and also does clay assist the viscous flow sintering. Clear differences found in the microstructures of mullite could be related to the different sintering aids, which have apparent effect on the densification behavior. SrO doped samples show a duplex microstructure of fine equiaxed and elongated grain morphology, while the clay doped samples show equiaxed and acicular morphology.