Preparation of Dense Mullite Ceramics through Gelcasting of Alumina Slurry Dispersed in Silica Sol

A nano‐SiO₂‐coated‐Al₂O₃ (NSCA) micro‐composite system has been derived from silica sol and alumina to prepare dense mullite ceramics by gelcasting technology. Concentrated NSCA suspension with high solid loading but low viscosity was successfully prepared, utilizing the stabilization mechanism of silica nanoparticle halos around alumina particles at pH 9.5. The defect‐free dried green body with bending strength of 5.85 MPa was obtained with only 7.7 wt.% of monomer and dense mullite ceramics was sintered at 1550°C for 2 h, through initial densification by transient viscous sintering, followed by a densification arrest and then a further densification by solid‐state diffusion at higher temperatures.