Synthesis and sintering of pre-mullite powders obtained via carbonate precipitation

Ultrafine pre-mullite powders, which yield mullite at high temperatures, have been prepared from colloidal silica and aluminium nitrate via carbonate coprecipitation and followed by calcination. The chemical and structural evolutions of the as-prepared precipitation powder during thermal treatment were studied and the sinterability of pre-mullite powders were investigated. The as-prepared powders are comprised of ammonium aluminum carbonate hydroxide and amorphous silica, which convert to mullite via the Al–Si spinel phase at 1250 °C. Calcination of the as-prepared powders at 1000 °C gives a very active powder which can be reactively sintered to 98.2% theoretical density at 1550 °C. The sintered body possesses a relatively uniform chemical composition with Al₂O₃/SiO₂ mole ratio of 1.48 and exhibits a very fine interlocking equiaxed and polygonal grain morphology with grain size of 100–200 nm.