Synthesis behavior and grain morphology in mullite ceramics with precursor pH and sintering temperature

The effects of the precursor pH and sintering temperature on the synthesizing behavior and morphology of mullite were studied using a stoichiometric mullite (3Al₂O₃ · 2SiO₂) precursor sol. The mullite precursor sol was prepared by the dissolution of aluminum nitrate enneahydrate [Al(NO₃)₃ · 9H₂O] into the mixture of silica sol. The precursor pH of the sols was controlled to the acidic (pH ≈ 1.5 to 2), intermediate (pH ≈ 4.5 to 5) and basic (pH ≈ 8.5 to 9) conditions. The gels dried from the synthesized aluminosilicate sols were formed into a disk shape under 20 MPa pressure; then the green bodies were sintered for 3 h in the temperature range of 1100–1600 °C. The synthesizing temperature of mullite phase was found to be above 1200 °C for pH ≈ 1.5 to 2, and above 1300 °C for pH ≈ 4.5 to 5 and pH ≈ 8.5 to 9. The grain morphology of the synthesized mullite was changed to a rod shape for pH ≈ 1.5 to 2, and granulate shape for pH ≈ 4.5 to 5 and pH ≈ 8.5 to 9 with increasing sintering temperature. It was found that the morphology of mullite particle was predominantly governed by precursor pH and sintering temperature. However, at higher pH, the precursor pH and sintering temperature did not affect the synthesis behavior and grain morphology.