Effect of precursor pH on the formation characteristics of sol-gel mullite

Mullite precursor gel of stoichiometric composition 3Al 2O 3 · 2SiO 2 has been prepared by reacting boehmite sol and tetraethyl orthosilicate (TEOS) in a stoichiometric ratio. The pH of the preparation medium was varied from pH 3.5–8. The sol to gel transformation of the precursor was studied using optical microscopy. Phase formation characteristics of the heated gels studied from X-ray frequencies indicate that the precursor gel prepared under low pH conditions forms crystalline mullite at 1250°C. However at higher pH conditions α-Al 2O 3 was the major phase. The mullite phase was also formed but the α-Al 2O 3 phase was retained even at high temperatures (1500 °C). The results show that the pH of the precursor at the time of gelation can highly influence the gelation characteristics, mullite formation and microstructural features.