Mullite Precursor Synthesis in Aqueous Conditions: Dependence of Mullite Crystallization and Grain Size and Morphology on Solution pH and Precursor Salt

The synthesis behavior of mullite in an aqueous system was investigated. Two kinds of aluminosilicate precursor sols were prepared by the dissolution of two kinds of salt (aluminum chloride hexahydrate, AlCl₃·6H₂O, and aluminum sulfate with 14–18 molecules of water, Al₂[SO₄]₃·14–18H₂O) into the mixture of colloidal silica sol. The effects of the solution pH, which was controlled by addition of ammonium hydroxide (NH₄OH), and the type of aluminum salt on the mullite crystallization and the grain size and morphology in the sintered materials are discussed in this paper. It has been discovered that in an aqueous system, the mullitization temperature strongly depends on the solubility and decomposition temperature of the aluminum salt used and that the pH of the initial sol impacts the grain size and morphology of mullite, in addition to the homogeneity of mixture. Mullite is synthesized at 850 °C without resorting to the addition of an organic additive or nonaqueous solvent.