Low temperature mullite crystallization in Al- and Si-alkoxide derived homogeneous gels

A homogeneous mullite gel (3:2 Al₂O₃:SiO₂ molar ratio) was made from tetraethoxysilane (TEOS) and aluminum isopropoxide (Al(OCH(CH₃)₂)₃) at room temperature within a relatively quick three days. Mullite was the only crystalline phase to form during calcination; metastable phases like aluminosilicate spinel did not appear. When heated at 20 °C/min, crystalline mullite (65 mol% Al₂O₃) started forming at 575 °C and reached 27 wt.% by 900 °C. Major crystallization occurred at ~ 1000 °C with a concurrent increase of Al₂O₃ concentration (68-69 mol%) in the mullite phase. The alumina content decreased towards stoichiometric (3Al₂O₃·2SiO₂) mullite at even higher temperatures. When the gel remained in the amorphous state, low temperature preheating significantly improved crystallization at higher temperatures. After preheating at 425 °C for 24 h, 78 wt.% of the final product was crystallized when it was subsequently annealed at 750 °C for 5 min. Only 20 wt.% crystallized without preheating.