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.