Chemistry-crystallization relations in molecular mullite gels

Sol-gel chemistry profoundly affects the crystallization of molecular aluminosilicate gels. Segregation of transition alumina-yielding species during gel synthesis results in the epitaxial nucleation of spinel at 980°C. When synthesis conditions leading to alumina segregation are avoided, t-mullite crystallizes at 980°C. However, it is shown, for the first time, that these gels are phase-separated prior to crystallization by a hydrothermal reaction known as dealumination. When the molecular gels are synthesized and heated to avoid this mechanism of phase separation, the gels crystallize directly to o-mullite at 700°C. This crystallization temperature for o-mullite in aluminosilicate gels is ∼ 500°C lower than those previously reported.