Crystallization of a coprecipitated mullite precursor during heat treatment

Abstract

Powder of mullite composition (3Al₂O₃·2SiO₂) has been made by a coprecipitation method. The evolution of mullite in this precursor powder during heat treatment has been studied using differential thermal analysis, electron microscopy and X-ray diffraction techniques. It is shown that during calcination below 1100°C the coprecipitate develops γ-Al₂O₃ and perhaps cristobalite crystallites within the basic grains, whose morphology is otherwise invariant with temperature. Mullite forms above 1100°C by reaction of these γ-Al₂O₃ and SiO₂ crystallites, and the grain morphology changes markedly. Small exothermic events occur at 1000 and 1250 °C. The former is associated with the decomposition of a small content of aluminosilicate or perhaps with the conversion of γ- to θ-Al₂O₃, and the latter with mullite formation. For comparison, the behaviour of a polymeric mullite precursor during calcination is also examined. This material showed a large exothermic event at 1000°C which could be associated with the decomposition of the (amorphous) aluminosilicate to crystalline γ-Al₂O₃ and SiO₂, and a small exothermic event at 1250° C due to mullite formation.