Sintering and crystallization of mullite powder prepared by sol-gel processing

Abstract

Mullite powder with the stoichiometric composition (3Al₂O₃.2SiO₂) was synthesized by a sol-gel process, followed by hypercritical drying with CO₂. Within the limits of detection by X-ray diffraction, the powder was amorphous. Crystallization of the powder commenced at ≈1200 °C and was completed after 1 h at 1350 °C. In situ X-ray analysis showed no intermediate crystalline phases prior to the onset of mullite crystallization and the pattern of the fully crystallized powder was almost identical to that of stoichiometric mullite. The synthesized powder was compacted and sintered to nearly theoretical density below ≈1250 °C. The microstructure of the sintered sample consisted of nearly equiaxial grains with an average size of ≈0.2 μm. The effect of heating rate (1–15 °C min⁻¹) on the sintering of the compacted powder was investigated. The sintering rate increased with increasing heating rate, and the maximum in the sintering curve shifted to higher temperatures. The sintering kinetics below ≈1150 °C can be described by available models for viscous sintering.