ULTRAFINE PURE MULLITE POWDER PREPARED BY SOL-GEL METHOD

Ultrafine pure mullite powder (3Al2O3.2SiO2) was prepared by the sol-gel method using silicon tetraethoxide and aluminum nitrate as starting materials. The phase transformation process and the effects of a catalyst on mullite gels were determined by differential thermal analysis (DTA) and X-ray diffraction (XRD). The results show that a pure mullite phase was obtained at 980-degrees-C in a nitric acid-catalyzed gel. In an ammonia-catalyzed gel, a pure mullite phase forms at 1200-degrees-C, while a spinel appears at 980-degrees-C. In an ammonium acetate-catalyzed gel, mullite and alpha-alumina form at 980-degrees-C and the main phase mullite appears at 1200-degrees-C. A pure mullite phase was obtained at 1300-degrees-C. The mean particle size of the mullite powder obtained from the nitric acid-catalyzed gel is < 0.5 mum and its particle size distribution is narrow. Mullite ceramics hot-pressed at 1600-degrees-C under 25 MPa for 90 min have a density of 3.168 g/ml, and flexural strength at 1200-degrees-C of 280 MPa.