SYNTHESIS AND SINTERED PROPERTIES OF MULLITE POWDER FROM SEEDED DIPHASIC AL2O3-SIO2 GEL

Diphasic Al2O3-SiO2 gel was prepared by using AlCl3 and fumed silica as the two starting materials through coprecipitation. The seeded gel was produced by incorporating a small amount of crystalline mullite particles into the diphasic Al2O3-SiO2 gel. Phase evolution as a function of heat treatment in the unseeded and seeded diphasic gels were characterized by differential thermal analysis (DTA) and X-ray diffraction (XRD). The seeding was found to promote the formation of crystals, but, in spite of that, the crystallization temperature could not be lowered. A diphasic Al2O3SiO2 gel with 4 wt% seeds could yield a well crystallized mullite without any other residual phases after calcination at 1300 degrees C for 2 h, while the unseeded gel did not show complete crystallization even though the calcination temperature was raised further. The mullite powder from seeded diphasic gel is of high purity and high activity, and can be sintered at 1650 degrees C for 4 h to over 99.0% of the theoretical density, showing a high flexural strength of 310 MPa.