SOME PROPERTIES OF MULLITE POWDERS PREPARED BY CHEMICAL-VAPOR-DEPOSITION .1. PREPARATION OF MULLITE POWDER

The chemical vapour deposition (CVD) technique based upon reaction amo ng aluminium chloride (AlCl3), silicon chloride (SiCl4) and oxygen was applied to produce submicrometre-sized mullite (3Al(2)O(3) . 2SiO(2)) powder. The conditions for preparing the best crystalline mullite wer e as follows: (i) the reaction temperature, 1200 degrees C; (ii) the f low rate of carrier gas (Ar) of AlCl3, 0.3 dm(3) min(-1), and that of SiCl4, 0.3 dm(3) min(-1); (iii) the sublimation temperature of AlCl3, 180 degrees C, and the evaporation temperature of SiCl4, 25 degrees C; and (iv) the flow rate of oxygen, 0.9 dm(3) min(-1). The as-prepared powder contained mullite, a small amount of gamma-Al2O3 (Al-Si spinel) and amorphous material; this powder was composed of spherical primary particles of similar to 0.05 mu m diameter. Although only mullite was present at the calcination temperature of 1300 degrees C, a small amo unt of alpha-Al2O3 was formed at 1400-1700 degrees C. Agglomeration du e to primary particle growth started at temperatures exceeding 1400 de grees C.