Mullite fine powders were prepared from the water solution of aluminum nitrate and ethyl silicate hydrolyzed with an HNO3 catalyst by spray decomposition using an ultrasonic atomizer. 0.05-0.1mol/dm3 of HNO3 concentration was necessary to obtain the stoichiometric mullite powders. As-prepared particles were porous, spherical and were of a sub-micrometer size with narrow size distribution. The average particle size increased with increasing concentration of the starting solution and carrier gas flow rate. On the other hand, the average particle size decreased with increasing decomposition temperature. The size distribution and morphology of the particles did not change with the experimental factors, such as concentration of starting solution, carrier gas flow rate and decomposition temperature. The Al2O3/SiO2 ratio of as-prepared powders determined by fluorescence X-ray analysis was about 3:2. Powder X-ray diffraction showed that as-prepared powders were amorphous and crystallized to mullite at 1000°C. The spherical morphology was also retained after the crystallization.
