SPHERICAL MULLITE PARTICLES PREPARED BY HYDROLYSIS OF AEROSOLS

Nonhollow spherical particles were prepared by hydrolysis of liquid aerosols consisting of mixtures of silicon methoxide and aluminum sec-butoxide. In order to obtain particles with mullite composition, some chloroform had to be added to the alkoxides mixture. TEM microanalyses carried out in single particles revealed a rather narrow distribution in composition around the mullite nominal value. The particles, as prepared, were amorphous. Their thermal evolution was studied by thermal analyses, X-ray diffraction, and infrared and Al-27, Si-29 NMR spectroscopies. Crystallization of pseudo-tetragonal mullite was detected at 950-degrees-C, suggesting a high degree of Al-Si mixing in the original particles. At the same time, some silica segregation was also observed. Between 1200-degrees-1400-degrees-C, the pseudo-tetragonal/orthorhombic mullite transformation took place, accompanied by the incorporation of segregated silica into the originally formed mullite.