The system Al2O3-SiO2 is studied using co-precipitated gels with pH values in the range 5.0-8.0, obtained by the addition of urea to alcohol solution of mixed siliconethylester and aluminiumethylester.Following the combustion of urea between 500°-600°C, the system appears to be amorphous up to about 1000°C. At this temperature an exothermic reaction occurs and a spinel-type phase is formed. It has various amounts of Si4+ substituted with Al3+ in the position of four co-ordination and its lattice parapeter increases as the Al2O3/SiO2 ratio increases.Mullite is formed at higher temperatures by the thermal decomposition of the spineltype phase. In the temperature range 1000°-1200°C mullite exhibits two-dimensional grain growth. The exothermic peak of the DTA-curve at about 1200°C is related to the velocity of formation of mullite and at temperatures higher than 1200°C mullite shows a remarkable grain growth. Chemical composition of mullite is not always constant and the possible existence of mullite containing more silica than 3:2 is suggested without saying the existence of the ratio Al2O3/SiO2=3/2-2/1. According to the electron-microscopic studies mullite showed always a needlelike morphology irrespective of the mode of formation.It is suggested from the measurement of the lattice spacing that the cristobalite formed in the silica-rich gel has some solid solution of alumina and is different from that produced from silica ge lalone.
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Yamada et al. (1962) studied this question.