Spherical fine particles of SiO2 were prepared from a partially hydrolyzed tetra-ethyl-ortho-silicate (TEOS) solution by an ultrasonic spray pyrolysis method at 400°-700°C. The starting solution was hydrolyzed with NH4OH catalyst at 55°C in N2 atmosphere. Its viscosity was measured by an Ostwald-type viscometer in order to calculate the intrinsic viscosity [η]. The powders prepared from solutions with [η] less than 0.009dl/g were the mixture of spherical and unspherical particles. While in the case of [η] larger than 0.009dl/g, the particles were spherical and their diameter was 0.36-0.56μm. In the case of the solutions of [η] larger than 0.015dl/g, it was observed by TEM that very fine particles (diameter: -0.03μm) precipitated from the solution during the refluxing. The particle from above solution is the aggregate of the primary particles which have similar size to the particles present in above solution. The effects of pre-hydrolysis of starting solutions on the shape and microstructure of paricles were discussed.
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Nogami et al. (1987) studied this question.