Ferric oxide particles were coated with silica by hydrolysis of ethyl silicate. When ethyl silicate (TEOS) was hydrolyzed in an ammonia alkaline ethanol suspension of Fe2O3 under ultrasonic agitation, silica precipitated on the surface of ferric oxide particles. The obtained coated particles were in monodispersed state. The coating conditions were affected by the amount of dispersed ferric oxide, the concentrations of TEOS, NH3 and H2O. Especially, when TEOS concentration became too high, silica particles precipitated also in the dispersing media. And a linear relationship was found between the limit of concentration of TEOS (CT) against silica precipitation and the concentration of dispersed ferric oxide (CF). Therefore, in order to prevent the separation of silica particles, TEOS concentration must be kept lower than this limiting line. The specific surface area and intensity of XRD for ferric oxide were greatly decreased by silica coating. The electrophoretic mobility curve of the coated particles was in agreement with that of silica particles. From these results, it was estimated that the surface of coated particles was perfectly covered with silica.
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Azuma et al. (1992) studied this question.