The process of coating on ultrafine particles by surface hydrolysis reaction in a fluidized bed reactor

The processes of dehydration of alpha -FeOOH and the hydrolysis of tetraethylorthosilicate (TEOS) to form a SiO2 coating on the surface of particles in a fluidized bed reactor (FBR) was investigated. The influence of the bed temperature, reaction time, TEOS concentration and the ammonia concentration were studied. Using the method of dehydration, SiO2 coating and reduction were completed in one step, which led to the production of SiO2-coated Fe3O4 magnetic powders with improved characteristics. The oxidation mechanism of the Fe3O4 magnetic powders, coated with a SiO2 thin-layer produced by FBR-CVD, has been identified as a three-dimensional spherical symmetry diffusion reaction. The activation energy is increased with increasing silica content. The protective silica layer on the Fe3O4 magnetic powder, formed by the FBR-CVD technique, improves the oxidation resistance of Fe3O4. (C) 2000 Elsevier Science B.V. All rights reserved.