Spherical alumina (hydrous) oxide particles of narrow size distribution were prepared by first hydrolyzing aluminum sulfate solutions at elevated temperatures in the presence of urea. The system was quenched before any solids were formed. Dilution of the prepared solutions at room temperature led to sol formation. The same procedure produced surface coatings on admixed, preformed solid particles. Uniform hematite, chromium hydroxide, and titania were used as core materials. The thickness of the coatings could be varied by proper adjustment of the concentrations of particles, aluminum salt solutions, and reaction time.
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Kratohvil et al. (1987) studied this question.
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