| The formation of ultrafine hydrous oxide particles by the hydrolysis of metal alkoxides was investigated. Initially, the growth of Stober silica spheres was studied experimentally and modeled by solving the hydrolysis and condensation kinetic expressions and applying the method of moments to expedite the solution of the governing population balance equation. The model was applied over a wide range of TEOS, water, and ammonia concentrations. A comparison of the experimental and theoretical results indicated that the particles grew by controlled aggregation at high water concentrations and ammonia concentrations. At low water and ammonia concentrations, the particles grew by the addition of soluble, or ramified, species to the particle surface.; A qualitative evaluation of titania, zirconia, titania-zirconia and titania-alumina systems shows that, like silica, the formation of roughly spherical particles involved four steps: (a) formation of soluble polymers, (b) precipitation and collapse of polymers, and (c) particle growth by controlled aggregation. However, contrary to silica where spherical particles were formed in all low molecular weight alcohols, solvent greatly affected the morphology of these particles. Spherical particles were easily formed by the hydrolysis of titanium, zirconium, and mixed titanium-zirconium alkoxides in primary alcohols. Irregular shaped gelatinous precipitates were formed in tert-butyl alcohol. Steric effects played an important role in particle morphology when hydrolysis rates were very fast. The hydrolysis of titanium-aluminum alkoxides resulted in gelatinous precipitate for high aluminum content to roughly spherical particles for low aluminum concentrations.; As an outgrowth of the homogeneous precipitation studies, a new reactor, the electric dispersion reactor (EDR), was invented to produce ultrafine powders by electrodispersion and the subsequent reaction of aqueous drops emanating from a nozzle. By varying reactant concentrations and by strategically locating reactants in the proper phase, porous and dense silica particles have been made as well as the composite material of 1:2:3 yttrium, barium and copper. Finally, extensive electrohydrostatic computations elucidate the effect of reactor geometry and drop size on the stability of pendant drops hanging from a conducting pipe. |
