Preparation And Characterization Of Nanometer And Micrometer Silicon Dioxide Spherical Particles

Silicon dioxide spherical particles are widely used in many fields of advanced technology, such as photonic crystals, catalyst supports, chromatographic packing materials and standard materials in measurement etc. In this article, nanometer and micrometer SiOi spherical particles were synthesized by hydrolysis of alkoxide and polymerization-induced colloid aggregation process, respectively. Also the structures of microspheres and preparation conditions were detailed studied.Monodispersed silicon dioxide spherical particles were prepared by hydrolysis of tetraethoxy silane (TEOS) in alcohol-water mixed solvents using ammonia as a catalyst. Effects of the type of solvents, the amount of water, the concentration of ammonia and temperature of hydrolysis etc. on the particle size and morphology of SiOa were investigated by transmission electron microscopy (TEM). The results show that the minodispersed silicon dioxide spherical particles could be obtained hi methanol or ethanol solvent, while agglomerated silicon dioxide particles were formed in n-propanol and n-butanol solvent. When the concentration of water and TEOS is increased or the temperature of hydrolysis is raised, the particle sizes increase. The formation mechanisms of the resultant SiCh particles under different reaction conditions were carefully investigated and discussed.The effects of acid and base catalyst on the rate of hydrolysis and morphologies of sol were investigated, the results show that monodispersed silicon dioxide spherical particles were obtained using base as catalyst, while silica sol of highly condensated three-dimensional network were obtained with acid as catalyst This is ascribed to the fact that during the hydrolysis of TEOS the rate of hydrolysis was slower and slower when acid is used as catalyst, at the same time, the midterm products began to condense and then formed highly condensated three-dimensional network. While when base is used as catalyst, the rate of hydrolysis was faster and faster, during hydrolyzing the midterm productsbegan to condense and form monodispersed silicon dioxide spherical particles.Porous silicon dioxide spherical particles were synthesized by polymerization- induced colloid aggregation (PICA). Polymer-based composite microspheres (urea-formaldehyde /SiOa) were obtained in silicon dioxide sol by the polymerization reaction of urea and formaldehyde in acid condition, and then formed porous microspheres after sintering. The morphology, size and size distribution of microspheres before and after sintering were investigated by using scanning electron microscopy (SEM). The desorption of water and ethanol and decompostion of polymers in microspheres were analyzed by using TG-DSC machine. The specific surface area before and after sintering were measured by nitrogen adsorption. The average particle size and size distribution were measured with laser particle size analyzer. The results show that urea and formaldehyde in acid solution can polymerize and form polymer microspheres with broad size distribution, however, many particles clustered together. In contrast, in the PICA process, composite microspheres appear a very narrow particle size distribution and have minimal particle aggregation. Mechanical agitation during particle growth can inhibit clustering of the aggregates. Microspheres will be destroyed due to over-agitation. The temperature of dry and sintering rose slowly in order to prevent the forming of clusters and crack of microspheres. By analysis of microsphere formation mechanisms, the results show that urea and formaldehyde undergo acid-catalyzed polymerization, and oligomers gradually separate from solution due to phase separation. In silicon dioxide sol, the oligomer is adsorbed on the surface of colloids by Wan der walls force and hydrogen bonding, then forms Polymer-based organic/inorganic composite microspheres in solution.