Colloidal silica can be prepared by various methods and starting materials including ion exchange of aqueous silicates, hydrolysis and condensation of silicon compounds, direct oxidation of silicon, and milling and peptization of silica powder. Various silica sols having particle sizes of 10-60 nm prepared by these methods and the preparation methods have been compared on the basis of their shape, size uniformity, sphericity, stability against pH variation, cation concentration, and price, etc. Silica sol prepared from tetraethoxysilane affords uniform size control and growth, and high purity, despite the relatively high costs. Silica sol prepared from liquid silicates affords relatively easy size and shape control; however, it is difficult to lower the alkali content to a level that is appropriate for carrying out semiconductor chemical mechanical polishing processes; in addition, the waste water treatment carried out for recovering the ion exchange resin gives rise environmental consideration. The properties of colloidal silica prepared from fumed silica powder by milling and dispersion depend on the starting silica source and it is relatively difficult to obtain monodispersed particles using this method. Colloidal silica prepared from silicon by direct oxidation has a monodispersed spherical shape and purity control with reasonable prices. It generates less waste water because it can be directly produced in relatively high concentrations. The cation fraction located in the particle relative to the free cation in the fluid is relatively lower in the silica sol prepared by the direct oxidation than others. A careful comparison of colloidal silica and the preparation methods may help in choosing the proper colloidal silica that is the most appropriate for the application being considered.
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