Preparation of silica micro spheres via a semibatch sol–gel method

Silica micro spheres could be obtained through a semibatch sol–gel process, which was carried out by adding a solution of tetraethyl orthosilicate in ethanol into a mixture batch of KCl, ethanol, NH₃•H₂O, and water. Compared with other methods the semibatch sol–gel method could produce micro size silica spheres with narrow size distribution. The influence of reaction factors to the size and size distribution of silica spheres were studied. K⁺ could help obtaining large size silica but also could reduce the charge shielding effect, which caused the collision of silica spheres. Water and NH₃•H₂O were necessary for the hydrolysis of tetraethyl orthosilicate. However, too much water could form thicker hydration shell around K⁺, and more NH₃•H₂O in mixture batch would increase number of the nuclei of the solution. When the consumption rate of active silanol for the growing of silica could not offset its formation rate, a small silica size, and broad size distribution would occour. Therefore, the amount of tetraethyl orthosilicate in the reaction and its adding rate should be careful controlled. It was also found that the reaction temperature and holding time, which is the time for stirring of solution after tetraethyl orthosilicate injection, had no significant influence on the particle size.