Preparation And Characterization Of Mesoporous 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, micrometer SiO2 spherical particles were synthesized by polymerization-induced colloid aggregation process. Also the structures of microspheres and preparation conditions were detailed studied.The urea-formaldehyde (UF) polymer/SiO2 composite microspheres were prepared by polymerization-induced colloid aggregation (PICA) method. The formation mechanism of composite microspheres is attributed to the fact that that urea and formaldehyde firstly undergo acid-catalyzed polymerization to form oligomers, and then the sol particles are adsorbed on the chain of oligomers by Wan der walls force and hydrogen bonding. When the oligomers reach the critical chain length, they separate from solution due to phase separation. Polymer-based UF/SiO2 composite microspheres are obtained in the solution.The effects of formaldehyde/urea molar ratio (R), pH value, ethanol volume percent amount (Ve) and reaction temperature on the resulting microspheres have been investigated with scanning electron microscopy (SEM) and optical microscope. The results show that monodisperse UF/SiO2 composite microspheres with a mean diameter of about 4 m can be repeatedly obtained at the optimum conditions, which include the pH value of 0.3 ~ 0.8, the R range from 1 to 4 and the Ve of 10% ~ 30%. The composite microspheres will break or be involved in the result gel when R value is out of the range. And the surface of microspheres will become coarser with the increase of pH value, resulting in breakage. The variation of Ve cause the great of microspheres gradually. As for the change of reaction temperature, a good appearance of microspheres will be observed.After heat treatment we obtain the mesoporous SiO2 microspheres, the morphology, size and size distribution of microspheres before and after sinteringwere investigated by using the scanning electron microscopy (SEM). The desorption of water and ethanol and decompostion of polymers in microspheres were analyzed by using TG-DSC machine. The changes of specific surface area before and after sintering were measured by nitrogen adsorption.The results show that the surface of SiOi particles become coarser along with the remove of polymer template, but the particle size keep no change. The composite microspheres include more UF polymer under low pH value but turn into more looser under high pH value, so the result mesoporous SiOi microspheres have big pore volume and average pore size. And the value drop to the lowest at pH=0.8. As the increase of heat treatment temperature, the pore size distribution peak of SiO2 microspheres is very narrow, meanwhile the specific surface area is the smallest before 160 C. The polymer template is removed at 350 C, so the pore structure of SiO2 particles has a big change which involve the increase of specific surface area and the broaden of pore size distribution peak. With the continuous raise of temperature the SiO2 network will shrink little, as a result the average pore size will decrease, but the specific surface area has no obvious change.