Sol–gel technology as representative processing for nanomaterials: case studies on the starting solution

In order to fabricate sol–gel products with desired microstructure in the form of bulk, fiber and coating film, the appropriate selection of the composition of the starting solution is of primary importance. In this paper, the effects of the composition of the starting solution on the reaction in alkoxysilane solutions, the formation of bulk and fiber, and the microstructure of a particular coating film are reviewed, based on our experiences. It is shown in the alkoxysilane and alkylalkoxysilane solutions that, besides hydrolysis and random polymerization, various reactions take place. Among them, the formation of a four-membered ring molecule in dimethyldialkoxysilane solution, formation of a cage-like cubic octamer in an tetraalkoxysilane solution containing, for example, tetramethylammonium ion and stabilization of a solution for polycomponent oxides by the addition of tartaric acid are discussed. It is also shown that the composition of the starting solution suitable for fiber drawing is different from that for the formation of crack-free bulk gels: for the fiber drawing acid catalyst and low water content are required in various oxide systems including silica, while for the bulk silica gels ammonia-catalyzed alkoxysilane solution with dimethylformamide solvent or highly acidic solution works well for bulk silica gel. Finally, it is shown that the control of microstructure of coating films can be realized by selecting the composition of the starting solution. As an example, the change of the acid content of the starting solution produces three different microstructures of polycrystalline, crystal-oriented and amorphous structure in the lithium borate coating film. As another example, the size of precipitated noble metal particles in the coating film is controlled by the water and acid content of the solution. The mechanism of the above phenomena is also discussed.