38. Sol-Gel Glasses

Sol–gel processing is a nonmelting path to forming primarily silicate glasses. The most widely used precursors for the sol–gel process are metal alkoxides that undergo hydrolysis and condensation polymerization. Pure silica, binary compositions and multicomponent compositions are reacted to generate oxide polymers in the presence of water and alcohols. The oxide polymers grow and crosslink to produce a gel network at the sol–gel transition. After gelation, the solvents are removed, leaving behind a microporous skeleton that can be collapsed to a chemical and physical duplicate of a melted glass. The sol–gel process also refers to solution routes that involve soluble salts and colloidal routes that involve metastable suspensions of oxide nanoparticles. Combinations of alkoxides, salts and colloids are all considered sol–gel routes. The advantage of the sol–gel process, compared to melting and quenching, is that the process is carried out largely at room temperature. The low temperature makes the sol–gel process compatible with organic polymers, which enables formation of organic–inorganic hybrids. Also, when it is not necessary to remove the porosity, the sol–gel process is a means to form microporous and macroporous glasses.