Recent advances in sol–gel synthesis of monolithic silica and silica-based glasses

Studies on sol–gel synthesis of monolithic silica and silicate glasses are summarized. Major research efforts have been devoted to avoid the fracture problems, associated with the extensive shrinkage of wet gels during drying. Supercritical drying, incorporation of silica fillers, and reduction of the surface tension of pore liquid have been used to form monolithic dried gels without fracture. More recent studies have shown that wet gels derived from alkoxides can be dried in a relatively short time without fracture at ambient pressure, by employing controlled drying accompanied by the cavitation of the pore liquid, macroscopic phase separation in parallel with gelation, and fluorine modification. By optimizing the synthesis procedure, it is also possible to reduce the use of reagents such as external alcohols, organic solvents, and other additives, which are removed during synthesis and are unnecessary in the final products. Another subject of considerable interest is the development of silica-based functional glasses that are difficult to form by conventional melt-quench and vapor-phase methods, by taking advantage of melt-free processes conducted at relatively low temperatures. Most of the recent studies on the functionalization of sol–gel-derived silica glasses are concerned with the doping of nanoparticles, rare earth ions, and/or fluorine.