Aerogels are a special class of open-cell foams derived from the supercritical drying of highly cross-linked inorganic or organic gels. These materials have ultrafine cell/pore sizes (less than 1,000 Å), continuous porosity, high surface area (400–1000 m2/g), and a microstructure composed of interconnected colloidal-like particles or polymeric chains with characteristic diameters of 100 Å. This microstructure is responsible for the unusual optical, acoustic, thermal, and mechanical properties of aerogels. For example, aerogels can be prepared as transparent, porous solids because their ultrafine cell/pore size minimizes light scattering in the visible spectrum. Figure 4.1 shows the different aerogels that will be discussed in this article.

The hydrolysis and condensation of metal alkoxides is the most common synthetic route for the formation of inorganic aerogels. Inorganic aerogels have been prepared from monomers such as tetraisopropoxy titanate, aluminum secbutylate, and zirconium isopropoxide. Nevertheless, the majority of scientific research has concentrated on the sol-gel polymerization of tetramethoxysilane (TMOS), or the less toxic tetraethoxysilane (TEOS). The resultant silica aerogels are being investigated for applications ranging from window insulation to the collection of hypervelocity partis cles in space.

The sol-gel polymerization of a multifunctional monomer in solution, leading to the formation of an aerogel, is not unique to metal alkoxides. Organic reactions that proceed through a sol-gel transition have been discovered recently.