AEROGELS

The first aerogels were made by S. S. Kistler in 1931. Aerogels are produced by a sol-gel process followed by super-critical drying of the wet gel in an autoclave. Their porosity extends from about 1 to 100 nm, which is the reason why properly made aerogels are highly transparent. Aerogels have a spongelike, open-pore structure with a large inner surface. Monolithic aerogels are used in Cerenkov detectors in high-energy physics; several houses have been insulated with layers of translucent granular aerogels for the purpose of passive solar energy usage; opacified aerogel powders are being tested as substitutes for chlorofluorocarbon-blown polyurethane foams and as thermal superinsulations in heat-storage systems; experiments have been performed with aerogels as catalytic substrates, acoustic impedance matching layers, precursors for high-quality glasses, containment for fusion fuels, and gas filters; and aerogels are used in radioluminescent light and energy sources. The attraction that these low-density materials exert on physicists, chemists, and material scientists is recognized from the upsurge of publications on aerogels: about 200 papers are currently published per year. Recently, the periodical Science has rated aerogels as one of the top ten scientific and technological developments.