aerogel, any of a group of extremely light and porous solid materials; the lightest is less than four times as dense as dry air. Aerogels are produced from certain gels (see colloid ) by heating the gel under pressure, which causes the liquid in the gel to become supercritical (in a state between a liquid and a gas) and lose its surface tension. In this state, the liquid may be removed from the gel by applying additional heat, without disrupting the porous network formed by the gel's solid component. Silica-, melamine-, and carbon-based aerogels have been produced. Silica-based aerogels are among the lightest, and some, nicknamed solid smoke or frozen smoke, are nearly transparent. Heavier aerogels were first developed in 1931 and have been used to detect high-energy particles emitted by particle accelerators . Newer, lighter aerogels with relatively high insulating properties are being tested as substitutes for the chlorofluorocarbon foams used as refrigerator insulation and as replacements for the air between the panes of double-glazed windows; other aerogels are being used as filters.

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