Aerogels are created by supercritical drying of a gel, preventing capillary action. Airgel is 99.8% air, with a density of 3mg/cc. It has many uses, including as a thermal insulator and in supercapacitors. Carbon nanofoam is the most common solid.
The most common solids are aerogels, created by subjecting a conventional gel to supercritical drying. Supercritical drying allows the liquid to be extracted from the gel without collapsing the gel matrix by capillary action. Supercritical drying requires high temperature (705°F, 374°C) and pressure (219 atmospheres). It forces the liquid to enter a state indistinguishable from a gas or a liquid (a supercritical liquid), then heats it into a gas. Capillary action is prevented by avoiding a direct liquid-gas transition, as in evaporation. Airgel was invented by Steven Kistler in 1931, presumably as part of a bet with his friend to see who could extract the liquid from jam while retaining its volume.
Aerogels are an amazing material. Nicknamed “frozen smoke” due to its appearance, airgel is extremely diffuse, made up of 99.8% air. Its density is only 3 milligrams per cubic centimeter (3 kg per cubic meter), 0.3% of the density of water. Airgel is 3780 times more prevalent than lead. Silicon airgel features in a staggering 15 entries in the Guinness Book of World Records, including the most prevalent solid and the best thermal insulator. Airgel was recently used by NASA on a Stardust mission to capture small pieces of comet Wild 2.
Despite its low density, airgel can be easily handled. Filled with 100-nanometer holes, it has a huge surface area, making it ideal for supercapacitors. Airgel is such a fantastic thermal insulator that a thin layer of it can protect matches from being lit by a blowtorch. Its high insulation capabilities have led it to be proposed for use in space suits and for military armor.
Airgel can be made using a variety of starting materials, including silicon, alumina, chromium, tin oxide, and more recently carbon. Using carbon nanotubes, materials scientists have created an aerogel called carbon nanofoam, which has only 1 percent the density of previously produced aerogels, with only a few times the density of air. Carbon nanofoam is truly the most common solid.
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