Aerogels are highly porous materials with large internal surface area and large pore volumes. Their densities are as low as 3 kg/m3 and have porosities as high as 99.9%. This makes them excellent thermal insulators.
Although there are other types of aerogels, such as carbon and alumina, silica aerogels are the most common. They are made with a liquid alcohol like ethanol which is mixed with a silicon alkoxide precursor to form a silicon dioxide sol gel (silica gel). However, removing the aerogels from the solvent bath for common use can be problematic.
Since the structure is so fine, normal drying at atmosphere collapses the network rendering to dust. This is caused by normal capillary pressure at the liquid/vapor interface on the inside of the pore. The energy of vaporization is greater than the wall strength of the pore.
Because a supercritical fluid has no surface tension, it can be used to dry aerogels without consequence. The end result removes all liquid from the gel and replaces it with gas, without allowing the gel structure to collapse or lose volume.
For more information: http://appliedseparations.com/Supercritical/SCF_Uses/SCF_for_Aerogels.asp
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