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Aerogel is composed of 99.8% air with a typical density of 3 mg/cm3. It feels like hard foam. Pressing softly won't leave any mark; pressing harder will leave a permanent dimple. Pressing hard enough will cause a catastrophic breakdown in the sparse structure causing it to shatter like glass (known as friability). Despite the fact that it is prone to shattering, it is very strong structurally, able to hold over 2000 times its own weight. Its impressive load bearing abilities are due to the dendritic microstructure, with spherical particles of average size 2-5 nm fused together to clusters, forming a three-dimensional highly porous structure of fractal-like chains with pores smaller than 100 nanometers. The average size and density of pores can be controlled during manufacture.
Aerogel is a remarkable insulator because it almost nullifies three methods of heat transfer (convection, conduction or radiation). It is a good convective inhibitor because air cannot circulate throughout the lattice. Silica aerogel is a good conductive insulator because silica is a poor conductor of heat. (Metallic aerogel, on the other hand, is a better heat conductor.) Carbon aerogel is a good radiative insulator because carbon absorbs the infrared radiation that transfers heat. The most insulative aerogel is silica aerogel with carbon added to it. SEAgel is a material similar to organic aerogel, made of agar, with a taste and consistency similar to rice cakes.
Silica aerogel is the most common type of aerogel and the most extensively studied and used. It is a silica-based substance and the world's lowest-density solid. It is derived from silica gel. The latest and lightest versions of this substance have a density 1.9 mg/cm3 (i.e., 1/530 as dense as water), and are produced by the Lawrence Livermore National Laboratory.
Silica aerogel strongly absorbs infrared radiation. It allows construction of materials that let the light into the buildings, but traps heat for solar heating.
It has extremely low thermal conductivity (approx. 0.017 W/(m·K)), which gives it remarkable insulative properties. Its melting point is 1,200 °C (2,192 °F).
Silica aerogel holds 15 entries in the Guinness Book of Records for material properties, including best insulator and lowest-density solid.