Aerogel

Aerogel is a silicon-based substance and the world's lowest-density solid. It is composed of 99.8% air and is a stiff foam with a typical density of 3 mg/cm³. 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. It is nicknamed frozen smoke, solid smoke or blue smoke.

It appears bluish because the silicon dioxide scatters shorter wavelengths of light much like air in the daytime sky. Despite its diaphanous appearance, it feels like hard plastic foam.

Aerogel has many interesting properties:

a highly dendritic structure
extremely low thermal conductivity (approx. 0.017 W/(m·K)), which gives it remarkable insulative properties
a melting point of 1,200 °C

Aerogel holds 15 entries in the Guinness Book of Records for material properties, including best insulator and lowest-density solid. Aerogel can support 2000 times its own weight without collapsing.

Aerogel was first created by Steven Kistler in 1931.

Aerogel can be made of many different materials; Kistler's work involved aerogels based on silica, alumina, chromia, tin and carbon.

Uses

There are a variety of tasks for which aerogel is used. Commercially, aerogel has been used in granular form to add insulation to skylights. Its high surface area leads to many applications, such as a chemical absorber for cleaning up spills (see adsorption). This feature also gives it great potential as a catalyst or a catalyst carrier. Aerogel particles are also used as thickening agents in some paints and cosmetics.

NASA used it to trap space dust particles aboard the Stardust spacecraft. NASA also used aerogel to insulate the Mars Rover.

Due to their extremely high surface area (about 800 m²/g), carbon aerogels are used in supercapacitors, ranging up to thousands of farads of capacity.

Finally, aerogels are used in particle physics as radiators in Cherenkov detectors. ACC system of the Belle detector at KEKB is a recent example of such use.

Production

Aerogel is made by drying a gel composed of colloidal silica in an extreme environment. Specifically, scientists start with a liquid alcohol like ethanol and mix it with a silicon alcoxide precursor to form a silicon dioxide gel (silica gel). Then, through a process called supercritical drying, the alcohol is removed from the gel. This is typically done by exchanging the ethanol for liquid carbon dioxide and then bringing the carbon dioxide above its critical point. The end result removes all liquid from the gel and replaces it with gas, without allowing the gel structure to collapse or lose volume.

Commercial manufacture of aerogel blankets began around the year 2000. This blanket is a composite of silica aerogel and fiberous reinforcement that turns the brittle aerogel into a durable, flexible material. The mechanical and thermal properties of the product may be varied based upon the choice of reinforcing fibers, the aerogel matrix, and opacification additives included in the composite. One manufacturer of this aerogel composite may be found in the link below.

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