Aerogel

Aerogel is a silicon-based substance often called frozen smoke or blue smoke. It is composed of 99.8% air and is a stiff foam with a density of 3 mg per cm³ (depending on amount of solid present), which makes it the world's lowest-density solid.

This substance has many interesting properties including a highly dendritic structure, extremely low thermal conductivity (approx. 0.017 W/mK), which gives it remarkable insulative properties, a melting point of 1,200 °C.

There are a variety of tasks for which aerogel is used. One use is by NASA for trapping dust particles, as it did aboard the Stardust spacecraft. NASA also used aerogel to insulate the Mars Rover. 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. This feature also gives it great potential as a catalyst.

Aerogel is not a recent invention, as it was first created by Steven Kistler in 1931. It 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 silicon dioxide to form a 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 (a step first included by scientists at Berkeley National Lab) 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. 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 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.

External links

• Nasa photos of aerogel.
• Lawrence Berkeley National Laboratory page on the Thermal Properties of Silica Aerogels
• Another LBL article covering the development of aerogels