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 per cm³. It is nicknamed frozen 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/mK), 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.

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. This feature also gives it great potential as a catalyst.

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

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 (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.

Additional photographs from NASA

External links

• NASA photos of aerogel
• Thermal Properties of Silica Aerogels from the Lawrence Berkeley National Laboratory (LBL) website
• Aerogel Research at LBL: From the Lab to the Marketplace, another article from the LBL website