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Aerogels

Dateline: 12/07/98

By Alan Bruzel

A typical aerogel is a solid foam consisting of 5% silica (SiO₂) and 95% air-filled pores. The silica particles are 2 to 5 nanometers (nm) in diameter; the pores are about 20 nm wide. Some silica aerogels contain as little as 0.13% silica with the remainder (99.87%) being air. Aerogels consequently inherit characteristics from both silica and air. This combination leads to some rather striking properties.

Air has a dielectric constant of 1. Being mostly air, aerogels have a dielectric constant of 1.1. Aerogels are thus being touted as replacements for present printed circuit board insulators whose dielectric constants are 2.5. Use of aerogel insulators may allow current carrying components to lie closer to one another, squeezing more calculating power from smaller computer chip areas.

The refractive index of air is 1.00029; silica aerogels (transparent to visible and infrared light) have a refractive index of 1.05. Add to this feature their heat insulating ability (R values of 10 to 15 per inch), and aerogels become prime candidates as transparent, insulating fillers of double-pane windows.

Aerogels also insulate against noise. The speed of sound in air is about 350 meters per second (m/s). Sound velocity in aerogels is about 100 m/s making these materials suitable for use in speaker cabinets.

How are silica aerogels produced? React tetraethylorthosilicate with water in the presence of a catalyst.

Si(OCH₂CH₃)₄ + 2H₂OSiO₂ + 4CH₃CH₂OH

Then use liquid carbon dioxide to replace the ethanol formed by the hydrolysis reaction. Remove the carbon dioxide and replace with air. A solid, transparent, bluish aerogel results. Organic aerogels of resorcinol-formaldehyde (red) and carbon (black) have also been prepared, and share many similar physical characteristics with the silica aerogels.

Because of their extremely low densities – from 0.35 gram per cubic centimeter (g/cm³) down to 0.003 g/cm³ (the density of air is 0.001 g/cm³) – aerogels have seen use in space applications where payload weight is a paramount consideration. Aerogels have landed on Mars as insulation on the rover, and have collected space dust as part of a series of experiments on the Mir space station. In addition to their other impressive attributes, aerogels have enormous surface area – up to 1000 square meters per gram.

Recommended Web resources for additional information:

Aerogel and the Mars Rover
Article from NASA’s Marshall Space Flight Center.

Capture of Fossil Fuels Waste Gases with Advanced Aerogels
Information from the US National Technology Transfer Center.

Creating the World's Best Insulator Out of Virtually Nothing
Aerogel research from the University of Virginia.

Matsushita Improves Silica Aerogel
Technology news article from Photonics Spectra.

Silica Aerogels
Detailed report of aerogel properties. From Ernest Orlando Lawrence Berkeley National Laboratory.

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