- lightweight. It is, in fact, the lowest-density solid on the planet. Some types are composed of more than 99% air, yet they still function as solids;
- extremely high in surface area. It can have a surface area up to 3,000 square meters per gram, meaning that a cubic inch of aerogel, if flattened out, could cover an entire football field; and
- strong. It can support up to 4,000 times its own weight. In the picture at right, a 2-gram piece of the material is supporting a 5-pound brick.
The following qualities will also assist with identification. Aerogel:
- appears blue due to Rayleigh scattering, the same phenomenon that colors the sky;
- feels like Styrofoam® to the touch. Although a slight touch will not leave a mark, pressing more firmly will leave a lasting depression or even produce a catastrophic breakdown in the structure, causing it to shatter like glass; and
- is rigid. Despite its name, it is hard and dry, little resembling the gel from which it was derived.
Performance as an Insulator
Composed almost entirely of gas, which is a poor heat conductor, aerogel can almost nullify the three methods of heat transfer (conduction, convection and radiation). Boasting an R-value of 10 to 30, NASA has used the material to protect astronauts and equipment, such as the Mars Rover, from the extreme cold of space. As compared to conventional insulation material, the R-values of vermiculite, rockwool, fiberglass and cellulose are approximately 2.13, 3.1, 3 and 3.1, respectively. Silica aerogel is especially valuable because silica is also a poor conductor of heat. A metallic aerogel, on the other hand, would be less useful as an insulator.
Manufacturers offer the material in a variety of forms, such as the granules pictured at right, made by Cabot, which are sometimes used as insulation in skylights. Aspen Aerogel® offers 57-inch wide rolls of the material in 0.2- and 0.4-inch thicknesses, while Thermoblok® comes in 1.5-inch wide strips that are used to cover framing studs and help prevent thermal bridging at a cost of about $1.99 per foot.
Aerogel safety is dependent on the safety of the gel from which it was made; it will be carcinogenic, for instance, if the gel from which it was derived had this quality. Fortunately, silica-based aerogel is not known to be dangerous, although it may irritate skin, mucous membranes, eyes, the respiratory tract, and the digestive system. Aerogel is hydroscopic and extremely dry to the touch, which can, in turn, cause it to dry out unprotected skin. Gloves and goggles are recommended for inspectors and contractors who must handle the material.