Aerogel is an advanced material which holds 15 entries in the Guinness Book of Records for properties such as lowest density solid and best insulator. It is a silica-based substance, consisting of a loose dendritic network of the atom silicon. Aerogel is manufactured by delicately removing the liquid, usually ethanol, from a silica alcogel, replacing it with nothing but air, which makes up 99.8% of the final product. Some aerogels have a density as low as .001 grams per cubic centimeter (.0005 ounces per cubic inch).
The tactile sensation of aerogel is like that of hard plastic foam. Its physical appearance is semi-transparent like that of solidified fog, earning it the nickname "frozen smoke." Aerogel shatters easily, making it unsuitable as a stand-alone insulator for windows, but it can support up to 2000 times its own weight. It is very expensive due to the chemicals and processes underlying its manufacture, costing as much as $300 per cubic inch (2.54 cubic centimeters), though prices are falling. Different varieties of aerogel exist, with slightly different colors and strengths, based on variations in the manufacturing process.
Aerogel was created by Steven Kistler in 1931, but its applications were not truly realized until the sixties and seventies. The 1980s saw a revival in the field of aerogels, with top-notch institutions such as NASA and Berkeley Labs working on research in the field. They have been considered as a replacement for polyurethane foam in refrigerators, and as an insulator for windows. Aerogel has been used in NASA space missions to catch micrometeorite particles for study without damaging their structure.
Aerogel is produced by releasing all the water from a colloidal silica gel without disturbing its overall structure. When gels die out under normal temperatures and pressures, the surface tension in tiny pores of the gel causes the structure to constrict and reduce itself to roughly 10 times its original volume. In aerogel manufacture, a gel is placed in a vessel of high heat (280°C or 536°F) and pressure (1800 pounds per square inch, or 1241 Newtons per square centimeter). This causes the liquid within the gel to go into a supercritical state, allowing a phase transition from liquid to gas without the accompanying shrinkage which causes a fine gel structure to collapse. This process is known as supercritical drying. At first the process took days to create aerogels, but improvements have compressed the drying time to a few hours. The process is still an energy-hungry one, leading to the high cost of aerogels.
Aerogels truly qualify as a "space age material," an advanced arrangement of matter with no analog in nature. It may one day be used in a range of applications from insulation for housing to new forms of artwork. Many young researchers are focused on the stuff, mixing aerogel with additives such as carbon to increase its insulating properties, or working to minimize pore size to make aerogel as transparent as possible. There are many directions for future research and many potential applications if this research bears fruit.
By Michael Anissimov