An aerogel is a porous nanostructure of silica molecules that is approximately 90-99% air by volume. It is formed by a highly cross-linked polymerization reaction and a careful drying phase. The polymerization process forms a solid silica network surrounded by a sol-gel liquid.
The drying process removes the liquid and leaves behind a delicate structure with nanoscale sized pores. This structure is responsible for giving aerogels the lowest know density, index of refraction, thermal, electrical, and acoustical conductivities of any solid material.
There are a wide variety of uses for aerogels. NASA has used them on the Stardust mission to collect comet dust samples, as well as to insulate the electronics onboard the Mars Pathfinder Rover. Due to their high insulating properties, aerogels have been experimentally used in apparel such as jackets and blankets. Because of their high surface area and porosity, allowing easy diffusion of gas into the matrix, aerogels have been used as sensors for detecting chemical species. In other areas, aerogels have been utilized as the dielectric material in electronic capacitors. There are many other potential applications yet to be discovered.
Aerogels originate as sol-gels. A Sol-gel is a silicon oxygen matrix formed through a polymerization reaction and surrounded by methanol and deionized water. To make an aerogel, the sol-gel solvent must be extracted and replaced with air. This process, though sounds simple is the challenge to fabricating monolithic aerogels. If the sol-gel solution is left to dry naturally it will form a xerogel, which is a high density aerogel. Formation of a xerogel is characterized by shrinkage of the gel. Shrinkage occurs due to the liquid-vapor interface of the receding solvent which exerts capillary forces on the pore walls. These forces cause the pores to collapse in on themselves and the aerogel will shrink. Cracking usually accompanies this shrinkage. In order to avoid the liquid-vapor interface, the methanol solvent must be taken to supercritical conditions so the liquid turns directly into a supercritical fluid (gas) without becoming a mixture of liquid and gas. The supercritical point of the solvent methanol is about 470 degrees F and 1160 psi. Once the solvent is in its supercritical state, the gas can be evacuated, leaving behind an aerogel.
The first aerogels were made by S.S. Kistler at Stanford University. He was working on colloidal substances (gels) that were dispersed in liquid solvents. The gels were interconnected and were stiff enough to support their own weight. Kistler was the first to successfully dry a ‘wet’ gel without collapsing the fragile network of the solid. He achieved this by repeated solvent exchange of interstitial water with alcohol in the gel and removed the alcohol at supercritical temperature and pressure to avoid the liquid-gas interface. It was Kisler who gave the remaining gels the name ‘aerogel,’ since the solvent in the sol-gel was replaced with air.
For more information see the blog by Dr. Stephen Steiner at: http://www.aerogel.org/?cat=7
Or watch a movie: https://ww2.kqed.org/quest/2008/04/01/quest-lab-aerogel/