In general, gels are three-dimensional networks of solid, spanning the entire volume of a fluid medium. You're probably most familiar with hydrogels, where the fluid is water - think of Jell-O. Aerogels are composed of a solid network spanning a gaseous medium, instead of a liquid one. The substitution of gas for liquid means that the density of standard aerogel is the lowest of any known bulk solid.
The most common aerogels are made from silica, although metal oxides and organic materials have also been used. Silica aerogels have very, very low thermal and electrical conductivity, making them wonderful insulators. Metal oxide aerogels have similarly low density and high porosity, and are often used as chemical catalysts.
Aerogels are made by supercritically drying a liquid-phase gel, which ensures that the structure of the aerogel is maintained throughout the drying process. Supercritical drying avoids the liquid-gas phase transition by heating and pressurizing the material to above its critical point, whereupon it becomes a supercritical fluid. The supercritical fluid has properties of both liquid and gas: very compressible, has no surface tension (due to lack of liquid-gas interface), diffuses through solids, and is still dense enough to support a solid structure. The supercritical fluid is then cooled and depressurized into the gas phase. If one were to evaporatively dry the liquid solvent - where the liquid is simply heated until it becomes a gas - the surface tension between the gas and the liquid would be too strong for the aerogel's delicate structure, and the gel would collapse. We built our own supercritical dryer for purposes of this project.
With all their useful properties taken into account, aerogels are still far from ideal. Standard aerogels are so hydrophilic that they collapse upon contact with water. In addition, the structure of the aerogel does not respond well to impact or flexing: too much pressure causes the gel to shatter like glass. Fortunately, modifying the structure to be more forgiving is easily accomplished by a few different processes, including liquid- and vapor-phase crosslinking, physical reinforcement, and reduced bonding. We focused on applying the pricniples of reduced bonding.
Aerogels with an alumina network, as opposed to a silica one, are typically used as catalysts for chemical reactions. They can be doped with a wide variety of metals to change their catalytic properties. Their optical properties are quite similar to those of silica aerogels; both types appear slightly blue due to Rayleigh scattering. Crosslinked alumina (x-alumina) aerogels have been produced that have greatly increased durability, but this has come at the cost of low density and optical effects. We attempted to apply the principles of reduced bonding to alumina aerogels, to create a durable, compliant gel that maintains its other desirable properties.