Aerogels are highly porous ultralight materials in which the liquid component of a gel has been replaced with a gas, and are noteworthy for being solids that are extremely effective thermal insulators with very low density. Aerogels can be prepared in a variety of ways, and though most have been based on silica, materials based on zirconia, titania, cellulose, polyurethane, and resorcinol—formaldehyde systems, amongst others, have been reported and explored. The prime disadvantage of a silica-based aerogel is its fragility, though NASA has used them for insulation on Mars rovers, the Mars Pathfinder and they have been used commercially for insulating blankets and between glass panes for translucent day-lighting panels. Particulate gels prepared by the Stöber process can be dehydrated rapidly to produce highly effective silica aerogels, as well as xerogels. They key step is the use of supercritical fluid extraction to remove water from the gel while maintaining the gel structure, which is typically done with supercritical carbon dioxide, as NASA does. The resulting aerogels are very effective thermal insulators because of their high porosity with very small pores (in the nanometre range). Conduction of heat through the gas phase is poor, and as the structure greatly inhibits movement of air molecules through the structure, heat transfer through the material is poor, as can be seen in the image at right where heat from a Bunsen burner transfers so poorly that crayons resting on the aerogel do not melt. Due to their low density, aerogels have also been used to capture interstellar dust particles with minimal heat changes in slowing them down (to prevent heat-induced changes in the particles) as part of the Stardust mission.
One method to produce a silica aerogel uses a modified Stöber process and supercritical drying. The product appears translucent with a blue tinge as a consequence of Rayleigh scattering; when placed in front of a light source, it becomes yellowish due to Mie scattering. This aerogel has a surface area of 700 m g and a density of 0.040 g cm; by way of contrast, the density of air is 0.0012 g cm (at 15 °C and 1 atm).
Silica aerogels held 15 entries for materials properties in the Guinness World Records in 2011, including for best insulator and lowest-density solid, though aerographite took the latter title in 2012.
Aerographene, with a density of just 13% of that of room temperature air and less dense than helium gas, became the lowest-density solid yet developed in 2013. Stöber-like methods have been applied in the preparation of aerogels in non-silica systems. NASA has developed silica aerogels with a polymer coating to reinforce the structure, producing a material roughly two orders of magnitude stronger for the same density, and also polymer aerogels, which are flexible and can be formed into a bendable thin film.