What are silica aerogels, and where are they used?

A silica aerogel is a solid that is mostly empty space. NASA's Jet Propulsion Laboratory (JPL) describes it as a silicon-based solid with a porous, sponge-like structure in which 99.8 percent of the volume is empty space, and says it is about 1,000 times less dense than glass (JPL). JPL has used it to insulate a Mars rover and to catch comet dust, both described below. Our technology page on silica aerogels tracks the wider evidence.

How it works

An aerogel starts as a gel, a solid network filled with liquid, somewhat like jelly. Making an aerogel means removing that liquid and letting air take its place, which leaves a solid that is mostly empty space. Drying is the step that needs care.

A 1995 paper in Nature by Prakash and colleagues explains why. Its abstract says aerogels are normally made by supercritical extraction of the liquid from a wet gel, which stops capillary forces from collapsing the network. The authors instead added surface groups to the gel so that drying shrinkage became reversible: as the solvent is withdrawn, the gel springs back to a porous state. Using a simple dip-coating method at ambient pressure, they report aerogel films with 98.5 percent porosity (Prakash et al., 1995).

Thermal conductivity is the measure of how easily heat flows through a material. The lower the value, the better the insulation.

What has been demonstrated

"Demonstrated" here means the result was measured in a mission, a laboratory or a published study, and the source reports it.

Spacecraft insulation. JPL used aerogel to insulate the electronics box of the Mars Pathfinder Sojourner rover, which explored the red planet in 1997 (JPL).

Dust capture. On the Stardust mission, the collector used aerogel to catch comet dust. The University of Washington describes the tennis-racquet-shaped collector as using "a remarkably light and porous material called aerogel to capture the particles" from comet Wild 2, which the spacecraft met on 2 January 2004 (University of Washington, 2006). Writing in 2002, before the comet encounter, JPL said the particles would be travelling up to six times the speed of a rifle bullet, that the aerogel would slow them gradually to a stop without damaging them, and that each particle would bury itself in a carrot-shaped track up to 200 times its own length (JPL).

Ceramic aerogels in the laboratory. A 2019 study in Science by Xu and colleagues reports ceramic aerogels made using three-dimensional graphene structures as a template. The journal's summary says the authors designed a mechanical metamaterial that pinches in a small amount when compressed, and that this dramatically improves mechanical stability (Xu et al., 2019). The summary gives no density, conductivity or thermal shock figures, so this article does not quote any. These are laboratory results for a ceramic material, not for silica aerogel products.

What is reported

"Reported" here means a claim from a project, agency or author that this article has not checked independently.

JPL describes aerogel as the world's lightest solid and says it was discovered in the 1930s by a Stanford University researcher (JPL).

The summary of the same Science study describes aerogels as holding promise as lightweight replacements for thermal insulation (Xu et al., 2019). This is a general claim about the material, not a measured result.

A review titled "Aerogel insulation for building applications: A state-of-the-art review" by Baetens and colleagues appeared in Energy and Buildings in 2011 (Baetens et al., 2011). We could not read its abstract, so this article does not use its performance figures.

What is speculative

Several uses are described as possibilities rather than as products in service. These include thin aerogel layers to add insulation to older buildings, translucent glazing that admits daylight, protection for electric vehicle battery packs, and insulation for habitats on the Moon or Mars. The 1995 Nature paper lists super-insulating architectural glazing among potential applications of highly porous films (Prakash et al., 1995), but none of the sources we opened report any of these uses as installed, so each is a possibility only.

Limits and open problems

Brittleness. Silica aerogel is brittle. A review of silica aerogel nanocomposites states that it is brittle in nature and that this drawback needs to be improved. It names polymer modification and fibre additives as approaches (Ślosarczyk, 2017).

Ceramic aerogels. The summary of the Science study says that poor mechanical stability has hampered progress in moving toward commercialisation (Xu et al., 2019).

Production. The 1995 Nature abstract describes the usual supercritical drying route as expensive, hazardous and incompatible with many potential applications, and says this severely restricts commercial use. Its authors offered ambient-pressure drying as an alternative (Prakash et al., 1995). The sources opened here do not give figures comparing the cost or scale of the two routes.

Cost. None of the sources opened gives a verified cost figure for aerogel insulation against conventional insulation, so this article makes no cost claim.

Dust and handling. The sources opened do not cover dust, health or handling in detail, so this article makes no claims about them. Anyone working with aerogel should follow the supplier's safety guidance.

What to watch next

Two questions matter most for wider use. The first is whether toughness improves in products that combine silica with fibres or polymers, which addresses the brittleness problem. The second is whether production routes can bring cost down. Independent tests of the laboratory ceramic aerogels would also show whether the reported results hold outside the original study.