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Aerogels are a class of synthetic porous ultralight material derived from a gel, in which the liquid component for the gel has been replaced with a gas, without significant collapse of the gel structure.

The result is a solid with extremely low density and extremely low thermal conductivity. Aerogels can be made from a variety of chemical compounds. Silica aerogels feel like fragile styrofoam to the touch, while some polymer-based aerogels feel like rigid foams. Aerogels are produced by extracting the liquid component of a gel through supercritical drying or freeze-drying. This allows the liquid to be slowly dried off without causing the solid matrix in the gel to collapse from capillary action, as would happen with conventional evaporation. The first aerogels were produced from silica gels. Kistler's later work involved aerogels based on alumina, chromia, and tin dioxide. Carbon aerogels were first developed in the late 1980s.

The first documented example of an aerogel was created by Samuel Stephens Kistler in 1931, as a result of a bet with Charles Learned over who could replace the liquid in "jellies" with gas without causing shrinkage.

They are good conductive insulators because they are composed almost entirely of gases, which are very poor heat conductors. (Silica aerogel is an especially good insulator because silica is also a poor conductor of heat; a metallic or carbon aerogel, on the other hand, would be less effective.) They are good convective inhibitors because air cannot circulate through the lattice.

In mercury porosimetry, the mercury is forced into the aerogel porous system to determine the pores' size, but this method is highly inefficient since the solid frame of aerogel will collapse from the high compressive force. Aerogel is also an open porous network: the difference between an open porous network and a closed porous network is that in the open network, gases can enter and leave the substance without any limitation, while a closed porous network traps the gases within the material forcing them to stay within the pores.

One way to waterproof the hydrophilic aerogel is by soaking the aerogel with some chemical base that will replace the surface hydroxyl groups (–OH) with non-polar groups (–OR), a process which is most effective when R is an aliphatic group.

To dry the gel, while preserving the highly porous network of an aerogel, supercritical drying employs the use of the liquid-gas transition that occurs beyond the critical point of a substance.

This often leads to freeze-drying being used for the creation of aerogel powders or as a framework for composite aerogels.

Resorcinol–formaldehyde aerogel (RF aerogel) is made in a way similar to production of silica aerogel. A carbon aerogel can then be made from this resorcinol–formaldehyde aerogel by pyrolysis in an inert gas atmosphere, leaving a matrix of carbon.

Silica aerogels are the most common type of aerogel, and the primary type in use or study. This aerogel has remarkable thermal insulative properties, although its thermal conductivity depends on formulation, density, pressure and measurement method. In one transient hot-wire study of monolithic silica aerogel, Cohen and Glicksman reported that the thermal conductivity decreased from 9.3 mW·m−1·K−1 (0.0093 W·m−1·K−1) at 1 atm to 3.2 mW·m−1·K−1 (0.0032 W·m−1·K−1) at 0.1 atm; the same aerogel in granular form had a thermal conductivity of 15.0 mW·m−1·K−1 at ambient gas pressure under modest compression.

One of the most notable differences between silica aerogels and metal oxide aerogel is that metal oxide aerogels are often variedly colored.

Polyimide aerogel films derived from NASA-developed aerogel technology have been commercialized for such applications; examples include the polyimide aerogel film AeroZero.

Textiles and other flexible materials: "Commercial manufacture of aerogel 'blankets' began around the year 2000, combining silica aerogel and fibrous reinforcement that turns the brittle aerogel into a durable, flexible material.

Quick Facts

• Resorcinol–formaldehyde aerogel (RF aerogel) is made in a way similar to production of silica aerogel.
• Aerogels are produced by extracting the liquid component of a gel through supercritical drying or freeze-drying.
• The result is a solid with extremely low density and extremely low thermal conductivity.
• The first aerogels were produced from silica gels.
• Polyimide aerogel films derived from NASA-developed aerogel technology have been commercialized for such applications; examples include the polyimide aerogel film AeroZero.

Source material: Wikipedia - "Aerogel". Adapted and summarized for DiscoverScroll. Original contributors are credited through the linked Wikipedia article. Read original on Wikipedia. CC BY-SA 4.0. Changes were made from the original.