Technology

The aerogel was first created by Samuel Stephens Kistler in 1931 when he assumed and tested that it was possible to dry a "gel" (a colloidal suspension that consists of a liquid dispersed phase in a solid continuous phase) in order to obtain only the solid structure while preserving, unchanged, its own original shape and size.
Intuitively it would have been necessary to remove all the liquid making it evaporate, but the high surface tension generated in micropores, as a result of the phase change from liquid to gas, would have destroyed the gel solid structure making it collapse or lose volume.
The revolutionary insight, progenitor of the current production methods, was to remove the liquid at high pressures and temperatures in a "supercritical environment" where the distinction between gas and liquid ceases to apply, densities of the liquid phase and vapour phase becomes equal (at critical point of drying) and surface tension is effectively absent. Thus were born the first samples of aerogel, but due to the expensive production costs the research was soon abandoned and forgotten.
The interest in aerogel was revived in the late seventies when the materials and methods were replaced by processes more efficient and economical.
Since then, continuous research in this direction has resulted in increasingly affordable and safer production methods that do not involve the use of harmful or highly reactive substances.
Today, thanks to the commercialization on a large scale, production costs have further dropped down allowing the use of aerogels in a wide range of applications.

Aerogel starts out as a "wet gel" obtained by hydrolysis through the "sol-gel" chemical technique during which a colloidal suspension is produced using a solution that consists of metal alkoxides like tetraethyl orthosilicate (TEOS), water, alcohol (such as ethanol) and catalysts (acid, basic, or both in succession) that allow the reaction within hours. The obtained product, known as "alcogel", is placed in an autoclave in order to remove all alcohol from the gel through a "supercritical drying" process. This is achieved using, for example, acetone to solubilise the ethanol, then they will be both removed by means of liquid carbon dioxide brought to supercritical conditions too. The end result of either process removes all liquid from the gel and replaces it with gas, without allowing the gel structure to collapse or lose volume. What remains is just the solid structure, made of silica, which is called an aerogel.
It should be noted that the use of catalysts, such as ammonium fluoride, for the synthesis of silica aerogels by a "one-step" or "two-step" sol–gel method, allows considerable control over what will be the physical and structural characteristics of the final product.