Aerogels. Material properties

The first "aerogels" were prepared in 1931, when Kistler, from the University of the Pacific in Stockton, California, sought to demonstrate that a gel contained a continuous solid network of the same size and shape as the gel itself. Kistler conjectured that, during drying, the liquid-vapor interface of the evaporating liquid exerted strong surface tension forces leading to the collapse of the solid structure. He then discovered the key aspect of aerogel production: hypercritical drying, in which the liquid phase is continuously converted into its gaseous form, i.e. without any change of state.

The first gels studied by Kistler were silica gels prepared by acid condensation of an aqueous sodium silicate solution. The water in the solution was exchanged with an alcohol for hypercritical drying, resulting in transparent, low-density, highly porous aerogels.

Although interesting in many respects, aerogels were nevertheless neglected due to the excessively long preparation time, and it was not until the late 1970s that interest was renewed by the development of a new manufacturing process by the Claude Bernard University in Lyon. By replacing sodium silicate with an alkoxysilane, tetramethoxysilane (TMOS), and hydrolyzing this TMOS in a methanol solution, a one-step gel was produced, known as an "alkogel", since the liquid filling the pores was none other than an alcohol.

This technique was gradually applied to other organometallic precursors leading to mineral aerogels other than silica (alumina, zirconia, TiO ₂ ...). In the 1990s, it was also proposed for the synthesis of organic aerogels derived from the polycondensation of resorcinol and formaldehyde. Carbon aerogels were then obtained by pyrolysis of organic aerogels. The appeal of aerogels' distinctive texture has prompted the scientific community to develop them in a wide variety of compositions for applications in fields as diverse as thermal insulation, electrochemistry, catalysis, particle detection, acoustics, nuclear waste containment, astrophysics and biosciences.

In the remainder of this article, we'll look at some of the properties of aerogels, whether organic or, more specifically, silica (single oxide), the latter having been the most extensively studied. In the final part of the article, we present various applications for these materials.