Aerogels. Fundamentals

Since the first international conference held in Padua in 1981, it has been well established that sol-gel processes can be a new route to synthesizing materials as diverse as glasses, ceramics or composites, enabling the synthesis of materials in the form of films, fibers or solid products.

What exactly is the sol-gel process, and why is it so attractive to materials scientists? A precise definition of the sol-gel method is difficult, as it encompasses a wide variety of synthesis techniques and processes in which one of the steps uses the material in the form of a gel. To get round this difficulty, it makes more sense to spell out what is meant by a gel in the remainder of this article: a gel is a two-phase medium made up of a solid and a fluid obtained by a progressive, isotropic aggregation process from a solution.

The first reason for the scientific community's interest in gels was technological: the aim was to synthesize high value-added materials. To this end, the main advantages of the sol-gel process are :

• the high purity of the precursor compounds, which can be preserved during the process;

• for multi-component chemical systems, the homogeneity of the different species can be achieved by chemical reaction in the stock solution;

• it would be possible to control the morphology of the final product (film, fibre, solid material) by adjusting the viscosity;

• in the case of ceramic powders, there would be an improvement in sintering reactivity thanks to their large specific surface area.

The second reason is a matter for materials physicists rather than engineers. It is dictated by the interest of these media as models. In particular, gels have been used to test theoretical models of aggregation and structures obtained by numerical simulation or the "gelification-percolation" analogy. It was envisaged that gels and aerogels could develop a solid phase network with a fractal structure within a certain scale domain, this structure being linked to the type of gelling mechanism involved. The ability to control these mechanisms would enable the fractal characteristics of the materials to be fine-tuned.

But what is an aerogel? An often-used image is to describe aerogel as "frozen smoke. In fact, the volume fraction of material contained in the aerogel with the lowest density is less than 0.14%. This means that air occupies 99.8% of the aerogel's volume. The lightest aerogel has a density barely three times that of air. As a result, this solid has a number of unique properties, which are described in this article.

It goes without saying that,...