ABSTRACT
Aerogels are a class of solid nano-porous materials characterized by open pore structure and a very high specic surface area. The term aerogel was originally proposed by Kistler (1931, 1932) to describe gels subjected to supercritical drying condition, where the liquid within the skeleton of the gel was, by a suitable process, replaced by air. In his original works, Kistler used supercritical water condition in autoclave to obtain aerogels from oxide hydrogels. However, at the high pressure and temperature used to reach the supercritical water condition, the stability of the gel was impaired by the mineralizing effect of water. To overcome this problem, Kistler was then forced to exchange water with solvents having milder critical value, such as alcohol or ether. Successive studies on the topic, by Teichner and coworkers (Nicolaon and Teichner 1968; Teichner et al. 1976), have enlarged the technique of synthesis allowing the oxide gel to be formed directly in an alcoholic medium; consequently, exchange of solvent was no longer necessary. It is worth noticing that the earlier techniques where all ending to a process of supercritical drying involving an alcohol, whose critical condition are in the range Tc = 512-514 K and Pc = 6-8 MPa.
