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N. Hüsing, U. Schubert:
"Aerogels";
in: "Ullmann's Encyclopedia of Industrial Chemistry", issued by: Wiley-VCH; Wiley-VCH, 2012, 621 - 646.

Aerogels are unique among solid materials. They have extremely low densities (up to 95 % of their volume is air), large open pores, and a high inner surface area. This combination of properties results in interesting physical properties, e.g., for silica aerogels extremely low thermal conductivities and low sound velocities are combined with optical transparency. Aerogels are typically prepared via wet chemical processes, such as sol - gel processing, and thus their pore system is initially filled with liquid. Only special drying techniques allow for exchange of the pore liquid against air while maintaining the filigrane solid framework. The most common drying technique is supercritical extraction, and sometimes chemical modification of the inner surface can be used to facilitate drying. The structure of the gel network, and thus the physical properties of aerogels, decisively depends on the choice of the precursors and the chemical reaction parameters for preparing the gels. Therefore, the later materials properties can be deliberately designed in the starting reaction solution.

http://dx.doi.org/10.1002/14356007.c01_c01.pub2