Physical Properties and Some Related Applications

One of the major characteristics of silica aerogels is their very low thermal conductivity, typically of the order of 0.015 W m at ambient temperature, pressure, and relative humidity. These values are significantly lower than the conductivity of air under the same conditions, e.g., 0.025 W m K . Thus, silica aerogels are among the best-known thermal insulating materials [124]. Besides, silica is nonflammable and silica aerogels are amorphous. 

Of course, it must be underlined here that appUcarimis in space are not at aU Umited to thermal insulation. Indeed, silica aerogels can be applied to collect aerosol particles [129], to protect space mirrors, or to design tank trifles [130, 131]. The appUcarimis in space were reviewed by Jones [132] (Part XI). The most recent project. Stardust, successfully returned to earth in January 2006. This mission provided samples of a recently deflected comet named Wild-2, which are being examined in various laboratories all over the world. 

The refractive index of silica aerogels increases with their density p (kg m ) accord- 

In spite of their low density and brittle characteristics, the mechanical properties of silica aerogels are quite promising for some rare but specific applications. As an illustration, their good compressibility can be used advantageously for absorbing the kinetic energy 

The relative dielectric constant of silica aerogels can be as low as 1.1 [165]. Hence, thin film silica aerogels could be and are being considered as super-low dielectric constant material for integrated circuits in computers. It is also possible to modify the surface of silica aerogel to obtain good electret materials (i.e., materials that produce a permanent external electric field) [166]. 

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