DIELECTRIC-PROPERTIES OF AEROGELS

We have measured the real (dielectric constant) and imaginary (loss fa ctor) components of the complex relative permittivity at 298 K using m icrowave frequencies (2, 10, and 18-40 GHz) for bulk SiO2-aerogels and for two types of organic aerogels, resorcinol-formaldehyde (RF) and m elamine-formaldehyde (MF). Measured dielectric constants are found to vary linearly between values of 1.0 and 2.0 for aerogel densities from 10 to 500 kg/m3. For the same range of densities, the measured loss t angents vary linearly between values of 2 x 10(-4) and 7 X 10(-2). The observed linearity of the dielectric properties with density in aerog els at microwave frequencies shows that their dielectric behavior is m ore gas-like than solid-like. The dielectric properties of aerogels ar e shown to be significantly affected by the adsorbed water internal to the bulk material. For example, water accounts for 7% of the dielectr ic constant and 70% of the loss at microwave frequencies for silica ae rogels. Because of their very high porosity, even with the water conte nt, the aerogels are among the few materials exhibiting such low diele ctric properties. Our measurements show that aerogels with greater tha n 99% porosity have dielectric constants less than 1.03; these are the lowest values ever reported for a bulk solid material.