The response function method as a novel technique to determine the dielectric permittivity of highly porous materials

Abstract

We measured the real and imaginary part of the dielectric permittivity ɛ of organic resorcinol-formaldehyde (RF) aerogels. The samples were used as the capacitive part of a high pass falter. This filter was exposed to white noise and the complex and frequency dependent response function, was measured. Values of e were obtained in the frequency range from 1 Hz - 50 kHz. In order to determine the low ε values of highly porous aerogels great care was taken to optimize the sensitivity of the measurement set-up. The main sources for errors such as non-ideal capacitors, variable cable positions etc. were determined and investigated. The e data were derived with an accuracy of 6%. The impedance spectrum of the RF aerogels in the density range of 340 kg/m 3 up to 880 kg/m 3 is clearly dominated by losses due to relaxation processes. Considering the so called Maxwell-Wagner polarisation we were able to attribute these losses to adsorbed water layers.