Dielectric properties of fast sol-gel glasses

The broad-band dielectric spectroscopy method was employed to investigate g lasses of fine porosity produced via the fast sol-gel route. The study was carried out in the frequency range 20 Hz to 1 MHz and temperature interval -100 to +120 degrees C on the sol-gel glasses prepared at temperatures betw een 60 and 100 degrees C. The dielectric response data were analyzed both i n frequency and time domains and interpreted in terms of the various non-De bye relaxation processes. A superposition of the Havriliak-Negami formula D elta epsilon/[1 + (i omega tau)(alpha)](beta) and Jonscher's term (i omega) ((n-1)) was used for the fitting procedure and the quantitative analysis of the dielectric spectra. Here, Delta epsilon is the dielectric strength and tau is the characteristic relaxation time. The parameters alpha and beta d escribe the symmetric and asymmetric broadening of the relaxation process, and n is a Jonscher parameter for the high-frequency part of the relaxation process. It was shown that the complex dielectric behavior of the sol-gel glasses could be described in terms of several distributed dielectric relax ation processes and the de conductivity part. At the low-temperature wing t he strong relaxation process can be observed for all the samples. It can be attributed to the ion motion of the mobile molecules and ions anchored on the methoxy residual groups in the matrix. The second process is associated with percolation of excitation along the developed fractal structure of co nnected pores. The excitation is coursed by the movements of charge carrier s within the pores. The dielectric relaxation response of this process in f requency and/or time domains can be used in order to calculate the porosity of the materials and fractal dimension of the porous space. The nature of the third process is different for various samples. It can be attributed to the fast mobility of the terminal oxygen groups and to the local mobility of the chain fragments located between neighboring knots of the entangled t hree-dimensional network.