Dielectric properties of fast sol-gel glasses

The broad-band dielectric spectroscopy method was employed to investigate glasses 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°C on the sol-gel glasses prepared at temperatures between 60 and 100°C. The dielectric response data were analyzed both in frequency and time domains and interpreted in terms of the various non-Debye relaxation processes. A superposition of the Havriliak-Negami formula Δ∈/[1 + (iωτ)^α]^β and Jonscher's term (iω)^(n-1) was used for the fitting procedure and the quantitative analysis of the dielectric spectra. Here, Δ∈ is the dielectric strength and τ is the characteristic relaxation time. The parameters α and β describe 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 relaxation processes and the dc conductivity part. At the low-temperature wing the 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 connected pores. The excitation is coursed by the movements of charge carriers within the pores. The dielectric relaxation response of this process in frequency 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 three-dimensional network.