Various Dielectric Relaxation and Polarization in Epoxy Resin Dielectric Materials

Abstract

Epoxy resin possesses excellent electrical insulation, mechanical, and thermal properties and has been widely used in gas insulated switchgear, bushings, solid state transformers, and power cable accessories, and so forth. The dielectric and electrical behaviors of polymeric dielectrics are closely related to their molecular motion and charge transport features, so it is significantly important to unravel the motion of molecular chains and charge carriers in epoxy resin at various ranges of frequencies and temperatures. Dielectric spectra or impedance spectra including the responses of vibrational polarization, orientational polarization due to dipolar molecules, and space charge polarization due to charge carrier hopping or interfacial charge accumulation is a powerful tool to investigate the dielectric properties. In the present work, we analyzed the dielectric relaxation properties in epoxy resin by utilizing the dielectric and electric modulus spectra simultaneously. Dielectric properties were measured in epoxy resin in a wide frequency range from at various temperatures. Based on the frequency spectra of complex conductivity, permittivity, and electric modulus, relaxation phenomena were analyzed. The existance of two relaxation peaks and one peak was confirmed at temperatures below and above 120 °C, respectively. The occurrence of electrode polarization was also observed. The two relaxation peaks at relatively low temperatures obey the Debye and Cole-Cole equations, which are presumably caused by the orientation of dipoles or the movement of chain segments. A Cole-Cole like equation was derived for the new relaxation peak in complex electric modulus at relatively high temperatures, which agrees well with the experimental results. Comparison of the macroscopic conductivity derived based on the relaxation of electric modulus and the microscopic conductivity derived from continuous time random walk theory indicates that the relaxation time constant of electric modulus is directly in proportion to the average time of ionic hopping. Futhermore, it was found that the real part of complex electric modulus is influenced by both electrode polarization and dc conductivity, while the imaginary part of complex electric modulus at relatively high temperatrues is predominated by dc conductivity.