Dielectric relaxation and carrier transport in epoxy resin

Epoxy resin (EP) is widely used as an insulating material in power equipment. Its dielectric relaxation and carrier transport properties are important factors affecting breakdown and surface flashover performance. The dielectric relaxation and carrier transport properties of EP based alumina (Al₂O₃) microcomposite were investigated through broadband dielectric spectroscopy. The glass transition temperature was measured by differential scanning calorimetry (DSC), which is about 120 °C. Gold electrodes with a diameter of 30 mm were sputtering on two sides of the samples. A broadband dielectric spectrometer (Concept 80 Novocontrol) was used to measure the dielectric relaxation properties at an ac voltage of 1 V_rms in a frequency range from 10^-1 to 10⁷ Hz at various temperatures. Above the glass transition temperature, a relaxation peak occurs at high frequencies due to the motion of molecular chains or segmental chains, and a dc conductivity resulted by the migration of charge carriers appears at low frequencies. In addition, molecular chains with different scales have different relaxation times. It was found that EP microcomposite has a very broad distribution of relaxation time. We calculated the distribution of relaxation time at various temperatures. Furthermore, the temperature dependences of molecular relaxation and dc conductivity satisfy the Vogel-Tammann-Fulcher equation. Fitting the experimental results, we obtained the Vogel temperatures and strength parameters of molecular relaxation and dc conductivity. From the Vogel temperatures, we estimated the glass transition temperature to be 117 °C, which is consistent with the DSC result. It means that free volume increases with increasing temperature, facilitating the motion of molecular chains and the migration of charge carriers.