Electric-Field- and Temperature-Dependent Nonlinear Conductivity of SiC/Epoxy Resin Composites at Cryogenic Temperature

Nonlinear conductive composite insulation has been widely used to reduce nonuniformities in the local high electric field of high-voltage direct-current equipment. The silicon carbide (SiC)/epoxy resin composites with nonlinear conductivities have great potential applications in high temperature superconducting equipment. In this article, SiC/epoxy resin composites with different filler loading are fabricated, and the nonlinear conductivities are measured at cryogenic temperature. The results indicate that when the filler loading is higher than 17.32 vol%, the composites present field-dependent nonlinear conductivity (FDC) properties. The nonlinear conductivity shows an increasing trend with filler loading increasing or temperature rising. Besides, the temperature-dependent nonlinear conductivity (TDC) is proposed, the mechanism of which can be attributed to the following. With the decrease of temperature, the amplitude of carrier thermal vibration decreases, which make the carriers harder to cross the Schottky barrier in the SiC-epoxy interface and the Poole-Frenkel barrier in epoxy matrix. On the other hand, the carrier-freeze-out effect significantly weakens the impurity ionization in SiC fillers at lower temperature, leading to a decrease of ionized carriers in SiC. The studies herein make a recommendation that the SiC/epoxy composites with a filler content of 29.52 vol% have a larger field-dependent nonlinear coefficient and lower temperature dependence, and play a more effective role in reducing the nonuniformities of electric field distribution in the current leads of HTS dc cable.