Structure and Electrical Properties of Silicone Rubber Filled with Thermally Reduced Graphene Oxide

Graphene oxide (GO) was heat treated at different temperatures between 120 and 220 degrees C and the structural changes were assessed by thermogravimetry, infrared spectroscopy, X-ray photoelectron spectroscopy and scanning electron microscopy (SEM). The resulting reduced graphene oxide (rGO) fillers showed markedly lower oxygen contents (primarily by reduction of epoxide and hydroxyl groups) than GO. SEM of silicone rubber composites containing 3 wt.% rGO or GO filler showed that the nanoparticles were uniformly distributed in the polymer. The rGO-filled composites exhibited electric field-dependent resistivity; the resistivity decreased from 10(14) to 10(11) ohm m as the electric field was increased from 0.2 to 6 kV (mm)(-1). The composites exhibited an increased resistivity after being exposed to a combined thermal cycling and electrical field. An increase in the resistivity of samples aged at 120 degrees C for more than 17 h was observed; the resistivity-electric field behavior and the dielectric constant of the aged composite resembled that of GO-filled composite. The composites exhibited dielectric constant values between 4.0 and 5.2 and a low tan delta (<= 0.015) at frequencies between 10(-2) and 10(4) Hz. The results suggest that the resistivity of the composites can be tuned by adjusting the degree of reduction of GO. The low rGO-filler content that was required to achieve this adequate property profile is attractive, which makes these composites potentially useful as electric field-grading material in HVDC cable accessories. However, this requires that the long-term stability problem can be sensible addressed.