Evaluation of cohesive property of carbon black in epoxy resin composite from the equivalent circuit in high-frequency region

Abstract

When carbon black (CB) with high cohesiveness is used for a carbon-black resin composite, the temperature coefficient of resin resistivity is negative and the increase in resin resistivity due to humidity is small. These symptoms are due to the balance between the mechanical change, such as thermal expansion and volume increase caused by humidity absorption and the bond strength between CBs. The bond strength between CBs is dependent on curing temperature. In this study, the cohesiveness of CBs which scatter in resin will be investigated by evaluating the electrical property in the high-frequency region. The impedance of a CB-resin composite was measured at frequencies up to 3 GHz and was expressed by the parallel equivalent circuit with resistance Rs and static capacitance Ce. Since residual resistance is present at frequencies up to 3 GHz, the equivalent circuit was replaced with the circuit with series resistance Rs and a parallel circuit with resistance RC and static capacitance CC. This equivalent circuit was investigated by the S parameter method. As the concentration CB increased, RS and RC rapidly decreased and CC increased. When CB particles were easily bonded together, RC was small and CC was high even if the concentration of CB was low. From the dependencies of RC and CC on the CB concentration and the temperature dependence of resistance, it was concluded that RC and CC provided information on the cohesiveness of CB.