Surface charge and dc flashover characteristics of direct-fluorinated SiR/SiO2 nanocomposites

Silicone rubber (SiR), as a basic insulating material, is widely used in insulators for HVDC transmission lines. Incorporation of nanoparticles could not only improve the insulating properties, but also alter surface charge behavior as a consequence. In HVDC transmission lines, corona discharge could even occur on well-designed insulators, which can inject charge into the insulator surface and influence dc flashover voltage. Direct fluorination is a method to change the surface chemical component of polymers, which could induce the corresponding changes in electrical properties of the surface layer, thus influencing surface charge behavior and dc flashover voltage. This paper tries to study the effects of fluorination time and mass fraction of nanoparticles on surface charge behavior and dc flashover voltage of SiR/SiO₂ nanocomposites. Samples were prepared by dispersing nano-SiO₂ into SiR matrix with the fraction of 0, 1, 3 and 5 wt% respectively. Then the direct fluorination of the samples were conducted in a stainless reaction vessel at 25 °C using a F₂/N₂ mixture gas with 12% F₂ by volume at 0.05 MPa for 5, 10, 15 and 20 minutes. After corona discharge tests performed at room temperature with a relative humidity of ~ 40%, the decay curves of the surface potential were measured using an electrostatic voltmeter, and flashover tests were carried out under dc stress, of which the polarity was identical with the corona discharge. Obtained results show that the dc flashover voltage is affected by the surface charge, which varies with the fluorination time and mass fraction of nanoparticles. It is indicated that the direct fluorination can markedly improve the decay rate of surface charge, and raise the dc flashover voltage of SiR/SiO₂ nanocomposites.