Mechanism of F₂/N₂ fluorination mitigating vacuum flashover of polymers

Polymer materials usually present lower surface flashover strength compared with their bulk breakdown strength, particuarly in a vacuum. Flashover along the solid insulation surface may pose a significant challenge to the safety of electrical equipment. In this paper, two kinds of polymers, epoxy resin and polyethylene, have been modified with F₂/N₂ fluorination to decrease the secondary electron yield (SEY) and further improve the surface electric withstanding strength. Experimental results have proved the effectiveness of this method in overall perspectives. The surface properties of modified samples are evaluated by comprehensive methods, i.e. surface roughness, scanning electron microscopy, charge trap characteristics and x-ray photoelectron spectroscopy. It is considered that physical factors such as roughness and chemical factors such as fluorine groups play a significant role in the modified mechanism. Reduction of SEY is linked with flashover voltage quantitively using multipactor and outgassing theory. In addition, we perform a simulation to illustrate multipactor dynamics in the presence of a surface barrier which represents specific surface roughness. The influence of trap characteristics is also analyzed theoretically to corroborate the experiment results. It is concluded that SEY is influenced by surface roughness and shallow traps. The reduction of SEY is directly linked with improvement of flashover voltage. Trap characteristics are changed by introducing fluorine groups. A comprehensive model of fluorination-based flashover mitigation is proposed considering the above features.