Enhancement of DC Flashover Voltage of Epoxy Insulators and Gap Breakdown Voltage in SF6 Gas by Direct Fluorination of Metal Electrodes

304 stainless steel electrodes together with the stainless steel sheets of the same material were surface fluorinated in the laboratory using the fluorine source of 12.5% F ₂ in N ₂ at 0.1 MPa and 250 °C for 10 h, to investigate the effects of the electrode fluorination on the insulator flashover voltage and the gap breakdown voltage between two electrodes. DC flashover tests in 0.1 MPa SF ₆ gas on the stainless steel finger electrode-epoxy sheet sample configuration indicate that the electrode fluorination increased the Weibull characteristic DC flashover voltage by 9.76%, and also reduced the insulator damage caused by flashover. Gap breakdown tests on the stainless steel sphere-plate electrode configuration show that the electrode fluorination also enhanced the gap characteristic DC breakdown voltage by 4.71% in 0.1 MPa SF ₆ gas. SEM observations show that the fluorination roughened the stainless steel surface and reduced its surface conductivity. The increase in surface roughness and the decrease in surface conductivity are quantified by the measurements of the roughness and conductivity. Further, EDS and XPS analyses indicate that the fluorination introduced a large number of fluorine atoms into the stainless steel surface layer and also some oxygen impurities in its surface, forming the M (metal)-F bonds in the surface layer and the M-F and F-M-O bonds in the surface. XPS and UPS valence band analyses consistently reveal that the fluorination changed the stainless steel surface from conductive to semiconductive or even insulating, increased the energy required for the electron emission from the stainless steel electrode, and greatly reduced the electron density near the valence band top. This is the cause for the enhancement of the flashover voltage and the breakdown voltage.