Zhenlian An’s research while affiliated with Tongji University and other places

Various composite films with high energy storage performance and/or dielectric strength have been fabricated. However, due to some reasons, few of them have been used in practice, and BOPP film is still regarded as the state-of-the-art capacitor dielectric film. Previous work has indicated that direct fluorination can improve the dielectric strength of BOPP films. Herein, as a continuation of previous work, the energy storage performance and basic dielectric properties of the surface fluorinated BOPP film are investigated. The charge-discharge experimental results show that the surface fluorinated BOPP sample has the maximum energy storage or release density of 5.27 or 4.43 J/cm ³ , which is 59.8% or 44.6% higher than that of the virgin BOPP sample. The dielectric measurement results indicate that the surface fluorinated BOPP sample has slightly higher dielectric constant and dielectric loss than the virgin BOPP sample. The analyses of physicochemical characteristics reveal that the 0.75 μm thick fluorinated surface layer has a high degree of fluorination. The results of the calculation based on the three-layer dielectric model and the dielectric measurements further show that the fluorinated surface layer has high dielectric constant and loss factor, similar to non-perfluoropolymers. The enhancement of energy storage performance and dielectric strength is mainly attributed to the blocking of the fluorinated layer to electron injection from the cathode, due to a 60% reduction of the interfacial electric field and a decrease in free volume of the surface layer by the fluorination.

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% $\text{F}_{{2}}$ in $\text{N}_{{2}}$ 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 SF6 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 SF6 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, energy dispersive X-ray spectroscopy (EDS) and X-ray photoelectron spectroscopy (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 ultraviolet photoelectron spectroscopy (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.

In order to verify the effectiveness of direct fluorination in improving the dc flashover performance of gas insulated switchgear (GIS) actual spacers and explore the effect of fluorination conditions, 126-kV disk spacers were surface fluorinated in the laboratory at the different temperatures of 35 °C, 55 °C, or 80 °C. The evaluation of surface physicochemical characteristics and basic electrical properties of the fluorinated disk spacers are realized by using the epoxy samples of the same material and simultaneously fluorinated with the disk spacers. Flashover tests indicate that direct fluorination significantly increases the dc flashover voltage of the actual spacer in SF ₆ gas and reduces the dispersion of flashover voltage, fluorination temperature has a significant effect on the enhancement of the flashover voltage, and the fluorination at higher temperatures can produce a larger improvement in the flashover performance. In addition, these fluorinated spacers can withstand heat treatment at 120 °C, with unchanged flashover performance. Infrared analysis and scanning electron microscopy observation show that the fluorination at different temperatures changes surface chemistry of the disk spacer substantially, forming the fluorinated layers with different thicknesses, compositions, and structures. Measurements of surface conductivity and surface potential decay consistently show that direct fluorination increases surface conduction of the spacer, and the fluorination at higher temperatures leads to a more significant increase in surface conduction. The improvement in dc flashover performance, caused by direct fluorination, is mainly attributed to the suppression of surface charge accumulation by the increase in surface conduction of the spacer.