Gaseous and solid by-products of SF/sub 6/ discharges

Corona and glow discharges have been maintained in sulphur hexafluoride (SF/sub 6/) atmospheres over varying periods of time, under various conditions of pressure, voltage and current, and in discharge chambers with walls and electrodes of different materials. As a result of dissociation of SF/sub 6/ and subsequent reactions of the dissociation products, both with gaseous impurities present in the SF/sub 6/ atmosphere and released from walls and electrodes, and with solid surfaces, a wide range of stable gaseous solid by-products is produced. The gaseous by-products have been studied by extracting discharge samples and subjecting them to mass spectrometry and Fourier Transform Infra Red (FTIR) analysis. Stable solid materials deposited onto surfaces have been investigated using X-ray analysis and an optical fibre technique. As expected, the nature and amounts of by-products vary significantly with the type of discharge, duration of discharge, and materials of chamber and electrode assemblies. Because, invariably, oxygen exists as an impurity, one or more of the oxyfluorides (SOF/sub 2/, SOF/sub 4/ and SO/sub 2/F/sub 2/) appears as a gaseous by-product. Other gaseous by-products include SO/sub 2/, CO, probably due to the presence of insulating materials within the discharge chamber, and SiF/sub 4/ when the chamber has a glass window. Stable solid deposits build up over time on surfaces. Preliminary analysis indicates that metal fluorides form on electrodes, while powders containing sulphur deposit preferentially on any insulating surfaces that exist within the discharge chamber. The study reveals the complexity of chemical processes that take place when SF/sub 6/ is dissociated in a discharge, and the subsequent degradation of the integrity of the SF/sub 6/ insulation that must follow. The results of the present study suggest that optical absorption techniques may be the preferred option for on-site monitoring of practical SF/sub 6/ gas-insulated systems.<>