Analysis of decomposition products of sulfur hexafluoride in negative dc corona with special emphasis on content of H/sub 2/O and O/sub 2/

We are able to analyse the SF/sub 6/ decomposition directly during a discharge by means of a FT-IR (Fourier transform infrared) spectrometer and a 2.51 stainless steel discharge cell with infrared windows and a path length of 0.175 m. With this cell we investigated the influence of water vapour and oxygen on the production and persistence of the most important by-products from negative DC corona in mixtures based on "pure" SF/sub 6/ (99.995%) at a total pressure of 100 kPa. SF/sub 6/ mixtures without oxygen addition ([O/sub 2/]<10 ppm/sub v/), with 1% (volume) oxygen and with different moisture contents (280 ppm/sub v/<[H/sub 2/O]<4350 ppm/sub v/) were analysed. 12 hours after preparation of the mixtures, when a certain equilibrium between residual water vapour and the walls had been reached, the moisture was measured and then the negative corona discharge was started in a point to plane gap. With a mean current of I=50 /spl mu/A the discharge was maintained for a duration in the range from 29 h to 55 h. The moisture and the decomposition products (S/sub 2/F/sub 10/, SF/sub 4/, SOF/sub 2/, SOF/sub 4/, SO/sub 2/F/sub 2/, SiF/sub 4/, COF/sub 2/, CF/sub 4/, CO and HF) were monitored by IR-absorption during and after the discharge. Quantitative analysis was made, by simple spectra subtraction, for the concentrations of H/sub 2/O and of the decomposition products S/sub 2/F/sub 10/, SF/sub 4/, SOF/sub 4/, SO/sub 2/F/sub 2/, and SOF/sub 2/. The results indicate an important effect of water and oxygen on the production of SF/sub 6/ decomposition products. In general, the S/sub 2/F/sub 10/ yield was reduced with increasing the initial moisture content. In the presence of 1% oxygen the production rate of S/sub 2/F/sub 10/ under "very dry" conditions (initial [H/sub 2/O]<280 ppm/sub v/) is reduced by a factor of 10 compared to mixtures with no oxygen added, [O/sub 2/]<10 ppm/sub v/. The experiments show a good stability of S/sub 2/F/sub 10/ at least for the first 30 hours after the discharge for all mixtures investigated.<
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