SPARK DECOMPOSITION OF SF6 AND SF6-PERCENT CF4 MIXTURES(50)

The spark decomposition of SF6 and of SF6 + 50% CF4 mixtures was studi ed principally at a gas pressure of 200 kPa. The sparks were generated between a point and a plane either under 50 Hz ac voltage (0.09 J per spark) or by discharging a capacitor (3.59 J per spark). Our attentio n was only focused on the gaseous SF6 by-products: SOF2 + SF4, SOF4, S O2F2, S2F10, S2OF10, S2O2F10 and S2O3F6 which were assayed by gas chro matography. The last three compounds were principally observed under t he higher energy sparks. Their yields were studied varying the cell pr eparation technique, the metal constituting the plane electrode (alumi nium, copper, stainless steel) and the concentrations of two additives , O-2 (between 0 and 1%) and H2O (between 0 and 0.2%). The cell prepar ation procedure had a strong effect on the formation of all products e xcept SOF2; the S2F10 yield was for example multiplied by similar or e qual to 17 when the cell was carefully dried and outgassed and with an aluminium electrode. The aluminium led, whatever the procedure used, to the highest levels of products. Under the high-energy sparks an inc rease of the oxygen content of SF6 or of the SF6-CF4 mixture led to a decrease of the SOF2 + SF4 and S2F10 formation rates and to an increas e of that of the other compounds. An increase of the H2O content had v ery little effect on SOF2 + SF4 production and led to increased produc tion of S2OF10 and SO2F2 + SOF4 and to a lowering of the formation of other compounds. Under the low-energy sparks the addition of O-2 to SF 6 or to the SF6-CF4 mixture led to a lowering of the SOF2 + SF4 and S2 F10 yields like under high-energy sparks and to an increase of S2OF10 (which became observable) and of SO2F2 + SOF4. Addition of water resul ted in an increase of the SOF2 + SF4 and SO2F2 + SOF4 yields, in a low ering of S2F10 and had no effect on S2OF10 which remained unobservable . Finally it should be noted that the addition of 50% of CF4 to the SF 6 had very little effect on the rates of formation of the gaseous SF6 by-products except under low-energy sparks where the SF6-SF4 mixture l ed to lower production rates for SOF2 + (SF4) and S2F10.