Influence of O2 and H2O on the spark decomposition of SF6 and SF6+50% CF4 mixtures

The spark decomposition of SF₆ and SF₆+ 50% CF₄ mixtures was studied 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). The authors´ attention was only focused on the gaseous SF₆ by-products: SOF₂+SF₄, SOF₄, SO ₂F₂, S₂F₁₀, S₂OF ₁₀, S₂O₂F₁₀ and S₂O₃F₆ which were assayed by gas chromatography. Their yields were studied varying the cell preparation technique, the metal constituting the plane electrode (aluminum, copper, stainless-steel) and the concentrations of two additives, O₂ (between 0 and 1%) and H₂O (between 0 and 0.2%). The cell preparation procedure had a strong effect on the formation of all products except SOF₂+SF₄. The aluminum led, whatever the procedure used, to the highest levels of products. With SF ₆ as well as with the SF₆+50% CF₄ mixtures and whatever the spark energy, an increase of the oxygen content led to a decrease of SOF₂+SF₄ and S₂ F₁₀ production rates and to an increase of that of SOF ₄+SO₂F₂. Moreover, an increase of the H ₂O content induced a lowering of the S₂F₁₀ production rate and an increase of those of SOF₂+SF ₄ and SOF₄+SO₂F₂. With the SF₆+50% CF₄ mixture the by-products production rates were lower (10 to 50% depending on the by-product considered and on the O₂ or H₂O concentration) than those measured with pure SF₆