Kinetics of the arc decomposition of SF6 — Gas mixtures

Kinetic investigation of the degradation reactions of SF
6
— gas mixtures has been performed by the application of a low power (60 W) electric arc. The influence of eight gaseous contaminants (H
2,
O
2
, N
2
, CO, CO
2
, CH
4
, C
2
H
4
and H
2
O) was studied as a function of their concentration and arcing time. Concentration variations of both initial constituants and of numerous reaction products were followed by the combined gas Chromatograph — mass spectrometry analytical technique. In all cases, thionyl fluouride (SOF
2
formed one of the principal decomposition products. Its rate of formation and similarly the rate of consumption of SF, and the added gas were seen to obey quite well pseudo first-order kinetics. One notable exception was hydrogen which influenced differently the decomposition rate of SF
6
depending on its concentration. A general reaction mechanism implying in the first step the consumption of fluorine atoms by the added gas is proposed. Quite unexpectedly water vapor did not produce the faster rate of SF
6
decomposition, in that hydrocarbons were found to be the most active catalysts. On the contrary, the addition of nitrogen produced almost no significant changes. Individual reaction schemes are also proposed for each gas based on the relative rate measurements and the identity of particular reaction products. The implication of these results regarding the development of new gaseous dielectric mixtures is discussed.