INFLUENCE OF A SOLID INSULATOR ON THE SPARK DECOMPOSITION OF SF6 AND 50-PERCENT SF6 PLUS 50-PERCENT CF4 MIXTURES

The spark decomposition of SF6 and 50% SF6 + 50% CF4 mixtures (100 kPa ) in the presence of a solid insulator struck by an electric are is st udied. The sparks were generated in a cylindrical Monel 400 cell (340 cm(3)) between a stainless steel point and a stainless steel plane (ga p space 0.9 mm) either under a 50 Hz ac voltage (90 J per disconnectio n) or by discharging a capacitor (3.59 J per spark). The gaseous by-pr oducts SO2F2, SOF4, SF4 + SOF2, S2F10, S2OF10, S2O2F10, S2O3F6 and CF4 were assayed by gas chromatography and their yields studied by varyin g the nature of the insulator material (Megelit, Kel-F (polytrifluoroc hloroethylene), Teflon, polypropylene, polyethylene, nylon) and the co ncentration of two additives: O-2 (0 and 0.2%) and H2O (0 and 0.2%). A s it becomes vaporized under the effect of the discharges, the insulat or produces species that can trap the fluorine atoms released from SF6 decomposition and enhance the formation of SF6 by-products. This expl ains the main results of this work which can be summarized as follows. (i) Enhancement, by an order of magnitude or more, of (SF4 + SOF2) by the insulators. (ii) A quantity of CF4 termed in the presence of the insulator proportional to the enhancement of (SF4 + SOF2). The ratio b etween the levels of these compounds proved to be the same regardless of the initial concentration of H2O or O-2 added to the SF6 and the in tensity of the sparks. (iii) Enhancement of the quantities of S2F10 (u p to fivefold), S2OF10, S2O2F10 and S2O3F6 formed in the presence of a n insulator whatever the gas sample studied. In the presence of an ins ulator, the SFG-CF4 mixture led to results rather similar to those of SF6 concerning the revels of SF6 decomposition products formed. In all cases however, i.e. in the presence or absence of insulator or added H2O or O-2, the mixture led to decreased formation of the main product s: (SF4 + SOF2) and S2F10 Interference was seen to occur between the a ction of the insulator and that of H2O and O-2; this is ascribed to re actions taking place between these molecules or their dissociation fra gments.