Synergism in SF₆ mixtures with C=C−C backbone compounds

Abstract

A positive synergy in the electric strength was observed in a previous study in SF6/HFO1234ze(E) mixtures which was shown to result from a strong electron energy moderation capability of HFO1234ze(E) combined with thermal electron attachment of SF6 [1]. In the present work, the electron energy moderation properties of compounds having similar C=C-C backbone is investigated. Swarm and breakdown measurements are performed in pure gases and in mixtures with SF6. Compounds having a trifluoromethyl group (-CF3) showed lower characteristic energy and as a consequence a positive synergism with SF6. Descriptors related to electron energy moderation are identified and computed; a clear trend is found from the analysis of descriptors related to inelastic processes which suggest(s) that vibrational excitations may be the main source of electron energy loss in the compounds showing positive synergy.

Robust approximation rules for critical electric field of dielectric gas mixtures

“…Previous studies have highlighted shortcomings in the Wieland approximation [15,16], with one notable limitation being an inability to predict positive synergism in gas mixtures-where a higher critical electric field of the mixture, above that of either constituent gases, is obtained. To remedy shortcomings, extensions on this style of approximation, allowing for a non-linear perturbation around the basic linear combination, have been explored [7,8]. Indeed, van Brunt [16] proposed a novel alternative parameterisation of swarm properties via consideration of electron temperatures, similar to what is presented in this current study.…”

“…Crucial to this CME approach is the recognition that, unlike the Wieland approximation or Blanc's Law, the reduced electric field in the mixture, Ěm , may not be the same as the reduced electric field in either pure gases at the common mean energy, ⟨ϵ⟩, considered. With ( 7) and ( 9) both explicitly referring to the value of E/n 0 in the mixture, we can combine (7) and ( 9) to obtain either an expression for the drift velocity if one desires…”

“…These points can be projected onto a more convenient 2D plot, like that commonly presented in literature, in figure 8. For comparison the pulsed Townsend swarm measurements of Egüzet al [7] have been plotted. Further, the critical electron mean energy, ⟨ϵ⟩ c , at a given critical electric field is plotted against the right-hand axis of figure 8.…”

“…Driving the effort towards replacing SF 6 is considerable investment in academic and commercial research performing dielectric breakdown experiments [5,6] or swarm experiments [7,8] to characterise insulating gases. There are many complexities and challenges in adequately performing and interpreting measurements of such experiments in their varying geometries (e.g.…”