On Dissociation Processes in Certain Gases of High Dielectric Strength

Assessment of Eco-friendly Gases for Electrical Insulation to Replace the Most Potent Industrial Greenhouse Gas SF6

“…It was not until the 1950s, supported by mass spectrometry studies, 44,45 that the halogenated molecules' ability to effectively attach free electrons became generally recognized as responsible for the small first Townsend coefficient and the high electric strength of halogenated gases. 46,47 It was found that negative ions are created by the two-body process of dissociative attachment such as in CCl 4 , 47 SF 6 , 47,48 and trifluoroiodomethane CF 3 I 30 or by the three-body process of electron attachment where a dissociation process does not occur. 48,49 From all halogenated substances, SF 6 turned out to be the best insulation gas in terms of stability, toxicity, and liquefaction temperature, although other gases revealed higher electric strengths.…”

“…However, other complex molecules such as hydrocarbons did not show such a high electric strength. It was not until the 1950s, supported by mass spectrometry studies, , that the halogenated molecules’ ability to effectively attach free electrons became generally recognized as responsible for the small first Townsend coefficient and the high electric strength of halogenated gases. , It was found that negative ions are created by the two-body process of dissociative attachment such as in CCl 4 , SF 6 , , and trifluoroiodomethane CF 3 I or by the three-body process of electron attachment where a dissociation process does not occur. , …”

The Determination of Electron Affinities.

“…Whilst the appearance potentials of positive ions in a mass-spectrometer are usually of the order of 10 or 15 v., the appearance potentials of negative ions may be as low as 1 v., and the peak intensity of negative ions is seldom as much as 1 per cent of the parent positive-ion peak intensity. (The abundance of the negative halogen ions in the mass spectra of the halomethanes is exceptional in this respect (6,7,39,177).) In many cases, negative ions formed in this way possess considerable kinetic energies: for example, the appearance potential of NH7 from NH3 is about 6 v. (109); the endothermicity of the reaction NH3 -> NH2 + H, i.e., Z)(H-NH2), is about 4 v. but as the reaction NH2 + e -> NH7 would be expected to be some 1 or 2 v. exothermic, it would appear that the overall reaction…”

Comparison of gases for electrical insulation: Fundamental concepts

“…For the relevant conditions of high pressures and large insulation gaps typically encountered in electrical apparatuses, the single most important parameter determining the onset voltage of the discharge is the number of free electrons in the gas, according to the classical avalanche picture and the subsequent streamer mechanism [37]- [40]. The growth of an electron avalanche is mainly determined by the kinetics of electrons and short-lived metastable anions that are created in attachment events [37]- [39], [41], [42]. When the applied electric field equals the critical electric field strength, the number of ionization and attachment events per unit length in an electron avalanche (given by the ionization and attachment coefficient and , respectively) are equal = .…”