Electrical breakdown of CF3I and CF3I-N2 gas mixtures

Trifluoroiodomethane CF₃I has been recently considered as a potential alternative to SF₆, thanks to its high electronegativity and extremely low environmental effect. However, due to its high boiling point, CF₃I should be mixed with other gases such as N₂ or CO₂ in order to avoid the liquefaction at low temperature. In this paper, we present breakdown measurements carried out in CF₃I and CF₃I-N₂ gas mixtures, under dc voltage, and with a pressure-distance product (p.d) up to 5 mmbar. Pure CF₃I has a higher breakdown voltage (Vb) than SF₆. However, the favourable synergistic effect observed in SF₆ gas mixtures with N2, at low SF₆ content, is not observed in CF₃I gas mixtures. In CF₃I-N₂ mixtures, the dc breakdown voltage increases proportionally to the CF₃I content. As a consequence, breakdown voltages for mixtures containing up to 50% CF₃I are lower than with SF₆ at the same ratio. These results are in good agreement with calculated values using breakdown model and reduced critical field strength (E/N)^ reported by others authors. In CF₃I, the appearance of solid by-products (particles) generated by breakdown is also observed. The nature of these particles is tentatively attributed to the formation of iodine. In our sphere-sphere geometry, the accumulation of such solid particles leads to a measurable conductivity.