Electrical breakdown of CF3I and CF3I-N2 gas mixtures

摘要

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)crit 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.