Electrical breakdown of CF3I and CF3I-N2 gas mixtures

Trifluoroiodomethane CF
₃
I has been recently considered as a potential alternative to SF
₆
, thanks to its high electronegativity and extremely low environmental effect. However, due to its high boiling point, CF
₃
I should be mixed with other gases such as N
₂
or CO
₂
in order to avoid the liquefaction at low temperature. In this paper, we present breakdown measurements carried out in CF
₃
I and CF
₃
I-N
₂
gas mixtures, under dc voltage, and with a pressure-distance product (p.d) up to 5 mmbar. Pure CF
₃
I has a higher breakdown voltage (Vb) than SF
₆
. However, the favourable synergistic effect observed in SF
₆
gas mixtures with N2, at low SF
₆
content, is not observed in CF
₃
I gas mixtures. In CF
₃
I-N
₂
mixtures, the dc breakdown voltage increases proportionally to the CF
₃
I content. As a consequence, breakdown voltages for mixtures containing up to 50% CF
₃
I are lower than with SF
₆
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 CF
₃
I, 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.