Effective ionization coefficient of C5 perfluorinated ketone and its mixtures with air

C5 perfluorinated ketone (C5 PFK with UIPAC chemical name 1,1,1,3,4,4,4-heptafluoro-3-(trifluoromethyl)-2-butanone and sold by 3M as Novec^™ 5110) has a high dielectric strength and a low global warming potential, which makes it interesting as an insulating gas in medium and high-voltage applications. The study was carried out to determine the effective Townsend ionization coefficient α _eff as a function of electric field strength and gas density for C5 PFK and for its mixtures with air. The non-self-sustained Townsend discharge between parallel plate electrodes was initiated by illuminating the cathode by UV radiation. The discharge current, I, was measured as a function of inter-electrode distance, d, at different gas densities, N, and electric field strengths, E. The effective ionization coefficient α _eff was determined from the semi-logarithmic plots of I/I ₀ against d. For each tested gas mixture, the density normalized effective ionization coefficient α _eff/N was found to be a unique function of reduced electric field strength E/N. The measurements were carried out in the absolute pressure range of 0.05–1.3 bar and E/N range of 150–1200 Td. The increasing fraction of C5 PFK in air resulted in the decrease of effective ionization coefficient. The limiting electric field strength (E/N)_lim where the effective ionization coefficient α _eff became zero was 770 Td (190 kV cm⁻¹ at 1 bar) for pure C5 PFK and decreased to 225 Td (78 kV cm⁻¹ at 1.4 bar) for 7.6% C5 PFK/air mixture. The latter value of (E/N)_lim is still more than two times higher than the (E/N)_lim value of synthetic air and about two-thirds of the value corresponding to pure SF₆. The investigated gas mixtures have the potential to become an alternative to SF₆ in numerous high- and medium-voltage applications. (paper)