Positive synergy of SF6 and HFO1234ze(E)

We measured mixtures of SF
6
and HFO1234ze(E) (C
3
H
2
F
4
) and determined the density-reduced critical electric field strength (E/N)
crit
in a pulsed Townsend experiment. HFO1234ze(E) is an environmentally friendly alternative to SF
6
since contrary to SF
6
, its global warming potential is negligible. While SF
6
is unsurpassed in its efficiency of attaching thermal electrons, HFO1234ze(E) appears to moderate electron energies very efficiently. Electron attachment to HFO1234ze(E), on the other hand, is much weaker. We find a positive synergism in SF
6
/HFO1234ze(E) mixtures in the sense that the mixture of both compounds has a higher electric field strength than each gas separately. A maximum (E/N)
crit
of 425 Td is reached for a mixture of 40% SF
6
and 60% HFO1234ze(E). A mixture of 90% HFO1234ze(E) and 10% SF
6
has a critical electric field strength of 360 Td, similar to pure SF
6
. We attribute this effect to the high efficiency of HFO1234ze(E) in moderating down the electron energies, which enhances electron attachment to SF
6
and reduces electron impact ionization.