Effects of additives on prebreakdown phenomena in liquid cyclohexane:: I.: Streamer initiation

Conduction currents and streamer initiation in cyclohexane with various additives have been investigated with fast impulses applied to a point-to-plane gap. The studied additives are perfluoro-n-hexane, N, N-dimethylaniline, 1-methylnaphthalene, di-n-propylether, 1,1-difluorocyclopentane and 1,4-benzoquinone, representing molecules with different electrochemical properties. We have used shadowgraphic imaging and a differential charge-measurement technique with a sensitivity of 0.1 pC to integrate currents induced to the point electrode, below and in the same voltage range where streamers first appeared. Effects on streamer initiation are found only for point anode streamers with the additive 1-methylnaphthalene, requiring higher voltages for initiation. We have observed transient conduction currents of measurable magnitudes in the nano to microampere range for voltages around initiation in all samples with a point anode and in 10 out of 21 samples with a point cathode. For a point anode, the more easily ionizable additives N, N-dimethylaniline and 1-methylnaphthalene induced somewhat larger currents. The results may be considered to support a hypothesis of electron avalanche induced streamer initiation, in that transient currents are induced by the ions left behind by the avalanches that are not sufficiently developed to initiate streamers.