A review is given on the physical properties of electronic charge carriers (electrons, holes) in non-polar and polar liquids. The results are used to explain or discuss certain electric conduction phenomena, which occur when these liquids are subjected to a high electric field strength. Electronic conduction in ultrapure liquids is finding an increasing number of applications.

Electronic Conduction Processes in Dielectric Liquids

A model is presented for the development of breakdown in liquids subjected to uniform high amplitude electric field stresses with duration of microseconds or less. In this model, breakdown proceeds in four stages: (i) formation of a low density site (nucleation) in the liquid near an electrode, (ii) growth and expansion of this site until the local density is reduced below a critical density for electron impact ionization to take place, (iii) growth of an electron avalanche and its transformation into an ionizing front, and (iv) propagation of the ionization front via a sequence of processes occurring in the region ahead of the front; namely, heating by electron injection, lowering of the liquid density and avalanche growth and retardation. Expressions for the duration of each of these stages are derived and their behaviour with pressure and field strength analysed for cathode initiated breakdown. From this, a critical energy criterion for breakdown is obtained. Comparison is made with experimental results for water, salt solutions, and the liquid noble gases and for pulse durations in the microsecond and sub-microsecond time scales. This model serves to elucidate the dynamics of pulsed breakdown of liquids.

https://iopscience.iop.org/article/10.1088/0022-3727/28/1/025/pdf

Development of pulsed dielectric breakdown in liquids

Properties of cryogenic insulants

The influence of hydrostatic pressure and liquid conductivity on the dielectric breakdown of water solutions subjected to high amplitude electric fields of sub-microsecond duration has been investigated. Well-defined pulses (80 kV, 3 ns risetime, 100 ns duration) have been applied to a gap (0.4 to 2.1 mm), between Rogowski profile electrodes (thus ensuring a uniform electric field), containing de-ionized water (non-distilled, and distilled and ultrasonically treated), sodium chloride solutions (0.001 to 1.0 molar), or magnesium sulfate solutions (0.01 to 0.1 molar). Breakdown in these liquids has been studied at pressures from atmospheric up to 40 MPa. The inter-electrode potential and the current response were measured indicating the time lag to breakdown, breakdown voltage, and temporal characteristics of the breakdown process. The breakdown time lag increases with increasing pressure, and is insensitive to the liquid conductivity. These findings have relevance to the ongoing discussion concerning 'thermal' vs. 'electronic' mechanisms for dielectric breakdown in liquids. In particular, the results suggest that breakdown evolves via 'bubble' formation by field emitted currents near asperities on the cathode, and that the time for the change in liquid conductivity as a result of breakdown is limited by processes other than ionization growth (due to electron impact ionization of molecules in the 'bubble') of prebreakdown electron currents. >

The influence of pressure and conductivity on the pulsed breakdown of water

The present paper is mainly devoted to phenomena occurring in point-plane electrode geometry, where breakdown is the result of the initiation and propagation of prebreakdown phenomena called "streamers". In this configuration, an investigative study of the streamer initiation processes, requiring very high electric field strengths (~ MV/cm), and of propagation (requiring low electric field, ~ kV/cm) can easily be carried out for negative streamer development as well as for the positive case. From analysis of experimental results in pure liquids the physical processes connected with streamer initiation and propagation, particularly the electronic ones, are presented and discussed. Estimations of the main parameters of slower subsonic streamers and of the faster filamentary ones (such as field strength at the streamer tip, field inside the channel, charge density, etc.) have been obtained from qualitative considerations and compared to experimental data

High field conduction and prebreakdown phenomena in dielectric liquids

Dielectric breakdown of cryogenic liquids in terms of pressure, polarity, pulse width and impurity

Dependence of polarity effect of dielectric breakdown on molecular structure of liquids

Breakdown characteristics of liq.Ar,Kr,Xe,N2,He and Ne are studied. In liquids with large electron mobility (Ar,Kr,Xe),electron avalanche seems to play important role at breakdown. In liq.Np, bubble process initiates breakdown at long time range, under low pressure, and at short electrode distance, but electronic process plays predominant role at short time range, under high pressure and at long electrode distance. In liq.He and Ne in which electron localizes in electron bubble state, anomalous polarity effect that non-uniform field with positive needle indicates higher strength than the uniform field is observed. Electron injection from cathode should trigger breakdown in this case.

Effects of impurity on breakdown of cryogenic liquids are also studied.

M. Terauchi

Papers

Dielectric breakdown of cryogenic liquids in terms of pressure, polarity, pulse width and impurity

Dependence of polarity effect of dielectric breakdown on molecular structure of liquids

Breakdown characteristics and mechanisms of cryogenic liquids