Conducting liquids, dielectric breakdown

I. Adamczewski, Ionization, Conductivity and Dielectric Breakdown in Dielectric Liquids, Taylor and Francis, London, 1969. 

Irradiated liquids do possess electrical conductivity, clearly indicating the existence of ionic species as in gaseous systems. Some results for cyclohexane23 are shown in Fig. 2. Dielectric breakdown of the liquid occurs, however, before a saturation value of the current is reached. 

The dielectric properties of oligodimethylsiloxanes and their dependence on temperature point to good dielectric characteristics of PMS liquids. Taking into consideration that PMS do not form conductive carbon particles in case of electric breakdown or sparking, it is obvious why they are used as liquid dielectrics in transformers and other electric devices. There is a law in Japan and the USA which forbids the use of nonflammable yet toxic pentachlorodiphenyl and leaves room for oligodimethylsiloxanes. This has stimulated a much more active production of PMS liquids in Japan, Germany and other countries. 

A solid or liquid dielectric inserted between two electrodes can support only a limited voltage. Several physical mechanisms can lead to a current instability and to breakdown, e.g., thermal instabilities in materials with thermally activated conductivities, transitions from trap-controlled transport to band transport, impact ionization, etc. (Zeller, 1987). Which one of the different mechanisms ultimately determines the dielectric strength depends on materials parameters, geometry, voltage pulse forms (including history), temperature, etc. (O Dwyer, 1973). 

All calculations in this chapter were carried out for water, because water has the highest ionic concentration and conductivity in comparison with other liquids used in studies of the dielectric breakdown phenomenon. Thus, all calculated quantities are larger than would be for other liquids. It means that the view presented here can also represent solvents other than water. 

There are two principal views on dielectric breakdown generation. The first favors the cavitation-bubble mechanism, and the second involves electronic phenomena occurring in the studied system. The major differences between the bubble and electronic models of electrical breakdown lie in the importance attached to the temporal development of events which precede a spark, i.e., a moment considered as the breakdown. In the former case, it is proposed that ionization and current growth begin to occur in the gaseous phase after nucleation of a bubble, whereas, in the latter case, these processes begin first in the liquid areas. Among these two schools several models based on a different approach to the source of the increased conductivity of Uquids under electrical stress have been proposed. 

Warman, J.M. de Haas, M.P. Hummel, A. Conduction and Breakdown in Dielectric Liquid Goldschwartz, J.M., Niessen, A.K., Boone, W., Eds. Delft Univ. Press Delft, Netherlands, 1975 70. 

Al-Arainy, A.A., Jayaram, S., Cross, ID. (1996), 12th Int. Conf. on Conduction and Breakdown in Dielectric Liquids, Rome, Italy. 

Sowada, U., Bakale, G., Yoshino, K., and Schmidt, W. E, Electron transport in high-mobility liquid hydrocarbons and tetramethylsilane, in Conduction and Breakdown in Dielectric Liquids, Goldschvartz, J. M., Niessen, A. K., and Boone, W, Eds., Delft University Press, Delft,... 

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