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A critical assessment of electrical breakdown in liquid dielectrics

The early literature on electrical breakdown in the condensed phase invokes three primary processes to explain the mechanism of breakdown. These are electron emission from the cathode, possibly via a Schottky or Fowler-Nordheim process, interaction of the electrons with the bulk of the dielectric resulting in energy transfer to the latter as proposed by von Hippel, and electron multiplication by some ionization process leading to Townsend type avalanches and eventually to complete breakdown. All these processes had been proposed earlier for gases and crystalline solids. Their application to amorphous condensed matter represented, however, certain problems. For example Lewis(1) tried to explain breakdown in saturated hydrocarbons in this manner. He assumed that near breakdown electron emission occurred from the cathode via a Schottky process. The injected electrons drifting in the liquid under the influence of the applied field, were visualized to lose their energy to the solvent molecules thereby exciting vibrational modes of the latter. Collisions of such excited molecules were then able to cause ionization processes to take place. The positive ions so produced were to lead to an instability such as an avalanche and hence to breakdown. Lewis' theory appeared to explain the experimental data he had obtained under uniform field conditions using electrode separations of 10 to 80 μm and dc stresses. It was not clear from this paper how the hydrocarbon molecules would acquire sufficient energy for dissociation since the proposed vibrational processes had associated with them less than 0.5 eV. Also there was great uncertainty of the mean free path an electron could travel in a liquid to gain sufficient energy from the applied field. The use of relatively narrow gaps, the assumption that the applied field was the same as the local field, and the lack of information on the electron injection mechanisms appeared to be responsible for leading Lewis to incorrect conclusions. To overcome some of these difficulties Watson and Sharbough considered the formation of bubbles at the cathode surface. These authors such as asperities, electron emission would occur leading to high current flow. The Joule heating resulting from this current flow could provide the energy necessary to vaporize the liquid in the immediate vicinity of these injection sites in times as short as 1 × 10-6s. The authors reasoned that once such a bubble was formed it could grow across the gap and form a spark channel. They attributed these high currents to be due to field-enhanced thermionic emission. They calculate a field enhancement factor of 5 and postulate a metal work function of between 0.25 and 5 eVe The bubble model thus avoided the need for an electron multiplication process. These conclusions were based on highly purified n-hexane as the test liquid, two parallel flat electrodes with rounded edges ranging in separation from 60 to over 600 μm, using rectangular voltage pulses as well dc conditions. The attractive features of this bubble model caused Krasucki(3) to investigate the influence of viscosity on this mechanisms. He extended the bubble concept to include initiation of these bubbles by submicroscopic particle impurities in the liquid. The author was able to actually photograph the bubble development in very viscous hexachloro biphenyl under uniform field conditions using parallel plate electrodes separated 100 μm and an applied voltage of 22 kV. The attractive feature of Krasucki IS development was its ability to predict correctly both temperature and pressure effects.