Electronic Models of Electrical Breakdown

The formation of electronic models of electrical breakdown by various authors is based on their different views of the mechanism of electronic injection from the cathode into the solution, and on distinct ideas as to what processes electrons undergo while in solution. Lewis, on the basis of observation of dielectric breakdown in -alkanes, has proposed a model based on an increase in electronic mobility leading to breakdown. Wong and Forster, on the basis of results obtained by the ultrahigh-speed laser schlieren system, have proposed a model based on observation of the formation of conductive channel columns bridging cathode and anode. An electrochemical explanation has been offered on the basis of the influence of the electrode material on the potential of electrical breakdown in water. 

Lewis has assumed that an efficient electron injection at the cathode is unlikely to occur by the Schottky or Fowler-Nordheim transfer processes, but if it is to occur it must come about by sufficient lowering of the energetic barrier to permit direct transfer of metal electrons of Fermi level energy into the free-electron states 

The electrons transferred to the solution would gain energy because interaction with the electric field and their mobility would increase, respectively. It was proposed that for electric fields of about 10 Vcm electron mobility could be as much as 

Lewis proposed that the energy of high-mobility electrons can be transferred to molecules of the liquid by interaction with molecular vibrational modes of energy of tenths of electronvolts, usually. For n-hexane it can be as high as 0.37 eV in the infrared. The transfer could take place in either of two ways. In the first, called the weak interaction mode, a quasicontinuous loss along the electron trajectory is envisaged, resulting from the dielectric response of the liquid to the electric field of the electron. It was estimated that the time required for excitation of the liquid is 10 s, and the interaction distance would be approximately 0.6 nm. 

The electrochemical model has been developed on the basis of such experimental facts as the dependence of the electrical breakdown phenomenon or processes leading to it on the type of the electrode material,on exchange current density for the electrochemical process taking place at the electrode surface, on the electrode gap, and on the ionic impurities present in the 

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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. 

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