Mechanism of Electrical Breakdown in Saturated Hydrocarbon Liquids

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Abstract
There have been several investigations of the electrical breakdown strength of hydrocarbon liquids in a high state of purity in which attempts have been made to correlate the strength with density or chain length. A simple relationship between these quantities did not exist when branched‐chain isomers of the n‐paraffins were included. The present paper develops the theory of breakdown outlined by the author in a previous publication in which molecular vibrations in these liquids can provide an energy barrier to ionization by electrons and final breakdown. It is suggested that three major processes occur, namely, electron emission from the cathode, energy loss to molecular vibrations, and ionization of the liquid. The instability which marks breakdown is then induced by positive ions which result from ionization. Provided a suitable test procedure is employed it is possible to compare the energy‐loss mechanism in the various liquids by comparing their electric strengths. The nature of the molecular vibrations is discussed, and it is suggested that the infrared frequencies of the CH3, CH2, and CH groups undergoing bond stretching are excited by the electron motion. Such a frequency, characteristic of the carbon‐hydrogen bond would be common to all hydrocarbon liquids. Collision cross sections for these groups can then be assigned and a breakdown criterion set up which is analogous to that proposed by von Hippel for condensed phases. The general correctness of this theory is illustrated by reference to the measurements of Crowe, Sharbaugh, and Bragg and to new measurements obtained by the author. It is found to be possible to correlate not only the electric strength of straight‐chain hydrocarbons but branched‐chain isomers as well. Finally it is suggested that the current pulses frequently observed in conduction measurements in n‐hexane are possibly associated with bond deformation vibrations.

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