Polarity Effect on Breakdown Voltage in Organic Liquids

The polarity effect on breakdown strength is a well-known phenomenon using a nonuniform electrode system, especially point versus plate electrodes. When the point electrode is negative, particularly, free electrons move in a liquid to form negative ions due to the possibility of electron attachment to electrically neutral atoms or molecules. Negative ions form space charges, by which field stress around the point electrode relaxes. Breakdown voltages have been measured in four liquids having chemical structures differing markedly from each other. It has been found that the breakdown voltage with the point negative is higher than that with the point positive in toluene and n-octhane which have a methyl group in their molecular structure; but there is no such polarity effect in chlorobenzene and carbon tetrachloride which have chlorine atoms. It may be concluded that the polarity effect depends on molecular structure: Electrons attach to methyl groups to make negative ions which may form space charges.