Charge injection into dielectric liquids

Abstract

This work was aimed at finding suitable conditions for increased charge injection into liquid hexane. Experiments have been carried out on charge injection by both photoelectric and high-field processes. Photoelectric injection in very pure and highly degassed hexane depends mainly on the condition of the photocathode. Immediately after the preparation of the photocathode, the current increases continuously with applied voltage and it is shown to be mainly due to the emission of photo- electrons. Both cathode and liquid deteriorate over several days and a larger proportion of the current becomes due to bulk ionisation. Simultaneous d.c. and pulse measurements are used to prove that the initial photocurrent not only originates at the cathode but that its magnitude is also limited by the rate of emission of photoelectrons which subsequently form heavy negative carriers. The field dependence of the photocurrent has been explained in terms of the probability of heavy carrier formation by the electrons passing over an image barrier. The observed carrier mobility is in close agreement with that of other workers. Field-injection from razor blade electrodes resulted in high conduction currents at comparatively low applied voltages. The current is only slightly higher when the blade ^s negative. In pure hexane the current is found to be independent of bulk movement of the liquid so that it must be limited by electrode processes which are probably electron emission and electron extraction for negative and positive polarities respectively. In commercial and water or alcohol doped hexane, the current increases with liquid flow when the blade is negative but decreases when it is positive. These effects are explained in terms of field dissociation of the impurities and the formation of space charge at the high-field electrode. The present measurements suggest that the high injection currents reported elsewhere are mainly due to field ionisation of impurities rather than electron emission.