The mobility of charge-carriers in dielectric liquids

This thesis describes work carried out in the Department of Applied Physics, Durham University, from October 1964 to September 1967. The work was a continuation of investigations, under the supervision of Dr. M.J. Mo rant, into the electrical properties of dielectric liquids. The work of Sletten in 1959 and that of Morant and Kahan in 1964 has shown that dissolved oxygen has a marked effect on the breakdown strength and the conductivity of a typical dielectric liquid, n-hexane. The present investigation shows that the reduced breakdown strength and increased conductivity observed on de-gassing hexane may not be explained in terms of an increase in carrier mobility. The mobility of photo-injected carriers in highly de-gassed hexane is found to be no greater than the mobility of the same carriers in air-saturated liquid. Further, no increase of mobility with applied field is observed in the de-gassed liquid for fields up to 140 KV/cm; nor is there any indication, under these conditions, of the existence of an additional 'fast' carrier. The variation of the injected current with both the applied field and the degree of oxidation of the cathode, however, indicates that the 'oxygen effect' referred to above is an electrode, surface phenomenon and not a property of the liquid itself. The current transients from which the mobility determinations were made are not exactly of the form predicted by simple theory. This and other anomalous observations indicate a strong interaction between the charge-carriers and the neutral liquid molecules. Such an interaction is consistent with self-trapping of the injected carriers by polarization of the surrounding medium. Suggestions for further work on the identification of the charge-carriers in dielectric liquids and on the factors affecting breakdown and conductivity are made in the text.