The influence of oxidation on space charge formation in gamma-irradiated low-density polyethylene

The research presented in this paper investigates the role of oxidation in the formation of space charge in gammairradiated low-density polyethylene after being electrically stressed under dc voltage. Polyethylene plaques both with and without antioxidant were irradiated up to 500 kGy using a Co gamma source and space charge distributions were measured using the piezoelectric induced pressure wave propagation method. It has been found that a large amount of positive charge evolved adjacent to the cathode in the sample without antioxidant and was clearly associated with oxidation of the surface. The amount of charge formed for a given applied stress increased with the dose absorbed by the material. A model has been proposed to explain the formation of space charge and its profile. The charge decay after the removal of the external applied stress is dominated by a process being controlled by the cathode interfacial stress (charge injection) rather than a conventional RC circuit model. On the other hand, space charge in a sample containing antioxidant under the same applied electric stress was negligible even in the sample exposed to 500 kGy. The main process to form space charge is via charge injection rather than charge separation in the sample without antioxidant. r 2003 Elsevier Science Ltd. All rights reserved.