The influence of oxidation on space charge formation and electrical performance in γ-irradiated low-density polyethylene

The research presented in this paper investigates the role of oxidation in the formation of space charge in gamma-irradiated 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 ⁶⁰Co 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 based on charge separation has been proposed to explain the formation of space charge and its profile. 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. Electrical breakdown strengths in these two types of samples were also measured.