Study on the Difference of Space Charge Distribution in EPDM under DC and AC Electric Fields

In order to compare the difference on space charge formation mechanism under DC and AC fields before and after thermal aging, the space charge behavior in Ethylene-Propylene-Diene Monomer(EPDM) is investigated in this paper. Part of the samples degassed in advance were thermal aged at 200 °C for 9 h. The space charge behavior was detected under 1~25 kV/mm DC field with polarity reversal and AC field, respectively. For unaged sample, heterocharge and homocharge appears near the top electrode with semi-conducting material and the bottom electrode with aluminum material, respectively. The heterocharge accumulation area near the top electrode narrows, while the distribution range of homocharge near the the bottom electrode expands after thermal aging at 200 °C. In the condition of AC field pressure, heterocharge and homocharge accumulates near the top electrode and the bottom electrode inside unaged sample, respectively. After thermal aging at 200 °C, the internal charge distribution of the sample tends to be consistent with the phenomenon under DC field. Based on the characteristics of charge diffusion and migration, it is inferred that the heterocharge in the unaged sample mainly originates from the dissociation of impurities, and the homocharge mainly due to the electrode injection. Thermal aging at 200 °C may weaken the impurity dissociation and enhance charge injection. Moreover, some deep traps may be introduced during thermal aging at 200 °C, resulting in a more difficult detrapping process. Compared the phenomena of space charge behavior under DC and AC fields, the differences are mainly due to the fact that the space charge inside the sample under DC field mainly comes from electrode injection, while the space charge under AC field mainly comes from impurity dissociation.