Research on simultaneous measurement of space charge and conduction current for thermally aged cross-linked polyethylene

Abstract

To evaluate the thermal aging effect on space charge dynamics and carrier transport characteristics in cross-linked polyethylene (XLPE), films with the thickness of ∼300 μm are thermally aged at 120 °C for 36, 84, 168 and 252 hours, respectively. And then, the simultaneous measurement of space charge and conduction current is employed by our developed system, which overcomes the defect of the weak correlation of experimental results caused by individually measurement of space charge and conduction current. Fourier transform infrared (FTIR) spectroscopy experiment is carried out to observe the functional group change due to the thermal aging. Finally, based on the simultaneous measurement results, the spatial and temporal distribution of space charge are obtained, as well as the trap energy distribution. It is shown that the carbonyl index increases with aging time, which demonstrates that oxidation occurred obviously. However, the average density of the accumulated space charge slightly decreases at the first aging stage and then rises again with the extension of aging time under various DC electric fields. As for external current density, it decreases gradually with the aging time. Besides, the traps in the samples are divided into two types, one is depicted as shallow trap, and the other is the kind with deeper depth according to the isothermal relaxation current obtained in the simultaneous measurement. With the increase of aging time, the depths of both two types of traps slightly change, while the densities of both two types of traps in aged samples are greater than that in the unaged sample. Therefore, the increase of the trap density may retard the growth of carriers in aged samples, which could explain why the external current decreases with the aging time. In addition, the increase of the trap density may restrict the injection of space charge from the electrodes, which results in the experimentally observed decrease of space charge injection depth.