Effect of Deep Traps and Molecular Motion on Dc Breakdown of Polyethylene Nanocomposites

Low-density polyethylene (LDPE) has been widely used as an insulating material in high-voltage direct-current power cables. In this research, we investigate how to improve the electrical breakdown strength of LDPE by nanodoping method and the mechanism of improvement. MgO particles with an average diameter of 50 nm are mixed with LDPE to fabricate nanocomposites by using a toque rheometer. Five kinds of nanocomposite samples are fabricated with nanofiller loadings of 0.25 wt%, 0.5 wt%, 1 wt%, 2 wt%, 3 wt% and pure LDPE is made as the contrast. Then the nanocomposites are pressed into sheet samples about 100 μm by plate vulcanizing machine. The images observed by scanning electron microscope show nanoparticles are dispersed uniformly in LDPE matrix. X-ray diffraction is used to measure the bonding effect between nanoparticles and polymer matrix as well as the morphology of nanocomposites. The trap parameters such as trap levels are characterized by thermally stimulated depolarization current. The dc breakdown experiments indicate that the dc breakdown strength increases firstly and then decreases with an increase in nanofiller loading. The dc breakdown strength is enhanced by incorporating nano MgO and reaches the maximum value 377.06 kV/mm at around 0.5 wt%, which is 17.61% higher than the breakdown field of pure LDPE. The influences of bonding effect, morphology, and trap properties on dc electrical breakdown strength of LDPE nanocomposites are analyzed. It is found that incorporating a small amount of MgO nanoparticles into LDPE matrix enhance the bonding effect between nanoparticles and polymer matrix and establish isolated interfacial regions around nanoparticles. Then, deep traps are formed in the interfacial regions and molecular chains with occupied deep charges are difficult to move under electric force. Consequently, the dc electrical breakdown performance is improved. At higher nanofiller loadings, bonding effect is weakened and interfacial regions are overlapped so that carriers can migrate more easily and the dc electrical breakdown field is reduced.