Surface treeing and segmented worm model of tracking behavior in MgO/Epoxy nanocomposites

Abstract

Surface tracking experiments were conducted on MgO/epoxy nanocomposites according to IEC60112. Treeing phenomenon, which was commonly observed in bulk insulators, was detected on the surface of tracked samples via optical microscopy. Tree branches were further found to be segmented tracking channels by scanning electron microscopy, which consisted of a series of ablation holes along electric field, i.e. wormholes. According to distribution of surface trees, tracking area was divided into three typical regions. Region A, close to electrodes, was barely ablated. Region B, which directly connects electrodes, exhibited paralleled tracking channels. Region C, which is the outer annular ablated region, is full of crossed tracking channels. At the beginning, the initial tracking channels were demonstrated to appear at the triple junction of Region A, B and C, where electric field was supposed to be the highest. Then tracking regions spread from the junction to Region B and Region C with tracking channels segmentally growing longer and wider, indicating a segmented energy accumulation process. Tracking trees were finally formed connecting two electrodes, leading to sample failure. With addition of nano-MgO particles, wide and deep channels turned into slight but dense ones. Correspondingly, comparative tracking index (CTI) increased from 400 to 475 V with increasing nano-MgO from 0 to 10 wt%. Improved tracking resistance was obtained in nanocomposites while reduced thermal conductivity and pyrolysis characteristics were observed. Combining with results above, a segmented worm model was thus proposed to explain surface tracking performance in MgO/epoxy nanocomposites.