Effect of traps on DC surface flashover characteristics of polymer in vacuum

Vacuum is an excellent electrical insulating material with well dielectric properties and low cost price. It has been widely applied in aerospace industry, power system insulation, marine electrical equipment and microelectronics. When the solid insulator is introduced into the vacuum gap as an insulating support, the solid insulating material applied in power and electronics system insulation will occur discharge breakdown phenomenon among the triple junction of metal-vacuum interface-dielectric surface, namely surface flashover. The surface flashover phenomenon in low dc electric field of polymer material is a key factor restricting the power system insulation strength, which is also a bottleneck problem for improving the reliability and life of solid insulating medium in vacuum. It is well known that the solid materials flashover phenomenon is very complicated and influenced by many factors, such as vacuum degree, applied voltage waveform, shape of material, surface condition, surface charge, temperature and so on. In order to reveal the traps effect on surface flashover performance, the PI/ZnO composite sample was prepared by blending physical-chemical method and the DC surface flashover testing system was established. The trap levels were obtained from thermally stimulated current (TSC) method. It was found that the DC surface flashover voltage presented first increased and then decreased in high vacuum with the increasing of nano ZnO mass fraction, reaching maximum at 3 wt%. It is obtained that the reasonable mass fraction of nano-fillers can significantly improve the DC flashover voltage in vacuum. The experiments results can be explained as that the doped ZnO nano-particles introduced deeper traps, which can reduce charge carriers mobility, secondary electron emission and space charge accumulation. It was also found that the average deep trap levels of composite increased at first then decreased with the increasing of nano ZnO mass fraction. At the same time, the trap level density of nano-composite was significantly higher than that of pure PI. It presented first increased and then decreased with the increasing of nano ZnO mass fraction as well. The deep trap density is much higher than the shallow trap density, which also presented first increased and then decreased. Experiment result demonstrated that ZnO nano-particles doping can remarkably improve the surface flashover performance. This has been attributed to the introduction of deep traps due to enormous interface areas caused by nanometer ZnO doping. Firstly, the electrons (or holes) are easily captured by deep trap in the interface regions and difficult to escape from the deep trap center, reducing the charge carriers mobility, kinetic energy and secondary electron emission of insulator. Secondly, the increased trap depth and density will result in the trapped homocharges near the cathode form a charge injection blocking, which will reduce the efficient electrical field in the interface regions, causing the reduction of the local electrical field at the cathode triple junction (CTJ). CTJ is the critical region in which the surface flashover is initiated; therefore, a larger overall stress must be applied before the flashover occurs. In addition, electrons and positive ions due to impurities ionization are limited in the interface regions as a result of low carriers mobility and kinetic energy. Thus the development of flashover process is restricted and the surface performance is promoted.