Dynamic Behavior Of Surface Charge On Polymer Insulating Materials

Surface charge accumulated on polymer insulating material can be held for a long time. The reduced dielectric strength has been attributed to the presence of such charge. During the past decades, a large number of papers relating to surface charge accumulation and decay process have been published, and the importance of this information for employing polymers in designing electrical devices of high reliability has been established.Polymer insulating materials are used in a broad range of applications in electrical and electronic devices, in which the polymers are possibly subjected to complicated working conditions including corona/contact charging and energetic irradiation. However, previous studies fell short of taking proper account of effect of such conditions on dynamic behavior of surface charging and decay. In this paper, Polyethylene (PE) and Polybutylene Naphthalate (PBN) were employed as samples to investigate the behavior by using a newly designed electrostatic probe with a charge resolution of 0.018 pC/ mm2.Surface charging behavior was investigated by applying a DC voltage between two patterns of parallel-electrode, one of which was achieved by directly pressed aluminum plates that provided non-compact contact. The other one was formed by attached copper foil with compact contact. Obtained results show that homocharge and hetrocharge are respectively accumulated in the case of non-compact and compact contact charging. For PE, negative charge is deposited more easily than that of positive charge. For PBN, positive charge is deposited with larger number than that of negative charge. It is proposed that surface charge accumulation is arised from contact charging onto localized surface states, and the subsequently hopping transportation gives rise to the characteristics of charge distribution. The charging process depends upon surface electronic structure of the polymer and contact method between electrode and sample.Short-circuit decay of surface charge was performed on corona-charged sample by employing the mentioned patterns of electrode. The results show that decay rate is high at the initial stage and tends to be slower with time. The rate appears to be dependent upon the contact method varying as a function of polarity of charging particles. It is suggested that the charge decay is initially induced by recombination between de-trapped charges and ions with the opposite sign in the air. With the lapse of time, the lateral motion of de-trapped charge under its self-field becomes dominant in the decay process. Furthermore, the decay behavior possibly depends upon the energy level of surface trap.Effect of gamma-ray irradiation on the formation of surface charge of PE was studied by dc corona charging method. Prior to the test, the samples were irradiated in air up to 100 kGy, 1000 kGy with dose rate of 10 kGy/ h by using a 60Co gamma- source. Obtained results show the dependence of charateristics of charge distribution upon the radiation dose. With the growth of the radiation dose, the total amount of charge initially decreases then tends to increase. Capacity of the charge is limited by radiation induced chemical reactions through mechanisms like cross-linking and degradation. It is proposed that gamma-ray irradiation with approparite radiation dose is available for inhibiting charge accumulation on cross-linking type polymers, and the effectiveness of such inhibition is investigated by employing PBN as the sample.