High-field electrical conduction and breakdown in insulating polymers

The current-voltage (I-V) characteristics of polyethylene films fabricated by plasma polymerization have been measured at very high electric fields using linear ramp voltages. Experimental results show that at average fields higher than 1 MV/cm, electrical conduction is mainly due to the Fowler-Nordheim type tunneling injection of holes from the anode and the high hole mobility. There is no evidence of impact ionization at fields close to the breakdown strength. High field conduction is filamentary and governed by the trapped hole space charge. Internal discharges such as electrical treeing and breakdown are initiated by thermal instability within high-current density regions of main conduction filaments (or channels), and then followed by the creation of low-density domains to provide large mean free paths for subsequent impact ionization which leads to an indefinite increase in carrier multiplication and final destruction of the material inside the filaments.; The same techniques have also been employed for the study of the effect of silicon incorporation in polyethylene. The results show that the incorporation of silicon in polyethylene creates hole traps, thus suppressing the conduction current and enhancing the breakdown strength. The amount of positive space charge resulting from the hole trapping increases with increasing magnitude and duration of the applied field for a fixed silicon content, and increases with increasing silicon content for a fixed magnitude and duration of the applied field. This positive space charge tends to suppress the actual field at the hole-injecting contact and to enhance the actual field at the electron-injecting contact. For fields higher than a certain critical value, the rate of the current increase with field changes rapidly. This phenomenon is attributed to the onset of double injection. If a proper amount of silicon is incorporated in polyethylene, the overall effect would be to reduce the conduction and to increase its breakdown strength.; We have also studied the chemically polymerized polypropylene. The results show that the high field conduction in the polypropylene is also due to the hole tunneling injection. A thin SiPE, PE, or SiO{dollar}sb2{dollar} layer deposited on the injecting contact as an emission shield suppresses hole injection, and in turn, increases the breakdown strength of the polypropylene. These important new findings should lead to a better understanding of insulating polymers and a potential application in polymeric insulated systems.; The structure of plasma-polymerized polyethylene films with and without silicon incorporation has been studied on the basis of infrared spectroscopy. The structure of the films without silicon incorporation is similar to that of polyethylene but with CH and CH{dollar}sb3{dollar} elements. The incorporation of silicon introduces SiH, SiH{dollar}sb2{dollar} and SiH{dollar}sb3{dollar} elements into the structure.