Carrier traps in polymeric insulating materials

Abstract

Recently, space charge in insulating materials has been pointed out to play an important role in the dielectric breakdown, especially in the DC insulation system. The nature of space charge is closely related to the carrier trapping phenomena. In this paper, natures of carrier traps in various polymers were investigated by using x-ray induced thermally stimulated current (TSC) and thermoluminescence (TL) techniques. The TSC and TL techniques are powerful especially in high and low temperature regions, respectively. Most polymers except those of high ionic conduction such as polyvinyl chloride show remarkable TSC and TL peaks due to trapped carriers, suggesting an abundance of carrier traps. Most of these peaks correspond to the onset of molecular motions in polymers, suggesting that the carrier detrapping is closely related to molecular motions. There exist many traps in crystallites and/or in their interfacial regions. In polyethylene (PE), there are five or six kinds of carrier traps, which are considered to be due to physical defects such as cavities. Both oxidation and γ-irradiation introduce new deep traps into PE and they greatly influence the electrical conduction in PE.