Trap and polarization of polyethylene terephthalate influenced by electron beam irradiation

Trap and polarization greatly affect dielectric breakdown and surface flashover behavior of insulating materials. Trap and polarization may be modified by electron beam irradiation via breaking polymeric molecular chains or triggering cross-linking. Polyethylene terephthalate (PET) with a thickness of 75 μm from DuPont was used to investigate the modifications of trap and polarization by electron beam irradiation. PET samples were irradiated by an electron beam with an energy of 30 keV and a beam current of 5 μA for 5 minutes in a vacuum chamber. Experiments of thermally stimulated depolarization current (TSDC) and broadband dielectric relaxation spectrometer (DRS) were conducted on samples before and after the electron beam irradiation. Three TSDC peaks were observed above glass transition temperature on both samples before and after irradiation. A TSDC current fitting method was used to extract trap parameters of as-received and irradiated samples. Trap energy and trap density are clearly increased after irradiation compared to the as-received sample. In addition, a significant crossover can be seen in the spectra of relative dielectric constant before and after irradiation. Dielectric loss factor is clearly increased after irradiation. Comparative experiments proved that trap parameters and polarization are changed by electron beam irradiation. Analyzing the energy deposition properties in PET, we found that the penetration range of electrons with the energy of 30 keV is about 12 μm. Electron beam irradiation transfers energy to molecular chains, causing ionization and producing free radicals, which can change the trap and polarization of PET.