Charge trapping and stability in dielectrics deduced from isothermal and non-isothermal measurements

The mechanisms of charge transport and trapping, in highly insulating polymers, can not be investigated properly by a unique (isothermal or nonisothermal) technique. Should the trapping time be high, the isothermal discharging time can be very long with the consequence that the discharging current becomes very low but there is still charge trapped in the sample. By sequential use of the isothermal charging, discharging, the final thermally stimulate discharge current and the final isothermal discharging current techniques, the charge dynamic in highly insulating materials can be investigated. A sequential protocol for the investigation of the trapping sites and trapped charge stability is proposed. The method is demonstrated for Teflon FEP-A. By using a selective charging, thermally stimulated discharge current peaks are obtained whose position shifts with formation conditions and are characterized by activation energies around 1.3-1.4 eV. The relaxation time of the trapped charge, at 523 K, increases as the amount of charge stored in the material decreases, providing evidence of a field-assisted detrapping process. The method is particularly useful for the temperature range where the thermal movement is important and/or the contributions from dipoles and space charge are equally important.