Discrimination between dipolar and space charge relaxation using isothermal and non-isothermal measurements

A combined isothermal and non-isothermal measurements technique, consisting of four steps: the isothermal charging current, the isothermal discharging current, the final thermally stimulated discharge current and the final isothermal discharging current is used to analyze space charge movement, trapping and detrapping in polyethylene terephthalate. By corroborating the results obtained during the four measurement steps a better understanding of the involved processes is achieved. An apparent peak was observed at about 8 to 12 °C above the charging temperature whose maximum current scales linearly with the field, similarly as in the case of dipolar peaks. The landscape of trapping sites, associated with amorphous fraction, amorphous-crystalline boundaries and crystalline fraction was explored in the range from 0.53 to 2.97 eV. The shape of the isothermal charging currents change with temperature and field, reflecting that at low fields and/or low temperatures the charge is trapped mainly in the amorphous fraction but for higher fields and/or temperatures the charge penetrates into and is trapped in the crystalline fraction. The shape of the isothermal discharging current is the same between 40 and 105 °C suggesting that the charge is released from the traps in the amorphous fraction near the metal-dielectric interface.