The nature of the thermally stimulated discharge current (TSDC) for polyethylene terephthalate samples in the temperature range from room temperature to above glass-rubber transition temperature of the amorphous phase is analyzed. The well conditioning of the sample is strictly necessary in order to have a good reproducibility and accuracy of results. A main peak was observed whose maximum temperature moves towards a lower value with the decreasing of the amount of charge that flows through the sample during polarization. The peak position changes as well, if the sample is polarized in air or in oxygen and the nature of change is more important in the case of oxygen. The shape of the peak is complex and at least four shoulders have been identified around 85, 90, 105, and 125 °C using the cleaning technique. The activation energy tends to increase with repetition of the TSDC runs, in the glass-rubber transition temperature range, in the case when the cleaning technique is used for the peaks separation. For the conditioned samples, there is a good agreement between the experimental results and the analytical expression of the current, particularly in the region where it reaches a maximum, and so relevant values for the characteristic parameters of the peak are determined. The time interval of the short circuiting of the sample, at room temperature, before the TSDC measurement, strongly influences the initial rise of the current and consequently the parameters of the peak. A possible redistribution of the internal field arising from the injected charge, the heterocharge, and the existing charge in the sample as received, has been put forward to account for the experimental evidences. The conclusion is that the current is mainly determined by the space-charge released from the traps that are likely continuously distributed in energy. For the stated polarization conditions, the charge is released from the shallow traps with an activation energy in the range 0.23–0.32 eV and a concentration of ∼1018/m3. The dipolar charge is of little importance.
Articles you may be interested inThe contribution of dipoles and space charge to low temperature relaxation in polyethylene terephthalate Combined isothermal and nonisothermal dc measurements to analyze space-charge behavior in dielectric materials J. Appl. Phys. 97, 044103 (2005); 10.1063/1.1847703On the nature of thermally stimulated discharge current spectra in polyethylene terephthalate Formation and relaxation of poled order in dye doped polystyrene probed by isothermal and nonisothermal current measurements Analysis of the thermally stimulated discharge current around glass-rubber transition temperature in polyethylene terephthalateThe thermally stimulated discharge current and the isothermally final discharging current have been measured, in vacuum and in different ambient gases for ''as-received'' polyethylene terephthalate specimens, in order to understand the nature of the origin of the released current in the temperature range from glass-rubber transition temperature up to 220°C. The behavior of the samples thermally treated in oxygen, in nitrogen and in ambient air was analyzed, the gases have been used for detecting the localized states in the material. The current spectrum is determined by the space-charge existing in the as-received sample, and by the adsorbed and/or absorbed gases and water vapors. The movement of the ions, resulting from the interaction of the adsorbed and absorbed gases with the parasitic space charge, in the field produced by the space charge, is responsible for observed change in polarity of the current during nonisothermal and/or isothermal measurements and for the appearance of the or space-charge peak. This movement is considered to be thermally activated with a field-modified activation energy. The calculated activation energy, for the sample thermally treated in oxygen at different temperatures, was in the range (0.9-2.3)Ϯ0.1 eV. From the isothermal discharging current measurements, values for the exponent of time in the range from 0.04 to 0.7 were obtained suggesting a dispersive transport of the charge. The total charge density stored in the material is about 4ϫ10 Ϫ5 C and the corresponding trap density approximately 10 23 /m 3 . This charge is substantially larger than that determined by the pulsed electroacoustic method.
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