Study of thermally poled PET electrets using the pulsed electroacoustic and thermally stimulated depolarization current techniques.

We have studied by pulsed electroacoustic method (PEA) and thermally stimulated depolarization current (TSDC) a set of PET electret samples. Experimental parameters such as the combined thermal and electrical history and the electrode type have been selected in order to correlate the polarization mechanisms revealed by TSDC with the charge profile measured by PEA in five different cases. Existing deconvolution procedures for PEA have been improved in order to calibrate successfully PEA signals. Samples where the $\alpha$ dipolar relaxation or the $\rho$ space charge relaxation is activated show a uniform polarization that manifests itself as image charge at the electrodes. In the experiments where external charge carriers are injected in the sample the same poling procedure has been tested under different electrode configurations. Charge profiles are similar in all of them but the depolarization currents show clearly different behavior. These differences are explained by the different blocking behavior of vacuum-deposited aluminium electrodes with regards electrodes with a thin air gap, on the one hand, and the distinct behavior of electrodes with air gap for electrons or vacancies, on the other hand. Numerical analysis of the polarization of TSDC peaks and charge per unit area of charge profiles back up this interpretation and confirm the relationship between both measurement types. All in all, PEA turns out to be a useful technique in the study of thermally poled electrets, either in the study of relaxations or of external charge.