Electric relaxation study in CdSe/PVA nanocomposites by the thermally stimulated discharge technique

The study of the dielectric relaxation processes in polymer nanocomposites is a useful tool to learn electrophysical characteristics of these materials. The experimental method usually employed for this analysis is thermally stimulated depolarization currents (TSDC). This measurement (TSDC) is one of the powerful tools for identifying and characterizing relaxation processes of materials [1, 2].The sample, supplied with suitable electrodes in a sandwich configuration, is heated up to a temperature Tp (polarization temperature) above the room temperature. Then, an electric field Ep (polarizing field) is applied to the sample for a time tp (isothermal polarization time). After this, the sample is cooled and subjected to the polarizing field.The aim of this paper was to study the electrical polarization and relaxation phenomena in PVA and CdSe/PVA nanocomposite to better understand these processes. Special attention was paid to the influence of addition of CdSe nanocrystals to PVA matrice.Two samples, pure PVA and CdSe/PVA nanocomposites, and with respectively 80, 120 micrometer thick in the film form were prepared mixing CdSe nanopowder in aqueous PVA. The CdSe nanopowder used for this experiment was synthezied by sonochemistry method in our previous work [].Circular aluminum electrodes of 10mm diameter were kept on both sides of all samples. The typical area of the specimens used for TSDC measurements was ....mm. chatacterization of samples were performed using an electrometer , Model y5-11 from Russia, sample chamber and a plotter , model H306, Russia.Thermally stimulated depolarization currents (TSDC) spectrum of the PVA and CdSe/PVA nanocomposite has been analyzed by inducing a polarization temperature of about 460 K. The relaxation zones are defined. In both samples relaxation peaks observed. In the case of pure PVA, the first peak, the sample was polarized at 373 K and was cooled at room temperature. The rate of increasing in temperature in all experiment was 5K/min. TSDC plot for this polarization shows one peak up to some degrees above T=430 K. the small bend in relaxation process is shown in figure 1 (I). The sample was polarized and cooled at 373 K for two times and the plot of relaxation process from TSDC technique were shown in figure1 (II and III is related to second and third polarization plot. the observed small bend in the first polarization shows that the uncompleted phase changing in PVA polymer but in second and third polarization the peaks likes together and are sharp. There are some possibilities for this observation, the first is existence the third materials like water in this sample but the most probable reason for this observation is change from amorphous to crystalline phase for PVA and also transition from different crystalline structure. Increaseing in degree of crystallinity with increasing in temperature is also affect on relaxation process. It is clear that the orientation and reorientation of dipoles in crystalline form are easier than amorphous phase. In the case of CdSe/ PVA nanocompoites, the relaxation takes place in the crystalline-amorphous interface. In this case crystalline CdSe particles in amorphous PVA were polarized at 373. the analysis reveals two change in peak at different temperatures for each polarization. For first polarization the peak were formed at about 413K but for second polarization the peak shifted to lower temperatures. Both of these peaks are in the up side of plot. In the down side of plot also two peaks observed at 443K. In the down side, a relaxation at the crystalline phase is identified in the high temperature region. In addition, a shoulder localized in a narrow temperature interval (220-240 K) of the TSDC spectrum between the first and second zones is observed. This suggests a relation between the anomalous relaxation and the motion of the polymer chains bound to the crystalline-amorphous interface.