Charge storage capability of cross-linked polypropylene electret films

Piezoelectrets, based on the bipolarity space-charge electret, become a new class of artificial piezoelectric functional materials. Their piezoelectric performances are relative to the charge storage capability of the material directly. In this paper, the storage stability and dynamic properties of the charges in the cross-linked polypropylene (XPP) electret films, treated by a hot-pressing process, are investigated by using the measurements of isothermal decay of the surface potential and open-circuit thermally stimulated discharge (TSD) current spectra. The results show that the charge storage stability of the positively charged samples is better than that of the negatively charged samples. When the pre-ageing temperatures are less than 90degC, the surface potentials of positively charged samples still keep above 90% of the initial value after annealing for 5120 min; while the values for the negatively charged samples are below 86%. The results also indicate that most of detrapped charges during TSD are retrapped in deeper traps. Therefore charge transport is controlled by a fast retrapping effect.