Influence of macroscopic dipoles on the charge storage and charge dynamics of polypropylene ferroelectret films

In this work, influence of the formation of macroscopic dipoles with space charges enclosed in a void (electric domain) and their polarization enhancement on the charging current characteristic and the conductivity of cellular polypropylene (PP) film were studied by the corona charging with a grid in combination with the reverse-polarity corona charging compensation method. By measuring the isothermal surface potential decay and analyses of open-circuit and short-circuit thermally stimulated discharging current spectra, the charge stability and charge dynamics of PP ferroelectrets in relation to the formation and density variety of macroscopy dipoles were also discussed. The experimental results indicate that the decrease of charge stability of cellular PP film with high polarization should be attributed to the increase of the conductivity induced by the self-field of macroscopic dipoles. For cellular electret film with weak polarization, macroscopic dipoles formed by space charges should only be trapped at two sharp ends of lens-like void and, therefore, detrapped charges will transport along the side surfaces at both ends of the void. When the sample has high polarization, the trapped charges are distributed in the whole surface layer of the void. Most of the detrapped charges excited by the electret field drift through the solid dielectric layer between adjacent voids, which leads to their decay.