Piezoelectric Properties of Ferroelectret

Ferroelectret films (FFs) were fabricated using porous polyethylene (p-PE) film with various polarities treated by corona discharge. Their piezoelectric constant in the direction perpendicular to their surface was over 100 pC/N Hence, in order to clarify the piezoelectric motion, Piezo-Response Microscope (PRM) measurements and perpendicular elasticity measurements of the ferroelectret p-PE film were carried out. As a result, first, we found that there is a strong correlation between elastic and piezoelectric constants, and second, the areas surrounding the pores shift in one direction following the application of ac voltage, but areas far from the pores cannot move freely. For the mechanism behind expression of the piezoelectric phenomenon, these results indicate that a large decrease in thickness occurs predominantly across the pores upon the application of stress and, as a result, the macroscopic polarization caused by trapped electric charges surrounding pores increases considerably following the change in thickness. Therefore, a large piezoelectric constant is observed in ferroelectret p-PE film.