Charging mechanism of fibrous PTFE films

Recently air-breakdown charging has gained increasing interest with respect to porous electrets for application in novel piezoelectric transducers. A strong discrepancy between porous (closed-porous) polypropylene (PP) and fibrous (open-porous) teflon PTFE has been recognized concerning the obtainable polarization and the related piezoelectric d₃₃ coefficients. The present study aims to clarify the reasons for the poor charge separation ability of fibrous materials when exposed to electrical breakdown in their air voids. It will be demonstrated that a breakdown-induced conductivity hinders the formation of a large internal polarization by mutual charge compensation and therewith accounts for the obtainable low piezoelectric d₃₃ coefficients in fibrous materials. It will be shown that this limitation can be overcome by utilizing a sandwich structure of two FEP charge blocking layers incorporating a fibrous PTFE film.