Air-breakdown charging mechanism of fibrous polytetrafluoroethylene films

Recently air-breakdown phenomena have gained increasing interest with respect to porous electrets for use as piezoelectric polymers. A strong discrepancy between porous (closed-porous) polypropylene and fibrous. (open-porous) polytetrafluoroethylene (PTFE) has been recognized concerning the obtainable polarization and the related piezoelectric d(33) coefficients. The present study aims to clarify the reasons for a poor charge separation ability of fibrous materials when exposed to electrical breakdown in its air voids. Therefore air-breakdown phenomenon of 91% fibrous PTFE films has been studied in detail, utilizing a corona triode for voltage application. At sufficient surface potential air breakdown sets in at an average field strength of 11.7 MV/m, which corresponds to the value expected by Paschen breakdown in air. For higher applied voltages and therewith higher electric-field strengths in the film, the current generated by such electrical breakdown limits the maximum obtainable surface potential. It will be shown that this limitation is caused by a "breakdown-induced conductivity," which increases with increasing grid voltage. Such 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. (c) 2005 American Institute of Physics.