Barrier discharges in cellular polypropylene ferroelectrets: How do they influence the electromechanical properties?

Ferroelectrets (i.e., charged cellular polymers) are rendered piezoelectric by means of barrier discharges inside the air-filled voids. The light emission from barrier discharges in cellular polypropylene ferroelectrets was quantitatively studied. Light emission typically occurs above a threshold voltage of 3 kV and then significantly increases with the applied voltage. Time-resolved images reveal discharge processes in individual voids. In addition, a second "back discharge" emission is observed when the voltage is reduced to zero. The buildup of the "effective polarization" in cellular PP ferroelectrets was studied by an acoustic method and dielectric resonance spectroscopy. A polarization-voltage (P-V) hysteresis loop was obtained by analyzing the data with an existing model for the piezoelectric d33 coefficient of ferroelectrets, from which a threshold charging voltage of 3 kV and the back barrier discharges were confirmed and a zero-field "effective polarization" of 0.5 mC/ m2 was determined. However, charge densities of up to 2 mC/ m2 were measured under an applied bias voltage, leading to the conclusion that the observed back discharges destroy a significant fraction of the effective charge density.