Piezoelectric Properties in Porous Fluoropolymer Having Isolated Voids

Abstract

The effects of the poling and the measuring conditions on the piezoelectricity were studied in foamed-fluorinated-ethylene- propylene-copolymer films having isolated voids with the average diameter of 0.05 mm. The piezoelectric constant in the thickness direction, d₃₃ larger than 300 pC/N was observed in the samples poled at 16 kV for one minute. Because of the breakdown in the void walls, d₃₃ decreased when the applied voltage was higher than 16 kV, or the poling period was longer than one minute. Even at the temperature higher than 170°C, the poled samples maintained d₃₃ larger than 80 % of the value in the samples poled and measured at room temperature. The d₃₃ considerably decreased under the static stress higher than 30 kPa. The pretreatments before the poling, such as a helium-gas displacement in the voids, a uniaxial compression, and an RF O₂ plasma radiation greatly improved the piezoelectricity.