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 O
2 plasma radiation greatly improved the piezoelectricity.
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