DC-biased ferroelectrets with large piezoelectric d33-coefficients

The application of external dc voltages significantly increases the piezoelectric d33-coefficients of ferroelectret films and also enhances their thermal stability. In this paper, interferometric measurements and model calculations with such dc-biased ferroelectret films are reported. It is shown experimentally and theoretically that the d33-coefficient of the inverse piezoelectric effect of these films actually consists of a part depending on internal electret charges and a part linearly depending on the external dc voltage. Very high d33-coefficients of up to 1200pC∕N at a frequency of 5kHz are determined interferometrically. These are explained quite well by the model calculations. The model departs from an existing theoretical description by assuming that the forces generated by the electric field act on the entire cellular structure and not only on the gas voids.