Increased piezoelectric d33-coefficients of ferroelectrets with high external DC-voltages

The application of external DC-voltages linearly increases the piezoelectric d₃₃-coefficients of conventional ferroelectret films. Since the external DC-voltage is obviously thermally stable whereas the internal electret polarization decreases at elevated temperatures, the application of the external DC-voltage not only increases the d₃₃-coefficients but also enhances their thermal stability. Moreover, ferroelectret films show relatively high nonlinear distortion when used in the actor mode. These nonlinear effects are also reduced by the external DC-voltage. In this paper, interferometric measurements of ferroelectret films operated with high DC-voltages are reported.