Ferroelectrets (electrically charged voided polymers) have been studied intensively in recent years due to their high piezoelectric coefficients (typical d33 coefficients reach several hundred pC/N), their mechanical flexibility and their capability to cover large areas, making them ideal candidates for a wide range of sensor and actuator applications. The piezoelectric activity of ferroelectrets results from engineered electric dipoles, formed by space charges deposited on the inner surfaces of the lens-like or box-shaped voids. The voids are electrically charged by applying a high electric field to the foam in the direction perpendicular to the film surface. As the electric field in the gas-filled voids reaches the critical field Ec for Paschen breakdown, a dielectric barrier discharge occurs, generating charge pairs that are separated and trapped at the internal surfaces of the voids. In the present work, this model was used to evaluate the dependence of the piezoelectric d33 coefficient on the gas pressure during the charging process, with the aim of predicting optimum charging parameters.

The piezoelectric activity of charged cellular foams (so-called ferroelectrets) is compared against simulations based on a multi-layer electromechanical model and Townsend's model of Paschen breakdown, with the distribution of void heights determined from scanning electron micrographs. While the calculated space charge hysteresis curves are in good agreement with experimental data, the onset of piezoelectric activity is observed at significantly higher electric fields than predicted by Paschen's law. One likely explanation is that the commonly accepted Paschen curve for electric breakdown in air poorly describes the critical electric field for dielectric barrier discharges in micrometer-size cavities.

The piezoelectric activity of charged cellular foams (so-called ferroelectrets) is compared against simulations based on a multi-layer electromechanical model and Townsend's model of Paschen breakdown, with the distribution of void heights determined from scanning electron micrographs. While the calculated space charge hysteresis curves are in good agreement with experimental data, the onset of piezoelectric activity is observed at significantly higher electric fields than predicted by Paschen's law. In addition, the model suggests that optimized charging should be possible by varying the gas pressure while the charging field is applied.