Contribution of flexoelectricity in piezoelectric PVDF films for EH applications

A new branch of research in energy harvesting is flexoelectricity of solids. The flexoelectric effect is an electromechanical conversion mechanism that occurs in all dielectric materials. This can lead to new applications especially due to the larger number of possible materials. Therefore, investigation of properties of dielectric materials and specific conversion conditions are of great importance. Of importance is also finding out the significance of flexoelectric in relation to the piezoelectric energy conversion and possible other conversion mechanisms. This paper seeks to verify the flexoelectric effect in solid piezoelectric materials and to discover the usability for energy harvesting purposes. It follows earlier studies in this area only proving the flexoelectricity in non-piezoelectric materials. In our investigations, PolyVinylidene DiFluoride (PVDF) polymer films were used in a plate capacitor configuration. A measurement setup was built to enable the evaluation of polymer films under different temperature levels. Thereby, the pure flexoelectric behavior above the Curie temperature could be separated from the combined piezoelectric and flexoelectric effect at room temperature. In this way, an impression of the contribution of the flexoelectric effect in the energy conversion should be gained. As main parameters we used the change of electrical voltage and drift current in a differential measurement setup by means of a Lock-In Amplifier. The strain gradients in the material were generated by needles of steel with a vertical oscillation of the polymer film in relation to the needle tip. The results clearly show the presence of the flexoelectric effect and in this way also its contribution for energy conversion. The relative voltage and current drift response of the samples offer a great contribution of flexoelectric energy conversion under inhomogeneous mechanical loads.