Figure 5.

The working principle and operation modes of the PENGs. a) Electrical poling direction and preferential chain direction of the piezoelectric materials. For instance, in poly(vinylidene fluoride) (PVDF), the polar axis (labeled as direction “3”) is the direction of the applied electrical poling field. The polymer stretch direction or the preferential direction of the aligned polymer chains is denominated as direction “1” and is perpendicular to the polar axis. the axis orthogonal to the stretch direction “1” is labeled as “2.” The shear planes of piezoelectricity are indicated by the directions “4,” “5,” “6,” and are perpendicular to the directions to “1,” “2,” “3,” respectively.^[238] The direction of the applied mechanical stress relative to the polar axis largely affects the performance of the piezoelectric energy harvesting device. b) The schematics of direct and converse piezoelectric effects. The direct piezoelectric effect appears when a mechanical stress is applied to a material, and the electric charges are generated proportional to the applied mechanical stress. Before the external stress is applied, the centers of the positive and negative charges of each molecule coincide and the material is in a neutral net electrical polarization. When a mechanical stress is applied and deforms the structure of the material, the positive and negative charges inside of the molecule will be separated and this leads to the generation of dipolar moments. When a mechanical stress is reversed, the polarity of dipolar moments will be reversed. This polarization generates an electric voltage output, which is the transformation of the mechanical vibration applied to the material into useful electrical energy to power electronic devices. The converse piezoelectric effect occurs when the electric field is applied to the piezoelectric material. The external electric field will change the position of electrons and nuclei in each molecule and dipoles will be created. These dipoles will result in the polarization of the material and ultimately induce the deformation of the material. When the electrical field is removed or reversed, the electrons and nuclei will move back to their original position, and the material will return to their initial geometry.