The researchers have built a thin polypropylene ferroelectret (PPFE) by creating a cheap polypropylene foam whose empty voids they charged through microplasma discharges (by applying a large electric field to the PP film). The artificial voids in the foam, spanning from the 1μm scale to the 100μm scale then form highly oriented giant dipoles across the 80μm thick film. Two conductive silver layers sputtered on the surfaces of the PPFE film complete the device, turning it into a sandwich-like metal-insulator-metal (MIM) structure.
Supporting their practical experimentations with finite element method (FEM) analysis, the researchers explain that as the charged voids change their thickness and thus their dipole moments under mechanical stress (compression for example), the change of dipole moments is capable of driving the electrons from the electrode with negative charge to the electrode with positive charge, generating a voltage under open circuit conditions, or generating current under short circuit condition (a flow of charge from one electrode to the other). They also highlight the PPFE films' piezoelectric coefficient (d33∼400 pC/N) as being significantly greater than that of typical piezopolymers like PVDF (d33∼15pC/N) or parylene-C (d33∼2 pC/N).