Boosting Piezoelectricity via a Bimodal Pore Distribution: The Role of Small Pores

Structural elements play an important role for improving mechanical-to-electrical energy conversion of piezoelectric devices with a hierarchical configuration. However, the state-of-the-art manufacturing method fails to ensure plenty of structural elements for better piezoelectricity. In this work, piezoelectric poly(vinylidene fluoride) (PVDF) foam with an extremely high content of structural elements was fabricated by applying bimodal pore distributions via supercritical carbon dioxide foaming and template-leaching methods, and the mechanisms for improved piezoelectricity were systematically researched. By removal of the preadded templates, the structural element density can be modulated from ∼187 to ∼2.6 × 106 cells/mm2. It was found that the small pores in a foam with bimodal pore distributions can diminish small or medium stress and convert it to high stress in the rest area of the large pores, thus greatly enlarging the ratio of stress concentration. Impressively, the stress concentration degree is improved from 0.19 of bulk PVDF to 5.3 and 13.3 of PVDF foam with unimodal and bimodal pore distributions, respectively. As a result, the piezoelectric outputs increase correspondingly from ∼5 V to ∼8.5 and ∼13 V. This work reveals the strain–charge correlations by applying structural elements and can be extended widely for studies on a three-dimensional piezoelectric device.