Experimental and theoretical study on time dependence of the quasi-piezoelectric d33 coefficients of cellular piezoelectret film

The voided charged polymer film, also called piezoelectret, has a very large quasi-piezoelectric coefficient in the thickness direction, and has emerged as a new kind of electromechanical transducer materials. Piezoelectret film is usually prepared from polymer by means of biaxial stretching and electric charging process. Due to the inherent viscosity of polymer, the quasi-piezoelectric d33 coefficient of cellular piezoelectret film usually depends on the pressure and time of the measurement. In this article, experiments were carried out on the time spectra of quasi-piezoelectric d33 coefficient in the thickness direction for cellular linear Polypropylene piezoelectret film. To study the effect of void microstructures on the time-dependence of quasi-piezoelectric d33 coefficient, samples of three different thicknesses were tested under two different pressures. The micromechanical theory for viscoelastic composite was extended to predict the electromechanical properties of voided charged polymer film. The voids with surplus charges, which can be piezoelectriclike under deformation, are considered as ellipsoidal heterogeneous piezoelectric inclusions, while the viscous polymer is taken as the matrix. In Laplace transformed space, the generalized Eshelby tensor is formulated for the isotropic nonpolar matrix as well as for the anisotropic matrix. The Mori–Tanaka average scheme is used to find the overall electromechanical properties. Time dependence of the effective properties in real space can be studied by Laplacian inversion. Sensitivity analysis to various parameters is investigated for time dependence of the effective properties, including effective elastic moduli and the quasi-piezoelectric coefficients. Theoretical simulation was presented and comparison with experimental results was conducted. Both qualitative analysis and quantitative comparison with experiments show that this theoretical formulation can predict the time dependence of the effective properties of voided charged piezoelectret film.