Modeling Of Piezoelectric Coupling Coefficients Of Soft Ferroelectrets For Energy Harvesting

Recently, Fluoroethylenepropylene (FEP) tubes have been thermally fused to form ferroelectrets with a parallel-tunnel structure with well-defined air gaps thereby improving their applicability for energy harvesting applications. These structures exhibit mechanical and electric properties, mostly depending on their geometry, which result in structure-specific longitudinal and transversal piezoelectric coupling coefficients (d(33) and d(31)). This work presents a finite element model able to describe the mechanical-to-electrical coupling of FEP parallel tunnel structures. The focus of the presented paper, however, lies singularly on the coefficient d(33), neglecting the transversal coupling coefficient d(31). The coupling coefficient d(33) is calculated in three main steps. First, the induced charge on the electrode's surface for the unloaded state is determined. Second, the deformation of the structure due to an external force applied in thickness direction is computed. Third, the modified shape is used to determine the electrode's charge variation. The model correctly predicts the experimental values for the Young's moduli in the range of 0.6MPa to 0.9MPa, within 10 %. The piezoelectric coefficient d(33) is predicted for two internal charge densities, namely 3 x 10(-4) C/m(2) and 5 x 10(-4) C/m(2). The respective d(33) coefficients are 110 and 240 pC/N. This is in excellent agreement with measured values of 120 and 180 pC/N, respectively. The results prove the finite element model to be suitable to calculate the piezoelectric coupling coefficients for different types of geometries.