Abstract
A nanogenerator is a nanodevice which converts ambient mechanical energy into electrical energy. A piezoelectric nanocomposite, composed of vertical arrays of piezoelectric zinc oxide (ZnO) nanowires, encapsulated in a compliant polymeric matrix, is one of most common configurations of a nanogenerator. Knowledge of the effective elastic, piezoelectric, and dielectric material properties of the piezoelectric nanocomposite is critical in the design of a nanogenerator. In this work, the effective material properties of a unidirectional, unimodal, continuous piezoelectric composite, consisting of SU8 photoresist as matrix and vertical array of ZnO nanowires as reinforcement, are systematically evaluated using finite element method (FEM). The FEM simulations were carried out on cubic representative volume elements (RVEs). Four different types of arrangements of ZnO nanowires and three sizes of RVEs have been considered. The volume fraction of ZnO nanowires is varied from 0 to a maximum of 0.7. Homogeneous displacement and electric potential are prescribed as boundary conditions. The material properties are evaluated as functions of reinforcement volume fraction. The values obtained through FEM simulations are compared with the results obtained via the Eshelby-Mori-Tanaka micromechanics. The results demonstrate the significant effects of ZnO arrangement, ZnO volume fraction, and size of RVE on the material properties.
Highlights
Mechanical energy is one of the most ubiquitous and abundant among sources of energy
A mechanical energy-harvesting nanodevice based on piezoelectric zinc oxide (ZnO) nanowires was first demonstrated by Wang and Song in 2006 [5]
The effective material properties are evaluated as a function of ZnO volume fraction for all six types of representative volume elements (RVEs) described in Section 3 using the finite element method (FEM) and the analytical EMT micromechanics-based approach
Summary
Mechanical energy is one of the most ubiquitous and abundant among sources of energy. Using asymptotic homogenization method Guinovart-Dıaz et al have derived exact expressions for binary piezoelectric composites where both components have 6 mm symmetry for cases where the reinforcement is a piezoelectric ceramic and the polymeric matrix may or may not be piezoelectrically active [26] In addition to these analytical and semianalytical approaches, computational methods based on finite element modeling have been employed to evaluate effective properties of piezoelectric composites. Previous work on nanogenerators with piezoelectric ZnO nanowires embedded in a PMMA matrix used simplistic Reuss and Voigt analyses to predict effective properties of the composite, assuming both the matrix and reinforcement to be isotropic [8]. The effective properties of the SU8/ZnO nanocomposite were obtained by FEM and by an approach based on Eshelby-MoriTanaka (EMT) micromechanics.
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