Abstract
Piezoelectrics are an important class of materials for mechanical energy harvesting technologies. In this paper we evaluate the piezoelectric harvesting process and define the key material properties that should be considered for effective material design and selection. Porous piezoceramics have been shown previously to display improved harvesting properties compared to their dense counterparts due to the reduction in permittivity associated with the introduction of porosity. We further this concept by considering the effect of the increased mechanical compliance of porous piezoceramics on the energy conversion efficiency and output electrical power. Finite element modelling is used to investigate the effect of porosity on relevant energy harvesting figures of merit. The increase in compliance due to porosity is shown to increase both the amount of mechanical energy transmitted into the system under stress-driven conditions, and the stress-driven figure of merit, FoM33X, despite a reduction in the electromechanical coupling coefficient. We show the importance of understanding whether a piezoelectric energy harvester is stress- or strain-driven, and demonstrate how porosity can be used to tailor the electrical and mechanical properties of piezoceramic harvesters. Finally, we derive two new figures of merit based on the consideration of each stage in the piezoelectric harvesting process and whether the system is stress- (FijX), or strain-driven (Fijx).
Highlights
The European Physical Journal Special Topics the electromechanical coupling coefficient, which is a measure of the efficiency of conversion of input mechanical energy into stored electrical energy, and vice versa: k2 =Stored electrical energyStored mechanical energy = (1)Input mechanical energy Input electrical energy and in terms of material properties, the coupling coefficient is given by: ki2j =d2ij εXii sEjj (2)where dij is the piezoelectric strain coefficient, εXii is the permittivity at constant stress and sEjj is the mechanical compliance of the material under constant electric field conditions
Input mechanical energy Input electrical energy and in terms of material properties, the coupling coefficient is given by: ki2j d2ij εXii sEjj where dij is the piezoelectric strain coefficient, εXii is the permittivity at constant stress and sEjj is the mechanical compliance of the material under constant electric field conditions
A detailed overview of the complete piezoelectric energy harvesting process has been described to aid with effective materials selection and design
Summary
The difference between the two merit indices in equations (2) and (3) is the lack of a compliance term in equation (3), which has been postulated to be a potential benefit for polymer-based piezoelectrics that exhibit high FoMXij values, despite their low coupling coefficients, as a result of their high compliance and low piezoelectric coefficients (dij) [3]. This concept is relevant to the use of porosity to enhance the performance of piezoelectric materials, whereby pores are intentionally introduced into a piezoceramic in order to reduce the effective permittivity, whilst maintaining relatively high piezoelectric strain coefficients. This leads to increased longitudinal harvesting figures of merit, FoMX33, with increasing porosity [5,6]
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