Tapered Cantilevered Bimorphs for Piezoelectric Energy Harvesting: Characterization with Impedance Spectroscopy

Cantilevered piezoelectric bimorphs resonating in the fundamental mode have previously been explored for energy harvesting, where it has been reported that tapering the geometry into triangular shapes provides a constant strain distribution over the surface of cantilevered bimorphs, unlike conventional rectangular geometries with a large tip mass. This enhances the k(31) electromechanical coupling coefficients in the devices, an aspect which is studied using impedance spectroscopy in this paper; a tool that can measure the exact values for the optimal load resistance which can be applied to maximize power output from piezoelectric bimorphs. It is found that tapering the devices into triangular geometries lowers the internal impedance of the device at the resonance frequency; and increases the impedance of the device at the anti-resonance frequency when compared to rectangular counterparts with the same device volume. The peak value for the phase angle shift is also larger in triangular bimorphs, indicating a larger dielectric loss at the median of the resonance and anti-resonance frequencies; implying that the devices here would be more compliant, i.e. provide a wider resonance frequency bandwidth. Therefore, the advantages provided by altering geometry to improve the strain distribution in the devices are reflected here from an electronic standpoint.