Piezoelectric Energy Harvester Design and Characterization

Abstract

The harvesting of kinetic energy from moving or vibrating sources has received increased attention over the last decade due to the abundance of such sources and the need to use their energy for powering autonomous sensors for health, environmental and structural monitoring. The main two modalities for the harvesting of mechanical energy are electromagnetic and piezoelectric with the latter being more amenable to miniaturization using micro and nano electromechanical technologies. Piezoelectric vibrational energy harvesters provide larger power densities, smaller footprint, and are easier to integrate with integrated circuits. The objective of this thesis is to design high-power density piezoelectric energy harvesters using a novel micromachining platform made available to the Masdar Institute by the Singapore Institute of Microelectronics. Our designs are based on the development and validation of accurate yet efficient computational electromechanical models of the piezoelectric harvesting element in the device. In particular, a distributed-parameter electromechanical model of a doubly-clamped multielectrode energy harvester (EH) is developed which leads to a closed-form, electromechanically-coupled analytical solution that is validated against finite-element models. The model is then translated into a single-mode equivalent electrical circuit that is used for efficiently co-simulating the energy harvester along with its loading network. A process-aware piezoelectric vibrational EH design workflow is devised and used to design optimized cantilever EH devices that satisfy the fabrication process requirements. The design process includes modeling and simulation of the candidate devices with CoventorWare, MEMS+ and Cadence Virtuoso. Equivalent circuit representations of the complex harvester shapes have been identified and used in circuit simulation tools to speed up the simulation. Moreover, an electrode placement optimization workflow has been devised and used to design doubly-clamped EH devices with optimally placed electrodes. The fabrication process considered for the design flow uses Aluminium Nitride (AlN) for the piezoelectric material, which is environmentally safer than the toxic lead-based piezoelectric material. In order to characterize the fabricated designs, several optical measurements have been taken to ensure that the device dimensions are in-line with the process design rules. Deflection measurements from a digital microscope have been used together with the device models to estimate the AlN layer residual stress present in the EH devices. A mechanical shaker along with its accompanying acquisition and analysis modules has been used to record the upper limit of acceleration excitation, open circuit voltage and maximum power collected from the EH devices. The frequency test range for the optimized impedance has been obtained using an impedance analyzer and an LCR meter.

Date of Award	Dec 2016
Original language	American English
Supervisor	Ibrahim Elfadel (Supervisor)