Mechanical-to-Electrical Energy Conversion of Thin-Film Piezoelectric Diaphragms
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0973-BB06-04
Mechanical-to-Electrical Energy Conversion of Thin-Film Piezoelectric Diaphragms Dylan J. Morris, Michelle C. Robinson, Leland W. Weiss, Cecilia D. Richards, Robert F. Richards, and David F. Bahr Mechanical and Materials Engineering, Washington State University, Pullman, WA, 99164
ABSTRACT A micro (~1 cm3) dynamic heat engine, capable of producing electrical power from lowgrade heat sources, utilizes a micro-machined diaphragm with a piezoelectric element as a The electromechanical coupling of a piezoelectric diaphragm under large initial stresses and/or large deflections – in the membrane limit – is described here. A simple model is derived for electromechanical transduction of a pressurized piezoelectric membrane and an experiment is described to measure it. Electromechanical coupling initially increases as the square of the center-point deflection as the residual stress is overcome. In the limit of large pressures, the electromechanical coupling approaches a limit that is predicted by the model. INTRODUCTION It has long been recognized that the power densities of electrochemical batteries is too low for sustained operation of mobile devices for long periods. One tack is to integrate energyscavenging devices, such as vibration harvesters, thermoelectric materials, or small heat engines in parallel with high energy-density components. The P3 micro heat engine under development at Washington State University is a dynamic heat engine meant to utilize external-combustion or waste heat, such as that rejected from a high-temperature fuel cell or micro-turbine, further increasing the overall efficiency of the primary micro energy source. The P3 utilizes a novel thermodynamic cycle to convert periodic heat pulses to a mechanical work in the form of a pressure pulse. The size of the engine is designed to take advantage of microfabrication construction techniques, by replacing the piston of a conventional engine with a very flexible silicon-based membrane that has an integrated piezoelectric element. Furthemore, the engine is modular – as few or as many engines as needed may be operated in series or parallel to take advantage of the quality of the waste heat source, within the space contraints. This piezoelectric element converts the pressure to electrical power with a thin-film laminated diaphragm that includes a piezoelectric element, electrodes, and support structure [1]. The purpose here is to examine the construction of a piezoelectric diaphragm with a view towards maximizing the transduction of mechanical to electrical energy, directly impacting the efficacy of the mobile heat-scavenging device. As shown in [2], typical operating pressures for the engine are 5 – 25 kPag. First, the most appropriate mechanical treatment for the piezoelectric diaphragm in the P3 engine – either as a plate, or a membrane – must be decided. The explicit transition from plate to membrane behavior has only been recently studied [3,4,5]. Komaragiri et al. [5] makes the choice of the applicable mechanics (plate vs. membrane) mo
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