Electro-Mechanical Coupling and Power Generation in a Pzt Micro-Engine

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ELECTRO-MECHANICAL COUPLING AND POWER GENERATION IN A PZT MICRO-ENGINE D.F. Bahr, K.R. Bruce, B.W. Olson, L.M. Eakins, C.D. Richards, and R.F. Richards Mechanical and Materials Engineering, Washington State University, Pullman WA 99164-2920 ABSTRACT A piezoelectric thin film MEMS device for generating power from a novel heat engine which approaches a Carnot cycle has been developed. The structure of the underlying electrode and PZT thin film generator has been optimized for increased adhesion. Atomic force microscopy was used to track electrode grain size and roughness; generating grain sizes of approximately 100 and 200 nm in diameter and a roughness of about 14-20 nm provide substantial improvements in film adhesion over systems with smaller grains and smoother surfaces. This has led to the ability to operate the engine at frequencies between 10 and 1500 Hz. The system of interest (a fluid filled cavity sealed by a micromachined silicon membrane and the PZT film) shows increased deflections for a given pressure applied to the membrane at frequencies where the system resonates. By operating the system dynamically, it is possible to generate more than 2 V from a single generator structure. INTRODUCTION The design of a microengine based on a generic, two-dimensional, modular architecture, the P microengine, consists of a unit cell engine of a cavity filled with a working fluid. The top and bottom of the cavity are bounded by thin membranes, which seal in a two-phase working fluid. The top membrane of the cavity consists of a thin (lead zirconate titanate, PZT) film generator on top of a bulk micromachined silicon membrane. The working fluid is chosen such that it is a saturated liquid-vapor mixture throughout the engine cycle. The working fluid does not circulate through the piston (cavity) of the engine. Instead, heat is alternately conducted in and out of the engine through the thin membranes capping the cylinder at top and bottom. As heat is conducted first into and then out of the working fluid, the quality (ratio of vapor to liquid) and volume of the saturated mixture first increases and then decreases. As a consequence, the upper membrane, which is fabricated of a PZT thin film, acts like a piston in a conventional large-scale engine. Instead of sliding in and out, however, the piezoelectric thin film membrane flexes in and out. In this way, the piezoelectric membrane acts alternately as a generator/expander and as an actuator/compressor during the engine cycle, shown schematically in Figure 1. The useful output of a unit cell engine is the electrical power generated by the piezoelectric membrane as it is strained during expansion of the two-phase working fluid, minus the power consumed by the membrane during the compression of the working fluid. Many researchers who use PZT films have demonstrated that controlling the structure of the PZT film and the underlying electrode can impact the electrical performance of the resulting PZT devices [1-4]. In this paper we will demonstrate how controlling the structure of