Cycle work from a MEMS heat engine and characterization of the liquid-vapor phase change actuation mechanism

This dissertation presents a MEMS-based thermopneumatic actuator that produces electrical power by flexing a piezoelectric membrane. Operating the device at the resonant frequency of the piezoelectric generator results in the first documented production of cycle work from a dynamic micro fabricated heat engine. At resonance the pressure-volume diagram is an open loop curve indicating the production of cycle work and allowing for classification as an engine. The engine generates a mechanical power of 26.6muW and an electrical power of .05muW while operating at 240Hz and consuming 1.31W. This corresponds to a thermal to mechanical efficiency of .002%, mechanical to electrical efficiency of .19%, and thermal to electrical efficiency of 3.8mu%.; Improvement of thermal to mechanical efficiency is accomplished by implementing a novel capillary wicking structure consisting of an array of open groove rectangular micro channels fabricated from SU-8. The wicking structure provides a method for delivering a controlled liquid layer to the heat addition region of the device.; A slow filling annular wick is used to investigate performance for single pulse operation. Implementing a 7.1mum thick wick increases efficiency by a factor of 60 from .0015% to .088% for an energy input of 7.8mJ. A numerical model of the device containing the annular wick is developed to determine the energy budget and parameters controlling efficiency for operation on the millisecond time scale. Simulations indicate that liquid thickness, thermal mass, and membrane compliance have a significant impact on efficiency. Predictions are verified experimentally by testing silicon nitride and SU-8 membranes. An 8mum thick SU-8 membrane reduces efficiency due primarily to the increase in thermal mass. A 200nm thick silicon nitride membrane increases efficiency due to a decrease in thermal mass and increase in compliance.; Incorporation of a fast filling 10.3mum thick radial wicking structure dramatically increases cyclic performance. This is because capillary pumping wets the heat addition region during the heat rejection process. At 10 Hz the device generates a peak to peak pressure of 148kPa with an efficiency of .11%. In comparison, the no wick case produces a pressure differential of 16.6kPa with an efficiency of .005%. The radial wicking structure produces significantly higher pressure excursions than all known cyclic thermal membrane actuators.