Design, fabrication, and characterization of a MEMS thermal switch and integration with a dynamic micro heat engine.

摘要

In this work, a MEMS scale thermal switch was designed, fabricated, and tested. The switch was shown to contol heat transfer over millisecond time scales and micrometer length scales. The switch was then integrated into a micro power generation system to enable waste heat harvesting.; Two test stands were used to determine two figures of merit characterizing a thermal switch: the ratio of thermal resistance of the switch in the off and on states, Roff/Ron, and the time required to switch from the off to the on state, tauswitch . For the thermal resistance ratio measurements, three switch conductor materials, polished silicon surfaces, arrays of Hg micro droplets, and vertically aligned carbon nanotubes were tested at steady state conditions. The liquid-metal switch has a thermal resistance off to on ratio of 168. A thin-film radial heat flux sensor was then used to characterize the transient thermal behavior of the liquid-metal microdroplet thermal switch at dynamic state conditions. The switching speed of the Hg micro-droplet switch was shown to be on the order of 0.01 seconds, realizing heating rates of 1800°C/sec.; A cantilever piezoelectric actuator was selected for the active thermal switch. The relationship among deflection, driving frequency, input voltage, and input electric power was developed. This model gives an estimation of the required input voltage or electrical power for piezoelectric devices of any application.; Power production by a dynamic micro heat engine with an integrated thermal switch was demonstrated. The thermal switch was used to control heat addition into the microengine from a constant heat source at 60-70°C. Net power output (mechanical power out over electrical power in) was demonstrated. The maximum power output from a passively cooled microengine was 364 muW. Under this condition, the power required to run the thermal switch was 23 muW.; The feasability of using electrowetting to actuate a MEMS scale thermal switch was investigated. A switch based on this concept was designed and fabricated. These tests indicate that this approach is feasible.