Combustion-based micro power generation: Thermoelectric and thermionic approaches.

Abstract

The goal of this doctoral research is to develop combustion-based micro thermoelectric (TE) and thermionic (TI) power sources that can achieve high power density and reasonable conversion efficiency. The ultimate goal is to realize a combustion-based micro power source capable of replacing conventional batteries or powering MEMS/IC devices. The key issues in this project are the successful development and integration of the micro combustor, micro energy converter and thermal management components. A basic configuration of micromachined combustor has been designed, which includes a thermally-insulated catalytic combustion chamber. The high temperature regions in the combustor are isolated either by a thin diaphragm or thick SiO₂ rings. Using this configuration, very high combustion temperatures (1000&deg;C) and large temperature gradients (50--100K per 100mum distance) have been achieved in millimeter-size combustors. A TE converter has been integrated with the combustor structure by incorporating polysilicon-metal thermopiles into a thin dielectric diaphragm. The doping concentration of polysilicon is optimized to achieve a maximum conversion efficiency of &sim;3%. The combustion of hydrogen and air mixture is self-sustained inside the 2mm x 8--12mm x 0.5mm combustor after ignition. An average output power density of 1.6mW/cm<super>3</super> has been demonstrated. A TI converter could achieve better efficiency and power density than a TE converter at high combustion temperatures. Technologies for TI emission, thermal isolation, vacuum packaging and sensing, and micro combustion have been developed and integrated into a fabrication process for a combustion-based micro TI power generator (muTIP). The TI device integrates a closely-spaced emitter-collector pair in vacuum on top of a micromachined combustor. Thick SiO₂ rings are used to support the emitter for excellent thermal isolation. Low work-function thin-films of BaO/SrO/CaO are formed on the surfaces of microfabricated emitter and collector through the coating, decomposition and activation of their carbonates. Power of &sim;30muW/cm<super>2</super> has been generated in the TI converter tests at an emitter temperature &sim;900&deg;C. Combustion of hydrogen and air mixture has been achieved in the 2mm x 2--6mm x 0.5mm combustors of the muTIPs. Improvements in device structure to help increase maximum temperature and thermal isolation during combustion are needed to approach the theoretical power levels of 0.1--1 W/cm<super> 2</super>.