Samuel B. Schaevitz

A microfabricated suspended-tube chemical reactor for thermally efficient fuel processing

We present a suspended-tube chemical reactor/heat exchanger for high-temperature fuel processing in micro energy conversion systems, primarily for hydrogen production in portable fuel cell systems. This reactor, designed to thermally isolate a high-temperature reaction zone, consists of four free-standing silicon nitride tubes comprising two independent U-shaped fluidic channels. Portions of the tubes are encased in silicon to enable heat exchange between the fluids in these channels. A thin-film platinum resistor is embedded for localized heating and temperature sensing. This paper describes the design and fabrication process for the MEMS fuel processor. Fluidic testing, thermal characterization up to 825/spl deg/C, and preparation of catalyst washcoats in the reactor microchannels are discussed. In addition, results from catalytic autothermal butane combustion and ammonia cracking in the reactor are presented.

A Combustion-Based MEMS Thermoelectric Power Generator

A thermoelectric generator with integrated catalytic combustion has been microfabricated and successfully tested. The device consists of a high-temperature silicon-germanium thermopile supported on a thermally insulating silicon nitride membrane. Heat is supplied by catalytic combustion of fuels on the underside of the membrane. Power output has been generated from on-chip autothermal combustion of hydrogen, ammonia and butane, with external power used only to drive mass-flow controllers. The device was stable at temperatures up to 500°C, with thermopile voltages up to 7V and device thermal efficiencies up to 0.02%.

A microfabricated suspended-tube chemical reactor for fuel processing

A microfabricated suspended-tube reactor has been developed and demonstrated to operate at temperatures over 900/spl deg/C for efficient thermal processing of chemical fuels. This reactor uses thin-walled SiN tubes to directly address the most significant problem in small-scale fuel processors: thermal management. It efficiently isolates a high-temperature zone while maintaining a temperature gradient of up to 2000/spl deg/C/mm. This design is ideally suited to serve as a combustor/recuperator for thermoelectric (TE) and thermophotovoltaic (TPV) generators, and as a reformer to produce hydrogen for portable fuel cell systems. Using the integrated heaters, catalytic ammonia cracking has been carried out to produce up to 1.6 W (9 sccm) of hydrogen with 97% fuel utilization.

Passive precharge and rippled power logic (PPRPL)

A low-power, high-speed logic style using Passive Precharge and Rippled Power (PPRPL) is proposed. Ultra-low threshold voltage (Vt) devices permit high speed operation, while the heavy leakage current pre-charges dynamic nodes. High Vt devices prevent leakage through the logic. The high Vt devices provide power to evaluate a sequence of logic gates and are activated in series for periods of time which are short relative to the clock period. The power effectively ripples through the logic path. These innovations combine to produce low power circuits that maintain very high speeds. A 16 bit by 16 bit multiplier was simulated in HSPICE using this logic style. We achieved a clock rate of 1 GHz with a latency of 1.3 ns. At that clock frequency the power dissipation is 10.9 mW.