Aleksander J. Franz

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%.

Design issues for membrane-based, gas phase microchemical systems

The use of finite element simulations to characterize the operating behavior of a microchemical reactor and enhance its design is described. Important design issues for a microreactor, specifically heat and mass transfer, are explored with the design of a micro-flow sensor and the redesign of the reactor heater. The impact of the design choices for a flow sensor is quantitatively evaluated and an optimal design is proposed. This design was fabricated and testing confirmed the simulation work. The reaction channel heater was redesigned to improve thermal uniformity in the reaction zone. Simulations showed that before ignition significant non-uniformity still existed, but this was dramatically reduced after ignition. The use of previously reported kinetic models for ammonia oxidation in microreactor simulations is also discussed. It was found that kinetic models evaluated in conventional reactor experiments are inadequate for use in microreactor simulations because of the lower operating temperature and broader range of temperatures of the microreactor.

Isotropic etching of silicon in fluorine gas for MEMS micromachining

Etching of silicon with molecular fluorine for the micromachining of micro electro mechanical systems (MEMS) has been evaluated. The etching process is carried out in a continuous flow etching system that uses a 25 vol% mixture of F2 in N2 and operates at room temperature and atmospheric pressure. Fluorine etches silicon isotropically at a rate of 0.2 µm min−1 and is a viable etchant for bulk silicon micromachining. The F2 etch results in the formation of pits about 10–50 µm in size within the silicon features and roughness in the micrometer and sub-micrometer length scales. SiO2, Pt, Ni, Al and Ta do not etch or roughen after several hours of F2 exposure. In addition, F2 does not etch low-stress silicon nitride; a solid layer forms on the silicon nitride surface upon exposure to F2 that protects the underlying silicon nitride from further attack. In contrast, a commercial XeF2 etching system etches low-stress silicon nitride with a silicon nitride:silicon selectivity of approximately 1:200. The F2 etching system is used to release tubes of a MEMS fuel processor that have 2 µm thick low-stress silicon nitride walls.

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.

Palladium Membrane Microreactors

Novel palladium membrane microreactors have been designed and microfabricated. Selective hydrogen flux has been demonstrated through the palladium membrane in the microreactor. The membrane microreactors can be used for a variety of hydrogenation and dehydrogenation reactions, as well as in hydrogen purification applications. The microfabrication process allows for integration of heaters and temperature sensors into the device. The e-beam deposited thin Pd film achieves excellent hydrogen selectivity at high permeation rates. The membrane has good mechanical strength and has been shown to withstand pressure gradients of over 5 atm. A model hydrogenation reaction has been demonstrated in the palladium membrane reactor. Microfabricated palladium membrane reactors enable new applications because of their small size, fast thermal response times, and high efficiency.

Palladium based micromembranes for hydrogen separation and hydrogenation/dehydrogenation reactions

Novel palladium-based micromembranes have been microfabricated to allow for controlled selective hydrogen flux. These micromembranes have the potential to be used in a variety of hydrogen purification applications, as well as for hydrogenation and dehydrogenation reactions. The microfabrication process allows for integration of heaters and temperature sensors into the device. The e-beam deposited thin Pd film achieves excellent hydrogen selectivity at high permeation rates. The membrane has good mechanical strength and has been shown to withstand pressure gradients of over 5 atm. Microfabricated palladium membranes enable new applications because of their small size, fast thermal response times, and high efficiency.

Expansion of Microreactor Capabilities through Improved Thermal Management and Catalyst Deposition

Capabilities of a gas phase membrane-based microreactor have been expanded, enabling new chemistries and applications. The expanded capabilities include improved control of temperature and selectivity for highly exothermic partial oxidation reactions, a larger thermal and pressure operating window, and the development of a catalyst library.

Integrated Gas Phase Microreactors

Microfabricated chemical reactors could be efficient experimental platforms by allowing process development on a single unit and subsequent scale-up by replication. The ability to integrate control, sensor, and reactor functionality offers advantages over the current complex, multi-unit pilot-plant environment. Assemblies of microfabricated reactors may provide point-of-use generation of hazardous chemical intermediates with storage and transportation restrictions. Fabrication, application, and materials issues in the development of microchemical reactor technology are presented. Microreactor feasibility is demonstrated with examples of partial oxidation reactions. The utility of physically-based, predictive simulation tools in guiding microreactor design is addressed as well.