Microcombustor

Details Microcombustor Microcombustor

2003-02-18

2004-09-07

Cocks, Josiah (Department: 3749)

Combustion

Frictional, chemical or percussive type igniter

Catalytic

C431S350000, C431S075000

Reexamination Certificate

active

06786716

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a microcombustor for on-chip thermal management and sensor applications.
BACKGROUND OF THE INVENTION
Most Microsystems currently use macroscopic power supplies and energy sources that are external to the Microsystems device. However, the use of macroscopic power supplies places severe limitations on the functionality of Microsystems for many applications. Therefore, a microsystem comprising an integrated; compact, and flexible power supply is highly desirable. Such an integral microscale power supply would typically need to store energy at a high density and discharge the stored energy at a high rate. A number of microscale power supply concepts have been considered, including microcombustors, electrochemical batteries, fuel cells, storage in magnetic or electric fields, storage as elastic strain energy, etc.
Microcombustors are becoming increasingly important for microsystems applications. Such microcombustors may be useful as Microsystems power supplies, for example, to convert chemical energy to electricity via thermoelectric or thermophotovoltaic generators or to produce hydrogen for fuel cells. In addition, the development of a small and stable on-chip microcombustor would permit the adaptation or translation of several very useful macroscopic devices into the microsystem domain, including on-chip flame ionization detectors (microFiDs), microreactors, micropropulsion, energy conversion and, importantly, heating and thermal management of microsystems. Microcombustors offer several advantages over other microscale power supply concepts for these applications. Microcombustion systems can provide on-demand, instantaneous power. Furthermore, hydrocarbon fuels offer approximately order of magnitude greater energy storage per mass than batteries. For example, the energy density of butane, including storage cylinder mass, is 50 times that of the best high-output batteries (e.g., nonrechargable LiMnO
2
batteries). Hydrocarbon fuels are cheap and readily available and may present fewer environment concerns than batteries. Thus, a tiny fuel tank could replace several bulky batteries in hand-held microanalytical systems and could supply a microcombustor for efficient heating of essential components in a microsystem.
To sustain combustion in a microcombustor, the reactants must remain in the combustion chamber long enough to react and the temperature must not exceed the structural limits of the microcombustor materials. Reaction and residence times are effected by the choice of fuel, the fuel-to-air ratio, the size and geometry of the combustion chamber, and the gas-flow rate through the microcombustor. The scalability of combustion systems can be limited due to the increased surface-to-volume ratio at small combustor dimensions. In particular, thermal quenching due to heat losses to the walls and chemical quenching of reactive free radicals at surfaces become problematic as the dimensions of the combustor decrease, thereby limiting propagation of the combustion flame.
Prior art microcombustors having millimetric dimensions have been developed for power generation for microsystem devices. Cohen et al. in U.S. patent application Ser. No. 2001/0029974, discloses a microcombustor that relies on a toroidal counterflow heat exchanger to reduce heat loss from the combustor and to preheat the reactant gases. This microcombustor uses an external heater or an igniter internal to the heat exchanger to ignite combustion and is further configured with a thermoelectric material to generate electrical current. Masel et al., in U.S. Pat. No. 6,193,501, discloses a microcombustor having a combustion chamber that uses catalysts to get the reactants hot, ignited, and burning. Thermal barriers and an isolation cavity are used to minimize heat loss from a serpentine combustion chamber. Neither of these microcombustors use a microhotplate to minimize heat loss from the combustion chamber.
Microhotplates have been developed for micro-chemical reactors for partial oxidation synthesis and hydrogen reforming and for gas sensing. However, such microhotplates have typically been used to promote or sense reactions at the surface of the microhotplate and not to generate self-propagating combustion flames. See R. Srinivasan et al., “Micromachined chemical reactors for surface catalyzed oxidation reactions,”
Tech. Digest
1996
Sol
.-
State Sensor and Actuator Workshop
, pp. 15-18 (1996); L. R. Arana et al., “A microfabricated suspended-tube chemical reactor for fuel processing,”
MEMS
2002, pp. 232-235 (2002); M. Gall, “The Si-planar-peilistor array, a detection unit for combustible gases,”
Sensors and Actuators B
16. 260 (1993); R. P. Manginell et al., “Selective, pulsed CVD of platinum on microfilament gas sensors,”
Tech. Digest
1996
Sol
-
State Sensor and Actuator Workshop
, pp. 23-27 (1996); R. E. Cavicchi et al., “Microhotplate gas sensor,”
Tech. Digest
1994
Sol
.-
State Sensor and Actuator Workshop
, pp. 53-56 (1994); and M. Zanni et al., “Fabrication and properties of a Si-based high sensitivity microcalorimetric gas sensor,”
Tech. Digest
1994
Sol
.-
State Sensor and Actuator Workshop
, pp. 176-179 (1994).
Finally, microFID systems created by other groups have used micromachined nozzles to anchor an oxyhydrogen diffusion flame, which is essentially a miniaturization of existing technology. Zimmerman et al., “Micro flame ionization detector and micro flame spectrometer,”
Sensors and Actuators B
63, 159 (2000) and Zimmerman et al., “Miniaturized flame ionization detector for gas chromatography,”
Sensors and Actuators B
83, 285 (2002) describe a miniaturized flame ionization detector that comprises a micro burner unit with a nozzle diameter of less than 100 &mgr;m to produce a stable miniature flame. Oxyhydrogen flow rates on the order of 35 ml/min were required for flame stabilization in this design.
There remains a need for an integrated, flexible, and efficient microcombustor that can be used for power generation, heating and thermal management of on-chip Microsystems, and for other sensor applications. Unlike the prior art, the present invention satisfies this need by providing a microcombustor comprising a microhotplate with a very low heat capacity and thermal conductivity to minimize heat loss from the combustion chamber and a surface catalyst for flame ignition and stabilization.
SUMMARY OF THE INVENTION
The microcombustor of the present invention combines a microhotplate and catalyst materials for sustained combustion on the microscale. The microhotplate comprises a thin-film heater/thermal sensor patterned on a thin insulating support membrane that is suspended from its edges over a substrate frame. This microhotplate has very low heat capacity and thermal conductivity and is an ideal platform for heating catalytic materials placed on the surface of the support membrane. The free-standing platform used in the microcombustor mitigates large heat losses arising from large surface-to-volume ratios typical of the microdomain, and, together with the heated catalyst, permits combustion on the microscale.
The heated catalyst enables flame stabilization, even in spaces with large surface/volume ratios; permits combustion with lean fuel/air mixtures; extends a hydrocarbon's limits of flammability; and lowers the combustion temperature. Surface oxidation, flame ignition, and flame stabilization have been achieved for hydrogen and hydrocarbon fuels premixed with air. Flame stabilization via catalytic surfaces permits stable combustion at hydrogen flows less than 5 ml/min and under lean conditions. In addition to providing for stable flames in the microdomain, the microcombustor expands the limit of flammability (LoF) for many hydrocarbon fuels, as compared with diffusion flames. For example, the LoF of the microcombustor for natural gas in air is 1-35%, as compared to the 4-16% typically observed. The LoF for hydrogen, methane, propane and ethane are likewise expanded. This expanded LoF has important consequences for microanalytical

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