demonstration of a thermoelectric MEMS generator that directly converts heat from catalytic combustion into electrical power without electrical input.
A COMBUSTION-BASED MEMS THERMOELECTRIC POWER GENERATOR Samuel B. Schaevitz*, Aleksander J. Franz**, Klavs F. Jensen**, and Martin A. Schmidt* Massachusetts Institute of Technology, Microsystems Technology Laboratories, *Department of Electrical Engineering and Computer Science **Department of Chemical Engineering 77 Massachusetts Ave., Cambridge, MA (a)
SUMMARY 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 onchip 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 500ºC, with thermopile voltages up to 7V and device thermal efficiencies up to 0.02%. Keywords: Thermoelectrics, Power Source, Combustion
(b)
TE Material B
TE Material A
Metalization
INTRODUCTION The increasing use of portable electronics has driven research in the area of portable electrical generators. Chemical fuels, e.g. hydrocarbons, when reacted with air release 10-100 times more energy per unit mass than is released from state of the art batteries. If a portion of that chemical energy could be converted to electrical energy in a compact generator, the resulting system would surpass batteries for many applications. Different schemes have been proposed for the conversion, including thermoelectrics, thermo photovoltaics, and fuel processing with fuel cells. Here we present the first demonstration of a thermoelectric MEMS generator that directly converts heat from catalytic combustion into electrical power without electrical input. Our TE generator takes advantage of several previous MEMS developments. Catalytic combustion on a membrane has been demonstrated in our group [1], and thermoelectric elements have been integrated with a number of MEMS comp onents [2, 3]. Our device consists of a channel etched through a silicon wafer and capped by a thin membrane. An aluminum plate seals the channel. A thermopile spans from the center of the
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Catalyst
Gas Flow Figure 1: (a) Micrograph of Device (b) Diagram of Device membrane (hot junctions) to the edge of the silicon (cold junctions). A catalyst, usually platinum, is deposited on the channel side of the membrane and is aligned with the hot end of the thermopile. An image and diagram of our device are shown if Figure 1. In operation, a fuel-air or fuel-oxygen mixture flows through the channel. The fuel and oxygen diffuse to the membrane where they react on the catalyst and release heat. The selective deposition of the catalyst confines the reaction to the thermopile region, and creates a temperature gradient between the thermopile junctions in the center of the membrane and the silicon channel walls.
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Heat flows to the silicon through the thermopile and through parasitic conduction of the membrane. The reaction products, usually water and carbon dioxide, diffuse back into the gas stream and exit at the end of the channel. As a net result, the fuel has been combusted and part of the heat released has been converted into electrical energy.
THERMOELECTRIC GENERATION Thermoelectric (TE) generators consist essentially of three parts: a heat source, a heat sink and a thermopile. The heat source and heat sink provide the energy to the system by creating a temperature gradient across the thermopile. The thermopile serves to convert some of the thermal energy contained in the thermal reservoirs into electrical energy. The advantage of TE generators over other heat engines resides in their simplicity. This simplicity and lack of moving parts make TE generators a good target for miniaturization. A simplified picture of an ideal thermocouple is illustrated in Figure 2. When current flows through the circuit, heat is absorbed at the hot junctions and converted to electrical energy at a rate proportional to current times the Seebeck coefficient (α) for the material. Heat is generated within the thermoelement by Joule heating, which is proportional to the dimensions of the structure. In parallel, heat is conducted down the length of the thermoelement at a rate inversely proportional to the dimensions of the structure. At the cold junction some of the electrical energy is converted back to heat,
Heat Conduction
Material A (α < 0)
Heat Absorbed: Peltier Effect ~ I Material B (α > 0)
Resistive Heat Generation Joule Heating ~ I 2
Heat Source (T = THot)
Heat Released: Peltier Effect ~ I
Current
_
+ Heat Sink (T = T Cold )
also due to the Peltier effect. Two different materials with opposing Seebeck coefficients are needed to generate power from the circuit. A single pair of these thermoelements (as shown in Figure 2) forms a thermocouple, with a net voltage at the terminals typically on the order of 300-500 µVolts per degree Kelvin for a highly efficient material system. In order to increase the terminal voltage to a useful range, thermocouples are often connected in series to form a thermopile, with a terminal voltage that is multiplied by the number of thermocouples. When the parasitic conduction of the support structure and resistance of the contacts and leads are included, the maximum efficiency is obtained when the effective load resistance (R) and efficiency (η) are, respectively [4]:
RMax η = rD ⋅ 1 + Z D ⋅ T Ave η
Max
=
(TH Ave −TC Ave )⋅ ( 1 + Z D ⋅TAve −1) TC Ave T H Ave ⋅ 1 + Z D ⋅ T Ave + TH Ave
(2)
Where Z D is the “Device Figure of Merit” :
α2 Eff ZD = rD ⋅ K Eff
(3)
rD is the total resistance of the device. THAve and TCAve are the average temperatures at the hot and cold junctions, respectively, of the thermoelements; TAve is their average. KEff is the effective conductivity of the structure, including thermopile and supports. αEff is the effective Seebeck coefficient; this is approximately the total Seebeck coefficient of one thermocouple times the number of thermocouples in the device. (Similar formulations without consideration of parasitic losses can be found in [5, 6].) Equations 2 and 3 can be used to optimize the device by optimizing η, usually through ZD. For example, the efficiency is strongly dependent on colocating the thermal and electrical ends of the thermopile. Any missalignment will increase rD, increase KEff, or decrease the temperature difference at the junctions, all resulting in lower efficiencies.
Electrical Load Figure 2: Thermoelectric Generation
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(1)
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The 11th International Conference on Solid-State Sensors and Actuators, Munich, Germany, June 10 – 14, 2001
Low-Stress SiN
much lower efficiencies, however higher levels should be achievable with improved processing.
EXPERIMENTAL RESULTS AND DISCUSSION
n-type p-type SiGe Layers TiN / Pt Metalization
KOH Etched Channel Catalyst
Shadowmask Figure 3: Device Fabrication
To assess the efficiency of the membrane/thermopile structure, we used an integrated electrical heater to supply a precise quantity of heat. The voltage and shortcircuit current of the thermopile was measured, and a linear resistance was assumed in order to calculate the maximum efficiency. Figure 4a shows the efficiency and power output of the thermopile vs. temperature as the membrane is heated. For this demonstration device, the thermopile is much thinner than the membrane, and the efficiency is dramatically reduced by conduction through the membrane support. To access the efficiency of the thermopile itself, the power needed to heat a control device with no thermopile was subtracted, and Figure 4b shows the measured efficiency of the thermopile alone. These curves only extend to approximately 500ºC because of contact degradation above that temperature. The use of a TiN barrier layer improves the maximum
The thermoelectric materials chosen for this device are polycrystalline silicon-germanium alloys with ~20 atomic % germanium, with one material doped with phosphorus and the other material doped with boron. Previous work with these materials has shown efficiencies of up to 8% for doping levels above 1020 cm-3 [7]. Our preliminary depositions have resulted in lower doping levels of approximately 4x1019 for the boron alloy and 5x1017 and for the phosphorus alloy. These lighter dopings result in
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75
(a)
60
0.015%
45 0.010%
30 0.005%
15
0.000%
0 0
0.600% TE Thermal Efficiency
TE Material
0.020%
Power Out (µW)
The fabrication sequence for the device is shown schematically in Figure 3. We begin with a 100 mm diameter silicon wafer coated with low-stress silicon nitride (SiN). The backside of the wafer is patterned for later use as a KOH etch mask. Two layers of silicongermanium (SiGe) are sequentially deposited and patterned into thermoelements on the front surface (SiGe reference). We use LPCVD silicon dioxide as a mask and an etchant of 1:50:50 49%HF:70%HNO3:H2O. Electrical contact is made using a PVD TiN barrier layer and ebeam deposited Pt metalization. The channels are etched into the wafer using KOH in a single-sided etch setup. The catalyst is e-beam deposed through a specially microfabricated, self-aligned shadowmask.
Total Thermal Efficiency
FABRICATION
100
200 300 400 Temperature (ºC)
500
100
200 300 400 Temperature (ºC)
500
(b)
0.500% 0.400% 0.300% 0.200% 0.100% 0.000% 0
Figure 4: (a) Efficiency and Power Out of Device (b) Measured Efficiency of TE Alone
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TE Voltage Generated (V)
3
ACKNOWLEDGEMENTS
2.5
Autothermal Operation
2 1.5
Ignition
1
The authors would like to thank Prof. Fitzgerald’s group for supplying the SiGe, MTL staff for their assistance in device processing, DARPA for their support of the project under contract number F30602-99-2-0544, and the Fannie and John Hertz Foundation for their fellowship support of the first author.
0.5
REFERENCES
0 0
20
40 60 80 100 120 Start- up Heater Power (mW) Figure 5: Autothermal Power Generation device operation from approximately 300ºC to 500ºC, but the interdiffusion of the metal and SiGe becomes very rapid above 500ºC. We expect that improved barriers like TaN, TaSiN or TiSiN will extend this range up to 700900ºC [8, 9]. Conversion of chemical to electrical energy is shown in Figure 5. The fuel for this test is H2 combined with air. Shown is the TE voltage vs. heater power as the device is heated to ignition and the heater is subsequently turned off. The remaining voltage, and resulting power output, is due to autothermal combustion of the fuel. The only external electrical power source is used to drive the mass-flow controllers. Similar results have been obtained while burning ammonia and butane with pure oxygen. Pure oxygen was used in the butane and ammonia tests because we were not able to achieve autothermal operation (zero heater power) using air. The thermal losses of the device were larger than the heat generated by combustion of butane and ammonia in air. In order to produce power from a butane/air mixture, the membrane geometry must be redesigned. Autothermal combustion with air has been previously demonstrated by our group in similar thin-wall reactors [10].
CONCLUSIONS A thermoelectric generator with integrated catalytic combustion has been microfabricated and has demonstrated positive net power output from on-chip autothermal combustion of hydrogen, ammonia and butane. The device was stably operated at temperatures up to 500ºC, with thermopile voltages up to 7V, and thermal efficiencies up to 0.02%.
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[1]
R. Srinivasan, I.-M. Hsing, and P. E. Berger, "Micromachined Reactors for Catalytic Partial Oxidation Reactions," AICHE Journal, vol. 43 pp. 3059-3069, 1997. [2] E. Yoon and K. D. Wise, "A Wideband Monolithic RMS-DC Converter Using Micromachined Diaphragm Structures," IEEE Transactions on Electron Devices, vol. 41, no. 9, pp. 1666-1668, 1994. [3] D. D. L. Wijingaards, S. H. Kong, M. Bartek, and R. F. Wolffenbuttel, "Design and Fabrication of OnChip Integrated PolySiGe and PolySi Peltier Devices," Sensors and Actuators, vol. 85 pp. 316323, 2000. [4] S. B. Schaevitz, A MEMS Thermoelectric Generator, M.Eng. in Electrical Engineering and Computer Science, Massachusetts Institute of Technology, Cambridge, MA: 2000. [5] A. F. Ioffe, Semiconductor Thermoelements and Thermoelectric Cooling. London: Infosearch Ltd., 1956. [6] J. M. Borrego, H. A. Lynden, and J. Blair, The Efficiency of Thermoelectric Generators. Cambridge, MA: Massachusetts Institute of Technology Industrial Liaison Office, 1958. [7] C. B. Vining, "Silicon Germanium," in [McNaughton, 1994 #5], D. M. Rowe, Ed. New York: CRC Press, 1994, pp. 329-337. [8] J. S. Reid, E. Kolawa, C. M. Garland, and M.-A. Nicolet, "Amorphous (Mo, Ta, or W)-Si-N Diffusion Barriers for Al Metallizations," Journal of Applied Physics, vol. 79, no. 2, pp. 1109-1115, 1996. [9] M. E. Thomas, M. P. Hartnet, J. E. McKay, A. K. Kapoor, and J. D. Chinn, "The Potential of Using Refractory Metals and Barrier Layers to Generate High Temperature Interconnects," VLSI Multilevel Interconnection Conference Proceedings, Fifth International IEEE, 1988. [10] K. F. Jensen, “Microchemical Systems: Status, Challenges, and Opportunities,” AICHE Journal, Vol. 45, no. 10, pp.2051-3, 1999.
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