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Design, Fabrication, and Characterization of a Micromachined Heat Exchanger Platform for Thermoelectric Power Generation

Permanent Link: http://ufdc.ufl.edu/UFE0022709/00001

Material Information

Title: Design, Fabrication, and Characterization of a Micromachined Heat Exchanger Platform for Thermoelectric Power Generation
Physical Description: 1 online resource (89 p.)
Language: english
Creator: Masilamani, Sivaraman
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2008

Subjects

Subjects / Keywords: exchanger, generation, heat, micro, multichip, power, stacking, thermoelectric, waste
Electrical and Computer Engineering -- Dissertations, Academic -- UF
Genre: Electrical and Computer Engineering thesis, M.S.
bibliography   ( marcgt )
theses   ( marcgt )
government publication (state, provincial, terriorial, dependent)   ( marcgt )
born-digital   ( sobekcm )
Electronic Thesis or Dissertation

Notes

Abstract: The ever-continuing trend in miniaturization and ever-growing power density requirement of portable electronics has necessitated the search for alternate sources of compact power with high energy content. To this end, new technologies such as microscale heat engines, micro fuel cells, micro-thermo photovoltaic and micro-thermoelectric generation are being developed as possible alternatives to traditional battery technologies. Among these, thermoelectric generators boast many key advantages such as robustness, reliability and long-life. Thermoelectrics devices enable the direct conversion of heat energy into electrical energy. Recent advancements in thermoelectric materials and increased demand for power in the microwatt to milliwatt range have fueled wider research on microscale thermoelectric generators. Traditionally, when scaled down, these generators suffer from fairly large thermal leakage; it is difficult to maintain a large temperature differential because of the small physical dimensions. This thesis presents the design, fabrication and characterization of a heat exchanger platform for thermoelectric power generation using waste heat from a small combustion engine while attempting to mitigate the thermal leakage problem. After evaluating several different heat exchanger device structures based on the desired performance parameters, a stacked radial in-plane structure was chosen. The design consists of several heat exchanger modules stacked vertically to form a tubular structure. Each module has two concentric silicon rings connected only by a 5 ?m thin supporting membrane to achieve low thermal leakage. This design offers an ideal compromise between thermal leakage, mechanical robustness and fabrication complexity. An analytical model of the device was built to predict the device performance, and a process flow was developed to fabricate the proposed radial in-plane heat exchanger. With a simple two mask process, devices were fabricated using planar microfabrication and bulk-micromachining techniques. Heat exchanger devices were then characterized in the laboratory under varied test conditions. The experimentally obtained data was found to agree well with analytically predicted performance. In conclusion, the contributions of this research work are twofold. First, it proves the feasibility of a mechanically robust radial in-plane heat exchanger structure for thermoelectric power generation from hot exhaust gasses. Second, it demonstrates the possibility of achieving low thermal leakage in an appropriately designed, ~1 cubic centimeter device.
General Note: In the series University of Florida Digital Collections.
General Note: Includes vita.
Bibliography: Includes bibliographical references.
Source of Description: Description based on online resource; title from PDF title page.
Source of Description: This bibliographic record is available under the Creative Commons CC0 public domain dedication. The University of Florida Libraries, as creator of this bibliographic record, has waived all rights to it worldwide under copyright law, including all related and neighboring rights, to the extent allowed by law.
Statement of Responsibility: by Sivaraman Masilamani.
Thesis: Thesis (M.S.)--University of Florida, 2008.
Local: Adviser: Arnold, David.

Record Information

Source Institution: UFRGP
Rights Management: Applicable rights reserved.
Classification: lcc - LD1780 2008
System ID: UFE0022709:00001

Permanent Link: http://ufdc.ufl.edu/UFE0022709/00001

Material Information

Title: Design, Fabrication, and Characterization of a Micromachined Heat Exchanger Platform for Thermoelectric Power Generation
Physical Description: 1 online resource (89 p.)
Language: english
Creator: Masilamani, Sivaraman
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2008

Subjects

Subjects / Keywords: exchanger, generation, heat, micro, multichip, power, stacking, thermoelectric, waste
Electrical and Computer Engineering -- Dissertations, Academic -- UF
Genre: Electrical and Computer Engineering thesis, M.S.
bibliography   ( marcgt )
theses   ( marcgt )
government publication (state, provincial, terriorial, dependent)   ( marcgt )
born-digital   ( sobekcm )
Electronic Thesis or Dissertation

Notes

Abstract: The ever-continuing trend in miniaturization and ever-growing power density requirement of portable electronics has necessitated the search for alternate sources of compact power with high energy content. To this end, new technologies such as microscale heat engines, micro fuel cells, micro-thermo photovoltaic and micro-thermoelectric generation are being developed as possible alternatives to traditional battery technologies. Among these, thermoelectric generators boast many key advantages such as robustness, reliability and long-life. Thermoelectrics devices enable the direct conversion of heat energy into electrical energy. Recent advancements in thermoelectric materials and increased demand for power in the microwatt to milliwatt range have fueled wider research on microscale thermoelectric generators. Traditionally, when scaled down, these generators suffer from fairly large thermal leakage; it is difficult to maintain a large temperature differential because of the small physical dimensions. This thesis presents the design, fabrication and characterization of a heat exchanger platform for thermoelectric power generation using waste heat from a small combustion engine while attempting to mitigate the thermal leakage problem. After evaluating several different heat exchanger device structures based on the desired performance parameters, a stacked radial in-plane structure was chosen. The design consists of several heat exchanger modules stacked vertically to form a tubular structure. Each module has two concentric silicon rings connected only by a 5 ?m thin supporting membrane to achieve low thermal leakage. This design offers an ideal compromise between thermal leakage, mechanical robustness and fabrication complexity. An analytical model of the device was built to predict the device performance, and a process flow was developed to fabricate the proposed radial in-plane heat exchanger. With a simple two mask process, devices were fabricated using planar microfabrication and bulk-micromachining techniques. Heat exchanger devices were then characterized in the laboratory under varied test conditions. The experimentally obtained data was found to agree well with analytically predicted performance. In conclusion, the contributions of this research work are twofold. First, it proves the feasibility of a mechanically robust radial in-plane heat exchanger structure for thermoelectric power generation from hot exhaust gasses. Second, it demonstrates the possibility of achieving low thermal leakage in an appropriately designed, ~1 cubic centimeter device.
General Note: In the series University of Florida Digital Collections.
General Note: Includes vita.
Bibliography: Includes bibliographical references.
Source of Description: Description based on online resource; title from PDF title page.
Source of Description: This bibliographic record is available under the Creative Commons CC0 public domain dedication. The University of Florida Libraries, as creator of this bibliographic record, has waived all rights to it worldwide under copyright law, including all related and neighboring rights, to the extent allowed by law.
Statement of Responsibility: by Sivaraman Masilamani.
Thesis: Thesis (M.S.)--University of Florida, 2008.
Local: Adviser: Arnold, David.

Record Information

Source Institution: UFRGP
Rights Management: Applicable rights reserved.
Classification: lcc - LD1780 2008
System ID: UFE0022709:00001


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DESIGN, FABRICATION, AND CHARACTERI ZATION OF A MICROMACHINED HEAT EXCHANGER PLATFORM FOR THERMO ELECTRIC POWER GENERATION By SIVARAMAN MASILAMANI A THESIS PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLOR IDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE UNIVERSITY OF FLORIDA 2008 1

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2008 Sivaraman Masilamani 2

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To my mother, Chitra Masilamani, and to the memory of my father, Masilamani Sambandhamoorthy 3

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ACKNOWLEDGMENTS I would like to express my deepest thanks a nd gratitude to my advisor, Dr. David Arnold for the invaluable opportunities, career and personal advice, a nd for his compassion during the most difficult times. Without his endless pa tience, persistent motivation and continued appreciation for hard work, this research work would have not been possible. I also extend my sincere thanks to Dr. Toshikazu Nishida a nd Dr. David Hahn for serving on my thesis committee. I am also grateful to Army Resear ch Laboratories for the financial support they provided for this research. I would like to cordially tha nk my project colleagues Israel Boniche, Christopher Meyer, and Eric Viale for their insight and assistance duri ng the course of this re search. Special thanks go to Ryan Durscher for his help with device modeling and for the many weekends he assisted me in the laboratory. Several other IMG-ers ha ve been of genuine help to me during my graduate study. In particular, I would like to thank Janhavi Agashe, for her cheerful encouragement, academic and personal advice, Sheetal Shetye, for her friendly guidance and cleanroom help, Erin Patrick, for help with th e microdispenser, Mingliang Wang and Yawei Li for their valuable microfabrication insights. I also acknowledge Al Ogden for his help with cleanroom equipment. I also owe special thanks to my undergraduate friends that lived with me in Bangalore, for their encouragement and support. Finally, I would like to thank my parents, Masilamani Sambandhamoorthy and Chitra Masilamani, for their uncountable love and suppor t all my life. The great human values, work ethics, and perseverance that they inculcated in me are the reasons behind everything that I achieve. I also thank my brother, Raghuraman Ma silamani, for his care and affection. Last but not least, I thank my fiance, Gayathri Devi Sridharan, for giving me the strength to face challenges; and for her fondness and love. 4

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TABLE OF CONTENTS page ACKNOWLEDGMENTS...............................................................................................................4 LIST OF TABLES................................................................................................................. ..........7 LIST OF FIGURES.........................................................................................................................8 ABSTRACT...................................................................................................................................10 CHAPTER 1 INTRODUCTION AND BACKGROUND...........................................................................12 1.1 Heat Exchangers............................................................................................................ 13 1.2 MEMS Heat Exchangers and Applications..................................................................15 1.3 Thermoelectric Power Generation................................................................................18 1.3.1 Seebeck Effect...................................................................................................19 1.3.2 Peltier Effect.....................................................................................................19 1.3.3 Thomson Effect.................................................................................................20 1.3.4 Thermoelectric Generator.................................................................................20 1.4 Microscale TE Generators.............................................................................................22 1.5 Research Goals............................................................................................................. .25 1.6 Thesis Outline............................................................................................................. ..25 2 HEAT EXCHANGER DEVICE DESIGN AND MODELING.............................................27 2.1 Thermopile Material and Arrangement.........................................................................28 2.2 Structure of Thermoelectric Microgenerator................................................................29 2.2.1 Simple in-Plane TEG Structure.........................................................................34 2.2.2 Out-of-Plane Flip-Chip Bonded Structure........................................................35 2.2.3 Vertically Stacked Thermopile Structure..........................................................37 2.2.4 Vertically Stacked Radial In-plane Structure....................................................38 2.3 Device Design...............................................................................................................40 2.3.1 Fin Geometry Optimization..............................................................................42 2.3.2 Exhaust Gas Channel Design............................................................................45 2.3.3 Ring Thickness and Space between Rings........................................................46 2.4 Thermal Modeling.........................................................................................................48 2.5 Final Dimensions of the Radial In-plane TE Modules..................................................52 2.6 Summary.................................................................................................................... ...52 3 FABRICATION AND STACKING OF THE HEAT EXCHANGER MODULES..............53 3.1 Through-Etching of Wafers..........................................................................................53 3.2 Membrane Strength Evaluation.....................................................................................54 3.3 Process Flow Description..............................................................................................55 5

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3.4 Mask Making................................................................................................................ 56 3.5 Stacking and Bonding of Heat Exchanger Unit Modules.............................................58 3.6 Final Device Photographs.............................................................................................61 3.7 Summary.................................................................................................................... ...61 4 CHARACTERIZATION OF TH E HEAT EXCHANGER DEVICE....................................62 4.1 Experimental Setup and Procedure...............................................................................62 4.1.1 Flow Measurements..........................................................................................64 4.1.2 Temperature Measurements..............................................................................67 4.2 Test Matrix and Ac tual Test Results.............................................................................69 4.3 Comparison with Predicted Results..............................................................................75 4.4 Limitations of the Experimental Setup.........................................................................77 4.5 Summary.................................................................................................................... ...78 5 CONCLUSIONS AND FUTURE WORK.............................................................................79 5.1 Conclusions................................................................................................................ ...79 5.2 Future Work................................................................................................................ ..80 5.2.1 Eutectic Bonding of TE Modules......................................................................81 5.2.2 Integrated Temperature Measurement..............................................................82 LIST OF REFERENCES...............................................................................................................85 BIOGRAPHICAL SKETCH.........................................................................................................89 6

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LIST OF TABLES Table page 1-1 Heat exchanger classification............................................................................................14 3-1 Heat exchanger process flow.............................................................................................56 4-1 Flow velocity measuremen t procedure using the CTA......................................................66 4-2 Heat exchanger characterization matrix.............................................................................70 4-3 Temperatures measured during heat exchanger characterization......................................70 7

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LIST OF FIGURES Figure page 1-1 Common example of a heat exchanger..............................................................................13 1-2 Extended surface heat exchanger used in electronic cooling.............................................14 1-3 Different fin configurations...............................................................................................15 1-4 Compact heat sink implemen ted by Tuckerman and Pease...............................................16 1-5 Improved MEMS electronic cooling methods...................................................................16 1-6 Micro heat exchangers for heat engines.............................................................................17 1-7 Thermoelectric effect.........................................................................................................19 1-8 Basic thermoelectric generator..........................................................................................21 1-9 TE generator with inte grated heat exchangers...................................................................22 1-10 Examples of micro TEGs...................................................................................................2 4 1-11 Concept of thermoelectric cooling.....................................................................................24 2-1 Thin-film TEG.............................................................................................................. .....28 2-2 Planar TEG structure..........................................................................................................30 2-3 In-plane TEG............................................................................................................... ......35 2-4 Out-of-plane TEG with top Si plate removed....................................................................36 2-5 Thermopile formation in out-of-plane TEG......................................................................37 2-6 Vertically stacked thermopile structure.............................................................................38 2-7 Stacked radial in-plane structure........................................................................................39 2-8 First-order equivalent thermal circ uit of a radial in-plane TE module..............................40 2-9 Dimensions of a single radial thermocouple leg................................................................41 2-10 Fin geometry optimization................................................................................................. 44 2-11 Detailed thermal model of radial in-plane strucuture........................................................48 2-12 Final device dimensions................................................................................................... ..52 8

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3-1 Heat exchanger module to be fabricated............................................................................54 3-2 Device cross-sections du ring various process steps...........................................................57 3-3 Mask patterns.............................................................................................................. .......58 3-4 Assembly jig............................................................................................................... .......59 3-5 Epoxies for bonding heat exchanger modules...................................................................60 3-6 Epoxy application methods................................................................................................60 3-7 Heat exchanger stack built with square shaped modules...................................................61 3-8 Heat exchanger built with circular modules......................................................................61 4-1 Experimental setup to test the heat exchanger device.......................................................62 4-2 Heat exchanger device bonde d with the fluidic coupler....................................................63 4-3 System schematic of a cons tant temperature anemometer................................................64 4-4 Temperature m easurement tools........................................................................................67 4-5 Device temperature measurement points...........................................................................68 4-6 Variation of inner and outer ring temperat ures with increasing hot-air temperature........71 4-7 Variation in inner silicon ring temperature with hot-air velocity......................................74 4-8 Comparison of experimental results with results predic ted by analytical model..............76 5-1 Eutectic bonding of TE modules........................................................................................81 5-2 Top view of the TE module with integrated RTDs............................................................83 9

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Abstract of Thesis Presen ted to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Master of Science DESIGN, FABRICATION, AND CHARACTERI ZATION OF A MICROMACHINED HEAT EXCHANGER PLATFORM FOR THERMO ELECTRIC POWER GENERATION By Sivaraman Masilamani August 2008 Chair: David Arnold Major: Electrical and Computer Engineering The ever-continuing trend in miniaturization and ever-growing power density requirement of portable electronics has necessitated the search for alternate sources of compact power with high energy content. To this e nd, new technologies such as micros cale heat engines, micro fuel cells, micro-thermo photovoltaic and micro-ther moelectric generation are being developed as possible alternatives to traditional battery technologies. Amo ng these, thermoelectric generators boast many key advantages such as robustness, re liability and long-life. Thermoelectrics devices enable the direct conversion of heat energy into electrical energy. Recent advancements in thermoelectric materi als and increased demand for power in the microwatt to milliwatt range have fueled wider research on microscale thermoelectric generators. Traditionally, when scaled down, th ese generators suffer from fairly large thermal leakage; it is difficult to maintain a large temperature differen tial because of the small physical dimensions. This thesis presents the design, fabrication and ch aracterization of a heat exchanger platform for thermoelectric power generation using waste he at from a small combustion engine while attempting to mitigate the thermal leakage problem. After evaluating several different heat excha nger device structures based on the desired performance parameters, a stacked radial in-plane structure was chosen. The design consists of 10

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11 several heat exchanger modules stacked vertically to form a tubul ar structure. Each module has two concentric silicon rings connected only by a 5 m thin supporting membrane to achieve low thermal leakage. This design offers an ideal compromise between thermal leakage, mechanical robustness and fabrication complexity. An analytical model of the device was built to predict the device performance, and a process flow was developed to fabricate the propos ed radial in-plane heat exchanger. With a simple two mask process, devices were fabr icated using planar mi crofabrication and bulkmicromachining techniques. Heat exchanger devi ces were then characterized in the laboratory under varied test conditions. The experimenta lly obtained data was found to agree well with analytically predicte d performance. In conclusion, the contributions of this res earch work are twofold. First, it proves the feasibility of a mechanically r obust radial in-plane heat excha nger structure for thermoelectric power generation from hot exhaust gasses. Seco nd, it demonstrates the possibility of achieving low thermal leakage in an appropriately designed, ~1 cm3 device.

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CHAPTER 1 INTRODUCTION AND BACKGROUND The recent boom in the wireless communica tions industry and the continuing trend in miniaturization of portable electronics have pl aced a large and expanding demand for portable power sources. Batteries, in spite of their la test advancements, fall short to meet the rising energy densities asked of them. Attention has, therefor e, turned to other sources of high energy content such as hydrocarbon and alcohol fuels. This paradigm shift in the portable power industry has provided the impetus to the development of many new technologies. Some of the common approaches adopted to the exploit the high specific energy of hydrocarbon fuels are miniaturization of existing large scale systems such as gas turbines and internal combustion engines, direct energy conve rsion schemes such as thermo photovoltaic and thermoelectric generation, and fuel cells. Th e remarkable advancement in MEMS technology over the years has enabled pursuit of successf ul research in each of these areas. The focus of this research is thermoelectri c (TE) power generation. Bulk TE generators have been in usage for more than half-a-century in many niche appli cations, yet only recently thin thermoelectric materials with reasonable effi ciency, suitable for microscale integration were available [ 1]. Following this, a host of research initi atives were launched, aiming at different applications of microscale TE pow er generation either as sta nd alone compact power sources or as a part of a system such as microscale heat engine, to improve the overall efficiency. For a microscale TE generator to achieve best performance, a high temperature difference needs to be created across a ve ry short distance in the range of hundreds of micrometers. Depending on the application, heat exchangers can be integrated with th e generator to improve the temperature difference. The aim of this wo rk is to design and fabricate a heat exchanger 12

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platform that can extract heat energy from the exhaust gas of an intern al combustion engine, and develop the temperature differentia l required for a TE microgenerator. 1.1 Heat Exchangers A heat exchanger may be defined as a device that enables efficient h eat transfer between two fluids at different temperatures. The most common example of a heat exchanger is the automobile radiator (Figure 1-1 ) [ 2] in which heat from liquid cool ant is absorbed and dissipated into the cold air blowing through the radiator. The coolant in turn removes heat from the engine and keeps its temperature under control. Cold coolant outlet Cold air flowing through the radiator Inlet for hot coolant Figure 1-1. Common example of a heat exchanger: Auto mobile radiator. [Source: http://www.answers.com/topic/radiator ] Heat exchangers are ubiquitous in the refrigeration and air-conditioning industry, power plants, chemical plants, oil re fineries and the manufacturing i ndustry. They also find other applications such as waste heat recovery, space heating and electronic co oling. Several different heat exchangers of varied forms and structures are currently being used in the industry. The five main classification criteria for heat exchangers [ 3] and examples for each are listed in Table 1-1 13

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Of particular interest to us is the cons truction geometry known as extended surfaces, commonly referred to as heat fins Heat fins are attachments or extrusions on a primary heat transfer surface that serve to increase the overall surface area, resulting in higher heat transfer. Figure 1-2 is a picture of a finned surface [ 4] used to cool integrated circuit (IC) chips. Table 1-1. Heat Exchanger Classification. Classification criteria Types Basic operation Recuperators and regenerators Geometry of construction Tubes, plates and extended surfaces Flow arrangements Parallel-flo w, counter-flow and cross-flow Transfer processes Direct contact and indirect contact Heat transfer mechanism Single-phase and two-phase Primary surface Heat fins Figure 1-2. Extended surface heat exchanger used in electronic cooling. [Source: http://npowertek.trustpas s.alibaba.com/product/11645462/ NP_Skived_Fin_Heat_Sink .html ] Apart from their usage in gasto-gas and gas-to-liq uid heat exchangers, extended surfaces are also used to enhance heat transfer between a solid and a fluid for in stance in electric power transformers. In the strictest sense, such kind of device does not qualify as a heat exchanger as there is no fluid-to-fluid heat transfer. However, it is common practice in the literature [ 5], [ 6] to refer to these devices as heat exchange rs anyways. The example in Figure 1-2 a heat sink, is one such device which enhances heat transfer fr om an IC (solid) to th e ambient air (fluid). 14

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Figure 1.2 is one such example; sp ecifically it is called a heat sink Figure 1-3. Different fin configurations. A) Rectangular fin. B) Triangular fin. C) Annular fin. D) Pin fin [Adapted from F. P. Incropera and D. P. DeWitt, Fundamentals of Heat and Mass Transfer (Page 128, Figure 3-14). John Wiley & Sons Inc., 2002] Several different fin configur ations are possible (Figure 1-3 ) [ 7], each catering to a different need. Typically, the fin material has high thermal conductivity and fins are commonly used in arrays rather than as single fins. 1.2 MEMS Heat Exchangers and Applications A tremendous amount of research effort is devoted to the implementation of heat exchangers at the microscale for a wide range of applications. One of the oldest and widest application area for micro heat exchangers is mi croelectronic device coo ling. As early as in 1981, Tuckerman and Pease [ 5] demonstrated the possibility of integrating a liquid-cooled micro heat exchanger (Figure 1-4 ) within the Si substrate, thereby eliminating the need for an external heat sink. Consequently, the concept of microc hannel heat sinking was used for other similar applications such as cooling of laser diode arrays [ 8] and monochromator crystals [9]. Today, microelectronic cooling with integrated heat ex changers has evolved as a significant research topic in itself. The DARPA HERETIC (Heat Re moval by Thermo-Integrated Circuits) program was commissioned to investigate this topic specifically. 15

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Figure 1-4. Compact heat sink im plemented by Tuckerman and Pease. [Adapted from D. B. Tuckerman and R. F. W. Pease, Hig h-performance heat sinking for VLSI, IEEE Electron Device Lett. (Figure 1), vol. EDL-2, no. 5, pp. 126-129, 1981] A C B Figure 1-5. Improved MEMS electro nic cooling methods. A) Jet impingement cooling. B) Spray cooling. [Reprinted with permission from Michael J. Ellsworth, Jr. and Robert E. Simons, High Powered Chip Cooling Air and Beyond, ElectronicsCooling (Figures 6 & 5), Volume 11, Number 3, pp. 14 22, August 2005] C) Droplet cooling [Adapted from C. H. Amon, J. Murthy, S. C. Yao, S. Narumanchi, C. F. Wu, and C. C. Hsieh, MEMS-enabled thermal manageme nt of high-heat-flux devices EDIFICE: embedded droplet impingement for inte grated cooling of electronics, Experimental Thermal and Fluid Science (Figure 2), vol. 25, pp. 231-242, 2001] Many alternate and improved heat removal techniques have been and are being developed. MEMS impinging jet cooli ng, illustrated by Wu [ 10] makes use of the fact that the heat transfer 16

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coefficient of an impinging jet is an order of ma gnitude larger than conve ntional tangential fluid cooling. Research at Carnegie Mellon Univers ity focuses on the development of an embedded impingement cooling device using tiny droplets of dielectric coolants [ 11]. These micro-droplets are impinged from an array of micro-nozzles ach ieving a very high chip heat transfer rate. Another interesting technique is to spray the dielectric co olant liquid over the chip again using micro-nozzles [ 12] and allowing it to evaporate. Schematic views of the three heat removal techniques just descri bed are shown in Figure 1-5 B A Figure 1-6. Micro heat exchangers for heat e ngines. A) Layered view of heat exchanger developed by Sullivan [S. Sullivan, X. Zhang, A. A. Ayon, J. G. Brisson, Demonstration of a Microscale Heat Exch anger for a Silicon Micro Gas Turbine Engine, Transducers 01 IEEE Piscataway, NJ, (Figure 2a) pp. 1606, 2001], B) Swiss roll type combustor [L. Sitzki, K. Borer, E. Schuster, P.D. Ronney, and S. Wussow, Combustion in microscale heat-recirculating burners, in Proc. of the Third Asia-Pacific Conference on Combustion Seoul, Korea, 24 June 2001, pp. 473] An emerging application area where micro heat exchangers are increasingly used is micro heat engines. The purpose of heat exchangers in these devices is to absorb or deliver heat energy to a working fluid from external sources or to recuperate heat from exhaust gases. The microfabricated rankine cycle steam tu rbine demonstrated by Frechette [ 13] integrates two microchannel two-phase heat exchangers that func tion as an evaporator and condenser. As part 17

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of the MITs micro gas turbine project, a rec uperative micro heat exchanger was developed and demonstrated by Sullivan (Figure 1-6(A) ) [ 14]. In this design, heat is exchanged between the exhaust gases and the pre-combusted compressed ai r entering the engine in a radial counter-flow configuration. Using the same principle of recuperation, the Swiss-roll type combustor (Figure 1-6(B) ) was built in USC [ 15]. The device has multiple windings of the reactant and the exhaust gas channel in a spiral configur ation resulting in a very high heat transfer surface area [ 16]. Both 2D and 3D type exchangers were demonstrated. Yet another important and promising use of mi croscale heat exchangers is thermoelectric (TE) cooling and power generation. As TE power generation forms the focus of the thesis, various existing implementations of micro heat exchangers for TE generation are reviewed in a later section. 1.3 Thermoelectric Power Generation Thermoelectric power generation is the process of direct conversion of thermal energy in the form of a temperature gradient into elec tricity. It works base d on the principle of thermoelectric effect which can be described as follows: the junction between two dissimilar metals generates a voltage when the junction temper ature is higher than the ambient temperature. The principle is illustrated in Figure 1-7 Similarly when an electri c current is passed through the junction of dissimilar materials, it results in a temperature change at the junction. Although the above is a simplified statement of the thermoel ectric effect, the term t hermoelectric effect is actually a single term that represents the combination of three separately identified physical effects namely the Seebeck effect, Peltier effect and the Thomson effect These three effects [ 17] are described below: 18

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Figure 1-7. Thermoelectric effect 1.3.1 Seebeck Effect When junctions of two dissimilar conductors ar e maintained at diffe rent temperatures, a voltage is developed between them. The develo ped voltage is a function of the difference in temperature and the material properties of the conductors. VT (1-1) where, V is the voltage developed because of Seebeck effect (V), is the Seebeck coefficient (V/K), a constant dependent on material properties, and T is difference in temperature between the junctions (K). 1.3.2 Peltier Effect When electric current flows between two di ssimilar conductors held at a constant temperature, heat is either absorbed or released at the junction depending on the direction of the current flow. The amount of heat is again dependent on the materials and the magnitude of current flowing through the junction. QI (1-2) where, Q is heat flow at the juncti on due to Peltier effect (W), is the Peltier coefficient (W/A), a constant dependent on material properties, and I is the current flowing through the junction (A). 19

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1.3.3 Thomson Effect When electric current flows through a single material under a given temperature gradient, heat is either absorbed or rele ased by the material depending on the direction of current flow. The amount of heat is proportional to the magnitude of current, the material and the temperature gradient across the material. dQdT I dxdx (1-3) where, dQ dx is the heat flow pe r unit length due to Thomson effect (W/m), denotes Thomson coefficient (V/K), a constant de pendent on material properties, I is the current flowing through the material (A), and dT dx represents the rate of change of temperature with respect to length of the material (K/m). 1.3.4 Thermoelectric Generator A thermoelectric generator (TEG) is a device wh ich converts thermal energy into electrical energy based on the principle of the thermoelectric e ffect. In its simplest form, it has three main elements, namely the heat source, the heat sink and the thermopile. The heat source is at a higher temperature than the heat sink, and the temperature difference between them creates the required temperature gradient across the thermopile The thermopile is made up of large number of alternating thermoelectric materials connected el ectrically in series an d thermally in parallel; each pair of different TE materials is called a thermocouple. In the presence of a temperature gradient, an electric potential is generated betwee n the ends of the thermopile and current flows through any electrical load that is connected in seri es with the thermopile. 20

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Heat Source, ThotHeat Sink, TcoldThermoelectric Material A Thermoelectric Material B Metal Interconnects V+VA B Figure 1-8. Basic thermoelectric generator. A) Schematic view. B) Photograph [Adapted from Thin Film Peltier Cooler, MPC-D901 Datasheet, www.micropelt.com ] Figure 1-8 shows the schematic of a standard parallel plate TE device and a commercial TE device [ 18]. The heat source and heat sink plates are generally highly thermally conductive, and thus are usually assumed to each be at a unif orm temperature. In real world implementations of TEG, the source of heat could be radioac tive decay, hydrocarbon fuel combustion or even automobile exhaust. The heat sink is typically interfaced to ambient air, but in special cases, it could be a coolant such as wate r or helium. For the particular configuration shown, voltage is generated with polarities as indi cated; however, in reality the volta ge polarity is dependent on the material properties of the constituent thermoelectric materials. For this simple TEG, the generated open circuit voltage, is given by ocV 21

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ocVn T n, (1-4) where, is the number of thermocouples, is the Seebeck coefficient, constant for a given pair of TE materials, and is the difference between Thot and Tcold. T As is obvious, for a given material and numb er of thermocouples, the generated voltage, is directly proportional to the temperature difference, ocVT between the heat source and heat sink. In order to efficiently transfer the heat energy from the source to the hot side of the thermopile and to reject heat from the cold side of the thermopile to the sink, heat exchangers can be attached to the hot and cold plates. These heat exchangers thus serve to enhance T and thereby improve overall TEG efficiency. Figure 1-9 shows a TEG with integrated extended surface heat exchangers. Heat Exchangers Thermopile Hot Side Cold SideFigure 1-9. TE generator with in tegrated heat exchangers 1.4 Microscale TE Generators With the recent developments in MEMS t echnology and the advent of high performance thin film TE materials [ 1], microscale TE generators ar e becoming increasingly popular especially due to the growing need for portable power sources. TE generation stands to benefit at a microscale especially because of the increase in the surface area-to-volume ratio [ 19] a 22

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greater surface area means higher exposure to th e heat source and sink resulting in a higherT This advantage is countered, however, by the increasing thermal leakage between the hot and cold sides of the thermopile as they get clos er and closer in a microscale device. Again, by integrating heat exchangers with the TEG device, some efficiency improvement can be achieved. There have been several implementations of microscale TE generators aimed at generating electrical energy from a wide variety of heat sources. One example is the combustion-based TE power generator [ 20] demonstrated by Schaevitz at MIT (Figure 1-10(A) ). The device used silicon-germanium thermopiles on bulk micromachin ed Si substrate to generate power while the source of heat was catalytic combustion of hydr ocarbon fuels such as hydrogen, ammonia, and butane. Reportedly, the efficiency of the device was very low because of thermal leakage 0.01% for a of 400 C. The poly-Si based micro TEG developed by Strasser [ 21] at Infineon, on the other hand, used simple BiCMOS surface micromachining techniques to fabricate the device. The device (Figure 1-10(B) ) achieves an open circu it voltage of less than 200 mV/K, limited by the electrical resistance of the thermocouple legs. The Swiss-roll combustor based thermoelectric power generation system, microFIRE [ 16] developed by Cohen and others at USC is yet another TEG that uses combustion as its energy source. The salient feature of this implementation is that both the thermoelectric ge nerator and the heat exchanger are integrated in a single system there by enhancing the overall system efficiency. TAs can be seen from the examples cited, TE generators generally suffer from low conversion efficiencies at micros cale due to issues such as ther mal leakage and high electrical resistance. Although not always feasible, integration of heat exchangers within the TE device can result in a higher temperature gradient and hence an improved system efficiency. 23

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B A Figure 1-10. Examples of micro TEGs. A) Co mbustion based TEG developed by Schaevitz [S. B. Schaevitz, A. J. Franz, K. F. Jensen, and M. A. Schmidt, A combustion-based MEMS thermoelectric power generator, in Proc.th Int. Conf. on Solid-State Sens. and Act. (Transducers ), (Figure 1b), 2001], and B) polysilicon poly SiGe TEG developed by Strasser [M. Strasser, R. Aigner, M. Franosch, G. Wachutka, Miniaturized thermoelectric generators based on poly-Si and poly-SiGe surface micromachining, in Proc.th Int. Conf. on Solid-State Sens. and Act. (Transducers ), (Figure 6, Page 539), 2001] Figure 1-11. Concept of thermoelectric cooling. [D .-Y. Yao, C.-J. Kim, and G. Chen, Design of Thin-Film Thermoelectric Microcoolers, in ASME International Mechanical Engineering Congress & Exposition, (Figure 1, Page 2), Orlando, Florida, November 5-10, 2000] Microscale thermoelectric coolers (TECs) al so find extensive applications including cooling of microelectronics, charge coupled de vices (CCDs) and other MEMS devices such as resonators and infra-red sensors [ 22][ 24]. TECs are devices that operate in Peltier mode, in 24

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contrast to TEGs which operate in Seebeck mode. However, the design considerations and challenges for TECs and TEGs are similar; especially, thermal isolation between the ends of thermocouples is a must in both devices. Figure 1-11 illustrates the concept of thermoelectric cooling. 1.5 Research Goals The goal of this research work is to design, fa bricate and test a micr oscale heat exchanger platform that can extract waste heat energy fr om the exhaust of sma ll combustion engines and maintain a temperature gradient across a thermo pile. The thermopile, in turn, converts the temperature gradient into electric potential. This work is part of a larg er research effort to develop a thermoelectric microgenerator that could serve as a small and efficient portable soldier power system. Therefore, the heat exchanger de sign needs to meet the constraints placed by the thermopile design. By design, the thermopile is made of thin f ilm materials that require a suitable substrate such as silicon for deposition and processing. This implies that th e heat exchanger platform also should be made out of silicon for ease of fabricat ion. Another important criterion is to minimize the leakage between the hot and cold side of the thermopile while still keeping the device structurally stable. In addition to these, the heat exchanger should improve system performance by coupling the maximum possible heat ener gy from the exhaust to the thermopile. 1.6 Thesis Outline The thesis is organized in five chapters. This chapter has provided the necessary background on heat exchangers and thermoelectric power generation. It reviewed a few of the several existing implementations of microscale heat exchangers, TE generators, and their attributes, and also identified the goals of the research, the co nstraints and challenges involved. 25

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26 Chapter 2 elaborates on the various different device design requirements and describes how a particular device structure is chosen amongst others. It al so explains the how the device dimensions are arrived at based on a first-order thermal circuit model; later, a detailed thermal model of the device to be built is also devel oped. This is followed by a discussion of the theoretical model based performance predictions. Chapter 3 gives a detailed description about the fabrication of the heat exchanger device. It explains the choice of diffe rent materials and methods and also discusses about the bonding and stacking procedures. Chapter 4 details the experimental setup and proc edure and presents the results obtained through laboratory testin g of the device for different temp erature and flow conditions. The results are compared with the predicti ons of the theoretical MATLAB model. Chapter 5 summarizes the major results, draws co nclusions and gives suggestions for improvements and future work.

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CHAPTER 2 HEAT EXCHANGER DEVICE DESIGN AND MODELING This chapter focuses on the design and mode ling of the heat exchanger platform onto which the thermopile is to be incorporated. As described earlier, the h eat exchanger is designed to both absorb heat energy from engine exhaus ts and create a temperature gradient across a thermopile. The thermopile consists of multiple legs of dissimilar materi als connected thermally in parallel but electrically in series. By virtue of the thermoelectric prop erties of the materials, this thermopile produces electric power from th e temperature gradient established by the heat exchanger. In a thermoelectric generator (TEG ), the thermopile and the heat exchanger are integrated into the same device, thus forming a complete system that converts waste heat into useful power. The heat exchanger design for such a system should therefore be optimized for the best performance of the overall system, i.e. to generate maximum power ra ther than to create maximum temperature gradient. Also, because of the small physical dimensions at the microscale, thermal leakage from the hot region to the cold region of the heat exch anger becomes significant and presents a serious design issue. A high thermal leakage (resulting in low temperature differentials) causes the heat exchanger efficiency to fall to extremely low va lues. The problem of high thermal leakage in microscale TEGs and the resultant low thermal effi ciency is widely acknowledged in literature [ 20], [ 21]. To avoid this leakage, it becomes nece ssary to thermally isolate the hot and cold regions of the heat exchanger. However, by definition, the TEG has multiple thermocouple legs extending from the hot region to th e cold region, thereby offering a direct thermal path between them. The thermocouple legs are obviously necessary, and materials that offer high thermoelectric function with low thermal conductiv ity are the focus for material optimization. From the heat exchanger design standpoint, the goal then is to eliminate or minimize all thermal 27

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paths other than the thermoelements themselves. Once this is achieved, the overall thermoelectric generator can be designed to gene rate reasonable amount of electric power. The design of the structure and layout of the thermopile is beyond the sc ope of this thesis. However, since the design of heat exchanger and thermop ile are strongly interrela ted, assumptions and details on thermopile design will be described wherever necessary. 2.1 Thermopile Material and Arrangement For the design of this heat exchanger, the following assumptions are made with regard to the thermopile: First, the thermocouples are made of two different thin-film thermoelectric materials such as pand n-type thin film IV -VI semiconductor or alte rnatively two dissimilar metals. Typical examples of IV-VI semiconductors that have good thermoelectric properties are alloys such as Bi2Te3, BiSbTe, PbTe, and PbSeTe. It should also be mentioned here that these doped semiconductors offer better thermoelectric performance compared to typical metals. Second, it is assumed the thermopile requires a silicon substrate for deposition and processing [ 25]. A thin layer of thermally grown SiO2 electrically insulates the thermopiles from the conductive Si substrate; the oxide layer also serves as the buf fer layer required for vapor deposition of IV-VI semiconductor [ 26]. Heat Source, ThotHeat Sink, TcoldV+VElectrical Load, RloadCurrent Flow Heat Flow A Figure 2-1. Thin-film TEG. A) Out-of-plane configuration B) In-plane configuration 28

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B Figure 2-1. Continued. The thermocouples can either be arranged in an in-plane fashion where current and heat flow parallel to the substrate or out-of-plane wh ere current and heat flow perpendicular to the substrate. Typical implementations of the in-plane and out-of-plane configurations are shown in Figure 2-1 2.2 Structure of Thermoelectric Microgenerator A variety of requirements need to be satisfied while decidi ng the structure of the TEG. Some of the important requirements can be expl ained by referring to the simple in-plane TE generator (Figure 2-2(A) ). The TEG is assumed to consist of multiple thermocouple legs on top of a silicon substrate; temperature gradient is cr eated by the flow of hot and cold fluids through channels at both ends of th e device. A thermally conductiv e but electrica lly insulating supporting membrane separates the thermocouples from the substrate; there is also a thin sheet of silicon underneath the membrane to provide ad ditional support to the thermocouples. The first-order model (Figure 2-2(B) ) consists of an electrical and a thermal equivalent circuit. However, since the focus of this work is on the thermal perfor mance of the device, the electrical part of the model will be discounted fo r the rest of the thesis This simplified model assumes 1-D heat transfer from hot to cold si de and ignores convection and radiation from the device surfaces. Each element in the model corre sponds to a lumped thermal resistance in the 29

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device: convh and convc represent the thermal re sistance due to convective heat transfer from the hot and cold fluids, respectively, to the silicon substrate, TEG is the effectiv e conductive thermal resistance of the thermocouple legs, leakage corresponds to the conductiv e thermal leakage of the supporting structures between th e hot and cold sides, and Si models the conductive thermal resistance from the fluid channels to thermocouple ends on both sides. and represent hot and cold fluid temperatures respectively. hfluidTcfluidT + Hot fluid Cold fluid Supporting membrane Silicon substrate Thermocouple legs A B Thfluid TEG leakage convh Thot Tcold Tcfluid Si Si convc Thermal equivalent circuit Relec + + Voc=n (Thot-Tcold) Electrical equivalent circuit Vout Figure 2-2. Planar TEG structure. A) Sc hematic view. B) Equivalent model. For the simple TEG shown above, the expression for the maximum generated output power is given by outP 30

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24Roc out elecV P, (2-1) where, is the generated open circuit voltage and is the electrical resistance of the thermopile. The open circuit volta ge is in turn given by Equation 1-4 and is repeated here for convenience ocVRelec ocVn T n; (2-2) where, represents the number of thermocouple legs, is the Seebeck coefficient and T is the difference between and It can be seen from Equations 2-1 and 2-2 that the output power of the TEG has a quadratic dependence on the temperature gr adient across the thermopile. Thus, is an important factor that affects the device power output. hotTcoldTT Based on the thermal equivale nt circuit shown in Figure 2-2(B) T can be expressed in terms of the various device thermal resistance com ponents. In the equations that follow, and in rest of the thes is, the symbol || denotes a parallel combination of resistances. For example, the parallel combination of thermal resistances A and B is given by the expression ||AB AB A B || 2||TEGleakagehfluidcfluid convhconvcSiTEGleakageTT T (2-3) Equation 2-3 shows increases with the temperature difference between the heat source, and heat sink, which is rather intuitive. It also shows that ThfluidTcfluidT T is inversely proportional to the convective thermal resistances, convh and convc and Si indicating that these resistances should be minimized. It can also be inferred that T increases as ||TEGleakage increases relative to the sum of thermal resistances, convh convc and Si indicating that TE G and 31

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leakage should be maximized. The thermocouple resistance, TEG is a function of the dimensions and number of the thermocouple legs, and is fixed fo r a given thermopile design. It is important to mention here that if TEG is increased with the aim of achieving higher T there is also an indirect increase in th e electrical resistance which results in decreased This implies that a maximum across the thermopile need not necessarily imply maximum A better way to improve and achieve high output power is to minimize Relec outPT T outPleakage or the convective thermal resistances convh and convc as these do not affect Relecleakage is the thermal resistance contributed by the thermal leakage paths through the thin silicon undernea th the thermopile and the supporting membrane. Leakage due to conduc tion and convection of ai r between the hot and cold sides is ignored in this model. The first requirement of a TEG structure is that the thermal leakage between the hot and cold sides of the TEG be minimal. It should be noted that a low thermal leakage implies high leakage thermal resistance, leakage ; an ideal device with zero th ermal leakage would therefore have infinite le a kage With an increase in le akage the term ||EGl T eakage approaches its maximum value of TEG resulting in enhanced T Expressions for both TEG and leakage can be obtained from Fouriers law for 1-D longitudinal heat conduction [ 7]. For a general case, the conductive thermal resistance, of a material is given by l kA (2-4) where l is the length of the material, is its thermal conductivity, and k A is the area of crosssection. Based on Equation 2-4 can be written as TEG 32

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1TEG TEG TEGTEGl nkA (2-5) where, and represent corresponding qua ntities for the individual thermoelements. Division by the number of thermocouple legs is to account for the parallel thermal paths between the hot and cold regions. Similarly, TEGlTEGkTEGAn nleakage is given by ||mem Si leakage memmemSiSill kAkA (2-6) where, and are the length, thermal conductivity and cross-sectional area of the supporting membrane, while, and denote similar quantities for the thin sheet of silicon underneath the membrane. memlmemkmemASilSikSiA The second requirement of a TEG structure is good fluid-solid convective heat transfer at the two ends of the thermopile. When stated in terms of the thermal resistances, the requirement is to minimize the convective thermal resistances convh and convc and Si From heat transfer theory [ 7], the convective thermal resistance is given by 1conv s hA, (2-7) where is the convection heat tr ansfer coefficient and h s A is the surface area where convection heat transfer takes place. For the TEG in Figure 2-2(A) the convection surface area is the sum of inner peripheral surface areas of the fluid channels. A comm on method employed to meet this requirement is to increase the convective surface area s A by using heat fins. For the planar TEG, this can be achieved by creating multiple fluidic channels as shown and/or introducing internal fins in the fluid channels. Alternatively, can also be increased; is generally dependent on fluid properties such as velocity, viscosity and th ermal conductivity, and the nature of fluid flow h h 33

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such as natural convection or forced convection. In natural c onvection, the fluid motion occurs without any external source and offers only a low values. In contrast, a forced convection can achieve very high but requires an external source such as a pump or fan, thereby reducing the net power efficiency of the TEG. hhSome of the other critical requirements includ e the structural stability of the device and ease of fabrication and integration of the heat ex changer with the thermopile. Another practical requirement arises from devices intended application energy harvesting. The fluid channel that extracts the heat energy from the exhaust gas also creates a fluidic backpressure on the exhaust outlet of the small combustion engine. Beyond a certain limit, this backpressure would affect normal operation of the engine. Therefor e, the fluidic resistance offered by the hot gas channels should be sufficiently low. Finally while meeting all these requirements, the dimensions of the device should still be in the microscale. A few different TEG device structures were evaluated based on these requirements. Following is a brief description of each of the structures, an outline of their fabrication strategy, and their pros and cons as TE generators. 2.2.1 Simple in-Plane TEG Structure The simplest form of TE generator is the in-plane structure (Figure 2-2(A)). Fabrication of the thermopile in this TEG is straightforwar d, and involves patterni ng and deposition of the chosen thermoelectric materials on the Si substrat e. On the other hand, formation of the closed fluid channels on either side of the device is involved and requires selective etching and bonding of multiple micromachined substrates to form the closed channels. The major disadvantage of this TEG structure, however, is the existence of the thin sheet of silicon underneath the thermocouples. It offers a large thermal path for the heat flux from the hot side to cold side of the devi ce resulting in huge thermal leakage. This silicon is essential for 34

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the mechanical stability of the entire structure and hence cannot be removed; without it, the rectangular structure would simply break. Some of the other concerns include large fluidic resistance of the channels the channel height is limited by the thickness of Si wafer which is usually only 500-600 m, and difficulty in coupling the exhaust gas from the engine to the TEG. Top and cross sectiona l views of the device are shown in Figure 2-3 Pads Buffer Layer Metal Semiconductor Fluid Channels A B Figure 2-3. In-plane TEG. A) T op view. B) Cross sectional view. 2.2.2 Out-of-Plane Flip-Chip Bonded Structure The next structure that was evaluated is an out-of-plane structure, as employed almost exclusively in macroscale devices. The schema tic view of the out-of-plane TEG is shown in Figure 2-4. The thermopile is made up of alterna ting pand n-type th ermoelectric pillars 35

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connected by metal interconnects and arranged in m eanders for area efficiency In contrast to the in-plane structure, the device he re is vertically configured with both heat and current flowing perpendicular to the surface of th e semiconductor thin film. The en tire structure is sandwiched between two silicon plates and is suitably insulated by buffer layers. A temperature gradient is created by the hot exhaust gas flowing through channe ls in top plate while the bottom plate is at room temperature; alternatively the bottom plate can also have channels or fins to improve the temperature gradient. Metal Interconnects p-type TE leg n-type TE leg Bottom Si plate Figure 2-4. Out-of-plane TEG with top Si plate removed. One key advantage of the out-of-plane structure is that the hot and cold silicon plates are connected only by the thermocouple legs and hence can result in low thermal leakage. However, it should be noted that the plates themselves are separated only by the height of thin semiconductor film (10 to 50 m), and radiative or convective h eat transfer could supersede the conductive pathway. From a fabric ation perspective, the complex ity involved is much greater than the in-plane design. The pand n-type TE legs have to be deposited on differe nt substrates and then flip-chip bonded. The integration of the fluid channels necessitate s a selective etch and another step of wafer bonding. Concerns related to fluidic coupling and channel fluidic resistance apply to this structure as well. 36

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Fluid Channels n-type TE leg p-type TE leg Buffer layer Figure 2-5. Thermopile form ation in out-of-plane TEG. 2.2.3 Vertically Stacked Thermopile Structure A stacked thermopile structure (Figure 2-6 ) takes advantage of the planar processing techniques to build several alternate layers of nand p-type thin film TE material sandwiched between buffer layers on top of a silicon substrat e. Metal interconnects serve to complete the electrical connectivity. When in tegrated with a suitable heat ex changer that creates a lengthwise temperature gradient, the structure would have the highest power density among the various structures discussed. The greatest disadvantage of this structure is the difficulty in integrating a suitable heat exchanger, which also makes it inappropriate for the application in hand. Thermal leakage from the hot region to the cold region is mostly li mited to the thermocouples under the assumption that the silicon substrate is et ched away. However, complete removal of silicon renders the 37

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device fragile, implying a trade-off between th ermal leakage and mechanical robustness. Another concern is the excessive process time re quired to deposit multiple layers of TE material. Metal contact n-type TE leg p-type TE leg Si substrate Buffer layerFigure 2-6. Vertically stacked thermopile structure. Hot region Cold region 2.2.4 Vertically Stacked Radial In-plane Structure The stacked radial in-plane device exploits the concept of stacking to form two coaxial silicon pipes connected by several layers of ra dially oriented thermocouples. The inner pipe serves as a passage for the hot exhaust gas, while the outer pipe is at ambient temperature. By virtue of this arrangement, a radially-direc ted temperature gradient is created across the thermocouples. To realize the tubular structure described a bove, multiple thermoelectric (TE) modules are fabricated and stacked one above the other. E ach TE module in the stack comprises of an inner and outer silicon ring connected by thermocouples. The rings are, in essence, formed by etching away the silicon underneath the thermocouples during fabrication. The supporting membrane on the top of the thermopile serves as a mechani cal connection between the silicon rings and also improves the structural stability of the device. Figure 2-7(A) shows the schematic of the stacked device; top and cross-sectiona l views of the individual TE modules are shown in Figure 2-7(B) and Figure 2-7(C) respectively. 38

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Heat fins on the inner and outer silicon ri ngs enhance the fluid-solid heat transfer, augmenting the temperature gradient across the thermopile. The inner silicon fins extrude longitudinally into the exhaust gas channel, wh ile the outer annular fins enable cross-flow cooling. The outer annular fins are formed by making every fourth TE module in the stack to have a larger diameter. A Silicon 0.5mm Hot gas channel Thermocouple legs Metal Interconnec t Polyimide 5mm 1mm 0.5mm Inner Silicon Fins Outer Silicon Ring B C Figure 2-7. Stacked radial in-plane structure. A) Schematic view. B) Top view of a single radial in-plane TE module. C) Cross-sectional view of a single radial in-plane TE module. This structure offers a good compromise betw een thermoelectric performance, fabrication complexity, and mechanical robustness. The absence of silicon undern eath the thermopile greatly reduces thermal leakage. Additionally, by stacking of multiple modules, the output power is significantly enhanced as it is the sum of power gene rated by the individual modules. The stacking also results in a structure that is mech anically very stable. The circular shape of the center exhaust gas channel lends itself to easy flui dic coupling with the engines exhaust outlet. 39

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The fluidic resistance of the channel can be de signed to be low enough as it is not limited anymore by wafer thickness. Owing to the various advantages of the radial in -plane design, it is chosen as the structure for the TEG to be built. In the following sections, design parameters and the thermal modeling of the radial in-plane TEG are discussed in detail. 2.3 Device Design Device design involves choice of materials, id entification of key de sign parameters and determination of various device dimensions wh ile meeting all the requirements outlined in Section 2.2. For the heat exchanger device, the most important design parameter is the maximum output power that can be achieved when integrated with a thermopile. From Equations 2-1 and 2-2 it becomes essential that outPhotcoldTTT be increased for maximum outPA simple expression for can be developed from the firstorder analytical heat transfer model for stacked radial in-plane TEG. Since th e stack is made up of several TE modules that are schematically the same, the thermal model of a single TE module would suffice for analysis. The thermal circuit model of the radial in-plane TE module is shown in Figure 2-8 Here, 1-D radial heat transfer is assumed, and end effects at the top and bo ttom of the stack are ignored. It can be seen that the model is generally the same as that of the simple in-plane TEG except that TSiRing replaces Si Obviously the individual thermal resi stance values are slightly different. Tambientconvh leakage TEGThotTcoldSi Ring Si Ringconvc Texhaust Figure 2-8. First-order equiva lent thermal circuit of a radial in-plane TE module. 40

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The thermal resistance component corresponding to the silicon rings, SiRing is negligible due to the large surface area, shor t distance, and relatively high thermal conductivity of Si, and hence can be eliminated. convh and convc represent the convective thermal resistances on the inner and outer silicon rings respectively. TEG is the effective conducti on thermal resistance of the thermocouple legs and leakag e corresponds to the thermal leakage through the supporting membrane. It should be noted that since the aim of this research work is to demonstrate only a heat exchanger device, thermocouple legs will no t be included in the design. Therefore, TEG is eliminated from the thermal model. Nevertheless, expression for TEG and how it affects the thermal performance of the final device are pr esented for the sake of completeness. The temperature difference across the annular thermopile, hotTTcoldT is given by || ||TEGleakagehfluidcfluid convhconvcTEGleakageTT T (2-8) routerrinnerTEG Figure 2-9. Dimensions of a si ngle radial thermocouple leg. The discussion in Section 2.2 regarding increase in T with change in different thermal resistances applies to the radial in-plane design as well. Nevertheless, unlike in the in-plane design, TEG and leakage now represent thermal resistance to 1-D radial heat conduction [ 7]. TEG can be expressed as 41

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ln 1outerouter TEG TEGTEGTEGrr ntk (2-9) where, and are the outer and inner radii of the radial thermocouple legs, outerrinnerrTEG is the angle subtended by the thermocouple leg at the center. Figure 2-9 shows the corresponding dimensions. Also, is the thickness of the thermocouple leg, is the thermal conductivity of the thermocouples, and is the number of thermocouples. TEGtTEGknleakage on the other hand, is given by ln 2outerinner leakage memmemrr tk (2-10) where, and are the thickness and thermal conduc tivity of the annular supporting membrane. memtmemkIn the following subsections, details are pr esented on how the device dimensions are chosen based on the simple thermal model of Figure 2-8 2.3.1 Fin Geometry Optimization The convective thermal resistance of the inne r and outer ring surfaces is given by Equation 2-4 and is repeated here for convenience. 1conv s hA (2-7) As shown in Figure 2-7(B) the inner ring surface of the ra dial TE module is longitudinally finned to achieve high er convection surface area sA. But when sA is increased arbitrarily, say by increasing the number of fins, the fluid flow through the center channel is constricted, reducing the flow velocity and hence th e convective heat transfer coefficient [ 7]. Therefore, h 42

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the fin geometry number and dimensions of the fins, have to be optimized for lowest convection thermal resistance. The heat transfer coefficient, by definition, is an empiri cal parameter, and so any optimal solution for low hconv can be achieved only based on expe rimental data. Fin correlations for a variety of fin geometries and channe l configurations exist in literature [ 27], [ 28]. For the radial in-plane TE module, optimization of th e inner ring longitudinal fins was accomplished using correlations for internally finned tubes obtained by Hu and Chang [ 27 ]. These correlations list the Nusselts numbers and friction factor-R eynolds number products fo r different number of fins and different lengths in circular ducts. Before the actual fin optimization is presen ted, a few important terms are defined: (A) Friction factor, f it is a dimensionless quantity given by the expression 2 s f u (2-11) where, s is the surface shear stress on pipe walls, is the density of the fluid, uis the free stream fluid velocity. It can also be shown that the friction factor is directly proportional to the pressure gradient needed to sustain the flow. Therefore, for the center channel of the radial TE device, a high friction factor w ould be undesirable as it would ex ert backpressure on the exhaust outlet of the combustion engine. (B) Nusselts number, this is defined as the dimens ionless temperature gradient at the convective surface of interest, and given by the expression Nu hD Nu k, (2-12) where, is the convection heat transfer coefficient, is the diameter of the channel, and is hDk 43

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the thermal conductivity of the fluid. As is obvious, a high implies high and is needed to achieve low Nu hconv From the correlation data, friction factors and heat transfer coefficients are determined assuming a fully developed laminar flow (Reynolds number, ) of air in a finned channel of diameter 3 mm. The fully-developed laminar flow assumption is likely not satisfied in the actual design, but this serves as a starti ng point for the design. The thermal conductivity of air, W/mK is used for the calculations. The computed values are plotted against number of fins for different fin lengths (Figure 2-10 ). 31.5310 Re0.0323 k A B Figure 2-10. Fin geometry optimization. A) Plot of Friction factor Vs Number of fins. B) Plot of Heat transfer coefficient Vs Number of fins for different fin lengths. 44

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The plots in Figure 2-10 show that with a fin length, 0.8lR high values of could be achieved as is increased. However, for any number of fins above hn 8 n the friction factor becomes prohibitively large. Thus, a length, 0.8 lR and 8 n were chosen for the longitudinal fins on the inner ring. To achieve a low convc annular fins are formed on the outer side by inserting a TE module with a larger outer diameter every fourth module in the st ack. In the final structure, this creates an annularly finned outer shell, wher e the fins are highly conductive silicon. The advantage of annular fins as compared to long itudinal fins on the outer side is fabrication simplicity. Moreover, the cross-flow cooling would likely be transv erse to the exhaust gas flow in an actual system, e.g. small combustion engine Unlike the longitudinal fins on the inner ring, the dimensions of the annular outer fins are not optimized but simply selected. A fin length of 2 mm is chosen to provide a reasonable aspect ratio (the fin thickness is the wafer thickness of 300-500 m) without creating excessive ly large modules. Large di es require more area on the silicon wafer and thus limit the number of de vices that can be fit on a given substrate for microfabrication. 2.3.2 Exhaust Gas Channel Design The diameter of the center exhaust gas channe l needs to be large enough to meet the low fluidic resistance requirement. To this end, a m odel airplane engine is selected as a candidate exhaust gas device and characteri zed to determine the maximum fluid backpressure that it can withstand. The experiment is done by blocking the exhaust outlet of the engine with a metal piece containing a circular hole. The procedure is repeated with smaller hole diameters until the engine starts choking. It was found that the engine started choki ng for hole diameters less than 5 mm. Therefore, the diameter, of the exhaust gas channel is chosen as 5 mm. d 45

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2.3.3 Ring Thickness and Space between Rings The only constraint for the inne r and outer Si rings is that they should be wide enough for the TE module to be structurally strong. Due to its high thermal conductivity, the thermal resistance of the Si rings is ne gligible. Based on these considerat ions, the ring width is chosen as 0.5 mm. It should be noted that for the larger TE modules that form the annular outer fin, the outer ring width is 2.5 mm. The inner ring width is 0.5 mm for both TE modules. Referring to Figure 2-7(B) thus far, all the device dime nsions are fixed except for the distance between the silicon rings. If the di stance between the rings is assumed to be then the overall radius of the smaller TE module can be written as below: s 2 2ringd R ts (2-13) where, R is the overall device radius, d is the diameter of exhaust gas channel, is the Si ring thickness and the space between the rings. By limiting the maximum overall device diameter to 10 mm (excluding the outer fins ), the maximum device radius is set to ringt Rsmax5 mm. Rewriting Equation 2-13 to determine the maximu m space between the rings masx maxmax2 2ringd sRt (2-14) max1.5s mm Thus the silicon rings can be anywhere from 0 1.5 mm apart. While determining an optimized value of s demands the knowledge of thermopile design (thermocouple leg dimensions and material propert ies), a reasonable value for can be chosen based on simple analysis. s 46

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A small value of yields only low thermal resistances sTEG and leakage This can be illustrated easily by recalling the expressions for TEG and leakage from Equations 2-9 and 2-10. Repeating TEG here for convenience, ln 1outerinner TEG TEGTEGTEGrr ntk (2-9) For the thermocouple le gs which contributes TEG and the supporting membrane which contributes leakage the space between the rings, corresponds to the difference between and souterrinnerrouterinnersrr (2-10) Since is set when the radius of the exha ust gas channel is fixed, decreasing results in a small and hence low innerroutersrTEG The same argument is applicable to leakage also. Therefore, shorter distance between silicon rings degrades T and outPOn the other hand, when is on the higher side, sT will be large but at the cost of increased electrical resistance resulting in a lower With increased distance between the rings, the device would also be mechanically weaker. Based on the trad e-offs discussed here, the distance between the silicon rings is chosen to be 1 mm. Rel ec ouPtLastly, the properties of the supporting membrane need to be mentioned. As it forms the only the leakage path between the hot and cold sides, the membrane should have low thermal conductivity. It should also offer sufficient mech anical strength to support the thermocouples and to hold the two silicon rings together. Photodefinable polymer epoxies form potential candidates for this purpose, as the deposition an d patterning steps can be seamlessly integrated 47

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into the heat exchanger process flow. Details re garding material selecti on and necessary process steps to integrate the supporting me mbrane are discussed in Chapter 3. 2.4 Thermal Modeling In the previous section, a simple first-order he at transfer model is presented to gain insight into the different performance trade-offs and to choose heat exchanger device dimensions. However, the model may not be adequate enough to accurately predict the that would be generated for a given exhaust temperature. With this objective, a more detailed thermal model is developed in this section. T mem Conduction through air Exhaust gas flow Cross-flow B air convc Si RingTambientconvh ThotTcoldSi Ring A Texhaust Figure 2-11. Detailed thermal model of radial in-plane strucuture. A) Thermal equivalent circuit of the radial in-plane heat exchanger modul e. B) Cross-sectional view of the heat exchanger stack showing different thermal paths. Figure 2-11(A) shows the detailed thermal model. The additional component that is modeled here is the conduction th ermal path through air between the silicon rings, depicted in Figure 2-11(B) The conduction resistances of the i nner and outer silicon rings are also considered. In what follows, expressions for each thermal resistance com ponent in the model of 48

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Figure 2-11(A) are presented. The equations for conduction thermal resistances TEG and mem have already been presented; air can also be expressed in a similar fashion. All the three are listed for the sake of completeness. ln 1outerinner TEG TEGTEGTEGrr ntk (2-9) ln 2outerinner mem memmemrr tk (2-10) ln 2outerinner air airairrr tk (2-15) where, and are the outer and inner radii of the radial thermocouple legs, outerrinnerrTEG is the angle subtended by the thermocouple leg at the center, and represent the thicknesses and thermal conductivities of the respective material s. In all previous discussions, the thermal conductivities of different materials are assumed to be constant. In reality, the thermal conductivities do change with temperatur e and the temperature dependence of for standard materials can be found in literature [ 7]. This variation in k needs take into account when computing the thermal resistances listed above. ktkNext, the convection thermal resistances are given by ,1convh hshhA, and (2-16) ,1convc cschA. (2-17) where, and are the convection heat transfer coefficients for longitudinal flow on the inner channel and cross-flow over the ou ter ring respectively; similarly, hhch, s hA and s cArepresent 49

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convection surface areas on the hot and cold sides. As the inner fin design is based on empirical correlations, the heat transfer coefficient, can directly obtained from the correlations. Empirical data indicate that for a fin length, hh0.8lR and number of fins, the Nusselts number, is 30.65 [ 27]. can be obtained by rearranging Equation 2-12. 8nNuhh aiD airkNur h innerkNu h (2-18) where, is the thermal conductivity of air, again a functi on of temperature, and is the diameter of the center channel. innerDTo compute the convection coefficient, for the outer ring, two different cases need to be considered with and without ex ternal cross-flow. Again, standard textbook correlations to compute are available for both natural and forced cross-flow convection over long cylinders. From values, can be obtained using Equation 2-12. For the sake of simplicity, annular fins on the outer side are ignored. chNuchFrom Incropera [ 7], the expression for Nusselt number for free convection around a horizontal cylinder is given by 8 9 27 161 60.387 0.6 0.559 1Nu Ra Pr (2-19) where, R a and represent the Rayleigh and Prand tl numbers for air around the cylinder calculated a given temperature. These are given by Pr 3coldambientoutergTTD Ra and (2-20) 50

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Pr (2-21) where, is acceleration due to gravity, is the outer ring temperature, is the ambient temperature, and is the diameter of the outer ring. gcoldTambientTouterD and are fluidic properties of air, that are generally depe ndent only on temperature. is called the volumetric thermal expansion coefficient of the fluid and determines how density of the fluid changes with temperature. is called kinematic viscosity and is simply the ratio of the absolute viscosity of the fluid to its density. Lastly, is called thermal diffusivity, defined as the ratio of thermal conductiv ity to heat capacity of the fluid. Nusselts number for cross-flow over a cylinder is a function of the fl ow velocity. Using correlation suggested by C hurchill and Bernstein [ 7], for cross-flow case is given by Nu 4 5 5 8 1 2 4 31 1 3 20.62 0.3 1 282000 0.4 1 RePrRe Nu Pr (2-22) where R e is the Reynolds number for the cross-flow and is the Prandtl number for air. Pr R e can be obtained using crossflowouterVD Re. (2-23) The analytical model just describe d was implemented in MATLAB, and and T for a range of exhaust gas temperatuust were estimated. The temperature dependence of the fluidic properties such as hotTcoldTres, exhaT and is accounted for by calculating their for each exhaustT. The results predicted by the MATLAB model is later used to with experimentally obtained val values mpare ues. co 51

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52 2.5 Final Dimensions of the Radial In-plane TE Modules Figure 2-12 shows the device dimensions for the small and large heat exchanger modules that need to be fabricated. As can be seen, th e thickness of outer Si ring is the only difference between the two modules. The thickness of both th e modules is just the thickness of a standard silicon wafer (300-500 m). Figure 2-12. Final device dimensions. A) Sm all TE module. B) Large TE module with wider outer Si ring. 13 mm 1 mm 5 mm 2.4 mm 9 mm 1 mm 5 mm 2.4 mm A B 2.6 Summary In this chapter, all aspect s of heat exchanger device design were discussed. The performance requirements were identified, and ba sed on that, different device structures were evaluated. The stacked radial in -plane design was chosen as the structure to be fabricated. With the aid of a first-order heat transfer model, th e device dimensions were established considering various design trade-offs. Also, a detailed ther mal model was developed to predict the thermal isolation performance of the designed heat exchanger.

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CHAPTER 3 FABRICATION AND STACKING OF THE HEAT EXCHANGER MODULES This chapter discusses in detail the pr ocess steps involved in fabricating the individual heat exchanger uni t modules and how the modules are stacked. As discussed in Chapter 2 the thermal leakage between the hot and cold sides of the heat exchanger should be as small as possible for the heat exchanger to be most efficient. This necessitates removal of all th e silicon underneath the thermocouples thereby restricting the thermal path primarily to the thin polyimide membrane. The resultant device has two concentric Si rings connected by the polyimide membrane. The cross-section of the unit heat exchanger module is shown in Figure 3-1 3.1 Through-Etching of Wafers Removal of bulk silicon underneath the ther mopile implies that the wafer needs to be etched through its entire thickness of around 500 m to 600 m. A high etch rate is therefore essential to avoid excessive process time. Also, a high degree of anisotropy is desired to realize a center exha ust channel of uniform diameter. Options for anisotropic etch of bulk silicon include wet etching tec hniques such as the KOH based etch or dry etching techniques such as the plasma based reactive ion etching (RIE ) and deep reactive ion etching (DRIE). However, because of its high etching speed, better anisotropy, vertical side-wall profile and ease of wafer handling, Boschs DRIE technique [ 29] becomes the obvious choice. The DRIE process utilizes alternate etch and passivation cycles to achieve high anisotropy. During passivation, a chemically inert compound similar to Teflon is coated on side-walls to prevent undercutting. Typi cally, the etch and passivation cycles last several seconds, and the fine ba lance between durations of eac h cycle is what determines 53

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the side-wall profile. The technique uses SF6 and O2 during the etch cycle and C4F8 for the passivation cycle [ 30 ]. For heat exchanger device fa brication, etching is performed using Surface Technology Systems (STS ) Multiplex ICP ASE (Inductively Coupled Plasma Advanced Silicon Etcher). Polyimide Membrane Exhaust Gas Channel Inner Si ring Outer Si ring A Polyimide Inner Si ring Oxide Figure 3-1. Heat exchanger module to be fa bricated. A) Top View B) Cross-sectional view 3.2 Membrane Strength Evaluation HD Microsystems product HD-8820 Aqueous Positive Polyimide [ 31 ] is chosen as the material for the membrane connecting the Si rings. It offers good mechanical strength, low thermal conductiv ity, and temperature compatibility up to 450 C. In addition, its photodefinability is used to pattern the membrane without the need for photoresist and thereby reducing the number of process steps. The polyimide membrane thickness is a critical parameter in the design of the heat exchanger. A thin membrane would offer low thermal leakage, but the result ing device may not be structurally stable. Outer Si ring B Exhaust gas channel 54

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On the other hand, a thick membrane would co mpromise the thermal isolation of the heat exchanger. For this reason, the optimum pol yimide thickness is found by trial and error; multiple devices are fabricated with different polyimide thicknesses 5 m and 10 m of HD8820. Although both devices are mechanically robust, some devices with 5 m polyimide had issues like cracks and membrane peel-off. However, through fine control of the number of DRIE cycles and careful hand ling of the devices during final release, an overall yield of more than 90% could be achieved. Thus, the thinner 5 m is chosen as the polyimide thickness. 3.3 Process Flow Description The heat exchanger process flow is devise d with the target thermoelectric generator (TEG) in mind so that process steps pertaini ng to thermopile fabrication can be readily accommodated (in future builds). Table 3-1 lists all the processing steps in detail, and Figure 3-2 shows the device cross-sections at each step. The process starts with a double side polished (DSP) (100) 100-mm Si wafer of thickness 500 m with 300 nm of thermal oxide on both sides. A 5 m thick polyimide layer is deposited, patterned and cured on the front-side. This laye r acts as the supporting membra ne for the thermoelectric elements in the TEG. The exposed thermal oxide on the front-side and the oxide layer on the back-side are removed using 6:1 buffered oxide etch (BOE). The front-side oxide layer mimics the buffer layer required in future builds to grow the thermoelectric PbTe themoelements. It also plays the role of et ch stop during the final DRIE step. After the oxide removal, photoresist is spun on the b ack-side and patterned after front-to-back alignment using EVG 620 Precision Mask Aligner. The wafer is then attached to a handle wafer using AZ9260 photoresist, and back-s ide through-wafer DRIE is performed. This step removes the bulk silicon under the thermopile, creates the central aperture, and 55

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56 forms an inner and an outer silicon ring. In addition, the DRIE step singulates the individual heat exchanger modul es, but they remain attached to the handle wafer. The modules are released from the handle wafer in an acetone bath that also removes the photoresist on the back-side. Square shaped longitudinal fins are formed on the inner silicon ring, also during the DRIE step; patterns for these are included in the back-side mask. On the other hand, outer silicon rings are formed during th e stacking process presented in Section 3.5. Initially, square shaped unit modules with longitudinal inner a nd outer fins were fabricated. The dimensions of this square device were similar to the circular shaped devices. These square devices served as a te st vehicle to evaluate the process flow, and fix process parameters such as the DRIE chamber pressure and gas flow rates. Table 3-1. Heat Exchanger Process Flow. Step Process Description 1 Start with double side polished (DSP), <100> crystal orientation, n-type Si wafers with 300nm thermal oxide on both sides 2 Spin deposit a 5m thick polyimide (HD8820) layer on the front-side 3 Pattern and cure polyimide layer 4 Remove exposed front-side oxide and back-side thermal oxide through a 6:1 BOE 5 Deposit and pattern 10m thick AZ 5260 photoresist on back-side after front-to-back alignment 6 Attach handle wafer on the front-side using AZ9260 photoresist 7 DRIE back-side to form the central aperture and the Si rings 8 Release heat exchanger modules by removing photoresist in acetone bath 3.4 Mask Making Heat exchanger module fabrication re quires only two photomasks one for patterning the front-side polyimi de and another for the back -side through-wafer etch. The masks are designed using AutoCAD 2008 (Figure 3-3 ). These masks are printed using emulsion on polyester films at J.D. Phot o Tools, U.K. The mask patterns are later

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a) Start with DSP Si wafers with 300nm thermal oxide on both sides b) Deposit HD8820 polyimide c) Pattern polyimide d) Remove thermal oxide through BOE e) Deposit photoresist on back-side f) Attach handle wafer on the front-side with AZ9260 photoresist g) Through-wafer DRIE from back-side h) Release heat ex changer modules in acetone bath Figure 3-2. Device cross-sections during various process steps. 57

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transferred on to 5 x 5 chrome/soda lime gla ss plates using MA6 Mask Aligner. These glass plates serve as the master-masks used in patterning of photoresist and polyimide. Both masks have patterns for fabricating 12 large modules of 13 mm diameter and 36 small modules of 9 mm diameter, yielding enough devices to make two 12 mm long tubular thermoelectric generators. Certain heat exchan ger modules are designed with access windows of 5.6 mm on the outer Si ring to enable temperatur e measurements using external thermocouples. An array of alignment marks is added in the mask to facilitate easy front-to-back alignment. Also, the back-side mask patterns are designed to match with the mirror image of the front side patterns to account for the wafer flippi ng involved in front-to-back processing. Figure 3-3. Mask patterns. A) Front-side ma sk pattern. B) Back-side mask pattern. 3.5 Stacking and Bonding of He at Exchanger Unit Modules The singulated heat exchanger modules are st acked and bonded to form the tubular heat exchanger. Stacking is done usi ng a simple assembly jig with two parallel metal rods that are spaced precisely to match th e device dimensions. Figure 3-4 shows both a top view and a photograph of the stacking setup. The diameter of the rods is accurately chosen so that it fits closely between two opposite pairs of inner fins of the heat exchanger module. The individual 58

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modules are slid one by one over the alignmen t rods while a thermally conductive epoxy is applied on each module to facilitate bonding. The annular outer fins are formed by inserting one large (13 mm) module for every three small (9 mm) modules. Figure 3-4. Assembly jig. A) Sche matic top view. B) Photograph. Two different thermally conductive epoxies ar e tried in attempts to bond the individual modules. First, a Pb-Sn based solder epoxy (Figure 3-5(A)), is deposited in tiny droplets around the periphery of the inner and outer Si rings. The deposition is done using EFD Fluid Dispenser and the droplet size is fixed at 100-15 0 m using appropriate dispensing tips. The stacked device is then cured in an oven at 200 C After curing, the device is structurally strong at room temperature; however it is experime ntally found that bonding between the individual modules weakened at te mperatures above 250 C. The second bonding method is attempted us ing a high temperature epoxy (Figure 3-5(B) ) from J-B Weld; the epoxy is ma de by mixing equal parts of two different pastes liquid steel epoxy resin and a hardener. The mixture is app lied carefully around the i nner and outer Si rings on each heat exchanger module as they are stacked. Figure 3-6 shows the application method employed for each kind of epoxy. Curing is done at room temperature over a period of 24 hours. A Alignment rods B 59

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The cured device is found to be mechanically ro bust even at temperatures close to 400 C. Therefore, this epoxy is chosen to bond the m odules of the actual heat exchanger device. Figure 3-5. Epoxies for bonding heat exchanger modules. A) Solder epoxy from EFD. [Adapted from http://www.efd-inc.com/Solder/Dispensing ] B) High-temperature epoxy from JB Weld. [Adapted from http://www.jbweld.net/products/jbweld.php ] Figure 3-6. Epoxy application methods. A) Solder epoxy. B) High temperature epoxy (JB Weld). After curing, the stack possesses a sealed inne r channel for the passage of the hot exhaust gas with longitudinal fins extending into it. The thermally conductive nature of the epoxy enables thermal conduction between the rings in the vertical direction. This helps maintain a uniform inner ring temperature and a uniform ou ter ring temperature al ong the length of the tubular device. B A A B 60

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61 3.6 Final Device Photographs Figure 3-7. Heat exchanger stack built with square shaped modules. Figure 3-8. Heat exchanger built with circular modules. 3.7 Summary The proposed heat exchanger device was built by fabricating and stacking individual TE modules. For TE module fabrication, polyimide was chosen as the supporting membrane, and DRIE was used to remove bulk silicon and form the center exhaust channel. The modules were stacked using a simple assembly setup, while bonding was achieved using a high temperature epoxy from JB Weld.

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CHAPTER 4 CHARACTERIZATION OF THE HEAT EXCHANGER DEVICE In this chapter, the experimental setup and procedure used to characterize the stacked radial in-plane heat exchanger device are described. Following this, the test results obtained from the experiments are presented in various graphs and tables; the results are also compared with the theoretical model predictions. The chapter concludes with a discussion on the limitations of the test procedure, and reasons fo r deviation of the expe rimental results from theoretical results. 4.1 Experimental Setup and Procedure Tests are performed on the heat exchanger device to characterize the thermal isolation temperature difference, that is created between the inner hot Si ring and the outer Si ring for various temp eratures and velocities of hot gas flowing through the center channel. Sustaining a high temperature across the thermocouple legs in the final TEG is critical for power generation. hotcoldTTT T Figure 4-1. Experimental setup to test the heat exchanger device. The schematic of the experimental setup is shown in Figure 4-1 A commercially available hot air gun is used as the source of hot gas; the outlet of the heat gun is connected to the sealed 62

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inner gas channel of the h eat exchanger under te st through high temperature tubing and appropriate fluidic couplers. Master Appliances Proheat Variair Heat Gun, PH-1300 [ 32] is used for this purpose; the variable temperature, variable air flow rate capability of the heat gun enables device testing under different conditions The shrink tubing att achment is used to confine the hot air flow only to the connecting tube; mo reover, the connecting tube is glued to the heat gun outlet to prevent detachment during the experiment. Prior to the experiment, an aluminium fluidic coupler is bonded to the stack ed heat exchanger using the high temperature epoxy, J-B Weld. Figure 4-2 shows the heat exchanger device with the fluidic coupler bonded at the bottom end. The coupler is machined accurate ly to make a snug fit with the connecting tube, thereby ensuring a leakage free c onduit for the hot gas from th e heat gun outlet to the center channel of the heat exchanger. Figure 4-2. Heat exchanger device bonded with the fluidic coupler. The device is tested under two different confi gurations in the firs t, the outer ring is cooled only by natural convection wh ile in the second, forced conv ection is used for cross-flow cooling of the outer ring. In both cases, the inner silicon ring is heated by the hot air from heat gun. The cross-flow for the forced convection te sts is created by a miniat ure fan, positioned to provide a fairly uniform flow around the device. As part of the experiment, the temperature of 63

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the hot air through the center channel, velocity of the hot air, and the cross-flow velocity, are changed, and temperatures of the inner and outer Si ring, and are measured. The tools and methods employed to measure these experimental parameters are described in the sub-sections that follow. exhaustTexhaustVcrossflowVhotTcoldT4.1.1 Flow Measurements A hot-wire anemometer is used to measure the ve locity of the hot air and that of the crossflow. Hot-wire anemometry involves exposure of an electrically heated element probe to a fluid medium in order to measure the medium propertie s such as velocity and composition. For the heat exchanger experiments, a constant temp erature anemometer (CTA ) with tungsten wire probe is used. The principle of operation of a CT A is to maintain a constant wire temperature using an internal negative feedback system comprised of a wheatstone bridge and a servo amplifier. The system schematic of the CTA is shown in Figure 4-3 Error Voltage Servo Amplifier Hot-wire probe R R Adjustable resistor Figure 4-3. System schematic of a constant temperature anemometer. When introduced into a fluid flow, the temp erature of the wire drops due to convective cooling and consequently its electr ical resistance decreases, due to the temperature coefficient of 64

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resistance of the tungsten metal. This causes an imbalance in the bridge circuit creating a finite error voltage across th e input terminals of the se rvo amplifier. The amp lifier acts to counter the imbalance by forcing more current through the ho t-wire, which, by virtue of joule heating increases the wire temperature. With increasing flow velocity, the error voltage and hence the amplifier output voltage both increase. Thus, the amplifier output serves as a direct measure of the fluid flow velocity. In practical measurements, the fluid velo city is determined using Kings law [ 33], which is given by 2 nEABuE A (4-1) where is the analog output voltage of the CTA, is the velocity of the flow normal to the wire; u B and n are constants. Before actual veloci ty measurements are performed, the hotwire anemometer is calibrated to ascertain the constants A B and in Equation 4-1 The calibration process involves expos ing the hot-wire probe to a se t of known velocities and noting the output voltages. The Kings law calibration constants can then be calculated using a powerlaw curve fit on the calibration data. nDuring heat exchanger testing, as the temperat ure of the center channel hot-air is varied, the measured hot-wire anemometer output voltage s need to be temperature corrected before computing flow velocities. The modified King s law with temperature correction is given by 2 n wireflowE ABu TT (4-2) where, is the hot-wire temperature, and wireT f lowT is the temperature of the fluid flow. The hotwire temperature, can be found using wireT 65

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0 0 wirea T T (4-3) where is the reference ambient temperature during calibration, 0T0 is the temperature coefficient of resistance (TCR) of the hot-wire probe at temperature and is a constant called overheat ratio that is established at the time of calibration. The ratio 0Ta 0a is usually defined as the overtemperature and denoted by overtempT 0overtempa T (4-4) Once the calibration constants A B and n and overtemperature, are determined, the fluid velocity values can be obtained using the expression overtempT 2 flow n flow wireflowE u TT A B (4-5) where f lowE is anemometer output voltage for a flow with velocity f lowu and temperature f lowT. Table 4-1 lists in a step-by-step fa shion all the steps involved in hot-wire calibration and flow velocity measurement. It should be added that for cross-flow velocity measurement, steps pertaining to temperature correcti on are not required as the flow te mperature is the same as the ambient temperature. Table 4-1. Flow velocity measurement procedure using the CTA. Step Description 1 Calibrate the hot-wire probe by forci ng air streams of known velocities, and measure CTA output voltages u E 2 Measure the ambient temperature during calibration process. This forms the reference temperature, 0T3 Determine from the overheat ratio Normally, is directly made available by the calibration software. overtempTaovertempT 66

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Table 4-1. Continued. 4 Calculate the working temperature of the sensor, from and the overtemperature, wT0TovertempT 5 Determine the calibration constants A B and n by power-law curve fitting of the calibration data. 6 For actual flow measurements, r ecord the flow temperature, f lowT and CTA output voltage, f lowE 7 Compute flow velocity f lowu from Equation 4-5 4.1.2 Temperature Measurements Thermocouples are used to measure the temperat ures of the inner and outer Si rings of the heat exchanger device, and that of the hot air ex iting the center channel. Faster response time and maximum operating temperature range are the tw o important considerati ons for the selection of the thermocouples. Accordingly, K-type ther mocouples made of nickel alloys Chromega and Alomega [ 34 ] are chosen. Specifically, the unsheathed fine-gage thermocouple of wire diameter 250 m, CHAL-010, having a response time of 2 seconds and a maximum working temperature of around 800 C is used for the temperature measurements. The thermocouple outputs are connected to different input channels of a temperature measurement module, USB-TEMP, from Measurement Computing [ 35]. The measurement module and the thermocouples used are shown B A Figure 4-4. Temperature measurement tools. A) Temperature module. [Figure courtesy Measurement Computing] B) Thermocoupl es, CHAL-010. [Figure courtesy of Omega] 67

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in Figure 4-4 The temperature data from the USB output of the temperature module is recorded in a PC using LabView software and is used later for various analyses. Outer silicon ring temperatures are measur ed at three different points around the circumference of the device to get a sense of the temperature profile around the device, especially for the experiments with external cross-flow. Temperature measurements were recorded at the 9o clock, 12o clock and 3o clock positions, as these cover the minimum and maximum temperatures around the circumference of the device. The temperature measurement points and the direction of cross-flow are shown in Figure 4-5 It should be noted that, with ease and accuracy of measurements in mind, the oute r ring temperatures are measured on the surface of the outer silicon ring. For the measurement of inner silicon ring temperature, a thermocouple is made to contact the inne r ring through the access window de signed for this purpose, as described in Section 3.4. Measurement of hot-air temperature is achieved by means of another thermocouple held close to the exit of the center channel of the device. A separate thermocouple is used to measure the ambient temperature. In order to make accurate temperature measurements, all the thermocouples are held stab ly in place using externally supported clips. Figure 4-5. Device temperature measurement points. 68

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4.2 Test Matrix and Actual Test Results A battery of experiments is performed to characterize the heat exchanger device under different temperatures and flow velocities of the hot-air. The heat gun is configured to operate in the variable temperature/variable flow rate m ode. In this mode, eight different temperature settings and eight different flow settings, constituting a total of sixty-four combinations are possible. Among these, a set of twenty-four combinations a ll of the eight flow settings repeated for three different temperature settings, ar e chosen. This set of experiments is repeated, in turn, for three different cross-flow conditio ns (zero cross-flow, and two settings of the miniature fan) making the total number of experiments seventy-two. Table 4-2 makes a list of all the experiments that are performed. For each experiment, five different steady-state temperatures are measured simultaneously; these temperatures are listed in Table 4-3 In addition to these, a one-time measurement of the ambient temperature, is also made. Hot-air flow velocities through the center channel ( through V), are measured independently with the lowest temperature setting on the heat gun. The flow velocity measurements are not repeated for higher temperature settings as the flow temperature exceeds the operating range of the cross-wire pr obe. The hot-air flow velocities are assumed to remain the same across the various temperature settings, but this assumption was not explicitly verified. The cross-flow velocity (ambientT8 ust1 exhaustVexha 1 crossflowV ) measurements are also performed independently. 2 crossflowVThe measured temperature data is analyzed by plotting various graphs The first set of plots (Figure 4-6 ) shows the variation in the inner and outer silicon ring temperatures with increasing hot-air temperature. 69

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Table 4-2. Heat Exchanger Characterization Matrix. Set Number Cross-flow velocity Hot-air temperature Hot-air velocity 1 exhaustT 1 exhaustV through 8 exhaustV 2 exhaustT 1 exhaustV through 8 exhaustV1 No cross-flow 3 exhaustT 1 exhaustV through 8 exhaustV1 exhaustT 1 exhaustV through 8 exhaustV2 exhaustT 1 exhaustV through 8 exhaustV2 1 crossflowV 3 exhaustT 1 exhaustV through 8 exhaustV1 exhaustT 1 exhaustV through 8 exhaustV2 exhaustT 1 exhaustV through 8 exhaustV3 2 crossflowV 3 exhaustT 1 exhaustV through 8 exhaustV Table 4-3. Temperatures measured du ring heat exchanger characterization. Serial Number Temperature measured 1 Inner silicon ring temperature, hotT 2 Outer silicon ring temperature at 9o clock position, coldaT3 Outer silicon ring temperature at 12o clock position, coldbT4 Outer silicon ring temperature at 3o clock position, coldcT5 Temperature of hot-air ex iting the center channel, exhaustT Max T 125 C A 70

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Max T ~ 140 C B Max T ~140 C C Figure 4-6. Variation of inner a nd outer ring temperatures with increasing hot-air temperature. A) No cross-flow case. B) High crossflow case. C) Low cross-flow case. Referring to Figure 4-6(A) the zero cross-flow case, a st eady increase in both inner and outer ring temperatures w ith hot-air temperature, is noticeable; also, the difference between ring temperatures, increases linearly with In this plot, only one exhaustThotcoldTTT exhaustT 71

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outer ring temperature data is shown as, without cross-flow, the outer ring temperature around the circumference of the device is f ound to be fairly uniform. Figures 4-6(B) and (C) show ring temperature variation in the presence of cro ss-flow. Apparently, a larger temperature differential, can be observed without cross-flow cooling, the maximum achieved is 125 C, whereas with cross-flow cooling the maximum T T T achieved is approximately 140 C. However, no striking improvement in T is observed as the cross-flow velocity is increased from low to high. This is mainly due to the fact that the difference in cross-flow velocities for the low and high cases is only 0.3 m/s. From cr oss-flow velocity measurements using hot-wire anemometer, = 2.6 m/s and 1 ossflowcrV2 sflow crosV = 2.9 m/s. From the experimentally observed maximum temperature differential, best case thermal ratio of the heat exchanger can be calcula ted. Thermal ratio is a measure of the thermal isolation performance of the heat exchanger, a nd is defined as the ra tio of the temperature difference between the hot and cold sides to the maximum temperature difference possible. It is given by T hot col amTT TTd bient therma ambientexhaustl exhaustT TT (4-6) For the radial in-plane heat exchanger characterized here, the thermal ratio is 140C 0.56250Ccold therma ambienthot exhaustTT TT l. (4-7) Also, the maximum heat transfer rate, Q, from the hot-air into the heat exchanger can be determined from the measured ring temperature values: 0.144exhaust convhhot SiRingTT Q W. (4-8) 72

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This implies that a maximum power of 144 mW is ex tracted from the hot-air stream. This can be compared with energy transfer rate of the hot-air flow in the center channel of the device. Using thermophysical properties of dry air from standard literature, and assuming slug flow through the center channel, the mass flow rate, dm dt, and hence the energy transfer rate can be calculated as follows: exhaustcrosssectiondm VA dt (4-9) where, is the density of air, is the hot-air velocity, and is the cross-sectional area of the center exhaust channel. The energy transfer rate of the hot -air flow is given by exhaustVcrosssectionA hotair pdm QC dt T W, (4-10) where, is the specific heat capacity of air. For the flow conditions under which the heat exchanger device is tested, this computes to pC58hotairQ indicating that only a small fraction of the heat energy is extracted from the hot-air stream. With regard to variation in outer ring temp erature around the circumference of the device, the temperature at the 3o clock position, coldcT is found to be the highest followed by coldbT and then High outer ring temperature at the 3o cl ock position on the device is expected as it is completely hidden from the cross-flow by the h eat exchanger device itself. On the other hand, the 9o clock position shows low temperatures as it receives maximum cross-flow. Moreover, with higher cross-flow velocity, a larger circumferential variati on in outer ring temperature can be noticed as a result of increased convection. coldaT 73

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A B Figure 4-7. Variation in inner silicon ring temperature with ho t-air velocity. A) No cross-flow case. B) High cross-flow case. C) Low cross-flow case. Figure 4-7 shows the variation in inne r silicon ring temperature, with increasing hotair velocity, at three different temperatures, hotTexhaustV1exhaustT through T3exhaust Referring to Figure 4-7(A) the zero cross-flow case, it can be seen that there is a small (around 20 C), but perceptible increase in inner ring temperature as th e flow velocity is increased. The same trend is observed for other hot-air temper atures as well, and also in the presence of cross-flow cooling. However, with cross-flow, the curves are shifte d down because of external cooling. The main 74

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observation from these plots is th at the inner ring temperature in creases with increasing hot-air velocity. This is expected because a higher flow velocity generally results in a higher convection heat transfer coefficient, h which implies lower convh and larger Nevertheless, in light of the fact that the hot-air temperature increases sligh tly with hot-air velocity (due to non-ideal heat gun), it can be inferred that the net increase in ring temperature because of increasing hot-air velocity is small. hotT C Figure 4-7. Continued. 4.3 Comparison with Predicted Results The experimental results obtai ned from heat exchanger char acterization desc ribed in the previous section are compared with results predicted by the detailed analytical model of the heat exchanger device developed in Section 2.5. Specifically, all the th ermal resistances in the equivalent circuit shown in Figure 2-11(A) are computed, using which estimates for the inner and outer silicon ring temperat ures are obtained for a range of hot-air temperatures, from 0 to 270 C. The device dime nsions, hot-air velocity, and cross-flow velocity, exhaustTcrossVexhaustVflow 75

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are the other input parameters used in this pr ediction. For calculating thermal conductivity and other fluid properties that are temp erature dependent, the measured and are used. hotTcoldT A B Figure 4-8. Comparison of experime ntal results with results predicted by analytical model. A) No cross-flow case. B) High cross-fl ow case. C) Low cross-flow case. The theoretically estimated results are plotte d along with the expe rimental results for comparison (Figure 4-8 ). From the plots, it can be seen that the analytical and experimental results match closely for the no cross flow case; however, with non-zero cross-flow velocity, the 76

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experimental results deviate significantly from the theoretical predictions especially for high hotair temperatures. The maximum deviation of 50 C is observed for the low cross-flow case. C Figure 4-8. Continued. 4.4 Limitations of the Experimental Setup One main drawback of the measurement st rategy employed to characterize the heat exchanger device is the usage of thermocouples to measure the various temperatures. The accuracy of a thermocouple temperature meas urement largely depends on the how well the thermocouple is in contact with the surface of interest. Although efforts are made to position and hold the thermocouples in place using external supports, and these connections are monitored periodically during heat excha nger device testing, there still remains a degree of uncertainty related to thermocouple position and orientation especially in the pres ence of cross-flow. The next source of error in h eat exchanger testing is the h eat gun. Ideally, in variable temperature/variable flow rate mode, the temperature of the hot-air is in dependent of the flow rate setting. However, the hot -air temperature is found to vary by at least 30 C between the lowest and highest flow rate setting. This makes it difficult to understand how responds to changes in hot-air flow veloci ty at a given temperature. T 77

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78 The other limitation of this experimental setup is the fairly large window in the outer silicon ring that is designed to provide access to inner ring for temperature measurement. When the device is tested with cross-fl ow cooling, a portion of the i nner ring gets indirectly exposed the cross-flow resulting in lower inner ring temperature than its tr ue value. This helps account for the disparity between the theoretical predic tion and actual experimental results evident in Figures 4-8(B) and 4-8(C) 4.5 Summary The thermal isolation perform ance of the radial in-plane heat exchanger device is characterized under different condi tions. The experimental results obtained are found to agree well with results from analytical model except for the inner ring temperature in presence of cross-flow. The possible reasons of the deviation are also examined.

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CHAPTER 5 CONCLUSIONS AND FUTURE WORK This chapter recapitulates the key goals and accomplishments of this research work. It also summarizes important results a nd suggests possible improvements and future work for heat exchanger device design, fabri cation and characterization. 5.1 Conclusions Microscale thermoelectric (TE) power generation has, of late, gained increased attention due to two important reasons: growing demand fo r portable power in the microwatt to milliwatt range [ 19], and availability of micromachinable TE materials with improved efficiency [ 1 ]. The principal challenge confronting microscale TEG design is to achieve a high temperature differential across thermopile ends that are only hundreds of micrometers apart [ 20], [ 21]. The main goal of this research work is to build a heat exchanger platform for a TE microgenerator that converts waste heat from exhaust gas to useful power, while attempting to meet the challenge above. To this end, the concept of a stacked silicon tubular heat exchanger device was proposed. By design, the stack is made of multiple TE mo dules, each comprising of an inner and an outer silicon ring connected only by a thin supporting membrane. The st ructural stability of such a device was, however, questionable. Through actual fabrication of the proposed heat exchanger device using polyimide for the supporting membrane, th e idea is proven to be indeed feasible. In the TE modules, the absence of silicon underneath the thermocouples limits thermal leakage to the low conductivity polyimide resulting in high thermal efficiency; whereas, silicon heat fins formed on the inner and outer sides serv e to enhance convective heat transfer. The fabricated heat exchanger device was ch aracterized in the laboratory under different conditions varying temp eratures and flow velocities of the exhaust gas, with and without 79

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external cross-flow cooling. A maximum temperature differe nce of 140 C between the inner and the outer rings was achieved for an exhaus t gas temperature of around 250 C, indicating a thermal ratio close to 60 %. An analytical he at transfer model of the device was developed in MATLAB to corroborate experimental results. The model predictions match reasonably well with measured temperatures for the test withou t cross-flow. There ar e significant deviations observed in the cross-flow case, but this is attributed to limitations of the experimental setup. In summary, a micro heat exchanger device fo r thermoelectric waste heat power generation was designed, fabricated and demonstrat ed to achieve high thermal efficiency. 5.2 Future Work Although successful in creating a fairly large temperature differential, the heat exchanger device developed in this research work has its own shortcomings. Firstly, the device does not lend itself to easy temperature measurements (necessary for thermal modeling validation and future device designs). Partic ularly, measuring inner ring temper ature is quite challenging since it is enclosed completely within the outer ring. The second disadva ntage is the procedure used to bond the TE modules. While it is true that manua l application of epoxy around the silicon rings offers a quick and dirty way to build the stack, it is labor intensive, demands considerable caution, and the method is prone to defects. An inadequate amount of epoxy can cause air gaps, resulting in thermal leakage, whereas an exce ss of the same could clog the center exhaust gas channel. Based on these thoughts, future i nvestigations in this research work can be pursued in two areas: a better and more reliable bonding/stackin g technique and structur al modifications to enable convenient temperature measurements. The following subsect ions suggest possible options in each area. 80

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5.2.1 Eutectic Bonding of TE Modules From a bonding perspective, the silicon substrates of the TE modules offer an advantage in that many of the emerging solutions for 3D integration of ICs [ 36] apply directly to bonding TE modules as well. On these line s, eutectic bonding is an establis hed method used in silicon wafer bonding. The principle here is to use an intermed iate bonding material, in this case a eutectic alloy from two or more metals. A eutectic alloy combination offers a combined melting point that is much lower than the individual melti ng points of the constituent elements, enabling relatively low-temperature bonding. Common eutectic pairs found in the litera ture include copper-tin, gold-tin, gold-sili con and gold-indium. Deposited Au Annular Polyimide Indent in Si rings A B Polyimide Membrane Deposited Gold for Eutectic bonding Figure 5-1. Eutectic bonding of TE modules. A) Cross-section of TE modules with deposited Au. B) Top view of TE modu le with deposited Au. 81

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The obvious choice for bonding of heat exch anger modules is Au-Si. Previous implementations [ 37], [ 38 ] of Au-Si eutectic bonding report an alloy formation temperature of less than 400 C, well within the operating temper ature range of polyimide The only additional process steps necessary for eute ctic bonding of TE modules are deposition of chromium (for adhesion) and gold on the front-sid e and nested DRIE on the back-s ide to realize a cross-section (Figure 5-1(A) ). Once the TE modules are fabricated and stacked, bonding of the entire stack can be achieved in a single instance by heati ng to the eutectic temperature. The primary challenge here would be to ensu re electrical insulation between the thermoelectric devices and the conductive bonding layer. 5.2.2 Integrated Temperature Measurement Integrated temperature measurement using resi stance temperature dete ctors (RTDs) present a promising alternative to the use of externally applied ther mocouples. RTDs respond to a change in temperature with a ch ange in resistance; the amount of resistance change for a given temperature change is dependent on a material property called the temperature coefficient of resistance (TCR). Resistance of any material is given by 010 R TRTTT (5-1) where R T is the resistance at temperature T 0 R T is the resistance at reference temperature and 0T is the temperature coefficient of resistance of the material. Usually, materials with high TCR such as platinum and nickel form good RTDs. RTDs can easily be integrated into the heat exchanger modules by in corporating additional process steps for metal deposition and patterning prior to polyimide patterning. RTDs can be added to the inner and outer silicon rings (Figure 5-2 ). There may be some loss of thermal efficiency because of leakage through the RTD; but this can be minimized by careful design. 82

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The real challenge, however, lies in performing the back-side DRIE without destroying the metal resistors that may only be a fe w hundred nanometers thick. Figure 5-2. Top view of the TE module with integrated RTDs. As final part of this research, attempts are made to implement platinum RTDs on the large 13 mm heat exchanger modules. A seed layer of chromium is first deposited to provide good adhesion to silicon substrate, fo llowed by deposition of the platinum RTD and a very thin gold layer to enable easy wire bonding to RTD pads. Consecutively, the regular process steps for heat exchanger module fabrication are pe rformed. After fabrication, it is observed that the inner ring RTDs of all the modules invariably fail continuity test. Careful scrutiny reveals fracturing of the RTDs mostly in the region be tween silicon rings where the RTD is supported only by the polyimide membrane on top. Although, the failure mechanism is not thoroughly understood at this point in time, cracks formed in the polyimi de due to high film stress developed during DRIE are speculated as a possible reason. This subj ect therefore offers a worthy subject for future examination. In conclusion, although the stacked radial heat exchanger platform is designed to specifically for thermoelectric power generation, the scope of the principles and fabrication methods developed reach out to several other applications requiring high thermal isolation such 83

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84 as thermoelectric cooling, micro reactors and the like [ 23], [ 39 ]. With MEMS technology making new strides by the day, an extension of th e concepts developed in this research may lead to ground-breaking new innovations.

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LIST OF REFERENCES [1] R. Venkatasubramanian, E. Siivola, T. Colpitts, and B. O'Quinn, Thin-film thermoelectric devices with high room-temperature figures of merit, Nature, vol. 413, no. 6856, pp. 597-602, 2001. [2] Wikipedia. (2008, Jul.). Radiator [Online], Available: http://www.wikipedia.org/wiki/Radiator [3] S. Kaka and H. Liu, Heat Exchangers Selection, Rating and Thermal Design Boca Raton, FL: CRC Press, 2002. [4] Alibaba.com. NP Skived Fin Heat Sink [Online], Available: http://npowertek.trustpass.alibaba.com/product/11645462/ NP_Skived_Fin_Heat_Sink.html [5] D. B. Tuckerman and R. F. W. Pease, High-performance heat sinking for VLSI, IEEE Electron Device Lett. vol. EDL-2, no. 5, pp. 126-129, May 1981. [6] D. T. Crane and G. S. Jackson, Optimization of cross flow heat exchangers for thermoelectric waste heat recovery, Energy Conversion and Management vol. 45, pp. 1565-1582, 2004. [7] F. P. Incropera and D. P. DeWitt, Fundamentals of Heat and Mass Transfer, 5th ed., Hoboken, NJ: John Wiley & Sons Inc., 2002. [8] D. Mundinger, R. Beach, W. Benett, R. So larz, W. Krupke, R. Staver, and D. Tuckerman, Demonstration of high performance silicon microchannel heat exchangers for laser diode array cooling, Appl. Phys. Lett. vol. 53, pp. 1030-1032, 1988. [9] J. Arthur, W. H. Tompkins, C. Troxel, Jr., R. J. Contolini, E. Schmitt, D. H. Bilderback, C. Henderson, J. White, and T. Settersten, Microchannel water cooling of silicon X-ray monochromator crystals, Rev. Sci. Instrum. vol. 63, pp. 433-436, 1992. [10] S. Wu, J. Mai, Y. C. Tai, and C. M. Ho, Micro heat exchanger by using MEMS impinging jets, in Proc. IEEE Micro Electro Mechanical Systems 1999, pp. 171-176. [11] C. H. Amon, J. Murthy, S. C. Yao, S. Narumanchi, C. F. Wu, and C. C. Hsieh, MEMSenabled thermal management of high-heat-f lux devices EDIFICE: embedded droplet impingement for integrated cooling of electronics, Experimental Thermal and Fluid Science vol. 25, pp. 231-242, 2001. [12] M. J. Ellsworth, Jr. and R. E. Simons High powered chip cooling air and beyond, Electronics Cooling vol. 11, no. 3, pp. 14, Aug. 2005. 85

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[13] L. G. Frchette, C. Lee, S. Arslan, Y.-C. Liu, Design of a microf abricated rankine cycle steam turbine for power generation, in Proc. ASME International Mechanical Engineering Congress Washington, DC, Nov. 15-21, 2003, pp. 335. [14] S. Sullivan, X. Zhang, A. A. Ayon, J. G. Brisson, Demonstration of a microscale heat exchanger for a silicon micr o gas turbine engine, in Proc. 11th International Conf. SolidState Sensors, Actuators and Microsyst. (Transducers), Munich, Germany, Jun. 2001, pp. 1606. [15] L. Sitzki, K. Borer, E. Schuster, P. D. Ronney, and S. Wussow, Combustion in microscale heat-recirculating burners, in Proc. Third Asia-Pacific Conference on Combustion, Seoul, Korea, Jun. 24-27, 2001, pp. 473. [16] A. Cohen, Microcombustor and Combus tion-based Thermoelectric Microgenerator, U.S. Patent 6 613 972, Sep. 2, 2003. [17] S. B. Schaevitz, A MEMS Thermoelectric Generator. Cambridge, MA, Massachusetts Institute of Technology, 2000, Ph.D. [18] Thin Film Peltier Cooler, MPC-D901 Datasheet [Online], Available: www.micropelt.com [19] S. A. Jacobson and A. H. Epstein, An informal survey of power MEMS, in Proc. Int. Symp. Micro-Mechanical Engineering, Tsuchiura and Tsukuba, Japan, Dec. 1, 2003, pp. 513. [20] S. B. Schaevitz, A. J. Franz, K. F. Je nsen, and M. A. Schmidt, A combustion-based MEMS thermoelectric power generator, in Proc. 11th International Conf. Solid-State Sensors, Actuators and Microsyst. (Transducers), Munich, Germany, Jun. 2001, pp. 30-33. [21] M. Strasser, R. Aigner, M. Franosch, G. Wachutka, Miniaturized thermoelectric generators based on poly-Si and po ly-SiGe surface mi cromachining, in Proc. 11th International Conf. Solid-State Sensors, Actuators and Microsyst. (Transducers), Munich, Germany, Jun. 2001, pp. 535-542. [22] G. Min, D. M. Rowe, and F. Volklein, Integrated thin film thermoelectric cooler, Electronics Letters vol. 34, no. 2, pp. 222-223, 1998. [23] A. Gross, H. Baoling, G.-S. Hwang, C. Lawrence, N. Ghafouri, S. W. Lee, H. Kim, C. Uher, M. Kaviany, K. Najafi, A multistage in-plane micro-thermoelectric cooler, in Proc. 21st IEEE Int. Conf. MEMS Tucson, AZ, Jan. 13-17, 2008, pp. 840-843. [24] D.-Y. Yao, C.-J. Kim, and G. Chen, Des ign of thin-film thermoelectric microcoolers, in Proc. ASME International Mechanical Engineering Congress & Exposition Orlando, FL, Nov. 5-10, 2000. 86

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[25] I. Boniche, C.D. Meyer, P.J. Taylor, N.K. Dhar, D.P. Arnold, Progress towards a micromachined thermoelectric generator using PbTe and PbSnSeTe films, in Tech. Dig. 6th Int. Workshop Micro Nanotechnology for Power Generation and Energy Conversion Apps. (PowerMEMS 2006) Berkeley, CA, Nov. 2006, pp. 195-198. [26] I. Boniche, B. C. Morgan, P. J. Tayl or, C. D. Meyer, and D. P. Arnold, Process development and material charact erization of polycrystalline Bi2Te3, PbTe, and PbSnSeTe thin films on silicon fo r MEMS thermoelectric generators, J. Vacuum Sci. Tech. A (in press), Jul./Aug. 2008. [27] W. M. Rohsenow, J. P. Hartnett, and Y. I. Cho, Handbook of Heat Transfer 3rd Ed., New York, NY: McGraw Hill, 1998. [28] R. L. Webb, Principles of Enhanced Heat Transfer New York, NY: John Wiley & Sons Inc., 1994. [29] M. A. Douglas, Trench etch process for a single-wafer RIE dry etch reactor, U.S. Patent 4 855 017, Aug. 8, 1989. [30] S. D. Senturia, Microsystem Design, New York, NY: Springer Science & Business Media Inc., 2001. [31] HD Microsystems, HD-8820 Aqueous Positive Polyimide Process Guide [Online], Available: http://www.hdmicrosystems.com [32] Master Appliances Product Catalog [Online], Available: www.masterappliance.com/Master _Appliance_50th_Catalog.pdf [33] F. E. Jorgensen, How to measure turbulence with hot-wire anemometers a practical guide Skovlunde, Denmark: Dantek Dynamics, 2002. [34] Thermoelectric Alloy Proper ty Data [Online], Available: http://www.omega.com/temperature/z/pdf/z049-050.pdf [35] Specifications USBTEMP [Online], Available: www.measurementcomputing.com/pdfs/usb-temp.pdf [36] P. Garrou, Future ICs go vertical, Semiconductor International Feb. 2005. [37] R. F. Wolfenbuttel, Low-temperature intermediate Au-Si wafer bonding:eutectic or silicide bond, Sensors and Actuators A-Physical vol. 62, pp. 680-686, 1997. [38] D.-J. Yao, G. Chen, C.-J. Kim, Low te mperature eutectic bonding for in-plane type micro thermoelectric cooler, in Proc. ASME International Mechanical Engineering Congress and Exposition New York, NY, Nov. 11-16, 2001. 87

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88 [39] A. Kasuga, S. Tanaka, and M. Eshashi, Vacuum-packaged micro fuel reformer for high thermal efficiency and low package temperature, in Proc. 21st IEEE Int. Conf. MEMS Tucson, AZ, Jan. 13-17, 2008, pp. 968-971.

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BIOGRAPHICAL SKETCH Sivaraman Masilamani was born in Edaiyur, a small village in Thiruvarur district of Tamilnadu, India. He received his bachel ors degree in electr onics and communication engineering from College of Engineering, A nna University, Chennai in the year 2001. Subsequently, he joined Alliance Semiconductor, Bangalore as a IC Design Engineer, designing analog and digital subcircuits for SRAMs and power management ICs. He started his graduate study in August 2005 at University of Florida, Gainesville. He joined the Interdisciplinary Microsystems Grou p under the supervision of Dr. David Arnold, focusing his research in the area of microsystems and microfabrica tion. He also gained industry experience in analog circuit design through in ternships at Qualcomm, San Diego and Texas Instruments, Melbourne. He received his Master of Science degree in el ectrical and computer engineering from University of Florida in August 2008. He has since accepted an Analog IC Design Engineer position at Intel, Oregon. His research interests include analog circu it design for power management ICs, power MEMS, and circuit design for MEMS. 89