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ABSTRACT

SARGOLZAEIAVAL, YASAMAN. Flexible Thermoelectric Energy Harvesters Using Bulk Thermoelectric Legs and Low-Resistivity Liquid Metal Interconnects with High Thermal Conductivity Encapsulation. (Under the direction of Dr. Mehmet C. Öztürk.)

The human body continuously generates bio-heat through metabolic reactions. The

metabolic heat which is dissipated from the human skin surface, is a renewable source of

en-ergy, which can be harvested to provide sufficient power for miniaturized electronic devices.

Thermoelectric generators (TEGs) are excellent candidates for conversion of metabolic heat

to electrical power. When a temperature gradient is established across a TEG, an electrical

voltage is generated due to the Seebeck effect. Since there exists a natural temperature

difference between the human body and the ambient, TEGs can be integrated into wearable

electronics, such as fitness gadgets, as the energy source to generate the required energy. In

general, wearable TEGs can generate micro-watts to milli-watts of electrical power, which

is sufficient to continuously run wearables with low power electronics. There is a growing

interest in long-term vigilant monitoring of human health parameters as well as

environ-mental factors using wearable electronics. However, the need for frequent replacement or

recharging of the batteries is a huge challenge which makes wearable monitoring systems

less appealing to a large population of users. Recent advances in low power wearable

elec-tronics makes it feasible to realize fully self-powered systems that solely rely on harvested

energy from the human body. Therefore, incorporating flexible TEGs as the power source

into wearable electronics is a promising way to eliminate the need for batteries. With low

power sensors, electronics, and flexible TEGs, fully self-powered wearable systems can be

achievable. The focus of this thesis was to minimize the parasitic losses that occurred due

to the presence of an encapsulation layer above the EGaIn interconnects..

The main goal of this work is to improve the performance of flexible TEGs using eutectic

gallium indium (EGaIn) interconnects. Initially, the performance of flexible TEGs was

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encasing layer of the EGaIn interconnects should have a thermal conductivity of about

1 W/mK (or higher) to avoid the impact of the parasitic thermal series resistance of the

encapsulation layer. Additionally, our modeling showed that incorporation of a thin metal

spreader with a thickness of less than 10µm on the cold side of the TEGs improves the

output power by enhancing heat rejection.

In order to produce flexible TEGs that can compete with commercial-off-the-shelf

(COTS) rigid TEGs, it is essential to to minimize the parasitic thermal resistances via new

materials and device design. We were able to successfully develop a high thermal

conductiv-ity (HTC) elastomer using Polydimethylsiloxane (PDMS) doped with EGaIn and graphene

nanoplatelets (GnPs). The thermal conductivity of this new elastomer was about 6X higher

than that of pure PDMS. We showed that the incorporation of the HTC elastomer as the

encasing layer of the EGaIn interconnects increased the output power of the flexible TEGs

by 1.7X compared to devices created with pure PDMS as the encapsulation layer. Finally,

we developed a method to electroplate copper on flexible TEGs that led to a further

en-hancement of 1.3X in the output power density with the presence of air flow at normal

walking speeds.

Finally, the performance of different flexible TEGs were evaluated and reported at

different ambient conditions showing best-in-class performance compared to other flexible

TEGs developed for body heat harvesting. Lastly, we identify and discuss future design

improvements and modeling pathways that can potentially impact the design, fabrication,

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© Copyright 2019 by Yasaman Sargolzaeiaval

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Flexible Thermoelectric Energy Harvesters Using Bulk Thermoelectric Legs and Low-Resistivity Liquid Metal Interconnects with High

Thermal Conductivity Encapsulation

by

Yasaman Sargolzaeiaval

A dissertation submitted to the Graduate Faculty of North Carolina State University

in partial fulfillment of the requirements for the Degree of

Doctor of Philosophy

Electrical Engineering

Raleigh, North Carolina

2019

APPROVED BY:

Dr. Michael Dickey Dr. Veena Misra

Dr. Alper Bozkurt Dr. Mehmet C. Öztürk

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DEDICATION

I dedicate this thesis to my parents for all the support and unconditional love they

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BIOGRAPHY

Yasaman Sargolzaeiaval was born in Zahedan located in southwest Iran. She completed

her Bachelors of Science in Electrical Engineering, with a focus in Electronics, at Sharif

University of Technology, Tehran, Iran. She then received her Masters of Science degree in

Electrical Engineering, majoring in nanoelectronic devices, also at Sharif University. She

started her PhD studies at NC State University in 2015 under the direction of Prof. Mehmet

C. Öztürk. In 2016, she joined the NSF funded Advanced Self-Powered Systems of Integrated

Sensors and Technologies (ASSIST) engineering research center and started working on her

PhD thesis. Her research interests include renewable energy, energy harvesting, low/

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ACKNOWLEDGEMENTS

I would first like to thank my parents for the unconditional love they gave me throughout

my entire life. Without their support and guidance, I would not have been able to be the

person I am today.

I want to also express my gratitude to the students and postdocs, who have helped and

supported me through out this work including Taylor V. Neumann, Abhishek Malhotra,

Ozkan Yildiz, Farzad Mohaddes, Amin Nozariasbmarz, and Michael Lim. I would also like

to thank Francisco Suarez for initiating this work and helping me with my work during

the early days. I am forever thankful to my friend and colleague, Viswanath Padmanabhan

Ramesh, for his friendship, help, and support during these four years.

I would like to thank the ASSIST Engineering Research Center for supporting this project

and giving me the resources I needed to complete my work. I would like to thank the director

and the staff of the NCSU Nanofabrication Facility (NNF); Dr. Philip Barletta, Nicole Hedges

and Marcio Cerullo for all of their help and guidance in the cleanroom. I want to also thank

Dr. Daryoosh Vashaee for his support and help during the course of my PhD.

I would like to thank Dr. Michael Dickey for his expertise and support of my work. He

not only provided much useful feedback, but he also opened his lab to share the equipment.

I would also like to thank Dr. Veena Misra for her grand vision for the ASSIST center; and

Dr. Alper Bozkurt for his insightful comments on my work.

I would like to express my deepest thanks to my friends, Amirhassan Fallah, Elnaz

Shabani, Ghazal Kamyabjoo, Alireza Garmabi, and Mohammad Salamati who have always

been a major source of support when I needed them. My friends, who are like family to me

now, helped me get through my hardest times during these four years.

Finally, I would like to express my sincere gratitude to my advisor Prof. Mehmet C.

Öztürk for his continuous support of my work, for his patience, motivation, enthusiasm,

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His technical and editorial advice was essential to the completion of this dissertation and

has taught me innumerable lessons and insights on the workings of academic research in

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TABLE OF CONTENTS

LIST OF TABLES . . . viii

LIST OF FIGURES. . . ix

Chapter 1 INTRODUCTION. . . 1

1.1 History of Thermoelectric Effect and Thermoelectric Devices . . . 1

1.2 Thermoelectric Modules . . . 8

1.3 TEGs for Body Heat Harvesting . . . 11

1.4 Summary of the Thesis . . . 14

Chapter 2 Review of the Existing Methods for Fabrication of Flexible TEGs . . . . 17

2.1 Introduction . . . 17

2.2 Synthesis of Thermoelectric Materials for Flexible TEGs . . . 18

2.2.1 Physical Vapor Deposition (PVD) . . . 18

2.2.2 Electrochemical deposition . . . 20

2.2.3 Screen Printing . . . 23

2.2.4 Dispenser Printing . . . 27

2.2.5 Organic and Solution-processable Materials for Fabrication of TEGs 29 2.2.6 Fabrics . . . 32

2.3 Flexible TEGs Using Bulk Materials . . . 34

2.3.1 Flexible TEGs Using Bulk Materials and Liquid Metal Interconnects 40 2.4 Summary . . . 43

Chapter 3 High Thermal Conductivity Silicone Elastomer Doped with Graphene Nanoplatelets and Eutectic GaIn Liquid Metal Alloy (Published) . . . . 44

3.1 Abstract . . . 44

3.2 Introduction . . . 45

3.3 Experimental . . . 47

3.4 Results and Discussion . . . 49

3.5 Conclusion . . . 60

Chapter 4 Flexible Thermoelectric Generators for Heat Harvesting From the Hu-man Body - Enhanced Device PerforHu-mance Using High Thermal Con-ductivity Elastomer Encapsulation on Stretchable Liquid Metal Inter-connects . . . 62

4.1 Abstract . . . 63

4.2 Introduction . . . 63

4.3 Experimental . . . 67

4.4 Results . . . 72

4.5 Conclusion . . . 82

Chapter 5 Incorporation of Flexible Heatsinks onto TEGs . . . 86

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5.2 Fabrication Process of Flexible Heatsinks . . . 86

5.3 Characterization of Flexible Heatsinks . . . 87

5.4 Incorporation of Flexible Heatsinks onto Flexible TEGs . . . 91

5.5 3D COMSOL Simulation of Flexible Heatsinks . . . 92

5.6 Conclusion . . . 94

Chapter 6 Modeling of the Human Body as the Heat source for TEGs . . . 95

6.1 introduction . . . 95

6.2 Modeling Results . . . 96

6.3 Conclusions . . . 102

Chapter 7 Conclusions and Future Work . . . 103

7.1 Conclusions . . . 103

7.2 Future Work . . . 104

BIBLIOGRAPHY . . . 106

APPENDICES . . . 123

Appendix A Steam Treatment of the Elastomer Surface for Durable Electro-plating . . . 124

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LIST OF TABLES

Table 1.1 Performance of some of the best rigid TEGs for body heat harvesting. 14

Table 4.1 Properties of thermoelectric legs. . . 67 Table 4.2 Parameters used in COMSOL simulation. . . 75

Table 5.1 Total thermal resistance of the hot side of a rigid TEG with the in-corporation of different flexible heatsinks measured at different air velocities. . . 91

Table 6.1 Metabolism rate of a average-sized person at different activity levels [218]. . . 98 Table 6.2 Equations for calculating RMR from sex, age, and body mass[219].

(RMR is inO2uptake units (ml.kg´1.min´1 . . . 100 Table B.1 Properties of the conductive powders mixed with PDMS to create

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LIST OF FIGURES

Figure 1.1 a) Thomas J. Seebeck. b) An illustration of the original apparatus used by Thomas Seebeck which led to the discovery of the thermo-electric effect. A and B are the junctions formed by two dissimilar conductors. The compass needle moves in response to heating one of these junctions. . . 2 Figure 1.2 a) Jean Charles Peltier. b) William Thompson (Lord Kelvin). . . 3 Figure 1.3 Dependence of the thermoelectric figure of merit (z T) of some

thermoelectric materials on the temperature[9]. a) N-type materials. b) P-type materials. . . 4 Figure 1.4 Dependence of electrical conductivity,thermal conductivity,

See-beck coefficient, and thermoelectric figure of merit, z T, on the carrier concentration for a typical thermoelectric material[9]. . . . 5 Figure 1.5 a) Maria Telkes, inventor of first TEG in 1947, and b) Abraham Ioffe

who developed the theory of thermoelectricity for semiconductors. 6 Figure 1.6 Curiosity Rover exploring Mars and relies solely on RTEGs for power. 7 Figure 1.7 Key developments in history of thermoelectrics. . . 7 Figure 1.8 Schematic of a simple thermoelectric module in a) cooling mode,

and b) power generation. . . 9 Figure 1.9 A commercial TEG made of many N-type and P-type thermoelectric

legs. a) Schematic[31]b) Real image protect[32]. . . 10 Figure 1.10 Human skin temperatures at different body locations for varying

ambient temperatures[36]. . . 12 Figure 1.11 An electrical circuit representative of a TEG on the human body . . 13

Figure 2.1 Flexible thin film TEG made with DC-sputtered thermoelectric ma-terial[95]. . . 20 Figure 2.2 Vertical flexible TEG made with planar ECD deposited

thermoelec-tric materials and vertical (through-plane) heat guides.[100]. . . 23 Figure 2.3 Vertical uni-polar flexible TEG made with planar screen-printed

thermoelectric materials. a) Schematic of the TEG. b) Fabricated TEG.[101] . . . 25 Figure 2.4 Screen-printed TEGs placed vertically on a steam pipe for waste

heat recovery.[105] . . . 25 Figure 2.5 Flexible TEG made with screen-printed thermoelectric material[106]. 27 Figure 2.6 Flexible TEG made with ink-jet printed thermoelectric material[75]. 29 Figure 2.7 A flexible and transparent TEG made with PEDOT:PSS and

ITO-PEDOT:PSS[118]. . . 31 Figure 2.8 A flexible TEG made withC60{T i S2nanosheets[120]. . . 32 Figure 2.9 Thermoelectric fiber made with thermal evaporation of nickel and

silver[94]. . . 34 Figure 2.10 a) Schematic, and b) Photograph of a woven TEG made with fibers

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Figure 2.11 flexible TEG made with planar bulk thermoelectric legs and heat pipes for maintaining the temperature differential[124]. . . 36 Figure 2.12 Schematic diagram of the designed wearable TEG using bulk legs

and flexible PCB: a) TEG wears on the wrist, b) schematic view of the wearable TEG on the skin.[91]. . . 37 Figure 2.13 A flexible TEG made with bulk legs and flexible PCB[90]. . . 37 Figure 2.14 A flexible TEG made with bulk thermoelectric legs, soldered copper

strip interconnections, and PDMS encapsulation[125]. . . 38 Figure 2.15 A flexible TEG made with bulk thermoelectric legs and FPCB

elec-trodes interconnections. Bakelite holders and flexible wires were used to achieve flexibility[126]. . . 39 Figure 2.16 A bulk flexible TEG made with copper interconnects in a flexible

PDMS substrate[128]. . . 39 Figure 2.17 A bulk flexible TEG made with liquid metal (EGaIn) interconnects

[53]. . . 41 Figure 2.18 a) and b) Experimental setup for bending test. c) AC resistance of

the flexible TEG made with liquid metal interconnects as a function of number of bending cycles. “Self-healing” properties of EGaIn is highlighted in this figure.[53]. . . 42 Figure 2.19 Predicted performance of a flexible TEG made with 64

thermoelec-tric legs with an area of 4 c m2 at room temperature of 24˝C for

several potential modifications[53]. . . 43

Figure 3.1 Measured total resistance of different EGaIn/PDMS plotted against sample thicknesses for five different weight percentages of GnP (Inset:Cured GnP/EGaIn/PDMS sample.). . . 50 Figure 3.2 Thermal conductivity of samples with different GnP weight

concen-trations with and without EGaIn (50 weight%). . . 52 Figure 3.3 Thermal conductivity enhancement relative to PDMS of samples

with different GnP weight concentrations with and without EGaIn (50 weight%). . . 53 Figure 3.4 EGaIn dispersion in PDMS. . . 53 Figure 3.5 Cross-sectional SEM images of elastomers with a) GnP and b) GnP

and EGaIn inclusions. . . 55 Figure 3.6 High magnification image of an EGaIn region in a sample doped

with GnPs and EGaIn. . . 56 Figure 3.7 Young’s modulus of PDMS with EGaIn (50 %wt) and GnP inclusion. 58 Figure 3.8 Maximum strain at break of PDMS with EGaIn and GnP inclusion. . 58 Figure 3.9 Tensile strength of PDMS with EGaIn and GnP inclusion. . . 59 Figure 3.10 Viscosity measurements of PDMS with EGaIn and GnP inclusion. . 60

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Figure 4.2 The process flow used in fabrication of flexible thermoelectric gener-ators. (a)The legs are placed through a template; (b) PDMS is poured between the legs, planarized and cured; (c)EGaIn interconnects are sprayed through a hard mask; (d) A thin („50µm) layer of PDMS is sprayed as temporary encapsulation; (e) The device is flipped and liquid metal interconnects are sprayed and encapsulated on the backside; (f ) Final high thermal conductivity encapsulation elas-tomer is drop-casted on both sides of the device and cured. . . 70 Figure 4.3 Thermal conductivity of the encapsulation elastomer with and

with-out EGaIn inclusion as a function of the GnP weight percentage. (Inset: A cured sample doped with 50 wt% liquid metal (EGaIn) and 2.2 wt% GnP.) . . . 71 Figure 4.4 a) A thermoelectric generator made with high thermal conductivity

encapsulation. b) cross-sectional image of the device. . . 71 Figure 4.5 a) Setup used to characterize the thermoelectric generators On the

hot plate. b) TEG worn on the wrist inside and outside of the wind tunnel. . . 73 Figure 4.6 Device performance measured for different encapsulation layers.

a) Measured open-circuit voltage versus air velocity. b) Calculated power density of TEGs with 20% fill factor. The numbers in the labels refer to the thickness of the encasing layer. The lines are just used to guide the eye. Decreasing the thickness of PDMS or increasing the thermal conductivity helps improve the power density. . . 74 Figure 4.7 a) The 3-D structure used in COMSOL simulations; b) Simulated

temperature drop across a single TE leg as a function encapsulating elastomer thickness for different elastomer thermal conductivities. 76 Figure 4.8 a) Open circuit voltage measured for thermoelectric generators

made high thermal conductivity and PDMS encapsulation with dif-ferent fill factors. b) Corresponding power density for those devices. The lines are used just to guide the eye. . . 77 Figure 4.9 ModeledT across a single leg in a 64 leg TEG plotted as a function

of the copper spreader thickness. . . 78 Figure 4.10 TEG with an electroplated copper spreader layer on top of HTC

elastomer. . . 79 Figure 4.11 a) Open circuit voltage measured for thermoelectric generators

made with high thermal conductivity encapsulation with and with-out a copper spreader layer. b) Open circuit voltage measured for TEGs with PDMS encapsulation with and without copper spreader. The lines are used just to guide the eye. . . 80 Figure 4.12 a) Power density for devices fabricated using high thermal

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Figure 4.13 a) Image of the TEG worn on the wrist b) Open circuit voltage mea-sured on the wrist for the thermoelectric generators made with high thermal conductivity encapsulation and copper spreader with a fill factor of 20%. c) Corresponding power density for those devices. The lines are used just to guide the eye. . . 84 Figure 4.14 Setup used to measure force required to bend the devices around a

cylinder. . . 85 Figure 4.15 Force required to keep the TEGs and control samples bent around a

cylinder. . . 85

Figure 5.1 Fabrication process of flexible heatsinks. . . 87 Figure 5.2 a) Flexible heatsink and the 3D printed mold used to create it; b) A

flexible TEG with the incorporated flexible heatsink. . . 88 Figure 5.3 The rigid TEG with incorporated flexible heatsink . . . 90 Figure 5.4 The wind tunnel designed to measure interface thermal resistance

of the rigid TEG with incorporated flexible heatsink . . . 90 Figure 5.5 a) Open circuit voltage and b) power density of a TEG with 20% fill

factor with and without the flexible heatsink. . . 92 Figure 5.6 The 3D structure used to model heatsinks in COMSOL. . . 93 Figure 5.7 Dependence of theT across a single thermoelectric leg as a

func-tion of a) radius of the heatsink cones; b) height of the heatsink cones. . . 93

Figure 6.1 Variation of skin temperature at the wrist, forehead and fingertips with respect to environmental temperature 20˝C[44]. . . . 97

Figure 6.2 Illustration of different aspects of modeling of human body[215]. . 97 Figure 6.3 Skin thermal conductivity at an ambient temperature of 20˝C

plot-ted against different metabolic rates. . . 99 Figure 6.4 Mean skin temperature at different ambient temperatures[216]. . . 99 Figure 6.5 Skin thermal conductivity at different metabolic rates plotted against

different ambient temperatures. . . 101 Figure 6.6 Metabolic heat generation of man at the age of 25 at different walking

speeds. . . 101

Figure 7.1 Comparison between the state-of-the-art flexible TEGs and the TEGs presented in this thesis. . . 105

Figure A.1 A porous PDMS surface created by water vapor[220]. . . 125 Figure A.2 (a)The schematic and (b) the actual setup of the steaming process.

(c) An steamed sample after curing. (d) A microscope image of a steamed PDMS sample (20X magnification). . . 126

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CHAPTER 1

INTRODUCTION

1.1

History of Thermoelectric Effect and Thermoelectric

De-vices

The origins of the thermoelectric effect dates back to 1821 when Thomas Johann Seebeck

(Fig. 1.1.a), a German Scientist, observed the deflection of a compass needle placed between

two dissimilar conductors. The conductors were connected together as shown in Fig. 1.1.b,

and heated by a candle at one end[1]. The deflection was explained to be due to the change

in the magnetic field produced by the current flowing through the closed electrical circuit

formed by the two electrodes[2]. Generation of an electrical voltage at the junction of two

dissimilar conductors in the presence of a temperature gradient is due to the displacement

of electrical charge carriers, which gain energy by absorbing the heat and travel to the

opposite side. The voltage generated in materials experiencing a temperature gradient is

called the “Seebeck voltage”.

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(a) (b)

Figure 1.1a) Thomas J. Seebeck. b) An illustration of the original apparatus used by Thomas Seebeck which led to the discovery of the thermoelectric effect. A and B are the junctions formed by two dissimilar conductors. The compass needle moves in response to heating one of these junctions.

discovered that whenever two dissimilar conductors came into contact to form a junction

with a current passing through, heat is rejected or absorbed at the junction resulting in

local cooling or heating[3]. This mechanism is the reverse of the Seebeck effect. It was

discovered that when carriers travel across a junction of two conductors, they either need

to overcome a potential barrier or relax to a lower energy state by absorbing or releasing

heat as phonons. This mechanism is called the “Peltier effect”.

21 years later, in 1855, the connection between Peltier and Seebeck effects was

dis-covered by a British scientist, William Thompson (Fig. 1.2.b)[3]. His discovery led to the

third groundwork of the thermoelectric effect which is known as the “Thompson effect”.

Thompson discovered that when a temperature difference existed across a block of material,

the electric current absorbed the heat on the hot side and carried it to the cold side. This

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The combination of Seebeck, Peltier, and Thompson effects is called the “Thermoelectric

effect" and defines the relation between the heat and the electrical energy transformation

in materials.

(a) (b)

Figure 1.2a) Jean Charles Peltier. b) William Thompson (Lord Kelvin).

The efficiency of a thermoelectric material is summarized in a parameter called

“ther-moelectric figure-of-merit: (zT)”, which was defined in 1909 by a German physicist, named

Edmund Altenkirch[2]. The figure-of-merit is expressed as, Eq. 1.1:

z TS

2σ

κ T (1.1)

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each thermoelectric material had an optimum operating temperature wherez T reached a maximum. This discovery led to extensive research to develop materials for applications

requiring different operation temperatures[4–8]. Fig. 1.3 shows the dependence ofz T on temperature for some of the popular N-type and P-type thermoelectric materials. A good

thermoelectric material must have a high Seebeck coefficient, a low thermal conductivity,

and a low electrical resistivity. Each of these three parameters are functions of carrier

con-centration and cannot be optimized independently. Fig. 1.4 shows the dependence ofσ,κ,

S, andz T on the carrier concentration for a typical thermoelectric material[9]. Shown in Fig. 1.4, as the carrier concentration increases, the Seebeck coefficient decreases while the

electrical and thermal conductivity increase. Therefore, for a maximumz T, an optimum value exists for a given carrier concentration. Early research on thermoelectric materials

was carried out by characterizing thermoelectric properties of metals and alloys which

were not suitable for the majority of applications of thermoelectric devices we have today.

(a) (b)

Figure 1.3Dependence of the thermoelectric figure of merit (z T) of some thermoelectric materi-als on the temperature[9]. a) N-type materials. b) P-type materials.

The first thermoelectric generator (TEG) was realized by a Hungarian Scientist, Maria

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thermoelec-Figure 1.4Dependence of electrical conductivity,thermal conductivity, Seebeck coefficient, and thermoelectric figure of merit,z T, on the carrier concentration for a typical thermoelectric material[9].

tric effect was discovered in the early 18t hcentury, it did not have any feasible applications

until after realization of doped small band-gap semiconductors with superior

thermo-electric performances in 1950. During this period, Abraham Ioffe (Fig. 1.5.b) developed

the fundamental theory for the thermoelectric effect in semiconductors that led to

break-throughs in the development of high efficiency thermoelectric materials and modules for a

variety of applications[3].

The first application of TEGs for heat harvesting was realized in 1957 by Ken Jordan and

John Birden when they used radioisotope thermoelectric generators (RTEGs) to generate

electric power from the heat produced by the decay of radioactive materials[11]. Radioactive

materials have the capability to generate a temperature gradient as high as 600K across

a thermoelectric module[12–14]. Because of the long lifetime of such materials, they are

able to provide the required heat for many years. NASA developed several satellites and

spacecrafts that entirely relied on RTEGs as the power source. One famous example is

the “Curiosity Rover” (Fig. 1.6) that landed successfully on Mars and is currently exploring

the planet. Generally, for space exploration missions, especially at distant locations far

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(a) (b)

Figure 1.5a) Maria Telkes, inventor of first TEG in 1947, and b) Abraham Ioffe who developed the theory of thermoelectricity for semiconductors.

weak sunlight rays reaching the panels; hence, RTEGs are better suited for this particular

application. Soon after realization of RTEGs, low temperature applications such as recovery

of waste heat drew attention and led to the development of new materials for efficient

performance at room temperature[15].

One of the main limitations of thermoelectric devices is their low efficiency which is

often as low as 10% of the Carnot efficiency defined as the theoretical maximum efficiency

of a heat engine working between two temperatures, which can be expresses as Eq. 1.2:

ηTH o t ´TC o l d

TH o t

(1.2)

whereTH o t andTC o l d are the temperatures on the hot and cold sides of the module

respec-tively.

This efficiency can be improved by materials optimization, which has been the main

focus of numerous studies[9, 16–20]. In 1960’s, several studies predicted the upper limit for

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Figure 1.6Curiosity Rover exploring Mars and relies solely on RTEGs for power.

at the time[22]. In 1993, it was predicted that an order of magnitude improvement inz T could be possible by nanostructuring of the thermoelectric materials[23]. This work led to

an exponential increase in the amount research on the subject[24–30].

Fig. 1.7 illustrates the history of Thermoelectric effect and key devices relying on the

thermoelectric effect.

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1.2

Thermoelectric Modules

Thermoelectric effect enables cooling or heating without incorporation of any moving

parts, which makes thermoelectric devices less susceptible to failure over a long period of

time. Thermoelectric devices can be used in compressorless refrigerators, which can be

very small and light weight, or as the generators of electrical energy by recovery of waste

heat.

Thermoelectric devices are typically made of N-type (where electrons are the majority

carriers) and P-type (where holes, deficiency of electrons, are the majority carriers)

semi-conductor blocks (which are referred to as thermoelectric “legs”) connected electrically

in series and thermally in parallel. When a doped semiconductor is heated on one side,

the majority carriers (electrons in N-type semiconductors and holes in P-type

semicon-ductors) absorb the thermal energy and leave the hot side resulting in accumulation of

carriers on the other side. This charge build-up generates an electric potential difference

across the semiconductor which is the basis of power generation in TEGs as described

before. The generated voltage can be increased by connecting multiple semiconductor

blocks (thermoelectric legs) in series. In the presence of the same temperature gradient,

N-type and P-type semiconductors show opposite polarities of generated Seebeck voltages.

Therefore, N-type and P-type semiconductors can be connected in series in alternating

order to achieve a net generated voltage, which is equal to sum of the voltages generated

by each individual thermoelectric leg. A simple thermoelectric couple made of a single

N-type and a single P-type semiconductor block is shown in Fig. 1.8. If the thermoelectric

couple is used as a cooler, a current (or a voltage) is applied to the module resulting in the

flow of majority careers toward one side of the semiconductor. When carriers move, they

transfer energy resulting in heat absorption at one side (cold side) and heat generation at

the other side (hot side) as shown in Fig. 1.8.a. In the power generation mode, a temperature

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load as shown in Fig. 1.8.b. More N-type and P-type thermoelectric legs can be added to a

thermoelectric module to increase the output voltage.

(a) (b)

Figure 1.8Schematic of a simple thermoelectric module in a) cooling mode, and b) power gener-ation.

As shown in Fig. 1.9, rigid TEGs are made of many thermoelectric legs connected in

series and sandwiched between two thermally conductive plates (commonly referred to

as headers) made of thermally conductive ceramics such as aluminum oxide (Al2O3) or aluminum nitride (Al N) to minimize the temperature loss due to packaging. Since effective heat dissipation from the cold side is necessary to maintain a temperature differential, a

heatsink attached to the TEG is typically used, to improve heat rejection via convection.

The maximum power generated by a TEG is obtained when it is connected to a matched

load and can be calculated using Eq. 1.3:

Pm a x

V2

O C

4RT E Ge f f

(1.3)

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(a) (b)

Figure 1.9A commercial TEG made of many N-type and P-type thermoelectric legs. a) Schematic [31]b) Real image protect[32].

resistance of the TEG. Eq. 1.3 can be re-written as Eq. 1.4:

Pm a x

z∆T2 4RT h e r m a l

(1.4)

wherez can be calculated using Eq. 1.1 and 4RT h e r m a l is the effective thermal resistance of

the TEG. Finally, the maximum conversion efficiency (ηM a x) of a TEG can be calculated

using Eq. 1.5:

ηM a xηC a r n o t

a

1`z Ta v g´1

a

1`z Ta v g` TC o l d

TH o t

(1.5)

wherez Ta v g is the average thermoelectric figure of merit of N-type and P-type

thermoelec-tric legs.

Even though there are many studies focused on improving the properties of

thermo-electric materials, the device design is just as important to realize a high performance TEG.

To obtain an enhanced performance, there are many challenges associated with the design

and engineering of TEGs. These challenges include electrical contact and interconnect

resistances, optimal spacing between the thermoelectric legs for a specific application (or

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ther-mal parasitic resistances introduced by the package, and the therther-mal contact resistances

between the device and the heat source or the cold side and the ambient[33]. Therefore, in

order to fully benefit from high efficiency thermoelectric materials, it is essential to optimize

the design of thermoelectric devices for an application. Ideally, to maximize the output

power of a TEG, the interconnects between the thermoelectric legs should have a negligible

electrical resistance compared to the electrical resistance of the thermoelectric legs (which

minimizes the total resistance of the TEG), the top and bottom substrates should be as

thermally conductive as possible (which minimizes the parasitic thermal resistances), and

the filler material between the legs should be as thermally insulating as possible (to guide

the heat flow through the thermoelectric legs). The density of the legs is determined by the

specific application, which requires an optimization between the electrical and thermal

resistances of the module.

In wearable electronics, the main advantage of TEGs over other mature energy

har-vesting technologies such as photovoltaics is the ability of the TEGs to work continuously

during a day which would be beneficial for self-powered systems that require continuous

operation. As the technologies for internet of things (IoT) continue to flourish, there is an

inevitable need for uninterrupted communication among many electronic devices,

includ-ing sensors, storage devices, and circuits. A reliable operation of connected networks and

systems depends upon power sources, which can continuously provide the required energy

levels. Given the recent developments in low-power electronic circuits and systems, using

TEGs to harvest heat as the power source is a very promising way to realize self-powered

continuous monitoring systems.

1.3

TEGs for Body Heat Harvesting

As discussed above, successful operation of a TEG relies on establishing a temperature

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much smaller temperature differentials. Given the fact that there is generally a temperature

gradient between the human body and the surrounding environment, TEGs can be utilized

to harvest the body heat and generate electric power. The first commercial device powered

by a body heat harvesting TEG was realized in 1998 by Seiko. The “Seiko Thermic” watch

solely relied on TEGs embedded in the watch as the power source[34].

Development of efficient thermoelectric materials such as Bismuth Chalcogenides

that worked at room temperature, led to the realization of TEGs as promising body heat

harvesters for powering wearable electronics. These materials possess az T value around 1 at room temperature[35]. For energy harvesting from the human body, TEGs can be

attached to various body locations to generate power. The higher the temperature at a

specific location on the body, the higher the power generated by TEGs. Fig. 1.10 shows the

temperature at different body locations for various ambient temperatures[36]. According to

Fig. 1.10, forehead, back of the neck, and chest are warmer than any other body locations.

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Using TEGs to harvest body heat however, has its own challenges, since there are several

parasitic thermal resistances involved including the contact resistance between the TEG

and the skin, the contact resistance between the TEG and the ambient, and the skin thermal

resistance. Each of these resistances also vary with ambient conditions and activity levels.

To overcome these challenges, careful material and device engineering is required.

A TEG can be represented by an equivalent electrical circuit consisting of resistors and

current/voltage sources. An analogous electrical circuit of a TEG on the human body is

shown in Fig. 1.11. The temperature difference between the skin and the ambient drops

across five thermal resistors in series including the resistance of the thermoelectric legs

in parallel with the filler material, two resistors which represent the thermal resistances

of the top and the bottom headers, and the two contact resistances which represent the

contacts between the TEG and the skin, and that between the ambient and the skin. In

order to maximize the temperature differential across the legs, the parallel combination of

the legs and filler thermal resistances should be as large as possible compared to the other

thermal resistances in the circuit.

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A number of studies focused on optimization of thermoelectric modules for body heat

harvesting. These studies include modeling the performance of TEGs on the human body

[33], studying the effects of the packaging on the performance[37–40], identifying the

impor-tance of thermal resisimpor-tances presented in the whole module[33, 41], maximizing the thermal

resistances of the active regions of TEGs[42], varying the geometry of the thermoelectric

legs[43], developing multistage TEGs[44], designing heatsinks for improved performance

[45, 46], identifying the optimum load resistance for maximum power generation[41], and

developing external circuitry for optimum performance[44, 47–49].

Some of the best rigid TEGs for heat harvesting from the human body are reported in

Table 1.1. In this table, reported open circuit voltages and corresponding power densities are

summarized. All the data reported in this table was obtained relying on natural convection

for heat rejection on the cold side.

Table 1.1Performance of some of the best rigid TEGs for body heat harvesting.

Year Power (µW/c m2) Ambient Temperature (˝C) Reference

2016 6.1 18.3 [38]

2014 2.2 22 [50]

2013 60 20 [51]

2012 28.5 23 [43]

2010 7.3 22 [52]

2007 20 22 [44]

1999 5.6 22 [34]

1.4

Summary of the Thesis

Due to exponential growth in the IoT industry, there is an interest in using renewable energy

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an abundant source of thermal energy, flexible TEGs have been explored in depth during

the recent decade for applications in wearable electronics. The main focus of this thesis is

to improve the performance of the flexible TEGs realized in this group in 2016 by F. Suarez

[53].

In this thesis, chapter 2 will cover a comprehensive review of the existing methods for

creation of flexible TEGs including both thin film and bulk fabrication techniques. For each

method, few examples will be provided and the pros and cons of each technique will be

discussed. Finally, the methods that provide the best thermoelectric performance will be

reviewed and explained as the starting point for the direction of this thesis.

Chapter 3 will discuss the work done on development and characterization of a high

thermal conductivity (HTC) elastomer, which is compatible with the first generation of

EGaIn-based flexible TEGs. This work provides a comprehensive study of thermal as well

as mechanical properties of the developed HTC elastomer made with inclusion of EGaIn

and graphene nano-platelets (GnPs) into polydimethylsiloxane (PDMS).

Chapter 4 will cover the development of the second generation TEGs made with the

developed HTC elastomer. Results from comprehensive COMSOL modeling will be

dis-cussed to clarify the role of the HTC elastomer and a metal spreader in performance of

flexible TEGs. The characteristics and performance of flexible TEGs made with both PDMS

and HTC elastomer with different thicknesses will be reported. Additionally, TEGs with

different fill factors will be compared in terms of both voltage and power output. Finally,

the TEG that provided the best power density is characterized on the human wrist at room

temperature and the characterization results are summarized.

Chapter 5 will summarize the development and characteristics of flexible heatsinks

made with the same elastomer as the top and and bottom encapsulation for improved

performance of TEGs.

Chapter 6 will cover the efforts on the modeling of the human body as the heat source

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for varying ambient conditions and activity levels. The effect of age, sex, weight, and height

of a person on the generated heat from the body was also investigated. Finally, the thermal

resistivity of the skin was calculated as a function of the ambient temperature and the

metabolic heat generated by the human body.

Finally, chapter 7 will conclude this work with a summary of the major contributions of

this thesis and directions for future studies.

Appendices A-C will summarize other studies done related to this thesis, including

steam treatment of the elastomer surface for durable electroplating, other attempts to

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CHAPTER 2

REVIEW OF THE EXISTING METHODS

FOR FABRICATION OF FLEXIBLE TEGS

2.1

Introduction

Although using commercial TEGs as body heat harvesters appears as a practical solution,

they do not provide optimal thermal contact to the human body due to their rigid nature.

Consequently, the interest in flexible TEGs that can form a conformal contact to the skin has

been rapidly growing in recent years. Aesthetically pleasing flexible TEGs can be integrated

into clothing, wristbands, armbands, or other form factors, enabling harvesting from a large

area on the body. However, it is challenging to produce flexible TEGs that can compete with

the performance of their rigid counterparts due to a variety of factors including lower quality

of the new flexible thermoelectric materials, limited electrical conductivity and flexibility

of interconnect materials, parasitic thermal resistances arising from flexible packaging

materials used as headers or as the filler material between the legs.

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ma-terials on flexible substrates such as fabrics[54–62], polydimethylsiloxane (PDMS)[63–69],

polyethylene terephthalate (PET)[70–73], and polyimide (PI)[74–81]. Since neither typical

fabrics nor flexible substrates could withstand elevated temperatures, low-temperature

deposition techniques, which did not require subsequent annealing at elevated

tempera-tures were employed. Different organic thermoelectric materials were also reported, which

showed promising results[82–88]. Finally, several groups took a different approach and

in-tegrated rigid thermoelectric legs in a flexible package[53, 89–91]. Flexible TEGs fabricated

using these techniques are reviewed in the following section.

2.2

Synthesis of Thermoelectric Materials for Flexible TEGs

2.2.1

Physical Vapor Deposition (PVD)

In 2005, the first flexible TEG created with PVD technique was demonstrated using thermal

evaporation of copper and nickel lines onto a flexible substrate[92]. While the generated

output power level was very low (16p W{c m2), this study provided a first example of a flexible TEG fabricated using conventional PVD techniques. Shortly afterwards, a flexible

TEG with the same architecture was demonstrated using sputteredB i2T e3that showed orders of magnitude improvement in output voltage[93]. Since then, a number of studies

focused on PVD techniques intended for flexible TEGs. These include, evaporation[78, 94],

ultra-low power sputtering[95], magnetron sputtering[79], and virtual cathode deposition

[80].

Kong et. al. used magnetron sputtering to deposit thin films ofB i2T e3andS e2T e3on polyimide substrates[79]. The entire device was then coated with the PDMS elastomer

for protection and mechanical strength, which also suppressed microcrack formation.

However, the effects of the parasitic thermal resistances introduced by the elastomer was not

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µV{K and 166µV{K respectively. The electrical properties of the thermoelectric films were

tested during repeated flexing cycles with a bending radius of 7m m. These measurements revealed less than 5% variation in electrical resistance after 2000 bending cycles. The thermal

conductivity andz T values were not reported for the deposited thermoelectric films in this work. A TEG fabricated using this technique with 13 pairs of thermoelectric legs produced

an output power of„700n W at a temperature differential of∆T=24˝C.

In another study, Sevilla et. al. depositedB i2T e3andS b2T e3films by ultra-low power DC sputtering. The material was deposited on a rigid silicon substrate coated with a thermally

grownSi O2layer[95]. Photolithography and etching was used to create planar legs with very large aspect ratios (2c mˆ4µm). Subsequently, Ti/Al contacts were deposited to serve as the

electrical interconnects between the thermoelectric legs. Lithography and isotropic etching

was used to form trenches in the underlying substrate, which allowed delamination of the

18µmthick device layer. The resulting TEG is shown in Fig. 2.1. Because the deposition was

carried on a Si wafer, the technique eliminated the drawbacks of flexible substrates due to

their limited thermal stability. Reported electrical resistivity values for the depositedB i2T e3 andS e2T e3films were 638µΩ.m and 2.22µΩ.m respectively. The corresponding Seebeck coefficients were and´37µV{K and 240µV{K respectively. Thermal conductivity and

z T values for the deposited films were not reported. The open circuit voltage and the corresponding output power (per thermocouple) generated by the TEG was reported to be

approximately 40m V and 0.15µW respectively at∆T “20˝C.

Morgan et.al. used virtual cathode deposition (VCD) to depositB i2T e3/G e T e films on a flexible polyimide substrate[80]. Since the technique provided deposition rates as high as

1µm{m i n at low temperatures (below 60˝C), the authors claimed its compatibility with

roll-to-roll manufacturing. The Seebeck coefficients of N-type and P-type materials were

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mechan-ical properties of the deposited films were also not reported. TEGs fabricated using this

technique exhibited an output power of 0.2n W per thermoelectric pair at a temperature differential of∆T “20˝C.

In general, the thermoelectric materials deposited using PVD techniques could not

match the properties of the best bulk materials. Furthermore, these studies suggested that

the materials were prone to crack formation since the materials are rigid and brittle in

nature.

Figure 2.1Flexible thin film TEG made with DC-sputtered thermoelectric material[95].

2.2.2

Electrochemical deposition

Electrochemical deposition (ECD), which can be used to selectively deposit thermoelectric

materials in windows defined in large-area flexible substrates has been considered by

several groups. Since ECD allows deposition of the thermoelectric legs perpendicular to the

plane of the device, stresses developed during flexing can be absorbed by the surrounding

flexible material, thus making the device less prone to crack formation.

Glatz et.al. used ECD to selectively grow copper and nickel films in windows defined in

a flexible SU-8 mold[96]. In a follow-up study, the same group used ECD to growB i2T e3, which yielded significant improved devices. The electroplating was achieved usingH T e O`

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120µm were deposited. Electrical interconnections between the thermoelectric legs were created using an electroplated Au layer. The device was tested with a bending radius of 7.5

m m, no changes in electrical resistance was reported. For 99 thermocouples, the output power of„750µW/c m2was reported atT 51.2˝C [97].

Yousef et. al. used ion-irradiation and etching to create submicron vias on a 75µmthick

polyimide foil (Kapton HN)[81]. The ion-irradiated polyimide could be selectively etched

in a sodium hypochlorite solution. A copper layer was then evaporated in the vias to act as

the seed layer for electrochemical deposition. Antimony and nickel were then selectively

deposited in the vias to form the thermoelectric legs. Evaporated copper strips were used

as the electrical interconnections between the legs. The electrical resistance of the TEG was

reported to be 54per thermocouple. The measured Seebeck coefficient of a thermocouple

was 30µV/K which is comparatively lower than those of other reported thermoelectric

materials deposited with ECD technique. The power generated by the fabricated TEG and

mechanical properties of the device were not studied.

Yamamuro et. al. used potentiostatic ECD using a nitric acid-based solution to deposit

thin films (3-4µm) ofB i2T e3andS e2T e3on a stainless steel substrate[98]. The deposited films were then transferred to a thermoplastic resin substrate using a Kapton tape.

Subse-quently, the thermoelectric legs were connected in series using Ag paste. The deposited

material suffered from small grains and cracks that developed during deposition. The

mea-sured Seebeck coefficient of N-type and P-type materials were´63µV{K and 172µV{K respectively. The electrical resistivities of the deposited materials were 79S{c mand 3.2 S{c m forB i2T e3andS e2T e3respectively. The thermal conductivity andz T values of the deposited materials were not measured in this study. An output power level of 5n W was obtained for a pair of thermoelectric legs atT=60˝C. The performance of this device

under mechanical stress was not studied.

Using the ECD technique, other thermoelectric materials were also explored to create

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polyethylene glycol terephthalate (PET) substrate using poly[Kx(Ni-ett)]as the

thermoelec-tric material[99]. A thin layer of sputtered Pt was used as the seed layer for electroplating .

thermoelectric legs with a thickness of 3µm and an area of 1.5ˆ2.5m m2were deposited on the substrate using a patterned PDMS mask. Evaporated Au electrodes were used as

the electrical interconnects between the thermoelectric legs. This was a planar device with

both hot and cold junctions on the substrate. The material was reported to have a Seebeck

coefficient and electrical conductivity of„ ´120µV{K and 250S{c mrespectively. Ther-mal conductivity andz T of the deposited material were not reported in the study. Output power of 26n W per leg was achieved at∆T “12˝C. Performance of the device under

bending condition was not studied.

Huu et. al. used a different device architecture, which employed thermoelectric legs

parallel to the plane of the substrate, while providing off-plane heat paths on both hot

and cold sides of the device using copper heat guides[100]. The TEG was made on a

sacrificial oxidized silicon substrate using pulsed electrochemical deposition ofB i2T e3 andS e2T e3 films with a thickness of 100 µm. The deposited thermoelectric materials were then annealed at a temperature of 250˝C. Stacks of Metal layers (Ti-TiN-Au-Cu) was

deposited on the thermocouples to connect them electricaly in series. Copper heat guides

were deposited on top of the device and the topside of the device was coated with a layer of

PDMS for mechanical stability.Si O2substrate was patterned and etched on the back side of the device and heat guides were deposited on this side as well. Finally, the substrate was

etched away completely and the backside of the device was coated with another layer of

PDMS. The fabricated device is shown in Fig. 2.2. The Seebeck coefficients of N-type and

P-type materials were´150µV{K and 170µV{K respectively. The electrical resistivities

of the deposited materials were 15µΩ.m (electrical conductivity of 6.67ˆ104S{m) and 25

µΩ.m(electrical conductivity of 4ˆ104S{m) forB i

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of 3µW{c m2.

The main advantage of ECD technique for fabrication of flexible TEGs is the ability of

this approach in deposition of thick films of thermoelectic materials compared to PVD

techniques.

Figure 2.2Vertical flexible TEG made with planar ECD deposited thermoelectric materials and vertical (through-plane) heat guides.[100].

2.2.3

Screen Printing

Screen printing is a low cost, simple and highly scalable technique for depositing

thermo-electric materials on flexible substrates.

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measured Seebeck coefficients of the materials were´119µV{K and 134µV{K forB i2T e3 andS b2T e3respectively. The electrical resistivities of the N-type and P-type materials were 0.35Ωc mand 0.02Ωc mrespectively. The reported electrical resistivities were higher than those of the bulk materials resulting in an output power of 48n W per thermocouple at

∆T “20˝C. Thermal conductivities,z T values, and mechanical properties of the deposited

materials were not reported.

Thermoelectric legs perpendicular to the plane of the TEG were also realized using

screen printing. Suemori et. al. created a vertical TEG using screen-printed CNT-polystyrene

composite on a polyethylene naphthalate film substrate[101]. The deposited material

consisted of 35 vol.% voids, leading to a light-weight TEG. The Seebeck coefficient and

electrical conductivity of the material were 57µV{K and 2.1S{c m respectively. A uni-polar TEG (using only N-type legs) composed of 1985 individual legs was made using this

material. Au was deposited as the electrical interconnects using PVD technique. This device

is shown in Fig. 2.3. This TEG was able to generate an output power of 5.5µW{c m2at

∆T “70˝C. No changes in electrical resistance of the TEG was observed during bending

cycles with a radius of 2mm. The properties of the screen-printed thermoelectric paste was

further improved by the addition of organic materials (PEDOT:PSS)[102]as well as using

new binding materials[103].

In another attempt, several planar devices were arranged vertically between two copper

substrates[104]. The thermoelectric material was screen-printed on a paper that showed a

Seebeck coefficient of 18µV{K and an electrical conductivity of 800S{c m. The reported thermoelectric figure of merit,z T, for this material was only 0.01 which is very low com-pared to the efficient thermoelectric materials. This TEG was capable of generating an

output power 0.1µW{c m2at∆T “50˝C.

In another approach, Iezzi et. al. created flexible TEGs using screen-printing for waste

heat recovery from steam pipes. Inexpensive metallic inks of silver and nickel were deposited

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Figure 2.3Vertical uni-polar flexible TEG made with planar screen-printed thermoelectric mate-rials. a) Schematic of the TEG. b) Fabricated TEG.[101]

Ag and Ni films were 4.5Ωm and 1Ωm respectively. Other thermoelectric properties of the films such as Seebeck coefficients, thermal conductivity, andz T were not reported. Several devices were placed vertically on a steam pipe as shown in Fig. 2.4. These TEGs

were capable of generating an output power of 0.73µW per thermocouple at∆T “127˝C.

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In another device, thermoelectric paste made ofB i2T e3andS b2T e3was pressed into a glass fabric followed by an annealing step at 530C in nitrogen ambient to improve the

thermoelectric properties of the legs[55]. Using this technique, thermoelectric materials

infiltrated the fabric and formed vertical (through-plane) legs through the substrate. The

reported Seebeck coefficient for N-type and P-type materials were -141µV{K and 98 µV{K respectively. Electrical resistivities of theB i2T e3andS b2T e3films were 0.67ˆ105 S{m and 1.5ˆ105S{m respectively. Thermal conductivies of the screen-printed materials were 1W{m K and 1.5W{m K for N-type and P-type materials leading toz T values of 0.35 and 0.3 respectively. Electrical interconnects were then screen printed on a sacrificial

silicon wafer coated with nickel, and transferred to the legs using silver paste. Finally, the

device was coated with PDMS for mechanical integrity. This device showed orders of

mag-nitude improvement in the output power compared to the previous screen printed devices

due to a significant reduction in the total electrical resistance of the device („0.35), as well as lower thermal conductivity of the thermoelectric material („1 W/mK). Worn on the wrist, the TEG showed a power density of 1µW{c m2at an ambient temperature of 15˝C. Further improvement on the same device was achieved by using ball-milled ternary

bulk thermoelectric materials, as well as replacing the silver paste with a gold paste as

the bonding material[106]. Another improvement on this device was reported later by

adopting a post-annealing step at 500˝C followed by application of an external mechanical

pressure to further densify the screen printed thermoelectric material for improved

ther-moelectric properties[107]. With these modifications as well as altering the composition

of thermoelectric materials, the group was able to reachz T values of 0.89 and 0.57 for screen-printed P-type (B i0.5S b1.5T e3) and N-type (B i2S e0.3T e2.7) materials respectively. The same group also reported an improvement on the thermoelectric properties of the screen

printed N-typeB i2S e0.3T e2.7(z T “0.9) by annealing the material in a hydrogen-rich am-bient[108]. However, the hydrogen annealing step was later found to be detrimental to

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in reduced reliability and reproducibility of the material[109]. This problem was resolved

later by adding different amounts of tellurium to the thermoelectric paste and optimizing

the doping concentration. A flexible TEG made with the screen printing technique is shown

in figure Fig. 2.5.

Figure 2.5Flexible TEG made with screen-printed thermoelectric material[106].

2.2.4

Dispenser Printing

Dispenser printing is a technique that allows depositing and patterning of materials without

using a mold. This method is attractive for the fabrication of flexible TEGs due to its low

cost. Dispenser printing of thermoelectric materials was first used to printB i2T e3powder mixed with a polymer binder into the holes in a PDMS substrate[64]. The electrical contacts

were created later by using flexible PCBs attached to the top and the bottom of the TEG.

Thermoelectric properties of the deposited materials such as electrical resistivity, thermal

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successful in creation of a dispenser-printed TEGs, its performance was limited due the

high electrical resistance of the device (170) leading to an output power density of 0.08

µW{c m2atT 19˝C. Number of thermoelectric legs used in the device and their sizes

were not reported. This technique was also later explored using fabric [110]as well as

polyimide[111]substrates. The output voltage generated by these two TEGs was significant

(tens of milli-volts); however, due to very high electrical resistances, both devices suffered

from output power levels less than a nano-watt for each thermoelectric pair.

An effort to use commercially available fabrics to create flexible TEGs led to the

realiza-tion of a silk-based TEG in 2016[54]. Using dispenser printing technique, N-type (B i2T e3) and P-type (S b2T e3) thermoelectric pastes were deposited onto the holes created in a silk sheet to form vertical columns of thermoelectric materials. The Seebeck coefficients for

N-type and P-type materials were -57µV{K and 111µV{K respectively. Other thermo-electric properties were not reported for the printed materials. The thermothermo-electric legs

were connected by Ag foil and Ag paste to achieve an electrically continuous device. In this

device, the high contact resistance between the legs and silver paste (with a total device

resistance of„7 k), as well as the low density of the thermoelectric materials embedded

in the silk fabric resulted in a TEG with poor performance. The TEG reported in this work

was fabricated with 11 thermocouples, and generated an output power of 15n W for a temperature differential of∆T “35˝C. The device showed less than 10% increase in the

electrical resistance during 100 bending cycles.

In another study, a mixture of thermoelectric powders (B i2S e0.3T e2.7orB i0.5S b1.5T e3) and a stabilizer solvent was deposited on a Kapton substrate using ink-jet printing (Fig. 2.6)

[75]. This method was an attractive way to use ink-jet printers for fabrication of flexible

TEGs. Using this technique, N-type and P-type thermoelectric materials showed Seebeck

coefficients of -140µV{K and 177µV{K respectively. Thermal conductivities andz T values of the thermoelectric materials were not reported. Electrical conductivities of N-type

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values (compared to those of bulk materials) for both electrical conductivity and Seebeck

coefficient were attributed to the low density of the deposited films.

In conclusion, the main issue associated with dispenser printed flexible TEGs is the

high electrical resistances of the thermoelectric materials which requires further studies

for improvement.

Figure 2.6Flexible TEG made with ink-jet printed thermoelectric material[75].

2.2.5

Organic and Solution-processable Materials for Fabrication of TEGs

There is an interest in developing organic-based thermoelectric element due to their

flexi-bility, low cost, low thermal conductivity, and non-toxicity. There are many studies focused

on understanding and improving the properties of organic thermoelectric materials[84,

112–117]. Solution based processes can be used to deposit organic materials onto various

flexible substrates[117].

The first organic-based TEG was made using poly (3, 4-ethylenedioxythiophene)

polysty-rene sulfonate (PEDOT:PSS), which is a P-type material, with an electrical conductivity

of 300S{c m, a Seebeck coefficient of 780µV{K (after exposing the polymer to tetrakis

(dimethylamino) ethylene vapor), a thermal conductivity of 0.3W{m K, and az T of 0.25 at room temperature[117]. For TEG fabrication, an organic conducting salt,

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the N-type material (with a Seebeck coefficient of -48µV{K and an electrical resistivity of 11

kΩ{c m). The TEG was fabricated on an SU-8 mold using Au as the electrical interconnects.

The fabricated TEG was capable of generating an output power of 0.128µW (25 nW/c m2) at∆T “10˝C. While the initial thermoelectric performance was encouraging, the oxidation

of the organic material over time caused appreciable degradation in the device properties.

Massonnet et. al. reported fabrication of a flexible TEG using organic materials to

create a flexible heat flux sensor[115]. In this study, organic thermoelectric materials were

drop-casted onto a temporary polymer substrate. After drying, the thermoelectric legs were

transferred to a flexible substrate. The initial Seebeck coefficient measured for the deposited

thermoelectric material (PEDOT:PSS layer) was 70µV{K which rapidly decreased over

time and reached a value of„40µV{K after 400 hours. These materials are currently at their infancy and much work has to be done before they can be a viable alternative to

conventional materials[94].

Another organic-based TEG was realized in 2015 by dip-coating of a polyester fabric in a

PEDOT:PSS solution[56]. A larger plain polyester substrate was used as a platform to attach

the coated polyester strips. The electrical interconnects between the strips were created

subsequently using woven silver wires. The TEG made with five thermoelectric strips (with

an area of 40m m ˆ5m m for each strip) was able to generate an output power of 12.29 n W at∆T “75.2˝C. As expected, the main limitation to the performance of the device was

the planar orientation, making it challenging for integration into wearable systems where

the temperature gradient is usually vertical to plane of the device. Other disadvantages

of the TEG include the use of single P-type legs (a unipolar TEG), poor thermoelectric

characteristics of the material (with a Seebeck coefficient of„15µV{K), and high electrical

resistance of the device („2500).

Solution-processable materials can also be considered to create flexible TEGs. Examples

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S{m, and N-typeW S2nanosheet with a Seebeck coefficient of -75µV{K and an electrical conductivity of 1.4ˆ103S{m) spin coated on top of an octadecyltrichlorosilane

(OTS)-treated silicon substrate, transferred to a PDMS layer, and connected electrically by Ag and

Au wires[65]; conductive PEDOT:PSS as the P-type and indium tin oxide (ITO) coated with

PEDOT:PSS as the N-type materials (with an average Seebeck coefficient of„38µV{K and an average electrical resistance of 1.7 kper N-P pair) (Fig. 2.7)[118]; poly(vinyl alcohol)

(PVA) gel electrolyte doped with ferric chloride and potassium ferricyanide (with an average

Seebeck Coefficient of„1.1m V{K and an average electrical conductivity of 0.008S{c m) [119]; a solution ofT i S2nanosheets exfoliated from layered polycrystalline powders, andc60 nanoparticles as a hybrid material as N-type thermoelectric legs (with a maximum Seebeck

coefficient of 115µV{K and a maximum electrical conductivity of 480 S/cm) deposited on a polyimide substrate (Fig. 2.8)[120].

Figure 2.7A flexible and transparent TEG made with PEDOT:PSS and ITO-PEDOT:PSS[118].

All of these studies demonstrated the feasibility of creation of organic-based as well

as solution-processable flexible TEGs. However, more work is needed to either lower the

(48)

Figure 2.8A flexible TEG made withC60{T i S2nanosheets[120].

2.2.6

Fabrics

For heat harvesting from the human body,TEGs with larger surface areas are desirable .

Therefore, TEGs made on a fabric platform are attractive since they can be worn comfortably

while covering a large area on the human body.

In 2002, Yamamoto et. al. realized the first fabric-based flexible TEG using Knitted metal

wires[121]. The TEG was constructed with knitted chromel and Alumel thermocouple wires

through a perforated glass-based resin. This TEG was capable of generating an output

power as high as 0.166µW (for a thermocouple pair) at∆T “26˝C.

In 2012, Hewitt et. al. created a fabric-like flexible TEG using alternating stacks of doped

multi-wall carbon nanotube (MWCNT)/polyvinylidene fluoride (PVDF) sheets[122]. The

Seebeck coefficient measured for a stack made of 72 layers of fabric was 550µV{K. This fabric-based device (with an area of„40c m2) showed an electrical resistance of 1270 leading to a maximum output power of 137n W at∆T “50˝C. While promising, the main

disadvantage of this TEG was the planar orientation of the thermoelectric legs with respect

to the substrate, which made it challenging to integrate the device into clothing where the

heat flow from the hot side to the cold side is perpendicular to the orientation of the legs.

In another work, Yadav et. al. created a flexible fiber coated with alternating P-type and

N-type thermoelectric segments as a single TEG[94]. In this study, Ni and Ag electrodes

Figure

Table 1.1Performance of some of the best rigid TEGs for body heat harvesting. 14
Figure 1.1 a) Thomas J. Seebeck. b) An illustration of the original apparatus used by ThomasSeebeck which led to the discovery of the thermoelectric effect
Figure 1.2 a) Jean Charles Peltier. b) William Thompson (Lord Kelvin).
Figure 1.4 Dependence of electrical conductivity,thermal conductivity, Seebeck coefficient, andthermoelectric figure of merit, zT , on the carrier concentration for a typical thermoelectricmaterial [9] .
+7

References

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