Study of Chalcogenide Based Thermoelectric Materials for Waste Heat Recovery

University of Wollongong Research Online

University of Wollongong Thesis Collection 2017+ University of Wollongong Thesis Collections

2018

Study of Chalcogenide based Thermoelectric Materials for Waste Heat Recovery

Sheik Md Kazi Nazrul Islam University of Wollongong

Follow this and additional works at: https://ro.uow.edu.au/theses1

University of Wollongong Copyright Warning You may print or download ONE copy of this document for the purpose of your own research or study. The University does not authorise you to copy, communicate or otherwise make available electronically to any other person any copyright material contained on this site. You are reminded of the following: This work is copyright. Apart from any use permitted under the Copyright Act 1968, no part of this work may be reproduced by any process, nor may any other exclusive right be exercised, without the permission of the author. Copyright owners are entitled to take legal action against persons who infringe their copyright. A reproduction of material that is protected by copyright may be a copyright infringement. A court may impose penalties and award damages in relation to offences and infringements relating to copyright material. Higher penalties may apply, and higher damages may be awarded, for offences and infringements involving the conversion of material into digital or electronic form. Unless otherwise indicated, the views expressed in this thesis are those of the author and do not necessarily represent the views of the University of Wollongong.

Recommended Citation Nazrul Islam, Sheik Md Kazi, Study of Chalcogenide based Thermoelectric Materials for Waste Heat Recovery, Doctor of Philosophy thesis, Institute for Superconducting and Electronic Materials, University of Wollongong, 2018. https://ro.uow.edu.au/theses1/347

Research Online is the open access institutional repository for the University of Wollongong. For further information contact the UOW Library: [email protected]

Institute for Superconducting and Electronic Materials

Study of Chalcogenide based Thermoelectric Materials for Waste Heat Recovery

SHEIK MD KAZI NAZRUL ISLAM

This thesis is presented as part of the requirements for the Award of the Degree of ‘Doctor of Philosophy’ of the University of Wollongong

March 2018

CERTIFICATE OF RESEARCH

This is to certify that the work presented in this thesis is carried out by the author under the supervision of Dr. Xiaolin Wang, Professor of the Institute for Superconducting and Electronic Materials, Australian Institute for Innovative Materials, University of Wollongong, Australia.

Senior Professor Xiaolin Wang Sheik Md Kazi Nazrul Islam

DECLARATION

I, Sheik Md Kazi Nazrul Islam, declare that this thesis, submitted in fulfilment of the requirements for the award of Doctor of Philosophy, in the Institute for Superconducting and Electronic Materials, Australian Institute for Innovative Materials, University of Wollongong, is wholly original work unless otherwise referenced or acknowledged. This thesis has not been submitted for qualifications at any other academic institution.

Sheik Md. Kazi Nazrul Islam March 2018

ABSTRACT

When fossil fuel resources are becoming exhausted due to huge consumption, followed by the environmental concerns according to IEA Key World energy statistics published in 2017, sustainable and environment friendly equivalent energy resources are desperately required for power generation. Thermoelectric (TE) devices have attracted extensive interest as power generators (through temperature gradient into electrical energy based on “Seebeck effect”) or cooling devices (current flows through a conducting circuit, heat is either absorbed or generated based on

“Peltier effect”). Despite relatively low conversion efficiency and the high cost of TE materials, TE could be a promising alternative for power generation after being resolved some features which are under development. TE generators (TEGs) have the ability to harvest useful electrical energy from waste heat if cheap, chemically stable and efficient novel materials can be developed.

My doctoral research includes methods of making Cu2Se based thermoelectric materials, improving their TE performance by using nanotechnology engineering, and study of their chemical and physical properties.

The thesis is divided into six chapters. Chapter 1 includes background and literature review. Chapter 2 outlines experimental details including both sample fabrications and characterizations. Chapter 3 discusses carbon doping effects on TE properties of

Cu2Se using different types of carbon sources such as carbon nanotube, graphite, hard carbon, diamond carbon fiber, and Super P. Chapter 4 demonstrates a systematic study of boron doping effect on the Cu2Se using spark plasma sintering iii method. Chapter 5 represents the various characterizations of carbon fiber incorporated Cu2Se to further understand the mechanism for the enhanced TE performance. Chapter 6 outlines the motivation of using liquid carbon source to dope into Cu2Se in order to achieve homogeneously mixing of carbon with Cu2Se for further enhancement of thermoelectric performance. The overall conclusion of the thesis is given in Chapter 7.

In chapter 3 I demonstrate that incorporating a small weight fraction of carbon using various carbon sources can significantly enhance the zT of Cu2Se at both middle and high temperatures. All the carbon-doped Cu2Se samples exhibit weak temperature dependent zT higher than 1.0 over a broad temperature range from 600 to 900 K, with the 0.6wt% Super P doped Cu2Se sample achieving a zT of 1.85 at 900 K.

Furthermore, the 0.3wt% carbon fiber doped Cu2Se shows zT > 1 for T > 520 K and reaches a record level of zT of ~ 2.4 at 850 K. These values for the carbon doped

Cu2Se are comparable or superior to those for the current state-of-the-art thermoelectric materials. Microstructure studies on graphite incorporated Cu2Se revealed that graphite nanostructures interspace between Cu2Se nanoscale grains being responsible for the significantly enhanced zT. The low thermal conductivity in the nanostructured composite is attributed to the high density of interfaces caused by the small grain diameters (30 - 60 nm), along with the strong acoustic mismatch between Cu2Se and carbon phonon states which enhances the thermal boundary resistance. This discovery indicates strong prospects for engineering carbon thermoelectric nanocomposites for a range of energy applications.

In chapter 4, I report on the realization of a record high zT of up to 2.6 in a carbon fiber doped Cu2Se composite fabricated using the spark plasma sintering method. A comprehensive investigation on atomic scale microstructures and crystal structures up to

1000 K, as well as thermoelectric performance, was conducted using high resolution transmission electron microscopy, high-temperature X-ray diffraction, and Seebeck, electrical, Hall effect, and thermal conductivity measurements. The origin of ultrahigh zT is that the carbon fiber addition creates a large density of boundaries, which remarkably reduces the thermal-conductivity by up to 60% through blocking the phonon transport, while the Seebeck coefficients and electrical conductivity change only slightly with doping. As a result, 0.75wt% carbon fiber doped Cu2Se composite exhibits the record zT of ~2.6 and an estimated ultra-high efficiency of 31%. Our results represent a remarkable advance in thermoelectric performance and hold great potential for intermediate temperature applications in thermoelectric devices, which will benefit the current thermoelectric industry.

In chapter 5, I demonstrate that insulating-boron nano-particles inclusion in Cu2Se has little effect on overall power factor, but can significantly reduce the thermal conductivity, resulting in great improvement in zT, by a factor of 1.6-2.6 over a wide range of temperature compared to undoped samples. Microstructure studies by high resolution transmission electron microscopy revealed that boron nanostructures interspaced between Cu2Se microscale grains are responsible for the great reduction in thermal conductivity and, in turn, the significantly enhanced zT. High-temperature synchrotron X-ray diffraction experiments show little change to the lattice from nano-boron doping as compared to undoped Cu2Se. The enhancement of thermal boundary resistance ascribed to the strong acoustic mismatch between Cu2Se and

v boron is responsible for the low thermal conductivity of the microstructured composite. Our findings offer an effective approach of using insulating nanoparticles to significantly improve Seebeck coefficient and significantly reduce lattice thermal conductivity for achieving high ZT in Cu2Se and many other types of thermoelectric composites.

In chapter 6, I used grape juice as a liquid source of carbon to investigate its effects on crystal structures, lattice parameters, microstructures, chemical stability using both scanning transmission electron microscopy (STEM) and synchrotron high- temperature X-ray diffraction (XRD) analysis. The motivation of using grape juice is to produce carbon and mix with Cu2Se uniformly and homogeneously during sample sintering process. Results show that the electrical conductivity of 0.60 wt% G doped

Cu2Se samples is significantly increased compared to undoped Cu2Se while keeping thermal conductivity unchanged. This has led to great enhancement of figure of merit by a factor of ~2.9 to ~2.2 over the temperature ranges from 400 to 850 K, respectively. From the microstructure study, I observed carbon nano-particles embedded inside Cu2Se grains and CuO nanoparticles are located at the interface between carbon and Cu2Se. It is proposed that the formation of CuO induces Cu deficiencies in Cu2Se resulting in greater enhancement of electrical conductivity which in turn significantly enhances the ZT.

ACKNOWLEDGEMENTS

First and foremost, all praise and gratitude to Allah, the Almighty, for giving me the strength and patience to complete this thesis work.

I would like to express my profound indebtedness and heartfelt gratitude to my supervisor Prof Xiaolin Wang for his valuable suggestions, constant guidance, great encouragement and kind in various ways during my PhD study at the Institute for

Superconducting & Electronic Materials (ISEM), the University of Wollongong

(UOW), Australia.

I am thankful for my International Postgraduate Tuition Award and University

Postgraduate Award from the University of Wollongong, Australia and Prof Xiaolin

Wang’s Faculty Scholarship of financial support towards finishing my Ph.D.

I would like to express my appreciation to Dr. Mitchell Nancarrow, Mr. Tony

Romeo, Dr. David R. G. Mitchell, Dr. Gilberto Casillas, Dr. Tomas Katkus, Dr.

Dongqi Shi, Dr Germanas Peleckis, Dr Sima Aminorroaya Yamini for their contributions to providing trainings and maintenance of relevant instruments and for doing measurements for some samples. My special thanks is owed to Dr. Tania

Silver for critical reading of the technical papers.

I would like to thank my dear friends and colleagues including Dr Mohammad

Rejaul Kaiser, Dr. Lanling Zhao, Dr David L. Cortie, Dr Majharul Haque Khan, Dr.

vii

Zhi Li, Ms. Lijuan Zhang, Mr. Meng Li, Mr. Zhenwei Yu, Mr. Frank Fei Yun, Mr.

Shohanur Mahmud, Dr. Feixiang Xiang, Mr, Guangsai Yang, Ms. Lina Sang, Dr.

Zengji Yue, Dr. Monirul Islam, Dr. Mansur Ahmed, Mr. Shaon Barua, Dr. Shazed

Aziz, Mrs. Faizunnesa, Mr. Al Jumlat Ahmed and other students and staff in ISEM.

This thesis could not have been completed without the contributions from my parents and my brother and sisters. They continuously encouraged me to obtain this higher degree. Their unfailing supports and advice helped me to accomplish this study.

viii

TABLE OF CONTENTS

CERTIFICATE OF RESEARCH ...... i

DECLARATION ...... ii

ABSTRACT ...... iii

ACKNOWLEDGEMENTS ...... vii

TABLE OF CONTENTS ...... ix

LIST OF FIGURES ...... xiv

LIST OF TABLES ...... xxvi

LIST OF ABBREVIATIONS ...... xxvii

Chapter 1 ...... 1

1. Introduction ...... 1

1.1. Background ...... 1

1.2. Literature and technology review ...... 3

1.2.1. Working principles of thermoelectric devices ...... 3

1.2.2. Seebeck Effect ...... 3

1.2.3. Peltier Effect ...... 5

1.2.4. Thermoelectric Figure of Merit ...... 6

1.2.5. Coefficient of performance (COP) ...... 9

1.3. Advantages offered by thermoelectric technology compared to alternative

technologies ...... 9

1.4. Review of applications of TE and assessment of their economic value .... 10

1.4.1. Application of the Seebeck effect: Power generation ...... 10

1.5. History and Development ...... 15

1.6. Figure of Merit in TE Materials ...... 17

1.7. Conflicting issues and strategies to improve ZT ...... 19

1.7.1. Carrier concentration tuning ...... 20

1.7.2. Motivations for nanotechnology in thermoelectricity ...... 24

1.8. Representative of high performance thermoelectric materials ...... 25

1.8.1. Skutterudites ...... 25

1.8.2. Clathrates ...... 28

1.8.3. Mg2Sn ...... 31

1.8.4. Chalcogenide compounds ...... 33

1.8.5. Bismuth Telluride (Bi2Te3) ...... 34

1.8.6. Copper selenide (Cu2Se) ...... 36

1.8.6.5. Thermoelectric properties of copper selenide with ordered

selenium layer and disordered copper layer ...... 44

1.8.6.6. Superior intrinsic thermoelectric performance in single-crystal

and melt-quenched highly dense Cu2-xSe bulks ...... 45

1.8.6.7. The effects of Te2− and I− substitutions on the electronic

structures, thermoelectric performance, and hardness in melt-quenched

highly dense Cu2-XSe ...... 46

1.9. References ...... 48

Chapter 2 ...... 61

2. Experimental methods ...... 61

2.1. Materials fabrication ...... 61

2.2. Spark plasma sintering (SPS) method ...... 61

2.3. Melt solidification technique ...... 61

2.4. Materials characterisation techniques ...... 62

2.4.1. Sample preparation ...... 62

2.4.2. X-ray powder diffraction (XRD) ...... 63

2.4.3. Scanning electronic microscopy ...... 64

2.4.4. Scanning transmission electronic microscopy ...... 65

2.4.5. Electrical transport measurements ...... 65

2.4.6. Thermal transport measurements ...... 66

Chapter 3 ...... 71

3. Significant enhancement of figure-of-merit in carbon-reinforced Cu2Se nanocrystalline solids ...... 71

3.1. Introduction ...... 71

3.2. Experimental Section ...... 73

3.2.1. Synthesis ...... 73

3.2.2. Measurements ...... 73

3.3. Results and discussions ...... 74

3.4. Conclusions: ...... 94

3.5. References: ...... 96

Chapter 4 ...... 102

4. Giant enhancement of the figure-of-merit over a broad temperature range in nano-boron incorporated Cu2Se ...... 102

4.1. Introduction ...... 102 xi

4.2. Sample fabrication ...... 104

4.2.1. Measurement details ...... 104

4.3. Results and discussions ...... 105

4.4. Conclusion ...... 124

4.5. References ...... 125

Chapter 5...... 130

5. Ultra-high figure-of-merit of 2.6 in carbon fiber doped Cu2Se fabricated using spark plasma sintering method ...... 130

5.1. Introduction ...... 130

5.2. Sample fabrications ...... 131

5.2.1. Thermoelectric measurements ...... 132

5.2.2. Temperature-dependent X-ray diffraction measurements ...... 133

5.3. Results ...... 133

5.3.1. Sample preparation and characterization: ...... 133

5.4. Thermoelectric measurements and performance: ...... 141

5.5. Analysis and discussion ...... 143

5.6. Conclusions: ...... 146

5.7. References ...... 147

Chapter 6...... 152

6. Enhancement of thermoelectric performance of Cu2Se using a liquid source of carbon ...... 152

6.1. INTRODUCTION ...... 152

6.2. Experimental details ...... 153

6.2.1. Synthesis: ...... 153 xii

6.2.2. Measurements ...... 154

6.3. RESULTS AND DISCUSSION: ...... 155

6.4. Conclusions ...... 166

6.5. Reference ...... 168

Chapter 7 ...... 172

7. General conclusions and outlook ...... 172

7.1. General conclusions ...... 172

7.2. Outlook ...... 175

xiii

LIST OF FIGURES

Figure 1-1 (a) World total primary energy supply by fuel (Mtoe); (b) World CO2 emissions by fuel (Mt of CO2) [IEA Key World energy statistics 2017] ...... 1

Figure 1-2 Schematic diagram showing the working of a thermoelectric generator

(Seebeck effect) ...... 4

Figure 1-3 Peltier effect ...... 5

Figure 1-4 Principle of Radioisotope thermoelectric generators ...... 11

Figure 1-5 Energy losses in a car (internal combustion engine) and the TE module associated to a car’s exhaust pipe ...... 12

Figure 1-6 DC to DC converter (Step up) ...... 14

Figure 1-7 Schematic representation of the current state of the figure of merit obtained in bulk thermoelectrics ...... 16

Figure 1-8 Thermoelectric figure of merit with respect to temperature for varies bulk materials; (a) n-Type, (b) p-Type ...... 19

Figure 1-9 ZT vs temperature of the state-of-the-art TE materials highlighting up-to- date improvements in ZT values above 1 ...... 20

Figure 1-10 Evaluation of the ZTmax with respect to time. The blue dots represent materials used for cooling application while the red triangles for power generation. 21

Figure 1-11 Optimization of carrier concentration for maximizing ZT) ...... 22

Figure 1-12 The figure of merit of Ca5ZnxIn2−xSb6 samples a) within the measured temperature range. b) ZT is optimised at 700 K at 2 × 1020 cm−3...... 23

Figure 1-13 Different types of Point defect in a material ...... 24

xiv

Figure 1-14 TE materials structure in different scales demonstrated in a nutshell. (a– d) Different dimensionalities: (a) bulk (3D), (b) thin ﬁlm (2D-30nm), (c) nanowire

(1D-100nm), (d) atomic cluster (0D-quantum dots). (e–h) variation of grain: (e) micron sized grained only, (f ) coarse and fine grained, (g) nanosized grained, (h) amorphous...... 24

Figure 1-15 Phonon scattering effect in a void in Skutterudites ...... 26

Figure 1-16 The CoSb3 Skutterudites structure shown the void cages filled with guest atoms ...... 26

Figure 1-17 Phonon engineering by internal Nano-structuring (NdyFe4Sb12) Some single, double and triple-filled skutterudites have demonstrated extremely low lattice thermal conductivities...... 28

Figure 1-18 Partially filled based on Co4Sb12 ...... 29

Figure 1-19 Clathrates crystal structure (type I). Two pentagonal dodecahedra (E20) and six tetrakaidecahedra (E24) contained in the cubic unit cell...... 30

Figure 1-20 Temperature dependent of (a) figure of merit and, (b) thermal conductivity for Ba8Ga16Ge30, Hf0.75Zr0.25NiSn0.975Sb0.025, 훽-Zn4Sb3, Zintl phase

Yb14MnSb11, and PbTe...... 30

Figure 1-21 crystal structure of Mg2Sn and Mg2Sn0.75Ge0.25 ...... 31

Figure 1-22 Thermoelectric transport (electrical) properties of Mg2Sn1−xGex with respect to temperature: (A) Electrical resistivity (B) Seebeck coefficient. (C) Power factor (PF). (D) PF of Mg2Sn0.75Ge0.25 from different sample batches...... 32

Figure 1-23 Thermoelectric transport (thermal) properties and ZT values of undoped and Ge doped Mg2Sn with respect to temperature (A-B) Thermal conductivity, (C )

Heat capacity, (D) Figure of merit compared with the Bi2Te2.7Se0.3 ...... 33

Figure 1-24 Bismuth Telluride (Bi2Te3) crystal structure ...... 35

Figure 1-25 Temperature dependent of ZT for the Bi2Te3 compounds with different fabrication methods...... 36

Figure 1-26 Sextuple layers of Se-Cu-Cu-Cu-Cu-Se are repeating at low temperature monoclinic Cu2Se phase...... 37

Figure 1-27 Cu2Se cubic (β-phase) unit cell at high temperatures above 400 K ...... 38

Figure 1-28 Two types of atomic vibration that carry heat: (a) A row of atoms at rest

(b) Longitudinal waves (c) Transverse waves. (Source: inventor.grantadesign.com) 38

Figure 1-29 The arrows indicate that the Cu ions can freely travel among the interstitial sites in Cu2 -xSe compounds with a cubic antifluorite structure...... 39

Figure 1-30 Temperature dependent thermoelectric transport properties of the Cu2Se and Cu1.98Se. (a) electrical resistivity (b) Seebeck coefficient (c) thermal conductivity

(d) figure of merit (ZT) [...... 41

Figure 1-31 Thermoelectric transport properties of the iodine doped Cu2Se with respect to temperature. (a) figure of merit (ZT) (b) electrical resistivity (𝜌) (c)

Seebeck coefficient (S) and (d) thermal conductivity (κ)...... 42

Figure 1-32 Thermoelectric transport properties with respect to temperature of the

Ag doped Cu2Se. (a) Figure of merit (ZT); (b) electrical conductivity (σ); (c) Seebeck coefficient (S); (d) thermal conductivity (κ); and (e) Lattice thermal conductivity ( ).

...... 43

Figure 1-33 Temperature dependent TE transport properties of Cu2Se. figure of merit

(ZT)...... 44

Figure 1-34 Temperature dependent TE transport properties of Cu2Se. (b) Electrical resistivity (𝜌); (c) Seebeck coefficient (S); (d) Thermal conductivity (κ); and (e)

Lattice thermal conductivity ( )...... 45

xvi

Figure 1-35 TE transport properties showing the (a) electrical resistivity (ρ), (b)

Seebeck coefficient (S), (c) total thermal conductivity (κ), (d) lattice thermal conductivity ( ), (e) power factor (PF), and (f) thermoelectric figure-of-merit (zT)

of the ultrafast formed Cu2-xSe and single crystal sample...... 46

Figure 1-36 Temperature dependent TE transport properties of the tellurium and

Iodine substituted Cu2Se. (a) figure of merit (ZT) (b) Electrical conductivity (σ); (c)

Seebeck coefficient (S); (d) Thermal conductivity (κ); and (e) Lattice thermal conductivity ( )...... 47

Figure 2-1 (a) Spark Plasma Sintering (Thermal Technology SPS model 10-4); (b)

Oxy-Acetyline Kit; (c) Vertical Mosilli furnace...... 62

Figure 2-2 (a) Struers accutom 50 (inset: after cut sample); (b) Struers Citropress 25;

Figure 2-3 (a) GBC MMA XRD measurements; (b) High temperature synchrotron

XRD (10-BM-1)...... 64

Figure 2-4 (a) SEM (JEOL JSM-6490LV); (b) FE-SEM (JEOL JSM-7001F); (c) FIB

(FEI Helios G3 CX) ...... 66

Figure 2-5 (a) Seebeck measurements (RZ 2001i), (b) Heat capacity measurements

(Differential Scanning Calorimetry - NETZSCH DSC 204 F1 Phoenix), (c) Thermal diffusivity measurements (Linseis Laserflash - LFA 1000) ...... 68

Figure 2-6 Schematic illustration of the Linseis LFA 1000 thermal diffusivity instrument showing the operation in the vertical mode...... 69

Figure 2-7 Physical properties measurement system (Quantum Design PPMS-9) ... 70

Figure 3-1 Schematic representation of the fabrication process of the graphite doped

Cu2Se samples(a), crystal structure for the low temperature α-phase Cu2Se (b) and X- ray diffraction patterns(c) for hexagonal structured carbon, monoclinic structured xvii

Cu2Se, and as-fabricated carbon doped Cu2Se samples with different carbon sources such as graphite(G), super P(SP), carbon nanotubes(CNT), and hard carbon (H). ... 75

Figure 3-2 Low magnification TEM image for the fabricated graphite doped Cu2Se

(0.8wt%) bulk sample(a), and high resolution TEM images showing the lattice and interface between graphite nanoclusters and Cu2Se ...... 76

Figure 3-3 Fractured surface morphology and EDS mapping for the Cu, Se, and C in the fabricated 8wt% graphite doped Cu2Se sample, respectively...... 78

Figure 3-4 Temperature dependence of the (a) electrical conductivity (), (b)

Seebeck coefficient (S), (c) thermal conductivity (κ), and (d) figure-of-merit (zT) of typical Cu2Se samples with a carbon doping level of 0.6wt% from various carbon sources, and 0.3wt% in the case of carbon fibers. The zT values for the state-of-the- art p-type middle temperature thermoelectric materials of PbTe and PbTeSe are provided for comparison...... 79

Figure 3-5 Field emission scanning electron microscope (FE-SEM) images of the commercial (a, e) carbon nanotubes, (b) graphite, (c) hard carbon, and (d, f) super P.

...... 81

Figure 3-6 Temperature dependence of the electrical conductivity (), Seebeck coefficient (S), thermal conductivity (κ), and figure-of-merit (zT) of typical samples with carbon doping level of 0.1wt% from various carbon sources...... 82

Figure 3-7 Temperature dependence of the electrical conductivity (), Seebeck coefficient (S), thermal conductivity (κ), and figure-of-merit (zT) of typical samples with carbon doping level of 0.2wt% from various carbon sources...... 83

Figure 3-8 Temperature dependence of the electrical conductivity (), Seebeck coefficient (S), thermal conductivity (κ), and figure-of-merit (zT) of typical samples with carbon doping level of 0.4wt% from various carbon sources...... 84

xviii

Figure 3-9 Temperature dependence of the electrical conductivity (), Seebeck coefficient (S), thermal conductivity (κ), and figure-of-merit (zT) of typical samples with carbon doping level of 0.8wt% from various carbon sources...... 85

Figure 3-10 Temperature dependence of the electrical conductivity (), Seebeck coefficient (S), thermal conductivity (κ), and figure-of-merit (zT) of carbon fiber doped samples...... 86

Figure 3-11 Temperature dependence of the (a) /0.0, (b) S/S0.0, (c) κ / κ 0.0, and (d) zT/zT0.0 for the carbon doped Cu2Se samples with a doping level of 0.6wt% from various carbon sources such as CNTs, graphite, hard carbon and super P, and 0.3wt% in the case of carbon fibers...... 87

Figure 3-12 Schematic illustration showing the microstructure and phonon transportation in the graphite doped Cu2Se bulks...... 90

Figure 3-13 Temperature dependence of the electrical and thermal transport properties over four repeated trials for a 0.3wt% CF doped Cu2Se sample: (a) electrical conductivity (); (b) Seebeck coefficient (S); (c) thermal diffusivity (D). 91

Figure 3-14 Temperature dependence of the power factor for typical Cu2Se samples with 0.1wt%, 0.2wt%, 0.4wt%, and 0.8wt% doping levels from various carbon sources...... 92

Figure 3-15 Temperature dependence of the thermoelectric compatibility factor (s) for the melt-solidified un-doped and carbon doped Cu2Se samples with different carbon sources and doping levels...... 93

Figure 3-16 Hardness of the melt-solidified selected carbon doped Cu2Se polycrystalline samples...... 94

Figure 4-1 (a) Schematic illustration showing the formation mechanism of the B- doped Cu2Se bulks with the fabrication process. (b) Low-temperature monoclinic

xix phase (left) and high-temperature cubic phase (right) of the Cu2Se crystal structure.

...... 106

Figure 4-2 (a) Schematic illustration of crystal structure of α-phase Cu2Se, and (b) X- ray diffraction patterns for pure Cu2Se and boron doped Cu2Se, along with (c) enlarged peaks for the undoped and boron doped Cu2Se samples e samples (x = 0, 0.14, 0.28, and 0.42) at room temperature...... 107

Figure 4-3 Synchrotron radiation X-ray diffraction (SR-XRD) profiles of (a) pure

Cu2Se and (b) 0.42 wt% boron doped Cu2Se obtained upon heating to different temperatures; (c) Temperature dependence of the heat capacity (Cp) for the undoped and boron doped Cu2Se samples; (d) XRD patterns for Cu2Se/0.42wt% boron obtained at 300 K at the beginning of the measurement (1) and after the measurement

(2), respectively...... 108

Figure 4-4 Rietveld refinement of 0.42 wt% boron doped Cu2Se (a-c) and undoped

Cu2Se (d-f) at 624 K, 512 K, and 403 K, respectively. The refined parameters are shown in Table 2...... 109

Figure 4-5 Powder diffraction patterns over the entire measured temperature range of

~300–773 K for 2휃 = a) 2.5°–30° (Undoped), b) 2.5°–30° (0.42 wt% Boron) c) 9.5°–

10.5° (Undoped), d) 16°–17° (Undoped), e) 18.75°–19.75° (Undoped), f) 9.5°–10.5°

(Boron), g) 16°–17° (Boron), h) 18.75°–19.75° (Boron). The wavelength is

0.5897313 Å...... 112

Figure 4-6 SEM images of fractured surface morphology of (a) pure Cu2Se, and b)

0.14 wt%, (c) 0.28 wt%m and (d) 0.42 wt% boron-doped polycrystalline Cu2Se fabricated by the spark plasma sintering (SPS) method...... 113

Figure 4-7 Energy-dispersive X-ray spectroscopy (EDS) of boron doped Cu2Se. (a)

SEM image showing the area (arrow sign) examined by EDS, and (b, c) EDS spectrum of the selected region...... 114

Figure 4-8 Structural characterization of 0.42 wt.% boron-doped Cu2Se polycrystals. a) Ball model of the monoclinic unit cell of Cu2Se representing the dashed square of the main panel of (b). b) Atomic resolution HAADF image at room temperature along the [0 -1 0] zone axis, where the Se atoms are represented by the yellow balls and the weak contrast Cu atoms among Se layers are represented by the red balls in the bottom inset; Top inset: the line profile along the dotted line AB in the main panel, showing the spacing between two Se. c) The SAED pattern along the [0 −1 0] orientation. d) HAADF STEM image of the FIB milled sample, showing boron-rich precipitates within the Cu2Se-rich matrix along with the corresponding EDS elemental mapping for Cu, Se and B. Average Boron particle sizes are 80 nm. e)

Magnified observation of boron-rich precipitates within the Cu2Se-rich matrix. .... 115

Figure 4-9 Low magnification TEM image of the boron doped Cu2Se bulk sample showing the presence of boron across the grain boundary...... 116

Figure 4-10 Temperature dependence of the thermoelectric transport properties of the samples: (a) electrical conductivity (σ), (b) Seebeck coefficient (S), (c) Power factor

(PF); (d) Carrier concentration and carrier mobility for the pure, 0.28 wt.% boron and

0.42 wt.% boron doped Cu2Se samples; (e) Thermal conductivity (κ) as a function of temperature; and (f) Schematic diagram of B-doped Cu2Se composite, showing the microstructure and phonon transportation...... 117

Figure 4-11 Thermoelectric figure of merit zT for the undoped and B-doped Cu2Se with different wt% B. b) Thermoelectric efficiency of the undoped and B-doped

Cu2Se samples, in comparison with the previously reported Cu2Se data ...... 119

xxi

Figure 4-12 Temperature dependence of (a) σ/ , (b) S/S0.0, (c) κ/κ0.0, and (d)

zT/zT0.0 for the boron doped Cu2Se samples with doping levels of 0.14, 028, and 0.42 wt%...... 120

Figure 4-13 Temperature dependence of the thermal transport properties for the undoped and boron doped polycrystalline samples: a) The calculated Lorenz number

(L) of undoped and boron doped Cu2Se.; b) total thermal conductivity (κ) and electronic thermal conductivity ( ); c) lattice thermal conductivity ( ) with total

thermal conductivity (κ); d) comparison of the boron doped κlat values and κtot values

(dotted lines with those of state-of-the-art Cu2Se [18, 20, 24], PbTeSe [25], PbTe

[26] polycrystals; ...... 121

Figure 4-14 Hardness of the undoped and boron doped Cu2Se polycrystalline samples in comparison with data for Cu2Se polycrystalline bulks ...... 123

Figure 4-15 The electrical and thermal transport properties with respect to temperature during heating up and cooling down for a 0.42 wt% Boron doped Cu2Se sample: (a) electrical conductivity (); (b) Seebeck coefficient (S); (c) thermal diffusivity (D)...... 124

Figure 5-1 (a) Schematic illustration showing the formation mechanism and fabrication process of the CF-doped Cu2Se bulks. (b) Crystal structure of low- temperature monoclinic phase (left) and high-temperature cubic phase (right) of

Cu2Se matrix...... 134

Figure 5-2 Synchrotron radiation X-ray diffraction (SR-XRD) profiles of (a) pure

Cu2Se and (b) 0.75 wt% CF-doped Cu2Se obtained upon heating, showing the disappearance of the extra peaks around 4.97° and 15° as the lower-symmetry α- phase is transformed to the higher-symmetry cubic β-phase. (c) Temperature

xxii dependence of the specific heat (Cp) for the undoped and CF doped Cu2Se samples.

(d) XRD patterns for Cu2Se/0.75wt% CF obtained at 300 K, at the beginning of the

DSC measurement (1) and after the measurement (2), respectively...... 135

Figure 5-3 (a, b) Rietveld refinement XRD patterns for the undoped and 0.75wt% CF doped Cu2Se samples, respectively; (c, d) Comparison of peak shifts of undoped and

CF-doped Cu2Se samples with temperature; (e, f) change in lattice parameters of the

훼-phase with temperature compared with [29] and 훽-phase transition for undoped and 0.75wt% CF doped samples...... 137

Figure 5-4 SEM-STEM image characterization of the densely distributed CFs inside the Cu2Se matrix: (a) Undoped Cu2Se. B) 0.75wt% CF at and inside the Cu2Se grain boundaries. (c-f) Low-magnification TEM image and corresponding EDX mapping for elements C, Cu, and Se, respectively. (g) Atomic resolution HAADF image at room temperature along the [2 0 1] zone axis, where the large bright dots refer to Se atoms and the weak contrast along the Se layers refers to Cu atoms. (h) The corresponding fast Fourier transform (FFT) pattern along the [2 0 1] zone axis. .... 138

Figure 5-5 Focused ion beam (FIB) cut image shows the carbon fiber in the Cu2Se sample...... 139

Figure 5-6 STEM-EDS analysis of 0.75wt% CF in Cu2Se: (a) Low magnification

TEM image; (b-d) the corresponding EDS elemental analyses of carbon, Cu, and Se, respectively...... 140

Figure 5-7 Temperature dependence of the (a) electrical conductivity, (b) Seebeck coefficient, (c) power factor, (d) carrier concentration and carrier mobility as functions of the doped CF ratio at different temperatures, in comparison with data for

Cu2Se [15], and (e) thermal conductivity for Cu2Se-xwt% CF (x = 0, 0.35, 0.55, 0.75,

xxiii and 0.85) polycrystals; and (f) Schematic diagram of CF-doped Cu2Se composite, showing the microstructure and phonon transportation...... 141

Figure 5-8 a) Thermoelectric figure-of-merit zT of CF-doped Cu2Se composite for different wt% CF. b) Thermoelectric efficiency of the undoped and CF-doped Cu2Se samples, in comparison with the previously reported Cu2Se data ...... 144

Figure 5-9 Temperature dependence of (a) σ/ , (b) S/S0.0, (c) κ/κ0.0, and (d) zT/zT0.0

for the carbon fiber doped Cu2Se samples, where 0.0 refers to the undoped sample.

...... 145

Figure 5-10 Temperature dependence of (a) the electrical conductivity and (b) the

Seebeck coefficient over four repeated trials for a 0.75wt% CF doped Cu2Se sample.

...... 146

Figure 6-1 Synchrotron X-ray diffraction profile of the (a) undoped Cu2Se and (b)

0 0 0.60G doped Cu2Se, notice the disappearance of the extra peaks around 5 and 15 upon heating due to the transformation of monoclinic α-phase to the cubic β-phase.

(c, d) Rietveld refinement of XRD patterns for the undoped and 0.6G doped Cu2Se samples, respectively; The refined parameters are shown in Table S1...... 156

Figure 6-2 Temperature dependence of specific heat (Cp) for the undoped and grape juice doped Cu2Se samples...... 158

Figure 6-3 FE-SEM cross-sectional micrographs of the (a) undoped and (b) 0.6G doped Cu2Se polycrystalline samples (inset as-prepared fabricated droplet). TEM micrograph of 0.6G for (c) Low, and (d) high magnification images showing the carbon inclusion inside the sample and the details of carbon-rich area A, enlarged views of areas A and B shown to the right of the image (d)...... 159

xxiv

Figure 6-4 STEM micrograph of a grape doped Cu2Se sample. (a) Low magnification

...... 160

Figure 6-5 EDS spectrum of the (a) Cu2Se phase, (b) C phase, and (c) CuO phase.

...... 162

Figure 6-6 Temperature-dependent of (a) electrical conductivity 𝜎, (b) Seebeck coefficient 횜, (c) thermal conductivity , and (d) figure of merit zT of the undoped and grape doped polycrystalline samples, respectively of Cu2Se + xwt%Grape (x = 0,

0.15, 0.30, 0.45 and 0.60)...... 163

Figure 6-7 Temperature dependence of the (a) σ/ , (b) S/S0.0, (c) κ/κ0.0, and (d)

zT/zT0.0 for the grape doped Cu2Se samples with a doping level of 0.15, 0.30, 0.45 and 0.60 wt%...... 164

Figure 6-8 Thermoelectric efficiency of the undoped and 0.6G doped Cu2Se samples, in comparison with the previously reported Cu2Se data ...... 165

Figure 6-9 Vickers Hardness of (a) 0.15 G (b) 0.30 G (c) 0.45 G (d) 0.60 G doped

Cu2Se sample under 0.245 N load in Lens (60x)...... 166

xxv

LIST OF TABLES

Table 1 Power output from human body heat ...... 13

Table 2 Parameters for the refinement of boron doped and undoped Cu2Se at 624 K,

512 K, and 403 K. Rp and Rwp are the profile and weighted profile R-factors, respectively, and 2 is the goodness-of-fit...... 110

Table 3 Rietveld Refinement parameters of grape-doped and undoped Cu2Se at different temperatures...... 157

xxvi

LIST OF ABBREVIATIONS

XRD X-ray diffraction SHT-XRD Synchrotron high temperature X-ray diffraction FE-SEM Field emission scanning electron microscopy EDS Energy-dispersive X-ray spectroscopy STEM Scanning transmission electron microscopy PLD Pulsed laser deposition SPS Spark plasma sintering DSC Differential scanning calorimetry PPMS Physical properties measurement system TE Thermoelectric TEG Thermoelectric generator TEC Thermoelectric cooler Rp Profile R-factor Rwp Weighted Profile R-factor GOF Goodness of fit LN Liquid nitrogen σ Electrical conductivity S Seebeck co-efficient D Thermal diffusivity Κ Thermal conductivity Fcc Face-centred cubic L Lorentz factor ξ Interaction parameter r Scattering parameter kB Boltzmann constant e Electron charge Fn (x) Fermi integral

xxvii

Chapter 1

1. Introduction

1.1. Background

The rapid consumption of limited natural energy resources as well as greenhouse gas emission (figure 1-1), the world desperately needs sustainable energy sources for the power generation as the fossil fuel reserve may diminish within 50 years [1, 2]. The thermoelectric (TE) technology comes as a blessing for its simplicity and gives environmentally friendly solution for power generation from ubiquitous waste heat.

Figure 1-1 (a) World total primary energy supply by fuel (Mtoe); (b) World CO2

emissions by fuel (Mt of CO2) [IEA Key World energy statistics 2017]

Currently, due to the relatively high cost and low efficiency in TE material applications, TE materials are employed in only limited applications. Consequently, more efficient TE materials needed to be identified in order to extend their applications. There are varies important applications for TE materials, to name a few: spacecraft applications, harvesting waste heat from power plants and automobiles for power generation, as well as great applications on cooling such as refrigerator etc. [3-

5] TE materials are classified according to their range of application temperatures.

For example, for low range temperature applications Bi2Te3, Bi2Se3 type materials available. For mid-temperature range examples, PbTe, Mg2Si, Yb12MnSb11, MnSi1.75, and liquid-like type TE materials available. For high temperature range application

TAGS, SiGe, BiCuOS, SrTiO3 half-Heuslers types materials available. Additionally, the superionic conductors in the chalcogenide-based copper selenide family has recently becoming a promising class of thermoelectric materials due to their liquid- like behaviour in their cubic structured phase.

The general objectives of this study including: a contribution in enhancing the knowledge in the field of bulk thermoelectric materials, and to employ time-efficient and low-cost methods of fabrication and searching for best dopants to enhance the efficiency of the current thermoelectric materials. One of our aim of the dissertation is to fabricate highly dense Cu2Se bulk sample using melt-solidification and spark plasma sintering method. I have investigated different doping effects on Cu2Se (solid doping: diamond carbon fiber, carbon nanotube, graphite, hard carbon, Super P, amorphous boron; liquid doping: grape, honey, sugar, pulm sugar, malik acid) to achieve fundamental understandings of the structures and thermoelectric properties of these materials, as well as in obtaining highly efficient thermoelectric materials with possibilities of commercialization and mass production for power generation applications.

1.2. Literature and technology review

1.2.1. Working principles of thermoelectric devices TE devices consists of two different semiconducting legs with no moving parts and it is generally designed in an extremely simple way by connecting two semiconducting legs electrically in series but thermally in parallel. The power generation from the TE device is due to the diffusion of charge carrier which comes with the temperature gradient [6].

1.2.2. Seebeck Effect a. Working principle

In 1821, Thomas Johann Seebeck discovered that

. When a pair of n-type and p-type TE semiconductors is connected electrically

in series with a temperature difference, a voltage is generated across them

(figure 1-2).

. The voltage across two different thermoelectric legs adds up and drives the

charge carrier (current) through the load.

. Finally, the electrical power is generated which is the product of the voltage

and the current [7].

The Seebeck coefficient is expressed as S = , where V and T represents voltage and temperature, respectively. The typical unit of S is μV/K. The sign of S comes positive for P-type and negative for n-type semiconductors, respectively. In a p-type semiconductor, holes diffuse from the hot side to the cold side, and vice versa

Figure 1-2 Schematic diagram showing the working of a thermoelectric generator

(Seebeck effect) [7]

for the n-type semiconductors, electrons move from the hot to the cold end 6,8. The S value for metals or degenerate semiconductors is given by [8]:

(1)

Where, n, and m* is the carrier concentration and the effective mass, respectively.

So, the Seebeck coefficient is increases if n and m* of carrier is decreases.

The electrical conductivity (σ) is proportional to n and the carrier mobility µ:

(2)

Where, e is the elementary charge. The σ for n-type and p-type semiconductors is dominated by electrons and holes, respectively and is expressed as:

(for n-type) (3)

(for p-type) (4)

1.2.3. Peltier Effect b. Working principle

Figure 1-3 Peltier effect [9]

Thermoelectric (TE) powered by electricity can be used for cooling or heating applications. In 1834 Peltier discovered this effect where he described that TE devices can convert electricity into a temperature gradient: The heat transfer is achieved when a DC current is passed through two different types of materials ( n- type and p-type semiconductor materials) (figure 1-3).

. The electrons travel from a low energy level (p-type material) to a higher

energy level (n-type semiconductor) leg. 5

. They gain energy by absorbing heat at the conductor interconnect between

the two legs (heat is absorbed from the crystal lattice) thereby reducing the

temperature, Tc, of this interconnect.

. During the returning of electrons to a lower energy level it releases heat at the

other end of the leg and dissipating it in a heat sink. The temperature TH of

this other junction is increased.

However, the efficiency of this heat transfer is reduced by two effects:

. Heat conduction in the legs. Heat diffuses from the hot side to the cold side

due to the temperature difference between the two sides of the

semiconductor.

. The other loss, Joule heating, occurs as current is run through the material

and energy is dissipated at a rate that is proportional to the square of the

current.

This phenomenon can eventually become the dominant factor; as current is increased, a point is reached at which an increase in current results in less net cooling. That’s why I have to choose thermally insulating materials that is electrically conducting.

1.2.4. Thermoelectric Figure of Merit The TE figure of merit (ZT), is determined by its transport properties which is expressed as follows:

(5) where, σ is the electrical conductivity, S is the Seebeck coefficient, T is the temperature, and κ is the thermal conductivity. The product of electrical conductivity and Seebeck coefficient is called the power factor (PF). To achieve an excellent ZT, 6 it is required to generate higher output power by increasing electrical conductivity and Seebeck coefficient and minimizing thermal conductivity to get large temperature difference across the hot and cold ends. The thermal conduction in a material is due to the contribution of electrons and phonons. Therefore, the total thermal conductivity, K, can be expressed as:

(6)

Where, , represents electronic and lattice thermal conductivity, respectively.

The lattice contribution to the thermal conductivity is obtained in TE materials by subtracting the electronic term (kel = LσT where L, σ and T are the Lorenz factor, electrical conductivity and absolute temperature, respectively) from the total thermal conductivity according to the Wiedemann–Franz law. The Lorenz factor is generally used to isolated the and lattice contributions to the thermal conductivity and is calculated using the below equations [10];

(7)

Where, r, kB, e and Fn (ξ) is the scattering parameter, Boltzmann constant, electron charge, and the Fermi Integral respectively. The Fermi Intergral is given by:

(8)

Here, the reduced Fermi energy can be calculated from the S and the scattering parameter (r) according to

(9)

L is taken to be 2.45 x 10-8 V2K-2 for metals and 1.49 x 10-8 V2K-2 for intrinsic semiconductors. However, it can vary dramatically (1.6 to 2.2 x 10-8 V2K-2) depending on the temperature and the specific material.

The is dominated primarily by the phonon–phonon scattering according to the

Keyes expression [11]:

(10)

Where R, Tm, , N0, and A is the ideal gas constant, the melting point, the density, the Grneisen constant, the Avogadros number, the fractional amplitude of interatomic thermal vibration, and the mean atomic weight, respectively.

1.2.5. Coefficient of performance (COP) The efficiency of the TE couple can be maximized by employing a current flow which gives the maximum efficiency, And the coefficient of performance (COP) of a TE device is a function of ZT, COP = f(ZT) in the following ways:

Power generation mode:

(11)

Cooling mode:

(12)

Where, and is Carnot efficiency, and and

is an average thermoelectric figure of merit.

1.3. Advantages offered by thermoelectric technology compared to alternative technologies

. Thermoelectric devices require little or no maintenance due to the motionless

parts hence it’s without mechanical vibration, therefore they are silent.

. This capability of steady state operation exceeds 10000000 hours while doing

life time testing.

. The same thermoelectric system can perform two functions - cooling and

heating: by switching the power supply polarity, the cooling mode or the

heating mode can be switched accordingly. A precision temperature controls

up to +0.1 oC can be obtained when a TE device is mounted with the

appropriate electronic circuitry.

. They contain no chlorofluorocarbons (compared to most compressor-based

refrigerators) or other materials that are environmentally harmful and may

require periodic replenishment.

. They are not position-dependent.

. TE devices can function in environments that are too severe or too confined

to allow the use of conventional refrigeration systems.

1.4. Review of applications of TE and assessment of their economic value

1.4.1. Application of the Seebeck effect: Power generation

1.4.1.1. High-power generation a. Space missions power generation-

Thermoelectric have decisive competitive advantages for powering space missions:

TE materials have demonstrated reliability and a very long-life cycle.

. Thermoelectric do not generate any mechanical vibrations.

. They can power a satellite for remote missions where solar irradiance might

not be sufficient.

For space mission’s power applications, TE materials is coupled with a nuclear heat source and converting energy into electricity (figure 1-4). Radioisotope thermoelectric generators (RTG) have been used in 25 missions by NASA since

1961. These generators operated between a large temperature differential, typically

300 K-1000 K. Thermoelectric for power generation on space missions are currently being industrialised by the Jet Propulsion Laboratory at Caltech, CA, USA.

Figure 1-4 Principle of Radioisotope thermoelectric generators [12]

b. Waste heat recovery from car exhaust

Waste heat recovery from car exhaust (figure 1-5) is a high volume, low profit market and thus apparently not a good entry-point for a new technology. However, there are markets inside the automobile market, and thermoelectric could be successfully introduced in certain segments such as luxury vehicles, "green" vehicles and commercial vehicles before spreading to consumer automobiles.

Estimation of the economic value of fuel saved through the use of TE:

. Assume average driving distance for a driver 15000 miles/year

. And fuel use 25 mpg (mile per gallon) and cost $3 per gallon for gas.

. According to LaGrandeur, TE can increase fuel efficiency by as much as 10

percent, and he replaces the alternator (a car's electrical generator) entirely in

his BMW with an internal combustion engine, which contributed to a

reduction of environmental impact of automobiles and increasing the fuel

efficiency. So, this experiment makes a saving of $180/year.

Figure 1-5 Energy losses in a car (internal combustion engine) and the TE module

associated to a car’s exhaust pipe [12]

Somewhere between 20 to 50% of industrial energy input is lost as waste heat (US department of Energy, 2008) where industrial sector consumed 31% of energy in the 12 world. Consequently, the application of TE technology in waste heat recovery is crucial.

1.4.1.2. Low power generation

The use of thermoelectric for low power generation applications has been reported.

Microwatt or milliwatt power could be generated by TE devices converting the body heat flow or the external side’s heat of a water pipe into electricity. This is a form of waste heat recovery, with free energy supply and for which performance is generally not measured in terms of COP = Electrical Energy Output/Waste Heat Input. The decision criteria here will be the cost of supplying energy to the device considered over its lifetime.

Estimation for typical wrist watches (Table 1):

. The typical current drain: 10 to 40µA with operating voltage: 1.5V

. Power needed is thus in the range: 15 µW to 60 µW. (a typical wrist watch

needs up to 100 µW)

* 2 TH(k)= 303; Input (Heat flow) = 18.8 mW/cm

** 2 TC(k) = 295 Heat flow range on a person’s wrist: 8.9~35.8 mW/cm

2 η (Carnot) = 2.6% with an average of 18.8 mW/cm .

Contact area = 1cm2

“ZT” 0.6 0.8 1 1.6 2 3 5

Actual COP 0.27% 0.33% 0.39% 0.53% 0.61% 0.75% 0.95%

Output (µW) 27 33 39 53 61 75 95

Table 1 Power output from human body heat [12]

And if the system needs more power it is possible to increase it by using step up DC chopper circuit (figure 1-6). And such types of chopper circuit has been reported in

Carlson et al. [13-17] A conventional chopper circuit is shown in Fig 5 where

when CH is closed and VS = VL and when CH is open.

* For least body heat generating persons, input (heat flow) = 10mW/cm2 at the beck of the wrist (i.e. the area of watch replacement.)

**Leonov et al, from the IMEC in Leuven measured the heat flux on a wrist, on the radial artery, along with the psychological reaction to the presence of a thermoelectric generator placed on the skin.

Figure 1-6 DC to DC converter (Step up)

1.4.1.3. Application of Peltier effect

A. Peltier effect: Cooling and Heating

1. Car seat heating/cooling

2. Automobile Beverage heater/Cooler

3. Motorcycle helmet and clothing

B. Peltier effect: Cooling

1. Room air-conditioning

2. Refrigerators

3. Electronics cooling

4. Laser cooling

5. Target performance of thin-film TE coolers for IC cooling

6. Recent research breakthrough in IC cooling using TE

1.5. History and Development

The wide study of thermoelectric materials has started since the 20th century. To understand and to tune the figure of merit, it is important to go back to the history of thermoelectric materials. Initially, metals were used as a thermos-element to develop

Seebeck, Peltier and Thompson effects. However, theoretical prediction by

Altenkrich clearly explained that metals were not efficient for the thermoelectric applications [18, 19]. Investigation on group III‐ V and II‐ VI semiconductors was done by Abram Fedorovich Ioffe to find out thermoelectric properties and he discovered a prototype of TE generator by identifying n‐ and p‐ type semiconductors [20]. Goldsmid and Wright have explored excellent thermoelectric materials (Bismuth telluride) [21, 22] known as the first generation TEs gives the device energy conversion efficiency of 4-6% with an average figure of merit ~1.

Thermoelectric research was hindered due to the lack of efficiency during and after the 1950s. Rowe and his colleagues at 1980s proposed a technique to improve thermoelectric performance by the phonon scattering at the grain boundaries [23].

Thereafter, nanostructuring concepts was revived by Dresselhaus and Hicks who contributes to the enhancement of power factor by explaining the quantum

15 confinement effect [24]. This concept helps in further investigations on low dimensional TE materials like superlattices. Using this approach, reduction of lattice thermal conductivity observed due to the grain boundary inclusions, compositional inhomogeneity, utilisation of nanoscale in homogeneity [25-28]. The continuous and wide variety of research contributing to the double of the figure of merit at high temperature, which resulting to the conversion efficiency of ~10 to 12% from the bulk thermoelectric which named as second generation TE materials such as lead tellurides, skutterudite compounds, half‐ Heuslers, clathrates, and ZT values as high as 1.7 has been reported [25, 29].

Figure 1-7 Schematic representation of the current state of the figure of merit

obtained in bulk thermoelectric [30]

Figure 1-7 demonstrates the comprehensive scenery so far established for different

TE materials. It reflexes the improvement of thermoelectric performance in various

16 materials by nanostructuring effect. However, the best bulk materials for the real world application in cooling purposes and energy harvesting so far is still

Bi0.5Sb1.5Te3 (with a ZT value of approx. 1.5 at 300 K [31] due to the following:

. Complications arise during the device fabrication with cost and small-scale

production.

. Efficient design and large quantities productions are the vital issues to be

overcomed at present.

. Poor mechanical properties also observed in most thermoelectric materials

[32, 33].

1.6. Figure of Merit in TE Materials

Generally, thermoelectric materials can be categorised based on their operating temperature range. Very few low temperature thermoelectric materials are discovered (figure 1-8) till now compared with the mid and high temperature thermoelectric materials [34]. For refrigeration devices, cryogenic temperature (4 –

250 K) range materials were used. Such as, Cesium Bismuth Telluride (CsBi4Te6) is a p-type alkali-metal bismuth chalcogenides-based material with a ZT value of 0.8, which has been widely used for this purpose [35-37].

Recently, scientists have discovered new types of cryogenic materials. One of such is the iron antimonide (FeSb2), which is a marcasite crystal structure showed very

-1 -2 -1 large power factor (2300µW cm k ) (seeback coefficient: -45000µV K at 10 K) which is 65 times larger than that of the Bi2Te3 but the drawback is its large lattice thermal conductivity, hence results to a ZT value of only 0.005 [38]. Chalcogenides

17 based Bismuth Telluride (Bi2Te3) materials have shown decent performance for both of n-type and p-type from room temperature to up to 500 K [39] and exhibited about

4-5% energy conversion efficiency [40, 41].

Lead chalcogenides, clathrates, skutterudites TE materials have shown the best performance in the temperature range from 500 to 900 K [42-45]. For the temperature range of 600 to 800 K, Cobalt antimonide (CoSb3) based skutterudites have illustrated the best ZT values [46-49], and clathrates and PbTe at and in between

700 to 900 K shown the best TE performance [50-52]. However, all these materials are comparatively expensive and are not earth abundant, that’s why silicides are comes in attention for the environment friendly TE materials [53]. The N-type

Magnesium Silicides (Mg2Si) [54, 55] and the p-type Manganese Silicides (MnSix)

[56] have becoming more popular in the mid-temperature range applications. For high temperature ranges i.e. above 900 K, metal oxides (e.g. NaCo2O4) [57-60] and half‐ Heusler intermetallics (e.g. Hf0.6Zr0.4NiSn0.98Sb0.02) [61-64] have become more favourable. Figure 1-9 and 1-10 demonstrated the figure of merit of various class of materials corresponding to their appropriate temperature range for TE applications

[38, 65, 66, 8, 67].

Figure 1-8 Thermoelectric figure of merit with respect to temperature for varies bulk

materials; (a) n-Type, (b) p-Type [8]

1.7. Conflicting issues and strategies to improve ZT

In practical experiment I have seen that if the Seebeck is increases then the electrical conductivity goes to decrease, otherwise the thermal conductivity increases. So, the strategy for achieving higher ZT values in TE materials is as the following [38, 12]-

. Maximizing the power factor which includes optimising existing

materials by carrier concentration tuning.

. Development of new materials.

. Minimizing the thermal conductivity where the lattice conductivity must

be diminished while keeping the carrier concentration unaffected by

. Fabricating isostructural solid solution by adding point defects.

. Introducing intrinsically low thermal conducting materials.

. Nanostructuring.

Figure 1-9 ZT vs temperature of the state-of-the-art TE materials highlighting up-to-

date improvements in ZT values above 1 [38, 65, 66, 8, 67-76]

1.7.1. Carrier concentration tuning Figure 1-11 shown that when an Al3+ is substituted by a Zn2+, one hole is generated resulting in the increasing of the hole concentration by Zn2+ + h+ for Al3+ (or In3+).

For almost all typical thermoelectric materials, doping changes the electrical conductivity to be higher while the the seeback coefficient is reduced. So maximisation of ZT is possible by optimising the dopant concentration (figure 1-12)

[77].

Figure 1-10 Evaluation of the ZTmax with respect to time. The blue dots represent

materials used for cooling application while the red triangles for power generation

[78]

Point defects includes illustrated in figure 1-13:

. An extra self-interstitial atom exhibits an interstitial void when the

concentration is Low. So, the atoms in the lattice structure is tightly packed

so high stress is produced. An interstitial atom fits in between the bulk atoms

i.e. into the open space, and it is smaller than the bulk matrix atoms. For

example, the radius of a carbon atom is 0.071nm, so it can have fitted nicely

in between the iron atoms (radius of iron atom 0.124 nm) in the lattice

structure of iron. In this way, steel is formed by the addition of interstitial

carbon atom in the iron atom.

Figure 1-11 Optimization of carrier concentration for maximizing ZT (G. Jeffrey

Snyder, ISEM Conf. UoW, 2014)

Figure 1-12 The figure of merit of Ca5ZnxIn2−xSb6 samples a) within the measured

temperature range. b) ZT is optimised at 700 K at 2 × 1020 cm−3 [77]

. When a different type of atom (which is close in size of not more than 15% in

difference) replaced one of the bulk atom, it is called a substitutional impurity

atom. For example, when cu atoms (radius ~0.128 nm) substituted by zinc

atoms (radius ~ 0.133 nm) it formed a brass.

. When some atoms are missing from its lattice points, these points is called

vacancies. Especially, when atoms randomly and frequently change their

positions due to the diffusion, it can cause vacancies to occur.

. Due to the quantum confinement it enhanced the density of states (DOS) by

improving the Seebeck coefficient while the electrical conductivity is unchanged.

Figure 1-13 Different types of Point defect in a material (source: nde-ed.org)

1.7.2. Motivations for nanotechnology in thermoelectricity

Figure 1-14 TE materials structure in different scales demonstrated in a nutshell. (a–

d) Different dimensionalities: (a) bulk (3D), (b) thin ﬁlm (2D-30nm), (c) nanowire

(1D-100nm), (d) atomic cluster (0D-quantum dots). (e–h) variation of grain: (e)

micron sized grained only, (f ) coarse and fine grained, (g) nanosized grained, (h)

amorphous [66].

. The thermal conductivity is also reduced more rapidly than the increasing in the

electrical conductivity due to the average mean path of phonons is higher in

lower dimensions (fig a-d).

. By introducing the grain boundaries, the thermal conductivity also reducing

which is different from the surface scattering.

1.8. Representative of high performance thermoelectric materials

Thermoelectric materials will be discussed based on their crystal structure, physical and chemical properties, and their optimal thermoelectric performance.

1.8.1. Skutterudites Skutterudites has an open cage unit cell structure where atoms inserted are weekly bonded with the frame and it can vibrate from their central position. The phonon which carries heat experiences damping in an Einstein mode (1-15).

The General formula of Skutterudites is MX3 where M (transition metal) = Cobalt

(Co), Rhodium (Rh) or Iridium (Ir); X (Pnictogen) = Phosphorus (P), Arsenic (As) or

Antimony (Sb).

Figure 1-15 Phonon scattering effect in a void in Skutterudites

In the case of CoSb3, voids is with an open crystal structure and it has higher electron mobility (µe) and higher effective mass (m*), has also with higher thermal conductivity (휅) and it contains (figure 1-16):

Figure 1-16 The CoSb3 Skutterudites structure shown the void cages filled with

guest atoms [79]

• Six squares and eight cubes formed by Sb and Co respectively.

• Cage size is 1.89 Å [80] or 0.189 nm.

• Void filing according to the formula RyMX3 (0 ), where R = Positive

element of La, Ac, In, Tl, or G 1, 2, 4 elements. 26

• Covalent bond is formed by accepting valance electrons from the

electropositive guest atoms.

• Filling limit: [81].

According to Slack, neutral atoms were proposed as fillers because Nd3+ and Sm3+ in

Ir4NdGe3Sb9 and Ir4SmGe3Sb9 has reduced thermal conductivity (K) Compared to

IrSb3, and the electronic properties is not changed, so the ZT values is unchanged

[65]. Improvements in ZT values is slower for p-type Skutterudites due to the fact that usually the filling makes it a n-type Skutterudites. For example, the n-type double filled skutterudites (BaxYbyCo4Sb12) materials shows a reduced thermal conductivity (figure 1-17) of about 0.9 Wm-1K-1 [82]. Moreover, n-type triple filled

-1 -1 BaxLayYbzCo4Sb12 shows extremely low lattice thermal conductivity ~0.2 Wm K and the resultant ZT comes to 1.7 [83]. However, p-type skutterudites shows slower improvements and the best ZT observed ~1 for the p-type skutterudites.

Figure 1-17 Phonon engineering by internal Nano-structuring (NdyFe4Sb12) Some

single, double and triple-filled skutterudites have demonstrated extremely low lattice

thermal conductivities. (source: www.fzu.cz)

1.8.2. Clathrates Clathrates is another type of compound containing open structures in their unit cell and contains a large number of atoms. The thermal conductivity is comparable with the amorphous germanium which is about 1 Wm-1k-1 (figure 1-18-20). There are nine different types of clathrate available [84], here type I will be considered in this review. Type I Clathrates composed of 92 covalent bonds in its unit cell, and it is with 184 valance electrons that fill the valance band completely [85].

Figure 1-18 Partially filled based on Co4Sb12

General formula Clathrate of type I: A8E46 where, A (Guest atoms) = Alkali metals

(Na, K, Rb, Cs) or Alkali-earth metals (Ba, Sr), and E = Group 14 elements Si, Ge or

Sn. Type I ternaries is in the form of A8B16E30 where B (Guest atoms) = Al, Bi, As,

Ga, Zn, Sb, In, or Cd.

Figure 1-19 Clathrates crystal structure (type I). Two pentagonal dodecahedra (E20)

and six tetrakaidecahedra (E24) contained in the cubic unit cell [85, 38]

Figure 1-20 Temperature dependent of (a) figure of merit and, (b) thermal

conductivity for Ba8Ga16Ge30, Hf0.75Zr0.25NiSn0.975Sb0.025, 훽-Zn4Sb3, Zintl phase

Yb14MnSb11, and PbTe.

1.8.3. Mg2Sn

Mg2Sn’s unit cell is a cubic structure and it contains 4 Sn atoms in the fcc array and

8 Mg atoms in the tetrahedral position (figure 1-21).

Figure 1-22 (B) shows the lack of semiconducting behaviour for the sample due to the linear curve shape of the electric resistivity from room temperature up to 300 0C.

0 For Mg2Sn, the Seebeck saturated after 300 C due to the bipolar effect of the narrow band gap (0.26 eV) of this compound. But doping maintains the linearity and the bandgap is increases to 0.41 ev and the carrier concentration increases from 1.8 to

20 -3 3.0 x 10 cm for x = 0 to 0.25 in Mg2Sn1-xGex.

Figure 1-21 crystal structure of Mg2Sn and Mg2Sn0.75Ge0.25 [86]

0 −1 −2 So, the Power Factor at 350 C observed to be 55 μW·cm ·K , which is due to the alloy scattering as the Ge atom is smaller than the Sn atom, so the bond strength changes, also the carrier concentration m* increases from 2 to 3.5mo and the carrier

Figure 1-22 Thermoelectric transport (electrical) properties of Mg2Sn1−xGex with

respect to temperature: (A) Electrical resistivity (B) Seebeck coefficient. (C) Power

factor (PF). (D) PF of Mg2Sn0.75Ge0.25 from different sample batches [86]

mobility μ decreases from 86 to 46 cm2V-1S-1 from x = 0 to 0.25, consequently the thermal conductivity decreases.

The output power density and leg efficiency were found to be 6.6 W·cm−2 and

10.5%, respectively where TH = 400 °C and Tc = 50 °C. All the change of thermal conductivity with Ge doping in Mg2Sn illustrated in figure 1-23.

Figure 1-23 Thermoelectric transport (thermal) properties and ZT values of undoped

and Ge doped Mg2Sn with respect to temperature (A-B) Thermal conductivity, (C )

Heat capacity, (D) Figure of merit compared with the Bi2Te2.7Se0.3 [26, 86]

1.8.4. Chalcogenide compounds When a group of 16 elements and one electropositive elements formed a chemical compound, it is called a chalcogenide [87]. Rock salt crystal structure of PbTe, PbS and PbSe are well known thermoelectric chalcogenides. Furthermore, Molybdenium

Disulfide and Bismuth Telluride are the other thermoelectric chalcogenides and dichalcogenides hexagonal crystal structure compounds [88, 89].

Chalcogenides compounds are semiconductors with high melting points, they are stable in air and they can melt congruently. They have good adaptability with other elements and are possible to be used in semiconductors for TE applications over a wide range of temperatures. The N-type Bi2Te3xSex and P-type Bi2xSbxTe3 have been widely used for thermoelectric cooling systems [90]. For the power generation purpose, PbTe and germanium-based materials are mostly used. Though the Ge based TAGS is more efficient than PbTe, however it is hindered for commercialisation due to their high cost, phase transition at low temperatures and with high sublimation rates.

1.8.5. Bismuth Telluride (Bi2Te3)

Bi2Te3 compound has a hexagonal crystal structure and of group V-VI semiconductors. As shown in figure 1-24 its crystal structure is stacked due to the

Vander Waals interactions along the c-axis and it’s stacked with five atomic layers

(Te1-Bi-Te2-Bi-Te1) [91].

Figure 1-24 Bismuth Telluride (Bi2Te3) crystal structure [92]

Bismuth Telluride has a narrow indirect gap of ~0.15eV. In the octahedral geometry, six Te atoms are coordinated with one Bi atom. The optimal composition for the TE

Cooling is Bi2Te2.7Se0.3 (n-type) and Bi0.5Sb1.5Te3 (p-type). The ZT-value enhancement is obtained by mixing nanograins and macrograins together. Figure 1-

25 shows the temperature dependent of figure of merit (ZT) for the bulk Bi2Te3 materials with different fabrication methods.

Figure 1-25 Temperature dependent of ZT for the Bi2Te3 compounds with different

fabrication methods

1.8.6. Copper selenide (Cu2Se)

Cu2Se is a chalcogenide based TE material, and an interesting semiconductor with potential applications such as in solar cells, photocatalytic applications or in thermoelectric converters as it has a large energy band gap (1.2 eV), a high melting point (11070C), a high effective mass (6.2), a small hole mobility (1.9 cm2V-1S-1), and a high mechanical strength (0.46 GPa), such unique qualities making Cu2Se an interesting compound for researchers, as well as for technical applications in semiconductor converters etc [67, 93-96].

Thus, for each individual unit cell there will be

The crystal structure of Cu2Se changes with temperature. Below 400 K it shows monoclinic structure (1-26) and transit to cubic structure after the phase transition temperature 400 K (figure 1-27). Generally, in a crystal structure, the motion of atoms generates and carries heat. Atoms vibrating about their mean positions with amplitude that increases with temperature. There are mainly two types of atomic vibration that carry heat as shown in figure 1-28.

Figure 1-26 Sextuple layers of Se-Cu-Cu-Cu-Cu-Se are repeating at low temperature

monoclinic Cu2Se phase [97]

And to reducing heat conductivity in a crystal structure of a material, the following methods can be adopted:

1. By decreasing the phonon mean free path through introducing defect and disrupting the periodicity in the crystal structure.

2. Eliminating atomic vibrational waves.

Figure 1-27 Cu2Se cubic (β-phase) unit cell at high temperatures above 400 K [67]

Figure 1-28 Two types of atomic vibration that carry heat: (a) A row of atoms at rest

(b) Longitudinal waves (c) Transverse waves. (Source: inventor.grantadesign.com)

A transverse wave travel in a solid material due to the friction between the atoms, i.e. adjacent atoms move with the moving atom when it moves up and down hence the wave propagates. However, in a liquid, friction is negligible where neighbouring atoms just slides up and down with the vibrating atoms. That’s why inside the liquid transverse wave cannot travel.

Figure 1-29 The arrows indicate that the Cu ions can freely travel among the

interstitial sites in Cu2 -xSe compounds with a cubic antifluorite structure [67]

In the crystal structure of copper-selenium as shown in figure 1-29, the thermal conductivity is reduced due to the free-flowing copper atoms and the electrical conductivity reduced due to the selenium. So, the figure of merit observed at 1000 K is 1.5 [67] which is the largest among all the bulk materials.

1.8.6.1. Cu2Se related research

1.8.6.2. Liu et al have described Cu2Se in their paper of “Copper ion liquid-like thermoelectric”:

This paper first introduces the liquid like behaviour of Cu in Cu2Se. And they described thermoelectric transport property and proposed the phonon-liquid electron- crystal (PLEC) thermoelectric concept which is an extension of the phonon-glass electron-crystal (PGEC) thermoelectric concept. Figure 1-30 shown the thermoelectric transport properties with respect to temperature of the Cu2Se and

Cu1.98Se compounds.

1.8.6.3. Liu et al have also studied the compound of Cu2Se1-xIx, as described in their

“ paper Ultrahigh Thermoelectric Performance by Electron and Phonon Critical

” Scattering in Cu2Se1-xIx :

Cu2Se has a phase transition around 400 K and at this temperature researcher found extremely high thermoelectric performance due to the iodine doping. Strong critical scattering in presence of iodine makes remarkable thermopower with the significantly reduction of thermal conductivity led to a record ZT value of 2.3 at 400

K. Figure 1-31 shows the temperature dependent thermoelectric performance of the iodine doped copper selenide.

Figure 1-30 Temperature dependent thermoelectric transport properties of the Cu2Se and Cu1.98Se. (a) electrical resistivity (b) Seebeck coefficient (c) thermal conductivity

(d) figure of merit (ZT) [67]

Figure 1-31 Thermoelectric transport properties of the iodine doped Cu2Se with

respect to temperature. (a) figure of merit (ZT) (b) electrical resistivity (𝜌) (c)

Seebeck coefficient (S) and (d) thermal conductivity (κ) [98]

1.8.6.4. Ag doped Cu2Se and Cu2Te based Thermoelectric materials

Figure 1-32 illustrates the temperature dependent thermoelectric transport properties of the Ag doped Cu2Se and Cu2Te. The addition of Ag in the Cu2Se and Cu2Te strongly suppresses the thermal conductivity by reducing the density of holes also enhancing the point defect scattering. Due to the different size and mass of Ag and

Cu atoms that reduces the lattice thermal conductivity.

Figure 1-32 Thermoelectric transport properties with respect to temperature of the

Ag doped Cu2Se. (a) Figure of merit (ZT); (b) electrical conductivity (σ); (c) Seebeck

coefficient (S); (d) thermal conductivity (κ); and (e) Lattice thermal conductivity ( )

[99]

1.8.6.5. Thermoelectric properties of copper selenide with ordered selenium layer and disordered copper layer

In their paper, Yu B et al has focused on the β-phase of pure Cu2Se with different concentration of Se in this compound. So, there is the ordered selenium (Se) and the disordered copper (Cu) layers in its unit cell, resulting in a low lattice thermal conductivity of 0.4–0.5 Wm-1K-1. The temperature dependent electrical conductivity,

Seebeck coefficient, thermal conductivity as well as the figure of merit results are shown in figure 1-33-34.

Figure 1-33 Temperature dependent TE transport properties of Cu2Se. figure of merit

(ZT) [100]

1.8.6.6. Superior intrinsic thermoelectric performance in single-crystal and melt- quenched highly dense Cu2-xSe bulks

Figure 1-35 shows the thermoelectric transport properties of the ultrafast-formed

Cu2-xSe bulk and single crystal samples. This study demonstrated that the performance of the Cu2Se system is not dependent on the grain size. Rather it is due to the random motion of Cu atoms at high temperature.

Figure 1-34 Temperature dependent TE transport properties of Cu2Se. (b) Electrical

resistivity (𝜌); (c) Seebeck coefficient (S); (d) Thermal conductivity (κ); and (e)

Lattice thermal conductivity ( ) [100]

Figure 1-35 TE transport properties showing the (a) electrical resistivity (ρ), (b)

Seebeck coefficient (S), (c) total thermal conductivity (κ), (d) lattice thermal

conductivity ( ), (e) power factor (PF), and (f) thermoelectric figure-of-merit (zT)

of the ultrafast formed Cu2-xSe and single crystal sample [101]

1.8.6.7. The effects of Te2− and I− substitutions on the electronic structures, thermoelectric performance, and hardness in melt-quenched highly dense Cu2-XSe

2- - Figure 1-36 represents Te and I substituted Cu2Se TE properties with respect to temperature. This study reveals from the density functional theory calculation that

Cu2Se is a zero-gap material and the Cu deficient in a Copper selenide making it a p- type conductor. This study has discovered a fast and economical method of

46 fabrication (melt quenching) that takes only a couple of minutes of heat treatment in producing the TE material.

Figure 1-36 Temperature dependent TE transport properties of the tellurium and

Iodine substituted Cu2Se. (a) figure of merit (ZT) (b) Electrical conductivity (σ); (c)

Seebeck coefficient (S); (d) Thermal conductivity (κ); and (e) Lattice thermal

conductivity ( ) [102]

1.9. References

[1] F. Birol, "Key world energy statistics," ed: IEA Publications, International

Energy Agency, rue de la Federation, Paris, France, 2017.

[2] G. S. Alemán-Nava et al., "Renewable energy research progress in Mexico:

A review," Renewable and Sustainable Energy Reviews, vol. 32, pp. 140-153,

2014/04/01/ 2014.

[3] J.-P. Fleurial, A. Borshchevsky, and T. Caillat, "New thermoelectric materials

and devices for terrestrial power generators," in AIP Conference Proceedings,

1997, vol. 387, no. 1, pp. 293-298: AIP.

[4] Y. W. Tung and M. L. Cohen, "Relativistic Band Structure and Electronic

Properties of SnTe, GeTe, and PbTe," Physical Review, vol. 180, no. 3, pp.

823-826, 04/15/ 1969.

[5] Y. Du et al., "Thermoelectric fabrics: toward power generating clothing,"

Scientific reports, vol. 5, 2015.

[6] T. M. Tritt, "Thermoelectric phenomena, materials, and applications," Annual

Review of Materials Research, vol. 41, pp. 433-448, 2011.

[7] J. P. Heremans, "Thermoelectricity: The ugly duckling," Nature, vol. 508, no.

7496, p. 327, 2014.

[8] G. J. Snyder and E. S. Toberer, "Complex thermoelectric materials," Nature

materials, vol. 7, no. 2, pp. 105-114, 2008.

[9] B. C. Sales, "Smaller is cooler," Science, vol. 295, no. 5558, pp. 1248-1249,

2002.

[10] S. Ballikaya, H. Chi, J. R. Salvador, and C. Uher, "Thermoelectric properties

of Ag-doped Cu2Se and Cu2Te," Journal of Materials Chemistry A,

10.1039/C3TA12508D vol. 1, no. 40, pp. 12478-12484, 2013.

[11] R. W. Keyes, "High-Temperature Thermal Conductivity of Insulating

Crystals: Relationship to the Melting Point," Physical Review, vol. 115, no. 3,

pp. 564-567, 08/01/ 1959.

[12] P. Bertreau, "Novel thermoelectric materials development, existing and

potential applications, and commercialization routes," Massachusetts Institute

of Technology, 2006.

[13] E. J. Carlson, K. Strunz, and B. P. Otis, "A 20 mV input boost converter with

efficient digital control for thermoelectric energy harvesting," Solid-State

Circuits, IEEE Journal of, vol. 45, no. 4, pp. 741-750, 2010.

[14] I. Chowdhury, R. Amin, S. M. K. N. Islam, M. S. Rahman, and S. Binzaid,

"Mixed-Signal VLSI Design in 0.5 µm Process of Nano-Power Subcompact

Mirror-Amplifier for AccuSensor," in Computational Intelligence and

Communication Networks (CICN), 2011 International Conference on, 2011,

pp. 522-526: IEEE.

[15] S. Binzaid, I. Chowdhury, M. S. Rahman, and S. M. K. N. Islam, "Nano-

Power Sensor Applications in VLSI Multi-die Tiny Chip," in Computational

Intelligence and Communication Networks (CICN), 2011 International

Conference on, 2011, pp. 554-558: IEEE.

[16] A. J. Hoque, S. M. Kazi, N. Islam, M. A. Siddik, and S. Ahamed, "Design

and Implementation of a Microcontroller Based 12V-7A/10A Smart Solar

Battery Charge Controller."

[17] T. Ahmed, S. M. K. N. Islam, I. Chowdhury, and S. Binzaid, "Sustainable

powered microcontroller-based intelligent security system for local and

remote area applications," in Informatics, Electronics & Vision (ICIEV), 2012

International Conference on, 2012, pp. 276-280: IEEE.

[18] E. Altenkirch, "Über den Nutzeffekt der Thermosäule," Physikalische

Zeitschrift, vol. 10, p. 560, 1909.

[19] E. Altenkirch, "Elektrothermische Kälteerzeugung und reversible elektrische

Heizung," Physikalische Zeitschrift, vol. 12, pp. 920-924, 1911.

[20] A. F. Joffe, "The revival of thermoelectricity," Scientific American, vol. 199,

pp. 31-37, 1958.

[21] H. Goldsmid and R. Douglas, "The use of semiconductors in thermoelectric

refrigeration," British Journal of Applied Physics, vol. 5, no. 11, p. 386, 1954.

[22] H. Goldsmid and A. Penn, "Boundary scattering of phonons in solid

solutions," Physics Letters A, vol. 27, no. 8, pp. 523-524, 1968.

[23] D. Rowe, V. Shukla, and N. Savvides, "Phonon scattering at grain boundaries

in heavily doped fine-grained silicon–germanium alloys," 1981.

[24] L. Hicks, T. Harman, and M. Dresselhaus, "Use of quantum‐ well

superlattices to obtain a high figure of merit from nonconventional

thermoelectric materials," Applied physics letters, vol. 63, no. 23, pp. 3230-

3232, 1993.

[25] G. S. Nolas, J. Poon, and M. Kanatzidis, "Recent developments in bulk

thermoelectric materials," MRS bulletin, vol. 31, no. 03, pp. 199-205, 2006.

[26] W. Liu et al., "Studies on the Bi 2 Te 3–Bi 2 Se 3–Bi 2 S 3 system for mid-

temperature thermoelectric energy conversion," Energy & Environmental

Science, vol. 6, no. 2, pp. 552-560, 2013.

[27] R. J. Mehta, C. Karthik, B. Singh, R. Teki, T. Borca-Tasciuc, and G.

Ramanath, "Seebeck tuning in chalcogenide nanoplate assemblies by

nanoscale heterostructuring," ACS nano, vol. 4, no. 9, pp. 5055-5060, 2010.

[28] G. Nolas, D. Morelli, and T. M. Tritt, "Skutterudites: A phonon-glass-

electron crystal approach to advanced thermoelectric energy conversion

applications," Annual Review of Materials Science, vol. 29, no. 1, pp. 89-116,

1999.

[29] A. Majumdar, "Thermoelectricity in semiconductor nanostructures," Science,

vol. 303, no. 5659, pp. 777-778, 2004.

[30] M. Saleemi, "Nano-Engineered Thermoelectric Materials for Waste Heat

Recovery," 2014.

[31] W. Xie, X. Tang, Y. Yan, Q. Zhang, and T. M. Tritt, "High thermoelectric

performance BiSbTe alloy with unique low-dimensional structure," Journal

of Applied Physics, vol. 105, no. 11, p. 113713, 2009.

[32] H. Alam and S. Ramakrishna, "A review on the enhancement of figure of

merit from bulk to nano-thermoelectric materials," Nano Energy, vol. 2, no.

2, pp. 190-212, 2013.

[33] G. G. Yadav, J. A. Susoreny, G. Zhang, H. Yang, and Y. Wu,

"Nanostructure-based thermoelectric conversion: an insight into the

feasibility and sustainability for large-scale deployment," Nanoscale, vol. 3,

no. 9, pp. 3555-3562, 2011.

[34] M. S. Dresselhaus et al., "New Directions for Low‐ Dimensional

Thermoelectric Materials," Advanced Materials, vol. 19, no. 8, pp. 1043-

1053, 2007.

[35] D.-Y. Chung et al., "A new thermoelectric material: CsBi4Te6," Journal of

the American Chemical Society, vol. 126, no. 20, pp. 6414-6428, 2004.

[36] C. D. Malliakas, D. Y. Chung, H. Claus, and M. G. Kanatzidis,

"Superconductivity in the Narrow-Gap Semiconductor CsBi4Te6," Journal of

the American Chemical Society, vol. 135, no. 39, pp. 14540-14543, 2013.

[37] D.-Y. Chung, S. Mahanti, W. Chen, C. Uher, and M. G. Kanatzidis,

"Anisotropy in Thermoelectric Properties of CsBi4Te6," in MRS

Proceedings, 2003, vol. 793, p. S6. 1: Cambridge Univ Press.

[38] J. R. Sootsman, D. Y. Chung, and M. G. Kanatzidis, "New and old concepts

in thermoelectric materials," Angewandte Chemie International Edition, vol.

48, no. 46, pp. 8616-8639, 2009.

[39] M. Toprak, Y. Zhang, and M. Muhammed, "Chemical alloying and

characterization of nanocrystalline bismuth telluride," Materials Letters, vol.

57, no. 24, pp. 3976-3982, 2003.

[40] Y. Ma et al., "Enhanced thermoelectric figure-of-merit in p-type

nanostructured bismuth antimony tellurium alloys made from elemental

chunks," Nano Letters, vol. 8, no. 8, pp. 2580-2584, 2008.

[41] X. Yan et al., "Experimental studies on anisotropic thermoelectric properties

and structures of n-type Bi2Te2. 7Se0. 3," Nano letters, vol. 10, no. 9, pp.

3373-3378, 2010.

[42] C. B. Vining, "Thermoelectrics: Half-full glasses," Nature materials, vol. 7,

no. 10, pp. 765-766, 2008.

[43] J. H. Roudebush, M. Orellana, S. Bux, T. Yi, and S. M. Kauzlarich, "Crystal

structure and thermoelectric properties of clathrate, Ba 8 Ni 3.5 Si 42.0:

Small cage volume and large disorder of the guest atom," Journal of Solid

State Chemistry, vol. 192, pp. 102-108, 2012.

[44] C. W. Myles, K. Biswas, and E. Nenghabi, "Rattling “guest” impurities in Si

and Ge clathrate semiconductors," Physica B: Condensed Matter, vol. 401,

pp. 695-698, 2007.

[45] D. Arčon et al., "Rattler Site Selectivity and Covalency Effects in Type-I

Clathrates," Journal of the Physical Society of Japan, vol. 82, no. 1, p.

014703, 2013/01/15 2012.

[46] C. Stiewe et al., "Nanostructured Co1− xNix (Sb1− yTey) 3 skutterudites:

theoretical modeling, synthesis and thermoelectric properties," Journal of

applied physics, vol. 97, no. 4, p. 044317, 2005.

[47] Z. He et al., "Effect of ceramic dispersion on thermoelectric properties of

nano-ZrO 2/CoSb 3 composites," Journal of applied physics, vol. 101, no. 4,

pp. 043707-043707-7, 2007.

[48] M. S. Toprak et al., "The impact of nanostructuring on the thermal

conductivity of thermoelectric CoSb3," Advanced Functional Materials, vol.

14, no. 12, pp. 1189-1196, 2004.

[49] L. Bertini et al., "Nanostructured Co1− xNixSb3 skutterudites: Synthesis,

thermoelectric properties, and theoretical modeling," Journal of applied

physics, vol. 93, no. 1, pp. 438-447, 2003.

[50] K. Ahn, C. Li, C. Uher, and M. G. Kanatzidis, "Improvement in the

thermoelectric figure of merit by La/Ag cosubstitution in PbTe," Chemistry of

Materials, vol. 21, no. 7, pp. 1361-1367, 2009.

[51] J. Androulakis, I. Todorov, J. He, D.-Y. Chung, V. Dravid, and M.

Kanatzidis, "Thermoelectrics from Abundant Chemical Elements: High-

Performance Nanostructured PbSe–PbS," Journal of the American Chemical

Society, vol. 133, no. 28, pp. 10920-10927, 2011.

[52] C.-I. Wu et al., "Thermoelectric Properties of Pulsed Electric Current

Sintered Samples of AgPb m SbSe17 (m= 16 or 17)," Journal of electronic

materials, vol. 41, no. 6, pp. 1579-1582, 2012.

[53] M. Fedorov, "THERMOELECTRIC SILICIDES: PAST, PRESENT AND

FUTURE," J Thermoelectr, pp. 51-60, 2009.

[54] S. Battiston et al., "Synthesis and characterization of Al-Doped Mg2Si

thermoelectric materials," Journal of electronic materials, vol. 42, no. 7, pp.

1956-1959, 2013.

[55] M. Saleemi et al., "Spark plasma sintering and thermoelectric evaluation of

nanocrystalline magnesium silicide (Mg2Si)," Journal of Materials Science,

vol. 48, no. 5, pp. 1940-1946, 2013.

[56] A. Famengo et al., "Phase Content Influence on Thermoelectric Properties of

Manganese Silicide-Based Materials for Middle-High Temperatures,"

Journal of electronic materials, vol. 42, no. 7, pp. 2020-2024, 2013.

[57] P. Jood et al., "Al-doped zinc oxide nanocomposites with enhanced

thermoelectric properties," Nano letters, vol. 11, no. 10, pp. 4337-4342, 2011.

[58] K. H. Jung, S. M. Choi, C. H. Lim, W. S. Seo, and H. H. Park, "Phase

analysis and thermoelectric properties of Zn1− xMxO (M= Al, Ga) samples,"

Surface and Interface Analysis, vol. 44, no. 11-12, pp. 1507-1510, 2012.

[59] J. He, Y. Liu, and R. Funahashi, "Oxide thermoelectrics: the challenges,

progress, and outlook," Journal of Materials Research, vol. 26, no. 15, pp.

1762-1772, 2011.

[60] H. Yamaguchi, Y. Chonan, M. Oda, T. Komiyama, T. Aoyama, and S.

Sugiyama, "Thermoelectric properties of ZnO ceramics co-doped with Al and

transition metals," Journal of electronic materials, vol. 40, no. 5, pp. 723-

727, 2011.

[61] A. P. Gonçalves and C. Godart, "New promising bulk thermoelectrics:

intermetallics, pnictides and chalcogenides," The European Physical Journal

B, vol. 87, no. 2, pp. 1-29, 2014.

[62] O. Appel, M. Schwall, D. Mogilyansky, M. Köhne, B. Balke, and Y.

Gelbstein, "Effects of Microstructural Evolution on the Thermoelectric

Properties of Spark-Plasma-Sintered Ti0. 3Zr0. 35Hf0. 35NiSn Half-Heusler

Compound," Journal of electronic materials, vol. 42, no. 7, pp. 1340-1345,

2013.

[63] C. S. Birkel et al., "Improving the thermoelectric properties of half-Heusler

TiNiSn through inclusion of a second full-Heusler phase: microwave

preparation and spark plasma sintering of TiNi 1+ x Sn," Physical Chemistry

Chemical Physics, vol. 15, no. 18, pp. 6990-6997, 2013.

[64] B. Balke, J. Barth, M. Schwall, G. H. Fecher, and C. Felser, "An alternative

approach to improve the thermoelectric properties of half-Heusler

compounds," Journal of electronic materials, vol. 40, no. 5, pp. 702-706,

2011.

[65] Z. Tian, S. Lee, and G. Chen, "A comprehensive review of heat transfer in

thermoelectric materials and devices," Ann. Rev. Heat Transfer, vol. 17, pp.

425-483, 2014.

[66] J.-F. Li, W.-S. Liu, L.-D. Zhao, and M. Zhou, "High-performance

nanostructured thermoelectric materials," NPG Asia Materials, vol. 2, no. 4,

pp. 152-158, 2010.

[67] H. Liu et al., "Copper ion liquid-like thermoelectrics," Nature materials, vol.

11, no. 5, pp. 422-425, 2012.

[68] M. Li, S. M. Kazi Nazrul Islam, S. Dou, and X. Wang, "Significantly

enhanced figure-of-merit in graphene nanoplate incorporated Cu2Se

fabricated by spark plasma sintering," Journal of Alloys and Compounds, vol.

769, pp. 59-64, 2018/11/15/ 2018.

[69] L. Zhao et al., "Significant enhancement of figure-of-merit in carbon-

reinforced Cu2Se nanocrystalline solids," Nano Energy, vol. 41, no.

Supplement C, pp. 164-171, 2017/11/01/ 2017.

[70] G. Tan et al., "Non-equilibrium processing leads to record high

thermoelectric figure of merit in PbTe–SrTe," Nature communications, vol. 7,

p. 12167, 2016.

[71] A. T. Duong et al., "Achieving ZT=2.2 with Bi-doped n-type SnSe single

crystals," Nature Communications, Article vol. 7, p. 13713, 12/12/online

2016.

[72] L.-D. Zhao et al., "Ultralow thermal conductivity and high thermoelectric

figure of merit in SnSe crystals," Nature, vol. 508, no. 7496, p. 373, 2014.

[73] H. Liu et al., "Copper ion liquid-like thermoelectrics," Nat Mater,

10.1038/nmat3273 vol. 11, no. 5, pp. 422-425, 05//print 2012.

[74] L.-l. Zhao et al., "Superior intrinsic thermoelectric performance with zT of

1.8 in single-crystal and melt-quenched highly dense Cu2-xSe bulks,"

Scientific reports, vol. 5, p. 7671, 2015.

[75] Q. Zhang et al., "Enhancement of thermoelectric figure-of-merit by resonant

states of aluminium doping in lead selenide," Energy & Environmental

Science, vol. 5, no. 1, pp. 5246-5251, 2012.

[76] M. Hong et al., "Realizing zT of 2.3 in Ge1− x− ySbxInyTe via Reducing the

Phase‐ Transition Temperature and Introducing Resonant Energy Doping,"

Advanced Materials, vol. 30, no. 11, p. 1705942, 2018.

[77] A. Zevalkink, J. Swallow, and G. J. Snyder, "Thermoelectric properties of

Zn-doped Ca 5 In 2 Sb 6," Dalton Transactions, vol. 42, no. 26, pp. 9713-

9719, 2013.

[78] J. P. Heremans, M. S. Dresselhaus, L. E. Bell, and D. T. Morelli, "When

thermoelectrics reached the nanoscale," Nature nanotechnology, vol. 8, no. 7,

pp. 471-473, 2013.

[79] J. Yang, W. Zhang, L. Chen, and G. P. Meisner, "Potassium and Sodium

Filled Skutterudites," ed: Google Patents, 2006.

[80] G. A. Slack and D. Rowe, "CRC handbook of thermoelectrics," CRC, Boca

Raton, FL, vol. 407440, 1995.

[81] X. Shi, W. Zhang, L. Chen, and J. Yang, "Filling fraction limit for intrinsic

voids in crystals: Doping in skutterudites," Physical review letters, vol. 95,

no. 18, p. 185503, 2005.

[82] X. Shi et al., "Low thermal conductivity and high thermoelectric figure of

merit in n-type BaxYbyCo4Sb12 double-filled skutterudites," Applied

Physics Letters, vol. 92, no. 18, p. 182101, 2008.

[83] X. Shi et al., "Multiple-filled skutterudites: high thermoelectric figure of

merit through separately optimizing electrical and thermal transports,"

Journal of the American Chemical Society, vol. 133, no. 20, pp. 7837-7846,

2011.

[84] G. S. Nolas, G. A. Slack, and S. B. Schujman, "Semiconductor clathrates: A

phonon-glass electron-crystal material with potential for thermoelectric

applications," Semiconductors and semimetals, vol. 69, pp. 255-300, 2000.

[85] S. Stefanoski, "Synthesis and Physical Properties Investigations of

Intermetallic Clathrates," 2010.

[86] W. Liu et al., "n-type thermoelectric material Mg2Sn0. 75Ge0. 25 for high

power generation," Proceedings of the National Academy of Sciences, vol.

112, no. 11, pp. 3269-3274, 2015.

[87] N. N. Greenwood and A. Earnshaw, Chemistry of the Elements. Elsevier,

2012.

[88] M. Francombe, "Structure-cell data and expansion coefficients of bismuth

telluride," British Journal of Applied Physics, vol. 9, no. 10, p. 415, 1958.

[89] W. Huang, H. Da, and G. Liang, "Thermoelectric performance of mx2 (m=

mo, w; x= s, se) monolayers," Journal of Applied Physics, vol. 113, no. 10, p.

104304, 2013.

[90] A. Bentien, G. K. H. Madsen, S. Johnsen, and B. B. Iversen, "Experimental

and theoretical investigations of strongly correlated ${\mathrm{FeSb}}_{2$-

${}x}{\mathrm{Sn}}_{x}$," Physical Review B, vol. 74, no. 20, p. 205105,

11/07/ 2006.

[91] B. Huang, X. Yang, L. Liu, and P. Zhai, "Effects of Van der Waals Bonding

on the Compressive Mechanical Behavior of Bulk Bi2Te3: A Molecular

Dynamics Study," (in English), Journal of Electronic Materials, vol. 44, no.

6, pp. 1668-1673, 2015/06/01 2015.

[92] J. Ko, J.-Y. Kim, S.-M. Choi, Y. S. Lim, W.-S. Seo, and K. H. Lee,

"Nanograined thermoelectric Bi 2 Te 2.7 Se 0.3 with ultralow phonon

transport prepared from chemically exfoliated nanoplatelets," Journal of

Materials Chemistry A, vol. 1, no. 41, pp. 12791-12796, 2013.

[93] S. M. Nazrul Islam, "Effectiveness of mill scale on the decomposition of dye

ingredients for the treatment of textile effluent," 2013.

[94] S. M. K. N. Islam, A. Kurny, and F. Gulshan, "Photocatalytic efficiency of

mill scale for the degradation of textile dye by photo Fenton and photo-

ferrioxalate system under UV and sunlight," Environment and Ecology

Research, vol. 1, no. 3, pp. 129-134, 2013.

[95] S. M. K. N. Islam, A. Kurny, and F. Gulshan, "Photocatalytic Degradation of

Methylene Blue By H2o2 And Oxalic Acid in Presence 0f Mill Scale."

[96] S. K. Islam, A. S. W. Kurny, and F. Gulshan, "Degradation of Commercial

Dyes Using Mill Scale by photo-Fenton," (in English), Environmental

Processes, vol. 2, no. 1, pp. 215-224, 2015/03/01 2015.

[97] H. Chi et al., "Low-temperature structural and transport anomalies in

${\mathrm{Cu}}_{2}\mathrm{Se}$," Physical Review B, vol. 89, no. 19, p.

195209, 05/28/ 2014.

[98] H. Liu et al., "Ultrahigh Thermoelectric Performance by Electron and Phonon

Critical Scattering in Cu2Se1‐ xIx," Advanced Materials, vol. 25, no. 45, pp.

6607-6612, 2013.

[99] S. Ballikaya, H. Chi, J. R. Salvador, and C. Uher, "Thermoelectric properties

of Ag-doped Cu 2 Se and Cu 2 Te," Journal of Materials Chemistry A, vol. 1,

no. 40, pp. 12478-12484, 2013.

[100] B. Yu et al., "Thermoelectric properties of copper selenide with ordered

selenium layer and disordered copper layer," Nano Energy, vol. 1, no. 3, pp.

472-478, 5// 2012.

[101] L.-l. Zhao et al., "Superior intrinsic thermoelectric performance with zT of

1.8 in single-crystal and melt-quenched highly dense Cu2-xSe bulks,"

Scientific reports, vol. 5, 2015.

[102] L. Zhao et al., "The Effects of Te2− and I− Substitutions on the Electronic

Structures, Thermoelectric Performance, and Hardness in Melt‐ Quenched

Highly Dense Cu2‐ xSe," Advanced Electronic Materials, vol. 1, no. 3, 2015.

Chapter 2

2. Experimental methods

2.1. Materials fabrication

Samples were fabricated using two different approaches: one is the melt solidification technique and the other is the spark plasma sintering method.

2.2. Spark plasma sintering (SPS) method

Firstly, the polycrystalline Cu2Se powder was mixed by hand using an agate mortar and a pestle accordingly Next, the well mixed powder was put in a 20 mm diameter graphite die under a pressure of about 40 MPa in vacuum with a temperature of 773

K for 10 minutes using SPS model 10-4 (figure 2-1 a). Finally, the highly dense

20mm diameter and 2 mm thick solid sample was formed.

2.3. Melt solidification technique

Firstly, the raw materials were mixed together by hand using an agate mortar and a pestle. The well mixed powder was made into pellets in an stainless steel die with a hydraulic hand pump. The pellets were then sealed in an evacuated quartz tube using oxyacetylene flame (figure 2-1b). Finally, the desired samples were obtained upon heating up using a vertical mosilli furnace (figure 2-1c) in a temperature above its melting point. In this thesis, the copper selenide powders were heated to above

1200℃ for just 10 min with a heating rate of 10 K/min, followed by a furnace cooling to room temperature.

Figure 2-1 (a) Spark Plasma Sintering (Thermal Technology SPS model 10-4); (b)

Oxy-Acetyline Kit; (c) Vertical Mosilli furnace

2.4. Materials characterisation techniques

2.4.1. Sample preparation

After fabrication, samples were then cut into rectangular bars and round disks using a cutting machine (Struers Accutom-50) (figure 2-2 a) for electrical and thermal transport measurements, respectively. The Struers Citropress 25 (figure 2-2 b) was used to mount the samples with Polyfast before being polishing with Struers

Tegramin-20 (figure 2-2 c). Then the samples were used for the microscopy and

Hardness measurements. Vickers microhardness tester (Struers DuraScan-70

Hardness, Figure 2-2 d) was used with a 0.245 N load in Lens (60x).

Figure 2-2 (a) Struers accutom 50 (inset: after cut sample); (b) Struers Citropress 25;

2.4.2. X-ray powder diffraction (XRD)

The room temperature powder diffraction patterns were determined by X-ray diffractometry (Cu Kα, GBC MMA,λ=1.5418 Å), with 10º ≤ 2θ ≤ 60°, in step scan mode with a step size of 0.014° and speed of 1 degree per min (figure 2-3 a). The

Australian Synchrotron was used to collect high resolution data using a wavelength of λ = 0.58973 Å, where the detectors covered an angular range of 2.5º ≤ 2θ ≤ 80°, simultaneously every 30 seconds and the sample was rotated at ~1 Hz with a heating rate of 4 deg./min under a helium gas ﬂow over the 300−774 K temperature range

(figure 2-3 b). The sample for the synchrotron measurement was prepared as follows:

A piece of the consolidated material about 60 mg was chipped off and ground using a mortar and a pestle. The resulting powder was transferred into a 0.5 mm outer- diameter quartz capillary tube. The capillary was ﬁlled up to about 20 mm in height and, importantly, the powder was tightly packed by placing in a sonicator (while holding the capillary lightly in hand).

Figure 2-3 (a) GBC MMA XRD measurements; (b) High temperature synchrotron

XRD (10-BM-1)

2.4.3. Scanning electronic microscopy

Two types of scanning electronic microscopy SEM (JEOL JSM-6490LV) and FE-

SEM (JEOL JSM-7001F) were used to investigate the samples' morphologies and microstructures in room temperature under vacuum (Figure 2-4 a and b). The

64 conductive carbon tape was used to stuck samples with the holder to check the characterised the freshly fractured samples. Polyfast were used to mount samples using a struers citopress-20 equipment and polished with a struers tegramin-20 water equipment, then used for scanning electron microscopy for the surface morphology.

Energy-dispersive X-ray spectroscopy (EDS) was used to study the compositions of the samples using. A 15 KV voltage and a 20mA current were used for conducting the EDS measurements.

2.4.4. Scanning transmission electronic microscopy

Scanning transmission electron microscopy (JEOL ARM200F) was used to check the general morphology in high-resolution images. The thin TEM specimens were prepared using focused ion beam milling (FIB) (figure 2-4 c) after being mounted in a Poly-fast using Struers Citopress-20 equipment and polished with a Struers

Tegramin-20 water equipment, the prepared sample then used for scanning transmission electron microscopy/x-ray spectroscopy (STEM/x-ray mapping) investigations (figure 2-4 d). The Fast Fourier Transforms (FFTs) of high resolution

TEM (HRTEM) images were used to calculate the Lattice fringes using the Gatan software.

2.4.5. Electrical transport measurements

2.4.5.1. Electrical conductivity and Seebeck coefficients measurements

The electrical conductivity and Seebeck coefficient were measured simultaneously using the direct current method under an Ozawa Rz2001i system (figure 2-5 a). The temperature range used was from 300 to 1000 K. The four-point method was used to

Figure 2-4 (a) SEM (JEOL JSM-6490LV); (b) FE-SEM (JEOL JSM-7001F); (c) FIB

(FEI Helios G3 CX) fabricate samples cut into rectangular bar and wrapped using parallel platinum wires for obtaining the voltage during the measurements.

2.4.6. Thermal transport measurements

2.4.6.1. Differential Scanning Calorimetry (DSC)

The DSC measurements were conducted to find the heat capacity (Cp) of the samples. Final DSC data of the sample is obtained by collecting data of blank crucible and reference samples (single crystal of Al2O3) by a DSC-204F1 Phoenix calorimeter under an argon gas flow, with a rate of 50 ml/min. Heat capacity (Cp) was calculated using equation 1 where Ae, Ae+r, Ae+S, are the measured heat of empty crucible, empty crucible with reference and empty crucible with sample, respectively followed by same heating process. , , and are the sample mass, reference samples mass and heat capacity of the reference samples, respectively. All the measurements were done in argon atmosphere so to protect the sample from oxidation.

(1)

2.4.6.2. Thermal diffusivity measurements

The thermal diffusivity measurements were accomplished using Linseis Laserflash -

LFA 1000 (figure 2-5c). the measurement temperature range was from 300 to 1000

K. Thin round disc shapes were cut from the fabricated samples and placed in a high intensity short-duration radiant energy pulse inside the thermal diffusivity instrument. The samples first absorbed energy from the front surface after the laser pulse hit the samples and travelled through it and finally reached to the rear face of the sample. The resultant temperature value, which is measured as a percentage to its maximum value is recorded by the liquid nitrogen cooled IR detector (figure 2-6).

During the measurement obtaining results were repeatedly checked to ensure the accuracy of the measurements.

Figure 2-5 (a) Seebeck measurements (RZ 2001i), (b) Heat capacity measurements

(Differential Scanning Calorimetry - NETZSCH DSC 204 F1 Phoenix), (c) Thermal

diffusivity measurements (Linseis Laserflash - LFA 1000)

Figure 2-6 Schematic illustration of the Linseis LFA 1000 thermal diffusivity

instrument showing the operation in the vertical mode

2.4.6.3. Physical property measurement system

Room temperature Hall coefficient measurements were performed to determine the carrier concentration and the carrier mobility using a physical properties measurement system (Quantum Design PPMS-9) shown in figure 2-7 over the temperature of 5 to 300 K and with -8 to 8 Tesla magnetic fields range. The carrier concentration p and mobility were calculated using the formula ,

where e represents the elemental charge and RH the Hall coefficient.

Figure 2-7 Physical properties measurement system (Quantum Design PPMS-9)

Chapter 3

3. Significant enhancement of figure-of-merit in carbon-reinforced Cu2Se nanocrystalline solids

3.1. Introduction

To meet global challenges and reduce the ongoing depletion of fossil fuels, thermoelectric (TE) energy conversion has been extensively explored to harvest thermal energy and produce electricity.[1-3] Solid-state thermoelectric generators have the advantages of high reliability, long operational lifetimes, vibration-free operation, and environmental friendliness[4-6]. The thermoelectric performance of specific materials, however, must be matched to the operating conditions where they will be deployed. This has motivated a widespread search for suitable materials for conditions ranging from deep space missions[7], harvesting waste heat from power plants[8] and automotive exhaust systems[9]. Many important energy-intensive technological processes generate thermal gradients between 500 and 750 K, for example, the boiling of biodiesel fuels, and commercial ammonia synthesis reactions. It is therefore critical to find optimal thermoelectric materials that operate in the latter temperature range.

The perfect thermoelectric materials should be environmentally friendly, inexpensive and high efficiency however existing materials (eg. Mg2Sn0.75Ge0.25[10, 11], PbTe based materials[12-14]) only partly fulfil these criteria in this region of interest. In contrast, Cu2-xSe is one of the most promising thermoelectric materials with an outstanding figure-of-merit (zT > 1.4) at high temperatures (> 800 K).[15, 16]

Unfortunately, the enhancement of zT has been moderate in the temperature range below 800 K. Furthermore, at high temperatures, Cu2-xSe is a superionic conductor in

the β-phase cubic structure, and the migration of copper ions plays a dominant role in its electrical and thermal conductivity, however, it has been difficult to utilize the material because copper segregation leads to reduced long-term performance.[17]

So far, there have been some efforts to further improve the thermoelectric performance and control the copper motion of the Cu2Se system through using various strategies including doping[18-23], manipulation of the electronic structure[20] and shrinking the grain size of pure Cu2Se to sub-micron dimensions[16]. In contrast, relatively few studies have analysed the effect of composite nanostructures containing Cu2Se and a second phase[24, 25] although this is a well-known strategy in other thermoelectric materials[26].

In this work, I conceptually propose that the strategic design of nano-interfaces in

Cu2Se with a second nanophase can lead to enhanced thermoelectric performance and potentially reduce long-range copper diffusion by acting as a confinement barrier for heat and ion motion. I propose that a strong and light element, such as carbon, is ideal for this purpose. To demonstrate this experimentally, I report a facile synthesis method that generates a composite of nanocrystalline Cu2Se solid with a high interface-to-bulk ratio containing carbon-graphite nanostructure inclusions. I demonstrate that addition using various solid carbon sources can significantly enhance the thermoelectric performance of the Cu2Se system over a wide temperature range. zT as high as 1.85 at

900 K and 2.4 at 850 K was achieved for the Super P and carbon fiber doped Cu2Se samples, respectively. These values for the carbon doped Cu2Se samples are comparable or superior to those for the current state-of-the-art thermoelectric materials.

3.2. Experimental Section

3.2.1. Synthesis

The raw materials used for fabricating the carbon doped Cu2Se composites were powders of Cu2Se, carbon nanotubes, graphite, hard carbon, Super P, and diamond carbon fibers. Firstly, the Cu2Se and the carbon powders were mixed in the weight ratios of 1: x (x = 0.1%, 0.2%, 0.4%, 0.6%, 0.8%). Then, the mixtures were pressed into pellets and sealed in evacuated quartz tubes before being heated to above 1200 K for just 10 min with a heating rate of 10 K/min, followed by a furnace cooling to room temperature. Finally, the obtained polycrystalline bulks were shaped into round disks and rectangular bars for thermal diffusivity and electrical conductivity measurements, respectively. The carbon-doped Cu2Se samples are denoted by their carbon weight percent and their carbon source, so that the sample with 0.1wt% carbon nanotubes was denoted as 0.1 CNT, etc.

3.2.2. Measurements X-ray diffraction (XRD) patterns were collected on a GBC MMA system using Cu

Kαradiation within the 2Ө range from 10° to 60°. Field emission scanning electron microscopy (FE-SEM, JEOL 7500) and transmission electron microscopy (TEM,

JEOL EM 2010) were used to reveal the phase composition and microstructure of as- synthesized bulk samples. The electrical conductivity and Seebeck coefficient were measured simultaneously under vacuum from room temperature to 900 K using a commercial RZ2001i system. The thermal diffusivity (D) was measured by the laser flash method (LINSEIS LFA 1000) under vacuum conditions. The specific heat (Cp) was determined by differential scanning calorimetry on a DSC-204F1 Phoenix under argon atmosphere with a flow rate of 50 ml/min. The sample density (dd) was

73 calculated using the measured weight and dimensions, and for some samples, dd was also determined by the Archimedes method. The thermal conductivity (κ) was calculated by κ= D × Cp × dd. All the electrical conductivity and thermal diffusivity measurements were repeated several times to confirm the reproducibility of as- prepared polycrystalline bulks.

3.3. Results and discussions

Using the melt-solidification method similar to our previous work [16], I investigated several carbon sources to determine the resulting crystal structure and microstructure.

Briefly, the bulk samples were fabricated by a solid state reaction followed by one melt-solidification approach, which is efficient and low-cost. Figure 3-1a shows the schematic representation of the fabrication process of the graphite doped Cu2Se samples. Figure 3-1b shows the crystal structure for the low temperature α-phase

Cu2Se. The low temperature α-phase Cu2Se is crystallized in a monoclinic structure with a space group of C 2/c and lattice parameters of a=7.1379(4) Å, b=12.3823(7) Å, c=27.3904(9) Å, and β=94.308(5)º.[27] X-ray diffraction (XRD) patterns for hexagonal structured carbon, monoclinic structured Cu2Se, and as-fabricated carbon doped Cu2Se samples with different carbon sources, such as graphite(G), super P(SP), carbon nanotubes(CNT)

Figure 3-1 Schematic representation of the fabrication process of the graphite doped

Cu2Se samples(a), crystal structure for the low temperature α-phase Cu2Se (b) and X-

ray diffraction patterns(c) for hexagonal structured carbon, monoclinic structured

Cu2Se, and as-fabricated carbon doped Cu2Se samples with different carbon sources

such as graphite(G), super P(SP), carbon nanotubes(CNT), and hard carbon (H)

and hard carbon(H), and various weight doping levels of 0.1wt%, 0.2wt%, 0.4wt%,

0.6wt%, and 0.8wt% are displayed in figure 3-1c. The XRD pattern for the monoclinic structured Cu2Se was generated from the unit cell displayed in figure 3-1b using

Materials Studio. The characteristic peak for hexagonal structured carbon located around a 2θ degree of 26.4º is marked with a* symbol in the enlarged XRD pattern displayed in figure 3-1c. The patterns indicate that all carbon doped Cu2Se samples are composites of hexagonal structured carbon and monoclinic structured Cu2Se. The 75 characteristic peak for hexagonal structured carbon can be easily found in almost all the carbon doped samples except the super P and carbon nanotubes doped samples with the doping levels of 0.4wt%, 0.6wt%, and 0.8wt%. For the super P and carbon nanotube doped Cu2Se samples with the carbon doping level over 0.4wt%，the characteristic peak of carbon exhibit almost the same 2θ degree with the characteristic peaks for monoclinic structured Cu2Se, hence, it cannot be detected separately. It indicates that the resulting material does not depend closely on the solid carbon precursor, and the carbon-doped Cu2Se (xwt%C + Cu2Se) using graphite(G), carbon nanotubes (CNTs), hard carbon (H), and Super P (SP) precursors are all composites of

Cu2Se and carbon.

Figure 3-2 Low magnification TEM image for the fabricated graphite doped Cu2Se

(0.8wt%) bulk sample(a), and high resolution TEM images showing the lattice and

interface between graphite nanoclusters and Cu2Se (b)

Transmission electron microscopy (TEM) was used to clarify the local structures of the samples and indicates a fine-grained nanostructured Cu2Se material containing a small amount of carbon inclusions. Figure 3-2a shows the low magnification TEM image for the fabricated graphite doped Cu2Se bulk sample at a weight fraction of

0.8% conducted in a HAADF mode. The carbon regions appear as light patches near the interfaces within the primary phase, with the carbon component confirmed by EDS mapping shown in figure 3-3. The clusters tend to appear at the grain boundary regions of Cu2Se indicating precipitation of dissolved carbon out when the liquid-phase Cu2Se is cooled. It is clear from the enlarged image (Figure 3-2b) that the grain size for the

Cu2Se is small (d = 30-60 nm), indicating the carbon precipitates acted as nucleation sites and prevent grain ripening. As the density of interfaces approximately scales as

1/d, the small grain size indicates a high-volume fraction of atoms within a nanometre of interface (9%). As the volume fraction of carbon (~3%) is insufficient to achieve complete encapsulation of the Cu2Se grains, the graphite appears as nanoparticle inclusions that partially cover the interfaces. High resolution TEM images were conducted to focus on the boundary regions and show the lattice and interface between graphite nanoclusters and Cu2Se. The (004), (131) and (135) planes with lattice distance around 0.68nm, 0.35nm and 0.29nm, respectively, for the monoclinic structured Cu2Se were marked in (Figure 3-2b). In addition, hexagonal graphite structure [28, 29] could be clearly identified near a boundary region. Figure 3-2b also shows obvious dislocations in the graphite doped Cu2Se bulk sample, which can affect the lattice thermal conductivity [30].

Figure 3-3 Fractured surface morphology and EDS mapping for the Cu, Se, and C in

the fabricated 8wt% graphite doped Cu2Se sample, respectively

To experimentally determine how the carbon-decorated nano-interfaces affect the thermoelectric performance in Cu2Se, I have studied the temperature dependent electrical conductivity, thermal conductivity, Seebeck coefficient, thermoelectric

1200 300 (a) 0.0 (b) 1000 0.6 CNT 250 0.6 G

0.6 H )

) 800 200 -1

-1 0.6 SP

K 

cm 0.3 CF 

600 V 150



(

( 

 400 S 100

200 50

0 0 300 400 500 600 700 800 900 300 400 500 600 700 800 900 T (K) T (K) 3.0 2.5

2.5 (c) (d)

2.0 PbTeSe )

-1 2.0 K

 1.5

-1 m

1.5 zT PbTe 

W 1.0 (

 1.0

0.5 0.5

0.0 0.0 300 400 500 600 700 800 900 300 400 500 600 700 800 900 T (K) T (K)

Figure 3-4 Temperature dependence of the (a) electrical conductivity (), (b) Seebeck

coefficient (S), (c) thermal conductivity (κ), and (d) figure-of-merit (zT) of typical

Cu2Se samples with a carbon doping level of 0.6wt% from various carbon sources, and

0.3wt% in the case of carbon fibers. The zT values for the state-of-the-art p-type

middle temperature thermoelectric materials of PbTe and PbTeSe are provided for

comparison [31-33]

compatibility as well as the micro hardness for carbon-free and all carbon-doped samples. Figure 3-4 shows the temperature dependence of the electrical conductivity,

Seebeck coefficient, thermal conductivity, and figure-of-merit for a few typical Cu2Se samples with 0.6 and 0.3wt% doping levels of various sources such as carbon nanotubes, graphite, super P, hard carbon and carbon fibers.

Figure 3-4a demonstrates that all samples show decreased electrical conductivity with 79 temperature. Specifically, compared to the carbon-free Cu2Se sample, the CNT and graphite doped samples have comparable and slightly reduced electrical conductivity, respectively, while all the hard carbon, super P, and carbon fiber doped samples exhibit much higher electrical conductivity over the whole measured temperature range, indicating that these carbon inclusions increase the electrical conductivity, σ.

These differences in σ may be related to intrinsic high electrical conductivity of carbon as well as the different microstructures. Figure 3-5 shows the field emission scanning electron microscope (FE-SEM) images of the commercial carbon nanotubes (CNTs), graphite, hard carbon and super P. It demonstrates that both CNTs and super P show typical structures of nanotubes and nanoparticles, respectively, while the graphite and hard carbon are micro structured layers and particles, respectively. It is well known that carbon materials show quite good electrical conductivity, and carbon particles have been successfully applied as conductive fillers to improve the electrical conductivity of polymers.[34-38] The low dimensional carbon materials such as carbon nanotubes and graphite, however, exhibit highly anisotropic electrical and thermal conductivity along the parallel and perpendicular direction of the tube/layer.[39, 40] Extremely high electrical and thermal conductivity could be found along the tube/layer direction, while greatly decreased electrical and thermal conductivity could be observed along the perpendicular direction of the tube/layer.

Additionally, crystallographic defects could strongly affect their electrical and thermal properties. Therefore, different variation trends could be observed for the temperature dependent electrical conductivity of these carbon doped Cu2Se composites with different carbon sources.

Figure 3-5 Field emission scanning electron microscope (FE-SEM) images of the commercial (a, e) carbon nanotubes, (b) graphite, (c) hard carbon, and (d, f) super P

Figure 3-4b shows that all the carbon-doped Cu2Se samples have comparable Seebeck coefficients to the carbon-free Cu2Se for temperatures from 450 to 900 K. It is noted that all the carbon-doped Cu2Se samples exhibit greatly decreased thermal conductivity (~0.5 W∙m-1∙K-1) compared to the carbon-free sample (~0.8 W∙m-1∙K-1 displayed in Figure 3-4c in the same temperature range. Similar results are seen from the temperature dependence of , S, κ, and zT for the carbon-doped Cu2Se samples with other doping levels (0.1wt%, 0.2wt%, 0.4wt%, and 0.8wt%) plotted in Figure 3-6 to 3-10.

1200 300 (a) 0.0 (b) 1000 0.1 CNT 250 0.1 G

800 0.1 H 200

) )

0.1 SP -1

-1



cm 

600 V 150



(



 400 S 100

200 50

0 0 300 400 500 600 700 800 900 300 400 500 600 700 800 900 T (K) T (K) 3.0 2.0 (c) (d)

2.5 1.6 )

-1 2.0 K

 1.2

-1 m

1.5 zT

 W

( 0.8

 1.0

0.5 0.4

0.0 0.0 300 400 500 600 700 800 900 300 400 500 600 700 800 900 T (K) T (K)

Figure 3-6 Temperature dependence of the electrical conductivity (), Seebeck

coefficient (S), thermal conductivity (κ), and figure-of-merit (zT) of typical samples

with carbon doping level of 0.1wt% from various carbon sources

Figure 3-4d indicates that compared to the carbon-free Cu2Se, all the carbon-doped

samples exhibit greatly enhanced zT values in the temperature range from 450 to 900

K. The zT values are over 1.0 for T > 600 K for all the carbon-doped Cu2Se samples.

Remarkably,

1200 300 (a) 0.0 (b) 1000 0.2 CNT 250 0.2 G

800 0.2 H 200

)

) -1

-1 0.2 SP



cm 

600 V 150



( (  400 S 100

200 50

0 0 300 400 500 600 700 800 900 300 400 500 600 700 800 900 T (K) T (K) 3.0 2.0 (c) (d)

2.5 1.6 )

-1 2.0 K

 1.2

-1 m

1.5 zT

 W

( 0.8

 1.0

0.5 0.4

0.0 0.0 300 400 500 600 700 800 900 300 400 500 600 700 800 900 T (K) T (K)

Figure 3-7 Temperature dependence of the electrical conductivity (), Seebeck

coefficient (S), thermal conductivity (κ), and figure-of-merit (zT) of typical samples

with carbon doping level of 0.2wt% from various carbon sources

the 0.6wt% super P doped Cu2Se sample has a zT of 1.85 at 900 K. More excitingly,

the 0.3wt% CF doped Cu2Se shows zT > 1 for T > 520 K and reaches a record level of

zT of ~ 2.4 at 850 K. The enhanced thermoelectric performance of the carbon-

containing Cu2Se nanocomposites rivals state-of-the art thermoelectric materials. For

example, the zT values for the p-type thermoelectric materials PbTe and PbTeSe are

also provided in Figure 3-4d for comparison. The data indicates that the carbon-

containing Cu2Se nanocomposites have comparable or even higher zT values from 450

to 900 K than those of the PbTe and PbTeSe polycrystalline bulks.

1200 300 (a) (b) 1000 0.0 250 0.4 CNT 0.4 G

800 200 )

) 0.4 H -1

-1 0.4 SP



cm 

600 V 150



( (  400 S 100

200 50

0 0 300 400 500 600 700 800 900 300 400 500 600 700 800 900 T (K) T (K) 3.0 2.0 (c) (d)

2.5 1.6 )

-1 2.0 K

 1.2

-1 m

1.5 zT

 W

( 0.8

 1.0

0.5 0.4

0.0 0.0 300 400 500 600 700 800 900 300 400 500 600 700 800 900 T (K) T (K)

Figure 3-8 Temperature dependence of the electrical conductivity (), Seebeck

coefficient (S), thermal conductivity (κ), and figure-of-merit (zT) of typical samples

with carbon doping level of 0.4wt% from various carbon sources

Figure 3-11 shows the temperature dependence of the (a) /0.0, (b) S/S0.0, (c) κ / κ 0.0,

and (d) zT/zT0.0 for the carbon doped Cu2Se samples with a doping level of 0.6wt%

from various carbon sources such as CNTs, graphite, hard carbon and super P, and

0.3wt% in the case of carbon fibres. It indicates that the hard carbon, super P and

carbon fiber doped samples show greatly increased electrical conductivity compared to 84 the pure Cu2Se sample, with the carbon fiber doped sample exhibiting the highest electrical conductivity around 1.350.0. All the carbon doped samples show comparable Seebeck coefficient values (around 0.9-1.1S0.0) with that of carbon-free

Cu2Se sample. Additionally, compared to the carbon-free Cu2Se sample, an obvious decrease in thermal conductivity can be found for all the carbon-containing Cu2Se

1200 300

(a) 0.0 (b) 1000 0.8 CNT 250 0.8 G

800 0.8 H 200

)

) -1

-1 0.8 SP



cm 

600 V 150



 (

 400 S 100

200 50

0 0 300 400 500 600 700 800 900 300 400 500 600 700 800 900 T (K) T (K) 3.0 2.0 (c) (d)

2.5 1.6 )

-1 2.0 K

 1.2

-1 m

1.5 zT

 W

( 0.8

 1.0

0.5 0.4

0.0 0.0 300 400 500 600 700 800 900 300 400 500 600 700 800 900 T (K) T (K)

Figure 3-9 Temperature dependence of the electrical conductivity (), Seebeck

coefficient (S), thermal conductivity (κ), and figure-of-merit (zT) of typical samples

with carbon doping level of 0.8wt% from various carbon sources

samples, with the lowest value around 0.5 κ / κ 0.0 observed for the 0.3CF sample. It should be noted that, the most significant change in the thermal conductivity occurs among all the thermoelectric parameters of electrical conductivity, Seebeck coefficient

85 and thermal conductivity. Furthermore, giant enhanced zT values were obtained for all the carbon-containing samples especially for the samples of 0.3CF and 0.6SP primarily thanks to the greatly decreased thermal conductivity.

The main mechanism for the greatly enhanced thermoelectric performance appears to be mainly caused by the modification of thermal conductivity. This is attributed to the reduction of transmission of heat by lattice vibrations through the complex heterogeneous nanostructure containing carbon precipitates at the grain boundaries of

Figure 3-10 Temperature dependence of the electrical conductivity (), Seebeck coefficient (S), thermal conductivity (κ), and figure-of-merit (zT) of carbon fiber doped

samples

Cu2Se. Using a simplified model, the lattice thermal conductivity in a polycrystalline solid is expressed as:

Where is the intrinsic conductivity in a single crystal, whereas is the

reduced conductivity in a nanogranular solid with average grainsize , and interface

resistance Rκ. In this model, the thermal resistance across the interface reduces the

1.6 1.6 (a) (b) 1.4 1.4

1.2 1.2

0.0 

 S

1.0 / 1.0 /

 S 0.0 0.8 0.6CNT 0.8 0.6G 0.6 0.6H 0.6 0.6SP 0.3CF 0.4 0.4 400 500 600 700 800 900 400 500 600 700 800 900 T (K) T (K) 1.6 (c) 2.4 (d) 1.4

1.2 2.0

0.0 0.0

 zT / / 1.0

 1.6 zT 0.8 1.2 0.6

0.4 0.8 400 500 600 700 800 900 400 500 600 700 800 900 T (K) T (K)

Figure 3-11 Temperature dependence of the (a) /0.0, (b) S/S0.0, (c) κ / κ 0.0, and (d)

zT/zT0.0 for the carbon doped Cu2Se samples with a doping level of 0.6wt% from

various carbon sources such as CNTs, graphite, hard carbon and super P, and 0.3wt%

in the case of carbon fibers 87 transmission of heat phonons, and this effect is amplified for nanocrystalline samples which contain many more interfaces. Rκ is an empirical parameter that model many effects including the percentage of the interface that is coated by the carbon inclusions, the strength of anharmonic interfacial bonding and the degree to which specular or diffuse reflectivity modifies phonon transport. Past work in pure Cu2Se has indicated

-9 2 -1 a negligible value of Rκ< 1×10 m ∙K∙W for the intrinsic Cu2Se:Cu2Se grain boundaries because large crystallites, micrograins and nanograins effectively produced nearly the same [16]. However, the inclusion of carbon near the interfaces

appears to greatly modify Rκ, in agreement with recent results on other thermoelectric materials[26]. Using the mean value of , and assuming that reduction from

-1 -1 -1 -1 -1 = 0.8 W∙m ∙K to poly = 0.5 W∙m ∙K is purely from increased interfacial

-7 2 -1 resistance, I can estimate the upper limit of Rκ = 0.45×10 m ∙K∙W . This value is very large but is smaller than comparable values for completely carbon-coated

-7 2 -1 graphene/skutteride micrograined composites (Rκ = 3.8 ×10 m ∙K∙W ) [26].

A complete mechanistic understanding of the enhanced Rκ at solid-solid interfaces is challenging, because many factors influence the empirical Rκ parameter including the phonon-density of states, local interface nanostructure, surface disorder and interface bonding configurations[41, 42]. It is, however, possible to make some general statements using the simple acoustic mismatch model to qualitatively explain why the graphite: Cu2Se interface has a high value of Rκ. Within this model, phonons undergo specular reflections at the interface of two dissimilar materials with a reflection rate determined by the acoustic impedance (ZA) of each layer, which in turns depends on mass density (ρ) and average phonon group velocity (c). The carbon phases have very

-3 -3 different mass densities of ( = 2.33 g∙cm ) compared to Cu2Se ( = 7.1 g∙cm ).

The longitudinal speed of sound in each media is also different

), as are the transverse speeds of sound ( . For the simplest model of perfect planar interfaces, where no diffuse scattering occurs, the thermal resistivity can be calculated from the ratios of , , , using the model according Ref.[41], assisted by the numerical integrals tabulated by Cheek[43].

Using this approximate theory, the resistivity of idealized Cu2Se:C interface is

3 4 2 -1 expected to scale as Rκ T =126 K cm W which is significantly larger than the acoustic impedance reported for other solid state interfaces in the same model

3 framework (for example aluminium: sapphire only gives a value of Rκ T =21

4 2 -1 K cm W ) [41]. The high thermal resistivity for the model Cu2Se:C interface is however, only slightly higher than other idealised interfaces using carbon allotropes

3 4 2 -1 such as the diamond: indium interface (Rκ T =88 K cm W )[41]. While the model therefore correctly predicts the Cu2Se:C interface is particularly good at reflecting phonons, it is noteworthy that the simple model underestimate the Rκ values significantly because it neglects important factors such as the diffusive scattering of phonons and local bonding effects in complex nano-morphologies, which can greatly enhance the high temperature thermal boundary resistance, and prevent the T-3 scaling

[41]. In particular, past work showed van der Waals interactions play an important role in enhancing thermal boundary resistance, and these will introduce additional enhancement for carbon allotropes [42]. Based on this understanding, Figure 3-12 schematically indicates the resulting picture of how graphite inclusion effects the

89 microstructure and phonon transport property of the nanocomposite sample to reduce thermal conductivity and enhance the thermoelectric figure-of-merit.

Figure 3-12 Schematic illustration showing the microstructure and phonon

transportation in the graphite doped Cu2Se bulks

Figure 3-13 shows the temperature dependence of the electrical and thermal transport properties over four repeated trials for the 0.3CF sample, revealing that the sample shows excellent reproducibility of electric and thermal transport properties over repeated measurement cycles. Similar results can also be found for the other carbon doped Cu2Se samples, too. The temperature dependence of the power factor (PF) is shown in Figure 3-14. Collectively, the data for many samples show that the carbon nanoinclusions reduce the thermal conductivity and enhance or retain the excellent

Seebeck coefficient and electrical conductivity values compared to the carbon-free

Cu2Se. Moreover, the temperature dependence of the thermoelectric compatibility factor (s) for un-doped and carbon doped Cu2Se. It reveals that all the CNT, graphite, hard carbon and super P doped Cu2Se samples exhibit little temperature dependence in their thermoelectric compatibility factors in the temperature range from 450 to 900 K, especially when the doping level is low Figure 3-15. This will greatly benefit the 90 practical application of the carbon doped Cu2Se materials. Thermoelectric materials with good mechanical properties can better sustain strong mechanical and thermal stresses and improve the reliability of segmented thermoelectric modules [44-46].

Figure 3-16 shows the hardness values of the super P, graphite, and carbon fiber doped

Figure 3-13 Temperature dependence of the electrical and thermal transport properties

over four repeated trials for a 0.3wt% CF doped Cu2Se sample: (a) electrical

conductivity (); (b) Seebeck coefficient (S); (c) thermal diffusivity (D)

Cu2Se polycrystalline bulks. The results indicate that all the carbon doped Cu2Se polycrystalline samples exhibit similar hardness, with measured hardness values are

0.299, 0.382, 0.293, 0.322, 0.391, 0.283, 0.421, 0.387, 0.369, and 0.312 GPa for the

0.1 SP, 0.2 SP, 0.4 SP, 0.1 G, 0.2 G, 0.4 G, 0.1 CF, 0.2 CF, 0.3 CF, and 0.4 CF doped

Cu2Se samples, respectively.

12 12

10 10

)

-1

m m

8  8



-2

K 

 6 6

W -4

-4 0 0

10 10

0.1 CNT ( 0.2 CNT ( 4 4

0.1 G 0.2 G PF PF PF PF 0.1 H 0.2 H 2 0.1 SP 2 0.2 SP 0 0 300 400 500 600 700 800 900 300 400 500 600 700 800 900 T (K) T (K) 12 12

10 10

) )

-1 -1

m m 

 8 8

-2 -2

K K 

 6 6

W W -4

-4 0 0

10 10 ( ( 4 0.4 CNT 4 0.8 CNT

0.4 G 0.8 G PF PF PF PF 0.4 H 0.8 H 2 0.4 SP 2 0.8 SP

0 0 300 400 500 600 700 800 900 300 400 500 600 700 800 900 T (K) T (K)

Figure 3-14 Temperature dependence of the power factor for typical Cu2Se samples

with 0.1wt%, 0.2wt%, 0.4wt%, and 0.8wt% doping levels from various carbon sources

It should be noted that the carbon doped Cu2Se polycrystalline bulks exhibit comparable mechanical properties with the well-known thermoelectric materials of

PbTe polycrystalline bulks (around 0.40 GPa) [47]. It further indicates that the carbon

92 doped Cu2Se materials show potential to be successfully applied in thermoelectric devices. These results reveal that the carbon-doped Cu2Se samples possess excellent potential for mid-temperature thermoelectric device applications due to their high zT

10 10 0.0 0.1CNT 0.0 0.1G (a) 0.2CNT 0.4CNT (b) 0.2G 0.4G 8 0.6CNT 0.8CNT 8 0.6G 0.8G

6 6

s s 4 4

2 2

0 0 400 500 600 700 800 900 400 500 600 700 800 900 T (K) T (K) 12 10 (c) 0.0 0.1H (d) 0.0 0.1SP 10 0.2H 0.4H 0.2SP 0.4SP 0.6H 0.8H 8 0.6SP 0.8SP 8

6 s s 6 4 4

2 2

0 0 400 500 600 700 800 900 400 500 600 700 800 900 T (K) T (K)

Figure 3-15 Temperature dependence of the thermoelectric compatibility factor (s) for

the melt-solidified un-doped and carbon doped Cu2Se samples with different carbon

sources and doping levels

values, which are also less temperature dependent than for lead-based TE materials, as well as their good electrical and thermal stability over a wide temperature range.

Figure 3-16 Hardness of the melt-solidified selected carbon doped Cu2Se

polycrystalline samples

3.4. Conclusions:

All the carbon doped Cu2Se nanocomposites exhibit figure-of-merit, zT, higher than

1.0 over a broad temperature range from 600 to 900 K, with the 0.6wt% Super P doped

Cu2Se sample achieving a highest zT of around 1.85 at 900 K. Furthermore, zT is found to be as high as 1.1 at 555 K and reaches 2.4 at 850 K in carbon fiber doped

Cu2Se sample, with values at or exceeding the state-of-the-art materials. The mechanism for this enhancement is that the carbon-decorated nanostructured interfaces strongly reduce heat conductivity by increasing the boundary resistance, whereas they only moderately affect electrical conductivity. Collectively our results indicate that

Cu2-xSe:C nanocomposites will be an excellent candidate for middle temperature thermoelectric applications, as they are less toxic than lead-based alternatives and low- cost in terms of both materials and fabrication methods. Furthermore, our findings reveal that there is considerable scope to improve and refine the mechanistic

94 understanding of phonon scattering at carbon-decorated nano-interfaces to broaden the horizons of next-generation thermoelectric materials.

3.5. References:

[1] Y. Yang, Z.-H. Lin, T. Hou, F. Zhang, and Z. L. Wang, "Nanowire-composite

based flexible thermoelectric nanogenerators and self-powered temperature

sensors," Nano Res., vol. 5, no. 12, pp. 888-895, 2012.

[2] Y. Yang et al., "Pyroelectric nanogenerators for harvesting thermoelectric

energy," Nano Lett., vol. 12, no. 6, pp. 2833-8, Jun 13 2012.

[3] Y. Yang et al., "Thermoelectric Nanogenerators Based on Single Sb-Doped

ZnO Micro/Nanobelts," ACS nano, vol. 6, pp. 6984-6989, 2012.

[4] L. E. Bell, "Cooling, heating, generating power, and recovering waste heat

with thermoelectric systems," Science, vol. 321, no. 5895, pp. 1457-61, Sep 12

2008.

[5] X. Wang, Z. L. Wang, and Y. Yang, "Hybridized nanogenerator for

simultaneously scavenging mechanical and thermal energies by

electromagnetic-triboelectric-thermoelectric effects," Nano Energy, vol. 26, pp.

164-171, 2016.

[6] D. M. Rowe, CRC Handbook of thermoelectrics, CRC press, FL, 1995.

[7] J.-P. Fleurial, A. Borshchevsky, and T. Caillat, "New thermoelectric materials

and devices for terrestrial power generators,," Proceedings of the 1st

Conference on Synergistic Power and Propulsion Systems Technology Ed. by

M. S. El-Genk, AIP Conf. Proc., vol. 387, pp. 293-298, 1997.

[8] Y. Du et al., "Thermoelectric fabrics: Toward power generating clothing," Sci.

Rep., vol. 5, p. 6411, 2015.

[9] Y. Tung and M. Cohen, "Relativistic Band Structure and Electronic Properties

of SnTe, GeTe, and PbTe," Phys. Rev., vol. 180, no. 3, pp. 823-826, 1969.

[10] W. Liu et al., "n-type thermoelectric material Mg2Sn0.75Ge0.25for high power

generation," Proceedings of the National Academy of Sciences, vol. 112, pp.

3269-3274, 2015.

[11] W. liu et al., "New insight into the material parameter B to understand the

enhanced thermoelectric performance of Mg2Sn1-x-yGexSby," Energy Environ.

Sci., vol. 9, pp. 530-539, 2016.

[12] Q. Zhang et al., "Heavy doping and band engineering by potassium to improve

the thermoelectric figure of merit in p-type PbTe, PbSe, and PbTe(1-y)Se(y)," J.

Am. Chem. Soc., vol. 134, no. 24, pp. 10031-8, Jun 20 2012.

[13] C. Zhou et al., "Scalable solution-based synthesis of component-controllable

ultrathin PbTe1-xSex nanowires with high n-type thermoelectric performance,"

J. Mater. Chem. A, vol. 5, pp. 2876-2884, 2017.

[14] Z. Chen et al., "Lattice Dislocations Enhancing Thermoelectric PbTe in

Addition to Band Convergence," Adv. Mater., vol. 29, p. 1606768, Apr 11

2017.

[15] H. Liu et al., "Copper ion liquid-like thermoelectrics," Nat. Mater., vol. 11, no.

5, pp. 422-425, May 2012.

[16] L. Zhao et al., "Superior intrinsic thermoelectric performance with zT of 1.8 in

single-crystal and melt-quenched highly dense Cu2-xSe bulks," Sci. Rep., vol. 5,

p. 7671, 2015.

[17] G. Dennler et al., "Are binary copper sulfides/selenides really new and

promising thermoelectric materials?," Adv. Energy Mater., vol. 4, no. 9, p.

1301581, 2014.

[18] H. Liu et al., "Ultrahigh thermoelectric performance by electron and phonon

critical scattering in Cu2Se1-xIx," Adv. Mater., vol. 25, no. 45, pp. 6607-6612,

Dec 3 2013.

[19] H. Chen, H. Lin, Z. X. Lin, J. N. Shen, L. Chen, and L. M. Wu, "Superionic

adjustment leading to weakly temperature-dependent ZT values in bulk

thermoelectrics," Inorg. Chem., vol. 54, pp. 867-871, Nov 24 2014.

[20] L. Zhao et al., "The effects of Te2− and I− substitutions on the electronic

structures, thermoelectric performance, and hardness in melt-quenched highly

dense Cu2-xSe," Adv. Electron. Mater., vol. 1, p. 1400015, 2015.

[21] L. Yang, Z. G. Chen, G. Han, M. Hong, and J. Zou, "Impacts of Cu deficiency

on the thermoelectric properties of Cu2-xSe nanoplates," Acta Materialia, vol.

113, pp. 140-146, Jul 2016.

[22] T. W. Day et al., "Influence of compensating defect formation on the doping

efficiency and thermoelectric properties of Cu2-ySe1-xBrx," Chem. Mater.,

2015.

[23] T. W. Day, W. G. Zeier, D. R. Brown, B. C. Melot, and G. J. Snyder,

"Determining conductivity and mobility values of individual components in

multiphase composite Cu1.97Ag0.03Se," Appl. Phys. Lett., vol. 105, no. 17, p.

172103, 2014.

[24] Y. Y. Li et al., "Enhanced thermoelectric performance of Cu2Se/Bi0.4Sb1.6Te3

nanocomposites at elevated temperatures," Appl. Phys. Lett., vol. 108, no. 6, p.

062104, 2016.

[25] L. Zhu et al., "One-step room temperature rapid synthesis of Cu2Se

nanostructures, phase transformation, and formation of p-Cu2Se/p-

Cu3Se2heterojunctions," CrystEngComm, vol. 18, no. 27, pp. 5202-5208,

2016.

[26] P.-a. Zong et al., "Skutterudite with graphene-modified grain-boundary

complexion enhances zT enabling high-efficiency thermoelectric device,"

Energy Environ. Sci., vol. 10, no. 1, pp. 183-191, 2017.

[27] L. Gulay, M. Daszkiewicz, O. Strok, and A. Pietraszko, "Crystal structure of

Cu2Se," Chem. Met. Alloys vol. 4, pp. 200-205, 2011.

[28] Y. Jia et al., "Defect Graphene as a Trifunctional Catalyst for Electrochemical

Reactions," Adv. Mater., vol. 28, no. 43, pp. 9532-9538, Nov 2016.

[29] T. Ma et al., "Tailoring the thermal and electrical transport properties of

graphene films by grain size engineering," Nat. Commun., vol. 8, p. 14486,

2017.

[30] H.-S. Kim, S. I. Kim, K. H. Lee, S. W. Kim, and G. J. Snyder, "Phonon

scattering by dislocations at grain boundaries in polycrystalline BiSb1.5Te3,"

Phys. Status Solidi B, vol. 254, no. 5, p. 1600103, 2017.

[31] Y. Pei, A. LaLonde, S. Iwanaga, and G. J. Snyder, "High thermoelectric figure

of merit in heavy hole dominated PbTe," Energy Environ. Sci., vol. 4, no. 6, p.

2085, 2011.

[32] S. N. Girard et al., "High performance Na-doped PbTe-PbS thermoelectric

materials: electronic density of states modification and shape-controlled

nanostructures," J. Am. Chem. Soc., vol. 133, no. 41, pp. 16588-97, Oct 19

2011.

[33] Y. Pei, X. Shi, A. Lalonde, H. Wang, L. Chen, and G. J. Snyder, "Convergence

of electronic bands for high performance bulk thermoelectrics," Nature, vol.

473, no. 7345, pp. 66-69, May 5 2011.

[34] Q. Cao, Y. Song, Y. Tan, and Q. Zheng, "Conductive and viscoelastic

behaviors of carbon black filled polystyrene during annealing," Carbon, vol.

48, no. 15, pp. 4268-4275, 2010.

[35] M. Hadadian, E. K. Goharshadi, and A. Youssefi, "Electrical conductivity,

thermal conductivity, and rheological properties of graphene oxide-based

nanofluids," J. Nanopart. Res., vol. 16, no. 12, p. 2788, 2014.

[36] S. E. Bourdo, B. A. Warford, and T. Viswanathan, "Electrical and thermal

properties of graphite/polyaniline composites," J. Solid State Chem., vol. 196,

pp. 309-313, 2012.

[37] A. M. F. Lima, V. G. d. Castro, R. S. Borges, and G. G. Silva, "Electrical

conductivity and thermal properties of functionalized carbon

nanotubes/polyurethane composites," Polímeros, vol. 22, no. 2, pp. 117-124,

2012.

[38] A. W. Smith and N. S. Rasor, "Observed Dependence of the Low-Temperature

Thermal and Electrical Conductivity of Graphite on Temperature, Type,

Neutron Irradiation, and Bromination," Phys. Rev., vol. 104, no. 4, pp. 885-

891, 1956.

[39] H. Liang and M. Upmanyu, "Elastic self-healing during shear accommodation

in crystalline nanotube ropes," Phys. Rev. Lett., vol. 94, no. 6, p. 065502, Feb

18 2005.

[40] D.-X. Yan, K. Dai, Z.-D. Xiang, Z.-M. Li, X. Ji, and W.-Q. Zhang, "Electrical

conductivity and major mechanical and thermal properties of carbon nanotube-

filled polyurethane foams," Journal of Applied Polymer Science, vol. 120, no.

5, pp. 3014-3019, 2011.

100

[41] E. T. Swartz and R. O. Pohl, "Thermal boundary resistance," Rev. Mod. Phys.,

vol. 61, no. 3, pp. 605-668, 07/01/ 1989.

[42] D. G. Cahill et al., "Nanoscale thermal transport," Journal of Applied Physics,

vol. 93, no. 2, pp. 793-818, 2003.

[43] J. D. N. Cheeke, H. Ettinger, and B. Hebral, "Analysis of heat transfer between

solids at low temperatures," Can. J. Phys., vol. 54, no. 17, pp. 1949-1771,

1976.

[44] K. Ueno, A. Yamamoto, T. Noguchi, T. Inoue, S. Sodeoka, and H. Obara,

"Optimization of hot-press conditions of Zn4Sb3 for high thermoelectric

performance. II. Mechanical properties," J. Alloys Compd., vol. 388, no. 1, pp.

118-121, 2005.

[45] F. Ren, H. Wang, P. A. Menchhofer, and J. O. Kiggans, "Thermoelectric and

mechanical properties of multi-walled carbon nanotube doped Bi0.4Sb1.6Te3

thermoelectric material," Appl. Phys. Lett., vol. 103, no. 22, p. 221907, 2013.

[46] J. E. Ni et al., "Room temperature Young's modulus, shear modulus, Poisson's

ratio and hardness of PbTe–PbS thermoelectric materials," Materials Science

and Engineering: B, vol. 170, no. 1-3, pp. 58-66, 2010.

[47] Y. Gelbstein, G. Gotesman, Y. Lishzinker, Z. Dashevsky, and M. P. Dariel,

"Mechanical properties of PbTe-based thermoelectric semiconductors," Scripta

Mater., vol. 58, no. 4, pp. 251-254, 2008.

101

Chapter 4

4. Giant enhancement of the figure-of-merit over a broad temperature range in nano-boron incorporated Cu2Se

4.1. Introduction

Thermoelectric (TE) technology offers a simple and environment-friendly solution for directly recovering waste heat as usable electricity leading to reduce the global energy crisis [1-3]. These devices operate with high reliability, with no moving parts or emission of greenhouse gasses [4]. TE materials are classified with respect to their intended operating temperature range, with applications varying from spacecraft [5] to harvesting waste heat from industrial appliances [6] and automobile exhaust systems [7]. The figure of merit, , is the key factor for the energy conversion efficiency of thermoelectric generators, where 𝜎, S, , T are the electrical conductivity, Seebeck coefficient, lattice thermal conductivity, charge carrier thermal conductivity, and absolute temperature, respectively. The total thermal conductivity () expressed by , is mainly dominated by lattice vibrations compared to those of the charge carriers at high temperature. Since there are strong correlations between thermal and electrical transport in solids, achieving high zT is extremely challenging. So, optimizing the imperfectly independent factors

, 𝜎, and 횜 has been the key focus in research to improve zT. The following techniques have been successfully implemented so far to enhance zT in a wide range of high-temperature TE materials: 1) substitution that changes 𝜎 due to carrier

concentration as well as  due to different atomic mass; 2) reduction of particle size by a nano-engineering approach, which gives rise to confinement of both intragrain and intergrain phonons; 3) reducing the dimensionality of TE materials by creating multilayer thin films; and 4) introducing inclusion of a secondary phase that does not change the carrier concentration, but does reduce  due to the blocking of phonons.

Among all the Cu-based TE materials, Cu2Se has received great attention recently, as it has good thermoelectric performance. To date, the electronic structures and thermal conductivity of Cu2Se have been modified by doping using Al, Sn, Sb, Ag, and Li in the Cu sites [8-12], and Te, S, I, and Br in the Se sites [13-16]. Recently, I discovered that carbon is a very effective dopant that can significantly enhance the

TE performance in Cu2Se, which has a zT greater than 1 for temperatures above 520

K and reaches a record level of zT of ~ 2.4 at 850 K [17]. The key factor for the great enhancement of zT by carbon incorporation is the giant reduction of thermal conductivity. Since any enhancement in electrical conductivity often accompanies with reduction in Seebeck coefficient or thermal power or vise-versa, significant reduction of thermal conductivity becomes an important strategy for improvement of

ZT. In this work, I propose to use another non-toxic, lightweight, but insulating dopant, boron-nano-particles, to incorporate into Cu2Se. It is well known that boron has remarkable stability in corrosive and acidic environments.

In this Chapter, I report that using the light element boron as a second nanophase inclusion in the Cu2Se can give significantly enhanced thermoelectric performance. I demonstrate that boron addition can remarkably reduce  and enhances 횜 over a

103 wide range of temperature, leading to the enhancement of zT by a factor of 1.6 - 2.6 over a wide temperature range compared to undoped samples.

4.2. Sample fabrication

Firstly, the polycrystalline Cu2Se powders were mixed with amorphous boron (B) powders in the weight ratios of 1: x (x = 0.14%, 0.28%, and 0.42%) followed by spark plasma sintering (SPS; Thermal Technology SPS model 10-4) at 773 K for just

10 min under a pressure of 40 MPa in vacuum. Finally, the highly dense 20 mm diameter and 2 mm thick solids were formed into a rectangular bar and round disks using a cutting machine (Struers Accutom-50) for electrical and thermal transport measurements, respectively. The boron-doped Cu2Se samples are symbolized by their boron weight percent and represent as 0.14 B, etc.

4.2.1. Measurement details

The room temperature powder diffraction patterns were determined by X-ray diffractometry (Cu Kα, GBC MMA) (λ=1.5418 Å), 10º ≤ 2θ ≤ 60°, in step scan mode with a step size of 0.014° and speed of 1 degree per min. The Australian Synchrotron was used to collect high resolution data using a wavelength of λ = 0.58973 Å, where the detectors covered an angular range of 2.5º ≤ 2θ ≤ 80°, simultaneously every 30 seconds and the sample was rotated at ~1 Hz with a heating rate of 4 deg./min under helium gas ﬂow over the 300−774 K temperature range. The sample morphologies and microstructures were investigated with field emission scanning electron microscopy (FE-SEM, JEOL 7500), and transmission electron microscopy (TEM,

JEOL ARM200F) was used to investigate the phase composition and microstructure of the undoped and boron doped Cu2Se samples. The thin TEM specimens were

104 prepared using focused ion beam milling (FIB) after being mounted in Polyfast® using Struers Citopress-20 equipment and polished with Struers Tegramin-20 water equipment, before they were used for scanning transmission electron microscopy/X- ray spectroscopy (STEM/X-ray mapping) investigations. Room temperature Hall coefficient measurements were performed to determine the carrier concentration and carrier mobility using a physical properties measurement system (Quantum Design

PPMS-9). The σ and S were simultaneously measured from room temperature to 852

K under vacuum using a commercial RZ2001i system. The laser flash method

(LINSEIS LFA 1000) was used to measure thermal diffusivity under vacuum conditions. The heat capacity (Cp) was measured by differential scanning calorimetry

(DSC-204F1 Phoenix) under argon atmosphere. The weight and dimensions were used to measure the sample density (dd). The total thermal conductivity (κ) was calculated using the formula κ = D × Cp × dd. All the transport measurements were repeated several times to check the consistency of the polycrystalline samples.

4.3. Results and discussions

The fabrication process for the boron doped Cu2Se is illustrated by schematic representation (Figure 4-1a) and details are mentioned in sample preparation section.

Figure 4-1 b in shows the Cu2Se structure, with the monoclinic α-phase (left side) at room temperature and the cubic 훽-phase (right side) at high temperature, respectively.

Powder X-ray diffraction (XRD) patterns (Figure 4-2b) show that the boron doped samples at room temperature is 훼-phase. I noted that there are almost no changes to the peak positions from boron nanoparticle inclusions. (Figure 4-2b), strongly suggesting that there is little substitution effect from boron.

105

Temperature-dependent synchrotron powder X-ray diffraction was performed in order to further investigate the boron doping effect over the change of phase transition with temperature and to identify the change in lattice parameters with temperature. Heat-map images with respect to temperature for undoped Cu2Se and a

Figure 4-1 (a) Schematic illustration showing the formation mechanism of the B-

doped Cu2Se bulks with the fabrication process. (b) Low-temperature monoclinic

phase (left) and high-temperature cubic phase (right) of the Cu2Se crystal structure

0.42wt% nano-boron doped Cu2Se sample from room temperature to 774 K are shown in Figure 4-3a and Figure 4-3b, respectively. The mapping shows that there are α to β phase transitions for both undoped and boron doped Cu2Se within this

106

Figure 4-2 (a) Schematic illustration of crystal structure of α-phase Cu2Se, and (b) X-

ray diffraction patterns for pure Cu2Se and boron doped Cu2Se, along with (c)

enlarged peaks for the undoped and boron doped Cu2Se samples e

samples (x = 0, 0.14, 0.28, and 0.42) at room temperature

temperature range. This transformation can be observed in differential scanning calorimetry (DSC) measurements (Figure 4-3c), where the phase transition takes place at 400 K for undoped Cu2Se and 417 K for 0.42 wt% boron doped Cu2Se.

107

Figure 4-3 Synchrotron radiation X-ray diffraction (SR-XRD) profiles of (a) pure

Cu2Se and (b) 0.42 wt% boron doped Cu2Se obtained upon heating to different

temperatures; (c) Temperature dependence of the heat capacity (Cp) for the undoped

and boron doped Cu2Se samples; (d) XRD patterns for Cu2Se/0.42wt% boron obtained at 300 K at the beginning of the measurement (1) and after the measurement

(2), respectively

The Rietveld refinements and high-resolution synchrotron XRD data of undoped and

0.42wt% boron doped Cu2Se samples are shown in Figure 4-4 and Figure 4-5, respectively. The lattice parameters, and R-factors of the undoped and boron doped

Cu2Se samples deduced from the Rietveld reﬁnement of the synchrotron XRD patterns are listed in Tables 2, respectively. I found that lattice parameter for the 108 boron doped Cu2Se has 5.8507 Å, 5.8388 Å, 5.8277 Å where as it was observed

5.8611 Å, 5.8427

Figure 4-4 Rietveld refinement of 0.42 wt% boron doped Cu2Se (a-c) and undoped

Cu2Se (d-f) at 624 K, 512 K, and 403 K, respectively. The refined parameters are

shown in Table 2

Å and 5.8316 Å at 624 K, 512 K and 403 K, respectively for the undoped sample.

From this result, it is unlikely that boron atoms go into the Cu2Se crystal lattice. I

109 noted that the XRD patterns recorded before and after heating and cooling are almost identical (Fig. 3d), indicating that the phase transformation process is reversible. This also confirms that the boron doped samples have good thermal stability, which is in conformity with the reproducibility of σ, S, and the thermal diffusivity (D) (Figure 4-

15).

Lattice parameter 2 Temp (K) a (Å) Rp Rwp  Sample 624 (a) 0.42 wt% Boron 5.85070 9.867 13.313 2.609 512 (b) 0.42 wt% Boron 5.83880 8.560 11.077 1.853 403 (c) 0.42wt% Boron 5.82770 9.858 12.999 2.579

624 (d) Cu2Se 5.86110 9.777 12.962 1.242

512 (e) Cu2Se 5.84270 9.325 12.175 1.157

403 (f) Cu2Se 5.83160 9.517 12.165 1.283

Table 2 Parameters for the refinement of boron doped and undoped Cu2Se at 624 K,

512 K, and 403 K. Rp and Rwp are the profile and weighted profile R-factors,

respectively, and 2 is the goodness-of-fit

Field emission scanning electron microscope (FE-SEM) images of the boron doped

Cu2Se and pure Cu2Se bulks are shown in Figure 4-6. All the samples were found to be highly dense with no porosity. The processed high-angle annular dark field – scanning transmission electron microscope (HAADF-STEM) images of monoclinic

Cu2Se, their ball models and selected area electron diffraction (SAED) pattern are presented in Figure 4-8.

Figure 4-8b shows the structure of boron doped Cu2Se as determined from the

HAADF-STEM images. The signal-to-noise ratio of the image was improved by

110 using an average filter. The top inset is the distance profile for Se-Se, showing that the space between Se-Se is around 0.899 nm along the marked dotted yellow line AB in the image. The bottom inset is the corresponding atomic model, indicating a perfect monoclinic structure along the [0 -1 0] orientation. Due to the very much lower Z contrast of B (atomic number contrast), it is impossible to distinguish B atoms in the HAADF-STEM image from the [0 -1 0] orientation. Figure 4-8c presents the SAED pattern along the [0 -1 0] orientation. Transmission electron microscope (TEM) images and the high resolution TEM images show (Figure 4-8 d- e) the boron regions, which appear as black patches and dots embedded inside the

Cu2Se grains, as determined by energy dispersive spectroscopy (EDS) mapping. The

Moiré fringes evident within precipitates are shown in Figure 4-8e. The EDS spectrum collected in conjunction with SEM is also shown in Figure 4-7. The presence of boron inclusions across the grain boundary is also shown in Figure 4-9. I can see that the boron particle sizes are on average 80 nm. These nano-boron inclusions are expected to play a significant role in blocking the phonons.

111

Figure 4-5 Powder diffraction patterns over the entire measured temperature range of

~300–773 K for 2휃 = a) 2.5°–30° (Undoped), b) 2.5°–30° (0.42 wt% Boron) c) 9.5°–

10.5° (Undoped), d) 16°–17° (Undoped), e) 18.75°–19.75° (Undoped), f) 9.5°–10.5°

(Boron), g) 16°–17° (Boron), h) 18.75°–19.75° (Boron). The wavelength is

0.5897313 Å

112

Figure 4-6 SEM images of fractured surface morphology of (a) pure Cu2Se, and b)

0.14 wt%, (c) 0.28 wt%m and (d) 0.42 wt% boron-doped polycrystalline Cu2Se

fabricated by the spark plasma sintering (SPS) method

I studied the effects of nano-boron addition on the 𝜎, 횜, and , as well as the microhardness. The temperature dependence of 𝜎, 횜, , and zT for an undoped and boron doped Cu2Se samples is shown in Figure 4-10. The σ of all samples decreases with increasing temperature over the whole temperature range (Figure 4-10a). I can

113

Figure 4-7 Energy-dispersive X-ray spectroscopy (EDS) of boron doped Cu2Se. (a)

SEM image showing the area (arrow sign) examined by EDS, and (b, c) EDS spectrum of the selected region in a

see that the boron-doped Cu2Se sample shows lower electrical conductivity than the undoped sample for both α- and 훽-phases. This trend becomes more obvious for high boron doping levels. The electrical conductivity is reduced from ~ 458 S cm-1 for the

-1 undoped sample to 207 S cm for the 0.42wt% boron doped Cu2Se at 852 K. The carrier concentration p and mobility were determined using Hall effect measurements from the formula: , where represents the elemental

charge and RH the Hall coefficient. I found from the Hall effect measurements that hole concentration (p) for the pure Cu2Se, 0.28 wt.% B and 0.42 wt.% B are

, and , respectively. The

mobility ( ) for the pure Cu2Se, 0.28 wt.% B and 0.42 wt.% B are

114

Figure 4-8 Structural characterization of 0.42 wt.% boron-doped Cu2Se polycrystals.

a) Ball model of the monoclinic unit cell of Cu2Se representing the dashed square of

the main panel of (b). b) Atomic resolution HAADF image at room temperature

along the [0 -1 0] zone axis, where the Se atoms are represented by the yellow balls

and the weak contrast Cu atoms among Se layers are represented by the red balls in

the bottom inset; Top inset: the line profile along the dotted line AB in the main

panel, showing the spacing between two Se. c) The SAED pattern along the [0 −1 0]

orientation. d) HAADF STEM image of the FIB milled sample, showing boron-rich

precipitates within the Cu2Se-rich matrix along with the corresponding EDS

elemental mapping for Cu, Se and B. Average Boron particle sizes are 80 nm. e)

Magnified observation of boron-rich precipitates within the Cu2Se-rich matrix

, and , respectively. The hole carrier concentration is decreases with the amount of boron and for the 0.42 wt.% boron doped samples which is ~37% lower than the pure Cu2Se. This result is consistent with the electrical conductivity and Seebeck coefficient measurements.

115

This may indicate that either B dope into the Cu sites or fill the Cu vacancies. Both can lead to the decrease in the carrier concentrations. The carrier mobility for the

Figure 4-9 Low magnification TEM image of the boron doped Cu2Se bulk sample

showing the presence of boron across the grain boundary

0.42 wt.% B doped sample is , which is about 54% lower than for the

undoped Cu2Se (Figure 4-10d). As σ = n , where n is the carrier concentration, the overall reduction in σ is mainly caused by the decreased hole concentration as well as carrier mobility in the B added sample. In addition, the electron scattering occurring at the boron/Cu2Se interface may also play an important role in reducing the σ. Since there is little substitution effect from boron, the possible main reason for the reduction in σ should be due to the significant electron scattering occurring at the boron/Cu2Se interface.

116

Figure 4-10 Temperature dependence of the thermoelectric transport properties of the

samples: (a) electrical conductivity (σ), (b) Seebeck coefficient (S), (c) Power factor

(PF); (d) Carrier concentration and carrier mobility for the pure, 0.28 wt.% boron and

0.42 wt.% boron doped Cu2Se samples; (e) Thermal conductivity (κ) as a function of

temperature; and (f) Schematic diagram of B-doped Cu2Se composite, showing the

microstructure and phonon transportation

The reduction in electrical conductivity often accompanies with enhancement of in

Seebeck coefficient. Figure 4-10b shows the temperature dependence of the Seebeck

117 coefficient for both boron doped and undoped samples. It can be seen that S increases with boron doping over a wide range of temperature. For the 0.42 wt% boron doped

−1 Cu2Se, the maximum S value is 245 μV K at 852 K, which is about 48% higher than that of the undoped sample.

The power factor (PF) = S2 is for the undoped 훼-phase and for the 훽-phase, which is almost the same as in previously reported results [13, 18] (Figure 4-10c). All the boron-doped samples show the same trend in the power factor with increasing temperature, and the highest values obtained were for the 훼-phase and

for the 훽-phase of 0.42wt% B doped Cu2Se, which is

about 39% larger as compared to the pristine Cu2Se sample at 456 K, although there was no significant difference at high temperature. Now, our expectations on any enhancement on ZT completely rely on whether or not the thermal conductivity can be reduced by the boron nano-inclusions.

Remarkably, the boron doped Cu2Se samples exhibit greatly reduced thermal conductivity compared to the boron-free sample, as displayed in Figure 4-10e over a temperature range from 300-850 K. The for the undoped sample is 1.4-

from 456 K - 852 K, while the value is as small as ~0.66 W·m-1·K-

1 for the 0.42 wt% boron doped Cu2Se over the whole temperature range. This value is half that of the undoped sample and is also the lowest among almost all the undoped Cu2Se samples reported so far [19, 14, 18, 20, 21, 15]. The microstructure and phonon transport property of the boron precipitated Cu2Se in the microcomposite 118 samples are illustrated in Figure 4-10f. The reduction of thermal conductivity and enhancement of the thermoelectric figure-of-merit are absolutely due to boron.

Figure 4-11 Thermoelectric figure of merit zT for the undoped and B-doped Cu2Se

with different wt% B. b) Thermoelectric efficiency of the undoped and B-doped

Cu2Se samples, in comparison with the previously reported Cu2Se data [22]

The zT is found to increase with the boron doping concentration for all temperatures

(Figure 4-11a). zT is 1.0 at T = 600 K and reaches 1.6 at 852 K for the 0.42 wt% boron doped sample as compared to zT of 0.46 at 600 K and 0.99 at 852 K for the undoped sample. It is remarkable that the overall zT enhancement ratio is 1.6 to 2.6 for the optimum boron doped sample compared to the undoped (Figure 4-12a). The efficiency is calculated according to the literature [23]. It was found that efficiency is

~15 % at 852 K for the 0.42 wt.% B sample, whereas it is only 9.4 % for the undoped

Cu2Se at 852 K (Figure 4-11b).

119

Figure 4-12 Temperature dependence of (a) σ/ , (b) S/S0.0, (c) κ/κ0.0, and (d)

zT/zT0.0 for the boron doped Cu2Se samples with doping levels of 0.14, 028, and 0.42

wt%

I calculated according to the Wiedemann-Franz law ( ), where L represents the Lorentz number. In this work, L is calculated according to the equation

, where L is in and S in

. The calculated results show that L varies in the range of

with different types of boron doping and with the temperature increasing from 350 to 852 K (Figure 4-13a). The total  in Figure 4-13b

120

Figure 4-13 Temperature dependence of the thermal transport properties for the

undoped and boron doped polycrystalline samples: a) The calculated Lorenz number

(L) of undoped and boron doped Cu2Se.; b) total thermal conductivity (κ) and

electronic thermal conductivity ( ); c) lattice thermal conductivity ( ) with total

thermal conductivity (κ); d) comparison of the boron doped κlat values and κtot values

(dotted lines with those of state-of-the-art Cu2Se [18, 20, 24], PbTeSe [25], PbTe

[26] polycrystals

shows a decrease to ~ 0.66 - 0.73 W m-1 K-1 with temperature for the 0.42 wt% boron

-1 -1 doped Cu2Se, while it was ~1.08-1.40 W m K for the undoped Cu2Se. The

-1 -1 calculated was 0.68 -1 W m k for the undoped Cu2Se, and it decreased to 0.27-

121

-1 -1 0.39 W m K for the 0.42 wt% boron doped Cu2Se. The dependence of of all

samples on temperature is shown in Figure 4-13c. The value is significantly lower than for most the reported polycrystalline samples at 0.23 W m-1 K-1 [18, 20, 24-26], as shown in Figure 4-13d. The reduced transmission of heat is most obviously correlated with extremely low along with and is due to the decreased charge carrier as well as the lattice vibrations through the complex heterogeneous microstructure of our boron doped polycrystalline Cu2Se with boron nano-inclusions at the grain boundaries. The of a polycrystalline solid can be determined using the following relation:

-1 = -1 +

Where , , and Rκ are the intrinsic thermal conductivity of Cu2Se, the change in thermal conductivity of a doped sample, the inverse average grain size, and the grain boundary resistance, respectively. The reduction of transmission of heat observed using this model is due to the thermal resistance developed across the interface, and it becomes more strengthened as the number of interfaces increase in microcrystalline solids. The key parameter Rκ expressed the percentage of grain boundary covered by the boron inclusions and the reflection of phonons instead of transport through the edge of the grain. A negligible value of ~1×10−9 m2·K·W−1

is found in the intrinsic Cu2Se solid due to the large crystalline grain size, while micro- and nanograins have almost the same [27], whereas it becomes significant with the boron inclusions near the edge, in conformity with other thermoelectric 122 results [28]. The average grain size of the boron doped sample was found to be 0.4

µm with and of 1.41 W∙m−1∙K−1 and 0.73 W·m−1·K−1, respectively, which makes the ultra-high value 2.63 × 10−7 m2·K·W−1. This value is comparable to the results for recently reported graphene doped thermoelectric materials [28].

Figure 4-14 Hardness of the undoped and boron doped Cu2Se polycrystalline

samples in comparison with data for Cu2Se polycrystalline bulks [29]

Thermoelectric materials with good mechanical properties can better sustain strong mechanical and thermal stresses and improve the reliability of segmented thermoelectric modules [30-32]. Figure 4-14 shows the microhardness values of the pure Cu2Se and boron doped Cu2Se bulks. The hardness value of the pure Cu2Se bulk is found to be ~0.38 GPa, and this mechanical performance gradually increases with increasing content of boron. The measured hardness values are 0.40, 0.43, and 0.45

123

GPa for the 0.14 wt% B, 0.28 wt% B, and 0.42 wt% B doped Cu2Se samples, respectively. These values are comparable to those for reported polycrystalline

Cu2Se solids (~0.40 GPa) prepared using melt-quenching approach [29].

Figure 4-15 The electrical and thermal transport properties with respect to

temperature during heating up and cooling down for a 0.42 wt% Boron doped Cu2Se

sample: (a) electrical conductivity (); (b) Seebeck coefficient (S); (c) thermal

diffusivity (D)

4.4. Conclusion

I found that nano-boron particles can significantly enhance the zT of Cu2Se. Our studies indicate that boron inclusion can reduce the thermal conductivity, increase the Seebeck coefficient, and reduce the electrical conductivity, resulting in enhancement of zT by a factor of 1.6-2.6 over a wide temperature range compared to undoped samples. The mechanism for this improvement of zT is that the reduction of heat conduction by boron-decorated microstructure edges increases the boundary resistance with decreasing carrier concentration. Our findings offer an effective approach of using insulating nano-particles to significantly improve Seebeck coefficient and significantly reduce lattice thermal conductivity for achieving high

ZT in Cu2Se and many other types of thermoelectric composites. 124

4.5. References

[1] F. J. DiSalvo, "Thermoelectric Cooling and Power Generation," Science, vol.

285, no. 5428, pp. 703-706, 1999.

[2] L. E. Bell, "Cooling, Heating, Generating Power, and Recovering Waste Heat

with Thermoelectric Systems," Science, vol. 321, no. 5895, pp. 1457-1461,

2008.

[3] A. I. Hochbaum et al., "Enhanced thermoelectric performance of rough

silicon nanowires," Nature, 10.1038/nature06381 vol. 451, no. 7175, pp. 163-

167, 01/10/print 2008.

[4] J. P. Heremans et al., "Enhancement of thermoelectric efficiency in PbTe by

distortion of the electronic density of states," Science, vol. 321, no. 5888, pp.

554-557, 2008.

[5] J.-P. Fleurial, A. Borshchevsky, and T. Caillat, "New thermoelectric materials

and devices for terrestrial power generators," in AIP Conference Proceedings,

1997, vol. 387, no. 1, pp. 293-298: AIP.

[6] Y. Du et al., "Thermoelectric fabrics: toward power generating clothing,"

Scientific reports, vol. 5, p. 6411, 2015.

[7] Y. W. Tung and M. L. Cohen, "Relativistic Band Structure and Electronic

Properties of SnTe, GeTe, and PbTe," Physical Review, vol. 180, no. 3, pp.

823-826, 04/15/ 1969.

[8] S. D. Kang et al., "Enhanced stability and thermoelectric figure-of-merit in

copper selenide by lithium doping," Materials Today Physics, vol. 1, pp. 7-

13, 2017.

125

[9] A. Suzumura, M. Watanabe, N. Nagasako, and R. Asahi, "Improvement in

thermoelectric properties of Se-free Cu3SbS4 compound," Journal of

Electronic Materials, vol. 43, no. 6, pp. 2356-2361, 2014.

[10] B. Zhong et al., "High superionic conduction arising from aligned large

lamellae and large figure of merit in bulk Cu1. 94Al0. 02Se," Applied Physics

Letters, vol. 105, no. 12, p. 123902, 2014.

[11] J. Fan et al., "Crystal Structure and Physical Properties of Ternary Phases

around the Composition Cu5Sn2Se7 with Tetrahedral Coordination of

Atoms," Chemistry of Materials, vol. 26, no. 18, pp. 5244-5251, 2014.

[12] S. Ballikaya, H. Chi, J. R. Salvador, and C. Uher, "Thermoelectric properties

of Ag-doped Cu 2 Se and Cu 2 Te," Journal of Materials Chemistry A, vol. 1,

no. 40, pp. 12478-12484, 2013.

[13] L. Zhao et al., "The Effects of Te2− and I− Substitutions on the Electronic

Structures, Thermoelectric Performance, and Hardness in Melt-Quenched

Highly Dense Cu2-xSe," Advanced Electronic Materials, vol. 1, no. 3, pp.

n/a-n/a, 2015.

[14] L. Zhao et al., "Improvement of thermoelectric properties and their

correlations with electron effective mass in Cu(1.98)S(x)Se(1−x)," Scientific

Reports, vol. 7, p. 40436, 01/16 10/10/received 12/06/accepted 2017.

[15] H. Liu et al., "Ultrahigh Thermoelectric Performance by Electron and Phonon

Critical Scattering in Cu2Se1‐ xIx," Advanced Materials, vol. 25, no. 45, pp.

6607-6612, 2013.

[16] T. W. Day et al., "Influence of Compensating Defect Formation on the

Doping Efficiency and Thermoelectric Properties of Cu2-ySe1–xBrx,"

Chemistry of Materials, vol. 27, no. 20, pp. 7018-7027, 2015/10/27 2015.

126

[17] L. Zhao et al., "Significant enhancement of figure-of-merit in carbon-

reinforced Cu2Se nanocrystalline solids," Nano Energy, vol. 41, no.

Supplement C, pp. 164-171, 2017/11/01/ 2017.

[18] B. Yu et al., "Thermoelectric properties of copper selenide with ordered

selenium layer and disordered copper layer," Nano Energy, vol. 1, no. 3, pp.

472-478, 5// 2012.

[19] Y.-B. Zhu, B.-P. Zhang, and Y. Liu, "Enhancing thermoelectric performance

of Cu2Se by doping Te," Physical Chemistry Chemical Physics,

10.1039/C7CP05149B vol. 19, no. 40, pp. 27664-27669, 2017.

[20] S. Ballikaya, H. Chi, J. R. Salvador, and C. Uher, "Thermoelectric properties

of Ag-doped Cu2Se and Cu2Te," Journal of Materials Chemistry A,

10.1039/C3TA12508D vol. 1, no. 40, pp. 12478-12484, 2013.

[21] D. Li et al., "Chemical synthesis of nanostructured Cu 2 Se with high

thermoelectric performance," Rsc Advances, vol. 4, no. 17, pp. 8638-8644,

2014.

[22] H. Liu et al., "Copper ion liquid-like thermoelectrics," Nature Materials, vol.

11, p. 422, 03/11/online 2012.

[23] H. S. Kim, W. Liu, G. Chen, C.-W. Chu, and Z. Ren, "Relationship between

thermoelectric figure of merit and energy conversion efficiency,"

Proceedings of the National Academy of Sciences, vol. 112, no. 27, pp. 8205-

8210, 2015.

[24] B. Gahtori et al., "Giant enhancement in thermoelectric performance of

copper selenide by incorporation of different nanoscale dimensional defect

features," Nano Energy, vol. 13, pp. 36-46, 2015.

127

[25] Y. Pei, X. Shi, A. LaLonde, H. Wang, L. Chen, and G. J. Snyder,

"Convergence of electronic bands for high performance bulk

thermoelectrics," Nature, vol. 473, no. 7345, pp. 66-69, 2011.

[26] K. Biswas et al., "High-performance bulk thermoelectrics with all-scale

hierarchical architectures," Nature, vol. 489, no. 7416, pp. 414-418, 2012.

[27] L.-l. Zhao et al., "Superior intrinsic thermoelectric performance with zT of

1.8 in single-crystal and melt-quenched highly dense Cu2-xSe bulks,"

Scientific reports, vol. 5, p. 7671, 2015.

[28] P.-a. Zong et al., "Skutterudite with graphene-modified grain-boundary

complexion enhances zT enabling high-efficiency thermoelectric device,"

Energy & Environmental Science, 10.1039/C6EE02467J vol. 10, no. 1, pp.

183-191, 2017.

[29] L. Zhao et al., "The Effects of Te2− and I− Substitutions on the Electronic

Structures, Thermoelectric Performance, and Hardness in Melt‐ Quenched

Highly Dense Cu2‐ xSe," Advanced Electronic Materials, vol. 1, no. 3, p.

1400015, 2015.

[30] K. Ueno, A. Yamamoto, T. Noguchi, T. Inoue, S. Sodeoka, and H. Obara,

"Optimization of hot-press conditions of Zn 4 Sb 3 for high thermoelectric

performance. II. Mechanical properties," Journal of alloys and compounds,

vol. 388, no. 1, pp. 118-121, 2005.

[31] F. Ren, H. Wang, P. A. Menchhofer, and J. O. Kiggans, "Thermoelectric and

mechanical properties of multi-walled carbon nanotube doped Bi0. 4Sb1.

6Te3 thermoelectric material," Applied Physics Letters, vol. 103, no. 22, p.

221907, 2013.

128

[32] J. E. Ni et al., "Room temperature Young's modulus, shear modulus,

Poisson's ratio and hardness of PbTe–PbS thermoelectric materials,"

Materials Science and Engineering: B, vol. 170, no. 1, pp. 58-66, 2010.

129

Chapter 5.

5. Ultra-high figure-of-merit of 2.6 in carbon fiber doped Cu2Se fabricated using spark plasma sintering method

5.1. Introduction

To overcome the energy crisis from rapid consumption of fossil fuel energy sources and to mitigate greenhouse gas emissions, sustainable and environment-friendly energy solutions are in great demand. The thermoelectric (TE) power generator, one such renewable energy solution, is quite attractive as a promising alternative green technology owing to its potential to directly harness electrical power from waste-heat energy [1-4]. The efficiency (η) of a TE generator is defined as

[5], which is determined by the material’s dimensionless figure of merit, where

𝜎, 횜, and  are the electrical conductivity, Seebeck coefficient, and thermal conductivity, respectively. The fraction representing the Carnot efficiency,

is determined by the hot (TH) and cold (TC) junction temperatures. Despite their great advantages of scalability and reliability [4], as well as their great potential in deep space missions, automobiles, microprocessors, and industrial processes [6, 7], thermoelectric generators have been mostly limited to a narrow range of applications due to their low efficiencies. In the past decade, great efforts have been made to overcome these, which have led to significantly improved thermoelectric performance and opened up avenues for new technological applications [8, 9, 1, 10,

11, 3, 12-19]. Recently, I discovered that various types of carbon doped Cu2Se composite exhibited high zT of ~ 2-2.4 at 850 K [20] using a melt fabrication process above the melting point of Cu2Se [21]. Inspired by our previous works, I further investigated carbon fiber (CF) doped Cu2Se composites using spark plasma sintering

(SPS) method with low fabrication temperature.

In addition to characterizations of thermoelectric performance, I investigated chemical stability as well as changes in the lattice parameters and microstructure, using both scanning transmission electron microscopy (STEM) and synchrotron high-temperature X-ray diffraction (XRD). Significant reduction of thermal conductivity (κ) from ~0.9 to ~0.4 due to CF doping was observed, while the power factor remained comparable to that of the undoped sample. This led to a record high zT of ~2.6 at 1000 K. Finally, the composite exhibited the ultrahigh efficiency of 31% at 1000 K, which was calculated according to the literature [22-24].

5.2. Sample fabrications

The polycrystalline Cu2Se powders were mixed with CF in the weight ratios of 1: x

(x = 0.35%, 0.55%, 0.75%, and 0.85%). The products were then compacted into dense pellets with a diameter of 20 mm and a thickness of ~2 mm by spark plasma sintering (Thermal Technology SPS model 10-4) at 500 K under a uniaxial pressure of 40 MPa for 10 minutes. Finally, the obtained highly dense polycrystalline bulks were shaped into round disks and rectangular bars using a cutting machine (Struers

Accutom-50) for thermal and electrical transport measurements, respectively. The

131

CF-doped Cu2Se samples are denoted by their CF weight percent, so that the sample with 0.75 wt% CF is denoted as CF 0.75, etc.

5.2.1. Thermoelectric measurements

The room temperature powder constituent phase was determined by X-ray diffractometry (Cu Kα, GBC MMA) equipped with Cu Kα radiation (λ = 1.5418 Å),

10º ≤ 2θ ≤ 600, and step scan mode (step size 0.014° and speed 1 degree/min). The sample morphologies and microstructures were investigated with a field emission scanning electron microscopy (FE-SEM, JEOL 7500), and a transmission electron microscope (JEOL ARM200F) was used to reveal the phase composition and microstructure of the as-synthesized bulk samples. The thin TEM specimens were prepared using focused ion beam milling (FIB) after being mounted in PolyFast® using Struers Citopress-20 equipment and polished with Struers Tegramin-20 water equipment. These samples were then used for scanning transmission electron microscopy/X-ray spectroscopy (STEM/X-ray mapping) investigation. The carrier concentration and carrier mobility were determined by Hall coefficient measurements at room temperature using a physical properties measurement system

(Quantum Design PPMS-9). The four-contact Hall-bar geometry was used for the measurements. At 300 K, the carrier concentration (p) and carrier mobility (μ) were estimated from the formula: Np = 1/(eRH) and μ = σRH, where e and σ are the elemental charge and the electrical conductivity, respectively, and RH is the Hall coefficient for holes [25-27]. The electrical conductivity and Seebeck coefficient were measured simultaneously under vacuum from room temperature to 1000 K using a commercial RZ2001i system. The thermal diffusivity (D) was measured by the laser flash method (LINSEIS LFA 1000) under vacuum conditions. The specific

132 heat (Cp) was determined by differential scanning calorimetry on a DSC-204F1

Phoenix under argon atmosphere with a flow rate of 50 ml/min. The sample density

(dd) was calculated using the measured weight and dimensions. The thermal conductivity (κ) was calculated by κ = D × Cp × dd. All the electrical conductivity and thermal diffusivity measurements were repeated several times to confirm the reproducibility of the as-prepared polycrystalline bulks.

5.2.2. Temperature-dependent X-ray diffraction measurements

High-resolution synchrotron powder diffraction data were collected at the beamline

10-BM-1, Advanced Photon Source, Australian Synchrotron using a wavelength of λ

= 0.58973 Å. Detectors covering an angular range of 2θ = 2.5° were scanned up to 2θ

= 80°, simultaneously every 30 seconds, and the sample was rotated at ~1 Hz with a heating rate of 4 deg./min under a helium gas ﬂow over the 300−774 K temperature range. The measured samples were prepared as follows. A piece of the consolidated materials with a mass of about 60 mg were chipped of and ground using a mortar and pestle. The resulting powder was transferred into a 0.5 mm outer-diameter quartz capillary tube. The capillary was ﬁlled up to about 20 mm in height, and importantly, the powder was tightly packed by placing it in a sonicator (while holding the capillary lightly between the fingers).

5.3. Results

5.3.1. Sample preparation and characterization:

The fabrication process for the CF doped Cu2Se composite is illustrated in Figure 5-1a

(see details in the sample preparation section). The temperature dependent crystal

133 structure is shown in Figure 5-1b, which illustrates the monoclinic α-phase (left side) at room -temperature and the cubic 훽-phase (right side) at high-temperature. The α-phase

Cu2Se is

Figure 5-1 (a) Schematic illustration showing the formation mechanism and

fabrication process of the CF-doped Cu2Se bulks. (b) Crystal structure of low-

temperature monoclinic phase (left) and high-temperature cubic phase (right) of

Cu2Se matrix

crystallized in space group C2/c with lattice parameters of a = 7.101(3) Å, b =

12.316(2) Å, c = 27.235 (6) Å, and β = 93.831(8) °. 훽-phase Cu2Se is crystallized in space group Fm3m with the lattice parameter a = 5.831(4) (Å). Synchrotron powder

X-ray diffraction was performed in order to investigate the CF doping effect in the 훼- and 훽-phases through the change of phase transition with temperature and to identify lattice parameter changes and test the 훽-phase stability at high temperatures. 134

Figure 5-2a and Figure 5-2b shows temperature dependent heat-map images of undoped Cu2Se and 0.75wt% CF-doped Cu2Se samples from room temperature to

774 K, respectively. The mapping shows that there is a gradual and continuous phase transition when the sample is heated to high temperature. The peaks of the α-phase at

Figure 5-2 Synchrotron radiation X-ray diffraction (SR-XRD) profiles of (a) pure

Cu2Se and (b) 0.75 wt% CF-doped Cu2Se obtained upon heating, showing the

disappearance of the extra peaks around 4.97° and 15° as the lower-symmetry α-

phase is transformed to the higher-symmetry cubic β-phase. (c) Temperature

dependence of the specific heat (Cp) for the undoped and CF doped Cu2Se samples.

(d) XRD patterns for Cu2Se/0.75wt% CF obtained at 300 K, at the beginning of the

DSC measurement (1) and after the measurement (2), respectively

135

4.97° and around 15° gradually weaken and then disappear. Finally, the peaks indicate a transition to the cubic phase. This transformation was also observed by differential scanning calorimetry (DSC) measurements (Figure 5-2c) and in the literature [28]. Note that the temperature of the phase transition (Tc) is at 400 K for the undoped Cu2Se and increases to the higher temperature of 413 K with 0.75 wt%

CF.

Figure 5-3c and Figure 5-3d show the peak shift with temperature for the doped sample in comparison with the undoped sample before and after the phase transition as well as at the beginning and after the synchrotron measurements. The changes in lattice parameters of the 훼-phase and 훽-phase for the undoped and 0.75 wt% CF- doped Cu2Se are shown in Figure 3e and f. I found that the lattice parameters of undoped sample at room temperature are: a = 7.101(3) Å, b = 12.316(2) Å, c =

27.235 (6) Å, and β = 93.831(8) °, and for the CF doped sample, they are: a =

7.124(6) Å, b = 12.354(9) Å, c = 27.278(5) Å, β = 94.292(3) °. In the cubic phase, the lattice parameters become a = 5.8314 Å, 5.8427 Å, 5.8611 Å, and 5.8795 Å for the undoped sample, and they change to 5.838(1) Å, 5.854(8), Å 5.872(1) Å, and

5.891(1) Å for the CF-doped sample at T = 402, 512, 622, and 773 K, respectively.

The nature of the second-order structural phase transition [18] and the increase in the lattice parameter for the doped sample compared with undoped sample are effected by variations in the sample density, concentration, and structure, which could lead to extremely large electron and phonon critical scattering. There are two possible reasons for the lattice expansion. One possibility is that the carbon atoms from CFs enter into the Cu2Se lattice, causing enlargement of the lattice constant of Cu2Se.

136

This cannot be concluded, however, until C atoms can be clearly identified inside the unit cell by further study.

Figure 5-3 (a, b) Rietveld refinement XRD patterns for the undoped and 0.75wt% CF

doped Cu2Se samples, respectively; (c, d) Comparison of peak shifts of undoped and

CF-doped Cu2Se samples with temperature; (e, f) change in lattice parameters of the

137

훼-phase with temperature compared with [29] and 훽-phase transition for undoped

and 0.75wt% CF doped samples

Figure 5-4 SEM-STEM image characterization of the densely distributed CFs inside

the Cu2Se matrix: (a) Undoped Cu2Se. B) 0.75wt% CF at and inside the Cu2Se grain

boundaries. (c-f) Low-magnification TEM image and corresponding EDX mapping

for elements C, Cu, and Se, respectively. (g) Atomic resolution HAADF image at

room temperature along the [2 0 1] zone axis, where the large bright dots refer to Se

atoms and the weak contrast along the Se layers refers to Cu atoms. (h) The

corresponding fast Fourier transform (FFT) pattern along the [2 0 1] zone axis

Field emission scanning electron microscope (FE-SEM) images of the undoped

(Figure 5-4a) and CF-doped Cu2Se bulks (Figure 5-4b) demonstrate the high concentration of CFs, which are uniformly distributed in the Cu2Se matrix without any agglomeration. The diameters of the CFs are around 6 휇m (Figure 5-5) and their average lengths are around 50 휇m. Figure 5-4c-f shows a low-magnification transmi-

138 ssion electron microscope (TEM) image of the CF and Cu2Se interface with the correspond--ing energy dispersive X-ray spectroscopy (EDX) mapping for the elements C, Cu, and Se, respectively. Figure 5-4g shows an atomic resolution high- angle annular dark field (HAADF) image at room temperature along the [2 0 1] zone axis, where the large bright dots refer to Se atoms and the weak contrast along the Se layers refers to Cu atoms. The STEM − energy dispersive spectroscopy (EDS) three- dimensional (3D) visualization of the elemental analysis of the 0.75wt% CF in Cu2Se is illustrated in Figure 5-6. The 3D maps represent carbon (Figure 5-6(b)), copper

(Figure 5-6(c)), and Selenium (Figure 5-6(d)), respectively. These CF inclusions are expected to play an important role in blocking phonons.

Figure 5-5 Focused ion beam (FIB) cut image shows the carbon fiber in the Cu2Se

sample

139

Figure 5-6 STEM-EDS analysis of 0.75wt% CF in Cu2Se: (a) Low magnification

TEM image; (b-d) the corresponding EDS elemental analyses of carbon, Cu, and Se,

respectively

140

5.4. Thermoelectric measurements and performance:

Figure 5-7 Temperature dependence of the (a) electrical conductivity, (b) Seebeck

coefficient, (c) power factor, (d) carrier concentration and carrier mobility as

functions of the doped CF ratio at different temperatures, in comparison with data for

Cu2Se [15], and (e) thermal conductivity for Cu2Se-xwt% CF (x = 0, 0.35, 0.55, 0.75,

and 0.85) polycrystals; and (f) Schematic diagram of CF-doped Cu2Se composite,

showing the microstructure and phonon transportation

141

I investigated the effect of CF addition on 𝜎, 횜, and , as well as the microhardness.

Figure 5-7 shows the temperature dependence of the electrical conductivity (𝜎),

Seebeck coefficient (횜), Power factor (PF), and thermal conductivity () for undoped and CF-doped Cu2Se samples. Figure 5-7a demonstrates that the σ of all samples decreases with increasing temperature over the whole temperature range. I can see that the CF-doped Cu2Se sample shows lower electrical conductivity than the undoped sample for both the α- and the 훽-phases. The electrical conductivity is reduced from ~ 224 S·cm-1 for the undoped sample to 164 S·cm-1 for the 0.75wt%

CF-doped Cu2Se at 1000 K. Using the Hall effect, I determined the carrier concentration p and mobility using the formula: , where is the

elemental charge and RH is the Hall coefficient. I found that the hole concentration for the undoped Cu2Se is which is comparable with what has

been reported for Cu2Se [15]. For the 0.75wt% CF doped Cu2Se, p is

at room temperature, which is ~31% lower than that in the

pristine Cu2Se. The mobility is for the 0.75wt% CF doped

sample, which is higher than for the undoped Cu2Se (Figure 5d). As σ = p , the overall reduction in σ is be mainly caused by the greatly decreased hole concentration. Figure 5-7b shows the temperature dependence of the Seebeck coefficient for the both the CF doped and the undoped samples. It can be seen that the 횜 increases with 0.75 wt% carbon fiber doping over a wide range temperature.

−1 For the 0.75 wt% CF doped Cu2Se, the 횜 is 266 μV K at 1000 K, which is about

10% larger than that of the undoped sample.

142

The power factor, PF = S2𝜎, is for the undoped 훼-phase and is for the high temperature 훽-phase, which is comparable to the previously reported values [16, 30] (Figure 5-7c). All the CF doped samples show the same trend in PF with increasing temperature. The PF for the 0.75 wt% CF doped samples is for the 훼-phase and

for the 훽-phase, which is about ~10% lower than that of the undoped sample at 1000 K.

5.5. Analysis and discussion

Remarkably, the CF-doped Cu2Se samples exhibit greatly reduced thermal conductivity compared to the CF-free sample over a wide temperature range, as displayed in Figure 5-7e. The  for the undoped sample is 1.1 - 0 from

456 K - 1000 K, while it is as low as ~0.4 W·m-1·K-1 for the 0.75 wt% CF-doped

Cu2Se for the entire temperature range. This value is about 60% lower than for the undoped sample and is also the lowest among all the undoped Cu2Se samples reported so far [31, 17, 30, 32, 33, 18]. The extremely low lattice thermal conductivity for our CF-doped Cu2Se composites is purely from increased interfacial thermal resistance which is calculated using a simplified model , where are the reduced thermal conductivity with an average grain size . and

are the intrinsic thermal conductivity and interface thermal resistance, respectively. Using the mean value of d̅ = 1.36 µm, and assuming that reduction from = 1.1 W∙m−1∙K−1 to = 0.4 W·m−1·K−1 is purely from increased interfacial

143 resistance, I can estimate the upper limit of = 1.56×10−6 m2·K·W−1. Our previous

−9 2 −1 work indicates negligible value < 1×10 m ·K·W for the intrinsic Cu2Se:Cu2Se grain boundaries [21]. Therefore, The greatly increased comes from the CF inclusion exhibit near the edge which reduces transmission of heat phonon and is agreement with recently published results [20] and micrograined graphene/skutteride composites ( = 11 × 10−7 m2·K·W−1) [34]. Figure 5-7f illustrates schematically how CF inclusion affects the microstructure and phonon transport property of the micro-composite samples.

Figure 5-8 a) Thermoelectric figure-of-merit zT of CF-doped Cu2Se composite for

different wt% CF. b) Thermoelectric efficiency of the undoped and CF-doped Cu2Se

samples, in comparison with the previously reported Cu2Se data [28]

The record high figure-of-merit zT is observed when CF is increased in the Cu2Se over the whole temperature range, as shown in Figure 5-8(a). The zT is 1.0 at T =

550 K and increases to ~2.6 at 1000 K for the 0.75 wt% CF doped sample as compared to zT of 0.44 at 550 K and 1.5 at 1000 K for the undoped sample. It is

144 remarkable that the overall zT enhancement factor for the optimum CF doped sample is above 1.8 to 2.4 compared to the undoped sample (Figure 5-9d). The efficiency is calculated according to the literature [22, 23] and found 15% at 650 K and reaches the ultra-high efficiency of ~31% at 1000 K (figure 6(b)). Finally, the overall results show that CF reinforced Cu2Se composites open up a new horizon for the next generation of thermoelectric power generation.

Figure 5-9 Temperature dependence of (a) σ/ , (b) S/S0.0, (c) κ/κ0.0, and (d) zT/zT0.0

for the carbon fiber doped Cu2Se samples, where 0.0 refers to the undoped sample

145

All the electrical conductivity and thermal diffusivity measurements were repeated several times shown in figure 5-10 which confirms the reproducibility of the as- prepared polycrystalline bulks.

Figure 5-10 Temperature dependence of (a) the electrical conductivity and (b) the

Seebeck coefficient over four repeated trials for a 0.75wt% CF doped Cu2Se sample

5.6. Conclusions:

Significant enhancement of ZT of up to 2.6 is also observed using spark plasma sintering approach. The origin of the ultrahigh zT is that the carbon fiber addition creates a large density of boundaries, which remarkably reduces the thermal-conductivity by up to 60% through blocking the phonon transport, while the Seebeck coefficients and electrical conductivity change only slightly with doping. As a result, 0.75wt% carbon fiber doped

Cu2Se composite exhibits the record zT of ~2.6 and an estimated ultra-high efficiency of 31%. Our results represent a remarkable advance in thermoelectric performance and hold great potential for intermediate temperature applications in thermoelectric devices, which will benefit the current thermoelectric industry.

146

5.7. References

[1] G. J. Snyder and E. S. Toberer, "Complex thermoelectric materials," Nature

materials, vol. 7, no. 2, pp. 105-114, 2008.

[2] F. J. DiSalvo, "Thermoelectric Cooling and Power Generation," Science, vol.

285, no. 5428, pp. 703-706, 1999.

[3] L. E. Bell, "Cooling, Heating, Generating Power, and Recovering Waste Heat

with Thermoelectric Systems," Science, vol. 321, no. 5895, pp. 1457-1461,

2008.

[4] J. P. Heremans et al., "Enhancement of thermoelectric efficiency in PbTe by

distortion of the electronic density of states," Science, vol. 321, no. 5888, pp.

554-557, 2008.

[5] D. Kraemer et al., "High thermoelectric conversion efficiency of MgAgSb-

based material with hot-pressed contacts," Energy & Environmental Science,

vol. 8, no. 4, pp. 1299-1308, 2015.

[6] Y. Du et al., "Thermoelectric fabrics: toward power generating clothing,"

Scientific reports, vol. 5, 2015.

[7] Y. W. Tung and M. L. Cohen, "Relativistic Band Structure and Electronic

Properties of SnTe, GeTe, and PbTe," Physical Review, vol. 180, no. 3, pp.

823-826, 04/15/ 1969.

[8] J. R. Sootsman, D. Y. Chung, and M. G. Kanatzidis, "New and old concepts

in thermoelectric materials," Angewandte Chemie International Edition, vol.

48, no. 46, pp. 8616-8639, 2009.

147

[9] M. S. Dresselhaus et al., "New directions for low‐ dimensional

thermoelectric materials," Advanced Materials, vol. 19, no. 8, pp. 1043-1053,

2007.

[10] M. Zebarjadi, K. Esfarjani, M. Dresselhaus, Z. Ren, and G. Chen,

"Perspectives on thermoelectrics: from fundamentals to device applications,"

Energy & Environmental Science, vol. 5, no. 1, pp. 5147-5162, 2012.

[11] D. Kraemer et al., "High-performance flat-panel solar thermoelectric

generators with high thermal concentration," Nature materials, vol. 10, no. 7,

pp. 532-538, 2011.

[12] A. Suzumura, M. Watanabe, N. Nagasako, and R. Asahi, "Improvement in

thermoelectric properties of Se-free Cu3SbS4 compound," Journal of

Electronic Materials, vol. 43, no. 6, pp. 2356-2361, 2014.

[13] B. Zhong et al., "High superionic conduction arising from aligned large

lamellae and large figure of merit in bulk Cu1. 94Al0. 02Se," Applied Physics

Letters, vol. 105, no. 12, p. 123902, 2014.

[14] J. Fan et al., "Crystal Structure and Physical Properties of Ternary Phases

around the Composition Cu5Sn2Se7 with Tetrahedral Coordination of

Atoms," Chemistry of Materials, vol. 26, no. 18, pp. 5244-5251, 2014.

[15] S. Ballikaya, H. Chi, J. R. Salvador, and C. Uher, "Thermoelectric properties

of Ag-doped Cu 2 Se and Cu 2 Te," Journal of Materials Chemistry A, vol. 1,

no. 40, pp. 12478-12484, 2013.

[16] L. Zhao et al., "The Effects of Te2− and I− Substitutions on the Electronic

Structures, Thermoelectric Performance, and Hardness in Melt-Quenched

Highly Dense Cu2-xSe," Advanced Electronic Materials, vol. 1, no. 3, pp.

n/a-n/a, 2015.

148

[17] L. Zhao et al., "Improvement of thermoelectric properties and their

correlations with electron effective mass in Cu(1.98)S(x)Se(1−x)," Scientific

Reports, vol. 7, p. 40436, 01/16 01/16 10/10/received 12/06/accepted 2017.

[18] H. Liu et al., "Ultrahigh Thermoelectric Performance by Electron and Phonon

Critical Scattering in Cu2Se1-xIx," Advanced Materials, vol. 25, no. 45, pp.

6607-6612, 2013.

[19] T. W. Day et al., "Influence of Compensating Defect Formation on the

Doping Efficiency and Thermoelectric Properties of Cu2-ySe1–xBrx,"

Chemistry of Materials, vol. 27, no. 20, pp. 7018-7027, 2015/10/27 2015.

[20] L. Zhao et al., "Significant enhancement of figure-of-merit in carbon-

reinforced Cu2Se nanocrystalline solids," Nano Energy, vol. 41, no.

Supplement C, pp. 164-171, 2017/11/01/ 2017.

[21] L.-l. Zhao et al., "Superior intrinsic thermoelectric performance with zT of

1.8 in single-crystal and melt-quenched highly dense Cu2-xSe bulks,"

Scientific reports, vol. 5, p. 7671, 2015.

[22] G. J. Snyder and A. H. Snyder, "Figure of merit ZT of a thermoelectric

device defined from materials properties," Energy & Environmental Science,

10.1039/C7EE02007D vol. 10, no. 11, pp. 2280-2283, 2017.

[23] G. J. Snyder, "Thermoelectric power generation: Efficiency and

compatibility," Thermoelectrics Handbook: Macro to Nano, 2006.

[24] G. J. Snyder and T. S. Ursell, "Thermoelectric efficiency and compatibility,"

Physical review letters, vol. 91, no. 14, p. 148301, 2003.

[25] K. A. Borup et al., "Measuring thermoelectric transport properties of

materials," Energy & Environmental Science, vol. 8, no. 2, pp. 423-435,

2015.

149

[26] Y. Pei, A. D. LaLonde, H. Wang, and G. J. Snyder, "Low effective mass

leading to high thermoelectric performance," Energy & Environmental

Science, vol. 5, no. 7, pp. 7963-7969, 2012.

[27] X.-Q. Huang, Y.-X. Chen, M. Yin, D. Feng, and J. He, "Origin of the

enhancement in transport properties on polycrystalline SnSe with

compositing two-dimensional material MoSe2," Nanotechnology, vol. 28, no.

10, p. 105708, 2017.

[28] H. Liu et al., "Copper ion liquid-like thermoelectrics," Nature Materials, vol.

11, p. 422, 03/11/online 2012.

[29] L. Gulay, M. Daszkiewicz, O. Strok, and A. Pietraszko, "Crystal structure of

Cu2Se," Chemistry of metals and alloys, no. 4,№ 3-4, pp. 200-205, 2011.

[30] B. Yu et al., "Thermoelectric properties of copper selenide with ordered

selenium layer and disordered copper layer," Nano Energy, vol. 1, no. 3, pp.

472-478, 5// 2012.

[31] Y.-B. Zhu, B.-P. Zhang, and Y. Liu, "Enhancing thermoelectric performance

of Cu2Se by doping Te," Physical Chemistry Chemical Physics,

10.1039/C7CP05149B vol. 19, no. 40, pp. 27664-27669, 2017.

[32] S. Ballikaya, H. Chi, J. R. Salvador, and C. Uher, "Thermoelectric properties

of Ag-doped Cu2Se and Cu2Te," Journal of Materials Chemistry A,

10.1039/C3TA12508D vol. 1, no. 40, pp. 12478-12484, 2013.

[33] D. Li et al., "Chemical synthesis of nanostructured Cu 2 Se with high

thermoelectric performance," Rsc Advances, vol. 4, no. 17, pp. 8638-8644,

2014.

[34] P.-a. Zong et al., "Skutterudite with graphene-modified grain-boundary

complexion enhances zT enabling high-efficiency thermoelectric device,"

150

Energy & Environmental Science, 10.1039/C6EE02467J vol. 10, no. 1, pp.

183-191, 2017.

151

Chapter 6.

6. Enhancement of thermoelectric performance of

Cu2Se using a liquid source of carbon

6.1. INTRODUCTION

Thermoelectric (TE) materials have become the new stars in the field of renewable energy as the thermoelectric power generation are the greenest renewable technology which allows the conversion of waste heat into electricity. Many TE materials such as Pb chalcogenide have demonstrated high figure of merit [1-4], however, large- scale applications for the Pb chalcogenide have become restricted due to the environmental concern of Pb [5-9]. Thus, copper selenide TE materials becoming more popular as a promising material for the large-scale applications, especially in the mid-temperatures range, as they have remarkably high zT values ~1.4 at 900 K

[10, 11]. Nevertheless, most of the current processing techniques involve costly instruments that required high temperature and pressure with long heat treatments as well as the complicated chemical process, making them less suitable for practical applications. Among all the different doping approaches, the nanosize precursor particles techniques were preferred to further enhancement of TE performance.

However, agglomeration of the nano-additives during solid state mixing is the vital bottleneck in achieving a homogeneous mixing. Therefore, it comes to our attention in finding effective fabrication process and doping technique that would be time- efficient and cost-effective for achieving a homogeneous mixing.

In this chapter, I report that the grape juice is used as a homogeneous carbon additive to the Cu2Se. We’ve investigated the grape juice doping effect on TE properties of

Cu2Se in terms of crystal structures, lattice parameters, microstructures, chemical stability, electrical and thermal transport properties with the Scanning Transmission

Electron Microscopy (STEM), synchrotron high-temperature X-ray Diffraction

(XRD) analysis, as well as their electrical and thermal conductivity properties.

Significant enhancement of electrical conductivity from 250 S·cm-1 to 483 S·cm-1 was observed at 850 K due to the grape juice doping, while the thermal conductivity remained comparable to the undoped Cu2Se compound, leading to the great enhancement of the figure of merit by a factor of 2.9 to 2.2 over the temperature ranges from 400 to 850 K, respectively.

6.2. Experimental details

6.2.1. Synthesis:

First, The Cu2Se Powders were mixed with grape juice (with the appropriate amount calculated to corresponding to about 0.15 wt%, 0.30 wt%, 0.45 wt%, and 0.60 wt% C in the compound) in deionized water by hot plate magnetic stirrers with the aim of achieving a more uniform distribution and sample drying. These mixed powders were further dried in a vertical furnace for about 2 hours in 2000C. Then, the mixtures were sealed in an evacuated quartz tube, before being heated to 12000C for10 min with a heating rate of 100C/min, followed by a furnace cooling to room temperature. Finally, the obtained polycrystalline bulks were shaped into round disks and rectangular bars using a Struers Accutom-50 machine for thermal diffusivity and electrical conductivity measurements. In this report, the grape juice doped samples

153 are denoted by their carbon weight percentage, so that the samples with 0.15 wt% carbon was denoted as 0.15G, etc.

6.2.2. Measurements

High resolution synchrotron powder X-ray diffraction were performed using a wavelength of λ = 0.58973 Å at the beamline 10-BM-1, with 2.5º ≤ 2θ ≤ 800, step scan mode simultaneously in every 30 seconds and with a helium gas flow rotated at a speed of 1Hz over the temperature range of 300-520 K. The sample preparations for the synchrotron measurement are as follows: First, about 60mg of the as-prepared sample was chipped off then ground using a mortar and a pestle. The quartz capillary tube (0.5mm in diameter and 20mm in length) was used to transfer the resulting fine powder. Importantly, the powder was tightly packed by placing it in a sonicator

(while holding the capillary lightly with a hand). All the Rietveld refinements were performed using the Rietica software. Field Emission Scanning Electron Microscopy

(FE-SEM, JEOL 7500) and Transmission Electron Microscopy (JEOL ARM200F) were used to investigate the samples’ morphology and microstructures of the undoped and grape doped Cu2Se samples. The thin TEM specimens were prepared using the Focused Ion Beam milling (FIB) for the Scanning Transmission Electron

Microscopy/x-ray spectroscopy (STEM/x-ray mapping) investigations. The FIB samples were prepared by mounting it in a Polyfast using a Struers Citopress-20 equipment and it was polished with a Struers Tegramin-20 equipment. The electrical conductivity and Seebeck coefficient were measured simultaneously under a helium atmosphere from room temp to 850 K using a commercial RZ2001i system. The thermal diffusivity (D) was measured by the laser flash method with an LFA 1000 analyser under the vacuum conditions. The specific heat (Cp) was determined by

154 differential scanning calorimetry on a DSC-204F1-Phoenix calorimeter under an argon atmosphere with a flow rate of 50ml/min. The samples density (dd) was calculated using the measured weight and dimensions. The thermal conductivity () was calculated by The hardness measurements were done using a Struers DuraScan-70 Hardness tester after being mounted in a Struers Citopress-20 and polished by a Struers Tegramin-20in water.

6.3. RESULTS AND DISCUSSION:

Figure 6-1a and Figure 6-1b illustrate the heat map image measured using synchrotron powder X-ray diffraction from room temperature to 520 K, respectively, and Figure Figure 6-1c and Figure 6-1d show the Rietveld refinements of the undoped and 0.6G doped Cu2Se samples, respectively. The heat map image was performed to determine the changes in the crystal structure vs temperature of the undoped and grape doped Cu2Se. It was observed that the peak of the α-phase at around 50 and 150 were gradually weakened and then disappears with temperature and finally transit to the cubic phase from the monoclinic phase. This transformation was also observed by our Differential Scanning Calorimetry (DSC) measurements

(Figure 6-2), and it is in agreement with what has been reported by H Liu et al [12].

The Rietveld refinements show that the samples were crystallized with cubic β-phase upon heating with the space group Fm3m. Slight variation of lattice parameters was observed for the doping. As shown in Table 3, the parameter a was found to be

5.79960 Å, 5.79850 Å, 5.79060 Å and

155

Figure 6-1 Synchrotron X-ray diffraction profile of the (a) undoped Cu2Se and (b)

0 0 0.60G doped Cu2Se, notice the disappearance of the extra peaks around 5 and 15

upon heating due to the transformation of monoclinic α-phase to the cubic β-phase.

(c, d) Rietveld refinement of XRD patterns for the undoped and 0.6G doped Cu2Se

samples, respectively; The refined parameters are shown in Table S1

5.78340 Å for the grape doped Cu2Se at 520, 500, 450, and 400 K respectively, whereas it was 5.81130 Å 5.80730 Å, 5.80110 Å and 5.79380 Å for the undoped

Cu2Se at the corresponding temperatures. The character of the second-order structural phase transition [13] and the change of lattice parameter with doping

156 effects on variations in the sample density, concentration, and structure could lead to extremely large electron and phonon critical scattering [13].

Figure 6-3a and Figure 6-3b show the Field Emission Scanning Electron Microscope

(FE-SEM) images of the pure and the 0.6G doped Cu2Se bulks, respectively. The as- prepared fabricated droplet of the undoped and grape doped samples shown in the

Lattice parameter

2 Temp (k) a (Å) Rp Rwp 

Sample

520 0.6 wt% G 5.79960 8.800 11.667 1.788

500 0.6 wt% G 5.79850 8.379 11.279 1.664

450 0.6 wt% G 5.79060 8.439 11.160 1.648

400 0.6 wt% G 5.78340 8.731 11.404 1.728

520 Cu2Se 5.81130 8.031 11.066 1.840

500 Cu2Se 5.80730 8.081 10.908 1.790

450 Cu2Se 5.80110 7.897 10.640 1.753

400 Cu2Se 5.79380 7.745 10.351 1.704

Table 3 Rietveld Refinement parameters of grape-doped and undoped Cu2Se at different temperatures

insets of the FE-SEM images respectively. All the samples were found to be highly dense without any visible void or porosity. The HRTEM was performed to clarify the crystal structures of the samples and results indicated that carbon inclusion happened 157 inside the fine-grained nanostructured. Low-resolution TEM images showed that the carbon regions appear as light patches within the 0.6G doped Cu2Se as shown in

Figure 6-3c. The high resolution TEM image showing the detail of a carbon-rich area in Figure 3d alongside with the enlarged view of areas A and B to the right of 6-3d.

The interplanar distance was calculated as 0.32 nm, corresponding to the (133) plane of the α-Cu2Se phase matched with the monoclinic crystal structure with space group

C2/C [14]. The Fast Fourier Transform (FFT) patters also shows the periodicity of the interplanar distance accomplished in the selected area.

Figure 6-2 Temperature dependence of specific heat (Cp) for the undoped and grape

juice doped Cu2Se samples

158

STEM micrograph of a grape doped Cu2Se sample illustrated in Figure 4 with the corresponding EDS mappings of Cu2Se (Figure 6-4b), C phase (Figure 4c), CuO phase (Figure 6-4d). STEM image showing the area (dash square) of the CuO nanoparticles embedded in the carbon phase. The high-resolution image showing the black area of CuO embedded in C of the selected region of (Figure 6-4e) illustrated in Figure 6-4f. The corresponding Fast Fourier Transform (FFT) patterns shown in the insets taken from the marked area in Figure 4f. The inset B matches with the

159 crystal plane (200) of the CuO with the PDF card No: 01-078-0428. The STEM/EDS spectrum is also shown in Figure 6-5.

Figure 6-4 STEM micrograph of a grape doped Cu2Se sample. (a) Low magnification

TEM image used for phase analysis using EDS mapping; (b) Cu2Se phase; (c) C

phase; (d) CuO phase; (e) STEM image showing the area (dash square) of the CuO

nano-cluster embedded in carbon phase studied for high resolution image; (f) high

resolution image showing the black area of CuO embedded in C of the selected

region of (e). The corresponding Fast Fourier Transform (FFT) patterns shown in the

insets taken from the marked area in Figure 4f. The inset B matches with the crystal

plane (200) of the CuO with the PDF card No: 01-078-0428

I have studied the grape doping effect over the electrical conductivity 𝜎, the Seebeck coefficient 횜, thermal conductivity , figure-of-merit zT as well as the

160 microhardness. The 𝜎, 횜, , and zT with respect to temperature for an undoped and grape doped Cu2Se samples are shown in Figure 6-6. The electrical conductivity 𝜎 decreases with temperature for all the grape doped and undoped samples from 350 to

850 K showing a degenerate semiconducting behaviour. The highest electrical conductivity of 2032 Scm-1 observed at 410 K for the 0.30G which increased more than a factor of 2 compared with the undoped sample (Figure 6-6a). It is found to be the highest electrical conductivity value amongst all of the state-of-the art copper selenide family [15, 16, 13, 17-25].

The Seebeck coefficient S shown in Figure 6-6b found to increase with temperature for the both the undoped and grape doped samples. It can be seen that the Seebeck coefficient with grape doped samples is comparable with the undoped sample. It can be seen in Figure 6-6c that the higher wt% grape doped samples (for 0.45 wt% G and

0.6 wt% G) showing very similar thermal conductivity behaviour to the undoped sample over the whole measuring temperature range which is roughly ~0.5 W m−1

K−1 and this value is comparable with the previously reported results [15, 16, 13, 17,

19, 20, 23, 25]. However, the lower wt% grape doped samples shown higher thermal conductivity than the undoped sample which is consistent with the σ values (Figure

6-6a). Finally, the calculated figure of merit plotted in Figure 6-6d with respect to temperature, it shows the improving values of the figure of merit for most of the grape doped samples.

161

Figure 6-5 EDS spectrum of the (a) Cu2Se phase, (b) C phase, and (c) CuO phase

162

0.15, 0.30, 0.45 and 0.60)

Most of the grape doped samples showed higher zT values and it increases with temperature. The zT value was found to be ~1 at 600 K and it reaches an ultra-high value of ~2 at 850 K for the 0.6G doped sample. The overall zT improvement factor observed to be ~2.9 to ~2.2 for the 0.6 G doped sample over the temperature ranges from 400 to 850 K (Figure 6-7d). The thermoelectric efficiency of the 0.6G doped

163

Figure 6-7 Temperature dependence of the (a) σ/ , (b) S/S0.0, (c) κ/κ0.0, and (d)

zT/zT0.0 for the grape doped Cu2Se samples with a doping level of 0.15, 0.30, 0.45 and 0.60 wt%

sample illustrated in figure 6-8 found to be 27% at 850 K and 15% at 600 K, whereas it was 16% at 850 K and 8% at 600 K for the undoped sample. The thermoelectric efficiency is calculated according to the literature [26, 27]. So, the great enhancement

164

Figure 6-8 Thermoelectric efficiency of the undoped and 0.6G doped Cu2Se samples,

in comparison with the previously reported Cu2Se data [12]

of the TE figure of merit values and the thermoelectric efficiency is due to the formation of CuO (revealed from the microstructure analysis) which induces Cu deficiencies in the Cu2Se compound, resulting in greater enhancement of electrical conductivity which in turn significantly enhances the ZT values.

From the microstructure study, I’ve observed that carbon nanoparticles were embedded inside the Cu2Se grains and the CuO nanoparticles were located at the interface between the carbon particles and Cu2Se. The Vickers hardness of grape doped Cu2Se with different concentration illustrated in Figure 6-9 (a-d) was measured under 0.245 N load in Lens (60x). The average hardness of grape doped

165 samples was found to be ~0.40 GPa which is about 10% higher than the undoped sample. These values are superior to the polycrystalline PbTe solids (~0.30 GPa)

[28]. Overall, the improved thermoelectric performance of the grape doped Cu2Se samples has opened up a new direction for the next generation economical thermoelectric materials for power generation.

Figure 6-9 Vickers Hardness of (a) 0.15 G (b) 0.30 G (c) 0.45 G (d) 0.60 G doped

Cu2Se sample under 0.245 N load in Lens (60x)

6.4. Conclusions

The electrical conductivity for 0.60 wt% G doped Cu2Se samples is significantly increased compared to undoped Cu2Se while keeping thermal conductivity

166 unchanged. This has led to great enhancement of the figure-of-merit by a factor of

~2.9 to ~2.2 over the temperature ranges from 400 to 850 K, respectively. From the microstructure study, I observed carbon nanoparticles embedded inside Cu2Se grains and Cu2O nanoparticles are located at the interface between carbon and Cu2Se. It is proposed that the formation of Cu2O induces Cu deficiencies in Cu2Se resulting in greatly enhancement of electrical conductivity which in turn significantly enhances the ZT.

167

6.5. Reference

[1] R. J. Korkosz et al., "High ZT in p-Type (PbTe) 1–2 x (PbSe) x (PbS) x

Thermoelectric Materials," Journal of the American Chemical Society, vol.

136, no. 8, pp. 3225-3237, 2014.

[2] G. Tan et al., "Non-equilibrium processing leads to record high

thermoelectric figure of merit in PbTe–SrTe," Nature communications, vol. 7,

p. 12167, 2016.

[3] Y. Lee et al., "Contrasting role of antimony and bismuth dopants on the

thermoelectric performance of lead selenide," Nature communications, vol. 5,

p. 3640, 2014.

[4] C. M. Jaworski, B. Wiendlocha, V. Jovovic, and J. P. Heremans, "Combining

alloy scattering of phonons and resonant electronic levels to reach a high

thermoelectric figure of merit in PbTeSe and PbTeS alloys," Energy &

Environmental Science, vol. 4, no. 10, pp. 4155-4162, 2011.

[5] A. Banik and K. Biswas, "Lead-free thermoelectrics: promising

thermoelectric performance in p-type SnTe 1− x Se x system," Journal of

Materials Chemistry A, vol. 2, no. 25, pp. 9620-9625, 2014.

[6] M. H. Lee, K.-R. Kim, J.-S. Rhyee, S.-D. Park, and G. J. Snyder, "High

thermoelectric figure-of-merit in Sb 2 Te 3/Ag 2 Te bulk composites as Pb-

free p-type thermoelectric materials," Journal of Materials Chemistry C, vol.

3, no. 40, pp. 10494-10499, 2015.

[7] L.-D. Zhao et al., "BiCuSeO oxyselenides: new promising thermoelectric

materials," Energy & Environmental Science, vol. 7, no. 9, pp. 2900-2924,

2014.

168

[8] J. Mao et al., "Thermoelectric performance enhancement of Mg 2 Sn based

solid solutions by band convergence and phonon scattering via Pb and Si/Ge

substitution for Sn," Physical Chemistry Chemical Physics, vol. 18, no. 30,

pp. 20726-20737, 2016.

[9] S. N. Guin, A. Chatterjee, D. S. Negi, R. Datta, and K. Biswas, "High

thermoelectric performance in tellurium free p-type AgSbSe 2," Energy &

Environmental Science, vol. 6, no. 9, pp. 2603-2608, 2013.

[10] H. Liu et al., "Copper ion liquid-like thermoelectrics," Nat. Mater., vol. 11,

no. 5, pp. 422-425, May 2012.

[11] L. Zhao et al., "Superior intrinsic thermoelectric performance with zT of 1.8

in single-crystal and melt-quenched highly dense Cu2-xSe bulks," Sci. Rep.,

vol. 5, p. 7671, 2015.

[12] H. Liu et al., "Copper ion liquid-like thermoelectrics," Nature Materials, vol.

11, p. 422, 03/11/online 2012.

[13] H. Liu et al., "Ultrahigh Thermoelectric Performance by Electron and Phonon

Critical Scattering in Cu2Se1‐ xIx," Advanced Materials, vol. 25, no. 45, pp.

6607-6612, 2013.

[14] L. Gulay, M. Daszkiewicz, O. Strok, and A. Pietraszko, "Crystal structure of

Cu2Se," Chemistry of metals and alloys, no. 4,№ 3-4, pp. 200-205, 2011.

[15] B. Gahtori et al., "Giant enhancement in thermoelectric performance of

copper selenide by incorporation of different nanoscale dimensional defect

features," Nano Energy, vol. 13, pp. 36-46, 2015.

[16] A. Olvera et al., "Partial indium solubility induces chemical stability and

colossal thermoelectric figure of merit in Cu 2 Se," Energy & Environmental

Science, vol. 10, no. 7, pp. 1668-1676, 2017.

169

[17] T. W. Day et al., "Influence of Compensating Defect Formation on the

Doping Efficiency and Thermoelectric Properties of Cu2-ySe1–x Br x,"

Chemistry of Materials, vol. 27, no. 20, pp. 7018-7027, 2015.

[18] H. Liu et al., "Copper ion liquid-like thermoelectrics," Nature materials, vol.

11, no. 5, pp. 422-425, 2012.

[19] K. Zhao et al., "High thermoelectric performance and low thermal

conductivity in Cu2− yS1/3Se1/3Te1/3 liquid-like materials with nanoscale

mosaic structures," Nano Energy, vol. 42, pp. 43-50, 2017.

[20] L. Zhao et al., "The Effects of Te2− and I− Substitutions on the Electronic

Structures, Thermoelectric Performance, and Hardness in Melt-Quenched

Highly Dense Cu2-xSe," Advanced Electronic Materials, vol. 1, no. 3, pp.

n/a-n/a, 2015.

[21] L.-l. Zhao et al., "Superior intrinsic thermoelectric performance with zT of

1.8 in single-crystal and melt-quenched highly dense Cu2-xSe bulks,"

Scientific reports, vol. 5, p. 7671, 2015.

[22] L. Zhao et al., "Significant enhancement of figure-of-merit in carbon-

reinforced Cu2Se nanocrystalline solids," Nano Energy, vol. 41, no.

Supplement C, pp. 164-171, 2017/11/01/ 2017.

[23] M. Y. Tafti et al., "Promising bulk nanostructured Cu 2 Se thermoelectrics

via high throughput and rapid chemical synthesis," Rsc Advances, vol. 6, no.

112, pp. 111457-111464, 2016.

[24] B. Zhong et al., "High superionic conduction arising from aligned large

lamellae and large figure of merit in bulk Cu1. 94Al0. 02Se," Applied Physics

Letters, vol. 105, no. 12, p. 123902, 2014.

170

[25] S. D. Kang et al., "Enhanced stability and thermoelectric figure-of-merit in

copper selenide by lithium doping," Materials Today Physics, vol. 1, pp. 7-

13, 2017.

[26] G. J. Snyder and A. H. Snyder, "Figure of merit ZT of a thermoelectric

device defined from materials properties," Energy & Environmental Science,

10.1039/C7EE02007D vol. 10, no. 11, pp. 2280-2283, 2017.

[27] G. J. Snyder, "Thermoelectric power generation: Efficiency and

compatibility," Thermoelectrics Handbook: Macro to Nano, 2006.

[28] M. Ablova, M. Vinogradova, and M. Karklina, "Effect of donor and acceptor

impurities on microhardness of PbTe," SOV PHYS SOLID STATE, vol. 10,

no. 8, 1969.

171

Chapter 7

7. General conclusions and outlook

7.1. General conclusions

In this doctoral work, I successfully fabricated highly dense Cu2Se based TE materials using both melt solidification technique and spark plasma sintering method.

I have investigated the microstructures, thermoelectric and mechanical properties of undoped Cu2Se and carbon doped Cu2Se using various types of carbon sources and melt-solidification method. I then demonstrated the doping effects of amorphous boron on Cu2Se’s thermoelectric properties. I further investigated carbon fiber incorporation into the Cu2Se using spark plasma sintering method with the detailed characterizations of high-temperature synchrotron X-ray diffraction, STEM, along with thermoelectric properties with the extended temperature range. Finally, I present a study of using grape juice as a liquid source of carbon to homogeneously mixing with Cu2Se powder which offers the cheap way to significantly enhance the ZT of

Cu2Se and related compounds.

For the study of significant improvement of ZT in melt-solidification processed

Cu2Se using different types of solid carbon such as carbon nanotube, graphite, hard carbon, diamond carbon fiber, and Super P on system, the main findings are robust in that, although I used several different carbon source precursors, collectively the results showed several common features: 1) The thermal conductivity decreases greatly with carbon incorporation. 2) The Seebeck coefficient remains almost the same as for low levels of carbon incorporation. 3) Electrical

172 conductivity increases slightly with doping over the middle-temperature range.

Consequently, all the carbon doped Cu2Se nanocomposites exhibit figure-of-merit, zT, higher than 1.0 over a broad temperature range from 600 to 900 K, with the

0.6wt% Super P doped Cu2Se sample achieving a highest zT of around 1.85 at 900 K.

Furthermore, zT is found to be as high as 1.1 at 555 K and reaches 2.4 at 850 K in carbon fiber doped Cu2Se sample, with values at or exceeding the state-of-the-art materials. The mechanism for this enhancement is that the carbon-decorated nanostructured interfaces strongly reduce heat conductivity by increasing the boundary resistance, whereas they only moderately affect electrical conductivity.

Collectively our results indicate that Cu2-xSe:C nanocomposites will be an excellent candidate for middle-temperature thermoelectric applications, as they are less toxic than lead-based alternatives and low-cost in terms of both materials and fabrication methods. Furthermore, our findings reveal that there is considerable scope to improve and refine the mechanistic understanding of phonon scattering at carbon- decorated nano-interfaces to broaden the horizons of next-generation thermoelectric materials.

For the Cu2Se with the boron addition, I found that nano-boron particle can significantly enhance the zT of Cu2Se. Our studies indicate that boron inclusion can reduce the thermal conductivity, increase the Seebeck coefficient, and reduce the electrical conductivity, resulting in enhancement of zT by a factor of 1.6-2.6 over a wide temperature range compared to undoped samples. The mechanism for this improvement of zT is that the reduction of heat conduction by boron-decorated microstructure edges increases the boundary resistance with decreasing carrier concentration. Our findings offer an effective approach of using insulating nano-

173 particles to significantly improve Seebeck coefficient and significantly reduce lattice thermal conductivity for achieving high ZT in Cu2Se and many other types of thermoelectric composites.

For the Cu2Se with carbon fiber doping, the sample fabrication was done using spark plasma sintering approach, significant enhancement of ZT of up to 2.6 is also observed.

The origin of the ultrahigh zT is that the carbon fiber addition creates a large density of boundaries, which remarkably reduces the thermal-conductivity by up to 60% through blocking the phonon transport, while the Seebeck coefficients and electrical conductivity change only slightly with doping. As a result, 0.75wt% carbon fiber doped Cu2Se composite exhibits the record zT of ~2.6 and an estimated ultra-high efficiency of

31%. Our results represent a remarkable advance in thermoelectric performance and hold great potential for intermediate temperature applications in thermoelectric devices, which will benefit the current thermoelectric industry.

For the Cu2Se doping with the grape juice and fabricated using melt solidification technique, our results show that the electrical conductivity for 0.60 wt% G doped Cu2Se samples is significantly increased compared to undoped Cu2Se while keeping thermal conductivity unchanged. This has led to great enhancement of the figure-of-merit by a factor of ~2.9 to ~2.2 over the temperature ranges from 400 to 850 K, respectively. From the microstructure study, I observed carbon nanoparticles embedded inside Cu2Se grains and Cu2O nanoparticles are located at the interface between carbon and Cu2Se. It is proposed that the formation of Cu2O induces Cu deficiencies in Cu2Se resulting in greater enhancement of electrical conductivity which in turn significantly enhances the ZT.

174

7.2. Outlook

This doctoral thesis work has been mainly focused on the increase of performance using different fabrication techniques and doping approach to the Cu2Se materials, especially melt-solidification and spark plasma sintering method. The structure- property relationship of the synthesized samples has been investigated thoroughly, and the thermoelectric performance was studied as well. At present, we have got outstanding performance on Cu2Se by increasing its zT to 2.6, now it comes to our aim to further increase its figure-of-merit by the co-doping effect on Cu2Se as well as to develop a theory with fast experimental characterization and device fabrication.

175