SYNTHESIS, CHARACTERIZATION AND APPLICATIONS OF NANOSTRUCTURED

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SYNTHESIS, CHARACTERIZATION AND APPLICATIONS OF NANOSTRUCTURED

CHALCOGENIDE THERMOELECTRIC MATERIALS

HARJEET KAUR

DEPARTMENT OF PHYSICS

INDIAN INSTITUTE OF TECHNOLOGY DELHI

JULY 2018

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©Indian Institute of Technology Delhi (IITD), New Delhi, 2018

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SYNTHESIS, CHARACTERIZATION AND APPLICATIONS OF NANOSTRUCTURED

CHALCOGENIDE THERMOELECTRIC MATERIALS

by

HARJEET KAUR DEPARMTENT OF PHYSICS

Submitted

in fulfillment of the requirements for the degree of Doctor of philosophy

to the

INDIAN INSTITUTE OF TECHNOLOGY DELHI

JULY 2018

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DEDICATED TO MUMMY, DADDY

and CHANNI

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CERTIFICATE

This is to certify that the Thesis entitled “SYNTHESIS, CHARACTERIZATION

AND APPLICATIONS OF NANOSTRUCTURED CHALCOGENIDE

THERMOELECTRIC MATERIALS” being submitted by Ms. HARJEET KAUR to the Department of Physics, Indian Institute of Technology Delhi for the award of the degree of

‘Doctor of Philosophy’ is a record of bonafide work carried by her. She has worked under our supervision and guidance and has fulfilled the requirements, which in our opinion have reached the requisite standard for the submission of this Thesis.

The results contained in this work have not been submitted, in part or full, to any other University or Institute for the award of any degree or diploma.

Dr. G. B. REDDY Dr. T. D. SENGUTTUVAN Professor Principal Scientist

Department of Physics National Physical Laboratory Indian Institute of Technology Delhi Delhi, India

Delhi, India

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ACKNOWLEDGEMENTS

My aim to complete this thesis is accomplished with the blessings of GOD and my loving parents. Also, completion of this doctoral thesis was possible with the support of several people. I would like to express my sincere gratitude to all of them.

Firstly, I take an opportunity to express my sincere gratitude to my respected supervisor Prof. G. B. Reddy. It is my pleasure to work with him who always put me on the right track in handling many problems. His overall guidance has always given me the strength and motivation. The discussions we had were always helpful in improving my writing skills and thought process along with exploring new possibilities in this research field.

I also take this opportunity to express heartfelt gratitude to my respected co- supervisor Dr. T. D. Senguttuvan, Principal Scientist at NPL Delhi, for his invaluable guidance, immense interest and constant encouragement for the successful completion of the present work. I owe the greatest debt of gratitude to him for suggesting the research problem followed by meticulous suggestions and ever-willing help during my entire research period. I am grateful to him for giving me this research topic, Thermoelectrics, which I always found interesting. His enthusiastic attitude cherished me during the tenure of my Ph.D. In my failures, he always boosted my confidence and enlightened me with his valuable suggestions.

He always helped me to improve my presentation skills a lot. His presentation skills always impressed me.

I am thankful to my student research committee (SRC) members: Prof. Sujeet Chaudhary, Dr. Rajendra Singh from Physics Department and Prof. Arunachalam Ramanan, Chemistry Department, IIT Delhi for their valuable comments and support that helped me improve during my thesis work. I am thankful to my host institute IIT Delhi and Director of IIT Delhi for providing me an opportunity to pursue Ph.D in Physics.

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I am also thankful to Prof. R. C. Budhani, (Former Director NPL) for giving valuable inputs to my work and Dr. Ajay Dhar (Chief Scientist, NPL) for providing the necessary facilities required during the course of my research work. I pay my humble regards to technical staff in NPL: Mr. K. N. Sood, Mr. Naval Kishore, Mr. Radheshyam, Mr. Jai Tawale for their constant and never ending support. I am thankful to Council of Scientific and Industrial Research (CSIR) for their financial support through fellowship and also providing me financial assistance to attend and present my work abroad in international conference.

I greatly appreciate the help and support received from my lab mates Dr. Ranjit Kumar, Dr. Ravindra Sharma, Dr. Veeresh Yadav, Ms. Shubhda Srivastava during my Ph.D.

I especially pay my thanks to Lalit Kumar, Shahnawaz, Dalvir Singh Rajput, Prabhat Kumar and Megha Singh for their help and support whenever/wherever I required.

Friendship and social support provided by my two beloved Mukesh Kumari and Savvi Mishra is highly appreciated. The moral support by Prabal Pratap Singh Badhauria, Simrjit, Jeevan Jyoti Sharma, Aniket Rana and Pawan Badhan is also acknowledged. All of them have helped me in one or the other way during my work or otherwise. I am always grateful to all of them.

I would also like to express my sincere gratitude to Dr. Harbhajan Thakral who taught me Physics during my school. He is the one because of whom I could find this subject interesting and decided to pursue PhD. He always believed in me and immensely supported me since my school days and even today. Although he is far from me, but never let me think so because he is always there to help, suggest or support me as and when required. He is a constant source of inspiration for me.

Without love and support of my family, it was very difficult to complete this journey.

I pay my sincere regards to my mummy, daddy and beera (Channi) for their encouragement,

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moral and emotional support. Thank you for showing faith in me and giving me liberty to choose what I desired. I consider myself the luckiest in the world to have such a supportive family, standing behind me with their love and support. I will try my best to make you proud of me!

I wish to express my gratitude to the Almighty for giving me the wisdom, health and strength to undertake this research task and enabling me to its completion.

HARJEET KAUR

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ABSTRACT

To reduce the dependency on non-renewable energy sources and to reduce harmful gas emissions, alternative sustainable energy sources or technologies need to be developed. One such low cost untapped energy source is heat. Most of the home appliances, industrial processes and automobiles produce a large amount of waste heat. The noticeable contributions are made through thermoelectric (TE) technology that converts directly waste heat energy into useful electrical energy. TE materials have attracted great attention owing to their potential application in power generation. The main focus in thermoelectric research is on improving the performance of TE material in terms of a dimensionless figure of merit (ZT=S²σ/κ). High ZT is achieved through enhancing electron transport and suppressing phonon transport in a material.

The present thesis aims at developing n-type and p-type TE materials with enhanced ZT by employing nanocomposite approach i.e. incorporating nanoinclusions into TE host matrix and measure thermo- emf using these two TE materials.

The motivation behind this work is based on the fact that by incorporating suitable nanoinclusions of variable sizes into the host matrix, one can achieve phonon scattering on all hierarchical length scale. This significantly reduces the thermal conductivity and has an overwhelming effect on enhancing ZT. For n-type system, Bi2Te3 acts as host matrix and BiTe is taken as nanoinclusion whereas for p-type system, antimony telluride (Sb2Te3) and PbTe are taken as host matrix and nanoinclusion respectively.

The thesis is organized in seven chapters: Chapter-1 (Introduction) gives an overview of the current research status in this field of work. Chapter-2 (Experimental Techniques) contains the details of synthesis routes, sintering techniques and property measurement details used in the present study. Chapter-3 is divided in two parts. The first partdescribes

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the synthesis of nanostructured BiTe by microwave assisted flash combustion (MAFC) route using citric acid and urea as fuels in different oxidant to fuel (O/F) ratios. The role of fuel as well as the effect of O/F ratio on the final product has been explored. It has been found that citric acid as a fuel results in better phase purity of the material as compared to urea. The second part of this chapter discusses about the synthesis of Bi2Te3 by solvothermal route using water and methanol as solvents and varying amounts of reducing agent (NaBH4). The powder prepared using methanol with optimized 2.3 g of NaBH4 results in phase pure hexagonal Bi2Te3. Chapter-4 contains three sections. The first two sections discuss about temperature dependent TE properties of nanostructured BiTe and Bi2Te3 respectively.

Compaction of powders into pellets is done using SPS technique wherein optimization of sintering time and temperature is described in detail. The third section describes TE properties of Bi2Te3/BiTe (2,4,6,8 and 10mol%) composite. An enhanced ZT value of ~1.1 at 470 K is obtained for Bi2Te3/BiTe(8mol%) composite sample in comparison to ZT=0.33 at 470 K for Bi2Te3. Similarly, Chapter-5 describes the synthesis of nanostructured PbTe by MAFC route and Sb2Te3 by solvothermal route in two different sections. Citric acid proves to a suitable fuel to get the desired phase. A hexagonal Sb2Te3 phase is achieved using methanol as solvent with an optimized NaBH4 quantity. Chapter-6 discusses about temperature dependent TE properties of PbTe, Sb2Te3, and Sb2Te3/PbTe composite each divided into three separate parts. An optimized composite sample Sb2Te3/PbTe (5mol%) resulted in a high ZT of 1.52 at 586 K which is 52% higher than ZT for Sb2Te3. Chapter-7 gives details about the fabrication of TE module and measurement of thermo- emf generated across TE module.

The design aspects in fabricating a module and its assembly are described. The voltage across the module and the corresponding current for varying temperature differences are given.

Chapter-8 highlights the important conclusions drawn from the present work and scope of future work in this area.

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सार

गैर नवीकरणीय ऊर्जा स्रोतोों पर ननर्ारतज को कम करने और हजननकजरक गैस उत्सर्ान को कम करने के निए, वैकल्पिक निकजऊ ऊर्जा स्रोत यज प्रौद्योनगनकयोों को नवकनसत करने की आवश्यकतज है। ऐसी कम िजगत वजिी

अपररपक्व ऊर्जा स्रोत गमी है। अनिकजोंश घरेिू उपकरण, औद्योनगक प्रनियजएों और ऑिोमोबजइि बडी मजत्रज में

अपनशष्ट तजप उत्पन्न करते हैं। ध्यजन देने योग्य योगदजन थमोइिेल्परिक (िीई) तकनीक के मजध्यम से नकए र्जते हैं

र्ो सीिे नवद्युत ऊर्जा को उपयोगी नवद्युत ऊर्जा में पररवनतात कर देतज है। नबर्िी उत्पजदन में उनके सोंर्जनवत अनुप्रयोग के कजरण िीई सजमनियोों ने बहुत ध्यजन आकनषात नकयज है। थमजाइिेल्परिक शोि में मुख्य फोकस मेररि

(र्ेडिी = एस 2 सीसी / κ) के आयजमी आोंकडे के सोंदर्ा में TE सजमिी के प्रदशान में सुिजर करनज है। इिेरिॉन पररवहन को बढजने और सजमिी में फोनोन पररवहन को दबजने के मजध्यम से उच्च र्ेडिी हजनसि नकयज र्जतज है।

वतामजन थीनसस कज िक्ष्य एन-प्रकजर और पी-प्रकजर िीई सजमिी नवकनसत करनज है र्ो एनएनओसीओमोसजइि

दृनष्टकोण को ननयोनर्त करके बढजए गए र्ेडिी के सजथ नवकनसत करतज है यजनी िीई होस्ट मैनििक्स में

नैनोइल्पिओशन शजनमि करतज है और इन दो िीई सजमिी कज उपयोग करके थमो-एमएम को मजपतज है।

इस कजम के पीछे प्रेरणज इस तथ्य पर आिजररत है नक मेर्बजन मैनििक्स में चर आकजरोों के उपयुक्त नैनोइनल्पियोंस को शजनमि करके, कोई र्ी सर्ी पदजनुिनमक िोंबजई पैमजने पर फोनन स्कैिररोंग प्रजप्त कर सकतज है।

यह थमाि चजिकतज को कजफी कम करतज है और जेडिी को बढजने पर इसकज असर पडतज है। एन-िजइप नसस्टम के निए, Bi2Te3 मेर्बजन मैनििक्स के रूप में कजया करतज है और बीईिी को नैनोइनक्यूशन के रूप में नियज र्जतज है

र्बनक पी-प्रकजर प्रणजिी के निए, एोंिीमोनी िेल्यरजइड (एसबी 2 िीई 3) और पीबीिी को िमशः मेर्बजन मैनििक्स और नैनोइनक्यूशन के रूप में नियज र्जतज है।

थीनसस सजत अध्यजयोों में आयोनर्त नकयज गयज है: अध्यजय -1 (पररचय) इस कजम के क्षेत्र में वतामजन शोि ल्पथथनत कज एक नसोंहजविोकन देतज है। अध्यजय -2 (प्रजयोनगक तकनीक) में वतामजन अध्ययन में इस्तेमजि सोंश्लेषण मजगा,

sintering तकनीक और सोंपनि मजप नववरण कज नववरण शजनमि है। अध्यजय -3 दो र्जगोों में बजोंिज गयज है। पहिज

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र्जग नवनर्न्न ऑक्सीडेंि में ईोंिन (ओ / एफ) अनुपजत में ईोंिन के रूप में सजइनििक एनसड और यूररयज कज उपयोग करके मजइिोवेव सहजयक फ्लैश दहन (एमएएफसी) मजगा द्वजरज नैनोस्टिक्चर बीआईिी के सोंश्लेषण कज वणान करतज है। अोंनतम उत्पजद पर ईोंिन की र्ूनमकज के सजथ-सजथ ओ / एफ अनुपजत के प्रर्जव की खोर् की गई है।

यह पजयज गयज है नक ईोंिन के रूप में सजइनििक एनसड के पररणजमस्वरूप सजमिी की बेहतर चरण शुद्धतज में

पररणजम होतज है। इस अध्यजय कज दूसरज र्जग सॉल्वैथमाि मजगा द्वजरज बजय 2 िी 3 के सोंश्लेषण के बजरे में चचजा

करतज है र्ो सॉल्वैंि्स के रूप में पजनी और मेथनॉि कज उपयोग करके और घिती एर्ेंि (NaBH4) की अिग- अिग मजत्रज के रूप में उपयोग करतज है। पजउडर शुद्ध हेक्सजगोनि Bi2Te3 में NaBH4 के अनुकूनित 2.3 िजम के सजथ मेथनॉि कज उपयोग करके तैयजर पजउडर। अध्यजय -4 में तीन खोंड हैं। पहिे दो खोंड िमशः

नैनोस्टिक्चर बीिीई और बीआई 2 िी 3 के तजपमजन ननर्ार िी गुणोों के बजरे में चचजा करते हैं। नछद्ोों में पजउडर की

कॉम्पैक्शन एसपीएस तकनीक कज उपयोग करके नकयज र्जतज है नर्समें नसन्टररोंग समय और तजपमजन कज अनुकूिन नवस्तजर से वनणात नकयज र्जतज है। तीसरज खोंड Bi2Te3 / BiTe (2,4,6,8 और 10mol%) समि के TE गुणोों कज वणान करतज है। Z2Te3 के निए 470 के सजथ ZT = 0.33 की तुिनज में Bi2Te3 / BiTe (8mol%) समि नमूनज के निए ~ 1.1 पर 470 के एक उन्नत र्ेडिी मजन प्रजप्त नकयज र्जतज है। इसी तरह, अध्यजय -5 एमएएफसी मजगा और एसबी 2 िी 3 द्वजरज दो अिग-अिग वगों में सोिवथमाि मजगा द्वजरज नैनोस्टिक्चर पीबीिी के

सोंश्लेषण कज वणान करतज है। वजोंनछत चरण प्रजप्त करने के निए सजइनििक एनसड एक उपयुक्त ईोंिन सजनबत होतज है। एक हेक्सजगोनि Sb2Te3 चरण मेथनॉि कज उपयोग एक अनुकूनित NaBH4 मजत्रज के सजथ नविजयक के

रूप में नकयज र्जतज है। अध्यजय -6 पीबीिीई, एसबी 2 िीई 3, और एसबी 2 िीई 3 / पीबीिीई समि तजपमजन तजपमजन िीई गुणोों के बजरे में चचजा करतज है र्ो प्रत्येक को तीन अिग-अिग नहस्ोों में नवर्जनर्त नकयज र्जतज है।

एक अनुकूनित समि नमूनज एसबी 2 िीई 3 / पीबीिीई (5 एमओएि%) के पररणजमस्वरूप 586 के 1.52 के

उच्च र्ेडिी के पररणजमस्वरूप एसबी 2 िी 3 के निए र्ेडिी से 52% अनिक है। अध्यजय -7 TE मॉड्यूि के

ननमजाण और िीई मॉड्यूि में उत्पन्न थमो-एएमएफ के मजप के बजरे में नववरण देतज है। एक मॉड्यूि और इसकी

असेंबिी बनजने में नडर्जइन पहिुओों कज वणान नकयज गयज है। मॉड्यूि र्र में वोल्टेर् और नवनर्न्न तजपमजन अोंतर के निए इसी िजरज को नदयज र्जतज है। अध्यजय -8 इस क्षेत्र में वतामजन कजया और र्नवष्य के कजम के दजयरे से

ननकजिे गए महत्वपूणा ननष्कषों पर प्रकजश डजिज गयज है।

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LIST OF FIGURES

Chapter-1

Fig. No. Figure Caption Page No.

Fig. 1.1 World’s total energy consumption and losses from 1990 to 2016

2

Fig. 1.2 Schematic diagram to illustrate Seebeck effect for a metal bar, subjected to ∆T, showing (a) electrons having more velocities at hot end than at cold end; and (b) ∆V developed between two ends

4

Fig. 1.3 Seebeck effect illustration for two conductors 4 Fig. 1.4 TE parameters: S, σ, S²σ, κe and κl plotted as a function of n 13 Fig. 1.5 The n- and p-type material with ohmic contacts used to make

a TE module

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Fig. 1.6 A schematic diagram for TE module 16

Fig. 1.7 Schematic for commercial TE device 16

Fig. 1.8 Efficiency (%) vs. ZT keeping fixed TC=323 K and different values of TH

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Fig. 1.9 ZT vs. temperature in bulk TE materials (a) n-type, (b) p-type 22 Fig. 1.10Schematic showing local increase in DOS around EF for Tl:PbTe as

compared to bulk PbTe

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Fig. 1.11 Schematic illustration of Skutterudite (CoSb3) crystal 26 Fig. 1.12 Density of states (DOS) for a) a bulk 3D crystalline

semiconductor, b) a 2D quantum well, c) a 1D nanowire, and d) a 0D quantum dot

28

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Fig. 1.13 Schematic diagram showing scattering mechanisms in a nanocomposite

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Fig. 1.14 Schematic for the crystal structure of Bi2Te3 35 Fig. 1.15 Schematic for the crystal structure of Sb2Te3 37

Chapter-2

Fig. 2.1 Flowchart for steps involved and microwave oven used for MAFC route

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Fig. 2.2 Hydro/solvothermal setup used for synthesis 49 Fig. 2.3 Flow diagram for the hydro/solvothermal synthesis of

nanomaterials

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Fig. 2.4 The hot press unit 52

Fig. 2.5 Schematic diagram showing basic configuration of SPS 54 Fig. 2.6 Spark Plasma Sintering (SPS) Unit used for sintering 55

Fig. 2.7 Bragg diffraction from crystal plane 56

Fig. 2.8 Energy level diagram showing Raman Effect 58 Fig. 2.9 SEM equipment and its schematic diagram 59 Fig. 2.10 TEM equipment used and its schematic diagram 61 Fig. 2.11 DSC equipment used to measure specific heat values 63 Fig. 2.12 Archimedes kit used to determine the density 64

Fig. 2.13 S and ρ measurements system (ZEM-3) 65

Fig. 2.14 Laser Flash equipment used and its schematic diagram 67

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Chapter-3

Fig. 3.1 Flow chart depicting steps involved for synthesis of BiTe by MAFC

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Fig. 3.2 Structural formulae of fuels: urea and citric acid 71 Fig. 3.3 XRD plots of as-synthesized sample using urea and citric acid

as fuels keeping e=1

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Fig. 3.4 XRD plots of as-synthesized samples (citric acid as a fuel) with different e values

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Fig. 3.5 Raman spectrum of as-synthesized powder from citric acid for

e =1.05

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Fig. 3.6 SEM images depicting (a) plate like and (b) wire like morphology for as-synthesized powder prepared from citric acid for ϕe =1.05

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Fig. 3.7 TEM image of the as-synthesized sample showing different structures in (a)- (b); (c) HRTEM image from nanoplate’s region. The insets: (a) TEM image of wire like structure and (c) SAED pattern

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Fig. 3.8 Flowchart for the solvothermal synthesis of Bi2Te3 79 Fig. 3.9 XRD patterns of powders synthesized by varying NaBH4 with

water as solvent

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Fig. 3.10 XRD patterns of powders synthesized by varying NaBH4 with methanol as solvent

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Fig. 3.11 Raman spectrum of powder synthesized with optimized 83

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NaBH4 =2.3 g and methanol as solvent

Fig. 3.12 SEM image of powder synthesized with optimized NaBH4=2.3g using methanol as solvent

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Fig. 3.13 Microstructures recorded by (a) TEM; (b) HRTEM from plate region, of powder synthesized with optimized NaBH4 =2.3g and methanol as solvent. The inset (b) HRTEM image from wire like structure

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Chapter-4

Fig. 4.1 XRD patterns for powders: as-synthesized and annealed at 523 K and 623 K

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Fig. 4.2 SEM images of sintered pellets from (a) as-synthesized (S1) and; (b) annealed (S2) powders

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Fig. 4.3(a)-(e) TE properties for BiTe sintered pellets from as-synthesized (S1) and annealed (S2) powders

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Fig. 4.4 Flowchart for SPS procedure to consolidate Bi2Te3 powder into pellet

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Fig. 4.5 TE properties of Bi2Te3 sintered pellet 94 Fig. 4.6 XRD pattern of Bi2Te3/BiTe (different mol%) sintered pellets 97 Fig. 4.7(a)-(e) TE properties for composite samples. The inset in every figure

shows TE parameters of pure BiTe

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Fig. 4.8 TE properties at 470 K as a function of BiTe in different mol% into Bi2Te3

103

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Fig. 4.9 Reitveld refinement of XRD patterns for (a) Bi2Te3 (b) Bi2Te3/BiTe(8mol%) sintered pellets

104

Fig. 4.10 Raman Spectra of Bi2Te3, Bi2Te3+BiTe(8mol%) and BiTe sintered pellet

105

Fig. 4.11 Microstructural recordings from (a) TEM of Bi2Te3 sintered pellet. The inset shows SAED pattern and HRTEM from same sample; (b) TEM; (c) HRTEM; (d)-(e) SAED patterns from marked areas of Bi2Te3/BiTe(8mol%) composite sintered sample

106

Chapter-5

Fig. 5.1 Flowchart for synthesis process of PbTe powder 110 Fig. 5.2 XRD plots of samples prepared using citric acid as fuel and

different ϕe values

111

Fig. 5.3 TEM recordings of samples prepared using citric acid as fuel and ϕe values as: (a) Stoichiometric (ϕe=1.0); (b) Fuel excess (ϕe =0.95); and (c) Fuel deficient (ϕe=1.05) ratios. The inset shows SAED pattern from marked region

113

Fig. 5.4 Steps followed for synthesis of Sb2Te3 powder 115 Fig.5.5 XRD plots of as-synthesized powder prepared using different

quantities of NaBH4 and water as solvent

116

Fig. 5.6 XRD plots of as-synthesized powder prepared using different quantities of NaBH4 and methanol as solvent

117

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Fig. 5.7 XRD plots of as-synthesized powder with optimized quantity of NaBH4 using water and methanol as solvents

118

Fig.5.8 SEM images of powder from methanol as solvent with optimized NaBH4=1.1 g at (a) low; (b) high magnification

119

Fig. 5.9 Microstructure details from (a) TEM; (b) HRTEM; and (c) SAED pattern of powder synthesized using methanol and optimized NaBH4=1.1 g

120

Chapter-6

Fig. 6.1 Temperature dependent TE properties of PbTe sintered pellet 125 Fig. 6.2 SPS conditions for consolidation of Sb2Te3 powder into pellet 126 Fig. 6.3 Temperature dependent TE properties of Sb2Te3 sintered

pellet

127

Fig. 6.4 XRD pattern of Sb2Te3 /PbTe composite pellets with different mol% of PbTe into Sb2Te3 powder

130

Fig. 6.5(a)-(f) TE properties of Sb2Te3 /PbTe composite samples. The inset shows TE parameters of pure PbTe

137

Fig. 6.6 Reitveld refinement of XRD patterns for (a) Sb2Te3 (b) Sb2Te3/PbTe(5mol%) sintered pellets along with lattice and GoF parameters

139

Fig.6.7 (a) HRTEM image of pure Sb2Te3 SPS’ed pellet. The inset shows SAED pattern from the same sample; (b) HRTEM image of Sb2Te3/PbTe(5mol%) composite SPS’ed pellet;

140

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SAED patterns from areas in (b) having lattice fringes corresponding to (c) Sb2Te3 (d) PbTe phase

Chapter-7

Fig. 7.1 Schematic for a TE module 143

Fig. 7.2 TE legs with contact layer after hot press 146 Fig. 7.3 SEM image depicting Ni coating into TE leg 147 Fig. 7.4 Schematic illustration for assembling TE module 147

Fig. 7.5 The whole set up for TE module 148

Fig. 7.6 The voltage (V) and current (I) values for different temperature gradients for modules consisting of pure and composite legs

149

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LIST OF TABLES

Table No. Table Caption Page No.

Table 1.1 List of ZT values for Bi2Te3 and Sb2Te3 using different approaches

38

Table 1.2 (a) Comparison of various parameters for Bi2Te3 and BiTe (b) Comparison of various parameters for Sb2Te3 and PbTe

39

Table 3.1 Different O/F ratio combinations for citric acid and urea as fuel 73 Table 4 1 Densities of BiTe sintered pellets at different sintering

temperature and holding time

88

Table 4.2 Density of all sintered composite samples 97 Table 4.3 Carrier concentration (n) and mobility (μ) values for all

composite samples

102

Table 4.4 Wyckoff positions of refined data along with lattice and GoF parameters

104

Table 5.1 Different ϕe values for citric acid and urea as fuels 110

Table 6.1 Density of PbTe sintered pellets at different sintering temperature and holding time

123

Table 6.2 Measured density of all sintered composite samples 129

Table 6.3 Carrier concentration (n) and mobility (μ) values for Sb2Te3

/PbTe composite samples

138

Table 7.1 CTE values for different materials 145

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LIST OF SYMBOLS

ρ : Electrical resistivity

σ : Electrical conductivity

S : Seebeck coefficient

S²σ : Power factor

μ : Electrical mobility

n : Carrier concentration

m* : Effective mass

kB : Boltzmann constant

EF : Fermi energy

d : Thermal diffusivity

κ : Thermal conductivity

κe : Electronic component of thermal conductivity κl : Lattice component of thermal conductivity

Cp : Specific heat

D : Density

l : Mean free path

L : Lorenz number

T : Temperature

ZT : Figure of merit

η : Efficiency of power conversion

ηC : Carnot efficiency

TH : Hot side temperature

TC : Cold side temperature

A : Area of TE leg

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L : Length of TE leg

K : Thermal conductance

R : Electrical resistance

e : Charge of electron

ε : Dielectric constant

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LIST OF ABBREVIATIONS

TE : Thermoelectric

TEG : Thermoelectric generator

MFP : Mean free path

MAFC : Microwave assisted flash combustion P.F. : Power factor

SPS : Spark plasma sintering

SEM : Scanning electron microscopy TEM : Transmission electron microscopy

HRTEM : High resolution transmission electron microscopy SAED : Selected area electron diffraction

XRD : X-ray diffraction

SYNTHESIS, CHARACTERIZATION AND APPLICATIONS OF NANOSTRUCTURED

SYNTHESIS, CHARACTERIZATION AND APPLICATIONS OF NANOSTRUCTURED

CHALCOGENIDE THERMOELECTRIC MATERIALS

HARJEET KAUR

DEPARTMENT OF PHYSICS

INDIAN INSTITUTE OF TECHNOLOGY DELHI

JULY 2018

SYNTHESIS, CHARACTERIZATION AND APPLICATIONS OF NANOSTRUCTURED

CHALCOGENIDE THERMOELECTRIC MATERIALS

HARJEET KAUR DEPARMTENT OF PHYSICS

Submitted

in fulfillment of the requirements for the degree of Doctor of philosophy

INDIAN INSTITUTE OF TECHNOLOGY DELHI

JULY 2018

DEDICATED TO MUMMY, DADDY

and CHANNI

CERTIFICATE

ACKNOWLEDGEMENTS

ABSTRACT

सार

CONTENTS

LIST OF FIGURES

Chapter-1

Chapter-2

Chapter-3

Chapter-4

Chapter-5

Chapter-6

Chapter-7

LIST OF TABLES

LIST OF SYMBOLS

LIST OF ABBREVIATIONS