Exploration and Optimization of Tellurium‐Based Thermoelectrics

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Chapter 8. Tuning of the Physical Properties for SnBi2Te4 Zhukova et al. [193] discovered the crystal structure of SnBi2Te4 in 1971, solved in 3 having a unit cell of 4.41×4.41×41.51 Å 3 . This structure is a layered variant of the Bi2Te3‐type crystal structure, shown previously. Placing the Sn atoms on the 0 0 0 Wyckoff position (3a) yields the diagram below (Figure 8.1). Bi sits on the 0 0 0.4278 (6c) site and Te occupies two sites: 0 0 0.136 and 0 0 0.29, both 6c. As can be seen, the unit cell is comprised of covalently bonded sections separated by van der Waals gaps (dTe–Te = 3.6 Å) with Sn atoms in the centre of each block. Due to each block occurring as alternating cis then trans, each unit cell is comprised of three blocks. While several models propose a mixed occupancy between Sn and Bi, data has been solved in both for example PbBi2Te4 where indistinguishable Pb, is usually fixed on the 3a position. Shown in Figure 8.1 below is a representation of a triple unit cell along a: Figure 8.1 Triple unit cell of SnBi2Te4 along a. Both crystal structure and physical properties of SnBi2Te4 have been studied to certain extents by a variety of research groups. Most recently, detailed melting studies predict SnBi2Te4 melts eutectically at 840 K at composition of approximately 45 % SnTe : 55 % Bi2Te3. [181] Additionally, studies [178, 180, 193] have explored the phase diagram of the system with attempts at indexing and unit cell size; thermoelectric properties of the stoichiometric ternary were discussed in Chapter 7; [179, 183, 188, 194] and electron microscope studies on alloying, solubility, and precipitates within the SnTe/PbTe‐Bi2Te3 system [192, 206] have been undertaken. 80
From the understanding of this compound gained in the literature as well as the known benefits of heavy‐element layered structures, one is left with numerous queries including structure and improvement of its properties. In 2010, similar studies were also investigated in the Kleinke group [189] on the sister compound PbBi2Te4 with information regarding the phase range, XRD studies, and thermoelectric properties. Such studies as understanding the compound’s phase range and attempts at doping to optimize charge carrier concentrations will greatly aid in a thorough understanding of SnBi2Te4. Through the gathering of crystallographic and DFT information on this system, one aims to understand the effects of tuning via additional elements on the cation sites. Doping and substitution experiments will ideally reflect some of these behaviours through cumulative physical property studies. 8.1. Synthesis and Structural Analysis Compounds were synthesized according to 7.2 and were weighed to a minimum of ±0.0005 g. While it was necessary to optimize the heating conditions for this system to avoid primarily Bi2Te3 or SnBi4Te7, reactions were heated slowly to 923 K where they were left for 12 or 24 hours. The samples were then quickly cooled to 843 K and left for 1 to 2 weeks before cooling to room temperature. Thus, a typical heating scheme for this system can be depicted as: Mixed phases were returned to 843 K in sealed tubes for approximately 2 weeks. Powdered samples of pure phase were typically cold‐pressed into 10×2×2 mm 3 bars, annealed for 12 hours in sealed tubes at 723 K and measured by the ZEM instrument. Attempts throughout this work to obtain a single crystal of SnBi2Te4 were generally in vain despite numerous strategic attempts including cooling rates of 1 °/hr near its melting point, various salt fluxes, or transport reactions involving TeCl4/I2; the fine plate‐like crystals turned out to be powder when cut and twinning made any obtainable data near‐impossible to properly analyze. One exception 81
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Exploration and Optimization of Tel
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Abstract Thermoelectric materials a
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Acknowledgements To begin, I would
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Table of Contents List of Figures..
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9.3.1. Heating, XRD, and Structural
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List of Figures Figure 1.1 (a) Pelt
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Figure 9.2 Selected DOS calculation
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List of Equations Equation 1.1 Dime
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Table B.1 Atomic positions, isotrop
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Section I: Fundamentals Background
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Opposing the Peltier Effect is the
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Hence it follows that a good thermo
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a b ∗ Equation 1.4 (a) Ele
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Historically, thermoelectric materi
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1.3.2. Advantages and Disadvantages
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common stoichiometry studied with c
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Figure 1.8 (a) Ge Quantum Dots [74]
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Chapter 2. Background of the Solid
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2.2. Defects, Non‐Stoichiometry a
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electric field draws electrons towa
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∑ known as a Bloch function, w
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orbital Hamilton population [96] i
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2.5.1. Doping and Substitution: Tun
Page 47 and 48: The major problem with exclusively
Page 49 and 50: Chapter 3. Synthesis Techniques All
Page 51 and 52: Since reactions are driven by both
Page 53 and 54: Table 3.1 Commonly used fluxes. [12
Page 55 and 56: The procedure is carried‐out on a
Page 57 and 58: Chapter 4. X‐Ray Diffraction Anal
Page 59 and 60: The closely related electron densit
Page 61 and 62: The majority of samples emerge from
Page 63 and 64: int ∑ mean ∑ | | Equat
Page 65 and 66: 4.3.1. Rietveld Method In 1967, Hug
Page 67 and 68: Chapter 5. Physical Property Measur
Page 69 and 70: The electrical conductivity, σ, is
Page 71 and 72: a p b 0.1388 53 c Equati
Page 73 and 74: measurement implies that there was
Page 75 and 76: Figure 5.6 Home‐made electrical c
Page 77 and 78: The SEM is constructed from an elec
Page 79 and 80: The EDX measurement system utilized
Page 81 and 82: 2 4 ext Equation 6.2 Ma
Page 83 and 84: Section III - Layered Bi2Te3 Compou
Page 85 and 86: aforementioned studies address the
Page 87 and 88: Pb‐based compound (~‐60 V∙K
Page 89 and 90: also differ in 2 there, while the r
Page 91 and 92: Table 7.2 LeBail data for phase‐p
Page 93 and 94: The physical properties, as mention
Page 95 and 96: Upon examination of the power facto
Page 97: Sn2Bi2Te5 was eventually found pure
Page 101 and 102: Table 8.1 Rietveld refinements on S
Page 103 and 104: Although there is evidence here of
Page 105 and 106: Figure 8.3 DOS calculation comparis
Page 107 and 108: In order to better determine if the
Page 109 and 110: 8.3.3. Physical Property Measuremen
Page 111 and 112: Figure 8.7 Thermal conductivity (le
Page 113 and 114: the thermal conductivity slope that
Page 115 and 116: 8.4. Conclusions and Future Work A
Page 117 and 118: in Chapter 3 (except Arc Melting) w
Page 119 and 120: Table 9.2 LeBail refinements on var
Page 121 and 122: 9.2.3.1. Doping Trials: [Tr]xSn1‐
Page 123 and 124: 9.2.3.2. Substitution Trials: [Tr]x
Page 125 and 126: One can observe a variety of grain
Page 127 and 128: 9.2.5. Conclusions Drawn from SnBi4
Page 129 and 130: SnBi4Te7, but suggest similar albei
Page 131 and 132: 9.4. Project Summary and Future Wor
Page 133 and 134: Chapter 10. Introduction to Tl5Te3
Page 135 and 136: Figure 10.2 DOS calculations for Tl
Page 137 and 138: Thallium‐based chalcogenides clea
Page 139 and 140: : 36.3 (expected: 48.8 : 13.8 : 37.
Page 141 and 142: sintering of these pellets. A cold
Page 143 and 144: Figure 11.2 Thermoelectric properti
Page 145 and 146: at 319 K, increasing smoothly to 0.
Page 147 and 148: purity upon the first heating with
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Tl/Sn/Bi present in the 4c Wyckoff
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V∙K ‐1 at 592 K. As the bismuth
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Electrical conductivity values are
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Chapter 13. Modifications of Tl9SbT
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Table 13.1 Rietveld refinements for
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make an observable trend despite th
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The electrical conductivity values
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Section V - Ba Late‐Transition‐
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Obviously, compounds possessing the
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Table 14.1 Known Ba‐Cg‐Q compou
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All samples were analyzed by means
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crystal structure study of the Se
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As emphasized in Figure 15.2 (b), t
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The Cu-Cu COHP curve, cumulated ove
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Chapter 16. Ba3Cu17‐x(S,Te)11 and
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the selenide‐telluride before, th
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surrounded by three Q1 atoms. Besid
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Figure 16.3 The three‐dimensional
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Figure 16.5 Seebeck coefficient (le
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Chapter 17. Ba2Cu7‐xTe6 17.1. Int
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Table 17.1 Crystallographic Data fo
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Table 17.2 Atomic coordinates, Ueq
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17.3.2. Electronic Structure Calcul
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Figure 17.6 Crystal orbital Hamilto
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Section VI - Supplementary Informat
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[33] Rowe, D. M.; Kuznetsov, V. L.;
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[90] Albright, T. A.; Burdett, J. K
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[156] Cottenier, S., Density Functi
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[219] Toure, A. A.; Kra, G.; Eholie
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[294] Kanno, R.; Ohno, K.; Kawamoto
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Table A.3 Atomic positions and Ueq
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Table B.2 Selected interatomic dist
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Table B.5 Atomic coordinates and Ue
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Table B.7 Selected interatomic dist
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Exploration and Optimization of Tellurium‐Based Thermoelectrics

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