Exploration and Optimization of Tellurium‐Based Thermoelectrics

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Chapter 7. Layered Ternary Structures in (SnTe)x(Bi2Te3)y 7.1. Introduction to Layered Bismuth Tellurides Since the discovery in the 1950s of bismuth telluride’s thermoelectric potential, researchers have been attempting to understand why Bi2Te3 is such an effective TE material and expanding on ways to further improve it. With all of the studies performed, Bi2Te3 and its composites warrant a book of their own. Now with respect to thermoelectrics, one can consider such contributions as solid solution work like (Sb,Bi)2Te3 or Bi2(Se,Te)3, [13] extensive studies on doping, [166‐168] crystal structures, [169] calculations, [170, 171] phase diagrams and of course, physical properties such as Seebeck coefficient and charge carrier concentration (Hall measurements). [81, 166, 167] Recent efforts towards nanoscopic studies have produced thin films, nanowires, nanocrystals and numerous other property‐related studies. [172‐175] In this chapter, new compounds with similar features are formed based on the parent Bi2Te3 structure by combining it with other (in this case) binaries expecting the child compounds to show similar characteristics and therefore similar, possibly enhanced physical properties. Bi2Te3 is a layered compound of the rhombohedral space group 3. Forming perpendicular to the c‐axis, are alternating layers of covalently bonded Bi and Te atoms in five segments (a block) starting [169, 176] and ending with Te: Te–Bi–Te–Bi–Te. The Bi–Te bond lengths in this layer are 3.07 and 3.26 Å. The interactions between blocks are predominantly van der Waals forces. A Te–Te van der Waals interaction of 3.65 Å connects the blocks in a stacked structure. This can be seen on the left side of Figure 7.1 displayed below. The search for related layered compounds led to investigations into the (TtQ)x(Pn2Q3)y (Tt = Ge, Sn, Pb; Pn = Sb, Bi; Q = Se, [177] Te) materials, where x and y are integers. The resulting studies uncover a series of increasingly longer unit cells with similar layering to that of bismuth telluride. Further discussion regarding layering in the crystal structures is discussed below. It has since been postulated that the lead‐based system’s phase diagram [178, 179] includes (PbTe)‐(Bi2Te3), (PbTe)‐(Bi2Te3)2, (PbTe)‐ (Bi2Te3)3, (PbTe)2‐(Bi2Te3) and arguably (PbTe)‐(Bi2Te3)4 which can be written as proper ternary compounds: PbBi2Te4, PbBi4Te7, PbBi6Te10, Pb2Bi2Te5, and PbBi8Te13 respectively. Due to the extensive layering along the c‐axis and the widespread occurrence of heavy p‐block elements, said structures may have potential for thermoelectric applications. [16] Though crystal structures have been predicted for this system, ordering beyond PbBi2Te4 cannot be verified due to the indistinguishable nature of Pb versus Bi in XRD studies – a feature that could be more apparent in the Sn‐Bi‐Te system. Though many of the 66
aforementioned studies address the Pb‐Bi‐Te system, its structure, and its properties, little is known on the behaviour and thermoelectric properties of the Sn‐Bi‐Te system. The suggested layering of the three best‐known ternaries in that system (versus Bi2Te3) is displayed below: Figure 7.1 Layering motifs of Bi 2Te 3 and compounds in (SnTe) 1(Bi 2Te 3) y; y = 1, 2, 3. It can be seen from the diagram that all compounds in this family are composed of repeating patterns of covalently bonded blocks connected by Te–Te van der Waals interactions. Bi2Te3 and SnBi2Te4 are the simplest of the family, being made up of 5‐member Bi‐Te blocks for Bi2Te3 (5‐5), and 7‐member Sn‐Bi‐Te blocks for SnBi2Te4 (7‐7). The larger compounds are then created by mixing blocks: The SnBi4Te7 motif is comprised of a 5‐member block sandwiched between two 7‐member blocks (7‐5‐7), whilst the SnBi6Te10 motif comprises two 5‐member blocks flanked by two 7‐member blocks (7‐5‐5‐7). It can therefore be seen that as x and y are altered throughout the system, one simply takes different combinations of the Bi2Te3 and SnBi2Te4 motifs to build a plethora of plate‐like ternary compounds – each with very similar a/b dimensions, differentiated by the c length which varies from structure to structure. As mentioned above, in lieu of many studies revolving around Pb‐Bi‐Te, there is still uncertainty about the 7‐member blocks and whether there is mixing between Bi and Tt or if it is fully ordered as shown in Figure 7.1. It is also worth mentioning that when x > 1, the layering motifs are not as above. With the majority of studies focusing on this group of materials, studies included this work were performed on each of SnBi2Te4, SnBi4Te7, and SnBi6Te10. Chapters 8 and 9 detail much of the work featuring individual ternary compounds and the resulting findings. 67
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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
Page 33 and 34: Figure 1.8 (a) Ge Quantum Dots [74]
Page 35 and 36: Chapter 2. Background of the Solid
Page 37 and 38: 2.2. Defects, Non‐Stoichiometry a
Page 39 and 40: electric field draws electrons towa
Page 41 and 42: ∑ known as a Bloch function, w
Page 43 and 44: orbital Hamilton population [96] i
Page 45 and 46: 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: Section III - Layered Bi2Te3 Compou
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 and 98: Sn2Bi2Te5 was eventually found pure
Page 99 and 100: From the understanding of this comp
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
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Figure 10.2 DOS calculations for Tl
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Thallium‐based chalcogenides clea
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: 36.3 (expected: 48.8 : 13.8 : 37.
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sintering of these pellets. A cold
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Figure 11.2 Thermoelectric properti
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at 319 K, increasing smoothly to 0.
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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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