Exploration and Optimization of Tellurium‐Based Thermoelectrics

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15.4. Conclusions A new structure type was uncovered, adopted by Ba3Cu17‐x(Se,Te)11. This structure apparently exists only when both selenium and tellurium are used. Although the Se:Te ratio may vary as determined via different single crystal structure and EDX studies, one of the five chalcogen sites is always mixed occupied by Se and Te, while a second site is solely occupied by Te, with the remaining sites being almost exclusively Se positions. The large differences in the coordination spheres of the chalcogen sites are concluded to be responsible for the different occupancy ratios. According to the electronic structure calculations, Ba3Cu17‐x(Se,Te)11 would be an intrinsic semiconductor when x = 1; however, the experiments indicate that x > 2. This is in agreement with Ba3Cu14.4Se8.5Te2.5 being classified as a heavily doped p‐type semiconductor. The absolute numbers and temperature dependencies of the Seebeck coefficient and electrical conductivity are comparable to those of (also cold‐pressed pellets of) Mo3Sb5.4Te1.6 [59] and Re3Ge0.6As6.4, [301] which reach values between 0.3 and 1.0 at elevated temperatures. [302] Hypothetically one might expect Ba3Cu14.4Se8.5Te2.5 to have higher values because of its likely lower thermal conductivity, as e.g. a comparison with the related Ba3Cu14xTe12 implies. [266] However the experimentally proven mobility of the Cu ions militates against a use of Ba3Cu17‐x(Se,Te)11 in a thermoelectric device, as such a device would deteriorate within days. 158
Chapter 16. Ba3Cu17‐x(S,Te)11 and Ba3Cu17‐x(S,Te)11.5 16.1. Introduction By again following the concept of differential fractional site occupancy (DFSO) [278, 279] with respect to the anionic components [284, 285] – the same approach that also led to the discovery of Nb1.72Ta3.28S2, [280] Nb0.95Ta1.05S, [281] Nb4.92Ta6.08S4 [282] and Nb6.74Ta5.26S4 [283] with fractional occupancies of the cations – we were able to discover another unique structure type in the plentiful Ba‐Cu‐Q system: Ba3Cu17‐x(Se,Te)11 [276] with 1 < x ≤ 2.7 (discussed in Chapter 15). Through replacement of selenium by sulfur, the existence of a new structure type, Ba3Cu17‐xQ11.5, not found in the selenide‐telluride system is revealed to occur when [S] < [Te] and a structure analogous to Ba3Cu17‐x(Se,Te)11 when [Te] < [S]. These two different sulfide‐tellurides are henceforth introduced. 16.2. Experimental Section 16.2.1.1. Synthesis All reactions commenced from the elements (Ba: 99 %, pieces, Aldrich; Cu: 99.5 %, powder ‐150 mesh; Alfa Aesar; S: 99.98 %, powder, Aldrich; Te: 99.999 %, ingot, Aldrich), with sample masses of approximately 600 mg. The elements were stored and handled in a glove box filled with argon. Inside the box, the elements were placed into silica tubes, which were subsequently sealed under vacuum (10 ‐3 mbar). The tubes were then heated in a resistance furnace at 1073 K for six hours. Thereafter, the furnace was slowly cooled down to 873 K and left for a period of 100 hours to allow for homogenization. Lastly, the furnace was switched off prompting fast cooling. The rhombohedral variant was first encountered in a similar reaction with the Ba : Cu : S : Te ratio of 1 : 4 : 2 : 1, and identified as Ba3Cu15.1(1)S7.92(2)Te3.08 via the single crystal structure determination described below. The cubic variant was discovered in a reaction with the Ba : Cu : S : Te ratio of 1 : 4 : 1 : 2, and subsequently identified as Ba3Cu16.0(1)S4.45(4)Te7.05. Since its formula contained Cu deficient sites and mixed S/Te occupancies, several reactions were carried out to analyze the phase range of Ba3Cu17‐xS11‐yTey. This formula results hypothetically with x = 0, when all filled Wyckoff sites are fully occupied. Variations of S/Te were studied with x = 0 or 1.0, while varying y between 2 and 9. Holding a constant y = 4 allowed for an additional study where x = 0, 0.5, 1.0, 1.5, 2. All samples were analyzed by means of X‐ray powder diffraction, utilizing the INEL diffractometer with Cu‐Kα1 radiation. Samples were found pure‐phase for x = 0.5, 1 or 1.5 with 2 ≤ y ≤ 5, producing the rhombohedral variant. For y = 6 and 7, respectively, the cubic variant was the primary phase with purity > 95 %, while larger values of y 159
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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
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The major problem with exclusively
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Chapter 3. Synthesis Techniques All
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Since reactions are driven by both
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Table 3.1 Commonly used fluxes. [12
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The procedure is carried‐out on a
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Chapter 4. X‐Ray Diffraction Anal
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The closely related electron densit
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The majority of samples emerge from
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int ∑ mean ∑ | | Equat
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4.3.1. Rietveld Method In 1967, Hug
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Chapter 5. Physical Property Measur
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The electrical conductivity, σ, is
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a p b 0.1388 53 c Equati
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measurement implies that there was
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Figure 5.6 Home‐made electrical c
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The SEM is constructed from an elec
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The EDX measurement system utilized
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2 4 ext Equation 6.2 Ma
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Section III - Layered Bi2Te3 Compou
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aforementioned studies address the
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Pb‐based compound (~‐60 V∙K
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also differ in 2 there, while the r
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Table 7.2 LeBail data for phase‐p
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The physical properties, as mention
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Upon examination of the power facto
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Sn2Bi2Te5 was eventually found pure
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From the understanding of this comp
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Table 8.1 Rietveld refinements on S
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Although there is evidence here of
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Figure 8.3 DOS calculation comparis
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In order to better determine if the
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8.3.3. Physical Property Measuremen
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Figure 8.7 Thermal conductivity (le
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the thermal conductivity slope that
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8.4. Conclusions and Future Work A
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in Chapter 3 (except Arc Melting) w
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Table 9.2 LeBail refinements on var
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9.2.3.1. Doping Trials: [Tr]xSn1‐
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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
Page 149 and 150: Tl/Sn/Bi present in the 4c Wyckoff
Page 151 and 152: V∙K ‐1 at 592 K. As the bismuth
Page 153 and 154: Electrical conductivity values are
Page 155 and 156: Chapter 13. Modifications of Tl9SbT
Page 157 and 158: Table 13.1 Rietveld refinements for
Page 159 and 160: make an observable trend despite th
Page 161 and 162: The electrical conductivity values
Page 163 and 164: Section V - Ba Late‐Transition‐
Page 165 and 166: Obviously, compounds possessing the
Page 167 and 168: Table 14.1 Known Ba‐Cg‐Q compou
Page 169 and 170: All samples were analyzed by means
Page 171 and 172: crystal structure study of the Se
Page 173 and 174: As emphasized in Figure 15.2 (b), t
Page 175: The Cu-Cu COHP curve, cumulated ove
Page 179 and 180: the selenide‐telluride before, th
Page 181 and 182: surrounded by three Q1 atoms. Besid
Page 183 and 184: Figure 16.3 The three‐dimensional
Page 185 and 186: Figure 16.5 Seebeck coefficient (le
Page 187 and 188: Chapter 17. Ba2Cu7‐xTe6 17.1. Int
Page 189 and 190: Table 17.1 Crystallographic Data fo
Page 191 and 192: Table 17.2 Atomic coordinates, Ueq
Page 193 and 194: 17.3.2. Electronic Structure Calcul
Page 195 and 196: Figure 17.6 Crystal orbital Hamilto
Page 197 and 198: Section VI - Supplementary Informat
Page 199 and 200: [33] Rowe, D. M.; Kuznetsov, V. L.;
Page 201 and 202: [90] Albright, T. A.; Burdett, J. K
Page 203 and 204: [156] Cottenier, S., Density Functi
Page 205 and 206: [219] Toure, A. A.; Kra, G.; Eholie
Page 207 and 208: [294] Kanno, R.; Ohno, K.; Kawamoto
Page 209 and 210: Table A.3 Atomic positions and Ueq
Page 211 and 212: Table B.2 Selected interatomic dist
Page 213 and 214: Table B.5 Atomic coordinates and Ue
Page 215 and 216: Table B.7 Selected interatomic dist
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Exploration and Optimization of Tellurium‐Based Thermoelectrics

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