Exploration and Optimization of Tellurium‐Based Thermoelectrics

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The 300 K S value of +50 V∙K ‐1 obtained from the sintered pellet of Tl8Sn2Te6 is significantly below the 80 V∙K ‐1 obtained from (low temperature measurements on) a polycrystalline ingot [248] and the 126 V∙K ‐1 obtained at 323 K from a hot‐pressed pellet of 91 % density. [237] The Seebeck coefficient of the latter continues to increase until 673 K (the end of the measurement), exceeding +200 V∙K ‐1 . The differences at room temperature may be due to slight variations in the exact Tl:Sn ratio and hence the charge carrier concentration, implying that the sample investigated here had higher charge carrier concentration while nominally the same formula. Interestingly, the polycrystalline ingot with an experimentally determined carrier concentration of 2.22·10 19 cm ‐3 has the same room temperature Seebeck coefficient as Tl7.8Sn2.2Te6. The electrical conductivity, σ, for the Tl10‐xSnxTe6 samples with x < 2.1 decreases with increasing temperature (top right of Figure 3), again evidence of a relatively high carrier concentration. Tl7.8Sn2.2Te6 is again the exception, its value reaching a minimum of 121 ‐1 cm ‐1 at 526 K. Thereafter, its increasing number of intrinsic, thermally activated carriers reverses the trend, causing increasing electrical conductivity. This change was not observed by Kosuga et al. in case of the hot‐pressed pellet of Tl8Sn2Te6. [237] Increasing x causes a decrease in σ, in accord with our expectations and the opposite trend in S. Tl9SnTe6 displays the highest value (1630 ‐1 cm ‐1 ) at room temperature, and Tl7.8Sn2.2Te6 the lowest with 250 ‐1 cm ‐1 , and the samples with x = 2.05 and 2.1 have almost identical values of 540 ‐1 cm ‐1 and 532 ‐1 cm ‐1 , respectively. For comparison, the corresponding values for the Tl8Sn2Te6 ingot and hot‐pressed pellet are 275 ‐1 cm ‐1 and 272 ‐1 cm ‐1 (at 323 K), respectively. The thermal conductivity curves (bottom left of Figure 11.2) roughly follow the same trend as the electrical conductivity, with Tl9SnTe6 displaying the highest and Tl7.8Sn2.2Te6 the lowest values. The values around 317 K, range from 1.57 W∙m ‐1 K ‐1 to 0.49 W∙m ‐1 K ‐1 which over the range of measurement, decreases slightly (for the most part) as the temperature increases. The thermal conductivity of the hot‐ pressed pellet was determined to be basically temperature independent around 0.5 W·m ‐1 K ‐1 , [237] while the one of the polycrystalline ingot was not measured. The thermoelectric (dimensionless) figure‐of‐merit, ZT, was calculated from the above‐ mentioned S, σ, and data, by computing polynomial fits for S 2σ, because was measured at different temperatures. In order to avoid extrapolation, only the data points that were both within the temperature range of S 2σ and were considered. Overall, the ZT trends display a gradual increase with increasing temperature (bottom right of Figure 3). Tl9SnTe6 exhibits the lowest figure‐of‐merit ZT = 0.05 126
at 319 K, increasing smoothly to 0.17 at 572 K. For the most part, ZT gradually increases with increasing x such that Tl7.8Sn2.2Te6 attains 0.12 at 316 K and 0.60 at 617 K. Hot‐pressed Tl8Sn2Te6 exhibited ZT = 0.28 at 323 K, which increased to 0.67 at 623 K and culminated in 0.74 at 673 K. [237] 11.4. Conclusion Various members of the Tl10‐xSnxTe6 series were successfully synthesized, analyzed and their physical properties measured for x = 1.0, 1.7, 1.9, 2.0, 2.05, 2.1 and 2.2. The materials are isostructural with the binary telluride Tl5Te3, and the crystal structure of Tl9SnTe6 was determined with the experimental formula Tl9.05Sn0.95Te6, possessing the same space group (4/) as Tl5Te3 and Tl8Sn2Te6, in contrast to Tl9SbTe6 that adopts the space group 4/. The band structure calculations for Tl9SnTe6 and Tl8Sn2Te6 suggest that Tl9SnTe6 is a heavily doped p‐type semiconductor with a partially empty valence band, while the Fermi level in Tl8Sn2Te6 is located in the band gap. In accord with these calculations, both thermal and electrical conductivity measurements indicate that increasing Sn results in lower values, as determined on sintered polycrystalline samples. The opposite trend is visible with respect to the Seebeck coefficient, as increasing x will result in higher values. The figure‐of‐merit reaches a maximum at the Sn‐rich end of the phase width: Tl7.8Sn2.2Te6 reaches its maximum ZT = 0.60 at 617 K (end of measurement), significantly larger than ZT of Tl8Sn2Te6 prepared and treated analogously, which remains under 50 % of the ZT values of Tl7.8Sn2.2Te6 at elevated temperatures. The fact that hot‐pressed Tl8Sn2Te6 was reported to reach 0.67 at 623 K, [237] implies that hot‐pressing of Tl7.8Sn2.2Te6 should ultimately result in higher values. 127
Page 1 and 2:
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
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
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: Figure 11.2 Thermoelectric properti
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 and 176: The Cu-Cu COHP curve, cumulated ove
Page 177 and 178: Chapter 16. Ba3Cu17‐x(S,Te)11 and
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
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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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