Exploration and Optimization of Tellurium‐Based Thermoelectrics

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Most Seebeck coefficients measured are essentially the same with values around 125 V∙K ‐1 , but it should be noted that all thallium‐based trials reduce S, likely due to its larger relativistic effects and/or preferential Tl + as opposed to Ga 3+ /In 3+ . The observation thusly results in power factors below 4.34 W∙cm ‐1 K ‐2 (SnBi4Te7) for the majority with two outlier compounds displaying 6.2 W∙cm ‐1 K ‐2 . Though not performed here, it can be presumed and ZT values on the same order of magnitude as 9.2.3.1. 9.2.4. Towards Nanoscopic Improvement of ZT The first study towards nanoscopic improvements was performed on a quaternary ZEM pellet via an extended SEM/EDX scanning period to verify homogeneity and therefore determine if any sections of the pellet’s surface showed non‐uniform elemental distributions (i.e. matrix encapsulation). Such a study can determine if Ga is concentrating on various portions of the surface, such as the impurity clustering [212] found to significantly impact the figure of merit in LAST materials and the like. The particular study was performed at 1000x magnification and 25,000x magnification. The additional EDX results show that the pellets surface was quite uniform. The data collected at this point was 0.5, 8.9, 32.9, and 57.7 % respectively based on the Ga0.05Sn0.95Bi4Te7 stoichiometry over six spots. Though uniform appearance in the pellet bodes well for homogeneity and property reproducibility, there is little indication of unusual behaviour – be it good or bad. TEM studies were also carried out at McMaster University to further probe compounds’ potential nanodomains and provide an understanding of the grain boundaries or defects that may contribute towards its behaviour during measurements. The results for the ternary SnBi4Te7 are thus displayed below in Figure 9.6 and Figure 9.7: Figure 9.6 TEM grain boundary observations. 106
One can observe a variety of grain boundaries on the left of Figure 9.6 all on the nanoscopic scale. The very top left shows the inclusion of a nanodomain of approximately 20 nm × 20 nm which occurs on the end of the studied material, as the carbon support can be seen in the top right corner of the image. The bottom left image seems to imply thickening of the layers which ought to affect heat transport in the material, should enough of these domains exist. The other pictures show standard grain boundries in the samples. The rightmost capture in Figure 9.6 shows two distinct regions separated by a boundary of approximately 7 nm – grain 1 shows tightly packed ordered rows (6 per 2 nm), whilst grain 2 shows more disordered growth (~4.5 ‘rows’ in 1 nm). Figure 9.7 below also shows a different kind of defect where it is noticed there is a potential missing atom in the network (top right of the figure) while potentially similar to a Schöttky defect, the feature was not readily observed in any sample trials that were attempted. Figure 9.7 TEM interval studies and atomic defects. The final image attempts to explain the nature of the rows in a particular nanodomain. Measurements of distance were taken on the rows, which can be seen as periodically light and dark. Coincidentally, the difference between repeating “light” rows is approximately 24 Å which is the same length as the c axis of the proposed unit cell. Thus, if light rows are ordered Sn atoms, this observation supports the proposed high degree of ordering of Sn in the unit cell. This measurement is in the lower right of the figure corresponding to the capture on the left. Considering ball‐milling experiments on the layered Bi2Te3 compound which not only led to an increase in ZT by 30 %, but a significant reduction in lat and the bipolar effect, [122] preliminary studies have been initiated for other layered compounds. The experiments entail a physical mixture of SnBi4Te7 107
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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
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 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: 9.2.3.2. Substitution Trials: [Tr]x
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
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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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