Exploration and Optimization of Tellurium‐Based Thermoelectrics

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W∙cm ‐1 K ‐2 , where decreasing values appear to taper. In this case, the largest power factor, found for 0.03 In, was 6.0 W∙cm ‐1 K ‐2 at 300 K. With respect to thermal conductivity of the system (Figure 9.4), one can see that improvements are observed in almost every case though Ga trials, specifically 0.05 Ga, show the most promising decrease in . With a room temperature value of 0.31 W∙m ‐1 K ‐1 , this trial displays a thermal conductivity reduction of 40 % from the ternary compound. Like the other layered materials, the doped trials display a continuously increasing with a slope change occurring around 470 K where the bipolar conductivity, that is mobile positive and negative charge carriers, begins to control the properties. To reiterate, this behaviour can also be observed as the change in direction for the electrical conductivities above. A significant suppression of the bipolar heat conduction cannot be observed with In or Ga unfortunately, though the overall has been lowered in addition to the slopes’ reduced incline at higher temperatures. Figure 9.4 Thermal conductivity (left) and ZT (right) for doped [Tr] xSn 1‐xBi 4Te 7, Tr = Ga, In. The dimensionless figures of merit for the doped SnBi4Te7 compound show a decrease in magnitude from 0.26 (SnBi4Te7) to below 0.16 (0.10 Ga). The values were calculated by fitting the power factors with a sixth‐order polynomial and combining it with the data as per the ZT formula. Ga trials indicate that more gallium further decreases the figure of merit and In trials, while less indicative systematic reduction, still prove that added dopants have a hampering effect on ZT. It is noteworthy to observe that due to its significantly lower thermal conductivity combined with a relatively trivial decrease in power factor, small quantities of Ga in this structure actually lead to an overall enhancement in ZT beyond 470 K – a more significant temperature range for power‐generation thermoelectrics. Ga0.05Sn0.95Bi4Te7 shows a fourfold increase from 0.009 at 670 K to 0.036. 104
9.2.3.2. Substitution Trials: [Tr]xSnBi4‐xTe7, Tr = Ga, In, Tl Procedurally identical, measurements were also performed on the samples incorporating x triel element on the Bi site. Assuming the triel elements are more stable in their Tr 3+ form, replacement of Bi 3+ atoms would leave the compound ideally in a charge‐balanced state. Figure 9.5 Substitution results (ZEM) for [Tr] xSnBi 4‐xTe 7. Substitution experiments paint a picture of charge carriers randomly distributed across Sn, Bi, and maybe Te sites displaying the same bipolar conductivity discussed in Chapter 7. Searching for trends in the electrical conductivity, shown above in the top left and right of Figure 9.5, shows little evidence for logical changes – though the majority of samples indicate that additional triel elements lower the electrical conductivity beyond that of the ternary (~300 ‐1 cm ‐1 at room temperature). Should the DOS calculations serve correct, one may be observing near‐similar n with oppression of the mobility factor, ; if additional Tr + is found, this may simultaneously cause a change in n. Furthermore, n possibly changes based on a Te‐loss hypothesis (i.e. …Te7‐y), should Te vaporize out of the structure. The legends in this case are displayed in order of decreasing σ. While Ga shows no order whatsoever, the heavier elements again suggest that additional substituents further lower σ. 0.03 Tl, breaking the hypothesized trend, likely had traces of impurity compounds (i.e. TlBiTe2) which were not apparent in the p‐XRD data. 105
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
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 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: 9.2.3.1. Doping Trials: [Tr]xSn1‐
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
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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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