Exploration and Optimization of Tellurium‐Based Thermoelectrics

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The resultant voltages created from the power function are quite small. Combine this with the fact that substantial experimental error can be encountered from calculation of a single point or from thermo‐ voltage contributions (∆ therm) by the wires/contacts, the measurement is necessarily taken at two different power settings with two different ∆ values (Equation 5.3) which eliminates ∆ therm via subtraction of both powers 1/2 yielding a more complex Equation 5.3 (b): Δsample Δsample sample ref Δref Δref Equation 5.4 MMR Seebeck coefficient. With the thermal contributions effectively eliminated and the measured with respect to a known constantan reference, one achieves an accurate measurement at regular temperature intervals. This is conditional on the pellet density and proper 1/2 inputs of course, but produces reproducible values comparable to that of the ZEM instrument. 5.5. Electrical Conductivity Measurements (low‐temperature) Those samples (again) measured before July 2009 or requiring data below ambient temperature or with less than 200 mg of powder can also be measured for σ utilizing a homemade measurement system displayed in Figure 5.6 (a) from ambient temperature to approximately 10 K. Pellets required for this setup are the same as those used in 5.4 and are similarly connected via silver paint to the voltage and current leads which can be seen in Figure 5.6 (b). Like the ZEM, this instrument uses the four‐point method for measurement via Ohm’s law: Equation 5.1 (a). The pellet is placed under vacuum (~10 –3 mbar) and slowly cooled via a closed‐cycle helium refrigerator using the Joule‐Thomson effect in the possible temperature range of 320 – 5 K. Any and all necessary readings, power/temperature changes and voltage adjustments are made through manual controllers attached to the instrument. The experimental setup is as follows: 56
Figure 5.6 Home‐made electrical conductivity: (a) σ experimental setup (b) σ pellet mount The power source used on the instrument is a Keithly 2400 with an input impedance of 1 GΩ to eliminate the wire contact terms mentioned in 5.1, which ensures the proper measuring of semiconducting materials which rarely reach even 1 kΩ. A Lakeshore 330 temperature controller is used for assessing the temperature throughout the measurement. During the measurements, samples are measured with both negative and positive voltage to ensure the instrument is accurately measuring correct values; another undesirable voltage is the thermal voltage, therm, (or additionally a thermoelectric effect) caused by dissimilar circuit junctions such as copper wires on silver paint, which can be written as meas therm. By reversing the current flow however, we can obtain meas2 therm such that subtraction of the two equations yields which eliminates the thermal effects that lead to measurement error. Likewise to avoid Joule heating (), one must choose as low a current as possible to still observe four significant voltage figures – in this work, around 10 mA; activation and deactivation of the measurement instrument must also be quick (say, ≤ 1 second) or heating effects on voltage will lead to an erroneous measurement. Lastly measurements are typically executed in both heating directions, that is, cycling cooling and successive warming to ensure experimental consistency through averaging both cycles. Following measurement of the pellet with a Vernier calliper, the electrical conductivity can be calculated. This strategy (Equation 5.1) is the same as that of the ZEM setup. 57
Page 1 and 2:
Exploration and Optimization of Tel
Page 3 and 4:
Abstract Thermoelectric materials a
Page 5 and 6:
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
Page 23 and 24: Hence it follows that a good thermo
Page 25 and 26: a b ∗ Equation 1.4 (a) Ele
Page 27 and 28: Historically, thermoelectric materi
Page 29 and 30: 1.3.2. Advantages and Disadvantages
Page 31 and 32: 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: measurement implies that there was
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 and 124: 9.2.3.2. Substitution Trials: [Tr]x
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One can observe a variety of grain
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9.2.5. Conclusions Drawn from SnBi4
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SnBi4Te7, but suggest similar albei
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9.4. Project Summary and Future Wor
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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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