Exploration and Optimization of Tellurium‐Based Thermoelectrics

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Chapter 1. Introduction to Thermoelectric Materials The world is at a crossroads. Fossil fuels are quickly drying up, global warming and environmental pollution are now a cause for worldwide concern, and industries are under pressure to make old products less damaging and new products “green”, clean, and efficient. Wind, solar, hydrogen and other forms of clean energy are being installed wherever possible, including roofs of buildings (aka green roofs), [1] factories, locomotives and, of course, automobiles. Politicians and scientists alike are beginning to understand (and explore) the need for alternative automotive fuel sources and ways to conserve energy that prolong and effectively utilize diminishing fossil fuels. Thermoelectric (TE) materials are the only known solids in existence that are capable of direct conversion of heat into electricity, [2] which is achieved via a temperature gradient across the material. These solid‐state devices have the added advantage that they can be used alongside any other power generation material, such as combustion engines, to further improve the energy yield from said device; any lost heat recycled into electricity, is energy the device would have otherwise never recovered! Take, for example, automotive designs for hybrid, electric or even conventional starter batteries: their internal chemistry is optimized to a certain temperature range which needs to be maintained in order to sustain a long battery life. Thermoelectric materials can regulate very hot or very cold batteries and even optimize them; a lead‐ acid battery at 294 K generates 13.5 V, but the same battery at 266 K generates 14.5 V. [3] In short, they are complementary materials that make green technology (e.g. batteries) even greener! When an n‐type and a p‐type thermoelectric material are paired together, attached to a circuit, and electrical current is moved through the device, a temperature gradient is created at both ends of said device; a system capable of removing heat from one end and depositing it on the other is the result. Known as the Peltier Effect (Figure 1.1 (a)), it is typically thought of as thermoelectric cooling. [4] Figure 1.1 (a) Peltier Effect (b) Seebeck Effect 2
Opposing the Peltier Effect is the Seebeck Effect or thermoelectric power generation, also shown above (Figure 1.1 (b)). To observe the Seebeck Effect, one side of the thermoelectric device must be cooled while the opposite side is simultaneously heated. The resulting temperature gradient causes the dominant charge carrier (n‐type = electrons; p‐type = holes, not positrons) to migrate from regions of high‐density (i.e. the hot side of the material) to regions of low‐density (cold side). [4] Due to the pairing of both types of semiconductors, migrations appear opposite to each other, or in other words, constituting a complete electrical circuit – the generation of electricity from heat! For the purposes of the work contained in this document however, we are concerned only with the latter of the two effects: power generation. 1.1. History of Thermoelectrics The beginnings of this field of study can be traced back as early as 1821 when German physicist Thomas Johann Seebeck (Figure 1.2 (a)) discovered that the formation of a circuit with two non‐identical bismuth and copper wires, each with a different temperature, would deflect the needle of a compass. [5] Eventually he also discovered that intensifying the temperature gradient would cause a greater deflection in the compass needle. This deflection was eventually found to have originated from an [5, 6] electric field as opposed to Seebeck’s original hypothesis – the “thermomagnetism” phenomenon. The ratio of the voltage difference produced between the dissimilar wires, ∆, to the temperature gradient, ∆, became the thermoelectric coefficient, ∆ . After Thomas Seebeck’s passing, the phenomenon was coined “the Seebeck Effect”. Figure 1.2 (a) T.J. Seebeck (b) J.C.A. Peltier (c) W. Thompson (Lord Kelvin) (1770 – 1831) (1785 – 1845) (1824 – 1907) Merely 13 years later in France, a meteorologist and physicist Jean Charles Athanase Peltier (Figure 1.2 (b)) encountered a temperature difference at the junctions of two different conductors as an electrical current was moving through. [7] Experimentally, Peltier was able to show the freezing of water at one junction, and upon reversal of the current flow, the melting of the ice. Despite the knowledge of the thermoelectric effect, the relationship between these discoveries remained elusive until Russian 3 ∆
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
Page 7 and 8: Table of Contents List of Figures..
Page 9 and 10: 9.3.1. Heating, XRD, and Structural
Page 11 and 12: List of Figures Figure 1.1 (a) Pelt
Page 13 and 14: Figure 9.2 Selected DOS calculation
Page 15 and 16: List of Equations Equation 1.1 Dime
Page 17 and 18: Table B.1 Atomic positions, isotrop
Page 19: Section I: Fundamentals Background
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
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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
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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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