Exploration and Optimization of Tellurium‐Based Thermoelectrics

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The logical rise in dimensionality that follows increasing a 0‐dimensional point defect into a 1‐D line (i.e. edge dislocation), 2‐D plane (i.e. grain boundary), or 3‐D volume (i.e. nanoregions) leads the idea of imperfections at the macro‐ and microscopic or even nanoscopic levels, driving properties such as mechanical strain or even colour. Consider for example diamonds (band gap ~5 eV) with boron or nitrogen impurities causing blue or yellow tints from acceptor (0.4 eV) and donor (4 eV) states respectively, via energy transitions in the band gap. [85] Examining now a 2‐dimensional defect, grain boundaries are frequently encountered in crystalline materials due to manufacturing inconsistencies, inhomogeneity in powder size, and growth in multiple directions, yielding some or all of the boundary types illustrated in Figure 2.3 (a). Grain boundaries play a large role in thermoelectrics due to their effect on carrier mobility throughout the crystal structure, which is demonstrated in studies on PbTe; several types of boundaries are examined (coherent, incoherent and semi‐coherent) on various off‐ stoichiometry defect interfaces and their effective reduction of the lattice thermal conductivity, ph. [88] TEM at the atomic scale (~1 nm) reveals one such boundary in the following image: Figure 2.3 (a) Different grain boundary types (b) Grain boundary in PbTe–2%Pb via TEM [88] 2.3. Charge Carriers and their Effects Charge carriers are any free‐moving particles that carry an electric charge. When it comes to metals – and every solid to an extent – charge carriers are electrons (i.e. recall Equation 1.4 (a)). With extrinsic semiconductors, charge carriers are considered in two categories: free electrons (n‐type) in the crystal lattice and a deficiency of electrons (p‐type), or a positive charge moving in a direction opposite of electron‐flow. Free electrons produce additional “donor states” close to the conduction band, to be released with very little energy (i.e. P0.01Si0.99); p‐type semiconductors however, can only achieve a full octet by breaking a neighbouring bond in the lattice. Additional “acceptor states” are therefore created close above the valence band for energetically inexpensive conduction (i.e. Al0.01Si0.99). P‐type conduction is referred to as “holes” due to the appearance of vacancies being promoted, since an 20
electric field draws electrons towards it, filling in the deficiency as they move, the hole drifts away from the field just like a positron. [89] This concept will again be discussed in 2.5.1 and Figure 2.9 (a). In the process of tuning the thermoelectric figure of merit, one must consider the charge carrier concentration and how it varies, in order to see improvements in the ZT factor. Charge carriers govern most of the physical properties and must be understood to properly design materials. Typically, insulators are considered to have a charge carrier concentration less than 10 18 (per cm 3 ), while metals should have > 10 20 . As depicted in Figure 2.4, the tuning of ZT becomes a difficult task as insulators boast a high Seebeck coefficient whilst metals have extremely high electrical and thermal conductivity. Researchers therefore look to semiconductors with ~10 19 < < ~10 20 for a reasonable compromise between the three primary components of ZT. Figure 2.4 ZT as a function of carrier concentration (n) [31] 2.3.1. Electrical Conductivity (σ) as a function of charge carriers (n) A brief summary of different materials’ behaviours according to Equation 1.4 (a): A metal has the highest concentration of charge carriers of all known materials; copper, for example, having 8.42∙10 22 cm ‐3 electrons, [26] means can be considered constant (or at least negligibly variant) and as temperature increases, mobility is obstructed by increased atom vibrations, and thus decreases slightly with increasing temperature. This is observed with vs. as a linear decrease. An intrinsic semiconductor can be understood to have no mobile carriers 0 at absolute zero, but as the temperature increases, carriers are increasingly able to cross the energy gap between valence and conduction bands, thus leading to a dominant term with increasing temperatures. An extrinsic semiconductor can now be considered as a material with a limited number of additional charge carriers to donate to the conduction band. As these extrinsic carriers begin crossing the energy band gap with a rising temperature, the material is behaving as a typical semiconductor (albeit higher σ) by exponentially 21
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 and 20: Section I: Fundamentals Background
Page 21 and 22: 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: 2.2. Defects, Non‐Stoichiometry a
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 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
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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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