Exploration and Optimization of Tellurium‐Based Thermoelectrics

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8.3. Doping and Substitution Experiments on SnBi2Te4 For thermoelectric applications, one would expect a compound to show an ability to improve its properties via an optimization process. In this sense, a traditional solid state approach can be utilized to in order to determine whether or not SnBi2Te4 has more to gain as a thermoelectric material. Doping experiments can therefore be employed to adjust its physical properties and figure of merit. Based on studies in Chapter 7, SnBi2Te4 with its narrow band gap, reasonable Seebeck magnitude and heavy atoms is in good standing for optimization. Likewise, the phase range experiments support the material’s ability to host atoms of varying concentrations. Little is known with respect to the doping potential of SnBi2Te4 with the possible exception of a study on Te‐doped PbSb2Te4, showing a marginal increase in S and decrease of σ. [208] Studies done by, for example, Heremans et al. [104] and Gelbstein et al. [190] show the unique potential of the triel (Tr) elements generating impurity states in the band gap and Fermi level pinning – since triel elements are often Tr 3+ , their introduction to the Sn 2+ site should lead to a one‐electron difference in counting. It should also be noted the tendency for Tl + and in certain cases, In + as a possibility. 8.3.1. Synthesis and Structural Analysis As expected, all samples in this system can be synthesized from the elements and doped samples were no exception. The dopants utilized were as follows: Ga: 99.999 %, ingot (metals basis), ALFA AESAR; In: 99.999 %, ingot (metals basis), ALFA AESAR. Substituents have a wider range of elements for the Bi site including Ga and In, but also the following: Tl: 99.99 %, granules 1‐5mm, ALDRICH; Nb: 99.8 %, powder ‐325 mesh, ALFA AESAR; Ta: 99.98 %, powder ‐100 mesh, ALFA AESAR. Upon their removal from the furnaces, many samples were inhomogeneous and required grindings followed by sintering at 843 K for approximately ten days up to a maximum of three reheats. Utilizing either dopant, samples were synthesized according to TrxSn1‐xBi2Te4 with increments of x = 0.01 until x = 0.05 and additional x = 0.07, 0.10, 0.12, and 0.15 to identify the primary impurity phases from excess dopant. The substituents (V/TrxSnBi2‐xTe4) followed the same increments with additional x = 0.20, and 0.25 to identify primary impurity phases and determine the maximum amount of impurity. XRD yielded, eventually, phase‐pure samples up to x = 0.07 for doping and x = 0.15 for substitution. EDX was run on selected phase‐pure samples, yielding acceptable atomic ratios. 88
In order to better determine if the elements were successfully incorporated, refinements were attempted on a series of compounds and their respective LeBail findings are displayed, in Table 8.5. All refinements on doped samples were performed with the proposed single crystal model [193] and followed a similar refinement “recipe” for consistency. Table 8.5 Summary of LeBail refinements for Tr xSn 1‐xBi 2Te 4 and Tr/V xSnBi 2‐xTe 4. Target Compound Space Group (Å) = (Å) (Å 3 ) RP a \ RB b SnBi2Te4 (cif) [193] 3 4.411 41.51099 699.47 3 \ ––––––– –––––– ––––––– ––––––– ––––––– ––– ––––––– Ga0.02Sn0.98Bi2Te4 3 4.402(1) 41.66(1) 699.1(5) 3 4.40 \ 7.73 Ga0.07Sn0.93Bi2Te4 3 4.395(2) 41.77(2) 698.7(9) 3 7.85 \ 8.05 In0.02Sn0.98Bi2Te4 3 4.394(2) 41.62(2) 696.1(7) 3 5.42 \ 6.89 In0.07Sn0.93Bi2Te4 3 4.4041(8) 41.590(7) 698.6(3) 3 5.16 \ 7.67 ––––––– –––––– ––––––– ––––––– ––––––– ––– ––––––– Target Compound Space Group 89 (%) (Å) = (Å) (Å 3 ) RP a \ RB b Ga0.02SnBi1.98Te4 3 4.3961(3) 41.644(2) 696.99(9) 3 3.98 \ 6.59 Ga0.05SnBi1.95Te4 3 4.37259(8) 41.3911(8) 685.35(2) 3 4.00 \ 5.61 In0.05SnBi1.95Te4 3 4.375(1) 41.39(1) 686.0(6) 3 6.08 \ 7.45 In0.07SnBi1.93Te4 3 4.3966(2) 41.584(1) 696.12(5) 3 3.70 \ 4.18 In0.15SnBi1.85Te4 3 4.37380(7) 41.4294(7) 686.37(2) 3 3.48 \ 5.21 Tl0.05SnBi1.95Te4 3 4.39009(6) 41.5146(7) 692.91(1) 3 3.81 \ 4.73 Nb0.05SnBi1.95Te4 3 4.4012(2) 41.627(2) 698.29(8) 3 4.29 \ 7.81 Ta0.05SnBi1.95Te4 3 4.387(1) 41.56(1) 693.0(6) 3 6.08 \ 6.81 a RP |yo ‐ yc| / |yo| b RB |Io ‐Ic| / |Io| It is apparent from the structural refinements that clear changes to the cell size and volume have occurred with several different impurity elements. With all refinement showing R‐values below 10 %, impurity inclusions should be considered successful. As intended, it can also be said that the Bi‐site replacements generally produced smaller unit cell sizes than their Sn counterparts. 8.3.2. Electronic Structure Calculations Via the standard LMTO software, predictions of the quaternary SnBi2Te4 samples could be performed and compared to one another. Of these calculations, several were selected and presented below in Figure 8.4 including a comparison of three triel elements on the Bi site, followed by increasing [Ga] on the Sn site and Bi site respectively. Calculations were run on a basis of 1000 evenly dispersed k points in the following symmetries: 3, 3, and 2/ depending on the size of the supercell used. (%)
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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
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
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: Figure 8.3 DOS calculation comparis
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
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
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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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