Exploration and Optimization of Tellurium‐Based Thermoelectrics

More documents

Recommendations

Info

$uWaterloo LaTeX Thesis Template - UWSpace - University of Waterloo$

The electronic structure calculations suggest that Ba3Cu17‐x(S,Te)11 would be an intrinsic p‐type semiconductor when x = 1, which is in agreement with the measurement of three stoichiometric variations thereof. The highest power factor was found to be 2.38 W∙cm ‐1 K ‐2 at 700 K, but like in case of its Se‐counterpart, high Cu ion mobility makes this material a poor choice for thermoelectric materials. The calculations for Ba3Cu17‐x(S,Te)11.5 revealed a significant albeit small number of states around the Fermi level, which concurs well with its smaller Seebeck coefficient and larger electrical conductivity. The highest power factor of this series, 2.12 W∙cm ‐1 K ‐2 at 750 K, is very comparable to the maximum measured for Ba3Cu17‐x(S,Te)11. Here as well, the structure refinements suggest the possibility of a high Cu mobility that makes it unappealing from a thermoelectric perspective. 168
Chapter 17. Ba2Cu7‐xTe6 17.1. Introduction All of the compounds we discovered previously discussed in Section V were characterized as Cu‐ deficient thus p‐type semiconductors, with only exception thus far, namely the metallic sulphide‐ telluride BaCu17‐xS11.5‐yTey. [277] As a high Ba:Cu ratio proved to be disadvantageous for thermoelectric properties (because of low electrical conductivity), we have increasingly concentrated on smaller Ba:Cu ratios, along with an excess of tellurium in order to obtain polytellurides, expected to cause the desired small band gaps. [24, 309] During this process, we discovered the new telluride Ba2Cu7‐xTe6 with significant Te–Te interactions, which is introduced with this chapter. 17.2. Experimental Section 17.2.1.1. Synthesis All reactions commenced from the elements (Ba: 99 %, pieces, Aldrich; Cu: 99.5 %, powder ‐150 mesh; Alfa Aesar; Te: 99.999 %, ingot, Aldrich), with sample masses of approximately 600 mg. The elements were stored and handled in a glove box filled with argon. Within, the elements were placed into glassy carbon crucibles inside silica tubes, which were subsequently sealed under vacuum (10 ‐3 mbar). The tubes were then heated in a resistance furnace at 1073 K for six hours. Thereafter, the furnace was slowly cooled down to 923 K and left for a period of 60 hours to allow for homogenization. Lastly, the furnace was switched off prompting fast cooling. Investigations on the phase range of Ba2Cu7–xTe6 were carried out with 0 ≤ x ≤ 1 in increments of 0.2. Therein, the sample with x = 0.4 was found pure phase, and the samples with x = 0.2 and 0.6 contained less than 5 % side products, hinting towards the existence of a rather small phase range. 17.2.1.2. Analysis The sample of nominal composition Ba2Cu6.6Te6 was analyzed via EDX. No heteroelements were detected during the examination. The obtained Ba : Cu : Te ratios were very homogenous, yielding 14 : 45:41 at‐%, as averaged over six crystals, compared to the nominal 13.7 : 45.2 : 41.1. Differential scanning calorimetry was also utilized to study the thermal stability of Ba2Cu6.6Te6. The compound was heated from room temperature to 1123 K under argon flow against a sapphire reference. Upon heating at a rate of 10 K·min ‐1 , the experiment showed a single endothermic peak around 1000 K corresponding to the melting point; upon cooling at a rate of 20 K·min ‐1 , the opposite 169
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 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 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 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
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
Page 173 and 174: As emphasized in Figure 15.2 (b), t
Page 175 and 176: The Cu-Cu COHP curve, cumulated ove
Page 177 and 178: Chapter 16. Ba3Cu17‐x(S,Te)11 and
Page 179 and 180: the selenide‐telluride before, th
Page 181 and 182: surrounded by three Q1 atoms. Besid
Page 183 and 184: Figure 16.3 The three‐dimensional
Page 185: Figure 16.5 Seebeck coefficient (le
Page 189 and 190: Table 17.1 Crystallographic Data fo
Page 191 and 192: Table 17.2 Atomic coordinates, Ueq
Page 193 and 194: 17.3.2. Electronic Structure Calcul
Page 195 and 196: Figure 17.6 Crystal orbital Hamilto
Page 197 and 198: Section VI - Supplementary Informat
Page 199 and 200: [33] Rowe, D. M.; Kuznetsov, V. L.;
Page 201 and 202: [90] Albright, T. A.; Burdett, J. K
Page 203 and 204: [156] Cottenier, S., Density Functi
Page 205 and 206: [219] Toure, A. A.; Kra, G.; Eholie
Page 207 and 208: [294] Kanno, R.; Ohno, K.; Kawamoto
Page 209 and 210: Table A.3 Atomic positions and Ueq
Page 211 and 212: Table B.2 Selected interatomic dist
Page 213 and 214: Table B.5 Atomic coordinates and Ue
Page 215 and 216: Table B.7 Selected interatomic dist
show all

Exploration and Optimization of Tellurium‐Based Thermoelectrics

You also want an ePaper? Increase the reach of your titles

Delete template?

Save as template?