Today the research for good thermoelectric compounds within the class of half-Heusler compounds is especially focused on two system based on TiNiSn for the n-type and ZrCoSb for the p-type materials. The main challenge for this compounds is the reduction of the thermal conductivity. This can be easily be done by introducing grain boundaries into system, which will considerably influence the phonon scattering. One way is the introduction of nanostructures by creating nanopowders by ball milling bulk samples and finally obtaining dense bulk material by SPS. Another way is based on exploiting a phase separation of the solid solution.
The newest directions in TE materials research is the concept of
nanostructure-engineering. Certainly nanostructuring or nanocomposites is the new paradigm of TE materials research. In the HH nanocomposite materials, the TE performance can be significantly enhanced by decreasing the lattice thermal
conductivity as well as enhancing the power factor. Nanoinclusions that can induce both a boundary scattering effect and energy filtering effect can effectively decouple the interrelated thermal and electrical transport properties.
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Although to date, the half-Heusler TE materials cannot be in used in very broad applications due to their low conversion efficiency. However, if the combination of combined mechanisms to enhance ZT in HH alloys, such as nanostructuring and band structure engineering, are able to achieve values of the ZT to 2 or even higher, improve the mechanical properties and maintain their good thermal stability with the
nanostructuring processes, then the broad range of potential applications for waste heat recovery using TE devices based on this next generation of HH alloys will be very promising.
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Figure 1.1- A thermoelectric module
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Figure 1.2- Schematic of the refrigeration and power generation mode of thermoelectric module.
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Figure 1.3- Diagram of a thermocouple showing how the Seebeck Effect works.
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Figure 1.4- Diagram showing how heat is expelled or absorbed during the Peltier Effect.
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Figure 1.5- Generating efficiency as a function of temperature and figure of merit.
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Figure 1.6- Overview of ZT vs temperature for different thermoelectric materials [1].
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Figure 1.7- Schematic dependence of electrical conductivity, Seebeck coefficient, power factor, and thermal conductivity on concentration of free carriers.
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Figure 2.1-The crystal structure of a typical Half-Heusler compound ABC. The red, blue, and green dots correspond to A, B, and C atoms in half-Heusler.
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Figure 2.2-Overview of the most promising doped n and p-type doped HH compounds [2]:
(a) n-type, (b) p-type. (1) ErNi1−xPdxSb ; (2) ZrNi0.8Ir0.2Sn ; (3) TiCo0.85Fe0.15Sb ; (4) ZrCoSn0.1Sb0.9 ; (5) Zr0.5Hf0.5CoSb0.8Sn0.2 ; (6) Ti0.5Zr0.25Hf0.25Co0.95Ni0.05Sb ; (7) Ti0.6Hf0.4Co0.87Ni0.13Sb ; (8) Zr0.75Hf0.25NiSn0.975Sb0.025 ; (9) Zr0.4Hf0.6NiSn0.98Sb0.02 ; (10) Zr0.25Hf0.25Ti0.5NiSn0.998Sb0.002 .
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Figure 2.3-Two types of HH nanocomposites: (a) micro-scale HH matrix with nano-scale inclusions; (b) nano-scale HH phase with nanoinclusions.
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Figure 3.8- Equipment of arc melting
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Figure 3.9- Equipment of ball milling
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Figure 3.10-Spark Plasma Sintering (SPS-515S) System
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Figure 3.4- Powder X-ray Diffractometer (X’PERT PRO)
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Figure 3.5-JEOL JSM-6500F scanning electron microscope (SEM)
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Figure 3.6- ZEM3 apparatus
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Figure 3.7-TC-9000 apparatus
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Figure 3.8- Metter DSC821 apparatus
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Figure 4.1- XRD pattern of ZrCo1-xFexSbafter arc melting.
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Figure 4.2- SEM micrographs of ZrCoSbafter arc melting under BEI mode.
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Figure 4.3- The backscattered electron image of ZrCo1-xFexSbafter arc melting for (a) x = 0.1; (b) x = 0.2; (c) x = 0.3; (d) x = 0.6.
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Figure 4.4- SEM micrographs of ZrFe0.6Co0.4Sb after arc melting: (a) under SEI mode, (b) under BEI mode.
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Figure 4.5- The Seebeck coefficient as a function of Fe content (x) for ZrFexCo1-xSb at room temperature.
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Figure 4.6- The temperature dependence of the Seebeck coefficient of ZrFexCo1-xSb alloys.
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Figure 4.7- The temperature dependence of the electrical conductivity of ZrFexCo1-xSb alloys.
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Figure 4.8- The temperature dependence of the thermal conductivity of ZrFexCo1-xSb alloys.
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Figure 4.9- The temperature dependence of ZT of ZrFexCo1-xSb alloys.
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Figure 4.10- XRD pattern of ZrCoSb1-xSnx (x = 0, 0.1 and 0.2) after SPS.
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Figure 4.11- SEM micrographs of ZrCoSb1-xSnx after SPS: (a) x = 0 under SEI mode and (b) x = 0 under BEI mode.
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Figure 4.12- SEM micrographs of ZrCoSb1-xSnx after SPS: (a) x = 0.1 under SEI mode and (b) x = 0.1 under BEI mode.
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Figure 4.13- SEM micrographs of ZrCoSb1-xSnx after SPS: (a) x = 0.2 under SEI mode and (b) x = 0.2 under BEI mode.
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Figure 4.14- The temperature dependence of the Seebeck coefficient of ZrCoSb1-xSnx
alloys.
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Figure 4.15- The temperature dependence of the electrical conductivity of ZrCoSb1-xSnx
alloys.
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Figure 4.16- The temperature dependence of the thermal conductivity of ZrCoSb1-xSnx
alloys.
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Figure 4.17- The temperature dependence of ZT of ZrCoSb1-xSnx alloys.
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Figure 4.18- SEM photograph of ball-milled powders of Zr0.5Hf0.5CoSb1-xSnx before SPS:
(a) x = 0.1, (b) x = 0.2.
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Figure 4.19- SEM micrographs of Zr0.5Hf0.5CoSb1-xSnx after SPS: (a) x = 0.1 under SEI mode, (b) x = 0.1 under BEI mode.
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Figure 4.20- SEM micrographs of Zr0.5Hf0.5CoSb1-xSnx after SPS: (a) x = 0.2 under SEI mode, (b) x = 0.2 under BEI mode.
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Figure 4.21- XRD pattern of Zr0.5Hf0.5CoSb1-xSnx (x = 0.1 and 0.2)after SPS.
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Figure 4.22- The detailed observations of mark Z in fig. 3(b) under different tilted angles: (a) without tilting, (b) the tilted angle = 2 ﾟ, (c) the tilted angle = 4 ﾟ.
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Figure 4.23- Total thermal conductivity of the Zr0.5Hf0.5Co Sb1-xSnx/HfO2 alloysas a function of temperature.
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Figure 4.24- Lattice part of thermal conductivity of the Zr0.5Hf0.5Co Sb1-xSnx/HfO2 alloys as a function of temperature.
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Figure 4.25- The temperature dependence of the Seebeck coefficient of Zr0.5Hf0.5CoSb1-xSnx/HfO2 alloys.
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Figure 4.26- The temperature dependence of electrical conductivity of Zr0.5Hf0.5CoSb1-xSnx/HfO2 alloys.
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Figure 4.27- ZT of the Zr0.5Hf0.5Co Sb1-xSnx/HfO2 alloysas a function of temperature.
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Figure 4.28- XRD pattern of Zr0.5Hf0.5FexCo1-xSn0.2Sb0.8 (x = 0.05, 0.1 and 0.2)after arc melting.
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Figure 4.29- SEM image of a characteristic region of Zr0.5Hf0.5Fe0.05Co0.95Sn0.2Sb0.8 after arc melting. (a) The backscattered electron image. (b) EDX result for Fe K signal. (c) EDX result for Sn L signal.
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Figure 4.30- SEM image of a characteristic region of Zr0.5Hf0.5Fe0.1Co0.9Sn0.2Sb0.8 after arc melting. (a) The backscattered electron image. (b) EDX result for Fe K signal. (c) EDX result for Sn L signal.
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Figure 4.31- SEM image of a characteristic region of Zr0.5Hf0.5Fe0.2Co0.8Sn0.2Sb0.8 after arc melting. (a) The backscattered electron image. (b) EDX result for Fe K signal. (c) EDX result for Sn L signal.
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Figure 4.32- The backscattered electron image of Zr0.5Hf0.5FeXCo1-XSn0.2Sb0.8 after arc melting for (a) x = 0.05; (b) x = 0.1; (c) x = 0.2.
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Figure 4.33- SEM micrographs of Zr0.5Hf0.5FexCo1-xSb0.8Sn0.2 after SPS: (a) x = 0.1 under SEI mode, (b) x = 0.1 under BEI mode and (c) EDX result for Fe K signal.
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Figure 4.34- The temperature dependence of the Seebeck coefficient of Zr0.5Hf0.5FexCo1-xSn0.2Sb0.8/Fe3Sn2 composites.
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Figure 4.35- The temperature dependence of the electrical conductivity of Zr0.5Hf0.5FexCo1-xSn0.2Sb0.8/Fe3Sn2 composites.
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Figure 4.36- Thermal conductivity of the Zr0.5Hf0.5FexCo1-xSn0.2Sb0.8/Fe3Sn2 composites as a function of temperature.
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Figure 4.37- Lattice thermal conductivity of the Zr0.5Hf0.5FexCo1-xSn0.2Sb0.8/Fe3Sn2
composites as a function of temperature.
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Figure 4.38- ZT of the Zr0.5Hf0.5FexCo1-xSn0.2Sb0.8/Fe3Sn2 composites as a function of temperature.
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Table 1-1 summary of composition, synthesis condition, and cost ($/kg) of the best n- and p- type Bi-Te, Pb-Te, skutterudites, half-Heusler, and Si-Ge nanocomposites.
DC-HP, RF-HP, and SPS correspond to direct current induced-hot press, RF induction-hot press, and spark plasma sintering, respectively [1].
Composition Powder synthesis Sintering Cost
Cu0.01Bi2Te2.7Se0.35 Mechanical alloying by ball milling DC-HP 171
BiXSb2-xTe3 Ball milling bulk alloy ingots DC-HP 231
PbTe0.9988I0.0012 Furnace melting → quenching→annealing →
hand grinding
RF-HP 135
K0.02Pb0.98Te0.15Se0.85 Furnace melting → slow cooling→ hand grinding DC-HP 59
Ba0.08La0.05Yb0.04Co4Sb12 Induction melting → annealing → grinding SPS 64
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Table 3-1. Experimental procedure for producinghalf-Heusler.
Process Conditions Environment
Arc melting melting four times argon atmosphere (99.9999 %)
Ball milling 5 mm WC balls in diameter for
90 minutes
1 atm, 25 ℃ and 40 %RH
SPS 80 MPa at 1373 K for 15 min vacuum atmosphere (40 Pa)
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Table 4.1- Distribution of elements as obtained by EDX analysis for ZrFexCo1-xSb.
Zr
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Table 4.2- Distribution of elements as obtained by EDX analysis for ZrCoSb1- xSnx. Zr Atomic% Co Atomic% Sb Atomic% Sn Atomic% O
Atomic%
A (x = 0) 33.63 66.37
B (x = 0) 35.80 30.26 33.94
C (x = 0) 35.96 30.25 33.79
D (x = 0.1) 35.57 29.78 30.70 3.95
E(x = 0.1) 34.81 30.16 31.35 3.69
F(x = 0.2) 35.43 30.72 28.59 5.26
G(x = 0.2) 35.84 29.95 28.56 5.65
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Table 4.3.- Distribution of elements as obtained by EDX analysis for Zr0.5Hf0.5CoSb1-xSnx (x = 0.1 and 0.2).
Zr(at.%) Hf(at.%) Co(at.%) Sb(at.%) Sn(at.%) O(at.%) EPMA composition
A ( x = 0.1) 34.07 65.93 HfO1.93
B ( x = 0.1) 17.81 16.90 31.66 30.69 2.94 Zr0.51Hf0.49CoSb0.91Sn0.09
C ( x = 0.1) 18.35 16.40 31.27 30.97 3.01 Zr0.53Hf0.47CoSb0.91Sn0.09
D ( x = 0.2) 17.89 16.56 31.82 28.87 4.85 Zr0.52Hf0.48CoSb0.86Sn0.14
E ( x = 0.2) 17.89 17.03 31.40 28.31 5.38 Zr0.51Hf0.49CoSb0.84Sn0.16
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Table 4.4- Oxygen content of experimental procedures as obtained by EDX analysis.
Oxygen content (wt%)
Raw metal 0~0.5
After Arc melting 0.63
After ball milling 1.93
After SPS 2.08
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Table 4.5- Distribution of elements as obtained by EDX analysis for
Zr0.5Hf0.5FexCo1-xSb0.8Sn0.2 (x = 0.05, 0.1 and 0.2) after arc melting.
156
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