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Wu D.,South University of Science and Technology of China | Wu D.,Shenzhen Key Laboratory of Thermoelectric Materials | Zhao L.-D.,Beihang University | Zhao L.-D.,Northwestern University | And 13 more authors.
Energy and Environmental Science | Year: 2015

Lead chalcogenides are dominant thermoelectric materials in the medium-temperature range due to their highly favorable electronic band structures and low thermal conductivities. An important system is the PbTe-PbS pseudo-binary, and its low thermal conductivity originates largely from the coexistence of both alloying and nanostructuring through phase-separation. To better understand the competition between the alloying and phase separation and its pronounced effects on the thermoelectric performance in PbTe-PbS, we systematically studied, via transmission electron microscopy (TEM) observations and theoretical calculations, the samples of Spark Plasma Sintered (SPSed) 3 at% Na-doped (PbTe)1-x(PbS)x with x = 10%, 15%, 20%, 25%, 30% and 35%. The highest figure of merit, viz., ZT ∼ 2.3 was obtained at 923 K, when the PbS phase-fraction, x, was 20%, which corresponds to the lowest lattice thermal conductivity of the series. The consistently lower lattice thermal conductivities in the SPSed samples as compared with the corresponding ingots originates from the mesostructured nature of the former, which contributes significantly to their superior ZT. We also studied the onset of carrier concentration modulation at ∼600 K, which leads to the observed saturation of electrical transport properties due to the diffusion and re-dissolution of excessive Na into the PbTe-PbS matrix. This carrier concentration modulation is equally crucial to achieve very high power factors (up to 26.5 μW cm-1 K-2 at 623 K) and outstanding thermoelectric performances in SPSed PbTe-PbS binaries. © 2015 The Royal Society of Chemistry.

Xiao Y.,Beihang University | Chang C.,Beihang University | Pei Y.,Beihang University | Wu D.,South University of Science and Technology of China | And 11 more authors.
Physical Review B - Condensed Matter and Materials Physics | Year: 2016

We provide direct evidence to understand the origin of low thermal conductivity of SnSe using elastic measurements. Compared to state-of-the-art lead chalcogenides PbQ(Q=Te, Se, S), SnSe exhibits low values of sound velocity (∼1420m/s), Young's modulus (E∼27.7GPa), and shear modulus (G∼9.6GPa), which are ascribed to the extremely weak Sn-Se atomic interactions (or bonds between layers); meanwhile, the deduced average Grüneisen parameter γ of SnSe is as large as ∼3.13, originating from the strong anharmonicity of the bonding arrangement. The calculated phonon mean free path (l ∼ 0.84 nm) at 300 K is comparable to the lattice parameters of SnSe, indicating little room is left for further reduction of the thermal conductivity through introducing nanoscale microstructures and microscale grain boundaries. The low elastic properties indicate that the weak chemical bonding stiffness of SnSe generally causes phonon modes softening which eventually slows down phonon propagation. This work provides insightful data to understand the low lattice thermal conductivity of SnSe. © 2016 American Physical Society.

Wu H.,South University of Science and Technology of China | Wu H.,Shenzhen Key Laboratory of Thermoelectric Materials | Zheng F.,South University of Science and Technology of China | Zheng F.,Shenzhen Key Laboratory of Thermoelectric Materials | And 7 more authors.
Nano Energy | Year: 2015

Thermoelectric (TE) materials can interconvert waste heat into electricity, thus are promising for power generation and solid-state refrigeration. The thermoelectric properties of a certain material strongly correlate with its chemical, structural and electronic features; therefore, a thorough characterization of these features is not only crucial to profoundly understand the material itself, but also helps to design new materials with desired properties. Under this circumstance, various electron microscopy (EM) techniques are developed, from micro-scale to atomic-scale, two-dimensional (2-D) to 3-D, and static to dynamic. In this review, we review advanced EM techniques already applied in and also look into the perspective of introducing more EM techniques into the field of thermoelectrics. Specifically, we firstly summarize "what have been done" involving: structural and chemical characterizations of all-scale "imperfectness", electronic structure investigation, 3-D morphology and dynamic evolution of nanostructures, and atomic-scale mapping of Seebeck coefficient and defects; based on these characterized features, we then briefly review the calculations on electrical and thermal transport properties to illustrate the structure-property correlations. In what follows, we propose "what can be done" in TEs via EM techniques including: valence-electron distribution, quantitative measurement of atomic displacement, point defect characterization, local band gap measurement, phonon excitation detection, electrostatic potential determination, thermal stability of nanostructures, and in-situ observation and measurement of local TE effects. © 2015 Elsevier Ltd.

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