Thermoelectric Figure-of-Merit of Fully Dense Single-Crystalline SnSe

Wei, Pai‐Chun; Bhattacharya, Sriparna; Liu, Yu Fei; Liu, Fengjiao; He, Jian; Tung, Yung Hsiang; Yang, Chun; Hsing, Cheng‐Rong; Nguyen, Duc-Long; Wei, C. M.; Chou, M. Y.; Lai, Yen‐Chung; Hung, Tsu‐Lien; Guan, Syu You; Chang, Chia Seng; Wu, Hsin‐Jay; Lee, Chi Hung; Li, Wen Hsien; Hermann, Raphaël P.; Chen, Yang Yuan; Rao, Apparao M.

doi:10.1021/acsomega.8b03323

Cited by 42 publications

(45 citation statements)

References 49 publications

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“…Because of strongly anharmonic bonding, both compounds exhibit very low thermal conductivities (single crystal κ at 300 K -SnSe: 1.6 (ref. 19 ), SnS: 2.42 (this work) Wm −1 K −1 with both compounds reaching κ < 1 Wm −1 K −1 at high temperature), which motivated numerous studies 12,15,17,20,21 . Despite huge interest in the low κ for thermoelectrics, very little experimental information is available on phonons at high-temperature, especially in the Cmcm phase.…”

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confidence: 68%

Extended anharmonic collapse of phonon dispersions in SnS and SnSe

et al. 2020

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The lattice dynamics and high-temperature structural transition in SnS and SnSe are investigated via inelastic neutron scattering, high-resolution Raman spectroscopy and anharmonic first-principles simulations. We uncover a spectacular, extreme softening and reconstruction of an entire manifold of low-energy acoustic and optic branches across a structural transition, reflecting strong directionality in bonding strength and anharmonicity. Further, our results solve a prior controversy by revealing the soft-mode mechanism of the phase transition that impacts thermal transport and thermoelectric efficiency. Our simulations of anharmonic phonon renormalization go beyond low-order perturbation theory and capture these striking effects, showing that the large phonon shifts directly affect the thermal conductivity by altering both the phonon scattering phase space and the group velocities. These results provide a detailed microscopic understanding of phase stability and thermal transport in technologically important materials, providing further insights on ways to control phonon propagation in thermoelectrics, photovoltaics, and other materials requiring thermal management.

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confidence: 68%

Extended anharmonic collapse of phonon dispersions in SnS and SnSe

et al. 2020

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“…Since the reported thermal conductivity increases and zT decreases after the phase transition, there is a good indication that thermal diffusivity is reduced by the total heat capacity through the phase transformation region (illustrated in Figure a). Calorimetry measurements of SnSe materials show a peak in heat capacity through the phase transition, which is expected since X‐ray diffraction results indicate that (∂φ/∂ T ) p ≠ 0 in Equation . The peak in the total heat capacity reported from calorimetry is ≈1.5 times the Dulong–Petit value, which can explain the difference between the reported κ and the trend‐line in Figure a (Note S7, Supporting Information).…”

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confidence: 71%

Phase Transformation Contributions to Heat Capacity and Impact on Thermal Diffusivity, Thermal Conductivity, and Thermoelectric Performance

2019

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Although direct measurements of κ are possible, [2] it is more frequent in measurement above room temperature to calculate thermal conductivity through the relation (Note S1, Supporting Information)Here, D [m 2 s −1 ] is the thermal diffusivity and ρc p = (∂H/∂T) p is the constant pressure heat capacity [J m −3 K −1 ] typically calculated from experimental bulk density, ρ [kg m −3 ] and the mass specific heat c p [J kg −1 K −1 ]. Equation (1) is utilized largely because measurements of thermal diffusivity, D, are accurate (within ≈3%) and easily accessible. [2] Thus, the uncertainty in calculating κ (often 10% or more) is frequently attributed to measurement of heat capacity. [2,3] Due to the uncertainty in the absolute magnitude of high temperature heat capacity measurements, it is often more accurate to use a model. Usually, the temperature independent Dulong-Petit heat capacity is a good approximation (within ≈5%) near room temperature. At higher temperatures, a model that incorporates a linear temperature dependence may be appropriate. [2,4,5] However, an incomplete understanding of heat capacity measurements and models can lead to inaccurate estimations of κ in some systems, especially those having substantial latent heats (e.g., during phase transitions). The recent debate surrounding the thermoelectric material, Cu 2 Se, is an excellent example. [6][7][8][9][10][11] In this material, and others, [12][13][14] the thermal diffusivity drops markedly as the material undergoes a phase transition. Depending on the heat capacity used to calculate κ, a maximum zT between 0.6 [7] and 2.3 [6] has been reported due to the superionic phase transition in Cu 2 Se. As exemplified in Figure 1, the choice of heat capacity can have a drastic impact on zT values.To properly characterize the behavior of thermal conductivity through a phase transition, it is paramount to understand the concurrent behavior of heat capacity. Recognizing that the total capacity of a material to absorb heat includes both the intrinsic heat capacity of the phases present and the enthalpy (heat) of transformation ΔH that is required to maintain equilibrium (characterized by the order parameter φ) as the temperature is changedThe accurate characterization of thermal conductivity κ, particularly at high temperature, is of paramount importance to many materials, thermoelectrics in particular. The ease and access of thermal diffusivity D measurements allows for the calculation of κ when the volumetric heat capacity, ρc p , of the material is known. However, in the relation κ = ρc p D, there is some confusion as to what value of c p should be used in materials undergoing phase transformations. Herein, it is demonstrated that the Dulong-Petit estimate of c p at high temperature is not appropriate for materials having phase transformations with kinetic timescales relevant to thermal transport. In these materials, there is an additional capacity to store heat in the material through the enthalpy of transformation ΔH. This can be described using a generalize...

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“…Therefore, there have been extensive follow-up investigations on SnSe and related materials aiming to further improve their TE performances through various approaches. [7][8][9][10][11][12][13][14][15][16][17] Among them, the most commonly used one is the chemical substitutions for Sn/Se or introducing vacancies.…”

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confidence: 99%