Remarkable Improvement of Thermoelectric Figure-of-Merit in SnTe through In Situ-Created Te Nanoinclusions

Abstract

SnTe exhibits very low Seebeck coefficient along with high electrical–thermal conductivities owing to very high hole concentration that results from intrinsic Sn vacancies. All these unfavorable thermoelectric parameters restrict pristine SnTe from becoming an outstanding thermoelectric material. In this work, we demonstrate the improvement in figure-of-merit of SnTe through in situ creation of Te nanoinclusions in the SnTe matrix. We have intentionally added excess Te in stoichiometric Sn/Te (1:1) for the synthesis of SnTe. During solidification of SnTe from the melt, the excess Te in the liquid state gets expelled to the grain boundaries. On further cooling, when Te starts to solidify, it exerts strain in the already solidified SnTe grains and creates extensive structural defects including dislocations, twin boundaries, and subgrain boundaries that scatter phonons of midwavelength. The point defect due to intrinsic Sn vacancies and SnTe/Te grain boundaries respectively scatters phonons of low and high wavelengths. Effective scattering of the entire spectrum of phonons through the hierarchical defects gives rise to an ultralow lattice thermal conductivity of 0.5 W m–1 K–1 that is close to the theoretical minimum limit. The contribution from the heavy hole valence band in transport along with energy filtering of charge carriers at the SnTe/Te interface results in improvement of the Seebeck coefficient near room temperature. The cumulative effect of improved Seebeck coefficient and suppressed thermal conductivity gives rise to a high figure-of-merit (ZT) value of ∼1.5 at 750 K and an average ZT of ∼0.48 for the Sn0.9Te sample.

A synergistic approach to achieving the high thermoelectric performance of La-doped SnTe using resonance state and partial band convergence

“…The EDS spectra of all the compositions of La-doped SnTe, Sn SnTe. 21 The composition Sn 0.92 La 0.08 Te shows the presence of grain boundaries, nanoprecipitates of La due to the solubility limit and defects although Fig 4(c) represents that all the three compositions of La-doped SnTe exhibit defects of various length scales and grain boundaries. These grain boundaries along with the defects present in the sample contribute to phonon scattering and thus lattice thermal conductivity is reduced.…”

“…The uniqueness of the planes is that they are the most active planes for the slip to occur and arises due to the anisotropic growth of the sample. 21 The study by Yang et al 36 reported that these kink bands serve as scattering centres for the phonons and thus suppress the thermal conductivity (Fig. 3(d)).…”

“…However, the large energy separation (ΔE=0.35 eV) [18][19][20] between the light and heavy holes inhibit the ∑ band to contribute to the transport properties at room temperature whereas, at high temperature, the L band moves toward the ∑ band, and thus there occurs the band convergence. [18][19][20][21][22][23] Pei et al have explained explicitly that in the two-valence band model of PbTe, the energy offset between conduction band (CB) and L-band of VB is temperaturedependent. However, the energy offset between CB and the heavy ∑ band of VB is independent of temperature but moves gradually with the addition of the dopants.…”

A Review on Fundamentals, Design and Optimization to High ZT of Thermoelectric Materials for Application to Thermoelectric Technology

“…There are various approaches to increase the ZT value of materials. These are alloying, [83][84][85][86][87] PGEC, 24,25,88 band engineering, [89][90][91] and nanostructuring, [92][93][94][95] which are discussed below.…”

“…[165][166][167] For instance, GaAs nanoinclusions dispersed in n-type Bi 2 Te 2.7 Se 0.3 lead to reduced thermal conductivity while maintaining a large power factor. 168 Ahmad et al 93 achieved a high ZT of 1.5 at 750 K owing to reduced lattice thermal conductivity due to hierarchical scattering of phonons and an improved Seebeck coefficient via energy filtering for Te nanoinclusions created in-situ in SnTe. Kong et al 169 introduced nano-TiO 2 particles in the Cu 2 Te matrix in order to reduce the thermal conductivity.…”