Understanding of the Extremely Low Thermal Conductivity in High‐Performance Polycrystalline SnSe through Potassium Doping

Abstract

P‐type polycrystalline SnSe and K0.01Sn0.99Se are prepared by combining mechanical alloying (MA) and spark plasma sintering (SPS). The highest ZT of ≈0.65 is obtained at 773 K for undoped SnSe by optimizing the MA time. To enhance the electrical transport properties of SnSe, K is selected as an effective dopant. It is found that the maximal power factor can be enhanced significantly from ≈280 μW m−1 K−2 for undoped SnSe to ≈350 μW m−1 K−2 for K‐doped SnSe. It is also observed that the thermal conductivity of polycrystalline SnSe can be enhanced if the SnSe powders are slightly oxidized. Surprisingly, after K doping, the absence of Sn oxides at grain boundaries and the presence of coherent nanoprecipitates in the SnSe matrix contribute to an impressively low lattice thermal conductivity of ≈0.20 W m−1 K−1 at 773 K along the sample section perpendicular to pressing direction of SPS. This extremely low lattice thermal conductivity coupled with the enhanced power factor results in a record high ZT of ≈1.1 at 773 K along this direction in polycrystalline SnSe.

Three‐Stage Inter‐Orthorhombic Evolution and High Thermoelectric Performance in Ag‐Doped Nanolaminar SnSe Polycrystals

“…[14] The above examples clearly illustrate that SnSe polycrystals could be engineered into promising thermoelectric materials by diverse approaches, such as n-type and p-type doping, [12,[14][15][16][17] alloying, [13,14,16] nanostructuring [11,13] and microstructure modulation. [14] The above examples clearly illustrate that SnSe polycrystals could be engineered into promising thermoelectric materials by diverse approaches, such as n-type and p-type doping, [12,[14][15][16][17] alloying, [13,14,16] nanostructuring [11,13] and microstructure modulation.…”

“…[1,7] Recently, SnSe single crystals have produced a surge in the field of thermoelectrics as a new type of promising TE materials because of the intrinsically ultralow thermal conductivity (<0.4 W m −1 K −1 at 923 K) and high ZT along the b-and c-crystallographic directions (>2.3 at 923 K). [10][11][12][13][14][15] For example, Wei et al [12] reported a ZT of ≈0.8 at 800 K in 1% Na-and K-doped SnSe, which was produced by conventional solid state reaction followed by a spark plasma sintering (SPS) treatment. [9] Nevertheless, the difficulties in large-scale synthesis of single crystals limit their practical applications, and extensive efforts have been devoted to the fabrication of high-performance polycrystalline counterparts.…”

“…[8] A new record of ZT dev ≈ 1.34 from 300-773 K along the b-crystallographic direction was achieved in thermoelectric device fabricated from hole-doped SnSe single crystal. [11] Apart from p-type alkali dopants (Li, Na, K), n-type dopants such as I and BiCl 3 are also adopted to improve the performance of polycrystalline SnSe. [10][11][12][13][14][15] For example, Wei et al [12] reported a ZT of ≈0.8 at 800 K in 1% Na-and K-doped SnSe, which was produced by conventional solid state reaction followed by a spark plasma sintering (SPS) treatment.…”

Evidence for hard and soft substructures in thermoelectric SnSe

“…21,23 The measured j lat values have also been reported to be sensitive to oxidation. 24 However, the promise of SnSe as an outstanding thermoelectric material is not in doubt as Na-doped single crystals show greatly improved thermoelectric power factors near room temperature and compete with the best known Bi 2 Te 3 based alloys in terms of their efficiency. 25,26 In this Letter, we report an investigation into the lowtemperature heat capacity of SnSe and combine this with crystal structure data to yield important insights into the thermal behaviour of SnSe.…”

Multi-Layer Metallization Structure Development for Highly Efficient Polycrystalline SnSe Thermoelectric Devices

“…The electrical conductivities of skutterudite and half-Heusler, both well-known intermediate-temperature thermoelectric materials, are above 1000 S/cm in overall temperature ranges [1,14]. The electrical conductivity of SnSe is only 18.1 S/cm, indicating that it is not necessary to have low SCR as in other materials [15]. As a result, several 10 −3 Ω·cm 2 SCR of SnSe thermoelectric material could be approximated to be the same as the 10 −4~1 0 −5 Ω·cm 2 SCR of skutterudite and half-Heusler thermoelectric materials.…”