Optimization of sodium hydroxide for securing high thermoelectric performance in polycrystalline Sn_{1 − x}Se via anisotropy and vacancy synergy

Abstract

The morphology and composition are two key factors to determine the thermoelectric performance of aqueously synthesized tin selenide (SnSe) crystals; however, their controlling is still under exploring. In this study, we report a high figure‐of‐merit (ZT) of ∼1.5 at 823 K in p‐type polycrystalline Sn1 − xSe resulted from a synergy of morphology control and vacancy optimization, realized by carefully tuning the sodium hydroxide (NaOH) concentration during solvothermal synthesis. After a comprehensive investigation on various NaOH concentrations, it was found that an optimized NaOH amount of 10 mL with a concentration of 10 mol L−1 can simultaneously achieve a large average crystal size and a high Sn vacancy concentration of ∼2.5%. The large microplate‐like crystals lead to a considerable anisotropy in the sintered pellets, and the high Sn vacancy level contributes to an optimum hole concentration to the level of ∼2.3 × 1019 cm−3, and in turn a high power factor of ∼7.4 μW cm−1 K−2 at 823 K, measured along the direction perpendicular to the sintering pressure. In addition, a low thermal conductivity of ∼0.41 W m−1 K−1 is achieved by effective phonon scattering at localized crystal imperfections including lattice distortions, grain boundaries, and vacancy domains, as observed by detailed structural characterizations. Furthermore, a competitive compressive strength of ∼52.1 MPa can be achieved along the direction of high thermoelectric performance, indicating a mechanically robust feature. This study provides a new avenue in achieving high thermoelectric performance in SnSe‐based thermoelectric materials.

High‐Performance Thermoelectric SnSe: Aqueous Synthesis, Innovations, and Challenges

“…Reproduced with permission. [170] Copyright 2019, Wiley. h) HRTEM image and inset SAED pattern of SnSe pellets viewed along the c-axis.…”

“…However, solvothermal is a good choice to meet the goal of simultaneously improving the thermoelectric and mechanical properties. It was reported that at a strain rate of 2.5 × 10 −4 s −1 , competitive compressive strength of ≈52.1 and ≈77.0 MPa can be achieved along the ⊥ and // directions for solvothermally synthesized Sn 1− x Se ( x = 0.025),170 respectively, and both of which are very competitive with reported record value of 74.4 MPa achieved by a combustion method 95. This value is also comparable to the other commercial thermoelectric materials, such as PbTe and Bi 2 Te 3 312,313.…”

“…g) SEM image of a cracked pellet by grinding. Reproduced with permission 170. Copyright 2019, Wiley.…”

“…A summary of ZTs for SnSe‐based thermoelectric materials. a) The timeline for state‐of‐the‐art SnSe bulks thermoelectric materials,11–124,169–182 the performance achieved by solution route are circled by yellow. b) Temperature‐dependent ZT and c) corresponding peak and average ZT values for polycrystalline SnSe through different fabrication techniques 13,16,22,46,58,62,95,99,101.…”

High‐Performance GeTe‐Based Thermoelectrics: from Materials to Devices

“…[ 26–30 ] High‐performance thermoelectric materials require high S , high σ, and low κ. [ 24,31–34 ] However, due to the strong coupling between these parameters, the optimization of thermal/electrical transport properties is the key for ZT enhancement. [ 35–40 ] The performance of thermoelectric materials can be enhanced from the two aspects: 1) optimization of electrical transportation performance and 2) minimization of thermal transport properties.…”