In recent years, the thermoelectric properties of SnSe-based bulk materials have received much attention because of their ultra low thermal conductivity. Zhao et al. reported that the thermal conductivity of SnSe single crystal at 923 K is 0.23 W/mK, and the corresponding ZT is 2.6 (Nature. 2014, 508:373). By further studying, it is revealed that its low thermal conductivity is mainly induced by their non-harmonic bonds. Inspired by this fact, we speculate that SnSe2-based materials should also have low thermal conductivity because of the similar Sn-Se bond and weak van der Waals force between Se-Sn-Se layers. In our case, the point defect was regulated by annealing nanopowders prepared by a wet-chemical method to improve electrical conductivity as well as power factor. By further Te doping, we found that lower thermal conductivity can be obtained, leading to improved thermoelectric properties.
Owing to the fact that Se element would sublimate at high temperature, it is difficult to synthesize pure SnSe2 samples by means of solid state reaction. Here, polycrystalline SnSe2 nanopowders were successfully synthesized by a hydrothermal reaction at 200 oC. Subsequently, the samples were heat treated at 400 oC to regulate the point defects. Finally, the polycrystalline bulk materials were obtained by a plasma spark sintering process. their structure, composition and morphology were characterized by X-ray diffraction (XRD), X-ray fluorescence (XRF) and scanning electron microscopy (SEM), respectively. Their electrical conductivity and Seebeck coefficient were measured by ZEM-3 (ULVAC) and CTA-3 (CRYALL). The thermal diffusion coefficient and specific heat were characterized by LFA-457 and DSC-404 (NETZSCH), respectively.
The XRD results indicates that the product of the hydrothermal reaction are mainly SnSe2 with a little amount of Sn impurity. After heat treating, the impurity is successfully removed. Meanwhile, the XRF characterization presents that the Se atomic percent decreases with the increasing of temperature, suggesting point defect generated during heat treating. Furthermore, SEM observation shows that the morphology on fracture surface is anisotropic, depending on the pressing direction. As a result, the electric conductivity can be optimized to 103 S/m at 673.15K while the Seebeck coefficient is -400*10-6 V/K. By Te-doping, the power factor nearly retain unchanged (1.7*10-4 W/mK2 at 673.15 K), but the thermal conductivity decreases from 0.8 W/mK to 0.64 W/mK.
The above result reveals that SnSe2 is a promising thermoelectric materials at middle temperature range. Our work may also pave a way for looking and developing thermoelectric materials with low thermal conductivity.