Energy Conversion Enhancement of Tin Selenide Thermoelectric Materials Using Graphene

Tin selenide-based thermoelectric materials are gaining significant importance in moderate- to high-temperature thermoelectric applications because of their single crystals outstanding high ZT of 2.6 along a specific crystallographic direction. Therefore, much attention has been paid to the optimization of polycrystalline tin selenide (SnSe) to improve its thermoelectric performance as they exhibit better mechanical properties and easily controllable production. In this study, we evaluate the thermoelectric performance of polycrystalline SnSe-based compound by using a combination of nano-structuring, graphene (GNP) nano-compositing, and germanium (Ge) doping/alloying. The impact of several factors such as the SnSe matrix milling duration, GNP milling duration, GNP concentration, Ge concentration, and the combination of Ge doping with GNP compositing on the thermoelectric behavior, mechanical behavior, and thermal stability of the compounds were studied. The results indicate the successful production of polycrystalline nano-bulk SnSe based compounds using high-energy ball milling followed by hot compaction. Graphene significantly enhanced the thermoelectric performance of SnSe. In addition, graphene improved the materials’ reliability by enhancing the consolidation process, mechanical hardness, thermal stability, isotropy, and oxidation behavior. However, it is important to optimize the duration of graphene milling and graphene concentration to realize maximum enhancements. An enhancement in power factor ratio of 18 % has been achieved for 0.05 wt.% graphene milled for 10 minutes in comparison to that of the pristine SnSe sample at 773 K. However, 315 % enhancement in ZT has been achieved for 0.1 wt.% graphene milled for 10 minutes at 573 K. In addition, hardness was increased by 11 % for 1 wt.% GNP SnSe composite. On the other hand, the result showed that Ge doping alone failed to contribute significantly to enhance the thermoelectric performance of SnSe. The poor electrical behavior of the Ge doped samples was significantly enhanced by the incorporation of graphene which resulted in an overall power factor enhancement of 16 % for Sn_0.975Ge_0.025Se/0.05 GNP at 773 K and an overall 163 % enhancement in ZT at 573 K for Sn_0.95Ge_0.05Se/0.05 GNP. These results clarify the importance of graphene in enhancing the thermoelectric performance and confirm the feasibility of using graphene as a nanofiller for enhancing the reliability of hot compacted polycrystalline SnSe-based materials.