Microstructure And Thermoelectric Properties Of The Se-Doping Bi₂Te₃-In₂Te₃ Alloys Via Zone Melting Directional Solidification

Bi₂Te₃ systems are one of the promising thermoelectric materials at room temperature region, which potentially can be used in thermoelectric refrigeration and electric generation driven by temperature difference. Developing Bi₂Te₃ based alloys with high ZT value is one of the most important topics in the thermoelectric field. In this work, a new quaternary Bi-In-Te-Se eutectic alloy system was developed, and the coupled growth of both Bi₂Te₃ and In2Te3 phases and thus resulted thermoelectric properties were further investigated for the quaternary eutectic alloy by zone melting directional solidification（ZMDS）.The eutectic composition of ternary Bi-In-Te changes with the addition of Se. Two quaternary eutectic alloys of Bi-In-Te-Se were developed by the modulation of the content of each element, which are Bi_22.5In_17.5Te_58.5Se_1.5 and Bi25In15Te54Se6. ZMDS was conducted for the Bi_22.5In_17.5Te_58.5Se_1.5 at 3μm/s, 5μm/s, 10μm/s and 20μm/s. It shows that the eutectic morphology changes from lamellae to rods with Se-doping in Bi-In-Te alloy. At relatively low growth rates such as 3μm/s and 5μm/s, a well-aligned coupled growth structure was observed. However, at growth rates higher than 10μm/s, some misoriented grains appeared which deviated from the growth direction of ZMDS. Especially at 20μm/s, a primary Bi₂Te₃ was found which deviated from the fully eutectic solidification. Overall, a low growth rate is central to the regular coupled growth of Bi₂Te₃ and In2Te3 phases. Correspondingly, Bi25In15Te54Se6 was directionally solidified at 3μm/s and a coupled growth structure was also found.The thermoelectric transportation properties were measured for the eutectic Bi-In-Te-Se alloys after ZMDS. It shows that the Se-doping alloys exhibit extremely low lattice thermal conductivity. This can be attributed to the largely introduced interfaces between Bi₂Te₃ and In2Te3 phases which significantly enhance the scattering of phonics. Additionally, a significantly improved ZT value was found in ZMDS Bi_22.5In_17.5Te_58.5Se_1.5, whose average ZT is 60% magnitude larger than that of eutectic Bi23In17Te60. However, a further increase of the doping Se to 6at.% results in an obvious reduction of ZT value.