Attaining ultrahigh thermoelectric performance of direction-solidified bulk n-type Bi2Te2.4Se0.6 via its liquid state treatment

Bi2Te3-based alloys are the best thermoelectric materials near room temperature and Bi2(TeSe)3 alloys are their typical n-type ones. However, the dimensionless figure of merit ZT of the bulk Bi2(TeSe)3 is normally less than 1.0 despite various delicate and time-consuming preparations associated with complex post-treatments. Here, via liquid state manipulation (LSM), we obtained a high ZT for bulk n-type Bi2Te2.4Se0.6 ingot direction-solidified with high temperature-gradient at a suitable growth rate. With the maximum ZT = 1.22 at 477 K, its average value (1.09) between 300 and 575 K is the highest among those of all the bulk Bi2(TeSe)3 fabricated so far. Specifically, with proper structure texturing, LSM obviously raises the Seebeck coefficient due to the increased effective mass, with almost unaltered electrical conductivity. Moreover, as growth rate decreases, the enhanced texturing boosts the electrical conductivity, but raises the thermal conductivity along the growth direction. More significantly, at the same direction growth condition, LSM can increase nanoscale particles, dislocations and atomic-scale lattice distortions, resulting in substantially reduced lattice thermal conductivity. Consequently, via LSM and suitable texturing, the bulk n-type Bi2Te2.4Se0.6 with outstanding ZT is directly obtained without complex post-treatments. This work provides a novel strategy for optimizing performance of thermoelectric materials with the merit of easy large-scale fabrication.