Highly anisotropic thermoelectric transport properties responsible for enhanced thermoelectric performance in the hot-deformed tetradymite Bi2Te2S

Abstract In general, Bi 2 Te 3 and related alloys show the best thermoelectric performance at near-room temperature region below about 423 K, which is in some cases not high enough for waste heat recovery. In order to raise the operation temperature on the basis of Bi 2 Te 3 technology, S element was introduced into Bi 2 Te 3 to synthesize a ternary tetradymite compound, Bi 2 Te 2 S, and the thermoelectric properties were analyzed. In this study, Bi 2 S 3 was considered, instead of using elemental S, to avoid the issues arising from the volatility of S. Compared with Bi 2 Te 3 , undoped Bi 2 Te 2 S shows higher Seebeck coefficients ( S max  = −210.9 μV K −1 at 523 K) but also higher resistivity ( ρ min  = 26.5 μΩ m at 323 K), resulting in decreased power factor ( PF max  = 1.49 mW K −2 m −1  at 373 K). However, low thermal conductivity ( κ min  = 0.987 WK −1 m −1  at 473 K) produced higher ZT at T  ≥ 423 K than Bi 2 Te 3 , reaching about 0.66 at 473 K. At the same time, the bipolar conduction was reduced due to the enlarged band gap of about 0.22 eV, which was estimated from the S - T curve. After additional hot deformation at 793 K, undoped Bi 2 Te 2 S shows remarkably improved values of power factor ( PF max  = 2.62 mW K −2 m −1  at 323 K) and ZT ( ZT max  = 0.86 at 473 K), which was ascribed to increased carrier concentration. In addition, the highly anisotropic transport properties of Bi 2 Te 2 S were suggested to be another origin, which was revealed by the first-principles calculations. In summary, Bi 2 Te 2 S is a promising candidate for thermoelectric generation at the intermediate temperature region 473–573 K where Bi 2 Te 3 shows degraded performances.