Probing the thermoelectric transport properties of n-type Bi ₂ Te ₃ close to the limit of constitutional undercooling

Abstract

Bulk n-type Bi 2 Te 3 single crystals with optimized chemical composition were successfully prepared by a high temperature-gradient directional solidification method. We investigate the influence of alloy microstructure, chemical composition, and growth orientation on the thermoelectric transport properties. The results show that the composition of single-crystal Bi 2 Te 3 alloy, along the c axis direction, could be slightly tuned by changing the growth rate of the crystal. At a rate of 18 mm/h, the formed Bi 2 Te 3 crystal exhibits good thermoelectric properties. At 300 K, a maximum Seebeck coefficient of −245 µV/K and an electrical conductivity of 5.6 × 10 4 S/m are acquired. The optimal power factor is obtained as 3.3 × 10 −3 W/K 2 m, with a figure of merit of 0.74. It can be attributed to the increased tellurium allocation in the Bi 2 Te 3 alloys, as verified well by the density functional theory calculations.

Nanostructured State-of-the-Art Thermoelectric Materials Prepared by Straight-Forward Arc-Melting Method

“…The number of thermally excited charge carriers increases with temperature, as shown by Hall concentration measurements. Charge carriers' density at 310 K is equal to 7.46 × 10 18 cm −3 , similar to other literature values [35]. Hall mobility of charge carriers is calculated by μ H = R H σ, resulting in 4514 cm 2 V −1 s −1 , which is an extremely high value due to low resistivity.…”

“…There is small improvement in thermopower regarding pure Bi 2 Te 3 samples, but it is still low in contrast to other Bi 2 (Te 1−x Se x ) 3 alloys, where values of −190 μV K −1 are reported [46] or even −259 μV K −1 at room temperature in samples with optimized composition [47]. Hall concentration of charge carriers is determined as 3.1 × 10 19 cm −3 at 300 K (inset in Figure 13a), which is somewhat higher than in pure Bi 2 Te 3 [20,35], as a result of donor feature of Bi 2 (Te 1−x Se x ) 3 alloys.…”

“…In bismuth telluride samples, charge carriers concentration is strongly affected by antisite Bi Te and Te Bi defects, which are randomly created during synthesis process. This feature yields a wide range of Seebeck coefficient values measured in samples prepared by different methods [15,[34][35][36][37][38][39].…”

Influence of Doping and Nanostructuration on n-Type Bi2(Te0.8Se0.2)3 Alloys Synthesized by Arc Melting

“…These results were checked in numerous samples. In comparison, the parent compound Bi 2 Te 3 shows similar n-type semimetallic behavior, with reported values in the range of −50 to −260 μV K −1 for samples prepared by different chemical and physical methods [15, 42, 45]. In particular, arc melting yields samples with around −50 μV/K [36].…”

“…4a) increases with increasing temperature due to thermal excitation, typical of semiconducting behavior. We find the charge carrier density at 300 K to be −3.1∙10 19  cm −3 , slightly higher than pure Bi 2 Te 3 [36, 42]. The mobility is quite low at 300 K, determined using μ H  =  R H  *  σ , which results in 10.1 cm 2  V −1 s −1 [35, 41].…”