Anisotropic thermoelectric transport properties of Bi0.5Sb1.5Te2.96+x zone melted ingots

In this study, p-type Bi0.5Sb1.5Te3 ingots were fabricated with a wide range of carrier concentrations through zone melting method. The anisotropy of thermoelectric properties was systematically investigated. The layered structure with weak van der Waals bonding, which selectively scatters charge carrier and phonon, produces the anisotropic thermoelectric properties. The intrinsic excitation generates electron-hole pairs with highly anisotropic transport behavior, resulting in a remarkable anisotropic Seebeck coefficient and bipolar thermal conductivity. The out-of-plane Seebeck coefficient is remarkably higher than that in-plane Seebeck coefficient in the intrinsic excitation regime. Doping with Te decreases the extrinsic carrier concentration of BiSbTe alloy, strengthening the intrinsic excitation and the contribution of the bipolar thermal conductivity. Optimization of carrier concentration via adjusting Te content significantly improves the thermoelectric performance. The maximum power factor of 5.2 ​mW ​m−1 ​K−2 is achieved for the Bi0.5Sb1.5Te3.08 sample at room temperature along the in-plane direction. A high ZT value of 1.06 ​at 375 ​K is attained for the Bi0.5Sb1.5Te3.04 sample along the in-plane direction, while the maximum out-of-plane ZT value is achieved as much as 0.94 ​at 450 ​K for Bi0.5Sb1.5Te3.02 sample. The highest in-plane ZT value of 1.06 is obtained at 375 ​K for Bi0.5Sb1.5Te3.04 sample, while the highest out-of-plane ZT value of 0.94 is obtained at 450 ​K for Bi0.5Sb1.5Te3.02 sample. [Display omitted] •p-Bi0.5Sb1.5Te2.96+x zone melted ingots with high quality and almost full texture were obtained.•Systematical investigation of anisotropic TE properties of these samples is carried out.•The mechanism of the anisotropic properties of these samples is revealed.•The intrinsic excitation enhances anisotropy in Seebeck coefficient and Bipolar thermal conductivity.