Charge transport anisotropy in hot extruded bismuth telluride: Scattering by acoustic phonons

A model of the thermoelectric transport properties of hot extruded bulk polycrystalline (HEBP) n-type Bi2Te3 has been developed based on the marked anisotropy of its properties induced by extrusion. HEBP Bi2Te3 is a highly textured material that allows treating electrical and thermal transport in the plane perpendicular to the extrusion axis as approximately isotropic. The model of this transport is based on the simplified electronic density of states suggested by ab initio calculations of the band structure of crystalline Bi2Te3 near the band edges. The proposed model can also be applied to transport along the extrusion axis in a temperature range when there is only one limiting charge carrier scattering mechanism. It provides excellent accord to the Hall effect (perpendicular) and the Harman (along extrusion) measurement of mobility, and the Seebeck coefficient of HEBP n-type Bi2Te3 in the temperature range from ∼200 to 460 K. It can be concluded that acoustic phonon scattering provides the limiting scattering mechanism for this temperature range. The model is also applied to elucidate the important role of thermodiffusion, suggesting that the usual approach to experimentally obtain this contribution to the thermal conductivity can only give approximate rough estimates.