Thermoelectric properties of Bi₂Te₃-related materials finely grained by mechanical alloying and high pressure torsion

Bi₂Te₃-related thermoelectric semiconductors were prepared by the mechanical alloying (MA) and the vertical Bridgman (VBM)-High Pressure Torsion (HPT) methods. Structures and electric properties of these alloys were then investigated. These samples had a nominal composition of Bi_0.5Sb_1.5Te_3.0 with 0.07mass% excess Te. The MA sample was sintered into a compact after mechanical alloying while the VBM-HPT sample was deformed by HPT after being cut into a disk from a melt-grown ingot by the vertical Bridgman method (VBM). The orientation factors of the MA and VBM-HPT samples were 0.054 and 0.525, respectively, indicating that the preferred orientation was formed by VBM-HPT. Average grain sizes of both samples were approximately 2μm. The VBM-HPT sample was shown low electrical conductivity than the MA one in spite of the high orientation factor. By estimating the slope of carrier mobility versus temperature the carrier scattering factor of the MA and VBM-HPT samples were estimated to be-1.3 and +0.1. These values are indicative of carrier scatterings by acoustic phonons and by the interaction between acoustic and optical phonons, respectively. The maximum power factor obtained for the isotropic MA sample was 4.1 x 10 ^-3 W m^-1 K^-2 at 310 K and this was about twice the 2.1 x 10^-3 Wm^-1K^-2 at 440K obtained for the anisotropic VBM-HPT sample.