Mechanical and thermoelectric properties of Bi_2−xSb_xTe₃ prepared by using encapsulated melting and hot pressing

Abstract

Bi_2-xSb_xTe₃ (x = 1.4 - 1.7) solid solutions were synthesized by using encapsulated melting (EM) and were consolidated by using hot pressing (HP). The lattice constants decreased with increasing Sb content, which indicated that solid solutions were successfully synthesized. The relative densities of all the hot-pressed specimens were higher than 97.9 %. X-ray diffraction patterns, pole figures and electron backscattered diffraction spectra of all the hot-pressed specimens indicated randomly oriented textures. Very low values of the orientation factor (F) were obtained, F_min = 0.008 for Bi_0.4Sb_1.6Te₃ hot-pressed at 673 K and F_max = 0.115 for Bi_0.4Sb_1.6Te₃ hot-pressed at 723 K, which implied that the microstructures were highly isotropic. A bending strength of 46 MPa and a Vickers hardness of 94 Hv were attained for Bi_0.4Sb_1.6Te₃ hot-pressed at 648 K. However, the mechanical properties were degraded with increasing HP temperature owing to grain growth. An increased HP temperature did not affect the electrical properties significanctly. The carrier concentration increased with increasing Sb content, and the specimen with x = 1.4 showed nondegenerate semiconductor behavior whereas those with x ≥ 1.5 behaved as degenerate semiconductors. All specimens showed p-type conduction, which was confirmed from the positive signs of the Seebeck coefficient and the Hall coefficient. The increased Sb content caused a shift in the peak values of the Seebeck coefficient to higher temperatures and an enhancement of the power factor. The electronic thermal conductivity decreased, but the lattice thermal conductivity increased, with increasing Sb content. The lowest theraml conductivity of 0.83 Wm ^-1K ^-1 was obtained at 373 K for Bi_0.4Sb_1.6Te₃. The maximum dimensionless figure of merit, ZT _max = 1.1, was achieved at 323 K for Bi_0.4Sb_1.6Te₃.