Thermoelectric and mechanical properties of Bi0.4Sb1.6Te3 prepared by using encapsulated melting and hot extrusion

Abstract

Bi0.4Sb1.6Te3 solid solutions were synthesized by using encapsulated melting (EM) and were consolidated by using hot extrusion (HE), and their thermoelectric and mechanical properties were examined and compared with those of specimens prepared by using hot pressing (HP). The relative densities of all hot-extruded specimens were higher than 97%. The basal plane (00l) was preferentially orientated parallel, rather than perpendicular, to the HE direction. The values of the orientation factor (F) were as follows: F max = 0.082 for specimens hot-extruded at 723 K and F max = 0.115 for specimens hot-pressed at 723 K. Thus, the crystallographic anisotropy was not high. From the pole figure and the electron backscatter diffraction analyses, the microstructures and the crystallinities of the HE specimens were found to be similar and comparable to those of the HP specimens. A bending strength of 57 MPa and a Vickers hardness of 74 Hv were obtained for Bi0.4Sb1.6Te3 hot-extruded at 698 K, which were superior to the corresponding properties of the HP specimens. However, the mechanical properties degraded with increasing HE temperature owing to the occurrence of grain growth. All specimens exhibited a negative temperature dependence for the electrical conductivity, indicating a degenerate semiconductor behavior. All Seebeck coefficients showed positive signs (p-type conduction); they attained peak values at 373 K and then decreased with increasing temperature owing to intrinsic conduction. The HE specimens exhibited lower values of the power factor than the HP specimens did because of the lower electrical conductivity and Seebeck coefficient of the former. The thermal conductivity was lower than 1.0 Wm−1K−1 at room temperature, and it increased slightly with increasing temperature. The maximum figure of merit, ZT max = 1.04, and the average figure of merit, ZT ave = 0.87, were achieved at 323 K for Bi0.4Sb1.6Te3 hot-extruded at 698 K.