Structural and Thermoelectric Properties of Bi85Sb15 Prepared by Non-equal Channel Angular Extrusion

We report on the mechanical and transport properties of polycrystalline bulk Bi85Sb15, as a low-temperature thermoelectric material. Bi85Sb15 samples were prepared by mechanical alloying and hot isostatic pressing, followed by sever plastic deformation (SPD). SPD was applied by either equal channel angular extrusion (ECAE) or non-equal channel angular extrusion (NECAE), at two different temperatures (373 K and 423 K). X-ray diffraction and scanning electron microscopy were used to characterize the prepared samples. The transport properties including the electrical conductivity, Seebeck coefficient and thermal conductivity were investigated, and correlated with the microstructure over the temperature range of 160–360 K. NECAE was found to be more effective than ECAE in enhancing bulk density, grain refinement and preferential grain orientation along the extrusion direction, particularly at higher processing temperatures. This is attributed to the better grain alignment and the creation of more intense grain boundaries and dislocation density, which resulted in an enhancement in carrier mobility and phonon scattering and hence a higher Z value. The highest Z value was achieved via NECAE at 423 K, and had a value of 0.39 × 10−3 K−1 at 250 K, which is 55% higher than that of the hot-pressed sample, 0.22 × 10−3 K−1 at 270 K. Also, the micro-hardness of the hot-pressed sample increases by at least 20% by SPD processes. Accordingly, optimized SPD can be classified as an effective processing tool for feasible mass production of bulk Bi85Sb15 alloy with better thermoelectric performance.