Fabrication of Bi<sub>2</sub>Te<sub>3</sub>-based Bulk Thermoelectric Materials by a Powder Extrusion Technique

From the viewpoints of grain refinement and preferred orientation, a mechanical alloying (MA) and hot-extrusion technique has been proposed to fabricate Bi2Te3-based bulk materials. In this paper, the MA and hot-extrusion process, microstructure and texture, thermoelectric and mechanical properties of extruded samples, and the effect of Cu-doping in n-type samples have been reviewed. The extruded samples exhibited fine-grained microstructure, and their basal planes were predominantly oriented parallel to the extrusion direction. The p-type Bi0.4Sb1.6Te3 samples showed significant enhancements in both thermoelectric and mechanical properties. For example, the sample extruded at 400°C had a maximum value of ZT = 1.2 at room temperature and a much higher hardness than unidirectionally solidified samples. However, the n-type Bi2Te2.85Se0.15 samples showed lower thermoelectric properties due to their higher carrier concentrations and smaller fractions of twin boundaries. Cu-doping in n-type samples resulted in a reduction in carrier concentration, and thus increases in related Seebeck coefficient and electrical resistivity and decrease in carrier thermal conductivity. The resulting ZT value of 0.86 for Cu0.05Bi2Te2.85Se0.15 sample was obtained.