Bi₂Te₃-based alloys are the most well-known thermoelectric materials for room-temperature applications. The existing Bi₂Te₃-based thermoelectric materials produced commercially are prepared by a zone melting method, resulting in highly oriented materials with high thermoelectric properties. However, the ingots produced by the zone melting method are prone to disintegration due to the weak van der Waals forces between the layers of Te atoms, leading to the poor mechanical properties and the proclivity for fracture during processing and practical applications. It is thus important to improve the mechanical properties and machinability of Bi₂Te₃-based materials with the excellent thermoelectric properties. This work was to prepare a n-type Bi₂Te_2.7Se_0.3 thermoelectric material via spark plasma sintering combined with hot extrusion (SPS+HE). The parameters affacting hot extrusion process and preparation of n-type Bi₂Te_2.7Se_0.3 materials under different process conditions were optimized, and the thermoelectric performance and mechanical properties were investigated. In addition, the machinability of the extruded Bi₂Te_2.7Se_0.3 material was also analyzed.

The commercial zone-melting crystal rods were pulverized and sintered via spark plasma sintering (SPS) at 350 ℃ and 20 MPa for 5 min. The ingot was then put into a hot extrusion mold, and treated at 390-550 ℃, extrusion speed of 0.05-0.50 mm/s, extrusion ratio of 9∶1, and extrusion angle of 30°. Under the determined hot extrusion parameters, n-type Bi₂Te_2.7Se_0.3 materials were prepared by different processes, i.e., the raw materials were ground for 1 h and then sintered by SPS and subsequent hot extrusion (HE); the raw materials were melted at 700 ℃ for 5 h and then cold-pressed (MT), and sintered by subsequent hot extrusion (HE); the raw materials were melted and cold-pressed, followed by annealing and hot extrusion (MT+AN+HE); and the raw materials were ground for 1.5 h and cold-pressed, followed by hot extrusion (BM+HE). The phase structure was analyzed by X-ray diffraction (XRD, D8 Advance, Bruker Co. Ltd., USA) at room temperature. The microstructure and fracture morphology were determined by scanning electron microscopy (SEM, Supra 55, ZEISS Co., Germany). The electrical conductivity and the Seebeck coefficient were measured by ZEM-3 (ULVAC Co. Ltd., Japan). The thermal diffusivity (λ) was measured by a laser flash method (LFA457, Netzsch Co., Germany). The thermal conductivity was calculated according to κ=λρC_p, where C_p was the heat capacity according to the Neumann-Kopp rule, and the density ρ was measured based on the Archimedes prinicple.

The results show that n-type Bi₂Te_2.7Se_0.3 thermoelectric materials can be prepared under optimum process conditions (i.e., extrusion temperature of 400 ℃ and extrusion speed of 0.05 mm/s). The XRD patterns of all the hot-extruded samples agree well with a rhombohedral structure (JCPDS 50-0954) of Bi₂Te_2.7Se_0.3. No impurity peaks appear. The SEM images reveal that Bi, Te, and Se are homogeneously distributed in the materials without any impurity phases. The electrical conductivity σ of all the samples decreases with increasing temperature, having a metallic conduction behavior. The σ of the MT+AN+HE sample is the maximum value of 1.4×10⁵ S/m. All the samples have negative Seebeck coefficients S, indicating that electrons are the major charge carriers. The S of the SPS+HE sample is the maximum value of 191.0 μV/K. At 300 K, the SPS+HE sample has the maximum power factor PF of 37.0 μW/(cm·K²). However, the PF of the material gradually decreases with the increase of temperature, which is consistent with the change of electrical conductivity and Seebeck coefficient due to the intrinsic excitation. The lattice thermal conductivity of MT+HE sample is the minimum value (i.e., 0.9 W·m^-1·K^-1). The maximum zT value reaches 0.83 at 400 K for SPS+HE sample and its Vickers hardness is 602.5 N/mm², which is twice greater than that of the zone-melting sample. This is mainly due to the dynamic recrystallization process of grain refinement during the hot extrusion process. The machinability of the extruded Bi₂Te_2.7Se_0.3 material are improved. The thermoelectric micron-sized particles with a minimum size of 147 μm can be achieved, providing a promising material for the development of Bi₂Te₃-based thermoelectric micro-devices.

The optimal hot extrusion process parameters for n-type Bi₂Te_2.7Se_0.3 thermoelectric materials were obtained (i.e., extrusion ratio 9∶1, extrusion angle 30°, extrusion temperature 400 K and extrusion speed 0.05 mm/s). A series of n-type Bi₂Te_2.7Se_0.3 thermoelectric material samples were prepared via hot extrusion under the optimum process conditions. The Bi₂Te_2.7Se_0.3 sample prepared by SPS and sebsequent hot extrusion (SPS+HE) had the maximum zT value 0.83 at 400 K, and its mechanical properties were improved. The Vickers hardness of the SPS+HE sample could reach 602.5 N/mm², which was twice greater than that of the zone-melting Bi₂Te_2.7Se_0.3 material. In addition, the machining performance was also improved, and the thermoelectric micron-sized particles with the minimum size of 147 μm could be machined for the extruded Bi₂Te_2.7Se_0.3 sample, providing a promising material for the Bi₂Te₃-based thermoelectric micro-devices.