Preparation of vertical corner extrusion and thermoelectric properties of p-type Bi<sub>2</sub>Te<sub>3</sub> based materials

The preparation technology of powder metallurgy is an important way to prepare Bi<sub>2</sub>Te<sub>3</sub>-based bulk materials with excellent mechanical properties and thermoelectric properties. However, the loss of sample orientation during the preparation of powder metallurgy results in low thermoelectric properties of the materials. The development of high-performance Bi<sub>2</sub>Te<sub>3</sub>-based thermoelectric materials with strong plate texture and fine grains is the focus of research on high-performance Bi<sub>2</sub>Te<sub>3</sub>-based thermoelectric materials. In this paper, a series of p-type Bi<sub>2</sub>Te<sub>3</sub>-based materials is prepared by vertical corner extrusion preparation technology. The influences of extrusion temperature on the microstructure and texture characteristics of the material and its influence on the thermoelectric properties of the material are systematically studied. In the vertical corner extrusion process, grains preferentially grow along the minimum resistance direction perpendicular to the pressure, that is, along the extrusion direction, thereby further enhancing the (00<i>l</i>) texture of the original hot-pressed sample; in the direction parallel to the pressure, due to friction with the inner wall of the die in the extrusion process, this frictional resistance will promote the inversion of the grains, so that the grains are arranged in a directional manner to reduce the frictional resistance, thus forming the (110) texture, which is not present in the original hot-pressed sample, in the extruded sample, and finally completing the transition from the hot-pressed sample to the plate texture of the extruded sample. When the extrusion temperature is low, the atomic diffusion rate is low, which limits the dynamic recrystallization of the grain, the grain growth process, and the grain deflection speed. With the increase of the extrusion temperature, these processes can be carried out rapidly, thus forming a more obvious plate texture characteristic. The 773 K extruded sample achieves high orientation factors of <i>F</i><sub>(00<i>l</i>)</sub> = 0.51 and <i>F</i><sub>(110)</sub> = 0.30 in the directions perpendicular to the pressure and parallel to the pressure, respectively, and the carrier mobility is as high as 345.4 cm<sup>2</sup>·V<sup>–1</sup>·s<sup>–1</sup> at room temperature, which is comparable to the carrier mobility of the zone melt sample, showing excellent electrical transport performance. The power factor reaches 4.43 mW·m<sup>–1</sup>·K<sup>–2</sup> at room temperature. At the same time, the sum of lattice thermal conductivity and bipolar thermal conductivity of the 773 K extruded sample decreases to a minimum value of 0.78 W·m<sup>–1</sup>·K<sup>–1</sup> at 323 K. Finally, the 773 K extruded sample obtains a maximum <i>ZT</i> value of 1.13 at 323 K, which is nearly 70% higher than that of the hot-pressed sample. This research provides a new way for preparing high-performance strong plate textures and fine-grained Bi<sub>2</sub>Te<sub>3</sub>-based thermoelectric materials, and lays an important foundation for fabricating micro thermoelectric devices.