Electronic Transport Properties of Bi₂Te_2.7Se_0.3 Fabricated by Hot Extrusion

Abstract

Herein we report the optimized processing conditions of hot extrusion for fabricating an <i>n</i>-type Bi<sub>2</sub>Te<sub>2.7</sub>Se<sub>0.3</sub> thermoelectric compound, with high electronic transport properties as well as improved mechanical reliability. We fabricated a Bi<sub>2</sub>Te<sub>2.7</sub>Se<sub>0.3</sub> extrudate that was 3.8 mm in diameter and 700 mm in length by controlling the processing parameters of temperature and pressure. A 3-point bending strength of over 70 MPa, which is 7 times higher that of the commercial zone melting ingot, was obtained in the samples prepared at 460 <sup>o</sup>C temperature under 6–6.5 MPa pressure. The samples benefitted from the formation of a highly-dense microstructure (relative density > 98%). It is noted that the electronic transport properties (electrical conductivity and Seebeck coefficient) could be manipulated by controlling the applied pressure of hot extrusion at 460 <sup>o</sup>C, mainly due to the change in the characteristics of the 00<i>l</i> crystal orientation, which originated from grain rotation and rearrangement. Power factor values of ~2.9 mW/mK<sup>2</sup> at 300 K and ~2.95 mW/mK<sup>2</sup> at 320 K, similar to those of sintered bulks, were obtained in the hot extrudate fabricated under processing parameters of 460 <sup>o</sup>C and 6 MPa. Moreover, a high power factor value of 2.25 mW/mK<sup>2</sup> was observed even at the high temperature of 480 K, which is 70% higher than that of Bi<sub>2</sub>Te<sub>2.7</sub>Se<sub>0.3</sub> bulk fabricated by hot pressing.