Effects of Low Rotational Speed on Crystal Orientation of Bi2Te3-Based Thermoelectric Semiconductors Deformed by High-Pressure Torsion

Bi2Te3-based thermoelectric semiconductors were deformed by high-pressure torsion (HPT) using a low rotational speed of 0.1 rpm, which is less than the speed of 1 rpm used in our previous studies. The effects of different rotational speeds were investigated by metallographic and thermoelectric studies. Sample disks of p-type Bi0.5Sb1.5Te3.0 were cut from sintered compacts made by mechanical alloying (MA) followed by hot-pressing. The disks were deformed by HPT with 1, 3, and 5 turns at 473 K under 6.0 GPa of pressure at a rotational speed of 0.1 rpm. The preferred orientation was investigated using X-ray diffraction. The orientation factors of the disks changed from 0.054 for pre-rotation up to 0.653 for post-rotation samples. The maximum power factor of the disk using 5 turns and a speed of 0.1 rpm was 6.6 x 10(-3) W m(-1) K-2 at 363 K, which was larger than the reported power factors of 4.3 x 10(-3) W m(-1) K-2 for a disk using 5 turns and a speed of 1 rpm, and 4.6 x 10(-3) W m(-1) K-2 for melt-grown materials. Slow deformation by HPT was found to enhance the electrical conductivities and Seebeck coefficients of Bi2Te3-based thermoelectric semiconductors by producing a preferred orientation and grain refinement. [doi:10.2320/matertrans.ME201101]