Effects of low rotational speed on crystal orientation of Bi ₂Te ₃-based thermoelectric semiconductors deformed by high-pressure torsion

Bi ₂Te ₃-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 Bi _0.5Sb _1.5Te _3.0 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 473K 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 × 10 ^-3W ^-1K ^-2 at 363 K, which was larger than the reported power factors of 4.3 × 10 ^-3Wm ^-1K ^-2 for a disk using 5 turns and a speed of 1 rpm, and 4.6 × 10 ^-3Wm ^-1K ^-2 for melt-grown materials. Slow deformation by HPT was found to enhance the electrical conductivities and Seebeck coefficients of Bi ₂Te ₃-based thermoelectric semiconductors by producing a preferred orientation and grain refinement.