Experimental studies on anisotropic thermoelectric properties and structures of n-type Bi₂Te_2.7Se_0.3

The peak dimensionless thermoelectric figure-of-merit (ZT) of Bi ₂Te₃-based n-type single crystals is about 0.85 in the ab plane at room temperature, which has not been improved over the last 50 years due to the high thermal conductivity of 1.65 W m^-1 K^-1 even though the power factor is 47 × 10^-4 W m^-1 K^-2. In samples with random grain orientations, we found that the thermal conductivity can be decreased by making grain size smaller through ball milling and hot pressing, but the power factor decreased with a similar percentage, resulting in no gain in ZT. Reorienting the ab planes of the small crystals by repressing the as-pressed samples enhanced the peak ZT from 0.85 to 1.04 at about 125 °C, a 22% improvement, mainly due to the more increase on power factor than on thermal conductivity. Further improvement is expected when the ab plane of most of the small crystals is reoriented to the direction perpendicular to the press direction and grains are made even smaller.