Improved power factor of Bi_0.4Sb_1.6Te₃ - Based samples prepared by cold pressing and sintering

Anisotropic thermoelectrical Bi_0.4Sb_1.6Te₃ samples were prepared by powder metallurgy procedures. A strong correlation was observed between the morphological anisotropy of the powder particles and the crystal anisotropy of the compound. By using this correlation and taking a particle size range in which the morphological anisotropy was dominant, the thermoelectrical anisotropy was maximized. This desirable particle size was determined by SEM and XRD measurements, while sample anisotropy was determined by XRD measurements on sample facets that had been oriented in parallel and in perpendicular to the applied compression, respectively. Two types of samples, one anisotropic with a high orientation factor (∼0.77) of the c-axis parallel to the compressing direction and the other pseudo-isotropic, were fabricated. This study is the first step toward a fabrication of functionally graded (FGM) thermoelectric samples, based on two Bi_0.4Sb _1.6Te₃ ingots, each with a different carrier concentration. The first ingot was designed so that its maximum efficiency is at room temperature, while the second ingot, doped with Pb, is designed for a higher temperature. The results indicate that a power factor of 40 μWCm ^-1K'^-2 can be achieved for a sample aimed to operate at the lower temperature. This value is comparable to that observed in single crystals.