Research on nano-microstructure control of the thermoelectric semiconductor by a solid phase process

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Bismuth telluride (Bi_2Te_3) based compounds have a rhombohedral structure at room temperature and are distinguished by a high degree of anisotropy for most of the physical properties. Since the compounds have distinct (111) cleavage planes perpendicular to the c-aixs, they have low mechanical properties and poor ability in micro-processing for fabricating the miniature thermoelectric modules.The ingot and powder of Bi_2Te_3 was sealed in the sheath pipes and groove-rolled at 300K〜673K. The sheath pipe was modified to the dual structure, which was the Al pipe inside and Cu pipe outside. After the groove-rolled, the diameter of crystal grains became small, and the crystal direction along the rolling direction became the re-orientation to the a-axis. It proved that nano-micro structure control is possible. The groove-rolled Bi_2Te_3 with non-doping changed from the p-type conduction of 137.0μV/K (at 473K) to n-type conduction of -142.2μV/K (at 673K) by the processing-temperature. Moreover, post heat treatment changed thermoelectric properties of the groove-rolled Bi_2Te_3 (at 300K). As for the anisotropy of the properties, the Seebeck coefficient and thermal conductivity showed high anisotropy, and the degree of anisotropy was the same as the ingot or the sintered compact. The value of Seebeck coefficient was equivalent to the sintered compact, and resistivity became about 2 times. And thermal conductivity was lowered about 20%. The refinement of a crystal grain was accelerated by the groove-rolling. The patent of this manufacturing process applied.