Effect of the Crystal Alignment and Grain Size on the Thermoelectric Properties of Bi 0.4 Sb 1.6 Te 3 Sintered Materials

Sintered polycrystalline Bi0.4Sb1.6Te3 thermoelectric materials with various degrees of crystal alignment and grain sizes were prepared by pulse-current sintering under cyclic uniaxial pressure at sintering temperatures of 350–425 °C for holding times of 0–60 min to clarify the relationship between the microstructure and the thermoelectric properties. The degree of crystal alignment was enhanced with an increase of both the sintering temperature and holding time, and grain growth was also confirmed in the high temperature or long-time sintering process. The thermoelectric properties were measured perpendicular to the pressing direction, which corresponds to the crystal alignment direction. The electrical resistivity decreased and the thermal conductivity slightly increased with increasing sintering temperature and holding time. As a result, the figures of merit of the crystal-aligned samples sintered at high temperatures or for long holding times tended to reach 0.9–1 with large power factors because of the small electrical resistivity. The relationships between the quantified microstructure parameters and thermoelectric properties are discussed. The electrical resistivity decreased with increasing degree of crystal alignment and it saturated at a certain degree of crystal alignment, indicating that perfect crystal alignment is not necessary to obtain the lower limit of the electrical resistivity. Conversely, no significant change of the lattice thermal conductivity was observed in the grain size range 0.6–9.7 μm. This means that the lattice thermal conductivity of crystal aligned Bi0.4Sb1.6Te3 is almost independent of the degree of crystal alignment and grain size in the measured range.