Highly textured Bi₂ Te₃ -based materials for thermoelectric energy conversion

This work is concerned with Bi₂Te₃ -based compounds known as being highly effective materials for thermoelectric applications near room temperature. These compounds are characterized by a remarkable anisotropy linked to their R 3^- m crystal structure. Two textured p -type Bi_0.4 Sb_1.6 Te₃ samples were prepared using a powder metallurgy approach, with the c axis parallel to the pressing direction. One sample was undoped while the second was doped with Pb which acts as an acceptor. The electrical conductivity, Hall coefficient, and magnetoresistivity were measured from room temperature down to 6 K. The Seebeck coefficient α and electrical conductivity were measured along and perpendicular to the c axis from 300 up to 550 K, and the thermal conductivity κ was measured at 300 K. Different values of Seebeck coefficient were observed along and perpendicular to the c axis at temperatures above Ti, the beginning of intrinsic region in which the influence of the minority carriers becomes significant. Below Ti, the Seebeck coefficient was isotropic. The maximal power factor P= α², calculated on the basis of the experimental results, was about 40 μW cm-1 K^-2 for the direction perpendicular to the c axis. The thermal conductivity values for the temperature domain above 300 K were calculated on the basis of a physical model and the measured values at 300 K. The calculated values of the figure of merit Z=Pκ were 3× 10^-3 and 2× 10^-3 K^-1 at 300 and 400 K for the two samples, respectively. These values are comparable to those observed in Bi₂ Te₃ -based single crystals, thus making the powder metallurgy approach appropriate for thermoelectric conversion applications.