Thermoelectric properties and transport phenomena in (Bi1-xSbx)2(Te1-ySey)3 quaternary n-type alloys produced by powder metallurgy and extrusion

The use of (Bi_1-xSb_x)₂(Te_1-ySe_y)₃ solid solution alloys instead of the Bi₂Te₃ binary compound allows a significant increase of its thermoelectric performance. This improvement is mainly related to the reduction of thermal conductivity of the crystal lattice due to increased phonon scattering obtained from the alloying effect. However the optimum amount of Sb and Se resulting in higher alloy performance remains uncertain. Moreover, it is not clear if the reduction of mobility of free charge carriers in alloys containing large amounts of Sb and Se is the only cause of thermoelectric performance degradation of such alloys. Other phenomena have been suggested to explain this behavior. In this research project, a series of quaternary alloys of n-type (Bi_1-xSb_x)₂(Te_1-ySe_y)₃ have been characterized with Sb and Se content varying from 0 to 10%. The specimens were produced by mechanical alloying of powders and hot extrusion. The thermoelectric properties have been studied in the temperature range from 230 to 350 K. The mobility and concentration of charge carriers were evaluated as a function of temperature from 15 to 300 K. The results indicate that, for quaternary alloys in the higher end of the concentrations of Se and Sb studied here, the hypothesis of charge carrier mobility degradation due only to alloying effect is not sufficient to explain the data. We will discuss the effect of additional generation of defect complexes for these cases.