| We have determined the thermoelectric properties of n-type (Bi1-xSb x)2 (Te1- ySey)3 alloys with 0 &le x, y &le 0.1 for temperatures between room temperature and 420 K. Our objective is to obtain the optimal alloy composition for waste heat recovery applications in a temperature range between room temperature and 420 K. These alloys are obtained by powder metallurgy followed by hot extrusion. This production process allowed us to easily obtain polycristalline alloys with varying stoechiometries. Because of their polycristalline character and texture, the alloys obtained by extrusion show an increased mechanical strength compared to those obtained by conventional solidification methods, which gives them a considerable advantage for high power-density thermoelectric generators and cooling modules.The analysis of the thermal conductivity, the electronic contribution to the thermal conductivity, calculated with the Wiedemann-Franz law, and the lattice thermal conductivity has shown the existence of a third heat transport mechanism, namely the ambipolar thermal conductivity. This mechanism involves energy transport by diffusion of electron-hole pairs and only becomes significant in the intrinsic domain, which is inferred to begin at approximately 400 K, from the results of resistivity and Seebeck coefficient measurements. The ambipolar thermal conductivity leads to a hid increase in the total thermal conductivity. As this increase is combined with the decrease of the Seebeck coefficient due to the intrinsic behaviour of the material at high temperatures, the figure of merit ZT strongly decreases above 400 K. This sets a limit to the use of bismuth telluride based materials at temperatures above 450 K.The figure of merit ZT, the Seebeck coefficient alpha, the electrical resistivity rho and the thermal conductivity lambda were measured following the Harman method. We have also carried out temperature dependant Hall measurements to obtain the carrier concentration and mobility between 15 K and 420 K. We found that the alloy (Bi 0.97Sb0.03)3 (Te 0.93Se0.07)3 shows the optimum thermoelectric performance for waste heat recovery between room temperature and 420 K, with a average figure of merit &langZT&rang= 0.93. |
