Effect of annealing on thermoelectric properties of bismuth telluride compounds doped with various additives

The p-type (Bi0.25Sb0.75)2Te3 doped with 8 wt % excess Te and 0.3 wt % Se and the n-type Bi2(Te0.94Se0.06)3 doped with 0.08 wt % SbI3, 0.02 wt % Te, and 0.01 wt % CuBr were grown by the Bridgman method at a rate of 6 cm/h. The thermoelectric properties of these specimens were measured before and after annealing, where annealing was done in the temperature range from 473 up to 673 K for 2-5 h in a vacuum and a hydrogen stream. When the p- and n-type specimens were annealed at 673 K for 2 h in a hydrogen stream and a vacuum, respectively, the thermoelectric figures of merit ZT at 298 K for the p- and n-type specimens reached the greatest values of 0.97 and 1.00, respectively. The rate of enhancement in the ZT due to the annealing, however, was only about 3% for both specimens, which is about a tenth of those of our previous specimens doped with different types of dopants. Combining with our previous results of bismuth telluride compounds, it was found that there is a strong correlation between the rate of enhancement in the ZT due to the annealing and the electrical resistivity ρ of the as-grown specimen, and that the rate of enhancement reaches a maximum when the as-grown bismuth telluride ingots have a ρ value of about 12 μΩ m, at least in the present fabrication method.