Effect of Annealing on Thermoelectric Properties of Bismuth Telluride Compounds

The thermoelectric properties of the p-type (Bi0.25Sb0.75)2Te3 doped with 3 wt% excess Te and the n-type Bi2(Te0.94Se0.06)3 doped substantially with 0.07 wt% I and 0.02 wt% Te which were grown by the Bridgman method at a rate of 6 cm/h were measured before and after annealing, where the annealing was done in the temperature range from 473 up to 673 K for 1–10 h in vacuum and in a hydrogen stream. When annealing was done at 673 K for 2 h in vacuum, the power factor at 298 K for the p-type specimen increased by 35% from 4.25×10-3 to 5.72×10-3 W/mK2. In contrast, when annealing was done at 673 K for 5 h in a hydrogen stream, the power factor at 298 K for the n-type specimen increased by 24% from 4.82×10-3 to 5.99×10-3 K-1. The thermal conductivities κ at 298 K for both annealed specimens then decreased by about 5%. As a result, the maximum thermoelectric figures of merit Z at 298 K for the annealed p- and n-type specimens reached surprisingly large values of 4.16×10-3 K-1 and 3.99×10-3 K-1, respectively, with corresponding ZT values of 1.26 and 1.19. The reason that the Z values of bismuth telluride compounds were significantly enhanced by such an annealing is also discussed.