Effect of annealing on thermoelectric properties of bismuth telluride compounds

The thermoelectric properties of the p-type (Bi0.25Sb0.75)(2)Te-3 doped with 3 wt% excess Te and the n-type Bi-2(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 x 10(-3) to 5.72 x 10(-3) W/mK(2). 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 x 10(-3) to 5.99 x 10(-3) K-1. The thermal conductivities K 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 x 10(-3) K-1 and 3.99 x 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.