The thermoelectric properties of the p -type (Bi 0.25 Sb 0.75 ) 2 Te 3 doped with 3 wt% excess Te and the n -type Bi 2 (Te 0.94 Se 0.06 ) 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/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 4.82×10 -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 Z T 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.
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Yamashita et al. (2003) studied this question.