Influence of annealing on the distribution of thermoelectric figure of merit in bismuth-telluride ingots

The p-type (Bi0.25Sb0.75)2Te3 doped with 8 wt% excess Te alone and n-type Bi2(Te0.94Se0.06)3 codoped with 0.017 wt% Te and 0.068 wt% I were grown by the Bridgman method and were cut into a parallelepiped of 5 × 5 × 15 mm3, where the length of 15 mm is parallel to the freezing direction. The specimen is mounted on an X-Y stage and the temperature difference between two probes set at an interval of 1 mm was approximately 2.6 K. The local Seebeck coefficient α and local electrical resistivity ρ along the freezing direction were measured at a scan step of 1 mm before and after annealing at 673 K for 2 h in vacuum. The local thermal conductivity κ was calculated from ρ using the relation between κ and ρ obtained previously for a series of bismuth-telluride compounds. As a result, α ρ and κ before and after annealing changed significantly from place to place, so that the effect of annealing on the local thermoelectric figure of merit (ZT) was never uniform throughout the specimen surface. The maximum (ZT) of the as-grown p- and n-type specimens reached surprisingly great values of 1.88 and 1.59 at 298 K, respectively, which correspond to about twice as large as those observed macroscopically by the conventional Seebeck coefficient apparatus with an interval (between two probes) of 8 mm. Probably, these maximum values of (ZT) would be an upper limit of ZT for the p- and n-type bismuth- telluride bulk compounds, at least in the present fabrication method. C 2006 Springer Science + Business Media, Inc.