Effect of the cooling rate on the thermoelectric properties of p-type (Bi0.25Sb0.75)2Te3 and n-type Bi2(Te0.94Se0.06)3 after melting in the bismuth–telluride system

The p-type (Bi0.25Sb0.75)2Te3 and n-type Bi2(Te0.94Se0.06)3 ingots were prepared by cooling at various cooling rates C after melting so that they have an intermediate state between the polycrystalline and Bridgman ingots which lowers their thermal conductivity κ, where C was changed from 0.10 to 2375 K/min in an evacuated glass tube. When the ingots were cooled at C = 0.50 K/min under the uniaxial temperature gradient of 5 K/cm, it was observed that the c axis of some grains points to the freezing direction. The electrical resistivity ρ, Seebeck coefficient α and κ of ingots were measured at 298 K along the freezing direction, so that ρ and κ at C = 0.50 K/min were lower by 20–30% and 9% than those of the corresponding Bridgman ingots. The thermoelectric figure of merits ZT(=α2T/ρκ) estimated for the p- and n-type ingots then reached high values of 1.27 and 1.25 at 298 K, respectively.