Effect of anisotropy of the Seebeck coefficient on the thermal conductivity of polycrystalline BiSb alloys

Abstract

In 1961, Cosgrove, McHugh and Tiller reported the effect of micro-segregation in fast-grown bismuth telluride on the thermal conductivity. They attributed a significant increase in this property to the presence of circulating electric currents associated with the non-uniform Seebeck coefficient in the inhomogeneous material. If this effect is an important one, then it could seriously impair the performance of thermoelements made from polycrystalline BiSb alloys since, even if such materials are chemically homogeneous, they will have local variations of the Seebeck coefficient associated with the anisotropy of this parameter in single crystals. We now think that circulating thermoelectric currents are not the main reason for the increase of thermal conductivity in inhomogeneous bismuth telluride. Such currents should have an even smaller effect in BiSb alloys and should not cause any substantial decrease in the figure of merit. This is borne out by measurements on the thermal conductivity of large-grained polycrystalline BiSb alloys. Using a technique for the determination of the electronic component that is described elsewhere, it has been found that the residual thermal conductivity is very close to the lattice component that is predicted form observations on single crystals. This would not be so if there were any significant heat transfer by internal circulating currents.