Observation of the effect of grain size on the lattice thermal conductivity of polycrystalline bismuth antimony

The thermal conductivity of polycrystalline sintered BiSb has been measured from 80 to 300 K. The samples had mean grain sizes between 1 /spl mu/m and 19 /spl mu/m. The electronic contribution to the thermal conductivity was determined by a technique that involves the comparison of the magneto-resistance and the magneto-thermal resistance effects in transverse fields of up to 1.0 tesla. The electronic thermal conductivity seems to be somewhat smaller than expected, bearing in mind that there should be a significant bipolar contribution, yet the technique is believed to be sound since it yields a lattice thermal conductivity for large-grained samples that is intermediate between the values for single crystals in the principal crystal directions. The lattice thermal conductivity is found to decrease as the grain size is reduced. Such a decrease has been predicted on the basis of grain boundary scattering of phonons but it is occurring at grain sizes somewhat larger than expected. It is possible that the effect may be due to the scattering of phonons on point or line defects that have been introduced as a consequence of powder metallurgy. Nevertheless, grain boundary scattering cannot be ruled out as the cause, in view of the simplifications that were made in the theory.