The effect of phonon-grain boundary scattering on the lattice thermal conductivity and thermoelectric conversion efficiency of heavily doped fine-grained, hot-pressed silicon germanium alloy

The preparation of fine-grained high-density compacts of heavily doped n-type Si63.5Ge36.5 alloy and the measurement of their thermal diffusivity over the temperature range 300 to 1150 K are reported. The compacts investigated possessed grain sizes (L) in the range 10<L<25 μm, 5<L<10 μm, and <5 μm. Within experimental error (<5%) the Seebeck coefficient and electrical resistivity do not change with grain size. The electronic and lattice thermal conductivity are obtained by combining the results with electrical conductivity data, which has been adjusted to take into account precipitation effects that occur during the period of measurement at high temperature. The lattice thermal conductivity decreases with a reduction in grain size. At room temperature the lattice thermal conductivity of the ’’single-crystal’’ starting material is equal to 4.31 W m−1K−1 and falls to 3.10 W m−1K−1 in the compact with a grain size <5 μm, a reduction of ∼28%. The reduction in lattice thermal conductivity is almost 35% at 1000 K. The thermoelectric conversion efficiency of the compact with a grain size <5 μm, when operating between room temperature and 1000 K, is computed to be 17.5% compared to an efficiency of 14.3% for ’’single-crystal’’ material, an improvement of ∼22%. It is concluded that the performance of a thermoelectric generator employing silicon-germanium thermocouples would be significantly improved by the use of fine-grained material.