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

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Abstract
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<LL−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.