Phonon scattering at grain boundaries in heavily doped fine-grained silicon–germanium alloys

Abstract

Thermocouples made from heavily doped n - and p-type silicon-germanium alloys are used in the nuclear powered thermoelectric generators which provides onboard power to the American Voyager spacecraft. The isotopic heat source is expensive and any improvement in the generators' heat energy to electrical energy conversion efficiency would save both money and weight. The possibility of reducing the thermal conductivity of thermoelectric semiconductor alloys by compacting fine grained material has been considered and various estimates have been made of the reduction in thermal conductivity with grain size. The electrical properties of hot pressed compacts of heavily doped silicon germanium alloys which possess density values dose to that of single crystal are unaffected by grain boundary scattering effects. Consequently, this method of preparation holds out a possibility of improving the ‘figure of merit’ of the thermocouple material and hence the conversion efficiency of the generator. Here the first measurements of the thermal diffusivity of fine grained hot pressed compacts of heavily doped n-type silicon germanium alloy are reported. The compacts investigated possessed grain sizes (L) in the range 10<L<25 µm, 5<L<10 µm and <5 µm. The results confirm that the lattice thermal conductivity decreases with a reduction in grain size. In the compact with L<5 µm, this reduction is ∼28% compared with ‘single crystal’ material. Within experimental error (<5%) the Seebeck coefficient and electrical resistivity do not change with grain size. This implies that the conversion efficiency of a thermoelectric generator employing silicon–germanium thermocouples would be significantly improved through the use of fine-grained thermocouple material.