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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.

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The time dependences of the carrier concentration, electrical resistivity, and Seebeck coefficient of boron doped Si63.5Ge36.5 alloy are investigated a t heat treatment temperatures of 1000, 1110, and 1200 K for periods of up to 2000 h. The carrier concentration decreases with time of heat treatment due to the precipitation of dopant and this is reflected in an increase in the electrical resistivity and Seebeck coefficient. Good agreement is obtained between the Lifshitz-Slyozov precipitation model and the experimental data. The activation energies of phosphorus and boron in the alloy are estimated to be rz 30 kcal/mol and = 56 kcal/mol, respectively. It is concluded that although the diffusion coefficient of boron in silicon-germanium alloys over the temperature range of device operation is substantially lower than that of phosphorus, precipitation of boron does occur as a function of time. The rate of precipitation maximises in the temperature range lo00 to 1200 K and is sufficiently fast to significantly affect the transport properties of the material and hence the thermoelectric conversion efficiency of a device, based upon silicon-germanium technology, during its period of operation.Die Zeitabhiingigkeiten der Ladungstriigerkonzentration, des elektrischen Widerstands und des Seebeck-Koeffizienten von Bor-dotierten Si63,6Ge36,5-Legierungen werden nach Temperaturbehandlungen von 1000, 110 und 1200 K fiir Zeiten bis 2000 h untersucht. Die Ladungstriigerkonzentration nimmt mit der Temperungsdauer infolge der Ausfiillung von Doponden ab, was sich im Anwachsen des elektrischen Widerstands und des Seebeck-Koeffizienten widerspiegelt. Zwischen dem Prazipitationsmodell von Lifshitz-Slyozov und den experimentellen Werten wird gute vbereinstimmung erhalten. Die Aktivierungsenergien von Phosphor und Bor in der Legierung werden zu = 30 kcal/Mol bzw. = 56 kcal/Mol berechnet. Es wird angenommen, daB, obwohl der Diffusionskoeffizient von Bor in der Silizium-Germanium-Legierung wesentlich niedriger als der von Phosphor ist, eine Ausfiillung von Bor in Abhiingigkeit von der Zeit stattfindet. Die Priizipitationsgeschwindigkeit nimmt im Temperaturbereich von lo00 bis 1200 K geniigend schnell zu und beeinflul3t damit die Transporteigenschaften des Materials und die thermoelektrische Konversionseffiziens eines Bauelements, das auf Silizium-Germanium-Technologie basiert, wiihrend der Betriebsdauer wesentlich. 1) King Edward VII Avenue, Cardiff CFl 3NU, Great Britain. 16.