THE MAXIMUM POSSIBLE CONVERSION EFFICIENCY OF SILICON-GERMANIUM THERMOELECTRIC GENERATORS

The thermoelectric properties of N-type and P-type Si-Ge alloys have been reviewed and detailed calculations for the efficiency of a thermoelectric generator made from a 70% Si-30% Ge alloy have been made over the temperature range from 300 to 1300 K. A model employing one valence band and two conduction bands has been used. A generator of standard material, optimally doped, and infinitely segmented will have an efficiency of 12.1% operating over this range. If the lattice thermal conductivity can be reduced to its minimum value without upsetting the electrical properties, then the efficiency can be raised to an ultimate maximum of 23.3%. A more modest increase in efficiency to 14.7% could be obtained by a 2.4 volume percent of finely dispersed second-phase precipitates which would act as phonon scatterers. The utility/futility of GaP additions and grain-boundary scattering as methods to increase the efficiency is discussed.