Development of the discrete electric power-generator from exhaust or waste heat sources using ecologically friendly semiconductors

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Mg2Si, an ecologically friendly semiconductor, is a promising thermoelectric material at temperatures ranging 'rom 500 to 800 K.Mg2Si crystals were grown by the vertical Bridgman method in various crucibles made of quartz, alumina, SiNx-coated quartz, CVD pyrolytic graphite, CVD SiC-coated graphite in order to minimize the reaction and sticking of molten Mg-Si during growth.Congruent crystallization was derived from a stoichiometric melt of Mg2Si and incongruent crystallization was derived from nonstoichiometric melts having Mg : Si ratios of 85 : 1 5, 70:30, and 60:40.Grown samples were characterizedby x-ray diffraction and electron-probe microanalysis, and their power factors were calculated from the Seebeck coefficients and electrical-conductivities measured from room temperature to 773 K.The grown crystals were single crystal and showed n-type conductivity in undoped condition. A sample derived from a stoichiometric melt had a Seebeck coefficient of -470 mV/K, while the highest power factor, 1.1x10-5 W/cmK2 at 373 K, was obtained for the sample derived from an incongruent melt with an Mg : Siratio of 70:30.The use of the graphite based crucibles such as pyrolytic and SiC coated graphites caused the carbon incorporati in the grown crystals, resulting in an increase in the Seebeck coefficient and the electrical conductivity.