SI80GE20 THERMOELECTRIC ALLOYS PREPARED WITH GAP ADDITIONS

Controlled amounts of GaP and P were added to a Si0.8Ge0.2 matrix by a powder-metallurgical technique in order to evaluate the optimum composition for thermoelectric applications. Bulk determination of the gallium and phosphorus content in fully dense, hot pressed compacts was performed by inductively coupled plasma atomic emission spectroscopy. The transport properties of the compacts were characterized by Hall effect measurements at room temperature and by measurements of electrical resistivity, Seebeck coefficient, and thermal diffusivity to 1000 degrees C. Considerable variation in the electrical transport properties were found to accompany changes in the Ga/P ratio, in the total amount of dopant, and changes in other preparation conditions. Alloys with gallium phosphide additions exhibit carrier concentrations higher than those obtained in alloys doped only with phosphorus. Alloys with a nominal phosphorus content greater than 2.0 at. % were found to be overdoped and those containing less than 0.6 at. % phosphorus were found to be underdoped relative to the material's maximum figure of merit. Room temperature electron mobilities greater than 40 cm(2)/V s were commonly obtained in samples with a carrier concentration of 3X10(20) cm(-3) or greater. A 300-1000 degrees C integrated average figure of merit (Z=S-2/p Lambda) of 0.93x10(-3)degrees C-1 was achieved within an optimum composition range of 0.8-1.2 at. % P and 0.6-0.8 mol % GaP, which is 20% higher than current n-type Si-Ge alloys doped with 0.59 at. % P as used in the Voyager, Galileo, and Ulysses missions. (C) 1995 American Institute of Physics.