Technology and physical properties of FeSi2-thermocouples

The application of doped FeSi2for thermoelectric generation was first discussed by R. M. Ware and D. J. McNeill. We have extended thier investigations by studying the influence of sample preparation and doping more closely. By a systematic variation of the parameters for powder metallurgic preparation and by application of hydro-static pressure these properties could be improved considerably. They were in no way inferior to those found for samples prepared by normal freezing methods. Further, the influence of various dopants on the thermo-electric properties was determined. It was found that Al, Ga, Mn and Cr act as acceptors and P, Co and Ni as donors. For these dopants we have measured the temperature dependence of Seebeck coefficient, electrical conductivity and Hall coefficient in detail. The most important result is the strong increse of the thermoelectric figure of merit with temperature for n-FeSi2to 10^-3grad^-1at 1000°K. This behaviour can be explained by the assumption that the electrical conduction is caused by small polarons, as discussed by U. Birkholz and J. Schelm. On the other hand, for p-FeSi2, only a weak temperature dependence was observed, as expected for conduction according to a band model. Maximum figure of merit values were obtained with Al or Cr for p-type and Co for n-type material. Thermocouples made of these materials can operate up to 1000°K. For example, with a hot junction temperature of 1000°K and a cold junction temperature of 400°K and using cube shaped legs (0.6 cm) a power output of about 0.7 W was obtained.