Seebeck, electrical, and thermal conductivity 'data are reported on CoSb,, and doped and undoped alloys of Co,-&,Sb, .-,Asy from-20 to 700 K. n-type semiconductors were obtained by doping with Ni, Te, or Pd, and the hole concentration in p-type samples was increased by substitution of Fe, Ru, OS, and Ge. An estimated maximum value for ZT of 0.6 (Z is the figure of merit) was found for a Te-doped (n-type) alloy at 700 K. For p-type alloys, the maximum value of ZT was found to be 0.3 at 550 K. Electrical and thermal transport data also are reported for CoAss , RhSb, , and IrSb,. Most of the samples investigated were polycrystalline, but a few measurements on CoSb, single crystals also are discussed.
Magnetic susceptibility, resistivity, Seebeck, Hall, and powder x-ray and neutron-diffraction measurements were used to characterize single crystals of FeSi and polycrystalline samples of Fe& "Ir"Sifor x (0.2. The Rietveld refinement of low-temperature powder neutron-diffraction data on FeSi showed no change in the space group and no structural anomalies from 4 to 300 K. Magnetic and transport data from 4 to 700 K are consistent with the characterization of FeSi as a narrow-gap semiconductor (Eg 1200 K) with strong intrasite correlations for the states just below and above the gap. Fits to the magnetic susceptibility and resistivity data suggest that the magnetic (or direct) gap may be larger than the transport (indirect) gap. Electron mobilities in FeSi are very low (3 -5 cm /V s). The thermopower of FeSi has a large positive peak (500 pV/K) at 50 K that is attributed to an unusually strong phonondrag mechanism. Iridium acts as an electron donor in the Fe& "Ir"Sialloys. As the iridium doping level is increased, there is a rapid decrease in the low-temperature resistivity and a large negative ( -140 pV/K) phonon-drag contribution to the thermopower. For Peltier cooling applications, a maximum value for ZT of 0.07 was found for a Fep 95Irp p5Si alloy at 100 K.
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