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We report on the influence of boron segregation on the thermoelectric properties of CoSi. Contrary to previous suggestions, and in stark contrast to aluminum substitution, boron does not enter the lattice on the Si site, but rather segregates to the grain boundaries in these alloys. Through a combination of x-ray diffraction, scanning electron microscope, and scanning Auger techniques, we present clear evidence of the formation of a CoB phase at the grain boundaries. Consistent with the failure of B to substitute for Si, we observe no changes in the electron concentration or the Seebeck coefficient under boron substitution. The electrical resistivity, on the other hand, displays a non-monotonic behavior with increasing boron concentration, first decreasing for small amounts of boron, before increasing at higher levels of substitution. We attribute this behavior to a combination of an initial healing effect of boron on microcracks, followed by the eventual increase in electron scattering by the secondary CoB phase at higher concentrations. [ABSTRACT FROM AUTHOR]

The compositional dependence of the crystal structure and lattice thermal conductivity in the Cu3SbSe4-Cu3SbS4 system has been studied. The lattice parameters of the Cu3SbSe4-xSx compounds decrease linearly with x, and the tetragonal structure (space group 142m no. 121) of the end compounds is maintained at all compositions. The lattice thermal conductivity is much lower than that predicted by a simple rule of mixtures, which is typical for a solid solution. The Debye model produces a very reasonable fit to the experimental lattice thermal conductivity data when phonon scattering due to atomic mass and size differences between Se and S is taken into account. Compounds in this series are likely to improve upon the thermoelectric performance of Cu3SbSe4, which has shown ZT = 0.72 when optimized. [ABSTRACT FROM AUTHOR]

We report on the influence of sintering gas atmosphere on the thermoelectric (TE) performance of Ca3Co4O9 + δ ceramics made from precursor powders synthesized using chemical sol-gel route. Two sets of polycrystalline pellets were sintered in air and oxygen atmosphere, respectively. Sintering gas atmosphere and sintering temperature were found to have great influence on the Ca3Co4O9 + δ crystal orientation and formation of the Co3O4 secondary phase. All samples, regardless of sintering atmosphere, had similar Seebeck coefficients S that increased with measurement temperature. By contrast, the electrical resistivity ρ changed dramatically as the sintering temperature and atmosphere changed. For samples sintered in air, when the sintering temperature is above 1193 K, ρ increased as the sintering temperature increased. For samples sintering in oxygen, ρ decreased monotonically as the sintering temperature increased up to 1253 K. Benefited from the reduced ρ, the electrical power factor of the sample sintered in the oxygen at 1253 K is 0.539 mW m−1 K−2, and 40% greater than that from the sample sintered at 1193 K in air. Dimensionless figure-of-merit ZT of the sample sintered at 1253 K in oxygen is 0.28 at 1073 K, which is 20% greater than that of samples sintered in air at 1193 K. Overall, the highest TE performance was obtained for Ca3Co4O9 + δceramics sintered in oxygen at 1253 K, which is significantly higher than the Ca3Co4O9 + δ decomposition temperature of 1199 K in air. [ABSTRACT FROM AUTHOR]