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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]

Higher manganese silicides (HMS) made of earth-abundant and non-toxic elements are regarded as promising p-type thermoelectric materials because their complex crystal structure results in low lattice thermal conductivity. It is shown here that the already low thermal conductivity of HMS can be reduced further to approach the minimum thermal conductivity via partial substitution of Mn with heavier rhenium (Re) to increase point defect scattering. The solubility limit of Re in the obtained Re xMn1- xSi1.8 is determined to be about x = 0.18. Elemental inhomogeneity and the formation of ReSi1.75 inclusions with 50−200 nm size are found within the HMS matrix. It is found that the power factor does not change markedly at low Re content of x ≤ 0.04 before it drops considerably at higher Re contents. Compared to pure HMS, the reduced lattice thermal conductivity in Re xMn1- xSi1.8 results in a 25% increase of the peak figure of merit ZT to reach 0.57 ± 0.08 at 800 K for x = 0.04. The suppressed thermal conductivity in the pure Re xMn1- xSi1.8 can enable further investigations of the ZT limit of this system by exploring different impurity doping strategies to optimize the carrier concentration and power factor. [ABSTRACT FROM AUTHOR]