Thermoelectric properties of the skutterudite Co1−xFexSb3 system

We have examined the phase equilibrium and thermoelectric properties of ${\mathrm{Co}}_{\text{1−x}}{\mathrm{Fe}}_x{\mathrm{Sb}}_3$ ternary system up to high iron context $\text{x=0.40.}$ Traces of Sb were observed in the hot-pressed samples with $\text{x⩾0.06,}$ and ${\mathrm{FeSb}}_{\mathrm{2}}$ $({\mathrm{Fe}}_{\mathrm{0.73}}{\mathrm{Co}}_{\mathrm{0.27}}{\mathrm{Sb}}_{\mathrm{2}})$ compound with marcasite structure was also observed in the samples with $\text{x⩾0.25}$ by x-ray diffraction. The lattice parameter of ${\mathrm{Co}}_{\text{1−x}}{\mathrm{Fe}}_x{\mathrm{Sb}}_{\mathrm{3}}$ is slightly larger than that of the binary compound ${\mathrm{CoSb}}_{\mathrm{3}}.$ The Seebeck coefficient and the electrical resistivity are generally reduced by the substitution for Co by Fe. The thermal conductivity is also reduced by the substitution especially at high iron content region. These behaviors of the thermoelectric properties in the samples with low iron content are ascribed to the substituted Fe, while those in the samples with high iron content are ascribed to the precipitated ${\mathrm{FeSb}}_{\mathrm{2}}$ $({\mathrm{Fe}}_{\mathrm{0.73}}{\mathrm{Co}}_{\mathrm{0.27}}{\mathrm{Sb}}_{\mathrm{2}})$ compound. For $\text{x⩽0.04,}$ the figure of merit for ${\mathrm{Co}}_{\text{1−x}}{\mathrm{Fe}}_x{\mathrm{Sb}}_{\mathrm{3}}$ decreases with increasing x. However, above $\text{x=0.06}$ the figure of merit increases with x and as a result, ${\mathrm{Co}}_{\mathrm{0.75}}{\mathrm{Fe}}_{\mathrm{0.25}}{\mathrm{Sb}}_{\mathrm{3}}$ has a maximum value of figure of merit of ${\text{4.8×10}}^{\text{−4}}\hspace{0.5em}{\mathrm{K}}^{\mathrm{-1}}$ at 773 K. This value is much larger than the maximum value for ${\mathrm{CoSb}}_{\mathrm{3}}$ ${\text{3.2×10}}^{\text{−4}}\hspace{0.5em}{\mathrm{K}}^{\mathrm{-1}}$ at 482 K.