Thermoelectric properties of Fe0.2Co3.8Sb12−xTex skutterudites

Skutterudites Fe0.2Co3.8Sb12−xTex (x = 0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6) were synthesized by induction melting at 1273 K, followed by annealing at 923 K for 144 h. X-ray powder diffraction and electron microprobe analysis confirmed the presence of the skutterudite phase as the main phase. The temperature-dependent transport properties were measured for all the samples from 300 to 818 K. A positive Seebeck coefficient (holes are majority carriers) was obtained in Fe0.2Co3.8Sb12 in the whole temperature range. Thermally excited carriers changed from n-type to p-type in Fe0.2Co3.8Sb11.9Te0.1 at 570 K, while in all the other samples, Fe0.2Co3.8Sb12−xTex (x = 0.2, 0.3, 0.4, 0.5, 0.6) exhibited negative Seebeck coefficients in the entire temperature range measured. Whereas for the alloys up to x = 0.2 (Fe0.2Co3.8Sb11.8Te0.2) the electrical resistivity decreased by charge compensation, it increased for x > 0.2 with an increase in Te content as a result of an increase in the electron concentration. The thermal conductivity decreased with Te substitution owing to carrier–phonon scattering and point defect scattering. The maximum dimensionless thermoelectric figure of merit, ZT = 1.04 at 818 K, was obtained with an optimized Te content for Fe0.2Co3.8Sb11.5Te0.5 and a carrier concentration of ∼n = 3.0 × 1020 cm−3 at room temperature. Thermal expansion (α = 8.8 × 10−6 K−1), as measured for Fe0.2Co3.8Sb11.5Te0.5, compared well with that of undoped Co4Sb12. A further increase in the thermoelectric figure of merit up to ZT = 1.3 at 820 K was achieved for Fe0.2Co3.8Sb11.5Te0.5, applying severe plastic deformation in terms of a high-pressure torsion process.