Void filling in (I) Bi(x)-added Co(4)Sb(12) or (II) Sb/Bi substitution of Co(4)Sb(12-x)Bi(x) has been investigated for structural and thermoelectric properties evaluation. X-ray powder data Rietveld refinements combined with electron probe microanalyses showed a polycrystalline and practically Bi-free CoSb(3) skutterudite phase as the major constituent as well as a secondary Bi phase in the grain boundaries. For series I alloys, the electrical conductivity, Seebeck coefficient and thermal conductivity were measured as a function of temperature in the range from 450 to 750 K. The electrical conductivity of all the samples increased with increasing temperature, showing a semiconducting nature with smaller values of the Seebeck coefficient for higher Bi fractions. Conduction over the entire temperature range was found to arise from a single p-type carrier. Thermal conductivity showed a reduction with Bi added in all the samples, except for Bi(0.75)Co(4)Sb(12), and the lowest lattice thermal conductivity was found for a Bi-added fraction of 0.5. The maximum zT value of 0.53 at 632 K is higher than that of Co(4)Sb(12).
The electronegativity difference between the filler and the host atom of skutterudites determines the bonding type, which effects the thermoelectric properties. The electropositive elements with low electronegativity compared to Sb atom can form n-type filled skutterudites. In this study, the electronegative element sulphur has filled into voids. A series of S x Co 4 Sb 12 (x= 0.05 to 0.4) compounds were synthesized by the solid-state method. CoSbS secondary phase was observed for x > 0.05. Filling fraction of sulphur into the void (̴ 0.04) was calculated using electron probe microanalyzer which agrees with Rietveld refinement of the X-ray diffraction pattern. The transport properties were measured from 350 K to 723 K. The positive Seebeck coefficient (S) confirms p-type semiconducting nature. S and the electrical resistivity (ρ) decreased with increasing x due to increased carrier concentration (n). All the samples except x=0.05 possess almost same S and ρ due to saturation of filling fraction. The enhanced phonon scattering in S x Co 4 Sb 12 resulted in low thermal conductivity (κ). The highest zT of 0.18 at 623 K was achieved for S 0.15 Co 4 Sb 12 .
CoSb 3 skutterudites are established thermoelectric materials in the 500-800K temperature range. Undoped and Bi filled CoSb 3 samples were synthesized by induction melting-annealing process and phase confirmation done by X-Ray diffraction. The role of bismuth as a filler in CoSb 3
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