Effect of Refractory Tantalum Metal Filling on the Microstructure and Thermoelectric Properties of Co(4)Sb(12) Skutterudites

[Image: see text] We report a systematic investigation of the microstructure and thermoelectric properties of refractory element-filled nanostructured Co(4)Sb(12) skutterudites. The refractory tantalum (Ta) metal-filled Co(4)Sb(12) samples (Ta(x)Co(4)Sb(12) (x = 0, 0.4, 0.6, and 0.8)) are synthesized using a solid-state synthesis route. All the samples are composed of a single skutterudite phase. Meanwhile, nanometer-sized equiaxed grains are present in the Ta(0.2)Co(4)Sb(12) and Ta(0.4)Co(4)Sb(12) samples, and bimodal distributions of equiaxed grains and elongated grains are observed in Ta(0.6)Co(4)Sb(12) and Ta(0.8)Co(4)Sb(12) samples. The dominant carrier type changes from electrons (n-type) to holes (p-type) with an increase in Ta concentration in the samples. The power factor of the Ta(0.6)Co(4)Sb(12) sample is increased to 2.12 mW/mK(2) at 623 K due to the 10-fold reduction in electrical resistivity. The lowest lattice thermal conductivity observed for Ta(0.6)Co(4)Sb(12) indicates the rattling action of Ta atoms and grain boundary scattering. Rietveld refinement of XRD data and the analysis of lattice thermal conductivity data using the Debye model confirm that Ta occupies at the voids as well as the Co site. The figure of merit (ZT) of ∼0.4 is obtained in the Ta(0.6)Co(4)Sb(12) sample, which is comparable to single metal-filled p-type skutterudites reported to date. The thermoelectric properties of the refractory Ta metal-filled skutterudites might be useful to achieve both n-type and p-type thermoelectric legs using a single filler atom and could be one of replacements of the rare earth-filled skutterudites with improved thermoelectric properties.