Effect of Refractory Tantalum Metal Filling on the Microstructure and Thermoelectric Properties of CoSb Skutterudites.

We report a systematic investigation of the microstructure and thermoelectric properties of refractory element-filled nanostructured CoSb skutterudites. The refractory tantalum (Ta) metal-filled CoSb samples (Ta CoSb ( = 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 TaCoSb and TaCoSb samples, and bimodal distributions of equiaxed grains and elongated grains are observed in TaCoSb and TaCoSb 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 TaCoSb sample is increased to 2.12 mW/mK at 623 K due to the 10-fold reduction in electrical resistivity. The lowest lattice thermal conductivity observed for TaCoSb 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 TaCoSb 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.