The advantage of secondary-phase induced carrier filtering on the thermoelectric properties has paved the way for developing cost-effective, highly efficient thermoelectric materials. Here, we report a very high thermoelectric figure-of-merit of skutterudite nanocomposites achieved by tailoring interface carrier filtering. The single-filled skutterudite nanocomposites are prepared by dispersing rare-earth oxides nanoparticles (Yb 2 O 3 , Sm 2 O 3 , La 2 O 3 ) in the skutterudite (Dy 0.4 Co 3.2 Ni 0.8 Sb 12 ) matrix. The nanoparticles/skutterudite interfaces act as efficient carrier filters, thereby significantly enhancing the Seebeck coefficient without compromising the electrical conductivity. As a result, the highest power factor of ∼6.5 W/mK 2 is achieved in the skutterudite nanocomposites. The nonuniform strain distribution near the nanoparticles due to the local lattice misfit and concentration fluctuations affect the heat carriers and thereby reduce the lattice thermal conductivity. Moreover, the three-dimensional atom probe analysis reveals the formation of Ni-rich grain boundaries in the skutterudite matrix, which also facilitates the reduction of lattice thermal conductivity. Both the factors, i.e., the reduction in lattice thermal conductivity and the enhancement of the power factor, lead to an enormous increase in ZT up to ∼1.84 at 723 K and an average ZT of about 1.56 in the temperature range from 523 to 723 K, the highest among the single-filled skutterudites reported so far.
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 12sample 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 metalfilled 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.
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