Compared to Te, Ni is found in abundance in the earth's crust. At the same time, the mechanical properties of Ni atom-substituted skutterudite are significantly improved. In this study, a series of S-filled Ni-substituted skutterudite compounds were synthesized by a high-pressure and high-temperature (HPHT) method in the synthesis pressure range of 1.0–3.0 GPa. The phase composition, microscopic morphology, and electrothermal transmission properties of the S x Co 3.6 Ni 0.4 Sb 12 ( x = 0, 0.05, 0.10, 0.20) samples were systematically characterized. Phase composition analysis showed that the introduction of S atoms into the intrinsic pores of skutterudite can improve the solid solution limit of Ni atoms in the Co sites of skutterudite. The filling limit of S increased with the synthetic pressure. Moreover, microscopic morphology analysis revealed that the filling of S atoms inhibited the growth of grains. A large number of lattice fringes in different directions were found in the sample, containing abundant microstructures such as lattice distortions and dislocation defects. Compared with the synthesized samples, S 0.05 Co 3.6 Ni 0.4 Sb 12 synthesized at 1.0 GPa had a maximum room temperature power factor of 7.98 × 10 −4 Wm −1 K −2 . The electrical properties of S 0.05 Co 3.6 Ni 0.4 Sb 12 samples stored for 6 months without any protective measures were tested at room temperature. No obvious changes in performance were observed, which proved that the HPHT method can synthesize stable samples. After several thermal cycles, the electrical properties of the S 0.05 Co 3.6 Ni 0.4 Sb 12 sample was tested for variable temperature, and it was found that the sample still had good stability. How the substitution of S atoms with Ni atoms reduces the lattice thermal conductivity can be explained by fitting the Callaway model. The S 0.05 Co 3.6 Ni 0.4 Sb 12 sample synthesized at 1.0 GPa had a maximum zT value of 0.46 at the test temperature of 773 K, which decreased to 0.43 after multiple thermal cycles at the same temperature.
In recent years, skutterudite filled with electronegative elements (S, Se, Cl, Br) has attracted the extensive attention of researchers. By doping electron donors (Pb, Ni, or Te, S, Se) at the Co or Sb sites, the electronegative elements can form thermodynamically stable compounds in the intrinsic pores of the skutterudite, substantially expanding the research scope of skutterudite. In this study, S0.05Co4Sb11.3Te0.6Se0.1 skutterudite was synthesized at high pressure and high temperature, with a pressure range of 2.0–3.5 GPa. The phase composition, micro-morphology, and electrical and thermal transport properties were systematically characterized. The micromorphology analysis shows that the introduction of S element and substituting Te and Se at the Sb sites inhibit the grain growth in a suitable high-pressure environment. Substantial differences are observed in the directions of the lattice stripes in the samples, and rich grain boundaries and many lattice distortions and dislocation defects occur. The carrier concentration can be optimized by filling the voids of the skutterudite with a few S atoms, and the thermoelectric properties can be optimized by scattering phonons through resonance scattering and defect scattering. The samples synthesized at a pressure of 3.0 GPa and a temperature of 900 K have a maximum power factor of 23.85 × 10–4 W m–1 K–2 and a maximum zT value of 1.30 at a test temperature of 773 K.
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