Nanostructuring is considered one of the key approaches to achieve highly efficient thermoelectric alloys by reducing thermal conductivity. In this study, we investigated the effect of oxide (ZnO and SnO₂) nanolayers at the grain boundaries of polycrystalline In_0.2Yb_0.1Co₄Sb₁₂ skutterudites on their electrical and thermal transport properties. Skutterudite powders with oxide nanolayers were prepared by atomic layer deposition method, and the number of deposition cycles was varied to control the coating thickness. The coated powders were consolidated by spark plasma sintering. With increasing number of deposition cycle, the electrical conductivity gradually decreased, while the Seebeck coefficient changed insignificantly; this indicates that the carrier mobility decreased due to the oxide nanolayers. In contrast, the lattice thermal conductivity increased with an increase in the number of deposition cycles, demonstrating the reduction in phonon scattering by grain boundaries owing to the oxide nanolayers. Thus, we could easily control the thermoelectric properties of skutterudite materials through adjusting the oxide nanolayer by atomic layer deposition method.

The formation of secondary phases of NiTe₂ and NbTe₂ in p-type Bi_0.5Sb_1.5Te₃ thermoelectric alloys was investigated through in situ phase separation by using the melt spinning process. Adding stoichiometric Ni, Nb, and Te in a solid-state synthesis process of Bi_0.5Sb_1.5Te₃, followed by rapid solidification by melt spinning, successfully segregated NiTe₂ and NbTe₂ in the Bi_0.5Sb_1.5Te₃ matrix. Since heterointerfaces of Bi_0.5Sb_1.5Te₃ with NiTe₂ and NbTe₂ form potential barriers of 0.26 and 0.08 eV, respectively, a low energy carrier filtering effect can be expected; higher Seebeck coefficients and power factors were achieved for Bi_0.5Sb_1.5Te₃(NiTe₂)_0.01 (250 μV/K and 3.15 mW/mK²), compared to those of Bi_0.5Sb_1.5Te₃ (240 μV/K and 2.69 mW/mK²). However, there was no power factor increase for NbTe₂ segregated samples. The decrease in thermal conductivity was seen due to the possible additional phonon scattering by the phase segregations. Consequently, zT at room temperature was enhanced to 0.98 and 0.94 for Bi_0.5Sb_1.5Te₃(NiTe₂)_0.01 and Bi_0.5Sb_1.5Te₃(NbTe₂)_0.01, respectively, compared to 0.79 for Bi_0.5Sb_1.5Te₃. The carrier filtering effect induced by NiTe₂ segregations with an interface potential barrier of 0.26 eV effectively increased the Seebeck coefficient and power factor, thus improving the zT of p-type Bi_0.5Sb_1.5Te₃, while the interface potential barrier of 0.08 eV of NbTe₂ segregation appeared to be too small to induce an effective carrier filtering effect.