Thermoelectrics are an important class of materials with great potential in alternative energy applications. In this study, two-dimensional (2D) nanoplates of the layered chalcogenides, Sb₂Te₃ and Bi₂Te₃, are synthesized and composites of the two are investigated for their thermoelectric properties. The two materials, Sb₂Te₃ and Bi₂Te₃, were synthesized as hexagonal, 2D nanoplates via a colloidal polyol route. The as-synthesized Sb₂Te₃ and Bi₂Te₃ vary drastically from one another in their lateral and vertical dimensions as revealed by scanning electron microscopy and atomic force microscopy. The single crystalline nanoplate nature is deduced by high-resolution transmission electron microscopy and selected area electron diffraction. Nanoplates have well-defined hexagonal facets as seen in the scanning and transmission electron microscopy images. The nanoplates were consolidated as an anisotropic nanostructured pellet via spark plasma sintering. Preferred orientation observed in the powder X-ray diffraction pattern and scanning electron microscopy images of the fractured pellets confirm the anisotropic structure of the nanoplates. Thermoelectric properties in the parallel and perpendicular directions were measured, revealing strong anisotropy with a significant reduction to thermal conductivity in the perpendicular direction due to increased phonon scattering at nanoplate interfaces. All compositions, except that of the 25% Bi₂Te₃ nanoplate composite, behave as degenerate semiconductors with increasing electrical resistivity as the temperature increases. The Seebeck coefficient is also increased dramatically in the nanocomposites, the highest reaching 210 μV/K for 15% Bi₂Te₃. The increase in Seebeck is attributed to energy carrier filtering at the nanoplate interfaces. Overall, these enhanced thermoelectric properties lead to a drastic increase in the thermoelectric performance in the perpendicular direction, with zT ∼ 1.26, for the 15% Bi₂Te₃ nanoplate composite at 450 K.

Thermoelectric properties of the half-Heusler phase ScNiSb (space group F $\overline{4}$ 3m) were studied on a polycrystalline single-phase sample obtained by arc-melting and spark-plasma-sintering techniques. Measurements of the thermopower, electrical resistivity, and thermal conductivity were performed in the wide temperature range 2–950 K. The material appeared as a p-type conductor, with a fairly large, positive Seebeck coefficient of about 240 μV K⁻¹ near 450 K. Nevertheless, the measured electrical resistivity values were relatively high (83 μΩm at 350 K), resulting in a rather small magnitude of the power factor (less than 1 × 10⁻³ W m⁻¹ K⁻²) in the temperature range examined. Furthermore, the thermal conductivity was high, with a local minimum of about 6 W m⁻¹ K⁻¹ occurring near 600 K. As a result, the dimensionless thermoelectric figure of merit showed a maximum of 0.1 at 810 K. This work suggests that ScNiSb could be a promising base compound for obtaining thermoelectric materials for energy conversion at high temperatures.

Deeper understanding of electrical and thermal transport is critical for further development of thermoelectric materials. Here we describe the thermoelectric performance of a group of rare-earth-bearing half-Heusler phases determined in a wide temperature range. Polycrystalline samples of $\mathrm{Sc}\mathrm{Ni}\mathrm{Sb}$, $\mathrm{Dy}\mathrm{Ni}\mathrm{Sb}$, $\mathrm{Er}\mathrm{Ni}\mathrm{Sb}$, $\mathrm{Tm}\mathrm{Ni}\mathrm{Sb}$, and $\mathrm{Lu}\mathrm{Ni}\mathrm{Sb}$ are synthesized by arc melting and densified by spark plasma sintering. They are characterized by powder x-ray diffraction and scanning electron microscopy. The physical properties are studied by means of heat-capacity and Hall-effect measurements performed in the temperature range from 2 to 300 K, as well as electrical-resistivity, Seebeck-coefficient, and thermal-conductivity measurements performed in the temperature range from 2 to 950 K. All the materials except $\mathrm{Tm}\mathrm{Ni}\mathrm{Sb}$ are found to be narrow-gap intrinsic $p$-type semiconductors with rather light charge carriers. In $\mathrm{Tm}\mathrm{Ni}\mathrm{Sb}$, the presence of heavy holes with large weighted mobility is evidenced by the highest power factor among the series ($17\mu \mathrm{W}{\mathrm{K}}^{-2}{\mathrm{cm}}^{-1}$ at 700 K). The experimental electronic relaxation time calculated with the parabolic band formalism is found to range from $0.8\times {10}^{-14}$ to $2.8\times {10}^{-14}$ s. In all the materials studied, the thermal conductivity is between 3 and $6{\mathrm{W\; m}}^{-1}{\mathrm{K}}^{-1}$ near room temperature (i.e., smaller than in other pristine $d$-electron half-Heusler phases reported in the literature). The experimental observation of the reduced thermal conductivity appears fully consistent with the estimated low sound velocity as well as strong point-defect scattering revealed by Debye-Callaway modeling. Furthermore, analysis of the bipolar contribution to the measured thermal conductivity yields abnormally large differences between the mobilities of $n$-type and $p$-type carriers. The latter feature makes the compounds examined excellent candidates for further optimization of their thermoelectric performance via electron doping.