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In this study, a series of Cu2–x SnSe3 (x = 0.075–0.175) and Cu2Sn1–y Se3 (y = 0.06–0.1) compounds were synthesized by self-propagating high-temperature synthesis combined with plasma-activated sintering. The effects of different cation vacancies (Cu vacancies and Sn vacancies) on the thermoelectric properties are systematically studied. Both Cu vacancies and Sn vacancies enhance the carrier densities and move the Fermi level deeply into the valence band, promoting the multiband from Γ and S points involved in the electrical transport and increasing the effective mass, which is further corroborated by the theoretical calculation. Due to the stronger carrier scattering caused by Cu vacancies, the mobilities of samples with Cu deficiencies reduce leading to decreased power factors. The power factors of samples with Sn deficiencies increase owing to the increased carrier concentration and attaining a maximum power factor of 10.17 μW cm–1 K–2 at 800 K. Besides, the deficiencies of both Cu and Sn strengthen the phonon scattering, and samples with Cu deficiencies obtain lower thermal conductivity than samples with Sn deficiencies due to the lower electronic thermal conductivity. All the samples with cation deficiencies have improved thermoelectric properties. For Cu1.875SnSe3, ZT reaches 0.95 at 800 K, which is 83% higher than those of undoped samples, while for Cu2Sn0.93Se3, ZT reaches 0.87 at 800 K, which is a 67% improvement.

The ZrNiSn alloy, a member of the half-Heusler family of thermoelectric materials, shows great potential for mid-to-high-temperature power generation applications due to its excellent thermoelectric properties, robust mechanical properties, and good thermal stability. The existing synthesis processes of half-Heusler alloys are, however, rather time and energy intensive. In this study, single-phase ZrNiSn bulk materials were prepared by self-propagating high-temperature synthesis (SHS) combined with spark plasma sintering (SPS) for the first time. The analysis of thermodynamic and kinetic processes shows that the SHS reaction in the ternary ZrNiSn alloy is different from the more usual binary systems. It consists of a series of SHS reactions and mass transfers triggered by the SHS fusion of the binary Ni-Sn system that eventually culminates in the formation of single-phase ternary ZrNiSn in a very short time, which reduced the synthesis period from few days to less than an hour. Moreover, the nonequilibrium feature induces Ni interstitials in the structure, which simultaneously enhances the electrical conductivity and decreases the thermal conductivity, which is favorable for thermoelectric properties. The maximum thermoelectric figure of merit ZT of the SHS + SPS-processed ZrNiSnSb alloy reached 0.7 at 870 K. This study opens a new avenue for the fast and low-cost fabrication of half-Heusler thermoelectric materials.

One-step plasma-activated sintering (OS-PAS) fabrication of single-phase high-performance CoSb 3 -based skutterudite thermoelectric material with a hierarchical structure on a time scale of a few minutes is first reported here. The formation mechanism of the CoSb 3 phase and the effects of the current and pressure fields on the phase transformation and microstructure evolution are studied in the one-step PAS process. The application of the panoscopic approach to this system and its effect on the transport properties are investigated. The results show that the hierarchical structure forms during the formation of the skutterudite phase under the effects of both current and sintering pressure. The samples fabricated by the OS-PAS technique have defined hierarchical structures, which scatter phonons more intensely over a broader range of frequencies and significantly reduce the lattice thermal conductivity. High-performance bulk Te-doped skutterudite with the maximum ZT of 1.1 at 820 K for the composition CoSb 2.875 Te 0.125 was obtained. Such high ZT values rival those obtained from single filled skutterudites. This newly developed OS-PAS technique enhances the thermoelectric performance, dramatically shortens the synthesis period and provides a facile method for obtaining hierarchical thermoelectric materials on a large scale. NPG Asia Materials (2017) 9, e352; doi:10.1038/am.2017.1; published online 24 February 2017 INTRODUCTION Thermoelectric technology uses solid-state semiconductors, which can directly convert heat into electricity and vice versa using the Seebeck effect for power generation and Peltier effect for cooling. The efficiency of thermoelectric materials is governed by the dimensionless figure of merit ZT = α 2 σT/κ, where α, σ, T and κ are the Seebeck coefficient, electrical conductivity, absolute temperature and thermal conductivity, respectively. Many studies have been conducted in the past decades 1-5 to enhance the thermoelectric properties with a focus on improving the power factor and decreasing the thermal conductivity. Because of the high electrical transport performance 6,7 and relatively good mechanical properties, 8 skutterudites are considered notably promising for commercial power generation thermoelectric applications 9,10 in the temperature range of 500-900 K. 6,7,11,12 However, the disadvantage of skutterudites is their notably high lattice thermal conductivity of 410 W m − 1 K − 1 . To obtain a higher conversion efficiency, the thermal conductivity must be further reduced.The thermal conductivity of skutterudites derives from the contributions of phonons with a notably broad range of frequencies and mean free paths (MFPs). [13][14][15][16][17] Those phonons are primarily scattered by features in the structure (dopant, nanostructures and so on) that are comparable in size to the MFP of the phonons. For example, the high-frequency (short-wavelength) phonons tend to be scattered more

Polycrystalline TiS (0.111 ≤ x ≤ 0.161) with high density and controllable composition were successfully prepared using solid-state reaction combined with plasma-activated sintering. TiS showed strong (00 l) preferred orientation with Lotgering factor of 0.32-0.60 perpendicular to the pressing direction (⊥), whereas the preferred orientation was not obvious along the pressing direction (∥). This structural anisotropy resulted in distinct anisotropic thermoelectric transport properties in TiS. At 300 K, while the Seebeck coefficient was weak anisotropic, the power factor and lattice thermal conductivity of TiS was much larger in the perpendicular direction as compared to that of the parallel direction, with an anisotropic ratio of 1.8-2.7 and 1.3-1.7, respectively. Theoretical calculations of formation energy of defects suggested that the excess Ti was most probably intercalated into the van der Waals gaps in metal-rich TiS, consistent with X-ray diffraction, high-resolution transmission electron microscopy characterization and transport measurements. With increasing x, the carrier concentration and power factor of TiS dramatically increased because of the donor behavior of Ti interstitials, which was accompanied by a significant decrease in the lattice thermal conductivity owing to the strengthened phonon scattering from structural disorder. Because of its strongest (00 l) preferred orientation and largest carrier mobility among all samples, TiS had the highest power factor of 22 μW cm K at 350 K perpendicular to the pressing direction, close to the value (37.1 μW cm K) achieved in single-crystal TiS. We found out that the maximum power factor and dimensionless figure of merit ZT could be achieved at an optimum carrier concentration of about 5.0 × 10 cm. Finally, TiS acquired the highest ZT value of 0.40 at 725 K perpendicular to the pressing direction because of the beneficial preferred orientation, improved power factor, and reduced lattice thermal conductivity.

The dependence of the electronic band structure of Mg2Si0.3–x Ge x Sn0.7 and Mg2Si0.3Ge y Sn0.7–y (0 ≤ x, and y ≤ 0.05) ternary solid solutions on composition and temperature is explained by a simple linear model, and the lattice thermal conductivity of solid solutions with different Si/Ge/Sn ratios is predicted by the Adachi model. The experimental results show excellent consistency with the calculations, which suggests that the approach might be suitable for describing the electronic band structure and the lattice thermal conductivity of other solid solutions using these simple calculations. Beyond this, it is observed that the immiscible gap in the Mg2Si1–x Sn x binary system is narrowed via the introduction of Mg2Ge. Moreover, for the Sb-doped solid solutions Mg2.16(Si0.3Ge y Sn0.7–y )0.98Sb0.02 (0 ≤ y ≤ 0.05), the energy offset between the light conduction band and the heavy conduction band at higher temperatures (500–800 K) will decrease with an increase in Ge content, thus making a contribution to the conduction band degeneracy and enhancing the power factor in turn. Meanwhile, mass fluctuation and strain field scattering processes are enhanced when Ge is substituted for Sn in Mg2.16(Si0.3Ge y Sn0.7–y )0.98Sb0.02 (0 ≤ y ≤ 0.05) because of the large discrepancy between the mass and size of Ge and Sn, and the lattice thermal conductivity is decreased as a consequence. Thus, the thermoelectric performance is improved, with the figure of merit ZT being >1.45 at ∼750 K and the average ZT value being between 0.9 and 1.0 in the range of 300–800 K, which is one of the best results for Sb-doped Mg2Si1–x–y Ge x Sn y systems with a single phase.

S substitution improves the ZT value of Cr2Se3−3xS3x which has a two-dimensional layered structure by 32%.

A series of Sb-doped Mg2Si(1-x)Sb(x) compounds with the Sb content x within 0 ≤ x ≤ 0.025 were prepared by self-propagating high-temperature synthesis (SHS) combined with plasma activated sintering (PAS) method in less than 20 min. Thermodynamic parameters of the SHS process, such as adiabatic temperature, ignition temperature, combustion temperature, and propagation speed of the combustion wave, were determined for the first time. Nanoprecipitates were observed for the samples doped with Sb. Thermoelectric properties were characterized in the temperature range of 300-875 K. With the increasing content of Sb, the electrical conductivity σ rises markedly while the Seebeck coefficient α decreases, which is attributed to the increase in carrier concentration. The carrier mobility μ(H) decreases slightly with the increasing carrier concentration but remains larger than the Sb-doped samples prepared by other methods, which is ascribed to the self-purification process associated with the SHS synthesis. In spite of the increasing electrical conductivity with the increasing Sb content x, the overall thermal conductivity κ decreases on account of a significantly falled lattice thermal conductivity κ(L) due to the strong point defect scattering on Sb impurities and possibly enhanced interface scattering on nanoprecipitates. As a result, the sample with x = 0.02 achieves the thermoelectric figure of merit ZT ∼ 0.65 at 873 K, one of the highest values for the Sb-doped binary Mg2Si compounds investigated so far. A subsequent annealing treatment on the sample with x = 0.02 at 773 K for 7 days has resulted in no noticeble changes in the thermoelectric transport properties, indicating an excellent thermal stability of the compounds prepared by the SHS method. Therefore, SHS method can serve as an effective alternative fabrication route to synthesize Mg-Si based themoelectrics and some other functional materials due to the resulting high performance, perfect thermal stability, and feasible production in large scale for commercial application.