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Vanadium sulfides with lower oxidation states are expected to be an interesting choice for metal-ion batteries due to their high theoretical capacity and lower redox potential. Among these vanadium sulfides, V3S4 having oxidation states of +2 and +3 has rarely been explored as an electrochemical energy storage material. Herein, we demonstrate a unique synthesis strategy of obtaining V3S4 nanoflakes from bulk V2O5via a two-step synthesis route where the hydrothermal treatment in the presence of ammonium sulfide modifies the bulk V2O5 into a VO2 nanosheet and the solid–gas reaction completely sulfurizes the chemically modified compound at 500 °C resulting in the formation of V3S4 nanoflakes. The solid–gas reaction synthesis strategy under controlled pressure–temperature conditions using a chemically modified precursor results in the formation of V3S4 at a relatively low temperature. Detailed physicochemical characterization indicates the phase purity and high air stability of the material. The electrochemical characterization indicates the anodic behavior of the as-prepared material as a Li-ion battery anode having a high reversible capacity of 781 mA h g−1 at 25 mA g−1. [ABSTRACT FROM AUTHOR]

Ti2−xNiCoSnSb (x = 0.125, 0.250, 0.375, and 0.500) half Heusler (HH) high-entropy thermoelectric alloys were synthesized by the arc melting—ball milling—spark plasma sintering route. The impact of secondary phase content on the thermoelectric properties in these alloys was examined. Ni-rich intermetallic (Ni3Sn2, Ni3Sn4) compounds were observed; the intermetallic content increased for lower Ti content, e.g., Ti1.5NiCoSnSb. A Ni-rich full Heusler (FH) secondary phase was also observed. These results were consistent with first-principles calculations that show that the formation enthalpy of Ti1.5NiCoSnSb was higher than that of Ti2NiCoSnSb and the full Heusler (FH) TiNi2Sn phase. In lower Ti content samples, the electrical conductivity increased, and lattice thermal conductivity decreased at the expense of thermopower owing to higher FH and the Ni3Sn2 phase content. Ti1.5NiCoSnSb exhibited lower lattice thermal conductivity of 3.5 W/mK, compared to 5.4 W/mK at 823 K for Ti2NiCoSnSb due to increased phonon scattering at HH/Ni3Sn2 interfaces. But considering the decreasing power factor with lower Ti content, the maximum ZT obtained in Ti1.875NiCoSnSb (0.171 at 973 K) was only marginally higher than the value for Ti2NiCoSnSb. Further, compositional tuning is hence necessary to maximize the power factor. [ABSTRACT FROM AUTHOR]

Sn-doped TiFe0.5Ni0.5Sb1−xSnx (x = 0, 0.05, 0.1, 0.2) were synthesized by vacuum arc melting (VAM). In addition to the half-Heusler phase, secondary phases of Fe–Sb-rich compound and Ti-rich compounds were obtained after VAM. The alloys were then subjected to ball milling for 1 h and 5 h. Ball milling for 1h led to microcrystalline grains, while that for 5 h led to nanocrystalline grains. Ball milling followed by spark plasma sintering (SPS) at 1173 K led to significant reduction in size of secondary phases in the microstructure. The undoped sample exhibited a ZT of 0.008 at 873 K for both 1h and 5h BM-SPS samples. [ABSTRACT FROM AUTHOR]

Filling the voids of cage forming compounds with loosely bound electropositive elements and by incorporating nano-sized secondary phases are promising approaches to enhance the thermoelectric figure of merit of these materials. Hence, in this work, by combining these two approaches—inserting In into the voids of skutterudite Co4Sb12 as well as dispersing nanoparticles (GaSb)—we have synthesized various samples via ball-milling and spark plasma sintering. InSb as the secondary phase of the matrix, mixed with GaSb, forms the solid solution Ga1−xInxSb. Nanocrystalline grains together with a few larger grains (10–30 μm) are found to be spread in In0.2Co4Sb12. The former is comprised of either InSb, GaSb or Ga1−xInxSb. Because of their identical space group and similar lattice parameters, InSb, GaSb and Ga1−xInxSb could not be detected separately in EBSD. High resolution transmission electron microscopy (HRTEM) was used to resolve different phases, which showed GaSb grains of size ∼10–30 nm and InSb grains of size ∼30–100 nm. Scattering of charge carriers at the interfaces of InSb, GaSb and Ga1−xInxSb as well as the matrix phases increased both the electrical resistivity and Seebeck coefficient. The multi-scale size distribution of grains, of both the matrix phase and the secondary phases, scattered phonons within a broad wavelength range, resulting in very low lattice thermal conductivities. As a result, an enhanced figure of merit of 1.4 was achieved for the (GaSb)0.1 + In0.2Co4Sb12 nanocomposite at 773 K. [ABSTRACT FROM AUTHOR]

In the present work, the concept of mechanically activated annealing (MAA) has been applied to produce nanocrystalline Al0.3CoCrFeNi high-entropy alloys (HEAs) with reduced contamination levels. Phase evolution during conventional mechanical alloying (MA), MAA and subsequent consolidation by spark plasma sintering (SPS) have been studied in detail. Complete alloying is obtained after 15 h of MA, while milling time of 5 h and annealing at 1100 °C for 1 h have been used to achieve alloy formation during MAA. Both the MA–SPS and MAA–SPS routes have shown major FCC phase. The contamination of WC observed during MA was successfully eliminated during MAA, while the volume fraction of Cr7C3 reduced from 20% during MA–SPS to 10% after MAA–SPS. This method can serve as an effective way to produce nanostructured HEAs with minimum contamination. [ABSTRACT FROM AUTHOR]

A new single phase high entropy alloy, Ti2NiCoSnSb with half-Heusler (HH) structure is synthesized for the first time by vacuum arc melting (VAM) followed by ball-milling (BM). The BM step is necessary to obtain the single phase. Local electrode atom probe (LEAP) analysis showed that the elements are homogeneously and randomly distributed in the HH phase without any clustering tendency. When the BM was carried out for 1 hour on the VAM alloy, microcrystalline alloy is obtained with traces of Sn as secondary phase. When BM was carried out for 5 h, single HH phase formation is realized in nanocrystalline form. However, when the BM samples were subjected to Spark plasma sintering (SPS), secondary phases were formed by the decomposition of primary phase. Nanostructuring leads to simultaneous increase in S and σ with increasing temperature. The micro (1 h BM-SPS) and nanocrystalline (5 h BM-SPS) alloys exhibited a power factor (S2σ) of 0.57 and 1.02 mWm−1K−2, respectively, at 860 K. The microcrystalline sample had a total thermal conductivity similar to bulk TiNiSn sample. The nanocrystalline alloy exhibited a ZT of 0.047 at 860 K. The microcrystalline alloy showed a ZT to 0.144 at 860 K, in comparison to the nanocrystalline alloy. [ABSTRACT FROM AUTHOR]

Filling the voids of cage forming compounds with loosely bound electropositive elements and by incorporating nano-sized secondary phases are promising approaches to enhance the thermoelectric figure of merit of these materials. Hence, in this work, by combining these two approaches—inserting In into the voids of skutterudite Co4Sb12 as well as dispersing nanoparticles (GaSb)—we have synthesized various samples via ball-milling and spark plasma sintering. InSb as the secondary phase of the matrix, mixed with GaSb, forms the solid solution Ga1−xInxSb. Nanocrystalline grains together with a few larger grains (10–30 μm) are found to be spread in In0.2Co4Sb12. The former is comprised of either InSb, GaSb or Ga1−xInxSb. Because of their identical space group and similar lattice parameters, InSb, GaSb and Ga1−xInxSb could not be detected separately in EBSD. High resolution transmission electron microscopy (HRTEM) was used to resolve different phases, which showed GaSb grains of size ∼10–30 nm and InSb grains of size ∼30–100 nm. Scattering of charge carriers at the interfaces of InSb, GaSb and Ga1−xInxSb as well as the matrix phases increased both the electrical resistivity and Seebeck coefficient. The multi-scale size distribution of grains, of both the matrix phase and the secondary phases, scattered phonons within a broad wavelength range, resulting in very low lattice thermal conductivities. As a result, an enhanced figure of merit of 1.4 was achieved for the (GaSb)0.1 + In0.2Co4Sb12 nanocomposite at 773 K. [ABSTRACT FROM AUTHOR]

The sintering behavior and structural stability of two-dimensional (2D) WS 2 nanosheet powder under high-temperature pressure-assisted spark plasma sintering (SPS) were investigated. Transformation of the 2D particle morphology to coarse WS 2 platelets was observed during sintering at a higher temperature of and above 1000 oC. Particle coarsening was caused by diffusion bonding of multiple 2D-WS 2 nanosheet particles on the basal plane surface. Consolidation of the 2D-WS 2 nanosheet particles at lower sintering temperatures primarily involved particle-particle interlocking and limited particle-particle diffusion bonding. About 94% of the theoretical density was achieved without any sintering additives on SPS at 1200 oC. The WS 2 stoichiometry was retained even at 1200 °C with no indication of any thermal degradation. A fraction of the hexagonal 2H-type structure of the 2D-WS 2 transformed to rhombohedral 3R phase during SPS at and above 800 °C. The as-sintered 2D-WS 2 showed a low coefficient of friction in the range of 0.06–0.1. The observed tribological behavior was correlated with its thermo-mechanical properties and sintering characteristics at different temperatures. [ABSTRACT FROM AUTHOR]