Search

The synthesis of a novel multicomponent perovskite was carried out by substituting 5 cations in the A-site of BaTiO 3 lattice. The mechanochemical synthesis involved 5 h of ball milling followed by annealing at 800 °C for 2 h and the compound was sintered at 1200 °C for 2 h to form the cubic perovskite phase adopting the P m 3 ‾ m space group. The lattice parameter was found to be 0.391 nm. The reduction in the tetragonality and the local distortion in the compound were observed in the 2D charge distribution maps. The band gap of the compound was found to be 2.94 eV with a dielectric constant value of 270. Ferroelectric hysteresis loops were recorded and the leakage current density was found to be in the order of 10-5 A/cm2. [ABSTRACT FROM AUTHOR]

A new set of half-Heusler high-entropy alloys Ti 2 NiCoSn 1-x Sb 1+ x (x = 0.5, 1), with a valence electron count higher than 18, were investigated for thermoelectric applications. Vacuum arc melting was employed for synthesis. Atom probe analysis confirmed single-phase at atomic level. The alloys were subsequently ball milled for 1 h followed by spark plasma sintering for consolidation. In 1 h BM cases, the alloy with x = 0.5 exhibited a low lattice thermal conductivity of 2.48 Wm−1K−1, and a ZT of 0.29 at 873 K. Image, graphical abstract [ABSTRACT FROM AUTHOR]

Entropy stabilization was attempted through the addition of 5 different rare earth cations, 5 different transition metal cations and all these 10 cations in the form of three compounds based on NiFe 2 O 4. Equiatomic ratios of these cations were used to maximize the entropy stabilization effect. The compounds attempted and investigated were NiFe 1.9 (Dy 0.02 Er 0.02 Gd 0.02 Ho 0.02 Tb 0.02)O 4 , (Co 0.2 Cr 0.2 Fe 0.2 Mn 0.2 Ni 0.2)Fe 2 O 4 and (Co 0.2 Cr 0.2 Fe 0.2 Mn 0.2 Ni 0.2)Fe 1.9 (Dy 0.02 Er 0.02 Gd 0.02 Ho 0.02 Tb 0.02)O 4. Synthesis of these compounds were carried out by sol–gel technique and all the compounds were found to crystallize in F d 3 ¯ m spinel structure indicating the entropy stabilization. An appropriate calculation of configurational entropy of mixing employing the sublattice model is demonstrated for such entropy stabilized oxides. Image, graphical abstract [ABSTRACT FROM AUTHOR]

Abstract Low-temperature synthesis of aluminium boride (AlB 12) powders by mechanochemical synthesis of Al and B powder blend has been carried out. The influence of milling time on the phase formation temperature and annealing time of AlB 12 was investigated. X-ray diffraction of the milled and annealed powder samples revealed the formation of AlB 2 without any oxide phases. AlB 12 powder was synthesized at a significantly low temperature of 1030 K. The processed AlB 12 powder was nanocrystalline in nature confirmed by transmission electron microscopy investigations. It has been shown that mechanochemical synthesis could be an attractive route for the synthesis of borides at low temperatures. [ABSTRACT FROM AUTHOR]

Abstract The present study reports a modification in the composition of the well-studied AlCoCrFeNi high-entropy alloy (HEA), by replacing Cr with Mn. AlCoFeMnNi alloy was made by vacuum arc melting and its thermal stability behaviour was studied. The as-cast structure exhibited B2 phase. Upon heat treatment at 1050 °C for 50 h, it gave rise to a disordered B2 and FCC phases. Calphad (CALculation of PHAse Diagrams) studies using TCHEA (v2.1) database predicted formation of B2 and FCC phases. Due to absence of embrittling σ-phase, wider applicability of the alloy is possible. Graphical abstract Unlabelled Image [ABSTRACT FROM AUTHOR]

A half-Heusler (HH) type high entropy alloy (HEA) Ti 2 NiCoSnSb has been synthesized by a fast powder metallurgy route for the first time. Mechanical alloying (MA) by wet milling produced a powder with a minor fraction of the HH phase. The dry milling route resulted in the desired single-phase HH material. Consolidation of the nanocrystalline mechanically alloyed (MA) powder by spark plasma sintering (SPS) resulted in a majority HH phase. Interestingly, the nanocrystalline alloy exhibited simultaneous enhancement in the Seebeck coefficient and electrical conductivity, with a maximum ZT of 0.13 at 973 K observed for the dry milled alloy. The band structure obtained by density functional theory (DFT) was in good agreement with the ultraviolet-visible-near infrared (UV-Vis-NIR) absorption spectroscopy results. The DFT calculations and microstructural analysis suggest that phase separation strongly influenced the thermoelectric properties. The band structure calculations provided a good rationale for the phase evolution and thermoelectric properties. [Display omitted] • High entropy half-Heusler alloy was synthesized by mechanical alloying. • Milling was done under Toluene (wet MA) and under Argon (dry MA). • The dry MA alloy exhibited higher figure of merit than reported arc melted alloys. • The electronic band structure was obtained by first principles calculations. • It was found to be in good agreement with UV-Vis-NIR spectroscopy results. [ABSTRACT FROM AUTHOR]

• First report on multi element substitution in layered delafossite type oxide. • Significant bond angle distortion originating from multiple element substitution. • Band gap tuning obtained by increase in number of elements in lattice. • All the compounds exhibit activity for photocatalytic degradation of methylene blue. A series of novel mixed oxides with honeycomb layered structure viz. delafossite-type Na 3 M 2 SbO 6 (where M = Cu; Ni; Cu, Ni; Cu, Ni, Co; Cu, Ni Co, Fe; Cu, Ni Co, Fe, Mn) were synthesized by a facile method. The synthesis involved heating of the metal precursors at 1100 °C followed by air quenching. The structure was analogous to that of Na 3 Ni 2 SbO 6 (space group: C2/m). The effect of increased entropy on the Jahn-Teller distortion effect, as previously reported in M = Cu system, was evident from the X-ray diffraction (XRD)/Rietveld structural analysis. The as-synthesized oxides showed photocatalytic activity as confirmed by the degradation of methylene blue. This was correlated to a drop in the optical band-gap from 2.41 to 1.96 eV on increasing the number of elements as well as its effect on the band energies are proposed here. [ABSTRACT FROM AUTHOR]

CoCrFeMnNiV high entropy alloy (HEA) exhibits a high content of σ-phase (70 vol%) when produced by casting route. In the present work, a combination of mechanical alloying (MA) and spark plasma sintering (SPS) has been used to synthesize nanocrystalline CoCrFeMnNiV HEA where the formation of σ-phase has been avoided. Electron microscopy and atom probe tomography analysis indicated the formation of FCC structured HEA matrix along with (Cr,V) carbide (15 vol%) precipitation, without the presence of σ-phase in SPS processed alloy. Gibbs energy vs composition (G-x) diagrams of binary subsystems and possible carbides and oxides substantiate the absence of σ-phase during SPS of CoCrFeMnNiV alloy. Thus, the unsolicited contamination during MA-SPS route proves beneficial in suppressing the complex phase formation. Image 1 • Cast CoCrFeMnNiV shows 71 vol% of sigma (σ) phase enriched in Cr and V. • σ-phase is avoided in nanocrystalline CoCrFeMnNiV formed by MA-SPS route. • Un-solicited contamination of during MA-SPS leads to suppression of σ phase. • Calphad based G vs composition plots are used to explain absence of σ phase. [ABSTRACT FROM AUTHOR]

Y 2 Ti 2 O 7 , Y 2 Zr 2 O 7 , Y 2 Hf 2 O 7 have pyrochlore structure and are found to be promising dispersoids for steel-based oxide dispersed strengthened (ODS) alloys, which are used in nuclear reactor and supercritical boilers applications. They should be nanocrystalline and possess high strength for better performance of ODS alloys. In the present study, oxides are successfully synthesized through Mechanically Activated Annealing (MAA) and Reverse Co-precipitation (RCP) techniques. The RCP technique could yield pyrochlore phase at lower temperatures with finer crystalline size. The structural analysis was done through X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and Raman spectra to confirm the presence of single phase and nanocrystallinity of oxides. Raman spectra probes into local disorderness in the structure and bond length. Y 2 Hf 2 O 7 synthesized through MAA has the highest hardness (20 GPA) among all the oxides synthesized in the present study and those reported so far. Further, Y 2 Ti 2 O 7 has higher Young's modulus than the Y 2 Zr 2 O 7, Y 2 Hf 2 O 7 due to the orderness in the structure. Image 1 • The mechanical properties of Y 2 Zr 2 O 7 and Y 2 Hf 2 O 7 are reported experimentally and compared with Y 2 Ti 2 O 7 for the first time. • The synthesis parameters of MAA and RCP were investigated to establish the optimum phase formation time and temperature. • The consolidation (SPS) time and temperature are lower compared to other consolidation techniques reported earlier. • Y 2 Hf 2 O 7 (by MAA route) has the highest hardness of 20 GPa and Y 2 Ti 2 O 7 has the highest Young's modulus (256 GPa). [ABSTRACT FROM AUTHOR]

Eutectic high entropy alloys synthesized via conventional casting have been intensely studied recently owing to their superior mechanical properties. In the current work, AlCoCrFeNi 2.1 eutectic high entropy alloy has been synthesized for the first time via a combination of mechanical alloying and spark plasma sintering (MA-SPS) route. The alloy primarily showed FCC and B2 phases in addition to minor secondary phases. As opposed to previous literature on MA-SPS, WC contamination from milling media was eliminated by adopting a shorter milling time. Apart from the elimination of WC, the adopted strategy is an effective method to obtain a heterogeneous microstructure, which has been demonstrated to enhance both strength and compressive strain to failure. Heat treatment of the alloys for a short duration of 10 min at 1200 °C was found to improve both strength and compressive plasticity. This was attributed to the reprecipitation of secondary phases and the presence of heterogeneous microstructure. Image 1 • Minimum WC contamination due to shorter milling durat. • MA+SPS resulted in bimodal (hierarchical) microstructure (FCC, B2,carbides). • Bimodal and multiphase microstructure resulted in superior strength but brittle carbide and oxides limited the plasticity. • Heat treatment of MA+SPS resulted in increase in strain to failure. • Strengthening contributions was quantified and the prime strengthening factor was found to be grain refinement. [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]

The combined effects of Ba-filling the voids and GaSb nanophase incorporation in the matrix of Co 4 Sb 12 have been studied for thermoelectric properties. High energy ball-milling was used to disperse GaSb in Ba 0·2 Co 4 Sb 12. A slight off-stoichiometry between Co and Sb generated CoSb 2 and CoSb phases. Electron back-scattered diffraction showed nanocrystalline (50–200 nm) GaSb grains uniformly distributed on the matrix along with a few large grains (1–3 μm). The low electrical resistivity (ρ) of Ba 0·2 Co 4 Sb 12 can be attributed to the +2 oxidation state of Ba. The increase in ρ of composites occurred due to the enhanced scattering of charge carriers at interfaces. No significant change in Seebeck coefficients was found in composites. The simultaneous effect of anharmonicity induced by Ba-filler in the voids and the enhanced interfaces between GaSb and the matrix reduced lattice thermal conductivity and brought about an improvement in zT from 0.75 to ~1.0 for (GaSb) 0.4 +Ba 0·2 Co 4 Sb 12 at 773 K. [ABSTRACT FROM AUTHOR]

• First report on Al-W-Cu ternary phase formation in 2D-WS 2 reinforced Al-4Cu. • Formation of Al-W-Cu ternary phase confirmed by STEM analysis. • Thermodynamics of Al-W-Cu phase formation studied by Calphad approach. • 41% enhancement in hardness of Al-4Cu composites due to 2D-WS 2 and Al-W-Cu phases. • Fraction of Cu affects the hardness and Young's modulus of Al-W-Cu ternary phase. Nanostructured two-dimensional (2D) WS 2 reinforced Al-4Cu alloy matrix composites were prepared via spark plasma sintering (SPS) at 550 and 570 °C. The microstructure and interface characteristics of the composites were investigated. In situ formation of Al-W-Cu ternary phase at the grain boundaries of the Al-rich matrix phase was observed on sintering. High-resolution transmission electron microscopy (HRTEM) and scanning TEM (STEM) with quantitative mapping confirmed that the structure of ternary Al-W-Cu (Al 12 W 0.6 Cu 0.4) belongs to bcc phase (space group: Im 3 ̅ (204), similar to that of binary bcc-Al 12 W phase. Such bcc intermetallic phase was absent in pure Al-matrix under identical sintering conditions. The ternary intermetallic phase along with the retained 2 D -WS 2 contributed to about 41% increase in the hardness of the alloy composite. Nanoindentation hardness and Young's modulus of the Al-W-Cu ternary intermetallic were found to be around 4.6 GPa and 104 GPa, respectively. The formation mechanism of Al-W-Cu intermetallic phase from 2 D -WS 2 nanosheets and Al-4Cu matrix was elucidated with Calphad (Calculation of Phase Diagrams)-based Gibbs energy analysis. [ABSTRACT FROM AUTHOR]

• The nanocomposite of Yb 0.2 Co 4 Sb 12 and InSb showed a maximum zT of ~1.2 at 773 K. • Uniform distribution of nanocrystalline InSb grains on the Yb 0.2 Co 4 Sb 12 matrix. • Combined filling and nanocomposite result in low lattice thermal conductivity. • Composite approach is effective in improving thermoelectric efficiency of materials. Out of several methods, one of the most explored strategies to decrease the lattice thermal conductivity of Co 4 Sb 12 -based materials are either filling suitable electropositive elements into the voids or the formation of nanocomposites. These two approaches were combined in this work by filling Yb into the void of Co 4 Sb 12 and preparing nanocomposites of Yb 0.2 Co 4 Sb 12 and InSb according to the formula (InSb) x + Yb 0.2 Co 4 Sb 12 (where x = 0.1, 0.2, 0.3, 0.4), via ball-milling and spark plasma sintering. Yb 2 O 3 and CoSb 2 as impurity phases were found at the grain boundaries. EBSD and TEM micrographs showed nanocrystalline InSb phase (20–200 nm) dispersed in the matrix grains. The charge transfer from Yb filler with an oxidation state of +3 to Co 4 Sb 12 yielded a low electrical resistivity (ρ) of the matrix. An increase in ρ and Seebeck coefficient (S) in the composites with x = 0.1 and 0.3 occurred due to the higher amount of oxide impurities in these two samples and the scattering of charge carriers at the interfaces induced by the secondary phases. The other two composites with x = 0.2 and 0.4 exhibited ρ(T) and S(T) similar to the Yb 0.2 Co 4 Sb 12 matrix. The dispersion of the InSb and Yb 2 O 3 phases at the grain boundaries combined with the anharmonicity introduced by the fillers (Yb) in the voids enhanced the scattering of phonons within a broad wavelength range and reduced the lattice thermal conductivity significantly. Hence, a highest zT of ~1.2 at 773 K with a thermoelectric efficiency of 8.89% and 8.28% (423–773 K) were obtained for (InSb) 0.1 + Yb 0.2 Co 4 Sb 12 and (InSb) 0.2 + Yb 0.2 Co 4 Sb 12 nanocomposites, respectively. [ABSTRACT FROM AUTHOR]