Subramshu Shekar Bhattacharya

For the first time, this study shows that distortion in a crystal structure due to magnetic effects is possible in a lattice with extreme chemical disorder. The transition metal-high entropy oxide (TM-HEO), (Co,Cu,Mg,Ni,Zn)O, has been attracting a lot of attention due to its unique application potential in many fields including electrochemical energy storage. In the present investigation, nanocrystalline TM-HEO was synthesised by three bottom-up methods. The presence of distortion in the rocksalt crystal structure, revealed by X-ray diffraction and Raman spectroscopy, and correlated with magnetic measurements from Superconducting quantum interference device (SQUID) and Electron paramagnetic resonance (EPR) studies could be attributed to the additive effects of exchange striction (from the magnetic constituents) and magnetic anisotropy (from the decreased crystallite size). Iron has been added to the TM-HEO to show that a higher amount of magnetic constituent increases the distortion in the lattice. Nanocrystalline TM-HEO also showed a “core-shell” magnetic behaviour below the bifurcation temperature arising from the uncompensated or canted spin at the surface. Néel temperature of the nanocrystalline TM-HEO is reported for first time to be as high as 700 K. This study helps unravel the structure and magnetic properties of such high entropy materials, and augurs a definite scope for better understanding of the factors influencing the crystal structure in high entropy oxides.

Technologically relevant strongly correlated phenomena such as colossal magnetoresistance (CMR) and metal-insulator transitions (MIT) exhibited by perovskite manganites are driven and enhanced by the coexistence of multiple competing magneto-electronic phases. Such magneto-electronic inhomogeneity is governed by the intrinsic lattice-charge-spin-orbital correlations, which, in turn, are conventionally tailored in manganites via chemical substitution, charge doping, or strain engineering. Alternately, the recently discovered high entropy oxides (HEOs), owing to the presence of multiple-principal cations on a given sub-lattice, exhibit indications of an inherent magneto-electronic phase separation encapsulated in a single crystallographic phase. Here, the high entropy (HE) concept is combined with standard property control by hole doping in a series of single-phase orthorhombic HE-manganites (HE-Mn), (Gd0.25La0.25Nd0.25Sm0.25)1-xSrxMnO3 (x = 0–0.5). High-resolution transmission microscopy reveals hitherto-unknown lattice imperfections in HEOs: twins, stacking faults, and missing planes. Magnetometry and electrical measurements infer three distinct ground states—insulating antiferromagnetic, unpercolated metallic ferromagnetic, and long-range metallic ferromagnetic—coexisting or/and competing as a result of hole doping and multi-cation complexity. Consequently, CMR ≈1550% stemming from an MIT is observed in polycrystalline pellets, matching the best-known values for bulk conventional manganites. Hence, this initial case study highlights the potential for a synergetic development of strongly correlated oxides offered by the high entropy design approach.

Superplastic behaviour is well established in fine-grained zirconia ceramics at high temperatures (well above 0.5Tm, where Tm is the absolute melting point). In this study, an attempt was made to develop a nanocrystalline 5 mol.% yttria partially stabilised zirconia ceramic (5Y-PSZ) capable of exhibiting superplastic flow at relatively lower temperatures. A physical vapour processing route was used to synthesise the powders which was consolidated and subjected to a pressureless sintering route. Dense specimens with grain sizes in the nanometer range were subjected to tensile as well as compressive tests in the temperature range of 1283-1523K at different stress or strain rate levels in order to determine the superplastic deformation behaviour. The stress - strain - strain rate response of the material was analysed by a model for grain boundary sliding controlled superplastic flow. It was demonstrated that the strain rates predicted by the model are in close agreement with the experimentally observed ones.

Conversion of Cr(VI) to Cr(III) in mitigating pollution of water bodies is of significant importance to public health due to the fact that Cr(VI) is known to be a potent carcinogen, while Cr(III) is relatively low in toxicity. Photocatalytic approaches are considered as important means to achieve this reduction. Here, TiO2/SnS2 core-shell nanostructures have been produced using a single-step hydrothermal method and its photocatalytic activity is tested for the reduction of aqueous Cr(VI). The structural and optical properties of the as-synthesized products are characterized by XRD, HRTEM, Raman, FTIR, XPS and DRS techniques. The present work reveals that by calcining the core-shell nanoparticles in Ar atmosphere a defective Ti3O5 phase is formed as the core with low band gap, and hence, offers improved light absorption in the visible range. However, its photoactivity was found to be lower than that of the core-shell nanoparticles annealed in oxidizing atmosphere. The observed lower photoreduction was due to the presence of midgap states which acted as recombination centres and hence, reduced the photocatalytic activity.

Multicomponent rare earth oxide (REO) nanocrystalline powders containing up to seven equiatomic rare earth elements were successfully synthesized in a single-phase CaF2-type (Fm-3 m) structure. The addition of more than six elements resulted in the formation of a secondary phase. Annealing at 1000°C for 1 h led to the formation of a single-phase (Ia-3) even in the 7-component system. In the absence of cerium (Ce4+), secondary phases were observed irrespective of the number of cations or the extent of thermal treatment indicating that cerium cations played a crucial role in stabilizing the multicomponent REOs into a phase pure structure.

In recent years, various boron nitride (BN) based nanostructures have emerged as drug carriers for biomedical applications. Herein, the drug carrier potential of spherical BN nanoparticles was investigated with curcumin as a drug in the treatment of A549 lung cancer cells. Spherical boron nitride nanoparticles were synthesized in a single step by a nebulized spray pyrolysis (NSP) technique using boron trioxide [B2O3] and urea [CO (NH2)2] as precursors. Rietveld refinement revealed that the synthesized nanoparticles had hexagonal crystal structures with space group P63mc. The specific vibrational modes (B–N: 781 cm−1 and 1358 cm−1) and binding energy (B–N: 398.3 eV) confirmed BN formation. The NSP technique ensured that the nanoparticles had a spherical morphology with sizes in the range of 20–180 nm. Excellent curcumin encapsulation/loading efficiency was achieved and the bioavailability improved using a simple chemical surface modification of the BN nanoparticles. The cytocompatibility and cytotoxicity of the encapsulated BN nanoparticles in A549 lung cancer cells was evaluated in vitro using MTT assays. Further, the assessment of apoptosis in cells through flow cytometry confirmed that curcumin encapsulated BN nanoparticles showed significant A549 cell death, suggesting that BN nanoparticles can be a promising material as a drug carrier. Graphical abstract: [Figure not available: see fulltext.].

Phase pure tin oxide (SnO2) and indium doped SnO2 nanocrystalline powders were synthesized in a single step by a flame spray pyrolysis method. The as-synthesized powders were characterized by standard techniques of x-ray diffraction, scanning and transmission electron microscopy, x-ray photoelectron spectroscopy and absorption spectroscopy. Using x-ray diffraction, it was established that the powders had the rutile (cassiterite) structure with tetragonal unit cells in the space group P42/mnm. Using the Rietveld refinement method, structural analysis was carried out in order to obtain the lattice parameters, volume and density. X-ray photoelectron spectra confirmed the presence of indium in the doped samples. Absorption spectra revealed that the powders were transparent to the visible spectrum with a sharp absorption below 350 nm. Energy bandgaps, estimated by Tauc plots, established that increasing the doping concentration reduced the bandgap.

Superplasticity is the ability of a polycrystalline material to exhibit, in a relatively isotropic manner, large elongations when deformed in tension. This property is exploited during superplastic forming in the fabrication of complex-shaped components which are otherwise technically difficult or economically costly to form by conventional methods. The ability of some titanium alloys to undergo superplastic deformation coupled with their diffusion bonding capability provides excellent opportunities to fabricate intricate parts resulting in significant cost and weight savings, particularly in the manufacture of aerospace structures. In the present work, experimental studies on the superplastic forming of square shaped components from titanium alloy Ti-6Al-4V sheets of 2 mm thickness that are commonly used in aerospace structural applications are reported. Superplastic forming of suitably sized blanks was carried out at temperatures of 1,148 K (875 C), 1,173 K (900 C) and 1,200 K (927 C) using constant argon gas pressures of 1, 1.4 and 1.8 MPa. The formed components were characterized for their thickness distribution, mechanical and metallurgical properties. Diffusion bonding characteristics of the alloy sheet of 1 mm thickness were investigated for varying time durations at different temperatures and 4 MPa stress under an argon atmosphere and lap shear strength values of the joints are reported. Efforts were then made to carry out diffusion bonding concurrent with superplastic forming (SPF/DB). For these experiments, two sheets of 1 mm thickness each were superplastically formed into square components of size 80 mm square and 60 mm deep with an initial forming cycle followed by a diffusion bonding cycle by subjecting the component to a static pressure (higher than the forming pressure) for a specified period of time, which ensured good bonding between the two sheets. The components formed by the SPF/DB process were compared with those formed from the monolithic 2 mm sheet and the results are presented. © 2013 Indian Institute of Metals.

Nanocrystalline alumina particles were successfully synthesized in an indigenously built flame-assisted spray pyrolysis (FASP) setup. Aluminium nitrate nonahydrate was dissolved in deionized water in the desired amount to get a precursor solute concentration of 0.01 mol L-1. The solution was then atomized by using a jet nebulizer. The droplets were then pyrolyzed in an oxy-LPG (liquifiedpetroleum gas) flame producing nanocrystalline alumina particles which were confirmed by x-ray diffraction (XRD) and transmission electron microscopy (TEM) analysis. Particle morphology was studied using scanning electron microscopy (SEM). The synthesized powders were predominantly metastable transition aluminas exhibiting spherical morphology and were unagglomerated. The synthesized powders were then calcined at 1473 K for 2 hour to transform into the stable α-Al2O3 (corundum) phase.

Multilayer diamond films were deposited on a reaction bonded silicon carbide (RB-SiC) substrate by a single-step continuous process as well as a three-step intermittent process in a hot filament chemical vapour deposition (HF-CVD) equipment. The recipes of HF-CVD during the single-step multilayer film (C-MC) were selected by varying the CH4/H2 concentration and chamber pressure such that the initially deposited diamond grains were microcrystalline, which became progressively finer with further deposition. In the case of the intermittently grown films (Ix-MC), a one, two and three-layered structure, each layer having a gradual decrease in grain size was deposited with the same recipes as that of the C-MC. All the films were characterised using SEM, AFM and Raman spectroscopy. Fine grains in the range of 0.2–0.7 μm were identified in the C-MC and sharply faceted diamond crystallites having sizes between 0.5- 2 μm were seen in the intermittently deposited coatings. The hardness values were 71 GPa, 56 GPa and 46 GPa for the intermittent coatings. In contrast, a lower hardness of 29 GPa was obtained in C-MC. The friction coefficients of all the diamond films were measured up to 106 sliding cycles (530 m) using a ball-on-disc universal tribometer. In the initial stages of sliding, the continuously grown C-MC exhibited a friction coefficient ~0.17, while the intermittently coated three-layered film (I3-MC) showed a friction coefficient of 0.28. However, beyond 40,000 sliding cycles, the friction coefficient of I3-MC showed a value of ~0.05, which was slightly lower than C-MC (~0.06).