Sudakshina Roy

THE STRUCTURAL TRANSFORMATION OF ANATASE TIO2 BY HIGH-ENERGY VIBRATIONAL BALL MILLING

The structural transformation of anatase TiO2 by high-energy vibrational ball milling was studied in detail by different analytical methods of x-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). This structural transformation involves both phase transition and nanoparticle formation, and no amorphization was observed. The crystallite size was found to decrease with milling time down to nanometer size similar to 13 nm and approaching saturation, accompanied by phase transformation to metastable phases, i.e., TiO2(II), which is a high-pressure phase and TiO2(B), which was identified in ball-milled powder reported for the first time in this paper. These phases eventually started transforming to rutile by further milling.

High emissivity coating on C-263 substrate for high temperature applications

An aluminium phosphate based high emissivity coating has been deposited on Ni based superalloy C-263 by dip coating method wherein the substrate was immersed in a chemical sol followed by curing of the green coating. Deposition of the coating by spraying the sol on the substrate surface has also been explored. The presence of carbon imparted high emissivity to the coating. Depending on the thickness of the coating, it exhibited an emissivity value of 0·6–0·9 in the wavelength range of 2.5–25 μm. The coating was found to offer good oxidation resistance to the substrate. Cyclic oxidation performance of uncoated and coated substrates has been evaluated at 800 and 1000°C for 100 h in air.

AlPO4–C composite coating for high emissivity and oxidation protection applications

Abstract AlPO4–C coating deposited by sol–gel technique on Ni base superalloy substrate Nimonic-75 has been examined for high emissivity applications. The coating showed a spectral (2–25 μm wavelength) emissivity in the range of 0·8–0·91, and the emissivity increased with coating thickness. The coating exhibited a reasonably good adhesion with the substrate, as determined from nanoscratch test. The above coating also had a good oxidation resistance in air at 800 and 900°C under cyclic heating and cooling conditions.

Optical inhomogeneities in nickel ion containing silica gels

Abstract Lateral variation in the concentration of Ni2+ in a nickel-containing, flat silica gel plate, caused by migration of the dopant during drying (60°C), was measured spectrophotometrically as a function of absorption of Ni2+ at 393 nm. Compositional variations along individual lines of measurement could be expressed as second order polynomials, and the overall distribution took the form of a shallow basin with a minimum at the central region of the gel plate.

Property mapping of polycrystalline diamond coatings over large area

Large-area polycrystalline diamond (PCD) coatings are important for fields such as thermal management, optical windows, tribological moving mechanical assemblies, harsh chemical environments, biological sensors, etc. Microwave plasma chemical vapor deposition (MPCVD) is a standard technique to grow high-quality PCD films over large area due to the absence of contact between the reactive species and the filament or the chamber wall. However, the existence of temperature gradients during growth may compromise the desired uniformity of the final diamond coatings. In the present work, a thick PCD coating was deposited on a 100-mm silicon substrate inside a 915-MHz reactor; the temperature gradient resulted in a non-uniform diamond coating. An attempt was made to relate the local temperature variation during deposition and the different properties of the final coating. It was found that there was large instability inside the system, in terms of substrate temperature (as high as ΔT = 212 °C), that resulted in a large dispersion of the diamond coating’s final properties: residual stress (∼15.8 GPa to +6.2 GPa), surface morphology (octahedral pyramids with (111) planes to cubo-octahedrals with (100) flat top surfaces), thickness (190 μm to 245 μm), columnar growth of diamond (with appearance of variety of nanostructures), nucleation side hardness (17 GPa to 48 GPa), quality (Raman peak FWHM varying from 5.1 cm−1 to 12.4 cm−1 with occasional splitting). This random variation in properties over large-area PCD coating may hamper reproducible diamond growth for any meaningful technological application.

Synthesis, consolidation and dielectric properties of BLT ceramics processed by M-PVA gel ignition technique

Abstract Abstract Polycrystalline samples of lanthanum modified bismuth titanate ceramics with composition Bi3·25La0·75Ti3O12 (BLT) were prepared at three different sintering temperatures (1050, 1100 and 1150°C) using M-PVA gel ignition technique. The samples were characterised for structural and electrical properties. A significant amount of as burnt BLT powder was found to be crystalline in nature having a yellowish colour. Very fine BLT nanoparticles were characterised by TEM analysis. Single phase with orthorhombic crystal structure was observed for all the samples, confirmed by X-ray analysis. Dielectric properties were studied as a function of frequency and temperature. More than 95% relative density was observed for all the three sintered samples. The sample sintered at 1100°C showed highest dielectric constant and relative density and lowest loss.

Improvement in hardness of soda-lime-silica glass

Hardness is a key design parameter for structural application of brittle solids like glass. Here we report for the first time the significant improvement of about 10% in Vickers hardness of a soda-lime-silica glass with loading rate in the range of 0.1-10 N.s−1. Corroborative dark field optical and scanning electron microscopy provided clue to this improvement through evidence of variations in spatial density of shear deformation band formation as a function of loading rate.