Chanchal Ghosh

Structural characterization of electrodeposited boron

Structural characterization of electrodeposited boron was carried out by using transmission electron microscopy and Raman spectroscopy. Electron diffraction and phase contrast imaging were carried out by using transmission electron microscopy. Phase identification was done based on the analysis of electron diffraction patterns and the power spectrum calculated from the lattice images from thin regions of the sample. Raman spectroscopic examination was carried out to study the nature of bonding and the allotropic form of boron obtained after electrodeposition. The results obtained from transmission electron microscopy showed the presence of nanocrystallites embedded in an amorphous mass of boron. Raman microscopic studies showed that amorphous boron could be converted to its crystalline form at high temperatures.

Structure imaging and vanadium substitution in cubic TiCr2 Laves phase

Properties of Laves phase compounds can be tailored by alloying and microstructural engineering. V-substituted cubic TiCr2 Laves phase has been studied to understand the location of V atoms in the lattice, by structural imaging and first-principle computations. Even though Ti, V and Cr appear next to each other in the periodic table, V preferentially replaces the Ti lattice producing anti-site defects. The defect formation energy for V substitution in Ti and in Cr lattice is 0.29 and 0.40 eV, respectively. V replacement in the Ti lattice generates atomic scale strain. Atomic numbers of V, Ti and Cr being very close, this phase is not quite suitable for incoherent imaging for understanding the structure and the chemistry. Instead, difference in channelling behaviour of electron waves along the Ti columns and along the Cr columns could be exploited to preferentially image the individual atom columns. Nature of the exit phase wave, phase and amplitude has been used to understand the contrast qualitatively. The intensity distribution of any particular atom column that is disturbed by the presence of foreign atom has been used to detect the position of V atoms. This method could be extended to study other Laves phases and complex intermetallic structures to understand their structure, defects and interfaces.

Electron Microscopy Studies on Oxide Dispersion Strengthened Steels

The 9Cr ODS steel, a candidate material for fast fission and fusion reactor applications, derives its superior irradiation performance due to the dispersoids in ferrite matrix. Electron microscopy studies on mechanically alloyed Fe- Y2O3-Ti model alloys and 9Cr yttria/Ti dispersion strengthened (ODS) ferritic steels are discussed in this paper. The size distribution of dispersoid in the consolidated model alloy and the ODS steel were found to be peaking at ~ 15 nm and 5 nm, respectively. The porosity in the ODS steel was greatly reduced in comparison to the model alloy owing to a superior milling process. The dispersoids were identified as Y-Ti-O complexes. An orientation relationship between the yttria dispersoid and the ferrite grains, in which they are embedded, was observed in the model alloy. Microtexture analysis on sections of consolidated ODS alloy rods showed a [110] fiber texture typical of rods of bcc metals. The ODS steel tube however contained randomly oriented ferrite grains.

Direct structure imaging of partially collapsed omega domains in phase-separated V–Ti alloy through atom column contrast interpretation

The metastable ω phase stabilizes either in hexagonal (P6/mmm) or trigonal (

HRTEM investigation of phase stability in alumina–zirconia multilayer thin films

P\overline{3} m1

Blue green and UV emitting ZnO nanoparticles synthesized through a non-aqueous route

P3¯m1) structures depending upon the extent of collapse of the atomic planes. The stability between these two structures depends on the atomic scale shear mechanism of the ω formation and also on the relative composition of the bcc stabilizer. Detailed interpretation of the bcc → ω phase transformation demands understanding of structure, chemistry and interface of ω phase along with the bcc matrix down to the atomic level. Present paper deals with the structural imaging of partially collapsed ω structure and its lattice correspondence with the bcc phase in a phase-separated V–Ti alloy. High-resolution TEM characterization with the aid of phase-contrast image simulation and atomic structure modeling has been systematically carried out to study the structural aspects of the nanostructured ω. The contrast of the collapsed atoms and the corner atoms are understood through systematic studies of the experimental images and their corresponding column intensities, as seen along simulated exit waves and their phase and amplitude parts along different zone axis. Attempts have also been made to understand the qualitative nature of the electron channeling behavior along the different atomic positions of the ω structure.

Synthesis and characterization of SnS nanosheets through simple chemical route

Oxide dispersion strengthened (ODS) steels are being developed as promising core component structural materials for future generation fast breeder and fusion reactors because of their better high temperature thermal stability and neutron irradiation void swelling resistance as compared to currently used austenitic steels. The type of oxide dispersoids, their size distribution in metallic matrix and stability at adverse service conditions (such as high temperature along with fast neutron irradiation) governs the physical and mechanical properties of the steel. Yttria (Y2O3), is the most preferred oxide dispersoid being used in the ODS steels, because of its superior thermal and neutron irradiation stability. However there are reports that show that these oxides either dissolve or dissociate or even become amorphous during mechanical milling and reprecipitate as coarse particles during high temperature consolidation process, in absence of Ti in a model Fe-15Y2O3 system [1]. It is believed that Ti can inhibit the growth of nano-dispersoid during annealing by formation of Y-Ti-O complex oxides such as Y2Ti2O7 or Y2TiO5 or YTiO3 [2]. The Y2O3 to Ti weight ratio in the alloy is critical in determining the chemical composition of the dispersoid and the Y2Ti2O7 oxides are finer and most stable oxide among all combinations of Y-Ti-O complex oxides whose size which varies in the range of 2-15 nm in ODS steel. Synchrotron XRD is used to characterize the dispersoids in the ODS steel, with 0.35 wt% of yttria and 0.2 wt% of Ti, due to the low volume fraction. TEM has been found more suitable for complete characterization of the nano-sized (~2-5 nm) dispersoids w.r.t size, distribution, morphology, chemical composition and crystal structure. However, characterisation of samples prepared by conventional methods for TEM studies continues to be difficult owing to magnetic nature of ferritic steel. Hence FIB was employed to extract electron transparent samples which are of micrometer dimensions. In order to understand the structural evolution of the Y2Ti2O7 oxide in ODS steel, a concentrated alloys of Fe-15wt%Y2O3-Xwt%Ti (X=0, 5, 10, 15) were synthesized by mechanical milling and subsequently annealed. Figure 1(a) and (b) represents the typical bright field (BF) TEM micrographs of Fe-15wt%Y2O3-15wt%Ti model ODS after 60h of milling and subsequent annealing at 1273K respectively, the corresponding SAD patterns are shown as inset. The analysis of SADP reveals amorphisation of yttria upon milling and recrystallisation of Y2Ti2O7, in annealed alloy powder. Interestingly, it was observed when the Y2O3 to Ti weight ratio is 1:1, the oxide phase formed upon annealing is only Y2Ti2O7 and are very finer in size (varies in the range of 2-30 nm). Details of these studies will be presented in the paper. 1. Pradyumna Kumar Parida, Arup Dasgupta, K. Jayasankar, M. Kamruddin, S. Saroja, J. Nucl. Mater. 441 (2013) 331–336. 2. P.K. Parida, A. Dasgupta, K.G. Raghavendra, K. Jayasankar, S. Saroja, Trans IIM (2016) 1-7.

Effect of substrate heating and microwave attenuation on the catalyst free growth and field emission of carbon nanotubes

Phase stability of nanostructured thin films can be significantly different from the stability of the same materials in bulk form because of the increased contribution from surface and interface effects. Zirconia (ZrO2), stabilized in tetragonal and cubic phases, is a technologically important material and is used for most high temperature applications. In literature, zirconia can be found to be stabilized in its high temperature phases down to room temperature via two routes, doping with divalent or trivalent cations and crystallite size controls. Apart from these, in the alumina/zirconia thin-film multilayer system, a constraining effect on the zirconia layers provides another route to stabilization of the tetragonal zirconia phase at room temperature. However, in such nanostructured geometries, at high temperatures, the small diffusion lengths involved can influence the phase stability. The present work deals with the high-resolution transmission electron microscope (HRTEM) studies of pulsed laser ablated alumina–zirconia thin-film multilayers in the as deposited state and annealed up to 1473 K at 2 × 10−5 mbar. Conventional techniques such as X-ray diffraction lack the ability to detect localized phase changes at nanometre length scales and also for the low volume fraction of newly formed phases. Cross-sectional HRTEM techniques have been successful in detecting and characterizing these interactions.