IIT Madras is a public technical and research university located in Chennai, Tamil Nadu. Founded in 1959, it is recognised as an Institute of National Importance.

IIT Madras has been ranked as the top engineering institute in India for four years in a row by the National Institutional Ranking Framework of the MHRD

It currently offers undergraduate, postgraduate and research degrees across 16 disciplines in Engineering, Sciences, Humanities and Management. About 596 faculty belonging to science and engineering departments and centres of the Institute are engaged in teaching, research and industrial consultancy.

IIT Madras

Various transmission electron microscopy techniques which are useful for the characterization of phase-separation phenomena in glass materials are exemplarily demonstrated at a 40 SiO2-30 Al2O3-18 Na2O-12 LaF3 glass. It is shown that direct imaging of phase-separated regions possesses manifold advantages over the traditional replica technique, particular if the feature size approaches the nano-scale. Although surface replica can be accomplished effortless and made a great leap in glass structure research possible, the method is indirect, i.e. requires a deep understanding of the chemical attack leading to the surface topography eventually imaged. Moreover, surface replicas reach their limits for nanosized features due to the inherent structure of the deposited replica film. © 2009 Elsevier B.V. All rights reserved.

Department of Metallurgical and Materials Engineering

World University root node

Journal of Non-Crystalline Solids

Various transmission electron microscopic techniques to characterize phase separation - illustrated using a LaF3 containing aluminosilicate glass

Aiming at tailoring optical properties, the precipitation of LaF3 nano-crystals in LaF3-Na2O-Al2O3-SiO2 glass-ceramics is studied thoroughly on the nano-scale using advanced transmission electron microscopic techniques. Nano-sized phase-separation droplets enriched in lanthanum and silicon are formed already in the base glass. Within these less than 20 nm large droplets, LaF3 crystallizes upon heat treatment. The nano-crystallization mechanism revealed is self-limited since growth is restricted by the size of the droplets. An average crystallite size of around 12 nm is achieved with a narrow size distribution since the phase-separation droplets also contain silicon not incorporated into the growing crystal. Instead, excess silicon relocated to the periphery of the pre-existing phase-separation droplets forms a diffusion barrier around the LaF3 nano-crystals preventing further crystal growth and/or ripening. © 2009 Elsevier B.V. All rights reserved.

Journal of Crystal Growth

Nano-crystallization in LaF3-Na2O-Al2O3-SiO2 glass

Few nanometer large (<2 nm) inclusions of D O3 -phase structure have been identified in the A2 matrix of highly magnetostrictive Fe-19% Ga alloys by high-resolution transmission electron microscopy (HRTEM). In addition, we have found that these nanostructures include a high density of {100} line defects with a Burgers vector of abcc 2 100. This dispersion of D O3 -phase nanostructures formed within the main A2 matrix and the lifting of their coherency by defects are consistent with a recent theory for the structure and properties of magnetostrictive Fe-Ga and Fe-Al alloys. © 2008 The American Physical Society.

Fulltext

Physical Review B - Condensed Matter and Materials Physics

Nanodispersed D O3 -phase nanostructures observed in magnetostrictive Fe-19% Ga Galfenol alloys

A method is presented for retrieving the variation of the absorptive potential in electron beam sensitive soft material. The absorptive potential is directly related to the variations in atomic density and in chemical composition fluctuation. In our approach only a single zero-loss energy-filtered bright-field transmission electron microscopy (TEM) image is sufficient to obtain this information. The accuracy and the effectiveness of this proposed method has been tested using both simulation and experiment. As an experimental example of soft (organic) material, the variation in absorptive potential in an unstained tissue sample of zebrafish gill has been determined. Microsc. Res. Tech., 2008. © 2008 Wiley-Liss, Inc.

Microscopy Research and Technique

Retrieval of absorptive potential variation in electron beam sensitive specimens using a single energy-filtered bright-field TEM image

We report on the direct observation of tetragonal nanodomains within an average monoclinic MC phase of Pb (Mg13 Nb23) O3 -0.35PbTi O3 single crystals by high-resolution transmission electron microscopy. These nanodomains are geometrically arranged in alternating layers of twins. The findings are consistent with the fundamental underlying assumptions of the ferroelectric adaptive phase theory [Y. M. Jin, Phys. Rev. Lett. 91, 197601 (2003)]. © 2008 American Institute of Physics.

Applied Physics Letters

Direct high-resolution transmission electron microscopy observation of tetragonal nanotwins within the monoclinic MC phase of Pb (Mg13 Nb23) O3 -0.35PbTi O3 crystals

Extended abstract of a paper presented at Microscopy and Microanalysis 2007 in Ft. Lauderdale, Florida, USA, August 5 – August 9, 2007

Microscopy and Microanalysis

Investigating nano-dispersed DO3 structures in galfenol alloy

Materials such as Si3N4, SiC and SrTiO3 can have grain boundaries characterized by the presence of a thin intergranular amorphous film of nearly constant thickness, in some cases (e.g. Si 3N4) almost independent of the orientation of the bounding grains, but dependent on the composition of the ceramic. Microscopy techniques such as high-resolution lattice fringe imaging, Fresnel fringe imaging and diffuse dark field imaging have been applied to the study of intergranular glassy films. The theme of the current investigation is the use of Fresnel fringes and Fourier filtering for the measurement of the thickness of intergranular glassy films. Fresnel fringes hidden in high-resolution micrographs can be used to objectively demarcate the glass-crystal interface and to measure the thickness of intergranular glassy films. Image line profiles obtained from Fourier filtering the high-resolution micrographs can yield better estimates of the thickness. Using image simulation, various kinds of deviation from an ideal square-well potential profile and their effects on the Fresnel image contrast are considered. A method is also put forth to objectively retrieve Fresnel fringe spacing data by Fourier filtering Fresnel contrast images. Difficulties arising from the use of the standard Fresnel fringe extrapolation technique are outlined and an alternative method for the measurement of the thickness of intergranular glassy films, based on zero-defocus (in-focus) Fresnel contrast images is suggested. The experimental work is from two ceramic systems: Lu-Mg-doped Si3N4 and SrTiO3 (stoichiometric and nonstoichiometric). Further, a comparison is made between the standard high-resolution lattice fringe technique, the standard Fresnel fringe extrapolation technique and the methods of analyses introduced in the current work, to illustrate their utility and merits. Taking experimental difficulties into account, this work is intended to be a practical tool kit for the study of intergranular glassy films. © 2006 The Authors.

Journal of Microscopy

Aspects regarding measurement of thickness of intergranular glassy films

An iterative method for reconstructing the exit face wave function from a through focal series of transmission electron microscopy image line profiles across an interface is presented. Apart from high-resolution images recorded with small changes in defocus, this method works also well for a large defocus range as used for Fresnel imaging. Using the phase-object approximation the projected electrostatic as well as the absorptive potential profiles across an interface are determined from this exit face wave function. A new experimental image alignment procedure was developed in order to align images with large relative defocus shift. The performance of this procedure is shown to be superior to other image alignment procedures existing in the literature. The reconstruction method is applied to both simulated and experimental images. © 2006 Elsevier B.V. All rights reserved.

Ultramicroscopy

Projected potential profiles across interfaces obtained by reconstructing the exit face wave function from through focal series

Amorphous 1 - 2-nm-wide intergranular films in ceramics dictate many of their properties. The detailed investigation of structure and chemistry of these films pushes the limits of today's transmission electron microscopy. We report on the reconstruction of the one-dimensional potential profile across the film from an experimentally acquired tilt series of energy-filtered electron diffraction patterns. Along with the potential profile, the specimen thickness, film orientation with respect to the grain lattice and specimen surface, and the absolute specimen orientation with respect to the laboratory frame of reference are retrieved. © Microscopy Society of America 2006.

Measuring electrostatic potential profiles across amorphous intergranular films by electron diffraction

The presence of 1-2 nm wide intergranular glassy films (IGFs) found in ceramics such as Si3N4 and SiC strongly influences the properties of the material, including its fracture and creep behaviour. It is therefore important to know the evolution of the film as a function of temperature. Experiments, wherein liquid phase sintered Si3N4 samples were quenched from high temperatures, have been carried out before and shown intriguing results. In the current investigation, in-situ heating experiments have been conducted in 120 kV and 400 kV microscopes, in order to separate beam irradiation from temperature effects. Based on the literature, it was expected that no changes would occur to the IGF below 1000 °C. However, it was surprising to note that the thickness measured at 950 °C was higher than that at room temperature. The correlation in trends observed in both microscopes shows that electron radiation has a minimal contribution to the change in IGF width at 950 °C. No change to the thickness was observed when heating up to 650 °C. After cooling back to room temperature there is a reduction in the thickness and thus a tendency to regain the original value of the thickness before heating. We conclude from these observations that certain material transport processes could be active at rather low temperatures (for Si3N4). Possible mechanisms and sources of artifacts are also discussed. © 2006 Elsevier B.V. All rights reserved.

Materials Science and Engineering A

The evolution of amorphous grain boundaries during in-situ heating experiments in Lu-Mg doped Si3N4

Projected electrostatic and absorptive potential profiles across intergranular glassy films (IGFs) and interfaces between grains and glassy pockets in silicon nitride ceramics were obtained by reconstructing the electron exit face wave function from a series of defocused TEM images. The phase-object approximation (POA) was used for extracting the projected potential from the complex-valued exit-face wave function. The electrostatic as well as the absorptive potentials were scaled with respect to vacuum. For La 2O3-MgO doped Si3N4, the potential profiles across the grain/glassy pocket interface and the IGF were observed to be very similar and give very strong evidence for the existence of a space charge layer at the interface.

Journal of the Ceramic Society of Japan

Projected Potential Profiles across Intergranular Glassy Films

Potential profiles across interfaces can provide useful information about variations in structure, composition, density, ionicity of their constituents, the presence of space charge layers, etc, showing that the ability to measure potential profiles across interfaces is of great importance in the field of material science.&nbsp;

Determination of projected potential profiles across interfaces using through focal series reconstruction

Abstract is unavailable for this paper.

Assessing thermodynamic properties of amorphous nanostructures by energy-filtered electron diffraction

The liquid holdup study has been carried out on a two-dimensional cold model under non-wetting condition with lateral gas injection as it is done in the blast furnace. Non-dimensional numbers for both, cold model and blast furnace have been determined. The measurements of static and dynamic liquid holdups were made under different gas and liquid flow rates using two particulate materials of various diameters. Few experiments were performed under dry and wet bed conditions to ascertain the effect of bed history. Holdups results have been compared with the existing correlations available in the open literature. It is found that existing correlations are unable to predict the correct liquid holdup under the blast furnace conditions.