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Dutta J.,Indian Institute of Science | Ananthakrishna G.,Indian Institute of Science | Banerjee S.,Indian Department of Atomic Energy
Acta Materialia | Year: 2012

One characteristic feature of the athermal β → ω transformation is the short time scale of the transformation. So far, no clear understanding of this issue exists. Here we construct a model that includes contributions from a Landau sixth-order free energy density, kinetic energy due to displacement, and the Rayleigh dissipation function to account for the dissipation arising from the rapid movement of the parent-product interface during rapid nucleation. We also include the contribution from ω-like fluctuations to local stress. The model shows that the transformation is complete on a time scale comparable to the velocity of sound. The estimated nucleation rate is several orders higher than that for diffusion-controlled transformations. The model predicts that the athermal ω phase is limited to a certain range of alloying composition. The estimated nucleation rate and the size of "isothermal" particles beyond 17% Nb are also consistent with experimental results. The model provides an explanation for the reprecipitation process of the ω particles in the "cleared" channels formed during deformation of ω-forming alloys. The model also predicts that acoustic emission should be detectable during the formation of the athermal phase. © 2011 Published by Elsevier Ltd. on behalf of Acta Materialia Inc. All rights reserved. Source


Chakraborty B.,Bhabha Atomic Research Center | Modak P.,Bhabha Atomic Research Center | Banerjee S.,Indian Department of Atomic Energy
Journal of Physical Chemistry C | Year: 2012

Applying first principles electronic structure calculations and molecular dynamics (MD) simulations we have studied the structural stability, hydrogen adsorption capability and hydrogen desorption kinetics of Y-decorated single walled carbon nanotube (SWCNT). We have predicted that a single Y atom attached on SWCNT can physisorb up to six hydrogen molecules which is not reported so far. Our MD simulations with four Y atoms placed at the alternate hexagons of SWCNT showed no clustering effect of Y atoms at room temperature and also we found that the system is stable even at higher temperature (700 K). For the first time we showed that 100% desorption at comparatively lower temperature can be achieved in a transition metal-decorated SWCNT system. Therefore the Y-decorated SWCNT has the potential to become a promising hydrogen storage device. © 2012 American Chemical Society. Source


Vinod V.T.P.,Indian Department of Atomic Energy | Sashidhar R.B.,Osmania University | Sreedhar B.,Indian Institute of Chemical Technology
Journal of Hazardous Materials | Year: 2010

Gum kondagogu (Cochlospermum gossypium), an exudates tree gum from India was explored for its potential to decontaminate toxic metal ions in aqueous solution. The toxic metal ions nickel and total chromium biosorption capacity of the gum kondagogu were studied in the batch experimental mode. The optimum conditions of biosorption were determined by investigating pH, contact time, and initial metal ion and biosorbent concentrations. The Freundlich and Langmuir adsorption models were used for the mathematical description of biosorption equilibrium and the data were analyzed on the basis of pseudo-second-order kinetic model. The maximum biosorption capacity of gum kondagogu as calculated by Langmuir model were found to be 50.5mgg-1 for nickel at pH 5.0±0.1 and 129.8mgg-1 for total chromium at pH 2.0±0.1, respectively. FTIR, SEM-EDXA and XPS analysis were used to evaluate the binding characteristics of gum kondagogu with metals. The experimental results demonstrate that the metal-ion interaction occurs through ion-exchange, adsorption and precipitation mechanisms. © 2010 Elsevier B.V. Source


Oxalate precipitation of lanthanides and thorium in acidic medium is a widely used group separation method at percentage to trace levels in different types of samples. In this report, a comparative study was made with the earlier reported conditions of trace level lanthanide separation as insoluble oxalates from a geological matrix at different pH values using calcium as carrier. The lanthanides and thorium are recovered quantitatively at trace levels as oxalates, using 300 mg calcium as a carrier at pH 1. The calcium is further removed by ammonium hydroxide precipitation in the presence of either stannic tin or ferric iron as the carrier. The combination of the two precipitative separations removed most of the matrix elements completely from the Ian-thanides and thorium at trace levels in different types of geological samples, such as igneous rocks, soils, and refractory minerals like ilmenite, rutile, columbite-tantalite, garnet and silliminite, as well as in certain environmental and industrial waste materials. Accuracy of the method was checked by analyzing some Canadian Certified Reference Project Materials such as syenite samples SY-2 and SY-3, gabro sample MRG-1, soil samples SO-1, SO-2 and iron formation sample FeR-2, and also synthetic samples. The lanthanide and thorium values obtained for the reference materials is comparable with the recommended values, indicating that the method is fairly accurate and reproducibility is characterized by a relative standard deviation CRSD) of 1 to 6% (n=4). Source


Complete and unequivocal preservation of natural water samples is a practical impossibility. The physico-chemical and biological changes continue inevitably after sample collection. In this paper, the various aspects which cause changes in the content of uranium, major cations and anions with reference to the time interval between sample collection and analysis are presented. These parameters warrant the need and use of Mobile Geochemical Laboratory for quick analysis of water samples. The reliability/quality of measurement results of water samples depends on strict adherence to each step of sampling, preservation of samples, time-interval between sampling and analysis for filtered but un-acidified water samples, and on the methodology adopted, and not simply analyzed by any person or lab or any technique. Interpretation and conclusions of hydrogeochemical reconnaissance survey will depend on the quality of measurement results. In addition to this, self-evaluation of data from collecting samples to reporting results should be carried out to ensure reliability and accuracy of analytical data of water samples. Source

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