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Chakraborty S.,Bhabha Atomic Research Center | Arora A.K.,Bhabha Atomic Research Center | Sivasubramanian V.,Bhabha Atomic Research Center | Krishna P.S.R.,Solid State Physics Division | Venkata Krishnan R.,Bhabha Atomic Research Center
Journal of Physics Condensed Matter | Year: 2012

Processes occurring at macroscopic and microscopic length scales across the glass transition (Tg) in lead-tellurite glass (PbO) x(TeO2)1-x (x = 0.1-0.3) are investigated using Brillouin and Raman spectroscopy, respectively. For all the samples, the temperature dependence of the longitudinal acoustic (LA) mode is found to exhibit a universal scaling below Tg and a rapid softening above Tg. The lower value of elastic modulus at a higher concentration of network modifier PbO, estimated from Brillouin data, arises due to loss of network rigidity. From quantitative analysis of the reduced Raman spectra, several modes are found to exhibit anomalous changes across Tg. Instead of the expected anharmonic behaviour, several modes exhibit hardening, suggesting stiffening of the stretching force constants with temperature, the effect being more pronounced in glasses with higher x. In addition, incorporation of PbO in the glass is also found to narrow down the bond-length distribution, as evident from the sharpening of the Raman bands. The stiffening of the force constants of molecular units at a microscopic length scale and the decrease of elastic constant attributed to loss of network rigidity on a macroscopic length scale appear to be opposite. These different behaviours at two length scales are understood on the basis of a microscopic model involving TeOn and PbO units in the structure. © 2012 IOP Publishing Ltd.

Rose J L.,National Institute of Technology Calicut | Tata B.V.R.,Bhabha Atomic Research Center | Aswal V.K.,Solid State Physics Division | Hassan P.A.,Chemistry Division | And 2 more authors.
European Physical Journal E | Year: 2015

Abstract.: Structural transitions triggered by p H in an aqueous micellar system comprising of a cationic surfactant (cetylpyridinium chloride) and an aromatic dibasic acid (phthalic acid) was investigated. Reversible switching between liquid-like and gel-like states was exhibited by the system on adjusting the solution p H. Self-assembled structures, responsible for the changes in flow properties were identified using rheology, light scattering techniques and cryogenic Transmission Electron Microscopy (cryo-TEM). High-viscosity, shear-thinning behavior and Maxwell-type dynamic rheology shown by the system at certain p H values suggested the growth of spheroidal/short cylindrical micelles into long and entangled structures. Light scattering profiles also supported the notion of p H-induced microstructural transitions in the solution. Cryo-TEM images confirmed the presence of spheroidal/short cylindrical micelles in the low-viscosity sample whereas very long and entangled thread-like micelles in the peak viscosity sample. p H-dependent changes in the micellar binding ability of phthalic acid is proposed as the key factor regulating the morphological transformations and related flow properties of the system.Graphical abstract: [Figure not available: see fulltext.]. © 2015, EDP Sciences, SIF, Springer-Verlag Berlin Heidelberg.

Vekariya R.L.,Sardar Patel University | Aswal V.K.,Solid State Physics Division | Hassan P.A.,Bhabha Atomic Research Center | Soni S.S.,Sardar Patel University
Langmuir | Year: 2014

The isotropic micellar state of Pluronic P123 in the presence and absence of N-alkylpyridinium halide ionic liquids (ILs) is investigated using SANS, DLS, and 1H NMR studies. The micellar structural parameters are obtained as a function of variation in alkyl chain length, anions, and concentrations of ILs by fitting the SANS scattering data with a model composed of core-shell form factor and a hard sphere structure factor of interaction. Addition of ILs decreases the micellar core, aggregation number, and hard sphere radius of P123 micelles. From quantitative analysis, we determined the amount of solvent (D2O + IL) present inside the core and the core-shell interface along with cationic head groups. This is further supported by monitoring interaction between ILs and polymer micelle using 1H NMR spectroscopy. The results are discussed and explained as a function of concentration of C8PyCl, alkyl chain length, and anions of N-alkylpyridinium halides. © 2014 American Chemical Society.

Linet Rose J.,National Institute of Technology Calicut | Tata B.V.R.,Bhabha Atomic Research Center | Talmon Y.,Technion - Israel Institute of Technology | Aswal V.K.,Solid State Physics Division | And 2 more authors.
RSC Advances | Year: 2015

Macroscopic properties of amphiphilic systems can be reversibly controlled by tailoring micellar morphology via appropriate choice of additive and external stimulus. In this work we report an aqueous micellar system showing pH responsive viscoelasticity and optical properties. pH sensitive behaviour of a phenolic acid, namely, ortho-coumaric acid (OCA) is effectively utilized to tune self-assembly in cetyltrimethylammonium bromide (CTAB) solution. Reversible switching between colourless, gel-like state and fluorescent green colour, liquid-like state can be attained by pH adjustment. pH dependent changes in bulk properties and the microstructures responsible for the behaviour were studied by means of rheology, UV-vis and fluorescence spectroscopic techniques, small angle neutron scattering (SANS) and cryogenic transmission electron microscopy (cryo-TEM). Rheological studies suggested transition between viscoelastic fluid and Newtonian liquid or vice versa, with specific changes in pH. Viscoelasticity of the system is attributed to the presence of entangled threadlike micelles. pH sensitive interactions between surfactant micelles and phenolic additive is regarded as the key factor regulating the morphological transitions and related flow behaviour in CTAB-OCA solution. © The Royal Society of Chemistry 2015.

Choudhuri G.,Quality Assurance Division | Jagannath,Theoretical Physics Division | Kiran Kumar M.,Material Science Division | Kain V.,Material Science Division | And 5 more authors.
Journal of Nuclear Materials | Year: 2013

The effects of Fe addition in the range of 300-1250 ppm in cold worked stress-relieved Zr-2.5Nb pressure tube on oxidation and hydrogen pick up behavior have been studied after 415 C steam autoclaving. Microstructure and micro-chemistry of second phase and precipitates were characterized using electron microscope. Addition of 800 ppm Fe in Zr-2.5Nb alloy led to better oxidation resistance. With further addition of Fe no significant improvement of oxidation resistance was observed but hydrogen-pickup was found to increase. Zr-Nb-Fe bearing precipitates were observed in Zr-2.5Nb alloy containing 800 ppm Fe. Further addition of Fe led to formation of Zr-Fe intermetallic. The chemical state of oxide has been determined by X-ray photo electron spectroscopy. Grazing Incidence X-ray Diffraction revealed that oxide in alloys with higher Fe, contained a higher fraction of tetragonal-Zirconia which is indicative of a protective oxide film and hence better oxidation resistance of the alloy. © 2013 Elsevier B.V. All rights reserved.

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