Department of Physics

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Core-shell material with hollow cenospheres of low density as the core has been synthesized by depositing a layer of iron oxide shell on the surface of seed hollow cenospheres. The phase structure, morphology, particle size, chemical composition, and magnetic properties of the synthesized material have been characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), energy dispersive X-ray spectroscopy (EDS), and vibrating sample magnetometer (VSM). Results confirm the coating of iron oxide on hollow cenospheres. Electromagnetic characterization of synthesized core-shell material was carried out in X band. Dilectric loss (tanδe) and magnetic loss (tanδm) were found in the range of~0.2 and 0.05, respectively, over 8-12 GHz. Simulation studies show the effective reflection loss (RL) is more than 10 dB over frequency range 9-10.5 GHz at a thickness of 2.2 mm. The low density iron oxide coated cenospheres have potential applications as microwave absorbing materials. © Taylor & Francis Group, LLC.

World University root node

International Journal of Green Nanotechnology: Biomedicine

Synthesis and microwave absorption property of the core-shell absorber with hollow cenosphere as core

Core-shell iron nanoparticles have been synthesized by pyrolysis of nanocomposite of oxides of iron-tin (Fe-Sn). The core-shell nanoparticles of phase pure iron in carbonaceous shell are formed only at very low concentration of tin (0.0011 mol) in the nanocomposite sample. From different studies viz. X-ray diffraction, high-resolution transmission electron microscopy, atomic force microscopy, and Raman spectroscopy, it has been established that core-shell nanostructures have been formed with Fe as core and amorphous carbon as the shell. The heating of nanocomposite at different temperatures up to 900 °C revealed very interesting dynamics of formation of core-shell structure wherein above 650 °C the iron carbide phase decomposes and carbon atoms move out to form an amorphous shell around iron nanoparticles. This process of formation of core-shell structures is quite different fromconventionalwaywherein synthesis of core material precedes formation of shell in two different steps. The microwave absorption properties of core-shell nanoparticles have been studied by making their composites in nitrile butadiene rubber. Reflection loss simulation studies showhigh values in theXandKu bands of microwave region. The frequency ofmaximum return loss can be tuned through variation of composition and thickness of composite layer. © Springer Science+Business Media Dordrecht 2012.

Journal of Nanoparticle Research

Synthesis of core-shell iron nanoparticles from decomposition of Fe-Sn nanocomposite and studies on their microwave absorption properties

We report the enhancement of the effect of transverse localization of light (TL) in presence of a weak longitudinal modulation of refractive index in disordered waveguide lattices. In our chosen lattices, tunneling inhibition along length favors to achieve the diffraction-free propagation along with the simultaneous presence of transverse disorder. Results will be useful to tune the threshold value of disorder to achieve localized light. © 2012 IEEE.

2012 Photonics Global Conference, PGC 2012

Effect of a weak longitudinal modulation in refractive index on transverse localization of light in 1D disordered waveguide lattices

Design of a mid-wave IR broadband (3.1 - 3.8 μm) fiber-based light source exploiting FWM in a meter long suitably designed chalcogenide microstructured fiber is reported. An achievable gain more than 35 dB is demonstrated. © 2012 OSA.

2012 International Conference on Fiber Optics and Photonics, PHOTONICS 2012

Design of an efficient mid-wave IR fiber optic light source

Tetrapyrazinoporphyrazine substituted at its periphery with eight antioxidant 3,5-di-t-butyl-4-hydroxyphenyl groups behaves as a turn-on fluorescent sensor for fluoride anions. Conversely, the precursor antioxidant-substituted 1,2-phthalonitrile was found to act in turn-off mode suggesting that the origin of the phenomenon lies at the phenolate-substituted 1,4-pyrazinyl moiety. © 2012 The Royal Society of Chemistry.

Chemical Communications

Antioxidant-substituted tetrapyrazinoporphyrazine as a fluorescent sensor for basic anions

Phase separation kinetics of a binary (A,B) mixture contained in a thin film of thickness D induced by a quench from the one-phase region into the miscibility gap is studied by simulations using a Cahn-Hilliard-Cook model. The initial randomly mixed state (50 A, 50 B) contains a concentration gradient perpendicular to the film, while the surfaces of the film are neutral (no preference for either A or B). In thermal equilibrium, a pattern of large A-rich and B-rich domains must result, separated by domain walls oriented perpendicularly to the external surfaces of the thin film. However, it is shown that for many choices of D and the strength of the initial gradient Ψ g, instead a very long-lived metastable layered structure forms, with two domains separated by a single interface parallel to the external walls. The transient time evolution that leads to this structure is interpreted in terms of a competition between domain growth in the bulk and surface-directed spinodal decomposition caused by the gradient during the initial stages. A surprising and potentially useful finding is that a moderate concentration gradient perpendicular to the film does not favor the layered structure but facilitates the approach toward the true equilibrium with just two domain walls perpendicular to the film. This mechanism may have useful applications in producing layered materials. © 2012 American Institute of Physics.

Journal of Chemical Physics

Formation of metastable structures by phase separation triggered by initial composition gradients in thin films

We use molecular dynamics simulations to study surface-directed spinodal decomposition in unstable binary AB fluid mixtures at wetting surfaces. The thickness of the wetting layer R 1 grows with time t as a power law (R 1∼tθ). We find that hydrodynamic effects result in a crossover of the growth exponent from θ 1/3 to 1. We also present results for the layerwise correlation functions and domain length scales. © 2012 American Physical Society.

Fulltext

Physical Review E - Statistical, Nonlinear, and Soft Matter Physics

Surface-directed spinodal decomposition: A molecular dynamics study

We study the kinetics of phase separation of a binary (A,B) mixture confined in a thin film of thickness D by numerical simulations of the corresponding Cahn-Hilliard-Cook (CHC) model. The initial state consisted of 50% A:50% B with a concentration gradient across the film, i.e., the average order parameter profile is Ψ av(z,t=0)=(2z/D-1)Ψ g, 0≤z≤D, for various choices of Ψ g and D. The equilibrium state (for time t→) consists of coexisting A-rich and B-rich domains separated by interfaces oriented perpendicular to the surfaces. However, for sufficiently large Ψ g, a (metastable) layered state is formed with a single interface parallel to the surfaces. This phenomenon is explained in terms of a competition between domain growth in the bulk and surface-directed spinodal decomposition (SDSD) that is caused by the gradient. Thus, gradients in the initial state can stabilize thin-film morphologies which are not stable in full equilibrium. This offers interesting possibilities as a method for preparing novel materials. © 2012 American Physical Society.

Phase separation of binary mixtures in thin films: Effects of an initial concentration gradient across the film

Pseudospark discharge based plasma cathode has capability to provide high current density electron beam during discharge process. In this paper an effort has been made to simulate the breakdown processes in the pseudospark discharge based plasma cathode electron gun. The two-dimensional plasma simulation codes VORPAL and OOPIC-Pro have been used and results are compared. The peak discharge current in the plasma cathode electron gun is found to be dependent on aperture size, hollow cathode dimensions, anode voltage and seed electrons energy. The effect of these design parameters on the peak anode current has been analysed by both the codes and results matches well within 10% variation. For the seed electron generation an electron beam trigger source is used to control the discharge process in the hollow cathode cavity. The time span of trigger source has been varied from 1-100 ns to analyze the effect on the peak anode current. © Published under licence by IOP Publishing Ltd.

Journal of Physics: Conference Series

Comparative simulation studies of plasma cathode electron (PCE) gun

We have investigated temperature, frequency, and magnetic field dependent dielectric properties of polycrystalline Bi 0.9Ca 0.1FeO 2.95. Two dielectric anomalies, near 65 K and 260 K, were observed with the anomaly near 65 K exhibiting distinct frequency dependence as the peak temperature shifts with increasing frequency. The low-temperature dielectric relaxation data that can be fitted to a Vogel-Fulcher expression yielding a characteristic relaxation time of ∼10 -8 s are four orders larger than that of pure BiFeO 3 which may be the resultant of the chemical pressure induced by Ca doping. We also observed a switchable magneto-dielectric response in Bi 0.9Ca 0.1FeO 2.95 at room temperature. © 2012 American Institute of Physics.