Kerala State Council for Science

Thiruvananthapuram, India

Kerala State Council for Science

Thiruvananthapuram, India

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Megha U.,University of Calicut | Shijina K.,University of Calicut | Varghese G.,Kerala State Council for Science
Processing and Application of Ceramics | Year: 2014

LaCo0.6Fe0.4O3 (LCFO) nanopowder was synthesized from constituent metal nitrates, citric acid and ethylene glycol by citrate sol gel autocombustion method and calcined at different temperatures. The powders were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDAX) and Fourier transform infrared spectra (FTIR), whereas dielectric properties were investigated with LCR-meter. The FTIR spectra, taken for the xerogel and the sample calcined at 1000 °C, confirm that the organic groups were removed during calcination and oxide structure was formed. The XRD result shows that LCFO has rhombhohedral crystal structure with R-3C space group and forms single phase after calcination at 600 °C. The activation energy of crystallite growth, determined from the Arrhenius plot, was 17±2 kJ/mol. Surface feature studies of the powders were obtained from SEM. At 1000 °C, dense microstructure with well-shaped grain boundaries was obtained and the average grain size was around 400 nm. EDAX confirms the elemental composition. Finally, from the dielectric studies, it was found that the dielectric constant (εr) as well as dielectric loss tangent (tan δ) decreases with increase in frequency.


Sijo S.,Mahatma Gandhi University | Manesh M.,Mahatma Gandhi University | Sreekala G.,Mahatma Gandhi University | Neethu T.W.,Mahatma Gandhi University | And 2 more authors.
Physics of Plasmas | Year: 2015

We investigate the influence of a second electron component on oblique dust ion acoustic solitary waves in a five component plasma consisting of positively and negatively charged dust, hydrogen ions, and hotter and colder electrons. Of these, the heavier dust and colder photo-electrons are of cometary origin while the other two are of solar origin; electron components are described by kappa distributions. The K-dV equation is derived, and different attributes of the soliton such as amplitude and width are plotted for parameters relevant to comet Halley. We find that the second electron component has a profound influence on the solitary wave, decreasing both its amplitude and width. The normalized hydrogen density strongly influences the solitary wave by decreasing its width; the amplitude of the solitary wave, however, increases with increasing solar electron temperatures. © 2015 AIP Publishing LLC.


Shijina K.,University of Calicut | Varghese G.,Kerala State Council for Science | Megha U.,University of Calicut
Materials Science in Semiconductor Processing | Year: 2015

Markedly enhanced near band edge ultraviolet fluorescence emission and significantly reduced defect related green emissions were also observed in ZnNiPdO nanorods capped with polyvinyl pyrrolidone (PVP). Under excitation at 325 nm, this rod like composites exhibited violet-blue defect emission. It is demonstrated that the surface modification by the capping agent PVP can dramatically change the blue emission of the nanorods. The strong UV extended violet-blue emission owes to the recombination of an electron in the zinc interstitial and a hole in the valence band with the addition of strong blue emission of PVP. Tailing effect in the bandgap was observed and found to obey Urbach rule. The X-ray diffraction studies confirmed the phase of formation and SEM demonstrates the hexagonal geometry of the nanorods. © 2015 Elsevier Ltd. All rights reserved.


Shijina K.,University of Calicut | Varghese G.,Kerala State Council for Science | Megha U.,University of Calicut
Materials Science and Engineering B: Solid-State Materials for Advanced Technology | Year: 2015

Abstract Synthesis of rare-earth metals (Dy, Ce) doped ZnNiO (Dy:ZnNiO and Ce:ZnNiO) nanocomposites using polyvinylpyrrolidone (PVP) as the capping molecules has been accomplished using the wet chemical route. Structural and optical properties of surface modified Dy:ZnNiO and Ce:ZnNiO are examined. X-ray diffraction indicates that all of the as-obtained samples can be indexed to hexagonal structure. Correlation between the structural and optical properties was studied for the synthesized nanoparticles. The crystallite sizes of the composites depend on the level of disorder, albeit to a limited extent, where minimum disorder matches with high crystallinity. Significant functional correlation could be found between the differences in experimentally observed crystallite size and Urbach energy. Specific surface area and pore size of the composites confirmed the mesoporous nature of the samples. The fluorescence spectra of the samples show, the effects of surface passivation in the ultraviolet emission region. © 2015 Elsevier B.V.


Sreekala G.,Mahatma Gandhi University | Sebastian S.,Mahatma Gandhi University | Michael M.,Mahatma Gandhi University | Abraham N.P.,Mahatma Gandhi University | And 3 more authors.
Advances in Space Research | Year: 2015

The two potential theory of Hasegawa has been used to derive the dispersion relation for the kinetic Alfven wave (KAW) in a plasma composed of hydrogen, oxygen and electrons. All three components have been modeled by ring distributions (obtained by subtracting two Maxwellian distributions with different temperatures) with the hydrogen and electrons drifting, respectively, with velocities VdH and Vde. For the most general case, the dispersion relation is a polynomial equation of order five; it reduces to a relation which supports only one mode when VdH = 0. For typical parameters at comet Halley, we find that both VdH and Vde can drive the wave unstable; the KAW is thus driven unstable in a current-less plasma. Such an instability was found for the ion acoustic wave by Vranjes et al. (2009). © 2015 COSPAR.


Megha U.,University of Calicut | Varghese G.,Kerala State Council for Science | Shijina K.,University of Calicut
Bulletin of Materials Science | Year: 2016

Nanopowders of La1-xBixCo0.6Fe0.4O3 (x = 0, 0.1, 0.2) and La1-2xBixSrxCo0.6Fe0.4O3 (x = 0.1) multinary perovskites were synthesized by citrate sol-gel autocombustion method. Crystalline phase and the lattice parameters were obtained from X-ray diffraction pattern. The XRD result shows that all compounds have rhombhohedral crystal structure with R3c space group and Bi (x = 0.2) have the presence of secondary peaks. Crystallite size, dislocation density, specific area and strain were calculated from XRD. The elemental composition and micrographs of grain were obtained from EDAX (energy dispersive X-ray analysis) and SEM (scanning electron microscopy), with an average grain size below 400 nm. Surface morphological studies using XPS (X-ray photoelectron spectroscopy) were used to find out the chemical states and surface proportion of oxygen present in samples. Finally, using the vibrating sample magnetometer the room temperature magnetic behaviour of compounds was studied and it was observed that the ferromagnetic behaviour of LaCo0.6Fe0.4O3 was reduced by Bi and Sr doping. © 2016 Indian Academy of Sciences.


Abraham N.P.,Mahatma Gandhi University | George S.,Mahatma Gandhi University | Sreekala G.,Mahatma Gandhi University | Sebastian S.,Mahatma Gandhi University | And 2 more authors.
Earth, Moon and Planets | Year: 2014

We have studied the stability of the electrostatic electron cyclotron wave in a plasma composed of hydrogen, oxygen and electrons. To conform to satellite observations in the low latitude boundary layer we model both the ionic components as drifting perpendicular to the magnetic field. Expressions for the frequency and the growth rate of the wave have been derived. We find that the plasma can support electron cyclotron waves with a frequency slightly greater than the electron cyclotron frequency ω ce; these waves can be driven unstable when the drift velocities of both the ions are greater than the phase velocity of the wave. We thus introduce another source of instability for these waves namely multiple ion beams drifting perpendicular to the magnetic field. © 2014 Springer Science+Business Media Dordrecht.


Sebastian S.,Mahatma Gandhi University | Sreekala G.,Mahatma Gandhi University | Michael M.,Mahatma Gandhi University | Abraham N.P.,Mahatma Gandhi University | And 2 more authors.
Physica Scripta | Year: 2015

We investigate the existence of compressive and rarefactive solitary waves in a five-component plasma. Positively and negatively multiply charged heavier ions (dust), kappa function described photo-electrons, hot electrons and ions form the five components. The pseudo-potential approach is used to determine the existence of a soliton. We find that the Sagdeev potential as well as the amplitude of the solitary wave for both compressive and rarefactive solitons increases with increasing spectral indices of the kappa distributions describing the cometary species. The magnitude of the amplitude of the solitary wave increases with increasing positively charged ion densities and charge numbers, but decreases with increasing charge numbers of the negative ions for both type of solitons. © 2015 The Royal Swedish Academy of Sciences.


Abraham N.P.,Mahatma Gandhi University | Sebastian S.,Mahatma Gandhi University | Sreekala G.,Mahatma Gandhi University | Devi S.E.,Mahatma Gandhi University | And 2 more authors.
Astrophysics and Space Science | Year: 2014

A popular model of a cometary plasma is hydrogen (H+) with positively charged oxygen (O+) as a heavier ion component. However, the discovery of negatively charged oxygen (O-) ions enables one to model a cometary plasma as a pair-ion plasma (of O+ and O-) with hydrogen as a third ion constituent. We have, therefore, studied the stability of the ion-acoustic wave in such a pair-ion plasma with hydrogen and electrons streaming with velocities VdH+ and Vde, respectively, relative to the oxygen ions. We find the calculated frequency of the ion-acoustic wave with this model to be in good agreement with the observed frequencies. The ion-acoustic wave can also be driven unstable by the streaming velocity of the hydrogen ions. The growth rate increases with increasing hydrogen density nH+, and streaming velocities VdH+ and Vde. It, however, decreases with increasing oxygen ion densities nO+ and nO-. © 2013 Springer Science+Business Media Dordrecht.

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