Evaluation of fuels for the synthesis of Li2CoMn3O8

Li₂CoMn₃O₈, a 5 V cathode material used in rechargeable lithium batteries, has been synthesized by adopting a novel technique of using fuels along with the nitrate reactants. The effect of the fuel on the synthesis of Li₂CoMn₃O₈ has been analyzed in terms of the physical and electrochemical properties of the final product formed by various methods such as solid-state carbonate fusion and the solution route using acetate and nitrate precursors. Powder X-ray diffraction FT IR spectrum, particle size, surface area and SEM analysis were carried out. The combustion method, also known as selfpropagating high temperature (SPHT) method, has been employed in the present study by using nitrate mixtures of the respective salts and a nitrogeneous fuel (urea or glycine) at a temperature of 300 °C for 3 hrs. The nitrate reactants without the addition of fuel gave only a deliquescent product even at elevated temperature (600 °C) thus indicating the necessity of fuels. Similar attempts using acetate reactants with and without the addition of nitrogeneous fuels were made separately in order to find out the necessity of fuel also in this case. The characterization of the product in terms of purity, single-phase formation and surface morphology suggested that the fuel played no role in the case of the acetate precursors. A comparative study was made on the products obtained by the acetate precursor, combustion method and the conventional carbonate method. Among the three methods, the combustion method with glycine as fuel yielded the spinel phase with high purity Li₂CoMn₃O₈ with superior electrochemical behavior both in terms of high cell voltage and good cycle life behavior.     

Finite size effect on Gd doped CoGdxFe2−xO4 (0.0≤x≤0.5) particles

Nanoparticles of CoGd_xFe_2−xO₄ (where x=0.0, 0.1, 0.3, 0.5) series have been prepared by chemical co-precipitation. The effect of Gd³⁺ ion concentration on crystalline phase, crystallinity, crystallite size, molecular vibrations and magnetic resonance has been investigated in detail. The crystallinity decreases with an increase in Gd³⁺ ion concentration and changes the structural parameters. The spin lattice relaxation has been correlated with the doping ion concentration. Similarly, the superparamagnetic behavior of these particles has been observed with EPR spectroscopy.     

Probing the ferroelectric phase transition in sol–gel‐derived polycrystalline bismuth ferrite thin films

Polycrystalline BiFeO₃ (BFO) thin films were successfully grown on Pt/Ti/SiO₂/Si(100) and SrTiO₃ (STO) (100) substrates using the chemical solution deposition (CSD) technique. X‐ray diffraction (XRD) patterns indicate the polycrystalline nature of the films with rhombohedrally distorted perovskite crystal structure. Differential thermal analysis (DTA) was performed on the sol–gel‐derived powder to countercheck the crystal structure, ferroelectric (FE) to paraelectric (PE) phase transition, and melting point of bismuth ferrite. We observed a significant exothermic peak at 840 °C in DTA graphs, which corresponds to an FE–PE phase transition. Raman spectroscopy studies were carried out on BFO thin films prepared on both the substrates over a wide range of temperature. The room‐temperature unpolarized Raman spectra of BFO thin films indicate the presence of 13 Raman active modes, of which five strong modes were in the low‐wavenumber region and eight weak Raman active modes above 250 cm⁻¹. We observed slight shifts in the lower wavenumbers towards lower values with increase in temperature. The temperature‐dependent Raman spectra indicate a complete disappearance of all Raman active modes at 840 °C corresponding to the FE–PE phase transitions. There is no evidence of soft mode phonons. Copyright © 2008 John Wiley & Sons, Ltd.     

Synchronized whistlers recorded at Varanasi

Some interesting events of synchronized whistlers recorded at low latitude station Varanasi during magnetic storm period of the year 1977 are presented. The dynamic spectrum analysis shows that the component whistlers are Eckersley whistlers having dispersion 10 s^1/2 and 30 s^1/2. An attempt has been made to explain the dynamic spectra using lightning discharge generated from magnetospheric sources     

Scattering parameters from an analysis of the hall electron mobility in Ga1−xAlxAs alloys

Hall mobilities in high purity epitaxial layers of n-type Ga_1?xA_xAs alloys with room-temperature electron concentrations in the range (5–10) × 10¹⁵ cm^?3 have been measured at 300 K for alloy compositions in the range 0? x ? 0.78. Models for the variation of various material parameters with composition are developed. The acoustic deformation potentials for the Π and X conduction band minima, the intervalley coupling constants for electron scattering among the X-X and L-X minima and the longitudinal optical phonon temperature for these scatterings and the polar optical scattering in the X minima have been determined from a theoretical fit to the experimental data. The electron mobilities in the Π, L and X minima are found to decrease with the composition in the range 0≦ x ? 0.45, show a minimum at x ? 0.50 and then increase again with composition. These mobility variations are reflected in the minimum in the Hall mobility at x ? 0.5 due to intense non-equivalent electron scattering involving the L and x minima. The minimum in the electron mobilities in the Π and X minima have been identified to occur mainly due to the space charge and intervalley scattering among the L and X minima, respectively. The number of equivalent x minima in Ga_1?x Al_x As alloys are estimated to be three and are, thus, situated at the zone edge.     

Algebra of charges in the supersymmetric nonlinear σ model

We examine the algebra of the nonlocal charges in the supersymmetric nonlinear σ model and show that they satisfy a nonlinear algebra at the tree-level. We also discuss other interesting questions like the transformation of these charges under a supersymmetry transformation and speculate that this algebra possibly continues to hold in the full quantum theory.