Electrical conductivity of transition metal oxide glasses in the system V2O5B2O3TeO2 at high temperatures

Bulk d.c. conductivities of the electronically conducting transition metal oxide glasses in the system xV₂O₅50B₂O₃(50?x)TeO₂ were measured at different temperatures in the range 310–623 K. The dependence of conductivity on composition is explained on the basis of the role of TeO₂ as a modifier or impurity. The observed kink in the log σ vs 1/T characteristic of the glasses is interpreted as being due to switching over of the conduction mechanism from one type to the other. The activation energies are fitted in a linear relation of the type E_g = A ? BP, where A and B are constants and P, the composition parameter. Comparing the results obtained with those of previous workers in the V₂O₅P₂O₅ and V₂O₅B₂O₃ systems, a physical analysis is presented.     

NIR to visible upconversion in Er/Yb co-doped CaYAl3O7 phosphor obtained by solution combustion process

Using the combustion synthesis, CaYAl₃O₇:Er³⁺ phosphor powders co-doped with Yb³⁺ have been prepared at low temperatures (550 °C) in a few minutes. Formation of the compound was confirmed by X-ray powder diffraction. Near-infrared to visible upconversion fluorescence emission in the Er³⁺ doped CaYAl₃O₇ phosphor powder has been observed. The effect of co-doping with triply ionized ytterbium in the CaYAl₃O₇:Er³⁺ phosphor has been studied and the process involved is discussed.     

Designing of endlessly single mode polarization maintaining highly birefringent nonlinear micro-structure fiber at telecommunication window by FV-FEM

An endlessly single mode highly polarization maintaining nonlinear microstructure fiber at telecommunication window is reported via full-vector finite element method. By taking three ring hexagonal PCF with suitable fiber parameter such as air hole diameter in cladding region d = 0.8 μm, pitch 2.3 μm and introducing four symmetrical large air holes near core region d′ = 2 μm, single mode (V_eff ≤ π), small effective mode area 2.7 μm², nonlinear co-efficient 44.39 W⁻¹ km⁻¹, high phase birefringence of the order of 10⁻³ and group birefringence of the order of 10⁻⁴ with beat length 0.3 μm at wavelength 1.55 μm are achieved.     

Structural,electronic, optical,elastic and thermal properties of ZnXAs<Subscript>2</Subscript> (X = Si and Ge) chalcopyrite semiconductors

We report first principles calculations of solid state properties of ZnSiAs₂ and ZnGeAs₂ chalcopyrite semiconductors. The structural properties are calculated using a Full Potential Linearized Augmented Plane Wave method (FP-LAPW) of the Density Functional Theory (DFT). A Generalized Gradient Approximation (GGA) scheme proposed by Wu and Cohen (WC) has been chosen to calculate electronic and optical properties. Optical features such as dielectric functions, refractive indices, extinction coefficient, optical reflectivity, absorption coefficients and optical conductivities were calculated for photon energies up to 30 eV. The elastic constants at equilibrium in tetragonal structure are also determined. Temperature effect on the volume, thermal expansion, heat capacity, Debye temperature, entropy, Grüneisen parameter and bulk modulus were calculated employing the quasi-harmonic Debye model at different temperatures and pressures and the silent results were interpreted. Finally using semi-empirical relation, we determined the hardness of the materials which attributed to different covalent bonding strengths.     

Influence of small DC bias field on the electrical behaviour of Sr- and Mg-doped lanthanum gallate

One of the promising electrolyte materials for solid oxide fuel cells application, Sr- and Mg-doped lanthanum gallate La_0.9Sr_0.1Ga_0.8Mg_0.2O_3-δ (LSGM), is synthesized by conventional solid state ceramic route. X-ray Rietveld analysis confirms the formation of main orthorhombic phase at room temperature along with a few minor secondary phases. SEM micrograph reveals the grain and grainboundary morphology of the system. Electrical conductivity of the LSGM sample is measured in the temperature range 573–873 K and in the frequency range 20 Hz–1 MHz at a few small DC bias fields (at 0.0, 0.5, 1.0, 1.5 and 2.0 V). The conductivity spectra show power-law behaviour. Electrical conductivity of the sample is found to be weakly dependent on DC bias field. This is attributed to field-dependent bulk and grainboundary conduction processes. In the present system, under investigated bias field range, the possibility of formation of Schottky barrier is ruled out. The concept of grainboundary channel (pathway) modulation on the application of bias field is proposed.     

Preparation of highly crystalline blue emitting MVO4:Tm (M=Gd, Y) spherical nanoparticles: Effects of activator concentration and annealing temperature on luminescence, lifetime and quantum yield

Highly crystalline spherical nanoparticles of MVO₄:Tm³⁺ (M=Gd, Y) having a size of 20–45 nm were prepared using ethylene glycol as both capping agent and reaction medium. X-ray diffraction study shows linear decrease in the unit cell volume with an increase in Tm³⁺ concentrations in MVO₄ (M=Gd, Y; Tm³⁺=0, 2, 5, 7, 10, 15, 20, 40, 60, 80 and 100 at%), indicative of quantitative substitution of Gd³⁺/Y³⁺ lattice sites by Tm³⁺ ions in MVO₄. Blue light emission at 475 nm is observed after excitation at 310 nm due to energy transfer from VO₄³⁻ absorption band to Tm³⁺. Emission intensity and average decay lifetime increase with an increase in heat treatment from 500 to 900 °C. This has been attributed to an extent of reduction in non-radiative process arising from surface. The emission intensity of Tm³⁺ in GdVO₄ host is higher than that in YVO₄ and thus the former host is better. Quantum yield increases with increase in heat-treatment temperature. This material will be the alternative blue light emitter.     

Design studies of a high-current radiofrequency quadrupole for accelerator-driven systems programme

A 3 MeV, 30 mA radiofrequency quadrupole (RFQ) accelerator has been designed for the low-energy high-intensity proton accelerator (LEHIPA) project at BARC, India. The beam and cavity dynamics studies were performed using the computer codes LIDOS, TOUTATIS, SUPERFISH and CST microwave studio. We have followed the conventional design technique with slight modifications and compared that with the equipartitioned (EP) type of design. The sensitivity of the RFQ to the variation of input beam Twiss-Courant parameters and emittance has also been studied. In this article we discuss both design strategies and the details of the 3D cavity simulation studies.     

Consistency and variations in optical characteristics of four-layer polymeric waveguides

In the present work, four-layer polymeric waveguides have been fabricated and characterized by interchanging the film layer. Using polyvinyl alcohol (PVA) and styrene acrylonitrile (SAN) polymer it has been shown that polymers do not loose their own property in multilayer structure and better efficiency can be achieved in the form of low propagation losses and mode filtration. Due to high contrast of PVA and SAN films, a broad spectrum of refractive index in four-layer structure has been achieved.     

On the variable-charge Coulomb-projected Born approximation

Total cross-sections for electron impact excitation of 1¹ S – 2³ S transition in helium have been calculated using variable-charge Coulomb-projected Born approximation and also using a distorted wave model in which the prior form of theT-matrix is used. The comparison of the two sets of results enables us to make certain observations about the suitability of the variable-charge Coulomb-projected Born approximation.     

Characterization and photoluminescence properties of some MLn2(1−x)O4:2xEu or 2xTb systems (M=Ba or Sr, Ln=Gd or La)

BaGd₂O₄, BaLa₂O₄ and SrLa₂O₄ powders doped with different concentrations of Eu³⁺ or Tb³⁺ are prepared by combustion synthesis method and the samples were further heated to 500, 700 and 900 °C to improve the crystallinity of the materials. The structure and morphology of materials have been examined by X-ray diffraction and scanning electron microscopy. It is remarkable that all the samples of BaGd₂O₄, BaLa₂O₄ and SrLa₂O₄ have similar morphology. The SEM images show homogeneous aggregates of varying shapes and sizes, which are composed of a large number of small elliptical shaped crystallites with an average diameter of about 0.5-3.0 μm. Photoluminescence for all materials increases with increase of temperature and shows a maximum for the samples heated to 900 °C with 4 mole% doping of Eu³⁺ or Tb³⁺ ions. The luminescence is almost same for all powders when doped with same concentration of Eu³⁺.