Showing papers in "Physica B-condensed Matter in 2009"

Ionic conductivity by correlated barrier hopping in NH4I doped chitosan solid electrolyte

Abstract: Chitosan–NH 4 I and chitosan–NH 4 I–EC films have been prepared by solution cast technique. The sample containing 45 wt% ammonium iodide (NH 4 I) exhibited the highest room temperature conductivity of 3.7×10 −7 S cm −1 . The conductivity of the sample increased to 7.6×10 −6 S cm −1 when 40 wt% ethylene carbonate (EC) was added to the 55 wt% chitosan-45 wt% NH 4 I sample. The conductivity–temperature relationship is Arrhenian. From dielectric loss variation with frequency, the power law exponent was obtained. The temperature dependence of the power law exponent for chitosan–NH 4 I system follows the correlated barrier hopping (CBH) model while conduction mechanism of the plasticized system can be represented by the small polaron hopping (SPH) model.

Enhanced effective mass in doped SrTiO3 and related perovskites

Abstract: The effective mass is one of the main factors determining the Seebeck coefficient and electrical conductivity of thermo-electrics. In this ab-initio LDA-GGA study the effective mass is estimated from the curvature of electronic bands by one-band-approximation and is in excellent agreement with experimental data of Nb- and La-doped SrTiO 3 . It is clarified that the deformation of SrTiO 3 crystals has a significant influence on the bandgap, effective electronic DOS-mass and band-mass, but the electronic effect due to the e g -band flattening near the Γ -point due to Nb-doping up to 0.2 at% is the main factor for the effective mass increase. Doping of La shows a linear decrease of the effective mass; this can be explained by the different surroundings of A- and B-sites in perovskite. Substitution with other elements such as Ba on the A-site and V on the B-site in SrTiO 3 increases the effective mass as well.

Describing Dzyaloshinskii–Moriya spirals from first principles

Abstract: In magnetic systems lacking spatial inversion symmetry it is observed that the Dzyaloshinskii–Moriya interaction can entail long-ranged spin spirals with a unique sense of rotation. Here, we present a computationally efficient scheme to calculate these large magnetic structures within density functional theory and extract the interaction parameters using perturbative approaches of different order. We analyze the accuracy of these methods by investigating thin Fe films on W(1 1 0) and Mo(1 1 0) substrates.

Thermal and electronic transport in CeRu2Al10: Evidence for a metal-insulator transition

Abstract: A first report of physical properties of the ternary intermetallic compound CeRu2Al10 is given. The electrical resistivity below room temperature shows activated behaviour with a narrow gap of Δ ≃ 20 K before the onset of a sharp peak in ρ ( T ) below T * = 27 K . The Hall coefficient as well as the thermoelectric power are overall positive, and both increase in a similarly sharp manner below T * . The lattice part of the thermal conductivity indicates phonon coupling of the heat transport at T * , possibly underlying a lattice transformation that accompanies the putative metal-to-insulator and magnetic phase transitions.

Structural analysis of nickel doped cobalt ferrite nanoparticles prepared by coprecipitation route

Abstract: Magnetic nanoparticles of nickel substituted cobalt ferrite (Ni x Co 1− x Fe 2 O 4 :0≤ x ≤1) have been synthesized by co-precipitation route. Particles size as estimated by the full width half maximum (FWHM) of the strongest X-ray diffraction (XRD) peak and transmission electron microscopy (TEM) techniques was found in the range 18–28±4 nm. Energy dispersive X-ray (EDX) analysis confirms the presence of Co, Ni, Fe and oxygen as well as the desired phases in the prepared nanoparticles. The selective area electron diffraction (SAED) analysis confirms the crystalline nature of the prepared nanoparticles. Data collected from the magnetization hysteresis loops of the samples show that the prepared nanoparticles are highly magnetic at room temperature. Both coercivity and saturation magnetization of the samples were found to decrease linearly with increasing Ni-concentration in cobalt ferrite. Superparamagnetic blocking temperature as determined from the zero field cooled (ZFC) magnetization curve shows a decreasing trend with increasing Ni-concentration in cobalt ferrite nanoparticles.

Band parameters for cadmium and zinc chalcogenide compounds

Abstract: Theoretical investigations of the electronic, optical and elastic properties of cadmium and zinc chalcogenides in the zinc-blende structure are performed using a pseudopotential formalism. Our results are in reasonable agreement with the available experimental data. Polynomial expressions are obtained for the electron effective mass and the static dielectric constant as a function of the fundamental energy band-gap. Relations of elastic constants ratio to the ionicity are also examined and discussed.

Microwave-absorbing properties of shape-optimized carbonyl iron particles with maximum microwave permeability

Abstract: The microwave-absorbing properties for different shapes of carbonyl-iron particles prepared by the high-energy planetary ball milling with 40 vol% in epoxy resin matrix have been investigated. Higher value of magnetic permeability and permittivity can be obtained in the composites for thin flake carbonyl iron than spherical powders. The results are attributed to reduction of eddy current loss, orientation of magnetic moment and space–charge polarization with the shape change from spherical powders to thin flake particles. As the iron flakes with 0.4 μm in thickness as the absorbent fillers, the minimum RL value of −6.20 dB was observed at 4.57 GHz with thickness of 1 mm. The minimum reflection loss (RL) shifts to lower frequency and the value declines with change from spherical powders to thin flakes. It results from the considerable dielectric loss in the absorbing materials.

Effects of cobalt doping on the microstructure and magnetic properties of Mn–Zn ferrites prepared by the co-precipitation method

Abstract: Mn–Zn ferrite nanoparticles with various amounts of cobalt doping have been synthesized by the co-precipitation method. The structure and morphology of the nanoparticles have been characterized by X-ray diffraction and transmission electron microscopy. The effects of cobalt ions on the crystallization behavior, lattice parameters and magnetic properties of Mn–Zn ferrites have been investigated. All the Co-doped ferrite nanoparticles calcined at 1150 °C possess a simple spinel structure and have an approximately spherical shape. The lattice parameters increase almost linearly with increasing Co content. The studies of magnetic properties show that the saturation magnetization Ms strongly depends on the Co content, having a maximum Ms value of 73 emu/g at a Co content of 1.0 at%, and all the Co-doped ferrites, with the average crystallite sizes ranging from 24.5 to 27.0 nm, exhibit superparamagnetism at room temperature.

Preparation and microwave–absorbing properties of NiFe2O4-polystyrene composites

Abstract: NiFe 2 O 4 nanoparticles were synthesized by the polyacrylamide gel method with acrylamide as the monomer and N,N ′-methylenediacrylamide as lattice agent. The average crystallite sizes of the nickel ferrites annealed at 500, 600 and 800 °C are about 10, 30 and 50 nm, respectively. Ferrite–polystyrene composites were made by hot pressing, and microwave-absorbing properties of the composites with different contents of 35, 45, 55 and 65 wt% ferrite were investigated by testing complex permeability and complex permittivity in the X-band (8.2–12.4 GHz) frequency range. All the parameters, e ′, e ″, μ ′ and μ ″, increase with increasing ferrite content. The reflection losses were calculated based on a model of a single-layered plane wave absorber backed by a perfect conductor. The composite with 65 wt% ferrite content shows a minimum reflection loss of −13 dB at 11.5 GHz with a −10 dB bandwidth over the extended frequency range of 10.3–13 GHz for an absorber thickness of 2 mm.

Highlights from the magnetism reflectometer at the SNS

Abstract: The magnetism reflectometer at the SNS has passed the phase of commissioning and is in operation for users. The high power of the neutron source demands that special attention be paid to the optimization of the background in order to be able to measure the two-dimensional maps of reflected and scattered intensities with polarized neutrons in a broad range of momentum transfer. It implies an effective separation of the magnetic and non-magnetic reflectivity, off-specular scattering and the grazing incidence SANS in a high range of momentum transfer. Therefore the polarizing and the analyzing efficiencies are of particular importance on this time-of-flight instrument. At the beginning of July 2008 the world's first 3He neutron analyzer with on-line pump-up polarization was successfully installed and the tests with a magnetic multilayer film showing a strong off-specular spin-flip scattering were made. The performance of the instrument is under constant improvement in order to make it an effective and optimal instrument for the applications in nanosciences.

Structural, electronic and magnetic properties of η carbides (Fe3W3C, Fe6W6C, Co3W3C and Co6W6C) from first principles calculations

Abstract: First-principles FLAPW-GGA calculations have been performed with the purpose to determine the peculiarities of the structural, electronic, magnetic properties and stability for a family of related η carbides M3W3C and M6W6C (where M=Fe and Co). The geometries of all phases were optimized and their structural parameters, theoretical density, cohesive and formation energies, total and partial densities of states, atomic magnetic moments have been obtained and analyzed in comparison with available theoretical and experimental data.

Luminescence and laser transition studies of Dy3+:K–Mg–Al fluorophosphate glasses

Abstract: Dysprosium ion doped fluorophosphate glasses with compositions of PKMAFDy: (56−x/2) P2O5+17K2O+8Al2O3+(15−x/2)MgO+4AlF3+xDy2O3 (x=0.01, 0.05, 0.1, 1.0 and 2.0 mol%) have been prepared by melt quenching technique and are characterized by optical absorption, emission spectra and fluorescence lifetime measurements. The observed bands in the absorption spectrum are analysed by using free-ion Hamiltonian (HFI) model. The Judd–Ofelt (JO) analysis has been performed and the intensity parameters (Ωλ, λ=2, 4, 6) have been evaluated that are used to predict radiative properties. From emission spectra, the effective bandwidth (Δλeff) and the stimulated emission cross-section (σ(λp)) were evaluated. The fluorescence decay from the 4F9/2 level of Dy3+ ions have been measured by monitoring the intense 4F9/2→6H13/2 transition (573 nm). The lifetimes (τ) are found to decrease with increasing concentration due to concentration quenching. The decay curves are single exponential for lower concentrations and gradually changes to non-exponential for higher concentrations. The non-exponential decay curves are well fitted to the Inokuti–Hirayama (IH) model for S=6 which indicates that the energy transfer between the donor and acceptor is of dipole–dipole type. The systematic analysis on decay measurements reveals that the energy transfer mechanism strongly depends on concentration as well as glass composition.

Analysis of temperature dependent I―V measurements on Pd/ZnO Schottky barrier diodes and the determination of the Richardson constant

Abstract: Temperature dependent current–voltage ( I – V ) and Hall measurements were performed on Pd/ZnO Schottky barrier diodes in the range 20–300 K The apparent Richardson constant was found to be 860 × 10 - 9 A K - 2 cm - 2 in the 60–160 K temperature range, and mean barrier height of 050 eV in the 180–300 K temperature range After barrier height inhomogeneities correction, the Richardson constant and the mean barrier height were obtained as 167 A K - 2 cm - 2 and 061 eV in the temperature range 80–180 K, respectively A defect level with energy at 012 eV below the conduction band was observed using the saturation current plot and ( 011 ± 001 ) eV using deep level transient spectroscopy measurements

Magnetocaloric properties in the Cr-doped La0.7Sr0.3MnO3 manganites

Abstract: Single-phase polycrystalline samples of La 0.7 Sr 0.3 Mn 1– x Cr x O 3 with nominal composition of x =0.00, 0.20, 0.40 and 0.50 were prepared by a conventional solid-state reaction method in air. Investigations of magnetization were carried out in the temperature range 5–400 K and magnetic field range 0–8 T. It was found that the Curie temperature T C decreases with increasing x and the maximum magnetic entropy change (−Δ S M ) for x =0.20 is ∼1.203 and ∼2.653 J/kg K, respectively for 2 and 6 T magnetic field near the temperature of 280 K.

Microwave-absorbing characteristics of epoxy resin composites containing nanoparticles of NiZn- and NiCuZn-ferrites

Abstract: NiZn- and NiCuZn-ferrite nanoparticles (50–70 nm) with the chemical formula Ni 0.5 Zn 0.5 Fe 2 O 4 (NiZn) and Ni 0.35 Cu 0.15 Zn 0.5 Fe 2 O 4 (NiCuZn) were synthesized by a combustion synthesis method. The nanocrystallite of these materials was characterized by structural and magnetic methods. Saturation magnetization increases from 83 emu/g (NiZn) to 91 emu/g (NiCuZn). Magnetic permeability and dielectric permittivity were measured on sintered samples (pellets and toroids) in the frequency range of 1 MHz–1.8 GHz. Reflection losses ( R L ) for both samples were calculated from complex permeability and permittivity. Cu substitution in NiZn-ferrite enhances permeability and R L . In order to explore microwave-absorbing properties in X-band, magnetic nanoparticles were mixed with an epoxy resin to be converted into a microwave-absorbing composite and microwave behaviors of both materials were studied using a microwave vector network analyzer from 7.5 to 13.5 GHz. Cu substitution diminishes absorption intensity in the range 11.5–12.5 GHz.

Electrical conductivity study in pure and doped ZnO ceramic system

Abstract: We report here the electrical conductivities measurements of Zn 1− x Fe x O ceramic samples with various x values (0.00⩽ x ⩽0.50). The measurements are made in both high and low temperature cases (300 K⩽ T ⩽523 K) and (20 K⩽ T ⩽300 K), respectively. Furthermore, the electrical conductivity data are well enough to calculate the activation energies E a for the considered samples. The character of the conductivities curves is not absolutely symmetric to the response of temperature in both cases. Interestingly, two jumps are observed only in the conductivity curves at low temperatures. The onset temperatures of these jumps are increased by Fe up to 0.30, followed by a decrease at Fe=0.50. At constant Fe content, the conductivities are found to be increased over the temperature range in both cases. While, these values are decreased with the increase in the Fe content up to Fe=0.50. The conductivity character is divided into discrete regions over the temperature range corresponding to different activation energies. In high temperature case, the values of E a are increased with increasing Fe up to 0.30, followed by a decrease at 0.50. While, in low temperature cases, the values of E a are generally increased with increasing Fe up to 0.50. Our results are discussed in terms of potential barrier, donor concentration, point defects and adsorption–desorption of oxygen which are affected by Fe doping in ZnO ceramic system.

Photoluminescence of Pr3+, Sm3+ and Dy3+: SiO2–Al2O3–LiF–GdF3 glass ceramics and Sm3+, Dy3+: GeO2–B2O3–ZnO–LaF3 glasses

Abstract: In this paper, we present the absorption and photoluminescence properties of Pr 3+ , Sm 3+ and Dy 3+ -doped transparent oxyfluoride alumino-silicate glass ceramics and Sm 3+ and Dy 3+ -doped oxyfluoride germanate glasses. The X-ray diffraction (XRD) and differential thermal analysis (DTA) profiles of these glasses have been carried out, to confirm their structure and thermal stability. These glasses or glass ceramics have shown strong emission and absorption bands in visible and near-infrared (NIR) region. From the measured absorption spectra, Judd–Ofelt (J–O) intensity parameters ( Ω 2 , Ω 4 and Ω 6 ) have been calculated for all the studied ions. Compared to Pr 3+ , Sm 3+ and Dy 3+ doped glasses, their respective glass ceramics have shown stronger emissions due to LiGdF 4 crystalline phase in alumino-silicate glass ceramics. For Pr 3+ doped glass and glass ceramic, emission bands centered at 527 nm( 3 P 0 → 3 H 5 ), 610 nm( 3 P 0 → 3 H 6 ), 640 nm( 3 P 0 → 3 F 2 ), 678 nm( 3 P 1 → 3 F 3 ), 702 nm( 3 P 0 → 3 F 3 ) and 721 nm( 3 P 0 → 3 F 4 ) have been observed with 445 nm( 3 H 4 → 3 P 2 ) excitation wavelength. Of them, 640 nm( 3 P 0 → 3 F 2 ) has shown bright red emission. Emission bands of 4 G 5/2 → 6 H 5/2 (562 nm), 4 G 5/2 → 6 H 7/2 (598 nm), 4 G 5/2 → 6 H 9/2 (644 nm) and 706 nm( 4 G 5/2 → 6 H 11/2 ) for the Sm 3+ : glass and glass ceramic, with excitation at 6 H 5/2 → 4 F 7/2 (402 nm) have been recorded. Of them, 4 G 5/2 → 6 H 7/2 (598 nm) has shown a bright orange emission. With regard to the Dy 3+ : glass and glass ceramic, a bright fluorescent yellow emission at 576 nm( 4 F 9/2 → 6 H 13/2 ) and blue emission at 485 nm( 4 F 9/2 → 6 H 15/2 ) have been observed, apart from 4 F 9/2 → 6 H 11/2 (662 nm) emission transition for glass ceramic with an excitation at 388 nm( 6 H 15/2 → 4 I 13/2 + 4 F 7/2 ) wavelength. For germanate glasses, emission bands of 4 G 5/2 → 6 H 5/2 (564 nm), 4 G 5/2 → 6 H 7/2 (601 nm), 4 G 5/2 → 6 H 9/2 (644 nm) and 705 nm( 4 G 5/2 → 6 H 11/2 ) for the Sm 3+ : glass, with an excitation at 6 H 5/2 → 4 F 7/2 (401 nm) have been recorded. Of them, 4 G 5/2 → 6 H 7/2 (601 nm) has shown a bright orange emission. With regard to the Dy 3+ : glass, a bright fluorescent yellow emission at 576 nm( 4 F 9/2 → 6 H 13/2 ) and blue emission at 482 nm( 4 F 9/2 → 6 H 15/2 ) have been observed, apart from 4 F 9/2 → 6 H 11/2 (661 nm) emission transition with an excitation at 386 nm( 6 H 15/2 → 4 I 13/2 + 4 F 7/2 ) wavelength. The stimulated emission cross-sections of all the emission bands of Pr 3+ , Sm 3+ and Dy 3+ : glasses and glass ceramics have been computed based on their measured full-width at half maxima (FWHM, Δ λ ) and measured lifetimes ( τ m ).

Photoluminescence and phosphorescence properties of MAl2O4:Eu2+, Dy3+ (M=Ca, Ba, Sr) phosphors prepared at an initiating combustion temperature of 500 °C

Abstract: Eu2+ and Dy3+ co-doped calcium aluminate, barium aluminate and strontium aluminate phosphors were synthesized at an initiating combustion temperature of 500 °C using urea as an organic fuel. The crystallinity of the phosphors was investigated by using X-ray diffraction (XRD) and the morphology was determined by a scanning electron microscope (SEM). The low temperature monoclinic structure for both CaAl2O4 and SrAl2O4 and the hexagonal structure of BaAl2O4 were observed. The effect of the host materials on the photoluminescence (PL) and phosphorescence properties were investigated by using a He–Cd Laser and a Cary Eclipse fluorescence spectrophotometer, respectively. The broad band emission spectra observed at 449 nm for CaAl2O4:Eu2+, Dy3+, 450 nm (with a shoulder-peak at 500 nm) for BaAl2O4:Eu2+, Dy3+ and 528 nm for SrAl2O4:Eu2+, Dy3+ are attributed to the 4f65d1 to 4f7 transition in the Eu2+ ion in the different hosts.

Microstructural parameters and optical constants of ZnTe thin films with various thicknesses

Abstract: Different thickness of polycrystalline ZnTe films have been deposited onto glass substrates at room temperature by vacuum evaporation technique. The structural characteristics studied by X-ray diffraction (XRD) showed that the films are polycrystalline and have a zinc blende (cubic) structure. The calculated microstructure parameters revealed that the crystallite size increases and microstrain decreases with increasing film thickness. The transmittance and reflectance have been measured at normal and near normal incidence, respectively, in the spectral range 400–2500 nm. For ZnTe films of different thicknesses, the dependence of absorption coefficient, α on the photon energy showed the occurrence of a direct transition with band gap energy E g opt = 2.21 ± 0.01 eV (For ZnTe films of different thicknesses) confirming the independency of deduced energy gap on film thickness. The refractive indices have been evaluated in terms of envelope method, which has been suggested by Swanepoul in the transparent region. The refractive index could be extrapolated by Cauchy dispersion relationship over the whole spectra range, which extended from 400 to 2500 nm. It was observed that the refractive index, n increased upon increasing the film thickness up to 508 nm, lying within the experimental error for further increases in film thickness.

Spectroscopic and optical properties of Nd3+ doped fluorine containing alkali and alkaline earth zinc-aluminophosphate optical glasses

Abstract: Nd3+ doped fluorine containing zinc-aluminophosphate glasses have been prepared with alkali and alkaline earth content to understand the effect of network modifiers on radiative process. The physical and optical properties of these glasses have been evaluated. The Judd–Ofelt model for the intensity analysis of induced electric dipole transitions has been applied to the measured oscillator strengths of the absorption bands to determine the three phenomenological intensity parameters Ω2, Ω4 and Ω6 for each glass. Using these parameters, transition probability (A), total transition probability (AT), branching ratios (βR) radiative life times (τR) and integrated cross-section (σa) for the stimulated emission have been theoretically calculated for certain excited Nd3+ fluorescent levels. From the obtained results the conclusion is made about the possibility of using these glasses as laser material.

Enhanced visible light emission from Co2+ doped ZnS nanoparticles

Abstract: ZnS nanoparticles with Co 2+ doping have been prepared at room temperature through a soft chemical route, namely the chemical co-precipitation method. The nanostructures of the prepared nanoparticles have been analyzed using X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM), selected-area electron diffraction (SAED), and UV-vis spectrophotometer. The sizes of as prepared nanoparticles are found to be in 1–4 nm range. Room-temperature photoluminescence (PL) spectrum of the undoped sample exhibits emission in the blue region with multiple peaks under UV excitation. On the other hand, in the Co 2+ doped ZnS samples enhanced visible light emissions with emission intensities of ~35 times larger than that of the undoped sample are observed under the same UV excitation wavelength of 280 nm.

The Silicon Vacancy in SiC

Abstract: A model is presented for the silicon vacancy in SiC. The previously reported photoluminescence spectra in 4H and 6H SiC attributed to the silicon vacancy are in this model due to internal transitions in the negative charge state of the silicon vacancy. The magnetic resonance signals observed are due to the initial and final states of these transitions.

Effect of Zn2+ substitution on the magnetic properties of Mg1−xZnxFe2O4 ferrites

Abstract: Polycrystalline Mg1−xZnxFe2O4 (x=0.0–0.6) ferrites have been prepared using solid-state reaction technique. The X-ray diffraction analysis revealed that the samples crystallize in a single-phase cubic spinel structure. The lattice parameter increases linearly with increase in zinc content obeying Vegard's law. The continuous decrease in Curie temperature (Tc) with an increase in Zn content is attributed to the weakening of A–B exchange interaction. Saturation magnetization (Ms) and magnetic moment are observed to increase up to x=0.4, and thereafter decrease due to the spin canting in B-sites. The initial permeability is found to increase with the addition of Zn2+ ions but the resonance frequency shifts towards the lower frequency.

A comparative study of the microstructures and optical properties of Cu- and Ag-doped ZnO thin films

Abstract: Cu- and Ag-doped ZnO films were deposited by direct current co-reactive magnetron sputtering technique. The microstructure, the chemical states of the oxygen, zinc, copper and silver and the optical properties in doped ZnO films were investigated by X-ray diffraction spectroscopy (XRD), X-ray photoelectron spectroscopy (XPS) and UV–Visible spectroscopy. XRD analysis revealed that both of Cu- and Ag-doped ZnO films consist of single phase ZnO with zincite structure while the doping elements had an evident effect on the (0 0 2) preferential orientation. The XPS spectra showed that the chemical states of oxygen were different in Cu- and Ag-doped ZnO thin films, which may lead to the shift of the band gap as can be observed in the transmittance and absorption spectra. Meanwhile, the widths of band tails of ZnO films became larger after Cu and Ag doping.

Shallow defects in Cu2ZnSnS4

Abstract: Cu 2 SnZnS 4 is a promising candidate for thin film solar cells with absorbers made of non-toxic and abundant elements. So far very little effort has been put into understanding the basic material properties. We investigate vapour phase grown Cu 2 SnZnS 4 crystals by temperature and intensity dependent photoluminescence measurements. We observe for the first time narrow photoluminescence peaks, which allow us to determine defect levels and to propose a defect recombination model for Cu 2 SnZnS 4 . Assuming an exciton binding energy of 10 meV, we find an energy gap of 1.519 eV at 10 K. From the observed DA transitions we derive the energies of two shallow acceptor states 10±5 and 30+5 meV above the valence band and one shallow donor state 5±3 meV below the conduction band.

Structure and crystallization of B2O3–Al2O3–SiO2 glasses

Abstract: B 2 O 3 –Al 2 O 3 –SiO 2 glasses with different B 2 O 3 /Al 2 O 3 ratios of 0.4–1.3 were prepared by the melting–quenching method at 1500–1600 °C for 2 h. Fragility index F was used to estimate the glass-forming ability. The infrared (IR) absorption curves and differential scanning calorimetry (DSC) curves of the glasses have been investigated for estimating the influence of the B 2 O 3 /Al 2 O 3 ratio on glass structure and crystallization of the B 2 O 3 –Al 2 O 3 –SiO 2 glass system. The crystallization kinetics of the glasses were described by activation energy ( E ) for crystallization and calculated by the Kissinger method. X-ray diffraction (XRD) and SEM analyses were also used to describe the types and morphologies of the crystals precipitated from the B 2 O 3 –Al 2 O 3 –SiO 2 glasses. The results show that with the increase of B 2 O 3 /Al 2 O 3 ratio, glass stability improves and the trend of crystallization decreases relatively. However, when the B 2 O 3 /Al 2 O 3 ratio reaches 1.3, boron-abnormal phenomenon appears and results in the raising trend of crystallization. Rod-like crystals of Al 4 B 2 O 9 and Al 20 B 4 O 36 were observed in the crystallized samples.

Structural, optical and thermal investigations on Dy3+ doped NaF–Li2O–B2O3 glasses

Abstract: Structural, optical and thermal properties of Dy 3+ doped lithium fluoroborate glasses have been studied for various concentrations of Dy 3+ from 0.5 to 5 wt%. The XRD studies confirm the amorphous nature of the glasses while the FTIR spectra reveal the presence of BO 3 and BO 4 local structural units. The UV–VIS–NIR absorption studies were carried out to calculate the bonding parameters ( β ¯ and δ ), to identify the ionic/covalent nature of the glasses. The JO parameters, experimental and theoretical oscillator strengths were also determined and reported. The luminescence spectra have been studied to determine the radiative transition probability ( A ), stimulated emission cross section ( σ P E ) and the experimental and calculated branching ratios ( β R ) for the excited levels that include 4 F 9/2 → 6 H 11/2 , 6 H 13/2 , and 6 H 15/2 transitions. The variation of optical properties with varying concentrations of dysprosium oxide content in the glasses are reported and discussed. The thermal behavior of Dy 3+ doped lithium fluoroborate glasses have been reported by recording DSC thermograms.

Enhancement of Tb3+ emission by non-radiative energy transfer from Dy3+ in silicate glass

Abstract: A study of energy transfer was performed in dysprosium–terbium-doped silicate glasses at room temperature. Enhancement of the Tb 3+ emission and a decrease in the Dy 3+ emission are observed as a result of energy transfer from Dy 3+ ions to Tb 3+ ions. The energy transfer efficiencies, transfer probabilities, as well as average donor–acceptor distances were also calculated. It is concluded that the energy transfer mechanism between Dy 3+ and Tb 3+ ion is mainly electric dipole–dipole in nature.

AC conductivity and dielectric behavior of polyaniline/sodium metavenadate (PANI/NaVO3) composites

Abstract: The conducting polyaniline/sodium metavenadate (PANI/NaVO 3 ) composites were synthesized by single step in situ polymerization technique by placing finely grinded powder of NaVO 3 during the polymerization of aniline. The formation of mixed phases of the polymer together with the conducting emeraldine salt phase was confirmed by spectroscopic techniques like FTIR. SEM images indicated a systematic morphological variation of particles aggregated in the composite matrix as compared to the pristine PANI. AC conductivity and dielectric behavior of these composites were investigated in the frequency range 50 Hz to 5 MHz. It is found that AC conductivity obeyed the power law index and the variation of conductivity with wt% of NaVO 3 could be related to conductivity relaxation phenomenon. These composites have shown high dielectric constant, which is related to polarization. It is seen that both dielectric constant and dielectric loss decrease with increase in frequency. Variations in measured parameters of AC response with increasing frequency of these composites are found to follow systematic trends that are similar to those observed with temperature and doping.

Study of optical band gap, carbonaceous clusters and structuring in CR-39 and PET polymers irradiated by 100 MeV O7+ ions

Abstract: Commercially purchased CR-39 and PET polymers were irradiated by 100 MeV O7+ ions of varying fluences, ranging from 1×1011 to 1×1013 ions/cm2. The effects of swift heavy ions (SHI) on the structural, optical and chemical properties of CR-39 and PET polymers were studied using X-ray diffraction (XRD), UV–visible spectroscopy and Fourier transform infrared (FTIR) spectroscopy. The XRD patterns of CR-39 show that the intensity of the peak decreases with increasing ion fluence, which indicates that the semicrystalline structure of polymer changes to amorphous with increasing fluences. The XRD patterns of PET show a slight increase in the intensity of the peaks, indicating an increase in the crystallinity. The UV–visible spectra show the shift in the absorbance edge towards the higher wavelength, indicating the change in band gap. Band gap in PET and CR-39 found to be decrease from 3.87 to 2.91 and 5.3–3.5 eV, respectively. The cluster size also shows a variation in the carbon atoms per cluster that varies from 42 to 96 in CR-39 and from 78 to 139 in PET. The FTIR spectra show an overall reduction in intensity of the typical bands, indicating the degradation of polymers after irradiation.