Synthesis and magnetic properties of FeNi<Subscript>3</Subscript>/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> core-shell nanocomposites

In this study, FeNi₃/Al₂O₃ core-shell nanocomposites, where individual FeNi₃ nanoparticles were coated with a thin layer of alumina, were fabricated by a modified sol-gel method. Several physical characterizations were performed on the samples of FeNi₃/Al₂O₃ nanocomposites with different thickness of Al₂O₃ shell. The encapsulation of FeNi₃ nanoparticles with alumina stops FeNi₃ agglomeration during heat treatment, and prevents interaction among the closely spaced magnetic FeNi₃ nanoparticles. The Al₂O₃ insulating shell improves the soft magnetic properties of FeNi₃. The study of the complex permeability of the samples shows that the real part μ’ of the permeability of the sample with Al molar content of 20% (Al/(Fe+Ni)) is as high as 12, and independent of frequency up to at least 1 GHz. The tunneling magnetoresistance arising from the presence of the Al₂O₃ shell have also been studied.     

Improvement of the intergranular pinning energy in uniaxially compacting (Bi-Pb)<Subscript>2</Subscript>Sr<Subscript>2</Subscript>Ca<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>10+δ</Subscript> ceramic samples

Measurements of the electrical resistivity as a function of temperature, ρ(T), for different values of applied magnetic field, B_a (0 ≤ B_a ≤ 50 mT), were performed in polycrystalline samples of Bi_1.65Pb_0.35Sr₂Ca₂ Cu₃O_10+δ subjected to different uniaxial compacting pressure (UCP). We have found appreciable differences in the grain orientation between samples by using X-ray diffractometry. From the X-ray diffraction patterns performed, in powder and pellet samples, we have estimated the Lotgering factor along the (00l) direction, F_(00l). The results indicate that F_(00l) increases ~23% with increasing UCP suggesting that grains of these samples are preferentially aligned along the c-axis, which is parallel to the compacting direction. The resistive transition of the samples have been interpreted in terms of the thermally activated flux-creep model. In addition, the effective intergranular pinning energy, U₀, have been determined for different applied magnetic field. The magnetic field dependence of U₀, for B_a > 8 mT, was found to follow a H^{- α} dependence with α = 0.5 for all samples. The analysis of the experimental data strongly suggested that increasing UCP results in appreciable changes in both the grain alignment and the grain connectivity of the samples. We have successfully interpreted the data by considering the existence of three different superconducting levels within the samples: the superconducting grains, the weak-links, and the superconducting clusters.     

Luminescence in Dy<Superscript>3+</Superscript> and Eu<Superscript>3+</Superscript> Activated K<Subscript>3</Subscript>Al<Subscript>2</Subscript>(PO<Subscript>4</Subscript>)<Subscript>3</Subscript>

The Dy³⁺ and Eu³⁺ activated K₃Al₂ (PO₄)₃ phosphors were prepared by a combustion synthesis. From a powder X-ray diffraction (XRD) analysis the formation of K₃Al₂ (PO₄)₃ was confirmed. In the photoluminescence emission spectra, the K₃Al₂(PO₄)₃:Dy³⁺ phosphor emits two distinctive colors: blue and yellow whereas K₃Al₂(PO₄)₃:Eu³⁺ emits red color. Thus the combination of colors gives BYR (blue–yellow–red) emissions can produce white light. These phosphors exhibit a strong absorption between 340 and 400 nm which suggest that present phosphor is a promising candidate for producing white light-emitting diodes (LED).     

Enhanced Up-Conversion Emission in Al<Superscript>3+</Superscript> Co-Doped ZnGa<Subscript>2</Subscript>O<Subscript>4</Subscript>:Yb<Superscript>3+</Superscript>,Tm<Superscript>3+</Superscript> Powder Phosphors

Yb³⁺-Tm³⁺ co-doped up-conversion powder phosphors using Zn(Al_xGa_1-x)₂O₄ (ZAGO) as the host materials were synthesized via solid-state reaction successfully. In addition, the morphology, structural characterization and up-conversion luminescent properties were all investigated by scanning electron microscope (SEM), x-ray diffraction (XRD) and fluorescence spectrophotometer (F-7000), respectively. Under the excitation of a 980 nm laser, all as-prepared powders can carry out blue emission at about 477 nm (corresponding to ¹G₄ → ³H₆ transition of Tm³⁺ ions), and red emission at about 691 nm (attributed to ³F₃ → ³H₆ transition of Tm³⁺ ions). Also, the influence of doping Al³⁺ ions were investigated. In brief, the doping of Al³⁺ ions has no effect on the position of emission peak. Howbeit the up-conversion efficiency and intensity of ZAGO:Yb,Tm phosphors are stronger than ZGO:Yb,Tm and ZAO:Yb,Tm phosphors, while the crystallinity is the opposite. More particularly, all as-prepared powder phosphors emit strong luminescence, which is observable by the naked eye, demonstrating the potential applications in luminous paint, luminescent dye, etc.     

Grain size effects on dynamics of Li-ions in Li<Subscript>3</Subscript>V<Subscript>2</Subscript>(PO<Subscript>4</Subscript>)<Subscript>3</Subscript> glass-ceramic nanocomposites

Li₃V₂(PO₄)₃ glass-ceramic nanocomposites, based on 37.5Li₂O-25V₂O₅-37.5P₂O₅ mol% glass, were successfully prepared via heat treatment (HT) process. The structure and morphology were investigated by X-ray diffraction (XRD) and scanning electron microscope (SEM). XRD patterns exhibit the formation of Li₃V₂(PO₄)₃ NASICON type with monoclinic structure. The grain sizes were found to be in the range 32–56 nm. The effect of grain size on the dynamics of Li⁺ ions in these glass-ceramic nanocomposites has been studied in the frequency range of 20 Hz–1 MHz and in the temperature range of 333–373 K and analyzed by using both the conductivity and modulus formalisms. The frequency exponent obtained from the power law decreases with the increase of temperature, suggesting a weaker correlation among the Li⁺ ions. Scaling of the conductivity spectra has also been performed in order to obtain insight into the relaxation mechanisms. The imaginary modulus spectra are broader than the Debye peak-width, but are asymmetric and distorted toward the high frequency region of the maxima. The electric modulus data have been fitted to the non-exponential Kohlrausch–Williams–Watts (KWW) function and the value of the stretched exponent β is fairly low, suggesting a higher ionic conductivity in the glass and its glass-ceramic nanocomposites. The advantages of these glass-ceramic nanocomposites as cathode materials in Li-ion batteries are shortened diffusion paths for Li⁺ ions/electrons and higher surface area of contact between cathode and electrolyte.     

Investigation on structural characteristics of PVDF–AgCF<Subscript>3</Subscript>SO<Subscript>3</Subscript>–Al<Subscript>2</Subscript>O<Subscript>3</Subscript> nanocomposite solid polymer electrolyte system

There has been an increasing interest towards the incorporation of nanosize ceramic fillers in polymer electrolytes. Solid polymer electrolytes based on polyvinylidene fluoride (PVDF), silver triflate (AgCF₃SO₃), and x wt% of aluminum oxide (Al₂O₃) nanopowders (where x = 1, 3, 5, and 10, respectively) have been prepared using solution casting technique. The structural characteristics of these thin film specimens were studied using Fourier transform infrared (FTIR) and X-ray diffraction (XRD) patterns at room temperature. The appearance of new absorption bands and gradual shifts observed in some characteristic peaks confirmed the complex formation between polyvinylidene fluoride and silver triflate. Furthermore, the addition of nanosized filler Al₂O₃ has also indicated the interaction of the filler with the polymer salt complex. The XRD patterns obtained for all these samples in the 2θ range 10° to 70° showed the amorphous nature of these samples. Paper presented at the Third International Conference on Ionic Devices (ICID 2006), Chennai, Tamilnadu, India, December 7–9, 2006.     

Luminescence and electronic excitations in Li<Subscript>6</Subscript>Gd(BO<Subscript>3</Subscript>)<Subscript>3</Subscript>: Ce<Superscript>3+</Superscript> crystals

This paper reports on a study of the luminescence emitted by Li₆Gd(BO₃)₃: Ce³⁺ crystals under selective photoexcitation to lower excited states of the host ion Gd³⁺ and impurity ion Ce³⁺ within the 100–500-K temperature interval, where the mechanisms of migration and relaxation of electronic excitation energy have been shown to undergo noticeable changes. The monotonic 10–15-fold increase in intensity of the luminescence band at 3.97 eV has been explained within a model describing two competing processes, namely, migration of electronic excitation energy over chains of Gd³⁺ ions and vibrational energy relaxation between the ⁶ I _j and ⁶ P _j levels. It has been shown that radiative transitions in Ce³⁺ ions from the lower excited state 5d ¹ to ² F _5/2 and ² F _7/2 levels of the ground state produce two photoluminescence bands, at 2.08 and 2.38 eV (Ce1 center) and 2.88 and 3.13 eV (Ce2 center). Possible models of the Ce1 and Ce2 luminescence centers have been discussed.     

On the Luminescence of Ce<Superscript>3+</Superscript>, Eu<Superscript>3+</Superscript>, and Tb<Superscript>3+</Superscript> in Novel Borate LiSr<Subscript>4</Subscript>(BO<Subscript>3</Subscript>)<Subscript>3</Subscript>

This paper reports on the photoluminescence (PL) and time-resolved properties of Ce³⁺, Eu³⁺, and Tb³⁺ in novel LiSr₄(BO₃)₃ powder phosphors. Ce³⁺ shows an emission band peaking at 420 nm under 350-nm UV excitation. Energy transfer from Ce³⁺ to Mn²⁺ takes place in the co-doped samples. Eu³⁺ shows red emission under near UV excitation. LiSr₄(BO₃)₃:Eu³⁺ phosphor could be a suitable candidate for phosphor-converted solid state lighting. The luminescence lifetime is 2.13 ms for Eu³⁺ in LiSr₄(BO₃)₃:0.001Eu³⁺. As Eu³⁺ concentration increasing, the decay curves deviate from exponential behavior. Tb³⁺ shows the strongest ⁵D₄→⁷ F₅ emission line at 540 nm. Decay curves of ⁵D₄→⁷ F₅ and ⁵D₃→⁷ F₅ emission with different Tb³⁺ concentrations were also measured. Cross-relaxation process is discussed based on the decay curves.     

Mössbauer studies of magnetic Fe<Subscript>2</Subscript>O<Subscript>3</Subscript>/SiO<Subscript>2</Subscript> nanocomposites

A large variety of glass and glass ceramics may be obtained by sol-gel process from hydrolysis of tetraethoxysilane. The transformation involves hydrolysis and polycondensation reactions leading to the growth of clusters that eventually collide together to form a gel. The structure and properties of the final product have been found to be strongly dependent on the initial conditions of preparation. Silica nanocomposites based on Fe₂O₃/SiO₂ were prepared with the help of ultrasonic activation and subsequent annealing in nitrogen atmosphere or air with concentrations of iron oxide of about 20 to 30wt.%.     

Green emission of Ca<Subscript>3</Subscript>La<Subscript>3(1-x)</Subscript>Tb<Subscript>3x</Subscript>(BO<Subscript>3</Subscript>)<Subscript>5</Subscript> under VUV-UV excitation

A series of phosphors Ca₃La_3(1-x)Tb_3x(BO₃)₅ were synthesized by using a high-temperature solid-state reaction technique, and their VUV-vis luminescence properties were investigated. Strong host-related absorption is observed in the VUV region, and the f–d transitions of Tb³⁺ in the host lattice are assigned and discussed. The influence of both the doping concentration and the temperature on the spectroscopic properties that include the relative emission intensity and the decay time is investigated. PACS 61.72.Ww; 78.55.-m; 78.47.+p     

Tunable emission and efficient energy-transfer properties of Ce<Superscript>3+</Superscript> and Mn<Superscript>2+</Superscript> co-doped Ba<Subscript>3</Subscript>Gd(PO<Subscript>4</Subscript>)<Subscript>3</Subscript> phosphors

MgO powders subjected to Q-switched laser pulses in water were characterized by X-ray/electron diffraction and optical spectroscopy to have a significant optical property change with accompanied transformation into the phase assemblages of periclase, brucite and liquid-crystalline lamella. The periclase nanoparticles tended to have {111} habit planes parallel to the basal layers of lamella and brucite flakes more or less rolled as fibers, ribbons or tubes. A significant internal compressive stress was built up for periclase and brucite but not the more flexible lamellar phase. The colloidal suspension containing the densified periclase nanoparticles within the rolled brucite/lamellae flakes showed UV–visible absorption corresponding to a minimum band gap of ca. 5 eV.     

One-pot fabrication and enhanced thermoelectric properties of poly(3,4-ethylenedioxythiophene)-Bi<Subscript>2</Subscript>S<Subscript>3</Subscript> nanocomposites

Bi₂S₃ nanotubes and de-doped poly(3,4-ethylenedioxythiophene) (PEDOT) composite nanopowders were synchronously synthesized by a one-pot self-assembly method. The powders were characterized by X-ray powder diffraction, infrared spectroscopy, and transmission electron microscopy, respectively. Thermoelectric properties of the Bi₂S₃–PEDOT composite nanopowders with different Bi₂S₃ contents after being cold pressed into pellets were measured at room temperature. The sample with 36.1 wt% Bi₂S₃ has a highest power factor of 2.3 μWm⁻¹K⁻², which is higher than that of both pure PEDOT (0.445 μWm⁻¹K⁻²) and Bi₂S₃ (1.94 μWm⁻¹K⁻²).     

Enhanced photoluminescence of Ca<Subscript>2</Subscript>Al<Subscript>2</Subscript>SiO<Subscript>7</Subscript>:Eu<Superscript>3+</Superscript> by charge compensation method

The effect of compensator on optical properties of Ca₂Al₂SiO₇:Eu³⁺ is systematically investigated by the X-ray powder diffraction, photo-luminescence (PL) properties and lifetime. It is obviously observed that the PL intensity of Eu³⁺ under 394 nm excitation increases in the order of Ca_1.86Eu_0.14Al₂SiO₇ (CAS), Ca_1.72Na_0.14Eu_0.14Al₂SiO₇ (CASNa) and Ca_1.86Eu_0.14Al_2.14Si_0.86O₇ (CASAl), the intensity of Eu³⁺ are 100%, 134%, 184%, and the lifetime of Eu³⁺ are 0.75 ms, 1.28 ms and 1.39 ms, respectively. A charge compensation model is proposed to explain the changes in the emission intensity and lifetime of Eu³⁺ in Ca₂Al₂SiO₇ with different compensation methods. PACS 78.55.-m; 61.72.Ji; 61.43.Gt; 42.70.-a; 74.62.Dh     

Spectroscopy of NaLa(MoO<Subscript>4</Subscript>)<Subscript>2</Subscript>: Tm<Superscript>3+</Superscript> and NaGd(MoO<Subscript>4</Subscript>)<Subscript>2</Subscript>: Tm<Superscript>3+</Superscript> crystals as advanced laser materials

Single crystals of double sodium-containing lanthanum and gadolinium molybdates doped with Tm³⁺ ions were synthesized by the Czochralski method. The spectroscopic properties of these crystals were investigated from the viewpoint of their use as active media in diode-pumped lasers. The polarized spectra of absorption on the ³ H ₄ and ³ F ₄ levels and the polarized spectra of luminescence due to the ³ F ₄-³ H ₆ laser transition were recorded, and the lifetimes of the ³ H ₄ and ³ F ₄ excited states of the Tm³⁺ ions were determined. The luminescence cross sections were calculated using the Füchtbauer-Ladenburg formula. The simulation of the decay curve of the ³ H ₄ excited state according to the Golubov-Konobeev-Sakun method revealed that, in the crystals under investigation, the interaction between Tm³⁺ ions predominantly occurs through the dipole-dipole mechanism.     

Shear-induced low-dimension electron transport in (LaMnO<Subscript>3</Subscript>)<Subscript>2</Subscript>/(SrMnO<Subscript>3</Subscript>)<Subscript>2</Subscript> superlattice

Effects of a mechanical shear on the electron transport properties of a (LaMnO₃)₂/(SrMnO₃)₂ superlattice are investigated using first-principle DFT calculations. While the unstrained superlattice is a 3-D conducting half metal, application of a pyramidal shear transforms it into a non-spin-polarized conductor. Depending on whether the out-of-plane component of the shear is tensile or compressive the conductivity is 1-D out-of-plane or 2-D in-plane. The shear-induced low-dimensional conductivity is also associated with the FM-AFM transition.     

Comparison of Eu(NO<Subscript>3</Subscript>)<Subscript>3</Subscript> and Eu(acac)<Subscript>3</Subscript> precursors for doping luminescent silica nanoparticles

In this study, we report the comparison between Eu³⁺-doped silica nanoparticles synthesized by Stöber method using Eu(NO₃)₃ or Eu(acac)₃ as precursors. The impact of different europium species on the properties of the final silica nanospheres is investigated in details in terms of size, morphology, reachable doping amount, and luminescence efficiency. Moreover, the results obtained for different thermal treatments are presented and discussed. It is shown that the organic complex modify the silica growing process, leading to bigger and irregular nanoparticles (500–800 nm) with respect to the perfectly spherical ones (400 nm) obtained by the nitrate salt, but their luminescence intensity and lifetime is significantly higher when 800–900 °C annealing is performed.     

Luminescence and thermally stimulated recombination processes in Li<Subscript>6</Subscript>Gd(BO<Subscript>3</Subscript>)<Subscript>3</Subscript>:Ce<Superscript>3+</Superscript> crystals

The luminescence and thermally stimulated recombination processes in lithium borate crystals Li₆Gd(BO₃)₃ and Li₆Gd(BO₃)₃:Ce have been studied. The steady-state luminescence spectra under X-ray excitation (X-ray luminescence), temperature dependences of the intensity of steady-state X-ray luminescence (XL), and thermally stimulated luminescence (TSL) spectra of these compounds have been investigated in the temperature range of 90–500 K. The intrinsic-luminescence 312-nm band, which is due to the ⁶ P _J → ⁸ S _7/2 transitions in Gd³⁺ matrix ions, dominates in the X-ray luminescence spectra of these crystals; in addition, there is a wide complex band at 400–420 nm, which is due to the d → f transitions in Ce³⁺ impurity ions. It is found that the steady-state XL intensity in these bands increases several times upon heating from 100 to 400 K. The possible mechanisms of the observed temperature dependence of the steady-state XL intensity and their correlation with the features of electronic-excitation energy transfer in these crystals are discussed. The main complex TSL peak at 110–160 K and a number of minor peaks, whose composition and structure depend on the crystal type, have been found in all crystals studied. The nature of the shallow traps that are responsible for TSL at temperatures below room temperature and their relation with defects in the lithium cation sublattice are discussed.     

Vibrational spectra of disordered oxyfluoride (NH<Subscript>4</Subscript>)<Subscript>3</Subscript>WO<Subscript>3</Subscript>F<Subscript>3</Subscript>

Comparative analysis of the IR absorption and Raman scattering spectra of a polycrystalline sample of perovskite-type oxyfluoride (NH₄)₃WO₃F₃ has been performed in the frequency range 370–4000 cm^?1 at temperatures from 92 to 303 K, including the transition between the orientationally disordered cubic and low-symmetry phases. The conformation of WO₃F₃ octahedral groups is established and transitional anomalies of the internal modes of these groups and ammonium ions are revealed. Comparative analysis of the IR and Raman spectra suggests that the phase transition under study is mainly related to the ordering of octahedral groups and formation of W-O…H-N hydrogen bonds.