Investigation of thermal decomposition of yttrium–aluminum-based precursors for YAG phosphors

Three types of precursors were prepared using the wet-chemical synthesis route, starting from yttrium?Ceuropium?Caluminum nitrate solution and different precipitating agents (urea, oxalic acid, and ammonium carbonate). The precursors were fired at 1200?°C in nitrogen atmosphere in order to obtain europium-doped yttrium aluminate Y₃Al₅O₁₂:Eu³⁺ phosphor with garnet structure (YAG:Eu). The processes involved in the thermal decomposition of precursors and their composition were put in evidence using thermal analysis (TG?CDTA) and FT-IR spectroscopy. The GA?CDTA curves possess typical features for basic-oxalate, -nitrate, and -carbonates as formed with oxalic acid, urea, and ammonium carbonate, respectively. Correlation between the thermal decomposition steps, mass loss, and composition of gases evolved during the thermal treatment was established using TG?CDTA?CFT-IR coupling. It was found that the different composition of precursors reflects on the luminescent characteristics of the corresponding phosphors. Urea and ammonium carbonate lead to the formation of YAG type phosphors, with garnet structure and specific red emission. As for the oxalic acid, this precipitating agent generates a non-homogeneous powder that contains yttrium oxide as impurity phase. This phosphor is a mixture of Y₂O₃:Eu³⁺, Y₄Al₂O₉:Eu³⁺, and Y₃Al₅O₁₂:Eu³⁺ that explain the relative higher emission intensity.     

Magnetic field and particle concentration competitive effects on ferrofluid based silicone elastomer microstructure

Structural peculiarities of ferroelastomers composed of polydimetylsiloxane with embedded magnetite particles during polymerization have been studied by small-angle neutron scattering. The effects of ferroparticles’ concentration in the range of C = 0–6 wt %, and external magnetic fields of induction B = 0−1 kG applied during the polymerization on the structure of polymeric matrix and particles distribution in polymer were analyzed.     

Fitting data using optimal Hermite type cubic interpolating splines

In this work we obtain a new optimal property for cubic interpolating splines of Hermite type applied to data-fitting problems. The existence and uniqueness of the Hermite type cubic spline with minimal quadratic oscillation in average are proved.     

Estimation of magnetic particle clustering in magnetic fluids from static magnetization experiments

The volume fraction dependence of the static magnetization of two magnetic fluids with different degrees of steric stabilization was measured at low field values (0-10 kA/m) and it was found to be nonlinear for both magnetic fluids. The nonlinearity is more pronounced in the case of the less stabilized magnetic fluid. The experimental data were processed by nonlinear regression using an analytical model for the formation of chain-like magnetic particle aggregates in magnetic fluids. The calculated dependence on the degree of steric stabilization, magnetic field, and sample concentration of the mean number of particles per chain was in the range (1-1.04).     

Electrokinetic capillary chromatography in a polar continuous-phase water-in-oil microemulsion constituted by water, sodium dodecyl sulfate, and n-pentanol

A water-in-oil (w/o) microemulsion (ME) constituted by 15% Tris buffer, pH 8.4, in water and 85% sodium dodecyl sulfate (SDS)/n-pentanol 1:4 mixture, capable of dissolving up to 30% vegetable oils and lard, was used as background electrolyte in reverse microemulsion electrokinetic capillary chromatography (RMEEKC). Owing to the free SDS ions in the continuous phase and some degree of percolation, the ME showed a high conductivity (0.65 mS. cm(-1) at 25 degrees C) and sustained a very stable capillary current. Previous rinsing of the capillary with a quaternary ammonium salt for electroosmotic flow (EOF) reduction, a series of nonionic and anionic solutes dissolved either in the ME or in fat samples diluted with the ME (1:4 ratio), were injected. Using -20 kV, fair separations of the solutes in the migration time order singly charged anions < nonionic solutes < doubly charged anions approximately pyromellitate were obtained, salicylate (I) showing by far the shortest migration time, and phthalate (II) and pyromellitate the longest. Separation was attributed to partition between the aqueous droplets, where pyromellitate and II were assumed to be trapped, and the n-pentanol continuous phase, where the mobilitites could be also modified by association of the solutes with SDS ions. Adequate EOF markers were not found, thus the relative mobility of any solute with respect to the mobility of the droplets, mu(r), was expressed as a fraction of the mobility of I with respect to that of the droplets, being mu(r) = (t(II) - t(R)) t(I) / [(t(II) - t(I)) t(R)], where t(R), t(I), and t(II) are the migration times of the solutes I and II, respectively. The application of RMEEKC to the analysis of both hydrophilic and hydrophobic samples, including edible fats, was demonstrated.     

Synthesis of Polymer Films by an Electron Emission CVD Technique

A low-pressure chemical vapor deposition (CVD) technique based on the formation of reactive film precursors by dissociation of gas-phase reactants by electron impact is described. The electrons are emitted by a hot filament and a positive bias voltage applied to the substrate provides control of the emission current. The emitted electrons are primarily responsible for the gas dissociation, but secondary electrons resulting from electron-impact ionization of the gas molecules also produce reactive species, contributing to the deposition process. This technique was used to synthesize polymer films from C₂H₂–N₂ mixtures at pressures ranging from 1–10 Pa. The dependencies of the current collected by the substrate, I_S, on the substrate bias voltage, V_S, and on the gas pressure were determined. The film deposition rate was measured as a function of several deposition variables such as I_S, V_S, and the N₂ to C₂H₂ flow rate ratio. Oxygen was present in the films as a contaminant. Analyses by Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy (XPS) provided insight into the film molecular structure, allowing identification of various functional groups and binding states of the C- and N–atoms. From the XPS spectra, the N/C and O/C ratios were determined.     

Application of meso-hydrobenzoin-derived chiral auxiliaries for the stereoselective synthesis of highly substituted pyrrolidines by 1,3-dipolar cycloaddition of azomethine ylides

The metal-catalyzed stereoselective 1,3-dipolar cycloaddition of azomethine ylides and acrylates using recyclable meso-hydrobenzoin-derived chiral auxiliaries is described. Cleavage of the auxiliary leads to highly substituted pyrrolidines in up to 87% enantiomeric excess.     

The numerical method of successive interpolations for Fredholm functional integral equations

A new numerical method for Fredholm functional integral equations is proposed. The method combines the fixed point technique with numerical integration and cubic spline interpolation. The convergence and the numerical stability of the method are proved and tested on some numerical examples.     

Magnetic field intensity effect on plane electric capacitor characteristics and viscoelasticity of magnetorheological elastomer

Magnetorheological (MR) elastomer was prepared using silicone rubber and soft magnetic carbonyl iron microspheres, and then examined as dielectric materials for manufacturing electric capacitors. As a specific element, capacity of the capacitors located in a magnetic field was found to be sensitive to both the MR suspension proportion to the silicone rubber and the intensity of the applied magnetic field. Viscoelastic characteristics of the MR elastomer, represented by storage modulus and creep behavior, were also studied.     

Micellar catalysis in aqueous-ionic liquid systems

We present the application of ionic liquid-aqueous micellar solutions as reaction media for Diels-Alder reaction and found that reaction rates could be significantly increased compared to the reaction in water.