Publications

The xZnO-(1-x)alpha-Fe2O3 nanoparticles system has been obtained by mechanochemical activation for x = 0.1, 0.3 and 0.5 and for ball milling times ranging from 2 to 24 h. Structural and morphological characteristics of the zinc-doped hematite system were investigated by X-ray diffraction (XRD) and Mossbauer spectroscopy. The Rietveld structure of the XRD spectra yielded the dependence of the particle size and lattice constant on the amount x of Zn substitutions and as function of the ball milling time. The x = 0.1 XRD spectra are consistent with line broadening as Zn substitutes Fe in the hematite structure and the appearance of the zinc ferrite phase at milling times longer than 4 h. Similar results were obtained for x = 0.3, while for x = 0.5 the zinc ferrite phase occurred at 2 h and entirely dominated the spectrum at 24 h milling time. The Mossbauer spectra corresponding to x = 0.1 exhibit line broadening as the ball milling time increases, in agreement with the model of local atomic environment. Because of this reason, the Mossbauer spectrum for 12 h of milling had to be fitted with two sextets. For x = 0.3 and 12 milling hours, the Mossbauer spectrum reveals the occurrence of a quadrupole-split doublet, with the hyperfine parameters characteristic to zinc ferrite, ZnFe2O4. This doublet clearly dominates the Mossbauer spectrum for x = 0.5 and 24 h of milling, demonstrating that the entire system of nanoparticles consists finally of zinc ferrite. As ZnO is not soluble in hematite in the bulk form, the present study clearly demonstrates that the solubility limits of an immiscible system can be extended beyond the limits in the solid state by mechanochemical activation. Moreover, this synthesis route allowed us to reach nanometric particle dimensions, which would make the materials very important for gas sensing applications. (c) 2006 Elsevier Ltd. All rights reserved.

Using an acetate-alkoxide sol-gel route in which the precursors are barium acetate, yttriurn isopropoxide and titanium diisopropoxide bis-acetylacetonate, we prepared a ferroelectric material with the formula: Ba1-xYxTiO3, x = 0.005. SEM analysis showed a polymeric microstructure of the gel due to the chelated titanium alkoxide precursor used as starting materials. The evolution of the structure and microstructure of the precursor gel heated at temperatures up to 1000 degrees C was studied by various techniques. The powder obtained by heating the gel at 1100 degrees C presented a homogeneous structure consisting of submicronic particles (similar to 200 mm). XRD and SAED analyses revealed that Ba0.995Y0.005TiO3 nanocrystals of about 5-10 rim appeared at 600 degrees C, together with BaCO3. The presence of barium carbonate was identified also by IR spectroscopy and thermal analyses. The ceramics obtained from the as-prepared powder presented good dielectric properties (capacitance = 840 pF/dielectric constant = 3860 and dielectric loss (tan delta) 0.078 at Curie temperatures of 120-121 degrees C).

Samples of R2Fe14Si2 (R = Y, Nd, Dy and Er) solid solutions were prepared by arc melting and exposed to hydrogenation. Characterization of hydrogenation effects was done using X-ray diffraction (XRD) and Mossbauer spectroscopy. Rietveld refining of the XRD spectra showed that both lattice parameters a and c increased after hydrogenation, with the exception of Er2Fe14Si2, for which they decreased slightly. The values of the magnetic hyperfine fields corresponding to the four inequivalent lattice sites were found to increase after hydrogenation in all offstoichiometric compounds investigated. Changes in the relative site occupancies were observed. Our study is the first to evidence hydrogenation effects on the hyperfine magnetic fields in intermetallics. (c) 2006 Elsevier Ltd. All rights reserved.

Six practical composite wires of Nb3Sn (Furukawa Electric Co, Ltd) with different architectures ( design and reinforcement) were compared from the normal state resistance R, critical temperatures T-c(onset), T-c(offset) and Delta T-c, upper critical field B-c2 ( at 4.2 K) and critical current density J(c) points of view. Wires were as follows: three of near-the-edge reinforcement design with Nb reinforcement of 0, 21, 50 vol% in the CuNb region, and two of central reinforcement design with Nb of 21 and 50 vol%. One wire with near the edge 50% vol Nb reinforcement had a different reinforcement/superconductor ratio. As-reacted wires show very different patterns of R, T-c(onset), T-c(offset), Delta T-c, B-c2 and J(c). For the superconducting parameters this is probably due to different 3D thermal residual strains. Data suggest that the architecture of the as-reacted wire can control residual strain values and distribution. During multiple bending of the wires at room temperature ( named pre-bending), introduction of the reinforcement improves relaxation of the 3D residual strain and especially of the lateral components. As a consequence, B-c2 and J(c) versus pre-bending strain, epsilon(pb), are enhanced to values closer to those of the Nb3Sn in the stress-free state. Relative enhancement of these critical parameters for the reinforced wires is higher than for the reinforcement-free wire. Evolution during pre-bending and maximum attained absolute values of the superconducting parameters can be grouped roughly as a function of near-the-edge or central reinforcement design. Variation of the superconducting parameters suggests that the pre-bent state may depend on the as-reacted one ( which is a function of wire architecture and processing history); when J(c) was low as in the case of as-reacted wires with central reinforcement, during pre-bending, despite a relatively high J(c) enhancement, this parameter attained lower absolute maximum values than for the other wires. Depending on the wire, when pre-bending strain epsilon(pb) = 0.8 - 1%, the enhancement of non-Cu J(c) at 4.2 K in the pre-bent wire is by up to 43.5% at 15 T compared to the as-reacted case. The maximum absolute value of J(c) is obtained for the near-the-edge reinforcement pre-bent wires with 21 and 50% vol Nb. B-c2 of these wires was lower than for the other reinforced wires and higher than for the reinforcement-free wire. Pre-bent wires with near-the-edge reinforcement are identified as new and promising candidates for fabrication of react-and-wind coils with improved performance.

We describe an experiment of inducing structural and oxygenation changes in small volumes of a thick YBa2Cu3O7-delta (YBCO) film by subjecting the film to a focused Ar laser beam used as the pump source for a depth profiling Raman scattering system. The laser-induced changes were evidenced by high spectral and spatial resolution Raman spectra taken at different depths of the film, before and after the films' exposure. The irradiation and the measurement were done at room temperature in a regular air environment.

This paper reports about the synthesis and characterization of a novel Fe/Fe2O3/polyoxocarbosilane core shell nanocomposite as a material for gas-sensing applications. The nanocomposites (Fe-based nano cores enveloped with polymeric polyoxocarbosilane shells) were prepared by the IR laser co-pyrolysis of iron pentacarbonyl and hexamethyldisiloxane. The iron-based cores become superficially oxidized in atmosphere. The morphologies, chemical content and thermal behavior were studied by different analytical techniques. The sensing properties of the polymer-poor nanocomposite thick films were tested by measuring the variation of the electrical resistance in presence of CO and CH4, at a working temperature of 450 degrees C. Preliminary results indicate that for CO toxic gas, an n-p transitional character of the semiconducting iron oxide appears in humid relatively to dry air.

A series of ferrites was prepared using the microwave method, starting with hematite as precursor in the Ni1-xZnxFe2O4 (x =0-1) system. After synthesis, the NiZn ferrite samples were irradiated with a microwave field for 90 s. X-ray diffraction (XRD) measurements were performed to yield the lattice constant as function of the amount x of Zn substitution, before and after irradiation. The lattice parameter was found to increase much slower with increasing x for the microwave irradiated specimens. Mossbauer spectroscopy was performed at room temperature and spectra were analyzed using the binomial distribution. For x = 0.4, 0.5 and 0.6 the resonance lines are much sharper for the irradiated samples and indicate that a microwave-induced recrystallization of the NiZn ferrites occurs in the composition range specified. (c) 2006 Elsevier B.V. All rights reserved.

New polymeric structures obtained by chemical transformations of maleic anhydride/dicyclopentadiene copolymer with triethylenetetraamine, p-aminobenzoic acid, and p-aminophenylacetic acid were used for the removal Cu(II) ions from aqueous solutions. The experimental values prove the importance of the chelator nature and of the macromolecular chain geometry for the retention efficiency. The retention efficiency (eta(r)), the retention capacity (Q(e)), and the distribution coefficient of the metal ion into the polymer matrix (K-d) are realized by evaluation of residual Cu(II) ions in the effluent waters, by atomic adsorption. Also are discussed the influence of pH, the thermal stability of the polymer, and their polymer-metal complex, as well as the particular aspects regarding the contact procedure and the batch time. Based on the polymers and polymer-metal complexes characterization a potential retention mechanism is proposed. All polymer supports as well theirs metal-complexes are characterized by ATD and FTIR measurements. (c) 2006 Wiley Periodicals, Inc.

We develop an approach to calculate the admittance of effectively one-dimensional open quantum systems in random-phase approximation (RPA). The stationary, unperturbed system is described within the Landauer-Buttiker formalism taking into account the Coulomb interaction in the Hartree approximation. The dynamic changes in the effective potential are calculated microscopically from the charge-charge correlation function resulting from the stationary scattering states. We provide explicit RPA expressions for the quantum admittance. As a first example the case of a quantum capacitor is considered where we can derive a small-frequency expansion for the admittance which lends itself to an experimental testing of the theory. A comparison of the low-frequency expansion with the complete RPA expression shows that for a quantum capacitor a simple classical equivalent circuit with frequency-independent elements does not describe satisfactorily the quantum admittance with increasing frequency.

Analytic forms are found for resolution functions of small-angle neutron scattering instruments. The expressions are developed as a function of momentum transfer (Q) rather than separately in terms of neutron wavelength (lambda) and scattering angle (theta). Effects caused by the gravitational field as well as by quasi-elastic scattering are included. Explicit analytic forms for the transmission functions are proposed for both the incident and scattered beams, enabling careful analysis of any problem regarding small-angle scattering experiments. Due to the reciprocal mathematical relationship between lambda, theta and Q, [lambda, theta] space is employed to approach different aspects of the topic. Applications to time-of-flight instruments with rectangular apertures, including the choice of the most convenient instrumental parameters, the analysis of smearing effects and the data reduction to Q space, are presented.