Publications

Analyses of standard current-voltage (I-V) characteristics of disordered high-temperature superconductors (HTS) indicate that the vortex phase at high magnetic fields H should be an elastic vortex glass, where the vortex pinning barriers diverge at low current densities, whereas do magnetization relaxation measurements reveal the presence of nondiverging (plastic) pinning barriers in a wide H-T domain. We show that the different conclusions concerning the nature of the vortex phase at high H in disordered HTS seem to be due to the ordering effect of the driving force existing in various experiments. (C) 2008 Elsevier B.V. All rights reserved.

A recent report on multilayered superconductors establishes unique constant T-c although the number of inner CuO2 plane (IP) increases. Many hypotheses proposed that the irreversibility field (B-irr) and critical current density (J(c)) are depends on the crystal structures. We have measured the J(c) and B-irr of the HgBa2Can-1CunO2n+2+delta sample containing n = 6-15 phases to investigate the effect of the number of IP's on the B-irr and J(c) and its pinning properties. The rate of fall of J(c) increases and irreversible lines (IL's) shift to lower temperatures with increasing the number of IP's, which suggests that anisotropy increases with n. The IL settled in between those for Bi2Sr2CaCu2Oy with high anisotropy value and YBa2Cu3O7 with less anisotropy value. The double logarithmic plot of irreversibility field versus [1 - (T/T-c)] analysis suggests that the flux line melting model is adopted. The flux pinning force density F-p (approximate to J(c)B) exhibits scaling behavior when the magnetic field B is normalized by the irreversibility field B;- Analysis of the normalized pinning force reveals that a surface pinning mechanism is dominant and reduced magnetic field b(max) = 0.2 agree with surface pinning mechanism with closely spaced pins. (C) 2008 Elsevier B.V. All rights reserved.

The effects of heating-induced crystallization on the structural and mechanical properties of Mg-Ni-Fe amorphous ribbons were studied by anelastic spectroscopy, differential scanning calorimetry (DSC) and X-ray diffraction. DSC results show that the crystallization occurs through several non-reversible steps, which correspond to significant changes in the Young's modulus and concomitant irreversible elastic energy loss peaks. Moreover, an anelastic peak is found at 215 K, which for the first time indicates the presence of some dynamical process related to the simultaneous presence of different phases. The formation of a metastable Mg(6)Ni phase is detected, which transforms into Mg and Mg(2)Ni stable phases. A quantitative analysis of the different phases present at the different steps was also carried out. (C) 2007 Elsevier B.V. All rights reserved.

Nano-scale ordering may be revealed in transmission electron microscopy (TEM) by at least three techniques that will be presented in this work: selected area electron diffraction, conventional TEM and high-resolution TEM. Digital image processing is used to extract additional information from the high-resolution micrographs. The described methods are illustrated in a microstructural and compositional study of a 90%Pb(Mg1/3Nb2/3)O-3-10%PbTiO2 ceramic sample. High-resolution images reveal the presence of ordered compositional nano-domains, observable in two specific crystallographic orientations. Antiphase boundaries lying in the (111) planes separate them, while (100) and (111) facets separate the ordered domains from the disordered matrix.

In this work we report the synthesis and characterization of the polypyrrole - porous silicon nanocomposites. Our main goal is to control the growth of conducting polypyrrole into the alveolar pores of porous silicon, in order to get a functionalized nanocomposite with the required properties for application as electrode in microfluidic devices. Porous silicon was electrochemically etched from (100) p-type silicon wafers. Different silicon resistivities (from 1.8 to 11 Omega cm) and preparation conditions (different electrolyte concentrations and current densities) were used. The electrodeposition of polypyrrole into porous silicon was carried out galvanostatically, at a constant current density 2.5 mA/cm(2) from a solution containing acetonitrile, the monomer, pyrrole and p-toluensulfonic acid as electrolyte. The polymerization time was varied in order to investigate the growing process of polypyrrole into the pores. The morphology and of the as-prepared nanocomposites was investigated by SEM. The existence of PPY into porous silicon is evidenced by the EDX spectra of the nanocomposite. The electrical characterization evidences the relative contributions of the polypyrrole and the substrate resistivity, respectively.

We have measured the vortex melting lines of high-pressure-synthesized Ba2Ca3Cu4O8(O1-yFy)(2) (nominal composition, 2y = 1.3, 1.6 and 2.0) F(2y)-0234 multilayered high-T-c superconductor using fundamental and third harmonic susceptibility responses carried out on preferentially oriented crystallites with very low ac field amplitude (5 mu T) and in applied dc fields up to 6 T. The vortex melting lines of all three F-substituted samples show interesting doping dependence and are very well described by the commonly accepted theory of melting lines. Interestingly, we discovered that the models describing the temperature dependence of the in-plane London penetration depth also depend on the doping level: the 3D XY model for the nearly-optimum-doped sample [F(1.3)], the mean-field theory for the under-doped sample [F(1.6)] and the two-fluid model for the heavily-under-doped sample [F(2.0)]. We found that the vortex melting lines in F-0234 are determined by the interplay in the coupling of pancake vortices through the charge reservoir layer (CRL), and through the block of inner CuO2 planes (IPs), respectively. The anisotropy values (47 for the near-optimally-doped sample) derived from experimental vortex melting lines are consistent with the values obtained by first-principles electronic-band-structure calculations.

The paper reports the computer simulation of nanoparticle formation based on classical homogenous condensation theory with the purpose of understanding nanoparticle formation and the dependence of their size distribution on process parameters. The simulation has been performed for the case of silicon. The process of condensation with the formation of clusters due to the high cooling speed (similar to 10(4)-10(5) K/s) is considered.

Water-soluble gold nanoparticles, capped with captopril, have been synthesized and characterized. Their average size is 2.3 nm, with a spherical shape. These gold nanoparticles can be easily labeled with stable free radicals (4-amino-tempo) by a coupling reaction performed in the presence of 2-ethoxy-1-ethoxycarbonyl-1,2-dihydroquinoline (EEDQ). Both synthesized and spin-labeled gold nanoparticles can be incorporated into much bigger (100 nm) silica nanoparticles using the Stober method, thus forming hybrid metal (gold)-inorganic (silica) nanoparticles. These hybrid silica nanoparticles (containing or not spin-labeled gold nanoparticles) can be easily spin-labeled with another stable free radical (4-isocyanato-tempo), leading to the formation of a double spin-labeled material. In this way, some stable free radicals are attached on the gold surface while others are attached on the silica surface. Three types of EPR spectra were recorded and discussed for the hybrid gold-silica nanoparticles: (1) where the spin labels are attached to the embedded gold nanoparticles, (2) where the spin labels are attached to the silica nanoparticles, and (3) in the case of the double spin labeled material where both gold and silica nanoparticles are spin-labeled. Influence of different solvents on the EPR spectra is also discussed.

Fe3O4 colloids modified by the chiral ligand cinchonidine were prepared with the goal of obtaining a magnetic and catalytic nano-material and were subsequently embedded in silica to form a heterogeneous catalyst. The systems were characterized by TEM and XRD measurements, while the Mossbauer technique was applied for measuring the magnetic properties of the Fe3O4 colloids. The hyperfine magnetic field distribution was consistent with one type of Fe oxide, namely, the magnetite (Fe304). These colloids, both as 'quasi-homogenous catalysts' (or soluble heterogeneous catalysts) and embedded in silica (heterogeneous catalysts) were employed in the selective hydrogenolysis of complex bicyclo[2.2.2]oct-7-enes (bicyclo[2.2.2]oct-2-enes when unsubstituted). (C) 2008 Published by Elsevier B.V.

The compounds YNi(4-x)Al(x)Mg with 0 <= x <= 1.5, crystallizing in the C15 structure type (MgCu(4)Sn-type), have been elaborated by mechanical milling and a subsequent heat treatment at 650 degrees C for 1 h. It was possible to synthesize the compounds YNi(4-x)Al(x)Mg with x up to approximately 1.15 and the lattice parameters was in good agreement with those already reported. All the samples, excepting the YNi(4)Mg compound, absorbed and desorbed hydrogen reversibly and no structural change is reported after the hydrogenation. A direct relationship between the lattice parameters and the equilibrium plateau pressure was found. (C) 2007 Elsevier B.V. All rights reserved.