Publications

Frequency-dependent AC susceptibility measurements have been used to estimate the critical current densities and pinning potential of two 1.1-mu m-thick YBCO films with nanoengineered pinning centres. The films were grown by pulsed laser deposition using a target with 4 % BaZrO3 nanoinclusions. Data were analysed in the framework of the Anderson-Kim model, collective pinning model, and a model due to Zeldov that implies a logarithmic dependence of pinning potential on the current density. It was found that the latter describes better our experimental data, and that the resulting pinning potential has reasonable high values in high magnetic field up to 9 T, indicating a high current-carrying capability in high magnetic fields.

Cobalt chromite (CoCr2O4) was synthesized through the precursor method. The precursors: (NH4)(3)[CoCr2(C4O6H4)(4)(OH)(3)]center dot 4H(2)O, (NH4)(3)[CoCr2(C6O7H10)(4)(C6O7H9)]center dot 5H(2)O were characterized by elemental chemical analysis, infrared (IR) and ultraviolet-visible (UV-vis) spectroscopy, and thermal analysis. The final oxides were characterized by X-ray diffraction (XRD), scanning and transmission electron microscopy (SEM/TEM), UV-vis, IR, Raman spectroscopy (RS), magnetic measurements, N-2 adsorption-desorption analyses and X-ray photoelectron spectroscopy (XPS). XRD confirmed the cubic CoCr2O4 phase only and determined average crystallite sizes between 14 and 21 nm. Electron microscopy revealed morphology corresponding to the complete crystallization into cubic CoCr2O4. All the samples presented ferrimagnetic ordering below the Currie temperature (T-c), and a phase transition at T-s similar to 26 K attributed to the onset of long-range spiral magnetic order. The CoCr2O4 nanoparticles generated through the gluconate route following calcination at 700 degrees C for 1 h were found to have the best catalytic activity in the total oxidation of methane. (C) 2014 Elsevier Ltd. All rights reserved.

Spark plasma sintering (SPS) was applied for fabrication of dense (relative density > 97 %) bulk MgB2 samples added with Ge. Mixtures of MgB2 and Ge powders with starting compositions (MgB2) Ge (x) , x=0.005, 0.01, and 0.03 were used. Added samples show enhancement of the critical current density at high magnetic fields when compared to a pristine reference sample. The optimum composition is for x=0.005 and for this sample J (c)(20 K) =10(2)A/cm (2) is obtained at 5 T, while for the reference sample is obtained at 3.9 T. Ge does not substitute in the crystal lattice of MgB2 and T (c,onset) or T (c,midpoint) from magnetization measurements scatter within 0.15 or 0.3 K, respectively.

AC susceptibility and DC magnetization relaxation measurements have been performed for YBa2Cu3O7 films containing BaZrO3 nanorods preferentially oriented along the c axis, with the external magnetic field perpendicular to the film surface. It was found that the effective vortex activation energy U (e) supplied by the standard analysis of the frequency dependent AC magnetic response remains very high even in the vicinity of the irreversibility line indicated by the DC relaxation. We show that the large U (e) values appearing in the AC signal are related to the pinning enhanced viscous drag and significant vortex velocities.

The Mossbauer spectra of a FeSe0.3Te0.7 single crystal grown by the Bridgman method were analysed across the superconducting transition by considering the interplay between the structure and electron configuration of the transition metal. The magnetically determined superconducting critical temperature is T-C similar to 14 K. The Fe-57 Mossbauer spectra collected in the temperature range from 5 to 200K mainly have an asymmetric doublet pattern, which was conveniently fitted by the full Hamiltonian method. No effective magnetic moment ascribed to the superconducting phase was observed down to 5K. The unusual behaviour observed below similar to 17K for the chemical isomer shift and quadrupole splitting may be associated with an electron reconfiguration process intimately related to an unusual lattice distortion accompanying the superconducting transition. The decreasing trend of the total absorption spectral area and second-order Doppler shift during cooling the sample below the critical temperature, point to enhanced phonon activation in the superconducting state.

In this paper we report conformational changes recorded on a protein molecule (alpha-amylase) under green light irradiation. In order to explain the experimental results we advanced the hypothesis that green light induces electric dipoles in the protein, which interact with each other, generating conformational modifications toward a more compact design, with different physical properties. The experiments were carried out with un-polarized light (lambda = 520 nm) from a light-emitting-diode (1000 lm, 20 W, 105 mW on the target). In view of the character of our hypothesis, and corroborated with all our experimental results, we suggest that this phenomenon may be more extended and general, specific for a larger class of proteins, occurring on the protein macromolecules under the green light. The effects of a-amylase protein irradiation were revealed by circular dichroism, fluorescence, Raman and FTIR-spectroscopies, zeta potential, cyclic voltammetry, electric impedance spectroscopy and atomic force microscopy. Tentatively, we term the novel conformations as P* (polarized) proteins.

Quasi-static polarization induced by dc electric field was studied in a broad range of temperatures in epitaxial films of relaxor ferroelectric PbMg1/3Nb2/3O3 and PbSc1/2Nb1/2O3. The electric field was applied and the response was measured along the out-of-plane crystal direction of the epitaxial perovskite-structure (001) oriented films. The films remained in the relaxor state in zero-field cooling. A new polar state can be induced by electric field at a critical temperature below 100 K. The critical field and the induced polarization increased on cooling and reached the bulk-like values at 20 K. The orientational anisotropy of thin-film dipolar systems is discussed as a possible reason for the observed stable relaxor state.

ZnO crystallites were grown by electroless deposition on poly(methyl methacrylate) (PMMA) fiber mats prepared by an electrospinning technique. The electroless deposition involves three steps: sensitization, activation and deposition, which were performed by subsequently dipping the PMMA fiber mats in the appropriate solutions. After the deposition the PMMA fibers are uniformly coated with ZnO prisms which show hexagonal wurtzite structure and optical signatures (band-gap value and emission bands) typical for this semiconductor. By combining electroless deposition and electrospinning, different semiconductor coated polymer fibers can be obtained for a wide range of applications. Both methods are appropriate for large scale production, being scalable, cheap, efficient and suitable for large-area covering techniques. (C) 2014 Elsevier B.V. All rights reserved.

The electrochemical polymerization of diphenylamine (DPA) onto electrodes of Pt coated with highly separated metallic (98%) or semiconducting (99%) single-walled carbon nanotubes (SWCNTs) in the presence of H3PW12O40 was performed by cyclic voltammetry in order to obtain composite materials based on polydiphenylamine (PDPA) doped with heteropolyanions of H3PW12O40 and carbon nanotubes. Our data demonstrate that the photoluminescence quenching effect of the PDPA doped with H3PW12O40 heteropolyanions in the presence of SWCNTs is due to the metallic component. Under UV irradiation of SWCNTs highly separated in metallic and semiconducting tubes functionalized with PDPA doped with heteropolyanions of H3PW12O40 new photochemical reactions are evidenced by photoluminescence studies. These reactions lead to a shortening of the macromolecular chain of PDPA. The photochemical process is more intense in the case semiconducting SWCNTs functionalized with PDPA doped with heteropolyanions in comparison to metallic SWCNTs functionalized with PDPA in doped state, it being a consequence of an additional chemical interaction between the DPA dimer doped with H3PW12O40 heteropolyanions and semiconducting SWCNTs. (C) 2014 Elsevier Ltd. All rights reserved.

We report on thin film deposition by matrix-assisted pulsed laser evaporation of simple hydroxyapatite (HA) or silver (Ag) doped HA combined with the natural biopolymer organosolv lignin (Lig) (Ag:HA-Lig). Solid cryogenic target of aqueous dispersions of Ag:HA-Lig composite and its counterpart without silver (HA-Lig) were prepared for evaporation using a KrF* excimer laser source. The expulsed material was assembled onto TiO2/Ti substrata or silicon wafers and subjected to physical-chemical investigations. Smooth, uniform films adherent to substratum were observed. The chemical analyses confirmed the presence of the HA components, but also evidenced traces of Ag and Lig. Deposited HA was Ca deficient, which is indicative of a film with increased solubility. Recorded X-ray Diffraction patterns were characteristic for amorphous films. Lig presence in thin films was undoubtedly proved by both X-ray Photoelectron and Fourier Transform Infra-Red Spectroscopy analyses. The microbiological evaluation showed that the newly assembled surfaces exhibited an inhibitory activity both on the initial steps of biofilm forming, and on mature bacterial and fungal biofilm development. The intensity of the antibiofilm activity was positively influenced by the presence of the Lig and/or Ag, in the case of Staphylococcus aureus, Pseudomonas aeruginosa and Candida famata biofilms. The obtained surfaces exhibited a low cytotoxicity toward human mesenchymal stem cells, being therefore promising candidates for fabricating implantable biomaterials with increased biocompatibility and resistance to microbial colonization and further biofilm development.