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Now showing items 1 - 4 of 4

  • Improvement in varistor properties of CaCu3Ti4O12 ceramics by chromium addition

    Edson Cezar Grzebielucka   Jo?o Frederico Haas Leandro Monteiro   Eder Carlos Ferreira de Souza   Christiane Philippini Ferreira Borges   AndréVitor Chaves de Andrade   Eloísa Cordoncillo   Héctor Beltrán-Mir   Sandra Regina Masetto Antunes  

    The effect of chromium addition on the structure and electrical properties of CaCu3Ti4O12 is studied.Compositions based on the formula CaCu3Ti4-xCrxO12-σ(x = 0, 0.025, 0.050 and 0.075 mol%) are prepared by solid-state reaction. Pellets sintered at 1070℃ for 12 h are characterised by X-ray diffraction, scanning electron microscopy/energy dispersive spectroscopy and impedance spectroscopy. Current densityelectric field, dielectric loss and permittivity measurements are also carried out. Rietveld refinement showed the substitution of Cr(Ⅲ) on the Ti(Ⅳ) site, with the maximum substitution for x = 0.025 mol%.The generation of electrical defects in the grain boundary region by chromium doping is responsible for increasing the electric breakdown field strength and nonlinearity coefficient at room temperature from 1723 V/cm and 5.29 for x = 0 mol% to 3431 V/cm and 8.16 for x = 0.025 mol%. This composition shows the greatest improvement in ceramic varistor parameters.
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  • Improvement in varistor properties of CaCu_3Ti_4O_(12) ceramics by chromium addition

    Edson Cezar Grzebielucka   Jo?o Frederico Haas Leandro Monteiro   Eder Carlos Ferreira de Souza   Christiane Philippini Ferreira Borges   André Vitor Chaves de Andrade   Eloísa Cordoncillo   Héctor Beltrán-Mir   Sandra Regina Masetto Antunes  

    The effect of chromium addition on the structure and electrical properties of CaCu_3Ti_4O_(12) is studied.Compositions based on the formula CaCu_3Ti_(4-x)Cr_xO_(12-σ)(x = 0, 0.025, 0.050 and 0.075 mol%) are prepared by solid-state reaction. Pellets sintered at 1070℃ for 12 h are characterised by X-ray diffraction, scanning electron microscopy/energy dispersive spectroscopy and impedance spectroscopy. Current densityelectric field, dielectric loss and permittivity measurements are also carried out. Rietveld refinement showed the substitution of Cr(Ⅲ) on the Ti(Ⅳ) site, with the maximum substitution for x = 0.025 mol%.The generation of electrical defects in the grain boundary region by chromium doping is responsible for increasing the electric breakdown field strength and nonlinearity coefficient at room temperature from 1723 V/cm and 5.29 for x = 0 mol% to 3431 V/cm and 8.16 for x = 0.025 mol%. This composition shows the greatest improvement in ceramic varistor parameters.
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  • Charge- and pH-dependent binding sites of dipyridamole in ionic micelles: A fluorescence study

    Christiane Philippini Ferreira Borges   Iourii E. Borissevitch   Marcel Tabak  

    Interaction of the coronary vasodilator dipyridamole (DIP) with micelles of cationic, anionic and zwitterionic detergents has been studied through the use of fluorescence spectroscopy. The aim of this study was to gain insight on the interaction of the drug with the cell membrane. Change of fluorescence emission upon binding allowed to obtain the association constants of both protonated and neutral forms of the drug. The association constants, K b, were in the range 1.6-3.1times10 3 M -1. The constants for the neutral form of the drug are quite similar for the three micellar systems suggesting a nonspecific binding and a small effect of the charged interface. For the protonated drug, the surface charges modulate the binding, increasing or decreasing it, as a function of electrostatic interactions. The binding constants decrease both for the cationic and zwitterionic micelles implying similar mode of binding while for the anionic micelle the constant increases. Fluorescence quenching using acrylamide and iodide allowed further localization of the drug inside the micelles. In general, binding to micelles decreases significantly the accessibility of the drug as compared to the buffer. For protonated drug, the decrease is similar for the three micelles; in the case of neutral drug for CTAC and HPS the quenching is similar to that of protonated drug suggesting a similar localization in the micelle independently of the charge of the drug, while for SDS the drug is more exposed in the micelle surface. Analysis of the quantum yields and fluorescence lifetimes suggest a significant intramolecular static quenching in the protonated drug
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  • Spectroscopic studies of dipyridamole derivatives in homogeneous solutions: Effects of solution composition on the electronic absorption and emission

    Christiane Philippini Ferreira Borges   Marcel Tabak  

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