Photoluminescence behavior of Eu(3+) ion doped into gamma- and alpha-alumina systems prepared by combustion, ceramic and Pechini methods

Al(2)O(3):Eu(3+)(1%) samples were prepared by combustion, ceramic, and Pechini methods annealed from 400 to 1400 degrees C. XRD patterns indicate that samples heated up to 1000 degrees C present disordered character of activated alumina (gamma-Al(2)O(3)). However, alpha-Al(2)O(3) phase showed high crystallinity and thermostability at 1200-1400 degrees C. The sample characterizations were also carried out by means of infrared spectroscopy (IR), scanning electron microscopy (SEM) and specific surface areas analysis (BET method). Excitation spectra of Al(2)O(3):Eu(3+) samples present broaden bands attributed to defects of Al(2)O(3) matrices and to LMCT state of O -> Eu(3+), however, the narrow bands are assigned to (7)F(0) -> (5)D(J),(5)H(J) and (5)L(J) transitions of Eu(3+) ion. Emission spectra of samples calcined up to 1000 degrees C show broaden bands for (5)D(0) -> (7)F(J) transitions of Eu(3+) ion suggesting that the rare earth ion is in different symmetry sites showed by inhomogeneous line broadening of bands, confirming the predominance of the gamma-alumina phase. For all samples heated from 1200 to 1400 degrees C the spectra exhibit narrow (5)D(0) -> (7)F(J) transitions of Eu(3+) ion indicating the conversion of gamma to alpha-Al(2)O(3) phases, a high intensity narrow peak around 695 nm assigned to R lines of Cr(3+) ion is shown. Al(2)O(3):Eu(3+) heated up to 1100 degrees C presents an increase in the Omega(2) intensity parameter with the increase of temperatures enhancing the covalent character of metal-donor interaction. The disordered structural systems present the highest values of emission quantum efficiencies (eta). CIE coordinates of Al(2)O(3):Eu(3+) are also discussed. (C) 2007 Elsevier Inc. All rights reserved.

Confinement-induced phase transition in a DNA-lipid hydrated complex

We study the effect of the soft confinement by fluid lipid bilayers on the spatial organisation of DNA molecules in a DNA-zwitterionic lipid hydrated lamellar complex. The confinement is increased by dehydrating the complex in a controlled way, which leads to a decrease of the water channel thickness separating the periodically stacked bilayers. Using grazing-incidence small-angle X-ray scattering on an oriented thin film, we probe in situ as dehydration proceeds the structure of the DNA-lipid complex. A structural phase transition is evidenced, where an apparently disordered phase of DNA rods embedded within the one-dimensionally ordered lipid lamellar phase observed at high hydration is replaced by a 2D hexagonal structure of DNA molecules intercalated between the lipid bilayers. Copyright (C) EPLA, 2010

Critical discontinuous phase transition in the threshold contact process

We analyze a threshold contact process on a square lattice in which particles are created on empty sites with at least two neighboring particles and are annihilated spontaneously. We show by means of Monte Carlo simulations that the process undergoes a discontinuous phase transition at a definite value of the annihilation parameter, in accordance with the Gibbs phase rule, and that the discontinuous transition exhibits critical behavior. The simulations were performed by using boundary conditions in which the sites of the border of the lattice are permanently occupied by particles.

Ionization and Structural Changes of the DMPG Vesicle along Its Anomalous Gel-Fluid Phase Transition: A Study with Different Lipid Concentrations

Dispersions of saturated anionic phospholipid dimyristoyl phosphatidylglycerol (DMPG) have been extensively studied regarding their peculiar thermostructural behavior. At low ionic strength, the gel-fluid transition is spread along nearly 17 degrees C, displaying several thermal events in the calorimetric profile that is quite different from the single sharp peak around 23 degrees C found for higher ionic strength DMPG dispersions. To investigate the role of charge in the bilayer transition, we carefully examine the temperature dependence of the electrical conductivity of DMPG dispersions at different concentrations, correlating the data with the corresponding differential scanning calorimetry (DSC) traces. Electrical conductivity together with electrophoretic mobility measurements allowed the calculation of the dependence of the degree of ionization of DMPG vesicles on lipid concentration and temperature. It was shown that there is a decrease in vesicle charge as the lipid concentration increases, which is probably correlated with the increase in the concentration of bulk Na(+). Apart from the known increase in the electrical conductivity along the DMPG temperature transition region, a sharp rise was observed at the bilayer pretransition for all lipid concentrations studied, possibly indicating that the beginning of the chain melting process is associated with an increase in bilayer ionization. It is confirmed here that the gel-fluid transition of DMPG at low ionic strength is accompanied by a huge increase in the dispersion viscosity. However, it is shown that this measured macroviscosity is distinct from the local viscosity felt by either charged ions or DMPG charged aggregates in measurements of electrical conductivity or electrophoretic mobility, Data presented here give support to the idea that DMPG vesicles, at low ionic strength, get more ionized along the temperature transition region and could be perforated and/or deformed vesicle structures.

CRITICAL BEHAVIOR AND THRESHOLD OF COEXISTENCE OF A PREDATOR-PREY STOCHASTIC MODEL IN A 2D LATTICE

We investigate the critical behavior of a stochastic lattice model describing a predator-prey system. By means of Monte Carlo procedure we simulate the model defined on a regular square lattice and determine the threshold of species coexistence, that is, the critical phase boundaries related to the transition between an active state, where both species coexist and an absorbing state where one of the species is extinct. A finite size scaling analysis is employed to determine the order parameter, order parameter fluctuations, correlation length and the critical exponents. Our numerical results for the critical exponents agree with those of the directed percolation universality class. We also check the validity of the hyperscaling relation and present the data collapse curves.

Conformational behavior of different possible ways of oligoglycine formation in a solvent-free environment

The possible ways for glycine oligopeptide formation in gas phase, both in the extended P-strand like conformation and folded 2(7)-ribbon like conformations are analyzed using quantum chemical calculations. We focus on the sequential formation of peptide bond through upgradation of the immediate lower order molecule and observe the consequences in other related processes like oligoglycine formation through simultaneous peptide linkage of n glycine monomers and interchange of molecular conformation through peptide linkage. A comparison is made between the structures and binding energies obtained for both conformers. All binding energies are increased by the zero-point energy contribution. The role of electron correlation effects is briefly analyzed. The folded 2(7)-ribbon-like conformations in vacuo are found to be more stable in comparison to the extended structure. (c) 2007 Elsevier B.V. All rights reserved.

Critical behavior of a contact process with aperiodic transition rates

We performed Monte Carlo simulations to investigate the steady-state critical behavior of a one-dimensional contact process with an aperiodic distribution of rates of transition. As in the presence of randomness, spatial fluctuations can lead to changes of critical behavior. For sufficiently weak fluctuations, we give numerical evidence to show that there is no departure from the universal critical behavior of the underlying uniform model. For strong spatial fluctuations, the analysis of the data indicates a change of critical universality class.

The magnetic phase diagram of Gd(2)Sn(2)O(7)

Measurements of the magnetic susceptibility of the frustrated pyrochlore magnet Gd(2)Sn(2)O(7) have been performed at temperatures below T = 5 K and in magnetic fields up to H = 12 T. The phase boundaries determined from these measurements are mapped out in an H-T phase diagram. In this gadolinium compound, where the crystal-field splitting is small and the exchange and dipolar energy are comparable, the Zeeman energy overcomes these competing energies, resulting in at least four magnetic phase transitions below 1 K. These data are compared against those for Gd(2)Ti(2)O(7) and will, we hope, stimulate further studies.

Non-Fermi-liquid behavior in UCu(4+x)Al(8-x) compounds

We report on experimental studies of the Kondo physics and the development of non-Fermi-liquid scaling in UCu(4+x)Al(8-x) family. We studied 7 different compounds with compositions between x = 0 and 2. We measured electrical transport (down to 65 mK) and thermoelectric power (down to 1.8 K) as a function of temperature, hydrostatic pressure, and/or magnetic field. Compounds with Cu content below x = 1.25 exhibit long-range antiferromagnetic order at low temperatures. Magnetic order is suppressed with increasing Cu content and our data indicate a possible quantum critical point at x(cr) approximate to 1.15. For compounds with higher Cu content, non-Fermi-liquid behavior is observed. Non-Fermi-liquid scaling is inferred from electrical resistivity results for the x = 1.25 and 1.5 compounds. For compounds with even higher Cu content, a sharp kink occurs in the resistivity data at low temperatures, and this may be indicative of another quantum critical point that occurs at higher Cu compositions. For the magnetically ordered compounds, hydrostatic pressure is found to increase the Neel temperature, which can be understood in terms of the Kondo physics. For the non-magnetic compounds, application of a magnetic field promotes a tendency toward Fermi-liquid behavior. Thermoelectric power was analyzed using a two-band Lorentzian model, and the results indicate one fairly narrow band (10 meV and below) and a second broad band (around hundred meV). The results imply that there are two relevant energy scales that need to be considered for the physics in this family of compounds. (C) 2011 Elsevier B.V. All rights reserved.

Experimental characterization of nonlinear systems: a real-time evaluation of the analogous Chua`s circuit behavior

This paper presents an experimental characterization of the behavior of an analogous version of the Chua`s circuit. The electronic circuit signals are captured using a data acquisition board (DAQ) and processed using LabVIEW environment. The following aspects of the time series analysis are analyzed: time waveforms, phase portraits, frequency spectra, Poincar, sections, and bifurcation diagram. The circuit behavior is experimentally mapped with the parameter variations, where are identified equilibrium points, periodic and chaotic attractors, and bifurcations. These analysis techniques are performed in real-time and can be applied to characterize, with precision, several nonlinear systems.

In situ X-ray diffraction studies of phase transition in Pb1-x La x Zr0.40Ti0.60O3 ferroelectric ceramics

The temperature dependence of the crystalline structure and the lattice parameters of Pb1-xLaxZr0.40Ti0.60O3 ferroelectric ceramic system with 0.00 x 0.21 was determined. The samples with x 0.11 show a cubic-to-tetragonal phase transition at the maximum dielectric permittivity, Tmax. Above this amount and especially for the x = 0.12 sample, a spontaneous phase transition from a relaxor ferroelectric state (cubic phase) to a ferroelectric state (tetragonal phase) is observed upon cooling below the Tmax. Unlike what has been reported in other studies, the x = 0.13, 0.14, and 0.15 samples, which present a more pronounced relaxor behavior, also presents a spontaneous normal-to-relaxor transition, indicated by a cubic to tetragonal symmetry below the Tmax. The origin of this anomaly has been associated with an increase in the degree of tetragonality, confirmed by the measurements of the X-ray diffraction patterns. The differential thermal analysis (DSC) measurements also confirm the existence of these phase transitions.

Structural Phase Transition Studies on PMN-0.35PT using Infrared Spectroscopy

The purpose of the present work is to report studies on structural phase transition for PMN-xPT ferroelectric, with melt PbTiO3 composition around the MPB (x = 0.35 mol %), using infrared spectroscopy technique. The study was centered on monitoring the behavior of the 1-(NbO), 1-(TiO) and 1-(MgO) stretching modes as a function of temperature. The increasing as a function of temperature for 1-(TiO) and 1-(MgO) modes, observed between 230 and 300 K, can be related to the monoclinic (MC) + tetragonal (T) phase coexistence in the PMN-PT.

Phase-transition studies of Ba(0.90)Ca(0.10)(Ti(1-x)Zr(x))O(3) ferroelectric ceramic compounds

Perovskite-structured Ba(0.90)Ca(0.10)(Ti(1-x)Zr(x))O(3) ceramics were prepared in this work and subsequently studied in terms of composition-dependent dielectric and high-resolution long-range order structural properties from 30 to 450 K. The dielectric response of these materials was measured at several frequencies in the range from 1 kHz to 1 MHz. Combining both techniques, including Rietveld refinement of the X-ray diffraction data, allowed observing that, when increasing Zr(4+) content, the materials change from conventional to diffuse and relaxor ferroelectric compounds, the transition occurring spontaneously at the x = 0.18 composition. Interestingly, this spontaneous transition turned out to be prevented for a further increase of Zr(4+). On the basis of all the dielectric and structural results processed, a phase diagram of this system is presented. (C) 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Adsorption Behavior of 5-Fluorouracil on Au(111): An In Situ STM Study

The biological effects of chemical substitution of DNA bases triggered several investigations of their physicochemical properties This paper studies the adsorption behavior of a halogenated uracil, 5-fluorouracil (5FU). at the electrochemical interface of Au(111) and sulfuric acid solution. Upon modulation of the electric field across the interface, four distinct phases could be inferred by means of cyclic voltammetry (CV) At negative potentials relative to the SCE electrode, limited by the threshold of hydrogen evolution, no molecular species could be detected by scanning tunneling microscopy (STM) at the reconstructed Au(111)-(23 x root 3) surface, indicating that any physisorbed molecules are randomly distributed Incursion into more positive potentials increases the surface population but doer not form any two-dimensional (2D) physisorbed ordered structure Instead, we observed metastable structures that are only detectable. on surfaces with high defect density At sufficiently high positive potentials. limited by gold oxidation, the molecules are chemisorbed in a (3 x 2 root 3) ordered structure. with the aromatic ring perpendicular to the surface We report the densest chemisorbed monolayer for pyrimidine-derivative molecules (area per molecule 0 14 +/- 0 04 nm(2)). A comparison of the adsorption behavior of uracil derivatives has been made based on recent results of chemical substitution and solvent effects. We propose that pi-stacking is enhanced when halogens are incorporated in the uracil structure, in a similar fashion to what is observed in then crystal structure

High specific surface area LaFeCo perovskites-Synthesis by nanocasting and catalytic behavior in the reduction of NO with CO

LaFe(1-x)CO(x)O(3) perovskites were conventionally or nanocasting synthesized. The nanocasting involved the preparation of a micro-mesoporous carbon mould using a Silica Aerosil 200 and a carbon source. Then, perovskites were carbon cast at 800 degrees C. The solids were characterized by XRD, N(2) sorption, FTIR, TGA/DTG, SEM and TEM. N(2) sorption evidenced that the nanocast perovskites did not show significant intraparticle porosity in despite of their enhanced (30-50 m(2)/g) specific surface area (SSA). Nevertheless, TEM images, XRD and Rietveld refinement data showed that the solids are constituted at least by 97 wt% of perovskite phase and by agglomerates smaller than 100 nm constituted by crystallites of about 6 nm. TGA/DTG results demonstrated carbon oxidation during the perovskite formation, thus eliminating the template effect and facilitating the occurrence of sintering, which limited the SSA increase. The nanocast perovskites were more active in the reduction of NO than the uncast ones, behavior that was attributed to the increase in their SSA that allows the exposure of a higher number of accessible active sites. However, the perovskite composition and the presence of impurities can reduce the effect of the improvement of the textural properties. The nanocast perovskites also showed high thermal and catalytic stability, corroborating their potential as catalysts for the studied reaction. (C) 2009 Elsevier B.V. All rights reserved.