Microstructural, structural and dielectric properties of Er(3+)-modified BaTi(0.85)Zr(0.15)O(3) ceramics

The (micro)structural and electrical properties of undoped and Er(3+)-doped BaTi(0.85)Zr(0.15)O(3) ceramics were studied in this work for both nominal Ba(2+) and Ti(4+) substitution formulations. The ceramics were produced from solid-state reaction and sintered at 1400 degrees C for 3 h. For those materials prepared following the donor-type nominal Ba(1-x)Er(x)(Ti(0.85)Zr(0.15))O(3) composition, especially, Er(3+) however showed a preferential substitution for the (Ti,Zr)(4+) lattice sites. This allowed synthesis of a finally acceptor-like, highly resistive Ba(Ti,Zr,Er)O(3-delta)-like system, with a solubility limit below but close to 3 cat.% Er(3+). The overall phase development is discussed in terms of the amphoteric nature of Er(3+), and appears to mainly or, at least, partially also involve a minimization of stress effects from the ion size mismatch between the dopant and host cations. Further results presented here include a comparative analysis of the behavior of the materials` grain size, electrical properties and nature of the ferroelectric-to-paraelectric phase transition upon variation of the formulation and Er(3+) content. (C) 2008 Elsevier Ltd. All rights reserved.

Microstructural, structural and electrical properties of La(3+)-modified Bi(4)Ti(3)O(12) ferroelectric ceramics

Bi(4-x)La(x)Ti(3)O(12) (BLT) ceramics were prepared and studied in this work in terms of La(3+)-modified microstructure and phase development as well as electrical response. According to the results processed from X-ray diffraction and electrical measurements, the solubility limit (XL) of La(3+) into the Bi(4)Ti(3)O(12) (BIT) matrix was here found to locate slightly above x = 1.5. Further, La(3+) had the effect of reducing the material grain size, while changing its morphology from the plate-like form, typical of BIT ceramics, to a spherical-like one. The electrical results presented and discussed here also include the behavior of the temperature of the ferroelectric-paraelectric phase transition as well as the normal or diffuse and/or relaxor nature of this transition depending on the La(3+) content. (c) 2008 Elsevier Ltd. All fights reserved.

Structure and optical properties of [Ba(1-x)Y(2x/3)](Zr(0.25)Ti(0.75))O(3) powders

[Ba(1-x)Y(2x/3)](Zr(0.25)Ti(0.75))O(3) powders with different yttrium concentrations (x = 0, 0.025 and 0.05) were prepared by solid state reaction. These powders were analyzed by X-ray diffraction (XRD). Fourier transform Raman scattering (FT-RS), Fourier transform infrared (FT-IR) and X-ray absorption near-edge (XANES) spectroscopies. The optical properties were investigated by means of ultraviolet-visible (UV-vis) absorption spectroscopy and photoluminescence (PL) measurements. Even with the addition of yttrium, the XRD patterns revealed that all powders crystallize in a perovskite-type cubic structure. FT-RS and FT-IR spectra indicated that the presence of [YO(6)] clusters is able to change the interaction forces between the O-Ti-O and O-Zr-O bonds. XANES spectra were used to obtain information on the off-center Ti displacements or distortion effects on the [TiO(6)] clusters. The different optical band gap values estimated from UV-vis spectra suggested the existence of intermediary energy levels (shallow or deep holes) within the band gap. The PL measurements carried out with a 350 nm wavelength at room temperature showed that all powders present typical broad band emissions in the blue region. (C) 2010 Elsevier Masson SAS. All rights reserved.

The n-site approximation for the triplet-creation model

The nonequilibrium phase transition of the one-dimensional triplet-creation model is investigated using the n-site approximation scheme. We find that the phase diagram in the space of parameters (gamma, D), where gamma is the particle decay probability and D is the diffusion probability, exhibits a tricritical point for n >= 4. However, the fitting of the tricritical coordinates (gamma(t), D(t)) using data for 4 <= n <= 13 predicts that gamma(t) becomes negative for n >= 26, indicating thus that the phase transition is always continuous in the limit n -> infinity. However, the large discrepancies between the critical parameters obtained in this limit and those obtained by Monte Carlo simulations, as well as a puzzling non-monotonic dependence of these parameters on the order of the approximation n, argue for the inadequacy of the n-site approximation to study the triplet-creation model for computationally feasible values of n.

SPECTRUM OF SCREENING MASSES FOR SU(2) GAUGE THEORY: UNIVERSALITY CLASS

In this work we study the spectrum of the lowest screening masses for Yang-Mills theories on the lattice. We used the SU(2) gauge group in (3 + 1) dmensions. We adopted the multiple exponential method and the so-called ""variational"" method, in order to detect possible excited states. The calculations were done near the critical temperature of the confinement-deconfinement phase transition. We obtained values for the ratios of the screening masses consistent with predictions from universality arguments. A Monte Carlo evolution of the screening masses in the gauge theory confirms the validity of the predictions.

The mass media destabilizes the cultural homogenous regime in Axelrod`s model

An important feature of Axelrod`s model for culture dissemination or social influence is the emergence of many multicultural absorbing states, despite the fact that the local rules that specify the agents interactions are explicitly designed to decrease the cultural differences between agents. Here we re-examine the problem of introducing an external, global interaction-the mass media-in the rules of Axelrod`s model: in addition to their nearest neighbors, each agent has a certain probability p to interact with a virtual neighbor whose cultural features are fixed from the outset. Most surprisingly, this apparently homogenizing effect actually increases the cultural diversity of the population. We show that, contrary to previous claims in the literature, even a vanishingly small value of p is sufficient to destabilize the homogeneous regime for very large lattice sizes.

Random walks systems with killing on Z

We study random walks systems on Z whose general description follows. At time zero, there is a number N >= 1 of particles at each vertex of N, all being inactive, except for those placed at the vertex one. Each active particle performs a simple random walk on Z and, up to the time it dies, it activates all inactive particles that it meets along its way. An active particle dies at the instant it reaches a certain fixed total of jumps (L >= 1) without activating any particle, so that its lifetime depends strongly on the past of the process. We investigate how the probability of survival of the process depends on L and on the jumping probabilities of the active particles.

Cellular automaton model for molecular traffic jams

We consider the time evolution of an exactly solvable cellular automaton with random initial conditions both in the large-scale hydrodynamic limit and on the microscopic level. This model is a version of the totally asymmetric simple exclusion process with sublattice parallel update and thus may serve as a model for studying traffic jams in systems of self-driven particles. We study the emergence of shocks from the microscopic dynamics of the model. In particular, we introduce shock measures whose time evolution we can compute explicitly, both in the thermodynamic limit and for open boundaries where a boundary-induced phase transition driven by the motion of a shock occurs. The motion of the shock, which results from the collective dynamics of the exclusion particles, is a random walk with an internal degree of freedom that determines the jump direction. This type of hopping dynamics is reminiscent of some transport phenomena in biological systems.

The disk-percolation model on graphs

We study a long-range percolation model whose dynamics describe the spreading of an infection on an infinite graph. We obtain a sufficient condition for phase transition and prove all upper bound for the critical parameter of spherically symmetric trees. (C) 2008 Elsevier B.V. All rights reserved.

Characterization of DODAB/DPPC vesicles

Dioctadecyldimethylammonium bromide (DODA B)/dipalmitoylphosphatidylcholine (DPPC) large and cationic vesicles obtained by vortexing a lipid film in aqueous solution and above the mean phase transition temperature (T-m) are characterized by means of determination of phase behaviour, size distribution, zeta-potential analysis and colloid stability. The effect of increasing % DODAB over the 0-100% range was a nonmonotonic phase behaviour. At 50% DODAB, the mean phase transition temperature and the colloid stability were at maximum. There is an intimate relationship between stability of the bilayer structure and colloid stability. In 1, 50 and 150 mM NaCl, the colloid stability for pure DPPC or pure DODAB vesicles was very low as observed by sedimentation or flocculation, respectively. In contrast, at 50% DODAB, remarkable colloid stability was achieved in 1, 50 or 150 mM NaCl for the DODAB/DPPC composite vesicles. Vesicle size decreased but the zeta-potential remained constant with % DODAB, due to a decrease of counterion binding with vesicle size. This might be important for several biotechnological applications currently being attempted with cationic bilayer systems. (c) 2008 Elsevier Ireland Ltd. All rights reserved.

Leakage current, ferroelectric and structural properties in Pb(1-x)Ba(x)TiO(3) thin films prepared by chemical route

Single-phase perovskite structure Pb(1-x)Ba(x)TiO(3) thin films (x = 0.30, 0.50 and 0.70) were deposited on Pt/Ti/SiO(2)/Si substrates by the spin-coating technique. The dielectric study reveals that the thin films undergo a diffuse type ferroelectric phase transition, which shows a broad peak. An increase of the diffusivity degree with the increasing Barium contents was observed, and it was associated to a grain decrease in the studied composition range. The temperature dependence of the phonon frequencies was used to characterize the phase transition temperatures. Raman modes persist above tetragonal to cubic phase transition temperature, although all optical modes should be Raman inactive. The origin of these modes was interpreted in terms of breakdown of the local cubic symmetry by chemical disorder. The absence of a well-defined transition temperature and the presence of broad bands in some interval temperature above FE-PE phase transition temperature Suggested a diffuse type phase transition. This result corroborates the dielectric constant versus temperature data, which showed a broad ferroelectric phase transition in these thin films. The leakage Current density of the PBT thin films was studied at different temperatures and the data follow the Schottky emission model. Through this analysis the Schottky barrier height values 0.75, 0.53 and 0.34 eV were obtained to the PBT70, PBT50 and PBT30 thin films, respectively. (C) 2008 Elsevier Ltd. All rights reserved.

Ageing effects on knee and ankle joint angles at key events and phases of the gait cycle.

The objective of this research was to determine whether joint angles at critical gait events and during major energy generation/absorption phases of the gait cycle would reliably discriminate age-related degeneration during unobstructed walking. The gaits of 24 healthy adults (12 young and 12 elderly) were analysed using the PEAK Motus motion analysis system. The elderly participants showed significantly greater single (60.3% versus 62.3%, p < 0.01) and double ( p < 0.05) support times, reduced knee flexion (47.7° versus 43.0°, p < 0.05) and ankle plantarflexion (16.8° compared to 3.3°, p = 0.053) at toe off, reduced knee flexion during push-off and reduced ankle dorsiflexion (16.8° compared to 22.0°, p < 0.05) during the swing phase. The plantarflexing ankle joint motion during the stance to swing phase transition (A₂) for the young group (31.3°) was about twice ( p < 0.05) that of the elderly (16.9°). Reduced knee extension range of motion suggests that the elderly favoured a flexed-knee gait to assist in weight acceptance. Reduced dorsiflexion by the elderly in the swing phase implies greater risk of toe contact with obstacles. Overall, the results suggest that joint angle measures at critical events/phases in the gait cycle provide a useful indication of age-related degeneration in the control of lower limb trajectories during unobstructed walking.

Luminescence and microstructure of Sr2Mg(BO3)2 doped with Eu

Sr₂Mg(B0₃)₂ doped with Eu was synthesized respectively in air and weak reducing atmosphere (combustion of carbon particle), whose photoluminescence characteristics and structure were also studied at room-temperature. In air, the fluorescent body's color was white for different synthesized temperatures. At room temperature, the sample was excited and showed red typical emission spectrum of Eu³⁺ whose emission apex were sharp near 612 nm and emission spect~m was made up of the charge transformation band (CTB) of Eu^{3 +} and excitation spectrum of 4f→4f high energy level transition, then reached the area of VUV. However, under reducing atmosphere (combustion of carbon particles), the color of the sample yielded was yellow, whose color became deeper with increasing temperature and showed phase transition. Using UV excitation, the luminescence of yellow sample was very weak. In a complicated broad spectrum at visible light area, the red emission spectrum of Eu²⁺ was not observed. Crystal structure and luminescence of the sample were completely different from the results of Diaz and Keszler. Two samples were prepared under oxidation and reducing atmosphere at high temperature, which were different on crystal structure and microstructure. By studying Sr₂Mg(B0₃)₂:Eu³⁺ a series of directional faults or educts were found, because Eu^{3 +} ions substituted for Sr^{2 +} ions. However, microstructure of Sr₂Mg(B0₃ )₂: Eu^{2 +} is more complicated, whose excitation spectrum might be excited by Eu^{2 +}. By XRD patten of the samples, phase transitibn could be found. Twins and clusters that were formed from point defect such as interstitial atom and big angle crystal boundary could be found by TEM.

Ab initio calculations, Raman and NMR investigation of the plastic crystal di-methyl pyrrolidinium iodide

Above 110 °C the symmetric di-methyl-pyrrolidinium iodide salt forms a plastic crystal phase of interest in the area of new electrolyte materials. In this study ab initio calculations of this material has been conducted in order to assign the vibrational spectra. Raman spectroscopy measurements on the solid salt as well as on the salt dissolved in different solvents has been performed and these have been compared to the theoretical spectra. Furthermore, Raman spectra as a function of temperature have been recorded to investigate possible changes in inter-ionic interaction and/or structure through the phase transition. 1H NMR linewidth measurements as a function of temperature showed a large decrease in linewidth above 100 °C, attributed here to an increase in mobility in agreement with a previously reported phase transition at ~110 °C.

Fast ion conduction in an acid doped pentaglycerine plastic crystal

High proton conductivity has been achieved in the high temperature plastic crystal phase of pentaglycerine when doped with strong acids, including trifluoromethanesulfonic acid (triflic acid) and methanesulfonic acid. The solid–solid phase transition from the ordered to plastic phase in this material occurs at 86 °C and conductivities of 10^{− 3} S/cm were measured in the high temperature plastic phase on the addition of 1 mol% triflic acid. In the case of methanesulfonic acid, the conductivities showed a greater dependence on acid concentration and were lower than for triflic acid, as expected on the basis of acid strengths. Electrochemical characterisation shows a clear hydrogen reduction process indicating that the proton is the mobile species in the plastic phase.