Vol. 59 No. 5 (2013): Revista Mexicana de Física.

Published: 2013-01-01

Articles

  • Avances en el desarrollo y conocimiento del cátodo Ba$_{0.5}$Sr$_{0.5}$Co$_{0.8}$Fe$_{0.2}$O$_{3 - \delta }$ para celdas de combustible de óxido sólido de temperatura intermedia IT-SOFC

    J. Alvarado-Flores, L. Ávalos-Rodríguez
    380-0
    Abstract:
    Solid oxide fuel cells (SOFCs) convert chemical energy directly into electric power in a highly efficient way. Lowering the operating temperature of SOFCs to around 500-800$^{\circ}$C is one of the main goals in current SOFC research. The associated benefits include reducing the difficulties associated with sealing and thermal degradation, allowing the use of low cost metallic interconnectors and suppressing reactions between the cell components. However, the electrochemical activity of the cathode deteriorates dramatically with decreasing temperature for the typical La$_{0.8}$Sr$_{0.2}$MnO$_{3}$ based electrodes. The cathode becomes the limiting factor in determining the overall cell performance. Therefore, the development of new electrodes with high electrocatalytic activity for oxygen reduction becomes a critical issue for intermediate temperature (IT)-SOFCs. Ba$_{0.5}$Sr$_{0.5}$Co$_{0.8}$Fe$_{0.2}$O$_{3 - \delta }$ (BSCF) perovskite oxide was first reported as a potential IT-SOFC cathode material in 2004 by Shao and Haile. After that, the BSCF cathode has attracted considerable attention. This paper reviews the current research activities on BSCF based cathodes for IT-SOFCs. Emphasis will be placed on the understanding and optimization of BSCF based materials. The issues raised by the BSCF cathode are also presented and analyzed to provide some guidelines in the search for the new generation of cathode materials for IT-SOFCs.
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  • Electrical and structural characteristics of spray deposited (ZnO)$_{X}$-(CdO)$_{1 - X}$ thin films

    G. Alarcón-Flores, B. Vásquez-Pérez, A. Peláez-Rodríguez, M. Villa-Garcia, S. Carmona-Téllez, J. A, C. Falcon, M. Aguilar-Frutis, .
    403-0
    Abstract:
    (ZnO)$_{X}$(CdO)$_{1 - X}$ thin films were deposited on glass substrates at 300$^{\circ}$C and 400$^{\circ}$C by ultrasonic spray pyrolysis with compositions ranging from CdO to ZnO. The electrical properties were obtained by impedance spectroscopy and Hall Effect measurements. Scanning electron microscopy, energy dispersive spectroscopy, and X-ray diffraction, were used to study the structural characteristics of the films. Ellipsometry, in addition, was used to confirm the structural characteristics. The films as deposited resulted mainly polycrystalline and dense, depending on the substrate temperature and on their relative composition. All the films showed n-type conductivity and the films with intermediate compositions resulted in a mixture of both phases; CdO and ZnO. Hall Effect measurements showed that the highest conductivity of CdO was close to 1$\times$10$^{3}$ ($\Omega $-cm)$^{ - 1}$, the highest value obtained for CdO, without doping. Impedance spectroscopy confirmed the Hall Effect results, showing that the highly conducting character of CdO influenced dramatically the conductivity of the (ZnO)$_{X}$(CdO)$_{1 - X}$ films. In addition, depending on the substrate temperature and on the relative composition of the films, both, the bulk or grains, as well as the grain boundaries properties limit the conductivity in them.
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  • Análisis estocástico de la dimensión fractal en electrodeposición de zinc en capa delgada

    E. J, J.A. Llanos-Pérez, J.A. Betancourt Mar, J.M. Nieto-Villar, E. Izquierdo-Kulich
    412-0
    Abstract:
    From a mesoscopic modeling electrodeposition process in a thin layer cell, we propose a stochastic model that relates the expected value of the area fraction occupied by zinc particles formed during electrodeposition and the fractal dimension of the pattern formed. Experiments were conducted at constant voltage in a circular cell with a cathode as a center point. It was found that the predictions made by our model are in keeping with experimental results obtained and reported in literature, showing that the fractal dimension increases with the initial concentration of zinc ions to a certain limit from which~ it then decreases. This behavior is explained through the proposed model taking into account the mechanism that determines pattern formation.
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  • Efectos de dispersión a primer orden y srs sobre un pulso con polarización circular dentro de un nolm con fibra torcida en lazo

    B. A, E.A. Kuzin, B. Ibarra-Escamilla, N. Korneev, A. Flores-Rosas.
    419-0
    Abstract:
    A theoretical analysis is presented in order to study the influence of both First Order Dispersion, and Stimulated Raman Scattering (SRS) on the transmittance of a Nonlinear Optical Loop Mirror (NOLM) made of twisted fiber with low birefringence. The system NLS equations were modified in order to introduce the FOD and SRS effects, then they were solved numerically take into account the effects of first order dispersion and SRS on a circularly polarized pulse within a NOLM based on the principle of polarization's asymmetry. The simulation results show that the first order dispersion effect is to increase the transmittance of the NOLM for pulses with input power lower than soliton power. This effect results in decrease of the nonlinear switch contrast. For input power higher than soliton power, SRS avoid the coherence between pulses. That destroys the NOLM operation.
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  • Investigation on the structure-electrical property relationship of hydrolyzed poly(vinyl alcohol) membranes

    J.F. Jurado, O. Checa, R.A. Vargas, .
    426-0
    Abstract:
    This investigation explored the effects of the pre-treatment temperature on the molecular conformations and electrical performance of poly(vinyl alcohol) (PVOH) membranes. The structure and properties of the membranes were characterized by X-ray diffraction (XRD), differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), Raman scattering (RS) and impedance measurements (IE). Water molecules absorbed by the PVOH membranes, which decreased in quantity as the temperature increased caused drastic change to be observed in the relative band intensities of the OH and CH$_2$ bonds with respect to the C-C bonds. The observations for the hydrated PVOH were correlated with the proton transport behavior, which were inferred from conductivity relaxation measurements over various temperature regions and were dependent on the water content in the membrane. The results were corroborated by DSC and TGA. For example, the temperature dependence of the conductivity relaxation frequency, $\omega_{\text{max}}$, followed different Arrhenius-type thermally activated processes at low and high temperatures. The corresponding activation energies in the low and high temperature regions were: 1.42$\pm$0.02 and 0.23$\pm$0.02~eV, respectively. In addition, the selected fitting temperature regions and activation energies for the $\omega_{\text{max}}$ data were equivalent (within experimental error) to the values for the dc-conductivity, $\sigma_0$(T). This result indicates that the mechanisms for long range ion displacement (dc conductivity) and ion-ion or ion-polymer chain correlations are identical, (i.e., an ion-hoping occurred in the various hydrated phases of PVOH).
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  • Atomic radiative corrections without QED: role of the zero-point field

    A. M, L. de la Peña, A. Valdés-Hernández
    433-0
    Abstract:
    We derive the atomic radiative corrections predicted by \textsc{qed} using an alternative approach that offers the advantage of physical clarity and transparency. The element that gives rise to these corrections is the fluctuating zero-point radiation field (\textsc{zpf}) of average energy $% \hbar \omega /2$ per mode, which ---in contrast with \textsc{qed}--- is taken here as a primordial real entity in permanent interaction with matter and responsible for its quantization. After briefly recalling how quantum mechanics itself emerges as a result of the balance between the \textsc{zpf} and radiation reaction, the most important higher-order effects of the radiative terms on the atom are studied. The nonrelativistic \textsc{qed} formulas for the lifetimes and the Lamb shift, as well as the corrections to the latter due to external factors that modify the vacuum field, are thus obtained in a self-consistent approach and without the need to resort to second quantization to the present order of approximation.
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  • Three-dimensional PIV measurements of bubble drag and lift coefficients in restricted media

    J. Ortiz-Villafuerte, W. D, Y. A
    444-0
    Abstract:
    A hybrid scheme combining Particle Image Velocimetry and Shadow Image Velocimetry has been used for a full-volume, three-dimensional, transient study of the shape, trajectory and forces acting on air bubbles rising in stagnant tap water in restricted media. The bubble Reynolds number ranged from 400 to 650. The three-dimensional reconstruction of the bubble was accomplished by combining images from two orthogonal views. This reconstruction process allowed for measurement of dimensions, orientation, trajectory, rotation, velocity and acceleration of an individual rising bubble. These parameters were then used to compute drag and lift forces acting on the bubble. Instantaneous values of drag and lift coefficients were then determined. These experimental results were compared to known experimental data and values obtained from correlations found in scientific literature. It was found that correlations intended for determining drag coefficient values should be adequately modified when necessary to account for wall impact, since the drag coefficient magnitude is considerably higher than that predicted by such correlations at Re below 550. Regarding the bubble lift coefficient, instantaneous data scatter noticeably as a function of Re, but average values agree within the range of known data. The major contributors to the uncertainty in this experiment were the capability of accurately reconstructing the 2D shape of the bubbles from distorted and/or incomplete PIV images and determining the bubble centroid. An overall error of 7% was computed for the drag coefficient, but it rises up to 44% for the lift coefficient.
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  • The band gap problem: the accuracy of the Wien2k code confronted

    J. A, R. Baquero
    453-0
    Abstract:
    This paper is a continuation of our detailed study [Phys. Rev. B 86, 195106 (2012)] of the performance of the recently proposed modified Becke-Johnson potential (mBJLDA) within the known Wien2k code. From the 41 semiconductors that we have considered in our previous paper to compute the band gap value, we selected 27 for which we found low temperature experimental data in order to pinpoint the relative situation of the newly proposed Wien2k(mBJLDA) method as compared to other methods in the literature. We found that the GWA gives the most accurate predictions. The Wien2k (mBJLDA) code is slightly less precise, in general. The Hybrid functionals are less accurate, on the overall. The GWA is definitely the most precise existing method nowadays. In 88% of the semiconductors considered the error was less than 10%. Both, the GWA and the mBJLDA potential, reproduce the band gap of 15 of the 27 semiconductors considered with a 5% error or less. An extra factor to be taken into account is the computational cost. If one would seek for precision without taking this factor into account, the GWA is the method to use. If one would prefer to sacrifice a little the precision obtained against the savings in computational cost, the empirical mBJLDA potential seems to be the appropriate method. We include a graph that compares directly the performance of the best three methods, according to our analysis, for each of the 27 semiconductors studied. The situation is encouraging but the problem is not yet a closed issue.
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  • Quartic couplings, masses and thresholds in the basic extension of the Standard Model

    S.R. Juárez, D. Morales, P. Kielanowski
    460-0
    Abstract:
    The Standard Model (SM), despite of being phenomenologically extremely successful, presents some fundamental questions, like the big differences in the values of the masses of the quarks (hierarchy of masses), and the possible generation of flavour changing neutral currents (inspired by the evidence about the oscillations of neutrinos). Hints how these questions might be answered may be obtained by the study of the Higgs sector of models beyond the standard model. The simplest extension of the SM known as the two-Higgs-doublet-model (2HDM) involves a second Higgs doublet and predicts the existence of five scalar particles: three neutral ($A^{0}$), ($h^{0}$, $H^{0}$) and two charged ($H^{\pm}$). In this paper we focus our attention on the basic results of the model, the masses of the five particles, and the theoretical constraints imposed by vacuum stability and the triviality principle. We address, on one side, the range of validity in the energy scale of the 2HDM by means of the renormalization group equations, and on the other, the consequences of a top/bottom Yukawa coupling unification, assuming that the hierarchy of the quark masses is atributted to the vacuum expectation values $v_{1}$ and $v_{2}$ of the Higgs fields and not to the Yukawa couplings.
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  • Trajectory tracking for the chaotic pendulum using PI control law

    J. Perez, J. P, F. Rdz, A. Flores
    471-0
    Abstract:
    This paper presents the application of trajectory tracking using adaptive neural networks to the double chaotic pendulum. The controller structure proposed is composed by a neural identifier and a PI Control Law. Experimental results with the chaotic pendulum showed the usefulness of the proposed approach. To verify the analytical results, an example of a dynamical network is simulated and a theorem is proposed to ensure the tracking of the nonlinear system.
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  • Coordinate systems adapted to constants of motion

    G.F. Torres del Castillo
    478-0
    Abstract:
    We present some examples of mechanical systems such that given $n$ constants of motion in involution (where $n$ is the number of degrees of freedom), we can identify a coordinate system in which the Hamilton--Jacobi equation is separable (or $R$-separable), with the separation constants being the values of the given constants of motion. Analogous results for the Schrödinger equation are also given.
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  • Operator folding and matrix product states in linearly-coupled bosonic arrays

    J. Reslen
    482-0
    Abstract:
    A protocol to obtain the matrix product state representation of a class of boson states is introduced. The proposal is presented in the context of linear systems and is tested by performing simulations of a reference model. The method can be applied regardless of the details of the coupling among modes and can be used to extract the most significant contribution of the tensorial representation. Characteristic issues as well as potential variants of the proposed protocol are discussed. Se introduce una técnica para obtener la representación en términos de productos de matrices de una clase de estados bosónicos. La técnica se presenta en el contexto de sistemas lineales y se verifica realizando simulaciones de un sistema conocido. Este método se puede aplicar independientemente del tipo de acoplamiento entre modos y se puede usar para extraer la parte mas significativa de la representación tensorial del estado. Se discuten tando las características mas importantes como las posibles extensiones de la propuesta.
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  • DFT study of the pressure influence on the electronic and magnetic properties of Ga$_{x}$Mn$_{1-x}$N compound

    Miguel J. Espitia R., Octavio Salcedo Parra., John H. Díaz F.
    488-0
    Abstract:
    We report a firt-principles study of the pressure dependence of electronic and the magnetic properties of Ga$_{x}$Mn$_{1-x}$N compounds ($x$ = 0.25, 0.50 and 0.75) in wurtzite-derived structures. We use the full-potential linearized augmented plane wave method (FP-LAPW) within of the density functional theory framework. We found that, the lattice constant vary linearly with Ga-concentration. The magnetic moment changes for a critical pressure. At $x$ = 0.75, a rather abrupt onset of the magnetic moment from 0 to 6.02 $\mu_{B}$ at $Pcr = 26.50$ GPa is observed. For \linebreak $x$ = 0.25 and 0.50 Ga concentrations, the magnetic moment increases gradually when the pressure decreases toward the equilibrium value. We study the transition pressure dependence to a ferromagnetic phase near the onset of magnetic moment for each Ga$_{x}$Mn$_{1-x}$N compounds. The calculation of the density of states with Ga concentration is carried out considering two spin polarizations. The results reveal that for\linebreak $x$ = 0.75 the compound behaves as a conductor for the spin-up polarization and that the density of states for spin-down polarization is zero at the Fermi level. At this concentration the compound presents a half metallic behavior; therefore this material could be potentially useful as spin injector. At high pressures $P > P_{cr}$ the compounds exhibit a metallic behavior.
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  • Metallic behaviour at YBaCuO7/Zas interfaces (G=Ga, Al)

    R. Torres, R. Baquero
    493-0
    Abstract:
    We present the electronic band structure of the interfaces YBa$_2$Cu$_3$O$_7$/GaAs (direct gap) and YBa$_2$Cu$_3$O$_7$/AlAs (indirect gap) in different configurations calculated using the Density Functional Theory as in the Wien2k code within the local density approximation. We have projected the density of states at the atomic layers forming the interface. We concentrated on the semiconductor side. The four first atomic layers in the semiconductor side of the interface present a clear metallic behaviour. We found for both semiconductors considered that it converges towards the bulk atomic-layer projected density of states a few atomic layers from the interface. We considered an ideal non-reconstructed interface in the (001) direction first and let it relax using the corresponding option in the Wien2k code. The behaviour does not change in an important way and we found but small deviations from the ideal case in the Density of States of the relaxed interface. It is important to relax the interface since the metallic behaviour of the semiconductor side of the could have been suppressed which is the most interesting result of this work. The behaviour at the interface is interesting and could be used in several technological applications and it opens, for example, the possibility to induce superconductivity on the semiconductor side of the metal/semiconductor interface.
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  • Experimental considerations on the determination of radiation fields in an electron accelerator

    L. Mondragón-Contreras, F. J, J. M, M. A, R. López Callejas, R. Peña Eguiluz
    498-0
    Abstract:
    The determination of the different radiation fields in an electron accelerator requires the use of selected radiation detectors, in this work we describe the experimental considerations on the determination of the intensity of electrons and X-rays generated by bremsstrahlung in an experimental electron accelerator covering the energy range from 80 keV to 485 keV. A lithium- drifted silicon detector, a high-purity germanium detector, a scintillation detector and a PIN diode were used in the experiments. Spectroscopic measurements allowed us to verify the terminal voltage of the accelerator. The PIN photodiode can measure the intensity of X-rays produced, with this information, we could determine its relationship with both the electron beam current and the accelerating voltage of the accelerator.
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