Vol. 61 No. 1 Jan-Feb (2015): Revista Mexicana de Física.

Published: 2015-01-01

Articles

  • Mesoscopic simulation of Brownian particles confined in harmonic traps and sheared fluids

    J. Fernández, H. Híjar
    1-0
    Abstract:
    We consider the motion of a Brownian particle bound by a harmonic force in a thermal bath driven from equilibrium by a uniform shear imposed externally. We extend the classical theory of Brownian motion to calculate the probability distribution function for finding the Brownian particle in a phase-space volume element when it is in the presence of the external shear. We find the explicit form of the reduced distribution for velocities in the stationary limit and show that it becomes anisotropic by extending itself over the direction of the imposed shear. We also consider the effects of the imposed shear on the time correlation functions of the Brownian particle and show that these quantities acquire contributions depending exclusively on the nonequilibrium state of the solvent, which render them non symmetric and time-irreversible. In order to verify these conclusions we develop a hybrid mesoscopic simulation technique based on Molecular Dynamics and Multi-particle Collision Dynamics. We observe a very good agreement between the predictions of the model and the results obtained independently from the simulation method, thus suggesting that the latter could be used as a complement to current experimental procedures.
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  • Stable soliton-like train pulses in an active fiber laser system

    M. Wilson, P. Rangel-Fonseca, A. González-García
    14-0
    Abstract:
    Stable soliton-like pattern formation in a compact erbium-doped all fiber laser system is reported. The windows where these bounded states can be achieved were founded varying the cavity losses actively with the help of an electro-optic modulator. The fiber laser worked with a slope efficiency of 24 percent and threshold of less than 16 mW allowing a maximum output power of 3.83 mW. This fiber laser system can be used to obtain non-distortion pulses for optical communication purposes.
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  • Aproximando el flujo de fluidos viscoelásticos mediante ecuaciones diferenciales con retardo

    L. G, A. E
    18-0
    Abstract:
    Using simple models to obtain useful answers, including those dealing with an acceptable error degree, are mainly important when data estimation is required for purposes such as experiment and equipment design, or even some properties that are non-process oriented. Modern computers currently have data handling capabilities that lead to see shortcut methods as archaic solutions, yet from time to time it remains as the only way to perform assessment evaluations that may occur from the theoretical phase to the experimental one at some procedure point. Therefore, this work features one shortcut method through delay differential equations to shortcut viscoelastic fluid equations.
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  • Invarianza de las ecuaciones de movimiento bajo transformaciones de escala espacio-temporales en la dinámica de Newton modificada (MOND)

    R. Acosta, E. Tuirán, U. Molina Redondo
    25-0
    Abstract:
    The basic principles that originated the Modified Newtonian Dynamics MOND, are shown, as well as a description of the fundamentals aspects of the theory: modification of gravity and modification of inertia. Also, it is considered the behaviour of the movement equations under space-temporal scale transformations of the movement equations, that is, transformations that have the form $(t,\vec{r})\rightarrow(\lambda t, \vec{r})$. It was observed in this way that the MOND regime comes from the requirement of the invariance of the movement equations with respect to this transformations.
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  • Análisis de materiales catódicos de estructura perovskita para celdas de combustible de óxido sólido, sofc'\lowercase{s}

    J. Alvarado-Flores J, L. Ávalos-Rodríguez
    32-0
    Abstract:
    Fuel cells directly and efficiently convert the chemical energy of a fuel into electrical energy. Of the various types of fuel cells, the solid oxide (SOFC), combine the advantages in environmentally benign energy generation with fuel flexibility. However, the need for high operating temperatures (800 - 1000$^{\circ}$C) has resulted in high costs and major challenges in relation to the compatibility the cathode materials. As a result, there have been significant efforts in the development of intermediate temperature SOFC (500 - 700$^{\circ}$C). A key obstacle for operation in this temperature range is the limited activity of traditional cathode materials for electrochemical reduction of oxygen. In this article, the progress of recent years is discussed in cathodes for SOFC perovskite structure (ABO$_3$), more efficient than the traditionally used La$_{1-x}$Sr$_x$MnO$_{3-\delta}$ (LSM) or (La, Sr) CoO$_3$. Such is the case of mixed conductors (MIEC) double perovskite structure (AA'B$_2$O$_{5+\delta}$) using different doping elements as La, Sr, Fe, Ti, Cr, Sm, Co, Cu, Pr, Nd, Gd, dy, Mn, among others, which could improve the operational performance of existing cathode materials, promoting the development of optimized intermediate temperature SOFC designs.
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  • Investigation of electrical RC circuit within the framework of fractional calculus

    H. Ertik, A.E. \c{C}al\i k, H. \c{S}irin, M. \c{S}en, B. Öder
    58-0
    Abstract:
    In this paper, charging and discharging processes of different capacitors in electrical RC circuit are considered theoretically and experimentally. The non-local behaviors in these processes, arising from the time fractality, are investigated via fractional calculus. In this context, the time fractional differential equation related to electrical RC circuit is proposed by making use of Caputo fractional derivative. The resulting solution exhibits a feature in between power law and exponential law forms, and is obtained in terms of Mittag-Leffler function which describes physical systems with memory. The order of fractional derivative characterizes the fractality of time and being considered in the interval $0<\alpha \leq 1$. The traditional conclusions are recovered for $\alpha =1$, where time becomes homogenous and system has Markovian nature. By using time fractional approach, the discrepancies between the experimentally measured data and the theoretical calculations have been removed.
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  • 1-(2-furoyl)-3,3-(diphenyl)thiourea: spectroscopic characterization and structural study from x-ray powder diffraction using simulated annealing

    O. Estévez-Hernández, J. Rodríguez-Hernández, H. Yee-Madeira, J. Duque
    64-0
    Abstract:
    1-Furoyl-3,3-diphenylthiourea (FDFT) was synthesized, and characterized by FTIR, {}$^{1}$H and {}$^{13}$C NMR and ab initio X-ray powder structure analysis. FDFT crystallizes in the monoclinic space group P2$_1$ with $a = 12.691$(1), $b = 6.026$(2), $c = 11.861$(1) \AA, $\beta = 117.95$(2) and $V= 801.5$(3) \AA$^{3}$. The crystal structure has been determined from laboratory X-ray powder diffraction data using direct space global optimization strategy (simulated annealing) followed by the Rietveld refinement. The thiourea group makes a dihedral angle of 73.8(6)$^{\circ}$ with the furoyl group. In the crystal structure, molecules are linked by van der Waals interactions, forming one-dimensional chains along the a axis.
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  • Perturbation method applied to a basic diode circuit

    H. Vazquez-Leal, Y. Khan, G. Fern, ez-Ana., a., U. Filobello-Nino, V.M. Jimenez-Fern, ez., A. Herrera-Ma, A. Diaz-Sanchez, A. Marin-Hern, ez., J. Huerta-Chua, .
    69-0
    Abstract:
    Because of the exponential characteristic of silicon diodes, exact solutions cannot be established when operating point and transient analysis are computed. To overcome that problem, the present work proposes a perturbation method which allows obtaining approximated analytic expressions of diode-based circuits. Simulation results show that numerical solutions obtained by using the proposed method are similar to those reported in literature, with the advantage of not requiring a user-selected arbitrary expansion point. Additionally, the method does not use the Lambert function $W$, reducing the proposed solution complexity, which makes it suitable for engineering applications.
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