Vol. 53 No. 3 (2007): Revista Mexicana de Física.

Published: 2007-01-01

Articles

  • Obtención de una microestructura nueva en la aleación Zn - 40%at. Al - 1. 5%at. Cu

    J.A. Aragón, J.R. Mir, a., A. García - Bórquez
    149-0
    Abstract:
    In a previous investigation, some acicular particles were found in the microstructure of Zn - 40 %at. Al - 1.5 %at. Cu alloy, when it is maintained at 376$^\circ$C and later cooled in the furnace. The aims of the present work are to establish the optimal treatment time to obtaining a microstructure with such micro - constituent in a high quantity and uniformly distributed, to realize relative mechanical basic characterization and by X - rays diffraction during its formation, and EDS microanalysis in integrants of the microstructure finally obtained in Zn - 40%at. Al - 1.5%at. Cu alloy. A plate of this alloy in as - cast condition was rolled in several steps for eliminate more easily its structure with heat treatments applied later. Little sheets of this alloy were maintained different times at 359, 376 and 380$^\circ$C, and they were cooled off later in the furnace. The microstructures produced were analysed and photographed, and their Rockwell 15T hardness and Vickers micro-hardnesses in their components settled down. It was established that wished microstructure is obtained with the thermal treatment by 50 hours at 376$^\circ$C. It is a new microstructure because one of its component, the acicular integrant, has a distribution, size, form, quantity and type that are different to those of any integrant of microstuctures of alloy treated different times at 359 and 380$^\circ$C. The acicular integrant increases the hardness of most components imbibed it, and it is the high - temperature $\beta$ phase. The treatment at 376$^\circ$C diminishes the relative softening that is normally caused by the increase of treatment time at constant temperature in metals previously rolled, and produce an alloy that is slightly more hard.
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  • Nonlinear element of a chaotic generator

    E. Campos-Cantón, J.S. Murguía, I. Campos Cantón, M. Chavira-Rodríguez
    159-0
    Abstract:
    A mathematical model of a nonlinear electronic circuit, which is the core of the electronic chaotic oscillator, is presented. This mathematical model or nonlinear function has a direct relationship with the values of the components used to build an experimental electronic system. In order to get a good approximation to the characteristic response curve of the nonlinear circuit, the mathematic model considers the current-voltage curve of nonlinear elements.
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  • Improved bounds for the effective energy of nonlinear 3D conducting composites

    A. León-Mecías, J. Bravo-Castillero, A. Mesejo-Chiong, L.D. Pérez-Fernández, F.J. Sabina
    164-0
    Abstract:
    Recent variational inequalities of Talbot are used to improve the lower % and upper bounds for the effective energy of nonlinear 3-D two-phase conducting composites. The effective conductivity of the linear isotropic two-phase periodic conducting composite used as comparison material in the inequalities is computed through an asymptotic homogenization model by finite element analysis of the local problem on the three-dimensional cubic unit cell with one spherical inclusion. A brief mathematical description of the numerical method is included. Numerical calculations of the effective conducting linear property are compared with % Bruno's bounds. It shows that the numerical solution for the limit cases of superconducting % and empty inclusions improves the bounds when the inclusion volume fraction is greater than about $0.4$. It is natural to expect an improvement in the whole volume % fraction of Talbot's bounds for nonlinear % conducting composites when the numerical calculation is used instead of % bounds for the linear comparison problem, as is the case here.
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  • Hybrid - block copolymer nanocomposites. characterization of nanostructure by small-angle X-ray scattering (SAXS)*

    A. Romo-Uribe
    171-0
    Abstract:
    The nanoscopic order of a series of block copolymer-inorganic nanocomposites was characterized using small-angle X-ray scattering (SAXS). The nanostructures were obtained via a diblock copolymer directed sol-gel synthesis. The copolymer consists of blocks of poly(isoprene) -PI- and blocks of poly(ethylene oxide) -PEO. The inorganic material consists of a crosslinked sol of\linebreak 3-glycidoxypropyltrimethoxysilane and aluminum-tri-sec-butoxide in a 4:1 mole ratio, to generate an aluminosilicate ceramic. The PEO block is swollen by the ceramic precursor and acts as a nanoreactor for their sol-gel synthesis. The resulting nanostructured hybrid has PI as the majority phase. Two series of nanocomposites, designated PI-b-PEO-D and PI-b-PEO-E, were studied; these correspond to 15 wt% PEO and 13 wt% PEO, respectively. The results showed that the nanoscale order characteristic of block copolymers (lamellar, spherical, and cubic) is not only achieved in these hybrid nanocomposites, but the molecular assembly offers the possibility of being utilized as a template for highly ordered inorganic nanostructures. The amount of inorganic nanofiller and the molecular weight of the blocks define the type of morphology assumed by the nanostructure.
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  • Análisis del comportamiento presión-temperatura y otras propiedades termodinámicas para los cristales líquidos PAA, 5CB y HOAOB utilizando el modelo convex peg y la teoría de los funcionales de la densidad en la transición isótropa-nemático

    E. García-Sánchez, L.H. Mendoza-Huizar, J. Álvarez Lozano, C. Rentaría Muñoz, M.Á. Flores Gómez
    179-0
    Abstract:
    In this work, we employed the Density Functional Theory to determine thermodynamic properties of isotropic and nematic phases of liquid crystals, based on a ``Convex Peg'' model. The Helmholtz-free energy of the molecular system, a convex hard core within an encircling spherical square-well (SW) potential, is obtained from a second-order perturbation theory for SW nonspherical particles, combined with the Parsons decoupling approximation of the translational and rotational degrees of freedom, and a long-range approximation for the evaluation of the perturbation terms. The theory is applied to predict the phase diagram and isotropic-nematic transition of p-azoxyanisole (PAA), 4 -- pentyl -- 4' - cyanobiphenyl (5CB) and 4 - 4'- bis (heptiloxy) azoxybenzene (HOAOB). To do this, an estimation of the volume and k of a Liquid Crystal molecule was derived using IPCM calculations; the volume obtained is mapped into a hard ellipsoid revolution volume of a Convex Peg molecule. A very good agreement in the prediction of the thermodynamic properties is obtained when they are compared with experimental data.
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  • The binding energy of light excitons in spherical quantum dots under hydrostatic pressure

    C.A. Moscoso-Moreno, R. Franco, J. Silva-Valencia
    189-0
    Abstract:
    We study the effects of hydrostatic pressure over the ground state binding energy of light hole excitons confined in $GaAs-Ga_{1-x}Al_{x}As$ spherical quantum dots. We applied the variational method using $1s$-hydrogenic wavefunctions, in the framework of the effective mass approximation. We computed the exciton binding energy as a function of the dot radius, $Al$ concentrations and pressures. Our results show that (i) the hydrostatic pressure increases the binding energy, for all quantum dot radii; (ii) the binding energy is an increasing function of the $Al$ concentration, for fixed radius and pressure, especially for a smaller dot; (iii) the binding energy follows approximately a linear dependence with the pressure, for fixed radius and $Al$ concentration.
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  • Bimetallic Pd-Pt films prepared by MOCVD

    R. Martinez Guerrero, E.G. Palacios, J.R. Vargas Garcia, V.F. Santes Hern, ez., A.B. Soto Guzman, E. Ramirez Meneses
    194-0
    Abstract:
    Bimetallic Pd-Pt films have been prepared on amorphous planar substrates by MOCVD using metal-acetylacetonate precursors. The effects of the MOCVD experimental conditions on the surface morphology, crystallite size and chemical composition of the bimetallic films have been analyzed. X-ray diffraction results show that bimetallic films consist of Pd-Pt solid solutions of variable composition depending on the initial metallic content in the inlet gases. Thus Pd-rich films can be obtained as preferred for the catalysts industry. The surface morphology of the bimetallic films varied from smooth to rough surfaces with the increase in total pressure. The crystallite size in the bimetallic films varies from 40 to 100 nm. Therefore, the MOCVD technique could be considered to be a viable alternative route for the preparation of bimetallic Pd-Pt films, which are in demand in the technology of supported bimetallic catalysts.
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  • On the Application of the numerical Laplace transform for accurate electromagnetic transient analysis

    P. Gómez Zamorano, F.A. Uribe Campos
    198-0
    Abstract:
    This work presents an overview of a methodology based on the Numerical Laplace Transform (NLT) and applied to the analysis of electromagnetic transient phenomena in power systems. The basic development of the method is described, with its main qualitative advantages as compared to conventional time domain methods, such as the method of characteristics and professional programs for transient simulation such as EMTDC and ATP/EMTP. Current practices for reducing errors derived from the truncation and discretization of the analytical equations are also discussed. Finally, some important results obtained recently with this tool are shown. Comparisons with time domain methods reveal a high accuracy of the Numerical Laplace Transform in several studies.
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  • Effect of particle size and Mg content on the processing parameters of Al-Si-Mg/SiC$_{p}$ composites processed by pressureless infiltration

    J.A. Aguilar-Martínez, M.B. Hernández, J. Castillo-Torres, M.I. Pech-Canul
    205-0
    Abstract:
    The effect of the following processing parameters, namely SiC particle size (20 and 75$\mu $m) and Mg content (3 and 6 wt.%), on residual porosity and on the degree of infiltration for SiC$_{p}$ preforms by Al-Si-Mg alloys were investigated and quantified (here the subscript p means particles). The contribution of each of these parameters to the infiltration mechanism was determined employing an analysis of variance (ANOVA), and the effect of the levels of each parameter was examined using a surface response analysis. ANOVA results show that particle size is the parameter that most significantly affects the degree of infiltration and residual porosity. A surface response analysis shows that the degree of infiltration increases with an increase in magnesium content. The increasing rate in the degree of infiltration is higher for 20~$\mu $m than for 75 $\mu $m of SiC, whereas residual porosity diminishes with decreasing particle size; these two effects occur for both Mg concentrations in the alloy. Accordingly, the optimum parameters for a maximum degree of infiltration as well as for a minimum residual porosity are 20~$\mu $m SiC powders and 6 wt.% Mg in the alloy.
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  • Instrumentación electrónica de la función potencial tipo Sombrero Mexicano

    I. Campos-Cantón, E. Campos-Cantón, L.F. Villanueva Ortega
    210-0
    Abstract:
    We present in this work the electronic implementation of Mexican Hat potentials using the Mexican Hat function. The construction of this potential function (Mexican Hat) is accomplished using integrated circuits and passive elements.
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  • Sound speed resolved by photoacoustic technique

    S.J. Pérez Ruíz, S. Alcántara Iniesta ; P.R, R. Castañeda-Guzmán
    213-0
    Abstract:
    In this work a new method for measuring the speed of sound in materials is reported. This method uses the photoacoustic effect, which is the generation of sound waves by pulsed optical radiation incident on a material sample. The sound waves generated on the surface of the sample travel through the material and are detected with two piezoelectric sensors separated by a known distance. An appropriate processing of the photoacoustic signal permits the separation of the information of the generated longitudinal waves, of their reflections, as well as of other types of waves generated (shear, surface, etc). The advantages and disadvantages, of this method are discussed in comparison with standard methods.
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  • A 540$\mu $T$^{ - 1}$ silicon-based MAGFET

    M.A. Dávalos-Santana, F. S, oval-Ibarra., E. Monto, a-Suárez.
    218-0
    Abstract:
    This paper describes an MOS transistor-based transducer used for measuring magnetic fields. The setup, the electric/magnetic characterization, and an equivalent circuit for transistor level simulations are presented. The sensor (also called MAGFET), designed in a 1.5$\mu $m CMOS process, presents a relative magnetic sensitivity S$_{r}$=540$\mu $T$^{ - 1}$ at room temperature.
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  • Power dissipation in a capacitive coupled 450 khz discharge set up for a CO$_{2}$ laser

    J. de la Rosa, J. Corredor, J. Yaljá, F. Gallegos, P.A. Calva
    222-0
    Abstract:
    In order to find the power dissipation in a 450 kHz capacitive coupled CO$_2 $ laser system using a 50 $\Omega$ $\pi $ matching network, we investigate its electrical behavior. Current and voltage evolution and the phase delay in all the circuit elements were measured with calibrated Rogowski coils and high voltage probes. The signals were registered using a digital oscilloscope interfaced to a PC. When the supplied power lay between 0.5 and 1 kW, the signals had a non-distorted sinusoidal form. This allowed to make the power dissipation estimation in the circuit elements using their current and voltage peak values, and their phase delay. Phase measurements were made with $\pm $ 0.3$^{\circ}$ accuracy, which gives an accuracy of $\pm $ 1.5% in the laser chamber power dissipation estimation. The laser chamber power dissipation is estimated at between 30 and 50% of the supplied power, and the rest is consumed in the matching circuit. The power losses in the matching circuit greatly contribute to the poor total system efficiency ($\approx$ 0.8%). The estimated resistance and capacitance values of the laser chamber are also presented.
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