Vol. 52 No. 5 (2006): Revista Mexicana de Física.

Published: 2006-01-01

Articles

  • An ab initio study of platinum hydrogen interaction

    J.H. Pacheco, A. Bravo, O. Novaro
    394-0
    Abstract:
    Potential energy surfaces of the ground state $^{3}$D(5d$^{9}$6s$^{1}$) and the first excited state $^{1}$S(5d$^{10}$) of the Pt-H$_{2}$ interaction are calculated by using variational and perturbative MRCI and pseudopotential relativistic methodologies in order to obtain the distances and the energies of adsorption. Reaction mechanisms between Pt and H$_{2}$ are established. Hydrogen H$_{2}$ and oxygen O$_{2}$ are the fuels used in a commercial fuel cell, where the proper mechanism of hydrogen storage is actually a process that is complicated to find. Thus, theoretical calculations can be very useful in these designs. The potential energy surfaces involved in these processes are obtained, and the probability of transition between them is obtained by means of Landau-Zener Theory. Futhermore, the physisorption of the hydrogen molecule H$_{2}$ on platinum Pt metal atom, and the chemisorption of the two hydrogen atoms on Pt are obtained and explained.
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  • Performance of a petal resonator surface (PERES) coil via equivalent circuit simulation

    A.O. Rodríguez, S.E. Solís, M.A. López, M.C. Mantaras, S.S. Hidalgo
    398-0
    Abstract:
    MRI coil parameters can be estimated via simulation using an equivalent RLC circuit to investigate coil performance. The Spice Opus simulator was used to simulate the loss return coefficients of a circular-shaped coil and a petal resonator surface (PERES) coil via equivalent (RLC) circuit. Simulated coefficient spectra were obtained and compared with experimentally-acquired spectra generated by both coils. From these spectra, resonant modes and quality factors of both coil prototypes were computed at 64 MHz and compared. Impedance and resonant frequency of the 8 petal-PERES coil design were computed and compared against those obtained with the circuit simulation. PERES coil design produced an impedance value of 54 $\Omega$, and an experimental resonant frequency differing by less than 1% from that predicted by the circuit simulator. The quality factor of the coil prototype differs by only 8% from that obtained with the simulation method. Due to construction imperfections in the coil design, it showed a drop of 8.84 dB in attenuation compared with the simulation results obtained with the aid of an equivalent circuit. This scheme may serve as an alternative to the trial-and-error method usually used to develop dedicated RF coils for magnetic resonance imaging.
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  • Elastic electron scattering by water molecules

    J.L.S. Lino
    404-0
    Abstract:
    We report an application of the Schwinger variational principle with plane waves as a trial basis set. Differential cross sections are obtained for e$^-$- H$_2$O from 10 to 50~eV. In these studies the exchange potential is evaluated by the Born-Ochkur model and our results are found to be in reasonable agreement with experimental data and theoretical studies.
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  • Cobalt nitride films produced by reactive pulsed laser deposition

    W. De La Cruz, O. Contreras, G. Soto, E. Perez-Tijerina
    409-0
    Abstract:
    The nitrides of magnetic metal are becoming important due to their potential technological applications. In this work cobalt nitride thin films are deposited by reactive pulsed laser deposition (nitrogen environments) on silicon substrates at room temperature. The resultant films are characterized in-situ by Auger, X-Ray Photoelectron and Electron Energy Loss Spectroscopies. The chemical bond of the CoN$_{x}$ is strongly linked to the stoichiometry, and it can be controlled by the N background pressure. We conclude that this deposition method offers a means for fine-tuning the properties of cobalt nitride.
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  • Calibración de polvo TLD-100 para energías de $^{60}$Co, $^{137}$Cs, $^{192}$Ir y RX de 250, 50 kVp en dosis absorbida en agua con fines de control de calidad dosimétrico para braquiterapia de alta tasa de dosis

    S.P. Loaiza Calderón, J.T. Álvarez Romero
    413-0
    Abstract:
    To help solve the traceability and quality control dosimetric problems for the users of $^{192}$Ir sources in the Mexican Republic, the Secondary Standard Dosimetric Laboratory at ININ to calibrated a batch of powder TLD-100 (LiF:Mg,Ti) in terms of absorbed dose to water D$_{w}$ for the following radiation sources: $^{60}$Co, $^{137}$Cs and RX 250 and 50 kVp. Later on, the calibration is interpolated to obtain the $^{192}$Ir. The calibration radiation field is carried out with the following protocols: For the $^{60}$Co, OIEA TRS 398 protocol employing a secondary standard Farmer chamber PTW N30013, calibrated on D$_{w}$ at the NRC (Canada). For $^{137}$Cs the AAPM TG 43 protocol is used, in terms of air kerma strength S$_{k}$ determined by the air kerma K$_{a}$ measured with a secondary standard chamber type thimble NE2611 traceable to the NIST (USA). For Rays X 250 and 50 kVp, the protocol AAPM TG 61 using a tertiary standard Farmer chamber PTW 30001, with traceability to the LCIE (France) on air kerma K$_{a}$. The calibration curves are built for the TLD response R$_{TLD}$ vs D$_{w}$, they are fitted by means of a least squares fit technique with a second degree polynomial that corrects the supralinearity response. The curves are validated by the lack of fit test, and the Anderson Darling normality test. Later on, the sensibility factors are interpolated for the sources of $^{192}$Ir: MicroSelectron and VariSource. Two capsules are sent to two hospitals to verify a nominal D$_{w }$= 2 Gy, in the first one an underestimate of the D$_{w}$ is obtained, and in other one an overestimation is presented. Finally, the expanded uncertainty associated to D$_{W}$ and the F$_{s}$ are calculated.
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  • On the morse potential in liquid phase and at liquid-vapor interface

    U.F. Galicia-Pimentel, D. Osorio-González, J. López-Lemus
    422-0
    Abstract:
    Canonical Molecular Dynamics simulations have been performed to calculate thermodynamic properties in the liquid phase and at the liquid-vapor interface for fluids interacting by Morse potential. Transport properties such as self diffusion and shear viscosity have been calculated in one phase. Self diffusion shows an important dependence on particle number whereas shear viscosity does not show such dependence. At the liquid-vapor interface, properties such as orthobaric densities, vapor pressure, and surface tension were calculated. Equilibrium densities were compared with results obtained by NpT plus test particle method, and an excellent agreement was found. The surface tension and the vapor pressure are computed for the first time in this work. We also analyzed the cut-off distance dependence in both bulk and interfacial properties. No significant difference was found in the data obtained when two different cut-off distances were used, $R_c=2.5\sigma$ and $R_c=4.0\sigma$. This is a consequence of the short-range nature of the potential.
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  • Hamiltonians and Lagrangians of non-autonomous one-dimensional mechanical systems

    G.F. Torres del Castillo, I. Rubalcava García
    429-0
    Abstract:
    It is shown that a given non-autonomous system of two first-order ordinary differential equations can be expressed in Hamiltonian form. The derivation presented here allows us to obtain previously known results such as the infinite number of Hamiltonians in the autonomous case and the Helmholtz condition for the existence of a Lagrangian.
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  • Cinética de la transformación de fase \pmb{$\alpha +\eta \to \beta $}, por microscopía electrónica de transmisión, de una aleación eutectoide Zn-Al solidificada rápidamente

    A. S, oval Jiménez., J. Negrete Sánchez, G. Torres Villaseñor
    433-0
    Abstract:
    By using transmission electron microscopy (TEM) technique was established the phase transformations kinetic of a Zn-Al eutectoid alloy solidified rapidly. The phase transformations experiments were realized and filmed in situ, at heating and cooling, into a transmission electron microscope. The kinetic was established considering only the Johnson-Mehl mathematical model, $y = 1 - e^{ - kt^n}$, where $y$ is the fraction transformed, $k $is the rate constant reaction and $n$ is the reaction order; $y$ was determined by measuring area increase of the high temperature $\beta $ phase, with regard to time, at video images of phase transformations; $n$ and $k$ are functions of $y$ and $t$.
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  • On the scaling properties of the total $\gamma^*\mathrm{p}$ cross section

    M.N. Mondragon, J.G. Contreras
    438-0
    Abstract:
    We perform a detailed analysis on the scaling properties of the total $\gamma^*\mathrm{p}$ cross section, $\sigma_{\gamma^*\mathrm{p}}$. We write the cross section as a product of two functions $W$ and $V$ representing, respectively, the dynamical degrees of freedom and the contribution from the valence partons. Analyzing data from HERA and fixed target experiments, we find that $V$ is independent of $Q^2$ and concentrated at large $x$, while $W$ carries all the information on the $Q^2$ evolution of $\gamma^*\mathrm{p}$. We define the reduced cross section $\tilde{\sigma}_{\gamma^*\mathrm{p}} \equiv W=\sigma_{\gamma^*\mathrm{p}}/V$, and show that it is very close to a generalized homogeneous function. This property gives rise to geometric scaling for $\tilde{\sigma}_{\gamma^*\mathrm{p}}$ and it also explains the known geometric scaling of $\sigma_{\gamma^*\mathrm{p}}$ at low $x$. As a consequence of our {\em Ansatz}, we also obtain a compact parameterization of $\sigma_{\gamma^*\mathrm{p}}$ describing all data above $Q^2=1$~GeV$^2$.
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  • Reduction of friction in fluid transport: experimental investigation

    G.Aguilar., K. Gasljevic, E.F. Matth, s.
    444-0
    Abstract:
    Drag reduction (DR) by the use of polymer and surfactant solutions is by far the most effective drag-reducing technique for turbulent flows (up to 8-fold reduction in friction coefficients is possible on straight pipes). From a fundamental point of view, the study of the DR phenomenon offers an opportunity for a better understanding of turbulence in general; from a practical point of view, DR can be used to save pumping power. Commercial implementation of drag-reducing fluids has proved successful for oil pipeline transportation, and looks promising for many other applications that are still under investigation, $e.g.$ district heating or cooling systems, hydronic systems in buildings, sewers, irrigation, industrial processes, etc. Our efforts have focused on two main areas: (A) experimental research on momentum and heat transfer of turbulent flows of drag-reducing solutions, and (B) implementation of these solutions in hydronic cooling systems in buildings for energy conservation purposes. This paper describes an overview of the typical experimental research that we conduct in our non-Newtonian fluid mechanics, rheology, and heat transfer laboratory at UCSB.
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  • Analysis of integrated optics elements based on photonic crystals

    I. Gur, ev., I.A. Sukhoivanov, S. Alej, ro-Izquierdo., M. Trejo-Durán, J.M. Estudillo-A, ala., J.A. Andrade-Lucio, E. Alvarado-Méndez
    453-0
    Abstract:
    We present the design of a demultiplexer based on photonic crystals (PC). The control of the refractive index, periodicity, geometry and size of the structure, are very important to light propagation in PC. The demultiplexer is intended to separate pulse channels with 1.3 and 1.55~$\mu$m wavelength into wide pass band wavelength division multiplexing (WDM) optical telecommunications systems. The dependence of the band gap width on the refractive index $\triangle n$ between background and road lays is shown. The results of FDTD numerical simulation of wavelength channel splitting and the spectral analysis for the maximum efficacy of transfer energy are also presented.
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  • Influence of the carrier gas in the growth kinetics of TiO$_{2}$ films deposited by aerosol assisted chemical vapor deposition with titanium-diisopropoxide as precursor

    A. Conde-Gallardo, M. Guerrero, A. B Soto, R. Fragoso, N. Castillo
    459-0
    Abstract:
    Titanium dioxide thin films were deposited on crystalline silicon (100) substrates by delivering a liquid aerosol of titanium-diisopropoxide and by using oxygen and nitrogen as carrier gases. The crystalline and morphological features indicate that the films are deposited by a metal organic chemical vapor deposition process. This is strongly supported by the behavior of the growth rate r$_{g}$ as a function of the deposition temperature, which indicates that the film formation is limited by both, the diffusion in gas phase of the precursor species to the surface substrate and reaction of those species at that surface. Even though the r$_{g}$ line shape does not depend on the kind of carrier gas used to transport the aerosol, its absolute value and the activation energy E$_{A}$ that characterizes the surface reaction do. A fitting procedure to an equation that takes into account both limiting mechanisms (gas phase diffusion and surface reaction) yields: E$_{A} \cong $26.4 kJ/mol or E$_{A}$=21.4~kJ/mol when oxygen or nitrogen is employed as carrier gas, respectively.
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  • Caracterización de un mensajero caótico binario con ruido en el canal: simulación y experimentación

    R. Núñez-Pérez
    464-0
    Abstract:
    A binary chaotic messenger is characterized, based on parameters' commutation, using simulation and experimental procedures to explore his real encrypted communication capacity. The characteristics to find out are: a) Parameter minimum change in such a way the change, of synchronization chaotic signal, be invisible to some spy at the noisy channel, and b) Noise maximum level and c) Time delays allow just to make reliable the binary information recovery. Because, the noisy channel allow hardly the messenger studied right performance, the solution is through a new digital chaotic messenger
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  • Instrumentación virtual de un pulsioxímetro

    I. Campos-Cantón, L.A. Martínez Garza, V. Vinaja Nuño, P.C. Rodríguez López
    474-0
    Abstract:
    A pulse Oximeter is developed using virtual instrumentation. This instrument is useful to analyze the oxygen saturation percentage in blood (SpO$_{2})$ without invasion. This approach measure blood's oxygenation based on illuminate with red and infrared light and sense the absorption of the two wavelength due to pass to hemoglobin. The instrumentation is based in the LabView's software of National Instruments Corporation and the 306-003549 sensor of CSI Company. This system of development has the advance of work in any pc with low cost.
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