Vol. 58 No. 6 (2012): Revista Mexicana de Física.

Published: 2012-01-01

Articles

  • Application of nanosize polycrystalline SnO$_{2}$-WO$_{3}$ solid material as CO$_{2}$ gas sensor

    S. B, S. S, S.B.Unhale., S.P. Yawale
    445-450
    Abstract:
    Among binary metal oxide sensors, tin oxide based sensors have received more attention, due to its high reactivity to many gases. The nanosize SnO$_{2}$-WO$_{3}$ polycrystalline solid material of different mol% was prepared to form thin films by screen printing method. The sensitivity of the films is measured for different concentration of CO$_{2}$ gas at room temperature (303K). The variation of sensitivity with CO$_{2}$ gas concentration is found to be linear. The sensor 40SnO$_{2}$-60WO$_{3}$ shows maximum sensitivity in presence of CO$_{2}$ gas. The static and dynamic response of the sensors was studied. The reproducibility and stability for all sensors was good. The average crystallite size was calculated from XRD spectra and found to be of the order of 17.11 to 17.17 nm except pure WO$_{3}$ film has 22.91 nm.
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  • Adsorción e incorporación de Cu en la superficie GaN(0001)

    J. Alberto Nieto, D. Alej, ro Rasero., C. Ortega López
    451-458
    Abstract:
    In this work we report first principles calculations to analyze the copper adsorption and incorporation on GaN (0001) surface, and on the deeper layers of a GaN slab in a 2$\times$2 geometry. The calculations were performed using the plane-wave pseudopotential method within the framework of the density functional theory (DFT). In the description of the electron-electron interaction is used generalized gradient approximation (GGA). To study the most favorable Cu adsorption model we considered T1, T4, H3 and Br special sites. We find that the most energetically favorable structure corresponds to the 1Cu-H3 model, while the Cu adsorption on top of a Ga atom (T1 position) is totally unfavorable. Furthermore, in the Cu incorporation on the deeper layers of the studied slab, we found that these impurities prefer substitutional sites of Ga in the upper bilayers of slab, indicating that they are unlikely to Cu migrate into the volume of GaN. Finally is analyzed the density of states with and without ad-atoms of Cu.
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  • TiO$_{2}$ and Al$_{2}$O$_{3}$ ultra thin nanolaminates growth by ALD; instrument automation and films characterization

    H. Tiznado, D. Domínguez, W. de la Cruz, R. Machorro, M. Curiel, G. Soto
    459-465
    Abstract:
    We report on the development of a fully operational atomic layer deposition (ALD) system. This system is computer-controlled and can deposit multilayered systems without user intervention. We describe the design of manifold, reaction chamber and exhaust. Additionally we give some features of the automatization software and electronics. To evaluate the ALD performance we used as precursor trymethyl aluminum (TMA) and tetrakis (dimethylamino) titanium (TDMAT) to deposit Al$_{2}$O$_{3 }$ and TiO$_{2}$, respectively, in nanolaminated film structures. The thicknesses and composition of the films are precisely controlled, as determined by spectroscopic ellipsometry, and the nanolaminates have a sharp interface as indicated by Auger depth profile.
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  • Simulación atomística de la fractura dinámica en un material frágil

    J. Aldaco, M. Hinojosa
    466-471
    Abstract:
    In this work molecular dynamics simulations (MD) of a model of uniaxial tension at were made rates of deformation ($\dot\varepsilon \approx$ $3.9\times 10^{11}{\mathrm { s}}^{ - 1}$), that allowed the generation and the characterization of the dynamic fracture in a previously cracked ideal brittle nanomaterial (silicon). These simulations generated crack propagation at an average velocity of $\approx 2$ km/s with characteristics similar to those observed in experiments at the macroscopic scale, in other simulations (and theory). Some of these similarities were the brittle propagation in the cleavage planes (110) and (111), the influence of the stress tensor in the crack path, average and instantaneous values of the crack speed and the morphology of the simulated samples; this last qualitative similarity with the fracture surfaces obtained in experiments on a macroscopic scale evokes the concept of self-affinity of the surfaces.
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  • Encrypted audio transmission via synchronized chaotic Nd:YAG lasers

    L. Cardoza-Avendaño, R.M. López-Gutiérrez, C. Cruz-Hernández, V.V. Spirin, R.A. Chávez-Pérez, A. Arellano-Delgado
    472-480
    Abstract:
    In this paper, encrypted audio transmission via synchronization of coupled chaotic Nd:YAG lasers in master-slave configuration is numerically studied. In particular, we resort to recent results from complex systems theory to achieve chaos synchronization. So that, enconding, transmission, and decoding of confidential audio messeges in chaotic optical communications are presented. In addition, we show transmission of encrypted audio messages when parameter mismatch and channel noise are considered. In order to illustrate this robustness synchronization property, we present the encrypted transmission of audio messages via an improved chaotic transmitted signal, and we obtain at the receiver that, the original audio message is appropriatelly recovered.
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  • La ecuación Seno-Gordon perturbada en la dinámica no lineal del ADN

    A.H. Salas, J.E. Castillo.
    481-487
    Abstract:
    We make use of a linear chain of coupled oscillators to construct the Lagrangian in order to give a first approximation for a nonlinear dynamic model of the DNA molecule through the perturbed sine-Gordon equation. By using semi analytic methods, we give exact solutions to perturbed sine-Gordon equation in a travelling wave form. These solutions have some relevant implications in the study of DNA open states. Some of these solutions are new in the open literature.
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  • Análisis de activación neutrónica y actividad en el acero de la vasija de un reactor nuclear tipo BWR para su estudio sin riesgos radiológicos en microscopía y espectrometría

    M. Moranchel, A. García Bórquez, L. C
    488-496
    Abstract:
    The vessel material of nuclear reactors is subject to irradiation damage induced by the bombardment of neutrons coming from the reactor core. Neutrons are classified as fast and thermal, which produce different effects. Fast neutrons cause damage to the material by dislocation or displacement of atoms in the crystal structure, while the effect of thermal neutrons is a nuclear transmutation that can significantly change the properties of the material. The type and intensity of damage is based on the characteristics of the material, the flow of neutrons and the modes of interaction of neutrons with the atomic structures of the material, among others. This work, alluding to nuclear transmutation, makes an analysis of neutron activation of all isotopes in a steel boiling water nuclear reactor (BWR) vessel. An analytical expression is obtained in order to model the activity of steel, on the basis of the weight percentage of its atomic components. Its activity is theoretically estimated in a witness sample of the same material as that of the vessel, placed within the nuclear reactor since the beginning of its commercial operation in April 1995, up to August 2010. It was theoretically determined that the witness sample, with a 0.56 g mass (1$\times$1$\times$0.07 cm$^{3}$ dimensions or equivalent) does not present a radiological risks during the stage of preparation, observation and analysis of it in electron microscopy and X-ray diffraction equipments. The theoretical results were checked experimentally by measuring the activity of the sample by means of gamma spectrometry, measurement of the exposure levels around the sample, as well as the induced level to whole body and limbs, using thermo-luminescent dosimetry (TLD). As a result of the theoretical analysis, new chemical elements are predicted, as a result of the activation phenomena and radioactive decay, whose presence can be a fundamental factor of change in the properties of the vessel. This work is a preamble to the investigation of the damage of neutron irradiation on the steel of the vessel of boiling water reactors (BWR) using electron microscopy, spectroscopy and X-ray diffraction techniques.
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  • Aplicación del Método Dobson a la estimación del ozono total utilizando un radiómetro ultravioleta

    J. C, J. Salamanca
    497-503
    Abstract:
    A procedure to compute total ozone estimation by applying Dobson method, with a data set obtained from a global ultraviolet radiometer -GUV2511 Biospherical Instruments- is presented here. Dobson method is applied by using four different spectral lines in the UV-B range that are measured by the radiometer. This procedure involves a proposal for a calculation of direct -and diffuse-spectral radiation from a global spectral radiation measurement. The enhancement of this total ozone calculation model can also provide elements to study clouds behavior in tropical high mountain zones. In our case, it is affected furthermore by the intertropical confluence zone.
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  • Structural properties of WO$_{3}$ dependent of the annealing temperature deposited by hot-filament metal oxide deposition

    J. E, J. Díaz-Re, es., J. A
    504-509
    Abstract:
    In this work presents a study of the effect of the annealing temperature on structural and optical properties of WO$_{3}$ that has been grown by hot-filament metal oxide deposition (HFMOD). The chemical stoichiometry was determined by X-ray Photoelectron Spectroscopy (XPS). By X-ray diffraction obtained that the as-deposited WO$_{3}$ films present mainly monoclinic crystalline phase. WO$_{3}$ optical band gap energy can be varied from 2.92 to 3.15 eV obtained by transmittance measurements by annealing WO$_{3}$ from 100 to 500$^{\circ}$C. The Raman spectrum of the as-deposited WO$_{3}$ film shows four intense peaks that are typical Raman peaks of crystalline WO$_{3}$ (m-phase) that corresponds to the stretching vibrations of the bridging oxygen that are assigned to W-O stretching ($\nu$) and W-O bending ($\delta$) modes, respectively, which enhanced and increased their intensity with the annealing temperature.
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  • Optimization of a cubic equation of state and van der Waals mixing rules for modeling the phase behavior of complex mixtures

    J.A. Lazzús
    510-514
    Abstract:
    A thermodynamic modeling for the vapor--liquid equilibrium of binary systems of supercritical fluids and ionic liquids is presented. The van der Waals mixing rules and a cubic equation of state are used to evaluate the fugacity coefficient on the systems. Then, a particle swarm algorithm was used to minimize the difference between calculated and experimental bubble pressure, and calculate the interaction parameters for all systems used. The results show that the bubble pressures were correlated with low deviations between experimental and calculated values. These deviations show that the proposed model is a good technique to optimize the interaction parameters of the phase equilibrium of binary systems containing supercritical fluids and ionic liquids.
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  • Modelo de radio propagación a través de un canal con ruido Gaussiano

    José R. Fermín, Juan C. Pizarro
    515-520
    Abstract:
    We present a physical model of radio waves propagation through a channel with white Gaussian noise. The continuous channel is simulated using a dissipative linear model, which consists of a flexible string of uniform density and immersed into a thermal bath. The channel is characterized by signal to noise ratio, which is calculated as a function of temperature, frequency of the signal and the propagation distance. For some value of temperature, propagation distance and frequency, the SNR exceeds the threshold below which Shannon's error-free transmission is not possible.
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