Vol. 49 No. 6 (2003): Revista Mexicana de Física.

Published: 2003-01-01

Articles

  • Superconducting transmission 23kV/2kA cable -- first in Latin America

    M. Jergel, A. Morales, C. Falcon, V. S, tnikov., P. Dolgosheev, D.I. Bel, i., A. Sierra, A. Pérez, J.L. Nieto, A. González, M. Ma, a., F. Ortiz, .
    489-0
    Abstract:
    Two superconducting models of transmission cables were designed, constructed and tested for the first time in Mexico, the 1m model and 5m model with parameters of practical applications 23 kV/2kA. For this purpose also a new cryogenic laboratory was built and equipped with all necessary devices and instruments. For construction of both models, a high critical temperature superconducting Bi-Sr-Ca-Cu-O tape was used. The design, construction and performed tests are briefly described and some preliminary results of test measurements are presented.
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  • Existencia y perturbación de solitones ``embebidos'', gobernados por una extensión de la ecuación NLS

    A. Espinosa-Cerón, J. Fujioka, A. Gómez-Rodríguez
    493-0
    Abstract:
    We determine the conditions for the existence of ``embedded solitons'' (ES), and conventional bright and dark pulses, in an extension of the cubic nonlinear Schrödinger (NLS) equation with higher-order dispersive and nonlinear terms. The stability of these SE is studied numerically, and it is found that these solitons are semi-stable. The damped oscillatory behavior of the perturbed SE is then analyzed by a variational method, and it is shown that this damping is a consequence of the emission of radiation. Finally, it is shown that the uniqueness of these SE is due to a delicate balance between nonlinearity and dispersion.
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  • Estructura (4$\times $3) inducida por la adsorción de los metales del grupo III sobre la superficie (001) del silicio

    J. Cotzomi-Paleta, G.H. Cocoletzi, N. Takeuchi
    506-0
    Abstract:
    A comparative study of the (4$\times $3) structure induced by the adsorption of group III metals (In, Al and Ga) on Si(001)-(4x3) is performed using first principles total energy calculations. The study corresponds to the deposition of metals at high temperature. Different models have been used to investigate the surface reconstructions and it has been found that the one proposed by Bunk et al. yielded the most stable configuration for all three cases. The structure showed the formation of pyramid-like metal-subunits with a trimer on top of it. This is composed of a Si atom occupying the highest vertical position and bound to two metal atoms with lower vertical positions. For the case of In, calculated bond lengths according to Bunk's model are similar to the sum of covalent radii. This is not the case of Al and Ga, explaining the experimental results showing that the (4$\times $3) structure remains as the stable structure at full In coverage, while for Ga and Al become (8$\times $5) and c(4$\times $2n), respectively.
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  • Circular ultrasonic transducer characterization: theoretical and experimental results

    L. Medina, E. Moreno, G. González, L. Leija
    511-0
    Abstract:
    Acoustic pressure fields generated by pulsed ultrasonic transducers under different boundary conditions are analyzed. Numerical simulations of the near-field pressure were evaluated considering rigid and soft baffles as boundary conditions. These field simulations were perfomed using the temporal convolution between the numerical derivative of the impulse response and the longitudinal wave velocity for both cases. Experimental pressure data were obtained by measuring the peak, peak to peak and root mean squared voltages. Simulated and experimental results were compared to investigate the temporal behavior of the acoustic signal as well as their spatial distribution on planes parallel to the transducer face. Special attention is given to the Fresnel region where the diffraction effect affects the pressure field measurements. Experimental readings were done using circular transducers with the same geometric characteristics and with resonant frequencies of 3.5~MHz and 5~MHz.
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  • Optical properties and Judd-Ofelt intensity parameters of Eu\pmb{$^{3 + }$} in PMMA:PAAc copolymer samples

    R. Sosa F., M. Flores H., R. Rodríguez T., A. Muñoz F.
    519-0
    Abstract:
    Eu$^{3 + }$-activated PMMA:PAAc copolymers samples with different concentrations of Eu$^{3 + }$ ions were prepared and the spectroscopic properties investigated. The mass density, refractive index, UV--Visible absorption and emission spectra and fluorescence decay times were experimentally obtained. These data were used with the Judd-Ofelt theory to calculate the oscillator strengths, the spontaneous emission probabilities and the branching ratios as a function of the europium content; the Eu concentration was varied in the range from 0.1 to 3 mol %. The europium ions affect the polymerization mechanism since their presence produced a steric effect, which reduces the reactivity of the acrylic acid. We analyzed the optical interaction mechanism between the RE$^{3+}$ and the polymeric matrix, which is of radiative type.
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  • Three-wave mixing with high amplification in a photorefractive polymer

    A. Apolinar Iribe, R.F. Domínguez Cruz
    525-0
    Abstract:
    We demonstrate that in photorefractive polymer by crossing a strong and a weak beams one can achieve high intensity gains when the angle between them is small. The amplification of the weak wave is accompanied by the formation of the third beam produced by the strong beam diffraction into the --1 order. The gain obtained by the mechanism of three-wave mixing are much higher than the gains produced by earlier reported mechanisms. The experimental results are agree with the theoretical results obtained with the earlier for photorefractive crystals.
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  • Dielectric permittivity and AC conductivity in polycrystalline and amorphous $\pmb{C_{60}}$

    J. Ortiz-López, R. Gómez-Aguilar
    529-0
    Abstract:
    The dielectric permittivity and AC conductivity of polycrystalline and amorphous $C_{60}$ samples were measured at temperatures between 75 and 300 K and frequencies in the range 100 Hz to 1 MHz. For polycrystalline samples we observe effects caused by $O_{2}$ molecular oxygen intercalation because prolonged exposure to ambient air. The conductivity $\sigma$ of these samples around 300 K depends on the measuring frequency $\nu$ as $\sigma$ $\sim$ $\nu^{n}$ with n $\approx$ 1 implying a strong reduction of DC conductivity to less than $10^{-12}$ S/cm. Dielectric permittivity in polycrystalline samples shows an anomaly around 258 K due to its order-disorder phase transition and dielectric relaxation phenomena is observed in the range 130-200 K with an activation energy of 0.237 eV. In contrast with the polycrystalline samples, the amorphous $C_{60}$ samples prepared by sublimation do not contain interstitial $O_{2}$, their DC conductivity at 300 K is of about $10^{-6}$ S/cm, is independent of frequency, and is well described by the hopping mechanism (Davis-Mott $T^{1/4}$ law) in the 200-300 K range. All evidence of phase transitions and/or dielectric relaxation disappears in the amorphous samples.
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  • Changes in the physical and chemical properties of PVDF irradiated by 4 MeV protons

    E. Adem, J. Rickards, E. Muñoz, G. Burillo, L. Cota, M. Avalos-Borja
    537-0
    Abstract:
    The behavior of poly-vinylidene fluoride (PVDF) under bombardment of different kinds of radiation is of interest due to the polymer's unique piezo-electric properties and various crystalline forms. In this work, PVDF film samples of 0.3 mm thickness and density 1.76 g/cm$^3$ were irradiated with 4 MeV protons from the Instituto de Física 9SDH-2 Pelletron Accelerator. Changes in the physical and chemical properties were investigated using Fourier Transform Infrared Spectroscopy (FTIR) with ATR, X-ray Diffraction (XRD), Differential Scanning Calorimetry (DSC) and gel fraction measurements. The gel percent increases with fluence to almost 60% for a fluence of 10$^{13}$ cm$^{-2}$, and then more slowly up to 100% for a fluence of 3 $\times$ 10$^{14}$ cm$^{-2}$. The DSC curve of the control sample shows a sharp band at 176 $^\circ$C with a shoulder on the low temperature side. A well defined lower temperature peak, related to the relaxation of inter-crystalline regions appears at a fluence of 10$^{13}$ cm$^{-2}$. At 3 $\times$ 10$^{14}$ cm$^{-2}$ there are no melting peaks, due to complete destruction of the crystalline structure. This was confirmed at this high fluence by the FTIR spectra, and by x-ray diffraction, where an amorphous structure was observed.
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  • Sensor distribuido para la detección de esfuerzos de umbral mediante rejillas de Bragg

    I. Márquez Borbón, M.G. Shl, agin., S.V. Miridonov, V.V. Spirin, R.M. López, E.A. Kuzin, M. Ma, Alarcón.
    542-0
    Abstract:
    For early warning and alarm systems, it is required to detect appearance of the monitored parameter, such as temperature or strain, within some predetermined interval. A system must generate an alarm signal when the monitored parameter approaches near or crosses the limit of the dangerous zone. For such applications, we propose a simple distributed sensor based on a large array of equal Bragg gratings. An interrogation of the sensor is performed at a fixed wavelength. We discuss a principle of operation of the sensor and present results of experimental demonstration of this technique for distributed detection of excess strain.
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  • Análisis numérico de la prueba de la estrella

    O. Totolhua Paleta, F. Renero Carrillo, F. Gracia Témich, A. Cornejo Rodríguez
    549-0
    Abstract:
    In this paper, we present one method of numeric analysis for the star test with MatLab (ANPEM), applied to reflecting telescope. ANPEM finds the difference between centers of the images of the annulus obtained with a Newtonian telescope and a Ritchey Chretiêne telescope with 14 and 211 cm of diameter respectively. ANPEM allows to identify quantitative the symmetry of the annulus for each reflector telescope with a central obstruction. ANPEM finds the centres of the annulus with diference between 12 to 60$\mu $m compared with the ideal numerical differences calculated with PC for the Newtonian telescope.
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  • Medición del módulo de Young en el hule látex usando ESPI

    J.A. Ra, as., R. Rodríguez-Vera, A. Martínez
    555-0
    Abstract:
    It is well know that the module of elasticity or Young's module relates the stress to the strain that suffers a object, which is yielded to external loads. In this work we show the implementation of the Electronic Speckle Pattern Interferometry (ESPI) as a technique to measure deformations on a piece of latex when it is subjected to external loads. Along with the measurement of tension loads (by means of a digital dynamometer), we determine the Young's module of the object under test. The value determined by using the ESPI technique is compared with that obtained in a device designed and constructed in our laboratory (which is equivalent to a commercial testing machine). It is demonstrated the veracity of the ESPI technique applied as an interferometric extensometer.
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  • Método alternativo para evaluar la velocidad instantánea en un laboratorio de docencia, mediante el uso de nuevas tecnologías

    S. Robles G., H. Villa M., E. Álvarez R., I. Morales F.
    565-0
    Abstract:
    In these days, new technological tools are driving changes in most of the disciplines forcing a revision in the traditional processes of teaching and learning and experimental physics is not out of this trend. In this work, we are reporting a computational tool to measure the instant speed allowing us to develop a set of experiments about kinematic in one and two dimensions, about dynamic and about mechanical energy. The program has been tested with positive results during the last two academic periods in the classical mechanics courses we taught and represents one of the main results in the revision of the experimental guide for the laboratory of the classical mechanics courses of the Physics Department at Universidad de Sonora.
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