Vol. 59 No. 6 Nov-Dec (2013): Revista Mexicana de Física.

Published: 2013-01-01

Articles

  • Frequency behavior of saturated nonlinear function series based on opamps

    E. Ortega-Torres, C. Sánchez-López, J. Mendoza-López
    504-0
    Abstract:
    In multiscroll chaotic circuit design based on active devices, piece-wise linear (PWL) approaches are often used to model the behavior of nonlinear functions, thereby that the behavior of a chaotic system can be forecasted through numerical simulations. However, although PWL models are relatively easy to build, they do not include any information related on the performance parameters of the active devices to be used. This a serious shortcoming, since PWL-models introduces a level of inaccuracy into a numerical analysis which is more evident when numerical simulations and experimental results are compared. These differences are more pronounced when the chaotic waveforms to be generated are pushed to operate at high-frequency. This paper introduces experimental results on the frequency behavior of a nonlinear function called saturated nonlinear function series based on operational amplifiers. These new results are key not only on the automatic synthesis of chaotic attractors and on the synchronization schemes used in secure communication systems based on chaos, but also on the metrics used to evaluate the complexity of a chaotic system. A mathematical model to characterize the behavior of the nonlinear function is also derived, showing a better accuracy compared with the PWL approach. The theoretical derivations and related results are experimentally validated through implementations from commercially available devices.
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  • Predicting the piezoresistance contribution of carbon nanotubes in a polymer matrix through finite element modeling

    A.I. Oliva-Avilés, V. Sosa, F. Avilés
    511-0
    Abstract:
    The change in electrical resistance due to mechanical deformation of carbon nanotube (CNT)/polymer composites can be rationalized in terms of two main effects: i) changes in the composite electrical resistivity due to changes in the CNT network configuration, and ii) deformation of the CNTs themselves. The contribution of CNT dimensional changes (ii) to the piezoresistivity of CNT/polymer composites is investigated here. A model based on a representative volume element which describes the CNT geometrical contribution to the composite electromechanical response (piezoresistivity) in terms of the CNT and matrix deformations is proposed. Finite element analysis is performed to correlate the macroscale composite strain to the individual CNT strain. The CNT geometric contribution to the piezoresistivity of the composite is quantified for a range of matrix elastic modulus and different CNT orientations. Based on the model predictions and previous experimental results, it is estimated that the contribution of the CNT deformation to the composite piezoresistivity is only about 5%, indicating that the dominant effect in the piezoresistivity of CNT/polymer composites is the change in the CNT network configuration.
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  • Study of plasma displacement and $\beta_{p}+l_{i}/2$ by the simplest Grad-Shafranov equation solution for circular cross section tokamak

    M. Asif
    517-0
    Abstract:
    In this work we present the plasma displacement and $\beta _p + l_{i}/2$ by the Simplest Grad-Shafranov Equation (GSE) solution using Solov'ev assumption for circular cross section HT-7 tokamak. Using diamagnetic and compensation loop, combining with poloidal magnetic probe array signals, plasma displacement and $\beta _p + l_{i}/2$ are measured. In this paper, theoretical and experimental results in determining plasma displacement and $\beta_p + l_{i}/2$ are presented. We have seen that the calculated plasma displacement and the calculated $\beta _p + {l_i }/{2}$ depend on the kind of discharge or plasma current.
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  • Propiedades magnéticas del sistema de aleaciones CuAl$_{1 - x}$Cr$_{x}$S$_{2}$ (x = 0.50, 0.75)

    M. A, P. Grima, M. Quintero, E. Moreno, J. Fernández, P. Silva, J. Villegas
    521-0
    Abstract:
    The synthesis, structural characterization and magnetic properties of alloy system CuAl$_{1 - x}$Cr$_{x}$S$_{2}$ (x = 0.50, 0.75) are reported. The samples were synthesized by using the direct fusion technique. The chemical analysis (EDX) confirmed the stoichiometric ratio for the concentrations. The powder diffraction patterns were indexed and the principal phases crystallizes with tetragonal symmetry with unit cell parameters\linebreak $a$ = 5.312(1) {\AA}, $c$ = 10.389(2) {\AA} for x = 0.50 and $a$ = 5.314(2) {\AA}, $c$ = 10.393(2) {\AA} for x = 0.75. Theses alloys behave as antiferromagnetic, with Néel temperatures of 37 K and 39 K, respectively. The EPR linewidth for these alloys shows a paramagnetic behavior between 100 and 610 K.The resonance field and the g factor show a slight variation with temperature. These results are discussed in terms of nearest-neighbor Cr$^{ + 3}$ (S=3/2) spin-coupled pairs.
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  • Una aproximación del criterio evolutivo de prigogine a sistemas químicos

    J.M. Nieto-Villar, E. Izquierdo-Kulich, R. Quintana, J. Rieumont
    527-0
    Abstract:
    Prigogine's evolution criterion is illustrated non-linear state nonlinear state in chemical reaction systems. It was proved that how at least in chemical reactions; the entropy production per unit time accords to the control parameters is in fact a Lyapunov function. This is indeed an evolution criterion in the nonlinear region and also the global stability is properly assured. Finally, it was shown that for fixed values of the control parameters, the system corresponding dynamical state exhibits a higher value of the entropy production per unit time, which can be considered in fact, an extension of the non-variational principle of maximum entropy.
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  • Aggregate structures of the sorbitan monooleate (SPAN80) surfactant at TiO$_{2}$(rutile)/water interfaces by computer simulations

    E. Núñez-Rojas, H. Domínguez
    530-0
    Abstract:
    Molecular dynamics simulations were carried out to study the behavior of a nonionic surfactant close to TiO$_{2}$ surfaces at three different crystallographic orientations of rutile, (001), (100) and (110). Sorbitan Monooleate (SPAN80) molecule was used as nonionic surfactant and it was observed that these molecules seemed to aggregate in similar ways. Namely, the hydrocarbon chains of the surfactant molecules were attached at the solid surfaces. Structure of the molecules and surfactant adsorption on the surfaces were studied in terms of tails and head groups density profiles as well as surface coverage. From density profiles and angular distributions it was possible to determine the influence of the solid surface. For instance, on the three surfaces the surfactant molecules formed molecular layers parallel to the surface. Besides, it was found that in the solids (100) and (110), where there were oxygen atoms exposed on the surface, surfactants were attached to the surfaces along the sites between the lines of these oxygen atoms. Finally, diffusion coefficients for the aggregates were calculated in order to determine molecular mobility on the surfaces. These results were compared with those of the Sodium Dodecylsulfate (SDS) molecules on the same rutile surfaces and it was found that the SPAN80 molecules were more attached to surfaces than the SDS molecules. On the other hand the diffusion coefficients calculated in the present work were also compared with those obtained in a SDS/graphite system and we observed that SDS molecules on graphite showed a significant mobility compared with the same molecules on rutile.
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  • Origin of dielectric relaxations in polycrystalline RbHSeO$_{4}$ above room temperature

    O. Checa, R. A, J. E
    540-0
    Abstract:
    In the present paper, the dielectric relaxation properties of RbHSeO$_{4}$ have been studied by means of impedance spectroscopy measurements over wide ranges of frequencies at several isotherms (T$< $415 K). The frequency dependence of the permittivity data reveal a distinct dielectric relaxation at low frequency, which is about 385 Hz at 310 K, then it shifts to higher frequencies ($\sim $40 kHz) as the temperature increases. The f$_{\max}$ vs. reciprocal T shows an activated relaxation process with an activation energy of 0.9 eV, which is in close agreement with that associated with transport of charge carriers. We suggest that the observed dielectric relaxation could be attributed to polarization induced by the proton jump and selenate tetrahedral reorientations. The displacement of mobile H$^{ + }$ proton accompanied by SeO$_4^{ - 2} $ tetrahedra reorientations create structural distortion in both sublattices which induce localized dipoles like HSeO$_4^ - $.
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  • Rediseño, análisis y simulación mecánica del comportamiento de una lente sólida elástica SEL de apertura máxima

    J. González-García, F. Iturbide-Jiménez, A. Santiago-Alvarado, V. H, es., R. M, V. I, E. A
    545-0
    Abstract:
    One type of adaptive lens known as the Solid Elastic Lens (SEL) is composed mainly of PDMS sylgar 184 elastic body and a mount made of aluminum. Radial forces can be applied to this elastic body thereby allowing the geometric parameters of the lens to change. Previous studies reported the construction and the optical performance of a SEL; the mount of this lens has some inconvenient mechanical drawbacks which cause additional aberrations in the image produced when the entire aperture of the lens is considered. In order to improve the performance of this type of adaptive lens, a study is presented of the optimization and redesign of the mechanical mount to enable the radial forces applied to be more homogenous on the elastic material for maximum aperture of the SEL, as shown in the results of the mechanical analysis done in Solidwork$^{\mbox{\textregistered }}$.
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  • Frustración en redes arquimedianas antiferromagnéticas

    W. Lebrecht, J. F
    554-0
    Abstract:
    This paper tackles the problem of studying anatically the phenomena associated to magnetic frustations for different Archimedean lattices with periodic boundary conditions, all of them in the antiferromagnetic phase. A lattice formed by N spins is considered and modeled through $\pm J$ Ising model. Antiferrogmagnetic phase is restricted to $+J$ interactions between two near-nearest neighbor spin sites. Topological parameters such as frustrated plaquettes, frustration length and energy associated to the ground level at T=0 are reported. The relative position of a triangular interaction for each lattice is as much or more important than the coordination number for the same lattice.
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  • Common-source cold-FET used to validate noise figure measurements and on-wafer FET noise parameters

    B. E, M. C, a Sánchez., J. A, noso Hernández.
    560-0
    Abstract:
    This work proposes the use of a common-source cold-FET with gate forward biased to validate the noise figure measurements and the noise parameters of on-wafer transistors. Since a common-source cold-FET behaves as an attenuator, its noise figure and noise parameters can be determined from S-parameters measurements. Three methods for determining the noise parameters of the common-source cold-FET are investigated. The first one uses the noise correlation matrix for passive devices (the S-parameters), the second one is the tuner method and the third one is the F$_{50}$ method. The noise figure measured and the noise figure computed from S-parameters agree quite well. The noise parameters extracted with the tuner method and the F$_{50}$ method show good correlation with the noise parameters computed with the S-parameters. These results validate both the noise figure measurements and the noise parameters extraction.
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  • A simple de-dembedding method for on-wafer RF CMOS FET using two microstrip lines

    H. J, J. R, J. A, noso-Hernández., P. Moreno, F. S, oval-Ibarra., S. Ortega-Cisneros.
    570-0
    Abstract:
    This letter deals with the de-embedding of on-wafer CMOS FETs embedded in symmetrical and reciprocals pads. A de-embedding method, that uses a calibrated vector network analyzer and two microstrip lines fabricated on a lossy SiO$_{2}$-Si substrate, is introduced. The proposed method not only allows the characterization on the interconnection lines but also allows the characterization of the CMOS pads. Our results demonstrate that a shunt admittance does not suffice to properly model CMOS pads. Experimental S-parameters data of on-wafer CMOS FETs de-embedded with the proposed L-L method, Mangan and the Pad-Open-Short De-embedded (PSOD) methods are compared. The S-parameter data, de-embedded with the PSOD and the proposed two-tier L-L show high correlation, validating the proposed de-embedding method.
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  • Gas-solid phase equilibrium of biosubstances by two biological algorithms

    J.A. Lazzús, M. Rivera
    577-0
    Abstract:
    Particle swarm optimization (PSO) and genetic algorithm (GA) are applied to the gas-solid phase equilibrium of biosubstances and to estimate their sublimation pressures ($P^s$). Four binary systems of supercritical carbon dioxide + biosubstances are considered in this study. The Peng--Robinson equation-of-state with the Wong--Sandler mixing rules, are used as a thermodynamic model to evaluate the fugacity coefficients in the classical solubility equation, and the van Laar model was incorporated to evaluate the excess Gibbs free energy included in the mixing rules. Then, the $P^s$ is calculated from regression analysis of solubility data ($y$). $P^s$ is usually small for most solid biosubstances and in many cases available experimental techniques cannot be used to obtain accurate values. Therefore, estimation methods must be used to obtain these data. PSO and GA are used for minimize the difference between calculated and experimental solubility. Comparing PSO with GA, it is shown that the results of PSO are better than that of GA, and provide a preferable method to estimate $y$ and $P^s$ of any biosubstances with high accuracy.
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  • Tricritical phenomena in asphaltene/aromatic hydrocarbon systems

    J. H, G. Ali Mansoori
    584-0
    Abstract:
    The calculation of phase behavior performed for asphaltene-micelles systems that assume several forms when they are mixed with petroleum fluid depending on its aromaticity, and on the relative sizes and polarities of the particles present in such fluid mixtures, is extended to tricritical phenomena. Asphaltene monomers form micelles in the presence of excess amounts of aromatic hydrocarbons and polar solvents, and they are dispersed in a petroleum fluid. The coupling between the micellization and phase separation may, in principle, lead to a near-tricritical coexistence of a monomer phase and a micellar phase; however, this tricritical phenomenon has not been experimentally observed. In this report, such tricritical phenomenon for asphaltene / aromatic systems is predicted.
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  • Numerical study of the Boussinesq approach validity for natural convection and surface thermal radiation in an open cavity

    M. Montiel Gonzalez, J. Hinojosa Palafox, C. Estrada Gasca
    594-0
    Abstract:
    In this work the validity of the Boussinesq approach for the heat transfer calculations in an open cavity considering natural convection and surface thermal radiation is studied. Numerical calculations were conducted for Rayleigh number (Ra) values in the range of 10$^{4}$ -10$^{6}$. The temperature difference between the hot wall and the bulk fluid ($\Delta $T) was varied between 10 and 100 K, and was represented as a dimensionless temperature difference ($\varphi )$. The deviations for the temperature fields and fluid flow patterns, between variable properties and Boussinesq approach are noticeable for $\varphi $=0.333. For total Nusselt numbers, the results with Boussinesq approach and variable properties, indicates deviations within 0.22 % (Ra=10$^{5}$ and $\varphi $=0.033) and 5 % (Ra=10$^{4}$ and $\varphi $=0.333).
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  • Dirac comb with a periodic mass jump

    J.J. Alvarez, M. Gadella, L.M. Nieto
    606-0
    Abstract:
    We discuss some of the properties of the spectrum of a Dirac comb with periodic mass discontinuity. Based on the relationship between the two different masses, we derive the general behavior of the spectra for both cases $E>0$ and $E<0$. The relationship with the constant mass model for the Dirac comb and the generalization to periodic quantum chains with $n$ different masses are also discussed.
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