Vol. 57 No. 2 (2011): Revista Mexicana de Física.

Published: 2011-01-01

Articles

  • Técnica fotoacústica aplicada a la determinación de propiedades térmicas de muestras de silicio poroso

    A. Gutiérrez, J. Giraldo, M.E. Rodríguez-García
    99-0
    Abstract:
    The photoacoustic technique is a technique that studies the radiation-matter interaction through a well-known phenomenon as photoacoustic effect. In this work using a theoretical fitting of the photoacoustic signal amplitude the effective thermal effusivity of porous silicon samples was determined. The porous samples were obtained by means of photoelectrochemical etching of (100) $n$-type silicon wafers with different resistivity values, all in the range of 1-25 $\Omega$ cm. The samples were formed at room temperature in an electrolytic bath composed by a mixture of hydrofluoric acid ($48%$) and ethanol having a composition ratio of 1:1 in volume under potentiostatic condition (20 V) and an etching time of 2 minutes using back illumination provided by a laser beam with a wavelength of 808nm. The obtained values for the effective thermal effusivity of the porous samples diminished respect to the corresponding value of single-crystalline silicon, this decrease can be attributed to the restriction of the mean free path of phonons in the porous network.
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  • Desarrollo de celdas lógicas por medio del espacio de estados en un sistema bidimensional

    I. Campos-Cantón
    106-0
    Abstract:
    This work shows the construction of the basic logic OR and AND gates in a bidimensional system with the use of the state space. The particular case of a stable linear system with distinct eigenvalues is analyzed.
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  • Evolutionary Associative Memories Through Genetic Programming

    J. Villegas-Cortez, J.H. Sossa, C. Avilés-Cruz, G. Olague
    110-0
    Abstract:
    Associative Memories (AMs) are useful devices designed to recall output patterns from input patterns. Each input-output pair forms an association. Thus, AMs store associations among pairs of patterns. An important feature is that since its origins AMs have been manually designed. This way, during the last 50 years about 26 different models and variations have been reported. In this paper, we illustrate how new models of AMs can be automatically generated through Genetic Programming (GP) based methodology. In particular, GP provides a way to successfully facilitate the search for an AM in the form of a computer program. The efficiency of the proposal was conducted by means of two tests based on binary and real-valued patterns. The experimental results show that it is possible to automatically generate AMs that achieve good results for the selected pattern recognition problems. This opens a new research area that allows, for the first time, synthesizing new AMs to solve specific problems.
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  • An exact solution of delay-differential equations in association models

    J.F. Rojas, I. Torres
    117-0
    Abstract:
    In modeling automatic engines, like in physiological or biological systems or ecology dynamics, oftently is necessary to include delay effects in the equations. This effect is related to reaction or transfer times and can be extended to the spatial case, for example, in cases such as the influence in local green biomass density due to dispersal of seeds. Spatial delay effects are present in liquid mixtures models such as the Cummings-Stell model (CSM) for associating molecules: in a series of publications they solve, for particular cases, an equation with spatial delay that must be satisfied by an auxiliary (Baxter's) function. In this paper, we present an analytical and general solution of the first order Delay-Differential Equation (or Differential-Difference Equation) DDE, for the auxiliary Baxter's function that appears in the CSM. A $n$-partition of the domain leaves a set of DDE's defined in the subintervals. We use recursive properties of these auxiliary functions and a matrix composed by differential and shift operators (MDSO) in order to obtain the solution of the original problem with an arbitrary value of $n$. The problem of solving spatial DDE's is common to other models of associative fluids, such as homogeneous and inhomogeneous mixtures of sticky shielded hard spheres, or models of chemical ion association and dipolar dumbbells and polymers. In all the cases the location of the potential, $L = m \sigma / n$, has different physical effects.
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  • Ecuación de estado cúbica para un fluido simple

    F. de J
    125-0
    Abstract:
    In this work, a method to build the Cubic Equation of State of a pure substance is presented. The Equation of State is determined with the values of the pressure $P$, the molar volume $v$, and the temperature $T$ in two states, namely, the critical point and the saturated vapor at temperature $T / T_{c} = 0.7$ (where $T_{c} $ is the critical temperature). The resulting Equation is unique for each substance and, in general, different to other known Cubic Equation of State in the literature.
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  • Impact of the base doping concentration on the transport mechanisms in n-type a-SiGe:H/p-type c-Silicon Heterojunctions

    P. Rosales-Quintero, M. Moreno-Moreno, A. Torres-Jacome, F.J. De la Hidalga Wade, J. Molina-Re, es., W. Calleja-Arriaga, C. Zuñiga-Islas
    133-0
    Abstract:
    The charge transport mechanisms occurring in n-type a-SiGe:H on p-type c-Si heterojunctions were determined by analyzing the temperature dependence of the current-voltage characteristics in structures with four different peak base doping concentrations ($N_{B}$ = 1$\times $10$^{15}$, 7$\times $10$^{16}$, 7$\times $10$^{17}$ and 5$\times $10$^{18}$ cm$^{ - 3})$. From the experimental results, we observed that at low forward bias (V$ < 0.45 $V) the current is determined by electron diffusion from the n-type amorphous film to the p-type c-Si for the heterojunction with $N_{B}$ = 1$\times $10$^{15 }$cm$^{ - 3}$, whereas the Multi-Tunneling Capture Emission (MTCE) was identified as the main transport mechanism for the other base doping concentrations. On the other hand, at high forward bias (V$ > 0.45 $V), the space charge limited current effect became the dominant transport mechanism for all the measured devices. Under reverse bias the transport mechanisms depends on the peak base doping, going from carrier generation inside the space charge region for the lowest doping, to hopping and thermionic field emission as the base doping concentration is increased.
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  • Esfuerzo triaxial ideal de Ti, Zr y Hf con estructura fcc: un estudio de primeros principios

    A. Bautista Hernández, J.H. Camacho García, M. Salazar Villanueva, E. Chigo Anota, A. Macias Cervantes
    140-0
    Abstract:
    At room temperature and zero pressure Ti, Zr and Hf crystallize in the hexagonal close-packed phase. Recent reports have shown that these elements are elastically metastable [14] in the structure face-centered cubic (fcc). We present the ideal triaxial strength of Ti, Zr and Hf metals with fcc structure, obtained by first principles calculations using the Density Functional Theory. It was used the Generalized Gradient Approximation and Local Density Approximation for exchange-correlation energy and the electron-nucleus interaction we use pseudopotentials. From the calculation of total energy for each element were obtained lattice parameters, bulk, shear and Young modulus, as well as elastic constants in the absence of stress and as a function of applied stress. In the ground state, we obtain good agreement in lattice parameters and elastic constants with experimental data. From the Born-Wang criteria we studied the ideal mechanical resistance to compression of each element. The maximum values of the stress (15, 13.5 and 22.8 GPa for Ti, Zr and Hf, respectively) are explained in terms of the density of states and charge density at the Fermi level.
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  • Recursive parametrization of quark flavour mixing matrices

    S.~Chaturvedi., V.~Gupta., G.~Sánchez-Colón., N. Mukunda
    146-0
    Abstract:
    We examine quark flavour mixing matrices for three and four generations using the recursive parametrization of $U(n)$ and $SU(n)$ matrices developed earlier. After a brief summary of the recursive parametrization, we obtain expressions for the independent rephasing invariants and also the constraints on them that arise from the requirement of mod symmetry of the flavour mixing matrix.
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  • A generalized rule of average for glow peak temperature of ternary alkali halide systems

    G. Moro, oqui-Estrella., R. Pérez-Salas, R. Rodríguez-Mijangos
    154-0
    Abstract:
    The anion composition dependence of the characteristic glow peak temperature of europium doped KCl$_{0.50}$KBr$_{0.25}$RbX$_{0.25}$:Eu$^{2 + }$ (X=Cl, Br) mixed crystals are studied. Each material shows a thermoluminescence glow curve consisting of two main glow peaks with a temperature is between KCl:Eu$^{2 + }$ and KBr:Eu$^{2 + }$. The most intense is related to the F center destruction as occur in the case of KCl:Eu$^{2 + }$ crystals and its characteristic temperature depends strongly on the halogen composition. This behavior confirms that the temperature of the F-H centers recombination in this type of materials depends more on the proportion of anions. From these results, a generalized rule to obtain the temperature from averaging the characteristic temperature as function of the composition is discussed.
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  • Finite photonic lattices: a solution using characteristic polynomials

    F. Soto-Eguibar, O. Aguilar-Loreto, A. Perez-Leija, H. Mo, a-Cessa., D.N. Christodoulides
    158-0
    Abstract:
    We give an analytic solution for the propagation of light in a finite waveguide array when the interaction coefficients are constant. We show how to excite the different sites to allow propagation without perturbation.
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  • Propiedades eléctricas, ópticas y estructurales de películas delgadas de SnO$_{2}$ nanoestructuradas, depositadas a partir de acetilacetonatos

    R. Vázquez Arreguín, M.A. Aguilar Frutis, J. Guzmán Mendoza, M. García Hipólito, R. Fragoso-Soriano, C. Falcon, Guajardo.
    162-0
    Abstract:
    Tin oxide (SnO$_{2})$ thin films, were deposited using ultrasonic spray pyrolysis technique. The films were deposited on glass substrates using tin acetylacetone dichloride [(C$_{5}$H$_{8}$O$_{2})_{2}$SnCl$_{2}$], as raw material, and, N, N-DMF as solvent, at temperatures from 400 to 550$^{\circ}$C. The optical, electric and structural properties of thin films obtained using an organometalic precursor are presented.
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  • Ab initio study of structural, electronic and magnetic properties of iron clusters Fe$_{n}$ (n=2-13)

    M. Samah, B. Moula
    166-0
    Abstract:
    Structures, binding energies, and magnetic moments of Fe$_{n}$ (n = 2--13) clusters have been obtained by pseudopotential density functional theory. A Troullier-Martin scheme was used to generate the iron's pseudopotential and we have successfully reproduced results (lattice constant and magnetic moment) for the bulk BCC iron. The results indicate that the magnetic moment per atom varies slowly around a mean value 3.0 $\mu _{B}$/atom. With increasing atom number the mean binding energy monotonically decreases.
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  • Propiedades ferroeléctricas de películas delgadas de GeSbTe

    J.J. Gervacio Arciniega, E. Prokhorov, F.J. Espinoza Beltrán
    172-0
    Abstract:
    The aim of this work is to investigate and compare ferroelectrical properties of thin GeSbTe films with composition Ge$_{4}$Sb$_{1}$Te$_{5}$ (with well defined ferroelectrical properties) and Ge$_{2}$Sb$_{2}$Te$_{5}$ using impedance, optical reflection, XRD, DSC and Piezoresponse Force Microscopy techniques. The temperature dependence of the capacitance in both materials shows an abrupt change at the temperature corresponding to ferroelectric-paraelectric transition and the Curie-Weiss dependence. In Ge$_{2}$Sb$_{2}$Te$_{5}$ films this transition corresponds to the end from a NaCl-type to a hexagonal transformation. Piezoresponse Force Microscopy measurements found ferroelectric domains with dimension approximately equal to the dimension of grains.
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  • Structural evolution of Ni-20Cr alloy during ball milling of elemental powders

    I. López-Báez, E. Martínez-Franco, H. Zoz, L.G. Trápaga-Martínez
    176-0
    Abstract:
    The ball milling (BM) of blended Ni and Cr elemental powders was carried out in a Simoloyer performing on high-energy scale mode at maximum production to obtain a nanostructured Ni-20Cr alloy. The phase transformations and structural changes occurring during mechanical alloying were investigated by X-ray diffraction (XRD) and optical microscopy (OM). A gradual solid solubility of Cr and the subsequent formation of crystalline metastable solid solutions described in terms of the Avrami-Erofe'ev kinetics model were calculated. The XRD analysis of the structure indicates that cumulative lattice strain contributes to the driving force for solid solution between Ni and Cr during BM. Microstructure evolution has shown, additionally to the lamellar length refinement commonly observed, the folding of lamellae in the final processing stage. OM observations revealed that the lamellar spacing of Ni rich zones reaches a steady value near 500 nm and almost disappears after 30 h of milling.
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