Vol. 54 No. 2 (2008): Revista Mexicana de Física.

Published: 2008-01-01

Articles

  • Stability of thermal structures with an internal heating source

    N. Sánchez, E. López
    86-0
    Abstract:
    We study the thermal equilibrium and stability of isobaric, spherical structures having a radiation source located at their center. The thermal conduction coefficient, external heating and cooling rates are represented as power laws of the temperature. The internal heating decreases with distance from the source $r$ approximately as $\sim \exp(-\tau)/r^2$, $\tau$ being the optical depth. We find that the influence of the radiation source is important only in the central region, but its effect is enough to make the system thermally unstable above a certain threshold central temperature. This threshold temperature decreases as the internal heating efficiency increases, but, otherwise, it does not depend on the structure size. Our results suggest that a solar-like star migrating into a diffuse interstellar region may destabilize the surrounding medium.
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  • Proton induced luminescence of minerals

    H. Calvo del Castillo, A. Millán, P. Beneítez, J.L. Ruvalcaba-Sil, T. Calderón
    93-0
    Abstract:
    This paper presents a summary of Ionoluminescence (IL) for several minerals commonly found in jewellery pieces and/or artefacts of historical interest. Samples including silicates and non-silicates (native elements, halide, oxide, carbonate and phosphate groups) have been excited with a 1.8 MeV proton beam, and IL spectra in the range of 200- 900 nm have been collected for each one using a fiber optic coupled spectrometer. Light emissions have been related to Cr $^{3 + }$, Mn$^{2 + }$ and Pr$^{3 + }$ ions, as well as intrinsic defects in these minerals. Results show the potential of IL for impurity characterization with high detection limits, local symmetry studies, and the study of the origin of minerals.
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  • Multichannel image storage with application to color multiplexing

    M. Araiza E., S. Guel S., C. Sifuentes G., I. de la Rosa V., G. Miramontes de L.
    100-0
    Abstract:
    This paper presents an improvement in the method for storing information using detour type multichannel holograms. This makes in possible to increase the number of image-processing operations at the retrieval stage. One of the main contributions of this work is in showing that this type of hologram is sensitive to color processing. As far as we know, this is the first effort made toward implementing color in binary holograms as shown in the experimental results.
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  • A theoretical quantum study on the distribution of electrophilic and nucleophilic active sites on Cu(100) surfaces modeled as finite clusters

    C.H. Rios-Re, es., A. Ponce-Rodriguez, M. Romero-Romo, L.H. Mendoza-Huizar
    104-0
    Abstract:
    In this work, it is shown a theoretical quantum study of the active sites distribution on a monocrystalline surface of Cu(100). The copper surface was modeled as finite clusters of 14, 23, 38 and 53 atoms. We performed Hartree-Fock and Density Functional Theory (B3LYP) abinitio calculations employing the pseudopotentials of Hay and Wadt (LANL1MB y LANL2DZ). From calculations, we found a work function value of 4.1 eV. The mapping of the HOMO and LUMO in the frozen core approximation, allowed us finding the electrophilic and nucleophilic active sites distribution, respectively. The results indicated that electrophilic sites on the Cu(100) surface were located on hollow position and its numerical density was 8.6$\times$10$^{16}$ sitescm$^{ - 2}$. From the nucleophilic local softness study, it was found that the nucleophilic sites were formed by a group of atoms and it had a numerical density of 2.4$\times$10$^{16}$ sitescm$^{ - 2}$. Last results indicated that adsorptions with 2$\times$2 and 3$\times$3 distributions can be favored onto a Cu(100) surface for the electrophilic and nucleophilic cases, respectively.
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  • Structural, optical and electrical properties of CdS thin films obtained by spray pyrolysis

    C. Santiago Tepantlán, A.M. Pérez González, I. Valeriano Arreola
    112-0
    Abstract:
    Cadmium sulphide (CdS) thin films were prepared by means of the chemical spray pyrolysis technique. The substrate temperature was varied in the range from 200 to 400$^{\circ}$C. The structural properties of the semiconductor were characterized by X-ray diffraction; XRD patterns indicated the presence of single-phase hexagonal CdS. Direct band gap values of 2.37-2.41 eV were obtained. The refractive index is reported on depending on the substrate temperature, and was obtained from transmission spectra and from spectroellipsometry measurements. As a consequence, the optical parameters of the films were determined using the Swanepoel, Cauchy, Sellmeier and Wemple models. The resistivity of the films was found to vary in the range 10$^{3}-$10$^{5} \Omega $.cm, depending on the substrate temperature.
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  • Fugas de calor y aprovechamiento de efluentes en la optimización de ciclos Brayton totalmente irreversibles

    C.A. Herrera, M.E. Rosillo, L. Castaño
    118-0
    Abstract:
    Optimization of maximum power and minimum rate of entropy generation is performed on a complete and realistic Brayton cycle--irreversible components, dissipative flows, power consumption on fluids movement, heat exchange through finite temperature gradients, heat leaks, and effluents unrecovered exergy-. The sets of values that produce optimum objective results are determined using fixed design parameters and variables under typical operational ranges. This work analyzes the heat leaks and effluents exergy recovery level on the optimizing set and on the system behavior. An illustrative example is presented as a study case, showing difficult-to-predict performances. This article illustrates a method which can be implemented in a computer and allows to asses existing systems and upgrade designs.
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  • Making hidden symmetries obvious

    G.F. Torres del Castillo, J.L. Calvario Acócal
    127-0
    Abstract:
    It is shown that the Hamiltonian of a particle in a uniform gravitational field which possesses a constant of motion not related to transformations in the configuration space, can be expressed in a system of canonical coordinates such that a maximal set of independent constants of motion follows from the existence of ignorable coordinates.
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  • Eliminación de ruido de ASE en una fibra dopada con erbio empleando un filtro basado en el interferómetro de Sagnac con fibra de Hi-Bi en el lazo

    A. Flores-Rosas, E.A. Kuzin, B. Ibarra-Escamilla, M. Bello-Jiménez
    130-0
    Abstract:
    We experimentally investigate the application of a thermically tunable filter based on a Sagnac interferometer with high birefringence (Hi-Bi) fiber in the loop to eliminate the biggest contribution in the amplified spontaneous emission (ASE) noise in an Erbium-doped fiber (EDF). The Sagnac interferometer provides a periodic transmitance that depends on the wavelength, which is used as a bandpass filter if it is selected the length of the Hi-Bi fiber appropriately in the loop. Furthermore due to its high sensibility to temperature variations, it is possible to adjust the transmitance of the interferometer and to use it like thermically tunable filter. The experimental results show that it is possible to eliminate the biggest contribution of ASE (peak around 1530 nm) and teherefore this device offers a great potential to be used in Erbium-doped fiber amplifiers (EDFAs) like suppressive of ASE noise.
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  • Numerical simulation of the dynamical properties of the human tympanum

    E. Alvarado-Anell, M. Sosa, M.A. Moreles
    135-0
    Abstract:
    A numerical simulation of the dynamical properties of the tympanic membrane is presented. A simple and different simulation of the vibratory patterns of the coupled system of the tympanum-malleus have been assessed by proposing the modeling of the tympanum through the vibrations of a forced elastic membrane, whereas the effect of the manubrium is introduced through a forced semi-membrane. We propose the superposition of these waveforms as a model for describing the vibrations of the coupled system of the tympanum-malleus. Both waveforms have analytical representations leading to simple computations. The results of the simulation for the vibrational mode (1,1) show an amplitude for the membrane larger than those for the handle of the malleus. The maximum amplitude obtained was around 1 $\mu $m, at a test frequency of 2 kHz. Also, level curves corresponding to the simulated vibrational modes were obtained. The numerical model presented can be easily handled to change input parameters, such as sound pressure and frequency. Also, other situations such as the conical shape of the tympanum or some asymmetries could be considered.
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  • Calculation of temporal spreading of ultrashort pulses propagating through optical glasses

    M. Rosete-Aguilar, F.C. Estrada-Silva, N.C. Bruce, C.J. Román-Moreno, R. Ortega-Martínez
    141-0
    Abstract:
    The chromatic dispersion of optical materials causes an optical pulse to spread as it propagates through the material. The pulse spreading is produced by the dependence of the group velocity on the frequency. In this paper we evaluate the temporal spreading of a pulse as it propagates through optical glass. We evaluate the dependence of group velocity on frequency in terms of the dependence of the phase refractive index of the glass on the wavelength of light. The dependence of the refractive index on the wavelength in glass is well known through the Sellmeier formula. Results are presented for 50, 80 and 100 fs pulses propagating a distance $L$, in BK7, SF14 and Fused Silica Schott optical glasses and verified by a model of the sum of Gaussian modulated frequencies.
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  • Estudio de las propiedades electrónicas de cúmulos de Pd: un estudio comparativo usando distintas técnicas y aproximaciones

    F. Aguilera-Granja, J.M. Montejano-Carrizales, A. Vega, J. Ferrer, J. Rogan, G. García
    149-0
    Abstract:
    A systematic study of the colineal and no-colineal magnetic properties and the metallic behavior in free standing clusters of Pd$_N$ ($2{\le} N{\le} 21$) is carried out. We also study the effect of the twinning in clusters on the magnetism in clusters up to $N=116$ atoms. To this aim we use two techniques: in one hand the ab initio based on the density functional theory (DFT) and on another hand semi-empirical based on the tight binding (TB) Hamiltonian. These are two of the most common used techniques to study the electronic properties of clusters. We found that the colineal magnetism has a no-monotonic decreasing behavior as a function of the cluster size. We observe that the twinning of clusters in some cases can increase the magnetic moment of relatively large size clusters. We found that the no-colineal magnetism always corresponds to a low energy excited state, and that the ground state always presents a colineal magnetic order. Respect to the methods, in general we found that the semi-empirical ones describe very well the electronic properties, as the magnetism; that are obtained from the difference between electronic populations. Although the metallicity requires of the exact determination of the position of the electronic states, it is razonably described by the semi-empirical method if the clusters size is big enough. We compare our results with previous calculations as well as with some experiments reported in the literature.
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  • Rotational states of asymmetric molecules revisited: matrix evaluation and generating function of Lamé functions

    E. Le, -Koo., R. Méndez-Fragoso
    162-0
    Abstract:
    The solutions of the Schrödinger equation describing rotational states of asymmetric molecules are known to be separable as products of Lamé functions in spheroconal coordinates. However, the numerical evaluation of such solutions has not been implemented in efficient and practical ways. This paper overcomes this deficiency by presenting a matrix method for evaluating the needed Lamé functions, in their two kinds and eight species, to obtain accurate energy eigenvalues and eigenfunctions for the rotational states of molecules with any asymmetry. A generating function for these eigenfunctions, including its series expansions for any asymmetry, is also presented.
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  • Non-laser approach to photochemical tissue bonding

    M. Martínez-Escanamé, J.D. Martínez-Ramírez, G.A. Aguilar-Hernández, C. Fuentes-Ahumada, B. Torres-Alvarez, B. Moncada, J.P. Castanedo-Cázares, F.J. González
    173-0
    Abstract:
    Lasers have been used in the past to activate chemical cross-linking agents in photochemical tissue bonding; however, lasers are expensive and bound to only one wavelength, so that they cannot be used with bonding agents that have different activation wavelengths. Since light activation does not rely on the coherence of the radiation, but only on the wavelength and energy density, it is possible to use light sources other than lasers to generate this activation energy. In this paper the effectiveness of three bonding agents (methylene blue, rose bengal and fluoroscein) on human skin is tested using a non-laser light source. Tension and skin temperature measurements showed that skin adherence is as good as previously published laser-irradiated experiments, and that the light source does not induce thermal-related skin damage.
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