Vol. 61 No. 4 Jul-Aug (2015): Revista Mexicana de Física.

Published: 2015-01-01

Articles

  • Sistema de microscopía de fuerza atómica basada en una unidad de lectura óptica digital y un escáner-zumbador

    Reza Dabirian., Wei-Min Wang., David Loza Matovelle., En-Te Hwu.
    238-0
    Abstract:
    An astigmatic detection system (ADS) based on a compact disk/digital-versatile-disk (CD/DVD) astigmatic optical pickup unit (OPU) is presented. It can achieve a resolution better than 0.3 nm in detection of the vertical displacement and is able to detect the two-dimensional angular tilt of the object surface. Furthermore, a novel scanner design actuated by piezoelectric disk buzzers is presented. The scanner is composed of a quad-rod actuation structure and several piezoelectric disks. It can be driven directly with low-voltage and low-current sources, such as analogue outputs of a data acquisition (DAQ) card and enables a sufficient scanning range of up to 15 $\mu$m. In addition, an economic, high-performance streamlined atomic force microscope (AFM) was constructed, using the buzzer-scanner to move the sample relative to the probe, and using a CD/DVD OPU to detect the mechanical resonance of a microfabricated cantilever. The performance of the AFM is evaluated. The high sensitivity and high bandwidth of the detection system makes the equipment suitable for characterizing nanoscale elements. An AFM using our detection system for detecting the deflection of microfabricated cantilevers can resolve individual atomic steps on graphite surfaces.
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  • Refractive index expressions for Ga$_{1-x}$ln$_{x}$As, GaAs$_{1-x}$N$_x$ and Ga$_{1-x}$ln$_{x}$N$_y$As$_{1-y}$ alloys

    J.A. Martín, M. Sánchez
    245-0
    Abstract:
    An expression suitable to estimate the refractive index of Ga$_{1-x}$ln$_{x}$N$_y$As$_{1-y}$ with $0\leq x\leq 0.4$ and $0\leq y\leq 0.04$ for band gap energies from 0.8 to 1.1 eV is presented. In case of Ga$_{1-x}$ln$_{x}$As, an improved expression, which shows better agreement with experimental data than previously reported expression, is proposed.
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  • Microestructura, propagación y emisión acústica de grietas en dos papeles de celulosa

    D. Aguirre, J. Aldaco, M. Hinojosa
    254-0
    Abstract:
    We report the comparative study of crack propagation on two types of paper by monitoring both the crack advance and the acoustic emision signal while considering the possible effect of the structure of the materials.The papers studied were a type of Bond paper identified as ``Alrey paper'' and a chemically treated paper known as ``tracing paper''. The microstructure was characterized and it was found that in the first case it exhibited relatively long microfibers whereas in the second case, as a result of the chemical treatment during its maufacture, the microfibers were shorter and appeared arranged in a continous way, the Alrey paper exhibited more porosity and was less homogeneous. Image analysis of the tension fracture for both paper using notched specimens resulted in two scenarios related to the microstructural differencies: In the Alrey, microcraking was produced and ocassionally this resulted in crack branching whereas in the tracing paper secondary cracks were nearly absent. In both cases, the crack growth dynamics starts with a steep acceleration followed by a steady stage, finishing with a prominent acceleration. This behavior is consistent with the Fiber Bundle Model, though a behavior with more dispersion was observed for the Alrey paper, this can be associated to the effect of the microstructure, since in the tracing paper the structure is more continuous and less random, thus preventing significant secondary cracks. The statistical analysis of waiting times between the acoustic emission events, $wt_{EA}$, and the energy of these events, $s_{EA}$, generated by the deformation and fracture of both papers corroborated that the behaviors correspond to power laws: Omori-type for the waiting times, with $\alpha = 1.46 \pm 0.05$ and $\alpha = 1.27 \pm 0.05$ for the Alrey and tracing papers, respectively and Gutenberg-Richter-type for the energy of the AE events, with $\beta = 1.8 \pm 0.2$ and $\beta = 2 \pm 0.2$, for the Alrey and tracing papers, respectively. The statistically different $ \alpha $ exponents for the Omori law appear to reflect the effect of the different fracture mechanism as a result of the microstructural differences on the distributions of waiting times. On the other hand, the effect of the microstructure on the $ \beta $ exponents in the Gutenberg-Richter law is not clear.
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  • Fractional viscoelastic models applied to biomechanical constitutive equations

    J.E. Palomares-Ruiz, M. Rodriguez-Madrigal, J.G. Castro Lugo, A.A. Rodriguez-Soto
    261-0
    Abstract:
    The aim of this work consist to compare the traditional viscoelastic material models vs the fractional ones, determinate the fractional order of the differential operator that characterize the mechanical stress-strain relation, the stress relaxation and the creep compliance of this models for a biological soft tissue, in particular a femoral artery. Apply the Laplace transform for Mittag-Leffler function type and the convolution on fractional standard lineal solid differential equation, known as Zener model, to obtain analytical solution. Simulated the force-pressure related by singular blood flow pulse and the displacement response.
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  • The Liouville theorem as a problem of common eigenfunctions

    G.F. Torres del Castillo
    268-0
    Abstract:
    It is shown that, by appropriately defining the eigenfunctions of a function defined on the extended phase space, the Liouville theorem on solutions of the Hamilton--Jacobi equation can be formulated as the problem of finding common eigenfunctions of $n$ constants of motion in involution, where $n$ is the number of degrees of freedom of the system.
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  • La extinción primaria y el factor estático de Debye-Waller en la caracterización de níquel con textura mediante difracción de rayos X

    T. Kr, shtab., A. Cadena Arenas, A. Kr, vko., J. Palacios Gómez
    272-0
    Abstract:
    The texture analysis using X-ray diffraction (XRD) implies measurement of pole figures (PFs) from the diffracted intensities considering the model of kinematical dispersion. The extinction phenomenon results in a decrease of diffracted intensity and that in turn in a decrease of pole densities (PDs). The phenomenon appears in the kinematical theory of XRD as the primary extinction and the secondary extinction to characterize the loss of intensity of kinematical dispersion. In turn, the static Debye-Waller factor is an integral characteristic of defects in crystals that is introduced in the kinematical theory of XRD and also is used in dynamical theory of XRD. In this work the correlation between the primary extinction coefficient and the static Debye-Waller factor in the case of textured nickel was determined. The value of static Debye-Waller factor was determined from the value of the calculated primary extinction coefficient. For the evaluation there were used PDs in the maxima of PFs obtained for 111 and 200 reflections with MoK$\alpha$ radiation, and the PDs in the maxima of PFs obtained for the first and second orders of these reflections with Cu K$\alpha$ and Co K$\alpha$ radiations. There were calculated the dislocation densities in grains using values of static Debye-Waller factor and the extinction coefficients. The dislocation densities calculated from these two characteristics are practically equal.
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  • Simple algebraic method to study the effects of hydrostatic pressure on the fundamental parameters of a Schottky barrier of metal/$n$-GaAs

    O. Oubram, L.M. Gaggero-Sager, I. Rodríguez-Vargas
    281-0
    Abstract:
    The effects of hydrostatic pressure on the fundamental parameters of a Schottky barrier diode of metal/{$n$-GaAs} are studied using a simple algebraic method. The method relies on the dependence of the parameters of the semiconductor (effective mass, dielectric constant and band gap) with the hydrostatic pressure. We obtain simple expressions for the Schottky Barrier Height, Background Density and Differential Capacity that account of the hydrostatic pressure readily. In particular, the Schottky Barrier Height expression agrees qualitatively with the experimental results available. The Differential Capacity expression depends directly on the effective mass, opening the possibility of determined the effective mass through capacitance measurements. Due to its simplicity the algebraic method could be useful in the design of devices that exploit hydrostatic pressure effects.
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  • Anomalous photon emission from a solid

    Boris I. Ivlev
    287-0
    Abstract:
    The solution of the wave equation for electron in a solid can be formally singular on some line. The singularity is smeared out within the distance $\sim 10^{-11}{\rm cm}$ due to electron ``vibrations'' caused by its interaction with photons. Because of this localization, the large momentum uncertainty results in the local increase of the electron kinetic energy $\sim 1~{\rm MeV}$. This energy enhancement is compensated by the local reduction of zero point energy of photons which can be considered as a potential well producing anomalous electron binding. The electron in this well gets to its bottom emitting photons of the total energy $\sim 1~{\rm MeV}$ (anomalous emission). These effects can occur in a solid when its surface is bombarded by ions with the energy $\sim 1~{\rm keV}$. Photons, produced inside the solid, escape from it and can be detected in addition to the usual Bremsstrahlung of incident ions.
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  • Contributions to the defocusing effect on pole figure measurements by X-ray diffraction

    J. Palacios Gómez, R.S. Salat Figols, A. Jiménez Jiménez, T. Kr, shtab.
    296-0
    Abstract:
    A simple method, considering a parallel beam approximation has been made to reproduce the main features of the defocusing effect, observed when pole figures are measured with the Schulz reflection technique using X-ray diffraction. A Lorentzian curve was used to approximate the primary beam profile. This method applied to low index reflections of copper and silver shows qualitatively and partially quantitatively, the extent the elongation of the ellipse resulting from the intersection of the beam with the tilted sample causes the defocusing effect. Differences observed experimentally are attributed mainly to the divergence of the beam, but also partially to the particular primary beam profile. Additionally, measurements with two different vertical heights of the receiving slit, i.e. the measured arch length of the Debye-Scherrer ring, indicate that this parameter plays no role in defocusing.
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  • Método de lente térmica resuelta en frecuencia para medir coeficientes de difusión térmica en muestras líquidas

    L.G. Rodriguez, L.P. Cantini, E. Granizo, D.P. Guanga, A. Díaz-Barrios, A. Debut, J.L. Paz, C. Costa Vera
    301-0
    Abstract:
    An experiment for measuring the thermal diffusivity of a liquid sample and nanoparticle colloidal systems using the thermal lens effect, induced by a frequency modulated laser diode, is proposed. The method consists in measuring the photothermal signal depending on the modulation frequency of the laser. From the experimental data is possible to estimate the thermal diffusivity of the sample if the radius of the laser beam is known. The experiment is calibrated by estimating the thermal diffusivity of ethanol. Thermal diffusivity coefficients of aqueous solutions of Brilliant Blue FCF and silver nanoparticles systems are also estimated indicating an increment of an order of magnitude of the nanoparticles system relative to the base liquid. The results show that the method is robust, with a reduced implementation cost and sensitivity enough to thermally characterize transparent and semi-transparent liquid materials.
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  • Evaluación de la resolución frecuencial en osciloscopios comerciales

    C. Stern Forgach, J.M. Alvarado Re, es.
    307-0
    Abstract:
    Digital and analogue oscilloscopes are necessary instruments in any teaching and research laboratory. Today's oscilloscopes can acquire signals and represent them in the time and frequency domains. User manuals describe well the time base and the storage capability, but they do not mention the important relationship between the number of data and the sampling frequency. These parameters are directly responsible of the frequency resolution that can be attained in a Fourier analysis. In this paper we compare various oscilloscopes with respect to the capacity they give the user to choose the number of data and the sampling frequency independently, and thus determine the frequency resolution, and avoid large unnecessary data files. The analysis was performed acquiring a mono chromatic signal with different oscilloscopes and different time basis. Results for a 10 Khz signal showed that when the auto button was used, there was up to a 50% error in the expected frequency, while only a 0.9% error was obtained with oscilloscopes that allow the user to choose independently the sampling frequency and the number of data.
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  • Optical and structural properties of CdS:Pb$^{2+}$ nanocrystals

    R. Gutiérrez, O. Portillo, M. Chávez, H. Juárez, M. Pacio, L. Chaltel, M. Zamora, M. Lazcano, E. Rubio, G. Hern, ez.
    312-0
    Abstract:
    The goal of this work is to study the effects of doping on structural, morphological and optical properties of CdS thin films as a function of Pb$^{2+}$ concentration. Thus, nanoparticles were synthesized by chemical bath and a thickness decrease of $\sim$575-200 nm range was observed. In Fourier-transform infrared spectroscopy, all the samples showed a sharp stretching mode observed at $\sim$1384 cm$^{-1}$ corresponding to the vibration mode of CO$_3^{-2}$. X-ray diffraction studies show that the size of crystallites is in the $\sim$33-12 nm range. The peaks belonging to primary phase are identified at the $2\theta = 26.5^{\circ}$ and $2\theta = 26.00^{\circ}$, corresponding to CdS and PbS respectively. Thus, a shift in maximum intensity peak from $2\theta = 26.4^{\circ}$ to 28.2$^{\circ}$ is observed. Likewise peaks at $2\theta = 13.57^{\circ}$, 15.91$^{\circ}$ correspond to lead perchlorate thiourea. The optical, absorption, and transmission properties of the films were determined by UV-vis spectrophotometry, optical energy gap was found to range from 2.1 to 2.4 eV. Raman spectroscopy on doped films showed a shifting of these modes that can be attributed to strain, stress effects, defects, phonon confinement, and variation in phonon relaxation with grain size.
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