Scielo RSS <![CDATA[Revista mexicana de física]]> http://www.scielo.org.mx/rss.php?pid=0035-001X20140003&lang=en vol. 60 num. 3 lang. en <![CDATA[SciELO Logo]]> http://www.scielo.org.mx/img/en/fbpelogp.gif http://www.scielo.org.mx <![CDATA[<b>Determinación de la cinética de crecimiento de grano durante la solidificación de una aleación eutéctica Al-Cu</b>]]> http://www.scielo.org.mx/scielo.php?script=sci_arttext&pid=S0035-001X2014000300001&lng=en&nrm=iso&tlng=en En este trabajo se aplica una metodología para describir cuantitativamente el crecimiento del grano durante la solidificación de una aleación de Al-Cu con composición cercana a la eutéctica, utilizando curvas de enfriamiento, densidad de granos por unidad de volumen y datos de evolución de la fracción sólida. La metodología supone el libre crecimiento del grano y una dependencia exponencial de la velocidad de crecimiento de los granos con respecto al subenfriamiento. La información recopilada se procesa numéricamente con el fin de encontrar los valores numéricos de los parámetros pre-exponencial y exponencial que caracterizan la cinética de crecimiento de grano en función del subenfriamiento. La evolución de la fracción sólida en la muestra durante la solidificación se determina aplicando el análisis térmico de Fourier (FTA) así como el método de análisis térmico de Newton (NTA), con el fin de comparar sus mediciones. La densidad de granos por unidad de volumen se obtiene utilizando métodos metalográficos. Todos los parámetros obtenidos fueron alimentados en un modelo mecanístico elaborado como parte de este trabajo para validar la metodología. Se encontró una buena concordancia entre los datos experimentales y las curvas de enfriamiento simuladas, sugiriendo que esta metodología podría ser utilizada para generar información útil necesaria para simular la solidificación eutéctica. El método de NTA es menos eficaz que el método de FTA para obtener los parámetros de crecimiento, pero su simplicidad justifica su uso como una primera aproximación.<hr/>In this work a methodology is applied to describe quantitatively grain growth during solidification of a near eutectic Al-Cu alloy using cooling curves, volume grain density and solid fraction evolution data. The methodology assumes free grain growth before impingement and an exponential dependence of the grain growth rate on undercooling. Information collected is processed numerically in order to find numerical values for the pre-exponential and exponential parameters that characterize the grain growth kinetics as a function of undercooling. The evolution of solid fraction in the sample during solidification is determined by applying the Fourier (FTA) and the Newton (NTA) thermal analysis methods in order to compare their performances. The volume grain density is obtained using metallographic methods. All the obtained parameters were feed into a mechanistic model, elaborated as a part of this work, to validate the methodology. It was found a relatively good agreement between the experimental and the predicted cooling curves, which suggest that this methodology could be used to generate useful information needed to simulate eutectic solidification. NTA method is less effective than FTA method to obtain the growth parameters but its simplicity justifies its use as a first approach. <![CDATA[<b>A numerical calculation of the electronic specific heat for the compound Sr<sub>2</sub>RuO<sub>4</sub> below its superconducting transition temperature</b>]]> http://www.scielo.org.mx/scielo.php?script=sci_arttext&pid=S0035-001X2014000300002&lng=en&nrm=iso&tlng=en In this work, a numerical study of the superconducting specific heat of the unconventional multiband superconductor Strontium Ruthenate, Sr2RuO4, is performed. Two band gaps models are employed, and the results rendered for each of them are compared. One of the models, previously proposed by one of the authors to explain the experimental temperature behavior of the ultrasound attenuation, considers two gaps with point nodes of different magnitude on different gap surface sheets, while the other one is an isotropic and line node model, reported in the literature for describing quantitatively experimental specific heat data. The Sr2RuO4 superconducting density of states, DOS, is computed by employing these two models and then, a detailed numerical study of the electronic specific heat, that includes the contribution from the different Fermi sheets, is carried out. It is found that the calculated point node model specific heat temperature behavior shows an excellent agreement with the existent Sr2RuO4 experimental data at zero field, particularly, it is obtained that the observed specific heat jump at Tc is precisely reproduced. Also, it is found that the sum of the contributions from the different bands fits quantitatively the measured specific heat data. The results in this work evidence that the Sr2RuO4 superconducting states are of unconventional nature, corresponding to those of a point node superconductor, and show the importance of taking into account the multiband nature of the material when calculating thermodynamic superconducting quantities.<hr/>En este trabajo se presenta un estudio numérico de la variación del calor específico electrónico del Rutenio de Estroncio, Sr2RuO4 , en su estado superconductor en función de la temperatura. Dos modelos para la descripción de la estructura de bandas de este compuesto son considerados: un modelo de nodos puntuales y otro de nodos lineales. Para cada uno de estos modelos, la densidad de estados superconductores es calculada y un estudio detallado del calor específico electrónico es llevado a cabo. Este estudio incluye la contribución de cada una de las bandas que componen la superficie de Fermi del material al calor específico del sistema. Los resultados numéricos muestran que el modelo de nodos puntuales describe cuantitativamente los datos experimentales existentes para la variacióon del calor específico del Sr2RuO4 en función de la temperatura, hallándose que los valores del calor específico corresponden a la suma de las contribuciones de cada una de las bandas. En particular, se encuentra que este modelo es capaz de reproducir precisamente el salto mostrado por el calor específico electrónico a Tc. Finalmente, los resultados evidencian la naturaleza no convencional del estado superconductor del Sr2RuO4 , el cual está asociado a una brecha con nodos de tipo puntual. Además, se demuestra la importancia de tener en cuenta la naturaleza multibandas del material considerado para el cálculo de sus variables termodinámicas en el estado superconductor. <![CDATA[<b>Retardo temporal en las lentes por galaxias en el contexto de Reissner - Nordstrom</b>]]> http://www.scielo.org.mx/scielo.php?script=sci_arttext&pid=S0035-001X2014000300003&lng=en&nrm=iso&tlng=en En este trabajo se deducen las expresiones matemáticas para el retardo temporal entre dos imágenes debidas a una lente gravitacional con distribuciones simétricas tanto de masa como de carga eléctrica a partir del elemento de línea en el contexto de Reissner - Nordstrom (RN). Para esto, se estudia el movimiento de fotones que se curvan cerca de la distribución de masa cargada, mediante la métrica de RN, obteniendo las expresiones para el ángulo de desviación y el potencial de deflexión, estudiando así el fenómeno de lentes gravitacionales tanto para una masa puntual como para una distribución de masa. Luego se aplican los resultados obtenidos, al modelo de la esfera singular isoterma (ESI), y se comparan con aquellos en los cuales técnicamente se considera galaxias con distribución de masa. Pero, cabe destacar que esta expresión del retardo temporal es aplicable a cualquier modelo de lente gravitacional cargado.<hr/>In this work we derive the mathematical expressions for the time delay between two images due to gravitational lensing with symmetric distributions of both: mass and electric charge from the line element in the context of Reissner - Nordstrom (RN). For this, is important to study the motion of photons that are curved around the mass distribution charged by RN metric, obtaining expressions for the deflection angle and deflection potential studying the phenomenon of so much for a lensing precise mass to mass distribution. Then applied the results, the singular sphere model isotherm (ESI), and compared with those in which only considers galaxies mass distribution. But, that this expression of the time delay is applicable to any gravitational lens model loaded too. <![CDATA[<b>Expanded use of a fast photography technique to characterize laser-induced plasma plumes</b>]]> http://www.scielo.org.mx/scielo.php?script=sci_arttext&pid=S0035-001X2014000300004&lng=en&nrm=iso&tlng=en Expanded use of fast photography is proposed to characterize laser ablation plasma plumes by the analysis of a set of photographs by means of appropriate mathematical algorithms. The laser ablation plasma plumes studied were generated by ablation of both a multicomponent target of the nominal composition Ni50Mn37Sn13 and a highly pure Cu target (Cu) using a Q-switched Nd-YAG laser system. The experiments were conducted under different background argon pressures. Several photograph parameters such as intensity per unit time of exposure for a pixel, mean intensity per pixel per unit time of exposure, integrated intensity and cross correlation were studied. The intensity per unit time of exposure allowed for identification of the fast component of the triple structure of the expanding plasma into the background gas (that travels at a speed close to the one measured in vacuum). This parameter together with the use of cross correlation enabled the identification of regions of the expanded plasma plume with higher and lower similarities in their optical emission behavior. The mean intensity per pixel per unit time of exposure can be used as a measurement of the amount of light emitted by the plume as a function of time. <![CDATA[<b>Gravitational waves bounds in Brane-Worlds</b>]]> http://www.scielo.org.mx/scielo.php?script=sci_arttext&pid=S0035-001X2014000300005&lng=en&nrm=iso&tlng=en This paper is dedicated to investigate an astrophysical method to obtain the new dynamics generated by extra dimensions as well as bounds for the brane tension. Using the modified Einstein equations in the brane with a vanishing non-local effects, we study the contributions of the modified radiated power by gravitational waves and the stellar period modified by branes in a binary system composed by two neutron stars. Finally we propose two lower energy bounds, using these astrophysical methods. <![CDATA[<b>Adherence and electrochemical behavior of calcium titanate coatings</b> <b>onto 304 stainless steel substrate</b>]]> http://www.scielo.org.mx/scielo.php?script=sci_arttext&pid=S0035-001X2014000300006&lng=en&nrm=iso&tlng=en Calcium titanate has been proposed as a coating for biomedical applications but it has not been reported characterization of adhesion failure mechanisms or electrochemical properties in time. In this work have been studied these properties of a calcium titanate coating growth onto AISI 304 steel deposited by r.f. magnetron sputtering. It was found that the coating has a critical adhesive load of 6.53 ± 0.14 N. With respect to its electrochemical properties potentiodynamic polarization curves show that the calcium titanate coating provides protection to AISI 304 steel. However, EIS indicates that even though metal dissolution occur through the pores in the coating, this leads to the precipitation of salts that block pores; this precipitates layer acts like an additional barrier to the metal dissolution in the system. <![CDATA[<b>Optical characterization of amber of Chiapas</b>]]> http://www.scielo.org.mx/scielo.php?script=sci_arttext&pid=S0035-001X2014000300007&lng=en&nrm=iso&tlng=en We show the refractive index value is a vulnerable test to discriminate true amber from false amber. In this sense, the fluorescence spectral signal is also a vulnerable test if the illuminating source has a broad spectral emission. We present results that allow us to conclude the Raman spectroscopy is a reliable test to distinguish amber of Chiapas from amber of the Baltic regions, and from false amber. We suggest the inclusion of the Raman spectroscopy to the Official Mexican Norm, NOM-152-SCFI-2003, to authenticate the amber of Chiapas.<hr/>Mostramos que el valor del índice de refracción es una prueba vulnerable para discriminar ámbar verdadero de ámbar falso. En este sentido, la señal espectral fluorescente es también una prueba vulnerable si la fuente de iluminación posee un amplio espectro de emisión. Presentamos resultados que permiten concluir que la espectroscopia Raman es una prueba confiable para distinguir ámbar de Chiapas de ámbar de las regiones del Báltico o de ámbar falso. Se sugiere la incorporación de la espectroscopia Raman a la Norma Oficial Mexicana, NOM-152-SCFI-2003, para autentificar el ámbar de Chiapas. <![CDATA[<b>An approximation of tribological behavior of Ti<sub>1-x</sub>Al<sub>x</sub>N coatings against animal bone in ringer's solution</b>]]> http://www.scielo.org.mx/scielo.php?script=sci_arttext&pid=S0035-001X2014000300008&lng=en&nrm=iso&tlng=en Due to their excellent properties, Ti-Al-N coatings have become attractive for biomedical applications. In this paper, friction and wear properties of Ti1-xAl xN films having various aluminum contents, x, have been studied. Adhesion was measured by the scratch test technique; friction was carried out by a pin-on-disk tribometer using an animal bone-pin as counterpart and Ringer's solution as simulated body fluid; and wear mechanisms were identified by SEM and EDS. It was found that the coating with x = 0.41 exhibited the highest COF, conserves its integrity as a coating, and causes the lowest wear on the bone in Ringer's solution. <![CDATA[<b>Growth of metal micro and/or nanoparticles utilizing arc-discharge immersed in liquid</b>]]> http://www.scielo.org.mx/scielo.php?script=sci_arttext&pid=S0035-001X2014000300009&lng=en&nrm=iso&tlng=en In this paper, we present results on the metal microcrystals and nanoparticles of Al, Cu and Al-Cu composite growth by arc-discharge with the system immersed in distilled water, under different conditions and varying the current from 50 to 150 A with constant voltage (27 V). These structures are characterized using X-ray diffraction, scanning electron microscopy, Raman spectroscopy and Uv-Vis spectroscopy. Our results demonstrate that metal micro and nanostructures can be prepared at low cost with high quality. <![CDATA[<b>Structural changes in ZrO<i><sub>x</sub></i>N<i><sub>y</sub></i>/ZrO<sub>2</sub> coatings deposited through spray pyrolisis-nitriding</b>]]> http://www.scielo.org.mx/scielo.php?script=sci_arttext&pid=S0035-001X2014000300010&lng=en&nrm=iso&tlng=en Thin films of zirconium oxynitride were deposited on stainless steel 316L and characterized through ultrasonic spray pyrolysis-nitriding (UPS-N). Initially, thin films of ZrO2 are deposited using ultrasonic spray pyrolysis, and later ZrO2 films were nitrided in a NH3 atmosphere. We analyzed the effect of some variables, such as substrate temperature, flow ratio, and time of the production of coatings and their influence on the structure of the films. The characterization was carried out using X-ray diffraction (XRD) spectroscopy, X-ray photoelectron (XPS), and scanning electron microscopy (SEM). Films that were grown using the UPS method exhibited a tetragonal zirconia polycrystalline structure with preferential orientation in plane (101). These films, after being nitrided in an atmosphere of anhydrous ammonia at 1023 K, go through two processes: a phase transition from tetragonal to monoclinic, and later the formation of zirconium oxynitride rhombohedral ZrOxNy. <![CDATA[<b>Dependence of the mechanical and electrical properties on the acid content in PVA+H<sub>3</sub>PO<sub>2</sub>+H<sub>2</sub>O membranes</b>]]> http://www.scielo.org.mx/scielo.php?script=sci_arttext&pid=S0035-001X2014000300011&lng=en&nrm=iso&tlng=en In this paper we report elastic modulus and ionic conductivity measurements in proton-exchange membranes based on poly (vinyl alcohol) (PVA), as a function of the acid (H3PO2) concentration and temperature. Tensile stress-strain test (nominal stress) was performed at low rate deformation, with acid concentration between 0.0 to 3.0 given in P/OH ratios. Results show that the elastic modulus, the elastoplastic modulus and the yield stress decrease as acid concentration increases. Ionic conductivity was obtained using the impedance spectroscopy technique on samples with the same acid concentrations, which shows an improvement in ionic conductivity of PVA+H3PO2+H2O within the range 10-3 - 10-1 S cm-1 as the acid concentration increases. The results obtained by comparing mechanical and electric properties, show that they are both directly linked to the acid content and water activity, thus these properties can be engineered to meet the appropriate conditions for an efficient fuel cell operation.<hr/>En este trabajo reportamos medidas del módulo elástico y conductividad iónica en membranas intercambiadoras de protones basadas en alcohol polivinílico (PVA) y ácido hipofosforoso (H3PO2). Estas medidas se realizaron como función de la concentración de ácido y la temperatura. Las pruebas de esfuerzo de fluencia se realizaron para las relaciones P/OH: 0.0 a 3.0 a una baja velocidad de deformación. Los resultados muestran que el módulo elástico, el módulo elastoplástico y la tensión de fluencia decrecen al aumentar la concentración de ácido. Las medidas de conductividad iónica, se obtuvieron por espectroscopía de impedancia, para muestras con las mismas relaciones P/OH, las cuales muestran que la conductividad mejora en un rango entre 10-3 - 10-1 S cm-1 con el incremento del ácido. Los resultados obtenidos al comparar las propiedades mecánicas y eléctricas muestran que ambas están directamente relacionados con el contenido de ácido y la actividad del agua, por lo tanto presentan propiedades adecuadas para operación en celdas de combustible. <![CDATA[<b>Interaction of light with gravitational waves</b>]]> http://www.scielo.org.mx/scielo.php?script=sci_arttext&pid=S0035-001X2014000300012&lng=en&nrm=iso&tlng=en The physical properties of electromagnetic waves in the presence of a gravitational plane wave are analyzed. Formulas for the Stokes parameters describing the polarization of light are obtained in closed form. The particular case of a constant amplitude gravitational wave is worked out explicitly and it is shown that it produces a linear polarization of light. <![CDATA[<b>Modeling surface processes and kinetics of compound layer formation during plasma nitriding of pure iron</b>]]> http://www.scielo.org.mx/scielo.php?script=sci_arttext&pid=S0035-001X2014000300013&lng=en&nrm=iso&tlng=en Different approaches have been developed concerning growth description of the compact nitride layers, especially those produced by ammonia. Nitriding by plasma uses a glow discharge technology to introduce nitrogen to the surface which in turn diffuses itself into the material. During this process, the ion bombardment causes sputtering of the specimen surface. This paper presents a mathematical model of compound layer formation during plasma nitriding of pure iron. The model takes into account the erosion effect at the plasma-solid interface due to sputtering. This erosion effect is computer simulated and adjusted in order to consider its contribution to the study of layer growth kinetics. The model is presented as a moving boundary diffusion problem, which considers the observed qualitative behavior of the process. <![CDATA[<b>Brownian motion of a colloidal particle immersed in a polymeric solution near a rigid wall</b>]]> http://www.scielo.org.mx/scielo.php?script=sci_arttext&pid=S0035-001X2014000300014&lng=en&nrm=iso&tlng=en By using three-dimensional digital video microscopy (DVM-3D), we study the displacement of a Brownian particle immersed in a polymeric solution located near a rigid wall. The technique takes advantage of the diffraction pattern generated by a fluorescent particle that is found below the focal plane of an optical microscope. The particle is then tracked from the analysis of a sequence of digitized images to reconstruct its trajectory, which provides relevant information about the properties of the system. In a first stage, we obtain the mean square displacement (MSD) of a spherical probe dissolved in a viscoelastic solution. This MSD is then used to determine the elastic and viscous moduli of the suspension. Such measurements are consistent with bulk measurements performed by means of two techniques, namely, diffusing wave spectroscopy and mechanical rheology. Near the rigid wall, the motion of the probe particle can be split in two directions, i.e., parallel and perpendicular to the surface. For short times (but still in the Brownian regime), such motion can be characterized by means of two distance dependent friction coefficients. We observe deviations of the measured friction coefficients in comparison with the Newtonian behavior.