Scielo RSS <![CDATA[Revista mexicana de física]]> vol. 63 num. 1 lang. en <![CDATA[SciELO Logo]]> <![CDATA[Simple harmonic oscillator with fractional electric potential]]> Abstract In this work we demonstrate the effect of an evolved electric potential on a charged particle placed in a harmonic oscillator. The effect of the evolved potential on the wave function and energy is shown for different states. We also show how the potential itself develops fractionally. <![CDATA[Synthesis and characterization of Co-doped Lanthanum Nickelate perovskites for solid oxide fuel cell cathode material]]> Abstract In the perovskite structures widely investigated and used as Solid Oxide Fuel Cells (SOFC) cathodes, oxygen reduction is mainly limited to the triple phase boundary (TPB), where oxygen (air), electrode and electrolyte are in contact. It is possible via the sol-gel modified Pechini method to: 1) control the material grain size, which can increase TPBs, 2) produce a homogenous material and 3) obtain a cathode material in a faster way compared with the solid state route. LaNi x ,Co 1-x O3 (x=0.3, 0.5, 0.7) were synthesized by the modified Pechini method. The perovskite phase formation began at 350°C and the presence of pure LaNi0.7Co0.3O3 LaNi0.5Co0.5O3 and LaNi0.3Co0.7O3 structures was evidenced by High Temperature X-ray diffraction (HT-XRD) measurements. Scanning Electron Microscopy (SEM) micrographs showed that the microstructure evolves with the amount of cobalt from a coalesced to an open structure. Electrochemical impedance spectroscopy (EIS) on symmetrical cells LaNi x Co 1-x O3/YSZ (yttria-stabilized zirconia)/LaNi x Co 1-x O3 showed that the highest ASR (area specific resistance) is obtained with x = 0.3, whereas ASR values are similar for x = 0.5 and 0.7 at temperatures higher than 600°C. At temperatures lower than 600°C, ASR is the lowest for LaNi0.5Co0.5O3, showing that this composition with intermediate porosity appears as a good choice for an intermediate-temperature solid oxide fuel cell (SOFC). <![CDATA[An estimation method of fractal dimension of self-avoiding roughened interfaces]]> Abstract: Two kinds of methods (graphical and statistical) commonly used for the estimation of fractal dimension of self-avoiding interfaces were investigated. It was determined that the current methods of both kinds have significant errors for this type of profiles. In the present work a novel efficient method for the estimation of fractal dimension of self-avoiding curves embedded in the space R 2 based on the Box-count and Hall-Wood estimators is developed. Some physical implications are discussed. <![CDATA[New insight into water-obsidian interaction]]> Abstract Hydration of obsidian samples in isotopic water H 2 18 O was performed at temperatures from 75°C to 200°C. The depth distributions of 1H and 18O in the obsidians measured by SIMS were found to be drastically different. This led us to the conclusion that water molecules split into hydrogen and oxygen at the obsidian surface and then the atomic hydrogen and 18O isotope diffused into the obsidian via two different mechanisms. The hydrogenated obsidians were heated in vacuum without hydrogen emission. A hypothesis is put forward that the obsidian hydrogenation results from a chemical reaction between atomic hydrogen and a glass network. A new linear-parabolic equation is suggested for obsidian hydrogenation dating. Aluminosilicate glass is considered as a promising material for hydrogen fusion cells. <![CDATA[Geometry of classical particles on curved surfaces]]> Abstract In this paper we consider a particle moving on a curved surface. From a variational principle, we write the equation of motion and the constraining force, both in terms of the Darboux frame adapted to the trajectory, that involves geometric information of the surface. By deformation of the trajectory on the surface, the constraining force and equation of motion of the perturbation are obtained. We show that the transversal deformation follows a generalized Raychaudhuri equation that contains extrinsic information besides the geodesic curvature. Results in the case of surface with axial symmetry can be parametrized in terms of the angular momenta.<hr/>Resumen En este artículo consideramos el movimiento de una partícula sobre una superficie curvada. De un principio variacional, encontramos las ecuaciones de movimiento y la fuerza de constricción, en el marco de Darboux adaptado a la trayectoria. Deformando estas ecuaciones, encontramos que la perturbación transversal satisface una ecuación de Raychaudhuri generalizada que contiene información extrínseca además de la curvatura geodésica. En el caso de superficies con simetría axial, los resultados se pueden parametrizar en términos del momento angular. <![CDATA[Structural refinement, photoluminescence and Raman spectroscopy of Wurtzite Mn-doped ZnO pellets]]> Abstract We report the results of the Rietveld refinement, photoluminescence and Raman spectroscopy of Mn-doped ZnO ceramic pellets. Rietveld refinement shows that samples crystallize in the wurtzite structure and for the Mn-doped sample indicates that the Mn atoms substitute the Zn tetrahedral crystallographic sites in the ZnO host lattice. The emission and absorption spectra of Mn-doped ZnO have been investigated in the Visible-UV region and the data have been interpreted in terms of the wurtzite ZnO electronic structure. Two broad bands, one due to superposition between donor bound excitons (DX) and free excitons (FX) and other due free-to-bond excitonic recombination (FB) dominates the low-temperature photoluminescence spectra of Mn-doped ZnO bulk. In the Raman spectrum, an extra mode at ∼520 cm-1 has been observed in agreement with earlier works, and it is an indicator for the incorporation of Mn+2 ions into the ZnO host matrix since it is not is observed in ZnO pristine. <![CDATA[On the anisotropic advection-diffusion equation with time dependent coefficients]]> Abstract The advection-diffusion equation with time dependent velocity and anisotropic time dependent diffusion tensor is examined in regard to its non-classical transport features and to the use of a non-orthogonal coordinate system. Although this equation appears in diverse physical problems, particularly in particle transport in stochastic velocity fields and in underground porous media, a detailed analysis of its solutions is lacking. In order to study the effects of the time-dependent coefficients and the anisotropic diffusion on transport, we solve analytically the equation for an initial Dirac delta pulse. We discuss the solutions to three cases: one based on power-law correlation functions where the pulse diffuses faster than the classical rate ∼ t, a second case specifically designed to display slower rate of diffusion than the classical one, and a third case to describe hydrodynamic dispersion in porous media.<hr/>Resumen En este trabajo se analiza la ecuación de advección-difusión en la cual se tiene la velocidad y el tensor de difusión anisotrópico dependientes del tiempo, y se examinan los efectos no-clásicos del transporte y el uso de una base vectorial no ortogonal. Esta ecuación aparece en diversas áreas de la física, particularmente en transporte de partículas en campos estocásticos de velocidad y en medios porosos subterraneos, sin embargo, hace falta un análisis más profundo de sus soluciones. A fin de examinar el efecto de los coeficientes dependientes del tiempo y de la anisotropía en la difusión hemos obtenido analíticamente la solución general del modelo para el caso de un pulso inicial tipo delta de Dirac. Aplicamos la ecuación a tres casos: uno basado en funciones de correlación que siguen leyes de potencias que da lugar a super-difusión, el cual ha sido resuelto numéricamente con anterioridad, otro que hemos construido específicamente para exhibir sub-difusión, y un tercero desarrollado para describir dispersión hidrodinámica en medios porosos. <![CDATA[Design and development of an elastic-scattering lidar for the study of the atmospheric structure]]> Abstract Light detection and ranging (lidar) technology has become a powerful tool to investigate the structure and air quality of the atmosphere with satisfactory spatial and temporal resolution. Due to its importance for atmospheric sciences this work presents the development and testing of a lidar based on the atmospheric elastic-scattering in Mexico. After a description of the elastic-lidar, a simple lidar signal processing scheme is presented. Quantitative evidence of potential applications of the lidar described in this work is demonstrated by a set of zenithal probing of the atmosphere over the city of Cuernavaca, Mexico. <![CDATA[Characterization of highly doped Ga<sub>0.86</sub> In<sub>0.14</sub>As<sub>0.13</sub>Sb<sub>0.87</sub> grown by liquid phase epitaxy]]> Abstract Ga0.86In0.14As0.13Sb0.87 layers lattice-matched to (100) Te-GaSb have been grown using the liquid phase epitaxy technique under supercooling conditions. N and p type layers were grown by adding tellurium or zinc in a wide range of molar fraction in the growth solution. By Raman spectroscopy, the structural quality of the epilayers was characterized. The Raman spectra show that the layers become more defective as the dopant molar fraction is increased, n- or p-type. Two main bands are observed in the Raman spectra centered at 230 and 245 cm-1 that depend strongly on the incorporated dopant molar concentration (Te or Zn), which are assigned to the observed vibrational modes of GaAs-like and (GaSb+InAs)-like mixture. The low-temperature photoluminescence of n (or p)-type GaInAsSb was measured as function of dopant concentration added to the melt solution. The photoluminescence spectra were interpreted taking into account the nonparabolicity of the conduction (or valence) band. Both the band filled as well as band tailing effects due to Coulomb interaction of free carriers with ionized impurities and shrinkage due to exchange interaction between free carriers were considered in order to properly be taken into account for the observed features of the photoluminescence spectra. It is shown that the band-to-band transition energy can be used to estimate the free carrier concentration in GaInAsSb for a wide range of dopant concentration.<hr/>Resumen Capas de Ga0.86In0.14As0.13Sb0.87 con coincidencia de parámetros de red a Te-GaSb (100) han sido crecidas mediante la técnica de epitaxia en fase líquida en condiciones de sobre-enfriamiento. Capas tipo n y p fueron crecidas mediante la adición de teluro y zinc en un amplio rango de fracción molar en la solución de crecimiento. Por espectroscopia Raman, se caracterizó la calidad estructural de los epicapas. Los espectros de Raman muestran que las capas se hacen más imperfectas a medida que aumenta la fracción molar de impurificante, tipo-n o -p. Dos bandas principales se observaron en los espectros Raman centradas en 230 y 245 cm-1 que dependen fuertemente de la concentración molar del impurificante (Te o Zn), que son asignados a los modos de vibración observados de GaAs-like y a la mezcla (GaSb + InAs)-like. La fotoluminiscencia a baja temperatura de GaInAsSb tipo-n (o p) se midió como función de la concentración de impurificante añadido a la solución en estado fundido. Los espectros de fotoluminiscencia se interpretaron teniendo en cuenta la no-parabolicidad de la banda de conducción (o de valencia). Tanto el llenado de la banda, así como los efectos de asimetría de la banda debido a la interacción coulombica de portadores libres con impurezas ionizadas y contracción debido a la interacción de intercambio entre portadores libres fueron considerados para tener debidamente en cuenta las características observadas en los espectros de fotoluminiscencia. Se muestra que la energía de transición de banda a banda puede ser usada para estimar la concentración de portadores libres en GaInAsSb para un amplio rango de concentración de impurificante. <![CDATA[Síntesis y caracterización de nanopartículas de paladio, depositadas sobre un soporte modificado de TiO<sub>2</sub> y su estudio en la reacción de oxidación de CO a CO<sub>2</sub>]]> Resumen: En este artículo, se reporta la síntesis de catalizadores heterogéneos con baja carga de paladio (Pd/TiO2), usando el método de depósito-precipitación con urea (DPU) y la evaluación de la actividad catalítica mediante la reacción de oxidación de CO a CO2; mostrando conversión de CO a temperatura ambiente. Los resultados indican, que el catalizador Pd/TiO2 con 2% de Pd exhibe excelente actividad para la oxidación de CO y alta estabilidad, durante un periodo de cuarenta y nueve días.<hr/>Abstract In this article, we report the synthesis of heterogeneous catalysts with low palladium loading (Pd/TiO2) by deposition-precipitation using urea (DPU) and the catalyst activity that was evaluated by means of the CO- CO2 oxidation reaction, showing CO conversion at room temperature. The results indicate that Pd/TiO2 catalyst with 2% Pd exhibits excellent activity for CO oxidation and high stability during forty-nine days. <![CDATA[Raman spectroscopy of carbon nano-particles synthesized by laser ablation of graphite in water]]> Abstract Carbon nanoparticles (CNPs) have been synthesized by laser ablation of polycrystalline graphite in water using a pulsed Nd:YAG laser (1064 nm) with a width of 8 ns. Structural and mesoscopic characterization of the CNPs in the supernatant by Raman spectroscopy provide evidence for the presence of mainly two ranges of particle sizes: 1-5 nm and 10-50 nm corresponding to amorphous carbon and graphite NPs, respectively. These results are corroborated by complementary characterization using atomic force microscopy (AFM) and transmission electron microscopy (TEM). In addition, large (10-100 μm) graphite particles removed from the surface are essentially unmodified (in structure and topology) by the laser as confirmed by Raman analysis. <![CDATA[Hardness optimization of boride diffusion layer on ASTM F-75 alloy using response surface methodology]]> Abstract In this study, the Response Surface Methodology (RSM) and Central Composite Design (CCD) were used to optimize the hardness of boride diffusion layer on ASTM F-75 alloy (also called Haynes alloy). A boronizing thermochemical treatment was carried out at different temperatures and for different time periods. Hardness tests were conducted. The boride diffusion layer was verified by the X-ray diffraction (XRD) analysis indicating the formation of CoB, Co2B, CrB and Mo2B phases. An optimal hardness of 3139.7 HV was obtained for the samples subjected to the boriding process for a duration of 6.86 h at 802.4°C. <![CDATA[Continued fraction approximations to the temperature integral]]> Abstract The simplest phenomenological model describing thermoluminescence becomes the one-trap-one recombination center model (OTOR). As it is known, the analysis of thermoluminescence data greatly benefits from separation of a glow curve into separate glow peaks. The shape of the peaks in thermoluminescence dosimetry can be represented by the so-called temperature integral. The highly important and close relation between the complementary incomplete gamma function Γ ( a , z ) and the temperature integral in thermoluminescence dosimetry has engaged specialists to pursue a more accurate calculation of Γ ( a , z ). In this work, new approximations for the temperature integral, considering the fast convergence of continued fractions and the straightforward evaluation of the algebraic expressions are obtained. The new reported approximants improve the calculation efficiency of the temperature integral. The simplicity or precision of resent approximations, have a strong effect on values of physical quantities obtained from thermoluminescence data. <![CDATA[Nanosecond laser pulse propagating through turbid media: a numerical analysis]]> Abstract A short pulse transmitted by a turbid medium is both distorted in shape and shifted temporally as its passes through. If the incident pulse is short enough an analytical expression of the deformed pulse can be calculated using the diffusion approximation of the radiative transport equation (RTE), and the optical properties of the medium can be recovered from the transmitted pulse. In this work, the effect of a homogeneous turbid medium on the transmitted temporal profile of nanosecond laser pulses was studied both experimentally and numerically for samples with different optical properties and various laser pulse widths. The numerical results showed a dependence of the pulse distortion on the variables tested, finding that this dependence dramatically changes upon varying pulse width. This work contributes to the field by showing that the interaction between turbid media phantoms and nanosecond laser pulses can also be analyzed, as it is the case for ultrashort laser pulses, by using the diffusion approximation of the RTE in certain regimes. This finding is significant and useful for applications when a laser pulse must attain to certain duration and shape after propagation through a turbid medium or when at a certain time an intensity threshold is required. <![CDATA[Interaction of small carbon molecules and zinc dichloride: DFT study]]> Abstract The interaction between carbon molecules and zinc dichloride molecules (ZnCl2 - zinc salt) comes to be evident by means of small molecular systems of six-carbon (linear or rings) and zinc salt. The interaction is evident through the potential energy surface (PES) of the interacting system at the ground state, starting with a molecular geometry optimization of the energy in order to get equilibrium energy of potential energy surfaces PES. This is one way to start understanding of the porous form of an adsorbent material known as activated carbon. We recognize six-carbon fragments as amorphous activated carbon after geometry optimization with zinc dichloride, which appear in several planar and non-planar forms. An adsorbent material of pollution known as activated carbon can be obtained when carbon molecules are exposed to zinc salt (ZnCl2) molecules. Some cases of the morphology of these systems are accomplished at the density functional theory level (DFT) using GGA-PW91 for exchange and correlation with dnd basis functions.<hr/>Resumen La interacción entre moléculas de carbón y moléculas de dicloruro de zinc (ZnCl2) se hace evidente por medio de sistemas moleculares de pequeñas moléculas de seis-carbones (lineales o anillos) y sal de zinc. La interacción es evidente a través de superficies de energía potencial (PES) del sistema interactuante en el estado base, empezando con una optimización de geometría molecular de la energía para obtener la energía de equilibrio de la superficie de energía potencial PES. Esta es una manera de empezar a entender la forma porosa de un material adsorbente conocido como carbón activado. Reconocemos fragmentos de seis-carbones como carbón activado amorfo con sal de zinc, que aparecen en varias formas planas y no-planas. Un material adsorbente de contaminantes se puede obtener cuando las moléculas de carbón se exponen a moléculas de sal de zinc (ZnCl2). Algunos casos de la morfología de estos sistemas se logran con un nivel de teoría de funcionales de la densidad (DFT) usando GGA-PW91 para intercambio y correlación con funciones base dnd.