Scielo RSS <![CDATA[Revista mexicana de física]]> vol. 59 num. 2 lang. en <![CDATA[SciELO Logo]]> <![CDATA[<b>Critical and compensation temperatures for the mixed spin-3/2 and spin-5/2 Ising model</b>]]> We have studied the critical and compensation temperatures of a ferrimagnetic Ising system with mixed spins SiA= ±3/2, ±1/2 and σjB = ±5/2, ±3/2, ±1/2, by using Monte Carlo simulations. The spins are alternated on a square lattice, such that nearest neighbor interactions occur between different spins (SiA ↔ σjB) and next nearest neighbors interactions between spins of the same type (SiA ↔ SiA). We investigate the effects of crystal field D and the J2 ferromagnetic coupling of spins SiA on the critical and compensation temperatures of the system, calculating the phase diagrams at finite temperature at the (D/|J1| ,kB T/|J1|) and (J2/|J1|,kB T/|J1|) planes. When the Hamiltonian includes antiferromagnetic couplings between spins SiA and σjB, ferromagnetic between spins SiA and the term of single ion anisotropy D, the system presents compensation temperatures in a certain range of parameters, which depend on the intensity of the ferromagnetic interaction of spins SiA. <![CDATA[<b>A program for phase identification using diffractograms obtained from TEM structure images</b>]]> In this work a computer program for the indexing of diffractograms is presented. The diffractograms are obtained by means of a discrete Fourier transform from high resolution electron microscope images. The program requires the use of x-ray diffraction data files together with a fast Fourier transform program, for this purpose we used the Digital Micrograph software.<hr/>Se presenta un programa de computo que ayuda a identificar los planos cristalográficos (indexar difractogramas) que aparecen al aplicar la Transformada Rápida de Fourier (FFT) a una imagen digital de alta resolución obtenida con un microscopio electrónico de transmisión de alta resolución. El programa requiere el uso de los archivos de Rayos X de uso común y de un programa que obtenga la FFT, para este caso se utiliza el software de Digital Micrograph. <![CDATA[<b>Estructura reconfigurable</b>: <b>comparador/multiplexor/demultiplexor</b>]]> La presente investigación da a conocer una estructura electrónica reconfigurable mediante la cual es posible obtener diferentes funciones lógicas, principalmente circuitos combinacionales como comparadores, multiplexores y demultiplexores. Se realiza el análisis matemático de estos sistemas, comprobando su veracidad con resultados físicos.<hr/>This research shows a reconfigurable electronic structure, which it could obtain different logic functions and many combinational logic circuits as comparators, multiplexers and demultiplexers. We perform the mathematical analysis of the systems and by physical results we check its veracity. <![CDATA[<b>Exploring solutions for Type-II superconductors in critical state</b>]]> An analytical solution is found for magnetic induction in a type-II superconducting plate in parallel geometry. In this study, critical current density is modeled following the Ming Xu et al., approach which requires the adjusting parameters n and B*. A symmetry relation between the generating equations of the H &gt; 0 and H < 0 cases is presented, as well as an equivalence between vertical and power laws for a superconductor in steady state. We present a systematic procedure for identifying magnetic induction profiles and we analyze the physical characteristics of the superconducting plate when n and B* vary. Specifically, we present flat surface plots of the penetration field, double penetration field, critical current density and, finally, an application of our results to calculate the shielding field, which prevents the occurrence flux jumps. <![CDATA[<b>Phenomenological model for yield stress based on the distribution of chain lengths in a dilute magnetorheological fluid under an oscillatory magnetic field</b>]]> We study the distribution of chain lengths in a magnetorheological fluid based on mineral magnetite particles dispersed in a dexrontype oil exposed simultaneously to a static magnetic field and a sinusoidal field of low amplitude, the fields are transverse to each other. We experimentally determined the general behavior of the distributions under several conditions of intensities of the fields, particle concentration, liquid viscosity, and frequency of the sinusoidal field. We found that in all cases exponential fits describe well these distributions. Based on this result we propose a simple yield stress model for a magnetorheological fluid which differs from other models that posit all chains are of the same length. We compare this model with experimental results and found that there is a well qualitative agreement with them and in some cases also there is a quantitative agreement. In the case of using only static field, the model is in accord with the existence of a magnetic field threshold above which the yield stress increases faster than below it. <![CDATA[<b>A theorem allowing the derivation of deterministic evolution equations from stochastic evolution equations</b>: <b>Tensorial, spinorial, and other extensions</b>]]> The proof of a new extension of a theorem that allows to construct deterministic evolution equations from a set of discrete stochastic evolution equation is developed. The present extension allows to handle evolution equations of dynamical variables that are tensors of any rank. Due that the almost paradigmatic field that uses tensors is relativity, an illustrative example is given and the equations that allows to find the geodesics is derived from a set of discrete stochastic evolution equations. Extension to dynamical variables described by spinor indices or "arbitrary labels" are given.<hr/>La demostración de una nueva extensión de un teorema, que permite la construcción de ecuaciones de evolución deterministas a partir de un conjunto de ecuaciones de evolución discretas estocásticas, es desarrollada. La extensión presente, permite manejar ecuaciones de evolución de variables dinámicas que son tensores de cualquier rango. Como el más paradigmático campo que usa tensores es la relatividad, un ejemplo ilustrativo es dado y las ecuaciones que permiten hallar las geodésicas es derivado de un conjunto de ecuaciones de evolución discretas estocásticas. Extensiones a variables dinámicas descriptas por índices espinoriales o "etiquetas arbitrarias" son dados. <![CDATA[<b>Microwave noise sources contributions to SiGe</b>: <b>C/Si and InP/InGaAs HBT's performances</b>]]> The present work describes the quantification of the noise sources contributions to the microwave transistor noise performance, particularly focusing on the minimum noise factor (Fmin) and on the equivalent noise resistance (Rn). For this analysis microwave noise small-signal modeling is used. This study is performed for one SiGe:C/Si and one InP/InGaAs heterojunction bipolar transistor (HBT) at several bias points and at two operation frequencies. It is shown that some parameters usually neglected to develop simplified formulas for noise analysis have a non-negligible contribution to Fmin and Rn. This demonstrates that for other HBT technologies it is necessary to carry out a similar study in order to determine whether noise sources can be neglected or not. This procedure may be useful when deriving simplified and accurate models of microwave noise analysis. The development of accurate and simplified analytical models for noise analysis for many other HBT (III-V and IV-IV) technologies may benefit from this procedure.<hr/>En este trabajo se describe la cuantificación de las diferentes fuentes de ruido que contribuyen al funcionamiento en ruido del transistor, orientándose particularmente sobre el factor de ruido mínimo (Fmin) y la resistencia de ruido equivalente (Rn), para ello nos basamos en el modelado en pequeña señal de altas frecuencias con ayuda del circuito eléctrico equivalente. El análisis es llevado a cabo para dos transistores bipolares de heterounión (TBH), uno SiGe:C y otro InP/InGaAs. Este estudio es realizado bajo diferentes niveles de polarización y para dos frecuencias de operación. Los resultados muestran que algunos parámetros usualmente despreciados para el análisis de ruido de microondas tienen una contribución no despreciable sobre Fmin y Rn. Esto es un indicador de que es necesario realizar un estudio similar al descrito en este artículo para determinar si una fuente de ruido puede ser despreciada o no. Este procedimiento puede ser aplicado para el desarrollo de modelos de análisis de ruido microondas simplificados y precisos que podrían ser útiles para una gran gama de TBH (III-V and IV-IV). <![CDATA[<b>Ecuación de Boltzmann de discos rígidos auto-impulsados para peatones en contraflujo</b>]]> Presentamos una ecuación cinética de Boltzmann para describir un conglomerado que camina en contraflujo sobre un corredor. Se considera a los peatones como partículas auto-impulsadas con velocidades de caminado balístico y perturbaciones aleatorias. Los cambios de velocidad durante los encuentros de caminado se representan como potenciales de discos rígidos. Por ser partículas auto-impulsadas los peatones se comportan como agentes reactivos que pueden cambiar voluntariamente la dirección de sus velocidades de caminado durante sus maniobras de evasión. La solución analítica de la ecuación de Boltzmann, en estado estacionario, se determina con base a la función de distribución de velocidades de caminado. La simulación del termino colisional de Boltzmann considera dos modos de operación, caminado libre y caminado con encuentros. Los resultados muestran que se presenta auto-organización colectiva de caminado, a cualquier densidad, lo que resulta ser una estrategia emergente que sirve para mejorar el flujo del conglomerado. Otro resultado importante es el diagrama fundamental, la curva de velocidad de flujo contra densidad, el cual reproduce correctamente los resultados experimentales. Con esto se confirma que nuestro modelo es adecuado para describir el transporte de un conglomerado de peatones.<hr/>We present a Boltzmann kinetic equation to describe a crowd in counter flow walking on a corridor. Pedestrians are considered as self-propelled particles with ballistic walking speeds and random perturbations. Speed changes during encounters are represented as walking hard disks potentials. Because pedestrians are considered self-propelled particles they behave like reactive agents which can change their walking speeds direction voluntarily during the evasive maneuvers. Analytical solutions of the Boltzmann equation, at steady state flow, are determined based on the speed distribution function of walking. The simulation algorithm of the Boltzmann collisional term considers two operation modes, free walking and walking with encounters. The results show collective self-organizing motion, at any density, which turn out to be an emerging strategy used to improve the flow efficiency of the crowd. Another important result is the fundamental diagram, the curve of average flow versus density, which correctly reproduces the experimental results. This confirms that our model is suitable to describe transport of pedestrian crowds. <![CDATA[<b>Microwave assisted synthesis of CdS nanoparticles and their size evolution</b>]]> The study of the size evolution of CdS nanoparticles in aqueous dispersion is presented in this paper. The sodium citrate was employed as stabilizer of CdS nanoparticles synthesized by microwave assisted synthesis. Analysis of this study was carried out by UV-Vis spectrophotometry, by comparison of the band gap energy using theoretical and empirical models. Results obtained show that the synthesis conditions produce CdS nanoparticles with diameters below of 6 nm, which remains stabilized by at least 14 days. These characteristics were confirmed by transmission electron microscopy. The X-ray diffraction pattern confirms cubic phase of the CdS nanoparticles. <![CDATA[<b>The effects of the Bragg curve on the nuclear track formation in CR-39 polycarbonate, with the atomic force microscopy approach</b>]]> The etching nuclear track parameters were analyzed, using atomic force microscopy (AFM), allowing the simulation of the nuclear track profiles evolution. For these experiments, CR-39 (LantrackTM) was chosen, because the excellent energy response to alpha particles. Due to the AFM limitations, it was necessary to reduce the incident particle energy in order to reach the Bragg peak region in the AFM scanning process. The different profile shapes of the etched tracks were clearly observed in the evolution process. <![CDATA[<b>Formation of copper nanoparticles in mordenites with variable SiO<sub>2</sub>/Al<sub>2</sub>O<sub>3</sub> molar ratios under redox treatments</b>]]> A series of protonated copper-containing mordenites with different SiO2/Al2O3 molar ratios (MR) in the range of 10≤MR≤206 was prepared by ion exchange in copper nitrate aqueous solution. The electron paramagnetic resonance of hydrated copper Mordenites series testifies of several Cu²+ ions sites. Hydrogen reduction of copper ions incorporated into the mordenites was shown to lead to different reduced copper species including small metallic particles inter alia. The structural properties and acidity of mordenites were characterized. The optical appearance of the copper particles showed strong but nonmonotonic dependence on the MR value, in line with the variation in acidity of this series of mordenites. Correlations between mordenite properties and the formation of different reduced copper species are discussed.<hr/>Un conjunto de zeolitas mordenitas protonadas e intercambiadas con cobre y con diferentes relaciones molares (RM) de SiO2/Al2O3 en el intervalo 10 ≤MR≤ 206 ha sido preparado por intercambio iónico en solución acuosa de nitrato de cobre. La resonancia paramagnética electrónica del conjunto de mordenitas con cobre en su estado hidratado muestra varios sitios de iones Cu²+. La reducción por hidrógeno de los iones de cobre incorporados en las mordenitas ha demostrado que la reducción produce diferentes especies de cobre, incluyendo la reducción de pequerñas partículas metálicas inter alia. Se han caracterizado las propiedades estructurales y la acidez del conjunto de mordenitas intercambiadas. La serial (óptica de nanoparticulas de cobre mostró fuerte dependencia monotónica, pero no con el valor RM, en línea con la variación de la acidez de este conjunto de mordenitas. Las correlaciones entre las propiedades de las mordenitas y la formación de diferentes especies reducidas de cobre son discutidas. <![CDATA[<b>Evaluation of a grid-connected photovoltaic system and in-situ characterization of photovoltaic modules under the environmental conditions of Mexico City</b>]]> In this work we present the results of the monitoring and statistics of a grid connected photovoltaic (PV) system located on the roof of the building occupied by the "Instituto de Ciencia y Tecnologia del Distrito Federal" (ICyTDF), at the historical center of Mexico-City (DF). The on-grid connected photovoltaic system has 6150 Watts peak (Wp). This system ensures a daily mean reduction of 30 kW.h in the consumption of electric energy at the ICyTDF facilities; which results in a significant saving of fossil fuels and avoiding the emission of greenhouse gases. This system can generate an annual average of 10.8 MW.h of electrical energy, equivalent to burning 0.87 tonnes of oil which would lead to the emission of approximately 2.1 tonne/year of CO2 into the atmosphere. The ecological impact of the photovoltaic system installed in the ICyTDF is equivalent to a forest area with more than 700 adult trees (7 hectares of forest). We also report the analysis of the PV-performance in real time of several commercial solar modules, manufactured with different materials and technologies, these results may give an indication of the comparative performance of the technologies under the environmental conditions of México City. Four different solar modules made of monocrystalline silicon, polycrystalline silicon, amorphous silicon and CdS/CdTe technologies were tested. The outcome of this study is that a better performance was noted for modules made of amorphous silicon technology under temperature variations typical of México City's environmental conditions. <![CDATA[<b>IHD para medir simultáneamente en 2D, usando dos fuentes independientes de luz laser con longitud de coherencia grande</b>]]> Se presenta un arreglo para medir simultáneamente dos componentes ortogonales de deformación en la superficie de un objeto. El arreglo, basado en interferometría holográfica digital, usa dos láseres para grabar independientemente dos hologramas en un sólo cuadro de la cámara CCD. El arreglo se prueba con una placa metálica que cambia ligeramente sus dimensiones al ser calentada Trabajos recientes en el desarrollo de este tipo de arreglos, muestran limitantes en el hecho de no poder usar fuentes de luz con longitud de coherencia relativamente grandes. Este arreglo esta exento de esa limitante.<hr/>One presents an arrangement to measure simultaneously two orthogonal components of deformation in the surface of an object. The arrange-ment, based in digital holographic interferometry, uses two lasers to record two holograms independently in a single frame from the camera CCD. The arrangement is proved by a metallic plate that changes lightly his dimensions to be heated. Recent works in the development of this type of arrangement, show constraints in the fact to not use light sources with relatively large coherence length. This arrangement is exempt from this limitation.