Scielo RSS <![CDATA[Superficies y vacío]]> http://www.scielo.org.mx/rss.php?pid=1665-352120140002&lang=pt vol. 27 num. 2 lang. pt <![CDATA[SciELO Logo]]> http://www.scielo.org.mx/img/en/fbpelogp.gif http://www.scielo.org.mx <![CDATA[<b>Nanopartículas de Au soportadas sobre Al<sub>2</sub>O<sub>3</sub> sólido de alta rugosidad</b>]]> http://www.scielo.org.mx/scielo.php?script=sci_arttext&pid=S1665-35212014000200001&lng=pt&nrm=iso&tlng=pt Convencionalmente los convertidores catalíticos utilizan monolitos cerámicos como soporte de la fase activa, dada su alta área superficial, sin embargo, son muy frágiles. Por ello, el objetivo de este trabajo es obtener sobre una superficie metálica, características de rugosidad apropiadas para aplicaciones catalíticas. Así mismo depositar sobre ella nanopartículas de Au por un método no convencional: descarga de plasma. Para lograr el primer objetivo, se creció por métodos mecano-térmicos una superficie de alúmina sobre un sustrato metálico de FeCrAl. La rugosidad superficial de la alúmina fue cuantificada determinando su dimensión fractal. Para un crecimiento durante 24h a 900 °C, la dimensión fractal fue de 2.59. Difracción de rayos X en haz rasante revela la presencia de las fases θ y α en la capa de óxido crecida, la cual tiene un espesor de 4-5 µm según medidas en sección transversal realizadas por SEM. El mapeo de R-X en sección transversal revelan la presencia solo de Al y O en la capa. Finalmente, nanopartículas de Au fueron depositadas sobre la superficie rugosa de la alúmina y su presencia controlada por SEM-EDXS. Para el conteo de tamaño de partículas se empleo un STEM revelando una distribución de tamaño de nanopartículas estrecha, +/- 1 nm, alrededor de un valor medio de 3.2 nm, con una alta densidad de dispersión, alrededor del 50 % y sin coalescencia significativa. Como conclusión, las características peculiares de soportes catalíticos fueron reproducidas sobre un sustrato metálico. Adicionalmente, un método matemático riguroso se introdujo por primera vez, usando dimensión fractal para caracterizar la rugosidad superficial. El tamaño y la distribución de tamaños pequeños de las nanopartículas de Au depositadas por descarga de plasma, así como su alta dispersión sobre la alúmina crecida, superan las obtenidas por otros métodos y aseguran una buena aplicación catalítica.<hr/>Conventionally catalytic converters are made of ceramic monoliths as active phase support, because they have high surface area, however on the other side, they are very brittle. Thinking about this problem, one task of this work consists in obtaining a metallic monolith, with suitable roughness for catalytic applications; as well as to deposit on it, Au-nanoparticles by plasma discharge, a nonconventional method. In this way, alumina whiskers were grown on FeCrAl alloy by thermomechanical processes, and its surface roughness was quantified by fractal dimension, namely 2.59, for 24 h growth at 900 °C. Grazing X-Ray Diffraction reveals the presence of θ and α alumina phases making up the 4-5 µm thickness layer measured in cross-section by SEM, moreover, the X-Ray elements mapping highlights only Al and O on the layer. The Au-nanoparticles deposition on the rough alumina layer was controlled by EBSD-EDXS and for statistical size distribution it was employed an STEM. The average nanoparticle size was of 3.2 nm with a spread of +/- 1 nm and a near 50 % occupancy, this without significant coalescence. As conclusion, peculiar characteristics of a catalytic support were achieved on a metallic substrate and by first time, fractal dimension, as a rigorous mathematical method, was introduced to quantify the substrate surface roughness. The small size and the narrow size distribution of the Au-nanoparticles deposited by plasma, as well as their high dispersion on the alumina grown, overcome the results obtained by conventional methods and promise good catalytic applications. <![CDATA[<b>Effect of sintering temperature on the structure and mean crystallite size of Zn<sub>1-x</sub>Co<sub>x</sub>O (x = 0.01 - 0.05) samples</b>]]> http://www.scielo.org.mx/scielo.php?script=sci_arttext&pid=S1665-35212014000200002&lng=pt&nrm=iso&tlng=pt In this work, powders were prepared Zn1-xCo xO (0.00 ≤ x ≤ 0.05) at temperatures of calcination of 773, 823, and 873 K by sol gel methods (citrate route). In the XRD patterns, all the observed diffraction peaks could be indexed to ZnO wurtzite structure, and no other impurity phase was found, leading to the conclusion that the Co ions in fact entered into ZnO crystal. Rietveld analysis shows that all samples have a hexagonal wurtzite structure with mean lattice constants a = 3.2507 A˚ and c = 5.2073 A˚, and that no important changes of the lattice parameters were observed as a consequence of Co doping or temperature of calcination. The average crystallite size was measured using Scherrer's method, and the results show that when Co concentration increased from 0 to 5%, crystallite size varied depending on the sample calcination temperature (Tc). In general, the lower the Tc, the higher the crystallite size, and at a particular Tc, the higher the Co concentration, the higher the crystallite size. The last one suggests the promotion of crystal growth as a result of Co doping in ZnO at least for the synthesis method used here. Raman spectroscopy results showed an increase of the defect concentration as a consequence of an increase in Co contents. <![CDATA[<b>Optimización para la generación de modos de superficie en la configuración de Kretschmann empleando LHM</b>]]> http://www.scielo.org.mx/scielo.php?script=sci_arttext&pid=S1665-35212014000200003&lng=pt&nrm=iso&tlng=pt En este trabajo se plantea una metodología para generar las condiciones óptimas para excitar en general los modos de superficie en las polarizaciones P y Sen un sistema vidrio-metamaterial-aire (configuración de Kretschmann). Para ello se calculan las relaciones de dispersión electromagnética que derivan de resolver las ecuaciones de Maxwell para una interfazmetamaterial-aire para ambas polarizaciones, las cuales nos indican las regiones de excitación, en particular de los modos de superficie (Polarización S) y plasmones de superficie (Polarización P). Para este trabajo se presentan espectros de reflexión, la simulación numérica de la técnica de ATR y la variación del campo eléctrico dentro del sistema estudiado.<hr/>We describe a methodology to explore optimal conditions to excite surface modes in a glass-metamaterial-vacuum system (Kretschmann configuration) using both P and S polarized light. We calculate the electromagnetic dispersion relation of a metamaterial surface by solving Maxwell equations for both polarizations with appropriate boundary conditions. The dispersion relations define the frequency domain of the surface mode existence for the surface modes (with S polarization) and surface plasmons (with P polarization). In this work we report reflectivity spectra calculations, the numerical simulation of the attenuated total reflectivity technique and the electric field variations within the layer system. <![CDATA[<b>On the properties and resistance to abrasive wear of surface-modified Ti6Al4V alloy by laser shock processing</b>]]> http://www.scielo.org.mx/scielo.php?script=sci_arttext&pid=S1665-35212014000200004&lng=pt&nrm=iso&tlng=pt The application of traditional surface modification techniques to improve mechanical properties of a wide range of materials has been used for at least three decades with important results. More recently, newer and innovative techniques such as Laser Shock Processing (LSP) have gained popularity due to the benefits offered. In this work, Ti6Al4V alloy was treated under several conditions of laser density and wavelength during the treatment. The roughness of the samples before and after treatment was measured by perfilometry. The resultant surface roughness average (Ra) is in the range for biomedical implants. The microhardness values were taken from the sample cross-section showing no increment after being treated with LSP. X-ray diffraction (XRD) was used for phase identification and possible changes in the lattice parameters. The abrasive wear resistance was evaluated by means of ball cratering tests, the wear volume was assessed measuring wear scars using profilometry from which the wear rate was calculated. Samples treated with LSP were in some cases more susceptible to abrasive wear that the untreated material. <![CDATA[<b>Diseño de un dispositivo sensor de masa tipo MEMS basado en microvigas en voladizo de polisilicio</b>]]> http://www.scielo.org.mx/scielo.php?script=sci_arttext&pid=S1665-35212014000200005&lng=pt&nrm=iso&tlng=pt Se presentan tres diseños de sensores de masa MEMS basados en vigas en voladizo de polisilicio, los cuales utilizan el principio de cambio de frecuencia debido a una masa añadida mientras operan en oscilación libre. Las estructuras se accionan mediante un actuador electroestático, que consiste en un peine de capacitores interdigitados, con una orientación tal que se maximiza la amplitud de oscilación del voladizo. En consecuencia se abordan problemas de integración entre las estructuras y un circuito electrónico de acondicionamiento. Los diseños se fabricarán por micro maquinado de superficie mediante el proceso SUMMiT-V y pueden ser adecuados para aplicaciones biológicas o químicas. La viabilidad de las propuestas se sustenta mediante análisis de elemento finito (FEA) realizados mediante la herramienta de COMSOL. Se obtuvo una sensitividad de 3.43 pg/Hz, 6.71 pg/Hz y 17.31 pg/Hz para cada uno de los tres diseños.<hr/>This work presents three MEMS mass sensor devices based in oscillating polysilicon cantilevers whose oscillation frequency shifts, due to corresponding changes of an added mass in the reaction container/area, can be better measured. An electrostatic actuator is used which consists of interdigitated electrodes configured to maximize the cantilever's oscillation amplitude. In consequence, integration design issues between the microelectromechanical device and the signal conditioning and acquisition circuitry are addressed. These designs are to be fabricated in the SUMMiT-V process and they can be suitable for biological or chemical applications. The feasibility of the proposed designs is supported by a mathematical model and FEA calculations performed in COMSOL and ANSYS and mass resolutions of de 3.43 pg/Hz, 6.71 pg/Hz y 17.31 pg/Hz were obtained. <![CDATA[<b>Diseño integrado de un Amplificador Lock-in compacto de bajo consumo para aplicaciones portátiles</b>]]> http://www.scielo.org.mx/scielo.php?script=sci_arttext&pid=S1665-35212014000200006&lng=pt&nrm=iso&tlng=pt Una técnica para la medición de la amplitud de una señal, incluso si el ruido y las interferencias superpuestas son mayores que la propia señal, es la técnica de detección sensible a fase, que es la base de los amplificadores lock-in. En este trabajo se presenta un nuevo diseño de amplificador lock-in en modo corriente, integrado en una tecnología CMOS de 0.18µm con voltaje de alimentación de 1.8V, y que es adecuado para aplicaciones portátiles gracias a su reducido consumo de potencia y voltaje de alimentación único. La arquitectura propuesta es capaz de recuperar una señal de interés de ambientes ruidosos con errores por debajo del 6.1% para una reserva dinámica de 42.7dB, está provista de una ganancia entre 1.2 y 20.3dB programable de forma digital, ruido referido a la entrada de @1kHz y consumo de potencia de 237µW.<hr/>A technique for measuring the amplitude of a signal, even if the superimposed noise and interferences are higher than the signal itself, is the technique of phase sensitive detection, which is the basis of lock-in amplifiers. In this work a novel design of an integrated current mode lock-in amplifier in 0.18μm CMOS technology with 1.8V supply voltage is presented. It is suitable for portable applications thanks to its reduced low power consumption and single supply voltage. The proposed architecture is capable of recovering a signal of interest from noisy environments with errors below 6.1% for a dynamic reserve of 42.7dB. It is provided of a digitally programmable gain ranging from 1.2 to 20.3dB, the input referred noise is @1kHz and power consumption is 237μW. <![CDATA[<b>Matching of p-n extruded materials based on Bi-Sb chalcogenides for thermoelement</b>]]> http://www.scielo.org.mx/scielo.php?script=sci_arttext&pid=S1665-35212014000200007&lng=pt&nrm=iso&tlng=pt Extruded thermoelectric materials of different conductivity types based on bismuth and antimony chalcogenides have quite various thermoelectric properties as shown by the experimental data given. The paper discusses how to match thermocouple pellets of such materials optimally. It is shown that the condition of equality of absolute values of the Seebeck coefficients at room temperature is a good criterion to match thermocouple materials for obtaining the maximum possible temperature difference.