Scielo RSS <![CDATA[Superficies y vacío]]> http://www.scielo.org.mx/rss.php?pid=1665-352120090001&lang=pt vol. 22 num. 1 lang. pt <![CDATA[SciELO Logo]]> http://www.scielo.org.mx/img/en/fbpelogp.gif http://www.scielo.org.mx <![CDATA[<b>Effect of duty cycle on surface properties of AISI 4340 using a pulsed plasma nitriding process</b>]]> http://www.scielo.org.mx/scielo.php?script=sci_arttext&pid=S1665-35212009000100001&lng=pt&nrm=iso&tlng=pt Plasma nitriding is a glow discharge technique, which is widely used to improve the surface properties of steel and its alloys, such as wear, corrosion resistance and hardness; additionally, it can increase fatigue strength. The aim of this work is to evaluate the effect of duty cycle on the surface properties of AISI 4340 low alloy steel, in a pulsed plasma nitriding process. The effect of duty cycle (applying a 10 - 80 % tON interval) on the surface hardness, treatment depth, compounds zone width and crystalline phase composition will be discussed. It has been found that for nitriding process at 540 °C in a gas mixture 50 % N2 - 50 % H2 at a constant pressure, using a discharge voltage of -800 V, duty cycle has a negligible effect on crystalline phase composition, even though the influence on surface hardness, treatment depth and compounds zone width is evident. <![CDATA[<b>Characterization of linseed oil epoxidized at different percentages</b>]]> http://www.scielo.org.mx/scielo.php?script=sci_arttext&pid=S1665-35212009000100002&lng=pt&nrm=iso&tlng=pt Several degree-epoxidized linseed oils (8, 20, 35, 40, 47 and 54%) were prepared for changing different reaction parameters such as temperature, amount of peroxide and enzyme of the well studied chemoenzymatic epoxidation method. The epoxidation reaction following was carried out by Infrared spectroscopy (FTIR) and Proton Nuclear Magnetic Resonance (H¹NMR) which are the most usual spectroscopes for this propose. However, microRaman spectroscopy and Differential Scanning Calorimetry (DSC) characterization were used in this work as complementary techniques. Particularly, DSC permitted to correlate the epoxy-aperture energy to each epoxidation percentage making it another optional technique for quantify epoxidation levels in triglycerides. <![CDATA[<b>FTIR and photoluminescence of annealed silicon rich oxide films</b>]]> http://www.scielo.org.mx/scielo.php?script=sci_arttext&pid=S1665-35212009000100003&lng=pt&nrm=iso&tlng=pt In order to have optoelectronic function integrated in a single chip, it is very important to obtain a silicon compatible material with an optimal Photoluminescence (PL) response. The Silicon Rich Oxide (SRO) has shown intense PL and is also compatible with silicon technology. In this work, the composition and optical properties of the SRO films are studied using null Ellipsometry, Fourier Transformed Infrared spectroscopy (FTIR), and Photoluminescence (PL). The SRO films were annealed at high temperature during different times. The IR absorption spectra show the presence of three characteristics Si-O-Si vibrations modes in SiO2. However, changes in their intensity and position were observed when annealing time and silicon excess were varied. These changes are directly related with structural variation in the SRO films. PL spectra show a considerable emission in the range 650 to 850 nm that varies with different thermal treatment times. <![CDATA[<b>Anisotropía en la propagación del sonido de un cristal fonónico desordenado</b>]]> http://www.scielo.org.mx/scielo.php?script=sci_arttext&pid=S1665-35212009000100004&lng=pt&nrm=iso&tlng=pt En este trabajo se presentan resultados de cálculos teóricos de la velocidad efectiva del sonido de un sistema bidimensional desordenado de cilindros de aluminio en aire en una estructura hexagonal. Al comparar con el caso simétrico se encuentra una ligera anisotropía inducida por el desorden. También se analiza el caso en el que el desorden se introduce mediante una variación aleatoria de los radios de los cilindros y se encuentra que la anisotropía es más marcada en este último caso. Utilizando el método de expansión de ondas planas en el límite de grandes longitudes de onda (homogenización) desarrollado previamente y, tomando una supercelda de 25 cilindros ligeramente desordenados, calculamos la velocidad del sonido en sistemas periódicos.<hr/>We present theoretical results for the effective sound velocity of a two-dimensional disordered system composed by a hexagonal array of aluminum cylinders in air. Compared with the symmetric case we observe a small anisotropy induced by the disorder. We analyze a different type of disorder considering a random variation of the cylinders radii and we observe a bigger anisotropy for this case. Using the plane wave expansion method in the long wavelength limit (homogenization) previously developed together with a supercell containing 25 cylinders lightly disordered, we calculate the sound velocity in periodic systems. <![CDATA[<b>Análisis DFT de las propiedades electrónicas de las hojas de grafeno y de nitruro de boro dopadas</b>]]> http://www.scielo.org.mx/scielo.php?script=sci_arttext&pid=S1665-35212009000100005&lng=pt&nrm=iso&tlng=pt Se investigan las propiedades electrónicas de la hoja de grafeno y hoja de nitruro de boro tipo coroneno (C24H12) dopadas con un hexágono central de nitrógeno y carbono respectivamente, esto se hace usando la Teoría del Funcional de la Densidad (DFT) en la parametrización para el término de intercambio-correlación B3PW91 y base atómica de valencia dividida 6-31G(d). Para la estabilidad estructural se sigue el criterio de obtener frecuencias de vibración positivas. Además se reportan los parámetros de reactividad (potencial químico, dureza química e índice electrofílico), y densidad de estados electrónicos (DOS). Los resultados nos indican que la hoja de grafeno es semiconductor tanto antes como después de doparla, pero la de nitruro de boro pasa de aislante a semiconductor. Además según los parámetros de reactividad química dichos sistemas dopados favorecen más el proceso de fisisorción.<hr/>We have investigated the electronic properties of graphene and boron nitride sheet coronene-like (C24H12) through the Density Functional Theory (DFT) with the functional of exchange-correlation of B3PW91 and the basis 6-31G(d). It has been calculated the optimal geometry, vibration frequencies (criterion of stability), dipolar moment, and reactivity parameters as chemical potential, hardness and electrofily index. The parameters obtained suggest that the graphene sheet shows semiconductor-like behavior despite of doping and the boron nitride sheet suffers a transformation from insulator-like to a semiconductor-like. In addition according to the parameters of chemical reactivity of these doped systems drive us better the process of physisorption.