Menú secundario

Artículos SCI


Mechanochemical preparation of BaTiO3-Ni nanocomposites with high dielectric constant

Sanchez-Jimenez, PE; Perez-Maqueda, LA; Dianez, MJ; Perejon, A; Criado, JM
Composite Structures, 92 (2010) 2236-2240
Reactividad de Sólidos


A mechanochemical procedure is proposed for an easy preparation of a BaTiO3-Ni composite in a single step. BaTiO3 and Ni powders available in the market are mixed by dry ball milling producing a decrease of particle size and an evenly distribution of both phases. In the sintered pellets the nickel particles are homogeneously distributed into the BaTiO3 matrix and isolated from others Ni particles. The dielectric constant of the composite is considerably higher than that of the barium titanate. Moreover, the temperature of the ferroelectric <-> paraelectric transition of the BaTiO3-Ni composite here prepared is much lower than the one of the pure BaTiO3 single phase.

Agosto, 2010 | DOI: 10.1016/j.compstruct.2009.08.011

Study of nanostructured Ni/CeO2 catalysts prepared by combustion synthesis in dry reforming of methane

Gonzalez-Delacruz, VM; Ternero, F; Pereniguez, R; Caballero, A; Holgado, JP
Applied Catalysis A-General, 384 (2010) 1-9
Materiales y Procesos Catalíticos de Interés Ambiental y Energético


This work reports the study of several catalysts of Ni-CeO2 active for dry methane reforming process (CH4 + CO2 -> 2CO + 2H(2)). The use of Ni as active phase is highly preferred, due to its availability, high activity and low cost, although its main lack is the coke formation on the surface of Ni metal particles, resulting in a severe deactivation. Here we report a new synthesis method that allows a simple, effective and fast way to prepare Ni-CeO2 catalysts, in a wide range of metallic loadings, resulting in all the cases in well-formed NiO crystallites with sizes in the range of 12-18 nm. The use of CeO2 as a support has been based on its massive use in TWC catalysts formulations, where it is recognized to activate CH4 and lower hydrocarbon dissociation. Moreover, CeO2 has been reported to have an intrinsic activity in the CH4 reforming reaction. Besides the metallic loading, several factors that control the preparation method of the catalyst have been varied, in order to optimize their performance. Most of the catalysts prepared show activity and selectivity values close to thermodynamic ones, maintaining a good stability on long periods of time and severe conditions. Nevertheless, formation of some carbon nano-fibers has been observed, which could result in a drawback for their application at large scale.

Agosto, 2010 | DOI: 10.1016/j.apcata.2010.05.027

Flexible, Adhesive, and Biocompatible Bragg Mirrors Based on Polydimethylsiloxane Infiltrated Nanoparticle Multilayers

Calvo, ME, Miguez, H
Chemistry of Materials, 22 (2010) 3909-3915
Materiales Ópticos Multifuncionales


Herein we present a series of self-standing, flexible, and biocompatible optical interference filters obtained through infiltration and polymerization of an elastomer (polydimethylsiloxane) in a porous Bragg mirror prepared by alternating deposition of layers of TiO2 and SiO2 nanoparticles. The method proposed yields the uniform filling of the nanopores of the multilayer by the polymer, which allows lifting off the hybrid structure as long as the ensemble is cooled to temperatures below the glass transition of the polymer. This multifunctional material combines the optical properties of the periodic nanoporous multilayer and the structural and physicochemical characteristics of polydimethylsiloxane. Experimental demonstrations of their potential use as flexible and adhesive UV-protecting filters, as well as of light, highly-efficient conformal back reflectors to enhance the efficiency of photovoltaic devices are provided.

Julio, 2010 | DOI: 10.1021/cm1001016

Anomalous group velocity at the high energy range of a 3D photonic nanostructure

Botey, M; Martorell, J; Dorado, LA; Depine, RA; Lozano, G; Miguez, H
Optics Express, 18 (2010) 15682-15690
Materiales Ópticos Multifuncionales


We report on a study of electromagnetic waves propagation in thin periodically ordered photonic nanostructures in the spectral range where the light wavelength is on the order of the lattice parameter. The vector KKR method we use allows us to determine the group index from finite photonic structures including extinction providing confirmation of recently emerged results. We show that for certain frequencies the group velocity of opal slabs can either be superluminal or approach zero depending on the crystal thickness and the unavoidable presence of losses. In some cases, group velocity can be negative. Such behavior can be clearly attributed to the finite character of the three-dimensional structure and reproduces previously reported experimental observations. Calculations show that contrary to the predictions of extraordinary group velocity reductions for infinite periodic structures, the group velocity of real opals may exhibit strong fluctuations at the high energy range. Hence, a direct identification between the calculated anomalous group velocities, for an actual opal film, and the predicted propagating low dispersion modes for an ideal infinite ordered structure seems difficult to establish.

Julio, 2010 | DOI: 10.1364/OE.18.015682

Steering the Self-Assembly of Octadecylamine Monolayers on Mica by Controlled Mechanical Energy Transfer from the AFM Tip

Benitez, JJ; Heredia-Guerrero, JA; Salmeron, M
Journal of Physical Chemistry C, 114 (2010) 12630-12634
Materiales de Diseño para la Energía y Medioambiente


We have studied the effect of mechanical energy transfer from the tip of an atomic force microscope on the dynamics of self-assembly of monolayer films of octadecylamine on mica. The formation of the self-assembled film proceeds in two successive stages, the first being a fast adsorption from solution that follows a Langmuir isotherm. The second is a slower process of island growth by aggregation of the molecules dispersed on the surface. We found that the dynamics of aggregation can be altered substantially by the addition of mechanical energy into the system through controlled tip surface interactions. This leads to both the creation of pinholes in existing islands as a consequence of vacancy concentration and to the assembly of residual molecules into more compact islands.

Julio, 2010 | DOI: 10.1021/jp102813s

Preparation and structural properties of YBCO films grown on GaN/c-sapphire hexagonal substrate

Chromik, S; Gierlowski, P; Spankova, M; Dobrocka, E; Vavra, I; Strbik, V; Lalinsky, T; Sojkova, M; Liday, J; Vogrincic, P; Espinos, JP
Applied Surface Science, 256 (2010) 5618-5622
Nanotecnología en Superficies y Plasma


Epitaxial YBCO thin films have been grown on hexagonal GaN/c-sapphire substrates using DC magnetron sputtering and pulsed laser deposition. An MgO buffer layer has been inserted between the substrate and the YBCO film as a diffusion barrier. X-ray diffraction analysis indicates a c-axis oriented growth of the YBCO films. &#934;-scan shows surprisingly twelve maxima. Transmission electron microscopy analyses confirm an epitaxial growth of the YBCO blocks with a superposition of three a&#8211;b YBCO planes rotated by 120° to each other. Auger electron spectroscopy and X-ray photoelectron spectroscopy reveal no surface contamination with Ga even if a maximum substrate temperature of 700 °C is applied.

Julio, 2010 | DOI: 10.1016/j.apsusc.2010.03.035

Porous One-Dimensional Photonic Crystal Coatings for Gas Detection

Hidalgo, N; Calvo, ME; Colodrero, S; Miguez, H
IEEE Sensors Journal, 10 (2010) 1206-1212
Materiales Ópticos Multifuncionales


Herein, we present an overview of recent progress on the development of different types of porous 1-D photonic crystal coatings which are optically responsive to gas pressure changes in the environment. Modification of the surrounding vapor pressure gives rise to adsorption and condensation phenomena within the porous networks of the photonic crystal building blocks, varying their refractive index and hence their optical features. This effect can be put into practice to precisely detect and monitor changes in the ambient through the spectral shift of either the photonic bandgap of the structure or of some other optical features. Our results demonstrate the potential of these optical coatings as new materials for gas sensing devices.

Julio, 2010 | DOI: 10.1109/JSEN.2010.2043525

Structure of Glancing Incidence Deposited TiO2 Thin Films as Revealed by Grazing Incidence Small-Angle X-ray Scattering

Gonzalez-Garcia, L; Barranco, A; Paez, AM; Gonzalez-Elipe, AR; Garcia-Gutierrez, MC; Hernandez, JJ; Rueda, DR; Ezquerra, TA; Babonneaum, D
Chemphyschem, 11 (2010) 2205-2208
Nanotecnología en Superficies y Plasma


For the first time, grazing incidence small-angle X-ray scattering (GISAXS) analysis is used to characterize the Morphology of TiO2 thin films grown by glancing angle physical vapor deposition (GLAD). According to cross-section scanning electron microscopy (SEM) images, the films consist of near isotilted TiO2 columns of different length and width depending on film thickness. The obtained GISAXS patterns show a characteristic asymmetry with respect to the incidence plane, which is associated with the titled of the TiO2 columns. The patterns also show the existence of two populations of columns in these GLAD TiO2 films. The population of the thinnest columns appears related to the first grown layer and is common for all the films investigated, while the second population of columns grows with the thickness of the films and has been related to wider columns formed by shadowing at the expense of the initially formed columns.

Julio, 2010 | DOI: 10.1002/cphc.201000136

Sc-45 Spectroscopy of Solids: Interpretation of Quadrupole Interaction Parameters and Chemical Shifts

Alba, MD; Chain, P; Florian, P; Massiot, D
Journal of Physical Chemistry C, 114 (2010) 12125-12132
Materiales de Diseño para la Energía y Medioambiente


The aims of the present study is to describe for the first time the Sc-45 MAS NMR spectra of X-2-Sc2SiO5 and C-Sc2Si2O7, to combine the spectroscopic information with the structures published from diffraction data, and to propose a rational interpretation of the chemical shifts and quadrupolar parameters. For that purposed, we have correlated the experimental quadrupole coupling parameters of Sc-45 determined for a number of scandium compounds to those found by a simple electrostatic calculation and we have found that the isotropic chemical shift of the Sc-45 is linearly correlated to the shift parameter, calculated by bond-valence theory. We also show that a simple point charge calculation can approximate the electric field gradient to a sufficiently good approximation that it provides a valuable mean to assign the NMR spectra.

Julio, 2010 | DOI: 10.1021/jp1036525

Influence of carbon chemical bonding on the tribological behavior of sputtered nanocomposite TiBC/a-C coatings

Abad, MD; Sanchez-Lopez, JC; Brizuela, M; Garcia-Luis, A; Shtansky, DV
Thin Solid Films, 518 (2010) 5546-5552
Materiales Nanoestructurados y Microestructura


The tribological performance of nanocomposite coatings containing Ti-B-C phases and amorphous carbon (a-C) are studied. The coatings are deposited by a sputtering process from a sintered TiB2:TiC target and graphite, using pulsed direct current and radio frequency sources. By varying the sputtering power ratio, the amorphous carbon content of the coatings can be tuned, as observed by X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. The crystalline component consists of very disordered crystals with a mixture of TiB2/TiC or TiBxCy phases. A slight increase in crystalline order is detected with the incorporation of carbon in the coatings that is attributed to the formation of a ternary TiBxCy phase. An estimation of the carbon present in the form of carbide (TiBxCy or TiC) and amorphous (a-C) is performed using fitting analysis of the C 1s XPS peak. The film hardness (22 to 31 GPa) correlates with the fraction of the TiBxCy phase that exists in the coatings. The tribological properties were measured by a pin-on-disk tribometer in ambient conditions, using 6 mm tungsten carbide balls at 1 N. The friction coefficients and the wear rates show similar behavior, exhibiting an optimum when the fraction of C atoms in the amorphous phase is near 50%. This composition enables significant improvement of the friction coefficients and wear rates (mu similar to 0.1; k < 1 x 10(-6) mm(3)/Nm), while maintaining a good value of hardness (24.6 GPa). Establishing the correlation between the lubricant properties and the fraction of a-C is very useful for purposes of tailoring the protective character of these nanocomposite coatings to engineering applications.

Julio, 2010 | DOI: 10.1016/j.tsf.2010.04.038

Improved wear performance of ultra high molecular weight polyethylene coated with hydrogenated diamond like carbon

Puertolas, JA; Martinez-Nogues, V; Martinez-Morlanes, MJ; Mariscal, MD; Medel, FJ; Lopez-Santos, C; Yubero, F
WEAR, 269 (2010) 458-465
Nanotecnología en Superficies y Plasma


Hydrogenated diamond like carbon (DLCH) thin films were deposited on medical grade ultra high molecular weight polyethylene (UHMWPE) by radio frequency plasma enhanced chemical vapor deposition. The DLCH coating thicknesses ranged from 250 to 700 nm The substrates were disks made of UHMWPEs typically used for soft components in artificial Joints, namely virgin GUR 1050 and highly crosslinked (gamma irradiated in air to 100 kGy) UHMWPEs Mechanical and tribological properties under bovine serum lubrication at body temperature were assessed on coated and uncoated polyethylenes by means of nano-hardness and ball-on-disk tests, respectively Morphological features of the worn surfaces were obtained by confocal microscopy and scanning electron microscopy This study confirms an increase in surface hardness and good wear resistance for coated materials after 24 h of sliding test compared to uncoated polyethylene. These results point out that to coat UHMWPE with DLCH films could be a potential method to reduce backside wear in total hip and knee arthroplasties.

Julio, 2010 | DOI: 10.1016/j.wear.2010.04.033

Effect of surface roughness and sterilization on bacterial adherence to ultra-high molecular weight polyethylene

Kinnari, TJ; Esteban, J; Zamora, N; Fernandez, R; Lopez-Santos, C; Yubero, F; Mariscal, D; Puertolas, JA; Gomez-Barrena, E
Clinical Microbiology and Infection, 16 (2010) 1036-1041
Nanotecnología en Superficies y Plasma


Sterilization with ethylene oxide (EO) and gas plasma (GP) are well-known methods applied to ultra-high molecular weight polyethylene (UHMWPE) surfaces in the belief that they prevent major material changes caused by gamma irradiation. However, the influence of these surface sterilization methods on bacterial adherence to UHMWPE is unknown. UHMWPE samples with various degrees of roughness (0.3, 0.8 and 2.0 mu m) were sterilized with either GP or EO. The variations in hydrophobicity, surface free energy and surface functional groups were investigated before and after sterilization. Sterilized samples were incubated with either Staphylococcus aureus or Staphylococcus epidermidis in order to study bacterial adherence to these materials. Fewer bacteria adhered to UHMWPE after sterilization with EO than after sterilization with GP, especially to the smoothest surfaces. No changes in chemical composition of the UHMWPE surface due to sterilization were observed using X-ray photoemission spectroscopy analysis. The decreased bacterial adherence to UHMWPE found at the smoothest surfaces after sterilization with EO was not directly related to changes in chemical composition. Increased bacterial adherence to rougher surfaces was associated with increased polar surface energy of EO-sterilized surfaces.

Julio, 2010 | DOI: 10.1111/j.1469-0691.2009.02995.x/full

Sunlight highly photoactive Bi2WO6-TiO2 heterostructures for rhodamine B degradation

Colon, G; Lopez, SM; Hidalgo, MC; Navio, JA
Chemical Communications, 46 (2010) 4809-4811
Fotocatálisis Heterogénea: Aplicaciones


Highly efficient Bi2WO6-TiO2 heterostructures are synthesized by means of a hydrothermal method; they have high photoactivity for the degradation of rhodamine B under sunlike irradiation. An interesting synergetic effect between TiO2 and Bi2WO6 leads to an improved charge carrier separation mechanism, causing the excellent photocatalytic performance.

Julio, 2010 | DOI: 10.1039/c0cc00058b

Microstructural Effects on the Creep Deformation of Alumina/Single-Wall Carbon Nanotubes Composites

Gomez-Garcia, D; Poyato, R; Lee, Z; Castillo-Rodriguez, M; Dominguez-Rodriguez, A; Radmilovic, V; Padture, NP
Journal of the American Ceramic Society, 93 (2010) 2042-2047
Reactividad de Sólidos, Propiedades Mecánicas de Sólidos


The enhanced high-temperature creep resistance in alumina/single-wall carbon nanotubes (SWNTs) composites has been attributed to the unprecedented grain-boundary structure of these composites, where the SWNTs bundles segregated at the alumina grain boundaries partially impede grain-boundary sliding. In this study, the effect of SWNTs distributions at alumina grain boundaries on the creep behavior of alumina/SWNTs composites has been investigated. Microstructures of two different alumina/10 vol% SWNTs composites, one with heterogeneous and the other with homogenous distributions of SWNTs at grain boundaries, have been characterized quantitatively. The steady-state creep rate (uniaxial compression) in the heterogeneous composite has been found to be over three times higher than that in the homogeneous composite at 1300° and 1350°C (argon atmosphere). It is argued that the less uniform distribution of SWNTs at the alumina grain boundaries in the heterogeneous composite results in less effective obstruction of grain-boundary sliding, and attendant higher creep rate. This also results in more efficient recovery in that composite.

Julio, 2010 | DOI: 10.1111/j.1551-2916.2010.03681.x

Computational fluid dynamics study of heat transfer in a microchannel reactor for low-temperature Fischer-Tropsch synthesis

Arzamendi, G; Dieguez, PM; Montes, M; Odriozola, JA; Sousa-Aguiar, EF; Gandia, LM
Chemical Engineering Journal, 160 (2010) 915-922
Química de Superficies y Catálisis


A three-dimensional computational fluid dynamics (CFD) study of heat transfer in a microchannel reactor for the low-temperature Fischer-Tropsch synthesis (FTS) is presented. The microreactor studied is a steel block with 80 square microchannels of 1 mm of side arranged in cross-flow configuration for the transport of syngas and cooling water. Syngas space velocities in the 5000-30,000 h(-1) (SIP) range have been considered. The microreactor exhibited good isothermicity under most simulated conditions. The FTS can be conducted with very low-temperature change between 483 and 523 K within a wide range of CO conversions using boiling water as coolant. To this end the pressure has to be set at the appropriate value between about Sand 35 atm. The pressure would have to be reduced as the CO conversion increases which might have a negative effect on the FTS selectivity to middle distillates. However, adjusting the cooling water flow rate in the range 0.25-250 g min(-1) allows maintaining the FTS temperature at suitable values while avoiding the use of low pressures. Relatively high values of the overall heat transfer coefficient in the 20-320 W m(-2) K-1 range have been obtained. A significant effect of the buoyancy forces on the thermal performance of the microreactor has been found.

Junio, 2010 | DOI: 10.1016/j.cej.2009.12.028

Gallium Arsenide Infiltration of Nanoporous Multi layers: A Route to High-Dielectric-Contrast One-Dimensional Photonic Crystals

Sanchez-Sobrado, O; Thomas, K; Povey, I; Pemble, ME; Miguez, H
Small, 6 (2010) 1283-1287
Materiales Ópticos Multifuncionales


Periodic multilayers of wide photonic bandgap and high reflectance in the visible and near infrared regions are fabricated. Optical properties show that reflectance intensities close to 90% are reached for stacks of only six layers, as well as gap-to-midgap ratios of 50%. The optical response of the hybrid ensemble can be accurately tuned through the number of infiltration cycles performed.

Junio, 2010 | DOI: 10.1002/smll.200902190

Murillo's paintings revealed by spectroscopic techniques and dedicated laboratory-made micro X-ray diffraction

Duran, A; Siguenza, MB; Franquelo, ML; de Haro, MCJ; Justo, A; Perez-Rodriguez, JL
Analytica Chimica Acta, 671 (2010) 1-8


This paper describes one of the first case studies using micro-diffraction laboratory-made systems to analyse painting cross-sections. Pigments, such as lead white, vermilion, red ochre, red lac, lapis lazuli, smalt, lead tin yellow type I, massicot, ivory black, lamp black and malachite, were detected in cross-sections prepared from six Bartolome Esteban Murillo paintings by micro-Raman and micro-XRD combined with complementary techniques (optical microscopy, SEM-EDS, and FT-IR). The use of micro-XRD was necessary due to the poor results obtained with conventional XRD. In some cases, pigment identification was only possible by combining results from the different analytical techniques utilised in this study.

Junio, 2010 | DOI: 10.1016/j.aca.2010.05.004

Environmentally responsive nanoparticle-based luminescent optical resonators

Sanchez-Sobrado, O; Calvo, ME; Nunez, N; Ocana, M; Lozano, G; Miguez, H
Nanoscale, 2 (2010) 936-941
Materiales Coloidales, Materiales Ópticos Multifuncionales


In this work, we demonstrate that optical resonators built using all-nanoparticle-based porous building blocks provide a responsive multifunctional matrix, totally different emission spectra being attained from the same embedded luminescent nanophosphors under varying environmental conditions. We show a clear correlation between modifications in the ambient surroundings, the induced changes of the resonant modes, and the resulting variations in the emission response. The method is versatile and allows nanophosphors of arbitrary shape to be integrated in the cavity. By precise control of the spectral features of the optical resonances, luminescence is strongly modulated in selected and tuneable wavelength ranges. Applications in the fields of sensing and detection are foreseen for these materials.

Junio, 2010 | DOI: 10.1039/b9nr00338j

Nonisothermal calorimetric study of the precipitation processes in a Cu-1Co-0.5Ti alloy

Donoso, E; Zuniga, A; Dianez, MJ; Criado, JM
Journal of Thermal Analysis and Calorimetry, 100 (2010) 975-980
Reactividad de Sólidos


The precipitation processes in a Cu-1.0 at.%Co-0.5 at.%Ti (Cu-1.5 at.%Co2Ti) alloy were studied using differential scanning calorimetry (DSC), transmission electron microscopy (TEM), and microhardeness measurements. The analysis of the calorimetric curves from room temperature to 900 K shows the presence of two exothermic reactions attributed to the formation of CoTi and Co2Ti particles in the copper matrix. On the basis of enthalpy calculations, it was found that the decomposition begins with the precipitation of CoTi, followed by the formation of Co2Ti particles. The activation energies calculated using the modified Kissinger method were lower than the ones corresponding to diffusion of cobalt and titanium in copper. Kinetic parameters were obtained by a convolution method based on the Johnson-Mehl-Avrami (JMA) formalism. The values obtained for the parameter n were indicative of a particle nucleation process from preexistent nuclei. Microhardness measurements and TEM micrographs confirmed the formation of the mentioned phases.

Junio, 2010 | DOI: 10.1007/s10973-009-0642-y

Self-assembly of supramolecular lipid nanoparticles in the formation of plant biopolyester cutin

Dominguez, E; Heredia-Guerrero, JA; Benitez, JJ; Heredia, A
Molecular Biosystems, 6 (2010) 948-950
Materiales de Diseño para la Energía y Medioambiente


The implication of a self-assembly process in the early stages of cutin biosynthesis has been shown by means of antibodies raised against polyhydroxy fatty acid nanoparticles (cutinsomes).

Junio, 2010 | DOI: 10.1039/b927186d