Menú secundario

Artículos SCI



2009


Electrical characteristics of mixed Zr–Si oxide thin films prepared by ion beam induced chemical vapor deposition at room temperature


Ferrer, FJ; Frutos, F; Garcia-Lopez, J; Jimenez, C; Yubero, F
Thin Solid Films, 517 (2009) 5446-5452

ABSTRACT

Mixed Zr–Si oxide thin films have been prepared at room temperature by ion beam decomposition of organometallic volatile precursors. The films were flat and amorphous. They did not present phase segregation of the pure single oxides. A significant amount of impurities (–C–, –CHx, –OH, and other radicals coming from partially decomposed precursors) remained incorporated in the films after the deposition process. This effect is minimized if the Ar content in the O2/Ar bombarding gas is maximized. Static permittivity and breakdown electrical field of the films were determined by capacitance–voltage and current–voltage electrical measurements. It is found that the static permittivity increases non-linearly from ~ 4 for pure SiO2 to ~ 15 for pure ZrO2. Most of the dielectric failures in the films were due to extrinsic breakdown failures. The maximum breakdown electrical field decreases from ~ 10.5 MV/cm for pure SiO2 to ~ 45 MV/cm for pure ZrO2. These characteristics are justified by high impurity content of the thin films. In addition, the analysis of the conduction mechanisms in the formed dielectrics is consistent to Schottky and Poole-Frenkel emission for low and high electric fields applied, respectively.


Julio, 2009 | DOI: 10.1016/j.tsf.2009.01.099

Degradation of n-Butyl tin Chlorides in Waters. A Comparative Assessment of the Process by Photo-assisted and Chemical- treatment Methods


Navio, JA; Cerrillos, C; Macias, M
Journal of Advanced Oxidation Technologies, 12 (2009) 158-163

ABSTRACT

Study of degradation processes of metals used in some artworks from the cultural heritage of Andalusia, Spain


Duran, A; Herrera, LK; de Haro, MCJ; Perez-Rodriguez, JL; Justo, A
Revista de Metalurgia, 45 (2009) 277-286

ABSTRACT

The study of the alteration processes of metals, such as lead, bronze, iron and tin-mercury alloys, used in some of the
most important chosen artefacts of Andalusian Cultural Heritage is the main objective of this paper. Hydrocerussite
and cerussite were detected in lead seals stored in a hole of cardboard. Bronze is altered to atacamite by environmental
contamination, which is also responsible for the formation of rust from iron. Corrosion of the tin-mercury surface
of amalgam mirrors produces tin monoxide and tin dioxide and releases liquid mercury from the solid phase.


Julio, 2009 | DOI: 10.3989/revmetalm.0827

Growth of Crystalline TiO2 by Plasma Enhanced Chemical Vapor Deposition


Borras, A; Sanchez-Valencia, JR; Widmer, R; Rico, VJ; Justo, A; Gonzalez-Elipe, AR
Crystal Growth & Design, 9 (2009) 2868-2876

ABSTRACT

TiO2 thin films in the form of anatase have been prepared by plasma enhanced chemical vapor deposition (PECVD) at 523 K as the substrate temperature and a low working pressure. The study of the microstructure and texture of the films at different stages of deposition show that their growth follows the Kolmogorov’s model developed to describe the evolution of crystalline films from a saturated homogeneous medium. An additional characteristic feature of the growth process by PECVD is the formation of different crystalline domains, particularly at low deposition rates. The effects of this parameter and of the characteristics of the substrate on the growing process are also addressed.


Junio, 2009 | DOI: 10.1021/cg9001779

Thermal Evolution of WC/C Nanostructured Coatings by Raman and In Situ XRD Analysis


El Mrabet, S; Abad, MD; Lopez-Cartes, C; Martinez-Martinez, D; Sanchez-Lopez, JC
Plasma Processes and Polymers, 6 (2009) S444-S449

ABSTRACT

In this work, a series of WC/C nanostructured films were deposited on silicon substrates by changing the ratio of sputtering power applied to graphite and WC magnetron sources (PC/PWC: 0, 0.1, 0.5, 1). The thermal stability of WC/C coatings was followed in situ by means of X-ray diffraction measurements up to 1 100 °C in vacuum (10−1 Pa). Initially, the film microstructure is composed of nanocrystalline WC1−x and W2C phases. As the PC/PWC ratio increases the crystallinity decreases, and WC1−x becomes the predominant phase from PC/PWC = 0.1. The results show that the structural evolution with temperature of all studied layers depends essentially on their initial phase and chemical composition (determined by the synthesis conditions: ratio PC/PWC). The coating deposited at PC/PWC = 0 reveals a transformation of W2C phase into W and W3C phases at 400 °C. However, the samples with PC/PWC greater than 0 exhibits an improved thermal stability up to 600–700 °C where the WC1−x begins to transform into W2C and WC phases. At 900 °C, WC is the predominant phase, especially for those coatings prepared with higher ratios. Further annealing above 1 000 °C yields W as the foremost phase. The thermal behaviour was later studied by means of Raman spectroscopy measurements at certain temperatures where the main changes in phase composition were observed. Particularly, a fitting analysis was carried out on the D and G bands typical of disordered and amorphous carbon. The changes induced during heating are discussed in terms of the positions of D and G lines, and full width at half maximum (FWHM).


Junio, 2009 | DOI: 10.1002/ppap.200931004

Chemical Reactions in 2D: Self-Assembly and Self-Esterification of 9(10),16-Dihydroxypalmitic Acid on Mica Surface


Heredia-Guerrero, JA; San-Miguel, MA; Sansom, MSP; Heredia, A; Benitez, JJ
Langmuir, 25 (2009) 6869-6874

ABSTRACT

9(10),16-Dihydroxypalmitic acid (diHPA) is a particularly interesting polyhydroxylated fatty acid (1) because it is the main monomer of cutin, the most abundant biopolyester in nature, and (2) because the presence of a terminal and a secondary hydroxyl group in midchain positions provides an excellent model to study their intermolecular interactions in a confined phase such as self-assembled layers. In this study we have combined atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), attenuated total reflection Fourier transform infrared (ATR-FT-IR) spectroscopy, as well as molecular dynamics (MD) simulations to conclude that the self-assembling of diHPA molecules on mica is a layer by layer process following a Brunauer−Emmett−Teller (BET) type isotherm and with the first layer growing much faster than the rest. Interactions between secondary hydroxyls reinforce the cohesive energy of the monolayer, while the presence of the terminal hydroxyl group is necessary to trigger the multilayered growth. Besides, XPS and ATR-FT-IR spectroscopies clearly indicate that spontaneous self-esterification occurs upon self-assembling. The esterification reaction is a prerequisite to propose a self-assembly route for the biosynthesis of cutin in nature. Molecular dynamics simulations have shown that internal molecular reorganization within the self-assembled layers provides the appropriate intermolecular orientation to facilitate the nucleophilic attack and the release of a water molecule required by the esterification reaction.


Junio, 2009 | DOI: 10.1021/la9001412

M-Doped Al2TiO5 (M=Cr, Mn, Co) Solid Solutions and their Use as Ceramic Pigments


Dondi, M; Lyubenova, TS; Carda, JB; Ocaña, M
Journal of the American Ceramic Society, 92 (2009) 1972-1980

ABSTRACT

New ceramic pigments based on the tialite (Al2TiO5) structure, doped with Co (pink), Cr (green), or Mn (brown), were prepared through the pyrolysis of aerosols followed by calcination of the obtained powders at 1400°C. The expected decomposition of Al2TiO5 into a mixture of Al2O3 and TiO2 on refiring was inhibited by Cr-doping and also by co-doping with Mg the Mn- or Co-doped samples. Microstructure and phase evolution during pigment preparation were monitored by scanning electron microscopy and XRPD. Unit cell parameters of tialite were determined by Rietveld refinement of the X-ray diffraction patterns, revealing in all cases the formation of solid solutions where the solubility of dopants in the Al2TiO5 lattice followed the trend Co<Mn<Cr. The valence state and possible location of dopants in the tialite lattice were investigated by X-ray photoelectron spectra and diffuse reflectance spectroscopies, which suggested the presence of Cr3+ ions in a large interstitial site of the tialite lattice with a distorted octahedral geometry, and of Mn3+ and Co2+ ions in the Al3+ octahedral sites of the tialite lattice in the former case, and in both Al3+ and Ti4+ octahedral sites in the latter. Testing the ceramic glazes assessed the technological behavior of pigments, which found that the color stability was reasonably good for the Mn-doped tialite and the Cr-doped pigment, although the latter suffered a small loss of green hue. The Co-doped pigment was found to be not stable in glazes, undergoing a cobalt-leaching effect.


Junio, 2009 | DOI: 10.1111/j.1551-2916.2009.03172.x

Phyllites used as waterproofing layer materials for greenhouses crops in Spain: multivariate statistical analysis applied to their classification based on X-ray fluorescence analysis


Garzon, E; Garcia-Rodriguez, IG; Ruiz-Conde, A; Sanchez-Soto, PJ
X-Ray Spectrometry, 38 (2009) 429-438

ABSTRACT

The results are presented for the chemical characterization carried out with 53 raw material samples with phyllosilicates from outcrops located between the provinces of Almería and Granada (SE Spain) in order to use them as waterproof layers under the floors of greenhouses. For this purpose, the samples have been studied by these techniques: X-Ray Diffraction (XRD), X-Ray Fluorescence (WDXRF), Scanning Electron Microscopy (SEM), chemical analysis by energy-dispersive X-Ray detection (EDX), and thermogravimetry. To isolate groups of phyllite samples with similar chemical profiles and correlations between the samples, the WDXRF data (11 chemical elements) have been processed using the methodology of exploratory multivariate statistical analyses: cluster analysis, main-component analysis, and discriminating canon analysis. This study was performed as a screening test and as a means of finding similarities and correlations among all 53 phyllite samples, allowing the isolation of groups of phyllite samples with similar chemical profiles. The results indicate that the 53 phyllite samples can be divided into two main groups. The first group is subdivided into two subgroups (1 and 2), one of which includes most of the samples. The latter is further classified into three blocks with the same chemical composition. This allows to search for the raw material with potentially the best waterproof characteristics within the five groups. The classification is of validity as a screening test for subsequent experimental determinations concerning the physical properties of these samples.


Junio, 2009 | DOI: 10.1002/xrs.1199

Fibrous MnO2 Nanoparticles with (2 × 2) Tunnel Structures. Catalytic Activity in the Total Oxidation of Volatile Organic Compounds


Dominguez, MI; Navarro, P; Romero-Sarria, F; Frias, D; Cruz, SA; Delgado, JJ; Centeno, MA; Montes, M; Odriozola, JA
Journal of Nanoscience and Nanotechnology, 9 (2009) 3837-3842

ABSTRACT

Manganese oxides having 2 × 2 tunnel structures (cryptomelanes) have been synthesized by a milling method in order to test their efficiency as catalysts for the abatement of volatile organic compounds, using toluene as probe molecule. These materials present excellent textural properties, arising from the nanofiber morphology and were active for toluene total oxidation. DRIFTS of the adsorbed phase allow proposing the role of lattice oxygen in the catalytic reaction.


Junio, 2009 | DOI: 10.1166/jnn.2009.NS76

Application of 29Si and 27Al MAS NMR Spectroscopy to the Study of the Reaction Mechanism of Kaolinite to Illite/Muscovite


Mantovani, M; Escudero, A; Becerro, AI
Clays and Clay Minerals, 57 (2009) 302-310

ABSTRACT

Understanding the mechanisms for illitization of clay minerals has important applications in reconstructing geologic histories and determining the origins of physical and chemical characteristics of buried sediments. While many studies have been carried out on this topic, few have focused on the mechanism of illite formation from kaolinite. The purpose of this study was to investigate more deeply the illitization of kaolinite in KOH solution at a high solid/liquid ratio (1000 mg/mL). X-ray diffraction (XRD) and infrared spectroscopy were used to follow the formation of new crystalline phases and the composition of the octahedral sheet, while the transformation of the Si and Al local environments was analyzed by 29Si and 27Al magic angle spinning nuclear magnetic resonance spectroscopy (MAS NMR). The results revealed that the first reaction stage consists of the diffusion of Al from the octahedral to the tetrahedral sheet of the kaolinite TO layers, giving rise to the precursors of the illite/muscovite nuclei. Combination of XRD with 27Al MAS NMR measurements indicated that a minimum amount of tetrahedral Al is required in the original TO layer before condensation of a second tetrahedral sheet occurs to complete the formation of the illite/muscovite TOT layers.


Junio, 2009 | DOI: 10.1346/CCMN.2009.0570303

Determination of nitrogen partitioning coefficients in superduplex stainless steels by NRA using a nuclear microprobe


Munoz, C; Morilla, Y; Lopez, JG; Paul, A; Odriozola, JA
Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, 267 (2009) 2208-2211

ABSTRACT

Superduplex stainless steels (SDSSs) combine the good mechanical behavior and the high corrosion resistance of the ferrite (α-Fe) and austenite (γ-Fe) phases. The SDSSs properties depend strongly on the partitioning of the elements that form the alloy. The ferrite is generally enriched in P, Si, Cr and Mo while the content of Ni, Mn, Cu and N in the austenite phase is higher. Nitrogen is known to be a strong austenite stabilizer and its presence increases the strength and the pitting corrosion resistance of the stainless steels. While the global nitrogen content in SDSSs can be readily determined using elemental analyzers, it cannot be measured at a microscopic scale.

In this work, the nuclear microprobe of the Centro Nacional de Aceleradores (Sevilla) was used to obtain the quantitative distribution of nitrogen in SDSSs. A deuteron beam of 1.8 MeV was employed to determine the overall elemental concentration of the matrix by deuteron-induced X-ray emission, whereas the nitrogen partitioning coefficients were obtained by using the 14N(d, α0)12C nuclear reaction. Mappings of this element show that the nitrogen ratio between the ferrite and austenite phases ranges from 0.3 to 0.6 in the analyzed samples.


Junio, 2009 | DOI: 10.1016/j.nimb.2009.03.093

Molding with nanoparticle-based one-dimensional photonic crystals: a route to flexible and transferable Bragg mirrors of high dielectric contrast


Calvo, ME; Sobrado, OS; Lozano, G; Miguez, H
Journal of Materials Chemistry, 19 (2009) 3144-3148

ABSTRACT

Self-standing, flexible Bragg mirror films of high refractive index contrast and showing intense and wide Bragg peaks are herein presented. Nanoparticle-based one-dimensional photonic crystals are used as templates to infiltrate a polymer, which provides the multilayer with mechanical stability while preserving the dielectric contrast existing in the mold. Such films can be lifted off the substrate and used to coat another surface of arbitrary shape.


Mayo, 2009 | DOI: 10.1039/B902090J

W,N-Codoped TiO2-Anatase: A Sunlight-Operated Catalyst for Efficient and Selective Aromatic Hydrocarbons Photo-Oxidation


Kubacka, A; Bachiller-Baeza, B; Colon, G; Fernandez-Garcia, M
Journal of Physical Chemistry C, 113 (2009) 8553-8555

ABSTRACT

New W,N-doped TiO2 anatase-based materials are synthesized having both unprecedent high activity and selectivity in the gas-phase partial oxidation of aromatic hydrocarbons using sunlight as excitation energy and molecular oxygen as oxidant.


Mayo, 2009 | DOI: 10.1021/jp902618g

Thermal Stability and Oxidation Resistance of Nanocomposite TiC/a-C Protective Coatings


Martinez-Martinez, D; Lopez-Cartes, C; Gago, R; Fernandez, A; Sanchez-Lopez, JC
Plasma Processes and Polymers, 6 (2009) S462-S467

ABSTRACT

Nanocomposite films composed by small crystallites of hard phases embedded in an amorphous lubricant matrix have been extensively studied as protective coatings. These kinds of coatings have often to work in extreme environments, exposed to high temperatures (above 800–900 °C), and/or oxidizing/corrosive atmospheres, which may resist. As a result, it is important to study the behavior of such coatings at high temperatures (thermal stability) and in the presence of oxygen (oxidation resistance). In this sense, we have selected a TiC/a-C nanocomposite coating with good mechanical and tribological properties in order to do several thermal tests under three different environments: high vacuum (10−6 mbar), low vacuum (10−1 mbar), and air. Our observations allow us to establish that the film microstructure is stable at least up to 1 000 °C in high vacuum. When oxygen is present, the practical temperature of use is reduced at 700 °C (low partial pressure) and 300 °C (air) by formation of Ti oxides and C removal.


Mayo, 2009 | DOI: 10.1002/ppap.200931002

Cationic (V, Mo, Nb, W) doping of TiO2–anatase: A real alternative for visible light-driven photocatalysts


Kubacka, A; Colon, G; Fernandez-Garcia, M
Catalysis Today, 143 (2009) 286-292

ABSTRACT

In this article we investigate the structure–activity link of anatase-type Ti–M (M = V, Mo, Nb, and W) mixed oxides used for toluene photo-oxidation under sunlight-type excitation. An analysis of the local and long-range structural and electronic characteristics of the mixed oxides show that only structurally highly homogeneous anatase-type oxides with electronic properties exclusively leading to a band gap decrease drive to efficient visible light-driven photocatalysts. Within our microemulsion preparation method, this only occurs for Ti–V and Ti–W series of samples. The isoelectronic (V4+) substitution of Ti4+ ions at the anatase lattice is characterized by a low solubility limit (ca. 2.5 at. %), and drives to a limited modification of the band gap and to a moderate enhancement of the photo-activity with respect to bare titania reference systems. W presence at anatase cation positions occurs with concomitant presence of cation vacancies derived by the charge imbalance between the W6+ and Ti4+ species. A unique W-vacancy local arrangement is detected by the structural characterization, leading to both an important band gap decrease and enhancement of the photo-activity upon sunlight excitation.


Mayo, 2009 | DOI: 10.1016/j.cattod.2008.09.028

Wetting angles and photocatalytic activities of illuminated TiO2 thin films


Rico, V; Romero, P; Hueso, JL; Espinos, JP; Gonzalez-Elipe, AR
Catalysis Today, 143 (2009) 347-354

ABSTRACT

TiO2 thin films have been prepared by physical vapour deposition (PVD) and plasma enhanced chemical vapour deposition (PECVD) to study the UV-induced photo-activity of this material. Wetting angle variations and photo-catalytic activity for the degradation of dyes upon UV illumination have been compared for thin films with different crystalline structure (amorphous, rutile and anatase), microstructure (columnar, compact, etc.) and porosities as estimated from the values of their refraction indices and their direct assessment with a quartz crystal monitor. The surface of the thin films became superhydrophilic upon UV light irradiation and then it recovered its original state by keeping the samples in the dark. Wetting angle decays follow very similar kinetics for amorphous and crystalline films, independently of their actual porosities. By contrast the photo-catalytic activity was very dependent on the crystalline structure of the films (anatase > rutile > amorphous) and on their porosities. The different behaviour depicted by the films with regard to these two properties suggests that they respond to different though related mechanisms and that they cannot be considered as equivalent when trying to prove the photo-activity of TiO2.


Mayo, 2009 | DOI: 10.1016/j.cattod.2008.09.037

Integration of methanol steam reforming and combustion in a microchannel reactor for H2 production: A CFD simulation study


Arzamendi, G; Dieguez, PM; Montes, M; Centeno, MA; Odriozola, JA; Gandia, LM
Catalysis Today, 143 (2009) 25-31

ABSTRACT

A computational fluid dynamics (CFD) study of the thermal integration of the steam reforming of methanol (SRM) and the combustion of methanol in a catalytic microchannel reactor is presented. This issue is of interest for in situ H2 production for portable power units based on low-temperature PEM fuel cells. Three-dimensional simulations have been carried out under relevant conditions for the SRM reaction that have shown that microreactors allow achieving complete methanol reforming and combustion at space velocities as high as 50,000 h−1, with selectivities for H2 above 99% at relatively low temperatures in the 270–290 °C range.


Mayo, 2009 | DOI: 10.1016/j.cattod.2008.09.034

AISI 304 Austenitic stainless steels monoliths for catalytic applications


Martinez, LM; Sanz, O; Dominguez, MI; Centeno, MA; Odriozola, JA
Chemical Engineering Journal, 148 (2009) 191-200

ABSTRACT

The thermal treatments of austenitic stainless steels monoliths were studied in order to generate a highly homogeneous and rough oxide scale strongly attached to the base alloy, which will subsequently ensure the good adherence of the catalysts. In this work it has been shown that the morphology, integrity and homogeneity of the scale are strongly influenced by the temperature and time of treatment. Washcoating method was used to deposit on the monolith surface a Au/CeO2 catalyst. The drying procedure turned out to be the most critical variable for the adherence and homogeneity of the catalytic ceria layer, while the ceria colloid concentrations in the starting aqueous dispersion seems to have only a threshold effect. The monolithic reactors containing Au/CeO2 layers are active in the oxidation of CO.


Mayo, 2009 | DOI: 10.1016/j.cej.2008.12.030

High deposition rates of uniform films in tetramethylsilane-based plasmas generated by elementary microwave sources in matrix configuration


Latrasse, L; Lacoste, A; Sanchez-Lopez, JC; Bes, A; Rayar, M; Pelletier, J
Surface and Coatings Technology, 203 (2009) 2343-2349

ABSTRACT

Plasma scaling up can be achieved by distributing elementary microwave plasma sources on planar rectangular networks. These so-called matrix plasmas can generate uniform sheets of plasma over a wide argon pressure range, from 7.5 to 750 Pa, with densities between 1012 and 1013 cm− 3. In order to estimate the capabilities of matrix plasmas for PACVD processing in terms of deposition rate and uniformity, SiOCH and SiNCH films were deposited using TMS (tetramethylsilane), as the organic gas precursor of silicon, mixed with oxygen or nitrogen flows. Plasmas of O2 / TMS and N2 / TMS gas mixtures can be sustained between 5 and 25 Pa. Variations in the deposition rate as a function of microwave power and nitrogen partial pressure are reported. Thickness uniformity of SiOCH and SiNCH films was measured across a silicon wafer. The obtained deposition rates exceed 1.3 μm/min and the films present a uniformity better than 5% on 75 mm diameter silicon wafers. Composition of the films has also been analyzed by XPS as a function of process parameters: microwave input power, composition of gas mixture, and N2 partial pressure. In particular, these analyses have shown a very low yield of nitrogen incorporation when using N2 gas as nitrogen precursor and high Si and Si–Si bonding contents in the films, probably due to a strong fragmentation of the TMS precursor in the high density plasma.


Mayo, 2009 | DOI: 10.1016/j.surfcoat.2009.02.121

Molecular dynamics simulation of the effect of pH on the adsorption of rhodamine laser dyes on TiO2 hydroxylated surfaces


Hamad, S; Sanchez-Valencia, JR; Barranco, A; Mejias, JA; Gonzalez-Elipe, AR
Molecular Simulation, 35 (2009) 1140-1151

ABSTRACT

We have carried out a study of adsorption, on the (1 0 1) surface of anatase TiO2, of two industrially relevant rhodamine molecules [rhodamine 6G (R6G) and rhodamine 800 (R800)] employing molecular dynamics. These theoretical studies have shown that R6G must adsorb on surfaces under basic conditions. Moreover, the adsorption of this molecule shows a strong dependence upon the pH of the system, i.e. under basic conditions the adsorption energy is quite high, under neutral conditions the adsorption energy is lower and under acidic conditions an even lower adsorption energy indicates that there must be very little adsorption under such conditions. By contrast, for R800, there is little dependence of the adsorption energy upon the pH, suggesting that the amount of adsorption of these molecules is little affected by this parameter. These theoretical results are in qualitative agreement with the experimental results consisting of the incorporation of these dye molecules into porous thin films.


Mayo, 2009 | DOI: 10.1080/08927020903108083

Páginas

icms