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Artículos SCI



2009


Combined kinetic analysis of thermal degradation of polymeric materials under any thermal pathway


Sanchez-Jimenez, PE; Perez-Maqueda, LA; Perejon, A; Criado, JM
Polymer Degradation and Stability, 94 (2009) 2079-2085
Reactividad de Sólidos

ABSTRACT

Combined kinetic analysis has been applied for the first time to the thermal degradation of polymeric materials. The combined kinetic analysis allows the determination of the kinetic parameters from the simultaneous analysis of a set of experimental curves recorded under any thermal schedule. The method does not make any assumptions about the kinetic model or activation energy and allows analysis even when the process does not follow one of the ideal kinetic models already proposed in the literature. In the present paper the kinetics of the thermal degradation of both polytetrafluoroethylene (PTFE) and polyethylene (PE) have been analysed. It has been concluded, without previous assumptions on the kinetic model, that the thermal degradation of PTFE obeys a first order kinetic law, while the thermal degradation of PE follows a diffusion-controlled kinetic model.


Noviembre, 2009 | DOI: 10.1016/j.polymdegradstab.2009.07.006

Vibrational spectroscopy characterization of magnetron sputtered silicon oxide and silicon oxynitride films


Godinho, V; Denisov, VN; Mavrin, BN; Novikova, NN; Vinogradov, EA; Yakovlev, VA; Fernandez-Ramos, C; de Haro, MCJ; Fernandez, A
Applied Surface Science
Materiales Nanoestructurados y Microestructura

ABSTRACT

Vibrational (infrared and Raman) spectroscopy has been used to characterize SiOxNy and SiOx films prepared by magnetron sputtering on steel and silicon substrates. Interference bands in the infrared reflectivity measurements provided the film thickness and the dielectric function of the films. Vibrational modes bands were obtained both from infrared and Raman spectra providing useful information on the bonding structure and the microstructure (formation of nano-voids in some coatings) for these amorphous (or nanocrystalline) coatings. X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM) analysis have also been carried out to determine the composition and texture of the films, and to correlate these data with the vibrational spectroscopy studies. The angular dependence of the reflectivity spectra provides the dispersion of vibrational and interference polaritons modes, what allows to separate these two types of bands especially in the frequency regions where overlaps/resonances occurred. Finally the attenuated total reflection Fourier transform infrared measurements have been also carried out demonstrating the feasibility and high sensitivity of the technique. Comparison of the spectra of the SiOxNy films prepared in various conditions demonstrates how films can be prepared from pure silicon oxide to silicon oxynitride with reduced oxygen content.


Octubre, 2009 | DOI: 10.1016/j.apsusc.2009.07.101

Mesostructured Thin Films as Responsive Optical Coatings of Photonic Crystals


Hidalgo, N; Calvo, ME; Miguez, H
Small, 5 (2009) 2309-2315
Materiales Ópticos Multifuncionales

ABSTRACT

A synthetic route is presented to attain high-optical-quality multilayered structures that residtfront coupling ordered n7esoporous tilaniuni oxide thin films to the surface of a dense one-dimensional photonic crystal. Such architectures present spectrally well-defined photon resonant modes localized in the outer coating that finely respond to physicochemically induced modifications of its pore volume. The potential of these porous coatings in detection of environmental changes through variations of the photonic response of the ensemble is demonstrated by performing isothermal optical reflectance measurements under controlled vapor-pressure conditions.


Octubre, 2009 | DOI: 10.1002/smll.200900411

Hydrothermal Synthesis of Kalsilite: A Simple and Economical Method


Becerro, AI; Mantovani, M; Escudero, A
Journal of the American Ceramic Society, 92 (2009) 2204-2206
Materiales de Diseño para la Energía y Medioambiente

ABSTRACT

This study reports a simple method to synthesize pure kalsilite (KAlSiO4) using readily available precursors, kaolinite and KOH solution, after only 12 h of hydrothermal treatment in mild conditions. A structural refinement has been carried out using the Rietveld method to obtain unit cell parameters, and the 29Si and 27Al magic angle spinning nuclear magnetic resonance spectra have shown the purity and complete Si/Al ordering of the kalsilite structure obtained. Finally, the morphology of the particles has been analyzed by scanning electron microscopy.


Octubre, 2009 | DOI: 10.1111/j.1551-2916.2009.03232.x

Hg/Sn amalgam degradation of ancient glass mirrors


Herrera, LK; Duran, A; Franquelo, ML; Justo, A; Perez-Rodriguez, JL
Journal of Non-Crystalline Solids, 355 (2009) 1980-1983

ABSTRACT

Tin amalgam, which is obtained by pouring mercury onto a sheet of tin, has been used in the production of reflective coatings for mirrors. The corrosion processes of the amalgam layer were investigated in several mirrors from historical buildings located in southern Spain using SEM/EDS, XPS, and GID. Mercury and Sn4+ are present as spheres on the amalgam surface due to the evaporation process (∼5 nm). The profile shows a mixture of Sn2+ and Sn4+. The original amalgam was composed of a binary alloy of tin and mercury (Hg0.1Sn0.9) and metallic tin. In this paper the tin oxidation mechanism of the amalgam is described. Liquid mercury is volatile and evaporates slowly, leaving fine tin particles that oxidize easily, forming tin monoxide (SnO) and tin dioxide (SnO2). The mercury-rich phase accelerates the corrosion of the tin-rich phase.


Octubre, 2009 | DOI: 10.1016/j.jnoncrysol.2008.11.045

Optical Analysis of the Fine Crystalline Structure of Artificial Opal Films


Lozano, G; Dorado, LA; Schinca, D; Depine, RA; Miguez, H
Langmuir, 25 (2009) 12860-12864
Materiales Ópticos Multifuncionales

ABSTRACT

Herein, we present a detailed analysis of the structure of artificial opal films. We demonstrate that, rather than the generally assumed face centered cubic lattice of spheres, opal films are better approximated by rhombohedral assemblies of distorted colloids. Detailed analysis of the optical response in a very wide spectral range (0.4 ≤ a/λ ≤ 2, where a is the conventional lattice constant), as well as at perpendicular and off-normal directions, unambiguously shows that the interparticle distance coincides very approximately with the expected diameter only along directions contained in the same close-packed plane but differs significantly in directions oblique to the [111] one. A full description of the real and reciprocal lattices of actual opal films is provided, as well as of the photonic band structure of the proposed arrangement. The implications of this distortion in the optical response of the lattice are discussed.


Octubre, 2009 | DOI: 10.1021/la903077r

Manganese and iron oxides as combustion catalysts of volatile organic compounds


Duran, FG; Barbero, BP; Cadus, LE; Rojas, C; Centeno, MA; Odriozola, JA
Applied Catalysis B-Environmental, 92 (2009) 194-201
Química de Superficies y Catálisis, Materiales Nanoestructurados y Microestructura

ABSTRACT

FeMn mixed oxides were prepared by the citrate method with Fe:Mn atomic ratio equal to 1:1, 1:3 and 3:1. The sample was characterized by means of specific surface area measurements, X-ray diffractometry (XRD), temperature programmed desorption of oxygen (O2-DTP), temperature programmed reduction (TPR), X-ray fluorescence (XRF), transmission electron microscopy (TEM and SAED) and high resolution TEM (HREM). The characterization results demonstrated the formation of a Mn2O3–Fe2O3 solid solution. The catalytic performance in ethanol, ethyl acetate and toluene total oxidation on these samples was better than on Fe2O3 and Mn2O3 pure oxides.


Octubre, 2009 | DOI: 10.1016/j.apcatb.2009.07.010

Rare-earth disilicate formation under Deep Geological Repository approach conditions


Alba, MD; Chain, P; Orta, MM
Applied Clay Science, 46 (2009) 63-68
Materiales de Diseño para la Energía y Medioambiente

ABSTRACT

The Deep Geological Repository (DGR) concept involves the placement of long-lived radioactive waste in rooms excavated deep. The major responsibility of the disposal safety falls on the Engineered Barrier System (EBS). The main constituent of EBS is bentonite that prevents the release of radiactive nuclei by physical and chemical mechanisms. The physical mechanism is expected to fault with the weathering of the bentonite while the chemical mechanisms have been only proved at 300 °C. It is the aim of this paper to explore the feasibility of the chemical mechanism at temperatures closer to the DGP conditions and to shed light on the mechanism of transformation of the argillaceous materials of the EBS in rare-earth disilicate phases. Saponite was submitted to hydrothermal reaction at 175 °C and 150 °C with different solutions of REE3+ cations (REE = Sc, Lu, Y, Sm, Nd and La). The products were analyzed by XRD, NMR and electron microscopy. At conditions close to the DGP, the saponite was able to form rare-earth silicates. The formation of the disilicate phase, as final product, needs a set of stages and oxyorthosilicate as precursor.


Septiembre, 2009 | DOI: 10.1016/j.clay.2009.07.012

Formation of Nitrogen Functional Groups on Plasma Treated DLC


Lopez-Santos, C; Yubero, F; Cotrino, J; Contreras, L; Barranco, A; Gonzalez-Elipe, AR
Plasma Processes and Polymers, 6 (2009) 555-565
Nanotecnología en Superficies y Plasma

ABSTRACT

Diamond like carbon (DLC) thin films have been exposed to different nitrogen containing plasmas. A dielectric barrier discharge (DBD) at atmospheric pressure and a microwave discharge (MW) at low pressure using N2 and mixtures Ar + NH3 have been compared. Optical Emission and X-ray Photoelectron spectroscopies, Atomic Force Microscopy and contact angle measurements have been used for this study. A DBD with Ar + NH3 is the most efficient method for DLC functionalization. Films treated with this plasma presented the highest concentration of amine groups as determined by derivatization with 4-chlorobenzaldehyde. All the treated samples underwent a significant aging with time. The efficiency of the different plasmas for DLC functionalization is discussed in the light of the intermediate species detected in the plasma.


Septiembre, 2009 | DOI: 10.1002/ppap.200900019

Near-ambient X-ray photoemission spectroscopy and kinetic approach to the mechanism of carbon monoxide oxidation over lanthanum substituted cobaltites


Hueso, JL; Martinez-Martinez, D; Caballero, A; Gonzalez-Elipe, AR; Mun, BS; Salmeron, M
Catalysis Communications, 10 (2009) 1898-1902
Nanotecnología en Superficies y Plasma, Materiales y Procesos Catalíticos de Interés Ambiental y Energético

ABSTRACT

We have studied the oxidation of carbon monoxide over a lanthanum substituted perovskite (La0.5Sr0.5CoO3−d) catalyst prepared by spray pyrolysis. Under the assumption of a first-order kinetics mechanism for CO, it has been found that the activation energy barrier of the reaction changes from ∼80 to ∼40 kJ mol−1 at a threshold temperature of ca. 320 °C. In situ XPS near-ambient pressure (∼0.2 torr) shows that the gas phase oxygen concentration over the sample decreases sharply at ca. 300 °C. These two observations suggest that the oxidation of CO undergoes a change of mechanism at temperatures higher than 300 °C.


Agosto, 2009 | DOI: 10.1016/j.catcom.2009.06.022

Comparative investigation of TiAlC(N), TiCrAlC(N), and CrAlC(N) coatings deposited by sputtering of МАХ-phase Ti2 − хCrхAlC targets


Shtansky, DV; Kiryukhantsev-Korneev, PV; Sheveyko, AN; Mavrin, BN; Rojas, C; Fernandez, A; Levashov, EA
Surface and Coatings Technology, 203 (2009) 3595-3609
Materiales Nanoestructurados y Microestructura

ABSTRACT

A comparative investigation of the structure and properties of TiAlC(N), TiCrAlC(N), and CrAlC(N) coatings deposited by sputtering of МАХ-phase Ti2 − хCrхAlC targets (where x = 0, 0.5, 1.5, and 2) in an Ar atmosphere or in a gaseous mixture of Ar + N2 is presented. The coatings were characterized in terms of their structure, elemental and phase composition, hardness, elastic modulus, elastic recovery, thermal stability, friction coefficient, wear rate, corrosion, and high-temperature oxidation resistance. The structure of the coatings was studied by means of X-ray diffraction, scanning and transmission electron microscopy, X-ray photoelectron spectroscopy, glow discharge optical emission spectroscopy, electron energy loss spectroscopy, and Raman spectroscopy. To evaluate the thermal stability and oxidation resistance, the coatings were annealed either in vacuum or in air at temperatures 600–1200 °C. The results obtained show that the TiAlCN coatings possess high hardness of 32–35 GPa, low friction coefficient against WC–Co well below 0.25, high thermal stability up to 1200 °C, and superior performance in dry milling tests against high Cr steel. Meanwhile, the coatings with high Cr content demonstrated improved oxidation resistance up to 1000 °C and superior electrochemical behavior, but their mechanical and tribological properties were deteriorated.


Agosto, 2009 | DOI: 10.1016/j.surfcoat.2009.05.036

Reactivity of LaNi1−y Co y O3−δ Perovskite Systems in the Deep Oxidation of Toluene


Pereniguez, R; Hueso, JL; Holgado, JP; Gaillard, F; Caballero, A
Catalysis Letters, 131 (2009) 164-169
Materiales y Procesos Catalíticos de Interés Ambiental y Energético

ABSTRACT

In the present work we have evaluated the oxidation of toluene over different lanthanum perovskites with a general composition of LaNi1−y Co y O3−δ. These catalysts, prepared by a spray pyrolysis method, have been characterised by XRD, BET and FE-SEM techniques. Additional experiments of temperature programmed desorption of O2, reduction in H2 and X-ray absorption spectroscopy were also performed in order to identify the main surface oxygen species and the reducibility of the different perovskites. The catalytic behaviour toward the oxidation of toluene (as a model for VOCs compounds) was evaluated in the range 100–600 °C, detecting a total conversion for all the samples below 400 °C and higher activities for the cobalt-containing perovskites. The catalytic behaviour of these samples is consistent with a suprafacial mechanism, with the α-type oxygen playing an active role in the oxidation reaction.


Agosto, 2009 | DOI: 10.1007/s10562-009-9968-0

Structural characteristics and morphology of SmxCe1−xO2−x/2 thin films


Hartmanova, M; Jergel, M; Mansilla, C; Holgado, JP; Zemek, J; Jurek, K; Kundracik, F
Applied Surface Science, 255 (2009) 9085-9091
Materiales y Procesos Catalíticos de Interés Ambiental y Energético

ABSTRACT

Effect of the deposition temperature (200 and 500 °C) and composition of SmxCe1−xO2−x/2 (x = 0, 10.9–15.9 mol%) thin films prepared by electron beam physical vapor deposition (EB-PVD) and Ar+ ion beam assisted deposition (IBAD) combined with EB-PVD on structural characteristics and morphology/microstructure was investigated. The X-ray photoelectron spectroscopy (XPS) of the surface and electron probe microanalysis (EPMA) of the bulk of the film revealed the dominant occurrence of Ce4+ oxidation state, suggesting the presence of CeO2 phase, which was confirmed by X-ray diffraction (XRD). The Ce3+ oxidation states corresponding to Ce2O3 phase were in minority. The XRD and scanning electron microscopy (SEM) showed the polycrystalline columnar structure and a rooftop morphology of the surface. Effects of the preparation conditions (temperature, composition, IBAD) on the lattice parameter, grain size, perfection of the columnar growth and its impact on the surface morphology are analyzed and discussed.


Agosto, 2009 | DOI: 10.1016/j.apsusc.2009.06.108

Chemical and electronic interface structure of spray pyrolysis deposited undoped and Al-doped ZnO thin films on a commercial Cz-Si solar cell substrate


Gabas, M; Barrett, NT; Ramos-Barrado, JR; Gota, S; Rojas, TC; Lopez-Escalante, MC
Solar Energy Materials and Solar Cells, 93 (2009) 1356-1365
Materiales Nanoestructurados y Microestructura

ABSTRACT

We have studied differences in the interface between undoped and Al-doped ZnO thin films deposited on commercial Si solar cell substrates. The undoped ZnO film is significantly thicker than the Al-doped film for the same deposition time. An extended silicate-like interface is present in both samples. Transmission electron microscopy (TEM) and photoelectron spectroscopy (PES) probe the presence of a zinc silicate and several Si oxides in both cases. Although Al doping improves the conductivity of ZnO, we present evidence for Al segregation at the interface during deposition on the Si substrate and suggest the presence of considerable fixed charge near the oxidized Si interface layer. The induced distortion in the valence band, compared to that of undoped ZnO, could be responsible for considerable reduction in the solar cell performance.


Agosto, 2009 | DOI: 10.1016/j.solmat.2009.02.018

Aluminium anodisation for Au-CeO2/Al2O3-Al monoliths preparation


Sanz, O; Martinez, LM; Echave, FJ; Dominguez, MI; Centeno, MA; Odriozola, JA; Montes, M
Chemical Engineering Journal, 151 (2009) 324-332
Química de Superficies y Catálisis

ABSTRACT

The anodisation of aluminium monoliths was performed in order to generate an alumina layer that ensures a good adherence of the catalysts. In this study, it is demonstrated that the morphology of the produced alumina layer depends on time, temperature, current density and concentration of the selected electrolyte. When anodisation process with the extreme conditions was applied (30 °C, 50 min, 2 A dm−2 and 2.6 M of sulphuric acid) a significant cracks were obtained and used to fix the subsequent catalytic coatings. The washcoating method was used to cover the monoliths with colloidal solutions of CeO2 and/or Au-CeO2 catalysts. The resulting monolithic catalysts were tested in the CO oxidation reaction being 1%Au-CeO2 containing system the most active. The structured catalyst prepared this way changed neither the textural nor the catalytic properties of the deposited catalytic powders.


Agosto, 2009 | DOI: 10.1016/j.cej.2009.03.062

Identification of hydrogen and deuterium at the surface of water ice by reflection electron energy loss spectroscopy


Yubero, F; Tokesi, K
Applied Physics Letters, 95 (2009) 084101
Nanotecnología en Superficies y Plasma

ABSTRACT

A nondestructive method to distinguish between hydrogen (H) and deuterium (D) at surfaces by reflection electron energy loss spectroscopy is presented. It is based on the analysis of the energy distributions of electrons elastically backscattered from surfaces containing H or D. We consider standard and deuterated water ices as test surfaces. The recoil energy of the backscattered electrons depends on the atomic mass of the targets, and the contributions of H, D, and O to the measured spectra can be easily separated. The results of Monte Carlo simulations corroborate the experimental findings.


Agosto, 2009 | DOI: 10.1063/1.3202402

High surface area α-alumina preparation by using urban waste


Martin-Ruiz, MM; Perez-Maqueda, LA; Cordero, T; Balek, V; Subrt, J; Murafa, N; Pascual-Cosp, J
Ceramics International, 35 (2009) 2111-2117
Reactividad de Sólidos

ABSTRACT

A new method for preparing high surface area α-alumina from urban waste is proposed. The method consists of the precipitation of a precursor that contains bohemite mixed with a linear polymer and subsequently the thermal decomposition of the precursor by heating in nitrogen and air to 1200 °C. The resulting α-alumina consists of nanocrystals of about 100 nm aggregated into larger particles with relatively high surface area (12 m2 g−1) and a significant macropore volume of 0.545 cm3 g. Methods of X-ray diffraction (XRD), scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM) and selected area electron diffraction (SAED) were used to characterize microstructure of prepared materials. Results of differential thermal analysis, thermogravimetry and emanation thermal analysis characterized the thermal behaviour of α-alumina precursors.


Agosto, 2009 | DOI: 10.1016/j.ceramint.2008.11.011

Synthesis by pyrolysis of aerosols and ceramic application of Cr-doped CaYAlO4 red–orange pigments


Lyubenova, TS; Carda, JB; Ocana, M
Journal of the European Ceramic Society, 29 (2009) 2193-2198
Materiales Coloidales

ABSTRACT

The synthesis of red–orange Cr-doped YCaAlO4 pigments has been improved (softer thermal conditions and lower environmental impact) and optimised by using the pyrolysis of aerosols method. We also study the crystallochemical features of the Cr chromophore with special emphasis on its oxidation state which has not been yet clarified, finding that Cr(III) and Cr(IV) species are present in the octahedral and interstitial tetrahedral sites of the YCaAlO4 lattice, respectively. Finally, the applicability of this system as ceramic pigment was tested using conventional industrial glazes. A change from orange to pink shades was detected after glaze firing, which is mainly attributed to the Cr3+ to Cr4+ oxidation.


Agosto, 2009 | DOI: 10.1016/j.jeurceramsoc.2009.01.020

Influence of sulfur on the structural, surface properties and photocatalytic activity of sulfated TiO2


Colon, G; Hidalgo, MC; Navio, JA; Kubacka, A; Fernandez-Garcia, M
Applied Catalysis B-Environmental, 90 (2009) 633-641
Fotocatálisis Heterogénea: Aplicaciones

ABSTRACT

TiO2 materials were prepared by sol–gel method and then impregnated with sulfuric acid and calcined using different temperatures and atmosphere (air and nitrogen). Systematic variation of these two experimental parameters makes possible to modulate the amount of surface sulfur from the impregnation procedure. The best photocatalyst for liquid phenol degradation was obtained after calcination at 700 °C in air, while gas toluene degradation optimum performance is obtained by calcination at 700 °C in nitrogen from 500 °C. Structural analysis of these materials by XRD, micro-Raman spectroscopy and FE-SEM shows that once calcined at 700 °C the material was a well-crystallized, high surface area anatase structure in all cases. The surface characterization by FTIR and XPS confirms the presence of a higher amount of sulfur species and acidic OH groups in samples partially calcined in nitrogen, and a low XPS O/Ti-atomic ratio with the O 1s peak shifted to higher binding energies (1.8 vs. 2 ± 0.1 and 530.4 eV vs. 529.8 eV, respectively, against the reference materials) for samples calcined at 700 °C, temperature at which most of sulfate species have been evolved. The paper presents an attempt to correlate the contribution of the observed structural defects within the anatase sub-surface layers and surface acidity to the different photoactivity behaviour exhibited for phenol liquid phase and toluene gas phase photodegradation.


Agosto, 2009 | DOI: 10.1016/j.apcatb.2009.04.026

Mineralogical stability of phyllosilicates in hyperalkaline fluids: Influence of layer nature, octahedral occupation and presence of tetrahedral Al


Becerro, AI; Mantovani, M; Escudero, A
American Mineralogist, 94 (2009) 1187-1197
Materiales de Diseño para la Energía y Medioambiente

ABSTRACT

Mineralogical changes in a set of phyllosilicates, differing in their layer nature, chemical composition, octahedral character, and Al content of the tetrahedral sheet, were analyzed after hydrothermal reaction in an alkaline solution. The composition of the alkaline solution was selected to simulate the first stage of cement degradation [NaOH-KOH-Ca(OH)2]. The reaction products have been analyzed by XRD, 29Si and 27Al MAS NMR spectroscopy, SEM/EDX, and TEM. The results indicate that the main factor influencing the stability of the clays is the occupation of the octahedral sheet such that all trioctahedral members withstand the alkaline attack, whereas most of the dioctahedral clays suffer a complete dissolution and crystallization of new phases. Second, clays with Al in the tetrahedral sheet of their layers are shown to be less stable than those with a pure Si tetrahedral sheet.


Agosto, 2009 | DOI: 10.2138/am.2009.3164

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