Scientific Papers in SCI

Abstract

We have studied low refractive index fluorine doped silica thin films prepared by reactive magnetron sputtering. Two experimental parameters were varied to increase the porosity of the films, the geometry of the deposition process (i.e., the use of glancing angle deposition) and the presence of chemical etching agents (fluorine species) at the plasma discharge during film growth. The microstructure, chemistry, optical properties, and porosity of the films have been characterized by scanning electron and atomic force microscopy, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, UV–vis, and spectroscopic ellipsometry. It is found that either the deposition at glancing angles or the incorporation of CFx species in the plasma discharge during film growth produces a decrease in the refractive index of the deposited films. The combined effect of the two experimental approaches further enhances the porosity of the films. Finally, the films prepared in a glancing geometry exhibit negative uniaxial birefringence.

Abstract

The concept of generalized time θ=∫exp(−Ea/RT)dt θ = ∫ exp ( − E a / RT ) d t in non-isothermal kinetics was introduced by Ozawa in 1965, together with the well-known isoconversional plot, i.e., Ozawa plot. The generalized time is the key concept to tie the kinetic data under varying temperature to the kinetic relationship at a constant temperature. It is well known that many processes studied by thermal analysis and calorimetry reveal a complex nature. Therefore, the generalized time concept seems to be very useful for the description of the change in the rate behavior depending on the fractional conversion. Using the concept of θ, three kinds of experimental master plots can be formalized in differential, integral, and multiplied forms. Among others, combination of the differential and multiplied master plots, y(α) = (dα/dθ) and z(α) = (dα/dθ)θ, show a high performance to discriminate the kinetic model based on the maxima condition of y(α y*) and z(α z*). The α y* − α y* kinetic plot is a useful tool to visualize the complexity of the kinetic process and to determine the most suitable kinetic model. The usefulness of α y* − α y* kinetic plot and the YZ-master plots is illustrated as exemplified by the kinetic analyses of complex crystallization processes of the as-prepared, thermally and mechanically treated amorphous zirconia.

Abstract

The thermal degradation kinetics of several ethylene–propylene copolymers (EPM) and ethylene–propylene–diene terpolymers (EPDM), with different chemical compositions, have been studied by means of the combined kinetic analysis. Until now, attempts to establish the kinetic model for the process have been unsuccessful and previous reports suggest that a model other than a conventional nth order might be responsible. Here, a random scission kinetic model, based on the breakage and evaporation of cleavaged fragments, is found to describe the degradation of all compositions studied. The suitability of the kinetic parameters resulting from the analysis has been asserted by successfully reconstructing the experimental curves. Additionally, it has been shown that the activation energy for the pyrolysis of the EPM copolymers decreases by increasing the propylene content. An explanation for this behavior is given. A low dependence of the EPDM chemical composition on the activation energy for the pyrolysis has been reported, although the thermal stability is influenced by the composition of the diene used.

Abstract

A series of synthesised TiO2-based and commercial photocatalysts were modified by Pt photodeposition and a study made of their photocatalytic activity in hydrogen production. The modified commercial photocatalysts were Evonik P25, Kronos vlp7000 and Hombikat UV-100, and the other modified photocatalysts were synthesised by our group using sol–gel and sol–gel hydrothermal processes (SG400, SG750 and HT). Pt weight percentages used in the study were 0.5, 1.0 and 2.1 wt.% (Pt/TiO2). The photocatalysts were extensively characterised by X-ray diffraction (XRD), UV–vis diffuse reflectance, Brunauer–Emmett–Teller (BET) surface area measurement, transmission electron microscopy (TEM), scanning electron microscopy (SEM–EDX), Fourier transform infrared spectroscopy (FTIR) and laser light dispersion. Methanol (25% vol.) was used as sacrificial agent over the 8 h of the hydrogen production tests and measurements were taken of the final concentrations of formaldehyde and formic acid as well as initial and final TOC. Photoactivity of all photocatalysts increased in the presence of Pt. The most efficient of the synthesised photocatalysts was SG750 and of the commercial photocatalysts P25. Maximum production of SG750 was 1846 μmol h−1 at 1.0 wt.% Pt and its production per surface unit was notably higher than that of P25.

Abstract

A comparative study of both conventional rising temperature and sample-controlled methods, like constant rate thermal analysis (CRTA), is carried out after analyzing a set of solid state reactions using both methods. It is shown that CRTA avoids the influence of heat and mass transfer phenomena for a wide range of sample sizes leading to reliable kinetic parameters. On the other hand, conventional rising temperature methods yield α–T plots dependent on experimental conditions, even when using samples sizes smaller than 2 mg. Moreover, it is shown that the discrimination of overlapping processes is dramatically improved using sample-controlled methods instead of conventional heating procedures. An advanced method for performing the kinetic analysis of complex processes from a single CRTA experiment is proposed.

Abstract

Which mechanisms mediate cell attachment to biomaterials? What role does the surface charge or wettability play on cell–material anchorage? What are the currently investigated strategies to modify cell–matrix adherence spatiotemporally? Considering the development of scaffolds made of biocompatible materials to temporarily replace the structure and/or function of the extracellular matrix, focus is given to the analysis of the specific (i.e., cell adhesive peptide sequences) and unspecific (i.e., surface charge, wettability) mechanisms mediating cell-matrix interactions. Furthermore, because natural tissue regeneration is characterized by the dynamic attachment/detachment of different cell populations, the design of advanced scaffolds for tissue engineering, based in the spatiotemporal tuning of cell–matrix anchorage is discussed.

Abstract

Erbium doped TiO2–Bi2WO6 have been synthesized by means of a surfactant free hydrothermal method having good photoactivities under sun-like excitation for the degradation of Rhodamine B. From the structural and morphological characterization it has been stated that the presence of Er3+ induces a progressive russelite cell contraction due to its incorporation in the Bi2WO6 lattice in substitutional sites. The best photocatalytic performance was attained for the samples with 1 at% of Er. From the study of the photocatalytic activity under different irradiation conditions it can be inferred that Er3+ presence induces a significant improvement of the photoactivity in the UV range. The evolution of band-gap values seems to be similarly related with the reaction rate progression. Thus, the higher band-gap values in lower Er doped systems would be the cause of a better electron hole separation under UV irradiation.

Abstract

A computerized database was created, based on a Geographic Information System (GIS), with hyperlinks to the website for a Rural Development Association (Almería province, Andalusia, Spain). Thus, a catalogue of traditional rural buildings in this particular area was compiled, identifying and characterizing each one, establishing criteria for a dynamic and rational selection. The purpose to select this example was to facilitate their management by public organizations or private individuals, for their reuse, restoration or both. The cataloguing and promotion of rural architecture will contribute to creating jobs by stimulating new economic activity, such as the promotion of cultural tourism, while preserving a valuable source of information on rural culture, recovering local construction techniques, encouraging a sense of community, and making villages and rural areas more attractive to visitors. The assessment of the rehabilitation potential of rural buildings in this region has helped to establish a priority order for their reuse, and so an intervention map has been devised in terms of a “Decision Index” corresponding to each considered building.

Abstract

Experimental results are reported on the (Ca-looping) multicyclic CO2 capture of CaO and nanosilica/CaO composites derived from Ca(OH)2 and nanosilica/Ca(OH)2 dry mixtures subjected in situ to linear and constant rate thermal analysis (CRTA) preheating programs in either air or air/CO2 atmospheres. By means of CRTA preheating the rates of the reactions taking place during pretreatment are kept at a constant and small value along the entire process. In agreement with a pore skeleton model, previously proposed in the literature for explaining the behavior of natural limestones thermally pretreated, our results suggest that air/CO2-CRTA pretreatment yields a thermally stable hard skeleton of poorly reactive CaO on which a soft skeleton of reactive CaO would be supported. The sorbent subjected to this preheating program exhibits a reactivation in the very first carbonation/calcination cycles, after which CaO conversion decays slowly with the cycle number. In contrast, linearly or air-CRTA preheated sorbents show a significant decrease of CaO conversion within the first cycles. In the latter case, CaO multicyclic conversion fits well to a model where it is assumed that the progressive reduction of surface area as the number of carbonation/calcination cycles is increased obeys to sintering of the preheated sorbent skeleton as it is subjected to repeated calcinations during cycling. In the former case, CaO conversion data conforms to the prediction by a model in which the loss of surface area is mainly due to sintering of a nascent CaO soft skeleton regenerated in the diffusive carbonation phase, which is enhanced by the air/CO2-CRTA pretreatment. As regards the effect of nanosilica, the results indicate that it slows down CaO sintering during pretreatment, which hinders the development of a stable CaO skeleton thus hampering reactivation and stabilization of conversion. On the other hand, as CaO sintering is also lessened during looping calcination, nanosilica is useful to increase the absolute values of CaO conversion.

Abstract

The use of RE2Si2O7 materials as environmental barrier coatings (EBCs) and in the sintering process of advanced ceramics demands a precise knowledge of the coefficient of thermal expansion of the RE2Si2O7. High-temperature X-ray diffraction (HTXRD) patterns were collected on different RE2Si2O7 polymorphs, namely A, G, α, β, γ, and δ, to determine the changes in unit cell dimensions. RE2Si2O7 compounds belonging to the same polymorph showed, qualitatively, very similar unit cell parameters behavior with temperature, whereas the different polymorphs of a given RE2Si2O7 compound exhibited markedly different thermal expansion evolution. The isotropy of thermal expansion was demonstrated for the A-RE2Si2O7 polymorph while the rest of polymorphs exhibited an anisotropic unit cell expansion with the biggest expansion directed along the REOx polyhedral chains. The apparent bulk thermal expansion coeficcients (ABCTE) were calculated from the unit cell volume expansion for each RE2Si2O7 compound. All compounds belonging to the same polymorph exhibited similar ABCTE values. However, the ABCTE values differ significantly from one polymorph to the other. The highest ABCTE values correspond to A-RE2Si2O7 compounds, with an average of 12.1 × 10−6 K−1, whereas the lowest values are those of β- and γ-RE2Si2O7, which showed average ABCTE values of ~4.0 × 10−6 K−1.

Abstract

Previous studies have indicated that long-chain linear carboxylic acids form commensurate packed crystalline monolayers on graphite even at temperatures above their melting point. This study examines the effect on the monolayer formation and structure of adding one or more secondary hydroxyl, functional groups to the stearic acid skeleton (namely, 12-hydroxystearic and 9,10-dihydroxystearic acid). Moreover, a comparative study of the monolayer formation on recompressed and monocrystalline graphite has been performed through X-ray diffraction (XRD) and Scanning Tunneling Microscopy (STM), respectively. The Differential Scanning Calorimetry (DSC) and XRD data were used to confirm the formation of solid monolayers and XRD data have provided a detailed structural analysis of the monolayers in good correspondence with obtained STM images. DSC and XRD have demonstrated that, in stearic acid and 12-hydroxystearic acid adsorbed onto graphite, the monolayer melted at a higher temperature than the bulk form of the carboxylic acid. However, no difference was observed between the melting point of the monolayer and the bulk form for 9,10-dihydroxystearic acid adsorbed onto graphite. STM results indicated that all acids on the surface have a rectangular p2 monolayer structure, whose lattice parameters were uniaxially commensurate on the a-axis. This structure does not correlate with the initial structure of the pure compounds after dissolving, but it is conditioned to favor a) hydrogen bond formation between the carboxylic groups and b) formation of hydrogen bonds between secondary hydroxyl groups, if spatially permissible. Therefore, the presence of hydroxyl functional groups affects the secondary structure and behavior of stearic acid in the monolayer.

Abstract

Highly oriented CrO₂ thin films have been heteroepitaxially grown on TiO₂ rutile (110), (100) and (001) single crystalline substrates, by Low Pressure Chemical Vapor Deposition from CrO₃ as precursor and flowing oxygen as carrier gas, under a pressure of 67 Pa. The experimental conditions were fine tuned by depositing on polycrystalline Ti foils, to improve the purity of the films and the deposition rate. A maximum deposition rate of 175 nm h^− 1 was obtained.

The composition and texture of films, up to 2 μm thick, have been determined by X-ray diffraction (XRD) and Micro Raman, while their microstructure has been examined by Scanning Electron Microcopy and Atomic Force Microscopy, and their magnetic behavior has been tested by superconducting quantum interference device magnetometry. These techniques reveal that the phase purity, texture, microstructure and thickness of these films are dependent on the crystalline face of the rutile substrate and the deposition temperature. Thus, microscopy techniques, XRD and Raman spectroscopy confirm that highly textured CrO₂ films were always obtained on the three rutile substrate faces when deposition temperature ranges between 616 K and 636 K. But these techniques also show that CrO₂ films are unpurified with inclusions or patches of Cr₂O₃, for the most of the substrates and especially at high deposition temperatures. Magnetic measurements conclusively demonstrate that pure CrO₂ films are only obtained when TiO₂ (110) is used as a substrate.

Abstract

We study in this paper the conversion of CaO-based CO2 sorbents when subjected to repeated carbonation–calcination cycles with a focus on thermally pretreated/doped sorbents. Analytical equations are derived to describe the evolution of conversion with the cycle number from a unifying model based on the balance between surface area loss due to sintering in the looping-calcination stage and surface area regeneration as a consequence of solid-state diffusion during the looping-carbonation stage. Multicyclic CaO conversion is governed by the evolution of surface area loss/regeneration that strongly depends on the initial state of the pore skeleton. In the case of thermally pretreated sorbents, the initial pore skeleton is highly sintered and regeneration is relevant, whereas for nonpretreated sorbents the initial pore skeleton is soft and regeneration is negligible. Experimental results are obtained for sorbents subjected to a preheating controlled rate thermal analysis (CRTA) program. By applying this preheating program in a CO2 enriched atmosphere, CaO can be subjected to a rapid carbonation followed by a slow rate controlled decarbonation, which yields a highly sintered skeleton displaying a small conversion in the first cycle and self-reactivation in the next ones. Conversely, carbonation of the sorbent at a slow controlled rate enhances CO2 solid-state diffusion, which gives rise, after a quick decarbonation, to a highly porous skeleton. In this case, CaO conversion in the first cycle is very large but it decays abruptly in subsequent cycles. Data for CaO conversion retrieved from the literature and from further experimental measurements performed in our work are analyzed as influenced by a variety of experimental variables such as preheating temperature program, preheating exposition time, atmosphere composition, presence of additives, and carbonation–calcination conditions. Conversion data are well fitted by the proposed model equations, which are of help for a quantitative interpretation of the effect of experimental conditions on the multicyclic sorbent performance as a function of sintering/regeneration parameters inferred from the fittings and allow foreseeing the critical conditions to promote reactivation. The peculiar behavior of some pretreated sorbents, showing a maximum conversion in a small number of cycles, is explained in light of the model.

Abstract

Clay-based minerals are considered to be an important component in backfill barriers due to both their ability to seal and adsorb radioactive waste and to interact chemically with it under subcritical conditions. Herein, we describe a systematic study of the properties of layered silicates that could affect their hydrothermal reactivity, namely type of layers, octahedral occupancy, origin and total amount of the layer charge, and nature of the interlayer cation. The silicates studied were selected on the basis of their different characteristics associated with these properties and were treated hydrothermally at 300 °C for 48 h in a 7.3 · 10− 2 M Lu(NO3)3 · 3.6H2O solution. The final products were analyzed by X-ray diffraction and solid-state NMR spectroscopy. All the layered silicates studied were found to be able to generate a Lu2Si2O7 phase after hydrothermal treatment under subcritical conditions, thereby confirming the participation of a chemical mechanism of the clay barrier generating phases being stables with temperature and pH conditions. However, the extent of this reaction depends to a large extent on the physicochemical properties of the framework and the interlayer space composition, such as the presence or absence of an octahedral sheet, the degree of occupancy of this sheet, and the origin and total layer charge. Therefore, this study allows tuning the clay mineral framework characteristic that favors the rare earth cations (as trivalent actinide simulator) immobilization.

Abstract

Gas-solid heterogeneous photocatalytic oxidation (PCO) of cyclohexane in humidified air over TiO2 and Pt/TiO2 catalyst was studied.

Pt/TiO2 photocatalysts were synthesized by photodeposition method at different Pt loadings (0.5–2 wt.%). The addition of 0.5 wt.% Pt does not significantly modify the TiO2 properties. The increase in Pt loading induces to an aggregation of metallic particles on TiO2 surface.

The cyclohexane PCO was performed in a fluidized bed photoreactor at 60 and 100 °C. Pure TiO2 was more active than 1 and 2 wt.% Pt/TiO2 samples at 60 °C. Nevertheless, the conversion level increases with temperature on Pt/TiO2 photocatalysts. The cyclohexane was mineralized into CO2, water and low amount of CO. A beneficial effect of Pt addition was found, since total CO2 selectivity was obtained. The Pt/TiO2 photocatalysts prepared by photodeposition provide the total cyclohexane PCO without CO production. Photocatalysts deactivation was not observed in any performed test. Evidence of an opportune tuning of temperature is highlighted.

Abstract

The conversion of thermally pretreated CaO along successive carbonation/calcination cycles has been investigated, as affected by looping-calcination conditions, by means of Thermogravimetric Analysis (TGA). Sorbent samples have been subjected in situ to a thermal preheating program based on Constant Rate Thermal Analysis (CRTA) by virtue of which decarbonation is carried out at a low controlled rate, which is able to promote self-reactivation in the first carbonation/calcination cycles. Our observations support a pore-skeleton model according to which solid-state diffusion in the first carbonation stages, which is enhanced by thermal pretreatment, gives rise to a soft skeleton with increased surface area. Yet, the results show that self-reactivation is hindered as looping-calcination conditions are harshened. Increasing the looping-calcination temperature and/or the looping calcination time period favors sintering of the soft skeleton and eventually self-reactivation is precluded. A model is developed that retrieves the main features of multicyclic conversion of thermally pretreated sorbents in the first cycles based on the balance between surface area gain due to promoted solid-state diffusion carbonation and surface area loss due to sintering of the soft skeleton in the looping-calcination stage, which can be useful to investigate the critical looping-calcination conditions that nullify self-reactivation. The proposed model allows envisaging the behavior of the sorbent performance as a function of the pretreatment conditions.

Abstract

In this work, we report on the determination of the infrared Yb3+ → Tm3+ energy transfer efficiency in YF3:Yb3+/Tm3+ nanocrystals through the study of Yb3+ dynamics. The obtained results are compared to those previously reported in macrocrystals to analyze possible changes related to size reduction. Luminescence lifetimes are much shorter in the nanoparticles than in bulk samples, a behavior that can be related to Yb3+ → Yb3+ migration and the enhanced surface/volume ratio of the nanoparticles. On the other hand, Yb3+ → Tm3+ energy transfer macroparameter remains unaltered, demonstrating that spectroscopic intrinsic parameters such as radiative and non-radiative probabilities are not affected by size reduction. Finally, a formula that describes Yb3+ lifetime dependence with Yb3+ and Tm3+ concentration is proposed, considering both the effects produced by migration between Yb3+ ions and energy transfer from Yb3+ to Tm3+ ions.

Abstract

Kanemite belongs to the group of naturally occurring sodium silicate minerals that was first found in Kanem, at the edge of the Lake Chad, and has been synthesized in different ways from NaOH-SiO2 mixtures and used as precursor for the design of microporous and mesoporous materials. The fluoride route to the synthesis of microporous materials is based on the substitution of OH− anions by fluoride anions, which may subsequently also play a mineralizing role, and gives rise to materials with higher hydrophobicity and thermal and hydrothermal stability. Moreover, F− plays an important role in the incorporation of framework heteroatoms, thereby affecting the activity of the final material. The aim of this study was to synthesize fluorokanemite using different synthetic routes and different F− source. The final product was characterized by a combination of methods that provided information regarding the incorporation of fluorine into the framework and the short- and long-range structural order of the fluorosilicate. Kanemite with water content close to ideal was obtained in all cases. The washing process was found to have no effect in the long- or short-range structural order of the layer framework, although it did affect the structure of the cation in the interlayer space of kanemite. The mineralizing agent therefore appears to be the key to the synthesis. Furthermore, it governs the resulting kanemite structure by controlling the formation of hydrogen bonds in the framework, and therefore the degree of lamellar structure condensation.

Abstract

ZrSiO₄ and HfSiO₄ are of considerable interest because of their low thermal expansions, thermal conductivities, and the optical properties of HfSiO₄. In addition, silicate phases of both are studied as model radioactive waste disposal materials. Previous first principles calculations reported near ideal mixing in the Zr_1–xHf_xSiO₄ system, with a very weak propensity for phase separation. Density functional theory (DFT)/cluster-expansion first principles calculations presented in this work indicate near ideal mixing with a very weak propensity for ordering. Zr_1–xHf_xSiO₄ samples (x = 0, 0.25, 0.5, 0.75, and 1.0) were synthesized from intimate stoichiometric mixtures of constituent-oxides and annealing at 1823 K for 20 days in a platinum crucible. Samples were characterized by X-ray diffraction (XRD; Rietveld analysis) and ²⁹Si MAS NMR. The XRD data exhibited a pronounced negative deviation from Vegard’s law in the excess volume of mixing, and the ²⁹Si MAS NMR spectra also suggest nonideal mixing. Given the very weak energetics that favor cation ordering, it is clear that there must be some other cause(s) for the observed deviations from ideal mixing behavior.

Abstract

In this paper, some clarifications regarding the use of model-fitting methods of kinetic analysis are provided in response to the lack of plot linearity and dispersion in the activation energy values for the thermal degradation of polystyrene found in the literature and some results proposing an nth order model as the most suitable one. In the present work, two model-fitting methods based on the differential and integral forms of the general kinetic equation are evaluated using both simulated and experimental data, showing that the differential method is recommended due to its higher discrimination power. Moreover, the intrinsic limitations of model-fitting methods are highlighted: the use of a limited set of kinetic models to fit experimental data and the ideal nature of such models. Finally, it is concluded that a chain scission model is more appropriate than first order.

Abstract

The influence of the synthesis method and Sn addition on Ni/CeO2–MgO–Al2O3 catalyst is correlated to its catalytic behavior in the reaction of methanol steam reforming. The catalysts prepared by impregnation method are compared to samples obtained by deposition of previously obtained nanoparticles by the polyol method. X-ray diffraction (XRD), specific surface area measurements and H2-temperature programmed reduction (TPR) were used to characterize the catalysts. The differences of the structure, phase transformation and reduction behavior are discussed and related to the catalytic performance of the samples as well as the nature of the carbonaceous deposits formed during the reaction.

Abstract

The preparation of hard coatings with low friction coefficient over a wide temperature range is still a challenge for the tribological community. The development of new nanocomposite materials consisting of different metal-ceramic phases, each of which exhibiting self-lubricating characteristics at different temperatures, may help to solve this problem. We report on the structure and tribological properties of MoCN-Ag coatings deposited by magnetron co-sputtering of Mo and C (graphite) targets and simultaneous sputtering of an Ag target either in pure nitrogen or in a gaseous mixture of Ar + N2. The structure and elemental composition of the coatings were studied by means of X-ray diffraction, scanning and transmission electron microscopy, X-ray photoelectron spectroscopy, energy dispersive spectroscopy, Raman spectroscopy, and glow discharge optical emission spectroscopy. The tribological properties of the coatings against an Al2O3 ball were investigated first at discrete temperatures of 25, 500, and 700 °C, and then during continuous heating in the temperature range of 25–700 °C. The coating structure and their respective wear tracks were also examined to elucidate their phase transformations during heat treatments. The lowest friction coefficients (<0.4) were observed in the temperature ranges of 25–100 °C and 400–700 °C and can be explained by the presence of a free amorphous carbon phase, which served as a lubricant at low temperatures, and by a positive role of silver and two phases forming at elevated temperatures, molybdenum oxide and silver molybdate, which provided lubrication above 400 °C. In the temperature range between 100 and 400 °C, the friction coefficient was relatively high. This problem is to be addressed in future works.

Abstract

This paper focuses on the physical–chemical and mechanical characterization of the building materials of a precious damaged structure, the Salares tower, a Hispanic–Islamic medieval construction located in the South of Spain, an active seismic area. An exhaustive description of the materials has been obtained, enriching the knowledge on historical construction materials. Sophisticated laboratory tests (XRD, SEM, EDX, FTIR, Micro-Raman spectroscopy, DTA and TG) have provided crucial complementary data to use together with traditional laboratory procedures, especially when the damage processes are the main concern. The experimental investigation has allowed a better understanding of the remote origin and of the failure mechanisms of this damaged structure. On the basis of these results, the most adequate repair materials are selected.

Abstract

A Molecular Dynamics study about the segregation of yttrium at 1500 K to a Σ5 grain boundary in 8 mol% YSZ has been performed. Segregation has been induced by explicitly taking into account the excess energy associated to the elastic misfit effect for yttrium cations located nearby the grain boundary planes. After an initial transient, a steady regime is reached, in which the number of yttrium cations does not increase with time. Accumulation of yttrium cations is accompanied by that of zirconium ones and oxygen vacancies at some distance of the grain boundary planes. The changes in the radial distribution functions for different ionic pairs are discussed, as also the effect of segregation on oxygen diffusion along the grain boundaries and in volume. Finally, the possibility that segregated yttrium located at available free sites at the grain boundaries is pointed out.

Abstract

Smectite is a family of clay minerals that have important applications. In the majority of these clay minerals, the hydrated interlayer cations play a crucial role on the properties of the clay. Moreover, many studies have revealed that both thermal and grinding treatments affect the MMT structure and that interlayer cations play an important role in the degradation of the structure, primarily after mechanical treatment. In this study, the effects of these treatments on MMTs homoionized with mono (Na+, Li+ or K+) or polyvalent (Ca2+ or Al3+) cations were analyzed by the combination of a set of techniques that can reveal the difference of bulk phenomena from those produced on the surface of the particles. The thermal and mechanical (in an oscillating mill) treatments affected the framework composition and structure of the MMT, and the thermal treatment caused less drastic changes that the mechanical one. The effect of the interlayer cations is primarily due to the oxidation state and, to the size of the cations, which also influenced the disappearance of aluminum in the MMT tetrahedral sheet. These treatments caused a decrease in the surface area and an increase in the particle agglomeration and the isoelectric point. Both treatments caused the leaching of the framework aluminum. Furthermore, the mechanical treatment induced structural defects, such as the breakup of the particles, which favored the dehydroxylation and the increase of the isoelectric points of the montmorillonites.

Abstract

We have deposited TiO2 and SiO2 thin films by techniques as different as plasma-enhanced chemical vapor deposition (PECVD) and reactive magnetron sputtering under experimental conditions where highly directional deposition fluxes are avoided. The results indicate that whatever the deposition technique employed or even the precursor gas in the PECVD technique, films share common microstructural features: a mounded surface topography and a columnar arrangement in the bulk, with the column width growing linearly with film thickness. With the help of a Monte Carlo model of the deposition, we conclude that these common aspects are explained by solely taking into consideration the incorporation of a low-energy, isotropically directed, deposition flux onto a substrate at low temperature and under a weak plasma/surface interaction environment.

Abstract

Metallic supported structured catalysts were obtained by washcoating AluchromYHf monoliths with an Au/TiO2 catalyst. The powder catalyst was synthesized by DAE (direct anionic exchange) method. Using this catalyst, a stable slurry was prepared and used to washcoat the monoliths. TEM and SEM studies revealed that gold nanoparticles in the Au/TiO2 powder catalyst had an average diameter of 3–4 nm, but during the preparation of the structured catalyst, aggregate Au particles of the slurry reached diameters of 9 nm. Before coating, Aluchrom YHf monoliths were thermally treated to generate a homogeneous and well-adhered oxide rough surface layer, mainly composed of α-Al2O3 whiskers, which favored the anchoring of the catalyst. The catalytic layer deposited was well attached and contained not only the Au/TiO2 catalyst but also metallic oxides formed from stainless steel components that diffused through the oxide scale. The structural characterization was performed by XRD, XRF, TEM, SEM, GD-OES and SBET.

The catalytic activity of the powder and structured catalysts was tested in the oxidation of the CO reaction. Catalysts demonstrated to be active at room temperature. After a first activation run, and in spite of their larger gold particle size, the catalytic activities of the structured catalysts overcame those of the powder catalyst. This improvement is probably due to the segregation of the transition metal oxides toward the surface oxide scale.

Abstract

A procedure for the synthesis of multifunctional europium(III)-doped gadolinium(III) fluoride (Eu:GdF3) nanoparticles (85 nm) with quasispherical shape by precipitation at 120 °C from diethylene glycol solutions containing lanthanide chlorides and an ionic liquid (1-Butyl, 2-methylimidazolium tetrafluoroborate) as fluoride source has been developed. These nanoparticles were polycrystalline and crystallized into a hexagonal structure, which is unusual for GdF3. They were also mesoporous (pore size = 3.5 Å), having a rather high BET surface area (75 m2 g–1). The luminescent and magnetic (relaxivity) properties of the Eu:GdF3 nanoparticles have been also evaluated in order to assess their potentiality as “in vitro” optical biolabels and contrast agent for magnetic resonance imaging. Finally, a procedure for their functionalization with aspartic-dextran polymers is also reported. The functionalized Eu:GdF3 nanoparticles presented negligible toxicity for Vero cells, which make them suitable for biotecnological applications.

Abstract

Er-doped LaPO4 microspheres have been synthesized by spray pyrolysis and the near infrared (NIR) properties have been characterized. It has been found that, following an adequate post-annealing treatment, the emission properties are remarkably improved. The NIR luminescence intensity is highly enhanced and its decay time increases to a value almost coincident with the reported radiative lifetime, which implies that the quantum yield approaches η ≈ 100%. This improvement in luminescence characteristics is probably related to the suppression of residual OH− radicals, that otherwise act as NIR luminescence quenchers, and to the increase in material's crystallinity.