R. Moreno

Institute of Ceramics and Glass, Madrid, Madrid, Spain

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Publications (152)286.96 Total impact

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    ABSTRACT: TiO2 photocatalytic fine-structured coatings were deposited by atmospheric plasma spraying (APS) on austenitic stainless steel coupons. The feedstock materials used were: one commercial TiO2 nanostructured spray-dried powder and five spray-dried powders obtained from suspensions comprising mixtures of submicrometric and nanometric TiO2 particles. Coating microstructure and phase composition were characterised using FEG-SEM and XRD techniques. The photocatalytic activity of the coatings was determined by measuring the degradation of methylene blue dye in aqueous solution. It was observed that the experimental data followed a pseudo first-order kinetic. Moreover, the kinetic constants displayed a clear relation with anatase content in the coatings. The photocatalytic decontamination capability of one of these APS coatings was subsequently investigated by treating a real industrial wastewater from tannery sector. A self-designed reactor to carry out photocatalytic experiments with TiO2 APS coated coupons was used. Results showed the partial degradation of pollutants by a decrease of Total Organic Carbon (TOC) and a significant decolouration effect which mainly relates to the removal of aromatic organic compounds typically present in this type of wastewaters. These findings prove the potentiality of plasma-sprayed photocatalytic TiO2 coatings for real wastewater treatments.
    No preview · Article · Sep 2015 · Ceramics International
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    ABSTRACT: The influence of two different additives, glycerol and polyvinyl alcohol (PVA), on the rheological behaviour and freeze casting performance of 35 vol.-% aqueous alumina suspensions is studied. Suspensions with PVA were prepared by either mixing all the components together or adding the PVA in a second step on the dispersed slurry. Although the slight increase in viscosity suggests that competitive adsorption occurs, the microstructure seems not to change depending on the order of addition of additives. Considering its lower molecular weight, glycerol provides lower viscosity, and as a consequence of its cryoprotective character, sintering leads to smaller pore size, being the porosity 35% for an added content of 10 wt-% with respect to solids. In the case of PVA, contents of 2 wt-% are enough to obtain solid firm bodies with a porosity of 48%. The porosity and the size of the pores decrease with increasing concentration of PVA.
    No preview · Article · May 2015 · Advances in Applied Ceramics
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    ABSTRACT: In the present work homogeneous mullite and mullite-alumina composites reinforced with carbon nanofibers (CNFs) were prepared. The stability of aqueous mullite and mullite-CNFs suspensions was studied through zeta potential and rheological behaviour of concentrated slurries. Slurries were optimised in terms of dispersant concentration and solids content and further used for the preparation of homogeneous mixtures of powders by freeze drying. The obtained powders were characterised through the determination of particle size distribution and morphology by Field Emission Scanning Electron Microscopy (FSEM) and Laser Scattering. The freeze dried powders were used to obtain dense composites by spark plasma sintering. Their microstructure and mechanical properties were evaluated. It was found that mullite and mullite-alumina composites reinforced with 2 vol.% CNFs maintain the strength and hardness of the ceramic matrix and presents higher values of toughness. The results indicated the homogeneous distribution and the survival of CNFs after consolidation.
    No preview · Article · May 2015 · Journal of the European Ceramic Society
  • R. Moreno · E. Bannier
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    ABSTRACT: In suspension and solution precursor plasma spraying, everything begins with the feedstock, which characteristics should be optimized as in any conformation process. However, in most published studies, commercial or highly diluted nanosuspensions are used, due to the difficulty to prepare well-dispersed, low-viscosity and stable suspensions. This chapter describes the fundamentals of the colloid chemistry and the rheology of ceramic suspensions that should be used in order to overcome these difficulties and to develop adequate SPS feedstocks.
    No preview · Article · Jan 2015
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    ABSTRACT: The process and impact of shaping mixed vanadium aluminum (hydr)oxides, VAlOH and VAlO, respectively, and BiMo catalysts by tableting and slip-casting were examined. Graphite (G) was employed as a shaping agent for tableting. Without it tableting was impracticable. Graphite was found to enhance the mechanical resistance of VAlOH-xG and BiMo-xG and changed the surface area by increasing it for the non-porous BiMo and by decreasing it for the mesoporous VAlOH. In addition, graphite modified the catalytic performance despite changing neither the chemical nor the structural state of the base VAlO(H) and BiMo catalysts. A positive effect on the performance of VAlO-xG in propane oxidative dehydrogenation was found. It was proposed that catalytic active sites are formed on graphite during calcination. Conversely, graphite was harmful for non-calcined VAlOH-xG and BiMo-xG. On the other hand, the preparation of chemically and physically stable VAlO(H) suspensions for slip-casting was accomplished. Chemical stability was achieved at pHs near the isoelectric point of these catalysts. For physical stability, the use of a dispersing agent, poly(acrylic acid), combined with a control of the solids concentration was necessary. A simple and reliable method for preparing VAlOH and BiMo pellets by slip-casting was thus developed with the use of colloidal silica as binding agent. The so prepared pellets were mechanically resistant and kept the surface area of the base materials. A decrease in the surface concentration of the active metals due to surface active site masking by silicon for VAlOH-xSi and BiMo-xSi led to an inferior catalytic performance.
    Full-text · Article · Sep 2014 · Catalysis Today
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    ABSTRACT: The influence was investigated of a graphite nanodispersoid addition on the lubricated sliding-wear behaviour of liquid-phase-sintered (LPS) SiC ceramics fabricated by spark-plasma sintering (SPS). The graphite nanodispersoids, introduced into the microstructure of the LPS SiC ceramic to act as self-lubricating phase, were obtained by graphitization of diamond nanoparticles during the SPS. It was found that the graphite nanodispersoid addition results in a lower resistance to mild wear, attributable to the lower hardness of graphite and the null lubrication it provides. Moreover, the graphite nanodispersoids, which mostly locate at grain boundaries/faces, worsen the cohesion of SiC grains, contributing together with the higher mild-wear rate to an early transition to the severe-wear stage. On the contrary, the graphite nanodispersoids were effective at improving the resistance to severe wear because they increase the fracture toughness while providing some external lubrication. Relevant implications for the microstructural design of advanced triboceramics are discussed.
    Full-text · Article · Sep 2014 · Journal of the European Ceramic Society
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    ABSTRACT: Atmospheric plasma spraying (APS) has been proved useful in obtaining TiO2 coatings with effective photocatalytic activity. However, the influence of feedstock characteristics on photocatalytic activity has hardly been addressed. This study was undertaken to prepare TiO2 photocatalytic coatings by APS from different feedstocks. The feedstocks were obtained by spray drying suspensions of a nano- and a submicron-sized TiO2 powder, with different solids contents and/or ratios of the nano- to submicron-sized particles. All the resulting powders were fully characterised to assess their suitability for use in APS processes. Feedstock powders were then deposited on steel coupons by APS using hydrogen or helium as secondary plasma gas. Coating microstructure and phase composition were characterised. Coating photocatalytic activity was determined by measuring the degradation of methylene blue dye in an aqueous solution. Powder characterisation showed that all feedstocks met plasma spray operation requirements with regard to agglomerate size distribution and density, as well as to powder flowability. Optimum agglomerate density was obtained when a well-balanced mixture of nano- and submicron-sized particles was used. All coatings displayed a bimodal microstructure with partially melted agglomerates that retained the initial nano- or submicron-sized structure and composition (anatase phase) of the feedstock, surrounded by a fully melted matrix mainly formed by rutile. As expected, coating porosity as well as the amount of partially melted areas depended strongly on plasma spray conditions and on feedstock characteristics. With regard to photocatalytic activity, a reasonably good fit of a first-order kinetic model to the experimental data was found for all coatings. The kinetic constants obtained displayed higher values than those of a commercial sol-gel coating. The values of these constants were related to feedstock characteristics as well as to plasma spraying conditions on the basis of anatase content. The contribution of other factors to the resulting coating surface is also discussed
    No preview · Article · Jul 2014 · Ceramics International
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    ABSTRACT: The introduction of carbon derivatives (nanotubes, graphene, etc.) as a second phase in ceramic matrices has limitations arising from their difficult processing. This paper studies the colloidal stability and the rheological behaviour of concentrated suspensions of alumina with 5 vol.% Y-TZP (AZ) and the effect of the addition of 2 vol.% of graphene oxide (AZGO) on the suspension stability, rheological behaviour and tape casting performance. The colloidal stability was studied using zeta potential measurements in terms of concentration of deflocculants and pH and homogenisation was optimised adjusting the sonication mode and time. The best results were obtained for pulsed mode. The optimum rheological properties were obtained for solid loadings of 53 vol.% and 40 vol.% for AZ and AZGO. Homogeneous, flexible tapes with thickness of ∼120 μm were obtained reaching densities of >60% of theoretical density in which secondary phases are well dispersed.
    No preview · Article · Jul 2014 · Journal of the European Ceramic Society
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    ABSTRACT: Rare-earth doped TiO2 nanoparticles via colloidal so-gel route have been the focus of intense research due to their potential optical application in filters, solar cells and light emitters. These applications require the preparation of good quality thin films from the colloids. Therefore different procedures have been used ranging from simple dipping and spin coating to electrophoretic deposition. However, these techniques do not allow a precise, nanometric control of the thickness of the deposited layers required for the targeted applications. In this work we report the successful production of optically active thin films using a non-conventional laser based method, called matrix assisted pulsed laser evaporation (MAPLE). TiO2 nanoparticles doped with Eu have been produced using a colloidal sol gel route [1]. The obtained solutions were frozen in liquid nitrogen to form a solid target further used in the MAPLE experiments. [1] M. Borlaf, et al., J. Phys. Chem. C. B 117, 1556 (2013).
    Full-text · Conference Paper · May 2014
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    ABSTRACT: This work deals with the preparation of copper dense compacts using an aqueous colloidal route. Concentrated suspensions of Cu powders were prepared in water with a polyelectrolyte and tetramethylammonium hydroxide at alkaline pH as dispersing agents. Suspensions were optimized by measuring the rheological behavior as a function of pH and volume fraction of solids. The slurries showed a pseudoplastic behavior with thixotropy due to the additives added. The suspensions were slip cast in plaster molds leading to high green densities (~ 61% TD), as a consequence of the particles' shape and size distribution packing characteristics. Sintering studies were performed at 900 °C in flowing Ar/5% H2 atmosphere. The green and dense compacts shaped by this route were characterized by SEM and Archimedes' density.
    Full-text · Article · Apr 2014 · Powder Technology
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    ABSTRACT: In this work the manufacture of commercial La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF) by aqueous colloidal processing is presented. The surface behavior of LSCF as a function of pH and the effect of a polyelectrolyte (Duramax D3005) on the stability are studied using measurements of zeta potential. Concentrated suspensions were prepared to solid content as high as 35 vol.%. The best dispersing conditions were determined by means of rheological measurements for obtaining stable and fluid slurry for tape casting technique. Different relative densities of the tapes were obtained at different temperatures. The LSCF tapes are good candidates for using as gas separation membrane or cathode for SOFC.
    No preview · Article · Apr 2014 · Journal of the European Ceramic Society
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    ABSTRACT: In this study, the influence of mechanical activation by intensive ball milling of a stoichiometric mixture of talc, kaolin, and alumina on the mechanism and kinetics of cordierite (2MgO center dot 2Al(2)O(3)center dot 5SiO(2)) formation was evaluated. The raw materials were characterized by chemical analysis, X-ray diffraction (XRD), laser diffraction, and helium pycnometry. The kinetics and mechanism of cordierite formation were studied by XRD, differential thermal analysis, and dilatometry in order to describe the phase formation as a function of temperature (1000-1400 degrees C), time of thermochemical treatment (0-4 h), and grinding time of the mixture (0-45 min). Finally, the optimal conditions of the thermochemical treatment that ensured the formation of cordierite were determined: milling time of 45 min and thermal treatment at 1280 degrees C for 1 h.
    No preview · Article · Apr 2014 · Química Nova
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    Mario Borlaf · Rodrigo Moreno · Angel L. Ortiz · Maria T. Colomer
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    ABSTRACT: The synthesis of nanoparticulate TiO2 sols without and with Eu3+ doping (1, 2, or 3 mol%) by the colloidal sol–gel method in aqueous media was investigated, with emphasis on the effect of the Eu3+ doping on the peptization time and rheological properties of the sols. It was found that the addition of Eu3+ increasingly retards the peptization process, and also results in sols with greater aggregate sizes which are therefore more viscous, although in all cases the distributions of aggregate sizes are unimodal and the flow behavior is Newtonian. The shifting of the isoelectric point of the sols toward greater pH with increasing Eu3+ doping indicates that the aforementioned trends are due to the chemical adsorption of europium ionic complexes in the form of solvated species. Furthermore, the effect of Eu3+ doping on the ultraviolet–visible spectrum and photocatalytic activity of the peptized sols was also explored. It was found that the Eu3+ doping increasingly shifts slightly the absorption edge from the ultraviolet to the visible range, and that its effect on the photocatalytic activity is certainly complex because this is enhanced only if the Eu3+ cations have some electronic transition (charge transfer transition or transitions between the ground state and the excited states) at the wavelength of the incident radiation, in which case the photocatalytic activity first increases with increasing Eu3+ content and then decreases perhaps due to occurrence of Eu–Eu interactions or simply to the greater aggregation state. Finally, the influence of the Eu3+ doping on the thermal stability of the nanoparticulate xerogels resulting from the drying of the peptized sols was also examined by X-ray thermo-diffractometry together with transmission electron microscopy, selected area electron diffractometry, and X-ray energy-dispersive spectrometry. It was found that although the xerogels crystallize all as anatase phase, this is increasingly more thermally stable with increasing Eu3+ doping, displaying a slowed down nanocrystallite growth, delayed onset temperature of the anatase-to-rutile phase transformation, and extended retention temperature of anatase phase.
    Full-text · Article · Mar 2014 · Materials Chemistry and Physics
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    Victor M. Candelario · Rodrigo Moreno · Angel L. Ortiz
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    ABSTRACT: Equiaxed ceramic nanoparticles and their mixtures are expected to exhibit shear-thinning behaviour when dispersed colloidally in aqueous media, whereas shear-thickening is the expectation for large aspect ratio phases such as, for example, carbon nanotubes (CNTs). Here, contrary experimental evidence is presented demonstrating the occurrence of severe coagulation at high shear rates in colloidally stable, semi-concentrated, aqueous suspensions of equiaxed SiC nanoparticles (major phase) mixed with equiaxed Y3Al5O12 nanoparticles (liquid-phase sintering additive), and how CNT addition prevents this coagulation if sufficient sonication is applied. It is also shown that although shear-thinning is the natural behaviour of the ceramic suspension up to moderate shear rates, coagulation is eventually a phenomenon inherent to the aqueous colloidal processing of these suspensions, with the critical shear rate for coagulation increasing and the rheopexy decreasing the better is the initial dispersion state achieved with the sonication. It is also shown that the critical shear rate for coagulation depends on the exact condition of shear rate increase, and that the re-sheared suspensions coagulate more significantly and at lower shear rates than the fresh suspensions. The mechanisms by which this coagulation occurs and is impeded by the CNTs are discussed, together with broader implications of these phenomena for the environmentally friendly processing of nanostructured ceramics and ceramic composites.
    Full-text · Article · Mar 2014 · Journal of the European Ceramic Society
  • M.I. Nieto · I. Santacruz · R. Moreno
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    ABSTRACT: Near-net shaping of ceramics is a key step in ceramic processing, because in this step a mass of powder is transformed into a piece with the desired geometry and size. During consolidation many defects can be introduced in the piece, so that the control of any stage is necessary to reduce or avoid the presence of defects. The gelation of polysaccharides constitutes one the most interesting consolidation mechanisms for shaping of ceramics due to the relative simplicity of the method, the use of water as liquid medium and the low concentration of additives necessary to obtain the bodies. In this work the main applications of polysaccharides in ceramic manufacturing through a gelation mechanism are reviewed. The chemical structure of the most common polysaccharides and their gelation mechanisms are described, including those yielding chemical gelation, as in the case of alginates in the presence of divalent ions, as well as polysaccharides that gel by a temperature change, either on heating as in cellulose derivatives, or on cooling, as in the case of agar derivatives and carrageenans. The influence of other polysaccharides used for their thickening properties but having an important synergy with gelling polysaccharides is also analyzed. In addition to the structure and the basic gelation mechanisms, several examples showing the influence of polysaccharides on the rheology and shaping of ceramic slurries are presented.
    No preview · Article · Jan 2014 · Revista Latinoamericana de Metalurgia y Materiales
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    ABSTRACT: Zirconium titanate-zirconia composites have potential for applications involving variations of temperature. Elastic characterization is necessary to evaluate stresses developed in materials which may be used in these kinds of applications. In this work, Young's and shear modulus and Poisson's ratio of two zirconium titanate-zirconia bulk composites (Z(Y)T70 and Z(Y)T50) have been determined at room temperature by the Impulse Excitation Technique (IET). Furthermore, Young's modulus (E) has been determined at high temperature (up to 1400 °C) for both composites. Young's modulus of Z(Y)T70 composite decreases ≈6% between room temperature and 400 °C due to the presence of zirconia. From 400 to 1400 °C, the decrease of E (≈14%) is due to the presence of zirconium titanate. Young's modulus behaviour at high temperature of Z(Y)T50 composite is determined by the degree of microcrack healing, which depends on the maximum temperature reached.
    Full-text · Article · Dec 2013 · Journal of the European Ceramic Society
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    ABSTRACT: Pb(Mg1/3Nb2/3)O3–PbTiO3 is used as a model system of perovskite solid solutions with very high piezoelectric response at tailored morphotropic phase boundaries to demonstrate the processing of textured ceramics by ceramic-only technology. A novel homogeneous templated grain growth approach that uses conventional ceramic procedures and a single-source nanocrystalline powder for the matrix and also for obtaining the templates is described. Two batches of (100) faceted cube-shaped microcrystals with average sizes of 27 and 10 μm were successfully used as templates, and aligned by tape casting for the processing of -textured Pb(Mg1/3Nb2/3)O3–PbTiO3 piezoelectric ceramics. Materials with effective piezoelectric coefficients up to 1000 pC/N and ferroelectric properties approaching those of single crystals are obtained.
    No preview · Article · Dec 2013 · Journal of the American Ceramic Society
  • M. Vicent · E. Sánchez · T. Molina · M. I. Nieto · R. Moreno
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    ABSTRACT: The stability and rheological behaviour of bimodal titania suspensions was studied. Bimodal mixtures were prepared by mixing nanosized TiO2 powders with an average primary size of ∼20-40 nm and surface area of ∼50 m2 g−1 and/or a colloidal titania suspension of the same nanopowders dispersed in water with a submicrometer sized titania. The dispersing conditions were studied as a function of pH, type and content of dispersant, and sonication time for a constant solids content of 30 vol% (62 wt%). The mixtures were slip cast and presintered at low temperatures (800-1000 °C) in order to obtain porous materials with anatase as the major phase. The pore size distribution, microstructure and phase composition were characterised using MIP, SEM and XRD techniques, respectively.
    No preview · Article · Dec 2013 · Ceramics International
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    ABSTRACT: Aqueous colloidal processing was used for the environmentally friendly preparation of well-dispersed concentrated suspensions and powder mixtures of submicrometric SiC powders with submicrometric Y3Al5O12 as sintering additive plus diamond nanoparticles as reinforcing phase. It is shown that the addition of nano-diamond markedly increases the viscosity and thixotropy of the SiC + Y3Al5O12 suspensions, and also that, by adjusting the pH, deflocculant content, and sonication time it is possible to co-disperse these three rheologically different ceramic phases (i.e., non-oxide, oxide, and hydrophobic compounds) in aqueous media, thereby avoiding the otherwise irremediable severe hetero-aggregation. Moreover, the microstructural characterization of the powder mixtures obtained by freeze-drying the suspensions confirmed the homogeneous dispersion of the diamond nanoparticles among the submicrometric SiC and Y3Al5O12 particles in the form of isolated or adhered nanoclusters and nanodeposits. Implications for engineering the microstructure of non-oxide ceramics with diamond nanodispersoids are discussed.
    Full-text · Article · Nov 2013 · Journal of the European Ceramic Society
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    ABSTRACT: The aqueous colloidal processing of SiC with Y3Al5O12 liquid-phase sintering additives was investigated for two different additive systems, one the mixture of Y2O3 and Al2O3 in a 3:5 molar ratio and the other directly Y3Al5O12. The investigation involved the study of the colloidal stability of the different components, and the comparison of the rheological behaviour of concentrated suspensions of SiC, SiC + 3Y2O3:5Al2O3, and SiC + Y3Al5O12 as a function of the sonication condition, dispersant content, and solid loading. This allowed appropriate conditions for the preparation of well-dispersed, single-phase, and multi-component concentrated suspensions of SiC to be identified. It was found that the multi-component suspensions have better rheological behaviour than the single-phase ones, and that in terms of rheology and slip casting the Y3Al5O12 additives are more functional than the conventional 3Y2O3 + 5Al2O3 additives. It was also demonstrated that the Y3Al5O12 additive is as effective as the 3Y2O3 + 5Al2O3 additive in densifying SiC via liquid-phase sintering, with there existing no differences either in the microstructure or in room-temperature mechanical properties (hardness, toughness, and fracture mode). Implications of interest for the wet-shaping of complex SiC parts are discussed.
    Full-text · Article · Sep 2013 · Journal of the European Ceramic Society