R. Moreno

Spanish National Research Council, Madrid, Madrid, Spain

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Publications (133)237.38 Total impact

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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.
    Catalysis Today 09/2014; In press(XXX):XXX. DOI:10.1016/j.cattod.2014.08.030 · 3.31 Impact Factor
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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
    Ceramics International 07/2014; DOI:10.1016/j.ceramint.2014.07.057 · 2.09 Impact Factor
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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.
    Journal of the European Ceramic Society 07/2014; 34(7):1819–1827. DOI:10.1016/j.jeurceramsoc.2013.12.027 · 2.31 Impact Factor
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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).
    EMRS Spring 2014, Lille, France; 05/2014
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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.
    Journal of the European Ceramic Society 04/2014; 34(4):953–959. DOI:10.1016/j.jeurceramsoc.2013.10.023 · 2.31 Impact Factor
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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.
    Powder Technology 04/2014; 256:540-544. DOI:10.1016/j.powtec.2014.01.059 · 2.27 Impact Factor
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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.
    Materials Chemistry and Physics 03/2014; 144:8-16. DOI:10.1016/j.matchemphys.2013.11.058 · 2.13 Impact Factor
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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.
    Journal of the European Ceramic Society 03/2014; 34(3):555–563. DOI:10.1016/j.jeurceramsoc.2013.09.003 · 2.31 Impact Factor
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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.
    Journal of the European Ceramic Society 01/2014; 34(10):2597–2602. · 2.31 Impact Factor
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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.
    Química Nova 01/2014; 37(2). DOI:10.5935/0100-4042.20140034 · 0.66 Impact Factor
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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.
    Ceramics International 12/2013; 39(8):9091-9097. DOI:10.1016/j.ceramint.2013.05.005 · 2.09 Impact Factor
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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.
    Journal of the European Ceramic Society 12/2013; 33(15-16):3195–3200. DOI:10.1016/j.jeurceramsoc.2013.06.026 · 2.31 Impact Factor
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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.
    Journal of the American Ceramic Society 12/2013; 97(2). DOI:10.1111/jace.12721 · 2.43 Impact Factor
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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.
    Journal of the European Ceramic Society 11/2013; 33(s 13–14):2473–2482. DOI:10.1016/j.jeurceramsoc.2013.04.016 · 2.31 Impact Factor
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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.
    Journal of the European Ceramic Society 09/2013; 33(10):1685–1694. DOI:10.1016/j.jeurceramsoc.2013.01.030 · 2.31 Impact Factor
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    ABSTRACT: Lanthanum chromite to be used as interconnector in high temperature solid oxide fuel cells (HTSOFCs) must have adequate electrical conductivity and high density to avoid the mixture of employed gases. Sr and Co doped-LaCrO3 synthesized by combustion reaction was used to study the effect of the sintering atmospheres, inert (Ar) and oxidant (air) on the sintering/densification behavior. The pellets were sintered in a dilatometer and characterized by X-ray diffraction. The sintered densities were determined by Archimedes method, and Scanning Electronic Microscopic was used for microstructure observation. The results shown that inert and oxidant atmospheres promote densification but after thermal cycles, in oxidant media, theHTSOFCs performance may be impaired
    Cerâmica 09/2013; 59(351):366-371. DOI:10.1590/S0366-69132013000300004
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    ABSTRACT: Shaping is often disregarded in catalysis science. Aiming to fill the gap between catalysis in academy and industry, the objective of this work was to systematically study the shaping of vanadium aluminum(hydr)oxides (VAlO) via wet methods. These materials are formulations active in propane ODH andammoxidation. The physical and chemical stability of suspended VAlO powders was evaluated alongwith the development of a method to prepare rheologically homogeneous concentrated suspensions ofVAlO. Their physical stability was assessed by following changes in relevant properties; namely, theirparticle size distribution, density, surface area and porosity, and isoelectric point (IEP), by considering the sedimentation of VAlO in aqueous medium, and by evaluating preparation parameters such as: effectand concentration of dispersant, powder content, and sonication time on the rheology of the suspensions.Results showed that due to their physical properties, the preparation of concentrated VAlO suspensionsis impracticable without the use of a dispersant, herein poly(acrylic acid) (PAA), that counteracts particlesettling. The chemical stability of suspended VAlO powders was determined by preparing diluted sus-pensions at different pHs and then analyzing the phenomena behind the dissolution of vanadium andaluminum and its correlation with the surface chemistry of the recovered powders. On the basis of theabove analysis, appropriate conditions for having physically and chemically stable concentrated VAlOsuspensions were established: particularly, pH near the IEP for minimum metal dissolution, VAlO con-centration ≤50 wt.%, PAA concentration 0.2–0.5 wt.%, and sonication time of 1 min. Slip casted cylindersof VAlO were obtained using these conditions.
    Applied Catalysis A General 08/2013; 468:190–203. DOI:10.1016/j.apcata.2013.08.041 · 3.67 Impact Factor
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    ABSTRACT: Concentrated aqueous suspensions of iron powder were developed to produce carriers for delivering silica nanoparticles, which may act as inoculants in cast iron during solidification. Those iron matrix nanocomposites may present higher strength and hardness due to enhanced nucleation and grain refinement. Moreover a nanosized dispersed ceramic phase may improve chemical and wear resistance of the metal matrix. In this work, colloid stability and the rheological behavior of iron aqueous suspensions have been studied. Under acidic conditions, iron readily dissolves, but no dissolution takes place at basic pH. Zeta potential studies showed that the isoelectric point occurs at pH ∼ 7. Concentrated iron and iron–silica suspensions were prepared to solids loading up to 40 vol% using ultrasound and ball mill homogenization. Deflocculant content, mixing time and SiO2 nanoparticles content were optimized. Stable suspensions were achieved using an ammonium salt of a polyacrylic acid-based polyelectrolyte as a dispersant, KOH for adjusting the pH at 10, and SiO2 nanoparticles solids loading from 1.0 to 12.5 vol%. Those suspensions were processed via slip casting and microstructures of the shaped carriers were characterized through FE-SEM.
    Materials Research Bulletin 07/2013; 48(7):2430–2436. DOI:10.1016/j.materresbull.2013.02.010 · 1.97 Impact Factor
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    ABSTRACT: The effect of Er3+ doping on the thermal stability of TiO2 nanoparticulate xerogels prepared by a colloidal sol–gel route was investigated. It was found that the as-synthesized xerogels crystallize as anatase phase with crystallite sizes in the low nanoscale range (<7 nm) and high-specific surface areas (>100 ± 5 m2/g). Nevertheless, it was also found that the Er3+ cations are deposited on the surface of TiO2 nanocrystallites thus resulting in xerogels with smaller and more uniform nanoaggregates. Most importantly, detailed analyses using X-ray thermo-diffractometry together with selective analyses by transmission electron microscopy, selected area electron diffractometry, and X-ray energy-dispersive spectrometry showed that the thermal stability of these TiO2 nanoparticulate xerogels increases with increasing Er3+ doping. Specifically, the nanocrystallite growth is slowed down, the onset temperature of the anatase-to-rutile phase transformation is delayed, and the anatase phase is retained up to greater temperatures as the Er3+ content increases, which are all phenomena attributable to the formation at low temperatures (i.e. ~200 °C) of solid solutions with Er3+ solutes within the TiO2 host. Moreover, these solid solutions also have lower thermal expansion coefficient than the undoped crystal structure. Finally, Er3+ doping increasingly promotes the precipitation at high temperatures of Er2Ti2O7 from the rutile TiO2 solid solution, with the precipitation temperature decreasing with increasing Er3+ doping content.
    Journal of Nanoparticle Research 06/2013; 15(6). DOI:10.1007/s11051-013-1752-9 · 2.28 Impact Factor
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    ABSTRACT: In recent years, intense research has shown that thermal spray techniques, especially atmospheric plasma spraying (APS), can be used to obtain nanostructured TiO2 coatings with effective photocatalytic activity.This study compares the photocatalytic activity of APS coatings obtained from different powders: two nanostructured TiO2 powders produced by spray-drying of two TiO2 nanosuspensions with different solids contents, one spray-dried powder obtained from a suspension comprising a mixture of submicronic and nanometric TiO2 particles and finally one commercial, nanostructured, TiO2 spray-dried powder. All powders were characterised by XRD, FEG-ESEM, granule size distribution, and a flowability evaluation. Feedstock powders were then deposited on austenitic stainless steel coupons using APS. Hydrogen or helium was used as secondary plasma gas. Coating microstructure and phase composition were characterised using FEG-ESEM and XRD techniques; coating anatase content was quantified by the Rietveld method.A significant amount of anatase to rutile transformation was found to take place during the plasma spraying process. In general, the coatings had a bimodal microstructure characterised by the presence of completely fused areas in addition to non-molten areas consisting of agglomerates of anatase nanoparticles. Results also showed that anatase content and porosity of the coatings largely depend on the secondary plasma gas nature, as well as on the characteristics of the feedstock.Finally the photocatalytic activity of the coatings was determined by measuring the degradation of methylene blue dye in an aqueous solution. A reasonably good fit of a first-order kinetic model to the experimental data was found for all coatings. The values of the kinetic constant were related to feedstock characteristics as well as to plasma spraying conditions
    Surface and Coatings Technology 04/2013; 220:179-186. DOI:10.1016/j.surfcoat.2012.08.059 · 2.20 Impact Factor