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[EN] Chitosan-coated Fe3O4 nanoparticles were prepared by coprecipitation followed by reaction with chitosan and different volumes of glutaraldehyde. Coating was modified by varying the volume of glutaraldehyde and reaction time. XRD pattern shows maxima compatible Fe3O4 structure. FTIR spectroscopía confirms the presence of chitosan, more evident in samples with higher chitosan content, as denoted by TGA. TEM images of samples with low glutaraldehyde content reveal particles coated with a homogeneous chitosan shell. Meanwhile, those prepared with high glutaraldehyde volume show nanoparticles dispersed in an organic matrix. Samples present almost superparamagnetic behavior with magnetization saturation values that are reduced as the content of organic matter increases. Regarding the stability of these samples in solution, the presence of a homogeneous coating improves the initial suspension, although it does not prevent its subsequent aggregation over time. However, this aggregation process is reduced for sample synthesized with 1 mL of glutaraldehyde after 6 h of reaction., [ES] Se han preparado nanopartículas de Fe3O4 recubiertas con quitosano mediante coprecipitación seguida de reacción con quitosano y diferentes volúmenes de glutaraldehído. El recubrimiento fue modificado a través de la variación del volumen de glutaraldehído y el tiempo de reacción. Los patrones de DRX muestran máximos compatibles con la estructura del Fe3O4. La espectroscopía IRTF confirma la presencia de quitosano, que se hace más evidente para muestras con altos contenidos de quitosano, como se observa mediante ATG. Las imágenes de MET con volumenes pequeños de glutaraldehído revelan partículas recubiertas homogéneamente con una coraza de quitosano. Por su parte, aquellas preparadas con grandes cantidades exhiben nanopartículas dispersas en la matriz orgánica. Las muestras presentan un comportamiento prácticamente superparamagnéticos con valores de magnetización a la saturación que se reducen cuanto mayor es le contenido de materia orgánica. Atendiendo a la estabilidad de las muestras en disolución, la presencia de un recubrimiento homogéneo mejora la suspensión inicial del material, pero no previene la agregación con el tiempo. Sin embargo, este proceso de agregación se ve reducido para la muestra sintetizada con 1 ml de glutaraldehído con un tiempo de reacción de 6 horas.

In recent decades, the demand for biomedical imaging tools has grown very rapidly as a key feature for biomedical research and diagnostic applications. Particularly, fluorescence imaging has gained increased attention as a non-invasive, inexpensive technique that allows real-time imaging. However, tissue auto-fluorescence under external illumination, together with a weak tissue penetration of low wavelength excitation light, largely restricts the application of the technique. Accordingly, new types of fluorescent labels are currently being investigated and, in this search, phosphorescent nanoparticles promise great potential, as they combine the interesting size-dependent properties of nanoscale materials with a long-lasting phosphorescence-type emission that allows optical imaging well after excitation (so avoiding autofluorescence). In this work, core-shell structures consisting of SrAlO:Eu,Dy luminescent cores encapsulated within a biocompatible silica shell were prepared, showing a green persistent phosphorescence with an afterglow time of more than 1000 s. A high-energy ball milling procedure was used to reduce the size of the starting phosphors to a size suitable for cellular uptake, while the silica coating was produced by a reverse micelle methodology that eventually allows the excitation and emission light to pass efficiently through the shell. Confocal fluorescence microscopy using HeLa cancer cells confirmed the potential of the all-ceramic composites produced as feasible labels for in vitro optical imaging.

[EN] The metastability impediment which usually prevents the obtaining of a phase-pure BiFeO3 material can be dramatically stressed when taking the system to the thin film configuration. In order to preserve the stoichiometry, the films need to be processed at low temperatures and hence the solid-state diffusion processes which usually govern the microstructural evolution in bulk cannot be expected to also rule the development of the functional films. All these circumstances were presumed when exploring the possibilities of an aqueous solution–gel process plus spin-coating deposition method to reproduce, in thin film dimensions, the excellent multiferroic properties that have been previously observed with an optimized rare-earth and Ti4+-codoped BiFeO3 bulk composition. The experiments indicate high reliability for the tested methodology, allowing for the obtaining of homogeneous dense films at temperatures as low as 600 1C and with a tunable multiferroic response depending on the formulated rare-earth (Sm or Nd). Thorough structural characterization of the films reveals that despite the low temperature processing restrictions, effective microstructural control is achieved at the nanoscale, which is attributed to effective retention (pinning) of the dopants inside the perovskite structure of BiFeO3.

[EN] Anatase TiO2 has become a material of great interest for photocatalytic production of hydrogen, environmental purification and solar energy conversion. Among the key parameters boosting the photocatalytic efficiency of the anatase nanoparticles, an increased light absorption to expand its optical response to the visible region, together with an improved charge separation of the photo- generated electrons and holes, can be enumerated. In this work, yellow-coloured, single-phase anatase nanoparticles have been obtained using a simple two-step solvothermal routine which requires no external addition of dopants, nor the use of a harassing/aggressive synthesis atmosphere. The obtained powders display a lowered bandgap (<3.0 eV) and significantly reduce the recombination processes, eventually leading to an improved photocatalytic performance under visible light, as exemplified by an enhanced degradation of phenol. This exceptional response is linked to the presence of intrinsic defects in the yellowish particles and, hence, the specific conditions of the proposed methodology become crucial to produce a propitious TiO2-defective nanomaterial capable of photo-degrade the phenol molecule, in contrast with the lack of photocatalytic activity currently exhibited by commercial photocatalysts under visible light.

[EN] A novel, bioinspired approach to water remediation is presented in which poly-peptide molecules with specific affinity for non-biological inorganic materials are used to immobilize metal oxide nanoparticles suspended in an aqueous batch. Based on the concept of molecular recognition, the proof of concept demonstrated here comprises a two-stage incubation methodology, whereby TiO nanoparticles in very low concentrations (0.04 mg/ml and below) can be effectively removed from a water suspension after being recognized and selectively bound to a properly engineered inorganic-binding peptide. Operating all the time under extraordinary soft conditions of temperature and pH (T = 37 °C, pH 7), the specific methodology involves first attaching the innocuous biomolecules to a substrate and then using the entire assembly to immobilize and remove the suspended nanoparticles. The whole procedure is systematically monitored by FESEM, microRaman and UV–vis analytical measurements and reveal that the immobilization mechanism fits widely in Langmuir's isothermal adsorption model, with the biomolecule acting as a kind of molecular glue that ensures a stable adhesion with the noxious nanoparticles.

[EN] In the search of multiferroic materials with enhanced magnetoelectric response, the BiFeO–BiTiO composite system has been proposed as a promising candidate. However, in order to ensure an effective coupling between the antiferromagnetic and the ferroelectric-order parameters, a high structural quality of the oxide heterostructure (well-matched interface) must be attained. This implies the absence of any inter-diffusion process across the interface during the consolidation of the composite assembly. In this contribution, we have analysed the different diffusion scenarios that could be established in this nominal BiFeO–BiTiO system as a function of the specific reactivity of the involved compounds. The obtained results clearly identify how and when that diffusion is produced, so now it can be controlled to ensure the maximum exploitation of the potential multiferroic properties of this candidate system.

[EN] Different dopant strategies are currently under investigation in order to overcome the many problems that limit the commercial viability of BiFeO-based ceramic devices. Neodymium substitution onto the A site of the perovskite lattice provokes significant changes in the crystal structure of the parent material which can derive in enhanced multiferroic properties, but the conductivity in the bulk system is still too high. Titanium doping on the other hand generates a distinctive micro-nanostructure in the consolidated ceramics which can largely increase the dc resistivity of the whole material. A combination of these two effects is here attempted in a co-doping approach which evidences that the microstructural effect caused upon Ti-doping, provoking a reduction of the leakage currents, eventually allows the co-doped material to capitalize on the unique piezoelectric and magnetic properties structurally triggered by the Nd-doping.

[EN] BiFeO ceramics have been consolidated applying an electric current activated/assisted sintering (ECAS) methodology under different electrical conditions. DC experiments produce a flash sintering regime by which the bulk cylindrical specimens densify in a few seconds; however, this goes together with a strong localization of the current flow within the material, leading to a dramatic lack of microstructural and compositional homogeneity. The situation changes under the alternate field; a more gradual FAST (Field Assisted Sintering) event is produced which allows the attainment of an exceptional microstructural homogeneity through the whole sintered compacts. Upon Ti-doping the overall diffusivity of the system is delayed but the AC conditions again yield a remarkable microstructural homogeneity in the consolidated material, this time even at the nanoscale level. Accordingly, bulk BiFeO ceramics with homogeneous micro-nanostructure can be successfully prepared by an ECAS methodology and at lower temperatures and much shorter times than by conventional solid-state sintering.

The formulation, development and optimization of water-based inks of platelet-like nanoparticles are the mainobjective of this work. As-synthesized Ni(OH)2nanoparticles were dispersed and stabilized in aqueous sus-pension by PEI addition. The combination of DEG (cosolvent) with H2O shows the ideal values of surface tensionand viscosity for piezoelectric inkjet printing, which exhibits a homogeneous jettingflow of the nanoplateletssuspension. The printed nanostructure was sintered at low temperature (325 °C) and the electrochemicaloverview of NiO electrode behavior was described. These printable pseudocapacitors tested by a three-electrodecell have showed a competitive specific capacitance, leading 92% and 78% of capacitance retention for 2000cycles at scan rates of 1 and 2 A/g respectively, and a coulombic efficiency of 100%. The initial performance ofthis printed NiO pseudocapacitor can be compared with others prepared by conventional methods. This newfinding is expected to be particularly useful for the designing of micro-pseudocapacitors.

Highly homogeneous gallium doped ZnO assembled nanostructures comprised by thin hexagonal platelets showing a striking preferred orientation along the ab plane have been obtained by an ultra-fast and eco-friendly microwave-assisted hydrothermal synthesis method. Although the incorporation of gallium into the ZnO lattice is found to be relatively low, namely ≤1 mol.%, the gallium nitrate addition plays a key role in the size and morphology of the obtained ZnO nanostructures. The observed morphology is attributed to the selective adsorption of the gallium species on the basal planes of ZnO, which inhibits the growth along the ± [0001] direction. Additionally, the optical band-gap and the fluorescence emission are modified upon gallium addition. The observed changes in the optical properties are ascribed to both the decreased crystal size and to the gallium incorporation into the ZnO structure, which lead to the modification in the energy levels.

[EN] Titanium substitution can improve the multiferroic properties of bismuth ferrite materials, albeit the structural changes that this dopant produces in the perovskite structure of BiFeO are still not well understood. Due to its high sensitivity, Raman spectroscopy can be a useful tool for trailing the dopant incorporation. In this work, we study the Raman spectra of undoped and Ti-doped BiFeO ceramics that also contain small amounts of secondary phases – as it is usual for materials of this system. The micro-Raman study together with a rigorous derivative analysis of the experimental Raman spectra recorded at low temperature (73 K) enables, for the first time, the identification of the Raman bands associated with the BiFeO phase together with those belonging to the minor secondary phases. The comparison between the Raman spectra of the undoped and the Ti-doped materials indicates that some of the titanium added to the composition interacts with the secondary phases while only a partial amount of this dopant actually incorporates into the BiFeO phase. Copyright © 2017 John Wiley & Sons, Ltd.

The electrochemical behaviour of ceramic semiconductors not only depends on the characteristics of the electroactive material but also on the processing method, the nanoparticles arrangement and the consolidation degree of the formed microstructure. In this sense, the use of nanoparticles with plane morphologies (disc, platelets, etc.) results interesting due to the formation of conduction pathways produced as a consequence of their laminar structures. Electrophoretic Deposition (EPD) is a shaping methodology which allows achieving high degrees in nanoplatelets packing by controlling their alignment during the coating process specifically over 3D substrates. In this work, we have studied the effect of a moderate nanoplatelets agglomeration, by tuning their surfaces with a polyelectrolyte multilayer following a Layer-by-Layer (LbL) methodology and fixing the electric conditions of the EPD process. Overcoming the destructive effects of the full agglomeration of nanoplatelets, NiO films with a stable and extremely open macroporous structure were processed to coat Ni foams, improving the capacitive performance of pseudocapacitors leading to values of specific capacitances of 650 F/g. Results collected in this work also evidence that an efficient ordering and orientation of nanoplatelets in EPD mainly depends on tuning the suspension parameters (solid contents, conductivity, electrophoretic mobility, etc.) to avoid the massive flux and interactions among interparticles and electro-hydrodynamic forces, as well as the interference of collateral electrode phenomena.

[EN] Nb or W donor-doped powders were synthesized by applying a mechanochemical synthesis route, in an attempt to improve the multiferroic properties of BiFeO3 based materials. The as-synthesized samples evidence a highly efficient incorporation of Nb or W into the BiFeO perovskite lattice, together with a low presence of secondary phases. The relatively high purity of the mechanosynthetized products is even more appealing when it is compared with the multiphase scenario obtained by a mixed oxides route for these compositions. This confirms this technique as a good alternative to conventional solid state processing; however the thermal evolution of the as-prepared powders also reveals a metastable nature of the synthesized phases which upon an external energy input leads to an unwelcome multiphase system.

[EN] Samples of nominal composition LiFeTi(PO) with a NASICON-type crystal structure, can be sintered up to 95% of the theoretical density at only 600 °C using spark plasma sintering, SPS. The final pellets however show different colouration in their upper and lower faces, being the upper face white and the lower one pink. The analysis of both surfaces by XPS shows a higher concentration of lithium on the white upper face, as a consequence of the polar diffusion of ions from the positive to the negative rams during SPS. On the other hand the study by XRD and SEM shows the formation of an extra phase in the upper face with nominal composition LiOPO. A change in the cell parameters of the original NASICON structure is also observed, confirming the diffusion of lithium ions., [ES] Muestras de composición nominal Li1.3Fe0.3Ti1.7(PO4)3 y estructura tipo NASICON se han logrado consolidar mediante SPS (spark plasma sintering) a tan sólo 600 °C y alcanzando una densidad del 95% de la densidad teórica. No obstante, las pastillas obtenidas muestran una diferente coloración en sus caras, siendo la cara superior de colour blanco y la cara inferior de colour rosado. El análisis de ambas superficies mediante XPS evidencia una mayor concentración de litio en la cara superior blanquecina, como resultado de un proceso de difusión polar de iones entre los polos positivo y negativo durante el tratamiento por SPS. Por su parte el estudio mediante DRX y SEM de ambas caras indica la formación de una fase extra en dicha cara superior, con una composición nominal LiOPO4. Además se observa un cambio en los parámetros de red de la estructura NASICON original, confirmando dicha difusión de los iones litio.

An effective synthesis strategy for the extensive fluorination of the TiO2 anatase lattice has been developed which provides a highly stable blue-colored titania powder. The process also produces a convenient faceted morphology of the doped nanoparticles. Both theoretical and experimental data indicate an ordered atomic structure, in which an exceptionally high amount of fluorine ions substitute oxygen ions in the TiO2 lattice. The extra-electrons borne by fluorine are stabilized by lattice Ti cations via a mechanism of valence induction, eventually leading to a consequent high amount of reduced Ti3+ centers. Such structure, whose general formula can be expressed as Ti4+(1-x)Ti3+xO2–(2-x)F–x, confers an excellent stability to the as-synthesized nanoparticles (in spite of the excess electrons), explaining for example why the blue color is retained even upon storage in ambient atmosphere. But moreover, the high concentration of the relatively shallow Ti3+ generated states also form a sort of sub-band close to the bottom of the conduction band itself. The whole effect results in a tangible decrease (0.2 eV) of the anatase band gap which allows an inspiring upgrading of its UV photocatalytic activity. Our results also suggest that narrowing the band gap is insufficient for a substantial improvement in the visible light harvesting.

The fabrication of hierarchical anatase microspheres with potential photocatalytic properties eventually comprises a consolidation step in which a high degree of crystalline order is typically achieved through conventional electric heating treatments. This however entails a substantial reduction in the specific surface area and porosity of the powders, with the consequent deterioration in their photocatalytic response. Here, we have tested the employ of microwave heating as an alternative energy-saving sintering method to promote fast crystallization. The results obtained suggest that under the microwave radiation, the TiO2 hierarchical structures can effectively crystallize in a drastically reduced heating time, allowing the specific surface area and the porosity to be kept in the high values required for an improved photocatalytic performance.

TiO2 nanoparticles with tailored morphology have been synthesized under exceptionally soft conditions. The strategy is based on the use of a non-aqueous alcoholic reaction medium in which water traces, coming either from the air (atmospheric water) or from an ethanol–water azeotropic mixture (ethanol 96%), are incorporated in order to accelerate hydrolysis of the Ti–precursor. Moreover, organic surfactants have been used as capping agents so as to tailor crystal growth in certain preferential directions. Combinations of oleic acid and oleylamine, which lead to the formation of another surfactant, dioleamide, are employed instead of fluorine-based compounds, thus increasing the sustainability of the process. As a result, TiO2 nanostructured hierarchical microspheres and individual nanoparticles with exposed high-energy facets can be obtained at atmospheric pressure and temperatures as low as 78 °C.

Fluorine species adsorbed on the surface of TiO2 anatase nanoparticles improve their photocatalytic performance by reducing the recombination rate of photogenerated electrons and holes. Trifluoroacetic acid (TFAA) has been recently proposed as a promising harmless substitute for hydrofluoric acid (HF) in preparing fluorine-modified anatase with good scavenger properties. However the photocatalytic performance of the TiO 2 nanoparticles also depends on their specific morphology, by which the number and type of crystal facets, which remain exposed, are determined. A comprehensive study is presented in this contribution which for the first time describes the role of TFAA in stabilizing the highly photoactive {001} TiO 2 facets. Furthermore, by reducing the amount of water incorporated into the reaction medium, a simple one-step synthesis method is also proposed that allows the preparation of TFAA-modified anatase nanoparticles with specific morphology and selected stabilized facets, eventually yielding a semiconductor material with excellent photocatalytic properties. © 2013 The Royal Society of Chemistry.

Ti-doped BiFeO3 ceramics prepared by a mixed-oxide route were structurally characterized by X-ray diffraction (XRD), field-emission scanning electron microscopy (SEM), and high-resolution transmission electron microscopy (HRTEM), giving evidence of the formation of an inner structure at the nanometric scale. The observed nanograins are separated by Ti-rich areas that originate due to the tendency of the titanium dopant to segregate from the perovskite lattice. Such a peculiar nanostructure is responsible for the changes produced in both the electrical and the magnetic properties of BiFeO3 upon titanium doping: the Ti-rich interfaces act as resistive layers that increase the direct-current (dc) resistivity of the material, while the existence of structural domains in the scale of tens of nanometers causes a ferrimagnetic-like behavior with a huge coercive field (on the order of 20 kOe), even at room temperature.

[EN] A versatile synthesis method is proposed to fabricate hierarchical anatase microspheres under soft conditions and rendering batch volumes significantly large for practical scale-up preparation. The strategy is based on the use of non-aqueous conditions to prevent the accelerated hydrolysis of the Titanium precursor. Water traces from the air atmosphere are only incorporated during the vaporization stage, allowing a controlled precipitation process. Diluted precursor solutions and ethanolic medium promote the self-organization of the precipitated nanoparticles into stable units of spherical shape. The calcination of the obtained powder finally leads to a homogenous, monodisperse and easy to handle population of nanostructured anatase microspheres with high crystallinity and large surface area. © 2012 Elsevier B.V. All rights reserved.

[EN] Mechanochemical processing has been demonstrated as a powerful tool for preparing metastable phases. In this work we have successfully applied it to synthesize a novel Bi6Ti5WO22 phase belonging to the Bi2O3-TiO2-WO3 ternary system. Attempts to obtain this phase by a solid state reaction method as well as by chemical precipitation have failed, confirming that cubic Bi 6Ti5WO22 shows a metastable nature. The employ of high-energy milling allows a reduction in both the calcination temperature and time required to obtain a crystalline sample (thereby reducing the Bi-losses by vaporization). As a consequence a pure Bi6Ti5WO 22 phase has been prepared which remains stable up to temperatures around 800 °C. © 2012 Elsevier B.V. All rights reserved.

The parameters that control the stability of ZnO-nanoparticles suspensions and their deposition by electrophoretic deposition were studied, so as to organize the assembly and compaction of nanoparticles. The addition of cationic polyelectrolyte - Polyethylenimine (PEI) - with different molecular weights was investigated, in order to study their effectiveness and the influence of the molecular weight of the organic chain on suspensions dispersion. It was found that PEI with the highest molecular weight provided better dispersion conditions. Cathodic EPD was performed under previously optimized suspensions conditions and over electropolished stainless steel substrates. Experimental results showed that the EPD process in these conditions allows obtaining dense transparent ZnO thin films. Deposition times and intensities were optimized by analyzing the resulting thin films characteristics. Finally, the deposits were characterized by FE-SEM, AFM, and different spectroscopic techniques.

Comprende 70 a 0% en peso de Pt, 20 a 5% en peso de C y 10 a 95% en peso de WO3 o WC respecto al peso total de sólidos de la composición y un vehículo orgánico o en base acuosa, donde dicha composición es capaz de ser sinterizada a una temperatura superior a 1000ºC, en particular superior a 1200ºC. También se refiere a un procedimiento para la integración de dicha composición en un material cerámico en base porcelana, gres, gres porcelánico y/o cerámicas similares. La invención también se refiere al sensor capacitivo obtenido con dicha composición sobre una cinta de composición cerámica y al material cerámico que integra dicha composición o dicho sensor.

[EN] The invention relates to a composition comprising between 70 and Owt. % of Pt, between 20 and 5 wt. % of C, and between 10 and 95 wt. % ofW03 or WC in relation to the total weight of solids ofthe composition and an organic or aqueous-based vehicle, where said composition can be synthesised at a temperature higher than 1000°C, in particular higher than 1200°C. The invention also relates to a method for integrating said composition into a ceramic material based on porcelain, stoneware, porcelain stoneware and/or similar ceramics. The invention further relates to the capacitive sensor obtained with said composition on a strip made of a ceramic composition, and to the ceramic material comprising said composition or said sensor., [ES] Comprende 70 a 0% en peso de Pt, 20 a 5% en peso de C y lOa 95% en peso de WO3 o WC respecto al peso total de sólidos de la composición y un vehiculo orgánico o en base acuosa, donde dicha composición es capaz de ser sintetizada a una temperatura superior a 1000°C, en particular superior a 1200°C. También se refiere a un procedimiento para la integración de dicha composición en un material cerámico en base porcelana, gres, gres porcelánico y/o cerámicas similares. La invención también se refiere al sensor capacitivo obtenido con dicha composición sobre una cinta de composición cerámica y al material cerámico que integra dicha composición o dicho sensor.

[EN] The energetic losses associated with the leakage currents can represent a serious problem for the efficient performance of ZnO-based varistor ceramics. However, by properly tailoring the microstructure of these ceramics, the leakage currents can be drastically reduced. Pursuing this goal, two intended doping strategies have been attempted over a standard high-voltage varistor formulation based on the ZnO-Bi 2O 3-Sb 2O 3 ternary system. The first strategy focuses on the stabilization of the secondary Zn 2.33Sb 0.67O 4 spinel particles; on cooling the ceramic, this spinel phase reversely transforms into a pyrochlore conductive structure whose presence favors the current flow in the varistor prebreakdown region (i.e., high leakage current values). A certain amount of TiO 2 incorporated into the spinel lattice impedes its reverse transformation to pyrochlore, thus reducing the varistor leakage current. The second strategy leans on the insertion of an insulating phase in between the skeleton of Bismuth-rich phases. This skeleton provides a continuous network through the whole varistor microstructure for the current to flow, so the inclusion of an insulating phase may serve as a physical blockade to break off that connectivity. Such a phase is yielded by properly doping with SiO 2. Eventually, the combination of both strategies leads to a decrease of two orders of magnitude in the varistor leakage current. © 2012 The American Ceramic Society.

The preparation of LiNbO3:Er3+/Yb3+ nanocrystals and their up-conversion properties have been studied. It is demonstrated that polyethyleneimine- (PEI) assisted dispersion procedures allow obtaining stable aqueous LiNbO3:Er3+/Yb3+ powder suspensions, with average size particles well below the micron range (100–200 nm) and the isoelectric point of the suspension reaching values well above pH 7. After excitation of Yb3+ ions at a wavelength of 980 nm, the suspensions exhibit efficient, and stable, IR-to-visible (green and red) up-conversion properties, easily observed by the naked eye, very similar to those of the starting crystalline bulk material.

The sintering behaviour and the microstructural evolution of W6+, Nb5+ and Ti4+iron-substituted BiFeO3 ceramics have been analyzed. The obtained results show that W6+ and Nb5+ ions interact with the secondary phases usually present in these materials, thus altering the solid state formation of the BiFeO3 phase. In contrast, Ti4+ ions incorporate into the perovskite structure, leading to an exceptionally low proportion of secondary phases. In addition to this, BiFe0.95Ti0.05O3 materials present a dense microstructure with submicronic and nanostructured grains, clearly smaller than those in the undoped materials.

The obtaining of multiferroicBiFeO3 as a pure single-phase product is particularly complex since the formation of secondary phases seems to be unavoidable. The process by which these secondary impurities are formed is studied by analyzing the diffusion and solidstate reactivity of the Bi2O3–Fe2O3 system. Experimental evidence is reported which indicates that the progressive diffusion of Bi3+ ions into the Fe2O3 particles governs the solidstatesynthesis of the perovskite BiFeO3 phase. However a competition is established between the diffusion process which tends to complete the formation of BiFeO3, and the crystallization of stable Bi2Fe4O9 mullite crystals, which tend to block that formation reaction.

The obtaining of multiferroic BiFeO3 as a pure single-phase product is particularly complex since the formation of secondary phases seems to be unavoidable. The process by which these secondary impurities are formed is studied by analyzing the diffusion and solid state reactivity of the Bi2O3?Fe2O3 system. Experimental evidence is reported which indicates that the progressive diffusion of Bi3+ ions into the Fe2O3 particles governs the solid state synthesis of the perovskite BiFeO3 phase. However a competition is established between the diffusion process which tends to complete the formation of BiFeO3, and the crystallization of stable Bi2Fe4O9 mullite crystals, which tend to block that formation reaction.

The aim of the present work is to explore the possibility of incorporate a small amount of ZnO to improve the microstructure control of W-doped BIT based materials. Two different processing routes have been used according to previous results reported for other materials: reaction and sintering in one single step and a previous calcination step. The sintering behaviour of the samples, the obtained crystalline phases and the microstructure analysis indicate that the reaction between ZnO and Bi2O3 plays a critical role during sintering. Both Bi2Ti2O7 and Zn2TiO4 secondary phases are stabilised when adding ZnO. Actually, when WO3 and ZnO are incorporated simultaneously to BIT materials, they interact stabilizing the Bi2Ti2O7 phase and avoiding the incorporation of W6+ into the BIT lattice. As a consequence, the electrical conductivity of the samples with ZnO is two orders of magnitude higher than that of the samples doped only with WO3, suggesting that WO3 does not form a solid solution with BIT. The curve dielectric constant vs temperature also reveals the role played by the Bi2Ti2O7 phase.