Síntesi i caracterització dels materials multiferroics BiFeO3, La: BiFeO3 i Sr: BiFeO3

Recentment, ha crescut un notable interès pels materials multiferroics (o materials que presenten simultàniament propietats elèctriques i magnètiques), a causa de les seves múltiples aplicacions, sobretot en el camp de l'electrònica. Aquest article pretén donar a conèixer els materials multiferroics, tant des del punt de vista de la física fonamental com de la química de l'estat sòlid. Tanmateix, es presentarà un dels materials multiferroics actualment més estudiats i s'exposaran els detalls de la seva síntesi i caracterització habituals en la química de l'estat sòlid.Multiferroic materials, or materials that present both electric and magnetic ordering simultaneously, have attracted enormous attention recently due to their application in electronic devices. This paper aims to highlight such materials, as much for solid state chemistry considerations as for fundamental physics. At the same time, one of the most studied multiferroic materials will be presented together with the synthetic details and characterization techniques most commonly used in solid state chemistry

Els signes d’autisme durant el primer any de vida: la detecció a través d’un cas clínic

A partir de les transcripcions dels vídeos domèstics del primer any d'un nen posteriorment diagnosticat de TEA, ens anirem endinsant en la comprensió del funcionament mental i la descripció dels signes d'alarma d'autisme. Analitzarem a nivell qualitatiu l'estil relacional dels pares i aquells senyals que,tant per excés com per defecte, comprometen el desenvolupament del nen.En l’apartat de la discussió s’apunten algunes reflexionssobre la detecció, el diagnòstic i la necessitat d’unaintervenció precoç, per canviar la dinàmica relacionalestablerta amb els pares i afavorir el vincle

Improving the functional properties of (K0.5Na0.5)NbO3 piezoceramics by acceptor doping

ZrO2 and TiO2 modified lead-free (K0.5Na0.5)NbO3 (KNN) piezoelectric ceramics are prepared by a conventional solid-state reaction. The effect of acceptor doping on structural and functional properties is investigated. A decrease in the Curie temperature and an increase in the dielectric constant values are observed when doping. More interestingly, an increase in the coercive field E-c and remanent polarization P-r is observed. The piezoelectric properties are greatly increased when doping with small concentrations dopants. ZrO2 doped ceramic exhibits good piezoelectric properties with piezoelectric coefficient d(33) = 134 pC/N and electromechanical coupling factor k(p) = 35%. It is verified that nonlinearity is significantly reduced. Thus, the creation of complex defects capable of pinning the domain wall motion is enhanced with doping, probably due to the formation of oxygen vacancies. These results strongly suggest that compositional engineering using low concentrations of acceptor doping is a good means of improving the functional properties of KNN lead-free piezoceramic system. (C) 2014 Elsevier Ltd. All rights reserved.Postprint (published version

Insights into the use of Flash Sintering methods to prepare catalytic materials

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Elastic, dielectric and electromechanical properties of (Bi0.5Na0.5)TiO3-BaTiO3 piezoceramics at the morphotropic phase boundary region

A systematic study of the functional properties of the (1-x)(Bi0.5Na0.5)TiO3–xBaTiO3 (BNT-xBT) piezoceramic system for 0.05 = x = 0.07 is performed. The samples are obtained through the conventional solid-state route. The expected microstructure for these compounds, with no significant dependence on the composition, is verified by field-emission scanning electron microscopy. The morphotropic phase boundary (MPB) is detected for x = 0.06–0.07 by means of the Rietveld analysis of X-ray diffraction data. The dielectric spectra show a frequency-independent, completely diffuse phase transition with a composition-dependent diffusivity coefficient. The depolarization temperature is effectively evaluated from pyroelectric measurements, the value being strongly dependent on the composition. A significant contribution of the extrinsic effect to elastic, dielectric and electromechanical properties is revealed for MPB BNT-xBT. The Bi3+ substitution by Ba2+ leads to the formation of A-site vacancies, which give rise to the enhancement of domain wall motion, as occurs in other perovskite-type piezoelectrics. Good functional properties are achieved for x = 0.07 (d33 = 180 pC/N), which are similar or even better than those obtained by complex synthesis routes. This system exhibits a remarkable stability in the permittivity that has hitherto not been reported. This fact may open the way for BNT-BT compositions to be used in specific applications in which lead-free piezoceramics have previously been employed with little success, e.g. in high power devices.Postprint (author's final draft

Efecto de la sustitución catiónica y aniónica en las transiciones de fase del LiNH4SO4

El ß LÍNH4SO4 (ß LAS) presenta dos transiciones de fase, una alrededor de 10 °C y la otra a 186 °C. La fase intermedia es ferroeléctrica. En este trabajo se estudia el efecto que produce la sustitución parcial del catión amonio por rubidio y del anión sulfato por seleniato en dichas transiciones de fase. Se establece la zona de existencia de las soluciones sólidas Li(NH4)-^_^Rb^S04 y LiNH4(SO^)^_^(Se04)^ en las que se mantiene la estructura del ß LAS. La presencia de aniones seleniato o de cationes rubidio afecta a las transiciones de fase de alta y baja temperatura, por lo que, tanto los tetraedros sulfato como el catión amonio intervienen en dichas transiciones. Hay un efecto cooperativo entre el desorden de los grupos sulfato y las distorsiones de los tetraedros amonio. El mecanismo que puede explicar estas transiciones es del tipo orden-desorden

CO oxidation on ceria studied by electrochemical impedance spectroscopy

Electrochemical impedance spectroscopy technique has been widely used to analyze the electrical properties of a large number of materials. In this study, the electrical properties of CeO2 pellets under CO oxidation conditions have been analyzed by electrochemical impedance spectroscopy. CeO2 pellets have been prepared by a conventional precipitation method and sintered at low temperature to satisfy a compromise between large surface area and a high relative density of the pellet. The electrical properties of CeO2 have been investigated under different atmosphere conditions such as N2, O2, CO, CO2, or selected combinations. The electrical sensitivity of CeO2 to the surrounding atmosphere allows to follow the catalytic reaction as a function of the CO : O2 ratio and temperature. The appropriate analysis of the electrical response by electro-chemical impedance spectroscopy could open a new insight to monitor the catalytic response and behavior of any catalyst

Microstructure and Defect Formation in BaTiO3 Ceramics Obtained by Flash Sintering of Micro and Nanopowders

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Induced p-type semiconductivity in Mg-doped Nd2Zr2O7 pyrochlore system

Heterovalent B-site MgO substitution in the Nd2Zr2O7-system (Nd2Zr2−xMgxO7−x) has been explored. The pyrochlores were synthesized by a polymeric sol-gel method and characterized by X-ray diffraction (XRD), Raman spectroscopy and Scanning Electron Microscopy to determine structure, phase composition and microstructure. Impedance Spectroscopy (IS) was employed to study the electrical behavior of the ceramics over the ranges 200–800 °C and under pure N2 and O2. The XRD showed that the solid solution limit was x > 0.02 and all the materials show a cubic ̅ structure. The Raman results confirm the structural disorder created by the introduction of Mg2+ and the subsequent generation of oxygen vacancies. The IS data shows a dramatical increase of the oxide-ion conductivity when doping and that the conductivity depends strongly on the atmosphere, leading to p-type semiconductivity under pure O2 atmosphere. The present study highlights the use of heterovalent dopants to drastically increase the oxide-ion conductivity of pyrochlore-like materials

Structural and electrical properties of Zr-doped K0.48 Na 0.52 NbO 3 ceramics: “Hard” lead-free piezoelectric

The structural and electrical properties of K0.48Na0.52Nb1−xZrxO3−δ (x = 0–0.04) ceramics prepared by the conventional solid-state reaction method were studied. Pellets with composition x ≤ 0.03 sintered at 1125 °C for 2 h showed single-phase of potassium sodium niobate (KNN) perovskite structure. Based on X-ray diffraction and Raman results, a mixture of orthorhombic and monoclinic phases was observed in intermediate compositions. The addition of Zr improved the sinterability and the “hard” piezoelectric properties of KNN, increasing the Ec and Qm values. The composition with x = 0.03 presented the highest permittivity at room temperature, ɛr′ = 363 and the lowest dielectric losses, tan δ = 0.027. Moreover, it was the sample with the highest Qm and d33 values, with Qm = 1781 and d33 = 82 pC/N. It was therefore the best compositions to obtain a “hard” piezoelectric material based on Zr-doped KNN, which makes it promising candidate for use as “hard” lead-free piezoelectric material for high power applications