Materiales piezoeléctricos derivados del (Bi0,5Na0,5)TiO3-BaTiO3 : preparación y estudio de las propiedades funcionales

[spa] Los materiales piezoeléctricos basados en el titanato zirconato de plomo (PZT) son ampliamente utilizados en una gran cantidad de dispositivos electrónicos debido a sus excelentes propiedades eléctricas. Esta mejora de las propiedades tiene lugar en cierta región del diagrama de fases que se relaciona con lo que se denomina transición de fase morfotrópica y depende de la composición. No obstante, el mayor inconveniente que presenta es la presencia de plomo debido a su elevada toxicidad, perjudicial para la salud y el medio ambiente. Por lo tanto, en la actualidad y durante los últimos años, la comunidad científica ha centrado su interés en la búsqueda de nuevos materiales piezoeléctricos libres de plomo capaces de sustituir al PZT. La familia de materiales basados en el titanato de bismuto y sodio (BNT) ha despertado un considerable interés desde los años 60 puesto que presenta características similares a las del PZT y es respetuoso con el medio ambiente. No obstante, las propiedades piezoeléctricas son bajas debido al elevado campo coercitivo que presentan estos materiales y la volatilización de sodio y bismuto durante la preparación, llevan a la obtención de cerámicas con elevadas conductividades. Por ello, con el objetivo de solucionar los inconvenientes presentados por el BNT, el interés se centró en cerámicas basadas en (Bi0,5Na0,5)TiO3-BaTiO3 (BNT-BT). Este sistema presenta una transición de fase morfotrópica donde se obtiene una buena respuesta piezoeléctrica y por lo tanto, es considerado un candidato prometedor para sustituir los materiales piezoeléctricos basados en plomo. Es importante destacar que se han llevado a cabo un gran número de estudios de este sistema pero no existe una composición exacta en la que se obtienen las mejores propiedades, los resultados obtenidos por distintos grupos de investigación varían. Por lo tanto, es necesario continuar investigando sobre este sistema e intentar mejorar las propiedades funcionales. Así, el principal objetivo en esta tesis ha sido el estudio del sistema (Bi0,5Na0,5)TiO3-BaTiO3 para tener un mejor conocimiento de su comportamiento que permita mejorar sus propiedades funcionales. En este trabajo se ha optimizado la preparación de cerámicas basadas en el BNT-BT mediante reacción en estado sólido y el método Pechini. Estos métodos de síntesis son reproducibles y escalables a nivel industrial para la obtención de cerámicas densas. En concreto, mediante reacción en estado sólido se han obtenido cerámicas con densidades relativas superiores al 97 % y mediante el método Pechini se ha conseguido disminuir la temperatura de calcinación respecto a la empleada en la reacción en estado sólido. Con la finalidad de mejorar las propiedades funcionales de estos materiales, además de mediante la variación del método de preparación, se ha estudiado el efecto de la incorporación de dopajes de tipo dador y de tipo aceptor, evaluando tanto el efecto de diferentes dopantes como de la concentración. Mediante el dopaje se ha conseguido mejorar las propiedades dieléctricas, ferroeléctricas y piezoeléctricas respecto al BNT-BT sin dopar así como, modificar las temperaturas de las transiciones de fase y mejorar su estabilidad dieléctrica frente al campo eléctrico. Finalmente, mediante la preparación de las soluciones precursoras y un control de la estequiometria debido a la presencia de especies volátiles, se han preparado capas delgadas de BNT-BT. Se han logrado obtener capas delgadas de BNT-BT sin dopar y dopadas sin la presencia de fases secundarias de una manera reproducible y escalable industrialmente que nos permitirán estudiar su comportamiento eléctrico.[eng] Piezoelectric materials based on lead zirconate titanate (PZT) are widely used in a large number of electronic devices due to their excellent properties. This improvement occurs in a region of the phase diagram related to a morphotropic phase boundary. However, the major drawback is the environmental and health problems arising from the presence of lead due to its high toxicity. Therefore, nowadays and during last years, the scientific community has focused on the search for lead-free piezoelectric materials. The family of lead-free bismuth sodium titanate (BNT) based materials has attracted considerable interest since the 60s attributable to their similar characteristics to PZT. Nevertheless, the piezoelectric properties are low as a result of a high coercive field and the volatilization of sodium and bismuth during the ceramic processing lead to obtain conductive ceramics. Consequently, in order to solve these problems, the attention was centred on (Bi0.5Na0.5)TiO3-BaTiO3 based ceramics (BNT-BT). This system has a morphotropic phase boundary where a great piezoelectric response is obtained and for this reason, BNT-BT system is considered a promising candidate to replace lead-based piezoelectric materials. Even the large number of studies carried out by different research groups based on this system, there is no exact composition where the best properties are obtained. So, it is necessary to continue doing research and try to improve the functional properties. Hence, the principal goal of this thesis has been the study of (Bi0.5Na0.5)TiO3- BaTiO3 system in order to have a better understanding to improve the functional properties. In this work, the processing of BNT-BT ceramics by solid state reaction and Pechini method has been optimized. These methods are reproducible and industrially scalable in order to obtain dense ceramics. Specifically, the densities of the ceramics obtained by solid state reaction are higher than 97 % and the calcination temperature has been reduced by Pechini method. In order to improve the functional properties of these materials, besides varying the preparation method, the effect of donor and acceptor dopants has been studied, focusing on the influence of different dopants and their concentration. The dielectric, ferroelectric and piezoelectric properties of the BNT-BT based ceramics have been improved when doping. Moreover, the phase transition temperatures have been modified and the dielectric stability under an electric field has been enhanced. Finally, the processing of BNT-BT based thin films has been successfully achieved by preparing the precursor solutions and controlling the stoichiometry due to the presence of volatile species. A reproducible and scalable method has been developed to obtain excellent BNT-BT based thin films with no secondary phases to study their electrical properties

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

Effect of lanthanide doping on structural, microstructural and functional properties of K0.5Na0.5NbO3 lead-free piezoceramics

Lanthanides-doped K0.5Na0.5NbO3(KNN) lead-free piezoelectric ceramics are prepared by conventional solid-state reaction. The effects of lanthanum concentration and the lanthanide type on the structure, microstructure and ferro-piezoelectric properties are evaluated. Ln3+ doping has a slight effect on the structure, but greatly inhibits the grain growth. Moreover, a decrease in the Curie temperature and an increase in the dielectric constant values are observed when doping. Large amounts of lanthanum induce a diffuse phase transition and an increase of the dielectric losses. The piezoelectric properties are greatly improved when doping with small amounts of dopants. As the ionic radii of the lanthanide is reduced, the piezoelectric properties of the ceramics are increased, the Eu3+-doped ceramics show an increase of 29% of the piezoelectric coefficient d33 with respect to pure KNN. The results show that low concentrations of lanthanides improve significantly the functional properties of KNN lead-free piezoceramics. Doping with lanthanides should be taken into consideration in some other compositions based on KNN.Postprint (author's final draft

A joint experimental and theoretical study on the electronic structure and photoluminescence properties of Al2(WO4)3 powders

In this paper, aluminum tungstate Al2(WO4)3 powders were synthesized using the co-precipitation method at room temperature and then submitted to heat treatment processes at different temperatures (100, 200, 400, 800, and 1000 °C) for 2 h. The structure and morphology of the powders were characterized by means of X-ray diffraction (XRD), Rietveld refinement data, and field emission scanning electron microscopy (FE-SEM) images. Their optical properties were examined with ultraviolet-visible (UV-vis) diffuse reflectance spectroscopy and photoluminescence (PL) measurements. XRD patterns and Rietveld refinement data showed that Al2(WO4)3 powders heat treated at 1000 °C for 2 h have a orthorhombic structure with a space group (Pnca) without the presence of deleterious phases. FE-SEM images revealed that these powders are formed by the aggregation of several nanoparticles leading to the growth of microparticles with irregular morphologies and an agglomerated nature. UV-vis spectra indicated that optical band gap energy increased from 3.16 to 3.48 eV) as the processing temperature rose, which was in turn associated with a reduction in intermediary energy levels. First-principle calculations were performed in order to understand the behavior of the PL properties using density functional theory at the B3LYP calculation level on periodic model systems and indicate the presence of stable electronic excited states (singlet). The analyses of the band structures and density of states at both ground and first excited electronic states provide insight into the main features, based on structural and electronic order-disorder effects in octahedral [AlO6] clusters and tetrahedral [WO4] clusters, as constituent building units of this material

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High stability of properties in morphotropic phase boundary Bi0.5Na0.5TiO3-BaTiO3 piezoceramics

The (1-x)Bi0.5Na0.5TiO3–xBaTiO3 (BNT-xBT) system with composition at its morphotropic phase boundary (MPB) has received significant attention because of their attractive properties as lead-free piezoceramics. Although the basic properties of this system are well-established, reports about the stability of the functional properties of these piezoelectric materials are still lacking. A study on the dielectric and piezoelectric properties of BNT-xBT close to their MPB, with emphasis on material response under high electric field or mechanical stress, is performed in this work. The results indicate that the BNT-BT system exhibits a high stability of dielectric and piezoelectric properties, making it potentially interesting for specific applications. A direct correlation between piezoelectric properties and nonlinear response is evidenced for a wide number of piezoceramics, which is expected due to the extrinsic nature of the piezoelectric response. Finding compositions that show high electromechanical properties and low nonlinear behavior is a challenge in the search for competitive lead-free piezoceramics

Efeito do dopante Ba sobre as propriedades estruturais e microestruturais de ceramicas Bi0.5Na0.5TiO3

Recently, Bi0,5Na0,5TiO3 (BNT) based ceramics has been widely studied as one promising material lead-free for piezoelectric applications. The Ba addition in BNT ((Bi0,5Na0,5)1-xBaxTiO3 - BNT-xBT) has attracted considerable attention due to the existence of a rhombohedral-tetragonal morphotropic phase boundary (MPB) near x = 0.06, where it shows maximized piezoelectric properties. In this study, BNT-XBT ceramics near the MPB (0.05 = x = 0.07), were prepared by solid-state reaction method. The effect of Ba doping on structure and microstructure of the BNT-xBT ceramics was investigated. The X-ray diffraction results show MPB for 0.06 = x <0.07. Raman spectroscopy suggests tetragonal symmetry, wich presence of the band at 300 cm-1 for composition x = 0.07, confirming the presence of the MPB region below this composition. Scanning electron microscopy images indicate apparent decrease of the grain size with increasing Ba concentration