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 conventional solid-state reaction. The effect of acceptor doping on structural and functional properties are 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 Ec and remanent polarization Pr is observed. The piezoelectric properties are greatly increased when doping with small concentrations dopants. ZrO2 doped ceramic exhibits good piezoelectric properties with piezoelectric coefficient d33=134 pC/N and electromechanical coupling factor kp=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 by 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

Synthèse par voie chimique et caractérisation de poudres nanocristallisées de formule La(1-x)MnO(3+delta)

Le premier objectif du présent travail était de synthétiser à basse température et par voie chimique, des mangnites de lanthane de formule Lal-xMn03+d. Le choix d'une méthode de syntèse par voie chimique était motivé par la volonté d'obtenir des poudres homogènes de manière simple et reproductible. Les méthodes pyrosol et sol-gel utilisées, avec lesquelles le mélange des espèces est assuré au niveau moléculaire par dissolution des cations en solution aqueuse, nous ont permis de synthétiser des poutres de formule Lal-xMn03+d à une température de 800ʿC. Le deuxième objectif était d'étudier dans ces composés dits "lacunaires", la nature et l'influence des défauts sur les propriétés structurales et magnétiques. L'analyse minutieuse des données de composition et de diffraction rayon X nous a permis de proposer différents modèles de défauts en fonction du rapport La/Mn. Nous avons ainsi montré que pour les rapports La/Mn < 0.9, les poudres présentent une démixtion entre une phrase pérovskite de formule La 0.9Mn03 et L'oxyde de manganèse Mn304. La constance des températures de Curie observées pour cette gamme de composition, et des mesures d'absorption des rayons X et de diffraction de neutrons nous ont permis de conforter cette hypothèse. Pour 0.9<La/Mn<1, l'excès d'oxygène observé a pu être expliqué en prenant en compte la présence de lacunes sur les deux sites du lanthane et du manganèse. La présence de lacunes sur le site du manganèse mène à une frustration des interactions magnétiques et à une chute générale des températures de Curie.GRENOBLE1-BU Sciences (384212103) / SudocSudocFranceF

Competition between elastic and chemical effects in the doping, defect association, and hydration of barium stannate

International audienceDensity-functional theory calculations are performed to examine how two characteristics of a trivalent dopant (the one, physical – the ionic radius, the other, chemical – the electronegativity) impact the thermodynamics of doping, the defect association energy and the hydration energy in barium stannate, a perovskite oxide candidate as an electrolyte for Solid Oxide or Protonic Ceramic Fuel Cells. The formation energies of several trivalent dopants currently used in experimental works are computed in different external conditions and on the two possible sites (Ba, Sn), in their ionized state. These dopants cover a wide range of ionic radii (from 0.62 to 1.03 Å) and can be divided in two families according to their electronegativity: elements of group IIIA (Ga, In), versus IIIB transitions metals and Rare Earths (Sc, Lu, Y, Gd, Sm, La). The oxygen vacancy and the protonic defect are also studied, either isolated or in the vicinity of the dopant substituted on Sn site (1st & 2nd neighbors). The association energy between the dopant and both the oxygen vacancy and the proton, as well as the formation energy of the dopant on the Ba site, are mainly governed by the ionic radius of the dopant, with the exception that electronegative dopants stabilize more the oxygen vacancy in their vicinity. Therefore, a subtle interplay between elastic effects and chemistry is found to control the hydration energy/enthalpy, the more electronegative dopants – indium particularly – producing, at given radius, a less stable hydrated state. We provide with general trends likely to help experimentalists in the design of new materials, regarding the choice of dopant

Hydration, oxidation, and reduction of GdBaCo 2 O 5.5 from first-principles

International audienceStructural, magnetic and chemical properties of GdBaCo 2 O 5.5 regarding hydration, oxidation and reduction are studied by density-functional calculations in the DFT + U formalism. Besides the orthorhombic Pmmm structure, a lower energy, tilted, distorted structure is found. The hydrated configurations obtained by water dissociation in the oxygen vacancies of the GdO 0.5 plane exhibit strong distortions with respect to the Pmmm structure. Simulation of protonic defects provides the energy landscape of incorporated protons, which preferentially bind to oxygens of the CoO 2 planes, suggesting their possible bidimensional diffusion in this plane. We also studied oxygen incorporation (oxidation) in the oxygen vacancies of the GdO 0.5 planes, and oxygen removal (reduction) from BaO, CoO 2 and GdO 0.5 planes. The oxidized compound, GdBaCo 2 O 5.75 , is rather p-type metallic, while the reduced compound, GdBaCo 2 O 5.25 , remains an insulator, due to electronic localization (Co 3+ + e À / Co 2+). Taking Pmmm as the reference, both hydration at high water concentration (one H 2 O per 38-atom supercell) and oxidation are found exothermic. However, if the distorted structure is chosen as the reference, these reactions become endothermic, at least at the high water/oxygen concentration studied. Reduction is, in both cases, endothermic. Nevertheless, negative formation energies of the protonic defects suggest the possibility of hydration at lower water concentration

Effects of biaxial strain on bulk 8\% yttria-stabilised zirconia ion conduction through molecular dynamics

Tensile strain is thought to give rise to enhanced conduction properties in ion conducting compounds. However, most experimental studies in the field involve simultaneous presence of interface structures and strain, thus complicating separation of the individual effects. Here, we present molecular dynamics calculations that clarify the influence of biaxial strain in bulk yttria-stabilised zirconia. Such a study mimics what may be experimentally observed in epitaxially deposited films. We show that, as expected, tensile strain leads to enhanced ion conduction properties. The maximum enhancement is observed for a 2-3\% tensile strain. We show that the increase of bulk diffusion is in part due to an opening of the Zr-Zr and Zr-Y distances induced by tensile strain, leading to a smaller oxygen migration energy. Above a 3\% tensile strain, the diffusion coefficient of oxygen is strongly reduced, reaching values even lower than without strain. This decrease is associated with important structural changes of the cation and oxygen network. Also, we show that the diffusion coefficient increases by less than a factor 2 at 833 K for the optimal strain value. This confirms that the great increase of conductivity observed in zirconia/strontium titanate multilayers was due either to an electron contribution from strontium titanate or to the presence of interfaces, but not to the direct influence of strain on the oxygen diffusion coefficient in zirconia. Copyright \textcopyright 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved

Simulations of REBaCo2_2O5.5_{5.5} (REGd, La, Y) cathode materials through energy minimisation and molecular dynamics

The GdBaCo2O5+x oxide has been presented as a promising cathode material for solid oxide fuel cells. It presents very high oxygen exchange and diffusion coefficients, two characteristics of utmost importance for an efficient cathode material. Yet the understanding at atomic scale of these two properties is rather limited. Here, we performed calculations to understand the influence of rare-earth nature in REBaCo2O5.5 (REGd, La, Y) on material stability and oxygen diffusion properties. Through energy minimisation, we determined the most energetically favourable distribution of A-site cations and oxygen vacancies. We also investigated with Molecular Dynamics simulations the mechanisms of oxygen diffusion in A-site ordered REBaCo2O5.5. The results confirm that oxygen vacancies essentially lie in the RE-plane and that diffusion is mainly two-dimensional with oxygen moving in the (a,b) plane while diffusion along the c axis is strongly hindered. Between 1300 and 1900K, the activation energy for oxygen diffusion lies in the range 0.69–0.83eV depending on the RE cation nature, values in good agreement with the experimental ones. We show that, in the double perovskite structure, the replacement of Gd by a larger rare-earth ion enhances oxygen diffusion properties but also reduces the stability of the double perovskite structure

Identification of oxygen diffusion mechanisms in Nd 1−x AE x BaInO 4−x/2 (AE = Ca, Sr, Ba) compounds through molecular dynamics

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Insight into the synthesis and electrical properties of alkali-earth-substituted Gd3GaO6 oxide-ion and proton conductors

International audienceNovel ionic conductors were prepared by substituting Ca2+ and Sr2+ for Gd3+ in Gd3GaO6. A microwave-assisted combustion technique was used to synthesize these compounds at 900 °C. SEM observations showed that both substituents promote grain growth during sintering. XRD proved that the Gd3-x(Ca,Sr)xGaO6-x/2 solid solutions are formed up to x = 0.10. Below 600 °C, the level of conductivity under wet Ar is higher than that of measured under dry atmospheres, thereby demonstrating the contribution of proton defects to the overall conductivity. The highest level of proton conduction, i.e. σ600°C = 1 × 10−3 S cm−1, was measured for Gd2.9Sr0.1GaO5.95 at 600 °C in wet Ar. At higher temperatures, only oxygen ions contribute to the conductivity. At 800 °C, a total oxide-ion conductivity of σ800°C = 1 × 10−2 S cm−1 was measured for the highest substitution level, i.e. x = 0.10. In both temperature ranges, activation energy associated with ionic transport decreases with the Me content as a result of an increase in grain size. Stability tests were successfully achieved as the structure of materials remains unchanged after different treatment under severe conditions. Conductivity measurements under varying oxygen partial pressures demonstrated that materials are purely oxide-ion conductors up to pO2 = 1 × 10−5 atm. At higher pO2, a p-type contribution appears