Axial hypertoroidal moment in a ferroelectric nanotorus: A way to switch local polarization

International audienc

Original Reaction Sequence of Pb(Yb_1/2Nb_1/2)O₃-PbTiO₃:Consequences on dielectric properties and chemical order

International audienceThe solid solution [Pb(Yb 1/2 Nb 1/2)O 3 ] 1−x-[PbTiO 3 ] x was synthesized with í µí±¥ ≤ 60%, using several high-temperature techniques as well as room-temperature mechanosynthesis. The high-temperature synthesis reveals a reaction path involving the synthesis first of the end-members before the solid solution. The density and dielectric constant measured on the ceramics prepared from these powders indicate the crucial role of the synthesis technique in the subsequent properties. Mechanosynthesis results in ceramics with higher density and dielectric constant. Identical optimized sintering conditions were then applied to all investigated compositions and the resulting dielectric properties and chemical orders were compared. All polar orders (antiferroelectricity, ferroelectricity, and relaxor behavior) were evidenced. The 1 : 1 chemical order on the B-site of Pb(Yb 1/2 Nb 1/2)O 3 results in the formation of a double perovskite Pb 2 YbNbO 6 , and the superstructures in the X-ray diagrams signing the existence of this order persist up to 30% PbTiO 3. The underlying mechanism for substitution of Yb or Nb by Ti is presented

Densification par Spark Plasma Sintering (SPS) de matériaux d’électrolytes, difficilement densifiables, pour piles à combustible

Des matériaux tels que les apatites à base d’oxydes de lanthane et de silicium ou des pérovskites conductrices protoniques, potentiellement utilisables au sein de piles à combustible, présentent une grande résistance au frittage. Celle-ci limite d’autant plus leur utilisation au sein de piles à combustible, surtout s’ils doivent être employés comme électrolytes. Plusieurs stratégies peuvent être envisagées pour remédier à ce problème parmi lesquelles l’emploi de nouvelles méthodes de frittage ou le choix d’une méthode de synthèse efficace (permettant par exemple de diminuer la taille des grains ou de limiter celle des agrégats souvent rédhibitoires au moment du frittage). Nous présentons ici les résultats de frittage par Spark Plasma Sintering (appelé par la suite SPS) en les comparants à ceux obtenus par frittage conventionnel haute température. Les matériaux étudiés ont des compositions dérivées des phases La9,330,67Si6O26 et BaZr0,9Y0,1O2,950,05 pour lesquelles des problèmes de frittage ont été rencontrés. Nous insisterons sur les particularités des matériaux obtenus par SPS en termes de structure et microstructure et des conséquences sur les propriétés de transport anionique

Influence of synthesis route and composition on electrical properties of La9.33 + xSi6O26 + 3x/2 oxy-apatite compounds

Oxy-apatite materials La9.33 + xSi6O26 + 3x/2 are thought as zirconia-substitutes in Solid Oxide Fuel Cells due to their fast ionic conduction. However, the well-known difficulties related to their densification prevent them from being used as such. This paper presents strategies to obtain oxyapatite dense materials. First, freeze-drying has been optimized to obtain ultrafine and very homogeneous La9.33 + xSi6O26 + 3x/2 (0≤x≤0.67) nanopowders. From these powders, conventional and Spark Plasma Sintering (SPS) have been used leading to very dense samples obtained at temperatures rather lower than those previously reported. For instance, SPS has allowed to prepare fully dense and transparent ceramics from 1200 °C under 100 MPa. The microstructure and transport properties of such samples have been then evaluated as a function of sintering conditions and lanthanum content. It will be show that for lanthanum content higher than 9.60 per unit formula, the parasitic phase La2SiO5 appears leading to a egradation of conduction properties.We also show that grain boundaries and porosity (for conventionally-sintered materials) seem to have blocking effects on oxygen transport. The highest overall conductivity values at 700 °C, i.e. σ700 °C=7.33.10−3 S cm−1, were measured for La9.33Si6O26 material conventionally-sintered at 1500 °C which contains bigger grains' size by comparison with σ700 °C=4.77.10−3 S cm−1 for SPS-sintered materials at the same temperature but for few minutes. These values are associated with activation energies close to 0.83–0.91 eV, regardless of sintering condition, which are commonly encountered for anionic conductivity into such materials

Two-photon real-time device for single-particle holographic tracking (red shot)

Three-dimension real-time tracking of single emitters is an emerging tool for assessment of biological behavior as intraneuronal transport, for which spatiotemporal resolution is crucial to understand the microscopic interactions between molecular motors. We report the use of second harmonic signal from nonlinear nanoparticles to localize them in a super-localization regime, down to 15 nm precision, and at high refreshing rates, up to 1.1 kHz, allowing us to track the particles in real-time. Holograms dynamically displayed on a digital micro-mirror device are used to steer the excitation laser focus in 3D around the particle on a specific pattern. The particle position is inferred from the collected intensities using a maximum likelihood approach. The holograms are also used to compensate for optical aberrations of the optical system. We report tracking of particles moving faster than 30 $\mu$m/s with an uncertainty on the localization around 40 nm. We have been able to track freely moving particles over tens of micrometers, and directional intracellular transport in neurites

Lowering of ground state induced by core-shell structure in strontium titanate

International audienc

Local Immiscibility Control on Shape and Size of Nano-Objects in Solvo-Thermal Process: Implications for Ferroelectric Nano-Particles

International audienceSolvo-thermal synthesis is an effective method to obtain ferroelectric nano-particles. The morphology and size of these nano-particles dramatically impact the dielectric, ferroelectric, piezoelectric, optical properties of these materials at the nanoscale. This is of primary importance for miniaturized electro-active devices based on these materials. Herein, we propose to harness the full potential of water-ethanol mixture solvents to control the shape and size of nano-particles of a model ferroelectric, barium titanate. To take advantage of the local inhomogeneities that exist even in fully miscible liquids such as water and ethanol, we use a barium precursor that is only soluble in water together with a titanium precursor that is present in both components. Indeed, ethanol having hydrophilic and hydrophobic parts the two liquids remain distinct at the local scale. A threshold molar fraction exists at x EtOH =0.3 in water-ethanol mixtures; below this threshold, the water H-bonds network percolates throughout the sample whereas above water forms clusters that act as "nano-reactors". If below the concentration 1 threshold SEM images reveal that only elongated shapes (at best mixed with nanospheres) are obtained, just above the threshold, nano-tori and nano-cubes are observed; even further above the threshold, the reaction is limited to the aggregation-nucleation stage and only nano-cubes are obtained. This illustrates the fact that, in solvo-thermal syntheses, when the solvent is a mixture and one the reactant is confined in one of the constituents of the solvent, it is possible to control the morphology and size of the synthesized nano-objects

Alginate beads and epoxy resin composites as candidates for microwave absorbers

This paper presents a new composite material, which is developed by mixing calcium alginate spheres with commercially available epoxies Stycas 2850 FT (s2850) and Stycast W19 (W19). The resulting composite material is examined in terms of transmission and re°ection coe±cients in microwave frequencies (26 to 40 GHz, 70 to 110 GHz and 300 to 320 GHz). The study reveals that the new material exhibits re°ection coe±cients much lower than some commercial CR absorbers from the Eccosorb group. The experimental results justify the use of the new composite material as absorber at microwave frequencies

Electrocaloric effect in core-shell ferroelectric ceramics: Theoretical approach and practical conclusions

International audienc