Perovskite particles and nanostructures by self-assembly

Controlled self-organization of nanocrystals in aqueous media can be a powerful tool to obtain (nano)particles and more complex architectures with well-defined morphology and new modified properties. Aggregation of nanocrystals produces polycrystalline assemblies which can be ordered or disordered. The oriented aggregation of nano buildings blocks overcomes the classic concept of crystal growth, which is typically thought to accur via atom-by-atom or monomer-to-monomer addition of existing nucleous. Secondary nucleation on the surface of existing crystals represents a further mechanism for the growth of particles with some level of internal organization. We will show some examples of self-assembly processes in the synthesis of BaTiO(3) and SrTiO(3) mesocrystals from aqueous suspensions of amorphous titanium hydroxide. The assembly process can be controlled by varying the temperature and the concentration of the solution as well as by introducing suitable organic molecules. Core-shell structures can be obtained when the assembly process occurs at the surface of template particles suspended in the solution. The coating of BaTiO(3) spherical particles with SrTiO(3) and BaZrO(3) nanocrystals and the possible application of this process in the field of dielectric materials will be discussed

Preparation and characterisation of the Ba(Zr,Ti)O(3) ceramics with relaxor properties

Ba(Zr(x)Ti(1-x))O(3) ceramics with various compositions x in the range (0, 0.5) have been prepared via solid state reaction. Optimum parameters for calcination and sintering have been found in order to obtain pure perovskite phase and high density ceramics. The dielectric data showed a transition from ferroelectric towards relaxor state and a shift of the Curie temperature towards lower values with increasing x. Using the modified Landau model for relaxors, the local order parameter has been calculated. Its temperature dependence shows the increasing of the degree of diffuseness of the phase transition with increasing Zr with a maximum for the composition x=0.35. The model also shows that in the relaxor state the local order parameter has non-zero values even at a few hundreds degrees above the temperature corresponding to the maximum of the dielectric constant. Further, the dielectric data obtained for x=0.35 under field cooling (FC) and zero-field cooling (ZFC) conditions shows a splitting characteristic to the relaxors and spin-glass systems

Properties of compositionally graded Ba(1-x)SrxTiO(3) thick films

Compositionally graded thick films (0.4 mm) have been fabricated using the airflow deposition method. Films were made of five layers with different composition Ba1-xSrxTiO(3) (BST, x=0, 0.1, 0.2, 0.3 and 0.4). The layers presented different thicknesses, ranging from 80 to 30 microns, but similar Vickers microhardness. The average particle size of deposited layers was below 500 nm and the density of asdeposited films was about 80% of theoretical. After sintering at 1350 ?C samples presented increaseddensity (>90%) and maintained a compositional gradient. When compared to single-composition BST ceramics, permittivity of graded films was much less dependent on temperature over a wide range, from -50 to 250 ?C. In addition, the films displayed polarisation offset when driven by an alternating field and heated above 50?C

Dense dielectric ceramics with local graded structure from core-shell particles: preparation and properties

The modification of the surface properties of particles by coating with a different material, resulting in the formation of core-shell structures, is a well-known process. However, the consolidation of core-shell particles in bulk ceramics has not been extensively investigated yet, mainly because of the difficulty in controlling interdiffusion and interface reactions. In this study, we have coated BaTiO(3) spherical templates with SrTiO(3) and BaZrO(3) using a precipitation process from inorganic precursors. The size of the particles as well as the overall composition can be tailored over a wide range. Densification of the resulting core-shell particles was accomplished either using conventional sintering or spark plasma sintering. Dense ceramics with a graded composition at the level of the single grains could only be obtained by careful choice of the sintering conditions. The final ceramics show strongly modified dielectric properties in comparison to both the parent compounds and the homogeneous solid solutions. The proposed approach is quite generic and suggests new possibilities for the realization of polycrystalline materials with local graded structure by the controlled sintering of core-shell particles

Inter-related magnetic and ferroelectric domain structures in BaTiO(3) - (Ni(0.5)Zn(0.5))Fe(2)O(4) multiferroic ceramics

Multiferroic ceramics (1-x)BaTiO(3)-x(Ni(0.5)Zn(0.5))Fe(2)O(4) with various compositions x are investigated in the present work. The processing parameters were adapted in order to obtain pure diphasic ceramics without reactions at the interfaces. The macroscopic ferroelectric behaviour was proved by the existence of the ferroelectric-paraelectric dielectric and calorimetric anomaly of BaTiO(3) around 125-130?C. The magnetic activity with a concentration influence ("dilution" effect) due to the presence of the non-magnetic phase was found by measuring the M(H) loops at various temperatures. The existence of both magnetic and ferroelectric domain structure and their interdependence was proved by local MFM/AFM-piezoresponse experiments

Nanoferroelectric perovskite oxides with unusual morphology produced by different synthesis procedures

We report in the present paper some original results of a joint research performed in the framework of the COST Action 539 ELENA. In search of higher miniaturisation of electroceramic devices a new outlook seems to arise from ceramics with unusual morphology that might present a new kind of circular or toroidal ferroelectric ordering of dipoles. Completely new perspectives in data storage can be expected if a close control of size confinement and dimensionality as well as of the chemical composition and the phase purity is reached. We succeeded in the fabrication of BaTiO3 hollow nanoparticles and nanowires, and Bi4Ti3O12 platelets. The use of soft chemistry and solid state methods allowed to produce coreshell powders and ferroelectric-ferromagnetic composites with completely new functional properties

Polymer-assisted precipitation of ZnO nanoparticles with narrow particle size distribution

Zinc oxide nanoparticles were produced by aqueous precipitation in mild hydrothermal conditions (90 ◦C). Well crystallized regular roundish particles were obtained, with a very narrow size distribution. The influence of soft templates (organic species) to control size and size distribution in the final product was investigated. No effect was observed with polyvinylpyrrolidone (PVP Mw 8000 and 40,000), (d−)fructose and hydroxypropylmethylcellulose (HPMC). A significant decrease of the particle size towards 100 nm, while conserving a very narrow size distribution was observed for poly(acrylic acid) (PAA Mw 2000), Dispex A40 (commercial PAA from CIBA Mw ∼10,000), and sodium dodecylsulfate (SDS). The influence of the pHwas also investigated with PAA. By increasing the pH from slightly acidic to basic conditions, the morphologywas spectacularly modified from roundish nanoparticles to micronic branched flower-like particles, with an elongated primary crystal morphology

Grain size determination of superconducting MgB2 powders from magnetization curve, image analysis and surface area measurement

The present article reports a method for the average grain size evaluation of superconducting nano-particles through their magnetic properties. The use of SQUID magnetometry to determine the average MgB2 particle size was investigated and the results compared with those achieved through other techniques. In particular the data obtained from zero field cooled magnetization measurement as function of the temperature were compared with the results obtained by scanning electron microscopy and Brunauer-Emmett-Teller techniques. The particle magnetization was measured by a commercial SQUID magnetometer in magnetic field (1 mT) and temperatures ranging from 5 to 50 K dispersing the powders in a grease medium. The grain size is obtained by fitting the data taking into account the Ginzburg-Landau temperature dependence of the London penetration depth. Variations on typical modeling parameters were explored in order to gain a better picture of the average grain size and the effectiveness of various measurement techniques. We find that it is possible to use the magnetization measurements to determine the average grain size even if the SEM image analysis allows extracting more information about the grain size distribution. Furthermore a Matlab routine has been developed in order to get automatic analysis of SEM images.Comment: 12 pages, 10 figures, 5 table

BaTiO(3)-(Ni(0.5)Zn(0.5))Fe(2)O(4) ceramic composites with ferroelectric and magnetic properties

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