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The grain size influence on the dielectric relaxation and nonlinear dielectric properties of BaTiO3 ceramics with grain size in the range of (92÷936) nm densified by Spark Plasma Sintering ( SPS) from ultrafine powders were investigated. The progressive reduction of the Curie temperature and of the effective permittivity results from a combination of intrinsic size effects and low-permittivity grain boundary layer. A model of dielectric cylindrical cavities was employed in order to calculate intrinsic effective permittivity values in GHz range. An interesting feature is the presence of a thermally activated Debye-like relaxation in the ferroelectric state of ceramics with grain size above 300 nm, with activation energies of 0.45-0.49 eV, which seems to be related to the domain walls forced motion under the applied field. By diminishing grain size, a progressive reduction of the ferroelectric nonlinear character was obtained, reaching a macroscopic non-switching character and a linear permittivity versus field dependence for the finest ceramics (grain size of 90-100 nm) until very high values of the applied field. The observed behavior supports the idea of frozen polarization induced by pinning centers as due to a large number of grain boundaries and charged defects in the fine structures. [ABSTRACT FROM AUTHOR]

Nanocoatings (5–20 nm) of different compounds on fine BaTiO3 particles were obtained by means of precipitation processes. Homogeneous and smooth shells of Y(OH)CO3 and Dy(OH)CO3 were grown from nitrate solutions in the presence of urea. An irregular coating consisting of zirconia nanoparticles was produced from zirconyl nitrate solution using ammonia as a precipitating agent after adsorption of a polymeric polyelectrolyte on the BaTiO3 surface. Composite particles with a peculiar morphology were obtained by inducing heterogeneous nucleation and growth of Ni(OH)2 lamellae on the BaTiO3 surface. The different shells can be transformed in a nanocrystalline coating of the corresponding oxide (Y2O3, Dy2O3, ZrO2, NiO) by calcination at moderate temperatures (400–700 °C). The overall results indicate that precipitation from solution represents a versatile process to grow a second-phase layer on the surface of BaTiO3 particles. This approach can be used as an alternative to mechanical wet mixing for controlled doping of ferroelectric materials and for the fabrication of composite materials with specific geometry of the two-phase assembly. [ABSTRACT FROM AUTHOR]

Barium titanate has been prepared by solid-state reaction of nanocrystalline TiO2 (70 nm) with BaCO3 of different particle size (650, 140, and 50 nm). The results give evidence of a strong effect of the size of BaCO3 in the solid-state synthesis of barium titanate. The use of nanocrystalline BaCO3 already leads to formation of the single-phase BaTiO3 after calcination for 8 h at 800°C. The final powder consists of primary particles of ≈100 nm, has a narrow particle size distribution with d50=270 nm, and no agglomerates larger than 800 nm. For the coarser carbonate, 4 h calcination at 1000°C are required and the final powder is much coarser. Solid-state reaction of nanocrystalline BaCO3 and TiO2 represents an alternative to chemical preparation routes for the production of barium titanate ultrafine powders. [ABSTRACT FROM AUTHOR]

The switching properties of dense BaTiO[sub 3] ceramics with 50 nm average grain size were investigated at local scale by piezoresponse force microscopy. Large areas with low piezoelectrical activity beside islands with strong piezoresponse were found. The application of electric fields induces stable domain structures and changes in the polarization state far away from the probing area, probably via trans-granular dipole interactions. Piezoelectric hysteresis loops were recorded on various positions, even in regions with initial zero piezoresponse, which possibly showed a superparaelectric behavior. The results are incontestable proof that 50 nm BaTiO[sub 3] ceramics retain ferroelectricity at a local scale. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Barium titanate has been prepared by solid-state reaction of nanocrystalline TiO2 (70 nm) with BaCO3 of different particle size (650, 140, and 50 nm). The results give evidence of a strong effect of the size of BaCO3 in the solid-state synthesis of barium titanate. The use of nanocrystalline BaCO3 already leads to formation of the single-phase BaTiO3 after calcination for 8 h at 800°C. The final powder consists of primary particles of ≈100 nm, has a narrow particle size distribution with d50=270 nm, and no agglomerates larger than 800 nm. For the coarser carbonate, 4 h calcination at 1000°C are required and the final powder is much coarser. Solid-state reaction of nanocrystalline BaCO3 and TiO2 represents an alternative to chemical preparation routes for the production of barium titanate ultrafine powders. [ABSTRACT FROM AUTHOR]

Abstract: A series of La-doped (10at.%) SrTiO3 ceramics with grain size ranging from 6μm to 24nm was prepared from nanocrystalline powders using high-pressure field assisted sintering (HP-FAST). A progressive reduction of thermal conductivity κ with decreasing grain size was observed. At room temperature, κ of the ceramic with grain size of 24nm (1.2Wm−1 K−1) is one order of magnitude lower than that of undoped single crystals. The strong suppression of κ can be ascribed to (i) the high concentration of lattice defects, (ii) the increasing contribution of grain boundaries to phonon scattering when the grain size is decreased to the nanoscale and (iii) a moderate amount (10–15vol.%) of nanopores. These results demonstrate that nanostructuration can be a successful strategy to attain a considerable reduction of κ in heavily doped bulk oxide ceramics. The low electrical conductivity of the La:SrTiO3 nanoceramics represents a major obstacle for thermoelectric applications. [Copyright &y& Elsevier]

The mechanisms driving the mutual crystallographic alignmentofnanocrystals in mesocrystals are various and not yet fully understood.As discussed in this paper, formation of mesocrystals can result froma topochemical reaction between single crystal particles or templatessuspended in a liquid phase and ionic/molecular species in solution.In such a case, the initial particle morphology is retained in thefinal mesocrystal. If the transformation is not topotactic, the finalproduct will maintain no memory of the precursor morphology. The magnitudeof the lattice mismatch as well as the defective state of the precursorsurface probably determine the degree of mutual crystallographic alignmentof the nanocrystals which nucleate and grow on the substrate. Althoughidentified by studying the hydrothermal crystallization of SrTiO3from different single crystal titania precursors, this mechanismis very general and applicable to a variety of compounds and experimentalsituations, including solvothermal and molten salt syntheses. [ABSTRACT FROM AUTHOR]