Influence of Ferroelectric Filler Size and Clustering on the Electrical Properties of (Ag-BaTiO3)-PVDF Sub-Percolative Hybrid Composites.

The paper presents a study concerning the role of ferroelectric filler size and clustering in the dielectric properties of 20%BaTiO3-80%PVDF and of 20% (2%Ag-98%BaTiO3)-PVDF hybrid nanocomposites. By finite element calculations, it was shown that using fillers with ε > 103 does not provide a permittivity rise in the composites and the effective dielectric constant tends to saturate to specific values determined by the filler size and agglomeration degree. Irrespective of the ferroelectric filler sizes, the addition of metallic ultrafine nanoparticles (Ag) results in permittivity intensification and the effect is even stronger if the metallic nanoparticles are connected to a higher degree with the ferroelectric particles' surfaces. When using coarse ferroelectric fillers, the probability of clustering is higher, thus favoring the permittivity increase by field concentration in small regions close to the interfaces separating dissimilar materials. The modeling results were validated by an experimental dielectric analysis performed in a series of PVDF-based thick films with the same amount of BaTiO3 fillers or with Ag-BaTiO3 hybrid fillers. Similar trends as predicted by simulations were found experimentally but with slightly higher permittivity values which were assigned to the modifications of the polymer phase composition due to the presence of nanofillers and the local sample inhomogeneity (the presence of clustering, in particular for coarse BaTiO3 grains), which create regions with enhanced local fields.

Mesoscale Models for Describing the Formation of Anisotropic Porosity and Strain-Stress Distributions during the Pressing Step in Electroceramics.

Porous ceramics are often produced by using pyrolisable additives to generate porosity during the sintering step. The examination of the experimental microstructures of the resulted porous ceramics revealed certain levels of anisotropy, even if the original soft additives used as pore forming agents were spherical. The paper shows that anisotropic porosity may result in ceramics when using equiaxed soft polymeric additives for generating porosity, due to the deformation of soft inclusions during the pressing step. It has been found, by means of analytical and numerical calculations, that uniaxial pressing of a mixture of solid particles with contrasting mechanical properties (hard/soft) generates modifications in the shape of the soft phase. As a result, anisotropic shape distribution of the soft inclusions in the green ceramic body and elongated porosity in the final ceramic product are obtained. The elongated pores are statistically oriented with the major axes perpendicular to the pressing direction and will generate anisotropy-related functional properties. Analytical calculations indicate the deformation of a single soft inclusion inside a continuum solid. Further, by finite element simulations performed in 2D planes along the transversal and radial directions of the pressing axis, a bimodal angular distribution of the long axes of the soft inclusions has been found.