The grain size (GS) effect on the electric field dependence of the dielectric constant in ferroelectric nanoceramics was studied using a detailed model and the predicted behavior compared to experimental data obtained for BaTiO3. In the finest ceramics the permittivity reduces below 1000 and a tendency to linearization of the permittivity vs field dependence is observed. The composite structure of ferroelectric ceramics corresponding to ferroelectric grain cores separated by nonferroelectric grain boundaries determines a progressive increase of the electrical inhomogeneity of the system with decreasing GS. The local electric field was computed at various applied voltages for different GSs by a finite element approach, and then the effective permittivity-field response $\varepsilon${}eff(E) was determined by taking into consideration the local field distribution. A remarkable agreement between the model and experimental data was obtained for BaTiO3 ceramics. The local field distribution explains with considerable accuracy the reduction of permittivity and tunability observed with decreasing GS as well as the tendency towards linearization of $\varepsilon${}eff(E) in nanoceramics.

The present paper reports the dependence of dielectric, ferroelectric and piezoelectric properties on the porosity level in BaZr_0.15Ti_0.85O₃ ceramics with porosity from 5% to 21%. Microporosity with 0–3 connectivity has been produced using PMMA microspheres as a sacrificial template. The functional properties (dielectric, ferroelectric and piezoelectric effect) are mostly affected by the “dilution effect”: permittivity decreases by 40% when porosity increases by 21%, and P_max decreases from 13 to 5 µC/cm² while the P_rem is in the range of (2–8) µC/cm². However, the reduction of the zero-field permittivity and hysteretic behaviour of ε(E) while the tunability level is still high makes from porous ceramics interesting materials for tunability application.

3D Finite Element Method simulations were employed in order to describe tunability properties in anisotropic porous paraelectric structures. The simulations predicted that properties of a ceramic can be tailored by using various levels of porosity. Porous Ba0.6Sr0.4TiO3 (BST) ceramics have been studied in order to investigate the influence of porosity on their functional properties. The BST ceramics with various porosity levels have been obtained by solid-state reaction. Lamellar graphite in different concentration of 10, 20 and 35 vol.%was added as sacrificial pore forming agent. The structural, microstructural, dielectric and tunability properties were investigated. By comparison with dense BST ceramic, porous samples present a fracture mode transformation from intragranular to an intergranular fracture and a decrease of grain size. Lower dielectric constants, low dielectric losses, but higher values of tunability than in the dense material were obtained in the porous BST structures as a result of local field inhomogeneity generated by the presence of air pores-ceramic interfaces.

Pure perovskite Ba_0.90Ca_0.10TiO₃ ceramics, with a relative density of between 79 and 98% and grain sizes larger than 1 µm, were prepared by solid-state reaction. The dielectric and electrocaloric properties were investigated and discussed considering the density and grain size of the samples. Room temperature impedance measurements show good dielectric properties for all ceramics with relative permittivity between 800 and 1100 and losses of <5%. Polarization vs. E loops indicates regular variation with increasing sintering temperature (grain size and density), an increase in loop area, and remanent and saturation polarization (from P_sat = 7.2 µC/cm² to P_sat = 16 µC/cm²). The largest electrocaloric effect was 1.67 K for ceramic with GS = 3 µm at 363 K and electrocaloric responsivity (ζ) was 0.56 K mm/kV. These values are larger than in the case of other similar materials; thus, Ba_0.90Ca_0.10TiO₃ ceramics with a density larger than 90% and grain sizes of a few µms are suitable materials for electrocaloric devices.

In the present work, BaZr_0.05Ti_0.95O₃ ceramics with grain sizes between 0.45 and 135 µm were prepared by solid-state reaction and classical sintering. The effect of grain size on dielectric properties was systematically explored, and it was found that dielectric permittivity reaches a maximum value for grain sizes between 1.5 and 10 µm and then rapidly drops for larger grain sizes. A numerical finite element method was employed to eliminate the effect of porosity on the effective values of permittivity. The results indicate that it is possible to have a critical size in slightly doped barium titanate ceramics with enhanced functional properties for a grain size between 1.5 and 10 µm.