Relaxor-like behavior of the dielectric response of dense ferroelectric composites

We study experimentally and theoretically the influence of size effects and structural factors on the dielectric and ferroelectric properties of the dense ferroelectric composites. The composites have the form of the tape-casted films with 28 vol% of nanosized or submicron BaTiO3 particles dispersed in the polyvinyl butyral. We measured and analyzed the temperature dependences of the capacitance and dielectric losses in the temperature range (-200 - +200)C-o and frequency range (10(2)-10(5)) Hz. The nonmonotonic temperature dependences of dielectric permittivity of the studied composite films have a wide plateau-like maxima located between 80(o)& Scy; and 160(o)& Scy;, in contrast to the relatively sharp maximum near the Curie temperature (similar to 124(o)C) observed in the fine-grained and coarse-grained ceramics, sintered from the same nanosized or submicron BaTiO3 particles at 1250(o)C. The shift of the dielectric permittivity and losses maxima (in more than 40(o)C) and their strong frequency dispersion do not agree with the behavior expected for the systems with a weak interaction between ferroelectric particles and a polymeric matrix. In contrast, we reveal the relaxor-like behavior of the composite dielectric response. To explain the observed results, we propose the analytical model, which considers the strong dipole-dipole cross-interaction of the BaTiO3 particles in the dense nanocomposite. The analytical model is corroborated by the structural studies of the BaTiO3 nanosized and submicron particles by X-ray phase analysis and static Ba-137 NMR spectra, as well as by the finite element modelling of the composite polar properties, which confirms the strong cross-interactions between the dipole moments of single-domain nanoparticles and/or between the domain walls of different submicron polydomain particles. The obtained results may be useful for development of cheap and flexible lead-free dense ferroelectric nanocomposites with relaxor-like behavior of the dielectric response, which are promising for non-volatile memory and energy-saving elements, modulators, electrical converters and piezoresistive elements.