Ultrahigh permittivity in core-shell ferroelectric ceramics: Theoretical approach and practical conclusions

Ferroelectric anomalies in most classical ferroelectric materials such as BaTiO3 (BT) and related compounds tend to become diffuse and eventually vanish when the grain size decreases. This has led to question the very existence of ferroelectricity for very small particles. However there are also reports in the literature of ultra-high values of permittivity when specific processes or doping are performed, for instance in the BT-family, or in the CaCu3Ti4O12- family and interpretations using (IBLC) have been proposed. We have considered these effects in the framework of a core-shell model of ceramics. The ultrahigh values mentioned above could be reproduced by considering that the core (be it dielectric or ferroelectric) have non-zero conductivity. In particular a huge background of permittivity and a Maxwell Wagner relaxation were obtained in agreement with the experimental situation. Different types of inhomogeneity (grain size distribution, mixing of conductive and non-conductive phases, porosity) have also been considered and their effects are discussed. Finally we have evaluated the potentiality of energy storage in IBLC compounds and observed a reduction (compared with non-conductive ferroelectrics) by a factor 2-4 depending on the structuration of the ceramic, which shows that the performance of such compounds are lower than in optimal insulating core-shell structures.