Engineered ferroelectric PVDF composites containing BaTiO3@AO2 (A = Ti, Si) inclusions: effective dielectric properties and 3D FEM modelling of field distribution

The demand for high dielectric constant materials and high energy density capacitors has rapidly increased in recent years due to the continuous and rapid development of the electronic industry and the need to store electrostatic energy more efficiently. The introduction of ferroelectric inclusions in a polymeric matrix enables the effective dielectric constant of the composite to be significantly increased while preserving the high breakdown field typical of polymers, thus improving the stored energy density. By varying the size, shape and volume fraction of the inclusions, the dielectric properties of the composite can be tailored, as a consequence of the alteration of the electric field distribution inside the material. A further approach for the modification of the electric field in the composite is coating the particles used as inclusions with an oxide layer with a different dielectric constant. In this work we have fabricated polyvinyldene fluoride (PVDF) composites containing 30 vol.% BaTiO3@AO2 (A = Si, Ti) particles. Barium titanate particles with a diameter of ?100 nm were synthesized using a hydrothermal-like method and coated with a thin shell (10 nm) of SiO2 or TiO2 by means of colloidal chemistry methods. The composites were prepared by solution casting and subsequent compression moulding. Reference composites were prepared by the same process using uncoated BaTiO3 particles. In order to understand the effect of interfacial modification by coating of the filler, the dielectric properties of the composites (dielectric constant and loss tangent) were measured at different frequencies (1 - 106 Hz) in the temperature range 25-150 °C. The coating results in a significant modification on the effective permittivity, which is mainly determined by the value of its dielectric constant. To gain insight into the role of the coating layer, the electric field distribution and the effective dielectric constant were calculated for the different composites using a 3D finite element modelling and compared to experimental results. This study provides some guidelines for tuning the dielectric properties of ferroelectric PVDF-ceramic composites. Acknowledgements: This work was funded by Fondazione Bancaria Compagnia di San Paolo, under the framework of the Project "Polycom".