Preparation, characterisation and dielectric properties of PVDF - BaTiO3 composites

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 of storing electrostatic energy more efficiently. The combination of dissimilar materials in a composite represents an effective approach for the optimization of the dielectric properties. In particular, the addition of ferroelectric (barium titanate, BaTiO3) nanoparticles with high dielectric constant (k ?1000) enables the relative dielectric constant of the polymer (usually in the range 3-10) to be significantly increased without compromising some of the most useful properties of the material, i.e. its flexibility and the high dielectric breakdown field. Polymer composites were prepared using poly(vinylidene fluoride) (PVDF, SOLEF 6008 Solvay) as a matrix and BaTiO3 nanoparticles (diameter: 100 nm) as inclusions. The BaTiO3 particles were prepared by a hydrothermal-like method starting from BaCl2 and TiCl4 precursors. The composites were fabricated by a two-step process. Firstly, PVDF and BaTiO3 particles were intimately melt-blended at 200 °C in an internal batch-mixer. Batches of about 100 g of composites containing 20-40 vol.% BaTiO3 were prepared each time. Films with a thickness of 0.5-1 mm were then fabricated by compression moulding. The composites were fabricated using bare and surface-modified BaTiO3 particles. Surface modification included functionalisation with coupling molecules (silane derivatives) and coating with a thin TiO2 shell (thickness: 20-30 nm). These engineered polymer/ceramic interfaces should facilitate the homogeneous dispersion of the inclusions and realise a more homogeneous distribution of the electric field in the ensuing composite. The composites were characterised with different techniques. The microstructure was observed on fragile-fracture surfaces by scanning electron microscopy. The thermal behaviour was investigated by TG and DSC analyses. The amount of the ferroelectric crystalline ? phase in the PVDF matrix was determined by FT-IR spectroscopy. The dielectric constant and loss tangent were measured at different frequencies by impedance spectroscopy. Simulation of the field distribution in model composites was performed by 3D finite element modelling. Acknowledgement: Work carried out in the framework of project Polycom funded by Fondazione Bancaria Compagnia di San Paolo.