Study On Preparation And Dielectric Property Of Percolative β-Polyvinylidene Fluoride Matrix Composite Film

With fast development of the electronic equipment, high-performance capacitors call for smaller size and multi-function. And the flexible composites with a high dielectric constant have a great future in the electronic industry field. The composites may be used to prepare a shape-bend mutilayer chip capacitors. Therefore, it is very significant for improving the dielectric constant of the composites.According to the percolation theory, as for the conductor/polymer composites, when the volume fraction of the conductor is close to the percolation threshold, the dielectric constant of such material will undergo a nonlinear transition and increases sharply. Therefore, it is meaningful to use the percolation theory to prepare the material with high dielectric constant.The microstructure and dielectric constant of composite films were studied, which were made of polymer matrix, such as polyvinylene fluoride (PVDF) or PVDF/BaTiO₃ (BT) , and cheap conductor filler, such as acetylene black (AB). PVDF/AB composite film, PVDF/BaTiO₃ composite film, and PVDF/ BaTiO₃/AB composite film were prepared by using dip-coating method with N,N-dimethyllformamide (DMF) as solvent. Phase structures and morphologies of the composite films were characterized by X-ray diffraction and Field Emission-Scan Electron Microscopy (FE-SEM) respectively. The dielectric properties of samples were measured using Agilent 4294A precision impedance analyzer in the frequency range of 100 Hz -10 MHz at room temperature.The experimental results reveal that, as for PVDF films, at lower heat treatment temperature (60°C) ,β-PVDF forms, and its dielectric constant is relatively higher; while, at higher heat treatment temperature (100°C), α-PVDF forms, and its dielectric constant is relatively lower. And the time for heat treatment has no effect on the crystalline phase formation, crystalline content and dielectric property of the PVDF film. As for PVDF/AB composite film, when AB volume fraction is close to the percolation threshold (f=1.3%), the dielectric constant of PVDF/AB composite film reach about 56, which is 7-8 times higher than that of PVDF film. Due to the longchain structure of AB particles, the percolation threshold (f_c=1.3%) of PVDF/AB film is much lower than the typical one (f_c=16%), which ensures the excellent flexibility and processing property. In addition, due to the existence of amorphous state, the dielectric loss of PVDF/AB film is below 0.15, when AB volume fraction is close to the percolation threshold. The fact that, dielectric constant of PVDF/AB film is independent of low frequency, shows no space charge polarization occurs.The experimental results also show that, when BaTiO₃ volume fraction is 30%, the dielectric constant of PVDF/ BaTiO₃ composite film is highest. When BaTiO₃ volume fraction is lower than 30%, the variation of dielectric constant of PVDF/ BaTiO₃ film with BaTiO₃ volume fraction is in good agreement with the data calculated by Maxwell-Garrett equation; when BaTiO₃ volume fraction is higher than 30%, the variation of dielectric constant of PVDF/BaTiO₃ film with BaTiO₃ volume fraction is not in agreement with the data calculated by Bruggemant equation. This is because when BaTiO₃ volume fraction is higher than 50%, the crystallinity of PVDF is low and the film quality deceases. When AB volume fraction is close to the percolation threshold (f=3.3%), the dielectric constant of PVDF/BaTiO₃/AB composite film reach about 652, which is 90 times higher than that of PVDF film and 40 times higher than that of PVDF/BT film.The problem of BaTiO₃ precipitation is solved by surface treating of BT particles using KH550. This method can not only solve the BT aggregation problem, but also enhance the dielectric constant of PVDF/BT/AB composite films.