The Preparation And Characterization Of PVDF/Ceramic Composite Film With High Dielectric Contant

In this paper, Poly(vinylidene fluoride)(PVDF) was chosen as the polymer matrix,while nanometer barium titanate (BaTiO3, abbreviated BT) and copper calcium titanate(CCTO) were choosen as the filler. The two-phase or three-phase composite films wereprepared through solution cast method. Their dielectric properties, crystallizationbehaviors and microstructures were characterized, followed by the study of influence ofceramic powders, preparation technology on the performance of the composite films.The composite films with the BT content is0,10wt%,30wt%,50wt%were preparedrespectively. It is found that the dielectric constant of the composite films increased withthe increase of the content of BT, When the content of BT up to50wt%, the dielectricconstant of the composite films reaches about10under100Hz at room temperature.Dielectric loss is hardly changed along with the content of ceramics powder. Thebreakdown strength of composite films decreases from251kV/mm to213kV/mm rapidlywith the addition of BT. The internal defects such as holes resulted from agglomerationand uneven dispersion are the main reason.The BT ceramic powders were modified with fluorine silane coupling agent. Thelipophilic degree was choosen as evaluation standard. The surface modification conditionswere optimized through single factor parallel test and orthogonal test. The results showedthe optimal modification conditions are that the dosage of coupling agent is5%,60℃,3h,pH=5respectively.In order to improve the dispersion of ceramic powders in matrix, the BT ceramicspowders were modified according to the optimal modification conditions, and then thecomposite films were prepared by using modified ceramics powders. The results showedthat the compatibility between PVDF and BT powders was inproved dramatically, and BTpowders were dispersed more uniformly. Compared with the unmodified composite films,the dielectric constent and breakdown strength were improved significantly. The dielectricconstant of modified50wt%PVDF/BT composite film increases to more than12at100Hzunder the room temperature. The breakdown strength also improves from10to20kV/mm. Dielectric loss is nearly unchanged.The composite films were quenched at room temperature, in ice water and liquidnitrogen. The results showed that the crystal type and crystallinity of composite filmchanged significantly because of different cooling rate in the different cooling medium,thus causing different performances of the composite film. Besides, the crystallizationbehavior of PVDF/BT composite films is controlled by the temperature. It was easy tocrystallize at a faster cooling rate, and different quenching condition could affect thecomposition of crystal deeply. The proportion of β crystalline form was increasedobviously under ice water quenching.Three phase composite films were prepared through solution cast method with BT andCCTO powders added into the PVDF matrix at the same time with different mixproportion. Compared with two-phase composite films which was filled with singlepowders, their dielectric performance had a certain improvement. Among them,three-phase composite film with compound ratio of3:1showed the best comprehensiveperformance. Its dielectric constant is about12, and it still keep breakdown strength morethan150kV/mm. Small size of powders can fill into the pore between large size ofpowders, improving the filling density, and then improving the performance of thematerials.