The Investigation Of Dielectric Properties Of Ceramic-polymer Composites For Energy Storage

The development of high-technology fields requires building new generational dielectric matericals with both high dielectric properties and good processing performance. As we know, ceramic and polymer are two popular dielectric materials. The former has a high dielectric constant and low loss while the latter possesses high breakdown strength and good plasticity. Based on the synergy principle, the composites constructed by using ceramic as filler and polymer as matrix can achevive relative high dielectric properties and at the same time, retain the good processing performance. This paper has mainly concentrated on the investigation of the microstructure and dielectric behaviour of BaTiO3-polyvinylidene fluoride（BT-PVDF） composites. The influence factors of dielectric properties have been analyzed with respect to size dependent polarization and interface compatibility. By tailoring the structure and processing procedure, the dielectric properties of composites have been improved greatly. In addition, another new type composite, FeTiNbO₆-polyvinylidene fluoride（FTN-PVDF）, has been prepared by using giant dielectric material FTN as filler, and the dependence of dielectric properties on filling content has been studied.BT-PVDF is the extensive reported composite material for application in modern electric devices. However, there still exists some obstacles prohibiting the further improvement of dielectric performance, such as poor interfacial compatibility and low dielectric constant. Therefore, in depth study of the size dependent polarization and surface modification of BT particle is of technological importance in developing high performance BT/PVDF composites. Here, a facile molten-salt synthetic method has been applied to prepare different grain sized BT particles through tailoring the calcination temperature. The size dependent spontaneous polarization of BT particle was thoroughly investigated by theoretical calculation based on powder X-ray diffraction Rietveld refinement data. The results revealed that 600 nm sized BT particles possess the strong polarization, ascribing to the ferroelectric size effect. Furthermore, the surface of optimal BT particles has been modified by water-soluble polyvinylprrolidone（PVP） agent, and the coated particles exhibited fine core-shell structure and homogeneous dispersion in the PVDF matrix. The dielectric constant of the resulted composites increased significantly, compared with the literature values of BT-PVDF at the same concentration of filler. Moreover, the energy storage density of the composites with tailored structure was largely enhanced at the low electric field.On the other hand, it is reported that compared to particle shaped ferroelectric filler, one-dimensional fillers with the same composition are more conductive to the improvement of dielectric properties. Because of good plasticity of the polymer matrix, one-dimensional ferroelectric material can realize the orientated arrangement in composites, resulting in high polarization under an electric field. However, with respect to the particle shaped filler, the synthesis of the one-dimensional ferroelectric filler is more complex. Therefore, it is important to find a simple and feasible method that can prepare one-dimensional ferroelectric material in a large quantity. In our work, based on chemical principle of structure transformation, one-dimensional BT materials have been successfully prepared by molten salt method and the related topological crystal growth mechanism has been proposed. Moreover, the BT fillers with different morphology have been added into PVDF matrix for comparison, and the dielectric and storage properties of corresponding composites have been investigated with respect to microstructure evolution.FeTiNbO₆（denoted as FTN） is a recently discovered giant dielectric material with high Curie temperature（550 K）. In the subsequent work, well dispersive FTN particles with mean particle size about 500 nm were synthesized by the improved solid-state reaction method. Using FTN particles as fillers and PVDF polymer as matrix, homogeneous ceramic/polymer composites have been prepared through a hot-pressing technology. At the same filling content, the FTN-PVDF composites display higher dielectric permittivity compared to the normal reported BT-PVDF composites. Through the theoretical analysis correlated with structure evolution observation, it is deduced that the high dielectric performance originates from the combined effect of a variety of factors, such as giant dielectric semiconducting FTN particles, the micro-capacitor structure and percolation phenomenon at appropriate volume fraction of FTN.In summary, the structure design and the related dielectric theoretical analysis in this thesis show the good refereeing value to developing new generational composite materials, and the proposed BT-PVDF and FTN-PVDF systems are potentional in application as embedded capacitor materials.