Study On Preparation And Property Of Poly(Vinlidene Fluoride) Based Composites With High Dielectric Constant

Electronics industry is progressing toward higher functionality. And the number of passive component of circuit board grows steadily, which hinders the development of electronic devices. The most effective way to solve the obstacle is the application of embedded passive component. However, embedded passive technology has not been commercialized for electronic packages yet due to materials and process issues. Therefore, it is necessary to develop materials that satisfy the requirements of fabrication as well as electrical and mechanical performances to enable embedded passive technology to be commercialized. With the advantages of good mechanical and dielectric performance, a great deal of efforts has been devoted to polymer based-composites. In the thesis, by employing semicrystalline poly(vinylidene fluoride) as the polymer host, BFN powders as the ceramic filler, and Ni powders as the conductive filler, a series of PVDF based composites are prepared and investigated. Our aim is to fabricate a novel polymer based composites with high dielectric permittivity and low dielectric loss. The main contents of the thesis are as follows:The micro-sized BFN particles (μ-BFN) were prepared by conventional solid-state method,μ-BFN/PVDF composites with μ-BFN content ranging from 0% to 60% by volume were fabricated and investigated. The dielectric constant and dielectric loss of the μ-BFN/PVDF composites are found to increase with the increase in volume fraction of μ-BFN filler. The highest value of dielectric constant of the composites as high as 57 at 100 Hz is achieved, which is nearly six times higher than that of pure PVDF. The introduction of μ-BFN powders could obviously enhance the dielectric constant of PVDF matrix and maintain the low value of dielectric loss, which provides a novel ceramic filler of PVDF based composites.The nano-sized BFN particles (n-BFN) were prepared by oxalate coprecipitation method, n-BFN/PVDF composites with enhanced dielectric permittivity were prepared by a physical mixing and a cold press technique. As expected, the effective dielectric permittivity increases with increasing concentrations of n-BFN particles at all frequency. The n-BFN/PVDF composites with 60 vol.% n-BFN loading exhibit enhanced dielectric permittivity～93, which is nine times higher than the value of dielectric permittivity of pure PVDF. Among various models, the large enhancement in dielectric permittivity matches well with theoretical values by the Yamada model. Accordingly, the dielectric properties of PVDF based composites can be enhanced by n-BFN powders. The microstructure and dielectric properties of μ-BFN/PVDF and n-BFN/PVDF composites have been characterized and investigated comparatively. It is found that the n-BFN/PVDF composites show much higher values of permittivity than μ-BFN/PVDF composites in all concentrations, which is due to the more interfacial polarization effect causing by larger interfacial areas in the composites containing nanometer scale BFN fillers. These results demonstrate that decreasing the size of fillers is an effective way to improve the dielectric properties of polymer based composites.According to percolation theory, Ni/PVDF composites with various volume fractions of Ni powders were prepared and investigated. When the volume fraction of Ni is 25 vol.%, the dielectric permittivity and electrical conductivity of the composites significantly increase due to the form of conductive pathway. For 25 vol.% Ni/PVDF composite, the dielectric constant is 301, however, the dielectric loss increases to 5.2. To further decrease the dielectric loss of Ni/PVDF composites, by employing the n-BFN as the third phase, n-BFN/Ni/PVDF three-phase composites were prepared and investigated. The 50 vol.% n-BFN/25 vol.% Ni/PVDF composites exhibit significantly enhanced dielectric permittivity (-475) and relatively low loss (-0.61). The results indicate that n-BFN/Ni/PVDF composites possess better dielectric property compared with Ni/PVDF composites. The introduction of n-BFN powders not only further improves the dielectric constant of Ni/PVDF composite, but also decreases the dielectric loss of Ni/PVDF composites.