Dielectric properties of carbon nanotube-BaTiO₃ hybrids reinforced PVDF composites

The dispersibility of carbon nanotube (CNT) in polyvinylidene fluoride (PVDF) is always a big challenge for the high dielectric property. Barium titanate (BT)-CNT hybrids with the special structure are proved to be effective for improving the dispersion of CNT in the polymer matrix and reduce the percolation threshold of the composite. This thesis aims to achieve high dielectric performance of composites via designing fillers with the favorable structure as well as comprehensively study the interaction between CNT and semi-crystalline polymer matrix.In chapter 1, we provide a general introduction about dielectric material's background knowledge. Meanwhile the development including recent breakthroughs and their applications for the dielectric field are also provided in this chapter.In chapter 2, we prepare two hybrids with different structures. The first hybrids are prepared by chemical vapor deposition (CVD) method. It is with the structure of BT as a core and CNTs growing outsides (H-CNT-BT). The second hybrids are prepared by hydrothermal reaction where BT particles coats outside CNT (H-BT-CNT). Meanwhile, we fabricate hybrids reinforced PVDF matrix composites by solution casting plus extrusion-injection way. Additionally, methods for characterization involving morphology, thermal and dielectric properties as well as crystallization are also introduced in this chapter.In chapter 3, the dielectric behaviors of H-CNT-BT/PVDF are studied concretely. A dramatic increment on dielectric permittivity is observed after the thermal treatment. This change may result from the reformation of CNT's conductive network and the behavior of PVDF's re-crystallization. By modeling work and experimental characterization, the shrinkage of the neighboring CNT's distance in PVDF's amorphous layers and the induced β polymorph at the CNT-PVDF interface may cause the significant increment in dielectric permittivity after the thermal treatment.In chapter 4, the CNT's dispersibility in PVDF matrix composites is studied by designing different structures. Firstly, a comparison between calculated and experimental percolation threshold of H-CNT-BT/PVDF is conducted for studying the morphology parameters of H-CNT-BT. Afterwards, two comparisons are conducted: one is between H-CNT-BT/PVDF and CNT/PVDF. The processing factors for the CNT's dispersibility are discussed via measuring the different layer's AC conductivity. The other is among three hybrids reinforced PVDF composites. The hybrids structure's effect the CNT's dispersibility is discussed via comparing the dielectric property of the composites with the same volume fraction of CNT and BT but different structures.In chapter 5, a general conclusion is formed according to the works and the perspective is provided for the improvement of the future work.