The Relationship Between Microstructure Control And Dielectric Properties Of Multi-Walled Carbon Nanotube/Polyvin-Ylidene Fluoride Nanocomposites With High Dielectric Constant

High-dielectric-constant polymer-matrix composites play a very important role in both electrical and electronic fields. The development of this kind of composites will reduce the size of electronic devices and improve their efficiencies. However, the polymer matrix usually exhibits relatively low dielectric constant. To obtain the high dielectric constant, polymer matrices need be incorporated by other fillers such as ceramic particles and conductive fillers, among which the carbon nanotube is considered as ideal filler due to its large aspect ratio and high electrical conductivity.In this thesis, to obtain the dielectric composite, multi-walled carbon nanotube (MWNT) and the ferroelectric polyvinylidene fluoride (PVDF) were selected as the conductive filler and polymer host respectively. The preparation process and relationship between microstructures and dielectric properties were discussed.Firstly, five approaches (Solution Method, Hot Stage Heating Method, Flocculation Method, Grinding Methods and Extrusion Method) were adopted to fabricate the MWNT/PVDF composites. The results showed that the composites prepared by solution method exhibited the lowest percolation threshold (0.0077) ascribed to the special sheet structure. However, the composites fabricated by extruder reached the high percolation threshold (0.072) due to the realization of good dispersion of MWNT.For the percolation composites, the dielectric properties were influenced significantly by the shape and size of conductive fillers. Thereby, the effect of aspect ratio of MWNT on the percolation threshold and critical exponent was also studied. The result showed that the change of percolation threshold displayed the same rule with that of critical exponent, and both of them increased with increase of AR in some extent. Meanwhile, the composites with larger AR possessed higher dielectric constant.Thirdly, to explore the effect of strain on dielectric properties, MWNT/PVDF composites were stretched to different lengths of tensile strain with and without thermal treatment. Second order orientation tensor was used to describe the Carbon nanotube orientation. Meanwhile, impedance analysis was done by giving equivalent circuit to simulate the physical response of the system.Fourthly, to reduce the dielectric loss and enhance further the dielectric constant of MWNT/PVDF composites, the third phase (ceramic particle) was incorporated into the composites. The results showed that the addition of barium titanate would be beneficial for the formation of conductive network of MWNT in PVDF matrix, but relatively high loading of barium titanate would destroy the formed network and accompany with the high dielectric constant and low dielectric loss.At last, to ameliorate the interface between MWNT filler and PVDF matrix, the MWNT was modified by KH550, NaDDBS, PVP and PVA respectively. And the FTIR, Raman, TG and DTA measurement were also characterized on the modified MWNTs. After that, the effect of modification of MWNT on the dielectric and thermal properties was studied. The results showed that for the system with good dispersion of MWNTs, the dielectric constant and thermal conductivity could increase abruptly at the same concentration range of MWNT.