Preparation And Dielectric Properties Of Poly(Vinylidene Fluoride) Composites Filled With Novel Nanocarbon Materials

Energy storage capacitor with high energy density,which allows adequate electrical energy to be stored and released under controlled conditions,has attracted significant attention for their application in many fields,such as portable electronics,hybrid electric vehicles,and pulse power applications.According to the capacitor theory,the energy density of capacitors is mainly dependent on the corresponding dielectric materials.Recently,polymer-based composites filled with carbon materials,such as carbon fibers(CFs),multi-walled carbon nanotubes(MWCNTs),and graphene,has become a research focus for dielectric materials.However,the aggregation and direct connection of carbon materials and their weak interfacial bonding with polymer matrix impede the formation of homogeneous composites,challenging to the enhancement of dielectric properties of such systems.In this thesis,novel amorphous carbon coated MWCNT(MWCNT@AC)core/shell nanohybrids and carbonized polyacrylonitrile/polyethylene glycol copolymer fibers(CPCFs)were fabricated by hydrothermal method and electrospinning method,respectively.Both of them were added into poly(vinylidene fluoride)(PVDF)to prepare composites.The electric and dielectric properties of these composites were investigated systematically.(1)Novel core/shell structured MWCNT@AC nanohybrids were fabricated by a hydrothermal method.The TEM images revealed that the MWCNTs were coated by an ultrathin amorphous carbon shell and the thickness of the layer is below than 2 nm.The FT-IR spectroscopy indicated that there were no new peaks appearing in the spectrum of the MWCNT@AC nanohybrids compared with pristine MWCNTs while the dispersibility of MWCNT@AC nanohybrids in solvent was enhanced significantly.This can be attributed to the isolation effect of the amorphous carbon shell and is beneficial to the formation of homogeneous composites.(2)MWCNT@AC/PVDF composites and MWCNT/PVDF composites with different filler loadings were produced by simple blending and subsequent molding.Compared with MWCNT/PVDF composites,the percolation threshold of MWCNT@AC/PVDF composites was increased from 1.01 vol% to 8.53 vol%,which arises from the insolation effect of amorphous carbon.As a result,the conductivity of MWCNT@AC/PVDF composites was far less than MWCNT/PVDF composites in the same content.On the other hand,a gigantic enhancement in the dielectric properties was achieved in MWCNT@AC/PVDF composites.The largest dielectric constant of the MWCNT@AC/PVDF composites could reach 5910 when the dielectric loss was about 2,which is about 5 times higher than that of the MWCNT/PVDF composites(ε = 1040).(3)Novel CPCFs were fabricated by a electrospinning method and then used them as fillers to enhance the dielectric properties of PVDF-based composites.These CPCFs are rich in nitrogen(8.55 %)and oxygen(3.94 %)atoms on the surface of them.The results of molecular dynamic(MD)simulations indicate that the existence of these atoms significantly increase the interaction energy between CPCFs and PVDF matrix from-45.13 Kcal/mol to-62.22 Kcal/mol,which promotes the intercalation of conductive CPCFs into insulated PVDF matrix to form ultrathin microcapacitors.As a result,the largest dielectric constant of CPCFs/PVDF composites can reach 1583(1 kHz),which is about 150 times higher than that of pure PVDF.