Surface Modification Of Exfoliated Graphite And Dielectric Properties Of Exfoliated Graphite Reinforced Composites

Ceramic materials have the disadvantage of high density,fragile,hard to mold,high temperature process and low breakdown strength so that they can not meet the requirements posed by the storage device of higher performance and miniaturization.Many researches have been focused on the synthesis of high energy density polymeric composites by incorporating conductive fillers into polymer matrix.Exfoliated graphite is a novel soft mineral material.It not only has the advantage of excellent conductivity,thermal stability,chemical stability,self-lubricating property and no toxicity possessed by natural graphite but also the advantage of low density,good biological compatibility,high aspect ratio,low cost and no secondary contamination ascribed to special molding technology and microstructure of exfoliated graphite.As a result,it can be considered as an ideal conductive filler to improve the dielectric constant of the polymer.In this dissertation,morphology,structure,composition and surface chemical properties of exfoliated graphite were firstly investigated by SEM,XRD,Raman,FTIR and XPS.Exfoliated graphite/PI and exfoliated graphite/PBS nanocomposites were synthesized by in-situ polymerization and combination of solution blending and mould pressing,respectively.The relationships between microstructure of composites and preparation process and effects of exfoliated graphite volume fraction and dispersion on dielectric properties of composites were discussed.Result indicated that percolation threshold of exfoliated graphite/PI and exfoliated graphite/PBS nanocomposites were fc=3.1±0.1 vol%and fc=6.0±0.1vol%,respectively.They both exhibited low percolation threshold due to high aspect ratio of exfoliated graphite 149?296.When volume fraction of exfoliated graphite neared the percolation threshold,dielectric constant of exfoliated graphite/PI and exfoliated graphite/PBS nanocomposites were 1.8×104 and 113,respectively.The large distinction of dielectric constant between exfoliated graphite/PI and exfoliated graphite/PBS ascribed to the micro structure of composites.Compared to exfoliated graphite/PBS composites,exfoliated graphite displayed a certain orientation in the exfoliated graphite/PI composites resulting in a large number of microcapacitors.In order to prove the compound dielectric effect of exfoliated graphite,polyaniline/PBS nanocomposites were synthesized by combination of solution blending and mould pressing.By comparing the dielectric properties of exfoliated graphite/PBS and polyaniline/PBS nanocomposites,polyaniline/PBS nanocomposites exhibited high percolation threshold fc=19.7vol%due to the low aspect ratio of polyaniline about 1.However,when volume fraction of polyaniline neared the percolation threshold,dielectric constant and dielectric loss of polyaniline/PBS nanocomposites were 44 and 0.16Exfoliated graphite reinforced polymeric composites had a low percolation threshold.Dielectric constant and dielectric loss both dramatically increased when volume fraction of exfoliated graphite neared the percolation threshold.Aiming at decrease dielectric loss of composites by preventing the direct contact of exfoliated graphite flakes leading the formation of conductive network,exfoliated graphite@polyaniline/PVDF composites were prepared in which core/shell structured exfoliated graphite@polyaniline composite particles were synthesized by chemical oxidation method.When concentration of exfoliated graphite@polyaniline composite particles was 3wt%,the dielectric constant and dielectric loss were 17 and 0.06,respectively.The result indicated that dielectric constant of composites can be improved while maintaining low dielectric loss as they were reinforced by exfoliated graphite@polyaniline composite particles.The phenomenon can be explained by that insulating ployaniline on the surface of exfoliated graphite flakes can prevent direct contact of exfoliated graphite flakes and formation of conductive network in the polymer matrix.The low thermal decomposition temperature of polyaniline limited exfoliated graphite@polyaniline composite particles to reinforce polymeric composites with high processing temperature.As a result,Ti02 was selected to modify exfoliated graphite due to its excellent thermal stability and dielectric properties.Ti02/exfoliated graphite composite particles were prepared by liquid phase deposition in which Ti02 were spherical particles and irregular flakes.Because exfoliated graphite surface had rare-COOH and-OH groups,little interaction displayed between TiO2 and exfoliated graphite.Thus,TiO2/graphite composite particles were prepared by control synthesis of TiO2 on the surface of graphite oxide and subsequently reduction treatment by hydrazine hydrate.When concentration of TiO2/graphite composite particles was 4wt%,the dielectric constant and dielectric loss were 17 and 0.09,respectively.In the study of dielectric properties of TiO2/graphite/PVDF,it can be concluded that TiO2 on the surface of graphite flakes can also prevent direct contact of graphite flakes and formation of conductive network in the polymer matrix.