Preparation And Properties Of Modified Graphene/poly (Vinylidene Fluoride-co-Hexafluoropropylene) Composite Films

With the generation and consumption of energy are often being out of sync,an efficient,environmental-friendly and inexpensive energy conversion and storage system become an effective way to increase the efficiency.Dielectric capacitor,one of energy storage device,with high power density,rapid charge-discharge rate,wide operating temperature range and good stability,draws people's attention.However,it is difficult for a dielectric material to have good dielectric,mechanical,and thermal conductivity properties spontaneously.Therefore,developing a dielectric material with these three properties is of great significance for both fundamental research and application.In this work,graphene was modified by two different materials.Then the modified graphene was filled into poly(vinylidene fluoride-co-hexafluoropropylene)(P(VDF-HFP))to study the dielectric,mechanical and thermal properties of the composite.The main results were listed as follow:(1)Tridecafluorooctyltriethoxysilane-modified reduced graphene oxide(FAS-rGO)was obtained by the chemical reduction of the graphene oxide(GO),whose hydroxyl groups on the surface had reacted with hydrolytic tridecafluorooctyltriethoxysilane.Fourier transform infrared(FT-IR)and X-ray photoelectron spectroscopy(XPS)indicated that FAS was grafted on the surface of GO and the reduction of GO was complete.Transmission electron microscopy(TEM)showed that the wrinkles of rGO decorated with FAS were reduced,and most of them were single layer.The results of dielectric test revealed that the dielectric constant of FAS-rGO/P(VDF-HFP)composite reached 43 at 1 kHz and the dielectric loss was 0.27 when the volume fraction of FAS-rGO was 1.68 %,and the percolation threshold of this composite was 1.35 vol%.Correspondingly,the dielectric loss of rGO/P(VDF-HFP)system was higher than 1 at the percolation threshold,which told that the introduction of FAS could reduce the dielectric loss of the system significantly.Tensile test results showed that the tensile strength of FAS-rGO/P(VDF-HFP)was slightly lower than that of rGO/P(VDF-HFP).Because FAS destroyed the six-membered ring structure of rGO,making the strength of rGO decreased.(2)Nano-silver-paritcles(AgNPs)-coated reduced graphene oxide(Ag-rGO)was obtained by reducing the mixture of silver nitrate and GO through one-step method.FT-IR and XPS results indicated that both silver nitrate and GO were fully reduced.And x-ray diffraction(XRD)results showed that the AgNPs crystallized completely,most of them were cubic phase.TEM results displayed that the nano-silver ball,uniform size,were dispersed on the rGO sheet surface evenly.Conductivity test results showed that when the mass fraction of AgNO3 was 75 %,Ag-rGO got the best conductivity,which was 253 S/m.AgNPs could block the rGO and inhibit the aggregation of rGO.And higher conductivity of Ag-rGO promoted the improvement of dielectric properties of the composite.The results of dielectric test revealed that the dielectric constant of Ag-rGO/P(VDF-HFP)composite thin film reached 262 at 1 kHz when the volume fraction of Ag-rGO was 0.59 %,which was 20.7 times to the pure P(VDF-HFP),and the dielectric loss was about 0.24.The percolation threshold of this composite was 0.55 vol%.Tensile test results showed that the tensile strength of Ag-rGO/P(VDF-HFP)composite film was 73 MPa when the volume fraction of Ag-rGO was 0.42 %,which were 174 % of pure P(VDF-HFP).Thermal conductivity test results indicated that the introduction of nano-silver could improve the thermal conductivity of the composite film.(3)Nano-silver and FAS co-modified reduced graphene oxide(Ag-FrGO)was obtained by the reduction of the this rGO reacted with FAS,which was reducd after modified with silver nitrate.FT-IR and XPS results indicated that FAS was grafted on the surface of rGO.Moreover,silver nitrate and GO were fully reduced.TEM and scanning electron microscopy(SEM)showed that the size of silver nanoparticles was uniform and it dispersed on the rGO surface uniformly.AgNPs retained the high conductivity of rGO,and FAS grafted on the surface of rGO after hydrolysis,which enhanced the interface binding between rGO and P(VDF-HFP).The cross-sectional SEM results of the films displayed that Ag-FrGO was dispersed in P(VDF-HFP)in parallel structure evenly,which endowed Ag-FrGO/P(VDF-HFP)composite with excellent dielectric,mechanical and thermal properties.The results of dielectric test revealed that the dielectric constant of Ag-FrGO/P(VDF-HFP)composite reached 412 at 1 kHz when the volume fraction of Ag-FrGO was 0.86 %,and the dielectric loss of the composite system was 0.29.The percolation threshold of this composite was 0.87 vol%.The tensile strength of Ag-rGO/P(VDF-HFP)composite film reached 87.4 MPa when the volume fraction of Ag-FrGO was 0.46 %.And when the volume fraction of Ag-FrGO was 0.86 %,the thermal conductivity of the composite reached 1.42 W/mK,which was 6.5 times higher than that of pure P(VDF-HFP).