Preparation And Mechanism Of Graphene/Resin Composites With Negative Permittivity

In recent years,epsilon negative materials(ENMs)which are used to describe materials with negative permittivity have attracted a lot of concern of researchers.Due to the unique negative dielectric properties,ENMs show potential applications in the field of superconductors,filters,capacitors,electromagnetic shielding and absorption.In this work,resin was selected as a insulating matrix and graphene(GR)was used as conductive nanofillers to prepare GR-resin composites by using the mechanical pressing method,to obtain the negative permittivity in radio frequency.Tunable negative permittivity was achieved by adjusting microstructures and compositions in composites.The microstructures and compositions of the composites were tested and analyzed by the field emission scanning electron microscope,X-ray diffraction and X-ray photoelectron spectroscopy.Dielectric properties including alternating current conductivity(σac),impedance(Z),dielectric permittivity(ε’ and ε")and dielectric loss tangent(tanδ)were investigated by using the impedance analyzer,and Zsimpwin impedance analysis software was used to fit equivalent circuits inside the composites.The mechanism of n’gative permittivity was studied by combining the percolation theory,Lorentz model,Drude model and the equivalent circuits model.The research contents and results were as follows.(1)Using phenolic resin(PR)as the matrix and GR as conductive nanofillers,GR-PR composites were prepared by the mechanical pressing method,to obtain tunable and weakly negative permittivity.When the GR content increased from 8 to 13 vol%,a significant increase of σac was observed,indicating a percolation phenomenon.It was found that the negative permittivity appeared when the GR content exceeded 13 vol%,resulting from the formation of three-dimensional(3D)interconnected GR network.Moreover,the negative permittivity was easily adjusted by controlling the GR content.Compared with the reported metal ENMs,absolute values of negative permittivity were low,which was in favor of impedance matching.For the GR content of 13 vol%,high dielectric loss above 25 over the whole frequency was also observed.(2)Carbon nanotubes-graphene-phenolic resin(CNTs-GR-PR)composites were prepared to investigate the effects of CNTs on negative dielectric properties of GR-PR composites.The results showed that microstructures were easily adjusted by the mass ratio of CNTs to GR and total carbon content,which had an impact on the negative dielectric properties of the composites.The negative permittivity from the CNT1.6-GR5-PR was observed with the total carbon content of 6.6 wt%,but the permittivity of the GR6.6-PR still remained positive,indicating that the addition of CNTs was beneficial to the appearance of negative permittivity.When the total carbon content exceeded 6.6 wt%,the addition of appropriate amount of CNTs(the mass ratio of CNTs to GR is 0.33:1 and l:1)improved the σac and ε"(3)GR-acrylic polyurethane(APU)composites with low percolation threshold and tunable negative permittivity were prepared by coating and pressing method.A percolation phenomenon from σac was observed when the GR content was increased from 0.9 to 6 vol%,and the percolation threshold was 1.8 vol%.The percolation threshold was obviously lower than that from GR-PR composites,which was possibly attributed to the well dispersion of GR in APU.As a result,the negative permittivity was obtained from the much lower GR content(only 3 vol%).