Tunable Negative Petmittivity And Mechanism Of Graphene Composite Ceramics

In recent years,the composite materials with negative permittivity have attracted extensive attention.It can be obtained by introducing conductive functional phase into the insulating matrix,and the negative permittivity is sensitive to the composition and microstructure of the composite material,so that the numerical value and frequency dispersion characteristics of the negative permittivity can be effectively regulated and designed by adjusting the phase composition and microstructure of the composite materials.The unique and novel negative permittivity properties make these kind of composites widely used in wireless communication,sensor,electromagnetic shielding and microwave absorption.Based on alumina(Al2O3)and aluminum nitride(AIN)ceramics for insulating substrate,with graphene(GR)and reduced graphene oxide(RGO)as(?)conductive function,using dry grinding and freeze-drying preparation of composite powders,the graphene/alumina(GR/Al2O3)composite ceramics,graphene/aluminum nitride(GR/AIN)composite ceramics and reduced graphene oxide/alumina(RGO/Al2O3)composite ceramics were fabricated by the spark plasma sintering(SPS).And the negative permittivity were obtained in all the composite ceramics.The negative permittivity of composite ceramics can be controlled by adjusting the phase composition,microstructure and distribution of conductive functional phase.The field emission scanning electron microscope(SEM),Raman spectroscopy and X-ray diffraction analyzer were used to characterize the microstructures and phase compositions of the composite ceramics,the apparent porosity and relative density of composite ceramics were tested and analyzed by the Archimedes' method.The specific heat capacity and thee the thermal diffusivity of the composite ceramics were tested at room temperature by the Laser Thermal Conductivity Analyzer.To test and investigate the dielectric properties(real permittivity,imaginary permittivity,dielectric loss tangent,ac conductivity,impedance)of the composite ceramics,the Precision Impedance Analyzer was used.The Zsimpwin impedance analysis software was used to fit the equivalent circuit of composite ceramics,and the generation mechanism and regulation mechanism of negative permittivity of the graphene composite ceramics were studied and analyzed by combining percolation theory and equivalent circuit fitting.The main research contents and results are as follows:(1)The GR and Al2O3 composite powders were obtained by dry grinding,and GR/Al2O3 composite ceramics were prepared by spark plasma sintering.With the increase of GR content,the three-dimensional conductive network was gradually formed in the GR/Al2O3 composite ceramics.When the GR content exceeded 15.38wt%,a negative permittivity phenomenon occurred.The real permittivity of GR15.38/Al2O3 and GR18.46/Al2O3 composite ceramics perform the transition from negative to positive at around 40MHz and 70MHz respectively,which is caused by LC resonance.At the same time,the maximum dielectric loss tangents of GR15.38/Al2O3 and GR,8.46/Al2O3 composite ceramics were found near 40MHz and 70MHz,respectively.(2)The GR/AlN composite ceramics were prepared by spark plasma sintering process.The percolation threshold of thermal conductivity was 19.27wt%,and that of ac conductivity was 19.03wt%according to the percolation theory fitting.When the content of GR reached 19.50wt%,the negative permittivity of GR19.50/AIN composite ceramics appeared during the test range from 10MHz to 1 GHz,and the conductive GR network was formed in the GR19.50/AIN composite ceramics.When the content of GR reached 19.50wt%,that is,when the percolation threshold of ac conductivity was exceeded,the reactance of GR/AIN composite ceramics changed from negative value to positive value,realizing the capacitive-inductive transformation,which corresponds to the appearance of negative permittivity.(3)The composite powders of GO and Al2O3 were prepared by freeze-drying method to improve the dispersion effect of GO in Al2O3 powders.RGO/Al2O3 composite ceramics were obtained by spark plasma sintering,and the negative permittivity was obtained.The conductivity percolation was achieved when the conductive functional phase content of RGO/Al2O3 composite ceramics increased from 13.50wt%to 15.38wt%.According to the percolation theory fitting,the percolation threshold of ac conductivity was 14.31wt%.When the amount of GO in the initial powder exceeds 11.54wt%,the negative permittivity appeared during the test range of 10MHz to 1GHz of RGO/Al2O3 composite ceramics.The real permittivity of RGO13 50/Al2O3 composite ceramics and RGO15.38/Al2O3 composite ceramics realized the conversion from negative to positive at 501 MHz and 299MHz,respectively.Compared with dry grinding,the application of freeze-drying method makes it easier to obtain negative permittivity at relatively low conductive functional phase content.