Preparation And Study Of Conventional Graphene Composite Ceramics With Tunable Negative Permittivity

Since the 21 st century,metamaterials have been more and more popular with many scientific researchers,especially conventional metacomposites.Because of their unique physical properties,flexibility,and controllable preparation methods,they have great potential applications in wireless communication transmission,capacitors,electromagnetic absorption and shielding,and other fields.In this paper,metacomposites with negative dielectric properties have been successfully prepared by using insulating alumina(Al2O3)and semiconductor silicon carbide(SiC)as the matrix,and reduced graphene oxide(RGO),graphene(GR)and carbon-iron(C-Fe)as the functional phases.The GO-Al2O3 composite powder was obtained by freeze-drying,and the RGO-alumina composite ceramic(RGO-Al2O3)was prepared by spark plasma sintering(SPS);The GR-silicon carbide composite ceramic(GR-SiC)was prepared by mechanical ball milling and SPS sintering;The alumina composite ceramic loading with C-Fe(C-Fe-Al2O3)was prepared by dipping and reduction process.By adjusting the preparation conditions(sintering temperature and pressure),functional phase content,microstructure and dispersion of functional phase,the negative dielectric properties can be controlled.Field emission scanning electron microscopy(FESEM),Raman spectroscopy(Raman)and X-ray photoelectron spectroscopy(XPS)were used to analyze the microstructure and composition of the material.E4991A impedance analyzer was used to characterize the dielectric properties of the materials,including real and image permittivity(?'and ?"),AC conductivity(aac),impedance(z)and dielectric loss(tan?).The negative dielectric mechanism of the materials was studied by Lorentz and Drude models.The details are as follows:(1)Freeze-drying technology was used to obtain the conventional metacomposites with uniform distribution of RGO.The technology improved the dispersion of RGO in Al2O3 ceramic matrix,which made negative permittivity appear when the content of RGO was only 8.91 vol%and reduced the percolation value of functional phase.In addition,when the content of graphene oxide was 8.91 and 15.01 vol%,the dielectric resonance between the matrix phase and functional phase produced interference superposition at the frequency of about 200 MHZ,resulting in an asymmetrical Fano resonance phenomenon.When Fano resonance occurred,its dielectric loss would also realize the superposition of two forms of loss.The influence of sintering conditions on dielectric properties is not a simple linear relationship.The sintering temperature can regulate the permittivity value by changing the "surface effect" and the content of the micro-capacitance inside the material,and the resonance frequency can be regulated by changing the comprehensive effect(LC)of the equivalent capacitance and the equivalent inductance inside the material;the sintering pressure can control the permittivity by changing the distance between the micro electric containers,as well as by the comprehensive·influence on(LC)to realize the regulation of resonance frequency.(2)With silicon carbide(SiC)as the matrix and GR as the conductive functional phase,GR-SiC composite ceramics with different GR content were prepared by SPS,and negative permittivity was achieved by adjusting the content of GR.When the GR content was more than 6.30wt%,the AC conductivity ?ac appeared obvious electroosmotic phenomenon.It was speculated that the electric percolation value of GR was 6.30?10.81 wt%.In addition,with the increase of GR content,the natural resonance of SiC decreased obviously due to the mutual dielectric shielding between SiC and GR.The content continued to increase,the resonance gradually disappeared and transformed into Lorentz model.So,the addition of GR can affect and adjust the special dielectric properties of SiC.(3)Alumina ceramics with different contents of carbon-iron(C-Fe-Al2O3)were prepared by impregnation and sintering process using sucrose and ferric chloride(FeC13)as carbon and iron sources respectively.The quality increment of(C-Fe)in composite ceramics was controlled by controlling the number of impregnation sintering.During the sintering process,iron oxide(main Fe2O3 and Fe3O4)and Iron carbon compound were formed from high valent iron.Amorphous carbon is favorable for graphitization to form GR structure under the catalysis of Iron element.In addition,under high temperature,iron oxide reacted with carbon to form iron element.As the conductive functional phase,the synergistic effect of zero dimensional Fe and two-dimensional graphitized carbon film can not only improve the concentration of effective electrons but also improve the transmission efficiency of electrons,so that the permittivity of C-Fe-Al2O3 composite ceramics changed from positive to negative at 700 MHz when the content of C-Fe was only 1.58wt%.When the content of C-Fe was 5.99wt%,the permittivity was all negative in the whole test band.The permittivity curve changed from Lorentz model to Drude model with further increase of the content of C-Fe.