Mechanism Of Tunable Negative Permittivity Behavior In Conductor-Insulator Percolative Composites

Materials with negative permittivity have shown good application prospects in electromagnetic cloaking and shielding,sub-wavelength imaging,filters,microwave absorption,sensors and new antennas.Negative permittivity can be achieved by artificial array elements with periodic structure as electromagnetic medium,or by percolation composites with random distribution of functional bodies.The latter has the advantages of simple process,easy regulation,low cost and high application frequency band.In this paper,metal-resin composites,conductive ceramic-nickel oxide composites,conductive ceramic-polymer composites,carbon-nickel oxide composites and carbon-calcium titanate composites were prepared by mechanical moulding and traditional ceramic preparation processes.The functional body was modified by chemical method,and the negative dielectric constant was regulated by different preparation techniques compared with different insulating substrates.XRD,SEM,FI-IR,TEM,TG-DSC,Raman and other means were used to characterize the microstructures and compositions of the composites.The electrical and dielectric properties of the composites were characterized by Agilent impedance analyzer.The equivalent electrical properties were analyzed by Zsimpwin software.The mechanism of negative dielectric generation is explored by means of percolation model,free electron model(Drude model),electric dipole model(Lorentz model)and equivalent circuit model.Specific research contents are as follows:(1)Iron(Fe)and epoxy resin(Epoxy)are used as raw materials.Firstly,SiO2 coated insulating iron powder(Coated-Fe)was prepared by optimum hydrolysis of TEOS.Then Fe/Epoxy,Fe/Coated-Fe/Epoxy and Coated-Fe/Epoxy composites were prepared.In Fe/Epoxy composites,Drude-type negative dielectric constant spectra can be observed at radio frequency band(10MHz-1GHz)when Fe content reaches 55 vol%.The total iron content is controlled at 60 vol%.The negative dielectric constant can be controlled by adj usting the ratio of Fe to Coated-Fe in Fe/Coated-Fe/Epoxy composites.At the same time,it is found that the permeability spectra do not change with the ratio of Fe to Coated-Fe.When the content of Fe is low,the hopping conductance is dominant for electrons in the composites,and when the content of Fe is higher than the percolation threshold,the metal-like conductance is dominant.(2)TiN/NiO composites,Ti3SiC2/PVDF composites and Ti3SiC2/PMMA composites were prepared from TiN.NiO,Ti3SiC2 MAX phase,PMMA and PVDF.When the content of conductive functional body is lower than the percolation threshold,all the composites exhibit hopping conductance behavior,which conforms to Jonscher's power law.When the content is higher than the percolation threshold,the composites exhibit metal-like conductance and show skin effect.When TiN reaches 50 wt%,TiN/NiO composites exhibit negative permittivity at 690 MHz-1 GHz band.If TiN content continues to increase,the permittivity will be negative at 20 MHz-1 GHz.Ti3SiC2/PVDF composites exhibit negative permittivity when Ti3SiC2 content is 85 wt%,while Ti3SiC2/PMMA exhibits negative permittivity when Ti3SiC2 content is 60 wt%,and the absolute value of permittivity is small.(3)CNTs/N-CFs/NiO three-phase composites,GR/CCTO composites and CNTs-CB/CCTO three-phase composites were prepared from carbon nanotubes(CNTs),carbon fibers(N-CFs),carbon black(CBs),graphene(GR),nickel oxide(NiO)and calcium copper titanate(CCTO).When the content of CNTs reaches 4 wt%.all CNTs/NiO composites exhibit stable weak negative permittivity properties at 20 MHz-1 GHz.When the content of CNTs continues to increase to 6 wt%,the negative permittivity spectra can be well fitted by Drude model.When the content of N-CFs is 12 wt%and CNTs is 4 wt%,CNTs/N-CFs/NiO composites exhibit weak negative permittivity properties in 430 MHz-1 GHz band.When GR content is 8 wt%,the negative permittivity properties of GR/CCTO composites appear in 350 MHz-1 GHz band.When GR content continues to increase,the negative permittivity spectra first decrease and then increase.Lorentz+Drude model is used to calculate and fit the results.When GR content is lower than percolation threshold,the composite material is capacitive,and when GR content is higher than percolation threshold,it is inductive.CNTs-CBs were fabricated by one-step electrostatic self-assembly.When CNTs-CBs were increased to 22 wt%,CNTs-CB/CCTO composites exhibited stable negative permittivity constant spectra at 20 MHz-1 GHz.