Preparation And Property Of The Epsilon-Negative Materials

Dielectric constant(permittivity)is the basic parameter for electromagnetic materials.The dielectric materials can be applied in different fields,where the requirements about the dielectric property of these materials are different.When the dielectric materials are used in capacitors,high permittivity and low dielectric loss are required.Microwave absorbers needs to have low permittivity but high dielectric loss,while the wave-transmitting materials have low permittivity as well as low dielectric loss.When the dielectric materials work as microwave medium.high-permittivity materials and low-permittivity materials can both have contributions,as long as the dilectric loss is low.Therefore,controlling the dielectric property has great significance in electromagnetic materials.Generally,the dielectric constant is positive for conventional dielctric materials.In recent years,the negative permittivity can also be achieved.The demonstration of the negative permittivity can make the design of electromagnetic property become more flexible,the negative permittivity materials gradually become a research hotspot.Negative permittivity can be realized in metamaterials with the periodic arrays structure and be obtained by the plasma in some materials without obvious periodic structure.With the advantange of the diversity of component materials,percolative negative permittivity materials can not only provide more flexible method for the design and regulation of electromagnetic properties and thus facilitate the multi-function of negative dielectric materials,but also promote the development of metamaterials by providing the optimal design according to the material science.As a result,more attention is paid to the percolative negative permittivity materials.However,there are still many scientific problems in the research of the percolative negative permittivity materials.The influence of the chemical composition and geometrical structure of the functional fillers on the negative permittivity is still not clear.The microstructural characteristics of the negative permittivity materials have not been proposed.It is necessary to establish the relationship between negative permittivity and carrier parameters.The negative permeability in negative permittivity materials can not be explained well.Novel negative permittivity materials need to be exploited.In this paper,the percolative composites with different compositions and microstructures are fabricated,and different theoretical models are used to investigate the mechanism of negative permittivity,the main strudy results are as follows:(1)Percolative composites with metallic fillers,carbon nanotubes and MXenes are prepared by the pressure forming.The percolative network is gradually formed with increasing the content of functional fillers,as a result,the percolative phenomenon is observed.When the filler content is below percolation threshold,the dielctric constant is positive,the conductivity mechanism is hopping conductivity.When the filler content is above the percolation threshold,the dielectric constant is negative,the conductivity mechanism is conductor-type conductivity with obvious skin effect.The chemical composition and aspect ratio of fillers can influence the percolation threshold,and thus influence the negative permittivity behavior.The Ni/MnO nanocomposites are prepared by the bio-gel derived strategy,when the nickel content is 60%,nickel nanoparticles are isolated with each other,however,the negative permittivity is achieved by the tunnelling effect according to the first principles calculation.(2)Epoxy,Fe and coated Fe powder are used to construct the ternary percolative composites.The distribution of fillers is adjusted by controlling the ratio of isolated metallic particles and metallic network,where the frequency dispersion of negative permittivity is investigated.According to the Drude model,Lorentz model and equivalent circuit analysis,negative permittivity behavior is attributed to the plasma effect,the percolative network is the basic microstructure with the inductive character,while the isolated fillers have capacitive character and adjust the frequency dispersion of negative permittivity by LC resonance.(3)The graphene is chosen as fillers to construct the percolative negative pennittivity materials,their carrier concentration and mobility are measured by the Hall effect.When the carrier concentration and mobility are both low,epsilon-near-zero property and weakly negative permittivity with low frequency dispersion are achieved.When the carrier concentration and mobility are relatively high,Drude-type negative permittivity can be obtained.Negative permittivity can be adjusted by changing the temperature due to the change of carrier concentration and mobility.(4)The conductive polyaniline and the ferromagnetic yttrium iron garnet areused to construct the negative permittivity materials.Negative permittivity is attributed to the plasma effect in the polyaniline.Negative permeability can be obtained by the domain wall resonance and spin resonance.The Ag/SiO2 microsphere composites are prepared by the self-assembly and template method.Since there is no magnetic material in the composites,the negative permeability comes from the magnetic response of induced currents,the model and equation of this kind of magnetic response are proposed in this work,the negative permeability has the linear relation with the f0.5(where f is the frequency of magnetic field).(5)The negative permittivity is precisely controlled and novel negativepermittivity materials are prepared according to the mechanism of negative penmittivity and the characteric microstructure of negative permittivity materials.The C/SiO2 microsphere composites are prepared by the self-assembly and template method,in which the negative permittivity is no longer sensitive to the change of the microstructure.Moreover,carbon aerogels are prepared via sol-gel polymerization,negative permittivity and negative permeability are simultaneously realized in the carbon aerogels by designing the microporous and mesoporous structures,which is the lightest negative permittivity materials reported.