In-situ Preparation And Double Negative Property Of Heterogeneous Composites

When permittivity and permeability are simultaneously negative values in electromagnetic media,they are termed as double negative materials?DNMs?,which possess novel electromagnetic properties,such as reversed Doppler effect,reversed Vavilov-Cherenkov effect and negative refraction index,etc.DNMs have promising applications in the field of subwavelength imaging,invisible cloak,perfect lens and microstrip antenna,etc.Actually,the negative electromagnetic parameters were initially achieved in artificial media,i.e.,metamaterials,which composed of periodic structure units.The tunable electromagnetic properties were controlled by adjusting the shape,size and orientation of unit rather than the chemical composition and microstructure in materials.Meanwhile,the development and applications of metamaterials were restricted in the field of engineering appilactions due to the expensive cost,complicated manufacture and narrow bandwidth.In recent years,it has become a new research hotspot to fabricate random metacomposites based on intrinsic properties of materials.It can provide significant research theory and technique support for metamaterials via designing and tailoring the compositon and microstructure.Metallic materials are regarded as excellent conductive materials,which can result in negative permittivity owing to their plasma oscillation.On the other hand,ferrimagnetic yttrium iron garnet can realize negative permeability by taking advantage of domain-wall resonance or nature resonance.Hence,in this investigation,the tunable negative permittivity and negative permeability were achieved by adjusting the composition and microstructure of metal/ceramic composites,which were fabricated by in-situ synthesis process.Moreover,the preparation and electromagnetic property of flexible metacomposites were investigated in this paper to expand the scope of metamaterials.The main research content and the experimental results were as follows.?1?Fe₂O₃ and Al₂O₃ were selected as raw materials,the alumina-matrix metal/ceramic composites?Fe-Fe3O4-Al₂O₃ multiphases composites and Fe/Al₂O₃ composites?were fabricated by presureless sintering and hydrogen reduction process.Additionally,the effects of reduced temperatures on the phase compositions,microstructures and electromagnetic property were investigated.The experimental results were shown that Fe₂O₃ was decomposed into Fe and Fe3O4,when reduced temperature was below 700 ?.When reduced temperature reached to 700 ?,Fe₂O₃ was totally reduced into Fe.It was demonstrated that the conductive mechanism of Fe/Al₂O₃ composites with lower Fe content was dominated by electrons hopping conduction,conforming to power law.When conductive fillers reached up tp percolation threshold,electrical conduction played a primary role in conduction and the resultant composites exhibited skin effect.When Fe content was beyond percolation threshold,there was negative permittivity behavior was observed,which was in good accordance with Drude model.Meanwhile,the negative permittivity was also attained in high frequency region.?2?CuO and ferromagnetic yttrium iron garnet?Y3Fe5O12,YIG?were chosen as raw materials,Cu/YIG composites were fabricated by presureless sintering and hydrogen reduction process.The effects of composition and microstructure on their electromagnetic property were investigated in detail.It was shown that the Cu/YIG can be achieved at a 500 ? reduced temperature.When copper content reached up to percolation threshold,the negative permittivity with a Fano-like resonance was obtained,where the permittivity changed from negative to positive,corresponding to the inductive-capacitive transition.Meanwhile,the combined contributions of the magnetic resonance of ferrimagnetic YIG particles and the diamagnetic response of the current loop bring about negative permeability in high frequency.?3?In order to improve the conductivity and simplify preparation technique,Ag2O and YIG were selected as raw materials and Ag/YIG composites were fabricated by one-step solid sintering method.In addition,the electromagnetic property of Ag/YIG composites were systematically investigated.It was indicated that the dielectric loss of Ag/YIG composites were ascribed to the combined contributions of conduction and polarization in the vicinity of percolation threshold.When silver content was above percolation threshold,the permittivity spectra with Fano-like were achieved in Ag/YIG composites;the permittivity switched from negative to positive at the zero-crossing point,where the resultant composites changed from inductive to capacitive character.Additionally,the frequency dispersion behavior of permeability spectra resulted from domain walls movement of Ag/YIG composites.Further increasing silver content,the microeddy current got enhanced,which brought about the relaxation-type spectra of permeability gradually became indistinct.?4?Polydimethylsiloxane?PDMS?was chosen as flexible matrix and multiwalled carbon nanotubes?MWCNTs?were regarded as functional fillers.The flexible PDMS/MWCNTs composites membranes were fabricated by in-situ polymerization process.It was presented that the dielectric constant got enhanced due to the Maxwell-Wagner-Sillars effect with the increase of MWCNTs content.When MWCNTs mass fraction reached to 5 wt%,the Lorentz-type permittivity spectrum was achieved in PDMS/MWCNTs composites and the real permittivity switched from positive to negative values.