| Recently,electromagnetic metamaterials(EMs)with negative permittivity or/and permeability have received great attention.While most of the conventional electromagnetic media have positive permittivity and permeability,the EMs offer unprecedented electromagnetic properties,such as negative refraction,perfect imaging and super absorption,and are very promising for applications in the field of optical cloaking,imaging,radio communications,sensors,microwave absorbing and shielding,among others.Until now,the study of EMs focused on two kinds of materials:the periodic metamaterials and the random metamaterials.The periodic metamaterials are constituted by the periodic sub-wavelength unit cells.Their negative electromagnetic parameters mainly originate from the periodic structures,and closely associated with shape,size and spacing of unit cells,rather than being inherent in the material.Therefore,the considerable efforts were directed towards the study of the realization and controlling of negative parameters in the random metamaterials without clearly periodic structures.Several studies have found that the negative parameters could be achieved in the random metamaterials by adjusting their compositions and microstructures,and relied on the intrinsic property of nature materials.It is of great theoretical significance to adjust the negative parameters,and the tunable negative parameters are also needed for many practical applications.For instance,small negative values of permittivity in broad frequency range benefit their application in the microwave absorbing and capacitor fields.Tunable frequency band of negative parameters is desired in a broad passband filter to make ensuring its multi-frequency operations.Hence controlling the negative parameters have been one of the main goals of EMs studies.In this dissertation.various random composite metamaterials were designed and prepared.and the author put forward the academic idea of constructing EMs by using percolative composites.The effects of chemical compositions and microstructures on the electromagnetic properties of the percolating ceramic matrix composites were systematically investigated,revealing the realization and regulation mechanism of negative parameters.The main research contents of the paper include the following aspects:(1)Cobalt/silicon nitride composites were synthesized by an impregnation-calcination process,and the sub-micron cobalt particles randomly distributed in the pore canals of porous silicon nitride matrixes.As the cobalt content inceraing,the three-dimensional cobalt networks were formed in the comopsites,leading to the appearance of percolation phenomenon.When the cobalt content is low,the composites exhibited a hopping conduction behavior and a capacitive character.The composites with the cobalt content above the percolation threshold showed a metal-like conduction behavior and an inductive character.The negative permittivity and permeability appeared in the composites with high cobalt content.Negative permittivity was arise from the low frequency plasmonic state generated by the formative conducting cobalt networks,while negative permeability was caused by the strong diamagnetic response of current loops in metallic networks and magnetic resonance of cobalt particles.The frequency dispersions of negative permittivity were well described by the Drude model.In the composites with the cobalt content of 35 wt%,simultaneous negative permittivity and negative permeability were realized in the frequency range from 550 MHz to 1 GHz.(2)The alumina nanocomposites with multi-walled carbon nanotubes uniformly dispersed in the alumina matrix were prepared by hot-pressing sintering.Two different types of negative permittivity(i.e.,resonance-induced and plasma-like)were observed in the composites.The resonance-induced negative permittivity behavior in the composite with low nanotube contents was ascribed to the induced electric dipole generated from the isolated carbon nanotubes,while the observed plasma-like negative permittivity behavior in the composites with high nanotube contents could be well explained by the low frequency plasmonic state generated from conductive nanotube networks.The equivalent circuit model was used to analyze the impedance response of the composites.The composites with low nanotube contents were equivalent to a circuit consisting of resistors and a capacitor.The composites beyond the percolation threshold were equivalent to a circuit consisting of resistors,capacitor and inductances.The introduced inductance element,in the equivalent circuit for the composites with high nanotube contents,was attributed to the formation of conductive nanotube networks and the generation of current loops in the conductive networks under the high frequency electric field.We believed that the inductive character always accompanied by a negative permittivity behavior.(3)Graphene/silicon nitride ceramic composites with uniformly dispersed graphene sheets were prepared using spark plasma sintering.When the graphene content was low,the isolated graphene sheets were dispersed in the silicon nitride matrixes,and the composites possessed positive permittivity.As graphene content reached the percolation threshold,the continuous conductive graphene networks were formed in the composites,leading to the occurrence of a plasma-like negative permittivity behavior.The increasing graphene content resulted in a larger magnitude of negative permittivity and the higher frequency of the negative-positive transitions for permittivity.Besides,the composites showed a high dielectric loss,which mainly derived from the conduction and polarization process.The clear dielectric loss peaks were observed near the frequency points where the permittivity switched negative to positive.(4)Pyrolytic carbon/silicon nitride composites were fabricated using a feasible impregnation-pyrolysis method.The carbon derived from the high temperature pyrolysis of sucrose powder was amorphous and was film-like.The carbon layer spreaded out on the silicon nitride grains and located in the pores.As the carbon content increased,the carbon films become more obvious in the composites.The elevated heat treatment temperatures resulted in the rapid increase in electrical conductivity of the amorphous carbon,which was attributed to the crystal growth and the higher graphitization degree.When the conducting passway was formed in carbon networks.a negative permittivity behavior appeared in the composites.Small magnitude of negative permittivity in the tested frequency was observed,showing a weak dispersion characteristic.The weakly negative permittivity behavior was ascribed to a low carrier concentration provided by the carbon networks.A tunable and weakly negative permittivity behavior could be achieved in the composites by adjusting the heat treatment temperatures and carbon contents,and the increased carbon content and heat treatment temperature could reduce carrier concentration,leading to the increase of the negative permittivity amplitude.(5)Yttrium iron garnet composites incorporating polypyrrole were prepared by an in-situ oxidative polymerization method,and the formative granular polypyrrole adhered to the surface of yttrium iron garnet particles.A negative permittivity behavior combined with metal-like conduction was observed in all the composites.The negative permittivity behavior could be effectively adjusted by controlling the polypyrrole content.The increase of polypyrrole loading led to largen the effective density of free carriers owing to the formation of less polypyrrole networks,while the effective mass of the carriers become smaller due to the reduced constraint of yttrium iron game particles on the movement of carriers.Hence,as the polypyrrole loading increased,plasma frequency,at which the permittivity changed from negative to positive,shifted to a higher frequency,and the magnitude of negative permittivity also become larger.In addition,the permeability of composites showed a relaxation-type frequency dispersion,which was attributed to the magnetic resonance of yttrium iron game particles.It is worth mentioning that the permeability was less than 1 at the high frequency regime,exhibiting diamagnetic responses,which resulted from the formative polypyrrole conducting networks in the composites. |
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