| The permittivity is a basic physical parameter which reflects the response mode and ability of materials in electric field.Permittivity of most dielectrics is usually positive and related to polarization process of materials,the stronger the polarization,the higher the permittivity.Moreover,applications of materials are always influenced by their permittivity,for instance,materials with high permittivity are usually used for capacitors,and those with low permittivity can be used for wave absorption or transmission according to their loss ability,and so on.Negative permittivity has been considered as a novel property of metamaterials,which depends on periodical building blocks,rather than being determined by the chemical composition and microstructure of materials.Recently,it has aroused great interest to study negative permittivity based on the intrinsic properties of materials.Materials with negative permittivity(Epsilon-negative materials,ENMs)have shown great potential in various applications,such as multilayered capacitors,coil-less inductors,electromagnetic interference shielding,and wave-absorbing.Though ENMs are expected to innovate some electronic devices and electromagnetic equipment,the operating mechanism and influencing factors for their applications are still unclear.Thus,it is necessary to investigate the mechanism of negative permittivity and the physical properties of ENMs,so as to guide the performance design and applications of these new kind functional materials.This dissertation focuses on the preparation,mechanism,and physical properties of ENMs,taking ceramic composites,cermets,and single-phase ceramics as research objects.Composition and microstructure of the materials were characterized,and the epsilon-negative properties,magnetic properties,impedance,conductivity and carriers'characters have all been studied based on the free electron theory,Drude model,Lorentz model,percolation theory,equivalent circuit analysis,and other relevant methods.The main research content is listed as follows.(1)Ceramic-based ENMs were prepared and tunable epsilon-negative properties were realized.Ceramic composites and cermets were prepared by means of heterogeneous composites,of which the electrical performance was improved via adding conductive ceramics or metallic fillers.Negative permittivity was observed at radio frequencies and the influence of fillers' content on epsilon-negative properties of the ceramic matrix composites were investigated.It's found that negative permittivity was determined by the percolation of conductive fillers within the insulating ceramic matrix.In addition,from the viewpoint of homogeneous construction,doped ceramics with single-phase were sintered to be ENMs.Influence of dopants amount and sintering atmosphere on the negative permittivity was studied.It's found that dopants amount directly affected electron concentration,and the varied sintering atmosphere affected electron concentration by affecting the equilibrium conditions of doping reactions.Plasma frequency and negative permittivity of single-phase ceramics were determined by electron concentration.Based on the research results of ceramic composites,cermets,and single-phase ceramics,realization and regulation mechanism of negative permittivity were further clarified.Negative permittivity was attributed to plasma oscillations of free electrons within materials,and the epsilon-negative properties could be regulated through changing fillers'content,dopants amount,or even the preparation conditions due to the effective electron consentration is usually affected by these factors.(2)It's found that ENMs are electric inductive in the investigations of ceramic composites.Due to that conductivity of TiN is similar to that of metals,ceramic composites with TiN fillers are expected to be ENMs.Thus,epsilon-negative ceramic composites with TiN fillers were prepared.On the one hand,TiN powders were oxized and then sintered into ceramics that contains TiN and TiOx.Influence of oxidation temperature on negative permittivity was studied.On the other hand,TiN/Al2O3 duplex ceramics were sintered,of which the conductivity was sharply increased as electric percolation occurred.Negative permittivity was observed in ceramic composites obtained by the above two methods,which was caused by the plasma oscillations of free electrons in TiN ceramics.Moreover,impedance behaviors of ceramics were closely related to permittivity.When permittivity was positive,Z"was negative,the voltage phase lagged the current phase,in this case the ceramics were electric capacitive.Otherwise,the ceramics were electric inductivie when the permittivity was negative.The positive-negative transition of permittivity made the ceramics changed from capacitive to inductive.Thus,inductive character can be considered as an inherent property of ENMs.(3)Doping-dependent negative permittivity and loss properties of ENMs were studied in single-phase ceramics.Antimony tin oxide(ATO)ceramic powders were synthesized by hydrothermal reactions and then sintered into single-phase ceramics.Sb impurities with+5 valence state were donors that provide electron in the ceramics.When Sb doping content increased from 0 to 8 mol%,carrier concentration and conductivity both increased.Permittivity of ATO ceramics was positive when the doping amount of Sb was low,and became negative as the doping amount reached up to 6 mol%due to the plasma oscillations of free electrons.The higher electron concentration caused the greater values of negative permittivity.For indium tin oxide ceramics sintered by soild state reactions,free electrons were introduced by the donor Sn atoms,and plasma oscillations of free electron caused permittivity to be negative.Equilibrium conditions of Sn-doping reactions were affected by the varied sintering atmosphere,of which lower oxygen partial pressure was favorable for Sn-doping,resulting in high carrier concentration and large values of negative permittivity.Ceramics with negative permittivity held high dielectric loss,which was attributed to the high electric conductivity contributed by free electrons within the ceramics.According to free electron theory,dielectric loss of ENMs that realized based on electron'plasma oscillations are entirely attributed to conduction loss.Thus,it's confimed that ENMs are obvious lossy materials.(4)It's found that ENMs are diamagnetic at high frequcy in the investigations of cermets.Percolative cermets with copper/nickel as fillers and rutile ceramics as matrix were sintered.Influence of metal fillers' content on conductivity,permittivity,and permeability of the sintered cermets were studied.Metal fillers became interconnected when content exceeded percolation threshold,leading to conductivity increased dramastically.Meanwhile,insulator-metal transition occurred and conduction mechanism of the percolative cermets changed from hopping conduction to metallic conduction.Negative permittivity was attributed to the plasmonic state of electrons in percolating pathways.In addition,magnetic properties of nickel/rutile cermets were further discussed.Permeability of the epsilon-negative cermets was smaller than 1,which was resulted from the induced high-frequency diamagnetic response of eddy currents within the interconnected conductive networks.Negative permittivity and diamagnetic performance were all determined by percolation of nickel fillers.Thus,ENMs that built based on percolated structure of metallic fillers are diamagnetic at high frequency,even though the materials contain ferromagnetic constituents,high-frequency diamagnetism is still an inherent property determined by the microstructure of the materials themselves. |
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