Study On The Preparation And Electrical Properties Of Carbon/alumina Ceramic Composites

Electromagnetic metamaterials are a new class of electromagnetic media with negative electromagnetic parameters,which show broad application prospects in electromagnetic shielding,stealth,antennas,capacitors,and other fields.Metacomposites are a branch of metamaterials,whose structural composition is different from conventional metamaterials.Conventional metamaterials generally consist of periodic arrangements of subwavelength structural units and their negative electromagnetic parameters are mainly determined by artificially designed periodic units.The electromagnetic properties of metacomposites do not depend on the periodic units but are mainly derived from the chemical composition and microstructure of the material.From the materials science perspective,the development of metacomposites based on the material’s intrinsic properties is very meaningful research work,because the materials community is more concerned with the relationship between material microstructure composition and properties.However,compared to conventionally ordered structure metamaterials,the research on metacomposites started late and there are fewer related studies.The regulation of negative permittivity is the focus of research in the field of electromagnetic metamaterials,which has significant theoretical and practical application value.For example,regulating the frequency band of negative permittivity is beneficial for the materials to be used in microwave filters,new antennas,and so on;materials with large values of negative permittivity can be used in the field of electromagnetic shielding;small values of negative permittivity with weak dielectric dispersion properties help them to be used in stacked capacitors.For metacomposites,the negative permittivity can be tuned by component designing and microstructure tailoring.Therefore,in this thesis,using carbon materials of different sizes and spatial dimensions(carbon black particles,carbon fibers,graphene)as the functional phase and alumina ceramics as the insulating matrix,multiple types of carbon/alumina ceramic composites were prepared by hot-press sintering process,and studied around the design of material composition structure and negative permittivity realization,regulation mechanism,and combined with the percolation theory,free electron theory,and equivalent circuit analysis,to deeply explore the negative permittivity,impedance,conductivity and other dispersion characteristics of the composites in the frequency band below 10 MHz.The main research works carried out in this thesis are as follows:(1)Various classes of carbon/alumina ceramic composites were prepared with negative permittivity in the tested frequency band,which is attributed to the plasma oscillations of free electrons in the percolation network constructed by the carbon functional phase,the negative permittivity dispersion properties were consistent with the Drude model.The effective electron concentration inside the material determines the overall plasma frequency of the material and thus affects its negative permittivity dispersion characteristics,which increase in absolute value with increasing content of the carbon functional phase.Therefore,the component design of the composite material can be used to regulate the effective electron concentration inside the material,which can effectively regulate the negative permittivity behavior.The size and spatial dimension of the functional phase contribute to the negative permittivity behavior of the composites,and the nano size and high dimensionality contribute to the construction of the percolation network of the functional phase,which is conducive to the negative permittivity behavior of the composites.In addition,the external temperature field also affects the electrical properties of the carbon composites.The external temperature affects the free electron concentration and electron motion state of the carbon composites,which in consequence affects the negative permittivity of the material.It is important to note that the value of the negative permittivity varies less with temperature,which provides the possibility to precisely regulate the negative permittivity behavior.(2)The conductivity dispersion characteristics are related to the number and degree of distribution of carbon functional phases.With the increase of the addition of carbon functional phases,the conductivity mechanism of the composite material transitions from jumping conductivity to metal-like conductivity,and the phenomenon of percolation appears.When the addition amount of carbon functional phase is lower than the threshold of percolation,the carbon functional phase shows an isolated distribution,the conductivity is low,and the material exhibits jump conductivity behavior.When the addition amount of carbon functional phase exceeds the threshold of percolation,the carbon functional phases start to contact each other to build the percolation network,the conductivity increases and the material has metal-like conductivity.(3)The content of the carbon functional phase affects the impedance characteristics of the composite material.As the carbon content increases,the composite material changes from capacitive to inductive,and the impedance characteristics of the material can be accurately analyzed using an equivalent circuit.When the amount of carbon functional phase added is lower than the percolation threshold,the material can be equated to a circuit containing resistance and capacitance,when the permittivity of the material is positive and has capacitive properties.When the amount of carbon functional phase added exceeds the percolation threshold,the dielectric constant of the material is a negative number,and the material has inductive properties and can be equated to a circuit containing inductance,capacitance,and resistance.The transition between positive and negative permittivity of composites is accompanied by a shift in its capacitive-inductive properties,and the appearance of negative permittivity behavior of the material is often accompanied by inductive properties.(4)The dielectric loss of percolation composites mainly comes from the conductivity loss and polarization loss,with the increase of carbon content,the imaginary permittivity of the material increases rapidly,which is due to the rapid increase of conductivity loss and the enhancement of polarization loss.The dielectric loss of the composite material with negative permittivity mainly comes from the conductivity of the large number of free electrons in the percolation network formed by the electric field,and a great conductivity loss will be generated in the process.Combined with the linearity of the fitting of the conductivity loss equation,it shows that the dielectric loss is dominated by the conductivity loss,thus demonstrating that plasma oscillations’ negative permittivity behavior is generally accompanied by greater dielectric loss.