Study On The Dielectric Properties In Percolative Alumina Ceramic Composite

This dissertation mainly focuses on the metacomposites that people have paid more attention to in recent years.From the perspective of performance,metacomposites usually refer to composite materials with many extraordinary physical properties,such as negative refractive index,negative dielectric constant,negative Poisson's ratio,and negative magnetic permeability.Military radar,electronic communication,optical stealth and other fields have broad development space.At present,according to the structural characteristics of metacomposites,they can be divided into periodic structure metamaterials and random structure metamaterials.Periodic metamaterials are mainly realized by adjusting the morphology or arrangement of the structural unit of the material on the basis of artificial design and synthesis.Compared with periodic metamaterials,random structure metamaterials mainly use the intrinsic properties of materials,which can largely overcome the problem that periodic materials must rely on manual design and the preparation process is more complicated.The design idea is to add a conductive phase material to the insulator material to obtain a conductor-insulator heterogeneous composite material,and tailor the microstructure of the composite material by rationally controlling the chemical composition of the conductive phase material to achieve a negative physical parameter.This design idea has attracted widespread attention because it can accelerate the marketization of metamaterials.In this thesis,based on the design ideas of random structure metamaterials,a conductor-insulator heterogeneous electromagnetic metacomposites is synthesized.By changing the amount of conductive filler or sintering temperature,the chemical composition and microstructure of the material are systematically investigated.The law of its dielectric properties is studied by studying the mechanism of the material's negative dielectric constant,so as to better realize the metacomposites materials with adjustable negative parameters.The achieved research conclusions as following:(1)Molybdenum/alumina percolation composites were prepared by atmospheric pressure sintering.The effects of different molybdenum metal content and microstructure on the dielectric properties of the composites were systematically studied.The study found that with the increase of the molybdenum content in the system,a threedimensional metal network gradually formed in the material,and percolation occurred.When the content of molybdenum metal is low,the conductivity mechanism of the material is electron hopping conductivity.When the content of molybdenum metal exceeds the percolation threshold,the formed metal molybdenum network makes the material have metal-like properties,and its conductivity mechanism becomes metal conductivity.When the molybdenum content in the system exceeds 50 wt%,the lowfrequency plasma state of free electrons in the molybdenum metal network leads to the appearance of negative dielectric constant,and its dielectric dispersion characteristics conform to the Drude-Lorentz model.This unique negative parameter property will provide new ideas for research in the fields of electromagnetic shielding,stealth and electromagnetic wave absorption.(2)Alumina and graphene prepared by Hummers method were combined with each other to prepare a composite of graphene and alumina ceramics by sintering at atmospheric pressure.By setting different mass fractions of of graphene components,the influence of different graphene content on dielectric properties was explored.The study found that when the graphene content exceeds 20 wt%,the composite material exhibits a negative dielectric constant.(3)The cracked carbon/alumina composite was prepared by impregnation carbonization method,and the effects of different carbon content or different carbonization temperature on the microstructure and dielectric properties of the composite were studied.The cracked carbon produced after the resin is calcined at high temperature is amorphous carbon,and is distributed in the form of a carbon film in the porous alumina ceramics.As the content of cracked carbon increases,the distribution of the carbon film becomes more pronounced;and the higher carbonization temperature will also cause the cracked carbon to form more ordered structures,the degree of graphitization becomes higher,and thus the conductivity increases.When a continuous conductive carbon network is formed in the system,the composite material has a negative dielectric constant.This experiment effectively controls the value of the dielectric constant by controlling different carbon content and carbonization temperature,in order to apply it more effectively to the field of electronic applications...