Synthesis And Microwave Absorption Properties Of Hollow Carbon Nano-composites

With the rapid development of communication and radar detection technology,electrical productions have been widely utilized in defense,medical,military,civil and other fields.However,electromagnetic pollution caused by the ensuring electromagnetic radiation and electromagnetic interference would bring great threat to information security and human health.The microwave absorption materials have become a research hotspot due to that they can convert and dissipate the electromagnetic energy into heat energy directly.In recent years,carbon-based microwave absorption materials with lightweight feature have attracted wide attention from researchers.However,due to the high electrical conductivity,poor impedance matching characteristics and sole loss mechanism,single carbon nano-materials will lead to narrow effective absorption bandwidth and weak absorption performance,which could limit their practical application.Generally speaking,rational structural design,component regulation and adjustment of electromagnetic parameters of microwave absorption materials are various of effective ways to solve above problems.Among a variety of nanostructures,hollow structures can reduce the filler ratio and optimize the impedance matching degree,which is attributed to their larger specific surface area,lower mass density,and multiple reflection in the interior cavity.However,the loss mechanisms of hollow carbon nano-materials are relatively simple,which cannot provide enough polarization sites in the absorption process.Therefore,it is necessary to combine hollow carbon nano-materials with dielectric/magnetic materials to prepare hollow carbon nano-composites,in order to increase the loss mechanisms and improve the microwave absorption performance.In this thesis,a variety of hollow carbon nano-composites were designed and prepared.The main contents are as follows.（1）The Ni S₂ spheres were used as a template to design and fabricate“non-standard”nickel sulfide/carbon sphere composites with yolk-shell structure.The large cavity exists inside the composites and inner nickel sulfide can always contact tightly with outer carbon shell.In the microwave absorption process,n-Ni_xS_y@NSC has the structural characteristics of both core-shell and yolk-shell structure:heterogeneous interfaces and free space inside,which could synergically improve the microwave absorption performance of the composites.The effective absorption bandwidth can reach 3.84 GHz with the matching thickness of 1.5 mm.（2）The ZIF-67 nanoplates were used as a template.The NCNTs arrays were grown on the surface of the hollow carbon nanoplates through relatively low carbonized temperature（480℃）and the in-situ method.The small-sized（<5 nm）magnetic Co particles were encapsulated inside the NCNTs.The introduction of NCNTs would improve the interfacial polarization,the construction of hierarchical structure could enhance the conduction loss and improve impedance matching characteristics.Meanwhile,the existence of magnetic particles could increase the magnetic loss capability and improve the attenuation mechanism.The effective absorption bandwidth can reach 4.26 GHz with the matching thickness of 1.8 mm.（3）The Si O₂ spheres were used as a template and small-sized Ni O/Ni heterojunction particles were loaded on the surface of hollow carbon spheres.The experimental results indicate that the introduction of Ni O/Ni heterojunction can enhance interfacial polarization and lead to stronger dielectric loss ability.Besides,the larger internal cavity would optimize the impedance matching characteristics.The unique structure and component endow composites with excellent microwave absorption performance.The effective absorption bandwidth could achieve 4.38GHz with the matching thickness of 1.7 mm.（4）To reduce the density of hollow carbon nano-composites further,cobalt single atoms were distributed uniformly on the surface of hollow carbon sphere using Si O₂ spheres as a template.The synthesize process of cobalt single atoms was achieved by the introduction of cobalt ions,subsequent annealing,and acid leaching treatment.The experimental results demonstrated that the introduction of cobalt single atoms can adjust the electronic structure of adjacent atoms and improve the dipole polarization.The minimum reflection loss was-44.96d B and the effective absorption bandwidth of prepared hollow carbon nano-composites could reach 5.25 GHz with the filler ratio of 8 wt.%.In addition,the relationship between single atoms,defects and microwave absorption mechanism was elaborated.Several kinds of hollow carbon nano-composites were designed and constructed as microwave absorption materials in this thesis.The as-prepared composites exhibited hollow,porous structures and multiple heterogeneous interfaces existed.These advantages improve the shortcomings such as high electrical conductivity,single loss mechanism and poor impedance matching characteristics of single carbon nano-materials.Furthermore,the microwave absorption mechanisms of composites were analyzed,which lays the foundation for the application of hollow carbon nano-composites in the field of microwave absorption.