Study On The Preparation And Electromagnetic Wave Absorption Properties Of Nitrogen-Doped Carbon Nanotube/Silica Composites

Electromagnetic wave absorbing materials have a very broad application prospect in the fields of anti-electromagnetic interference of electronic communication equipment,eliminating the harm of electromagnetic radiation to human body and stealth of radar wave by aircraft and ships.Therefore,the development of light,efficient and cheap electromagnetic absorption materials has become a research hotspot.Carbon nanotube（CNT）is a typical hollow structure with high electrical conductivity.Compared with metal or metal oxide,CNT has larger specific surface area and lighter properties.Therefore,CNT has potential application value in the field of lightweight electromagnetic absorbing materials.However,the high conductivity of carbon nanotubes lead to skin effect,and it is difficult to achieve the ideal impedance matching characteristics,which result is that leads to the electromagnetic wave absorption performance can not meet the requirements of practical applications.In addition,the direct application of metal compounds as catalysts,supplemented by organic reagents containing carbon and nitrogen,is a commonly used method for the preparation of nitrogen-doped carbon nanotubes（NCNTs）.However,the agglomeration of metal composite powders usually leads to uneven growth of carbon nanotubes,and larger metal particle aggregates are formed in the final products.In this thesis,a spatial separation strategy of silica is proposed to synthesize a series of nitrogen-doped carbon nanotubes/silica composites.Due to the introduction of non-conductive silica spheres that play role of the spatial separation,the agglomeration of metal particles and the generation of large size carbon nanotubes is avoided efficiently,but also the impedance matching characteristics of materials are improved,thus significantly improving the electromagnetic wave absorption performance of the resulant composites.On the basis,the electromagnetic wave absorption mechanism of composites is studied based on dielectric relaxation theory.The main research contents of this thesis are as follows:（1）A spatial separation strategy of silicon dioxide is proposed,by which a series of nitrogen-doped carbon nanotubes（Ni@NCNT@SiO₂,Fe₃C/Fe@NCNT@SiO₂and Co@NCNT@SiO₂）embedded with magnetic nanoparticles are synthesized on silica spheres,which effectively avoide the formation of large metal aggregates and large size nitrogen-doped carbon nanotubes in the product.In addition,due to the introduction of silica spheres,the impedance matching characteristics of the composites are effectively controlled.Among the composites,the Ni@NCNT@SiO₂has better electromagnetic wave absorption properties.When the mass loading is 30 wt.%and the matching thickness is 1.5 mm,its effective absorption bandwidth is 4.14 GHz,and the minimum reflection loss value is-39.58 d B at the frequency of 5.60 GHz.（2）In order to control the composition of the composite,Fe_xNi_y@NCNT@SiO₂composites are constructed through the alloying strategy based on the spatial separation strategy of silica.The density functional theory calculation shows that the electron interaction between Fe and Ni can adjust the polarization loss and conduction loss of Fe_xNi_y@NCNT@SiO₂after Ni is introduced.The optimized three-dimensional structure Fe₂Ni₁@NCNT@SiO₂has excellent electromagnetic wave absorption performance.When the mass loading is 25 wt.%and the matching thickness is 1.6 mm,its effective absorption bandwidth is 4.32 GHz,and the minimum reflection loss is-49.39 d B at the frequency of7.68 GHz.（3）In order to control the morphology of carbon nanotubes,organic reagent adsorption strategy is proposed based on the spatial separation strategy of silica.The diameter of CNT in composite samples is adjusted and is tuned the conduction loss,the polarization loss and impedance matching characteristics,thus improving the absorption performance of the Fe₃C/Fe@NCNT@SiO₂-GT sample.Its effective absorption bandwidth is 4.51 GHz at 25wt.%and the matching thickness is 1.5 mm,and the minimum reflection loss is-48.43 d B at5.12 GHz.（4）In order to effectively control the microstructure of the composite,the etching strategy of organic weak base reagent is proposed to synthesise yolk-shelled-sphere-supported and Ni-embedded NCNTs composite（NCSiNi@NCNT@SiO₂）.In the composite,the yolk material is SiO₂sphere,and the shell material is composed of N-doped carbon layer containing Ni,Siand O elements.The NCNT array encapsulating Ni nanoparticles is radiated from the yolk-shelled spheres.The structure has the advantages of both yolk-shelled structure and three-dimensional structure,which improves polarization loss and conductivity loss.The optimized sample has a minimum reflection loss of-59.90 d B,a corresponding absorption frequency of 15.04 GHz,and a matching thickness of only 1.4 mm when the mass loading is20 wt.%.