The Preparation Of Carbon Nanotube Composites And Their Microwave Absorption Properties

In recent years,with the rapid development of electronic information technology and phased radar,improving the stelth of aircraft,missiles and other weapons,microwave absorption materials have a wide range of demands in the military field.Moreover,the popularization of electronic products has led to a large number of electromagnetic pollution in our daily life,which not only causes harm to the environment and human health,but also has a great impact on the normal operation of electronic products due to the highly integrated development of electronic devices.Therefore,the design and preparation of microwave absorption materials with light weight and high performance have been widely paid attention.Carbon nanotube（CNT）has attracted much attention because of their light weight,high aspect ratio,high electrical conductivity and high specific surface area.However,due to its own high conductivity,its impedance matching performance is poor,a large numbers of incident microwaves reflected on its surface,resulting in its limited microwave absorption performance.At present,the researchers mainly improve the microwave absorption performance of CNT by designing and manipulating the structure of the material to increase multiple reflection and introducing good impedance matching layer,such as porous structure and multi-layer structure.In addition,the microwave absorption performance of CNT can be improved by combining with magnetic materials with strong magnetic loss to improve their magnetic permeability.However,due to the limitation of preparation methods and technology,the improvement of performance by these methods is still not obvious.Moreover,the influencing factors of CNT microwave absorption performance are still not clear enough.In this paper,the effects of different types of impedance matching layers and material structures on the microwave absorption performance of CNT were studied by modifying the CNT composite and adjusting the distribution structure of CNT.The existing preparation methods and processes of different composite materials were tried and optimized.Focusing on the topic of"Preparation and Microwave Absorption Properties of Carbon Nanotube Composites",this paper has carried out research from the following three aspects:（1）By combining with ferrite and conductive polymer,the effects of the composites on the electromagnetic loss capacity and impedance matching of CNT have been studied,and the existing reported preparation methods and processes have been optimized;（2）Through the freeze-drying process,the influence of the structure on the microwave absorption performance was studied with cellulose-based carbon as the skeleton,and the main roles of CNT and carbon skeleton in the microwave absorption materials were identified.（3）Based on the above research results,using polyimide as the matrix framework and CNT as the microwave absorber,a lightweight microwave absorption foam with excellent mechanical properties was prepared.The main research work and conclusions are as follows:1.Preparation of CNT/BaFe₁₂O₁₉ composites and its microwave absorption performanceCNT/BaFe₁₂O₁₉ composite absorbing agent was prepared by self-propagating combustion method.The effects of preparation technology and the content of the absorbing agent on the absorbing property were studied systematically.The results showed that:（1）By changing the calcination process of BaFe₁₂O₁₉ precursor,the dispersion of CNT in BaFe₁₂O₁₉ could be improved,and a wave absorber with excellent wave absorption performance could be obtained at a lower CNT content.（2）When the CNT content is only 2%of the theoretical yield of barium ferrite,the maximum reflection loss（RLmin）is-43.6 d B,and the effective absorption bandwidth（RL≦-10 d B）is 3.9 GHz.It effectively solves the phenomenon that CNT is easy to agglomerate when barium ferrite composites are prepared by ball milling technology in the literature,and shows good wave absorption performance.（3）BaFe₁₂O₁₉ has a high density and is easy to agglomerate,so it needs a higher content（70 wt%）to get better absorbing property.Therefore,when the CNT absorbing property is improved by composite with magnetic materials,the density of CNT will inevitably increase.2.Preparation of CNT/PANI composites and its microwave absorption performanceCompared with barium ferrite,polyaniline（PANI）can effectively improve the impedance matching performance of CNT due to its smaller density,high temperature resistance and adjustable electrical conductivity.By introducing polyvinylpyrrolidone（PVP）into the in-situ polymerization process of PANI,the dispersion of CNT can be improved effectively.Moreover,as a space stabilizer,Pani micelle particles formed by hydrogen bonding with Pani uniformly coated CNT surface and further polymerized to form uniform Pani coating layer.The effects of PANI content on the structure and wave absorption performance of the composite were studied.The results showed that:（1）in the presence of PVP,the uniform PANI coating layer could be formed regardless of the content of PANI.（2）By changing the content of PANI,the dielectric parameters of the composites can be effectively regulated,and the dielectric loss performance and impedance matching performance can be optimized.（3）When the mass ratio of CNT to PANI is 1:2,it shows good microwave absorption performance.When the content is only 10 wt%,the RL_min can reach-43.6 d B,and the effective absorbing bandwidth（RL≦-10 d B）can reach 5.1 GHz.3.Preparation of CNT/amorphous carbon foams and its microwave absorption performanceSince amorphous carbon has good impedance matching performance and adjustable dielectric properties,coating amorphous carbon on CNT surface can effectively improve the impedance matching performance of CNT,and obtain a light absorbent with excellent wave absorbing performance.Due to the excellent coating performance of PVP on CNT and the strong hydrogen bonding between PVP and cellulose,a light carbon foam was obtained by freeze-drying the cellulose dispersion containing CNT.When used as absorbing agent after grinding,the RLmin can reach-43.6d B and the effective absorbing bandwidth（RL≦-10d B）can reach 7.42GHz at the carbonization temperature of 550℃.When used as a lightweight absorbing foam,due to the integrity of its multistage pore structure,when the carbonization temperature is 550℃,its RL_min can reach-41.6 d B,the effective absorbing bandwidth（RL≦-10d B）up to 7.42 GHz.Because of its low density（0.05g/m L）,it has great application potential as a lightweight absorbing material.4.Preparation of CNT/polyimide foam and its microwave absorption performanceA water-soluble polyimide acid（PAA）aqueous solution was prepared,and a certain amount of CNT dispersion was mixed evenly,and then CNT/polyimide（PI）foam was prepared by freeze-drying and thermal imination process.The CNT/PI density was about 0.035 g/m L,the RL_min was up to-44.7 d B,and the effective absorbing bandwidth（RL≦-10 d B）was up to 7.41 GHz,showing excellent absorbing performance.5.Preparation of CNT/graphene/polyimide foam and its microwave absorption performanceTo further reduce the density,CNT/RGO/PI foam was prepared by using GO as dispersant,and then mixed with PAA dispersion,which was prepared by freeze-drying and thermal imination.The foam density is about 0.02g/m L,its RL_min is up to-32.5d B,and its effective absorbing bandwidth（RL≦-10 d B）is up to 8.51GHz.Moreover,in the whole X and Ku band,its reflection loss is≦-5 d B,which can well meet the practical use requirements.