Microstructure Design And Dielectric Properties Of Carbon Nanofiber Composites

Electromagnetic wave absorbing materials play an increasingly important role in the coming 5G intelligent era.While blindly improving the material properties,the ability to dynamically change its electromagnetic wave absorption performance according to real-time requirements is also of great research significance in the future civil and military fields.Carbon-based absorbing materials have become one of the most popular dielectric absorbing materials in today’s research field due to their excellent surface properties,excellent electrical properties and not being limited by Curie temperature like magnetic materials.Since conventional carbon materials usually have high conductivity,it will lead to high electromagnetic wave reflectivity,which is not conducive to impedance matching.Therefore,the study of amorphous carbon with low conductivity loss is particularly important.However,a single amorphous carbon material cannot efficiently attenuate electromagnetic waves due to its low conductivity and weak dielectric loss.Therefore,it is usually compounded with high-loss dielectrics,and its microstructure and crystallization degree are regulated.Excellent microwave absorption performance is achieved through multi-scale structural design and synergy of multiple loss mechanisms.In this paper,bacterial cellulose(BC)-derived carbon nano fiber(CNF)is used as an impedance-matched absorber matrix.By compounding zero-dimensional metal particles and carbon nanotubes(CNTs)on it,the electromagnetic wave loss ability is optimized through the construction of heterogeneous interfaces and the improvement of electrical properties,and the two loss mechanisms of polarization loss and conduction loss are coordinated.The absorber is constructed into a three-dimensional macroscopic body by changing the stress.The intelligent adjustment of the dielectric properties and effective absorption bandwidth of the absorber is realized.The main results are as follows:(1)Through the reduction of metal salts by BC,the high loss particle Co is evenly distributed on the surface of CNF after heat treatment,and the Co@CNF sample is prepared.A rich heterogeneous interface is constructed on the micro level,and the conductivity is improved.At the macro level,porous aerogels are beneficial to the multiple reflection of electromagnetic waves,promote the energy dissipation of electromagnetic waves,and achieve the goal of light weight.The density of Co@CNF aerogels is as low as 8.1 mg/cm3.At a frequency of 9.9 GHz and a thickness of 4.5 mm,Co@CNF aerogel with 0.11 wt%Co content and heat treatment temperature of 900℃ exhibits a minimum reflection loss of-81.0 dB.The entire test band is encompassed by the effective absorption bandwidth,and the full frequency is below-12.1 dB.Results demonstrate that designing a sensible structure at multiple scales is a successful approach to resolving the issue of materials’ inadequate absorption capacity.(2)Inspired by the pine branch structure,CNTs/CNF composites with pine branch-like hierarchical structure were constructed on the nanoscale by using one-dimensional BC-derived CNF as the matrix and in-situ growing CNTs on its surface as the loss medium.It not only avoids the disadvantage of easy agglomeration of CNTs,but also the growth of amorphous CNTs does not damage the good impedance matching performance,accompanied by the introduction of lots of defects and nano-interfaces.The synergistic loss mechanism effectively improves the electromagnetic wave absorption capacity of CNTs/CNF.The minimum reflection loss of CNTs/CNF with a filler content of 20 wt%and a CNTs content of 63.2 wt%can reach-68.2 dB at a frequency of 11.4 GHz and a thickness of 2.7 mm,and the maximum effective absorption bandwidth is 5.4 GHz.(3)CNTs/CNF absorbing agent was dispersed in waterborne polyurethane(WPU)to construct a layered and resilient CNTs/CNF-WPU aerogel by bidirectional freeze-drying method.Under the alternating electromagnetic field,multiple micro-capacitors are formed between the layers.In addition,inspired by the sensor,this paper adjusts the interlayer distance of CNTs/CNF-WPU aerogel by applying stress,so that the storage charge capacity in the layer changes,so that the complex permittivity can change with the amount of compression,and the effective absorption bandwidth of CNTs/CNF-WPU acrogel is realized.Intelligent control provides an effective way to explore new intelligent absorbers.At a frequency of 9.4 GHz and a thickness of 5.1 mm,the minimum reflection loss of CNTs/CNF-WPU aerogel with 2.5 wt%CNTs/CNF content is-77.3 dB.This effective absorption bandwidth can cover the entire X-band and has remarkable absorption performance.