Design And Study On Electromagnetic Wave Absorption Properties Of Porous Carbon Matrix Composites Based On Precursor Pyrolysis

Developing new lightweight,high efficiency,and wideband electromagnetic wave（EMW）absorbers is an effective way to address electromagnetic pollution,electromagnetic interference and improve the stealth and penetration abilities of military equipment.Carbon materials are widely used in electromagnetic absorbers because of their light weight,high efficiency,excellent conductivity,and adjustable dielectric properties.However,carbon materials fail to meet the requirements of modern applications due to their poor impedance matching performance and single absorption mechanism.In response to this problem,the research ideas of“component optimization,structure regulation,structure-activity relationship and performance improvement”were adopted.In this study,a series of carbon-based composites were synthesized by pyrolyzing metal-organic frameworks（MOFs）.The effects of morphology,composition,and micro-nano structures on the electromagnetic parameters,attenuation characteristics,impedance matching,and EMW absorption performance of the composites were studied.At the same time,the mechanism of EMW absorption under multiple synergistic effects was revealed systematically.Three porous carbon-based magnetic materials（Co/p NC）were prepared by pyrolyzing zeolitic imidazolate frameworks（ZIFs）controlled by different solvents.All the composites could achieve a certain degree of effective EMW absorption in the frequency range of 2–18GHz and the minimum reflection loss(RL_min)values are close to-20 d B.The relationship between the filler loading of the absorber and the dielectric parameters,dielectric loss,and EMW absorption performance was illustrated by the Maxwell-Wagner-Sillars polarization,percolation theory,debye relaxation polarization theory and the characteristics of attenuation and impedance matching,which provided guidance for rapidly selecting the appropriate filler loading for EMW absorbers.Epitaxial growth of carbon nanotubes（CNTs）on nitrogen-doped porous carbon materials（Co/p NC@CNTs）were prepared by the thermal decomposition of bimetallic imidazole framework materials with the introduction of zinc salt.CNTs are grown in-situ on the product’s surface controlled by changing the mole ratio of Zn²⁺:Co²⁺in the precursor and the calcination temperature.The effects of calcination temperature on the degree of graphitization,specific surface area,etc,as well as the effects of these parameters on the microwave absorbing properties of the samples were studied.Through the strategy of constructing a crystal-amorphous heterogenous carbon matrix,the attenuation ability and EMW absorption performance are greatly improved.Among them,the sample Co/p NC@CNTs-600 obtained by calcination at 600°C reaches the minimum reflection loss(RL_min)of-50.7 d B when the coating thickness is 2.5 mm,and the effective absorption bandwidth(EAB_max)reaches 5.5 GHz when the coating thickness is 2.0 mm.The RL_min values of sample Co/p NC@CNTs-700 is-25 d B with a coating thickness of 1.2 mm.Two ternary heterogeneous composites with different core-shell structures were constructed via a two-step solid-phase heating method.Studying the difference of micro-nano structures and the microwave absorbing properties of the two complexes.Maxwell’s equation and Maxwell-Garnett theory were used to reveal the effects of micro-nano structures on the electromagnetic parameters of materials.The absorption mechanism of the material was elucidated based on the theory of multiple dielectric loss and the quarter wavelength model.The experimental results show that the pore structures can optimize the electromagnetic parameters and the cavity structures between the heterogeneous core and the carbon shell could trap and dissipate EMW through multiple reflections.The Sn O₂/Co₃Sn₂@C hybrid shows excellent absorption performance with an RL_min of-56.2 d B at 6.8 GHz.In addition to strong absorption,Sn O₂/Co₃Sn₂@Air@C composite possesses superior broadband absorption characteristics with an ultrabroad bandwidth from 11.0 to17.1 GHz due to impedance matching mediators of the pore/void.An“All in One”EMW absorber design strategy was proposed based on the advantages of the above composites and the absorbing mechanism.Namely,integrating the dominant elements such as dielectric materials,magnetic materials,porous structures,core-shell structures,and cavity structures into one carbon-based composite.According to the characteristics of high-temperature evaporation of zinc,we doped Ni-MOFs precursors with different contents of zinc salt and calcined them at 600℃and 900℃respectively to obtain six kinds of materials with different components,morphologies and micro-nano structures.Then,the effects of these variables on the electromagnetic parameters and EMW absorption performance in the same system were systematically studied.The RL_min of sample S3-900reaches-64.4 d B at 9.92 GHz.With a coating thickness of 1.95 mm,the EAB_max reaches 6.9GHz,covering the entire Ku band.The excellent properties of the composite are attributed to the synergistic effects of multiple magnetic loss,heterogeneous dielectric materials,abundant interfaces polarization and unique structure.In summary,precursors were pyrolyzed in this study to prepare porous carbon-based composites.The regulation of structure-function integration was achieved,high-performance EMW absorbing materials were obtained,and the multiple cooperative absorption mechanism was revealed.This study provides a theoretical basis for the design of efficient EMW absorbers.