Preparation Of Lightweight And Efficient Biomass Carbon Composites And Its Application In Wave Absorbers

With the development of radio technologies in 5G communication,precision guidance and military radar,electromagnetic wave（EMW）absorption is an important prerequisite for protecting military facilities,civilian equipment,and human health.Currently,researchers are dedicated to manufacturing high-performance Microwave absorbing materials（MAMs）for electronic information security,anti-surveillance stealth and human health protection,but most absorbers are still plagued by cumbersome synthesis processes,high costs,impedance mismatches,weak attenuation capabilities and other problems,which further hinder the wide application of advanced MAMs.In contrast,biomass-derived carbon（BDC）materials converted from natural resources are more environmental-friendly and readily available in large quantities.Carbon materials with porous structures can be obtained by high-temperature cracking and alkali activation processes,which provide abundant dipole polarization sites and facilitate improvement of microwave absorption.In view of this,moderate magnetization strength and double loss mechanism are key factors to effectively enhance the performance of MAMs.That’s to say,the rational design of magnetic components with dielectric materials can improve the impedance matching characteristics of absorbers,while the synergistic effect between magnetic and dielectric losses enhances the attenuation ability of EMW.Therefore,we have developed next-generation MAMs with various functions for practical applications in harsh environments.（1）Biomass porous carbon（BPC）converted from longan shells by alkaline（KOH）activation and high temperature cracking process presents a honeycomb porous structure,followed by the introduction of magnetic ZnFe₂O₄@C particles with core-shell structure into the porous carbon and the use of mechanical co-mingling to obtain composite materials（ZnFe₂O₄@C@BPC）.The results demonstrate that the BPC was rich in pores and the magnetic ZnFe₂O₄@C nanospheres were uniformly anchored in the surface of BPC and the three-dimensional porous structure of the carbon material did not collapse.Given the suitable synergy between BPC and magnetic ZnFe₂O₄@C nanoparticles,the impedance matching characteristics and attenuation ability of ZnFe₂O₄@C@BPC composite were close to ideal states.The synthesized ZnFe₂O₄@C@BPC composite exhibits excellent electromagnetic absorption properties,with an minimal reflection loss(RL_min)value reached-58.6 dB at 12.9 GHz and a wide effective absorption bandwidth（EAB）achieved 9.1 GHz at 3.4 mm.（2）Long rod-shaped biomass carbon fibers was originated from cotton under inert（N₂）atmosphere after continuous heating at 800℃for 60 min.A novel multi-heterostructure carbon fiber@graphene@layer double hydroxide（CF/RGO/LDH）composite was prepared by typical electrostatic self-assembly and solvothermal method.Specific morphological structure demonstrated that graphene sheets were tightly wrapped around cotton-derived carbon fibers,while two-dimensional layered double hydroxides were tightly arranged on the graphene surface,forming a hierarchical yolk-shell structure.Specifically,the CF/RGO/LDH multistage composite reached strong absorption capacity(RL_min=-60.9 dB)and a wide EAB（6.1 GHz）with a filling ratio of only 20 wt%.（3）A multi-dimensional iron-cobalt@carbon@porous carbon（FeCo@C@BDC）aerogel was synthesized by the solvothermal and subsequent freeze-drying process.The carbon aerogel derived from Pleurotus eryngii（PE）was modified by cationic polyelectrolyte polyethyleneimine（PEI）.Simultaneously,FeCo@PDA was obtained by polymerization reaction of as-prepared FeCo alloy with dopamine（DA）at room temperature,and then FeCo@C was obtained by heating at 500°C for 90 min under N₂ atmosphere.Finally,FeCo@C nanocages with core-shell structure were uniformly anchored on the carbon skeleton by electrostatic interactions.Thanks to the rational structural design of the hybrid,FeCo@C@BDC aerogel presented light weight,good compression and corrosion resistance.Remarkably,the FeCo@C@BDC aerogel achieves a RL_min of-69.5 dB at 12.2 GHz and an EAB of 8.6 GHz at 2.9 mm,which is better than most reported carbon-based materials.