Preparation And Microwave Absorption Properties Of ZIFs Derived Carbon@Carbon Core-shell Composites

Facing the increasingly tense international military situation and the rapid development of electronic industry,it is urgent to explore high-performance microwave absorbing materials（MAMs）for strengthening military stealth technology and eliminating electromagnetic（EM）pollution.Due to the advantages of low density,stable chemical properties and outstanding dielectric characteristic,carbon/carbon composites have gradually become the research hotspot of lightweight,stable and qualified candidates.Nevertheless,the characteristic impedance of these carbon/carbon composites is poor,and thus they fail to display the broad response bandwidth.In addition,most of carbon/carbon MAMs reported in previous works are simply compounding two kinds of carbon components with dielectric characteristics,lacking a guiding design.In core-shell nanocomposites,through the targeted selection of core and shell components,the composites not only have the unique characteristics of different components,but also show some new characteristics that cannot be found in a single component,which provides a theoretical basis for the controllable preparation of new high-performance functional materials.In this study,a variety of graphite carbon@amorphous carbon composites with core-shell configuration are constructed by using ZIFs derivation strategy.Through the systematic analysis towards the EM properties of these as-prepared composites,the structure-activity relationship between chemical composition,compounding style,microstructure and property is explored,and the microwave absorption mechanism of different composites is further understood.By depositing SiO₂ layer with controllable thickness on the surface of ZIF-67 cube,hollow porous carbon microboxes（HPCMBs）are successfully fabricated due to the counteraction of SiO₂ layer towards organic frameworks during high-temperature pyrolysis.Compared with the carbon material derived from pure ZIF-67,HPCMBs display larger specific surface area and pore volume.The analysis of EM parameters uncovers that the large pore volume is conducive to enhance conductivity loss,interface polarization and optimize impedance matching.In addition,hollow cavity also promotes the dissipation of EM energy by providing additional path for multiple reflection of EM waves,and thus HPCMCs-2 show excellent microwave absorption performance,whose minimum reflection loss（RL）value can reach-60.7 d B（f=6.9 GHz,d=3.2 mm）,which is better than most reported porous carbon materials.ZIF-67@SiO₂ is used as precursor and phenolic resin（PR）as coating to frabricate CNTs@CMCs composite with yolk-shell microstructure through high temperature pyrolysis at 800 ^oC and template removal,where CMCs are composed of amorphous carbon shell and graphite carbon core.The introduction of amorphous carbon shell can significantly improve the impedance matching and enrich the dielectric loss mechanism,and meanwhile,the formation of graphite carbon enhances the conductivity loss of composites.As a result,CNTs@CMCs displays powerful microwave absorption,whose RL value can reach-72.5 d B（f=8.4 GHz,d=2.6 mm）,and the effective absorption bandwidth（EAB）value can reach 4.1 GHz（d=1.5 mm）.With commercial CNTs as precursor,we direct the in situ growth of ZIF-8 particles on its surface,and finally obtain porous carbon nanoparticles/CNTs（CNTs@PCNs）composites with unique core-sheath structure through high-temperature pyrolysis.By comparing dielectric parameters and microwave absorption properties between CNTs@PCNs and individual CNTs,porous carbon nanocubes（PCNCs）,physically mixed CNTs/PCNs（PM-CNTs/PCNs）composites,and large-size CNTs@PCNs（CNTs@PCNs（L））composites,it can be calculated that the effective regulation of compounding style can strengthen the synergistic effect between PCNs（amorphous carbon）and CNTs（graphite carbon）.Therefore,the microwave absorption performance of CNTs@PCNs-2 composite is significantly reinforced,whose the strongest RL value can reach up to-71.5 d B with the filling of 10 wt%and its absorber thickness of 1.1 mm.Such a property is superior to those of most carbon/carbon composites ever reported.Combined with self-assembly,in-situ growth and high-temperature pyrolysis,the r GO@NPCNs aerogel with crosslinked three-dimensional macropores and long channel is obtained.The microwave absorption mechanism of the composites was explored by using numerical simulation（NS）technology and theoretical calculation of Debye relaxation model.It is found that the introduction of NPCNs not only significantly improves the impedance matching,but also induces a large number of defects,and as a result,the polarization loss value increases from 4 to 20 in the low frequency region.At the same time,the introduction of crosslinked macropores structure can strengthen conductivity loss,and the value of conductivity loss can be maintained at 4.The synergistic effect between material loss and structural loss makes r GO@NPCNs aerogel achieve broad response bandwidth,and the EAB value is 5.1 GHz（d=1.3 mm）,covering85%of the Ku band.