Preparation Of MOFs Derived Carbon Based Nanomaterials For Electromagnetic Absorption

With the continuous advances in high-tech communication and radar detection technology,the accompanying problems of electromagnetic interference,electromagnetic radiation and the existential threat to weapons and equipment need to be resolved immediately.The development of electromagnetic absorbing materials is significant for their essential role in solving the above problems.It could effectively convert incident electromagnetic wave into heat or other forms of energy,so as to achieve the purpose of absorbing electromagnetic wave.Recently,MOFs derived carbon-based electromagnetic absorbing materials have attracted attention abroad on account of their advantage of simple synthetic methods,porous structure as well as adjustable composition and structure.Although MOFs derived carbon-based electromagnetic absorbing materials have achieved preliminary results at present,it should be noted that most MOFs derived carbon nanomaterials present the low aspect ratio polyhedral structure,and the microstructure and composition of their derivatives depend on the original MOFs structure.Thus,most MOFs derived carbon-based materials could not simultaneously realize the strong absorption capacity and wide absorption owing to the insufficient electromagnetic wave loss capacity and less transmission paths of polyhedral structure materials.To well resolve these issues and boost the electromagnetic absorbing performances,we have rationally designed and fabricated a series of MOFs derived carbon nanomaterials with unique microstructure.The series of leaf-like bimetallic CoZn-ZIF-L with adjustable ratio of Co and Zn were chosen as precursor to fabricate the hierarchical Co/N-decorated carbon architecture comprising of carbon nanotubes grafted on carbon nanoflakes(abbreviated as Co NC/CNTs)through one-step pyrolysis.Co NC/CNTs integrate the advantages of in-situ generated nanotubes for multiple polarization loss of electromagnetic wave,and nanoflakes with interconnected porous structure for multiple reflection losses of electromagnetic wave as well as optimization of impedance matching.The resultant Co NC/CNTs demonstrate excellent electromagnetic absorbing performance.The largest effective bandwidth range of Co NC/CNT-3/1 achieved 4.5 GHz with a thickness of only 1.5 mm when the weight ratio of 20 wt%.Its maximum reflection loss reached up to-44.6 d B at 5.20 GHz when the weight ratio of 20 wt%.Moreover,the Co/N co-doped hybrid carbon networks consisting of different structural units,including carbon nanosheet network(H-700),carbon nanotube interconnected carbon nanosheet(H-800),and interweaved carbon nanotube network attached carbon nanosheet(H-900)were constructed through pyrolysis of acidified ZIF-L at different temperatures.Hybrid network structures enriched the electromagnetic wave transmission path,enhanced the multiple scattering and optimized impedance matching.Consequently,H-800exhibited strong absorption ability with maximum reflection loss of-56.2 d B.H-900 had broad absorption bandwidth with the effective absorption bandwidth of 5.2GHz when weight ratio was only 10 wt%.To further enhance the electromagnetic transmission path of carbon-based absorbent materials derived from low aspect ratio polyhedral MOFs,it is necessary to connect MOFs derived polyhedral structures with low aspect ratio materials through an effective method.Inspired from the above in-situ formation of carbon nanotubes,the additional carbon source melamine was introduced to FeNi-MIL-88B as the precursor.After one-step pyrolysis,FeNi-MIL-88B derived nanostructure were in situ anchored into interconnected carbon nanotube networks,constructing FeNi₃and N embedded carbon-based absorbing material with interconnected networks(FeNi@CNT/CNRs).It could not only construct the hybrid network structure,but also avoid the collapse and agglomeration of the pore structure of FeNi-MIL-88B derived carbon materials through rationally controlling the mass ratio between FeNi-MIL-88B and melamine.So,FeNi@CNT/CNRs have achieved a high specific surface area and porosity,which has effectively optimized the impedance matching.On account of multiple transmission paths and multiple dielectric loss of synergistic contributions of carbon nanotube networks and MOFs derived carbon-based materials,FeNi@CNT/CNRs nanoarchitectures delivered excellent electromagnetic absorbing performances.In particular,the optimized FeNi@CNT/CNR-0.9 obtained maximum reflection loss of-47.0 d B at the thickness of 2.3 mm,and broadband effective absorption of 4.5 GHz at thickness of1.6 mm.To simultaneously achieve the strong electromagnetic wave dissipation ability and electromagnetic wave transmission path,the composite CoNi-BTC/graphene was chosen as precursor to fabricate the carbon-based absorbing material CoNi@NC/rGO-600 with hierarchical structure and hybrid network together through one-step pyrolysis.CoNi-BTC derived pomegranate-like hierarchical CoNi@NC nanosphere were uniformly dispersed on the surfaces of three-dimensional graphene network modified by small CoNi nanoparticles.Herein,the multiple heterogeneous interfaces resulted from graphitized carbon shell and small CoNi nanoparticles in CoNi@NC nanospheres effectively promoted interfacial polarization loss.The graphene network supplied multiple electron transportation paths.Moreover,the combination of the two structures well optimized the impedance matching of CoNi@NC/rGO-600.Consequently,the CoNi@NC/rGO-600 exhibited strong wideband electromagnetic absorbing performances.Its maximum reflection loss and effective absorption bandwidth reaches-68.0 d B and6.7 GHz,respectively.To further extend the application of MOFs derived flexible electromagnetic absorbing materials,the nano-arrays and second assembly structures of ZIFs unit were grown on the surface of cotton fibers cloth to prepare the MOFs based flexible precursors with hierarchical structure.After two-step morphology preserving pyrolysis process,the ZIFs unit derived hollow Co₃O₄ nanoparticles doped carbon frameworks were uniformly attached on flexible carbon-fiber cloth,obtaining the flexible carbon-based films(CC/ZIFs-300)with three-dimensional braided network structure and hierarchical structure.The hierarchical structure coupled with carbon-fiber cloth effectively promoted electron transportation,dielectric loss,multi-scattering and impedance matching.Furthermore,flexible electromagnetic absorption films were prepared using the carbon-based film and PDMS as absorbent and substrate material,respectively.As expected,the obtained flexible electromagnetic absorbing films exhibit broadband absorption performance,and the largest effective absorption bandwidth reached 11.6 GHz.