Preparation And Microwave Absorption Properties Of Rare Earth Synergistic Cobalt-based MOFs Derivatives

With the widespread use of all kinds of electronic devices in people’s daily lives,the global electromagnetic environment is becoming increasingly severe.Electromagnetic radiation pollution not only has a serious impact on human health,but also interferes with the normal operation of some important facilities.In order to improve the electromagnetic environment,electromagnetic wave（EMW）absorbing materials have attracted great research interest in both civilian and military fields.In recent years,metal-organic frameworks（MOFs）have made great contributions in the field of microwave absorption due to their ability to develop porous carbon-based MOFs derivatives with multiple features such as controllable defects,tunable structures,and variable compositions.However,studies on MOFs derivatives mainly focus on transition metals,while rare earth metals are rarely studied.To this end,the research of this paper focuses on the combination of cobalt magnetic nanoparticles and rare earth metal oxides through the unique structure of MOFs,and the study of its electromagnetic wave absorption properties.Based on this idea,two kinds of absorbing materials were prepared,and their properties were studied in this paper.（1）In this paper,a large number of uniformly sized La₂O₃/Co@N-doped porous carbon composites with regular icosahedral structure were synthesized by a two-step process of simple co-precipitation and pyrolysis using a mixture of ethanol and deionized water as solvent（1:1 v/v）and polyvinylpyrrolidone（PVP）as surfactant.The morphology of the product was analyzed by using scanning electron microscopy（SEM）and transmission electron microscopy（TEM）,and the results showed that the La₂O₃/Co@NCNTs composite obtained by one-step annealing had obvious carbon nanotubes（CNTs）on the sample surface generated,and its minimum reflection loss at 2.45 mm is-53.40 d B,and its effective absorption bandwidth（EAB）is 4.34 GHz.In addition,the comparison shows that no CNTs generation on the surface of the pretreated La₂O₃/Co@NPC composite,and its reflection loss at 4.10 mm is-20.10 d B,and the EAB is 1.45 GHz.Therefore,the emergence of carbon nanotubes has an important impact on the microwave absorption performance of La₂O₃/Co@NCNTs composites,which can effectively improve the conduction loss.In addition,the presence of La₂O₃ nanoparticles in La₂O₃/Co@NCNTs optimizes impedance matching,while Co metal nanoparticles can increase magnetic loss.The synergy of metal oxides,magnetic nanoparticles and carbon nanotubes gives La₂O₃/Co@NCNTs excellent microwave absorption properties.（2）Co Ce-BDC was synthesized by hydrothermal reaction with the organic solvent N,N-dimethylformamide（DMF）as the solvent and assisted by PVP,and then subjected to pyrolysis treatment to obtain Co/Ce O₂/NC composites.Through thermogravimetric analysis,we obtained the pyrolysis law of the sample and studied the structure,morphology,and microwave properties of the composite material at different temperatures.In addition,the microstructure of the samples was characterized by X-ray diffractometer（XRD）,SEM,and TEM.When the pyrolysis temperature was850 ^oC,the Co/Ce O₂/C-850 composites exhibited the outstanding microwave absorption in the frequency range of 2-18 GHz with a minimum reflection loss(RL_min)of-45.22 d B and an effective absorption bandwidth（EAB）of 4.61 GHz at a thin thickness of 1.75 mm.The microwave absorption performance of Co/Ce O₂/C composites is mainly attributed to the dielectric loss due to interfacial polarization originating from different interfaces and dipole polarization caused by the oxygen vacancies in Ce O₂.In addition,the introduction of the Co nanoparticles not only provides the magnetic loss but also modulate the impendence matching for the current magnetoelectric coupling system.