The Preparation And Properity Study Of Molybdenum Carbide-based Microwave Absorbers

The development of electronic information technology,especially the emerging communication technologies,has brought great convenience to human life,improved the timeliness of network communication and intelligence of military operations.However,the pollution of excessive electromagnetic energy also brings unprecedented risks in information security and physical health of human beings.Electromagnetic absorption and shielding are two common strategies to prevent electromagnetic pollution.Among them,as an important component of electromagnetic absorption,electromagnetic absorption materials(EAMs)can directly convert electromagnetic energy into heat,and avoid secondary pollution caused by electromagnetic shielding.Magnetic metal-based materials are a kind of EAMs with better performance in previous research.However,magnetic metal has characteristics of high density,high filling volume and easy oxidization and corrodation.It is difficult to show strong magnetic loss characteristics in the process of compounding with carbon matrix.It is mainly used to adjust impedance matching and enhance dielectric loss in composites.In contrast,molybdenum carbide is an ideal dielectric loss material with high melting point,moderate dielectric constants,good mechanical and chemical stability.Molybdenum carbide composites were prepared by metal organic frameworks(MOFs)derivatization method with regular morphology,good dielectric performance,uniform and adjustable chemical composition.In this paper,the preparation and microwave absorption properties of molybdenum carbide-based composites were studied.The microwave absorption properties of composites were optimized and the internal mechanism of electromagnetic attenuation was explored by adjusting the content of molybdenum carbide,optimizing the microstructure and introducing different components.A series of Mo₂C@C nanospheres with core-shell structure were prepared by using phosphomolybdic acid and pyrrole monomer as raw materials.The polymerization of pyrrole monomer was induced by phosphomolybdic acid,along with the carbothermal reduction process at high temperature.The optimal ratio of PMo₁₂/PPy was determined by adjusting the relative content of phosphomolybdic acid and pyrrole.PL-600,PL-700,and PL-800 nanospheres were obtained by carbothermal reduction of the precursors at600,700,and 800?,respectively.When the carbothermal reduction temperature is 700?,PL-700 shows remarkable microwave absorption performance,and reaches the minimum reflection loss(RL)value of<-48 dB at 12.5 GHz with the effective absorption bandwidth(the frequency range where reflection loss is<-10 dB)of 4.1 GHz.The excellent microwave absorption properties of the core-shell Mo₂C@C nanospheres mainly come from the abundant dipole polarization provided by the defects of carbon matrix,and the interfacial polarization between the carbon shell and Mo₂C nanoparticles.Meanwhile,the micro conductive network formed by the core-shell structure can effectively bind the incident electromagnetic waves and enhance multiple reflection.Molybdic acid substituted ZIF-8 precursors were prepared using MOFs derivatization strategy and the molar ratios of 2-methylimidazole to molybdic acid(MIM/Mo)were 1.5:1,3.0:1,6.0:1,and 9.0:1,respectively.A series of Mo₂C/C polyhedral composites with ultra-small Mo₂C nanoparticles(?4.5 nm)were prepared by high temperature carbothermal reduction process,which maintained the original morphology and microstructure of pristine ZIF-8.When MIM/Mo=6.0,S-Mo₂C/C-6.0shows excellent impedance matching characteristics and electromagnetic attenuation ability.The minimum RL value is<-60 dB,and the corresponding coating thickness is1.5 mm with the effective absorption bandwidth of 4.8 GHz,which can cover the whole Ku band.The excellent microwave absorption properties are mainly due to the introduction of Mo₂C nanoparticles,which can effectively adjust the impedance matching of carbon matrix.And the ultra-small Mo₂C nanoparticles are uniformly dispersed in the carbon matrix to enhance the polarization loss.Compared with Mo₂C@C nanospheres obtained by pyrrole polymerization induced by phosphomolybdic acid,it is confirmed that the introduction of ultra-small Mo₂C nanoparticles is conducive to the improvement of microwave absorbing properties of carbon matrix.After 24 h treatment in 3 M HCl and473 K,Mo₂C/C polyhedral composites still have good microwave absorbing properties,acid corrosion resistance,and high temperature resistance.Ternary Mo₂C/Co/C nanorods were synthesized by solvothermal synthesis of MoO₃nanorods and ZIF-67.These as-synthesis composites through adjusting the relative contant of MoO₃ and Co(NO₃)₂ were named as MCC-25,MCC-50,and MCC-75,respectively,which exhibited adjustable microwave absorption properties.When the amount of MoO₃ is 30%,the minimum RL values of these samples are-44.8 dB(4.2 GHz,5.0 mm),-48.0 dB(6.6 GHz,3.0 mm),and-21.4 dB(13.8 GHz,1.5 mm),respectively.MCC-50 shows good impedance matching characteristics and low-frequency attenuation strength.The effective bandwidth can cover 3.0-18.0 GHz when the thickness of absorbing coating is between 1.5 to 5.0 mm.Compared with core-shell Mo₂C@C nanospheres and Mo₂C/C polyhedron,ternary Mo₂C/Co/C nanorods not only exhibit excellent effective absorption bandwidth,but also have strong low-frequency absorption characteristics.The excellent microwave absorption properties of these composites mainly come from abundant dipole polarization centers and interface polarization effects provided by the dispersed Mo₂C and Co nanoparticles in carbon matrix.Meanwhile,magnetic Co nanoparticles enhance the dielectric loss ability of the material by providing magnetic loss and bring the electron transmission efficiency between Mo and Co,indicating that metal nanoparticles can improve the impedance matching.Based on the above absorption mechanism,it is found that the dielectric loss is the main attenuation of the three composite materials.Ultra-small Mo₂C nanoparticles dispersed in carbon matrix bring enhanced absorption performance,and the effective absorption bandwidth covers X and Ku bands.The ternary Mo₂C/Co/C with trace magnetic metals can optimize impedance matching characteristic,showing good low-frequency absorption characteristic,covering C-band,and effectively extend the electromagnetic attenuation performance in low-frequency band.