Synthesis And Microwave Absorption Properties Of Hierarchical Nickel,Carbon And Their Composites

Accompanied with the rapid development of radio technology in the current world and the wide application of electronic products in the industrial production and daily life,a large number of electromagnetic waves have been entering into our living environment,which results in a series of problems,such as electromagnetic pollution and electromagnetic interference.In order to effectively prevent excessive electromagnetic radiation,researchers have devoted a lot of efforts into the research and development of new efficient microwave absorption materials(MAMs).This dissertation aims to investigate the effect of hierarchical microstructure construction and chemical composition regulation on microwave absorption properties,and demonstrates the design and fabrication of flower-like Ni microspheres(FNMs),waxberry-like Ni@C microspheres,carbon nanotubes wrapping waxberry-like Ni@C microspheres(NC@NCNTs),and multi-chamber carbon microspheres(MCCMs)with special hierarchical microstructure.Their micro-morphology and-structure,physicochemical properties,electromagnetic parameters,and reflection loss characteristics have been deeply investigated and discussed.In addition,by comparing with a series of control samples,the acting mechanism of the hierarchical microstructure in improving the performance of these MAMs has been clarified.Hierarchical three-dimensional FNMs are prepared by a hydrothermal reduction with flower-like Ni(OH)₂ microspheres as precursor.The results show that FNMs inherit the micro-morphology and-structure of the precursor well,and the nanosheets are assembled by many cross-linked Ni nanoparticles.The three-dimensional flower-like hierarchical microstructure is conducive to the formation of the internal conductive network and the improvement of the conductivity loss of magnetic metal materials,and thus significantly increases the complex permittivity and attenuation constant.Compared with home-made Ni particles,commercial Ni powders,and porous Ni assemblies with damaged hierarchical microstructure,FNMs can show superior microwave absorption performance.When the thickness is 1.1 mm,the effective absorption bandwidth is 3.5 GHz and the maximum reflection loss intensity is-56.8 d B.Hierarchical waxberry-like Ni@C microspheres are prepared by MOFs-derived method.The results showed that waxberry-like Ni@C microspheres are composed of many core-shell Ni@C nanoparticles with very uniform particle size.The uniform distribution of magnetic particles and high chemical homogeneity are favorable for obtaining strong interfacial polarization relaxation and good impedance matching characteristic.Benefitted from the admirable attenuation ability and impedance matching characteristics,Ni@C-700 obtained at the pyrolysis temperature of 700?behaves the best microwave absorption performance.When the thickness is 1.8 mm,the maximum effective absorption bandwidth is 4.8 GHz and the maximum reflection loss intensity can reach up to-73.2 d B.The electromagnetic analysis shows that compared with traditional core-shell Ni@C composites,hierarchical microstructure brings optimized impedance matching characteristic to waxberry-like Ni@C microspheres,which favors the improved microwave absorption performance of Ni@C-700.In order to regulate the chemical composition proportion of magnetic metal/carbon composites derived from MOFs precursors,and make microstructure construction and chemical composition regulation take their synergistic effects on improving the microwave absorption properties of composites,NC@NCNTs are designed and fabricated,whose carbon nanotubes in situ grow on waxberry-like Ni@C microspheres.By changing the mass ratio of melamine to Ni@C microspheres in the synthesis process,the loading amount of carbon nanotubes can be controllably adjusted,so as to achieve the control of the chemical components proportion and the modulation of electromagnetic properties in Ni/C MAMs derived from MOFs precursors.When carbon content is 51.1 wt%,NC@NCNTs exhibit further enhanced microwave absorption performance.With the thickness of 1.7 mm,the maximum effective absorption bandwidth is 5.2 GHz and the maximum reflection loss intensity is-41.5 d B.The electromagnetic analysis shows that multiple polarization relaxation caused by the multi-component medium and good conductivity resulted from the cross-linked carbon nanotube network make NC@NCNTs show sufficient dielectric loss ability with low filling ratio.Hierarchical multi-chamber carbon microspheres(MCCMs)are obtained by MOFs-derived method and subsequent acid etching treatment.Transmission electronic microscope analysis shows that MCCMs are composed of large quantities of small hollow carbon chambers with a pore size of about 20 nm,and the abundant mesoporous holes in the microspheres are very conducive to the multiple reflection behavior of electromagnetic waves and the improvement of effective absorption bandwidth.In consequence,MCCMs show excellent reflection loss characteristics.When the thickness is 1.8 mm,the effective absorption bandwidth can reach up to 5.7GHz,and the maximum reflection loss intensity is-28.5 d B,which is obviously superior than the other three MAMs mentioned above and many previously reported carbon MAMs.Through investigating the influence of internal structure of carbon microspheres on the electromagnetic properties,it is found that the multi-chamber hierarchical structure can improve the dielectric loss of carbon microspheres while maintaining good impedance matching,and it is also confirmed that this structure has the unique advantage of strengthening the multiple reflection and absorption effect of electromagnetic waves.