Preparation Of Magnetic Particles Coated By Polydopamine-carbon And Study On Microwave Absorption Performance

Nowadays,the excessive use of electronic equipment has brought about tremendous electromagnetic radiation.Then there are many subsequent problems appearing,such as,be serious harmful to human beings,having strong interference with other equipment,and having greater harm in the military filed.Therefore,the research of microwave absorbers（MAs）has become more and more significant.At present,the research hotspots of MAs are mainly based on magnetism/carbon materials.For magnetism/carbon composite,undergoing special design in structure can dramatically boost microwave absorption performance（MAP）.Nowadays,the design of core-shell structure seems to popular.At the same time,the core-shell structure can be divided into three types:（1）solid type;（2）egg-shell type;（3）hollow type.Compared to egg-shell and solid types,the hollow type has the largest void space.Therefore,there are many advantages in designing and fabricating this type of materials.Therefore,in this paper,we use two different methods（soft template and hard template）to synthesize two types of MAs with hollow structure.（1）We use CoNi-glycerate solid spheres（CoNi-G）as the soft template to synthesize hierarchical CoNi-based carbonate hydroxide hollow microspheres（CoNi-GH）through simple hydrothermal process.Then,the polydopamine（PDA）is chosen as the shell and the CoNi-GH is selected as the core,the hierarchical carbon-coated CoNi alloy hollow microspheres are fabricated through a two-step process,including in-situ polymerization and pyrolysis.Different calcination temperatures make a difference in morphology,microstructure,the size of CoNi alloy and electromagnetic parameters.Appropriate calcination temperature（700℃）endows CoNi@NC-700（the sample with the calcination temperature of 700℃）the best MAP.The minimum reflection loss exhibited by CoNi@NC-700 can unexpectedly achieve-69.58 d B.At the same time,the thickness and frequency of this material are 2.99 mm and 10.09 GHz,respectively.Meanwhile,the entire X band can be covered by this material and the corresponding thickness is 3.08 mm.The maximum effective absorption band width（MEAB）achieved by CoNi@NC-700 is 5.3 GHz（12.7-18 GHz）with a thinner thickness（2.13 mm）.The best MAP can be due to two factors:（1）the strongest attenuation ability;（2）the best impedance matching.（2）Based on one-pot method,we employ silica as the hard template.In the reaction process of tetraethoxysilane,the dopamine and nickel chloride are poured into the system simultaneously,and the silica nanospheres coated by Ni-polydopamine complex are fabricated successfully.The hollow carbon spheres/Ni composite（HNC€Ni）are synthesized after high-temperature calcination and etch.The introduction of Ni Cl₂·6H₂O has three positive effects:（1）May reduce the reaction rate of dopamine,thereby reducing the degree of agglomeration;（2）Improve the thermal stability;（3）Be beneficial to boost the degree of graphitization of carbon.The product named as HNC€Ni-2,which means the addition amount of Ni Cl₂·6H₂O is 1.0 g,possesses the minimum reflection loss of-62.96 d B,and the thickness is 4.2 mm at the same time.The MEAB with a value of 6.8 GHz（11.2-18 GHz）can appear at the thickness of 2.7 mm,whose coverage area includes the whole Ku band.The excellent MAP can be derived from two advantages:（1）Higher interfacial polarization;（2）Higher conduction loss.