| Nowadays,the use of many electronic devices enriches our daily lives,but the electromagnetic pollution caused by them needs to be solved urgently.Microwave absorbing materials are one of the effective means to solve electromagnetic interference.In addition,with the development of radar detection technology,higher requirements have been placed on the survival and stealth capabilities of military weapons.The disadvantages of single loss mechanism,high density and weak attenuation ability of traditional absorbing materials have restricted their development.The effective solution is the reasonable design of the material microstructure and the compounding of materials with different loss mechanisms to meet the goal of "low density,broadband,strong absorption,and electromagnetic matching".In addition,the air embedded in the hollow structure is beneficial to improve the impedance matching,and the existing cavity is beneficial to the reflection of electromagnetic waves,extend the propagation path of electromagnetic waves for the loss ability.The residual carbon rate of the phenolic resin is moderate,and the preparation conditions are mild and controllable.Hence,we have constructed a series of composite materials of hollow carbon and small-sized magnetic nanomaterials and studied the influence of the structure and composition of the material on its absorbing properties.The main contents are:1.A bird-nest-patterned C@Fe3O4(BN-HDCF)composite material with a hollow doubleshell structure composed of bowl-shaped carbon as the core and magnetic Fe3O4 nanosheets as the shell was prepared by the St?ber method and the solvothermal method.The composite structure of different shell thickness can be obtained by adjusting the addition amount of the precursor,and then the electromagnetic parameters and microwave absorption performance can be adjusted.The results show that when the precursor content is 500 mg and the filling content is 20 wt%,the maximum effective absorption bandwidth(EAB)is 6.1 GHz with the thickness of 2.23 mm,and the reflection loss(RLmin)value as high as-80 d B with the thickness of 2.45 mm,more importantly,the entire X-band absorption can be achieved with the thickness of 3.1mm,which is significantly enhanced compared to pure carbon(P-C)and pure Fe3O4(P-Fe3O4).Its excellent microwave absorption ability is mainly attributed to the existence of multiple interfaces which lead to the enhancement of interface polarization.The existence of hollow and bowl-shaped structural grooves increases the scattering and reflection of electromagnetic waves and improves the dielectric loss ability of the material.The suppression of the eddy current effect by the magnetic Fe3O4 is beneficial to impedance matching and the contribution to the magnetic loss mechanism such as resonance.2.A C/MQDs composite material with a hollow structure composed of hollow carbon as the shell layer and magnetic Fe3O4 quantum dots(MQDs)scattered on the outer and inner walls of it,was prepared by a hard-template method and solvothermal method.The number of magnetic quantum dots on the surface of the carbon shell is controlled by adjusting the amount of ferric nitrate added,thereby adjusting its electromagnetic parameters and microwave absorption performance.The results show that when the additional amount of ferric nitrate is200 mg,at a low filling amount of 15 wt%,the EAB is as broad as 7.06 GHz with a thickness of 2.55 mm,and the RLmin can reach-43 d B with the thickness of 2.35 mm.In addition,the EAB of 5.02 GHz at 3.3 mm can completely cover the full X-band.The excellent performance can be attributed to the large number of dangling bonds in the magnetic quantum dots and the presence of defects in the amorphous carbon leading to the enhancement of dipole polarization,the suppression of the eddy current effect by the small size effect of Fe3O4 MQDs,and the contribution of the magnetic loss mechanism.Moreover,the hollow structure increases the propagation path of electromagnetic waves and improves the electromagnetic wave dissipation capability.Such an impressive structure provides a new direction for the design of new composite absorbing materials. |
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