Study On Microwave Loss Mechanisms Regulation And Microwave Absorption Properties Of Magnetic Dielectric/carbon Composites

Hollow,yolk-shell,and core-shell structure magnetic dielectric/carbon composites have the advantages of large specific surface area,light weight,unique electromagnetic properties,etc.,which are of great interest in the current research of microwave absorption materials.However,the poor electromagnetic synergy also results in the low microwave absorption strength of these special structural composites,with effective absorption bandwidths generally less than 6.0 GHz,which is difficult to meet the microwave absorption in the currently widely used 2-18 GHz band.The main reasons for the poor electromagnetic synergy of these composites are as follows:（1）the uncontrollable graphitization of the carbon components of the composite makes it difficult to generate multiple polarization relaxation processes in the high-frequency band（10-18 GHz）,reducing the frequency range of electromagnetic synergy;（2）the compact hierarchical structure makes the composite exhibit uncontrollable multiple exchange resonances in the high-frequency band,making it difficult to form effective co-frequency synergy with polarization relaxation;（3）the single scale of the magnetic medium（nano or submicron）prevents the composites from simultaneously producing strong exchange resonances and natural resonances in the high and low-frequency bands（2-10 GHz）,reducing the electromagnetic synergy of the materials throughout 2-18 GHz.To improve the electromagnetic synergy and microwave absorption properties of specially structured magnetic dielectric/carbon composites,the following studies have been carried out in this paper:（1）Hollow Ni/C microspheres were prepared by vacuum heat treatment using Ni（OH）₂/C microspheres as precursors.The effects of different heat treatment temperatures（650,700,and 750℃）on the high-frequency polarization relaxation and microwave absorption properties of Ni/C microspheres were studied.At the heat treatment temperature of 700°C,the Ni/C microspheres exhibit a dual polarization relaxation process（dipole polarization and interfacial polarization）at high frequencies due to the formation of equivalent electric dipoles from carbon defects,oxygen-containing groups,and Ni-C heterogeneous interfaces.Meanwhile,the local magnetic moments generated by the carbon defects,oxygen-containing groups,and Ni-C heterogeneous interface also enable the Ni/C microspheres to acquire dual exchange resonance in the high-frequency band and form a co-frequency synergy with the dual polarization relaxation,effectively enhancing the high-frequency electromagnetic synergy of Ni/C microspheres.The good electromagnetic synergy not only optimizes the impedance matching of Ni/C microspheres but also improves their attenuation coefficient.In comparison with the homologous composites,the prepared Ni/C microspheres exhibit more excellent microwave absorption properties.At 11.60 GHz,Ni/C microspheres obtain a minimum reflection loss of-48.74 d B and the effective absorption bandwidth reaches 7.35 GHz（10.65-18 GHz）.（2）Hollow（Ni/C）/Zn Fe₂O₄ composites were prepared by introducing submicron hollow Zn Fe₂O₄ into the prepared hollow Ni/C microspheres.The effects of different Zn Fe₂O₄ additions（0,6,12,and 18 wt%）on the low-frequency microwave loss mechanism and microwave absorption properties of the composites were investigated.The results show that the space charge polarization and natural resonance of Zn Fe₂O₄ enhance the low-frequency electromagnetic synergy and microwave absorption strength of the composites.When the addition of Zn Fe₂O₄ is 12 wt%,the resulting（Ni/C）/Zn Fe₂O₄composite obtains a minimum reflection loss of-38.64 d B and an effective absorption bandwidth of 2.16 GHz（4.33-6.49 GHz）in the low-frequency band.Meanwhile,the composite also maintains good high-frequency properties,with a minimum reflection loss of-43.07 d B at 12.67 GHz,corresponding to an effective absorption bandwidth of 5.63 GHz（10.30-15.93 GHz）.（3）The hierarchical hollow structure and yolk-shell structure Co@C composites were prepared by vacuum heat treatment using Co organometallic framework（Co-MOF）and Co-MOF@phenolic resin（Co-MOF@PR）as precursors.The squeezing force generated by the PR carbonization shrinkage disperses the structural units of the internally hierarchical hollow structure Co@C,weakening the effect of the exchange magnetic coupling between the structural units on the exchange resonance,so that the high-frequency exchange resonance of the yolk-shell structured Co@C composite is dominated by the local magnetic moments formed by the carbon defects,oxygen-containing groups,and the Ni-C heterogeneous interface,realizing the co-frequency synergy between its dual exchange resonance and polarization relaxation.This co-frequency synergy enhances the impedance matching and attenuation coefficients of the yolk-shell structured Co@C composite,enhancing its microwave absorption properties.At 12.44 GHz,the yolk-shell structured Co@C composite achieves a minimum reflection loss of-48.02 d B and an ultra-wide effective absorption bandwidth of 8.20 GHz（9.65-17.85 GHz）.（4）Core-shell Fe₃O₄@C composite with multi-scale（nano and submicron）core structure was prepared by vacuum heat treatment using Fe₃O₄@PR as the precursor.The effects of different scale Fe₃O₄ core structures on the ferromagnetic resonance and microwave absorption properties of the composite were investigated.The results show that the multi-scale core structures enable the composite to exhibit strong natural and exchange resonances at 2-18 GHz,forming a good synergy with the conductivity loss and polarization relaxation in the same frequency band.This effectively enhances the impedance matching and attenuation coefficients of the composite.In comparison with homologous composites,the prepared Fe₃O₄@C composite exhibits a stronger microwave absorption capacity and a wider effective absorption bandwidth.Its minimum reflection loss and effective absorption bandwidth are-54.34 d B and 6.42 GHz（11.58-18 GHz）for high frequencies,and-53.36 d B and 3.45 GHz（5.48-8.93 GHz）for low frequencies,respectively.The above studies suggest that the dielectric and magnetic loss mechanisms of the composites can be effectively regulated through the adjustment of the graphitization degree,the group element types,and scale,enhancing the electromagnetic synergy and microwave absorption properties of the composites.These studies provide effective ideas for the design of high-performance microwave absorption materials,and the resulting composites can be applied to microwave absorption in the 2-18 GHz frequency band.