Preparation And Absorbing Properties Of Cobalt Ferrite And Its Core-shell Structure Composites

The requirements of electromagnetic pollution prevention and military stealth technology for absorbing materials are gradually increasing.Therefore,it is of great significance to develop absorbing materials with both low reflection loss and effective absorption frequency bandwidth.Although cobalt ferrite has been widely studied as a wave absorbing material,it still has many defects,such as poor impedance matching and high density.Therefore,this paper studies the preparation of cobalt ferrite and its wave-absorbing performance enhancement.The specific research contents and conclusions are as follows:（1）Nano-cobalt ferrites with mixed spinel structure prepared by the coprecipitation-hydrothermal method,and the effects of reaction raw materials,crystallization temperature,and crystallization time on the microstructure and wave-absorbing properties of cobalt ferrite were investigated.The results show that the spherical cobalt ferrite prepared at about 35.00 nm has the best absorption performance under the conditions of CTAB as the surfactant,cobalt acetate hexahydrate and ferric chloride hexahydrate as reaction materials,p H=11,crystallization temperature 180℃and crystallization time 8 h,and the minimum reflection loss is-8.03 d B when the thickness of absorption layer is 2.50 mm and the frequency is 13.50 GHz.The reaction materials,crystallization temperature,and crystallization time mainly affect the grain size of cobalt ferrite and the formation ofα-Fe₂O₃impurity phase to make the difference in absorption properties.（2）The effects of the substitution of two rare earth elements,neodymium,and gadolinium,on the microstructure and absorbing properties of cobalt ferrites,were investigated.The results show that the optimal doping molar amount of neodymium and gadolinium are both x=0.04,Both rare-earth ions can reduce the grain size of cobalt ferrite and increase the lattice constant.The substitution of both rare-earth ions will reduce the saturation magnetization of cobalt ferrite and enhance the coercive force and magnetocrystalline anisotropy.The reason why the doping effect of gadolinium ion is better than that of neodymium ion is that gadolinium ion can make the grain size of cobalt ferrite smaller,and the higher interface polarization increases the dielectric loss.The enhancement of coercivity and magnetocrystalline anisotropy also increases the magnetic loss.When the thickness of the absorption layer of Co Fe_1.96Gd_0.04O₄is 2.50mm and the frequency is 13.75 GHz,the minimum reflection loss value is-14.83 d B,and the effective absorption bandwidth is 2.04 GHz（12.73～14.77 GHz）.Compared with cobalt ferrite,the reflection loss value is increased by 84.68%,and the effective absorption band is widened by 2.04 GHz.（3）Cobalt ferrite@carbon core-shell composites were prepared by a two-step hydrothermal method.The results show that the carbon shell is an amorphous structure.When the carbon content is 6.01%,and the thickness of the absorption layer is 2.00 mm,the reflection loss reaches a minimum value of-25.66 d B at a frequency of 13.92 GHz,and the effective absorption bandwidth is 4.59 GHz（11.20～15.79 GHz）.Compared with cobalt ferrite,the density is reduced by 20.78%,the reflection loss value is increased by219.55%,and the effective absorption band is widened by 4.59 GHz.The main reason for the enhancement of wave absorption performance is that the interface polarization formed by cobalt ferrite and carbon shell and the dipole polarization formed by abundant functional groups in the carbon shell greatly enhance the dielectric loss,and at the same time,the composite material forms a good impedance matching.（4）Gd-Co ferrite@silica@carbon core-shell-shell composites were prepared by in-situ polymerization of silica.The results show that the silicon dioxide layer has an amorphous structure.When the thickness of the absorption layer is 2.50 mm,the reflection loss value reaches a minimum value of-46.85 d B at a frequency of 9.93 GHz,and the effective absorption bandwidth is 3.14 GHz（8.74～11.88 GHz）.Compared with Gd-Co ferrite,the reflection loss is increased by 215.91%,and the effective absorption band is widened by 1.10 GHz.The mechanism of the enhancement of wave absorbing performance is that the multi-shell structure can form multiple interface polarizations,further enhance the dielectric loss,and form a good impedance matching.In addition,the close packing between the multi-layer core-shell structure and the particles increases the propagation path of the incident electromagnetic wave and causes multiple scattering,which intensifies the attenuation of the incident electromagnetic wave.This paper has 92 figures,19 tables,and 203 references.