Characterization And Microwave Electromagnetic Properties Of Core-shell Structured CoFe?Ni?@MO₂?M=Si,Ti?

With the rapid development of modern new weapons,high-performance electromagnetic wave absorbing?EMA?materials with strong absorption properties,wide absorption frequency and good oxidation resistance is highly demanded.Ferromagnetic metal/alloy particles have high-performance broadband microwave absorption ability as microwave absorbers due to their unique ferromagnetic features.However,the excellent electromagnetic properties of this kind of materials have not been fully revealed because of the eddy current effect and low filling ratio.To solve these problems,one effective method is to use dielectric coating to fabricate core-shell structured composite that integrate the ferromagnetic components and dielectric components together.The existence of ferromagnetic/dielectric interfaces is likely to generate new electromagnetic losses and further improve the electromagnetic wave absorption performance.Herein,aiming at ferromagnetic/dielectric composites with high-performance,Co Fe?Ni?were firstly synthesized.The electromagnetic properties were regulated by the introducting of Fe and Ni.Then,Co Fe?Ni?@MO₂ composite with excellent antioxidant capacity were obtained by dielectric coating.Subsequently,the electromagnetic properties were further optimized through hydrogen thermal annealing.Firstly,cobalt microspheres with diameters of 0.5–3.5 ?m,Co Ni microspheres with conical bulges and uniform Co₇Fe₃ nanospheres with diameters ranging from 350 nm to 2 ?m were synthesized by liquid phase reduction method.The introduction of Fe element can improve the saturation magnetization.The introduction of Ni element can produce the ferromagnetic resonance frequency at 5.1 GHz,thus the magnetic loss performance of Co Ni particles is significantly enhanced compared with that of Co particles.Co Fe?Ni?@Mi O₂ core-shell composite particles with different shell thicknesses were fabricated using a sol-gel method.The oxidation temperature considerably increases after Co Fe?Ni?particles are cladded by Si O₂ shells.The influence of filling ratio on the electromagnetic properties of specimens containing Co and Co@Si O₂ microspheres as fillers was examined.It is found that the effect of filling ratio on the electromagnetic properties presented a linear rule.The dielectric relaxation and permittivity increase with the increase of filling ratio.The influence of MO₂ coating on the electromagnetic properties of specimens containing Co Fe?Ni?microspheres as fillers was examined.It is found that microspheres are separated completely by the induction of MO₂ coating in Co and Co₇Fe₃,which can effectively avoid local agglomeration and weaken the electric dipole polarization,leading to decreased permittivity.However,the Co Ni microspheres in a small range were completely wrapped and formed into particles with a certain aspect ratio during the coating process,which enhanced the dipole polarization and local eddy current effect.The influence of annealing on the microstructure evolution and electromagnetic properties of Co Fe?Ni?@MO₂ composite microspheres were investigated.Upon annealing,the Ms increased due to the elimination of crystal defect and improvement in crystallinity.The significantly increased Ms contributes to an increase in the permeability.In addition,Ti O₂ was crystallized to anatase phases after annealing,which is contributed to a greatly increase in conductivity.Thus,the permittivity is increased.Analysis of electromagnetic wave absorption performance of Co Fe?Ni?revealed that the EMA performance of coating containing Co₇Fe₃ and Co Ni as fillers was significantly enhanced compared with that of Co.When Co₇Fe₃ with diameter of 350 nm is used as the fillers,the coating with thickness of 1.59 mm possesses a maximum reflection loss of 78.4d B and an effective absorption bandwidth of 6.7 GHz?10–16.7 GHz?.When Co Ni microspheres is used as the fillers,the absorption bandwidth with RL higher than 10 d B is 9.6 GHz?8.4–18 GHz?,covering the whole X-Ku band with a matching thickness of 2.5 mm.Upon Ti O₂ coating and hydrogen thermal annealing,the EMA performance is significantly improved.For example,an effective absorption band is 2.0 GHz in width within C band at a thickness of 2.62 mm when Co₇Fe₃@Ti O₂ is used as the fillers.The maximum reflection loss is 76.6 d B,the effective absorption band is 1.2 GHz within S band when Co Ni@Ti O₂ is used as the fillers.