Structures And Electromagnetic Properties Of Dielectric Shell Coated Ferromagnetic Metal Micro-and Nano-particles

The ferromagnetic metal micro/nanoparticles with diferent elements and components were synthesized by a simple, high-yield and low cost chemical reduction method at the temperature of 80 oC. The ferromagneic metal micro/nanoparticles possess the sphere-, flower-, leaf- and chain-like structures. The ferromagneic metal micro/nanoparticles were coated by silica and carbon dielectric materials using a sol-gel method and a hydrothermal processs, respectively. In additional, FeNi₃@C core shell structured chains were obtained with a simple one-pot hydrothermal method. The morphology, composition and microstructure of the as-prepared samples were characterized with SEM, TEM, EDS and XRD, respectively. The static magnetic properties were measured by PPMS at room temperature. The electromagnetic properties of the paraffin matrix compositions containing ferromagnetic metal micro/nanoparticles as fillers were measured by VNA at the 2-18 GHz frequency region. The electromagnetic wave absorption performances were calculated and valued from electromagnetic paramters by transmission line theory.The investigation of static magnetic properties clearly shows that the static magnetic properties of ferromagnetic alloy micro/nanoparticles are related to the element type and component. FeNi alloy micro/nanoparticles show low coercivity. In contrast, CoNi alloy microparticles present relatively high coercivity. The FeCo alloy micro/nanoparticles possess very high saturation magnetization, which is higher than bulk iron or cobalt. The high saturation magnetization reveals that the alloy micro/nanoparticles prepared by self-catalyzed reduction method at a relatively low temperature are FeCo alloy phase, rather than the simple mixtures between iron and cobalt microparticles. If the element types were given, the static magnetic properties are relatived with the alloy element molar ratio. For example, for Co_(1-x)Ni_x alloy micro/nanoparticles, the coercivity, saturation magnetization and remanance increase with the decreasing of Ni conent regularly.Studies of electromagnetic properties for ferromagnetic matal micro/naonparticles revealed that the size, morpholoy and structures can effectively influence their electromagnetic properties. The ferromagnetic matal microparticles possessing smaller size, larger surface area and shape anisotropy have higher complex permittivity and more remarkable dieletric polarization and relaxation. For the given weight ratio, the samples with smaller size and larger surface area have larger volume ratio, which results in higher complex permittivity. They also have larger conductor/insulator interfaces, which bring more remarkable dielectric polarization and relaxation. In addition, the samples with smaller size, larger surface area and shape anisotropy have higher complex permeability and natural resonance frequency, which comes from several reasons, such as high surface ion, large magnetic anisotropy field and the effective suppressing of eddy current effect. The remarkable dielectric relaxation and high natural resonance result in high dielectric and magnetic loss, which are favorable to the enhanced electromagnetic absorption performances.According to transimission line theory, the electromagnetic absorption performance of ferromagnetic alloy micro/nanoparticles was calculated, calculated results revealed that ferromagnetic alloy micro/nanoparticles are ideal candidates for electromagnetic absorption materials. For example, the Fe_0.14(CoNi)_0.86 alloy nanoparticles present excellent electromagnetic absorption performance. A reflection loss exceeding -10 dB can be obtained within the whole 2-18 GHz frequency range choosing an appropriate matching thickness. The effective absorption band (>-10 dB) is wide up to 8 GHz at a given thin absorping thickness of 1.8 mm. In particular, the minimiu reflection loss of -58.9 dB is obtained with thin layer thickness of 2.3 mm. In addtion, hererin the electromagnetic parameters of complex permittivity and permeability were adjusted by the element type and component of ferromagnetic alloy micro/nanoparticles. Because the electromagnetic absorption performance is dependenced on the electromagnetic parameters, the electromagnetic absorption performance were adjusted by the element type and component. For example, for flower-like Fe_(1-x)Co_x alloy micro/nanoparticles, the electromagnetic absorption performance were tuned by the molar ratio of Fe/Co. The effective absorption band locates at S-band (2-4 GHz) and C-band (4-8 GHz) for the Fe_0.4Co_0.6 alloy, X-band (8-12.4 GHz) and Ku-band (12.4-18 GHz) for the Fe_0.5Co_0.5 alloy, C-band and X-band for the Fe_0.6Co_0.4 alloy micro/nanoparticles, respectively. The significant influence of component on the intensity and location of effective electromagnetic absorption band was investigated in this Fe_(1-x)Co_x alloy system.The investigation of SEM and TEM clearly revealed that the introduction of silica and carbon dielectric shells. The thickness of SiO₂ shell is about 25 nm for FeNi₃@SiO₂ core shell structures, and the thickness of carbon shell is about 10 nm for FeNi₃@C core shell structures. The dielectric shells can effectively isolate and insulate ferromagnetic alloy micro/nanoparticle from agglomeration, oxidation and corrosion. It was found that magnetic coupling between ferromagnetic alloy micro/nanoparticles was restrained. The dielectric coating can decrease the conductivity of ferromagnetic alloy micro/nanoparticles, so the eddy current effect can be suppressed effectively. As well known, the disadvantage of high density limited the application potential of ferromagnetic alloy micro/nanoparticles, while the compositions of ferromagnetic alloy microparticles and dielectric possess lower density than metal materials. For EMA materials, the complex permeability (εr=ε'- jε"), permittivity (μ= rμ'- jμ") and their electromagnetic impedance match determine the reflection and attenuation characteristics. Comparing with individual metal and dielectric materials, the compositions of metal and dielectric materials present more appropriate impedance match, which results in the enhanced electromagnetic absorption performance.