Synthesis And Electromagnetic Properties Of Fe-based Nanocomposites With Magnetic Core And Dielectric Shell

Gigahertz-band microwave paraphernalia,such as mobile phones,air traffic/railway controls systems,local area networks,radar systems,Wi-Fi,W-LAN,computers,etc.,are being used more and more in modern days.Due to the enormous application of electromagnetic waves(EMW),signals released from these devices have become a major threat to public/natural health and other electronic devices in the form of radiation contamination.Nanocomposite materials become the critical research subject due to their great potential in solving a lot of modern area problems such as energy,climate,safety,microwave emissions,and health.Nanocomposites have a stronger synergetic effect and physicochemical efficacy that depends on the interface structures of the core and shell.In this context,the core/shell type nanocomposites are highly demanded in the field of microwave absorption,to combine the magnetic/dielectric phases into the core/shell nanostructure which could consume the microwave energy through the magnetic resonances,dielectric interfacial polarizations,relaxation polarizations,eddy current effect or conduction loss effect.In the past,three main forms of microwave absorbing materials were used for micro wave absorption:dielectric,magnetic,and conductive loss materials;though the protection against EMW damage was obtained partly by either absorbing or shielding from these absorbents.But the development of microwave absorbing materials with high absorption,lightweight and low cost with a wider effective bandwidth is a matter of urgency to reduce ever-increasing EMW pollution.Generally,in core/shell style nanocomposite,EMW performance is governed largely by morphology,size distribution,surface,composition and microstructure,etc.As EMW pollution problems are fatal for human health as well as the electronic devices’ functionalities,thus for the solutions of EMW pollution;this dissertation makes an attempt to focus on the fabrication of magnetic/dielectric type nanocomposites,characterization and understanding their electromagnetic behaviors.Fe-based magnetic cores coated or combined with the dielectric shells of SiO2,FeSi/SiO2,SiC@C,ZrO2 at the nanometer-scale,were synthesized by a simple facile route of the arc-discharge plasma.Using an online optical emission spectroscopy diagnosis,the formation mechanisms of Fe@SiO2 nanocapsules and Fe@FeSi/SiO2 nanocomposites were understood with the information about higher energetic/temperature ions within the arc plasma.The oxidation potentials of metallic elements in Ellingham diagram were also referred for the formation of dielectric shells on the magnetic Fe core.The experimental results favor to establish the corresponding relationships between the preparation condition and the resultant structure/morphology of nanocomposites,also provide a guide to the design and control of pertinent nanostructures.Subsequent measurements of the complex permittivity and permeability in 2-18 GHz frequency range were performed,and the reflections losses of all composites were also estimated and analyzed.The static magnetism and thermal stability of all composites were also studied in detail.Key achievements in this dissertation are summarized as below:(1)Spherical Fe@SiO2 nanocapsules(NCs)are synthesized by arc-discharge plasma under a mixture atmosphere of H2 and Ar.The high energy states of excited ions(Ar,H,Fe,Si)within the plasma region are in situ recorded by online optical emission spectroscopy(OES)and established visible evidence for the energy conditions for the formation of Fe@SiO2 NCs.The estimated electron temperature of the plasma is～3×104 K(～2.6 eV)in formation of the Fe@SiO2 NCs.Ferromagnetic Fe@SiO2 NCs demonstrates a partial transition into the superparamagnetism at TB=160 K.Electromagnetic parameters in frequency range of 2-18 GHz display that an appropriate impedance matching has been well established in the Fe@SiO2/paraffin composite,which brings an excellent reflection loss(RL)of-61.6 dB at 9.3 GHz with a thickness of 2.81 mm,a wide bandwidth from 6 to 18 GHz with RL≤-10 dB.(2)The Fe/FeSi@SiO2 nanocomposites are fabricated by arc-discharge plasma under a pure argon atmosphere,using compacted Fe and Si powders as the raw target.Online OES diagnoses reveal the energy states of Ar/Fe/Si ions in the plasma,the determined electron temperatures referring spectral lines of Ar,Fe,and Si ions are 23513 K(2.02 eV),23225 K(2.00 eV)and 23063K(1.99 eV),respectively.Three prominent resonance peaks at 9.7,14.3 and 16.8 GHz,are thought from the synergetic effect of heterogeneous interfaces inside the Fe@FeSi/SiO2 nanocomposite.Higher reflection losses of-33,-20 and-38 dB at these resonant frequencies are obtained in optimized thicknesses,respectively.The excellent microwave absorption of Fe@FeSi/SiO2 nanocomposite is readily tunable by the multi-resonance behavior and the electromagnetic synergetic effect in the interface-rich nanocomposite.The multi-resonance phenomena are significant in design and fabrication of electromagnetic materials or the correlative devices,with higher losses at certain frequencies.(3)Unique Fe@ZrO2 nanochains fabricated by the arc discharge plasma shows a higher saturation magnetization(Ms)of 188 emu/g and a coercivity of 253 Oe.The Fe@ZrO2 nanochains exhibits the stability up to～437℃ in air,implying higher anti-oxidization ability in an austere atmosphere.The minimum reflection loss value of Fe@ZrO2 nanochains reaches to-45.36 dB in a thickness of 3 mm,and the frequency band covers 10～18 GHz with RLs less than-10 dB in a thickness of 1.5 mm.The excellent microwave absorption ability is attributed to the high magnetic/dielectric losses caused by multi-resonances and interfacial polarization in the linear morphology of SiC nanowires,as well a proper matching established between the electromagnetic phases.(4)Electromagnetic absorbents with beneficial microstructure,wide broadband,adequate corrosion resistance,and by a facile synthesis route,are always highly practical in the telecom and stealth fields.In this regard,the carbon-coated(SiC/Fe)@C nanowires(NWs)are effectively synthesized through in-situ vapor-liquid-solid(VLS)growth confined at the arc-plasma region,using a merger of Fe/Si bulks as the target material under the atmosphere of CH4/Ar mixture gases.The interconnected network of randomly oriented nanowires features the dielectric SiC NWs with Fe nanoparticles at the tips,both tightly encapsulated by conductive graphene-like carbon nanolayers at the surface.Such a capsule of nanowire can endow proper impedance matching conditions and anti-corrosion ability in the harsh environment.With absorbent loadings of 15 wt.%,25 wt.%,35 wt.%and 50 wt.%into paraffin matrix,the electromagnetic parameters and reflection loss(RL)are scrutinized in gigahertz frequency range(2-18 GHz).Among these composites,loading of 25 wt.%(SiC/Fe)@C NWs is shown as the best composite,which exhibits a minimum RL of-63.44 dB at 12.4 GHz,and a wider effective absorption band of 7 dB(RL≤-10 dB)in sheet thickness of 1.95 mm.