Studies On Preparation And Electromagnetic Properties Of Microwave Absorption Composite In Low Frequency

It would have great significance to study the microwave absorption material, which isapplied widely to national defense and civil industry. At present, the microwave absorbingmaterial is developing towards "thin, light, wide and strong". But single absorbing materialhas been difficult to meet these requirements at the same time. Moreover, the microwaveabsorption composite, which overcomes the disadvantage of the single absorbing material, isone of development directions of microwave absorbing material in the future. The microwaveabsorption composite in low frequency has been researched very little in our country atpresent. Therefore, the magnetic loss nano ferrite, carbonyl iron, dielectric loss barium titanateand polyaniline are chose for research in this work. And due to the advantages of theirdifferent loss mechanism, the composites Fe₃O₄-SrFe₁₂O₁₉, Ni_0.5Zn_0.5Fe₂O₄-PANI,BaTiO₃-PANI and Fe（CO）₅/PANI, which display excellent microwave absorption ability inlow frequency, are designed and developed.Nanoparticles Fe₃O₄were prepared by coprecipitation method in the absence of nitrogen.The effect of different molar ratio of Fe³⁺to Fe²⁺on magnetic performance of sample hasbeen investigated in detail. On this basis, the composite Fe₃O₄-SrFe₁₂O₁₉was prepared byinterface method. The effect of different mass ratio of Fe₃O₄and SrFe₁₂O₁₉on magnetic andmicrowave absorbing properties had been studied. The results show that, when the molar ratioof Fe³⁺to Fe²⁺is1:3.5, the spherical Fe₃O₄exhibits superparamagnetic and grain size isbetween10¹5nm; when the mass ratio of Fe₃O₄to SrFe₁₂O₁₉is1:0.3, the SrFe₁₂O₁₉wascoated uniformly with Fe₃O₄. Meanwhile, the composite Fe₃O₄-SrFe₁₂O₁₉not only exhibitssuperparamagnetic, but also has the best wave absorbing property, which of the maximumabsorption can reach-17.7dB, the-5dB bandwidth is1300MHz （2130MHz³430MHz）.Coprecipitation and sol-gel combined method was first used to prepare acicularnano-Ni_0.5Zn_0.5Fe₂O₄. The influence of Ni and Zn addition on aspect ratio of nano-Ni0.5Zn0.5Fe2O4was studied. The aspect ratio of nano-Ni_0.5Zn_0.5Fe₂O₄were controlled by the dopedmass of Ni and Zn in the coprecipitation process. The effect of calcination temperature on the phase of samples was studied. The results show that, as the molar ratio of Ni:Zn:Fe equals to0.3:0.3:2, the aspect ratio is at its maximum （about20）. And the aspect ratio of samplesreduces with the increasing calcination temperature. Then, the compositeNi_0.5Zn_0.5Fe₂O₄-PANI was prepared by in situ polymerization. The bonding action betweenNi_0.5Zn_0.5Fe₂O₄and PANI and different mass ratio of Ni_0.5Zn_0.5Fe₂O₄and PANI on magneticand microwave absorbing properties have been studied. The FT-IR spectrogram indicatesthat the characteristic absorption peaks of composite show bathochromic shift because ofstronger interaction between Ni_0.5Zn_0.5Fe₂O₄and PANI. The performance of composite wereanalyzed and characterized by VSM and PNA. The results show that, the coercive force andsaturation magnetization of composite decrease with the increase of amount of PANI. Thewave absorbing properties of composite first increase and then decrease with the increase ofamount of PANI in0⁶000MHz. When the mass ratio of Ni_0.5Zn_0.5Fe₂O₄to PANI is1:10, ithas the best wave absorbing property, which of the maximum absorption can reach-21.5dB,the-5dB bandwidth is2900MHz （3100MHz⁶000MHz）.Nanoparticles BaTiO₃were prepared by sol-gel method and hydrothermal methodrespectively. The grain average size of square crystalloid BaTiO₃is about60nm, which waswas synthesized from the hydrothermal reaction for24h. On this basis, the surface of BaTiO₃was modified by silane coupling agent KH570, and the BaTiO₃-PANI composite with core-shell structure was prepared by in-situ polymerization. The composite was characterized andanalysed by the aid of XRD, TEM, FT-IR and PNA. The mechanism of silane coupling agentKH570in preparing process had been discussesed.The effect of the mass ratio of BaTiO₃toPANI and absorption thickness on microwave absorbing properties of samples had beenresearched. The results show that KH570playes a bridge role for aniline and BaTiO₃, andmakes the BaTiO₃coated with PANI, so BaTiO₃-PANI with core-shell structure is resulted, inwhich the core and shell is BaTiO₃and PANI respectively, shell layer is40nm. Besides, theBaTiO₃after being coated has better dispersion due to the steric hindrance. Also the PNAresults show that the reflectivity RLof composite presented the trend of first decrease thenincrease with the increase of the PANI content and absorption thickness during the range of0⁶000MHz. When the mass ratio of BaTiO₃to PANI is4:1and absorption thickness is3mm,BaTiO₃-PANI exhibits the best wave absorbing property with the maximum absorption of-14.4dB and the-5dB bandwidth of1114MHz （4798MHz⁵912MHz）.The Fe（CO）₅/PANI composite was prepared by physical blending. The effect of the mass ratio of Fe（CO）₅to PANI and absorption thickness on microwave absorbing properties ofcomposite had been studied. And the magnetic property of Fe（CO）₅/PANI composite hadbeen disscussed. The results show that the coercive force of Fe（CO）₅/PANI composite isslightly larger than that of pure Fe（CO）₅, but the its saturation magnetization decreasecompared with pure Fe（CO）₅. The reflectivity RLof composite decreases with the increasingPANI content and absorption thickness in the range of0⁶000MHz, and then increases. Whenthe PANI content is6%and absorption thickness is3mm, the wave absorbing property ofcomposite is the best, and the maximum absorption can reach-39.1dB, and the-10dBbandwidth is1639MHz （4361MHz⁶000MHz）.