Study On The Controllable Preparation And Electromagnetic Properties Of One Dimensional Porous/hollow Magnetic Materials

The development of effective and new microwave absorbers is an effective way to solve the increasingly electromagnetic pollution at present.The traditional microwave absorbers(i.e.,Fe3O4)have some inevitable problems,such as high density,narrow absorption band,poor impedance matching,etc.Therefore,it is difficult for Fe3O4 absorbers to meet the high requirements of light weight,thin thickness,strong absorption,and wide frequency band.Numerous researches have demonstrated that the combination of a variety of materials or other metal ions doping can break the limit of electromagnetic wave absorption from single materials.One dimensional(1D)materials(nanorods,nanotubes,nanowires,nanofibers,and nanosprings)with shape anisotropy have novel and adjustable properties.Owing to large specific surface area,high porosity,high permeability,assembly,high adsorption,and low density,the porous or hollow materials exhibit excellent microwave absorbing properties.Therefore,we have designed and prepared three kinds of 1D porous or hollow structure magnetic materials;the effects of morphology,composition,and structure on the static magnetic and microwave absorption properties were studied.The main research contents include as follow:1.Controllable preparation and electromagnetic properties of Fe3O4/NiFe2O4/Ni heterogeneous structure porous microrods.We developed a coordinated self-assembly/precipitate transfer/sintering method that allows the controllable synthesis of Fe3O4/NiFe2O4/Ni heterogeneous structure porous microrods(HPRs).A series of characterizations confirms that changing[Ni2+]can effectively control the crystal size,internal strain,composition,textural characteristics,and properties of HPRs.Molar percentages of Ni and NiFe2O4 in HPRs increase with[Ni2+]in various Boltzmann function modes.Saturation magnetization Ms and coercivity Hc show U-shaped change trends because of crystal size,composition,and interface magnetic coupling.High magnetic loss is maintained after decorating NiFe2O4 and Ni on the surface of Fe3O4 PRs.Controlling the NiFe2O4 interface layers and Ni content can improve impedance matching and dielectric losses,thereby leading to lighter weight,stronger absorption,and broader absorption band of Fe3O4/NiFe2O4/Ni HPRs than Fe3O4 PRs.An optimum EM wave absorbing property was exhibited by Fe3O4/NiFe2O4/Ni HPRs formed at[Ni2+]= 0.05 M.The maximum reflection loss(RL)reaches-58.4 dB at 13.68 GHz,which corresponds to a 2.1 mm matching thickness.The absorbing bandwidth(RL ≤-20 dB)reaches 14.4 GHz with the sample thickness at 1.6-2.4 and 2.8-10.0 mm.These excellent properties verify that Fe3O4/NiFe2O4/Ni HPRs are promising candidates for new and effective absorptive materials.2.The preparation and electromagnetic characteristics research on the MnxFe3-xO4 hollow or porous spherical chains.MnxFe3-xO4(0≤ x ≤ 1.09)hollow or porous spherical chains(H/PSCs)were prepared via a facile one-pot solvothermal approach and magnetic field induced-Oswald ripening mechanism.The external magnetic field induced nanocrystals to aggregate into microspheres along the[111]direction,and these microspheres further assembled into 1D H/PSCs.Characterization confirmed that an increase in the Mn2+/Fe3+ atom ratio decreased crystal size,diameter,and aspect ratio as well as increased internal strain,lattice constant,Mn doping amount,and BET specific surface area.Consequently,saturation magnetization and coercivity decreased because of crystal size and Mn2+ substitution.Mn2+ substitution induced a dual-frequency absorption at 2-18 GHz,in which the absorption band at λ/4 decreased and that at 3 A/4 increased with increasing x.Compared with Fe3O4 solid spheres and hollow spheres,Mn0.746Fe2.254O4 H/PSCs exhibited a broader absorption band(RL ≤-20 dB)of 9.86 GHz(2.05-7.91 and 14-18 GHz,respectively).The enhanced absorption performance may be related to hollow and porous structures,oriented arrangement of nanocrystals,and Mn2+substitution.3.Controllable Synthesis of CuxFe3-xO4@Cu Core-Shell Hollow Spherical Chains for Broadband,Lightweight Microwave Absorption.CuxFe3-xO4@Cu(0<x<1.30)core-shell hollow spherical chains(HSCs)with tunable composition were prepared through solvothermal liquid-phase reduction with magnetic field induction.The influence of[Cu2+]on composition,phase purity,morphology,and microstructure of the products was confirmed by energy-dispersive X-ray spectrometer,X-ray diffraction,X-ray photoelectron spectrometer,scanning electron microscope,and transmission electron microscope analyses.Cu0 content,Cu2+ doping,and crystal size in CuxFe3-xO4@Cu HSCs can be adjusted by changing[Cu2+].Increasing[Cu2+]decreases the mean crystal size and increases the Cu0 content and Cu2+ substitution.The saturation magnetization(Ms)of CuxFe3-xO4@Cu HSCs linearly decreases,and the coercivity(Hc)varies in Boltzmann mode.The microwave absorbing properties of CuxFe3-xO4@Cu HSCs decline first and then increase,and sample with x = 1.30 exhibits the optimal properties.Cu1.300Fe1.700O4@Cu HSCs with 44 wt.%mass fraction obtain the maximum RL of-45.1 dB at 3.44 GHz and the absorption bandwidth(RL ≤-20 dB)of 9.9 GHz.Cu1.300Fe1.700O4@Cu HSCs exhibit stronger absorption,lighter weight,and broader absorption band than the other samples.The improved absorption performance may be ascribed to the hollow/porous core-shell structures,Cu2+ substitution,and Cu0 shell with high conductivity,leading to enhanced permittivity,matching,and resonance absorption.