Preparation And Microwave Absorption Property Of Magnetic Fe₃O₄Nanocomposites

Recently, microwave absorbing materials have aroused extensive interest since the electromagnetic radiation and electromagnetic interference phenomenon are becoming more and more serious. Magnetite (Fe3O4), as a basic ferrite, has early been used in electromagnetic interference shielding (EMI) and electromagnetic wave (EMW) absorbing applications because of abundant natural resources, environment-friendly as well as a typical double-complex medium with both dielectric loss and magnetic loss. In this paper, we have successfully synthesized the hollow Fe3O4hemispheres/r-GO composites, the hollow Fe3O4hemispheres, hollow Fe3O4/Ni composites and carbon-encapsulated Fe3O4nanoparticles via a solvothermal method, respectively. Moreover, their microwave absorption properties and microwave absorbing mechanism has been discussed.The main works can be summarized as follows:1. Preparation and microwave absorption property of hollow Fe3O4hemispheres Hollow Fe3O4hemispheres/r-GO composites were successfully synthesized by using P123as a soft template and FeCl3·6H2O as the precursor via a solvothermal method in EG solvent, which show good dispersity. And then, the as-synthesized hollow Fe3O4hemispheres/r-GO composites were calcinated in air at an appropriate temperature of380℃for120min to remove the r-GO while to prevent the Fe3O4from further oxidation. After calcination, pure hollow Fe3O4hemispheres with an average diameter of250nm and a shell thickness of80nm can be obtained. The morphology and microstructure of the products were studied by employing scanning electron microscopy (SEM), transmission electron microscopy (HRTEM), X-ray diffractometer (XRD). In addition, we studied the microwave absorption properties of the hollow Fe3O4hemispheres by using an Agilent E5071C vector network analyzer. Results show that the as-prepared hollow Fe3O4hemispheres exhibit improved microwave absorbing property, compared to the Fe3O4hemispheres/r-GO composites. Particularly, the sample containing60wt%hollow Fe3O4hemispheres with the thickness of4mm exhibits a maximum absorption of27.84dB at7.6GHz and the bandwidth of reflection loss below-10dB can reach2.72GHz (from6.16to8.88GHz). Two Cole-Cole semicircles reveal that there are dual dielectric relaxation losses:one is interfacial polarization relaxation, the other is dipolar relaxation. X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared (FT-IR) spectra of the products calcinated at different temperatures reveal the enhanced interfacial polarization relaxation effect due to the higher concentration of Fe2+ions in the surface layer and the increased interfaces among Fe3O4nanoparticles that constructing submicron-sized hollow Fe3O4hemispheres. In addition, the fitted results of complex permeability demonstrate that the magnetic loss is mainly caused by Kittel natural resonance.2. Preparation and microwave absorption property of hollow Fe3O4hemispheres/Ni compositesThe hollow Fe3O4hemispheres/Ni composites can be obtained by dipping the hollow Fe3O4hemispheres into the plating solution to make the inner surface and outer surface of the hollow Fe3O4hemispheres coating a thin nickel layer with the thickness of50nm using chemical nickel deposition. The microwave absorption property of hollow Fe3O4hemispheres/Ni composites has been studied. Results show that the dielectric relaxation loss is mainly caused by interfacial polarization relaxation while the magnetic loss is caused both by Kittel natural resonance and exchange coupling resonance.3. Preparation and microwave absorption property of carbon-encapsulated Fe3O4nanoparticlesCarbon-encapsulated Fe3O4nanoparticles have been synthesized by using ferrocene (Fe(C5H5)2) as the precursor via a solvothermal method in acetone (C3H6O) solvent. The influence of different reaction temperature on the product has been studied. Results show that the higher reaction temperature results in the smaller size of the carbon-encapsulated Fe3O4nanoparticles. The carbon-encapsulated Fe3O4nanoparticles in a mean size of120nm were obtained at the reaction temperature of210℃while the size increases to200nm as the reaction temperature arising to180℃. Moreover, we have studied the microwave absorption property of carbon-encapsulated Fe3O4nanoparticles. Results reveal that the carbon-encapsulated Fe3O4nanoparticles show good microwave absorption property, which exhibits a maximum absorption of-24.2dB at8.56GHz. Compared to that of the hollow Fe3O4hemispheres, the bandwidth below-10dB has been broaden obviously, which reaches3.52GHz (from7.0to10.52GHz).