Preparation Of Magnetic Materials Based On Fe₃O₄ And Its Composites Film Electromagnetic Properties

The problems of electromagnetic interference (EMI), damage of ecological environment caused by electromagnetic radiation pollution and threat to the health of human have become more serious due to wide applications of electromagnetic waves in GHz range for mobile phone, local area network, radar systems and so on. The application of microwave absorber can not only solve the above problems, but also realize the electromagnetic compatibility and stealth in military. Therefore, it is very important to synthesize effective electromagnetic wave absorbers. The film composed of microwave absorbers can keep functionality of the absorbers and own speciality of film, which may be used as EMI shielding and thin microwave absorbing materials.The aim of this research is to prepare magnetic film composed by ferromagnetic materials dispersed in organic surpport with excellent microwave absorbing properties. The main contents of the thesis contain controllable synthesis of 0D nanoparticles, 1D nano-necklace chains, 2D nanopetals and 3D flower structure Fe₃O₄ particles in solution, synthesis of amorphous CoNi alloy particles with different mol ratio of Co to Ni, preparation of composites of Ni, Co and Fe₃O₄, Co coated on hollow microspheres, Fe₃O₄-graphite, respectively, preparation of magnetic microwave absorbing film, the study about the influence of external magnetic field and design architecture on the microwave absorbing properties of the magnetic film.The magnetic and electromagnetic property is seriously affected by the morphology of materials. 0D nanoparticles, 1D nano-necklace chains, 2D nanopetals and 3D flower structure Fe₃O₄ particles were prepared by co-precipitation and reflux of ethylene glycol (EG) solution. Several factors including crystal-face attraction, van der Waals forces, hydrophobic interactions, hydrogen bonds, electrostatic and dipolar fields associated with the magnetic nanoparticles may have effects on the formation of Fe₃O₄ with different structure under different conditions. The 0D Fe₃O₄ nanoparticles show superparamagnetic property due to the particle size smaller than superparamagnetic critical size, which indicates relatively weak microwave absorbing capability. The Fe₃O₄ nanochains with aspect ratio of more than 5 have larger coercivity and remanence magnetization than that of nanoparticles because of the size of particle in nanochains distribute between critical value (the particles change from one state with multiple magnetic domains to another with a single domain) and surparamagnetic critical value and the shape anisotropy of chain structure. The Fe₃O₄ with chain structure shows better microwave absorbing property than that of surparamagnetic nanopartcles. The dielectric relaxation of Fe₃O₄ nanoparticles in 2-18GHz is mainly caused by intrinsic electric dipole orientation polarization and interface polarization. The Fe₃O₄ with smaller size shows larger complex permittivity for the increament of polarization caused by larger surface. The magnetic dissipation of Fe₃O₄ nanoparticles mainly contains domain wall resonance and skin effect. Theμ" curves of the prepared Fe₃O₄ with the flower structure composed of porous petals exhibit peaks at about 3.0 GHz, indicating that the Fe₃O₄ can break through the Snoek's limits due to large specific surface area and shape anisotropy of flower petal. The flower structure Fe₃O₄ has good impedance match and good attenuation because of the geometrical effect and blocking effect caused by flower structure and the pore, respectively. For frequency dispersion and dielectric and magnetic loss, the Fe₃O₄ with flower structure exhibits good ability of microwave absorption with a match thickness of 1.5 mm, the maximum reflection loss of -47.4dB. At the thickness between 1.5 and 3.0 mm, the bandwidth corresponding to over 70% microwave absorption is ca. 10.0 GHz.The amorphous CoNi with double metal spheres with mol ratio of Co to Ni at 7:3 indicates the match thickness of 1.2mm and the maxmum RL of -32.4dB. The RL is over -10 dB in frequency range of 3.6-17GHz with thickness of 1.1-3.2mm.The composites of crystalline and amorphous Co and Fe₃O₄ were prepared by double reductant and single reductant KBH4, respectively. The face centered cubicα-Co and FeCo alloy with nanofiber structures were obtained by heat treatment of composites of amorphous and Fe₃O₄ nanoparticles. The natural resonance frequency of the composites was modified in 2-18GHz, so the composites show excellent microwave absorbing properties for the addition of magnetic loss. At the thickness of 1.2 mm, the bandwidth corresponding to reflection loss below -10 dB (over 90% microwave absorption) is higher than 4.3 GHz.The single layer magnetic film was obtained by dispersion of ferromagnetic particles to chloroprene rubber (CR). The single layer film composed of Co and FeCo alloy fibers and carbonyl iron dispersed in CR showed maximum reflection loss of -22.86dB at thickness of 0.44mm, the bandwidth of crosponding RL over -8dB reached 5.6 GHz, the RL is over -4dB over range of 7.7-18GHz. The magnetic anisotropy of two orientations in plane surface of single film made under parallel external magnetic field is larger than that of film made without external magnetic field, but the RL of the film was less affected by parallel external magnetic field. The RL of film made under vertical external magnetic field improved due to the decrease of sedimentation rate of magnetic particles in film. The film composed of amorphous Co and Fe₃O₄ and carbonyl iron made under vertical external magnetic field shows maximum RL of -15.61dB at thickness of 0.43mm, According to effective architecture designed for microwave absorption, the films composed of transmission layer, absorption layer and reflection layer were made, the materials with dielectric and magnetic loss were chosen in each layer. The maximum RL of three layers film reached -26.59dB at the thickness of 1.23mm, the bandwidth corresponding to RL below -8 dB is higher than 6.6 GHz, RL is over -4dB in frequency range of 6.24-18GHz.