Fabrication, Structure And Electromagnetic Property Of Quasi One-dimensional Ferrites Based Soft Hard Magnetic Composite Materials

Electromagnetic interference (EMI) has been becoming a serious problem due to extensive utilization of electronic devices, local computer networks, mobile phones and the essential part of a stealth defense system for all military platforms. To resolve the EMI problem, electromagnetic wave absorbers are required technologically. For an ideal EM wave absorbing material, it should be characterized with a strong absorption, wide working frequency range, thin layer and light weight. However, it is hard to meet all these characteristics for conventional materials.Nanocomposite magnets offer a possibility to create unified systems from components whose properties are complementary or even mutually exclusive. According to the exchange spring concept, when both phases are sufficiently exchange-coupled with each other, the high saturation magnetization of the soft phase and the high coercivity of the hard phase will be utilized for giant energy product and enhance the absorbing microwave property of the nanocomposite system. However, for nanocomposite powders, as nanoparticles have a strong tendency to aggregate, the aggregated particles are usually beyond the exchange length and the powders so that could not be sufficiently exchange-coupled.Aligned one-dimensional (ID) magnetic nanostructures have attracted a great attention due to their novel chemical and physical properties and potential applications for ultrahigh-density data storages, sensors, micro/nano devices, catalytic and magnetic composites. For 1D materials, they could restrain the aggregation of particles due to shape characteristics and improve the exchange-coupled interaction effectively. In addition, among various 1D nanostructures, nanofibers with a high aspect ratio and anisotropic characteristic are helpful to broad the microwave absorption bandwidth due to polarization effects and improve the absorption strengthIn this dissertation, the soft/hard composite microfibers and nanofibers were prepared by the sol-gel and electrospinning respectively with inorganic metal salts as the main raw materials, and they were characterized by the analysis methods of TG, XRD, SEM, TEM, FTIR, VSM and NA techniques. According to the characteristics of the as-prepared fibers, the feasibility of their applications in the microwave absorption was studied, and the main results are as follows:1. Quasi one-dimensional ferrites based soft hard magnetic composite materials. (1) Utilizing the sol-gel method and electrospining method, we have successfully prepared a series of ferrites based soft hard magnetic composite microfibers and nanofibers respectively (BaxSr1-xFe12O19 (0≤x≤1) ferrite hollow microfibers; composite ferrite nanofibers; Alinged MeFe12O19/Ni0.5Zn0.5Fe2O4 (MFO/NZFO, Me=Ba2+、Sr2+) composite ferrite hollow microfibers and BaFe12O19/Fe composite nanofibers). (2) The gel-precursor decomposition and ferrite formation were analyzed by thermo-gravimetric and differential scanning calorimetry, infrared spectroscopy and X-ray diffraction. The microfibers and nanofibers as-prepared were also characterized with scanning electron microscopy and vibrating sample magnetometer. The diameter of composite nanofibers is range from 100 to 150 nm. The microfibers have obvious structure and the ratio of hollow diameter to micro fiber diameter is about 2/3. It is interesting to find that small Nio.5Zno.5Fe204 grains are adsorbed on the surface of lager MeFe12O19 grains in the nanocomposite ferrite microfibers. (3) The mechanism of hollow structure was also discussed.2. Exchange-coupling interaction of quasi one-dimensional ferrites based soft hard magnetic composite microfibers and nanofibers. (1) The effects of phase, structure and morphology on the exchange-coupling interaction were studied in detail, and the related physical mechanisms were also discussed. Especially, the soft and hard phases are distrubuted in theoretical exchange-coupling length, which plays an important role in improvement of exchange-coupling interactrion in composite phase. (2) The magnetic properties of ferrite based composite microfibers and nanofibers were studied. It is found that the competition of exchange-coupling interaction and dipolar interaction in composite fibers determined the mechanism of magnetic domains spins.3. Microwave absorption of quasi one-dimensional ferrites based soft hard magnetic composite microfibers and nanofibers. (1) We revealed the relation of phase composition, microstructure and microwave absorption and further discussed the absorption property of quasi one-dimensional ferrites based soft hard magnetic composite microfibers. (2) The difference of absorption property between composite microfibers and nanofibers was also studied and the dependent relationships of microwave absorption on magnetic domain reverse were further established.4. Design and microwave absorption of multilayer absorption materials. For three layered absorption materials, out layer as BaFe12O19/Fe, middle layer as F and inner layer as Ni0.5Zn0.5Fe2O4, the composite materials show the excellent absoption property and the minimum reflection loss (RL) with thickness (out layer as 1.6 mm, middle layer as 0.1 mm and inner layer as 1.0 mm) reaches the optimized value of -98.9 dB at 12.6 GHz. The microwave absorption bandwidth exceeding-10 dB covers whole G-band, X-band and Ku-band.