Studies On Preparation And Electromagnetic Properties Of Oxide-based Magnetic Composite Materials

Due to the increasing use of gigahertz electromagnetic waves in military, industrial and commercial applications, more and more electromagnetic waves have penetrated our living space, the electromagnetic radiation has become a serious problem, it has caused serious harm to human health and electronic devices and apparatus. In order to overcome and eliminate this problem, the special attention was focused on the microwave absorbing materials, which have the low reflection and high absorption properties. Oxide-based microwave absorption materials are of great interest because of some significant characteristics such as low cost, high oxidation resistance, temperature consistent properties and reasonably facile processibility. So the magnetic oxide is still the most chose material for microwave absorption application at present. As a representative magnetic oxide, the microwave loss of ferrite is mainly depends on magnetic loss. On the basis of the detailed review of the electromagnetic wave absorption materials, we first developed a novel preparation route of barium ferrite, and researched on the microstructure and properties of Ni²⁺Ti⁴⁺substituted barium ferrite. Then titania（TiO₂）, Fe-doped zinc oxide（ZnO） and conducting polymer have been selected to modify ferrite by coating method in order to improve the microwave electromagnetic properties of ferrite, respectively. Finally, a three phase composite absorber has been prepared so that it can great absorb microwave energy in the lower frequency band.First of all, barium hexaferrite powders with a suitable size and magnetic properties were successfully synthesized by heating the precursors, which were prepared using sugar as a source of chelating agent. The effects of molar ratio of Fe/Ba, calcination temperature and time, on the phase component and the magnetic properties of hexaferrite particles were studied in detail by means of X-ray diffraction and physical property measurement system. When the molar ratio of Fe/Ba was 11.5, pH was 6.5, the specific saturation magnetization of the hexagonal platelet barium ferrite particles increase from 62.2 to 64.5emu/g, and coercive force decrease from 2546.6 to1848.9Oe with increasing the calcination time from 1.5 to 2.0h at 1100℃. According to the TG/DTA and FT-IR, the process mechanism was also suggested.The Ni²⁺Ti⁴⁺ substituted barium ferrites have been prepared by means of the above obtained process parameters. The XRD results show that the substitution ions have entered into the lattice of barium ferrite and a single-phase Ni²⁺Ti⁴⁺ substituted hexaferrite(BaFe_12-2x(Ni_0.8Ti_0.7)_xO_19-0.8x) can be obtained for x≤1.2. The M?ssbauer parameters were obtained by means of Mossbauer spectra, it shows that the quadrupole splitting increases for the Ni²⁺Ti⁴⁺ substituted barium ferrite, which means that the electric density around atomic nucleus increases, and the fraction of spectral area decreases, as well as the hyperfine magnetic field have great changes in the site of 2b and 4f1, which shows the substituted ions enter the site of 2b and 4f1. The microwave measurement results show that the Ni²⁺Ti⁴⁺ substitution has significantly effect on theε′,ε″,μ′andμ″in the frequency range from 2.0 to 12.0GHz. With increasing substitution, the real and imaginary parts of complex permeability and permittivity increase first, then decrease, with the maximum for x=1.2. The maximum loss increases first, and then decrease, with the maximum -11.92dB when the substitution amount x is 0.8 and the thickness is 2.0mm. The peak positions of the maximum loss drift to the lower band. With the increase of frequency, the real part of complex permeability and permittivity all decrease. The substitution or vacancy content is possibly responsible for this microwave response property.The anatase titania（TiO2） and Fe-doped zinc oxide（ZnO） coated barium ferrite composites were prepared by a heterogeneous precipitation method and heat treatment process, and obtained the core-shell microstructure composite particles, the results show that the coverage has a great influence on microwave response of barium ferrite, the formation of coating layer on the surface of a ferrite particle changes the character of the frequency dispersion of the complex permittivity. As for the titania-coated barium ferrite composites, the complex permeability of the composites was found to decrease with an increase in frequency as well as with the molar ratio of Ti/Ba. There is a critic content in coating layer for wave absorbing, the maximum reflection loss was obtained at the Ti/Ba ratio of 1/10, and the peak of the maximum reflection loss shifts to a lower frequency value with increasing titania fraction. As for the ZnO-coated barium ferrite composites, the real part of the complex permeability of composites was found to decrease with an increase in frequency as well as with the weight ratio of ZnO. However, the imaginary part of the complex relative permeability of composites was found to enhance with an increase in the molar ratio of ZnO except for the sample ZB3, the peak of theμ″shifts to a lower frequency value with increasing ZnO fraction. When the molar ratio of ZnO is 1/10, the imaginary part of the complex permeability of ZnO/barium ferrite–resin composites shows the maximum value, and the maximum reflection loss was obtained. By changing the thickness of coverage, the frequency dependence of the complex permittivity could be adjusted, which provides us an opportunity for the synthesis of tailored particles. A composite of polyaniline（PANI）-coated barium ferrite was prepared by an in-situ polymerization of an aniline monomer in the presence of barium ferrite particles. The result indicated the existence of an interaction at the interface of polyaniline macromolecule and ferrite particle, which influenced the composite’s physical and chemical properties. The surface modification of barium ferrite by polyaniline largely improved the complex permittivity in the most frequency range. The microwave measurement results show that the microwave absorption is low, and the peak of the reflection loss shifts to a higher frequency value in the frequency range of 2-12GHz.In order to get suitable composites, the three phase composite absorber （Fe-TiO₂-FeTiO₃） has been prepared under reduction atmosphere through heat treatment precursor. The electromagnetic properties of composite particles were discussed in this dissertation. The XRD and M?ssbauer spectra were applied to analyze the electrovalence of Fe ion in composites. The content of Fe or TiO₂ increases, while the content of FeTiO₃ decreases with increasing reduction time, there are only Fe and TiO₂ phases in the XRD spectrum when the reduction time was 4h, however, the M?ssbauer spectrum shows that there exists Fe²⁺ in the composites with the formation of FeTiO₃, the coercive force of composite is low, which is character of soft magnetic materials. The real and imaginary parts of complex permittivity increase first, then decrease, the complex permeability increases, and the peak of theμ″shifts to a lower frequency value with reduction time increment. When the thickness of coating is thinner than 1.5mm, these materials do not work well. The effects of thickness and reduction time on absorbing properties were studied. It indicates that the performance could be adjusted by changing the relative content of composites and the coating thickness.