Preparation And Electromagnetic Properties Of Modified Fe₃O₄ As Microwave Absorbing Materials

The electromagnetic wave pollution destroyed the good entiroment badly, threaten the health of the human being. The application of microwave absorbing materials is effective measure to settle the electromagnetic contamination. It is often important to study and apply the microwave absorbing materials.The magnetite Fe3O4 particles were prepared by chemical co-precipitation. According to the contribution of impedance matching to the microwave absorbing, Ni-B coated Fe3O4 (Fe3O4/Ni-B) and Co-B coated Fe3O4 (Fe3O4/Ni-B) were prepared via electroless plating and Fe3O4-CB composites were prepared by in-situ chemical route.The effect of drying temperature on the microwave absorbing properties of Fe3O4 was researched, which showed that with a thickness of 6 mm, the maximum reflection loss was -28.38 dB for the Fe3O4 sample dried at 50oC . The bandwidth corresponding to the reflection loss over 10 dB was about 4GHz.XRD patterns of Fe3O4/Ni-B and Fe3O4/Co-B particles indicated that the Ni-B and Co-B films were amorphous alloys, with a thickness of about 20 nm as shown by SEM and TEM photographs. Ni-B alloy film was composed of granular Ni-B particles and fibrilar structure was found for the Co-B alloy. The hysteresis loops of Fe3O4/Ni-B and Fe3O4/Co-B particles indicated that the saturated magnetization was decreased, the coercivity value of Fe3O4/Ni-B was decreased and that of Fe3O4/Co-B was increased, compared with bare Fe3O4 particles. The complex permittivity increased after coated with alloys, so the dielectric dissipation was intensified. The imaginary part of complex permeability of Fe3O4/Co-B particles showed a maximum value at 4GHz, indicating that the resonance frequency of the materials shifted to the range of 2~18GHz, which would increase the absorbing capability in the whole frequency band. When the sample thickness was 4mm, Fe3O4/Ni-B particles showed an absorbing peak of -32.67dB, and the band width of the microwave absorbing over 10dB was 1.0GHz; Fe3O4/Co-B particles showed an absorbing peak of -36.07dB and the band width of the microwave absorbing over 10dB was 2.6GHz. The absorbing properties were improved by calculation, for the impedance matching and attenuation of the Fe3O4 particles were improved by coating with Ni-B or Co-B alloys.The electric conductivity of carbon black is 0.655 S·cm-1. The complex permittivity of carbon black was very high, and the real part of complex permeability in 2~18 GHz was in the range of 0.3~1.1, while the imaginary part of complex permeability was about zero, so the pure carbon black attenuates electromagnetic wave energy mainly by electric dissipation. The impedance matching of poorly conductive Fe3O4 would be improved via adding dielectric carbon black. The composites with different mass ratios of Fe3O4 and carbon black were prepared by in-situ chemical route. When the mass content of carbon black was 20%, and the sample thickness was 2mm, the microwave absorbing peak was -37.3dB, and the band width over 10dB was 3.0GHz.With the thickness of 4mm, the microwave absorbing properties of Fe3O4/Co-B particles were better than other modified materials. But when the thickness was 2mm, the Fe3O4-CB composites with 20wt% CB have more powerful absorbing ability.