Synthesis, Characterization And The Study Of Electromagnetic Parameters Of MWNTs-BaFe₁₂O₁₉ Composite

In the field of microwave absorbing material, carbon nanotubes (CNTs), as a novel microwave absorber, have been extensively studied, which show a strong microwave absorbing capability in a relatively wide band, due to their electric conductivity and the properties of surface effect, small size effect, quantum size effect and macroscopic quantum tunneling effect as one dimensional nano material. However, the poor impedance match is the bottle-neck of CNTs in the application as microwave absorber, for it has a relatively large complex permittivity and pure CNTs has no magnetism, which cause a very strong reflection of microwave on the surface of CNTs absorber. Now a lot of study has focus on how to improve the impedance match of CNTs by modification.This paper is aim to synthesize a novel MWNTs-BaFe₁₂O₁₉ composite with MWNTs coated by BaFe₁₂O₁₉ in nanometer scale, in which magnetic BaFe₁₂O₁₉ with a poor electric conductivity is utilized to improve the impedance match and finally strengthen the microwave-absorbing capability of MWNTs, by optimizing its magnetic property and electric conductivity.Firstly, this paper has studied the hydrothermal oxidizing treatment to MWNTs, for the oxidation of MWNTs directly affects the following coating performance. The content of oxygen element in hydrothermal oxidized MWNTs is measured by XPS, which is found to have a positive correlation with the reaction temperature (T), reaction time (t) and the concentration of mixed acid (c) in the hydrothermal oxidizing treatment. In a optimized condition of T=200℃, t=5h and c=70%, the hydrothermal oxidized MWNTs with an oxygen element content of 22.6% can be gained, which is much more higher than the content of oxygen in regular concentrated acid oxidized MWNTs (around 6%). The micrograph of hydrothermal oxidized MWNTs are observed using SEM. It is found that the effect to MWNTs mechanic properties is supposed to be under control, for the tube structure and the tube length is maintained during the hydrothermal oxidizing process.Secondly, this paper has done a preparative study of synthesizing nanosized BaFe₁₂O₁₉ applying a hydrothermal process. Nanosized M type BaFe₁₂O₁₉ has been synthesized at 140℃, 160℃and 180℃, respectively. The obtained barium ferrite powder is characterized using XRD and TEM. It is found that the grain size of BaFe₁₂O₁₉ increases as the synthesis temperature increases and the barium ferrite synthesized at 180℃has a more perfect cystalinity with a grain size at 60nm. Complex permittivity and permeability are measured using a vector network analyzer in a scan range of 2-18GHz. It is found that the barium ferrite so synthesized is a good magnetic loss type microwave absorber with a relatively high imaginary part of complex permeability (μ"max=0.5) and can be used to improve the magnetic properties of MWNTs in the following chapter.Lastly, this paper has successfully synthesized a kind of MWNTs-BaFe₁₂O₁₉ composite by bringing hydrothermal oxidized MWNTs in the hydrothermal synthesis reaction of BaFe₁₂O₁₉. The composite powder is characterized by XRD and TEM. It is found that the MWNTs are well coated by BaFe₁₂O₁₉. The complex permittivity and permeability of the composite is measured using a vector network analyzer in a scan range of 2-18GHz. It is found that the impedance match of MWNTs has been greatly improved after coated with BaFe₁₂O₁₉. Compared with hydrothermal oxidized MWNTs (ε'max=170,ε"max=130,μ'max=0.85,μ"max=0.25), the coating of BaFe₁₂O₁₉ has effectively lowered the dielectric properties of MWNTs (ε'max=60-100,ε"max=45-55) and increased its imaginary part of complex permeability (μ"max=0.4) in the same time. The decrease of relative content of MWNTs in the composite is conducive to lower the value of composite'complex permittivity and increase the value of the imaginary part of its complex permeability.