Study On Preparation And Electromagnetic Properties Of Radar Absorbing Materials

The purpose of this thesis is to prepare and investigate the microwave absorbing materials of excellent absorption properties in low frequencies, which include: Carbon NanoTubes (CNTs) composites, Fe/Al₂O₃ magnetic composites, Fe-Co-Ni magnetic alloys and their composites.CNTs prepared by thermal CVD method, were treated by high-temperature oxidation method, and by concentrated HC1 method, or by HNO₃-H₂SO₄ mixture in different conditions. FTIR, TEM and EDS were employed to study the effect of these treatments on external structure of CNTs. The complex permittivity and permeability were measured by vector network analyzer in the range of 0.5～6.0 GHz.Fe/Al₂O₃ magnetic composites of different components were produced through the method of Self-propagating High-temperature Synthesis (SHS) and hydrogen-deoxidization. Their detailed characterization and anti-oxidation were investigated by XRD, SEM, DSC-TGA, and the electromagnetic characterization were measured by agilent microwave network analyzer in the range of 2.0～6.0GHz. The binary and ternary alloys of Fe, Co and Ni were also prepared by SHS method. The components with the best electromagnetic parameters have been found by studying the relations of components, phase structures and electromagnetic parameters.In order to research the intrinsic reasons for microwave absorption of composites, simulation program of reflectivity basing on electromagnetic parameters were completed and run by the software of MATLAB. The equivalent electromagnetic parameters of different basic materials, multi- RAM were studied, too. Finally, the model of Microwave Transmission-Line was examined by comparing the simulated results and actual measurement of single-layer and two-layer coatings.The experiments showed that existence of amorphous carbon in CNT samples could improve the dielectric dissipation. And concentrated HC1 or HNO₃-H₂SO₄ mixture treatments for long time would change the polarization performance and external structure of CNTs, and affect the permittivity of CNTs in Low-frequency bands finally.The ability of anti-oxidation was improved greatly, when the ratio (atoms) of Al-to-Fe is (or over) 1: 9 in Fe/Al₂O₃ composites. Adding different Al₂O₃ concentration would effectively modify the real and imaginary permittivity value of the composites, because of the low dielectric performance and insulation of Al₂O₃.Ni_0.8Co_0.2 and Fe_0.5Ni_0.5 had better magnetic loss than other Fe, Co, Ni binary alloys. And their phase structures were Ni-Co solid solution, and Fe-Ni Taenite solution individually. The ternary Fe_0.5Ni_0.4Co_0.1 whose imaginary permeability reached the maximum value of 1.9, had the best magnetic property among any unitary and binary magnetic alloys. Its main phase structure also was Fe-Ni Taenite solution, but Co was dissolving into Fe-Ni basic solid solution.15wt%CNTs has been incorporated into CIP-A/wax composite. And the results showed that the real permittivity of composite had effectively increased by 9. When 5wt%CNTs mixed with Fe_0.5Ni_0.5/epoxy composite, the permittivity also increased, too.By comparing the simulation and actual measurements of CNT single-layer coating and two-layer coating, we had found that the model of Transmission-Line could accord with the actual measurements very well.