| Recently, electromagnetic (EM) wave devices have been widely used in both military and civil applications:radar, wireless communication tools, local area net works, personal digital assistant, etc. The frequency of EM wave has reached gigahertz (GHz) range. However, the increasing usage of EM waves in the GHz electromagnetic interference (EMI) and electromagnetic compatibility (EMC) problems which have drawn more attention as a specific kind of environmental pollution. To solve this problem, it is necessary to exploit a type of microwave absorption materials with thinner thickness, lighter weight, wider frequency band of microwave loss and more intensity of the microwave sbsorption. Complex permittivity and permeability of the absorbers play key roles in determining the absorption properties. According to the quarter-wavelength model, for metal magnetic absorbers, higher permeability and appropriate permittivity values are needed to satisfy the above demands. According to the bianisotropy theory, the easy-plane anisotropy materials can exceed the limit of the Snoek’s limit and obtain a higher permeability in higher frequency range.The shape planar anisotropy carbonyl-iron powder and magnetocrystalline easy-plane anisotropy rare-earth3d transition intermetallics have been prepared based on the bianisotropy theory. The dependence of the complex permeability, permittivity and the microwave absorption properties of these materials were investigated. On the other hand, we discussed the relation between the frequency of microwave loss, the thickness of the absorber and the complex permeability and permittivity. Moreover, the reflection of the reflective wave by the absorber and the perfect conductor to the microwave absorption properties was also investigated. According to the reflective loss model, we further improved the microwave absorption properties of the absorbers. The details and results of our research are listed as follows:1. The choice of the easy-plane anisotropy materials. The morphology, static magnetic properties and high frequency properties of the ball milling particles will be different even in the same ball milling method, due to the different types of carbonyl-iron particles have different surface treatment. The RXEG and RXEL sphere-shaped carbonyl-iron particles were ball milling with400r/min for12hours. The ball milled RXEG particles have irregular shape, while the ball milled RXEL particles is flake-shaped. The thickness of the ball milled RXEL particles is lower than the skin depth of the iron particles, which induces that the permeability of the particles is almost constant up to2GHz. On the other hand, the permeability of the ball milling RXEG particles decreased rapidly with the increasing frequency.2. To optimize the ball milling method. The RXEL sphere-shaped carbonyl-iron particles were ball milled under400r/min for8hs,12hs and16hs. The raw powders are deformed into thin flake particles after ball milling, and the flaky appears cold welding state for16hs ball milled particles. The permeability of the particles decreases and the resonance frequency shifts to lower frequency with increasing ball milling time. When ball milling time is used as8hs, the product of the permeability and the resonance frequency can maximum exceed the Snoek’s limit. Then, the raw particles were ball milled under different speeds (200r/min,350r/min,400r/min,450r/min,500r/min,550r/min.) for8hs. The permeability of the samples are first increase and then decreased with increasing the ball milling speeds, which is consist with the change of the morphology and static magnetic properties. In summary, the raw particles are ball milled under500r/min for8hs possess a best frequency property.3. In order to further optimize the high frequency properties and the microwave absorption properties of the easy-plane anisotropy canbonyl-iron particles. The easy-plane anisotropy canbonyl-iron/paraffin composite was oriented in an external magnetic field. The permeability of composite has an increasing factor of0.2-0.3than the nonoriented one. The resonance frequency and the permittivity of the oriented composite increase compared with the nonoriented one, and thus the matching thickness and the matching frequency decrease for the oriented composite.4. To reduce the permittivity of the easy-plane anisotropy carbonyl-iron particles, the surface of the particles were coated with a thin layer of amorphous SiO2by Stober method. After coating SiO2layer, the permittivity of the easy-plane anisotropy carbonyl-iron/SiO2decreases significantly while the permeability shows a small decrease, which increase the threshold value of the volume fractions. By change the volume rate of the absolute ethyl alcohol/the water, the amount of the TEOS and the ammonia, the optimized condition of coating SiO2layer are obtained.5. For the aim of thin electromagnetic wave absorbers used in quasimicrowave frequency band, easy-plane anisotropy carbonyl-iron (PACI) particles coated with ZnO nanoshells were prepared by ball milling technique and chemical precipitation method. Compared with the as-milled PACI/paraffin composite, lower electric constant was obtained for the composite containing PACI at ZnO particles, and hence a dramatic enhancement of reflection loss (RL) was obtained. The minimum RL of PACI at ZnO composite reaches-31.93dB at1.96GHz with the matching thickness of2.5mm. The PACI at ZnO core-shell particles exhibit great potential in application of the thin absorber in the1-4GHz frequency range.6. easy-plane anisotropy carbonyl-iron (PACI)/Nio.5Zno.5Fe204composite as absorbent filler in quasimicrowave band has been synthesized via ball-milling technique and solvothermal method. The effective permeability of the composite was measured and calculated. The result indicates that the magnetic loss in the composite is mainly caused by the natural resonance. Compared with the uncoated PACI particles, the permittivity of the composite decreased dramatically, and hence a dramatic enhancement of reflection loss (RL) was obtained in quasimicrowave band. This result indicates that our PACI/ferrite composite can be used as potential microwave absorbers in quasimicrowave band for its novel microwave properties.7. A new easy-plane anisotropy Ce2Fe17N3-δ compound as an electromagnetic absorption material was prepared by arc melting method. The influence of rotational orientation in various magnetic fields on the complex permeability and orientation degrees of the compound/paraffin composites were systematically studied. It is found that the orientation plays an important role in complex permeability and orientation degrees. For the composite with rotational orientation in1.6T, the real permeability reaches a large value of4.8at2GHz and the imaginary part reaches2.6at5.5GHz, on the other hand, the orientation degree reaches62.4%. It is evident that the oriented Ce2Fe17N3-δ composite with easy-plane anisotropy may have potential applications as microwave absorption materials.8. A new easy-plane anisotropy Sm2Fe14B nanocrystal as an electromagnetic absorption material was prepared by melt spinning method. The electromagnetic and microwave absorbing properties of Sm2Fe14B nanocrystal/nonmagnetic matrix composite in the frequency range of0.1-10GHz were measured and calculated. At the perfect matching point (2.9GHz, vol.50%); the minimum reflection loss reaches-42.0dB at the matching thickness of3.1mm. Furthermore, the calculation shows that the normalized input impedance Zin/Zo equals to1, but the modulus of the ratio between the complex permittivity and permeability|ε/μ|is far away from unity at the perfect matching point.9. The matching frequency and the intensity of reflection loss for the absorbers obey the quarter-wavelength model and the reflective loss model. The results show that the peak intensity of the RL is directly affected by the intensity of the reflective wave from the absorber layer and the emerging wave from the metal layer. The intensity of these two reflective waves dominates the intensity of the reflection loss even at the perfect matching point.10. Coating a conductor layer in the front layer of the absorber can change the intensity of the reflective wave from the absorber layer interface and improve the microwave absorption properties of the absorbers. |
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