Research On The Electromagnetic Properties Of CB,BaM Doping With MnO₂

This dissertation commits itself to develop high quality microwave absorbing materials. Carbon Black (CB), as a kind of the conventional absorber, has been extensively used due to its high electric loss. But some disadvantages are still needed to be overcome, such as the narrow band of absorbption, the low microwave absorption and so forth. Another kind of magnetic loss absorber, epitomized by the M-type barium ferrite (BaM), has been applied in the low frequency due to their natural resonance. Therefore, based on former research, this dissertation aims at the development of novel absorber through doping of manganese dioxide (MnO₂) in the CB and BaM ferrite. The composites are analysed by the advanced method of XRD, SEM, and VSM. The electromagnetic parameters and reflection loss are also measured.To begin with, the electromagnetic and microwave absorbing properties of CB doping with different amount of MnO₂, have been researched. The results show that MnO₂ belongs to dielectric loss medium with flake and strip shaped while CB belongs to erratic current loss medium with sphere and multi-hole shaped. It is notable that MnO₂ has a great effort to intensify microwave absorption capability of CB due to not only its high resistance which can improve the impedance matching of samples but also its flake and strip shaped microstructure which can increase the reflection and scatter cross section of electromagnetic wave.Furthermore, M-type barium ferrite (BaFe₁₂O₁₉) was synthesized using the conventional ceramic technique. The crystalline structure and magnetic properties of M-type barium ferrite doped with small amounts of MnO₂ have been investigated by means of XRD, SEM and VSM. The results show that the crystalline structures of barium ferrite are still M-type hexagonal structure. The particles are approximately hexagonal-shaped and the average grain size is about 2-4μm. Both the saturation magnetization and magnetocrystalline anisotropy constants reach a maximum at x=0.75, which are closely related to the displacement of Fe³⁺ ions from 4f₁ site or 4f₂ site to 2b site. And at x>0.75, the decrement of anisotropy constants has its origin in the displacement of Fe³⁺ ions from 2b site to 12k site. Whereas, the magnetization also decreases rapidly, this can be attributed to the attenuation of superexchange interaction of Fe ions. In addition, the coercivity decreases almost linearly with x. The permeability of BaM has shown a wide peak at about 13GHz. It may lead to the good performance of microwave absorption during this band of frequency.Finally, the computer simulation of microwave absorption property for BaM has been investigated using Matlab software, and the predication of matching thickness has also been discussed. All the microwave absorption peaks correspond with the frequency which emerges the maxium value of tangent loss. Also, the thickness of slab is an important factor to...