| As the large-scale application of communication technology and radar detection technology in modern society,the excessive electromagnetic wave existed in the free space gives rise to electromagnetic wave pollution,which threatens health of people and safe operation of important components.Facing the increasing challenge of electromagnetic wave pollution,it is the main way to solve this problem researching on electromagnetic wave absorbing materials,which can convert electromagnetic energy into thermal or other energy,so as to completely eliminate the electromagnetic pollution.Based on the application requirements,the absorbing materials should have the characteristics of thinner absorbing thickness,lighter weight,wider effective absorbing bandwidth and stronger absorbing energy.Manganese dioxide,as a kind of dielectric loss material,has a great potential with the characteristics of resistance and low density,but a narrow effective absorption bandwidth,which cannot agree to the requirements.In order to broaden the absorption bandwidth of materials,both the attenuation ability and impedance matching should be considered.This thesis,which based on the researches of the manganese dioxide dielectric absorption material in the frequency range of 2?18 GHz,explored the relationship between crystal structure and microstructure and microwave absorbing ability.Through the structural control of element doping,the synergistic effect between different crystal phases and morphologies can be exerted.While maintaining the high dielectric loss capability,the impedance matching is improved to broaden the effective absorption bandwidth.The main contents in this thesis are shown as follows:(1)Crystal structure and morphology are two main factors that determine the microwave absorbing properties of MnO2.The dominant morphologies nanowires and nanosheets as well as the two common manganese dioxide,?-MnO2 and ?-MnO2,have been studied and their effects on absorption performance have been discussed.Due to the higher distortion degree of MnO6 octahedron,the dipole moment in ?-MnO2 is 1 order of magnitude larger than that in ?MnO2,so there exists higher dipole polarization in ?-MnO2.The conductivity of ?-MnO2 are 2?3 orders of magnitude larger than ?-MnO2,so that ?-MnO2 owns larger conductive loss.From the perspective of morphology,MnO2 with nanowires have larger aspect ratio,which build a conductive network,the conductive of nanowires is 1 order of magnitude higher than nanosheets.Nanowires ?-MnO2 shows the best attenuation performance,of which attenuation coefficient is 21.8?435.9,while due to the poor intrinsic impedance(the intrinsic impedance coefficient is 0.15-0.16).As a result,the loss function cannot play a significant role,the effective absorption bandwidth is only 1.52 GHz.Nanosheets ?-MnO2 shows better impedance matching(the intrinsic impedance coefficient is 0.50?0.53),but owns weak attenuation ability,which is unable to achieve effective absorption.Further research shows that the mechanical mixing method cannot effectively combine high loss of nanowires ?-MnO2 and high impedance matching of ?-MnO2.(2)A series of different content of Fe3+ doped manganese dioxide samples were prepared by hydrothermal reaction,and the effect of Fe3+ on the growth and properties of 1anganese dioxide was explored.In the reaction process,Fe3+ and manganese oxide groups can form a complex with Fe3+ as the center,forming the nucleating core of ?-MnO2,and forming stable nanowires ?-MnO2.As the Fe3+ ions increase,the products show as follow:nanosheets ?-MnO2?nanowires ?-MnO2/nanosheets ?-MnO2 composites?nanowires ?-MnO2.According to the first-principles calculation,Fe can change the distribution of the electrons of the system,increase the average bond length and bond energy,which can improve the dipole polarization ability,so that the dielectric loss performance can be enhanced.The nanowire ?MnO2/nanosheet ?-MnO2 composite prepared under the action of Fe can give full play to the synergistic effect of the two components,and effectively combine the high loss performance and high impedance matching.When the doping ratio of Fe3+ is 7%,the attenuation coefficient of the composite is 14.77?194.35,the intrinsic impedance coefficient is 0.29?0.40,and the effective absorption bandwidth can reach 5.44 GHz when the thickness of the absorbing layer is 2 mm.The microwave absorbing properties of MnO2 materials are greatly broadened.(3)The effect of different proportion of Co2+ on the phase control and microwave absorbing properties of MnO2 was studied.During the reaction process,Co2+ ions can be oxidized and generate Co3+,and Co3+ ions act the same function of Fe3+ to lead to the generation of ?-MnO2.When the doping amount of Co2+ is excessive,there are not enough manganese oxide groups,and Co3+ ions are unstable in an aqueous solution,?-MnO2 gradually collapse.The product transforms into the more stable nanosheet ?-MnO2,which can accommodate more doped ions.Therefore,as increasing the doping amount of Co2+,the product can be:nanosheets?-MnO2?nanowires ?-MnO2/nanosheets ?-MnO2 composites?nanowires ?-MnO2?nanowires ?-MnO2/nanosheets ?-MnO2 composites?nanosheets ?-MnO2.Because of the special transformation mechanism,there are more Co ions and other ions(such as K+)can be accommodated,which can significantly improve the dielectric loss of MnO2 and the microwave absorption performance.When the doping ratio of Co is 28.6%,the attenuation coefficient and impedance matching coefficient are 36.86?209.60 and 0.28?0.41,respectively.When the thickness of the absorption layer is 2.2 mm,the effective absorption band can reach 6 GHz,which can achieve effective absorption in Ku band.When the thickness is 3.1 mm,the effective absorption can be achieved in X band.(4)The loss layer@matching layer composite is constructed to effectively improve the electromagnetic wave absorption performance.Firstly,the samples of Ag-doped nanowires ?MnO2 were prepared by hydrothermal reaction.According to the first-principles calculation,Ag+with larger atomic radius can greatly improve the average bond length and bond energy of?-MnO2,which can enhance the dielectric loss ability.The experimental results show that after Ag doped the dielectric loss factor increases to 0.16?0.46,compared with 0.50?1.05 of the pure?-MnO2.The attenuation coefficient of Ag-doped ?-MnO2 can reach 67.76?636.37.Using this high loss material as the core layer,nanosheet ?-MnO2 with excellent matching performance was compounded on its surface by low temperature chemical deposition reaction.The nanosheets grow vertically on the surface of nano wires,and the interface lattice mismatch ratio of ?(201)//?(211)is 0.417%.The Ag-doped ?-MnO2@?-MnO2 composite has high loss performance(attenuation coefficient is 18.46?272.98)and good impedance matching(intrinsic impedance matching coefficient is 0.26?0.34).When the thickness is only 1.6 mm,the effective absorption bandwidth can reach 5.2 GHz,which improves the broadband absorption performance of MnO2 in thin thickness. |
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