Research On Electromagnetic Parameters Modulation Of Carbonyl Iron Absorbents And Bandwidth Extention Of Metamaterial Absorbers By In-plain Design

Microave absorption materials have been widely applied in military applications,such as reduction of radar signature of aircraft,ships,tanks,and targets,as well as in civilian applications,such as reduction of electromagnetic interference between components and circuits,reduction of the back-radiation of microstrip radiators.In recent years,with the fast development of electrical technology,the demand for high performance microwave absorbers and radar-absorbing materials is ever growing.In this dissertation,we focus on the research of microwave absorbing materials.To realize the ultrathin broadband microwave absorption,we conduct research works on modulation of electromagnetic parameters of traditional magnetic materials,conbination of magnetic absorber with metamaterial absorber and broadening the absorption bandwidth of metamaterial absorbers.In Chaper 2,When Ce acted as additive in the high energy ball milling process,the Fe/CeO₂ composites were prepared.The structures,magnetic and microwave properties of the samples have been investigated.Resultant showed that,Ce mixed with the carbonly iron particles?CIPs?matrix uniformly and interacted with the Fe oxides druing the high energy ball milling process.Thus,Ce was oxided to CeO₂ and dispersed in the CIPs uniformly,blocking the transfer of electron and increasing the intrinsic resistivity,which suppressed eddy current effect but decreased the saturation magnetization of the CIPs.Therefore,with the increase of Ce contents,the permeability first increased and then decreased.When the Ce content was 1%,the maximum permeability was obtained,which increased by 10%at 1 GHz comparing with the CIPs without Ce.The paraffin wax-based composites containing 28 vol%Fe/CeO₂ composite with 1%Ce can exhibit reflection loss?RL?<-8 dB in 8-20 GHz and minimum RL of-15 dB when the thickness was 1.4 mm.When ZnO acted as additive,Fe/ZnO core shell praticles were prepared due to the hexagonal wurtzite layered structure of ZnO.The insulating ZnO shell restrained the cold welding of the CIPs and decreased eddy current effect between the CIPs,which decreased permittivity and enhancement permeability of the particles.The paraffin wax-based composites containing 15 vol%Fe/ZnO particles with 10 wt%ZnO exhibited RL<-8dB in 8.5-18 GHz and minimum RL of-19 dB at a thickness of 1.9 mm.This chaper has provided a methord to control the shape,content,size,surface state and ununiform structures.Chaper 3 has studied the effect of the initial grain size and surface in-situ oxidation on the electromagnetic properties of the CIPs.First,effect of aspect ratio of the flaky CIPs on maximum filling ratio in a dielectric matrix was studied.Prolong the milling time increased the aspect ratio and specific surface area of the CIPs.This greatly enhanced the intrinsic electromagnetic parameters but simultaneously reduces the filling capability in composites,resulting in the occurrence of the largest permeability of the composites at a moderate aspect ratio of the CIPs.The subsequent heat-treatment in air made surface in-situ oxidation of the flaky CIPs,dramatically decreasing the dielectric loss while keeping the high permeability due to the formation of a Fe3O4 and?-FeOOH shell.Consequently,the as-designed composites with flaky CIPs exhibited reflection loss?RL?<-6 dB in 2.5-8 GHz and a minimum RL of-11 dB at 4 GHz when the thickness was 1.2 mm.This work provided an effective and convenient route to prepare a high performance thin absorber in low frequency region.Second,the effect of grain size on grain orientation has studied.The orientation of?100?crystal face originates from the slip and rotation of the grains along the easy slip plane when the grains were teared by the compressive force exerted by the milling balls in the plastic deformation process of the particles.The increase of the initial grain size of the particles increased contents of the slipped and twined grains in the milling process.Based on the mechanism,the orientation factor of?100?crystal face could be modulated by controlling the initial grain size and milling process.Consequently,the enhancement of the permeability in 0.1-8 GHz was obtained and the maximum imaginary part of permeability increased from 2.7 to3.3 due to the enhancement of magnetocrystalline anisotropy field resulted by the orientation of?100?crystal face,benefiting of which the composite even at the thickness of 1 mm exhibited reflection loss?RL?<-8 dB in 2-4 GHz,<-3 dB in 1-10 GHz and a minimum RL of-13.8 dB at 3 GHz.This work provided an effective route to enhance permeability of the magnetic absorbents and prepare high performance thin absorbers in 1-8 GHz.In Chaper 4,an phase coupling mechanism to enhance microwave absorption was developed based on the antenna array phase overlap principle.The critical value of abnormal reflection converting to reflection cancellation was analyzed theoretically.A sample of chessboard structure consisting of FR4 units and CIP composites unit was demonstrated to verify the absorbing mechanism.Resultant showed that the reflection loss enhancement occured on the frequency of anti-phase of FR4 cell and CIP cell,which is agree with the theoretical prediction.Furthermore,we fabricated an metamaterial absorber?MMA?based on the anti-phase coupling mechanism,which exhibited RL<-12 dB in 8-18 GHz at the thickness of 2 mm.The anti-phase coupling mechanism provided a new route to enhance absorption by phase design in horizontal direction without increasing the thickness of the MMAs for microwave and other frequency band.Second,by investigating a square-shaped metamaterial structure we discovered that wave diffraction at diagonal corners of such a structure excited transverse magnetic harmonics of 210 mode(TM₂₁₀ harmonics).Multi-layer overlapping and deliberately regulating period length between adjacent unit cells were able to significantly enhance TM₂₁₀ harmonics,leading to a strong absorption waveband.On such a basis,a design strategy was proposed to achieve broadband,thin-thickness multi-layered MMA.In this strategy,the big pyramidal arrays placed in the“white blanks”of a chessboard exhibited two isolated absorption bands due to their fundamental and TM₂₁₀ harmonics,which were further connected by another absorption band from the small pyramidal arrays in the“black blanks”of the chessboard.The as-designed MMA at a total thickness?h?of 4.36 mm showed an absorption of above 0.9 in the whole frequency range of 7-18 GHz,whose bandwidth was tripled with respect to the original simple configuration which only used the fundamental harmonics.This strategy provided an effective route to extend the absorption bandwidth of MMAs without increasing h.This dissertation has proposed additive assisted high energy ball milling methord,surface in-situ oxidation methord in air and grain orientation methord of carbolyl iron particle,which have extended the preparation methord of high permeability microwave absorbents.This dissertation has also developed phase coupling mechanism to enhance microwave absorption and extention absorption bandwidth of metamaterial by ultilizing high order harmonics,which have profound guiding significance for designing metamaterial absorbers.