| The increasingly serious electromagnetic radiation pollution has caused serious harm to healthy life,electronic information transmission and national defense security.It is urgent to solve the serious challenges brought by electromagnetic radiation,in which microwave absorbers can effectively attenuate the incident electromagnetic wave energy to mitigate electromagnetic radiation pollution,so as to protect the human body from electromagnetic radiation hazards,maintain the safety of electronic information transmission and ensure the concealment of military equipment.Therefore,the study of microwave absorbers has important research significance and application value in both civil and military fields.Traditional single-layer microwave absorber can only achieve single effective absorption peak,and the absorption bandwidth is very narrow.The design of structures based on microwave absorbing materials can break through the limitations between the electromagnetic parameters of the material,thus broadening the effective absorption bandwidth.With the increasing complexity of the electromagnetic environment,the performance of artificial microwave absorption structures is not only measured by the thickness,weight,absorption bandwidth and strength of the structure,but also by the tunable properties and functional indicators such as optics and mechanics.This dissertation focuses on the application requirements of broadband absorption,tunable performance and multifunctional integration of artificial microwave absorbing structures.Two artificial microwave absorption structures,double-layer and pyramidal,are constructed to broaden the effective absorption bandwidth.Further,the effective absorption bandwidth is tuned based on origami technique.Finally,the design of multifunctional integrated artificial microwave absorption structure based on hydrogel material is explored and studied.The main research contents of this dissertation are as follows.(1)A two-layer artificial microwave absorbing structure composed of carbonyl iron powder layer/polyethylene layer is constructed,and the broadband microwave absorption effect is finally achieved by optimizing the structure and dimensional parameters.The impedance matching band of a single layer of carbonyl iron powder is limited.In contrast,loading the impedance-matching layer polyethylene above the carbonyl iron powder layer can realize the gradient change of impedance,which can substantially increase the electromagnetic wave energy entering inside the absorbing structure,and finally broaden the absorption bandwidth of carbonyl iron powder to13.6 GHz.Next,a pyramidal artificial microwave absorption structure composed of doped acrylonitrile/butadiene/styrene copolymer(ABS)material was constructed,and the final broadband microwave absorption effect was achieved by optimizing the structural dimensional parameters.Unlike the double-layer artificial microwave absorption structure,the pyramidal structure exhibits gradient impedance due to its top-down gradient intrinsic structural properties.Combining the material’s own dielectric loss with the structural resonance effect,the broadband microwave absorber can cover the effective absorption in the frequency range of 5.3-18 GHz.(2)A foldable artificial microwave absorber structure composed of doped polylactic acid(PLA)material was constructed,and the absorption bandwidth was tuned by folding the thin layer of doped PLA into different structures.As the basic configuration of the folded structure,the thin layer PLA has almost no absorption capability;while the single-arch structure can obtain single effective absorption peak,but the effective absorption bandwidth is only 3.3 GHz;the double-arch structure can obtain double effective absorption peak,and the absorption bandwidth is broadened to6 GHz;the double-arch structure is filled with U-shaped strips to obtain an improved double-arch structure,which can achieve three effective absorption peak with broadband absorption,and the effective absorption band covers the range of 3.4-18 GHz.From the mechanism analysis,it can be seen that the single-arch structure can provide a single wall for resonance,producing a single peak of effective absorption.While the double-arch structure can provide the inner and outer walls to resonate separately,thus producing two effective absorption peaks.The loss capability of the improved double-arch structure is enhanced overall,and thus the effective absorption band is further broadened.(3)An artificial microwave absorbing structure consisting of polyacrylamide(PAM)hydrogel was constructed.The structure achieved optical and mechanical properties at different temperature conditions while broadband absorption.The electromagnetic parameters of hydrogels with different water contents were studied and tested.The absorbing structure is designed and optimized based on the material parameters.Firstly,a hydrogel thin layer is introduced to realize the tunable optical properties,and secondly,a hydrogel lattice is introduced to realize the tunable mechanical properties,and finally,an artificial microwave absorbing structure is constructed based on the combined structure of thin layer+lattice hydrogel with simultaneous temperature tunable optical and mechanical properties.Further,the broadband absorption effect is obtained by optimizing the structural size parameters.Hydrogel-based artificial microwave absorbing structures were prepared experimentally.The mechanical,optical and absorbing properties of the structure was characterized under room temperature and freezing condition.The artificial microwave absorbing structure was realized to exhibit multifunctional integrated properties of absorbing/flexible/transparent at room temperature and absorbing/rigid/opaque at freezing condition.Finally,the broadband absorption of the designed absorbing structure in the frequency range of 5.7-18 GHz was finally verified. |
