Microstructure Design Of Microwave Absorption Materials

The worsing electromagnetic pollution has caused irretrievable damages to healthcare,electronic safety,and national defense security.In order to solve this problem,tremendous efforts have been dedicated to manufacturing microwave absorption?MA?materials with features of strong absorption ability,broad working band,thin thickness,and light weight.Recently,vast advances in MA theory and fabrication technology have demonstrated that the superb MA performance undoubtedly benefits from the delicately designed microstructure,such as core-shell,sandwich,and porous structure.However,there are still challenges,including simplification of preparation process and reduction of cost,in the construction of MA material microstructure.Moreover,the influence of microstructure on MA performance is still under studies.In this thesis,various excellent microwave absorbents with different structures have been prepared on the basis of facile methods and economical raw materials,and the contribution of structure to MA performance has been investigation thoroughly.The main content of this thesis is divided into following three parts:?1?A series of hierarchically porous carbons?HPCs?with first-rate MA performance have been prepared by single-step carbonization of intentionally selected biomass precursors.During the carbonization,the micro-scale pores templating from the inherent porous tissues and the nano-scale pores produced by the alkali metal activation constitute the hierarchical pores.Among as-prepared samples,the spinach-derived HPC exhibits a maximum reflection loss(RL_max)of-62.2 dB and a broad effective absorption bandwidth?EAB?of 7.3 GHz.Particularly,such excellent MA performance can be reproduced using precursors harvested in different seasons,indicating the good reproducibility of this HPC.Study on the mechanism suggests that the superimposed effect of micro-and nano-sacle pores together with the well-adjustable components contributes to the impressive MA performance.?2?Core-shell Fe₃O₄@polypyrrole?PPy?composites with superior MA performance have been successfully manufactured by a facile sequential process including etching,polymerization,and in-suit replication.Starting from the templates of pre-prepared Fe₃O₄ microspheres,ferric ions are released from the skin layer of microspheres by hydrochloric acid etching to initiate the oxidative polymerization of pyrrole in suit,resulting in the formation of PPy shell.The thickness of PPy shell and mass fraction of Fe₃O₄ core in the composite can be elaborately tuned by simply adjusting the etching time.Eventually,the core-shell Fe₃O₄@PPy composites prepared at the etching time of 5 min domanstrate a RL_max of as much as-1.9 dB and an ultra-broad EAB covering the entire Ku band?12.0-18.0 GHz?.The excellent MA performance of core-shell Fe₃O₄@PPy composites is mainly attributed to the enhanced dielectric loss from the PPy shell.?3?The ternary composite of carbon/FeO_x/graphene?CFG?with sandwich structure has been successfully synthesized utilizing a facile two-step method for the first time.After being immersed in the solution containing oxide graphene and ferric nitrate,the melamine foam is dried and subsequently carbonized in the nitrogen atmosphere.During the pyrolysis process,the reduced oxide graphene covers the carbon framework,the iron oxide micro-and nanoparticles derived from the ferric nitrate adhere to the reduced oxide graphene,resulting the formation of the sandwiched composite.The CFG exhibits a RL_max of-51.2 dB and a broad EAB of 8.7 GHz,which covers entire Ku band and most of X band.Such surpeb MA performance originates from the multiple reflection of microwave and the interfacial polarization enhanced by the sandwich structure.