Structural Design And Microwave Absorption Properties Of Ppy Derived Core-shell Composites

With the widespread uses of electronic equipment,electromagnetic radiations have brought more and more harm to people.The absorbing materials coated on the surface of the object could transform electromagnetic energy of incident electromagnetic wave into thermal energy and other forms of energy and dissipate.Absorbing materials are widely used in military stealth protection,civil antielectromagnetic radiation and microwave darkroom,and other fields.The pursuit on thin,light,wide and strong properties are the basic requirements for absorbing materials.The traditional single component and structure materials have various disadvantages such as large density,narrow absorption band,and low absorption strength.Therefore,the current focus researchees are foacused on the developments of several loss mechanisms into one material,which can effectively adjust the electromagnetic parameters,and optimize the impedance matching characteristics.The core-shell composites can effectively adjust the electromagnetic parameters,optimize the impedance matching characteristics,and enhance the absorbing properties through rational collocation between internal core and external shell materials.Reasonable regulation of microstructure and material composition are important approaches to improve the electromagnetic absorption of materials.Based on the core-shell structure and polypyrrole PPy materials,several core-shell composites were designed,and the absorbing properties in the frequency range of 2.0-18.0 GHz were analyzed in this thesis.By controlling the mass ratio of PPy to aniline,the homogeneous core-shell composites PPy@PANI with controllable shell thickness were prepared by in-situ polymerization.The impedance matching characteristics of PPy@PANI were optimized,resulted in better absorbing characteristics than that of PPy,PANI and physical mixture PPy/PANI with the thicknesses of PANI shells of 60,85,105 and 120 nm.With an absorber thickness of 2.0 mm,the maximum reflection losses were-23.0,-34.8,-31.5,and-27.3 d B and the effective absorption bandwidth over-10.0 d B were 4.5,4.7,4.7 and 4.5 GHz,respectively.Those materials can effectively enhance the interface polarization effects,optimize the impedance matching characteristics,and improve the absorption performance.The regulate absorption characteristics can be controled by changing the thickness of PANI shell and the coating.The yolk-shell composites Fe3O4@C with magnetic and dielectric loss properties were fabricated through a “coating-coating-pyrolysis-etching” route.The absorbing characteristics of yolk-shell composites Fe3O4@C were superior to the single component Fe3O4,carbon Cppy and core-shell composites Fe3O4@C.With an absorber thickness of 2.0 mm,the maximum reflection losses were-73.1 d B and the effective absorption bandwidth over-10.0 d B was 5.7 GHz.The absorbing characteristics of yolk-shell composites Fe3O4@C were superior to that of core-shell composites Fe3O4@C with same chemical composition and different structures.This phenomenon indicates that the yolk-shell structure is superior to core-shell structure in electromagnetic wave attenuation.The yolk-shell composites Fe3O4@C optimize the impedance matching characteristic and effectively improve the absorbing performance under the synergistic effect of the internal reflection and interfacial polarization of Fe3O4 core,external carbon shell and yolk-shell structure.The yolk-shell composites C@C were prepared through the differential shrinkage in the internal cores and external shells of PPy microspheres with the assistance of outermost SiO2 coating at high temperature pyrolysis.The traditional coating-coating-pyrolysis-etching process was simplified this method to coatingpyrolysis-etching.The absorbing performance analyses show that the absorbing performances of the material with good impedance matching characteristics are good with the similar loss ability of the two material.With an absorber thickness of 2.0 mm,the effective absorption bandwidth over-10.0 d B was 4.9 GHz,which was better than that of Cm.It indicated that the yolk-shell structure with single component can also effectively adjust the electromagnetic wave parameter,optimizes the impedance matching characteristics,and improve the absorption performance.The double-shelled yolk-shell composites C@C@TiO2 were derived prepared by non-synchronous shrinkage induction method.The existence of TiO2 shell can effectively stabilize the external carbon shell structure and control the electromagnetic parameters.The absorbing properties of double-shelled yolk-shell composites C@C@TiO2 were better than those of yolk-shell composites C@C and Cm.The coating layer of double-shelled yolk-shell composites C@C@TiO2 was thinner than that of yolk-shell composites C@C under the same absorption bandwidth ranges in S,C,X and Ku frequency region,which indicated that the attenuation ability of double-shelled yolk-shell structure was better than that of yolkshell structure.With the absorber thicknesses of 1.0-5.0 mm,the maximum effective absorption bandwidth over-10.0 d B was 5.3 GHz with the thickness of only 1.7 mm.The absorption abilities are gradually enhanced with the increase of the thickness under the low-frequency region,which indicats that double-shelled yolk-shell composites C@C@Ti O2 have the advantages of thin coating and strong absorption ability in low-frequency region.