Core-shell Electromagnetic Nanocomposites:Synthesis And Microwave Absorbing Properties

As electromagnetic environment pollution problem is increasingly serious and modern military equipment stealth technology is continuously improved,traditional electromagnetic wave absorbing materials can not meet the current needs in the civil,commercial,military and other fields.Hence,it is very urgent to develop ideal electromagnetic wave absorbing materials with strong absorption,wide absorbing frequency band and light weight features.After lots of exploration and accumulation in the past several decades,development of electromagnetic wave absorbing materials has tended to be complex,diversified and nano-sized.The combination of dielectric materials and magnetic materials not only contributes to the improvement of impedance matching,but also promotes the increase of attenuation coefficient.Diversified design can produce more heterojunctions and enhance the interfacial polarization of materials.Due to the special nanostructures,nanomaterials can produce higher absorption capacity for electromagnetic waves than conventional materials.Up to now,some research progress has been made on core-shell nanomaterials as microwave absorbents,but several important issues have not been clear,which need to be further expanded and deepened.For example,what are the advantages of core-shell structured nanomaterials rather than the physical blending materials for improving microwave absorbing performance,whether the conjugation of hierarchical structure and the core-shell structure can further enhance the microwave absorbing performance,what is the effect of various porosity on the microwave absorbing properties,whether the construction of one-dimensional core-shell electromagnetic nanomaterials can achieve broadband absorption.Along with above questions,this thesis have carried out the synthesis and properties of new electromagnetic wave absorbing nanomateirals.The main research contents and conclusions are as follows:Highly regular core-shell Fe₃O₄@PPy composite microspheres have been successfully prepared by surfactant-induced chemical oxidative polymerization.The formation mechanism analysis reveals that p-toluenesulfonic acid acts as an imperative guider of pyrrole monomer,facilitating the growth of PPy shell,and polyvinyl alcohol promotes the dispersion and stability of as-synthesized composite microspheres.Through changing the amount of pyrrole monomer,three composite microspheres with different shell thicknesses were obtained.The study results of electrical properties and magnetic properties indicate that as the thickness of polypyrrole shell increases,the conductivity of composite microspheres gradually increases,while the magnetic properties gradually decrease.The investigations of microwave absorbing properties indicate that as-obtained Fe₃O₄@PPy composite microspheres have stronger microwave absorption capability than pure Fe₃O₄ microspheres.When the shell thickness is 80 nm,Fe₃O₄@PPy composite microspheres exhibit a maximum reflection loss peak of-31.5 d B(15.42 GHz)and an effective absorption bandwidth about 5.2 GHz(12.8-18.0 GHz)at the layer thickness of 2.5mm,which already cover the whole Ku band.The comparison between physically blended Fe₃O₄/PPy composites and core-shell Fe₃O₄@PPy composite microspheres as microwave absorbing wave materials is conducted.The results demonstrate that under the same ingredients,composite microspheres show intensiver microwave absorption ability and wider effective absorption frequency band than physically blended composites,which strongly confirms the superiority of the core-shell structured design in improving microwave absorption performance.Core-shell Fe₃O₄@MnO₂ composite microspheres were prepared by a simple two-step method,and the MnO₂ shells were formed from the assembly of numerous two-dimensional nanosheets.Through adjusting the reaction temperature and p H values of hydrothermal system,three kinds of core-shell Fe₃O₄@MnO₂ composite microspheres with different surface morphology were obtained,namely mushroom-like,honeycomb-like and corolla-like composite microspheres.The investigations of microwave absorbing properties indicate that three composite microspheres have stronger microwave absorption capability than pure Fe₃O₄ microspheres.The minimum reflection coefficients of mushroom-like and honeycomb-like composite microspheres are-33.4 d B(12.2 GHz)and-24.2 d B(17.6 GHz),and the corolla-like composite microspheres exhibit the strongest absorption capacity with a minimum reflection loss value of-48.5 d B(11.2 GHz).This results demonstrate that hierarchically structured MnO₂ shells can improve the absorbing properties of magnetic materials and the surface architecture of hierarchial MnO₂ shells have great influence on microwave absorbing properties.Novel core-shell Fe₃O₄@MnO₂ composite microspheres were prepared by a two-step hydrothermal method.The porous Fe₃O₄ magnetic microspheres acted as the cores and the hierarchically structured MnO₂ acted as the outer shells.For these composite microspheres,the BET specific surface area is 167 m²/g and the saturation magnetization value is about 35 emu/g.Investigations of microwave absorbing properties manifest that Fe₃O₄@MnO₂ composite microspheres with loading 10%weight fraction possess the strongest reflection loss peak of42.6 d B(5.7 GHz)at the matching thickness 4.0 mm,and Fe₃O₄@MnO₂ composite microspheres with loading 20%weight fraction have a broader absorbing bandwidth(10.2-15.0GHz and 15.5-17.3 GHz)at the matching thickness 1.5 mm.Achieving the advantages of intensive absorption,wide effective absorbing bandwidth band and intrinsic lightweight feature,porous core-shell Fe₃O₄@MnO₂ composite microspheres are believed to be more promising in the application of microwave absorption.The microwave absorbing mechanisms include dielectric loss,magnetic loss,good impedance matching,multiple reflection and multiple absorption in the porous Fe₃O₄@MnO₂ composite microspheres.The results demonstrate that the conjugation of core-shell structure,pore structure and hierarchical structure can further improve the microwave absorbing properties.The porous Fe₃O₄ magnetic microspheres were used as the assemble units,and yolk-shell Fe₃O₄@void@Si O2 nanochains were prepared by magnetic-field-induced distillation-precipitation polymerization together with self-sacrificing template method,which is first reported at present.The length of nanochains is concentrated at 10?15 um,BET specific surface area is about 49 m²/g,and saturation magnetization is about 42.3 emu/g.The investigations of microwave absorbing properties indicate that Fe₃O₄@void@Si O2 nanochains can achieve a minimum reflection reflection loss value of-24.7 d B and an effective absorption frequency bandwidth of 2.41 GHz(15.40-17.81 GHz).When Fe₃O₄@void@Si O2 nanochains are coated by polypyrrole shells,as-obtained Fe₃O₄@void@Si O2@PPy nanochains exhibit superior microwave absorbing properties with a minimum reflection reflection loss value of-54.2 d B(17.70 GHz)and an effective absorption bandwidth of 3.94 GHz(8.4-12.34 GHz),almost covering the entire X band.This result indicates that the design of yolk-shell structured nanochains is an effective way to develop highly effective microwave absorbing materials.Microwave absorbing mechanism analysis reveals that the introduction of polypyrrole shells contributes to enhancing the conduction loss,dipole polarization,interfacial polarization and relaxation loss of Fe₃O₄@void@Si O2 nanochains.Yolk-shell Fe₃O₄@void@N-doped carbon nanochains,as novel microwave absorbing materials,have been developed for the first time.The length of nanochains is mainly distributed at 10?30um,the shell thickness of nitrogen-doped carbon is about 64 nm,BET specific surface area is about 74 m²/g,and the saturation magnetization is about 36 emu/g.Some special features,such as high aspect ratio,yolk–shell structure,numerous pores and spaces,endow Fe₃O₄@void@N-doped carbon nanochains with superior microwave absorption performance.At the matching layer thickness of 3.1 mm,paraffin-based composites with loading 20 wt%yolk–shell Fe₃O₄@void@N-doped carbon nanochains can have a minimum reflection loss value of-63.09d B(11.91 GHz)and an effective absorption bandwidth of 5.34 GHz.Through changing the layer thicknesses,the effective absorption frequency bands can be tuned in the C,X,and Ku bands,respectively.Acquiring these advantages of stronger absorption capability,broad absorption bandwidth,low loading,thin layers,and intrinsic light weight,yolk–shell Fe₃O₄@void@N-doped carbon nanochains can be regarded as being more desirable candidates in the practical application of microwave absorption.Detailed microwave absorption mechanisms include quarter-wavelength cancellation theory,good impedance matching,intense conductive loss,multiple reflections and scatterings,microwave plasma loss,dielectric loss,and magnetic loss.Furthermore,the dielectric loss mainly originates from the various interfacial polarizations,plentiful dipole polarizations,and large amounts of relaxation loss,while magnetic loss mostly derives from natural resonance and the eddy current effect.This result further demonstrates yolk-shell nanochains have the unique structural advantages for the dissipation of electromagnetic wave energy,which contributes to openning up a new strategy for the exploitation of optimal microwave absorption materials.