The Construction And Electromagnetic Wave Absorption Properties Of TiO₂-and ZnO-based Core Shell Structures

Electromagnetic（EM）pollution and interference problems from the wide application of communication devices have led to the significant interest in developing high-performance microwave absorption materials（MAMs）with strong absorption and broad bandwidth.Indeed,great advances have been achieved on model absorbers,and insights into the correlations of the material morphology,size as well as chemical composition and absorption characteristics are crucial for not only understanding the EM wave absorption mechanism,but also generating new concepts to guide the rational design of practical microwave absorbers.Research confirmed that nanostructured core-shell absorbers could combine multiple wave loss mechanisms and achieve high-efficiency wave absorption performance.In this thesis,several novel core-shell structured MAMs were developed in order to achieve excellent EM wave absorption performance.The main contents and results are listed as follows:（1）Carbon-coated TiO₂（TiO₂@C）core–shell nanocrystals have been synthesized by a simple acetylene decomposition method and further explored for the microwave absorbing application.Results demonstrate that a well-graphitized carbon layer with a thickness of about 3.5 nm at 500°C can be uniformly coated on the surface of TiO₂.The thickness of carbon layer can be adjusted by changing the reaction temperature.It is found that the microwave absorption properties of TiO₂@C are remarkably enhanced compared to the bare Ti O₂.The optimal RL calculated from the measured complex permittivity and permeability is–58.2 dB at 7.6 GHz for TiO₂@C with a loading of 40wt%.Whereas for TiO₂@C with a loading of 60 wt%,the effective bandwidth of less than–10 dB is found to reach 5.0 GHz with the coating thickness of 2.2 mm.The enhanced performance can be attributed to the increased dielectric properties and the multiple relaxation processes caused by the core–shell composite materials.（2）We further synthesized carbon-coated ZnO（ZnO@C）nanorods by the similar acetylene decomposition method.The characterization of morphology and structure analysis demonstrated that ZnO nanorod was well coated by an amorphous carbon shell with a thickness of about 20 nm.Comparted with ZnO,ZnO@C exhibit significantly enhanced microwave absorption properties.The effective absorption bandwidth with RL values exceeding–10 dB can reach 5.3 GHz for ZnO@C with a matching thickness of 2.5 mm.The excellent microwave absorption arises from enhanced dielectric loss caused by interfacial polarization,dipole polarization and the formation of conductive network.（3）ZnO is nonmagnetic and thus its electromagnetic absorption properties mostly derive from the dielectric loss.For achieving strong absorption and broad bandwidth,one of the effective ways to solve the problem is to integrate Zn O with magnetic materials.Therefore,an elegant atomic layer deposition（ALD）method has been employed for controllable preparation of a uniform Fe₃O₄-coated ZnO（ZnO@Fe₃O₄）core–shell flower-like nanostructure.The Fe₃O₄ coating thickness of the ZnO@Fe₃O₄nanostructure can be tuned by varying the cycle number of ALD Fe₂O₃.When serving as additives for microwave absorption,the Zn O@Fe₃O₄–paraffin composites exhibit a higher absorption capacity than the ZnO–paraffin composites.For ZnO@500-Fe₃O₄,the effective absorption bandwidth below-10 d B can reach 5.2 GHz and the RL values below-20 dB also cover a wide frequency range of 11.6–14.2 GHz when the coating thickness is 2.3 mm,suggesting its potential application in the treatment of the electromagnetic pollution problem.On the basis of experimental observations,a mechanism has been proposed to understand the enhanced microwave absorption properties of the ZnO@Fe₃O₄ composites.（4）Magnetic metal nanoparticles have a higher complex permeability compared to ferrite materials in the GHz band range.Therefore,we employed the ALD technology to fabricate the interesting Ni-coated ZnO（ZnO@Ni）core–shell composites.The structure and EM absorption properties of the as-prepared samples are investigated.By applying 800 ALD cycles of NiO deposition followed by a reduction process,the surfaces of ZnO are densely covered by Ni nanoparticles with a narrow particle size distribution and an average size of 13.1 nm.ZnO@Ni exhibits remarkably improved EM absorption properties compared to ZnO.The optimal RL calculated from the measured complex permittivity and permeability is–48.0 dB at 10.4 GHz.Moreover,the EM wave absorption less than–10 dB is found to reach 5.3 GHz for an absorber thickness of 1.5 mm.The enhanced absorption ability arises from the effective combination of multiple dielectric–magnetic loss mechanisms.（5）Co Fe/C core-shell structured nanocomposites（CoFe@C）have been fabricated through the thermal decomposition of acetylene with CoFe₂O₄ as precursor.The as-prepared CoFe@C was characterized by X-ray powder diffraction,X-ray photoelectron spectroscopy,Raman spectroscopy,transmission electron microscopy and thermogravimetric analysis.The results demonstrate that the carbon shell in CoFe@C has a poor crystallization with a thickness about 5-30 nm and a content approximately48.5 wt.%.Due to a good combination between intrinsic magnetic properties and high electrical conductivity,the CoFe@C exhibits not only excellent absorption intensity but also wide frequency bandwidth.The minimum RL value of CoFe@C can reach–44 dB at a thickness of 4.0 mm and RL values below–10 dB is up to 4.3 GHz at a thickness of 2.5 mm.The present CoFe@C may be a potential candidate for microwave absorption application.