The Research On Microwave Absorbing Properties Of Nitrogen-doped Carbon Coated Magnetic Nanoparticle Composite Microspheres

When the world is immersed in the convenience from the prosperous development of electronic devices and communication technology,consequent pollution known as electromagnetic wave interference?EMI?has already caused adverse impacts in many aspects included healthcare,electronic safety and national defense security.In order to reverse the unfavorable situation,more attention has been paid to develop high-efficiency measures to solve this problem.Microwave absorption?MA?is one of the accepted approaches to eliminate the threat from EMI.Herein,it is desired for researchers to design and fabricate microwave absorption materials with light weight,thin thickness,wide efficient absorption and strong absorption properties.For microwave absorption materials?MAMs?,subtle structural designing and chemical doping are essential approaches in improving their microwave absorption properties.In this thesis,we innovatively used metal-based alkoxide sphere as core and polypyrrole?PPy?as shell to synthesize two kinds of nitrogen doped magnetic carbon spheres with special structure for application as a microwave absorber.Fe-based alkoxide sphere was used as core and PPy was used as shell to synthesize Fe/Fe₃O₄@N-doped carbon?FNC?with unique plum pudding-like structure via in-situ polymerization followed by carbothermal reduction process.Different calcination temperatures were investigated to show influence on the components of magnetic cores.Our results show that the precursor transforms to Fe₃O₄ at 600°C and Fe₃O₄ will be gradually reduced to Fe when the temperature exceeds 600°C.Microwave absorption properties are evaluated in the frequency range of 2.0?18.0 GHz.The microspheres formed at 750°C with a low filler loading ratio of only 15%exhibit a minimum reflection loss?RL?value of-67.02 dB at 2.15 mm,and the widest effective absorption bandwidth??-10 dB?6.16 GHz could be obtained at 2.3 mm.The enhanced microwave absorption performance can be ascribed to the strengthened interfacial polarization from multi-core/shell structure,dipole polarization and defect polarization induced by nitrogen and collective of multiple loss mechanism.These results support the approaches of fine structural designing and chemical doping to enhance the microwave absorption properties.Fe₇Co₃@N-doped carbon with yolk-shell structure?YS-Fe₇Co₃@C?was synthesized through a two-step method.First of all,Fe₇Co₃-based alkoxide sphere was coated with pyrrole via in-situ polymerization.Then the MAM with special yolk-shell structure could be obtained after a calcination at 750°C of above sample.To reflect the excellent MA properties of composites,the MA performance of pure Fe₇Co₃ and N-doped carbon were studied.The results show YS-Fe₇Co₃@C possess superior MA properties than each single material.An effective absorption bandwidth of 6.6 GHz?11.4-18.0GHz?has been achieved at the thickness of 2.17mm with a low filler loading ratio of 20%,while the minimum RL value of-41.7dB is obtained at the thickness of 1.78mm.Furthermore,the additive proportion of Fe₇Co₃-based alkoxide sphere and pyrrole was researched.Only a proper raw material ratio can guarantee the effective MA,and acquire the yolk-shell sturture in the same time.The enhanced microwave absorption performance can be ascribed to the strengthened interfacial polarization from yolk-shell structure,dipole polarization and defect polarization induced by nitrogen and collective of multiple loss mechanism.