Preparation and Microwave Absorption of CNT Films/Magnetic Metals Nanowires Composites

Author:Shen Jun Yao

Supervisor:leng jin song

Database:Doctor

Degree Year:2019

Download:120

Pages:127

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Microwave absorbing material is a kind of materials to absorb external electromagnetic wave(EMW)and attenuate the energy with different energy forms of losses(e.g.,dielectric loss,magnetic loss),which has been wildly applied in military strikes and civilian health-guards.Among them,carbon nanotube(CNT)/nanosized magnetic metals composites absorbers can efficiently amend the many problems such as poor absorption,narrow absorption frequencies and low loss attenuations in the sole CNT and traditional magnetic metals absorbers.However,it still exists many problems including unique magnetic micro/nanostructured metals,weight optimization of magnetic metals and CNT for improving microwave absorption and optimization of absorbing structures based on CNT/heterostructured magnetic metals absorbers,which urgently calls for further studying.This thesis is concentrated on the topic of CNTs/magnetic metals nanowires(NWs)composites absorbers.We have prepared and analyzed the microwave absorption of CNT films,Fe NWs,FeCo NWs,and their related composites step by step.We also study the stack-layered absorbing structures base on this two CNT films/magnetic metals NWs(Fe NWs and FeCo NWs)composites absorbers,which might give some valuable contributions for the development of related microwave absorption application.The highlights of this paper are as follows:CNT films are grown via injection-assisted chemical vapor deposition(CVD)method.The outer diameter is 50-80 nm and the specific surface is up to 371.31 m2g-1 in the CNT films.Both CNT films/SR composite and commercial CNT powders/SR composites are subsequently prepared for the investigation the effect on the thermal,mechanical and electrical properties in the different CNT macrostructures.The CNT films/SR composite also exhibits better EMW absorbing performance than CNT powders/SR composites.For example,the CNT films/SR composite shows the maximum reflection loss(RL)value of-26.55 dB and the efficient absorbing frequency of 3.57 GHz under 1 mm coating thickness.It is thanks to the synergism effects of electrical and dielectric losses from CNT films.Fe NWs with an average diameter of 100 nm and the aspect ratio of 50 are prepared via magnetic-field assisted method.The length of Fe NWs is positively correlated with the applied magnetic field,and the microstructure of the NWs appears as necklace-like.RL parameters of Fe NWs are enhanced with-27.28 dB.Three absorbing structures based on CNT films/Fe NWs composites are constructed for EMW absorption modeling which are denoted as UCDF type,UFDC type,and UMCF type,respectively.The stiumlated results show that UMCF type obtains the optimized EMW absorption performance which holds the uniform arrangement of CNT films and Fe NWs in the composite.CNT films/Fe NWs composites absorbers with different weight ratios of Fe NWs/CNT films are subsequently prepared to investigate the relationship between weight ratios of Fe NWs/CNT films and the microwave absorption performances.All the CNT films/Fe NWs composites absorbers show good RL abilities.The CNT films/Fe NWs absorber with the weight ratios of 1:1 Fe NWs/CNT,for instance,shows the maximum reflection loss(RL)value of-29.15 dB under 1-5 mm coating thickness.The related efficient absorbing frequency covers2.35-16.60 GHz.FeCo NWs including Fe3Co7 NWs,Fe5Co5 NWs and Fe7Co3 NWs are prepared by the aid of an external magnetic field.Facilitation of tuning EMW absorption is reached to meet requirements of different frequency bands with changing molar ratios of Fe/Co atoms.As a case,the maximum reflection losses of Fe3Co7 NWs,Fe5Co5NWs and Fe7Co3 NWs NWs are-25.88 dB,-19.06 dB and-21.98 dB under 3 mm coating thickness,respectively.Then the CNT film/FeCo NWs composites are prepared and we also investigate the effects of different weight ratios of FeCo NWs and CNT films for microwave absorption performances.The three absorbers present excellent RL values and broad EW absorption band absorption.The CNT films/Fe NWs absorber with the weight ratio of 1.5:1 Fe NWs/CNT,as a case,shows the maximum reflection loss(RL)value of-42.21 dB and efficient absorbing frequency of 13.36 GHz under 1-5 mm coating thickness.The unit absorbing structures are built referred to CNT films/Fe NWs composites and CNT film/FeCo NWs composites absorbers,respectively.The four types of double-layered absorbing structures are constructed and their microwave absorbing performances are also modeled for the optimal forecast of stack-layered absorbing structures.