| Due to the rapid development of electronic science and technological industry,electromagnetic technology has been applied in almost every aspect of human life,aerospace,military and many other areas.Therefore,the demands for absorbing materials are becoming stronger and stronger.Carbon nanotubes?CNTs?and graphene possess unique structures and excellent dielectric properties.Owing to their light weight and adjustable conductivity,CNTs and graphene are proved to be good microwave absorbing materials with dielectric loss.Magnetic metal oxides with peculiar structures,such as hollow spherical ferrite and layered double oxides?LDOs?display excellent magnetic properties and show potential applications in microwave absorbing field.The combination of CNTs or graphene with special structured magnetic metal oxides is expected to exhibit desirable microwave absorbing performance.As-prepared composites could meet the requirement of wide bandwidth.This research will display important theoretical significance and practical value in the control of electronic pollution.In this thesis,core-shell structured CoFe2O4 hollow sphere@CNT composites,Mg/Fe LDO/helical CNT?HCNT?composites and layered porous CoFe2O4 rugby/graphene composites were synthesized and their microwave absorption performance were investigated.The details are as follows:?1?CoFe2O4 hollow spheres were synthesized by a hydrothermal method,using?NH4?2Fe?SO4?2ˇ6H2O and CoSO4ˇ7H2O as starting materials,deionized water as a solvent,glucose as a template precursor.Thereafter core-shell structured CoFe2O4 hollow sphere@CNT composites were in situ prepared using a chemical vapor deposition method,with CoFe2O4 hollow spheres as catalysts and magnetic ingredients,ethanol as carbon source.The influence of reaction temperatures,times and flow rate of carrier gas on the morphologies of the composites were studied.Optimal conditions for the preparation of core-shell structured CoFe2O4 hollow sphere@CNT composites were reaction temperature of 550?,reaction time of 17 min,ethanol flow rate of 0.2 mLˇmin-1,H2 flow rate of 80mLˇmin-1 and Ar flow rate of 70 mLˇmin-1.The minimum reflection loss?RL?of core-shell structured composites was-32.8 dB with a thickness of 2 mm at 11.7 GHz and the effective absorption bandwidth?RL<-10 dB?was 5.7 GHz.?2?Mg/Fe LDHs were obtained using a urea-assisted co-precipitation process,with FeCl3ˇ6H2O and MgCl2ˇ6H2O as starting materials,NaOH as a precipitant.Mg/Fe LDO/HCNT composites were in situ synthesized by a chemical vapor deposition method,with Mg/Fe LDHs as catalysts and precursors of Mg/Fe LDOs which acted as magnetic ingredients.The influence of reaction temperatures,times,and flow rate of H2 were investigated.Results showed that longer reaction time and higher flow rate of H2 could promote the amount of HCNTs,but distroyed the layered structure of Mg/Fe LDOs at the same time.Optimal conditions for the preparation of Mg/Fe LDO/HCNT composites were reaction temperature of 500?,reaction time of 30 min,ethanol flow rate of 0.3 mLˇmin-1,H2 flow rate of 90 mLˇmin-1 and Ar flow rate of 100 mLˇmin-1.The minimum RL of as-synthesized composites was-35.0 dB with a thickness of 3 mm at 11.9 GHz,and the effective absorption bandwidth was 5.2 GHz.?3?Layered porous CoFe2O4 rugby/graphene composites were prepared by a vapor diffusion-assisted solvethermal method,with FeCl3ˇ6H2O and CoCl2ˇ6H2O as starting materials,ethylene glycol as a solvent,urea as a precipitant,PVP as templates and graphene oxides as dielectric ingredients.Optimal conditions for the preparation of CoFe2O4rugby/graphene composites were reaction temperature of 160?and reaction time of 24 h.Layered porous CoFe2O4 rugbies were distributed on the silk-like sheets of graphene.As-obtained composites showed better microwave absorbing performance at a thinner thickness and a wider bandwidth compared with single dielectric or magnetic absorbing materials.The minimum RL of layered porous CoFe2O4 rugby/graphene composites was-39.0 dB with a thickness of 2 mm and the effective absorption bandwidth was 4.7 GHz. |
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