Research On Micro-structure Adjustment And Microwave Absorption Properties Of Carbon-based Composite Meterials

The fast development of radar detection technology has been pushing the new generation warplanes to have broadband electromagnetic wave stealth ability.Meanwhile,the increasingly serious electromagnetic interference pollution has become a major concern for human health and reliability of integrated electron devices.Therefore,high efficiency electromagnetic wave(EMW)absorption materials is in urgent need in both military and civil applications.Ideal EMW absorption material is demanded to be lightweight,thin and have broadband yet strong absorption capbility.However,the existing EMW absorption materials,such as ferrite,are still suffering from narrow absorption bandwidth and high density.Carbon-based nanomaterials have the obvious advantage of being lightweight,dielectric lossy and easy to be modified.Especially,their EMW absorption properties and density can be easily optimised by adjusting their microstructures,thus very promissing for fabricating ideal EMW absorption materials.In this thesis,three kinds of EMW absorption materials with special microstructures are fabricated with hydrothermal synthesis,in-situ catalytic growth and unidirectional freeze casting respectively.Their EMW absorption properties are characterized and optimised.EMW absorption machenisum are analysised to clarify how their micro-structure affect the EMW absorption properties.These nanostructures are very promissing for high performace broadband EMW absorprion applications.The main contents are as follows.In order to broaden the effective absorption bandwidth(EAB,?-10dB),Co/CoO@C nanocomposite particles with a novel bowknot-like microstructure is fabricated through hydrothermal synthesis and post-annealing.Co(NO₃)₂·6H₂O was used as the Co²⁺doner and gelatin was slected as the molecular template to build up the bowknot-like microstructure during hydrothermal assemble.Electromagnetic parameters are measured and the reflection loss(RL)was calculated in 2-18GHz.The results indicate that by adjusting the annealing temperature the EMW absorption properties is optimized effectively.When annealed at 600°C,the Co/CoO@C-600 nanocomposite shows excellent broadband EMW absorprion ability.A 9.9GHz(8.1–18GHz)EAB and a-33.6dB RL_min are achieved with the thickness of 3mm.when the thickness is increased to 6mm,EAB is further enlargered to 13.6GHz(4.4–18GHz),covering 85%of 2-18GHz,togather with a-45.0dB RL_min.The excellent broadband EMW absorprion is attributed to the bowknot-like microstructure,surface/interface polarization as well as the synergistic effect of magnetic and dielectric loss,which make the bowknot-like Co/CoO@C nanocomposite a promising canidate for broadband EMW absorber.Being lightweight is another critical requirement for ideal EMW absorprion materials.Therefore,we fabricated a lightweight hierarchical hollow carbon fiber coated with Co/C particles and villous-like carbon nanotubes(CNTs).Cotton fiber is selected as lightweight biomass carbon source which is coated with Co containing ZIF-67 matel-origanic-frameworks(MOFs)nanoparticles.A high temperature calcination in H₂/Ar flow is employed for the carbonization of cotton fibers and ZIF-67 and the in-situ catalytic growth of CNTs,yielding the lightweight hierarchical CNT/Co/C composite hollow fiber with ultra-low apparent density(0.0198g/cm³).The lightweight CNT/Co/C fiber shows an 8.02GHz EAB with the thickness of 2mm.Compared to carbonized cotton fibers(EAB=2.26GHz),the hierarchical CNT/Co/C fiber exhibit dramatically enhanced broadband EMW absorption capbility.The idea of combaining biomass derived carbon material with molecular level porous MOFs also illuminates a new strategy for fabricating lightweight broadband EMW absorprion materialsAssembling low dimensional electromagnetic lossy nanomaterial into three dimensional(3D)porous architecture is a effective way to reduce material density and optimize EMW absorption properties.To explore the effect of microstructure on the EMW absorption properties,we fabricated a highly aligned porous Ti₃C₂T_xMXene/gelatin composite(M@G)aerogel with a unidirectional freeze casting method.The new two dimensional(2D)nanomaterial—MXene nanosheet is selected as nano build block.Gelatin molecule is introduced to enhance the interconnection between Ti₃C₂T_xnanosheets,yielding Ti₃C₂T_x/gelatin composite aerogel with enhanced and anistropic mechanical,thermal and electromagnetic properties.The Ti₃C₂T_x/gelatin aerogels exhibit large compression strength and negative Poisson's ratio and in axial direction,but in radial direction the composite aerogel shows lower compression strength and high elasticity and nearly zero Poisson ratio.The aligned composite aerogel also shows anistropic thermal conductivity and the minimum thermal conductivity in radial direction is as low as 0.008W/m·K(M@G-45).M@G-45 also shows-57.3dB RL_min at 4.08GHz and 0.9GHz(3.7-4.6GHz)EAB in radial direction.But in axial direction,the EAB is effectively enlarged to 6.24GHz(11.76-18GHz)and a-59.5dB RL_min peak shift to much higher frequency range(14.04GHz).The highly aligned porous Ti₃C₂T_x/gelatin aerogels shows the possibility to fabricate EMW absorprion materials with anistropic EMW absorprion ability.