Preparation And Electromagnetic Properties Of Electromagnetic Wave Absorbers With In-situ Grown Carbon Nanotubes

Carbon nanotubes(CNTs)have excellent electrical conductivity and high dielectric loss,and can be used as reinforcing materials to strengthen the electromagnetic wave absorption performance of the matrix.In this paper,carbon fiber(CF)and Ti3C2Tx MXene-based electromagnetic wave absorbing materials were prepared by chemical vapor deposition(CVD)using CNTs as reinforcements;the effects of process parameters on their structures and properties were explored,and the structure protection strategy of MXene during CVD process was explored for the first time,including:(1)the influence of pretreatment on the surface chemistry of CF and MXene;(2)the adjustment of CVD process parameters and its influence on the matrix and CNTs;(3)the effect of the catalyst precursors for CNTs growth;(4)electromagnetic wave absorption performance and other characterization of CF/CNT and CNT/MXene composites.In ters of surface chemistry,electrochemical anodic oxidation(EAO)was used to effectively activate the inert surface of CF.Compared with the original CF,the C content of CF-EAO was reduced from 82.82%to 64.51%,and the O content was increased from 11.57%to 31.77%.A new C-O bond peak located at 286.1 eV appeared in the C 1s spectrum of CFEAO,and an O-C=O bond absorption peak was found at 288.4 eV.After the etching process,the absorption peaks of O 1s and F 1s were found at 526.6 eV and 681.2 eV,respectively.After the CVD process,the Ti-C bond signal in the Ti 2p spectrum of MXene was significantly weakened and the peak of the Ti-O bond was observed.In terms of CVD process parameters,the carbon source cracked into activated carbon atoms at a growth temperature of 600℃ and properly repaired the surface defects of CF.However,the structure of MXene had collapsed at 600℃.After lowering the temperature to 550℃,500℃,and 450 ℃,the degree of structural collapse of MXene was significantly reduced,and the structural integrity of MXene was well protected at 450℃.As the temperature decreased,the crystallinity of CNTs decreased and certain defects occured.The defects and the conductivity corresponding to the structural integrity of MXene affect the electromagnetic wave absorption performance of the CNT/MXene composites.The CNT/MXene prepared at 500℃has the best electromagnetic wave absorption performance.The lowest reflection loss(RLmin)reaches-47.28 dB when the sample thickness is 1.36 mm,and the maximum effective absorption bandwidth(EAB)reaches 4.16 GHz when the sample thickness is as low as 1.16 mm.The ratio of the gas introduced in the CVD process also affects the morphology of CNTs.When N2:C2H2=2:1,graphite ring stacked CNTs can be grown on the surface of MXene.When N2:C2H2=10:1,the CNTs grown on the surface of MXene have a bamboo-like structure.The bamboo-shaped structure effectively strengthens the electromagnetic wave absorption performance of the CNT/MXene composites,and the RLmin reaches-52.56 dB at a sample thickness of 2.5 mm.In terms of catalyst precursors,the influence of different concentrations of Co-Ni catalyst precursor on CNTs growth and electromagnetic wave absorption performance were explored.When the concentration of the precursor solution was 0.03 mol/L,the CNTs on the CF surface were denser with a small number of clusters,but the length of the CNTs was short.When the solution concentration was 0.05 mol/L,the growth density of CNTs decreased but the length increased significantly,and they cross-linked each other to form a network.When the solution concentration was 0.07 mol/L and 0.09 mol/L,the length of CNTs gradually decreased with the increase of concentration,and individual CNTs with a larger diameter were observed.Owing to the three-dimensional conductive network composed of CNTs and CF,CF/CNT@0.05 exhibits the best electromagnetic wave absorption performance.When the sample thickness is 1.54 mm,the RLmin reaches-58.75 dB at a sample thickness of 1.25 mm,and the EAB reaches 4.24 GHz.The effects of monometallic and bimetallic catalysts on the growth of CNTs on the surface of MXene were explored.The single-component Fe catalyst cannot catalyze the growth of CNTs at any temperature.The single-component Co catalyst can only catalyze a small amount of sparse CNTs at 600℃.The two-component Fe-Co catalyst can catalyze a large amount of CNTs at 450℃,indicating that the bimetallic catalyst has a synergistic effect.In terms of other characterization and microstructure of CF/CNT and CNT/MXene composite materials,CF/CNT@0.09 has the best monofilament tensile strength,which is 11.7%higher than the original CF.After the etching process,the Al peak in the XRD pattern of MXene completely disappeared,and the peak of(002)crystal plane moved from 2θ=9.5° to 2θ=7°.After the CVD process,a new peak appeared at 2θ=25.3°,which corresponds to the(002)plane of graphite.Moreover,multiple diffraction peaks corresponding to TiO2 were found in the XRD pattern of the CNT/MXene prepared at 600℃.The number of semicircles in the ColeCole diagram of the CNT/MXene composites has increased significantly,indicating that a variety of polarization relaxation processes occurred inside the material,which effectively improves the electromagnetic wave absorption performance of CNT/MXene.