| Carbon nanocoil(CNC),one of the distinctive types of three-dimensional helical nanomaterials,have attracted wide interests due to their unique morphology and attractive applications.Although some improvements were made in raising the purity of CNC by using different methods,but there is still the problem of by-products in the synthesis process.On the other hand,all studies thus far have been focused on single catalyst particle suitable for CNC growth,and the inevitable interaction between adjacent catalyst particles in the process of chemical vapor deposition(CVD)has been ignored.Meanwhile,the synergy effect played by Sn and Fe in a binary catalyst are insufficiently clear enough.To solve these problems,this thesis systematically studies the preparation methods of high-purity CNC,reveals the structureperformance relationship between the catalyst aggregation and the morphology of the CNC,and clarifies the relationship between the respective effects of the Fe/Sn catalyst and the anisotropy of the catalyst.In addition,as a typical chiral dielectric material,CNC is expected to be a promising candidate in the field of advanced electromagnetic wave absorbing materials with the development goal of "thin,wide,light,and strong".Although the current research on CNC-based absorbing materials has made certain progress,there are still problems such as low attenuation strength,thick thickness,high filling rate and difficulty in adjusting the absorption frequency band.To this end,the development of CNC-based derivative composites,which combine helical chiral structure with dielectric-magnetic composite,has important guiding significance for the design of high-performance multi-mechanism synergistically enhanced absorbing materials.The main contents are as follows:Firstly,for the synthesis of high-purity CNC,we prepared catalyst particles with different Fe/Sn molar ratios and morphologies through a simple one-step solvothermal method,and systematically studied the effects of Fe/Sn molar ratio on the growing purity of CNC.CNCs with high purity of～99%have been synthesized by using catalyst particles under Fe/Sn molar ratio of 10:1.Meanwhile,the yield of the CNC reaches 9098%in a 6 h CVD reaction.The appropriately aggregated catalyst-carbon fiber structure provides the necessary root fixation for the spiral growth of CNC.In addition,the short carbon fiber layer in the product originates from CNC initial growth stage.Secondly,the effect of the areal density of the catalyst on the growth of CNC was systematically studied.The experimental results show that with the increase of the catalyst areal density,the morphology of the product gradually changes from twisted nanofiber to spring-like CNC,and its wire diameter increases,but the crystallinity gradually decreases.Further studies reveal that with the increases of the catalyst areal density,the particles on the tip of the CNC transform from single one to multi-particle aggregates.It is concluded that Fe-containing particles are the main catalytic component in a Fe/Sn binary catalyst system,which plays the roles of catalytic decomposition of C2H2 and deposition of a carbon filament.The SnO2 prevent the excessive accumulation of carbon on the surface of catalysts and prolong their lives.Besides,the Sn and Fe components in the multiple contacting particles fuse with each other and transform into Fe3SnC during the CVD reaction.Therefore.the growth mechanism of CNC can be attributed to the multiple particles on the tip with different catalytic activities corresponding to different rates for the growth of the carbon filaments.Thirdly,a novel composite of helical CNC and Fe3O4@C core-shell structure is synthesized via sequential processes of CVD technique and hydrothermal reactions.By controlling the reaction temperature and time in CVD process,the composites with different morphologies and crystal structure are obtained.The electromagnetic parameters are controlled by constructing the heterointerfaces of CNC/Fe3O4,amorphous CNC/polycrystalline carbon shell,and Fe3O4@carbon shell.The minimum reflection loss of the CNC/Fe3O4@C composite reaches-47.5 dB with a thickness of only 1.7 mm.Meanwhile,the composite also exhibits an effective absorption frequency range(≤-10 dB)of as wide as 5.03 GHz at a thickness of only 1.5 mm.These fascinating results arise from the synergic dissipation effect of magnetic and dielectric loss,as well as the unique chiral CNC.Finally,we report a scalable strategy to fabricate hierarchical aerogels composed of CNCs,reduced graphene oxide,carbon nanofibers,and core-shell particles.By adjusting the preparation parameters,the morphology,electromagnetic parameters,and absorbing properties of aerogels were systematically studied.the existence of helical CNC effectively improves the stacking of graphene sheets.making the system rich in pore structure,and improving the impedance matching characteristics of the material while providing necessary growth space for the subsequent introduction of carbon nanofibers.By introducing multi-dimensional carbon nanostructures into the aerogel,the electromagnetic wave attenuation mechanism of the composite is greatly enriched,the electrical conductivity and impedance matching characteristics are orderly and controllable.As a result,the loss ability and impedance matching characteristics of the material are organically unified.Moreover,the minimum reflection loss value of optimized aerogel reaches-71.5 dB,and the corresponding effective absorption frequency range covers the whole X-band.The remarkable microwave absorption performance of the as-prepared aerogel is closely related to the multiple loss mechanisms of multidimensional gradient structures and the synergistic effect of each component.As a final comment,through the rational design of the catalyst,the controllable preparation of high-purity CNC is realized,and the mechanism of the synergistic growth of CNC with multi-particle catalysts is also revealed,which lays an important foundation for the practical application of CNC.On this basis,CNC is used as the skeleton to construct a multi-level composites,which realizes the efficient absorption of electromagnetic waves in different frequency bands.Meanwhile,the synergistic effect between chirality-dielectric-magnetic loss is revealed.The above works have significance for the design of high-performance multimechanism synergistically enhanced absorbing materials. |
