Optimization Of Properties Of Carbon Nanotube Fiber Fabricated By Floating Catalyst Chemical Vapor Deposition

The carbon nanotube(CNT)possesses outstanding features,such as high strength,excellent electrical conductivity and elevated thermal conductivity,therefore,it had attracted attentions throughout the globe wide.Composed by millions of carbon nanotubes,CNT fiber is an important kind of macroscopic materials with broad application prospects.However,due to unavoidable defects in the carbon nanotubes assembling process,the performance of CNT fiber on the macro scale is far from the ideal value of carbon nanotube.Therefore,preparation and strengthening for CNT fiber with high performance are the research hotspots at present.In this work,continuous and controllable preparation for CNT fibers was achieved by floating catalyst chemical vapor deposition(FCCVD)method,and the influence of process parameters on the structure of CNT fiber and its precursors was studied systematically.Strengthening treatment was applied on the CNT fiber to enhance its performance.The fracture mechanism of CNT fibers with different strengthening treatment was compared,and the main influencing factors of the strengthening process were defined.In addition,the CNT and copper were successfully compounded together by physical vapor deposition(PVD),and the influence of the copper contents on the mechanical and electrical properties of CNT was quantified.After the strengthening treatment to the CNT/Cu composite fiber,a new method of preparing CNT fiber material with high strength and high electrical conductivity was achieved eventually.Based on the basic principle of FCCVD process,effects of the process parameters to the carbon nanotubes growth were studied systemically in this work,including carrier gas flow,volume ratio of carrier gas,temperature at the injecting area,synthesis temperature and additional catalyst.The results were as follows:(1)the alignment of the CNT bundles in the CNT fiber was optimized as carrier gas flow and corresponding collecting speed increased,and the internal gaps and pores were decreased;(2)the increase of the volume ratio of H2 in the carrier gas was beneficial to eliminate the amorphous carbon,which attached on the surface of the carbon nanotube;(3)the crystalline degree of carbon nanotube was increased as the temperature increased,however,the content of the amorphous carbon was also increased;(4)adding a small amount of copper to the catalyst can reduce the number of walls of the carbon nanotubes.After systematic optimization experiments,the best parameters were as follows:the volume ratio of H2 should be more than 67%;the temperature at the injecting area should be kept between 350-400 ?;the most appropriate synthesis temperature was 1275-1325?;and the suitable content of the additive copper was(Cu):n(Fe)?1:4?1:2,where the content of the n(Fe)was a constant value.After optimization of growth process parameters,CNT fibers can grow continuously and steadily,and the diameter was controlled between 20-120?m stably.By statistical analysis results,it is discovered that there is an exponential relationship between the tensile strength and the diameter of the CNT fiber,i.e.(?)=15294.5×(?)-0.82,which may be helpful for the performance prediction of the CNT fiber.According to Van Der Waals force theory between CNT bundles,this work suggests that increasing the density and optimizing the alignment of CNT bundles are effective method to optimize the performance of CNT fiber.Therefore,strengthening treatments,including drawing and rolling,were performed on the CNT fibers to optimize the structures and performance.In the drawing process,the spring back phenomenon was likely to be happened if the the CNT fiber was drawn directly,and the strengthening effect was limited.Even worse,the surface of the CNT fiber could be easily damaged.Therefore,a new drawing process,i.e.drawing the CNT fiber with a capsule,was proposed in this work,which can inhibit the occurrence of spring back and surface damage,thus optimized the density and alignment of the CNT fiber effectively.Based on the drawing treatment,rolling treatment was carried on the CNT fiber for further strengthening.After rolling treatment,the internal pores of CNT fiber were eliminated greatly,and the roughness of the CNT fiber was decreased.As a result,the tensile strength of CNT fiber were enhanced greatly,as high as 2790± 180 MPa.It is found that the strengthening mechanism of CNT fiber was directly related to the increase of density of the CNT fiber.As the density increase,the Van Der Waals force and friction between the CNT bundles were enhanced significantly.The relationship of the tensile load function and density was obtained by statistical analysis as follow:F = F0+b×??p.In the same way,the function of the tensile strength and density can be drawn as ? = A×?+B×?2,which can provide a theoretical basis for further optimization of the CNT fiber.After strengthening treatment,the electrical conductivity of the CNT fiber can be increased to(1.3±0.16)×106S/m,but still one order of magnitude lower than that of the metal electrical conductor material.To further improve the electrical conductivity of the CNT fiber,consecutive and dense copper films were coated on the CNT fiber by PVD process,realizing the compounding of the CNT fiber and copper.After coating the copper film,the electrical conductivity was increased by 2 orders of magnitude,and the ampacity of the CNT fibers were enhanced greatly,while the tensile strength dropped significantly.For further enhancement,the drawing and rolling treatment were adopted to strengthen the CNT/Cu composite fiber.The quality of copper film and the structure of CNT fiber were optimized simultaneously,and the interaction between the copper film and CNT fiber was enhanced.Finally,both the mechanical property and electrical conductivity of the CNT/Cu composite fiber were improved significantly.After strengthening treatment,the CNT/Cu composite fiber possessed high effective tensile strength,i.e.1010MPa,more 3 times of the pure copper.Moreover,the electrical conductivity was as high as(2.6±0.3)×107S/m,about 45%of the pure copper.This work can provide theory and technology basis for the CNT/Cu composite wire,which possess low density,high strength and high conductivity.