A Study On Preparation, Property And Application Of Carbon Nanotube Macroscopic Materials

Carbon nanotube(CNT)has attracted great attention from academic and industrial fields since it was discovered in 1991 by Iijima of NEC Corporation.This material possesses excellent mechanical,electrical,and thermal properties,thus allowing it for a wide range of potential applications in areas such as reinforcement materials,field emission electronic materials,field effect transistors,lithium ion batteries,supercapacitors,and sensors.However,the as-synthesized CNT is mostly powder-like,has limited solubility,and tends to bundle together due to the strong van der Waals interactions,making it difficult to dissolve or disperse uniformly in all kinds of common solvents and thus hard to be manipulated for real applications.In order to fully utilize its superior properties and realize its diverse applications,CNT needs to be effectively assembled into macroscopic materials,such as fibers and films.To date,the main methods used for synthesis of CNT fibers/films include a CNT dispersion-based process,a spinnable CNT array-based process,and a floating-catalyst chemical vapor deposition(CVD)-based process.Among these methods,the floating-catalyst CVD-based process is the most promising way for one-step,continuous,and large-scale synthesis of CNT fibers/films.However,the previous CVD process involves hydrogen carrier gas and a sealed system,which brings out safety concern and processing complication.Moreover,the properties of the as-prepared CNT fiber/film are still far below those for individual CNT and even much lower than those for commercial carbon fibers.Finally,the appearance of CNT fiber/film provides a new design and research direction for preparing CNT composites.But these reported methods involve sophisticated procedures,time-consuming processes,and a limited size of the resultant composites,and thus are hard to be used for continuous and large-scale production.Considering the above problems,the present dissertation focuses on the continuous preparation and performance maximization of CNT fiber and film.The main results obtained in this dissertation are given below.(1)A macroscopic cylinder-like CNT assembly is continuously synthesized by a floating-catalyst CVD method at 1150-1300oC in a horizontal furnace,using ethanol,ferrocene,thiophene,alundum tube,and nitrogen as the carbon source,catalyst precursor,growth promoter,reactor,and carrier gas,respectively.The cylinder consists primarily of CNTs and a low content of Fe catalyst particles.The constituent CNTs have a wall number of 2-7,diameter of 2-7 nm,and a good graphitic structure.The content of Fe catalyst particles is in the range from 7 to 15 wt.%,depending upon the amount of ferrocene added into the reaction solution.(2)A CNT fiber is prepared by directly introducing the cylinder into a water or ethanol pool for condensation.The surface morphology and cross section of the fiber depends upon the wetting agent used for condensation.The water-condensed fiber exhibits a width of¹60?m and a thickness in the range of 5-9?m with a rough and uneven surface.The ethanol-condensed one has a width of⁴5?m,a thickness of 20?m,and an even surface.The as-prepared CNT fiber exhibits an average tensile strength of 360 MPa,elongation of 20-30%,and an electrical conductivity of 1.27×10³ S cm^-1 at room temperature.The packing density and thus mechanical and electrical properties of the resultant fiber can be significantly enhanced by mechanical rolling.After repeated mechanical rolling,the densified CNT fiber has a combination of high tensile strength(3.76-5.53 GPa),high tensile ductility(8-13%),and high electrical conductivity(1.82-2.24×10⁴ S cm^-1),which is superior to those for any other man made fibers and films currently available.(3)A CNT film is prepared by the in-situ layer-by-layer condensation and deposition of CNT cylinder using a paper strip and an ethanol solution as removable flexible substrate and wetting agent,respectively.The width of the CNT film depends on the width of the paper strip or the diameter of the cylinder used for deposition,the length of the film depends on the winding time,and the thickness of the film is determined by the number of winding.The as-prepared CNT film has a smooth surface and exhibits an average tensile strength of 520 MPa,elongation of 9-14%,and an average electrical conductivity of 530 S cm^-1 at room temperature.(4)A copper tartrate/CNT composite film with an alternating layered structure is prepared by the in-situ layer-by-layer condensation and deposition of CNT cylinder using a paper strip and copper tartrate/ethanol suspension as removable flexible substrate and wetting agent,respectively.The desired Cu/CNT composite film is subsequently obtained by the heat treatment of the copper tartrate/CNT composite film at 300oC for 30 min in vacuum.The relative compositions of the Cu/CNT composite film can be adjusted by tuning the concentration and flow rate of the copper tartrate/ethanol suspension.The proposed method can be used as a generical strategy for preparing various CNT-based composite films.(5)The as-prepared CNT film and Cu/CNT composite film is used as an alternative to platinum counter electrode(CE)for quantum dot-sensitized solar cells(QDSSCs).It is found that the power conversion efficiency(PCE)of the assembled cells is closely related to the layer number of CNT film and Cu content of Cu/CNT composite film.As for the CNT film CE,the PCE first increases with increasing the layer number,reaching a maximum of 3.17%at the layer number of 10,and then decreases with the further addition of layer number.As for the Cu/CNT composite film CE,the PCE monotonously increases with increasing Cu content.The highest PCE of 5.73%is achieved for the composite film CE with a Cu content of 63 wt.%,which is very close to that with Cu₂S CE and superior to those based on other Cu₂S/C CEs.Furthermore,the PCEs for the cells with flexible CNT film and Cu/CNT composite film CEs are 1.82%and 3.49%,respectively,demonstrating their great potential application in all-flexible QDSSCs.