Preparation And Formation Mechanism Of PAH/PI-based Graphite Fibers And Cones

Non-planar graphite single crystals are polyhedral nano-or micro-structures with various shapes such as needles,rods,and rings.This structure generally exhibits high electrical conductivity,mechanical strength and chemical stability.Some graphite cones and polyhedral crystals can be used as probes for atomic force microscopes and scanning tunneling microscopes,new functional fillers for nanocomposites,energy storage materials for nanoelectromechanical systems,and so on.However,as the first non-planar graphite single crystal discovered,the preparation method and growth mechanism of graphite cones are still unclear,leading to fewer practical applications.Therefore,proposing a stable preparation method with a clear mechanism is the key to popularizing the application of graphite cones.This article firstly uses organic precursors to prepare carbon fibers.Then,by controlling the appropriate conditions,a large number of graphite crystal cones were found to grow on the surface of the carbon fiber during the high-temperature graphitization process.And a clear growth mechanism was proposed,laying the foundation for the development and application of graphite cones.First,this study explored the preparation of polycyclic aromatic hydrocarbons(PAH)/polyimide(PI)based composite graphite fibers.In the experiment,the composite PI fiber was prepared by the dry-jet wet spinning process combined with the thermal imidization treatment.The composite carbon fiber and composite graphite fiber were obtained through carbonization and graphitization.Then,the structure and performance of the composite fiber materials at each stage were tested using characterization methods such as infrared spectroscopy,X-ray diffraction,and atomic force microscopy.Studies have shown that the addition of polycyclic aromatic hydrocarbons has a positive effect on improving the conductivity of polyimide-based graphite fibers.The thermal conductivity of the composite graphite fiber prepared when the addition amount is 3 wt%is 112.5%higher than that of the pure polyimide-based graphite fiber.This dramatic improvement in performance is due to the formation of π-π conjugation effect between the molecular chains of polyamic acid and polycyclic aromatic hydrocarbons in the polymerization stage,which realizes the up and down stacking between them.Effectively increase the flatness of the molecular chain of polyamic acid.In the subsequent spinning and thermal imidization process,the molecular chain of the polyamic acid can be effectively oriented in the plane of the polycyclic aromatic hydrocarbon due to the strong stretching.In the final graphitization process,molecular chains with high flatness are easier to transform into highly crystalline graphite layered structures,thereby improving fiber performance.Then,the experiment further explored the influence of different precursor structures of polyimide on the properties of composite graphite fibers.The results show that the polyimide whose precursor has rigid groups has a greater improvement in fiber conductivity after doping with polycyclic aromatic hydrocarbons.This is because polyimides with rigid groups have better flatness that are easier to produce π-π conjugate effects with polycyclic aromatic hydrocarbons,and ultimately increase the degree of fiber orientation.However,the existence of rigid groups makes the mechanical properties of composite fibers poor,which is not conducive to the subsequent widespread application of fibers.Finally,this experiment explored the growth mechanism of graphite cones on the surface of the composite carbon fiber during the graphitization process.In the process of preparing composite graphite fibers,it was found that a large number of graphite crystal cones grew on the surface and inside of composite graphite fibers when the amount of polycyclic aromatic hydrocarbons was more than 8 wt%.In-depth research on this growth phenomenon was carried out,the growth mechanism and key influencing factors of graphite cones were summarized.The results show that the molecular structures of PAH and PI during the high temperature treatment process are different in the degree of easy graphitization,so the graphitization time has a sequential difference.During the graphitization process,the easily graphitized PAH carbon first shrinks to form a regular graphitized carbon layer,and then the more difficult-to-graphitize polyimide carbon is graphitized at a higher temperature.In this process,the polyimide-based carbon structure produces a large shrinkage stress for the formation of a regular graphite layer.The PAH graphite layer in the polyimide carbon structure is therefore strongly squeezed and bent,forming the initial deposition point of the graphite crystal cone.At the same time,the pyrolytic carbon generated at high temperature is continuously deposited on the deposition point.Then shrinks and dislocation occurs during the stacking process,and finally forms the graphite crystal cone.During the research process,it was found that:1)The curved graphite layer formed by PAH serves as the starting point for the growth of graphite crystal cones,and its addition is a key factor affecting the growth of graphite crystal cones.In this experiment,more than 8 wt%of PAH was added to prepare a large number of graphite crystal cones on the surface of the composite graphite fiber;2)The fiber diameter is another key factor affecting the growth of graphite crystal cones.During the growth of the graphite crystal cones,the holes in the fibers act as reaction vessels.The pyrolysis gas generated during the graphitization process is retained in the fiber pores.When the concentration reaches a certain level,the pyrolysis gas rises along the graphite layer,and is finally deposited on the high-curvature curved PAH graphite layer to realize the growth of graphite crystal cones.The results show that when the inner diameter of the fiber is mostly mesopores,it is more conducive to the growth of graphite cones on the surface of the fiber.In short,this study improves the orientation and thermal conductivity of PI-based graphite fibers by adding PAH that is easy to graphitize.And this study provides a new idea for the preparation of PIbased high thermal conductivity graphite fibers.At the same time,the preparation method and growth mechanism of graphite cones were obtained during the fiber preparation process,which laid the foundation for further research and application.