Preparation And Mechanism Study Of Graphite Cones-A Kind Of Fibrous Graphite Crystal Materials

Fibrous graphite crystal materials have a large length-diameter ratio,high mechanical properties,rich and diverse appearance,and complex growth mechanisms.Due to its complex preparation method,high temperature and high voltage arc discharge and other conditions,and the difficulty of separation and purification,graphite cones have not been applied in practice.In the process of preparing polyimide(PI)-based graphite fibers with polycyclic aromatic hydrocarbons(PAH),we found that a large number of graphite cones were grown in situ on its surface,and their appearance and growth mechanism were different from those of the currently known similar materials.Therefore,we conducted a more indepth exploration and research on this type of fibrous graphite crystal materials.This includes exploring the growth conditions of graphite cones,studying the internal structure of graphite cones,exploring their growth mechanisms,and predicting their potential applications.Specifically,it includes the following three aspects:(1)First of all,using pyromellitic anhydride(PMDA)and 4,4-diaminodiphenyl ether(ODA)as monomers,different contents of PAH were added.Graphite fibers were prepared by dry jet wet spinning and high-temperature heat treatment.When the amount of PAH added is greater than 8wt% and graphitized at 2200-2800 ℃,a large number of spiral graphite cones(SGCs)are grown in situ on the surface of the graphite fibers.The growth mechanism of graphite cones,including the formation of initial deposition sites and the spiral growth process of graphite cones,was proposed through molecular simulation and some characterization methods.The results show that the formation of the initial deposition sites may be due to the shrinkage force caused by the different graphitization properties of PAH and PI during high-temperature treatment.The spiral growth is due to the important role played by the spiral dislocation between the carbon layers inside the graphite cone.(2)On the basis of(1),graphite cones are prepared by changing the precursor of graphite fibers.Polyimides prepared by doping PAH with 3,3’,4,4’-biphenylenedicarboxylic anhydride(BPDA)/ODA,BPDA/pphenylenediamine(PDA),and polyacrylonitrile(PAN)doped with PAH were used for spinning,respectively.In addition to using fibers as the substrate,this section also uses membranes,powders,electrospinning films,and substrates in the form of electrospun micro and nanoparticles.The results show that the growth of graphite cones has little effect on the form of the matrix,but has a great relationship with the molecular structure of the precursor monomer,mainly manifested in the ability to form carbon deposition sites.Among them,the ether oxygen bond in the ODA molecule plays a key role in the formation of carbon deposition sites,which is reflected in the fact that it can break during low-temperature carbonization,causing the PI macromolecular chain to break into multiple small molecular chains,providing more possibilities for carbon chain rearrangement and regular arrangement during high-temperature treatment at 2200 to 2800 ℃,which means there are a large number of deposition sites and empty bonds generated,providing more conditions for the growth of graphite cones.(3)The graphite cones successfully prepared were separated and purified,and the operation method for graphite cones separation was explored,resulting in the graphite cones with a purity of about 90%.Then the separated cones are assembled into lithium batteries for electrochemical testing.The results show that the initial specific capacity of the graphite cones reaches 280.6 m Ah/g,which is lower than the theoretical specific capacity of traditional graphite.It is due to the carbon layer of the turbine layer inside the graphite cone.The carbon layer is folded up inside the graphite cone,and the edge parts are in a closed loop state,resulting in difficulty in lithium-ion insertion and little lithium storage.However,the cycle stability of graphite cones is good,which indicates that they have potential application in batteries,and can be used as an additive for other materials to improve the electrical properties of this material.In summary,graphite cones have been successfully prepared using simple and novel methods in this thesis.Their structural characteristics,growth conditions,and growth mechanism have been studied and explored,which provides a certain basis for the future mass production and application of this kind of fibrous graphite crystal material.