In-situ Electrothermal Method For Preparing High Strength-High Modulus Carbon Fiber And Study On Its Structure-Property Relationship

Carbon fiber has attracted extensive attention and application in both scientific research and industry due to its excellent mechanical and physicochemical properties.The increasing variety of products and application fields continuously drives the demand for carbon fiber year by year.However,the high cost of production to some extent limits further development of carbon fiber.Therefore,finding an efficient preparation process is of great practical significance.In terms of performance,high modulus carbon fiber often lacks high tensile strength due to excessively large grain size.Thus,simultaneously improving the tensile strength and modulus of carbon fiber has always been a key focus and challenge in this field.This thesis explores an efficient process for carbon fiber production utilizing the inherent electrothermal effects of carbon fibers.It utilizes electric field modulation to control microcrystalline structure and achieve high-strength,high-modulus carbon fibers.A comparison of the structure and properties between carbon fibers prepared using traditional methods and those prepared using the in-situ electrothermal method is conducted to investigate the advantages and specific structure-property relationships of the latter.The main innovative research contents are as follows:1.In-situ electrothermal graphitization and microstructural evolution of carbon fibers: Firstly,commercial carbon fibers are subjected to in-situ electrothermal graphiteization treatment.The in-situ electrothermal method utilizes the rapid temperature increase from the fiber’s own Joule heating effect,greatly improving production efficiency.Additionally,compared to carbon fibers prepared by traditional methods,those obtained through in-situ electrothermal exhibit 7.8% increase in tensile strength and 9.2% increase in tensile modulus.Characterization techniques such as Raman spectroscopy,transmission electron microscopy(TEM),X-ray diffraction(XRD),combined with density functional theory(DFT)calculations,reveal the mechanism of microstructural regulation by electric fields,elucidating the unique microstructure and mechanical property enhancement principles of carbon fibers prepared by in-situ electrothermal.The next part of the research aims to extend the in-situ electrothermal method to the early stages of carbonization to amplify these advantageous microstructures,thereby further improving fiber performance.2.Mean Field Theory Reveals Microstructural Evolution and Origin of Conductivity during Precursor Fiber Pre-carbonization Process: Based on the investigation of the structure-property relationship of carbon fiber graphiteization under in-situ electrothermal,attempts are made to advance this process to the early stages of carbonization.Initially,the origin of conductivity during the precursor fiber carbonization process needs exploration.Therefore,this section establishes a microstructural model for pre-oxidized precursor fiber(POF)during the pre-carbonization process.Based on TEM and XRD results,the microstructure of low-temperature carbonized POF is approximated as a nanocomposite material consisting of pseudo-graphitic layers(PGS)and an amorphous carbon base.The electrical conductivity of POF at different pre-carbonization temperatures is tested using a four-probe apparatus.Utilizing mean-field theory analysis of the microstructural model of this nanocomposite material and employing effective medium approximation,the evolution of electrical conductivity of low-temperature carbonized POF with microstructure variation is investigated.Subsequently,in conjunction with experimental results,the existence of infiltration effects during the precursor fiber pre-carbonization process is determined.The results indicate the formation of a carbon scaffold structure consisting of interconnected PGS during the pre-carbonization process of precursor fibers.3.In-situ Electrothermal Graphitization and Microstructural Evolution of Precursor Fibers during Pre-carbonization: Based on the investigation of the structure-property relationship and the origin of electrical conductivity of carbon fibers prepared through in-situ electrothermal graphiteization,this section extends the in-situ electrothermal process to the early stages of carbonization.Laboratory-produced precursor fibers subjected to low-temperature carbonization are subsequently treated with in-situ electrothermal graphiteization.The fibers prepared exhibit a 50.3% increase in tensile strength and a 9.2% increase in tensile modulus compared to fibers processed using traditional methods.Characterization techniques such as Raman spectroscopy,transmission electron microscopy(TEM),X-ray diffraction(XRD),along with molecular dynamics simulations,provide a detailed understanding of the unique microstructure of carbon fibers produced via in-situ electrothermal,further revealing the underlying mechanisms of the in-situ electrothermal process.4.Preparation of conductive precursor fibers and achievement of chemical stabilization: This section attempts to prepare conductive precursor fibers,thereby replacing the entire carbon fiber production process with an in-situ electrothermal method.The traditional carbon fiber stabilization process requires prolonged heating,consuming significant energy.Achieving room temperature chemical stabilization would effectively control the cost of carbon fiber production.Using density functional theory(DFT)calculations at the Becke3LYP(B3LYP)level combined with ab initio molecular dynamics(AIMD),the feasibility of chemical stabilization is determined,along with the production of conductive precursor fibers.Simulation results indicate that the energy barrier for the cyclization reaction of graphene oxide/polyacrylonitrile(GO/PAN)composite fibers is reduced to 3.76 kcal/mol when treated with hydrazine hydrate,enabling this reaction to occur spontaneously at room temperature.Subsequently,GO/PAN composite fibers are prepared using a wet-spinning method and treated with hydrazine hydrate.XRD,Raman spectroscopy,Fourier-transform infrared spectroscopy(FT-IR),and differential scanning calorimetry(DSC)results confirm the completion of pre-oxidation in the fibers.This ambient-temperature chemical stabilization method significantly improves the efficiency of carbon fiber production and effectively reduces energy consumption.The prepared conductive precursor fibers can be utilized for subsequent in-situ electrothermal carbonization/graphitization treatment.