Preparation process,reaction mechanism,and control method of pitch-based carbon fiber
Pitch-based carbon fiber is a crucial engineering material extensively utilized in aerospace,national defense,and military in-dustries,etc.However,the lengthy preparation process and numerous influencing factors hinder the production of pitch-based carbon fiber with exceptional mechanical performance.This work delved into the current state,methods,mechanisms,and regulation of raw material pretreatment,pitch precursor preparation and control,melt-spinning,stabilization,carbonization,and graphitization to address these challenges comprehensively.It emphasized that the breakthrough direction for pitch-based carbon fiber lies in its molecular-level di-rectional and controllable preparation.The raw materials for pitch-based carbon fiber include coal-based,petroleum-based,biomass-based,and polymer-based materials as well as pure compounds.Distillation and extraction are the common methods for the pretreatment of raw materials.The preparation of pitch precursor is essentially a liquid-phase carbonization reaction that involves parallel hydrogen transfer reactions and free radical reactions,including complex reaction processes such as cyclization,aromatization,oligomerization,and conden-sation.Aromatic oligomers condense at high temperatures to form the basic structural units of polycyclic aromatic hydrocarbon flakes,which continuously aggregate through thermal effect to form aggregates.The basic structural units stack together with the force of van der Waals,forming parallel stacking bodies to convert to basic building unit aggregates.The basic building unit aggregates can transform into long-range disordered and short-range ordered non-graphitization structures under different reaction conditions,as well as into long-range ordered and short-range ordered graphitization structures.When the pitch precursors derive from coal-based,petroleum-based,and biomass-based materials,the thermal condensation method with simple equipment,easy operation,and low cost is usually selected owing to their high reaction activity.In contrast,higher temperatures are required to initiate free radical reactions when pure compounds with low reactivity such as naphthalene and methyl naphthalene are used as raw materials.Therefore,catalytic synthesis is usually used.Melt-spin-ning plays a role in shaping,and the diameter and shape of carbon fibers can be adjusted by changing the spinning parameters and the shape of the spinneret.Moreover,the molecular rearrangement effect of melt-spinning on the precursor of mesophase pitch is particularly significant,making it easier for graphite microcrystals to form ordered graphite crystal structures along the axial one-dimensional arrange-ment.To prevent the melting of pitch fibers during the carbonization process,it is necessary to transform the pitch fibers from thermoplas-tic to thermosetting,that is,through a slight oxidation reaction,the polycyclic aromatic hydrocarbon molecules in the pitch undergo oxida-tion,condensation,decomposition,and dehydration,forming a specific cross-linked structure,and ultimately generating a certain amount of oxygen-containing functional groups in the fiber structure.Carbonization is the formation of carbon fibers with molecular structural char-acteristics of carbon materials through complex chemical reactions and structural transformations of stabilized fibers.During the carboniza-tion process,graphite microcrystals undergo rearrangement,non-carbon atoms are removed,and the carbon element content further in-creases.After graphitization,the graphite crystal structure becomes more complete and highly oriented,which transforms carbon fibers in-to graphite fibers and enhances their strength performance.In addition,it provides clear guidance for optimizing and improving time-con-suming and energy-intensive stabilization steps in carbon fiber production.
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