Study On The Time-Combination Effect In The Evolution Of Graphite Crystallite Of Pan-Based Carbon Fiber

Graphite crystal structure is reasonable for the excellent mechanical properties,electrical and thermal conductivity of PAN-based carbon fibers.Its growth behavior is not only connected with the structure of fiber during high temperature treatment,but also related to process parameters such as time,temperature,and tension.The graphite crystallite grow in a threedimensional order through thermal activation during graphitization of PAN-based carbon fiber.Its growth kinetics and growth mechanism are important components of the key scientific issues of high modulus carbon fiberMost of the current work in this field tend to focus on the effects of graphitization temperature,time,and draft on the microstructure of carbon fibers.Based on the study of the time-temperature effect of graphite microcrystal evolution during graphitization of PAN carbon fiber,this paper further explores the influence of the arithmetic and iteration time combination modes on the graphite crystal structure.In this paper,the evolution of graphite-like structure of PAN carbon fiber during high temperature graphitization was studied by means of Xray diffraction and Raman spectroscopy.Based on the time-temperature effect of high temperature graphitization of PAN carbon fiber,the concept of time combination in graphitization process is proposed.Explore a new time management process that can effectively control the formation of preoxidized structure and the growth of graphite crystallite during the continuous preparation of PAN high modulus carbon fiber.By studying the effect of time combination on the graphite microcrystal of PAN carbon fiber,the internal mechanism of discontinuous growth of graphite microcrystal is revealed.Through the coupling study of time combination and temperature,the role of temperature is revealed when the time combination effect affects the evolution of graphite crystallite.The results show that:1.With increasing heat-treatment temperature,both the crystallite size(La,Lc)and the degree of preferred orientation(?)increase,while the crystallite interlayer spacing(d002)decreases.Compared with as-received carbon fiber,after treatment at 2600?,d002 decreased by 3.2%,Lc increased by 219%,and La increased by 112%.The graphite crystallite in carbon fiber is more regular and orderly,showing the characteristics of high bulk density(1.9017g/cm3)and high tensile modulus(433GPa).2.Prolonging the graphitization time at 2200?,the graphite crystallite size of PAN carbon fiber increases,but the the preferred orientation degree of crystallites remains almost constant.When the graphitization time is extended from 30s to 180s,the crystallite thickness Lc increases from 3.28nm to 4.12nm,the layer size perpendicular to the fiber axis,La? increases from 6.57nm to 7.20nm,and the layer size along the fiber axis La//increases from 11.42nm to 13.23nm.3.When the carbon fiber was heat-treated by equal difference time combination,the change of times has a negative effect on the growth of microcrystals by affecting the accumulation of activation energy.However,the processing time increases exponentially with the increase of the number of times,and its promotion for the growth of crystallites is far greater than the negative effect caused by the increase of times.Compared with one 60s treatment,the crystallite size Lc,La?,La//increased by 11.4%,4.3%,1.5%,respectively,and the graphitization degree increased by 6.1%after the two 60s treatments.4.When the graphitization process is carried out in the iterative time combination mode,the microscopic chemical reaction process is extremely short compared with the macroscopic processing time.Therefore,the crystallite size and graphitization degree of carbon fiber remain almost unchanged.5.The average width of the carbon base surface La(8.43nm)measured by powder XRD was compared with the radial width La?(7.92nm)and axial width La//(15.37nm)measured by fiber XRD.It is revealed that most of the crystallites in carbon fiber tend to be distributed perpendicular to the fiber axis,and only a few crystallites are oriented with respect to the fiber axis.