Study On Structure Evolution And Performance Of High Thermal Conductivity Mesophase Pitch-based Carbon Fiber

Mesophase pitch-based graphite fiber with high thermal conductivity is a key material for thermal management components in many important fields and has great value.The physico-chemical properties of mesophase pitch,spinning parameters(spinneret structure,spinning pressure and temperature,etc.)have an important influence on the structure of fiber.In the follow-up process,the MPGF with better performance can be obtained after appropriate preoxidation,carbonization and graphitization treatment.To study the effect of MP,spinning pressure and final temperature of preoxidation on the texture and conductivity properties of MPGF,two kinds of mesophase pitch MP-1 and MP-2 were used as raw materials for melt spinning.Then the MP-2 was used as raw material to prepare many groups of MPGF through different nozzle holes(circular,ribbon and Y-shaped),the relationship between thermal conductivity and diameter,cross-section texture,microcrystalline size of MPGF was discussed.The fiber obtained by Y-shaped nozzle hole with high thermal conductivity after heat treatment was selected as the research object,the content of soluble fraction in the precursor and oxidized fiber was changed in order to explore the development of texture of the fiber and the role of soluble fraction in the process.The main results are as follows:(1)Both types of MP were mainly composed by C and H elements,(the sum of H and C contents was greater than 98%).Compared with MP-1,MP-2 had more contents of C and H elements and a larger H/C value.MP-2 possessed more branched chains or naphthenic structures,the stacking of aromatic molecules was more compact.(2)When the spinning pressure was 0.04 MPa,the cross-sectional texture of MPGF prepared by MP-2 showed a radial-folded texture with good regularity and symmetry.The cross-sectional texture of MPGF prepared by MP-1 and MP-2 under different spinning pressure showed a radial texture under other conditions.Under the same spinning pressure,MPGF prepared by MP-2 had larger microcrystalline size and higher thermal conductivity.Besides,the thermal conductivity of MPGF obtained from MP-1 and MP-2 increased with the increasing of spinning pressure.The thermal conductivity of MPGF prepared by MP-2 reached to the maximum value when the spinning pressure is 0.04 MPa,the corresponding above said value was 742 W/(m·K).(3)The fiber prepared by MP-2 at a spinning pressure of 0.03 MPa was selected as the research object.With the increasing of final temperature of preoxidation,the distribution area of folded structure in the cross-section of MPGF gradually increased,the cross-sectional texture of MPGF changed from the internal radical and external onion-skin texture to the radial-folded structure with good regularity and symmetry.The thermal conductivity reached to the maximum value when the final temperature of preoxidation was 320?,and the corresponding above said value was 750 W/(m·K).(4)The MPGF tended to have higher thermal conductivity when the cross-sectional texture showed a radial texture and the layer arrangement was relatively compact and regular.When the cross-section of MPGF prepared by Y-shaped nozzle hole was divided into three parts that arranged in a regular radial texture,it had the highest thermal conductivity which can reach 802 W/(m·K).The range of thermal conductivity of the prepared groups of MPGFs was 450?810 W/(m·K),with the increase of thermal conductivity,the microcrystalline size Lc showed an upward trend,La showed a trend of first rised and then falled,while d002 had no obvious trend.(5)On the cross-section surface of the precursor fibers,there were spherical or elliptical microdomains with a length of 50-100 nm.After preoxidation,the microdomains formed a plate structure on the cross-section surface of the fibers.After carbonization,the microdomains shrinked and its curved periphery became straighter,and the microdomains connected with each other to form linear domains which length was above 200 nm or formed curved or loop-shaped domains through combination,The microdomains were further shrinked and straightened after the graphitization,but there were still many curved domains in the cross-section of MPGF.At this time,the MPGF showed an obvious mesoscopic texture,which was divided into divided into three parts that arranged in a regular radial texture.(6)The soluble fraction played a connecting role in the fibers.The decrease of soluble fraction led the microcrystalline layer of MPGF became more straighter and denser,but the soluble fraction can not change the layers arrangement of the cross-section of MPGF.The soluble fraction contributed to the density of the fibers before graphitization treatment,but the existence of soluble fraction limited the growth of microcrystalline thickness and the stacking of layers in the process of graphitization.