Preparation And Structural Regulation Of High Thermal Conductivity Mesophase Pitch-based Carbon Fibers

Mesophase pitch-based carbon fibers have higher thermal conductivity and Young’s modulus compared with polyacrylonitrile-based carbon fibers.These superior properties allow mesophase pitch-based carbon fibers to be widely used in the fields of aerospace vehicles,electronic devices,robotic arms and so on.However,the preparation of mesophase pitch-based carbon fibers is a long and multidisciplinary process.The synthesis of the high-purity spinnable mesophase pitch,the microstructural control of the continuous fiber,and the continuous graphitization treatment are more difficult in this process.In this dissertation,firstly,the relationship between mesophase pitch’s rheological properties and the spinning process as well as the continuous process for removing volatile matter in mesophase pitch were studied.secondly,the microstructure modification and the effect of the continuous graphitization on the structure and properties of the pitch-based carbon fibers were investigated systematically.Finally,the analysis of typical structural characteristics for carbon fibers with high thermal conductivity were systematically discussed.（1）The rheological properties of the home-made petroleum-based pitch（MP）and the domestic naphthyl-based pitch（SD）were studied by a rotary rheometer.The linear viscoelastic region of SD is obtained by amplitude sweep,frequency sweep and temperature sweep in oscillation mode.The storage modulus and loss modulus increase with increasing angular frequency and decrease with increasing temperature.The variation of the shear rate scan in the rotating mode shows that the SD exhibits shear thinning behavior of non-Newtonian fluids.The effect of shear rate on the viscosity is higher at low temperatures than it is at higher temperatures.On the basis of the above results and by reference to the thermal weight loss curve combined with temperature scanning,the low-viscosity stationary regions of MP and SD are obtained,which are312³24°C（4¹0 Pa·s）and 345³60°C（4¹0 Pa·s）,respectively.The effect of the spinning temperature（312°C,316°C,320°C and 324°C）on the microstructure of MP fibers was investigated by single-hole spinning in the low-viscosity stationary regions.The results show that all the MP carbon fibers present similar split-radical microstructure,which indicates that the MP has a relatively wide spinning process region of about 12°C.The effect of the length to diameter ratio（L/D）on the microstructure,mechanical properties and thermal conductivity of MP and SD fibers at 320°C（5 Pa·s）and 355°C（5 Pa·s）spinning temperature were also studied.As the L/D increasing from 1 to 5,the MP and SD based carbon fibers are the same split-radical microstructure.Besides,the mechanical properties and thermal conductivity are basically unchanged,indicating that the smaller L/D at lower viscosities is sufficient to obtain a highly oriented pitch-based carbon fibers.The relationship between spinning temperature,melt viscosity,spinneret structural design,structure of carbon fibers and properties was constructed.（2）The effects of the vacuum degree and filling rate on softening point,thermal stability,rheological properties and polarized texture of the MP before and after the continuous devolatilization process by twin-screw extruder were researched in order to solve the problem of the few isotropic components and high volatile matter in the MP.The results show that the thermal stability of MP is improved and its isotropic components are reduced by devolatilization process with a filling rate of 64%and a vacuum of-0.09 MPa.On this basis,the MP obtained by the two devolatilization processes with a filling rate of 64%and a vacuum degree of 0 and-0.09 MPa were used in the 1K continuous melt spinning.When the vacuum degree is 0 MPa,the pitch fiber has a large number of holes and protrusions on the surface.When the vacuum degree is-0.09 MPa,the surface of the green fibers is smooth and the diameter is uniform,which indicating that the technology of MP continuous devolatilization and 1K melt spinning can be established.Based on the above process,the microstructure modification of the pitch-based carbon fiber was carried out.The controllable preparation of MP-A carbon fibers with‘split-radical’structure and the MP-B carbon fibers with‘outer fold-like radial middle onion skin-like’structure were realized.The flow behavior of the MP-A mesophase pitch liquid crystal in spinneret micropores belongs to the double action mechanism of‘shear induced orientation-tensile induced orientation’,and the flow behavior of the MP-B mesophase pitch liquid crystal in spinneret micropores belongs to the triple action mechanism of‘shear induced orientation-stress relaxation-tensile induced orientation’.At the same heat treatment temperature,the tensile strength of MP-B fiber is better than that of MP-A fiber.The Young’s modulus and thermal conductivity of MP-A fiber are better than that of MP-B fiber.Besides,because of the larger crystallite sizes,higher crystal orientation and better developed graphite sheet,the thermal conductivity of the MP-A-2800 is up to645 W·m^-1·K^-1 which is equivalent to the K13C2U,and the thermal conductivity of the MP-B-2800 is up to 512 W·m^-1·K^-1 which is equivalent to the XN-90.（3）The effect of continuous graphitization temperatures on the microstructure and properties of mesophase pitch and PAN-based carbon fibers was investigated.With the graphitization temperature increasing from 2300°C to 2600°C,the crystallite sizes,the preferential orientation of the graphite sheet along the fiber axis,the tensile strength,tensile modulus and thermal conductivity increase,while the distorted,cross-linking and other microcrystalline defects decrease as for the graphitized MP-A-1000carbon fibers.For the graphitized T1000G carbon fibers,the graphitization degree,the graphite crystallite size,the preferred orientation,the tensile modulus and the thermal conductivity are increased with the graphitization temperature increasing.Besides,the removal of the nitrogen atoms leads to forming microcrystalline defects in the fiber and decreasing the tensile strength.The thermal conductivity of the MP-A-2600 is up to 582 W·m^-1·K^-1 which is better than that of the XN-90.The mechanical properties of the T1000G-2600 are comparable to those of the M50JB.（4）In order to further clarify the relationship between the microstructure and thermal conductivity of mesophase pitch-based carbon fibers and guide the localization of high thermal conductivity mesophase pitch-based carbon fibers,five types of high thermal conductivity mesophase pitch carbon fibers(500¹127 W·m^-1·K^-1)which are commercially available and home-made were analyzed in detail in the aspect of microstructure,crystal size and orientation.The microstructures of high thermal conductivity mesophase pitch-based carbon fibers prepared by different manufacturers show great differences,and have certain typical"fingerprint"characteristics.However,all of them have relatively well-developed graphite microcrystalline structures.The carbon fibers with higher thermal conductivity show better developed graphite sheet,obvious radial microstructure,and more easily splitting.The split microstructure is believed to be the external representation of the high orientation of the graphite sheet.The carbon fibers with high thermal conductivity have a long and ordered lattice structure,and when the carbon fibers have higher thermal conductivity,they show the higher orientation along the fiber,the higher degree of graphitization,the larger graphite crystallite,the smaller interplanar spacing and the fewer crystallite defects.Based on these analysis,the structural model of the five high thermal conductive mesophase pitch-based carbon fibers is proposed.