Carbon fiber graphitization is the process of preparing high-modulus or high-strength high-modulus carbon fiber by using ultra-high-temperature heating or irradiating it with high-energy substances to form a regular three-dimensional graphite crystal structure within the carbon fiber from a random graphite sheet structure.
At present, only Japan and the United States have high-strength, high-modulus carbon fibers that achieve high productivity, excellent quality, and industrialization of a series of products. Toray's carbon fiber production in Japan ranks first in the world. Its M70J PAN-based graphite fiber has a tensile strength of 3.4 GPa and a tensile modulus of 690 GPa, which represents the current series of polyacrylonitrile (PAN) -based graphite fibers. The highest level. In the 1990s, high-performance asphalt-based graphite fibers began to develop rapidly. The main manufacturers are Japan's Mitsubishi, Amoco (now Cytec), and Nippon graphite fiber. The most representative ones are The product is Mesophase pitch-based graphite fiber of Thornel K1100 type from Cytec Company in the United States. Its tensile strength is 3.1 GPa, tensile modulus is up to 965 GPa, and axial thermal conductivity is up to 1100 W / (m · K).
The core of the carbon fiber graphitization technology lies in the high efficiency of ultra-high temperature heat treatment of carbon fibers by the graphitization equipment and the effective control of the optimal evolution of the fiber structure by the graphitization process. This article reviews the advanced graphitization processes such as carbon fiber graphitization equipment, new high-temperature heat treatment technology and high-energy substance radiation technology, catalytic graphitization during graphitization, and the relationship between the evolution of carbon fiber microstructure and macroscopic properties during graphitization, in order to understand Explain the mechanism of structural evolution at different levels, and then guide the carbon fiber graphitization process for related optimization.
1 carbon fiber graphitization equipment
1.1 Graphitization equipment comparison
In order to prepare high-strength and high-modulus carbon fibers, researchers at home and abroad have conducted extensive research on carbon fiber graphitization equipment and developed graphitization furnaces with different heating methods. Existing graphitization equipment is classified according to heating methods, such as tam-type resistance furnaces, induction furnaces, radio frequency furnaces, and plasma furnaces.
A variety of carbon fiber graphitization technologies proposed by scholars at home and abroad. The industrialization and widespread use are high-temperature tubular resistance indirect heating and electromagnetic induction indirect heating graphitization technologies, which are all graphite body indirect heating technologies. In these two graphitization heating methods, carbon fiber passes through the middle of the graphite tube, and direct current is applied to the two ends of the graphite tube or the graphite tube generates direct current through electromagnetic induction. When the current passes through the graphite heating element, the graphite heating element is heated as a resistance. The hot air flow indirectly heats the carbon fibers through the form of heat transfer between the graphite heating element and the carbon fibers. The temperature of the furnace of the graphitization furnace needs to be higher than the temperature of the carbon fiber, the heat transfer process is slow, most of the energy is dissipated, the thermal efficiency is low, the energy consumption is large, the temperature acting on the carbon fiber is difficult to control, and the cavity material is required to be high; High temperature, easy to be attacked by residual oxygen, short life and irreparable after burn out. At the same time, due to the limitation of the high temperature resistant material of the heating element, the graphite heating element begins to sublimate at 2800 ° C, and it cannot be stabilized at a very high temperature above 2800 ° C for a long time. The heat treatment temperature of carbon fiber is limited, which affects the evolution of the structure of the carbon fiber during the graphitization process. .
1.2 New method of carbon fiber graphitization
1.2.1 Laser ultra-high temperature graphitization technology
Beijing University of Chemical Technology Tan Jing and others developed a carbon fiber ultra-high temperature graphitized laser tunnel furnace, as shown in Figure 2. The equipment consists of a winding and unwinding device, a graphitization furnace body, a laser heating system, a sealing device and the like. The graphitization process is under the protection of argon gas. The carbon fiber is guided through the graphitization furnace. The rewinding and unwinding device controls the release of carbon fiber tow, winding collection and control the wire speed. The laser is directly irradiated on the carbon fiber. The laser and carbon fiber The interaction realizes energy transmission, converts the absorbed light energy into thermal energy to increase the temperature of the carbon fiber, and then realizes the graphitization process of the carbon fiber.
This device uses laser light to irradiate carbon fibers for radiant heating. Most of its energy is absorbed by carbon fibers, which can achieve long-term stable heating at ultra-high temperature above 3000 ° C. The laser acts on the carbon fiber with a large temperature rise gradient, and the instant temperature rise and heat conduction make the carbon fiber tow The whole is heated, and the graphitization efficiency is high; the graphitization process does not need to heat the entire furnace cavity, the design requirements of the furnace body structure and the material of the furnace body are low, and the manufacturing cost is low; at the same time, the space control and time control of the laser are strong, and the laser beam Easy to guide and focus. By changing the spot shape, spot size and energy distribution of the laser heating system by program, the temperature field distribution of the heating zone is changed, so that the carbon fiber can efficiently complete the graphitization process under the appropriate temperature field. Controllable preparation of high-temperature graphitization.
Yang Weimin and others used a CO2 laser to radiate PAN-based carbon fibers and studied the graphitization process. Under the irradiation of 63W laser beam, the degree of graphitization was reduced from 1.4515 to 0.0811, the distance between graphitized microcrystalline layers was reduced from 0.353nm to 0.345nm, and the distance between layers close to ideal graphite crystals was 0.335nm, confirming the realization of PAN-based carbon fibers The regular structure transition from the random layer graphite structure to the graphite microcrystals was described. It was observed in the scanning electron microscope characterization that the surface of the carbon fiber after laser irradiation was rougher, and the carbon fiber was exfoliated due to the instantaneous temperature rise and non-carbon atom overflow during the graphitization process, although the graphitized carbon fiber strength caused a certain degree However, a certain surface roughness is conducive to improving the bonding strength between the fiber and the resin. The graphite fibers prepared in the existing graphitization furnace have the defect of poor binding with the resin due to the smooth surface and lack of reactive functional groups. Laser ultra-high temperature graphitization Technology is expected to provide an improved solution for the preparation of carbon fiber composite materials.
1.2.2 Carbon fiber self-heating graphitization method
Matsuda to Kang proposed a carbon fiber continuous graphitization furnace. As shown in Figure 3, the pre-heating zone is heated with resistance wires to pre-heat the carbon fiber to about 1000 ° C. Then the carbon fiber enters the energized heating zone and passes through a pair of energized roller pairs. The carbon fiber applies current to heat itself to realize the graphitization process.
The device applies electric current to the carbon fiber to generate heat by itself. Compared with the conventional graphite tube indirect heating method, the device reduces power consumption, and the device structure is simple. The current can be efficiently controlled. After a certain process debugging, the carbonization and graphitization processes can be performed. It is integrated, that is, the carbonization process is performed in the pre-heating zone, and the ultra-high temperature graphitization process is performed in the heating zone to improve the production efficiency and quality of graphite fibers.
2 carbon fiber graphitization process
2.1 Effect of temperature on graphitization of carbon fibers
Temperature is the main factor affecting the graphitization of carbon fibers. Shen Zengmin et al. Studied the effect of graphitization temperature on the microstructure of high-modulus PAN-based carbon fibers. As the graphitization temperature increases, the degree of graphitization gradually increases, and the size of graphite microcrystals increases. As it increases, the graphite microcrystalline structure tends to be perfect, the arrangement is more regular, and the crystallinity is improved. Wang Haojing et al. Got the conclusion that as the temperature of PAN-based carbon fibers increases with temperature, the non-carbon element content gradually decreases, the preferred orientation of graphite crystallites increases, the tensile strength decreases, and the modulus increases. Defects such as micropores in the fiber structure during the graphitization process affect the mechanical properties to varying degrees. How to control the temperature rise during the graphitization process to reduce the reduction in the tensile strength of carbon fibers is very important.
2200 ° C is a sensitive temperature for carbon fiber graphitization. At this temperature, the carbon fiber has basically completed the denitrification process, and various microcrystalline structural parameters have changed significantly. Zhang Yonggang et al. Used self-made high-strength medium-mold carbon fibers to perform graphitization at different temperatures, and respectively studied low-temperature graphitization at 1600 ~ 2200 ℃ and high-temperature graphitization at 2500 ℃. During the low-temperature graphitization, the tensile strength of carbon fibers first showed a downward trend, and it began to increase significantly at 2000 ° C. It may be due to stress relaxation of the fibers under high temperature conditions, and the internal defects of the fibers decreased, and the strength increased. As the graphite crystallites increase in size, under certain thermal drafting, the creep behavior of graphite crystallites will eliminate internal interlayer stresses, and promote the preferred orientation of fibers along the fiber axis. As a result, the tensile modulus increases linearly. .
Under different temperature fields, the carbon fiber absorbs heat differently, which affects the evolution process of the microstructure, which will eventually lead to some differences in the degree of graphitization and fiber performance. Xu Lianghua et al. Studied the effects of gradient heating and one-step heating methods on the element content and microstructure of PAN-based carbon fibers, and found that the difference in elemental composition of graphite fibers prepared under the two temperature fields is small. The effective energy absorbed is higher, so the activity of carbon atoms is enhanced, which is conducive to the reorganization of carbon atoms and the rearrangement of graphite sheets, higher fiber orientation, regular arrangement of graphite crystallites, and higher degree of graphitization. Liu Fujie et al. Studied the effect of stepwise heat treatment on the microstructure and properties of graphite fibers. Under the same draft ratio, the carbon fibers were subjected to stepwise heat treatment and one step heat treatment to the same temperature. It was found that stepwise heat treatment is beneficial to the tensile strength of graphite fibers. The bulk density is improved, but the effect on the modulus is small, the degree of graphitization inside the fiber is high, and the overall graphitization homogeneity is good. Therefore, the step heat treatment is beneficial to improve the density and homogeneity of the fiber and ensure the modulus. As the amount increases, the tensile strength of the fiber also increases, which has great guiding significance for the preparation of high-strength and high-model graphite fibers.
2.2 Effect of heat treatment time on carbon fiber graphitization
The time control of carbon fiber graphitization is achieved by adjusting the wire feed rate. The graphitization time can generally complete the orderly evolution of three-dimensional graphite crystal structure within tens of seconds. The study of heat treatment time aims to master the graphitization process and ensure the graphite fiber. Reduce the heat treatment time while reducing the quality to save energy consumption.
Han Zan studied the effect of graphitization time on the microstructure parameters and mechanical properties of PAN-based carbon fibers. T800 was used as the raw material, and the time was 30 ~ 55s at a heat treatment temperature of 2500 ℃ and a constant draft rate (2.5%). A series of graphitization treatments characterize the microstructure and mechanical properties of the sample. It is found that the overall change of the microstructure parameters of the sample under different graphitization times is small. With the increase of the graphitization time, the crystallite size and fiber orientation degree , Graphitization degree, etc. increase slightly first, then tend to remain unchanged, strength decrease slightly, then stabilize, and modulus increase first, then stabilize. Greene et al. Studied the effect of short-term graphitization on the properties of pitch-based carbon fibers. At a graphitization time of 0.7s, the carbon fibers were significantly densified, leading to an increase in bulk density, with a degree of graphitization of 50%, electrical conductivity and thermal conductivity. Significant sexual improvement.
2.3 Effect of drafting force on carbon fiber graphitization
Carbon materials generally experience creep at high temperatures above 2000 ° C. PAN fibers are composed of linear molecules. After carbonization at 1000 ~ 1500 ° C, there are a large number of interwoven and wrinkled graphite microcrystalline ribbons inside the fibers. These microcrystalline ribbons creep under the high temperature graphitization heat drawing state. The change can eliminate and transfer the cross-linking between the microfibers, so that the graphite microcrystalline strips can be unwrinkled and entangled, and form a preferred orientation of the fiber axial direction in the fiber drafting direction, which improves the mechanical properties of the fiber.
Jin Yuwei et al. Studied the effect of drafting on the structure and mechanical properties of graphite fibers. Using domestic PAN-based carbon fibers as raw materials, graphitization was performed with different drafting forces at a certain heat treatment temperature. It was found that as the drafting force increased, Larger, the degree of fiber orientation is significantly improved, which makes the fiber structure from disorder to order, promotes the diffusion and closing of voids, and improves the lattice arrangement. Using a certain drafting force at an appropriate temperature can improve the microstructure of carbon fibers. To increase tensile strength and modulus. Yang Weimin and others studied the effect of drafting force on the chemical structure and microstructure of PAN-based carbon fibers in the process of laser graphite, and found that under a certain laser power, appropriately increasing the drafting force can increase the degree of graphitization of the fiber to a certain extent. Promote the preferred orientation of the fibers in the axial direction, and improve the regularity of the crystallite size and arrangement, but the effect of the drafting force on the mechanical properties of the fibers during laser graphitization needs to be further explored.
During the graphitization process, a certain drafting force is applied to form a complex thermo-solid-fluid coupling field. The evolution of the fiber structure is very complicated. The relevant mathematical and physical models can be established according to the actual process conditions and the microcrystalline parameters and properties of the fiber. Furthermore, the process parameters were optimized to optimize the performance of the prepared graphite fibers.
2.4 Effect of γ-ray radiation on mechanical properties of carbon fibers
Tianjin University of Technology used γ-rays to irradiate carbon fibers to realize the graphitization process. At room temperature, carbon fibers were irradiated with γ-rays excited by 60Co elements. X-ray diffraction was used to characterize the interlayer spacing of the graphite microchip layers in the carbon fibers after γ-ray irradiation. The carbon fiber sample had a significantly smaller interlayer distance than the graphite microcrystalline layer before irradiation. It was pointed out that the Compton effect and thermal effect of γ-rays are the reaction mechanism of carbon fiber graphitization.
Donghua University used γ-rays to treat carbon fibers, and studied the effect of radiation on its mechanical properties. As the amount of radiation increased, the tensile strength of carbon fibers increased first and then decreased, while the tensile modulus increased, which was characterized by scanning electron microscopy. Its surface morphology and its surface roughness increase with the amount of radiation. When the radiation dose is lower than 30kGy, the cross-linking of the internal structure of the carbon fiber dominates the tensile strength, and when the radiation dose exceeds 30kGy, defects on the surface of the carbon fiber begin to reduce the tensile strength.
As a high-energy substance, γ-rays interact with carbon fibers to promote the graphitization process. Under the comprehensive grasp of the mechanism of action, the appropriate radiation process conditions are adopted. The mechanical properties of carbon fibers after graphitization can reach a certain level. The conductivity of carbon fibers also deserves further investigation.
2.5 Catalyst's promotion of carbon fiber graphitization
In existing catalysts, only boron atoms can be combined with carbon atoms to form a solid solution. The methods for introducing boron into carbon fibers include indirect introduction method, liquid impregnation method, PAN copolymerization modification method and vapor deposition method. A large number of studies have shown that boron atoms have a strong promotion effect on the graphitization process of carbon fibers, reducing the fiber's thermal expansion coefficient and improving its oxidation resistance. Due to the complexity of the carbon fiber graphite microcrystalline structure, the reason why the catalytic effect of boron atoms on the mechanical properties of the fibers has not been identified.
Wang Huiqi and others introduced boron into carbon fibers by indirect method, and studied the effect of boron content on the mechanical properties of carbon fibers. With the increase of boron content, the tensile strength of carbon fibers increased after graphitization, and began to increase after a certain value It decreases, but the tensile modulus has been increasing, as shown in Figure 4. Therefore, reasonable control of the boron content of the carbon fiber can increase the tensile modulus while increasing the tensile strength.
Xu Shihai and others used the anodized PAN-based carbon fibers directly immersed in a uniformly dispersed Mo-B sol to obtain Mo-B coated carbon fibers. The graphite microcrystals were obtained by heating the graphite fibers in a helium atmosphere at 2400 ° C for 2 hours. The interlayer distance d002 is 0.3358 nm, the thickness of graphite crystallites is 28 nm, and the number of graphite crystallite layers is as high as 83 layers. It is confirmed that the graphitization is achieved by Mo and B catalysis, and the temperature is lower than the conventional graphitization temperature. The catalytic graphitization process follows a "dissolution-reprecipitation" replacement type solid solution mechanism. The size of boron atoms is equivalent to that of carbon atoms. The boron atoms fill the gaps in the graphite sheet by replacing the carbon atoms. Therefore, the arrangement of graphite crystallites is more regular, so that Carbon fiber surface cracks are eliminated, and internal structural defects are compensated. Bryan et al. Used polyethylene as a precursor to melt-spin carbon fiber precursors. They were heated in 20% fuming sulfuric acid to undergo sulfonation to obtain high-temperature infusible stability. The fibers were then immersed in a boric acid solution and subsequently heated. Graphitization was achieved at 2400 ° C, and graphite fibers with a tensile modulus exceeding 400 GPa were obtained, which were characterized by wide-angle XRD diffraction. The initial graphitization temperature was reduced by 400 ° C under the catalysis of boron, and the production cost of polyethylene fibers was low. High efficiency, this research opens a new path for the research of low-cost, high-efficiency, high-quality graphite fiber preparation.
Kuang Yafei et al. Studied the catalytic effect of electrodeposited Fe-P coating on the graphitization process. Under the catalytic action of Fe-P alloy, PAN-based carbon fibers were graphitized at 1000 ° C. This catalytic graphitization process is energy-efficient and efficient. Characteristics, has a very broad application prospect in the field of low-temperature graphitization of carbon materials.
3 Relationship between structure and performance
The microstructure is a key factor affecting the mechanical properties of carbon fibers. During the graphitization process, with the removal of non-carbon atoms, the disordered structure partly decreases and gradually shifts to an ordered structure. At the same time, the regularity of the graphite sheet layer improves, the distribution The formation of uneven microscopic pores and fiber surface defects has different degrees of influence on the macroscopic tensile strength and modulus, and the structural evolution of carbon fibers with different precursors during the graphitization process is diverse, leading to the mechanical properties of the prepared graphite fibers. There are big differences.
Lu Chunxiang et al. Discussed the relationship between graphite fiber structure and performance based on the "elastic wrinkle" model and Griffith microcrack theory. The recrystallization process of graphite crystallites changed the shear flexibility of the carbon lattice, and the graphite fiber elastically wrinkled. The process is not obvious, so the stress relaxation process is not significant compared to ungraphitized fibers under external force in the tensile state, as shown in Figures 5 and 6, which may have an adverse effect on the improvement of the strength of graphite fibers. A correction method was proposed based on the fractal characteristics of the graphite fiber surface, and related mathematical and physical models were established. It was found that the relationship between the tensile strength of carbon fibers and micropore parameters conformed to the Griffith equation. The specific factors affecting tensile strength are also: To be further explored. Xu Jian et al. Conducted a comparative study on the microstructures of domestic high-model graphite fibers and high-strength high-model graphite fibers with similar modulus, and found that the high-model graphite fibers have large intergranular cracks and void structures, resulting in low strength and high strength. High-model graphite fibers have multiple layers of microstructures, micropores, cracks, and other defects. More stress diffusion, energy storage, and dissipation paths are key to maintaining tensile strength and strain.
For PAN-based carbon fibers, the continuous removal of nitrogen atoms during graphitization will cause different degrees of defects in the fibers. The increase in tensile modulus is at the expense of its tensile strength, which severely restricts the performance of PAN-based carbon fibers to high strength and high modulus. Transformation, resulting in poor matching of tensile strength and tensile modulus. Bian Wenfeng et al. Studied the effect of PAN-based carbon fiber graphite microcrystalline structure on tensile properties. The tensile strength is related to both microcrystalline and amorphous carbon. The tensile load on the two is different. High strength and high model graphite The tensile strength of fibers decreases with the decrease of amorphous carbon, and the increase of crystallite size plays a decisive role in the reduction of fiber strength. The Mori-Tanaka method was used to establish a two-phase meso-mechanical model of carbon fibers. A meso-mechanical analysis of PAN-based high-modulus carbon fibers was performed. It was found that the main factors affecting the improvement of fiber modulus were graphite crystallite orientation, volume fraction, and length. For the fine ratio, only one of the influencing factors is considered, and the other factors are kept unchanged. The obtained relationship curve is shown in Fig. 7. The graphite crystallite orientation degree and volume fraction have a greater effect on the fiber modulus than the graphite crystallite length. The effect of the ratio on the fiber modulus, only when the crystallite orientation is close to 100%, the effect of the crystallite slenderness ratio on the fiber modulus may exceed the effect of the crystallite orientation on the fiber modulus.
Qin Xianying et al. Studied the differences in the microstructure and mechanical properties of PAN-based carbon fibers and mesophase pitch-based carbon fibers during high-temperature graphitization. Using T300 and homemade mesophase pitch-based carbon fibers as raw materials for high-temperature graphitization at different temperatures, the differences between the microstructure of PAN-based carbon fibers and pitch-based carbon fibers before and after graphitization, and the evolution of microstructure and macroscopic properties, such as Figure 8 shows. As the graphitization temperature increases, the modulus of PAN-based and pitch-based carbon fibers increases due to the growth of graphite crystallites and the preferred orientation. The tensile strength of PAN-based carbon fibers decreases as the graphitization temperature increases. It is due to the high shear stress caused by the covalent bonds cross-linked between the graphite sheets, and the microporous defects are caused by the overflow of non-carbon atoms, the distortion of crystal grains, and dislocation. For pitch-based carbon fibers, the tensile strength increases with increasing graphitization temperature, because the plane stress between graphite sheets and the size of graphite crystallites increase, the preferred orientation of graphite crystallites gradually increases, and the number of micropores decreases. . In addition, a small number of graphite microcrystalline defects are caused by physical entanglement and chemical cross-linking being destroyed during high temperature processing.
By comparing the graphitization process of PAN-based carbon fibers with pitch-based carbon fibers, it is found that the structural defect causes the tensile strength of PAN-based carbon fibers to decrease after graphitization at high temperatures. How to avoid the occurrence of defects is worth further investigation.
4 Theoretical analysis at the molecular level
In recent years, the rapid development of cutting-edge technology has put forward higher requirements on the properties of materials, and endless new materials have greatly promoted the progress of science and technology. However, the trial production of materials still relies on experience to prepare suitable materials after "selection and selection". Although effective, the experience is often subjective and blind, and the grasp of the molecular level of materials is not well understood. The same is true in the development of carbon fiber materials. Only by grasping the structural evolution of carbon fibers at the molecular level can high-performance carbon fibers be manufactured at the lowest cost.
For a long time, Chinese and foreign scholars have conducted in-depth and comprehensive explorations of the microstructure and macroscopic properties of carbon fibers, but the theoretical framework of carbon fibers at the molecular and atomic levels is still in its infancy. Molecular dynamics simulation is a scientific method that reveals the composition, structure and properties of substances on a molecular scale, which has developed rapidly in recent years. It has been widely used in the simulation research of biological macromolecules, polymer materials and nanomaterials. application. Nitant et al. Used molecular dynamics simulation to deeply analyze the molecular structural defects of the PAN-based carbon fiber during the high-temperature carbonization process, and pointed out that topological defects inevitably occur during the high-temperature pyrolysis process, which is accompanied by the evolution of the layered graphite-like structure. With regard to the formation of the "D-ring", as shown in Fig. 9, the tensile strength of the carbon fiber calculated by simulation is only 1/4 of the theoretical value. The molecular structure defects naturally formed during the carbonization process have not been improved during the ultra-high temperature graphitization process, so their strength cannot be further improved, but the strength of the defects will be reduced due to the expansion of the defects, so they must be avoided at the initial moment of defect formation. Only after carbonization and graphitization of carbon fiber can the mechanical properties be further improved and then tend to the theoretical value. At present, molecular simulation only involves the preparation of filaments, pre-oxidation and carbonization in the research of carbon fiber materials, and molecular simulation has not been performed in graphitization. It is urgent to simulate the dynamics of atoms in the process of graphitization at the molecular level for carbon fibers of different structural levels The state analyzes the cause from the root cause of the macro performance, and then "the right medicine", which will effectively solve the problem that the strength and modulus are far below the theoretical value.