Method for producing medium-to-high modulus large carbon fiber

Abstract

The present invention provides a method for producing medium- to high-modulus large-tow carbon fiber. The method includes the steps of: subjecting low-modulus large-tow carbon fiber to in-phase microwave heating and adjusting the surface current density of the carbon fiber to a current density range of 60 to 330 A / m, thereby rapidly raising the temperature of the carbon fiber to the graphitization temperature, thereby achieving a uniform and rapid graphitization process. The medium- to high-modulus large-tow carbon fiber obtained by this method has a tensile modulus of 300 to 600 GPa, a tensile strength of 3.5 to 5.0 GPa, and a variance of the tensile modulus of the carbon fiber of less than 1.5%.

Description

Cross-reference to related applications

[0001] This application claims the priority of a Chinese patent application with application number 202311268510.2 and invention title "Method for manufacturing large-tow carbon fiber with medium-high elastic modulus", which was filed with the China National Intellectual Property Administration on September 28, 2023, and all of its contents are incorporated herein by reference.

Technical field

[0002] This application belongs to the technical field of graphitization of large-tow carbon fiber, and particularly relates to a method for manufacturing large-tow carbon fiber with medium-high elastic modulus.

Background art

[0003] Carbon fiber is often used as a carbon reinforcement material for manufacturing high-performance composite materials due to its high strength and high elastic modulus. Usually, carbon fiber is divided into small-tow carbon fiber (1~24K) and large-tow carbon fiber (24~480K) according to the number of carbon fiber monofilaments in the carbon fiber bundle. Small-tow carbon fiber (1~24K) is a conventional carbon fiber product with excellent mechanical properties, having a tensile strength of 3500~7000 MPa and a tensile elastic modulus of 230~680 GPa. Among them, those with a tensile elastic modulus exceeding 280 GPa are generally called medium-elasticity carbon fiber, and those with a tensile elastic modulus exceeding 350 GPa are called high-elasticity carbon fiber. Medium-high elasticity carbon fiber is mainly used in the fields of aerospace, high-end industries, high-end sports and other products. Since the technical threshold is high, its price is usually 5 to 10 times or more that of ordinary low-elasticity carbon fiber and it is not easily available, it has high development potential.

[0004] On the other hand, large-tow carbon fiber (24-480K) has a tensile strength of 3500-5000 MPa and a tensile modulus of 230-280 GPa. Large-tow carbon fiber has advantages such as high single-line production capacity, a cost reduction of approximately 40%, fewer layers required for processing composite materials, and easy availability. In recent years, it has been gradually used as a substitute for small-tow carbon fiber in fields such as automobiles, wind turbine blades, energy buildings, shipbuilding, and sporting goods. However, large-tow carbon fiber has lower performance in terms of tensile strength and tensile modulus compared to small-tow carbon fiber, and all are low-modulus carbon fibers, so the application fields of large-tow carbon fiber are limited. Table 1 shows an overview of the performance of large-tow carbon fiber currently available domestically and internationally.

[0005] [Table 1]

[0006] To broaden the application fields of large-tow carbon fiber, this patent combines the low-cost characteristics of large-tow carbon fiber with innovative technology to lower the barrier to production of medium-to-high modulus carbon fiber, rapidly upgrading low-modulus (<280 GPa) large-tow carbon fiber to medium-to-high modulus (300-600 GPa) and significantly improving the cost-effectiveness of carbon fiber. Conventional technology requires high-temperature graphitization at 2000-3000°C under specific conditions in a high-temperature furnace under the protection of an inert gas. Current graphitization technology for medium-to-high modulus large-tow carbon fiber has been rarely reported domestically or internationally, mainly due to limitations of high-temperature graphitization furnaces. According to conventional heating methods, graphitization furnaces are divided into direct heating type and indirect heating type. In direct heating types, the large number and thickness of monofilament carbon fibers in large tow carbon fibers result in uneven heating of the monofilament carbon fibers when heat is transferred from the heating element in the graphitization furnace. This leads to variations in the degree of graphitization, causing localized stress concentration in the fibers, leading to brittle cracks, and ultimately, yarn breakage. Indirect graphitization furnaces, such as induction graphitization furnaces, use graphite muffles for induction heating to transfer heat to the carbon fibers. However, they have a short lifespan, high energy consumption in the graphitization process, making it difficult to reduce production costs. Furthermore, they can only process small batches, the heating temperature field is unevenly distributed across the fibers, resulting in highly discrete performance characteristics, making continuous production impossible, and making it difficult to improve mechanical properties.

[0007] Furthermore, the crystalline layer of large-tow carbon fiber contains many alignment defects, and the graphitization process is prone to fiber breakage. As a result, there are currently no records of successful continuous production of medium-to-high modulus large-tow carbon fiber, either domestically or internationally. [Overview of the project] [Problems that the invention aims to solve]

[0008] The objective of this invention is to obtain medium-to-high modulus large tow carbon fibers with uniform product performance by graphitizing low modulus large tow carbon fibers with microwaves, thereby achieving a tensile modulus of 300-600 GPa, maintaining a tensile strength of 3.5-5.0 GPa, and having a dispersion of the tensile modulus of the carbon fibers of less than 1.5%. [Means for solving the problem]

[0009] To achieve the above technical objectives, this application, The present invention provides a method for producing medium-to-high modulus large-saw carbon fibers, comprising the steps of: graphitizing low-modulus large-saw carbon fibers with microwaves; specifically, heating the low-modulus large-saw carbon fibers with in-phase microwaves in a microwave graphitization furnace; simultaneously adjusting the surface current density of the low-modulus large-saw carbon fibers in the microwave graphitization furnace so that the surface current density of each carbon fiber in the furnace is the same or tends to be the same, with the current density in the range of 60 to 330 A / m; thereby rapidly and uniformly raising the surface temperature of the large-saw carbon fibers in the microwave graphitization furnace to 2000 to 3000°C, thereby uniformly carrying out the graphitization process and obtaining medium-to-high modulus large-saw carbon fibers.

[0010] Definition of surface current density of carbon fiber: In a microwave graphitization furnace, electromagnetic waves accompanied by time-dependent electric and magnetic fields travel within a limited space. The superposition of these traveling and reflected waves generates specific resonant electric and magnetic field distributions. When a carbon fiber is under the resonance distribution of these electromagnetic waves, the time-dependent magnetic field at that location generates numerous localized eddy currents within the carbon fiber. At this time, the current flux per unit length (per meter) on the carbon fiber corresponds to the surface current density of the carbon fiber.

[0011] In the above technical solution, since carbon fiber is a high dielectric loss material in the microwave band, the transmitted microwave energy can be efficiently converted into heat, thereby achieving the advantages of rapid heating and energy saving. This heating mainly originates from the polarization loss of the dipoles and a small amount of electrical conduction loss. In a microwave electromagnetic field, the orientation of the dipoles within the carbon fiber tends to rotate and coincide, and the frequency in the polarization direction reaches several hundred million times per second, allowing the carbon fiber to be heated by the generated thermal kinetic energy. Heat is generated by the interaction between the microwaves and the carbon fiber as a whole, and the heating efficiency is greatly improved. Furthermore, the in-phase microwave heating used in this application ensures that the microwave signals entering the process cavity are in phase, achieving efficient and uniform heating of the carbon fiber in the furnace.

[0012] Furthermore, by adjusting the range of surface current density of the carbon fibers, the surface current density of each carbon fiber in the microwave graphitization furnace can be adjusted to be the same or tend to be the same, uniformly enhancing the heating effect of the ohmic loss of the carbon fibers, and improving the utilization rate of microwave energy and the efficiency of the process. The inventors have found that when the surface current density of the carbon fibers is adjusted to the range of 60 to 330 A / m, the high temperature generated by the heating effect of the same dielectric loss and ohmic loss causes the molecular structure of each carbon fiber to reorganize, the molecules crystallize and rapidly form a high modulus structure, the diameter decreases from 7.0 μm to 6.3 μm, and the tensile modulus of the carbon fiber rapidly increases to 300 to 600 GPa. However, if the surface current density is too high and exceeds 330 A / m, a significant tip discharge effect occurs in the carbon fiber in the microwave electric field, the local temperature exceeds the maximum allowable temperature of carbon fiber (3000°C), overheating occurs, the structure of the carbon fiber collapses, the tensile strength of the carbon fiber decreases significantly, and the effect of improving the tensile modulus weakens.

[0013] As described above, by controlling the adjustment range from 60 to 330 A / m through in-phase microwave heating and adjustment of the surface current density of the carbon fibers, the temperature of the large-tow carbon fibers can be uniformly raised to 2000 to 3000°C within 90 seconds. This achieves a uniform graphitization process and uniformly improves the tensile modulus of the low-modulus large-tow carbon fibers.

[0014] Furthermore, the microwave graphitization furnace employs an in-phase microwave design, and a dielectric periodic structure is provided within the microwave graphitization furnace.

[0015] By employing the above technical solutions, the microwave energy is stably concentrated and distributed within the distribution region of the dielectric periodic structure due to the in-phase microwave design and dielectric periodic structure, and the surface current density of the large tow carbon fibers in the microwave graphitization furnace is regulated by the dielectric periodic structure.

[0016] Furthermore, the microwave graphitization furnace includes a metal cavity, the metal cavity is provided with an even number of microwave feed ports connected to the same microwave generator, the even number of microwave feed ports are evenly distributed on the upper top surface and lower bottom surface of the metal cavity, supplying microwaves into the metal cavity, and the microwave feed ports are also provided with corresponding impedance matching regulators. Typically, in microwave heating, the microwave function equations generated by each magnetron are as follows:

[0017]

number

[0018] Here, A is amplitude, k is the number of waveguides, x is displacement, ω is angular frequency, t is time, and θ is the starting phase angle. When the power is turned on, high voltage electricity acts on the magnetron, causing it to oscillate via electron cyclotron resonance and generate an electromagnetic wave signal, stabilizing the output power within 2-3 seconds. Since the oscillation time of each magnetron is a random value, in a microwave system composed of multiple magnetrons, each magnetron transmitter radiates an electromagnetic wave signal at a different starting phase angle and at a different time. When each signal is transmitted to the internal space of the cavity through the microwave feed port, depending on the path and time the signal travels, a superposition of electromagnetic fields of different intensity and direction occurs at each point in the cavity space. Due to the phase difference between multiple microwave signals, the resulting electromagnetic field superposition can cause constructive or destructive cancellation, resulting in different electromagnetic field superposition results each time the power is turned on and microwaves are output. As a result, the reproducibility of the heating effect may decrease, and uneven heating may occur. In this invention, by providing an impedance matching regulator corresponding to the microwave feed port, the electromagnetic impedance of the microwave feed ports can be matched and adjusted so that the signals from each microwave feed port are radiated into the microwave process cavity with the same phase and intensity. This reduces electromagnetic field cancellation and uncontrollable electromagnetic field distribution caused by random phase differences, and achieves the best utilization efficiency of microwave energy.

[0019] Furthermore, the dielectric periodic structure comprises multiple dielectric units forming a cavity smaller in size than the metal cavity, and the carbon fibers can pass through the cavity of the dielectric periodic structure. The dielectric periodic structure includes an upper top surface, a lower bottom surface, a front side surface, and a rear side surface, and the upper top surface and the lower bottom surface each include multiple dielectric units arranged parallel to each other at equal intervals, and the direction in which the dielectric units are installed is perpendicular or parallel to the direction in which the carbon fibers advance, or at other angles.

[0020] By adopting the above technical solution, since the dielectric periodic structure affects the electromagnetic field distribution, by adjusting the position of the dielectric periodic structure within the metal cavity, a specific electromagnetic field distribution can be obtained, and microwave energy can be stably concentrated in the dielectric distribution region, thereby concentrating the microwave energy on the carbon fibers, making the surface current density of each carbon fiber tend to be the same or approximately the same, and controlling the range to 60 - 330 A / m.

[0021] Furthermore, the material of the dielectric unit is graphite, silicon carbide, or a combination related to carbides. When the dielectric material is graphite, the distance between the upper top surface and the lower bottom surface of the periodic structure and the carbon fibers is 10 - 35 mm, and the distance between the upper top surface and the lower bottom surface and the microwave feed port is 200 - 260 mm. When the dielectric material is silicon carbide, the distance between the upper top surface and the lower bottom surface of the periodic structure and the carbon fibers is 15 - 35 mm, and the distance between the upper top surface and the lower bottom surface and the microwave feed port is 180 - 260 mm.

[0022] By adopting the above technical solution, the surface current density of the carbon fibers in the microwave graphitization furnace can be adjusted within the range of 60 - 330 A / m, which is helpful for improving the tensile elastic modulus of the carbon fibers. Through experiments, it is found that the tensile elastic modulus of the carbon fibers can be improved to 300 - 600 GPa, and the dispersion degree of the tensile elastic modulus is less than 1.5%. Furthermore, in this graphitization process, the diameter of the carbon fibers decreases from 7.0 μm to 6.3 μm.

[0023] Furthermore, in order to maintain the purity of the carbon fibers and prevent other impurities from affecting the graphitization process, it is necessary to remove the sizing agent on the surface of the large tow carbon fibers before microwave graphitization.

[0024] Furthermore, after microwave graphitization, the large tow carbon fibers are surface-treated, sized again, dried, and wound up to obtain large tow carbon fibers with medium-high elastic modulus.

Advantages of the Invention

[0025] The beneficial effects of this invention are as follows: In this invention, in-phase microwave heating allows for rapid and uniform heating of carbon fibers from the inside out to the graphitization temperature. Because microwaves can self-heat internally, the system's heat retention requirements are lower compared to conventional heating methods. Furthermore, in the microwave electromagnetic field, heat is generated by the interaction between the microwaves and the entire carbon fiber, improving the uniformity of heating throughout the carbon fiber and allowing for rapid attainment of the graphitization temperature. Microwave heating increases the internal energy of carbon atoms (increases the transition frequency), accelerating the formation rate of the carbon fiber graphite layer, shortening the high-temperature processing time, and helping to maintain fiber strength. In addition, by adjusting the surface current density of the carbon fibers in the graphitization furnace using a dielectric periodic structure, and adjusting the surface current density of each carbon fiber to be the same or tend to be the same, a specific magnetic field can be formed, concentrating microwave energy on the carbon fibers and improving the utilization rate of microwave energy and the effectiveness of the process. This graphitization process allows the tensile modulus of large-tow carbon fibers to reach 300-600 GPa, while the tensile strength remains at 3.5-5.0 GPa. The dispersion of the tensile modulus of the carbon fibers is less than 1.5%, resulting in uniform product performance. [Brief explanation of the drawing]

[0026] [Figure 1] This is a comparison diagram of the heat conduction of carbon fiber using conventional heating and microwave heating. [Figure 2] This is a schematic front view of a microwave graphitization furnace. [Figure 3] This is a schematic diagram of the left side of a microwave graphitization furnace. [Figure 4] This is a schematic diagram of the non-thermal effects of periodic structures and in-phase microwaves. [Figure 5a] This is a schematic diagram simulating a case where the current density on the carbon fiber surface is 170 A / m. [Figure 5b] This figure, corresponding to Figure 5a, shows the actual heating effect of carbon fiber. [Figure 6] This figure shows the relationship between the tensile modulus of carbon fiber and the surface current density of carbon fiber. [Figure 7]This is a comparison chart of carbon fiber diameters before and after microwave graphitization. [Modes for carrying out the invention]

[0027] The technical solutions of the present application will be described clearly and completely below with reference to the drawings, but it is clear that the embodiments described are only some embodiments of the present application, not all embodiments. Any other embodiments obtained based on the embodiments of the present application, without the creative work of a person skilled in the art, are all within the scope of the protection of the present application.

[0028] Furthermore, the technical features of the different embodiments of the present application described below may be combined with each other, insofar as they do not contradict each other.

[0029] This invention provides a method for producing medium-to-high modulus large-saw carbon fibers, which involves graphitizing low-modulus large-saw carbon fibers with microwaves, specifically by heating the large-saw carbon fibers with in-phase microwaves in a microwave graphitization furnace, and simultaneously adjusting the surface current density of the low-modulus large-saw carbon fibers so that the surface current density of each carbon fiber is the same or tends to be the same, with the current density in the range of 60 to 330 A / m, thereby rapidly raising the surface temperature of the large-saw carbon fibers in the microwave graphitization furnace to 2000 to 3000°C within 90 seconds, completing the graphitization process, and obtaining medium-to-high modulus large-saw carbon fibers.

[0030] In this application, microwave heating differs from conventional heating in its heating process. As shown in Figures 1a and 1b, 1a represents conventional heating, where the heat flows from the outside to the inside, while 1b represents microwave heating, where the heat flows from the inside to the outside. Compared to conventional heating, microwave heating reduces the system's heat retention requirements, increases the internal energy of carbon atoms, increases the transition frequency of carbon atoms, promotes the formation of the carbon fiber graphite layer, shortens the high-temperature processing time, and helps maintain the strength of the carbon fiber. Furthermore, in this application, in-phase microwaves are used, meaning that signals from each microwave feed port are radiated into the microwave process cavity with the same phase and intensity. This reduces electromagnetic field cancellation due to random phase differences and uncontrollable electromagnetic field distribution, enabling uniform heating of carbon fibers in the microwave graphitization furnace.

[0031] Based on in-phase microwaves, the surface current density of carbon fibers in the microwave graphitization furnace is adjusted to a range of 60-330 A / m, and adjusted so that the surface current density of each carbon fiber is the same or tends to be the same. Due to the high temperature generated by the heating effect of the same dielectric loss and ohmic loss, the carbon fibers undergo molecular structure reorganization, the molecules crystallize and rapidly form a high modulus structure, the diameter of the carbon fibers decreases from 7.0 μm to 6.3 μm, and the tensile modulus of the carbon fibers rapidly increases to 300-600 GPa. However, if the surface current density exceeds 330 A / m, a significant tip discharge effect occurs in the carbon fibers in the microwave electric field, the local temperature exceeds the maximum temperature of the carbon fibers (3000°C), causing overheating, a significant decrease in the tensile strength of the carbon fibers, and weakening the effect of improving the tensile modulus.

[0032] The microwave graphitization furnace employs an in-phase microwave design, and a dielectric periodic structure is provided inside the microwave graphitization furnace. The surface current density of the carbon fibers is regulated by the dielectric periodic structure, and the in-phase microwaves and the dielectric periodic structure installed inside the furnace work together to stably concentrate the microwave energy in the distribution region of the dielectric periodic structure, thereby improving the stability and efficiency of heating the carbon fibers.

[0033] Referring to Figures 2 and 3, a microwave graphitization furnace used in the present invention is shown, which includes a metal cavity 2, and an even number of microwave feed ports 3 are provided in the metal cavity 2, and the even number of microwave feed ports 3 are evenly distributed on the upper top surface and lower bottom surface of the metal cavity 2, supplying microwaves into the metal cavity 2, and corresponding impedance matching regulators 4 are also provided in the microwave feed ports 3, which adjust the electromagnetic impedance input into the microwave graphitization furnace, make the microwave phases input into the furnace in phase, and achieve the best utilization efficiency of microwave energy. A ceramic support 6 is provided in the metal cavity 2, and a dielectric periodic structure is provided in the ceramic support 6, and the dielectric periodic structure is such that a plurality of dielectric units 5 constitute a cavity smaller in size than the metal cavity 2, and carbon fibers 1 can pass through the cavity made up of the dielectric units 5. The dielectric periodic structure includes an upper top surface, a lower bottom surface, a front side surface, and a rear side surface. The upper top surface and the lower bottom surface each consist of a plurality of dielectric units 5 arranged parallel to each other at equal intervals, and the direction in which the dielectric units 5 are installed is perpendicular to the forward direction of the carbon fibers 1. Furthermore, the direction in which the dielectric units 5 on the upper top surface and the lower bottom surface of the dielectric periodic structure are installed may also be parallel to the forward direction of the carbon fibers 1, or at other angles. The material of the periodic structure is graphite, silicon carbide, or a combination related to carbides.

[0034] Let d1 be the distance between the carbon fibers and the upper and lower top surfaces of the dielectric periodic structure inside the microwave graphitization furnace, and d2 be the distance between the upper and lower top surfaces of the dielectric periodic structure and the microwave feed port on the same side.

[0035] The dielectric periodic structure influences the electromagnetic field distribution, and by adjusting the position of the periodic structure within the metal cavity, a specific electromagnetic field distribution can be adjusted, allowing microwave energy to be concentrated on the carbon fibers. Taking the dielectric periodic structure shown in Figures 2 and 3 as an example, that is, taking the case where the orientation of the dielectric units on the upper and lower top surfaces of the dielectric periodic structure is perpendicular to the direction of advancement of the carbon fibers, refer to Figure 4 for a schematic diagram of the periodic structure and the non-thermal effects of in-phase microwaves. Inside the microwave graphitization furnace, the upper top surface incident electromagnetic wave 7 passes through the dielectric unit 5, forming an upper top surface incident electromagnetic wave diffraction peak 8 below the carbon fibers 1, and the lower top surface incident electromagnetic wave 9 passes through the dielectric unit 5, forming a lower top surface incident electromagnetic wave diffraction peak 10 above the carbon fibers 1. The vertical distance D from any point on the upper top surface incident electromagnetic wave diffraction peak 8 and the lower top surface incident electromagnetic wave diffraction peak 10 to the carbon fibers represents the intensity of the electromagnetic wave diffraction peak, and the greater the distance D, the greater the intensity of the diffraction peak. The upper top-plane incident electromagnetic wave diffraction peak 8 and the lower top-plane incident electromagnetic wave diffraction peak 10 are both projected onto the carbon fiber 1, forming a high-efficiency heating region 11 for the ohmic loss of the carbon fiber. Furthermore, as the carbon fiber advances horizontally, the ohmic loss effect of the carbon fiber is maximized while maintaining uniform heating. In addition, by adjusting the position of the dielectric periodic structure within the metal cavity, the surface current density of the carbon fiber is controlled to 60-330 A / m, enhancing the heating effect of the ohmic loss, improving energy utilization efficiency, and improving the tensile modulus of the carbon fiber.

[0036] Carbon fiber heating with in-phase microwave heating

number

[0037] Conventional heating of carbon fibers

number

[0038] Here ε s Emission coefficient of dielectric materials ds : The distance between the dielectric and the central horizontal plane of the periodic structure, i.e., d1 in this application. δ b :Stefan-Boltzmann constant A CF Area of ​​carbon fiber A s : Area of ​​dielectric I: Surface current of carbon fiber R: Radius of carbon fiber T S : Dielectric temperature

[0039] Comparing the above equations, it was found that the heating efficiency of in-phase microwaves, compared to conventional heating, includes ohmic loss heating. This significantly improves the heating efficiency and heating uniformity of carbon fibers.

[0040] Referring to Figures 5a and 5b, Figure 5a is a schematic diagram simulating a carbon fiber with a surface current density of 170 A / m, and Figure 5b is a diagram of the corresponding actual heating effect of carbon fiber in a microwave graphitization furnace, where the white areas are regions where thermal energy is concentrated. It was found that the surface current density of the simulated carbon fiber corresponds to the heating region of the carbon fiber in the microwave graphitization furnace.

[0041] Furthermore, the following embodiments further illustrate the effect of controlling the surface current density of carbon fibers on the tensile modulus of carbon fibers during the in-phase microwave heating process in this application.

[0042] The comparative example uses the original large-tow carbon fiber, specifically a PAN-based large-tow carbon fiber manufactured by SGL GmbH in Germany, with the model CT50-4.4, 50k, and a standard tensile modulus of 255 GPa. Ten samples were randomly selected, and performance data was tested and recorded. The examples are carbon fibers obtained by graphitizing the original large-tow carbon fiber with microwaves at different surface current densities. Ten samples were randomly selected for each example, and performance data was tested and recorded. A comparison of the performance data between the comparative example and the examples is shown in Table 2.

[0043] [Table 2] JPEG0007834261000006.jpg143123

[0044] According to Table 2, the relationship between the surface current density and tensile modulus of carbon fibers is specifically shown in Figure 6. Combining Table 2 and Figure 6, it was found that the tensile modulus of the carbon fibers changed as a result of graphitizing the original carbon fibers with microwaves. Specifically, (1) when the surface current density of the carbon fibers is set to less than 60 A / m for graphitization, the increase in the tensile modulus of the resulting carbon fibers is small. (2) When the surface current density of the carbon fibers is set in the range of 60 to 330 A / m for graphitization, the tensile modulus of the resulting carbon fibers increases significantly to 300 to 600 GPa, and the dispersion of the tensile modulus is kept within 1.5%. (3) When the surface current density of the carbon fibers is exceeded 330 A / m for graphitization, the effect of improving the tensile modulus of the carbon fibers begins to weaken.

[0045] Furthermore, the present inventors have shown that the microwave graphitization method of this application causes carbon fibers to undergo molecular structure reorganization due to the high temperature generated by the heating effect of the same dielectric loss and ohmic loss, leading to the rapid crystallization of molecules and the formation of a high modulus structure. As a result, the diameter changes from an average of 7.0 μm to 6.3 μm, and a comparison of the diameter change of carbon fibers before and after graphitization is shown in Figure 7.

[0046] Furthermore, regarding the adjustment of the surface current density of carbon fibers, Table 3 shows the positional relationship of the dielectric periodic structure within the metal cavity of the microwave graphitization furnace when the periodic structure material is graphite or silicon carbide.

[0047] [Table 3]

[0048] Table 3 shows that the surface current density of carbon fibers can be adjusted by adjusting the position of the dielectric material in the microwave graphitization furnace. When the dielectric material is graphite, the optimal adjustment range for d1 is 15-35 mm and the optimal adjustment range for d2 is 200-260 mm. When the dielectric material is silicon carbide, the optimal adjustment range for d1 is 30-35 mm and the optimal adjustment range for d2 is 200-210 mm.

[0049] The present invention also provides a medium-to-high modulus large tow carbon fiber having a tensile strength of 3.5 to 5.0 GPa, a tensile modulus of 300 to 600 GPa, and a dispersion of the tensile modulus of the carbon fiber of less than 1.5%.

[0050] As described above, the method for producing medium-to-high modulus large-saw carbon fibers according to this invention can stably improve the modulus of low-modulus large-saw carbon fibers to a medium-to-high modulus, representing a significant advance in the field of technology for producing medium-to-high modulus large-saw carbon fibers both domestically and internationally.

[0051] Clearly, the above embodiments are merely illustrative examples to clearly illustrate the present application and do not limit the embodiments. Those skilled in the art can make various other variations or modifications based on the above description. It is not necessary to cover all embodiments here, nor is it possible to do so. Any obvious variations or modifications derived here remain within the scope of the present invention. [Explanation of Symbols]

[0052] 1 Carbon fiber 2 Metal Cavity 3 Microwave feed port 4. Impedance Matching Regulator 5 Dielectric Unit 6. Ceramic support 7. Electromagnetic waves incident on the upper top surface 8. Upper top surface incident electromagnetic wave diffraction peak 9. Electromagnetic waves incident on the lower top surface 10 Lower top surface incident electromagnetic wave diffraction peak 11. High heating efficiency region of ohmic loss in carbon fibers

Claims

1. A method for producing medium-to-high modulus large carbon fiber, A method for producing medium-to-high modulus large-saw carbon fibers, comprising the steps of: graphitizing low-modulus large-saw carbon fibers with microwaves; specifically, heating the low-modulus large-saw carbon fibers with in-phase microwaves in a microwave graphitization furnace; simultaneously, adjusting the surface current density of the low-modulus large-saw carbon fibers in the microwave graphitization furnace so that the surface current density of each carbon fiber in the furnace is uniformly controlled to be the same or tend to be the same, with the current density in the range of 60 to 330 A / m; thereby uniformly raising the surface temperature of the large-saw carbon fibers in the microwave graphitization furnace to 2000 to 3000°C within 90 seconds, thereby uniformly carrying out the graphitization process and obtaining medium-to-high modulus large-saw carbon fibers.

2. The method for producing medium-to-high modulus large-saw carbon fibers according to claim 1, characterized in that the microwave graphitization furnace employs an in-phase microwave design, a dielectric periodic structure is provided inside the microwave graphitization furnace, the microwave energy is stably concentrated and distributed in the distribution area of ​​the dielectric periodic structure due to the in-phase microwave design and the dielectric periodic structure, and the current density on the surface of the large-saw carbon fibers inside the microwave graphitization furnace is regulated by the dielectric periodic structure.

3. The method for producing medium-high modulus large tow carbon fiber according to claim 2, characterized in that the microwave graphitization furnace includes a metal cavity, the metal cavity is provided with an even number of microwave feed ports, the even number of microwave feed ports are evenly distributed on the upper top surface and lower bottom surface of the metal cavity, and microwaves are supplied into the metal cavity, the microwave feed ports are also provided with corresponding impedance matching regulators, the electromagnetic impedances of the even number of microwave feed ports are matched and adjusted, the microwave phases input into the graphitization microwave furnace are made in phase, and the best utilization efficiency of microwave energy is achieved, and the dielectric periodic structure provided in the metal cavity is composed of a plurality of dielectric units forming a cavity smaller in size than the metal cavity, and carbon fibers can pass through the cavity of the dielectric periodic structure.

4. The method for producing medium-to-high modulus large tow carbon fiber according to claim 3, wherein the dielectric periodic structure includes an upper top surface, a lower bottom surface, a front side surface, and a rear side surface, and the upper top surface and the lower bottom surface each include a plurality of dielectric units provided parallel to each other at equal intervals.

5. The method for producing medium-to-high modulus large tow carbon fiber according to claim 4, characterized in that the material of the dielectric unit is graphite or silicon carbide.

6. The method for producing medium-to-high modulus large tow carbon fiber according to claim 5, characterized in that, when the dielectric material in the microwave graphitization furnace is graphite, the distance between the upper top surface and the lower bottom surface of the dielectric periodic structure and the carbon fiber is 10 to 35 mm, and the distance between the upper top surface and the lower bottom surface and the microwave feed port is 200 to 260 mm.

7. The method for producing medium-to-high modulus large tow carbon fiber according to claim 5, characterized in that, when the dielectric material in the microwave graphitization furnace is silicon carbide, the distance between the upper top surface and the lower bottom surface of the dielectric periodic structure and the carbon fiber is 15 to 35 mm, and the distance between the upper top surface and the lower bottom surface and the microwave feed port is 180 to 260 mm.

8. A method for producing medium-to-high modulus large-saw carbon fibers according to claim 1, characterized in that the sizing agent on the surface of the low-modulus large-saw carbon fibers is removed before the low-modulus large-saw carbon fibers are graphitized with microwaves.

9. A method for producing medium-to-high modulus large-saw carbon fibers according to claim 1, characterized in that low-modulus large-saw carbon fibers are graphitized with microwaves, medium-to-high modulus large-saw carbon fibers are surface-treated, sized again, dried, and then wound up.

Images