Manufacturing method for medium-high modulus large tow 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 priority to a Chinese patent application filed with the China Patent Office on September 28, 2023, bearing application number 202311268510.2 and entitled "Method for producing medium-high modulus large-tow carbon fiber," the entire contents of which are incorporated herein by reference. [Technical Field]

[0002] The present application belongs to the technical field of graphitization of large tow carbon fibers, and in particular to a method for producing medium- to high-modulus large tow carbon fibers. [Background technology]

[0003] Due to its high strength and high modulus, carbon fiber is often used as a carbon reinforcement material for producing high-performance composites. Depending on the number of carbon fiber monofilaments in the carbon fiber bundle, carbon fiber is typically divided into small-tow carbon fiber (1-24K) and large-tow carbon fiber (24-480K). Small-tow carbon fiber (1-24K) is a traditional carbon fiber product with excellent mechanical properties, such as a tensile strength of 3500-7000 MPa and a tensile modulus of 230-680 GPa. Among these, fibers with a tensile modulus of over 280 GPa are generally referred to as medium-modulus carbon fiber, while fibers with a tensile modulus of over 350 GPa are referred to as high-modulus carbon fiber. Medium- and high-modulus carbon fiber is primarily used in aerospace, high-end industry, high-end sports, and other products. However, due to its high technical barriers, high price (5-10 times higher than that of standard low-modulus carbon fiber), and limited availability, it offers significant development potential.

[0004] 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 offers advantages such as high single-line production capacity, reduced carbon fiber costs by approximately 40%, fewer layers required for composite processing, and ease of availability. In recent years, it has gradually replaced small-tow carbon fiber in areas such as automobiles, wind turbine blades, energy buildings, shipbuilding, and sporting goods. However, large-tow carbon fiber has lower tensile strength and modulus than small-tow carbon fiber. Since all large-tow carbon fibers are low-modulus carbon fibers, their application fields are limited. A summary of the current performance of large-tow carbon fiber, both domestically and internationally, is shown in Table 1.

[0005] [Table 1]

[0006] To expand 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 production threshold for medium- and high-modulus carbon fiber, rapidly upgrading low-modulus (<280 GPa) large-tow carbon fiber to medium- and high-modulus (300-600 GPa) carbon fiber, and significantly improving the cost performance of carbon fiber. Conventional technology for producing medium- and high-modulus carbon fiber requires high-temperature graphitization at 2000-3000°C under specific conditions in a high-temperature furnace under inert gas protection. Current graphitization technology for medium- and high-modulus large-tow carbon fiber has rarely been reported domestically or internationally, primarily due to the limitations of high-temperature graphitization furnaces. Based on the conventional heating method, graphitization furnaces can be divided into direct-heating and indirect-heating types. In direct-heating furnaces, the number of monofilament fibers in large-tow carbon fiber is too large and their thickness is too great, resulting in uneven heating of the monofilament carbon fiber when heat is transferred from the heating element in the graphitization furnace to the carbon fiber. This results in uneven graphitization, which tends to cause local stress concentration in the fiber, leading to brittle cracking and yarn breakage. Indirect graphitization furnaces, such as induction graphitization furnaces, use a graphite muffle to induce heating and transfer heat to the carbon fiber. However, they have a short lifespan and high energy consumption during the graphitization process, making it difficult to reduce production costs. Furthermore, they can only process small batches, and the heating temperature field is unevenly distributed on the fiber, resulting in very discrete and large product performance expressions, making continuous production impossible and making it difficult to improve mechanical properties.

[0007] Furthermore, the crystalline layer of large-tow carbon fiber itself has many alignment defects, and thread breakage is likely to occur during the graphitization process, so there are currently no records of successful continuous production of large-tow medium- to high-modulus carbon fiber, both domestically and internationally. Summary of the Invention [Problem to be solved by the invention]

[0008] The object of the present application is to obtain medium to high modulus large tow carbon fiber by graphitizing low modulus large tow carbon fiber with microwaves, so that the tensile modulus of the large tow carbon fiber reaches 300 to 600 GPa, the tensile strength is maintained at 3.5 to 5.0 GPa, and the variance of the tensile modulus of the carbon fiber is less than 1.5%, thereby providing a product with uniform performance. [Means for solving the problem]

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

[0010] Definition of surface current density of carbon fiber: In a microwave graphitization furnace, electromagnetic waves accompanied by a time-dependent electric field and a time-dependent magnetic field travel within a limited space, and the superposition of the forward and reflected waves generates a specific resonant electric field and resonant magnetic field distribution. When a carbon fiber is under the resonant distribution of the electromagnetic wave, the time-dependent magnetic field at that position generates many localized eddy-induced currents within the carbon fiber. In this case, the current magnetic 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, carbon fiber is a material with high dielectric loss in the microwave band, so it can efficiently convert the transmitted microwave energy into heat, thereby achieving the advantages of rapid heating and energy saving. The heating mainly comes from dipole polarization loss and small electrical conduction losses. In a microwave electromagnetic field, the dipoles in the carbon fiber tend to rotate and align, with a polarization frequency reaching hundreds of millions of times per second, generating thermal kinetic energy that heats the carbon fiber. Heat is generated by the interaction of the microwaves with the entire carbon fiber, greatly improving heating efficiency. 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, adjusting the surface current density range of the carbon fiber allows the surface current density of each carbon fiber strand in the microwave graphitization furnace to be uniform or tend to be uniform, thereby uniformly enhancing the ohmic loss heating effect of the carbon fiber and improving the microwave energy utilization rate and process efficiency. The inventors have found that adjusting the surface current density of the carbon fiber to a range of 60-330 A / m causes the molecular structure of each carbon fiber to reorganize due to the high temperature generated by the same dielectric loss and ohmic loss heating effect, resulting in molecular crystallization and the rapid formation of a high-modulus structure. The diameter of the carbon fiber decreases from 7.0 μm to 6.3 μm, and the tensile modulus of the carbon fiber rapidly increases to 300-600 GPa. However, if the surface current density is too high, exceeding 330 A / m, the microwave electric field will cause a serious tip discharge effect on the carbon fiber, causing local temperatures to exceed the maximum allowable temperature of 3000°C for the carbon fiber. This overheating will cause the carbon fiber structure to collapse, significantly reducing the tensile strength of the carbon fiber and weakening the effect of improving the tensile modulus.

[0013] As described above, by controlling the in-phase microwave heating and the surface current density of the carbon fiber within the range of 60 to 330 A / m, the temperature of the large tow carbon fiber 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 fiber.

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

[0015] By adopting the above technical solution, 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 surface current density of the large tow carbon fiber in the microwave graphitization furnace is adjusted by the dielectric periodic structure.

[0016] Furthermore, the microwave graphitization furnace includes a metal cavity, and 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 the lower bottom surface of the metal cavity to supply microwaves into the metal cavity, and the microwave feed ports are also provided with corresponding impedance matching regulators. Generally, in microwave heating, the microwave function equation generated by each magnetron is as follows:

[0017]

number

[0018] where A is the amplitude, k is the number of waveguides, x is the displacement, ω is the angular frequency, t is the time, and θ is the starting phase angle. When power is applied, high-voltage electricity acts on the magnetron, oscillating it through electron cyclotron resonance to generate an electromagnetic signal. The output power stabilizes within 2–3 seconds. Because the oscillation time of each magnetron is random, in a microwave system consisting of multiple magnetrons, each magnetron emits an electromagnetic signal at a different starting phase angle at a different time. When each signal is transmitted into the internal space of the cavity through the microwave feed port, electromagnetic field superpositions of different strengths and directions are ultimately generated at each point in the cavity space depending on the path and time the signal passes through. Phase differences between multiple microwave signals can cause constructive superposition or destructive cancellation, resulting in different electromagnetic field superposition results each time the power is turned on and microwaves are output. This can result in reduced reproducibility of the heating effect and uneven heating. In the present application, an impedance matching regulator corresponding to the microwave feed port is provided, so that the electromagnetic impedance of the microwave feed port 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, thereby reducing the electromagnetic field cancellation and uncontrollable electromagnetic field distribution caused by random phase difference, and realizing the best utilization efficiency of microwave energy.

[0019] Furthermore, the dielectric periodic structure has a cavity formed by a plurality of dielectric units, which is smaller than the metal cavity, and the carbon fiber can pass through the cavity of the dielectric periodic structure. The dielectric periodic structure has 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 arranged in parallel at equal intervals, and the installation direction of the dielectric units is perpendicular to or parallel to the forward direction of the carbon fiber, or is arranged at another angle.

[0020] By adopting the above technical solution, since the dielectric periodic structure affects the electromagnetic field distribution, a specific electromagnetic field distribution can be obtained by adjusting the position of the dielectric periodic structure in the metal cavity, and the microwave energy can be stably concentrated in the dielectric distribution area, thereby concentrating the microwave energy on the carbon fibers, so that the surface current density of each carbon fiber is the same or tends to be the same, and the range is controlled to be 60 to 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 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. 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 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.

[0022] By adopting the above technical solution, the surface current density of the carbon fiber in the microwave graphitization furnace can be adjusted to the range of 60-330 A / m, which helps improve the tensile modulus of the carbon fiber. Experiments have shown that the tensile modulus of the carbon fiber can be improved to 300-600 GPa, and the variance of the tensile modulus is less than 1.5%. Furthermore, this graphitization process reduces the diameter of the carbon fiber from 7.0 μm to 6.3 μm.

[0023] Furthermore, 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 from the surface of the large tow carbon fibers before microwave graphitization.

[0024] Furthermore, after microwave graphitization, the large tow carbon fiber is surface treated, sized again, dried and wound up to obtain a medium-high modulus large tow carbon fiber. [Effects of the Invention]

[0025] The beneficial effects of the present invention are as follows: In this invention, the in-phase microwave heating allows the carbon fiber to be heated to the graphitization temperature quickly and uniformly from the inside to the outside. Because microwaves can self-heat internally, the system requires less heat retention than conventional heating. Furthermore, the microwave electromagnetic field generates heat through the interaction of the microwaves with the entire carbon fiber, improving the uniformity of heating throughout the carbon fiber and allowing it to reach the graphitization temperature more quickly. Microwave heating increases the internal energy of carbon atoms (increasing the transition frequency), accelerating the formation rate of graphite layers in the carbon fiber, shortening the high-temperature treatment time, and helping to maintain fiber strength. Furthermore, the dielectric periodic structure controls the surface current density of the carbon fiber in the graphitization furnace, adjusting the surface current density of each carbon fiber so that it is uniform or tends to be uniform. This creates a specific magnetic field, concentrating microwave energy on the carbon fiber, improving microwave energy utilization and process efficiency. Through this graphitization process, the tensile modulus of the large-tow carbon fiber reaches 300-600 GPa, but the tensile strength remains at 3.5-5.0 GPa. The dispersion of the tensile modulus of the carbon fiber is less than 1.5%, and the product performance is uniform. [Brief explanation of the drawings]

[0026] [Figure 1] FIG. 1 is a comparison of the thermal conduction of carbon fiber between conventional heating and microwave heating. [Figure 2] FIG. 1 is a front schematic view of a microwave graphitization furnace. [Figure 3] FIG. 2 is a left-side schematic view of a microwave graphitization furnace. [Figure 4] Schematic diagram of the non-thermal effect of periodic structures and in-phase microwaves. [Figure 5a] FIG. 1 is a schematic diagram simulating a case where the current density on the surface of a carbon fiber is 170 A / m. [Figure 5b] FIG. 5b is a diagram of the actual heating effect of the carbon fiber corresponding to FIG. 5a. [Figure 6] FIG. 1 is a diagram showing the relationship between the tensile modulus of a carbon fiber and the surface current density of the carbon fiber. [Figure 7]FIG. 1 is a diagram comparing the diameters of carbon fibers before and after microwave graphitization. DETAILED DESCRIPTION OF THE INVENTION

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

[0028] Furthermore, the technical features according to different embodiments of the present application described below may be combined with each other unless they are inconsistent with each other.

[0029] The present application provides a method for producing medium- to high-modulus large tow carbon fibers, which comprises graphitizing low-modulus large tow carbon fibers with microwaves, specifically, by subjecting the large tow carbon fibers to in-phase microwave heating in a microwave graphitization furnace, and simultaneously adjusting the surface current density of the low-modulus large tow 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 being in the range of 60 to 330 A / m, thereby rapidly increasing the surface temperature of the large tow 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 tow carbon fibers.

[0030] The microwave heating used in this application differs from conventional heating in that the heating process is different. As shown in Figures 1a and 1b, 1a is conventional heating, where heat flow is from the outside to the inside, while 1b is microwave heating, where heat flow is 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, and increases the transition frequency of carbon atoms, promoting the formation of carbon fiber graphite layers, shortening the high-temperature treatment time and helping to maintain the strength of the carbon fiber. Furthermore, this application uses in-phase microwaves, meaning that signals from each microwave feed port are radiated into the microwave processing cavity with the same phase and intensity, reducing electromagnetic field cancellation and uncontrollable electromagnetic field distribution due to random phase differences, thereby achieving uniform heating of the carbon fiber in the microwave graphitization furnace.

[0031] By adjusting the surface current density of the carbon fiber in the microwave graphitization furnace to a range of 60-330 A / m using in-phase microwaves, and adjusting the surface current density of each carbon fiber to be the same or tend to be the same, the high temperature generated by the heating effects of the same dielectric loss and ohmic loss causes the carbon fiber to reorganize its molecular structure, crystallize, and quickly form a high-modulus structure, reducing the carbon fiber diameter from 7.0 μm to 6.3 μm and rapidly increasing the tensile modulus of the carbon fiber to 300-600 GPa. However, if the surface current density exceeds 330 A / m, the microwave electric field will cause a serious tip discharge effect on the carbon fiber, causing the local temperature to exceed 3000°C, resulting in overheating, significantly reducing the tensile strength of the carbon fiber, and weakening the improvement in tensile modulus.

[0032] The microwave graphitization furnace adopts an in-phase microwave design and is equipped with a dielectric periodic structure inside, which adjusts the surface current density of the carbon fiber, and the in-phase microwave and the dielectric periodic structure installed inside the furnace work together to stably concentrate microwave energy in the distribution area of ​​the dielectric periodic structure, thereby improving the stability and efficiency of carbon fiber heating.

[0033] 2 and 3 , a microwave graphitization furnace used in the present application is shown, comprising a metal cavity 2, an even number of microwave feed ports 3 evenly distributed on the top and bottom surfaces of the metal cavity 2 to supply microwaves into the metal cavity 2, and the microwave feed ports 3 are also provided with corresponding impedance matching regulators 4 to adjust the electromagnetic impedance input into the microwave graphitization furnace and synchronize the phase of the microwaves input into the furnace to achieve optimal utilization efficiency of microwave energy. A ceramic support 6 is provided within the metal cavity 2, and a dielectric periodic structure is provided on the ceramic support 6. The dielectric periodic structure includes a plurality of dielectric units 5 forming a cavity smaller in size than the metal cavity 2, and a carbon fiber 1 can pass through the cavity formed by 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, and the upper top surface and the lower bottom surface each include a plurality of dielectric units 5 arranged in parallel at equal intervals, and the installation direction of the dielectric units 5 is perpendicular to the forward movement direction of the carbon fibers 1. Furthermore, the installation direction of the dielectric units 5 on the upper top surface and the lower bottom surface of the dielectric periodic structure may also be parallel to the forward movement direction of the carbon fibers 1 or may be arranged at another angle. The material of the periodic structure is graphite, silicon carbide, or a combination related to carbides.

[0034] The distance between the top and bottom surfaces of the dielectric periodic structure in the microwave graphitization furnace and the carbon fiber is d1, and the distance between the top and bottom surfaces of the dielectric periodic structure and the microwave feed port on the same side is d2.

[0035] The dielectric periodic structure influences the electromagnetic field distribution. By adjusting the position of the periodic structure within the metal cavity, a specific electromagnetic field distribution can be tailored to focus microwave energy on the carbon fiber. Taking the dielectric periodic structure shown in Figures 2 and 3 as an example, i.e., the dielectric units on the top and bottom surfaces of the dielectric periodic structure are installed perpendicular to the direction of advancement of the carbon fiber. A schematic diagram of the periodic structure and the non-thermal effect of in-phase microwaves is shown in Figure 4. In a microwave graphitization furnace, the electromagnetic wave incident on the top surface 7 passes through the dielectric unit 5, forming a top-top incident electromagnetic diffraction peak 8 below the carbon fiber 1. The electromagnetic wave incident on the bottom surface 9 passes through the dielectric unit 5, forming a bottom-top incident electromagnetic diffraction peak 10 above the carbon fiber 1. The vertical distance D from any point on the top-top incident electromagnetic diffraction peak 8 or the bottom-top incident electromagnetic diffraction peak 10 to the carbon fiber represents the intensity of the electromagnetic diffraction peak; the greater the distance D, the greater the intensity of the diffraction peak. Both the top-incident electromagnetic wave diffraction peak 8 and the bottom-incident electromagnetic wave diffraction peak 10 are projected onto the carbon fiber 1, forming a region 11 with high ohmic loss heating efficiency. Furthermore, the carbon fiber advances horizontally, maintaining uniform heating and maximizing the ohmic loss effect of the carbon fiber. Furthermore, 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, strengthening the ohmic loss heating effect, improving energy utilization, and increasing the tensile modulus of the carbon fiber.

[0036] Carbon fibers in in-phase microwave heating.

number

[0037] Carbon fiber in conventional heating

number

[0038] where ε s : Radiation coefficient of dielectric material 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 ​​the dielectric I: Surface current of carbon fiber R: Radius of carbon fiber T S :Temperature of the dielectric

[0039] Comparing the above equations, it can be seen that the heating efficiency of in-phase microwaves includes ohmic loss heating, compared with conventional heating, which significantly improves the heating efficiency and heating uniformity of carbon fiber.

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

[0041] Additionally, the following examples 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 herein.

[0042] The comparative example is the original large tow carbon fiber, a PAN-based large tow carbon fiber manufactured by SGL, Germany, with a model of 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 using microwaves at different surface current densities. Ten samples of each type were randomly selected, 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] JPEG2025536483000007.jpg143123

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

[0045] Furthermore, the inventors have found that the high temperatures generated by the microwave graphitization method of the present invention, due to the heating effects of the same dielectric loss and ohmic loss, cause the molecular structure of the carbon fiber to reorganize, causing the molecules to crystallize and rapidly form a high-elasticity structure, which results in a decrease in the average diameter from 7.0 μm to 6.3 μm. A comparison of the diameter change of carbon fiber before and after graphitization is shown in FIG. 7.

[0046] Furthermore, with regard to the adjustment of the surface current density of the carbon fiber, when the periodic structure material is graphite or silicon carbide, the positional relationship of the dielectric periodic structure in the metal cavity of the microwave graphitization furnace is shown in Table 3.

[0047] [Table 3]

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

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

[0050] As described above, the method for producing medium-high modulus large tow carbon fiber according to the present invention can stably improve the modulus of low modulus large tow carbon fiber to a medium-high modulus, and has made great progress in the technical field of producing medium-high modulus large tow carbon fiber both domestically and internationally.

[0051] Obviously, the above examples are merely illustrative for clarifying the present application, and are not intended to limit the embodiments. Those skilled in the art can make various other changes or modifications based on the above description. This is not necessary, and it is not possible to cover all embodiments. And, obvious changes or modifications derived therefrom still fall within the scope of protection of the present invention. [Explanation of symbols]

[0052] 1. Carbon fiber 2 Metal Cavity 3 microwave feed ports 4 Impedance matching regulator 5 Dielectric Unit 6 Ceramic support 7. Electromagnetic waves incident on the upper surface 8. Electromagnetic wave diffraction peaks incident on the upper surface 9. Electromagnetic waves incident on the bottom surface 10 Electromagnetic wave diffraction peaks incident on the bottom surface 11. High-efficiency heating region of ohmic loss in carbon fiber

Claims

1. A method for producing medium-high modulus large tow carbon fiber, comprising:

1. A method for producing medium- to high-modulus large tow carbon fiber, comprising the steps of: graphitizing low-modulus large tow carbon fiber with microwaves; specifically, performing in-phase microwave heating on the low-modulus large tow carbon fiber in a microwave graphitization furnace; and simultaneously adjusting the surface current density of the low-modulus large tow carbon fiber 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 being in the range of 60 to 330 A / m, thereby rapidly and uniformly increasing the surface temperature of the large tow carbon fiber in the microwave graphitization furnace to 2000 to 3000°C, thereby uniformly carrying out the graphitization process and obtaining medium- to high-modulus large tow carbon fiber.

2. 2. The method for producing medium- to high-modulus large tow carbon fiber according to claim 1, wherein the microwave graphitization furnace adopts an in-phase microwave design and is provided with a dielectric periodic structure, the in-phase microwave design and the dielectric periodic structure allow microwave energy to be stably concentrated and distributed in the distribution area of ​​the dielectric periodic structure, and the current density on the surface of the large tow carbon fiber in the microwave graphitization furnace is adjusted by the dielectric periodic structure.

3. 3. The method for producing a medium-to-high modulus large tow carbon fiber according to claim 2, wherein 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 an upper top surface and a 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, which match and adjust the electromagnetic impedances of the even number of microwave feed ports, and then synchronize the microwave phases input into the graphitization microwave furnace to achieve optimal utilization efficiency of microwave energy. The dielectric periodic structure provided in the metal cavity includes a plurality of dielectric units to form a cavity smaller than the metal cavity, and the carbon fiber can pass through the cavity of the dielectric periodic structure.

4. 4. The method for producing a 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 arranged in parallel at equal intervals.

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

6. 6. The method for producing a medium-to-high modulus large tow carbon fiber according to claim 5, wherein 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. 6. The method for producing a medium-to-high elastic modulus large tow carbon fiber according to claim 5, wherein 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. 2. The method for producing medium- to high-modulus large tow carbon fibers according to claim 1, wherein the sizing agent on the surface of the low-modulus large tow carbon fibers must be removed before the low-modulus large tow carbon fibers are graphitized by microwaves.

9. 2. The method for producing medium- to high-modulus large tow carbon fiber according to claim 1, wherein the low-modulus large tow carbon fiber is graphitized by microwaves, and then the medium- to high-modulus large tow carbon fiber is surface-treated, sized again, dried, and wound up.

10. A medium-high elastic modulus large tow carbon fiber according to the manufacturing method of claims 1 to 9, A medium-high modulus large tow carbon fiber characterized in that the tensile modulus can reach 300-600 GPa, the tensile strength is 3.5-5.0 GPa, and the dispersion of the tensile modulus of the carbon fiber is less than 1.5%.

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