Preparation method for medium-high-modulus large-tow carbon fibers

Abstract

A preparation method for medium-high-modulus large-tow carbon fibers. The preparation method comprises: subjecting low-modulus large-tow carbon fibers to in-phase microwave heating and carbon fiber surface current density regulation, wherein the current density regulation is within a range of 60-330 A / m, such that the carbon fibers rapidly reach a graphitization temperature, thereby completing a uniform and rapid graphitization process. The tensile modulus of the medium-high-modulus large-tow carbon fibers obtained by using the preparation method can reach 300-600 GPa, the tensile strength is maintained at 3.5-5.0 GPa, and the dispersion of the tensile modulus of the carbon fiber is less than 1.5%.

Description

A method for preparing medium-high modulus large-tow carbon fiber

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims priority to the Chinese patent application filed with the Patent Office of China on September 28, 2023, with application number 202311268510.2 and invention name “A method for preparing medium- and high-modulus large-tow carbon fiber”, the entire contents of which are incorporated herein by reference. Technical Field

[0003] The present application belongs to the technical field of large-tow carbon fiber graphitization, and in particular relates to a method for preparing medium- and high-modulus large-tow carbon fibers. Background Art

[0004] Due to its high strength and high modulus, carbon fiber is often used as a carbon reinforcement material to prepare high-performance composite materials. Generally, carbon fiber can be divided into small-tow carbon fiber (1-24K) and large-tow carbon fiber (24-480K) according to the number of carbon fiber filaments in the carbon fiber bundle. Small-tow carbon fiber (1-24K) is a traditional carbon fiber product with excellent mechanical properties, a tensile strength of 3500-7000MPa, and a tensile modulus of 230-680GPa. Those with a tensile modulus greater than 280GPa are generally called medium-modulus carbon fibers, while those with a tensile modulus greater than 350GPa are called high-modulus carbon fibers. Medium- and high-modulus carbon fibers are mainly used in aerospace, high-end industry, high-end sports and other product fields; they have high technical barriers, their prices are more than 5-10 times that of general low-modulus carbon fibers, and they are not easy to obtain, so they have great development potential.

[0005] The tensile strength of large-tow carbon fiber (24-480K) is 3500-5000MPa, and the tensile modulus is 230-280GPa. Large-tow carbon fiber has the advantages of large single-line production capacity, can reduce the carbon fiber cost by about 40%, the number of layers required for processing composite materials is small, and it is easy to obtain. In recent years, it has gradually replaced small-tow carbon fiber in the fields of automobiles, wind turbine blades, energy buildings, shipbuilding industry and sporting goods. However, the tensile strength, tensile modulus and other properties of large-tow carbon fiber are weaker than those of small-tow carbon fiber. They are all low-modulus carbon fibers, which limits the application of large-tow carbon fiber. The performance overview of large tow at home and abroad is shown in Table 1.

[0006] Table 1

[0007] In order to broaden the application field of large-tow carbon fibers, this patent combines the low-cost characteristics of large-tow carbon fibers and adopts innovative technology to lower the production threshold of medium- and high-modulus carbon fibers, quickly increasing the low-modulus (<280GPa) large-tow carbon fibers to medium- and high-modulus (300-600GPa), significantly improving the performance-cost ratio of carbon fibers. In traditional technology, the manufacture of medium- and high-modulus carbon fibers requires high-temperature graphitization at 2000-3000°C under specific conditions in an inert gas-protected furnace. However, there are few reports on the graphitization technology of medium- and high-modulus large-tow carbon fibers at home and abroad today, mainly due to the limitations of high-temperature graphitization furnaces. According to traditional heating methods, graphitization furnaces are divided into direct and indirect heating types. In the direct heating type, because the number of single-filament fibers of large-tow carbon fibers is too large and too thick, when the heat source is transferred from the heating element in the graphitization furnace to the carbon fibers, the single-filament carbon fibers are unevenly heated, resulting in different degrees of graphitization, which is prone to partial stress concentration in the fibers, resulting in brittle cracks, and the formation of hairs or breaks. Indirect graphitization furnaces, such as induction graphitization furnaces, use a graphite muffle to induce heating and then transfer heat to the carbon fiber. They have a short service life and high energy consumption in the graphitization process, making it difficult to reduce production costs. They can only be processed in small batches, and the heating temperature field is unevenly distributed on the fiber. The product performance is highly discrete, making continuous production impossible and it is difficult to achieve improved mechanical properties.

[0008] In addition, the arrangement defects of the microcrystalline layer of large-tow carbon fiber itself are relatively large, and the yarn is easily broken during the graphitization process. Therefore, there is currently no record of successful continuous production of large-tow medium- and high-modulus carbon fiber at home and abroad.

[0009] Summary of the Invention

[0010] The purpose of this application is to perform a microwave graphitization process on low modulus large-tow carbon fibers so that the tensile modulus of the large-tow carbon fibers can reach 300-600 GPa, the tensile strength is maintained at 3.5-5.0 GPa, and the dispersion of the carbon fiber tensile modulus is less than 1.5%, thereby obtaining medium- and high-modulus large-tow carbon fibers with uniform product performance.

[0011] To achieve the above technical objectives, the present application provides a method for preparing medium-to-high modulus large-tow carbon fibers, comprising:

[0012] The microwave graphitization of low modulus large-tow carbon fibers is specifically carried out by performing in-phase microwave heating on the low modulus large-tow carbon fibers in a microwave graphitization furnace; at the same time, the current density on the surface of the low modulus large-tow carbon fibers in the graphitization furnace is regulated so that the current density on the surface of each carbon fiber in the microwave graphitization furnace is the same or tends to be the same, and the current density range is controlled within 60-330A / m; thereby, the surface temperature of the large-tow carbon fibers in the microwave graphitization furnace can be quickly and evenly increased to 2000-3000°C, completing a uniform graphitization process and obtaining medium- and high-modulus large-tow carbon fibers.

[0013] Definition of current density on the carbon fiber surface: In a microwave graphitization furnace, electromagnetic waves with time-varying electric fields and time-varying magnetic fields move in a limited space. The superposition of their forward waves and reflected waves forms a specific resonant electric field and resonant magnetic field distribution. When the carbon fiber is under the resonant distribution of electromagnetic waves, the time-varying magnetic field at its location generates many localized eddy induced currents inside it. At this time, the current flux on a unit length (1 meter) of carbon fiber is equivalent to the current density on the carbon fiber surface.

[0014] In the above technical solution, because carbon fiber is a high dielectric loss material in the microwave band, it can convert the penetrating microwave energy into heat with high efficiency, achieving the advantages of rapid temperature rise and energy saving. The heating is mainly due to the polarization loss of the dipole and a small amount of conductivity loss. In the microwave electromagnetic field, the orientation of the dipoles in the carbon fiber rotates and tends to be consistent, and the frequency of the polarization can reach hundreds of millions of times per second. The thermal kinetic energy generated can heat the carbon fiber. The heat is generated by the interaction between the microwave and the entire carbon fiber, so the heating efficiency can be greatly improved. In addition, the in-phase microwave heating adopted in this application can ensure that the microwave signal entering the process cavity is in phase, thereby achieving efficient and uniform heating of the carbon fiber in the furnace.

[0015] In addition, by adjusting the current density range on the carbon fiber surface, the current density on the surface of each carbon fiber in the microwave graphitization furnace can be controlled to be the same or approaching the same, which can evenly enhance the ohmic loss heating effect of the carbon fiber, improve the microwave energy utilization rate and process effect. The inventors found that when the current density on the carbon fiber surface is controlled within the range of 60-330A / m, each carbon fiber undergoes molecular structure reorganization due to the high temperature generated by the same dielectric loss and ohmic loss heating effect, and the molecular crystallization quickly forms a high modulus structure, with its diameter reduced from 7.0μm to 6.3μm, and the tensile modulus of the carbon fiber is rapidly increased to 300-600GPa. However, when the surface current density is too high, exceeding 330A / m, it will trigger a serious tip discharge effect of the carbon fiber in the microwave electric field, and the local temperature will exceed the maximum temperature of the carbon fiber of 3000℃, resulting in overburning, causing the carbon fiber structure to disintegrate, greatly reducing the tensile strength of the carbon fiber, and thus weakening the effect of improving the tensile modulus.

[0016] In summary, by using in-phase microwave heating and regulating the current density on the carbon fiber surface within a range of 60-330 A / m, the temperature of the large-tow carbon fiber can be uniformly increased to 2000-3000°C within 90 seconds; thereby completing a uniform graphitization process and uniformly improving the tensile modulus of the low-modulus large-tow carbon fiber.

[0017] Furthermore, the microwave graphitization furnace adopts a co-phase microwave design, and a dielectric periodic structure is provided in the microwave graphitization furnace.

[0018] By adopting the above technical solution, the microwave co-phase design and the dielectric periodic structure can stably concentrate the microwave energy in the distribution area of ​​the dielectric periodic structure; the current density on the surface of the large-tow carbon fiber in the microwave graphitization furnace is regulated by the dielectric periodic structure.

[0019] Furthermore, the microwave graphitization furnace includes a metal cavity, which 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 top and bottom surfaces of the metal cavity to provide microwaves into the metal cavity. The microwave feed ports are also provided with corresponding impedance matching adjusters. Generally, in microwave heating, the microwave function equation generated by each magnetron is as follows:

[0020] Where A is the amplitude, k is the waveguide number, x is the displacement, ω is the angular frequency, t is the time, and θ is the starting phase angle. When the power is turned on, high voltage is applied to the magnetron, generating an electromagnetic wave signal through electron cyclotron resonance (ECR). The output power reaches a stable level within 2-3 seconds. Because the start-up time of each magnetron is random, in a microwave system composed of multiple magnetrons, each magnetron emitter will radiate an electromagnetic wave signal with a different starting phase angle at a different time. After each signal is transmitted through the microwave feed port into the cavity, it will generate electromagnetic field superpositions of varying strengths and directions at various points within the cavity, depending on the path and time the signal travels. Due to the phase differences between multiple microwave signals, the resulting electromagnetic field superpositions may result in constructive addition or destructive cancellation. Therefore, each "power on - microwave output" cycle may produce different electromagnetic field superpositions, resulting in reduced heating reproducibility and uneven heating. In the present application, by providing a corresponding impedance matching adjuster at the microwave feed inlet, the electromagnetic impedance of the microwave feed inlet can be matched and adjusted so that the signal of each microwave feed inlet is radiated into the microwave process cavity with the same phase and intensity, reducing the electromagnetic field cancellation and uncontrollable electromagnetic field distribution caused by random phase differences, thereby achieving optimal microwave energy utilization efficiency.

[0021] Furthermore, the dielectric periodic structure is composed of a plurality of dielectric units forming a cavity smaller than the metal cavity; 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; the upper top surface and lower bottom surface each contain a plurality of equidistant and parallel dielectric units, with the dielectric units arranged perpendicularly, parallel, or at other angles to the direction of carbon fiber travel.

[0022] By adopting the above technical solution, the periodic structure of the dielectric affects the electromagnetic field distribution. By adjusting the position of the periodic structure of the dielectric in the metal cavity, a specific electromagnetic field distribution can be adjusted, and the microwave energy can be stably concentrated in the dielectric distribution area, thereby further concentrating the microwave energy on the carbon fiber, so that the current density on the surface of each carbon fiber can be the same or tend to be the same, and the range is controlled within 60-330A / m.

[0023] Furthermore, the dielectric unit is made of graphite, silicon carbide, or a combination of carbides. When the dielectric is made of graphite, the distance between the upper and lower surfaces of the periodic structure and the carbon fiber is 10-35 mm, and the distance between the upper and lower surfaces and the microwave feed port is 200-260 mm. When the dielectric is made of silicon carbide, the distance between the upper and lower surfaces of the periodic structure and the carbon fiber is 15-35 mm, and the distance between the upper and lower surfaces and the microwave feed port is 180-260 mm.

[0024] By adopting the above technical solution, the surface current density of carbon fibers in the microwave graphitization furnace can be controlled within the range of 60-330A / m, which is conducive to improving the tensile modulus of carbon fibers. Experiments have shown that the tensile modulus of carbon fibers can be increased to 300-600GPa, and the dispersion of the tensile modulus is less than 1.5%. In addition, during the graphitization process, the diameter of the carbon fibers is reduced from 7.0μm to 6.3μm.

[0025] Furthermore, before microwave graphitization, it is necessary to remove the sizing agent on the surface of the large-tow carbon fiber to maintain the purity of the carbon fiber and prevent other impurities from affecting the graphitization process.

[0026] Furthermore, after microwave graphitization, the large-tow carbon fiber needs to be surface treated, re-sized, dried and rolled up to obtain medium-to-high modulus large-tow carbon fiber.

[0027] Beneficial effects of the present application: In the present application, by heating with microwaves in phase, the carbon fiber can be quickly and evenly heated from the inside to the outside to the graphitization temperature; microwaves can generate internal self-heating, and the insulation requirement of the system is low compared to traditional heating; and in the microwave electromagnetic field, heat is generated by the interaction between microwaves and the entire carbon fiber, and the overall heating uniformity of the carbon fiber is improved, and the graphitization temperature can be quickly reached; microwave heating increases the internal energy of carbon atoms (transition frequency increases), accelerates the formation speed of the carbon fiber graphite layer, reduces the high-temperature treatment time, and is conducive to maintaining fiber strength; at the same time, a dielectric periodic structure is used to regulate the surface current density of the carbon fiber in the graphitization furnace, and the current density on the surface of each carbon fiber is regulated to be the same or tends to be the same, which can form a specific magnetic field, concentrate microwave energy on the carbon fiber, and improve microwave energy utilization and process effect. After the graphitization process, the tensile modulus of the large-tow carbon fiber can reach 300-600GPa, while the tensile strength is maintained at 3.5-5.0GPa, and the discreteness of the tensile modulus of the carbon fiber is less than 1.5%, and the product performance is uniform. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 is a comparison of heat conduction between conventional heating and microwave heating of carbon fibers;

[0029] FIG2 is a schematic front view of a microwave graphitization furnace;

[0030] FIG3 is a schematic diagram of a microwave graphitization furnace from the left;

[0031] FIG4 is a schematic diagram of the periodic structure and the athermal effect of in-phase microwaves;

[0032] Figure 5a is a schematic diagram of the simulation of the carbon fiber surface current density of 170A / m;

[0033] FIG5b is a diagram showing the actual heating effect of the carbon fiber corresponding to FIG5a;

[0034] FIG6 is a graph showing the relationship between the tensile modulus of carbon fiber and the surface current density of carbon fiber;

[0035] Figure 7: Diameter comparison of carbon fibers before and after microwave graphitization;

[0036] Among them: 1-carbon fiber; 2-metal cavity; 3-microwave feed port; 4-impedance matching adjuster; 5-dielectric unit; 6-ceramic support body; 7-upper top surface incident electromagnetic wave; 8-diffraction peak of upper top surface incident electromagnetic wave; 9-lower top surface incident electromagnetic wave; 10-diffraction peak of lower top surface incident electromagnetic wave; 11-carbon fiber ohmic loss heating high-efficiency area. DETAILED DESCRIPTION

[0037] The following will clearly and completely describe the technical solution of this application in conjunction with the accompanying drawings. Obviously, the embodiments described are part of the embodiments of this application, not all of them. Based on the embodiments in this application, all other embodiments obtained by ordinary technicians in this field without making any creative efforts are within the scope of protection of this application.

[0038] In addition, the technical features involved in the different embodiments of the present application described below can be combined with each other as long as they do not conflict with each other.

[0039] The present application provides a method for preparing medium-to-high modulus large-tow carbon fibers, comprising:

[0040] The microwave graphitization of low modulus large-tow carbon fibers is specifically carried out by performing in-phase microwave heating on the large-tow carbon fibers through a microwave graphitization furnace. At the same time, the current density on the surface of the low modulus large-tow carbon fibers is regulated so that the current density on the surface of each carbon fiber is the same or tends to be the same, and the current density range is controlled within 60-330A / m, so that the surface temperature of the large-tow carbon fibers in the microwave graphitization furnace is rapidly increased to 2000-3000°C within 90 seconds, completing the graphitization process and obtaining medium- and high-modulus large-tow carbon fibers.

[0041] The microwave heating in this application has a different heating process than traditional heating. See Figures 1a and 1b. 1a is traditional heating, and its heat flow is from outside to inside. 1b is microwave heating, and its heat flow is from inside to outside. Compared with traditional heating, microwave heating has low insulation requirements for the system, and microwave heating can increase the internal energy of carbon atoms, increase the transition frequency of carbon atoms, accelerate the formation speed of carbon fiber graphite layers, reduce high-temperature treatment time, and help maintain carbon fiber strength. In addition, this application uses in-phase microwaves, that is, the signal of each microwave feed port is radiated into the microwave process cavity with the same phase and intensity, reducing the electromagnetic field cancellation and uncontrollable electromagnetic field distribution caused by random phase differences, and can achieve uniform heating of carbon fibers in the microwave graphitization furnace.

[0042] On the basis of the same-phase microwave, by controlling the current density on the surface of the carbon fiber in the microwave graphitization furnace within the range of 60-330A / m, and controlling the current density on the surface of each carbon fiber to be the same or close to the same, the carbon fiber undergoes molecular structure reorganization at high temperatures generated by the same dielectric loss and ohmic loss heating effects, and the molecular crystallization quickly forms a high modulus structure. The diameter of the carbon fiber is reduced from 7.0μm to 6.3μm, and the tensile modulus of the carbon fiber is rapidly increased to 300-600GPa. However, when the surface current density exceeds 330A / m, it will trigger a severe tip discharge effect of the carbon fiber in the microwave electric field, and the local temperature will exceed the maximum temperature of the carbon fiber of 3000℃, resulting in overburning, which greatly reduces the tensile strength of the carbon fiber, thereby weakening the effect of improving the tensile modulus.

[0043] The microwave graphitization furnace uses a co-phase microwave design and is equipped with a periodic dielectric structure. This periodic dielectric structure allows for the regulation of the current density on the carbon fiber surface. Furthermore, the co-phase microwaves and the periodic dielectric structure within the furnace work together to stably concentrate microwave energy within the periodic dielectric structure, improving the stability and efficiency of carbon fiber heating.

[0044] Referring to Figures 2 and 3, the microwave graphitization furnace used in this application is shown, comprising a metal cavity 2, on which an even number of microwave feed ports 3 are provided. The even number of microwave feed ports 3 are evenly distributed on the upper and lower surfaces of the metal cavity 2 to provide microwaves into the metal cavity 2. The microwave feed ports 3 are also provided with corresponding impedance matching adjusters 4, which can adjust the electromagnetic impedance input into the microwave graphitization furnace so that the phase of the microwave input into the furnace is in phase, thereby achieving optimal microwave energy utilization efficiency. A ceramic support 6 is provided in the metal cavity 2, and a dielectric periodic structure is provided on the ceramic support 6. The dielectric periodic structure is composed of a plurality of dielectric units 5 forming a cavity smaller than the metal cavity 2, and the 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. The upper top surface and lower bottom surface are respectively composed of a plurality of equidistant and parallel dielectric units 5, and the arrangement direction of the dielectric units 5 is perpendicular to the travel direction of the carbon fibers 1. In addition, the arrangement direction of the dielectric units 5 on the upper top surface and lower bottom surface of the dielectric periodic structure can also be parallel to the travel direction of the carbon fibers 1, or arranged at other angles. The material of the periodic structure is graphite or silicon carbide, or a combination of carbides.

[0045] The distance between the upper top surface and lower bottom surface of the dielectric periodic structure in the microwave graphitization furnace and the carbon fiber is recorded as d1, and the distance between the upper top surface and lower bottom surface of the dielectric periodic structure and the microwave feeding port on the same side is recorded as d2.

[0046] The periodic structure of the dielectric can affect the distribution of the electromagnetic field. By adjusting the position of the periodic structure in the metal cavity, a specific electromagnetic field distribution can be adjusted to concentrate the microwave energy on the carbon fiber. Taking the periodic structure of the dielectric shown in Figures 2 and 3, that is, the arrangement direction of the dielectric units on the top and bottom surfaces of the periodic structure of the dielectric is perpendicular to the direction of travel of the carbon fiber as an example, its periodic structure and the schematic diagram of the non-thermal effect of the in-phase microwave are specifically shown in Figure 4. In the microwave graphitization furnace, the upper top surface incident electromagnetic wave 7 passes through the dielectric unit 5, and forms the upper top surface incident electromagnetic wave diffraction peak 8 below the carbon fiber 1. The lower top surface incident electromagnetic wave 9 passes through the dielectric unit 5, and forms the lower top surface incident electromagnetic wave diffraction peak 9 above the carbon fiber 1. Peak 10, 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 fiber represents the electromagnetic wave diffraction peak intensity. The larger the distance D, the greater the diffraction peak intensity. The upper top surface incident electromagnetic wave diffraction peak 8 and the lower top surface incident electromagnetic wave diffraction peak 10 are both projected onto the carbon fiber 1, forming a carbon fiber ohmic loss heating high-efficiency zone 11. Because the carbon fiber travels in a horizontal direction, the ohmic loss effect of the carbon fiber is maximized while maintaining heating uniformity. In addition, by regulating the position of the dielectric periodic structure within the metal cavity, the surface current density of the carbon fiber is controlled within a range of 60-330A / m, enhancing the ohmic loss heating effect, improving energy utilization, and increasing the tensile modulus of the carbon fiber.

[0047] Carbon fiber in in-phase microwave heating:

[0048] Carbon fiber in traditional heating:

[0049] in:

[0050] ε s : radiation coefficient of dielectric material;

[0051] d s : The horizontal distance between the dielectric and the center of the periodic structure, i.e., d1 in this application;

[0052] σ b : Stefan-Boltzmann constant;

[0053] A CF : carbon fiber area;

[0054] A s : dielectric area;

[0055] I: carbon fiber surface current;

[0056] R: carbon fiber radius;

[0057] T S: Dielectric temperature.

[0058] From the comparison of the above formulas, it can be seen that compared with traditional heating, the heating efficiency of in-phase microwaves also includes ohmic loss heating, which greatly improves the heating efficiency and heating uniformity of carbon fiber.

[0059] Referring to Figures 5a and 5b, Figure 5a shows a schematic diagram of a simulated carbon fiber surface current density of 170 A / m, while Figure 5b shows the actual heating effect of the carbon fiber in a microwave graphitization furnace. The white area represents the heat energy concentration zone. It can be seen that the simulated carbon fiber surface current density corresponds to the carbon fiber heating area in the microwave graphitization furnace.

[0060] Furthermore, the following examples further illustrate the effect of controlling the carbon fiber surface current density on the tensile modulus of the carbon fiber during the in-phase microwave heating process in this application:

[0061] The comparative example is a pristine large-tow carbon fiber, using PAN-based large-tow carbon fiber produced by SGL in Germany, model CT50-4.4, 50k, with a standard tensile modulus of 255 GPa. Ten samples were randomly selected and tested and recorded for performance data. The example is a carbon fiber obtained by microwave graphitizing the pristine large-tow carbon fiber at different carbon fiber surface current densities. Ten samples of each type were randomly selected and tested and recorded for performance data. A comparison of the performance data of the comparative example and the example is shown in Table 2.

[0062] Table 2

[0063] According to Table 2, a relationship diagram between the surface current density of carbon fiber and the tensile modulus is formed, as shown in Figure 6. Combining Table 2 and Figure 6, it can be seen that after the original carbon fiber is subjected to microwave graphitization, the tensile modulus of the carbon fiber changes, specifically: (1) When the surface current density of the carbon fiber is set to less than 60A / m for graphitization, the tensile modulus of the obtained carbon fiber is slightly improved; (2) When the surface current density of the carbon fiber is set to be in the range of 60-330A / m for graphitization, the tensile modulus of the obtained carbon fiber is significantly improved to 300-600GPa, and the discreteness of the tensile modulus can be controlled within 1.5%; (3) When the surface current density of the carbon fiber is set to be more than 330A / m for graphitization, the tensile modulus improvement effect of the carbon fiber begins to weaken.

[0064] In addition, the inventors found that through the microwave graphitization method of the present application, the carbon fiber undergoes molecular structure reorganization due to the high temperature generated by the same dielectric loss and ohmic loss heating effect, and the molecules crystallize quickly to form a high modulus structure, which causes the diameter to change. The diameter decreases from an average of 7.0 μm to 6.3 μm. The comparison of the diameter changes before and after graphitization of the carbon fiber is shown in Figure 7.

[0065] In addition, regarding the control of carbon fiber surface current density, when the periodic structure material is graphite or silicon carbide, the position relationship of the dielectric periodic structure in the metal cavity of the microwave graphitization furnace is shown in Table 3:

[0066] Table 3

[0067] It can be seen from Table 3 that by adjusting the position of the dielectric in the microwave graphitization furnace, the current density on the carbon fiber surface can be controlled. When the dielectric material is graphite, the optimal control range of d1 is 15-35 mm, and the optimal control range of d2 is 200-260 mm; when the dielectric material is silicon carbide, the optimal control range of d1 is 30-35 mm, and the optimal control range of d2 is 200-210 mm.

[0068] The present application also provides a medium-to-high modulus large-tow carbon fiber, whose tensile strength can reach 3.5-5.0 GPa, the tensile modulus can reach 300-600 GPa, and the dispersion of the carbon fiber tensile modulus is less than 1.5%.

[0069] In summary, the preparation method of medium- and high-modulus large-tow carbon fibers provided in this application can stably increase the modulus of low-modulus large-tow carbon fibers to medium- and high-modulus, which is a major breakthrough in the technical field of medium- and high-modulus large-tow carbon fibers produced at home and abroad.

[0070] Obviously, the above embodiments are merely examples for the purpose of clearly illustrating the present application and are not intended to limit the embodiments. Those skilled in the art will appreciate that other variations or modifications can be made based on the above description. It is not necessary and impossible to enumerate all embodiments here. Obvious variations or modifications derived therefrom remain within the scope of protection of the present invention.

Claims

1. A method for preparing medium-high modulus large-tow carbon fiber, characterized in that: include: The microwave graphitization of low modulus large tow carbon fiber is specifically carried out by performing in-phase microwave heating on the low modulus large tow carbon fiber through a microwave graphitization furnace; at the same time, the surface current density of the low modulus large tow carbon fiber in the microwave graphitization furnace is regulated so that the current density on the surface of each carbon fiber in the furnace is uniformly controlled to be the same or tends to be the same, and the current density ranges from 60 to 330 A / m, so that the surface temperature of the large tow carbon fiber in the microwave graphitization furnace is quickly and uniformly increased to 2000 to 3000°C, completing a uniform graphitization process and obtaining medium and high modulus large tow carbon fiber.

2. The method for preparing medium-high modulus large-tow carbon fiber according to claim 1, characterized in that: The microwave graphitization furnace adopts a co-phase microwave design, and a dielectric periodic structure is arranged in the microwave graphitization furnace; the microwave co-phase design and the dielectric periodic structure can stably and concentratedly distribute microwave energy in the dielectric periodic structure distribution area; the current density on the surface of the large-tow carbon fiber in the microwave graphitization furnace is regulated by the dielectric periodic structure.

3. The method for preparing medium-high modulus large-tow carbon fiber according to claim 2, characterized in that: The microwave graphitization furnace comprises a metal cavity, on which an even number of microwave feed ports are arranged, and 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 provide microwaves into the metal cavity; the microwave feed ports are also provided with corresponding impedance matching adjusters, and after matching and adjusting the electromagnetic impedances of the even number of microwave feed ports, the phases of the microwaves input into the graphitization microwave oven are made to be in the same phase, thereby achieving the best microwave energy utilization efficiency; the dielectric periodic structure arranged in the metal cavity is composed of a plurality of dielectric units to form a cavity smaller than the metal cavity, and carbon fibers can pass through the cavity of the dielectric periodic structure.

4. The method for preparing medium-high modulus large-tow carbon fiber according to claim 3, characterized in that: The dielectric periodic structure comprises 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 respectively comprise a plurality of dielectric units which are equidistantly and parallelly arranged.

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

6. The method for preparing medium-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 periodic structure and the carbon fiber 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.

7. The method for preparing medium-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 periodic structure and the carbon fiber is 15-35 mm, and the distance between the upper top surface and the lower bottom surface and the microwave feeding port is 180-260 mm.

8. The method for preparing medium-high modulus large-tow carbon fiber according to claim 1, characterized in that: Before microwave graphitization of low modulus large tow carbon fibers, the sizing agent on the surface of the low modulus large tow carbon fibers needs to be removed.

9. The method for preparing medium-high modulus large-tow carbon fiber according to claim 1, characterized in that: After microwave graphitization of low modulus large tow carbon fibers, medium and high modulus large tow carbon fibers need to be surface treated and re-sized before drying and winding.

10. A medium-high modulus large-tow carbon fiber, based on the preparation method of claims 1-9, 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 carbon fiber tensile modulus is less than 1.5%.

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