A modified polyacrylonitrile-based carbon fiber and a preparation method thereof

Abstract

The present invention provides a modified polyacrylonitrile-based carbon fiber and a preparation method thereof, and relates to the technical field of carbon fiber. The modified polyacrylonitrile-based carbon fiber preparation method comprises the following steps: step 1, mixing the graphene / bagasse mixture with the polyacrylonitrile spinning solution evenly to form a spinning solution; step 2, making the spinning solution into a modified polyacrylonitrile precursor through a spinning process; step 3, pre-oxidizing the modified polyacrylonitrile precursor, and then performing low-temperature carbonization and high-temperature carbonization treatment in a protective atmosphere to obtain a modified polyacrylonitrile-based carbon fiber. The present invention solves the problem of graphene agglomeration by mixing graphene with bagasse, and the graphene is attached to the reticular pores of the bagasse. The bagasse can be evenly mixed with the polyacrylonitrile spinning solution, so that the graphene is evenly dispersed, so that the graphene effectively improves the comprehensive performance of the carbon fiber.

Description

Technical Field

[0001] The invention relates to the technical field of carbon fiber, and in particular to a modified polyacrylonitrile-based carbon fiber and a preparation method thereof. Background Art

[0002] Carbon fiber refers to high-strength and high-modulus fibers with a carbon content of more than 90%. Carbon fiber is mainly composed of carbon elements and has the characteristics of high temperature resistance, friction resistance, thermal conductivity and corrosion resistance. It is fibrous, soft, and can be processed into various fabrics. Because its graphite microcrystalline structure is preferentially oriented along the fiber axis, it has high strength and modulus along the fiber axis. Carbon fiber has a low density, so its specific strength and specific modulus are high. The main use of carbon fiber is as a reinforcing material combined with resin, metal, ceramic and carbon to manufacture advanced composite materials. Carbon fiber reinforced epoxy resin composite materials have the highest specific strength and specific modulus among existing engineering materials. The diameter of carbon fiber is only 5 microns, which is equivalent to one tenth to one twelfth of a hair, but its strength is more than 4 times that of aluminum alloy.

[0003] At present, the main products of carbon fiber are divided into three categories: polyacrylonitrile-based, asphalt-based and viscose-based. Each type of product is divided into many varieties due to the different types of raw fibers, processes and final carbon fiber properties. Due to its excellent performance, polyacrylonitrile carbon fiber has become an advanced composite material reinforcement and has been widely used in aviation, aerospace, general industry and sports and leisure. In recent years, people's demand for new carbon fibers with special functions has continued to increase. Graphene, as a new type of carbon material, has excellent mechanical properties. Researchers have tried to evenly blend it with spinning solution to prepare composite carbon fiber materials in order to improve the comprehensive performance of carbon fiber materials. However, the study found that the graphene materials on the market are difficult to blend evenly with the spinning solution. There are problems such as uneven dispersion of graphene and easy agglomeration, which makes it difficult for graphene to effectively improve the comprehensive performance of carbon fiber. Summary of the invention

[0004] In order to solve the problem that the graphene materials on the market mentioned in the background technology are difficult to be evenly blended with the spinning solution, there are problems such as uneven dispersion of graphene and easy agglomeration, which makes it difficult for graphene to effectively improve the comprehensive performance of carbon fiber. The present invention solves the problem of graphene agglomeration by mixing graphene with bagasse, and the graphene is attached to the mesh pores of the bagasse. The bagasse can be evenly mixed with the polyacrylonitrile spinning solution, so that the graphene is evenly dispersed, so that the graphene effectively improves the comprehensive performance of carbon fiber.

[0005] The specific scheme is: a method for preparing modified polyacrylonitrile-based carbon fiber, comprising the following steps:

[0006] Step 1, mixing the graphene / bagasse mixture with the polyacrylonitrile spinning solution to form a spinning solution;

[0007] Step 2, preparing modified polyacrylonitrile precursor by spinning the spinning solution;

[0008] Step 3: pre-oxidize the modified polyacrylonitrile precursor, and then perform low-temperature carbonization and high-temperature carbonization treatment in a protective atmosphere to obtain modified polyacrylonitrile-based carbon fibers.

[0009] When implementing the above embodiment, preferably, the mass ratio of graphene to bagasse in the graphene / bagasse mixture is 0.1-0.5:2-3.5.

[0010] When implementing the above embodiment, preferably, the preparation process of the graphene / bagasse mixture is to treat the bagasse with boiling water, wash it with distilled water until it is neutral, dry it, and crush it; stir and mix the bagasse powder and nano-graphene according to a ratio, and obtain it after stirring in a stirring barrel.

[0011] When implementing the above embodiment, preferably, the mass ratio of the graphene / bagasse mixture to the polyacrylonitrile in the polyacrylonitrile spinning solution is 5 to 14:100.

[0012] When implementing the above embodiment, preferably, the spinning process in step 2 adopts a wet spinning process, and the process is as follows: the spinning solution is subjected to coagulation molding, primary drawing, water washing, oiling, drying and densification, secondary drawing and heat setting processes to obtain modified polyacrylonitrile raw fibers.

[0013] When implementing the above embodiment, preferably, in step three, the pre-oxidation temperature is 200-300° C., the time is 50-80 min, and the drawing ratio applied to the filament bundle during the pre-oxidation treatment is 1.0-1.1.

[0014] When implementing the above embodiment, preferably, in step three, the low-temperature carbonization process uses high-purity nitrogen as the protective gas, the low-temperature carbonization temperature is 300°C to 700°C, the time is 3 to 6 minutes, and the drawing ratio applied to the filament bundle during the carbonization treatment is 0.85 to 1.1.

[0015] When implementing the above embodiment, preferably, in step three, the high-temperature carbonization process uses high-purity nitrogen as the protective gas, the high-temperature carbonization temperature is 1100-1300°C, the time is 1-4 minutes, and the drawing ratio applied to the filament bundle during the carbonization treatment is 0.85-1.1.

[0016] On the other hand, a modified polyacrylonitrile-based carbon fiber of the present invention is prepared by the modified polyacrylonitrile-based carbon fiber preparation method as described above.

[0017] Furthermore, the modified polyacrylonitrile-based carbon fiber has a diameter of 4 to 7 μm, a tensile strength of 5.5 to 7 GPa, and a tensile modulus of 600 to 980 GPa.

[0018] Compared with the prior art, the beneficial features of the present invention include:

[0019] 1. The modified polyacrylonitrile-based carbon fiber preparation method of the present invention compounds nano-graphene with bagasse with a large sheet diameter ratio. The nano-graphene is evenly arranged on the sugarcane sheets, which can effectively solve the problem of serious agglomeration and difficulty in dispersion of nano-graphene. The bagasse powder can be evenly dispersed in the polyacrylonitrile spinning solution, so that the graphene can fully play a role in the carbon fiber modification, and the bagasse can be used as a raw material in the carbon fiber process.

[0020] 2. The modified polyacrylonitrile-based carbon fiber preparation method of the present invention adopts a wet spinning process, which can provide more overlapping points for the fibers, provide meshing effects between the fibers or increase the friction of the overlapping surfaces, and can improve the chemical stability, thermal stability and strength requirements of the polyacrylonitrile-based carbon fiber, and has a significant effect in improving the flexibility of the carbon fiber.

[0021] 3. The modified polyacrylonitrile-based carbon fiber preparation method of the present invention enables graphene to effectively improve the comprehensive performance of carbon fiber, wherein the tensile strength can reach 5.5-7GPa, and the tensile modulus is 600-980GPa, which is significantly improved compared with the existing carbon fiber materials. It has broad application prospects in aerospace, machinery, electronics, medical treatment, sports and other fields. At the same time, the preparation process of the graphene-modified composite carbon fiber provided by the present invention is simple, controllable, and suitable for large-scale production. DETAILED DESCRIPTION

[0022] In order to make the purpose, technical solution and advantages of the embodiments of the present invention clearer, the technical solution in the embodiments of the present invention will be clearly and completely described below in combination with the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0023] Some of the drugs and their actual descriptions of the embodiments of the present invention and their comparative examples are as follows:

[0024] Polyacrylonitrile spinning solution: 10kg acrylonitrile (Shandong Nice Chemical Co., Ltd.), 0.2kg itaconic acid (Shanghai Yien Chemical Technology Co., Ltd.), 0.06kg azobisisobutyronitrile (Shandong Wantai Chemical Co., Ltd.) and 40kg dimethyl sulfoxide (Shandong Fit Chemical Technology Co., Ltd.) were copolymerized at 52°C to obtain a copolymer with a solid content of 18.6% and a viscosity-average molecular weight of 147,000. After de-monomerization and degassing, the copolymer was placed in a storage kettle for standby use.

[0025] Graphene / bagasse mixture: boil bagasse in water for 1-5h, wash with distilled water until neutral, filter, and then dry at 60°C. After drying, crush and sieve through 80-200 mesh. Add graphene (nanograde, Henan Liugong Graphite Co., Ltd.) and bagasse powder in a mixing barrel at a ratio of 0.3:2, stir at 50rpm for 5min for use.

[0026] It should be noted that, if no specific techniques or conditions are specified in the examples, the techniques or conditions described in the literature in the field or the product instructions are used, and the reagents or instruments used without indicating the manufacturer are all conventional products that can be obtained commercially.

[0027] Example 1

[0028] A method for preparing modified polyacrylonitrile-based carbon fiber comprises the following steps:

[0029] Step 1: Mix the graphene / bagasse mixture with the polyacrylonitrile spinning solution at a stirring rate of 100 r / min, stir evenly, and form a spinning solution; the mass ratio of the graphene / bagasse mixture to the polyacrylonitrile in the polyacrylonitrile spinning solution is 5:100;

[0030] Step 2, adopting a wet spinning process, subjecting the spinning solution to coagulation, primary drawing, water washing, oiling, drying and densification, secondary drawing and heat setting processes to obtain modified polyacrylonitrile precursor; wherein the drawing medium of the primary drawing is water vapor at 100° C., and the drawing multiple is 6 times; the drawing medium of the secondary drawing is water vapor at 120° C., and the drawing multiple is 1.5 times; the heat setting temperature is 160° C.;

[0031] Step 3, (1) pre-oxidation treatment: the temperatures of the heating zones are 220, 230, 240, 250, and 260° C., respectively. The residence time of the modified polyacrylonitrile precursor in each heating zone is 10 min. The pre-oxidation treatment time is 50 min in total. The draft ratios applied to the modified polyacrylonitrile precursor in each heating zone are 1.01, 1.01, 1, 0.98, and 0.95, respectively.

[0032] (2) Low-temperature carbonization: high-purity nitrogen is used as the protective gas, and the oxygen content in the nitrogen is less than 5 ppm. The obtained pre-oxidized fiber is carbonized at a temperature of 500°C for 3 min. The draft ratio applied to the tow during the carbonization process is 0.93;

[0033] (3) High-temperature carbonization: High-purity nitrogen is used as the protective gas, and the oxygen content in the nitrogen is less than 3 ppm. The obtained low-temperature carbonized fiber is carbonized at a high temperature of 1100°C for 1 min. The drawing ratio applied to the tow during the carbonization treatment is 1.01 to obtain modified polyacrylonitrile-based carbon fiber.

[0034] Example 2

[0035] A method for preparing modified polyacrylonitrile-based carbon fiber comprises the following steps:

[0036] Step 1: Mix the graphene / bagasse mixture with the polyacrylonitrile spinning solution at a stirring rate of 100 r / min, stir evenly, and form a spinning solution; the mass ratio of the graphene / bagasse mixture to the polyacrylonitrile in the polyacrylonitrile spinning solution is 7:100;

[0037] Step 2, adopting a wet spinning process, subjecting the spinning solution to coagulation, primary drawing, water washing, oiling, drying and densification, secondary drawing and heat setting processes to obtain modified polyacrylonitrile precursor; wherein the drawing medium of the primary drawing is water vapor at 100° C., and the drawing multiple is 6 times; the drawing medium of the secondary drawing is water vapor at 120° C., and the drawing multiple is 1.5 times; the heat setting temperature is 160° C.;

[0038] Step 3, (1) pre-oxidation treatment: the temperatures of the heating zones are 220, 230, 240, 250, and 260° C., respectively. The residence time of the modified polyacrylonitrile precursor in each heating zone is 10 min. The pre-oxidation treatment time is 50 min in total. The draft ratios applied to the modified polyacrylonitrile precursor in each heating zone are 1.01, 1.01, 1, 0.98, and 0.95, respectively.

[0039] (2) Low-temperature carbonization: high-purity nitrogen is used as the protective gas, and the oxygen content in the nitrogen is less than 5 ppm. The obtained pre-oxidized fiber is carbonized at a temperature of 500°C for 3 min. The draft ratio applied to the tow during the carbonization process is 0.93;

[0040] (3) High-temperature carbonization: High-purity nitrogen is used as the protective gas, and the oxygen content in the nitrogen is less than 3 ppm. The obtained low-temperature carbonized fiber is carbonized at a high temperature of 1100°C for 1 min. The drawing ratio applied to the tow during the carbonization treatment is 1.01 to obtain modified polyacrylonitrile-based carbon fiber.

[0041] Example 3

[0042] A method for preparing modified polyacrylonitrile-based carbon fiber comprises the following steps:

[0043] Step 1: Mix the graphene / bagasse mixture with the polyacrylonitrile spinning solution at a stirring rate of 100 r / min and stir evenly to form a spinning solution; the mass ratio of the graphene / bagasse mixture to the polyacrylonitrile in the polyacrylonitrile spinning solution is 10:100;

[0044] Step 2, adopting a wet spinning process, subjecting the spinning solution to coagulation, primary drawing, water washing, oiling, drying and densification, secondary drawing and heat setting processes to obtain modified polyacrylonitrile precursor; wherein the drawing medium of the primary drawing is water vapor at 100° C., and the drawing multiple is 6 times; the drawing medium of the secondary drawing is water vapor at 120° C., and the drawing multiple is 1.5 times; the heat setting temperature is 160° C.;

[0045] Step 3, (1) pre-oxidation treatment: the temperatures of the heating zones are 220, 230, 240, 250, and 260° C., respectively. The residence time of the modified polyacrylonitrile precursor in each heating zone is 10 min. The pre-oxidation treatment time is 50 min in total. The draft ratios applied to the modified polyacrylonitrile precursor in each heating zone are 1.01, 1.01, 1, 0.98, and 0.95, respectively.

[0046] (2) Low-temperature carbonization: high-purity nitrogen is used as the protective gas, and the oxygen content in the nitrogen is less than 5 ppm. The obtained pre-oxidized fiber is carbonized at a temperature of 500°C for 3 min. The draft ratio applied to the tow during the carbonization process is 0.93;

[0047] (3) High-temperature carbonization: High-purity nitrogen is used as the protective gas, and the oxygen content in the nitrogen is less than 3 ppm. The obtained low-temperature carbonized fiber is carbonized at a high temperature of 1100°C for 1 min. The drawing ratio applied to the tow during the carbonization treatment is 1.01 to obtain modified polyacrylonitrile-based carbon fiber.

[0048] Example 4

[0049] A method for preparing modified polyacrylonitrile-based carbon fiber comprises the following steps:

[0050] Step 1: Mix the graphene / bagasse mixture with the polyacrylonitrile spinning solution at a stirring rate of 100 r / min, stir evenly, and form a spinning solution; the mass ratio of the graphene / bagasse mixture to the polyacrylonitrile in the polyacrylonitrile spinning solution is 14:100;

[0051] Step 2, adopting a wet spinning process, subjecting the spinning solution to coagulation, primary drawing, water washing, oiling, drying and densification, secondary drawing and heat setting processes to obtain modified polyacrylonitrile precursor; wherein the drawing medium of the primary drawing is water vapor at 100° C., and the drawing multiple is 6 times; the drawing medium of the secondary drawing is water vapor at 120° C., and the drawing multiple is 1.5 times; the heat setting temperature is 160° C.;

[0052] Step 3, (1) pre-oxidation treatment: the temperatures of the heating zones are 220, 230, 240, 250, and 260° C., respectively. The residence time of the modified polyacrylonitrile precursor in each heating zone is 10 min. The pre-oxidation treatment time is 50 min in total. The draft ratios applied to the modified polyacrylonitrile precursor in each heating zone are 1.01, 1.01, 1, 0.98, and 0.95, respectively.

[0053] (2) Low-temperature carbonization: high-purity nitrogen is used as the protective gas, and the oxygen content in the nitrogen is less than 5 ppm. The obtained pre-oxidized fiber is carbonized at a temperature of 500°C for 3 min. The draft ratio applied to the tow during the carbonization process is 0.93;

[0054] (3) High-temperature carbonization: High-purity nitrogen is used as the protective gas, and the oxygen content in the nitrogen is less than 3 ppm. The obtained low-temperature carbonized fiber is carbonized at a high temperature of 1100°C for 1 min. The drawing ratio applied to the tow during the carbonization treatment is 1.01 to obtain modified polyacrylonitrile-based carbon fiber.

[0055] Comparative Example 1

[0056] A method for preparing polyacrylonitrile-based carbon fiber comprises the following steps:

[0057] Step 1, adopting a wet spinning process, subjecting the polyacrylonitrile spinning solution to coagulation, primary drawing, water washing, oiling, drying and densification, secondary drawing and heat setting processes to obtain modified polyacrylonitrile precursor; wherein the drawing medium of the primary drawing is water vapor at 100° C., and the drawing multiple is 6 times; the drawing medium of the secondary drawing is water vapor at 120° C., and the drawing multiple is 1.5 times; the heat setting temperature is 160° C.;

[0058] Step 2, (1) pre-oxidation treatment: the temperatures of the heating zones are 220, 230, 240, 250, and 260° C., respectively. The residence time of the modified polyacrylonitrile precursor in each heating zone is 10 min. The pre-oxidation treatment time is 50 min in total. The draft ratios applied to the modified polyacrylonitrile precursor in each heating zone are 1.01, 1.01, 1, 0.98, and 0.95, respectively.

[0059] (2) Low-temperature carbonization: high-purity nitrogen is used as the protective gas, and the oxygen content in the nitrogen is less than 5 ppm. The obtained pre-oxidized fiber is carbonized at a temperature of 500°C for 3 min. The draft ratio applied to the tow during the carbonization process is 0.93;

[0060] (3) High-temperature carbonization: High-purity nitrogen is used as the protective gas, and the oxygen content in the nitrogen is less than 3 ppm. The obtained low-temperature carbonized fiber is carbonized at a high temperature of 1100°C for 1 min. The drawing ratio applied to the tow during the carbonization treatment is 1.01 to obtain polyacrylonitrile-based carbon fiber.

[0061] Comparative Example 2

[0062] A method for preparing polyacrylonitrile-based carbon fiber comprises the following steps:

[0063] Step 1: Mix graphene and polyacrylonitrile spinning solution at a stirring rate of 100 r / min, stir evenly, and form a spinning solution; the mass ratio of graphene to polyacrylonitrile in the polyacrylonitrile spinning solution is 5:100;

[0064] Step 2, adopting a wet spinning process, subjecting the spinning solution to coagulation, primary drawing, water washing, oiling, drying and densification, secondary drawing and heat setting processes to obtain modified polyacrylonitrile precursor; wherein the drawing medium of the primary drawing is water vapor at 100° C., and the drawing multiple is 6 times; the drawing medium of the secondary drawing is water vapor at 120° C., and the drawing multiple is 1.5 times; the heat setting temperature is 160° C.;

[0065] Step 3, (1) pre-oxidation treatment: the temperatures of the heating zones are 220, 230, 240, 250, and 260° C., respectively. The residence time of the modified polyacrylonitrile precursor in each heating zone is 10 min. The pre-oxidation treatment time is 50 min in total. The draft ratios applied to the modified polyacrylonitrile precursor in each heating zone are 1.01, 1.01, 1, 0.98, and 0.95, respectively.

[0066] (2) Low-temperature carbonization: high-purity nitrogen is used as the protective gas, and the oxygen content in the nitrogen is less than 5 ppm. The obtained pre-oxidized fiber is carbonized at a temperature of 500°C for 3 min. The draft ratio applied to the tow during the carbonization process is 0.93;

[0067] (3) High-temperature carbonization: High-purity nitrogen is used as the protective gas, and the oxygen content in the nitrogen is less than 3 ppm. The obtained low-temperature carbonized fiber is carbonized at a high temperature of 1100°C for 1 min. The drawing ratio applied to the tow during the carbonization treatment is 1.01 to obtain polyacrylonitrile-based carbon fiber.

[0068] The performance of the polyacrylonitrile-based carbon fibers obtained in Comparative Examples 1-2 and Examples 1-4 was tested, and the results are shown in the following table:

[0069] Diameter(μm) Tensile strength(GPa) Tensile modulus(GPa) Electrical conductivity (S / m) Comparative Example 1 6.8 4.5 503 <![CDATA[6.3×10 4 ]]> Example 1 6.9 6.1 871 <![CDATA[1.6×10 5 ]]> Example 2 7 5.9 837 <![CDATA[1.6×10 5 ]]> Example 3 6.9 5.8 839 <![CDATA[1.5×10 5 ]]> Example 4 6.9 6.1 867 <![CDATA[1.7×10 5 ]]> Comparative Example 2 7.0 4.6 496 <![CDATA[8.6×10 4 ]]>

[0070] According to the above table, it can be found that compared with the examples, comparative example 1 has no graphene modification, and its tensile strength, tensile modulus and conductivity are all inferior to those of examples 1 to 4. Although comparative example 2 has added graphene for modification, it has the problem of difficult dispersion of graphene as described in the background technology, and the modification effect is not good.

[0071] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or replace some or all of the technical features therein with equivalents. However, these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for preparing modified polyacrylonitrile-based carbon fiber, characterized in that: The steps include: Step 1, mixing the graphene / bagasse mixture with the polyacrylonitrile spinning solution to form a spinning solution; Step 2, preparing modified polyacrylonitrile precursor by spinning the spinning solution; Step 3, pre-oxidizing the modified polyacrylonitrile precursor, and then performing low-temperature carbonization and high-temperature carbonization treatment in a protective atmosphere to obtain modified polyacrylonitrile-based carbon fibers; The mass ratio of graphene to bagasse in the graphene / bagasse mixture is 0.1-0.5:2-3.5; The preparation process of the graphene / bagasse mixture is as follows: after treating the bagasse with boiling water, washing it with distilled water until it is neutral, drying it, and crushing it; mixing the bagasse powder and the nano-graphene according to a proportion, and obtaining the mixture after stirring in a stirring barrel; The mass ratio of the graphene / bagasse mixture to the polyacrylonitrile in the polyacrylonitrile spinning solution is 5 to 14:100; The modified polyacrylonitrile-based carbon fiber has a diameter of 4 to 7 μm, a tensile strength of 5.5 to 7 GPa, and a tensile modulus of 600 to 980 GPa.

2. The method for preparing modified polyacrylonitrile-based carbon fiber according to claim 1, characterized in that: The spinning process in step 2 adopts a wet spinning process, and the process is as follows: the spinning solution is spun through a spinneret, coagulated, drawn once, washed, oiled, dried, drawn twice and heat-set to obtain modified polyacrylonitrile precursor.

3. The method for preparing modified polyacrylonitrile-based carbon fiber according to claim 1, characterized in that: In step three, the pre-oxidation temperature is 200-300° C., the time is 50-80 min, and the draft ratio applied to the filament bundle during the pre-oxidation treatment is 1.0-1.

1.

4. The method for preparing modified polyacrylonitrile-based carbon fiber according to claim 1, characterized in that: In step 3, high-purity nitrogen is used as the protective gas in the low-temperature carbonization process, the low-temperature carbonization temperature is 300° C. to 700° C., the time is 3 to 6 minutes, and the drawing ratio applied to the filament bundle during the carbonization treatment is 0.85 to 1.

1.

5. The method for preparing modified polyacrylonitrile-based carbon fiber according to claim 1, characterized in that: In step 3, high-temperature carbonization process uses high-purity nitrogen as protective gas, the high-temperature carbonization temperature is 1100-1300° C., the time is 1-4 minutes, and the drawing ratio applied to the filament bundle during the carbonization treatment is 0.85-1.

1.

6. A modified polyacrylonitrile-based carbon fiber, characterized in that: The modified polyacrylonitrile-based carbon fiber is prepared by the method for preparing the modified polyacrylonitrile-based carbon fiber as described in any one of claims 1 to 2.