Pre-oxidized polyacrylonitrile fiber, preparation method therefor and use thereof

Abstract

The present disclosure provides a pre-oxidized polyacrylonitrile fiber, a preparation method therefor, and a use thereof. Within a strain range of 0-2.5%, the pre-oxidized polyacrylonitrile fiber has stress σ and strain ε values that satisfy the nonlinear fitting formula σ = Aε2 + Bε + C, and the elongation at break E of a pre-oxidized fiber monofilament satisfies the formula -150 ≤ (B * E * 100) / A ≤ -70, or the formula -150 ≤ (B * E * 100 + C) / A ≤ -70, A, B and C being coefficients of the quadratic function for stress σ and strain ε. Using a technical solution of the present invention, the pre-oxidized fiber has excellent mechanical properties and is easy to process, and carbon / carbon composite materials prepared therefrom can achieve XY-directional and Z-directional compressive strength of over 150 MPa.

Description

Polyacrylonitrile pre-oxidized fiber, its preparation method and application Technical Field

[0001] This invention relates to the field of pre-oxidized fiber technology, specifically to a polyacrylonitrile pre-oxidized fiber, its preparation method, and its application. Background Technology

[0002] Carbon / carbon composites are structural materials with advantages such as high specific strength, high thermal shock resistance, and high ablation resistance. Carbon / carbon composites can withstand temperatures up to 3000℃ and possess the unique property that their strength increases with increasing temperature.

[0003] The preparation process of carbon / carbon composite materials mainly includes three processes: preform molding, preform densification, and graphitization. The preform is the foundation for preparing carbon / carbon composite materials and is also one of the key factors affecting their performance. Polyacrylonitrile pre-oxidized fibers are chopped into short fibers, then dry-carded into a fiber web and reinforced with needle punching to form the preform. Carbon / carbon composite materials made from these preforms have advantages such as light weight, good impact resistance, and high damage tolerance.

[0004] Generally, pre-oxidized polyacrylonitrile (PA) fibers are produced by treating PA fibers with high-temperature air in an oxidation furnace. During the pre-oxidation process, PA fibers undergo oxidation and cyclization reactions, gradually transforming from a helical linear molecular structure to a heat-resistant ladder-shaped molecular structure. Simultaneously, the crystallinity and orientation of the PA fibers also change.

[0005] During the pre-oxidation process, since fibers are three-dimensional entities, the effects of mass and heat transfer, especially oxygen diffusion, are crucial. Poor control of mass and heat transfer can lead to severe core-sheath structure in the fibers. The core-sheath structure is essentially caused by differences in the degree of oxidation and cross-linking. This structure results in significant differences in the radial structure of the pre-oxidized fibers, leading to substantial losses in mechanical properties and making them difficult to process and apply in subsequent applications.

[0006] Currently, carbon / carbon composites are known to be prepared using carbon fiber as the main raw material, as disclosed in CN117776756A and CN118125852A. Since carbon fiber is a brittle material, processing is generally difficult in processes such as crimping, stub cutting, and needle punching. Therefore, pre-oxidized fibers with high elongation at break are selected to facilitate processing. However, the pre-oxidation process leads to a loss of fiber strength. Many studies have attempted to reduce the fiber strength loss during pre-oxidation. CN1478930A developed a method for manufacturing pre-oxidized fibers, which produces pre-oxidized fibers with high strength, but suffers from problems such as high temperature and low Z-axis strength of the resulting carbon / carbon composite. CN112708969A, CN115707801A, and CN114457469A only address the issues of production stability and low strength of pre-oxidized fibers, without considering the subsequent impact on the carbon / carbon composite preparation process. Summary of the Invention

[0007] To address the problems of poor mechanical properties of pre-oxidized fibers and low XY and Z-axis compressive strength of carbon / carbon composites made from them in the prior art, this invention provides a pre-oxidized polyacrylonitrile fiber, its preparation method, and its application.

[0008] The pre-oxidized fiber in this invention, within a strain range of 0-2.5%, exhibits stress σ and strain ε values ​​that satisfy the nonlinear fitting formula σ=Aε. 2 +Bε+C, and the breaking elongation E of the pre-oxidized fiber monofilament satisfies the formula -150≤(B*E*100) / A≤-70 or the formula -150≤(B*E*100+C) / A≤-70; where the unit of stress σ is GPa and the unit of strain ε is %, and where A, B, and C are the coefficients of the quadratic function of stress σ and strain ε. In some embodiments, -0.0420≤A≤-0.0140; 0.1450≤B≤0.2390; and 0.0620≤C≤0.0760. In some embodiments, 11.0%≤E≤16.0%.

[0009] This invention solves the above problems and can be used in the industrial production of pre-oxidized fibers for carbon / carbon composite materials.

[0010] One objective of this invention (a first aspect) is to provide a pre-oxidized polyacrylonitrile fiber in which the values ​​of stress σ and strain ε, within a strain range of 0-2.5%, satisfy the nonlinear fitting formula σ=Aε. 2 +Bε+C, and the breaking elongation E of the pre-oxidized fiber monofilament satisfies the formula -150≤(B*E*100) / A≤-70 or the formula -150≤(B*E*100+C) / A≤-70; where A, B, and C are the coefficients of the quadratic function of stress σ and strain ε.

[0011] In some implementations, -0.0420 ≤ A ≤ -0.0140; 0.1450 ≤ B ≤ 0.2390; and 0.0620 ≤ C ≤ 0.0760. In some implementations, 11.0% ≤ E ≤ 16.0%.

[0012] In this invention, the unit of stress σ is GPa, and the unit of strain ε is %.

[0013] In this invention, the tensile properties of the resin-impregnated fiber bundles are tested by performing a tensile test according to GB / T 26749-2022, that is, the stress-strain ε curve of the pre-oxidized fiber is determined (to obtain the values ​​of stress σ and strain ε).

[0014] In some embodiments, for the polyacrylonitrile pre-oxidized fibers of the present invention, the values ​​of stress σ and strain ε, within the strain range of 0-2.5%, satisfy the nonlinear fitting formula σ=Aε. 2 +Bε+C, and the breaking elongation E of the pre-oxidized fiber monofilament satisfies the formula -140≤(B*E*100) / A≤-70 or the formula -140≤(B*E*100+C) / A≤-70; where A, B, and C are the coefficients of the quadratic function of stress σ and strain ε.

[0015] In some embodiments, for the polyacrylonitrile pre-oxidized fibers of the present invention, the values ​​of stress σ and strain ε, within the strain range of 0-2.5%, satisfy the nonlinear fitting formula σ=Aε. 2 +Bε+C, and the breaking elongation E of the pre-oxidized fiber monofilament satisfies the formula -135≤(B*E*100) / A≤-75 or the formula -135≤(B*E*100+C) / A≤-75; where A, B, and C are the coefficients of the quadratic function of stress σ and strain ε.

[0016] In some embodiments of the present invention, the roundness of the monofilament cross-section of the pre-oxidized polyacrylonitrile fiber is 0.80-0.99, preferably 0.86-0.98, more preferably 0.87-0.98. For example, it can be 0.80, 0.81, 0.82, 0.83, 0.84, 0.85, 0.86, 0.87, 0.88, 0.89, 0.90, 0.91, 0.92, 0.93, 0.94, 0.95, 0.96, 0.97, or any intermediate value between any two values ​​or a range composed of any two values, such as 0.85-0.98.

[0017] In some embodiments of the present invention, the number of single bundles of polyacrylonitrile pre-oxidized fibers is 1,000-100,000, preferably 3,000-50,000, more preferably 3,000-48,000, for example 1,000, 2,000, 3,000, 4,000, 5,000, 6,000, 7,000, 8,000, 9,000, 10,000, 12,000, 15,000, 18,000, 20,000, 24,000, 30,000, 35,000, 40,000, 45,000, 47,000, 48,000, 49,000, 50,000, 60,000, 70,000, 80,000, 90,000, 100,000, or any intermediate value between any two values ​​or a range composed of any two values, for example 1,000-50,000.

[0018] In some embodiments of the present invention, the fineness of the polyacrylonitrile pre-oxidized fiber is 0.5-5.0 dtex, preferably 1.0-3.5 dtex, more preferably 1.1-3.0 dtex, and even more preferably 1.15-3.0 dtex. For example, 0.5, 0.6, 0.7, 0.8, 0.9, 1.0, 1.05, 1.1, 1.15, 1.2, 1.25, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9, 2.0, 2.1, 2.2, 2.3, 2.4, 2.5, 2.6, 2.7, 2.8, 2.9, 3.0, 4.0, 5.0, or any intermediate value between any two values ​​or a range composed of any two values, and even more preferably 1.25-2.5 dtex.

[0019] In some embodiments of the present invention, the bulk density of the pre-oxidized polyacrylonitrile fiber is 1.20-1.50 g / cm³. 3 The preferred value is 1.25-1.45 g / cm³. 3 The preferred value is 1.30-1.40 g / cm³. 3 For example, 1.20, 1.21, 1.22, 1.23, 1.24, 1.25, 1.26, 1.27, 1.28, 1.29, 1.30, 1.31, 1.32, 1.33, 1.34, 1.35, 1.36, 1.37, 1.38, 1.39, 1.40, 1.41, 1.42, 1.43, 1.44, 1.45, 1.46, 1.47, 1.48, 1.49, 1.50, or any intermediate value between two values, or a range composed of any two values, more preferably 1.33-1.37 g / cm³. 3 .

[0020] In some embodiments of the present invention, the monofilament strength of the pre-oxidized polyacrylonitrile fiber is 2.7-4.0 cN / dtex or 2.8-4.0 cN / dtex, preferably 3.0-3.3 cN / dtex, for example 2.7, 2.8, 2.9, 3.0, 3.1, 3.2, 3.3, 3.4, 3.5, 3.6, 3.7, 3.8, 3.9, 4.0 or an intermediate value between two values ​​or a range composed of any two values.

[0021] In some embodiments of the present invention, the monofilament breaking elongation E of the pre-oxidized polyacrylonitrile fiber is 10.0-20.0%, preferably 11.0-16.0%, more preferably 12.0-15.0%, and even more preferably 12.5-14.0%, for example, 12.5-13.9%, such as 10.5%, 11.0%, 12.0%, 12.1%, 12.2%, 12.3%, 12.4%, 12.5%, 1 2.6%, 12.7%, 12.8%, 12.9%, 13.0%, 13.1%, 13.2%, 13.3%, 13.4%, 13.5%, 13.6%, 13.7%, 13.8%, 13.9%, 14.0%, 14.5%, 15.0%, 15.5%, 16.0%, 17.0%, 18.0%, 19.0%, or the middle value between any two values ​​or a range of any two values.

[0022] In some embodiments of the present invention, the crystallinity of the polyacrylonitrile pre-oxidized fiber is 0.10-0.30, preferably 0.12-0.25 or 0.14-0.25. For example, the crystallinity of the polyacrylonitrile pre-oxidized fiber can be 0.10, 0.11, 0.12, 0.13, 0.14, 0.15, 0.16, 0.17, 0.18, 0.19, 0.20, 0.21, 0.22, 0.23, 0.24, 0.25, 0.26, 0.27, 0.28, 0.29, 0.30, or an intermediate value between any two values ​​or a range composed of any two values, for example, 0.13-0.24.

[0023] In some embodiments of the present invention, the grain size of the polyacrylonitrile pre-oxidized fiber is 1.80-3.50 nm, preferably 2.00-3.00 nm, and more preferably 2.10-3.00 nm. For example, the grain size of the polyacrylonitrile pre-oxidized fiber can be 1.80 nm, 1.90 nm, 2.00 nm, 2.10 nm, 2.20 nm, 2.30 nm, 2.40 nm, 2.50 nm, 2.60 nm, 2.70 nm, 2.80 nm, 2.90 nm, 3.00 nm, 3.10 nm, 3.20 nm, 3.30 nm, 3.40 nm, 3.50 nm, or any intermediate value between any two values, or a range composed of any two values, such as 2.30-3.00 nm.

[0024] In some embodiments of the present invention, the carbon to oxygen content ratio (mass content ratio) of the polyacrylonitrile pre-oxidized fiber is 0.50-3.00, preferably 0.70-3.00, and more preferably 0.70-2.50. For example, the carbon to oxygen content ratio of the polyacrylonitrile pre-oxidized fiber can be 0.50, 0.60, 0.70, 0.80, 0.90, 1.00, 1.10, 1.20, 1.30, 1.40, 1.50, 1.60, 1.70, 1.80, 1.90, 2.00, 2.10, 2.20, 2.30, 2.40, 2.50, 2.60, 2.70, 2.80, 2.90, 3.00, or any intermediate value between any two values ​​or a range composed of any two values, such as 0.80-2.40.

[0025] Through extensive experimental research, this invention unexpectedly discovered that for polyacrylonitrile pre-oxidized fibers, within the strain range of 0-2.5%, the values ​​of stress σ and strain ε, as well as the breaking elongation of the pre-oxidized polyacrylonitrile fiber monofilament, conform to the relationships expressed by the above formulas. This can solve the problems of poor mechanical properties of pre-oxidized fibers and low XY and Z-direction compressive strength of carbon / carbon composite materials made from them in the prior art.

[0026] More specifically, without any theoretical limitations, this application utilizes the nonlinear fitting formula σ=Aε 2+Bε+C describes the stress-strain relationship of the fiber; this formula is a quadratic function with its axis of symmetry located at ε=-B / 2A. The ratio B / A is -2 times the axis of symmetry, reflecting the shape characteristics of the curve: the stress-strain curve corresponding to a small B / A value exhibits a short and wide characteristic; the stress-strain curve corresponding to a large B / A value exhibits a tall and slender characteristic. The inventors of this application have found that both excessively large and small B / A values ​​are detrimental to improving the tensile strength of the fiber, while high fiber tensile strength is related to the high compressive strength in the XY and Z directions of carbon / carbon composite materials; in addition, a suitable elongation at break helps the pre-oxidized fiber maintain its stretchability, promote (integral felt) molding, and reduce defects in the preform preparation process such as needle punching; therefore, the formula (B*E*100 / A) integrates elongation at break and tensile strength to ensure the molding quality of the preform and the compressive performance of the carbon / carbon composite material; where the E*100 term is used to extract the pure numerical value of the elongation at break E (dimensionless). The formula (B*E*100) / A can be used to comprehensively evaluate pre-oxidized fibers, taking into account the synergistic effect of their tensile strength and elongation at break, ultimately achieving a significant improvement in the compressive performance of carbon / carbon composites. Furthermore, the coefficient C corresponds to the initial stress value when the strain ε is zero. Research in this application shows that the ratio C / A is related to the regularity of the pre-oxidized fiber's crystalline structure. C / A can further distinguish fibers with different structural characteristics, ensuring that the selected fibers experience less crystalline structure damage during pre-oxidation, retaining higher crystallinity and grain size, thereby endowing the fibers with excellent tensile strength. The formula (B*E*100+C) / A can be used to further comprehensively evaluate pre-oxidized fibers, taking into account the synergistic effect of their tensile strength and elongation at break, and considering the crystalline structure of the pre-oxidized fibers, ultimately achieving a significant improvement in the compressive performance of carbon / carbon composites.

[0027] In this invention, polyacrylonitrile pre-oxidized fibers can be obtained through the preparation method of the pre-oxidized fibers of this invention, wherein the pre-oxidized fibers have A, B, and C (i.e., within the strain range of 0-2.5%), and the nonlinear fitting formula for the stress σ and strain ε values ​​is σ=Aε. 2 The coefficients of +Bε+C) and the breaking elongation E of the pre-oxidized fiber monofilament can satisfy the formula of the present invention -150≤(B*E*100) / A≤-70 or the formula -150≤(B*E*100+C) / A≤-70.

[0028] A second objective of this invention is to provide a method for preparing polyacrylonitrile pre-oxidized fibers. For example, the method for preparing polyacrylonitrile pre-oxidized fibers according to one objective of this invention includes the step of obtaining polyacrylonitrile pre-oxidized fibers by sequentially passing polyacrylonitrile-based precursor fibers through a pre-oxidation process comprising m+n pre-oxidation temperature zones. The m pre-oxidation temperature zones are all heated in a hot oxygen-containing atmosphere, where m is an integer from 2 to 4, and the n pre-oxidation temperature zones are all heated using microwaves, where n is an integer from 2 to 6. In this invention, m can be 2, 3, or 4, and n can be 2, 3, 4, 5, or 6. In this invention, "m+n pre-oxidation temperature zones" means that the polyacrylonitrile-based precursor fibers first pass through the m pre-oxidation temperature zones (i.e., m zones heated in a hot oxygen-containing atmosphere), and then through the n pre-oxidation temperature zones (i.e., n zones heated by microwaves).

[0029] In some embodiments, the method for preparing the polyacrylonitrile pre-oxidized fiber includes the step of pre-oxidizing the polyacrylonitrile-based precursor fiber by passing it through at least four pre-oxidation temperature zones to obtain the polyacrylonitrile pre-oxidized fiber; wherein the first 2-4 pre-oxidation temperature zones are heated using a hot oxygen-containing atmosphere, and the remaining pre-oxidation temperature zones are heated using microwave heating, and the at least four pre-oxidation temperature zones include at least two pre-oxidation temperature zones heated by a hot oxygen-containing atmosphere and at least two pre-oxidation temperature zones heated by microwave heating; preferably, the polyacrylonitrile-based precursor fiber is pre-oxidized by passing it through 4-8 pre-oxidation temperature zones to obtain the polyacrylonitrile pre-oxidized fiber; wherein the first 2-4 pre-oxidation temperature zones... All pre-oxidation zones are heated in a hot oxygen-containing atmosphere, while the remaining pre-oxidation zones are heated by microwaves. The 4-8 pre-oxidation zones include at least two pre-oxidation zones heated in a hot oxygen-containing atmosphere and at least two pre-oxidation zones heated by microwaves. More preferably, polyacrylonitrile-based precursor fibers are pre-oxidized by passing them through 4-6 pre-oxidation zones to obtain pre-oxidized polyacrylonitrile fibers. Specifically, the first 2-3 pre-oxidation zones are heated in a hot oxygen-containing atmosphere, while the remaining pre-oxidation zones are heated by microwaves. The 4-6 pre-oxidation zones include at least two pre-oxidation zones heated in a hot oxygen-containing atmosphere and at least two pre-oxidation zones heated by microwaves.

[0030] As those skilled in the art will understand, during the pre-oxidation process, when the polyacrylonitrile precursor fiber passes through the pre-oxidation temperature zones, the fiber sequentially passes through each temperature zone. In the method of the present invention, the polyacrylonitrile precursor fiber first passes through at least two pre-oxidation temperature zones heated in a hot oxygen atmosphere, and then passes through at least two microwave-heated pre-oxidation temperature zones.

[0031] In this invention, "heating with a hot oxygen atmosphere" refers to heating the raw filament with a hot oxygen atmosphere.

[0032] In this invention, "microwave heating" refers to heating by applying microwaves to the raw filament or fiber.

[0033] In some embodiments of the present invention, polyacrylonitrile precursor fibers are pre-oxidized by sequentially passing through the following pre-oxidation temperature zones to obtain pre-oxidized polyacrylonitrile fibers:

[0034] Two pre-oxidation temperature zones heated by a hot oxygen-containing atmosphere and two pre-oxidation temperature zones heated by microwave;

[0035] Two pre-oxidation temperature zones heated by a hot oxygen-containing atmosphere and three pre-oxidation temperature zones heated by microwave;

[0036] Two pre-oxidation temperature zones heated by a hot oxygen-containing atmosphere and four pre-oxidation temperature zones heated by microwave;

[0037] Two pre-oxidation temperature zones heated by a hot oxygen-containing atmosphere and five pre-oxidation temperature zones heated by microwave;

[0038] Three pre-oxidation temperature zones heated by a hot oxygen-containing atmosphere and two pre-oxidation temperature zones heated by microwave;

[0039] Three pre-oxidation temperature zones heated by a hot oxygen-containing atmosphere and three pre-oxidation temperature zones heated by microwave;

[0040] Three pre-oxidation temperature zones heated by a hot oxygen-containing atmosphere and four pre-oxidation temperature zones heated by microwave;

[0041] Three pre-oxidation temperature zones heated by a hot oxygen-containing atmosphere and five pre-oxidation temperature zones heated by microwave;

[0042] Four pre-oxidation temperature zones heated by a hot oxygen-containing atmosphere and two pre-oxidation temperature zones heated by microwave;

[0043] Four pre-oxidation temperature zones heated by a hot oxygen-containing atmosphere and three pre-oxidation temperature zones heated by microwave;

[0044] Four pre-oxidation temperature zones heated by a hot oxygen-containing atmosphere and four pre-oxidation temperature zones heated by microwave; or

[0045] Four pre-oxidation temperature zones heated by a hot oxygen-containing atmosphere and five pre-oxidation temperature zones heated by microwave.

[0046] In some embodiments, the method for preparing the pre-oxidized polyacrylonitrile fiber includes the step of passing a polyacrylonitrile-based precursor fiber through a pre-oxidation process in at least four temperature zones to obtain the pre-oxidized polyacrylonitrile fiber; wherein, the first two temperature zones of the pre-oxidation process are heated in a hot oxygen-containing atmosphere, and the remaining temperature zones are heated by microwave; preferably, the method for preparing the pre-oxidized polyacrylonitrile fiber includes the step of passing a polyacrylonitrile-based precursor fiber through a pre-oxidation process in four to eight temperature zones to obtain the pre-oxidized polyacrylonitrile fiber; wherein, the first two or three temperature zones of the pre-oxidation process are heated in a hot oxygen-containing atmosphere, and the remaining temperature zones are heated by microwave.

[0047] In this invention, after the polyacrylonitrile precursor fiber has sequentially passed through the m+n pre-oxidation temperature zones of this invention, optionally, the polyacrylonitrile precursor fiber or fiber may further pass through one or more additional pre-oxidation temperature zones, which do not meet the requirements of the m+n pre-oxidation temperature zones described in this application. For example, in some embodiments, after the polyacrylonitrile precursor fiber has sequentially passed through two pre-oxidation temperature zones heated by a hot oxygen atmosphere and two pre-oxidation temperature zones heated by microwaves, i.e., after passing through the m+n pre-oxidation temperature zones of this invention (where m=2 and n=2), the polyacrylonitrile precursor fiber may further pass through a pre-oxidation temperature zone heated by hot air; such embodiments are within the scope of this invention.

[0048] In this invention, the inventors unexpectedly discovered that by employing the pre-oxidation process of this invention to pre-oxidize polyacrylonitrile-based precursor fibers to obtain pre-oxidized polyacrylonitrile fibers, the monofilament strength of the obtained pre-oxidized polyacrylonitrile fibers can be further improved, while essentially maintaining or preserving the monofilament breaking elongation. Therefore, the method of pre-oxidizing polyacrylonitrile-based precursor fibers of this invention is a method for improving the monofilament strength of pre-oxidized polyacrylonitrile fibers, wherein this method can essentially maintain or preserve the monofilament breaking elongation.

[0049] In some embodiments of the present invention, during the pre-oxidation process, the draw ratio of the pre-oxidation temperature zone heated by a hot oxygen-containing atmosphere is greater than or equal to the draw ratio of the pre-oxidation temperature zone heated by microwave. In some embodiments, preferably, the draw ratio of the pre-oxidation temperature zone heated by a hot oxygen-containing atmosphere can be 1.000-1.200, more preferably 1.000-1.080, more preferably 1.010-1.050, and more preferably 1.010-1.040, for example, 1.005, 1.010, 1.015, 1.020, 1.025, 1.030, 1.035, 1.040, 1.045, 1.050, 1.055, 1.060, 1.070, 1.080, 1.090, 1.100, 1.150, 1.200, or an intermediate value between two values, or a range consisting of any two values. In some embodiments, preferably, the draw ratio of the pre-oxidation temperature zone using microwave heating can be 0.950-1.000, more preferably 0.970-1.000, more preferably 0.980-1.000, and even more preferably 0.990-0.999, for example, 0.950, 0.955, 0.960, 0.965, 0.970, 0.975, 0.980, 0.985, 0.990, 0.991, 0.992, 0.993, 0.994, 0.995, 0.996, 0.997, 0.998, or any intermediate value between any two values, or a range composed of any two values.

[0050] In some embodiments of the present invention, during the pre-oxidation process, the temperatures of the pre-oxidation temperature zones heated by a hot oxygen-containing atmosphere may be the same or different, and may be independently 150-350°C, preferably 180-280°C, such as 190°C, 200°C, 210°C, 220°C, 230°C, 240°C, 250°C, 260°C, 270°C, or any intermediate value between any two values, or a range composed of any two values. In some embodiments, preferably, the temperature of the pre-oxidation temperature zones heated by a hot oxygen-containing atmosphere gradually increases; in some embodiments, more preferably, the temperature difference between adjacent pre-oxidation temperature zones heated by a hot oxygen-containing atmosphere is not less than 30°C, preferably 30-100°C, such as 30°C, 35°C, 40°C, 45°C, 50°C, 60°C, 70°C, 80°C, 90°C, or any intermediate value between any two values, or a range composed of any two values.

[0051] In some embodiments of the present invention, during the pre-oxidation process, the total temperature difference of the pre-oxidation temperature zone heated by a hot oxygen-containing atmosphere can be 30-150°C, preferably 50-70°C. In the present invention, during the pre-oxidation process, the total temperature difference of the pre-oxidation temperature zone heated by a hot oxygen-containing atmosphere can be 30°C, 40°C, 50°C, 60°C, 70°C, 80°C, 90°C, 100°C, 110°C, 120°C, 130°C, 140°C, 150°C, or any intermediate value between any two values, or a range composed of any two values, for example, 40-90°C.

[0052] In this invention, the temperature in the pre-oxidation zone, heated by a hot oxygen atmosphere, is measured by thermocouples positioned within the zone. The fibers pass through the pre-oxidation zone in a flat, laid-out state. The width D (in mm) of the flat lay is related to the number of fibers N and the fineness λ (in dtex) in a single fiber bundle by the relationship D = N * λ / 1800. In the hot oxygen atmosphere heating zone, three thermocouples are positioned parallel to the fiber bundle's centerline, spaced 1 meter apart along the fiber bundle's direction of movement. The middle thermocouple is located at the center of the zone, 25 cm away from the fiber bundle. The average temperature measured by the three thermocouples is the temperature of the zone.

[0053] In this invention, the oxygen-containing atmosphere in the pre-oxidation temperature zone heated by a hot oxygen-containing atmosphere can be any oxygen-containing atmosphere known in the art, such as air, a mixture of air and oxygen, a mixture of air and an inert gas, a mixture of oxygen and an inert gas, a mixture of air and oxygen and an inert gas, etc. In this invention, the oxygen volume concentration of the oxygen-containing atmosphere can be 5%-80%, preferably 15%-50%. In this invention, for example, the oxygen volume concentration of the oxygen-containing atmosphere can be 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, or any intermediate value between any two values ​​or a range composed of any two values, for example, 15%-45%. In some embodiments of this invention, during the pre-oxidation process, the oxygen-containing atmosphere in the pre-oxidation temperature zone heated by a hot oxygen-containing atmosphere is air, or a mixture of oxygen and an inert gas. In some embodiments, preferably, the volume fraction of the inert gas in the oxygen and inert gas mixture can be 50%-80%, for example, 50%, 55%, 60%, 65%, 70%, 75%, 80%, or any intermediate value between any two values ​​or a range composed of any two values. The inert gas can be, for example, nitrogen (N2), helium (He), neon (Ne), argon (Ar), etc.

[0054] In this invention, preferably, the atmosphere in the pre-oxidation temperature zone heated by a hot oxygen-containing atmosphere is air.

[0055] In some embodiments of the present invention, during the pre-oxidation process, the microwave frequencies of the pre-oxidation temperature zones heated by microwaves may be the same or different, and can be independently 1.0-10.0 GHz, preferably 2.0-7.0 GHz, more preferably 2.0-6.0 GHz, for example 1.0 GHz, 1.1 GHz, 1.2 GHz, 1.3 GHz, 1.4 GHz, 1.5 GHz, 1.6 GHz, 1.7 GHz, 1.8 GHz, 1.9 GHz, 2.0 GHz, 2.1 GHz, 2.2 GHz, 2.3 GHz, 2.4 GHz, 2.5 GHz, 2.6 GHz, 2.7 GHz, 2.8 GHz, 2.9 GHz. 3.0GHz, 3.1GHz, 3.2GHz, 3.3GHz, 3.4GHz, 3.5GHz, 3.6GHz, 3.7GHz, 3.8GHz, 3.9GHz, 4.0GHz, 4.1GHz, 4.2GHz, 4.3GHz, 4.4GHz, 4.5GHz, 4.6GHz, 4.7GHz, 4.8GHz, 4.9GHz, 5.0GHz, 5.5GHz, 6.0GHz, 6.5GHz, 7.0GHz, 7.5GHz, 8.0GHz, 8.5GHz, 9.0GHz, 9.5GHz, or any two values, or any range of two values.

[0056] In some embodiments of the present invention, preferably, the microwave frequency of the pre-oxidation temperature zone using microwave heating during the pre-oxidation process can be incremental; more preferably, in some embodiments, the microwave frequency difference between adjacent pre-oxidation temperature zones using microwave heating is not less than 0.5 GHz, preferably 0.5-3.0 GHz. For example, the microwave frequency difference between adjacent pre-oxidation temperature zones using microwave heating can be 0.5 GHz, 0.6 GHz, 0.7 GHz, 0.8 GHz, 0.9 GHz, 1.0 GHz, 1.1 GHz, 1.2 GHz, 1.3 GHz, 1.4 GHz, 1.5 GHz, 1.6 GHz, 1.7 GHz, 1.8 GHz, 1.9 GHz, 2.0 GHz, 2.1 GHz, 2.2 GHz, 2.3 GHz, 2.4 GHz, 2.5 GHz, 2.6 GHz, 2.7 GHz, 2.8 GHz, 2.9 GHz, 3.0 GHz, or any intermediate value between any two values, or a range composed of any two values, for example, 0.5-2.5 GHz.

[0057] In some embodiments of the present invention, during the pre-oxidation process, the total microwave frequency difference of the pre-oxidation temperature zone using microwave heating can be 0.5-5.0 GHz, preferably 0.6-3.5 GHz. For example, the total microwave frequency difference of the pre-oxidation temperature zone using microwave heating is 0.5 GHz, 0.6 GHz, 0.7 GHz, 0.8 GHz, 0.9 GHz, 1.0 GHz, 1.1 GHz, 1.2 GHz, 1.3 GHz, 1.4 GHz, 1.5 GHz, 1.6 GHz, 1.7 GHz, 1.8 GHz, 1.9 GHz, 2.0 GHz, 2.1 GHz, 2.2 GHz, 2.3 GHz, 2.4 GHz, 2.5 GHz, 2.6 GHz, 2.7 GHz, or 2.8 GHz. 2.9GHz, 3.0GHz, 3.1GHz, 3.2GHz, 3.3GHz, 3.4GHz, 3.5GHz, 3.6GHz, 3.7GHz, 3.8GHz, 3.9GHz, 4.0GHz, 4.1GHz, 4.2GHz, 4.3GHz, 4.4GHz, 4.5GHz, 4.6GHz, 4.7GHz, 4.8GHz, 4.9GHz, 5.0GHz, or any intermediate value between any two values, or any range of any two values, such as 0.5-4.0GHz.

[0058] In some embodiments of the present invention, during the pre-oxidation process, the temperature of each pre-oxidation zone heated by microwave can be 160-300°C, preferably 160-250°C. For example, the temperature of each pre-oxidation zone heated by microwave can be independently 160°C, 170°C, 180°C, 190°C, 200°C, 210°C, 220°C, 230°C, 240°C, 250°C, 260°C, 270°C, 280°C, 290°C, 300°C, or any intermediate value between any two values, or a range composed of any two values, such as 170-260°C.

[0059] In this invention, the temperature of the pre-oxidation zone, heated by microwave, is measured by an infrared thermometer within the zone. The fibers pass through the pre-oxidation zone in a flat, laid-out state. The width D (in mm) of the flat lay is related to the number of fibers in a single bundle N and the fineness λ (in dtex) by the formula D = N * λ / 1800. Within the microwave-heated zone, three infrared thermometers are positioned parallel to the fiber bundle's centerline, spaced 1 meter apart along the fiber bundle's direction of movement. The central infrared thermometer is located at the center of the zone, 20 cm away from the fiber bundle. The average temperature measured by the three infrared thermometers is the temperature of the zone.

[0060] In this invention, the atmosphere in the pre-oxidation temperature zone using microwave heating can be any atmosphere known in the art, such as air, a mixture of air and oxygen, a mixture of air and an inert gas, a mixture of oxygen and an inert gas, a mixture of air and oxygen and an inert gas, etc. In this invention, the oxygen volume concentration of the atmosphere can be 5%-80%, preferably 15%-50%. In this invention, for example, the oxygen volume concentration of the atmosphere can be 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, or any intermediate value between any two values, or a range composed of any two values, for example, 15%-45%. In some embodiments of this invention, during the pre-oxidation process, the atmosphere in the pre-oxidation temperature zone using microwave heating is air, or a mixture of oxygen and an inert gas. In some embodiments, preferably, the volume fraction of the inert gas in the oxygen and inert gas mixture can be 50%-80%, for example, 50%, 55%, 60%, 65%, 70%, 75%, 80%, or any intermediate value between any two values ​​or a range composed of any two values. The inert gas can be, for example, nitrogen (N2), helium (He), neon (Ne), argon (Ar), etc.

[0061] In this invention, preferably, the atmosphere in the pre-oxidation temperature zone heated by microwave is air.

[0062] In some embodiments of the present invention, during the pre-oxidation process, the residence time of the fiber in each pre-oxidation temperature zone may be the same or different, and may be 5-60 minutes, preferably 10-30 minutes, for example 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 35, 40, 45, 50, 55, 60 minutes or any intermediate value between any two values ​​or a range composed of any two values, more preferably 10-28 minutes.

[0063] In some embodiments of the present invention, during the pre-oxidation process, the total residence time of the fiber in the pre-oxidation temperature zone heated by a hot oxygen-containing atmosphere does not exceed 60 minutes, preferably 20-60 minutes or 25-60 minutes. For example, during the pre-oxidation process, the total residence time of the fiber in the pre-oxidation temperature zone heated by a hot oxygen-containing atmosphere is 20 minutes, 25 minutes, 30 minutes, 35 minutes, 40 minutes, 45 minutes, 50 minutes, 55 minutes, 60 minutes, or any intermediate value between any two values ​​or a range composed of any two values, such as 30-60 minutes.

[0064] In some embodiments of the present invention, during the pre-oxidation process, the total residence time of the fiber in the microwave-heated pre-oxidation temperature zone does not exceed 70 minutes, preferably 20-70 minutes or 30-65 minutes. For example, during the pre-oxidation process, the total residence time of the fiber in the microwave-heated pre-oxidation temperature zone is 20 minutes, 25 minutes, 30 minutes, 35 minutes, 40 minutes, 45 minutes, 50 minutes, 55 minutes, 60 minutes, 65 minutes, or 70 minutes, or any intermediate value between any two values, or a range composed of any two values, such as 25-70 minutes.

[0065] In this invention, there are no particular limitations on the polyacrylonitrile-based precursor fibers used in the pre-oxidation process; various polyacrylonitrile-based precursor fibers commonly used in the art for preparing pre-oxidized fibers can be used. In some embodiments, the polyacrylonitrile-based precursor fibers can be obtained from polyacrylonitrile dope via a spinning process.

[0066] In this invention, there are no special limitations on the preparation of polyacrylonitrile-based precursor fibers; they can be obtained from polyacrylonitrile stock solutions commonly used in the art through commonly used spinning processes. Furthermore, there are no special limitations on the polyacrylonitrile stock solutions or spinning processes in this invention.

[0067] In this invention, the polyacrylonitrile spinning solution contains polyacrylonitrile dissolved in a solvent.

[0068] The polyacrylonitrile used in this application can be a polyacrylonitrile homopolymer or a polyacrylonitrile copolymer. In some embodiments, preferably, the polyacrylonitrile used in this application is a polyacrylonitrile copolymer. The polyacrylonitrile copolymer can be a copolymer prepared from acrylonitrile and a comonomer commonly used in the art. For example, the polyacrylonitrile comonomer can be a vinyl-containing monomer, preferably one or more of acrylates, vinyl esters, acrylamides, sulfonates, carboxylic acids, and ammonium salts. In some embodiments, for example, the polyacrylonitrile comonomer can be itaconic acid.

[0069] In this invention, the type of solvent is not particularly important as long as the objective of the invention can be achieved. The following is an illustrative description, but it does not limit the scope of the invention. According to some embodiments of the invention, the solvent may be one or more of dimethyl sulfoxide, dimethylformamide, and dimethylacetamide, preferably dimethyl sulfoxide.

[0070] According to some embodiments of the present invention, the content of acrylonitrile structural units in the polyacrylonitrile copolymer can be 85-99.5 wt%, for example, the content can be 86 wt%, 87 wt%, 88 wt%, 89 wt%, 90 wt%, 91 wt%, 92 wt%, 93 wt%, 94 wt%, 95 wt%, 96 wt%, 97 wt%, 98 wt%, 98.5 wt%, and 99 wt%, preferably 95-99 wt%. According to some embodiments of the present invention, the content of copolymer structural units in the polyacrylonitrile copolymer can be 15-0.5 wt%, for example, the content can be 14 wt%, 13 wt%, 12 wt%, 11 wt%, 10 wt%, 9 wt%, 8 wt%, 7 wt%, 6 wt%, 5 wt%, 4 wt%, 3 wt%, 2 wt%, 1.5 wt%, and 1 wt%, preferably 5-1 wt%.

[0071] In some embodiments of the present invention, the polyacrylonitrile-based precursor fiber is obtained from polyacrylonitrile spinning solution through wet spinning, dry spinning, or dry-jet wet spinning, preferably through wet spinning. Preferably, the wet spinning process includes steps of multi-stage coagulation, hot water drawing, oiling, drying and densification, steam drawing, and steam heat setting; more preferably, the wet spinning process includes steps of polyacrylonitrile spinning solution delivery, precision filtration, multi-stage coagulation, hot water drawing, oiling, drying and densification, steam drawing, and steam heat setting.

[0072] In this invention, the steps of precision filtration, multi-stage coagulation, hot water drawing, oiling, drying and densification, steam drawing, and steam heat setting in the wet spinning process are steps known in the art for preparing polyacrylonitrile precursor fibers.

[0073] According to some embodiments of the present invention, the conditions for the precision filtration may include: a filtration accuracy of 2-10 micrometers, preferably 4-8 micrometers. In the present invention, filtration accuracy refers to the average pore size of the filter screen.

[0074] In this invention, the total draw ratio of the multi-stage solidification can be 0.3-1.5, preferably 0.5-1.3.

[0075] In this invention, the coagulation bath in the multi-stage coagulation process is a conventional choice in the art. In some embodiments of this invention, the coagulation bath can be an aqueous solution of dimethyl sulfoxide. In some preferred embodiments of this invention, the mass concentration of the coagulation bath can be 35-90%, for example, 35-90%. In some embodiments, the concentration of the dimethyl sulfoxide aqueous solution in the multi-stage coagulation bath can decrease sequentially.

[0076] In this invention, the temperature range of the coagulation bath is relatively wide. The following is an illustrative description, but it does not limit the scope of the invention. In this invention, the temperature of the coagulation bath during coagulation can be a conventional choice in the art. In some embodiments, the temperature of the coagulation bath can be 20–80°C, for example, 25–80°C. In some embodiments, the temperatures of the multi-stage coagulation baths can increase sequentially.

[0077] In this invention, there are no special requirements for the conditions of hot water stretching as long as the purpose of the invention can be achieved. The following is an illustrative description, but it does not limit the scope of the invention. According to some embodiments of the invention, the conditions for hot water stretching include a temperature of 80-100°C.

[0078] In this invention, there are no special requirements regarding the number of hot water stretching passes, as long as the objective of the invention can be achieved. The following is an illustrative description, but it does not limit the scope of the invention. According to some embodiments of the invention, the number of hot water stretching passes is 2-4.

[0079] According to some embodiments of the present invention, the conditions for hot water stretching include: a total stretch ratio of 2.5-6.

[0080] In some embodiments of the present invention, the hot water drawing is preferably multi-stage hot water drawing, for example, three-stage hot water drawing. In some embodiments, more preferably, the temperatures of the three hot water drawing stages are 90-99°C. In some embodiments, the draw ratios of the three hot water drawing stages are 1.2 to 2.0, for example, 1.3 to 1.9.

[0081] In this invention, there are no special requirements regarding the oiling conditions, as long as the objective of the invention can be achieved.

[0082] In the preparation of raw fibers, applying an oiling agent to the fibers via an oiling step is known in the art. Various oiling agents commonly known in the art can be used in this oiling step; for example, the oiling agents used may include silicone-free oiling agents, low-silicone oiling agents, and silicone-containing oiling agents, etc.

[0083] In this invention, the drying and densification conditions are conventionally chosen in the art, and the following is an illustrative description, but does not limit the scope of the invention. According to some embodiments of the invention, the drying and densification conditions include a temperature of 90-150°C, for example, 120-150°C.

[0084] In some embodiments, the present invention employs a two-stage drying densification process. In some embodiments, the temperature of the first stage of drying densification is lower than the temperature of the second stage of drying densification.

[0085] In this invention, the steam drawing pressure is a conventional choice in the art, and the following is an illustrative description, but does not limit the scope of the invention. According to some embodiments of the invention, the conditions for steam drawing include: a steam pressure of 0.2-0.6 MPa.

[0086] In this invention, the draw ratio of the steam drawing is a conventional choice in the art. The following is an illustrative description, but does not limit the scope of the invention. According to some embodiments of the invention, the conditions for steam drawing include a draw ratio of 1.0-3.0, for example, 1.0-2.5.

[0087] In this invention, the heat setting pressure is a conventional choice in the art, and the following is an illustrative description, but does not limit the scope of the invention. According to some embodiments of the invention, the heat setting conditions include a steam pressure of 0.05-0.15 MPa.

[0088] In this invention, the draw ratio for heat setting is a conventional choice in the art, and the following is an illustrative description, but does not limit the scope of the invention. According to some embodiments of the invention, the heat setting conditions include a draw ratio of 0.8-1.1.

[0089] The third objective of this invention is to provide a carbon / carbon composite material prepared from polyacrylonitrile pre-oxidized fiber obtained by one objective of this invention or by the preparation method of another objective of this invention.

[0090] This application does not specifically limit the preparation process of the carbon / carbon composite material, which may include (using methods commonly used in the art) preparing a preform (molding), densifying the preform, and graphitizing the preform, wherein the preparation of the preform includes using the polyacrylonitrile pre-oxidized fiber of the present invention to prepare the preform.

[0091] In some embodiments of the present invention, the preparation process of the carbon / carbon composite material includes the steps of preparing a preform (preform molding), densification of the preform, and graphitization treatment; wherein the preparation of the preform includes using polyacrylonitrile pre-oxidized fibers to prepare the preform, wherein the polyacrylonitrile pre-oxidized fibers are the polyacrylonitrile pre-oxidized fibers described in the first aspect of the present invention or the polyacrylonitrile pre-oxidized fibers prepared by the preparation method of the second aspect of the present invention. Preferably, in some embodiments, the preparation process of the carbon / carbon composite material includes preparing a preform by needle punching, chemical vapor deposition or chemical vapor infiltration, resin impregnation, and graphitization to obtain the carbon / carbon composite material, wherein the preparation of the preform includes using polyacrylonitrile pre-oxidized fibers to prepare the preform, wherein the polyacrylonitrile pre-oxidized fibers are the polyacrylonitrile pre-oxidized fibers described in the first aspect of the present invention or the polyacrylonitrile pre-oxidized fibers prepared by the preparation method of the second aspect of the present invention.

[0092] In some embodiments of the present invention, a method for preparing carbon / carbon composite materials is also provided, the method comprising the following steps: preparing a preform using the polyacrylonitrile pre-oxidized fiber of the present invention or the polyacrylonitrile pre-oxidized fiber obtained by the preparation method of the present invention; densifying the preform; and graphitizing the densified preform.

[0093] In some embodiments, the present invention provides a method for preparing carbon / carbon composite materials, the method comprising the following steps:

[0094] Polyacrylonitrile pre-oxidized fibers are prepared by the preparation method described in the second objective of this invention;

[0095] Preforms were prepared using the aforementioned polyacrylonitrile pre-oxidized fibers;

[0096] Densifying the preform; and

[0097] The densified preform is then graphitized.

[0098] In this invention, using the polyacrylonitrile pre-oxidized fiber of the present invention to prepare a preform means using the polyacrylonitrile pre-oxidized fiber of the present invention, as well as optionally one or more other fibers, to prepare the preform. In this invention, the other fibers may be fibers commonly used to prepare carbon / carbon composites, including but not limited to carbon fibers, polyacrylonitrile pre-oxidized fibers different from those of the present invention, etc.

[0099] As those skilled in the art will understand, the preparation of preforms from fibers, the densification of the preforms, and the graphitization of the densified preforms are known in the field of carbon / carbon composites. This disclosure does not specifically limit the use of various methods, processes, and operations known in the field of carbon / carbon composites to prepare preforms, densify preforms, and graphitize densified preforms.

[0100] In this application, there are no particular limitations on the shape, structure, etc., of the preform; various shapes and structures commonly known in the art can be used. In some embodiments, the density of the preform can be 0.1-0.5 g / cm³. 3 .

[0101] In some implementations, pre-oxidized polyacrylonitrile fibers can be chopped into short fibers, and then dry carded, laminated, and needle-punched to form a preform.

[0102] After obtaining the preform, it can be densified. In some embodiments, densification can be achieved, for example, via liquid phase impregnation, chemical vapor deposition (CVD), chemical vapor infiltration (CVI), etc.

[0103] After densification, the densified preform is subjected to graphitization treatment. The graphitization treatment can be carried out in an inert environment at 2000-3000℃.

[0104] In some embodiments, the carbon / carbon composite material of the present invention can be prepared by the following preparation process:

[0105] Pre-oxidized fibers were needle-punched to form a pre-oxidized fiber integral felt, resulting in a preform with a density of 0.1-0.5 g / cm³. 3 The integral felt is placed in a chemical vapor infiltration furnace, using propane as a precursor, and pyrolyzed at 500-1500℃, furnace pressure 0.5-5 kPa, gas flow rate 100-1000 mL / min, for 20-300 hours to obtain a preform treated with chemical vapor infiltration. A phenolic resin / ethanol impregnation solution with a concentration of 0.1-10 wt% is prepared, and the preform is placed in it. After standing for 1-60 minutes, it is removed and pre-cured in an oven at 80-150℃ for 1-100 minutes, followed by hot pressing at a pressure of 0.1-10 MPa, a temperature of 150-300℃, and a time of 5-200 minutes to obtain a carbon / carbon preform. The carbon / carbon preform is then graphitized at 2200-2800℃ for 100-300 minutes to obtain a carbon / carbon composite material.

[0106] In some embodiments of the present invention, the XY compressive strength of the carbon / carbon composite material is ≥150 MPa, preferably ≥200 MPa, and / or, the Z compressive strength is ≥150 MPa, preferably ≥200 MPa. In some embodiments of the present invention, preferably, the XY compressive strength of the carbon / carbon composite material is ≥150 MPa, preferably ≥200 MPa, and the Z compressive strength is ≥150 MPa, preferably ≥200 MPa. In some embodiments of the present invention, preferably, the XY compressive strength of the carbon / carbon composite material is from 150 MPa to 220 MPa, preferably from 200 MPa to 220 MPa. For example, the XY compressive strength of the carbon / carbon composite material can be 150 MPa, 155 MPa, 160 MPa, 165 MPa, 170 MPa, 175 MPa, 180 MPa, 185 MPa, 190 MPa, 195 MPa, 200 MPa, 205 MPa, 210 MPa, 215 MPa, 220 MPa, or any range of two of the above values. In some embodiments of the present invention, preferably, the Z compressive strength of the carbon / carbon composite material is 150 MPa to 220 MPa, more preferably 200 MPa to 220 MPa. For example, the Z-axis compressive strength of the carbon / carbon composite material can be 150 MPa, 155 MPa, 160 MPa, 165 MPa, 170 MPa, 175 MPa, 180 MPa, 185 MPa, 190 MPa, 195 MPa, 200 MPa, 205 MPa, 210 MPa, 215 MPa, 220 MPa, or any range of two of the above values.

[0107] In this invention, the XY and Z directions of the carbon / carbon composite material have meanings generally known in the art. Specifically, the XY direction of the carbon / carbon composite material is generally the direction of the non-woven fabric or fiber lay-up plane; the Z direction is the direction perpendicular to the XY direction.

[0108] A fourth objective of this invention is to provide a polyacrylonitrile pre-oxidized fiber according to one objective of this invention or a polyacrylonitrile pre-oxidized fiber prepared by the preparation method according to another objective of this invention for use in the preparation of carbon / carbon composite materials. When the polyacrylonitrile pre-oxidized fiber of this invention is used to prepare carbon / carbon composite materials, it can improve the XY compressive strength and Z compressive strength of the prepared carbon / carbon composite material. In some embodiments of this invention, preferably, the XY compressive strength of the prepared carbon / carbon composite material is 150 MPa to 220 MPa, more preferably 200 MPa to 220 MPa. For example, the XY compressive strength of the carbon / carbon composite material can be 150 MPa, 155 MPa, 160 MPa, 165 MPa, 170 MPa, 175 MPa, 180 MPa, 185 MPa, 190 MPa, 195 MPa, 200 MPa, 205 MPa, 210 MPa, 215 MPa, 220 MPa, or any range of two of the above values. In some embodiments of the present invention, preferably, the Z-axis compressive strength of the prepared carbon / carbon composite material is 150 MPa to 220 MPa, more preferably 200 MPa to 220 MPa. For example, the Z-axis compressive strength of the carbon / carbon composite material can be 150 MPa, 155 MPa, 160 MPa, 165 MPa, 170 MPa, 175 MPa, 180 MPa, 185 MPa, 190 MPa, 195 MPa, 200 MPa, 205 MPa, 210 MPa, 215 MPa, 220 MPa, or any range of two of the above values.

[0109] The pre-oxidized fiber, using the technical solution of this invention, exhibits excellent mechanical properties and is easy to process. The carbon / carbon composite material prepared from the pre-oxidized fiber of this invention can achieve compressive strengths of over 150 MPa in both the XY and Z directions, preferably over 200 MPa, demonstrating excellent technical results. Detailed Implementation

[0110] The present invention will now be described in detail with reference to specific embodiments. It should be noted that the following embodiments are only used to further illustrate the present invention and should not be construed as limiting the scope of protection of the present invention. Some non-essential improvements and adjustments made by those skilled in the art based on the content of the present invention are still within the scope of protection of the present invention.

[0111] The standard for testing the linear density (fineness) of pre-oxidized fibers in this invention is GB / T 14343-2008; the standard for testing the tensile properties of pre-oxidized fiber bundles is GB / T 26749-2022 (determination of the stress σ-strain ε curve of pre-oxidized fibers); the standard for testing the bulk density of pre-oxidized fibers is GB / T 30019-2013; the roundness of the pre-oxidized fiber monofilament cross-section is determined by the perimeter L and area A of the monofilament cross-section (roundness = 4πA / L). 2 The test standard for the strength and elongation at break of pre-oxidized fiber monofilaments is GB / T 14337-2008; the test standard for the XY and Z compressive strength of carbon / carbon composites is GB / T 34559-2017.

[0112] The crystallinity and grain size of the pre-oxidized fiber of this invention were obtained by X-ray diffraction (Bruker D8 Discover diffractometer). After subtracting the air background, peak separation was performed using PeakFit, and the crystallinity X of the fiber was calculated according to formula (1). c And the grain size L is calculated according to formula (2). (100) .

[0113] Among them, A a For the area of ​​the non-crystalline peak, A c λ is the area of ​​the crystallization peak, K is the Scherrer constant (taken as 0.89), λ is the wavelength of the X-ray (0.154184nm), β is the half-width at half maximum (FWHM) of the diffraction peak, and θ is the Bragg diffraction angle.

[0114] The carbon and oxygen content of the pre-oxidized fiber of this invention was determined by isotope ratio mass spectrometry coupled with elemental analysis (Thermo Scientific Flash 2000). 1.5-3 mg of polyacrylonitrile pre-oxidized fiber was cut, placed in tin / silver foil, folded into a 0.5 × 0.5 cm square, and tested at 950°C for 720 s. The same sample was tested three times, and the average value was taken. Carbon content was determined using tin foil under an oxygen atmosphere; oxygen content was determined using silver foil under a helium atmosphere.

[0115] In this invention, the nonlinear fitting is performed as follows: the stress σ-strain ε curve (with stress on the vertical axis and strain on the horizontal axis) is obtained by tensile testing of resin-impregnated wire harness according to GB / T26749-2022, and the fitting is performed using Microsoft "Excel" to obtain the calculated coefficients A, B, and C; the fitting is performed in the range of strain 0-2.5%, the trend line option is selected as a polynomial of order 2, and the trend prediction is all 0 period.

[0116] Unless otherwise specified, the raw materials used in the embodiments and comparative examples of this invention are all disclosed in the prior art, such as those that can be directly purchased or prepared according to the preparation methods disclosed in the prior art.

[0117]

Example 1

[0118] Preparation of polyacrylonitrile-based precursor fibers:

[0119] Acrylonitrile and itaconic acid were added to the reactor at a weight ratio of 98.2:1.8, with azobisisobutyronitrile (0.005% of the weight of acrylonitrile) as the initiator and dimethyl sulfoxide (5 times the total weight of acrylonitrile and itaconic acid) as the solvent. The reaction was carried out at 66°C for 22 hours under nitrogen protection. After removing monomers and bubbles, a dimethyl sulfoxide solution of polyacrylonitrile copolymer with a solid content of 20% was prepared.

[0120] Using wet spinning technology, the spinning solution is precisely metered and filtered by a metering pump, then extruded through a 3000-hole spinneret with a 60μm pore size. It is then sequentially coagulated in four coagulation baths of dimethyl sulfoxide aqueous solution at temperatures of 35℃, 60℃, 65℃, and 75℃, with a total draw ratio of 1.2 and concentrations of 85%, 67%, 53%, and 39%, respectively. Following this, it undergoes three stages of hot water drawing at temperatures of 95℃, 96℃, and 97℃, with draw ratios of 1.35, 1.65, and 1.85, respectively. After oiling, it undergoes two stages of drying and densification treatment without drawing, at temperatures of 130℃ and 140℃, respectively. Next, it undergoes steam drawing at a steam pressure of 0.3MPa, with a draw ratio of 1.25. Finally, it undergoes steam heat setting at a steam pressure of 0.12MPa, with a draw ratio of 0.98. The final product is polyacrylonitrile-based precursor yarn.

[0121] Preparation of pre-oxidized fibers:

[0122] The aforementioned polyacrylonitrile-based precursor fibers (3000 fibers per bundle) were sequentially subjected to four pre-oxidation temperature zones, sizing, and drying to obtain a fineness of 3.0 dtex and a bulk density of 1.30 g / cm³. 3The pre-oxidized fiber has a monofilament cross-sectional roundness of 0.86, a monofilament strength of 3.17 cN / dtex, a monofilament breaking elongation of 12.5%, a crystallinity of 0.198, a grain size of 2.787 nm, and a carbon-to-oxygen ratio of 1.84. The pre-oxidation temperature zones are as follows: First pre-oxidation zone: hot air heating at 200℃, draw ratio of 1.010, and residence time of 20 min; Second pre-oxidation zone: hot air heating at 250℃, draw ratio of 1.010, and residence time of 25 min; Third pre-oxidation zone: microwave heating at 2.1 GHz, temperature of 200℃, draw ratio of 0.990, and residence time of 20 min; Fourth pre-oxidation zone: microwave heating at 2.8 GHz, temperature of 220℃, draw ratio of 0.998, and residence time of 20 min. The atmosphere for both hot air and microwave heating is air.

[0123] After tensile property testing, within the strain range of 0-2.5%, the nonlinear fitting formula for the stress σ (in GPa) and strain ε (in %) of the pre-oxidized fiber is σ = Aε. 2 +Bε+C, where A=-0.0143, B=0.147, C=0.0754, that is, σ=-0.0143ε 2 +0.147ε+0.0754, the breaking elongation E of the pre-oxidized fiber monofilament satisfies the formulas -150≤(B*E*100) / A≤-70 and -150≤(B*E*100+C) / A≤-70, where the value of the intermediate term in the above formula is -134.

[0124] Preparation of carbon / carbon composite materials:

[0125] Pre-oxidized fibers were prepared into pre-oxidized fiber integral mats (cubic cuboid, length × width × height 400mm × 300mm × 210mm) using needle punching technology, with a density of 0.21g / cm³. 3 The integral felt was placed in a chemical vapor infiltration furnace and pyrolyzed at 1000℃ with propane as the precursor. The furnace pressure was 1.5 kPa, the propane gas flow rate was 500 mL / min, and the time was 180 hours to obtain a preform treated with chemical vapor infiltration. A phenolic resin / ethanol impregnation solution with a concentration of 1 wt% (PF3312 type, Jinan Shengquan Group Co., Ltd.) was prepared. The preform was placed in the solution, allowed to stand for 2 minutes, and then removed and pre-cured in a 120℃ oven for 10 minutes. Then, it was hot-pressed at a pressure of 2 MPa, a temperature of 240℃, and a time of 30 minutes to obtain a carbon / carbon preform. The carbon / carbon preform was graphitized at 2400℃ for 180 minutes to obtain a carbon / carbon composite material with compressive strengths of 208 MPa in the XY and Z directions, respectively.

[0126]

Example 2

[0127] Preparation of polyacrylonitrile-based precursor fibers:

[0128] Acrylonitrile and itaconic acid were added to the reactor at a weight ratio of 98.2:1.8, with azobisisobutyronitrile (0.005% of the weight of acrylonitrile) as the initiator and dimethyl sulfoxide (5 times the total weight of acrylonitrile and itaconic acid) as the solvent. The reaction was carried out at 66°C for 22 hours under nitrogen protection. After removing monomers and bubbles, a dimethyl sulfoxide solution of polyacrylonitrile copolymer with a solid content of 20% was prepared.

[0129] Using wet spinning technology, the spinning solution is precisely metered and filtered by a metering pump, then extruded through a 6000-hole spinneret with a 60μm pore size. It is then sequentially coagulated in four coagulation baths of dimethyl sulfoxide aqueous solution at temperatures of 35℃, 60℃, 65℃, and 75℃, with a total draw ratio of 1.2 and concentrations of 85%, 67%, 53%, and 39%, respectively. Following this, it undergoes three stages of hot water drawing at temperatures of 95℃, 96℃, and 97℃, with draw ratios of 1.35, 1.65, and 1.85, respectively. After oiling, it undergoes two stages of drying and densification treatment without drawing, at temperatures of 130℃ and 140℃, respectively. Next, it undergoes steam drawing at a steam pressure of 0.3MPa, with a draw ratio of 1.53. Finally, it undergoes steam heat setting at a steam pressure of 0.12MPa, with a draw ratio of 0.98. The final product is then wound to obtain polyacrylonitrile-based precursor fibers.

[0130] Preparation of pre-oxidized fibers:

[0131] The aforementioned polyacrylonitrile-based precursor fibers (6000 fibers per bundle) were subjected to pre-oxidation, sizing, and drying in four temperature zones to obtain a fineness of 2.50 dtex and a bulk density of 1.34 g / cm³. 3 The pre-oxidized fiber has a monofilament cross-sectional roundness of 0.90, a monofilament strength of 3.13 cN / dtex, a monofilament breaking elongation of 12.6%, a crystallinity of 0.192, a grain size of 2.766 nm, and a carbon-to-oxygen ratio of 1.87. The heating process involves four temperature zones: 1) hot air heating at 200°C with a draw ratio of 1.010 and a residence time of 20 min; 2) hot air heating at 250°C with a draw ratio of 1.010 and a residence time of 25 min; 3) microwave heating at 2.1 GHz with a temperature of 200°C and a draw ratio of 0.990 and a residence time of 20 min; and 4) microwave heating at 2.8 GHz with a temperature of 220°C and a draw ratio of 0.998 and a residence time of 20 min. Air is used for both the hot air and microwave heating processes.

[0132] After tensile property testing, within the strain range of 0-2.5%, the nonlinear fitting formula for the stress σ (in GPa) and strain ε (in %) of the pre-oxidized fiber is σ = Aε. 2 +Bε+C, where A=-0.0221, B=0.1668, C=0.068, that is, σ=-0.0221ε 2 +0.1668ε+0.068, the breaking elongation E of the pre-oxidized fiber monofilament satisfies the formulas -150≤(B*E*100) / A≤-70 and -150≤(B*E*100+C) / A≤-70, where the value of the intermediate term in the above formula is -98.

[0133] Preparation of carbon / carbon composite materials:

[0134] The preparation process was the same as in Example 1, and the XY and Z compressive strengths of the resulting carbon / carbon composite material were 201 and 207 MPa, respectively.

[0135]

Example 3

[0136] Preparation of polyacrylonitrile-based precursor fibers:

[0137] Acrylonitrile and itaconic acid were added to the reactor at a weight ratio of 98.2:1.8, with azobisisobutyronitrile (0.005% of the weight of acrylonitrile) as the initiator and dimethyl sulfoxide (5 times the total weight of acrylonitrile and itaconic acid) as the solvent. The reaction was carried out at 66°C for 22 hours under nitrogen protection. After removing monomers and bubbles, a dimethyl sulfoxide solution of polyacrylonitrile copolymer with a solid content of 20% was prepared.

[0138] Wet spinning technology is employed. The spinning solution is precisely metered and filtered by a metering pump, then extruded through a spinneret with 12,000 holes and a pore size of 60 μm. It is then sequentially coagulated in four coagulation baths of dimethyl sulfoxide aqueous solution at temperatures of 35℃, 60℃, 65℃, and 75℃, with a total draw ratio of 1.2 and concentrations of 85%, 67%, 53%, and 39%, respectively. Following this, it undergoes three stages of hot water drawing at temperatures of 95℃, 96℃, and 97℃, with draw ratios of 1.35, 1.65, and 1.85, respectively. After oiling, it undergoes two stages of drying and densification treatment without drawing, at temperatures of 130℃ and 140℃, respectively. Next, it undergoes steam drawing at a steam pressure of 0.3 MPa, with a draw ratio of 1.96. Finally, it undergoes steam heat setting at a steam pressure of 0.12 MPa, with a draw ratio of 0.98. The final product is then wound to obtain polyacrylonitrile-based precursor fibers.

[0139] Preparation of pre-oxidized fibers:

[0140] The aforementioned polyacrylonitrile-based precursor fibers (12,000 fibers per bundle) were subjected to pre-oxidation, sizing, and drying in four temperature zones to obtain a fineness of 1.50 dtex and a bulk density of 1.37 g / cm³. 3 The pre-oxidized fiber has a monofilament cross-sectional roundness of 0.95, a monofilament strength of 3.16 cN / dtex, a monofilament breaking elongation of 12.5%, a crystallinity of 0.215, a grain size of 2.795 nm, and a carbon-to-oxygen ratio of 1.82. The heating process involves four temperature zones: 1) hot air heating at 200°C with a draw ratio of 1.010 and a residence time of 20 min; 2) hot air heating at 250°C with a draw ratio of 1.010 and a residence time of 25 min; 3) microwave heating at 2.1 GHz with a temperature of 200°C and a draw ratio of 0.990 and a residence time of 20 min; and 4) microwave heating at 2.8 GHz with a temperature of 220°C and a draw ratio of 0.998 and a residence time of 20 min. Air is used for both the hot air and microwave heating processes.

[0141] After tensile property testing, within the strain range of 0-2.5%, the nonlinear fitting formula for the stress σ (in GPa) and strain ε (in %) of the pre-oxidized fiber is σ = Aε. 2 +Bε+C, where A=-0.0203, B=0.1617, C=0.0689, that is, σ=-0.0203ε 2 +0.1617ε+0.0689, the breaking elongation E of the pre-oxidized fiber monofilament satisfies the formulas -150≤(B*E*100) / A≤-70 and -150≤(B*E*100+C) / A≤-70, where the value of the intermediate term in the above formula is -103.

[0142] Preparation of carbon / carbon composite materials:

[0143] The preparation process was the same as in Example 1, and the XY and Z compressive strengths of the resulting carbon / carbon composite material were 200 and 204 MPa, respectively.

[0144]

Example 4

[0145] Preparation of polyacrylonitrile-based precursor fibers:

[0146] Acrylonitrile and itaconic acid were added to the reactor at a weight ratio of 98.2:1.8, with azobisisobutyronitrile (0.005% of the weight of acrylonitrile) as the initiator and dimethyl sulfoxide (5 times the total weight of acrylonitrile and itaconic acid) as the solvent. The reaction was carried out at 66°C for 22 hours under nitrogen protection. After removing monomers and bubbles, a dimethyl sulfoxide solution of polyacrylonitrile copolymer with a solid content of 20% was prepared.

[0147] Using wet spinning technology, the spinning solution is precisely metered and filtered by a metering pump, then extruded through a spinneret with 24,000 holes and a pore size of 60 μm. It then sequentially enters four coagulation baths containing dimethyl sulfoxide aqueous solution at temperatures of 35℃, 60℃, 65℃, and 75℃, with a total draw ratio of 1.2 and concentrations of 85%, 67%, 53%, and 39%, respectively. Following this, it undergoes three stages of hot water drawing at temperatures of 95℃, 96℃, and 97℃, with draw ratios of 1.35, 1.65, and 1.85, respectively. After oiling, it undergoes two stages of drying and densification treatment without drawing, at temperatures of 130℃ and 140℃, respectively. Next, it undergoes steam drawing at a steam pressure of 0.3 MPa, with a draw ratio of 2.14. Finally, it undergoes steam heat setting at a steam pressure of 0.12 MPa, with a draw ratio of 0.98. The final product is then wound to obtain polyacrylonitrile-based precursor fibers.

[0148] Preparation of pre-oxidized fibers:

[0149] The aforementioned polyacrylonitrile-based precursor fibers (with 24,000 fibers per bundle) were subjected to pre-oxidation, sizing, and drying in four temperature zones to obtain a fineness of 1.30 dtex and a bulk density of 1.35 g / cm³. 3 The pre-oxidized fiber has a monofilament cross-sectional roundness of 0.86, a monofilament strength of 3.12 cN / dtex, a monofilament breaking elongation of 12.7%, a crystallinity of 0.190, a grain size of 2.771 nm, and a carbon-to-oxygen ratio of 1.87. The heating process involves four temperature zones: 1) hot air heating at 200°C with a draw ratio of 1.010 and a residence time of 20 min; 2) hot air heating at 250°C with a draw ratio of 1.010 and a residence time of 25 min; 3) microwave heating at 2.1 GHz with a temperature of 200°C and a draw ratio of 0.990 and a residence time of 20 min; and 4) microwave heating at 2.8 GHz with a temperature of 220°C and a draw ratio of 0.998 and a residence time of 20 min. Air is used for both the hot air and microwave heating processes.

[0150] After tensile property testing, within the strain range of 0-2.5%, the nonlinear fitting formula for the stress σ (in GPa) and strain ε (in %) of the pre-oxidized fiber is σ = Aε. 2 +Bε+C, where A=-0.019, B=0.1618, C=0.0703, that is, σ=-0.019ε 2 +0.1618ε+0.0703, the breaking elongation E of the pre-oxidized fiber monofilament satisfies the formulas -150≤(B*E*100) / A≤-70 and -150≤(B*E*100+C) / A≤-70, where the value of the intermediate term in the above formula is -112.

[0151] Preparation of carbon / carbon composite materials:

[0152] The preparation process was the same as in Example 1, and the resulting carbon / carbon composite material had XY and Z compressive strengths of 204 and 201 MPa, respectively.

[0153]

Example 5

[0154] Preparation of polyacrylonitrile-based precursor fibers:

[0155] Acrylonitrile and itaconic acid were added to the reactor at a weight ratio of 98.2:1.8, with azobisisobutyronitrile (0.005% of the weight of acrylonitrile) as the initiator and dimethyl sulfoxide (5 times the total weight of acrylonitrile and itaconic acid) as the solvent. The reaction was carried out at 66°C for 22 hours under nitrogen protection. After removing monomers and bubbles, a dimethyl sulfoxide solution of polyacrylonitrile copolymer with a solid content of 20% was prepared.

[0156] Using wet spinning technology, the spinning solution is precisely metered and filtered by a metering pump, then extruded through a spinneret with 50,000 holes and a pore size of 60 μm. It then sequentially enters four coagulation baths containing dimethyl sulfoxide aqueous solution at temperatures of 35℃, 60℃, 65℃, and 75℃, with a total draw ratio of 1.2 and concentrations of 85%, 67%, 53%, and 39%, respectively. Following this, it undergoes three stages of hot water drawing at temperatures of 95℃, 96℃, and 97℃, with draw ratios of 1.35, 1.65, and 1.85, respectively. After oiling, it undergoes two stages of drying and densification treatment without drawing, at temperatures of 130℃ and 140℃, respectively. Next, it undergoes steam drawing at a steam pressure of 0.3 MPa, with a draw ratio of 2.27. Finally, it undergoes steam heat setting at a steam pressure of 0.12 MPa, with a draw ratio of 0.98. The final product is then wound to obtain polyacrylonitrile-based precursor fibers.

[0157] Preparation of pre-oxidized fibers:

[0158] The aforementioned polyacrylonitrile-based precursor fibers (50,000 fibers per bundle) were subjected to pre-oxidation, sizing, and drying in four temperature zones to obtain a fineness of 1.17 dtex and a bulk density of 1.33 g / cm³. 3The pre-oxidized fiber has a monofilament cross-sectional roundness of 0.96, a monofilament strength of 3.15 cN / dtex, a monofilament breaking elongation of 12.7%, a crystallinity of 0.194, a grain size of 2.763 nm, and a carbon-to-oxygen ratio of 1.85. The heating process involves four temperature zones: 1) hot air heating at 200°C with a draw ratio of 1.010 and a residence time of 20 min; 2) hot air heating at 250°C with a draw ratio of 1.010 and a residence time of 25 min; 3) microwave heating at 2.1 GHz with a temperature of 200°C and a draw ratio of 0.990 and a residence time of 20 min; and 4) microwave heating at 2.8 GHz with a temperature of 220°C and a draw ratio of 0.998 and a residence time of 20 min. Air is used for both the hot air and microwave heating processes.

[0159] After tensile property testing, within the strain range of 0-2.5%, the nonlinear fitting formula for the stress σ (in GPa) and strain ε (in %) of the pre-oxidized fiber is σ = Aε. 2 +Bε+C, where A=-0.0218, B=0.1644, C=0.0672, that is, σ=-0.0218ε 2 +0.1644ε+0.0672, the breaking elongation E of the pre-oxidized fiber monofilament satisfies the formulas -150≤(B*E*100) / A≤-70 and -150≤(B*E*100+C) / A≤-70, where the value of the intermediate term in the above formula is -99.

[0160] Preparation of carbon / carbon composite materials:

[0161] The preparation process was the same as in Example 1, and the XY and Z compressive strengths of the resulting carbon / carbon composite material were 200 and 200 MPa, respectively.

[0162]

Example 6

[0163] Preparation of polyacrylonitrile-based precursor fibers:

[0164] Acrylonitrile and itaconic acid were added to the reactor at a weight ratio of 98.2:1.8, with azobisisobutyronitrile (0.005% of the weight of acrylonitrile) as the initiator and dimethyl sulfoxide (5 times the total weight of acrylonitrile and itaconic acid) as the solvent. The reaction was carried out at 66°C for 22 hours under nitrogen protection. After removing monomers and bubbles, a dimethyl sulfoxide solution of polyacrylonitrile copolymer with a solid content of 20% was prepared.

[0165] Using wet spinning technology, the spinning solution is precisely metered and filtered by a metering pump, then extruded through a 6000-hole spinneret with a 60μm pore size. It is then sequentially coagulated in four coagulation baths of dimethyl sulfoxide aqueous solution at temperatures of 35℃, 60℃, 65℃, and 75℃, with a total draw ratio of 1.2 and concentrations of 85%, 67%, 53%, and 39%, respectively. Following this, it undergoes three stages of hot water drawing at temperatures of 95℃, 96℃, and 97℃, with draw ratios of 1.35, 1.65, and 1.85, respectively. After oiling, it undergoes two stages of drying and densification treatment without drawing, at temperatures of 130℃ and 140℃, respectively. Next, it undergoes steam drawing at a steam pressure of 0.3MPa, with a draw ratio of 2.17. Finally, it undergoes steam heat setting at a steam pressure of 0.12MPa, with a draw ratio of 0.98. The final product is then wound to obtain polyacrylonitrile-based precursor fibers.

[0166] Preparation of pre-oxidized fibers:

[0167] The aforementioned polyacrylonitrile-based precursor fibers (6000 fibers per bundle) were subjected to pre-oxidation, sizing, and drying in six temperature zones to obtain a fineness of 1.17 dtex and a bulk density of 1.34 g / cm³. 3 Pre-oxidized fiber with a monofilament cross-sectional roundness of 0.89, a monofilament strength of 3.02 cN / dtex, a monofilament breaking elongation of 14.0%, a crystallinity of 0.177, a grain size of 2.489 nm, and a carbon to oxygen ratio of 1.07. The heating process is divided into six zones: Zone 1 is hot air heating at 200°C, with a draw ratio of 1.010 and a residence time of 20 min; Zone 2 is hot air heating at 250°C, with a draw ratio of 1.010 and a residence time of 25 min; Zone 3 is microwave heating at 2.1 GHz, with a temperature of 200°C, a draw ratio of 0.990, and a residence time of 20 min; Zone 4 is microwave heating at 2.8 GHz, with a temperature of 220°C, a draw ratio of 0.998, and a residence time of 20 min; Zone 5 is microwave heating at 3.8 GHz, with a temperature of 230°C, a draw ratio of 0.986, and a residence time of 12 min; and Zone 6 is microwave heating at 4.5 GHz, with a temperature of 235°C, a draw ratio of 0.980, and a residence time of 10 min. The atmosphere for both hot air heating and microwave heating is air.

[0168] After tensile property testing, within the strain range of 0-2.5%, the nonlinear fitting formula for the stress σ (in GPa) and strain ε (in %) of the pre-oxidized fiber is σ = Aε. 2 +Bε+C, where A=-0.0325, B=0.1896, C=0.063, that is, σ=-0.0325ε 2+0.1896ε+0.063, the breaking elongation E of the pre-oxidized fiber monofilament satisfies the formulas -150≤(B*E*100) / A≤-70 and -150≤(B*E*100+C) / A≤-70, where the value of the intermediate term in the above formula is -84.

[0169] Preparation of carbon / carbon composite materials:

[0170] The preparation process was the same as in Example 1, and the XY and Z compressive strengths of the resulting carbon / carbon composite material were 201 and 210 MPa, respectively.

[0171]

Example 7

[0172] Preparation of polyacrylonitrile-based precursor fibers:

[0173] Acrylonitrile and itaconic acid were added to the reactor at a weight ratio of 98.2:1.8, with azobisisobutyronitrile (0.005% of the weight of acrylonitrile) as the initiator and dimethyl sulfoxide (5 times the total weight of acrylonitrile and itaconic acid) as the solvent. The reaction was carried out at 66°C for 22 hours under nitrogen protection. After removing monomers and bubbles, a dimethyl sulfoxide solution of polyacrylonitrile copolymer with a solid content of 20% was prepared.

[0174] Using wet spinning technology, the spinning solution is precisely metered and filtered by a metering pump, then extruded through a spinneret with 12,000 holes and a pore size of 60 μm. It is then sequentially coagulated in four coagulation baths of dimethyl sulfoxide aqueous solution at temperatures of 35℃, 60℃, 65℃, and 75℃, with a total draw ratio of 1.2 and concentrations of 85%, 67%, 53%, and 39%, respectively. Following this, it undergoes three stages of hot water drawing at temperatures of 95℃, 96℃, and 97℃, with draw ratios of 1.35, 1.65, and 1.85, respectively. After oiling, it undergoes two stages of drying and densification treatment without drawing, at temperatures of 130℃ and 140℃, respectively. Next, it undergoes steam drawing at a steam pressure of 0.3 MPa, with a draw ratio of 1.34. Finally, it undergoes steam heat setting at a steam pressure of 0.12 MPa, with a draw ratio of 0.98. The final product is then wound to obtain polyacrylonitrile-based precursor fibers.

[0175] Preparation of pre-oxidized fibers:

[0176] The aforementioned polyacrylonitrile-based precursor fibers (12,000 fibers per bundle) were subjected to pre-oxidation, sizing, and drying in four temperature zones to obtain a fineness of 2.85 dtex and a bulk density of 1.35 g / cm³. 3The pre-oxidized fiber has a monofilament cross-sectional roundness of 0.86, a monofilament strength of 3.08 cN / dtex, a monofilament breaking elongation of 13.9%, a crystallinity of 0.185, a grain size of 2.790 nm, and a carbon-to-oxygen ratio of 1.86. The heating process involves four temperature zones: 1) hot air heating at 200°C with a draw ratio of 1.010 and a residence time of 20 min; 2) hot air heating at 250°C with a draw ratio of 1.010 and a residence time of 25 min; 3) microwave heating at 2.1 GHz with a temperature of 200°C and a draw ratio of 0.990 and a residence time of 20 min; and 4) microwave heating at 2.8 GHz with a temperature of 220°C and a draw ratio of 0.998 and a residence time of 20 min. Air is used for both the hot air and microwave heating processes.

[0177] After tensile property testing, within the strain range of 0-2.5%, the nonlinear fitting formula for the stress σ (in GPa) and strain ε (in %) of the pre-oxidized fiber is σ = Aε. 2 +Bε+C, where A=-0.026, B=0.1736, C=0.0649, that is, σ=-0.026ε 2 +0.1736ε+0.0649, the breaking elongation E of the pre-oxidized fiber monofilament satisfies the formulas -150≤(B*E*100) / A≤-70 and -150≤(B*E*100+C) / A≤-70, where the value of the intermediate term in the above formula is -95.

[0178] Preparation of carbon / carbon composite materials:

[0179] The preparation process was the same as in Example 1, and the resulting carbon / carbon composite material had XY and Z compressive strengths of 207 and 202 MPa, respectively.

[0180]

Example 8

[0181] Preparation of polyacrylonitrile-based precursor fibers (same as Example 3).

[0182] Preparation of pre-oxidized fibers:

[0183] The aforementioned polyacrylonitrile-based precursor fibers (12,000 fibers per bundle) were subjected to pre-oxidation, sizing, and drying in four temperature zones to obtain a fineness of 1.50 dtex and a bulk density of 1.38 g / cm³. 3The pre-oxidized fiber has a monofilament cross-sectional roundness of 0.95, a monofilament strength of 3.10 cN / dtex, a monofilament breaking elongation of 12.8%, a crystallinity of 0.193, a grain size of 2.597 nm, and a carbon-to-oxygen ratio of 1.75. The heating process involves four temperature zones: 1) Hot air heating at 180°C, a draw ratio of 1.050, and a residence time of 30 min; 2) Hot air heating at 250°C, a draw ratio of 1.010, and a residence time of 25 min; 3) Microwave heating at 2.1 GHz, a temperature of 200°C, a draw ratio of 0.990, and a residence time of 20 min; and 4) Microwave heating at 2.8 GHz, a temperature of 220°C, a draw ratio of 0.998, and a residence time of 20 min. Air is used for both the hot air and microwave heating processes.

[0184] After tensile property testing, within the strain range of 0-2.5%, the nonlinear fitting formula for the stress σ (in GPa) and strain ε (in %) of the pre-oxidized fiber is σ = Aε. 2 +Bε+C, where A=-0.0271, B=0.1785, C=0.0649, that is, σ=-0.0271ε 2 +0.1785ε+0.0649, the breaking elongation E of the pre-oxidized fiber monofilament satisfies the formulas -150≤(B*E*100) / A≤-70 and -150≤(B*E*100+C) / A≤-70, where the value of the intermediate term in the above formula is -87.

[0185] Preparation of carbon / carbon composite materials:

[0186] The preparation process was the same as in Example 1, and the XY and Z compressive strengths of the resulting carbon / carbon composite material were 205 and 204 MPa, respectively.

[0187]

Example 9

[0188] Preparation of polyacrylonitrile-based precursor fibers (same as Example 3).

[0189] Preparation of pre-oxidized fibers:

[0190] The aforementioned polyacrylonitrile-based precursor fibers (12,000 fibers per bundle) were subjected to pre-oxidation, sizing, and drying in four temperature zones to obtain a fineness of 1.50 dtex and a bulk density of 1.36 g / cm³. 3The pre-oxidized fiber has a monofilament cross-sectional roundness of 0.95, a monofilament strength of 3.14 cN / dtex, a monofilament breaking elongation of 12.6%, a crystallinity of 0.207, a grain size of 2.799 nm, and a carbon-to-oxygen ratio of 1.96. The heating process involves four temperature zones: 1) hot air heating at 220°C with a draw ratio of 1.020 and a residence time of 10 min; 2) hot air heating at 280°C with a draw ratio of 1.030 and a residence time of 15 min; 3) microwave heating at 2.1 GHz with a temperature of 200°C and a draw ratio of 0.990 and a residence time of 20 min; and 4) microwave heating at 2.8 GHz with a temperature of 220°C and a draw ratio of 0.998 and a residence time of 20 min. Air is used for both the hot air and microwave heating processes.

[0191] After tensile property testing, within the strain range of 0-2.5%, the nonlinear fitting formula for the stress σ (in GPa) and strain ε (in %) of the pre-oxidized fiber is σ = Aε. 2 +Bε+C, where A=-0.0324, B=0.1901, C=0.0626, that is, σ=-0.0324ε 2 +0.1901ε+0.0626, the breaking elongation E of the pre-oxidized fiber monofilament satisfies the formulas -150≤(B*E*100) / A≤-70 and -150≤(B*E*100+C) / A≤-70, where the value of the intermediate term in the above formula is -76.

[0192] Preparation of carbon / carbon composite materials:

[0193] The preparation process was the same as in Example 1, and the XY and Z compressive strengths of the resulting carbon / carbon composite material were 207 and 208 MPa, respectively.

[0194]

Example 10

[0195] Preparation of polyacrylonitrile-based precursor fibers (same as Example 3).

[0196] Preparation of pre-oxidized fibers:

[0197] The aforementioned polyacrylonitrile-based precursor fibers (12,000 fibers per bundle) were subjected to pre-oxidation, sizing, and drying in four temperature zones to obtain a fineness of 1.50 dtex and a bulk density of 1.37 g / cm³. 3The pre-oxidized fiber has a monofilament cross-sectional roundness of 0.95, a monofilament strength of 3.09 cN / dtex, a monofilament breaking elongation of 13.0%, a crystallinity of 0.190, a grain size of 2.841 nm, and a carbon-to-oxygen ratio of 2.22. The heating process involves four temperature zones: 1) Hot air heating at 180℃, a draw ratio of 1.050, and a residence time of 30 min; 2) Hot air heating at 250℃, a draw ratio of 1.010, and a residence time of 25 min; 3) Microwave heating at 4.7 GHz, a temperature of 240℃, a draw ratio of 0.990, and a residence time of 20 min; and 4) Microwave heating at 5.7 GHz, a temperature of 245℃, a draw ratio of 0.998, and a residence time of 10 min. Air is used for both the hot air and microwave heating processes.

[0198] After tensile property testing, within the strain range of 0-2.5%, the nonlinear fitting formula for the stress σ (in GPa) and strain ε (in %) of the pre-oxidized fiber is σ = Aε. 2 +Bε+C, where A=-0.029, B=0.1842, C=0.064, that is, σ=-0.029ε 2 +0.1842ε+0.064, the breaking elongation E of the pre-oxidized fiber monofilament satisfies the formulas -150≤(B*E*100) / A≤-70 and -150≤(B*E*100+C) / A≤-70, where the value of the intermediate term in the above formula is -85.

[0199] Preparation of carbon / carbon composite materials:

[0200] The preparation process was the same as in Example 1, and the XY and Z compressive strengths of the resulting carbon / carbon composite material were 206 and 204 MPa, respectively.

[0201]

Example 11

[0202] Preparation of polyacrylonitrile-based precursor fibers (same as Example 3).

[0203] Preparation of pre-oxidized fibers:

[0204] The aforementioned polyacrylonitrile-based precursor fibers (12,000 fibers per bundle) were subjected to pre-oxidation, sizing, and drying in six temperature zones to obtain a fineness of 1.50 dtex and a bulk density of 1.38 g / cm³. 3Pre-oxidized fiber with a monofilament cross-sectional roundness of 0.95, a monofilament strength of 3.00 cN / dtex, a monofilament breaking elongation of 12.6%, a crystallinity of 0.173, a grain size of 2.433 nm, and a carbon to oxygen ratio of 0.94. The heating process is divided into six zones: Zone 1 is hot air heating at 220°C, with a draw ratio of 1.020 and a residence time of 10 min; Zone 2 is hot air heating at 280°C, with a draw ratio of 1.030 and a residence time of 15 min; Zone 3 is microwave heating at 2.4 GHz, with a temperature of 210°C, a draw ratio of 0.990, and a residence time of 20 min; Zone 4 is microwave heating at 3.7 GHz, with a temperature of 230°C, a draw ratio of 0.998, and a residence time of 12 min; Zone 5 is microwave heating at 4.8 GHz, with a temperature of 240°C, a draw ratio of 0.986, and a residence time of 10 min; and Zone 6 is microwave heating at 5.9 GHz, with a temperature of 250°C, a draw ratio of 0.980, and a residence time of 10 min. The atmosphere for both hot air heating and microwave heating is air.

[0205] After tensile property testing, within the strain range of 0-2.5%, the nonlinear fitting formula for the stress σ (in GPa) and strain ε (in %) of the pre-oxidized fiber is σ = Aε. 2 +Bε+C, where A=-0.0299, B=0.1877, C=0.0639, that is, σ=-0.0299ε 2 +0.1877ε+0.0639, the breaking elongation E of the pre-oxidized fiber monofilament satisfies the formulas -150≤(B*E*100) / A≤-70 and -150≤(B*E*100+C) / A≤-70, where the value of the intermediate term in the above formula is -82.

[0206] Preparation of carbon / carbon composite materials:

[0207] The preparation process was the same as in Example 1, and the XY and Z compressive strengths of the resulting carbon / carbon composite material were 203 and 207 MPa, respectively.

[0208]

Example 12

[0209] Preparation of polyacrylonitrile-based precursor fibers (same as Example 3).

[0210] Preparation of pre-oxidized fibers:

[0211] The aforementioned polyacrylonitrile-based precursor fibers (12,000 fibers per bundle) were subjected to pre-oxidation, sizing, and drying in four temperature zones to obtain a fineness of 1.50 dtex and a bulk density of 1.36 g / cm³. 3The pre-oxidized fiber has a monofilament cross-sectional roundness of 0.95, a monofilament strength of 2.84 cN / dtex, a monofilament breaking elongation of 13.4%, a crystallinity of 0.157, a grain size of 2.451 nm, and a carbon-to-oxygen ratio of 1.70. The heating process involves four temperature zones: 1) Hot air heating at 150°C with a draw ratio of 1.010 and a residence time of 30 min; 2) Hot air heating at 280°C with a draw ratio of 1.040 and a residence time of 30 min; 3) Microwave heating at 2.1 GHz with a temperature of 200°C and a draw ratio of 0.990 and a residence time of 20 min; and 4) Microwave heating at 2.8 GHz with a temperature of 220°C and a draw ratio of 0.998 and a residence time of 20 min. Air is used for both the hot air and microwave heating processes.

[0212] After tensile property testing, within the strain range of 0-2.5%, the nonlinear fitting formula for the stress σ (in GPa) and strain ε (in %) of the pre-oxidized fiber is σ = Aε. 2 +Bε+C, where A=-0.0168, B=0.1563, C=0.0723, that is, σ=-0.0168ε 2 +0.1563ε+0.0723, the breaking elongation E of the pre-oxidized fiber monofilament satisfies the formulas -150≤(B*E*100) / A≤-70 and -150≤(B*E*100+C) / A≤-70, where the value of the intermediate term in the above formula is -129.

[0213] Preparation of carbon / carbon composite materials:

[0214] The preparation process was the same as in Example 1, and the XY and Z compressive strengths of the resulting carbon / carbon composite material were 157 and 163 MPa, respectively.

[0215]

Example 13

[0216] Preparation of polyacrylonitrile-based precursor fibers (same as Example 4).

[0217] Preparation of pre-oxidized fibers:

[0218] The aforementioned polyacrylonitrile-based precursor fibers (with 24,000 fibers per bundle) were subjected to pre-oxidation, sizing, and drying in four temperature zones to obtain a fineness of 1.30 dtex and a bulk density of 1.36 g / cm³. 3The pre-oxidized fiber has a monofilament cross-sectional roundness of 0.86, a monofilament strength of 2.80 cN / dtex, a monofilament breaking elongation of 13.5%, a crystallinity of 0.150, a grain size of 2.482 nm, and a carbon-to-oxygen ratio of 1.75. The heating process involves four temperature zones: 1) hot air heating at 200°C, a draw ratio of 1.010, and a residence time of 20 min; 2) hot air heating at 250°C, a draw ratio of 1.010, and a residence time of 25 min; 3) microwave heating at 1.5 GHz, a temperature of 190°C, a draw ratio of 0.995, and a residence time of 30 min; and 4) microwave heating at 2.8 GHz, a temperature of 220°C, a draw ratio of 0.998, and a residence time of 20 min. Air is used for both the hot air and microwave heating processes.

[0219] After tensile property testing, within the strain range of 0-2.5%, the nonlinear fitting formula for the stress σ (in GPa) and strain ε (in %) of the pre-oxidized fiber is σ = Aε. 2 +Bε+C, where A=-0.0413, B=0.2353, C=0.0832, that is, σ=-0.0413ε 2 +0.2353ε+0.0832, the breaking elongation E of the pre-oxidized fiber monofilament satisfies the formulas -150≤(B*E*100) / A≤-70 and -150≤(B*E*100+C) / A≤-70, where the value of the intermediate term in the above formula is -79.

[0220] Preparation of carbon / carbon composite materials:

[0221] The preparation process was the same as in Example 1, and the XY and Z compressive strengths of the resulting carbon / carbon composite material were 164 and 176 MPa, respectively.

[0222]

Example 14

[0223] Preparation of polyacrylonitrile-based precursor fibers (same as Example 5).

[0224] Preparation of pre-oxidized fibers:

[0225] The aforementioned polyacrylonitrile-based precursor fibers (50,000 fibers per bundle) were subjected to pre-oxidation, sizing, and drying in four temperature zones to obtain a fineness of 1.17 dtex and a bulk density of 1.35 g / cm³. 3The pre-oxidized fiber has a monofilament cross-sectional roundness of 0.96, a monofilament strength of 2.81 cN / dtex, a monofilament breaking elongation of 13.7%, a crystallinity of 0.152, a grain size of 2.475 nm, and a carbon-to-oxygen ratio of 1.73. The heating process involves four temperature zones: 1) Hot air heating at 200°C with a draw ratio of 1.010 and a residence time of 35 min; 2) Hot air heating at 250°C with a draw ratio of 1.010 and a residence time of 25 min; 3) Microwave heating at 2.1 GHz with a temperature of 200°C and a draw ratio of 0.990 and a residence time of 20 min; and 4) Microwave heating at 2.8 GHz with a temperature of 220°C and a draw ratio of 0.998 and a residence time of 20 min. Air is used for both the hot air and microwave heating processes.

[0226] After tensile property testing, within the strain range of 0-2.5%, the nonlinear fitting formula for the stress σ (in GPa) and strain ε (in %) of the pre-oxidized fiber is σ = Aε. 2 +Bε+C, where A=-0.0304, B=0.1801, C=0.0754, that is, σ=-0.0304ε 2 +0.1801ε+0.0754, the breaking elongation E of the pre-oxidized fiber monofilament satisfies the formulas -150≤(B*E*100) / A≤-70 and -150≤(B*E*100+C) / A≤-70, where the value of the intermediate term in the above formula is -84.

[0227] Preparation of carbon / carbon composite materials:

[0228] The preparation process was the same as in Example 1, and the XY and Z compressive strengths of the resulting carbon / carbon composite material were 164 and 178 MPa, respectively.

[0229]

Example 15

[0230] Preparation of polyacrylonitrile-based precursor fibers (same as Example 3).

[0231] Preparation of pre-oxidized fibers:

[0232] The aforementioned polyacrylonitrile-based precursor fibers (12,000 fibers per bundle) were subjected to pre-oxidation, sizing, and drying in seven temperature zones to obtain a fineness of 1.50 dtex and a bulk density of 1.39 g / cm³. 3The pre-oxidized fiber has a monofilament cross-sectional roundness of 0.95, a monofilament strength of 2.82 cN / dtex, a monofilament breaking elongation of 13.7%, a crystallinity of 0.155, a grain size of 2.375 nm, and a carbon-to-oxygen ratio of 0.87. The heating process involves: a first temperature zone of 220°C (hot air heating), a draw ratio of 1.020, and a residence time of 10 min; a second temperature zone of 280°C (hot air heating), a draw ratio of 1.030, and a residence time of 15 min; a third temperature zone of 200°C (microwave heating), a draw ratio of 0.990, and a residence time of 20 min; and a fourth temperature zone of 225°C (microwave heating), a draw ratio of 0.998, and a residence time of 15 min. The fifth temperature zone is microwave heating at a frequency of 4.0 GHz, a temperature of 233°C, a draw ratio of 0.986, and a residence time of 12 min; the sixth temperature zone is microwave heating at a frequency of 5.2 GHz, a temperature of 243°C, a draw ratio of 0.980, and a residence time of 10 min; the seventh temperature zone is microwave heating at a frequency of 5.9 GHz, a temperature of 250°C, a draw ratio of 1.0, and a residence time of 10 min. The atmosphere for both hot air heating and microwave heating is air.

[0233] After tensile property testing, within the strain range of 0-2.5%, the nonlinear fitting formula for the stress σ (in GPa) and strain ε (in %) of the pre-oxidized fiber is σ = Aε. 2 +Bε+C, where A=-0.0298, B=0.1793, C=0.0831, that is, σ=-0.0298ε 2 +0.1793ε+0.0831, the breaking elongation E of the pre-oxidized fiber monofilament satisfies the formulas -150≤(B*E*100) / A≤-70 and -150≤(B*E*100+C) / A≤-70, where the value of the intermediate term in the above formula is -85.

[0234] Preparation of carbon / carbon composite materials:

[0235] The preparation process was the same as in Example 1, and the XY and Z compressive strengths of the resulting carbon / carbon composite material were 169 and 174 MPa, respectively.

[0236]

Example 16

[0237] Preparation of polyacrylonitrile-based precursor fibers (same as Example 3).

[0238] Preparation of pre-oxidized fibers:

[0239] The aforementioned polyacrylonitrile-based precursor fibers (12,000 fibers per bundle) were subjected to pre-oxidation, sizing, and drying in five temperature zones to obtain a fineness of 1.50 dtex and a bulk density of 1.41 g / cm³. 3 Pre-oxidized fiber with a monofilament cross-sectional roundness of 0.95, a monofilament strength of 2.87 cN / dtex, a monofilament breaking elongation of 13.7%, a crystallinity of 0.165, a grain size of 2.543 nm, and a carbon to oxygen ratio of 1.67. The heating process is divided into five zones: Zone 1 (hot air heating, 200℃, draw ratio 1.020, residence time 20 min); Zone 2 (hot air heating, 220℃, draw ratio 1.030, residence time 20 min); Zone 3 (hot air heating, 250℃, draw ratio 1.030, residence time 20 min); Zone 4 (microwave heating, 2.1GHz, 200℃, draw ratio 0.990, residence time 20 min); and Zone 5 (microwave heating, 2.8GHz, 220℃, draw ratio 0.998, residence time 20 min). The atmosphere for both hot air and microwave heating is air.

[0240] After tensile property testing, within the strain range of 0-2.5%, the nonlinear fitting formula for the stress σ (in GPa) and strain ε (in %) of the pre-oxidized fiber is σ = Aε. 2 +Bε+C, where A=-0.0242, B=0.1946, C=0.0643, that is, σ=-0.0242ε 2 +0.1946ε+0.0643, the breaking elongation E of the pre-oxidized fiber monofilament satisfies the formulas -150≤(B*E*100) / A≤-70 and -150≤(B*E*100+C) / A≤-70, where the value of the intermediate term in the above formula is -113.

[0241] Preparation of carbon / carbon composite materials:

[0242] The preparation process was the same as in Example 1, and the XY and Z compressive strengths of the resulting carbon / carbon composite material were 153 and 162 MPa, respectively.

[0243]

Example 17

[0244] Preparation of polyacrylonitrile-based precursor fibers (same as Example 3).

[0245] Preparation of pre-oxidized fibers:

[0246] The aforementioned polyacrylonitrile-based precursor fibers (12,000 fibers per bundle) were subjected to pre-oxidation, sizing, and drying in four temperature zones to obtain a fineness of 1.50 dtex and a bulk density of 1.36 g / cm³. 3The pre-oxidized fiber has a monofilament cross-sectional roundness of 0.97, a monofilament strength of 2.90 cN / dtex, a monofilament breaking elongation of 13.1%, a crystallinity of 0.170, a grain size of 2.559 nm, and a carbon-to-oxygen ratio of 1.73. The heating process involves four temperature zones: 1) hot air heating at 200°C with a draw ratio of 1.010 and a residence time of 20 min; 2) hot air heating at 250°C with a draw ratio of 1.010 and a residence time of 25 min; 3) microwave heating at 2.8 GHz with a temperature of 220°C and a draw ratio of 0.998 and a residence time of 20 min; and 4) microwave heating at 2.1 GHz with a temperature of 200°C and a draw ratio of 0.990 and a residence time of 20 min. Air is used for both the hot air and microwave heating processes.

[0247] After tensile property testing, within the strain range of 0-2.5%, the nonlinear fitting formula for the stress σ (in GPa) and strain ε (in %) of the pre-oxidized fiber is σ = Aε. 2 +Bε+C, where A=-0.0227, B=0.1624, C=0.0648, that is, σ=-0.0227ε 2 +0.1624ε+0.0648, the breaking elongation E of the pre-oxidized fiber monofilament satisfies the formulas -150≤(B*E*100) / A≤-70 and -150≤(B*E*100+C) / A≤-70, where the value of the intermediate term in the above formula is -97.

[0248] Preparation of carbon / carbon composite materials:

[0249] The preparation process was the same as in Example 1, and the XY and Z compressive strengths of the resulting carbon / carbon composite material were 158 and 161 MPa, respectively.

[0250]

Comparative Example 1

[0251] Preparation of polyacrylonitrile-based precursor fibers (same as in Example 1).

[0252] Preparation of pre-oxidized fibers:

[0253] The aforementioned polyacrylonitrile-based precursor fibers (3000 fibers per bundle) were subjected to pre-oxidation, sizing, and drying in three temperature zones to obtain a fineness of 3.00 dtex and a bulk density of 1.27 g / cm³. 3The pre-oxidized fiber has a monofilament cross-sectional roundness of 0.86, a monofilament strength of 2.65 cN / dtex, a monofilament breaking elongation of 13.6%, a crystallinity of 0.153, a grain size of 2.571 nm, and a carbon-to-oxygen ratio of 1.50. The heating process involves three temperature zones: a first zone of hot air heating at 200°C, a draw ratio of 1.010, and a residence time of 20 min; a second zone of hot air heating at 250°C, a draw ratio of 1.010, and a residence time of 25 min; and a third zone of microwave heating at 2.1 GHz, a temperature of 200°C, a draw ratio of 0.990, and a residence time of 20 min. The atmosphere for both hot air and microwave heating is air.

[0254] After tensile property testing, within the strain range of 0-2.5%, the nonlinear fitting formula for the stress σ (in GPa) and strain ε (in %) of the pre-oxidized fiber is σ = Aε. 2 +Bε+C, where A=-0.0106, B=0.1573, C=0.0576, that is, σ=-0.0106ε 2 +0.1573ε+0.0576, the breaking elongation E of the pre-oxidized fiber monofilament does not satisfy the formulas -150≤(B*E*100) / A≤-70 and -150≤(B*E*100+C) / A≤-70, where the value of the intermediate term in the above formula is -207.

[0255] Preparation of carbon / carbon composite materials:

[0256] The preparation process was the same as in Example 1, and the XY and Z compressive strengths of the resulting carbon / carbon composite material were 87 and 95 MPa, respectively.

[0257] [Comparative Example 2]

[0258] Preparation of polyacrylonitrile-based precursor fibers (same as Example 2).

[0259] Preparation of pre-oxidized fibers:

[0260] The aforementioned polyacrylonitrile-based precursor fibers (6000 fibers per bundle) were subjected to pre-oxidation, sizing, and drying in four temperature zones to obtain a fineness of 2.50 dtex and a bulk density of 1.36 g / cm³. 3The pre-oxidized fiber has a monofilament cross-sectional roundness of 0.90, a monofilament strength of 2.75 cN / dtex, a monofilament breaking elongation of 13.2%, a crystallinity of 0.171, a grain size of 2.735 nm, and a carbon-to-oxygen ratio of 1.79. The heating process involves four temperature zones: 1) hot air heating at 200°C (draw ratio 1.010, residence time 20 min); 2) hot air heating at 250°C (draw ratio 1.010, residence time 25 min); 3) hot air heating at 280°C (draw ratio 0.990, residence time 10 min); and 4) microwave heating at 2.8 GHz (220°C, draw ratio 0.998, residence time 20 min). Air is used for both the hot air and microwave heating processes.

[0261] After tensile property testing, within the strain range of 0-2.5%, the nonlinear fitting formula for the stress σ (in GPa) and strain ε (in %) of the pre-oxidized fiber is σ = Aε. 2 +Bε+C, where A=-0.014, B=0.1672, C=0.0498, that is, σ=-0.014ε 2 +0.1672ε+0.0498, the breaking elongation E of the pre-oxidized fiber monofilament does not satisfy the formulas -150≤(B*E*100) / A≤-70 and -150≤(B*E*100+C) / A≤-70, where the value of the intermediate term in the above formula is -161.

[0262] Preparation of carbon / carbon composite materials:

[0263] The preparation process was the same as in Example 1, and the XY and Z compressive strengths of the resulting carbon / carbon composite material were 103 and 118 MPa, respectively.

[0264] [Comparative Example 3]

[0265] Preparation of polyacrylonitrile-based precursor fibers (same as Example 3).

[0266] Preparation of pre-oxidized fibers:

[0267] The aforementioned polyacrylonitrile-based precursor fibers (12,000 fibers per bundle) were subjected to pre-oxidation, sizing, and drying in two temperature zones to obtain a fineness of 1.50 dtex and a bulk density of 1.21 g / cm³. 3The pre-oxidized fiber has a monofilament cross-sectional roundness of 0.95, a monofilament strength of 2.61 cN / dtex, a monofilament breaking elongation of 10.2%, a crystallinity of 0.150, a grain size of 2.568 nm, and a carbon-to-oxygen ratio of 1.47. The first temperature zone is heated by hot air at 220°C, with a draw ratio of 1.050 and a residence time of 15 min. The second temperature zone is heated by microwave at 3.5 GHz at 225°C, with a draw ratio of 0.950 and a residence time of 25 min. The atmosphere for both hot air and microwave heating is air.

[0268] After tensile property testing, within the strain range of 0-2.5%, the nonlinear fitting formula for the stress σ (in GPa) and strain ε (in %) of the pre-oxidized fiber is σ = Aε. 2 +Bε+C, where A=-0.0475, B=0.1576, C=0.0742, that is, σ=-0.0475ε 2 +0.1576ε+0.0742, the breaking elongation E of the pre-oxidized fiber monofilament does not satisfy the formulas -150≤(B*E*100) / A≤-70 and -150≤(B*E*100+C) / A≤-70, where the value of the intermediate term in the above formula is -52.

[0269] Preparation of carbon / carbon composite materials:

[0270] The preparation process was the same as in Example 1, and the XY and Z compressive strengths of the resulting carbon / carbon composite material were 76 and 82 MPa, respectively.

[0271] [Comparative Example 4]

[0272] Preparation of polyacrylonitrile-based precursor fibers (same as Example 4).

[0273] Preparation of pre-oxidized fibers:

[0274] The aforementioned polyacrylonitrile-based precursor fibers (with 24,000 fibers per bundle) were subjected to pre-oxidation, sizing, and drying in four temperature zones to obtain a fineness of 1.30 dtex and a bulk density of 1.41 g / cm³. 3The pre-oxidized fiber has a monofilament cross-sectional roundness of 0.86, a monofilament strength of 2.68 cN / dtex, a monofilament breaking elongation of 15.8%, a crystallinity of 0.176, a grain size of 2.516 nm, and a carbon-to-oxygen ratio of 1.43. The heating process involves four temperature zones: 1) Hot air heating at 180°C, a draw ratio of 1.150, and a residence time of 20 min; 2) Hot air heating at 220°C, a draw ratio of 1.100, and a residence time of 30 min; 3) Hot air heating at 250°C, a draw ratio of 1.050, and a residence time of 25 min; and 4) Hot air heating at 280°C, a draw ratio of 1.010, and a residence time of 15 min. The heating atmosphere is air.

[0275] After tensile property testing, within the strain range of 0-2.5%, the nonlinear fitting formula for the stress σ (in GPa) and strain ε (in %) of the pre-oxidized fiber is σ = Aε. 2 +Bε+C, where A=-0.0454, B=0.1942, C=0.0617, that is, σ=-0.0384ε 2 +0.1942ε+0.0617, the breaking elongation E of the pre-oxidized fiber monofilament does not satisfy the formulas -150≤(B*E*100) / A≤-70 and -150≤(B*E*100+C) / A≤-70, where the value of the intermediate term in the above formula is -69.

[0276] Preparation of carbon / carbon composite materials:

[0277] The preparation process was the same as in Example 1, and the XY and Z compressive strengths of the resulting carbon / carbon composite material were 78 and 85 MPa, respectively.

[0278] [Comparative Example 5]

[0279] Preparation of polyacrylonitrile-based precursor fibers (same as Example 3).

[0280] Preparation of pre-oxidized fibers:

[0281] The aforementioned polyacrylonitrile-based precursor fibers (12,000 fibers per bundle) were subjected to pre-oxidation, sizing, and drying in four temperature zones to obtain a fineness of 1.50 dtex and a bulk density of 1.31 g / cm³. 3The pre-oxidized fiber has a monofilament cross-sectional roundness of 0.95, a monofilament strength of 2.64 cN / dtex, a monofilament breaking elongation of 15.0%, a crystallinity of 0.157, a grain size of 2.576 nm, and a carbon-to-oxygen ratio of 1.57. The heating process involves: a first temperature zone (microwave heating) at 2.0 GHz, a temperature of 200°C, a draw ratio of 0.999, and a residence time of 30 min; a second temperature zone (microwave heating) at 4.5 GHz, a temperature of 235°C, a draw ratio of 1.000, and a residence time of 27 min; a third temperature zone (microwave heating) at 5.4 GHz, a temperature of 245°C, a draw ratio of 0.986, and a residence time of 15 min; and a fourth temperature zone (microwave heating) at 5.9 GHz, a temperature of 250°C, a draw ratio of 0.992, and a residence time of 13 min. The microwave heating atmosphere is air.

[0282] After tensile property testing, within the strain range of 0-2.5%, the nonlinear fitting formula for the stress σ (in GPa) and strain ε (in %) of the pre-oxidized fiber is σ = Aε. 2 +Bε+C, where A=-0.0359, B=0.1357, C=0.0518, that is, σ=-0.0359ε 2 +0.1357ε+0.0518, the breaking elongation E of the pre-oxidized fiber monofilament does not satisfy the formulas -150≤(B*E*100) / A≤-70 and -150≤(B*E*100+C) / A≤-70, where the value of the intermediate term in the above formula is -58.

[0283] Preparation of carbon / carbon composite materials:

[0284] The preparation process was the same as in Example 1, and the XY and Z compressive strengths of the resulting carbon / carbon composite material were 96 and 100 MPa, respectively.

[0285] [Comparative Example 6]

[0286] Preparation of polyacrylonitrile-based precursor fibers (same as Example 3).

[0287] Preparation of pre-oxidized fibers:

[0288] The aforementioned polyacrylonitrile-based precursor fibers (12,000 fibers per bundle) were subjected to pre-oxidation, sizing, and drying in four temperature zones to obtain a fineness of 1.50 dtex and a bulk density of 1.33 g / cm³. 3The pre-oxidized fiber has a monofilament cross-sectional roundness of 0.95, a monofilament strength of 2.67 cN / dtex, a monofilament breaking elongation of 13.4%, a crystallinity of 0.169, a grain size of 2.595 nm, and a carbon-to-oxygen ratio of 1.65. The pre-oxidation temperature zones are as follows: First pre-oxidation zone: microwave heating at 2.0 GHz, 200°C, draw ratio of 0.999, and residence time of 30 min; Second pre-oxidation zone: microwave heating at 5.9 GHz, 250°C, draw ratio of 1.000, and residence time of 27 min; Third pre-oxidation zone: hot air heating at 250°C, draw ratio of 1.050, and residence time of 25 min; Fourth pre-oxidation zone: hot air heating at 280°C, draw ratio of 1.010, and residence time of 15 min. The atmosphere for both hot air and microwave heating is air.

[0289] After tensile property testing, within the strain range of 0-2.5%, the nonlinear fitting formula for the stress σ (in GPa) and strain ε (in %) of the pre-oxidized fiber is σ = Aε. 2 +Bε+C, where A=-0.0538, B=0.1362, C=0.0936, that is, σ=-0.0538ε 2 +0.1362ε+0.0936, the breaking elongation E of the pre-oxidized fiber monofilament does not satisfy the formulas -150≤(B*E*100) / A≤-70 and -150≤(B*E*100+C) / A≤-70, where the value of the intermediate term in the above formula is -36.

[0290] Preparation of carbon / carbon composite materials:

[0291] The preparation process was the same as in Example 1, and the XY and Z compressive strengths of the resulting carbon / carbon composite material were 71 and 84 MPa, respectively.

[0292] [Comparative Example 7]

[0293] Preparation of polyacrylonitrile-based precursor fibers (same as Example 3).

[0294] Preparation of pre-oxidized fibers:

[0295] The aforementioned polyacrylonitrile-based precursor fibers (12,000 fibers per bundle) were subjected to pre-oxidation, sizing, and drying in four temperature zones to obtain a fineness of 1.50 dtex and a bulk density of 1.35 g / cm³. 3The pre-oxidized fiber has a monofilament cross-sectional roundness of 0.95, a monofilament strength of 2.60 cN / dtex, a monofilament breaking elongation of 14.1%, a crystallinity of 0.147, a grain size of 2.549 nm, and a carbon-to-oxygen ratio of 1.68. The pre-oxidation temperature zones are as follows: First pre-oxidation zone: hot air heating at 180℃, draw ratio of 1.150, and residence time of 20 min; Second pre-oxidation zone: microwave heating at 3.8 GHz, temperature of 230℃, draw ratio of 1.000, and residence time of 27 min; Third pre-oxidation zone: hot air heating at 250℃, draw ratio of 1.050, and residence time of 25 min; Fourth pre-oxidation zone: microwave heating at 4.5 GHz, temperature of 235℃, draw ratio of 0.992, and residence time of 13 min. The atmosphere for both hot air and microwave heating is air.

[0296] After tensile property testing, within the strain range of 0-2.5%, the nonlinear fitting formula for the stress σ (in GPa) and strain ε (in %) of the pre-oxidized fiber is σ = Aε. 2 +Bε+C, where A=-0.0147, B=0.1805, C=0.0886, that is, σ=-0.0147ε 2 +0.1805ε+0.0886, the breaking elongation E of the pre-oxidized fiber monofilament does not satisfy the formulas -150≤(B*E*100) / A≤-70 and -150≤(B*E*100+C) / A≤-70, where the value of the intermediate term in the above formula is -179.

[0297] Preparation of carbon / carbon composite materials:

[0298] The preparation process was the same as in Example 1, and the XY and Z compressive strengths of the resulting carbon / carbon composite material were 83 and 80 MPa, respectively.

[0299] [Comparative Example 8]

[0300] Preparation of polyacrylonitrile-based precursor fibers (same as Example 3).

[0301] Preparation of pre-oxidized fibers:

[0302] The aforementioned polyacrylonitrile-based precursor fibers (12,000 fibers per bundle) were subjected to pre-oxidation, sizing, and drying in two temperature zones to obtain a fineness of 1.50 dtex and a bulk density of 1.27 g / cm³. 3The pre-oxidized fiber has a monofilament cross-sectional roundness of 0.95, a monofilament strength of 2.70 cN / dtex, a monofilament breaking elongation of 10.7%, a crystallinity of 0.279, a grain size of 2.953 nm, and a carbon-to-oxygen ratio of 2.95. The first temperature zone is heated by hot air at 220°C, with a draw ratio of 1.050 and a residence time of 60 min. The second temperature zone is heated by microwave at 3.5 GHz at 225°C, with a draw ratio of 0.950 and a residence time of 25 min. The atmosphere for both hot air and microwave heating is air.

[0303] After tensile property testing, within the strain range of 0-2.5%, the nonlinear fitting formula for the stress σ (in GPa) and strain ε (in %) of the pre-oxidized fiber is σ = Aε. 2 +Bε+C, where A=-0.0129, B=0.1986, C=0.0779, that is, σ=-0.0129ε 2 +0.1986ε+0.0779, the breaking elongation E of the pre-oxidized fiber monofilament does not satisfy the formulas -150≤(B*E*100) / A≤-70 and -150≤(B*E*100+C) / A≤-70, where the value of the intermediate term in the above formula is -171.

[0304] Preparation of carbon / carbon composite materials:

[0305] The preparation process was the same as in Example 1, and the XY and Z compressive strengths of the resulting carbon / carbon composite material were 83 and 85 MPa, respectively.

[0306] [Comparative Example 9]

[0307] Preparation of polyacrylonitrile-based precursor fibers (same as Example 3).

[0308] Preparation of pre-oxidized fibers:

[0309] The aforementioned polyacrylonitrile-based precursor fibers (12,000 fibers per bundle) were subjected to pre-oxidation, sizing, and drying in five temperature zones to obtain a fineness of 1.50 dtex and a bulk density of 1.27 g / cm³. 3Pre-oxidized fiber with a monofilament cross-sectional roundness of 0.98, a monofilament strength of 2.83 cN / dtex, a monofilament breaking elongation of 9.5%, a crystallinity of 0.148, a grain size of 2.469 nm, and a carbon to oxygen ratio of 1.87. The heating process is divided into five zones: Zone 1 (hot air heating, 220℃, draw ratio 1.020, residence time 10 min); Zone 2 (hot air heating, 280℃, draw ratio 1.030, residence time 15 min); Zone 3 (microwave heating, 2.4GHz, 210℃, draw ratio 0.990, residence time 30 min); Zone 4 (microwave heating, 3.5GHz, 225℃, draw ratio 0.950, residence time 25 min); and Zone 5 (microwave heating, 5.9GHz, 250℃, draw ratio 0.980, residence time 20 min). The atmosphere for both hot air and microwave heating is air.

[0310] After tensile property testing, within the strain range of 0-2.5%, the nonlinear fitting formula for the stress σ (in GPa) and strain ε (in %) of the pre-oxidized fiber is σ = Aε. 2 +Bε+C, where A=-0.0289, B=0.1738, C=0.0583, that is, σ=-0.0289ε 2 +0.1738ε+0.0583, the breaking elongation E of the pre-oxidized fiber monofilament does not satisfy the formulas -150≤(B*E*100) / A≤-70 and -150≤(B*E*100+C) / A≤-70, where the value of the intermediate term in the above formula is -59.

[0311] Preparation of carbon / carbon composite materials:

[0312] The preparation process was the same as in Example 1, and the XY and Z compressive strengths of the resulting carbon / carbon composite material were 93 and 89 MPa, respectively.

[0313] As can be seen from Examples 1-17 and Comparative Examples 1-9, the polyacrylonitrile pre-oxidized fibers in Comparative Examples 1-9 did not undergo a pre-oxidation process with at least four temperature zones during preparation (as in Comparative Examples 1 and 3), or although the polyacrylonitrile pre-oxidized fibers underwent a pre-oxidation process with at least four temperature zones, the condition that the first m temperature zones were heated by hot air and the remaining n temperature zones were heated by microwave was not met (as in Comparative Examples 2 and 4-9). As a result, the breaking elongation E of the pre-oxidized fiber monofilaments did not satisfy the formulas -150≤(B*E*100) / A≤-70 and -150≤(B*E*100+C) / A≤-70, resulting in a significant decrease in the XY and Z compressive strength of the carbon / carbon composite material prepared from it compared to Examples 1-17.

Claims

1. A pre-oxidized polyacrylonitrile fiber, characterized in that: Within the strain range of 0-2.5%, the values ​​of stress σ and strain ε satisfy the nonlinear fitting formula σ=Aε. 2 +Bε+C, and the breaking elongation E of the pre-oxidized fiber monofilament satisfies the formula -150≤(B*E*100) / A≤-70; The unit of stress σ is GPa and the unit of strain ε is %, where A, B and C are the coefficients of the quadratic functions of stress σ and strain ε.

2. The polyacrylonitrile pre-oxidized fiber as described in claim 1, characterized in that: Within the strain range of 0-2.5%, the values ​​of stress σ and strain ε satisfy the nonlinear fitting formula σ=Aε. 2 +Bε+C, and the breaking elongation E of the pre-oxidized fiber monofilament satisfies the formula -140≤(B*E*100) / A≤-70; Where stress σ is in GPa and strain ε is in %, and A, B and C are the coefficients of the quadratic functions of stress σ and strain ε; Preferably, within the strain range of 0-2.5%, the values ​​of stress σ and strain ε satisfy the nonlinear fitting formula σ=Aε. 2 +Bε+C, and the breaking elongation E of the pre-oxidized fiber monofilament satisfies the formula -135≤(B*E*100) / A≤-75; The unit of stress σ is GPa and the unit of strain ε is %, where A, B and C are the coefficients of the quadratic functions of stress σ and strain ε.

3. The polyacrylonitrile pre-oxidized fiber as described in claim 1 or 2, characterized in that: Within the strain range of 0-2.5%, the values ​​of stress σ and strain ε satisfy the nonlinear fitting formula σ=Aε. 2 +Bε+C, and the breaking elongation E of the pre-oxidized fiber monofilament satisfies the formula -150≤(B*E*100+C) / A≤-70; Where stress σ is in GPa and strain ε is in %, and A, B and C are the coefficients of the quadratic functions of stress σ and strain ε; Preferably, within the strain range of 0-2.5%, the values ​​of stress σ and strain ε satisfy the nonlinear fitting formula σ=Aε. 2 +Bε+C, and the breaking elongation E of the pre-oxidized fiber monofilament satisfies the formula -140≤(B*E*100+C) / A≤-70; Where stress σ is in GPa and strain ε is in %, and A, B and C are the coefficients of the quadratic functions of stress σ and strain ε; More preferably, within the strain range of 0-2.5%, the values ​​of stress σ and strain ε satisfy the nonlinear fitting formula σ=Aε. 2 +Bε+C, and the breaking elongation E of the pre-oxidized fiber monofilament satisfies the formula -135≤(B*E*100+C) / A≤-75; The unit of stress σ is GPa and the unit of strain ε is %, where A, B and C are the coefficients of the quadratic functions of stress σ and strain ε.

4. The polyacrylonitrile pre-oxidized fiber according to any one of claims 1-3, characterized in that: -0.0420≤A≤-0.0140; 0.1450≤B≤0.2390 and 0.0620≤C≤0.0760; and / or 11.0% ≤ E ≤ 16.0%, preferably 12.0% ≤ E ≤ 15.0%, more preferably 12.5% ​​≤ E ≤ 14.0%.

5. The polyacrylonitrile pre-oxidized fiber according to any one of claims 1-4, characterized in that: The monofilament roundness of the pre-oxidized polyacrylonitrile fiber has a cross-sectional roundness of 0.80-0.99, preferably 0.85-0.98; and / or, The number of individual filament bundles of the pre-oxidized polyacrylonitrile fiber is 1,000-100,000, preferably 3,000-50,000; and / or, The fineness of the pre-oxidized polyacrylonitrile fiber is 0.5-5.0 dtex, preferably 1.0-3.5 dtex; and / or, The bulk density of the pre-oxidized polyacrylonitrile fiber is 1.20-1.50 g / cm³. 3 The preferred value is 1.25-1.45 g / cm³. 3 ; and / or, The monofilament strength of the pre-oxidized polyacrylonitrile fiber is 2.7-4.0 cN / dtex, preferably 3.0-3.3 cN / dtex; and / or, The crystallinity of the pre-oxidized polyacrylonitrile fiber is 0.10-0.30, preferably 0.14-0.25; and / or, The grain size of the polyacrylonitrile pre-oxidized fiber is 1.80-3.50 nm, preferably 2.00-3.00 nm; and / or, The carbon to oxygen content ratio of the pre-oxidized polyacrylonitrile fiber is 0.50-3.00, preferably 0.70-2.

50.

6. A method for preparing polyacrylonitrile pre-oxidized fiber according to any one of claims 1-5, comprising the step of obtaining polyacrylonitrile pre-oxidized fiber by sequentially passing polyacrylonitrile-based precursor fibers through a pre-oxidation process comprising m+n pre-oxidation temperature zones; wherein, The m pre-oxidation temperature zones are all heated by a hot oxygen-containing atmosphere, where m is an integer from 2 to 4, and the n pre-oxidation temperature zones are all heated by microwaves, where n is an integer from 2 to 6. Preferably, polyacrylonitrile precursor fibers are pre-oxidized by sequentially passing them through the following pre-oxidation temperature zones to obtain pre-oxidized polyacrylonitrile fibers: Two pre-oxidation temperature zones heated by a hot oxygen-containing atmosphere and two pre-oxidation temperature zones heated by microwave; Two pre-oxidation temperature zones heated by a hot oxygen-containing atmosphere and three pre-oxidation temperature zones heated by microwave; Two pre-oxidation temperature zones heated by a hot oxygen-containing atmosphere and four pre-oxidation temperature zones heated by microwave; Two pre-oxidation temperature zones heated by a hot oxygen-containing atmosphere and five pre-oxidation temperature zones heated by microwave; Three pre-oxidation temperature zones heated by a hot oxygen-containing atmosphere and two pre-oxidation temperature zones heated by microwave; Three pre-oxidation temperature zones heated by a hot oxygen-containing atmosphere and three pre-oxidation temperature zones heated by microwave; Three pre-oxidation temperature zones heated by a hot oxygen-containing atmosphere and four pre-oxidation temperature zones heated by microwave; Three pre-oxidation temperature zones heated by a hot oxygen-containing atmosphere and five pre-oxidation temperature zones heated by microwave; Four pre-oxidation temperature zones heated by a hot oxygen-containing atmosphere and two pre-oxidation temperature zones heated by microwave; Four pre-oxidation temperature zones heated by a hot oxygen-containing atmosphere and three pre-oxidation temperature zones heated by microwave; Four pre-oxidation temperature zones heated by a hot oxygen-containing atmosphere and four pre-oxidation temperature zones heated by microwave; Four pre-oxidation temperature zones heated by a hot oxygen-containing atmosphere and five pre-oxidation temperature zones heated by microwave.

7. The preparation method according to claim 6, characterized in that: The draw ratio of the pre-oxidation temperature zone heated by a hot oxygen-containing atmosphere is greater than or equal to the draw ratio of the pre-oxidation temperature zone heated by microwave. Preferably, the draw ratio of the pre-oxidation temperature zone heated by a hot oxygen-containing atmosphere is 1.000-1.200, more preferably 1.000-1.080; and / or, the draw ratio of the pre-oxidation temperature zone heated by microwave is 0.950-1.000, more preferably 0.970-1.

000.

8. The preparation method according to claim 6 or 7, characterized in that: The pre-oxidation temperature zones heated by a hot oxygen-containing atmosphere may have the same or different temperatures, each independently ranging from 150-350℃, preferably 180-280℃; preferably, the temperatures of the pre-oxidation temperature zones heated by a hot oxygen-containing atmosphere are increasing; more preferably, the temperature difference between adjacent pre-oxidation temperature zones heated by a hot oxygen-containing atmosphere is not less than 30℃, preferably 30-100℃; and / or, The total temperature difference of the pre-oxidation temperature zone heated by a hot oxygen-containing atmosphere is 30-150℃, preferably 50-70℃.

9. The preparation method according to any one of claims 6-8, characterized in that: The microwave frequencies of the pre-oxidation temperature zones using microwave heating may be the same or different, each independently ranging from 1.0 to 10.0 GHz, preferably from 2.0 to 7.0 GHz; and / or The microwave frequency of the pre-oxidation temperature zone using microwave heating increases progressively; preferably, the microwave frequency difference between adjacent pre-oxidation temperature zones is not less than 0.5 GHz, more preferably 0.5-3.0 GHz; and / or, The total microwave frequency difference in the pre-oxidation temperature zone using microwave heating is 0.5-5.0 GHz, preferably 0.6-3.5 GHz; and / or, The temperatures of the pre-oxidation zones using microwave heating are each 160-300℃, preferably 160-250℃.

10. The preparation method according to any one of claims 6-9, characterized in that: During the pre-oxidation process, the residence time of the fibers in each pre-oxidation temperature zone may be the same or different, each independently ranging from 5 to 60 minutes, preferably 10 to 30 minutes; and / or, The total residence time of the fibers in the pre-oxidation temperature zone heated in a hot oxygen-containing atmosphere shall not exceed 60 minutes, preferably 25-60 minutes; and / or, The total residence time of the fiber in the microwave-heated pre-oxidation temperature zone shall not exceed 70 minutes, preferably 30-65 minutes.

11. The preparation method according to any one of claims 6-10, characterized in that: The polyacrylonitrile-based precursor fiber is obtained by wet spinning, dry spinning or dry-jet wet spinning of polyacrylonitrile spinning solution, preferably by wet spinning. Preferably, the wet spinning process includes the steps of multi-stage coagulation, hot water drawing, oiling, drying and densification, steam drawing, and steam heat setting.

12. The preparation method according to any one of claims 6-11, characterized in that: The oxygen-containing atmosphere is selected from air, a mixture of air and oxygen, a mixture of air and an inert gas, a mixture of oxygen and an inert gas, and a mixture of air and oxygen and an inert gas. Preferably, the inert gas is selected from nitrogen (N2), helium (He), neon (Ne), argon (Ar), and mixtures thereof; Preferably, the oxygen volume concentration of the oxygen-containing atmosphere is 5%-80%, more preferably 15-50%.

13. A carbon / carbon composite material, which is prepared from polyacrylonitrile pre-oxidized fibers according to any one of claims 1-5 or from polyacrylonitrile pre-oxidized fibers obtained by any one of claims 6-12.

14. The carbon / carbon composite material as described in claim 13, characterized in that: The preparation process of the carbon / carbon composite material includes the steps of preparing a preform, densifying the preform, and graphitizing the preform, wherein the preparation of the preform includes using the polyacrylonitrile pre-oxidized fiber to prepare the preform.

15. The carbon / carbon composite material as described in claim 13 or 14, characterized in that: The carbon / carbon composite material has an XY compressive strength ≥150 MPa, preferably 150 MPa to 220 MPa, more preferably 200 MPa to 220 MPa, and / or, The Z-axis compressive strength of the carbon / carbon composite material is ≥150MPa, preferably 150MPa to 220MPa, and more preferably 200MPa to 220MPa.

16. The use of a pre-oxidized polyacrylonitrile fiber as described in any one of claims 1-5 or a pre-oxidized polyacrylonitrile fiber obtained by the preparation method as described in any one of claims 6-12 for the preparation of carbon / carbon composite materials.

17. A method for preparing carbon / carbon composite materials, the method comprising the following steps: Preparation of a preform; wherein the preparation of the preform includes using the polyacrylonitrile pre-oxidized fiber to prepare the preform, wherein the polyacrylonitrile pre-oxidized fiber is the polyacrylonitrile pre-oxidized fiber according to any one of claims 1-5 or prepared by the preparation method according to any one of claims 6-12; The preform is densified; and The densified preform is then graphitized.

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