Preparation process of high-strength CC composite material

Abstract

The invention relates to the field of CC composite materials, and discloses a high-strength CC composite material preparation process which comprises the following steps: step 1, heating a reaction container to melt asphalt; 2, introducing a gas medium into the reaction container to increase the internal pressure of the reaction container; enabling the gas medium to be in a supercritical state; step 3, introducing a gas medium into the reaction container, and oscillating the molten asphalt by using the gas medium; 4, after oscillation is finished, the reaction container is heated to the carbonization temperature, and in-situ carbonization and supercritical fluid extraction are carried out. Carbon dioxide is introduced and is in a supercritical state, so that the viscosity of the impregnated asphalt is reduced; and meanwhile, the pressure in the reaction container is periodically changed between the peak pressure and the valley pressure to generate a physical pumping effect, so that the low-viscosity molten asphalt can be forcibly pressed into tiny pores of the carbon fiber preform, and residual gas in the pores can be discharged.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of CC composite materials, in particular to a preparation process of high-strength CC composite materials. BACKGROUND

[0002] Carbon fiber is a kind of high-performance inorganic fiber, which is widely used as a reinforcing phase in advanced composite materials due to its high strength, high modulus, low density, high temperature resistance, chemical corrosion resistance and other characteristics, and plays a key role in high-tech fields such as aviation, aerospace and automobile industry.

[0003] In order to fully exert the performance of carbon fiber, it is usually combined with matrix material. Among them, the carbon / carbon (C / C) composite material with both reinforcing phase and matrix being carbon elements is concerned due to its excellent overall performance. This kind of material is usually prepared by impregnating pitch or other carbon precursors into a porous carbon fiber preform, and then through a series of heat treatment processes such as carbonization.

[0004] However, the existing liquid phase impregnation-carbonization process has several technical problems in preparing C / C composite materials. The pitch in molten state as a carbon precursor usually has high viscosity. When the impregnation temperature is low, the high viscosity of the pitch makes it difficult to effectively penetrate into the small pores inside the carbon fiber preform, resulting in insufficient impregnation. At the same time, the gas remaining in the preform pores also forms a "gas block" effect, further hindering the flow of molten pitch into the material interior. On the contrary, if the impregnation temperature is excessively increased to reduce the viscosity of the pitch, the pitch will be pyrolyzed too early, producing a large amount of small molecular volatile gas. When these gases escape from the inside of the blank, they will generate pressure inside, thereby introducing defects such as pores and micro-cracks in the matrix carbon formed finally. These factors together result in high porosity, low density and poor mechanical properties of the final composite material, which is difficult to meet the needs of high-performance applications. SUMMARY

[0005] In view of the deficiencies of the prior art, the present application provides a preparation process of high-strength CC composite materials, which solves the problem that the pitch viscosity and impregnation temperature are difficult to be considered in the traditional process, resulting in insufficient impregnation, easy formation of pyrolysis defects, and finally high porosity and poor mechanical properties of the material.

[0006] To achieve the above object, the present application is implemented by the following technical scheme: a preparation process of high-strength CC composite materials, comprising the following steps: Step S1: material loading and melting A carbon fiber preform with a porosity of 45-55% and solid pitch are placed in a reaction vessel. Subsequently, the reaction vessel is sealed and heated to a temperature of 250-350°C and maintained at this temperature for 0.5-1 h to melt the pitch completely and impregnate the carbon fiber preform.

[0007] Step S2: Establishing a supercritical environment A gaseous medium, which is carbon dioxide with a purity of more than 99.9%, is injected into the reaction vessel. The injection is continued until the internal pressure of the reaction vessel is increased to 10-15 MPa. At this pressure and the temperature of step S1, the carbon dioxide medium is transformed into a supercritical state. The carbon dioxide in the supercritical state acts as a plasticizer and can be dissolved in the molten pitch, thereby reducing the viscosity of the molten pitch.

[0008] Step S3: Pressure periodic variation for enhanced impregnation The pressure in the reaction vessel is periodically varied between a preset peak pressure and a preset valley pressure by an external pressure control system. The cycle period of the periodic variation of the pressure is 10-25 seconds. In one cycle, the pressure is first increased to a peak pressure of 15-25 MPa and then decreased to a valley pressure of 8-12 MPa. The periodic variation of the pressure produces a physical pumping effect: when the pressure is increased, the low-viscosity molten pitch is forced into the small pores of the carbon fiber preform; when the pressure is decreased, a small amount of gas or low-boiling components that may remain in the pores expand and escape from the pores due to the sudden decrease in external pressure. This step effectively overcomes the gas blocking phenomenon during impregnation and achieves deep and rapid impregnation of the preform.

[0009] Step S4: In-situ carbonization and supercritical fluid extraction After the periodic variation of the pressure in step S3 is stopped, the pressure in the reaction vessel is maintained within the pressure range of the supercritical state. A programmed temperature increase is started for in-situ carbonization. The temperature increase program is as follows: first heated to 600-650°C at a heating rate of 5-12°C / min, and then heated to a final carbonization temperature at a heating rate of 5-8°C / min. The final carbonization temperature is 900-1200°C. After reaching the final carbonization temperature, a holding period of 2-3 h is performed. During the entire temperature increase and holding period, the supercritical carbon dioxide fluid acts as an extractant to continuously dissolve and carry out of the reaction vessel the small molecular volatile substances produced by the pyrolysis of pitch from the inside of the body, avoiding the accumulation of volatile substances inside the body and the formation of high pressure.

[0010] Step S5: Repeated densification and final high-temperature treatment After the carbonization process in step S4 is completed, the reaction vessel is cooled. Without removing the carbonized green body, a new round of impregnation pitch is replenished into the reaction vessel. Subsequently, steps S2 to S4 are continuously repeated. The total number of repetitions of this process is 2-4 times.

[0011] After the last repeated densification cycle is completed, the atmosphere in the reaction vessel is replaced with argon. Subsequently, the reaction vessel is heated to 2200-2800℃ and held for 2-4 hours for high temperature treatment. After the high temperature treatment is completed, the reaction vessel is naturally cooled down in the furnace and finally removed to obtain a high-strength C / C composite material.

[0012] The present application provides a high-strength CC composite material preparation process. It has the following advantages: 1. The present application reduces the viscosity of the impregnation pitch by introducing carbon dioxide and making it in a supercritical state. At the same time, by periodically changing the pressure in the reaction vessel between the peak pressure and the valley pressure, a physical pumping effect is produced, which can force the low-viscosity molten pitch into the small pores of the carbon fiber preform and help to expel the residual gas in the pores.

[0013] 2. The present application heats the reaction vessel to carbonization temperature in two stages, and heats to 600-650℃ at a heating rate of 5-12℃ / min, which can match the generation rate of volatile matter with the extraction rate of supercritical fluid, avoiding the high pressure generated by the rapid accumulation of gas inside the green body, thereby effectively inhibiting the generation of cracks and pores in the matrix carbon. Subsequently, the temperature is raised to the carbonization temperature at a heating rate of 5-8℃ / min, and at this time the release of volatile matter has basically been completed, thus helping to obtain a dense matrix structure.

[0014] 3. The present application uses supercritical carbon dioxide as an extractant, which can continuously dissolve and remove small molecular volatile matter produced by pitch pyrolysis from the inside of the green body, avoiding the accumulation of volatile matter inside the green body to form high pressure, thereby inhibiting the formation of cracks and pores caused by the disordered escape of gas.

[0015] 4. The present application continuously performs impregnation and carbonization in the same reaction vessel, eliminating the cooling, transfer and reheating steps of the green body in different equipment in the traditional process, which shortens the overall production cycle, reduces energy consumption, and avoids the mechanical damage that the green body may suffer during the transfer process. DETAILED DESCRIPTION

[0016] The technical solutions in the embodiments of the present application will be clearly and completely described below with reference to the embodiments in the specification of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, rather than all the embodiments of the present application. Based on the embodiments in the present application, all other embodiments obtained by a person of ordinary skill in the art without creative effort belong to the protection scope of the present application.

[0017] The sources and specifications of the main raw materials used in the examples and comparative examples are as follows, and those not specifically described are commercially available or higher grade products.

[0018] Carbon fiber preform: T700 grade polyacrylonitrile-based carbon fiber was used as raw material, and a carbon fiber preform was prepared by a three-dimensional needling process, with a volume fraction of 45% and an initial porosity of 55%.

[0019] Pitch: medium temperature coal pitch, CAS No. 65996-93-2; its main performance indicators are: softening point (ring and ball method) 105℃; coking value ≥60%; quinoline insoluble (QI) content ≤0.2wt%; toluene insoluble (TI) content 25wt%.

[0020] Carbon dioxide (CO2), CAS No. 124-38-9, commercially available industrial grade product, purity ≥99.9%.

[0021] Argon (Ar), CAS No. 7440-37-1, commercially available high purity grade product, purity ≥99.999%. Example 1

[0022] The present embodiment provides a high-strength CC composite material preparation process, comprising: (1) Material loading and melting: a carbon fiber preform (porosity 55%) with a size of 100mm×30mm and 200g of medium temperature coal pitch were placed together in a 5L volume high-pressure impregnation and carbonization integrated reaction container. The reaction container was sealed, the heating program was started, the reaction container was heated to 300℃, and kept at this temperature for 0.9 hours to make the pitch completely melted.

[0023] (2) Establishing a supercritical environment: under the condition of maintaining 300℃ temperature, carbon dioxide gas with a purity of 99.9% was injected into the reaction container until the reference pressure in the reaction container reached 12MPa and remained stable.

[0024] (3) Pressure periodic variation strengthening impregnation: the pressure control system was started to make the pressure in the reaction container periodically vary above and below the reference pressure of 12MPa. The cycle period was set to 15 seconds / time, the peak pressure was set to 20MPa, and the valley pressure was set to 10MPa. This pressure periodic variation process lasted for 2 hours.

[0025] (4) In-situ carbonization and supercritical fluid extraction: After the pressure periodic variation is stopped, the programmed temperature is started. First, the temperature is raised to 630°C at a rate of 10°C / min, then the temperature is raised to the final carbonization temperature of 1000°C at a rate of 6°C / min, and the temperature is kept at 1000°C for 2.5 hours.

[0026] (5) Repeated densification: After the first carbonization is completed, the reaction container is cooled to 300°C. Without taking out the carbonized blank, 120g of medium temperature coal pitch is supplemented into the reaction container. Then, the above-mentioned operations of establishing a supercritical environment, in-situ carbonization and supercritical fluid extraction, and in-situ carbonization and supercritical fluid extraction are repeatedly performed; a total of 3 times.

[0027] (6) High temperature treatment: After the third carbonization cycle is completed, the reaction container is cooled to room temperature. The atmosphere in the reaction container is replaced with argon with a purity of 99.999%. Then, the heating program is started, the reaction container is heated to 2500°C, and the temperature is kept at this temperature for 3 hours. After the treatment is completed, the furnace is naturally cooled to room temperature, the sample is taken out, and a C / C composite material is obtained. Example 2:

[0028] The embodiment provides a high-strength CC composite material preparation process, which comprises the following steps: (1) Material loading and melting: a carbon fiber preform with a size of 100mm*30mm (porosity 55%) and 200g of medium temperature coal pitch are placed in a 5L volume high-pressure impregnation and carbonization integrated reaction container. The reaction container is sealed, the heating program is started, the reaction container is heated to 250°C, and the temperature is kept at this temperature for 0.5 hours to completely melt the pitch.

[0029] (2) Establishing a supercritical environment: under the condition of maintaining the temperature of 250°C, carbon dioxide gas with a purity of 99.9% is injected into the reaction container until the reference pressure in the reaction container reaches 10MPa and remains stable.

[0030] (3) Pressure periodic variation enhanced impregnation: the pressure control system is started to make the pressure in the reaction container periodically vary above and below the reference pressure of 10MPa. The cycle period is set to 10 seconds / time, the peak pressure is set to 15MPa, and the valley pressure is set to 8MPa. The pressure periodic variation process lasts for 1.5 hours.

[0031] (4) In-situ carbonization and supercritical fluid extraction: after the pressure periodic variation is stopped, the programmed temperature is started. First, the temperature is raised to 600°C at a rate of 5°C / min, then the temperature is raised to the final carbonization temperature of 900°C at a rate of 5°C / min, and the temperature is kept at 900°C for 2 hours.

[0032] (5) Repeated densification: After the first carbonization, the reaction vessel is cooled to 300°C. Without removing the carbonized green body, 120 g of medium temperature coal pitch is added to the reaction vessel. Subsequently, the above-mentioned operations of establishing supercritical environment, in-situ carbonization and supercritical fluid extraction, and in-situ carbonization and supercritical fluid extraction are repeated; a total of 2 times.

[0033] (6) High temperature treatment: After the third carbonization cycle, the reaction vessel is cooled to room temperature. The atmosphere in the reaction vessel is replaced with argon gas with a purity of 99.999%. Subsequently, the heating program is started, and the reaction vessel is heated to 2200°C, and kept at this temperature for 2 hours. After the treatment is completed, the furnace is naturally cooled to room temperature, and the sample is removed to obtain a C / C composite material. Example 3:

[0034] The present embodiment provides a high-strength CC composite material preparation process, comprising: (1) Material loading and melting: A piece of carbon fiber preform with a size of 100 mm x 30 mm (porosity 45%) and 200 g of medium temperature coal pitch are placed in a 5L volume high-pressure impregnation and carbonization integrated reaction vessel. The reaction vessel is sealed, and the heating program is started to heat the reaction vessel to 350°C and keep it at this temperature for 1 hour to completely melt the pitch.

[0035] (2) Establishing a supercritical environment: Under the condition of maintaining a temperature of 350°C, carbon dioxide gas with a purity of 99.9% is injected into the reaction vessel until the reference pressure in the reaction vessel reaches 15 MPa and remains stable.

[0036] (3) Pressure periodic variation enhanced impregnation: The pressure control system is started to make the pressure in the reaction vessel periodically vary above and below the reference pressure of 15 MPa. The cycle period is set to 25 seconds / time, the peak pressure is set to 25 MPa, and the valley pressure is set to 12 MPa. This pressure periodic variation process lasts for 3 hours.

[0037] (4) In-situ carbonization and supercritical fluid extraction: After stopping the pressure periodic variation, the temperature program is started. First, heat to 650°C at a heating rate of 12°C / min, then heat to the final carbonization temperature of 1200°C at a heating rate of 8°C / min, and keep it at 1200°C for 3 hours.

[0038] (5) Repeated densification: After the first carbonization, the reaction vessel is cooled to 300°C. Without removing the carbonized green body, 120 g of medium temperature coal pitch is added to the reaction vessel. Subsequently, the above-mentioned operations of establishing supercritical environment, in-situ carbonization and supercritical fluid extraction, and in-situ carbonization and supercritical fluid extraction are repeated; a total of 4 times.

[0039] (6) High temperature treatment: after the third carbonization cycle, the reaction vessel was cooled to room temperature. The atmosphere in the reaction vessel was replaced with argon gas with a purity of 99.999%. Then the heating program was started, and the reaction vessel was heated to 2800°C, and kept at this temperature for 4 hours. After the treatment, the furnace was naturally cooled to room temperature, and the sample was taken out to obtain the C / C composite material.

[0040] Comparative Example 1: Compared with Example 1, the difference is that the carbon fiber preform and the medium temperature coal pitch are directly placed in the high-pressure impregnation and carbonization integrated reaction vessel for heating without using carbon dioxide gas medium, and there is no baseline pressure for periodic change; the rest are the same.

[0041] Comparative Example 2: Compared with Example 2, the difference is that the supercritical environment is established, and the carbon dioxide gas is injected to reach a baseline pressure of 9 MPa; the rest are the same.

[0042] Comparative Example 3: Compared with Example 3, the difference is that the supercritical environment is established, and the carbon dioxide gas is injected to reach a baseline pressure of 16 MPa; the rest are the same.

[0043] Comparative Example 4: Compared with Example 1, the difference is that the carbon dioxide gas medium is introduced, but the baseline pressure is not periodically changed; the rest are the same.

[0044] Comparative Example 5: Compared with Example 2, the difference is that the carbon dioxide gas medium is introduced, and the baseline pressure is periodically changed, but the peak pressure is set to 14 MPa, the valley pressure is set to 7 MPa, and the periodic change process lasts for 1.2 hours; the rest are the same.

[0045] Comparative Example 6: Compared with Example 3, the difference is that the carbon dioxide gas medium is introduced, and the baseline pressure is periodically changed, but the peak pressure is set to 27 MPa, and the valley pressure is set to 14 MPa. The pressure periodic change process lasts for 3.2 hours; the rest are the same.

[0046] Comparative Example 7: Compared with Example 1, the difference is that in the in-situ carbonization and supercritical fluid extraction, the temperature is directly raised to 1000°C at a rate of 6°C / min without using staged heating; the rest are the same.

[0047] Comparative Example 8: The difference compared with Example 2 is that: in situ carbonization and supercritical fluid extraction, no stage heating is used, and the temperature is directly raised to 900℃ at a heating rate of 10℃ / min; the rest are the same.

[0048] Comparative Example 9: The difference compared with Example 3 is that: in situ carbonization and supercritical fluid extraction, no stage heating is used, and the temperature is directly raised to 1200℃ at a heating rate of 13℃ / min; the rest are the same.

[0049] Comparative Example 10: The difference compared with Example 1 is that: stage heating is used, first heated to 630℃ at a heating rate of 10℃ / min, then heated to the final carbonization temperature of 1000℃ at a heating rate of 10℃ / min, and held for 2.5 hours; the rest are the same.

[0050] Comparative Example 11: The difference compared with Example 2 is that: stage heating is used, first heated to 600℃ at a heating rate of 3℃ / min, then heated to the final carbonization temperature of 900℃ at a heating rate of 4℃ / min, and held for 2 hours at 900℃; the rest are the same.

[0051] Comparative Example 12: The difference compared with Example 3 is that: stage heating is used, first heated to 650℃ at a heating rate of 16℃ / min, then heated to the final carbonization temperature of 1200℃ at a heating rate of 10℃ / min, and held for 3 hours at 1200℃; the rest are the same.

[0052] Test Example 1 1. Purpose of the experiment The purpose of this test example is to verify the technical effects of the C / C composite material preparation process provided by the present application. The specific purposes include: (1) Verify the necessity and positive role of using supercritical carbon dioxide medium in the impregnation-carbonization integrated process.

[0053] (2) Verify the key influence of periodic pressure change on the final performance of the material.

[0054] (3) Verify the rationality and superiority of the process pressure parameter (including reference pressure, peak pressure and valley pressure) range defined by the present application.

[0055] 2. Experimental samples The experimental samples are C / C composites prepared by the methods described in Examples 1-3 and Comparative Examples 1-6.

[0056] 3. Performance test method (1) Bulk density and open porosity test: Archimedes drainage method was used to test each sample. Five samples with size of 10 mm x 10 mm x 10 mm were cut from each sample, and their dry weight, saturated weight and water weight were measured respectively, and the bulk density and open porosity were calculated, and the arithmetic mean was taken as the result.

[0057] (2) Bending strength test: three-point bending method was used to test each sample. Five samples with size of 60 mm x 10 mm x 4 mm were cut from each sample, and tested on a universal material testing machine. The loading span was set to 40 mm, and the loading rate was 5 mm / min. The maximum breaking load of each sample was recorded, and the bending strength was calculated, and the arithmetic mean was taken as the result.

[0058] 4. Test results The C / C composite materials prepared in Examples 1-3 and Comparative Examples 1-6 were subjected to the above performance tests, and the results are summarized in Table 1 below.

[0059] Table 1: Performance test results of each sample Sample Bulk density (g / cm 3 )]]> Open-cell porosity (%) Flexural strength (MPa) Example 1 1.86 3.8 252 Example 2 1.83 4.5 231 Example 3 1.88 3.2 265 Comparative Example 1 1.62 14.5 103 Comparative Example 2 1.74 7.9 165 Comparative Example 3 1.76 7.1 178 Comparative Example 4 1.71 9.2 151 Comparative Example 5 1.72 8.5 159 Comparative Example 6 1.75 7.7 171 5. Experimental analysis Comparison of the results of Example 1 and Comparative Examples 1 and 4: Comparative Example 1 (without carbon dioxide, without periodic change of pressure) has the lowest performance indicators. Comparative Example 4 (with carbon dioxide, but without periodic change of pressure) has certain performance improvement compared with Comparative Example 1, but compared with Example 1 (both), there is still a significant gap in performance. This shows that the introduction of supercritical carbon dioxide and the application of periodic change of pressure are key technical means to obtain high-performance C / C composite materials, and the synergistic effect of the two is the best.

[0060] Comparison of the results of Example 2 and Comparative Example 2, and Example 3 and Comparative Example 3: the reference pressure of Comparative Example 2 (9 MPa) is lower than the lower limit (10 MPa) defined in the present application, and the reference pressure of Comparative Example 3 (16 MPa) is higher than the upper limit (15 MPa) defined in the present application. The test results show that the density, porosity and strength of the samples of these two comparative examples are inferior to those of the corresponding examples. This shows that too low or too high reference pressure is not conducive to achieving the best impregnation effect, thereby confirming the rationality of the 10-15 MPa reference pressure range defined in the present application.

[0061] The results of Comparative Example 2 and Comparative Example 5, and Example 3 and Comparative Example 6: the peak pressure and the valley pressure (14 MPa / 7 MPa) of Comparative Example 5 are both lower than the lower limit of the range defined in the present application, and the peak pressure and the valley pressure (27 MPa / 14 MPa) of Comparative Example 6 are both higher than the upper limit of the range defined in the present application. The test results show that the performance indicators of the two comparative sample are also worse than the corresponding sample of the example. This shows that the pressure range of the periodic change of pressure is crucial, too low cannot produce enough pumping effect, and too high may cause disturbance to the material structure, confirming the effectiveness of the range of the peak pressure (15-25 MPa) and the valley pressure (8-12 MPa) defined in the present application.

[0062] In summary, the test results show that the technical solution provided by the present application can significantly improve the density and mechanical properties of C / C composite materials by using a specific supercritical pressure environment and combining the periodic change of a specific pressure range, and the effect is better than the preparation process lacking key technical features or process parameters exceeding the defined range.

[0063] Test Example 2 1. Experimental purpose The present test example aims to verify the necessity and technical advantages of the stepwise temperature rising procedure used in the process of the present application. By comparing the performance of the samples prepared by using stepwise temperature rising and single rate temperature rising, it is proved that the temperature rising method plays a key role in optimizing the carbon structure of the matrix, suppressing internal defects, and ultimately improving the comprehensive performance of the composite material: 2. Experimental samples The experimental samples are C / C composite materials prepared by using the methods described in Examples 1-3 and Comparative Examples 7-9.

[0064] 3. Performance test method (1) Volumetric density and open porosity test: Archimedes drainage method is used to test each sample. Five samples with a size of 10mmx10mmx10mm are cut from each sample, and their dry weight, saturated weight and water weight are measured respectively to calculate the volumetric density and open porosity, and the results are taken as the arithmetic mean.

[0065] (2) Bending strength test: three-point bending method is used to test each sample. Five samples with a size of 60mmx10mmx4mm are cut from each sample, and tested on a universal material testing machine. The loading span is set to 40mm, and the loading rate is 5mm / min. The maximum breaking load of each sample is recorded, and the bending strength is calculated, and the results are taken as the arithmetic mean.

[0066] 4. Test results The C / C composite materials prepared in Examples 1-3 and Comparative Examples 7-9 were tested for the above performance, and the results are summarized in Table 2 below.

[0067] Table 2: Performance test results of each sample Sample Bulk density (g / cm 3 )]]> Open-cell porosity (%) Flexural strength (MPa) Example 1 1.86 3.8 252 Example 2 1.84 4.4 233 Example 3 1.88 3.2 262 Comparative Example 7 1.77 7.3 181 Comparative Example 8 1.70 9.8 146 Comparative Example 9 1.73 8.4 162 5. Experimental analysis (1) Comparison of the results of Example 1 and Comparative Example 7: Example 1 uses stepwise heating, and the sample prepared by Example 1 is significantly better than Comparative Example 7 which uses single rate direct heating in terms of volume density, open porosity and bending strength.

[0068] (2) Comparison of the results of Example 2 and Comparative Example 8: Similarly, the sample performance of Example 2 (stepwise heating) is overall better than Comparative Example 8 (single rate direct heating). In particular, in Comparative Example 8, the use of a faster single heating rate of 10℃ / min results in a sharp increase in open porosity and a significant decrease in bending strength.

[0069] (3) Comparison of the results of Example 3 and Comparative Example 9: This group of comparisons further verifies the above conclusion. Comparative Example 9 uses the fastest single heating rate of 13℃ / min, and the performance of the final product is significantly different from Example 3 which uses stepwise heating.

[0070] In summary, during the carbonization process of pitch, especially at temperatures below 650℃, intense pyrolysis reactions occur and a large amount of small molecule gas volatiles are released. The use of a single, faster heating rate (such as Comparative Examples 7-9) will result in the explosive generation of these gases inside the body within a short period of time, and their escape rate will far exceed the diffusion rate, thereby forming high pressure inside and impacting the matrix structure, forming a large number of microcracks and pores. The stepwise heating scheme used in the present application uses a controlled heating rate at low temperatures, which matches the generation rate of gas volatiles with the extraction rate of supercritical fluid, achieving a gradual and orderly release of gas; at high temperatures, the pyrolysis reaction tends to be gentle, and the heating rate is adjusted again, thereby ensuring that the final obtained matrix carbon structure is dense and has few defects, and the macroscopic mechanical properties are significantly improved.

[0071] Test Example 3 1. Purpose of the experiment The purpose of this test example is to verify the rationality and superiority of the specific heating rate range of the stepwise heating program defined in the process of the present application. By comparing the performance of the samples prepared using the heating rate range defined in the present application and the heating rate exceeding the range, it is proved that only when the stepwise heating is performed within a specific rate range, the best material performance can be obtained.

[0072] 2. Experimental samples The experimental samples are C / C composites prepared by the methods described in Examples 1-3 and Comparative Examples 10-12.

[0073] 3. Performance test method (1) Bulk density and open porosity test: Archimedes method was used to test each sample. Five specimens with size of 10 mm x 10 mm x 10 mm were cut from each sample, and their dry weight, saturated weight and water weight were measured respectively. The bulk density and open porosity were calculated, and the results were taken as the arithmetic mean.

[0074] (2) Flexural strength test: Three-point bending method was used to test each sample. Five specimens with size of 60 mm x 10 mm x 4 mm were cut from each sample, and tested on a universal material testing machine. The loading span was set to 40 mm, and the loading rate was 5 mm / min. The maximum breaking load of each specimen was recorded, and the flexural strength was calculated. The results were taken as the arithmetic mean.

[0075] 4. Test results The C / C composites prepared in Examples 1-3 and Comparative Examples 10-12 were subjected to the above performance tests, and the results are summarized in Table 3 below.

[0076] Table 3: Performance test results of each sample Sample Bulk density (g / cm 3 )]]> Open-cell porosity (%) Flexural strength (MPa) Example 1 1.86 3.8 254 Example 2 1.83 4.5 231 Example 3 1.87 3.3 262 Comparative Example 10 1.78 6.9 189 Comparative Example 11 1.75 7.5 173 Comparative Example 12 1.74 8.1 168 5. Experimental analysis (1) Comparison of the results of Example 1 and Comparative Example 10: The rate of Comparative Example 10 in the second heating stage (10°C / min) exceeded the upper limit defined in the present application (8°C / min). The results showed that the sample prepared was inferior to Example 1 in terms of density, porosity and strength. This indicates that a too fast heating rate in the high-temperature carbonization stage is also not conducive to the stabilization and densification of the matrix carbon structure.

[0077] (2) Comparison of the results of Example 2 and Comparative Example 11: The rates of Comparative Example 11 in the two heating stages (3°C / min and 4°C / min) were lower than the lower limit defined in the present application (5°C / min). The sample performance was significantly worse than Example 2. This indicates that a too slow heating rate cannot achieve the best carbonization kinetics, which may lead to insufficient pyrolysis or the formation of an undesirable carbon structure, thereby affecting the final densification effect and mechanical properties.

[0078] (3) Comparison of the results of Example 3 and Comparative Example 12: The heating rate of Comparative Example 12 in the first stage (16°C / min) and the second stage (10°C / min) both exceeded the upper limit defined in the present application. The sample performance was one of the worst among all the comparative groups. Especially in the first stage where gas volatiles are produced intensively, a too high heating rate leads to a sharp increase in internal defects, which seriously damages the final performance of the material.

[0079] In summary, the test results show that only the technical scheme of stepwise heating is not enough to ensure the optimal effect. The selection of heating rate is crucial, and too fast or too slow will have a negative impact on the final performance of the material. The stepwise heating rate range defined by the present application (5-12℃ / min in the first stage, 5-8℃ / min in the second stage) is a necessary process condition to obtain high density and excellent mechanical properties of C / C composite materials.

Claims

1. A process for preparing high-strength CC composite material, characterized in that, Includes the following steps: Step 1: Place the carbon fiber preform and asphalt into the same reaction vessel, and then heat the reaction vessel to melt the asphalt; Step 2: Introduce a gaseous medium into the reaction vessel to increase the internal pressure of the reaction vessel; thus bringing the gaseous medium into a supercritical state. Step 3: By introducing a gaseous medium into the reaction vessel, the melted asphalt is agitated using the gaseous medium. Step 4: After oscillation, heat the reaction vessel to the carbonization temperature for in-situ carbonization and supercritical fluid extraction.

2. The preparation process of a high-strength CC composite material according to claim 1, characterized in that, In step 1, the reaction vessel is heated to 250-350°C and maintained at that temperature for 0.5-1 hour to allow the asphalt to melt completely.

3. The preparation process of a high-strength CC composite material according to claim 1, characterized in that, The gaseous medium is carbon dioxide, and the purity of the carbon dioxide gas is >99.9%.

4. The preparation process of a high-strength CC composite material according to claim 1, characterized in that, In step 2, the internal pressure of the reaction vessel is increased to 10-15 MPa.

5. The preparation process of a high-strength CC composite material according to claim 1, characterized in that, In step 3, the gas medium is introduced into the reaction vessel every 10-25 seconds, and the peak pressure in the reactor is 15-25 MPa and the valley pressure is 8-12 MPa when the gas medium is introduced, and the cycle lasts for 1.5-3 hours.

6. The preparation process of a high-strength CC composite material according to claim 1, characterized in that, In step 4, the reaction vessel is heated to 600-650°C at a heating rate of 5-12°C / min, and then heated to the carbonization temperature at a heating rate of 5-8°C / min, and held at this temperature for 2-3 hours.

7. The preparation process of a high-strength CC composite material according to claim 6, characterized in that, The carbonization temperature is 900-1200℃.

8. The preparation process of a high-strength CC composite material according to claim 1, characterized in that, After step 4 is completed, without removing the carbon fiber preform, a new round of impregnation with asphalt is applied, and steps 2 to 4 are repeated continuously for a total of 2-4 times.

9. The preparation process of a high-strength CC composite material according to claim 8, characterized in that, After the total number of repetitions is completed, a high-temperature treatment step is also included, which includes: Under an argon atmosphere, the reaction vessel is heated to 2200-2800℃ and held at that temperature for 2-4 hours. After the reaction is completed, it is allowed to cool naturally and then removed to obtain a high-strength CC composite material.

10. The preparation process of a high-strength CC composite material according to claim 1, characterized in that, The porosity of the carbon fiber preform used in step 1 is 45-55%.