A post-processing method for preparing low-oxygen gradient SiBNO fibers by integrating a single furnace with steam assistance

Abstract

This invention discloses a post-processing method for the integrated preparation of low-oxygen gradient SiBNO fibers in a single furnace using a time-sequential water-vapor assisted process, belonging to the field of ceramic fiber processing technology. The method mainly includes four steps: sealed vacuum pretreatment, three-way gas supply setup, segmented time-sequential heat treatment, and cooling and exhaust gas treatment. The segmented time-sequential heat treatment step includes five stages: low-temperature crosslinking, vacuum replacement, deep deoxidation, matrix transition, and surface modification. This method effectively overcomes the defects of traditional processes such as fiber fusion, high oxygen content, ammonia blockage, and oxygen absorption during multi-furnace transfer. The resulting low-oxygen gradient SiBNO fiber has a total oxygen content ≤1.1wt%, a strength retention rate ≥85% after holding at 1400℃ for 2 hours, a 70% reduction in single-batch processing time, and no corrosive exhaust gas generation. This fiber is suitable for hypersonic vehicle radomes, high-temperature ceramic matrix composite reinforcements above 1400℃, and high-temperature broadband stealth thermal protection components.

Description

Technical Field

[0001] This invention relates to the field of ceramic fiber processing, specifically to a post-processing method for the integrated preparation of low-oxygen gradient SiBNO fibers in a single furnace with time-sequential steam-assisted steam. Background Technology

[0002] SiBNO ceramic fibers possess characteristics such as low density, wide-bandwidth low dielectric constant, high-temperature self-healing, oxidation resistance, and thermal shock resistance, making them a core reinforcing material in the fields of high-temperature wave transmission and thermal protection for aerospace. This fiber combines the high strength of silicon nitride fibers with the excellent dielectric properties of boron nitride fibers, giving it irreplaceable application value in key components such as radomes and high-temperature windows for hypersonic aircraft. According to patents and core academic literature, existing heat treatment processes for SiBN-based fibers suffer from multiple technical bottlenecks, failing to simultaneously achieve formability, low oxygen content, mass production, and structural controllability. Specifically, these bottlenecks manifest in the following aspects: First, there is an inherent technical contradiction between fiber melting and crosslinking and low-oxygen preparation, and mainstream processes all have significant shortcomings. While air oxidation crosslinking processes (such as air-insoluble treatment) are the simplest and mildest, treating polycarbosilane fibrils above 200°C results in an oxygen content as high as approximately 20 wt% (CN106192078B). Even commercially available Nicalon NL202 and Tyranno LoxM fibers have an oxygen content as high as approximately 12 wt%. This excessively high oxygen content leads to a decrease in the SiC content within the fiber. x O y The phase decomposes violently above 1200℃, releasing CO and SiO gases, forming pores and defects, drastically reducing fiber strength, and severely deteriorating high-temperature stability. To address this, although some patents attempt to control the oxygen content to 4-10wt% through a combination of low-temperature air non-melting and thermal crosslinking, this only provides limited relief and cannot fundamentally resolve the structural contradiction of oxygen-containing matrix. While the pure nitrogen inert atmosphere heat treatment process avoids the introduction of external oxygen, the precursor lacks crosslinking active sites in the 200-500℃ temperature range, leading to severe thermal melting and adhesion of the fibers. Although the full-process ammonia heat treatment process (CN104831419A) can achieve crosslinking and decarburization, in practical applications it has been found that the low-temperature boron component introduced by fiber borylation reacts with ammonia to form boroammonium salt crystals, clogging the gas path and furnace cavity, resulting in poor mass production stability. Irradiation crosslinking processes (such as electron beam irradiation) utilize high-energy radiation to initiate crosslinking, avoiding the introduction of external oxygen, but the equipment investment is huge, the operation and maintenance costs are high, and it is difficult to efficiently match continuous production processes, limiting large-scale application.

[0003] Secondly, the trace moisture-assisted crosslinking process suffers from two major problems: residual moisture cannot be removed after crosslinking, and the mixing of moisture and ammonia easily clogs pipelines. Previous studies have confirmed that trace moisture can activate Si-N and BN bond crosslinking, solving the problem of fiber fusion during pure nitrogen heat treatment. However, current processes only introduce moisture during the crosslinking stage, without a post-crosslinking directional dehydration process. Residual moisture in the furnace continuously triggers matrix hydrolysis, generating oxygen-containing groups. Simultaneously, the lack of time- and zone-based gas supply logic accelerates the precipitation of boron ammonium salts when moisture and ammonia coexist in the low-temperature zone, exacerbating pipeline blockage.

[0004] Third, the industry faces challenges in producing covalently integrated gradient-protected SiBN fibers at low cost. Current gradient protection modification methods are limited to CVD coating and liquid-phase impregnation, resulting in low production capacity, high equipment costs, and the protective layer's physical bond with the matrix, making it prone to peeling due to high-temperature thermal mismatch. Existing processes cannot rely on controlled furnace atmosphere to generate a covalently bonded integrated SiBN gradient skin in situ through elemental thermal diffusion within the fiber itself.

[0005] In summary, there is currently no process that can simultaneously solve the four major pain points of fiber thermal melting and bonding, high oxygen content in the matrix, ammonium salt blockage, and easy peeling of the protective layer. A new integrated post-processing technology needs to be developed to solve these problems. Summary of the Invention

[0006] To address the aforementioned problems, this invention proposes a post-processing method for the integrated preparation of low-oxygen gradient SiBNO fibers using a time-series water-steam assisted single furnace, comprising the following steps: S1 closed vacuum pretreatment uses polyborosilicate melt-spun nascent fibers as raw materials. The fibers are sealed and placed in a tube furnace, and the air in the furnace is exhausted by a vacuum pump. Then, high-purity nitrogen is refilled to atmospheric pressure. The S2 three-way gas supply system is equipped with three independent gas delivery pipelines: a continuous high-purity nitrogen main line, a micro-water vapor mixed gas branch line that can be opened and closed, and a heated ammonia branch line that can be opened and closed. The water vapor mixed gas branch line is set to be opened only in the low-temperature range, and the ammonia branch line is set to be opened only in the high-temperature range. S3 segmented timing thermal processing, this step is divided into five stages: Low-temperature crosslinking: Nitrogen gas mixed with a small amount of water vapor is introduced into the furnace to raise the temperature and crosslink the fiber, thereby constructing a three-dimensional rigid skeleton of the fiber and avoiding melting and fiber fusion. The water vapor supply is cut off after the final crosslinking temperature is reached. Vacuum replacement: After crosslinking is completed, vacuuming and nitrogen purging are performed multiple times to completely remove residual water vapor in the furnace; Deep deoxidation: After introducing dry high-purity nitrogen into the furnace, the temperature is raised to remove hydroxyl groups and organic oxygen-containing impurities from the fibers; Matrix transition: The furnace is kept at a constant temperature by pure nitrogen gas, and the polyborosilicate is fully pyrolyzed to generate a homogeneous SiBN amorphous core layer; Surface modification: Nitrogen and ammonia are simultaneously introduced into the furnace, and a continuous SiBNO gradient protective thin layer is generated in situ on the fiber surface. S4 cooling and exhaust gas treatment: ammonia gas is shut off, and only nitrogen gas is passed through to cool the gas to room temperature at a uniform rate. The exhaust gas discharged from the furnace is washed with water to absorb trace amounts of ammonia gas before being discharged harmlessly.

[0007] The resulting fiber has a covalently bonded core-sheath integrated structure, with a low-oxygen SiBN matrix as the core layer and a SiBNO gradient layer as the surface layer, and possesses high-temperature antioxidant and self-healing properties.

[0008] Furthermore, the tubular furnace has a built-in fiber sample boat. The input end of the tubular furnace is connected to the outlet of the gas mixer. The first inlet of the mixer is connected to the nitrogen storage tank through the nitrogen main line. A nitrogen main line valve is installed on the nitrogen main line. The second inlet of the mixer is connected to the outlet pipe of the water vapor generator. The inlet pipe of the water vapor generator is connected to the nitrogen main line. Both the inlet and outlet pipes are equipped with water vapor branch valves as water vapor mixed gas branches. The third inlet of the mixer is connected to the ammonia storage tank through the ammonia branch. A heating tape and an ammonia branch valve are installed on the ammonia branch. The two branch pipes with control valves at the output end of the tubular furnace are connected to the vacuum pump and the tail gas washing tank, respectively.

[0009] Furthermore, in step S1, the tubular furnace is evacuated to a furnace pressure ≤ 0.001 MPa; in the vacuum replacement stage of step S3, the replacement cycle is performed 1-4 times, and the vacuum pressure of each vacuuming cycle is not higher than 0.001 MPa.

[0010] Furthermore, in step S2, the three gas supply parameters are as follows: nitrogen purity in the main gas supply is ≥99.999%; water vapor volume fraction in the water vapor mixture is 0.4-0.8 vol; ammonia branch gas is heated to 100-110℃ throughout; and in the surface modification stage of step S3, the nitrogen-ammonia volume ratio is 8:1-10:1.

[0011] Furthermore, in step S3, the heating, holding, and cooling parameters and the timing sequence of the vacuum replacement process are as follows: During the low-temperature crosslinking stage, the heating rate is 1-3℃ / min, the final temperature is 500-550℃, the holding time is 0.8-1.2h, and the vacuum replacement stage begins immediately after the holding time is completed; During the deep deoxidation stage, the heating rate is 3-5℃ / min, the final temperature is 1050-1100℃; During the matrix transition stage, the temperature is held for 20-40min; During the surface modification stage, the heating rate is 5-7℃ / min, the final temperature is 1280-1320℃, and the holding time is 1.2-1.8h; In step S4, the cooling rate is 4-6℃ / min.

[0012] The beneficial effects of this invention are as follows: This invention overturns the prevailing technical prejudice that water vapor crosslinking inevitably leads to high oxygen levels in SiBN-based fibers. For the first time, it integrates the entire process of crosslinking, dehydration, deoxidation, pyrolysis, and gradient modification within a closed tubular furnace, without opening the furnace door or transferring fibers, thus eliminating secondary oxygen absorption at the source. The processing time per batch is reduced by more than 70% compared to existing processes. Through the sequential combination of "short-time micro-volume water vapor directional crosslinking + post-crosslinking vacuum directional dehydration + high-temperature pure nitrogen deep deoxidation," it solves the problem of fiber thermal melting and bonding while controlling the total oxygen content of the finished product to ≤1.1wt%, far superior to the 3.5% oxygen content of the air oxidation crosslinking process. Oxygen content levels of wt%-8wt%; a unique timing control logic for the timed supply of water vapor and ammonia, with the two streams not being activated simultaneously, eliminates the coexistence of water vapor and ammonia in the low-temperature range, fundamentally preventing the problem of boron ammonium salt crystallization clogging the pipeline, and significantly improving mass production stability; ammonia in the high-temperature section utilizes the thermal diffusion of the fiber's own elements to generate a 100-200nm thick SiBNO gradient protective layer covalently bonded to the core layer in situ, eliminating the risk of interface defects and peeling, and providing long-term self-healing and antioxidant capabilities during high-temperature service; the resulting fiber bundle has a tensile strength ≥1.8GPa, a wideband dielectric constant of 2-18GHz that is stable at 2.85-2.95, and a tensile strength retention rate ≥80% after 2 hours of air insulation at 1400℃, making it suitable for extreme service environments such as hypersonic vehicle radomes and high-temperature ceramic matrix composite reinforcements above 1400℃. Attached Figure Description

[0013] Figure 1 This is a process flow diagram of the present invention; Figure 2 This is a schematic diagram of the connection structure of the tubular furnace and its associated equipment in this invention; Figure 3 This is a temperature-atmosphere coupling control curve diagram of Embodiment 1 of the present invention; Figure 4 This is a schematic cross-sectional view of the core-sheath structure of the low-oxygen gradient SiBNO fiber obtained in Example 1 of the present invention.

[0014] The following are the annotations for the attached diagrams: 1. Tube furnace; 2. Vacuum pump; 3. Fiber sample boat; 4. Mixer; 5. Nitrogen storage tank; 6. Nitrogen main valve; 7. Water vapor generator; 8. Water vapor branch valve; 9. Ammonia storage tank; 10. Heating tape; 11. Ammonia branch valve; 12. Control valve; 13. Tail gas washing tank. Detailed Implementation

[0015] The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of them. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0016] This invention uses polyborosilazane (PBSZ) melt-spun nascent fibers as raw material. The synthesis of the polyborosilazane precursor can be achieved using any method known in the art, such as polymerization of boron trichloride, monomethyldichlorosilane, and hexamethyldisilazane. After the precursor polymer is melt-spun to obtain nascent fibers, the method of this invention can be used for post-processing. The entire heat treatment is completed in a single closed tube furnace 1, without opening the furnace door or transferring the fibers throughout the process. Figure 2 As shown, the tubular furnace 1 has a built-in fiber sample boat 3. The input end of the tubular furnace 1 is connected to the outlet of the gas mixer 4. The first inlet of the mixer 4 is connected to the nitrogen storage tank 5 through the nitrogen main line. The nitrogen main line is equipped with a nitrogen main line valve 6. The second inlet of the mixer 4 is connected to the outlet pipe of the water vapor generator 7. The inlet pipe of the water vapor generator 7 is connected to the nitrogen main line. Both the inlet and outlet pipes are equipped with water vapor branch valves 8 as water vapor mixed gas branches. The third inlet of the mixer 4 is connected to the ammonia storage tank 9 through the ammonia branch line. The ammonia branch line is equipped with a heat tracing cable 10 and an ammonia branch valve 11. The two branch pipes with control valves 12 at the output end of the tubular furnace 1 are respectively connected to the vacuum pump 2 and the tail gas washing tank 13.

[0017] Example 1

[0018] like Figure 1 , Figure 3 As shown in the diagram, this embodiment prepares low-oxygen gradient SiBNO fibers according to the illustrated process. The specific steps are as follows: S1 Sealed Vacuum Pretreatment: Place the nascent polyborosilicate melt-spun fibers (approximately 15 μm in diameter) in fiber sample boat 3, spread them evenly, and place them in the center of the constant temperature zone of a single sealed tube furnace 1 (furnace chamber inner diameter 80 mm, constant temperature zone length 300 mm). Close the furnace door, start vacuum pump 2 to evacuate to furnace pressure ≤ 0.001 MPa, maintain for 5 min, and then refill with high-purity nitrogen (purity ≥ 99.999%) to atmospheric pressure (0.101 MPa).

[0019] S2 Three-way Gas Supply Setup: The main line continuously supplies high-purity nitrogen (purity ≥99.999%) at a flow rate of 150 mL / min; in the water vapor branch, high-purity nitrogen is bubbled into the constant-temperature water bath (water temperature 30 ℃) of the water vapor generator 7, carrying out water vapor formed from deionized water. The volume fraction of the water vapor is adjusted to 0.6 vol% by a mass flow meter. Only the low-temperature crosslinking zone of this branch is open. At this time, the valves 8 of the two water vapor branches are open, and the valve 11 of the ammonia branch is closed; in the ammonia branch, high-purity ammonia (purity ≥99.999%) is heated by the heating cable 10 (heating temperature 105 ℃) and then enters the furnace. Only the high-temperature surface modification zone of this branch is open. At this time, the valves 8 of the two water vapor branches are closed, and the valve 11 of the ammonia branch is open.

[0020] S3 segmented timing heat treatment: Low-temperature crosslinking: The temperature was increased from room temperature to 520 ℃ at a rate of 2 ℃ / min, and a mixture of nitrogen and water vapor (water vapor volume fraction 0.6 vol%) was introduced and held at this temperature for 1.0 h to construct a three-dimensional rigid crosslinked fiber skeleton. The water vapor supply was immediately cut off after the final crosslinking temperature was reached.

[0021] Vacuum replacement: After the cross-linking and heat preservation are completed, immediately close all air inlet valves, start vacuum pump 2 to evacuate to furnace pressure ≤0.001 MPa, maintain for 3 minutes and then turn off the vacuum pump. Open nitrogen main valve 6 to return nitrogen to atmospheric pressure, and repeat the above vacuum-nitrogen-filling cycle 3 times.

[0022] Deep deoxygenation: The temperature is increased from 520 ℃ to 1080 ℃ at a rate of 4 ℃ / min, and only dry high-purity nitrogen gas (flow rate 150 mL / min) is introduced to remove hydroxyl groups and organic oxygen-containing impurities from the fiber.

[0023] Matrix transition: Pure nitrogen gas is continuously introduced at 1080 ℃ and kept at a constant temperature for 30 min, so that the polyborosilicate is fully pyrolyzed to generate a homogeneous low-oxygen SiBN amorphous core layer.

[0024] Surface modification: The temperature was increased from 1080 ℃ to 1300 ℃ at a heating rate of 6 ℃ / min. When the furnace temperature reached 1280 ℃, the ammonia branch valve 11 was opened, and nitrogen and heated ammonia were simultaneously introduced (nitrogen-ammonia volume ratio 9:1, total flow rate 150 mL / min). After reaching 1300 ℃, the temperature was held for 1.5 h, and a continuous SiBNO gradient protective thin layer was formed in situ on the fiber surface.

[0025] S4 Cooling and Exhaust Gas Treatment: The ammonia branch is closed, and only high-purity nitrogen (flow rate 200 mL / min) is introduced to cool to room temperature at a uniform rate of 5 ℃ / min. The generated exhaust gas is introduced into the exhaust gas washing tank 13 through a pipeline, and after the trace amount of ammonia is absorbed by deionized water, it is discharged harmlessly.

[0026] The cross-section of the fiber core-sheath structure obtained in this embodiment is as follows: Figure 4 As shown, its bundle tensile strength is 1.92 GPa, total oxygen content is 0.95 wt%, dielectric constant is 2.88-2.92 at 2-18 GHz, skin thickness is about 150 nm, and strength retention rate is 84% ​​after being kept in air at 1400℃ for 2 hours.

[0027] Example 2

[0028] This embodiment follows the same steps and procedures as Example 1, except that the water vapor volume fraction in the water vapor branch of S2 is changed to investigate its effect on fiber properties. Six experimental groups were set up with water vapor volume fractions of 0 vol%, 0.2 vol%, 0.4 vol%, 0.6 vol%, 0.8 vol%, and 1.0 vol%, respectively. Three parallel samples were taken from each experimental group, and the other parameters were exactly the same as in Example 1.

[0029] The results are shown in Table 1. When the water vapor volume fraction is 0.4-0.8 vol%, the fiber does not melt completely and the overall performance of the finished product is optimal. The preferred water vapor volume fraction is 0.4-0.8 vol%, and more preferably 0.5-0.7 vol%.

[0030] Table 1. Effect of water vapor volume fraction on fiber properties

[0031] Example 3

[0032] This embodiment follows the same steps as Example 1, except that the number of vacuum-nitrogen replacement cycles in the vacuum replacement stage S3 is changed to examine its effect on the oxygen content of the fiber. Five experimental groups were set up with replacement cycles of 0, 1, 2, 3, and 4 times respectively, and all other parameters were exactly the same as in Example 1.

[0033] The results are shown in Table 2. One vacuum cycle reduced the residual water vapor to below 5% of its original level, and after two cycles it was reduced to below 1%. After three cycles, the oxygen content stabilized at around 0.85 wt%, and there was no further improvement after four cycles. Therefore, the optimal number of vacuum replacement cycles is 2-3.

[0034] Table 2. Effect of vacuum-nitrogen replacement cycle number on fiber properties

[0035] Example 4

[0036] This embodiment follows the same steps as Example 1, except that the surface modification temperature in the S3 surface modification stage is changed to investigate its effect on the gradient skin structure and the high-temperature performance of the fiber. Six sets of experiments were set with surface modification temperatures of 1240 ℃, 1260 ℃, 1280 ℃, 1300 ℃, 1320 ℃, and 1340 ℃, respectively, with all other parameters being exactly the same as in Example 1.

[0037] The results are shown in Table 3. When the modification temperature is 1280-1320 ℃, the skin layer is continuous and covalently bonded to the core layer, the skin layer thickness is 100-200 nm, and the high-temperature strength retention rate is ≥81%. The preferred surface modification temperature is 1280-1320 ℃, and more preferably 1290-1310 ℃.

[0038] Table 3 Effect of surface modification temperature on fiber properties

[0039] Example 5

[0040] This embodiment follows the same steps as Example 1, with a fixed water vapor volume fraction of 0.6 vol%, three vacuum replacements, and a surface modification temperature of 1300 ℃. The heating rate and holding time at each stage are varied to examine the process parameter window.

[0041] The results are shown in Table 4. All three schemes achieved low oxygen content (≤1.1 wt%) and high strength (≥1.8 GPa), with scheme B showing the best overall performance. Scheme A, due to its slower heating and longer holding time, had a slightly lower oxygen content but decreased production efficiency; Scheme C, due to its faster heating and shorter holding time, increased production efficiency but had a slightly lower degree of fiber densification. These parameters can be flexibly adjusted within the above parameter window according to actual production needs.

[0042] Table 4. Effects of heating rate and holding time on fiber properties

[0043] Comparative Example 1 The difference between this comparative example and Example 1 is that the vacuum replacement dehydration process in the vacuum replacement stage of S3 is completely omitted. After the crosslinking and heat preservation are completed, the temperature is directly increased from 520 ℃ to 1080 ℃ at 4 ℃ / min to enter the deep deoxygenation stage. The other parameters are exactly the same as those in Example 1.

[0044] Results: The moisture introduced during the crosslinking stage was not removed before heating, and the residual moisture continued to react with the fiber matrix as the temperature increased. Obvious surface corrosion marks appeared when the fiber temperature reached approximately 650 °C, resulting in a brittle and low-strength final product. Testing revealed that the total oxygen content of the finished product was 2.87 wt%, the tensile strength of the fiber bundle was only 0.83 GPa, and the strength retention rate at 1400 °C for 2 hours was only 52%.

[0045] Comparative Example 2 The difference between this comparative example and Example 1 is that, in the low-temperature crosslinking stage of S3, water vapor and ammonia (water vapor volume fraction 0.6 vol%, nitrogen-ammonia volume ratio 9:1) are introduced simultaneously, that is, the water vapor branch and the ammonia branch are opened at the same time instead of being supplied in stages. The other parameters are exactly the same as those in Example 1.

[0046] Results: When the furnace temperature reached approximately 200 °C, water vapor and ammonia coexisted in the low-temperature zone. The boron component reacted with ammonia to form white crystals of boron ammonium salts (NH4B5O8, etc.), which deposited on the walls of the furnace tubes in the low-temperature zone and the inner walls of the exhaust pipes. After 2 hours of operation, the pressure drop in the pipeline increased significantly, and exhaust became obstructed; after 4 hours, the pipeline became severely blocked, forcing the experiment to be stopped. Upon disassembly of the pipeline, a white crystalline deposit approximately 2 mm thick was found adhering to the pipe walls.

[0047] This comparative example demonstrates that the time-sharing gas supply timing control logic of the present invention, which states that "the water vapor branch and the ammonia branch are not opened at the same time," is necessary. When water vapor and ammonia coexist in the low-temperature range, boron ammonium salt crystals are inevitably generated, while the time-sharing supply strategy eliminates the conditions for crystal formation from the source.

[0048] Comparative Example 3 The difference between this comparative example and Example 1 is that: in the matrix transition stage of S3, after the matrix has stabilized, the gradient surface modification process of the surface modification stage is not performed. That is, after the heat preservation at 1080 ℃, it is directly cooled to room temperature at 5 ℃ / min without heating and ammonia is introduced. The other parameters are exactly the same as those in Example 1.

[0049] Results: The obtained fibers have a uniform SiBN single-phase structure without a gradient skin. The tensile strength of the fiber bundle was tested to be 1.78 GPa (slightly lower than that of Example 1, because it did not undergo high-temperature sintering densification). After being kept in air at 1400 °C for 2 hours, the strength retention rate was only 58%, and obvious oxidation pits appeared on the fiber surface, with a diameter reduction of about 15%.

[0050] This comparative example demonstrates that the covalently bonded SiBNO gradient skin generated by the high-temperature ammonia surface modification process is the key to the fiber's excellent high-temperature oxidation resistance—SiBN fibers without skin protection oxidize rapidly in air at 1400 ℃, resulting in a sharp decline in mechanical properties.

[0051] In summary, the results are shown in Table 5. The comparative examples demonstrate from the opposite perspective that the three processes of vacuum directional dehydration, timed supply of water vapor and ammonia, and high-temperature ammonia surface modification are indispensable. Only through the synergistic effect of the three processes can fiber anti-melting, low-oxygen preparation, non-clogging operation, and high-temperature anti-oxidation performance be achieved simultaneously.

[0052] Table 5 Summary of the effects of the examples and comparative examples

[0053] This invention effectively overcomes the defects of traditional processes, such as fiber fusion, high oxygen content, ammonia blockage, and oxygen absorption during multi-furnace transfer. The resulting low-oxygen gradient SiBNO fiber has a total oxygen content ≤1.1wt%, a strength retention rate ≥85% after holding at 1400℃ for 2 hours, a 70% reduction in single-batch processing time, and no corrosive exhaust gas generation. This fiber is suitable for hypersonic vehicle radomes, high-temperature ceramic matrix composite reinforcements above 1400℃, and high-temperature broadband stealth thermal protection components.

[0054] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The embodiments and descriptions in the specification are merely illustrative of the principles of the invention. Various changes and modifications can be made to the invention without departing from its spirit and scope, and all such changes and modifications fall within the scope of the claimed invention.

Claims

1. A post-processing method for the integrated preparation of low-oxygen gradient SiBNO fibers using a time-sequential steam-assisted single-furnace process, characterized in that, Includes the following steps: S1 closed vacuum pretreatment, using polyboron silazane melt-spun nascent fiber as raw material, the fiber is sealed and placed in a tube furnace (1), and the air in the furnace is exhausted by a vacuum pump (2), and then high-purity nitrogen is refilled to atmospheric pressure; S2 Three-way gas supply setting, the tubular furnace (1) is equipped with three independent gas delivery pipelines, namely a continuous high-purity nitrogen main line, a micro-water vapor mixed gas branch line that can be opened and closed, and a heated ammonia branch line that can be opened and closed. The water vapor mixed gas branch line is set to be opened only in the low temperature range, and the ammonia branch line is set to be opened only in the high temperature range. S3 segmented timing thermal processing, this step is divided into five stages: Low-temperature crosslinking: Nitrogen gas mixed with a small amount of water vapor is introduced into the furnace to raise the temperature and crosslink the fiber, thereby constructing a three-dimensional rigid skeleton of the fiber and avoiding melting and fiber fusion. The water vapor supply is cut off after the final crosslinking temperature is reached. Vacuum replacement: After crosslinking is completed, vacuuming and nitrogen purging are performed multiple times to completely remove residual water vapor in the furnace; Deep deoxidation: After introducing dry high-purity nitrogen into the furnace, the temperature is raised to remove hydroxyl groups and organic oxygen-containing impurities from the fibers; Matrix transition: The furnace is kept at a constant temperature by pure nitrogen gas, and the polyborosilicate is fully pyrolyzed to generate a homogeneous SiBN amorphous core layer; Surface modification: Nitrogen and ammonia are simultaneously introduced into the furnace, and a continuous SiBNO gradient protective thin layer is generated in situ on the fiber surface. S4 cooling and exhaust gas treatment: ammonia gas is shut off, and only nitrogen gas is passed through to cool the gas to room temperature at a uniform rate. The exhaust gas discharged from the furnace is washed with water to absorb trace amounts of ammonia gas before being discharged harmlessly. The resulting fiber has a covalently bonded core-sheath integrated structure, with a low-oxygen SiBN matrix as the core layer and a SiBNO gradient layer as the surface layer, and possesses high-temperature antioxidant and self-healing properties.

2. The post-processing method for the integrated preparation of low-oxygen gradient SiBNO fibers in a single furnace with time-sequential steam-assisted steam processing according to claim 1, characterized in that, The tubular furnace (1) has a built-in fiber sample boat (3). The input end of the tubular furnace (1) is connected to the outlet of the gas mixer (4). The first inlet of the mixer (4) is connected to the nitrogen storage tank (5) through the nitrogen main line. The nitrogen main line is equipped with a nitrogen main line valve (6). The second inlet of the mixer (4) is connected to the outlet pipe of the water vapor generator (7). The inlet pipe of the water vapor generator (7) is connected to the nitrogen main line. The inlet and outlet pipes are equipped with water vapor branch valves (8) as water vapor mixed gas branches. The third inlet of the mixer (4) is connected to the ammonia storage tank (9) through the ammonia branch. The ammonia branch is equipped with a heat tracing cable (10) and an ammonia branch valve (11). The two branch pipes with control valves (12) at the output end of the tubular furnace (1) are connected to the vacuum pump (2) and the tail gas washing tank (13) respectively.

3. The post-processing method for the integrated preparation of low-oxygen gradient SiBNO fibers in a single furnace with time-sequential steam-assisted steam processing according to claim 1, characterized in that, In step S1, the tubular furnace (1) is evacuated to a furnace pressure ≤ 0.001 MPa; in the vacuum replacement stage of step S3, the replacement cycle is performed 1-4 times, and the pressure of each vacuuming is not higher than 0.001 MPa.

4. The post-processing method for the integrated preparation of low-oxygen gradient SiBNO fibers in a single furnace with time-sequential steam-assisted steam processing according to claim 1, characterized in that, In step S2, the three gas supply parameters are as follows: nitrogen purity of the main gas supply is ≥99.999%; water vapor volume fraction in the water vapor mixture is 0.4-0.8 vol%; ammonia branch gas supply is heated to 100-110℃ throughout; and in the surface modification stage of step S3, the nitrogen-ammonia volume ratio is 8:1-10:

1.

5. The post-processing method for the integrated preparation of low-oxygen gradient SiBNO fibers in a single furnace with time-sequential steam-assisted steam processing according to claim 1, characterized in that, In step S3, the heating, holding, and cooling parameters and the timing of the vacuum replacement process are as follows: During the low-temperature crosslinking stage, the heating rate is 1-3℃ / min, the final temperature is 500-550℃, the holding time is 0.8-1.2h, and the vacuum replacement stage begins immediately after the holding time is completed; During the deep deoxidation stage, the heating rate is 3-5℃ / min, the final temperature is 1050-1100℃; During the matrix transition stage, the temperature is held for 20-40min; During the surface modification stage, the heating rate is 5-7℃ / min, the final temperature is 1280-1320℃, and the holding time is 1.2-1.8h; In step S4, the cooling rate is 4-6℃ / min.

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