High-temperature-resistant aerogel composite material and preparation method thereof

Abstract

This invention discloses a high-temperature resistant aerogel composite material and its preparation method. The material comprises an oriented mullite fiber skeleton, an alumina aerogel matrix filling the fiber gaps, and macroscopic channels extending along the fiber orientation. The axial to radial thermal conductivity ratio is greater than 2.0, and the density is 0.30 to 0.40 g / cm³. The preparation method includes directional cryogenic casting to construct the oriented fiber skeleton, vacuum pressure impregnation for selective sol filling, five-stage gradient solvent replacement, segmented atmospheric pressure drying, and sintering at 1350 degrees Celsius. By controlling the freezing rate to 20 to 50 micrometers per second to oriented the fibers, selective filling is achieved by utilizing the time difference in gelation of the sol within the fiber gaps and macroscopic channels. Combined with a four-stage drying process, large-size bulk materials are formed without cracks. This material can be used to prepare complete bulk materials larger than 600 mm in size, and its lifespan in a 1200 degree Celsius water-cooled thermal cycle exceeds 50 cycles.

Description

Technical Field

[0001] This invention belongs to the field of inorganic non-metallic materials, specifically relating to a high-temperature resistant aerogel composite material and its preparation method. Background Technology

[0002] Alumina aerogels possess a nanoporous structure and low thermal conductivity, making them valuable for high-temperature insulation applications above 1200 degrees Celsius. However, this field has long faced two mutually restrictive technical challenges.

[0003] The first challenge is the difficulty of drying and molding large-sized blocks at atmospheric pressure. Pure aerogel skeletons are extremely brittle; during atmospheric pressure drying, the capillary forces generated by solvent evaporation cause the skeleton to shrink. When preparing blocks larger than 300 mm, the difference in shrinkage rates between the central and surface regions generates enormous internal stress, leading to through-cracks. Industrially, supercritical drying processes are typically used to avoid capillary force damage; however, supercritical drying equipment is expensive and limited by the size of the autoclave, making it impossible to prepare large thermal insulation components larger than 500 mm. Furthermore, high-pressure processes pose safety risks and consume enormous amounts of energy.

[0004] The second challenge is the material's insufficient thermal shock resistance. Even with fiber reinforcement to improve room-temperature toughness, traditional isotropic aerogels exhibit uniform internal thermal stress distribution under rapid heating and cooling conditions, lacking effective stress release channels. When the material surface is subjected to high-temperature thermal shock, a temperature gradient is generated between the surface and the interior, causing thermal stress to accumulate within the material and leading to rapid cracking failure. Literature reports that conventional alumina aerogels typically develop through-cracks after only 3 to 5 cycles in water-cooled thermal cycling tests from 800 degrees Celsius to room temperature, failing to meet the requirements for use in industrial kilns and aerospace thermal protection systems.

[0005] To address the aforementioned issues, existing technologies have proposed several improvement schemes. Regarding the preparation process, conventional methods in this field reduce capillary forces during atmospheric pressure drying by optimizing solvent replacement and surface modification. However, this technique is only suitable for small-sized samples; the cracking rate remains high when preparing large-sized bulk materials, and the process window is narrow with poor repeatability. In terms of structural design, existing technologies use randomly distributed short-cut fibers to improve toughness. However, isotropic structures cannot solve the problem of thermal stress concentration, and the fiber-aerogel matrix interface has weak bonding, easily debonding at high temperatures and becoming a channel for crack propagation.

[0006] Furthermore, while directional cryo-casting utilizes the ice crystal template effect to prepare directional porous ceramics, those skilled in the art generally agree that the strength of aerogel matrices is far lower than that of dense ceramics. The fragile fibrous skeleton obtained through cryo-casting is extremely prone to breakage during subsequent sol impregnation and drying processes. Therefore, there is no incentive in existing technologies to use cryo-casting for aerogel preparation. More importantly, thermal insulation materials typically strive for isotropy to prevent heat leakage, and existing technologies do not disclose a technical solution that combines directional heat dissipation with radial insulation using anisotropic structures.

[0007] Therefore, there is an urgent need in this field for a high-temperature resistant aerogel composite material and its preparation method that can take into account both large-size fabrication capability and excellent thermal shock resistance. Summary of the Invention

[0008] The high-temperature resistant aerogel composite material of the present invention comprises three interdependent and synergistic structural components. The first part is an oriented mullite fiber skeleton, which is composed of mullite fibers with a length of 5 to 8 mm and a diameter of 3 to 5 micrometers arranged in an orderly manner along a specific direction. The fiber alignment in the orientation direction is greater than 80%, which is determined by scanning electron microscopy image analysis, calculating the percentage of fibers with an orientation angle of less than 15 degrees out of the total number of fibers. Mullite fibers are chosen because they maintain chemical stability above 1200 degrees Celsius, do not undergo volume changes due to crystal transformation, and their coefficient of thermal expansion matches that of the alumina aerogel matrix. The fiber length is limited to 5 to 8 mm; fibers shorter than 5 mm have insufficient reinforcement, while fibers longer than 8 mm are prone to entanglement and agglomeration in the slurry. The fiber volume fraction is controlled between 1% and 3%, preferably 2%; below 1%, an effective skeleton cannot be formed, and above 3%, the interaction between fibers is enhanced, hindering directional alignment. The second part is an alumina aerogel matrix, which fills the gaps in the fiber skeleton. Its main components are alumina and silicon oxide, with a molar ratio of aluminum to silicon of 3:1. This ratio ensures the formation of the 3Al₂O₃·2SiO₂ mullite crystalline phase during high-temperature sintering. 3% to 8% of a high-temperature stable aluminum phosphate phase is uniformly dispersed in the matrix; when the content is below 3%, the high-temperature stabilization effect is not significant, and when the content is above 8%, the high-temperature strength of the material decreases. The third part consists of macroscopic channels penetrating the material along the fiber orientation direction. These channels have a pore size of 50 to 200 micrometers and a porosity of 60% to 70%, forming a dual-scale pore structure with the nanopores in the fiber gaps. The overall density of the composite material is 0.30 to 0.40 g / cm³, the ratio of axial to radial thermal conductivity is greater than 2.0, the axial thermal conductivity is 0.15 W / (m·K), and the radial thermal conductivity is 0.30 W / (m·K).

[0009] The aforementioned composite material is prepared through the following five process steps, with each step having a close process logic and technical correlation, forming a complete technology chain.

[0010] The first process step is directional cryogenic casting. This step constructs a fiber skeleton with oriented alignment characteristics and simultaneously forms interconnected macroscopic channels, establishing a structural foundation for subsequent selective filling. During operation, mullite fibers are dispersed in deionized water to form a slurry with a solid content of 15% and a fiber volume fraction of 2%. 0.5 wt% ammonium polyacrylate dispersant is added to the slurry and stirred for 30 minutes. The ammonium polyacrylate, through electrostatic repulsion, imparts a negative charge to the fiber surface, creating a double-layer repulsion force between the fibers, ensuring uniform dispersion. The slurry is poured into a specialized mold. The bottom of the mold has a copper cooling end connected to a -40°C cold source, and the sidewalls are made of polytetrafluoroethylene (PTFE) insulation material to ensure unidirectional heat transfer, forming a bottom-up directional temperature gradient. The freezing rate is controlled at 20 to 50 micrometers per second, achieved by adjusting the cold source temperature and the slurry height. Within this rate range, ice crystals grow stably in a planar shape. As their leading edges move, they generate a physical displacement effect, pushing the fibers between the ice crystal layers and forcing them to align along the ice crystal growth direction. If the freezing rate is below 20 micrometers per second, ice crystal growth is too slow, resulting in low production efficiency, and the slow displacement effect leads to insufficient fiber alignment. If the freezing rate is above 50 micrometers per second, the ice crystals extend rapidly in a dendritic manner, forming a complex branched structure, leaving tortuous and non-connected channels after sublimation. After the ice crystals have fully grown, they are freeze-dried at -20 degrees Celsius for 24 hours. This temperature is below the triple point of water, and the vacuum level is maintained below 10 Pa, ensuring that the ice crystals sublimate directly without passing through a liquid water stage, thus avoiding capillary forces that damage the channel structure. The output of this step is a fiber skeleton with oriented channels, the channel direction being consistent with the fiber orientation, forming an anisotropic template.

[0011] The second process step is sol vacuum impregnation and selective filling. This step utilizes the differences in pore size and sol gelation kinetics established in the first step to achieve selective distribution, which is a key step in constructing the aerogel matrix. An alumina-silicon composite sol is prepared using aluminum isopropoxide as the aluminum source and tetraethyl orthosilicate as the silicon source, controlling the aluminum-to-silicon molar ratio at 3:1. Aluminum dihydrogen phosphate is added at 5% of the alumina weight, and this component is uniformly dispersed in the sol in an ionic state. After hydrolysis, the solid content is controlled at 20%, the viscosity at 8 mPa·s, and the pH at 3 to 4. Because the equivalent pore size of the fiber skeleton gaps is approximately 5 to 10 micrometers, with a specific surface area as high as 200 to 300 square meters per gram, the sol undergoes rapid condensation reaction within the gaps due to high surface catalysis, with a gelation time of approximately 10 to 15 minutes. However, the macroscopic pore size is 50 to 200 micrometers, with a specific surface area less than 5 square meters per gram, extending the gelation time of the sol within the pores to over 60 minutes while maintaining liquid fluidity. The fiber skeleton obtained in step one is placed in a vacuum impregnation tank, and a vacuum is drawn to a negative pressure of 0.095 MPa and maintained for 15 minutes to remove air from the skeleton. Then, a sol is injected and a pressure of 0.3 to 0.5 MPa is applied and maintained for 20 to 30 minutes. Under this pressure, the sol fully penetrates the fiber gaps and gels within 10 to 15 minutes, forming a wet gel precursor. Within the macroscopic channels, the sol remains liquid due to the longer gelation time and the limited duration of the pressure, allowing it to be displaced and removed in subsequent steps, thus achieving selective filling. If the impregnation pressure is below 0.2 MPa, the sol penetration is insufficient, resulting in incomplete filling. If the pressure is above 0.6 MPa or the impregnation time exceeds 40 minutes, the sol will also gel within the macroscopic channels, leading to channel blockage. This step is closely technically linked to step one; the difference in channel size formed in step one is a prerequisite for selective filling in step two, and the parameters of the two steps must be matched.

[0012] The third process step is solvent replacement and chemical modification, which creates conditions for atmospheric pressure drying in the fourth step. The wet preform obtained in step two is sequentially placed in 30%, 50%, 70%, 90%, and 100% ethanol aqueous solutions for 12 hours each to perform a five-level gradient solvent replacement. This five-level gradient design is based on the principle of surface tension gradient control. The surface tension of water is 72 mN / m, and that of ethanol is 22 mN / m. Direct replacement would cause a sudden change in surface tension, generating capillary forces exceeding 5 MPa, which would exceed the strength of the wet gel skeleton and cause the nanopores to collapse. Through five-level gradient replacement, the surface tension decreases step by step, with the difference between each level controlled within 10 to 15 mN / m, ensuring that the stress on the skeleton is lower than its fracture strength. After replacement, the aerogel is modified with a solution of trimethylchlorosilane and n-hexane in a volume ratio of 1:10 for 6 hours. The Si-Cl groups in the trimethylchlorosilane molecule undergo a condensation reaction with the Si-OH groups on the aerogel surface to generate Si-O-Si bonds and graft hydrophobic methyl groups. After modification, the preform is washed three times with anhydrous ethanol to remove residual hexane and unreacted trimethylchlorosilane, preventing safety hazards or residual carbon affecting material purity during subsequent drying and sintering. This step is technically necessary because unmodified wet gels contain a large amount of physically adsorbed water, which generates significant capillary forces during atmospheric pressure drying, leading to cracking. Unmodified aerogels have an excessively high hydroxyl density on their surface; during drying, the continuous condensation reaction causes skeletal shrinkage, and at high temperatures, they easily adsorb moisture, affecting thermal insulation performance.

[0013] The fourth process step is segmented atmospheric pressure drying. This step, based on the anisotropic structural characteristics, involves a differentiated drying program and is the core step for achieving crack-free molding of large-sized blocks. The first stage involves maintaining the material at 30°C under atmospheric pressure and ventilation for 12 hours. This low-temperature environment ensures slow solvent evaporation, preventing surface hardening. Furthermore, 30°C is below the boiling point of ethanol, ensuring high safety. This stage primarily removes free solvent from macroscopic channels. The second stage involves ventilation drying at 40-50°C for 8 hours, further removing deep-layer solvents and accelerating ethanol evaporation. The third stage involves drying at 80°C for 6 hours, achieving deep drying and removing residual modifiers. The fourth stage involves drying at 120°C to constant weight, completely removing residual solvent. This four-stage process ensures orderly solvent evaporation and avoids localized stress concentration. Due to the anisotropic structure of the material, the shrinkage rates differ in different directions. The slow removal rate in the first stage coordinates and synchronizes shrinkage in all directions, preventing interlayer shear stress that could lead to delamination. This step is closely linked to step three. Only after solvent replacement and modification are completed in step three can the wet gel withstand this drying process. At the same time, this step provides a dry green body for sintering in step five. If there is residual moisture in the green body, the moisture will instantly vaporize during sintering at 1350 degrees Celsius, expanding in volume by about 1600 times. Even a small amount of residual moisture will cause the material to crack.

[0014] The fifth process step is high-temperature sintering, which transforms the porous structure established in the previous four steps into a high-temperature stable state. The temperature is increased to 1350°C at a rate of 2°C per minute and held for 2 hours. This heating rate is calculated to match thermal expansion, as the thermal expansion coefficients of the fiber skeleton, aerogel matrix, and macroscopic channels differ. Excessive heating can generate thermal stress exceeding the material's strength, leading to cracking. During the heating process, residual organic groups decompose between 200 and 400°C, amorphous alumina begins to crystallize between 400 and 800°C, and aluminum dihydrogen phosphate decomposes and reacts with alumina between 800 and 1200°C to form a high-temperature resistant aluminophosphate bonded phase. This bonded phase is uniformly distributed within the alumina skeleton, inhibiting excessive grain growth and sintering shrinkage. During the 1350°C holding stage, the aluminum-silicon components fully react to form a mullite crystalline phase. Needle-like mullite whiskers grow in situ at the fiber-aerogel matrix interface, forming a riveted structure that firmly bonds the fiber to the matrix. This step utilizes the aluminum dihydrogen phosphate and aluminum-silicon raw materials introduced in step two to complete the crystal phase transformation and interface strengthening at high temperatures, enabling the material to achieve temperature resistance above 1200 degrees Celsius. Without this step, the material is merely a gel state dried at low temperatures, and the structure collapses rapidly at high temperatures. If the heating rate exceeds 3 degrees Celsius per minute, even after drying in step four, the thermal stress difference generated by the anisotropic structure may still cause the green body to crack. After sintering, the material forms a complex structure with oriented fibers, an aerogel matrix filling the gaps, interconnected macroscopic channels, and mullite interfacial bonding. The various components work synergistically to achieve the technical goals of large size, anisotropy, and high temperature resistance.

[0015] The five process steps described above form a complete technical chain, each step interconnected and indispensable. Step one establishes an anisotropic template; step two fills the gaps in the template with functional materials while maintaining pore continuity; step three removes physical obstacles for drying and enhances the toughness of the skeleton; step four achieves large-size, crack-free molding; and step five imparts high-temperature stability and interfacial bonding strength. The absence of any single step or deviation in parameters will prevent the final product from simultaneously achieving the technical goals of large-size fabrication, anisotropic structure, and high-temperature resistance. For example, skipping step three and proceeding directly to step four will inevitably cause the wet gel to crack. If a pressure control error in step two leads to pore blockage, even if subsequent steps are executed perfectly, the material will lose its anisotropy. This technical solution, through strict step sequence and parameter control, overcomes the technical bottleneck in this field where it is difficult to simultaneously achieve large-size aerogel fabrication and thermal shock resistance. Attached Figure Description

[0016] Figure 1 This is a flowchart of the overall process of the preparation method of the present invention.

[0017] Figure 2 This is a flowchart illustrating the mechanism of directional cryogenic casting and structure formation.

[0018] Figure 3This is a flowchart of a segmented atmospheric pressure drying process. Detailed Implementation

[0019] Example 1 20 grams of mullite fibers, 5 mm in length and 3 μm in diameter, were mixed with 980 grams of deionized water, resulting in a fiber volume fraction of approximately 2%. 5 grams of ammonium polyacrylate dispersant were added, and the mixture was stirred for 30 minutes to form a homogeneous slurry. The slurry was poured into a stainless steel mold. The bottom of the mold was a copper plate connected to a constant temperature cold source at -40°C, and the side walls were lined with polytetrafluoroethylene insulation. The mold's internal dimensions were 600 mm long, 300 mm wide, and 200 mm high. During freezing, the ice crystal growth rate was controlled at 30 μm per second. The ice crystals grew directionally upwards from the bottom copper plate, displacing the fibers and aligning them vertically. After freezing, the preform was transferred to a freeze dryer and freeze-dried at -20°C and a vacuum of 5 Pa for 24 hours to obtain a fiber skeleton with oriented channels.

[0020] An alumina-silicon composite sol was prepared by mixing aluminum isopropoxide and tetraethyl orthosilicate at a molar ratio of 3:1, adding 5% aluminum dihydrogen phosphate by weight of alumina, and hydrolyzing to obtain a sol with a solid content of 20% and a viscosity of 8 mPa·s. A fiber skeleton was placed in a vacuum impregnation tank, and a vacuum of 0.095 MPa was maintained for 15 minutes to remove air. The sol was then injected and a pressure of 0.4 MPa was applied for 25 minutes. The sol rapidly gelled within the fiber interstices to form a wet gel precursor, while remaining liquid within the macroscopic channels. The wet preform was sequentially immersed in 30%, 50%, 70%, 90%, and 100% ethanol aqueous solutions for 12 hours each for five stages of solvent replacement. It was then modified for 6 hours in a solution of trimethylchlorosilane and n-hexane at a volume ratio of 1:10. After modification, it was washed three times with anhydrous ethanol.

[0021] The modified preform was placed in a drying oven and dried under normal pressure at 30°C for 12 hours in the first stage, under ventilation at 45°C for 8 hours in the second stage, under ventilation at 80°C for 6 hours in the third stage, and under constant quality at 120°C in the fourth stage. Finally, it was sintered by raising the temperature to 1350°C at a rate of 2°C per minute and holding it at that temperature for 2 hours to obtain a high-temperature resistant aerogel composite material.

[0022] The obtained material has a density of 0.32 g / cm³, an axial thermal conductivity of 0.15 W / (m·K), a radial thermal conductivity of 0.30 W / (m·K), and a thermal conductivity ratio of 2.0. The prepared block, 600 mm long, 300 mm wide, and 200 mm thick, showed no surface cracks and retained its internal structure intact. After 50 cycles of water-cooled thermal cycling from 1200°C to room temperature, no through-cracks appeared, and the strength retention rate was 82%.

[0023] Example 2 Mullite fibers with a length of 8 mm were used, and the freezing rate was controlled at 20 micrometers per second. Other process parameters were the same as in Example 1. Longer fibers, at a slower freezing rate, achieved a higher degree of alignment, resulting in a more regular fiber arrangement within the ice crystal layers. The resulting material had a density of 0.35 g / cm³, an axial thermal conductivity of 0.14 W / (m·K), a radial thermal conductivity of 0.32 W / (m·K), and a thermal conductivity ratio of 2.3. It exhibited a thermal shock cycle life of 55 cycles, with the longer fiber skeleton providing better toughness. During preparation, a slight increase in slurry viscosity was observed due to the increased fiber length, but uniform dispersion could still be maintained by extending the stirring time to 40 minutes.

[0024] Example 3 The sol impregnation process parameters were changed, with the air pressure increased to 0.6 MPa and the holding time extended to 40 minutes, while the remaining process parameters remained consistent with Example 1. Under these conditions, the filling behavior of the sol within the fiber interstices changed significantly. Due to the impregnation pressure exceeding the preferred range and the excessive holding time, the sol overcame the physical barrier between the fiber interstices and the macroscopic channels, not only filling the fiber interstices but also penetrating extensively into the macroscopic channels with pore sizes of 50 to 200 micrometers. The sol formed a gel layer approximately 20 to 50 micrometers thick on the inner wall of the channels, resulting in approximately 40% of the macroscopic channels being partially blocked or completely sealed. This phenomenon disrupted the selective filling mechanism established in step two, causing the stress relief channels that should have been unobstructed to fail.

[0025] The density of the resulting material increased to 0.38 g / cm³, significantly higher than the 0.32 g / cm³ of Example 1, indicating that excess gel entered the macroscopic channels that should have maintained the cavity. Thermal conductivity tests showed that the axial thermal conductivity increased to 0.22 W / (m·K), a significant increase from 0.15 W / (m·K) in Example 1, while the radial thermal conductivity remained at 0.31 W / (m·K), resulting in a decrease in the axial to radial thermal conductivity ratio to 1.4, and a significant reduction in anisotropy. Thermal shock resistance tests showed that the material's lifespan in a 1200°C water-cooled thermal cycle decreased to 25 cycles, a 50% decrease from 50 cycles in Example 1. This is because after the macroscopic channels were blocked, thermal stress could not be quickly released through the axial channels, but instead accumulated randomly within the material, ultimately leading to crack initiation and propagation. This example fully demonstrates that the selective filling process parameters must be strictly controlled within a pressure range of 0.3 to 0.5 MPa and a time range of 20 to 30 minutes. Any parameters exceeding this range will lead to pore blockage, damage to the anisotropic structure of the material, and significant deterioration of its thermal shock resistance.

[0026] Example 4 Using a conventional isotropic preparation process as a comparative example, 5 mm long mullite fibers were randomly dispersed in deionized water to form a slurry. Without a directional cryogenic casting step, conventional gelation, solvent replacement, segmented drying, and high-temperature sintering were performed directly, with the remaining process parameters the same as in Example 1. Due to the lack of a directional cryogenic process, the fibers were randomly distributed in the slurry, without orientation or the formation of interconnected macroscopic channels. When the slurry was left to stand in the mold, the fibers easily settled and agglomerated, resulting in uneven distribution.

[0027] During the atmospheric pressure drying stage, due to the random distribution of fibers and the lack of through-holes as stress release channels, the capillary force generated by solvent evaporation produces randomly distributed tensile stress within the material. When preparing a block with dimensions of 300 mm long, 300 mm wide, and 200 mm, multiple through-cracks appear on the material surface during the second drying stage. The cracks are randomly oriented, approximately 1 to 3 mm wide, and penetrate the entire cross-section, making it impossible to obtain a complete large-sized material. Even when the sample is cut into smaller sizes (less than 100 mm), microcracks still exist within the material.

[0028] The resulting material exhibits isotropic thermal conductivity of 0.28 W / (m·K), higher than the radial thermal conductivity of Example 1. Thermal shock cyclic testing showed that the material developed penetrating cracks after only four cycles of water-cooled thermal cycling from 1200°C to room temperature. The cracks propagated along the randomly distributed fiber direction, representing a performance decrease of over 90% compared to the 50-cycle life of Example 1. This is because thermal stress is uniformly distributed in all directions in the isotropic structure, making it impossible to release through channels in specific directions. Furthermore, the randomly distributed fiber-matrix interface is prone to debonding under thermal shock, becoming a crack initiation point. This comparative example fully demonstrates that the directional cryogenic casting step plays an irreplaceable and decisive role in constructing anisotropic structures, achieving large-scale fabrication, and improving thermal shock resistance. Without this step, it is impossible to simultaneously meet the technical requirements of large-scale molding and high thermal shock resistance.

Claims

1. A high-temperature resistant aerogel composite material, characterized in that, The composite material includes an oriented mullite fiber skeleton, an alumina aerogel matrix filling the gaps between the fiber skeleton, and macroscopic channels penetrating the material along the fiber orientation direction. The ratio of the axial to radial thermal conductivity of the composite material is greater than 2.0, and the density is 0.30 to 0.40 g / cm³.

2. The composite material according to claim 1, characterized in that, The mullite fibers are 5 to 8 mm in length, 1% to 3% in volume, and have an alignment of more than 80% in the orientation direction.

3. The composite material according to claim 1, characterized in that, The macroscopic pore size is 50 to 200 micrometers, and the porosity is 60% to 70%.

4. The composite material according to claim 1, characterized in that, The ratio of axial to radial thermal conductivity is 2.0 to 3.0, with an axial thermal conductivity of 0.15 W / (m·K) and a radial thermal conductivity of 0.30 W / (m·K).

5. The composite material according to claim 1, characterized in that, The alumina aerogel matrix is ​​doped with a high-temperature stable phase of aluminum phosphate, with a content of 3% to 8%.

6. A method for preparing the composite material according to any one of claims 1 to 5, characterized in that, Includes the following steps: S1. Directional cryogenic casting: Mullite fibers are dispersed in deionized water to form a slurry with a fiber volume fraction of 1% to 3% and a slurry solid content of 15%. 0.5wt% ammonium polyacrylate dispersant is added and stirred for 30 minutes. The slurry is poured into a mold with a copper cooling end at the bottom and heat-insulated side walls. The ice crystal growth rate is controlled to be 20 to 50 micrometers per second during the freezing process. After freezing, the slurry is freeze-dried at -20 degrees Celsius and a vacuum degree of less than 10 Pa for 24 hours to obtain a fiber skeleton with directional channels. S2, Sol Vacuum Impregnation and Selective Filling: Alumina-silicon composite sol containing aluminum dihydrogen phosphate was prepared, with aluminum isopropoxide as the aluminum source and tetraethyl orthosilicate as the silicon source. The molar ratio of aluminum to silicon was 3:1, the amount of aluminum dihydrogen phosphate was 5% of the weight of alumina, the solid content of the sol was 20%, and the viscosity was 8 mPa·s. The fiber skeleton obtained in step S1 was placed in a vacuum impregnation tank, and a vacuum was drawn to a negative pressure of 0.095 MPa and maintained for 15 minutes to remove air. Then, the sol was injected and a gas pressure of 0.3 to 0.5 MPa was applied and maintained for 20 to 30 minutes, so that the sol would rapidly gel in the fiber gaps while remaining liquid in the macroscopic channels, thus achieving selective filling. S3, Solvent replacement and chemical modification: The wet preform obtained in step S2 was placed in 30%, 50%, 70%, 90% and 100% ethanol aqueous solutions for 12 hours each for five-level gradient solvent replacement. Then, it was modified with a solution of trimethylchlorosilane and n-hexane in a volume ratio of 1:10 for 6 hours. After the modification was completed, it was washed three times with anhydrous ethanol. S4. Segmented atmospheric pressure drying: A four-stage atmospheric pressure drying process is adopted. The first stage is drying at 30 degrees Celsius under atmospheric pressure and ventilation for 12 hours. The second stage is drying at 40 to 50 degrees Celsius under ventilation for 8 hours. The third stage is drying at 80 degrees Celsius for 6 hours. The fourth stage is drying at 120 degrees Celsius to constant weight. S5. High-temperature sintering: The preform dried in step S4 is heated to 1350 degrees Celsius at a rate of 2 degrees Celsius per minute and held for 2 hours to obtain a high-temperature resistant aerogel composite material.

7. The method according to claim 6, characterized in that, In step S1, the bottom of the mold is a copper plate connected to a constant temperature cold source of -40 degrees Celsius, and the side wall is a polytetrafluoroethylene insulation layer. Ice crystals grow directionally from the bottom up and displace the fibers so that they are oriented in the vertical direction. The fiber alignment in the orientation direction is greater than 80%.

8. The method according to claim 6, characterized in that, In step S2, the gelation time of the sol in the gaps of the fiber skeleton is 10 to 15 minutes, and the gelation time in the macroscopic channels is extended to more than 60 minutes, so as to achieve selective filling through the gelation time difference.

9. The method according to claim 6, characterized in that, During the heating process described in step S5, residual organic groups decompose in the range of 200 to 400 degrees Celsius, amorphous alumina begins to crystallize in the range of 400 to 800 degrees Celsius, aluminum dihydrogen phosphate decomposes and reacts with alumina to form a high-temperature resistant phosphoaluminate bonded phase in the range of 800 to 1200 degrees Celsius, and aluminum-silicon components react to form a mullite crystal phase during the 1350-degree Celsius holding stage. Needle-shaped mullite whiskers grow in situ at the interface between the fiber and the aerogel matrix to form a riveted structure.

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