Alumina-doped silicon-based aerogel composite and method for preparing the same

Abstract

This invention relates to the field of aerogel materials technology and discloses an alumina-doped silicon-based aerogel composite material and its preparation method, comprising the following steps: (1) obtaining a passivated modified pre-crosslinked silicon-based sol; (2) obtaining a composite wet gel; (3) subjecting the composite wet gel to segmented gradient aging treatment; (4) obtaining the alumina-doped silicon-based aerogel composite material. This invention achieves high-performance preparation of alumina-doped silicon-based aerogel composite materials through the synergistic effect of quantitative silanol passivation, alumina sol chelation modification, segmented gradient aging, gradient solvent replacement, and gradient temperature difference atmospheric pressure drying processes and key components. Since tetraethyl orthosilicate is used as a single main silicon source, the hydrolysis process is highly controllable and can stably generate linear siloxane prepolymers, avoiding the component fluctuations and structural inhomogeneities caused by multiple silicon sources, and providing a uniform basic framework for subsequent three-dimensional network construction.

Description

Technical Field

[0001] This invention relates to the field of aerogel materials technology, and more particularly to an alumina-doped silicon-based aerogel composite material. Background Technology

[0002] Silicon-based aerogels possess irreplaceable application value in fields such as high-temperature thermal insulation, heat insulation, aerospace, and industrial energy conservation due to their nanoscale continuous porous structure, extremely low thermal conductivity, and high specific surface area. However, pure silicon-based aerogels themselves have problems such as low skeleton strength, susceptibility to skeleton shrinkage and pore structure collapse under high-temperature environments, and insufficient thermal stability, which limit their use in higher temperature fields and harsher operating conditions.

[0003] To improve the overall performance of silicon-based aerogels, the industry often employs metal oxide doping modification. Among these methods, alumina doping can significantly enhance the strength of the aerogel framework, increase its heat resistance temperature, and suppress high-temperature shrinkage. Current conventional doping methods mostly involve direct blending or simple sol-gel mixing, which suffers from problems such as uneven dispersion of the aluminum component, weak bonding with the silicon-based framework, and a tendency for phase separation. Furthermore, traditional preparation processes lack precise control over the sol-gel crosslinking, gelation, aging, solvent replacement, and drying stages, easily leading to stress concentration within the gel, pore collapse, and a high cracking rate in the finished product, making it difficult to stably obtain alumina-doped silicon-based aerogel composites with uniform structure and reliable performance.

[0004] Therefore, developing a technology for preparing alumina-doped silicon-based aerogel composite materials with uniform doping, strong framework bonding, complete structure, stable performance, and suitability for large-scale preparation has become an urgent technical problem to be solved in this field. Summary of the Invention

[0005] To address the problems in the prior art, the present invention provides an alumina-doped silicon-based aerogel composite material.

[0006] The technical solution adopted by this invention to solve its technical problem is: a method for preparing an alumina-doped silicon-based aerogel composite material, comprising the following steps: (1) Prepare a pre-crosslinked silica sol and perform quantitative silanol passivation treatment on the pre-crosslinked silica sol to obtain a passivated modified pre-crosslinked silica sol; (2) Prepare chelated modified aluminum sol, add the chelated modified aluminum sol to the passivated modified pre-crosslinked silica sol at a uniform rate in a gradient, mix and stir evenly, adjust the pH value of the system, and carry out a constant temperature closed gelation reaction to obtain a composite wet gel. (3) The composite wet gel is subjected to a segmented gradient aging treatment. After aging, anhydrous ethanol and n-hexane are used to sequentially replace the wet gel with a gradient solvent to remove residual polar impurities. (4) After the solvent replacement is completed, the wet gel is dried under gradient temperature difference and normal pressure to release the capillary stress inside the gel throughout the process, and finally the alumina-doped silicon-based aerogel composite material is obtained.

[0007] As a further technical solution, step (1) specifically involves: using tetraethyl orthosilicate as the single main silicon source, anhydrous ethanol as the solvent, and 0.1 mol / L dilute hydrochloric acid as the acidic catalyst, stirring and hydrolyzing under constant temperature and humidity conditions to generate a linear siloxane prepolymer; adding methyltrimethoxysilane as a crosslinking agent to the system, stirring and reacting at constant temperature to form a three-dimensional network pre-crosslinked silica sol; adding vinyltriethoxysilane as a passivating coupling agent to the pre-crosslinked silica sol, stirring and reacting in a closed environment at constant temperature to precisely passivate excess active silanol groups in the system, and retaining a quantitative amount of active silanol groups as directional bonding sites for alumina.

[0008] As a further technical solution, the molar ratio of tetraethyl orthosilicate, anhydrous ethanol, water, and 0.1 mol / L dilute hydrochloric acid is 1:8:5:0.02; the hydrolysis temperature is 50℃, the relative humidity is constant at 50%, the hydrolysis time is 1.5 h, and the stirring is uniform throughout the process; the amount of methyltrimethoxysilane added is 6%-8% of the mass of tetraethyl orthosilicate, the crosslinking reaction temperature is 50℃, and the reaction time is 45 min; the amount of vinyltriethoxysilane added is 0.4% of the total mass of the pre-crosslinked silica sol, the passivation reaction temperature is 55℃, the reaction time is 20 min, and the silanol passivation ratio is controlled at 60%-65%.

[0009] As a further technical solution, step (2) is specifically as follows: using aluminum isopropoxide as the aluminum source, anhydrous ethanol as the solvent, adding ethyl acetoacetate as the chelating agent, and refluxing at constant temperature and normal pressure to block the active sites of aluminum ions. After the refluxing is completed, the mixture is cooled to room temperature, and deionized water is added and stirred at a constant speed to hydrolyze the mixture, thus preparing a uniform and stable chelated modified aluminum sol. The chelated modified aluminum sol is then added dropwise at a constant speed to the passivated silicon-based sol at a fixed volume ratio, and the mixture is stirred at a low speed throughout the process to achieve directional pre-bonding of aluminum oxygen groups and residual active silanol groups. After the addition is completed, dilute ammonia is slowly added to adjust the pH value of the system, and the mixture is allowed to stand and gel under closed constant temperature and humidity conditions to obtain a composite wet gel with a uniform structure.

[0010] As a further technical solution, the molar ratio of aluminum isopropoxide to ethyl acetoacetate is 1:0.35-0.4, the reflux temperature is 70℃, the reflux time is 2h, and the molar concentration of the chelated modified aluminum sol is 0.3-0.4mol / L; the volume ratio of passivated silica-based sol to chelated modified aluminum sol is 1:0.25-0.3, the dropping temperature is kept constant at 25±2℃, the total gradient dropping time is 90min, and the stirring speed is 120-150r / min throughout the process; the concentration of ammonia water used to adjust the pH is 0.5mol / L, the final pH value of the system is adjusted to 4.8-5.2, the gelation temperature is 35℃, the relative humidity is 60%-70%, and the gelation standing time is 2.5-3h.

[0011] As a further technical solution, the segmented gradient aging in step (3) is as follows: the composite wet gel is first placed in a constant temperature environment of 45-48℃ and aged at low temperature for 12-15h, and then heated to 55-58℃ at a uniform rate of 1℃ / min, and aged at high temperature for 6-8h; the gradient solvent replacement temperature is kept constant at 25±2℃, the replacement time of anhydrous ethanol is 8-10h, and the replacement time of n-hexane is 10-12h; the solvent replacement adopts a cyclic mode of soaking for 30min and low-speed stirring for 10min, with a stirring rate of 60-80r / min, and the replacement is stopped when the purity of the n-hexane replacement solution is ≥99%.

[0012] As a further technical solution, the gradient temperature difference atmospheric pressure drying in step (4) is as follows: the wet gel after solvent replacement is laid flat in a single layer in an atmospheric pressure drying oven, with a spacing of 3-5 cm between the single wet gel pieces and no overlap; the temperature is raised to 80°C at a uniform rate of 5°C / h, and a gradient temperature difference of 3-5°C is formed by using a micro-assisted heating at the bottom of the drying oven and natural heat dissipation at the top; high-purity dry nitrogen is introduced for laminar flow purging and heat preservation; the temperature is then raised to 110-120°C at a uniform rate of 10°C / h, the bottom auxiliary heating and nitrogen supply are turned off, and the whole temperature is kept constant to complete the stress-relieving drying process.

[0013] As a further technical solution, when drying and holding at 80℃, the auxiliary heating temperature at the bottom of the drying oven is controlled at 83-85℃, the nitrogen purging flow rate is 0.6-0.8L / min, the purging direction is parallel to the upper surface of the wet gel, and the holding time at 80℃ is 6-7h; after heating to 110-120℃, the holding time is 4-5h.

[0014] As a further technical solution, the single main silicon source can be replaced with a compound silicon source composed of tetraethyl orthosilicate and methyltriethoxysilane in a mass ratio of 1:1, while keeping the other preparation parameters unchanged; the composite material after being dried at normal pressure can also be placed in an argon inert atmosphere at 1000℃ for high-temperature solid-state heat treatment for 1 hour, with a heat treatment heating rate of 2-3℃ / min, to further enhance the interfacial bonding strength and high-temperature stability.

[0015] The preparation method described above yields an alumina-doped silicon-based aerogel composite material.

[0016] The beneficial effects of this invention are: This invention achieves high-performance preparation of alumina-doped silicon-based aerogel composite materials through the synergistic effect of key components and processes such as quantitative silanol passivation, alumina sol chelation modification, segmented gradient aging, gradient solvent replacement, and gradient temperature difference atmospheric pressure drying.

[0017] Using tetraethyl orthosilicate as the single primary silicon source ensures strong controllability of the hydrolysis process, stably generating linear siloxane prepolymers and avoiding component fluctuations and structural inhomogeneities caused by multiple silicon sources. This provides a uniform basic framework for subsequent three-dimensional network construction. Anhydrous ethanol is used as a solvent to ensure polarity matching and uniform dispersion, reducing side reactions during hydrolysis and crosslinking. 0.1 mol / L dilute hydrochloric acid acts as an acidic catalyst, gently catalyzing the hydrolysis of tetraethyl orthosilicate, controlling the hydrolysis rate and degree of polymerization, and preventing localized agglomeration and pore blockage caused by rapid polymerization. This results in a continuous and uniform prepolymer structure, providing a structurally stable reaction substrate for the pre-crosslinking and passivation steps.

[0018] By adding methyltrimethoxysilane as a crosslinking agent to the system, its molecular structure can covalently bond with the linear siloxane prepolymer, rapidly constructing a three-dimensional network pre-crosslinked silica sol, improving the structural strength and crosslinking density of the sol, and avoiding phase separation or sedimentation before gelation. Vinyltriethoxysilane, as a passivating coupling agent, can selectively react with excess active silanol groups in the system to achieve quantitative silanol passivation, stably controlling the silanol passivation ratio at 60%–65%. This eliminates the problems of excessive polycondensation and stress concentration caused by excess silanol groups, while retaining quantitative active silanol groups as directional bonding sites for alumina, allowing the aluminum component to be precisely anchored to the silica framework rather than physically attached, thereby improving the interfacial bonding force between the two phases and fundamentally solving the problems of uneven doping and low bonding strength.

[0019] Because aluminum isopropoxide is used as the aluminum source, its solubility and reactivity are well-suited to the silica-based sol system, allowing it to hydrolyze under mild conditions to form alumino groups. Ethyl acetoacetate, as a chelating agent, can form a stable chelate structure with aluminum ions, blocking the highly active sites of aluminum ions and inhibiting their self-hydrolysis and aggregation, ensuring that the aluminum component exists in a monodisperse or oligomeric state in the sol. Through gradient uniform dropwise addition and low-speed stirring, the chelated modified aluminum sol and the passivated silica-based sol can be fully contacted, and the alumino groups and residual silanol groups gradually undergo directional pre-bonding, avoiding aggregation and phase separation caused by excessively high local concentrations. Adjusting the pH to 4.8–5.2 with 0.5 mol / L dilute ammonia water can achieve mild and uniform gelation, avoiding uneven gel structure and internal cracks caused by sudden pH changes, thereby forming a composite wet gel with a uniform microstructure and no obvious defects, thus improving the overall consistency of the material.

[0020] The segmented gradient aging process employs a low-temperature static aging method to initially cross-link and stabilize the gel network structure. This is followed by uniformly increasing the temperature to a high temperature for aging, further enhancing the cross-linking density and skeletal strength, thus gradually strengthening the gel network structure and avoiding insufficient cross-linking or excessive shrinkage caused by aging at a single temperature. Gradient solvent replacement, using anhydrous ethanol and n-hexane sequentially, gradually reduces the system's polarity, gently removing polar impurities such as moisture and residual catalysts. Combined with a circulating soaking and stirring mode, impurities can be completely removed without damaging the pore structure, thereby reducing the risk of pore collapse during subsequent drying. Gradient temperature difference and atmospheric pressure drying, with the synergistic effects of gradient heating, bottom micro-heating, temperature difference between upper and lower parts, and nitrogen laminar flow purging, continuously and slowly releases capillary stress within the gel, preventing structural shrinkage, cracking, and pore collapse caused by rapid drying. This preserves the nanoporous structure intact, ensuring the material's high specific surface area, high porosity, and excellent mechanical properties.

[0021] This invention achieves synergistic effects by matching and optimizing the key components and process parameters, resulting in a composite material that combines high specific surface area, high porosity, low high-temperature shrinkage, excellent compressive strength, and long-term thermal stability. This completely solves the industry problems of traditional silicon-based aerogels, such as low strength, poor heat resistance, uneven alumina doping, easy cracking during drying, and poor batch stability. It can meet the stringent requirements of high-end applications such as aerospace, high-temperature insulation, and industrial energy conservation. Attached Figure Description

[0022] Figure 1 This is a comparison diagram of porosity between embodiments and comparative examples of the present invention. Detailed Implementation

[0023] The technical solutions of 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 some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0024] This invention provides a method for preparing alumina-doped silicon-based aerogel composite material, comprising the following steps: A pre-crosslinked silica-based sol was prepared, and the pre-crosslinked silica-based sol was subjected to quantitative silanol passivation treatment to obtain a passivated modified pre-crosslinked silica-based sol. A chelated modified aluminum sol was prepared by adding the chelated modified aluminum sol at a uniform gradient to a passivated modified pre-crosslinked silica-based sol, mixing and stirring until homogeneous, adjusting the pH value of the system, and carrying out a constant-temperature closed gelation reaction to obtain a composite wet gel. The composite wet gel was subjected to a segmented gradient aging treatment. After aging, the wet gel was successively replaced with anhydrous ethanol and n-hexane to remove residual polar impurities. After solvent replacement, the wet gel is dried under gradient temperature difference and normal pressure to release capillary stress inside the gel throughout the process, and finally alumina-doped silicon-based aerogel composite material is obtained.

[0025] This invention first prepares a pre-crosslinked silica-based sol and performs quantitative silanol passivation treatment. Using tetraethyl orthosilicate as the single main silicon source, anhydrous ethanol as the solvent, and 0.1 mol / L dilute hydrochloric acid as the acidic catalyst, hydrolysis is carried out under constant temperature and humidity conditions with stirring to generate a linear siloxane prepolymer. Methyltrimethoxysilane is added to the system as a crosslinking agent, and the reaction is carried out with constant temperature stirring to form a three-dimensional network pre-crosslinked silica-based sol. Vinyltriethoxysilane is added to the pre-crosslinked silica-based sol as a passivating coupling agent, and the reaction is carried out under closed, constant temperature stirring to precisely passivate excess active silanol groups in the system, retaining a quantitative amount of active silanol groups as directional bonding sites for alumina.

[0026] The molar ratio of tetraethyl orthosilicate, anhydrous ethanol, water, and 0.1 mol / L dilute hydrochloric acid was 1:8:5:0.02. Hydrolysis was carried out at 50°C with a constant relative humidity of 50% for 1.5 hours, with uniform stirring throughout. Methyltrimethoxysilane was added at 6% to 8% of the mass of tetraethyl orthosilicate, with a crosslinking reaction temperature of 50°C and a reaction time of 45 minutes. Vinyltriethoxysilane was added at 0.4% of the total mass of the pre-crosslinked silica sol, with a passivation reaction temperature of 55°C and a reaction time of 20 minutes, and the silanol passivation ratio was controlled at 60%-65%.

[0027] After obtaining the passivated and modified pre-crosslinked silica sol, a chelated modified aluminum sol was prepared. Aluminum isopropoxide was used as the aluminum source, anhydrous ethanol as the solvent, and ethyl acetoacetate as the chelating agent. The reaction was carried out under constant pressure and temperature reflux to block the active sites of aluminum ions. After reflux, the mixture was cooled to room temperature, and deionized water was added with uniform stirring to hydrolyze the mixture, resulting in a homogeneous and stable chelated modified aluminum sol. The chelated modified aluminum sol was then added dropwise at a fixed volume ratio to the passivated silica sol at a uniform rate, with low-speed stirring throughout to achieve directional pre-bonding of aluminum oxide groups and residual active silanol groups. After the addition was complete, dilute ammonia was slowly added to adjust the pH of the system. The mixture was then allowed to gel under sealed, constant temperature and humidity conditions to obtain a structurally homogeneous composite wet gel.

[0028] The molar ratio of aluminum isopropoxide to ethyl acetoacetate was 1:0.35-0.4, the reflux temperature was 70℃, the reflux time was 2h, and the molar concentration of the chelated modified aluminum sol was 0.3-0.4mol / L. After passivation, the volume ratio of silica-based sol to chelated modified aluminum sol was 1:0.25-0.3, the dropping temperature was kept constant at 25℃±2℃, the total gradient dropping time was 90min, and the stirring speed was 120-150r / min throughout. The ammonia concentration used to adjust the pH was 0.5mol / L, and the final pH value of the system was adjusted to 4.8-5.2. The gelation temperature was 35℃, the relative humidity was 60-70%, and the gelation standing time was 2.5h-3h.

[0029] After obtaining the composite wet gel, a segmented gradient aging treatment was performed. The composite wet gel was first placed in a constant temperature environment of 45-48℃ for low-temperature static aging for 12-15 hours, then the temperature was increased to 55-58℃ at a uniform rate of 1℃ / min, and the gel was statically aged at high temperature for 6-8 hours. The gradient solvent replacement temperature was kept constant at 25℃±2℃, with anhydrous ethanol replacement for 8-10 hours and n-hexane replacement for 10-12 hours. Solvent replacement adopted a circulation mode of immersion for 30 minutes followed by low-speed stirring for 10 minutes, with a stirring rate of 60-80 r / min. The circulation was stopped when the purity of the n-hexane replacement solution was ≥99%.

[0030] After aging and solvent replacement, gradient temperature difference drying at ambient pressure is performed. The solvent-replaced wet gel is laid flat in a single layer in an ambient pressure drying oven, with a spacing of 3-5 cm between individual wet gel pieces and no overlap. The temperature is increased to 80℃ at a uniform rate of 5℃ / h, using a combination of auxiliary heating at the bottom and natural heat dissipation from the top to create a 3-5℃ temperature gradient. High-purity dry nitrogen is then introduced for laminar flow purging and temperature maintenance. The temperature is then increased to 110-120℃ at a uniform rate of 10℃ / h, the bottom auxiliary heating and nitrogen supply are turned off, and the entire process is kept at a constant temperature to complete the stress-relieving drying process.

[0031] When drying and holding at 80℃, the auxiliary heating temperature at the bottom of the drying oven should be controlled at 83-85℃, the nitrogen purging flow rate should be 0.6-0.8 L / min, and the purging direction should be parallel to the upper surface of the wet gel. The holding time at 80℃ should be 6-7 hours. After heating to 110-120℃, the holding time should be 4-5 hours.

[0032] The single primary silicon source of this invention can be replaced with a composite silicon source consisting of tetraethyl orthosilicate and methyltriethoxysilane in a 1:1 mass ratio, while keeping the other preparation parameters unchanged. The composite material, after being dried at atmospheric pressure, can also be subjected to high-temperature solid-state heat treatment at 1000℃ under an argon inert atmosphere for 1 hour, with a heating rate of 2-3℃ / min.

[0033] This invention achieves directional bonding and uniform doping of alumina and silicon-based aerogels through a synergistic process of quantitative passivation of silanol groups, chelation modification of aluminum sol, gradient gelation, segmented aging, gradient solvent replacement, and gradient temperature difference atmospheric pressure drying. This improves the thermal stability, mechanical strength, and pore structure integrity of the composite material, solving the problems of easy cracking, poor heat resistance, and uneven doping of traditional silicon-based aerogels. The prepared aerogel composite material has high porosity, large specific surface area, low high-temperature shrinkage, and excellent compressive strength.

[0034] The following are specific examples: Example 1: The ingredients were prepared by mixing tetraethyl orthosilicate, anhydrous ethanol, water, and 0.1 mol / L dilute hydrochloric acid in a molar ratio of 1:8:5:0.02. The mixture was hydrolyzed at a constant stirring rate for 1.5 h at a hydrolysis temperature of 50 °C and a relative humidity of 50% to generate a linear siloxane prepolymer. 6% (by mass) of methyltrimethoxysilane was added to the mixture, and the mixture was stirred at 50 °C for 45 min to form a pre-crosslinked silica sol. 0.4% (by mass) of vinyltriethoxysilane was added to the pre-crosslinked silica sol, and the mixture was stirred in a sealed container at 55 °C for 20 min. The passivation ratio of silanols was controlled at 60% to obtain a passivated modified pre-crosslinked silica sol.

[0035] Aluminum isopropoxide and ethyl acetoacetate were prepared at a molar ratio of 1:0.35, using anhydrous ethanol as solvent. The mixture was refluxed at 70°C under normal pressure for 2 hours. After cooling to room temperature, deionized water was added and stirred at a constant speed to hydrolyze the mixture, resulting in a 0.3 mol / L chelated modified aluminum sol. At 25°C, the passivated silica-based sol and chelated modified aluminum sol were added dropwise at a volume ratio of 1:0.25 over a total time of 90 minutes, with a stirring speed of 120 r / min throughout. After the addition was complete, the pH of the system was adjusted to 4.8 with 0.5 mol / L dilute ammonia. The mixture was then allowed to stand at 35°C and 60% relative humidity for 2.5 hours to gel, yielding a composite wet gel.

[0036] The composite wet gel was placed in a constant temperature environment of 45℃ for low-temperature static aging for 12 hours, and then heated to 55℃ at a rate of 1℃ / min for high-temperature static aging for 6 hours. At 25℃, it was replaced with anhydrous ethanol for 8 hours and n-hexane for 10 hours. A circulation mode of soaking for 30 minutes and low-speed stirring for 10 minutes was adopted, with a stirring rate of 60 r / min, and the circulation was continued until the purity of the n-hexane replacement solution was ≥99%.

[0037] The wet gel was laid in a single layer in a drying oven with a spacing of 3 cm. The temperature was increased to 80°C at a rate of 5°C / h, and the bottom auxiliary heating temperature was controlled at 83°C to create a temperature difference of 3°C. High-purity nitrogen was purged parallel to the gel surface at a flow rate of 0.6 L / min and kept at a constant temperature for 6 h. Then the temperature was increased to 110°C at a rate of 10°C / h, the auxiliary heating and nitrogen were turned off, and the temperature was kept at a constant temperature for 4 h to obtain an alumina-doped silicon-based aerogel composite material.

[0038] Example 2: The ingredients were prepared by mixing tetraethyl orthosilicate, anhydrous ethanol, water, and 0.1 mol / L dilute hydrochloric acid in a molar ratio of 1:8:5:0.02. The mixture was hydrolyzed at a constant stirring rate for 1.5 h at a hydrolysis temperature of 50 °C and a relative humidity of 50% to generate a linear siloxane prepolymer. 8% (by weight of tetraethyl orthosilicate) of methyltrimethoxysilane was added, and the mixture was stirred at 50 °C for 45 min to form a pre-crosslinked silica sol. 0.4% (by weight of the total pre-crosslinked silica sol) of vinyltriethoxysilane was added, and the mixture was stirred in a sealed container at 55 °C for 20 min. The passivation ratio of silanols was controlled at 65% to obtain a passivated modified pre-crosslinked silica sol.

[0039] Aluminum isopropoxide and ethyl acetoacetate were prepared at a molar ratio of 1:0.4, using anhydrous ethanol as solvent. The mixture was refluxed at 70°C under normal pressure for 2 hours. After cooling to room temperature, deionized water was added and stirred at a constant speed to hydrolyze the mixture, resulting in a 0.4 mol / L chelated modified aluminum sol. At 25°C, the passivated silica-based sol and chelated modified aluminum sol were added dropwise at a gradient rate of 1:0.3 over a total time of 90 minutes, with a stirring speed of 150 r / min throughout. After the addition was complete, the pH of the system was adjusted to 5.2 with 0.5 mol / L dilute ammonia. The mixture was then allowed to stand at 35°C and 70% relative humidity for 3 hours to gel, yielding a composite wet gel.

[0040] The composite wet gel was placed in a constant temperature environment of 48℃ for low-temperature static aging for 15 hours, and then heated to 58℃ at a rate of 1℃ / min for high-temperature static aging for 8 hours. At 25℃, it was replaced with anhydrous ethanol for 10 hours and n-hexane for 12 hours. A circulation mode of soaking for 30 minutes and stirring at low speed for 10 minutes was adopted, with a stirring rate of 80 r / min, and the circulation was continued until the purity of the n-hexane replacement solution was ≥99%.

[0041] The wet gel was laid in a single layer in a drying oven with a spacing of 5 cm. The temperature was increased to 80°C at a rate of 5°C / h, and the bottom auxiliary heating temperature was controlled at 85°C to create a temperature difference of 5°C. High-purity nitrogen was purged parallel to the gel surface at a flow rate of 0.8 L / min and kept at a constant temperature for 7 h. Then the temperature was increased to 120°C at a rate of 10°C / h, the auxiliary heating and nitrogen were turned off, and the temperature was kept at a constant temperature for 5 h to obtain an alumina-doped silicon-based aerogel composite material.

[0042] Example 3: The ingredients were prepared by mixing tetraethyl orthosilicate, anhydrous ethanol, water, and 0.1 mol / L dilute hydrochloric acid in a molar ratio of 1:8:5:0.02. The mixture was hydrolyzed at a constant stirring rate for 1.5 h at a hydrolysis temperature of 50 °C and a relative humidity of 50% to generate a linear siloxane prepolymer. 7% (by mass) of methyltrimethoxysilane was added to the mixture, and the mixture was stirred at 50 °C for 45 min to form a pre-crosslinked silica sol. 0.4% (by mass) of vinyltriethoxysilane was added to the pre-crosslinked silica sol, and the mixture was stirred in a sealed container at 55 °C for 20 min. The passivation ratio of silanols was controlled at 62% to obtain a passivated modified pre-crosslinked silica sol.

[0043] Aluminum isopropoxide and ethyl acetoacetate were prepared at a molar ratio of 1:0.37, using anhydrous ethanol as solvent. The mixture was refluxed at 70°C under normal pressure for 2 hours. After cooling to room temperature, deionized water was added and stirred at a constant speed to hydrolyze the mixture, resulting in a 0.35 mol / L chelated modified aluminum sol. At 25°C, the passivated silica-based sol and chelated modified aluminum sol were added dropwise at a volume ratio of 1:0.27 over a total time of 90 minutes, with a stirring speed of 135 r / min throughout. After the addition was complete, the pH of the system was adjusted to 5.0 with 0.5 mol / L dilute ammonia. The mixture was then allowed to stand at 35°C and 65% relative humidity for 2.7 hours to gel, yielding a composite wet gel.

[0044] The composite wet gel was placed in a constant temperature environment of 46℃ for low-temperature static aging for 13 hours, and then heated to 56℃ at a rate of 1℃ / min for high-temperature static aging for 7 hours. At 25℃, it was replaced with anhydrous ethanol for 9 hours and n-hexane for 11 hours. A circulation mode of soaking for 30 minutes and low-speed stirring for 10 minutes was adopted, with a stirring rate of 70 r / min, and the circulation was continued until the purity of the n-hexane replacement solution was ≥99%.

[0045] The wet gel was laid in a single layer in a drying oven with a spacing of 4 cm. The temperature was increased to 80°C at a rate of 5°C / h, and the bottom auxiliary heating temperature was controlled at 84°C to create a temperature difference of 4°C. High-purity nitrogen was purged parallel to the gel surface at a flow rate of 0.7 L / min and kept at a constant temperature for 6.5 h. Then the temperature was increased to 115°C at a rate of 10°C / h, the auxiliary heating and nitrogen were turned off, and the temperature was kept at a constant temperature for 4.5 h to obtain an alumina-doped silicon-based aerogel composite material.

[0046] Comparative Example 1: Compared to Example 3, no silanol passivation treatment was performed, but the remaining steps and parameters were exactly the same.

[0047] The pre-crosslinked silica sol was prepared by mixing tetraethyl orthosilicate, anhydrous ethanol, water, and 0.1 mol / L dilute hydrochloric acid in a molar ratio of 1:8:5:0.02 and then hydrolyzing the mixture. No vinyltriethoxysilane was added for passivation treatment. The subsequent steps of aluminum sol preparation, gelation, aging, solvent replacement, and drying are the same as in Example 3.

[0048] Comparative Example 2: Compared with Example 3, no chelated modified aluminum sol was used; instead, unchelated ordinary aluminum sol was used directly. The remaining steps and parameters were exactly the same.

[0049] The preparation steps for passivated silicon-based sol are the same as in Example 3; Using aluminum isopropoxide as the aluminum source and anhydrous ethanol as the solvent, without adding ethyl acetoacetate for chelation, ordinary aluminum sol was prepared by direct hydrolysis. The subsequent steps of dropwise addition, gelation, aging, displacement, and drying were the same as in Example 3.

[0050] Comparative Example 3: Compared with Example 3, segmented gradient aging was not performed; instead, the aging was carried out directly at a constant temperature. The remaining steps and parameters were exactly the same.

[0051] The preparation and gelation steps of passivated silicon-based sol and chelated aluminum sol are the same as in Example 3; The composite wet gel was directly aged at a constant temperature of 50°C for 20 hours without staged temperature increases. The subsequent solvent replacement and drying steps were the same as in Example 3.

[0052] Comparative Example 4: Compared with Example 3, gradient temperature difference drying at normal pressure was not used; instead, constant temperature and normal pressure drying was performed directly. The remaining steps and parameters were exactly the same.

[0053] The preparation, gelation, aging, and solvent replacement steps of passivated silicon-based sol and chelated aluminum sol are the same as in Example 3; The wet gel was dried directly at a constant temperature and pressure of 100℃ without gradient heating, bottom auxiliary heating, or nitrogen purging, until it reached constant weight.

[0054] Verification experiment: Experiment 1: Porosity and specific surface area test of composite materials: Experimental objective: The porosity and specific surface area of ​​the alumina-doped silicon-based aerogel composite materials prepared in Examples 1-3 and Comparative Examples 1-4 were tested to verify the effects of silanol passivation, alumina sol chelation, segmented aging, and gradient drying on the pore structure.

[0055] Test method: Specific surface area was determined using nitrogen adsorption-desorption, and porosity was determined using Archimedes' displacement method. Samples of equal mass were vacuum dried at 200℃ for 2 hours, and the BET specific surface area was determined using a specific surface area and pore size analyzer. Porosity was calculated from the sample volume and mass.

[0056] Experimental data: Table 1

[0057] The specific surface area and porosity of Examples 1-3 are significantly better than those of the comparative examples, indicating that the process of the present invention can construct a complete aerogel pore structure. Comparative Example 1 lacks passivated silanol groups; excessive silanol groups lead to over-crosslinking of the sol, pore collapse, and a significant decrease in specific surface area and porosity. Comparative Example 2 lacks chelated aluminum sol, resulting in aluminum ion aggregation, disrupting the continuity of the silicon-based framework, and reducing the integrity of the pore structure. Comparative Example 3 lacks segmented aging; the gel network crosslinking is insufficient, pores are prone to shrinkage, and performance is lower than the examples. Comparative Example 4 lacks gradient drying; capillary stress cannot be relieved, large-area pore collapse occurs, and performance is the worst.

[0058] Experiment 2: High-Temperature Volume Shrinkage Rate Test of Composite Materials Experimental objective: The volume shrinkage rate of the test sample after high-temperature treatment at 1000℃ was tested to verify the high-temperature thermal stability of the composite material.

[0059] Test method: The sample was cut into regular cubes and the initial volume was measured. It was then placed in an argon atmosphere furnace and kept at 1000℃ for 1 hour. After cooling to room temperature, the final volume was measured and the volume shrinkage rate was calculated.

[0060] Experimental data: Table 2

[0061] Examples 1-3 all exhibited high-temperature shrinkage rates below 3.5%, demonstrating excellent thermal stability. Comparative Example 1 lacked silanol passivation, resulting in weak bonding between alumina and silicon, leading to easy deformation of the skeleton at high temperatures and a significant increase in shrinkage. Comparative Example 2 showed unchelated aluminum ions, exacerbating agglomeration at high temperatures, damaging the skeleton structure, and resulting in a high shrinkage rate. Comparative Example 3 lacked segmented aging, resulting in insufficient gel strength and higher high-temperature shrinkage than the examples. Comparative Example 4 lacked gradient drying, exhibiting internal stress defects, leading to rapid shrinkage at high temperatures and the worst stability.

[0062] Experiment 3: Compressive strength test of composite materials: Experimental objective: The compressive strength of the test specimens was tested to verify the effect of each process step on improving mechanical properties.

[0063] Test method: A universal testing machine was used to perform a compressive test on a cylindrical specimen at a loading rate of 1 mm / min. The maximum load at which the specimen failed was recorded, and the compressive strength was calculated.

[0064] Experimental data: Table 3

[0065] Examples 1-3 exhibit significantly higher compressive strength than the comparative examples, demonstrating excellent mechanical properties. Comparative Example 1 lacked silanol passivation, resulting in poor interfacial bonding, a easily fractured framework, and extremely low strength. Comparative Example 2 showed unchelated aluminum sol, uneven doping, localized stress concentration, and relatively low strength. Comparative Example 3 lacked segmented aging, resulting in insufficient gel cross-linking and overall weaker strength than the examples. Comparative Example 4 lacked gradient drying, exhibiting numerous internal cracks, poor structural integrity, and the lowest compressive strength.

[0066] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

[0067] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and variations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A method for preparing an alumina-doped silicon-based aerogel composite material, characterized in that, Includes the following steps: (1) Prepare a pre-crosslinked silica sol and perform quantitative silanol passivation treatment on the pre-crosslinked silica sol to obtain a passivated modified pre-crosslinked silica sol; (2) Prepare chelated modified aluminum sol, add the chelated modified aluminum sol to the passivated modified pre-crosslinked silica sol at a uniform rate in a gradient, mix and stir evenly, adjust the pH value of the system, and carry out a constant temperature closed gelation reaction to obtain a composite wet gel. (3) The composite wet gel is subjected to a segmented gradient aging treatment. After aging, anhydrous ethanol and n-hexane are used to replace the wet gel with gradient solvents in sequence to remove residual polar impurities inside. (4) After the solvent replacement is completed, the wet gel is dried under gradient temperature difference and normal pressure to release the capillary stress inside the gel throughout the process, and finally the alumina-doped silicon-based aerogel composite material is obtained.

2. The preparation method according to claim 1, characterized in that, Step (1) specifically involves: using tetraethyl orthosilicate as the single main silicon source, anhydrous ethanol as the solvent, and 0.1 mol / L dilute hydrochloric acid as the acidic catalyst, hydrolyzing under constant temperature and humidity conditions to generate a linear siloxane prepolymer; adding methyltrimethoxysilane as a crosslinking agent to the system, reacting under constant temperature and stirring to form a three-dimensional network pre-crosslinked silica sol; adding vinyltriethoxysilane as a passivating coupling agent to the pre-crosslinked silica sol, reacting under closed temperature and stirring to precisely passivate excess active silanol groups in the system, and retaining a quantitative amount of active silanol groups as directional bonding sites for alumina.

3. The preparation method according to claim 2, characterized in that, The molar ratio of tetraethyl orthosilicate, anhydrous ethanol, water, and 0.1 mol / L dilute hydrochloric acid is 1:8:5:0.02; the hydrolysis temperature is 50℃, the relative humidity is constant at 50%, the hydrolysis time is 1.5 h, and the stirring is uniform throughout the process; the amount of methyltrimethoxysilane added is 6%-8% of the mass of tetraethyl orthosilicate, the crosslinking reaction temperature is 50℃, and the reaction time is 45 min; the amount of vinyltriethoxysilane added is 0.4% of the total mass of the pre-crosslinked silica sol, the passivation reaction temperature is 55℃, the reaction time is 20 min, and the silanol passivation ratio is controlled at 60%-65%.

4. The preparation method according to claim 1, characterized in that, Step (2) is as follows: using aluminum isopropoxide as the aluminum source, anhydrous ethanol as the solvent, and ethyl acetoacetate as the chelating agent, the reaction is carried out under constant temperature and pressure at normal pressure to block the active sites of aluminum ions. After the reflux is completed, the mixture is cooled to room temperature, and deionized water is added and stirred at a constant speed to hydrolyze the mixture, thus preparing a uniform and stable chelated modified aluminum sol. The chelated modified aluminum sol is then added dropwise at a constant speed in a gradient to the passivated silica-based sol at a fixed volume ratio. The mixture is stirred at a low speed throughout the process to achieve directional pre-bonding of aluminum oxygen groups and residual active silanol groups. After the addition is completed, dilute ammonia is slowly added to adjust the pH value of the system. The mixture is then allowed to stand and gel under closed constant temperature and humidity conditions to obtain a composite wet gel with a uniform structure.

5. The preparation method according to claim 4, characterized in that, The molar ratio of aluminum isopropoxide to ethyl acetoacetate is 1:0.35-0.4, the reflux temperature is 70℃, the reflux time is 2h, and the molar concentration of chelated modified aluminum sol is 0.3-0.4mol / L. The volume ratio of passivated silica-based sol to chelated modified aluminum sol is 1:0.25-0.3, the dropping temperature is kept constant at 25±2℃, the total gradient dropping time is 90min, and the stirring speed is 120-150r / min throughout the process. The concentration of ammonia water used to adjust the pH is 0.5mol / L, the final pH value of the system is adjusted to 4.8-5.2, the gelation temperature is 35℃, the relative humidity is 60%-70%, and the gelation standing time is 2.5-3h.

6. The preparation method according to claim 1, characterized in that, The segmented gradient aging in step (3) is as follows: the composite wet gel is first placed in a constant temperature environment of 45-48℃ and aged at low temperature for 12-15h, and then the temperature is raised to 55-58℃ at a uniform rate of 1℃ / min and aged at high temperature for 6-8h; the gradient solvent replacement temperature is kept constant at 25±2℃, the replacement time of anhydrous ethanol is 8-10h and the replacement time of n-hexane is 10-12h; the solvent replacement adopts a circulation mode of soaking for 30min and low-speed stirring for 10min, with a stirring rate of 60-80r / min, and the circulation replacement is stopped when the purity of the n-hexane replacement solution is ≥99%.

7. The preparation method according to claim 1, characterized in that, The gradient temperature difference atmospheric pressure drying in step (4) is as follows: the solvent-replaced wet gel is laid flat in a single layer in an atmospheric pressure drying oven, with a spacing of 3-5 cm between the single wet gel pieces and no overlap; the temperature is raised to 80°C at a uniform rate of 5°C / h, and a temperature gradient of 3-5°C is formed by using a micro-assisted heating at the bottom of the drying oven and natural heat dissipation at the top; high-purity dry nitrogen is introduced for laminar flow purging and heat preservation; then the temperature is raised to 110-120°C at a uniform rate of 10°C / h, the bottom auxiliary heating and nitrogen supply are turned off, and the whole process is kept at a constant temperature to complete the stress-relieving drying process.

8. The preparation method according to claim 7, characterized in that, When drying and holding at 80℃, the auxiliary heating temperature at the bottom of the drying oven should be controlled at 83-85℃, the nitrogen purging flow rate should be 0.6-0.8L / min, and the purging direction should be parallel to the upper surface of the wet gel. The holding time at 80℃ should be 6-7h; after heating to 110-120℃, the holding time should be 4-5h.

9. The preparation method according to claim 1, characterized in that, The single primary silicon source can be replaced with a compound silicon source composed of tetraethyl orthosilicate and methyltriethoxysilane in a mass ratio of 1:1, while the other preparation parameters remain unchanged. The composite material after being dried at normal pressure can also be placed in an argon inert atmosphere at 1000℃ for high-temperature solid-state heat treatment for 1 hour, with a heat treatment heating rate of 2-3℃ / min.

10. An alumina-doped silicon-based aerogel composite material is prepared by any one of the preparation methods according to claims 1-9.

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