Dual-mechanism synergistic high-phase-transition-temperature Al2O3 aerogel and preparation method thereof

Abstract

The invention discloses a method for increasing the phase transition temperature of Al2O3 aerogel through grain boundary pinning and nano-structure strengthening, which comprises the following steps: co-hydrolyzing an aluminum source, yttrium salt, a silicon source and magnesium salt to form sol through the coordination of double mechanisms of grain boundary pinning and nano-domain isolation, and carrying out gradient heating aging, mixed medium supercritical drying and two-stage sintering to obtain the phase transition temperature of the Al2O3 aerogel. The gamma-alpha phase transition temperature of the Al2O3 aerogel is greater than or equal to 1300 DEG C. The specific surface area of the aerogel after heat treatment at 1300 DEG C is larger than 100 m / g, the process cost is controllable, and the aerogel is suitable for industrial production and can be applied to extreme environments such as a spacecraft thermal protection system and a high-temperature industrial furnace lining.

Description

Technical Field

[0001] The present invention relates to the technical field of high-temperature aerogel materials, and specifically to a method for increasing the γ→α phase transition temperature of Al2O3 aerogel from the conventional 1000-1100°C to above 1300°C through multi-element doping and microstructure design. The aerogel product prepared using this method is suitable for use in extreme environments such as spacecraft thermal protection systems and high-temperature industrial furnace linings. Background Art

[0002] Due to its excellent properties such as high temperature resistance, high porosity, and low density, Al2O3 aerogel is widely used in industrial furnaces, spacecraft thermal protection, and other fields. However, during use, as the ambient temperature rises, Al2O3 aerogel undergoes a series of phase transitions, resulting in a significant collapse of the pore structure, limiting its scope of use. Existing technologies have the following drawbacks: ① Pure Al2O3 aerogel undergoes an irreversible phase transition (γ→α) at ​​high temperatures, causing the pore structure to collapse (shrinkage rate >50%), thereby losing its thermal insulation properties; ② Traditional doping methods (such as single La2O3 or SiO2) have a threshold effect, and excessive doping can clog the aerogel pores (porosity decreases >40%), affecting material performance; ③ Some patents use ZrO2 for stabilization, but ZrO2 itself undergoes a phase transition (t→m) at high temperatures, causing microcracks. Therefore, there is an urgent need to develop a method that can overcome these technical bottlenecks to prepare a series of Al2O3 aerogel products to meet the demand for use as thermal protection materials in high-temperature applications. Summary of the Invention

[0003] To address the above-mentioned drawbacks, the present invention aims to provide a dual-mechanism synergistic high phase transition temperature Al2O3 aerogel and its preparation method. This method uses the dual mechanism of "grain boundary pinning + nano-domain isolation" to prepare an aerogel material that can be widely used as a thermal protection material in high-temperature fields. The material also meets the following requirements: ① Phase transition temperature ≥ 1300℃; ② Specific surface area > 100m after heat treatment at 1300℃ 2 / g; ③The process cost is controllable and suitable for industrial production.

[0004] In order to achieve the above technical objectives, the following technical solutions are adopted:

[0005] In a first aspect, a method for preparing a dual-mechanism synergistic high phase transition temperature Al2O3 aerogel comprises the following steps:

[0006] (1) adding an aluminum source into a water-alcohol mixed solvent and hydrolyzing it to form an aluminum sol;

[0007] (2) dissolving yttrium salt and magnesium salt in deionized water to obtain a dopant mixture, and then adding the mixture to aluminum sol to form an Al-Y-Mg composite sol;

[0008] (3) adding a silicon source into anhydrous alcohol and hydrolyzing it to form a silica sol;

[0009] (4) After the silica sol and the Al-Y-Mg composite sol are uniformly dispersed, a gelling agent is added to form a composite wet gel material having an Al-Si-Y-Mg network structure;

[0010] (5) subjecting the composite wet gel material to gradient temperature aging, and then supercritical drying through a mixed medium to obtain an Al2O3 aerogel material;

[0011] (6) The Al2O3 aerogel material is subjected to two-stage sintering to obtain Al2O3 aerogel with a γ→α phase transition temperature ≥1300℃.

[0012] Preferably, the aluminum source is aluminum isopropoxide.

[0013] Preferably, the yttrium salt is yttrium nitrate, and the doping amount is 10-40% of the molar ratio of aluminum oxide.

[0014] Preferably, the magnesium salt is magnesium nitrate, and the doping amount is 3-6% of the molar ratio of aluminum oxide.

[0015] Preferably, the silicon source is tetraethyl orthosilicate or polymethyl silicate, and the doping amount is 30-50% of the molar ratio of aluminum oxide.

[0016] Preferably, the gelling aid in step (4) is a homogeneous solution formed by dissolving polyethylene oxide powder in alcohol.

[0017] Preferably, the temperature of the gradient aging in step (5) is 50°C → 80°C → 110°C.

[0018] Preferably, the composite wet gel material in step (5) is placed in a nitrogen environment containing 5% water vapor for gradient temperature aging.

[0019] Preferably, the two-stage sintering in step (6) includes: keeping the temperature at 600°C for 2 hours, heating the temperature to 1300°C at a heating rate of 10°C / min, keeping the temperature for 1 hour, and then annealing.

[0020] Secondly, a dual-mechanism synergistic high phase transition temperature Al2O3 aerogel is prepared by the above preparation method. After heat treatment at 1300℃, its pore size distribution is 5-50nm, the thermal conductivity is ≤0.035W / m·k, and the phase transition temperature is ≥1300℃.

[0021] The beneficial effects achieved by the present invention are as follows:

[0022] Compared with the prior art, the present invention provides a dual-mechanism synergistic high phase transition temperature Al2O3 aerogel and its preparation method. The present invention adopts a dual-mechanism synergistic method of "grain boundary pinning + nano-domain isolation" to increase the phase transition temperature of Al2O3 aerogel. Among them, grain boundary pinning is achieved by rare earth elements (Y 3+ ) segregates at the Al2O3 interface, inhibiting grain growth and phase transition driving force; nano-domain isolation introduces Si 4+ and Mg 2+ Forming spinel (MgAl2O4) isolation phase, dividing Al2O3 crystal domains and delaying phase transition kinetics; Y 3+ With Al 3+ The difference in ionic radius induces lattice distortion, increases the phase transition activation energy, and further improves the material performance. In addition, process optimization and energy conservation are carried out, and water vapor is introduced into the nitrogen environment to assist aging and promote the formation of Si-O-Al network. Supercritical drying of the mixed medium reduces energy consumption by more than 30%. The two-stage high-temperature sintering heat treatment can not only remove excess organic matter, but also form a stable grain boundary structure. According to tests, the phase transition temperature of the aerogel prepared by the present invention is increased to above 1300℃, and the specific surface area after heat treatment at 1300℃ is greater than 100m 2 / g, can be compounded with fiber-reinforced materials (such as mullite fiber felt) to prepare flexible thermal insulation felt, which expands the scope of application and meets the demand for protective materials in high-temperature fields. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] The present invention will be described below with reference to the accompanying drawings.

[0024] Figure 1 This is a process flow chart for preparing the Al2O3 aerogel of the present invention.

[0025] Figure 2 This is a process roadmap for preparing Al2O3 aerogel in Example 1 of the present invention.

[0026] Figure 3 This is the process roadmap for preparing Al2O3 aerogel in Example 2 of the present invention. DETAILED DESCRIPTION

[0027] The following specific embodiments illustrate the embodiments of the present invention. Those skilled in the art can easily understand the other advantages and effects of the present invention from the contents disclosed in this specification. The described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0028] Example 1

[0029] The preparation method of Al2O3 aerogel described in this embodiment is as follows Figure 1 and Figure 2 As shown, the following steps are included:

[0030] (1) Preparation of a mixed solvent: ethanol and deionized water were mixed at a volume ratio of 4:1, and nitric acid (HNO3) was added dropwise to adjust the pH to 3.0-3.5 to form a mixed solvent.

[0031] (2) Preparation of aluminum sol: Aluminum isopropoxide was slowly added to the mixed solvent and stirred continuously at 200 rpm and 60°C for about 2 h until a transparent solution was formed.

[0032] (3) Preparation of dopant mixture: Yttrium nitrate (Y(NO3)3·6H2O) and magnesium nitrate (Mg(NO3)2·6H2O) were dissolved in a minimum amount of deionized water at a molar ratio of Al:Y:Mg=10:1:0.5 and ultrasonicated at 40 kHz for 10 min.

[0033] (4) Preparation of Al-Y-Mg composite sol: The dopant mixture was added dropwise to the aluminum sol and stirred continuously at 300 rpm and 70°C for 1 h to form an Al-Y-Mg composite sol.

[0034] (5) Preparation of silica sol: Tetraethyl orthosilicate (TEOS) and anhydrous ethanol were mixed in a volume ratio of 1:2, 0.1 mol / l nitric acid was added dropwise to adjust the pH to 4.0, and the mixture was hydrolyzed at 25°C and 200 rpm for 30 min.

[0035] (6) Composite sol mixing: Silica sol was slowly added to the Al-Y-Mg composite sol at a molar ratio of Al:Si = 3:1, and stirred continuously at 50 °C for 4 h to form an Al-Si-Y-Mg hybrid network.

[0036] (7) Addition and aging of flocculants: Polyethylene oxide (PEO) powder was added to ethanol at 1 wt% of the mass of the composite sol, and ultrasonic dispersion was performed for 30 min. After forming a homogeneous solution, it was slowly added dropwise to the composite sol. After stirring at 100 rpm for 1 h, it was allowed to stand at room temperature for 24 h to promote molecular self-assembly, and finally an Al2O3 wet gel material with an Al-Si-Y-Mg network structure was obtained.

[0037] (8) Gradient temperature aging: The wet gel material was subjected to gradient temperature aging from 50°C (12h) to 80°C (12h) to 110°C (6h) in a nitrogen environment containing 5% water vapor to promote the growth of Y 3+ Migrate to the grain boundaries.

[0038] (9) Supercritical drying: Using an ethanol-carbon dioxide mixed medium, supercritical drying was performed at 250 °C and 8 MPa for 8 h to reduce the capillary pressure and obtain Al2O3 aerogel material.

[0039] (10) Two-stage high-temperature sintering stabilization treatment: The Al2O3 aerogel material is subjected to high-temperature treatment at 600℃ for 2h to remove organic matter; then the temperature is raised to 1300℃ at 10℃ / min for 1h and then annealed to form a stable grain boundary structure, finally obtaining an Al2O3 aerogel material with a γ→α phase transition temperature ≥1300℃.

[0040] Example 2

[0041] The preparation method of Al2O3 aerogel described in this embodiment is as follows Figure 3 As shown, the following steps are included:

[0042] (1) Preparation of a mixed solvent: Methanol and deionized water were mixed at a volume ratio of 7:1, and nitric acid (HNO3) was added dropwise to adjust the pH to 3.0-3.5 to form a mixed solvent.

[0043] (2) Preparation of aluminum sol: Aluminum isopropoxide was slowly added to the mixed solvent and stirred continuously at 200 rpm and 60°C for about 2 h until a transparent solution was formed.

[0044] (3) Preparation of dopant mixture: Yttrium nitrate (Y(NO3)3·6H2O) and magnesium nitrate (Mg(NO3)2·6H2O) were dissolved in a minimum amount of deionized water at a molar ratio of Al:Y:Mg=10:1:0.5 and ultrasonicated at 40 kHz for 10 min.

[0045] (4) Preparation of Al-Y-Mg composite sol: The dopant mixture was added dropwise to the aluminum sol and stirred continuously at 300 rpm and 70°C for 1 h to form an Al-Y-Mg composite sol.

[0046] (5) Preparation of silica sol: polymethyl silicate and anhydrous methanol were mixed at a volume ratio of 1:7, 0.1 mol / l nitric acid was added dropwise to adjust the pH to 4.0, and hydrolyzed at 25°C and 200 rpm for 30 min.

[0047] (6) Composite sol mixing: Silica sol was slowly added to the Al-Y-Mg composite sol at a molar ratio of Al:Si = 3:1, and stirred continuously at 50 °C for 4 h to form an Al-Si-Y-Mg hybrid network.

[0048] (7) Addition and aging of flocculants: Polyethylene oxide (PEO) powder was dissolved in methanol at 1 wt% of the mass of the composite sol, and ultrasonically dispersed for 30 min. After forming a homogeneous solution, it was slowly added dropwise to the composite sol. After stirring at 100 rpm for 1 h, it was allowed to stand and age at room temperature for 24 h to promote molecular self-assembly, and finally an Al2O3 wet gel material with an Al-Si-Y-Mg network structure was obtained.

[0049] (8) Gradient temperature aging: The wet gel material was subjected to gradient temperature aging from 50°C (12h) to 80°C (12h) to 110°C (6h) in a nitrogen environment containing 5% water vapor to promote the growth of Y 3+ Migrate to the grain boundaries.

[0050] (9) Supercritical drying: Using an ethanol-carbon dioxide mixed medium, supercritical drying was performed at 250 °C and 8 MPa for 8 h to reduce the capillary pressure and obtain Al2O3 aerogel material.

[0051] (10) Two-stage high-temperature sintering stabilization treatment: The Al2O3 aerogel material is subjected to high-temperature treatment at 600℃ for 2h to remove organic matter; then the temperature is raised to 1300℃ at 10℃ / min for 1h and then annealed to form a stable grain boundary structure, finally obtaining an Al2O3 aerogel material with a γ→α phase transition temperature ≥1300℃.

[0052] Comparative Example 1

[0053] Preparation of pure Al2O3 aerogel:

[0054] (1) Aluminum source hydrolysis: Aluminum isopropoxide and ethanol were mixed in a molar ratio of 1:4, and magnetically stirred at 60°C until completely dissolved. 0.1 mol / l nitric acid was added dropwise to adjust the pH to 3.5-4.0. The hydrolysis reaction was carried out for 2 h to form a transparent aluminum sol.

[0055] (2) Inducing gel: Slowly add deionized water (water and Al) to the transparent aluminum sol formed in step (1). 3+ The temperature was raised to 80°C and stirred for about 30 to 60 min until the sol gradually transformed into a milky white gel.

[0056] (3) Gradient aging: The milky white gel was aged in an 80°C water bath for 12 h to strengthen the Al-O-Al network; then it was transferred to a mixture of ethanol and acetone (volume ratio 1:1) and aged at 40°C for 24 h to replace the pore water.

[0057] (4) Solvent replacement: Soak the gel in 50%, 70%, 90%, and 100% ethanol in sequence for 12 h each level. After each replacement, vacuum filtration (-0.8 MPa) was performed to remove the residual solvent. Soak the gel in acetone for 48 h (replaced every 12 h) to completely remove the ethanol.

[0058] (5) Supercritical drying: The gel was supercritically dried for 6 h using carbon dioxide as the medium at 10 MPa, with a pressure relief rate of ≤0.1 MPa / min to prevent sudden changes in structural stress, and pure Al2O3 aerogel material was obtained after supercritical drying.

[0059] (6) High temperature stabilization: The pure Al2O3 aerogel material was calcined in air at 400℃ for 2h (heating rate 2℃ / min) to remove residual organic matter.

[0060] Comparative Example 2

[0061] Preparation of Al2O3 aerogel doped only with yttrium:

[0062] (1) Preparation of yttrium salt-doped sol: aluminum isopropoxide and ethanol were mixed in a molar ratio of 1:4, magnetically stirred at 60°C until completely dissolved, 0.1 mol / l nitric acid was added dropwise to adjust the pH to 3.5-4.0, and hydrolysis reaction was carried out for 2 hours to form a transparent aluminum sol; yttrium nitrate (Y(NO3)3·6H2O) was weighed according to Al:Y=20:1-50:1 (molar ratio), dissolved in a minimum amount of deionized water to form a yttrium salt solution; the yttrium salt solution was added dropwise to the transparent aluminum sol, and stirred continuously at 70°C for 1 hour to form an Al-Y composite sol; 1 wt% PEO powder based on the total sol mass was added to the Al-Y composite sol, ultrasonically dispersed at 40 kHz for 30 minutes, and aged at room temperature for 12 hours to obtain a yttrium salt-doped sol.

[0063] (2) Gelation and gradient aging: Deionized water (water: Al 3+ The mixture was heated to 80°C and stirred continuously until the sol gradually transformed into a milky white gel (approximately 30 to 60 minutes). The mixture was aged in an 80°C water bath for 24 hours to strengthen the Al-OY network. The gel was transferred to a mixture of ethanol and acetone (volume ratio of 1:1) and aged at 50°C for 24 hours to replace the pore solvent, thereby obtaining a yttrium-doped Al2O3 wet gel material.

[0064] (3) Sol replacement and supercritical drying: The yttrium-doped Al2O3 wet gel material was soaked in 50%, 70%, 90%, and 100% ethanol in sequence, with each stage lasting 12 hours. After each replacement, the residual solvent was removed by vacuum filtration (-0.8 MPa); soaked in acetone for 48 hours (replaced every 12 hours) to completely remove the ethanol; supercritical drying was performed using carbon dioxide medium at 10 MPa for 8 hours, with a pressure release rate of ≤0.1 MPa / min to prevent sudden changes in structural stress, and the dried yttrium-doped Al2O3 aerogel material was obtained.

[0065] (4) High-temperature stabilization treatment: The yttrium-doped Al2O3 aerogel material was calcined in air at 400℃ for 2h (heating rate 2℃ / min) to remove residual organic matter; and calcined in argon at 1100℃ for 3h (heating rate 5℃ / min) to promote the Y 3+ Grain boundary segregation.

[0066] Based on the examples and comparative examples, the test results of the prepared gels are shown in Table 1.

[0067] Table 1 Comparison of performance data of examples and comparative examples

[0068] sample γ-α phase transition temperature 1300℃ specific surface area 1300℃ linear shrinkage High temperature thermal conductivity (800℃) Example 1 1325℃ <![CDATA[118m 2 / g]]> 9.2％ 0.024 Example 2 1337℃ <![CDATA[136m 2 / g]]> 10.5％ 0.026 Comparative Example 1 1050℃ <![CDATA[<5m 2 / g]]> 58％ 0.038 Comparative Example 2 1150℃ <![CDATA[32m 2 / g]]> 22％ 0.028

[0069] The Al2O3 aerogel in Example 1 of the present invention has a γ→α phase transition temperature increased to 1325°C. After heat treatment at 1300°C, the pore size distribution is 5-50nm and the specific surface area is 118m 2 / g, linear shrinkage is 9.2%; high temperature thermal conductivity at 800℃ is 0.024W / m·k; Al2O3 aerogel in Example 2 of the present invention, γ→α phase transition temperature is increased to 1337℃, and after heat treatment at 1300℃, the specific surface area is 136m 2 / g, linear shrinkage is 10.5%; high temperature thermal conductivity at 800℃ is 0.026W / m·k; in comparative example 1, the pure Al2O3 aerogel has a γ→α phase transition temperature of less than 1300℃, only 1050℃, and after heat treatment at 1300℃, the specific surface area is less than 5m 2 / g, linear shrinkage is 58%; high temperature thermal conductivity at 800℃ is 0.038W / m·k; in comparative example 2, the Al2O3 aerogel doped only with yttrium has a γ→α phase transition temperature of 1150℃, and after heat treatment at 1300℃, the specific surface area is 32m 2 / g, linear shrinkage is 22%; and high-temperature thermal conductivity at 800°C is 0.028W / m·k. Comparison of the performance data of the examples and comparative examples shows that the aerogel prepared by the present invention significantly outperforms aerogel materials prepared by existing technologies in key performance indicators such as phase transition temperature, specific surface area, linear shrinkage, and thermal conductivity, making it suitable for the preparation of protective materials in high-temperature fields.

[0070] Other aspects of the present invention that are not described in detail are all conventional techniques known to those skilled in the art.

[0071] It should be noted that the terms "comprises," "includes," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that includes a series of elements includes not only those elements, but also other elements not explicitly listed, or elements that are inherent to such process, method, article, or apparatus.

[0072] The protection scope of the present invention is not limited to the technical solutions disclosed in the specific implementation methods. Any modifications, equivalent replacements, improvements, etc. made to the above embodiments based on the technical essence of the present invention fall within the protection scope of the present invention.

Claims

1. A method for preparing a dual-mechanism synergistic high phase transition temperature Al2O3 aerogel, characterized by: The following steps are involved: (1) Adding an aluminum source into a water-alcohol mixed solvent to hydrolyze and form an aluminum sol; (2) Dissolving yttrium salt and magnesium salt in deionized water to obtain a dopant mixture, and then adding it to aluminum sol to form an Al-Y-Mg composite sol; (3) Adding a silicon source into anhydrous alcohol to hydrolyze and form a silica sol; (4) After the silica sol and the Al-Y-Mg composite sol are uniformly dispersed, a gelling agent is added to form a composite wet gel material with an Al-Si-Y-Mg network structure; (5) The composite wet gel material is subjected to gradient temperature aging and then supercritical drying in a mixed medium to obtain an Al2O3 aerogel material; (6) The Al2O3 aerogel material is subjected to two-stage sintering to obtain Al2O3 aerogel with a γ→α phase transition temperature ≥1300℃.

2. The method for preparing the dual-mechanism synergistic high phase transition temperature Al2O3 aerogel according to claim 1, characterized in that: The aluminum source is aluminum isopropoxide.

3. The method for preparing the dual-mechanism synergistic high phase transition temperature Al2O3 aerogel according to claim 1, characterized in that: The yttrium salt is yttrium nitrate, and the doping amount is 10-40% of the molar ratio of aluminum oxide.

4. The method for preparing the dual-mechanism synergistic high phase transition temperature Al2O3 aerogel according to claim 1, characterized in that: The magnesium salt is magnesium nitrate, and the doping amount is 3-6% of the molar ratio of aluminum oxide.

5. The method for preparing the dual-mechanism synergistic high phase transition temperature Al2O3 aerogel according to claim 1, characterized in that: The silicon source is tetraethyl orthosilicate or polymethyl silicate, and the doping amount is 30-50% of the molar ratio of aluminum oxide.

6. The method for preparing the dual-mechanism synergistic high phase transition temperature Al2O3 aerogel according to claim 1, characterized in that: The gelling aid in step (4) is a homogeneous solution formed by dissolving polyethylene oxide powder in alcohol.

7. The method for preparing the dual-mechanism synergistic high phase transition temperature Al2O3 aerogel according to claim 1, characterized in that: The temperatures for the gradient aging in step (5) are 50°C → 80°C → 110°C.

8. The method for preparing the dual-mechanism synergistic high phase transition temperature Al2O3 aerogel according to claim 1, characterized in that: The composite wet gel material in step (5) is placed in a nitrogen environment containing 5% water vapor for gradient temperature aging.

9. The method for preparing the dual-mechanism synergistic high phase transition temperature Al2O3 aerogel according to claim 1, characterized in that: The two-stage sintering in step (6) includes: keeping the temperature at 600°C for 2 hours, heating the temperature to 1300°C at a heating rate of 10°C / min, keeping the temperature for 1 hour, and then annealing.

10. A dual-mechanism synergistic high phase transition temperature Al2O3 aerogel, characterized by: The nanostructured carbon nanotube is prepared by the preparation method according to any one of claims 1 to 9, wherein after heat treatment at 1300°C, the pore size distribution is 5-50 nm, the thermal conductivity is ≤0.035 W / m·K, and the phase transition temperature is ≥1300°C.

Images