Preparation method of modified high-silica fiber reinforced SiO2 aerogel composite material
By modifying high-silica fiber-reinforced SiO2 aerogel composites through mild nitric acid oxidation and vacuum freeze-drying processes, the problems of low mechanical strength and high brittleness in high-temperature and high-pressure environments were solved, resulting in high-performance, low-density fiber-reinforced aerogel composites suitable for high-temperature environments.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- SOUTHEAST UNIV
- Filing Date
- 2026-01-27
- Publication Date
- 2026-05-26
AI Technical Summary
Existing silica aerogels exhibit low mechanical strength and high brittleness in high-temperature and high-pressure environments. The manufacturing defects and performance dispersion caused by fiber reinforcement methods make it difficult to achieve synergistic optimization of mechanical and thermal insulation properties in high-temperature environments.
A mild nitric acid oxidation combined with vacuum freeze-drying process was adopted to prepare modified high-silica fiber reinforced SiO2 aerogel composite material by interfacial bonding between modified high-silica fibers and aerogel matrix, using MTMS and TEOS as co-precursors and controlling the fiber content.
A high-performance, low-density, and high-temperature stable fiber-reinforced aerogel composite material has been developed, which improves the thermal stability and mechanical properties of the material, reduces the thermal conductivity, and is suitable for high-temperature environments.
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Figure CN122079595A_ABST
Abstract
Description
Technical Field
[0001] This invention relates to the field of fiber-reinforced aerogel composite material preparation technology, and more specifically, to a method for preparing modified high-silica fiber-reinforced SiO2 aerogel composite material. Background Technology
[0002] Silica aerogel is a three-dimensional network porous material with extremely low density, low thermal conductivity, large specific surface area, and high porosity, showing broad application prospects in thermal insulation, fire protection, acoustic insulation, and aerospace. Since Kistler first synthesized aerogel in the 1930s, this type of material has played an increasingly important role in energy conservation and environmental protection. With the continuous maturation of supercritical drying technology and freeze-drying processes, the capability for large-scale production of various aerogel composite materials is now available. Therefore, high-performance thermal insulation composite materials with aerogel as the core have been widely used globally, such as in the thermal protection systems of NASA spacecraft, insulation layers for industrial pipelines, insulation materials for high-temperature equipment, and insulation panels in building energy conservation. However, as a novel functional material, pure silica aerogel has inherent defects such as low mechanical strength and high brittleness, which severely limits its application in high-temperature and high-pressure environments and load-bearing structures. In particular, current research on its thermomechanical properties under high-temperature environments is insufficient. These properties are closely related to the material's microstructure and fiber reinforcement method, and thermal insulation performance cannot be improved by simply increasing the amount of material used; on the contrary, increasing density may lead to an increase in thermal conductivity. Inorganic fiber-reinforced aerogel composites can significantly improve the mechanical properties and thermal stability of aerogels by introducing reinforcing phases such as high-silica glass fibers into the aerogel matrix. However, aerogel materials with significant multi-scale structures are prone to manufacturing defects during preparation, such as uneven fiber distribution, fiber agglomeration, and poor bonding between the matrix and fiber interfaces, resulting in significant dispersion in material properties. Furthermore, to obtain specific properties, precise design of parameters such as fiber content, fiber surface treatment methods, and precursor composition is required.
[0003] Therefore, this patent proposes a method for preparing high-silica glass fiber reinforced aerogel composite materials based on dual surface treatment with nitric acid oxidation and silane coupling agents. By combining MTMS and TEOS as co-precursors and utilizing surface modification technology to enhance the interfacial bonding strength between the fiber and the aerogel matrix, the mechanical properties and thermal insulation properties of the composite material are balanced by controlling the fiber content. This achieves the preparation of high-performance, low-density, and high-temperature stable fiber-reinforced aerogel composite materials, providing a new approach to solving the problem of synergistic optimization of mechanical and thermal insulation properties of aerogel materials in high-temperature applications. Summary of the Invention
[0004] To address the above problems, this invention proposes a method for preparing modified high-silica fiber-reinforced SiO2 aerogel composite materials. This method can improve the interfacial interaction between high-silica fibers and aerogel, and produces high-performance fiber-reinforced aerogel composite materials using only a simple sol-gel method and freeze-drying process.
[0005] The technical solution adopted in this invention is: a method for preparing modified high-silica fiber-reinforced SiO2 aerogel composite material, comprising the following steps: Step 1: First, the high-silica fiber is modified and dried. After drying, it is subjected to ultrasonic desizing in acetone solution and then placed in nitric acid solution for oxidation treatment. Prepare a siloxane coupling agent hydrolysate and immerse the high-silica fiber in the siloxane coupling agent hydrolysate to obtain modified high-silica fiber with a long silicon chain structure on the surface. Step 2: The modified high-silica fiber is placed in a hydrolysate containing methoxytrimethylsilane and tetraethyl silicate as co-precursors. The molar ratio of methoxytrimethylsilane to tetraethyl silicate is 6:1. The mixture is stirred and undergoes five stages: aging, solvent replacement, surface modification, cleaning, and freeze-drying to obtain the modified high-silica fiber reinforced SiO2 aerogel composite material.
[0006] Furthermore, the step of ultrasonic desizing with acetone solution in step 1 includes: immersing the high-silica fiber in a mixed solution of acetone and ethanol with a volume ratio of 1:2 to 1:3, heating and ultrasonically treating it, and then vacuum drying it.
[0007] Furthermore, the step of surface oxidation of the fiber using nitric acid solution in step 1 includes: immersing the dried high-silica fiber in an aqueous nitric acid solution, oxidizing it in a water bath at 40°C, and then drying it under vacuum.
[0008] Furthermore, the hydrolysate of the siloxane coupling agent is KH550 hydrolysate and KH560 hydrolysate, respectively. The KH-550 hydrolysate is obtained by mixing KH-550, ethanol, deionized water and glacial acetic acid in a volume ratio, wherein the amount of KH-550 is 25-35 parts, ethanol is 50-60 parts, deionized water is 10-15 parts and glacial acetic acid is 20-30 parts. The KH-560 hydrolysate is obtained by mixing KH-560, ethanol, deionized water and glacial acetic acid in a volume ratio, wherein the amount of KH-560 is 15-25 parts, ethanol is 55-75 parts, deionized water is 15-20 parts and glacial acetic acid is 20-30 parts.
[0009] Furthermore, the surface modification of fibers using KH550 hydrolysate involves first subjecting the high-silica fibers to hydrothermal treatment in a 50°C water bath for 2 hours, then immersing the dried high-silica fibers in a siloxane coupling agent hydrolysate at 50°C for 12 hours, followed by vacuum drying. The surface modification of fibers using KH-560 hydrolysate includes: subjecting the high-silica fibers to hydrothermal treatment in a 50-60°C water bath for 2 hours, then immersing the dried high-silica fibers in a KH-560 hydrolysate solution at 50-60°C for 12 hours, followed by vacuum drying.
[0010] Furthermore, in step 2, the high-silica fiber and the precursor hydrolysate are stirred and decomposed on a homogenizer, and the added mass of the high-silica fiber is set to be 10 wt%~60 wt%.
[0011] Furthermore, in step 2, after the high-silica fiber and the precursor hydrolysate are stirred to form a gel, it is immersed in an ethanol solution for gel aging, then immersed in a hexane solution for solvent replacement, and then immersed in a trimethylchlorosilane / hexane mixed solution for surface modification of the gel. The volume ratio of trimethylchlorosilane to hexane is 1:9. Finally, it is immersed in a hexane solution for gel cleaning. Each step is set at an ambient temperature of 40 degrees Celsius for 24 hours.
[0012] Furthermore, the freeze-drying in step 2 includes a pre-freezing stage: freezing at -45°C for 4 hours; a first-stage drying stage: under vacuum conditions of <20Pa, the temperature is increased from -30°C to -10°C for 12 hours; and a second-stage drying stage: under vacuum conditions of <10Pa, the temperature is increased from -10°C to 20°C for 8 hours.
[0013] This invention represents a substantial improvement and innovation based on existing fiber-reinforced aerogel preparation technology. Unlike conventional techniques or the "high-temperature sulfuric acid oxidation combined with ambient pressure drying" approach used in previous explorations by the inventors, this invention creatively constructs a novel preparation system combining "mild nitric acid oxidation with vacuum freeze-drying." Specifically, this invention abandons the traditional process of 60°C sulfuric acid oxidation and ambient pressure gradient drying, instead employing a 40°C low-temperature nitric acid system for flexible fiber modification, coupled with a specific multi-stage vacuum freeze-drying process. Furthermore, this invention specifically optimizes the precursor ratio (adjusting the molar ratio of methoxytrimethylsilane to tetraethyl silicate to 6:1) and the surface modifier concentration (adjusting the ratio of trimethylchlorosilane to n-hexane to 1:9) specifically for the freeze-drying mechanism. The changes in the above process parameters and preparation route are not simply numerical adjustments, but are intended to effectively overcome the problems of aerogel skeleton shrinkage and pore structure collapse caused by excessive capillary force during atmospheric pressure drying. At the same time, mild oxidation is used to better preserve the bulk mechanical strength of the reinforcing fibers, thereby obtaining modified fiber-reinforced aerogel composite materials with lower density and more complete structure than existing technologies. Beneficial effects
[0014] (1) This invention modifies high-silica fibers, thereby increasing the functional groups of high-silica fibers and grafting long-chain chemical structures onto the surface of high-silica fibers. A large number of -OH groups increase the fluidity of high-silica fibers in MT sol.
[0015] (2) The -OH groups present on the modified high silica fiber of the present invention are connected with the aerogel during the polycondensation reaction of the aerogel, and a large number of chemically stable Si-O-Si bonds are constructed between the fiber and the aerogel.
[0016] (3) This invention achieves the simple preparation of modified high silica fiber reinforced SiO2 aerogel composite material through sol-gel method and freeze drying process, and obtains composite material with strong plasticity and strong heat resistance at extremely low production cost. Attached Figure Description
[0017] Figure 1 The sample image and SEM image of the composite material provided in Example 1 of this invention; Figure 2 The sample image and SEM image of the composite material provided in Example 2 of this invention; Figure 3 The sample image and SEM image of the composite material provided in Example 3 of this invention; Figure 4 The sample image and SEM image of the composite material provided in Comparative Example 4 of the present invention; Figure 5 The sample image and SEM image of the composite material provided in Comparative Example 5 of the present invention; Figure 6 The images shown are sample images and SEM images of the composite material provided in Comparative Example 6 of the present invention. Detailed Implementation
[0018] This invention discloses a method for preparing modified high-silica fiber-reinforced SiO2 aerogel composite materials, which improves the thermal stability of aerogel materials and meets the protective performance requirements of building structural engineering. To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions of this invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of this invention.
[0019] A method for preparing a modified high-silica fiber-reinforced SiO2 aerogel composite material includes the following two steps: Step 1: After desizing and nitric acid oxidation of high-silica fibers, surface modification grafting treatment is carried out sequentially using KH-550 hydrolysate and KH-560 hydrolysate to obtain modified carbon fibers. The specific operation process is as follows: S1. Desizing of High-Silica Fibers: Select high-silica glass fibers with a length of 1 mm and a diameter of 7 μm. First, dry the fibers in a forced-air drying oven at 90 degrees Celsius for 10 hours to obtain dry high-silica glass fibers. Then, place the fibers in a beaker containing a mixture of acetone and ethanol, immerse it in an ultrasonic bath, and continuously sonicate at 35 degrees Celsius for 6 hours. Rinse with ethanol, and then dry in a vacuum drying oven at 80 degrees Celsius for 6 hours.
[0020] S2. Oxidation treatment of high silica fiber: After desizing of high silica fiber, place the high silica fiber slurry in a 500ml beaker, pour in a mixed solution of nitric acid and deionized water, place it in a 40℃ water bath, and continue to oxidize for 6 hours. Rinse repeatedly with deionized water, and place the oxidized high silica fiber in a vacuum drying oven and dry continuously at 70 degrees for 12 hours.
[0021] S3. Grafting treatment of high silica fibers: First, prepare pre-hydrolyzed solutions of KH-550 and KH-560. Then, place the oxidized high silica fibers in the pre-hydrolyzed solutions for grafting treatment: First, put a mixed solution of KH-550, ethanol, deionized water and glacial acetic acid into a rotor and continuously stir and heat in a 50°C water bath for 2 hours to obtain KH-550 pre-hydrolyzed solution. Then, pour the oxidized high silica fibers into the KH-550 pre-hydrolyzed solution and place them in a 60°C constant temperature oven for 12 hours. After that, rinse them repeatedly with deionized water and dry them in a vacuum drying oven at 80°C for 12 hours.
[0022] S4. Next, a mixed solution of KH-560, ethanol, deionized water and glacial acetic acid is placed in a rotor and continuously stirred and heated in a 50°C water bath for 2 hours to obtain a KH-560 pre-hydrolyzed solution. The high-silica fibers grafted with KH-550 are poured into the KH-560 pre-hydrolyzed solution and placed in a 50°C constant temperature oven for 12 hours. After that, they are repeatedly rinsed with deionized water and dried in a vacuum drying oven at 80°C for 12 hours to finally obtain modified high-silica fibers.
[0023] Step 2: Modifying high-silica fiber-reinforced SiO2 aerogel composite materials, the specific operation process is as follows: S5. Methoxytrimethylsilane (MTMS) and tetraethyl silicate (TEOS) were used as co-precursors, EtOH was used as solvent, and the molar ratio of H2O was 6:1:35:40. MTMS and TEOS were subjected to acid-catalyzed hydrolysis using 0.1 M / L hydrochloric acid to adjust the pH of the overall solution to 4. The mixed solution was placed in a water bath at 50°C and the hydrolysis reaction was carried out continuously for 5 hours.
[0024] S6. Modified high-silica fibers were added to a hydrolysate containing MTMS and TEOS as co-precursors. The pH was adjusted to 9 using 2 M / L NH3•H2O solution, and the mixture was stirred continuously for 10 minutes until gel formation. Ethanol was added, and the mixture was placed in a constant temperature oven at 40°C for 24 hours to age the aerogel composite. Hexane was added, and the mixture was placed in a constant temperature oven at 40°C for 24 hours for solvent replacement. A 1:9 volume ratio of TMCS / hexane was added, and the mixture was placed in a constant temperature oven at 40°C for 24 hours to modify the surface of the aerogel composite. Hexane was added, and the mixture was placed in a constant temperature oven at 40°C for 24 hours. After 12 hours, fresh hexane was added again to remove chloride ions from the aerogel composite. The gel was then placed in a forced-air drying oven at 40°C for 2 hours, 60°C for 2 hours, and 100°C for 8 hours to obtain the modified high-silica fiber reinforced SiO2 aerogel composite.
[0025] Example 1: The modified high-silica fiber-reinforced SiO2 aerogel composite material will be processed according to the following steps: Step 1: Preparation of modified high-silica fibers (1) Fiber desizing: Desizing treatment of high silica fiber: Select high silica glass fibers with a length of 1 mm and a diameter of 7 μm. First, dry the fibers in a forced-air drying oven at 90 degrees Celsius for 10 hours to obtain dry high silica glass fibers. Then, place the fibers in a beaker and mix them with acetone and ethanol at a volume ratio of 1:2. Place the beaker in an ultrasonic bath and continue ultrasonic treatment at 35 degrees Celsius for 6 hours. Rinse with ethanol and then dry in a vacuum drying oven at 80 degrees Celsius for 6 hours.
[0026] (2) Fiber oxidation: After desizing the high-silica fiber, the high-silica fiber slurry was placed in a 500ml beaker, and a mixed solution of nitric acid and deionized water was poured in. The beaker was placed in a 40℃ water bath and oxidized for 6 hours. The beaker was then rinsed repeatedly with deionized water. The oxidized high-silica fiber was then placed in a vacuum drying oven and dried at 70 degrees Celsius for 12 hours.
[0027] (3) Fiber grafting: KH-550, ethanol, deionized water and glacial acetic acid were mixed and placed in a rotor. The mixture was stirred and heated in a 50°C water bath for 2 hours to obtain a KH-550 pre-hydrolyzed solution. Then, the oxidized high-silica fiber was poured into the KH-550 pre-hydrolyzed solution and placed in a 60°C constant temperature oven for 12 hours. After that, it was repeatedly rinsed with deionized water and dried in a vacuum drying oven at 80°C for 12 hours.
[0028] Next, a mixed solution of KH-560, ethanol, deionized water, and glacial acetic acid was placed in a rotor and continuously stirred and heated in a 50°C water bath for 2 hours to obtain a KH-560 pre-hydrolyzed solution. The high-silica fibers grafted with KH-550 were then poured into the KH-560 pre-hydrolyzed solution and placed in a pre-prepared 50°C constant temperature oven for 12 hours. After that, the fibers were repeatedly rinsed with deionized water and dried in a vacuum drying oven at 80°C for 12 hours to finally obtain modified high-silica fibers.
[0029] Step 2: Preparation of modified high-silica fibers with a fiber mass ratio of 10 wt%. MTMS and TEOS were used as co-precursors, EtOH as solvent, and H2O in a molar ratio of 6:1:35:40. MTMS and TEOS were subjected to acid-catalyzed hydrolysis using 0.1 M / L hydrochloric acid to adjust the pH of the overall solution to 4. The mixed solution was then placed in a water bath at 50°C and the hydrolysis reaction was continued for 5 hours.
[0030] Modified high-silica fibers, accounting for 10 wt% of the total mass of the fiber-reinforced aerogel composite, were added to the precursor solution. The pH was adjusted to 9 with 2 M / L NH3•H2O solution, and the mixture was stirred continuously for 10 minutes until gel formation. Ethanol was added, and the mixture was placed in an incubator at 40°C for 24 hours to age the aerogel composite. Hexane was added, and the mixture was placed in an incubator at 40°C for 24 hours to perform solvent replacement. A 1:9 volume ratio TMCS / hexane mixture was added, and the mixture was placed in an incubator at 40°C for 24 hours to perform surface modification of the aerogel composite. Hexane was added, and the mixture was placed in an incubator at 40°C for 24 hours. After 12 hours, fresh hexane was added again to remove chloride ions from the aerogel composite gel. The gel was placed in a forced-air drying oven and dried at 40℃ for 2 hours, 60℃ for 2 hours, and 100℃ for 8 hours respectively, finally obtaining a modified high-silica fiber-reinforced SiO2 aerogel composite material with a fiber mass ratio of 10 wt%. The obtained sample is shown in the figure. Figure 1 As shown.
[0031] Example 2: The other operating steps are the same as in Example 1, except that the high-silica fibers account for 20 wt% of the total fiber-reinforced aerogel composite material. See the specific sample images and SEM images for details. Figure 2 As shown. Example 3
[0032] The other operating steps are the same as in Example 1, except that the high-silica fibers account for 30 wt% of the total fiber-reinforced aerogel composite material. See the specific sample images and SEM images for details. Figure 3 As shown. Example 4
[0033] The other operating steps are the same as in Example 1, except that the high-silica fibers account for 40 wt% of the total fiber-reinforced aerogel composite material. See the specific sample images and SEM images for details. Figure 4 As shown. Example 5
[0034] The other operating steps are the same as in Example 1, except that the high-silica fibers account for 50 wt% of the total fiber-reinforced aerogel composite material. See the specific sample images and SEM images for details. Figure 5 As shown. Example 6
[0035] The other operating steps are the same as in Example 1, except that the high-silica fibers account for 60 wt% of the total fiber-reinforced aerogel composite material. See the specific sample images and SEM images for details. Figure 6 As shown.
[0036] Table 1. Physical properties, mechanical properties, and thermal properties of modified high-silica fiber reinforced SiO2 aerogel composites.
[0037] As shown in Table 1 above, the compressive strength gradually increases with the increase of fiber content, while the density and thermal conductivity gradually decrease. The modified high-silica fiber-reinforced SiO2 aerogel exhibits low density. This invention, through the introduction of an innovative cross-linking network construction strategy and interface strengthening technology, effectively suppresses the formation of heat conduction paths while enhancing the connection strength and energy absorption capacity between network nodes. The aerogel material maintains ultra-low thermal conductivity while exhibiting excellent resistance to compressive deformation.
[0038] The aerogel preparation process of this invention significantly enhances mechanical properties with only a slight reduction in thermal conductivity.
[0039] Although the invention has been illustrated and described with reference to specific embodiments, it should be understood that many other changes and modifications can be made without departing from the spirit and scope of the invention. Therefore, it is intended that all such changes and modifications falling within the scope of the invention be included in the appended claims.
Claims
1. A method for preparing a modified high-silica fiber-reinforced SiO2 aerogel composite material, characterized in that: Includes the following steps: Step 1: First, the high-silica fiber is modified and dried. After drying, it is subjected to ultrasonic desizing in acetone solution and then placed in nitric acid solution for oxidation treatment. Prepare a siloxane coupling agent hydrolysate and immerse the high-silica fiber in the siloxane coupling agent hydrolysate to obtain modified high-silica fiber with a long silicon chain structure on the surface. Step 2: The modified high-silica fiber is placed in a hydrolysate containing methoxytrimethylsilane and tetraethyl silicate as co-precursors. The molar ratio of methoxytrimethylsilane to tetraethyl silicate is 6:
1. The mixture is stirred and undergoes five stages: aging, solvent replacement, surface modification, cleaning, and freeze-drying to obtain the modified high-silica fiber reinforced SiO2 aerogel composite material.
2. The method for preparing the modified high-silica fiber-reinforced SiO2 aerogel composite material according to claim 1, characterized in that: The step of ultrasonic desizing with acetone solution in step 1 includes: immersing high-silica fibers in a mixed solution of acetone and ethanol with a volume ratio of 1:2 to 1:3, heating and ultrasonic treatment, and then vacuum drying.
3. The method for preparing the modified high-silica fiber-reinforced SiO2 aerogel composite material according to claim 2, characterized in that: The step 1, which involves surface oxidation of the fiber using nitric acid solution, includes: immersing the dried high-silica fiber in an aqueous nitric acid solution, oxidizing it in a 40°C water bath, and then drying it under vacuum.
4. The method for preparing the modified high-silica fiber-reinforced SiO2 aerogel composite material according to claim 3, characterized in that: The siloxane coupling agent hydrolysates are KH550 hydrolysate and KH560 hydrolysate. The KH-550 hydrolysate is obtained by mixing KH-550, ethanol, deionized water, and glacial acetic acid in a volume ratio, wherein the amount of KH-550 is 25-35 parts, ethanol is 50-60 parts, deionized water is 10-15 parts, and glacial acetic acid is 20-30 parts. The KH-560 hydrolysate is obtained by mixing KH-560, ethanol, deionized water, and glacial acetic acid in a volume ratio, wherein the amount of KH-560 is 15-25 parts, ethanol is 55-75 parts, deionized water is 15-20 parts, and glacial acetic acid is 20-30 parts.
5. The method for preparing the modified high-silica fiber-reinforced SiO2 aerogel composite material according to claim 4, characterized in that: The surface modification of fibers using KH550 hydrolysate involves first subjecting the high-silica fibers to hydrothermal treatment in a 50°C water bath for 2 hours, then immersing the dried high-silica fibers in a siloxane coupling agent hydrolysate at 50°C for 12 hours, followed by vacuum drying. The surface modification steps of the fiber using KH-560 hydrolysate include: hydrothermal treatment of the high-silica fiber in a water bath at 50-60℃ for 2 hours, followed by immersion of the dried high-silica fiber in KH-560 hydrolysate at 50-60℃ for 12 hours, and then vacuum drying.
6. The method for preparing the modified high-silica fiber-reinforced SiO2 aerogel composite material according to claim 5, characterized in that: In step 2, the high-silica fiber and the precursor hydrolysate are stirred and decomposed on a homogenizer, and the added mass of high-silica fiber is set to 10 wt%~60 wt%.
7. The method for preparing the modified high-silica fiber-reinforced SiO2 aerogel composite material according to claim 6, characterized in that: In step 2, after the high-silica fiber and the precursor hydrolysate are stirred to form a gel, it is immersed in an ethanol solution for gel aging, then immersed in a hexane solution for solvent replacement, and then immersed in a trimethylchlorosilane / hexane mixed solution for surface modification of the gel. The volume ratio of trimethylchlorosilane to hexane is 1:
9. Finally, it is immersed in a hexane solution for gel cleaning. The ambient temperature is set at 40 degrees Celsius and the duration is 24 hours for each step.
8. The method for preparing the modified high-silica fiber-reinforced SiO2 aerogel composite material according to claim 7, characterized in that: The freeze-drying in step 2 includes a pre-freezing stage: freezing at -45°C for 4 hours; a first-stage drying stage: under vacuum conditions of <20Pa, the temperature is raised from -30°C to -10°C for 12 hours; and a second-stage drying stage: under vacuum conditions of <10Pa, the temperature is raised from -10°C to 20°C for 8 hours.