Silica aerogel-aramid fiber composite material with gradient density and preparation method thereof

Abstract

The invention relates to the technical field of aerogel, in particular to a silicon dioxide aerogel-aramid fiber composite material with gradient density and a preparation method of the silicon dioxide aerogel-aramid fiber composite material. According to the method provided by the invention, the aramid fiber and the SiO2 / POSS matrix are combined to form the composite insulating material with a three-dimensional interpenetrating network structure through the process steps of sol preparation, gradient gelation and directional forming, and meanwhile, Fe3O4 nanoparticles are modified on the aramid fiber, so that magnetic field regulation and control of fiber orientation are realized. According to the preparation method provided by the invention, the problems of weak interface bonding force, non-uniform fiber orientation and poor performance stability of the aramid fiber-SiO2 composite material in related technologies are solved.

Description

Technical Field

[0001] This application relates to the field of aerogel technology, and in particular to a silica aerogel-aramid fiber composite material with gradient density and its preparation method. Background Technology

[0002] Silica aerogel is a porous material composed of a nanoscale three-dimensional network structure with a porosity of 80%–99.8%, making it one of the lightest known solid materials. Its unique nanoporous structure endows it with ultra-low thermal conductivity, thus making it valuable for applications in aerospace thermal protection, building energy conservation, and energy storage. However, pure silica aerogel has poor mechanical properties, exhibiting high brittleness, low toughness, and susceptibility to shrinkage and cracking. Therefore, composite reinforcement strategies are typically used to improve its mechanical properties. Common methods include fiber reinforcement (such as aramid, carbon fiber, and glass fiber), polymer modification (such as polyimide and polyvinyl alcohol), and nanoparticle doping (such as graphene and carbon nanotubes). Aramid fibers, due to their high strength, high modulus, excellent temperature resistance, and electrical insulation, have become an ideal reinforcing material for silica aerogel.

[0003] Currently, most related technologies employ simple mixing methods to prepare aramid fiber / SiO2 composite materials, such as directly adding aramid fibers to SiO2 sol and curing them. Without controlling the interface between the fiber and the matrix or optimizing the fiber orientation, the resulting material exhibits limited improvement in mechanical strength and insulation properties, and insufficient overall thermal insulation and flame retardant performance. This can easily lead to safety hazards in high-temperature, high-pressure, and flammable environments. For example, patent CN107051339A discloses a fiber-toughened SiO2 aerogel and its preparation method, which uses a sol-gel method to prepare fiber-composite SiO2 wet gel.

[0004] Therefore, there is an urgent need to develop a method for preparing composite materials that can enhance the interfacial bonding between aramid fibers and the SiO2 matrix, while improving the thermal insulation and flame retardant properties of the material. Summary of the Invention

[0005] This application provides a silica aerogel-aramid fiber composite material with gradient density and its preparation method, in order to solve the problems of weak interfacial bonding, uneven fiber orientation, and poor performance stability of aramid fiber-SiO2 composite materials in related technologies.

[0006] In a first aspect, this application provides a method for preparing a silica aerogel-aramid fiber composite material with gradient density, comprising the following steps: Step S101: Add aramid fiber to deionized water for ultrasonic cleaning, vacuum dry after cleaning, then add KH560 coupling agent solution for impregnation, and add dilute hydrochloric acid to adjust the pH of the system to 3.0-4.0. After impregnation, dry to obtain modified aramid fiber. In step S102, the modified aramid fiber is dispersed in deionized water, a mixture of ferrous salt and ferric salt is added, and ammonia is added dropwise under nitrogen protection to adjust the pH to 9.0-10.0. The temperature is raised to 70-80℃. After the reaction is completed, the mixture is cooled and filtered. It is then washed alternately with deionized water and anhydrous ethanol. After washing, it is vacuum dried to obtain Fe3O4 nanoparticle modified aramid fiber. In step S103, Fe3O4 nanoparticles modified aramid fibers are dispersed in anhydrous ethanol and ultrasonically dispersed to obtain a fiber suspension. The fiber suspension is added to a mixed sol composed of tetraethyl orthosilicate, anhydrous ethanol, deionized water, and octahydroxybutylated cage-like polysilsesquioxane. The mixture is magnetically stirred and ultrasonically treated. Then, dilute hydrochloric acid is added to adjust the pH of the system to 3.0-4.0. The mixture is heated to 35-45℃ for pre-gelling. Then, the temperature is raised to 70-80℃, and ammonia is added to adjust the pH of the system to 8.0-9.0 to obtain SiO2-aramid fiber gel. Step S104: Inject SiO2-aramid fiber gel into a mold with progressively increasing thickness along the thickness direction, apply gradient magnetic field and gradient pressure, dry under normal pressure, and cool to room temperature to obtain silica aerogel-aramid fiber composite material with gradient density.

[0007] In some embodiments, in step S101, the mass concentration of the KH560 coupling agent solution is 1%-2%. The epoxy groups contained in the KH560 coupling agent molecule can react with the active groups on the surface of the aramid fiber, while the siloxane groups can form chemical bonds with the SiO2 matrix, thereby enhancing the interfacial bonding force between the fiber and the matrix at the molecular level.

[0008] In some embodiments, in step S101, the aramid fiber is selected as para-aramid fiber.

[0009] In some embodiments, the molar ratio of ferrous salt to ferric salt is 1:2 to 1:3.

[0010] In some embodiments, the ferrous salt is selected from ferrous chloride or ferrous nitrate, and the ferric salt is selected from ferric chloride or ferric nitrate.

[0011] In some embodiments, in step S102, the amount of the mixture of ferrous salt and ferric salt added is 3%-8% of the mass of the modified aramid fiber.

[0012] In some embodiments, the molar ratio of tetraethyl orthosilicate, anhydrous ethanol, deionized water and octahydroxybutylated cage-like polysilsesquioxane is 1:3-10:3-5:0.1-0.3.

[0013] In some embodiments, in step S103, the mass concentration of the fiber suspension is 0.5%-1%.

[0014] In some embodiments, in step S103, the mass of the mixed sol is 10%-20% of the mass of the fiber suspension.

[0015] In some embodiments, in step S104, the temperature for atmospheric pressure drying is 60-80°C.

[0016] In some embodiments, in step S104, the gradient magnetic field is a magnetic field that linearly increases from 0.2T to 0.5T along the gradient direction; the gradient pressure is a pressure that increases from 0.05MPa to 0.15MPa along the magnetic field gradient direction.

[0017] Secondly, this application also provides a silica aerogel-aramid fiber composite material with gradient density prepared by the above preparation method.

[0018] The beneficial effects of the technical solution provided in this application include: 1. This application utilizes silane coupling agents to introduce active groups on the surface of aramid fibers, which enhances the interfacial bonding force between aramid fibers and the SiO2 / POSS matrix, avoids the delamination of fibers from the matrix during the composite process, and improves the mechanical properties and stability of the material. According to the test, the tensile strength of the material can reach more than 60 MPa, and the breaking strength retention rate is more than 92%. 2. This application generates magnetic nanoparticles on the surface of aramid fibers through a co-precipitation method. The magnetic field induces precise fiber orientation, achieving directional arrangement of aramid fibers. The anisotropy of the material can be controlled according to requirements, solving the performance fluctuation problem caused by uneven fiber orientation in traditional composite materials. At the same time, the regular fiber arrangement reduces heat transfer paths, further optimizing thermal insulation performance. The limiting oxygen index of the material reaches 38%-40%, the thermal conductivity at room temperature is ≤0.03W / (m·K), and a dense, crack-free char layer is formed after combustion. The flame retardant rating reaches UL94 V-0, demonstrating excellent flame retardant performance. 3. This application adopts a gradient gelation process, first pre-gelling under acidic conditions to allow the sol to initially encapsulate the fiber, and then accelerating polycondensation under alkaline conditions to form a three-dimensional interpenetrating network, which not only ensures the effective coating of the fiber by the matrix, but also improves the density of the matrix. 4. This application obtains a fiber composite material with gradient density through the synergistic effect of gradient magnetic field-induced fiber stacking and gradient pressure-assisted densification. In the low magnetic field region, the fibers are uniformly dispersed and loosely stacked, corresponding to low fiber content and low matrix density; in the high magnetic field region, the fibers are oriented and densely stacked, corresponding to high fiber content and high matrix density, and a continuous density transition is formed in the intermediate region. Attached Figure Description

[0019] To more clearly illustrate the technical solutions in the embodiments of this application, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0020] Figure 1 This is a schematic diagram illustrating the preparation method of the silica aerogel-aramid fiber composite material provided in this application. Detailed Implementation

[0021] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0022] This application provides a silica aerogel-aramid fiber composite material with gradient density and its preparation method, which can solve the problems of weak interfacial bonding, uneven fiber orientation, and poor performance stability of aramid fiber-SiO2 composite materials in the prior art.

[0023] refer to Figure 1 This application provides a method for preparing a silica aerogel-aramid fiber composite material with gradient density, comprising the following steps: Step S101: The aramid fiber is ultrasonically cleaned in deionized water for 20-30 minutes to remove surface impurities, and then vacuum dried at 50-60℃ for 2-3 hours. Afterwards, it is impregnated with a 1%-2% KH560 coupling agent solution for 1-2 hours, and dilute hydrochloric acid is added to adjust the pH of the system to 3.0-4.0, allowing the coupling agent to graft onto the surface of the aramid fiber. After impregnation, it is dried at 70-80℃ to obtain the modified aramid fiber. Modification with the coupling agent can improve the interfacial bonding force between the aramid fiber and SiO2 / POSS. Step S102: The modified aramid fiber is dispersed in deionized water, and ferrous salt and ferric salt are added in a molar ratio of 1:2-1:3. Under N2 protection, ammonia water is added dropwise to adjust the pH of the system to 9.0-10.0. The temperature is raised to 70-80℃ and the reaction is maintained at a constant temperature for 1-2 hours. After the reaction is completed, the mixture is cooled to room temperature, filtered, and washed alternately with deionized water and anhydrous ethanol 3-5 times. After washing, the mixture is vacuum dried at 60℃ for 2 hours to obtain Fe3O4 nanoparticle-modified aramid fiber. The amount of the mixture of ferrous salt and ferric salt added is 3%-8% of the mass of the modified aramid fiber. Step S103: Disperse Fe3O4 nanoparticle-modified aramid fibers in anhydrous ethanol and sonicate at 300W for 15-20 min to obtain a fiber suspension with a mass concentration of 0.5%-1%. Prepare a mixed sol according to the molar ratio of tetraethyl orthosilicate: anhydrous ethanol: deionized water: octahydroxybutylated cage-like polysilsesquioxane = 1:3-10:3-5:0.1-0.3. Add the fiber suspension dropwise to the mixed sol (the mass of the mixed sol is 10%-20% of the mass of the fiber suspension), and simultaneously stir magnetically at 500 rpm and sonicate at 300W for 10 min. Then add dilute hydrochloric acid to adjust the pH of the system to 3.0-4.0, heat to 35-45℃ for pre-gelation for 1-2 h, and then raise the temperature to 70-80℃ at a rate of 0.5-1 mL / min. Ammonia water was added dropwise at a certain rate to adjust the pH of the system to 8.0-9.0, resulting in SiO2-aramid fiber gel. The synergistic effect of stirring and ultrasound was used to prevent aramid fiber agglomeration, forming a uniform suspension system. Adding ammonia water to the reaction system created an alkaline environment, accelerating the polycondensation reaction and allowing SiO2 to form a cross-linked three-dimensional network structure with octahydroxybutylated cage-like polysilsesquioxane. This ultimately yielded a three-dimensional interpenetrating network gel with aramid fiber as the skeleton and SiO2 / POSS as the matrix. Controlling the rate of ammonia water addition prevented localized pH abrupt changes that could lead to uneven gel structure, ensuring the compactness of the matrix. In step S104, SiO2-aramid fiber gel is injected into a mold with progressively increasing thickness along the thickness direction. A vacuum pressure of -0.1 MPa is applied to remove air bubbles inside the gel. Simultaneously, a gradient magnetic field is applied, with the magnetic field increasing linearly from 0.2T to 0.5T along the gradient direction. Then, a gradient pressure is applied, increasing from 0.05 MPa to 0.15 MPa along the magnetic field gradient direction. The oriented gel is dried at 60-80℃ under normal pressure for 6-10 hours and finally naturally cooled to room temperature to obtain a silica aerogel-aramid fiber composite material with gradient density. Vacuum adsorption can promote the oriented alignment of aramid fibers along the mold direction, improving the anisotropic mechanical properties and thermal insulation properties of the material. During the vacuum-assisted molding process, the magnetic field induces the magnetic fibers to be precisely oriented along the magnetic field direction, resulting in low packing density in the low magnetic field region and high packing density in the high magnetic field region, achieving gradient fiber alignment.

[0024] The density of the silica aerogel-aramid fiber composite material prepared above exhibits a gradient change from low density (0.15-0.25 g / cm³) to high density (0.35-0.5 g / cm³).

[0025] The following describes in detail the silica aerogel-aramid fiber composite material with gradient density and its preparation method provided in this application, with reference to examples and comparative examples.

[0026] Example 1: (1) Place the para-aramid fiber in deionized water and ultrasonically clean it for 20 min to remove surface impurities. Then, vacuum dry it at 60℃ for 3 h. After that, impregnate it with a 2% KH560 coupling agent solution for 1 h and add dilute hydrochloric acid to adjust the pH of the system to 4.0. After impregnation, dry it at 80℃ to obtain the modified aramid fiber. (2) The modified aramid fiber was dispersed in deionized water, and ferrous chloride and ferric chloride were added in a molar ratio of 1:2. Under N2 protection, ammonia was added dropwise to adjust the pH of the system to 10.0. The temperature was raised to 70℃ and kept constant for 1 h. After the reaction was completed, the mixture was cooled to room temperature, filtered, and washed 5 times alternately with deionized water and anhydrous ethanol. After washing, the mixture was dried under vacuum at 60℃ for 2 h to obtain Fe3O4 nanoparticle modified aramid fiber. The amount of the mixture of ferrous chloride and ferric chloride added was 5% of the mass of the modified aramid fiber. (3) Fe3O4 nanoparticles modified aramid fibers were dispersed in anhydrous ethanol and ultrasonically dispersed at 300W for 15 min to obtain a fiber suspension with a mass concentration of 0.8%. A mixed sol was prepared according to the molar ratio of tetraethyl orthosilicate: anhydrous ethanol: deionized water: octahydroxybutylated cage polysilsesquioxane = 1:4:3.5:0.2. The fiber suspension was then added dropwise to the mixed sol (the mass of the mixed sol was 15% of the mass of the fiber suspension). At the same time, the mixture was magnetically stirred at 500 rpm and ultrasonically treated at 300W for 10 min. Then, dilute hydrochloric acid was added to adjust the pH of the system to 4.0. The system was heated to 40℃ for pre-gelation for 1 h. Then, the temperature was raised to 70℃ and ammonia was added dropwise at a rate of 0.5 mL / min to adjust the pH of the system to 9.0 to obtain SiO2-aramid fiber gel. (4) The SiO2-aramid fiber gel was injected into a mold with a thickness that gradually increases along the thickness direction. A vacuum pressure of -0.1MPa was applied to remove air bubbles inside the gel. At the same time, a gradient magnetic field was applied, which increased linearly from 0.2T to 0.5T along the gradient direction. Then, a gradient pressure was applied, which increased from 0.05MPa to 0.15MPa along the gradient direction of the magnetic field. The gel after orientation was dried at 60℃ under normal pressure for 7h and then naturally cooled to room temperature to obtain a silica aerogel-aramid fiber composite material with a density that varies from 0.15 to 0.45g / cm³.

[0027] Example 2: (1) Place the para-aramid fiber in deionized water and ultrasonically clean it for 30 min to remove surface impurities. Then, vacuum dry it at 55℃ for 2 h. After that, impregnate it with a 1.5% KH560 coupling agent solution for 1 h and add dilute hydrochloric acid to adjust the pH of the system to 3.0. After impregnation, dry it at 70℃ to obtain the modified aramid fiber. (2) The modified aramid fiber was dispersed in deionized water, and ferrous sulfate and ferric sulfate were added in a molar ratio of 1:3. Under N2 protection, ammonia was added dropwise to adjust the pH of the system to 10.0. The temperature was raised to 80℃ and kept constant for 2 hours. After the reaction was completed, the mixture was cooled to room temperature, filtered, and washed 5 times alternately with deionized water and anhydrous ethanol. After washing, the mixture was dried under vacuum at 60℃ for 2 hours to obtain Fe3O4 nanoparticle modified aramid fiber. The amount of the mixture of ferrous sulfate and ferric sulfate added was 6% of the mass of the modified aramid fiber. (3) Fe3O4 nanoparticles modified aramid fibers were dispersed in anhydrous ethanol and ultrasonically dispersed at 300W for 15 min to obtain a fiber suspension with a mass concentration of 1%. A mixed sol was prepared according to the molar ratio of tetraethyl orthosilicate: anhydrous ethanol: deionized water: octahydroxybutylated cage polysilsesquioxane = 1:3.5:5:0.3. The fiber suspension was then added dropwise to the mixed sol (the mass of the mixed sol was 10% of the mass of the fiber suspension). At the same time, the mixture was magnetically stirred at 500 rpm and ultrasonically treated at 300W for 10 min. Then, dilute hydrochloric acid was added to adjust the pH of the system to 4.0. The system was heated to 45℃ for pregelation for 2 h. Then, the temperature was raised to 80℃ and ammonia was added dropwise at a rate of 0.5 mL / min to adjust the pH of the system to 8.0 to obtain SiO2-aramid fiber gel. (4) The SiO2-aramid fiber gel was injected into a mold with a thickness that gradually increases along the thickness direction. A vacuum pressure of -0.1MPa was applied to remove air bubbles inside the gel. At the same time, a gradient magnetic field was applied, which increased linearly from 0.2T to 0.5T along the gradient direction. Then, a gradient pressure was applied, which increased from 0.05MPa to 0.15MPa along the gradient direction of the magnetic field. The gel after orientation was dried at 70℃ under normal pressure for 6 hours and then naturally cooled to room temperature to obtain a silica aerogel-aramid fiber composite material with a density that varies from 0.2 to 0.4 g / cm³.

[0028] Example 3: (1) Place the para-aramid fiber in deionized water and ultrasonically clean it for 25 min to remove surface impurities. Then, vacuum dry it at 60°C for 3 h. After that, impregnate it with a 2% KH560 coupling agent solution for 2 h and add dilute hydrochloric acid to adjust the pH of the system to 4.0. After impregnation, dry it at 75°C to obtain the modified aramid fiber. (2) The modified aramid fiber was dispersed in deionized water, and ferrous chloride and ferric chloride were added in a molar ratio of 1:3. Under N2 protection, ammonia was added dropwise to adjust the pH of the system to 10.0. The temperature was raised to 80℃ and kept constant for 1 h. After the reaction was completed, the mixture was cooled to room temperature, filtered, and washed three times alternately with deionized water and anhydrous ethanol. After washing, the mixture was dried under vacuum at 60℃ for 2 h to obtain Fe3O4 nanoparticle modified aramid fiber. The amount of the mixture of ferrous salt and ferric salt added was 5% of the mass of the modified aramid fiber. (3) Fe3O4 nanoparticles modified aramid fibers were dispersed in anhydrous ethanol and ultrasonically dispersed at 300W for 15 min to obtain a fiber suspension with a mass concentration of 0.8%. A mixed sol was prepared according to the molar ratio of tetraethyl orthosilicate: anhydrous ethanol: deionized water: octahydroxybutylated cage polysilsesquioxane = 1:4:5:0.1. The fiber suspension was then added dropwise to the mixed sol (the mass of the mixed sol was 16% of the mass of the fiber suspension). At the same time, the mixture was magnetically stirred at 500 rpm and ultrasonically treated at 300W for 10 min. Then, dilute hydrochloric acid was added to adjust the pH of the system to 4.0. The system was heated to 40℃ for pre-gelation for 1.5 h. Then, the temperature was raised to 75℃ and ammonia was added dropwise at a rate of 1 mL / min to adjust the pH of the system to 9.0 to obtain SiO2-aramid fiber gel. (4) The SiO2-aramid fiber gel was injected into a mold with a thickness that gradually increases along the thickness direction. A vacuum pressure of -0.1MPa was applied to remove air bubbles inside the gel. At the same time, a gradient magnetic field was applied, which increased linearly from 0.2T to 0.5T along the gradient direction. Then, a gradient pressure was applied, which increased from 0.05MPa to 0.15MPa along the gradient direction of the magnetic field. The gel after orientation was dried at 80℃ under normal pressure for 6 hours. Finally, it was naturally cooled to room temperature to obtain a silica aerogel-aramid fiber composite material with a density that showed a gradient change from 0.25-0.5g / cm³.

[0029] Comparative Example 1: (1) Aramid fibers were dispersed in deionized water, and ferrous chloride and ferric chloride were added in a molar ratio of 1:2. Under N2 protection, ammonia was added dropwise to adjust the pH of the system to 10.0. The temperature was raised to 70℃ and kept constant for 1 h. After the reaction was completed, the mixture was cooled to room temperature, filtered, and washed 5 times alternately with deionized water and anhydrous ethanol. After washing, the mixture was dried under vacuum at 60℃ for 2 h to obtain Fe3O4 nanoparticle-modified aramid fibers. The amount of the mixture of ferrous salt and ferric salt added was 5% of the mass of the aramid fibers. (2) Fe3O4 nanoparticles modified aramid fibers were dispersed in anhydrous ethanol and ultrasonically dispersed at 300W for 15 min to obtain a fiber suspension with a mass concentration of 0.8%. A mixed sol was prepared according to the molar ratio of tetraethyl orthosilicate: anhydrous ethanol: deionized water: octahydroxybutylated cage polysilsesquioxane = 1:4:3.5:0.2. The fiber suspension was then added dropwise to the mixed sol (the mass of the mixed sol was 15% of the mass of the fiber suspension). At the same time, the mixture was magnetically stirred at 500 rpm and ultrasonically treated at 300W for 10 min. Then, dilute hydrochloric acid was added to adjust the pH of the system to 4.0. The system was heated to 40℃ for pre-gelation for 1 h. Then, the temperature was raised to 70℃ and ammonia was added dropwise at a rate of 0.5 mL / min to adjust the pH of the system to 9.0 to obtain SiO2-aramid fiber gel. (3) The SiO2-aramid fiber gel was injected into a custom mold with a thickness that increases sequentially along the thickness direction. A vacuum pressure of -0.1MPa was applied to remove air bubbles inside the gel. At the same time, a gradient magnetic field was applied, which increased linearly from 0.2T to 0.5T along the gradient direction. Then, a gradient pressure was applied, which increased from 0.05MPa to 0.15MPa along the gradient direction of the magnetic field. The gel after orientation was dried at 60℃ under normal pressure for 7h and then naturally cooled to room temperature to obtain a silica aerogel-aramid fiber composite material with a density that varies from 0.15 to 0.45g / cm³.

[0030] Comparative Example 2: (1) Place the para-aramid fiber in deionized water and ultrasonically clean it for 20 min to remove surface impurities. Then, vacuum dry it at 60℃ for 3 h. After that, impregnate it with a 2% KH560 coupling agent solution for 1 h and add dilute hydrochloric acid to adjust the pH of the system to 4.0. After impregnation, dry it at 80℃ to obtain the modified aramid fiber. (2) The modified aramid fiber was dispersed in anhydrous ethanol and ultrasonically dispersed for 15 min at 300 W to obtain a fiber suspension with a mass concentration of 0.8%. A mixed sol was prepared according to the molar ratio of tetraethyl orthosilicate: anhydrous ethanol: deionized water: octahydroxybutylated cage polysilsesquioxane = 1:4:3.5:0.2. The fiber suspension was then added dropwise to the mixed sol (the mass of the mixed sol was 15% of the mass of the fiber suspension). At the same time, the mixture was magnetically stirred at 500 rpm and ultrasonically treated for 10 min at 300 W. Then, dilute hydrochloric acid was added to adjust the pH of the system to 4.0. The system was heated to 40℃ for pregelation for 1 h. Then, the temperature was raised to 70℃ and ammonia was added dropwise at a rate of 0.5 mL / min to adjust the pH of the system to 9.0, thus obtaining SiO2-aramid fiber gel. (3) Inject the SiO2-aramid fiber gel into a mold with increasing thickness along the thickness direction, apply a vacuum pressure of -0.1MPa to remove air bubbles inside the gel, and then dry the oriented gel at 60℃ under normal pressure for 7h. Finally, cool it naturally to room temperature to obtain the silica aerogel-aramid fiber composite material.

[0031] Comparative Example 3: (1) Place the para-aramid fiber in deionized water and ultrasonically clean it for 20 min to remove surface impurities. Then, vacuum dry it at 60℃ for 3 h. After that, impregnate it with a 2% KH560 coupling agent solution for 1 h and add dilute hydrochloric acid to adjust the pH of the system to 4.0. After impregnation, dry it at 80℃ to obtain the modified aramid fiber. (2) The modified aramid fiber was dispersed in deionized water, and ferrous chloride and ferric chloride were added in a molar ratio of 1:2. Under N2 protection, ammonia was added dropwise to adjust the pH of the system to 10.0. The temperature was raised to 70℃ and kept constant for 1 h. After the reaction was completed, the mixture was cooled to room temperature, filtered, and washed 5 times alternately with deionized water and anhydrous ethanol. After washing, the mixture was dried under vacuum at 60℃ for 2 h to obtain Fe3O4 nanoparticle modified aramid fiber. The amount of the mixture of ferrous salt and ferric salt added was 5% of the mass of the modified aramid fiber. (3) Fe3O4 nanoparticles modified aramid fibers were dispersed in anhydrous ethanol and ultrasonically dispersed at 300W for 15 min to obtain a fiber suspension with a mass concentration of 0.8%. A mixed sol was prepared according to the molar ratio of tetraethyl orthosilicate: anhydrous ethanol: deionized water: KH560 coupling agent = 1:4:3.5:0.2. The fiber suspension was then added dropwise to the mixed sol (the mass of the mixed sol was 15% of the mass of the fiber suspension). At the same time, the mixture was magnetically stirred at 500 rpm and ultrasonically treated at 300W for 10 min. Then, dilute hydrochloric acid was added to adjust the pH of the system to 4.0. The system was heated to 40℃ for pre-gelation for 1 h. Then, the temperature was raised to 70℃ and ammonia was added dropwise at a rate of 0.5 mL / min to adjust the pH of the system to 9.0 to obtain SiO2-aramid fiber gel. (4) The SiO2-aramid fiber gel was injected into a mold with a thickness that gradually increases along the thickness direction. A vacuum pressure of -0.1MPa was applied to remove air bubbles inside the gel. At the same time, a gradient magnetic field was applied, which increased linearly from 0.2T to 0.5T along the gradient direction. Then, a gradient pressure was applied, which increased from 0.05MPa to 0.15MPa along the gradient direction of the magnetic field. The gel after orientation was dried at 60℃ under normal pressure for 7h. Finally, it was naturally cooled to room temperature to obtain a silica aerogel-aramid fiber composite material with a density that showed a gradient change from 0.15-0.4g / cm³.

[0032] The performance of the composite materials prepared in Examples 1-3 and Comparative Examples 1-3 was tested, and the results are shown in Table 1.

[0033] Table 1: Performance test results of the composite materials prepared in Examples 1-3 and Comparative Examples 1-3 ; As can be seen from the data in Examples 1-3 and Comparative Example 1 in Table 1, the tensile strength, fracture strength retention rate, and peel strength of the composite material obtained without modification of aramid fibers all decreased. The applicant analyzed that this was because the interfacial bonding force between aramid fibers and the SiO2 / POSS matrix was weak in the composite material obtained without modification of aramid fibers, and the fibers and matrix were easily separated during the composite process, thereby reducing the mechanical properties and stability of the material.

[0034] As can be seen from the data in Examples 1-3 and Comparative Example 2 in Table 1, the flame retardant properties of the composite material obtained without the introduction of Fe3O4 nanoparticles decreased. The applicant analyzed that this was because the fiber orientation in the composite material obtained without the introduction of Fe3O4 nanoparticles was uneven, and the char layer formed after combustion was prone to cracking, thus affecting the flame retardant properties.

[0035] As can be seen from the data in Table 1 of Examples 1-3 and Comparative Example 3, the mechanical properties of the composite material obtained by replacing the octahydroxybutylated cage-like polysilsesquioxane with KH560 coupling agent decreased. The applicant analyzed that this was because replacing it with KH560 coupling agent could not form a stable three-dimensional interpenetrating network structure, thus affecting the mechanical properties of the material.

[0036] In the description of this specification, the references to terms such as "one embodiment / mode," "some embodiments / modes," "example," "specific example," or "some examples," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment / mode or example is included in at least one embodiment / mode or example of this application. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment / mode or example. Moreover, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments / modes or examples. Furthermore, without contradiction, those skilled in the art can combine and integrate the different embodiments / modes or examples described in this specification, as well as the features of different embodiments / modes or examples.

[0037] It should be noted that in this application, relational terms such as "first" and "second" are used merely to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. Without further limitations, an element defined by the phrase "comprising one..." does not exclude the presence of other identical elements in the process, method, article, or apparatus that includes said element. In this application, "a plurality of" means at least two, such as two, three, etc., unless otherwise expressly specified.

[0038] The above description is merely a specific embodiment of this application, enabling those skilled in the art to understand or implement this application. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of this application. Therefore, this application is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features claimed herein.

Claims

1. A method for preparing a silica aerogel-aramid fiber composite material with gradient density, characterized in that, Includes the following steps: S101, aramid fibers are modified using silane coupling agents to obtain modified aramid fibers; S102, Fe3O4 nanoparticles were modified on the surface of the modified aramid fiber by coprecipitation method to obtain Fe3O4 nanoparticle modified aramid fiber. S103, Fe3O4 nanoparticles modified aramid fibers were dispersed in anhydrous ethanol and ultrasonically dispersed to obtain a fiber suspension. The fiber suspension was added to a mixed sol composed of tetraethyl orthosilicate, anhydrous ethanol, deionized water and octahydroxybutylated cage-like polysilsesquioxane. The mixture was magnetically stirred and ultrasonically treated. Then, dilute hydrochloric acid was added to adjust the pH of the system to 3.0-4.

0. The system was heated to 35-45℃ for pre-gelling. Then, the temperature was raised to 70-80℃ and ammonia was added to adjust the pH of the system to 8.0-9.0 to obtain SiO2-aramid fiber gel. S1014, SiO2-aramid fiber gel is injected into a mold with progressively increasing thickness along the thickness direction, a gradient magnetic field and gradient pressure are applied, and the mixture is dried at normal pressure and cooled to room temperature to obtain a silica aerogel-aramid fiber composite material with gradient density.

2. The method for preparing the silica aerogel-aramid fiber composite material with gradient density according to claim 1, characterized in that, In step S101, the process of modifying aramid fibers using silane coupling agents is as follows: aramid fibers are added to deionized water for ultrasonic cleaning, vacuum dried after cleaning, then impregnated with KH560 coupling agent solution, and dilute hydrochloric acid is added to adjust the pH of the system to 3.0-4.

0. After impregnation, the fibers are dried to obtain modified aramid fibers.

3. The method for preparing the silica aerogel-aramid fiber composite material with gradient density according to claim 1, characterized in that, In step S102, the process of modifying the surface of the modified aramid fiber with Fe3O4 nanoparticles using the coprecipitation method is as follows: the modified aramid fiber is dispersed in deionized water, a mixture of ferrous salt and ferric salt is added, ammonia is added dropwise under nitrogen protection to adjust the pH value to 9.0-10.0, the temperature is raised to 70-80℃, after the reaction is completed, the fiber is cooled and filtered, and washed alternately with deionized water and anhydrous ethanol. After washing, the fiber is vacuum dried to obtain Fe3O4 nanoparticle-modified aramid fiber.

4. The method for preparing the silica aerogel-aramid fiber composite material with gradient density according to claim 3, characterized in that, Ferrous salts are selected from ferrous chloride or ferrous nitrate, and ferric salts are selected from ferric chloride or ferric nitrate.

5. The method for preparing the silica aerogel-aramid fiber composite material with gradient density according to claim 3, characterized in that, The amount of the mixture of ferrous salt and ferric salt added is 3%-8% of the mass of the modified aramid fiber.

6. The method for preparing the silica aerogel-aramid fiber composite material with gradient density according to claim 1, characterized in that, The molar ratio of tetraethyl orthosilicate, anhydrous ethanol, deionized water and octahydroxybutylated cage-like polysilsesquioxane is 1:3-10:3-5:0.1-0.

3.

7. The method for preparing the silica aerogel-aramid fiber composite material with gradient density according to claim 1, characterized in that, In step S103, the mass concentration of the fiber suspension is 0.5%-1%.

8. The method for preparing the silica aerogel-aramid fiber composite material with gradient density according to claim 1, characterized in that, In step S103, the mass of the mixed sol is 10%-20% of the mass of the fiber suspension.

9. The method for preparing the silica aerogel-aramid fiber composite material with gradient density according to claim 1, characterized in that, In step S104, the temperature for atmospheric pressure drying is 60-80℃.

10. A silica aerogel-aramid fiber composite material with gradient density, characterized in that, It is prepared by the preparation method according to any one of claims 1-9.

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