Preparation method of ceramic aerogel and ceramic aerogel

Abstract

This application discloses a method for preparing ceramic aerogels. The method includes dispersing cellulose nanosheets in water, adding silicon carbide nanoparticles, and mixing and dispersing to obtain a cellulose-silicon carbide mixed dispersion; freezing the cellulose-silicon carbide mixed dispersion and then freeze-drying it to obtain a cellulose-silicon carbide composite aerogel; dispersing graphene oxide in water to obtain a graphene oxide dispersion; immersing the cellulose-silicon carbide composite aerogel in the graphene oxide dispersion, performing vacuum-assisted infiltration, and then freeze-drying it to obtain a precursor composite aerogel; and subjecting the precursor composite aerogel to high-temperature heat treatment under an inert atmosphere to reduce the graphene oxide to graphene, thereby obtaining a cellulose-silicon carbide-graphene composite aerogel. The ceramic aerogel prepared in this application possesses multiple properties, including lightweight, high-temperature resistance, and electromagnetic wave attenuation, effectively meeting the electromagnetic protection requirements of aerospace and high-temperature electronic equipment.

Description

Technical Field

[0001] This application relates to the field of ceramic aerogel preparation technology, and in particular to a method for preparing ceramic aerogel and ceramic aerogel. Background Technology

[0002] With the rapid development of 5G, the Internet of Things, high-power electronic devices, and aerospace technology, electronic devices are evolving rapidly towards higher frequencies, higher integration, miniaturization, and higher power density. In this process, while the dense coverage and high-frequency transmission of electromagnetic signals have greatly improved information exchange efficiency, they have also triggered unprecedented electromagnetic interference (EMI) problems. Electromagnetic radiation can cause signal crosstalk, data errors, and even functional failures in precision electronic components, seriously affecting the reliability and stability of equipment. Long-term exposure to high-intensity electromagnetic radiation environments also poses a potential threat to human health.

[0003] In addition, the service environment of modern high-end equipment is becoming increasingly harsh, especially in the fields of hot-end components of aerospace engines, high-power microwave devices, and thermal management systems for new energy vehicle batteries. Electromagnetic shielding materials often need to work stably for a long time under high temperature or even extreme thermal shock environments.

[0004] However, most mainstream electromagnetic shielding materials on the market today use polymers (such as epoxy resin, polyimide, and silicone rubber) as the matrix, filled with carbon-based or metallic conductive fillers. Although these materials have advantages such as good processability and low density, they are still essentially organic polymers with limited heat resistance. Under high-temperature environments (usually above 250℃-300℃), the polymer matrix is ​​prone to softening, melting, or even thermal decomposition and carbonization, leading to the collapse of the internal conductive network and a sharp decline or even complete failure of shielding effectiveness. In addition, most polymers are inherently flammable, and their application is strictly limited in fields such as aerospace where flame retardancy requirements are extremely high.

[0005] While metallic materials (such as copper, aluminum, silver, and their alloys) have long been widely used as electromagnetic shielding materials due to their excellent electrical conductivity, their high density (heavy weight) contradicts the extreme pursuit of "lightweight" in aerospace and portable electronic devices. Furthermore, metallic materials are highly susceptible to oxidation and corrosion in complex environments such as humidity and acid / alkali conditions, leading to a decrease in surface conductivity and thus weakening shielding performance. More critically, the electromagnetic shielding mechanism of metallic materials relies primarily on high reflection of electromagnetic waves. While this "reflection-dominated" shielding mechanism can block electromagnetic wave penetration, it cannot eliminate electromagnetic waves, and may even cause serious secondary electromagnetic pollution, interfering with the normal operation of other sensitive electronic devices in the vicinity. Summary of the Invention

[0006] This application provides a method for preparing ceramic aerogel and the ceramic aerogel, aiming to solve the problem that existing battery shielding materials cannot simultaneously possess multiple properties such as lightweight, high temperature resistance, and effective attenuation of electromagnetic waves, thus failing to meet the electromagnetic protection requirements of aerospace and high-temperature electronic equipment.

[0007] In a first aspect, this application provides a method for preparing ceramic aerogel, the method comprising the following steps: Cellulose nanosheets were dispersed in water, and silicon carbide nanoparticles were added and mixed to obtain a cellulose-silicon carbide mixed dispersion. The cellulose-silicon carbide mixed dispersion was freeze-formed and then freeze-dried to obtain a cellulose-silicon carbide composite aerogel. Graphene oxide was dispersed in water to obtain a graphene oxide dispersion. The cellulose-silicon carbide composite aerogel was immersed in the graphene oxide dispersion, and after vacuum-assisted permeation, it was freeze-dried to obtain the precursor composite aerogel. The precursor composite aerogel was subjected to high-temperature heat treatment under an inert atmosphere to reduce graphene oxide to graphene, thereby obtaining a cellulose-silicon carbide-graphene composite aerogel.

[0008] In some embodiments, the mass ratio of the cellulose nanosheets to the silicon carbide nanoparticles is 1:1 to 1:4.

[0009] In some embodiments, the cellulose nanosheets have a diameter of 20 nm to 50 nm and a length of 5 μm to 10 μm; the silicon carbide nanoparticles have a particle size of 20 nm to 200 nm.

[0010] In some embodiments, when the cellulose-silicon carbide mixed dispersion is cryogenically molded, the cryogenic temperature is -10°C to -50°C, and the cryogenic time is 6 hours to 48 hours.

[0011] In some embodiments, when the freeze-formed cellulose-silicon carbide mixed dispersion is further freeze-dried, the freeze-drying temperature is -40°C to -60°C, the freeze-drying pressure is ≤10Pa, and the freeze-drying time is 24h to 72h.

[0012] In some embodiments, the concentration of the graphene oxide dispersion is from 1 mg / mL to 10 mg / mL.

[0013] In some embodiments, when the cellulose-silicon carbide composite aerogel is immersed in the graphene dispersion by vacuum-assisted infiltration, the vacuum-assisted infiltration time is 20 min to 90 min.

[0014] In some embodiments, when freeze-drying is performed after vacuum-assisted permeation, the freeze-drying temperature is -40°C to -60°C and the freeze-drying time is 12h to 48h.

[0015] In some embodiments, the high-temperature heat treatment is performed at a temperature of 300°C to 1000°C, for a duration of 1 hour to 3 hours, with a heating rate of 2°C / min to 10°C / min, and the inert atmosphere is argon or nitrogen.

[0016] On the other hand, this application also proposes a ceramic aerogel, which is prepared by the method for preparing ceramic aerogel as described in any one of the first aspects.

[0017] Unlike existing technologies, this application provides a method for preparing ceramic aerogels. Using cellulose nanosheets as a framework template, a three-dimensional porous network structure is constructed by interweaving SiC particles through hydrogen bonding. This unique structural design not only results in ceramic aerogels with extremely low density (less than 50 mg / cm³), meeting the stringent requirements for lightweight materials in aerospace and other fields, but also provides abundant multiple reflective interfaces through the cellulose framework. Combined with the continuous conductive network constructed from graphene, this significantly enhances the material's ability to attenuate electromagnetic waves, achieving excellent performance with an effective absorption bandwidth exceeding 6 GHz and a shielding effectiveness greater than 40 dB while maintaining extremely low density.

[0018] Secondly, this invention introduces silicon carbide (SiC), which possesses excellent high-temperature stability, as a high-temperature dielectric loss phase, fundamentally overcoming the technical bottleneck of traditional polymer-based shielding materials' easy decomposition and failure under high-temperature environments. SiC particles form a stable composite structure with cellulose and graphene, endowing the aerogel framework with outstanding thermal stability. Tests show that after heating at 250°C for 2 hours, this material still maintains its complete framework structure and effective electromagnetic shielding performance, greatly expanding the application scenarios of electromagnetic shielding materials in high-temperature and extreme thermal environments.

[0019] Furthermore, this invention utilizes a process of "freeze-drying to construct a cellulose-SiC framework-graphene composite" to establish a multiple loss mechanism, transforming the traditional primarily reflective shielding mechanism into one primarily absorbent, effectively preventing secondary electromagnetic pollution. Specifically, SiC particles provide dielectric loss capability at high temperatures, graphene uniformly coating the framework surface provides high conductivity to enhance ohmic loss, and the cellulose framework provides structural support and increases interfacial polarization. The synergistic effect of these three components achieves a wide-bandwidth, strong-absorption electromagnetic protection effect. Simultaneously, this preparation process is simple and controllable, with mild reaction conditions. It not only solves the technical challenges of graphene's easy agglomeration and difficulty in uniform dispersion but also achieves precise control over the material's microstructure, demonstrating promising prospects for industrial application.

[0020] Additional aspects and advantages of the embodiments of this application will be described or shown in part in the following description, or illustrated by practice of the embodiments of this application. Attached Figure Description

[0021] One or more embodiments are illustrated by way of example with reference to the accompanying drawings, which are not intended to limit the embodiments, and elements having the same reference numerals in the drawings are designated as similar elements.

[0022] Figure 1 This is a flowchart of a method for preparing ceramic aerogel according to this application. Detailed Implementation

[0023] 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 described below with reference to the accompanying drawings. Obviously, the described embodiments are some embodiments of this application, but not all embodiments.

[0024] In this application, the reference to "embodiment" means that a particular feature, structure, or characteristic described in connection with an embodiment may be included in at least one embodiment of this application. The appearance of this phrase in various places in the specification does not necessarily refer to the same embodiment, nor is it a separate or alternative embodiment that is mutually exclusive with other embodiments.

[0025] In the description of the embodiments of this application, technical terms such as "first" and "second" are used only to distinguish different objects and should not be construed as indicating or implying relative importance or implicitly specifying the number, specific order, or primary and secondary relationship of the indicated technical features. In the description of the embodiments of this application, "multiple" means two or more, unless otherwise explicitly defined.

[0026] In the description of the embodiments of this application, directional terms such as up, down, left, right, front, back, front, back, top, and bottom are defined relative to the structures shown in the accompanying drawings. The terms "inner" and "outer" refer to directions toward or away from the geometric center of a specific component, respectively. These are relative concepts and may therefore vary depending on their location and usage. Therefore, these or other directional terms should not be interpreted as restrictive terms.

[0027] The technical features involved in the different embodiments of this application described below can be combined with each other as long as they do not conflict with each other.

[0028] For your convenience, please refer to [the following]. Figure 1 This application provides a method for preparing ceramic aerogel, characterized in that the preparation method includes the following steps: Step S1: Disperse cellulose nanosheets in water, add silicon carbide nanoparticles, mix and disperse to obtain a cellulose-silicon carbide mixed dispersion.

[0029] Cellulose nanosheets are a type of natural one-dimensional nanomaterial with advantages such as wide availability, high aspect ratio, and rich hydroxyl content on the surface. In this application, cellulose nanosheets serve as a framework template, intertwining with silicon carbide particles through hydrogen bonding to construct a three-dimensional network structure. This three-dimensional network structure, after subsequent freeze-drying, forms a lightweight, porous composite aerogel framework. Cellulose itself can simultaneously serve as a structural support and mechanical reinforcement phase. Silicon carbide (SiC), as a wide-bandgap semiconductor ceramic material, possesses excellent high-temperature stability, low density, and good dielectric loss characteristics. As a high-temperature resistant dielectric loss phase, silicon carbide imparts high-temperature stability to the aerogel framework, enabling the material to maintain effective electromagnetic shielding performance under extreme thermal environments.

[0030] Cellulose nanosheets are rich in hydroxyl groups on their surface, while silicon carbide nanoparticles have silanol groups on their surface. The two form a stable interwoven network structure through hydrogen bonding, preventing the aggregation and sedimentation of the silicon carbide particles. Cellulose nanosheets are dispersed in water, and then silicon carbide nanoparticles are added. The mixture is then thoroughly mixed using ultrasonic dispersion and mechanical stirring to obtain a stable cellulose-silicon carbide dispersion.

[0031] In some embodiments, the solvent used to disperse cellulose nanosheets and silicon carbide nanoparticles in step S1 is water; water, as a dispersion medium, enables the cellulose nanosheets and silicon carbide nanoparticles to be uniformly dispersed, facilitating the subsequent construction of a three-dimensional network structure through hydrogen bonding. The water can be any one of deionized water, distilled water, or purified water, preferably deionized water; deionized water can reduce the influence of impurity ions on the dispersion system, ensuring the stability and uniformity of the dispersion.

[0032] In some embodiments, the mass ratio of cellulose nanosheets to silicon carbide nanoparticles is from 1:1 to 1:4. For example, the mass ratio of cellulose nanosheets to silicon carbide nanoparticles can be 1:1, 1:2, 1:3, or 1:4. If the proportion of cellulose nanosheets is too high, the silicon carbide content will be insufficient, and the improvement in the high-temperature stability of the aerogel will be limited. If the proportion of cellulose nanosheets is too low, the cellulose framework cannot form a complete three-dimensional porous network, and the mechanical integrity and structural stability of the aerogel will decrease. Within this mass ratio range, cellulose nanosheets and silicon carbide nanoparticles can synergistically construct a complete three-dimensional porous framework, while taking into account both high-temperature stability and lightweight properties.

[0033] In some embodiments, the diameter of the cellulose nanosheets ranges from 20 nm to 50 nm, and the length ranges from 5 μm to 10 μm. For example, the diameter of the cellulose nanosheets can be 20 nm, 30 nm, 40 nm, or 50 nm, and the length can be 5 μm, 6 μm, 7 μm, 8 μm, 9 μm, or 10 μm. Cellulose nanosheets in this size range have a high aspect ratio, which is beneficial for forming a stable three-dimensional network structure and avoids insufficient skeleton strength due to excessively small size or uneven dispersion due to excessively large size.

[0034] In some embodiments, the particle size of silicon carbide nanoparticles is from 20 nm to 200 nm. For example, the particle size of silicon carbide nanoparticles can be 20 nm, 110 nm, or 200 nm. Silicon carbide nanoparticles within this size range can be uniformly dispersed in the three-dimensional network composed of cellulose nanosheets and fill the pores of the framework, avoiding dispersion difficulties due to excessively large particle size or agglomeration caused by excessively small particle size. At the same time, silicon carbide nanoparticles within this size range have a large specific surface area and abundant interfacial polarization effects, which are beneficial to enhancing the dielectric loss capability of aerogel against electromagnetic waves.

[0035] Step S2: After freezing the cellulose-silicon carbide mixed dispersion into a solid state, freeze-dry it to obtain a cellulose-silicon carbide composite aerogel.

[0036] Cryoforming refers to placing a cellulose-silicon carbide mixed dispersion in a low-temperature environment, allowing the solvent (water) to crystallize and form an ice crystal template. During the freezing process, cellulose nanosheets and silicon carbide nanoparticles are squeezed into the gaps between the ice crystals, gradually condensing and self-assembling to form a three-dimensional network structure. After cryoforming is completed, the ice crystals are directly sublimated under low pressure and low temperature conditions through freeze drying, thereby leaving an interconnected porous structure inside the material to obtain a cellulose-silicon carbide composite aerogel.

[0037] Freeze-forming and freeze-drying processes are the core steps in constructing a three-dimensional porous network of cellulose-silicon carbide composite aerogels. By controlling the size and morphology of the ice crystal template, the pore structure, porosity, and skeleton morphology of the aerogel can be controlled, thereby affecting the material's density, mechanical properties, and electromagnetic wave multiple reflection loss capability.

[0038] In some embodiments, when the cellulose-silicon carbide mixed dispersion is cryogenically molded, the cryogenic temperature is -10°C to -50°C, and the cryogenic time is 6 hours to 48 hours. For example, the cryogenic temperature is -10°C and the cryogenic time is 48 hours, or the cryogenic temperature is -30°C and the cryogenic time is 26 hours, or the cryogenic temperature is -50°C and the cryogenic time is 6 hours. If the cryogenic temperature is too high or the time is too short, the ice crystal growth is insufficient, and the aerogel pore structure is incomplete. If the cryogenic temperature is too low or the time is too long, the ice crystal size is too small or too large, and the pore structure is difficult to control. Within this temperature and time range, a uniform and interconnected porous structure can be formed, which is beneficial for the uniform loading of graphene and the multiple reflection losses of electromagnetic waves.

[0039] Step S3: Disperse graphene oxide in water to obtain a graphene oxide dispersion.

[0040] Graphene oxide (GO) is a precursor to graphene, with abundant oxygen-containing functional groups (such as hydroxyl, carboxyl, and epoxy groups) on its surface. It exhibits good hydrophilicity and water dispersibility, enabling it to form stable dispersions in water. In this embodiment, by dispersing graphene oxide in water and then subjecting it to ultrasonic treatment, a uniform and stable graphene oxide dispersion can be obtained, facilitating subsequent uniform loading onto the framework surface of a cellulose-silicon carbide composite aerogel via an impregnation process.

[0041] In some embodiments, the concentration of the graphene oxide dispersion is from 1 mg / mL to 10 mg / mL. For example, the concentration of the graphene oxide dispersion is 1 mg / mL, 5.5 mg / mL, or 10 mg / mL. If the concentration of the graphene oxide dispersion is too low, the graphene loading on the surface of the aerogel after impregnation will be insufficient, making it difficult to form a continuous conductive network and affecting the electromagnetic shielding effectiveness. If the concentration of the graphene oxide dispersion is too high, the graphene oxide sheets are prone to agglomeration, making it difficult to uniformly coat the surface of the aerogel framework. Within this concentration range, graphene oxide can be uniformly dispersed and fully penetrate into the porous structure of the aerogel, providing a basis for the subsequent formation of a continuous graphene conductive network.

[0042] Step S4: Immerse the cellulose-silicon carbide composite aerogel in the graphene oxide dispersion, and after vacuum-assisted permeation, freeze-dry to obtain the precursor composite aerogel.

[0043] The cellulose-silicon carbide composite aerogel prepared in step S2 was immersed in the graphene oxide dispersion prepared in step S3. Vacuum-assisted infiltration was used to allow the graphene oxide dispersion to fully penetrate into the three-dimensional porous network of the aerogel. The vacuum environment eliminated air from the aerogel pores, allowing the graphene oxide dispersion to fully penetrate into each pore of the aerogel under the action of capillary force and pressure difference, achieving uniform loading of graphene oxide on the surface of the aerogel framework. Subsequently, freeze-drying was performed to remove moisture, resulting in a precursor composite aerogel with graphene oxide uniformly coated on the surface of the cellulose-silicon carbide framework.

[0044] In some embodiments, the vacuum-assisted infiltration time is between 20 and 90 minutes. If the infiltration time is too short, the graphene oxide dispersion cannot fully penetrate the pores inside the aerogel, resulting in uneven loading; if the infiltration time is too long, it may damage the porous structure of the aerogel. Within this time range, it is possible to ensure that the graphene oxide dispersion fully penetrates and is uniformly loaded on the surface of the aerogel framework, while maintaining the integrity of the aerogel structure.

[0045] It should be noted that the specific time for vacuum-assisted permeation is determined by ensuring that the graphene oxide dispersion fully wets the internal pores of the aerogel. Those skilled in the art can determine the appropriate permeation time by observing the color change of the aerogel mass and the escape of bubbles, based on the pore structure of the aerogel, the concentration and viscosity of the graphene oxide dispersion. In some embodiments of this application, when the concentration of the graphene oxide dispersion is between 1 mg / mL and 10 mg / mL, the vacuum-assisted permeation time is between 20 min and 90 min.

[0046] In some embodiments, after vacuum-assisted infiltration, freeze-drying is performed at a temperature of -40°C to -60°C for a time of 12 hours to 48 hours. Under these conditions, the moisture introduced by impregnation can be sufficiently removed, preventing the aerogel structure from collapsing due to incomplete drying, while maintaining a uniform coating of graphene oxide on the framework surface.

[0047] It's important to note that the freeze-drying temperature should be below the triple point of water to ensure that ice crystals sublimate directly without passing through the liquid phase. Temperatures that are too low (below -60°C) may reduce freeze-drying efficiency, while temperatures that are too high (above -40°C) may cause the ice crystals to melt, damaging the porous structure of the aerogel. Furthermore, the freeze-drying time should be determined when the sample is completely dry, meaning its weight no longer changes. A freeze-drying time that is too short (less than 12 hours) will result in residual moisture, which can damage the aerogel structure during subsequent high-temperature heat treatment; a freeze-drying time that is too long (more than 48 hours) will increase process costs.

[0048] Those skilled in the art can determine the appropriate freeze-drying time within the time range of 12h to 48h in the foregoing embodiments by monitoring changes in sample weight, based on the sample's size, thickness, and moisture content.

[0049] Step S5: The precursor composite aerogel is subjected to high-temperature heat treatment under an inert atmosphere to reduce graphene oxide to graphene, thereby obtaining cellulose-silicon carbide-graphene composite aerogel.

[0050] Under high-temperature conditions, the oxygen-containing functional groups on the surface of graphene oxide undergo thermal desorption, reducing the graphene oxide to graphene and forming a continuous conductive network on the surface of the cellulose-silicon carbide framework. The reduced graphene exhibits excellent conductivity and dielectric loss, significantly enhancing the aerogel's ability to attenuate electromagnetic waves. Simultaneously, during the high-temperature heat treatment, the cellulose nanosheets carbonize, transforming into a carbon framework with certain conductivity and mechanical strength, further enhancing the aerogel's structural stability and electromagnetic shielding performance.

[0051] In some embodiments, the high-temperature heat treatment temperature is 300°C to 1000°C, the heat treatment time is 1 hour to 3 hours, the heating rate is 2°C / min to 10°C / min, and the inert atmosphere is argon or nitrogen. If the heat treatment temperature is too low (below 300°C) or the time is too short (less than 1 hour), the reduction of graphene oxide will be insufficient, the conductive network will be incomplete, and the electromagnetic shielding effectiveness will be affected. If the heat treatment temperature is too high (above 800°C) or the time is too long (more than 3 hours), it may cause excessive shrinkage or even collapse of the aerogel framework, destroying the porous structure. Within this temperature and time, graphene oxide can be fully reduced to graphene while maintaining the porous structure and mechanical integrity of the aerogel. Controlling the heating rate within the range of 2°C / min to 10°C / min can avoid cracking or collapse of the aerogel structure due to thermal stress caused by excessively rapid heating.

[0052] Secondly, this application also provides a ceramic aerogel, which is prepared by the ceramic aerogel preparation method as described in any embodiment of the first aspect.

[0053] The following describes embodiments of this application. The embodiments described below are exemplary and are only used to explain this application, and should not be construed as limiting this application. Where specific techniques or conditions are not specified in the embodiments, they are performed according to the techniques or conditions described in the literature in this field or according to the product instructions. Reagents or instruments used, unless otherwise specified, are all conventional products that can be obtained commercially.

[0054] Example 1 (1) Preparation of cellulose-silicon carbide mixed dispersion: Take 0.5 g of cellulose nanosheets (diameter 20-50 nm, length 5-10 μm), add to 100 mL of deionized water, and ultrasonically disperse for 30 min to obtain a uniform cellulose dispersion (concentration 5 mg / mL). Weigh 1.0 g of silicon carbide nanoparticles (average particle size 50 nm), add to the above cellulose dispersion, magnetically stir for 2 h, and ultrasonically disperse for 30 min to obtain a cellulose-silicon carbide mixed dispersion (cellulose to silicon carbide mass ratio 1:2).

[0055] (2) Preparation of cellulose-silicon carbide composite aerogel: The above cellulose-silicon carbide mixed dispersion was poured into a mold and frozen in a -20℃ refrigerator for 12h. Then it was placed in a freeze dryer and freeze-dried at -50℃ and pressure <10 Pa for 48h to obtain cellulose-silicon carbide composite aerogel.

[0056] (3) Preparation of graphene oxide dispersion: Weigh 0.3g of graphene oxide (GO), add it to 100mL of deionized water, and ultrasonically exfoliate for 2h to obtain a GO dispersion of 3mg / mL.

[0057] (4) Preparation of precursor composite aerogel: The above cellulose-silicon carbide composite aerogel was immersed in GO dispersion and vacuum-assisted permeation for 30 min. After being taken out, it was freeze-dried again at -50℃ for 24 h to obtain precursor composite aerogel.

[0058] (5) High temperature heat treatment: The above precursor composite aerogel was placed in a tube furnace and heated to 600°C at a heating rate of 5°C / min under argon atmosphere protection. The temperature was held for 2 hours and then cooled to room temperature to obtain cellulose-silicon carbide-graphene composite aerogel.

[0059] (6) Performance testing: The density of the composite aerogel is 32 mg / cm³. Its electromagnetic shielding effectiveness in the X-band (8-12 GHz) is 45.6 dB, with an effective absorption bandwidth of 6.8 GHz (9.2-16.0 GHz). After heat treatment at 250℃ for 2 hours, the shielding effectiveness retention rate is 92.3%, and after exposure to a flame at 1200℃ for 10 seconds, the shielding effectiveness retention rate is 82.5%.

[0060] Example 2 (1) Preparation of cellulose-silicon carbide mixed dispersion: Take 0.8 g of cellulose nanosheets (diameter 20-50 nm, length 5-10 μm), add to 100 mL of deionized water, and ultrasonically disperse for 30 min. Weigh 2.4 g of silicon carbide nanoparticles (average particle size 100 nm), add to the above dispersion, magnetically stir for 3 h, and ultrasonically disperse for 40 min to obtain cellulose-silicon carbide mixed dispersion (cellulose to silicon carbide mass ratio 1:3).

[0061] (2) Preparation of cellulose-silicon carbide composite aerogel: The above cellulose-silicon carbide mixed dispersion was poured into a mold and frozen in a -40℃ refrigerator for 24 h. Then it was placed in a freeze dryer and freeze-dried at -60℃ and pressure <10 Pa for 48 h to obtain cellulose-silicon carbide composite aerogel.

[0062] (3) Preparation of graphene oxide dispersion: Weigh 0.5g of graphene oxide (GO), add it to 100mL of deionized water, and ultrasonically exfoliate for 2h to obtain a graphene oxide dispersion of 5mg / mL.

[0063] (4) Preparation of precursor composite aerogel: The above cellulose-silicon carbide composite aerogel was immersed in graphene oxide dispersion and vacuum-assisted permeation for 60 min. After being taken out, it was freeze-dried at -60℃ for 24 h to obtain precursor composite aerogel.

[0064] (5) High temperature heat treatment: The above precursor composite aerogel was placed in a tube furnace and heated to 800°C at a heating rate of 5°C / min under nitrogen atmosphere protection. The temperature was held for 2 hours and then cooled to room temperature to obtain cellulose-silicon carbide-graphene composite aerogel.

[0065] (6) Performance testing: The density of the composite aerogel is 38 mg / cm³. Its electromagnetic shielding effectiveness in the X-band (8-12 GHz) is 52.3 dB, with an effective absorption bandwidth of 7.5 GHz (8.5-16.0 GHz). After heat treatment at 250℃ for 2 hours, the shielding effectiveness retention rate is 94.5%, and after exposure to a flame at 1200℃ for 10 seconds, the shielding effectiveness retention rate is 84.3%.

[0066] Comparative Example 1 (Cellulose-graphene composite aerogel without silicon carbide) (1) Preparation of cellulose dispersion: Take 0.5g of cellulose nanosheets (diameter 20-50nm, length 5-10 μm), add to 100mL of deionized water, and ultrasonically disperse for 30min to obtain a uniform cellulose dispersion (concentration 5mg / mL).

[0067] (2) Preparation of cellulose aerogel: The above cellulose dispersion was poured into a mold and frozen at -20℃ for 12h. Then it was placed in a freeze dryer and freeze-dried at -50℃ and pressure <10 Pa for 48h to obtain cellulose aerogel.

[0068] (3) Preparation of graphene oxide dispersion: Weigh 0.3g of graphene oxide (GO), add it to 100mL of deionized water, and ultrasonically exfoliate for 2h to obtain a GO dispersion of 3mg / mL.

[0069] (4) Preparation of precursor composite aerogel: The above cellulose aerogel was immersed in GO dispersion and vacuum-assisted permeation for 30 min. After being taken out, it was freeze-dried at -50℃ for 24 h to obtain precursor composite aerogel.

[0070] (5) High temperature heat treatment: The above precursor composite aerogel is placed in a tube furnace and heated to 600°C at a heating rate of 5°C / min under argon atmosphere protection. It is held for 2 hours and then cooled to room temperature to obtain cellulose-graphene composite aerogel (without silicon carbide).

[0071] (6) Performance testing: The density of the composite aerogel is 25 mg / cm³. Its electromagnetic shielding effectiveness in the X-band (8-12 GHz) is 35.8 dB, with an effective absorption bandwidth of 4.2 GHz (10.8-15.0 GHz). After heat treatment at 250℃ for 2 hours, the shielding effectiveness retention rate is 70%; after exposure to a flame at 1200℃ for 10 seconds, the shielding effectiveness retention rate is 44.2%.

[0072] Performance testing The composite aerogels prepared in Example 1, Example 2, and Comparative Example 1 were tested as follows.

[0073] Density test: The mass of the aerogel was weighed using an electronic balance, the dimensions of the composite aerogel were measured using vernier calipers and the volume was calculated, and the density was calculated based on the mass-to-volume ratio.

[0074] Electromagnetic shielding effectiveness test: Using the waveguide method or flange coaxial method, the composite aerogel sample is placed in the test fixture, and the electromagnetic wave scattering parameters (S-parameters) are measured by a vector network analyzer. The electromagnetic shielding effectiveness (SE) is calculated based on the transmission coefficient.

[0075] Effective absorption bandwidth test: Based on the electromagnetic shielding effectiveness test results, the frequency range with shielding effectiveness greater than 20 dB (i.e., more than 90% of electromagnetic waves are attenuated) is defined as the effective absorption bandwidth.

[0076] High-temperature stability test: The composite aerogel sample was heated in a 250℃ oven for 2 hours, and after natural cooling, the electromagnetic shielding effectiveness was tested, and the shielding effectiveness retention rate was calculated (shielding effectiveness after heating / initial shielding effectiveness × 100%). The aerogel sample was exposed to a 1200℃ flame for 10 seconds, and after natural cooling, the electromagnetic shielding effectiveness was tested, and the shielding effectiveness retention rate was calculated.

[0077] The test results of the ceramic aerogels prepared in Example 1, Example 2 and Comparative Example 1 are shown in Table 1.

[0078] Table 1

[0079] As shown in Table 1, in terms of electromagnetic shielding effectiveness, the shielding effectiveness of Example 1 and Example 2 is 45.6 dB and 52.3 dB, respectively, and the effective absorption bandwidth is 6.8 GHz and 7.5 GHz, respectively. Both are significantly better than Comparative Example 1 (35.8 dB / 4.2 GHz), indicating that the introduction of silicon carbide particles effectively enhances the attenuation ability of aerogel to electromagnetic waves through interface polarization and dielectric loss.

[0080] Regarding high-temperature stability, the shielding effectiveness retention rates of Examples 1 and 2 after heating at 250℃ for 2 hours were 92.3% and 94.5%, respectively, and after exposure to a flame at 1200℃ for 10 seconds, the retention rates were 82.5% and 84.3%, respectively, significantly better than Comparative Example 1 (70% after 250℃ and 44.2% after 1200℃). This indicates that the introduction of silicon carbide particles significantly improves the high-temperature stability of the aerogel, enabling the material to maintain effective electromagnetic shielding performance under extreme thermal environments. Comparative Example 1, lacking silicon carbide, experienced pyrolysis and shrinkage of the cellulose skeleton at high temperatures, resulting in damage to the graphene conductive network and a sharp decline in shielding effectiveness.

[0081] In summary, this application utilizes a ternary composite of cellulose nanosheets, silicon carbide nanoparticles, and graphene oxide. The process involves freeze-drying to construct a three-dimensional porous framework, impregnating and loading graphene oxide, and then thermally reducing it to graphene at high temperatures. This approach achieves a synergistic balance between broadband strong absorption (shielding effectiveness >45dB, effective bandwidth >6GHz) and high thermal stability (maintaining effective shielding even after heating to 250℃ and exposure to a 1200℃ flame). Cellulose nanosheets form the three-dimensional porous framework and provide multiple reflective interfaces; silicon carbide particles impart excellent high-temperature stability and dielectric loss capability; and graphene constructs a continuous conductive network to enhance ohmic loss. The synergistic effect of these three components enables the ceramic aerogel of this application to maintain lightweight properties while possessing both broadband, high-efficiency electromagnetic shielding and extreme thermal stability. This provides a novel material solution for electromagnetic protection of aerospace and high-temperature electronic equipment in extreme thermal environments.

[0082] In this application, the mass ratio of cellulose nanosheets to silicon carbide nanoparticles can be adjusted within a certain range. When the proportion of cellulose nanosheets is high (e.g., a mass ratio of 1:1), the cellulose skeleton is more complete, which is beneficial for forming a lightweight porous structure and providing abundant multiple reflective interfaces, thereby enhancing the attenuation capability of electromagnetic waves. When the proportion of silicon carbide nanoparticles is high (e.g., a mass ratio of 1:4), silicon carbide imparts better high-temperature stability to the aerogel, while further enhancing dielectric loss through interfacial polarization. Within the above-mentioned mass ratio range, the integrity of the cellulose skeleton, the dielectric loss capability of silicon carbide, and the synergistic effect between the two show a monotonic changing trend, without any unpredictable abrupt changes. Therefore, based on the content disclosed in Example 1 (mass ratio 1:2) and Example 2 (mass ratio 1:3) of this application, those skilled in the art can reasonably expect that the technical effects of this application can be achieved within the range of 1:1 to 1:4, and the specific implementation within this range can be determined without excessive effort.

Claims

1. A method for preparing ceramic aerogel, characterized in that, The preparation method includes the following steps: Cellulose nanosheets were dispersed in water, and silicon carbide nanoparticles were added and mixed to obtain a cellulose-silicon carbide mixed dispersion. The cellulose-silicon carbide mixed dispersion was freeze-formed and then freeze-dried to obtain a cellulose-silicon carbide composite aerogel. Graphene oxide was dispersed in water to obtain a graphene oxide dispersion. The cellulose-silicon carbide composite aerogel was immersed in the graphene oxide dispersion, and after vacuum-assisted permeation, it was freeze-dried to obtain the precursor composite aerogel. The precursor composite aerogel was subjected to high-temperature heat treatment under an inert atmosphere to reduce graphene oxide to graphene, thereby obtaining a cellulose-silicon carbide-graphene composite aerogel.

2. The method as described in claim 1, characterized in that, The mass ratio of the cellulose nanosheets to the silicon carbide nanoparticles is 1:1 to 1:

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3. The method as described in claim 1, characterized in that, The cellulose nanosheets have a diameter of 20 nm to 50 nm and a length of 5 μm to 10 μm; the silicon carbide nanoparticles have a particle size of 20 nm to 200 nm.

4. The method as described in claim 1, characterized in that, When freezing the cellulose-silicon carbide mixed dispersion, the freezing temperature is -10℃ to -50℃, and the freezing time is 6h to 48h.

5. The method as described in claim 1, characterized in that, When the freeze-formed cellulose-silicon carbide mixed dispersion is further freeze-dried, the freeze-drying temperature is -40℃ to -60℃, the freeze-drying pressure is ≤10Pa, and the freeze-drying time is 24h to 72h.

6. The method as described in claim 1, characterized in that, The concentration of the graphene oxide dispersion is from 1 mg / mL to 10 mg / mL.

7. The method as described in claim 1, characterized in that, When the cellulose-silicon carbide composite aerogel is immersed in the graphene dispersion by vacuum-assisted infiltration, the vacuum-assisted infiltration time is 20 min to 90 min.

8. The method as described in claim 1, characterized in that, When freeze-drying is performed after vacuum-assisted permeation, the freeze-drying temperature is -40℃ to -60℃ and the freeze-drying time is 12h to 48h.

9. The method as described in claim 1, characterized in that, The high-temperature heat treatment is performed at a temperature of 300°C to 1000°C, for a duration of 1 hour to 3 hours, with a heating rate of 2°C / min to 10°C / min. The inert atmosphere is either argon or nitrogen.

10. A ceramic aerogel, characterized in that, It is prepared by the method of any one of claims 1-9 for preparing ceramic aerogel.

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