Flame-retardant high-toughness bio-based composite aerogel as well as preparation method and application thereof

Abstract

The present application relates to the technical field of aerogel, and discloses a kind of flame-retardant high toughness bio-based composite aerogel and its preparation method and application.Utilize cellulose acetate nanofiber and sodium alginate oxidation hemiacetal crosslinking, jointly construct the main structure of aerogel, then introduce and substrate between mutual action bio-based flame retardant phytic acid and thermal resistance response material expandable graphite, while enhancing the flame retardancy and mechanical properties of aerogel, construct stable continuous high conductive network, finally utilize directional freezing combining microwave vacuum freeze drying technology, and make flame-retardant high toughness bio-based composite aerogel.The preparation process of the present application is simple, reaction condition is controllable, the composite aerogel prepared has stable structure, high flexibility, excellent flame-retardant and heat-insulating, fire early warning response is sensitive, has good biocompatibility, degradability, has application potential in flame-retardant and heat-insulating material, fire early warning material, intelligent fire monitoring material, building heat-insulating and heat-preserving material and the like field.

Description

Technical Field

[0001] This invention relates to the field of aerogel technology, specifically to a flame-retardant, high-toughness bio-based composite aerogel, its preparation method, and its applications. Background Technology

[0002] Bio-based aerogels have attracted widespread attention due to their abundant raw material sources, low cost, environmental friendliness, and biodegradability. However, bio-based aerogels are flammable and pose a significant fire risk. Bio-based aerogels prepared using traditional methods are difficult to control in terms of structure and shape, generally exhibiting problems such as insufficient mechanical strength, low deformation recovery rate, and low electrical conductivity. Furthermore, traditional fire warning devices have poor compatibility with aerogel materials and are susceptible to environmental interference, severely limiting their application in the field of safety protection [Deng P, Liu X, Li Y, et al. Konjac glucomannan-based aerogels with excellent thermal stability and flame retardancy for thermal insulation application[J]. International Journal of Biological Macromolecules, 2024,254: 127814]. With the increasing demand for intelligent safety protection, the development of multifunctional aerogel materials that combine flame retardant properties with fire warning functions has become a trend. Bio-based aerogels, made from cellulose, chitosan, starch, protein, sodium alginate, and other raw materials, not only retain the inherent characteristics of high porosity and low density of aerogels but also possess the biocompatibility and environmental friendliness of natural polymer materials, making them an ideal alternative to traditional organic / inorganic aerogels. However, bio-based materials themselves lack flame retardant properties. Generally, methods such as adding inorganic flame retardants, phosphorus-based flame retardants, and bio-based flame retardants are used to promote the formation of a stable carbon layer, thereby improving their thermal stability and fire resistance at high temperatures. Guan et al. used sodium alginate (SA) as the matrix and introduced methyltrimethoxysilane (MTMS) as the crosslinking agent. They constructed a stable crosslinking network by silanization in situ crosslinking combined with unidirectional cryogenic casting, and prepared a low-temperature superelastic SA-based aerogel with anisotropic layered / honeycomb structure. This significantly improved the mechanical strength and thermal properties of the material. However, the aerogel material could not provide an early warning response to fire [Guan F, Feng S, Sun J, et al. Low-temperature superelastic, anisotropic, silane-crosslinked sodiumalginate aerogel for thermal insulation[J]. International Journal of Biological Macromolecules, 2024, 262: 129800].

[0003] Thermally triggered fire early warning sensors trigger alarms based on the principle that the resistance of materials decreases sharply with increasing temperature (thermal resistance effect). They provide a highly sensitive response to hazard signals, much faster than traditional sensors (including smoke detectors and infrared detectors). Currently, the thermal reduction properties of carbon-based nanomaterials such as carbon nanotubes, graphene oxide, and expandable graphite are widely used to prepare carbon-based composite aerogels that achieve rapid temperature response in the early stages of a fire. Expandable graphite (EG), in particular, is a highly efficient flame retardant and has electrical conductivity; its resistance decreases with increasing temperature, and it expands at high temperatures to form a dense carbon layer that isolates heat and oxygen. Liu et al. used expandable graphite (EG) and sodium carboxymethyl cellulose (CMC) as raw materials to construct a Janus aerogel with a heterostructure of electrical and thermal conductivity through freeze drying and metal ion crosslinking technology, achieving good flame retardant properties and rapid fire warning capability [Liu Y, Cheng F, Li K, et al. Lightweight, flame retardant Janus carboxymethyl cellulose aerogel with fire-warning properties for smart sensor[J]. Carbohydrate Polymers, 2024, 328:121730]. Chinese invention patent CN116874872A discloses a bio-based aerogel, its preparation method, and its applications. The method involves hydrolyzing an organoalkoxysilane in an acidic solution to obtain an organosilicon sol, which is then added to a dispersion of biomass (at least one of bacterial cellulose, microcrystalline cellulose, chitosan, alginate, and gelatin) to form a mixed solution. This mixed solution is then mixed with a dispersion of functional nanomaterials (at least one of aminated carbon nanotubes, graphene oxide, titanium carbide, and titanium nitride) and freeze-molded to obtain a bio-based aerogel with good mechanical properties, strong hydrophobicity, excellent flame retardancy, and functions including fire warning, piezoresistive sensing, and self-cleaning. However, the bio-based composite aerogel material obtained in the above research still faces a series of key challenges in practical applications. Its internal three-dimensional network structure often lacks stability, making it prone to deformation or collapse under external forces or complex environmental conditions. The mechanical properties of the material also urgently need improvement, exhibiting low resilience after compression and insufficient overall flexibility. Furthermore, when these aerogel materials are used for fire early warning functions, their effective signal response and continuous warning time are generally short, making it difficult to meet the needs of real-world scenarios for long-term, stable monitoring. Therefore, there is an urgent need to develop novel, highly sensitive fire early warning flame-retardant and high-toughness bio-based composite aerogel materials. Summary of the Invention

[0004] The purpose of this invention is to overcome the shortcomings of existing technologies and provide a flame-retardant, high-toughness bio-based composite aerogel, its preparation method, and applications. The aerogel's main structure is constructed by cross-linking cellulose acetate nanofibers with sodium alginate hemiacetal. Phytic acid, a bio-based flame retardant that interacts with the matrix, and expandable graphite, a thermally responsive material, are then introduced. This enhances the aerogel's flame retardancy and mechanical properties while building a stable and continuous highly conductive network. Finally, directional freezing combined with microwave vacuum freeze-drying technology is used to prepare the flame-retardant, high-toughness bio-based composite aerogel. The preparation process of this invention is simple, and the reaction conditions are controllable. The prepared composite aerogel exhibits stable structure, high flexibility, excellent flame retardancy and thermal insulation, and sensitive fire early warning response. It also possesses good biocompatibility, degradability, and environmental friendliness, showing potential applications in flame-retardant and thermal insulation materials, fire early warning materials, intelligent fire monitoring materials, and building thermal insulation materials.

[0005] To achieve the above objectives, the technical solution adopted by the present invention is as follows: A method for preparing a flame-retardant, high-toughness bio-based composite aerogel includes the following steps: Step (1): Preparation of CA nanofiber dispersion Cellulose acetate (CA) was dissolved in a mixed solvent of acetone and N,N-dimethylacetamide to prepare a cellulose acetate spinning solution. The cellulose acetate spinning solution was electrospun and dried to obtain a cellulose acetate nanofiber membrane. The cellulose acetate nanofiber membrane was pulverized and homogenized in water to obtain a CA nanofiber dispersion. Step (2): Preparation of sodium oxidized alginate Sodium alginate was added to 1-ethyl-3-methylimidazolium acetate ionic liquid and stirred continuously until sodium alginate was completely dissolved to obtain sodium alginate / ionic liquid solution. The sodium alginate / ionic liquid solution was adjusted to pH 4.0, potassium periodate was added, and an oxidation reaction was carried out. After purification, oxidized sodium alginate (OSA) was obtained. Step (3): Prepare CA / OSA / PA / EG mixed dispersion Sodium oxidized alginate was added to the CA nanofiber dispersion for mixing and reaction, and then phytic acid (PA) was added and stirred to dissolve, resulting in a CA / OSA / PA mixed dispersion. Powdered expandable graphite (EG) was added to the CA / OSA / PA mixed dispersion to obtain the CA / OSA / PA / EG mixed dispersion; Step (4): Preparation of CA / OSA / PA / EG composite aerogel The CA / OSA / PA / EG mixed dispersion was crosslinked and then freeze-dried in a directional manner to obtain a flame-retardant, high-toughness bio-based composite aerogel, denoted as CA / OSA / PA / EG composite aerogel.

[0006] Preferably, in step (1): the mass concentration of cellulose acetate in the cellulose acetate spinning solution is 12-26%, and the volume ratio of acetone to N,N-dimethylacetamide is 1-4:1.

[0007] Furthermore, in step (1), the acetyl content of cellulose acetate is 20.5-46.1%, and the hydroxyl content is 1.5-4.5%.

[0008] Preferably, in step (1), the electrospinning conditions are: electrospinning at a temperature of 20-40℃, a relative humidity of 60-75%, a spinning distance of 12-26cm, and a spinning voltage of 10-25kV.

[0009] Preferably, in step (1), the pulverization and homogenization operation includes: pulverizing the cellulose acetate nanofiber membrane and dispersing it in water to form a dispersion, using a cell disruptor to ultrasonically pulverize it at a power of 180-460W for 15-40 minutes, and then using a high-speed shear homogenizer with a rotation speed of 10000-30000rpm to shear homogenize the dispersion.

[0010] Preferably, in step (1), the mass concentration of CA nanofibers in the CA nanofiber dispersion is 0.2-1.2%.

[0011] Preferably, in step (2), the dissolution temperature is 80-95℃; the mass concentration of sodium alginate in the sodium alginate / ionic liquid solution is 0.8-2%.

[0012] Preferably, in step (2), the mass concentration of potassium periodate in the reaction solution is 0.5-2.5 g / L; the oxidation reaction conditions are: oxidation reaction for 30-90 min in a light-proof environment and under ultrasonic treatment conditions; wherein, the ultrasonic treatment is intermittent ultrasonic treatment, specifically: after each ultrasonic treatment for 15 min, a 1 min pause is taken, and the power of the ultrasonic treatment is 80-150 W.

[0013] Preferably, in step (2), the purification process includes: after adding ethanol to terminate the reaction, filtering to remove insoluble matter, placing the resulting filtrate into a dialysis bag with a molecular weight cutoff of 8-14 kD, dialyzing with running water for 3-4 days to remove residual oxidant and ionic liquid, concentrating the filtrate by rotary evaporation to precipitate the product, washing thoroughly with ethanol, and vacuum drying.

[0014] Preferably, in step (2), the aldehyde content at the C2 and C3 positions of sodium alginate is 14.52-33.27%, the viscosity-average molecular weight is 82,600-184,900, and the water solubility is 0.65-10.13 g / 100 mL.

[0015] Preferably, in step (3), the mixing reaction conditions are as follows: 0.1-0.2 mol / L of acetate / sodium acetate buffer solution is added dropwise to the CA nanofiber dispersion to adjust the pH of the solution to 4.0-5.5, and then sodium alginate is added. The mixture is stirred continuously at 25-75°C for 1-4 hours to achieve uniform mixing reaction. The stirring and dissolving conditions are as follows: the mixture is stirred continuously at 25-75°C for 2-4 hours to dissolve.

[0016] Preferably, in step (3), the mass ratio of CA nanofibers to sodium oxidized alginate is 1-3:1-3; and the mass concentration of phytic acid in the CA / OSA / PA mixed dispersion is 0.5-3%.

[0017] Preferably, in step (3), the mass concentration of expandable graphite in the CA / OSA / PA / EG mixed dispersion is 0.15 to 1.2%.

[0018] Preferably, in step (4), the crosslinking reaction conditions are: crosslinking reaction for 0.5 to 3 hours under microwave radiation power of 200 to 320 W and microwave radiation temperature of 45 to 65 °C.

[0019] Preferably, in step (4), the directional freeze-drying includes: transferring the reacted CA / OSA / PA / EG mixed dispersion into a mold, placing the mold on the upper surface of a copper block, and immersing the bottom of the copper block completely in liquid nitrogen for directional freezing. After freezing for 6 to 15 minutes, the CA / OSA / PA / EG mixed dispersion is completely frozen into ice. Then, the mold is transferred to a microwave vacuum freeze dryer and continuously freeze-dried for 20 to 60 hours under the conditions of microwave vacuum freeze-drying temperature of -70 to -55°C, microwave power of 960 to 1800W, and vacuum degree of 12 to 20Pa.

[0020] Preferably, a flame-retardant, high-toughness bio-based composite aerogel is prepared using the method described above.

[0021] Preferably, the application of the flame-retardant, high-toughness bio-based composite aerogel as described above in flame-retardant heat insulation materials, fire early warning materials, intelligent fire monitoring materials, or building heat insulation materials.

[0022] Compared with the prior art, the beneficial effects of the present invention are as follows: 1. This invention utilizes cellulose acetate nanofibers and sodium alginate hemiacetal crosslinking to construct the main structure of an aerogel. Phytic acid, a bio-based flame retardant that interacts with the matrix, and expandable graphite, a thermally responsive material, are then introduced. This enhances the flame retardancy and mechanical properties of the aerogel while constructing a stable and continuous highly conductive network. Finally, a flame-retardant, high-toughness bio-based composite aerogel is produced using directional freezing combined with microwave vacuum freeze-drying technology. The preparation process of this invention is simple, and the reaction conditions are controllable. The prepared composite aerogel exhibits stable structure, high flexibility, excellent flame retardancy and thermal insulation, and sensitive fire early warning response. It also possesses good biocompatibility, degradability, and environmental friendliness, showing potential applications in flame-retardant and thermal insulation materials, fire early warning materials, intelligent fire monitoring materials, and building thermal insulation materials.

[0023] 2. This invention uses electrospinning technology to prepare cellulose acetate (CA) nanofibers. By controlling the electrospinning conditions (voltage, solvent type, spinning solution concentration, receiving distance, etc.), CA nanofibers with small and uniform diameters are obtained. The hydroxyl groups on the CA nanofiber molecular chains can form hemiacetal chemical bonds with the aldehyde groups in the sodium alginate oxidized molecules. Furthermore, the acetyl and hydroxyl groups in the CA nanofiber molecules can form hydrogen bonds with the hydroxyl and carboxyl groups in the sodium alginate oxidized (OSA) molecules. The formation of multi-site interaction forces between the two components helps to construct a stable three-dimensional network framework, significantly improving the structural stability and mechanical properties of the aerogel material.

[0024] This invention uses phytic acid (PA), a bio-based flame retardant, as a crosslinking agent. The cyclohexanehexylene backbone (inositol ring) in the PA molecule serves as the linker for phosphate groups, forming a stable cyclic hexaphosphate ester structure. The six phosphate groups (-H₂PO₄) contained within have extremely strong electronegativity and the ability to chelate metal ions. The phosphate groups of PA can respectively bind with the acetyl and hydroxyl groups on CA nanofibers and the hydroxyl and carboxyl groups and positively charged sodium ions (Na₂O₃) in oxidized sodium alginate. +The introduction of polyphosphoric acid (PA) enhances the cross-linking effect between CA nanofibers and the sodium alginate matrix through hydrogen bonds, coordination bonds, and electrostatic interactions. This improves the strength and flexibility of the CA / SA aerogel network skeleton, enhancing the stability and durability of the aerogel in complex environments. When heated, the PA in the aerogel matrix decomposes to produce a large amount of polyphosphoric acid, promoting matrix dehydration and carbonization, reducing the formation of combustible small molecule products. The resulting char layer covers the aerogel surface, isolating oxygen and heat from the internal matrix, effectively preventing the spread of combustion. Simultaneously, the high phosphorus content in phytic acid molecules can capture highly reactive free radicals generated during combustion in the gas phase, interrupting the chain reaction of combustion and further inhibiting fire development, giving the aerogel material excellent flame-retardant properties. The bio-based composite aerogel does not use halogens, antimony oxide, or other substances added in conventional flame retardants during its preparation, and produces no toxic or corrosive gases, making it environmentally friendly and with broad application prospects.

[0025] 3. This invention uses 1-ethyl-3-methylimidazolium acetate ([EMIM][Ac]) ionic liquid to dissolve sodium alginate. The ionic liquid contains the positive ion [EMIM]+ and the negative ion [Ac]. － The process involves reacting with oxygen atoms and sodium ions in the hydroxyl groups of sodium alginate molecules, respectively, to rapidly dissolve the sodium alginate in an ionic liquid by breaking the intermolecular hydrogen bonds. This direct dissolution significantly reduces the degradation of the sodium alginate molecular chain during dissolution, and the ionic liquid is recyclable, making it environmentally friendly. Adding potassium periodate to oxidize chitosan under ultrasonic conditions creates ultrasonic cavitation, generating high pressure, dispersion, and activation effects in the reaction solution, improving reactant activity and oxidation efficiency, shortening the oxidation reaction time, and achieving uniform and controllable oxidation. This effectively reduces the oxidative degradation of sodium alginate, increases product yield, and prepares water-soluble oxidized sodium alginate that retains the good properties of the macromolecule, expanding the application range of oxidized sodium alginate. Simultaneously, it effectively avoids the drawbacks of traditional water bath heating methods for sodium alginate oxidation, such as long reaction time, low product yield, and severe oxidative degradation of sodium alginate.

[0026] 4. This invention introduces thermally responsive expandable graphite (EG) into an aerogel matrix. Through the formation of various hydrogen bonds between the polar groups (acetyl, hydroxyl, carboxyl, etc.) in CA nanofibers and sodium alginate molecules and the oxygen-containing functional groups (hydroxyl, epoxy, carboxyl) on EG sheets, uniform dispersion and strong cross-linking of EG in the aerogel network are achieved, avoiding the problems of easy agglomeration, weak interfacial bonding, and poor compatibility of EG inorganic fillers. By precisely controlling the amount of EG added, a three-dimensional network with a stable structure and high conductivity can be constructed. When exposed to high temperatures in a fire, EG can rapidly expand to form a dense expanded carbon layer, exerting a physical heat insulation and flame retardant effect. Moreover, the expansion of EG sheets can quickly form a more compact and coherent conductive path, expanding the conductive network inside the aerogel and improving its conductivity, effectively enhancing the thermal resistance response capability of the aerogel, and significantly improving the sensitivity of fire early warning and the duration of continuous alarm.

[0027] This invention involves adding different masses of expandable graphite (EG) powder to a CA / OSA / PA mixed solution and then reacting it under microwave irradiation. Due to the rapid heating speed, short reaction time, and good uniformity of microwave irradiation, the EG sheets achieve sufficient contact with the molecules of CA, OSA, and PA, promoting hydrogen bonding between the oxygen-containing functional groups (hydroxyl, epoxy, and carboxyl groups) on the EG sheets and the polar groups such as acetyl, hydroxyl, carboxyl, and phosphate groups in the CA, OSA, and PA molecules. Simultaneously, the microwave radiation enhances the energy absorption of the EG sheets and the CA, OSA, and PA molecules, promoting the movement between molecular chain segments and accelerating the multi-site cross-linking rate between the EG sheets and the CA, OSA, and PA molecules. This process is simple, saves reaction time, and reduces energy consumption.

[0028] 5. This invention employs liquid nitrogen-assisted directional freeze-drying technology. Utilizing the temperature gradient generated by rapid freezing with liquid nitrogen, water in the aerogel precursor is induced to crystallize directionally along the temperature gradient. The gradually forming ice crystal pillars compress other non-aqueous components to adjacent regions. After sublimation, the ice crystals form a unique continuous porous structure with a longitudinally ordered arrangement and a transverse honeycomb pattern. This structure effectively disperses stress under external compression, enhancing the mechanical stability of the matrix and facilitating the preparation of composite aerogel materials with excellent mechanical strength. Simultaneously, during directional freezing, CA nanofibers are not only interwoven between parallel micropores, but also embedded abundantly on the pore wall surface, enhancing the structural strength of the aerogel pore walls and providing a solid framework support for the entire 3D conductive framework. Furthermore, the CA nanofibers penetrating between adjacent pore walls act as bridging structures, effectively dispersing stress during compression and bending, thus dissipating energy and improving the mechanical durability and elastic recovery of the aerogel. This avoids the defects of traditional random freezing methods, where the aerogel pore structure is randomly arranged, failing to uniformly disperse stress under external force, easily leading to framework collapse and damage. In addition, CA nanofibers are rich in hydroxyl groups, which can be hydrogen bonded to OSA and PA, thereby improving the overall skeletal stability of the aerogel. OSA, as a physical crosslinking agent, effectively enhances the interfacial bonding between EG sheets and CA nanofibers. PA crosslinking improves the mechanical strength of the aerogel skeleton, while EG sheets construct the 3D conductive network of the aerogel.

[0029] This invention involves directional freeze-drying of a CA / OSA / PA / EG composite aerogel, followed by microwave vacuum freeze-drying. Combining microwave and vacuum drying leverages their respective advantages, allowing the ice crystals generated within the composite aerogel during directional freezing to sublimate and dry below the vacuum and eutectic temperature. A microwave generator provides the latent heat of sublimation to the frozen material, rapidly removing moisture from the composite. Compared to the slow heat conduction rate under conventional vacuum conditions, this method shortens drying time, improves drying efficiency, ensures uniform temperature during microwave vacuum freeze-drying, results in a uniform distribution of water molecules within the composite, and maintains a consistent moisture drying rate, thus enhancing the three-dimensional skeletal strength of the composite aerogel.

[0030] 6. The CA / OSA / PA / EG composite aerogel prepared by this invention exhibits excellent flame retardant properties, a high limiting oxygen index, and a low heat release rate during combustion. It forms a dense char layer during combustion, effectively blocking heat and oxygen transfer and inhibiting flame spread. Simultaneously, the phosphorus element in phytic acid can capture free radicals generated during combustion in the gas phase, interrupting the combustion chain reaction. Expandable graphite rapidly expands upon heating to form a loose, porous, worm-like insulating layer, further blocking heat and oxygen transfer to the internal matrix. The char layer formed by phytic acid fills the pores between the expandable graphite layers, making the barrier layer more dense. Furthermore, the expansion and support of the expandable graphite prevents the char layer from cracking and collapsing at high temperatures, synergistically constructing a more stable and efficient insulating barrier in the condensed phase. Combined with the free radical quenching effect in the gas phase, this multi-dimensionally enhances the flame retardant capability of the composite aerogel. Furthermore, the composite aerogel possesses good thermal stability, maintaining structural integrity even at high temperatures, providing reliable protection for fire early warning and achieving full-cycle safety protection from early fire warning to fire spread prevention. Attached Figure Description

[0031] Figure 1 These are SEM images of the composite aerogels prepared in Examples 1, 3, Comparative Example 1, and Comparative Example 2 of this invention; Figure 1 In the image, a is a SEM image of the CA / OSA / PA / EG composite aerogel prepared in Example 1, b is a SEM image of the CA / OSA / PA / EG composite aerogel prepared in Example 3, c is a SEM image of the CA / OSA / EG composite aerogel prepared in Comparative Example 1, and d is a SEM image of the CA / OSA / PA / EG composite aerogel prepared in Comparative Example 2. Figure 2 This refers to the apparatus and test digital diagram used to test the fire early warning response performance of the composite aerogel prepared in Example 3 of this invention. Figure 2 In the diagram, a is a physical image of the fire early warning device used to test the fire early warning response performance of the composite aerogel prepared in Example 3; a1 is a partial enlarged view of the physical image of the fire early warning device used to test the fire early warning response performance of the composite aerogel prepared in Example 3; b is a fire early warning test image of the composite aerogel prepared in Example 3 at 0s; c is a fire early warning test image of the composite aerogel prepared in Example 3 at 1.8s; d is a fire early warning test image of the composite aerogel prepared in Example 3 at 10s; e is a fire early warning test image of the composite aerogel prepared in Example 3 at 30s; and f is a fire early warning test image of the composite aerogel prepared in Example 3 at 150s. Figure 3 This is a schematic diagram of the cross-linking reaction and secondary bond interaction between CA and OSA during the preparation of flame-retardant, high-toughness bio-based composite aerogel in this invention; the molecular structure of sodium alginate is m = 80-171, and the molecular structure of cellulose acetate (CA) nanofibers is n = 65-192. Figure 4 This is a schematic diagram illustrating the interaction of secondary bonds between CA, OSA, and PA during the preparation of flame-retardant, high-toughness bio-based composite aerogel in this invention. Figure 5 This is a schematic diagram illustrating the interaction of secondary bonds between CA, OSA, PA, and EG during the preparation of flame-retardant, high-toughness bio-based composite aerogels in this invention. Detailed Implementation

[0032] The present invention will be further illustrated below through specific embodiments. The following embodiments are specific implementations of the present invention, but the implementation of the present invention is not limited to the following embodiments. Any changes, modifications, substitutions, combinations, or simplifications made without departing from the spirit and principle of the present invention shall be considered equivalent substitutions and are included within the protection scope of the present invention.

[0033] Example 1 This embodiment discloses a method for preparing a flame-retardant, high-toughness bio-based composite aerogel, comprising the following steps: Step (1): Preparation of CA nanofiber dispersion Powdered cellulose acetate (CA, with 26.4% acetyl content and 1.8% hydroxyl content, purchased from Shanghai Aladdin Biochemical Co., Ltd.) was dissolved at room temperature in a mixed solvent of acetone and N,N-dimethylacetamide (DMAC), with a volume ratio of acetone to DMAC of 1:1. After continuous stirring, a cellulose acetate spinning solution with a mass concentration of 15% was prepared. The cellulose acetate spinning solution was electrospun at a temperature of 20℃, a relative humidity of 60%, a spinning distance of 15cm, and a spinning voltage of 12kV. The solution was then dried in an oven at 45℃ for 10 h to remove residual organic solvents, thus obtaining a cellulose acetate nanofiber membrane. Cellulose acetate nanofiber membranes were pulverized using a pulverizer, and the pulverized fiber membrane fragments were dispersed in deionized water to form a dispersion. The dispersion was then ultrasonically pulverized for 18 minutes at 220W using a cell disruptor. The dispersion was then sheared and homogenized using a high-speed shear homogenizer at 12000rpm to obtain a uniformly distributed CA nanofiber dispersion with a mass concentration of 0.3%. Step (2): Preparation of sodium oxidized alginate Sodium alginate (Shanghai Maclean Biochemical Co., Ltd., viscosity-average molecular weight of 220,000) was added to 1-ethyl-3-methylimidazolium acetate ([EMIM][Ac]) ionic liquid and stirred continuously at 82°C to completely dissolve sodium alginate, resulting in a sodium alginate / ionic liquid solution with a mass concentration of 1%. The pH was adjusted to 4.0 by adding 0.5 mol / L acetic acid aqueous solution to sodium alginate / ionic liquid solution, and potassium periodate was added as an oxidant to obtain the reaction solution; The mass concentration of potassium periodate in the reaction solution is 0.6 g / L. The reaction solution was oxidized for 45 min under light-protected conditions and ultrasonic treatment. After the reaction was terminated by adding anhydrous ethanol, the insoluble matter was removed by filtration. The resulting filtrate was placed in a dialysis bag (the molecular weight cutoff of the dialysis bag was 12 kD) and dialyzed with running water for 3 days to remove residual oxidant and ionic liquid. The filtrate was concentrated by rotary evaporation to precipitate the product, which was then thoroughly washed with anhydrous ethanol and dried under vacuum to obtain oxidized sodium alginate (OSA). The ultrasonic treatment was intermittent, specifically: after 15 minutes of ultrasonic treatment, there was a 1-minute pause, and the power of the ultrasonic treatment was 96W; the content of aldehyde groups at the C2 and C3 positions of oxidized sodium alginate was 15.91%, the viscosity-average molecular weight was 173,500, and the solubility in water was 1.68g / 100mL. Step (3): Prepare CA / OSA / PA / EG mixed dispersion 0.1 mol / L acetate / sodium acetate buffer (Sinopharm Chemical Reagent Co., Ltd., pH=4.0) was added dropwise to the CA nanofiber dispersion to adjust the pH of the solution to 4.2. Then sodium oxidized alginate was added, and the mixture was stirred continuously at 30℃ for 1.5 h to mix and react evenly. Then phytic acid (PA) was added, and the mixture was stirred continuously at 30℃ for 2 h to dissolve, resulting in a uniform CA / OSA / PA mixed dispersion. The mass ratio of CA nanofibers to sodium oxidized alginate was 2:1; the mass concentration of phytic acid in the CA / OSA / PA mixed dispersion was 0.8%. Powdered expandable graphite (EG) was added to the CA / OSA / PA mixed dispersion to obtain the CA / OSA / PA / EG mixed dispersion; The mass concentration of expandable graphite in the CA / OSA / PA / EG mixed dispersion was 0.2%. Step (4): Preparation of CA / OSA / PA / EG composite aerogel The CA / OSA / PA / EG mixed dispersion was crosslinked for 0.5 h under microwave radiation power of 200 W and microwave radiation temperature of 45 °C. After the reaction, the reaction solution was transferred to a 3 cm × 3 cm × 5 cm polytetrafluoroethylene mold. The mold was placed on the upper surface of a copper block, and the bottom of the copper block was completely immersed in liquid nitrogen for directional freezing. After 8 min of freezing treatment, the CA / OSA / PA / EG mixed dispersion was completely frozen into ice. The mold was then transferred to a microwave vacuum freeze dryer and continuously freeze-dried for 24 h under microwave vacuum freeze-drying conditions of -58 °C, microwave power of 1100 W, and vacuum degree of 18 Pa to obtain a flame-retardant, high-toughness bio-based composite aerogel, denoted as CA / OSA / PA / EG composite aerogel.

[0034] Example 2 This embodiment discloses a method for preparing a flame-retardant, high-toughness bio-based composite aerogel, comprising the following steps: Step (1): Preparation of CA nanofiber dispersion Powdered cellulose acetate (CA, acetyl content 31.7%, hydroxyl content 2.6%, purchased from Shanghai Aladdin Biochemical Co., Ltd.) was dissolved at room temperature in a mixed solvent of acetone and N,N-dimethylacetamide (DMAC) with a volume ratio of acetone to DMAC of 1.5:1. After continuous stirring, a cellulose acetate spinning solution with a mass concentration of 18% was prepared. The cellulose acetate spinning solution was electrospun at a temperature of 25℃, a relative humidity of 65%, a spinning distance of 18cm, and a spinning voltage of 15kV. The solution was then dried in an oven at 50℃ for 12 h to remove residual organic solvents, resulting in a cellulose acetate nanofiber membrane. Cellulose acetate nanofiber membranes were pulverized using a pulverizer, and the pulverized fiber membrane fragments were dispersed in deionized water to form a dispersion. The dispersion was then ultrasonically pulverized for 25 minutes at 300W using a cell disruptor. The dispersion was then sheared and homogenized using a high-speed shear homogenizer at 18000rpm to obtain a uniformly distributed CA nanofiber dispersion with a mass concentration of 0.5%. Step (2): Preparation of sodium oxidized alginate Sodium alginate (Shanghai Maclean Biochemical Co., Ltd., viscosity-average molecular weight of 220,000) was added to 1-ethyl-3-methylimidazolium acetate ([EMIM][Ac]) ionic liquid and stirred continuously at 85°C to completely dissolve sodium alginate, resulting in a sodium alginate / ionic liquid solution with a mass concentration of 1%. The pH was adjusted to 4.0 by adding 0.5 mol / L acetic acid aqueous solution to sodium alginate / ionic liquid solution, and potassium periodate was added as an oxidant to obtain the reaction solution; The mass concentration of potassium periodate in the reaction solution is 0.8 g / L. The reaction solution was oxidized for 60 min in a light-protected environment under ultrasonic treatment. After the reaction was terminated by adding anhydrous ethanol, the insoluble matter was removed by filtration. The resulting filtrate was placed in a dialysis bag (the molecular weight cutoff of the dialysis bag was 11 kD) and dialyzed with running water for 3 days to remove residual oxidant and ionic liquid. The filtrate was concentrated by rotary evaporation to precipitate the product, which was then thoroughly washed with anhydrous ethanol and dried under vacuum to obtain oxidized sodium alginate (OSA). The ultrasonic treatment was intermittent, specifically: after 15 minutes of ultrasonic treatment, there was a 1-minute pause, and the power of the ultrasonic treatment was 110W; the content of aldehyde groups at the C2 and C3 positions of oxidized sodium alginate was 19.14%, the viscosity-average molecular weight was 122,700, and the solubility in water was 6.03g / 100mL. Step (3): Prepare CA / OSA / PA / EG mixed dispersion Add 0.1 mol / L acetate / sodium acetate buffer (Sinopharm Chemical Reagent Co., Ltd., pH=4.0) dropwise to the CA nanofiber dispersion to adjust the pH of the solution to 4.4, then add sodium alginate oxidase, and stir continuously at 40℃ for 2 hours to mix and react evenly. Then add phytic acid (PA), and stir continuously at 40℃ for 2.5 hours to dissolve, to obtain a uniform CA / OSA / PA mixed dispersion. The mass ratio of CA nanofibers to sodium alginate was 1:2; the mass concentration of phytic acid in the CA / OSA / PA mixed dispersion was 1.6%. Powdered expandable graphite (EG) was added to the CA / OSA / PA mixed dispersion to obtain the CA / OSA / PA / EG mixed dispersion; The mass concentration of expandable graphite in the CA / OSA / PA / EG mixed dispersion was 0.4%. Step (4): Preparation of CA / OSA / PA / EG composite aerogel The CA / OSA / PA / EG mixed dispersion was crosslinked for 1 hour under microwave radiation power of 260W and microwave radiation temperature of 55℃. After the reaction, the reaction solution was transferred to a 3cm×3cm×5cm polytetrafluoroethylene mold. The mold was placed on the upper surface of a copper block, and the bottom of the copper block was completely immersed in liquid nitrogen for directional freezing. After 10 minutes of freezing treatment, the CA / OSA / PA / EG mixed dispersion was completely frozen into ice. The mold was then transferred to a microwave vacuum freeze dryer and continuously freeze-dried for 36 hours under microwave vacuum freeze-drying conditions of -62℃, microwave power of 1400W, and vacuum degree of 15Pa to obtain a flame-retardant, high-toughness bio-based composite aerogel, denoted as CA / OSA / PA / EG composite aerogel.

[0035] Example 3 This embodiment discloses a method for preparing a flame-retardant, high-toughness bio-based composite aerogel, comprising the following steps: Step (1): Preparation of CA nanofiber dispersion Powdered cellulose acetate (CA, acetyl content 39.8%, hydroxyl content 3.5%, purchased from Shanghai Aladdin Biochemical Co., Ltd.) was dissolved at room temperature in a mixed solvent of acetone and N,N-dimethylacetamide (DMAC) with a volume ratio of acetone to DMAC of 2:1. After continuous stirring, a cellulose acetate spinning solution with a mass concentration of 22% was prepared. The cellulose acetate spinning solution was electrospun at a temperature of 25℃, a relative humidity of 70%, a spinning distance of 22cm, and a spinning voltage of 20kV. The solution was then dried in an oven at 50℃ for 14 h to remove residual organic solvents, thus obtaining a cellulose acetate nanofiber membrane. Cellulose acetate nanofiber membranes were pulverized using a pulverizer, and the pulverized fiber membrane fragments were dispersed in deionized water to form a dispersion. The dispersion was then ultrasonically pulverized for 30 minutes at 380W using a cell disruptor. The dispersion was then sheared and homogenized using a high-speed shear homogenizer at 24000rpm to obtain a uniformly distributed CA nanofiber dispersion with a mass concentration of 0.6%. Step (2): Preparation of sodium oxidized alginate Sodium alginate (Shanghai Maclean Biochemical Co., Ltd., viscosity-average molecular weight of 220,000) was added to 1-ethyl-3-methylimidazolium acetate ([EMIM][Ac]) ionic liquid and stirred continuously at 90°C to completely dissolve sodium alginate, resulting in a sodium alginate / ionic liquid solution with a mass concentration of 1.5%. The pH was adjusted to 4.0 by adding 0.5 mol / L acetic acid aqueous solution to sodium alginate / ionic liquid solution, and potassium periodate was added as an oxidant to obtain the reaction solution; The mass concentration of potassium periodate in the reaction solution is 1 g / L. The reaction solution was oxidized for 80 min in a light-protected environment under ultrasonic treatment. After the reaction was terminated by adding anhydrous ethanol, the insoluble matter was removed by filtration. The resulting filtrate was placed in a dialysis bag (the molecular weight cutoff of the dialysis bag was 10 kD) and dialyzed with running water for 4 days to remove residual oxidant and ionic liquid. The filtrate was concentrated by rotary evaporation to precipitate the product, which was then thoroughly washed with anhydrous ethanol and dried under vacuum to obtain oxidized sodium alginate (OSA). The ultrasonic treatment was intermittent, specifically: after 15 minutes of ultrasonic treatment, there was a 1-minute pause, and the power of the ultrasonic treatment was 125W; the aldehyde content at the C2 and C3 positions of the oxidized sodium alginate was 31.84%, the viscosity-average molecular weight was 100,800, and the solubility in water was 8.72g / 100mL. Step (3): Prepare CA / OSA / PA / EG mixed dispersion Add 0.2 mol / L acetate / sodium acetate buffer (Sinopharm Chemical Reagent Co., Ltd., pH=4.0) dropwise to the CA nanofiber dispersion to adjust the solution pH=4.6, then add sodium alginate oxide, and stir continuously at 60℃ for 3 hours to mix and react evenly, then add phytic acid (PA), and stir continuously at 60℃ for 3 hours to dissolve, to obtain a uniform CA / OSA / PA mixed dispersion; The mass ratio of CA nanofibers to sodium alginate was 1:1; the mass concentration of phytic acid in the CA / OSA / PA mixed dispersion was 2.4%. Powdered expandable graphite (EG) was added to the CA / OSA / PA mixed dispersion to obtain the CA / OSA / PA / EG mixed dispersion; The mass concentration of expandable graphite in the CA / OSA / PA / EG mixed dispersion was 0.6%. Step (4): Preparation of CA / OSA / PA / EG composite aerogel The CA / OSA / PA / EG mixed dispersion was crosslinked for 2 hours under microwave radiation power of 300W and microwave radiation temperature of 60℃. After the reaction, the reaction solution was transferred to a 3cm×3cm×5cm polytetrafluoroethylene mold. The mold was placed on the upper surface of a copper block, and the bottom of the copper block was completely immersed in liquid nitrogen for directional freezing. After freezing treatment for 12 minutes, the CA / OSA / PA / EG mixed dispersion was completely frozen into ice. The mold was then transferred to a microwave vacuum freeze dryer and continuously freeze-dried for 48 hours under microwave vacuum freeze-drying conditions of -68℃, microwave power of 1500W, and vacuum degree of 12Pa to obtain a flame-retardant, high-toughness bio-based composite aerogel, denoted as CA / OSA / PA / EG composite aerogel.

[0036] Example 4 This embodiment discloses a method for preparing a flame-retardant, high-toughness bio-based composite aerogel, comprising the following steps: Step (1): Preparation of CA nanofiber dispersion Powdered cellulose acetate (CA, acetyl content 43.5%, hydroxyl content 4.1%, purchased from Shanghai Aladdin Biochemical Co., Ltd.) was dissolved at room temperature in a mixed solvent of acetone and N,N-dimethylacetamide (DMAC) with a volume ratio of acetone to DMAC of 3:1. After continuous stirring, a cellulose acetate spinning solution with a mass concentration of 25% was prepared. The cellulose acetate spinning solution was electrospun at a temperature of 35℃, a relative humidity of 72%, a spinning distance of 24cm, and a spinning voltage of 23kV. The solution was then dried in an oven at 55℃ for 15 h to remove residual organic solvents, resulting in a cellulose acetate nanofiber membrane. Cellulose acetate nanofiber membranes were pulverized using a pulverizer, and the pulverized fiber membrane fragments were dispersed in deionized water to form a dispersion. The dispersion was then ultrasonically pulverized for 35 minutes at 430W using a cell disruptor. The dispersion was then sheared and homogenized using a high-speed shear homogenizer at 25000rpm to obtain a uniformly distributed CA nanofiber dispersion with a mass concentration of 0.8%. Step (2): Preparation of sodium oxidized alginate Sodium alginate (Shanghai Maclean Biochemical Co., Ltd., viscosity-average molecular weight of 220,000) was added to 1-ethyl-3-methylimidazolium acetate ([EMIM][Ac]) ionic liquid and stirred continuously at 90°C to completely dissolve sodium alginate, resulting in a sodium alginate / ionic liquid solution with a mass concentration of 1.5%. The pH was adjusted to 4.0 by adding 0.5 mol / L acetic acid aqueous solution to sodium alginate / ionic liquid solution, and potassium periodate was added as an oxidant to obtain the reaction solution; The mass concentration of potassium periodate in the reaction solution is 1 g / L. The reaction solution was oxidized for 80 min in a light-protected environment under ultrasonic treatment. After the reaction was terminated by adding anhydrous ethanol, the insoluble matter was removed by filtration. The resulting filtrate was placed in a dialysis bag (the molecular weight cutoff of the dialysis bag was 10 kD) and dialyzed with running water for 4 days to remove residual oxidant and ionic liquid. The filtrate was concentrated by rotary evaporation to precipitate the product, which was then thoroughly washed with anhydrous ethanol and dried under vacuum to obtain oxidized sodium alginate (OSA). The ultrasonic treatment was intermittent, specifically: after 15 minutes of ultrasonic treatment, there was a 1-minute pause, and the power of the ultrasonic treatment was 125W; the aldehyde content at the C2 and C3 positions of the oxidized sodium alginate was 31.84%, the viscosity-average molecular weight was 100,800, and the solubility in water was 8.72g / 100mL. Step (3): Prepare CA / OSA / PA / EG mixed dispersion Add 0.2 mol / L acetate / sodium acetate buffer (Sinopharm Chemical Reagent Co., Ltd., pH=4.0) dropwise to the CA nanofiber dispersion to adjust the pH of the solution to 5.1, then add sodium alginate oxide, and stir continuously at 70℃ for 2 hours to mix and react evenly. Then add phytic acid (PA), and stir continuously at 70℃ for 4 hours to dissolve, to obtain a uniform CA / OSA / PA mixed dispersion. The mass ratio of CA nanofibers to sodium oxidized alginate was 1:3; the mass concentration of phytic acid in the CA / OSA / PA mixed dispersion was 2.6%. Powdered expandable graphite (EG) was added to the CA / OSA / PA mixed dispersion to obtain the CA / OSA / PA / EG mixed dispersion; The mass concentration of expandable graphite in the CA / OSA / PA / EG mixed dispersion was 0.8%. Step (4): Preparation of CA / OSA / PA / EG composite aerogel The CA / OSA / PA / EG mixed dispersion was crosslinked for 2 hours under microwave radiation power of 320W and microwave radiation temperature of 65℃. After the reaction, the reaction solution was transferred to a 3cm×3cm×5cm polytetrafluoroethylene mold. The mold was placed on the upper surface of a copper block, and the bottom of the copper block was completely immersed in liquid nitrogen for directional freezing. After freezing treatment for 14 minutes, the CA / OSA / PA / EG mixed dispersion was completely frozen into ice. The mold was then transferred to a microwave vacuum freeze dryer and continuously freeze-dried for 54 hours under microwave vacuum freeze-drying conditions of -68℃, microwave power of 1500W, and vacuum degree of 12Pa to obtain a flame-retardant, high-toughness bio-based composite aerogel, denoted as CA / OSA / PA / EG composite aerogel.

[0037] Comparative Example 1 This comparative example discloses a method for preparing a composite aerogel, comprising the following steps: Step (1): Preparation of CA nanofiber dispersion Powdered cellulose acetate (CA, acetyl content 39.8%, hydroxyl content 3.5%, purchased from Shanghai Aladdin Biochemical Co., Ltd.) was dissolved at room temperature in a mixed solvent of acetone and N,N-dimethylacetamide (DMAC) with a volume ratio of acetone to DMAC of 2:1. After continuous stirring, a cellulose acetate spinning solution with a mass concentration of 22% was prepared. The cellulose acetate spinning solution was electrospun at a temperature of 25℃, a relative humidity of 70%, a spinning distance of 22cm, and a spinning voltage of 20kV. The solution was then dried in an oven at 50℃ for 14 h to remove residual organic solvents, thus obtaining a cellulose acetate nanofiber membrane. Cellulose acetate nanofiber membranes were pulverized using a pulverizer, and the pulverized fiber membrane fragments were dispersed in deionized water to form a dispersion. The dispersion was then ultrasonically pulverized for 30 minutes at 380W using a cell disruptor. The dispersion was then sheared and homogenized using a high-speed shear homogenizer at 24000rpm to obtain a uniformly distributed CA nanofiber dispersion with a mass concentration of 0.6%. Step (2): Preparation of sodium oxidized alginate Sodium alginate (Shanghai Maclean Biochemical Co., Ltd., viscosity-average molecular weight of 220,000) was added to 1-ethyl-3-methylimidazolium acetate ([EMIM][Ac]) ionic liquid and stirred continuously at 90°C to completely dissolve sodium alginate, resulting in a sodium alginate / ionic liquid solution with a mass concentration of 1.5%. The pH was adjusted to 4.0 by adding 0.5 mol / L acetic acid aqueous solution to sodium alginate / ionic liquid solution, and potassium periodate was added as an oxidant to obtain the reaction solution; The mass concentration of potassium periodate in the reaction solution is 1 g / L. The reaction solution was oxidized for 80 min in a light-protected environment under ultrasonic treatment. After the reaction was terminated by adding anhydrous ethanol, the insoluble matter was removed by filtration. The resulting filtrate was placed in a dialysis bag (the molecular weight cutoff of the dialysis bag was 10 kD) and dialyzed with running water for 4 days to remove residual oxidant and ionic liquid. The filtrate was concentrated by rotary evaporation to precipitate the product, which was then thoroughly washed with anhydrous ethanol and dried under vacuum to obtain oxidized sodium alginate (OSA). The ultrasonic treatment was intermittent, specifically: after 15 minutes of ultrasonic treatment, there was a 1-minute pause, and the power of the ultrasonic treatment was 125W; the aldehyde content at the C2 and C3 positions of the oxidized sodium alginate was 31.84%, the viscosity-average molecular weight was 100,800, and the solubility in water was 8.72g / 100mL. Step (3): Preparation of CA / OSA / EG mixed dispersion Add 0.2 mol / L acetate / sodium acetate buffer (Sinopharm Chemical Reagent Co., Ltd., pH=4.0) dropwise to the CA nanofiber dispersion to adjust the solution pH=4.6, then add oxidized sodium alginate, and stir continuously at 60℃ for 3 hours to mix and react evenly to obtain a uniform CA / OSA mixed dispersion. The mass ratio of CA nanofibers to sodium oxidized alginate is 1:1. Powdered expandable graphite (EG) was added to the CA / OSA mixed dispersion to obtain a CA / OSA / EG mixed dispersion; The mass concentration of expandable graphite in the CA / OSA / EG mixed dispersion was 0.6%. Step (4): Preparation of CA / OSA / EG composite aerogel The CA / OSA / EG mixed dispersion was subjected to a crosslinking reaction for 2 hours under microwave radiation power of 300W and microwave radiation temperature of 60℃. After the reaction, the reaction solution was transferred to a 3cm×3cm×5cm polytetrafluoroethylene mold. The mold was placed on the upper surface of a copper block, and the bottom of the copper block was completely immersed in liquid nitrogen for directional freezing. After freezing treatment for 12 minutes, the CA / OSA / EG mixed dispersion was completely frozen into ice. The mold was then transferred to a microwave vacuum freeze dryer and continuously freeze-dried for 48 hours under microwave vacuum freeze-drying conditions of -68℃, microwave power of 1500W, and vacuum degree of 12Pa to obtain a composite aerogel, denoted as CA / OSA / EG composite aerogel.

[0038] Comparative Example 2 This comparative example discloses a method for preparing a composite aerogel, comprising the following steps: Step (1): Preparation of CA nanofiber dispersion Powdered cellulose acetate (CA, acetyl content 39.8%, hydroxyl content 3.5%, purchased from Shanghai Aladdin Biochemical Co., Ltd.) was dissolved at room temperature in a mixed solvent of acetone and N,N-dimethylacetamide (DMAC) with a volume ratio of acetone to DMAC of 2:1. After continuous stirring, a cellulose acetate spinning solution with a mass concentration of 22% was prepared. The cellulose acetate spinning solution was electrospun at a temperature of 25℃, a relative humidity of 70%, a spinning distance of 22cm, and a spinning voltage of 20kV. The solution was then dried in an oven at 50℃ for 14 h to remove residual organic solvents, thus obtaining a cellulose acetate nanofiber membrane. Cellulose acetate nanofiber membranes were pulverized using a pulverizer, and the pulverized fiber membrane fragments were dispersed in deionized water to form a dispersion. The dispersion was then ultrasonically pulverized for 30 minutes at 380W using a cell disruptor. The dispersion was then sheared and homogenized using a high-speed shear homogenizer at 24000rpm to obtain a uniformly distributed CA nanofiber dispersion with a mass concentration of 0.6%. Step (2): Preparation of sodium oxidized alginate Sodium alginate (Shanghai Maclean Biochemical Co., Ltd., viscosity-average molecular weight of 220,000) was added to 1-ethyl-3-methylimidazolium acetate ([EMIM][Ac]) ionic liquid and stirred continuously at 90°C to completely dissolve sodium alginate, resulting in a sodium alginate / ionic liquid solution with a mass concentration of 1.5%. The pH was adjusted to 4.0 by adding 0.5 mol / L acetic acid aqueous solution to sodium alginate / ionic liquid solution, and potassium periodate was added as an oxidant to obtain the reaction solution; The mass concentration of potassium periodate in the reaction solution is 1 g / L. The reaction solution was oxidized for 80 min in a light-protected environment under ultrasonic treatment. After the reaction was terminated by adding anhydrous ethanol, the insoluble matter was removed by filtration. The resulting filtrate was placed in a dialysis bag (the molecular weight cutoff of the dialysis bag was 10 kD) and dialyzed with running water for 4 days to remove residual oxidant and ionic liquid. The filtrate was concentrated by rotary evaporation to precipitate the product, which was then thoroughly washed with anhydrous ethanol and dried under vacuum to obtain oxidized sodium alginate (OSA). The ultrasonic treatment was intermittent, specifically: after 15 minutes of ultrasonic treatment, there was a 1-minute pause, and the power of the ultrasonic treatment was 125W; the aldehyde content at the C2 and C3 positions of the oxidized sodium alginate was 31.84%, the viscosity-average molecular weight was 100,800, and the solubility in water was 8.72g / 100mL. Step (3): Prepare CA / OSA / PA / EG mixed dispersion Add 0.2 mol / L acetate / sodium acetate buffer (Sinopharm Chemical Reagent Co., Ltd., pH=4.0) dropwise to the CA nanofiber dispersion to adjust the solution pH=4.6, then add sodium alginate oxide, and stir continuously at 60℃ for 3 hours to mix and react evenly, then add phytic acid (PA), and stir continuously at 60℃ for 3 hours to dissolve, to obtain a uniform CA / OSA / PA mixed dispersion; The mass ratio of CA nanofibers to sodium alginate was 1:1; the mass concentration of phytic acid in the CA / OSA / PA mixed dispersion was 2.4%. Powdered expandable graphite (EG) was added to the CA / OSA / PA mixed dispersion to obtain the CA / OSA / PA / EG mixed dispersion; The mass concentration of expandable graphite in the CA / OSA / PA / EG mixed dispersion was 0.6%. Step (4): Preparation of CA / OSA / PA / EG composite aerogel The CA / OSA / PA / EG mixed dispersion was subjected to a crosslinking reaction for 2 hours under microwave radiation power of 300W and microwave radiation temperature of 60℃. After the reaction was completed, the reaction solution was transferred to a 3cm×3cm×5cm polytetrafluoroethylene mold, and then the mold was transferred to a microwave vacuum freeze dryer. The freeze dryer was continuously freeze-dried for 48 hours under microwave vacuum freeze-drying conditions of -68℃, microwave power of 1500W, and vacuum degree of 12Pa to obtain a composite aerogel, which is denoted as CA / OSA / PA / EG composite aerogel.

[0039] Comparative Example 3 This comparative example discloses a method for preparing a composite aerogel, comprising the following steps: Step (1): Preparation of CA nanofiber dispersion Powdered cellulose acetate (CA, acetyl content 39.8%, hydroxyl content 3.5%, purchased from Shanghai Aladdin Biochemical Co., Ltd.) was dissolved at room temperature in a mixed solvent of acetone and N,N-dimethylacetamide (DMAC) with a volume ratio of acetone to DMAC of 2:1. After continuous stirring, a cellulose acetate spinning solution with a mass concentration of 22% was prepared. The cellulose acetate spinning solution was electrospun at a temperature of 25℃, a relative humidity of 70%, a spinning distance of 22cm, and a spinning voltage of 20kV. The solution was then dried in an oven at 50℃ for 14 h to remove residual organic solvents, thus obtaining a cellulose acetate nanofiber membrane. Cellulose acetate nanofiber membranes were pulverized using a pulverizer, and the pulverized fiber membrane fragments were dispersed in deionized water to form a dispersion. The dispersion was then ultrasonically pulverized for 30 minutes at 380W using a cell disruptor. The dispersion was then sheared and homogenized using a high-speed shear homogenizer at 24000rpm to obtain a uniformly distributed CA nanofiber dispersion with a mass concentration of 0.6%. Step (2): Prepare a mixed dispersion of CA / SA / PA / EG; 0.2 mol / L acetate / sodium acetate buffer (Sinopharm Chemical Reagent Co., Ltd., pH=4.0) was added dropwise to the CA nanofiber dispersion to adjust the pH of the solution to 4.6. Then sodium alginate (SA, Shanghai Maclean Biochemical Co., Ltd., viscosity-average molecular weight of 220,000) was added. The mixture was stirred continuously at 60℃ for 3 hours to achieve uniform mixing and reaction. Then phytic acid (PA) was added and stirred continuously at 60℃ for 3 hours to dissolve, resulting in a uniform CA / SA / PA mixed dispersion. The mass ratio of CA nanofibers to sodium alginate was 1:1; the viscosity-average molecular weight of sodium alginate was 102,400, and its water solubility was 6.95 g / 100 mL; the mass concentration of phytic acid in the CA / SA / PA mixed dispersion was 2.4%. Powdered expandable graphite (EG) was added to the CA / SA / PA mixed dispersion to obtain the CA / SA / PA / EG mixed dispersion; The mass concentration of expandable graphite in the CA / SA / PA / EG mixed dispersion was 0.6%. Step (4): Preparation of CA / SA / PA / EG composite aerogel The CA / SA / PA / EG mixed dispersion was crosslinked for 2 hours under microwave radiation power of 300W and microwave radiation temperature of 60℃. After the reaction, the reaction solution was transferred to a 3cm×3cm×5cm polytetrafluoroethylene mold. The mold was placed on the upper surface of a copper block, and the bottom of the copper block was completely immersed in liquid nitrogen for directional freezing. After freezing treatment for 12 minutes, the CA / SA / PA / EG mixed dispersion was completely frozen into ice. The mold was then transferred to a microwave vacuum freeze dryer and continuously freeze-dried for 48 hours under microwave vacuum freeze-drying conditions of -68℃, microwave power of 1500W, and vacuum degree of 12Pa to obtain a composite aerogel, denoted as CA / SA / PA / EG composite aerogel.

[0040] Experimental data characterization and performance testing (1) The average diameter and fiber length of the CA nanofibers in the CA nanofiber dispersions prepared in the embodiments and comparative examples of the present invention were tested. The specific test results are shown in Table 1: Table 1

[0041] The testing methods for each indicator in Table 1 are as follows: surface morphology images of CA nanofibers in the CA nanofiber dispersion are obtained by scanning electron microscopy, and then the images are imported into Nano Measure analysis software, which automatically calculates the average diameter and length of the CA nanofibers.

[0042] The test results in Table 1 show that filamentous nanofibers can be formed by adjusting the mass concentration of cellulose acetate spinning solution and electrospinning parameters. However, the uniformity of fiber diameter varies with different spinning concentrations. For example, the nanofibers obtained by electrospinning with the 22% cellulose acetate spinning solution used in Example 3 and Comparative Examples 1-3 have a finer diameter, with an average diameter of 312 nm. The fiber size distribution is relatively concentrated and uniform, exhibiting a large specific surface area and a high aspect ratio. The length is mainly concentrated in the range of 62.7 μm. The excellent dispersion stability provides a good guarantee for the subsequent preparation of flexible aerogel materials with high skeleton strength.

[0043] (2) The pore diameter, specific surface area, density, and porosity of the composite aerogels prepared in the embodiments and comparative examples of the present invention were tested. The specific test results are shown in Table 2: Table 2

[0044] The testing methods for each index in Table 2 are as follows: The density of the composite aerogel was tested by the liquid displacement method, and the porosity of the composite aerogel was tested by the liquid displacement method (refer to the literature: Jiao Chenlu. Preparation of microcrystalline cellulose-based aerogel and study on adsorption and degradation of heavy metals and dyes [D]. Soochow University, 2017. DOI: CNKI:CDMD:1.1018.018375). The specific surface area and pore diameter changes of the composite aerogel samples were tested using a Micron Instruments ASAP 2460 surface area and pore size analyzer (BET).

[0045] The test results in Table 2 show that the density, porosity, pore diameter, and specific surface area distribution of the composite aerogel differ under different EG addition amounts. With the increase of EG addition in Examples 1-3, the density of the aerogel gradually decreases, while the pore diameter decreases, the porosity increases accordingly, and the specific surface area also increases. This indicates that EG sheets participate in the construction of the flexible aerogel framework, and the intercalation of sheet-like EG increases the porosity of the aerogel's three-dimensional structure, reduces the density, and increases the specific surface area. However, in Example 4, when more EG was added, the intercalation of EG sheets enhanced the porosity and reduced the density, but the excessive stacking of sheet-like EG led to larger pores and a lower specific surface area. The samples in Comparative Example 1 (lacking phytic acid crosslinking), Comparative Example 2 (conventionally freeze-dried), and Comparative Example 3 (using sodium alginate) showed a poorer porous structure due to the weakened stability of the three-dimensional network framework and the crosslinking structure of CA nanofibers penetrating between adjacent pore walls. Compared with Examples 1-4, these samples exhibited larger pore diameters and densities, and lower specific surface areas and porosities.

[0046] (3) The microstructure of the composite aerogels prepared in Examples 1, 3, Comparative Example 1, and Comparative Example 2 of this invention was observed using TEM. The specific test results are as follows: Figure 1 As shown.

[0047] Figure 1 In the image, a is a SEM image of the CA / OSA / PA / EG composite aerogel prepared in Example 1, b is a SEM image of the CA / OSA / PA / EG composite aerogel prepared in Example 3, c is a SEM image of the CA / OSA / EG composite aerogel prepared in Comparative Example 1, and d is a SEM image of the CA / OSA / PA / EG composite aerogel prepared in Comparative Example 2.

[0048] Figure 1In examples a and b, the longitudinal cross-sectional morphology of the CA / OSA / PA / EG composite aerogel shows that the CA / OSA / PA / EG composite aerogels prepared in Examples 1 and 3 of this invention contain a large number of parallel-arranged microporous structures. This is because, during the liquid nitrogen-assisted directional freezing process, water molecules crystallize along the temperature gradient, and the gradually formed ice crystal pillars squeeze other non-aqueous phase components to adjacent regions, causing these substances to be distributed in a fixed direction under the guidance of the ice crystals. After the ice crystals are removed during the freeze-drying process, the solid substances are retained, eventually forming a continuous longitudinal pore wall framework. At the same time, it can be observed that CA nanofibers are interlaced between the parallel pore walls, constructing a bridging structure that runs through adjacent pore walls. The introduction of CA nanofibers not only enhances the stability of the aerogel pore walls but also helps to disperse the stress of the pore walls during compression and bending, enhancing the mechanical durability and flexibility of the aerogel. Furthermore, the EG sheets participate in the construction of the aerogel framework, expanding the three-dimensional conductive pathways of the aerogel and improving conductivity and fire early warning sensitivity. In addition, CA nanofibers can interconnect with OSA and PA, significantly improving the overall structural stability and framework strength of the aerogel.

[0049] from Figure 1 As can be seen from c, the continuous longitudinal pore wall skeleton of CA / OSA / EG composite aerogel is relatively loose, the size is increased, and the number of CA nanofibers cross-linked between parallel pore walls is reduced. This indicates that the lack of cross-linking enhancement effect of phytic acid (PA) weakens the bonding force between CA nanofibers, OSA and EG sheets, resulting in poorer regularity and decreased stability of the aerogel skeleton structure.

[0050] from Figure 1 As can be seen from d, the random freeze-dried CA / OSA / PA / EG composite aerogel exhibits a disordered cross-section due to the increased randomness of ice crystal nucleation sites and initial growth directions. The pores are randomly distributed and partially collapsed, making it impossible to uniformly distribute stress when subjected to external forces. The material lacks structural stability and toughness.

[0051] (4) The height retention rate and stress retention rate of the composite aerogels prepared in the embodiments and comparative examples of the present invention were tested after 100 compression cycles under 40% deformation compression, and the number of times the composite aerogels showed no cracks during cyclic compression was also tested. The specific test results are shown in Table 3: Table 3

[0052] The test methods for each indicator in Table 3 are as follows: The axial static compression recovery performance (height and stress retention rate, number of compressions) of the composite aerogel is tested using a CMT6104 universal testing machine in accordance with the ASTM D3575-14 test standard, and the test speed is 10 mm / min.

[0053] The test results in Table 3 show that the composite aerogels prepared in Examples 1-3 exhibited high height and stress retention rates after 100 compression cycles under 40% deformation compression, and also demonstrated a high number of compression cycles. With increasing EG content, the composite aerogels maintained good mechanical recovery properties after compression. However, the samples in Comparative Example 1 (lacking phytic acid crosslinking), Comparative Example 2 (conventionally freeze-dried), and Comparative Example 3 (using sodium alginate) showed poorer compression recovery due to the weakened crosslinking structure and interfacial bonding effect of CA nanofibers penetrating adjacent pore walls.

[0054] (5) Using a cone calorimeter, the composite aerogels prepared in Examples 1, 3 and Comparative Example 1 of this invention were subjected to cone calorimetry analysis. The specific test results are shown in Table 4: Table 4

[0055] The test methods for each indicator in Table 4 are as follows: Referring to ISO 5660 standard, a Vouch 6810 cone calorimeter was used at 35 kW / m³. 2 The ignition time (TTI), heat release rate (PHRR), total heat release (THR), and heat release time (T) of the composite aerogel were tested under heat flux conditions. PHRR Indicators such as total smoke emission (TSR), carbon monoxide formation rate (PCO), and carbon dioxide formation rate (PCO2) are used.

[0056] The test results in Table 4 show that the CA / OSA / EG composite aerogel without phytic acid in Comparative Example 1 reached its peak HRR (PHRR) at an ignition time (TTI) of 14 s, with PHRR and total heat release (THR) values ​​of 63.139 kW / m³. 2 and 4.094 MJ / m 2 Because the aerogel matrix degrades rapidly during combustion, heat and combustible decomposition products can be continuously transferred to the outside of the combustion zone. The heat release rate and total heat release are both at a high level, indicating that the flame retardant properties of CA / OSA / EG aerogel are poor.

[0057] The CA / OSA / PA / EG composite aerogel in Example 3 achieved a pHRR of 24.508 kWh / m³ after 35 seconds. 2 and 1.695MJ / m 2 The PHRR and THR values ​​were reduced by 61.18% and 58.59% respectively compared to the CA / OSA / EG aerogel in Comparative Example 1. This indicates that the CA / OSA / PA / EG composite aerogel can effectively reduce PHRR and THR during combustion and prevent rapid exothermic reactions of the material.

[0058] Table 4 also shows that the total smoke production (TSP) of the CA / OSA / PA / EG composite aerogel in Example 3 is 0.051 m³. 2 Compared with the CA / OSA / EG composite aerogel of Comparative Example 1 (0.241m), 2 The CO and CO2 release (PCO, PCO2) of the CA / OSA / PA / EG composite aerogel in Example 3 was reduced by 78.84%. This is because the CA / OSA / PA / EG composite aerogel prepared in this embodiment contains acidic groups such as carboxyl and phosphoric acid. These groups can dehydrate and form a dense and stable carbon layer when exposed to high temperature or flame, thereby inhibiting smoke emission during combustion. At the same time, a large number of phosphoric acid derivatives are generated during pyrolysis, which can not only promote the dehydration and carbonization of carbon source, but also capture free radicals and improve flame retardant and heat insulation performance. Meanwhile, the EG in the composite aerogel expands rapidly after heating to form a loose and porous heat insulation layer, which can further block the transfer of heat and oxygen. The carbon layer formed by PA can fill the pores between the EG layers, making the barrier layer more compact and constructing a more stable and efficient heat insulation barrier. Thus, the flame retardant ability of the CA / OSA / PA / EG composite aerogel is improved in multiple dimensions.

[0059] (6) The limiting oxygen index (LOI) and the time (i.e., response time) for the composite aerogels prepared in the embodiments and comparative examples of the present invention to trigger the fire warning system upon contact with flame were tested. The specific test results are shown in Table 5 and Figure 2 : Table 5

[0060] The testing methods for each indicator in Table 5 are as follows: through the assembled fire alarm device (such as...) Figure 2 As shown), the fire early warning response performance of the composite aerogel prepared in Example 3 was tested; the limiting oxygen index (%) was tested by placing the aerogel sample in a mixture of oxygen and nitrogen in accordance with GB / T2406.2-2009 standard and testing it on a JF-3 oxygen index meter. The samples prepared in Examples 1-4 and Comparative Examples 1-3 were tested three times in parallel and the average value was taken.

[0061] According to Table 5 and Figure 2The test results show that as the amount of phytic acid and expandable graphite added increases, the limiting oxygen index of the composite aerogels in Examples 1 to 4 increases, and their flame retardant ability increases. The composite aerogels in Comparative Examples 2 and 3, containing phytic acid and expandable graphite, also show good flame retardant performance. However, Comparative Example 1 lacks phytic acid, resulting in a lower limiting oxygen index. This indicates that the PA in the aerogel matrix decomposes upon heating to produce a large amount of polyphosphoric acid, forming a dehydrated carbonized layer on the aerogel surface. Simultaneously, the phosphorus element in the phytic acid molecules captures highly reactive free radicals generated during combustion, interrupting the chain reaction of combustion. Furthermore, the EG in the aerogel expands rapidly upon exposure to high fire temperatures to form a dense carbon layer, providing physical insulation and flame retardancy.

[0062] Depend on Figure 2 As can be seen from c, the composite aerogel prepared in Example 3 can trigger the fire warning system within 1.8 s. This is because the EG in the CA / OSA / PA / EG composite aerogel expands rapidly upon heating, increasing the amount of carbon layer formed in the aerogel. Furthermore, the strength and density of the carbon layer are significantly enhanced, increasing the contact area between the aerogel layers, effectively expanding the conductive network, promoting the formation and stabilization of 3D conductive pathways within the aerogel, and improving the response sensitivity of the fire warning system. In addition, as... Figure 2 As shown in C, D, E, and F, the composite aerogel prepared in Example 3 exhibits a fire warning response time exceeding 150 s when continuously exposed to flame, demonstrating good alarm persistence and ensuring the sustainable monitoring and reliability of fire warnings. Table 5 shows that the fire warning response times of Examples 1, 2, and 4 are slightly delayed, especially the fire warning trigger times of Comparative Examples 1, 2, and 3, which are longer. This indicates that the three-dimensional network structure stability of the composite aerogel samples lacking phytic acid crosslinking, conventional random freeze-drying, and the use of sodium alginate is weakened, hindering the formation of tightly connected conductive pathways and leading to a decrease in the fire warning sensitivity of the aerogel matrix.

[0063] Figures 3 to 5 The preparation process of the flame-retardant, high-toughness bio-based composite aerogel of this invention is shown. From Figure 3 It is known that the hydroxyl groups on the CA molecular chain can cross-link with the aldehyde groups in OSA to form hemiacetal chemical bonds, thus constructing the basic framework of the aerogel. Figure 4 It can be seen that the hydroxyl and ester groups in CA molecules, and the carboxyl and sodium ions in OSA molecules can form secondary bond forces such as hydrogen bonds and ionic bonds with the phosphate groups on PA molecules, thereby strengthening the three-dimensional skeleton of the composite aerogel. Figure 5 The study demonstrates that various active groups in the CA, OSA, and PA components of the composite aerogel can combine with carboxyl groups, epoxy groups, etc. on the EG sheets to form secondary bonds, further constructing a stable three-dimensional network structure of the composite aerogel.

[0064] It should be noted that the embodiments described above are only for explaining the present invention and do not constitute any limitation on the present invention. The present invention has been described with reference to typical embodiments, but it should be understood that the words used therein are descriptive and explanatory terms, not limiting terms. Modifications can be made to the present invention within the scope of the claims, and revisions can be made to the present invention without departing from the scope and spirit of the present invention. Although the present invention described herein relates to specific methods, materials, and embodiments, it does not mean that the present invention is limited to the specific examples disclosed herein; on the contrary, the present invention can be extended to all other methods and applications with the same function.

Claims

1. A method for preparing a flame-retardant, high-toughness bio-based composite aerogel, characterized in that, Includes the following steps: Step (1): Add sodium alginate oxide to the cellulose acetate nanofiber dispersion and mix and react, then add phytic acid and stir to dissolve, to obtain a cellulose acetate nanofiber / sodium alginate oxide / phytic acid mixed dispersion. Expandable graphite was added to a mixed dispersion of cellulose acetate nanofibers / sodium alginate / phytic acid to obtain a mixed dispersion of cellulose acetate nanofibers / sodium alginate / phytic acid / expandable graphite. Step (2): Crosslink the cellulose acetate nanofiber / sodium alginate / phytic acid / expandable graphite mixed dispersion. After the reaction is completed, freeze-dry in a directional manner to obtain flame-retardant high-toughness bio-based composite aerogel.

2. The method for preparing a flame-retardant, high-toughness bio-based composite aerogel according to claim 1, characterized in that, In step (1), the cellulose acetate nanofiber dispersion is prepared by the following steps: Cellulose acetate was dissolved in a mixed solvent of acetone and N,N-dimethylacetamide to prepare a cellulose acetate spinning solution. The cellulose acetate spinning solution was electrospun and dried to obtain a cellulose acetate nanofiber membrane. The cellulose acetate nanofiber membrane was pulverized and homogenized in water to obtain a cellulose acetate nanofiber dispersion.

3. The method for preparing a flame-retardant, high-toughness bio-based composite aerogel according to claim 2, characterized in that, In step (1), when preparing the cellulose acetate nanofiber dispersion: In the cellulose acetate spinning solution, the mass concentration of cellulose acetate is 12-26%, and the volume ratio of acetone to N,N-dimethylacetamide is 1-4:

1. Electrospinning conditions are as follows: electrospinning at a temperature of 20–40℃, a relative humidity of 60–75%, a spinning distance of 12–26 cm, and a spinning voltage of 10–25 kV. The pulverization and homogenization process includes: pulverizing the cellulose acetate nanofiber membrane and dispersing it in water to form a dispersion; ultrasonically pulverizing the dispersion for 15 to 40 minutes using a cell disruptor at a power of 180 to 460 W; and then shearing and homogenizing the dispersion using a high-speed shear homogenizer at a speed of 10,000 to 30,000 rpm. The mass concentration of cellulose acetate nanofibers in the cellulose acetate nanofiber dispersion is 0.2–1.2%.

4. The method for preparing a flame-retardant, high-toughness bio-based composite aerogel according to claim 1, characterized in that, In step (1), sodium alginate is prepared by the following steps: Sodium alginate was added to 1-ethyl-3-methylimidazolium acetate ionic liquid and stirred continuously to dissolve, thus obtaining a sodium alginate / ionic liquid solution. The sodium alginate / ionic liquid solution was adjusted to pH 4.0, potassium periodate was added, and an oxidation reaction was carried out. After purification, oxidized sodium alginate was obtained.

5. The method for preparing a flame-retardant, high-toughness bio-based composite aerogel according to claim 4, characterized in that, In step (1), when preparing oxidized sodium alginate: the dissolution temperature is 80-95℃; the mass concentration of sodium alginate in the sodium alginate / ionic liquid solution is 0.8-2%; the mass concentration of potassium periodate in the reaction solution is 0.5-2.5 g / L; the oxidation reaction conditions are: oxidation reaction for 30-90 min in a light-proof environment under ultrasonic treatment; wherein, the ultrasonic treatment is intermittent ultrasonic treatment, specifically: after every 15 min of ultrasonic treatment, stop for 1 min, and the power of ultrasonic treatment is 80-150 W.

6. The method for preparing a flame-retardant, high-toughness bio-based composite aerogel according to claim 1, characterized in that, In step (1), the mixing reaction conditions are as follows: 0.1-0.2 mol / L of acetic acid / sodium acetate buffer solution is added dropwise to the cellulose acetate nanofiber dispersion to adjust the pH of the solution to 4.0-5.5, and then sodium alginate is added. The mixture is stirred continuously at 25-75℃ for 1-4 hours to achieve uniform mixing reaction. The stirring and dissolving conditions are as follows: the mixture is stirred continuously at 25-75℃ for 2-4 hours to dissolve.

7. The method for preparing a flame-retardant, high-toughness bio-based composite aerogel according to claim 1, characterized in that, In step (1), the mass ratio of cellulose acetate nanofibers to sodium alginate oxide is 1-3:1-3; the mass concentration of phytic acid in the cellulose acetate nanofibers / sodium alginate oxide / phytic acid mixed dispersion is 0.5-3%; and the mass concentration of expandable graphite in the cellulose acetate nanofibers / sodium alginate oxide / phytic acid / expandable graphite mixed dispersion is 0.15-1.2%.

8. The method for preparing a flame-retardant, high-toughness bio-based composite aerogel according to claim 1, characterized in that, In step (2), the crosslinking reaction conditions are: crosslinking reaction for 0.5 to 3 hours under microwave radiation power of 200 to 320 W and microwave radiation temperature of 45 to 65 °C; directional freeze drying includes: transferring the reacted cellulose acetate nanofiber / sodium alginate / phytic acid / expandable graphite mixed dispersion into a mold, placing the mold on the upper surface of a copper block, and immersing the bottom of the copper block completely in liquid nitrogen for directional freezing. After freezing treatment for 6 to 15 minutes, the cellulose acetate nanofiber / sodium alginate / phytic acid / expandable graphite mixed dispersion is completely frozen into ice. Then, the mold is transferred to a microwave vacuum freeze dryer and continuously freeze-dried for 20 to 60 hours under microwave vacuum freeze-drying conditions of -70 to -55 °C, microwave power of 960 to 1800 W, and vacuum degree of 12 to 20 Pa.

9. A flame-retardant, high-toughness bio-based composite aerogel prepared by the preparation method of the flame-retardant, high-toughness bio-based composite aerogel as described in any one of claims 1-8.

10. The application of the flame-retardant, high-toughness bio-based composite aerogel as described in claim 9 in flame-retardant heat insulation materials, fire early warning materials, intelligent fire monitoring materials, or building heat insulation materials.

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