Reversible composite aerogel for fire early warning and its preparation method

Abstract

To provide a reversible composite aerogel for fire early warning and a preparation method thereof.SOLUTION: A composite aerogel is magnetic graphene oxide / expanded graphite / biomass matrix composite aerogel. Functional bodies of the composite aerogel are uniformly dispersed in a biomass aqueous solution matrix, and a three-dimensional network structure is constructed by intermolecular hydrogen bond cross-linking. Reversible resistance-temperature response characteristics of Fe3O4 nanoparticles and semiconductor behaviors of cooperating with graphene oxide for early warning of fire are exploited, whereby the expanded graphite expands to form an insulating layer. The carbon-forming properties of biomass matrices enable preparation of the composite aerogel for fire early warning with excellent flame-retardant properties that can break through single-time temperature monitoring limit, and material can be used as a friction material for a positive electrode for constituting a friction-charged nano-generator to achieve self-power supply in the early warning process. Thus fire protection performance can be further improved and an application range in the fields of high-rise buildings, chemical industry, electricity, firefighting and the like is widened.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to the technical field of functional nanocomposite materials, in particular to a reversible composite aerogel for fire early warning and its preparation method. [Background technology]

[0002] In recent years, fires caused by organic flammable materials have frequently occurred in fields such as lithium batteries, high-rise building insulation, and electricity, resulting in significant damage to lives and property. For example, on September 20, 2021, a fire broke out in a community in Beijing's Tongzhou District due to thermal runaway of a lithium battery while an electric vehicle was charging indoors. On December 2, 2019, a fire broke out in the SR International New City in Shenyang's Hunnan New District due to the ignition of exterior wall insulation. On June 14, 2017, the exterior cladding of London's Grenfell Tower caught fire, causing a serious fire accident. These incidents have led to stricter requirements for the fire resistance of materials used. Due to their ultralight weight, high porosity, and excellent thermal insulation properties, aerogels are an ideal alternative to traditional flammable materials and have wide application potential in fields such as petrochemicals, military, aerospace, chemical engineering, construction, batteries, environmental protection, and transportation. However, organic aerogels pose certain fire hazards, significantly limiting their practical use in these fields, making improvements in their fire resistance urgently needed. Research has shown that the addition of inorganic fillers such as layered montmorillonite, sodium bicarbonate, expanded graphite, and graphene can significantly improve the flame-retardant properties of organic aerogels, making the preparation of flame-retardant gels a fundamental means of improving their fire protection properties. Furthermore, by monitoring changes in resistance, temperature, smoke density, or infrared intensity, specific functional materials can trigger early warning signals before a fire breaks out, thereby preventing fires in advance. Endowing aerogels with highly sensitive fire early warning capabilities is an effective way to further improve their fire protection performance.

[0003] Currently, research on flame-retardant aerogel systems is more systematic and complete, while research on early-warning aerogels with higher levels of fire protection is in its early stages. From 2020, "An ultrasensitive fire-warning chitosan / montmorillonite / carbon nanotube composite aerogel with high fire resistance" Chem. Eng. J., 399 (2020) 125729 and "Flame-retardant cellulose nanofiber aerogel modified with graphene oxide and sodium montmorillonite and its fire-alarm application" Polym. Adv. Technol.,32(2021)1877-1887 reported that researchers had, for the first time, prepared carbon-based composite aerogels capable of rapid temperature response at high temperatures / in the early stages of fires, based on the thermal reduction of carbon-based nanomaterials such as carbon nanotubes and graphene oxide. However, this type of aerogel's resistance no longer changes after thermal reduction at high temperatures, making it unable to repeatedly monitor temperature. Secondly, when used as a temperature sensor, it usually requires power from an external power source, which is easily damaged by high temperatures and can cause the fire early warning material to fail due to a power outage. Therefore, preparing reversible aerogels for fire early warning, which can be used to build self-powered sensors, remains a major challenge.

[0004] Chinese invention patent publication CN114210276A discloses a magnetic carbon-based composite aerogel that combines fire early warning function and flame retardancy, and a method for preparing the same. Through rational configuration, the prepared magnetic carbon-based composite aerogel can have both fire early warning function and flame retardancy, but does not have a reversible fire early warning function. Summary of the Invention

[0005] Considering the fact that prior art carbon-based composite aerogels can only achieve a rapid response during a single high temperature / fire early stage and that constructing a self-powered sensor capable of repeated temperature monitoring remains a challenge, the objective of the present invention is to provide a reversible composite aerogel for fire early warning and a method for preparing the same.

[0006] In order to achieve the object of the present invention, the technical solution of the present invention is as follows: A reversible composite aerogel for fire early warning, which is made from magnetic graphene oxide nanosheets, expanded graphite nanosheets, and a biomass matrix, which are cross-linked by intermolecular hydrogen bonds, self-assembled into a 3D structure, and freeze-dried.

[0007] The magnetic graphene oxide nanosheets are graphene oxide nanosheets modified with magnetic Fe3O4 nanoparticles.

[0008] When exposed to high temperatures, the expanded graphite nanosheets instantly expand in volume by 100 to 250 times, transforming from a sheet shape into a worm-like shape, and forming an excellent heat insulating layer.

[0009] The biomass matrix is ​​one or more of carboxymethyl chitosan, hydroxypropyl chitosan, hydroxypropyl methylcellulose, cellulose nanofibers, sodium alginate.

[0010] Preferably, the mass ratio of the magnetic graphene oxide nanosheets in the reversible fire early warning composite aerogel is 10-60 wt%, the content of the magnetic Fe3O4 nanoparticles is the upper limit of the magnetic Fe3O4 nanoparticles that can be grown and / or grafted by the graphene oxide sheets, and the mass ratio of the expanded graphite nanosheets in the reversible fire early warning composite aerogel is 0.5-6 wt%.

[0011] Preferably, the magnetic Fe3O4 nanoparticles and expanded graphite nanosheets are the flame-retardant functional materials of the reversible fire early warning composite aerogel, serving as a physical barrier against catalytic carbonization and expansion, respectively, while the expanded graphite nanosheets and biomass matrix serve as carbon sources to form a stable and dense carbon layer.

[0012] Preferably, the reversible composite aerogel for fire early warning is the reversible composite aerogel for fire early warning according to any one of claims 1 to 3, and the preparation method thereof comprises the following steps: (1) Magnetization modification of graphene oxide nanosheets: Magnetic graphene oxide nanosheets were obtained using one of two methods: grafting magnetic Fe3O4 nanoparticles by reaction between functional groups or in situ growth of magnetic Fe3O4 nanoparticles by coprecipitation. The method for grafting magnetic Fe3O4 nanoparticles using a reaction between the functional groups is as follows: the surface of existing magnetic Fe3O4 nanoparticles is amino-modified by silane ligand exchange, and then magnetic modification is performed using a reaction between the magnetic Fe3O4 nanoparticles and the carboxyl functional groups on the surface of the graphene oxide nanosheets to obtain magnetic graphene oxide nanosheets, and the size of the magnetic Fe3O4 nanoparticles is 10 to 200 nm; The method for in-situ growth of magnetic Fe3O4 nanoparticles by co-precipitation is as follows: 2+ and Fe 3+ The precursor is added to a graphene oxide dispersion, and magnetic graphene oxide nanosheets are grown in situ by coprecipitation under heating conditions using an alkali catalyst. (2) Preparation of reversible fire early warning composite aerogel: Under conditions of ultrasonic frequency 38-42 kHz and temperature 22-28°C, (1.12-17.65) wt% of expanded graphite nanosheets and (11.2-176.5) wt% of the magnetic graphene oxide nanosheets prepared in step (1) are added to the biomass matrix aqueous solution, and the mass ratio of the magnetic graphene oxide nanosheets to the expanded graphite nanosheets is (20-5):1. Ultrasonic dispersion is carried out for 0.5-1 hour, and intermolecular hydrogen bonding is used to induce self-assembly into a three-dimensional network structure. After aging for 2-6 hours, the composite aerogel for reversible fire early warning is obtained by freeze-drying at a temperature of -20 to (-80)°C for 12-24 hours.

[0013] Preferably, the method for grafting magnetic Fe3O4 nanoparticles using the reaction between the functional groups is as follows: I. Mix 0.18-0.22M aqueous sodium oleate solution with 0.18-0.22M aqueous anhydrous ferric chloride solution in a volume ratio of 1:(0.9-1.1), stir well to produce a reddish-brown precipitate, filter, rinse with deionized water, and then dry in a vacuum oven to obtain a waxy substance. After drying, the waxy substance is dissolved in ethanol whose volume is 55-65% of the volume of the sodium oleate aqueous solution. Oleic acid is then added to the ethanol and mixed uniformly. The amount of oleic acid added is 8-12% of the volume of the ethanol. The mixture is then transferred to a polytetrafluoroethylene high-pressure reactor and reacted at a temperature of 175-185°C for 4-6 hours. The mixture is washed with anhydrous ethanol, separated with a magnet, and dispersed in toluene to obtain a toluene dispersion of Fe3O4 nanoparticles. II. Modification using silane ligand exchange method, that is, adding 0.4-0.6% by volume of aminosilane and 0.008-0.015% by volume of acetic acid to the toluene dispersion of Fe3O4 nanoparticles obtained in step 1 under the conditions of ultrasonic frequency 38-43KHz and temperature 21-28°C, ultrasonic treatment for 15-30 minutes, stirring at room temperature for 22-48 hours to react, washing with toluene, separating with a magnet, and freeze-drying the separated product to obtain amino-modified magnetic Fe3O4 nanoparticles, thereby amino-modifying the surface of the existing magnetic Fe3O4 nanoparticles through silane ligand exchange. III. The reaction between the amino-modified magnetic Fe3O4 nanoparticles in step II and the carboxyl functional groups on the surface of the graphene oxide nanosheets is utilized to achieve magnetization modification, thereby obtaining magnetic graphene oxide nanosheets.

[0014] The reaction between the amino-modified magnetic Fe3O4 nanoparticles and the carboxyl functional groups on the surface of the graphene oxide nanosheets is preferably as follows: The graphene oxide nanosheets are uniformly dispersed in water using ultrasound (preferably, for example, 1 g of graphene oxide nanosheets is dispersed in 50 mL of water), 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride (EDC) and N-hydroxysuccinimide (NHS) are added (preferably, in a ratio of 1.5 to 2.5:1), and the activation reaction is carried out at room temperature for 10 to 18 minutes. After that, the amino-modified magnetic Fe3O4 nanoparticles prepared in step II are added (preferably, in a ratio of, for example, 8 to 12 wt% of the graphene oxide nanosheets), and the reaction is continued with ultrasound for 10 to 15 hours to obtain magnetic carbon nanotubes (i.e., magnetic graphene oxide nanosheets).

[0015] Preferably, the aminosilane is one or more of 3-aminopropyltrimethoxysilane, 3-aminopropyltriethoxysilane, 3-(2-aminoethylamino)propyltrimethoxysilane, 4-aminobutyldimethylmethoxysilane, 4-aminobutyltriethoxysilane, 3-[(2-aminoethylamino)propyl]dimethoxysilane, (3-aminopropyl)dimethylethoxysilane.

[0016] Preferably, the method for in-situ growth of magnetic Fe3O4 nanoparticles by coprecipitation is as follows: graphene oxide nanosheets are ultrasonically dispersed in deionized water under magnetic stirring at a rotation speed of 500-800 rpm to obtain a graphene oxide dispersion, in which the graphene oxide nanosheets:deionized water ratio is (1.5-2.5) g:(48-52) mL; then, an inert gas is introduced to deoxidize for 1.8-2.2 hours; and then, Fe3O4 nanoparticles are added in a molar ratio of (1.5-2.5):1, in an amount of 0.8-1.5% of the volume of the deionized water. 3+ and Fe 2+ After stirring at room temperature for 2-5 hours under inert gas protection, the temperature is raised to 70-90°C, aqueous ammonia is added to adjust the pH value of the solution to 9-12, and the temperature is maintained at 70-90°C for 1-2 hours to produce magnetic graphene oxide nanosheets.

[0017] Preferably, the inert gas is N2.

[0018] Preferably, the mass ratio of the magnetic graphene oxide nanosheets to the expanded graphite nanosheets is 10:1.

[0019] Applications of the reversible fire early warning composite aerogel, which are fire alarms for high-rise buildings and applications in the fields of flame retardancy, thermal insulation and fire protection, are the reversible fire early warning composite aerogel described above or the reversible fire early warning composite aerogel prepared by the above preparation method.

[0020] Preferably, the application uses magnetic Fe3O4 nanoparticles with reversible resistance-temperature response as a fire alarm functional body, and graphene oxide nanosheets with single-time thermal reduction and multiple-time thermal conductivity properties as a carrier for Fe3O4. This synergistic effect is achieved by breaking through the limitations of single-time temperature monitoring. Under normal conditions, the reversible fire early warning composite aerogel will not trigger a fire early warning or turn on the warning light. However, upon encountering a fire for the first time or being exposed to high temperatures, the resistance of the magnetic Fe3O4 nanoparticles will decrease. In both cases, the graphene oxide nanosheets undergo thermal reduction to remove their functional groups, resulting in a rapid decrease in resistance and a large change in the resistance and current in the closed circuit, triggering an early warning and / or illuminating an early warning light. Upon encountering a fire and high temperature for the Nth time, where N is greater than or equal to 2, the resistance of the magnetic Fe3O4 nanoparticles still reversibly decreases, but the graphene sheets only provide a continuous electron conduction path, allowing the magnetic Fe3O4 nanoparticles to repeatedly contribute to realizing a fire early warning and / or illuminating an early warning light.

[0021] The reversible fire early warning composite aerogel has excellent flame retardant properties. The inorganic Fe3O4 nanoparticles and expanded graphite nanosheets function as flame retardants, respectively, by catalytic carbonization and layered physical barrier functions. The cellulose or other biomass matrix promotes carbon formation, forming a stable and dense carbon layer. Each component acts synergistically to improve flame retardant performance. Excellent flame retardant properties are the basis for a material to provide fire early warning. To achieve rapid flame detection and fire early warning response, the material must be thermally stable and flame retardant during detection; otherwise, timely and reliable warning will not be achieved.

[0022] The reversible fire early warning composite aerogel exhibits repeatable temperature monitoring capabilities. It uses magnetic Fe3O4 nanoparticles with reversible resistance-temperature response as the fire warning functional body and graphene oxide nanosheets with single-cycle thermal reduction and multiple-cycle thermal conductivity as the Fe3O4 carrier. This synergistic effect breaks the limitations of single-cycle temperature monitoring. Under normal conditions, the composite aerogel does not trigger a fire early warning or activate the warning light. However, upon encountering a fire / high temperature for the first time, the resistance of the Fe3O4 nanoparticles decreases, and the graphene oxide nanosheets undergo thermal reduction to remove their functional groups, resulting in a rapid decrease in resistance and a large change in the resistance / current in the closed circuit, triggering the early warning and activating the early warning light. After encountering fire / high temperature for the Nth (N≧2) time, the resistance of the Fe3O4 nanoparticles continues to decrease reversibly. However, the graphene sheets simply provide a continuous electron conduction path, allowing the Fe3O4 nanoparticles to repeatedly realize fire early warning and activate the early warning light.

[0023] Because the reversible fire early warning performance of the composite aerogel for reversible fire early warning is affected by the inconstant resistance change of graphene oxide, a rational test method and a three-stage test procedure were required. Specifically, a four-point probe was used to measure the resistance-temperature response curves of three types of aerogel splines (GO-Fe3O4, GO only, and Fe3O4 only) between 30 and 400°C. Next, the splines were placed on a heating plate and alternately heated (300°C) and cooled (to room temperature). A digital multimeter was used to measure the output current curves to characterize the reversibility of the resistance-temperature response and determine the appropriate resistance value to trigger the early warning. Finally, a complete circuit including the aerogel spline, warning light, and power supply was designed. The spline was exposed to the flame of an alcohol lamp for 20 or 40 seconds, and the current change and the lighting status of the warning light were recorded.

[0024] The technical effects of the present invention are as follows:

[0025] The present invention rationally and specifically sets the composition of each component of the reversible fire early warning system and the manufacturing parameters of each step of the preparation method, prepares a series of composite aerogels, and establishes the internal relationship between material composition, structure, and performance, thereby realizing the optimized design of aerogel preparation parameters and processes with performance as the guide.

[0026] By investigating the mechanisms of flame retardancy and reversible fire alarm, the present invention clarifies the mechanisms of these performances at the molecular level, determines the specific product composition of the present invention, and further improves the fire protection performance of aerogel materials, which can be used in fields such as fire alarms, flame retardancy, thermal insulation, and fire protection in high-rise buildings. [Brief explanation of the drawings]

[0027] [Figure 1] Schematic structure of the composite aerogel (magnetic graphene oxide / expanded graphite / biomass matrix) for reversible fire early warning. [Figure 2] FIG. 1 is a schematic diagram of a reversible composite aerogel for fire early warning. [Figure 3] FIG. 10 is a schematic diagram of repeatable temperature monitoring of composite aerogels for reversible fire early warning. DETAILED DESCRIPTION OF THE INVENTION

[0028] The following detailed description will be given of preferred embodiments of the present invention in conjunction with examples. It should be understood that the following examples are for illustrative purposes only and do not limit the scope of the present invention. Those skilled in the art can make various modifications and substitutions to the present invention without departing from the spirit and scope of the present invention.

[0029] The experimental methods used in the following examples are conventional methods unless otherwise specified. All materials, reagents, etc. used in the following examples are commercially available unless otherwise specified.

[0030] Example 1 This example provides a method for preparing a composite aerogel with in situ magnetic modification by graphene oxide, which has high fire protection performance and reversible fire early warning function. The preparation structure is shown in Figure 1 and specifically includes the following steps: (1) Preparation of magnetic graphene oxide nanosheets It was synthesized by coprecipitation. Specifically, 2 g of graphene oxide nanosheets were ultrasonically dispersed in 50 mL of deionized water, and then N2 was introduced to deoxidize for 2 hours. After that, the graphene oxide nanosheets were mixed with Fe2O3 in a molar ratio of 2:1. 3+ / Fe 2+ Add 0.5 mL of (i.e., 1.6 g of FeCl3·6H2O and 0.93 g of FeSO4·7H2O) and stir at room temperature for 5 h under N2 protection. Then, heat to 80 °C, add aqueous ammonia, adjust to pH = 12, and react for 1 h until complete. Collect with a magnet and wash with deionized water three times to obtain magnetic graphene oxide nanosheets, designated as GO@Fe3O4. (2) Preparation of reversible composite aerogels for fire early warning 3 g of cellulose nanofibers were dispersed in 6 mL of deionized water and sonicated to homogenize the mixture. 0.5 g of GO@Fe3O4 and 0.25 g of expanded graphite nanosheets were added and sonicated for 2 h. The resulting crosslinked hydrogel was then aged at room temperature for 3 h and freeze-dried (-45 °C, 260 Pa) for 18 h to prepare a reversible composite aerogel for fire early warning.

[0031] Example 2 This example provides a method for preparing a composite aerogel modified with graphene oxide-grafted magnetic particles according to the present invention, which has high fire protection performance and reversible fire early warning function. The preparation process is as follows: (1) Preparation of Fe3O4 nanoparticles Mix 100 mL of 0.2 M sodium oleate solution with 100 mL of 0.2 M anhydrous ferric chloride solution and stir thoroughly to produce a reddish-brown precipitate. This precipitate is then filtered, rinsed with deionized water, and dried in a vacuum oven. The dried wax-like substance is dissolved in 60 mL of ethanol, 6 mL of oleic acid is added, and the mixture is mixed uniformly. The mixture is then transferred to a polytetrafluoroethylene high-pressure reactor and reacted at 180 °C for 5 h. The precipitate is washed with absolute ethanol, separated with a magnet, and dispersed in toluene. (2) Amino modification of the surface of Fe3O4 nanoparticles The nanoparticles were modified using the silane ligand exchange method by adding 0.5% (v / v) 3-aminopropyltriethoxysilane and 0.01% (v / v) acetic acid to 100 ml of a toluene dispersion of Fe3O4 nanoparticles (0.1 g) and stirring at room temperature for 24 h. After washing with toluene and separating with a magnet, the nanoparticles were freeze-dried and labeled as Fe3O4@NH2. (3) Preparation of magnetic graphene oxide nanosheets 1 g of graphene oxide nanosheets was uniformly dispersed in 50 mL of water by ultrasonication, and 0.62 g of 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride (EDC) and 0.31 g of N-hydroxysuccinimide (NHS) were added for 15 min activation reaction at room temperature. After that, 0.1 g of Fe3O4@NH2 nanoparticles were added, followed by 12 h ultrasonic reaction to obtain magnetic carbon nanotubes, designated as GO@Fe3O4. (4) Preparation of reversible composite aerogels for fire early warning 3 g of cellulose nanofibers were dispersed in 6 mL of deionized water and sonicated to homogenize the mixture. 0.75 g of GO@Fe3O4 and 0.375 g of expanded graphite nanosheets were added and sonicated for 2 h. The resulting crosslinked hydrogel was then aged at room temperature for 3 h and freeze-dried (-45 °C, 260 Pa) for 18 h to prepare the composite aerogel.

[0032] (Application example) The reversible fire early warning composite aerogel of the present invention takes full advantage of the reversible resistance-temperature response characteristics of Fe3O4 nanoparticles and the single-cycle thermal reduction and multiple-cycle thermal conductivity characteristics of graphene oxide nanosheets as the Fe3O4 carrier, thereby breaking through the limitations of single-cycle temperature monitoring. Under normal conditions, the composite aerogel does not trigger a fire alarm or light the warning light (as shown in Figure 2(a)). However, upon encountering a fire or high temperature for the first time, the resistance of the Fe3O4 nanoparticles decreases, and the graphene oxide undergoes thermal reduction to remove its functional groups, resulting in a rapid decrease in resistance and a large change in the resistance / current in the closed circuit, triggering an early warning and lighting the early warning light (as shown in Figure 2(b1)). After encountering fire and high temperature for the Nth (N≧2) time, the resistance of the Fe3O4 nanoparticles continues to decrease reversibly, but at this time, the graphene sheets simply provide a continuous electron conduction path, allowing the magnetic Fe3O4 nanoparticles to repeatedly trigger a fire early warning and light the early warning light (as shown in Figure 2(b2)).

[0033] This application example provides a method for evaluating the reversible fire alarm performance of composite aerogels with high fire protection performance. Because graphene oxide is subject to the inconstant resistance change discipline, a rational test method was designed and a "three-stage" test procedure was selected. Specifically, a four-point probe was used to measure the resistance-temperature response curves of three types of aerogel splines (GO-Fe3O4, GO only, and Fe3O4 only) between 30 and 400°C. The splines were then placed on a heating plate and alternately heated (300°C) and cooled (to room temperature). A digital multimeter was used to measure the output current curves to characterize the reversibility of the resistance-temperature response and determine the appropriate resistance value to trigger an early warning. Finally, a complete circuit including the aerogel spline, warning light, and power supply was designed. The splines were exposed to an alcohol lamp flame for 20 or 40 seconds, and the current change and the status of the warning light were recorded. The experimental results for graphene-based aerogels show a corresponding periodic change in current (right panel of Figure 3), which is significantly different from the single current change and subsequent constant result (left panel of Figure 3). This demonstrates the feasibility of repeated temperature monitoring. The left panel of Figure 3 shows that when graphene aerogels are first exposed to high temperatures, the resistance decreases and the current increases after functional groups such as amino and carboxyl groups are removed. However, because the functional group removal process is irreversible, upon the nth high temperature exposure, there are no functional groups to be removed, so the resistance remains unchanged, providing only a single early warning. The right panel of Figure 3 demonstrates the Fe3O4 semiconductor effect of the present invention. The resistance changes with temperature and is reversible, allowing for repeated monitoring.

[0034] The reversible composite aerogel for fire early warning described in the present invention has improved fire protection performance and can be used in the fields of fire alarms for high-rise buildings, flame retardancy, thermal insulation, fire protection, etc.

[0035] Although the present invention has been described in detail above using general descriptions and specific embodiments, it is obvious to those skilled in the art that some changes or improvements can be made based on the present invention. Therefore, all such changes or improvements made without departing from the spirit of the present invention are included in the scope of protection claimed by the present invention.

Claims

1. A reversible composite aerogel for fire early warning, the composite aerogel for fire early warning being made by cross-linking magnetic graphene oxide nanosheets, expanded graphite nanosheets and a biomass matrix through intermolecular hydrogen bonds, self-assembling into a 3D structure and freeze-drying; The magnetic graphene oxide nanosheets are magnetic Fe ３ O ４ Graphene oxide nanosheets modified with nanoparticles, When exposed to high temperatures, the expanded graphite nanosheet instantly expands in volume by 100 to 250 times, transforming from a sheet to a worm-like shape, and forming an excellent heat insulating layer. The biomass matrix is ​​one or more of carboxymethyl chitosan, hydroxypropyl chitosan, hydroxypropyl methylcellulose, cellulose nanofiber, and sodium alginate.

2. The mass ratio of the magnetic graphene oxide nanosheets in the reversible fire early warning composite aerogel is 10 to 60 wt %, and the magnetic Fe ３ O ４ The content of the nanoparticles is magnetic Fe, which can be grown and / or grafted by graphene oxide sheets. ３ O ４ 2. The reversible fire early warning composite aerogel according to claim 1, wherein the mass ratio of the expanded graphite nanosheets in the reversible fire early warning composite aerogel is 0.5 to 6 wt %, which is the upper limit of nanoparticles.

3. The magnetic Fe ３ O ４ The reversible composite aerogel for fire early warning according to claim 1 or 2, characterized in that the nanoparticles and expanded graphite nanosheets are flame-retardant functional bodies of the reversible composite aerogel for fire early warning, and serve as a physical barrier against catalytic carbonization and expansion, respectively, while the expanded graphite nanosheets and biomass matrix serve as carbon sources to form a stable and dense carbon layer.

4. The reversible composite aerogel for fire early warning according to claim 1, the preparation method of which comprises the following steps: (1) Magnetization modification of graphene oxide nanosheets: Magnetic Fe is produced by the reaction between functional groups. ３ O ４ Magnetic Fe nanoparticles were grafted or co-precipitated. ３ O ４ Magnetic graphene oxide nanosheets were obtained using one of two methods: in situ growth of nanoparticles; Magnetic Fe is produced by using the reaction between the functional groups. ３ O ４ The method for grafting nanoparticles is as follows: ３ O ４ The surface of the nanoparticles was amino-modified by silane ligand exchange to produce magnetic Fe ３ O ４ The magnetic properties of the graphene oxide nanosheets are modified by reacting the nanoparticles with the carboxyl functional groups on the surface of the graphene oxide nanosheets to obtain magnetic graphene oxide nanosheets. ３ O ４ The size of the nanoparticles is 10-200 nm, By the coprecipitation, magnetic Fe ３ O ４ The method for in situ growth of nanoparticles is as follows: Fe ２＋ and Fe ３＋ The precursor is added to a graphene oxide dispersion, and magnetic graphene oxide nanosheets are generated in situ by coprecipitation under heating conditions using an alkali catalyst. (2) Preparation of reversible composite aerogel for fire early warning: A method for preparing a reversible composite aerogel for fire early warning, comprising: adding (1.12 to 17.65) wt % of expanded graphite nanosheets and (11.2 to 176.5) wt % of the magnetic graphene oxide nanosheets prepared in step (1) to a biomass matrix aqueous solution under conditions of an ultrasonic frequency of 38 to 42 kHz and a temperature of 22 to 28°C, such that the mass ratio of the magnetic graphene oxide nanosheets to the expanded graphite nanosheets is (20 to 5):1; ultrasonic dispersion is carried out for 0.5 to 1 hour, and intermolecular hydrogen bonding is used to induce self-assembly to form a three-dimensional network structure; followed by aging for 2 to 6 hours; and freeze-drying at a temperature of -20 to -80°C for 12 to 24 hours to obtain a reversible composite aerogel for fire early warning.

5. Magnetic Fe is produced by using the reaction between the functional groups. ３ O ４ The method for grafting nanoparticles is as follows: I. A 0.18-0.22M aqueous solution of sodium oleate and a 0.18-0.22M aqueous solution of anhydrous ferric chloride were mixed in a volume ratio of 1:(0.9-1.1), and stirred thoroughly to produce a reddish-brown precipitate. The precipitate was filtered and rinsed with deionized water, then placed in a vacuum oven and dried to obtain a waxy substance. The waxy substance was then dissolved in ethanol whose volume was 55-65% of the volume of the aqueous solution of sodium oleate. Oleic acid was then added to the ethanol and mixed uniformly, with the amount of oleic acid being 8-12% of the volume of the ethanol. The mixture was then transferred to a polytetrafluoroethylene high-pressure reactor and reacted at a temperature of 175-185°C for 4-6 hours. The mixture was washed with absolute ethanol and separated using a magnet. The separated product was then dispersed in toluene to obtain Fe. ３ O ４ A toluene dispersion of nanoparticles was obtained. II. Modification by silane ligand exchange method, that is, 0.4-0.6% by volume of aminosilane and 0.008-0.015% by volume of acetic acid are added to the Fe obtained in step 1 under the conditions of ultrasonic frequency 38-43KHz and temperature 21-28°C. ３ O ４ The nanoparticles were added to the toluene dispersion, ultrasonicated for 15 to 30 minutes, and stirred at room temperature for 22 to 48 hours to react. The mixture was washed with toluene, separated with a magnet, and then freeze-dried to obtain amino-modified magnetic Fe nanoparticles. ３ O ４ nanoparticles, whereby the surface of the existing magnetic Fe3O4 nanoparticles is amino-modified by silane ligand exchange; III. Step II amino-modified magnetic Fe ３ O ４ The method for preparing a reversible composite aerogel for fire early warning, as described in claim 4, characterized in that the magnetic modification is achieved by utilizing the reaction between nanoparticles and carboxyl functional groups on the surface of the graphene oxide nanosheets to obtain magnetic graphene oxide nanosheets.

6. 6. The method for preparing a reversible composite aerogel for fire early warning according to claim 5, wherein the aminosilane is one or more of 3-aminopropyltrimethoxysilane, 3-aminopropyltriethoxysilane, 3-(2-aminoethylamino)propyltrimethoxysilane, 4-aminobutyldimethylmethoxysilane, 4-aminobutyltriethoxysilane, 3-[(2-aminoethylamino)propyl]dimethoxysilane, and (3-aminopropyl)dimethylethoxysilane.

7. By the coprecipitation, magnetic Fe ３ O ４ The method for in-situ growth of nanoparticles is as follows: graphene oxide nanosheets are ultrasonically dispersed in deionized water under magnetic stirring at a rotation speed of 500-800 rpm to obtain a graphene oxide dispersion, where the graphene oxide nanosheets:deionized water ratio is (1.5-2.5) g:(48-52) mL, and then an inert gas is introduced to deoxidize for 1.8-2.2 h. After that, a molar ratio of Fe2O3 of (1.5-2.5):1, which is 0.8-1.5% of the volume of the deionized water, is added. ３＋ and Fe ２＋ and stirring the mixture at room temperature for 2-5 hours under inert gas protection, followed by heating to 70-90°C, adding aqueous ammonia to adjust the pH of the solution to 9-12, and maintaining the temperature at 70-90°C for 1-2 hours to obtain magnetic graphene oxide nanosheets.

8. 5. The method for preparing a reversible composite aerogel for fire early warning as claimed in claim 4, wherein the mass ratio of the magnetic graphene oxide nanosheets to the expanded graphite nanosheets is 10:

1.

9. Use of a reversible fire early warning composite aerogel in the fields of fire alarms for high-rise buildings and flame retardancy, thermal insulation and fire protection, characterized in that the reversible fire early warning composite aerogel is the reversible fire early warning composite aerogel described in claim 1 or a reversible fire early warning composite aerogel prepared by the preparation method described in claim 4.

10. Magnetic Fe having reversible resistance-temperature response ３ O ４ The nanoparticles were used as fire alarm functional bodies, and graphene oxide nanosheets with single thermal reduction and multiple thermal conductivity properties were synthesized using Fe ３ O ４ This synergistic effect is achieved by breaking through the limitations of single-time temperature monitoring. Under normal conditions, the reversible fire alarm composite aerogel will not trigger a fire alarm or light up the warning light, but upon encountering a fire for the first time or being exposed to high temperatures, the magnetic Fe aerogel will ３ O ４ As the resistance of the nanoparticles decreases, the graphene oxide nanosheets can be thermally reduced to remove the functional groups, resulting in a rapid decrease in resistance, a large change in the resistance and current in the closed circuit, and trigger an early warning and / or light an early warning light when encountering fire and high temperature for the Nth time, where N≧2. ３ O ４ Although the resistance of the nanoparticles still reversibly decreases, the magnetic Fe ３ O ４ 10. Use according to claim 9, characterized in that the nanoparticles contribute to realizing repeated fire early warnings and / or to illuminating early warning lights.

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