MXene-based aerogel as well as normal-pressure preparation method and application thereof

Abstract

The invention belongs to the technical field of aerogel, and discloses MXene-based aerogel and a normal-pressure preparation method and application thereof.The preparation method comprises the steps that mixed dispersion liquid of chitosan and conductive filler is mixed with glutaraldehyde, standing and crosslinking are conducted, and hydrogel is obtained; in the hydrogel, the conductive filler accounts for 20-75% of the mass of a solid substance, the chitosan accounts for 15-55% of the mass of the solid substance, and the glutaraldehyde accounts for 1-20% of the mass of the solid substance; the filler is selected from MXene, a carbon nano tube, graphene, boron nitride or MOF (Metal Organic Framework); the hydrogel is frozen at the temperature ranging from-20 DEG C to-80 DEG C; and carrying out normal pressure drying on the frozen hydrogel at 25-80 DEG C to obtain the aerogel. The solvent exchange-free normal-pressure drying preparation process breaks through the limitation of solvent exchange on the size, the high cost of a large amount of solvent is saved, large-area extensible preparation can be realized on the basis of energy conservation and environmental protection, and meanwhile, the prepared aerogel has relatively high mechanical strength, and the application range is widened.

Description

Technical Field

[0001] This invention belongs to the field of aerogel preparation technology, specifically relating to an MXene-based aerogel and its preparation method and application under normal pressure. Background Technology

[0002] The statements herein provide only background information in relation to this invention and do not necessarily constitute prior art.

[0003] The main methods for preparing MXene aerogels include atmospheric pressure drying, freeze drying, and supercritical carbon dioxide drying. Compared to atmospheric pressure drying, other preparation processes are limited by the size constraints of energy-intensive equipment, affecting the size and scale of aerogel production and hindering their practical applications.

[0004] During the ordinary drying process, when the liquid in the gel pores evaporates, a huge capillary force is generated at the gas-liquid interface, which breaks down the fragile porous nano-network structure of the aerogel, causing the pores to collapse and the volume to shrink significantly. In the end, instead of obtaining an aerogel, a dense dry gel is obtained.

[0005] To eliminate capillary forces, during atmospheric pressure drying, solvent displacement is typically used to replace the water in the gel pores with an organic solvent with low surface tension, such as alcohol or acetone. Additionally, a silanizing agent is usually used to replace the hydrophilic groups on the gel backbone with hydrophobic organic groups, preventing the low surface tension solvent from wetting the gel backbone and thus minimizing capillary forces.

[0006] However, solvent replacement takes a long time, generally 1-7 days, and the solvent needs to be replaced regularly during the process to ensure complete water replacement. Therefore, this method is time-consuming, consumes a large amount of solvent, is costly, and is not environmentally friendly.

[0007] In addition, aerogels have problems such as low mechanical strength, high brittleness, low toughness and easy pulverization, making them difficult to use on a large scale. Summary of the Invention

[0008] To address the shortcomings of existing technologies, the purpose of this invention is to provide an MXene-based aerogel, its preparation method under ambient pressure, and its applications. The preparation process of this material is economical, energy-saving, and scalable, improving production efficiency while reducing production costs. Furthermore, the prepared aerogel exhibits good mechanical properties, laying the foundation for the practical application of aerogels.

[0009] To achieve the above objectives, the present invention is implemented through the following technical solution: In a first aspect, the present invention provides a method for preparing MXene-based aerogels under normal pressure, comprising the following steps: A mixture of chitosan and conductive filler dispersions was mixed with glutaraldehyde and allowed to stand for crosslinking to obtain a hydrogel. In the hydrogel, the conductive filler accounts for 20-75% of the solid matter by mass, chitosan accounts for 15-55% of the solid matter by mass, and glutaraldehyde accounts for 1%-20% of the solid matter by mass. The filler is selected from MXene, carbon nanotubes, graphene, boron nitride, or MOF; The hydrogel was frozen at -20℃ to -80℃; The frozen hydrogel is dried at normal pressure at 25℃-80℃ to obtain the aerogel.

[0010] Secondly, the present invention provides an MXene-based aerogel prepared by the aforementioned preparation method.

[0011] Thirdly, the present invention provides the application of the high mechanical strength aerogel in the preparation of electromagnetic wave shielding / absorbing devices or thermal protection elements.

[0012] The beneficial effects achieved by one or more embodiments of the present invention described above are as follows: (1) The solvent-free atmospheric pressure preparation method for crosslinked MXene aerogel proposed in this invention has the following advantages: First, the solvent-free atmospheric pressure drying preparation process breaks through the size limitation of solvent exchange, saves a lot of high solvent costs, and enables large-area scalable preparation on the basis of energy saving and environmental protection. At the same time, the prepared aerogel has high mechanical strength, which broadens the application range. Second, based on the adjustable content of fillers such as MXene, the interaction between the incident electromagnetic wave and the aerogel is different, thus realizing that the MXene aerogel has adjustable shielding and absorption characteristics.

[0013] (2) The preparation process of the present invention enhances the pore wall by means of the chitosan with intrinsic high strength and induces the hydrophobic interaction to construct hydrophobic micro-regions by crosslinking with glutaraldehyde, thus giving low surface tension. When chitosan, filler and glutaraldehyde are compounded in a specific mass ratio, they can resist capillary force and maintain the pore structure without collapsing and the aerogel maintains high strength when dried under normal pressure.

[0014] (3) Controlling the amount of glutaraldehyde added achieves control over the crosslinking density of the aerogel. When the glutaraldehyde content is ≥7.0%, the crosslinking density is high, the number of hydrophobic micro-regions is large, and the aerogel is hydrophobic. When the glutaraldehyde content is <7.0%, the crosslinking density is low, the number of hydrophobic micro-regions is small, and the aerogel is hydrophilic.

[0015] (4) Cross-linked MXene aerogels with different MXene contents prepared by solvent-free exchange and atmospheric pressure drying have the characteristics of switching between absorption and shielding. For example, when the MXene content is low, the aerogel exhibits wave absorption performance and shows the best reflection loss of -51.6 dB when the thickness is 2.8 mm. When the MXene content is high, the aerogel exhibits shielding performance and has a shielding performance of -76.0 dB when the aerogel thickness is 8 mm. It has a wide range of application value.

[0016] (5) The intrinsic flame-retardant properties of MXene and chitosan, combined with the chemical cross-linking structure of the aerogel, enable the aerogel to exhibit a high limiting oxygen index of 60% and a cyclic fire warning performance of up to 196 seconds per cycle. The synergy between the components and the structure also achieves excellent thermal insulation performance.

[0017] (6) The atmospheric pressure preparation process of the double crosslinked MXene aerogel without solvent exchange proposed in this invention can promote their large-scale application in the fields of electromagnetic wave protection and thermal protection due to its low cost and high efficiency. Attached Figure Description

[0018] The accompanying drawings, which form part of this invention, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an improper limitation of the invention.

[0019] Figure 1 Atomic force microscopy and transmission electron microscopy images of MXene used in Example 1.

[0020] Figure 2 Photograph of chitosan / MXene hydrogel after chemical cross-linking with glutaraldehyde.

[0021] Figure 3 Comparative Example 1 shows photos of each component added, directly frozen, and then dried at atmospheric pressure.

[0022] Figure 4 Photograph of the large-size cross-linked MXene aerogel prepared in Example 1.

[0023] Figure 5 Scanning electron microscope image of the cross-linked MXene aerogel prepared in Example 1.

[0024] Figure 6 Frequency-selective absorption characteristics of the cross-linked MXene aerogel prepared in Example 1.

[0025] Figure 7 Limiting oxygen index of pure chitosan aerogel and cross-linked MXene aerogel prepared in Example 1. Detailed Implementation

[0026] It should be noted that the following detailed description is illustrative and intended to provide further explanation of the invention. Unless otherwise specified, all technical and scientific terms used in this invention have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains.

[0027] In a first aspect, the present invention provides a method for preparing MXene-based aerogels under normal pressure, comprising the following steps: A mixture of chitosan and conductive filler dispersions was mixed with glutaraldehyde and allowed to stand for crosslinking to obtain a hydrogel. In the hydrogel, the conductive filler accounts for 20-75% of the solid matter by mass, chitosan accounts for 15-55% of the solid matter by mass, and glutaraldehyde accounts for 1%-20% of the solid matter by mass. The filler is selected from MXene, carbon nanotubes, graphene, boron nitride, or MOF; The hydrogel was frozen at -20℃ to -80℃; The frozen hydrogel is dried at normal pressure at 25℃-80℃ to obtain the aerogel.

[0028] Traditional atmospheric pressure drying processes using solvent replacement methods are time-consuming, consume large amounts of solvent, are costly, and are not environmentally friendly.

[0029] During their actual research, the inventors discovered that after solvent replacement, subsequent drying at normal pressure leads to a decrease in the mechanical properties of the prepared aerogel. The specific reasons are analyzed below: In MXene hydrogels, the surface of MXene nanosheets is rich in a large number of hydrophilic terminal functional groups. When the nanosheets self-assemble into a three-dimensional network in water, the connection between adjacent nanosheets is mainly through hydrogen bonds and electrostatic interactions, which gives the hydrogel a certain structural strength and elasticity.

[0030] When solvent replacement is performed, replacing the water in the gel pores with an organic solvent with a surface tension much lower than that of water will disrupt the strong hydrogen bonds originally formed between the nanosheets. This will isolate the connections between the nanosheets by the organic solvent molecules. After the organic solvent evaporates, the remaining interactions between the nanosheets are mainly dominated by weak van der Waals forces, resulting in aerogels with weak mechanical properties that are more easily compressed or broken.

[0031] Repeated experiments revealed that when the filler, chitosan, and glutaraldehyde in the hydrogel are mixed in the above proportions, and the hydrogel is frozen at -20℃ to -80℃, it can be dried at low temperature and normal pressure, resulting in an aerogel with good mechanical properties. Specific analysis is as follows: Chitosan molecules are rich in amino groups. The two aldehyde groups of glutaraldehyde can react with the two amino groups of chitosan to form a Schiff base reaction, forming a stable C=N double bond. This connects the linear chitosan molecules into a stable three-dimensional network, providing the basis for the mechanical strength of aerogels.

[0032] Fillers such as MXene and carbon nanotubes are nanomaterials with high intrinsic strength. When they are uniformly dispersed and integrated into the chitosan network, they play a role in strengthening and toughening, further enhancing the network structure.

[0033] -20℃ to -80℃ represents a relatively slow freezing rate, allowing ample time for ice crystals to grow in three dimensions during water crystallization, forming a large (micrometer-scale) and interconnected ice crystal framework. In contrast, liquid nitrogen freezing (-196℃) is extremely rapid, causing water to instantly solidify into numerous tiny (nanometer-scale) and disordered ice crystals. After sublimation, the former leaves larger pores, while the latter leaves smaller pores. The large pore structure allows water vapor to diffuse rapidly through larger channels during atmospheric pressure drying, reducing mass transfer resistance and drying time. Small pores, however, have tortuous paths, hindering water vapor diffusion and making them prone to structural collapse due to localized stress concentration. The cross-linked network formed by chitosan and glutaraldehyde (Schiff base reaction) and the reinforcing effect of conductive fillers (such as MXene) can stably support the large pore framework, preventing collapse during drying due to structural weight or surface tension. Small pores, on the other hand, require higher network support strength and are more susceptible to structural integrity damage due to stress accumulation. Therefore, freezing at -20℃ to -80℃ allows for the control of ice crystal size and distribution, thereby constructing a pore structure with large pore size and low resistance. Combined with the mechanical support of the cross-linked network, this provides structural stability and mass transfer efficiency for atmospheric pressure drying, and the prepared aerogel has good mechanical properties.

[0034] Furthermore, the inventors discovered through experiments that when the amount of glutaraldehyde added accounts for more than 7% of the mass percentage of the solid matter in the hydrogel, the cross-linking density of the gel network is high, the number of hydrophobic microregions is large, the gel strength is higher, and the hydrophobicity results in less force at the gas-liquid interface during the subsequent atmospheric pressure drying process, further ensuring the structural integrity of the aerogel.

[0035] In some embodiments, the solid content of the hydrogel is 2%-4%, where % is a mass percentage.

[0036] In some embodiments, during the atmospheric pressure drying process, the frozen hydrogel is placed in a ventilated environment such that the partial pressure of water vapor in the environment is lower than the saturated vapor pressure of ice at the drying temperature.

[0037] Preferably, during the atmospheric pressure drying process, a drying gas at a set temperature is continuously blown onto the frozen hydrogel surface. This gas provides the heat required for sublimation of the frozen hydrogel and rapidly removes water vapor from the gel surface, ensuring that the local water vapor partial pressure on the gel surface remains lower than the saturated vapor pressure of the ice surface.

[0038] In some embodiments, the filler is MXene.

[0039] In some embodiments, the freezing temperature is -20°C to -30°C.

[0040] Preferably, the freezing temperature is -20℃ to -25℃.

[0041] In some embodiments, the drying temperature is 25°C-50°C and the drying time is 24-72 hours.

[0042] Preferably, the drying temperature is 25℃-30℃ and the drying time is 48-72h.

[0043] Secondly, the present invention provides an MXene-based aerogel prepared by the aforementioned preparation method.

[0044] Thirdly, the present invention provides the application of the MXene-based aerogel in the preparation of electromagnetic wave absorbing devices or thermal protection elements.

[0045] The present invention will be further described below with reference to the embodiments.

[0046] Example 1 A method for preparing an MXene-based aerogel includes the following steps: (1) Mix 2.0% chitosan dispersion and 2.0% MXene dispersion in a certain proportion to obtain chitosan / MXene dispersion; After stirring for 10 minutes, a glutaraldehyde dispersion with a solid content of 25% was added. After rapid stirring, the mixture was allowed to stand and crosslink into a hydrogel. The mass ratio of chitosan, MXene, and glutaraldehyde in the hydrogel was approximately 42:42:16, and the solid content in the hydrogel was 2%. The mixture was then completely frozen in a low-temperature freezer at -20°C.

[0047] (2) The completely frozen chitosan / MXene hydrogel obtained in step (1) is dried at room temperature to obtain cross-linked MXene aerogel that is dried at normal pressure without solvent exchange.

[0048] (4) The electromagnetic parameters of the chitosan / MXene aerogel obtained in (3) were tested using an Agilent Technologies E8363A electromagnetic vector network analyzer to obtain the absorption performance of the material.

[0049] from Figure 1 It can be seen that the average size of the MXene sheet is 2~3 μm.

[0050] from Figure 2 As can be seen, after chemical cross-linking, a high-strength black hydrogel is formed due to the strong cross-linking network.

[0051] from Figure 3 It can be seen that when glutaraldehyde is not added to MXene / chitosan, the aerogel cannot maintain its porous structure during the atmospheric pressure drying process, indicating that it is necessary to use cross-linking to enhance the pore walls during the preparation process.

[0052] from Figure 4 It can be seen that, based on the characteristics of atmospheric pressure drying, sample preparation is not limited by the size of the instrument, making it very suitable for the large-scale preparation of aerogels.

[0053] from Figure 5 It can be seen that the aerogel exhibits isotropic pores, demonstrating the cross-linked microstructure of the aerogel.

[0054] from Figure 6 It can be seen that when electromagnetic waves are incident, the best reflection loss is -42.2 dB at a thickness of 3.8 mm, and -45.9 dB at a thickness of 4.7 mm. The best reflection loss is 6.4 GHz at a thickness of 4.3 mm, which means that the cross-linked MXene aerogel has excellent absorption properties.

[0055] from Figure 7 It can be seen that the limiting oxygen index of MXene aerogel dried at room pressure without solvent exchange is 60%. The limiting oxygen index is the lowest oxygen concentration, i.e., the volume fraction of oxygen, that an aerogel can support combustion in a mixture of oxygen and nitrogen. This is an index that characterizes the combustion behavior of materials.

[0056] Compression tests were conducted on the aerogel using a tensile-compression testing machine, and the stress-strain curve was tested. When the strain was 80%, the mechanical compressive strength of the aerogel was approximately 824 kPa.

[0057] Example 2 A method for preparing an MXene-based aerogel includes the following steps: (1) Mix 2.0% by mass chitosan dispersion and 2.0% by mass carbon nanotube dispersion in a certain proportion to obtain chitosan / carbon nanotube dispersion; After stirring for 10 minutes, a glutaraldehyde dispersion with a solid content of 25% was added. After rapid stirring, the mixture was allowed to stand and crosslink into a hydrogel. The mass ratio of chitosan, carbon nanotubes and glutaraldehyde in the hydrogel was approximately 19:74:7. The solid content of the hydrogel was 2%. The mixture was then completely frozen in a low-temperature freezer at -20°C.

[0058] (2) The completely frozen chitosan / carbon nanotube hydrogel obtained in step (1) is dried at room temperature to obtain a cross-linked aerogel that is dried at normal pressure without solvent exchange.

[0059] Compression tests were conducted on the aerogel using a tensile-compression testing machine, and the stress-strain curve was tested. When the strain was 80%, the mechanical compressive strength of the aerogel was approximately 178 kPa.

[0060] Example 3 A method for preparing an MXene-based aerogel includes the following steps: (1) A chitosan dispersion with a mass percentage of 2.0% and a graphene dispersion with a mass percentage of 2.0% were mixed in a certain proportion to obtain a chitosan / graphene dispersion. After stirring for 10 minutes, a glutaraldehyde dispersion with a solid content of 25% was added. After rapid stirring, the mixture was allowed to stand and crosslink into a hydrogel. The mass ratio of chitosan, graphene and glutaraldehyde in the hydrogel was approximately 19:74:7. The solid content of the hydrogel was 2%. The mixture was then completely frozen in a low-temperature freezer at -20°C.

[0061] (2) The completely frozen chitosan / graphene hydrogel obtained in step (1) is dried at room temperature to obtain a cross-linked aerogel that is dried at normal pressure without solvent exchange.

[0062] Compression tests were conducted on the aerogel using a tensile-compression testing machine, and the stress-strain curve was tested. When the strain was 80%, the mechanical compressive strength of the aerogel was approximately 153 kPa.

[0063] Example 4 A method for preparing an MXene-based aerogel includes the following steps: (1) Mix 2.0% by mass chitosan dispersion and 2.0% by mass boron nitride dispersion in a certain proportion to obtain chitosan / boron nitride dispersion; After stirring for 10 minutes, a glutaraldehyde dispersion with a solid content of 25% was added. After rapid stirring, the mixture was allowed to stand and crosslink into a hydrogel. The mass ratio of chitosan, boron nitride, and glutaraldehyde in the hydrogel was approximately 19:74:7. The solid content of the hydrogel was 2%. The mixture was then completely frozen in a low-temperature freezer at -20°C.

[0064] (2) The completely frozen chitosan / boron nitride hydrogel obtained in step (1) is dried at room temperature to obtain a cross-linked aerogel that is dried at normal pressure without solvent exchange.

[0065] Example 5 A method for preparing an MXene-based aerogel includes the following steps: (1) Mix 2.0% by mass chitosan dispersion and 2.0% by mass carbon nanotube dispersion in a certain proportion to obtain chitosan / carbon nanotube dispersion; After stirring for 10 minutes, a glutaraldehyde dispersion with a solid content of 25% was added. After rapid stirring, the mixture was allowed to stand and crosslink into a hydrogel. The mass ratio of chitosan, MXene, and glutaraldehyde was approximately 19:74:7. After rapid stirring and standing, the mixture was transferred to a -20°C freezer for freezing after crosslinking into a hydrogel.

[0066] (2) Dry the completely frozen hydrogel obtained in (1) at room temperature.

[0067] The mechanical compressive properties of Comparative Example 1 were tested using a tensile-compression testing machine. When the strain was 80%, the mechanical compressive strength of the aerogel was 184 kPa.

[0068] The difference from Example 1 is that the high concentration of MXene causes electromagnetic waves to tend to be reflected, and the aerogel exhibits electromagnetic wave shielding performance. The 2 mm thick aerogel exhibits a shielding performance of about 30 dB.

[0069] Comparative Example 1 The difference from Example 1 is that the step of "completely freezing in a low-temperature refrigerator at -20°C" in step (1) is omitted, while all other steps are the same as in Example 1.

[0070] As the hydrogel gradually loses moisture, the cross-linked network completely collapses, making it impossible to form an aerogel structure.

[0071] Comparative Example 2 The difference from Example 1 is that the freezing in step (1) is done in liquid nitrogen at a freezing temperature of -196°C, while the rest is the same as in Example 1.

[0072] The difference from Example 1 is that the aerogel structure collapsed after being frozen with liquid nitrogen.

[0073] Comparative Example 3 The difference from Example 1 is that the mass ratio of chitosan, MXene and glutaraldehyde in the hydrogel is approximately 19:75:2, and the mass percentage of glutaraldehyde in the solid material is 2%, which is less than 7%. All other aspects are the same as in Example 1.

[0074] Compared to the aerogel of Example 1, which showed almost no shrinkage, the low glutaraldehyde concentration caused partial collapse of the pore walls of the aerogel and significant shrinkage, approaching 40%, with the aerogel strength below 150 kPa.

[0075] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for preparing MXene-based aerogel under normal pressure, characterized in that: Includes the following steps: A mixture of chitosan and conductive filler dispersions was mixed with glutaraldehyde and allowed to stand for crosslinking to obtain a hydrogel. In the hydrogel, the conductive filler accounts for 20-75% of the solid matter by mass, chitosan accounts for 15-55% of the solid matter by mass, and glutaraldehyde accounts for 1%-20% of the solid matter by mass. The filler is selected from MXene, carbon nanotubes, graphene, boron nitride, or MOF; The hydrogel was frozen at -20℃ to -80℃; The frozen hydrogel is dried at normal pressure at 25℃-80℃ to obtain the aerogel.

2. The method for preparing MXene-based aerogel at atmospheric pressure according to claim 1, characterized in that: The solid content of the hydrogel is 2%-4%, where % is a mass percentage.

3. The method for preparing MXene-based aerogel at atmospheric pressure according to claim 1, characterized in that: The filler is MXene.

4. The method for preparing MXene-based aerogel at atmospheric pressure according to claim 1, characterized in that: During the atmospheric pressure drying process, the frozen hydrogel is placed in a ventilated environment so that the partial pressure of water vapor in the environment is lower than the saturated vapor pressure of ice at the drying temperature.

5. The method for preparing MXene-based aerogel at atmospheric pressure according to claim 4, characterized in that: The freezing temperature is -20℃ to -30℃.

6. The method for preparing MXene-based aerogel at atmospheric pressure according to claim 1, characterized in that: The freezing temperature is -20℃ to -25℃.

7. The method for preparing MXene-based aerogel at atmospheric pressure according to claim 1, characterized in that: The drying temperature is 25℃-50℃, and the drying time is 24-72h.

8. The method for preparing MXene-based aerogel at atmospheric pressure according to claim 7, characterized in that: The drying temperature is 25℃-30℃, and the drying time is 48-72h.

9. An MXene-based aerogel, characterized in that: It is prepared by any one of the preparation methods described in claims 1-8.

10. The application of the MXene-based aerogel of claim 9 in the preparation of electromagnetic wave absorbing devices or thermal protection elements.

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