Anisotropic graphene aerogel as well as normal-pressure preparation method and application thereof

Abstract

The invention discloses anisotropic graphene-based aerogel as well as a normal-pressure preparation method and application thereof, and the normal-pressure preparation method comprises the following steps: uniformly mixing a cellulose nanofiber aqueous dispersion and a graphene aqueous dispersion in proportion to obtain a mixed aqueous dispersion; the mixed dispersion liquid is contained in a mold, and a bottom plate of the mold is a metal bottom plate; the mold is placed in an environment with the temperature ranging from-40 DEG C to-60 DEG C to be frozen; soaking the completely frozen solid in an organic solvent, and replacing water in the solid with the organic solvent; and drying the replaced hydrogel at normal pressure and room temperature to obtain the anisotropic graphene aerogel. Through the enhancement effect of the high-strength cellulose nanofibers on the pore wall and low surface tension caused by solvent replacement, capillary force can be resisted and the pore structure can be kept from collapsing during normal-pressure drying.

Description

Technical Field

[0001] The present invention belongs to the technical field of aerogel preparation, and particularly relates to an anisotropic graphene-based aerogel, an atmospheric-pressure preparation method thereof, and applications thereof. Background Art

[0002] The statements herein only provide background art related to the present invention and do not necessarily constitute prior art.

[0003] The preparation process of graphene-based aerogels includes steps such as freeze-drying and supercritical carbon dioxide drying. However, these preparation processes need to be carried out under high-temperature or high-pressure environments, which requires special facilities such as autoclaves and various freeze dryers. Therefore, the size and large-scale production of graphene-based aerogels are both limited, hindering their practical applications. Summary of the Invention

[0004] Aiming at the deficiencies of the existing technology, the purpose of the present invention is to provide an anisotropic graphene-based aerogel, an atmospheric-pressure preparation method thereof, and applications thereof. The preparation process of this material has the characteristics of energy conservation and scalable preparation, while improving production efficiency and reducing production costs.

[0005] To achieve the above purpose, the present invention is realized through the following technical solutions:

[0006] In the first aspect, the present invention provides an atmospheric-pressure preparation method for an anisotropic graphene-based aerogel, including the following steps:

[0007] Uniformly mix an aqueous dispersion of cellulose nanofibers and an aqueous dispersion of graphene in proportion to obtain a mixed aqueous dispersion;

[0008] Place the mixed dispersion in a mold, and the bottom plate of the mold is a metal bottom plate;

[0009] Place the mold in an environment of -40°C to -60°C for freezing;

[0010] Immerse the completely frozen solid in ethanol to replace the water therein with ethanol;

[0011] Dry the replaced hydrogel at atmospheric pressure and room temperature to obtain an anisotropic graphene aerogel.

[0012] The room temperature generally refers to the indoor temperature of 20 - 30°C; atmospheric pressure refers to the state or environment under atmospheric pressure, that is, the atmospheric pressure situation in most regions of the earth.

[0013] The function of the freezing is, on the one hand, to expand the pore size so as to reduce the capillary force, and on the other hand, to utilize the ice template to create oriented pores.

[0014] Immerse the completely frozen cellulose / graphene in an organic solvent, and change the organic solvent every 4 hours. Changing it 4 times can ensure the complete replacement of the organic solvent.

[0015] In some embodiments, the organic solvent can be organic solvents such as ethanol, methanol, acetone, ether, n-hexane, toluene, tetrahydrofuran, dichloromethane, ethyl acetate, isopropanol, etc., as long as the surface tension is small enough, because if dried directly with water, due to the large surface tension of water, the pore structure will collapse during the drying process.

[0016] In some embodiments, the concentration of the cellulose aqueous dispersion is 2% - 4.5%; the concentration of the graphene aqueous dispersion is 8% - 12%; the mass ratio of the cellulose aqueous dispersion to the graphene aqueous dispersion is 2:8 - 8:2; % is the mass percentage.

[0017] The concentration of the cellulose aqueous dispersion can be 2.0%, 2.2%, 2.4%, 2.8%, 3.0%, 3.2%, 3.4%, 3.6%, 3.8%, 4.0%, 4.2%, 4.5%;

[0018] The concentration of the graphene aqueous dispersion can be 8.0%, 8.5%, 9.0%, 9.5%, 10.0%, 10.5%, 11.0%, 11.5%, 12.0%.

[0019] The freezing temperature can be -40°C, -45°C, -50°C, -55°C, -60°C.

[0020] Preferably, the concentration of the cellulose aqueous dispersion is 2% - 3%; the concentration of the graphene aqueous dispersion is 9% - 11%.

[0021] More preferably, the concentration of the cellulose aqueous dispersion is 2.4% - 2.6%; the concentration of the graphene aqueous dispersion is 10% - 11%.

[0022] In some embodiments, the average diameter of the cellulose nanofibers is 1 - 2 nm, and the average length is 400 - 600 nm.

[0023] In some embodiments, the material of the metal bottom plate is a highly thermally conductive metal such as an iron plate or a copper plate.

[0024] In some embodiments, the freezing temperature is -40°C to -50°C.

[0025] Preferably, the freezing temperature is -40°C to -45°C.

[0026] In a second aspect, the present invention provides an anisotropic graphene-based aerogel prepared by the preparation method.

[0027] Thirdly, the present invention provides an application of the anisotropic graphene-based aerogel in preparing a frequency selective electromagnetic wave absorption device.

[0028] The beneficial effects achieved by one or more of the above embodiments of the present invention are as follows:

[0029] (1) For the atmospheric pressure preparation method of the anisotropic graphene aerogel proposed by the present invention, on the one hand, the atmospheric pressure drying preparation process breaks through the limitations of equipment and instruments, and can achieve large-area scalable preparation on the basis of energy conservation and environmental protection; on the other hand, due to the anisotropic pores, the interaction between electromagnetic waves incident along the vertical pore structure and the parallel pore structure and the aerogel is different, resulting in the frequency selective absorption characteristics of the graphene aerogel.

[0030] (2) Compared with the prior art, the preparation process adopted by the present invention can resist capillary force and maintain the pore structure from collapsing during atmospheric pressure drying through the pore wall strengthening effect of high-strength cellulose nanofibers and the low surface tension caused by solvent replacement; if cellulose nanofibers are not added, during the solvent replacement process, graphene will be dispersed in ethanol and the porous structure cannot be maintained, that is, anisotropic graphene aerogel cannot be formed.

[0031] (3) The anisotropic graphene aerogel prepared under atmospheric pressure has the characteristics of frequency selective absorption of electromagnetic waves incident along different directions. For example, when the electromagnetic wave is incident along the direction perpendicular to the pore diameter, the best reflection loss of -52.48 dB and an effective absorption bandwidth of 3.92 GHz are shown at a thickness of 1.38 mm; when the electromagnetic wave is incident along the direction parallel to the pore diameter, the best reflection loss of -51.48 dB and an effective absorption bandwidth of 3.92 GHz are shown at a thickness of 2.65 mm, which has wide application value.

[0032] (4) The atmospheric pressure preparation process of the anisotropic graphene aerogel proposed by the present invention can promote their large-scale application in the field of electromagnetic wave absorption due to its low cost and high efficiency. Description of the Drawings

[0033] The specification drawings forming a part of the present invention are used to provide a further understanding of the present invention. The schematic embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute an improper limitation to the present invention.

[0034] Figure 1 Atomic force microscope photos and transmission electron microscope photos of cellulose and graphene used in Example 1.

[0035] Figure 2 Photo of directly drying the cellulose / graphene hydrogel without the solvent replacement process.

[0036] Figure 3Photograph of direct solvent replacement of pure graphene after complete freezing without adding cellulose for Comparative Example 1.

[0037] Figure 4 Photograph of the large-sized anisotropic graphene aerogel prepared in Example 1.

[0038] Figure 5 Scanning electron microscope photograph of the anisotropic graphene aerogel prepared in Example 1.

[0039] Figure 6 Frequency selective absorption characteristics of the anisotropic graphene aerogel prepared in Example 1.

[0040] Figure 7 Real part of the dielectric constant of the anisotropic graphene aerogel prepared in Example 1 in different directions.

[0041] Figure 8 Imaginary part of the dielectric constant of the anisotropic graphene aerogel prepared in Example 1 in different directions. Detailed implementation mode

[0042] It should be noted that the following detailed description is illustrative and is intended to provide further explanation of the present invention. Unless otherwise specified, all technical and scientific terms used in the present invention have the same meaning as commonly understood by those of ordinary skill in the technical field to which the present invention belongs.

[0043] The present invention will be further described below in conjunction with embodiments.

[0044] Example 1

[0045] (1) A cellulose / graphene mixed dispersion was prepared by mixing a 2.4% cellulose nanofiber dispersion and a 10% graphene dispersion at a mass ratio of 4:6. After stirring for 30 min, a homogeneous dispersion was obtained, which was then transferred to a polytetrafluoroethylene mold with a metal bottom plate and completely frozen in a low-temperature refrigerator at -40°C.

[0046] (2) The completely frozen cellulose / graphene mixed dispersion obtained in (1) was placed in an ethanol solution for solvent exchange.

[0047] (3) After the water in the original cellulose / graphene mixed dispersion was completely replaced by ethanol, it was dried at room temperature to obtain an anisotropic graphene aerogel dried under normal pressure.

[0048] (4) The cellulose / graphene aerogel obtained in (3) was tested for electromagnetic parameters using an Agilent Technologies E8363A electromagnetic wave vector network analyzer, and the wave absorption performance of the material was calculated based on the electromagnetic parameters.

[0049] Comparative Example 1

[0050] (1) Prepare a graphene dispersion with a solid content of 3% from a 10% graphene dispersion. After stirring for 30 min, a homogeneous dispersion is obtained, which is then transferred to a polytetrafluoroethylene mold with a metal bottom plate and completely frozen in a low-temperature refrigerator at -40 °C.

[0051] (2) Place the completely frozen graphene dispersion obtained in (1) in an ethanol solution for solvent exchange.

[0052] (3) After the water in the original graphene dispersion is completely replaced by ethanol, dry it at room temperature.

[0053] The difference from Example 1 is that in the first step of the preparation process, no cellulose nanofiber dispersion is added, only a single graphene dispersion.

[0054] From Figure 1 It can be seen that the average diameter of the cellulose nanofibers is 1.4 nm, the aspect ratio is ~300, and the average size of the graphene sheets is ~1 μm.

[0055] From Figure 2 It can be seen that when solvent exchange is not carried out, due to the too large surface tension of water, the pore structure will completely collapse during drying.

[0056] From Figure 3 It can be seen that when no cellulose nanofibers are added to the graphene, during the solvent exchange process, the graphene will disperse in ethanol and cannot maintain the porous structure, indicating that it is necessary to add cellulose nanofibers to strengthen the pore walls during the preparation process.

[0057] From Figure 4 It can be seen that based on the characteristics of atmospheric pressure drying, the sample preparation is not limited by the size of the instrument, so it is very suitable for the large-scale preparation of aerogels.

[0058] From Figure 5 It can be seen that orderly arranged pores and randomly isotropic pores can be observed in the longitudinal and transverse planes respectively, demonstrating the anisotropic microstructure of the aerogel.

[0059] From Figure 6 It can be seen that when the electromagnetic wave is incident along the direction perpendicular to the pore diameter, at a thickness of 1.38 mm, it shows an optimal reflection loss of -52.48 dB and an effective absorption bandwidth of 3.92 GHz; when the electromagnetic wave is incident along the direction parallel to the pore diameter, at a thickness of 2.65 mm, it shows an optimal reflection loss of -51.48 dB and an effective absorption bandwidth of 3.92 GHz, which means that the anisotropic graphene has the characteristic of frequency-selective absorption.

[0060] FromFigure 7 It can be seen that when the electromagnetic wave is incident along the direction perpendicular to the pore channel, the real part of the dielectric constant is greater than that when the electromagnetic wave is incident along the direction parallel to the pore channel.

[0061] From Figure 8 It can be seen that when the electromagnetic wave is incident along the direction perpendicular to the pore channel, the imaginary part of the dielectric constant is also greater than that when the electromagnetic wave is incident along the direction parallel to the pore channel.

[0062] Example 2

[0063] (1) A cellulose / graphene mixed dispersion was prepared by mixing a 2.6% cellulose nanofiber dispersion and an 11% graphene dispersion at a mass ratio of 4:8. After stirring for 40 min, a homogeneous dispersion was obtained, which was then transferred to a polytetrafluoroethylene mold with a metal bottom plate and completely frozen in a low-temperature refrigerator at -50 °C.

[0064] (2) The completely frozen cellulose / graphene mixed dispersion obtained in (1) was placed in an ethanol solution for solvent exchange.

[0065] (3) After the water in the original cellulose / graphene mixed dispersion was completely replaced by ethanol, it was dried at room temperature to obtain an anisotropic graphene aerogel dried under atmospheric pressure.

[0066] Example 3

[0067] (1) A cellulose / graphene mixed dispersion with a solid content of 3% was prepared by mixing a 3.5% cellulose nanofiber dispersion and an 8% graphene dispersion at a mass ratio of 4:6. After stirring for 30 min, a homogeneous dispersion was obtained, which was then transferred to a polytetrafluoroethylene mold with a metal bottom plate and completely frozen in a low-temperature refrigerator at -60 °C.

[0068] (2) The completely frozen cellulose / graphene mixed dispersion obtained in (1) was placed in an ethanol solution for solvent exchange.

[0069] (3) After the water in the original cellulose / graphene mixed dispersion was completely replaced by ethanol, it was dried at room temperature to obtain an anisotropic graphene aerogel dried under atmospheric pressure.

[0070] The above are only the preferred embodiments of the present invention and are not intended to limit the present invention. For those skilled in the art, the present invention can have various changes and modifications. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A method for preparing anisotropic graphene-based aerogel at normal pressure, characterized in that: The steps include: The cellulose nanofiber aqueous dispersion and the graphene aqueous dispersion are uniformly mixed according to a certain proportion to obtain a mixed aqueous dispersion; The mixed dispersion is placed in a mold, the bottom plate of the mold being a metal bottom plate; The mold is placed in a -40℃~-60℃ environment for freezing; The completely frozen solid is immersed in an organic solvent to replace the water therein with the organic solvent; The replaced hydrogel is dried under normal pressure and room temperature to obtain anisotropic graphene aerogel.

2. The method for preparing anisotropic graphene-based aerogel under normal pressure according to claim 1, characterized in that: The organic solvent is ethanol, methanol, acetone, ether, n-hexane, toluene, tetrahydrofuran, dichloromethane, ethyl acetate or isopropanol; Preferably, the concentration of the cellulose aqueous dispersion is 2%-4.5%; the concentration of the graphene aqueous dispersion is 8%-12%; the mass ratio of the cellulose aqueous dispersion to the graphene aqueous dispersion is 2:8-8:2; % is mass percentage.

3. The method for preparing anisotropic graphene-based aerogel under normal pressure according to claim 2, characterized in that: The concentration of the cellulose aqueous dispersion is 2%-3%; the concentration of the graphene aqueous dispersion is 9%-11%.

4. The method for preparing anisotropic graphene-based aerogel under normal pressure according to claim 3, characterized in that: The concentration of the cellulose aqueous dispersion is 2.4%-2.6%; the concentration of the graphene aqueous dispersion is 10%-11%.

5. The method for preparing anisotropic graphene-based aerogel under normal pressure according to claim 1, characterized in that: The average diameter of the cellulose nanofibers is 1-2 nm, and the average length is 400-600 nm.

6. The method for preparing anisotropic graphene-based aerogel under normal pressure according to claim 1, characterized in that: The material of the metal bottom plate is an iron plate or a copper plate.

7. The method for preparing anisotropic graphene-based aerogel under normal pressure according to claim 1, characterized in that: The freezing temperature is -40°C to -50°C.

8. The method for preparing anisotropic graphene-based aerogel under normal pressure according to claim 7, characterized in that: The freezing temperature is -40°C to -45°C.

9. An anisotropic graphene-based aerogel, characterized in that: Prepared by the preparation method described in any one of claims 1 to 8.

10. Use of the anisotropic graphene-based aerogel according to claim 9 in preparing a frequency selective electromagnetic wave absorption device.

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