Magnetic-doped nanofiber-derived carbon aerogel wave-absorbing material with gradient pore structure and preparation method of magnetic-doped nanofiber-derived carbon aerogel wave-absorbing material

Abstract

The invention discloses a preparation method of a magnetic doped nanofiber derived carbon aerogel wave-absorbing material with a gradient pore structure and application of carbon aerogel, and belongs to the field of microwave wave-absorbing materials. The aerogel microstructure is controlled by regulating and controlling the parameters of the arched ice template and the content of the magnetic material precursor, and the lightweight carbon aerogel material is obtained through arched ice template freezing, freeze drying and carbonization. The gradient pore structure of the carbon aerogel improves impedance matching and promotes microwave reflection and scattering; the nano cellulose, the aramid nano fiber and the metal organic framework cooperatively optimize the polarization loss and magnetic loss performance by constructing a multi-element heterogeneous interface. Due to the characteristics, the carbon aerogel has excellent wave absorbing performance. According to the present invention, the carbon aerogel has the optimal reflection loss value of-57.03 dB when the frequency is 2-18 GHz and the thickness is 2.8 mm, and the effective absorption bandwidth (RL < =-10 dB) of the microwave absorption is 6.76 GHz, such that the carbon aerogel has the good application prospect in the electromagnetic wave field, and can simultaneously achieve the broadband and the strong absorption of the microwave under the low thickness.

Description

Technical Field

[0001] The present invention relates to a preparation method of a magnetic-doped nanofiber-derived carbon aerogel microwave absorbing material with a gradient pore structure, and specifically belongs to the technical field of microwave absorbing materials. Background Art

[0002] With the advent of the intelligent era, wireless communication technology has brought great convenience to human production and life. However, the problem of electromagnetic (EM) interference caused by various electronic devices is becoming increasingly serious. Therefore, in order to cope with the increasingly complex electromagnetic environment, researchers have prepared many high-efficiency microwave absorbing materials, including magnetic materials such as iron oxide and conductive polymers such as polypyrrole. Generally, satisfactory microwave absorption materials (MAMs) need to meet conditions such as strong absorption intensity, wide effective absorption bandwidth, light weight, and thin thickness. At present, carbon nanomaterials (including graphene, carbon nanotubes, and porous carbon) have been recognized as ideal candidates for high-performance microwave absorbing materials due to their excellent electrical conductivity, tunable dielectric properties, and the ability to serve as carriers for other materials.

[0003] Carbon aerogels are considered a promising microwave absorbing material due to their ultra-low density, high porosity, and large specific surface area. The porous structure can reduce the dielectric constant, improve impedance matching, and allow more electromagnetic waves to enter. In addition, the tortuous pores and pore walls expand the transmission path of electromagnetic waves, providing rich interfaces for the attenuation of electromagnetic waves through multiple scattering. In addition, the rich solid-gas interfaces formed by its porous structure lead to the accumulation and asymmetric distribution of charges, enhancing the space polarization loss. Therefore, the pore structure is closely related to the microwave absorption performance of carbon aerogels, and reasonable design of the pore structure is the key to improving the microwave absorption performance of materials. At present, the ice template technology is considered the simplest and most suitable method for regulating the pore structure of aerogels. By controlling the growth direction, rate, and temperature gradient of ice crystals, aerogels with customized pore structures can be obtained, such as random pore structures, oriented microchannel structures, layered porous structures, honeycomb structures, etc. However, most current studies on the pore structure of aerogels focus on the regulation of uniform pore sizes. Considering that there is not a single corresponding relationship between impedance matching and multiple reflection and scattering and pore size. Therefore, the present invention is committed to preparing carbon aerogels with a gradient pore structure to coordinate the impedance matching of carbon aerogels and the microwave attenuation ability enhanced by multiple reflection and scattering. In addition, single carbon materials usually cannot meet the impedance matching requirements and need to be compounded with other materials to improve the dielectric loss and magnetic loss capabilities, and enhance the attenuation of electromagnetic waves through the synergistic action of multiple loss mechanisms. Summary of the Invention

[0004] Object of the Invention: In order to solve the above technical problems, the present invention aims to provide a preparation method of a magnetic-doped nanofiber-derived carbon aerogel microwave absorbing material with a gradient pore structure.

[0005] The present invention proposes an arched ice template method, designs an arched ice template freezing mold for preparing carbon aerogel with a gradient pore structure. At the same time, to enrich the loss mechanism of carbon aerogel, metal-organic frameworks are introduced to prepare a magnetic-doped nanofiber-derived carbon aerogel absorbing material with a gradient pore structure. The preparation process of this material is simple, with low cost and controllable structure. The absorbing performance can be regulated at multiple scales by controlling the parameters of the arched ice template freezing mold, the addition amount of magnetic materials, etc.

[0006] To achieve the above technical objectives, the technical solution adopted by the present invention is as follows: A preparation method of a magnetic-doped nanofiber-derived carbon aerogel absorbing material with a gradient pore structure, comprising the following preparation steps:

[0007] (1) Synthesize metal-organic framework materials by the solvothermal method;

[0008] (2) Add the metal-organic framework materials in step (1) to the nanofibrillated cellulose / aramid nanofiber mixed solution, and stir evenly to obtain a mixed dispersion;

[0009] (3) Subject the mixed dispersion in step (2) to arched ice template freezing technology and freeze-drying treatment to obtain an aerogel;

[0010] (4) Subject the aerogel obtained in step (3) to high-temperature carbonization in an inert gas atmosphere to obtain carbon aerogel.

[0011] Preferably, in step (1), the metal-organic framework nanoparticles are of the ZIF series, MIL series, UIO-66 series, and the central metal ions are Co 2+ , Ni 2+ , Fe 3+ , Cu 2+ and other monovalent or polyvalent metal ions, and the organic ligands are one or several of terephthalic acid, trimellitic acid, fumaric acid, 2-methylimidazole, benzimidazole, etc.

[0012] More preferably, in step (1), the metal-organic framework nanoparticles are ZIF-67, ZIF-8, MIL-88(Fe), and the central metal ions are Co 2+ , Ni 2+ , Fe 3+ metal ions, and one or several of trimellitic acid, fumaric acid, 2-methylimidazole, etc.

[0013] Preferably, in step (1), the molar ratio of the central metal ion to the organic ligand is 1:(1-8), the mass ratio of the solute to the solvent is 1:(10-30), and the mass ratio of the solute to the dispersant is 1:(1-5).

[0014] More preferably, the molar ratio of the central metal ion to the organic ligand is 1:(1 - 5), the mass ratio of the solute to the solvent is 1:(10 - 20), and the mass ratio of the solute to the dispersant is 1:(1 - 3).

[0015] Preferably, in step (1), the solvent is water, ethanol, methanol, N,N-dimethylformamide, and the dispersant is one or more of PVPK15, PVPK30, PVPK60, etc.

[0016] Preferably, in step (3), the arched ice template technology is to place the mixed dispersion in the upper freezing container, place an isosceles triangle mold at the bottom of the upper freezing container, place the upper freezing container on a metal block, and then place the metal block in a liquid nitrogen bath; the material of the triangle mold is PDMS and / or rubber, the apex angle is 60° - 150°, the height of the triangle is 5 - 50 mm, the bottom side length of the triangle is 10 - 200 mm, and the freezing temperature is -(110°C - 200°C).

[0017] More preferably, the material of the triangle mold is PDMS, the apex angle is 90° - 120°, the height of the triangle is 5 - 30 mm, the bottom side length of the triangle is 10 - 180 mm, and the freezing temperature is -(120°C - 200°C).

[0018] Preferably, in step (3), the freeze-drying temperature ≥ -40°C, the drying pressure ≤ 20 Pa, and the drying time is 36 - 72 h.

[0019] Preferably, in step (4), the inert gas is nitrogen and / or argon.

[0020] Preferably, in step (4), the high-temperature carbonization process is: heating from room temperature to 500°C at a rate of 2 - 15°C / min, and holding at 500°C for 1 - 4 h, then heating from 500°C to (700 - 1400)°C at a rate of 5 - 20°C / min and holding for 1 - 4 h.

[0021] More preferably, in step (4), the high-temperature carbonization process is: heating from room temperature to 500°C at a rate of 2 - 10°C / min, and holding at 500°C for 1 - 4 h, then heating from 500°C to (800 - 1000)°C at a rate of 5 - 10°C / min and holding for 1 - 4 h.

[0022] A carbon aerogel obtained by a preparation method of a magnetic-doped nanofiber-derived carbon aerogel absorbing material with a gradient pore structure.

[0023] A carbon aerogel obtained by the preparation method of the present invention.

[0024] The carbon aerogel obtained by the present invention is used for microwave absorption.

[0025] Principle of the invention: During the freezing process, the arch-shaped ice template realizes the redistribution of solute particles by adjusting the temperature gradient. Vertically, the temperature conducts gradually from bottom to top. Horizontally, the temperature gradually increases from both ends of the template towards the interior. Since the temperatures at both ends are relatively low, ice crystals grow rapidly and occupy space, forming a larger pore structure, while forcing solute particles to migrate towards the interior. The higher solute concentration on the inner side inhibits the growth of ice crystals, resulting in smaller pore sizes. Therefore, after freeze-drying, a gradient pore structure aerogel with large pores on the outer layer and small pores on the inner layer is prepared. The large pores on the outside of this carbon aerogel allow more microwaves to enter, while the inner pores effectively attenuate the incident microwaves through multiple reflections and scatterings, demonstrating excellent microwave absorption advantages. In addition, the multiple electromagnetic wave loss mechanisms are one of the key factors for achieving efficient broadband absorption. On the one hand, the continuous conductive network constructed by carbonized nanocellulose and aramid nanofibers provides an effective path for electron transport, thus significantly enhancing the conductive loss ability of the material. The 3D network structure can extend the propagation path of electromagnetic waves and further improve the electromagnetic wave loss efficiency through multiple reflections inside the material. On the other hand, the abundant heterogeneous interfaces can induce a large number of carrier accumulation behaviors, generate abundant space charges, and lead to strong interfacial polarization. The magnetic interfaces introduced by the multi-interface engineering can effectively regulate the magnetic response characteristics of the material to electromagnetic waves. At the same time, the natural resonance effect and eddy current loss induced by magnetic nanoparticles further enrich the magnetic loss mechanism, thus promoting the efficient construction and optimization of the magnetoelectric coupling network.

[0026] The present invention has the following beneficial effects compared with the prior art:

[0027] (1) The present invention uses the arch-shaped ice template method in combination with the high-temperature carbonization method to prepare carbon aerogel. The operation is simple, the structure of the carbon aerogel is controllable, and it is environmentally friendly; the prepared carbon aerogel with a gradient pore structure has the dual advantages of optimizing impedance matching and improving the attenuation ability;

[0028] The operation is simple, the structure of the carbon aerogel is controllable, and it is environmentally friendly; the prepared carbon aerogel with a gradient pore structure has the dual advantages of optimizing impedance matching and improving the attenuation ability;

[0029] The prepared carbon aerogel with a gradient pore structure has the dual advantages of optimizing impedance matching and improving the attenuation ability;

[0030] (2) The present invention selects two nanofibers, namely nanocellulose and aramid nanofibers, and metal-organic frameworks as microwave absorption materials. Due to the low density of nanofibers and the lightweight porous structure of metal-organic frameworks, the prepared aerogel has a low density; the nanofiber-derived carbon materials

[0031] Due to the low density of nanofibers and the lightweight porous structure of metal-organic frameworks, the prepared aerogel has a low density; the nanofiber-derived carbon materials

[0032] Due to the low density of nanofibers and the lightweight porous structure of metal-organic frameworks, the prepared aerogel has a low density; the nanofiber-derived carbon materials

[0033] The three-dimensional network structure is retained, and the continuous conductive network constructed by it provides an effective path for electron transport, thus significantly enhancing the conductive loss ability of the material; the metal-organic framework-derived carbon has both conductivity and magnetism, while avoiding the characteristics of high density and easy oxidation of metal particles; in addition, the interfacial polarization caused by the rich heterogeneous interfaces greatly improves the attenuation ability of the microwave absorption material and enhances the microwave absorption intensity of the material;

[0034] (3) The density of the carbon aerogel prepared in the present invention is 38.46 mg / cm 3 , and when the thickness is 2.8 mm within 2 - 18 GHz, the optimal reflection loss value can reach -57.03 dB, and the effective absorption bandwidth (RL ≤ -10 dB) of microwave absorption is 6.76 GHz. It better meets the application requirements of the current electromagnetic wave absorption materials of "light, thin, wide, and strong". Description of the Drawings

[0035] Figure 1 is the SEM image of the carbon aerogel prepared in Example 1;

[0036] Figure 2 is the SEM image of the pore wall of the carbon aerogel prepared in Example 2;

[0037] Figure 3 is the reflection loss curve graph of the carbon aerogel prepared in Example 1;

[0038] Figure 4 is the reflection loss curve graph of the carbon aerogel prepared in Example 2;

[0039] Figure 5 is the reflection loss curve graph of the carbon aerogel prepared in Comparative Example 1;

[0040] Figure 6 is the reflection loss curve graph of the carbon aerogel prepared in Comparative Example 2;

[0041] Figure 7 is the schematic diagram of the technical container device of the arched ice template method. Detailed Description of the Invention

[0042] Example 1

[0043] A method for preparing a microwave absorption material of carbon aerogel derived from magnetically doped nanofibers with a gradient pore structure, comprising the following steps:

[0044] (1) First, take 0.1 g of nanocellulose and add it to deionized water for ultrasonic dispersion to obtain a homogeneous solution, and then take 0.1 g of aramid nanofibers and add them to the nanocellulose solution for ultrasonic and stirring mixing dispersion to obtain a uniform mixture;

[0045] (2) Place the mixed dispersion obtained in step (1) in an upper-layer freezing container with a 90° triangular apex angle, a 10-mm triangular height, and a 20-mm base length, and a freezing temperature of -196 °C; place the obtained frozen sample in a freeze dryer and dry it at 8 Pa and -50 °C for 72 h to obtain a nanocellulose / aramid nanofiber gel;

[0046] (3) Place the nanocellulose / aramid nanofiber aerogel obtained in step (2) in a tubular furnace for carbonization. Heat it from room temperature to 500 °C at a rate of 2 °C / min and hold for 1 h, then heat it to 900 °C at a rate of 5 °C / min and hold for 1 h, and finally cool it at a rate of 5 °C / min to obtain a nanocellulose / aramid nanofiber-derived carbon aerogel, denoted as G-CA. Its microscopic morphology is as Figure 1 shown. At a thickness of 9 mm, the reflection loss value is -50.47 dB, and the effective absorption bandwidth is 7.15 GHz (8.5 mm).

[0047] Example 2

[0048] (1) First, dissolve 1.46 g of Co(NO3)2·6H2O in 40 mL of methanol solution and stir until clear. Subsequently, add 2.4 g of PVPK30 and stir until completely dissolved. Finally, add 1.64 g of 2-methylimidazole, and the solution turns purple. After stirring for 0.5 h, let it stand at room temperature for 24 h for self-precipitation. Wash it with methanol and ethanol and centrifuge 3 times to obtain a purple precipitate. Finally, dry it in vacuo at 60 °C for 12 h to obtain ZIF-67 nanoparticles;

[0049] (2) Take 0.1 g of nanocellulose and add it to deionized water for ultrasonic dispersion to obtain a homogeneous solution. Then, take 0.1 g of aramid nanofibers and add them to the nanocellulose solution and stir evenly. Finally, add 20 mg of ZIF-67 nanoparticles and ultrasonically mix and disperse them to obtain a homogeneous mixture;

[0050] (3) Place the mixed dispersion obtained in step (2) in an upper-layer freezing container with a 90° triangular apex angle, a 10-mm triangular height, and a 20-mm base length, and a freezing temperature of -196 °C; place the obtained frozen sample in a freeze dryer and dry it at 8 Pa and -50 °C for 72 h to obtain a nanocellulose / aramid nanofiber / ZIF-67 aerogel;

[0051] (4) The nanocellulose / aramid nanofiber / ZIF-67 aerogel obtained in step (3) was placed in a tube furnace for carbonization. It was heated from room temperature to 500 °C at a rate of 2 °C / min and held for 1 h, then heated to 900 °C at a rate of 5 °C / min and held for 1 h, and finally cooled at a rate of 5 °C / min to obtain the aramid nanofiber / nanocellulose / ZIF-67 derived carbon aerogel, denoted as G-CA-Co-1. Its morphological analysis is as shown in Figure 2 . At a thickness of 2.8 mm, the reflection loss value was -57.03 dB and the effective absorption bandwidth was 6.76 GHz (2.7 mm).

[0052] Example 3

[0053] (1) First, 1.46 g of Co(NO3)2·6H2O was dissolved in 40 mL of methanol solution and stirred until clear. Subsequently, 2.4 g of PVPK30 was added and stirred until completely dissolved. Finally, 1.64 g of 2-methylimidazole was added, and the solution turned purple. After stirring for 0.5 h, it was left to self-precipitate at room temperature for 24 h. It was washed with methanol and ethanol and centrifuged 3 times to obtain a purple precipitate. Finally, it was vacuum dried at 60 °C for 12 h to obtain ZIF-67 nanoparticles;

[0054] (2) 0.1 g of nanocellulose was taken and added to deionized water for ultrasonic dispersion to obtain a homogeneous solution. Then, 0.1 g of aramid nanofiber was taken and added to the nanocellulose solution and stirred evenly. Finally, 60 mg of ZIF-67 nanoparticles were added and ultrasonically mixed and dispersed to obtain a homogeneous mixture;

[0055] (3) The mixed dispersion obtained in step (2) was placed in an upper-layer freezing container with a triangular apex angle of 120°, a triangular height of 5 mm, and a bottom side length of 16 mm, and the freezing temperature was -196 °C; the obtained frozen sample was placed in a freeze dryer and dried at 8 Pa and -50 °C for 72 h to obtain the nanocellulose / aramid nanofiber / ZIF-67 aerogel;

[0056] (4) The nanocellulose / aramid nanofiber / ZIF-67 aerogel obtained in step (3) was placed in a tube furnace for carbonization. It was heated from room temperature to 500 °C at a rate of 2 °C / min and held for 1 h, then heated to 800 °C at a rate of 5 °C / min and held for 2 h, and finally cooled at a rate of 5 °C / min to obtain the aramid nanofiber / nanocellulose / ZIF-67 derived carbon aerogel, denoted as G-CA-Co-2. At a thickness of 3.5 mm, the reflection loss value was -53.52 dB and the effective absorption bandwidth was 6.89 GHz (3.2 mm).

[0057] Example 4

[0058] (1) First, dissolve 1.46 g of Co(NO3)2·6H2O in 40 mL of methanol solution and stir until clear. Subsequently, add 2.4 g of PVPK30 and stir until completely dissolved. Finally, add 1.64 g of 2-methylimidazole, and the solution turns purple. After stirring for 0.5 h, let it stand at room temperature for 24 h for self-precipitation. Wash with methanol and ethanol, and centrifuge 3 times to obtain a purple precipitate. Finally, dry it in vacuo at 60 °C for 12 h to obtain ZIF-67 nanoparticles;

[0059] (2) Take 0.1 g of nanocellulose and add it to deionized water for ultrasonic dispersion to obtain a homogeneous solution. Then, take 0.1 g of aramid nanofibers and add them to the nanocellulose solution and stir evenly. Finally, add 20 mg of ZIF-67 nanoparticles and ultrasonically mix and disperse to obtain a homogeneous mixture;

[0060] (3) Place the mixed dispersion obtained in step (2) in an upper-layer freezing container with a triangular apex angle of 120°, a triangular height of 60 mm, and a bottom side length of 160 mm, and a freezing temperature of -196 °C; place the obtained frozen sample in a freeze dryer and dry it at 8 Pa and -50 °C for 72 h to obtain a nanocellulose / aramid nanofiber / ZIF-67 aerogel;

[0061] (4) Place the nanocellulose / aramid nanofiber / ZIF-67 aerogel obtained in step (3) in a tube furnace for carbonization. Heat it from room temperature to 500 °C at a rate of 2 °C / min and hold for 2 h, then heat it to 900 °C at a rate of 5 °C / min and hold for 2 h, and finally cool it at a rate of 5 °C / min to obtain an aramid nanofiber / nanocellulose / ZIF-67-derived carbon aerogel, denoted as G-CA-Co-3. At a thickness of 4.2 mm, the reflection loss value is -45.68 dB, and the effective absorption bandwidth is 7.22 GHz (3.8 mm).

[0062] Example 5

[0063] (1) First, dissolve 1.46 g of Co(NO3)2·6H2O in 40 mL of methanol solution and stir until clear. Subsequently, add 2.4 g of PVPK30 and stir until completely dissolved. Finally, add 1.64 g of 2-methylimidazole, and the solution turns purple. After stirring for 0.5 h, let it stand at room temperature for 24 h for self-precipitation. Wash with methanol and ethanol, and centrifuge 3 times to obtain a purple precipitate. Finally, dry it in vacuo at 60 °C for 12 h to obtain ZIF-67 nanoparticles;

[0064] (2) Take 0.1 g of nanocellulose and add it to deionized water for ultrasonic dispersion to obtain a homogeneous solution. Then, take 0.1 g of aramid nanofibers and add them to the nanocellulose solution and stir evenly. Finally, add 20 mg of ZIF-67 nanoparticles and ultrasonically mix and disperse to obtain a homogeneous mixture;

[0065] (3) Place the mixed dispersion obtained in step (2) in an upper-layer freezing container with a triangular apex angle of 120°, a triangular height of 30 mm, and a bottom side length of 80 mm, and a freezing temperature of -196°C; place the obtained frozen sample in a freeze dryer and dry it at 8 Pa and -50°C for 72 h to obtain a nanocellulose / aramid nanofiber / ZIF-67 aerogel;

[0066] (4) Place the nanocellulose / aramid nanofiber / ZIF-67 aerogel obtained in step (3) in a tube furnace for carbonization. Heat it from room temperature to 500°C at a rate of 2°C / min and hold for 2 h, then heat it to 1000°C at a rate of 5°C / min and hold for 2 h, and finally cool it at a rate of 5°C / min to obtain an aramid nanofiber / nanocellulose / ZIF-67-derived carbon aerogel, denoted as G-CA-Co-4. At a thickness of 7.2 mm, the reflection loss value is -32.84 dB, and the effective absorption bandwidth is 6.86 GHz (6.5 mm).

[0067] Comparative Example 1

[0068] To prove the influence of the arch-shaped ice template freezing technology and the unidirectional freezing technology on the microstructure and mechanical properties of the nanocellulose / aramid nanofiber carbon aerogel, Comparative Example 1 is provided.

[0069] The steps not specifically described are the same as those in Example 1. The difference in Comparative Example 1 is that no step is added to the freezing and curing mold, and only a temperature gradient from bottom to top is provided, that is, unidirectional directional freezing. The obtained carbon aerogel is denoted as U-CA.

[0070] Unidirectional freezing only provides a temperature gradient from bottom to top, and there is no temperature control in the horizontal direction. Therefore, ice crystals grow disorderly in the horizontal direction, and the pores after ice crystal removal by freeze-drying are in a uniform and disordered distribution state; while the carbon aerogel prepared by the arch-shaped ice template has a gradient pore structure. Comparison Figure 3 and Figure 5 , the optimal reflection loss value of the U-CA carbon aerogel is -41.24 dB, while the optimal reflection loss value of the G-CA carbon aerogel can reach -50.47 dB. Compared with the uniform small pore structure, the gradient pore structure has two advantages. One is that the large pores on the outside increase the air density and optimize the impedance matching. The other is that it increases the multiple reflections and scattering of the incident wave inside, enhancing the microwave attenuation ability.

[0071] Comparative Example 2

[0072] To prove the influence of the arch-shaped ice template freezing technology and the bidirectional freezing technology on the microstructure and mechanical properties of the nanocellulose / aramid nanofiber carbon aerogel, Comparative Example 2 is provided.

[0073] The steps not specifically described are the same as those in Example 1. The difference in Comparative Example 2 lies in that a wedge-shaped frustum is placed in the freeze-curing mold, and at the same time, temperature gradients in both the bottom-up and single horizontal directions are provided, that is, bidirectional directional freezing. The carbon aerogel prepared is denoted as B-CA.

[0074] Bidirectional freezing provides a temperature gradient in a single horizontal direction. Therefore, ice crystals grow orderly in the horizontal direction, and the pores after the removal of ice crystals by freeze-drying are in a uniform and orderly layered state; while the carbon aerogel prepared by the arched ice template has a gradient pore structure. Comparison Figure 3 and Figure 6 , the optimal reflection loss value of the B-CA carbon aerogel is -27.11 dB, while the optimal reflection loss value of the G-CA carbon aerogel can reach -50.47 dB. Compared with the homogeneous small-pore structure, the gradient pore structure has two advantages. One is that the large pores on the outside increase the air density and optimize the impedance matching. The other is that it increases the multiple reflections and scattering of the incident wave inside, enhancing the microwave attenuation ability.

[0075] Comparative Example 3

[0076] To prove the influence of the arched ice template freezing technology and the bidirectional freezing technology on the microstructure and mechanical properties of the nanocellulose / aramid nanofiber carbon aerogel, Comparative Example 3 is provided.

[0077] The steps not specifically described are the same as those in Example 1. The difference in Comparative Example 3 lies in that the apex angle of the arched template parameter is 45°, and the bottom side length is 20 mm. Based on the principle analysis of the formation of gradient pores by the arched ice template, if the apex angle is too small and the bottom side length is too short, it is difficult to achieve a long-range effective action of the temperature gradient in the horizontal direction provided by the arched ice template, and it is difficult to play the role of redistributing solute particles. Therefore, the size difference of the ice crystals formed by freezing is small, and a pore structure with a good gradient distribution cannot be presented.

[0078] Comparative Example 4

[0079] To prove the influence of the arched ice template freezing technology and the bidirectional freezing technology on the microstructure and mechanical properties of the nanocellulose / aramid nanofiber carbon aerogel, Comparative Example 3 is provided.

[0080] The steps not specifically described are the same as those in Example 1. The difference in Comparative Example 3 lies in that the apex angle of the arched template parameter is 160°, and the bottom side length is 180 mm. Based on the principle analysis of the formation of gradient pores by the arched ice template, if the apex angle is too large and the bottom side length is too long, the temperature gradient in the vertical direction is smaller, weakening the temperature gradient in the horizontal direction, and it is difficult to play the role of redistributing solute particles. Therefore, the size difference of the ice crystals formed by freezing is small, and a pore structure with a good gradient distribution cannot be presented.

[0081] It should be noted that the above embodiments are only some of the embodiments of the preferred implementation manners of the present invention, rather than all embodiments. Obviously, all other embodiments obtained by those of ordinary skill in the art based on the above embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

Claims

1. A preparation method of a magnetic-doped nanofiber-derived carbon aerogel microwave absorption material with a gradient pore structure, characterized in that, It includes the following preparation steps: (1) Synthesize metal-organic framework nanoparticles by solvothermal method; (2) Add the metal-organic framework nanoparticles in step (1) into the nano-cellulose / aramid nanofiber composite dispersion liquid, and stir evenly to obtain a mixed dispersion liquid; (3) Subject the mixed dispersion liquid in step (2) to arched ice template freezing technology and freeze-drying treatment to obtain an aerogel; (4) Carbonize the aerogel obtained in step (3) at high temperature in an inert gas atmosphere to obtain a carbon aerogel.

2. The preparation method of the magnetic-doped nanofiber-derived carbon aerogel microwave absorption material with a gradient pore structure according to claim 1, characterized in that, In step (1), the metal-organic framework nanoparticles are from the ZIF series, MIL series, UIO-66 series, and the central metal ions are Co 2+ , Ni 2+ , Fe 3+ , Cu 2+ and other mono- or multi-metal ions, and the organic ligand is one or several of terephthalic acid, trimesic acid, fumaric acid, 2-methylimidazole, benzimidazole, etc.

3. The preparation method of the magnetic-doped nanofiber-derived carbon aerogel microwave absorption material with a gradient pore structure according to claim 1, wherein, In step (1), the molar ratio of the central metal ion to the organic ligand is 1:(1 - 8), the mass ratio of the solute to the solvent is 1:(10 - 30), and the mass ratio of the solute to the dispersant is 1:(1 - 5).

4. The preparation method of the magnetic-doped nanofiber-derived carbon aerogel microwave absorption material with a gradient pore structure according to claim 1, characterized in that, In step (1), the solvent is water, ethanol, methanol, N,N-dimethylformamide, and the dispersant is one or more of solvents such as PVPK15, PVPK30, PVPK60, PVPK90, etc.

5. The preparation method of the magnetic-doped nanofiber-derived carbon aerogel microwave absorption material with a gradient pore structure according to claim 1, characterized in that, The arched ice template technology in step (3) is to place the mixed dispersion liquid in the upper freezing container, place an isosceles triangle mold at the bottom of the upper freezing container, place the upper freezing container on a metal block, and then place the metal block in a liquid nitrogen bath; the material of the triangle mold is PDMS and / or rubber, the apex angle is 60° - 150°, the triangle height is 5 - 50 mm, the triangle bottom side length is 10 - 200 mm, and the freezing temperature is -(110°C - 200°C).

6. The preparation method of the magnetic-doped nanofiber-derived carbon aerogel microwave absorption material with a gradient pore structure according to claim 1, characterized in that, In step (3), the freeze-drying temperature ≥ -40°C, the drying pressure ≤ 20 Pa, and the drying time is 36 - 72 h.

7. The preparation method of the magnetic-doped nanofiber-derived carbon aerogel microwave absorption material with a gradient pore structure according to claim 1, characterized in that, In step (4), the inert gas is nitrogen and / or argon.

8. The preparation method of the magnetic-doped nanofiber-derived carbon aerogel microwave absorption material with a gradient pore structure according to claim 1, characterized in that, The high-temperature carbonization process in step (4) is: heat up from room temperature to 500°C at a rate of 2 - 15°C / min, and keep it at 500°C for 1 - 4 h, then heat up from 500°C to (700 - 1400)°C at a rate of 5 - 20°C / min and keep it for 1 - 4 h.

9. A carbon aerogel obtained by the preparation method of the magnetic-doped nanofiber-derived carbon aerogel microwave absorption material with a gradient pore structure according to any one of claims 1 - 8.

10. A carbon aerogel obtained by the preparation method of the microwave absorption material of the magnetic-doped nanofiber-derived carbon aerogel with a gradient pore structure as claimed in claim 9, characterized in that The obtained carbon aerogel is used for microwave absorption.

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