A magnetic-doped nanofiber-derived carbon aerogel wave-absorbing material with a gradient pore structure and a preparation method thereof

Abstract

This invention discloses a method for preparing a magnetically doped nanofiber-derived carbon aerogel microwave absorbing material with a gradient pore structure and its applications, belonging to the field of microwave absorbing materials. This invention controls the microstructure of the aerogel by adjusting the parameters of an arched ice template and the content of magnetic material precursors. A lightweight carbon aerogel material is obtained through freezing, freeze-drying, and carbonization using an arched ice template. The gradient pore structure of the carbon aerogel improves impedance matching and promotes microwave reflection and scattering. Nanocellulose, aramid nanofibers, and metal-organic frameworks synergistically optimize polarization loss and magnetic loss performance by constructing a multi-component heterogeneous interface. Due to these characteristics, this type of carbon aerogel exhibits excellent microwave absorption performance. In the 2-18 GHz range, with a thickness of 2.8 mm, the optimal reflection loss value can reach -57.03 dB, and the effective absorption bandwidth (RL≤-10 dB) is 6.76 GHz. This type of carbon aerogel has promising application prospects in the electromagnetic wave field, achieving both broadband and strong microwave absorption at a relatively low thickness.

Description

Technical Field

[0001] This invention relates to a method for preparing a magnetically doped nanofiber-derived carbon aerogel microwave absorbing material with a gradient pore structure, specifically belonging to the field of microwave absorbing material technology. Background Technology

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

[0003] Carbon aerogels are considered promising microwave absorbing materials 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. Furthermore, the tortuous pores and pore walls extend the transmission path of electromagnetic waves, providing abundant interfaces for attenuation through multiple scattering. In addition, the abundant solid-gas interfaces formed by the porous structure lead to charge accumulation and asymmetric distribution, enhancing spatial polarization loss. Therefore, the pore structure is closely related to the microwave absorption performance of carbon aerogels, and rationally designing the pore structure is key to improving the microwave absorption performance of the material. Currently, ice template technology is considered the simplest and most suitable method for controlling 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, and honeycomb structures. However, most current research on aerogel pore structures focuses on the control of uniform pore size, considering that impedance matching and multiple reflections and scattering are not a simple correspondence with pore size. Therefore, this invention aims to prepare carbon aerogels with a gradient pore structure, coordinating the impedance matching of carbon aerogels with microwave attenuation capabilities enhanced by multiple reflections and scattering. Furthermore, single carbon materials typically cannot meet impedance matching requirements, necessitating composites with other materials to improve dielectric and magnetic loss capabilities, thereby enhancing electromagnetic wave attenuation through the synergistic effect of multiple loss mechanisms. Summary of the Invention

[0004] Purpose of the invention: In order to solve the above-mentioned technical problems, the present invention aims to provide a method for preparing magnetically doped nanofiber-derived carbon aerogel microwave absorbing material with a gradient pore structure.

[0005] This invention proposes an arched ice template method, designing an arched ice template cryogenic mold for preparing carbon aerogels with a gradient pore structure. Simultaneously, to enrich the loss mechanism of the carbon aerogel, a metal-organic framework is introduced to prepare a magnetically doped nanofiber-derived carbon aerogel microwave absorbing material with a gradient pore structure. The material preparation process is simple, low-cost, and structurally controllable. Its microwave absorption performance can be multi-scale regulated by controlling the parameters of the arched ice template cryogenic mold and the amount of magnetic material added.

[0006] To achieve the above-mentioned technical objectives, the technical solution adopted by this invention is as follows: a method for preparing a magnetically doped nanofiber-derived carbon aerogel microwave absorbing material with a gradient pore structure, comprising the following preparation steps:

[0007] (1) Metal-organic framework materials were synthesized by a solvothermal method;

[0008] (2) Add the metal-organic framework material from step (1) to the nanocellulose / aramid nanofiber mixture and stir until uniform to obtain a mixed dispersion;

[0009] (3) The mixed dispersion in step (2) was subjected to arched ice template freezing technology and freeze drying to obtain aerogel;

[0010] (4) The aerogel obtained in step (3) is carbonized at high temperature in an inert gas atmosphere to obtain carbon aerogel.

[0011] Preferably, in step (1), the metal-organic framework nanoparticles are ZIF series, MIL series, or UIO-66 series, and the central metal ion is Co. 2+ Ni 2+ Fe 3+ Cu 2+ The organic ligands are one or more of the following: mono- or poly-metallic ions, such as terephthalic acid, pyromellitic acid, fumaric acid, 2-methylimidazole, and benzimidazole.

[0012] More preferably, in step (1), the metal-organic framework nanoparticles are ZIF-67, ZIF-8, or MIL-88(Fe), and the core metal ion is Co. 2+ Ni 2+ Fe 3+ Metal ions, such as pyromellitic acid, fumaric acid, 2-methylimidazole, etc.

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

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

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

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

[0017] More preferably, the triangular mold is made of PDMS, with a apex angle of 90°-120°, a triangle height of 5-30mm, a triangle base length of 10-180mm, and a freezing temperature of -(120℃-200℃).

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

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

[0020] Preferably, the high-temperature carbonization process in step (4) is as follows: at room temperature, the temperature is increased to 500℃ at a rate of 2-15℃ / min and held at 500℃ for 1-4 hours, and at 500℃, the temperature is increased to (700-1400)℃ at a rate of 5-20℃ / min and held for 1-4 hours.

[0021] More preferably, the high-temperature carbonization process in step (4) is as follows: at room temperature, the temperature is increased to 500°C at a rate of 2-10°C / min and held at 500°C for 1-4 hours, and at 500°C, the temperature is increased to (800-1000)°C at a rate of 5-10°C / min and held for 1-4 hours.

[0022] A method for preparing carbon aerogel derived from magnetically doped nanofibers with a gradient pore structure.

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

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

[0025] Invention Principle: During the freezing process, an arched ice template redistributes solute particles by adjusting the temperature gradient. Vertically, temperature gradually increases from bottom to top. Horizontally, temperature gradually increases from both ends of the template inwards. Due to the lower temperature at both ends, ice crystals grow rapidly and occupy space, forming a larger pore structure. This forces solute particles to migrate inwards, while the higher solute concentration on the inner side inhibits ice crystal growth, resulting in smaller pore sizes. Therefore, after freeze-drying, a gradient pore structure aerogel with large outer pores and small inner pores is prepared. The large outer pores of this carbon aerogel allow more microwaves to enter, while the inner pores effectively attenuate incident microwaves through multiple reflections and scattering, exhibiting excellent microwave absorption advantages. Furthermore, multiple electromagnetic wave loss mechanisms are one of the key factors for achieving efficient broadband absorption. On one hand, the continuous conductive network constructed from carbonized nanocellulose and aramid nanofibers provides an effective path for electron transport, thereby significantly enhancing the material's conductivity loss capability. The 3D network structure can extend the propagation path of electromagnetic waves and further improve electromagnetic wave loss efficiency through multiple reflections within the material. On the other hand, abundant heterogeneous interfaces can induce a large amount of carrier accumulation, generating abundant space charge and leading to strong interfacial polarization. Magnetic interfaces introduced by multi-interface engineering can effectively modulate the magnetic response characteristics of materials to electromagnetic waves. Meanwhile, the natural resonance effect and eddy current loss induced by magnetic nanoparticles further enrich the magnetic loss mechanism, thereby promoting the efficient construction and optimization of magnetoelectric coupling networks.

[0026] Compared with the prior art, the present invention has the following advantages:

[0027] (1) This invention uses an arched ice template method combined with a high-temperature carbonization method to prepare carbon aerogels.

[0028] The process is simple, the carbon aerogel structure is controllable, and it is environmentally friendly; the prepared carbon aerogel has a gradient pore structure.

[0029] Gels offer the dual advantages of optimized impedance matching and improved attenuation capability;

[0030] (2) This invention selects two types of nanofibers, nanocellulose and aramid nanofibers, and metal.

[0031] Organic frameworks, as microwave absorbing materials, benefit from the low density of nanofibers and the light weight of metal-organic frameworks.

[0032] The porous structure of the nanofibers resulted in aerogels with low density; the nanofibers derived from carbon materials...

[0033] The three-dimensional network structure is retained, and the continuous conductive network it constructs provides an effective path for electron transport, thereby significantly enhancing the material's conductivity and loss capability. The metal-organic framework-derived carbon has both conductivity and magnetism, while avoiding the characteristics of high metal particle density and easy oxidation. In addition, the interfacial polarization caused by the abundant heterogeneous interfaces greatly improves the attenuation capability of the absorbing material and enhances the material's absorption intensity.

[0034] (3) The carbon aerogel prepared in this invention has a density of 38.46 mg / cm³. 3 Within the 2-18 GHz range, with a thickness of 2.8 mm, the optimal reflection loss value can reach -57.03 dB, and the effective absorption bandwidth (RL≤-10 dB) for microwave absorption is 6.76 GHz. This well meets the current application requirements of electromagnetic wave absorbing materials for being "light, thin, wide, and strong". Attached Figure Description

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

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

[0037] Figure 3 The graph shows the reflection loss curve of the carbon aerogel prepared in Example 1.

[0038] Figure 4 The graph shows the reflection loss curve of the carbon aerogel prepared in Example 2.

[0039] Figure 5 The reflection loss curve of the carbon aerogel prepared in Comparative Example 1 is shown.

[0040] Figure 6 The reflection loss curve of the carbon aerogel prepared in Comparative Example 2 is shown.

[0041] Figure 7 This is a schematic diagram of the container device for the arched ice template method. Detailed Implementation

[0042] Example 1

[0043] A method for preparing a magnetically doped nanofiber-derived carbon aerogel microwave absorbing material with a gradient pore structure includes the following steps:

[0044] (1) First, take 0.1g of nanocellulose and add it to deionized water to disperse it by ultrasonication to obtain a uniform solution. Then, take 0.1g of aramid nanofiber and add it to the nanocellulose solution by ultrasonication and stirring to disperse it into a uniform mixture.

[0045] (2) Place the mixed dispersion obtained in step (1) in the upper freezing container with a triangle vertices of 90°, a triangle height of 10 mm, a base length of 20 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 nanocellulose / aramid nanofiber gel.

[0046] (3) The nanocellulose / aramid nanofiber aerogel obtained in step (2) was placed in a tube furnace for carbonization. The temperature was increased to 500℃ at a rate of 2℃ / min at room temperature and held for 1 h. Then, the temperature was increased to 900℃ at a rate of 5℃ / min and held for 1 h. Finally, the temperature was decreased at a rate of 5℃ / min to obtain the nanocellulose / aramid nanofiber derived carbon aerogel, denoted as G-CA. Its microstructure is as follows: Figure 1 As shown, with a thickness of 9 mm, the reflection loss is -50.47 dB, and the effective absorption bandwidth is 7.15 GHz (8.5 mm).

[0047] Example 2

[0048] (1) First, 1.46 g of Co(NO3)2·6H2O was dissolved in 40 mL of methanol solution and stirred until clear. Then, 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 at room temperature for 24 h to allow for self-precipitation. The solution was washed with methanol and ethanol, and centrifuged three times to obtain a purple precipitate. Finally, it was vacuum dried at 60 °C for 12 h to obtain ZIF-67 nanoparticles;

[0049] (2) Take 0.1g of nanocellulose and add it to deionized water to disperse it by ultrasonication to obtain a uniform solution. Then take 0.1g of aramid nanofiber and add it to the nanocellulose solution and stir evenly. Finally, add 20mg of ZIF-67 nanoparticles and disperse them by ultrasonication to obtain a uniform mixture.

[0050] (3) Place the mixed dispersion obtained in step (2) in the upper freezing container with a triangle vertices of 90°, a triangle height of 10 mm, a base length of 20 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 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. The temperature was increased to 500℃ at a rate of 2℃ / min at room temperature and held for 1 h. Then, the temperature was increased to 900℃ at a rate of 5℃ / min and held for 1 h. Finally, the temperature was decreased at a rate of 5℃ / min to obtain the aramid nanofiber / nanocellulose / ZIF-67 derived carbon aerogel, denoted as G-CA-Co-1. Its morphology analysis is as follows: Figure 2 As shown, with a thickness of 2.8 mm, the reflection loss is -57.03 dB, and the effective absorption bandwidth is 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. Then, 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 at room temperature for 24 h to allow for self-precipitation. The solution was washed with methanol and ethanol, and centrifuged three times to obtain a purple precipitate. Finally, it was vacuum dried at 60 °C for 12 h to obtain ZIF-67 nanoparticles;

[0054] (2) Take 0.1g of nanocellulose and add it to deionized water to disperse it by ultrasonication to obtain a uniform solution. Then take 0.1g of aramid nanofiber and add it to the nanocellulose solution and stir evenly. Finally, add 60mg of ZIF-67 nanoparticles and disperse them by ultrasonication to obtain a uniform mixture.

[0055] (3) Place the mixed dispersion obtained in step (2) in the upper freezing container with a triangle vertices of 120°, a triangle height of 5 mm, a base length of 16 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 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. The temperature was increased to 500℃ at a rate of 2℃ / min at room temperature and held for 1 h. Then, the temperature was increased to 800℃ at a rate of 5℃ / min and held for 2 h. Finally, the temperature was decreased at a rate of 5℃ / min to obtain 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, 1.46 g of Co(NO3)2·6H2O was dissolved in 40 mL of methanol solution and stirred until clear. Then, 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 at room temperature for 24 h to allow for self-precipitation. The solution was washed with methanol and ethanol, and centrifuged three times to obtain a purple precipitate. Finally, it was vacuum dried at 60 °C for 12 h to obtain ZIF-67 nanoparticles;

[0059] (2) Take 0.1g of nanocellulose and add it to deionized water to disperse it by ultrasonication to obtain a uniform solution. Then take 0.1g of aramid nanofiber and add it to the nanocellulose solution and stir evenly. Finally, add 20mg of ZIF-67 nanoparticles and disperse them by ultrasonication to obtain a uniform mixture.

[0060] (3) Place the mixed dispersion obtained in step (2) in the upper freezing container with a vertices of 120°, a height of 60 mm, a base 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 nanocellulose / aramid nanofiber / ZIF-67 aerogel.

[0061] (4) The nanocellulose / aramid nanofiber / ZIF-67 aerogel obtained in step (3) was placed in a tube furnace for carbonization. The temperature was increased to 500℃ at a rate of 2℃ / min at room temperature and held for 2h. Then, the temperature was increased to 900℃ at a rate of 5℃ / min and held for 2h. Finally, the temperature was decreased at a rate of 5℃ / min to obtain aramid nanofiber / nanocellulose / ZIF-67 derived carbon aerogel, denoted as G-CA-Co-3. At a thickness of 4.2mm, the reflection loss value was -45.68dB, and the effective absorption bandwidth was 7.22GHz (3.8mm).

[0062] Example 5

[0063] (1) First, 1.46 g of Co(NO3)2·6H2O was dissolved in 40 mL of methanol solution and stirred until clear. Then, 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 at room temperature for 24 h to allow for self-precipitation. The solution was washed with methanol and ethanol, and centrifuged three times to obtain a purple precipitate. Finally, it was vacuum dried at 60 °C for 12 h to obtain ZIF-67 nanoparticles;

[0064] (2) Take 0.1g of nanocellulose and add it to deionized water to disperse it by ultrasonication to obtain a uniform solution. Then take 0.1g of aramid nanofiber and add it to the nanocellulose solution and stir evenly. Finally, add 20mg of ZIF-67 nanoparticles and disperse them by ultrasonication to obtain a uniform mixture.

[0065] (3) Place the mixed dispersion obtained in step (2) in the upper freezing container with a triangle vertices of 120°, a triangle height of 30 mm, a base 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 nanocellulose / aramid nanofiber / ZIF-67 aerogel.

[0066] (4) The nanocellulose / aramid nanofiber / ZIF-67 aerogel obtained in step (3) was placed in a tube furnace for carbonization. The temperature was increased to 500℃ at a rate of 2℃ / min at room temperature and held for 2h. Then, the temperature was increased to 1000℃ at a rate of 5℃ / min and held for 2h. Finally, the temperature was decreased at a rate of 5℃ / min to obtain aramid nanofiber / nanocellulose / ZIF-67 derived carbon aerogel, denoted as G-CA-Co-4. At a thickness of 7.2mm, the reflection loss value was -32.84dB, and the effective absorption bandwidth was 6.86GHz (6.5mm).

[0067] Comparative Example 1

[0068] To demonstrate the effects of arched ice template freezing technology and unidirectional freezing technology on the microstructure and mechanical properties of nanocellulose / aramid nanofiber carbon aerogels, Comparative Example 1 is provided.

[0069] Unless otherwise specified, the steps are the same as in Example 1. The difference in Comparative Example 1 is that no ladder is added in the freeze-curing mold, and only a temperature gradient from bottom to top is provided, i.e., unidirectional directional freezing. The carbon aerogel prepared is denoted as U-CA.

[0070] One-way freezing provides only a bottom-up temperature gradient, without horizontal temperature control. Therefore, ice crystals grow randomly in the horizontal direction, resulting in a uniformly disordered pore distribution after freeze-drying. In contrast, carbon aerogels prepared using arched ice templates exhibit a gradient pore structure. Figure 3 and Figure 5 The optimal reflection loss of U-CA carbon aerogel is -41.24 dB, while that of G-CA carbon aerogel can reach -50.47 dB. Compared with uniform small-pore structures, gradient-pore structures have dual advantages: firstly, the large pores on the outside increase air density and optimize impedance matching; secondly, they increase the multiple reflections and scattering of incident waves inside, enhancing microwave attenuation capabilities.

[0071] Comparative Example 2

[0072] To demonstrate the effects of arched ice template freezing technology and bidirectional freezing technology on the microstructure and mechanical properties of nanocellulose / aramid nanofiber carbon aerogels, Comparative Example 2 is provided.

[0073] Unless otherwise specified, the steps are the same as in Example 1. The difference in Comparative Example 2 is that a wedge-shaped platform is placed in the freeze-curing mold, and a temperature gradient is provided from bottom to top and in a single horizontal direction, i.e., bidirectional directional freezing. The resulting carbon aerogel is denoted as B-CA.

[0074] Bidirectional freezing provides a single horizontal temperature gradient, resulting in orderly ice crystal growth in the horizontal direction. After freeze-drying, the pores after ice crystal removal exhibit a uniform, ordered layered structure; in contrast, carbon aerogels prepared using arched ice templates possess a gradient pore structure. Figure 3 and Figure 6 The optimal reflection loss of B-CA carbon aerogel is -27.11 dB, while that of G-CA carbon aerogel can reach -50.47 dB. Compared with uniform small-pore structures, gradient-pore structures have dual advantages: firstly, the large pores on the outside increase air density and optimize impedance matching; secondly, they increase the multiple reflections and scattering of incident waves inside, enhancing microwave attenuation capabilities.

[0075] Comparative Example 3

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

[0077] Unless otherwise specified, the steps are the same as in Example 1. The difference in Comparative Example 3 is that the apex angle of the arched template is 45° and the base length is 20mm. Based on the analysis of the gradient hole formation principle of the arched ice template, it can be seen that if the apex angle is too small and the base length is too short, the horizontal temperature gradient provided by the arched ice template is difficult to achieve a long-range effective effect, and it is difficult to play a role in redistributing solute particles. Therefore, the ice crystals formed by freezing have small size differences and cannot present a well-distributed pore structure.

[0078] Comparative Example 4

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

[0080] Unless otherwise specified, the steps are the same as in Example 1. The difference in Comparative Example 3 is that the apex angle of the arched template is 160° and the base length is 180mm. Based on the analysis of the gradient hole formation principle of the arched ice template, it can be seen that if the apex angle is too large and the base length is too long, the vertical temperature gradient is small, which weakens the horizontal temperature gradient. This makes it difficult to redistribute the solute particles, resulting in smaller differences in the size of the ice crystals formed by freezing, and thus failing to present a well-distributed pore structure.

[0081] It should be noted that the above embodiments are merely some preferred embodiments of the present invention, and not all embodiments. Obviously, based on the above embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort should fall within the scope of protection of the present invention.

Claims

1. A method for preparing a magnetically doped nanofiber-derived carbon aerogel microwave absorbing material with a gradient pore structure, characterized in that, The preparation steps include the following: (1) Metal-organic framework nanoparticles were synthesized by a solvothermal method; (2) Add the metal-organic framework nanoparticles from step (1) to the nanocellulose / aramid nanofiber composite dispersion and stir until uniform to obtain a mixed dispersion; (3) The mixed dispersion in step (2) is subjected to arched ice template freezing technology and freeze drying to obtain aerogel; the arched ice template technology in step (3) is to place the mixed dispersion in an upper freezing container, place an isosceles triangular 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 triangular mold is PDMS and / or rubber, the vertex angle is 60°-150°, the triangle height is 5-50 mm, the triangle base length is 10-200 mm, and the freezing temperature is -200 ℃~-110 ℃; (4) The aerogel obtained in step (3) is carbonized at high temperature in an inert gas atmosphere to obtain carbon aerogel.

2. The method for preparing the magnetically doped nanofiber-derived carbon aerogel microwave absorbing material with a gradient pore structure according to claim 1, characterized in that, In step (1), the metal-organic framework nanoparticles are ZIF series, MIL series, or UIO-66 series, and the central metal ion is Co. 2+ Ni 2+ Fe 3+ Cu 2+ The mono- or poly-metallic ions in the mixture contain one or more organic ligands selected from terephthalic acid, pyromellitic acid, fumaric acid, 2-methylimidazole, and benzimidazole.

3. The method for preparing the magnetically doped nanofiber-derived carbon aerogel microwave absorbing material with a gradient pore structure according to claim 1, characterized in that, Step (1) The molar ratio of central metal ion to organic ligand is 1:(1-8), the mass ratio of solute to solvent is 1:(10-30), and the mass ratio of solute to dispersant is 1:(1-5).

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

5. The method for preparing the magnetically doped nanofiber-derived carbon aerogel microwave absorbing material with a gradient pore structure according to claim 1, characterized in that, Step (3) Freeze-drying temperature ≥ -40 ℃, drying pressure ≤ 20 Pa, drying time 36-72 h.

6. The method for preparing the magnetically doped nanofiber-derived carbon aerogel microwave absorbing material with a gradient pore structure according to claim 1, characterized in that, Step (4) The inert gas is nitrogen and / or argon.

7. The method for preparing the magnetically doped nanofiber-derived carbon aerogel microwave absorbing material with a gradient pore structure according to claim 1, characterized in that, Step (4) High-temperature carbonization process: At room temperature, the temperature is increased to 500℃ at a rate of 2-15℃ / min and held at 500℃ for 1-4 h. At 500℃, the temperature is increased to 700℃-1400℃ at a rate of 5-20℃ / min and held for 1-4 h.

8. A carbon aerogel obtained by the method for preparing the magnetically doped nanofiber-derived carbon aerogel microwave absorbing material with a gradient pore structure according to any one of claims 1-7.

9. A carbon aerogel according to claim 8, characterized in that, The resulting carbon aerogel was used for microwave absorption.

Images