Nitrogen-doped carbon nanotube array loaded magnetic carbon aerogel and preparation method thereof

Abstract

The invention discloses nitrogen-doped carbon nanotube array loaded magnetic carbon aerogel and a preparation method thereof.The method comprises the steps that cobalt salt and nickel salt are evenly dissolved in a biomass precursor aqueous solution, and a homogeneous solution is formed; adding a cross-linking aid to construct a three-dimensional network, and wrapping metal ions; performing freeze drying after directional freezing to form a directional porous structure; carrying out heat preservation and carbonization, converting the biomass into a porous carbon matrix, and reducing the cobalt / nickel salt into a uniformly embedded CoNi alloy; through two-stage heat treatment, the nitrogen-rich organic matter is decomposed to release a nitrogen source, and the vertical carbon nanotube array grows in situ. The material integrates CoNi alloy magnetic loss, carbon matrix dielectric loss and multistage structure scattering effect, realizes dielectric-electromagnetic cooperation, optimizes impedance matching, avoids strong surface reflection caused by high conductivity of a single carbon material, introduces polarization loss, widens an absorption frequency band, has excellent broadband efficient electromagnetic wave absorption performance, and is suitable for a complex electromagnetic environment.

Description

Technical Field

[0001] This invention belongs to the field of electromagnetic wave absorbing materials technology, and relates to a nitrogen-doped carbon nanotube array-supported magnetic carbon aerogel and its preparation method. Background Technology

[0002] With the rapid popularization of fifth-generation communication technology (5G), high-frequency radar detection, and high-power electronic devices, electromagnetic waves, as the core carrier of information transmission and energy transfer, are experiencing exponential growth in application scope and intensity. This has led to increasingly prominent electromagnetic pollution problems (such as interference with electronic devices and bio-electromagnetic radiation hazards) and military stealth requirements (such as radar wave stealth for weapons and equipment), becoming key challenges restricting modern electronic technology and national defense security. Against this backdrop, electromagnetic wave absorbing materials, as a core technology for reducing electromagnetic radiation hazards and achieving target stealth through energy conversion (converting electromagnetic waves into heat energy or other forms of loss), have become a hot topic of interdisciplinary research in materials science, electromagnetics, and the defense industry. Their research and application are of strategic significance for promoting the greening of the electronic information industry and improving the stealth performance of national defense equipment.

[0003] Currently, research on electromagnetic wave absorbing materials mainly revolves around traditional magnetic absorbing materials and novel carbon-based materials. Traditional magnetic absorbing materials (such as ferrites and metal alloys) dominated the early field of wave absorption due to their excellent magnetic loss capabilities. However, their high density makes it difficult to achieve lightweight equipment, and their susceptibility to oxidation limits long-term stability in complex environments (such as high temperature and high humidity). Furthermore, their narrow absorption bandwidth makes it difficult to meet the absorption requirements of broadband electromagnetic waves (such as 5G multi-band and wideband radar detection). On the other hand, carbon-based materials, represented by graphene and carbon nanotubes, have advantages such as light weight and high chemical stability. However, due to the lack of a magnetic loss mechanism, they mainly rely on conductivity loss and dielectric polarization loss, making it difficult to achieve efficient absorption over a wide frequency range. Moreover, the single loss mode easily leads to impedance mismatch (i.e., an imbalance between surface reflection and internal absorption), further reducing the absorption performance.

[0004] In recent years, biomass-derived carbon materials have provided a unique platform for the design of lightweight magnetic carbon-based microwave absorbing materials due to their sustainability (widespread availability of raw materials), structural tunability (high-temperature carbonization can form a rich microporous / mesoporous three-dimensional framework), and ease of preparation (such as direct carbonization of biomass precursors). Their high specific surface area and defect structure can significantly enhance dielectric polarization relaxation loss and conductivity loss, potentially breaking through the performance bottlenecks of traditional materials. However, single biomass-derived carbon materials still face key challenges: high conductivity easily induces strong reflection of electromagnetic waves on the material surface, leading to impedance mismatch; relying solely on conductivity loss and dielectric polarization loss as a single energy conversion mode is insufficient to cover the absorption requirements of broadband electromagnetic waves, ultimately limiting their practical application in complex electromagnetic environments. Summary of the Invention

[0005] To address the problems existing in the prior art, this invention provides a nitrogen-doped carbon nanotube array-loaded magnetic carbon aerogel and its preparation method, thereby solving the technical problem that in the prior art, single carbon materials are prone to strong electromagnetic wave surface reflection due to high conductivity, and rely solely on a single energy conversion mode of conductivity loss and dielectric polarization loss, resulting in impedance mismatch and difficulty in achieving broadband and efficient absorption, thus limiting their practical application in complex electromagnetic environments.

[0006] This invention is achieved through the following technical solution: A method for preparing nitrogen-doped carbon nanotube array-supported magnetic carbon aerogel includes the following steps: S1: Add cobalt salt and nickel salt to the aqueous solution of biomass precursor, stir to dissolve, and obtain a homogeneous solution of biomass-metal salt; S2: Add a cross-linking aid to the biomass-metal salt homogeneous solution to obtain a cross-linked composite suspension; S3: The cross-linked composite suspension is subjected to directional freezing and then freeze-dried to obtain a directional porous aerogel precursor; S4: The directional porous aerogel precursor is placed in a nitrogen or inert atmosphere for heat preservation treatment to obtain biomass-derived magnetic carbon aerogel. S5: The nitrogen-rich organic matter and the biomass-derived magnetic carbon aerogel are placed in a tube furnace and subjected to a two-stage heat treatment in a nitrogen or inert atmosphere to obtain the nitrogen-doped carbon nanotube array-supported magnetic carbon aerogel.

[0007] Preferably, in step S1, the ratio of the cobalt salt, nickel salt and biomass precursor is 1 mmol:1 mmol:(0.1~1) g.

[0008] Preferably, in step S2, the ratio of the crosslinking aid to the biomass precursor is (0.1~1) mL:(0.1~1) g.

[0009] Preferably, in step S3, the freeze-drying time is 10-60 h.

[0010] Preferably, in step S4, during the heat preservation process, the heating rate is 2~10 ℃ / min, the heat preservation time is 1~6 h, preferably 1~5 h, and the heat preservation temperature is 600~1000 ℃.

[0011] Preferably, in step S5, the mass ratio of the biomass-derived magnetic carbon aerogel to the nitrogen-rich organic matter is 1:(10~40).

[0012] Preferably, in step S5, the nitrogen-rich organic matter and the biomass-derived magnetic carbon aerogel are placed in a tube furnace and subjected to a two-stage heat treatment in a nitrogen or inert atmosphere. Specifically, the nitrogen-rich organic matter is placed upstream of the tube furnace, and the biomass-derived magnetic carbon aerogel is placed downstream of the tube furnace.

[0013] Preferably, in step S5, the two-stage heat treatment process is as follows: in the first stage heat treatment process, the temperature is 300~600 ℃, the heating rate is 2~10 ℃ / min, and the time is 1~6 h, preferably 1~5 h; in the second stage heat treatment process, the temperature is 600~1000 ℃, the heating rate is 2~10 ℃ / min, and the time is 1~6 h, preferably 1~5 h.

[0014] A nitrogen-doped carbon nanotube array-supported magnetic carbon aerogel was prepared by the method described above.

[0015] The above describes the application of a nitrogen-doped carbon nanotube array-loaded magnetic carbon aerogel in the field of electromagnetic wave shielding.

[0016] Compared with the prior art, the present invention has the following beneficial technical effects: This invention discloses a method for preparing nitrogen-doped carbon nanotube array-supported magnetic carbon aerogel. The method first involves uniformly dissolving cobalt and nickel salts in an aqueous solution of a biomass precursor to form a homogeneous solution. The key to this step is ensuring high dispersion of metal ions in the biomass solution and avoiding local aggregation. Then, a cross-linking agent is added, causing the biomass molecular chains to form a three-dimensional network structure through a cross-linking reaction, "encapsulating" the metal ions within the cross-linked network. Next, directional freezing is used to allow ice crystals to grow along a specific direction, forming a directionally aligned ice crystal template. After freeze-drying, the ice crystals sublimate, leaving a directional porous structure complementary to the ice crystal template, i.e., a "directional honeycomb structure." Finally, a thermal insulation treatment is performed, carbonizing the biomass precursor to form a porous carbon matrix, while the cobalt and nickel salts are reduced to CoNi alloys. Gold is uniformly embedded in a carbon matrix. A two-stage heat treatment process decomposes nitrogen-rich organic matter, releasing nitrogen. Simultaneously, carbon nanotubes grow in situ on the surface of the magnetic carbon aerogel, forming a highly ordered array structure perpendicular to the substrate. This yields the nitrogen-doped carbon nanotube array-loaded magnetic carbon aerogel. The CoNi alloy in this array provides magnetic loss, which, together with the dielectric loss of the carbon aerogel, forms a dielectric-electromagnetic synergy. Impedance matching is adjusted through magnetic loss, avoiding impedance mismatch caused by single dielectric loss and optimizing impedance matching characteristics during electromagnetic wave incidence. Secondly, a nitrogen-doped carbon nanotube array is synthesized in situ on the surface of the magnetic carbon aerogel through a two-stage heat treatment, constructing a composite structure of "nitrogen-doped carbon nanotube array-oriented honeycomb structure magnetic carbon aerogel." The nitrogen-doped carbon nanotube array not only further dissipates electromagnetic waves through dielectric loss, but its array structure also induces multiple reflections and scattering of electromagnetic waves, extending the propagation path. The oriented honeycomb structure, through its special channel design, enhances multiple reflections of electromagnetic waves, promoting energy dissipation. Furthermore, nitrogen doping can adjust the conductivity of carbon materials, avoiding strong surface reflection caused by excessive conductivity, while introducing polarization loss and enriching energy conversion modes. In summary, this invention integrates dielectric loss, magnetic loss, and structural scattering effects through the synergistic design of structure (oriented honeycomb, carbon nanotube array) and composition (magnetic CoNi alloy, nitrogen doping), achieving multi-mechanism synergistic dissipation of electromagnetic waves. It significantly optimizes impedance matching and broadens the absorption bandwidth, solving the problems of poor impedance matching and single loss mode of single carbon materials. It possesses excellent broadband and high-efficiency electromagnetic wave absorption performance and is suitable for practical applications in complex electromagnetic environments.

[0017] Furthermore, in step S1, the ratio of cobalt salt, nickel salt, and biomass precursor is 1 mmol:1 mmol:(0.1~1) g. This ratio ensures that the CoNi alloy formed subsequently is uniformly dispersed in the carbon matrix by precisely controlling the ratio of metal salt to carbon source. The equimolar ratio of cobalt and nickel (1:1) is beneficial for forming a CoNi alloy with excellent magnetic properties. Its magnetic loss capability can effectively adjust the impedance matching of the material and avoid strong surface reflection caused by the high conductivity of single carbon material. At the same time, the amount range of biomass precursor (0.1~1 g) can adjust the porosity and carbon content of carbon aerogel, ensuring sufficient carbon source to form a continuous framework while avoiding excessive carbon source leading to structural densification. This provides a suitable substrate environment for the subsequent growth of nitrogen-doped carbon nanotube arrays, and ultimately optimizes electromagnetic wave absorption performance through the synergistic effect of magnetic loss and dielectric loss.

[0018] Furthermore, in step S2, the ratio of the crosslinking aid to the biomass precursor is (0.1~1) mL:(0.1~1) g. This ratio directly affects the structural stability of the aerogel precursor by controlling the degree of crosslinking. An appropriate amount of crosslinking aid can promote the formation of a crosslinked network of biomass molecular chains, avoiding structural collapse during freeze-drying due to insufficient crosslinking (such as uneven pore size or insufficient mechanical strength), or excessive crosslinking leading to excessive rigidity and reduced porosity of the aerogel. A stable crosslinked structure provides the foundation for the "directional honeycomb structure" formed by subsequent directional freezing, ensuring that electromagnetic waves can undergo multiple reflections and scatterings in the porous structure, enhancing dissipation capacity, and simultaneously providing support for the uniform loading of the magnetic CoNi alloy, ultimately improving the broadband absorption performance of the material.

[0019] Furthermore, in step S3, the freeze-drying time is 10–60 h. This time range directly affects the pore structure of the aerogel precursor by controlling the ice crystal sublimation rate. Too short a time (<10 h) will result in incomplete sublimation of the ice crystals, leaving residual moisture in the aerogel and creating defects that disrupt the uniformity of the oriented porous structure. Too long a time (>60 h), while ensuring complete drying, will increase energy consumption and production costs. A suitable freeze-drying time (10–60 h) can form a complete “oriented honeycomb structure.” This structure not only extends the propagation path of electromagnetic waves through multi-level channels (enhancing reflection / scattering), but also works in conjunction with the subsequent carbon nanotube array to construct a multi-level dissipation network, thereby optimizing impedance matching and broadening the absorption bandwidth.

[0020] Furthermore, in step S4, during the heat preservation process, the heating rate is 2~10 ℃ / min, the heat preservation time is 1~6 h, preferably 1~5 h, and the heat preservation temperature is 600~1000 ℃. An excessively fast heating rate (>10 ℃ / min) will cause the biomass precursor to shrink rapidly, generating cracks and damaging structural integrity; an excessively slow rate (<2 ℃ / min) will result in low efficiency. The heat preservation time (1~6 h) and temperature (600~1000 ℃) jointly determine the degree of carbonization and the formation of the CoNi alloy: 600~1000 ℃ ensures that the biomass is completely converted into a carbon matrix, while simultaneously promoting the reduction of cobalt and nickel salts to the CoNi alloy (the source of magnetism), while the 1~6 h heat preservation time ensures uniform alloy dispersion and a stable carbon structure. This combination of parameters avoids the high conductivity of a single carbon material (impedance matching is adjusted through magnetic alloying) and ensures the synergistic effect of magnetic loss and dielectric loss, improving broadband absorption capability.

[0021] Furthermore, in step S5, the mass ratio of the biomass-derived magnetic carbon aerogel to nitrogen-rich organic matter is 1:(10~40). This ratio, by controlling the amount of nitrogen source, directly affects the morphology and performance of the nitrogen-doped carbon nanotube array. Nitrogen-rich organic matter is a key raw material for nitrogen doping. An appropriate amount (10~40 times) can ensure sufficient nitrogen atoms are incorporated into the carbon nanotubes, adjusting their dielectric properties (introducing polarization loss), while avoiding excessive growth or aggregation of carbon nanotubes, which would disrupt the uniformity of the array structure. At this ratio, the nitrogen-doped carbon nanotube array can form a "dielectric-electromagnetic synergy" with the dielectric loss of the magnetic carbon aerogel and the magnetic loss of the CoNi alloy. By dissipating electromagnetic waves through multiple mechanisms, it solves the problem of the single loss mode of a single carbon material, significantly improving broadband absorption efficiency.

[0022] Furthermore, in step S5, the two-stage heat treatment process is as follows: In the first stage heat treatment, the temperature is 300~600 ℃, the heating rate is 2~10 ℃ / min, and the time is 1~6 h, preferably 1~5 h; In the second stage heat treatment, the temperature is 600~1000 ℃, the heating rate is 2~10 ℃ / min, and the time is 1~6 h, preferably 1~5 h. By controlling the pyrolysis process in stages, the growth of nitrogen-doped carbon nanotube arrays is optimized. The first stage (300~600 ℃) is mainly used for the preliminary decomposition of nitrogen-rich organic matter and the incorporation of nitrogen atoms, avoiding the direct high temperature from causing violent decomposition of organic matter and destroying the structure; The second stage (600~1000 ℃) promotes the nucleation and growth of carbon nanotubes to form an array structure. Staged processing ensures uniform nitrogen doping and highly ordered carbon nanotube arrays, thereby enhancing dielectric loss (the array structure extends the electromagnetic wave path) and scattering effect (the honeycomb structure and array work together), ultimately achieving broadband and efficient electromagnetic wave absorption and solving the problem of poor impedance matching in single carbon materials. Attached Figure Description

[0023] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings used in the embodiments will be briefly introduced below. It should be understood that the following drawings only show some embodiments of the present invention and should not be regarded as a limitation on the scope. For those skilled in the art, other related drawings can be obtained based on these drawings without creative effort.

[0024] Figure 1 The XRD comparison diagrams are of the biomass-derived magnetic carbon aerogel obtained in step (4) of Example 4 of the present invention and the nitrogen-doped carbon nanotube array-supported magnetic carbon aerogel obtained in step (5). Figure 2 SEM images of the biomass-derived magnetic carbon aerogel prepared in Example 4 of the present invention at different magnifications, wherein the scale bar of image a is 50 μm and the scale bar of image b is 1 μm. Figure 3 EDS image of the biomass-derived magnetic carbon aerogel prepared in Example 4 of this invention; Figure 4 SEM images of the nitrogen-doped carbon nanotube array-supported magnetic carbon aerogel prepared in Example 4 of this invention, wherein the scale bar of image a is 50 μm and the scale bar of image b is 1 μm. Figure 5 EDS image of the nitrogen-doped carbon nanotube array-supported magnetic carbon aerogel prepared in Example 4 of this invention; Figure 6 This is a three-dimensional reflection loss diagram of the nitrogen-doped carbon nanotube array-supported magnetic carbon aerogel prepared in Example 4 of the present invention. Figure 7 Contour plot of three-dimensional reflection loss of the nitrogen-doped carbon nanotube array loaded with magnetic carbon aerogel prepared in Example 4 of the present invention. Figure 8 This is a two-dimensional reflection loss diagram of the nitrogen-doped carbon nanotube array-supported magnetic carbon aerogel prepared in Example 4 of the present invention. Figure 9 The image shows a Cole-Cole semicircle of the nitrogen-doped carbon nanotube array-supported magnetic carbon aerogel prepared in Example 4 of this invention. Figure 10 This is an electron image of the biomass-derived magnetic carbon aerogel prepared in Example 4 of the present invention being adsorbed by a magnet; Figure 11 This is an electronic image of the nitrogen-doped carbon nanotube array-loaded magnetic carbon aerogel prepared in Example 4 of the present invention being adsorbed by a magnet. Detailed Implementation

[0025] To enable those skilled in the art to understand the features and effects of the present invention, the terms and expressions used in the specification and claims are explained and defined in general below. Unless otherwise specified, all technical and scientific terms used herein have the ordinary meaning understood by those skilled in the art regarding the present invention, and in case of conflict, the definitions in this specification shall prevail.

[0026] The theories or mechanisms described and disclosed herein, whether right or wrong, should not in any way limit the scope of the invention, that is, the contents of the invention can be implemented without being limited by any particular theory or mechanism.

[0027] In this document, all features defined by numerical ranges or percentage ranges, such as numerical values, quantities, contents, and concentrations, are for the sake of brevity and convenience only. Accordingly, descriptions of numerical ranges or percentage ranges should be considered as covering and specifically disclosing all possible sub-ranges and individual numerical values ​​(including integers and fractions) within those ranges.

[0028] In this article, unless otherwise specified, “contains,” “includes,” “containing,” “has,” or similar terms cover the meanings of “composed of” and “mainly composed of,” for example, “A contains a” covers the meanings of “A contains a and others” and “A contains only a.”

[0029] For the sake of brevity, not all possible combinations of the technical features in each implementation scheme or embodiment are described herein. Therefore, as long as there is no contradiction in the combination of these technical features, the technical features in each implementation scheme or embodiment can be combined arbitrarily, and all possible combinations should be considered within the scope of this specification.

[0030] This invention provides a method for preparing magnetic carbon aerogel supported on nitrogen-doped carbon nanotube arrays, comprising the following steps: S1: Add cobalt salt and nickel salt to the aqueous solution of biomass precursor, stir to dissolve, and obtain a homogeneous solution of biomass-metal salt; The biomass precursor is one of cellulose, glucose, and chitosan; The ratio of biomass precursor to water in the aqueous solution of the biomass precursor is (0.1~1) g:(10~100) mL; The cobalt salt is one of cobalt nitrate hexahydrate, cobalt chloride hexahydrate, cobalt acetate tetrahydrate, and cobalt sulfate heptahydrate; The nickel salt is one of nickel nitrate hexahydrate, nickel chloride hexahydrate, nickel acetate tetrahydrate, and nickel sulfate heptahydrate; The ratio of cobalt salt, nickel salt and biomass precursor is 1 mmol:1 mmol:(0.1~1) g.

[0031] In one specific embodiment, the cobalt salt is 1-3 mmol and the nickel salt is 1-3 mmol.

[0032] S2: Add a cross-linking aid to a biomass-metal salt homogeneous solution to obtain a cross-linked composite suspension; The crosslinking aid is citric acid or acetic acid, and the ratio of the crosslinking aid to the biomass precursor is (0.1~1) mL:(0.1~1) g.

[0033] S3: The cross-linked composite suspension is subjected to directional freezing and then freeze-drying to obtain a directional porous aerogel precursor; Specifically: the cross-linked composite suspension is transferred into a custom mold, the mold is placed on a copper column in liquid nitrogen for directional freezing, and the frozen sample is freeze-dried for 10-60 hours. S4: The directional porous aerogel precursor is placed in a nitrogen or inert atmosphere for heat preservation treatment to obtain biomass-derived magnetic carbon aerogel. During the heat preservation process, the heating rate is 2~10 ℃ / min, the heat preservation time is 1~6 h, preferably 1~5 h, and the heat preservation temperature is 600~1000 ℃; the inert atmosphere is argon.

[0034] S5: The nitrogen-rich organic matter and the biomass-derived magnetic carbon aerogel are placed in a tube furnace and subjected to a two-stage heat treatment in a nitrogen or inert atmosphere to obtain the nitrogen-doped carbon nanotube array-supported magnetic carbon aerogel.

[0035] The nitrogen-rich organic compound is at least one of urea, thiourea, melamine, dicyandiamide, and monocyanamide; The mass ratio of biomass-derived magnetic carbon aerogel to nitrogen-rich organic matter is 1:(10~40); The two-stage heat treatment process is as follows: In the first stage heat treatment, the temperature is 300~600 ℃, the heating rate is 2~10 ℃ / min, and the time is 1~6 h, preferably 1~5 h; In the second stage heat treatment, the temperature is 600~1000 ℃, the heating rate is 2~10 ℃ / min, and the time is 1~6 h, preferably 1~5 h; The inert atmosphere is argon.

[0036] In summary, this invention discloses a method for preparing nitrogen-doped carbon nanotube array-supported magnetic carbon aerogel, comprising the following steps: dissolving a biomass precursor in deionized water and stirring to dissolve; then adding cobalt salt and nickel salt to the above solution and stirring to dissolve; then adding a crosslinking aid and maintaining stirring to form a suspension; next, transferring the above suspension into a customized mold, placing the mold on a copper column in liquid nitrogen for directional freezing, and then freeze-drying the frozen sample; subsequently, incubating the freeze-dried sample in an inert atmosphere for a certain period of time to obtain a biomass-derived magnetic carbon aerogel; placing nitrogen-rich organic matter and biomass-derived magnetic carbon aerogel upstream and downstream of a tube furnace, respectively, and performing multi-stage heat treatment in an inert atmosphere to obtain nitrogen-doped carbon nanotube array-supported magnetic carbon aerogel. In summary, this invention employs a directional freezing method and a two-stage heat treatment to synthesize nitrogen-doped carbon nanotube arrays in situ on biomass-derived magnetic carbon aerogels, constructing a directional honeycomb structure magnetic carbon aerogel loaded with nitrogen-doped carbon nanotube arrays. This invention achieves a dielectric-electromagnetic synergy through the dielectric loss of the nitrogen-doped carbon nanotube arrays and the carbon aerogel, as well as the magnetic loss of the CoNi alloy in the magnetic carbon aerogel. This dielectric-electromagnetic synergy helps optimize the impedance matching of the absorbing material. Simultaneously, the multiple reflections and scattering of electromagnetic waves in the directional honeycomb structure and carbon nanotube arrays also contribute to electromagnetic wave dissipation. The fabrication process of this invention is simple, environmentally friendly, and suitable for mass production. The prepared nitrogen-doped carbon nanotube array-loaded magnetic carbon aerogel exhibits excellent electromagnetic wave absorption performance and can be widely used in the field of military and aerospace electromagnetic wave absorption.

[0037] The present invention will be further illustrated below with reference to specific embodiments. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of the invention. Furthermore, it should be understood that after reading the teachings of this invention, those skilled in the art can make various alterations or modifications to the invention, and these equivalent forms also fall within the scope defined by the appended claims.

[0038] The following examples use instruments and equipment conventional in the art. Experimental methods in the following examples, unless otherwise specified, are generally performed under conventional conditions or as recommended by the manufacturer. All raw materials used in the following examples are conventional commercially available products with specifications conventional in the art. In this specification and the following examples, unless otherwise specified, "%" refers to weight percentage, "parts" refers to parts by weight, and "ratio" refers to weight proportion.

[0039] Example 1 A method for preparing nitrogen-doped carbon nanotube array-supported magnetic carbon aerogel includes the following steps: (1) Dissolve 0.1 g of cellulose in 50 mL of deionized water and stir to dissolve. Then add 2 mmol of cobalt nitrate hexahydrate and 2 mmol of nickel nitrate hexahydrate to the above solution and stir to dissolve to obtain a homogeneous biomass-metal salt solution.

[0040] (2) Add 0.5 mL of citric acid to the biomass-metal salt homogeneous solution and keep stirring to form a cross-linked composite suspension.

[0041] (3) The cross-linked composite suspension was transferred to a custom mold, and the mold was placed on a copper column in liquid nitrogen for directional freezing. The frozen sample was freeze-dried for 20 h to obtain a directional porous aerogel precursor.

[0042] (4) The obtained directional porous aerogel precursor was kept at 600 °C for 1 h under argon protection, with a heating rate of 5 °C / min, to obtain biomass-derived magnetic carbon aerogel.

[0043] (5) 5 g of urea and biomass-derived magnetic carbon aerogel were placed upstream and downstream of a tube furnace, respectively, and subjected to two-stage heat treatment in a nitrogen atmosphere. In the first stage of heat treatment, the temperature was 400 ℃, the heating rate was 2 ℃ / min, and the time was 1 h. In the second stage of heat treatment, the temperature was 600 ℃, the heating rate was 2 ℃ / min, and the time was 2 h, thus obtaining nitrogen-doped carbon nanotube array-supported magnetic carbon aerogel.

[0044] Example 2 A method for preparing nitrogen-doped carbon nanotube array-supported magnetic carbon aerogel includes the following steps: (1) Dissolve 1 g of glucose in 100 mL of deionized water and stir to dissolve. Then add 1 mmol of cobalt sulfate heptahydrate and 1 mmol of nickel sulfate heptahydrate to the above solution and stir to dissolve to obtain a biomass-metal salt homogeneous solution.

[0045] (2) Add 1 mL of citric acid to the above-obtained biomass-metal salt homogeneous solution and keep stirring to form a cross-linked composite suspension.

[0046] (3) The above cross-linked composite suspension was transferred into a custom mold, and the mold was placed on a copper column in liquid nitrogen for directional freezing. The frozen sample was freeze-dried for 60 h to obtain a directional porous aerogel precursor.

[0047] (4) The obtained directional porous aerogel precursor was kept at 1000 °C for 2 h under argon protection, with a heating rate of 10 °C / min, to obtain biomass-derived magnetic carbon aerogel.

[0048] (5) 7 g of cyanamide and biomass-derived magnetic carbon aerogel were placed upstream and downstream of a tube furnace, respectively, and subjected to two-stage heat treatment in an argon atmosphere. In the first stage of heat treatment, the temperature was 600 ℃, the heating rate was 10 ℃ / min, and the time was 2 h. In the second stage of heat treatment, the temperature was 1000 ℃, the heating rate was 10 ℃ / min, and the time was 2 h, to obtain nitrogen-doped carbon nanotube array-supported magnetic carbon aerogel.

[0049] Example 3 A method for preparing nitrogen-doped carbon nanotube array-supported magnetic carbon aerogel includes the following steps: (1) Dissolve 0.1 g of cellulose in 20 mL of deionized water and stir to dissolve. Then add 3 mmol of cobalt nitrate hexahydrate and 3 mmol of nickel nitrate hexahydrate to the above solution and stir to dissolve to obtain a homogeneous biomass-metal salt solution.

[0050] (2) Add 0.8 mL of citric acid to the biomass-metal salt homogeneous solution and keep stirring to form a cross-linked composite suspension.

[0051] (3) The above cross-linked composite suspension was transferred into a custom mold, and the mold was placed on a copper column in liquid nitrogen for directional freezing. The frozen sample was freeze-dried for 30 h to obtain a directional porous aerogel precursor.

[0052] (4) The obtained directional porous aerogel precursor was kept at 700 °C for 5 h under nitrogen protection, with a heating rate of 8 °C / min, to obtain biomass-derived magnetic carbon aerogel.

[0053] (5) 5 g of dicyandiamide and biomass-derived magnetic carbon aerogel were placed upstream and downstream of a tube furnace, respectively, and subjected to two-stage heat treatment in a nitrogen atmosphere. In the first stage of heat treatment, the temperature was 300 ℃, the heating rate was 5 ℃ / min, and the holding time was 3 h. In the second stage of heat treatment, the temperature was 800 ℃, the heating rate was 5 ℃ / min, and the holding time was 2 h, thus obtaining nitrogen-doped carbon nanotube array-supported magnetic carbon aerogel.

[0054] Example 4 A method for preparing nitrogen-doped carbon nanotube array-supported magnetic carbon aerogel includes the following steps: (1) Dissolve 0.3 g of chitosan in 20 mL of deionized water and stir to dissolve. Then add 1 mmol of cobalt chloride hexahydrate and 1 mmol of nickel chloride hexahydrate to the above solution and stir to dissolve to obtain a biomass-metal salt homogeneous solution.

[0055] (2) Add 0.3 mL of acetic acid to the above biomass-metal salt homogeneous solution and keep stirring to form a cross-linked composite suspension.

[0056] (3) The above cross-linked composite suspension was transferred into a custom mold, and the mold was placed on a copper column in liquid nitrogen for directional freezing. The frozen sample was freeze-dried for 50 h to obtain a directional porous aerogel precursor.

[0057] (4) The obtained directional porous aerogel precursor was kept at 800 °C for 2 h under argon protection, with a heating rate of 2 °C / min, to obtain biomass-derived magnetic carbon aerogel.

[0058] (5) 10 g of dicyandiamide and biomass-derived magnetic carbon aerogel were placed upstream and downstream of a tube furnace, respectively, and subjected to two-stage heat treatment in a nitrogen atmosphere. In the first stage of heat treatment, the temperature was 500 ℃, the heating rate was 5 ℃ / min, and the time was 3 h. In the second stage of heat treatment, the temperature was 800 ℃, the heating rate was 5 ℃ / min, and the holding time was 2 h, thus obtaining nitrogen-doped carbon nanotube array-supported magnetic carbon aerogel.

[0059] Example 5 A method for preparing nitrogen-doped carbon nanotube array-supported magnetic carbon aerogel includes the following steps: (1) Dissolve 0.2 g of chitosan in 30 mL of deionized water and stir to dissolve. Then add 2 mmol of cobalt acetate tetrahydrate and 2 mmol of nickel acetate tetrahydrate to the above solution and stir to dissolve to obtain a biomass-metal salt homogeneous solution.

[0060] (2) Add 1 mL of acetic acid to the above biomass-metal salt homogeneous solution and keep stirring to form a cross-linked composite suspension.

[0061] (3) The above cross-linked composite suspension was transferred into a custom mold, and the mold was placed on a copper column in liquid nitrogen for directional freezing. The frozen sample was freeze-dried for 300 h to obtain a directional porous aerogel precursor.

[0062] (4) The obtained directional porous aerogel precursor was kept at 700 °C for 1 h under argon protection, with a heating rate of 5 °C / min, to obtain biomass-derived magnetic carbon aerogel.

[0063] (5) 10 g of melamine and biomass-derived magnetic carbon aerogel were placed upstream and downstream of a tube furnace, respectively, and subjected to two-stage heat treatment in a nitrogen atmosphere. In the first stage of heat treatment, the temperature was 400 ℃, the heating rate was 3 ℃ / min, and the holding time was 4 h. In the second stage of heat treatment, the temperature was 900 ℃, the heating rate was 5 ℃ / min, and the holding time was 3 h, thus obtaining nitrogen-doped carbon nanotube array-supported magnetic carbon aerogel.

[0064] The nitrogen-doped carbon nanotube array-supported magnetic carbon aerogels prepared in Examples 1-5 all exhibit good electromagnetic wave absorption performance. The following description uses the nitrogen-doped carbon nanotube array-supported magnetic carbon aerogel prepared in Example 4 as an example.

[0065] Figure 1 The XRD patterns of the biomass-derived magnetic carbon aerogel prepared in step (4) and the nitrogen-doped carbon nanotube array-supported magnetic carbon aerogel prepared in step (5) of Example 4 of this invention are shown in the figure. As can be seen from the figure, the peaks of the biomass-derived magnetic carbon aerogel at 44.48°, 51.88°, and 76.24° belong to the (111), (200), and (220) crystal planes, corresponding to the face-centered cubic CoNi metal alloy. In particular, it is clearly observed that the broad peak at approximately 23° belongs to the amorphous state of the biomass-derived carbon. In contrast, the XRD pattern of the nitrogen-doped carbon nanotube array-supported magnetic carbon aerogel not only contains the diffraction peaks of CoNi, but also clearly shows a diffraction peak at 26.88° belonging to the graphitic carbon of the nitrogen-doped carbon nanotubes (JCPDs No. 41-1487). In summary, the successful preparation of the magnetic carbon aerogel supported on a carbon nanotube array is demonstrated.

[0066] Figure 2 The SEM image of the biomass-derived magnetic carbon aerogel prepared in Example 4 of this invention clearly shows that the introduction of the directional freezing method successfully prepared a magnetic carbon aerogel with a directional honeycomb structure. Figure 2 As shown in Figure a. And from... Figure 2 The higher magnification scan in Figure b reveals that the biomass-derived carbon material framework has uniformly loaded magnetic CoNi particles. This oriented honeycomb structure facilitates various reflections and scatterings of electromagnetic waves, and the biomass-derived carbon on it helps increase the material's conductivity, thereby enhancing conductivity loss.

[0067] Figure 3The image shows the EDS (Energy Dispersion Study) image of the biomass-derived magnetic carbon aerogel prepared in Example 4 of this invention. As can be seen from the image, energy dispersion analysis of C, N, Co, and Ni elements reveals that the magnetic carbon aerogel with a directional honeycomb structure contains nitrogen doping, which facilitates polarization loss and thus aids in the dissipation of electromagnetic waves. Furthermore, the EDS shows that the Co and Ni elements are located in the same positions, proving that the magnetic carbon aerogel with a directional honeycomb structure is loaded with CoNi alloy particles. The abundant interface between the CoNi alloy and the biomass-derived magnetic carbon material is conducive to interfacial polarization, further aiding in the dissipation of electromagnetic waves.

[0068] Figure 4 This is a SEM image of the nitrogen-doped carbon nanotube array-supported magnetic carbon aerogel prepared in Example 4 of this invention. Figure 4 It can be observed that the magnetic carbon aerogel loaded with carbon nanotube arrays, prepared through a two-stage heat treatment, did not disrupt the original oriented honeycomb structure of the magnetic carbon aerogel. Furthermore, the carbon nanotube arrays were successfully loaded, exhibiting uniform diameter and clear morphology. The successful loading of the carbon nanotube array is beneficial because the high conductivity of the carbon nanotubes enhances conductivity loss. Simultaneously, this unique structure further facilitates the reflection and scattering of electromagnetic waves, thereby improving electromagnetic wave absorption performance.

[0069] Figure 5 The image shown is an EDS image of the nitrogen-doped carbon nanotube array-supported magnetic carbon aerogel prepared in Example 4 of this invention. Elemental analysis of C, N, Co, and Ni in the magnetic carbon aerogel supporting the carbon nanotube array confirmed that the carbon nanotubes are nitrogen-doped carbon nanotubes. It also confirmed that the special structure of the nitrogen-doped carbon nanotube array was formed on the oriented honeycomb magnetic carbon aerogel through this two-step method, catalyzed by the magnetic CoNi alloy, by the nitrogen-rich organic matter.

[0070] Figure 6 This is a three-dimensional reflection loss diagram of the nitrogen-doped carbon nanotube array-supported magnetic carbon aerogel prepared in Example 4 of the present invention. Figure 7 Contour plot of three-dimensional reflection loss of the nitrogen-doped carbon nanotube array loaded with magnetic carbon aerogel prepared in Example 4 of the present invention. Figure 8 This is a two-dimensional reflection loss diagram of the nitrogen-doped carbon nanotube array-supported magnetic carbon aerogel prepared in Example 4 of this invention. By measuring the electromagnetic parameters of the magnetic carbon aerogel supported on the carbon nanotube array, the reflection loss was calculated based on transmission line theory. The magnetic carbon aerogel supported on the carbon nanotube array exhibits excellent electromagnetic wave absorption performance. Furthermore, according to... Figure 7 and Figure 8It can be seen that the magnetic carbon aerogel loaded with carbon nanotube arrays has the minimum reflection loss value of -57.81dB at 4.04mm and the maximum absorption bandwidth of 3.28GHz when the matching thickness is 1.32mm. It is evident that the magnetic carbon aerogel loaded with carbon nanotube arrays achieves excellent electromagnetic wave absorption effect through structural design.

[0071] Figure 9 The figure shows the Cole-Cole semicircle diagram of the nitrogen-doped carbon nanotube array-supported magnetic carbon aerogel prepared in Example 4 of this invention. The Cole-Cole semicircle diagram has approximately three semicircles, which can be assigned to the carbon material and magnetic particles. These semicircles represent the interfacial polarization caused by uneven distribution of dissimilar electrons between the carbon nanotubes and magnetic particles, and between the carbon nanotubes and the carbon framework. Additionally, the carbon material possesses certain defects, which act as polarization centers, generating dipole polarization. Interfacial polarization and dipole polarization constitute the dielectric loss mechanism of the magnetic carbon aerogel supported on the carbon nanotube array, helping to dissipate electromagnetic waves.

[0072] Figure 10 The image shows the biomass-derived magnetic carbon aerogel prepared in Example 4 of this invention being attracted by a magnet. It can be clearly seen that the magnetic carbon aerogel exhibits magnetism due to the presence of CoNi alloy, and can be easily attracted by a magnet.

[0073] Figure 11 The image shows the magnetic carbon aerogel loaded with nitrogen-doped carbon nanotube arrays prepared in Example 4 of this invention being adsorbed by a magnet. It can be clearly seen that the magnetic carbon aerogel loaded with carbon nanotube arrays exhibits magnetism due to the presence of CoNi alloy. Furthermore, the magnetism was not destroyed during the loading of carbon nanotubes, and it can still be easily adsorbed by a magnet.

[0074] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

[0075] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit the scope of protection of the present invention. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the essence and scope of the technical solutions of the present invention.

Claims

1. A method for preparing magnetic carbon aerogel supported on a nitrogen-doped carbon nanotube array, characterized in that, Includes the following steps: S1: Add cobalt salt and nickel salt to the aqueous solution of biomass precursor, stir to dissolve, and obtain a homogeneous solution of biomass-metal salt; S2: Add a cross-linking aid to the biomass-metal salt homogeneous solution to obtain a cross-linked composite suspension; S3: The cross-linked composite suspension is subjected to directional freezing and then freeze-dried to obtain a directional porous aerogel precursor; S4: The directional porous aerogel precursor is placed in a nitrogen or inert atmosphere for heat preservation treatment to obtain biomass-derived magnetic carbon aerogel. S5: The nitrogen-rich organic matter and the biomass-derived magnetic carbon aerogel are placed in a tube furnace and subjected to a two-stage heat treatment in a nitrogen or inert atmosphere to obtain the nitrogen-doped carbon nanotube array-supported magnetic carbon aerogel.

2. The method for preparing a nitrogen-doped carbon nanotube array-supported magnetic carbon aerogel according to claim 1, characterized in that, In step S1, the ratio of cobalt salt, nickel salt and biomass precursor is 1 mmol:1 mmol:(0.1~1) g.

3. The method for preparing a nitrogen-doped carbon nanotube array-supported magnetic carbon aerogel according to claim 1, characterized in that, In step S2, the ratio of the crosslinking aid to the biomass precursor is (0.1~1) mL:(0.1~1) g.

4. The method for preparing a nitrogen-doped carbon nanotube array-supported magnetic carbon aerogel according to claim 1, characterized in that, In step S3, the freeze-drying time is 10~60 h.

5. The method for preparing a nitrogen-doped carbon nanotube array-supported magnetic carbon aerogel according to claim 1, characterized in that, In step S4, during the heat preservation process, the heating rate is 2~10 ℃ / min, the heat preservation time is 1~6 h, preferably 1~5 h, and the heat preservation temperature is 600~1000 ℃.

6. The method for preparing a nitrogen-doped carbon nanotube array-supported magnetic carbon aerogel according to claim 1, characterized in that, In step S5, the mass ratio of the biomass-derived magnetic carbon aerogel to nitrogen-rich organic matter is 1:(10~40).

7. The method for preparing a nitrogen-doped carbon nanotube array-supported magnetic carbon aerogel according to claim 1, characterized in that, In step S5, the nitrogen-rich organic matter and the biomass-derived magnetic carbon aerogel are placed in a tube furnace and subjected to a two-stage heat treatment in a nitrogen or inert atmosphere. Specifically, the nitrogen-rich organic matter is placed upstream of the tube furnace, and the biomass-derived magnetic carbon aerogel is placed downstream of the tube furnace.

8. The method for preparing a nitrogen-doped carbon nanotube array-supported magnetic carbon aerogel according to claim 1, characterized in that, In step S5, the two-stage heat treatment process is as follows: in the first stage heat treatment process, the temperature is 300~600 ℃, the heating rate is 2~10 ℃ / min, and the time is 1~6 h, preferably 1~5 h; in the second stage heat treatment process, the temperature is 600~1000 ℃, the heating rate is 2~10 ℃ / min, and the time is 1~6 h, preferably 1~5 h.

9. A nitrogen-doped carbon nanotube array-supported magnetic carbon aerogel, characterized in that, It is prepared by the method described in any one of claims 1 to 8.

10. The application of the nitrogen-doped carbon nanotube array-supported magnetic carbon aerogel as described in claim 9 in the field of electromagnetic wave shielding.

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