Nitrogen-doped carbon aerogel as well as preparation method and application thereof

Abstract

The invention discloses nitrogen-doped carbon aerogel and a preparation method and application thereof.The method comprises the steps that cobalt salt and nickel salt are added into a biomass solution and stirred to be dissolved, and a mixed solution containing metal ions is obtained; adding a nitrogen-rich organic matter into the mixed solution containing the metal ions, and continuously stirring to prepare a mixed solution containing the metal ions and the nitrogen-rich organic matter; the method comprises the following steps: adding a cross-linking aid into a mixed solution containing metal ions and nitrogen-rich organic matters, and stirring to form gel; after directional freezing is conducted on the gel, freeze drying is conducted, and doped precursor aerogel is prepared; and placing the doped precursor aerogel in an inert atmosphere, and carrying out heat treatment to prepare the nitrogen-doped carbon aerogel. According to the method, a multi-stage magnetic structure is constructed through in-situ reduction growth of magnetic nanowires and nanoparticles on a carbon skeleton in an in-situ growth method, and efficient dissipation of electromagnetic waves is performed through the synergistic effect of multi-dimensional magnetic loss of the nanowires and the nanoparticles and conductive loss and polarization loss of carbon aerogel.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of electromagnetic wave absorbing materials, and relates to a nitrogen-doped carbon aerogel as well as a preparation method and application thereof. BACKGROUND

[0002] With the rapid development of 5G communication, high-frequency radar and high-power electronic devices, the demand for electromagnetic pollution control and military stealth is increasingly urgent. Under this background, carbon aerogel has become an ideal choice for electromagnetic wave absorbing materials due to its ultra-low density, high specific surface area and controllable dielectric properties. Its unique three-dimensional porous structure can prolong the propagation path of electromagnetic waves through multiple scattering and reflection, thereby enhancing dielectric polarization relaxation and conductive loss. However, single carbon aerogel has significant limitations. High electrical conductivity can easily cause impedance mismatch, and due to the lack of magnetic loss ability of single loss mechanism, it is difficult to meet the performance requirements of electromagnetic wave absorbing materials such as thinness, lightness, wide bandwidth and strong absorption.

[0003] To overcome these defects, existing research mainly adopts two strategies to construct dielectric-magnetic composite carbon aerogel. The first is physical blending, which disperses ferrite or metal magnetic powder in the carbon precursor to provide magnetic loss. However, this method can easily cause the agglomeration of magnetic components, thereby weakening the interfacial polarization effect and affecting the wave absorption performance. The second is in-situ growth, which usually deposits magnetic nanoparticles on the carbon skeleton through high-temperature carbon thermal reaction. However, when preparing carbon aerogel wave absorbing materials by in-situ growth method, the magnetic loss units are not uniformly distributed, which leads to insufficient synergy between structure and function, and it is difficult to fully exert the wave absorption potential of the material. SUMMARY

[0004] In view of the problems in the prior art, the application provides a nitrogen-doped carbon aerogel, a preparation method and application thereof, thereby solving the technical problems that the physical blending method used in the prior art to construct dielectric-magnetic composite carbon aerogel can easily cause the agglomeration of magnetic components and weaken the interfacial polarization effect, and the in-situ growth method has uneven distribution of magnetic loss units and insufficient structure-function synergy.

[0005] The application is realized by the following technical scheme: A preparation method of a nitrogen-doped carbon aerogel, comprising the following steps: S1: dissolving a biomass precursor in water and stirring until dissolution to obtain a biomass solution; adding a cobalt salt and a nickel salt to the biomass solution and stirring until dissolution to obtain a mixed solution containing metal ions; S2: adding a nitrogen-rich organic matter to the mixed solution containing metal ions and continuously stirring to obtain a mixed solution containing metal ions and nitrogen-rich organic matter; S3: adding a cross-linking aid to the mixed solution containing metal ions and nitrogen-rich organic matter, and stirring to form a gel; S4: after directional freezing of the gel, freeze-drying is performed to obtain a doped precursor aerogel; S5: the doped precursor aerogel is placed in an inert atmosphere and heat-treated to obtain the nitrogen-doped carbon aerogel.

[0006] Preferably, the biomass precursor is cellulose or chitosan, and the biomass precursor is used in a ratio of (0.1-1) g to (10-100) mL of water.

[0007] Preferably, the mass ratio of the biomass precursor to the cobalt salt and the nickel salt is (0.1-1):(0.1-0.5):(0.1-0.5).

[0008] Preferably, the mass ratio of the nitrogen-rich organic matter to the biomass precursor is (1-5):1.

[0009] Preferably, in step S2, the stirring speed is 500-1000 rpm / min, and the stirring time is 30-120 min.

[0010] Preferably, in step S3, after the crosslinking aid is added to the mixed solution containing metal ions and nitrogen-rich organic matter, the stirring speed is 100-500 rpm / min, and the stirring time is 5-40 min.

[0011] Preferably, in step S4, the freeze-drying time is 10-60 h.

[0012] Preferably, in step S5, during the heat treatment, the heating rate is 2-10 ℃ / min, the heat treatment temperature is 600-1000 ℃, and the time is 1-6 h.

[0013] A nitrogen-doped carbon aerogel is prepared by the above method; when the matching thickness is 4.29 mm, the minimum reflection loss value of the nitrogen-doped carbon aerogel is -34.67 dB, and the maximum absorption bandwidth is 4.08 GHz.

[0014] The above nitrogen-doped carbon aerogel is used in the field of electromagnetic wave absorption.

[0015] Compared with the prior art, the present application has the following beneficial technical effects: The present application firstly adopts in-situ growth to grow CoNi alloy nanowires and CoNi nanoparticles on the biomass-derived carbon skeleton. Unlike physical blending, in-situ growth enables the magnetic components to be combined with the carbon skeleton at the molecular or atomic level, avoiding the phenomenon of agglomeration of magnetic components due to interaction in the physical blending process, and ensuring uniform distribution of the magnetic components in the material. In addition, the magnetic CoNi nanoparticles are uniformly anchored on the surface of the carbon skeleton, forming a continuous heterogeneous interface with the carbon skeleton. This uniform interface structure is conducive to the accumulation and transmission of charges at the interface, enhancing the interface polarization effect and solving the problem of weakened interface polarization effect due to agglomeration of magnetic components in the physical blending method.

[0016] Secondly, the present application prepares carbon aerogel through a directional freezing process. The growth direction of ice crystals controls the structure formation of the material, enabling the in-situ grown CoNi alloy nanowires and CoNi nanoparticles to be more uniformly distributed on the carbon skeleton, avoiding the uneven distribution of magnetic loss units that may occur in the in-situ growth method. At the same time, the prepared nitrogen-doped carbon aerogel has a felt-like structure, with the "fleece layer" composed of nanowires and particles. The in-situ grown CoNi alloy nanowires penetrate the skeleton, and the in-situ grown magnetic CoNi nanoparticles are uniformly anchored on the surface of the dielectric carbon skeleton, forming a multi-level magnetic unit (nanowire + particle) and a continuous heterogeneous interface with the dielectric skeleton. This felt-like multi-level magnetic structure organically combines the magnetic loss units (nanowires and nanoparticles) with the dielectric skeleton, constructing anisotropic magnetic loss and generating more interface polarization. This multi-level structure achieves the synergistic optimization of impedance matching and loss capacity, solving the problem of insufficient structure-function synergy in the in-situ growth method, enabling the material to effectively adjust impedance matching and improve electromagnetic wave absorption performance while having uniform magnetic loss unit distribution; and through the directional freezing process, the directional porous structure is created by the growth direction of ice crystals, which is conducive to multiple reflections and scattering of electromagnetic waves after incidence, helping to dissipate electromagnetic waves; At the same time, the addition of nitrogen-rich organic matter forms N-doping of the aerogel during the heat treatment process. N elements can act as polarization centers for polarization loss, which helps to dissipate electromagnetic waves; In summary, the present application discloses a preparation method of nitrogen-doped carbon aerogel, which adopts a directional freezing and in-situ growth method to prepare a nitrogen-doped carbon aerogel wave-absorbing material with a felt-like multi-level magnetic structure. Through in-situ reduction and growth of magnetic nanowires and nanoparticles on the carbon skeleton in the in-situ growth method, a multi-level magnetic structure is constructed, and through the synergistic effect of multi-dimensional magnetic loss of nanowires and nanoparticles and conductive loss and polarization loss of carbon aerogel, electromagnetic waves are efficiently dissipated.

[0017] Further, the biomass precursor is cellulose or chitosan, and the ratio of the biomass precursor to water is (0.1-1) g:(10-100) mL, which can ensure the strength of the gel skeleton and the formation of stable open directional channels after directional freezing, thereby providing a physical basis for electromagnetic wave multiple reflection.

[0018] Further, the mass ratio of the biomass precursor to cobalt salt and nickel salt is (0.1-1):(0.1-0.5):(0.1-0.5), which can ensure sufficient Co 2+ / Ni 2+ in-situ reduction to CoNi alloy to form an effective magnetic loss unit, and the ratio can uniformly anchor the CoNi nanowires / particles on the carbon skeleton, realize atomic-level coupling between the magnetic and dielectric components, and enhance the interface polarization.

[0019] Further, the mass ratio of the nitrogen-rich organic matter to the biomass precursor is (1-5):1, which balances the nitrogen doping level and the integrity of the carbon skeleton and cooperates with the magnetic unit to improve the dielectric loss.

[0020] Further, in step S2, the stirring speed is 500-1000 rpm / min, and the stirring time is 30-120 min, which can ensure the sufficient dispersion and pre-crosslinking of the nitrogen-rich organic matter and guarantee the continuity of the nanowire / particle distribution in the "felt-like" structure to avoid the aggregation of the magnetic unit.

[0021] Further, in step S3, after the crosslinking aid is added to the mixed solution containing metal ions and nitrogen-rich organic matter, the stirring speed is 100-500 rpm / min, and the stirring time is 5-40 min, which controls the crosslinking reaction rate and the gel strength to ensure that the gel can withstand the directional freezing stress.

[0022] Further, in step S4, the freeze-drying time is 10-60 h, which can ensure the complete sublimation of ice crystals and maintain the structural stability and the multi-level pore structure to support the electromagnetic wave multiple scattering path.

[0023] Further, in step S5, during the heat treatment, the heating rate is 2-10 ℃ / min, the heat treatment temperature is 600-1000 ℃, and the time is 1-6 h, which cooperatively controls the carbonization, doping, and magnetic phase growth. At the heating rate, the ordered reduction of Co / Ni ions can be promoted to form nanowires with a controllable aspect ratio, while the excessive growth of metal particles is inhibited to maintain the nanoscale size. The heat treatment temperature can retain a high nitrogen content, enhance the dipole polarization, and improve the graphitization degree and the crystallinity of the CoNi alloy. In addition, the control of the heat treatment time can promote the Ostwald ripening of the magnetic particles and optimize the magnetic anisotropy. BRIEF DESCRIPTION OF DRAWINGS

[0024] In order to more clearly illustrate the technical solutions of the embodiments of the present application, the drawings needed to be used in the embodiments will be briefly introduced as follows. It should be understood that the following drawings only show some of the embodiments of the present application, and therefore should not be considered as limiting the scope. For those skilled in the art, other related drawings can also be obtained without creative labor on the basis of these drawings.

[0025] Figure 1 XRD pattern of the nitrogen-doped carbon aerogel prepared for the present application embodiment 2; Figure 2 SEM patterns of the nitrogen-doped carbon aerogel prepared for the present application embodiment 2 at different magnifications, wherein the scale of (a) is 50 μm and the scale of (b) is 10 μm; Figure 3 High magnification SEM and its corresponding EDS pattern of the nitrogen-doped carbon aerogel prepared for the present application embodiment 2; Figure 4 Three-dimensional and two-dimensional reflection loss patterns of the nitrogen-doped carbon aerogel prepared for the present application embodiment 2, wherein (a) is the three-dimensional reflection loss pattern, (b) is the contour plot of the three-dimensional reflection loss pattern, and (c) is the two-dimensional reflection loss pattern; Figure 5 Cole-Cole semicircle pattern of the nitrogen-doped carbon aerogel prepared for the present application embodiment 2. DETAILED DESCRIPTION

[0026] In order for those skilled in the art to understand the characteristics and effects of the present application, the following will be a general description and definition of the terms and phrases mentioned in the specification and claims. Unless otherwise specified, all technical and scientific words used herein have their usual meanings to those skilled in the art of the present application, and in case of conflict, the definition in the specification shall prevail.

[0027] Theories or mechanisms described and disclosed herein, whether correct or not, should not be considered limiting the scope of the present application, i.e., the present application can be practiced without any particular theory or mechanism.

[0028] In this paper, all the characteristics defined in the form of numerical range or percentage range, such as numerical value, quantity, content and concentration, are only for the sake of brevity and convenience. Therefore, the description of numerical range or percentage range should be considered to have covered and specifically disclosed all possible sub-ranges and individual numerical values within the range (including integers and fractions).

[0029] In the present specification, unless particularly stated, "comprising", "including", "containing", "having" or like terms means "consisting essentially of and "consisting essentially of", for example, "A includes a" means "A includes a and other", and "A includes only a".

[0030] In the present specification, all possible combinations between technical features in various embodiments or examples are not described in order to simplify the description. Therefore, as long as the combinations of technical features do not contradict each other, the technical features in various embodiments or examples can be combined arbitrarily, and all possible combinations should be considered as within the scope of the present specification.

[0031] The present application provides a preparation method of nitrogen-doped carbon aerogel, comprising the following steps: S1: dissolving biomass precursor and metal salt: dissolving biomass precursor in water, stirring to dissolve, to obtain a biomass solution; adding cobalt salt and nickel salt to the biomass solution, stirring to dissolve, to obtain a mixed solution containing metal ions; Preferably, the biomass precursor is cellulose or chitosan, and the use amount ratio of biomass precursor to water is (0.1-1) g:(10-100) mL; here, the water is preferably deionized water.

[0032] The cobalt salt is at least one of cobalt chloride hexahydrate, cobalt acetylacetonate and cobalt nitrate hexahydrate; The nickel salt is at least one of nickel chloride hexahydrate, nickel acetylacetonate and nickel nitrate hexahydrate; The mass ratio of the biomass precursor to the cobalt salt and the nickel salt is (0.1-1):(0.1-0.5):(0.1-0.5).

[0033] S2: dispersing nitrogen-rich organic matter: adding uniformly ground nitrogen-rich organic matter to the mixed solution containing metal ions obtained in step S1, stirring, to obtain a mixed solution containing metal ions and nitrogen-rich organic matter; Preferably, the nitrogen-rich organic matter is at least one of melamine, dicyandiamide and monocyandiamide; The stirring speed is 500-1000 rpm / min, and the stirring time is 30-120 min; The mass ratio of the nitrogen-rich organic matter to the biomass precursor is (1-5):1; S3: crosslinking aid assisted gelation: adding a crosslinking aid to the mixed solution containing metal ions and nitrogen-rich organic matter obtained in step S2, keeping stirring, to form a gel.

[0034] Preferably, the crosslinking aid is citric acid or acetic acid; The crosslinking aid is 100-700 µL; The stirring speed is 100-500 rpm / min, and the stirring time is 5-40 min. S4: directional freezing and freeze-drying: the gel obtained in step S3 is transferred to a customized mold and subjected to directional freezing in liquid nitrogen; the frozen sample is subjected to freeze-drying for 10-60 h to obtain a doped precursor aerogel; S5: inert atmosphere one-step pyrolysis (carbonization and nitrogen doping): the doped precursor aerogel obtained in step S4 is subjected to heat treatment under the protection of an inert atmosphere; the heat treatment simultaneously realizes carbonization and nitrogen doping to obtain the nitrogen-doped carbon aerogel; Preferably, the inert atmosphere is nitrogen or argon. During the heat treatment, the heating rate is 2-10 ℃ / min; the heat treatment temperature is 600-1000 ℃, and the holding time is 1-6 h.

[0035] The present application discloses a preparation method of a nitrogen-doped carbon aerogel. The method, on the one hand, realizes multi-level hybridization of dielectric-magnetic materials, specifically, a three-dimensional conductive network is formed by biomass-derived carbon to provide polarization loss and charge transmission path; meanwhile, CoNi alloy nanowires are grown in situ and penetrate the framework, and magnetic CoNi nanoparticles are grown in situ and anchored uniformly on the surface of the dielectric material carbon framework, which effectively adjusts the impedance matching of the carbon material, constructs anisotropic magnetic loss and can produce more interface polarization. On the other hand, the obtained nitrogen-doped carbon aerogel has a felt-like structure, which simulates the "fleece layer" (nanowire+particle) structure of the felt, effectively prolongs the electromagnetic wave scattering path; the multi-level magnetic unit (nanowire+particle) and the dielectric framework form a continuous heterogeneous interface, which adjusts the impedance matching and can also induce more interface polarization and natural resonance; the directional porous structure in the nitrogen-doped carbon aerogel realizes electromagnetic wave incidence and multiple reflection scattering. The design breaks through the limitation of the separation of magnetic-dielectric components in traditional composite aerogels, realizes the synergistic optimization of impedance matching and loss capacity through the felt-like multi-level structure, and provides a simple preparation method for a new generation of lightweight wideband wave-absorbing materials.

[0036] The present application designs an innovative structure-felt-like multi-level magnetic structure, and designs a nitrogen-doped carbon aerogel framework as a dielectric component, and constructs dielectric-magnetic loss synergy with the felt-like multi-level magnetic structure. The preparation process of the present application is simple, low in cost, has excellent electromagnetic wave absorption performance, and has certain practical application value.

[0037] The present application will be further described in conjunction with specific examples. It should be understood that these examples are only used to illustrate the present application and not used to limit the scope of the present application. Furthermore, it should be understood that those skilled in the art can make various modifications or changes to the present application after reading the content taught by the present application, and these equivalent forms also fall within the scope defined by the appended claims of the present application.

[0038] The following examples use apparatus and equipment that are conventional in the art. The experimental methods in the following examples, unless otherwise specified, are generally conducted under conventional conditions, or under conditions recommended by the manufacturer. The following examples use various raw materials, unless otherwise specified, which are conventional commercially available products, and which are of conventional specifications in the art. In the specification of the present application and in the following examples, unless otherwise specified, "%" means percent by weight, "parts" means parts by weight, and ratios are by weight.

[0039] Example 1 A method for preparing a nitrogen-doped carbon aerogel, comprising the following steps: Step (1): 0.5 g of cellulose was added to 10 mL of deionized water, and magnetically stirred (500 rpm) for 60 min until completely dissolved to obtain a transparent biomass solution.

[0040] Step (2): 0.1 g of cobalt acetylacetonate and 0.1 g of nickel acetylacetonate were added to the biomass solution of step (1), and stirring was continued for 20 min until the metal salts were completely dissolved to obtain a mixed solution containing metal ions.

[0041] Step (3): 0.1 g of powdered melamine was added to the mixed solution containing metal ions of step (2), and stirred vigorously at a speed of 1000 rpm for 30 min.

[0042] Step (4): 500 μL of citric acid was added dropwise to the solution of step 3 while stirring at a speed of 500 rpm for 5 min to form a stable gel.

[0043] Step (5): The gel was poured into a cylindrical mold, immersed in liquid nitrogen for directional freezing for 30 min, and then transferred to a freeze dryer for drying for 10 h to obtain a doped precursor aerogel.

[0044] Step (6): The doped precursor aerogel was placed in a tube furnace and heated to 600°C at a rate of 10°C / min under a nitrogen atmosphere, and held for 1 h. After natural cooling, a black nitrogen-doped carbon aerogel was obtained.

[0045] Example 2 A method for preparing a nitrogen-doped carbon aerogel, comprising the following steps: Step (1): 0.3 g of chitosan was added to 15 mL of deionized water, and magnetically stirred (500 rpm) for 60 min until completely dissolved to obtain a transparent biomass solution.

[0046] Step (2): 0.24 g of cobalt chloride hexahydrate and 0.24 g of nickel chloride hexahydrate were added to the biomass solution of step (1), and stirred for 40 min until the solution was dark pink to obtain a mixed solution containing metal ions.

[0047] Step (3): 0.6 g of ground dicyandiamide powder was added into the metal ion containing mixed solution of step (2) and stirred at 800 rpm for 90 min.

[0048] Step (4): 300 μL of acetic acid was slowly added into the solution of step 3 and stirred at 200 rpm for 30 min to form a stable gel.

[0049] Step (5): The gel was poured into a cylindrical mold, immersed in liquid nitrogen for directional freezing for 30 min, and freeze-dried for 60 h to obtain a doped precursor aerogel.

[0050] Step (6): The doped precursor aerogel was placed in a tube furnace and heated to 800 ℃ at 2 ℃ / min under an argon atmosphere, and kept for 2 h to prepare a nitrogen-doped carbon aerogel.

[0051] Example 3 A method for preparing a nitrogen-doped carbon aerogel, comprising the following steps: Step (1): 0.5 g of cellulose was added to 50 mL of deionized water and magnetically stirred (300 rpm) for 60 min until completely dissolved to obtain a transparent biomass solution.

[0052] Step (2): 0.3 g of cobalt nitrate hexahydrate and 0.3 g of nickel nitrate hexahydrate were added to the biomass solution of step (1) and stirred for 30 min to obtain a mixed solution containing metal ions.

[0053] Step (3): 1.5 g of ground monocyandiamide powder was added to the metal ion containing mixed solution of step (2) and stirred at 700 rpm for 60 min.

[0054] Step (4): 400 μL of citric acid was added dropwise to the solution of step 3 and stirred at 300 rpm for 20 min to complete gelation to form a stable gel.

[0055] Step (5): The gel was poured into a cylindrical mold, immersed in liquid nitrogen for directional freezing for 30 min, and freeze-dried for 60 h to obtain a doped precursor aerogel.

[0056] Step (6): The doped precursor aerogel was placed in a tube furnace and heated to 800 ℃ at 5 ℃ / min under nitrogen protection, and kept for 3 h to prepare a nitrogen-doped carbon aerogel.

[0057] Example 4 A method for preparing a nitrogen-doped carbon aerogel, comprising the following steps: Step (1): 0.2 g of chitosan was added to 30 mL of deionized water and magnetically stirred (300 rpm) for 60 min until completely dissolved to obtain a transparent biomass solution.

[0058] Step (2): To the biomass solution of step (1), 0.2 g nickel acetylacetonate and 0.1 g cobalt nitrate hexahydrate were added and stirred for 25 min to obtain a mixed solution containing metal ions.

[0059] Step (3): To the mixed solution containing metal ions of step (2), 0.6 g of ground melamine powder was added and stirred at 700 rpm for 40 min.

[0060] Step (4): 200 μL of acetic acid was added dropwise to the solution of step (3) and stirred at 400 rpm for 10 min to form a stable gel.

[0061] Step (5): The gel was poured into a cylindrical mold, immersed in liquid nitrogen for directional freezing for 30 min, and freeze-dried for 12 h to obtain a doped precursor aerogel.

[0062] Step (6): The doped precursor aerogel was placed in a tube furnace and heated to 700 °C at 8 °C / min in argon, and held for 2 h to obtain a nitrogen-doped carbon aerogel.

[0063] Example 5 A method for preparing a nitrogen-doped carbon aerogel, comprising the following steps: Step (1): 0.15 g of cellulose was added to 15 mL of deionized water and magnetically stirred (300 rpm) for 60 min until completely dissolved to obtain a transparent biomass solution.

[0064] Step (2): To the biomass solution of step (1), 0.12 g of cobalt chloride hexahydrate and 0.08 g of nickel acetylacetonate were added and stirred for 30 min to obtain a mixed solution containing metal ions.

[0065] Step (3): To the mixed solution containing metal ions of step (2), 0.15 g of dicyandiamide powder was added and stirred at 900 rpm for 90 min.

[0066] Step (4): 500 μL of citric acid was added dropwise to the solution of step (3) and stirred at 200 rpm for 30 min to form a stable gel.

[0067] Step (5): The gel was poured into a cylindrical mold, immersed in liquid nitrogen for directional freezing for 30 min, and freeze-dried for 48 h to obtain a doped precursor aerogel.

[0068] Step (6): The doped precursor aerogel was placed in a tube furnace and heated to 900 °C at 3 °C / min in nitrogen, and held for 4 h to obtain a nitrogen-doped carbon aerogel.

[0069] Example 6 A method for preparing a nitrogen-doped carbon aerogel, comprising the following steps: S1: dissolving biomass precursor and metal salt: dissolving a biomass precursor in water, the biomass precursor being cellulose, stirring until dissolved to obtain a biomass solution; adding cobalt chloride hexahydrate and nickel chloride hexahydrate to the biomass solution, stirring until dissolved to obtain a mixed solution containing metal ions; the use amount ratio of the biomass precursor to water is 0.1 g:10 mL; the mass ratio of the biomass precursor to cobalt salt and nickel salt is 0.1:0.1:0.1.

[0070] S2: dispersing nitrogen-rich organic matter: adding uniformly ground nitrogen-rich organic matter, which is melamine, to the mixed solution containing metal ions obtained in step S1, the mass ratio of the nitrogen-rich organic matter to the biomass precursor being 1:1, stirring at a stirring speed of 500 rpm / min for 120 min to obtain a mixed solution containing metal ions and nitrogen-rich organic matter; S3: crosslinking aid gelation: adding 100 μL of crosslinking aid citric acid to the mixed solution containing metal ions and nitrogen-rich organic matter obtained in step S2, maintaining stirring at a stirring speed of 100 rpm / min for 40 min to form a gel.

[0071] S4: directional freezing and freeze-drying: transferring the gel obtained in step S3 to a customized mold, and placing it in liquid nitrogen for directional freezing; freeze-drying the frozen sample for 10 h to obtain a doped precursor aerogel; S5: inert atmosphere one-step pyrolysis: performing heat treatment on the doped precursor aerogel obtained in step S4 under nitrogen protection, the heat treatment being performed at a heating rate of 2 ℃ / min, the heat treatment temperature being 600 ℃, and the holding time being 6 h, wherein carbonization and nitrogen doping are simultaneously achieved by the heat treatment to obtain the nitrogen-doped carbon aerogel.

[0072] Example 7 A method for preparing a nitrogen-doped carbon aerogel, comprising the following steps: S1: dissolving biomass precursor and metal salt: dissolving a biomass precursor in water, the biomass precursor being chitosan, stirring until dissolved to obtain a biomass solution; adding cobalt acetylacetonate and nickel acetylacetonate to the biomass solution, stirring until dissolved to obtain a mixed solution containing metal ions; the use amount ratio of the biomass precursor to water is 1 g:100 mL; the mass ratio of the biomass precursor to cobalt salt and nickel salt is 1:0.5:0.5.

[0073] S2: dispersing nitrogen-rich organic matter: adding the ground and uniformly milled nitrogen-rich organic matter, namely dicyandiamide, to the mixed solution containing metal ions obtained in step S1, the mass ratio of the nitrogen-rich organic matter to the biomass precursor being 5:1, stirring at a stirring speed of 1000 rpm / min for 30 min, to obtain a mixed solution containing metal ions and nitrogen-rich organic matter; S3: cross-linking aid assisted gelation: adding 700 µL of the cross-linking aid acetic acid to the mixed solution containing metal ions and nitrogen-rich organic matter obtained in step S2, keeping stirring at a stirring speed of 500 rpm / min for 5 min, to form a gel.

[0074] S4: directional freezing and freeze-drying: transferring the gel obtained in step S3 to a customized mold, and placing it in liquid nitrogen for directional freezing; freeze-drying the frozen sample for 60 h, to obtain a doped precursor aerogel; S5: one-step pyrolysis in an inert atmosphere: performing heat treatment on the doped precursor aerogel obtained in step S4 under the protection of argon, the heat treatment being performed at a temperature increasing rate of 10 ℃ / min, the heat treatment temperature being 1000 ℃, and the holding time being 1 h, the heat treatment synchronously realizing carbonization and nitrogen doping, to obtain the nitrogen-doped carbon aerogel.

[0075] Example 8 A method for preparing a nitrogen-doped carbon aerogel, comprising the following steps: S1: dissolving a biomass precursor and metal salt: dissolving a biomass precursor, cellulose, in water, and stirring until dissolution, to obtain a biomass solution; adding cobalt nitrate hexahydrate and nickel salt, nickel nitrate hexahydrate, to the biomass solution, and stirring until dissolution, to obtain a mixed solution containing metal ions; the use amount ratio of the biomass precursor to water being 0.5 g:60 mL; the mass ratio of the biomass precursor to the cobalt salt and the nickel salt being 0.5:0.25:0.25.

[0076] S2: dispersing nitrogen-rich organic matter: adding the ground and uniformly milled nitrogen-rich organic matter, namely dicyandiamide, to the mixed solution containing metal ions obtained in step S1, the mass ratio of the nitrogen-rich organic matter to the biomass precursor being 5:1, stirring at a stirring speed of 1000 rpm / min for 30 min, to obtain a mixed solution containing metal ions and nitrogen-rich organic matter; S3: cross-linking aid assisted gelation: adding 700 µL of the cross-linking aid acetic acid to the mixed solution containing metal ions and nitrogen-rich organic matter obtained in step S2, keeping stirring at a stirring speed of 500 rpm / min for 5 min, to form a gel.

[0077] S4: directional freezing and freeze-drying: transferring the gel obtained in step S3 into a customized mold, and placing it in liquid nitrogen for directional freezing; freeze-drying the frozen sample for 40 h to obtain a doped precursor aerogel; S5: inert atmosphere one-step pyrolysis: under the protection of nitrogen, the doped precursor aerogel obtained in step S4 is subjected to heat treatment, wherein the heating rate is 8℃ / min; the heat treatment temperature is 800℃, and the holding time is 3 h; the heat treatment simultaneously realizes carbonization and nitrogen doping, and the nitrogen-doped carbon aerogel is obtained.

[0078] The nitrogen-doped carbon aerogel with the felt-like multi-level magnetic structure prepared in the application has good electromagnetic wave absorption performance, and the following is described by taking the nitrogen-doped carbon aerogel with the felt-like multi-level magnetic structure prepared in Example 2 as an example.

[0079] Figure 1 The XRD pattern of the nitrogen-doped carbon aerogel prepared in Example 2 of the application is shown in the figure. As can be seen from the figure, the peak values at 44.41°, 51.72° and 76.15° belong to (111), (200) and (220) crystal faces, respectively, corresponding to metallic cobalt (JCPDS No. 15-0806) and nickel (JCPDS No. 04-0850). The diffraction peak at about 25° belongs to the diffraction peak of biomass-derived carbon. It is proved that the nitrogen-doped carbon aerogel with the felt-like multi-level magnetic structure is composed of biomass-derived carbon and CoNi alloy.

[0080] Figure 2 The SEM images of the nitrogen-doped carbon aerogel prepared in Example 2 of the application at different magnifications are shown in the figure, wherein the scale of (a) is 50 μm, and the scale of (b) is 10 μm. As can be seen from the figure, the nitrogen-doped carbon aerogel prepared in the application has a felt-like multi-level magnetic structure and a directional porous structure, which is caused by the directional structure of the carbon aerogel prepared by the directional freezing process. At the same time, it can be seen from the b figure of Figure 2 , that a felt-like structure is formed on the biomass-derived carbon skeleton. This directional porous felt-like structure is beneficial to the multiple reflection and scattering of electromagnetic waves after entering the material, and is beneficial to the dissipation of electromagnetic waves.

[0081] Figure 3High magnification SEM of nitrogen-doped carbon aerogel prepared in Example 2 of the present application and its corresponding EDS map. As can be seen from the figure, the nanowires and nanoparticles are hybridized with each other and exist at the same time. Energy spectrum analysis of C, N, Co and Ni elements can find that the nitrogen-doped carbon aerogel with the felt-like multi-level magnetic structure is doped with N element due to the existence of biomass precursor, which helps to occur polarization loss as a polarization center and thus helps to dissipate electromagnetic waves. And from the EDS, it can be found that the Co element and the Ni element are located at the positions of the nanowires and the nanoparticles, and the positions are consistent, proving that the CoNi nanowires and nanoparticles are loaded in the nitrogen-doped carbon aerogel, and also proving the successful preparation of the felt-like multi-level magnetic structure.

[0082] Figure 4 Three-dimensional and two-dimensional reflection loss maps of the nitrogen-doped carbon aerogel prepared in Example 2 of the present application, (a) is a three-dimensional reflection loss map, (b) is a contour map of the three-dimensional reflection loss map, and (c) is a two-dimensional reflection loss map. By measuring the electromagnetic parameters of the nitrogen-doped carbon aerogel with the felt-like multi-level magnetic structure, the reflection loss is calculated according to the transmission line theory, as shown in Figure 4 The nitrogen-doped carbon aerogel prepared in the present application has excellent electromagnetic wave absorption performance. And the nitrogen-doped carbon aerogel has a minimum reflection loss value of -34.67 dB at 4.29 mm, and has a maximum absorption bandwidth of 4.08 GHz when the matching thickness is 4.79 mm. It can be seen that the nitrogen-doped carbon aerogel with the felt-like multi-level magnetic structure has obtained excellent electromagnetic wave absorption effect through the design of the felt-like multi-level magnetic structure.

[0083] Figure 5 Cole-Cole semicircle map of the nitrogen-doped carbon aerogel prepared in Example 2 of the present application. As can be seen from the figure, the Cole-Cole semicircle map has about three semicircles, which can be assigned to the interface polarization caused by uneven distribution of heterogeneous electrons between the nitrogen-doped carbon aerogel skeleton and the multi-level magnetic structure (nanowires and nanoparticles), and the dipole polarization caused by the nitrogen doping and defects of the nitrogen-doped carbon material as a polarization center. At the same time, the conductive loss provided by the three-dimensional conductive network provided by the nitrogen-doped carbon aerogel skeleton. These loss mechanisms together constitute the dielectric loss mechanism of the nitrogen-doped carbon aerogel with the felt-like multi-level magnetic structure, thereby efficiently dissipating electromagnetic waves.

[0084] Finally, it should be noted that the above examples are only used to illustrate the technical solutions of the present application and not to limit the protection scope of the present application. Although the present application has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solutions of the present application can be modified or replaced by equivalents without departing from the essence and scope of the technical solutions of the present application.

Claims

1. A method for preparing nitrogen-doped carbon aerogel, characterized in that: The following steps are involved: S1: dissolving the biomass precursor in water and stirring until dissolved to obtain a biomass solution; adding cobalt salt and nickel salt to the biomass solution, stirring until dissolved, to obtain a mixed solution containing metal ions; S2: adding nitrogen-rich organic matter to the mixed solution containing metal ions, and continuously stirring to obtain a mixed solution containing metal ions and nitrogen-rich organic matter; S3: adding a crosslinking agent to the mixed solution containing metal ions and nitrogen-rich organic matter, stirring to form a gel; S4: Directionally freezing the gel, and then freeze-drying it to obtain a doped precursor aerogel; S5: placing the doped precursor aerogel in an inert atmosphere and performing heat treatment to obtain the nitrogen-doped carbon aerogel.

2. The method for preparing nitrogen-doped carbon aerogel according to claim 1, wherein: The biomass precursor is cellulose or chitosan, and the usage ratio of the biomass precursor to water is (0.1-1) g: (10-100) mL.

3. The method for preparing nitrogen-doped carbon aerogel according to claim 1, wherein: The mass ratio of the biomass precursor to the cobalt salt and the nickel salt is (0.1-1):(0.1-0.5):(0.1-0.5).

4. The method for preparing nitrogen-doped carbon aerogel according to claim 1, wherein: The mass ratio of the nitrogen-rich organic matter to the biomass precursor is (1-5):

1.

5. The method for preparing nitrogen-doped carbon aerogel according to claim 1, wherein: In step S2, the stirring speed is continuously 500-1000 rpm / min, and the stirring time is 30-120 min.

6. The method for preparing nitrogen-doped carbon aerogel according to claim 1, wherein: In step S3, after adding the crosslinking auxiliary agent to the mixed solution containing metal ions and nitrogen-rich organic matter, the mixture is stirred at a stirring speed of 100 to 500 rpm / min and a stirring time of 5 to 40 minutes.

7. The method for preparing nitrogen-doped carbon aerogel according to claim 1, wherein: In step S4, the freeze-drying time is 10 to 60 hours.

8. The method for preparing nitrogen-doped carbon aerogel according to claim 1, wherein: In step S5, during the heat treatment, the heating rate is 2-10°C / min, the heat treatment temperature is 600-1000°C, and the time is 1-6 hours.

9. A nitrogen-doped carbon aerogel, characterized in that: It is prepared by the method according to any one of claims 1 to 8; when the matching thickness is 4.29 mm, the minimum reflection loss value of the nitrogen-doped carbon aerogel is -34.67 dB, and the maximum absorption bandwidth is 4.08 GHz.

10. Use of the nitrogen-doped carbon aerogel according to claim 9 in the field of electromagnetic wave absorption.

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