Nitrogen-doped carbon / cobalt-based wave absorber, preparation method and application thereof

Abstract

The present application relates to wave-absorbing material technical field, specifically to a kind of nitrogen-doped carbon / cobalt-based wave-absorbing agent and its preparation method and application.The specific preparation method includes the following steps: with nitrogen-doped carbon nanosheet as matrix, 2-methyl imidazole as organic ligand, under solvent system, with Co source is co-precipitated, make the ZIF-67 formed on nitrogen-doped carbon nanosheet, then under protective atmosphere, at 700 DEG C~900 DEG C pyrolysis, obtain nitrogen-doped carbon / cobalt-based wave-absorbing agent.The carbon nanosheet stable structure of preparation supported cubic ZIF-67, so that the Co / NCS composite material after pyrolysis exists heterostructure interface, strengthens multiple loss mechanism.The wave-absorbing agent can be used to prepare electromagnetic wave absorbing material, under lower filling amount, higher reflection loss and wider effective absorption bandwidth are realized.The existing wave-absorbing material exists the problem of narrow effective absorption bandwidth and weak electromagnetic wave loss capacity.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of wave-absorbing materials, and particularly relates to a nitrogen-doped carbon / cobalt-based wave-absorbing agent and a preparation method and application thereof. BACKGROUND

[0002] In the current era of rapid development of information technology, the deployment of 5G networks, the application of unmanned driving technology and the popularity of smart homes have greatly improved people's quality of life, but the development of information technology has brought great convenience to people's lives, while the use of a large number of electronic products has caused an explosive increase in electromagnetic radiation in space, which has gradually become a problem that cannot be ignored.

[0003] At present, there are mainly two effective solutions to the problem of electromagnetic wave protection, namely developing new wireless communication technology and preparing electromagnetic shielding / absorbing materials. According to the existing electromagnetic theory, it is still extremely difficult to achieve point-to-point directional propagation of electromagnetic waves, while the preparation of electromagnetic shielding / absorbing materials has made great progress. Electromagnetic wave absorbing materials can effectively convert the energy of incident electromagnetic waves into other forms of energy, achieving effective absorption of electromagnetic waves. The research goal of such materials is to reduce the adverse effects of electromagnetic radiation on human health and electronic devices, while enhancing the anti-interference performance of electronic devices. The mechanism of wave-absorbing materials mainly includes electric conduction loss, dielectric loss and magnetic loss, which are very effective in absorbing electromagnetic waves and can consume electromagnetic wave energy inside. Electromagnetic wave-absorbing materials have been widely used in electromagnetic noise suppression in complex circuits, stable transmission of wireless communication base stations or satellite communication terminal signals, electromagnetic interference protection of high-end electronic instruments, and even in key fields such as anti-radar detection systems for military confrontation. With the increasingly complex application environment of electromagnetic absorbing materials, various performances of wave-absorbing materials also face higher requirements, and how to achieve higher reflection loss and wider effective absorption bandwidth at a lower thickness has become a research hotspot in the field of electromagnetic wave absorbing materials.

[0004] Carbon materials have low density, large dielectric loss, high conductivity, good processing performance, corrosion resistance and other advantages, and have been widely concerned and applied in the field of electromagnetic wave absorbing materials. In terms of morphology, carbon nanomaterials, such as one-dimensional carbon nanotubes, two-dimensional graphene and three-dimensional porous carbon nanomaterials, have good dielectric properties and high magnetic loss tangent due to large surface area, rich dangling bonds and macroscopic quantum tunneling effect, and show great potential in electromagnetic protection. However, most carbon nanomaterials have excessive surface electron mobility and large dielectric constant, resulting in impedance mismatch between carbon nanomaterials and free space, and thus poor absorption performance of electromagnetic waves. Therefore, in the field of electromagnetic wave absorbing materials, the introduction of heteroatoms such as N, S, B, etc. is mainly used to destroy the electron symmetry of nanocarbon and form a large number of structural defects, thereby improving its dielectric loss. However, its high dielectric constant leads to characteristic impedance mismatch, hindering its wide application.

[0005] In recent years, it has become a research hotspot to develop wave-absorbing materials by pyrolyzing MOF materials as precursors. ZIF-67 is a widely used example of MOF materials, which uses cobalt as a metal node and 2-methylimidazole as an organic ligand. It inherits the general characteristics of MOF materials, such as designable pore structure and high specific surface area, and also exhibits unique properties similar to zeolites, such as chemical stability and thermal stability. However, ZIF-67 has its own defects, such as poor magnetic and electrical properties and poor wave-absorbing performance.

[0006] For example, prior art 1 is to use Co 2+ , Ni 2+ as metal source and trimer acid as ligand to develop CoNi-MOF, and obtain CoNi@C microsphere composite material with adjustable porous and hollow morphology by heat treatment of CoNi-MOF. The minimum reflection loss of the sample can reach -44.8 dB at 10.7 GHz. The effective wave-absorbing bandwidth of this method is relatively narrow, and the minimum reflection loss is low, resulting in poor microwave absorption performance.

[0007] Prior art 2 is to pyrolyze Co-based MOF precursor ZIF-67 in an Ar atmosphere to obtain Co / C porous composite material, which effectively improves the dielectric loss and magnetic loss capacity, and the reflection loss reaches -35.3 dB when the thickness is 4 mm. The matching thickness of this method is large, and the reflection loss is low, resulting in poor microwave absorption performance.

[0008] Prior Art 1: Liu Z, Duan Y, Deng B, et al. Synthesis of ultralight N-rich porous graphene nanosheets derived from fluid catalytic cracking slurry and their electromagnetic wave absorption properties [J]. Industrial & Engineering Chemistry Research, 2020, 59(17): 8243-8251.

[0009] Prior Art 2: Lü Y, Wang Y, Li H, et al. MOF-Derived Porous Co / C Nanocomposites with Excellent Electromagnetic Wave Absorption Properties [J]. ACS Applied Materials & Interfaces, 2015, 7(24): 13604-13611. SUMMARY

[0010] In order to solve the problem of narrow effective absorption bandwidth and weak electromagnetic wave loss capacity existing in the prior art wave-absorbing material, the purpose of the present application is to provide a nitrogen-doped carbon / cobalt-based wave-absorbing agent and a preparation method and application thereof.

[0011] In order to achieve the above-mentioned purpose, the technical scheme of the present application is as follows.

[0012] The first aspect of the present application provides a preparation method of a nitrogen-doped carbon / cobalt-based wave-absorbing agent, comprising the following steps:

[0013] The nitrogen-doped carbon nanosheet is used as a base body, 2-methyl imidazole is used as an organic ligand, and a coprecipitation reaction is carried out with a Co source in a solvent system, so that the formed ZIF-67 is loaded on the nitrogen-doped carbon nanosheet, and then pyrolysis is carried out at 700 DEG C to 900 DEG C under a protective atmosphere, to obtain the nitrogen-doped carbon / cobalt-based wave-absorbing agent.

[0014] The application mainly loads ZIF-67 on nitrogen-doped carbon nanosheets by heating co-precipitation method, prepares carbon nanosheet stable structure loaded with cubic ZIF-67, makes the Co / NCS composite material after pyrolysis exist heterojunction, and strengthens multiple loss mechanisms. In the case of full synergy of magnetic loss ability and dielectric loss ability, the wave absorbing material realizes higher reflection loss and wider effective absorption bandwidth at a lower filling amount. The method can not only provide a new design idea for the research of light wideband wave absorbing material, but also provide necessary innovative technical support and theoretical support for the development of long-distance detection technology, and has important academic significance and practical value.

[0015] Preferably, the pyrolysis temperature is preferably 800 DEG C; the pyrolysis time is 3h-5h. Through subsequent tests, it is shown that the pyrolysis temperature is 700 DEG C-900 DEG C, wherein the pyrolysis at 800 DEG C can obtain the electromagnetic wave absorption performance of a wider effective wave absorbing bandwidth at a thinner thickness of 2.83mm, reaching 7.52GHz. And the lowest reflection loss RL of-58.34dB is reached at the matching thickness of 2.63mm, far exceeding most reported composite wave absorbing materials, confirming the advantages of the composite material prepared at the temperature and the electromagnetic synergy effect in electromagnetic wave attenuation. min

[0016] Preferably, the Co source is Co(NO3)2·6H2O or CoCl2·6H2O; the mass ratio of the nitrogen-doped carbon nanosheet, 2-methylimidazole and the Co source is 50:500-600:300-360.

[0017] Preferably, the nitrogen-doped carbon nanosheet is prepared by the following method:

[0018] Inorganic salt is used as a template agent, the carbon source, the nitrogen source, the inorganic salt and water are mixed to prepare a solid mixture; the solid mixture is calcined under a protective atmosphere, and then etched in hydrochloric acid, and washed to remove the inorganic salt, thereby obtaining the nitrogen-doped carbon nanosheet.

[0019] The application prepares the porous nitrogen-doped carbon nanosheet by mixing the inorganic salt as a template agent with the carbon source and the nitrogen source to prepare a solid intermediate, and then calcining, etching and washing.

[0020] Further preferably, the carbon source is citric acid, the nitrogen source is urea, and the inorganic salt is potassium carbonate; the molar ratio of the carbon source, the nitrogen source and the inorganic salt is 0.01-0.015:0.02-0.021:0.05-0.07.

[0021] ​Further preferably, the calcination temperature is 600-800 DEG C; the calcination time is 1-3 hours.

[0022] Further preferably, the concentration of hydrochloric acid used in the etching is 0.8-1 M.

[0023] Preferably, the protective atmosphere is an argon atmosphere.

[0024] Preferably, the temperature of the co-precipitation reaction is 65-70 DEG C; the time of the co-precipitation reaction is 0.5-2 hours. At this temperature, it is helpful to load ZIF-67 on the porous carbon nanosheet to obtain a precursor by a heating co-precipitation method.

[0025] Preferably, the solvent is methanol; the ratio of the amount of use of the nitrogen-doped carbon nanosheet to methanol is 50 mg: 90-100 mL.

[0026] The second aspect of the application provides a nitrogen-doped carbon / cobalt-based wave-absorbing agent prepared by the preparation method of the first aspect.

[0027] The third aspect of the application provides an application of the nitrogen-doped carbon / cobalt-based wave-absorbing agent of the second aspect in the preparation of an electromagnetic wave absorbing material.

[0028] Preferably, the specific application method is: mixing the nitrogen-doped carbon / cobalt-based wave-absorbing agent with a polymer matrix to prepare an electromagnetic wave absorbing coating material; wherein the polymer matrix is polyurethane or epoxy resin.

[0029] Preferably, the mass percentage content of the nitrogen-doped carbon / cobalt-based wave-absorbing agent in the electromagnetic wave absorbing material is 5-20%.

[0030] The beneficial effects of the application are:

[0031] 1、The application mainly uses nitrogen-doped carbon nanosheets as a matrix, loads ZIF-67 by a heating co-precipitation method, and prepares carbon nanosheet stable structures loaded with cubic ZIF-67, so that the Co / NCS composite material after pyrolysis has a heterojunction interface, strengthens a multiple loss mechanism, and in the case of full synergy of magnetic loss ability and dielectric loss ability, realizes high reflection loss and wide effective absorption bandwidth at a low filling amount, and solves the problems of narrow effective absorption bandwidth and weak electromagnetic wave loss ability of existing wave-absorbing materials.

[0032] 2, The minimum reflection loss of the nitrogen-doped carbon / cobalt-based wave absorber prepared by the method reaches-58.34 dB, the effective wave absorption bandwidth can reach 7.52 GHz at a matching thickness of 2.83 mm, and 90% of electromagnetic waves in the frequency range of 10-18 GHz can be absorbed. It shows that the electromagnetic wave absorption performance of the nitrogen-doped carbon / cobalt-based wave absorber prepared by the method has an effective wave absorption bandwidth far exceeding most of the reported composite wave absorbing materials at a thinner thickness, further confirming the advantages of the nitrogen-doped carbon / cobalt-based wave absorber and electromagnetic synergistic effect in electromagnetic wave attenuation. BRIEF DESCRIPTION OF DRAWINGS

[0033] Figure 1 SEM and TEM images of samples Co / NCS-700, Co / NCS-800 and Co / NCS-900 prepared in Examples 1-3. Wherein (a1) is the SEM image of Co / NCS-700; (b1) is the SEM image of Co / NCS-800; (c1) is the SEM image of Co / NCS-900; (a2) is the TEM image of Co / NCS-700; (b2) is the TEM image of Co / NCS-800; (c2) is the TEM image of Co / NCS-900.

[0034] Figure 2 EDS images of the sample Co / NCS-800 prepared in Example 2. (d1)-(d4) are EDS images of Co / NCS-800.

[0035] Figure 3 HETEM image (e) and SAED image (f) of the sample Co / NCS-800 prepared in Example 2.

[0036] Figure 4 Raman spectrum (a) and XRD spectrum (b) of samples Co / NCS-700, Co / NCS-800 and Co / NCS-900 prepared in Examples 1-3.

[0037] Figure 5 Adsorption-desorption curve (a) and pore size distribution graph (b) of samples Co / NCS-700, Co / NCS-800 and Co / NCS-900 prepared in Examples 1-3.

[0038] Figure 6 Magnetic hysteresis loop graph (a) at room temperature and local amplified magnetic hysteresis loop graph (b) at room temperature of samples Co / NCS-700, Co / NCS-800 and Co / NCS-900 prepared in Examples 1-3.

[0039] Figure 7The electromagnetic parameter diagrams of the samples Co / NCS-700, Co / NCS-800 and Co / NCS-900 prepared in Example 1-Example 3. Among them, (a) is the real part of the dielectric constant; (b) is the imaginary part of the dielectric constant; (c) is the dielectric loss tangent; (d) is the real part of the magnetic permeability; (e) is the imaginary part of the magnetic permeability; (f) is the magnetic loss tangent.

[0040] Figure 8 The reflection loss diagrams of the samples Co / NCS-700, Co / NCS-800 and Co / NCS-900 prepared in Example 1-Example 3. Among them, (a) is the reflection loss diagram of Co / NCS-700; (b) is the reflection loss diagram of Co / NCS-800; (c) is the reflection loss diagram of Co / NCS-900. DETAILED DESCRIPTION

[0041] In order to make the objects, technical solutions and advantages of the present application clearer, the following will further describe the present application in combination with embodiments. It should be understood that the specific embodiments described herein are only used to explain the present application and not used to limit the present application.

[0042] Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without making creative efforts belong to the scope of protection of the present application.

[0043] In the field of electromagnetic wave absorbing materials, the introduction of heteroatoms such as N, S, B, etc. will destroy the electron symmetry of nano-carbon, resulting in the formation of a large number of electric dipoles, which attenuate electromagnetic energy through dipole polarization loss. At the same time, a large number of structural defects in doped carbon can serve as polarization centers for dipoles, thereby improving its dielectric loss.

[0044] Compared with other heteroatoms, the size of N atom is close to that of C atom, avoiding obvious lattice mismatch. Due to the more negative electronegativity of carbon atom, nitrogen-doped carbon atom can exhibit different electronic conductivity, which depends on the content and type of nitrogen in carbon atom. So far, the reports of various nitrogen-doped carbon materials have accounted for more than 90%, and the related research on nitrogen-doped carbon-based high-efficiency electron beam absorbers is also constantly innovating. Researchers have adjusted the structure of nitrogen-doped carbon nanomaterials through different synthesis methods, such as chemical vapor deposition CVD technology, high-temperature carbonization, and in-situ nitrogen doping, to optimize their microwave absorption performance. Among them, nitrogen-doped porous carbon nanosheets have shown some excellent performance in the wave absorption field, but the characteristic impedance mismatch caused by their high dielectric constant hinders their wide application.

[0045] Due to the low permeability and strong magnetization of magnetic materials, the relative complex permeability can be improved, resulting in strong magnetic loss, improving impedance matching, and therefore, carbon materials combined with magnetic metals, alloys and oxides have become the development direction of carbon materials.

[0046] In recent years, it has become a research hotspot to develop wave-absorbing materials by pyrolyzing MOF materials using MOF materials as precursors. ZIF-67 is a widely used example of MOF materials, which uses cobalt as a metal node and 2-methylimidazole as an organic ligand. It inherits the general properties of MOF materials, such as designable pore structure and high specific surface area, while also exhibiting unique properties similar to zeolites, such as chemical stability and thermal stability. In ZIF-67 material, Co metal exists in the form of ions, while carbon elements exist in the organic framework. Therefore, ZIF-67 itself has poor magnetic and conductive properties, and the wave-absorbing efficiency is not obvious.

[0047] Currently, by pyrolyzing ZIF-67 material at high temperature, metal ions can be converted into magnetic metal Co atoms to enhance magnetic loss, and the organic framework can be reduced to carbon or nitrogen-doped carbon to provide dielectric loss, thus obtaining Co / C composite materials with magnetic and electric synergies. The composite material combines the advantages of metal Co and carbon materials, and its structure and composition are easy to control, becoming a research hotspot in the field of wave-absorbing materials.

[0048] In the prior art, various ZIF-67 composite materials synthesized by ingenious methods show their broad prospects and effective competitiveness as lightweight microwave absorbing materials. The high specific surface area and ordered pore structure of ZIF-67 help to improve its performance in electromagnetic wave absorption, because these characteristics can increase the interaction between electromagnetic waves and materials, thereby improving the absorption efficiency. In addition, the cobalt metal center in ZIF-67 endows it with certain magnetism, which makes ZIF-67 contribute to electromagnetic wave absorption in terms of magnetic loss, especially in absorbing low-frequency electromagnetic waves. By combining ZIF-67 with other wave-absorbing materials, the wave-absorbing performance and stability of the material can be effectively improved, providing new ideas and methods for the design and preparation of electromagnetic wave absorbing materials. However, the existing technology generally has the problems of relatively narrow effective wave-absorbing bandwidth, large matching thickness, and low minimum reflection loss, resulting in poor microwave absorption performance.

[0049] For example: Prior art 1: Liu et al. synthesized Co 2+ , Ni 2+A CoNi-MOF was prepared with trimer acid as a ligand for a metal source, and CoNi@C microsphere composites with a porous and hollow morphology were obtained by heat treatment of the CoNi-MOF, and the minimum reflection loss of the sample at 10.7 GHz can reach-44.8 dB. Prior art 2: Lü et al. pyrolyzed Co-based MOF precursor ZIF-67 in an Ar atmosphere to obtain Co / C porous composites, which effectively improved the dielectric loss and magnetic loss capacity, and the reflection loss reached-35.3 dB when the thickness was 4 mm.

[0050] Therefore, in view of the problems of the existing wave-absorbing materials, such as narrow effective absorption bandwidth and weak electromagnetic wave loss capacity, the present application prepares a transition metal Co / NCS composite material obtained by loading ZIF-67 after high-temperature pyrolysis on a nitrogen-doped carbon nanosheet matrix.

[0051] The present application combines nitrogen-doped carbon nanosheets and ZIF-67 by reasonable component and structure design and temperature control, and prepares a structure-function integrated wave-absorbing material with light weight, wide frequency and strong absorption after high-temperature pyrolysis, which is likely to become a major breakthrough in the field of wide-band electromagnetic wave absorption. A series of transition metal / carbon composite materials with nitrogen atom doping and heterogeneous nanostructure are prepared by loading ZIF-67 on a nitrogen-doped carbon nanosheet matrix through a heating co-precipitation method and pyrolysis at different temperatures, which provides a model system and rich experience for in-depth understanding of the component and microstructure control of transition metal-doped hierarchical structure carbon composite materials.

[0052] The construction and frequency band widening mechanism of the nitrogen-doped carbon / cobalt-based wave-absorbing agent of the present application will be analyzed through specific examples below. The methods described in the following examples are conventional methods unless otherwise specified; and the reagents and materials can be purchased on the market unless otherwise specified.

[0053] In the following examples, NCS is a nitrogen-doped carbon nanosheet. NCS-600, NCS-700 and NCS-800 are a series of nitrogen-doped carbon nanosheets prepared at different pyrolysis temperatures; Co / NCS-700, Co / NCS-800 and Co / NCS-900 are a series of nitrogen-doped carbon / cobalt-based wave-absorbing materials prepared at different pyrolysis temperatures, namely Co / NCS composite materials.

[0054] Example 1

[0055] A preparation method of a nitrogen-doped carbon / cobalt-based wave-absorbing agent, comprising the following steps:

[0056] (1) Preparation of nitrogen-doped carbon nanosheets:

[0057] The resulting white solid was transferred to a tube furnace and heated to 700°C at a ramping rate of 5°C / min under Ar atmosphere and kept for 2h, followed by natural cooling to room temperature. -1

[0058] After the end of the heat preservation, the obtained black powder was etched with 1M HC1 and washed with deionized water several times to remove K2CO3 in the product. Finally, the obtained solution was vacuum filtered to obtain a solid, which was dried at 80°C overnight to obtain nitrogen-doped carbon nanosheets, denoted as NCS-700.

[0059] (2) Preparation of Co / NCS composite:

[0060] First, 50mg NCS-700 and 600mg 2-methylimidazole were dispersed in 50mL methanol in a 250mL round-bottom flask and ultrasonically treated, denoted as solution A. Then, 360mg Co(N03)2-6H20 was dispersed in 50mL methanol and ultrasonically treated, denoted as solution B. Solution B was added dropwise to solution A, and the color of the mixture changed from gray to purple. The mixed solution was then stirred at 70°C for 1h. The purple precipitate NCS / ZIF-67 was collected and dried at 60°C. Finally, the obtained purple powder was heated at 700°C under Ar protection at a ramping rate of 5°C / min for 4h to obtain a black powder product, which was a Co / NCS composite, denoted as Co / NCS-700.

[0061] Example 2

[0062] A method for preparing a nitrogen-doped carbon / cobalt-based wave-absorbing agent, comprising the following steps:

[0063] (1) Preparation of nitrogen-doped carbon nanosheets:

[0064] 0.012 mol of monohydrated citric acid, 0.0208 mol of urea and 0.06 mol of potassium carbonate were dissolved in 70mL deionized water, and after stirring for 30min, the obtained solution was freeze-dried at -80°C for 24h. The obtained white solid was transferred to a tube furnace and heated to 700°C at a ramping rate of 5°C / min under Ar atmosphere and kept for 2h, followed by natural cooling to room temperature. -1 ​The temperature was raised to 700 °C at a rate of 5 °C / min and kept for 2 h, and then naturally cooled to room temperature. After the end of the incubation, the black powder obtained was etched with 1 M HC1 and washed several times with deionized water to remove K2CO3 from the product. Finally, the solid obtained after vacuum filtration of the solution was dried at 80 °C overnight to obtain nitrogen-doped carbon nanosheets, denoted as NCS-700.

[0065] (2) Preparation of Co / NCS composite:

[0066] First, 50 mg of NCS-700 and 600 mg of 2-methylimidazole were dispersed in 50 mL of methanol in a 250 mL round-bottom flask and sonicated, denoted as solution A. Then, 360 mg of Co(N03)2-6H20 was dispersed in 50 mL of methanol and sonicated, denoted as solution B. Solution B was added dropwise to solution A, and the color of the mixture changed from gray to purple. The mixed solution was then stirred at a constant temperature of 70 °C for 1 h. The purple precipitate NCS / ZIF-67 was collected and dried at 60 °C. Finally, the purple powder obtained was heated at a rate of 5 °C / min under Ar protection at 800 °C for 4 h to obtain a black powder product, which is a Co / NCS composite, denoted as Co / NCS-800.

[0067] Example 3

[0068] A method for preparing a nitrogen-doped carbon / cobalt-based wave-absorbing agent, comprising the following steps:

[0069] (1) Preparation of nitrogen-doped carbon nanosheets:

[0070] 0.012 mol of monohydrated citric acid, 0.0208 mol of urea, and 0.06 mol of potassium carbonate were dissolved in 70 mL of deionized water, and after stirring for 30 min, the obtained solution was freeze-dried at -80 °C for 24 h. The white solid obtained after drying was transferred to a tube furnace and heated at a rate of 5 °C / min under Ar atmosphere to 700 °C and kept for 2 h, and then naturally cooled to room temperature. After the end of the incubation, the black powder obtained was etched with 1 M HC1 and washed several times with deionized water to remove K2CO3 from the product. Finally, the solid obtained after vacuum filtration of the solution was dried at 80 °C overnight to obtain nitrogen-doped carbon nanosheets, denoted as NCS-700. -1

[0071] (2) Preparation of Co / NCS composite:

[0072] ​First, in a 250 mL round-bottom flask, 50 mg of NCS-700 and 600 mg of 2-methylimidazole were dispersed in 50 mL of methanol and sonicated, labeled as solution A. Then, 360 mg of Co(NO3)2·6H2O was dispersed in 50 mL of methanol and sonicated, labeled as solution B. Solution B was added dropwise to solution A, and the mixture changed color from gray to purple. The mixture was then stirred at 70 °C for 1 hour. The purple precipitate NCS / ZIF-67 was collected and dried at 60 °C. Finally, the obtained purple powder was heated at 900 °C for 4 hours under Ar protection at a heating rate of 5 °C / min to obtain a black powder product, which is the Co / NCS composite material, denoted as Co / NCS-900.

[0073] Example 4

[0074] A method for preparing a nitrogen-doped carbon / cobalt-based microwave absorber includes the following steps:

[0075] (1) Preparation of nitrogen-doped carbon nanosheets:

[0076] 0.01 mol citric acid monohydrate, 0.02 mol urea, and 0.05 mol potassium carbonate were dissolved in 70 mL of deionized water. After stirring for 30 min, the resulting solution was freeze-dried at -80 °C for 20 h. The resulting white solid was transferred to a tube furnace and dried at 5 °C / min under an Ar atmosphere. -1 The temperature was increased to 600℃ and held for 1 hour, then allowed to cool naturally to room temperature. After the holding period, the resulting black powder was etched with 1M HCl and washed several times with deionized water to remove K2CO3 from the product. Finally, the resulting solution was vacuum filtered to obtain a solid, which was then dried overnight at 80℃ to obtain nitrogen-doped carbon nanosheets, designated NCS-600.

[0077] (2) Preparation of Co / NCS composite materials:

[0078] First, 50 mg NCS-600 and 500 mg 2-methylimidazole were dispersed in 45 mL methanol and ultrasonically treated in a 250 mL round-bottom flask, labeled as solution A. Then, 300 mg Co(N03)2-6H20 was dispersed in 45 mL methanol and ultrasonically treated, labeled as solution B. Solution B was added dropwise to solution A, and the color of the mixture changed from gray to purple. The mixed solution was then stirred at a constant temperature of 65 °C for 0.5 h. The purple precipitate NCS / ZIF-67 was collected and dried at 60 °C. Finally, the obtained purple powder was heated at a rate of 5 °C / min to 800 °C under Ar protection for 3 h, and a black powder product, i.e., Co / NCS composite material, was obtained. SEM and TEM images showed that Co particles were obviously loaded on the porous carbon nanosheets, and a nitrogen-doped carbon / cobalt-based wave-absorbing material was prepared.

[0079] Example 5

[0080] A preparation method of a nitrogen-doped carbon / cobalt-based wave-absorbing agent, comprising the following steps:

[0081] (1) Preparation of nitrogen-doped carbon nanosheets:

[0082] 0.015 mol of monohydrated citric acid, 0.021 mol of urea, and 0.07 mol of potassium carbonate were dissolved in 70 mL of deionized water, and after stirring for 30 min, the obtained solution was freeze-dried at -80 °C for 24 h. The obtained white solid was transferred to a tube furnace, heated to 800 °C at a rate of 5 °C / min under Ar atmosphere and kept for 3 h, and then naturally cooled to room temperature. After the incubation, the obtained black powder was etched with 1M HC1 and washed with deionized water several times to remove K2C03 in the product. Finally, the obtained solution was vacuum filtered to obtain a solid, which was dried at 80 °C overnight to obtain nitrogen-doped carbon nanosheets, labeled as NCS-800. -1

[0083] (2) Preparation of Co / NCS composite material:

[0084] ​First, 50 mg NCS-800 and 550 mg 2-methylimidazole were dispersed in 50 mL methanol and ultrasonically treated in a 250 mL round-bottom flask, labeled as solution A. Then, 330 mg Co(NO3)2·6H2O was dispersed in 45 mL methanol and ultrasonically treated, labeled as solution B. Solution B was added dropwise to solution A, and the color of the mixture changed from gray to purple. The mixed solution was then stirred at 70°C for 2 h. The purple precipitate NCS / ZIF-67 was collected and dried at 60°C. Finally, the obtained purple powder was heated at a rate of 5°C / min to 800°C for 5 h under Ar protection, and a black powder product, i.e., Co / NCS composite material, was obtained. SEM and TEM images showed that Co particles were obviously loaded on the porous carbon nanosheets, and a nitrogen-doped carbon / cobalt-based wave-absorbing material was prepared.

[0085] To explore the performance differences of Co / NCS composites prepared by pyrolysis of NCS / ZIF-67 at different temperatures, a series of Co / NCS composites were prepared by pyrolysis at 700°C, 800°C, and 900°C in Example 1-3, respectively, and the samples prepared at different pyrolysis temperatures were named Co / NCS-700, Co / NCS-800, and Co / NCS-900, respectively. The Co / NCS composites prepared in Examples 1-3 were analyzed by X-ray diffraction, vibrating sample magnetometer VSM, electromagnetic wave absorption performance, and other tests and characterizations.

[0086] Test 1: SEM analysis and TEM analysis.

[0087] The samples Co / NCS-700 prepared in Example 1, Co / NCS-800 prepared in Example 2, and Co / NCS-900 prepared in Example 3 were subjected to SEM analysis and TEM analysis, respectively, and the results are shown in Figure 1 SEM analysis is scanning electron microscope analysis; TEM analysis is transmission electron microscope analysis. SEM image is a scanning electron microscope image; TEM image is a transmission electron microscope image.

[0088] Figure 1 SEM images and TEM images of the samples Co / NCS-700, Co / NCS-800, and Co / NCS-900 prepared in Examples 1-3 are shown in FIGS. 1-3. In FIGS. 1-3, (a1) is the SEM image of Co / NCS-700; (b1) is the SEM image of Co / NCS-800; (c1) is the SEM image of Co / NCS-900; (a2) is the TEM image of Co / NCS-700; (b2) is the TEM image of Co / NCS-800; and (c2) is the TEM image of Co / NCS-900.

[0089] From the SEM images and TEM images of the samples Co / NCS-700, Co / NCS-800, and Co / NCS-900 prepared in Examples 1-3, it can be seen that the Co particles are obviously loaded on the porous carbon nanosheets, and the nitrogen-doped carbon / cobalt-based wave-absorbing material is prepared.Figure 1 The results show that Co particles are clearly loaded on porous carbon nanosheets, and the size of the magnetic particles decreases and the surface roughness increases with increasing pyrolysis temperature.

[0090] Test 2: EDS analysis, HETEM analysis, and SAED analysis.

[0091] The Co / NCS-800 sample prepared in Example 2 was subjected to EDS, HETEM, and SAED analyses, respectively. The results are shown in the figure. Figure 2 and Figure 3 EDS analysis is energy dispersive spectroscopy; HETEM analysis is high-resolution transmission electron microscopy; SAED analysis is selected area electron diffraction. EDS plots are energy dispersive spectroscopy; HETEM plots are high-resolution transmission electron microscopy images; SAED plots are selected area electron diffraction patterns.

[0092] Figure 2 The image shows the EDS diagram of the Co / NCS-800 sample prepared in Example 2. (d1) to (d4) are the EDS diagrams of Co / NCS-800. Figure 3 HETEM (e) and SAED (f) images of the sample Co / NCS-800 prepared for Example 2.

[0093] Depend on Figure 2 and Figure 3 The results show that the selected area electron diffraction pattern reveals diffraction rings of the Co(111) and C(002) crystal planes, consistent with the information in its high-resolution transmission electron microscopy pattern, indicating that C, Co, and N elements are uniformly distributed in the material.

[0094] Test 3: Raman spectroscopy and XRD analysis.

[0095] Raman spectroscopy and XRD analysis were performed on samples Co / NCS-700, Co / NCS-800, and Co / NCS-900 prepared in Examples 1 to 3, respectively. The results are shown in the figure. Figure 4 XRD analysis is X-ray diffraction analysis. The XRD pattern is an X-ray diffraction pattern.

[0096] Figure 4 Raman spectra (a) and XRD patterns (b) of samples Co / NCS-700, Co / NCS-800 and Co / NCS-900 prepared for Examples 1 to 3.

[0097] Depend on Figure 4 The results show that as the pyrolysis temperature increases, the ID / IG value of the sample increases, indicating that there are more defects in the material. The characteristic peak of Co is enhanced, indicating that the crystallinity of Co particles increases with the increase of pyrolysis temperature.

[0098] Test 4: Adsorption-desorption curve and pore size analysis.

[0099] The adsorption-desorption curve and pore size analysis of the samples Co / NCS-700, Co / NCS-800 and Co / NCS-900 prepared in Example 1-Example 3 were carried out, and the results are shown in Figure 5 .

[0100] Figure 5 The adsorption-desorption curve (a) and pore size distribution (b) of the samples Co / NCS-700, Co / NCS-800 and Co / NCS-900 prepared in Example 1-Example 3.

[0101] From Figure 5 The results show that the sample Co / NCS-800 has the largest specific surface area, and its pore size is concentrated in 2nm-4nm.

[0102] Test 5: Hysteresis loop analysis.

[0103] The hysteresis loop analysis of the samples Co / NCS-700, Co / NCS-800 and Co / NCS-900 prepared in Example 1-Example 3 was carried out, and the results are shown in Figure 6 .

[0104] Figure 6 The hysteresis loop diagram (a) at room temperature and the local enlarged hysteresis loop diagram (b) at room temperature of the samples Co / NCS-700, Co / NCS-800 and Co / NCS-900 prepared in Example 1-Example 3.

[0105] From Figure 6 The results show that the sample Co / NCS-800 has a lower coercivity, which is beneficial to the storage of magnetism and the attenuation of electromagnetic waves.

[0106] Test 6: Electromagnetic parameter analysis.

[0107] The electromagnetic parameter analysis of the samples Co / NCS-700, Co / NCS-800 and Co / NCS-900 prepared in Example 1-Example 3 was carried out, and the results are shown in Figure 7 .

[0108] Figure 7 The electromagnetic parameter diagram of the samples Co / NCS-700, Co / NCS-800 and Co / NCS-900 prepared in Example 1-Example 3. Among them, (a) is the real part of dielectric constant; (b) is the imaginary part of dielectric constant; (c) is the tangent of dielectric loss angle; (d) is the real part of magnetic permeability; (e) is the imaginary part of magnetic permeability; (f) is the tangent of magnetic loss angle.

[0109] From Figure 7The results show that the sample Co / NCS-800 has better dielectric loss performance and the best magnetic loss performance.

[0110] Test 7: Analysis of wave absorption performance.

[0111] The samples Co / NCS-700, Co / NCS-800 and Co / NCS-900 prepared in Example 1 to Example 3 are subjected to wave absorption performance analysis, the electromagnetic parameters of each sample are input in MATLAB, and the thickness is fitted between 1-5mm and the frequency range is between 2-18GHz. The reflectivity diagram of the three samples can be obtained by calculation, and the results are shown in Figure 8 .

[0112] Figure 8 The reflectance loss diagrams of the samples Co / NCS-700, Co / NCS-800 and Co / NCS-900 prepared in Example 1 to Example 3 are shown in Figure 6. Wherein, (a) is the reflectance loss diagram of Co / NCS-700; (b) is the reflectance loss diagram of Co / NCS-800; (c) is the reflectance loss diagram of Co / NCS-900.

[0113] From the reflectance loss diagrams of the samples Co / NCS-700, Co / NCS-800 and Co / NCS-900 prepared in Example 1 to Example 3, it can be seen that the sample Co / NCS-800 has the lowest reflectance loss. Figure 8 The results show that the sample Co / NCS-800 of Example 2 has the lowest reflectance loss RL min of-58.34dB at the matching thickness of 2.63mm, and the effective wave absorption bandwidth reaches 7.52GHz at the matching thickness of 2.83mm, which indicates that the sample Co / NCS-800 can absorb 90% of the electromagnetic waves in the frequency range of 10-18GHz. By comparison with the reported composite wave absorption materials, the electromagnetic wave absorption performance of the sample Co / NCS-800 of Example 2 has an effective wave absorption bandwidth at a relatively thin thickness of 2.83mm, which far exceeds most of the reported composite wave absorption materials, further proving the advantages of the composite material and electromagnetic synergistic effect of the embodiment of the present application in electromagnetic wave attenuation.

[0114] In summary, the above-mentioned embodiments of the present application prepare a magnetic-electric composite material by loading ZIF-67 on nitrogen-doped carbon nanosheets through high-temperature pyrolysis, so that the wave absorption material can realize wide-band strong absorption in a thin layer and low filling amount state.

[0115] The above-mentioned embodiments of the present application use Co 2+Co / NCS composite material prepared by nitrogen-doped carbon nanosheet supported ZIF-67 pyrolysis was prepared for electromagnetic wave absorption application. The multiple heterojunction formed by nitrogen-doped carbon nanosheet and Co magnetic particles further strengthens the interface polarization, defect polarization, dipole polarization mechanism, realizes multiple loss, and the Co magnetic particles existing in the Co / NCS composite material obtained by high temperature pyrolysis of nitrogen-doped carbon nanosheet supported ZIF-67 can induce magnetic loss, and synergize with the dielectric loss of the carbon matrix to promote the loss of electromagnetic waves. These are only preliminary results, and more systematic research is needed. Using nitrogen-doped carbon nanosheet and structure diverse MOFs to prepare magnetic transition metal / nitrogen-doped carbon composite material is expected to obtain high-efficiency wave-absorbing material.

[0116] The above only is the preferred embodiment of the present application, and does not limit the present application, any modification, equivalent replacement and improvement made within the spirit and principle of the present application should be included in the protection scope of the present application.

Claims

1. A method for preparing a nitrogen-doped carbon / cobalt-based wave-absorber, characterized in that, Comprise the following steps: The nitrogen-doped carbon nanosheet is used as a matrix, 2-methyl imidazole is used as an organic ligand, and a Co source is used for a co-precipitation reaction in a solvent system to form ZIF-67 loaded on the nitrogen-doped carbon nanosheet, which is then pyrolyzed at 700-900 DEG C under a protective atmosphere to form a multiple heterojunction of the nitrogen-doped carbon nanosheet and Co magnetic particles to obtain a nitrogen-doped carbon / cobalt-based wave absorber.

2. The method for preparing the nitrogen-doped carbon / cobalt-based microwave absorbing agent according to claim 1, characterized in that, The pyrolysis temperature is 800 DEG C, and the pyrolysis time is 3-5 h.

3. The method for preparing the nitrogen-doped carbon / cobalt-based microwave absorbing agent according to claim 1, characterized in that, The Co source is Co(NO3)2·6H2O or CoCl2·6H2O. The mass ratio of the nitrogen-doped carbon nanosheet, 2-methyl imidazole and the Co source is 50:500-600:300-360.

4. The method for preparing the nitrogen-doped carbon / cobalt-based microwave absorber according to claim 1, characterized in that, The nitrogen-doped carbon nanosheet is prepared by the following method: An inorganic salt is used as a template agent, a carbon source, a nitrogen source, the inorganic salt and water are mixed to prepare a solid mixture, the solid mixture is calcined under a protective atmosphere, then etched in hydrochloric acid, and the inorganic salt is removed by washing to prepare the nitrogen-doped carbon nanosheet.

5. The method for preparing the nitrogen-doped carbon / cobalt-based microwave absorber according to claim 4, characterized in that, The carbon source is citric acid, the nitrogen source is urea, and the inorganic salt is potassium carbonate. The molar ratio of the carbon source, the nitrogen source and the inorganic salt is 0.01-0.015:0.02-0.021:0.05-0.

07.

6. The method for preparing the nitrogen-doped carbon / cobalt-based microwave absorbing agent according to claim 4, characterized in that, The calcination temperature is 600-800 DEG C, and the calcination time is 1-3 h.

7. The method for preparing the nitrogen-doped carbon / cobalt-based microwave absorbing agent according to claim 1, characterized in that, The co-precipitation reaction temperature is 65-70 DEG C, and the co-precipitation reaction time is 0.5-2 h.

8. The method for preparing the nitrogen-doped carbon / cobalt-based microwave absorber according to claim 1, characterized in that, The solvent is methanol, and the dosage ratio of the nitrogen-doped carbon nanosheet and methanol is 50 mg:90-100 mL. 9.A nitrogen-doped carbon / cobalt-based wave absorber prepared by the preparation method in any one of claims 1-8. 10.Use of the nitrogen-doped carbon / cobalt-based wave absorber in claim 9 in the preparation of an electromagnetic wave absorbing material.

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