Nano-porous carbon / cobalt / cobalt oxide composite material, preparation method and application thereof

Abstract

The present invention belongs to the technical field of microwave absorbing materials, and particularly relates to a nano-porous carbon / cobalt / cobalt oxide composite material, a preparation method thereof and an application thereof, which include the following steps: S1. Dispersing nano-porous carbon powder, hexamethylenetetramine, trisodium citrate and soluble cobalt salt in water to form a dispersion liquid, and then carrying out a solvothermal reaction at 85-95 °C. After washing and drying, an NCP / Co precursor is obtained; S2. Pyrolyzing the NCP@Co precursor prepared in S1 at 700-900 °C to obtain a nano-porous carbon / cobalt / cobalt oxide composite material; The present invention utilizes the dielectric loss of the nano-porous carbon matrix and the magnetic loss of cobalt / cobalt oxide nanoparticles, and combines multiple loss mechanisms. The nano-porous carbon / cobalt / cobalt oxide nano-composite microwave absorbing material exhibits excellent performance in terms of absorption bandwidth and absorption intensity.

Description

Technical Field

[0001] The present invention belongs to the technical field of microwave absorbing materials, and particularly relates to a nano-porous carbon / cobalt / cobalt oxide composite material, a preparation method thereof and an application thereof. Background Art

[0002] Stealth technology is an important means to avoid detection and is crucial for enhancing the penetration ability of high-speed aircraft. To achieve the "stealth" effect, designing the aerodynamic shape of the aircraft to reduce the radar cross-section and using plasma shock waves or microwave absorbing materials on the skin and other parts are currently the main feasible methods, which can efficiently convert and dissipate the incident electromagnetic wave energy or interfere and cancel each other, thereby significantly reducing the intensity of the electromagnetic wave echo. Therefore, more and more scientific research work has tended to the field of microwave absorption. Using the structure / composition strategy to improve the stealth ability of microwave absorbing materials is an important frontier research direction in the field of aircraft stealth.

[0003] Magnetoelectric composite loss is one of the main loss mechanisms of electromagnetic waves in materials. It combines many loss characteristics of magnetic loss and electrical loss, such as resistive loss, hysteresis loss and damping loss. For the research on optimizing the microwave absorption performance of magnetoelectric loss-type microwave absorbing materials, it mainly focuses on the regulation of the complex permittivity and complex permeability of the materials. Among them, to improve the absorption performance of radar waves in the low-frequency band, the material needs to have high magnetic loss and good frequency response characteristics of the permittivity. Magnetic nanomaterials have high magnetic saturation and low coercivity, so they have excellent complex permeability, can achieve high magnetic loss, and can be used as potential microwave absorbers. In the past few decades, a large number of magnetic-based nanomaterials for microwave absorption have emerged, such as magnetic metals (Fe, Co, Ni), magnetic alloys (CoNi, FeCo, NiFe, NiCu), magnetic oxides (Fe 3 O 4 , Fe 2 O 3 , Co 3 O 4 , NiO, CoFe 2 O 4 ) and so on. However, when using such materials, the matrix filling amount is relatively high, and a large amount of binder matrix needs to be blended to form a continuous network structure inside the material to play a microwave absorption role.

[0004] Therefore, how to make the microwave absorbing material have excellent reflection loss and absorption bandwidth while reducing the filler loading percentage, and achieve the characteristics of thin, light, wide and strong is a problem to be solved by the present invention. Summary of the Invention

[0005] To solve the above technical problems, the present invention provides a nano-porous carbon / cobalt / cobalt oxide composite material, a preparation method thereof, and an application thereof. By combining nanotechnology with magnetic micropowders, a nano-magnetic composite material is prepared, enabling the wave-absorbing material to achieve strong broadband absorption in a thin layer state.

[0006] The present invention is specifically implemented through the following technical solutions.

[0007] The first object of the present invention is to provide a preparation method of a nano-porous carbon / cobalt / cobalt oxide composite material, comprising the following steps:

[0008] S1. Disperse nano-porous carbon powder NCP, hexamethylenetetramine, trisodium citrate, and soluble cobalt salt in water to form a dispersion liquid, and then perform a solvothermal reaction at 85 - 95 °C. After washing and drying, obtain the NCP@Co precursor.

[0009] S2. Pyrolyze the NCP@Co precursor prepared in S1 at 700 - 900 °C to obtain the nano-porous carbon / cobalt / cobalt oxide composite material.

[0010] Preferably, in S1, the mass ratio of nano-porous carbon powder NCP, hexamethylenetetramine, trisodium citrate, and soluble cobalt salt is 70:35:7.25:150.

[0011] Preferably, in S1, the time of the solvothermal reaction is 6 - 7 h.

[0012] Preferably, in S1, the soluble cobalt salt is cobalt fluoride, cobalt chloride, cobalt iodide, cobalt bromide, cobalt carbonate, cobalt sulfate, cobalt oxalate, cobalt acetate, or cobalt nitrate hexahydrate.

[0013] Preferably, in S1, after the reaction ends, wash the product with water and absolute ethanol.

[0014] Preferably, in S2, the pyrolysis time is 2 h.

[0015] Preferably, in S2, the heating rate is 5 °C / min. After the heat preservation ends, cool down to 400 °C at a cooling rate of 5 °C per minute and then naturally cool to room temperature.

[0016] The second object of the present invention is to provide a nano-porous carbon / cobalt / cobalt oxide composite material prepared by the above preparation method.

[0017] The third object of the present invention is to provide the application of the above nano-porous carbon / cobalt / cobalt oxide composite material in the field of wave-absorbing materials.

[0018] Compared with the prior art, the present invention has the following beneficial effects:

[0019] The object of the present invention is to endow the microwave absorbing material with excellent reflection loss and absorption bandwidth while reducing the filler loading percentage, which is achieved through the synergistic effect of the following aspects:

[0020] For the first time, the present invention uses nanoporous carbon powder (NCP) as a substrate for modification, and prepares a nanoporous carbon / cobalt / cobalt oxide (NCP / Co / CoO) composite microwave absorbing material through a simple thermal solvent and pyrolysis process. By utilizing the dielectric loss of the nanoporous carbon matrix and the magnetic loss of cobalt / cobalt oxide nanoparticles, multiple loss mechanisms are combined. On the one hand, the honeycomb structure of NCP can achieve uniform loading of magnetic nanoparticles, confine the metal nanoparticles in the pores, and prevent them from agglomerating, which is beneficial to improving the matching of the material. On the other hand, the high specific surface area and high porosity of the NCP / Co / CoO composite material can provide interfacial polarization and dipole polarization while effectively reducing the filling content of the microwave absorber (only 5 wt%).

[0021] The NCP / Co / CoO nanocomposite microwave absorbing material obtained by the above strategy of the present invention exhibits excellent performance in terms of absorption bandwidth and absorption intensity: among them, the NCP / Co-900 sample obtained at 900 °C has good impedance matching and excellent microwave absorbing performance, and can effectively absorb the X-band. When the matching thickness d = 2.5 mm, RL min = -52.84 dB, EAB = 3.8 GHz (8.6 - 12.4 GHz), covering 85% of the X-band. In addition, the excellent thermal stability of the NCP / Co / CoO composite material has great application potential in the field of high-temperature microwave absorption.

[0022] The present invention provides a simple experimental synthesis scheme, and this method can also be used to prepare high-performance microwave absorbers from other cobalt metal salts. Description of the Drawings

[0023] Figure 1 Schematic diagram of the preparation process of the NCP@Co precursor and the composite material;

[0024] Figure 2 Scanning electron microscope photos: SEM images of NCP (a)-(c), SEM image of the NCP@Co precursor (d);

[0025] Figure 3 Transmission electron microscope photos: TEM images of the composite material (a)-(c), HRTEM image of the composite material (d);

[0026] Figure 4 XRD pattern of the composite material (a), TGA curves of NCP and NCP / Co under argon (b);

[0027] Figure 5XRD pattern (a), Raman spectrum (b), BET curve (c) and pore size distribution diagram (d) of the composite material;

[0028] Figure 6 Electromagnetic parameters of the composite material: real part of dielectric constant (A), imaginary part of dielectric constant (B), real part of magnetic permeability (C), imaginary part of magnetic permeability (D), dielectric loss tangent (E) and magnetic loss tangent (F);

[0029] Figure 7 RL diagram and RL projection diagram of the composite material; among them, (a), (c), (e) are RL diagrams; (b), (d), (f) are RL projection diagrams. Detailed implementation manners

[0030] In order to enable those skilled in the art to better understand and implement the technical solutions of the present invention, the present invention will be further described below in conjunction with specific embodiments and drawings, but the specific embodiments cited do not limit the present invention.

[0031] In the following various embodiments, the experimental methods and detection methods are all conventional methods unless otherwise specified; the reagents and materials can all be purchased on the market unless otherwise specified.

[0032] A preparation method of a nano-porous carbon / cobalt / cobalt oxide composite material includes the following steps:

[0033] S1. Disperse nano-porous carbon powder NCP, hexamethylenetetramine, trisodium citrate, and soluble cobalt salt in water to form a dispersion liquid, and then carry out a solvothermal reaction at 85 - 95 °C. After washing and drying, an NCP@Co precursor is prepared;

[0034] S2. Pyrolyze the NCP@Co precursor prepared in S1 at 700 - 900 °C to obtain the nano-porous carbon / cobalt / cobalt oxide composite material.

[0035] The soluble cobalt salt is one of cobalt fluoride, cobalt chloride, cobalt iodide, cobalt bromide, cobalt carbonate, cobalt sulfate, cobalt oxalate, cobalt acetate, cobalt nitrate hexahydrate.

[0036] The content of the present invention will be specifically described below through the following specific embodiments.

[0037] Example 1

[0038] A preparation method of a nano-porous carbon / cobalt / cobalt oxide composite material, as Figure 1 shown, includes the following steps:

[0039] S1. Preparation of NCP@Co precursor

[0040] First, disperse 70 mg of nano-porous carbon powder NCP in 40 ml of deionized water, ultrasonically disperse it at room temperature for 10 min, then add 35 mg of hexamethylenetetramine, 7.25 mg of trisodium citrate and 150 mg of cobalt nitrate hexahydrate thereto, fully stir with a magnetic stirrer for 20 min, then react for 6 h under the condition of a 90 °C oil bath. Subsequently, wash the obtained black product several times with deionized water and absolute ethanol, and dry it at 60 °C for standby.

[0041] S2. Preparation of NCP / Co / CoO composite material

[0042] Pyrolyze the above-prepared NCP@Co precursor black powder in an argon atmosphere for 2 h, with a pyrolysis temperature of 700 °C and a heating rate of 5 °C / min. After the heat preservation is completed, cool it to 400 °C at a cooling rate of 5 °C per minute and then naturally cool it to room temperature. Name the sample at this temperature NCP / Co-700.

[0043] Example 2

[0044] A preparation method of a nano-porous carbon / cobalt / cobalt oxide composite material includes the following steps:

[0045] S1. Preparation of NCP@Co precursor

[0046] First, disperse 70 mg of nano-porous carbon powder NCP in 40 ml of deionized water, ultrasonically disperse it at room temperature for 10 min, then add 35 mg of hexamethylenetetramine, 7.25 mg of trisodium citrate and 150 mg of cobalt nitrate hexahydrate thereto, fully stir with a magnetic stirrer for 20 min, then react for 6 h under the condition of a 90 °C oil bath. Subsequently, wash the obtained black product several times with deionized water and absolute ethanol, and dry it at 60 °C for standby.

[0047] S2. Preparation of NCP / Co / CoO composite material

[0048] Pyrolyze the above-prepared NCP@Co precursor black powder in an argon atmosphere for 2 h, with a pyrolysis temperature of 800 °C and a heating rate of 5 °C / min. After the heat preservation is completed, cool it to 400 °C at a cooling rate of 5 °C per minute and then naturally cool it to room temperature. Name the sample at this temperature NCP / Co-800.

[0049] Example 3

[0050] A preparation method of a nano-porous carbon / cobalt / cobalt oxide composite material includes the following steps:

[0051] S1. Preparation of NCP@Co precursor

[0052] First, disperse 70 mg of nanoporous carbon powder NCP in 40 ml of deionized water, ultrasonically disperse it at room temperature for 10 min, then add 35 mg of hexamethylenetetramine, 7.25 mg of trisodium citrate, and 150 mg of cobalt nitrate hexahydrate thereto, and stir well with a magnetic stirrer for 20 min. Subsequently, react it under an oil bath condition at 90 °C for 6 h. Then, wash the obtained black product with deionized water and absolute ethanol several times, and dry it at 60 °C for standby.

[0053] S2. Preparation of NCP / Co / CoO composite

[0054] Pyrolyze the prepared NCP@Co precursor black powder under an argon atmosphere for 2 h, with a pyrolysis temperature of 900 °C and a heating rate of 5 °C / min. After the heat preservation is completed, cool it to 400 °C at a cooling rate of 5 °C per minute and then naturally cool it to room temperature. Name the sample at this temperature NCP / Co-900.

[0055] Test and characterize the pyrolyzed sample with a series of instruments such as X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Raman, specific surface area (BET), and electromagnetic wave absorption performance.

[0056] Figure 2 Shown in the scanning electron microscope photos are the SEM images of NCP (a)-(c), and the SEM image of the NCP@Co precursor (d). It can be seen that the surface of pure NCP is regular, with a rich porous structure. Each pore is interconnected and in a three-dimensional through state, which can effectively improve the conductivity of the material. The connecting pores between the pores are small, and after the metal particles are loaded, they can be restricted within the pores and are evenly distributed without agglomeration. By continuously adjusting the concentration of cobalt salt and nanoporous carbon NCP, when the mass concentration ratio of cobalt salt to NCP is 15:7, as shown in Figure 2 the (c) in the scanning electron microscope, obvious metal particles can be seen growing on the NCP substrate. Observed under the transmission electron microscope, as shown in Figure 3 , it can be seen that there are obvious color contrast changes in the NCP@Co precursor, which proves from the side that a layer of Co(OH) 2 nano-particles grows on the honeycomb-like NCP surface.

[0057] Figure 4 Shown in the XRD spectrum of the composite sample (a) and the TGA curves of NCP and NCP@Co under argon (b). The NCP@Co precursor is characterized and analyzed by XRD. The results are shown in Figure 4 the (a) in it. Through comparative analysis, it is found that obvious diffraction peaks appear at 2θ = 37.1° and 61.5°, corresponding to Co(OH) 2for the (101) and (111) crystal planes, further proving the successful loading of Co(OH) on nanoporous carbon 2 nanoparticles. Figure 4 (b) in it is the thermogravimetric curve of the precursor and the nanoporous carbon sample under an argon atmosphere. The heating rate during the test is 10 °C / min. It can be seen that before 348 °C, the mass of the NCP and NCP@Co precursors will show a continuous loss, which is due to the escape of unstable components in the nanoporous carbon skeleton or the oxidative decomposition loss caused by the residual air in the pores of the nanoporous carbon. The second weight loss peak that appears at 583 °C is mainly formed by the collapse and decomposition of the main skeleton of the nanoporous carbon and stabilizes after 750 °C. The NCP@Co precursor has good thermal stability, with a mass residue of about 88.5% and only a weight loss of 11.4%.

[0058] To further analyze the change of carbon components in the NCP / Co nanocomposite, Raman spectroscopy fitting was used to analyze the carbon morphology under different pyrolysis conditions of the NCP / Co nanocomposite. The results are as Figure 5 (b) shown in it. It can be clearly observed in the figure that the D peak and G peak near 1367 cm -1 and 1580 cm -1 are present in the three samples. The D peak is related to the k-point phonon of the A 1g mode. When disordered graphite or nanographite crystals exist, the D peak will become active. The G peak is the scattering result of the E 2 vibration mode of sp 2g carbon atoms and generally corresponds to ordered graphite carbon. Usually, the area ratio of the D peak to the G peak (I D / I G ) is used as an index to evaluate the graphitization degree of carbon materials. It can be seen that the I D / I G values of the NCP / Co-700, NCP / Co-800, and NCP / Co-900 samples are 1.0032, 1.0024, and 1.0018 respectively. It is found that with the increase of the pyrolysis temperature, the I D / I G value gradually decreases, indicating that the content of graphite carbon in the NCP / Co composite gradually increases and the content of disordered carbon decreases. Generally speaking, the I D / I G value changes little because the nanoporous carbon skeleton itself has a relatively high degree of graphite carbon and good thermal stability. High-temperature pyrolysis will not cause large-scale collapse of the mesoporous carbon skeleton to form a disordered structure, which helps to increase the conductivity of the NCP / Co nanocomposite and thus improve the dielectric loss ability of the material. The specific surface area and pore size distribution of the NCP / Co composite absorber were characterized by BET test, and the results are as Figure 5In (c) and (d), it can be clearly observed from the nitrogen adsorption and desorption curves that the adsorption amount of the wave absorber increases rapidly at relatively low relative pressures (P / P 0 <0.1), and between the relative pressures P / P 0 = 0.6 - 1.0, a typical H3-type hysteresis loop appears due to the separation of the adsorption and desorption branches. There is no adsorption saturation plateau at the relative pressure P / P 0 = 1, which is a typical characteristic of the Type IV adsorption isotherm curve. Due to the different sizes of the porous structures in the nanoporous carbon NCP, the hysteresis loop shows the H3 type. According to the BET multi-point method, the specific surface areas of the NCP / Co-700 / 800 / 900 composite wave absorbers are 689.61 m 2 / g, 672.38 m 2 / g, and 647.84 m 2 / g respectively. It is precisely because of its large specific surface area that the filling content of the wave absorber can be effectively reduced (the actual measured filling content is only 5 wt%). Through the pore size distribution Figure 5 in (d), it can be clearly observed that two obvious pore size distribution regions appear at 2.8 nm and 17.1 nm, further proving the existence of mesoporous structures with different sizes in the NCP / Co nanocomposite. This porous structure can be regarded as an effective medium for the combination of air and the matrix material, which can achieve the regulation of the dielectric constant, enabling the porous structure to have better impedance matching and Debye relaxation.

[0059] Figure 6 are the electromagnetic parameters of the NCP / Co composite: real part of dielectric (A), imaginary part of dielectric (B), real part of permeability (C), imaginary part of permeability (D), dielectric loss tangent (E), and magnetic loss tangent (F). As can be seen from Figure 6 , (a), (b), and (e) are the real part of dielectric, imaginary part of dielectric, and dielectric loss tangent angle respectively. It can be found that the of NCP / Co-700, NCP / Co-800, and NCP / Co-900 is positively correlated with the reaction temperature. In the range of 2 - 18 GHz, the real part of dielectric of the three samples shows a decreasing trend due to the influence of the dispersion effect. The of NCP / Co-700 gradually decreases from 7.7 to 4.7, the of NCP / Co-800 decreases from 9.8 to 5.5, and an obvious resonance peak appears in the range of 12 - 18 GHz. The of NCP / Co-900 decreases from 12.7 to 7.6 and an obvious resonance peak appears in the same frequency range. Through comparative analysis, it is found that the resonance peak of the NCP / Co-900 sample is the largest, indicating that the polarization relaxation of the sample pyrolyzed at 900 °C is the strongest.

[0060] Figure 7RL diagrams (a), (c), (e) and RL projection diagrams (b), (d), (f) of NCP / Co composite materials. (a) and (b), (c) and (d), (e) and (f) are the simulated RL and RL projection diagrams of NCP / Co-700, NCP / Co-800 and NCP / Co-900 samples at different thicknesses respectively. It can be observed that all three samples can produce effective absorption in the range of 4 - 18 GHz (the area of the black dotted line in the projection diagram is the effective absorption region). From (a), (c), (e), it can be found that the wave absorption performance of NCP / Co-700 is poor, and the minimum reflection loss RL min is only -19.8 dB, with a matching thickness of 3.1 mm. While the wave absorption performance of NCP / Co-800 and NCP / Co-900 is better, and they have strong absorption loss for electromagnetic waves in the X-band. Among them, the minimum reflection loss RLmin of NCP / Co-800 is -44.98 dB at 9.02 GHz, with a matching thickness of 3.3 mm. The NCP / Co-900 sample has the best wave absorption performance, with a minimum reflection loss RL min of -52.84 dB, indicating that 99.999% of the electromagnetic waves are absorbed. The matching thickness is only 2.5 mm, and the effective absorption bandwidth EAB is 3.8 GHz (8.6 - 12.4 GHz), which can cover 85% of the X-band (8 - 12 GHz). And it can be found that as the matching thickness increases, the minimum value of the reflection coefficient RLmin gradually moves towards the low-frequency direction.

[0061] Obviously, those skilled in the art can make various changes and modifications to the present invention without departing from the spirit and scope of the present invention. Thus, if these modifications and variations of the present invention fall within the scope of the claims of the present invention and its equivalent technologies, these changes and modifications are also intended to be included.

Claims

1. Preparation method of nano-porous carbon / cobalt / cobalt oxide composite material, characterized in that, it comprises the following steps: S1. Dispersing nano-porous carbon powder, hexamethylenetetramine, trisodium citrate and soluble cobalt salt in water to form a dispersion liquid, then carrying out solvothermal reaction at 85-95 °C, washing and drying to obtain a precursor; S2. Pyrolyzing the precursor prepared in S1 at 700-900 °C to obtain nano-porous carbon / cobalt / cobalt oxide composite material; In S1, the mass ratio of nano-porous carbon powder, hexamethylenetetramine, trisodium citrate and soluble cobalt salt is 70:35:7.25:150; The nano-porous carbon / cobalt / cobalt oxide composite material is used for absorbing X-band electromagnetic waves; There are mesoporous structures with different sizes in the nano-porous carbon / cobalt / cobalt oxide composite material.

2. The preparation method according to claim 1, characterized in that, in S1, the time of solvothermal reaction is 6-7 h.

3. The preparation method according to claim 1, characterized in that, in S1, the soluble cobalt salt is one of cobalt fluoride, cobalt chloride, cobalt iodide, cobalt bromide, cobalt carbonate, cobalt sulfate, cobalt oxalate, cobalt acetate, cobalt nitrate hexahydrate.

4. The preparation method according to claim 1, characterized in that, in S1, after the reaction ends, the product is washed with water and absolute ethanol.

5. The preparation method according to claim 1, characterized in that, in S2, the pyrolysis time is 2 h.

6. The preparation method according to claim 5, characterized in that, in S2, the heating rate is 5 °C / min. After the heat preservation ends, it is cooled to 400 °C at a cooling rate of 5 °C per minute and then naturally cooled to room temperature.

7. Nano-porous carbon / cobalt / cobalt oxide composite material prepared by the preparation method according to any one of claims 1-6.

8. Application of the nano-porous carbon / cobalt / cobalt oxide composite material according to claim 7 in the field of microwave absorbing materials.

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