Preparation Method and Application of a Hollow Spherical Nitrogen-Doped Molybdenum Carbide / Carbon Composite Absorbing Material

Abstract

Preparation method and application of a hollow spherical nitrogen-doped molybdenum carbide / carbon composite microwave absorption material, which relates to a preparation method and application of a microwave absorption material. The purpose of the present invention is to solve the problems existing in magnetic metal microwave absorption materials, such as large density, easy oxidation and corrosion, poor stability, insufficient attenuation loss ability, etc. Method: First, mix deionized water, absolute ethanol and ammonia water; Second, add tetraethyl orthosilicate, resorcinol and formaldehyde solution to Solution I; Third, add ammonium molybdate tetrahydrate and dopamine hydrochloride to Solution II; Fourth, perform centrifugal separation and washing; Fifth, perform high-temperature calcination to obtain Reaction Product II; Sixth, put Reaction Product II into an alkaline solution, stir, wash and dry to obtain a hollow spherical nitrogen-doped molybdenum carbide / carbon composite microwave absorption material. A hollow spherical nitrogen-doped molybdenum carbide / carbon composite microwave absorption material is used as a microwave absorption material, showing excellent attenuation loss ability and a relatively wide response frequency band.

Description

Technical Field

[0001] The present invention relates to a preparation method and application of a wave-absorbing material. Background Art

[0002] Problems of electromagnetic pollution and information leakage caused by excessive electromagnetic radiation are becoming increasingly serious. As a technology for eliminating electromagnetic radiation and alleviating electromagnetic pollution, electromagnetic wave absorbing materials (wave-absorbing materials) have attracted the attention of governments and researchers in various countries. The research hotspots are strong absorption performance, wide absorption frequency band, low density and low thickness. The studied magnetic metal wave-absorbing materials have problems such as large density, easy oxidation and corrosion. Due to their excellent chemical stability, low density and controllable dielectric properties, carbon materials have gradually become a hotspot in the wave-absorbing field. In order to achieve good absorption performance, carbon materials are combined with other magnetic or dielectric components to construct binary or ternary composite materials with significant synergistic effects, which can not only optimize the impedance matching characteristics, but also the construction of multiple components can significantly increase the electromagnetic loss mechanism. As is well known, in addition to the inherent loss characteristics of composite materials, their microstructure also plays an extremely important role in the consumption of incident electromagnetic waves. Therefore, reasonable design of the microstructure of carbon-based composite materials will be beneficial to improving microwave absorption performance.

[0003] At present, some carbon-based composite materials with porous, hollow and eggshell structures have played a positive role in promoting the attenuation of electromagnetic energy. However, the composite materials with a single structure still restrict the repeated attenuation and polarization loss of electromagnetic waves, and the attenuation effect of specific microstructures on electromagnetic waves still needs to be further strengthened to improve the electromagnetic wave absorption performance of carbon-based composite materials. Summary of the Invention

[0004] The purpose of the present invention is to solve the problems existing in magnetic metal wave-absorbing materials, such as large density, easy oxidation and corrosion, poor stability, insufficient attenuation loss ability, etc., and to provide a preparation method and application of a hollow spherical nitrogen-doped molybdenum carbide / carbon composite wave-absorbing material.

[0005] To solve the above technical problems, the present invention constructs a hollow nitrogen-doped carbon nanosphere-supported molybdenum carbide nanoparticle (HHMC) electromagnetic wave absorption composite material through an aldol condensation reaction and a polymerization process at room temperature; the hollow structure is beneficial to reducing the density of the material, enhancing the multiple reflections of electromagnetic waves inside the material, and the small-sized spheres on the surface can strengthen electromagnetic wave scattering, improving the electromagnetic wave absorption performance of the composite material. In addition, the air sealed inside the spheres can optimize the impedance matching characteristics; the molybdenum carbide material exhibits excellent performance in electromagnetic wave absorption due to its excellent dielectric properties and good chemical stability, and the molybdenum carbide material can obtain uniformly dispersed small-sized nanoparticles at a relatively low pyrolysis temperature, which can provide sufficient polarization loss for electromagnetic energy attenuation; moreover, the doping modification of nitrogen can regulate the surface structure of the carbon matrix, improve the electron density between the molybdenum carbide and the carbon matrix, and at the same time, the introduction of nitrogen can serve as a dipole center to strengthen the dipole polarization loss effect of electromagnetic waves and enhance electromagnetic energy attenuation;

[0006] A preparation method of a hollow spherical nitrogen-doped molybdenum carbide / carbon composite wave-absorbing material is specifically completed according to the following steps:

[0007] I. Mix deionized water, absolute ethanol, and ammonia water, and stir at room temperature to obtain Solution I;

[0008] II. Add tetraethyl orthosilicate to Solution I, stir at room temperature, then add resorcinol, stir to dissolve, and finally add formaldehyde solution and stir to obtain Solution II;

[0009] III. Add ammonium molybdate tetrahydrate to Solution II, stir, and then add dopamine hydrochloride and stir to obtain Solution III;

[0010] IV. Centrifuge Solution III to obtain a solid product; wash the solid product, and then dry it to obtain Reaction Product I;

[0011] V. Place Reaction Product I in a corundum boat, then place the corundum boat in the center of a tube furnace, introduce argon into the tube furnace, and then heat it at a heating rate of 2 °C / min to 5 °C / min to 700 °C to 900 °C under an argon atmosphere, and then keep it warm at 700 °C to 900 °C. After the heat preservation is completed, cool it to room temperature to obtain Reaction Product II;

[0012] VI. Place Reaction Product II in an alkaline solution, stir, wash it by centrifugation with water, and then dry it to obtain a hollow spherical nitrogen-doped molybdenum carbide / carbon composite wave-absorbing material.

[0013] A hollow spherical nitrogen-doped molybdenum carbide / carbon composite wave-absorbing material is used as a wave-absorbing material.

[0014] Advantages of the present invention:

[0015] The present invention provides a preparation method for an electromagnetic wave absorbing material of molybdenum carbide nanoparticles coated with a nitrogen-doped carbon skeleton having a hollow structure, which is simple, efficient, and highly reproducible; tetraethyl orthosilicate is added to an alkaline solution containing resorcinol and formaldehyde. Tetraethyl orthosilicate will hydrolyze into silicon dioxide in an alkaline environment. Resorcinol and formaldehyde coat silicon dioxide through an aldol condensation reaction to form phenolic resin microspheres. Subsequently, ammonium molybdate and hydrochloric acid dopamine are added. Polydopamine has the characteristics of good film-forming property and strong adhesion, and can establish interactions with organic-inorganic surfaces, thereby enhancing the coating ability of cross-linked polydopamine. Through a high-temperature pyrolysis process, the internal phenolic resin microspheres are carbonized and react with molybdate at the same time to generate molybdenum carbide nanoparticles. This process is accompanied by the formation of a hollow structure. Finally, the silicon dioxide is etched away with an alkaline solution to obtain a hollow-structured nitrogen-doped molybdenum carbide / carbon composite material. The innovation points are as follows:

[0016] (1) During the high-temperature pyrolysis process, while the internal phenolic resin spheres are carbonized, they react with the surface molybdate. According to the Kirkendall effect, while carbon species migrate to the outer layer, molybdenum atoms in the outer layer move inward, thus forming an internal hollow structure. After alkali treatment, the silicon dioxide particles are removed, forming numerous mesoporous structures;

[0017] (2) The mesoporous structure increases the heterointerfaces and optimizes the polarization loss characteristics of the material; nitrogen doping can not only adjust the graphitization degree of the carbon skeleton, but also, thanks to the molybdenum carbide particles and nitrogen doping, form abundant dipole polarization centers, enhancing the dipole polarization loss characteristics of the material;

[0018] (3) The hollow structure reduces the overall density of the material. The numerous mesoporous structures distributed on the surface act as airbags, and the air inside makes the overall impedance matching of the material closer to the external space, thereby enhancing the characteristic impedance matching characteristics of the material;

[0019] (4) Whether it is the numerous mesoporous structures on the surface or the internal hollow structure, they can confine electromagnetic waves in different frequency ranges, forming multiple scattering, thereby enhancing the attenuation loss of electromagnetic waves; the controllable preparation of the hollow-structured nitrogen-doped molybdenum carbide / carbon composite material is realized by using a liquid-phase polymerization and high-temperature pyrolysis process. The synthesis process is simple and highly quantifiable;

[0020] (5) The present invention constructs an electromagnetic wave absorbing material with a hollow-structured nitrogen-doped carbon skeleton coated with molybdenum carbide nanoparticles through a simple liquid-phase chemical synthesis and high-temperature carbonization process, reducing the material density, optimizing the impedance matching characteristics of the material, enhancing the multiple reflection and scattering of electromagnetic waves, and improving the electromagnetic wave attenuation and absorption performance. When the coating thickness is 1.7 mm, the minimum reflection loss value reaches -59.5 dB, and the maximum effective absorption bandwidth is 4.8 GHz, showing excellent attenuation loss ability and a relatively wide response frequency band, which has good inspiration and reference significance for the design of structural wave-absorbing materials and has good application prospects. Description of the Drawings

[0021] Figure 1 SEM and TEM images of the hollow spherical nitrogen-doped molybdenum carbide / carbon composite wave-absorbing material prepared in Example 2;

[0022] Figure 2 XRD pattern, where 1 is the hollow spherical nitrogen-doped molybdenum carbide / carbon composite wave-absorbing material prepared in Example 1, 2 is the hollow spherical nitrogen-doped molybdenum carbide / carbon composite wave-absorbing material prepared in Example 2, and 3 is the hollow spherical nitrogen-doped molybdenum carbide / carbon composite wave-absorbing material prepared in Example 3;

[0023] Figure 3 Raman spectrum, where 1 is the hollow spherical nitrogen-doped molybdenum carbide / carbon composite wave-absorbing material prepared in Example 1, 2 is the hollow spherical nitrogen-doped molybdenum carbide / carbon composite wave-absorbing material prepared in Example 2, and 3 is the hollow spherical nitrogen-doped molybdenum carbide / carbon composite wave-absorbing material prepared in Example 3;

[0024] Figure 4 Curves of the real part and imaginary part of the complex permittivity. In the figure, (a) is the real part of the complex permittivity, (b) is the imaginary part of the complex permittivity, 1 is the hollow spherical nitrogen-doped molybdenum carbide / carbon composite wave-absorbing material prepared in Example 1, 2 is the hollow spherical nitrogen-doped molybdenum carbide / carbon composite wave-absorbing material prepared in Example 2, and 3 is the hollow spherical nitrogen-doped molybdenum carbide / carbon composite wave-absorbing material prepared in Example 3;

[0025] Figure 5 Three-dimensional diagram of reflection loss. In the figure, (a) is the hollow spherical nitrogen-doped molybdenum carbide / carbon composite wave-absorbing material prepared in Example 1, (b) is the hollow spherical nitrogen-doped molybdenum carbide / carbon composite wave-absorbing material prepared in Example 2, and (c) is the hollow spherical nitrogen-doped molybdenum carbide / carbon composite wave-absorbing material prepared in Example 3;

[0026] Figure 6It is an impedance matching diagram. In the diagram, (a) is the hollow spherical nitrogen-doped molybdenum carbide / carbon composite microwave absorbing material prepared in Example 1, (b) is the hollow spherical nitrogen-doped molybdenum carbide / carbon composite microwave absorbing material prepared in Example 2, and (c) is the hollow spherical nitrogen-doped molybdenum carbide / carbon composite microwave absorbing material prepared in Example 3. Detailed implementation manners

[0027] Detailed implementation manner 1: The preparation method of a hollow spherical nitrogen-doped molybdenum carbide / carbon composite microwave absorbing material in this implementation manner is specifically completed according to the following steps:

[0028] 1. Mix deionized water, absolute ethanol, and ammonia water, and stir at room temperature to obtain Solution I;

[0029] 2. Add tetraethyl orthosilicate to Solution I, stir at room temperature, then add resorcinol, stir to dissolve, and finally add formaldehyde solution and stir to obtain Solution II;

[0030] 3. Add ammonium molybdate tetrahydrate to Solution II, stir, and then add dopamine hydrochloride and stir to obtain Solution III;

[0031] 4. Centrifuge Solution III to obtain a solid product; wash the solid product, and then dry it to obtain Reaction Product I;

[0032] 5. Place Reaction Product I in a corundum boat, then place the corundum boat in the center of a tube furnace, introduce argon into the tube furnace, and then heat it at a heating rate of 2 °C / min to 5 °C / min in an argon atmosphere to 700 °C to 900 °C, and then keep it warm at 700 °C to 900 °C. After the heat preservation is completed, cool it to room temperature to obtain Reaction Product II;

[0033] 6. Place Reaction Product II in an alkali solution, stir, wash by centrifugation with water, and then dry it to obtain a hollow spherical nitrogen-doped molybdenum carbide / carbon composite microwave absorbing material.

[0034] Detailed implementation manner 2: The difference between this implementation manner and Detailed implementation manner 1 is that: in Solution I in Step 1, the volume ratio of deionized water, absolute ethanol, and ammonia water is (15 mL to 25 mL):(45 mL to 55 mL):(2 mL to 4 mL); the mass fraction of ammonia water in Step 1 is 25%; the stirring time in Step 1 is 20 min to 40 min. Other steps are the same as those in Detailed implementation manner 1.

[0035] Specific Embodiment 3: The difference between this embodiment and one of Specific Embodiments 1 or 2 is as follows: In Step 2, the mass-to-volume ratio of resorcinol, tetraethyl orthosilicate, and formaldehyde solution in Solution II is (0.4 g - 0.6 g):(2 mL - 4 mL):(0.6 mL - 0.8 mL); the volume ratio of tetraethyl orthosilicate described in Step 2 to deionized water described in Step 1 is (2 mL - 4 mL):(15 mL - 25 mL). Other steps are the same as those in Specific Embodiment 1 or 2.

[0036] Specific Embodiment 4: The difference between this embodiment and one of Specific Embodiments 1 to 3 is as follows: In Step 2, the mass fraction of the formaldehyde solution is 37% - 40%; in Step 2, tetraethyl orthosilicate is added to Solution I, stirred at room temperature for 20 min - 40 min, then resorcinol is added and stirred until dissolved, and finally the formaldehyde solution is added and stirred for 22 h - 26 h to obtain Solution II. Other steps are the same as those in Specific Embodiments 1 to 3.

[0037] Specific Embodiment 5: The difference between this embodiment and one of Specific Embodiments 1 to 4 is as follows: In Step 3, the mass ratio of ammonium molybdate tetrahydrate to dopamine hydrochloride is (0.4 g - 0.5 g):(0.5 g - 0.6 g); the mass of ammonium molybdate tetrahydrate described in Step 3 to the volume of deionized water described in Step 1 is (0.4 g - 0.5 g):(15 mL - 25 mL); in Step 3, ammonium molybdate tetrahydrate is added to Solution II and stirred for 10 min - 20 min, then dopamine hydrochloride is added and stirred for 2 h - 4 h to obtain Solution III. Other steps are the same as those in Specific Embodiments 1 to 4.

[0038] Specific Embodiment 6: The difference between this embodiment and one of Specific Embodiments 1 to 5 is as follows: In Step 4, deionized water and absolute ethanol are used to repeatedly wash the solid product until the washing liquid is colorless, and then it is placed in an oven at 60°C - 80°C and dried for 10 h - 15 h. Other steps are the same as those in Specific Embodiments 1 to 5.

[0039] Specific Embodiment 7: The difference between this embodiment and one of Specific Embodiments 1 to 6 is as follows: In Step 5, the heat preservation time is 2 h - 4 h. Other steps are the same as those in Specific Embodiments 1 to 6.

[0040] Specific Embodiment 8: The difference between this embodiment and one of Specific Embodiments 1 to 7 is as follows: In Step 5, it is heated to 700°C - 800°C at a heating rate of 2°C / min - 5°C / min under an argon atmosphere, and then heat-preserved at 700°C - 800°C. Other steps are the same as those in Specific Embodiments 1 to 7.

[0041] Specific Embodiment Nine: The difference between this embodiment and any one of Specific Embodiments One to Eight is that: the alkali solution described in Step Six is sodium hydroxide solution or potassium hydroxide solution, with a concentration of 3 mol / L to 6 mol / L; in Step Six, the reaction product II is put into the alkali solution, stirred for 22 h to 26 h, centrifuged and washed 3 to 5 times with water, and then put into a vacuum oven at a temperature of 60°C to 80°C and dried for 10 h to 12 h to obtain a hollow spherical nitrogen-doped molybdenum carbide / carbon composite wave-absorbing material. Other steps are the same as those in Specific Embodiments One to Eight.

[0042] Specific Embodiment Ten: This embodiment is about using a hollow spherical nitrogen-doped molybdenum carbide / carbon composite wave-absorbing material as a wave-absorbing material.

[0043] The following examples are used to verify the beneficial effects of the present invention:

[0044] Example 1: A preparation method of a hollow spherical nitrogen-doped molybdenum carbide / carbon composite wave-absorbing material, which is specifically completed according to the following steps:

[0045] I. Preparation of Solution I:

[0046] Mix 20 mL of deionized water, 50 mL of absolute ethanol and 3 mL of ammonia water (mass fraction 25%), and stir at room temperature for 30 min to obtain Solution I;

[0047] II. Preparation of Solution II:

[0048] Add 3 mL of tetraethyl orthosilicate to Solution I, stir at room temperature for 30 min, then add 0.5 g of resorcinol, stir to dissolve, and finally add 0.74 mL of formaldehyde solution, and stir for 24 h to obtain Solution II;

[0049] The mass fraction of the formaldehyde solution described in Step II is 37%;

[0050] III. Preparation of Solution III:

[0051] Add 0.45 g of ammonium molybdate tetrahydrate to Solution II, stir for 10 min, then add 0.55 g of hydrochloric acid dopamine, and stir for 3 h to obtain Solution III;

[0052] IV. Centrifuge Solution III to obtain a solid product; repeatedly wash the solid product with deionized water and absolute ethanol until the washing liquid is colorless, and then put it into an oven at 80°C and bake for 10 h to obtain Reaction Product I;

[0053] V. Put Reaction Product I into a corundum boat, then put the corundum boat in the center of a tube furnace, introduce argon into the tube furnace for 30 min, then heat it to 700°C at a heating rate of 3°C / min under an argon atmosphere, and then keep it at 700°C for 3 h. After the heat preservation is completed, cool it to room temperature to obtain Reaction Product II;

[0054] 6. Put the reaction product II into a sodium hydroxide solution with a concentration of 4 mol / L, stir for 24 h, wash it by centrifugation with water 5 times, and then dry it at 80 °C for 10 h to obtain a hollow spherical nitrogen-doped molybdenum carbide / carbon composite microwave absorption material.

[0055] Example 2: The difference between this example and Example 1 is that in step 5, the reaction product I is put into a corundum boat, and then the corundum boat is placed in the center of a tubular furnace. Argon is introduced into the tubular furnace for 30 min, and then the temperature is raised to 800 °C at a heating rate of 3 °C / min under an argon atmosphere, and then kept at 800 °C for 3 h. After the heat preservation is completed, it is cooled to room temperature to obtain the reaction product II. Other steps and parameters are the same as those in Example 1.

[0056] Example 3: The difference between this example and Example 1 is that in step 5, the reaction product I is put into a corundum boat, and then the corundum boat is placed in the center of a tubular furnace. Argon is introduced into the tubular furnace for 30 min, and then the temperature is raised to 900 °C at a heating rate of 3 °C / min under an argon atmosphere, and then kept at 900 °C for 3 h. After the heat preservation is completed, it is cooled to room temperature to obtain the reaction product II. Other steps and parameters are the same as those in Example 1.

[0057] Figure 1 SEM and TEM images of the hollow spherical nitrogen-doped molybdenum carbide / carbon composite microwave absorption material prepared in Example 2;

[0058] As can be seen from the SEM image ( Figure 1 (a)), the hollow spherical nitrogen-doped molybdenum carbide / carbon composite microwave absorption material prepared in Example 2 shows a regular spherical morphology. The broken spheres clearly show that the interior of the spheres is a hollow structure, and the surface of the spheres is composed of small particles piled up; as can be further confirmed from the TEM image ( Figure 1 (b)) that the interior of the spheres is a hollow structure, and the small particles on the surface of the spheres also show a hollow structure. Figure 1 (c) is the EDS element mapping image. It can be seen from the image that carbon, molybdenum, and nitrogen elements are mainly distributed on the spherical shell and are evenly distributed throughout the sample, indicating that nitrogen elements are doped into the carbon skeleton, and Mo 2 C nanoparticles are evenly distributed in the carbon skeleton.

[0059] Figure 2 is the XRD pattern. In the figure, 1 is the hollow spherical nitrogen-doped molybdenum carbide / carbon composite microwave absorption material prepared in Example 1, 2 is the hollow spherical nitrogen-doped molybdenum carbide / carbon composite microwave absorption material prepared in Example 2, and 3 is the hollow spherical nitrogen-doped molybdenum carbide / carbon composite microwave absorption material prepared in Example 3;

[0060] From Figure 2It can be seen that characteristic diffraction peaks appear at 2θ = 37.7°, 43.7°, 63.4° and 75.1° for all three samples, which can be respectively attributed to the (111), (200), (220) and (311) crystal planes of cubic phase Mo 2 C (JCPDS 15 - 0457).

[0061] Figure 3 The figure shows the Raman spectra. In the figure, 1 is the hollow spherical nitrogen - doped molybdenum carbide / carbon composite microwave absorption material prepared in Example 1, 2 is the hollow spherical nitrogen - doped molybdenum carbide / carbon composite microwave absorption material prepared in Example 2, and 3 is the hollow spherical nitrogen - doped molybdenum carbide / carbon composite microwave absorption material prepared in Example 3;

[0062] From Figure 3 it can be seen that two characteristic displacement peaks appear near 1350 cm -1 and 1590 cm -1 for all three samples, which respectively belong to the D peak and the G peak. The D peak represents the sp 3 hybridization form of the carbon material, and the G peak represents the sp 2 hybridization form of carbon; usually, the intensity ratio (I D / I G ) of the D peak and the G peak is used to represent the graphitization degree of the carbon material. From Figure 3 it can be seen that as the temperature increases, I D / I G gradually increases. For the amorphous carbon component, an increase in I D / I G indicates an enhanced graphitization degree of the carbon material; the results show that an increase in temperature is beneficial to enhancing the graphitization degree of carbon, and the graphitization degree is positively correlated with the conductivity. Furthermore, it can be concluded that as the temperature increases, the conductivity of the composite material gradually increases, and its electromagnetic wave loss ability also gradually increases.

[0063] Figure 4 The figures show the real part and the imaginary part of the complex permittivity. In the figures, (a) is the real part of the complex permittivity, (b) is the imaginary part of the complex permittivity, 1 is the hollow spherical nitrogen - doped molybdenum carbide / carbon composite microwave absorption material prepared in Example 1, 2 is the hollow spherical nitrogen - doped molybdenum carbide / carbon composite microwave absorption material prepared in Example 2, and 3 is the hollow spherical nitrogen - doped molybdenum carbide / carbon composite microwave absorption material prepared in Example 3;

[0064] The real part of the complex dielectric constant represents the material's ability to store electromagnetic energy, while the imaginary part represents the ability to consume electromagnetic energy; when the carbonization temperature is 700°C, the hollow spherical nitrogen-doped molybdenum carbide / carbon composite absorbing material prepared in Example 1 exhibits the smallest dielectric constant value in the entire frequency range, and its imaginary part of the dielectric constant is less than 4 and remains basically unchanged, indicating that the hollow spherical nitrogen-doped molybdenum carbide / carbon composite absorbing material prepared in Example 1 has a weak electromagnetic loss capability. When the carbonization temperature is increased to 800°C, both the real and imaginary parts of the dielectric constant of the hollow spherical nitrogen-doped molybdenum carbide / carbon composite absorbing material prepared in Example 2 are significantly increased, especially when the carbonization temperature is 900°C, the dielectric constant of the hollow spherical nitrogen-doped molybdenum carbide / carbon composite absorbing material prepared in Example 3 is significantly enhanced, indicating that the hollow spherical nitrogen-doped molybdenum carbide / carbon composite absorbing material prepared in Example 3 has the strongest electromagnetic attenuation capability.

[0065] 30 mg of the hollow spherical nitrogen-doped molybdenum carbide / carbon composite absorbing material powder prepared in Example 1 and 70 mg of sliced ​​paraffin were placed in an agate mortar, and the mortar was placed in a 70°C oven and heated for 20 min. After the paraffin was completely melted, the mortar was taken out and quickly ground. The melted paraffin and the powder sample were fully mixed until the paraffin re-solidified. The mixed sample was then transferred to a sample preparation mold for tableting to obtain a concentric ring test sample; the ring specifications were: inner diameter 3 mm, outer diameter 7 mm, thickness 2-3 mm; the prepared concentric rings were placed in the concentric axis transmission line fixture of a vector network analyzer, the fixture was connected to the vector network analyzer, the electromagnetic parameters of the parameter test material were set, and the sample was taken out after the test was completed. The test data are shown in Figures 4(a) and 4(b).

[0066] 30 mg of the hollow spherical nitrogen-doped molybdenum carbide / carbon composite absorbing material powder prepared in Example 2 and 70 mg of sliced ​​paraffin were placed in an agate mortar, and the mortar was placed in a 70°C oven and heated for 20 min. After the paraffin was completely melted, the mortar was taken out and quickly ground. The melted paraffin and the powder sample were fully mixed until the paraffin re-solidified. The mixed sample was then transferred to a sample preparation mold for tableting to obtain a concentric ring test sample; the ring specifications were: inner diameter 3 mm, outer diameter 7 mm, thickness 2-3 mm; the prepared concentric rings were placed in the concentric axis transmission line fixture of a vector network analyzer, the fixture was connected to the vector network analyzer, the electromagnetic parameters of the parameter test material were set, and the sample was taken out after the test was completed. The test data are shown in Figures 4(a) and 4(b).

[0067] Put 30 mg of the hollow spherical nitrogen-doped molybdenum carbide / carbon composite microwave absorption material powder prepared in Example 3 and 70 mg of sliced paraffin into an agate mortar. Place the mortar in an oven at 70 °C and heat for 20 min. After the paraffin has completely melted, take out the mortar and quickly grind it to fully mix the melted paraffin with the powder sample until the paraffin solidifies again. Then transfer the mixed sample to a sample preparation mold for pressing to obtain a concentric circular ring test sample; the ring specifications are: inner diameter 3 mm, outer diameter 7 mm, and thickness 2 - 3 mm; put the prepared concentric ring into the coaxial transmission line fixture of a vector network analyzer, connect the fixture to the vector network analyzer, set the parameters to test the electromagnetic parameters of the material, take out the sample after the test is completed, and the test data obtained is shown in 4(a) and 4(b).

[0068] Figure 5 It is a three-dimensional map of reflection loss. In the figure, (a) is the hollow spherical nitrogen-doped molybdenum carbide / carbon composite microwave absorption material prepared in Example 1, (b) is the hollow spherical nitrogen-doped molybdenum carbide / carbon composite microwave absorption material prepared in Example 2, and (c) is the hollow spherical nitrogen-doped molybdenum carbide / carbon composite microwave absorption material prepared in Example 3;

[0069] From Figure 5 (a), it can be seen that the minimum reflection loss value of the hollow spherical nitrogen-doped molybdenum carbide / carbon composite microwave absorption material prepared in Example 1 is only -17.7 dB, and the corresponding maximum effective absorption bandwidth is less than 4 GHz. As the temperature increases, the hollow spherical nitrogen-doped molybdenum carbide / carbon composite microwave absorption material prepared in Example 2 shows enhanced microwave absorption performance, not only the reflection loss ability is enhanced, but also the maximum effective absorption bandwidth increases. Its minimum reflection loss value reaches -59.5 dB, and the maximum effective absorption bandwidth is 4.8 GHz; however, the hollow spherical nitrogen-doped molybdenum carbide / carbon composite microwave absorption material prepared in Example 3 with the largest dielectric constant does not show excellent microwave absorption performance, and its minimum reflection loss value even just reaches -10 dB. This indicates that the attenuation loss ability is not the only parameter determining the microwave absorption performance, and the microwave absorption performance of the material is also related to impedance matching.

[0070] Figure 6 It is an impedance matching diagram. In the figure, (a) is the hollow spherical nitrogen-doped molybdenum carbide / carbon composite microwave absorption material prepared in Example 1, (b) is the hollow spherical nitrogen-doped molybdenum carbide / carbon composite microwave absorption material prepared in Example 2, and (c) is the hollow spherical nitrogen-doped molybdenum carbide / carbon composite microwave absorption material prepared in Example 3;

[0071] Figure 6 The larger the area less than 0.4 in it indicates the better the impedance matching performance. Although the hollow spherical nitrogen-doped molybdenum carbide / carbon composite microwave absorption material prepared in Example 1 shows good impedance matching( Figure 6(a)), but its attenuation ability is insufficient and it cannot exhibit strong microwave absorption performance. The hollow spherical nitrogen-doped molybdenum carbide / carbon composite microwave absorption material prepared in Example 2 has the largest coverage area, indicating its excellent impedance matching characteristics. The main reason why the hollow spherical nitrogen-doped molybdenum carbide / carbon composite microwave absorption material prepared in Example 3 does not exhibit excellent microwave absorption performance is its poor impedance matching characteristics ( Figure 6 (c)). Considering the comprehensive results, the main reason why the hollow spherical nitrogen-doped molybdenum carbide / carbon composite microwave absorption material prepared in Example 2 exhibits the best microwave absorption performance is its excellent impedance matching characteristics and good attenuation ability.

Claims

1. Preparation method of hollow spherical nitrogen-doped molybdenum carbide / carbon composite microwave absorbing material, characterized in that the preparation method is specifically completed according to the following steps: I. Mix deionized water, absolute ethanol and ammonia water, and stir at room temperature to obtain solution I; II. Add tetraethyl orthosilicate to solution I, stir at room temperature, then add resorcinol, stir to dissolve, and finally add formaldehyde solution, stir to obtain solution II; III. Add ammonium molybdate tetrahydrate to solution II, stir, then add dopamine hydrochloride, stir to obtain solution III; IV. Centrifuge solution III to obtain a solid product; Wash the solid product, and then dry it to obtain reaction product I; V. Place reaction product I in a corundum boat, then place the corundum boat in the center of a tube furnace, introduce argon into the tube furnace, and then heat it at a heating rate of 2°C / min to 5°C / min to 700°C to 900°C under an argon atmosphere, then keep it warm at 700°C to 900°C, and cool to room temperature after the insulation is completed to obtain reaction product II; VI. Place reaction product II in an alkali solution, stir, wash by centrifugation with water, and then dry it to obtain a hollow spherical nitrogen-doped molybdenum carbide / carbon composite microwave absorbing material.

2. The preparation method of a hollow spherical nitrogen-doped molybdenum carbide / carbon composite microwave absorbing material according to claim 1, characterized in that in solution I in step I, the volume ratio of deionized water, absolute ethanol and ammonia water is (15 mL to 25 mL):(45 mL to 55 mL):(2 mL to 4 mL); the mass fraction of ammonia water in step I is 25%; the stirring time in step I is 20 min to 40 min.

3. The preparation method of a hollow spherical nitrogen-doped molybdenum carbide / carbon composite microwave absorbing material according to claim 1, characterized in that in solution II in step II, the mass-volume ratio of resorcinol, tetraethyl orthosilicate and formaldehyde solution is (0.4 g to 0.6 g):(2 mL to 4 mL):(0.6 mL to 0.8 mL); the volume ratio of tetraethyl orthosilicate to deionized water in step I in step II is (2 mL to 4 mL):(15 mL to 25 mL).

4. The preparation method of a hollow spherical nitrogen-doped molybdenum carbide / carbon composite microwave absorbing material according to claim 1, characterized in that the mass fraction of the formaldehyde solution in step II is 37%-40%; in step II, add tetraethyl orthosilicate to solution I, stir at room temperature for 20 min to 40 min, then add resorcinol, stir to dissolve, and finally add formaldehyde solution, stir for 22 h to 26 h to obtain solution II.

5. The preparation method of a hollow spherical nitrogen-doped molybdenum carbide / carbon composite microwave absorbing material according to claim 1, characterized in that In step three, the mass ratio of ammonium molybdate tetrahydrate to dopamine hydrochloride is (0.4 g - 0.5 g):(0.5 g - 0.6 g); in step three, the mass ratio of ammonium molybdate tetrahydrate to the volume of deionized water in step one is (0.4 g - 0.5 g):(15 mL - 25 mL); in step three, ammonium molybdate tetrahydrate is added to solution II, stirred for 10 min - 20 min, and then dopamine hydrochloride is added and stirred for 2 h - 4 h to obtain solution III.

6. The preparation method of a hollow spherical nitrogen-doped molybdenum carbide / carbon composite wave-absorbing material according to claim 1, characterized in that in step four, deionized water and absolute ethanol are used to repeatedly wash the solid product until the washing liquid is colorless, and then it is placed in an oven at 60 °C - 80 °C and dried for 10 h - 15 h.

7. The preparation method of a hollow spherical nitrogen-doped molybdenum carbide / carbon composite wave-absorbing material according to claim 1, characterized in that the heat preservation time in step five is 2 h - 4 h.

8. The preparation method of a hollow spherical nitrogen-doped molybdenum carbide / carbon composite wave-absorbing material according to claim 1, characterized in that in step five, it is heated to 700 °C - 800 °C at a heating rate of 2 °C / min - 5 °C / min under an argon atmosphere, and then heat-preserved at 700 °C - 800 °C.

9. The preparation method of a hollow spherical nitrogen-doped molybdenum carbide / carbon composite wave-absorbing material according to claim 1, characterized in that the alkali solution in step six is sodium hydroxide solution or potassium hydroxide solution with a concentration of 3 mol / L - 6 mol / L; in step six, the reaction product II is placed in the alkali solution, stirred for 22 h - 26 h, centrifuged and washed 3 - 5 times, and then dried in a vacuum oven at 60 °C - 80 °C for 10 h - 12 h to obtain the hollow spherical nitrogen-doped molybdenum carbide / carbon composite wave-absorbing material.

10. The application of a hollow spherical nitrogen-doped molybdenum carbide / carbon composite wave-absorbing material prepared by the preparation method according to claim 1, characterized in that a hollow spherical nitrogen-doped molybdenum carbide / carbon composite wave-absorbing material is used as a wave-absorbing material.

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