Frequency-selective wave-absorbing composite wave-absorbing material and preparation method thereof

Abstract

The invention discloses a frequency-selective wave-absorbing composite wave-absorbing material and a preparation method thereof, and belongs to the technical field of wave-absorbing materials, the preparation method comprises the steps of ultrasonic dispersion, binder preparation, high-speed shearing and high-temperature sintering; the ultrasonic dispersion comprises the following steps: mixing the carbon nano tube with a solvent and a surfactant, carrying out ultrasonic dispersion to obtain a carbon nano tube dispersion liquid, and drying the carbon nano tube dispersion liquid to obtain a pretreated carbon nano tube; in the step of high-speed shearing, the pretreated carbon nanotubes, graphene nanosheets and gas-phase silicon oxide are mixed to obtain a raw material; mixing the raw materials with a polyvinyl alcohol solution, and performing high-speed shearing to obtain solid powder; the prepared composite wave-absorbing material capable of selecting frequency and absorbing waves is small in density, has frequency-selecting wave-absorbing performance, is good in impedance matching performance, and also has excellent wave-absorbing performance.

Description

Technical Field

[0001] The present invention belongs to the technical field of microwave absorbing materials, and particularly relates to a composite microwave absorbing material with frequency-selective absorption and a preparation method thereof. Background Art

[0002] In recent years, with the rapid development of electronic devices and communication technologies, the problem of electromagnetic wave interference has become increasingly serious. Especially in the fields of military, aerospace, and civilian electronic devices, electromagnetic wave interference not only affects the normal operation of devices but also may pose a threat to information security. Microwave absorbing materials can convert the electromagnetic wave energy entering the material into heat energy or other forms of energy and dissipate it. The microwave absorbing materials should have the characteristics of wide absorption bandwidth and strong absorption performance. The frequency-selective absorption technology in microwave absorbing materials can precisely control the absorption of electromagnetic waves, allowing electromagnetic waves of specific frequencies to pass through while absorbing or blocking other frequencies. In the military field, the application of frequency-selective absorption materials can significantly reduce the radar cross-section of aircraft, ships, and other devices, enhancing their stealth performance and thus gaining a strategic advantage in modern warfare. In the civilian field, frequency-selective absorption technology helps manage the increasingly complex electromagnetic environment, ensuring clear signal transmission and reception while reducing the potential impact of electromagnetic pollution on human health and the environment. Therefore, the research and application of microwave absorbing materials with frequency-selective absorption function are of great importance.

[0003] Currently, the microwave absorbing materials on the market are mainly divided into several categories such as metal-based microwave absorbing materials, carbon-based microwave absorbing materials, and composite microwave absorbing materials. Although metal-based microwave absorbing materials have good microwave absorption performance, they have a large density, a complex preparation process, and are easily oxidized; carbon-based microwave absorbing materials have attracted wide attention due to their light weight, high strength, and good electrical conductivity, but there are certain deficiencies in their microwave absorption performance and impedance matching. It is urgent to improve the impedance of the materials to match the microwave absorption performance. For example, the main advantage of ordered mesoporous carbon as a microwave absorbing material lies in its porous structure, which is beneficial to reducing the density and complex dielectric constant, enabling most electromagnetic waves to enter the material interior. However, due to the relatively large dielectric constant of carbon materials, when used alone, the impedance matching characteristics of the microwave absorption layer are poor, and there are disadvantages such as a single loss mechanism, narrow absorption bandwidth, and weak absorption performance.

[0004] Optimization of electromagnetic absorption properties based on graphene, carbon nanotubes, and Fe3O4 multidimensional composites; Polymer Composites; Pang Xianke; 2024 discloses a graphene / carbon nanotube / ferrite (G / C / Fe3O4) composite material and its preparation method. This composite material forms a multidimensional structure by using one-dimensional tubular carbon nanotubes and two-dimensional graphene through a physical mixing method, enhancing the electromagnetic wave absorption ability of the material. Although the G / C / Fe3O4 composite material exhibits good wave absorption performance, there is a problem of poor frequency-selective wave absorption performance. This means that the material may perform well in a specific frequency range, but the absorption effect is not good at other frequencies, resulting in limited application scope. In addition, since ferrite is a metal composite material, it has a large density and weak antioxidant ability, which limits its application as a frequency-selective wave absorption material.

[0005] Research on electromagnetic wave loss materials based on graphene; Inorganic materials science; Kang Yue; In 2018, a three-dimensional ultra-light composite wave absorption material based on graphene and its preparation method were disclosed. By utilizing the synergistic effect of graphene and carbon nanotubes, this material realizes multiple reflections of electromagnetic waves within the wave absorption material, significantly improving the wave absorption performance of the material. However, the absorption performance in the low-frequency region is weak, the preparation process involves complex steps such as chemical vapor deposition or hydrothermal synthesis, resulting in high production costs, and the stability is poor in extreme environments.

[0006] Chinese Patent CN113480973B discloses a boron / nitrogen co-doped carbon nanotube-carbon nanosheet composite material and its preparation method. The composite material consists of ultrathin carbon nanosheets and bamboo-shaped carbon nanotubes, and metal nickel nanoparticles are wrapped inside the carbon nanotubes. The method includes: preparing nickel boride by a mild chemical reduction method. Mixing nickel boride, dicyandiamide, and sodium chloride in proportion and grinding them evenly. Calcining the mixture at high temperature under an inert atmosphere to obtain a black powdery material. The above method has simple process, strong repeatability, and the boron / nitrogen co-doped carbon nanotube-carbon nanosheet composite material prepared by one-step pyrolysis has characteristics such as light weight, small density, and excellent electromagnetic wave absorption performance, and has excellent absorption intensity and absorption bandwidth at different thicknesses.

[0007] Chinese Patent CN118894523A discloses a preparation method of a superstructure carbon nanotube absorbing material. In the above method, polyimide is coated on the outer surface of MoO3 nanorods, and through etching and carbonization, a superstructure carbon nanotube material is obtained. In addition to having good dispersibility, the superstructure carbon nanotubes obtained by the above method have a surface formed by assembling nanosheets, and the rich ordered mesopores effectively suppress the skin current on the surface of the carbon nanotubes, thus significantly improving the impedance matching.

[0008] However, in the above two patents, only the nanosheets and nanotubes are simply mixed or assembled. The nanosheets and nanotubes can only play a common wave-absorbing role and cannot achieve frequency-selective wave absorption.

[0009] Based on the above problems, it is of great significance to develop a composite absorbing material with low density, frequency-selective wave-absorbing performance, good impedance matching performance and excellent wave-absorbing performance. Summary of the Invention

[0010] In view of the above technical problems, the present invention proposes a frequency-selective wave-absorbing composite absorbing material and a preparation method thereof. The prepared frequency-selective wave-absorbing composite absorbing material has low density, frequency-selective wave-absorbing performance, good impedance matching performance, and excellent wave-absorbing performance.

[0011] To solve the above technical problems, the present invention adopts the following technical solutions: A preparation method of a frequency-selective wave-absorbing composite absorbing material, including: ultrasonic dispersion, preparing a binder, high-speed shearing, and high-temperature sintering; In the ultrasonic dispersion, after mixing carbon nanotubes with a solvent and a surfactant, ultrasonic dispersion is carried out to obtain a carbon nanotube dispersion liquid, and the carbon nanotube dispersion liquid is dried to obtain pretreated carbon nanotubes; In the ultrasonic dispersion, the dosage ratio of carbon nanotubes, solvent, and surfactant is 1.8 - 2.2 g: 95 - 105 mL: 45 - 55 mg; The solvent is one of ethyl acetate, ethanol, and deionized water; The surfactant is one of polyvinylpyrrolidone or cetyltrimethylammonium bromide; The power of the ultrasonic dispersion treatment is 500 W, and the time is 15 min; Preferably, the surfactant is polyvinylpyrrolidone; Preferably, the model of the carbon nanotubes is XMF25, the length is 10 μm, and the diameter is 30 - 50 nm; In the preparation of the binder, the binder is mixed with deionized water to obtain a binder solution; In the preparation of the binder, the dosage ratio of the binder to deionized water is 6 - 10 g: 100 mL; The binder is one of polyvinyl alcohol, sodium carboxymethyl cellulose, and polyacrylamide; Preferably, the binder is polyvinyl alcohol with the model number PVA-1788; For the high-speed shearing, the pretreated carbon nanotubes, graphene nanosheets, and fumed silica are mixed to obtain a raw material; the raw material is mixed with a polyvinyl alcohol solution and then subjected to high-speed shearing to obtain a solid powder; In the high-speed shearing, the mass ratio of the pretreated carbon nanotubes, graphene nanosheets, and fumed silica is 2:5 - 50:48 - 93; The dosage ratio of the raw material to the polyvinyl alcohol solution is 20 g:3 - 10 mL; Preferably, the dosage ratio of the raw material to the polyvinyl alcohol solution is 20 g:5 mL; The shearing rate of the high-speed shearing is 8000 - 15000 r / min, and the shearing time is 1 - 4 min; Preferably, the shearing rate of the high-speed shearing is 10000 r / min, and the shearing time is 2 min; The processing amount of the raw material in the high-speed shearing is 10 - 40 g / time; Preferably, the processing amount of the raw material in the high-speed shearing is 20 g / time; The density of the graphene nanosheets is 2.1 - 2.3 g / cm 3 , and the thickness is 0.55 - 3.74 nm; The density of the fumed silica is 2.6 g / cm 3 ; For the high-temperature sintering, in a protective gas atmosphere, the solid powder is heated to the sintering temperature for high-temperature sintering to obtain a frequency-selective absorbing composite absorbing material; In the high-temperature sintering, the protective gas is one of nitrogen, helium, and neon; The heating rate when heating to the sintering temperature is 3 - 7 °C / min, the sintering temperature is 450 - 550 °C, and the high-temperature sintering time at the sintering temperature is 2 - 5 h; Preferably, the heating rate when heating to the sintering temperature is 5 °C / min, the sintering temperature is 500 °C, and the high-temperature sintering time at the sintering temperature is 2 h.

[0012] A frequency-selective absorbing composite absorbing material prepared by the aforementioned preparation method.

[0013] The composite microwave absorbing material with optional frequency absorption has the following microwave absorption principle: Using graphene nanosheets and carbon nanotubes as microwave absorbing agents, carbon nanotubes bridge graphene nanosheets to form a conductive network. Fumed silica is used as the main filling medium, and polyvinyl alcohol is used as the binder. The three materials are assembled by high-speed shearing to obtain microspheres of fumed silica / carbon nanotube-bridged graphene nanosheets. Different materials have different ways of losing electromagnetic waves, and they can be converted between polarization loss and dielectric loss, so that electromagnetic waves in different frequency bands are lost in different ways.

[0014] Compared with the prior art, the present invention has the following advantages and technical effects: (1) For the preparation method of the composite microwave absorbing material with optional frequency absorption of the present invention, a high-speed shearing preparation process is adopted. The process is simple, with lower cost, shorter preparation cycle, and is environmentally friendly and pollution-free. The high-speed shearing self-assembly method has unique advantages. By adjusting the size and rotation speed of the rotor, the size of the microspheres obtained by shearing can be regulated within a certain range. At the same time, the high-speed shearing method has the advantages of simple operation, low energy consumption, simplicity, and high efficiency, and can quickly complete the forming of particles, showing great potential in constructing the structure of composite materials. (2) For the preparation method of the composite microwave absorbing material with optional frequency absorption of the present invention, graphene nanosheets, carbon nanotubes, and fumed silica form a spherical structure through high-speed shearing self-assembly. Through the hierarchical filling method, the stability of the material is enhanced. Carbon nanotubes bridge graphene nanosheets to form a conductive network, and fumed silica is filled between graphene nanosheets to prevent the impedance matching of the microspheres from deteriorating due to excessive stacking. The conductive network bridged by graphene nanosheets and carbon nanotubes not only provides a way of conductance loss to lose electromagnetic waves, but also forms a variety of heterointerfaces, enhancing the interfacial polarization and the ability of the material to absorb electromagnetic waves. (3) For the preparation method of the composite microwave absorbing material with optional frequency absorption of the present invention, the microstructure and properties of the prepared composite material are beneficial to optional frequency absorption. By regulating the filling amount of fumed silica and the content of graphene nanosheets, it is found that the wave-transparent medium can not only fill the pore structure, but also adjust the absorption frequency band of the material by changing the proportion of fumed silica and graphene nanosheets. When the content of graphene nanosheets is low, it shows excellent microwave absorption performance in the low-frequency band. As the content of graphene nanosheets increases, it shows excellent microwave absorption performance in both the middle-frequency and high-frequency bands. (4) For the preparation method of the composite microwave absorbing material with optional frequency absorption of the present invention, highly conductive carbon nanotubes can play a role in bridging graphene nanosheets to form a three-dimensional network structure, and nano fumed silica with high wave-transparent function can be filled between graphene nanosheets to regulate impedance matching, thereby generating excellent electromagnetic wave absorption performance and reducing the density. (5) The frequency-selective absorbing composite absorbing material prepared by the present invention is detected by the coaxial method of a vector network analyzer (detection standard: GJB5239-2004). When the addition amount of the absorbing material is 30% of the total mass of the test absorbing ring, when the content of graphene nanosheets is low (the mass fraction of graphene nanosheets in the total mass of pretreated carbon nanotubes, graphene nanosheets, and fumed silica is 20%), it shows excellent absorbing performance in the low-frequency band (4.12 GHz). When the thickness is 3.7 mm, the reflection loss value is -46.53 dB, and the effective absorbing bandwidth is 4.84 GHz; when the content of graphene nanosheets is medium (the mass fraction of graphene nanosheets in the total mass of pretreated carbon nanotubes, graphene nanosheets, and fumed silica is 30%), it shows excellent absorbing performance in the medium-frequency band (9.64 GHz). When the thickness is 5.4 mm, the reflection loss value is -53.61 dB, and the effective absorbing bandwidth is 5.20 GHz; when the content of graphene nanosheets is high (the mass fraction of graphene nanosheets in the total mass of pretreated carbon nanotubes, graphene nanosheets, and fumed silica is 40%), it shows excellent absorbing performance in the high-frequency band (15.68 GHz). When the thickness is 4.1 mm, the reflection loss value is -42.31 dB, and the effective absorbing bandwidth is 4.50 GHz. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] The drawings constituting a part of this application are used to provide a further understanding of this application. The schematic embodiments of this application and their descriptions are used to explain this application and do not constitute an improper limitation to this application.

[0016] Figure 1 It is a performance test diagram of the frequency-selective absorbing composite absorbing material prepared in Example 1; Among them, Figure 1 (a) is a scanning electron microscope photograph of the surface of the fumed silica / carbon nanotube bridged graphene nanosheet microspheres prepared in Example 1; Figure 1 (b) is a scanning electron microscope photograph of the inside of the fumed silica / carbon nanotube bridged graphene nanosheet microspheres prepared in Example 1; Figure 1 (c) is a transmission electron microscope photograph of the fumed silica / carbon nanotube bridged graphene nanosheet microspheres prepared in Example 1; Figure 1 (d) is a schematic diagram of the absorbing performance of the fumed silica / carbon nanotube bridged graphene nanosheet microspheres prepared in Example 1 in the frequency range of 2-18 GHz. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0017] The technical solution of the present invention will be described below with reference to the accompanying drawings through specific embodiments. It should be understood that one or more steps mentioned in the present invention do not exclude the existence of other methods and steps before and after the combined steps, or other methods and steps can be inserted between these explicitly mentioned steps. It should also be understood that these examples are only used to illustrate the present invention and not to limit the scope of the present invention. Unless otherwise specified, the numbers of each method step are only for the purpose of identifying each method step, rather than restricting the arrangement order of each method or limiting the scope of implementation of the present invention. The change or adjustment of their relative relationship can also be regarded as the scope in which the present invention can be implemented under the condition of no substantial change in technical content.

[0018] Unless otherwise specified, the raw materials in the present invention are all obtained through commercial channels.

[0019] The fumed silica, graphene nanosheets, carbon nanotubes, etc. used in the embodiments of the present invention are all commercially available analytical pure products, and the glass instruments and equipment used are the commonly used instruments and equipment in the laboratory; The normal temperature or room temperature in the embodiments of the present invention refers to 25 ± 2 °C.

[0020] Example 1 A preparation method of a frequency-selective absorbing composite absorbing material is specifically as follows: 1. Pretreatment of carbon nanotubes First, take 2 g of carbon nanotubes, mix them with 100 ml of ethyl acetate and add 50 mg of polyvinylpyrrolidone, and perform ultrasonic dispersion treatment for 15 min using an ultrasonic device at room temperature to obtain a carbon nanotube dispersion liquid. Then, place the carbon nanotube dispersion liquid in a blast drying oven for blast drying to obtain pretreated carbon nanotubes; The model of the carbon nanotubes is XMF25, the length is 10 μm, and the diameter is 30 - 50 nm; The power of the ultrasonic dispersion treatment is 500 W; 2. Preparation of polyvinyl alcohol solution Use a magnetic stirrer as a homogenizing machine to prepare a dilute polyvinyl alcohol solution. Add 8 g of polyvinyl alcohol (polyvinyl alcohol: deionized water = 8 g: 100 mL) to a beaker containing deionized water, and continuously stir to obtain a polyvinyl alcohol solution.

[0021] The model of the polyvinyl alcohol is PVA - 1788; The polyvinyl alcohol solution mainly plays a binding role; 3. Preparation of fumed silica / carbon nanotube bridged graphene nanosheet microspheres At room temperature, 2 g of pretreated carbon nanotubes, 30 g of graphene nanosheets, and 68 g of fumed silica were mixed to obtain the raw materials; 20 g of the raw materials were mixed with 5 mL of polyvinyl alcohol solution, and then added to a high-speed shearer for high-speed shearing. The shearing rate of the high-speed shearer was controlled to 10,000 r / min, and the shearing time was 2 min. After the shearing was completed, the above shearing method was repeated until all the raw materials were processed, and then dried to obtain solid powder; then the solid powder was transferred to a tubular furnace, and in a nitrogen atmosphere, it was heated to 500 °C at a heating rate of 5 °C / min and sintered at 500 °C for 2 h to obtain fumed silica / carbon nanotube bridged graphene nanosheet microspheres (i.e., the optional frequency absorbing composite absorbing material) with a particle size of 15-20 μm; The density of the graphene nanosheets is 2.1-2.3 g / cm 3 , and the thickness is 0.55-3.74 nm; The density of the fumed silica is 2.6 g / cm 3 ; The fumed silica mainly plays a supporting role.

[0022] This example also provides an optional frequency absorbing composite absorbing material prepared by the aforementioned preparation method.

[0023] Using the coaxial method of a vector network analyzer for detection (detection standard GJB5239-2004), it was finally found that when the addition amount of the absorbing material was 30% of the total mass of the test absorbing ring, it was measured that it had the best microwave absorption in the C band. When the thickness was 5.4 mm, the reflection loss value was -53.61 dB (4.12 GHz), and the effective absorbing bandwidth was 5.2 GHz.

[0024] Scanning electron microscopy tests were performed on the surface and inside of the fumed silica / carbon nanotube bridged graphene nanosheet microspheres prepared in this example, and the scanning electron microscopy photos obtained are shown in Figure 1 (a) and Figure 1 (b); Transmission electron microscopy tests were performed on the fumed silica / carbon nanotube bridged graphene nanosheet microspheres prepared in this example, and the transmission electron microscopy photos obtained are shown in Figure 1 (c); The schematic diagram of the absorbing performance of the fumed silica / carbon nanotube bridged graphene nanosheet microspheres prepared in this example in the frequency range of 2-18 GHz is shown in Figure 1 (d). It can be seen from Figure 1 (d) that the reflection loss of the fumed silica / carbon nanotube bridged graphene nanosheet microspheres prepared in Example 1 at 9.64 GHz is -53.61 dB, and the corresponding thickness is 5.4 mm.

[0025] Example 2 Same as Example 1, except that in Step 3, the mass ratio of carbon nanotubes:graphene nanosheets:fumed silica is changed to 2:20:78, and the others are the same as in Example 1.

[0026] Detection was carried out by the coaxial method using a vector network analyzer (detection standard GJB5239-2004). Finally, it was found that when the addition amount of the wave-absorbing material was 30% of the total mass of the test wave-absorbing ring, the best microwave absorption in the X band was measured. When the thickness was 3.7 mm, the reflection loss value was -46.53 dB (9.64 GHz), and the effective wave-absorbing bandwidth was 4.84 GHz.

[0027] Example 3 Same as Example 1, except that in Step 3, the mass ratio of carbon nanotubes:graphene nanosheets:fumed silica is changed to 2:40:58, and the others are the same as in Example 1.

[0028] Detection was carried out by the coaxial method using a vector network analyzer (detection standard GJB5239-2004). Finally, it was found that when the addition amount of the wave-absorbing material was 30% of the total mass of the test wave-absorbing ring, the best microwave absorption in the Ku band was measured. When the thickness was 4.1 mm, the reflection loss value was -42.31 dB (15.68 GHz), and the effective wave-absorbing bandwidth was 4.50 GHz.

[0029] Example 4 Same as Example 1, except that in the polyvinyl alcohol solution prepared in Step 2, the ratio of polyvinyl alcohol to deionized water is 24 g:100 mL, and the others are the same as in Example 1.

[0030] The fumed silica / carbon nanotube bridged graphene nanosheet composite material obtained in Step 3 did not form relatively uniform spheres.

[0031] Detection was carried out by the coaxial method using a vector network analyzer (detection standard GJB5239-2004). Finally, it was found that when the addition amount of the wave-absorbing material was 30% of the total mass of the test wave-absorbing ring, the best microwave absorption in the Ku band was measured. When the thickness was 2.73 mm, the reflection loss value was -40.96 dB (12.96 GHz), and the effective wave-absorbing bandwidth was 4.93 GHz.

[0032] Example 5 Same as Example 1, except that the shear rate in the high-speed shearing in Step 3 is 3000 r / min, and the others are the same as in Example 1. After high-speed shearing, the three materials did not achieve high-speed shearing into particles and did not reach the expected structure.

[0033] Detection was carried out by the coaxial method using a vector network analyzer (detection standard: GJB5239-2004). Finally, it was found that when the addition amount of the wave-absorbing material was 30% of the total mass of the test wave-absorbing ring, the best microwave absorption in the X band was measured. When the thickness was 1.98 mm, the reflection loss value was -30.38 dB (10.8 GHz), and the effective wave-absorbing bandwidth was 3.81 GHz.

[0034] Example 6 Same as Example 1, except that in step 3, the temperature of high-temperature sintering was increased from 500 °C to 550 °C, and the others were the same as in Example 1. The obtained silica / carbon nanotube-bridged graphene nanosheet microspheres had the same microscopic morphology as the silica / carbon nanotube-bridged graphene nanosheet microspheres in Example 1.

[0035] Detection was carried out by the coaxial method using a vector network analyzer (detection standard: GJB5239-2004). Finally, it was found that when the addition amount of the wave-absorbing material was 30% of the total mass of the test wave-absorbing ring, the best microwave absorption in the C band was measured. When the thickness was 2.69 mm, the reflection loss value was -42.83 dB (6.91 GHz), and the effective wave-absorbing bandwidth was 4.12 GHz.

[0036] Example 7 Same as Example 1, except that in step 3, the shearing time was shortened from 2 min to 1 min, and the others were the same as in Example 1. The obtained silica / carbon nanotube-bridged graphene nanosheet microspheres had a larger particle size, reaching 30 μm, which was different from the expectation.

[0037] Detection was carried out by the coaxial method using a vector network analyzer (detection standard: GJB5239-2004). Finally, it was found that when the addition amount of the wave-absorbing material was 30% of the total mass of the test wave-absorbing ring, the best microwave absorption in the C band was measured. When the thickness was 3.14 mm, the reflection loss value was -36.77 dB (5.69 GHz), and the effective wave-absorbing bandwidth was 3.43 GHz.

[0038] Example 8 Same as Example 1, except that in step 3 during heat treatment, the atmosphere in the tubular furnace was changed from a nitrogen atmosphere to an argon atmosphere, and the others were the same as in Example 1. The obtained silica / carbon nanotube-bridged graphene nanosheet microspheres were the same as the silica / carbon nanotube-bridged graphene nanosheet microspheres in Example 1.

[0039] Detection was carried out using the coaxial method of a vector network analyzer (detection standard: GJB5239-2004). Finally, it was found that when the addition amount of the microwave absorbing material was 30% of the total mass of the test microwave absorbing ring, the best microwave absorption in the X band was measured. When the thickness was 2.55 mm, the reflection loss value was -42.68 dB (10.5 GHz), and the effective microwave absorption bandwidth was 2.96 GHz.

[0040] Comparative Example 1 The difference from Example 1 was that in the second step, epoxy resin was used in equal mass to replace polyvinyl alcohol as the binder, and the dosage remained unchanged. Other process steps were the same as those in Example 1. During high-speed shearing, due to the too high viscosity of epoxy resin, the materials could not be connected during shear granulation, and finally, the obtained powder did not have the structure of carbon nanotube-bridged graphene nanosheets.

[0041] Comparative Example 2 The difference from Example 1 was that when the solid powder was sintered at high temperature in the third step, the gas atmosphere was changed from a nitrogen atmosphere to an oxygen atmosphere, and the rest of the operations were the same as those in Example 1.

[0042] The detection method was the same as that in Example 1. It was measured that when the silica-coated graphene composite microwave absorbing material prepared in this comparative example accounted for 30% of the total mass of the test microwave absorbing ring and the thickness was 4.06 mm, the reflection loss value was -29.01 dB (12.88 GHz), and the effective microwave absorption bandwidth was 4.21 GHz.

[0043] The above results showed that in an oxygen atmosphere, carbon nanotubes and graphene nanosheets would be oxidized during heat treatment, resulting in a reduction in the microwave absorbing agent in the gas-phase silica / carbon nanotube-bridged graphene nanosheet microspheres and a decrease in the microwave absorption performance of the material.

[0044] Comparative Example 3 The difference from Example 1 was that silicon nitride crystals were selected to replace gas-phase silica as the wave-transparent material in equal mass to improve the impedance matching of the carbon nanotube-bridged graphene nanosheet structure. However, during shear granulation, the silicon nitride crystals could not effectively fill between the graphene nanosheet layers, and the impedance matching of the carbon nanotube-bridged graphene nanosheet structure could not be improved.

[0045] The silicon nitride crystals were β-silicon nitride, with an aspect ratio of 1:10 - 15, a length of 2 - 3 μm, and a density of 3.12 g / cm 3 .

[0046] The detection method was the same as that in Example 1. When the silicon nitride / carbon nanotube bridged graphene nanoplate composite absorbing material prepared in this comparative example accounted for 30% of the total mass of the test absorbing ring and had a thickness of 3.75 mm, the reflection loss value was -36.47 dB (9.56 GHz), and the effective absorbing bandwidth was 2.79 GHz.

[0047] Comparative Example 4 The difference from Example 1 was that silicon carbide nanowires were selected to replace carbon nanotubes in equal mass as the material for bridging graphene nanoplate. After high-speed shearing, the silicon carbide nanowires could not effectively connect the graphene nanoplate, and the stacking phenomenon of the graphene nanoplate was serious, resulting in poor impedance matching and weak absorbing ability. The length of the silicon carbide nanowires was 20 μm.

[0048] The detection method was the same as that in Example 1. When the silicon dioxide / carbon nanotube bridged graphene nanoplate composite absorbing material prepared in this comparative example accounted for 30% of the total mass of the test absorbing ring and had a thickness of 2.38 mm, the reflection loss value was -23.19 dB (6.67 GHz), and the effective absorbing bandwidth was 3.58 GHz.

[0049] The absorbing abilities of the optional frequency absorbing composite absorbing materials obtained in Examples 1-8 and Comparative Examples 1-4 were summarized, and the summary results were as follows:

[0050] It can be seen from the above results that the silicon dioxide / carbon nanotube bridged graphene nanoplate microspheres prepared in Examples 1-3 all had strong absorbing abilities.

[0051] The above is only the preferred specific implementation manner of the present application, but the protection scope of the present application is not limited thereto. Any changes or substitutions that can be easily thought of by those skilled in the art within the technical scope disclosed in the present application should be covered by the protection scope of the present application. Therefore, the protection scope of the present application should be subject to the protection scope of the claims.

Claims

1. A method for preparing a composite wave absorbing material capable of selective frequency absorption, characterized in that: include: Ultrasonic dispersion, binder preparation, high-speed shearing, high-temperature sintering; The ultrasonic dispersion comprises mixing the carbon nanotubes with a solvent and a surfactant, and then ultrasonically dispersing the mixture to obtain a carbon nanotube dispersion liquid, and drying the carbon nanotube dispersion liquid to obtain pretreated carbon nanotubes; The binder is prepared by mixing the binder with deionized water to obtain a binder solution; The high-speed shearing is to mix the pretreated carbon nanotubes, graphene nanosheets and gas-phase silicon oxide to obtain raw materials; and the raw materials are mixed with a polyvinyl alcohol solution and then subjected to high-speed shearing to obtain a solid powder.

2. The method for preparing the frequency-selective wave-absorbing composite absorbing material according to claim 1, characterized in that: In the ultrasonic dispersion, the usage ratio of carbon nanotubes, solvent and surfactant is 1.8-2.2 g:95-105 mL:45-55 mg.

3. The method for preparing the frequency-selective wave-absorbing composite absorbing material according to claim 1, characterized in that: In the ultrasonic dispersion, the solvent is one of ethyl acetate, ethanol and deionized water; The surfactant is one of polyvinyl pyrrolidone or hexadecyl trimethyl ammonium bromide; The power of the ultrasonic dispersion treatment is 500W and the time is 15min; The carbon nanotubes have a model of XMF25, a length of 10 μm, and a diameter of 30-50 nm.

4. The method for preparing a frequency-selective wave-absorbing composite absorbing material according to claim 1, characterized in that: In the prepared binder, the ratio of binder to deionized water is 6-10 g:100 mL; The binder is one of polyvinyl alcohol, sodium carboxymethyl cellulose and polyacrylamide.

5. The method for preparing a frequency-selective wave-absorbing composite absorbing material according to claim 1, characterized in that: In the high-speed shearing, the mass ratio of the pretreated carbon nanotubes, graphene nanosheets, and fumed silicon oxide is 2:5-50:48-93; The dosage ratio of the raw material to the polyvinyl alcohol solution is 20g:3-10mL; The shear rate of the high-speed shearing is 8000-15000 r / min, and the shearing time is 1-4 min; The processing amount of the raw materials in the high-speed shearing is 10-40g / time.

6. The method for preparing a frequency-selective wave-absorbing composite absorbing material according to claim 1, characterized in that: The high temperature sintering is to heat the solid powder to a sintering temperature in a protective gas atmosphere and perform high temperature sintering to obtain a composite wave absorbing material with selectable frequency.

7. The method for preparing a frequency-selective wave-absorbing composite absorbing material according to claim 6, characterized in that: During the high temperature sintering, the protective gas is one of nitrogen, helium and neon; The heating rate when heating to the sintering temperature is 3-7°C / min, the sintering temperature is 450-550°C, and the high-temperature sintering time at the sintering temperature is 2-5h.

8. A frequency-selective wave-absorbing composite absorbing material prepared by the preparation method according to any one of claims 1 to 7.

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