A carbon composite microwave absorbing material and its preparation method

Abstract

This invention belongs to the field of microwave absorbing materials technology, specifically relating to a carbon composite microwave absorbing material and its preparation method. The invention uses degummed waste filter cotton as a carbon source, employing an impregnation method to impregnate the pretreated waste filter cotton in magnetic metal salts, followed by heat treatment to obtain the composite microwave absorbing material. The carbon skeleton formed by the heat-treated waste filter cotton constitutes a conductive network in the composite material, exhibiting good dielectric loss. Magnetic metal elements and their alloys possess high saturation magnetization, providing magnetic loss in the composite material. The synergistic effect of dielectric and magnetic losses solves the problem of poor impedance matching in single carbon materials. The preparation process of this invention is simple and pollution-free, solving the environmental pollution problems caused by traditional waste filter cotton treatment methods, and is easy for large-scale production. The resulting composite microwave absorbing material is lightweight, efficient, has good impedance matching, and possesses excellent microwave absorption performance.

Description

Technical Field

[0001] This invention belongs to the field of microwave absorbing materials technology, specifically relating to a carbon composite microwave absorbing material and its preparation method. Background Technology

[0002] With the changing seasons, the frequent use of air conditioners in high-speed trains, cars, homes, and commercial buildings generates a large amount of waste filter cotton. Waste filter cotton has a fibrous network structure, and its main component is polyester fiber. It is a high-molecular-weight waste that is not easily broken and readily produces large amounts of smoke and harmful gases when burned in air. The large quantities of waste filter cotton, when piled up and stored, occupy space, not only impacting the environment but also posing safety management risks due to the storage of lightweight, floating debris. Therefore, a scientific, effective, and harmless method is needed to treat waste filter cotton, avoid its impact on the environment and human health, and achieve resource-based recycling.

[0003] Currently, the main method for treating waste filter cotton is to send it to recycling facilities. This primarily includes mechanical separation and high-temperature incineration. Mechanical separation involves reprocessing and reusing the fiber materials in the waste filter cotton, but its overall utilization rate is not high, mainly focusing on low-value-added applications. High-temperature incineration uses high temperatures to burn off the combustible materials in the waste filter cotton, and then landfills the resulting waste. However, the waste gas and waste residue produced by high-temperature incineration pose certain environmental hazards. Therefore, while seeking harmless treatment and reuse of waste filter cotton, multiple aspects such as rationality, safety, and high added value should be comprehensively considered.

[0004] With the rapid advancement of information technology, electromagnetic interference and radiation generated by wireless communication devices have posed a certain threat to human health. Microwave-absorbing materials can convert electromagnetic radiation into heat energy, effectively weakening the impact of electromagnetic waves. They have attracted much attention because they can effectively protect human health and ensure the normal operation of electronic devices. In recent years, carbon-based composite materials, due to their low density, high conductivity, and numerous interface advantages, can efficiently absorb electromagnetic waves and are considered promising microwave absorbing materials. Despite their many advantages, the expensive and complex synthesis process of carbon materials (such as graphene and carbon nanotubes) severely limits their large-scale application.

[0005] In summary, designing a solution that utilizes waste filter cotton as a raw material to prepare low-cost and high-value-added microwave absorbing composite materials would not only avoid the harm of waste filter cotton to the environment and human health, but also realize the high-value resource utilization of waste filter cotton, which would be of great significance. Summary of the Invention

[0006] The purpose of this invention is to provide a method for preparing a carbon composite microwave absorbing material. This method can recycle waste filter cotton, achieving high-value utilization, reducing harm to the environment and human health, and exhibiting excellent microwave absorption performance.

[0007] To achieve the above objectives, the present invention adopts the following technical solution: a method for preparing a carbon composite microwave absorbing material, comprising the following steps:

[0008] S1. Cut the waste filter cotton into small pieces, soak it in an alkaline solution and treat it with a water bath. Then take out the treated waste filter cotton, wash it several times with deionized water until it is neutral, and dry it to obtain the pretreated waste filter cotton.

[0009] S2. Prepare a magnetic metal salt solution, add the pretreated waste filter cotton, let it stand to impregnate evenly, then take it out and dry it to obtain the precursor.

[0010] S3. Place the precursor in a protective atmosphere and sinter at 600℃-900℃ for 1-2 hours to obtain the composite microwave absorbing material.

[0011] Further improvements were made to the preparation method of carbon composite microwave absorbing materials:

[0012] Preferably, the alkaline solution is one of sodium hydroxide and potassium hydroxide, with a concentration of 5wt%-10wt%.

[0013] Preferably, the water bath temperature is 80-100℃ and the time is 6-10h, and the drying temperature is 40-80℃ and the time is 4-10h.

[0014] Preferably, the concentration of the magnetic metal salt is 0.1-1 mol / L, and the amount of pretreated waste filter cotton added to the magnetic metal salt solution is 0.01-0.1 g / ml.

[0015] Preferably, the magnetic metal salt is one or a combination of two or more of cobalt salt, iron salt, and nickel salt.

[0016] Preferably, when the magnetic metal salt is a combination of two salts, the molar ratio of the two salts is 0.1-10.

[0017] Preferably, in step S2, the soaking time is 6-24 hours, the drying temperature is 40-60°C, and the time is 8-12 hours.

[0018] Preferably, in step S3, the temperature is increased to the sintering temperature using a gradient heating method. Specifically, the temperature is increased from room temperature to 300-400°C at a heating rate of 5-10°C / min, and then increased to the sintering temperature at a heating rate of 5-10°C / min. The protective atmosphere is argon or an inert atmosphere.

[0019] The second objective of this invention is to provide a carbon composite absorbing material prepared by any of the above-mentioned methods.

[0020] Further improvements to carbon composite microwave absorbing materials:

[0021] Preferably, the carbon composite absorbing material is used in the fields of electromagnetic wave shielding and electromagnetic wave absorption.

[0022] The advantages of this invention compared to the prior art are as follows:

[0023] 1) This invention provides a method for preparing carbon composite microwave absorbing material, which uses waste filter cotton as raw material, soaks it in an alkaline solution for water bath treatment to remove the adhesive on the surface of the waste filter cotton, then impregnates it in a magnetic metal salt solution, and then sintersects it at high temperature to obtain carbon composite microwave absorbing material.

[0024] The superior electromagnetic wave absorption performance of composite absorbing materials stems from the uniform loading of magnetic metal ions onto the surface of waste filter cotton during the preparation process. This waste filter cotton, loaded with magnetic metal ions, is then sintered in an inert atmosphere, causing carbonization. Simultaneously, the magnetic metal ions undergo a carbothermic reduction reaction, transforming into nano-metal elemental or alloy particles loaded onto the carbon surface. The appropriate amount of metal elemental or alloy particles on the surface of the composite absorbing material effectively improves electromagnetic impedance matching. Furthermore, the abundant heterogeneous interfaces in the composite absorbing material allow for the formation of numerous interfacial polarizations, and the polar groups and defects in the carbon matrix can lead to dipole polarization. These factors all contribute to enhancing the absorption performance of the composite absorbing material.

[0025] This invention utilizes an impregnation method combined with heat treatment to prepare composite microwave absorbing materials. The preparation process is simple, easy to implement, and low in cost. The impregnation solution used in the product preparation process can be recycled, and no complex synthesis equipment or organic solvents are required, making it suitable for industrial production.

[0026] 2) The composite absorbing material prepared by this invention not only has a thin matching layer, a wide effective absorption bandwidth, and strong microwave absorption, with absorption capabilities almost covering the X-band and Ku-band, but its electromagnetic parameters and absorption performance can also be controlled by changing the concentration of the metal salt solution in the impregnation solution, sintering temperature or time, and the thickness of the matching layer to meet the needs of different application environments. Minimum reflection loss RL min Achieving a flux density of -67.46 dB and a maximum effective absorption bandwidth of 5.20 GHz, this invention utilizes readily available and inexpensive waste filter cotton as raw material, and its preparation process is pollution-free. This low-cost, high-efficiency composite absorbing material can be widely used in electromagnetic shielding of wireless communication devices, reducing signal reflection in radar systems, and constructing stealth materials for aerospace vehicles. Attached Figure Description

[0027] Figure 1This is the XRD pattern of the CoFe / C composite microwave absorbing material prepared in Example 4;

[0028] Figure 2 These are the SEM images and energy dispersive spectroscopy (EDS) images of the CoFe / C composite microwave absorbing material prepared in Example 4;

[0029] Figure 3 This is a diagram of the electromagnetic parameters of the CoFe / C composite absorbing material prepared in Example 4;

[0030] Figure 4 This is a 3D diagram of the reflection loss of the CoFe / C composite absorbing material prepared in Example 4. Detailed Implementation

[0031] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to embodiments. Obviously, the described embodiments are only some, not all, of the embodiments of this invention. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without creative effort are within the scope of protection of this invention. The waste filter cotton used in the following embodiments and comparative examples all originated from waste filter cotton generated during the maintenance of high-speed trains at the Nanjing Branch of Shanghai Railway Locomotive & Rolling Stock Development Co., Ltd. The air conditioning filter cotton was manufactured by Shanghai Railway Rolling Stock High-Tech Materials Jiangsu Co., Ltd., and the models were XK-200-L15, XK-200-L20, and XK-380B.

[0032] Example 1

[0033] This embodiment provides a method for preparing a carbon composite microwave absorbing material, which specifically includes the following steps:

[0034] S1. Take waste filter cotton (model XK-200-L15) and cut it into small pieces. Soak it in 5wt% NaOH solution. Then place the soaking solution in a 90℃ water bath for 6 hours. Take it out and wash it with deionized water until neutral. Place it in a 60℃ forced-air drying oven for 5 hours to obtain pretreated waste filter cotton.

[0035] S2. Add 10.1g of ferric nitrate nonahydrate to 50mL of deionized water and stir magnetically until completely dissolved to obtain a magnetic metal salt solution with a concentration of 0.5mol / L. Add 1.0g of pretreated waste filter cotton, let it stand and soak for 24h, then take it out and dry it. After filtration, put it in a 60℃ forced-air drying oven and dry for 10h to obtain the precursor.

[0036] S3. Place the precursor in a crucible, place the crucible in a tube furnace, and heat it from room temperature to 300℃ at a heating rate of 5℃ / min under a nitrogen atmosphere. Then heat it to 700℃ at the same heating rate. Hold it at this temperature for 2 hours and let it cool naturally to room temperature to obtain the Fe / C composite microwave absorbing material.

[0037] Using the coaxial method, the obtained composite absorbing material was mixed with paraffin wax at a mass ratio of 5.5:4.5 at 80℃ until homogeneous. The mixture was then pressed into paraffin rings of thickness d using a standard mold. The absorbing performance of paraffin rings of different thicknesses was calculated based on the electromagnetic parameters of the material using an Agilent vector network analyzer. The results are as follows: Minimum reflection loss RL was achieved at a matching thickness d of 1.7 mm. min The effective absorption bandwidth (RL < -10dB) is 3.12GHz, with a matching thickness d of 1.3mm. At this thickness, the effective absorption bandwidth reaches its maximum of 4.28GHz, and the minimum reflection loss RL is [value missing]. min It is -25.99dB.

[0038] Example 2

[0039] This embodiment provides a method for preparing a carbon composite microwave absorbing material. The specific steps are the same as in Example 1, except that in step S2, 7.27 g of nickel nitrate hexahydrate is added to 50 mL of deionized water and magnetically stirred until completely dissolved to form a magnetic metal salt solution with a concentration of 0.5 mol / L. The Ni / C composite microwave absorbing material is then obtained.

[0040] The obtained composite absorbing material was mixed with paraffin wax at a mass ratio of 5.5:4.5 at 80℃ until homogeneous. The mixture was then pressed into paraffin rings of a specific thickness using a standard mold. The absorbing performance of the paraffin rings with different thicknesses was calculated using an Agilent vector network analyzer based on the material's electromagnetic parameters. The results are as follows: Minimum reflection loss RL was achieved at a matching thickness d of 1.5 mm. min It has a strength of -56.82 dB and an effective absorption bandwidth of 4.4 GHz.

[0041] Example 3

[0042] This embodiment provides a method for preparing a carbon composite microwave absorbing material. The specific steps are the same as in Example 1, except that in step S2, 7.28 g of cobalt nitrate hexahydrate is added to 50 mL of deionized water and magnetically stirred until completely dissolved to form a magnetic metal salt solution with a concentration of 0.5 mol / L. The final Co / C composite microwave absorbing material is thus obtained.

[0043] The obtained composite absorbing material was mixed with paraffin wax at a mass ratio of 5.5:4.5 at 80℃ until homogeneous. The mixture was then pressed into paraffin rings of a specific thickness using a standard mold. The absorbing performance of the paraffin rings with different thicknesses was calculated based on the electromagnetic parameters of the material using an Agilent vector network analyzer. The results are as follows: Minimum reflection loss RL was achieved at a matching thickness d of 3.3 mm. min The effective absorption bandwidth is -47.39 dB, with a matching thickness d of 1.72 GHz. At a matching thickness d of 1.6 mm, the effective absorption bandwidth reaches its maximum of 5.20 GHz, with the minimum reflection loss RL. min It is -21.92dB.

[0044] Example 4

[0045] This embodiment provides a method for preparing a carbon composite microwave absorbing material. The specific steps are the same as in Example 1, except that in step S2, 7.28 g of cobalt nitrate hexahydrate and 10.1 g of ferric nitrate nonahydrate are added to 50 mL of deionized water and magnetically stirred until completely dissolved to form a magnetic metal salt solution. The molar ratio of the two salts is 1:1, and the total concentration is 1.0 mol / L. The final CoFe / C composite microwave absorbing material is obtained.

[0046] The obtained composite absorbing material was mixed with paraffin wax at a mass ratio of 5.5:4.5 at 80℃ until homogeneous. The mixture was then pressed into paraffin rings of a specific thickness using a standard mold. The absorbing performance of the paraffin rings with different thicknesses was calculated based on the electromagnetic parameters of the material using an Agilent vector network analyzer. The results are as follows: Minimum reflection loss RL was achieved at a matching thickness d of 1.7 mm. min The effective absorption bandwidth is -67.46 dB, with a matching thickness d of 1.5 mm. At this thickness, the effective absorption bandwidth reaches its maximum of 4.44 GHz, with the minimum reflection loss RL. min It is -39.29dB.

[0047] Example 5

[0048] This embodiment provides a method for preparing a carbon composite microwave absorbing material, which specifically includes the following steps:

[0049] S1. Take waste filter cotton (model XK-200-L20) and cut it into small pieces. Soak it in 10wt% NaOH solution. Then place the soaking solution in a 100℃ water bath for 6 hours. Take it out and wash it with deionized water until neutral. Place it in an 80℃ forced-air drying oven for 4 hours to obtain pretreated waste filter cotton.

[0050] S2. Add 7.27g of nickel nitrate hexahydrate and 10.1g of ferric nitrate nonahydrate to 50mL of deionized water and stir magnetically until completely dissolved to form a magnetic metal salt solution. The molar ratio of the two salts is 1:1 and the total concentration is 1.0mol / L. Add 5.0g of pretreated waste filter cotton, let it stand and soak for 10h, then take it out and dry it. After filtration, put it in a 60℃ forced-air drying oven and dry for 8h to obtain the precursor.

[0051] S3. Place the precursor in a crucible, place the crucible in a tube furnace, and heat it from room temperature to 350°C at a heating rate of 8°C / min under a nitrogen atmosphere. Then heat it to 900°C at a heating rate of 8°C / min. Hold the temperature at this temperature for 1 hour and let it cool naturally to room temperature to obtain the NiFe / C composite microwave absorbing material.

[0052] The obtained composite absorbing material was mixed with paraffin wax at a mass ratio of 5.5:4.5 at 80℃ until homogeneous. The mixture was then pressed into paraffin rings of a specific thickness using a standard mold. The absorbing performance of the paraffin rings with different thicknesses was calculated based on the electromagnetic parameters of the material using an Agilent vector network analyzer. The results are as follows: Minimum reflection loss RL was achieved at a matching thickness d of 1.8 mm. min The effective absorption bandwidth is -53.17dB, with a matching thickness d of 1.5mm. At this thickness, the effective absorption bandwidth reaches its maximum of 4.8GHz, with the minimum reflection loss RL. min It is -32.21dB.

[0053] Example 6

[0054] This embodiment provides a method for preparing a carbon composite microwave absorbing material, which specifically includes the following steps:

[0055] S1. Take waste filter cotton (model XK-380B) and cut it into small pieces. Soak it in 8wt% NaOH solution. Then place the soaking solution in a 90℃ water bath for 8 hours. Take it out and wash it with deionized water until neutral. Place it in a 70℃ forced-air drying oven for 5 hours to obtain pretreated waste filter cotton.

[0056] S2. Add 7.27g of nickel nitrate hexahydrate and 7.28g of cobalt nitrate nonahydrate to 50mL of deionized water and stir magnetically until completely dissolved to form a magnetic metal salt solution. The molar ratio of the two salts is 1:1 and the total concentration is 1.0mol / L. Add 2.0g of pretreated waste filter cotton, let it stand and soak for 6h, then take it out and dry it. After filtration, put it in a 50℃ forced-air drying oven and dry for 10h to obtain the precursor.

[0057] S3. Place the precursor in a crucible, place the crucible in a tube furnace, and heat it from room temperature to 400℃ at a heating rate of 6℃ / min under a nitrogen atmosphere. Then heat it to 800℃ at the same heating rate. Hold the temperature at this temperature for 1.5h and let it cool naturally to room temperature to obtain the NiCo / C composite microwave absorbing material.

[0058] The obtained composite absorbing material was mixed with paraffin wax at a mass ratio of 5.5:4.5 at 80℃ until homogeneous. The mixture was then pressed into paraffin rings of a specific thickness using a standard mold. The absorbing performance of the paraffin rings with different thicknesses was calculated based on the electromagnetic parameters of the material using an Agilent vector network analyzer. The results are as follows: The minimum reflection loss RL was achieved at a matching thickness d of 3.7 mm. min The effective absorption bandwidth is -57.77dB, with a matching thickness d of 1.96GHz. At a matching thickness d of 1.2mm, the effective absorption bandwidth reaches its maximum of 2.44GHz, with the minimum reflection loss RL. min It is -14.39dB.

[0059] Comparative Example 1

[0060] This comparative example provides a method for preparing carbon microwave absorbing material. The specific steps are the same as in Example 1, except that the waste filter cotton after pretreatment in step S1 is directly subjected to the operation in step S3 to obtain the waste filter cotton-derived carbon microwave absorbing material.

[0061] The obtained absorbing material was mixed with paraffin at a mass ratio of 5.5:4.5 at 80°C and then pressed into paraffin rings of a certain thickness using a standard mold. The absorbing performance of paraffin rings of different thicknesses was calculated based on the electromagnetic parameters of the material using an Agilent vector network analyzer. The results are as follows: the absorbing performance showed no effective waveband.

[0062] Comparative Example 2

[0063] This comparative example provides a method for preparing a carbon composite microwave absorbing material. The specific steps are the same as in Example 4, except that in this comparative example, the carbon microwave absorbing material derived from waste filter cotton is prepared by sintering at 600℃ for 2 hours.

[0064] The obtained composite absorbing material was mixed with paraffin at a mass ratio of 5.5:4.5 at 80°C and then pressed into paraffin rings of a certain thickness using a standard mold. The absorbing performance of paraffin rings of different thicknesses was calculated based on the electromagnetic parameters of the material using an Agilent vector network analyzer. The results are as follows: the absorbing performance showed no effective waveband.

[0065] Table 1 Comparison of microwave absorption performance between Examples 1-7 and the comparative examples

[0066] <![CDATA[RL min (dB)]]> Widest effective absorption bandwidth (GHz) Example 1 -46.26 4.28 Example 2 -56.82 4.40 Example 3 -47.39 5.20 Example 4 -67.46 4.44 Example 5 -53.17 4.8 Example 6 -57.77 2.44 Comparative Example 1 -9.53 0 Comparative Example 2 -5.91 0

[0067] As shown in Table 1, comparing the microwave absorption performance of Examples 1-6 with Comparative Examples 1-2, the waste filter cotton pretreatment followed by impregnation with loaded magnetic metal ions to obtain a precursor, which was then sintered to obtain the composite microwave absorbing material, exhibits excellent microwave absorption performance. This is because the waste filter cotton carbonizes after heat treatment to form a carbon conductive network structure, and magnetic metal ions form nano-metal elemental or alloy particles on the carbon surface. Appropriate content of nano-metal elemental or alloy particles can effectively improve electromagnetic impedance matching. Furthermore, the intertwining of the carbon conductive network structure not only induces multiple scattering and reflection of the incident electromagnetic waves, prolonging the dissipation path of electromagnetic wave energy, but more importantly, a large number of interfaces are formed between the components of the composite microwave absorbing material, promoting space charge transfer and leading to interface polarization. Polar groups and defects in the carbon matrix can act as polarization centers for dipoles, leading to dipole polarization and thus effectively improving the microwave absorption performance of the composite material. In contrast, the materials prepared in Comparative Examples 1-2 have poor microwave absorption performance. This is because the material obtained in Comparative Example 1 is a single carbon material with a high dielectric constant, resulting in poor impedance matching. This causes electromagnetic waves to be strongly reflected on its surface and cannot enter the material for effective absorption, thus resulting in poor wave absorption performance. In contrast, the sintering temperature in Comparative Example 2 is lower than the graphitization temperature of waste filter cotton, resulting in a material with poor conductivity and low dielectric loss, which makes it impossible for the material to effectively attenuate electromagnetic waves.

[0068] Figure 1 This is the XRD pattern of the composite absorbing material prepared in Example 4 of this invention; Figure 2 These are the SEM images and energy spectrum diagrams of the composite absorbing material prepared in Example 4 of this invention; Figure 3 This is a diagram showing the electromagnetic parameters of the composite absorbing material prepared in Example 4 of this invention; Figure 4 This is a 3D diagram of the reflection loss of the composite absorbing material prepared in Example 4 of the present invention.

[0069] Combination Figure 1 and Figure 2 The test results show that after heat treatment, the waste filter cotton is converted into C and Co. 2+ and Fe 3+ It is converted into a CoFe alloy through carbothermal reduction and uniformly loaded on the C surface.

[0070] Depend on Figure 3 The test results of the composite absorbing material prepared in Example 4 show that the real part ε' of the dielectric constant decreases with increasing frequency, which can be attributed to the influence of the dipole on the dielectric constant in the high-frequency range and the enhanced dispersion effect. The real part μ' and imaginary part μ" of the permeability are in the range of 1 and 0, respectively, exhibiting weak magnetism. The loss tangent value shows that the dielectric loss tangent tanδ... ε Greater than the magnetic loss tangent tanδμ This indicates that the microwave absorption performance of the composite microwave absorbing material prepared in Example 4 mainly depends on the dielectric loss.

[0071] Depend on Figure 4 The test results of the composite absorbing material prepared in Example 4 show that the composite absorbing material has excellent microwave absorption performance, and the minimum reflection loss RL is achieved when the matching thickness d is 1.7 mm. min Its value is -67.46 dB; similarly, the composite absorbing material prepared in Example 1 was tested, and the minimum reflection loss RL was achieved when the matching thickness was 1.7 mm. min Its value is -46.26 dB; similarly, the composite absorbing material prepared in Example 2 was tested, and the minimum reflection loss RL was achieved when the matching thickness was 1.5 mm. min Its value is -56.82dB; similarly, the composite absorbing material prepared in Example 3 was tested, and the minimum reflection loss RL was achieved when the matching thickness was 3.3mm. min Its value is -47.39 dB; similarly, the composite absorbing material prepared in Example 5 was tested, and the minimum reflection loss RL was achieved when the matching thickness was 2.4 mm. min Its value is -46.17 dB; similarly, the composite absorbing material prepared in Example 6 was tested, and the minimum reflection loss RL was achieved when the matching thickness was 1.8 mm. min Its value is -53.17 dB; similarly, the composite absorbing material prepared in Example 7 was tested, and the minimum reflection loss RL was achieved when the matching thickness was 3.7 mm. min The value is -57.77 dB. Test results prove that the composite absorbing materials prepared in Examples 1-7 can absorb more than 99.99% of electromagnetic waves.

[0072] Those skilled in the art should understand that the above descriptions are merely several specific embodiments of the present invention, and not all embodiments. It should be noted that many modifications and improvements can be made by those skilled in the art, and all modifications or improvements not exceeding the scope of the claims should be considered within the protection scope of the present invention.

Claims

1. A method for preparing a carbon composite microwave absorbing material, characterized in that, Includes the following steps: S1. Cut the waste filter cotton from the air conditioning filter system into small pieces, soak them in an alkaline solution and treat them with a water bath. Then, take out the treated waste filter cotton, wash it several times with deionized water until neutral, and dry it to obtain pretreated waste filter cotton. The alkaline solution is one of sodium hydroxide and potassium hydroxide, with a concentration of 5wt%-10wt%. The water bath temperature is 80-100 ℃ and the time is 6-10 h. The drying temperature is 40-80 ℃ and the time is 4-10 h. S2. Prepare a magnetic metal salt solution, add pretreated waste filter cotton, let it stand and soak evenly, then take it out and dry it to obtain the precursor; the magnetic metal salt is one or more of cobalt salt, iron salt and nickel salt, the concentration of the magnetic metal salt solution is 0.1-1 mol / L, and the amount of pretreated waste filter cotton added to the magnetic metal salt solution is 0.01-0.1 g / ml; S3. Place the precursor in a protective atmosphere and sinter at 600 ℃-900 ℃ for 1-2 h to obtain the composite microwave absorbing material.

2. The method for preparing the carbon composite microwave absorbing material according to claim 1, characterized in that, When the magnetic metal salt is a combination of two salts, the molar ratio of the two salts is 0.1-10.

3. The method for preparing the carbon composite microwave absorbing material according to claim 1, characterized in that, In step S2, the soaking time is 6-24 h, the drying temperature is 40-60 ℃, and the time is 8-12 h.

4. The method for preparing the carbon composite microwave absorbing material according to claim 1, characterized in that, In step S3, the temperature is raised to the sintering temperature by a gradient heating method. Specifically, the temperature is raised from room temperature to 300-400℃ at a heating rate of 5-10℃ / min, and then raised to the sintering temperature at a heating rate of 5-10℃ / min.

5. A carbon composite absorbing material prepared by the method of any one of claims 1-4.

6. The carbon composite microwave absorbing material according to claim 5, characterized in that, The carbon composite absorbing material is used in the fields of electromagnetic wave shielding and electromagnetic wave absorption.

Images