Activated coffee residue biochar wave-absorbing material and preparation method thereof

Abstract

The invention provides an activated coffee residue biochar wave-absorbing material which takes coffee residue as a raw material, and a preparation method of the wave-absorbing material comprises the following steps: cleaning, drying and grinding the coffee residue, then carrying out high-temperature pyrolysis on the ground coffee residue at 500-900 DEG C for 2 hours under the condition of oxygen isolation, taking out the ground coffee residue, mixing the ground coffee residue with sodium hydroxide powder, and carrying out high-temperature activation for 2 hours at 500-900 DEG C after mixing, so as to obtain the activated coffee residue biochar wave-absorbing material. And washing, filtering and drying the product to obtain the activated coffee residue biochar wave-absorbing material. Coffee grounds are rich in polysaccharide and can be used for preparing a high-purity biomass charcoal material and forming a charcoal conductive network structure after being carbonized and activated, so that the conductive loss of the material is improved, meanwhile, the temperature can be regulated and controlled, the pore diameter and the pore number of peanut residue biomass charcoal are increased, and the microwave absorption loss is improved. And according to the loss constant, the impedance is obviously changed along with the temperature, and the change regularity is strong, so that the microwave absorption performance of the material can be accurately regulated and controlled through the temperature.

Description

Technical Field

[0001] This invention relates to the field of microwave absorbing materials, and in particular to an activated preparation of coffee grounds biochar microwave absorbing material and its preparation method. Background Technology

[0002] Electromagnetic absorbing materials are materials that can absorb or attenuate incident electromagnetic waves, converting the absorbed electromagnetic wave energy into heat energy for dissipation. With the widespread use of electronic devices, electromagnetic pollution has become increasingly serious, posing a threat to human health. Electromagnetic absorbing materials can absorb electromagnetic waves, reducing electromagnetic pollution and thus minimizing the harm to the body. Simultaneously, in anti-interference scenarios for electronic devices, electromagnetic absorbing materials are needed to shield interfering electromagnetic waves. Therefore, electromagnetic absorbing materials are widely used in the civilian field. Furthermore, in the field of stealth technology in the military, electromagnetic absorbing materials also play a crucial role, especially in aircraft and missiles. The electromagnetic absorbing materials coated on the surface not only require strong absorption capacity and wide bandwidth but also good lightweight properties.

[0003] The performance of microwave absorbing materials is influenced by many factors. The loss of electromagnetic waves by the absorbing material is determined by the dielectric loss and magnetic loss. Furthermore, the impedance matching of the absorbing material changes with variations in the complex permittivity and complex permeability. Therefore, it is necessary to comprehensively consider impedance matching and electromagnetic loss to improve the overall absorption performance. Currently, multi-component composite materials are generally used to adjust electromagnetic parameters, thereby improving the absorption capacity of the absorbing material while maximizing impedance matching. For example, Sheng Zhi and Gao Dingwei disclosed an absorbing material based on biological waste and magnetic material waste, and its preparation method (Patent No.: 2020104024284). This absorbing material is prepared from biological waste and magnetic material waste. The specific preparation method includes: making biochar from biological waste, making fine magnetic powder from magnetic material waste, and mixing the biochar and fine magnetic powder together to obtain a microwave absorbing material based on biological waste and magnetic material waste. Although this material has good microwave absorption performance, the addition of other magnetic materials such as MnFe2O4 or ZnFe2O4 presents several problems. First, it increases the density of the material, which is not conducive to lightweighting. Second, the process involves ball milling, mixing, and other steps, making the process relatively complex. Third, due to the complex composition, the electromagnetic parameters are difficult to control precisely, making it difficult to accurately control various changing parameters and hindering the preparation of microwave absorbing materials with the desired absorption performance.

[0004] Biomass materials are abundant and inexpensive, thus having great application prospects in the preparation of carbonaceous materials. As coffee becomes increasingly popular in China, more and more coffee grounds are being discarded, causing pollution and waste. If coffee grounds are prepared into high-value functional microwave absorbing materials, it will provide a better way to comprehensively utilize coffee grounds. Summary of the Invention

[0005] In view of the problems in the background technology, the purpose of this invention is to provide an activated coffee ground biochar microwave absorbing material. This microwave absorbing material is prepared by high-temperature pyrolysis using coffee grounds as raw material. The preparation method of this microwave absorbing material includes the following steps: (1) Wash the collected coffee grounds with deionized water 3-5 times to remove surface impurities; (2) Dry the cleaned coffee grounds at 160°C for 4 hours to ensure that the moisture is completely removed; (3) Grind the dried coffee grounds from step (2) into a uniform powder to further refine them into uniform fine powder particles. (4) The obtained granular powder was placed in a tube furnace and subjected to high-temperature pyrolysis at 500-900℃ for 2 h under the condition of continuous argon gas to isolate oxygen, in order to prepare coffee grounds biochar. (5) The obtained coffee grounds biochar and sodium hydroxide powder are quickly mixed and rapidly ground to form a uniform mixture. The mixing ratio of peanut residue and sodium hydroxide is 1:1 by mass.

[0006] (6) The mixture from step (5) is placed back into a tube furnace and activated at 500-900 °C for 2 h under conditions of continuous argon gas to isolate oxygen, in order to prepare activated coffee grounds biochar. (7) The prepared activated coffee grounds biochar product was washed and filtered several times with deionized water until the solution was nearly neutral, and then the filtered wet product was collected. (8) The collected wet product is dried to make activated coffee grounds biochar microwave absorbing material.

[0007] Preferably, the temperature of the high-temperature pyrolysis condition in step (4) is 600 °C.

[0008] Preferably, the temperature of the high-temperature activation condition in step (6) is 600 °C.

[0009] This invention also provides a method for preparing a microwave absorbing coating using activated coffee grounds biochar. The prepared activated coffee grounds biochar material is mixed with a matrix at a mass ratio of 1:1, wherein the matrix is ​​either paraffin wax or epoxy resin.

[0010] Preferably, the thickness of the wave-absorbing coating is 2 mm.

[0011] Beneficial effects (1) The biomass used in this invention is waste coffee grounds. Using it as a carbon source for biomass can turn waste coffee grounds into treasure and greatly improve the utilization value of coffee grounds.

[0012] (2) Coffee grounds are rich in carbon. After carbonization, they are dehydrogenated and nitrogen-removed, and rearranged to produce a layered graphite structure. In this structure, hydrogen is generated into water vapor at high temperature, while most nitrogen is volatilized and removed. At the same time, coffee grounds contain 1.4% to 1.56% nitrogen compounds, 2.6% to 3.7% phosphides, and 0.32% to 0.37% potassium. They have a relatively complex composition and rich functional group structure. After being used to prepare activated biochar, it can form a large number of interfaces and dipoles with the graphite structure, which enhances the dielectric loss, including interface polarization and dipole polarization. (3) Coffee grounds are rich in polysaccharides, especially containing about 50% cellulose and hemicellulose. After carbonization and activation, they can be used to prepare high-purity biochar materials, which is conducive to the formation of carbon conductive network structure, thereby improving the conductivity loss of the material.

[0013] (4) Using coffee grounds as raw material, its cellulose has been damaged to a certain extent, and the particle size after crushing is small. During high-temperature pyrolysis, the small particle size is beneficial to the pyrolysis effect. After carbonization and activation, it is beneficial to prepare nanostructures and enhance the specific surface area, so that electromagnetic waves can be reflected at multiple interfaces, which is beneficial to improving the microwave loss of the material.

[0014] (5) The carbonized coffee grounds biochar is mixed with sodium hydroxide, so that the sodium hydroxide is embedded between the biochar, which increases the degree of dispersion. During activation, it is beneficial for the activated biochar to form a porous structure. Through the coordinated effect of "sodium hydroxide dispersion + high temperature pyrolysis + sodium hydroxide continuous activation", the pore size of peanut residue biochar is effectively increased and the number of pores is increased, which is beneficial for improving microwave absorption loss.

[0015] (6) The activated coffee grounds biochar microwave absorbing material prepared by the present invention has a loss constant and impedance that change significantly with temperature and have a strong regularity, which makes it possible to control the microwave absorption performance of the material by temperature, and provides a simple control method for preparing microwave absorbing materials with predetermined absorption performance.

[0016] (7) The process has the advantages of low cost and green environmental protection, making it suitable for industrial production. A search revealed that there are currently no reports on the preparation of adjustable biochar microwave absorbing materials using sodium hydroxide based on coffee grounds. Attached Figure Description

[0017] Figure 1 SEM images of activated coffee grounds biochar: (a) 500℃; (b) 600℃; (c) 700℃; (d) 800℃; (e) 900℃.

[0018] Figure 2 XRD pattern of activated coffee grounds biochar.

[0019] Figure 3Dielectric parameters and loss diagram of activated coffee grounds biochar: (a) Real part of dielectric constant ε′; (b) Imaginary part ε′′; (c) Dielectric loss tangent tanδ ε (d) Attenuation constant α.

[0020] Figure 4 Cole-Cole curves for activated coffee grounds biochar: (a) T500; (b) T600; (c) T700; (d) T800; (e) T900.

[0021] Figure 5 : RL variation curve and normalized input impedance diagram of activated coffee grounds biochar: (a) RL variation curve; (b) T500 normalized input impedance diagram; (c) T600 normalized input impedance diagram; (d) T700 normalized input impedance diagram; (e) T800 normalized input impedance diagram; (f) T900 normalized input impedance diagram.

[0022] Figure 6 Performance diagrams of T600 activated coffee grounds biochar: (a) Reflectance curves at different thicknesses; (b) Impedance diagrams at different thicknesses; (c) Quarter-wavelength curves; (d) Three-dimensional reflectance surface diagrams.

[0023] Figure 7 Reflectance diagrams of T600 activated coffee grounds biochar at different filling ratios. Detailed Implementation

[0024] Embodiments of the present invention are described in detail below, examples of which are illustrated in the accompanying drawings, wherein the same or similar reference numerals denote the same or similar elements or elements having the same or similar functions throughout. The embodiments described below with reference to the accompanying drawings are exemplary and intended to explain the present invention, and should not be construed as limiting the present invention.

[0025] Example 1 Collect waste coffee grounds from coffee shops and rinse them thoroughly with deionized water 3-5 times to remove surface impurities. Then, dry the washed coffee grounds at 160°C for 4 hours to ensure complete removal of moisture. Grind the dried material into a uniform fine powder using a mechanical grinder. Divide the powder into five portions and carbonize them in a high-temperature tube furnace under an argon (Ar) atmosphere at carbonization temperatures of 500°C, 600°C, 700°C, 800°C, and 900°C for 2 hours. After carbonization, mix the resulting samples with sodium hydroxide (NaOH) at a mass ratio of 1:1. Place the mixtures back into the tube furnace and activate them at high temperatures under an argon (Ar) atmosphere. The activation temperatures of each sample are the same as their carbonization temperatures, namely 500°C, 600°C, 700°C, 800°C, and 900°C. After high-temperature activation for 2 hours, the sample is thoroughly rinsed with deionized water to remove residual alkali and impurities until the solution pH value is close to neutral. Then, it is filtered under vacuum and dried in an oven to obtain activated coffee grounds biomass carbon microwave absorbing material.

[0026] Morphological and structural analysis: The activated coffee grounds biochar prepared in Example 1 were designated as T500, T600, T700, T800, and T900 according to their carbonization temperatures. Figure 1SEM images of activated coffee grounds biochar prepared at different temperatures are shown. Each sample exhibits a porous carbon structure to varying degrees, with significant differences in pore morphology. At 500℃ (T500), the surface appears relatively rough. Although some pores are observed, their development is limited, with small pore sizes and sparse distribution. This underdeveloped porosity can be attributed to incomplete decomposition of organic components and slow release of volatiles at the lower carbonization temperature. At 600℃ (T600), interconnected pore networks begin to appear, forming more complex and continuous porous channels. This indicates that increased thermal energy facilitates the decomposition of organic matter and the release of volatiles, promoting wall erosion between adjacent pores and enhancing connectivity. When the temperature is further increased to 700℃, the biochar surface exhibits a different pore size distribution, ranging from nanoscale pores to relatively large micron-scale channels. At 800℃ (T800), the material exhibits a highly porous structure with well-developed and numerous pores. This is due to the almost complete decomposition of organic matter, leading to the extensive evolution of volatiles and resulting in larger pore volumes. At 900°C (T900), deeper surface textures and irregular depressions appear around the pores. These features are likely due to the intense atomic motion and rearrangement at high temperatures, leading to localized collapse and deformation of the carbon structure. The observed multi-scale and heterogeneous porous structure is crucial for improving electromagnetic wave absorption because it increases the internal surface area, causing multiple internal reflections and scatterings of incident electromagnetic waves, thereby enhancing attenuation. The changes in biochar pore size reveal that temperature can significantly modulate the morphology of activated coffee grounds biochar, exhibiting a relatively monotonic control over pore size. This provides a pathway for morphology regulation of activated coffee grounds biochar and offers the possibility of temperature-based control of microwave absorption performance.

[0027] Figure 2XRD patterns of activated coffee grounds biochar prepared at different temperatures are shown. All samples exhibit broad diffraction peaks centered at 2θ≈24.5° and 43.6°, corresponding to the (002) and (100) planes of graphitic carbon (JCPDS No: 41-1487), respectively. The width of these peaks indicates a low degree of graphitization, suggesting the presence of amorphous or disordered carbon structures. The activation by sodium hydroxide not only creates porous biochar but also disrupts carbon chains and introduces structural defects, thus broadening the diffraction peaks. It can also be seen that as the carbonization temperature increases from 500℃ to 900℃, the intensity of the (100) peak at ~43.6℃ gradually increases, indicating that higher carbonization temperatures promote the rearrangement of carbon atoms, thereby increasing the crystallinity and graphitization degree of the biochar. Furthermore, it can be observed that as the carbonization temperature increases from 500℃ to 900℃, the diffraction peaks shift to lower angles. This is because, with increasing carbonization temperature, biomass char transforms into a regular, graphitized carbon structure, thereby reducing the interatomic spacing. Therefore, carbonization temperature significantly affects the structural evolution of biomass carbon, particularly the development of graphite domains. This is crucial for improving electrical conductivity and dielectric loss. A high degree of graphitization facilitates the formation of a better conductive network, which in turn improves the conductivity of activated coffee grounds biochar. It is evident that temperature can significantly regulate the structure of activated coffee grounds biochar, and the regulation of the graphitization degree and interatomic spacing exhibits a relatively good monotonicity, providing a pathway for structural regulation of activated coffee grounds biochar and offering the possibility of controlling microwave absorption performance through temperature.

[0028] Electromagnetic parameters and loss angle analysis: The activated coffee grounds biochar prepared in Example 1 was mixed with paraffin at a mass ratio of 1:1 to prepare coaxial rings with an inner diameter of 3.04 mm and an outer diameter of 7.0 mm. The dielectric constant (ε) of the material in the frequency range of 2-18 GHz was measured using the coaxial method. r , ε r =ε'-jε") and permeability (μ) r , ε r =μ'-jμ").

[0029] Figure 3 The dielectric parameters and loss diagrams of activated coffee grounds biomass prepared at different temperatures are shown. Figure 3 (a) is a graph showing the variation of the dielectric constant ε′, where ε′ represents the material's ability to store electrical energy. Figure 3 (a) It can be seen that ε′ decreases with increasing frequency. This is due to frequency dispersion behavior, where dipole polarization cannot follow the alternating field at high frequencies. The T700 sample has the largest ε′ value, which first decreases, then increases, and then decreases again with increasing carbonization temperature. Figure 3 (b) is a graph showing the variation of the dielectric constant ε′′, where ε′′ represents the material's ability to dissipate electrical energy. Figure 3(b) It can be seen that the trend of ε′′ is similar to that of ε′, and the change of ε′′ is consistent with the electrical conductivity of the material. The higher the value of ε′′, the higher the electrical conductivity. At 2.0 GHz, the ε′ values ​​of T500-T900 are 3.6, 13.4, 54.0, 21.9 and 34.4, respectively, and the ε′′ values ​​are 0.3, 6.7, 255.3, 158.1 and 335.9, respectively. The T900 sample has the highest ε′′ value, indicating that high-temperature treatment promotes graphitization, reduces structural defects, and improves carrier mobility, thereby significantly improving conductivity and thus increasing the dielectric loss of biochar.

[0030] There are three main mechanisms of dielectric loss: conduction loss, dipole polarization, and interface polarization. Figure 3 (c) is the tangent curve of biochar loss. The graph shows that as the carbonization temperature increases, the tanδ of the biochar decreases. ε The increase in value is due to the higher degree of graphitization at higher temperatures, which forms a better conductive network, increases conductivity, and thus increases the conductive loss of biochar, thereby enhancing dielectric loss.

[0031] The attenuation constant α is calculated by the following equation (1), which reflects the material's ability to attenuate electromagnetic waves.

[0032] (1) like Figure 3 As shown in (d), α increases significantly with increasing temperature, from 3.8 (T500) to 511.4 (T900). This trend is consistent with tanδ ε The increase in the value is consistent with the fact that graphitization and increased conductivity contribute to enhanced attenuation of electromagnetic waves.

[0033] To further analyze the relaxation behavior of the dielectric, according to the Debye theory (2), the ideal relaxation process produces a semicircle in the Cole-Cole diagram.

[0034] (2) In the formula ε s and ε ∞ These are the static and optical dielectric constants, respectively.

[0035] The Cole-Cole curve of the sample can be plotted based on the values ​​of ε′ and ε′, such as... Figure 4 The above refers to the Cole-Cole curves of activated coffee grounds biochar prepared under different carbonization temperature conditions. Figure 4Multiple distorted semicircles were observed in all samples, indicating the presence of multiple Debye relaxation processes. This is due to defect polarization caused by structural defects (e.g., vacancies, dislocations) and interfacial polarization caused by the layered porous structure. The existence of multiple relaxation paths dissipates energy through various mechanisms such as charge jumps, dipole rearrangements, and interfacial charge accumulation, thereby enhancing the attenuation of electromagnetic waves. Furthermore, all samples exhibited linear tails, a characteristic of conductive losses. With increasing carbonization temperature, the conductive network becomes more continuous, leading to an increase in free electron density and mobility. Under the influence of an alternating electric field, these electrons cause ohmic losses, further improving absorption efficiency.

[0036] The electromagnetic wave absorption performance of a material is evaluated by calculating the absorption capacity (RL), where RL represents the material's ability to absorb incident electromagnetic waves. An RL below -10 dB corresponds to an absorption efficiency of 90%. The bandwidth where RL remains below -10 dB is called the EAB. RL is calculated using transmission line theory, as shown in equations (3) and (4): (3) (4) In the formula, Z in Z0 and Z0 are the input impedance and free space impedance, respectively, and ε r μ is the complex permittivity. r denoted as complex permeability, d as the thickness of the absorber, f as the frequency of the electromagnetic wave, and c as the speed of light in free space.

[0037] Figure 5 (a) RL curves of coffee grounds biochar samples prepared under different temperature conditions at their respective optimal thicknesses. Among all samples, only T600 has an RL value below -10 dB, indicating effective electromagnetic wave absorption. Other samples, despite high dielectric loss, exhibit relatively poor absorption performance. This difference highlights the crucial role of impedance matching, using the normalized input impedance |Z in / Z0|Quantifies the impedance matching degree between the absorbing material and free space. A value close to 1 indicates a good match, allowing more electromagnetic waves to enter the material. Figure 5 (bf) represents |Z in T500-T900 in / Z0| Two-dimensional contour plots of Z with frequency and thickness. Samples T500 and T600 within a certain frequency and thickness range |Z in A value of / Z0| close to 1 indicates good impedance matching. However, the attenuation capability of T500 is insufficient, limiting its absorption. In contrast, T600 achieves optimal synergy between impedance matching and dielectric loss, resulting in superior absorption performance. For samples T700-T900, |Z0| inA / Z0| value below 0.5 across most of the measurement range indicates poor impedance matching. Despite their high dielectric loss, the low electromagnetic wave injection efficiency due to impedance mismatch negatively impacts their absorption performance. This underscores the crucial need for simultaneous optimization of impedance matching and dielectric loss to achieve efficient electromagnetic wave absorption.

[0038] The above analysis shows that, due to the good regulatory relationship between carbonization activation temperature and the morphology and structure of coffee grounds biochar, the dielectric loss capacity of coffee grounds biochar can be improved in a coordinated manner through two pathways: forming a porous structure and increasing the conductive network. At the same time, the carbonization activation temperature factor also has a good, opposite effect on the impedance matching ability of coffee grounds biochar. Therefore, T600 achieves synergy between impedance matching and dielectric loss, and obtains superior absorption performance. Thus, microwave absorbing materials with predetermined absorption performance can be prepared by controlling the single factor of carbonization activation temperature. The control method is simple and efficient, and has unexpected technical effects.

[0039] Figure 6 This further revealed the wave adsorption characteristics of the optimal T600 biochar. Figure 6 (a) shows the RL curves for different biochar thicknesses (1.4–2.6 mm). At a thickness of 1.7 mm, the minimum RL reaches -32.15 dB at 17.4 GHz. At a thickness of 2.0 mm, the effective absorption bandwidth (RL < -10 dB) reaches 5.8 GHz (12.2–18.0 GHz), exhibiting good broadband absorption. With increasing thickness, the minimum RL shifts to lower frequencies; this redshift is related to a quarter-wavelength (λ / 4) interference cancellation mechanism. Figure 6 As shown in (c), the minimum value of RL is closely consistent with the frequency predicted by the λ / 4 condition, confirming this absorption mechanism. Figure 6 (b) and (d) show the corresponding impedance matching conditions and the three-dimensional RL mapping. The RL peak always appears at |Z in The proximity of / Z0| to 1 confirms that optimal absorption occurs at frequencies where impedance matching and phase cancellation are both satisfied. These results highlight the flexibility and tunability of the T600 sample for targeted frequency absorption via thickness modulation.

[0040] Example 2 The T600 activated coffee grounds biochar prepared in Example 1 was mixed with paraffin at different mass ratios (activated coffee grounds biochar: paraffin = 0.8:1, 0.9:1, 1.1:1, 1.2:1) to prepare coaxial rings with an inner diameter of 3.04 mm and an outer diameter of 7.0 mm. The dielectric constant (ε) of the material in the frequency range of 2-18 GHz was measured using the coaxial method. r , ε r =ε'-jε") and permeability (μ)r , ε r =μ'-jμ"), and then simulate and calculate the RL value of each sample at a thickness of 2mm.

[0041] Electromagnetic parameters and loss angle analysis: like Figure 7 The figure shows the reflectance of T600 activated coffee grounds biochar at different mass filling ratios. As can be seen from the figure, the microwave absorption performance of the biochar gradually improves as the filling ratio increases from 0.8:1 to 1:1. The sample with a filling ratio of 1:1 achieves a minimum reflectance of 26.4 dB at 14.7 GHz. When the filling ratio continues to increase, the microwave absorption rate of the biochar decreases rapidly. In the entire 2-18 GHz band, the reflectance of samples with filling ratios of 1:1.1 and 1:1.2 does not reach -10 dB, indicating poor microwave absorption performance. Therefore, by adjusting the filling ratio, its microwave absorption performance can be further precisely controlled.

[0042] The above-disclosed embodiments are merely preferred embodiments of the present invention and should not be construed as limiting the scope of the invention. Those skilled in the art will understand that implementing all or part of the above-described embodiments and making equivalent changes in accordance with the claims of the present invention are still within the scope of the invention.

Claims

1. An activated coffee grounds biochar microwave absorbing material, wherein the microwave absorbing material is prepared by high-temperature pyrolysis using coffee grounds as raw material, characterized in that, The preparation method of this microwave absorbing material includes the following steps: (1) Wash the collected coffee grounds with deionized water 3-5 times to remove surface impurities; (2) Dry the cleaned coffee grounds at 160°C for 4 hours to ensure that the moisture is completely removed; (3) Grind the dried coffee grounds from step (2) into a uniform powder to further refine them into uniform fine powder particles. (4) The obtained granular powder was placed in a tube furnace and subjected to high-temperature pyrolysis at 500-900℃ for 2 h under the condition of continuous argon gas to isolate oxygen, in order to prepare coffee grounds biochar. (5) Mix the obtained coffee grounds biochar with sodium hydroxide powder and grind it quickly to make it uniform. The mixing ratio of peanut residue and sodium hydroxide is 1:1 by mass. (6) The ground mixture from step (5) is placed back into a tube furnace and activated at 500-900 °C for 2 h under conditions of continuous argon gas to isolate oxygen, in order to prepare activated coffee grounds biochar. (7) The prepared activated coffee grounds biochar product was washed and filtered several times with deionized water until the solution was nearly neutral, and then the filtered wet product was collected. (8) The collected wet products can be dried to make activated coffee grounds biochar microwave absorbing material.

2. An activated coffee grounds biochar microwave absorbing material as described in claim 1, characterized in that, The temperature of the high-temperature pyrolysis condition in step (4) is 600 ℃.

3. An activated coffee grounds biochar microwave absorbing material as described in claim 2, characterized in that, The temperature of the high-temperature activation condition in step (6) is 600 ℃.

4. A method for preparing a microwave absorbing coating using the activated coffee grounds biochar obtained according to claim 3, characterized in that, The activated coffee grounds biochar is mixed with a matrix at a mass ratio of 1:1, and the matrix is ​​either paraffin or epoxy resin.

5. A method for preparing a microwave absorbing coating as described in claim 4, characterized in that, The thickness of the wave-absorbing coating is 2 mm.

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