Preparation method of non-uniform pore structure biochar wave absorbing material

Abstract

The application relates to a preparation method of a non-uniform pore structure biochar wave absorbing material, and belongs to the fields of electromagnetic wave absorbing materials and advanced ceramic manufacturing technologies.The method comprises the following steps: firstly, freeze-dried biomass is immersed in an iron salt solution, and a multipole magnetic field with a magnetic induction intensity of 0.25 T is applied through an external electromagnetic coil, in-situ generated magnetic particles are induced to migrate along the magnetic field direction, and non-uniform enrichment is formed in the internal base tissue of the biomass pore channel; secondly, the biomass loaded with the magnetic particles is pyrolyzed to obtain biochar loaded with the magnetic particles; and finally, the non-uniform pore structure biochar wave absorbing material is obtained after the magnetic particles are acid washed and etched. The material has a micropore-mesopore-macropore non-uniform multistage pore structure, can effectively increase the propagation path and scattering times of electromagnetic waves in the material, enhances the dissipation and attenuation of wave energy, effectively absorbs electromagnetic waves in a wide frequency band, and the material is non-magnetic.

Description

Technical Field

[0001] This invention relates to a method for preparing a non-uniform porous biochar microwave absorbing material, belonging to the fields of electromagnetic wave absorbing materials and biochar materials. Background Technology

[0002] There is an urgent need to develop efficient, lightweight, and broadband electromagnetic wave absorbing materials. Carbon-based absorbing materials have become a research hotspot in recent years due to their advantages such as low density, strong chemical stability, and easy control of conductivity and dielectric properties. Biochar materials, in particular, are obtained from the pyrolysis or activation of natural resources (such as wood, straw, and coconut shells), offering advantages such as low cost, abundant sources, and environmental friendliness and renewability. The porous structure and carbon skeleton of biochar itself provide excellent channels for multiple scattering, loss, and conduction of electromagnetic waves, making it an ideal candidate for achieving lightweight, broadband, and efficient absorbing materials. However, the pore structure of traditional biochar is usually relatively uniform, making it difficult to form multi-scale synergistic effects, which is detrimental to electromagnetic wave gradient impedance matching and multiple reflections, thus limiting its absorption efficiency. Therefore, how to improve the absorption performance of biochar has become a problem that those skilled in the art want to solve. Summary of the Invention

[0003] To address the aforementioned problems, this invention provides a method for preparing a non-uniform porous biochar microwave absorbing material. By adding magnetic particles and introducing a magnetic field to in-situ regulate the biomass carbonization and pore structure formation process, a non-uniform pore network is constructed, enhancing the multiple reflections, scattering, and polarization losses of electromagnetic waves. Simultaneously, impedance matching performance is improved, and the effective absorption bandwidth is broadened. Furthermore, this technology is simple, controllable, and extensible to various biomass and magnetic particle systems. This invention achieves synergistic optimization of the non-uniform pore size distribution, structural morphology, and electromagnetic parameters of biochar, endowing biochar with excellent electromagnetic wave absorption performance, and possesses promising application prospects and engineering promotion value.

[0004] The present invention solves the above-mentioned technical problems through the following technical solution:

[0005] A method for preparing a non-uniform porous biochar microwave absorbing material, characterized by the following steps:

[0006] Step 1, pretreatment of biomass materials, which involves dividing biomass into small pieces, removing soluble impurities, and obtaining biomass precursors;

[0007] Step 2, in-situ preparation of reaction solution and synchronous control of magnetic field:

[0008] Add 250 mL of deionized water to an Erlenmeyer flask, then add 0.004-0.008 mol / L sodium citrate, 0.05-0.2 mol FeCl3·6H2O, and 0.025-0.1 mol FeCl2·4H2O in sequence. Stir at 300-700 rpm for 5-10 min until completely dissolved to obtain a mixed iron salt solution. Add the biomass precursor from step 1 and continue stirring under nitrogen protection for 30-60 min to allow it to soak.

[0009] Under nitrogen protection, the reaction system was stirred at 500 rpm and heated to 50-70℃. The pH was controlled between 9 and 10 using 25% wt ammonia water. Fe3O4 magnetic nanoparticles were generated in situ. The reaction was continued for 30-60 minutes. During this process, the particles were generated simultaneously in the solution and in the biomass pores and tissues.

[0010] Meanwhile, during the in-situ generation reaction of Fe3O4 magnetic nanoparticles, a stable multipolar magnetic field with a magnetic induction intensity of 0.1-0.3T was applied to the outside of the conical flask through an external electromagnetic coil, which induced the generated magnetic particles to migrate along the direction of the magnetic field and form uneven enrichment in the biomass pore matrix. The biomass was taken out and freeze-dried at -60℃ for 12-24h to obtain biomass with uneven magnetic particle loading for later use.

[0011] Step 3, Biomass pyrolysis: The biomass with unevenly loaded magnetic particles obtained in Step 3 is placed in a tube furnace for pyrolysis to obtain biochar with unevenly loaded magnetic particles.

[0012] Step 4, magnetic particle removal: Remove the magnetic particles from the biochar with unevenly loaded magnetic particles obtained in Step 3 to obtain biochar with a non-uniform porous structure.

[0013] The advantages of the above technical solution are: by introducing an external multipolar magnetic field during the in-situ generation of Fe3O4, magnetic particles are induced to migrate and accumulate in a directional manner, forming a non-uniform distribution in the pores and tissues of coconut shell biomass. The non-uniformly distributed magnetic particles are selectively removed during subsequent pyrolysis and acid washing, resulting in the retention of micropores, mesopores, mesopores, macropores and other pore sizes, thus constructing a non-uniform multi-scale pore structure. Combined with the natural three-dimensional structure of coconut shell biochar, this is beneficial for multiple scattering, loss and notch enhancement of electromagnetic waves, thereby significantly improving the absorption performance.

[0014] Based on the above technical solution, the following improvements are made to the application:

[0015] Furthermore, in step 1, the soaking time for removing soluble impurities is 12 hours, and the freeze-drying time is 36 hours.

[0016] Furthermore, in step 2, sodium citrate is used as a dispersant, with an addition amount of 0.006 mol / L. It can stabilize iron ions through chelation, preventing them from undergoing uneven hydrolysis or precipitation before the reaction. At the same time, it can prevent the agglomeration of the generated Fe3O4 magnetic particles, thereby controlling the particle size and dispersibility, making it easier for them to enter the interior of coconut shell biomass under the action of a magnetic field.

[0017] Furthermore, in step 2, the concentrations of FeCl3·6H2O and FeCl2·4H2O are 0.1 mol / L and 0.05 mol / L, respectively. The two iron salts are added sequentially to the solution containing the dispersant, and the mixture is magnetically stirred at 500 rpm for 5 minutes to obtain a homogeneous and stable solution. The stirring and wetting time under nitrogen protection is 40 minutes. The amount of sodium citrate added as the dispersant is 0.006 mol / L. It stabilizes iron ions through chelation, preventing uneven hydrolysis or precipitation before reaction. Simultaneously, it prevents the aggregation of the generated Fe3O4 magnetic particles, thereby controlling particle size and dispersibility, making it easier for the particles to penetrate the coconut shell biomass under the influence of a magnetic field.

[0018] Furthermore, in step 2, the magnetic induction intensity is 0.25T.

[0019] Further, in step 3, the biomass with unevenly loaded magnetic particles obtained in step 2 is placed in a tube furnace, heated to 600-900℃ at a heating rate of 3-5℃ / min under nitrogen protection, held at that temperature for 1-3 hours, and then cooled to 400℃ at a cooling rate of 3-5℃ / min before being cooled to room temperature with the furnace to obtain biochar with unevenly loaded magnetic particles, which is then ready for use.

[0020] Furthermore, in step 3, the heating rate is 3℃ / min and the cooling rate is 5℃ / min.

[0021] Furthermore, in step 3, the pyrolysis temperature is 800℃.

[0022] Further, the specific steps of step 4 are as follows: the biochar with unevenly loaded magnetic particles obtained in step 3 is ground and dispersed in a 1M HCl solution, and the solid-liquid ratio is 1g of biochar material is dissolved in 50-200mL of hydrochloric acid solution. The mixture is shaken in a constant temperature shaker at 25℃ for 1-3h, filtered to obtain a solid, and washed with deionized water until neutral. The obtained biochar is placed in a 60℃ oven and dried for 3-6h to obtain biochar with a non-uniform porous structure.

[0023] Furthermore, in step 4, the solid-liquid ratio of the magnetic particle-loaded biochar to hydrochloric acid is 1g:100mL; the isothermal shaking time is 2h; and the drying time is 5h.

[0024] The present invention has the following beneficial effects:

[0025] 1. This invention obtains a non-uniform porous biochar microwave absorbing material with controllable structure and excellent performance by regulating the uneven distribution of magnetic particles (Fe3O4) inside the biomass during in-situ growth and the changes in pore structure during carbonization to form a hierarchical pore structure in which micropores, mesopores and macropores coexist.

[0026] 2. The coconut shell biochar obtained by this invention possesses a non-uniform pore structure (a combination of micropores, mesopores, and macropores). This structure effectively increases the propagation path and scattering frequency of electromagnetic waves within the material, enhances wave energy dissipation and attenuation, significantly broadens the effective absorption bandwidth, reduces minimum reflection loss, improves impedance matching with free space, reduces electromagnetic wave surface reflection, and increases absorption efficiency. The preparation method described in this invention, through in-situ control of the carbonization and pore structure formation process, achieves synergistic optimization of pore size distribution, structural morphology, and electromagnetic parameters, endowing the biochar with excellent electromagnetic wave absorption performance, and has good application prospects and engineering promotion value.

[0027] 3. This invention uses an aqueous iron salt solution and natural biomass (coconut shell) for synergistic treatment, combined with a stable multipole magnetic field provided by an external electromagnetic coil. It does not require complicated post-processing steps or expensive equipment. The carbonization process is simple, the reaction conditions are mild, and it has good repeatability.

[0028] 4. The raw material uses renewable coconut shell biomass as the carbon source, combined with a simple impregnation-carbonization process, achieving high-value utilization of resources and aligning with current green and sustainable development trends. The non-uniform porous structure reduces material density while maintaining microwave absorption performance, meeting the needs of aerospace, electronic equipment, and other applications for lightweight microwave absorbing materials. Attached Figure Description

[0029] Figure 1 Scanning electron microscope image of the non-uniform porous biochar microwave absorbing material prepared in Example 1;

[0030] Figure 2 Transmission electron microscope image of coconut shell biochar microwave absorbing material with unevenly loaded magnetic particles synthesized in Comparative Example 3.

[0031] Figure 3 Transmission electron microscope image of the non-uniform porous biochar microwave absorbing material synthesized in Example 1;

[0032] Figure 4 The X-ray diffraction patterns of the microwave absorbing materials synthesized in Example 1 and Comparative Example 3 are shown below.

[0033] Figure 5 The image shows the three-dimensional reflection loss and two-dimensional projection of the non-uniform porous biochar absorbing material synthesized in Example 1.

[0034] Figure 6The three-dimensional reflection loss and two-dimensional projection diagram of the biochar microwave absorbing material synthesized in Comparative Example 2 are shown.

[0035] Figure 7 Impedance matching of the non-uniform porous biochar absorbing material synthesized in Example 1;

[0036] Figure 8 The impedance matching diagram is shown for the biochar synthesized in Comparative Example 1. Detailed Implementation

[0037] The following embodiments, in conjunction with the accompanying drawings, are merely illustrative of the technical solutions described in the claims and are not intended to limit the scope of protection of the claims. In this application, coconut shells are used as an example of biomass.

[0038] Example 1

[0039] A method for preparing a non-uniform porous biochar microwave absorbing material, the specific steps of which are as follows:

[0040] Step 1, Pretreatment of biomass materials:

[0041] The coconut shells were divided into 1×1cm pieces, soaked in deionized water for 15 hours to remove soluble impurities, and then freeze-dried at -60℃ for 36 hours to obtain biomass precursors. The obtained biomass precursors were stored in a vacuum protected from light for later use.

[0042] Step 2, in-situ preparation of reaction solution and synchronous control of magnetic field:

[0043] Add 250 mL of deionized water to an Erlenmeyer flask, then add 0.5 g of sodium citrate, 6.76 g of FeCl3·6H2O and 1.81 g of FeCl2·4H2O in sequence. Stir at 500 rpm for 5 min until completely dissolved to obtain a mixed iron salt solution for later use.

[0044] Ten biomass precursors from step 1 were placed into the mixed solution from step 2 and stirred for 40 minutes under nitrogen protection to allow them to soak.

[0045] The reaction system (under nitrogen protection, stirred at 500 rpm) was heated to 65°C, and the pH was controlled between 9 and 10 using ammonia water (25% wt) to generate Fe3O4 magnetic nanoparticles in situ. The reaction was continued for 50 min. During the reaction, a stable multipole magnetic field with a magnetic induction intensity of 0.25 T was continuously applied to the outside of the conical flask through an external electromagnetic coil.

[0046] The coconut shell biomass was extracted and freeze-dried at -60℃ for 20 hours to obtain coconut shell biomass with uneven magnetic particle loading, which was then set aside for later use.

[0047] Step 3, biomass pyrolysis:

[0048] The coconut shell biomass blank with unevenly loaded magnetic particles obtained in step 2 was placed in a tube furnace and heated from room temperature to 800°C at a heating rate of 3°C / min under nitrogen atmosphere protection and held for 2 hours. Then, it was cooled to 400°C at a cooling rate of 5°C / min and cooled to room temperature with the furnace to obtain coconut shell biochar with unevenly loaded magnetic particles for later use.

[0049] Step 4, Magnetic particle removal:

[0050] The coconut shell biochar with unevenly loaded magnetic particles obtained in step 3 was ground and dispersed in 1M HCl solution (solid-liquid ratio of 1g biochar material dissolved in 100mL hydrochloric acid solution), shaken in a constant temperature shaker at 25℃ for 3h, filtered to obtain solid and washed with deionized water until neutral, and set aside for later use.

[0051] The obtained biochar was dried in a 60℃ oven for 5 hours to obtain a biochar microwave absorbing material with a non-uniform porous structure.

[0052] Example 2

[0053] Step 1, Pretreatment of biomass materials:

[0054] The coconut shells were divided into 1×1cm pieces, soaked in deionized water for 15 hours to remove soluble impurities, and then freeze-dried at -60℃ for 36 hours to obtain biomass precursors. The obtained biomass precursors were stored in a vacuum protected from light for later use.

[0055] Step 2, in-situ preparation of reaction solution and synchronous control of magnetic field:

[0056] Add 250 mL of deionized water to an Erlenmeyer flask, then add 0.327 g of sodium citrate, 3.38 g of FeCl3·6H2O and 0.91 g of FeCl2·4H2O in sequence. Stir at 400 rpm for 5 min until completely dissolved to obtain a mixed iron salt solution for later use.

[0057] Ten biomass precursors from step 1 were placed into the mixed solution from step 2 and stirred for 30 minutes under nitrogen protection to allow them to soak.

[0058] The reaction system (under nitrogen protection, stirred at 500 rpm) was heated to 50°C, and the pH was controlled between 9 and 10 using ammonia water (25% wt) to generate Fe3O4 magnetic nanoparticles in situ. The reaction was continued for 50 min. During the reaction, a stable multipole magnetic field with a magnetic induction intensity of 0.1 T was continuously applied to the outside of the conical flask through an external electromagnetic coil.

[0059] The coconut shell biomass was extracted and freeze-dried at -60℃ for 20 hours to obtain coconut shell biomass with uneven magnetic particle loading, which was then set aside for later use.

[0060] Step 3, biomass pyrolysis:

[0061] The coconut shell biomass blank with unevenly loaded magnetic particles obtained in step 5 was placed in a tube furnace and heated from room temperature to 800°C at a heating rate of 3°C / min under nitrogen atmosphere protection and held for 2 hours. Then, it was cooled to 400°C at a cooling rate of 5°C / min and cooled to room temperature with the furnace to obtain coconut shell biochar with unevenly loaded magnetic particles for later use.

[0062] Step 4, Magnetic particle removal:

[0063] The coconut shell biochar with unevenly loaded magnetic particles obtained in step 6 was ground and dispersed in 1M HCl solution (solid-liquid ratio of 1g biochar material dissolved in 50mL hydrochloric acid solution), shaken in a constant temperature shaker at 25℃ for 3h, filtered to obtain solid and washed with deionized water until neutral, for later use.

[0064] The biochar obtained in step 6 was dried in a 60°C oven for 5 hours to obtain a biochar microwave absorbing material with a non-uniform porous structure.

[0065] Example 3

[0066] Step 1, Pretreatment of biomass materials:

[0067] The coconut shells were divided into 1×1cm pieces, soaked in deionized water for 15 hours to remove soluble impurities, and then freeze-dried at -60℃ for 36 hours to obtain biomass precursors. The obtained biomass precursors were stored in a vacuum protected from light for later use.

[0068] Step 2, in-situ preparation of reaction solution and synchronous control of magnetic field:

[0069] Add 250 mL of deionized water to an Erlenmeyer flask, then add 0.654 g of sodium citrate, 13.52 g of FeCl3·6H2O and 3.62 g of FeCl2·4H2O in sequence. Stir at 700 rpm for 10 min until completely dissolved to obtain a mixed iron salt solution for later use.

[0070] Ten biomass precursors from step 1 were placed into the mixed solution from step 2 and stirred for 60 minutes under nitrogen protection to allow them to soak.

[0071] The reaction system (under nitrogen protection, stirred at 500 rpm) was heated to 70°C, and the pH was controlled between 9 and 10 using ammonia water (25% wt) to generate Fe3O4 magnetic nanoparticles in situ. The reaction was continued for 50 min. During the reaction, a stable multipole magnetic field with a magnetic induction intensity of 0.3 T was continuously applied to the outside of the conical flask through an external electromagnetic coil.

[0072] The coconut shell biomass was extracted and freeze-dried at -60℃ for 20 hours to obtain coconut shell biomass with uneven magnetic particle loading, which was then set aside for later use.

[0073] Step 3, biomass pyrolysis:

[0074] The coconut shell biomass blank with unevenly loaded magnetic particles obtained in step 2 was placed in a tube furnace and heated from room temperature to 800°C at a heating rate of 3°C / min under nitrogen atmosphere protection and held for 3 hours. Then, it was cooled to 400°C at a cooling rate of 5°C / min and cooled to room temperature with the furnace to obtain coconut shell biochar with unevenly loaded magnetic particles for later use.

[0075] Step 4, Magnetic particle removal:

[0076] The coconut shell biochar with unevenly loaded magnetic particles obtained in step 3 was ground and dispersed in 1M HCl solution (solid-liquid ratio of 1g biochar material dissolved in 200mL hydrochloric acid solution), shaken in a constant temperature shaker at 25℃ for 3h, filtered to obtain solid and washed with deionized water until neutral, and set aside for later use.

[0077] The obtained biochar was dried in a 60℃ oven for 5 hours to obtain a biochar microwave absorbing material with a non-uniform porous structure.

[0078] Example 4

[0079] Step 1, Pretreatment of biomass materials:

[0080] The coconut shells were divided into 1×1cm pieces, soaked in deionized water for 15 hours to remove soluble impurities, and then freeze-dried at -60℃ for 36 hours to obtain biomass precursors. The obtained biomass precursors were stored in a vacuum protected from light for later use.

[0081] Step 2, in-situ preparation of reaction solution and synchronous control of magnetic field:

[0082] Add 250 mL of deionized water to an Erlenmeyer flask, then add 0.5 g of sodium citrate, 6.76 g of FeCl3·6H2O and 1.81 g of FeCl2·4H2O in sequence. Stir at 500 rpm for 5 min until completely dissolved to obtain a mixed iron salt solution for later use.

[0083] Ten biomass precursors from step 1 were placed into the mixed solution from step 2 and stirred for 40 minutes under nitrogen protection to allow them to soak.

[0084] The reaction system (under nitrogen protection, stirred at 500 rpm) was heated to 65°C, and the pH was controlled between 9 and 10 using ammonia water (25% wt) to generate Fe3O4 magnetic nanoparticles in situ. The reaction was continued for 50 min. During the reaction, a stable multipole magnetic field with a magnetic induction intensity of 0.25 T was continuously applied to the outside of the conical flask through an external electromagnetic coil.

[0085] The coconut shell biomass was extracted and freeze-dried at -60℃ for 12 hours to obtain coconut shell biomass with uneven magnetic particle loading, which was then set aside for later use.

[0086] Step 3, biomass pyrolysis:

[0087] The coconut shell biomass blank with unevenly loaded magnetic particles obtained in step 2 was placed in a tube furnace and heated from room temperature to 600°C at a heating rate of 3°C / min under nitrogen atmosphere protection and held for 3 hours. Then, it was cooled to 400°C at a cooling rate of 3°C / min and cooled to room temperature with the furnace to obtain coconut shell biochar with unevenly loaded magnetic particles for later use.

[0088] Step 4, Magnetic particle removal:

[0089] The coconut shell biochar with unevenly loaded magnetic particles obtained in step 3 was ground and dispersed in 1M HCl solution (solid-liquid ratio of 1g biochar material dissolved in 100mL hydrochloric acid solution), shaken in a constant temperature shaker at 25℃ for 3h, filtered to obtain solid and washed with deionized water until neutral, and set aside for later use.

[0090] The obtained biochar was dried in a 60℃ oven for 5 hours to obtain a biochar microwave absorbing material with a non-uniform porous structure.

[0091] Example 5

[0092] Step 1, Pretreatment of biomass materials:

[0093] The coconut shells were divided into 1×1cm pieces, soaked in deionized water for 15 hours to remove soluble impurities, and then freeze-dried at -60℃ for 36 hours to obtain biomass precursors. The obtained biomass precursors were stored in a vacuum protected from light for later use.

[0094] Step 2, in-situ preparation of reaction solution and synchronous control of magnetic field:

[0095] Add 250 mL of deionized water to an Erlenmeyer flask, then add 0.5 g of sodium citrate, 6.76 g of FeCl3·6H2O, and 1.81 g of FeCl2·4H2O in sequence. Stir at 300-700 rpm for 5-10 min until completely dissolved to obtain a mixed iron salt solution. Add the biological sample from step 1 and stir at 500 rpm for 5 min until completely dissolved to obtain a mixed iron salt solution for later use.

[0096] Ten biomass precursors from step 1 were placed into the mixed solution from step 2 and stirred for 40 minutes under nitrogen protection to allow them to soak.

[0097] The reaction system (under nitrogen protection, stirred at 500 rpm) was heated to 65°C, and the pH was controlled between 9 and 10 using ammonia water (25% wt) to generate Fe3O4 magnetic nanoparticles in situ. The reaction was continued for 50 min. During the reaction, a stable multipole magnetic field with a magnetic induction intensity of 0.25 T was continuously applied to the outside of the conical flask through an external electromagnetic coil.

[0098] The coconut shell biomass was extracted and freeze-dried at -60℃ for 24 hours to obtain coconut shell biomass with uneven magnetic particle loading, which was then set aside for later use.

[0099] Step 3, biomass pyrolysis:

[0100] The coconut shell biomass blank with unevenly loaded magnetic particles obtained in step 2 was placed in a tube furnace and heated from room temperature to 900°C at a heating rate of 5°C / min under nitrogen atmosphere protection and held for 1 hour. Then, it was cooled to 400°C at a cooling rate of 5°C / min and cooled to room temperature with the furnace to obtain coconut shell biochar with unevenly loaded magnetic particles for later use.

[0101] Step 4, Magnetic particle removal:

[0102] The coconut shell biochar with unevenly loaded magnetic particles obtained in step 3 was ground and dispersed in 1M HCl solution (solid-liquid ratio of 1g biochar material dissolved in 100mL hydrochloric acid solution), shaken in a constant temperature shaker at 25℃ for 3h, filtered to obtain solid and washed with deionized water until neutral, and set aside for later use.

[0103] The obtained biochar was dried in a 60℃ oven for 5 hours to obtain a biochar microwave absorbing material with a non-uniform porous structure.

[0104] Comparative Example 1

[0105] The preparation method of coconut shell biochar microwave absorbing material is as follows:

[0106] Step (1): Divide the coconut shell into 1×1cm pieces, soak them in deionized water for 15h to remove soluble impurities, and then freeze-dry them at -60℃ for 36h to obtain biomass precursors. Store the obtained biomass precursors in a vacuum protected from light for later use.

[0107] Step (2): Place the coconut shell biomass blank obtained in step 1 into a tube furnace, raise the temperature from room temperature to 800℃ at a rate of 3℃ / min under nitrogen atmosphere protection and hold for 2 hours, then lower the temperature to 400℃ at a rate of 5℃ / min and cool it to room temperature with the furnace to obtain coconut shell biochar.

[0108] Comparative Example 2

[0109] The preparation method of porous coconut shell biochar microwave absorbing material without external multipolar magnetic field is as follows:

[0110] Step (1): Divide the coconut shell into 1×1cm pieces, soak them in deionized water for 15h to remove soluble impurities, and then freeze-dry them at -60℃ for 36h to obtain biomass precursors. Store the obtained biomass precursors in a vacuum protected from light for later use.

[0111] Step (2): Add 250 mL of deionized water to the conical flask, then add 0.5 g of sodium citrate, 6.76 g of FeCl3·6H2O and 1.81 g of FeCl2·4H2O in sequence. Stir at 500 rpm for 5 min until completely dissolved to obtain a mixed solution of iron salts for later use.

[0112] Step (3): Place 10 biomass precursors from Step 1 into the mixed solution from Step 2 and stir for 40 minutes under nitrogen protection to allow them to soak.

[0113] Step (4): The reaction system (under nitrogen protection, stirred at 500 rpm) is heated to 65°C, and the pH is controlled between 9 and 10 using ammonia water (25% wt) to generate Fe3O4 magnetic nanoparticles in situ. The reaction is continued for 50 min.

[0114] Step (5): Take out the coconut shell biomass and freeze-dry it at -60℃ for 20h to obtain coconut shell biomass with uneven magnetic particle loading, for later use;

[0115] Step (6): The coconut shell biomass blank with unevenly loaded magnetic particles obtained in step 5 is placed in a tube furnace and heated from room temperature to 800°C at a heating rate of 3°C / min under nitrogen atmosphere protection and held for 2 hours. Then, it is cooled to 400°C at a cooling rate of 5°C / min and cooled to room temperature with the furnace to obtain coconut shell biochar with unevenly loaded magnetic particles for later use.

[0116] Step (7): Grind the coconut shell biochar with uneven magnetic particle loading obtained in step 6 and disperse it in 1M HCl solution (solid-liquid ratio is 1g biochar material dissolved in 100mL hydrochloric acid solution), shake in a constant temperature shaker at 25℃ for 3h, filter to obtain solid and wash with deionized water until neutral, and set aside.

[0117] Step (8): The biochar obtained in step 6 is placed in a 60°C oven and dried for 5 hours to obtain a biochar microwave absorbing material with a non-uniform pore structure.

[0118] Comparative Example 3

[0119] The preparation method of coconut shell biochar microwave absorbing material with non-uniform magnetic particle loading is as follows:

[0120] Step (1): Divide the coconut shell into 1×1cm pieces, soak them in deionized water for 15h to remove soluble impurities, and then freeze-dry them at -60℃ for 36h to obtain biomass precursors. Store the obtained biomass precursors in a vacuum protected from light for later use.

[0121] Step (2): Add 250 mL of deionized water to the conical flask, then add 0.5 g of sodium citrate, 6.76 g of FeCl3·6H2O and 1.81 g of FeCl2·4H2O in sequence. Stir at 500 rpm for 5 min until completely dissolved to obtain a mixed solution of iron salts for later use.

[0122] Step (3): Place 10 biomass precursors from Step 1 into the mixed solution from Step 2 and stir for 40 minutes under nitrogen protection to allow them to soak.

[0123] Step (4): The reaction system (nitrogen protection, stirring at 500 rpm) is heated to 65°C, and the pH is controlled between 9 and 10 using ammonia water (25% wt) to generate Fe3O4 magnetic nanoparticles in situ. The reaction is continued for 50 min. During the reaction, a stable multipole magnetic field is continuously applied to the outside of the conical flask through an external electromagnetic coil, with a magnetic induction intensity of 0.1–0.3 T.

[0124] Step (5): Take out the coconut shell biomass and freeze-dry it at -60℃ for 20h to obtain coconut shell biomass with uneven magnetic particle loading, for later use;

[0125] Step (6): The coconut shell biomass blank with unevenly loaded magnetic particles obtained in step 5 is placed in a tube furnace and heated from room temperature to 800°C at a heating rate of 3°C / min under nitrogen atmosphere protection and held for 2 hours. Then, it is cooled to 400°C at a cooling rate of 5°C / min and cooled to room temperature with the furnace to obtain coconut shell biochar with unevenly loaded magnetic particles.

[0126] The scanning electron microscope images of Example 1 are attached. Figure 1 As shown in the figure, the synthesized non-uniform porous biochar microwave absorbing material has a micron-scale non-uniform macroporous structure, which is beneficial to the multiple reflection loss of electromagnetic waves.

[0127] Appendix Figure 2 The image shows a transmission electron microscope image of the coconut shell biochar microwave absorbing material with unevenly distributed magnetic particles synthesized in Comparative Example 3. The image shows that nano- and micron-sized particles are unevenly dispersed on the surface of the biochar material. This indicates that unevenly distributed magnetic particles can be grown on the biomass surface by combining in-situ growth precipitation method with a stable multipole magnetic field provided by an external electromagnetic coil.

[0128] Appendix Figure 3 The image shows a transmission electron microscope image of the non-uniform porous biochar microwave absorbing material synthesized in Example 1. It can be seen that after the magnetic particles are etched, nanoscale micropores are generated on the surface of the biochar, which can effectively increase the propagation path of electromagnetic waves and thus improve the loss of electromagnetic waves.

[0129] Appendix Figure 4(The horizontal axis 2θ represents the diffraction angle (degree), and the vertical axis Intensity represents the intensity (au)) shows the X-ray diffraction patterns of the microwave absorbing materials synthesized in Example 1 and Comparative Example 3. The figures show that both materials obtained in Example 1 and Comparative Example 3 exhibit broad peaks of amorphous carbon, while the microwave absorbing material synthesized in Comparative Example 3 shows obvious crystalline peaks of Fe3O4 (JCPDS: 75-0033). This indicates that acid etching can completely etch the magnetic particles on the surface of biochar, thereby preserving the pore structure of the loading sites and forming a non-uniform pore structure on the biochar surface, thus improving electromagnetic wave absorption performance. Furthermore, after completely eliminating the magnetic particles, the biochar no longer possesses magnetism, allowing for applications in more scenarios.

[0130] Appendix Figure 5 (The horizontal axis represents thickness (mm), the vertical axis represents frequency (GHz), and the vertical axis represents reflection loss (dB)) This shows the electromagnetic wave reflection loss performance of Example 1 at different thicknesses and frequencies. Thanks to its excellent pore structure and high graphitization degree (dielectric loss), the non-uniform porous biochar material exhibits excellent wave absorption performance. When the material thickness is 1.9 mm, the absorption peak corresponds to a frequency of 5.81 GHz; when the thickness is 2.0 mm, the maximum reflection loss is -48.57 dB, indicating that the non-uniform porous biochar material has the best absorption effect at this thickness.

[0131] Appendix Figure 6 (The horizontal axis represents thickness (mm), the vertical axis represents frequency (GHz), and the vertical axis represents reflection loss (dB)) This shows the electromagnetic wave reflection loss performance of Comparative Example 2 at different thicknesses and frequencies. When the material thickness is 1.8 mm, the frequency corresponding to the absorption peak is 4.54 GHz; when the thickness is 3.8 mm, the maximum reflection loss is -40.32 dB. Compared to Example 1, the absorption bandwidth is significantly narrower, and the thickness is also greater, indicating that the non-uniform channel structure constructed by applying a multipolar magnetic field can effectively improve the wave absorption performance of the material.

[0132] Appendix Figure 7 and attached Figure 8 (The horizontal axis represents frequency (GHz), and the vertical axis represents the normalized input impedance magnitude |Z) in / Z0|,Z in(Z0 is the input impedance of the material, and Z0 is the characteristic impedance in free space) are the impedance matching diagrams for Example 1 and Comparative Example 1, respectively. Compared to pyrolytic biochar, the impedance matching of the non-uniform porous biochar material is near 1, indicating good impedance matching characteristics, which is beneficial for broadband and efficient absorption. This is because the non-uniform porous structure and the high degree of graphitization of biochar provide multiple reflection losses and dielectric losses, achieving excellent impedance matching and thus improving absorption performance.

Claims

1. A method for preparing a non-uniform porous biochar microwave absorbing material, characterized in that, The steps include the following: Step 1, pretreatment of biomass materials, which involves dividing biomass into small pieces, removing soluble impurities, and obtaining biomass precursors; Step 2, in-situ preparation of reaction solution and synchronous control of magnetic field: Add 250 mL of deionized water to an Erlenmeyer flask, then add 0.004-0.008 mol / L sodium citrate, 0.05-0.2 mol FeCl3·6H2O, and 0.025-0.1 mol FeCl2·4H2O in sequence. Stir at 300-700 rpm for 5-10 min until completely dissolved to obtain a mixed iron salt solution. Add the biomass precursor from step 1 and continue stirring under nitrogen protection for 30-60 min to allow it to soak. Under nitrogen protection, the reaction system was stirred at 500 rpm and heated to 50-70℃. The pH was controlled between 9 and 10 using 25% wt ammonia water. Fe3O4 magnetic nanoparticles were generated in situ. The reaction was continued for 30-60 minutes. During this process, the particles were generated simultaneously in the solution and in the biomass pores and tissues. Meanwhile, during the in-situ generation reaction of Fe3O4 magnetic nanoparticles, a stable multipolar magnetic field with a magnetic induction intensity of 0.1-0.3T was applied to the outside of the conical flask through an external electromagnetic coil, which induced the generated magnetic particles to migrate along the direction of the magnetic field and form uneven enrichment in the biomass pore matrix. The biomass was taken out and freeze-dried at -60℃ for 12-24h to obtain biomass with uneven magnetic particle loading for later use. Step 3, Biomass pyrolysis: The biomass with unevenly loaded magnetic particles obtained in Step 3 is placed in a tube furnace for pyrolysis to obtain biochar with unevenly loaded magnetic particles. Step 4, magnetic particle removal: Remove the magnetic particles from the biochar with unevenly loaded magnetic particles obtained in Step 3 to obtain biochar with a non-uniform porous structure.

2. The method for preparing the non-uniform porous biochar microwave absorbing material according to claim 1, characterized in that, In step 1, the soaking time for removing soluble impurities is 12 hours, and the freeze-drying time is 36 hours.

3. The method for preparing the non-uniform porous biochar microwave absorbing material according to claim 1, characterized in that, In step 2, sodium citrate is used as a dispersant, and the amount added is 0.006 mol / L.

4. The method for preparing the non-uniform porous biochar microwave absorbing material according to claim 1, characterized in that, In step 2, the concentrations of FeCl3·6H2O and FeCl2·4H2O are 0.1 mol / L and 0.05 mol / L, respectively. The two iron salts are added sequentially to the solution containing the dispersant, and the solution is magnetically stirred at 500 rpm for 5 min to obtain a uniform and stable solution. The stirring and wetting time under nitrogen protection is 40 min.

5. The method for preparing the non-uniform porous biochar microwave absorbing material according to claim 1, characterized in that, In step 2, the magnetic induction intensity is 0.25T.

6. The method for preparing the non-uniform porous biochar microwave absorbing material according to claim 1, characterized in that, In step 3, the biomass with unevenly loaded magnetic particles obtained in step 2 is placed in a tube furnace and heated to 600-900℃ at a heating rate of 3-5℃ / min under nitrogen protection, held at that temperature for 1-3 hours, and then cooled to 400℃ at a cooling rate of 3-5℃ / min before being cooled to room temperature with the furnace to obtain biochar with unevenly loaded magnetic particles, which is then ready for use.

7. The method for preparing the non-uniform porous biochar microwave absorbing material according to claim 1 or 6, characterized in that, In step 3, the heating rate is 3℃ / min and the cooling rate is 5℃ / min.

8. The method for preparing the non-uniform porous biochar microwave absorbing material according to claim 7, characterized in that, In step 3, the pyrolysis temperature is 800℃.

9. The method for preparing the non-uniform porous biochar microwave absorbing material according to claim 1, characterized in that, The specific steps of step 4 are as follows: the biochar with unevenly loaded magnetic particles obtained in step 3 is ground and dispersed in 1M HCl solution. The solid-liquid ratio is 1g of biochar material dissolved in 50-200mL hydrochloric acid solution. The mixture is shaken at 25℃ for 1-3h in a constant temperature shaker. The solid is obtained by suction filtration and washed with deionized water until neutral. The obtained biochar is placed in a 60℃ oven and dried for 3-6h to obtain biochar with a non-uniform pore structure.

10. The method for preparing the non-uniform porous biochar microwave absorbing material according to claim 9, characterized in that, In step 4, the solid-liquid ratio of the non-uniformly loaded biochar with magnetic particles to the HCl solution is 1g:100mL; the isothermal shaking time is 2h; and the drying time is 5h.

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