Biomass-derived carbon-based shielding materials, their preparation methods and applications, and electromagnetic shielding components.

Abstract

This invention discloses a biomass-derived carbon-based shielding material, its preparation method, applications, and electromagnetic shielding components. Using natural plant stems as raw materials, the material undergoes selective removal of certain components, directional freeze-drying, and controlled high-temperature carbonization, retaining the hierarchical porous structure and anisotropic pore arrangement of the biological template. By controlling the degree of carbon graphitization, a gradient distribution of electrical conductivity is achieved, forming an electromagnetic matching structure that gradually transitions from high impedance at the surface to low impedance at the interior. This material achieves an electromagnetic shielding effectiveness of 45-65 dB in the X-band (8.2-12.4 GHz), with an absorption ratio exceeding 75% and a density below 0.15 g / cm³. 3 The thickness is 2-5 mm. This invention makes full use of the natural porous structure of biomass, avoiding complex artificial pore-forming processes. The lightweight and efficient shielding material prepared has promising applications in fields such as electromagnetic compatibility of electronic devices and aerospace stealth.

Description

Technical Field

[0001] This invention relates to the field of electromagnetic shielding materials technology, specifically to a biomass-derived carbon-based shielding material, its preparation method and application, and electromagnetic shielding components. Background Technology

[0002] With the rapid development of technologies such as 5G communication, the Internet of Things, and artificial intelligence, electromagnetic compatibility (EMC) issues of electronic devices are becoming increasingly prominent. Electromagnetic radiation not only interferes with the normal operation of equipment but may also pose potential hazards to human health. Traditional metal shielding materials (such as copper foil and aluminum foil) have high shielding effectiveness, but they suffer from drawbacks such as high density, susceptibility to corrosion, complex processing, and difficulty in adapting to curved surfaces. Polymer-based conductive composite materials (such as carbon nanotubes / polymers and graphene / polymers) are lightweight and easy to mold, but high filler content (>10 wt%) is required to form a conductive network, leading to decreased mechanical properties, increased costs, and secondary pollution due to reflection loss.

[0003] Porous carbon materials have become a research hotspot in the field of electromagnetic shielding due to their lightweight, high specific surface area, tunable conductivity, and dielectric properties. An ideal porous shielding material should possess the characteristics of "primarily absorption, secondarily reflection," which requires impedance matching within the material to allow electromagnetic waves to efficiently penetrate the material rather than be reflected at the surface, and then dissipate as heat through dielectric and conduction losses. Currently, the main strategies for achieving impedance matching include: multilayer structure design (high impedance at the surface, low impedance at the bottom), gradient pore structure design, and conductivity gradient distribution, but these methods often involve complex artificial assembly or template fabrication processes.

[0004] Over millions of years of evolution, biomass materials have developed intricate hierarchical porous structures and anisotropic arrangements, such as the directional vascular structure of wood and the axial aeration tissue of reeds. These natural structures provide ideal templates for the preparation of porous carbon materials. In recent years, research on the preparation of carbon-based electromagnetic shielding materials using wood as a template has made progress, but the following problems exist: First, the presence of lignin leads to high brittleness after carbonization and difficulty in graphitization; second, the uneven distribution of natural pore size makes it difficult to optimize the electromagnetic response to specific frequency bands (such as the X-band and Ku-band); third, the anisotropic structure is prone to collapse during carbonization and is difficult to maintain; fourth, there is a lack of systematic design for impedance matching, and the shielding mechanism is mainly based on reflection.

[0005] Chinese patent CN110744567A discloses a wood-derived carbon-based electromagnetic shielding material, which removes lignin through chemical treatment and then carbonizes it, preserving the directional porous structure of the wood. However, this method does not involve impedance gradient design, and the high-temperature graphitization results in high overall conductivity, leading to a relatively high proportion of reflection loss. CN112410235A reports a gradient-pore carbon aerogel, but it is prepared using a sol-gel method, which is complex and has high raw material costs, failing to utilize the natural structural advantages of biomass.

[0006] Therefore, developing a carbon-based shielding material that can fully utilize the natural anisotropic structure of biomass and achieve impedance gradient and efficient absorption through a simple process has significant application value. Summary of the Invention

[0007] The present invention aims to at least solve one of the technical problems existing in the prior art, and provides a biomass-derived carbon-based shielding material, its preparation method and application, and an electromagnetic shielding component.

[0008] In a first aspect, embodiments of the present invention provide a biomass-derived carbon-based shielding material, comprising: A three-dimensional porous carbon framework, obtained by selectively removing lignin and some hemicellulose from natural plant stems and then carbonizing them, possesses axially penetrating macropores and a radially distributed mesoporous-microporous network. The macropores have a pore size of 50-200 μm, and the mesoporous-microporous network has a pore size of 2-50 nm. The porosity is 85-95%, and the density is 0.08-0.15 g / cm³. 3 ; The impedance gradient structure exhibits a gradient distribution of conductivity along the material thickness direction, with a surface conductivity of 10. -3 -10 -1 S / m, intermediate layer conductivity 10 1 -10 2 S / m, core layer conductivity 10 2 -10 3 S / m, forming a matching structure that causes electromagnetic waves to gradually absorb from the surface and be transmitted to the interior with losses; Anisotropic channels are arranged, with large channels oriented along the axial height, with an orientation degree greater than 80%, radial curvature of 1.2-2.0, and axial curvature of 1.0-1.1.

[0009] Optionally, the natural plant stem is selected from one or more of reed stalks, cattail stalks, sugarcane bagasse, bamboo joints, corn stalks, and sorghum stalks, with a diameter of 2-10 mm, a wall thickness of 0.5-2 mm, and a natural porosity of 60-75%.

[0010] Optionally, the impedance gradient structure is implemented in the following way: The surface layer is pre-oxidized at low temperature to introduce oxygen-containing functional groups to reduce conductivity, the core layer is graphitized at high temperature above 2000℃ to increase conductivity, and the intermediate layer remains in an intermediate state, forming a gradient distribution with orders of magnitude difference in conductivity.

[0011] Alternatively, the carbonization process may employ the following heating method: Dehydration is promoted by heating from room temperature to 400℃ at a low rate of 1-2℃ / min, carbonization is completed by heating from 400-800℃ at 3-5℃ / min, and structural rearrangement is carried out by heating from 800-1200℃ at 5-10℃ / min. The protective atmosphere is nitrogen or argon.

[0012] Secondly, embodiments of the present invention provide a method for preparing the biomass-derived carbon-based shielding material described above, comprising the following steps: (1) Raw material pretreatment: Cut the natural plant stems into sections, peel them, wash them, and dry them until the moisture content is <10%; (2) Selective removal: The lignin is selectively removed using a sodium chlorite-acetic acid system or an alkaline hydrogen peroxide system while retaining the cellulose skeleton. The removal rate is 60-85%, and a white porous cellulose template is obtained. (3) Directional freeze drying: The cellulose template is immersed in deionized water or tert-butanol and frozen axially at a freezing rate of 5-20℃ / min; then freeze-dried for 48-72 h to fix the anisotropic pore structure. (4) Surface pre-oxidation: Place the dried template in the air and treat it at 150-250℃ for 2-4 h to introduce oxygen-containing functional groups such as carbonyl and carboxyl groups; (5) Controllable carbonization: The pre-oxidized template is heated to 800-1200℃ under a protective atmosphere and held for 2-4 h to obtain a carbon skeleton with anisotropic porous channels. (6) Core graphitization: The carbon skeleton is embedded in graphite powder and heated to 2200-2600℃ at 10-15℃ / min under argon atmosphere and held for 1-2 h to achieve core graphitization. The surface layer maintains low electrical conductivity due to the defect layer formed by pre-oxidation, thus forming an impedance gradient structure.

[0013] Optionally, in the sodium chlorite-glacial acetic acid system described in step (2): Sodium chlorite concentration 1.0-1.5 wt%, pH adjusted to 3.5-4.5 with glacial acetic acid, temperature 70-90℃, treatment time 4-8h; in the alkaline hydrogen peroxide system, NaOH concentration 2-4 wt%, H2O2 concentration 3-5 wt%, temperature 50-70℃, treatment time 6-12h.

[0014] Optionally, the directional freezing in step (3) adopts a unidirectional freezing method with bottom copper plate cooling and top insulation. The freezing direction is parallel to the stem axis, and the ice crystals grow along the axis, squeezing the cellulose wall to form a directionally arranged layered structure.

[0015] Optionally, in step (6) during graphitization, graphite powder is used as a heat-conducting medium and a sacrificial layer to prevent the sample from being excessively sintered and deformed. The particle size of the graphite powder is 100-300 μm, and the amount used is 10-20 times the mass of the sample.

[0016] Thirdly, embodiments of the present invention provide an electromagnetic shielding component, including the biomass-derived carbon-based shielding material described above, with a thickness of 2-5 mm, which can be used as a single piece or in multiple stacks. When stacked, the axial channels of adjacent layers are arranged at an angle of 0-90°, and the total shielding effectiveness can reach 60-80 dB.

[0017] Fourthly, embodiments of the present invention provide an application of the biomass-derived carbon-based shielding material described above in electromagnetic shielding of electronic devices, stealth of aerospace structures, and electromagnetic protection of building walls, with a working frequency range of 100 MHz-40 GHz and an operating temperature range of -50°C to 200°C.

[0018] Compared with existing technologies, the biomass-derived carbon-based shielding material, its preparation method, its application, and electromagnetic shielding components proposed in this invention have the following beneficial effects: (1) Fully utilize the natural hierarchical porous structure and anisotropy of biomass, avoid complex artificial pore-forming processes, and the raw material cost is lower than that of synthetic polymer templates; (2) Impedance gradient is achieved through differential graphitization. The low conductivity of the surface layer promotes the entry of electromagnetic waves, while the high conductivity of the interior achieves efficient loss. The absorption ratio is >75%, which is better than that of a single high conductivity material. (3) Anisotropic structures can be designed, and the shielding performance in a specific direction can be optimized by adjusting the orientation; (4) Low density (<0.15 g / cm³) 3 It has higher shielding effectiveness (SSE, shielding effectiveness / density) and is suitable for weight-sensitive applications such as aerospace. Detailed Implementation

[0019] To enable those skilled in the art to better understand the technical solutions of the present invention, the present invention will be further described in detail below with reference to specific embodiments. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. Based on the described embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0020] Unless otherwise specifically stated, the technical or scientific terms used in the embodiments of this invention should be understood in their ordinary meaning as understood by one of ordinary skill in the art to which this invention pertains. The terms "comprising" or "including," as used in the embodiments of this invention, do not limit the shapes, numbers, steps, actions, operations, components, elements, and / or groups thereof mentioned, nor do they exclude the appearance or addition of one or more other different shapes, numbers, steps, actions, operations, components, elements, and / or groups thereof, or the inclusion of these.

[0021] In the description of the embodiments of the present invention, the terms "one embodiment," "some embodiments," "example," "specific example," or "some examples," etc., refer to specific features, structures, materials, or characteristics described in connection with that embodiment or example, which are included in at least one embodiment or example of the present invention. In the embodiments of the present invention, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials, or characteristics described may be combined in a suitable manner in any one or more embodiments or examples. Furthermore, without contradiction, those skilled in the art can combine and integrate the different embodiments or examples described in the embodiments of the present invention, as well as the features of different embodiments or examples.

[0022] The following will describe in detail exemplary embodiments according to the present invention. Obviously, the described embodiments are merely some embodiments of the present invention, and not all embodiments of the present invention. It should be understood that the present invention is not limited to the exemplary embodiments described herein.

[0023] This invention relates to a biomass-derived carbon-based shielding material, comprising: A three-dimensional porous carbon framework, obtained by selectively removing lignin and some hemicellulose from natural plant stems and then carbonizing them, possesses axially penetrating macropores and a radially distributed mesoporous-microporous network. The macropores have a pore size of 50-200 μm, and the mesoporous-microporous network has a pore size of 2-50 nm. The porosity is 85-95%, and the density is 0.08-0.15 g / cm³. 3 ; The impedance gradient structure exhibits a gradient distribution of conductivity along the material thickness direction, with a surface conductivity of 10. -3 -10 -1 S / m, intermediate layer conductivity 10 1 -10 2 S / m, core layer conductivity 10 2 -10 3 S / m, forming a matching structure that causes electromagnetic waves to gradually absorb from the surface and be transmitted to the interior with losses; Anisotropic channels are arranged, with large channels oriented along the axial height, with an orientation degree greater than 80%, radial curvature of 1.2-2.0, and axial curvature of 1.0-1.1.

[0024] In some embodiments, the natural plant stems are selected from one or more of reed stalks, cattail stalks, sugarcane bagasse, bamboo joints, corn stalks, and sorghum stalks, with a diameter of 2-10 mm, a wall thickness of 0.5-2 mm, and a natural porosity of 60-75%.

[0025] In some embodiments, the impedance gradient structure is implemented in the following manner: The surface layer is pre-oxidized at low temperature to introduce oxygen-containing functional groups to reduce conductivity, the core layer is graphitized at high temperature above 2000℃ to increase conductivity, and the intermediate layer remains in an intermediate state, forming a gradient distribution with orders of magnitude difference in conductivity.

[0026] In some embodiments, the carbonization process involves heating in the following manner: Dehydration is promoted by heating from room temperature to 400℃ at a low rate of 1-2℃ / min, carbonization is completed by heating from 400-800℃ at 3-5℃ / min, and structural rearrangement is carried out by heating from 800-1200℃ at 5-10℃ / min. The protective atmosphere is nitrogen or argon.

[0027] Based on the same inventive concept, this invention also provides a method for preparing the biomass-derived carbon-based shielding material described above, comprising the following steps: (1) Raw material pretreatment: Cut the natural plant stems into sections, peel them, wash them, and dry them until the moisture content is <10%; (2) Selective removal: The lignin is selectively removed using a sodium chlorite-acetic acid system or an alkaline hydrogen peroxide system while retaining the cellulose skeleton. The removal rate is 60-85%, and a white porous cellulose template is obtained. (3) Directional freeze drying: The cellulose template is immersed in deionized water or tert-butanol and frozen axially at a freezing rate of 5-20℃ / min; then freeze-dried for 48-72 h to fix the anisotropic pore structure. (4) Surface pre-oxidation: Place the dried template in the air and treat it at 150-250℃ for 2-4 h to introduce oxygen-containing functional groups such as carbonyl and carboxyl groups; (5) Controllable carbonization: The pre-oxidized template is heated to 800-1200℃ under a protective atmosphere and held for 2-4 h to obtain a carbon skeleton with anisotropic porous channels. (6) Core graphitization: The carbon skeleton is embedded in graphite powder and heated to 2200-2600℃ at 10-15℃ / min under argon atmosphere and held for 1-2 h to achieve core graphitization. The surface layer maintains low electrical conductivity due to the defect layer formed by pre-oxidation, thus forming an impedance gradient structure.

[0028] In some embodiments, in the sodium chlorite-glacial acetic acid system of step (2): Sodium chlorite concentration 1.0-1.5 wt%, pH adjusted to 3.5-4.5 with glacial acetic acid, temperature 70-90℃, treatment time 4-8h; in the alkaline hydrogen peroxide system, NaOH concentration 2-4 wt%, H2O2 concentration 3-5 wt%, temperature 50-70℃, treatment time 6-12h.

[0029] In some embodiments, the directional freezing in step (3) adopts a unidirectional freezing method with bottom copper plate cooling and top insulation. The freezing direction is parallel to the stem axis, and the ice crystals grow along the axis, squeezing the cellulose wall to form a directionally arranged layered structure.

[0030] In some embodiments, during the graphitization process in step (6), graphite powder serves as a heat-conducting medium and a sacrificial layer to prevent excessive sintering and deformation of the sample. The graphite powder has a particle size of 100-300 μm and is used in amounts 10-20 times the sample mass.

[0031] Based on the same inventive concept, this embodiment of the invention also provides an electromagnetic shielding component, including the biomass-derived carbon-based shielding material described above, with a thickness of 2-5 mm, which can be used as a single piece or in multiple stacked layers. When stacked, the axial channels of adjacent layers are arranged at an angle of 0-90°, and the total shielding effectiveness can reach 60-80 dB.

[0032] Based on the same inventive concept, this invention also provides an application of the biomass-derived carbon-based shielding material described above in electromagnetic shielding of electronic devices, stealth of aerospace structures, and electromagnetic protection of building walls, with a working frequency range of 100 MHz-40 GHz and an operating temperature range of -50℃ to 200℃.

[0033] The present invention will be described in detail below with reference to several embodiments.

[0034] Example 1: Reed stalk-based impedance-gradient carbon shielding material (R-MZG) Raw materials: Select one-year-old reed (Phragmites australis) stalks with a diameter of 4-6 mm, cut the middle section of 20 cm, peel and wash, and dry at 80℃ for 12 h.

[0035] Selective removal: A sodium chlorite-glacial acetic acid system was used, with 1.2 wt% NaClO2, pH 4.0 (adjusted with glacial acetic acid), and treatment at 85℃ for 6 h. The lignin removal rate was 78%, yielding a white cellulose template with a yield of 46%.

[0036] Directional freeze drying: The template is immersed in deionized water for 4 hours, then placed in a polypropylene mold (inner diameter 8 mm), with the bottom in contact with a -40℃ cold plate and the top insulated. The freezing rate is 10℃ / min, and the template is freeze-dried at -50℃ for 60 hours.

[0037] Surface pre-oxidation: After treatment in air at 200℃ for 3 hours, the surface color turns light brown with a depth of about 0.3 mm.

[0038] Controlled carbonization: Under nitrogen protection, the temperature was increased to 400℃ at a rate of 1℃ / min, to 800℃ at a rate of 3℃ / min, and to 1000℃ at a rate of 5℃ / min, and held for 3 hours. A black carbon skeleton was obtained, with a diameter shrinking to 2.5-3.5 mm and a density of 0.12 g / cm³. 3 .

[0039] Core graphitization: The carbon skeleton is embedded in 200-mesh graphite powder, protected by argon, heated to 2400℃ at 15℃ / min, and held for 1.5 h.

[0040] Structural characterization: SEM showed highly oriented axial macropores (average pore diameter 80 μm) and radially thin walls forming layered folds; TEM showed clear graphite striations in the core layer (d). 002 =0.338 nm), the surface layer is amorphous carbon. XPS shows that the oxygen content of the surface layer is 12.5 at%, and that of the core layer is 3.2 at%. The conductivity measured by the four-probe method is 0.05 S / m for the surface layer, 15 S / m for the middle layer, and 350 S / m for the core layer.

[0041] Electromagnetic performance testing: A vector network analyzer (Agilent N5247A) was used with the waveguide method to test in the X-band (8.2-12.4 GHz). The sample was cut to 22.86 × 10.16 mm (WR-90 waveguide size) with a thickness of 3 mm. The total shielding effectiveness SET = 58 dB (of which reflection SER = 12 dB and absorption SEA = 46 dB), with an absorption ratio of 79%. The bandwidth of reflection coefficient RL < -10 dB covers the X-band, indicating good impedance matching.

[0042] Anisotropy testing: Comparison of axial (channel parallel to electromagnetic wave propagation direction) and radial (perpendicular) tests on the sample. Axial SET=58 dB, radial SET=42 dB, the difference stems from the guiding effect of channel orientation on electromagnetic wave propagation.

[0043] Example 2: Cattail stalk-based shielding material (T-MZG) Raw material: Cattail (Typha orientalis) culms, 6-8 mm in diameter, with well-developed pith cavities. The pith cavities are prone to collapse during pyrolysis, so the heating rate needs to be reduced.

[0044] Process adjustment: During the carbonization stage, the temperature was reduced from room temperature to -400℃ to 0.5℃ / min, and the rest was the same as in Example 1.

[0045] Properties: Density 0.09 g / cm³ 3 SET=52 dB (X-band), absorption rate 76%. The low density is due to the high pith ratio of cattail, but the mechanical strength is low (compressive strength 0.8 MPa).

[0046] Example 3: Bamboo-based shielding material (B-MZG) Raw material: Internode segments of moso bamboo (Phyllostachys edulis), with a wall thickness of 1.5 mm, dense and with little pith.

[0047] Removal difficulties: Bamboo lignin and cellulose are tightly bound, and the removal rate of sodium chlorite system is only 55%. When the alkaline hydrogen peroxide system (3 wt% NaOH, 4 wt% H2O2, 60℃, 10 h) is used, the removal rate is 72%.

[0048] Energy: Density 0.18 g / cm³ 3 (Higher than reeds), SET=63 dB, absorption ratio 71%. High density and high shielding efficiency are due to dense structures, but the specific shielding efficiency SSE (=SET / density) is lower than that of Example 1.

[0049] Example 4: Layered Structure Design The material from Example 1 was cut into three layers, each 2 mm thick, with axial channel directions of 0°, 45°, and 90°, and stacked together, for a total thickness of 6 mm. SET=72 dB, an improvement of 14 dB compared to a single layer, and the performance in each direction tended to be consistent (anisotropy <15%).

[0050] Comparative Example 1: No pre-oxidation treatment (R-Nonox) The pre-oxidation step of Example 1 was omitted, and the entire product was directly carbonized and then graphitized. The overall conductivity was 280 S / m, SET=61dB, but the reflection SER=28 dB, the absorption ratio was only 54%, and the bandwidth of RL<-10 dB was only 3.2 GHz, resulting in poor impedance matching.

[0051] Comparative Example 2: R-Random Freezing Conventional freeze-drying (non-directional, pre-frozen at -50°C), with randomly arranged channels. SET = 48 dB (X-band), 10 dB lower than Example 1, and is isotropic.

[0052] Comparative Example 3: Lignin not removed (R-Lignin) Direct carbonization of reed stalks (without lignin removal). During carbonization, lignin melts and foams, destroying the pore structure, resulting in high brittleness (SET=31 dB) and uneven thickness.

[0053] Comparative Example 4: Comparison of Synthetic Templates Using polyurethane foam as a template, impregnated with phenolic resin and carbonized, the same density (0.12 g / cm³) was achieved. 3 At SET=38dB, the absorption rate is 62%, which is lower than that of biomass templates, and the process is complicated.

[0054] This invention relates to biomass-derived carbon-based shielding materials, their preparation methods, applications, and electromagnetic shielding components. It describes a method for preparing carbon-based electromagnetic shielding materials with impedance gradient structures and anisotropic porous channels using natural plant stems. Through selective removal, directional freezing, and differential graphitization, lightweight, high-absorption, and designably anisotropic shielding performance is achieved. However, the high-temperature graphitization step in the process is energy-intensive and costly, and the material's brittleness needs to be addressed through resin impregnation or flexible encapsulation; these are key challenges for future industrialization.

[0055] It is understood that the above embodiments are merely exemplary implementations used to illustrate the principles of the present invention, and the present invention is not limited thereto. For those skilled in the art, various modifications and improvements can be made without departing from the spirit and essence of the present invention, and these modifications and improvements are also considered to be within the scope of protection of the present invention.

Claims

1. A biomass-derived carbon-based shielding material, characterized in that, include: A three-dimensional porous carbon framework, obtained by selectively removing lignin and some hemicellulose from natural plant stems and then carbonizing them, possesses axially penetrating macropores and a radially distributed mesoporous-microporous network. The macropores have a pore size of 50-200 μm, and the mesoporous-microporous network has a pore size of 2-50 nm. The porosity is 85-95%, and the density is 0.08-0.15 g / cm³. 3 ; The impedance gradient structure exhibits a gradient distribution of conductivity along the material thickness direction, with a surface conductivity of 10. -3 -10 -1 S / m, intermediate layer conductivity 10 1 -10 2 S / m, core layer conductivity 10 2 -10 3 S / m, forming a matching structure that causes electromagnetic waves to gradually absorb from the surface and be transmitted to the interior with losses; Anisotropic channels are arranged, with large channels oriented along the axial height, with an orientation degree greater than 80%, radial curvature of 1.2-2.0, and axial curvature of 1.0-1.

1.

2. The biomass-derived carbon-based shielding material according to claim 1, characterized in that, The natural plant stems are selected from one or more of the following: reed stalks, cattail stalks, sugarcane bagasse, bamboo joints, corn stalks, and sorghum stalks. They have a diameter of 2-10 mm, a wall thickness of 0.5-2 mm, and a natural porosity of 60-75%.

3. The biomass-derived carbon-based shielding material according to claim 1, characterized in that, The impedance gradient structure is achieved in the following way: The surface layer is pre-oxidized at low temperature to introduce oxygen-containing functional groups to reduce conductivity, the core layer is graphitized at high temperature above 2000℃ to increase conductivity, and the intermediate layer remains in an intermediate state, forming a gradient distribution with orders of magnitude difference in conductivity.

4. The biomass-derived carbon-based shielding material according to claim 1, characterized in that, The carbonization process uses the following heating method: Dehydration is promoted by heating from room temperature to 400℃ at a low rate of 1-2℃ / min, carbonization is completed by heating from 400-800℃ at 3-5℃ / min, and structural rearrangement is carried out by heating from 800-1200℃ at 5-10℃ / min. The protective atmosphere is nitrogen or argon.

5. A method for preparing a biomass-derived carbon-based shielding material according to any one of claims 1-4, characterized in that, Includes the following steps: (1) Raw material pretreatment: Cut the natural plant stems into sections, peel them, wash them, and dry them until the moisture content is <10%; (2) Selective removal: The lignin is selectively removed using a sodium chlorite-acetic acid system or an alkaline hydrogen peroxide system while retaining the cellulose skeleton. The removal rate is 60-85%, and a white porous cellulose template is obtained. (3) Directional freeze drying: The cellulose template is immersed in deionized water or tert-butanol and frozen axially at a freezing rate of 5-20℃ / min; then freeze-dried for 48-72 h to fix the anisotropic pore structure. (4) Surface pre-oxidation: Place the dried template in the air and treat it at 150-250℃ for 2-4 h to introduce oxygen-containing functional groups such as carbonyl and carboxyl groups; (5) Controllable carbonization: The pre-oxidized template is heated to 800-1200℃ under a protective atmosphere and held for 2-4 h to obtain a carbon skeleton with anisotropic porous channels. (6) Core graphitization: The carbon skeleton is embedded in graphite powder and heated to 2200-2600℃ at 10-15℃ / min under argon atmosphere and held for 1-2 h to achieve core graphitization. The surface layer maintains low electrical conductivity due to the defect layer formed by pre-oxidation, thus forming an impedance gradient structure.

6. The preparation method according to claim 5, characterized in that, In the sodium chlorite-glacial acetic acid system described in step (2): Sodium chlorite concentration 1.0-1.5 wt%, pH adjusted to 3.5-4.5 with glacial acetic acid, temperature 70-90℃, treatment time 4-8 h; in the alkaline hydrogen peroxide system, NaOH concentration 2-4 wt%, H2O2 concentration 3-5 wt%, temperature 50-70℃, treatment time 6-12 h.

7. The preparation method according to claim 5, characterized in that, The directional freezing described in step (3) adopts a unidirectional freezing method with bottom copper plate cooling and top insulation. The freezing direction is parallel to the stem axis, and the ice crystals grow along the axis, squeezing the cellulose wall to form a directionally arranged layered structure.

8. The preparation method according to claim 5, characterized in that, In step (6) during graphitization, graphite powder serves as a heat-conducting medium and a sacrificial layer to prevent excessive sintering and deformation of the sample. The particle size of the graphite powder is 100-300 μm, and the amount used is 10-20 times the mass of the sample.

9. An electromagnetic shielding assembly, characterized in that, The material includes the biomass-derived carbon-based shielding material as described in any one of claims 1-4, with a thickness of 2-5 mm. It can be used as a single piece or in multiple stacks. When stacked, the axial channels of adjacent layers are arranged at an angle of 0-90°, and the total shielding effectiveness can reach 60-80 dB.

10. The application of the biomass-derived carbon-based shielding material according to any one of claims 1-4 in electromagnetic shielding of electronic equipment, stealth of aerospace structures, and electromagnetic protection of building walls, with a working frequency range of 100 MHz-40 GHz and an operating temperature range of -50℃ to 200℃.