A broadband microwave absorbing plate with low carbon tube loading and a preparation method thereof

Abstract

The present application discloses a broadband absorbing flat plate with low carbon tube loading and a preparation method thereof, which relates to the technical field of absorbing materials, and at least solves the technical problem that the broadband absorbing flat plate prepared by the prior art cannot meet the application requirements. The broadband absorbing flat plate with low carbon tube loading comprises an upper wave-transmitting structure and a lower wave-absorbing structure; the carbon nanotube content of the upper wave-transmitting structure is 1wt% to 20wt%, the thickness is 0.5mm to 4mm, and the single-walled carbon nanotubes are vertically oriented; the composite carbon nanotube content of the lower wave-absorbing structure is 1wt% to 10wt%, the thickness is 0.5mm to 4mm, and the single-walled carbon nanotubes are vertically and horizontally oriented.

Description

Technical Field

[0001] The present application relates to the technical field of absorbing materials, and in particular to a broadband absorbing flat plate with low carbon tube loading and a preparation method thereof. Background Art

[0002] Carbon nanotubes are considered to be highly efficient microwave absorbers due to their high electrical conductivity, specific surface area, thermal conductivity and low density. However, the electromagnetic incompatibility of the single component and structure of the original carbon nanotubes makes their impedance performance poor, and the electromagnetic waves are almost reflected instead of dissipated, resulting in extremely limited absorption performance. Common improvement strategies are to modify the surface of carbon nanotubes, such as loading magnetic particles and surface oxidation; or to construct special microstructures, such as porous structures and honeycomb structures, to improve interface properties and introduce multiple reflections, thereby improving the absorption performance of the overall structure.

[0003] However, all of the above rely on complex preparation processes and high amounts of carbon tube addition, and the prepared flat plates have poor mechanical properties and are difficult to meet actual application requirements. Summary of the invention

[0004] The present application provides a broadband absorbing flat plate with low carbon tube loading and a preparation method thereof, which at least solves the technical problem that the broadband absorbing flat plate prepared by the prior art cannot meet application requirements.

[0005] In order to solve the above technical problems, the embodiment of the present application provides: a broadband wave absorbing plate with low carbon tube loading, the broadband wave absorbing plate comprising an upper wave-transmitting structure and a lower wave-absorbing structure;

[0006] The carbon nanotube content of the upper wave-transmitting structure is 1wt% to 20wt%, the thickness is 0.5mm to 4mm, and the single-walled carbon nanotubes are vertically oriented;

[0007] The composite carbon nanotube content of the lower absorbing structure is 1wt% to 10wt%, the thickness is 0.5mm to 4mm, and the single-walled carbon nanotubes are oriented vertically and horizontally.

[0008] As some optional embodiments of the present application, the carbon nanotube content of the upper wave-transmitting structure is 6.7wt%, and the thickness is 0.7mm; the composite carbon nanotube content of the lower wave-absorbing structure is 4.8wt%, and the thickness is 1.8mm.

[0009] As some optional embodiments of the present application, the composite carbon nanotubes include original carbon nanotubes and modified carbon nanotubes, and the ratio of the original carbon nanotubes to the modified carbon nanotubes is 1-5:5-9.

[0010] As some optional embodiments of the present application, the ratio of the original carbon nanotubes to the modified carbon nanotubes is 2:8.

[0011] As some optional embodiments of the present application, the maximum tensile strength of the broadband absorbing flat plate is 95 MPa, and the maximum bending strength is 136 MPa.

[0012] As some optional implementations of the present application, the broadband absorbing plate can achieve full absorption in the 8 GHz to 18 GHz band, and the absorbing bandwidth less than -10 dB is as low as 8.5 GHz.

[0013] On the other hand, the present application provides a method for preparing the broadband absorbing plate with low carbon tube loading as described above, comprising the following steps:

[0014] The original carbon nanotubes are oxidized by the Hummers method to obtain oxidized carbon nanotubes with partially destroyed conjugated structures and oxygen-containing functional groups on the surface; the oxidized carbon nanotubes with partially destroyed conjugated structures and oxygen-containing functional groups on the surface are subjected to a derivatization reaction with active oxygen-containing functional groups to obtain modified carbon nanotubes; the modified carbon nanotubes have macromolecular chains grafted on the surface;

[0015] The original carbon nanotubes and the modified carbon nanotubes are dried, blended, kneaded and screw extruded with a thermoplastic matrix and a plasticizer to obtain a printable filament for FDM;

[0016] The FDM printable filament is placed in an FDM device, and printing is started along the X-0° direction according to the digital model to obtain a cube with a first target thickness, in which the modified carbon nanotubes are all oriented along the X-0° direction; and then the cube is cut perpendicularly to the X direction to obtain an upper plate with a first target thickness, in which the modified carbon nanotubes are all oriented along the Z-0° direction;

[0017] The FDM printable filament is placed in an FDM device, and the upper flat plate is used as a substrate. Printing is started along the 0°+90° or 45° direction according to the digital model to prepare a cube with a second target thickness in which the carbon nanotubes are all oriented along the XY direction; after printing, the cube is taken out and the substrate is placed upward to obtain a "wave-transmitting-wave-absorbing" heterostructure in which the upper modified carbon nanotubes are all oriented along the Z-0° direction and the lower carbon nanotubes are all oriented along the XY direction.

[0018] As some optional embodiments of the present application, the original carbon nanotubes are open-type carbon nanotubes.

[0019] As some optional implementations of the present application, when the original carbon nanotubes are oxidized by the Hummers method, the temperature is 50° C. and the reaction time is 8 hours to 72 hours.

[0020] As some optional embodiments of the present application, the macromolecular chain is a low-polarity molecular chain.

[0021] As some optional embodiments of the present application, the macromolecular chain includes any one of an alkane chain, a siloxane molecular chain and a fluorine-containing molecular chain.

[0022] As some optional embodiments of the present application, the thermoplastic matrix is ​​any one of polylactic acid, ABS, nylon, polycarbonate, polyphenylene sulfide, polyetherimide, polyetheretherketone and polyetherketone.

[0023] As some optional implementations of the present application, the first target thickness is 0.5 mm to 4 mm, and the second target thickness is 0.5 mm to 4 mm.

[0024] As some optional embodiments of the present application, the diameter of the printing needle used for printing is 0.25mm to 0.8mm.

[0025] It can be seen that the present application provides a broadband absorbing flat plate with a low carbon tube loading, and the broadband absorbing flat plate includes an upper wave-transmitting structure and a lower wave-absorbing structure; the carbon nanotube content of the upper wave-transmitting structure is 1wt% to 20wt%, the thickness is 0.5mm to 4mm, and the single-walled carbon nanotubes are vertically oriented; the composite carbon nanotube content of the lower wave-absorbing structure is 1wt% to 10wt%, the thickness is 0.5mm to 4mm, and the single-walled carbon nanotubes are vertically and horizontally oriented. Through the above-mentioned structural improvement, the broadband absorbing flat plate with a low carbon tube loading described in the present application has a very significant improvement compared with the composite materials prepared by traditional melt blending or solvent blending methods, and achieves full absorption in the 8-18GHz band, and the absorbing bandwidth less than -10dB can reach 8.5GHz. In addition, the tensile strength of the double-layer heterogeneous structure absorbing flat plate described in the present application reaches 95MPa, and the flexural strength reaches 136MPa, which is far superior to conventional porous structures. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] In order to more clearly illustrate the specific implementation of the present application or the technical solution in the prior art, the following is a brief introduction to the drawings required for the specific implementation or the prior art description. In all drawings, similar elements or parts are generally identified by similar reference numerals. In the drawings, each element or part is not necessarily drawn according to the actual scale.

[0027] Figure 1 This is a schematic diagram of the modification of single-walled carbon nanotubes described in the embodiments of the present application;

[0028] Figure 2 This is a schematic diagram of a wave-transmitting upper layer structure prepared by fused deposition 3D printing as described in an embodiment of the present application;

[0029] Figure 3 This is a schematic diagram of the second layer structure prepared by fused deposition 3D printing in an embodiment of the present application;

[0030] Figure 4 This is a schematic diagram of the "wave-transmitting-wave-absorbing" structure described in an embodiment of the present application.

[0031] The realization of the purpose, functional features and advantages of this application will be further explained in conjunction with embodiments and with reference to the accompanying drawings. DETAILED DESCRIPTION

[0032] The following will be combined with the drawings in the embodiments of the present application to clearly and completely describe the technical solutions in the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, not all of the embodiments. Based on the embodiments in the present application, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of this application.

[0033] In recent years, the emerging additive manufacturing (3D printing) technology is based on the principle of discrete stacking, using automatic control to stack materials layer by layer to achieve rapid manufacturing of complex parts. It has the advantages of flexible structural design, integrated processing and molding, high preparation accuracy, and short preparation cycle, providing a new way to prepare absorbing parts with complex structures. Among them, fused deposition 3D printing technology (FDM) is a method of melting various fusible filaments or granules by heating, solidifying and molding them layer by layer on the machine printing table to finally obtain a three-dimensional component. During the material melting and deposition process, the internal filler will be oriented along the path direction to form a special anisotropic configuration.

[0034] Based on this, the present application hopes to utilize the characteristics of FDM to first prepare a cube with a certain thickness of carbon nanotubes oriented along the X-0° direction, and then obtain a flat plate with a certain thickness by cutting perpendicular to the X direction, in which the carbon nanotubes are all oriented along the Z-0° direction. Then, the flat plate is used as a substrate to continue printing a material containing carbon tubes of a certain thickness, with a path of 0°+90° or 45°. After printing, the workpiece is taken out and the substrate is placed upward to obtain a "wave-transmitting-wave-absorbing" heterostructure in which the upper carbon nanotubes are oriented along the Z-0° direction and the lower carbon nanotubes are oriented along the XY direction. In addition, in order to further improve the wave transmission performance of the upper layer, it is necessary to solve the problems of agglomeration and high conductivity of the original carbon nanotubes to improve the impedance matching and orientation degree of the carbon tubes, so as to ultimately achieve broadband and efficient absorption of electromagnetic waves at a low carbon nanotube loading, and maintain good mechanical properties of the structure.

[0035] To solve the above problems, the present application provides a design and preparation method for constructing a broadband absorbing structure with low carbon tube loading based on fused deposition 3D printing, and the specific process steps and conditions are as follows:

[0036] 1) Modification of original carbon nanotubes (e.g. Figure 1 shown)

[0037] The original carbon nanotubes (CNTs) are oxidized by the Hummers method to obtain oxidized carbon nanotubes (CNT-O) with a partially destroyed conjugated structure and a large number of oxygen-containing functional groups on the surface; then, macromolecular chains are grafted on the surface of CNT-O through a derivatization reaction with active oxygen-containing functional groups (carboxyl or hydroxyl).

[0038] The carbon nanotubes used are open-type carbon nanotubes, preferably single-walled carbon nanotubes. The Hummers method used to modify carbon nanotubes is controlled at a temperature of 50°C and a reaction time of 8-72 hours, preferably 12 hours. This is because if the modification time is too short, the carbon nanotubes are not sufficiently oxidized, the grafting reaction sites are insufficient, and the density of the grafted molecular chains is low; if the time is too long, the carbon nanotubes will be excessively damaged, and the subsequent conjugated structure will be difficult to repair, affecting the electromagnetic wave absorption performance. The grafted macromolecular chain is a low-polarity molecular chain such as an alkane chain, a siloxane molecular chain, a fluorine-containing molecular chain, and preferably a fluorine-containing molecular chain.

[0039] 2) Preparation of printable filaments

[0040] The original carbon nanotubes or modified carbon nanotubes, thermoplastic matrix, plasticizer and other raw materials are dried, blended, kneaded and screw extruded to obtain printable filaments for FDM.

[0041] Through experimental comparison, it is found that under a specific content, the carbon nanotubes are arranged in parallel along the direction of electromagnetic wave transmission, with high impedance matching, which can play a role similar to honeycomb, and can effectively transmit and absorb certain electromagnetic waves. When the content is too high, the impedance is not matched, most of the electromagnetic waves are reflected, and the wave absorption effect cannot be played; when the content is too low, although the electromagnetic waves can be incident, the absorption efficiency is insufficient. Therefore, the content of modified carbon nanotubes used to prepare the upper wave-transmitting layer wire is 1wt% to 20wt%, preferably 6.7wt%. The carbon nanotubes used to prepare the wave-absorbing layer wire are original carbon nanotubes and modified carbon nanotubes, with a ratio of 5:5, 4:6, 3:7, 2:8, 1:9, preferably 2:8; the content is 1wt% to 10wt%, preferably 4.8wt%. The thermoplastic matrix used is thermoplastic resins such as polylactic acid, ABS, nylon, polycarbonate, polyphenylene sulfide, polyetherimide, polyetheretherketone, and polyetherketone, which can be selected according to temperature and environmental use requirements.

[0042] 3) Fused deposition 3D printing of the upper wave-transmitting structure (e.g. Figure 2 shown)

[0043] The printable filament containing modified carbon nanotubes obtained in step 2) is placed in an FDM device, and printing is started along the X-0° direction according to the digital model to prepare a cube with a certain thickness in which the modified carbon nanotubes are all oriented along the X-0° direction. Then, a flat plate with a certain thickness is obtained by cutting the printed cube perpendicular to the X direction, and the modified carbon nanotubes in the flat plate are all oriented along the Z-0° direction.

[0044] The diameter of the printing needle is 0.25mm, 0.4mm, 0.6mm and 0.8mm. Considering the printing efficiency and improving the orientation of carbon nanotubes, 0.4mm is preferred. This is because the narrower the diameter, the more obvious the shearing effect on the polymer chain, which can effectively improve the orientation of the internal carbon nanotubes; when the diameter is too low, such as 0.25mm, the internal filler is easily accumulated at the nozzle and causes blockage. Through theoretical calculations and actual experiments, it is concluded that if the thickness is too low, the absorption effect is not good; if it is too thick, it will affect the actual use, so a moderate thickness is taken; therefore, the thickness of the cutting plate is 0.5mm to 4mm, preferably 0.7mm.

[0045] 4) Fused deposition 3D printing of the lower absorbing structure (e.g. Figure 3 shown)

[0046] The printable filament containing the original carbon nanotubes obtained in step 2) is placed in the FDM device, and the flat plate obtained in step 3) is used as the substrate. Printing is started along the 0°+90° or 45° direction according to the digital model to prepare a cube with a certain thickness and carbon nanotubes oriented along the XY direction. After printing, the product is taken out and the substrate is placed upward to obtain a "wave-transmitting-wave-absorbing" heterostructure in which the upper modified carbon nanotubes are oriented along the Z-0° direction and the lower carbon nanotubes are oriented along the XY direction.

[0047] The diameter of the printing needle is 0.25mm, 0.4mm, 0.6mm and 0.8mm. Considering the printing efficiency, the poor dispersion of the unmodified original carbon nanotubes makes it easier to block the holes. At the same time, the parallelism perpendicular to the electromagnetic wave direction has no significant effect on the wave absorption performance. Therefore, 0.8mm is preferred. The printing thickness is 0.5mm to 4mm. Through theoretical calculation and actual experiments, it is found that if the thickness is too low, the absorption effect is not good; if it is too thick, it will affect the actual use. A moderate thickness is selected, that is, 1.8mm is preferred.

[0048] The double-layer heterogeneous structure absorbing plate (4) prepared by the above method provided in the present application has a very significant improvement over the carbon nanotube absorbing structure (1) prepared by the conventional method. The carbon nanotube absorbing structure (1) prepared by the conventional method has a thickness of 2.5 mm and refers to a disordered state in which the carbon nanotubes are randomly distributed in the matrix by melt blending or solvent blending. The comparative data are shown in Table 1, wherein the test standard of the data in Table 1 is GJB 2038A-2011, the bow method test system is used, the test frequency range is 2-18 GHz, and the sample size is . The results show that full absorption is achieved in the 8-18GHz band, and the minimum reflectivity can reach -67.2dB, and the absorption bandwidth less than -10dB can reach 8.5GHz. In addition, the tensile strength of the double-layer heterostructure absorbing plate reaches 95MPa, and the bending strength reaches 136MPa, which is far superior to the conventional porous structure. Among them, the standard method for testing tensile strength is GB / T 1447-2005. The speed control mode is: displacement speed control; test temperature: room temperature; loading test rate: 2mm / min. Bending is measured according to GB / T 1449-2005.

[0049] Table 1:

[0050]

[0051] Specifically, compared with the prior art, the present application has the following beneficial effects:

[0052] ① The present application innovatively constructs a "wave-transmitting" structure of single-walled carbon nanotubes with excellent impedance on the upper layer, which is conducive to the incidence of electromagnetic waves, and a "wave-absorbing" structure of single-walled carbon nanotubes with excellent absorption performance on the lower layer, which is oriented vertically and horizontally in the plane (such as Figure 4 As shown in the figure, the double-layer heterostructure composed of the two exhibits excellent broadband absorption capability (8.5GHz, RL<-10dB) at low carbon nanotube content (upper layer 6.7wt% + lower layer 4.8wt%) and low thickness (upper layer 0.7mm + lower layer 1.8mm).

[0053] ② This application uses carbon nanotubes as a "wave-transmitting" channel. Thanks to the excellent mechanical properties and dense composite structure of carbon nanotubes, it has excellent mechanical properties compared to the traditional method of introducing gaps or cavities and can meet the usage requirements of most scenarios.

[0054] ③ Since the method of this application is based on fused deposition 3D printing, the typical "wave absorbing" and "wave transparent" structure design and optimization are flexible, the material utilization is high, and the post-processing process is less, which can shorten the design and manufacturing cycle and reduce production costs, providing a fast way for the development and flexible batch production of wave absorbing components.

[0055] ④ Based on the method described in this application, in addition to being applicable to a variety of thermoplastic substrates, it can also be transformed into ceramic-based composite materials through precursors, such as absorbing silicon oxide, silicon nitride and aluminum oxide.

[0056] ⑤ The idea of ​​the method described in this application is also applicable to a variety of additive manufacturing processes, such as ink direct writing to form absorbing polyvinyl alcohol. At the same time, in addition to using filaments as printing materials, it can also be formed by granular melt deposition.

[0057] It can be seen that the present application has the following technical innovations: ① The "wave-transmitting" structure of the single-walled carbon nanotubes with excellent impedance in the upper layer, which is conducive to the incidence of electromagnetic waves, is innovatively constructed, and the "wave-absorbing" structure of the single-walled carbon nanotubes with excellent absorption performance in the lower layer is oriented vertically and horizontally in the plane, which shows excellent broadband absorption ability at low carbon nanotube content and low thickness; ② Through oxidation modification, not only can the large conjugated structure of the carbon tube be broken, the conductivity be regulated, and dipole polarization be introduced, but also the reaction site can be introduced to graft macromolecular chains, thereby improving the agglomeration between carbon nanotubes and improving the orientation degree of carbon nanotubes along the printing direction; ③ Thanks to the excellent mechanical properties and dense composite structure of carbon nanotubes, it has excellent mechanical properties compared with the traditional method of introducing gaps or cavities, and can meet the use requirements of most scenarios; ④ The double-layer heterogeneous structure is prepared by using the principle of additive manufacturing, which not only has high material utilization and less post-processing process, but also can shorten the design and manufacturing cycle and reduce production costs. At the same time, various complex structures can be designed according to service requirements, thereby realizing flexible and efficient customization of target components.

[0058] The technical solution described in this application will be described in detail below in conjunction with specific embodiments:

[0059] Example 1

[0060] 1) Modification of original carbon nanotubes

[0061] The original single-walled carbon nanotubes (SWCNTs) were oxidized according to the improved Hummers method. The oxidation process mainly has two stages:

[0062] Pre-oxidation stage: 3 g of single-walled carbon nanotubes, 10 mL of concentrated sulfuric acid, 1.2 g of potassium persulfate, and 0.5 g of phosphorus pentoxide were added to a beaker and stirred, heated to 80°C in an oil bath and maintained for 5 hours, naturally cooled to room temperature, and 500 mL of ice water was slowly added to dilute, and finally filtered and washed to a pH of 7, and dried at room temperature for 48 hours to obtain pre-oxidized single-walled carbon nanotubes.

[0063] Oxidation stage: Add 3.0g of pre-oxidized single-walled carbon nanotubes to 120mL of concentrated sulfuric acid, slowly add 9g of potassium permanganate, and stir in an ice bath for 1 hour. Adjust the temperature to 50°C and react for 12 hours. Put the product obtained so far into ice water, add 40mL of hydrogen peroxide and stir continuously. When the solution color begins to show bright yellow, it can be left to stand for 48 hours, and the supernatant can be poured out. The lower precipitate is washed with deionized water and hydrochloric acid respectively to make the pH of the product about 7, and dried at room temperature for 48 hours to obtain the desired oxidized single-walled carbon nanotubes (SWCNT-O).

[0064] Derivatization modification through active oxygen-containing functional groups: 3 g SWCNT-O was added to 500 mL of anhydrous ethanol, dispersed by ultrasound for 30 min, then 500 mg of fluorine-containing silane coupling agent was added and magnetically stirred, heated to 70 ° C in a water bath and maintained for 10 hours, cooled naturally to room temperature, and finally filtered and washed with anhydrous ethanol to remove the ungrafted coupling agent, and dried at room temperature for 48 hours to obtain modified single-walled carbon nanotubes (SWCNT-OF).

[0065] 2) Preparation of printable filaments

[0066] ① Mixing: Pour 100g of SWCNT-OF and 1492.5g of PEEK powder into the mixing barrel of the V-type mixer, add grinding balls of different diameters, start the drive motor, set the parameters: speed is 50r / min, mixing time is 8H; after mixing, take out the mixture from the outlet. ② Drying: Since the mixing time is as long as 8H, in order to ensure the quality of extrusion, the mixed powder needs to be dried, and the drying temperature is set to 120℃ and the drying time is 12H. ③ Screw extrusion: The blended, dispersed and dried SWCNT-OF / PEEK mixed powder is added to the feed port of the screw machine. After passing through different heating zones, it undergoes a transformation from solid to glassy state and then to viscous flow state. Finally, the desired composite material wire is extruded from the die. After being cooled by air in the air cooling tank, the product is obtained by winding it through the traction wheel. The temperatures of zones 1 to 7 are 327°C, 339°C, 343°C, 346°C, 345°C, 343°C and 341°C, respectively, and the screw speed is 35 r·min -1 , winding speed is 11mm·s -1 .

[0067] 3) Fused deposition 3D printing of the upper wave-transmitting structure

[0068] The model to be printed is obtained by drawing with Materialise Magics software, and then sliced ​​with Cura software. Before printing, the filament needs to be tested for printability to determine the appropriate printing rate and slice thickness. The printable filament containing 6.7wt% SWCNT-OF is placed in the FDM device, the nozzle diameter is selected to be 0.4mm, and printing is started along the X-0° direction according to the digital model to prepare a cube with a certain thickness of modified carbon nanotubes oriented along the X-0° direction. After that, a flat plate with a thickness of 0.7mm is obtained by cutting the printed cube perpendicular to the X direction, and the modified carbon nanotubes in the flat plate are all oriented along the Z-0° direction.

[0069] 4) Fused deposition 3D printing of the lower absorbing structure

[0070] The printable filament containing 0.96wt% original carbon nanotubes and 3.84wt% SWCNT-OF is placed in the FDM device, and the flat plate obtained in step 3) is used as the substrate. The nozzle diameter is selected to be 0.8mm, and printing is started along the 45° direction according to the digital model to prepare a cube with a thickness of 1.8mm and carbon nanotubes oriented along the XY direction. After printing, the product is taken out and the substrate is placed upward to obtain a "wave-transmitting-wave-absorbing" heterostructure in which the upper modified carbon nanotubes are oriented along the Z-0° direction and the lower carbon nanotubes are oriented along the XY direction.

[0071] The above are only preferred embodiments of the present application, and are not intended to limit the patent scope of the present application. Any equivalent structure or equivalent process transformation made using the contents of the present application specification and drawings, or directly or indirectly applied in other related technical fields, are also included in the patent protection scope of the present application.

Claims

1. A broadband microwave absorbing plate with low carbon tube loading, characterized in that: The broadband wave absorbing plate comprises an upper wave-transmitting structure and a lower wave-absorbing structure; The modified carbon nanotube content of the upper wave-transmitting structure is 1wt% to 20wt%, the thickness is 0.5mm to 4mm, and the single-walled carbon nanotubes are vertically oriented; The composite carbon nanotube content of the lower absorbing structure is 1wt% to 10wt%, the thickness is 0.5mm to 4mm, and the single-walled carbon nanotubes are oriented vertically and horizontally; the composite carbon nanotubes include original carbon nanotubes and modified carbon nanotubes, and the ratio of the original carbon nanotubes to the modified carbon nanotubes is 1 to 5:5 to 9.

2. The broadband microwave absorbing flat plate with low carbon tube loading according to claim 1, characterized in that: The modified carbon nanotube content of the upper wave-transmitting structure is 6.7wt%, and the thickness is 0.7mm; the composite carbon nanotube content of the lower wave-absorbing structure is 4.8wt%, and the thickness is 1.8mm.

3. The broadband microwave absorbing plate with low carbon tube loading according to claim 1, characterized in that: The ratio of the original carbon nanotubes to the modified carbon nanotubes is 2:

8.

4. The broadband microwave absorbing flat plate with low carbon tube loading according to claim 1, characterized in that: The maximum tensile strength of the broadband absorbing flat plate is 95 MPa, and the maximum bending strength is 136 MPa.

5. The broadband microwave absorbing flat plate with low carbon tube loading according to claim 1, characterized in that: The broadband wave-absorbing plate can achieve full absorption in the 8 GHz to 18 GHz band, and the wave-absorbing bandwidth less than -10 dB is as low as 8.5 GHz.

6. A method for preparing a broadband microwave absorbing flat plate with low carbon tube loading as claimed in any one of claims 1 to 5, characterized in that: The following steps are involved: The original carbon nanotubes are oxidized by Hummers method to obtain oxidized carbon nanotubes with partially destroyed conjugated structures and oxygen-containing functional groups on the surface. Then, the oxidized carbon nanotubes with oxygen-containing functional groups on the surface whose conjugated structure is partially destroyed undergo a derivatization reaction with the active oxygen-containing functional groups to obtain modified carbon nanotubes; The surface of the modified carbon nanotube is grafted with a macromolecular chain; The original carbon nanotubes and the modified carbon nanotubes are dried, blended, kneaded and screw extruded with a thermoplastic matrix and a plasticizer to obtain a printable filament for FDM; Placing the FDM printable filament in an FDM device, and starting printing along the X-0° direction according to the digital model to obtain a cube with a first target thickness in which all modified carbon nanotubes are oriented along the X-0° direction; Then, the cube is cut perpendicularly to the X direction to obtain an upper plate with a first target thickness, wherein the modified carbon nanotubes in the plate are all oriented along the Z-0° direction; Placing the FDM printable filament in an FDM device, taking the upper flat plate as a substrate, and starting printing along a direction of 0°+90° or 45° according to a digital model to prepare a cube having a second target thickness and in which carbon nanotubes are all oriented along the XY direction; After printing is completed, the cube is taken out and the substrate is placed upward to obtain a "wave-transmitting-wave-absorbing" heterostructure in which the upper modified carbon nanotubes are all oriented along the Z-0° direction and the lower carbon nanotubes are all oriented along the XY direction.

7. The method for preparing a broadband microwave absorbing flat plate with low carbon tube loading according to claim 6, characterized in that: The original carbon nanotubes are open-type carbon nanotubes.

8. The method for preparing a broadband microwave absorbing flat plate with low carbon tube loading according to claim 6, characterized in that: When the original carbon nanotubes are oxidized by the Hummers method, the temperature is 50° C. and the reaction time is 8 to 72 hours.

9. The method for preparing a broadband microwave absorbing flat plate with low carbon tube loading according to claim 6, characterized in that: The macromolecular chain is a low-polarity molecular chain.

10. The method for preparing a broadband microwave absorbing flat plate with low carbon tube loading according to claim 9, characterized in that: The macromolecular chain is any one of an alkane chain, a siloxane molecular chain and a fluorine-containing molecular chain.

11. The method for preparing a broadband microwave absorbing flat plate with low carbon tube loading according to claim 6, characterized in that: The thermoplastic matrix is ​​any one of polylactic acid, ABS, nylon, polycarbonate, polyphenylene sulfide, polyetherimide, polyetheretherketone and polyetherketone.

12. The method for preparing a broadband microwave absorbing flat plate with low carbon tube loading according to claim 6, characterized in that: The first target thickness is 0.5 mm to 4 mm, and the second target thickness is 0.5 mm to 4 mm.

13. The method for preparing a broadband microwave absorbing flat plate with low carbon tube loading according to claim 6, characterized in that: The diameter of the printing needle used for printing is 0.25mm-0.8mm.

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