A porous carbon fiber wave-absorbing material, a preparation method thereof and application thereof in a spacecraft shell

Abstract

This invention relates to the field of microwave absorbing materials, and discloses a porous carbon fiber microwave absorbing material, its preparation method, and its application in spacecraft shells. This porous carbon fiber microwave absorbing material is fibrous, with uniformly distributed nanoscale micropores within the fibers, and a BET value of 350-500 μm. 2 / g, and the 0.5-1.0nm portion of the microporous structure accounts for at least 75%. The porous carbon fiber microwave absorbing material of the present invention has a nanoscale microporous structure with specific BET and specific pore size distribution and no metal residue. This porous carbon fiber microwave absorbing material not only has good microwave absorption and light weight, but also has good weavability. It is expected to break through the previous application method of coating on spacecraft shells, and directly use this material to form a spacecraft shell with microwave absorption properties after weaving.

Description

Technical Field

[0001] This invention relates to the field of microwave absorbing materials, and more particularly to a porous carbon fiber microwave absorbing material, its preparation method, and its application in spacecraft shells. Background Technology

[0002] Carbon fiber composites, due to their high specific strength and modulus and their rust-free properties, have become an indispensable basic material in the structures of aircraft, missiles, launch vehicles, and artificial satellites. Using carbon fiber composites in aircraft main wings, horizontal and vertical tail fins and spars, decorative materials, and panel materials can significantly reduce the structural weight of aircraft and decrease fuel consumption.

[0003] As electromagnetic waves (EMW) serve as carriers of information transmission, the rapid development of new multi-band and broadband electronic instruments necessitates the development of high-performance materials with broadband electromagnetic wave absorption capabilities to effectively eliminate electromagnetic pollution in fields such as healthcare, electronic security, and national defense. Electromagnetic wave absorbing materials can effectively absorb electromagnetic waves. With the increasing demand for electronic security defense technologies, microwave absorbing devices with flexible properties, lightweight construction, and high efficiency are being actively sought and promoted in both civilian and military electronic instruments. Developing materials with highly efficient microwave attenuation properties is beneficial for preventing the harmful effects of electromagnetic waves emitted by electronic devices in both civilian and military fields.

[0004] Carbon-based materials are commonly used as lightweight electromagnetic wave absorbing materials due to their excellent properties such as improved electrical conductivity and good thermal stability. Many studies on microwave absorbing structures are based on the synergistic effect of materials and structures. Results show that the design of porous structures can not only reduce the weight of microwave absorbing composite materials but also alleviate impedance mismatch and enhance multiple scattering absorption within the pores.

[0005] Most existing electromagnetic wave absorbing materials achieve their purpose by coating the outer layer of the equipment with a wave-absorbing coating. However, this method inevitably adds weight to spacecraft and other equipment. If the outer shell of spacecraft and other equipment could have wave-absorbing properties, the structural weight of the spacecraft could be reduced, thus reducing fuel consumption. Summary of the Invention

[0006] To address the aforementioned technical problems, this invention provides a porous carbon fiber microwave absorbing material, its preparation method, and its application in spacecraft shells. The porous carbon fiber microwave absorbing material of this invention possesses a nanoscale microporous structure with specific BET and pore size distribution, and is free of metal residue. This porous carbon fiber microwave absorbing material not only exhibits excellent microwave absorption properties and is lightweight, but also has good weavability. It is expected to break through the previous application method of using it as a coating on spacecraft shells, allowing for the direct use of this material to form a spacecraft shell with microwave absorption properties through weaving.

[0007] The specific technical solution of this invention is as follows:

[0008] In a first aspect, the present invention provides a porous carbon fiber microwave absorbing material, which is fibrous in shape, with nanoscale micropores uniformly distributed within the fibers, having a BET of 350-500 μm. 2 / g, and the portion of the microporous structure between 0.5-1.0nm accounts for at least 75%.

[0009] The porous carbon fiber microwave absorbing material of this invention has a nanoscale microporous structure with specific BET and specific pore size distribution. Through research, this invention has found that porous carbon fibers with the above characteristics have the following advantages compared with traditional carbon fibers or porous carbon fibers: (1) In terms of microwave absorption: the high aspect ratio of carbon fibers is conducive to significantly increasing conduction loss and widening the absorption bandwidth; combined with the microporous structure with specific BET and specific pore size distribution, it is more conducive to the multi-interface reflection of incident electromagnetic waves. The porous structure introduces more interfaces and correspondingly enhances the interface polarization, resulting in a higher dielectric constant. On this basis, the porous carbon fiber microwave absorbing material can be woven to form a conductive network, further enhancing the dielectric loss. Therefore, the porous carbon fiber microwave absorbing material of this invention has excellent microwave absorption capability.

[0010] (2) In terms of weight: Compared with the prior art of coating porous absorbing materials as paint on the outer shell of spacecraft and other equipment, the present invention directly uses lightweight porous carbon fiber absorbing materials after weaving to form the outer shell of spacecraft and other equipment with its own wave-absorbing function, which can significantly reduce weight and reduce fuel consumption.

[0011] (3) Regarding weavability: Since the porous carbon fiber microwave absorbing material of this invention needs to be woven into a spacecraft shell, the weavability requirements for this fiber material are particularly high, mainly in terms of bending strength. Through research, this invention has found that if the BET and pore size distribution of the porous structure are not properly controlled during the introduction of a porous structure inside the carbon fiber, the carbon fiber is prone to bending breakage during weaving or breakage under external impact after weaving due to a significant decrease in bending strength. Therefore, this invention, combining theory and through numerous experiments, has found that controlling the BET and pore size distribution of the carbon fiber within the aforementioned range not only meets the high microwave absorption requirements but also does not reduce or even improves the weavability of the carbon fiber. Specifically: Under the BET and pore size distribution of this invention, the size and number of individual pores in the carbon fiber are extremely small. If the size or number of individual pores is large, the carbon fiber will be prone to bending and breakage under stress; if the size or number of individual pores is small, it will affect the microwave absorption performance and density of the material. Within the scope of the present invention, we have found that not only can the wave absorption properties of the material be ensured, but also that the appropriate microporous structure during the carbon fiber weaving process can help release the internal stress generated when the material is bent or folded, making the carbon fiber less prone to breakage and thus more suitable for weaving.

[0012] Preferably, the diameter of the fiber is 300-500 nm.

[0013] Secondly, the present invention provides a method for preparing porous carbon fiber microwave absorbing material, comprising the following steps:

[0014] (1) Mix zinc salt, polyacrylonitrile and solvent, heat and stir to obtain a clear and transparent solution, which is the spinning solution.

[0015] (2) The spinning solution is made into zinc salt / polyacrylonitrile composite fiber by electrospinning.

[0016] (3) The zinc salt / polyacrylonitrile composite fiber is dried and calcined in an inert atmosphere to convert polyacrylonitrile into carbon while volatile zinc to form a microporous structure, thus producing a porous carbon fiber microwave absorbing material.

[0017] To obtain the porous carbon fiber microwave absorbing material of this invention, the present invention provides the above-mentioned preparation method. First, zinc salt and polyacrylonitrile (PAT), a carbon fiber precursor, are prepared as a spinning solution. Then, zinc salt / PAT composite fibers are obtained through electrospinning. Finally, high-temperature calcination converts PAT into carbon fibers and sublimates zinc to create nanoscale micropores. Specifically, this invention adds zinc salt to the spinning solution and creates pores during the calcination process of carbon fiber preparation. Utilizing the characteristic of zinc's easy high-temperature sublimation, uniform nanoscale micropores can be formed, thereby endowing the porous carbon fiber with lightweight and strong microwave absorption properties, and leaving virtually no zinc residue in the carbon fiber. Compared with carbon fiber materials with metal residues, pure dielectric materials not only improve impedance matching, allowing more electromagnetic waves to enter the material and be dissipated, but also increase the material's thermal stability, protecting it from damage caused by high temperatures during ultra-high-temperature operation.

[0018] Preferably, in step (1), the zinc salt is zinc nitrate and the solvent is DMF.

[0019] Preferably, the heating and stirring temperature is 40-60℃ and the time is 20-40 min.

[0020] It is important to note that, in order to ensure that there is no zinc residue in the carbon fiber and that it has the specific range of BET and pore size distribution mentioned above, the following aspects are extremely critical:

[0021] (a) Ratio of zinc salt to polyacrylonitrile: The ratio of zinc salt to polyacrylonitrile directly determines the number of micropores after calcination and the BET of the carbon fiber. Preferably, in step (1), the ratio of zinc salt to polyacrylonitrile is 0.4–0.8 mmol:0.5 g; more preferably, it is 0.6 mmol:0.5 g. This invention has found that an ideal number of micropores can be obtained within the above ratio range, thereby obtaining a specific range of BET.

[0022] (b) Calcination process: We found that the calcination process significantly affects the pore size of the micropores after zinc volatilization and whether zinc can be completely volatilized. Preferably, in step (3), the calcination process includes:

[0023] The calcination process includes: pre-oxidation in air at 240-260℃ for 1.5-2.5 hours; pre-forming in an inert atmosphere at 750-850℃ for 1.5-2.5 hours; and final forming in an inert atmosphere at 1200-1300℃ for 3-5 hours, followed by cooling to room temperature to obtain porous carbon fiber microwave absorbing material. Compared with the traditional two-step process of low-temperature pre-oxidation followed by high-temperature calcination for polyacrylonitrile-based carbon fibers, this invention adopts a three-stage calcination process: low-temperature pre-oxidation at 240-260℃, medium-temperature pre-forming at 750-850℃, and final high-temperature final forming at 1200-1300℃. During the low-temperature pre-oxidation stage, the fiber undergoes a dehydrogenation cyclization reaction upon contact with oxygen, increasing the O content, while simultaneously undergoing partial fiber pyrolysis, decreasing the N content. During the medium-temperature pre-forming stage, N, H, and O in the fiber are gradually removed under an inert atmosphere, transforming it from a carbon source into a carbonized matrix. The temperature range of 750-850℃ is chosen because this temperature range is where zinc begins to volatilize, and it also coincides with a relatively ideal rigidity of the carbonized matrix. At this temperature, the plasticity of the carbonized matrix is ​​just right, allowing the micropore size formed during zinc volatilization to be in the range of 0.5-1 nm. If the rigidity of the carbonized matrix is ​​too high or too low, it can easily lead to micropores that are too small or too large, respectively. Furthermore, the carbonized matrix at this temperature is more conducive to zinc escape, resulting in less residue. In the high-temperature final forming stage, the carbonized substrate is completely formed, increasing its rigidity. Simultaneously, because this temperature is much higher than the boiling point of zinc, any trace amounts of residual zinc can be completely volatilized.

[0024] Preferably, the total mass concentration of zinc salt and polyacrylonitrile in the spinning solution is 0.1-0.3 g / mL.

[0025] Preferably, in step (2), the electrospinning specifically includes: loading the spinning solution into a syringe with an inner diameter of 0.5-0.7 mm at the needle outlet, with an electrostatic voltage of 15-19 kV and a feeding rate of 0.4-0.5 mL / h.

[0026] Preferably, in step (3), the drying is vacuum drying at a temperature of 50-70°C for 10-15 hours.

[0027] Thirdly, the present invention provides the application of the above-mentioned porous carbon fiber absorbing material in the outer shell of a spacecraft, specifically by weaving the above-mentioned porous carbon fiber absorbing material to form the outer shell of a spacecraft.

[0028] Compared with the prior art, the beneficial effects of the present invention are:

[0029] (1) The porous carbon fiber absorbing material of the present invention has a uniformly distributed nanoscale microporous structure and no metal residue. The porous carbon fiber absorbing material has the advantages of high absorption and low density. The material has a high electromagnetic wave absorption rate in the range of 2 to 18 GHz (S, C, X, Ku bands). When the thickness is only 2.0 mm, the minimum reflection loss (RLmin) is -37.36 dB and the effective absorption bandwidth (EAB) is 5.44 GHz.

[0030] (2) The porous carbon fiber absorbing material of the present invention has a nanoscale microporous structure with specific BET and specific pore size distribution. The porous carbon fiber absorbing material within the above range can overcome the negative impact of porous structure on the weavability of the material while ensuring high absorption and light weight. It is more conducive to weaving and forming, and is expected to break through the previous application method of coating on the outer shell of spacecraft. The material can be directly used to form a spacecraft outer shell with wave absorption performance after weaving, which can eliminate the step of coating the wave absorbing coating and greatly reduce the weight. Attached Figure Description

[0031] Figure 1 The image shows the reflection loss (RL) of the PCF obtained in Example 2.

[0032] Figure 2 The diagram shows the reflection loss (RL) of the pure carbon fiber absorbing material (CF) prepared in Comparative Example 1.

[0033] Figures 3(a) and (b) are transmission electron microscope (TEM) images of the PCF prepared in Example 2;

[0034] Figure 4 The pore size distribution diagram of the PCF obtained in Example 2 is shown.

[0035] Figure 5 A comparison diagram of the real dielectric part (ε′) of the CF prepared in Comparative Example 1 and the PCF prepared in Example 2;

[0036] Figure 6 A comparison diagram of the imaginary dielectric part (ε″) of the CF prepared in Comparative Example 1 and the PCF prepared in Example 2.

[0037] Figure 7 The elemental analysis diagram of PCF prepared in Comparative Example 4 is shown.

[0038] Figure 8 The elemental analysis diagram is shown for the PCF prepared in Example 2. Detailed Implementation

[0039] The present invention will be further described below with reference to embodiments.

[0040] General Implementation Examples

[0041] A porous carbon fiber microwave absorbing material, in the form of fibers with a diameter of 300-500 nm, has uniformly distributed nanoscale micropores within the fibers, with a BET of 350-500 nm. 2 / g, and the portion of the microporous structure between 0.5-1.0nm accounts for at least 75%.

[0042] A method for preparing a porous carbon fiber microwave absorbing material includes the following steps:

[0043] (1) A zinc salt (preferably zinc nitrate), polyacrylonitrile, and a solvent (preferably DMF) are mixed and heated and stirred (preferably at 40-60°C for 20-40 min) to obtain a clear and transparent solution, which is the spinning solution. Preferably, the ratio of zinc salt to polyacrylonitrile is 0.4-0.8 mmol:0.5 g, more preferably 0.4 mmol:0.5 g; the total mass concentration of zinc salt and polyacrylonitrile in the spinning solution is 0.1-0.3 g / mL.

[0044] (2) Electrospinning of the spinning solution: The spinning solution is loaded into a syringe with an inner diameter of 0.5-0.7 mm at the needle outlet, the electrostatic voltage is 15-19 kV, and the feeding rate is 0.4-0.5 mL / h to produce zinc salt / polyacrylonitrile composite fiber.

[0045] (3) The zinc salt / polyacrylonitrile composite fiber is vacuum dried (preferably 50-70℃, 10-15h) and then calcined: first, it is pre-oxidized in air at 240-260℃ for 1.5-2.5h; then, it is pre-formed in an inert atmosphere at 750-850℃ for 1.5-2.5h; finally, it is final-formed in an inert atmosphere at 1200-1300℃ for 3-5h, and cooled to room temperature to obtain porous carbon fiber microwave absorbing material.

[0046] Specific embodiments and comparative examples

[0047] (I) Effect of different zinc salt to polyacrylonitrile ratios on carbon fiber properties

[0048] (1) Preparation of spinning solution: Take an appropriate amount of zinc nitrate (see Table 1), add it to 5 ml of DMF and stir at room temperature; take 0.5 g of polyacrylonitrile powder and introduce it into the above solution; heat the mixed solution in a water bath at 50°C for 30 min to obtain the spinning solution.

[0049] (2) Preparation of nanofiber precursor: Using electrospinning technology, the spinning solution was loaded into a plastic syringe with a stainless steel needle of No. 20 (outlet inner diameter of 0.6 mm), the electrostatic voltage was 17 kV, and the feeding rate was 0.45 mL / h to obtain the nanofiber precursor.

[0050] (3) Drying the nanofiber precursor: Place the nanofiber precursor obtained by electrospinning into a vacuum drying oven and dry it at 60°C for 12 hours.

[0051] (4) The dried nanofiber precursor is calcined at high temperature in a tube furnace. First, it is pre-oxidized at 250°C for 2 hours in air; then it is pre-formed at 850°C for 2 hours in nitrogen atmosphere; finally, it is final-formed at 1250°C for 4 hours in nitrogen atmosphere and cooled to room temperature to obtain the black material, namely carbon fiber microwave absorbing material.

[0052] Table 1: Effect of different zinc salt to polyacrylonitrile ratios on the properties of porous carbon fibers

[0053]

[0054] As can be seen from the table above, the only difference between each comparative example and each embodiment is the different zinc nitrate content.

[0055] Regarding the comparison between Comparative Example 1 and Best Example 2: Comparative Example 1 did not add zinc nitrate, i.e., no microporous structure was introduced into the carbon fibers. Compared with Example 2, Comparative Example 1... Figure 1 To measure the reflection loss (RL) of the porous carbon fiber absorbing material (PCF) in Example 2, the electromagnetic wave absorption performance of the PCF prepared in Example 2 was tested. The material thickness ranged from 1.0 to 5.5 mm. Generally, an RL value below -10 dB indicates that 99% of the electromagnetic waves are absorbed. Figure 1 It can be seen that in the 2–18 GHz range, with a PCF of 2.0 mm, the RLmin is -37.36 dB, and the effective absorption bandwidth (RL value < -10 dB) is 5.44 GHz; while Figure 2 To measure the reflection loss (RL) of the pure carbon fiber absorbing material (CF) prepared in Comparative Example 1, the electromagnetic wave absorption performance of the CF prepared in Comparative Example 1 was tested. The material thickness ranged from 1.0 to 5.5 mm. Figure 2 It can be seen that in the 2–18 GHz range, the RLmin of CF at 2.5 mm is only -10.88 dB, which is weaker than that of Example 2. Figures 3(a) and (b) are transmission electron microscope (TEM) images of the PCF prepared in Example 2. As can be seen from Figure 3(a), the diameter of each porous carbon fiber is about 400 nm; as can be seen from Figure 3(b), the carbon fiber exhibits a dense pore structure after Zn sublimation. Figure 4 The image shows the pore size distribution of the PCF obtained in Example 2. Figure 4 It is clearly shown that the pores in this material are basically microporous structures of about 0.7 nm. Figure 5 , Figure 6The figures show a comparison of the real dielectric part (ε′) and imaginary dielectric part (ε″) of the CF prepared in Comparative Example 1 and the PCF prepared in Example 2, respectively. As can be seen from the figures, the real dielectric part (ε′) and imaginary dielectric part (ε″) of the PCF are significantly higher than those of the CF. This indicates that the porous structure of the PCF introduces more interfaces and correspondingly enhances interfacial polarization, resulting in a higher dielectric constant, increased dielectric loss, and a significant improvement in the material's ability to dissipate electromagnetic waves. Furthermore, in terms of bending strength, the PCF of Example 2 shows little difference from the CF of Comparative Example 1, and even shows a slight improvement, indicating that the introduction of the microporous structure did not lead to a decrease in the weavability of the PCF of Example 2.

[0056] Regarding Comparative Example 2, Examples 1-3, and Comparative Example 3: The difference between the above comparative examples and examples is that the zinc nitrate content increases sequentially. A comparison of the data in the table above shows that when the zinc nitrate dosage is in the range of 0.4-0.8 mmol, the specific BET range (350-500 mg / L) of this invention can be obtained. 2 Porous carbon fibers with a specific pore size distribution range (at least 75% of the microporous structure is 0.5-1.0 nm); if the zinc nitrate content is low, its BET is low, and correspondingly its wave absorption is also poor; if the zinc nitrate content is too high, its bending strength is poor, which is not conducive to weaving and forming.

[0057] (II) Effect of different calcination processes on the properties of porous carbon fibers The steps (1)-(3) of the following comparative examples and embodiments are the same as those in Example 2, and the difference in the calcination process of step (4) is shown in Table 2:

[0058]

[0059] As can be seen from the table above, the only difference between each comparative example and each embodiment lies in the calcination process.

[0060] Regarding Comparative Example 4 and Best Example 2: Comparative Example 4 employs a traditional two-step process of low pre-oxidation followed by high-temperature calcination. This method not only fails to obtain PCF with the target pore size distribution, but elemental analysis of the product also shows residual Zn. Figure 7 ), while the test results in Example 2 showed that almost no Zn residue was found in the product (), Figure 8 The above two aspects are the reasons why the wave absorption and bending strength of the PCF in Comparative Example 4 are not as good as those in Example 2.

[0061] Regarding Comparative Example 5, Example 4, Example 2, and Comparative Example 6: The difference between the above cases lies in the sequentially increasing preforming temperature. The results show that Comparative Example 5, preformed at 650°C, also had Zn residue in the product and could not achieve the target pore size distribution; Comparative Example 6, preformed at 950°C, although the product had almost no Zn residue, still could not achieve the target pore size distribution; while Examples 2 and 4, with preforming temperatures in the range of 750-850°C, could obtain PCFs with the target pore size distribution without Zn residue, and their wave absorption and flexural strength were significantly better than those of the comparative examples.

[0062] Unless otherwise specified, the raw materials and equipment used in this invention are all commonly used in the field; unless otherwise specified, the methods used in this invention are all conventional methods in the field.

[0063] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. Any simple modifications, alterations, and equivalent transformations made to the above embodiments based on the technical essence of the present invention shall still fall within the protection scope of the present invention.

Claims

1. A method for preparing a porous carbon fiber wave-absorbing material, characterized in that include: (1) Zinc salt, polyacrylonitrile and solvent are mixed, heated and stirred to obtain a clear and transparent solution, which is the spinning solution; The ratio of zinc salt to polyacrylonitrile is 0.4~0.8 mmol: 0.5 g; (2) The spinning solution is electrospinned to form zinc salt / polyacrylonitrile composite fibers; (3) The zinc salt / polyacrylonitrile composite fiber is dried and pre-oxidized in air; it is pre-formed at 750-850℃ for 1.5-2.5h in an inert atmosphere, and finally formed at 1200-1300℃ for 3-5h in an inert atmosphere, converting polyacrylonitrile into carbon while volatilizing zinc to form a microporous structure. After cooling, a fibrous porous carbon fiber microwave absorbing material is obtained; the fiber has a uniformly distributed nanoscale microporous structure with a BET of 350-500 μm. 2 / g, and the portion of the microporous structure with a diameter of 0.5-1.0nm accounts for at least 75%.

2. The production method according to claim 1, characterized by: The diameter of the fiber is 300-500 nm.

3. The production method according to claim 1, wherein: In step (1), the zinc salt is zinc nitrate and the solvent is DMF.

4. The production method according to claim 1, wherein: In step (1), the heating and stirring temperature is 40-60℃ and the time is 20-40min.

5. The production method according to claim 1, wherein: In step (1), the total mass concentration of zinc salt and polyacrylonitrile in the spinning solution is 0.1-0.3 g / mL.

6. The production method according to claim 3, characterized by: In step (2), the electrospinning specifically includes: loading the spinning solution into a syringe with an inner diameter of 0.5-0.7 mm at the needle outlet, with an electrostatic voltage of 15-19 kV and a feeding rate of 0.4-0.5 mL / h.

7. The production method according to claim 3, wherein: In step (3), the drying is vacuum drying at a temperature of 50-70℃ for 10-15 hours.

8. The preparation method according to claim 3 or 7, characterized in that: In step (3), the material is first pre-oxidized in air at 240-260℃ for 1.5-2.5h; then pre-formed in an inert atmosphere at 750-850℃ for 1.5-2.5h; and finally final-formed in an inert atmosphere at 1200-1300℃ for 3-5h, and cooled to room temperature to obtain porous carbon fiber microwave absorbing material.

9. The application of the porous carbon fiber microwave absorbing material obtained by the preparation method according to any one of claims 1-8 in the outer shell of a spacecraft, characterized in that: The porous carbon fiber absorbing material is woven to form the outer shell of a spacecraft.

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