Carbon fiber aerogel hollow co9s8 polyhedral material and preparation method and application thereof

Abstract

This invention relates to carbon fiber aerogel@hollow Co9S8 polyhedral materials, their preparation methods, and applications, belonging to the field of microwave absorbing materials technology. The technical problem solved by this invention is to provide a cobalt-based metal sulfide composite material with excellent microwave absorption performance in the low-frequency band. This material uses carbon fiber aerogel as a carrier, with hollow Co9S8 polyhedra loaded on the carrier. This invention uses chemical deposition technology to load ZIF-67 polyhedra onto the surface of the fiber aerogel, followed by vulcanization annealing to prepare a carbon fiber aerogel@hollow Co9S8 polyhedral hierarchical composite material. By constructing a hierarchical structure with a carbon fiber interconnect network as the skeleton, the composite material is endowed with richer heterojunction surfaces, improving multiple reflections and scattering of electromagnetic waves and enhancing polarization loss capability. The unique structural characteristics of the hollow Co9S8 polyhedra enable the composite material to exhibit special magnetic dielectric properties in the low-frequency electromagnetic band, achieving excellent microwave absorption performance of carbon fiber aerogel@hollow Co9S8 polyhedra in the low-frequency band with low filling amounts.

Description

Technical Field

[0001] This invention relates to carbon fiber aerogel@hollow Co9S8 polyhedral material, its preparation method, and its application, belonging to the field of microwave absorbing materials technology. Background Technology

[0002] Electronic devices and wireless communication systems have experienced rapid development and are widely used in various fields, such as radar detection, aerospace communication systems, intelligent driving, and 5G cellular networks. However, the resulting electromagnetic wave (EMW) radiation and electromagnetic interference have adverse effects on equipment performance, human health, and military security. Therefore, the development of high-performance electromagnetic wave absorbing materials is of great significance for both military and civilian applications. Much previous research has focused on enhancing the absorption of mid-to-high frequency electromagnetic waves to achieve wider effective bandwidth and higher absorption intensity. However, research on low-frequency (S-band and C-band) electromagnetic wave absorbing materials is relatively limited. Since the effective operating frequency band of most electronic devices is 2-8 GHz, the lack of absorption in the S-band and C-band may hinder the further development of the 5G era. Furthermore, with the gradual deployment of S-band active phased array radars on aircraft carriers and airborne early warning aircraft, there is an urgent need to develop efficient low-frequency electromagnetic wave absorbing materials.

[0003] Transition metal sulfides (TMS) are semiconductor materials that possess tunable composite dielectric constants and higher electronic conductivity compared to their corresponding metal oxides. Consequently, researchers are increasingly investigating the electromagnetic wave absorption performance of TMS. In particular, magnetic TMS are expected to exhibit exceptionally high permeability compared to their corresponding metal oxides. Therefore, their unique dielectric and magnetic properties inspire the design of magnetic transition metal sulfides for low-frequency electromagnetic wave absorption. Furthermore, it is worth noting that the morphology and phase composition of TMS significantly influence their electromagnetic wave attenuation characteristics and impedance matching due to different synthesis methods. Therefore, optimizing their microstructure and modulating their phase composition is crucial. However, synthesizing TMS with specific structures using traditional template methods is extremely cumbersome. Recently, metal-organic frameworks (MOFs) have been used as precursors or templates to prepare various metal sulfides with specific structures due to their large specific surface area, tunable morphology, and diverse compositions. In particular, cobalt (Co)-based metal sulfides derived from ZIF-67 with various structures have been reported to exhibit excellent absorption performance in the 8-18 GHz high-frequency band. However, as the frequency decreases, the wave impedance and dissipation of microwaves exhibit a denser increase in coupling, making it still a significant challenge to achieve efficient microwave absorption at low-frequency GHz using these Co-based metal sulfides. To address this issue, careful design of the structure and composition of cobalt-based metal sulfides is required.

[0004] Patent CN113943022A discloses a hollow spherical Co9S8 / Ni3S4 heterojunction material derived from cobalt-based MOFs, which can be applied in the field of supercapacitor technology. Patent CN111804313A discloses a method for preparing Fe2O3@Co9S8 double hollow core-shell structure nanocomposite materials and their applications; this material is used as an electrocatalyst in the oxygen evolution reaction of water electrolysis. Patent CN109192949A discloses a method for obtaining velvety hollow polyhedral Co9S8@MoS2 by derivatizing ZIF-67 polyhedra; this material is used as a negative electrode material for lithium-ion batteries. It is evident that the structures of these cobalt-based metal sulfides are unsuitable for use as microwave absorbing materials, let alone achieving efficient microwave absorption in the low-frequency band. Summary of the Invention

[0005] To address the above deficiencies, the technical problem solved by this invention is to provide a cobalt-based metal sulfide composite material with excellent wave absorption performance in the low-frequency band, namely carbon fiber aerogel@hollow Co9S8 polyhedral material.

[0006] This invention relates to a carbon fiber aerogel@hollow Co9S8 polyhedron material, which uses carbon fiber aerogel as a carrier and loads hollow Co9S8 polyhedra on the carrier.

[0007] This invention relates to a carbon fiber aerogel@hollow Co9S8 polyhedron material, in which hollow Co9S8 polyhedra are loaded onto a carbon fiber aerogel carrier. The special structural features of the hollow Co9S8 polyhedron enable the composite material to exhibit special magnetic dielectric properties in the low-frequency electromagnetic band, thus achieving excellent wave absorption performance of carbon fiber aerogel@hollow Co9S8 polyhedron in the low-frequency band with low filling amount.

[0008] The second technical problem solved by this invention is to provide a method for preparing carbon fiber aerogel@hollow Co9S8 polyhedral material.

[0009] The present invention discloses a method for preparing carbon fiber aerogel@hollow Co9S8 polyhedral material, comprising the following steps:

[0010] a. The fiber aerogel is placed in a mixed solution of soluble cobalt salt, dimethylimidazole and methanol to carry out a chemical deposition reaction, then removed and dried to obtain fiber aerogel@ZIF-67 dodecahedron;

[0011] b. The fiber aerogel@ZIF-67 dodecahedron and sulfur source solution were mixed and subjected to a solvothermal reaction. Then, the mixture was removed, dried, and annealed at high temperature under a protective atmosphere to obtain carbon fiber aerogel@hollow Co9S8 polyhedron material.

[0012] In some preferred embodiments, the fiber aerogel is bamboo fiber aerogel, cotton fiber aerogel, or lotus fiber aerogel.

[0013] In one specific embodiment, the soluble cobalt salt is cobalt nitrate hexahydrate or cobalt sulfate.

[0014] In some specific embodiments, the sulfur source is thioacetamide or thiourea.

[0015] In some specific embodiments, in step a, the ratio of soluble cobalt salt to dimethylimidazole is 1:6 to 8.

[0016] In a preferred embodiment, the ratio of soluble cobalt salt to dimethylimidazole is 1:7.

[0017] In some specific embodiments, in step b, the sulfur source solution is an ethanol solution of a sulfur source or an aqueous solution of an ethanol source, and the concentration of the sulfur source in the sulfur source solution is 3-6 mg / mL. Preferably, the concentration of the sulfur source is 4 mg / mL.

[0018] In some specific embodiments, in step a, the temperature of the chemical deposition reaction is 20–30°C, and the reaction time is 12–24 hours. In a preferred embodiment, the temperature of the chemical deposition reaction is room temperature, 25°C.

[0019] In some specific embodiments, in step b, the solvothermal reaction temperature is 90–120°C, and the time is 4–8 hours. In one specific embodiment, the solvothermal reaction temperature is 120°C, and the time is 8 hours.

[0020] In some embodiments of the present invention, the high-temperature annealing temperature is 300–400°C, and the time is 1–2 hours. In a preferred embodiment, the high-temperature annealing temperature is 350°C.

[0021] In one specific embodiment of the present invention, the fiber aerogel is bamboo fiber aerogel, and the preparation method of bamboo fiber aerogel includes:

[0022] 1) Bamboo strips are placed in an alkaline solution for hydrothermal reaction, then washed and bleached to obtain long bamboo fibers.

[0023] 2) Mix long bamboo fibers with an alkali / urea aqueous solution for 3-5 minutes, then heat to gel, then soak to remove excess alkali and urea, and freeze dry to obtain bamboo fiber aerogel.

[0024] Specifically, in step 1), the alkaline solution is KOH or NaOH, with a concentration of 4–8 wt%, and the bamboo strips have a mass fraction of 4–8 wt%; the hydrothermal reaction temperature is 130–150℃, and the reaction time is 12–18 h. Preferably, the concentration of the alkaline solution is 6 wt%.

[0025] Preferably, in step 1), a sodium chlorite solution is used for bleaching treatment, the concentration of the sodium chlorite solution is 0.5-3 wt%, and the pH of the sodium chlorite solution is adjusted to 4-5. As a preferred embodiment, the concentration of the sodium chlorite solution is 1 wt%, and the pH is 4.5.

[0026] In some embodiments of the present invention, in step 2), the weight ratio of alkali, urea and water in the alkali / urea aqueous solution is 5-10:10-15:80-85; the temperature of the alkali / urea aqueous solution is -20 to -10°C; the stirring time is 3-5 min; the heating temperature for the gelation reaction is 70-90°C; and the time is 24-36 h. As a preferred embodiment, the weight ratio of alkali, urea and water in the alkali / urea aqueous solution is 7:12:81; and the temperature of the alkali / urea aqueous solution is -12°C.

[0027] This invention also provides the application of the carbon fiber aerogel@hollow Co9S8 polyhedral material described herein in low-frequency electromagnetic wave absorbing materials.

[0028] The fiber aerogel@hollow Co9S8 polyhedral material of this invention has excellent wave absorption properties and can be used as a lightweight electromagnetic wave absorbing material at low frequencies. It has broad application prospects in radar stealth, electromagnetic compatibility of microelectronic components and other fields.

[0029] Compared with the prior art, the present invention has the following beneficial effects:

[0030] (1) This invention uses chemical deposition technology to load ZIF-67 polyhedra onto the surface of fiber aerogel, followed by vulcanization annealing to prepare a composite material with a carbon fiber aerogel@hollow Co9S8 polyhedron hierarchical structure. By constructing a hierarchical structure with a carbon fiber interconnect network as the skeleton, the composite material is endowed with richer heterojunction surfaces, which enhances the multiple reflection and scattering of electromagnetic waves and strengthens the polarization loss capability. The special structural characteristics of the hollow Co9S8 polyhedron enable the composite material to exhibit special magnetic dielectric properties in the low-frequency electromagnetic band, realizing the excellent wave absorption performance of carbon fiber aerogel@hollow Co9S8 polyhedron in the low-frequency band with low filling amount.

[0031] (2) This invention prepares low-cost fiber aerogels using an improved alkali / urea dissolution method. Short-term stirring promotes the dissolution of the cellulose portion on the surface of the bamboo fibers in the alkali / urea aqueous solution, while ensuring uniform dispersion of the bamboo fibers in the mixed solution system. The gel is then gently heated to form a gel, and finally soaked to remove residual alkali and urea before freeze-drying to obtain the bamboo fiber aerogel. This process is simple, allows for easy control of the bamboo fiber concentration, and exhibits high processing stability. Attached Figure Description

[0032] Figure 1 The images show the physical specimen and scanning electron microscope (SEM) image of the bamboo fiber aerogel in Example 1.

[0033] Figure 2 Physical image and scanning electron microscope image of bamboo fiber aerogel@ZIF-67 dodecahedron in Example 1.

[0034] Figure 3 The images shown are physical images and scanning electron microscope images of the carbon fiber aerogel@hollow Co9S8 polyhedron in Example 1.

[0035] Figure 4 This is a transmission electron microscope image of the carbon fiber aerogel@hollow Co9S8 polyhedron in Example 1.

[0036] Figure 5 The image shows the X-ray diffraction pattern of the bamboo fiber aerogel in Example 1.

[0037] Figure 6 The X-ray diffraction patterns are of bamboo fiber aerogel@ZIF-67 dodecahedron and carbon fiber aerogel@hollow Co9S8 polyhedron in Example 1.

[0038] Figure 7 The image shows a scanning electron microscope (SEM) image of the bamboo fiber aerogel@ZIF-67 nanosheet array in Comparative Example 1.

[0039] Figure 8 The image shows a scanning electron microscope (SEM) image of the carbon fiber aerogel@Co9S8 nanosheet array in Comparative Example 1.

[0040] Figure 9 The graph shows the reflection loss curves of carbon fiber aerogel@hollow Co9S8 polyhedron in Example 1 under different filling conditions.

[0041] Figure 10 The graph shows the reflection loss curves of the carbon fiber aerogel@Co9S8 nanosheet array in Comparative Example 1 under different filling conditions. Detailed Implementation

[0042] This invention relates to a carbon fiber aerogel@hollow Co9S8 polyhedron material, which uses carbon fiber aerogel as a carrier and loads hollow Co9S8 polyhedra on the carrier.

[0043] This invention relates to a carbon fiber aerogel@hollow Co9S8 polyhedron material, in which hollow Co9S8 polyhedra are loaded onto a carbon fiber aerogel carrier. The special structural features of the hollow Co9S8 polyhedron enable the composite material to exhibit special magnetic dielectric properties in the low-frequency electromagnetic band, thus achieving excellent wave absorption performance of carbon fiber aerogel@hollow Co9S8 polyhedron in the low-frequency band with low filling amount.

[0044] In various specific structures, hollow or porous materials can achieve good impedance matching characteristics because the air impedance within the cavity is closer to that of free space. Furthermore, hollow or porous structures can induce multiple scattering and reflection of electromagnetic waves, thereby extending their propagation path and converting electromagnetic wave energy into heat energy. Therefore, this invention constructs a hierarchical composite material containing a hollow Co-based sulfide multi-component structure, providing an effective solution for decoupling wave impedance and electromagnetic attenuation at low frequencies.

[0045] Aerogels, due to their porosity, high specific surface area, and lightweight nature, are commonly used to construct layered composite materials to optimize microwave absorption performance. In particular, carbon-based aerogels not only inherit these advantages but also possess good electrical conductivity, which can induce electromagnetic wave propagation losses. Because of their typical three-dimensional interconnected porous structure, low cost, abundant natural resources, and environmental friendliness, biomass-derived carbon fiber aerogels can serve as a porous framework to support hollow cobalt-based sulfides to construct hierarchical structures.

[0046] This invention utilizes an aerogel composite material composed of hollow Co9S8-loaded three-dimensional interconnected carbon fibers to achieve a hierarchical structure for low-frequency electromagnetic wave absorption. On one hand, the highly porous structure enhances impedance matching. On the other hand, the three-dimensional interconnected structure extends the electron transport path, thereby increasing conduction loss, while the cobalt-based sulfides loaded on the three-dimensional interconnected carbon fibers forming the framework strengthen the interfacial polarization effect, further inducing electromagnetic wave attenuation. Furthermore, the hierarchical structure design also facilitates the uniform distribution of the cobalt-based sulfides, inhibits their aggregation, and improves the electromagnetic properties of the material.

[0047] The second technical problem solved by this invention is to provide a method for preparing carbon fiber aerogel@hollow Co9S8 polyhedral material.

[0048] The present invention discloses a method for preparing carbon fiber aerogel@hollow Co9S8 polyhedral material, comprising the following steps:

[0049] a. The fiber aerogel is placed in a mixed solution of soluble cobalt salt, dimethylimidazole and methanol to carry out a chemical deposition reaction, then removed and dried to obtain fiber aerogel@ZIF-67 dodecahedron;

[0050] b. The fiber aerogel@ZIF-67 dodecahedron and sulfur source solution were mixed and subjected to a solvothermal reaction. Then, the mixture was removed, dried, and annealed at high temperature under a protective atmosphere to obtain carbon fiber aerogel@hollow Co9S8 polyhedron material.

[0051] This invention obtains fiber aerogel loaded with ZIF-67 dodecahedrons through in-situ chemical deposition, and prepares carbon fiber aerogel@hollow Co9S8 polyhedron composite material by using a simple vulcanization and pyrolysis process.

[0052] Step a involves the preparation of fiber aerogel@ZIF-67 dodecahedrons, which mainly employs a chemical deposition reaction to load ZIF-67 dodecahedrons onto the fiber aerogel.

[0053] Commonly used fiber aerogels in this field are applicable to this invention. In some preferred embodiments, the fiber aerogel is bamboo fiber aerogel, cotton fiber aerogel, or lotus fiber aerogel.

[0054] The soluble cobalt salt may also be a cobalt salt commonly used in the art. In one specific embodiment, the soluble cobalt salt is cobalt nitrate hexahydrate or cobalt sulfate.

[0055] The ZIF-67 material can be obtained by reacting a soluble cobalt salt with dimethylimidazole. In some specific embodiments, the ratio of the soluble cobalt salt to dimethylimidazole is 1:6 to 8. In one specific embodiment, the ratio of the soluble cobalt salt to dimethylimidazole is 1:7.

[0056] Chemical deposition occurs when the fiber aerogel is placed in a mixed solution of soluble cobalt salt, dimethylimidazole, and methanol, without special treatment. In some embodiments, the chemical deposition reaction occurs at a temperature of 20–30°C for 12–24 hours. In a preferred embodiment, the chemical deposition reaction occurs at room temperature (25°C).

[0057] Step b involves the preparation of carbon fiber aerogel@hollow Co9S8 polyhedral material. The fiber aerogel@ZIF-67 dodecahedral is mixed with a sulfur source solution, subjected to a solvothermal reaction, and then annealed at high temperature to obtain the final product.

[0058] Commonly used sulfur sources in this field are suitable for this invention. In some specific embodiments, the sulfur source is thioacetamide or thiourea.

[0059] In some specific embodiments, the sulfur source solution is an ethanol solution of the sulfur source or an aqueous solution of the sulfur source in ethanol. That is, ethanol can be used as a solvent, or a mixture of ethanol and water can be used as a solvent.

[0060] In some specific embodiments, the concentration of the sulfur source in the sulfur source solution is 3–6 mg / mL. As a preferred embodiment, the concentration of the sulfur source is 4 mg / mL.

[0061] The temperature and time of the solvothermal reaction can be those commonly used in the art. In some specific embodiments, the temperature of the solvothermal reaction is 90–120°C, and the time is 4–8 hours. In one specific embodiment, the temperature of the solvothermal reaction is 120°C, and the time is 8 hours.

[0062] To avoid oxidation, high-temperature annealing is performed under a protective atmosphere. This protective atmosphere is an atmosphere that does not participate in the reaction, including but not limited to nitrogen, argon, helium, neon, and krypton.

[0063] In some embodiments of the present invention, the high-temperature annealing temperature is 300–400°C, and the time is 1–2 hours. In a preferred embodiment, the high-temperature annealing temperature is 350°C.

[0064] In one specific embodiment of the present invention, the fiber aerogel is bamboo fiber aerogel, and the preparation method of bamboo fiber aerogel includes:

[0065] 1) Bamboo strips are placed in an alkaline solution for hydrothermal reaction, then washed and bleached to obtain long bamboo fibers.

[0066] 2) Long bamboo fibers are stirred and mixed with an alkali / urea aqueous solution for 3–5 minutes, then heated for a gelation reaction. Excess alkali and urea are removed by soaking, followed by freeze-drying to obtain bamboo fiber aerogel. This method employs a typical ice-template sacrificial method (freeze-drying) to prepare aerogels of bamboo cellulose fibers with 3D interconnections. Furthermore, the short mixing time between the long bamboo fibers and the alkali / urea aqueous solution prevents complete dissolution, resulting in an aerogel with irregularly structured pores formed by the fibers, which helps improve microwave absorption performance.

[0067] Specifically, in step 1), the alkaline solution is a KOH solution or a NaOH solution, the concentration of the alkaline solution is 4-8 wt%, and the mass fraction of bamboo strips is 4-8 wt%; the hydrothermal reaction temperature is 130-150℃, and the time is 12-18 h; preferably, the concentration of the alkaline solution is 6 wt%.

[0068] Preferably, in step 1), bleaching is performed using a sodium chlorite solution with a concentration of 0.5–3 wt%, and the pH of the sodium chlorite solution is adjusted to 4–5. As a preferred embodiment, the concentration of the sodium chlorite solution is 1 wt%, and the pH is 4.5. Acetic acid can be used to adjust the pH.

[0069] In some embodiments of the present invention, in step 2), the weight ratio of alkali, urea and water in the alkali / urea aqueous solution is 5-10:10-15:80-85; the temperature of the alkali / urea aqueous solution is -20 to -10°C; the stirring time is 3-5 min; the heating temperature for the gelation reaction is 70-90°C; and the time is 24-36 h. As a preferred embodiment, the weight ratio of alkali, urea and water in the alkali / urea aqueous solution is 7:12:81; and the temperature of the alkali / urea aqueous solution is -12°C.

[0070] This invention also provides the application of the carbon fiber aerogel@hollow Co9S8 polyhedral material described herein in low-frequency electromagnetic wave absorbing materials.

[0071] The fiber aerogel@hollow Co9S8 polyhedral material of this invention has excellent wave absorption properties and can be used as a lightweight electromagnetic wave absorbing material at low frequencies. It has broad application prospects in radar stealth, electromagnetic compatibility of microelectronic components and other fields.

[0072] The specific embodiments of the present invention will be further described below with reference to examples, but the present invention is not limited to the scope of the embodiments described herein.

[0073] Example 1

[0074] A method for preparing carbon fiber aerogel@hollow Co9S8 polyhedral material, comprising the following steps:

[0075] (1) 8g of bamboo strips were placed in 160mL of 6wt% KOH solution and then placed in a 250mL reactor for hydrothermal reaction at 130℃ for 18h. After the reaction, the strips were washed with water until neutral, then mechanically loosened to disperse the fibers. Finally, the strips were bleached in 200mL of 1wt% sodium chlorite solution (pH 4.5) for 6h to obtain long bamboo fibers with lignin and hemicellulose removed.

[0076] (2) Soak 3g of bamboo fiber in an alkali / urea aqueous solution (7g / 12g / 81mL) at -12℃ and stir rapidly for 3min to form a viscous, partially dissolved mixture. Pour the mixture into a mold and heat at 70℃ for 24h to gel. After gelation, soak the mixture in deionized water to remove residual alkali and urea, and finally freeze-dry to obtain bamboo fiber aerogel.

[0077] (3) A cylindrical aerogel with a height of 1 cm and a diameter of 3 cm was placed in a mixture of 0.291 g cobalt nitrate hexahydrate methanol solution (25 mL) and 0.657 g dimethyl imidazole methanol solution (25 mL) and reacted at room temperature (25 °C) for 24 h. After the reaction was completed, the product was washed with anhydrous ethanol 2 to 3 times and then dried in an oven at 70 °C to obtain bamboo fiber aerogel@ZIF-67 dodecahedron.

[0078] (4) Bamboo fiber aerogel@ZIF-67 dodecahedron was placed in 30 mL of 120 mg thioacetamide ethanol solution and transferred to a 50 mL reactor for solvothermal reaction at 120 °C for 8 h. After the reaction, the product was removed and washed with ethanol 2-3 times, then dried in a 70 °C oven, and finally annealed in a tube furnace at 350 °C for 2 h under a nitrogen atmosphere at a heating rate of 5 °C / min to obtain carbon fiber aerogel@hollow Co9S8 polyhedron composite material.

[0079] The bamboo fiber aerogel obtained in step (2) was observed by scanning electron microscopy, and the results are as follows: Figure 1 As shown. By Figure 1 It can be seen that bamboo fiber aerogel is composed of interwoven bamboo fibers and has a rich porous structure. XRD tests were performed on the bamboo fiber aerogel obtained in step (2), and the results are as follows: Figure 5 As shown, by Figure 5 It can be seen that bamboo fiber aerogel has typical cellulose crystallization peaks.

[0080] The bamboo fiber aerogel@ZIF-67 dodecahedron obtained in step (3) was observed by scanning electron microscopy, and the results are as follows: Figure 2 As shown, ZIF-67 dodecahedrons are uniformly loaded on the surface of the bamboo fiber aerogel, and the ZIF-67 dodecahedrons have a smooth surface morphology. XRD tests were performed on the bamboo fiber aerogel@ZIF-67 dodecahedrons obtained in step (3), as shown... Figure 6 As shown, by Figure 6 It can be seen that the XRD pattern of bamboo fiber aerogel@ZIF-67 dodecahedron has typical ZIF-67 diffraction peaks, indicating that ZIF-67 dodecahedrons were successfully loaded on the surface of bamboo fiber aerogel.

[0081] The carbon fiber aerogel@hollow Co9S8 polyhedron material obtained in step (4) was observed by scanning electron microscopy, and the results are as follows: Figure 3 As shown, hollow Co9S8 polyhedra with rough surface features can be observed loaded on the fiber surface. The carbon fiber aerogel@hollow Co9S8 polyhedral hierarchical composite material obtained in step (4) was observed by transmission electron microscopy, and the results are as follows: Figure 4 As shown, Co9S8 polyhedra with a significant hollow structure can be observed. Due to the decomposition of the dimethylimidazolium organic ligand during vulcanization, the edges of the dodecahedrons become less distinct, forming a hollow polyhedral structure. XRD tests were performed on the carbon fiber aerogel@hollow Co9S8 polyhedra obtained in step (4), as shown... Figure 6 As shown, by Figure 6 It can be seen that the crystalline peaks in the XRD pattern correspond to those on the Co9S8 standard card (PDF#19-0364), and an amorphous carbon peak can be observed near 26°. This indicates that its chemical composition is a composite material of carbon fiber aerogel / Co9S8.

[0082] Electromagnetic parameters of the prepared carbon fiber aerogel@hollow Co9S8 polyhedra were tested using a vector network analyzer. Reflection loss was further calculated based on transmission line theory. The carbon fiber aerogel@hollow Co9S8 polyhedra were uniformly mixed with paraffin wax at proportions of 5 wt%, 7.5 wt%, 10 wt%, and 15 wt%, and pressed into rings with an outer diameter of 7 mm and an inner diameter of 3.04 mm using a mold for testing. The electromagnetic parameters of the samples were measured in the 2-18 GHz range using a vector network analyzer, and finally, reflection loss curves at different thicknesses were calculated.

[0083] Reflection loss curves of carbon fiber aerogel@hollow Co9S8 polyhedra under different filling ratios are shown below. Figure 9 As shown. Figure 9 In the diagram, a represents a filling ratio of 5 wt%, b represents a filling ratio of 7.5 wt%, c represents a filling ratio of 10 wt%, and d represents a filling ratio of 15 wt%. It can be seen that the RLmin values ​​at filling ratios of 5 wt%, 7.5 wt%, 10 wt%, and 15 wt% are -19.4 dB, -44.5 dB, -31.2 dB, and -53.6 dB, respectively, corresponding to matching thicknesses of 3.0 mm, 1.9 mm, 5.2 mm, and 5.2 mm. At filling ratios of 10 wt% and 15 wt%, carbon fiber aerogel@hollow Co9S8 polyhedrons exhibit excellent low-frequency absorption performance.

[0084] Comparative Example 1

[0085] The preparation method of Example 1 is the same as that of Example 1, except that in step (3), a cylindrical aerogel with a height of 1 cm and a diameter of 3 cm is placed in a mixture of 0.291 g of cobalt nitrate hexahydrate deionized water (20 mL) and 0.657 g of dimethyl imidazole methanol solution (20 mL) and reacted at room temperature (25 °C) for 24 h. After the reaction is completed, the product is washed with anhydrous ethanol 2 to 3 times and then dried in an oven at 70 °C to obtain bamboo fiber aerogel@ZIF-67 nanosheet array.

[0086] The bamboo fiber aerogel@ZIF-67 nanosheet array was then placed in 30 mL of 120 mg thioacetamide ethanol solution and transferred to a 50 mL reactor for a solvothermal reaction at 120 °C for 8 h. After the reaction, the product was removed, washed 2–3 times with ethanol, dried in a 70 °C oven, and finally annealed in a tube furnace at 350 °C for 2 h under a nitrogen atmosphere at a heating rate of 5 °C / min to obtain the carbon fiber aerogel@Co9S8 nanosheet array composite material.

[0087] Scanning electron microscopy was performed on the bamboo fiber aerogel@ZIF-67 nanosheet array obtained in Comparative Example 1, and the results are as follows: Figure 7 As shown, the ZIF-67 nanosheet array is uniformly loaded on the surface of bamboo fiber. Scanning electron microscopy observation of the carbon fiber aerogel@Co9S8 nanosheet array yielded the following results: Figure 8 As shown, the original morphology of ZIF-67 nanosheets is destroyed after vulcanization and continuous annealing, forming irregular flakes loaded on the carbon fiber surface.

[0088] The reflection loss curves of carbon fiber aerogel@Co9S8 nanosheet arrays under different filling ratios are shown below. Figure 10As shown in the figure, a represents a filling ratio of 10 wt%, and b represents a filling ratio of 15%. It can be seen that the RLmin values ​​at 10 wt% and 15 wt% filling ratios are -10.1 dB and -30.1 dB, respectively, corresponding to matching thicknesses of 3.0 mm and 2.8 mm. In contrast, carbon fiber aerogel@hollow Co9S8 polyhedrons, due to their unique structural characteristics and multi-component synergy, endow the composite material with excellent low-frequency microwave absorption performance.

Claims

1. A method for preparing carbon fiber aerogel@hollow Co9S8 polyhedral material, characterized in that, Includes the following steps: a. The fiber aerogel is placed in a mixed solution of soluble cobalt salt, dimethylimidazole and methanol to carry out a chemical deposition reaction, taken out and dried to obtain fiber aerogel@ZIF-67 dodecahedron; wherein the molar ratio of soluble cobalt salt and dimethylimidazole is 1:6 to 8; b. Mix fiber aerogel@ZIF-67 dodecahedron with sulfur source solution, carry out solvothermal reaction, then take it out, dry it, and anneal it at high temperature of 300-400℃ for 1-2 hours under protective atmosphere to obtain carbon fiber aerogel@hollow Co9S8 polyhedron material. The sulfur source solution is an ethanol solution of sulfur source or an aqueous solution of sulfur source in ethanol, wherein the concentration of sulfur source in the sulfur source solution is 3-6 mg / mL.

2. The method for preparing carbon fiber aerogel@hollow Co9S8 polyhedral material according to claim 1, characterized in that: The soluble cobalt salt is cobalt nitrate hexahydrate or cobalt sulfate; the sulfur source is thioacetamide or thiourea.

3. The method for preparing carbon fiber aerogel@hollow Co9S8 polyhedral material according to claim 1, characterized in that: The molar ratio of soluble cobalt salt to dimethylimidazole is 1:

7.

4. The method for preparing carbon fiber aerogel@hollow Co9S8 polyhedral material according to claim 1, characterized in that: The concentration of the sulfur source was 4 mg / mL.

5. The method for preparing carbon fiber aerogel@hollow Co9S8 polyhedral material according to claim 1, characterized in that: In step a, the temperature of the chemical deposition reaction is 20–30°C and the reaction time is 12–24 h; in step b, the temperature of the solvothermal reaction is 90–120°C and the time is 4–8 h.

6. The method for preparing carbon fiber aerogel@hollow Co9S8 polyhedral material according to claim 5, characterized in that: The temperature for the chemical deposition reaction is 25℃.

7. The method for preparing carbon fiber aerogel@hollow Co9S8 polyhedral material according to claim 1, characterized in that: The high-temperature annealing temperature is 350℃.

8. The method for preparing carbon fiber aerogel@hollow Co9S8 polyhedral material according to claim 1, characterized in that: The fiber aerogel is bamboo fiber aerogel, cotton fiber aerogel, or lotus fiber aerogel.

9. The method for preparing carbon fiber aerogel@hollow Co9S8 polyhedral material according to claim 1, characterized in that: The fiber aerogel is bamboo fiber aerogel, and the preparation method of bamboo fiber aerogel includes: 1) Bamboo strips are placed in an alkaline solution for hydrothermal reaction, then washed and bleached to obtain long bamboo fibers; 2) Stir and mix long bamboo fibers with an alkali / urea aqueous solution for 3-5 minutes, then heat and gel the mixture, then soak to remove excess alkali and urea, and freeze dry to obtain bamboo fiber aerogel.

10. The method for preparing carbon fiber aerogel@hollow Co9S8 polyhedral material according to claim 9, characterized in that: In step 1), the alkaline solution is KOH or NaOH, the concentration of the alkaline solution is 4-8 wt%, and the mass fraction of bamboo strips is 4-8 wt%; the hydrothermal reaction temperature is 130-150℃, and the time is 12-18h.

11. The method for preparing carbon fiber aerogel@hollow Co9S8 polyhedral material according to claim 10, characterized in that: The concentration of the alkaline solution is 6 wt%.

12. The method for preparing carbon fiber aerogel@hollow Co9S8 polyhedral material according to claim 9, characterized in that: In step 1), a sodium chlorite solution is used for bleaching treatment. The concentration of the sodium chlorite solution is 0.5-3 wt%, and the pH of the sodium chlorite solution is adjusted to 4-5.

13. The method for preparing carbon fiber aerogel@hollow Co9S8 polyhedral material according to claim 12, characterized in that: The sodium chlorite concentration is 1 wt%, and the pH is 4.

5.

14. The method for preparing carbon fiber aerogel@hollow Co9S8 polyhedral material according to claim 9, characterized in that: In step 2), the weight ratio of alkali, urea and water in the alkali / urea aqueous solution is 5-10:10-15:80-85; the temperature of the alkali / urea aqueous solution is -20 to -10℃; the stirring time is 3-5 min; the heating temperature for the gel reaction is 70-90℃; and the time is 24-36 h.

15. The method for preparing carbon fiber aerogel@hollow Co9S8 polyhedral material according to claim 14, characterized in that: In the alkali / urea aqueous solution, the weight ratio of alkali, urea and water is 7:12:81; the temperature of the alkali / urea aqueous solution is -12℃.

16. The carbon fiber aerogel@hollow Co9S8 polyhedral material prepared by the preparation method according to any one of claims 1-15, characterized in that: The composite material uses carbon fiber aerogel with a three-dimensional interconnected porous structure as a carrier, and hollow Co9S8 polyhedra are loaded on the carrier to form a hierarchical structure; the minimum reflection loss of the composite material for 2-8GHz electromagnetic waves is ≤-31.2dB.

17. The application of the carbon fiber aerogel@hollow Co9S8 polyhedral material according to claim 16 in low-frequency electromagnetic wave absorbing materials.

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