Multistage-porous sibcn / sic / sio2 composite aerogel, preparation method and application thereof
Patent Information
- Application Number
- CN202610916907.5
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Filing Date
- 2026-06-24
- Publication Date
- 2026-09-15
Smart Images
Figure CN122745801A_ABST
Abstract
Description
Technical Field
[0001] This invention belongs to the technical field of microwave absorbing materials, specifically relating to a hierarchical porous SiBCN / SiC / SiO2 composite aerogel, its preparation method, and its application. Background Technology
[0002] Electromagnetic wave absorbing materials hold a crucial strategic position in defense stealth technology and civilian electromagnetic compatibility. With the rapid development of hypersonic vehicles, next-generation weaponry, and high-temperature electronic equipment, the service environments faced by absorbing materials are becoming increasingly demanding. Ideal absorbing materials not only need to achieve efficient attenuation over a wide frequency range but also must possess lightweight properties, excellent mechanical load-bearing capacity, and structural stability in high-temperature, oxygen-rich environments.
[0003] Polymer precursor ceramics, especially SiBCN ceramics, have attracted widespread attention in the field of high-temperature microwave absorption due to their advantages such as strong molecular structure designability, excellent high-temperature stability, and tunable dielectric properties. SiBCN ceramics not only inherit the excellent high-temperature creep resistance of amorphous ceramics but also exhibit outstanding high-temperature oxidation resistance. By controlling the heat treatment temperature, the precipitation of free carbon phases and SiC nanocrystals in the matrix can be induced, thereby regulating conductivity and polarization losses. However, pure SiBCN ceramics suffer from two major bottlenecks: firstly, their intrinsic attenuation capability is weak, resulting in low electromagnetic wave loss efficiency; secondly, poor impedance matching (large difference between dielectric constant and free space) makes it difficult for electromagnetic waves to effectively penetrate the material and dissipate, leading to a narrow effective absorption bandwidth. Constructing SiBCN into a three-dimensional aerogel structure can improve these problems to some extent. Its high porosity (>90%) and three-dimensional interconnected network can extend the electromagnetic wave propagation path and enhance multiple scattering, thereby optimizing impedance matching and broadening the absorption bandwidth. However, traditional SiBCN aerogels are mostly constructed by zero-dimensional particle stacking or sol-gel method, which have the following inherent defects: (a) the contact interface between particles is limited, making it difficult to provide sufficient interfacial polarization loss; (b) the pore structure is simple, and the impedance control space is insufficient; (c) stress concentration at the skeleton connection is prone to brittle fracture under load, and the compressive strength is generally lower than 0.3–0.5 MPa, which cannot meet the structural load-bearing requirements. Summary of the Invention
[0004] This invention addresses the shortcomings of existing technologies by providing a hierarchical porous SiBCN / SiC / SiO2 composite aerogel, its preparation method, and its applications. Through in-situ whisker reinforcement and interfacial oxidation strategies, a hierarchical porous core-shell structure SiBCN / SiC / SiO2 nanofiber composite aerogel is prepared, which significantly improves electromagnetic wave absorption performance, mechanical properties, and thermal stability.
[0005] To achieve the above objectives, the technical solution of the present invention is as follows:
[0006] A hierarchical porous SiBCN / SiC / SiO2 composite aerogel comprises: a three-dimensional network structure of SiBCN nanofiber skeleton, SiC nanowires grown in situ on the SiBCN nanofiber skeleton, and a porous SiO2 shell coating the surfaces of the SiBCN nanofiber skeleton and the SiC nanowires.
[0007] Optionally, the aerogel has a hierarchical porous structure including micron-sized pores and nano-sized pores; the density of the aerogel is 43–50 mg / cm³. 3 Its specific surface area is 58.6–90.1 m². 2 / g.
[0008] Furthermore, the SiBCN nanofibers have a diameter of 100–300 nm, the SiC nanowires are uniformly distributed in the pores of the fiber framework, and the SiO2 shell has a continuous porous structure with nanoscale closed pores inside the shell and at the fiber interface. Thus, the aerogel possesses a hierarchical porous structure, including: micron-sized macropores formed by ice crystal sublimation, nanoscale micropores formed by fiber entanglement, and closed nanopores within the SiO2 shell.
[0009] A method for preparing the above-mentioned hierarchical porous SiBCN / SiC / SiO2 composite aerogel includes the following steps:
[0010] S1: SiBCN polymer precursor nanofibers were prepared by electrospinning, and then obtained by pre-oxidative crosslinking and high-temperature pyrolysis. S2: Disperse the SiBCN nanofibers obtained in step S1 in water, add C / SiO2 sol and agarose, and prepare SiBCN nanofiber aerogel preform by directional freeze drying; S3: The aerogel preform obtained in step S2 is subjected to a carbothermal reduction reaction under an inert atmosphere to grow SiC nanowires in situ in the SiBCN nanofiber framework, thus obtaining SiBCN / SiC composite aerogel. S4: The SiBCN / SiC composite aerogel obtained in step S3 is subjected to thermal oxidation treatment in air atmosphere to form a porous SiO2 shell on the surface of SiBCN nanofiber skeleton and SiC nanowire, thus obtaining SiBCN / SiC / SiO2 composite aerogel.
[0011] Optionally, in step S1, the parameters of the electrospinning are: feed rate 0.5-1.5 mL / h, spinning distance 15-25 cm, and operating voltage 11-25 kV.
[0012] Optionally, the pre-oxidation temperature is 180-230 ℃; the high-temperature pyrolysis heating rate is 1-10 ℃ / min, the temperature is 1000-1300 ℃, and the holding time is 1-5 h; the diameter of the obtained SiBCN nanofibers is 100-300 nm.
[0013] Optionally, in step S2, the amount of SiBCN nanofibers added is 1.0-3.5 wt% of the water mass; the amount of C / SiO2 sol added is 2-20 wt% of the water mass; and the amount of agarose added is 0.1-1 wt% of the water mass.
[0014] The C / SiO2 sol mentioned here refers to a mixture of SiO2 sol and glucose solution.
[0015] Optionally, in step S2, the directional freeze-drying involves first freezing in liquid nitrogen for 10-20 min, followed by freeze-drying under vacuum conditions for 24-48 h.
[0016] Optionally, in step S3, the carbothermic reduction reaction is carried out by heating to 1350-1500 ℃ at a rate of 1-10 ℃ / min and holding at that temperature for 1-5 h.
[0017] Optionally, in step S4, the thermal oxidation treatment is carried out at 800-1000 ℃ for 5-60 min.
[0018] The above-mentioned hierarchical porous SiBCN / SiC / SiO2 composite aerogel is used in electromagnetic wave absorbing materials.
[0019] The beneficial effects of this invention are as follows:
[0020] Compared with existing technologies, this method significantly improves electromagnetic wave absorption performance, achieving synergistic optimization of multiple loss mechanisms (conductive loss, interface polarization, dipole polarization) and impedance matching, with a total effective absorption bandwidth covering the entire C, X, and Ku wavebands. The in-situ growth of SiC nanowires forms a three-dimensional reinforcing network, overcoming the low mechanical strength of traditional aerogels. The in-situ growth of SiC nanowires using a carbothermal reduction reaction avoids the problem of excessively high dielectric constant caused by an added second phase. The SiBCN ceramic framework and SiC nanowires endow the material with intrinsic high-temperature oxidation resistance, maintaining structural integrity after high-temperature heating. This integrated structural and functional design provides a novel technical route for developing a new generation of high-performance, long-life, lightweight, and high-strength microwave absorbing materials suitable for extreme high-temperature environments.
[0021] Other features and advantages of the invention will be set forth in the following description, and will be apparent in part from the description, or may be learned by practicing the invention. Attached Figure Description
[0022] Figure 1 This is a TEM image of SiBCN nanofibers in the hierarchical porous SiBCN / SiC / SiO2 composite aerogel of Example 1 after high-temperature oxidation.
[0023] Figure 2 The image shows the microstructure of the hierarchical porous SiBCN / SiC / SiO2 composite aerogel from Example 1.
[0024] Figure 3 Figure (a) shows the compressibility test results and the high-temperature thermal stability test results of the SiBCN / SiC / SiO2 composite aerogel in Example 1.
[0025] Figure 4 The image shows the reflection loss diagram of the microwave absorption performance of the hierarchical porous SiBCN / SiC / SiO2 composite aerogel in Example 1. Detailed Implementation
[0026] The present invention will be further explained below with reference to the accompanying drawings and specific embodiments.
[0027] The hierarchical porous SiBCN / SiC / SiO2 composite aerogel of this embodiment is constructed by using electrospun SiBCN nanofibers as the basic building blocks, assembling them into a lightweight porous aerogel through directional freeze-drying, then using a carbothermal reduction reaction to grow SiC nanowires in situ within the framework, constructing a fiber / nanowire hierarchical porous structure, and finally coating the fiber and nanowire surfaces with a porous SiO2 shell through interfacial oxidation treatment, obtaining a SiBCN / SiC / SiO2 nanofiber composite aerogel with a core-shell structure. The specific preparation method is described below:
[0028] S1: SiBCN polymer precursor nanofibers were prepared by electrospinning, followed by pre-oxidation crosslinking and high-temperature pyrolysis to obtain SiBCN nanofibers. The electrospinning parameters were: feed rate 0.5-1.5 mL / h, spinning distance 15-25 cm, and working voltage 11-25 kV; the pre-oxidation temperature was 180-230 ℃; the high-temperature pyrolysis heating rate was 1-10 ℃ / min, the temperature was 1000℃, and the holding time was 1-5 h; the diameter of the obtained SiBCN nanofibers was 100–300 nm. S2: The SiBCN nanofibers obtained in step S1 are dispersed in water, C / SiO2 sol and agarose are added, and SiBCN nanofiber aerogel preforms are prepared by directional freeze-drying; the amount of SiBCN nanofibers added is 1.3-3.5 wt% of water mass; the amount of C / SiO2 sol added is 2-20 wt% of water mass; the amount of agarose added is 0.1-1 wt% of water mass; the directional freeze-drying is to first freeze in liquid nitrogen for 10-20 min, and then freeze-dry under vacuum for 24-48 h; S3: The aerogel preform obtained in step S2 is heated to 1350-1500 ℃ at 1-10 ℃ / min under an inert atmosphere and held for 1-5 h to carry out a carbothermic reduction reaction, thereby growing SiC nanowires in situ in the SiBCN nanofiber framework to obtain SiBCN / SiC composite aerogel. The SiC nanowires are nucleated and grown in situ on the surface of SiBCN nanofibers through the carbothermic reduction reaction, forming a strong chemical bond with the matrix and avoiding the interface debonding problem caused by the added second phase. At the same time, the semiconductor properties of SiC nanowires provide moderate conductivity loss, and the abundant SiBCN / SiC heterostructure induces strong interface polarization, effectively avoiding the impedance mismatch problem caused by the high conductivity phase. S4: The SiBCN / SiC composite aerogel obtained in step S3 is subjected to thermal oxidation treatment at 800-1000 ℃ for 5-60 min in air atmosphere to form a porous SiO2 shell on the surface of the SiBCN nanofiber skeleton and SiC nanowires, thus obtaining SiBCN / SiC / SiO2 composite aerogel. Through short-time high-temperature oxidation, a continuous porous SiO2 shell is formed on the surface of SiBCN nanofibers and SiC nanowires. This SiO2 shell serves as an impedance gradient layer, reducing the equivalent dielectric constant and achieving asymptotic impedance matching between the material and free space. On the other hand, the nanoscale closed pores inside the SiO2 shell and at the fiber interface further weaken the dielectric constant. At the same time, the multiphase interfaces such as SiO2 / SiC and SiO2 / SiBCN contribute additional polarization relaxation losses.
[0029] In this SiBCN / SiC / SiO2 composite aerogel, a triple hierarchical pore system is formed by micron-nano hierarchical channels created by the entanglement of SiBCN nanofibers, secondary channels constructed by SiC nanowire bridging, and closed nanopores within the SiO2 shell. Incident electromagnetic waves undergo multiple reflections, scattering, and refractions within these complex channels, significantly extending the propagation path and greatly increasing the chance of energy dissipation. This achieves a synergistic enhancement of energy loss through the hierarchical pore structure.
[0030] Example 1
[0031] A method for preparing a hierarchical porous core-shell structured SiBCN / SiC / SiO2 nanofiber composite aerogel includes the following steps:
[0032] Preparation of SiBCN nanofibers: 0.5 g of polyborosilazane (PBSZ) and 0.5 g of polyvinylpyrrolidone (PVP) were dissolved in a mixed solvent of 3 g DMF and 7 g THF, and stirred for 6 h to obtain a homogeneous spinning solution. Electrospinning was performed at a feed rate of 0.5 mL / h, a spinning distance of 15 cm, and a working voltage of 11 kV. The resulting polymer fibers were dried in a 60 ℃ forced-air drying oven for 24 h, followed by pre-oxidative crosslinking in air at 210 ℃ for 2 h. Finally, the crosslinked fibers were placed in a high-temperature tube furnace and heated to 1000 ℃ at a rate of 5 ℃ / min under argon protection and held for 2 h to obtain SiBCN nanofibers.
[0033] Preparation of aerogel preform: 1.5 g of the above-mentioned SiBCN nanofibers were dispersed in 60 mL of deionized water and broken up for 5 min at 6000 r / min using a high-speed disperser. After magnetic stirring for 30 min, 3 mL of C / SiO2 sol (5 wt%) was added, and stirring was continued for another 30 min. Subsequently, 0.3 g of agarose was added, and the mixture was stirred in a 90 ℃ water bath for 1 h and then allowed to cool naturally. The mixed solution was transferred to liquid nitrogen and frozen for 10 min, and then freeze-dried in a freeze dryer for 48 h to obtain the SiBCN nanofiber aerogel preform.
[0034] In-situ growth of SiC nanowires: The preform was placed in a high-temperature tube furnace and heated to 1400 ℃ at a rate of 2 ℃ / min under an argon atmosphere and held for 2 h. The SiBCN / SiC composite aerogel with SiC nanowires grown inside was obtained by carbothermal reduction reaction.
[0035] Interface oxidation treatment: The above SiBCN / SiC composite aerogel was oxidized in air at 1000 °C for 30 min to finally obtain SiBCN / SiC / SiO2 composite aerogel.
[0036] Figure 1 This is a TEM image of SiBCN nanofibers after high-temperature oxidation. The size is approximately 200 nm. As can be seen from the image, the surface of the fiber is uniformly coated with a SiO2 shell, and the interior contains a large number of pores.
[0037] Figure 2 The image shows the microstructure of the prepared SiBCN / SiC / SiO2 composite aerogel. As can be seen, SiBCN nanofibers and SiC nanowires are interwoven and entangled, forming a rich porous structure. The density of this aerogel is 46.2 mg / cm³. 3 Its specific surface area is 78.5 m². 2 / g.
[0038] Figure 3To test the compressive strength and high-temperature thermal stability of the prepared SiBCN / SiC / SiO2 composite aerogel, stress-strain testing of the SiBCN / SiC / SiO2 composite aerogel showed that its compressive strength reached 0.69 MPa. Figure 3 a) Under the influence of a 100g weight, it was burned with a butane torch at 1200℃ without structural collapse. Figure 3 b).
[0039] Wave absorption performance and reflection loss were tested on samples of SiBCN / SiC / SiO2 composite aerogel of Example 1 with different thicknesses (one sample every 0.5 mm from 1.0 mm to 5.0 mm). Figure 4 As shown in the figure, the aerogel prepared in this embodiment exhibits a minimum reflection loss of -53.18 dB and an effective absorption bandwidth of 6.32 GHz at a thickness of 2.0 mm; the total effective absorption bandwidth reaches 13.52 GHz within a thickness range of 1.5–5.0 mm. This SiBCN / SiC / SiO2 composite aerogel maintains high loss capability while achieving near-perfect impedance matching (|Z0|0|0) at a thickness of 2.0 mm. in / Z0|≈1).
[0040] Furthermore, this aerogel effectively achieves load transfer and stress dispersion through a three-level reinforcement structure of "fiber skeleton-nanowire reinforcement-oxide layer bonding," achieving a compressive strength of 0.69 MPa, which is 132% higher than that of pure SiBCN aerogel. Figure 3 a); The intrinsic high-temperature oxidation resistance of SiBCN ceramics and SiC imparts structural stability to the material at 1200 ℃ ( Figure 3 (b) This completely solves the fatal flaw of traditional carbon-based carriers (such as carbon fibers and carbon nanotubes) that undergo severe oxidative degradation above 400 °C.
[0041] Example 2
[0042] A method for preparing a hierarchical porous core-shell structured SiBCN / SiC / SiO2 nanofiber composite aerogel includes the following steps:
[0043] Preparation of SiBCN nanofibers: 0.5 g of polyborosilazane (PBSZ) and 0.5 g of polyvinylpyrrolidone (PVP) were dissolved in a mixed solvent of 3 g DMF and 7 g THF, and stirred for 6 h to obtain a homogeneous spinning solution. Electrospinning was performed at a feed rate of 1.0 mL / h, a spinning distance of 20 cm, and a working voltage of 15 kV. The resulting polymer fibers were dried in a 60 ℃ forced-air drying oven for 24 h, followed by pre-oxidative crosslinking in air at 220 ℃ for 2 h. Finally, the crosslinked fibers were placed in a high-temperature tube furnace, heated to 1000 ℃ at a rate of 3 ℃ / min under argon protection, and held at that temperature for 3 h to obtain SiBCN nanofibers.
[0044] Preparation of aerogel preform: 1.5 g of the above-mentioned SiBCN nanofibers were dispersed in 60 mL of deionized water and broken up for 5 min at 6000 r / min using a high-speed disperser. After magnetic stirring for 30 min, 3 mL of C / SiO2 sol (5 wt%) was added, and stirring was continued for another 30 min. Subsequently, 0.3 g of agarose was added, and the mixture was stirred in a 90 ℃ water bath for 1 h and then allowed to cool naturally. The mixed solution was transferred to liquid nitrogen and frozen for 15 min, and then freeze-dried for 48 h to obtain the SiBCN nanofiber aerogel preform.
[0045] In-situ growth of SiC nanowires: The preform was placed in a high-temperature tube furnace and heated to 1450 ℃ at a rate of 2 ℃ / min under an argon atmosphere and held for 1 h. The SiBCN / SiC composite aerogel with SiC nanowires grown inside was obtained by carbothermal reduction reaction.
[0046] Interfacial oxidation treatment: The above SiBCN / SiC composite aerogel was oxidized in air at 1000 °C for 30 min to finally obtain a SiBCN / SiC / SiO2 composite aerogel. The density of this aerogel was 48.1 mg / cm³. 3 Its specific surface area is 72.9 m². 2 / g.
[0047] Tests showed that the aerogel prepared in this embodiment had a minimum reflection loss of -45.6 dB and an effective absorption bandwidth of 5.8 GHz at a thickness of 2.0 mm.
[0048] Example 3
[0049] A method for preparing a hierarchical porous core-shell structured SiBCN / SiC / SiO2 nanofiber composite aerogel includes the following steps:
[0050] Preparation of SiBCN nanofibers: 0.5 g of polyborosilazane (PBSZ) and 0.5 g of polyvinylpyrrolidone (PVP) were dissolved in a mixed solvent of 3 g DMF and 7 g THF, and stirred for 6 h to obtain a homogeneous spinning solution. Electrospinning was performed at a feed rate of 0.8 mL / h, a spinning distance of 18 cm, and a working voltage of 13 kV. The resulting polymer fibers were dried in a 60 ℃ forced-air drying oven for 24 h, followed by pre-oxidative crosslinking in air at 200 ℃ for 2 h. Finally, the crosslinked fibers were placed in a high-temperature tube furnace, heated to 1000 ℃ at a rate of 5 ℃ / min under argon protection, and held at that temperature for 2 h to obtain SiBCN nanofibers.
[0051] Preparation of aerogel preform: 1.5 g of the above-mentioned SiBCN nanofibers were dispersed in 60 mL of deionized water and crushed for 5 min at 6000 r / min using a high-speed disperser. After magnetic stirring for 30 min, 3 mL of C / SiO2 sol (5 wt%) was added, and stirring was continued for another 30 min. Subsequently, 0.3 g of agarose was added, and the mixture was stirred in a 90 ℃ water bath for 1 h and then allowed to cool naturally. The mixed solution was transferred to a freezing device and frozen for 10 min, followed by freeze-drying for 48 h to obtain the SiBCN nanofiber aerogel preform.
[0052] In-situ growth of SiC nanowires: The preform was placed in a high-temperature tube furnace and heated to 1400 ℃ at a rate of 2 ℃ / min under an argon atmosphere and held for 2 h. The SiBCN / SiC composite aerogel with SiC nanowires grown inside was obtained by carbothermal reduction reaction.
[0053] Interfacial oxidation treatment: The SiBCN / SiC composite aerogel was oxidized in air at 950 °C for 45 min to obtain a SiBCN / SiC / SiO2 composite aerogel. The density of this aerogel was 45.7 mg / cm³. 3 Its specific surface area is 83.7 m². 2 / g.
[0054] Tests showed that the aerogel prepared in this embodiment had a minimum reflection loss of -48.3 dB and an effective absorption bandwidth of 6.0 GHz at a thickness of 2.2 mm.
[0055] The aforementioned SiBCN / SiC / SiO2 composite aerogel, when applied to electromagnetic wave absorbing materials, significantly improves electromagnetic wave absorption performance compared to existing technologies, achieving synergistic optimization of multiple loss mechanisms (conductive loss, interfacial polarization, and dipole polarization) and impedance matching. Its overall absorption performance surpasses that of existing fiber-based aerogel absorbing materials. Furthermore, it possesses excellent high-temperature stability and mechanical properties, making it suitable for radar stealth materials in high-temperature extreme environments such as hypersonic vehicles and engine nacelles.
[0056] The above embodiments are only used to further illustrate the multi-level porous SiBCN / SiC / SiO2 composite aerogel of the present invention, its preparation method and application. However, the present invention is not limited to the embodiments. Any simple modifications, equivalent changes and alterations made to the above embodiments based on the technical essence of the present invention shall fall within the protection scope of the technical solution of the present invention.
Claims
1. A hierarchical porous SiBCN / SiC / SiO2 composite aerogel, characterized in that, include: The three-dimensional network structure of SiBCN nanofiber framework, SiC nanowires grown in situ on the SiBCN nanofiber framework, and porous SiO2 shells covering the surface of the SiBCN nanofiber framework and SiC nanowires.
2. The hierarchical porous SiBCN / SiC / SiO2 composite aerogel according to claim 1, characterized in that: The aerogel has a hierarchical porous structure including micron-sized pores and nano-sized pores; the density of the aerogel is 43–50 mg / cm³. 3 Its specific surface area is 58.6–90.1 m². 2 / g.
3. A method for preparing the hierarchical porous SiBCN / SiC / SiO2 composite aerogel according to any one of claims 1 to 2, characterized in that, Includes the following steps: S1: SiBCN polymer precursor nanofibers were prepared by electrospinning, and then obtained by pre-oxidative crosslinking and high-temperature pyrolysis. S2: Disperse the SiBCN nanofibers obtained in step S1 in water, add C / SiO2 sol and agarose, and prepare SiBCN nanofiber aerogel preform by directional freeze drying; S3: The aerogel preform obtained in step S2 is subjected to a carbothermal reduction reaction under an inert atmosphere to grow SiC nanowires in situ in the SiBCN nanofiber framework, thus obtaining SiBCN / SiC composite aerogel. S4: The SiBCN / SiC composite aerogel obtained in step S3 is subjected to thermal oxidation treatment in air atmosphere to form a porous SiO2 shell on the surface of SiBCN nanofiber skeleton and SiC nanowire, thus obtaining SiBCN / SiC / SiO2 composite aerogel.
4. The method for preparing the hierarchical porous SiBCN / SiC / SiO2 composite aerogel according to claim 3, characterized in that: In step S1, the parameters for electrospinning are: feed rate 0.5-1.5 mL / h, spinning distance 15-25 cm, and operating voltage 11-25 kV.
5. The method for preparing the hierarchical porous SiBCN / SiC / SiO2 composite aerogel according to claim 4, characterized in that: The pre-oxidation temperature is 180-230 ℃; the high-temperature pyrolysis has a heating rate of 1-10 ℃ / min, a temperature of 1000-1300 ℃, and a holding time of 1-5 h; the diameter of the obtained SiBCN nanofibers is 100-300 nm.
6. The method for preparing the hierarchical porous SiBCN / SiC / SiO2 composite aerogel according to claim 3, characterized in that: In step S2, the amount of SiBCN nanofibers added is 1.0-3.5 wt% of the water mass; the amount of C / SiO2 sol added is 2-20 wt% of the water mass; and the amount of agarose added is 0.1-1 wt% of the water mass.
7. The method for preparing the hierarchical porous SiBCN / SiC / SiO2 composite aerogel according to claim 6, characterized in that: In step S2, the directional freeze-drying involves first freezing in liquid nitrogen for 10-20 minutes, followed by freeze-drying under vacuum conditions for 24-48 hours.
8. The method for preparing the hierarchical porous SiBCN / SiC / SiO2 composite aerogel according to claim 3, characterized in that: In step S3, the carbothermic reduction reaction is carried out by heating to 1350-1500 ℃ at a rate of 1-10 ℃ / min and holding at that temperature for 1-5 h.
9. The method for preparing the hierarchical porous SiBCN / SiC / SiO2 composite aerogel according to claim 3, characterized in that: In step S4, the thermal oxidation treatment is carried out at 800-1000 ℃ for 5-60 min.
10. The application of the hierarchical porous SiBCN / SiC / SiO2 composite aerogel as described in claim 1 in electromagnetic wave absorbing materials.