A method for preparing BN fiber-based porous ceramics with a multilayer structure

Abstract

The present invention discloses a method for preparing a BN fiber-based porous ceramic with a multilayer structure. The method comprises the following steps: first, preparing BN fibers, and then using the fibers as a substrate to vertically grow cauliflower-shaped SiC on the surface through chemical vapor deposition and the action of a magnetic field. The porous structure of the polymer foam is utilized to generate porous carbon on the surface of the BN / SiC polymer and coat it therein. Finally, the ceramic is calcined to generate the BN fiber-based porous ceramic. The present invention can utilize the cauliflower-like structure formed by aggregation of fine flaky SiC to cause electromagnetic waves to reflect multiple times upon incident, thereby improving the wave absorption performance. The porous carbon structure coated on the surface of the BN / SiC polymer can increase the specific surface area of the material. The electromagnetic waves are scattered and reflected multiple times in the pores, thereby enhancing the interaction between the electromagnetic waves and the wave absorber. The multilayer structure of the BN fiber-based porous ceramic enables the energy of the electromagnetic waves to be continuously converted and absorbed upon incident, thereby improving the absorption effect. The ceramic has broad application prospects in the field of wave absorption.

Description

Technical Field

[0001] The invention belongs to the field of composite ceramic material preparation, and in particular relates to a method for preparing a BN fiber-based porous ceramic with a multi-layer structure. Background Art

[0002] With the rapid development of electronic communications, artificial intelligence, and the Internet of Things (IoT), and their widespread application in unmanned systems, rapid communications, industry, aerospace, and other fields, the level of development in electronic information technology has become a reflection of a country's comprehensive national strength. The widespread use of electromagnetic waves undoubtedly brings countless conveniences to our lives. However, electromagnetic radiation can interfere with the normal operation of precision electronic equipment, affecting its reliability. It can also heat human cells or interfere with the body's inherent electromagnetic field, adversely affecting human health. Therefore, how to fully utilize the advantages of electromagnetic waves while protecting people's health and the surrounding environment from their effects has become a research priority. Addressing the problem of electromagnetic pollution requires the development of high-performance microwave absorbing materials that can effectively convert electromagnetic wave energy into other forms of energy.

[0003] Boron nitride (BN) is a microwave-transmitting material with remarkable electrical insulation, a low dielectric constant, good chemical stability, and excellent microwave transmission performance. In recent years, a small number of studies have examined combining BN with conventional absorbing materials in the field of electromagnetic wave absorption, aiming to optimize the material's impedance matching and overall performance. However, BN suffers from poor mechanical properties and oxidation resistance. Furthermore, in high-temperature environments, its high surface temperature leads to a small temperature gradient across the thickness of the material, which causes a sharp deterioration in its high-temperature dielectric properties. Therefore, BN needs to be combined with other materials to modulate its dielectric properties and thus enhance the composite's microwave absorption performance. Carbon-based absorbing materials, with their high conductivity, are the first materials considered for forming composites with BN. For example, carbon fibers (CFs) offer lightweight, superior electrical and thermal properties, and good structural properties. However, CFs are limited by their high electrical conductivity, poor impedance matching due to their inherent lack of magnetic properties, and a single absorption mechanism, resulting in suboptimal absorption when used alone. The use of silicon carbide as an absorber has been extensively researched. Silicon carbide not only absorbs electromagnetic waves and infrared signals, but also offers advantages such as low density, good toughness, high-temperature resistance, and adjustable resistivity. However, existing research is limited by the fact that the composite method of boron nitride and absorber particles is still mainly a single coating structure, resulting in insufficient size control and dispersion of the absorber particles, and weak polarization losses such as interfacial polarization.

[0004] Ye et al. (Ye F, ZHANG L, YIN X, et al. The improvement of wave-absorbingability of silicon carbidefibers by depositing boron nitride coating [J]. Applied Surface Science, 2013, 270: 611-616.) prepared boron nitride-coated silicon carbide (SiC) fiber materials by chemical vapor infiltration (CVI). Compared with uncoated silicon carbide (SiC) fibers, the tensile strength of the BN-coated SiC fiber bundles was improved. In addition, BN has an extremely low dielectric constant and dielectric loss, which helps to enhance the introduction of electromagnetic waves and reduce electromagnetic wave reflection, thereby greatly improving the material's ability to absorb electromagnetic waves and achieving electromagnetic wave impedance matching. However, BN coating the surface of the SiC fiber does not take advantage of boron nitride's inherent advantages such as high thermal conductivity and high specific surface area, and suffers from problems such as interfacial polarization and weak polarization loss.

[0005] Zhou et al. (Zhou W, Xiao P, Li Y. Preparation and study on microwave absorbing materials of boronnitride coated pyrolytic carbon particles [J]. Applied Surface Science, 2012, 258 (22): 8455-8459.) synthesized a boron nitride coating on pyrolytic carbon (BN-coated PyC) particles through the chemical reaction of boric acid and urea in nitrogen, and obtained a high-temperature resistant and antioxidant composite absorber. The thermal stability and physical insulation of the material were improved. Compared with pyrolytic carbon particles, the BN-coated pyrolytic carbon particles had a lower dielectric constant and better microwave absorption performance, showing a strong absorption peak at 10.64 GHz, and obtained better impedance matching performance. However, due to the coating structure, the size control of the carbon particles is poor, and the dispersion is not strong enough, which limits the improvement of microwave absorption performance.

[0006] The Chinese patent, "Cr5Te8@Expanded Graphite Electromagnetic Absorbing Material, Preparation Method, and Application thereof" (Application Number: CN202311151311.3, Grant Number: CN116875958A, Publication Date: October 13, 2023), discloses an electromagnetic absorbing material, its preparation method, and its application. The patent utilizes a simple and efficient one-step chemical vapor deposition technique to prepare a heterogeneous Cr5Te8@Expanded Graphite (ECT) absorbing material. The resulting material exhibits excellent electromagnetic absorption and radar cross-sectional area reduction capabilities, effectively dissipating electromagnetic waves. However, the material suffers from poor particle size control and uneven particle size, which limits its improved absorption performance.

[0007] The Chinese patent "Method for preparing substrate-induced chemical vapor deposition absorbing SiC ceramics" (application number: CN202211483210.1, authorization number: CN115716750A, announcement date: 2023.02.28) discloses a method for preparing substrate-induced chemical vapor deposition absorbing SiC ceramics. Porous mullite is used as the substrate to deposit SiC ceramics to prepare porous mullite / BN / SiC composite ceramics with a layered structure. The composite ceramics have good electromagnetic wave absorption performance, but their surface is too flat and smooth, and there is still room for structural optimization of the absorption performance. Summary of the Invention

[0008] The purpose of the present invention is to provide a method for preparing BN fiber-based porous ceramics with a multilayer structure, so as to solve the problem that the existing technology of BN and absorber particle composite is still mainly based on a single coating structure, resulting in the control of absorber particle size and insufficient dispersion, which structurally limits the optimization of the material's absorbing performance.

[0009] The technical solution adopted in the present invention is:

[0010] A method for preparing a BN fiber-based porous ceramic with a multilayer structure is specifically implemented by the following steps:

[0011] Step 1, preparation of BN fibers

[0012] Boric acid and melamine are dissolved in deionized water in a certain ratio and heated and stirred. After cooling and drying, a precursor fiber is obtained. The precursor fiber is then placed in a tubular furnace and calcined under a nitrogen atmosphere and kept warm to obtain BN fiber.

[0013] Step 2: Depositing cauliflower-shaped SiC particles

[0014] The BN fiber obtained in step 1 is placed as a substrate in a chemical vapor deposition reaction chamber, and then nitrogen and hexamethyldisilazane gas are filled into the chemical vapor deposition reaction chamber to maintain a constant temperature. A pulsed magnetic field coil is installed outside the chemical vapor deposition reaction chamber, and a suitable frequency, intensity, and magnetic field direction are selected. Cauliflower-shaped BN / SiC is deposited at a certain temperature for a certain time.

[0015] Step 3: Constructing a porous BN / SiC / C precursor

[0016] Dispersing the BN / SiC obtained in step 2 in water to obtain a BN / SiC aqueous solution; weighing a water-soluble polymer and dissolving it in water, adding the BN / SiC aqueous solution and the polymer foam to the polymer aqueous solution, continuously stirring and heating, cooling, and freeze-drying to obtain a BN / SiC / C precursor;

[0017] Step 4, calcination

[0018] The BN / SiC / C precursor obtained in step 3 is placed in a tubular furnace for calcination to obtain BN fiber-based porous ceramics.

[0019] Furthermore, the specific method of step 1 is: adding boric acid and melamine in a molar ratio of 3:1 to deionized water, stirring at a temperature of 80°C to 90°C for 30min to 60min, cooling for 12h and then drying to obtain a precursor fiber; then placing it in a tubular furnace and heating it to 1400°C to 1600°C under a nitrogen atmosphere for calcination, keeping it warm for 3h to 4h to obtain BN fiber.

[0020] Furthermore, in step 2, the ratio of nitrogen and hexamethyldisilazane gas is 1:1.25~1.25:1, and the constant temperature is 50℃~60℃; the frequency of the pulsed magnetic field coil is adjusted to 50Hz, the induction power is 2.2kW~12kW, and the direction of the magnetic field is perpendicular to the direction of the BN substrate; and cauliflower-shaped BN / SiC is obtained by deposition at a temperature of 1010℃~1220℃ for 2h.

[0021] Furthermore, the specific steps of step 3 are as follows:

[0022] Step 3.1, adding the BN / SiC obtained in step 2 to water and sonicating for 30 minutes until completely dispersed to obtain a BN / SiC aqueous solution;

[0023] Step 3.2, weighing a water-soluble polymer and dissolving it in water, stirring at 80° C. for 30 to 60 minutes, adding the BN / SiC aqueous solution obtained in step 3.1 and stirring at 80° C. for 1 hour to obtain a BN / SiC polymer mixed aqueous solution;

[0024] In step 3.3, the polymer foam was cut into cubes with a side length of 1 to 2 cm, and immersed in anhydrous ethanol for ultrasonic treatment. After being completely dried at 60°C, it was added to the BN / SiC polymer mixed aqueous solution in step 3.2, and stirred and heated continuously. After the polymer foam was completely soaked, it was taken out and naturally cooled, and freeze-dried for 24 hours to obtain a BN / SiC / C precursor.

[0025] Furthermore, the water-soluble polymer is one of polyvinyl alcohol, polystyrene and polyvinylamine.

[0026] Furthermore, the polymer foam is one of melamine foam, polystyrene foam, polyethylene foam, polyurethane foam, and polypropylene foam.

[0027] Furthermore, the calcination treatment method in step 4 is as follows: the BN / SiC / C precursor obtained in step 3 is placed in a tube furnace and heated to 750° C. to 850° C. in an argon atmosphere for calcination, and kept warm for 2 h to 3 h to obtain BN fiber-based porous ceramics.

[0028] Compared with the prior art, the present invention has the following beneficial effects:

[0029] A method for preparing a multilayered BN fiber-based porous ceramic produces a multilayered microwave-absorbing composite material composed of porous carbon coated on BN fibers with cauliflower-like SiC particles growing on their surfaces. The cauliflower-like SiC particles, formed by agglomerating fine SiC flakes and oriented vertically, combine with the BN material to cause electromagnetic waves to reflect multiple times upon impact, enhancing their absorption performance. The porous carbon structure increases the material's specific surface area. Electromagnetic waves scatter and reflect multiple times within the pores, enhancing their interaction with the absorber. The multilayered structure allows for the continuous conversion and absorption of incident electromagnetic waves, thereby enhancing absorption efficiency. This material has broad application prospects in the microwave absorption field. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 It is a schematic diagram of the structure of the multi-layer porous wave-absorbing composite material prepared by the present invention.

[0031] Figure 2 This is a microscopic diagram of the cauliflower-like SiC structure.

[0032] In the figure, 1-cauliflower-like SiC, 2-BN fiber, 3-porous carbon. DETAILED DESCRIPTION

[0033] The present invention will be described in detail below with reference to the accompanying drawings and specific embodiments.

[0034] A method for preparing a BN fiber-based porous ceramic with a multilayer structure is specifically implemented by the following steps:

[0035] Step 1, preparation of BN fibers:

[0036] Boric acid and melamine are added to deionized water in a molar ratio of 3:1, stirred at 80°C to 90°C for 30min to 60min, cooled for 12h and then dried to obtain a precursor fiber; then placed in a tubular furnace and heated to 1400°C to 1600°C under a nitrogen atmosphere for calcination, and kept warm for 3h to 4h to obtain BN fiber.

[0037] Step 2: Depositing cauliflower-shaped SiC particles

[0038] First, the BN fiber obtained in step 1 is placed in a chemical vapor deposition reaction chamber as a substrate, and nitrogen and hexamethyldisilazane (HMDS) gas are sent into the chemical vapor deposition reaction chamber at a ratio of 1:1.25 to 1.25:1 at a constant temperature of 50°C to 60°C; a pulsed magnetic field coil is installed outside the chemical vapor deposition reaction chamber, the frequency is adjusted to 50 Hz, the induction power is 2.2 kW to 12 kW, and the direction of the magnetic field is perpendicular to the direction of the BN substrate; cauliflower-shaped BN / SiC is deposited at a temperature of 1010°C to 1220°C for 2 hours.

[0039] Step 3: Constructing porous BN / SiC / C precursor:

[0040] Step 3.1, adding the BN / SiC obtained in step 2 to water and sonicating for 30 minutes until completely dispersed to obtain a BN / SiC aqueous solution;

[0041] Step 3.2, weighing a water-soluble polymer and dissolving it in water, stirring at 80°C for 30 to 60 minutes, adding the BN / SiC aqueous solution obtained in step 3.1 and stirring at 80°C for 1 hour, wherein the water-soluble polymer is one of polyvinyl alcohol, polystyrene, and polyvinylamine;

[0042] In step 3.3, the polymer foam is cut into cubes with a side length of 1 to 2 cm, and is immersed in anhydrous ethanol for ultrasonic treatment. After being completely dried at 60°C, it is added to the BN / SiC polymer mixed aqueous solution in step 3.2, and is continuously stirred and heated. After the foam is completely soaked, it is taken out and naturally cooled, and freeze-dried for 24 hours to obtain a BN / SiC / C precursor. The polymer foam is one of melamine foam, polystyrene foam, polyethylene foam, polyurethane foam, and polypropylene foam.

[0043] Step 4, calcination treatment:

[0044] The BN / SiC / C precursor obtained in step 3 is placed in a tubular furnace and heated to 750° C. to 850° C. in an argon atmosphere for calcination, and kept at this temperature for 2 h to 3 h to obtain BN fiber-based porous ceramics.

[0045] The present invention selects hexamethyldisilazane (HMDS) gas as a silicon source and a carbon source and nitrogen in a ratio of 1.25:1 to 1:1.25 to prepare SiC, controls the volume ratio of the introduced gases to regulate the size of the grown cauliflower-shaped SiC particles, and controls the gas input temperature to disperse the hexamethyldisilazane (HMDS) gas more evenly, thereby making the SiC particles more evenly distributed and dense. A pulse coil is installed outside a chemical vapor deposition reaction chamber to control the direction and frequency of the magnetic field so that SiC grows vertically in a cauliflower shape on the surface of a BN fiber. The cauliflower-shaped structure is formed by the accumulation of small SiC flakes, and the structure can cause electromagnetic waves to be reflected multiple times when incident, thereby effectively improving the electromagnetic wave absorption efficiency. The deposition temperature and deposition time are controlled so that the cauliflower-shaped SiC grows more evenly on the BN surface, thereby enhancing the wave absorption performance of the BN fiber.

[0046] The present invention uses polymer foam and a water-soluble polymer as carbon sources to produce a porous honeycomb C coating on the surface of a BN / SiC composite material. The density of the pore structure is adjusted by controlling the calcination temperature. The pores in the porous structure cause multiple reflections and scattering of electromagnetic waves, complicating the propagation path of electromagnetic waves within the material and increasing the interaction between the electromagnetic waves and the material, thereby enhancing the absorption effect. Furthermore, the multilayer structure of the BN fiber-based porous ceramic ensures that the energy of incident electromagnetic waves is continuously converted and absorbed, thereby improving the absorption effect, and has broad application prospects in the field of microwave absorption.

[0047] from Figure 1-2 It can be seen from the figure that cauliflower-like SiC grows vertically on the surface of BN fiber. Cauliflower-like SiC is aggregated from small flake SiC. Porous carbon coats BN / SiC composite material to form a multilayer porous absorbing composite material.

[0048] Example 1

[0049] First, weigh 6.2g of boric acid and 3.4g of melamine into a beaker, then add 400ml of deionized water, seal with plastic wrap, heat and stir at 80℃ for 1h, cool to room temperature after complete dissolution, cool at room temperature for 12h, filter and dry, then place in a nitrogen tubular furnace for calcination, and keep warm at 1400℃ for 3h to obtain BN fiber.

[0050] First, a BN fiber substrate was placed in a chemical vapor deposition (CVD) chamber. Nitrogen and hexamethyldisilazane (HMDS) gases were maintained at a constant temperature of 50°C at a ratio of 1:1.25 and introduced into the CVD chamber, with a total pressure of 450 Pa. The magnetic field around the CVD chamber was adjusted to be perpendicular to the substrate, with a frequency of 50 Hz and an induction power of 2.2 kW. The BN / SiC composite was deposited at 1010°C for 2 hours.

[0051] Weigh 1g of BN / SiC and add it to 20ml of water and ultrasonicate for 30min until completely dispersed to obtain a BN / SiC aqueous solution; weigh 3g of polyvinyl alcohol and add it to 30ml of water, heat and stir in a water bath at 80℃ for 30min, after the polyvinyl alcohol is completely dissolved, add the BN / SiC aqueous solution and stir at 80℃ for 1h to obtain a BN / SiC polymer mixed aqueous solution; cut the melamine foam into cubes with a side length of 1cm, then soak it in anhydrous ethanol and ultrasonicate it, after it is completely dried at 60℃, add the BN / SiC polymer mixed aqueous solution, continue stirring and heating, take it out after the melamine foam is completely soaked and naturally cooled, and freeze-dry it for 24h to obtain a BN / SiC / C precursor.

[0052] The BN / SiC / C precursor was heated to 750°C in an argon atmosphere and kept at this temperature for 2 h to obtain BN fiber-based porous ceramics.

[0053] Example 2

[0054] First, weigh 6.2g of boric acid and 3.4g of melamine into a beaker, then add 400ml of deionized water, seal with plastic wrap, heat and stir at 90℃ for 30min, cool to room temperature after complete dissolution, cool at room temperature for 12h, filter and dry, then place in a nitrogen tubular furnace for calcination, and keep warm at 1600℃ for 4h to obtain BN fiber.

[0055] First, a BN fiber substrate was placed in a chemical vapor deposition (CVD) chamber. Nitrogen and hexamethyldisilazane (HMDS) gases were maintained at a constant temperature of 60°C at a ratio of 1.25:1 and introduced into the CVD chamber at a fixed total pressure of 450 Pa. The magnetic field around the CVD chamber was adjusted to be perpendicular to the substrate, with a frequency of 50 Hz and an induction power of 12 kW. The BN / SiC composite was deposited at 1220°C for 2 hours.

[0056] Weigh 1g of BN / SiC and add it to 20ml of water and ultrasonicate for 30min until completely dispersed to obtain a BN / SiC aqueous solution; weigh 3g of polystyrene and add it to 30ml of water, heat and stir in a water bath at 80℃ for 60min, and after the polyvinyl alcohol is completely dissolved, add the BN / SiC aqueous solution and stir at 80℃ for 1h to obtain a BN / SiC polymer mixed aqueous solution; cut the polyaniline foam into cubes with a side length of 2cm, then soak it in anhydrous ethanol for ultrasonic treatment, and after completely drying at 60℃, add the BN / SiC polymer mixed aqueous solution, continue stirring and heating, and take it out after the polyaniline foam is completely soaked and naturally cooled, and freeze-dried for 24h to obtain a BN / SiC / C precursor.

[0057] The BN / SiC / C precursor was heated to 850°C under an argon atmosphere and kept at this temperature for 3 h to obtain BN fiber-based porous ceramics.

[0058] Example 3

[0059] First, weigh 6.2g of boric acid and 3.4g of melamine into a beaker, then add 400ml of deionized water, seal with plastic wrap, heat and stir at 85℃ for 1h, cool to room temperature after complete dissolution, cool at room temperature for 12h, filter and dry, then place in a nitrogen tubular furnace for calcination, and keep warm at 1500℃ for 3h to obtain BN fiber.

[0060] First, a BN fiber substrate was placed in a chemical vapor deposition (CVD) chamber. Nitrogen and hexamethyldisilazane (HMDS) gases were maintained at a constant temperature of 55°C in a 1:1 ratio and introduced into the CVD chamber at a fixed total pressure of 450 Pa. The magnetic field around the CVD chamber was adjusted to be perpendicular to the substrate, with a frequency of 50 Hz and an induction power of 10 kW. The BN / SiC composite was deposited at 1130°C for 2 hours.

[0061] Weigh 1g of BN / SiC and add it to 20ml of water and ultrasonicate for 30min until completely dispersed to obtain a BN / SiC aqueous solution; weigh 3g of polyvinylamine and add it to 30ml of water, heat and stir in an 80℃ water bath for 30min, and after the polyvinyl alcohol is completely dissolved, add the BN / SiC aqueous solution and continue stirring for 1h to obtain a BN / SiC polymer mixed aqueous solution; cut the polyethylene foam into cubes with a side length of 1.5cm, then soak it in anhydrous ethanol and ultrasonicate it, and after it is completely dried at 60℃, add the BN / SiC polymer mixed aqueous solution, continue stirring and heating, and take it out after the polyethylene foam is completely soaked and naturally cooled, and freeze-dry it for 24h to obtain a BN / SiC / C precursor.

[0062] The BN / SiC / C precursor was heated to 800°C in an argon atmosphere and kept at this temperature for 3 h to obtain BN fiber-based porous ceramics.

[0063] Table 1 is a comparison of the minimum reflectivity (RL) of BN fiber-based porous ceramics, BN-coated PyC composite materials and graphite / BN composite materials in Example 2. As can be seen from Table 1, the minimum reflectivity of the BN-coated PyC composite material is the largest, and the minimum reflectivity of the graphite / BN composite material is slightly smaller than that of the BN-coated PyC composite material. The interface bonding of graphite and BN is more effective in improving the material's wave absorption performance, but its single structure limits the improvement of the wave absorption performance. The cauliflower-shaped SiC particles that grow vertically in the BN / SiC / C composite material in Example 2 can cause electromagnetic waves to reflect multiple times when incident, thereby improving the wave absorption performance. The porous structure of carbon can increase the specific surface area of the material, causing electromagnetic waves to scatter and reflect multiple times in the pores. The multilayer structure combined with the two structures allows the energy of electromagnetic waves to be continuously converted and absorbed when incident, thereby improving the electromagnetic wave absorption effect. Therefore, the minimum reflectivity of the BN / SiC / C composite material in Example 2 is the smallest.

[0064] Table 1

[0065] category Minimum reflectivity (dB) BN fiber-based porous ceramics in Example 2 -42.5 Graphite / BN composite materials -26.8 BN-coatedPyC composites -21.5

[0066] Although specific embodiments of the present invention have been described above, those skilled in the art will appreciate that these are merely illustrative and that the scope of the present invention is defined by the appended claims. Those skilled in the art may make various changes or modifications to these embodiments without departing from the principles and essence of the present invention, and such changes and modifications are intended to fall within the scope of the present invention.

Claims

1. A method for preparing a BN fiber-based porous ceramic having a multilayer structure, characterized in that: Please follow the steps below to implement: Step 1, preparation of BN fibers Boric acid and melamine are dissolved in deionized water in a certain ratio and heated and stirred. After cooling and drying, a precursor fiber is obtained. The precursor fiber is then placed in a tubular furnace and calcined under a nitrogen atmosphere and kept warm to obtain BN fiber. Step 2: Depositing cauliflower-shaped SiC particles The BN fiber obtained in step 1 is placed as a substrate in a chemical vapor deposition reaction chamber, and then nitrogen and hexamethyldisilazane gas are filled into the chemical vapor deposition reaction chamber to maintain a constant temperature. A pulsed magnetic field coil is installed outside the chemical vapor deposition reaction chamber, and a suitable frequency, intensity, and magnetic field direction are selected. Cauliflower-shaped BN / SiC is deposited at a certain temperature for a certain time. Step 3: Constructing a porous BN / SiC / C precursor Dispersing the BN / SiC obtained in step 2 in water to obtain a BN / SiC aqueous solution; weighing a water-soluble polymer and dissolving it in water, adding the BN / SiC aqueous solution and the polymer foam to the polymer aqueous solution, continuously stirring and heating, cooling, and freeze-drying to obtain a BN / SiC / C precursor; Step 4, calcination The BN / SiC / C precursor obtained in step 3 is placed in a tubular furnace for calcination to obtain BN fiber-based porous ceramics.

2. The method for preparing a BN fiber-based porous ceramic having a multilayer structure according to claim 1, characterized in that: The specific method of step 1 is as follows: adding boric acid and melamine to deionized water in a molar ratio of 3:1, stirring at a temperature of 80°C to 90°C for 30 minutes to 60 minutes, cooling for 12 hours and then drying to obtain a precursor fiber; then placing it in a tubular furnace and heating it to 1400°C to 1600°C under a nitrogen atmosphere for calcination, and keeping it warm for 3 hours to 4 hours to obtain BN fiber.

3. The method for preparing a BN fiber-based porous ceramic having a multilayer structure according to claim 1, characterized in that: In step 2, the ratio of nitrogen and hexamethyldisilazane gas is 1:1.25 to 1.25:1, and the constant temperature is 50°C to 60°C; the frequency of the pulsed magnetic field coil is adjusted to 50 Hz, the induction power is 2.2 kW to 12 kW, and the direction of the magnetic field is perpendicular to the direction of the BN substrate; and cauliflower-shaped BN / SiC is obtained by deposition at a temperature of 1010°C to 1220°C for 2 hours.

4. The method for preparing a BN fiber-based porous ceramic having a multilayer structure according to claim 1, characterized in that: The specific steps of step 3 are as follows: Step 3.1, adding the BN / SiC obtained in step 2 to water and sonicating for 30 minutes until completely dispersed to obtain a BN / SiC aqueous solution; Step 3.2, weighing a water-soluble polymer and dissolving it in water, stirring at 80° C. for 30 to 60 minutes, adding the BN / SiC aqueous solution obtained in step 3.1 and stirring at 80° C. for 1 hour to obtain a BN / SiC polymer mixed aqueous solution; In step 3.3, the polymer foam was cut into cubes with a side length of 1 to 2 cm, and immersed in anhydrous ethanol for ultrasonic treatment. After being completely dried at 60°C, it was added to the BN / SiC polymer mixed aqueous solution in step 3.2, and stirred and heated continuously. After the polymer foam was completely soaked, it was taken out and naturally cooled, and freeze-dried for 24 hours to obtain a BN / SiC / C precursor.

5. The method for preparing a BN fiber-based porous ceramic having a multilayer structure according to claim 4, characterized in that: The water-soluble polymer is one of polyvinyl alcohol, polystyrene and polyvinylamine.

6. The method for preparing a BN fiber-based porous ceramic having a multilayer structure according to claim 4, characterized in that: The polymer foam is one of melamine foam, polystyrene foam, polyethylene foam, polyurethane foam and polypropylene foam.

7. The method for preparing a BN fiber-based porous ceramic having a multilayer structure according to claim 1, characterized in that: The calcination treatment method in step 4 is as follows: the BN / SiC / C precursor obtained in step 3 is placed in a tube furnace and heated to 750° C. to 850° C. in an argon atmosphere for calcination, and kept at this temperature for 2 h to 3 h to obtain BN fiber-based porous ceramics.

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