Nanometer micropore heat insulation plate for energy storage of hydrogen fuel cell and preparation method of nanometer micropore heat insulation plate

Abstract

The invention discloses a hydrogen fuel cell energy storage nanometer micropore heat insulation plate and a preparation method thereof. The heat insulation plate has an open pore net structure, the volume density is 0.25-0.30 g / cm < 3 >, the room temperature heat conductivity coefficient is smaller than or equal to 0.020 W / m.K, and the compression strength is larger than or equal to 750 kPa when the compression deformation is 20%. The preparation method comprises the following steps: enabling pre-dried fibers to reach a fluidized dispersion state under the action of airflow, and then enabling the fibers to be single and surface-charged through a high-voltage electrostatic field, so as to obtain charged fibers; carrying out dry mixing on the charged fibers and other raw materials in sequence under vacuum; according to the invention, through an airflow and static synergetic dispersion technology, the problem that fibers are easy to agglomerate in a dry process is fundamentally solved, and uniform distribution of a fiber network is realized, so that the product is endowed with excellent heat insulation performance and mechanical strength at the same time.

Description

Technical Field

[0001] This invention relates to the field of fuel cell energy storage materials technology, and in particular to a nanoporous heat insulation plate for hydrogen fuel cell energy storage and its preparation method. Background Technology

[0002] Solid oxide fuel cells, as a highly efficient and clean energy conversion and storage system, rely heavily on the performance of key auxiliary materials—high-temperature insulation materials—for their efficient and stable operation. These materials need to have extremely low thermal conductivity over a wide temperature range (especially the medium- and high-temperature range) to minimize system heat loss, while also possessing sufficient mechanical strength to withstand stresses during assembly and operation, and exhibiting excellent long-term high-temperature dimensional stability.

[0003] In recent years, thermal insulation materials based on nanoporous structures have attracted widespread attention due to their superior thermal insulation performance. The principle lies in using nanoscale powders (such as fumed silica) to construct a large number of pores smaller than the mean free path of air molecules, thereby significantly inhibiting the thermal conduction of gas molecules. By adding light-blocking agents to scatter infrared radiation and introducing ceramic fibers as a reinforcing skeleton, it is theoretically possible to obtain thermal insulation panels with excellent overall performance.

[0004] Dry molding is a common process for preparing fiber-reinforced composite insulation panels due to its simplicity, high efficiency, and environmental friendliness. However, this process faces a long-standing and unresolved core technical bottleneck in practical applications: the uniform dispersion of the reinforcing fibers. The ceramic or high-silica fibers used for reinforcement have enormous specific surface areas and extremely high surface energies, exhibiting a strong tendency to agglomerate in the dry state. During dry mixing, these fibers readily adsorb and entangle with each other due to van der Waals forces, forming fiber agglomerates that are difficult to break apart. This inherent agglomeration phenomenon prevents the fibers from being uniformly distributed as individual fibers within the nanopowder substrate.

[0005] This is the fundamental reason why nanoporous thermal insulation materials prepared by traditional dry methods have uneven structures and poor performance. Fiber agglomeration directly causes the following serious consequences: (1) It forms local and macroscopic fiber-rich and fiber-poor areas inside the material, destroying the uniformity that the thermal insulation structure should have; (2) The fibers inside the agglomerates are tightly packed, which cannot effectively form a three-dimensional network skeleton to support the entire material, weakening the reinforcement effect; (3) The dense fiber agglomerates hinder the uniform filling of nanopowders, affecting the uniformity of the microporous structure. Ultimately, the thermal conductivity of the product is often higher than theoretically expected, the mechanical strength fluctuates greatly, the reliability is insufficient, and the dimensional stability at high temperatures is also difficult to guarantee, which cannot meet the stringent requirements of high-end equipment such as solid oxide hydrogen fuel cells for the consistency and reliability of thermal insulation materials.

[0006] Therefore, developing a new method that can fundamentally solve the problem of fiber agglomeration in dry processes, achieve highly individualized and uniformly dispersed fibers, and thus prepare nanoporous insulation boards with uniform structure, excellent performance, and stability has become an urgent technical need in this field. Summary of the Invention

[0007] To overcome the aforementioned shortcomings of the prior art, embodiments of the present invention provide a nanoporous insulation plate for hydrogen fuel cell energy storage, the innovation of which lies in:

[0008] Made by dry molding from raw materials containing the following components:

[0009] Nanoscale composite substrate composed of vapor-phase SiO2 and vapor-phase Al2O3;

[0010] Opacifier containing SiC and TiO2 particles;

[0011] Reinforcing fibers containing chopped high-silica fibers and Al2O3 ceramic fibers;

[0012] The insulation board has an open-pore mesh structure and a bulk density of 0.25–0.30 g / cm³. 3 The thermal conductivity at room temperature is not higher than 0.020 W / m·K, and the compressive strength at 20% compression deformation is not lower than 750 kPa.

[0013] Furthermore, the particle size of the above-mentioned gas-phase SiO2 is 2.0–4.5 nm, the particle size of the gas-phase Al2O3 is 2.0–9.0 nm, the particle size of SiC is 0.5–1.2 μm, and the particle size of TiO2 particles is 12.0–20.0 nm.

[0014] Furthermore, the diameter of the aforementioned short-cut high-silica fibers is 2.0–4.0 μm, and the aspect ratio is 10–20:1; the diameter of the Al2O3 ceramic fibers is 9.0–12.0 μm, and the aspect ratio is 15–25:1.

[0015] Furthermore, the permanent linear change rate of the above-mentioned insulation board after being treated at 1000°C for 12 hours is 1.0 to 2.0%.

[0016] This invention provides a method for preparing a nanoporous thermal insulation board, comprising the following steps:

[0017] (1) Fiber pretreatment and dispersion: After pre-drying the short high silica oxygen fiber and Al2O3 ceramic fiber, fiber agglomerates are formed. The fiber agglomerates are transported through the electrostatic dispersion chamber under the action of airflow. At the same time, a high voltage electrostatic field is applied in the electrostatic dispersion chamber to make the fiber single and charge the surface, and finally form several charged fibers.

[0018] (2) Dry mixing: Charged fibers, gas-phase SiO2, gas-phase Al2O3, SiC opacifier and TiO2 particles are sequentially added to the mixing equipment and mixed evenly under vacuum dehumidification to obtain dry mixture.

[0019] (3) Gradient molding: The dry mixture is filled into the mold. First, a first-stage pressure of 20 to 30 tons is applied to make the material flow and fill. Then, the pressure is stepped up to a second-stage pressure of 200 to 400 tons to degas and densify, forming a blank with a density gradient.

[0020] (4) Low temperature heat treatment: The billet is heat treated at a temperature of 150 to 300°C.

[0021] Furthermore, in step (1), pre-drying reduces the moisture content of the fiber to less than 0.5%.

[0022] Furthermore, in step (1), the airflow is configured to cause the fibers to be in a fluidized and dispersed state before entering the region of action of the high-voltage electrostatic field.

[0023] Furthermore, in step (2), the order of feeding materials is as follows: first, add gaseous SiO2 and gaseous Al2O3, then add SiC and TiO2 particles, and finally add the fiber treated in step (1).

[0024] The beneficial effects of this invention are:

[0025] 1. This invention fundamentally solves the bottleneck problem of uniform fiber dispersion in dry processes, achieving a revolutionary optimization of the microstructure of insulation materials: It innovatively employs a synergistic scheme of "airflow pre-dispersion combined with high-voltage electrostatic treatment." Before electrostatic treatment, precisely controlled airflow ensures the fiber aggregates reach a "fluidized dispersion state," creating an ideal prerequisite for subsequent efficient electrostatic charging and repulsive dispersion. The high-voltage electrostatic field causes individual fiber surfaces to carry the same charge, achieving permanent dispersion through Coulomb repulsion. The innovative use of airflow pre-dispersion to achieve a fluidized dispersion state ensures high efficiency in electrostatic dispersion, resulting in a uniform fiber network and fine pore structure within the prepared insulation board. This completely overcomes the structural inhomogeneity defects caused by fiber agglomeration in traditional dry processes, laying a crucial structural foundation for obtaining high-performance products.

[0026] 2. A nanoporous insulation board with both ultra-low thermal conductivity and high mechanical strength was successfully prepared, with comprehensive performance significantly surpassing existing products: Thanks to the uniform and stable three-dimensional fiber network and delicate nanoporous structure created by the above method, the insulation board prepared by this invention can simultaneously achieve ultra-low thermal conductivity (≤0.020W / m·K) and high compressive strength (≥750kPa when deformed by 20%), solving the contradiction that such materials usually struggle to balance thermal insulation and mechanical properties.Figure 5 As shown in the examples, its high-temperature (e.g., 1000℃) thermal insulation performance is improved by more than 30% compared to traditional ceramic fiber boards, and its stability over a wide temperature range is superior. This enables it to better meet the stringent requirements of high-end equipment such as solid oxide hydrogen fuel cells for lightweight, efficient thermal insulation, and structural reliability. Attached Figure Description

[0027] Figure 1 This is a flowchart illustrating the fabrication process of the nanoporous insulation board of this invention.

[0028] Figure 2 This is a schematic diagram illustrating the principle of fiber electrostatic dispersion treatment in this invention.

[0029] Figure 3 This is a schematic diagram of the pressure-time curve of the gradient molding process of the present invention.

[0030] Figure 4 This is a SEM image of the microstructure of the nanoporous insulation board prepared according to the present invention.

[0031] Figure 5 This is a comparison chart showing the thermal conductivity of the insulation board of this invention and the comparative product at different temperatures. Detailed Implementation

[0032] The nanoporous heat insulation plate for solid oxide hydrogen fuel cell energy storage of the present invention and its preparation method are described in detail below with reference to the accompanying drawings and embodiments. The scope of protection of the present invention is not limited to the described embodiments.

[0033] Working principle overview

[0034] This invention aims to provide a nanoporous insulation board that combines ultra-low thermal conductivity, high mechanical strength, and excellent high-temperature stability, making it particularly suitable for high-temperature energy storage systems such as solid oxide hydrogen fuel cells. Its core principle lies in:

[0035] 1. Nanoporous structure inhibits heat transfer: The nanoscale composite substrate composed of gas-phase SiO2 and gas-phase Al2O3 forms a large number of nanopores with a size smaller than the mean free path of air molecules, which greatly restricts the convective heat transfer of air molecules and the heat conduction of gas.

[0036] 2. Light-blocking agent inhibits radiative heat transfer: The addition of submicron-sized SiC and nano-sized TiO2 particles as light-blocking agents can effectively scatter and absorb infrared radiative heat in the medium and high temperature range (especially above 600℃), reducing the proportion of radiative heat transfer.

[0037] 3. Three-dimensional fiber network reinforcement and shaping: Through specific pretreatment and electrostatic dispersion techniques, short-cut high-silica fibers and Al2O3 ceramic fibers are uniformly monolithized and surface-charged, forming a uniform and stable three-dimensional network framework during subsequent mixing and molding. This framework not only provides mechanical support to prevent excessive compaction of nanoparticles but also fixes the open-pore network structure.

[0038] 4. Gradient molding achieves structural optimization: The gradient molding process, which first uses low-pressure flow filling and then high-pressure stepwise degassing and densification, ensures that the material is uniformly filled into the mold while gradually removing air, forming a blank with a reasonable density gradient from the surface to the interior, thereby balancing strength and thermal insulation performance.

[0039] 5. Low-temperature heat treatment stabilizes the structure: Low-temperature heat treatment further removes residual moisture and organic impurities, promotes the bonding between the fiber and particle interface, stabilizes the microstructure, and improves the dimensional stability and mechanical properties of the product.

[0040] The following examples will illustrate in detail how to implement the above principles.

[0041] Example 1: Main Example

[0042] This embodiment details a nanoporous thermal insulation board and its preparation method, the process flow diagram of which is shown below. Figure 1 As shown.

[0043] 1. Raw material preparation

[0044] Prepare the raw materials according to the following mass proportions:

[0045] Nanoscale composite substrate: 65 parts of vapor-phase SiO2 (particle size 3.0 nm) and 15 parts of vapor-phase Al2O3 (particle size 5.0 nm).

[0046] Opacifier: 10 parts SiC particles (particle size 0.8 μm), 5 parts TiO2 particles (particle size 15.0 nm).

[0047] Reinforcing fibers: 3 parts of short-cut high-silica fibers (diameter 3.0 μm, length 30-60 μm, aspect ratio about 15:1) and 2 parts of Al2O3 ceramic fibers (diameter 10.0 μm, length 150-250 μm, aspect ratio about 20:1).

[0048] 2. Fiber pretreatment and dispersion

[0049] First, the short-cut high-silica fibers and Al2O3 ceramic fibers were pre-dried in an oven at 120°C for 4 hours to reduce their moisture content to less than 0.5%, forming initial fiber aggregates 1.

[0050] Subsequently, the crucial electrostatic dispersion step is carried out, the principle of which is as follows: Figure 2 As shown. Figure 2 The principle of fiber electrostatic dispersion is illustrated schematically, where fiber aggregates 1 enter the electrostatic dispersion chamber 2 under the action of airflow, forming charged fibers 22 under the action of a high-voltage electrostatic field 21. The dried fiber aggregates 1 are transported into the electrostatic dispersion chamber 2 by airflow. Before entering the electrostatic field region, the fibers achieve a fluidized dispersion state by precisely controlling the airflow speed and direction, i.e., the fibers are separated from each other and dynamically suspended and tumbling. Next, the fibers flow through the high-voltage electrostatic field 21 region (in this embodiment, an 8kV DC voltage is applied using a pair of parallel metal plates). In the high-voltage electrostatic field 21, the surface of the fluidized fiber individual or extremely small clusters is uniformly charged with the same type of charge (e.g., negative charge). Due to Coulomb repulsion, the fibers are further repelled and separated, ultimately outputting fully single, surface-charged charged fibers 22. This step ensures that the fibers are uniformly dispersed in subsequent mixing, forming an ideal three-dimensional network.

[0051] 3. Dry mixing

[0052] In a vacuum mixer, the ingredients are added and mixed in the following order:

[0053] a. First, add gaseous SiO2 and gaseous Al2O3 and mix at low speed for 2 minutes.

[0054] b. Add SiC particles and TiO2 particles, and continue mixing at low speed for 3 minutes to ensure that the opacifier is evenly attached to the surface of the nanoparticles.

[0055] c. Finally, add the charged fibers 22 obtained in step 2. Under conditions of vacuum of -0.095 MPa and relative humidity <10%, mix at medium speed for 10 minutes to obtain a uniform dry mixture. Due to the surface charge, the charged fibers can penetrate more evenly into the powder particles, reducing agglomeration.

[0056] 4. Gradient molding

[0057] The dry mixture is filled into a 300mm × 300mm steel mold. A gradient molding process is used, and its pressure-time curve is shown in the figure below. Figure 3 As shown.

[0058] Phase 1 (Rapid Pressure Increase and Low-Pressure Holding): Starting from 0 seconds, within approximately 10 seconds (corresponding to...) Figure 3 From point 0 to point A, the pressure is rapidly and linearly increased to 25 tons (approximately 83 MPa). Then, the pressure is maintained at 25 tons for 20 seconds (from point A to point B), during which the material is allowed to flow fully and fill the mold cavity.

[0059] The second stage (stepped pressure increase and high-pressure holding): After the pressure holding period, a stepped pressure increase begins. Within 60 seconds (point B to C), the pressure is gradually increased from 25 tons to 300 tons (approximately 1000 MPa) through multiple steps of "pressure increase for 5-10 seconds, followed by brief pressure holding for 2-3 seconds." This stage facilitates the gradual expulsion of air from the mold cavity, preventing defects. After reaching the maximum pressure of 300 tons, the pressure is held for 120 seconds (point C to D) to achieve final densification of the billet.

[0060] Pressure relief and demolding: After the pressure holding period ends, the pressure is slowly released to 0 over approximately 30 seconds (points D→E), and then the mold is removed to obtain a plate-shaped blank with a density gradient.

[0061] 5. Low-temperature heat treatment

[0062] The demolded billet is placed in a programmable temperature controlled furnace and heated to 250°C at a rate of 2°C / min, and held at this temperature for 4 hours for heat treatment. It is then cooled to room temperature in the furnace. This step further stabilizes the billet structure.

[0063] 6. Finished product inspection and shaping

[0064] The dimensions of the heat-treated blank are inspected, and the edges are trimmed with sandpaper to obtain the final nanoporous insulation board.

[0065] 7. Product performance and structural characterization

[0066] The prepared insulation board was tested:

[0067] Physical properties: Bulk density is 0.28 g / cm³ 3 The thermal conductivity at room temperature (25°C) is 0.018 W / m·K; the compressive strength at 20% compression deformation is 800 kPa; and the permanent linear change rate after heating for 12 hours at 1000°C is 1.5%. All these indicators are better than those specified in this invention.

[0068] Microstructure: such as Figure 4 As shown in the SEM images, the product exhibits an ideal open-pore mesh structure. The three-dimensional network formed by the charged fibers 22 is uniformly distributed, with nanoparticles (vaporized SiO2 / Al2O3, and opacifier) ​​adhering to and filling the spaces between the fibers, forming numerous tiny nanopores. The fibers and particles are well bonded together, with clear interfaces.

[0069] Comparison of thermal conductivity: Figure 5 The thermal conductivity of the insulation board of this invention and two comparative products is compared at different temperatures. Specific data are shown in the table below:

[0070]

[0071] analyze Figure 5 It can be seen that at a high temperature of 1000℃, the thermal conductivity of the product of this invention is only 0.035 W / m·K, which is 30% higher than that of Comparative Example 1 (0.050 W / m·K). Even at room temperature, the thermal conductivity of the product of this invention is significantly lower. More importantly, the thermal conductivity curve of this invention is at its lowest point across the entire temperature range, and the curve rises most gently with increasing temperature, demonstrating its excellent wide-temperature-range thermal insulation performance and high-temperature stability. This strongly supports the performance parameter limitations in this invention and highlights the synergistic effect of the complete innovative process, including "electrostatic dispersion."

[0072] Example 2: Parameter Variation Example

[0073] This embodiment is used to illustrate the feasibility of the parameter range in this invention.

[0074] The preparation method is basically the same as in Example 1, except that the following raw material parameters are adjusted:

[0075] The particle size of SiO2 produced by the gas phase method was changed to 4.0 nm, and the particle size of Al2O3 produced by the gas phase method was changed to 8.0 nm.

[0076] The SiC particle size was changed to 1.0 μm, and the TiO2 particle size was changed to 18.0 nm.

[0077] The diameter of the short-cut high-silica fibers was changed to 3.5 μm, and the aspect ratio was controlled at approximately 18:1; the diameter of the Al2O3 ceramic fibers was changed to 11.0 μm, and the aspect ratio was controlled at approximately 22:1.

[0078] The prepared insulation board has a bulk density of 0.29 g / cm³. 3 The room temperature thermal conductivity is 0.019 W / m·K, and the compressive strength is 780 kPa, which still meets the core performance requirements of this invention.

[0079] Example 3: Method Variation Example

[0080] This embodiment focuses on demonstrating variations of some conditions in the preparation method, which still achieve the purpose of the invention.

[0081] The preparation process is the same as in Example 1, but with the following adjustments:

[0082] Fiber pretreatment: The fibers are pre-dried at 150°C for 2 hours to reduce the moisture content to 0.3%.

[0083] Airflow configuration: Before the fiber enters the electrostatic dispersion chamber 2, a vortex airflow is generated through the Venturi tube structure to ensure that the fiber reaches a better fluidized dispersion state, that is, the fiber is almost completely suspended as a single fiber before entering the high voltage electrostatic field 21.

[0084] The final product performance is comparable to that of Example 1, indicating that the specific means of achieving "fluidized dispersion" and low moisture content can be adjusted while meeting functional requirements.

[0085] Comparative Example A (used to demonstrate the necessity of electrostatic dispersion)

[0086] This comparative example is intended to illustrate that omitting the electrostatic dispersion treatment in step (1) of this invention leads to a decrease in performance.

[0087] The preparation method is basically the same as in Example 1, but in step (1), only the fibers are pre-dried, without passing through the electrostatic dispersion chamber 2, and the dried fiber aggregates 1 are directly added to the mixture. That is, the fibers are not treated by the high-voltage electrostatic field 21 and no charged fibers 22 are formed.

[0088] The resulting product exhibited a room temperature thermal conductivity of 0.023 W / m·K and a thermal conductivity of 0.047 W / m·K at 1000℃, while its compressive strength decreased to 650 kPa. SEM analysis of its microstructure revealed significant fiber agglomeration and uneven distribution. This comparative data corresponds to... Figure 5 The data points are marked with triangles (Comparative Example 2: Ordinary nano-insulation board (without electrostatic dispersion)). The comparison shows that omitting the electrostatic dispersion step directly leads to an increase in thermal conductivity and a decrease in compressive strength, thus demonstrating the necessity of the electrostatic dispersion step in this invention and the importance of the "fluidized dispersion state." The comparison highlights the crucial role of the fiber electrostatic dispersion step in obtaining a uniform three-dimensional network structure, thereby achieving ultra-low thermal conductivity and high strength.

[0089] In summary, as can be seen from the above embodiments and comparative examples, the nanoporous insulation board and its preparation method provided by the present invention, through specific raw material combinations, innovative fiber electrostatic dispersion pretreatment, optimized dry mixing sequence and gradient molding process, can reliably prepare insulation board products with ultra-low thermal conductivity, high mechanical strength and good high temperature stability, and fully realize the technical solution and beneficial effects defined by the present invention.

[0090] Finally, the following points should be noted: First, in the description of this application, it should be noted that, unless otherwise specified and limited, the terms "installation", "connection", and "linkage" should be interpreted broadly, and can be mechanical or electrical connections, or internal connections between two components, or direct connections. "Up", "down", "left", "right", etc. are only used to indicate relative positional relationships. When the absolute position of the described object changes, the relative positional relationship may change.

[0091] Secondly: The accompanying drawings of the embodiments disclosed in this invention only involve the structures involved in the embodiments disclosed in this invention. Other structures can refer to the general design. In the absence of conflict, the same embodiment and different embodiments of this invention can be combined with each other.

[0092] In conclusion, the above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A nanoporous insulating plate for hydrogen fuel cell energy storage, characterized in that, Made by dry molding from raw materials containing the following components: Nanoscale composite substrate composed of vapor-phase SiO2 and vapor-phase Al2O3; Opacifier containing SiC and TiO2 particles; Reinforcing fibers containing chopped high-silica fibers and Al2O3 ceramic fibers; The insulation board has an open mesh structure and a bulk density of 0.25–0.30 g / cm³. 3 The thermal conductivity at room temperature is not higher than 0.020 W / m·K, and the compressive strength at 20% compression deformation is not lower than 750 kPa.

2. The nanoporous insulation plate for hydrogen fuel cell energy storage according to claim 1, characterized in that, The particle size of the gas-phase SiO2 is 2.0–4.5 nm, the particle size of the gas-phase Al2O3 is 2.0–9.0 nm, the particle size of the SiC is 0.5–1.2 μm, and the particle size of the TiO2 particles is 12.0–20.0 nm.

3. The nanoporous insulation plate for hydrogen fuel cell energy storage according to claim 1, characterized in that, The chopped high-silica fibers have a diameter of 2.0–4.0 μm and an aspect ratio of 10–20:1; the Al2O3 ceramic fibers have a diameter of 9.0–12.0 μm and an aspect ratio of 15–25:

1.

4. The nanoporous insulation plate for hydrogen fuel cell energy storage according to claim 1, characterized in that, The permanent linear change rate of the insulation board after being treated at 1000°C for 12 hours is 1.0% to 2.0%.

5. A method for preparing a nanoporous thermal insulation board as described in any one of claims 1 to 4, characterized in that, Includes the following steps: (1) Fiber pretreatment and dispersion: After pre-drying the short-cut high-silica oxygen fiber and Al2O3 ceramic fiber, fiber agglomerates (1) are formed. The fiber agglomerates (1) are transported through the electrostatic dispersion chamber (2) under the action of airflow. At the same time, a high voltage electrostatic field (21) is applied in the electrostatic dispersion chamber (2) to make the fiber single and surface charged, and finally form several charged fibers (22). (2) Dry mixing: The charged fiber (22) is sequentially fed into a mixing device with gaseous SiO2, gaseous Al2O3, SiC opacifier and TiO2 particles, and mixed evenly under vacuum dehumidification conditions to obtain a dry mixture. (3) Gradient molding: The dry mixture is filled into the mold. First, a first-stage pressure of 20 to 30 tons is applied to make the material flow and fill. Then, the pressure is stepped up to a second-stage pressure of 200 to 400 tons to degas and densify, forming a blank with a density gradient. (4) Low-temperature heat treatment: The blank is heat-treated at a temperature of 150 to 300°C.

6. The preparation method according to claim 5, characterized in that, In step (1), the pre-drying process reduces the moisture content of the fiber to less than 0.5%.

7. The preparation method according to claim 5, characterized in that, In step (1), the airflow is configured to cause the fibers to be in a fluidized and dispersed state before entering the high-voltage electrostatic field region.

8. The preparation method according to claim 5, characterized in that, In step (2), the order of feeding materials is as follows: first, feed the gaseous SiO2 and gaseous Al2O3, then feed the SiC and TiO2 particles, and finally feed the fiber treated in step (1).

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