Hetero-folded hexagonal boron nitride / defect-free graphene thermally conductive insulating interface material

Abstract

The application discloses a heterogeneous folding hexagonal boron nitride / defect-free graphene heat-conducting insulating interface material, and belongs to the technical field of thermal management materials. The material takes a defect-free graphene skeleton containing macroscopic folding and microscopic folding as a base, and a hexagonal boron nitride nano layer uniformly coats the surface of the skeleton and penetrates into a groove, thereby forming a continuous heterogeneous coating structure. The folding structure converts the high in-plane thermal conductivity of the graphene into a thickness direction heat conduction path, and the in-situ deposited hexagonal boron nitride reduces the interface thermal resistance and improves the insulation. The out-of-plane thermal conductivity of the material is up to 402 W / mK, and the surface resistivity is up to 0.1 MΩ / sq. The material can significantly reduce the working temperature of electronic devices, and is suitable for high-integration electronic, LED and other thermal management scenes.

Description

Technical Field

[0001] This invention belongs to the field of thermal management materials technology, specifically relating to a heterogeneous folded hexagonal boron nitride / defect-free graphene thermally conductive and insulating interface material. Background Technology

[0002] With the rapid development of intelligent electronic devices, highly integrated chips, and high-power devices, the large amount of heat generated during operation is difficult to dissipate in a timely manner, easily leading to increased operating temperatures and affecting operational efficiency, service life, and stability. Therefore, developing thermal interface materials with both high thermal conductivity and high reliability is of great significance. Existing thermal interface materials mainly include polymeric, metallic, and inorganic types. Polymer-based thermal interface materials possess good flexibility, interfacial adhesion, and electrical insulation properties, but their intrinsic thermal conductivity is low, often making efficient heat dissipation difficult. Metallic thermal interface materials, while having high thermal conductivity, have low resistivity, easily causing leakage and short-circuit problems, usually requiring additional insulation treatment, thus affecting their thermal conductivity. In contrast, inorganic materials have the potential to utilize phonons for efficient heat conduction. Graphene and hexagonal boron nitride are both typical two-dimensional layered materials with high in-plane thermal conductivity, with hexagonal boron nitride also exhibiting good insulation properties. However, the high thermal conductivity of graphene and hexagonal boron nitride is mainly achieved along the in-plane direction, while their out-of-plane thermal conductivity is relatively low, limiting their application in scenarios where heat is transferred along the thickness direction. Furthermore, in the field of thermal interface materials, thermal conductivity and electrical insulation properties are often mutually restrictive. How to improve out-of-plane thermal conductivity while maintaining excellent insulation properties remains a pressing technical problem to be solved in this field. Summary of the Invention

[0003] To address the shortcomings of existing technologies, such as insufficient out-of-plane thermal conductivity, difficulty in balancing thermal conductivity and insulation performance, and high interfacial thermal resistance, this invention provides a heterogeneous folded hexagonal boron nitride / defect-free graphene thermally conductive and insulating interface material. By constructing a folded defect-free graphene framework with macroscopic folds and microscopic wrinkles, the original high in-plane thermal conductivity path of graphene is transformed into a thermally conductive path along the thickness direction. Simultaneously, by depositing a hexagonal boron nitride nanolayer in situ on the surface of the framework, a low lattice mismatch heterogeneous interface is formed, thereby reducing interfacial thermal resistance and improving electrical insulation performance.

[0004] To achieve the above objectives, the present invention adopts the following technical solution: a heterogeneous folded hexagonal boron nitride / defect-free graphene thermally conductive and insulating interface material, comprising a folded defect-free graphene framework, wherein the folded defect-free graphene framework has a macroscopic folded structure and a microscopic wrinkled structure, and the macroscopic folded structure is oriented along the thickness direction of the material, wherein the hexagonal boron nitride is a nanostructure layer uniformly covering the outer surface of the defect-free graphene framework and extending into the grooves of the microscopic wrinkled structure, forming a continuous heterogeneous coating structure.

[0005] Furthermore, the height of the macroscopic folded structure is 450 μm, the thickness of the microscopic wrinkled structure is 40 μm, and the width of the wrinkled structure is 1–2 μm.

[0006] Furthermore, the insulating material simultaneously possesses out-of-plane thermal conductivity and electrical insulation properties, with a surface resistivity of up to 0.1 MΩ / sq and an out-of-plane thermal conductivity of up to 402 W / m·K.

[0007] This invention also provides a method for preparing a heterogeneous folded hexagonal boron nitride / defect-free graphene thermally conductive and insulating interface material, comprising the following steps:

[0008] Step 1, Preparation of graphene oxide film: Inject the graphene oxide precursor solution into a mold and dry it to obtain graphene oxide film;

[0009] Step 2, preparation of defect-free graphene film: The graphene oxide film obtained in Step 1 is subjected to segmented high-temperature heat treatment in an inert atmosphere, and a preset static pressure is applied during the heat treatment process; then the obtained film is placed between two flat rigid plates for uniaxial graded pressing to obtain defect-free graphene film.

[0010] Step 3, prepare a folded defect-free graphene skeleton: sandwich the defect-free graphene film obtained in step 2 between elastic buffer materials and apply periodic compression force to shrink the film to a preset size ratio to obtain a pre-folded structure; then place the pre-folded structure between two flat rigid plates, limit the distance using limiting components, and repeatedly compress from both sides to the center for a preset number of times and time to obtain a folded defect-free graphene skeleton.

[0011] Step 4, in-situ deposition of hexagonal boron nitride nanolayers: The boron source is placed at both ends of the tube furnace, and the folded defect-free graphene skeleton obtained in step 3 is placed in the middle of the tube furnace. The furnace is heated in stages and kept at a constant temperature, so that the hexagonal boron nitride nanolayers are deposited in situ on the surface and internal groove region of the folded defect-free graphene skeleton, and finally a heterogeneous folded hexagonal boron nitride / defect-free graphene thermally conductive and insulating interface material is obtained.

[0012] Furthermore, in step one, a graphene oxide precursor solution with a concentration of 5.5 mg / mL is injected into a polytetrafluoroethylene mold, the liquid thickness reaches 1 cm, and it is dried at 30 ℃ for 72 h to obtain a graphene oxide film.

[0013] Furthermore, in step two, the inert atmosphere is an argon atmosphere, and the segmented high-temperature heat treatment includes: first heat treatment at 1000℃ for 2 h, then heating to 3000℃ at 10℃ / min and holding for 2 h; the static pressure is 560 Pa; the flat rigid plate is a mirror steel plate; the pressurization process of the graded pressing is as follows: 50 MPa for 5 min, 100 MPa for 10 min, 150 MPa for 15 min, 200 MPa for 60 min, and 300 MPa for 120 min.

[0014] Furthermore, in step three, the elastic buffer material is polyurethane foam, the periodic compression force is a bi-circular compression force, and the film shrinks to 40% of its original size; the limiting component is a steel sheet with a thickness of 450μm, a single compression time of 3min, and 8 repeated compressions; the flat rigid plate is a mirror steel plate.

[0015] Furthermore, the boron source is a borane-ammonia complex, and the amount of borane-ammonia complex in each crucible is 5 mg.

[0016] Furthermore, in step four, before the segmented heating, high-purity Ar gas is introduced for 10 minutes and a vacuum is drawn to make the pressure in the reaction chamber lower than 50 Pa.

[0017] Furthermore, in step four, the segmented heating and holding process includes the following method: heating at a rate of 10℃ / min, first heating to 130℃ and holding for 60 min, then heating to 500℃ and holding for 60 min, and finally heating to 1050℃ and holding for 120 min. In this step, the borane-ammonia complex is first dehydrogenated to generate an intermediate product, then polymerized to form an amorphous BN layer, and finally converted into a hexagonal boron nitride nanolayer at 1050℃.

[0018] Compared with existing technologies, this invention has at least the following beneficial effects: First, by constructing thermally conductive channels arranged along the thickness direction through a folded orientation structure, the original high in-plane thermal conductivity of graphene can be effectively converted into out-of-plane thermal conductivity, significantly improving the material's out-of-plane heat transfer performance. Second, by in-situ deposition of hexagonal boron nitride nanolayers on the surface and inside the trenches of the folded defect-free graphene framework, a heterogeneous interface is formed between hexagonal boron nitride and defect-free graphene, which helps to reduce interfacial thermal resistance and improve the material's insulation performance while maintaining high thermal conductivity pathways. In mobile phone heat dissipation tests, the device temperature can be stabilized at approximately 55 °C after using this material; in LED-related thermal management tests, the device center temperature can be reduced by approximately 30%, indicating its good practical application prospects. (See attached figures.)

[0019] Figure 1This is a schematic diagram illustrating the design and preparation process of the heterogeneous folded hexagonal boron nitride / defect-free graphene thermally conductive and insulating interface material of the present invention.

[0020] Figure 2 Schematic diagrams of the surface and cross-sectional morphology of folded defect-free graphene and heterofolded hexagonal boron nitride / defect-free graphene materials.

[0021] Figure 3 The graph shows the surface resistivity variation curve and mechanical property test results of the material of this invention.

[0022] Figure 4 This is a test diagram of the mechanical properties of the material of this invention.

[0023] Figure 5 The thermal conductivity and deformation effects of the material of this invention.

[0024] Figure 6 This is a diagram illustrating the application effect of the material of this invention in heat dissipation of a device. Detailed Implementation

[0025] The present invention will be further described below with reference to the accompanying drawings and specific embodiments, but the scope of protection of the present invention is not limited to the following embodiments:

[0026] Example 1

[0027] A method for preparing a heterogeneous folded hexagonal boron nitride / defect-free graphene thermally conductive and insulating interface material includes the following steps:

[0028] Step (1) Preparation of graphene oxide film: A graphene oxide precursor solution was prepared using the Hummers method. The concentration of the obtained graphene oxide precursor solution was 5.5 mg / mL. To ensure the quality of subsequent film formation, the precursor solution was placed in a vacuum oven before film formation to remove entrained air. Subsequently, a polytetrafluoroethylene mold with a diameter of 20 cm was used, and 320 mL of the graphene oxide precursor solution was injected into the mold, reaching a liquid thickness of 1 cm. The solution was then dried at 30°C for 72 h to obtain a large-area graphene oxide film. The planar dimensions of the graphene oxide sheets were mainly concentrated in the range of 20–80 μm, and the layer thickness was approximately 2 nm.

[0029] Step (2) Preparation of defect-free graphene film: The graphene oxide film obtained in step (1) was placed in an Ar atmosphere and heat-treated at 1000 °C for 2 h to remove most of the oxygen-containing functional groups and oxygen defects; then the temperature was increased to 3000 °C at a heating rate of 10 °C / min and held for 2 h. During this high-temperature heat treatment, a static pressure of 560 Pa was applied to further repair defects in the carbon framework and improve the orderliness of the graphene sheets. After the above treatment, impurity atoms inside the material escape in the form of gaseous oxides and form micro-gas bladder structures with a size of several micrometers to tens of micrometers between the sheets. To eliminate the aforementioned micro-air pockets and further improve the film density, the sample treated at 3000 °C was placed between two mirror steel plates and mechanically pressed. The pressing method was a step-by-step pressurization: first, it was held at 50 MPa for 5 min, then at 100 MPa for 10 min, then at 150 MPa for 15 min, then at 200 MPa for 60 min, and finally at 300 MPa for 120 min, to obtain a dense, defect-free graphene film.

[0030] After the aforementioned high-temperature repair and graded pressing, the resulting defect-free graphene film exhibits high crystallinity and a long-range ordered honeycomb lattice structure. Compared with the sample heat-treated at 1000 °C, the sample treated at 3000 °C shows further reductions in defects and more ordered layer stacking, which is beneficial for maintaining the intrinsic high thermal conductivity of graphene. The graded pressing process not only significantly reduces micro-air pockets but also increases the density of interlayer packing, providing a foundation for the subsequent construction of a folded thermally conductive framework.

[0031] Step (3) Preparation of folded defect-free graphene framework: The defect-free graphene film obtained in step (2) is sandwiched between two polyurethane foams, and a circumferential force is applied to the polyurethane foams to cause the defect-free graphene film sandwiched therein to shrink and deform. When the polyurethane foam shrinks to 40% of its original size, the graphene film is removed to obtain a pre-folded defect-free graphene film. Subsequently, the pre-folded defect-free graphene film is placed between two flat mirror steel plates, and the distance between the two steel plates is fixed by a steel sheet with a thickness of 450 μm. Under the condition of keeping the limiting distance unchanged, the sample is compressed from both sides to the center for 3 minutes to further gather, compact and fold the wrinkles on the film; the above operation is repeated 8 times to finally obtain a folded defect-free graphene framework.

[0032] After the above folding process, the resulting defect-free folded graphene framework forms a multi-level folded structure with both macroscopic and microscopic wrinkled structures. The macroscopic folded structure is mainly arranged along the material thickness direction, with a folding height of approximately 450 μm. Microscopic wrinkled structures with a width of 1–2 μm and a thickness of 40 μm are further formed on the surface and in local areas of the macroscopic folded structure. These microscopic wrinkled structures are induced by 300 MPa pressing, which improves the structural stability of the material while maintaining the continuity of the overall framework. The macroscopic folded structure causes a reorientation of the original graphene sheets that were originally aligned parallel to the film surface, thus making the heat conduction direction more aligned along the material thickness direction.

[0033] Step (4) In-situ deposition of hexagonal boron nitride nanolayers: Two crucibles were placed at opposite ends of a tube furnace, and 5 mg of borane-ammonia complex precursor was added to each crucible; the folded defect-free graphene framework obtained in step (3) was placed in the middle of the tube furnace. Before the reaction began, high-purity Ar gas was introduced into the reaction system for 10 min to replace the air, and then the system was evacuated to make the pressure in the reaction chamber lower than 50 Pa, so as to facilitate the uniform diffusion and deposition of the precursor pyrolysis products in the reaction space. Then the temperature was increased according to the following procedure: at a heating rate of 10℃ / min, the temperature was first increased to 130℃ and held for 60 min, then increased to 500℃ and held for 60 min, and finally increased to 1050℃ and held for 120 min. After the reaction was completed, the temperature was naturally cooled to room temperature to obtain a heterogeneous folded hexagonal boron nitride / defect-free graphene thermally conductive and insulating interface material.

[0034] In the aforementioned deposition process, the borane-ammonia complex precursor first undergoes dehydrogenation to generate intermediate products such as aminoborane and borazine. These intermediate products further undergo dehydrogenation and polymerization reactions to form polymer precursor layers such as polyaminoborane and polyborazine. At 500 °C, the polymer intermediate layer further transforms into an amorphous BN layer. Under a holding temperature of 1050 °C, the amorphous BN layer finally transforms into a hexagonal boron nitride nanolayer and is deposited on the outer surface and internal groove regions of the folded, defect-free graphene framework. Because the precursor pyrolysis products can diffuse sufficiently within the system under relatively low pressure conditions, the hexagonal boron nitride nanolayer can not only cover the framework surface but also extend inward along the fold gaps and grooves, thereby forming a relatively uniform coating deposition structure.

[0035] In the resulting heterostructured folded hexagonal boron nitride / defect-free graphene thermally conductive and insulating interface material, a heterostructure interface is formed between the hexagonal boron nitride nanolayers and the defect-free graphene. Transmission electron microscopy characterization results show that the interface region exhibits minimal distortion, with lattice perturbations mainly concentrated within 1–2 atomic layers, indicating a low lattice mismatch characteristic. Compared to traditional insulating polymers or amorphous insulating layers, this heterostructure interface is beneficial for reducing interfacial thermal resistance and maintaining the continuity of the efficient thermal conduction pathways of graphene.

[0036] Example 2

[0037] The morphology and structure of the material obtained in Example 1 were characterized, and the results are as follows:

[0038] like Figure 2 As shown, scanning electron microscopy (SEM) observation of the folded, defect-free graphene framework reveals continuous wrinkles and folded regions on its surface. Its cross-section exhibits a macroscopic folded layered structure aligned along the thickness direction, with clearly defined folding channels and a thermally conductive framework with vertical orientation between the layers. After hexagonal boron nitride deposition, the material's surface morphology gradually transforms from the original graphene folded texture into a continuous coated surface uniformly covered by nanolayers, with the deposited layer extending into the grooved regions.

[0039] Further observation using transmission electron microscopy revealed that the defect-free graphene sample treated at 3000℃ exhibited a clear long-range ordered planar honeycomb lattice. After hexagonal boron nitride deposition, a layered nano-coating structure was formed on the graphene surface, which together with the graphene formed a heterogeneous layered structure with a clear interface.

[0040] Example 3

[0041] The electrical insulation properties of the material obtained in Example 1 were measured, and the results are as follows:

[0042] like Figure 3 As shown, the surface resistivity of the material was measured using an electrochemical workstation. The sample size was 2 cm × 2 cm × 450 μm. The test results show that the surface resistivity of the material increases significantly with the increase of the thickness of the hexagonal boron nitride deposition layer, gradually increasing from an initial low level to 0.1 MΩ / sq. This indicates that the hexagonal boron nitride nanolayer forms an effective insulating coating on the graphene surface, which can significantly improve the conductivity characteristics of the graphene material surface, thereby improving its reliability in thermal interface applications of electronic devices.

[0043] Example 4

[0044] The sample was subjected to uniaxial tensile testing using a universal testing machine at a loading rate of 2 mm / min. The results showed that:

[0045] like Figure 4As shown, the mechanical properties of graphene oxide films are relatively limited. After heat treatment at 1000 ℃, the mechanical properties of defect-free graphene films begin to improve. After treatment at 3000 ℃ and pressing at 300 MPa, the tensile strength and fracture strain of the material are further improved. This indicates that high-temperature repair and pressing densification treatment help improve the layer connectivity and enhance the mechanical stability of the graphene framework through microstructure regulation, providing a basis for its use as a thermal interface material to withstand compression and bonding.

[0046] Example 5

[0047] The thermal conductivity of the material obtained in Example 1 was tested, and the results are as follows:

[0048] like Figure 5 As shown, the thermal conductivity of the samples was determined using the laser scintillation method. The test results indicate that the unoriented graphene film exhibits significant anisotropy in thermal conductivity, with heat primarily transferred along the in-plane direction, resulting in low out-of-plane thermal conductivity. After folding and orientation, the out-of-plane thermal conductivity of the material is significantly improved. The out-of-plane thermal conductivity of the folded, defect-free graphene framework reaches 468 ± 20 W / m·K. Further deposition of a hexagonal boron nitride nanolayer on the framework surface yields a heterogeneous folded hexagonal boron nitride / defect-free graphene thermally conductive and insulating interface material that maintains high out-of-plane thermal conductivity while also possessing excellent insulation properties, with an out-of-plane thermal conductivity reaching a maximum of 402 W / m·K.

[0049] Further testing of the thermal conductivity of the material under different strain conditions revealed that when the sample is subjected to a certain in-plane tensile deformation, its out-of-plane thermal conductivity gradually decreases with increasing deformation, but it still maintains a high level of thermal conductivity within a small deformation range. This indicates that the folded thermally conductive skeleton constructed in this invention still has good thermal conductivity stability under actual pressure bonding conditions.

[0050] Example 6

[0051] The material obtained in Example 1 was applied to thermal management testing of electronic devices, and the results are as follows:

[0052] like Figure 6 As shown, the material was further applied to thermal management tests between a semiconductor cooler and an LED lamp. The results showed that using the material of this invention as a thermal interface layer reduced the center temperature of the LED device by approximately 30%. This further demonstrates that the heterostructure folded hexagonal boron nitride / defect-free graphene thermally conductive and insulating interface material provided by this invention has promising application prospects in high heat flux density electronic devices.

[0053] In summary, this invention effectively transforms the high in-plane thermal conductivity of graphene into high out-of-plane thermal conductivity by constructing a folded, oriented, defect-free graphene thermally conductive framework and depositing hexagonal boron nitride nanolayers in situ on its surface and internal trench regions. Simultaneously, it increases the surface resistivity of the material and reduces interfacial thermal resistance, ultimately obtaining a thermal interface material with both high out-of-plane thermal conductivity and good electrical insulation properties, suitable for thermal management in mobile phones, LED devices, and other electronic devices.

Claims

1. A heterogeneous folded hexagonal boron nitride / defect-free graphene thermally conductive and insulating interface material, characterized in that, The invention includes a folded defect-free graphene framework, which has a macroscopic folded structure and a microscopic wrinkled structure. The macroscopic folded structure is oriented along the thickness direction of the material. The hexagonal boron nitride is a nanostructure layer that is uniformly coated on the outer surface of the defect-free graphene framework and extends into the grooves of the microscopic wrinkled structure to form a continuous heterogeneous coating structure.

2. The heterogeneous folded hexagonal boron nitride / defect-free graphene thermally conductive and insulating interface material according to claim 1, characterized in that, The height of the macroscopic folded structure is 450 μm, the thickness of the microscopic wrinkled structure is 40 μm, and the width of the wrinkled structure is 1–2 μm.

3. A heterogeneous folded hexagonal boron nitride / defect-free graphene thermally conductive and insulating interface material according to claim 1 or 2, characterized in that, The insulating material has both out-of-plane thermal conductivity and electrical insulation properties, with a maximum surface resistivity of 0.1 MΩ / sq and a maximum out-of-plane thermal conductivity of 402 W / m·K.

4. A method for preparing a heterogeneous folded hexagonal boron nitride / defect-free graphene thermally conductive and insulating interface material, characterized in that, Includes the following steps: Step 1, Preparation of graphene oxide film: Inject the graphene oxide precursor solution into a mold and dry it to obtain graphene oxide film; Step 2, Preparation of defect-free graphene film: The graphene oxide film obtained in Step 1 is subjected to segmented high-temperature heat treatment under an inert atmosphere, and a preset static pressure is applied during the heat treatment process. The resulting film was then placed between two flat, rigid plates for uniaxial graded pressing to obtain a defect-free graphene film. Step 3, prepare a folded defect-free graphene skeleton: sandwich the defect-free graphene film obtained in step 2 between elastic buffer materials and apply periodic compression force to shrink the film to a preset size ratio to obtain a pre-folded structure; then place the pre-folded structure between two flat rigid plates, limit the distance using limiting components, and repeatedly compress from both sides to the center for a preset number of times and time to obtain a folded defect-free graphene skeleton. Step 4, in-situ deposition of hexagonal boron nitride nanolayers: The boron source is placed at both ends of the tube furnace, and the folded defect-free graphene skeleton obtained in step 3 is placed in the middle of the tube furnace. The furnace is heated in stages and kept at a constant temperature, so that the hexagonal boron nitride nanolayers are deposited in situ on the surface and internal groove region of the folded defect-free graphene skeleton, and finally a heterogeneous folded hexagonal boron nitride / defect-free graphene thermally conductive and insulating interface material is obtained.

5. The method for preparing a heterogeneous folded hexagonal boron nitride / defect-free graphene thermally conductive and insulating interface material according to claim 4, characterized in that, In step one, a graphene oxide precursor solution with a concentration of 5.5 mg / mL was injected into a polytetrafluoroethylene mold, and the liquid thickness reached 1 cm. The solution was then dried at 30 °C for 72 h to obtain a graphene oxide film.

6. The method for preparing a heterogeneous folded hexagonal boron nitride / defect-free graphene thermally conductive and insulating interface material according to claim 4, characterized in that, In step two, the inert atmosphere is argon atmosphere, and the segmented high-temperature heat treatment includes: first heat treatment at 1000 ℃ for 2 h, then heating to 3000 ℃ at 10 ℃ / min and holding for 2 h; the static pressure is 560 Pa; the flat rigid plate is mirror steel plate; the pressurization process of the graded pressing is as follows: 50 MPa for 5 min, 100 MPa for 10 min, 150 MPa for 15 min, 200 MPa for 60 min, and 300 MPa for 120 min.

7. The method for preparing a heterogeneous folded hexagonal boron nitride / defect-free graphene thermally conductive and insulating interface material according to claim 4, characterized in that, In step three, the elastic buffer material is polyurethane foam, the periodic compression force is a bi-circular compression force, and the film shrinks to 40% of its original size; the limiting component is a steel sheet with a thickness of 450μm, a single compression time of 3min, and 8 repeated compressions; the flat rigid plate is a mirror steel plate.

8. In the preparation method of a heterogeneous folded hexagonal boron nitride / defect-free graphene thermally conductive and insulating interface material according to claim 4, in step four, the boron source is a borane-ammonia complex, and the amount of borane-ammonia complex in each crucible is 5 mg.

9. The method for preparing a heterogeneous folded hexagonal boron nitride / defect-free graphene thermally conductive and insulating interface material according to claim 4, characterized in that, In step four, before the segmented heating, high-purity Ar gas is introduced for 10 minutes and a vacuum is drawn to make the pressure in the reaction chamber lower than 50 Pa.

10. The method for preparing a heterogeneous folded hexagonal boron nitride / defect-free graphene thermally conductive and insulating interface material according to claim 9, characterized in that, In step four, the segmented heating and holding process includes the following method: heating at a rate of 10℃ / min, first heating to 130℃ and holding for 60 min, then heating to 500℃ and holding for 60 min, and finally heating to 1050℃ and holding for 120 min.

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