Splicing type mould pressing graphite sagger and preparation method thereof

Abstract

The invention discloses a spliced mould pressing graphite sagger and a preparation method thereof, and relates to the technical field of graphite saggers. The bowl body comprises the following preparation raw materials in parts by mass: 70-90 parts of phosphorus-doped graphite powder, 3-5 parts of silicon nitride whiskers and 5-8 parts of a composite reinforced phase; the partition plate comprises the following preparation raw materials in parts by mass: 50-80 parts of crystalline flake graphite, 5-20 parts of carbon fibers and 4-10 parts of aluminum nitride; the composite reinforcement phase is boron nitride nanotubes and zirconium oxide. The preparation method of the spliced mould pressing graphite sagger comprises the following steps: S1, carrying out ball milling on preparation raw materials of the sagger body, carrying out wet mixing to obtain a sagger body precursor, carrying out mould pressing on the sagger body precursor, and carrying out heat treatment to obtain the sagger body; s2, carrying out ball milling on the raw materials for preparing the partition plate to obtain a partition plate precursor, and carrying out mold pressing and heat treatment on the partition plate precursor to obtain the partition plate; and S3, preparing the spliced molded graphite saggar.

Description

Technical Field

[0001] The invention belongs to the technical field of graphite saggers, in particular to a spliced ​​molded graphite sagger and a preparation method thereof. Background Art

[0002] Graphite saggers and separators are key consumables in high-temperature industries, such as lithium battery sintering, photovoltaic silicon wafer heat treatment, and semiconductor processing. They are primarily used to support and protect the heated materials. Traditional graphite saggers have poor thermal shock and oxidation resistance, resulting in a short service life. They also rely on high-VOC asphalt binders.

[0003] In the prior art, silicon nitride is used to improve the oxidizability of the bowl, while phenolic resin is used to improve the mechanical properties of the bowl, but the durability is poor. In addition, when the bowl is loaded with too much material, the material in the middle and around the bowl is heated unevenly. Summary of the Invention

[0004] The present invention is to overcome the above technical problems and thus provides a spliced ​​molded graphite sagger and a preparation method thereof. The spliced ​​molded graphite sagger of the present invention has good durability and good thermal conductivity, which can make the heated materials more uniform.

[0005] The present invention solves the above technical problems through the following technical solutions.

[0006] The present invention discloses a spliced ​​molded graphite sagger, comprising a sagger body and a partition; The bowl body comprises the following raw materials in parts by weight: 70-90 parts of phosphorus-doped graphite powder, 3-5 parts of silicon nitride whiskers, and 5-8 parts of composite reinforcement phase; Preferably, the bowl body comprises the following raw materials in parts by weight: 75-85 parts of phosphorus-doped graphite powder, 3.5-4.5 parts of silicon nitride whiskers and 5.5-7.0 parts of composite reinforcement phase; The separator comprises the following raw materials in parts by mass: 50 to 80 parts of flake graphite, 5 to 20 parts of carbon fiber and 4 to 10 parts of aluminum nitride; Preferably, the separator comprises the following raw materials in parts by mass: 55-75 parts of flake graphite, 8-12 parts of carbon fiber, and 5-8 parts of aluminum nitride; The composite reinforcement phase is boron nitride nanotubes and zirconium oxide.

[0007] In the present invention, the mass ratio of boron nitride nanotubes to zirconium oxide in the composite reinforcement phase is 2 to 4:1. The boron nitride nanotubes in the composite reinforcement phase create fast heat conduction channels, and the ZrO2 can buffer thermal stress, allowing the sagger to maintain structural stability under rapid cooling and heating.

[0008] The method for preparing phosphorus-doped graphite powder comprises the following steps: impregnating graphite powder with a 3-7 wt% diammonium hydrogen phosphate solution and activating the powder at 750-850° C. to form phosphorus-doped graphite powder; wherein phosphorus doping can improve the oxidation resistance of graphite, improve the durability of a sagger, and optimize the thermal conductivity of the sagger; therefore, compared with a sagger prepared from ordinary graphite, the sagger can have a longer service life at high temperatures.

[0009] Furthermore, the material-liquid ratio of the graphite powder to the diammonium hydrogen phosphate solution is 1 / 2 to 1 / 5 g / L.

[0010] In the present invention, the silicon nitride whiskers are β -Si3N4; among them, β -Si3N4 does not undergo phase change at high temperatures, and β -Si3N4 can effectively hinder crack propagation and improve the strength of sagger; β -The Si-N bonds on the surface of Si3N4 easily react with the defective carbon in graphite to form a Si-CN transition layer.

[0011] In the present invention, the specific surface area of ​​the silicon nitride whiskers is ≥15m 2 / g; In the present invention, the bending strength of the silicon nitride whiskers is ≥1.0 GPa; In the present invention, the specific surface area of ​​the boron nitride nanotubes is ≥50m 2 / g, preferably 50~70 m 2 / g.

[0012] In the present invention, the thermal conductivity of the boron nitride nanotubes is ≥3000 W / (m·k), preferably 3000-3200 W / (m·k).

[0013] In the present invention, the mechanical strength of the boron nitride nanotubes is ≥1.8 TPa, preferably 1.8-2.0 TPa.

[0014] In the present invention, the composite reinforcement phase is formed by ball milling boron nitride nanotubes and zirconia.

[0015] In the present invention, the particle size of the flake graphite is 800-1000 mesh; In the present invention, the tensile strength of the carbon fiber is ≥4100 MPa, preferably 4100-4200 MPa.

[0016] In the present invention, the raw materials for preparing the bowl body further include 4 to 6 parts of a binder; In the present invention, the raw materials for preparing the separator further include 8 to 12 parts of a binder.

[0017] Furthermore, the binder is at least one of polyvinyl butyral (PSZ, CAS No.: 63148-65-2) and polysilazane (PVB, CAS No.: 89535-60-4).

[0018] Furthermore, 0.1-0.3 wt % of aluminum acetylacetonate may be added to the binder.

[0019] In the present invention, the bowl body is composed of the following raw materials in parts by mass: 70-90 parts of phosphorus-doped graphite powder, 3-5 parts of silicon nitride whiskers, 5-8 parts of composite reinforcement phase and 4-6 parts of binder.

[0020] In the present invention, the separator is composed of the following raw materials in parts by mass: 55-75 parts of flake graphite, 8-12 parts of carbon fiber, 5-8 parts of aluminum nitride and 8-12 parts of binder.

[0021] In the present invention, the partition is cross-shaped and is placed in the bowl body, so as to divide the internal space of the bowl body into four independent areas.

[0022] In the present invention, the bowl body has a thermal shock resistance of ≥ 200 times; In the present invention, the flexural strength of the bowl body is 19-23 MPa.

[0023] In the present invention, the thermal conductivity of the separator is 400-430 W / (m·K).

[0024] The present invention also discloses a method for preparing a spliced ​​molded graphite sagger, comprising the following steps: S1. Preparation of a bowl: The raw materials for preparing the bowl are ball-milled and wet-mixed to obtain a bowl precursor, the bowl precursor is molded and heat-treated to obtain a bowl; S2 preparation of the separator: the separator raw material is prepared after ball milling to obtain a separator precursor, the separator precursor is molded and heat treated to obtain a separator; S3. Preparation of spliced ​​molded graphite sagger: Assemble the bowl body and the partition to obtain a spliced ​​molded graphite sagger.

[0025] In S1, the solid content of the bowl precursor is 50-70%.

[0026] In S1, the molding pressure is 5 MPa to 15 MPa, and the molding time is 5 to 15 minutes; In S1, the heat treatment is sintering at 1000-1200° C. for 70-80 hours in a nitrogen atmosphere; In S2, the molding pressure is 5MPa~15Mpa, and the molding time is 5~15min; In S2, the heat treatment is sintering at 1000-1200° C. for 15-30 hours in a nitrogen atmosphere.

[0027] Application of the aforementioned spliced ​​molded graphite sagger, or the spliced ​​molded graphite sagger prepared by the aforementioned method for preparing the spliced ​​molded graphite sagger, in the fields of high-temperature heating and chemical reactions.

[0028] On the basis of conforming to the common sense in this field, the above-mentioned preferred conditions can be arbitrarily combined to obtain the preferred embodiments of the present invention.

[0029] Compared with the prior art, the present invention has the following beneficial effects: The bowl of the present invention adopts phosphorus-doped graphite powder, β - Combination of Si3N4 whiskers and composite reinforcement phase. Improvement of oxidation resistance of graphite powder by phosphorus doping, β The Si3N4 whiskers enhance the bowl's strength, while the composite reinforcement creates a highly efficient three-dimensional thermal network. The zirconia toughening effect also enhances thermal shock resistance. Consequently, the bowl's thermal shock resistance is ≥200 times, and in some preferred embodiments, it can reach 200-250 times. The bowl also boasts a flexural strength of ≥19 MPa, and in some preferred embodiments, it can reach 19-23 MPa.

[0030] The separator of the present invention uses flake graphite, carbon fiber and aluminum nitride as the core. The thermal conductivity of flake graphite and the radial reinforcement effect of carbon fiber synergistically solve the technical problem of insufficient thermal conductivity of graphite material. The introduction of aluminum nitride further optimizes the interfacial thermal resistance, and the resulting separator has a thermal conductivity of 410-430 W / (m·k). BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 This is a schematic diagram of the structural assembly of a spliced ​​molded graphite sagger. DETAILED DESCRIPTION

[0032] To facilitate understanding of the present invention, the present invention will be described more comprehensively and meticulously below in conjunction with preferred embodiments, but the protection scope of the present invention is not limited to the following specific embodiments.

[0033] Unless otherwise defined, all technical terms used hereinafter have the same meanings as those generally understood by those skilled in the art. The technical terms used herein are only for the purpose of describing specific embodiments and are not intended to limit the scope of protection of the present invention.

[0034] The "ranges" disclosed herein are defined in terms of lower and upper limits. A given range is defined by selecting a lower limit and an upper limit, and the selected lower and upper limits define the boundaries of the particular range. Ranges defined in this manner can be inclusive or exclusive of the end values ​​and can be combined in any manner, i.e., any lower limit can be combined with any upper limit to form a range. For example, if ranges of 60-120 and 80-110 are listed for a particular parameter, it is understood that ranges of 60-110 and 80-120 are also contemplated. Furthermore, if the minimum range values ​​listed are 1 and 2, and if the maximum range values ​​listed are 3, 4, and 5, then the following ranges are all contemplated: 1-3, 1-4, 1-5, 2-3, 2-4, and 2-5. In this disclosure, unless otherwise indicated, the numerical range "ab" is an abbreviation for any combination of real numbers between a and b, where a and b are both real numbers. For example, the numerical range "0-5" indicates that all real numbers between "0-5" have been listed herein, and "0-5" is merely an abbreviation for these numerical combinations. In addition, when a parameter is expressed as an integer ≥ 2, it is equivalent to disclosing that the parameter is, for example, an integer of 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0035] Unless otherwise specified, all embodiments and optional embodiments of the present invention can be combined with each other to form new technical solutions.

[0036] Unless otherwise specified, all technical features and optional technical features of the present invention can be combined with each other to form a new technical solution.

[0037] Unless otherwise specified, all steps of the present invention may be performed sequentially or randomly, preferably sequentially. For example, "the method includes steps (a) and (b)" indicates that the method may include steps (a) and (b) performed sequentially, or may include steps (b) and (a) performed sequentially. For example, "the method may further include step (c)" indicates that step (c) may be added to the method in any order, for example, the method may include steps (a), (b) and (c), or may include steps (a), (c) and (b), or may include steps (c), (a) and (b), etc.

[0038] Unless otherwise specified, the terms "include" and "comprising" used in the present invention may be open-ended or closed-ended. For example, "include" and "comprising" may mean that other components not listed may also be included or that only the listed components are included.

[0039] Unless otherwise specified, the term "or" is inclusive in this disclosure. For example, the phrase "A or B" means "A, B, or both A and B." More specifically, the condition "A or B" is satisfied if any of the following conditions are met: A is true (or exists) and B is false (or does not exist); A is false (or does not exist) and B is true (or exists); or both A and B are true (or exist).

[0040] The raw material information used in the following examples is as follows: Graphite powder was purchased from Qingdao Dongkai Graphite Co., Ltd. with a particle size of 800 mesh; Flake graphite was purchased from Qingdao Pingdu Fukang Graphite Processing Plant with a particle size of 1000 mesh; Silicon nitride whiskers were purchased from Shandong Starry Metal Materials Co., Ltd. β -Silicon nitride whiskers, with a bending strength of 1.0 GPa and a thermal expansion coefficient of 3.2×10 -6 / ℃, specific surface area is 15m 2 / g; Boron nitride nanotubes were purchased from Zhongke Leiming Technology Co., Ltd., with an average diameter of 50 nm and a specific surface area of ​​50 m 2 / g, thermal conductivity is 3000W / (m·k), expansion coefficient is 1×10 -6 / ℃, mechanical strength is 1.8TPa; Zirconia's D50 = 48 μm; The carbon fiber is short-cut carbon fiber purchased from Weihai Guangwei Composite Materials Co., Ltd. TZ50J, with a tensile strength of 4120 MPa, a tensile modulus of 475 GPa, and a bulk density of 1.88 g / cm 3 ; Including but not limited to the above manufacturers and models.

[0041] Example 1 The spliced ​​molded graphite crucible of this embodiment consists of a crucible body and a partition; The bowl body is composed of the following raw materials in parts by mass: 80 parts of phosphorus-doped graphite powder, 4.5 parts of silicon nitride whiskers, 6.3 parts of composite reinforcement phase and 5.4 parts of binder.

[0042] Preparation method of phosphorus-doped graphite powder: graphite powder is impregnated with 6.4wt% diammonium hydrogen phosphate solution with a material-liquid ratio of 1 / 3 g / L, and then activated at 800°C to form phosphorus-doped graphite powder.

[0043] The mass ratio of boron nitride nanotubes to zirconium oxide in the composite reinforcement phase is 3:1, and the composite reinforcement phase is obtained by mixing boron nitride nanotubes and zirconium oxide through ball milling.

[0044] The adhesive is polyvinyl butyral (PSZ), and 0.2 wt% aluminum acetylacetonate is also added to the adhesive.

[0045] The separator is composed of the following raw materials in parts by mass: 70 parts of flake graphite, 10 parts of carbon fiber, 6.3 parts of aluminum nitride, and 11.4 parts of a binder.

[0046] The preparation method of the spliced ​​molded graphite saggar of the present embodiment is as follows: S1. Preparation of a bowl: The raw materials for preparing the bowl were ball-milled and wet-mixed to obtain a bowl precursor having a solid content of 65%. The bowl precursor was molded at 10 MPa for 12 min and sintered at 1100 ° C for 75 h in a nitrogen atmosphere to obtain a bowl; S2 preparation of the separator: the separator raw materials were ball-milled to obtain a separator precursor, the separator precursor was molded at 10Mpa for 10min, and then sintered at 1150 ° C for 20h in a nitrogen atmosphere to obtain a separator after heat treatment; S3. Assemble the bowl and the partition to obtain Figure 1 Spliced ​​molded graphite sagger.

[0047] The connected molded graphite sagger prepared in this embodiment was subjected to thermal shock resistance test and thermal conductivity test; Thermal conductivity test method refers to GB / T 8722; The thermal shock test method refers to GB / T 30873. The specific process is to heat the sample to 1200℃, then quench the sample in water for 3 minutes, and repeat the hot and cold cycle until the heated end face of the sample is half broken. The number of thermal shock cycles is recorded. The bowl body of this embodiment has a thermal shock resistance cycle of 240 times and a flexural strength of 22.5 MPa; The thermal conductivity of the separator is 423 W / (m·K).

[0048] Example 2 The difference between this embodiment and embodiment 1 is that: The bowl body is composed of the following raw materials in parts by mass: 80 parts of phosphorus-doped graphite powder, 6 parts of silicon nitride whiskers, 4.2 parts of composite reinforcement phase and 5.6 parts of binder.

[0049] Other raw materials, steps and parameters are the same as in Example 1.

[0050] The number of thermal shock resistance cycles of the bowl body in this embodiment is 203 times, and the flexural strength of the bowl body is 19.7 MPa.

[0051] Example 3 The difference between this embodiment and embodiment 1 is that: The bowl body is composed of the following raw materials in parts by mass: 80 parts of phosphorus-doped graphite powder, 4.8 parts of silicon nitride whiskers, 5.7 parts of composite reinforcement phase and 5.1 parts of binder.

[0052] Preparation method of phosphorus-doped graphite powder: graphite powder is impregnated with 5wt% diammonium hydrogen phosphate solution with a material-liquid ratio of 1 / 5 g / L, and activated at 830°C to form phosphorus-doped graphite powder.

[0053] The number of thermal shock resistance cycles of the bowl body in this embodiment is 215 times, and the flexural strength of the bowl body is 21.6 MPa.

[0054] Example 4 The difference between this embodiment and embodiment 1 is that: The bowl body is composed of the following raw materials in parts by weight: 80 parts of phosphorus-doped graphite powder, 4.8 parts of silicon nitride whiskers, 7.2 parts of composite reinforcement phase and 4.8 parts of binder; The mass ratio of boron nitride nanotubes to zirconium oxide in the composite reinforcement phase is 2.4:1.

[0055] The number of thermal shock resistance cycles of the bowl body of this embodiment is 225 times, and the flexural strength of the bowl body is 22.8 MPa.

[0056] Example 5 The difference between this embodiment and embodiment 1 is that: The separator is composed of the following raw materials in parts by mass: 65 parts of flake graphite, 11.7 parts of carbon fiber, 5.7 parts of aluminum nitride and 8.4 parts of binder; The binder is polysilazane and polyvinyl butyral, and the mass ratio of polysilazane to polyvinyl butyral is 4.5:1.

[0057] The thermal conductivity of the separator in this embodiment is 416 W / (m·K).

[0058] Example 6 The difference between this embodiment and embodiment 1 is that: The mass ratio of boron nitride nanotubes to zirconium oxide in the composite reinforcement phase is 1:1.

[0059] The bowl body of this embodiment has a thermal shock resistance cycle number of 194 times, and a flexural strength of 18.1 MPa.

[0060] Other raw materials, steps and parameters are the same as in Example 1.

[0061] Comparative Example 1 The difference between this comparative example and Example 1 is: The graphite powder in the bowl is not doped with phosphorus, and the graphite powder not doped with phosphorus is used to replace the phosphorus-doped graphite powder; The bowl of this comparative example has a thermal shock resistance cycle of 182 times and a flexural strength of 16.1 MPa. The graphite in this comparative example is not doped with phosphorus, so the bowl has low strength, poor thermal conductivity, and correspondingly reduced thermal shock resistance.

[0062] Other raw materials, steps and parameters are the same as in Example 1.

[0063] Comparative Example 2 The difference between this comparative example and Example 1 is: The bowl body uses silicon nitride instead of silicon nitride whiskers. Silicon nitride was purchased from Jiangxi Silicon Nitride New Materials Co., Ltd. (D50≤10μm). The bowl in this comparative example resisted 176 thermal shock cycles and had a flexural strength of 17.9 MPa. Silicon nitride whiskers inhibit crack propagation through a bridging effect, increasing fracture toughness and promoting the formation of a continuous thermal conduction path. ZrO2 phase transition energy absorption relies on stress transfer through the whiskers, while granular silicon nitride, due to its short-range action, cannot effectively buffer thermal stress, resulting in poor thermal shock resistance.

[0064] Other raw materials, steps and parameters are the same as in Example 1.

[0065] Comparative Example 3 The difference between this comparative example and Example 1 is: The separator is composed of the following raw materials in parts by mass: 70 parts of flake graphite, 6.2 parts of aluminum nitride and 8.4 parts of a binder; The thermal conductivity of the separator of this comparative example is 374 W / (m·K).

[0066] Other raw materials, steps and parameters are the same as in Example 1.

[0067] Comparative Example 4 The difference between this comparative example and Example 1 is: The separator is composed of the following raw materials in parts by mass: 80 parts of flake graphite, 10 parts of carbon fiber, and 10 parts of a binder.

[0068] The thermal conductivity of the separator in this comparative example is 388 W / (m·K).

[0069] Other raw materials, steps and parameters are the same as in Example 1.

[0070] Application Examples Lithium iron phosphate was prepared using a solid-phase method, using the spliced ​​molded graphite sagger described in Example 1 as the experimental subject. Specifically, an iron compound, a lithium compound, and a phosphorus compound were placed in the sagger and sintered at 750°C for 12 hours. This served as the experimental group. A control group, in which the separators of the spliced ​​molded graphite sagger were removed and only the sagger itself was used as the container, remained identical to the experimental group.

[0071] After sintering, samples of lithium iron phosphate raw materials were taken from the experimental group and the control group respectively. The sampling method was to take 5 samples from the periphery and the center of the sagger respectively. After preparing half-cells, the discharge specific capacity of each sample was tested, and the variance was used to measure the difference in discharge specific capacity of the lithium iron phosphate samples in the center and around the sagger.

[0072]

[0073] Unless otherwise specified, all raw materials, reagents, instruments, and equipment used in the present invention can be purchased from the market or prepared by existing methods. The specific embodiments described above further illustrate the purpose, technical solutions, and beneficial effects of the present invention. It should be understood that the above description is only a specific 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 shall be included in the scope of protection of the present invention.

Claims

1. A spliced ​​molded graphite sagger, characterized in that: Including bowl body and partition; The bowl body comprises the following raw materials in parts by weight: 70-90 parts of phosphorus-doped graphite powder, 3-5 parts of silicon nitride whiskers, and 5-8 parts of composite reinforcement phase; The separator comprises the following raw materials in parts by mass: 50 to 80 parts of flake graphite, 5 to 20 parts of carbon fiber and 4 to 10 parts of aluminum nitride; The composite reinforcement phase is boron nitride nanotubes and zirconium oxide.

2. The spliced ​​molded graphite sagger according to claim 1, characterized in that: The preparation method of the phosphorus-doped graphite powder comprises the following steps: impregnating graphite powder with a 3-7 wt% diammonium hydrogen phosphate solution, and activating the solution at 750-850° C. to form the phosphorus-doped graphite powder.

3. The spliced ​​molded graphite sagger according to claim 2, characterized in that: The material-liquid ratio of the graphite powder to the diammonium hydrogen phosphate solution is 1 / 2 to 1 / 5 g / L.

4. The spliced ​​molded graphite sagger according to claim 1, characterized in that: Satisfy at least one of the following conditions a~b: a. The silicon nitride whiskers are β -Si3N4; b. The mass ratio of boron nitride nanotubes to zirconium oxide in the composite reinforcement phase is 2 to 4:

1.

5. The spliced ​​molded graphite sagger according to claim 4, characterized in that: Satisfy at least one of the following conditions a to d: a. The specific surface area of ​​the silicon nitride whiskers is ≥15m 2 / g; b. The bending strength of the silicon nitride whiskers is ≥1.0GPa; c. The specific surface area of ​​the boron nitride nanotubes is ≥50m 2 / g; d. The thermal conductivity of the boron nitride nanotubes is ≥3000 W / (m·K).

6. The spliced ​​molded graphite sagger according to claim 1, characterized in that: At least one of the following conditions a~b is met: a. The particle size of the flake graphite is 800~1000 mesh; b. The tensile strength of the carbon fiber is ≥4100 MPa.

7. The spliced ​​molded graphite sagger according to claim 1, characterized in that: The raw materials for preparing the bowl body also include 4 to 6 parts of a binder; And / or, the raw materials for preparing the separator further include 8 to 12 parts of a binder; Wherein, the binder is at least one of polyvinyl butyral and polysilazane.

8. The spliced ​​molded graphite sagger according to claim 1, characterized in that: The partition is in a cross shape and is placed in the bowl body to divide the internal space of the bowl body into four independent areas.

9. A method for preparing a spliced ​​molded graphite sagger, characterized in that: The following steps are involved: S1. Preparation of a bowl: The raw materials for preparing the bowl are ball-milled and wet-mixed to obtain a bowl precursor, the bowl precursor is molded and heat-treated to obtain a bowl; S2 preparation of the separator: the separator raw material is prepared after ball milling to obtain a separator precursor, the separator precursor is molded and heat treated to obtain a separator; S3. Preparation of spliced ​​molded graphite sagger: Assemble the bowl body and the partition to obtain a spliced ​​molded graphite sagger.

10. The method for preparing a spliced ​​molded graphite sagger according to claim 9, wherein: Satisfy at least one of the following conditions a to d: a. In S1, the molding pressure is 5MPa~15Mpa, and the molding time is 5~15min; b. In S1, the heat treatment is sintering at 1000-1200°C in a nitrogen atmosphere for 70-80h; c. In S2, the molding pressure is 5MPa~15Mpa, and the molding time is 5~15min; d. In S2, the heat treatment is sintering at 1000-1200°C for 15-30 hours in a nitrogen atmosphere.

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