Long-life high-strength graphite composite crucible for differential pressure casting and preparation method thereof

Abstract

The application discloses a long-life high-strength graphite composite crucible for differential pressure casting and a preparation method thereof, and belongs to the technical field of differential pressure casting and composite materials. The composite crucible is composed of a graphite composite matrix and a surface gradient coating, and the matrix component comprises high-purity flake graphite, silicon carbide, carbon nanotubes, graphene nanosheets, boron carbide and silicon powder. The application adopts a "melting silicon infiltration in-situ reaction" technology, realizes a "matrix densification + in-situ reinforcement + coating oxidation resistance" three-in-one synergistic protection mechanism through a chemical bonding interface between silicon carbide whiskers and the graphite matrix, a carbon nanotube and graphene nanosheet collaborative dispersion toughening network and a surface SiC-BN-Al2O3 gradient coating. The one-dimensional and two-dimensional collaborative reinforcement network of the carbon nanotube and the graphene nanosheet can effectively inhibit micro-crack propagation, and the B2O3 glass phase generated by the oxidation of boron carbide in service can realize the self-closing of the oxidation-resistant micropores. Test results show that the composite crucible has a porosity of less than or equal to 2%, a room temperature bending strength of more than or equal to 60 MPa, a fracture toughness of more than or equal to 3.5 MPa·, and a service life of more than 150 shifts under the differential pressure casting working condition, which is more than 2 times that of similar products, and has a wide application prospect.

Description

Technical Field

[0001] This invention belongs to the field of differential pressure casting and composite materials technology, specifically relating to a graphite composite crucible for differential pressure casting aluminum alloy melting, and more specifically to a long-life, high-strength graphite composite crucible for differential pressure casting and its preparation method. Background Technology

[0002] Differential pressure casting is currently the mainstream production process for high-end castings such as aluminum alloy steering knuckles. In differential pressure casting, the pressure inside the crucible is greater than the pressure inside the mold. Under the action of the pressure difference, the molten aluminum alloy in the crucible fills the mold through the riser pipe. During the sequential solidification process, the molten aluminum alloy in the crucible continuously feeds the casting and solidifies under high pressure. Therefore, the load-bearing capacity and service reliability of the crucible are directly related to the final quality of the casting.

[0003] Currently, crucibles used in differential pressure casting are mainly made of isostatic graphite or carbon-carbon composite materials. However, the working conditions of differential pressure casting crucibles are extremely harsh: on the one hand, the inner wall of the crucible is in direct contact with high-temperature molten aluminum (700-750℃) for a long time, enduring the scouring and erosion of the molten aluminum; on the other hand, the outside of the crucible is subjected to alternating gas pressure, which easily leads to fatigue damage under long-term reciprocating loading conditions. The porosity of traditional graphite crucibles is about 10%-20%. During service, oxygen diffuses from the outer wall of the crucible inward through the pores, reacting with the graphite matrix to generate CO2 gas, leading to continuous consumption of graphite; active elements such as Mg and Si in the molten aluminum also penetrate along the graphite grain boundaries, reacting with graphite to generate brittle phases such as Al4C3, accelerating crucible failure. In practice, the average service life of traditional graphite crucibles under differential pressure casting conditions is only about 60-80 shifts, and frequent replacements seriously affect the continuous operation efficiency of the production line.

[0004] Therefore, developing a long-life, high-strength graphite composite crucible that can be stably used for more than 150 shifts under the harsh conditions of differential pressure casting is of great engineering significance for solving the long-standing bottleneck of the lifespan of key consumables in the industry, reducing casting production costs, and improving the production efficiency of differential pressure casting. Summary of the Invention

[0005] The purpose of this invention is to overcome the technical defects of existing graphite composite crucibles, such as insufficient strength and poor oxidation resistance, and to provide a long-life, high-strength graphite composite crucible for differential pressure casting and its preparation method.

[0006] According to one aspect of the present invention, a graphite composite crucible for long-life, high-strength differential pressure casting is provided, characterized in that the crucible is composed of a graphite composite matrix and a surface gradient coating. The graphite composite matrix is ​​composed of the following components in mass percentage: 45-65% high-purity flake graphite, 8-18% silicon carbide particles, 1-4% carbon nanotubes, 0.5-2.5% graphene nanosheets, 3-7% boron carbide, 5-12% silicon powder, and the balance being nano-alumina and yttrium oxide; the surface gradient coating is composed of 40-60% silicon carbide, 10-20% boron nitride nanosheets, 15-25% aluminum dihydrogen phosphate, and 5-15% α-alumina nanoparticles.

[0007] Preferably, the mass ratio of carbon nanotubes to graphene nanosheets in the graphite composite matrix is ​​1:0.5 to 1:2.5.

[0008] Preferably, the mass ratio of boron nitride nanosheets to α-alumina nanoparticles in the surface gradient coating is 1:0.3 to 1:1.5.

[0009] Preferably, the high-purity flake graphite has a purity of ≥99.9%, the carbon nanotubes have a diameter of 10-30 nm and a length of 5-15 μm, the graphene nanosheets have a thickness of 1-5 nm and a sheet diameter of 1-5 μm, and the silicon powder has a purity of ≥99.5% and a particle size of 1-15 μm.

[0010] According to another aspect of the present invention, a method for preparing the above-mentioned graphite composite crucible is provided, characterized by comprising the following steps:

[0011] Step 1: Carbon nanotubes and graphene nanosheets are ultrasonically dispersed in anhydrous ethanol at an ultrasonic power of 300-500W for 30-60 minutes to obtain a uniform suspension.

[0012] Step 2: Weigh high-purity flake graphite, silicon carbide particles, boron carbide, silicon powder, nano-alumina, and yttrium oxide according to the specified ratio, and grind and mix them with ethanol as the medium for 6-12 hours. Add the carbon nanotube suspension and graphene nanosheet suspension obtained in Step 1 to the ground mixture, and stir and mix at a low speed of 100-150 r / min for 2-4 hours to obtain a composite powder in which the nano-carbon material is uniformly dispersed in the matrix material.

[0013] Step 3: After drying, the composite powder is placed into a cold isostatic pressing mold and held under pressure of 180-250 MPa for 10-20 minutes to obtain a crucible blank.

[0014] Step 4: Place the green blank in a high-temperature sintering furnace, and after degreasing at 0-350℃ at a rate of 1-2℃ / min, heat it to 1420-1700℃ and hold it for 3-5 hours for in-situ reaction sintering. After cooling, the graphite composite matrix is ​​obtained.

[0015] Step 5: Mix silicon carbide micro powder, boron nitride nanosheets, α-alumina nanoparticles, and aluminum dihydrogen phosphate to prepare a coating slurry. Spray the slurry onto the inner and outer surfaces of the crucible and dry it. Then, sinter it at 900-1100℃ in an argon atmosphere for 2-3 hours and cool it to obtain a surface gradient coating.

[0016] Preferably, in step one, high-purity flake graphite is placed in a vacuum drying oven and dried at 120–150°C for 12–24 hours for use in step two.

[0017] Preferably, in step two, a planetary ball mill is used for grinding at a speed of 300-400 r / min, and the ratio of anhydrous ethanol added is material:ball:ethanol = 1:2:1.

[0018] Preferably, the in-situ reaction sintering in step four is divided into three stages:

[0019] S1, Heating and Degreasing Section: Heating from room temperature to 350℃ at a rate of 1-2℃ / min and holding for 2 hours;

[0020] S2, Silicon Powder Melting and Penetration Section: The temperature is increased to 1420℃ at a rate of 3-5℃ / min and held for 1-2 hours to melt the silicon powder into a liquid phase;

[0021] S3, In-situ silicon carbide generation section: Heat to 1600-1700℃ at 2-3℃ / min and hold for 3-5 hours.

[0022] Preferably, the apparent porosity of the graphite composite matrix obtained in step four is ≤2%, and the bulk density is ≥2.1g / cm³.

[0023] Preferably, in step five, silicon carbide micro powder with an average particle size of 0.5-1 μm, boron nitride nanoparticles with an average particle size of 50-100 nm, α-alumina nanoparticles with an average particle size of 30-50 nm, and aluminum dihydrogen phosphate binder are mixed in proportion, deionized water is added, and the mixture is stirred and dispersed at 800-1200 r / min for 2-3 hours to obtain a uniform coating slurry with a solid content of 40-60 wt%. This slurry is then sprayed onto the inner and outer surfaces of the crucible with a thickness of 100-200 μm and allowed to dry naturally at room temperature.

[0024] This invention employs a "fused silicon in-situ reaction" technique, utilizing the chemical bonding interface between silicon carbide whiskers and the graphite matrix, the synergistic dispersion and toughening network of carbon nanotubes and graphene nanosheets, and a SiC-BN-Al2O3 gradient coating on the surface to achieve a three-in-one synergistic protection mechanism of "matrix densification, in-situ reinforcement, and coating oxidation resistance." Specifically, the one-dimensional and two-dimensional synergistic reinforcement network of carbon nanotubes and graphene nanosheets effectively inhibits microcrack propagation, while the B2O3 glass phase generated by the oxidation of boron carbide during service enables the self-closure of oxidation-resistant micropores. Testing shows that the composite crucible of this invention has an apparent porosity ≤2%, a room temperature flexural strength ≥60 MPa, and a fracture toughness ≥3.5 MPa. Its service life exceeds 150 shifts under differential pressure casting conditions, which is more than twice that of similar products, and it has broad application prospects. Attached Figure Description

[0025] Figure 1 A schematic diagram of a graphite composite crucible according to an embodiment of the present invention is shown.

[0026] Figure 2 The process for preparing a graphite composite crucible according to an embodiment of the present invention is shown. Detailed Implementation

[0027] Exemplary embodiments of the present invention will now be described in detail with reference to the accompanying drawings. The exemplary embodiments described below and illustrated in the drawings are intended to teach the principles of the invention, enabling those skilled in the art to implement and use the invention in various environments and for various applications. Therefore, the scope of protection of the invention is defined by the appended claims, and the exemplary embodiments are not intended, and should not be considered, a limiting description of the scope of protection of the invention. Furthermore, for ease of description, the dimensions of the various parts shown in the drawings are not necessarily drawn to actual scale. Any numerical range stated herein is intended to include all subranges contained therein, and a numerical range expressed as “value A to value B” refers to a range including endpoint values ​​A and B. Those skilled in the art will understand that terms such as “nth,” “Sn,” etc., in this invention are used only to distinguish different steps, devices, or modules, and do not represent any specific technical meaning, nor do they indicate a necessary logical order between them.

[0028] According to the present invention, a long-life, high-strength graphite composite crucible for differential pressure casting and its preparation method are provided. Based on the components and preparation process described below, the crucible is composed of a graphite composite matrix and a surface gradient coating. Through the three-in-one design of "multi-scale synergistic reinforcement - gradient coating anti-oxidation protection - dual-phase interface toughening" of the matrix, the crucible can be stably operated for more than 150 shifts under the long-term service conditions of differential pressure casting.

[0029] Composition of graphite-based composite materials

[0030] The graphite composite crucible matrix of the present invention is composed of the following components, as shown in Table 1 below.

[0031] Table 1: Composition of graphite composite crucible matrix (mass percentage: %)

[0032]

[0033] Thus, according to the present invention, in order to obtain a graphite composite matrix, two nanomaterials of different dimensions, carbon nanotubes (CNTs) and graphene nanosheets (GNs), are simultaneously introduced into the graphite matrix for the first time. The one-dimensional fibrous structure of CNTs forms a bridging effect through the network in the matrix, generating pull-out and fracture effects during crack propagation to consume fracture energy; the two-dimensional sheet structure of GNs induces crack deflection and bifurcation during crack propagation, while its interlayer lubrication properties can buffer the thermal mismatch stress between the matrix and SiC particles. Furthermore, the incorporation of CNTs and GNs can reduce the surface roughness of the graphite casting mold cavity. The synergistic effect of both constructs a dispersed, strong, and toughened network at both one-dimensional and two-dimensional scales, improving fracture toughness by more than 40% compared to traditional graphite / silicon carbide composites.

[0034] Furthermore, according to the present invention, a dual-phase reinforcement approach of "silicon carbide particles and in-situ reactive silicon carbide" is adopted. Boron carbide (B4C) reacts with silicon powder at high temperature: B4C + 5Si → 5SiC + B (solid solution). On the one hand, a dispersed secondary silicon carbide phase is generated in the graphite matrix; on the other hand, the generated B atoms are dissolved in the silicon carbide lattice, adjusting the silicon carbide grain boundary structure, improving the density and oxidation resistance of the silicon carbide layer, and reducing the diffusion rate of oxygen and aluminum liquid along the grain boundaries.

[0035] Thus, according to the material composition of the present invention, the graphite matrix provides the framework structure; micron / nano multi-scale SiC particles provide a hard reinforcing phase and an anti-erosion barrier; the one-dimensional bridging of CNTs and the two-dimensional interlayer lubrication of GNs synergistically improve the thermal shock resistance; B4C+Si reacts in situ to generate dispersed silicon carbide and achieves grain boundary B solid solution; silicon powder also acts as an active binder to achieve a chemical bonding interface between the graphite matrix and the reinforcing phase during sintering.

[0036] Preferably, the mass ratio of carbon nanotubes to graphene nanosheets in the graphite composite matrix is ​​1:0.5 to 1:2.5, and the two work together to construct a one-dimensional and two-dimensional nano-carbon dispersion toughening network.

[0037] Antioxidant Coating

[0038] Antioxidant coatings were prepared on the inner and outer surfaces of the graphite composite crucible body 1 by in-situ reaction method to obtain inner coating 2 and outer coating 3. The composition of the coatings is shown in Table 2 below.

[0039] Table 2: Composition of Antioxidant Coating

[0040]

[0041] The inner coating 2 and outer coating 3 both have a thickness of 50–150 μm. The silicon carbide layer in the coating forms a strong chemical bond with the graphite matrix, effectively preventing high-temperature oxidation of the graphite crucible and corrosion by molten aluminum during processing. Boron nitride (BN) is an inert inorganic high-temperature lubricant that does not adhere to or wet molten metal, completely protecting the crucible surface that is in direct contact with molten aluminum, magnesium, zinc alloys, and slag. Boron nitride nanosheets have a graphite-like layered hexagonal fine-grained structure with strong chemical inertness; alumina nanoparticles construct a dense nanoscale network in the coating, effectively blocking oxygen and molten aluminum penetration.

[0042] Preferably, the mass ratio of boron nitride nanosheets to α-alumina nanoparticles in the surface gradient coating is 1:0.3 to 1:1.5. The boron nitride nanosheets provide an inert interface that does not wet the molten aluminum, while the α-alumina nanoparticles disperse and fill the pores of the coating to improve its density.

[0043] <Preparation Process>

[0044] The method for preparing a long-life, high-strength differential pressure casting graphite composite crucible of the present invention includes the following steps:

[0045] Step 1: Raw material pretreatment

[0046] High-purity flake graphite (purity ≥99.9%, see GB / T 3518-2023 "Flake Graphite") was placed in a vacuum drying oven and dried at 120–150℃ for 12–24 hours to remove surface-adsorbed moisture and gases. Multi-walled carbon nanotubes (diameter 10–30 nm, length 5–15 μm) and graphene nanosheets (thickness 1–5 nm, sheet diameter 1–5 μm) were placed in a dispersion solvent (anhydrous ethanol) and ultrasonically dispersed in an ultrasonic cell disruptor at 300–500 W power for 30–60 minutes to obtain uniformly dispersed CNT and GNs suspensions.

[0047] Step 2: Mixing the Formula

[0048] Weigh the raw materials according to the proportions listed in Table 1. Place the weighed graphite powder (i.e., the high-purity flake graphite pretreated in step one), silicon carbide particles, boron carbide particles, and silicon powder into a grinding mill, such as a planetary ball mill, and add anhydrous ethanol (material:ball:ethanol = 1:2:1.0). Ball mill at 300–400 r / min for 6–12 hours to ensure uniform mixing of the components through wet grinding. After ball milling, add the ultrasonically dispersed CNTs and GNs suspension dropwise into the ball mill jar and continue stirring at a low speed of 100–150 r / min for 2–4 hours to uniformly disperse the nano-carbon materials in the matrix material, obtaining a uniformly mixed composite powder. This also avoids the damage to the structure of the nano-carbon materials caused by high-speed ball milling.

[0049] Since carbon nanotubes (CNTs and GNs) have a much larger specific surface area and interfacial bonding force than graphite particles, conventional ball milling processes are difficult to achieve uniform dispersion. According to the present invention, the micron-sized matrix components such as graphite / silicon carbide / silicon powder / boron carbide powder are first pre-ball-milled and mixed, and then the carbon nanotubes are introduced by low-speed stirring. This avoids the destruction of CNTs and GNs by the ball milling media and effectively ensures that carbon nanotubes and graphene nanosheets are dispersed in the composite matrix in their original form.

[0050] Step 3: Cold Isostatic Pressing

[0051] The uniformly mixed composite powder is loaded into a rubber or polyurethane elastic mold, vibrated to compact, and then sealed. The pressure of the cold isostatic press is set to 180–250 MPa, and the holding time is 10–20 minutes. The green blank formed by isostatic pressing has uniform dimensions and density, laying a good foundation for subsequent sintering.

[0052] Step 4: Heat treatment and in-situ reaction sintering

[0053] The isostatically pressed green body is placed in a high-temperature sintering furnace and heat-treated according to the following procedure:

[0054] S1, Heating and Degreasing Section: The temperature is raised from room temperature to 350°C at a rate of 1-2°C / min, and held for 2 hours to remove organic matter and adsorbed water;

[0055] S2, Silicon Powder Melting and Penetration Section: The temperature is raised to 1420℃ at a rate of 3-5℃ / min and held for 1-2 hours to melt the silicon powder into a liquid phase. The molten silicon then penetrates into the graphite pores under capillary action.

[0056] S3, In-situ silicon carbide generation section: The temperature is raised to 1600-1700℃ at a rate of 2-3℃ / min and held for 3-5 hours. Molten silicon reacts with graphite in situ to generate silicon carbide whiskers. At the same time, boron carbide reacts with silicon to generate dispersed silicon carbide and boron atoms are dissolved in the silicon carbide lattice.

[0057] More specifically, within this temperature range, molten silicon reacts in situ with graphite: Si + C → SiC, generating nanoscale whisker-like silicon carbide between graphite particles and at the graphite / silicon carbide particle interface. Boron carbide (B4C) reacts with silicon powder at high temperature: B4C + 5Si → 5SiC + B (solid solution), generating a dispersed secondary silicon carbide phase. Simultaneously, B atoms are dissolved in the silicon carbide lattice, further improving the density and oxidation resistance of the silicon carbide layer.

[0058] Cooling section: Slowly cool to room temperature at 2-4℃ / min to prevent thermal stress cracking.

[0059] According to the present invention, a "molten silicon infiltration in-situ silicon carbide generation" technology is employed. Compared to the simple mixing method of directly incorporating silicon carbide particles, the molten silicon infiltration method generates SiC whiskers in situ within the graphite matrix, forming a chemical bonding interface with the graphite matrix. The bonding strength is far higher than that of physical mixing, significantly improving the thermal shock resistance and fatigue resistance of the composite material. Simultaneously, molten silicon in its liquid state can fully fill the pores between graphite particles, achieving a matrix density of over 98%, reducing apparent porosity to below 2%, and a bulk density ≥2.1 g / cm³, thereby greatly enhancing oxidation resistance.

[0060] Step 5: Surface Coating Preparation

[0061] (S1) Slurry preparation: Silicon carbide micro powder (average particle size 0.5-1μm), nano boron nitride (average particle size 50-100nm), α-alumina nanoparticles (average particle size 30-50nm) and aluminum dihydrogen phosphate binder are mixed in proportion, and an appropriate amount of deionized water is added. The mixture is stirred in a high-speed disperser at 800-1200r / min for 2-3 hours to obtain a uniform coating slurry with a solid content of 40-60wt%.

[0062] (S2) Coating spraying: Use an air spray gun to evenly spray the slurry onto the inner and outer surfaces of the crucible, with the coating thickness controlled at 100-200 μm. After spraying, allow it to air dry at room temperature for 24 hours.

[0063] (S3) Coating Curing and Sintering: The dried crucible is placed in an atmosphere sintering furnace and heated to 900–1100°C at a rate of 5°C / min under an argon protective atmosphere. The temperature is held for 2–3 hours, and then cooled with the furnace. Within this temperature range, aluminum dihydrogen phosphate transforms into aluminum polyphosphate and aluminum phosphate networks during baking, bonding and anchoring SiC and BN particles. Al2O3 nanoparticles disperse and fill the pores, forming a dense coating. The silicon carbide layer of the coating is firmly bonded to the graphite matrix, and the SiC layer densely fills the internal pores of the graphite crucible (dense filling), preventing oxidizing gases from entering the interior of the graphite crucible.

[0064] Step Six: Precision Machining

[0065] After the coating has cured, the crucible is precision machined to ensure that the roundness and perpendicularity of the crucible meet the installation accuracy requirements of the differential pressure casting equipment.

[0066] <Example 1>

[0067] (1) Matrix material formulation

[0068] Weigh the raw materials according to the following mass percentages (total mass is calculated as 1000g):

[0069] High-purity flake graphite (25μm): 640g (64wt%)

[0070] Silicon carbide particles (average particle size 10μm): 140g (14wt%)

[0071] Boron carbide (B4C, particle size 2μm): 50g (5wt%)

[0072] Silicon powder (2μm): 100g (10wt%)

[0073] Carbon nanotubes (CNTs, 15nm in diameter): 25g (2.5wt%)

[0074] Graphene nanosheets (GNs, 2nm thick): 15g (1.5wt%)

[0075] α-Al₂O₃ nanoparticles: 20g (2wt%)

[0076] Y2O3 powder: 10g (1wt%)

[0077] (2) Preparation steps

[0078] Graphite, SiC, B4C, Si powder, Al2O3, and Y2O3 were added to a planetary ball mill, and ball milled at 300 rpm for 8 hours using ethanol as the medium. CNTs and GNs were ultrasonically dispersed separately in anhydrous ethanol at 400 W for 45 minutes. After ball milling, the CNTs and GNs suspension was added dropwise, and the mixture was stirred at low speed (120 rpm) for 3 hours. After drying the powder, it was loaded into a rubber mold and subjected to cold isostatic pressing at 200 MPa for 15 minutes.

[0079] Sintering process: room temperature → 350℃ (1℃ / min, hold for 2h) → 1420℃ (4℃ / min, hold for 1.5h) → 1650℃ (2.5℃ / min, hold for 4h) → slow cooling to room temperature (3℃ / min).

[0080] Coating preparation: A slurry was prepared with 45% SiC, 15% BN, 10% Al2O3, 20% aluminum dihydrogen phosphate, and the remainder deionized water. The slurry was sprayed onto the inner and outer surfaces of the crucible and sintered at 1000℃ under an argon atmosphere for 2.5 hours.

[0081] (3) Performance testing

[0082]

[0083] After 168 shifts, the crucible's outer wall structure was found to be intact, with no expansion or delamination caused by oxygen diffusion; the inner wall coating was slightly thinned but not peeled off; and no through cracks were found inside the substrate.

[0084] The above embodiments are only used to illustrate the technical solutions of this application, and are not intended to limit them. Although this application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features therein. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the various embodiments of this application.

Claims

1. A long-life, high-strength graphite composite crucible for differential pressure casting, characterized in that, The crucible is composed of a graphite composite matrix and a surface gradient coating. The graphite composite matrix consists of the following components by mass percentage: 45-65% high-purity flake graphite, 8-18% silicon carbide particles, 1-4% carbon nanotubes, 0.5-2.5% graphene nanosheets, 3-7% boron carbide, 5-12% silicon powder, and the balance being nano-alumina and yttrium oxide. The surface gradient coating consists of 40-60% silicon carbide, 10-20% boron nitride nanosheets, 15-25% aluminum dihydrogen phosphate, and 5-15% α-alumina nanoparticles.

2. The graphite composite crucible according to claim 1, characterized in that, The mass ratio of carbon nanotubes to graphene nanosheets in the graphite composite matrix is ​​1:0.5 to 1:2.

5.

3. The graphite composite crucible according to claim 1, characterized in that, The mass ratio of boron nitride nanosheets to α-alumina nanoparticles in the surface gradient coating is 1:0.3 to 1:1.

5.

4. The graphite composite crucible according to claim 1, characterized in that, The high-purity flake graphite has a purity of ≥99.9%, the carbon nanotubes have a diameter of 10–30 nm and a length of 5–15 μm, the graphene nanosheets have a thickness of 1–5 nm and a sheet diameter of 1–5 μm, and the silicon powder has a purity of ≥99.5% and a particle size of 1–15 μm.

5. A method for preparing the graphite composite crucible according to any one of claims 1 to 4, characterized in that... Includes the following steps: Step 1: Carbon nanotubes and graphene nanosheets are ultrasonically dispersed in anhydrous ethanol at an ultrasonic power of 300-500W for 30-60 minutes to obtain a uniform suspension. Step 2: Weigh high-purity flake graphite, silicon carbide particles, boron carbide, silicon powder, nano-alumina, and yttrium oxide according to the specified ratio, and grind and mix them with ethanol as the medium for 6-12 hours. Add the carbon nanotube suspension and graphene nanosheet suspension obtained in Step 1 to the ground mixture, and stir and mix at a low speed of 100-150 r / min for 2-4 hours to obtain a composite powder in which the nano-carbon material is uniformly dispersed in the matrix material. Step 3: After drying, the composite powder is placed into a cold isostatic pressing mold and held under pressure of 180-250 MPa for 10-20 minutes to obtain a crucible blank. Step 4: Place the green blank in a high-temperature sintering furnace, and after degreasing at 0-350℃ at a rate of 1-2℃ / min, heat it to 1420-1700℃ and hold it for 3-5 hours for in-situ reaction sintering. After cooling, the graphite composite matrix is ​​obtained. Step 5: Mix silicon carbide micro powder, boron nitride nanosheets, α-alumina nanoparticles, and aluminum dihydrogen phosphate to prepare a coating slurry. Spray the slurry onto the inner and outer surfaces of the crucible and dry it. Then, sinter it at 900-1100℃ in an argon atmosphere for 2-3 hours and cool it to obtain a surface gradient coating.

6. The preparation method according to claim 5, characterized in that, In step one, high-purity flake graphite is placed in a vacuum drying oven and dried at 120–150°C for 12–24 hours for use in step two.

7. The preparation method according to claim 5, characterized in that, In step two, a planetary ball mill is used to grind the material at a speed of 300-400 r / min, and the ratio of anhydrous ethanol added is material:ball:ethanol = 1:2:

1.

8. The preparation method according to claim 5, characterized in that, The in-situ reaction sintering in step four is divided into three stages: S1, Heating and Degreasing Section: Heating from room temperature to 350℃ at a rate of 1-2℃ / min and holding for 2 hours; S2, Silicon Powder Melting and Penetration Section: The temperature is increased to 1420℃ at a rate of 3-5℃ / min and held for 1-2 hours to melt the silicon powder into a liquid phase; S3, In-situ silicon carbide generation section: Heat to 1600-1700℃ at 2-3℃ / min and hold for 3-5 hours.

9. The preparation method according to claim 5 or 8, characterized in that, The apparent porosity of the graphite composite matrix obtained in step four is ≤2%, and the bulk density is ≥2.1g / cm³.

10. The preparation method according to claim 5, characterized in that, In step five, silicon carbide micro powder with an average particle size of 0.5–1 μm, boron nitride nanoparticles with an average particle size of 50–100 nm, α-alumina nanoparticles with an average particle size of 30–50 nm, and aluminum dihydrogen phosphate binder are mixed in proportion, deionized water is added, and the mixture is stirred and dispersed at 800–1200 r / min for 2–3 hours to obtain a uniform coating slurry with a solid content of 40–60 wt%. This slurry is then sprayed onto the inner and outer surfaces of the crucible with a thickness of 100–200 μm and allowed to dry naturally at room temperature.

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