High-purity graphite for silicon carbide crystal growth and purification process thereof

Abstract

The invention discloses high-purity graphite for silicon carbide crystal growth and a purification process thereof, and the purification process comprises the following steps: placing graphite in a purification furnace, and starting vacuum at the pressure of 0.1-100 Pa; the temperature is increased to 1900-2100 DEG C, heat preservation is conducted, mixed gas containing chlorine gas / argon gas is introduced during heat preservation, the volume content ratio is (2-10): 100, a gas inlet is closed after gas introduction is conducted for 10-20 min, heat preservation is conducted for 10-20 min, then vacuum is started, the pressure is 0.1-100 Pa, and circulation is conducted for multiple times; the temperature is increased to 2100-2300 DEG C, heat preservation is carried out, mixed gas of chlorine-containing gas / fluorine-containing gas / argon is introduced during heat preservation, the volume content ratio is (2-5): (4-10): 100, a gas inlet is closed after gas introduction is carried out for 10-20 min, the gas is kept for 10-20 min, vacuum is started, and circulation is carried out for multiple times; the temperature is increased to 2300-2500 DEG C, heat preservation is conducted, argon is introduced during heat preservation, vacuum is started, the pressure is 1-10 KPa, and after heat preservation is finished, natural cooling is conducted to the room temperature; the graphite cooled to the room temperature is ultrasonically cleaned and dried after being cleaned, the graphite material with the purity larger than or equal to 99.9999% is obtained, the energy consumption is low, and the purification efficiency is high.

Description

Technical Field

[0001] The present invention relates to the technical field of graphite purification, in particular to high-purity graphite for silicon carbide crystal growth and a purification process thereof. Background Art

[0002] In the semiconductor field, silicon carbide (SiC) offers irreplaceable advantages in high-temperature, high-frequency, and high-power electronic devices due to its wide bandgap, high breakdown field strength, and excellent thermal conductivity. However, various impurities may be unintentionally introduced during the growth of SiC single crystals. These impurities can significantly reduce the carrier mobility, breakdown voltage, and long-term reliability of SiC devices. Therefore, obtaining high-purity SiC is a key issue in its industrialization.

[0003] As a carbon-based material with excellent performance, graphite is widely used in metallurgy, electronics, aerospace, nuclear industry, new energy, and semiconductor fields due to its good electrical conductivity, thermal stability, corrosion resistance, and high-temperature strength. High-purity graphite is a key material for the thermal field in silicon carbide crystal growth furnaces. With the rapid development of the third-generation semiconductor industry, stringent requirements are placed on the control of graphite impurities. For example, the ash content of graphite required in the semiconductor production process must be below 5ppm. However, most natural graphite or traditional artificial graphite often contains metallic impurity elements such as aluminum, calcium, titanium, vanadium, and iron, as well as non-metallic impurity elements such as boron and chlorine, which restricts the application of graphite in the semiconductor field.

[0004] In the existing technology, the commonly used industrial graphite purification methods usually include chemical pickling, high-temperature heat treatment, halogen roasting, high-temperature halogen purification and other methods. Among them, although chemical pickling can effectively reduce the content of some metals, it will also produce a large amount of acidic waste liquid, which has a high risk of environmental pollution and has a poor effect on removing impurities wrapped between graphite layers; among them, high-temperature heat treatment generally heats graphite to 2600-3000℃ and removes impurities by high-temperature volatilization, but this method needs to maintain high temperature for a long time, consumes extremely high energy, and has low removal efficiency of refractory metal carbides (such as titanium carbide and vanadium carbide), making it difficult to meet high purity requirements; among them, the halogen roasting method is Due to the low reaction temperature, the reaction kinetics of halogens and impurities are slow, the treatment cycle is long, the halogen gas is highly toxic, the operating requirements for personnel are high, and the exhaust gas treatment is strict, which limits its application; among them, the high-temperature halogen purification method uses high temperature and halogen or halogen-containing hydrocarbon gas to simultaneously act to convert metal impurities into low-boiling point halogen compounds for discharge, but it cannot repair the microstructural defects caused by the escape of impurities between graphite layers / intracrystalline lattices during the purification process, and the tiny carbon particles produced by the decomposition of halogen-containing hydrocarbon gas at high temperature or the high-temperature pulverization of the graphite surface will adhere to the graphite surface, which has a negative impact on subsequent use, especially on crystal growth. Summary of the Invention

[0005] The embodiments of the present application provide high-purity graphite for silicon carbide crystal growth and a purification process thereof. The method of multi-stage precise control of high-temperature halogen is adopted, which can not only solve the problems of high energy consumption and low efficiency of traditional purification methods, but also realize self-repair of microscopic defects of graphite during the roasting process. Ultrasonic treatment can remove tiny carbon particles attached to the surface of graphite, thereby obtaining a graphite material with low impurity content (purity ≥99.9999%), few microscopic defects and surface particles.

[0006] A first aspect of an embodiment of the present application provides a high-purity graphite purification process for silicon carbide crystal growth, comprising the following steps in sequence: S10, placing graphite in a purification furnace, closing the furnace cover, starting a vacuum pump, and controlling the pressure in the furnace to 0.1-100 Pa; S20, slowly raising the temperature in the furnace to 1900-2100°C, then keeping the temperature for 1-3 hours, during which a chlorine-containing gas / argon mixed gas is introduced at a flow rate of 20-120 L / min, wherein the volume ratio of the mixed gas is chlorine-containing gas: argon = 2-10:100, after ventilation for 10-20 minutes, the air inlet is closed and maintained for 10-20 minutes, and then the vacuum pump is started to evacuate to the furnace pressure of 0.1-100 Pa, and so on, for several cycles; S30, continue to slowly increase the furnace temperature to 2100-2300°C, then keep it warm for 1-3 hours, during which a mixture of chlorine-containing gas / fluorine-containing gas / argon is introduced at a flow rate of 20-120 L / min, wherein the volume ratio of the mixed gas is chlorine-containing gas: fluorine-containing gas: argon = 2-5:4-10:100. After ventilation for 10-20 minutes, close the air inlet and maintain it for 10-20 minutes, then start the vacuum pump to evacuate to the furnace pressure of 0.1-100 Pa, and so on, for several cycles; S40, continue to slowly raise the furnace temperature to 2300-2500°C, then keep it at that temperature for 1-3 hours, during which time argon is introduced at a flow rate of 20-120 L / min, start the vacuum pump, and control the pressure in the furnace at 1-10 kPa. After completion, naturally cool it to room temperature; S50, ultrasonically cleaning the graphite cooled to room temperature, and drying after cleaning to obtain a graphite material with a purity of ≥99.9999%.

[0007] In a possible implementation, the chlorine-containing gas is hydrogen chloride, chloromethane, or chloroethane, and the fluorine-containing gas is hydrogen fluoride, fluorine-containing methane, or fluorine-containing ethane.

[0008] In a possible implementation, the chlorine-containing gas is methyl chloride, and the fluorine-containing gas is methyl fluoride.

[0009] In a possible implementation, in step S20, the volume ratio of the mixed gas is chlorine-containing gas: argon gas = 5-10:100.

[0010] In a possible implementation, in step S30, the volume ratio of the mixed gas is chlorine-containing gas: fluorine-containing gas: argon gas = 2-5:7-10:100.

[0011] In a possible implementation, in step S20, the number of cycles is 28 to 35 times.

[0012] In a possible implementation, in step S30, the number of cycles is 35 to 40 times.

[0013] A second aspect of an embodiment of the present application provides high-purity graphite, which is prepared according to the high-purity graphite purification process as described above.

[0014] Beneficial effect: Compared with the prior art, the high-purity graphite purification process for silicon carbide crystal growth provided by the present application first converts calcium, magnesium and other metal element compounds into chlorides that are volatilized and discharged through the first heating stage, thereby avoiding the subsequent fluorination process from generating high-boiling point compounds that are difficult to remove, and then removes carbides and remaining impurity element compounds through the second heating stage. Then, in the third heating stage, the microstructural defects caused by the escape of impurity elements are automatically healed in a high-temperature inert atmosphere, while avoiding the introduction of nitrogen elements. At the same time, the halides remaining in the graphite during the reaction process and light elements such as halogens and nitrogen elements adsorbed on the graphite are discharged. Finally, through ultrasonic cleaning, a graphite material for silicon carbide crystal growth with low impurity element content (purity ≥99.9999%, i.e., the corresponding impurity element residual content ≤1ppm), few microstructural defects, and a clean surface can be obtained, which can solve the problems of high energy consumption and low efficiency of traditional purification methods and has a lower impurity content.

[0015] The objects, features and advantages of the present invention are fully reflected in the following detailed description. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 The figure shows a schematic flow chart of the high-purity graphite purification process for silicon carbide crystal growth according to the present invention.

[0017] Figure 2 The boiling point values of carbides, oxides, chlorides and fluorides of some impurity elements in the graphite to be purified are shown. DETAILED DESCRIPTION

[0018] The following description is intended to disclose the present invention so that those skilled in the art can implement the present invention. The preferred embodiments described below are for illustrative purposes only, and those skilled in the art will readily appreciate other obvious variations. The basic principles of the present invention defined in the following description may be applied to other embodiments, variations, improvements, equivalents, and other technical solutions that do not depart from the spirit and scope of the present invention.

[0019] It should be understood by those skilled in the art that, in the disclosure of the specification, the terms "longitudinal", "transverse", "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inside", "outside" and the like indicating the orientation or position relationship are based on the orientation or position relationship shown in the accompanying drawings, which are only for the convenience of describing the present invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, be constructed and operate in a specific orientation. Therefore, the above terms should not be understood as limiting the present invention.

[0020] It is to be understood that the term "one" should be understood as "at least one" or "one or more", that is, in one embodiment, the number of an element may be one, while in another embodiment, the number of the elements may be multiple, and the term "one" should not be understood as a limitation on the quantity.

[0021] refer to Figure 1 and Figure 2 The present invention provides a high-purity graphite purification process for silicon carbide crystal growth, which comprises the following steps in sequence: S10, placing graphite in a purification furnace, closing the furnace cover, starting a vacuum pump, and controlling the pressure in the furnace to 0.1-100 Pa; S20, first heating stage: slowly increase the temperature in the furnace to 1900-2100°C, then keep it warm for 1-3 hours, and during the holding period, introduce a chlorine-containing gas / argon mixed gas at a flow rate of 20-120 L / min, wherein the volume ratio of the mixed gas is chlorine-containing gas: argon = 2-10:100. After ventilation for 10-20 minutes, close the air inlet and maintain it for 10-20 minutes, then start the vacuum pump and pump it down to the furnace pressure of 0.1-100 Pa, and so on, for several cycles, so as to convert calcium, magnesium and other metal element compounds into chlorides first, volatilize and discharge them, and avoid the generation of high-boiling-point compounds that are difficult to remove in the subsequent fluorination process; S30, second heating stage: continue to slowly increase the furnace temperature to 2100-2300°C, then keep it warm for 1-3 hours. During the holding period, introduce a mixture of chlorine-containing gas / fluorine-containing gas / argon at a flow rate of 20-120 L / min, wherein the volume ratio of the mixed gas is chlorine-containing gas: fluorine-containing gas: argon = 2-5:4-10:100. After ventilation for 10-20 minutes, close the air inlet and maintain it for 10-20 minutes. Then start the vacuum pump and pump it to the furnace pressure of 0.1-100 Pa. Repeat this cycle several times. Since it is difficult for the chlorination reaction to remove carbides in the first heating stage, fluorine-containing gas is introduced to remove carbides and remaining impurity element compounds. S40, third heating stage: continue to slowly raise the furnace temperature to 2300-2500°C, then keep it at that temperature for 1-3 hours, during which time argon is introduced at a flow rate of 20-120 L / min, and the vacuum pump is started to control the pressure in the furnace at 1-10 kPa. After the end, the furnace is naturally cooled to room temperature, so that the microstructural defects caused by the escape of impurity elements in the second and third heating stages can be automatically healed in the high-temperature inert atmosphere, and while avoiding the introduction of nitrogen, the halides remaining in the graphite during the reaction process and light elements such as halogens and nitrogen adsorbed on the graphite are discharged; S50, ultrasonically cleaning the graphite cooled to room temperature, and drying after cleaning to obtain a graphite material with a purity of ≥99.9999%, that is, the residual content of impurity elements in the graphite material is ≤1ppm, and the graphite material has fewer microstructural defects and a cleaner surface, which is fully suitable for the growth of silicon carbide crystals; Wherein, the chlorine-containing gas is hydrogen chloride, chloromethane or chloroethane; Wherein, the fluorine-containing gas is hydrogen fluoride, fluorine-containing methane or fluorine-containing ethane.

[0022] Specifically, the chlorine-containing gas may be CH3Cl (monochloromethane), and the fluorine-containing gas may be CH3F (monofluoromethane), wherein: CH3Cl→C+H2+HCl; CH3F→C+H2+HF At high temperatures, HCl and HF generated by the decomposition of the purified gas react with carbide / oxide impurities in the graphite to form low-boiling point chlorides / fluorides, which are discharged with the carrier gas during the vacuum process.

[0023] In one embodiment, in step S20, the volume ratio of the mixed gas is chlorine-containing gas: argon gas = 5-10:100.

[0024] In one embodiment, in step S30 , the volume ratio of the mixed gas is chlorine-containing gas: fluorine-containing gas: argon gas = 2-5:7-10:100.

[0025] In step S20, the number of cycles is preferably 28 to 35. In step S30, the number of cycles is preferably 35 to 40.

[0026] The calculation basis of the number of cycles in the above steps S20 and S30 is as follows: First, determine the ash content and furnace load of the graphite to be purified, and calculate the total content of impurity elements in the graphite to be purified; Second, determine the required amount of halogen based on the total impurity element content in the graphite and the following reaction equation example: XO y +yCl2+yC→XCl 2y +yCO XO y +yF2+yC→XF 2y +yCO; Third, the amount of gas required for the reaction is calculated based on the halogen dosage and the proportion of halogen elements in the purified gas used. Taking into account that some gas in the furnace fails to fully contact and react with the impurity elements in the graphite, the actual total amount of gas introduced needs to be increased by 10% to 20% on the basis of the calculated value. Fourth, the final number of cycles is determined based on the actual gas usage, combined with the gas input volume and input time of each cycle.

[0027] Therefore, in the high-purity graphite purification process for silicon carbide crystal growth provided in the present application, a chlorine-containing gas / argon mixed gas is used in a lower temperature range (1900-2100°C) to preferentially remove calcium, magnesium and other metal elements, and in a slightly higher temperature range (2100-2300°C), it is switched to a chlorine-containing gas / fluorine-containing gas / argon mixed gas to remove other impurity elements that are difficult to remove by chlorination reactions. Subsequently, in the third heating stage, in an argon environment at a temperature of 2300-2500°C, the graphite is rearranged in its crystal structure at high temperature to compensate for microscopic defects caused by impurity escape, while further discharging residual chlorides, fluorides and light elements such as halogens and nitrogen attached to the graphite. Finally, the purified graphite is placed in an ultrasonic field, and the energy of the ultrasonic field is used to remove tiny carbon particles attached to the surface during the purification process. The purity of the final graphite material can reach more than 99.9999%, and the corresponding residual content of impurity elements is 1ppm, not 99.999%.

[0028] Example 1: S10, placing graphite in a purification furnace, closing the furnace cover, starting the vacuum pump, and controlling the pressure in the furnace at 1 Pa; S20, first heating stage: slowly raise the temperature in the furnace to 2000°C, then keep it warm for 3 hours, and during the holding period, introduce a chlorine-containing gas / argon mixed gas at a flow rate of 80 L / min, wherein the volume ratio of the mixed gas is CH3Cl:argon = 2:100. After ventilating for 10 minutes, close the air inlet and maintain it for 10 minutes, then start the vacuum pump and pump it down to the furnace pressure of 10Pa. Repeat this cycle 30 times, thereby converting calcium, magnesium and other metal element compounds into chlorides that are volatilized and discharged, thereby avoiding the generation of high-boiling-point compounds that are difficult to remove in the subsequent fluorination process; S30, second heating stage: continue to slowly raise the furnace temperature to 2250°C, then keep it warm for 3 hours. During the holding period, a mixed gas of CH3Cl:CH3F:argon is introduced at a flow rate of 80L / min, wherein the volume ratio of the mixed gas is CH3Cl:CH3F:argon = 2:4:100. After ventilating for 10 minutes, the air inlet is closed and maintained for 10 minutes. Then, the vacuum pump is started and the furnace pressure is evacuated to 10Pa. This cycle is repeated 30 times. Since it is difficult to remove carbides by the chlorination reaction in the first heating stage, a fluorine-containing gas is introduced to remove carbides and remaining impurity element compounds. S40, third heating stage: continue to slowly raise the furnace temperature to 2400°C, then keep it at that temperature for 2 hours, during which time argon is introduced at a flow rate of 100L / min, and the vacuum pump is started to control the pressure in the furnace at 5KPa. After the end, the furnace is naturally cooled to room temperature, so that the microstructural defects caused by the escape of impurity elements in the second and third heating stages can be automatically healed in the high-temperature inert atmosphere, and while avoiding the introduction of nitrogen, the halides remaining in the graphite during the reaction process and light elements such as halogens and nitrogen adsorbed on the graphite are discharged; S50, ultrasonically cleaning the graphite cooled to room temperature, and drying after cleaning to obtain a graphite material with a purity of ≥99.9999%, with a specific residual content of impurity elements of 0.89 ppm.

[0029] Example 2: Different from Example 1, in Example 2, in the first temperature rising stage, the volume content ratio of the mixed gas is CH3Cl:Argon=5:100.

[0030] Example 3: Different from Example 1, in Example 3, in the first heating stage, the volume content ratio of the mixed gas is CH3Cl:Argon=8:100.

[0031] Example 4: Different from Example 1, in Example 4, in the first heating stage, the volume content ratio of the mixed gas is CH3Cl:Argon=10:100.

[0032] Comparative Example 1: Different from Example 1, in Comparative Example 1, in the first heating stage, no chlorine-containing gas was introduced into the mixed gas, but only argon gas was introduced, and the resulting residual content of impurity elements was 10.41 ppm.

[0033] It can be seen that in the first heating stage, after the mixed gas with different mixing ratios is introduced, the remaining content of impurity elements is different, as shown in Table 1 below:

[0034] Table 1 During the first heating stage, chlorine-containing gas and argon are introduced. The reasons for avoiding the introduction of fluorine-containing gas during this stage are as follows: ① The boiling point of some fluorides is higher than that of chlorides, and premature introduction of fluorine is not conducive to the discharge of impurities; ② At low temperatures, graphite will react with fluorine to form fluorinated graphite, which will have a negative impact on the surface of the graphite to be purified; In addition, the use of a lower temperature to pass the chlorine-containing gas can effectively reduce the power consumption. As the proportion of chlorine-containing gas increases, the impurity content inside the graphite gradually decreases and tends to be flat. However, as the chlorine-containing gas content continues to increase, the final purification effect does not change significantly. In other words, the trend of further reduction in impurity content is not obvious, and the excess gas will be wasted. In addition, as more chlorine-containing gas is introduced, some of the gas will be adsorbed on the insulation inside the furnace, which will prolong the cycle vacuuming time. At the same time, there will be an unbearable odor during the furnace opening process, which will threaten the health of the operators. Therefore, in the first heating stage, the content ratio of the mixed gas is chlorine-containing gas: argon gas = 2~10:100, and the content of elemental impurities can be significantly reduced. At the same time, the occurrence of the aforementioned various negative factors can be avoided, such as waste of resources, reduced work efficiency, harm to the health of operators, etc.

[0035] Example 5: Different from Example 1, in Example 5, in the second heating stage, the volume content ratio of the mixed gas is CH3Cl:CH3F:argon = 2:8:100, and the residual content of impurity elements in the final graphite material is 0.45ppm.

[0036] Example 6: Different from Example 1, in Example 6, in the second heating stage, the volume content ratio of the mixed gas is CH3Cl:CH3F:argon = 2:10:100, and the residual content of impurity elements in the final graphite material is 0.46 ppm.

[0037] Example 7: Different from Example 1, in Example 7, in the second heating stage, the volume content ratio of the mixed gas is CH3Cl:CH3F:argon = 4:5:100, and the residual content of impurity elements in the final graphite material is 0.70ppm.

[0038] Example 8: Different from Example 1, in Example 8, in the second heating stage, the volume content ratio of the mixed gas is CH3Cl:CH3F:argon = 4:7:100, and the residual content of impurity elements in the final graphite material is 0.49 ppm.

[0039] Example 9: Different from Example 1, in Example 9, in the second heating stage, the volume content ratio of the mixed gas is CH3Cl:CH3F:argon = 4:7.5:100, and the residual content of impurity elements in the final graphite material is 0.43ppm.

[0040] Example 10: Different from Example 1, in Example 10, in the second heating stage, the volume content ratio of the mixed gas is CH3Cl:CH3F:argon = 4:10:100, and the residual content of impurity elements in the final graphite material is 0.47ppm.

[0041] Example 11: Different from Example 1, in Example 11, in the second heating stage, the volume content ratio of the mixed gas is CH3Cl:CH3F:argon = 5:5:100, and the residual content of impurity elements in the final graphite material is 0.71ppm.

[0042] Example 12: Different from Example 1, in Example 12, in the second heating stage, the volume content ratio of the mixed gas is CH3Cl:CH3F:argon = 5:10:100, and the residual content of impurity elements in the final graphite material is 0.50ppm.

[0043] It can be seen that in the second heating stage, after the gas is introduced in different proportions, the content of impurity elements is different, as shown in the data in Table 2 below;

[0044] Table 2 Other comparative examples: Different from Example 1, in the second heating stage, the volume content ratios of chlorine-containing gas, fluorine-containing gas and argon gas are different, and the residual content values (ppm) of impurity elements in the resulting graphite materials are shown in Table 2.

[0045] Comparative Example 2: Different from Example 1, in the first heating stage, the furnace temperature is slowly increased to 1600°C instead of 2000°C. The final residual content of impurity elements is 1.52ppm, which is much larger than the final residual content of impurity elements of 0.89ppm in Example 1.

[0046] It can be seen from Table 2 above that in the second heating stage, when the volume ratio of the introduced chlorine-containing gas and the fluorine-containing gas is low, that is, within a volume ratio of 5, and the introduced fluorine-containing gas content ratio is less than 4, the remaining content of the impurity elements is greater than 1 ppm, that is, the purity is less than 99.9999%; In the second heating stage, fluorine-containing, chlorine-containing and argon gases are introduced simultaneously. This stage mainly removes the fluorine free radicals generated by the decomposition of the fluorine-containing gas at high temperature, which remove the impurity elements and carbides remaining in the graphite after the first heating stage. The proportion of chlorine-containing gas introduced in this stage has less influence on the final purity than the introduction of fluorine-containing gas. As the amount of fluorine-containing gas introduced increases, the impurity elements in the graphite further decrease and tend to be stable. However, since the temperature is raised again in this stage, it is conducive to the purification reaction. Therefore, when too much chlorine-containing gas and argon are introduced, the impurity content of the graphite is further reduced slightly, but the overall effect is limited. Therefore, the purification effect is limited and the resource waste rate is high. Therefore, in the second temperature rising stage, the content ratio of the mixed gas is chlorine-containing gas: fluorine-containing gas: argon gas = 2-5:4-10:100.

[0047] Argon primarily serves as a carrier gas. If it's not used in the incoming gas, problems such as insufficient diffusion of the purified gas and difficulty in discharging impurities can occur, leading to lower-than-expected purity or uneven purity across locations. Furthermore, purified gas decomposes at high temperatures to produce fluorine and chlorine. If these levels are too high, even extended pumping time can make it difficult to completely evacuate the gas. This can lead to a strong, unpleasant odor when the furnace is turned on, which is detrimental to operator health, increases production time, and wastes purified gas.

[0048] A second aspect of an embodiment of the present application provides high-purity graphite, which is prepared according to the high-purity graphite purification process as described above.

[0049] It should be noted that the terms "first, second, third and fourth" in this application are used for descriptive purposes only and do not indicate any order. They cannot be understood as indicating or implying relative importance. These terms can be interpreted as names.

[0050] Those skilled in the art will appreciate that the embodiments of the present invention described above and shown in the accompanying drawings are intended to be illustrative only and are not intended to limit the present invention. The advantages of the present invention have been fully and effectively achieved. The functional and structural principles of the present invention have been demonstrated and illustrated in the embodiments. Any variations or modifications may be made to the embodiments of the present invention without departing from the principles described.

Claims

1. A process for purifying high-purity graphite for growing silicon carbide crystals, characterized in that: The following steps are included in sequence: S10, placing graphite in a purification furnace, closing the furnace cover, starting a vacuum pump, and controlling the pressure in the furnace to 0.1-100 Pa; S20, slowly raising the temperature in the furnace to 1900-2100°C, then keeping the temperature for 1-3 hours, during which a chlorine-containing gas / argon mixed gas is introduced at a flow rate of 20-120 L / min, wherein the volume ratio of the mixed gas is chlorine-containing gas: argon = 2-10:100, after ventilation for 10-20 minutes, the air inlet is closed and maintained for 10-20 minutes, and then the vacuum pump is started to evacuate to the furnace pressure of 0.1-100 Pa, and so on, for several cycles; S30, continue to slowly increase the furnace temperature to 2100-2300°C, then keep it warm for 1-3 hours, during which a mixture of chlorine-containing gas / fluorine-containing gas / argon is introduced at a flow rate of 20-120 L / min, wherein the volume ratio of the mixed gas is chlorine-containing gas: fluorine-containing gas: argon = 2-5:4-10:

100. After ventilation for 10-20 minutes, close the air inlet and maintain it for 10-20 minutes, then start the vacuum pump to evacuate to the furnace pressure of 0.1-100 Pa, and so on, for several cycles; S40, continue to slowly raise the furnace temperature to 2300-2500°C, then keep it at that temperature for 1-3 hours, during which time argon is introduced at a flow rate of 20-120 L / min, start the vacuum pump, and control the pressure in the furnace at 1-10 kPa. After completion, naturally cool it to room temperature; S50, ultrasonically cleaning the graphite cooled to room temperature, and drying after cleaning to obtain a graphite material with a purity of ≥99.9999%.

2. The high-purity graphite purification process for silicon carbide crystal growth according to claim 1, wherein: The chlorine-containing gas is hydrogen chloride, chloromethane or chloroethane, and the fluorine-containing gas is hydrogen fluoride, fluorine-containing methane or fluorine-containing ethane.

3. The high-purity graphite purification process for silicon carbide crystal growth according to claim 2, wherein: The chlorine-containing gas is methyl chloride, and the fluorine-containing gas is methyl fluoride.

4. The high-purity graphite purification process for silicon carbide crystal growth according to claim 1, wherein: In step S20 , the volume ratio of the mixed gas is chlorine-containing gas: argon gas = 5-10:

100.

5. The high-purity graphite purification process for silicon carbide crystal growth according to claim 4, characterized in that: In step S30 , the volume ratio of the mixed gas is chlorine-containing gas: fluorine-containing gas: argon gas = 2-5:7-10:

100.

6. The high-purity graphite purification process for silicon carbide crystal growth according to claim 1, wherein: In step S20, the number of cycles is 28 to 35 times.

7. The process for purifying high-purity graphite for growing silicon carbide crystals according to claim 1 or 6, wherein: In step S30, the number of cycles is 35 to 40 times.

8. High purity graphite, characterized in that The high-purity graphite is prepared according to the high-purity graphite purification process according to any one of claims 1 to 7.

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