Method for controlling impurities during growth of sapphire crystals, and crucible for crystal growth

Abstract

The present invention belongs to the technical field of the growth of sapphire crystals, and specifically relates to a method for controlling impurities during the growth of sapphire crystals, and a crucible for crystal growth. The method for controlling impurities during the growth of sapphire crystals comprises the following steps: crucible impurity removal, involving spraying an aluminum oxide solution onto the inner wall of a molybdenum crucible, placing the crucible into a crystal growing furnace, sealing a furnace body, and then starting heating for sintering, followed by cooling; and crystal growing, involving filling the treated crucible with a raw material powder, i.e. aluminum oxide, sealing the furnace body, then starting heating to melt the raw material, performing seeding and crystal growing after the raw material has melted, and after the growth of crystals is completed, cooling same and then taking out the crystals. Mainly by designing and controlling the reaction of aluminum oxide at different temperatures, a crucible for crystal growth is obtained by means of an impurity removal operation on the crucible, thereby preventing impurity elements in a new crucible from being introduced into the growth process of sapphire crystals; therefore, impurities during the growth of sapphire crystals are controlled, and the quality and consistency of crystal growth are improved.

Description

A method for controlling impurities in sapphire crystal growth and a crucible for crystal growth

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] The present disclosure claims priority to the Chinese patent application No. 2024110922748, filed on August 9, 2024, and entitled "A method for controlling impurities in sapphire crystal growth and a crucible for crystal growth", the entire content of which is incorporated herein by reference. TECHNICAL FIELD

[0003] The present disclosure belongs to the technical field of sapphire crystal growth, and particularly relates to a method for controlling impurities in sapphire crystal growth and a crucible for crystal growth. BACKGROUND

[0004] In the crystal growth process, the crucible is a crucial component. Molybdenum crucible is commonly used for high-melting-point crystal growth due to its good thermal conductivity and low thermal expansion coefficient. Currently, sapphire crystals are mainly grown by the Kyropoulos crystal growth method, i.e., the Kyropoulos method (abbreviated as KY method). Since the crucible is usually made of molybdenum metal and its alloy powder through isostatic pressing, shaping and sintering, there are free impurity elements in the crucible during the process. Therefore, when a new molybdenum crucible is first used in the crystal growth process, it may introduce impurity elements, especially free molybdenum elements, into the crystal, which directly affects the quality of the sapphire crystal during the growth process.

[0005] Currently, in the industrial production process of sapphire crystals, the improvement of the quality of sapphire crystal growth is mainly the control of impurities in the crystal growth process, which is essentially the control of impurities in the new molybdenum crucible. The method mainly has two steps. First, before using the new crucible, the crucible is filled with pure water and then ultrasonically cleaned. Second, the crucible is placed in the crystal growth furnace and heated for air firing for more than once, at a temperature of about 2050°C for at least 24 hours. However, this method cannot completely remove the free molybdenum elements produced during the shaping process of the crucible, which will mix into the raw materials of the first few furnaces of the new crucible growth. During the crystal growth process, the molybdenum impurities enter the aluminum atom vacancies or interstitials, causing different degrees of distortion of the crystal lattice. Macroscopically, molybdenum impurities will cause the crystal to discolor, affecting the quality of the crystal and being not conducive to application.

[0006] Patents with publication numbers CN108727023A and CN110818413A disclose that aluminum oxide and molybdenum trioxide will undergo solid-phase reaction to generate aluminum molybdate at high temperature. It can be seen that free molybdenum elements may react with aluminum oxide crystal growth raw materials at high temperature, resulting in the generation of crystal boundaries and stress in the sapphire crystal growth process, and the appearance of bubbles in the grown sapphire crystal. In addition, excessive molybdenum elements and other impurity elements may also cause the crystal to present light green or other colors. To solve the above problems, a patent with publication number CN220116725U discloses a graphite heat field crystal growth furnace crucible, which is provided with an inner lining layer and a transition layer inside the molybdenum crucible to prevent the adhesion of crystal growth raw materials to molybdenum and affect the quality of the crystal, which belongs to a physical isolation method and does not involve spraying an aluminum oxide raw material coating on the inner wall of the crucible to control the chemical reaction between aluminum oxide and molybdenum elements. A patent with publication number CN103361722B provides a method for preparing a polycrystalline silicon ingot, which sprays a barrier coating on the inner wall of a crucible that has already been sprayed with a silicon nitride coating, to prepare a high-quality polycrystalline silicon ingot. It is aimed at the problem of the reaction between silicon material and the original silicon nitride coating on the inner wall of the crucible. The barrier layer is added under the premise of the existing coating, and the chemical reaction is avoided by means of barrier, which actually belongs to a physical isolation method.

[0007] Therefore, during the growth of sapphire crystals, the content of molybdenum elements needs to be strictly controlled to ensure the quality and stability of the crystal. In view of this problem, there is a need to disclose a sapphire crystal growth impurity control method. SUMMARY

[0008] To solve the above problems, the present disclosure provides a sapphire crystal growth impurity control method. This method mainly utilizes the reaction design and control of aluminum oxide at different temperatures, through crucible impurity removal operation, to avoid the introduction of impurity elements in the new crucible into the sapphire crystal growth process, thereby controlling the impurities of sapphire crystal growth, reducing the generation of crystal boundaries, stress and bubbles, avoiding the crystal to present light green or other colors, ensuring the quality and stability of crystal growth, and improving the quality and consistency of crystal growth.

[0009] The present disclosure provides a sapphire crystal growth impurity control method, which comprises the following steps:

[0010] S1, crucible impurity removal: spray an aluminum oxide solution on the inner wall of a molybdenum crucible, and place the crucible in a crystal growth furnace. After the furnace body is sealed, start temperature rising sintering and temperature falling cooling.

[0011] Among them, the temperature rising sintering: after the first stage of vacuumizing, the vacuum degree is controlled to be ≤500 Torr, then the temperature is raised to 400-500℃, and maintained for 0.5-3h. In the second stage, after filling with protective gas, the temperature is continuously raised to 700-750℃, and maintained for 0.5-3h. In the third stage, the temperature is continuously raised to 2055-2065℃, and maintained for 8-12h.

[0012] S2, crystal growth: fill the alumina raw material powder into the treated crucible, seal the furnace body, start to heat and melt the raw material, after the raw material is melted, perform crystal seeding and crystal growth, after the crystal growth is completed, cool down and take out the crystal.

[0013] In step S1, the crucible needs to be cleaned before impurity removal, first ultrasonic cleaning with pure water, shake out the impurities existing in the crucible or slit, pure water can also effectively avoid the introduction of other impurity elements in the cleaning solution during the cleaning of the crucible; after drying, use alcohol solution to clean the crucible, because the molybdenum crucible has the possibility of oxidation, its oxide is slightly soluble in water, use alcohol to clean the dried crucible, remove the impurities dissolved in water and the precipitate produced after drying.

[0014] Optionally, in step S1, the ultrasonic cleaning time of the crucible is 4-5h.

[0015] Optionally, in step S1, the alumina solution is a colorless transparent solution mixed by nano-alumina powder with a particle size of 5-10nm and a suspending agent, and the mass ratio is 3-6:1, and the suspending agent is a polyacrylic acid water-based solution. After mixing the nano-alumina powder with the suspending agent, the alumina solution is treated by hydrothermal method, and has certain adhesion, and is more easily attached to the surface of the crucible to form a reaction layer, and reacts with impurities at high temperature, thereby effectively controlling the content of impurities.

[0016] Optionally, in step S1, the temperature of the alumina solution is 40-50℃, too low temperature will cause poor flowability of the raw material, and too high temperature will cause decomposition of alumina, this temperature range helps to improve the flowability during spraying, forming a uniform alumina coating; the spraying pressure is set to 0.3-0.5MPa, and the spraying angle is set to 45-60°; the spraying thickness is 1-2mm, ensuring uniformity, density, no bubbles or falling off.

[0017] Optionally, in the temperature rising and sintering process in step S1, after vacuumizing in the first stage, the vacuum degree is controlled to ≤400Torr to ensure good airtightness of the crystal growing furnace, after vacuumizing, the temperature is raised to 430-480℃ and maintained for 1-2h; after filling the protective gas in the second stage, the temperature is continuously raised to 710-730℃ and maintained for 1-2h; in the third stage, the temperature is continuously raised to 2058-2062℃, and the temperature is maintained for 9-11h, which refers to the temperature at the position close to the liquid surface in the crucible.

[0018] Optionally, in the temperature rising and sintering process in step S1, the temperature rising time in the first stage is 6-10h, the temperature rising time in the second stage is 3-4h, and the temperature rising time in the third stage is 20-25h.

[0019] Optionally, in the temperature rising sintering process in step S1, the first stage has a temperature rising time of 7h, 8h or 9h, the second stage has a temperature rising time of 3.2h, 3.4h, 3.6h or 3.8h, and the third stage has a temperature rising time of 21h, 22h, 23h or 24h.

[0020] Optionally, the protective gas is hydrogen.

[0021] In the first stage of the temperature rising sintering process in step S1, after vacuumizing, the vacuum degree in the furnace is high, and there is a small amount of oxygen in the furnace. As the temperature of the molybdenum crucible rises, when the temperature is greater than 400℃, the molybdenum will undergo a slight and slow oxidation reaction.

[0022] In the second stage of the temperature rising sintering process in step S1, since the alumina decomposes to generate free oxygen atoms at high temperature, the molybdenum will react with the oxygen atoms to generate molybdenum trioxide solid, which is distributed on the inner wall of the crucible. As the temperature gradually rises, the molybdenum trioxide in the environment also reacts with the alumina to generate aluminum molybdate, which adheres to the inner wall of the crucible to form a protective layer.

[0023] Due to the protective layer adhering to the inner wall of the crucible, the alumina powder raw material is isolated from the inner wall of the molybdenum crucible during the crystal growth process, effectively controlling the molybdenum element in the crucible from being precipitated to affect the quality of the crystal growth.

[0024] Aluminum molybdate is tasteless and light green, with a density of 3.46-3.49g / cm 3 The density of artificial sapphire crystal is 3.4-4.1g / cm 3 The density of aluminum molybdate is close to that of sapphire. If there is no protective layer on the inner wall of the molybdenum crucible, the molybdenum element on the inner wall of the molybdenum crucible will react with the alumina raw material during the crystal growth process, causing the reaction product to mix into the sapphire crystal, resulting in the edges of the grown sapphire crystal appearing light green or other colors.

[0025] Optionally, in the third stage of the temperature rising sintering process in step S1, the temperature is raised to a high temperature for a long time, so that the remaining alumina that has not reacted with the molybdenum trioxide is completely melted.

[0026] Since the boiling point of molybdenum trioxide is 1155℃, in the third stage of the temperature rising sintering process in step S1, the unreacted molybdenum trioxide in the crucible will volatilize in this high temperature environment. Since the protective gas is continuously filled and discharged in the crucible, the gas circulation will discharge the volatilized molybdenum trioxide gas from the crucible, ensuring that the remaining molybdenum trioxide is discharged. Therefore, the temperature rising sintering process in step S1 can effectively remove the free impurity molybdenum element in the crucible.

[0027] In the cooling process in step S1, the temperature is lowered at a rate of 20-60℃ / h, and the crucible is taken out at room temperature.

[0028] In the cooling process in step S1, the temperature is lowered at a rate of 40-60℃ / h above 1200℃, 30-40℃ / h between 800-1200℃, and 20-35℃ / h below 800℃, and the power is reduced to zero, and then the crucible is naturally cooled to room temperature.

[0029] In step S2, the used alumina raw material powder has a purity of ≥99.9999%, an average particle size of ≤3μm, and a density of 3.9g / cm 3 .

[0030] In step S2, the temperature in the furnace is heated to 10-20℃ above the melting point of alumina, so that the alumina raw material is completely melted to form a melt, and the preferred temperature range is 2060-2065℃.

[0031] In step S2, after the raw material is melted, the seed crystal head on the seed crystal rod is uniformly eroded by 2-5mm, and then the seed crystal is lowered until it contacts the surface of the melt, and the crystal is started.

[0032] In step S2, after the crystal is introduced, the sapphire crystal nucleus is formed, the heating power is reduced at a rate of 2-5℃ / h, the cooling rate is controlled, and the sapphire single crystal is gradually crystallized and grown downward from the top.

[0033] In step S2, after the crystal growth is completed, the crystal is pulled upward at a rate of 10-12mm / min, so that the crystal is separated from the solution at the bottom of the crucible, the temperature is lowered at a rate of 15-20℃ / h, and the crystal is taken out after being lowered to room temperature.

[0034] The present disclosure provides a crucible for crystal growth, which is obtained after the crucible is purified in the above-mentioned sapphire crystal growth impurity control method.

[0035] In the crucible for crystal growth, an aluminum molybdate protective layer is attached to the inner wall of the crucible, and the thickness of the layer is 40-100μm.

[0036] The present disclosure provides a method for controlling impurities in sapphire crystal growth, which includes two parts of crucible decontamination and crystal growth. First, the crucible is cleaned, which can not only remove large particle impurities, but also effectively remove small particles and dirt attached to the inner wall of the crucible, ensuring the cleanliness of the crucible. Then, the molybdenum crucible is sintered at high temperature, so that the impurity molybdenum element in the inner wall of the molybdenum crucible can be fully reacted and released, effectively controlling the impurity elements in the crucible. In addition, during the heating process, the molybdenum impurities will react at each heating stage, and finally react with aluminum oxide on the inner wall of the crucible to form aluminum molybdate. In the subsequent crystal growth process, this protective layer will isolate the impurity molybdenum element on the inner wall of the crucible from the aluminum oxide raw material during crystal growth, preventing direct reaction and avoiding the reaction product mixing into the sapphire crystal, which is conducive to reducing the generation of crystal boundaries, stress and bubbles in the crystal, avoiding discoloration of the crystal, and improving the quality of sapphire crystal growth. At the same time, the present disclosure can further enhance the high-temperature resistance of the crucible, effectively improve the service life of the crucible, and reduce production costs.

[0037] The present disclosure has the following beneficial effects:

[0038] 1. Improve the quality of crystal growth: the aluminum molybdate protective layer formed by chemical reaction can reduce the release of impurities on the inner wall of the crucible, providing a purer environment for crystal growth, which helps to obtain higher quality and more uniform crystals. In the process of crystal growth, trace amounts of molybdenum elements will affect the electrical properties of the crystal, causing the crystal to discolor, and the protective layer can effectively reduce the influence of impurities;

[0039] 2. Improve the service life of the crucible: the aluminum molybdate protective layer attached to the inner wall of the crucible can effectively reduce the wear of the crucible in a high-temperature, chemically corrosive environment, thereby extending its service life. In the process of crystal growth, high temperature and chemicals will gradually erode the inner wall of the crucible, and the presence of the protective layer can greatly slow down this erosion rate;

[0040] 3. Reduce production costs: due to the extension of the service life of the crucible, the replacement frequency of the crucible is reduced, thereby reducing the equipment cost in the production process;

[0041] 4. Enhance process stability: the protective layer can make the physical and chemical properties of the inner wall of the crucible more stable, reducing process fluctuations caused by changes in the crucible. In the process of crystal growth, which has extremely stringent requirements on process conditions, stable crucible performance is crucial to ensure the consistency and reliability of the product. BRIEF DESCRIPTION OF DRAWINGS

[0042] Figure 1 is an electron microscope image of the inner wall of the crucible after decontamination of the crucible in the embodiment of the present disclosure. DETAILED DESCRIPTION

[0043] The implementation is as follows:

[0044] A method for controlling impurities in sapphire crystal growth, comprising the following steps:

[0045] S1, the crucible decontamination process is as follows:

[0046] Crucible cleaning: a new crucible is placed in a container filled with pure water by a mechanical arm (the new crucible is a molybdenum crucible that has never been used for crystal growth), an ultrasonic generator is arranged in the container, and the cleaning is continuously performed for 4 hours. Then, the crucible is placed in a drying room for drying treatment, and then cleaned with an alcohol solution to remove impurities dissolved in water and possible precipitates generated in the drying process. The alcohol has good solubility and volatility, and can effectively remove impurities on the surface of the crucible. Finally, the obtained crucible has high inner wall cleanliness, providing a good substrate for the alumina spraying step;

[0047] Alumina spraying: alumina solution is sprayed on the inner wall of the molybdenum crucible. The alumina solution is prepared by mixing nano-alumina powder with a particle size of 5-10 nm and polyacrylic acid water-based suspending agent at a mass ratio of 4:1. The solution is heated to 40-50°C and then loaded into a spray gun. The spraying pressure is set to 0.3-0.5 MPa, and the spraying angle is 45-60°. At this time, the crucible is slowly rotated at a speed of 3 r / min to allow the spray gun to uniformly spray the alumina solution on the inner wall of the crucible. The thickness of the alumina coating is controlled to be within 1-2 mm. The crucible is placed in a crystal growth furnace, and the furnace body is sealed and then subjected to temperature rising and sintering and temperature dropping and cooling, wherein,

[0048] Temperature rising and sintering: in the first stage, the vacuum pump is started to perform vacuum pumping, the vacuum degree in the furnace is maintained below 400 Torr, and then the temperature is raised at a rate of 70-100°C / h for 6-10 h until the temperature reaches 400-500°C. The temperature is maintained at this temperature for 1-2 h. In the second stage, hydrogen gas is filled into the furnace, and then the temperature is raised at a rate of 30-50°C / h for 3-4 h until the temperature reaches 700-750°C. The temperature is maintained at this temperature for 1-2 h. In the third stage, the temperature is continuously raised for 20-25 h at a rate of 50-70°C / h until the equipment output reaches 40-60% and the temperature reaches 2055-2065°C. The temperature is maintained at this temperature for 10 h.

[0049] Temperature dropping and cooling: the temperature dropping rate is 40-60°C / h above 1200°C, 30-40°C / h at 800-1200°C, and 20-35°C / h below 800°C. The power is reduced to zero to enter the natural cooling stage until the temperature drops to room temperature and the crucible is taken out.

[0050] After the new crucible was treated by impurity removal, the superficial surface of the inner wall of the crucible changed in appearance, and a shallow layer of aluminum molybdate film was formed by reaction, as shown in FIG. 1, with an immersion depth of 50 μm. The aluminum content in the film layer was detected to be 0.17%. This data proves that the aluminum oxide reacts with the molybdenum crucible to form a shallow layer of aluminum molybdate film, which adheres to the inner wall of the crucible to form a protective layer and inhibit the release of molybdenum elements;

[0051] S2, crystal growth: the alumina raw material powder with a purity of ≥99.9999% and an average particle size of ≤3 μm is filled into the treated crucible, and the furnace body is sealed. After the raw material is melted, seed crystal is introduced and crystal growth is carried out, and finally the crystal is removed by cooling.

[0052] Raw material melting: the vacuum pump is started to perform vacuum pumping, and the vacuum degree in the furnace is maintained below 400 Torr. Then, the temperature is raised at a rate of 50-100 ℃ / h until the equipment output reaches 40-60%, and the temperature in the furnace is heated to 10-20 ℃ above the melting point of alumina (2050 ℃) to completely melt the raw material to form a melt.

[0053] Seed crystal introduction: after the head of a single crystal seed crystal is uniformly dissolved by 2-5 mm, the seed crystal is lowered until it contacts the surface of the melt.

[0054] Crystal growth: after the seed crystal introduction is completed, the sapphire crystal nucleus is formed. The heating power is reduced at a rate of 2 ℃ / h to control the cooling rate, and the sapphire single crystal is gradually crystallized and grown downward from the top.

[0055] Cooling: after the crystal growth is completed, the crystal is pulled upward to separate the crystal from the solution at the bottom of the crucible, and the temperature is reduced at a rate of 15-20 ℃ / h until it reaches room temperature, and then the crystal is removed.

[0056] In the comparative example, the crucible was not treated by impurity removal, and the new crucible was directly used for crystal growth. The crystal growth parameters were the same as in the example. The effects of the example and the comparative example are compared as follows:

[0057] (1) Impurity element content test and effect: the first three batches of sapphire crystals grown in the new crucible without impurity removal treatment in the comparative example had a high content of molybdenum elements, and the first three batches of sapphire crystals grown in the new crucible treated by impurity removal in the example had a significantly lower content of molybdenum elements. The average removal efficiency of molybdenum elements was calculated to be more than 85%, as shown in Table 1. The removal efficiency = (original content - treated content) / original content.

[0058] Table 1 Comparison of molybdenum element content in sapphire crystals

[0059] (2) Sapphire crystal growth quality test and effect: the sapphire crystal grown in the comparative example had an average grain boundary density of about 50 / cm 2, the stress concentration density is reduced to an average of 5 / cm 2 , the bubble density is reduced to an average of 5 / cm 2 ; the sapphire crystal grown in the example has a grain boundary density reduced to an average of 10 / cm 2 , the stress concentration density is reduced to an average of 5 / cm 2 , the bubble density is reduced to an average of 5 / cm 2 .

[0060] (3) Crystal color test and effect: the sapphire crystal grown in the comparative example has a light green edge; the sapphire crystal grown in the example has no light green edge color.

[0061] (4) Crucible service life test and effect: the crucible without impurity removal in the comparative example is used continuously for crystal growth, and the service life of the crucible is about 20 crystal growth cycles; after more than 20 times, a new crucible needs to be replaced; the crucible with impurity removal in the example is used continuously for crystal growth, and the service life of the crucible reaches more than 30 crystal growth cycles.

[0062] In summary, the experimental data of the example fully prove the significant effect of the disclosed method in controlling impurity elements in a new crucible, improving the growth quality of sapphire crystals, and prolonging the service life of the crucible. Industrial applicability

[0063] The method provided by the disclosure mainly utilizes the reaction design and control of aluminum oxide at different temperatures to obtain a crucible for crystal growth through crucible impurity removal operation, avoids the introduction of impurity elements in the new crucible into the sapphire crystal growth process, thereby controlling the impurities of the sapphire crystal growth, and improving the quality and consistency of the crystal growth.

Claims

1. A method for controlling impurities in sapphire crystal growth, characterized by, The method comprises the following steps: S1, removing impurities from the crucible: spraying an alumina solution on the inner wall of the molybdenum crucible, placing the crucible in a crystal growing furnace, and starting to heat and sinter and cool after sealing the furnace body; In the heating and sintering process, the vacuum degree is controlled to be ≤400 Torr after vacuumizing in the first stage, the temperature is raised to 430-480°C after vacuumizing, and maintained for 1-2 h; the protective gas is filled in the second stage, and the temperature is continuously raised to 710-730°C, and maintained for 1-2 h; the temperature is continuously raised to 2058-2062°C in the third stage, and the temperature is maintained for 9-11 h, which refers to the temperature at the position close to the liquid surface in the crucible. S2, crystal growing: filling alumina raw material powder into the treated crucible, sealing the furnace body, starting to heat and melt the raw material, and then performing crystal pulling and crystal growing after the raw material is melted. The crystal is taken out after the crystal growing is completed.

2. The control method according to claim 1, characterized by, In the step S1, the crucible needs to be cleaned before removing impurities. First, ultrasonic cleaning is performed with pure water, and then the crucible is cleaned with an alcohol solution after drying.

3. The control method according to claim 1 or 2, characterized by, In the step S1, the alumina solution is a colorless transparent solution prepared by mixing nano-alumina powder with a particle size of 5-10 nm and a suspending agent at a mass ratio of 3-6:

1. The suspending agent is a polyacrylic acid water-based solution.

4. The control method according to any one of claims 1 to 3, characterized by, In the step S1, the temperature of the alumina solution is controlled to be 40-50°C, the spraying pressure is set to be 0.3-0.5 MPa, the spraying angle is set to be 45-60°, and the spraying thickness is 1-2 mm.

5. The control method according to any one of claims 1 to 4, characterized by, In the heating and sintering process in the step S1, the vacuum degree is controlled to be ≤400 Torr after vacuumizing in the first stage, the temperature is raised to 430-480°C after vacuumizing, and maintained for 1-2 h; the protective gas is filled in the second stage, and the temperature is continuously raised to 710-730°C, and maintained for 1-2 h; the temperature is continuously raised to 2058-2062°C in the third stage, and the temperature is maintained for 9-11 h, which refers to the temperature at the position close to the liquid surface in the crucible.

6. The control method according to claim 5, characterized by, In the heating and sintering process in the step S1, the temperature is raised for 6-10 h in the first stage, the temperature is raised for 3-4 h in the second stage, the protective gas is hydrogen, and the temperature is raised for 20-25 h in the third stage.

7. The control method according to any one of claims 1 to 6, characterized by, In the cooling process in the step S1, the temperature is lowered at a rate of 20-60°C / h, and the crucible is taken out after cooling to room temperature.

8. The control method according to claim 7, characterized by, In the cooling process in the step S1, the temperature is lowered at a rate of 40-60°C / h above 1200°C, at a rate of 30-40°C / h between 800-1200°C, and at a rate of 20-35°C / h below 800°C. The power is reduced to zero, and the temperature is naturally cooled to room temperature.

9. The control method according to any one of claims 1 to 8, characterized by, In the step S2, the temperature in the furnace is heated to 10-20℃ above the melting point of alumina, so that the alumina raw material is completely melted to form a melt; after the raw material is melted, the seed crystal head on the seed crystal rod is uniformly eroded by 2-5mm, then the seed crystal is sunk until it contacts the surface of the melt, and the crystal is induced; after the crystal induction is completed, the sapphire crystal nucleus is formed, the heating power is reduced at a speed of 2-5℃ / h, the cooling rate is controlled, and the sapphire single crystal is gradually crystallized and grown downward from the top; after the crystal growth is completed, the crystal is pulled upward at a speed of 10-12mm / min, so that the crystal is separated from the solution at the bottom of the crucible, the temperature is reduced at a speed of 15-20℃ / h, and the crystal is taken out after the temperature is reduced to room temperature.

10. A crucible for crystal growth, characterized by, The crucible is obtained after the crucible is purified by the control method in any one of claims 1-9.

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