Quartz crucible and manufacturing process thereof

Abstract

The invention provides a novel quartz crucible which comprises a crucible wall and is characterized in that a protective layer surrounding the crucible wall is arranged on the outer side of at least one part of the crucible wall, and the protective layer is a transparent layer. Compared with an existing quartz crucible structure, the quartz crucible provided by the invention has the advantages that the transparent state layer with compactness is additionally arranged, so that the strength of the outermost layer of the quartz crucible can be improved, the reaction probability of the quartz crucible and a carbon crucible can be reduced, and the service life of the quartz crucible is prolonged. The invention further provides a manufacturing process for the quartz crucible, the thickness of each layer and the distribution condition of quartz sand in each layer are controlled by adjusting the vacuum condition and the melting temperature during melting, and a uniform and compact transparent protection layer is obtained on the outer wall surface of the crucible wall of the existing quartz crucible; the reaction probability of the outermost side of the quartz crucible and the carbon crucible is reduced to the maximum extent, the strength of the quartz crucible is improved, and the service life of the quartz crucible is prolonged.

Description

Technical Field

[0001] The present application belongs to the technical field of quartz crucible manufacturing, and in particular, relates to a quartz crucible and a manufacturing process thereof. Background Art

[0002] The existing quartz crucible for crystal pulling is composed of two parts, including an inner transparent layer and an outer bubble layer. The outer bubble layer is a region with a high bubble density, also called a bubble composite layer (bubble state layer), and its main function is to improve the heat dissipation uniformity of the quartz crucible. The first layer is a transparent state layer, which is mainly used to improve the single crystal crystallization rate and quality, and at the same time has corrosion resistance and reduces the reaction rate of the quartz crucible.

[0003] Since silicon is highly chemically active in the molten state, its first layer will react with the quartz crucible, i.e., S i O2+S i →2S i O, this process will cause irreversible damage to the quartz crucible; at the same time, the outermost layer of the quartz crucible is in direct contact with the carbon-carbon crucible, and the following reaction occurs at high temperature: SiO2+2C→2CO↑+Si. This reaction causes the performance of the quartz crucible to change, directly affecting the service life of the quartz crucible. The released gas will also affect the overall performance of the quartz crucible, causing abnormal accidents. Summary of the invention

[0004] The present application provides a quartz crucible and a manufacturing process thereof, which solves the technical problem in the prior art that the service life of the quartz crucible is reduced due to phase change between impurities in the outer bubble layer of the quartz crucible and the carbon-carbon crucible outside the quartz crucible.

[0005] In order to solve at least one of the above technical problems, the technical solution adopted in this application is:

[0006] A quartz crucible comprises a crucible wall, wherein a protective layer surrounding the crucible wall is arranged on the outer side of at least a portion of the crucible wall, wherein the protective layer is a transparent layer.

[0007] Furthermore, the crucible wall has a bottom and a barrel-shaped portion with one end tightly connected to the bottom and the other end open, wherein at least a portion of the protective layer includes the barrel-shaped portion.

[0008] Furthermore, the protective layer also includes the bottom.

[0009] Furthermore, the crucible wall is sequentially constructed with a first layer and a second layer from inside to outside along the wall thickness direction, wherein the first layer is a transparent layer, the second layer is a bubble layer, and the protective layer is arranged outside the second layer.

[0010] A manufacturing process for manufacturing the quartz crucible as described above comprises the steps of controlling the temperature and the exhaust state during melting, and obtaining the crucible wall and the protective layer in sequence from the inside to the outside.

[0011] Furthermore, the method also includes a step of controlling the thickness of each layer based on controlling the time of opening or closing the vacuum state.

[0012] Furthermore, during the melting process of the crucible wall, the vacuum state is first turned on and then turned off to keep the melting temperature constant, wherein the melting temperature is 1500-1600°C.

[0013] Furthermore, when the protective layer is melted, the vacuum state is turned on, and the melting temperature thereof is lower than the melting temperature of the crucible wall.

[0014] Furthermore, the initial value of the melting temperature of the protective layer is the melting temperature of the crucible wall, and the temperature starts to drop when the temperature reaches 1400-1500° C., and the melting ends when the temperature reaches room temperature.

[0015] Furthermore, in the melting crucible wall and the protective layer, the time taken to start the exhaust state is different, and the time taken from the inside to the outside decreases successively;

[0016] Preferably, the time taken to start the exhaust state when melting the crucible wall is 0-500s, and the time taken to start the exhaust state when melting the protective layer is 0-100s.

[0017] The quartz crucible proposed in the present application has an additional dense transparent layer compared to the existing quartz crucible structure, which can not only improve the strength of the outermost layer of the quartz crucible, but also reduce the reaction probability of the quartz crucible and the carbon-carbon crucible, thereby extending the service life of the quartz crucible.

[0018] At the same time, based on the above-mentioned quartz crucible, the present application also proposes a manufacturing process for the quartz crucible, by adjusting the vacuum conditions and the melting temperature during melting to control the thickness of each layer and the distribution of quartz sand in each layer, a uniform, dense and transparent protective layer is obtained on the outer wall surface of the crucible wall of the existing quartz crucible, thereby minimizing the reaction probability between the outermost side of the quartz crucible and the carbon-carbon crucible, improving the strength of the quartz crucible and extending its service life. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 is a schematic structural diagram of a quartz crucible according to an embodiment of the present application;

[0020] Figure 2 is a structural schematic diagram of a quartz crucible according to another embodiment of the present application;

[0021] Figure 3 It is a schematic diagram of the mold structure for preparing a quartz crucible.

[0022] In the figure:

[0023] 10. Crucible wall 11. First layer 12. Second layer

[0024] 13. Protective layer 22. Barrel-shaped portion 33. Bottom

[0025] 20. Mould 30. Vacuum pump DETAILED DESCRIPTION

[0026] The present application is described in detail below with reference to the accompanying drawings and specific embodiments.

[0027] A quartz crucible, such as Figure 1-2 As shown, it includes a crucible wall 10, and a protective layer 13 surrounding the crucible wall 10 is arranged on the outside of at least a part of the crucible wall 10, wherein the protective layer 13 is a transparent layer. The crucible wall 10 is sequentially constructed with a first layer 11 and a second layer 12 along the wall thickness direction from the inside to the outside, and the crucible wall 10 has a bottom 33 and a barrel-shaped portion 22 with one end sealedly connected to the bottom 33 and the other end open. Among them, at least the outer side of the barrel-shaped portion 22 is provided with a protective layer 13. In this embodiment, the protective layer 13 is stacked on the outer side of the second layer 12, and the protective layer 13 at least includes a barrel-shaped portion 22 configured on the straight wall section of the crucible wall 10, and the first layer 11 and the second layer 12, and the second layer 12 and the protective layer 13 are different state layers from each other. Among them, the first layer 11 and the protective layer 13 are both transparent state layers, and the second layer 12 is a bubble state layer. As for the transparent state layer, it can be known that by vacuuming during melting, the bubbles between the quartz sand are reduced, resulting in a denser and more uniform distribution of the quartz sand. The improvement of the transparent state layer can not only improve the overall strength of the layer, but also have corrosion resistance, which can reduce the reaction between the inner wall of the crucible wall 10 and the molten silicon, and can also reduce the reaction between the outer wall of the crucible wall 10 and the carbon-carbon crucible (omitted in the figure) arranged outside it, thereby extending its service life. As for the bubble state layer, the number of bubbles in the layer can be controlled by turning off the vacuum pump to obtain a bubble composite layer with a high bubble density. The increase in the bubble content can improve the heat dissipation of the quartz crucible, so that the temperature of the quartz crucible is dissipated along the bubble holes to reduce the deformation of the quartz crucible.

[0028] Compared with the prior art, this embodiment adds a dense and uniform transparent protective layer 13 on the outer wall surface of the second layer 12 of the bubble state layer, which not only improves the supporting strength of the crucible wall 10, but also improves the corrosion resistance of its outer wall surface, and can also reduce the crystallization reaction rate between the crucible wall 10 and the carbon-carbon crucible, thereby extending the overall service life of the quartz crucible.

[0029] One embodiment, such as Figure 1As shown, the protective layer 13 is only a barrel-shaped portion 22 disposed on the straight wall section of the quartz crucible, and the barrel-shaped portion 22 is disposed from the top of the crucible wall 10 and completely covers the straight wall section of the crucible wall 10 along its height direction. In the process of melting silicon crystals, the heater is mainly disposed along the height direction of the crucible wall 10, that is, the straight wall section in the height direction is heated more and has a greater impact on the temperature gradient of the crystal pulling, and thus the height of the protective layer 13 covers the straight wall section of the crucible wall 10, which is a basic structure for improving the overall strength and corrosion resistance of the outer wall surface of the crucible wall 10.

[0030] One embodiment, such as Figure 2 As shown, the protective layer 13 has a barrel-shaped portion 22 and also includes a bottom 33 disposed on the bottom section of the crucible wall 10, and the barrel-shaped portion 22 and the bottom 33 are integrally configured and completely wrapped on the outer wall surface of the second layer 12. The thickness of each layer is uniformly configured, that is, the thickness of the first layer 11 is uniform and the same, the thickness of the second layer 12 is uniform and the same, and the thickness of the protective layer 13 is uniform and the same. Since the outermost layer of the crucible wall 10 is in direct contact with the carbon-carbon crucible, the following reactions are likely to occur at high temperatures:

[0031] S i O2+2C→2CO↑+S i ,

[0032] This reaction can easily cause the performance of the crucible wall 10 to change, directly affecting the service life of the crucible wall 10 . The released gas can also affect the overall performance of the crucible wall 10 , causing abnormal accidents.

[0033] Therefore, the protective layer 13 of the crucible wall 10 in this embodiment increases the contact area with the carbon-carbon crucible, which can not only further improve the overall strength of the crucible wall 10, but also further reduce the crystallization speed of the outermost part of the crucible wall 10 reacting with the carbon-carbon crucible.

[0034] Regardless of the structure of the crucible wall 10, the first layer 11, the second layer 12 and the protective layer 13 have different thicknesses due to their different positions and functions. The first layer 11 is close to the side of the molten silicon, and because silicon has high chemical activity in the molten state, it will react with the crucible wall 10, that is:

[0035] S i O2+S i →2S i Oh,

[0036] This process will cause irreversible damage to the crucible wall 10, so its thickness must be greater than the thickness of the outermost protective layer 13. Preferably, the thickness of the first layer 11 is 5-6 mm, because it is dense and uniform, so that its thickness within the range of 5-6 mm can obtain strong corrosion resistance and anti-crystallization reaction performance.

[0037] The second layer 12 in the middle is a bubble composite layer of a translucent structure. Since the second layer 12 in the middle needs to have strong heat dissipation and uniformity, its thickness needs to be maximized, which is not only convenient for temperature dissipation, but also has a strong supporting effect on the strength of the entire quartz crucible. Preferably, the thickness of the second layer is 7-8 mm.

[0038] As the outermost protective layer 13, its thickness does not need to be too thick. It only needs to attach a transparent layer on its surface to enhance the corrosion resistance of the outer wall of the crucible wall 10, and also separate the bubble layer of the second layer 12 from the external carbon-carbon crucible, thereby reducing the reaction rate of the crucible wall 10 and the carbon-carbon crucible, thereby protecting the overall strength of the quartz crucible and extending the service life of the quartz crucible. Preferably, the thickness of the protective layer 13 is 1-3 mm.

[0039] A manufacturing process for the quartz crucible as described above, wherein the mold structure during manufacturing is as follows Figure 3 As shown, the external vacuum pump 30 is connected to the channel in the mold 20. A number of evenly distributed exhaust holes are provided on the wall of the channel close to the inner cavity. The vacuum pump 30 is used to exhaust air between the layers of quartz sand in the quartz crucible, so that the quartz sand is tightly and evenly distributed in the inner cavity to obtain a dense and uniform state layer.

[0040] The manufacturing process proposed in this application includes:

[0041] The step of controlling the temperature and the exhaust state during melting to sequentially obtain the crucible wall 10 and the protective layer 13 from the inside to the outside along the direction of the quartz crucible wall thickness.

[0042] Furthermore, the method further includes a step of controlling the thickness of each layer based on controlling the time of opening or closing the vacuum state.

[0043] During melting, based on the structure of the mold 20 , a layer of quartz sand is first laid in the inner cavity thereof, and the quartz sand is evenly laid in the inner cavity thereof.

[0044] Then, by controlling the temperature and the exhaust state during melting, the first layer 11, the second layer 12 and the protective layer 13 are obtained in sequence from the inside to the outside along the thickness direction of the quartz crucible. The number and distribution state of the bubbles in each layer are controlled by the temperature during melting and the exhaust state of the vacuum pump 30, so that the transparent state layer and the bubble state layer at different positions can be obtained, wherein the bubble state layer is a bubble composite layer of a translucent structure. At the same time, during the melting process, by controlling the melting time, that is, by controlling the exhaust state or closing the exhaust state time, the thickness of the crucible wall 10 and the protective layer 13 is controlled, that is, by controlling the working time of the vacuum pump 30 when exhausting or not exhausting, to obtain state layers of different thicknesses, so that the protective layer 13 with an external transparent state layer required by the present application can be obtained to protect the corrosion resistance and anti-crystallization ability of the quartz crucible, to reduce its reaction rate, and to improve the overall service life of the quartz crucible. In the process of melting the crucible wall 10, it is necessary to first turn on the vacuum state and then turn off the vacuum state; in the process of melting the protective layer 13, it is necessary to turn on the vacuum state again. Specifically, controlling the vacuum state during melting includes turning on the vacuum pump 30 when melting the first layer 11 and the protective layer 13, and turning off the vacuum pump when melting the second layer 12; and controlling the thickness of each layer based on the time when the vacuum pump 30 is turned on. Based on the characteristics of the first layer 11 and the protective layer 13, it is necessary to vacuumize while melting and sintering; while when melting the second layer 12, it is only necessary to melt and sinter without vacuumizing.

[0045] That is, the vacuum pump 30 is turned on only when the first layer 11 and the protective layer 13 are melted, and the vacuum pump 30 is not turned on when the second layer 12 is melted, so that a transparent protective layer 13 with corrosion resistance is added on the basis of the existing two layers. The first layer 11 and the protective layer 13 are both transparent layers that can improve the crystallization quality of the single crystal and have corrosion resistance, and the second layer 12 is a bubble layer with high bubble density. Therefore, the vacuum pump 30 is not turned on continuously during the melting of these three layers.

[0046] During the melting process of the crucible wall 10 , the vacuum state needs to be turned on and then turned off to keep the melting temperature constant, wherein the melting temperature is 1500-1600° C. When melting the protective layer 13 , the vacuum state is turned on and the melting temperature is lower than the melting temperature of the crucible wall 10 .

[0047] The initial value of the melting temperature of the protective layer 13 is the melting temperature of the crucible wall. When the temperature reaches 1400-1500° C., the temperature starts to drop and the melting ends when the temperature reaches room temperature.

[0048] Specifically, during the melting process of the first layer 11 and the protective layer 13, the vacuum pump 30 is turned on and their respective melting temperatures are kept stable; during the melting process of the second layer 12, the vacuum pump 30 does not need to be turned on, but its melting temperature also needs to be kept stable. That is, the melting temperatures of the three layers are stable and unchanged, and the melting temperatures of the first layer 11 and the second layer 12 are the same. However, since the protective layer 13 is located at the outermost side, its melting temperature is slightly lower than that of the first layer 11. After the protective layer 13 is melted, its temperature will be gradually reduced until it reaches room temperature, and the melting temperature of the entire quartz crucible ends.

[0049] When melting the first layer 11, the purpose of turning on the vacuum pump is to remove all the bubbles in the first layer, thereby generating a dense and uniform transparent state layer in the melting first layer 11. Furthermore, the transparent state layer of the first layer 11 is mainly affected by vacuuming. After the quartz sand is melted, the vacuum pump is turned on to vacuumize from the inside to the outside, so that all the bubbles inside and between the quartz sand in the first layer 11 are extracted, thereby generating the transparent state layer 11.

[0050] After the first layer 11 is melted, the second layer 12 is melted. The second layer 12 is mainly a bubble composite layer. At this time, it is necessary to stop vacuuming, so that the bubbles on the quartz sand and the bubbles between the quartz sand can be retained inside, so that the second layer 12 can form a bubble state layer with a high bubble density. The presence of bubbles can improve the heat dissipation of the crucible wall 10. Since the bubble composite layer is more likely to undergo crystallization reaction than the transparent first layer 11 during the high-temperature crystal pulling process, and crystallization is one of the most important issues affecting the overall service life of the quartz crucible, it is necessary to add a transparent protective layer 13 on its outer side, so as to reduce the probability of crystallization reaction of the second layer 12, thereby improving the service life of the quartz crucible.

[0051] After the preparation of the second layer 12 is completed, the vacuum pump is controlled to be turned on, that is, when the protective layer 13 is melted, the purpose of turning on the vacuum pump is to remove all the bubbles in the first layer, so that a dense and uniform transparent layer can be produced in the melted protective layer 13. Furthermore, for the protective layer 13, it is mainly affected by vacuuming. After the quartz sand is melted, the vacuum pump is turned on to vacuum from the inside to the outside, so that the bubbles inside and between the quartz sand in the protective layer 13 are all extracted, and then a transparent protective layer 13 is produced. The addition of the protective layer 13 can not only improve the corrosion resistance of the quartz crucible, but also reduce the speed of the crystallization reaction of the quartz crucible, thereby increasing the overall service life of the quartz crucible.

[0052] During the melting process, the purity of each layer of quartz sand is generally greater than 99.999%. In order to ensure its melting quality, the melting temperature is required to be in the range of 1400-1600°C.

[0053] The melting temperature of the first layer 11 is greater than the melting temperature of the protective layer 13, and the melting temperature of the second layer 12 is the same as that of the first layer 11, both of which are 1500-1600°C. The starting value of the melting temperature of the protective layer 13 is the melting temperature of the second layer 12, and the temperature starts to drop when its temperature becomes 1400-1500°C, until the melting ends when the temperature reaches room temperature. That is, the temperature of melting the first layer 11 and the second layer 12 is the same, and its range is slightly greater than the temperature when melting the protective layer 13. Specifically, the melting temperatures of the first layer 11 and the second layer 12 are both 1500-1600°C. The starting value of the melting temperature of the protective layer 13 is the same as the temperature when the second layer 12 is melted, and then sintering and melting are carried out at this temperature. After the melting is completed, the temperature needs to be lowered, and the temperature starts to drop when its end temperature becomes 1400-1500°C, until the temperature reaches room temperature when the melting ends.

[0054] Furthermore, by controlling the melting time and the working time of the vacuum pump 30, the thickness of each layer can be controlled. The melting time of each layer is the working time of the vacuum pump 30, that is, the time of turning on or off the vacuum state. Accordingly, by controlling the thickness of the quartz sand in the straight wall section and the curved surface section, and the height position of the vacuum hole in the mold 20, the following can be obtained: Figure 1 The structure of the protective layer 13 shown in FIG. Figure 2 The structure of the protective layer 13 is shown.

[0055] In melting the crucible wall 10 and the protective layer 13, the time used to open the exhaust state is different, that is, the time used to melt the first layer 11, the second layer 12 and the protective layer 13 is different, and the time used from the inside to the outside decreases successively. That is, the melting time of the protective layer 13 is the shortest, and the melting time of the first layer 11 is longer than the melting time of the second layer 12. Preferably, the time used to open the exhaust state when melting the crucible wall 10 is 0-500s, that is, the melting time of the first layer 11 is 0-500s; the time used to open the exhaust state when melting the protective layer 13 is 0-100s, that is, the melting time of the protective layer 13 is 0-100s.

[0056] The quartz crucible prepared by the preparation method of the present application and the quartz crucible prepared by the prior art, that is, the conventional quartz crucible prepared by the same vacuum conditions and without quartz sand, for quartz crucibles of different diameters, such as 28-inch quartz crucible, 30-inch quartz crucible, 32-inch quartz crucible and 36-inch quartz crucible, the service life results are compared, as shown in Table 1. It can be seen from Table 1 that for quartz crucibles of the same size, when other conditions remain unchanged, the service life of the quartz crucible obtained by the preparation method of the present application is longer than that of the quartz crucible prepared by the prior art.

[0057] Table 1 Comparison of service life of quartz crucibles prepared by the preparation method of the present application and by the prior art

[0058]

[0059] The present application controls the working state of the vacuum pump that controls the vacuum state of each layer when melting the quartz crucible, the melting time and the melting temperature, and can obtain a quartz crucible with an inner and outer double transparent state layer structure, wherein the thickness of the transparent state layer of the protective layer 13 can be controlled within 1-3mm, and the purpose of increasing the life of the quartz crucible is achieved without affecting the thermal insulation performance and thermal conductivity performance of the quartz crucible. By adopting the above-mentioned manufacturing process, the protective layer 13 of the quartz crucible can be a dense and uniform transparent state layer, reducing the reaction speed of the quartz crucible and the carbon-carbon crucible, and extending the service life of the quartz crucible.

[0060] The present application optimizes the proportion and type of each layer of quartz sand in the quartz crucible, adds a vacuum step in the final stage of melting the quartz crucible, changes the physical properties of the outermost layer of the quartz crucible, changes the two layers of the traditional process into the existing three-layer structure, and adds a transparent protective layer 13 on the outside of the bubble state layer. It can not only reduce the crystallization reaction speed of the outermost protective layer 13 of the quartz crucible, extend the service life of the quartz crucible, reduce abnormal accidents in the silicon single crystal pulling process such as bulging and siliconization caused by the violent reaction of the protective layer 13, improve production efficiency and reduce production costs.

[0061] The quartz crucible proposed in the present application has an additional dense transparent layer compared to the existing quartz crucible structure, which can not only improve the strength of the outermost layer of the quartz crucible, but also reduce the reaction probability of the quartz crucible and the carbon-carbon crucible, thereby extending the service life of the quartz crucible.

[0062] At the same time, based on the above-mentioned quartz crucible, the present application also proposes a manufacturing process for the quartz crucible, by adjusting the vacuum conditions and the melting temperature during melting to control the thickness of each layer and the distribution of quartz sand in each layer, a uniform, dense and transparent protective layer is obtained on the outer wall surface of the crucible wall of the existing quartz crucible, thereby minimizing the reaction probability between the outermost side of the quartz crucible and the carbon-carbon crucible, improving the strength of the quartz crucible and extending its service life.

[0063] The above detailed description of the embodiments of the present application is only a preferred embodiment of the present application and cannot be considered to limit the scope of implementation of the present application. All equivalent changes and improvements made within the scope of the present application should still fall within the scope of the patent coverage of the present application.

Claims

1. A quartz crucible, comprising a crucible wall, characterized in that: A protective layer surrounding the crucible wall is arranged on the outer side of at least a portion of the crucible wall, wherein the protective layer is a transparent layer.

2. The quartz crucible according to claim 1, characterized in that: The crucible wall has a bottom and a barrel-shaped portion with one end tightly connected to the bottom and the other end open, wherein at least the outer side of the barrel-shaped portion is provided with the protective layer.

3. The quartz crucible according to claim 2, characterized in that: The protective layer is arranged on the outer side of the bottom.

4. The quartz crucible according to any one of claims 1 to 3, characterized in that: The crucible wall is sequentially constructed with a first layer and a second layer from the inside to the outside along the wall thickness direction, wherein the first layer is a transparent layer, the second layer is a bubble layer, and the protective layer is arranged outside the second layer; Preferably, the thickness of the second layer is greater than the thickness of the third layer and less than the thickness of the second layer.

5. A manufacturing process for manufacturing the quartz crucible according to any one of claims 1 to 4, characterized in that: Control the temperature and exhaust status during melting, and obtain the crucible wall and protective layer from the inside to the outside.

6. The manufacturing process according to claim 5, characterized in that: The thickness of the crucible wall and the protective layer is controlled by controlling the time of opening or closing the vacuum state.

7. The manufacturing process according to claim 5 or 6, characterized in that: When melting the protective layer, turn on the vacuum state and make its melting temperature lower than the melting temperature of the crucible wall.

8. The manufacturing process according to claim 7, characterized in that: During the melting process of the crucible wall, the melting temperature is constant, wherein the melting temperature is 1500-1600°C; Preferably, the initial value of the melting temperature of the protective layer is the melting temperature of the crucible wall, and the temperature starts to drop when the temperature reaches 1400-1500° C., and the melting ends when the temperature reaches room temperature.

9. The manufacturing process according to claim 8, characterized in that: During the melting process of the crucible wall, the vacuum state is first turned on and then turned off; Preferably, when melting the first layer, the vacuum state is turned on, and when melting the second layer, the vacuum state is turned off.

10. The manufacturing process according to any one of claims 5 to 9, characterized in that: In the melting crucible wall and the protective layer, the time used to open the exhaust state is different, and the time used from the inside to the outside decreases successively; Preferably, the time taken to start the exhaust state when melting the crucible wall is 0-500s, and the time taken to start the exhaust state when melting the protective layer is 0-100s.

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