A crucible and temperature field structure for growing crystals by the Czochralski method

Abstract

The application discloses a crucible and a temperature field structure for growing a crystal by a pulling method, the crucible is a cylinder with an open upper end and a closed lower end, the crucible comprises a crystal growth zone, a transition zone and a heat preservation zone from top to bottom, the transition zone and the heat preservation zone are used for containing a melt, and the transition zone and the heat preservation zone are buried in a zirconium sand layer of the temperature field structure when the crystal is grown, the crystal growth zone is used for growing the crystal, the heat preservation zone and the crystal growth zone are equal-diameter cylinders, the diameter of the heat preservation zone is smaller than that of the crystal growth zone, and the transition zone is connected between the heat preservation zone and the crystal growth zone and is a conical cylinder with a large upper end diameter and a small lower end diameter. The application solves the problem that the bottom of a large-size crucible for growing a large-size crystal is prone to spontaneous crystallization of the melt in the prior art, and ensures stable growth of the large-size crystal.

Description

Technical Field

[0001] The present invention relates to the technical field of crystal growth, and in particular to a crucible and a temperature field structure for growing crystals by a Czochralski method. Background Art

[0002] Currently, most oxide crystals are grown through the melt method, among which the melt method mainly includes the pulling method, crucible descent method, temperature gradient method, laser heating base method and zone melting method. Most of these growth methods require the use of a crucible.

[0003] Taking the Czochralski method for growing lithium niobate crystals as an example, to obtain lithium niobate crystals with uniform composition and good structure, the following basic conditions are ideal: ① The mass of the grown crystal should not exceed 8% of the melt mass, and the mole fraction should not deviate from the same composition by more than ±1%. If the lithium oxide content in the melt is relaxed to no more than 8% of the mole fraction of the same composition, the mass of the grown crystal generally does not exceed 2% of the melt mass. ② The diameter of the grown crystal should be significantly smaller than the diameter of the crucible. For a crucible with a diameter of 45mm-50mm, the diameter of the grown crystal generally does not exceed 10mm-12mm. Generally, the ratio of the grown crystal diameter to the crucible diameter ranges from 0.4-0.6, and the ratio of the melt level in the crucible to the crucible diameter should be controlled at approximately 1. Therefore, in order to obtain crystals with uniform composition, the utilization rate of the crucible and raw materials is extremely low, which undoubtedly increases production costs.

[0004] Furthermore, during crystal growth using RF heating, the Joule heating generated by the crucible is proportional to the square of the electromagnetic field intensity, with the crucible wall being the primary heat source. Because the electromagnetic field intensity decreases with increasing distance from the induction power source, and because of the skin effect of the crucible wall on the magnetic field lines, the Joule heating generated at the crucible bottom decreases dramatically with increasing distance from the crucible wall. Specifically, the closer to the crucible wall, the greater the heat, while the farther away from the crucible wall, the less heat. This change is more pronounced for crucibles with larger diameters.

[0005] Simulation results indicate that the temperature at the crucible's bottom, located at the center axis, is relatively low. For small-diameter crucibles, their higher thermal conductivity and more thorough stirring of the melt can compensate for the low bottom temperature. However, for large-diameter crucibles used to grow large crystals, even higher thermal conductivity and more thorough stirring cannot compensate for the low bottom temperature, often resulting in a lower melt temperature at the crucible's center axis. For melts with high density and poor thermal diffusivity, coupled with inadequate insulation at the crucible bottom, the melt at the crucible bottom is likely to overcool, even below the melting point, leading to spontaneous crystallization. This can disrupt stable crystal growth and prevent the production of crystals that meet operational requirements. SUMMARY

[0006] The embodiment of the present application provides a crucible for growing crystal by the Czochralski method and a temperature field structure, so as to solve the technical problem that the melt at the bottom of the crucible will generate spontaneous crystallization phenomenon, which destroys the stable growth of the crystal.

[0007] The embodiment of the present application provides a crucible for growing crystal by the Czochralski method, the crucible is a cylinder with an open upper end and a closed lower end, the crucible sequentially comprises a crystal growth zone, a transition zone and a heat preservation zone from top to bottom, the transition zone and the heat preservation zone are used for containing the melt, and the transition zone and the heat preservation zone are buried in the zirconium sand layer of the temperature field structure when growing the crystal, the crystal growth zone is used for growing the crystal, the heat preservation zone and the crystal growth zone are both equal-diameter cylinders, the diameter of the heat preservation zone is smaller than that of the crystal growth zone, the transition zone is connected between the heat preservation zone and the crystal growth zone, and the transition zone is a conical cylinder with a large upper end diameter and a small lower end diameter.

[0008] Further, the height of the crystal growth zone is H1, the diameter is D1, the height of the transition zone is H2, the height of the heat preservation zone is H3, and the diameter is D3, the diameter of the crystal grown by the crucible is d, d is greater than or equal to 160 mm, the crystal grown by the crucible has an equal-diameter part, the length of the equal-diameter part is h, 2d is greater than or equal to D1 and is greater than or equal to 1.5d, d is greater than or equal to D3 and is greater than or equal to 0.4d, h is greater than or equal to H1 and is greater than or equal to 0.6h, 0.5h is greater than or equal to H3 and is greater than or equal to 0.3h, and H3 is equal to 1.5H2.

[0009] A temperature field structure for growing crystal by the Czochralski method, comprising a heat preservation structure and a crucible located in the heat preservation structure, the heat preservation structure comprises a side cylinder and a cylinder bottom connected to the lower end of the side cylinder, the cylinder bottom is a multi-layer structure comprising a bottom heat preservation cotton layer and a zirconium sand layer, and the zirconium sand layer is located at the uppermost layer, the crucible is a cylinder with an open upper end and a closed lower end, the crucible sequentially comprises a crystal growth zone, a transition zone and a heat preservation zone from top to bottom, the transition zone and the heat preservation zone are used for containing the melt, and the transition zone and the heat preservation zone are buried in the uppermost zirconium sand layer of the cylinder bottom when growing the crystal, the crystal growth zone is used for growing the crystal, the heat preservation zone and the crystal growth zone are both equal-diameter cylinders, the diameter of the heat preservation zone is smaller than that of the crystal growth zone, the transition zone is connected between the heat preservation zone and the crystal growth zone, and the transition zone is a conical cylinder with a large upper end diameter and a small lower end diameter.

[0010] Furthermore, the height of the crystal growth zone is H1 and the diameter is D1, the height of the transition zone is H2, the height of the insulation zone is H3 and the diameter is D3, and the crystal grown in the crucible has an equal-diameter portion, the diameter of the equal-diameter portion is d, d ≥ 160 mm, and the length of the equal-diameter portion is h, 2d ≥ D1 ≥ 1.5d, d ≥ D3 ≥ 0.4d, h ≥ H1 ≥ 0.6h, 0.5h ≥ H3 ≥ 0.3h, H3 = 1.5H2.

[0011] Furthermore, a first heating coil and a second heating coil are sequentially provided on the periphery of the side cylinder from top to bottom, the first heating coil corresponds to the position of the crystal growth zone, and the second heating coil corresponds to the positions of the transition zone and the insulation zone.

[0012] Furthermore, the first heating coil and the second heating coil are connected to different controllers respectively. The first heating coil is connected to a first controller, and the second heating coil is connected to a second controller. The first controller and the second controller respectively control the heating temperatures of the first heating coil and the second heating coil.

[0013] Furthermore, the crystal growth zone is connected to a first thermocouple, and the bottom of the insulation zone is connected to a second thermocouple. The first thermocouple is connected to a first controller, and the second thermocouple is connected to a second controller. The first controller controls the heating temperature of the first heating coil according to the temperature of the crystal growth zone monitored by the first thermocouple, and the second controller controls the heating temperature of the second heating coil according to the temperature of the crystal growth zone monitored by the second thermocouple.

[0014] Furthermore, the side cylinder is a multi-layer structure, which includes a quartz layer, a side insulation cotton layer and a corundum layer from the outside to the inside.

[0015] Furthermore, the bottom of the cylinder includes two layers of bottom insulation cotton layers and zircon sand layers, the two layers of bottom insulation cotton layers and the two layers of zircon sand layers are alternately arranged, one of the two layers of bottom insulation cotton layers is located at the bottom layer, and one of the two layers of zircon sand layers is located at the top layer.

[0016] Furthermore, the zircon sand layer is a zirconium dioxide particle layer, and the thickness of the uppermost zircon sand layer is greater than the sum of the heights of the heat preservation zone and the transition zone of the crucible.

[0017] The beneficial effects of the present invention are as follows: the crucible of the present invention is used in a Czochralski crystal growth method to grow large-sized crystals with a diameter greater than 160 mm. Taking into account the phenomenon that the temperature of the melt at the bottom of the large-sized crucible is too cold, even below the melting point, and spontaneous crystallization occurs, the distance between the melt at the bottom of the crucible and the side wall of the crucible is shortened, and the bottom of the crucible is buried in a layer of zircon sand, thereby enhancing the thermal insulation performance of the bottom of the crucible and effectively raising the temperature at the center of the bottom of the crucible. This avoids the phenomenon that the heat convection circulation and heat conduction capacity of the bottom of the crucible are weak in the early stage of growth, and the spontaneous crystallization of the melt caused by the temperature of the center of the crucible being below the melting point due to power reduction in the later stage, thereby enabling stable crystal growth. In addition, the crucible is generally funnel-shaped, with a "large top and small bottom" shape. The crystal growth zone ensures the smooth growth of the equal-diameter portion of the crystal. At the same time, because the volume of the crucible bottom is reduced compared to the prior art, that is, the volume of the melt in the insulation zone and transition zone is relatively reduced, the input of growth raw materials is saved, the raw material utilization rate is improved, and the growth cost is reduced.

[0018] Secondly, the present invention uses segmented heating coils to heat different areas, which can ensure the smooth progress of the growth process. For sudden emergencies such as "interface inversion" and "frozen pot", they can be solved by increasing the power of the coil, while minimizing the impact on the grown crystals, thereby improving the success rate of crystal growth.

[0019] In addition, the present invention can formulate the structural parameters of the new crucible according to the characteristics of different crystal growth, thereby ensuring the stable growth of large-sized crystals under controllable conditions as much as possible, reducing the possibility of spontaneous crystallization at the bottom center of the crucible during the growth process, reducing the amount of raw material input, reducing production costs, and improving the yield rate. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the following briefly introduces the drawings required for use in the description of the embodiments of the present invention. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative labor.

[0021] Figure 1 This is a schematic structural diagram of an embodiment of a temperature field structure for growing crystals using the Czochralski method according to the present invention;

[0022] In the figure: 1. Crucible; 2. Crystal; 3. Corundum layer; 4. Side insulation cotton layer; 5. Quartz layer; 6. Zirconium sand layer; 7. Bottom insulation cotton layer; 8. Second thermocouple; 9. First thermocouple; 10. First heating coil; 11. Second heating coil; 12. First controller; 13. Second controller. DETAILED DESCRIPTION

[0023] In order to make the technical problems, technical solutions and beneficial effects solved by the present invention more clearly understood, the present invention is further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention.

[0024] In the description of the present invention, it should be understood that the terms "longitudinal", "radial", "length", "width", "thickness", "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inside", "outside", etc., indicating orientations or positional relationships, are based on the orientations or positional relationships shown in the accompanying drawings and are intended only to facilitate the description of the present invention and simplify the description. They do not indicate or imply that the devices or elements referred to must have a specific orientation, be constructed and operate in a specific orientation, and therefore should not be understood as limiting the present invention. In the description of the present invention, unless otherwise specified, "plurality" means two or more.

[0025] In the description of the present invention, it should be noted that, unless otherwise expressly specified or limited, the terms "mounted," "connected," and "connected" should be understood in a broad sense. For example, they may refer to fixed, detachable, or integral connections; mechanical or electrical connections; direct or indirect connections through an intermediate medium; and internal communication between two components. Those skilled in the art will understand the specific meanings of the above terms in the present invention based on the specific circumstances.

[0026] An embodiment of a temperature field structure for growing crystals by a Czochralski method according to the present invention is as follows: Figure 1 As shown, the temperature field structure includes an external insulation structure and a crucible 1 located inside the insulation structure, and the insulation structure includes a side cylinder and a cylinder bottom connected to the lower end of the side cylinder. The side cylinder and the cylinder bottom are both multi-layer structures, wherein the side cylinder includes a quartz layer 5, a side insulation cotton layer 4 and a corundum layer 3 from the outside to the inside. The cylinder bottom includes a bottom insulation cotton layer 7 and a zircon sand layer 6, and the zircon sand layer 6 is located at the top. In this embodiment, the cylinder bottom includes two layers of the bottom insulation cotton layer 7 and the zircon sand layer 6, and the two layers of bottom insulation cotton layers 7 and the two layers of zircon sand layers 6 are alternately arranged, one of the two layers of bottom insulation cotton layers 7 is located at the bottom layer, and one of the two layers of zircon sand layers 6 is located at the top layer.

[0027] The temperature field structure in the present invention is used in the crystal growth method of the pulling method. The crystal 2 grown by the crucible 1 has an equal-diameter portion, and the diameter of the equal-diameter portion is greater than 160 mm. The crucible 1 is a cylinder with an open upper end and a closed lower end. The crucible 1 includes a crystal growth zone, a transition zone and a heat preservation zone from top to bottom. The transition zone and the heat preservation zone are used to accommodate the melt, and the transition zone and the heat preservation zone are buried in the topmost zircon sand layer 6 at the bottom of the cylinder when growing crystals 2. In this embodiment, the zircon sand layer 6 is a zirconium dioxide particle layer, and the thickness of the zircon sand layer 6 located at the top is greater than the sum of the heights of the heat preservation zone and the transition zone of the crucible 1. The crystal growth zone is used for growing crystals 2, and the equal-diameter portion of the grown crystal 2 is located in the crystal growth zone, such as Figure 1 As shown, the transition zone and the holding zone contain the melt. The holding zone and the crystal growth zone are both cylindrical bodies of equal diameter, and the diameter of the holding zone is smaller than that of the crystal growth zone. The transition zone is connected between the holding zone and the crystal growth zone and is a conical cylinder with a larger diameter at the upper end and a smaller diameter at the lower end.

[0028] In this embodiment, the height of the crystal growth zone is H1 and the diameter is D1. The height of the transition zone is H2, and the height of the insulation zone is H3 and the diameter is D3. The crystal 2 grown in the crucible 1 has an equal diameter portion, the diameter of the equal diameter portion is d, d ≥ 160 mm, and the length of the equal diameter portion is h, 2d ≥ D1 ≥ 1.5d, d ≥ D3 ≥ 0.4d, h ≥ H1 ≥ 0.6h, 0.5h ≥ H3 ≥ 0.3h, and H3 = 1.5H2. It should be noted here that the height H1 of the crystal growth zone is the distance from the lower end to the upper end of the crystal growth zone, the height H2 of the transition zone is the distance from the lower end to the upper end of the transition zone, and the height H3 of the insulation zone is the distance from the lower end to the upper end of the insulation zone. The diameters of the crystal growth zone, transition zone, and insulation zone of the crucible 1 mentioned here are all inner diameters. The crucible 1 is a platinum crucible 1, and the thickness of each part of the crucible 1 is the same.

[0029] In this embodiment, using the growth of lithium niobate crystals as an example, the parameters of the various components of the crucible 1 meet the following requirements: D1 = 1.5 d, D2 = 0.4 d, H1 = 0.6 h, H3 = 1.5H2 = 0.3 h. Under these parameters, the raw material contained in the crystal growth zone of the crucible 1 can meet the requirements of lithium niobate crystals with a diameter of d and a height of h at the equal diameter portion. Of course, for other embodiments, appropriate parameters can be set based on the physical properties and growth habits of different crystals 2.

[0030] In this embodiment, the periphery of the side cylinder is provided with a first heating coil 10 and a second heating coil 11 from top to bottom. The first heating coil 10 corresponds to the position of the crystal growth zone, and the second heating coil 11 corresponds to the position of the transition zone and the insulation zone.

[0031] In this embodiment, the first heating coil 10 and the second heating coil 11 are respectively connected to different controllers. The first heating coil 10 is connected to a first controller 12, and the second heating coil 11 is connected to a second controller 13. The first controller 12 and the second controller 13 respectively control the heating temperatures of the first heating coil 10 and the second heating coil 11.

[0032] In this embodiment, the crystal growth zone is connected to a first thermocouple 9, and the bottom of the insulation zone is connected to a second thermocouple 8. The first thermocouple 9 is connected to a first controller 12, and the second thermocouple 8 is connected to a second controller 13. The first controller 12 controls the heating temperature of the first heating coil 10 based on the temperature of the crystal growth zone monitored by the first thermocouple 9, and the second controller 13 controls the heating temperature of the second heating coil 11 based on the temperature of the crystal growth zone monitored by the second thermocouple 8. The segmented heating coils heat different areas to ensure the smooth progress of the growth process. Sudden emergencies such as "interface inversion" and "frozen pot" can be resolved by increasing the power of the coil, while minimizing the impact on the grown crystal 2, thereby improving the success rate of crystal 2 growth.

[0033] An embodiment of a crucible for growing crystals by the Czochralski method of the present invention is the same as the crucible in the temperature field structure for growing crystals by the Czochralski method described above. The diameter of the equal diameter portion of the crystal grown by the crucible is greater than 160 mm. Figure 1 As shown, the crucible 1 is a cylinder with an open upper end and a closed lower end. The crucible 1 includes a crystal growth zone, a transition zone and a heat preservation zone from top to bottom. The transition zone and the heat preservation zone are used to accommodate the melt, and the transition zone and the heat preservation zone are buried in the zircon sand layer 6 of the temperature field structure when growing crystals 2. The crystal growth zone is used to grow crystals 2. The heat preservation zone and the crystal growth zone are both equal-diameter cylinders, and the diameter of the heat preservation zone is smaller than the diameter of the crystal growth zone. The transition zone is connected between the heat preservation zone and the crystal growth zone. The transition zone is a conical cylinder with a large diameter at the upper end and a small diameter at the lower end.

[0034] In this embodiment, the height of the crystal growth zone is H1 and the diameter is D1, the height of the transition zone is H2, the height of the insulation zone is H3 and the diameter is D3, the diameter of the crystal 2 grown in the crucible 1 is d, d ≥ 160 mm, and the crystal 2 grown in the crucible 1 has an equal-diameter portion, the length of the equal-diameter portion is h, 2d ≥ D1 ≥ 1.5d, d ≥ D3 ≥ 0.4d, h ≥ H1 ≥ 0.6h, 0.5h ≥ H3 ≥ 0.3h, and H3 = 1.5H2.

[0035] In this embodiment, using the growth of lithium niobate crystals as an example, the parameters of the various components of the crucible 1 meet the following requirements: D1 = 1.5 d, D2 = 0.4 d, H1 = 0.6 h, H3 = 1.5H2 = 0.3 h. Under these parameters, the raw material contained in the crystal growth zone of the crucible 1 can meet the requirements of lithium niobate crystals with a diameter of d and a height of h at the equal diameter portion. Of course, for other embodiments, appropriate parameters can be set based on the physical properties and growth habits of different crystals 2.

[0036] The embodiments described above are only used to illustrate the technical solutions of the present invention, rather than to limit the same. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or make equivalent replacements for some of the technical features therein. These modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions of the various embodiments of the present invention, and should all be included in the scope of protection of the present invention.

Claims

1. A crucible for growing crystals by the Czochralski method, characterized in that: The crucible (1) is a cylinder with an open upper end and a closed lower end. The crucible (1) includes a crystal growth zone, a transition zone and a heat preservation zone from top to bottom. The transition zone and the heat preservation zone are used to accommodate a melt, and the transition zone and the heat preservation zone are buried in a zircon sand layer (6) of a temperature field structure when growing crystals. The crystal growth zone is used to grow crystals (2). The heat preservation zone and the crystal growth zone are both equal-diameter cylinders, and the diameter of the heat preservation zone is smaller than the diameter of the crystal growth zone. The transition zone is connected between the heat preservation zone and the crystal growth zone. The transition zone is a conical cylinder with a large diameter at the upper end and a small diameter at the lower end.

2. The crucible according to claim 1, characterized in that The height of the crystal growth zone is H1 and the diameter is D1, the height of the transition zone is H2, the height of the insulation zone is H3 and the diameter is D3, the diameter of the crystal (2) grown by the crucible (1) is d, d≥160 mm, the crystal (2) grown by the crucible (1) has an equal diameter portion, the length of the equal diameter portion is h, 2d≥D1≥1.5d, d≥D3≥0.4d, h≥H1≥0.6h, 0.5h≥H3≥0.3h, H3=1.5H2.

3. A temperature field structure for growing crystals by a Czochralski method, comprising a heat-insulating structure and a crucible (1) located within the heat-insulating structure, characterized in that: The heat preservation structure comprises a side cylinder and a cylinder bottom connected to the lower end of the side cylinder. The cylinder bottom is a multi-layer structure, comprising a bottom heat preservation cotton layer (7) and a zircon sand layer (6). The zircon sand layer (6) is located at the top. The crucible (1) is a cylinder with an open upper end and a closed lower end. The crucible (1) comprises a crystal growth zone, a transition zone and a heat preservation zone from top to bottom. The transition zone and the heat preservation zone are used to accommodate the melt, and the transition zone and the heat preservation zone are buried in the top zircon sand layer (6) of the cylinder bottom when growing crystals. The crystal growth zone is used to grow crystals (2). The heat preservation zone and the crystal growth zone are both equal-diameter cylinders, and the diameter of the heat preservation zone is smaller than the diameter of the crystal growth zone. The transition zone is connected between the heat preservation zone and the crystal growth zone. The transition zone is a conical cylinder with a large diameter at the upper end and a small diameter at the lower end.

4. The temperature field structure according to claim 3, characterized in that: The height of the crystal growth zone is H1 and the diameter is D1, the height of the transition zone is H2, the height of the insulation zone is H3 and the diameter is D3, the crystal (2) grown by the crucible (1) has an equal diameter portion, the diameter of the equal diameter portion is d, d≥160 mm, the length of the equal diameter portion is h, 2d≥D1≥1.5d, d≥D3≥0.4d, h≥H1≥0.6h, 0.5h≥H3≥0.3h, H3=1.5H2.

5. The temperature field structure according to claim 3 or 4, characterized in that: A first heating coil and a second heating coil (11) are sequentially provided on the periphery of the side cylinder from top to bottom, the first heating coil corresponds to the position of the crystal growth zone, and the second heating coil (11) corresponds to the positions of the transition zone and the heat preservation zone.

6. The temperature field structure according to claim 5, characterized in that: The first heating coil and the second heating coil (11) are respectively connected to different controllers, the first heating coil is connected to a first controller (12), and the second heating coil (11) is connected to a second controller (13), and the first controller (12) and the second controller (13) respectively control the heating temperatures of the first heating coil and the second heating coil (11).

7. The temperature field structure according to claim 6, characterized in that: The crystal growth zone is connected to a first thermocouple, and the bottom of the insulation zone is connected to a second thermocouple (8). The first thermocouple is connected to a first controller (12), and the second thermocouple (8) is connected to a second controller (13). The first controller (12) controls the heating temperature of the first heating coil according to the temperature of the crystal growth zone monitored by the first thermocouple, and the second controller (13) controls the heating temperature of the second heating coil (11) according to the temperature of the crystal growth zone monitored by the second thermocouple (8).

8. The temperature field structure according to claim 3 or 4, characterized in that: The side cylinder is a multi-layer structure, which comprises, from the outside to the inside, a quartz layer (5), a side heat-insulating cotton layer, and a corundum layer (3).

9. The temperature field structure according to claim 3 or 4, characterized in that: The cylinder bottom comprises two layers of bottom thermal insulation cotton layers (7) and zircon sand layers (6), the two layers of bottom thermal insulation cotton layers (7) and the two layers of zircon sand layers (6) being arranged alternately, one of the two layers of bottom thermal insulation cotton layers (7) being located at the bottom layer, and one of the two layers of zircon sand layers (6) being located at the top layer.

10. The temperature field structure according to claim 9, characterized in that: The zircon sand layer (6) is a zirconium dioxide particle layer, and the thickness of the zircon sand layer (6) located at the top is greater than the sum of the heights of the heat preservation zone and the transition zone of the crucible (1).

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