Crucible and temperature field structure for growing crystals by Czochralski method

Abstract

The invention discloses a crucible and a temperature field structure for growing crystals by a Czochralski method, the crucible is a cylinder with an open upper end and a closed lower end, the crucible sequentially comprises a crystal growth area, a transition area and a heat preservation area from top to bottom, the transition area and the heat preservation area are used for accommodating a melt, and the transition area and the heat preservation area are communicated with the crystal growth area. The transition area and the heat preservation area are buried in a zirconium sand layer of the temperature field structure during crystal growth, the crystal growth area is used for crystal growth, the heat preservation area and the crystal growth area are both equal-diameter cylinders, the diameter of the heat preservation area is smaller than that of the crystal growth area, the transition area is connected between the heat preservation area and the crystal growth area, and the transition area is connected with the heat preservation area. The transition area is a conical barrel with the upper end large in diameter and the lower end small in diameter. According to the invention, the problem that the spontaneous crystallization phenomenon of a melt easily occurs at the bottom of a large-size crucible for growing large-size crystals in the prior art is solved, and stable growth of the large-size crystals is ensured.

Description

Technical Field

[0001] The 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 by the melt method, among which the melt method mainly includes the Czochralski method, the crucible descent method, the temperature gradient method, the laser heating susceptor method and the zone melting method. Most of these growth methods require the use of a crucible.

[0003] Taking the growth of lithium niobate crystals by the Czochralski method as an example, in order to obtain lithium niobate crystals with uniform composition and good structure, it is best to meet the following basic conditions: ① The mass of the grown crystal does not exceed 8% of the mass of the melt, and the molar fraction deviates from the same component by no more than ±1%. If the range of lithium oxide content in the melt is relaxed to no more than 8% of the molar fraction of the same component concentration, the mass of the grown crystal generally does not exceed 2% of the mass of the melt. ② The diameter of the grown crystal is much smaller than the diameter of the crucible. For a crucible with a diameter of 45mm-50 mm, the diameter of the grown crystal generally does not exceed 10mm-12 mm. In general, the ratio of the diameter of the grown crystal to the diameter of the crucible is in the range of 0.4-0.6, and the ratio of the liquid level of the melt in the crucible to the diameter of the crucible should be controlled at about 1. It can be seen that in order to obtain crystals with uniform composition, the utilization rate of the crucible and raw materials is extremely low, which undoubtedly increases the production cost.

[0004] In addition, for the crystal growth process under RF heating, the Joule heat value generated by the crucible is proportional to the square value of the electromagnetic field strength, and the crucible wall is the main heat source. Since the electromagnetic field strength decreases with the increase of the distance from the induction power supply, and due to the skin effect of the crucible wall on the magnetic field lines, the Joule heat generated at the bottom of the crucible decreases sharply with the increase of the distance from the crucible wall, that is, the closer to the crucible wall, the greater the heat, and the farther away from the crucible wall, the smaller the heat. This change is more obvious for large diameter crucibles.

[0005] Data simulation results show that the temperature of the bottom of the crucible 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 impact of the low temperature at the bottom of the crucible. However, for large-diameter crucibles for growing large-sized crystals, even higher thermal conductivity and stirring of the melt cannot compensate for the impact of the low temperature at the bottom of the crucible, which often results in a lower temperature of the melt at the center axis of the bottom of the crucible. Then, for melts with high density and poor thermal diffusivity, coupled with factors such as the failure to keep the bottom of the crucible warm, it is very likely that the temperature of the melt at the bottom of the crucible will be too cold, even below the melting point, resulting in spontaneous crystallization. These will destroy the stable growth of the crystal and fail to obtain crystals that meet the use requirements. Summary of the invention

[0006] The embodiment of the present invention provides a crucible and a temperature field structure for growing crystals by a Czochralski method, so as to solve the technical problem in the prior art that the melt at the bottom of the crucible may produce a spontaneous crystallization phenomenon, thereby destroying the stable growth of the crystal.

[0007] An embodiment of the present invention provides a crucible for growing crystals by a pulling method, wherein the crucible is a cylinder with an open upper end and a closed lower end, and the crucible includes, from top to bottom, a crystal growth zone, a transition zone, and a heat preservation zone, wherein 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 zirconium sand layer of a temperature field structure when growing crystals, and the crystal growth zone is used for crystal growth, and 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, and 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 diameter at the upper end and a small diameter at the lower end.

[0008] 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, the diameter of the crystal grown in the crucible is d, d≥160 mm, and the crystal grown in the crucible has an equal-diameter part, the length of the equal-diameter part is h, 2d≥D1≥1.5d, d≥D3≥0.4d, h≥H1≥0.6h, 0.5h≥H3≥0.3h, H3=1.5H2.

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

[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≥160mm, 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 zircon sand layer located at the top is greater than the sum of the heights of the insulation 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 the Czochralski crystal growth method to grow large-sized crystals with a diameter greater than 160 mm. The temperature of the melt at the bottom of the large-sized crucible is considered to be too cold, even below the melting point, resulting in spontaneous crystallization. By shortening the distance between the melt at the bottom of the crucible and the side wall of the crucible, and burying the bottom of the crucible in a zirconium sand layer, the heat preservation performance of the bottom of the crucible is enhanced, and the temperature at the center of the bottom of the crucible is effectively increased. The weak heat convection circulation and heat conduction capacity of the bottom of the crucible in the early stage of growth, and the phenomenon of spontaneous crystallization of the melt caused by the temperature of the center of the bottom of the crucible being lower than the melting point due to power reduction in the later stage are avoided, so that the crystal can grow stably. In addition, the crucible is overall in the shape of a "large top and small bottom" funnel, and the crystal growth zone ensures the smooth growth of the equal-diameter part of the crystal. At the same time, because the volume of the bottom of the crucible is reduced compared with the prior art, that is, the volume of the melt in the insulation zone and the transition zone is relatively reduced, the input of growth raw materials is saved, the utilization rate of raw materials is improved, and the growth cost is reduced.

[0018] Secondly, the present invention adopts segmented heating coils to heat different areas, which can ensure the smooth progress of the growth process. For sudden emergency situations 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, ensure the stable growth of large-sized crystals under controllable conditions as much as possible, reduce the possibility of spontaneous crystallization at the bottom center of the crucible during the growth process, reduce the amount of raw material input, reduce production costs, and improve 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 accompanying drawings required for use in the description of the embodiments of the present invention will be briefly introduced below. Obviously, the accompanying drawings in the following description are only some embodiments of the present invention. For ordinary technicians in this field, other accompanying drawings can be obtained based on these accompanying drawings without paying creative labor.

[0021] Figure 1 It is a structural schematic diagram of an embodiment of a temperature field structure for growing crystals by a Czochralski method according to the present invention; In the figure: 1. crucible; 2. crystal; 3. corundum layer; 4. side insulation cotton layer; 5. quartz layer; 6. zircon 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

[0022] 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 in conjunction with 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 used to limit the present invention.

[0023] 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" and the like indicate positions or positional relationships based on the positions or positional relationships shown in the drawings, and are only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as limiting the present invention. In the description of the present invention, unless otherwise specified, "plurality" means two or more.

[0024] In the description of the present invention, it should be noted that, unless otherwise clearly specified and limited, the terms "installed", "connected", and "connected" should be understood in a broad sense, for example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection, or it can be indirectly connected through an intermediate medium, or it can be the internal communication of two components. For ordinary technicians in this field, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

[0025] 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 in 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.

[0026] The temperature field structure in the present invention is used in the crystal growth method by 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 the crystal 2. In the present embodiment, the zircon sand layer 6 is a zirconium dioxide particle layer, and the thickness of the zircon sand layer 6 located at the topmost layer 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 the growth of crystal 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 heat preservation zone contain the melt. 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, and the transition zone is a conical cylinder with a large diameter at the upper end and a small diameter at the lower end.

[0027] 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 from 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. 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.

[0028] In this embodiment, taking the growth of lithium niobate crystal as an example, the parameters of each part of the crucible 1 meet the following conditions: 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 needs of lithium niobate crystals with a diameter of d and a height of h at the equal diameter part. Of course, for other embodiments, reasonable parameters can be set according to the physical properties and growth habits of different crystals 2.

[0029] In this embodiment, the periphery of the side cylinder is provided with a first heating coil 10 and a second heating coil 11 in sequence 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.

[0030] In this embodiment, the first heating coil 10 and the second heating coil 11 are connected to different controllers respectively. 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 control the heating temperatures of the first heating coil 10 and the second heating coil 11 respectively.

[0031] In this embodiment, the crystal growth zone is connected to a first thermocouple 9, the bottom of the insulation zone is connected to a second thermocouple 8, the first thermocouple 9 is connected to a first controller 12, 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 according to 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 according to the temperature of the crystal growth zone monitored by the second thermocouple 8. The segmented heating coil heats different areas to ensure the smooth progress of the growth process. For sudden emergencies such as "interface inversion" and "frozen pot", the problem can be solved by increasing the power of the coil, while minimizing the impact on the grown crystal 2 as much as possible, thereby improving the success rate of crystal 2 growth.

[0032] 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, and the diameter of the equal diameter part 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, and the transition zone is a conical cylinder with a large diameter at the upper end and a small diameter at the lower end.

[0033] 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, H3 = 1.5H2.

[0034] In this embodiment, taking the growth of lithium niobate crystal as an example, the parameters of each part of the crucible 1 meet the following conditions: 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 needs of lithium niobate crystals with a diameter of d and a height of h at the equal diameter part. Of course, for other embodiments, reasonable parameters can be set according to the physical properties and growth habits of different crystals 2.

[0035] 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 the technical solutions described in the aforementioned embodiments may still be modified, or some of the technical features may be replaced by equivalents. Such modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions of the embodiments of the present invention, and should all be included in the protection scope of the present invention.

Claims

1. A crucible for growing crystals by the Czochralski method, It is characterized in that The crucible (1) is a cylinder with an open upper end and a closed lower end. The crucible (1) comprises, from top to bottom, a crystal growth zone, a transition zone and a heat preservation zone. The transition zone and the heat preservation zone are used to contain a melt. When growing crystals, the transition zone and the heat preservation zone are buried in a zirconium sand layer (6) of a temperature field structure. The crystal growth zone is used to grow crystals (2). 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. 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, It is 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 preservation structure and a crucible (1) located in the heat preservation structure, It is characterized in that The heat-insulating 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-insulating cotton layer (7) and a zircon sand layer (6), the uppermost layer being the zircon sand layer (6). 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-insulating zone from top to bottom. The transition zone and the heat-insulating zone are used to accommodate a melt. When growing a crystal, the transition zone and the heat-insulating zone are buried in the uppermost zircon sand layer (6) of the cylinder bottom. The crystal growth zone is used to grow a crystal (2). The heat-insulating zone and the crystal growth zone are both equal-diameter cylinders. The diameter of the heat-insulating zone is smaller than the diameter of the crystal growth zone. The transition zone is connected between the heat-insulating 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, It is 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, It is characterized in that A first heating coil and a second heating coil (11) are arranged in sequence from top to bottom on the periphery of the side cylinder, 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, It is 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, It is characterized in that The crystal growth zone is connected to a first thermocouple, the bottom of the heat preservation zone is connected to a second thermocouple (8), the first thermocouple is connected to a first controller (12), 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, It is characterized in that The side cylinder is a multi-layer structure, and 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, It is 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, It is 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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