High quality magnesium oxide crystal and method for growing the same

Abstract

The application relates to high-quality magnesium oxide crystals and a growth method thereof, which adopts high-purity magnesium oxide polycrystal particles as raw materials; the raw materials are densely packed in a cold crucible furnace chamber with a certain thickness; on this basis, ignition medium is laid and a seeding crystal is pre-embedded; high-frequency induction heating is used to firstly form an arc through the ignition medium to form a molten pool; after all the raw materials are put in, the raw materials are completely melted by being kept at 2900-3300 DEG C for 5-10 hours to form a stable melt; the cold crucible furnace body is slowly lowered or the high-frequency induction coil is slowly raised, and the crystal growth is completed after 3-5 days; after natural cooling to room temperature, the crystal ingot is taken out from the furnace body to obtain high-quality magnesium oxide single crystals. Compared with the prior art, the overgrowth melting method can grow high-quality magnesium oxide single crystals with a purity of more than 4N and a size of more than 20 mm, the process is relatively simple and efficient, mass industrial production can be realized, and the method has high practical value.

Description

Technical Field

[0001] This invention relates to the field of crystal growth technology, and in particular to a high-quality magnesium oxide crystal and its growth method. Background Technology

[0002] Magnesium oxide (MgO) single crystals, as typical ionic crystals, possess high thermal conductivity, excellent optical properties, and good chemical stability, showing broad application prospects in fields such as high-temperature superconducting substrates, optical window materials, aerospace, medical devices, and plasma displays. In recent years, with the rapid development of communication technologies and related high-tech fields, the demand for high-quality, large-size magnesium oxide single crystals has been increasing. However, due to the extremely high melting point of magnesium oxide (2852℃) and the stringent requirements for crystal purity, the preparation of high-quality single crystals faces numerous technical and process challenges.

[0003] Currently, the most common method for preparing magnesium oxide single crystals is the arc melting method. This method typically uses carbon electrodes for heating, which can quickly reach the melting temperature. However, because the electrodes are in direct contact with the raw material, carbon impurities are easily introduced, affecting the final purity of the crystal. Currently, the countries capable of producing and applying magnesium oxide single crystals are mainly developed countries. These countries started researching magnesium oxide single crystals earlier, have accumulated extensive technical expertise, and possess the capability to produce large-sized magnesium oxide single crystals. However, their raw materials often use seawater magnesia, which not only affects the purity of the crystal but also incurs high pre-treatment costs, further limiting its economic viability and application expansion.

[0004] Shell melting, a crucible-free growth technique, operates on a principle similar to arc melting, but employs high-frequency induction heating, effectively avoiding the introduction of impurities from the electrodes or crucible. However, due to the extremely low conductivity of magnesium oxide at room temperature, it cannot be directly heated by high-frequency induction. Therefore, an ignition medium is necessary to initiate the heating process in the early stages of growth. The design of the ignition stage and the selection of the medium become key technical challenges in achieving stable molten pool formation and subsequent crystal growth. A stable molten pool must be formed within the furnace for the ignition stage to be considered successful. However, in existing technologies, using 2-3 large graphite electrodes in direct contact with the raw material, generating electric sparks or arcs between the electrodes for heating, causes the raw material to reach a locally high-temperature melting state instantaneously. While this results in rapid heating, the thermal field is difficult to control stably, leading to uneven temperature distribution. Furthermore, it is highly sensitive to the size of the ignition electrodes and the discharge gap during actual operation, exhibiting considerable instability. Simultaneously, the direct contact between the electrodes and the raw material increases the risk of impurity introduction, thus affecting crystal purity.

[0005] Furthermore, a Korean patent (KR100432907B1) proposes a similar high-frequency induction method for MgO crystal growth. While it includes a preliminary design of the crucible size and heating frequency based on the electrical conductivity characteristics of magnesium oxide, it does not disclose a clearly feasible ignition method, nor does it provide a complete crystal growth implementation plan or evidence of the quality of the resulting crystals. To date, commercially available MgO single crystals still primarily rely on arc melting for spontaneous nucleation growth. Because multiple nuclei grow simultaneously and compete with each other during nucleation, defects easily appear inside the crystal, making it difficult to meet the requirements of high-end applications such as high-temperature superconducting substrates in terms of crystal quality and size.

[0006] In summary, in order to overcome the bottlenecks in purity, size and crystal quality of existing magnesium oxide single crystal preparation technology, it is urgent to develop a method that can achieve stable growth of high-quality, large-size magnesium oxide single crystals. Summary of the Invention

[0007] The purpose of this invention is to provide a high-quality magnesium oxide crystal and its growth method. The magnesium oxide crystal prepared by this method has high purity, large size, and high crystal quality.

[0008] The objective of this invention can be achieved through the following technical solutions: This invention provides a method for growing high-quality magnesium oxide crystals, comprising the following steps: S1: Mix high-purity magnesium oxide polycrystalline raw materials of different particle sizes evenly according to the formula to obtain mixed polycrystalline material; S2: A portion of the mixed polycrystalline material is filled into the cold crucible furnace to form a densely packed raw material layer; S3: An ignition medium is laid in the central region inside the raw material layer, and a seed crystal is pre-embedded at the bottom of the cold crucible furnace. S4: High-frequency induction heating is used to heat the ignition medium and cause the surrounding mixed polycrystalline material to melt, gradually forming a molten pool; as the molten pool gradually expands, the remaining mixed polycrystalline material is added to the upper part of the raw material layer in small amounts and multiple times. S5: During the formation of the molten pool, the mixed polycrystalline material close to the inner wall of the cold crucible furnace forms a molten shell layer due to the cooling effect of the cold crucible. After all the mixed polycrystalline material except the molten shell layer has melted, heat preservation treatment is carried out to stabilize the molten pool. S6: Slowly lower the cold crucible furnace body or raise the high-frequency induction coil to allow the magnesium oxide crystal to grow directionally on the seed crystal. After the crystal growth is completed, stop heating and allow it to cool naturally to room temperature. S7: Peel off the fusion shell layer to obtain the high-quality magnesium oxide crystals.

[0009] Preferably, in step S1, the purity of the high-purity magnesium oxide polycrystalline raw material is ≥99.9%, and the mixed polycrystalline material is made by mixing two kinds of high-purity magnesium oxide polycrystalline raw materials with particle sizes of 1-3 mm and 5-8 mm in a mass ratio of 3:7-4:6.

[0010] More preferably, in step S1, the magnesium oxide polycrystalline raw material with a purity higher than 99.9% is pre-sieved to obtain polycrystalline raw materials with different particle size distributions of 1~3mm and 5~8mm.

[0011] In this invention, the magnesium oxide raw material is a polycrystalline particle with a purity of ≥99.9%, which avoids the formation of a large amount of powder suspension during ignition and feeding.

[0012] The particle size of MgO raw materials is one of the key issues in growing high-quality MgO single crystals. The particle size of magnesium oxide raw materials directly affects the formation of the molten pool. In particular, if the particle size is too small, the powder is prone to suspension under high-temperature heat flow, resulting in porosity between the powder and the melt, which can easily lead to ignition failure and affect the stability of the molten pool. Therefore, this invention adopts a raw material pretreatment mixing method of dense packing of multiple particle sizes, which can improve the contact between grains, improve heat transfer efficiency, and is more conducive to the stable formation of the molten pool.

[0013] Preferably, in step S2, the furnace chamber of the cold crucible furnace has a diameter of 300mm-600mm and a height of 300mm-500mm, and the specific dimensions can be adjusted according to actual needs.

[0014] Preferably, in step S2, the thickness of the raw material layer is greater than 60 mm, and more preferably 60 mm to 80 mm.

[0015] More preferably, in step S2, the portion of the mixed polycrystalline material used to form the raw material layer accounts for 12 wt% to 15 wt% of all the mixed polycrystalline materials in step S1.

[0016] Preferably, in step S3, the ignition medium is a conductive material.

[0017] Preferably, in step S3, the ignition medium is laid out in a ring, with the ends connected one after the other.

[0018] More preferably, in step S3, the ignition medium is either a graphite electrode or a magnesium strip.

[0019] More preferably, in step S3, the ignition medium is coated with a layer of magnesium oxide protective layer, which can meet the requirements of long-term heating without being consumed quickly.

[0020] More preferably, in step S3, the thickness of the magnesium oxide protective layer is 1 mm to 3 mm.

[0021] More preferably, in step S3, the magnesium oxide protective layer is prepared by plasma spraying.

[0022] Preferably, in step S3, the seed crystal is a columnar magnesium oxide single crystal grown perpendicular to the (100) plane, and the diameter of the columnar magnesium oxide single crystal is 5mm~10mm and the height is 10~15mm.

[0023] More preferably, in step S3, the seed crystal is pre-embedded in the bottom of the cold crucible furnace, specifically in one of the following two ways: Method 1: The seed crystal is pre-embedded in the center of the bottom of the cold crucible furnace chamber; Method 2: Seed crystals are pre-embedded at the bottom of the cold crucible furnace near both ends. Crystal nucleation begins from the part near the molten shell. Therefore, pre-embedding seed crystals at both ends is more conducive to the growth of large-sized crystals.

[0024] This invention introduces seed crystal-induced directional growth technology, which can promote crystal growth along a specific direction and effectively improve the size and crystal quality of magnesium oxide single crystals.

[0025] Preferably, in step S4, the temperature of the high-frequency induction heating is controlled at 2900~3300℃ and the frequency is 350kHz~400kHz.

[0026] Preferably, in step S4, the remaining mixed polycrystalline material accounts for 85 wt% to 88 wt% of all the mixed polycrystalline material in step S1.

[0027] More preferably, in step S4, the remaining mixed polycrystalline material is replenished to the upper part of the raw material layer in small amounts and multiple times, meaning that each replenishment is made in an amount of 5wt% of the total remaining mixed polycrystalline material.

[0028] Preferably, in step S5, the temperature of the heat preservation treatment is 2900~3300℃, and the heat preservation time is 5~10 hours.

[0029] Preferably, in step S6, the rate at which the cold crucible furnace body descends or the high-frequency induction coil rises is 2~4 mm / h, and the crystal growth time is 3~5 days, with the specific rate depending on the size of the cold crucible furnace body and the raw materials.

[0030] Preferably, the cold crucible furnace chamber refers to a cavity formed by several water-cooled copper pipes.

[0031] More preferably, multiple water-cooled copper tubes are arranged closely around the circumference to form the side wall of the furnace, and the bottom of the furnace is also formed by water-cooled copper tubes or in conjunction with the bottom tray to form a closed space; each water-cooled copper tube has an independent cooling water channel inside, with a cooling water inlet on one side and a cooling water outlet on the other side, and the copper tube is forcibly cooled by circulating cooling water.

[0032] In this invention, during high-frequency induction heating, the ignition medium located at the center of the furnace rapidly heats up under the influence of a high-frequency electromagnetic field. This heat conduction melts the surrounding high-purity polycrystalline magnesium oxide raw material, while the raw material layer close to the inner wall of the water-cooled copper tube remains solid due to the strong cooling effect of the copper tube, forming a self-supporting "melt shell layer." This melt shell layer serves two purposes: firstly, it replaces the traditional crucible in containing the melt; secondly, it isolates the melt from direct contact with the copper tube, thus completely preventing contamination of the melt by the crucible material.

[0033] Preferably, the method for growing high-quality magnesium oxide crystals includes the following steps: (1) High-purity magnesium oxide raw materials are subjected to particle size screening, and polycrystalline materials of different particle sizes are uniformly mixed in a close packing manner; (2) Use conductive materials such as graphite electrodes or magnesium strips as ignition medium, and pre-embed seed crystals; (3) An electric arc is generated by high-frequency induction heating, and a stable molten pool is gradually formed; (4) Crystal growth is achieved through precise temperature control and descent rate control.

[0034] More preferably, the method for growing high-quality magnesium oxide crystals includes the following steps: (1) Magnesium oxide polycrystalline raw materials with a purity higher than 99.9% are screened to obtain polycrystalline raw materials with different particle size distributions of 1~3mm and 3~5mm; (2) According to the principle of close packing, the magnesium oxide polycrystalline materials of different particle sizes are uniformly mixed to form a mixed polycrystalline material, and then a mixed polycrystalline material with a thickness of more than 60mm is filled in the furnace. (3) Lay a layer of graphite electrode or magnesium strip on the surface of polycrystalline material as an ignition medium, and pre-embed seed crystals, and then cover it with raw materials. (4) Start the high-frequency induction heating power supply. A molten pool gradually forms in the furnace, melting all the polycrystalline material and forming a stable melt. The temperature is controlled at 2900~3300℃. The power is adjusted according to the raw materials added. The molten pool is kept warm for 5~10 hours. (5) After the heat preservation is completed, the furnace body is lowered or the high-frequency induction coil is raised at a slow rate. The crystal growth stage is completed in 3 to 5 days, and then heating is stopped. (6) After naturally cooling to room temperature, the ingot is taken out of the furnace and stripped to obtain high-quality magnesium oxide single crystals.

[0035] The present invention also provides a high-quality magnesium oxide crystal prepared by the above-described growth method, wherein the high-quality magnesium oxide crystal has a purity greater than 4N, a size greater than 20 mm, and a full width at half maximum (FWHM) range of 0.03~0.035° for its X-ray twin rocking curve.

[0036] Preferably, the purity of the high-quality magnesium oxide crystals is greater than 4N, meaning the MgO content is ≥99.99%, and the content of major metal impurities is less than 10ppm.

[0037] Preferably, the dimension greater than 20mm refers to a crystal diameter greater than 20mm.

[0038] Compared with the prior art, the present invention has the following beneficial effects: (1) This invention provides a high-quality magnesium oxide crystal and its growth method by optimizing the selection of raw materials, mixing of raw material particle size, seed crystal inoculation and directional growth, and control of ignition process parameters. The magnesium oxide crystal prepared by this method has high purity, large size and high crystal quality.

[0039] (2) The present invention uses high-purity magnesium oxide polycrystalline raw material and combines the technical characteristics of crucible-free growth by shell melting method, so that the raw material does not come into contact with any crucible wall during the melting process, effectively avoiding external contamination introduced by crucible material; at the same time, by selecting the ignition medium and optimizing the ignition method, the risk of impurity introduction is minimized, and high-quality magnesium oxide single crystal with a purity greater than 99.99% can be obtained.

[0040] (3) The present invention uses graphite electrodes or magnesium strips as ignition medium, which utilizes their good conductivity to preferentially heat up under high frequency induction, and can stably initiate the melting of raw materials and form a molten pool; at the same time, a magnesium oxide protective layer formed by plasma spraying is coated on the outside of the ignition medium. This protective layer can effectively isolate the ignition medium from oxidation and erosion by the high temperature atmosphere, significantly slow down the consumption rate of the ignition medium during the heating process, and ensure that it can work stably up to the crystal growth temperature above 3000℃, thereby achieving efficient and long-lasting ignition start-up; this design not only further reduces the risk of impurity introduction, but also effectively solves the technical problems of easy oxidation and short life of traditional ignition medium, providing a reliable guarantee for the stable formation of the molten pool and high-quality crystal growth.

[0041] (4) The present invention uses multi-size densely packed mixed raw materials. By filling the material with different particle sizes, the packing density of the raw material layer is significantly improved. During the melting process, the molten pool is formed more uniformly and stably, which effectively avoids the problems of local voids or melt collapse that are easy to occur with single-size raw materials, which is conducive to improving the quality of crystals.

[0042] (5) The present invention achieves the seeding and directional growth of magnesium oxide crystals by pre-embedding seed crystals with specific crystal orientations at the bottom of the furnace. This method can precisely control the growth direction of the crystals and is the key to obtaining large-size, high-quality strip-shaped single crystals, which is significantly better than the traditional spontaneous nucleation process.

[0043] (6) The present invention uses graded polycrystalline raw materials, special ignition medium, seed crystal induced nucleation and other processes to grow magnesium oxide single crystals with a purity of 4N or higher, a size of 20mm or higher and high crystallinity in a cold crucible heated by high frequency. The process is relatively simple and efficient, and can be mass-produced in industry, and has high practical value.

[0044] (7) This invention utilizes an alternating magnetic field generated by an induction coil to couple with the overall thermal field of the ignition medium, thereby heating and melting the entire melt system components. It does not achieve localized instantaneous melting through direct discharge between electrodes or arc energy. This induction heating method can form a uniform and stable temperature field, with higher controllability and a more uniform thermal field distribution, thereby improving the stability of the melting process and the quality of crystal growth.

[0045] (8) The method of the present invention is simple to operate and the growth process is stable and controllable. Different specifications of crystals can be prepared by adjusting the size of the cold crucible furnace. It has good prospects for process scale-up and meets the needs of large-scale application of high-quality magnesium oxide single crystals in fields such as high-temperature superconducting substrates. Attached Figure Description

[0046] Figure 1 This is a schematic diagram of the cold crucible of the present invention; Figure 2 This refers to the magnesium oxide single crystal obtained in Example 3 of the present invention; Figure 3 This refers to the magnesium oxide single crystal obtained in Example 7 of the present invention; Figure 4 The X-ray double rocking curve of magnesium oxide single crystal obtained by growing it in Example 7 of this invention; Figure 1 In the middle: 1-Water-cooled copper pipe, 2-Fused shell layer, 3-Fused body, 4-Induction coil, 5-Magnesium oxide crystal, 6-Magnesium oxide seed crystal, 7-Inlet, 8-Outlet. Detailed Implementation

[0047] This embodiment is implemented based on the technical solution of the present invention, and provides detailed implementation methods and specific operation processes. However, the scope of protection of the present invention is not limited to the following embodiment.

[0048] Unless otherwise specified, the reagents, methods, instruments, and equipment used in this invention are conventional in the art. Unless otherwise specified, the reagents and materials used in the following examples are all commercially available.

[0049] This invention provides a method for growing high-quality magnesium oxide crystals, the method comprising the following steps: S1: Select high-purity magnesium oxide polycrystalline raw materials, preferably with a purity of ≥99.9%, and mix raw materials of different particle sizes evenly in a certain ratio to obtain mixed polycrystalline material. The particle size of the high-purity magnesium oxide polycrystalline raw materials includes two specifications: 1~3mm and 5~8mm, and the mixing ratio is 3:7~4:6.

[0050] S2: A portion of the mixed polycrystalline material is filled into the cold crucible furnace chamber to form a densely packed material layer. The furnace chamber of the cold crucible furnace has a diameter of 300mm-600mm and a height of 300mm-500mm. The thickness of the material layer is greater than 60mm.

[0051] S3: An ignition medium is laid in the central area inside the raw material layer, and a seed crystal is pre-embedded at the bottom of the cold crucible furnace. The ignition medium is either a graphite electrode or a magnesium strip. The seed crystal is a columnar magnesium oxide single crystal grown perpendicular to the (100) plane with a particle size of 5mm~10mm. The ignition medium is preferably coated with a magnesium oxide protective layer with a thickness of 1 mm to 3 mm, which is prepared by plasma spraying.

[0052] S4: High-frequency induction heating is used to heat the ignition medium and cause the surrounding mixed polycrystalline material to melt, gradually forming a molten pool. As the molten pool gradually expands, the remaining mixed polycrystalline material is added to the upper part of the raw material layer in small amounts and multiple times. The temperature of high-frequency induction heating is preferably controlled at 2900~3300°C and the frequency is 350kHz~400kHz.

[0053] S5: During the formation of the molten pool, the mixed polycrystalline material close to the inner wall of the cold crucible furnace forms a molten shell layer due to the cooling effect of the cold crucible. After all the mixed polycrystalline material except the molten shell layer has melted, a heat preservation treatment is performed to stabilize the molten pool. The preferred temperature for the heat preservation treatment is 2900~3300°C, and the heat preservation time is 5~10 hours.

[0054] S6: Slowly lower the cold crucible furnace body or raise the high-frequency induction coil to allow the magnesium oxide crystal to grow directionally on the seed crystal. After the crystal growth is completed, stop heating and allow it to cool naturally to room temperature. The rate at which the cold crucible furnace body is lowered or the high-frequency induction coil is raised is 2~4 mm / h, and the crystal growth time is 3~5 days.

[0055] S7: Peel off the fusion crust layer to obtain high-quality magnesium oxide crystals.

[0056] The present invention also provides a high-quality magnesium oxide crystal prepared by the above-described growth method, wherein the crystal has a purity greater than 4N, a size greater than 20 mm, and a full width at half maximum (FWHM) range of 0.03 to 0.035 for the X-ray twin rocking curve.

[0057] like Figure 1 As shown, in the following embodiments, the cold crucible furnace refers to a cavity formed by several water-cooled copper pipes 1.

[0058] Specifically, multiple water-cooled copper tubes 1 are arranged closely around the circumference to form the side wall of the furnace. The bottom of the furnace is also formed by water-cooled copper tubes 1 or in conjunction with the bottom tray to form a closed space. Each water-cooled copper tube 1 has a cooling water channel inside, with a cooling water inlet 7 on one side and a cooling water outlet 8 on the other side. The copper tube is forcibly cooled by circulating cooling water.

[0059] The furnace chamber is surrounded by a high-frequency induction coil 4, which is connected to a high-frequency power supply. The coil can be configured as a single-turn or multi-turn structure according to process requirements. The high-frequency induction coil 4 and the cold crucible furnace chamber maintain a certain electrical insulation distance. The induction coil is configured as a liftable structure according to the growth process requirements.

[0060] During high-frequency induction heating, the high-frequency electromagnetic field generated by the high-frequency induction coil 4 acts on the interior of the furnace. Since magnesium oxide has extremely low conductivity at room temperature, it cannot directly generate heat through inductive coupling. Therefore, the ignition medium (such as a graphite electrode or magnesium strip) embedded in the center of the raw material layer is heated first. Under the influence of the high-frequency electromagnetic field, the ignition medium rapidly heats up, drawing heat from the surrounding high-purity polycrystalline magnesium oxide raw material to above its melting point through heat conduction. This causes the solid crystal to gradually melt into melt 3, forming an initial molten pool. As the molten pool forms, the conductivity of the molten magnesium oxide increases, allowing it to directly absorb high-frequency energy, further expanding the molten pool and maintaining the temperature.

[0061] During this process, the raw material layer close to the inner wall of the water-cooled copper tube 1 remains solid due to the strong cooling effect of the copper tube, forming a self-supporting molten shell layer 2. This molten shell layer 2 serves two purposes: firstly, it replaces the traditional crucible in containing the melt 3; secondly, it isolates the melt 3 from direct contact with the water-cooled copper tube 1, thus completely avoiding contamination of the melt by the crucible material.

[0062] The furnace bottom center has a magnesium oxide seed crystal 6, which can promote the growth of magnesium oxide crystal 5 along a specific direction, effectively improving the size and crystal quality of magnesium oxide single crystals.

[0063] The present invention will now be described in detail with reference to the accompanying drawings and specific embodiments.

[0064] Example 1 Magnesium oxide polycrystalline material with a purity ≥99.9% was screened and classified to obtain 1-3 mm and 5-8 mm polycrystalline materials with a normal particle size distribution. These were then mixed uniformly in a 4:6 ratio according to the close-packing principle to obtain magnesium oxide polycrystalline material. A 70 mm thick layer of the mixed polycrystalline material was filled into a furnace chamber with dimensions φ300 mm × 400 mm. An ignition medium (graphite electrode) was then placed on its surface and covered with the raw material. A high-frequency induction heating power supply was started, and the frequency was adjusted to 400 kHz. As the temperature rose, a molten pool first formed around the graphite electrode and gradually expanded. During this process, it was necessary to maintain stable power while reducing the remaining... The remaining raw materials were added in small amounts multiple times until all the raw materials were melted. After the raw materials were completely melted, the holding time was controlled at 5 hours and the temperature was controlled at 3000℃, and the molten pool gradually stabilized. Then the crystal growth stage began. The furnace body was slowly lowered or the high-frequency induction coil was raised at a rate of 3.5mm / h, and the crystal grew for 72 hours. After the growth was completed, the heating power was turned off and the crystal was allowed to cool naturally to room temperature. The ingot was then removed from the furnace body using a hoisting device. The outer layer of magnesium oxide polycrystalline material was peeled off, and the crystal was crushed. After screening, about 220g of magnesium oxide single crystals with a purity of over 99.99%, colorless and transparent, and a size of over 20mm×20mm×10mm were obtained, in approximately 10 pieces.

[0065] The obtained magnesium oxide single crystal samples were sampled and purity was analyzed by ICP-OES (inductively coupled plasma optical emission spectrometry). According to GB / T 39807-2021, the results showed that the content of major metal impurities was less than 20 ppm and the purity of single crystal was ≥99.99%.

[0066] Comparative Example 1 In this comparative example, only polycrystalline material with a single particle size of 1 mm was used; the rest was the same as in Example 1. Approximately 150 g of six magnesium oxide single crystals with a purity of over 99.99%, colorless and transparent, and a size of over 10 mm × 10 mm × 5 mm were obtained through screening.

[0067] Example 2 Magnesium oxide polycrystalline material with a purity ≥99.9% was screened and classified to obtain 1-3mm and 5-8mm polycrystalline materials with a normal particle size distribution. These were then mixed uniformly in a 3:7 ratio according to the close-packing principle to obtain magnesium oxide polycrystalline material. A 70mm thick layer of the mixed polycrystalline material was filled into a furnace chamber with dimensions φ300mm×500mm. An ignition medium (graphite electrode) was then placed on its surface, and a seed crystal was pre-embedded at the center of the furnace chamber, which was then covered with the raw material. A high-frequency induction heating power supply was started, and the frequency was adjusted to 370kHz. As the temperature rose, a molten pool first formed around the high-purity magnesium strip and gradually expanded. During this process, the power needed to be kept stable while all remaining raw material was added. After all the raw material had melted, the holding time was controlled at 6 hours, and the temperature was controlled... The temperature was set at 3000℃, and the molten pool gradually stabilized. Then, the crystal growth stage began. The cold crucible furnace body was slowly lowered at a rate of 3 mm / h, or the high-frequency induction coil was raised. The crystal grew for 120 hours. After the growth was completed, the heating power was turned off, and the crystal was allowed to cool naturally to room temperature. The ingot was then removed from the furnace body using a hoisting device. The outer layer of magnesium oxide polycrystalline material was peeled off, and the crystal was broken. After screening, approximately 280g of magnesium oxide single crystals with a purity of over 99.99%, colorless and transparent, and a size of over 20mm×20mm×15mm were obtained, in 8 pieces.

[0068] The obtained magnesium oxide single crystal samples were sampled and purity was analyzed by ICP-OES method. According to GB / T 39807-2021, the results showed that the content of major metal impurities was less than 20 ppm and the purity of single crystal was ≥99.99%.

[0069] Comparative Example 2: In this comparative example, no seed crystal was pre-embedded, and the rest was the same as in Example 2. Approximately 220g of nine magnesium oxide single crystals with a purity of over 99.99%, colorless and transparent, and a size of over 20mm × 20mm × 15mm were obtained through screening.

[0070] Example 3 Magnesium oxide polycrystalline materials with a purity of ≥99.9% were screened and classified to obtain 1~3mm polycrystalline raw materials and 5~8mm polycrystalline raw materials with a normal particle size distribution. Then, the two were mixed evenly in a 4:6 ratio according to the close packing principle to obtain magnesium oxide polycrystalline material. A 60mm thick layer of mixed polycrystalline material is filled into a furnace chamber with dimensions of φ400mm×500mm. An ignition medium (high-purity magnesium oxide strip) is then placed on its surface, and a seed crystal is pre-embedded in the center of the furnace chamber, subsequently covered with the raw material. A high-frequency induction heating power supply is activated, and the frequency is adjusted to 350kHz. As the temperature rises, a molten pool first forms around the high-purity magnesium oxide strip and gradually expands. During this process, the power must be kept stable while all remaining raw material is added. After all the raw material has melted, the holding time is controlled at 7 hours, and the temperature is controlled at 3050℃, allowing the molten pool to gradually stabilize. The crystal growth stage then begins, with the cold crucible furnace body slowly lowered or the high-frequency induction coil raised at a rate of 3mm / h. Crystal growth lasts for 144 hours. After growth, the heating power is turned off, and the crystal is allowed to cool naturally to room temperature. The ingot is then removed from the furnace using a hoisting device, the outer layer of magnesium oxide polycrystalline material is peeled off, and the crystal is broken. Figure 2 As shown, approximately 360g of magnesium oxide single crystals with a purity of over 99.99%, colorless and transparent, and a size of over 25mm×25mm×10mm were obtained through screening. The number of crystals was 15, with the largest crystal size being 30mm×30mm×20mm.

[0071] The obtained magnesium oxide single crystal samples were sampled and purity was analyzed by ICP-OES method. According to GB / T 39807-2021, the results showed that the content of major metal impurities was less than 10 ppm and the purity of single crystal was ≥99.99%.

[0072] Comparative Example 3: In this comparative example, no seed crystal was pre-embedded, and the rest was the same as in Example 3. Approximately 220g of magnesium oxide single crystals with a purity of over 99.99%, colorless and transparent, and a size of over 20mm × 20mm × 15mm were obtained through screening. The total number of crystals was 9, with the largest crystal size being 25 × 25 × 15mm.

[0073] Example 4 Magnesium oxide polycrystalline material with a purity ≥99.9% was screened and classified to obtain 1-3mm and 5-8mm polycrystalline materials with a normal particle size distribution. These were then mixed uniformly in a 4:6 ratio according to the close-packing principle to obtain magnesium oxide polycrystalline material. A 60mm thick layer of the mixed polycrystalline material was filled into a furnace chamber with dimensions φ450mm×500mm. An ignition medium (graphite electrode) was then placed on its surface, and a seed crystal was pre-embedded at the center of the furnace chamber, which was then covered with the raw material. A high-frequency induction heating power supply was started, and the frequency was adjusted to 400kHz. As the temperature rose, a molten pool first formed around the graphite electrode and gradually expanded. During this process, the remaining raw material was added in small amounts multiple times while maintaining stable power. After all the raw material had melted, the holding time was controlled at 5 hours, and the temperature was controlled at 3000℃. The molten pool gradually stabilizes; then the crystal growth stage begins, with the furnace body slowly lowered or the high-frequency induction coil raised at a rate of 3.5 mm / h, and the crystal growth lasts for 72 hours; after growth, the heating power is turned off, and the crystal is allowed to cool naturally to room temperature. The ingot is then removed from the furnace using a hoisting device, the outer magnesium oxide shell is peeled off, and the crystal is broken. After screening, approximately 300g of magnesium oxide single crystals with a purity of over 99.99%, colorless and transparent, and a size of over 25mm×25mm×10mm are obtained, in approximately 12 pieces, with the largest crystal size being 30mm×30mm×20mm.

[0074] The obtained magnesium oxide single crystal samples were sampled and purity was analyzed by ICP-OES (inductively coupled plasma optical emission spectrometry). According to GB / T 39807-2021, the results showed that the content of major metal impurities was less than 20 ppm and the purity of single crystal was ≥99.99%.

[0075] Example 5 Magnesium oxide polycrystalline material with a purity ≥99.9% was screened and classified to obtain 1-3mm and 5-8mm polycrystalline materials with a normal particle size distribution. These were then mixed uniformly in a 4:6 ratio according to the close-packing principle to obtain magnesium oxide polycrystalline material. A 60mm thick layer of the mixed polycrystalline material was filled into a furnace chamber with dimensions φ300mm×400mm. An ignition medium (a 3mm thick graphite electrode) was then placed on its surface, and a seed crystal was pre-embedded at the center of the furnace chamber, which was then covered with the raw material. A high-frequency induction heating power supply was started, and the frequency was adjusted to 400kHz. As the temperature rose, a molten pool first formed around the graphite electrode and gradually expanded. During this process, the remaining raw material was added in small amounts multiple times while maintaining stable power. After all the raw material had melted, the process was carried out under heat preservation conditions. The furnace is kept at 3000℃ for 5 hours, and the molten pool gradually stabilizes. Then the crystal growth stage begins. The furnace body is slowly lowered or the high-frequency induction coil is raised at a rate of 3.5 mm / h, and the crystal grows for 72 hours. After the growth is completed, the heating power is turned off and the crystal is allowed to cool naturally to room temperature. The ingot is then removed from the furnace body using a hoisting device. The outer magnesium oxide shell is peeled off, and the crystal is broken. After screening, approximately 400g of magnesium oxide single crystals with a purity of over 99.99%, colorless and transparent, and a size of over 30×30×15mm are obtained, in 11 pieces.

[0076] The obtained magnesium oxide single crystal samples were sampled and purity was analyzed by ICP-OES (inductively coupled plasma optical emission spectrometry). According to GB / T 39807-2021, the results showed that the content of major metal impurities was less than 10 ppm and the purity of single crystal was ≥99.99%.

[0077] Example 6 Magnesium oxide polycrystalline material with a purity ≥99.9% was screened and classified to obtain 1-3mm and 5-8mm polycrystalline materials with a normal particle size distribution. These were then mixed uniformly in a 4:6 ratio according to the close-packing principle to obtain magnesium oxide polycrystalline material. A 70mm thick layer of the mixed polycrystalline material was filled into a furnace chamber with dimensions φ400mm×500mm. An ignition medium (high-purity magnesium metal strip) was then placed on its surface, and columnar magnesium oxide single crystals were pre-embedded near the edge of the furnace chamber as seed crystals, which were then covered with the raw material. A high-frequency induction heating power supply was started, and the frequency was adjusted to 370kHz. As the temperature rose, a molten pool first formed around the high-purity magnesium metal strip and gradually expanded. During this process, the remaining raw material was added while maintaining stable power. After all the raw material had melted, the holding time was controlled at [time missing]. For 6 hours, the temperature was controlled at 3000℃, and the molten pool gradually stabilized. Then, the crystal growth stage began. The cold crucible furnace body was slowly lowered or the high-frequency induction coil was raised at a rate of 3mm / h. The crystal grew for 120 hours. After the growth was completed, the heating power was turned off and the crystal was allowed to cool naturally to room temperature. The ingot was then removed from the furnace body using a hoisting device. The outer layer of magnesium oxide polycrystalline material was peeled off, and the crystal was broken. After screening, approximately 400g of magnesium oxide single crystals with a purity of over 99.99%, colorless and transparent, and a size of over 30×30×20mm were obtained, in 14 pieces.

[0078] Example 7 Magnesium oxide polycrystalline materials with a purity of ≥99.9% were screened and classified to obtain 1~3mm polycrystalline raw materials and 5~8mm polycrystalline raw materials with a normal particle size distribution. Then, the two were mixed evenly in a 4:6 ratio according to the close packing principle to obtain magnesium oxide polycrystalline material. A 70mm thick layer of mixed polycrystalline material is filled into a furnace chamber with dimensions of φ450mm×500mm. An ignition medium (a 3mm thick layer of magnesium metal strip) is then placed on its surface, and columnar magnesium oxide single crystals are pre-embedded near the edge of the furnace chamber as seed crystals, which are then covered with the raw material. A high-frequency induction heating power supply is activated, and the frequency is adjusted to 370kHz. As the temperature rises, a molten pool first forms around the high-purity magnesium metal strip and gradually expands. During this process, the power needs to be kept stable while all remaining raw material is added. After all the raw material has melted, the holding time is controlled at 6 hours, and the temperature is controlled at 3000℃, allowing the molten pool to gradually stabilize. The crystal growth stage then begins, with the cold crucible furnace body slowly lowered or the high-frequency induction coil raised at a rate of 3mm / h. Crystal growth lasts for 120 hours. After growth, the heating power is turned off, and the crystal is allowed to cool naturally to room temperature. The ingot is then removed from the furnace using a hoisting device, the outer layer of magnesium oxide polycrystalline material is peeled off, and the crystal is broken. Figure 3 As shown, approximately 440g of magnesium oxide single crystals with a purity of over 99.99%, colorless and transparent, and a size of over 35×30×20mm were obtained through screening. The quantity was 18 crystals.

[0079] By testing the X-ray double rocking curve of the crystal ( Figure 4 The results showed that the full width at half maximum (FWHM) of the crystal was 0.03º, indicating that the crystal quality was relatively high.

[0080] Comparative Example 4: This comparative example uses the existing arc melting method for growth. The crystal has a higher internal carbon content, resulting in lower purity compared to the crystal obtained in Example 7.

[0081] The above description of the embodiments is provided to enable those skilled in the art to understand and use the invention. It will be apparent to those skilled in the art that various modifications can be made to these embodiments, and the general principles described herein can be applied to other embodiments without inventive effort. Therefore, the present invention is not limited to the above embodiments, and any improvements and modifications made by those skilled in the art based on the disclosure of the present invention without departing from the scope of the invention should be within the protection scope of the present invention.

Claims

1. A method for growing high-quality magnesium oxide crystals, characterized in that, Includes the following steps: S1: Mix high-purity magnesium oxide polycrystalline raw materials of different particle sizes evenly according to the formula to obtain mixed polycrystalline material; S2: A portion of the mixed polycrystalline material is filled into the cold crucible furnace to form a densely packed raw material layer; S3: An ignition medium is laid in the central region inside the raw material layer, and a seed crystal is pre-embedded at the bottom of the cold crucible furnace. S4: High-frequency induction heating is used to heat the ignition medium and cause the surrounding mixed polycrystalline material to melt, gradually forming a molten pool; as the molten pool gradually expands, the remaining mixed polycrystalline material is added to the upper part of the raw material layer in small amounts and multiple times. S5: During the formation of the molten pool, the mixed polycrystalline material close to the inner wall of the cold crucible furnace forms a molten shell layer due to the cooling effect of the cold crucible. After all the mixed polycrystalline material except the molten shell layer has melted, heat preservation treatment is carried out to stabilize the molten pool. S6: Slowly lower the cold crucible furnace body or raise the high-frequency induction coil to make the magnesium oxide crystal grow in a directional manner on the seed crystal. After the crystal growth is completed, stop heating and let it cool naturally to room temperature. S7: Peel off the fusion shell layer to obtain the high-quality magnesium oxide crystals.

2. The method for growing high-quality magnesium oxide crystals according to claim 1, characterized in that, In step S1, the purity of the high-purity magnesium oxide polycrystalline raw material is ≥99.9%, and the mixed polycrystalline material is made by mixing two kinds of high-purity magnesium oxide polycrystalline raw materials with particle sizes of 1-3 mm and 5-8 mm in a mass ratio of 3:7-4:

6.

3. The method for growing high-quality magnesium oxide crystals according to claim 1, characterized in that, In step S2, the furnace chamber of the cold crucible furnace has a diameter of 300mm to 600mm and a height of 300mm to 500mm, and the thickness of the raw material layer is 60mm to 80mm.

4. The method for growing high-quality magnesium oxide crystals according to claim 1, characterized in that, In step S3, the ignition medium is laid out in a ring, with the ends connected one after the other. The ignition medium is either a graphite electrode or a magnesium strip.

5. The method for growing high-quality magnesium oxide crystals according to claim 1, characterized in that, In step S3, the ignition medium is covered with a magnesium oxide protective layer with a thickness of 1 mm to 3 mm, and the magnesium oxide protective layer is prepared by plasma spraying.

6. The method for growing high-quality magnesium oxide crystals according to claim 1, characterized in that, In step S3, the seed crystal is a columnar magnesium oxide single crystal grown perpendicular to the (100) plane, and the diameter of the columnar magnesium oxide single crystal is 5mm~10mm and the height is 10~15mm.

7. The method for growing high-quality magnesium oxide crystals according to claim 1, characterized in that, In step S4, the temperature of the high-frequency induction heating is controlled at 2900~3300℃ and the frequency is 350kHz~400kHz.

8. The method for growing high-quality magnesium oxide crystals according to claim 1, characterized in that, In step S5, the temperature of the heat preservation treatment is 2900~3300℃, and the heat preservation time is 5~10 hours.

9. The method for growing high-quality magnesium oxide crystals according to claim 1, characterized in that, In step S6, the rate at which the cold crucible furnace body descends or the high-frequency induction coil rises is 2-4 mm / h, and the crystal growth time is 3-5 days.

10. A high-quality magnesium oxide crystal, prepared by the growth method according to any one of claims 1 to 9, characterized in that, The high-quality magnesium oxide crystals have a purity greater than 4N, a size greater than 20mm, and a full width at half maximum (FWHM) range of 0.03~0.035° for the X-ray twin rocking curve.

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