Process for rapidly preparing large-size hexagonal boron nitride single crystal by high-temperature flux method

Abstract

The present application relates to the technical field of crystal growth, and particularly relates to a process for rapidly preparing large-size hexagonal boron nitride single crystal by high-temperature flux method. The process comprises the following steps: pre-activating LiF-NaF-KF ternary eutectic salt or LiF-CaF2 eutectic salt under vacuum condition, mixing the pre-activated salt with hexagonal boron nitride main raw material, and loading the mixture into a crucible; heating to 1450-1500 DEG C under a protective atmosphere, and applying rotating motion to the crucible in the heat preservation growth stage; washing and drying after cooling, and then performing high-temperature stabilization treatment at 1850-1900 DEG C to obtain hexagonal boron nitride single crystal. The present application provides a stable liquid phase growth environment through a fluorine salt eutectic system, can rapidly obtain hexagonal flaky boron nitride single crystal with a size of 100 microns or more under normal pressure, and is simple in process operation and easy to be industrialized and enlarged in production.

Description

Technical Field

[0001] This invention relates to the field of crystal growth technology, specifically to a process for rapidly preparing large-size hexagonal boron nitride single crystals using a high-temperature flux method. Background Technology

[0002] Hexagonal boron nitride (h-BN) single crystals, due to their ultra-wide bandgap of approximately 6 eV, excellent thermal conductivity, and chemical stability, hold significant promise for applications in deep-ultraviolet optoelectronic devices, neutron detection, and two-dimensional semiconductor packaging substrates. However, the atmospheric pressure preparation of high-quality, large-size h-BN single crystals has long been a technical challenge in this field. This is mainly because h-BN is difficult to melt at high temperatures, insoluble in conventional solvents, and lacks a suitable flux system for its crystal growth.

[0003] Currently, there are two main types of methods for preparing h-BN single crystals under normal pressure. One type is the metal flux method. For example, Chinese patent CN109695053A discloses a method using a cobalt-chromium alloy as a flux, in which h-BN powder is dissolved in molten cobalt-chromium alloy, and then cooled at an extremely low rate to allow crystal precipitation. Similarly, the literature "Characterization of bulkhexagonal boron nitride single crystals grown by the metal flux technique" (Journal of Crystal Growth) reports the use of a nickel-chromium flux to slowly grow h-BN single crystals at 1500℃ with a cooling rate of 2-4℃ / h. Although this type of method can obtain millimeter-sized crystals, the separation of the metal flux from the product requires strong acid corrosion, making the process complex and prone to introducing contamination. At the same time, the extremely slow cooling rate results in a growth cycle of tens of hours, leading to high energy consumption and low efficiency.

[0004] Another method is the molten salt method, which typically uses chloride molten salts (such as KCl-NaCl) to synthesize h-BN at relatively low temperatures (900-1300℃). However, this method is currently mainly used to prepare h-BN nanosheets or nanopowders, with product sizes usually in the nanometer to submicron range, and it is not yet possible to prepare single crystals larger than micrometers. The fundamental reason is that the melting point and operating temperature of existing molten salt systems are relatively low, resulting in limited solubility for BN and an inability to provide sufficient mass transport driving force for significant grain growth.

[0005] In summary, existing technologies present an irreconcilable contradiction between growth efficiency and crystal quality. While the metal flux method can yield larger crystals, its growth cycle is too long, and although the molten salt method is simple, the product size is limited. Therefore, there is an urgent need for a process that can rapidly prepare large-size, high-quality h-BN single crystals under ambient pressure. Summary of the Invention

[0006] This invention addresses the problems existing in the prior art by providing a process for the rapid preparation of large-size hexagonal boron nitride single crystals using a high-temperature flux method. This method significantly improves the crystal growth rate by constructing liquid phase material transport channels under high-temperature conditions, while avoiding the impurity contamination problem caused by metal fluxes. The aim is to achieve rapid preparation of large-size, high-quality hexagonal boron nitride single crystals under ambient pressure.

[0007] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0008] A process for rapidly preparing large-size hexagonal boron nitride single crystals using a high-temperature flux method includes the following steps: (1) The molten salt medium is pre-activated under vacuum conditions; (2) Mix the pre-activated molten salt medium with the hexagonal boron nitride main raw material and load it into a crucible; (3) Heat the crucible to 1450-1500℃ under a protective atmosphere. After reaching this temperature, enter the heat preservation stage and apply rotational motion to the crucible. Stop rotating after the heat preservation is completed. (4) After cooling, wash and dry the product. Then, heat the product to 1850-1900℃ under a protective atmosphere for high-temperature stabilization treatment. After cooling, obtain hexagonal boron nitride single crystal.

[0009] Further, the molten salt medium in step (1) is a LiF-NaF-KF ternary eutectic salt or a LiF-CaF2 eutectic salt.

[0010] Furthermore, the temperature of the pre-activation treatment in step (1) is 700-800℃, and the holding time is 2-4 hours.

[0011] Pre-activation of the molten salt medium, under high-temperature vacuum conditions, decomposes and volatilizes residual oxyanions and other impurities in the molten salt, preventing these impurities from forming oxygen-containing ion clusters during subsequent growth and becoming heterogeneous nucleation sites for boron nitride. This inhibits extensive spontaneous nucleation during the cooling process of the molten salt. The oxygen impurity content of the pre-activated molten salt is significantly reduced, and the transport and precipitation of boron nitride mainly occurs along the surface of the existing seed crystal, which is beneficial for continuous grain growth and ultimately obtaining large-size single crystals.

[0012] Furthermore, the main raw material for hexagonal boron nitride in step (2) is hexagonal boron nitride micro powder with a particle size of 1-5 μm, or a mixture of boron oxide and carbon powder.

[0013] Furthermore, the amount of molten salt medium added in step (2) is 10%-20% of the mass of the main raw material.

[0014] Furthermore, when the main raw material mentioned in step (2) is hexagonal boron nitride micro powder, B2O3 is also added during mixing, and the amount of B2O3 added is less than 1% of the mass of the molten salt medium.

[0015] Furthermore, the mixing method described in step (2) is ball milling.

[0016] The addition of trace amounts of B2O3 can adjust the viscosity of the fluoride salt melt and improve its flow behavior during crucible rotation. At the same time, B2O3 can interact with the h-BN surface at high temperatures, which is beneficial to improving the interfacial environment for crystal growth and promoting continuous grain growth.

[0017] Furthermore, the protective atmosphere in step (3) is a nitrogen atmosphere, the furnace pressure is 0.11-0.12 MPa, and the heating rate is 10-15 °C / min.

[0018] Furthermore, the rotational motion described in step (3) is a periodic forward and reverse rotation of the crucible along the axial direction, with a maximum rotational speed of ±50-100 rpm and a duration of 1.5-5 minutes for each rotational direction.

[0019] In step (3), the growth temperature is controlled within the range of 1450-1500℃. At this temperature, the molten salt medium is in a stable liquid state with low viscosity and high fluidity, which can fully wet the BN particles and provide a good material transport environment. If the temperature is too low, the molten salt's ability to dissolve BN is insufficient, and the driving force for grain growth is weak; if the temperature is too high, the molten salt volatilization intensifies, the melt composition deviates from the eutectic point, and the liquid phase stability decreases. Controlling the temperature within the range of 1450-1500℃ ensures that the molten salt has sufficient dissolving power and material transport rate, while avoiding compositional changes and process instability caused by excessive volatilization of the molten salt. Applying rotational motion to the crucible during the heat preservation stage can generate forced convection in the molten salt, accelerate the dissolution and transport of BN in the molten salt, and at the same time make the temperature and concentration distribution of the molten salt more uniform, which is conducive to the stable transport and continuous precipitation of BN material to the seed crystal surface, thereby improving the crystal growth efficiency and crystal size uniformity.

[0020] Furthermore, the heat preservation time mentioned in step (3) is 1.5-3 hours.

[0021] Compared with the prior art, the present invention has the following beneficial effects: This invention utilizes a fluoride eutectic system instead of traditional metal flux, avoiding the impact of metal residues on crystal quality. It also simplifies post-processing, requiring only ultrasonic washing with dilute hydrochloric acid to remove the molten salt medium, eliminating the need for strong acid corrosion. Vacuum pre-activation of the molten salt medium effectively eliminates spontaneous nucleation induced by impurities during growth, concentrating mass transport and crystal precipitation on the surface of existing seed crystals, thus promoting continuous grain growth. Applying rotational motion to the crucible during the growth stage enhances mass transport and convection within the molten salt, significantly improving crystal growth efficiency and dimensional uniformity. Furthermore, high-temperature stabilization after growth further eliminates lattice defects, enhancing the crystal integrity of the product. The process of this invention is easily scalable for industrial production, providing an effective solution to problems such as long growth cycles, small product sizes, and metal contamination in existing technologies. Attached Figure Description

[0022] Figure 1 SEM image of the product prepared in Example 1.

[0023] Figure 2 SEM image of the product prepared in Example 2.

[0024] Figure 3 SEM image of the product prepared in Example 3.

[0025] Figure 4 SEM image of the product prepared for Comparative Example 1.

[0026] Figure 5 SEM image of the product prepared for Comparative Example 2.

[0027] Figure 6 SEM image of the product prepared in Comparative Example 3.

[0028] Figure 7 SEM image of the product prepared in Comparative Example 4.

[0029] Figure 8 The XRD patterns are those of the products prepared in Example 1 and Comparative Examples 1-4. Detailed Implementation

[0030] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention. Unless otherwise specified, the raw materials used in the embodiments are all commercially available products.

[0031] Example 1 This embodiment provides a process for rapidly preparing large-size hexagonal boron nitride single crystals using a high-temperature flux method, including the following steps: (1) Preparation of eutectic salt: LiF, NaF, and KF were weighed in a molar ratio of 46.5:11.5:42 and placed in a mortar. The mixture was thoroughly ground and mixed in a glove box for 1 hour. The mixed powder was then loaded into a high-purity graphite crucible and compacted. The crucible was placed in a high-temperature furnace and heated to 600°C at a rate of 5°C / min under a nitrogen atmosphere. The temperature was maintained for 2 hours to allow the mixed salt to completely melt and be fully homogenized. After the temperature maintenance was completed, the mixture was cooled to room temperature with the furnace. The eutectic salt block was removed, crushed in a mortar, and passed through a 200-mesh sieve to obtain LiF-NaF-KF ternary eutectic salt powder.

[0032] The above-mentioned eutectic salt powder was placed into a high-purity graphite crucible and then placed in a vacuum heat treatment furnace, where a vacuum of 100 kWh was applied. -2 Pa, then the temperature is increased to 800℃ at a rate of 10℃ / min, and held for 4 hours for pre-activation treatment. After the holding period, the furnace is cooled under vacuum. (2) Weigh the pre-activated eutectic salt and hexagonal boron nitride micro powder with a particle size of 3 μm at a ratio of 15% of the mass of the eutectic salt to the mass of the hexagonal boron nitride micro powder, and mix them. Add 0.5% of the mass of the eutectic salt to B2O3. Use anhydrous ethanol as the medium to ball mill and mix in a planetary ball mill for 4 hours. The ball-to-material ratio is 3:1 and the rotation speed is 300 rpm. Dry the mixed slurry in an oven at 80℃ for 12 hours, pass it through a 100-mesh sieve, and put it into a BN-coated crucible.

[0033] (3) Push the crucible into the constant temperature zone of the high temperature atmosphere furnace, evacuate and fill it with high-purity nitrogen to 0.11 MPa. Repeat the evacuation and filling three times and keep the pressure inside the furnace at 0.11 MPa. Heat it to 1500℃ at a rate of 12℃ / min. After reaching this temperature, enter the heat preservation stage. At the same time, apply a periodic forward and reverse rotation motion along the axial direction to the crucible. The maximum speed is 80 rpm and the duration of each rotation direction is 3 minutes. Stop the rotation after heat preservation for 2 hours.

[0034] (4) Cool the sintered block to room temperature with the furnace at a rate of 4℃ / min, remove the block, and ultrasonically wash it in dilute hydrochloric acid for 30 minutes to remove residual molten salt. Then ultrasonically wash it with deionized water for 15 minutes each time until the washing solution is neutral. Then dry it in an oven at 120℃ for 6 hours. Place the dried product in a high-temperature atmosphere furnace, evacuate it, and fill it with high-purity nitrogen to 0.11MPa. Heat it to 1880℃ at a rate of 15℃ / min, hold it at that temperature for 1 hour, and then cool it to room temperature with the furnace to obtain hexagonal boron nitride single crystal.

[0035] Figure 1 SEM images of the product prepared in Example 1, by Figure 1As can be seen, the product exhibits a regular plate-like crystal structure with a smooth crystal surface and clear and complete edge contours. The crystal size in the sample is relatively uniform, and there are few broken and small crystal grains.

[0036] Example 2 This embodiment provides a process for rapidly preparing large-size hexagonal boron nitride single crystals using a high-temperature flux method, including the following steps: (1) Preparation of eutectic salt: LiF and CaF2 were weighed at a molar ratio of 78:22 and placed in a mortar. They were then ground and mixed thoroughly in a glove box for 1 hour. The mixed powder was then loaded into a high-purity graphite crucible and compacted. The crucible was placed in a high-temperature furnace and heated to 850°C at a rate of 5°C / min under a nitrogen atmosphere. The temperature was maintained for 2 hours to allow the mixed salt to completely melt and be fully homogenized. After the temperature maintenance was completed, the mixture was cooled to room temperature with the furnace. The eutectic salt block was then removed, crushed in a mortar, and passed through a 200-mesh sieve to obtain LiF-CaF2 eutectic salt powder.

[0037] The above-mentioned eutectic salt powder was placed into a high-purity graphite crucible and then placed in a vacuum heat treatment furnace, where a vacuum of 100 kWh was applied. -2 Pa was heated to 750°C at a rate of 10°C / min and held for 3 hours for pre-activation treatment. After holding, the furnace was cooled under vacuum.

[0038] (2) Weigh the pre-activated eutectic salt and hexagonal boron nitride micro powder with a particle size of 5 μm at a ratio of 12% of the mass of the eutectic salt to the mass of the hexagonal boron nitride micro powder, and mix them. Add 0.8% of the mass of the eutectic salt to B2O3. Use anhydrous ethanol as the medium to ball mill and mix in a planetary ball mill for 4 hours. The ball-to-material ratio is 3:1 and the rotation speed is 300 rpm. Dry the mixed slurry in an oven at 80℃ for 12 hours, pass it through a 100-mesh sieve, and put it into a BN-coated crucible.

[0039] (3) Push the crucible into the constant temperature zone of the high temperature atmosphere furnace, evacuate and fill it with high-purity nitrogen to 0.11 MPa. Repeat the evacuation and filling three times and keep the pressure inside the furnace at 0.11 MPa. Heat it to 1470℃ at a rate of 10℃ / min. After reaching this temperature, enter the heat preservation stage. At the same time, apply a periodic forward and reverse rotation motion along the axial direction to the crucible. The maximum speed is 60 rpm and the duration of each rotation direction is 4 minutes. Stop the rotation after heat preservation for 2.5 hours.

[0040] (4) Cool the sintered block to room temperature with the furnace at a rate of 3℃ / min, remove the block, and ultrasonically wash it in dilute hydrochloric acid for 30 minutes to remove residual molten salt. Then ultrasonically wash it with deionized water for 15 minutes each time until the washing solution is neutral. Then dry it in an oven at 120℃ for 6 hours. Place the dried product in a high-temperature atmosphere furnace, evacuate it, and fill it with high-purity nitrogen to 0.11MPa. Heat it to 1850℃ at a rate of 15℃ / min, hold it for 0.5 hours, and then cool it to room temperature with the furnace to obtain hexagonal boron nitride single crystal.

[0041] Figure 2 SEM images of the product prepared in Example 2, by Figure 2 It is evident that the product is mainly composed of plate-like crystals with smooth crystal surfaces, intact lamellar structure, and no agglomeration of fine powder particles.

[0042] Example 3 This embodiment provides a process for rapidly preparing large-size hexagonal boron nitride single crystals using a high-temperature flux method, including the following steps: (1) LiF-NaF-KF ternary eutectic salt powder was prepared according to the method in Example 1.

[0043] The above eutectic salt powder was placed into a high-purity graphite crucible and then placed in a vacuum heat treatment furnace, where a vacuum of 5 × 10⁻⁶ was applied. -3 Pa was heated to 720°C at a rate of 10°C / min and held for 4 hours for pre-activation treatment. After holding, the furnace was cooled under vacuum.

[0044] (2) Weigh the pre-activated eutectic salt and the mixture of boron oxide and carbon powder (the molar ratio of B2O3 to C is 1:3) according to the ratio of eutectic salt to 18% of the mass of the mixture, mix them, and ball mill them in a planetary ball mill for 4 hours with anhydrous ethanol as the medium. The ball-to-material ratio is 3:1 and the speed is 300 rpm. The mixed slurry is dried in an oven at 80°C for 12 hours, passed through a 100-mesh sieve, and loaded into a BN-coated crucible.

[0045] (3) Push the crucible into the constant temperature zone of the high temperature atmosphere furnace, evacuate and fill it with high-purity nitrogen to 0.12 MPa. Repeat the evacuation and filling three times and keep the pressure inside the furnace at 0.12 MPa. Heat it to 1460℃ at a rate of 14℃ / min. After reaching this temperature, enter the heat preservation stage. At the same time, apply periodic forward and reverse rotation motion along the axial direction to the crucible. The maximum speed is 90 rpm and the duration of each rotation direction is 2 minutes. Stop the rotation after heat preservation for 1.5 hours.

[0046] (4) Cool the sintered block to room temperature with the furnace at a rate of 5℃ / min, remove the block, and ultrasonically wash it in dilute hydrochloric acid for 30 minutes to remove residual molten salt. Then ultrasonically wash it with deionized water for 15 minutes each time until the washing solution is neutral. Then dry it in an oven at 120℃ for 6 hours. Place the dried product in a high-temperature atmosphere furnace, evacuate it, and fill it with high-purity nitrogen to 0.11MPa. Heat it to 1900℃ at a rate of 15℃ / min, hold it at that temperature for 1.5 hours, and then cool it to room temperature with the furnace to obtain hexagonal boron nitride single crystal.

[0047] Figure 3 SEM images of the product prepared in Example 3, by Figure 3 It is evident that the product can form large plate-like crystals with wide crystal sheet width and good crystal integrity; large plate-like single crystals exist in the field of view, with some single crystals having a diameter of over 190 μm, accompanied only by a small number of smaller fragment particles.

[0048] Comparative Example 1 The difference between this comparative example and Example 1 is that the LiF-NaF-KF ternary eutectic salt is replaced with the KF-LiF binary eutectic salt, and the temperature is raised to 1420℃ and held in step (3). The preparation method of the KF-LiF binary eutectic salt is as follows: KF and LiF were weighed at a molar ratio of 50:50 and placed in a mortar. The mixture was thoroughly ground and mixed in a glove box for 1 hour. The resulting powder was then packed into a high-purity graphite crucible and compacted. The crucible was placed in a high-temperature furnace and heated to 650°C at a rate of 5°C / min under a nitrogen atmosphere. The temperature was maintained for 2 hours to ensure complete melting and homogenization of the mixed salt. After the holding period, the mixture was cooled to room temperature with the furnace. The eutectic salt block was removed, crushed in a mortar, and passed through a 200-mesh sieve to obtain KF-LiF binary eutectic salt powder.

[0049] The remaining steps are the same as in Example 1.

[0050] Figure 4 SEM images of the product prepared in Comparative Example 1, by Figure 4 It is evident that the sample contains a large number of fine and tiny particles, with a low proportion of intact flaky grains; the overall grain size is much smaller than that of Example 1, and the grain development integrity is poor.

[0051] Comparative Example 2 The difference between this comparative example and Example 1 is that the LiF-NaF-KF ternary eutectic salt is replaced with a NaF-LiF binary eutectic salt. The preparation method of the NaF-LiF binary eutectic salt is as follows: NaF and LiF were weighed at a molar ratio of 39:61 and placed in a mortar. The mixture was thoroughly ground and mixed in a glove box for 1 hour. The resulting powder was then packed into a high-purity graphite crucible and compacted. The crucible was placed in a high-temperature furnace and heated to 750°C at a rate of 5°C / min under a nitrogen atmosphere. The temperature was maintained for 2 hours to ensure complete melting and homogenization of the mixed salt. After the holding period, the mixture was cooled to room temperature with the furnace. The eutectic salt block was removed, crushed in a mortar, and passed through a 200-mesh sieve to obtain NaF-LiF binary eutectic salt powder.

[0052] The remaining steps are the same as in Example 1.

[0053] Figure 5 SEM images of the product prepared in Comparative Example 2, by Figure 5 It is evident that the sample contains a large number of fine fragments and a small number of complete plate-like grains; compared with Example 1, the overall grain size of this sample is significantly smaller, making it difficult to generate large, shaped single crystals.

[0054] Comparative Example 3 The difference between this comparative example and Example 1 is that the pre-activation treatment in step (1) is omitted. That is, after the eutectic salt is prepared, it is not subjected to vacuum pre-activation treatment, but is directly mixed with hexagonal boron nitride micro powder and the subsequent steps are carried out. The rest is the same as Example 1.

[0055] Figure 6 SEM images of the product prepared in Comparative Example 3, by Figure 6 As can be seen, the sample is mainly composed of lamellar grains, and there is also a phenomenon of grain stacking and agglomeration. Compared with Example 1, the overall size of the grains in the sample is smaller, making it difficult to form complete lamellar grains with a large width.

[0056] Comparative Example 4 The difference between this comparative example and Example 1 is that the crucible remains stationary in step (3) and no rotational motion is applied; the rest is the same as in Example 1.

[0057] Figure 7 SEM images of the product prepared in Comparative Example 4, by Figure 7 It can be seen that the crystals in the sample are mainly thin-film hBN particles, but a large number of single crystals agglomerate to form dense flocculent aggregates; the statistically analyzed lateral size distribution of the grains is wide, with the grain size concentrated in the range of 2.8 to 5.9 μm, and the average size is significantly smaller.

[0058] Figure 8 The images show the XRD patterns of the products prepared in Example 1 and Comparative Examples 1-4. Figure 8As can be seen, the diffraction peak positions of the samples obtained in Example 1 and Comparative Examples 1-4 correspond one-to-one, and no obvious impurity diffraction peaks appear in the spectra, proving that the phases of the products in each group are consistent, all being hexagonal boron nitride. There are significant differences in the intensity and shape of the diffraction peaks of different samples; among them, the diffraction peak intensity of Example 1 is the highest and the peak shape is the sharpest; followed by Comparative Example 3, Comparative Example 2, and Comparative Example 4; Comparative Example 1 has the lowest diffraction peak intensity, significant peak broadening, and the worst crystal integrity.

[0059] In summary, the process of this invention can produce hexagonal boron nitride plate-like single crystals with regular morphology, large flake size, and complete crystallization under normal pressure conditions, with excellent crystallinity. When a binary fluorine salt molten salt system is used instead, the molten salt's ability to dissolve hexagonal boron nitride is insufficient, resulting in an increase in lattice defects within the crystal and a significant decrease in crystallinity. When the molten salt vacuum pre-activation treatment is omitted or the periodic rotation of the crucible for forced convection is eliminated, the number of heterogeneous nucleation sites in the melt increases significantly. A large number of small crystal nuclei compete for solute raw materials, making it impossible to develop large-sized single crystals. SEM observations show that the products are filled with a large number of fine grains, with severe grain agglomeration and poor size uniformity.

[0060] Finally, it should be noted that the above content is only used to illustrate the technical solution of the present invention, and is not intended to limit the scope of protection of the present invention. Simple modifications or equivalent substitutions made by those skilled in the art to the technical solution of the present invention do not depart from the essence and scope of the technical solution of the present invention.

Claims

1. A process for rapidly preparing large-size hexagonal boron nitride single crystals using a high-temperature flux method, characterized in that, Includes the following steps: (1) The molten salt medium is pre-activated under vacuum conditions; (2) Mix the pre-activated molten salt medium with the hexagonal boron nitride main raw material and load it into a crucible; (3) Heat the crucible to 1450-1500℃ under a protective atmosphere. After reaching this temperature, enter the heat preservation stage and apply rotational motion to the crucible. Stop rotating after the heat preservation is completed. (4) After cooling, wash and dry the product. Then, heat the product to 1850-1900℃ under a protective atmosphere for high-temperature stabilization treatment. After cooling, obtain hexagonal boron nitride single crystal.

2. The process for rapid preparation of large-size hexagonal boron nitride single crystals using the high-temperature flux method according to claim 1, characterized in that, The molten salt medium mentioned in step (1) is a LiF-NaF-KF ternary eutectic salt or a LiF-CaF2 eutectic salt.

3. The process for rapid preparation of large-size hexagonal boron nitride single crystals using the high-temperature flux method according to claim 1, characterized in that, The temperature of the pre-activation treatment in step (1) is 700-800℃, and the holding time is 2-4 hours.

4. The process for rapid preparation of large-size hexagonal boron nitride single crystals using the high-temperature flux method according to claim 1, characterized in that, The main raw material for hexagonal boron nitride in step (2) is hexagonal boron nitride micro powder with a particle size of 1-5 μm, or a mixture of boron oxide and carbon powder.

5. The process for rapid preparation of large-size hexagonal boron nitride single crystals using the high-temperature flux method according to claim 1, characterized in that, The amount of molten salt medium added in step (2) is 10%-20% of the mass of the main raw material.

6. The process for rapid preparation of large-size hexagonal boron nitride single crystals using the high-temperature flux method according to claim 1, characterized in that, When the main raw material mentioned in step (2) is hexagonal boron nitride micro powder, B2O3 is also added during mixing. The amount of B2O3 added is less than 1% of the mass of the molten salt medium.

7. The process for rapid preparation of large-size hexagonal boron nitride single crystals using the high-temperature flux method according to claim 1, characterized in that, The mixing method described in step (2) is ball milling.

8. The process for rapid preparation of large-size hexagonal boron nitride single crystals using the high-temperature flux method according to claim 1, characterized in that, The protective atmosphere in step (3) is a nitrogen atmosphere, the furnace pressure is 0.11-0.12 MPa, and the heating rate is 10-15℃ / min.

9. The process for rapid preparation of large-size hexagonal boron nitride single crystals using the high-temperature flux method according to claim 1, characterized in that, The rotational motion described in step (3) is a periodic forward and reverse rotation of the crucible along the axial direction, with a maximum speed of ±50-100 rpm and a duration of 1.5-5 minutes for each rotational direction.

10. The process for rapid preparation of large-size hexagonal boron nitride single crystals using the high-temperature flux method according to claim 1, characterized in that, The heat preservation time mentioned in step (3) is 1.5-3 hours.

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