Preparation method of industrial-grade large-size hexagonal boron nitride single crystal

Abstract

The application discloses a preparation method of an industrial-grade large-size hexagonal boron nitride monocrystal. The application adopts Fe, Ni, Cr, Fe / Ni, Fe / Cr and Ni / Cr alloys as metal melts, and utilizes high-purity boron nitride tubes for wrapping, so that the rapid growth of high-quality h-BN bulk monocrystals of centimeter level or even close to inch level can be realized under super-high temperature conditions. The contact mode provided by the application can realize the rapid dissolution of B and N atoms into the melt under high temperature and high pressure, reduce the holding time, effectively control the nucleation position and nucleation amount of hexagonal boron nitride, control the growth of the crystal from the contact position to the center of the melt, and fill the whole melt with the growth of the crystal, so that the yield of the monocrystal can be greatly improved. In addition, the melt only contacts with the parent material, so that the entry of external impurities can be prevented, and the high cleanliness of the monocrystal can be ensured.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of single crystal material preparation, and particularly relates to a preparation method of an industrial-grade large-size hexagonal boron nitride single crystal. BACKGROUND

[0002] Hexagonal boron nitride (h-BN) has a two-dimensional layered structure similar to graphite and is known as "white graphite". It has strong B-N sp2 hybridization covalent bonds within the layers and weak van der Waals forces between the layers, allowing it to be cleaved at the atomic level and produce single-layer, double-layer, and other two-dimensional (2D) materials, hence the name "white graphene". This material exhibits many important properties and is indispensable in scientific research and industrial production. More importantly, unlike graphite and other 2D materials, "white graphene" exhibits unique electrical insulation properties, with a bandgap of 5.9 eV and low dielectric loss, which is unique among 2D materials. In addition, it has high lattice matching with other 2D materials, making it indispensable as a substrate insulating material in graphene and other quantum devices and flexible electronic devices. With the development and application of two-dimensional quantum materials such as graphene, and the development of the flexible electronic device industry, this will bring important opportunities for the application of h-BN. However, these high-end applications usually require high-quality, large-size single crystal h-BN to produce large-area single "white graphene". Due to the high melting point of h-BN, growing single crystals under normal pressure is challenging, and the samples available on the market are usually low-quality micron-sized powders, which cannot meet the strict requirements of future high-end technology industries for material performance, so new preparation methods and approaches need to be explored.

[0003] The properties of h-BN single crystal layers vary with the change in crystal orientation. In polycrystalline h-BN, the crystal orientations of each grain are different, which significantly affects its macroscopic properties. In addition, the grain boundaries in polycrystalline materials also interfere with their physical properties. Therefore, in practical applications, the performance of single crystal samples is significantly better than that of polycrystalline samples. Ideally, h-BN with low defect density and high purity can exhibit the best performance, as defect density can adversely affect the hetero-layer, and high-purity single crystals can ensure maximum optical transparency. Therefore, the preparation of high-quality, large-size h-BN single crystals is crucial for future electronic device research.

[0004] The preparation of h-BN single crystals started in the 1970s, when Ishii et al. first obtained h-BN single crystals with a diameter of about 2 mm and a thickness of about 10 um using silicon as a flux in a nitrogen atmosphere. However, due to the influence of impurities, these h-BN crystals were yellow and contained a large number of nitrogen vacancy defects. Subsequently, Kubota et al. used nickel, nickel-molybdenum and nickel-chromium instead of silicon as a flux to study the growth of h-BN single crystals in different solvents. The results showed that, at atmospheric pressure, the growth rate was too low due to the low solubility of nitrogen in the Ni flux; when using a nickel-molybdenum solvent, the grown crystals were a few hundred um of polycrystals; and when using a nickel-chromium solvent, it was beneficial to increase the thickness of the crystals. In order to increase the size of the h-BN single crystals, Edgar et al. dissolved the polycrystalline powder in a nickel-chromium alloy flux and heated it to 1500 °C under a stream of nitrogen at atmospheric pressure, and then slowly cooled it to 1250 °C at a rate of 4 °C / h, finally obtaining h-BN single crystals with a diameter of 1-2 mm and a single particle diameter of 100-200 um. In order to prepare flaky h-BN under more simple conditions, Liang et al. used boric acid and melamine as reactants, NaCl-KCl as a molten salt system, and prepared single crystal h-BN by microwave heating of the molten salt method in air without nitrogen or ammonia protection. Studies have shown that after 60 minutes of microwave heating, complete hexagonal flaky h-BN crystals can be formed, making it possible to prepare flaky h-BN under relatively simple and mild conditions. However, the size of the h-BN prepared by this method is only um level. In addition to the above-mentioned flux system, the Ba-BN system is also an effective flux for growing single crystal h-BN, but its strong hygroscopicity and easy oxidation are a challenge. Since trace amounts of oxygen pollution can seriously interfere with the growth process of the crystal, the experiment needs to be carried out in a dry nitrogen atmosphere. Therefore, finding alternative solvents to obtain high-quality h-BN single crystals is still a major challenge in this method. The high defect concentration of samples prepared under atmospheric pressure is also an important factor limiting the application of this material in frontier applications.

[0005] So far, it is still difficult to prepare large-size h-BN single crystals. Researchers try to meet the use requirements through thin film preparation, including chemical vapor deposition, magnetron sputtering, molecular beam epitaxy growth and ion beam assisted electron beam deposition, etc. Dahal et al. used triethylboron and ammonia as boron and nitrogen precursors to successfully prepare h-BN thin film by metal organic chemical vapor deposition. Xiang et al. prepared thin film by magnetron sputtering deposition of bulk polycrystalline boron nitride target, and Sutter et al. sputtered boron onto Ru(0001) epitaxial thin film in a nitrogen / argon environment, also obtained a small amount of single crystal thin film. In the study of molecular beam epitaxy growth of h-BN thin film, Gupta et al. used NH3 as nitrogen source to deposit h-BN thin film on sapphire(0001), infrared spectrum confirmed that it was h-BN, but electron diffraction showed that it was polycrystalline structure. There are many problems in the preparation of h-BN thin film, such as the development and use of high-performance precursors. With the increasing requirements of h-BN coating quality, process controllability and safety, etc., the traditional halogen-containing two-component precursor has gradually been difficult to meet the demand. Due to the complexity of chemical vapor deposition, the deposition mechanism of h-BN is not fully clear, and it has a high dependence on the size and structure of the production equipment, the production process is complex, and the cost is high.

[0006] So far, the most successful method for h-BN single crystal preparation is high pressure high temperature method. Watanabe et al. used boron nitride powder as raw material, mixed with fluxing agent Ba-BN compound, heated to 1500-1750 ℃ under 5.5 GPa pressure, and kept for 20 hours. In order to reduce the introduction of impurities, the powder was first heat treated at 2100 ℃ for 2 hours before the experiment, and the physical adsorption of oxygen on the surface of the raw material was removed by nitrogen flow. After the experiment, the sample was dissolved with hot aqua regia to obtain the grown crystal, and the crystal size was about several hundred um. Similarly, Taniguchi et al. used Ba3B2N4 as flux, controlled the growth temperature between 1500-1650 ℃ under 4-5 GPa pressure, and kept for 20 to 80 hours. Although the single crystal size increases with the increase of holding time, it is still in the micron level. In recent years, Zhigadlo used high pressure-high temperature technology to grow transparent colorless h-BN single crystal from Mg-B-N, using magnesium sheet, amorphous boron powder and boron nitride powder as raw material, and controlling the molar ratio of Mg: B: BN to be 1:12:0.1. After removing volatile substances such as oxygen impurities, the crucible filled with Mg-B-BN mixture was placed in the graphite heater and the beryl cube. The sample was gradually compressed to the required pressure during preparation, and heated to 1800-2100 ℃ within 1 hour, kept for 0.5-2 hours, and then reduced to 20 ℃ within 1 hour. Subsequently, the BN crucible was broken, and the excess Mg was removed by heating at 750 ℃ in vacuum for 0.5 hour. Using this method, plate-shaped h-BN crystals with a maximum width of 2.5 mm and a thickness of 10 um can be grown, but the process is complicated, and the post-treatment may introduce impurities, affecting the quality of the final crystal. At present, among the preparation methods of single crystal h-BN, the h-BN single crystal prepared by molten salt precipitation method has small size and needs several hours of holding; the deposition method such as CVD is complex and high cost due to equipment restrictions; the h-BN single crystal prepared by high temperature-high pressure technology has a size of mm level, but needs tens of hours of holding, long cycle and high cost. Therefore, the preparation of fast, low-cost and large-size h-BN single crystal is still an urgent engineering and scientific problem to be solved.

[0007] Although Japanese scientists successfully prepared high-quality single crystal h-BN by high temperature and high pressure method, due to the size limitation of high pressure cavity and the limitation of high temperature and high pressure technology, the prepared single crystal is only 1-3 mm, and the yield is extremely low, which can only meet the research purpose at present. Similarly, Edgar et al. claimed to grow centimeter-level single crystal under normal pressure by using Fe under normal pressure, which can be obtained by adhesive tape peeling, and the single crystal size is questionable, and the yield is extremely low. Therefore, it is of great significance to develop a new high temperature and high pressure preparation method to prepare industrial-grade single crystal h-BN with a size of centimeter or more to inch. SUMMARY

[0008] The present application aims to overcome at least one deficiency of the prior art, and provide a preparation method of an industrial-grade large-size hexagonal boron nitride single crystal.

[0009] The technical scheme adopted by the present application is:

[0010] The present application provides a preparation method of an industrial-grade large-size hexagonal boron nitride single crystal, comprising the following steps:

[0011] 1) Place the metal core column in the boron nitride tube, pressurize to a growth pressure of 2-6 GPa, and after reaching the set pressure, enter the pressure maintaining stage;

[0012] 2) Heat at a rate of 10-20 ℃ / min to 1000-1500 ℃ and release internal stress;

[0013] 3) Continue to heat at a rate of 10-20 ℃ / min, reach a growth temperature of 1800-3000 ℃, and maintain at this temperature for 0.5-2 hours;

[0014] 4) Control the crystal growth by reducing the temperature at a rate of 1-5 ℃ / min, remove the metal after the crystal growth is completed, and wash and dry to obtain the hexagonal boron nitride single crystal.

[0015] In some examples, the diameter ratio of the metal core column to the boron nitride tube is 0.65-0.7.

[0016] In some examples, the height ratio of the metal core column to the boron nitride tube is 0.65-0.67.

[0017] In some examples, the diameter difference between the boron nitride tube and the metal core column is not less than 2 mm.

[0018] In some examples, the metal core column is made of Fe, Ni, Cr, or Fe / Ni, Fe / Cr, Ni / Cr alloy.

[0019] In some examples, the growth pressure is 2-5.5 GPa.

[0020] In some examples, the growth temperature is 2000-2200 ℃.

[0021] In some examples, the pressure for single crystal growth is 4 GPa, and the temperature is maintained at 2200 ℃ for 1 h.

[0022] In some examples, the purity of the boron nitride tube is not less than 99.999%.

[0023] In some examples, the metal is removed by acid washing.

[0024] The above features can be combined arbitrarily without conflict.

[0025] The beneficial effects of the present application are:

[0026] 1) The contact method provided by the present application can quickly realize the rapid dissolution of B and N atoms into the melt under high temperature and high pressure, reducing the holding time.

[0027] 2) The contact method provided by the present application can effectively control the nucleation position and nucleation amount of hexagonal boron nitride, and can control the growth of the crystal from the contact position to the center of the melt.

[0028] 3) The contact method provided by the present application can realize the growth of the crystal to fill the entire melt, which can greatly improve the yield of single crystals.

[0029] 4) The contact method provided by the present application can realize the contact of the melt only with the parent material, which can prevent the entry of external impurities and ensure the high cleanliness of the single crystal. BRIEF DESCRIPTION OF DRAWINGS

[0030] Figure 1 Assembled schematic diagram of high-pressure cavity required for h-BN single crystal growth, wherein: (a) schematic diagram of pressurization of high-pressure assembly; (b) assembled physical diagram of pyrophyllite; (c) h-BN tube wrapped around a metal core column (Fe or Ni, Cr, Fe / Ni, Fe / Cr, Ni / Cr column); (d) assembled schematic diagram of h-BN tube and internal metal core column (Fe or Ni, Cr, Fe / Ni, Fe / Cr, Ni / Cr column).

[0031] Figure 2 Assembled schematic diagram of high-pressure cavity required for h-BN single crystal growth, wherein: (a) schematic diagram of pressurization of high-pressure assembly; (b) assembled physical diagram of pyrophyllite; (c) h-BN tube wrapped around a metal core column (Fe or Ni, Cr, Fe / Ni, Fe / Cr, Ni / Cr column); (d) assembled schematic diagram of h-BN tube and internal metal core column (Fe or Ni, Cr, Fe / Ni, Fe / Cr, Ni / Cr column).

[0032] Figure 3 Hexagonal boron nitride obtained by using iron (Fe), nickel (Ni) and chromium (Cr) as the melt, respectively.

[0033] Figure 4 h-BN single crystal size and yield characterization, wherein: (a) h-BN single crystals grown at 5 GPa pressure and different temperatures; (b) h-BN single crystals obtained by holding at 2200 ℃ under different pressures; (c) h-BN single crystals obtained by holding at 5 GPa pressure and 2200 ℃ temperature for different times; (d) h-BN single crystal yield obtained by holding at 4 GPa pressure and 2200 ℃ temperature for 1 h; (e) h-BN single crystal size obtained by holding at 4 GPa pressure and 2200 ℃ temperature for 2 h.

[0034] Figure 5 Hexagonal boron nitride single crystals obtained by holding at 5 GPa, (a) 1800 ℃ and (b) 1950 ℃, respectively.

[0035] Figure 6 For h-BN single crystal quality characterization, (a) XRD spectrum; (b) low wave number Raman spectrum; (c) Raman spectrum E 2g vibration mode of h-BN single crystal sample; (d) relationship between peak width at half height and peak position of Raman spectrum E2g vibration mode of h-BN single crystal sample; (e) ultraviolet-visible spectrum of h-BN single crystal; and (f) dielectric characterization of thin layer h-BN single crystal sample. DETAILED DESCRIPTION

[0036] The application provides a preparation method of an industrial-grade large-size hexagonal boron nitride single crystal, comprising the following steps:

[0037] 1) placing a metal core column in a boron nitride tube, pressurizing to a growth pressure of 2-6 GPa, and entering a pressure maintaining stage after reaching the set pressure;

[0038] 2) heating at a rate of 10-20 ℃ / min to 1000-1500 ℃ and maintaining the temperature to release internal stress;

[0039] 3) continuing to heat at a rate of 10-20 ℃ / min, reaching a growth temperature of 1800-3000 ℃, and maintaining the temperature for 0.5-2 hours;

[0040] 4) reducing the temperature at a rate of 1-5 ℃ / min to control crystal growth, removing the metal after the crystal growth is completed, and washing and drying to obtain the hexagonal boron nitride single crystal.

[0041] The large size refers to that the length of the hexagonal boron nitride single crystal is not less than 1 mm.

[0042] In some examples, the diameter ratio of the metal core column to the boron nitride tube is 0.65-0.7. In this way, sufficient contact area between the metal core column and the boron nitride tube can be ensured, facilitating the growth of the hexagonal boron nitride single crystal.

[0043] In some examples, the height ratio of the metal core column to the boron nitride tube is 0.65-0.67. In this way, the boron nitride can sufficiently contact the metal core column, facilitating the growth of the hexagonal boron nitride single crystal.

[0044] In some examples, the diameter difference between the boron nitride tube and the metal core column is not less than 2 mm.

[0045] In some examples, the metal core column is made of Fe, Ni, Cr or Fe / Ni, Fe / Cr or Ni / Cr alloy. Experimental data show that the metal core column made of these metals or alloys can better dissolve boron nitride as a solute at high temperature and high pressure, which is conducive to obtaining high-quality and large-size hexagonal boron nitride single crystals.

[0046] In some examples, the growth pressure is 2-5.5 GPa.

[0047] In some examples, the growth temperature is 2000-2200 ℃. At this temperature, a better quality and larger size of h-BN single crystal can be obtained.

[0048] In some examples, the pressure for single crystal growth is 4 GPa, and the temperature is maintained at 2200 ℃ for 1 h. Data show that the h-BN single crystal under this condition has better performance.

[0049] In some examples, the purity of the BN tube is not less than 99.999%. A high-purity BN tube is more conducive to obtaining a high-quality h-BN single crystal.

[0050] The metal can be removed by any method that does not damage or less damages the h-BN single crystal. In some examples, the metal is removed by acid washing. The operation of acid washing is simple and effective. The metal can also be removed by electrolysis.

[0051] The above features can be combined arbitrarily without conflict.

[0052] The following disclosure provides many different embodiments or examples for implementing different aspects of the present application. Embodiments

[0053] The present application uses a high-temperature and high-pressure method to grow h-BN bulk single crystals. The experiment is carried out on a cubic press device (DS650). Figure 1 The assembly of the high-pressure chamber for h-BN single crystal growth is shown. Figure 1 (a) shows the assembled pyrophyllite assembly (see Figure 2 ) is placed between the six WC anvils of the cubic press. By applying pressure through the piston cylinder, the WC anvil slowly extrudes the pyrophyllite block, thereby generating high pressure inside it. The high-purity BN tube wraps the high-purity Fe column, and is assembled with other high-pressure chamber components in an argon-filled glove box to prevent the introduction of an oxidative atmosphere. The outer diameter and height of the h-BN tube are both 22 mm, the inner diameter is 15.1 mm, and the diameter of the high-purity Fe column is 15 mm and the height is 10 mm. A Fe column that is too high or too large in diameter can overflow the h-BN tube at high temperatures, resulting in instability of the high-pressure chamber and failure of the experiment.

[0054] Figure 2(a) shows the process curve during the growth of h-BN single crystals. In the experiment, the leaf waxite assembly block of the cubic press was first pressurized to a target pressure of 2-5.5 GPa. When the set pressure was reached, the pressure holding stage was entered and heating was started. The preliminary heating was carried out at a rate of 10-20 ℃ / min until 1500 ℃ was reached, and the temperature was kept at this temperature for 10 minutes to ensure the stability of the cavity and release the internal stress. Next, heating was continued at a rate of 5 ℃ / min to reach a range of 1800 to 2200 ℃, and the temperature was kept at this temperature for 10 to 30 minutes. During this process, the Fe column melted, and the boron (B) and nitrogen (N) atoms in the h-BN dissolved in the melt and formed nuclei at the contact surface between the Fe melt edge and the h-BN tube, as shown in Figure 2 (b) left. Subsequently, the crystal growth was controlled by cooling at a rate of 1-5 ℃ / min. Too fast cooling can affect the size and quality of the crystal, while too slow cooling contributes limitedly to the size growth, Figure 2 (b) right shows a schematic diagram after the crystal growth.

[0055] Figure 3 h-BN single crystals wrapped in Fe, Ni, and Cr columns, respectively, obtained under high temperature and high pressure conditions using iron (Fe), nickel (Ni), and chromium (Cr) as solutes, are shown. Then the metal Fe (Ni or Cr) was removed by dilute hydrochloric acid (HCl), and finally the pure h-BN single crystal sample was obtained by washing with pure water and anhydrous ethanol and drying in an oven at 70 ℃.

[0056] Figure 4 The influence of adjusting the growth temperature, pressure, and holding time on the size and yield of h-BN single crystals is shown. At 5 GPa, the holding temperature was set to 1800, 1950, 2100, and 2200 ℃, and the holding time was 30 minutes, and the grown h-BN single crystals are shown in Figure 4 (a). When the holding temperature was 1800 ℃, the single crystal size was very small, only 1-2 mm, mainly in the form of towers, pyramids, or cones (see the optical micrographs in Figure 5 (a)), and the color was gray, which may be due to the presence of more melt that was not excluded. When the temperature was raised to 1950 ℃, the color of the h-BN single crystal changed obviously from gray to white, and the morphology gradually changed to flaky (see Figure 5(b), which indicates that the growth of h-BN single crystal along the c-axis direction (perpendicular to the h-BN layer direction) is weakened, while the growth along the a, b-axis direction (parallel to the h-BN layer direction) is enhanced, as the temperature increases. When the temperature reaches 2100 ℃, the single crystal basically presents a flake-like morphology, and the size rapidly increases to about 5-10 mm, while at 2200 ℃, the crystal size reaches about 10 mm, and the yield is significantly improved, indicating that as the temperature approaches 2200 ℃, the solubility of B and N atoms in the Fe melt (or Cr, Ni, Fe / Cr, Fe / Ni and Cr / Ni melt) further increases, which is beneficial to the rapid growth of the crystal and the obtaining of large-size crystals.

[0057] Under the condition of a fixed temperature of 2200 ℃, the growth pressure is set to 2, 3, 4 and 5 GPa, and the results show that under the pressure of 2-4 GPa, the growth yield of h-BN single crystal is similar, while at 5 GPa, the yield decreases (see Figure 4 (b), which indicates that the solubility of B / N atoms in the high-temperature Fe melt may have a peak value. In addition, too high pressure may promote the conversion of h-BN to cubic boron nitride (c-BN), which may be the reason for the decrease in yield. It is worth noting that for the case of more melt, appropriately increasing the pressure helps to reduce the fluidity of the Fe melt to maintain the stability of the experiment. The inventors also explored the effect of holding time on the yield of the crystal, as shown in Figure 4 (c), increasing the holding time significantly improves the yield of h-BN single crystal. Figure 4 (d) and (e) respectively show the yield and size of h-BN single crystal obtained under the condition of 4 GPa pressure and 2200 ℃ temperature for 1 hour, and hundreds of 1 cm size and thousands of 1-3 mm size h-BN single crystals can be obtained in a single growth.

[0058] X-ray diffraction (XRD) characterization of single-layer hexagonal h-BN (as shown in Figure 6 (a)) shows that in the range of 10 to 90 degrees, only the diffraction peaks of 002, 004 and 006 are observed, and no diffraction peaks of other crystal faces are detected, which indicates the high crystallinity of the crystal. The half-peak width of the 006 diffraction peak is analyzed, and the width is 0.13°, which is comparable to the 0.14° of the high-quality h-BN single crystal reported in the current literature, reflecting that the h-BN single crystal prepared by the inventors has an international advanced level of quality. Figure 6 (b) shows the Raman spectrum of h-BN single crystal, and the peak values at frequencies of 52.5 cm⁻¹ and 1365.7 cm⁻¹ correspond to the interlayer shear low-frequency vibration mode (A g ) of h-BN and the in-plane reverse high-frequency vibration mode (E 2gAs shown in the illustration, the presence of low-frequency vibrational modes reflects the degree of crystallinity of h-BN. These modes typically only appear in single-crystal h-BN within the material, indicating that the h-BN single crystals prepared by the inventors have a significant thickness (~500 micrometers). According to E... 2g The half-width at half-maximum (WHM) of the vibrational modes can be used to assess the crystallinity of h-BN. The single-crystal boron nitride prepared by the inventors has a WHM of approximately 7.9 cm⁻¹ (e.g., Figure 6 (as shown in (c)), this is comparable to the highest quality h-BN single crystals reported worldwide (>7.8 cm⁻¹). Furthermore, the inventors discovered E 2g The half-width at half-maximum (WHM) and peak position show an approximately linear decreasing relationship. Figure 6 (d) shows that this may be related to phonon softening caused by the introduction of internal defects, a relationship that can also be used to evaluate the quality of the material. To assess the content of internal impurities such as oxygen and carbon, the inventors performed ultraviolet-visible absorption spectroscopy analysis on the sample (e.g., Figure 6 (e) shows this. Typically, the introduction of oxygen and carbon atoms creates specific pseudolevels in h-BN, lowering the intrinsic band gap. In the inventors' data, no significant absorption was observed throughout the 2 to 6 eV range, indicating high sample purity. Figure 6 As shown in (f), a 21 nm thick layer was prepared by mechanical exfoliation. The breakdown critical field strength was calculated to be 50% higher than the typical BN value of 800 MV / m. This indicates that it has excellent dielectric properties and reflects that the h-BN single crystal has extremely low impurity content.

[0059] The above is a further detailed description of the present invention and should not be considered as a limitation on the specific implementation of the present invention. For those skilled in the art, simple deductions or substitutions without departing from the concept of the present invention are all within the protection scope of the present invention.

Claims

1. A method for preparing industrial-grade large-size hexagonal boron nitride single crystals, characterized in that, Includes the following steps: 1) Place the metal core column inside the boron nitride tube and pressurize it to a growth pressure of 2-6 GPa. After reaching the set pressure, enter the pressure holding stage. 2) Heat to 1000-1500 ℃ at a rate of 10-20 ℃ / min and hold at that temperature to release internal stress; 3) Continue heating at a rate of 10–20 °C / min until a growth temperature of 1800–3000 °C is reached, and maintain this temperature for 0.5–2 hours; 4) Control the crystal growth by cooling at a rate of 1-5 °C / min. After the crystal growth is complete, remove the metal, wash and dry to obtain hexagonal boron nitride single crystal.

2. The preparation method according to claim 1, characterized in that, The diameter ratio of the metal core and the boron nitride tube is 0.65 to 0.

7.

3. The preparation method according to claim 1, characterized in that, The height ratio of the metal core column to the boron nitride tube is 0.65 to 0.

67.

4. The preparation method according to claim 1, characterized in that, The diameter difference between the boron nitride tube and the metal core column is not less than 2 mm.

5. The preparation method according to claim 1, characterized in that, The metal core is made of Fe, Ni, Cr or Fe / Ni, Fe / Cr, Cr / Ni alloy.

6. The preparation method according to claim 1, characterized in that, The growth pressure is 2–5.5 GPa.

7. The preparation method according to claim 1, characterized in that, The growth temperature is 2000–2200 ℃.

8. The preparation method according to any one of claims 1 to 7, characterized in that, The single crystal growth was carried out at a pressure of 4 GPa and maintained at 2200 °C for 1 hour.

9. The preparation method according to any one of claims 1 to 7, characterized in that, The purity of the boron nitride tube is not less than 99.999%.

10. The preparation method according to any one of claims 1 to 7, characterized in that, Metals are removed by acid pickling.

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