Preparation method for growing large-size isotope hexagonal boron nitride at normal pressure and high temperature and application of large-size isotope hexagonal boron nitride in neutron detection

Abstract

The invention discloses a preparation method for growing large-size isotope hexagonal boron nitride at normal pressure and high temperature and application of the large-size isotope hexagonal boron nitride in neutron detection, and the method comprises the following steps: uniformly mixing nickel powder, chromium powder and boron powder to obtain a mixture of boron and nickel-chromium alloy; then carrying out deoxidation treatment under the protection gas flow; reducing the pressure of the system, introducing mixed gas of protective gas and reactive protective gas to normal pressure, and protecting; finally, the deoxidized mixture is subjected to secondary stepped heating, heat preservation is carried out after primary heating, and the temperature of primary heating is higher than that of secondary heating; and naturally cooling to room temperature to generate the hexagonal boron nitride crystal. According to the method, the nickel-chromium alloy is adopted as the metal fluxing agent, the large-size and high-quality isotope hexagonal boron nitride single crystal is successfully prepared under the normal pressure condition through the optimized gas atmosphere and stepped temperature control, and the method has important application value in the fields of neutron detection, two-dimensional materials and the like.

Description

Technical Field

[0001] This invention belongs to the fields of single crystal material preparation technology and neutron detection technology, and relates to a method for preparing large-size isotope hexagonal boron nitride under normal pressure and high temperature and its application in neutron detection. Background Technology

[0002] Hexagonal boron nitride (h-BN) is known as "white graphene" due to its layered crystal structure similar to graphite. Its monolayer consists of boron (B) and nitrogen (N) atoms separated by sp... 2 Hybrid bonding forms a six-membered ring planar honeycomb structure with bond lengths of approximately 1.45 Å and bond angles close to 120°. Interlayer stacking relies on van der Waals forces, with a spacing of approximately 3.3 Å, significantly larger than the interlayer spacing of graphite, thus exhibiting excellent interlayer exfoliation properties. h-BN generally adopts an AA′ stacking mode, where boron atoms are directly above nitrogen atoms. This configuration effectively suppresses interlayer electronic coupling and enhances structural stability. In terms of physical properties, h-BN not only possesses high in-plane stiffness and thermal conductivity but also exhibits an ultra-wide bandgap semiconductor with a bandgap of approximately 5.9 eV and a resistivity as high as 10⁻⁶ eV. 16 -10 18 It exhibits a low Ω·m density and maintains good optical transparency in the deep ultraviolet band (200–300 nm). It possesses excellent thermal stability, with an oxidation initiation temperature of approximately 800 °C in air and a low coefficient of friction of around 0.03 at high temperatures, making it an ideal high-temperature lubricant. Chemically, h-BN can withstand temperatures up to 2270 °C in air, sublimates at approximately 3000 °C, and is inert to common acids, alkalis, and oxidants, reacting only under high-temperature or strong reducing conditions. These combined properties make it highly promising for applications in extreme environments such as two-dimensional electronic device substrates, deep ultraviolet optical windows, high-temperature protective coatings, and neutron shielding and detection in nuclear reactors.

[0003] In the field of neutron detection, h-BN has particularly promising prospects, mainly due to its isotope properties. 10 B possesses an extremely high thermal neutron capture cross section (σ = 3840 barns). When neutrons are... 10 After B is captured, the nuclear reaction releases high-energy alpha particles and 7 Li particles, as secondary particles, travel through the h-BN lattice, exciting a large number of electron-hole pairs. Under the influence of an applied electric field, the electrons and holes separate and are collected by electrodes, forming a detectable electrical signal, thus enabling the direct detection of neutrons. Compared to coated semiconductor neutron detectors, h-BN-based devices can effectively avoid self-absorption effects, improving detection efficiency and signal-to-noise ratio.

[0004] However, the preparation and crystal quality of h-BN single crystals remain key bottlenecks restricting their practical application in neutron detection. Currently, the mainstream methods include chemical vapor deposition (CVD) and metal solvent methods. CVD typically uses transition metals and their alloys (such as Cu, Ni, Pt, etc.) as substrates, and achieves the epitaxial growth of h-BN by transporting precursors through a carrier gas in a high-temperature region. Although CVD technology has made continuous progress in the preparation of high-quality h-BN thin films, it has inherent contradictions in controlling single crystal size and thickness: pursuing large-size single crystals (such as centimeter-scale) requires extended growth time, which easily leads to interlayer stress accumulation, causing edge curling or cracking; while controlling the thickness to a single layer or few layers easily results in thickness unevenness due to fluctuations in precursor concentration, and consequently, the sensitivity of h-BN insulation properties to thickness leads to local leakage current, affecting device stability.

[0005] In comparison, the metal solvent method exhibits significant advantages in crystal size and thickness control. This method can prepare h-BN single crystals with millimeter-scale diameters, and its thickness control capability is superior to CVD, which is beneficial for achieving uniform multilayer structures and suitable for applications requiring high thickness consistency. By optimizing the composition of the metal melt and process conditions, the metal solvent method holds promise for preparing large-area, high-thickness, and high-quality h-BN single crystals. However, the appropriate selection of the metal melt, limitations in experimental conditions, and the use of... 10 The unique challenges posed by using B isotopes as raw materials all affect h 10 BN and h 11 The growth of BN single crystals has placed higher demands on the process. Summary of the Invention

[0006] The purpose of this invention is to solve the technical problems of the contradiction between simultaneously satisfying the thickness and size requirements in the preparation of single crystal materials by existing technologies, and the harsh preparation conditions. This invention provides a method for preparing large-size isotope hexagonal boron nitride under normal pressure and high temperature, and its application in neutron detection.

[0007] To achieve the above objectives, the present invention employs the following technical solution: The first aspect of this invention provides a method for preparing large-size isotopic hexagonal boron nitride by growing it at ambient pressure and high temperature, comprising the following steps: S1, after uniformly mixing nickel powder, chromium powder and boron powder, a mixture of boron and nickel-chromium alloy is obtained; S2, a mixture of boron and nickel-chromium alloy is passed through a protective gas flow to deoxidize it, resulting in a deoxidized mixture; S3, for the deoxygenated mixture, reduce the pressure and introduce protective gas and reactive protective gas to atmospheric pressure, and then continue to introduce protective gas for protection; S4 involves a two-stage heating process on the deoxidized mixture. After the first heating, the mixture is held at a higher temperature than the second heating. After natural cooling to room temperature, hexagonal boron nitride crystals are formed.

[0008] Preferably, the nickel-chromium alloy contains 40-60 wt.% nickel.

[0009] Preferably, the nickel-chromium alloy has a nickel mass fraction of 50 wt.%.

[0010] Preferably, the mass ratio of nickel powder, chromium powder and boron powder is 9:9:(1.134-1.206).

[0011] Preferably, the protective gas is Ar / H2; the mass ratio of Ar to H2 is (14-19):(1-6).

[0012] Preferably, the deoxidation of the boron-nickel-chromium alloy mixture by passing it through a protective gas flow includes: After depressurizing the boron and nickel-chromium alloy mixture, a protective gas flow is passed through it for protection, and it is heated to 1400-1700℃ and held at that temperature. Then, it is cooled to 700-1000℃ and then allowed to cool naturally to room temperature to complete the deoxidation.

[0013] Preferably, the reactive protective gas is N2.

[0014] Preferably, the temperature of the first heating is 1300-1700℃; the temperature of the second heating is 700-1000℃.

[0015] Preferably, in step S3, the flow rate of the protective gas is 10-50 sccm, and the flow rate of the reactive protective gas is 100-200 sccm.

[0016] Preferably, in S2, during deoxygenation, the protective gas flow rate is 100-200 sccm.

[0017] Preferably, in step S3, the mass ratio of Ar to H2 in the protective gas flow Ar / H2 is 19:1.

[0018] Preferably, in S2, the deoxidation process is as follows: after mixing boron powder with Ni-Cr alloy evenly, the pressure inside the furnace tube is reduced to below 1 Pa, and then the pressure is reduced to below 0.01 Pa. Ar / H2 is introduced to atmospheric pressure, and this process is repeated 3-5 times. Then, under the protection of Ar / H2 gas flow, the furnace is heated to 1500℃, maintained for 48 h, and then naturally cooled to room temperature to complete the deoxidation.

[0019] Preferably, in S1, the mass ratio of boron powder to nickel powder and chromium powder is 1.17:9:9.

[0020] Preferably, in step S3, the pressure is reduced to 5 × 10⁻⁶. -2 If the pressure is below Pa, continue to pump the pressure down to below 0.01 Pa, then introduce Ar / H2 and N2 gas flow to normal pressure. Repeat this process 2-3 times. Finally, introduce Ar / H2 and N2 gas flow for protection. The Ar / H2 gas flow rate is 25 sccm and the N2 gas flow rate is 125 sccm.

[0021] A second aspect of the present invention is an isotopic hexagonal boron nitride single crystal prepared by the above method.

[0022] A third aspect of the present invention provides an application of the isotopic hexagonal boron nitride single crystal prepared by the above method in neutron detection.

[0023] Compared with the prior art, the present invention has the following beneficial effects: This invention discloses a method for preparing large-size isotopic hexagonal boron nitride at ambient pressure and high temperature. A boron-nickel-chromium alloy mixture is formed by mixing nickel powder, chromium powder, and boron powder. The nickel-chromium alloy not only serves as a dispersion carrier for boron but also reduces the melting point and diffusion resistance of boron through alloying, significantly enhancing the activity of boron in the subsequent nitriding reaction. Compared to traditional single-metal carrier systems, this mixed system improves the utilization rate of boron and avoids raw material waste. Deoxidation treatment using a protective gas flow efficiently removes impurities such as free oxygen and hydroxyl groups from the mixture. On the one hand, it avoids the formation of boron oxide (B2O3) impurities at high temperatures—the presence of boron oxide severely hinders the nitriding reaction and easily forms defects in the crystal. On the other hand, it reduces the impact of impurities on the electrical and thermal properties of the isotopic hexagonal boron nitride. An atmosphere control strategy of "pressure reduction - introduction of protective and reactive protective gases - repressurization to atmospheric pressure" is adopted. The pressure reduction process removes residual gases such as air and moisture from the reaction system, preventing side reactions with the raw materials at high temperatures. The subsequently introduced protective gas isolates the system from air, while the reactive protective gas (such as ammonia, providing a nitrogen source) is evenly distributed throughout the system, ensuring sufficient contact between nitrogen and boron elements. This solves the problem of discontinuous crystal growth caused by uneven nitrogen source supply in traditional atmospheric pressure preparation. Compared to high-pressure preparation processes, atmospheric pressure eliminates the need for complex high-pressure equipment, reducing equipment costs and operational risks. It also avoids the restriction of crystal growth direction by high pressure, facilitating the lateral growth of large-size crystals and providing feasibility for preparing inch-scale isotopic hexagonal boron nitride single crystals. Large-size, high-quality crystal growth is achieved through a stepped heating process. A high initial heating temperature rapidly activates the reactivity of the raw materials, promoting the chemical reaction between boron and nitrogen to form hexagonal boron nitride nuclei. Subsequent holding at this temperature ensures uniform nuclei growth, preventing excessive nuclei from competing for growth and resulting in small crystals or polycrystalline structures. A secondary heating process, at a lower temperature than the initial heating, slows the crystal growth rate, reduces defects during crystal growth (such as dislocations and interlayer dislocations), and promotes orderly growth along a specific direction (such as the c-axis), improving crystallinity and integrity. Finally, after natural cooling, isotopic hexagonal boron nitride crystals larger than 5 mm in size, with uniform thickness and a smooth surface are produced.

[0024] Furthermore, controlling the nickel mass fraction in the nickel-chromium alloy to 40-60 wt.% ensures excellent structural stability at high temperatures. When the nickel content is below 40 wt.%, the chromium content is too high, leading to increased brittleness and structural fracture at high temperatures. This makes it difficult to stably support the reaction process of boron powder and nitrogen source, potentially causing the reaction system to collapse and affecting crystal formation. Conversely, when the nickel content exceeds 60 wt.%, the alloy's high-temperature strength decreases, and it is prone to softening and deformation during prolonged high-temperature holding, making it difficult to maintain a stable reaction space. The nickel content range of 40-60 wt.% ensures that the nickel-chromium alloy possesses sufficient high-temperature strength to resist deformation and good toughness to prevent brittle fracture throughout the entire atmospheric pressure high-temperature preparation cycle, providing a stable and continuous matrix support for the full reaction of boron and nitrogen and the growth of hexagonal boron nitride crystals.

[0025] Furthermore, the protective gas uses a mixture of Ar and H2 with a mass ratio controlled at (14-19):(1-6), which significantly improves the deoxygenation effect and clears impurities for subsequent reactions. H2, as a reducing gas, can react with residual oxygen and oxidizing impurities (such as boron oxide and metal oxides) in the mixture to generate easily discharged H2O, thereby deeply removing oxygen impurities from the system; while Ar, as an inert gas, can provide a stable environment for the reduction reaction of H2 and prevent other gases from participating in side reactions. When the mass ratio of Ar to H2 is within this range, the H2 content ensures sufficient reducing activity to effectively remove oxygen impurities, while preventing the reaction from becoming too vigorous due to an excessively high H2 ratio (e.g., below 14:6), which could lead to localized overheating or the generation of excess hydride impurities. At the same time, the Ar ratio (14-19 parts) ensures that it can fully cover the reaction system, isolate air, and prevent external oxygen from re-entering. This synergizes with the deoxidation process in step S2, reducing the oxygen content of the system to an extremely low level and generating high-purity isotopic hexagonal boron nitride crystals.

[0026] Furthermore, the step-by-step temperature control strategy ensures the size and quality of the crystal. Specifically, high-temperature heat preservation matches the thermodynamic growth window of the h-BN crystal, promoting the full diffusion, migration, and alignment of boron and nitrogen atoms. Combined with long-term heat preservation, it provides sufficient time for the crystal to grow from the nanoscale to a large size (up to the millimeter scale), solving the problems of small size and easy agglomeration of h-BN crystals in traditional processes. Through the medium-temperature cooling stage, the concentration of thermal stress inside the crystal caused by direct cooling is avoided, reducing the probability of crystal cracking and deformation. At the same time, the crystal lattice arrangement can be further optimized, the defect density in the crystal can be reduced, the thermal conductivity of the h-BN crystal can be improved, and the insulation performance and chemical stability can also be significantly enhanced.

[0027] Furthermore, the initial high-temperature heating (1300-1700℃) significantly enhances the diffusion rate of boron atoms in the nickel-chromium alloy matrix, breaking the bonds between boron and metal atoms. This allows boron atoms to detach from the alloy lattice with sufficient energy and rapidly combine with the introduced nitrogen source (such as active nitrogen atoms) to form the initial nuclei of hexagonal boron nitride. If the temperature is below 1300℃, the diffusion activity of boron atoms is insufficient, resulting in a slow reaction rate with the nitrogen source and difficulty in forming a sufficient number of nuclei, leading to a lack of "seeds" for subsequent crystal growth. If the temperature is above 1700℃, it can cause excessive softening or even local melting of the nickel-chromium alloy, damaging the integrity of the matrix structure. It may also trigger excessive decomposition of the nitrogen source, generating excess gaseous impurities that interfere with nuclei formation. The temperature range of 1300-1700℃ achieves a balance between "rapid reaction and stable nucleation," providing a suitable number and uniformly distributed high-quality nuclei for subsequent large-size crystal growth. The second low-temperature heating (700-1000℃) is crucial after the initial high-temperature heating forms the crystal nucleus. During crystal growth, the growth rate needs to be controlled to prevent defects. This temperature range maintains the reactivity of boron and nitrogen atoms, ensuring continuous nucleus growth, while also slowing atomic diffusion. This allows sufficient time for atoms to align orderly along the optimal growth direction (e.g., the c-axis of hexagonal boron nitride), reducing defects such as interlayer dislocations and other defects caused by excessively rapid growth. If the second heating temperature is below 700℃, atomic reactivity is insufficient, crystal growth nearly stops, and large-size crystals cannot be formed. If the temperature is above 1000℃, atomic diffusion is too rapid, easily leading to disordered atomic accumulation on the nucleus surface, forming polycrystalline structures or irregular grains, thus compromising crystal integrity. The 700-1000℃ temperature design achieves an optimal balance between crystal growth rate and atomic arrangement order, ultimately producing hexagonal boron nitride crystals. The significant temperature gradient (difference of 300-1000℃) between the first high temperature and the second low temperature guides the crystal to grow laterally, creating conditions for large-size crystal fabrication. During the rapid temperature drop (from 1300-1700℃ to 700-1000℃) of crystal nuclei formed in the high-temperature stage, a concentration gradient arises due to the difference in temperature distribution within the system: boron and nitrogen atoms surrounding the crystal nuclei migrate towards the low-temperature region (the surface of the crystal nucleus), driving the crystal to expand laterally. Simultaneously, the stable environment in the low-temperature stage prevents the formation of new crystal nuclei, avoiding competition between new and existing nuclei for growth, ensuring that energy and raw materials are concentrated on supplying the existing nuclei, allowing them to continue to expand in size.

[0028] Furthermore, the deoxygenation pretreatment and crystal growth process can be integrated into the same reaction chamber and carried out continuously, avoiding the risk of contamination and oxidation of the sample during the transfer process, simplifying the operation, and enhancing the reliability and repeatability of the process. Attached Figure Description

[0029] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings used in the embodiments will be briefly introduced below. It should be understood that the following drawings only show some embodiments of the present invention and should not be regarded as a limitation on the scope. For those skilled in the art, other related drawings can be obtained based on these drawings without creative effort.

[0030] Figure 1 This is a schematic diagram of the tube furnace required for the growth of hBN single crystals in this invention. Figure 2 This is a metallographic microscope diagram of the hBN single crystal after growth in Example 1. Figure 3 Raman spectrum of the grown hBN single crystal Figure 4 XRD spectrum of the grown hBN single crystal Figure 5 XPS spectrum of the grown hBN single crystal Figure 6 h after growth 10 Raman spectrum of BN Figure 7 h after growth 10 XRD spectrum of BN Figure 8 h after growth 10 BN single crystal and h 11 Neutron detection efficiency of BN single crystal at 25 meV.

[0031] Among them, 1. Temperature control equipment; 2. Temperature control programming display; 3. Air inlet; 4. Tube furnace; 5. Alumina crucible; 6. Corundum furnace tube; 7. Air outlet. Detailed Implementation

[0032] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. The components of the embodiments of the present invention described and shown in the accompanying drawings can generally be arranged and designed in various different configurations.

[0033] Therefore, the following detailed description of the embodiments of the invention provided in the accompanying drawings is not intended to limit the scope of the claimed invention, but merely to illustrate selected embodiments of the invention. All other embodiments obtained by those skilled in the art based on the embodiments of the invention without inventive effort are within the scope of protection of the invention.

[0034] It should be noted that similar labels and letters in the following figures indicate similar items. Therefore, once an item is defined in one figure, it does not need to be further defined and explained in subsequent figures.

[0035] The present invention will now be described in further detail with reference to the accompanying drawings: This invention employs a tube furnace for hBN single crystal growth, comprising a temperature-controlled programming display 2, a temperature control device 1, an inlet 3, a tube furnace 4, an alumina crucible 5, an alumina furnace tube 6, and an outlet 7. The alumina furnace tube 6 penetrates the tube furnace 4; the inlet 3 of the alumina furnace tube 6 is used for ventilation; the outlet 7 of the alumina furnace tube 6 exhausts gas and is connected to a vacuum pump and a molecular pump for regulating the gas pressure during the experiment; the alumina crucible 5 is placed inside the alumina furnace tube 6; the temperature control device 1 is connected to the tube furnace 4 to control the experimental temperature; the temperature-controlled programming display 2 is integrated into the temperature control device 1 for setting the temperature control configuration. The physical separation of the temperature control device 1 from the heating element of the tube furnace 4 effectively isolates the mechanical vibration and electromagnetic interference generated during the operation of the control unit, creating a highly stable microenvironment for single crystal growth. This ensures the stability of the growth interface from a molecular dynamics perspective, thereby effectively suppressing defects and improving the structural integrity and performance consistency of the crystal. The deoxygenation pretreatment and crystal growth process are integrated into the same reaction chamber and carried out continuously, avoiding the risk of contamination and oxidation of the sample during the transfer process, simplifying the operation, and enhancing the reliability and repeatability of the process.

[0036] Example 1 Take 9g of nickel powder, 9g of chromium powder, and 1.17g of boron powder (abundance ≥ 80%). 11 B) Place the mixture into an alumina crucible, as per [reference]. Figure 1 The apparatus involves placing an alumina crucible 5 in the central heating zone of a tube furnace 4. A vacuum pump is used to evacuate the furnace tube pressure to below 1 Pa. A molecular pump is then activated to further evacuate the pressure to below 0.01 Pa. Both the vacuum pump and the molecular pump are then turned off. Ar / H2 is introduced to bring the pressure to atmospheric pressure, and this process is repeated 2-3 times. Finally, an Ar / H2 gas flow is introduced for protection, with H2 comprising 5%. The furnace is heated to 1550°C at a flow rate of 125 sccm and a rate not exceeding 5°C / min, and held at this temperature for 48 hours. After heating, the temperature is lowered to 900°C at a rate of 4°C / min, and then allowed to cool naturally to room temperature to complete the deoxygenation step. Subsequently, a vacuum pump is used to evacuate the furnace tube pressure to 5 × 10⁻⁶ Pa. -2Below Pa, turn on the molecular pump to continue pumping the pressure to below 0.01 Pa, then purge with Ar / H2 and N2 to atmospheric pressure, repeating 2-3 times. Finally, purge with Ar / H2 and N2 gas flow for protection, with an Ar / H2 flow rate of 25 sccm and an N2 flow rate of 125 sccm. Heat to 1550℃ at a rate not exceeding 5℃ / min and hold for 48h. Then cool to 1525℃ at a rate of 0.5℃ / h, then to 1350℃ at a rate of 2℃ / h, and finally to 900℃ at a rate of 4.5℃ / h. After naturally cooling to room temperature, remove the sample to obtain hexagonal boron nitride single crystals.

[0037] A large number of hexagonal boron nitride single crystals were grown on the alloy surface. After the single crystals were peeled off, they were characterized. Figure 2 As shown, the lateral dimension of a single crystal can reach 5 mm.

[0038] Raman spectroscopy was performed on the exfoliated single crystal, such as... Figure 3 As shown, the characteristic peak in the spectrum is located at 1366 cm⁻¹. -1 Half-height and width can reach 9.5cm -1 This indicates that the crystal quality is relatively high. For example... Figure 4 As shown, the X-ray diffraction pattern exhibits a significant diffraction peak at 26.7°, corresponding to the (002) crystal plane of hBN, and a diffraction peak of the (004) crystal plane is detected at 55.1°, confirming that the sample has a strictly periodic layered structure. Figure 5 As shown, X-ray photoelectron spectroscopy revealed characteristic peaks for multiple elements, including O 1s (~530 eV), C 1s (~284-285 eV), Cl 2p (~200 eV), Si 2p (~100 eV), N 1s (~400 eV), and B 1s (~190-200 eV). The O element primarily originates from air or impurities adsorbed on the sample surface, while the C element comes from surface-adsorbed carbon. Since the sample was characterized on a Si / SiO2 substrate, the Si element's presence is an intrinsic signal of the Si / SiO2 substrate. The presence of the N 1s and B 1s peaks confirms the presence of hBN in the sample.

[0039] Implementation Example 2 Take 9g of nickel powder, 9g of chromium powder, and 1.206g of boron powder (abundance ≥ 80%). 11 B) Place the mixture into an alumina crucible, as per [reference]. Figure 1The apparatus involves placing the crucible in the central heating zone of a tube furnace. A vacuum pump is used to evacuate the furnace tube pressure to below 1 Pa. A molecular pump is then activated to further evacuate the pressure to below 0.01 Pa. Both the vacuum pump and the molecular pump are then shut off. Ar / H2 is introduced to atmospheric pressure, with H2 comprising 30%. This process is repeated 2-3 times. Finally, an Ar / H2 gas flow is introduced for protection. The furnace is heated to 1550°C at a flow rate of 200 sccm and a rate not exceeding 5°C / min, and held at this temperature for 60 hours. After heating, the temperature is lowered to 900°C at a rate of 4°C / min, and then allowed to cool naturally to room temperature to complete the deoxygenation step. The pressure inside the furnace tube is then evacuated to 5 × 10⁻⁶ Pa using a vacuum pump. -2 Below Pa, the molecular pump was turned on to continue pumping the pressure to below 0.01 Pa. Ar / H2 and N2 were then introduced to bring the pressure back to ambient pressure, repeated 2-3 times. Finally, an Ar / H2 and N2 gas flow was introduced for protection, with an Ar / H2 flow rate of 10 sccm and an N2 flow rate of 100 sccm. The sample was heated to 1550℃ at a rate not exceeding 5℃ / min and held for 5 hours. Then, the temperature was lowered to 1525℃ at a rate of 0.5℃ / h, then to 1350℃ at a rate of 2℃ / h, and finally to 900℃ at a rate of 4.5℃ / h. After natural cooling to room temperature, the sample was removed, yielding a hexagonal boron nitride single crystal. Raman spectroscopy was performed on the exfoliated single crystal, and the characteristic peak in the spectrum was located at 1366 cm⁻¹. -1 Half-height and width can reach 10.2cm -1 This indicates that the crystal quality is relatively high.

[0040] Implementation Example 3 Take 9g of nickel powder, 9g of chromium powder, and 1.134g of boron powder (abundance ≥ 80%). 11 B) Place the mixture into an alumina crucible, as per [reference]. Figure 1 The apparatus involves placing the crucible in the central heating zone of a tube furnace. A vacuum pump is used to evacuate the furnace tube pressure to below 1 Pa. A molecular pump is then activated to further evacuate the pressure to below 0.01 Pa. Both the vacuum pump and the molecular pump are then shut off. Ar / H2 (15% H2) is introduced until atmospheric pressure is reached, and this process is repeated 2-3 times. Finally, an Ar / H2 gas flow is introduced for protection. The furnace is heated to 1550°C at a flow rate of no more than 5°C / min at a flow rate of 100 sccm, and held at this temperature for 5 hours. After heating, the temperature is lowered to 900°C at a rate of 4°C / min, and then allowed to cool naturally to room temperature to complete the deoxygenation step. The pressure inside the furnace tube is then evacuated to 5 × 10⁻⁶ Pa using a vacuum pump. -2Below Pa, the molecular pump was turned on to continue pumping the pressure to below 0.01 Pa. Ar / H2 and N2 were then introduced to bring the pressure back to ambient pressure, repeated 2-3 times. Finally, an Ar / H2 and N2 gas flow was introduced for protection, with an Ar / H2 flow rate of 50 sccm and an N2 flow rate of 200 sccm. The sample was heated to 1550℃ at a rate not exceeding 5℃ / min and held for 60 h. Then, the temperature was lowered to 1525℃ at a rate of 0.5℃ / h, then to 1350℃ at a rate of 2℃ / h, and finally to 900℃ at a rate of 4.5℃ / h. After natural cooling to room temperature, the sample was removed, yielding a hexagonal boron nitride single crystal. Raman spectroscopy was performed on the exfoliated single crystal, and the characteristic peak in the spectrum was located at 1366 cm⁻¹. -1 Half-height and width can reach 10.0cm -1 This indicates that the crystal quality is relatively high.

[0041] Implementation Example 4 Take 9g of nickel powder, 9g of chromium powder, and 1.17g of boron powder (abundance ≥ 80%). 11 B) Place the mixture into an alumina crucible, as per [reference]. Figure 1 The apparatus involves placing the crucible in the central heating zone of a tube furnace. A vacuum pump is used to evacuate the pressure inside the furnace tube to below 1 Pa. A molecular pump is then activated to further evacuate the pressure to below 0.01 Pa. Both the vacuum pump and the molecular pump are then turned off. Ar / H2 is introduced to bring the pressure to atmospheric pressure, and this process is repeated 2-3 times. Finally, an Ar / H2 gas flow is introduced for protection. The furnace is heated to 1700°C at a flow rate of no more than 5°C / min at a flow rate of 125 sccm, and held at this temperature for 48 hours. After heating, the temperature is lowered to 900°C at a rate of 4°C / min, and then allowed to cool naturally to room temperature to complete the deoxygenation step. The pressure inside the furnace tube is then evacuated to 5 × 10⁻⁶ Pa using a vacuum pump. -2 Below Pa, the molecular pump was turned on to continue pumping the pressure down to below 0.01 Pa. Ar / H2 and N2 were then introduced to bring the pressure back to ambient pressure, repeated 2-3 times. Finally, an Ar / H2 and N2 gas flow was introduced for protection, with an Ar / H2 flow rate of 30 sccm and an N2 flow rate of 150 sccm. The sample was heated to 1700℃ at a rate not exceeding 5℃ / min and held for 48 hours. Then, the temperature was lowered to 1525℃ at a rate of 2℃ / h, then to 1350℃ at a rate of 4℃ / h, and finally to 700℃ at a rate of 4.5℃ / h. After natural cooling to room temperature, the sample was removed, yielding a hexagonal boron nitride single crystal. Raman spectroscopy was performed on the exfoliated single crystal, and the characteristic peak in the spectrum was located at 1366 cm⁻¹. -1 .

[0042] Implementation Example 5 Take 9g of nickel powder, 9g of chromium powder, and 1.206g of boron powder (abundance ≥ 95%). 10 B) Place the mixture into an alumina crucible, as per [reference]. Figure 1The apparatus involves placing the crucible in the central heating zone of a tube furnace. A vacuum pump is used to evacuate the pressure inside the furnace tube to below 1 Pa. A molecular pump is then activated to further evacuate the pressure to below 0.01 Pa. Both the vacuum pump and the molecular pump are then turned off. Ar / H2 is introduced to bring the pressure to atmospheric pressure, and this process is repeated 2-3 times. Finally, an Ar / H2 gas flow is introduced for protection. The furnace is heated to 1400°C at a flow rate of 100 sccm and a rate not exceeding 5°C / min, and held at this temperature for 48 hours. After heating, the temperature is lowered to 900°C at a rate of 4°C / min, and then allowed to cool naturally to room temperature to complete the deoxygenation step. The pressure inside the furnace tube is then evacuated to 5 × 10⁻⁶ Pa using a vacuum pump. -2 Below Pa, the molecular pump was turned on to continue pumping the pressure to below 0.01 Pa. Ar / H2 and N2 were then introduced to bring the pressure back to ambient pressure, repeated 2-3 times. Finally, Ar / H2 and N2 gas flow was introduced for protection, with an Ar / H2 flow rate of 25 sccm and an N2 flow rate of 175 sccm. The sample was heated to 1300℃ at a rate not exceeding 5℃ / min and held for 48 hours. Then, the temperature was lowered to 1525℃ at a rate of 2℃ / h, then to 1350℃ at a rate of 1℃ / h, and finally to 1000℃ at a rate of 4.5℃ / h. After natural cooling to room temperature, the sample was removed, yielding a hexagonal boron nitride single crystal. Raman spectroscopy was performed on the exfoliated single crystal, and the characteristic peak in the spectrum was located at 1391 cm⁻¹. -1 .

[0043] Implementation Example 6 Take 9g of nickel powder, 9g of chromium powder, and 1.206g of boron (abundance ≥ 95%). 10 B) Place the mixture into an alumina crucible, as per [reference]. Figure 1 The apparatus involves placing the crucible in the central heating zone of a tube furnace. A vacuum pump is used to evacuate the pressure inside the furnace tube to below 1 Pa. A molecular pump is then activated to further evacuate the pressure to below 0.01 Pa. Both the vacuum pump and the molecular pump are then turned off. Ar / H2 is introduced to bring the pressure to atmospheric pressure, and this process is repeated 2-3 times. Finally, an Ar / H2 gas flow is introduced for protection. The furnace is heated to 1550°C at a flow rate of 135 sccm and a rate not exceeding 5°C / min, and held at this temperature for 48 hours. After heating, the temperature is lowered to 900°C at a rate of 4°C / min, and then allowed to cool naturally to room temperature to complete the deoxygenation step. The pressure inside the furnace tube is then evacuated to 5 × 10⁻⁶ Pa using a vacuum pump. -2Below Pa, the molecular pump was turned on to continue pumping the pressure to below 0.01 Pa. Ar / H2 and N2 were then introduced to bring the pressure back to ambient pressure, repeated 2-3 times. Finally, an Ar / H2 and N2 gas flow was introduced for protection, with an Ar / H2 flow rate of 25 sccm and an N2 flow rate of 125 sccm. The sample was heated to 1550℃ at a rate not exceeding 5℃ / min and held for 48 hours. Then, the temperature was lowered to 1525℃ at a rate of 2℃ / h, then to 1350℃ at a rate of 1℃ / h, and finally to 900℃ at a rate of 4.5℃ / h. After natural cooling to room temperature, the sample was removed, yielding a hexagonal boron nitride single crystal. Raman spectroscopy was performed on the exfoliated single crystal, and the characteristic peak in the spectrum was located at 1391 cm⁻¹. -1 .

[0044] Implementation Example 7 Take 9g of nickel powder, 9g of chromium powder, and 1.17g of boron powder (abundance ≥ 95%). 10 B) Place the mixture into an alumina crucible, as per [reference]. Figure 1 The apparatus involves placing the crucible in the central heating zone of a tube furnace. A vacuum pump is used to evacuate the pressure inside the furnace tube to below 1 Pa. A molecular pump is then activated to further evacuate the pressure to below 0.01 Pa. Both the vacuum pump and the molecular pump are then shut off. Ar / H2 is introduced to bring the pressure to atmospheric pressure, and this process is repeated 2-3 times. Finally, an Ar / H2 gas flow is introduced for protection. The furnace is heated to 1550°C at a flow rate of 125 sccm and a rate not exceeding 5°C / min, and held at this temperature for 48 hours. After heating, the temperature is lowered to 900°C at a rate of 4°C / min, and then allowed to cool naturally to room temperature to complete the deoxygenation step. The pressure inside the furnace tube is then evacuated to 5 × 10⁻⁶ Pa using a vacuum pump. -2 Below Pa, the molecular pump was turned on to continue pumping the pressure to below 0.01 Pa. Ar / H2 and N2 were then introduced to bring the pressure back to ambient pressure, repeated 2-3 times. Finally, an Ar / H2 and N2 gas flow was introduced for protection, with an Ar / H2 flow rate of 25 sccm and an N2 flow rate of 125 sccm. The sample was heated to 1550℃ at a rate not exceeding 5℃ / min and held for 48 hours. Then, the temperature was lowered to 1525℃ at a rate of 2℃ / h, then to 1350℃ at a rate of 1℃ / h, and finally to 900℃ at a rate of 4.5℃ / h. After natural cooling to room temperature, the sample was removed, yielding a hexagonal boron nitride single crystal. Raman spectroscopy was performed on the exfoliated single crystal. Figure 6 As shown, the characteristic peak in the spectrum is located at 1391 cm⁻¹. -1 The half-height and width can reach 7.84673cm. -1 This is consistent with the 7.8 cm reported in current literature. -1 A comparable quality indicates that the crystal is of high quality, such as... Figure 7 As shown, the X-ray diffraction pattern shows a significant diffraction peak at 26.7°, corresponding to the (002) crystal plane of hBN, and a (004) crystal plane diffraction peak is detected at 55.1°, confirming that the sample has a strictly periodic layered structure.

[0045] For the synthesized h 10 The neutron detection efficiency of BN single crystals and hBN single crystals at a 25 meV thermal neutron source with different thicknesses was tested, such as... Figure 8 As shown, h increases with thickness. 10 BN single crystal and h 11 The neutron absorption efficiency of BN single crystals gradually increases, h 10 BN achieves a neutron detection efficiency of 70% at a thickness of 50 μm, 95% at 150 μm, and 100% at 200 μm. With further increases in thickness to 300 μm, the neutron detection efficiency stabilizes at 100% and remains unchanged. 11 The neutron detection efficiency of BN increases continuously with increasing thickness without saturation. At a thickness of 50 μm, the neutron detection efficiency is 20%; at 150 μm, it reaches 50%; at 200 μm, it is 55%; and at the maximum thickness of 300 μm, it reaches 70%. This indicates that the neutron detection efficiency of the BN grown in this invention is... 10 BN single crystal and h 11 The thickness of a BN single crystal has a significant impact on neutron detection efficiency. Based on the data above, at the same thickness, h 10 BN single crystals have a neutron detection efficiency far higher than h 11 The neutron detection efficiency of BN single crystals indicates that the isotope hexagonal boron nitride (hN) grown by this method... 10 BN has good neutron detection performance.

[0046] The above are merely preferred embodiments of the present invention and are not intended to limit the present invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for preparing large-size isotopic hexagonal boron nitride by growing it at ambient pressure and high temperature, characterized in that, Includes the following steps: S1, after uniformly mixing nickel powder, chromium powder and boron powder, a mixture of boron and nickel-chromium alloy is obtained; S2, a mixture of boron and nickel-chromium alloy is passed through a protective gas flow to deoxidize it, resulting in a deoxidized mixture; S3, for the deoxygenated mixture, reduce the pressure and introduce protective gas and reactive protective gas to atmospheric pressure, and then continue to introduce protective gas for protection; S4 involves a two-stage heating process on the deoxidized mixture. After the first heating, the mixture is held at a higher temperature than the second heating. After natural cooling to room temperature, hexagonal boron nitride crystals are formed.

2. The method for preparing large-size isotopic hexagonal boron nitride by growing at ambient pressure and high temperature according to claim 1, characterized in that, The nickel-chromium alloy contains 40-60 wt.% nickel.

3. The method for preparing large-size isotopic hexagonal boron nitride by growing at ambient pressure and high temperature according to claim 1, characterized in that, The protective gas is Ar / H2; the mass ratio of Ar to H2 is (14-19):(1-6).

4. The method for preparing large-size isotopic hexagonal boron nitride by growing at ambient pressure and high temperature according to claim 1, characterized in that, The mass ratio of the nickel powder, chromium powder and boron powder is 9:9:(1.134-1.206).

5. The method for preparing large-size isotopic hexagonal boron nitride by growing at ambient pressure and high temperature according to claim 1, characterized in that, The process of deoxidizing the mixture of boron and nickel-chromium alloy by passing it through a protective gas flow includes: After depressurizing the boron and nickel-chromium alloy mixture, a protective gas flow is passed through it for protection, and it is heated to 1400-1700℃ and held at that temperature. Then, it is cooled to 700-1000℃ and then allowed to cool naturally to room temperature to complete the deoxidation.

6. The method for preparing large-size isotopic hexagonal boron nitride by growing at ambient pressure and high temperature according to claim 1, characterized in that, The reactive protective gas is N2.

7. The method for preparing large-size isotopic hexagonal boron nitride by growing at ambient pressure and high temperature according to claim 1, characterized in that, The temperature of the first heating is 1300-1700℃; the temperature of the second heating is 700-1000℃.

8. The method for preparing large-size isotopic hexagonal boron nitride by growing at ambient pressure and high temperature according to claim 1, characterized in that, In S3, the flow rate of the protective gas is 10-50 sccm, and the flow rate of the reactive protective gas is 100-200 sccm.

9. A single crystal of isotopic hexagonal boron nitride prepared by the method according to any one of claims 1-8.

10. An application of a single crystal of hexagonal boron nitride prepared by the method according to any one of claims 1-8 in neutron detection.

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