Hexagonal boron nitride synergistic exfoliation method based on thermal shock

Abstract

The application discloses a hexagonal boron nitride stripping method based on thermal shock, which comprises the following steps: pre-intercalation treatment, multiple washing, urea secondary intercalation, multiple washing to neutral, thermal shock stripping, gradient centrifugal sorting and vacuum or freeze drying. The application combines "sulfuric acid / hydrogen peroxide pre-intercalation" and "urea secondary intercalation" for the first time to form a ladder intercalation mechanism, and cooperates with the instantaneous high-temperature thermal shock in the vacuum environment, hydrogen production machine flame gun, 1500-2000 DEG C, the gas generated by the decomposition of urea expands instantaneously between the layers, generates great pressure, significantly weakens the lip-lip interaction of the B-N bond between the layers, realizes the efficient and low-damage stripping of the hexagonal boron nitride, and has the advantages of green environmental protection, no pollution, high stripping efficiency, small structure damage, controllable layer number, good reproducibility, simple process, easy enlargement, excellent product performance and wide application prospect.

Description

Technical Field

[0001] This invention relates to the field of two-dimensional material preparation technology, and in particular to a highly efficient synergistic exfoliation method for hexagonal boron nitride based on urea intercalation-assisted thermal shock. Background Technology

[0002] Hexagonal boron nitride (h-BN) possesses excellent thermal conductivity (>300 W / m·K), insulation (band gap ~5.9 eV), and chemical stability, earning it the title of "white graphene" and showing great potential in high-end heat dissipation and high-frequency electronic devices. However, due to its layered structure, h-BN exhibits significant anisotropy in thermal transfer, which depends on the number of layers. Its in-plane and cross-plane thermal conductivity (TC) vary considerably with the number of layers.

[0003] On the other hand, with the rapid development of AI technology, heat dissipation of AI chips (such as GPUs and TPUs) has become a critical challenge that urgently needs to be addressed. Based on the high computing power requirements of quartz AI chips, the power density of computing units exceeds 100 W / cm², resulting in high local hotspot temperatures. Chip heat dissipation needs to consider both the lateral heat diffusion of efficient in-plane thermal conductive materials and efficient vertical heat transfer to the heat sink. A conservative estimate for heat transfer temperature (TC) should be greater than 10 W / m·K or even higher, while the out-of-plane TC of traditional TIM1 materials such as silicone grease is <5 W / m·K. Although the TC of copper foil and graphene can reach as high as 400 W / m·K and 2000 W / m·K respectively, the risk of short circuits caused by single conductive elements is difficult to overcome. Therefore, low-layer (e.g., 2-5 layers) h-BN is expected to become a key material for solving the heat dissipation problem of AI chips.

[0004] However, the preparation of few-layer h-BN is not a simple task. Unlike the relatively weak van der Waals forces between graphite layers, the interlayer bonding of bulk h-BN involves both van der Waals forces and lip-lip interactions of polar BN bonds, making it difficult to efficiently exfoliate into few-layer (<10 layers) or monolayer structures, thus limiting its performance. Currently, mainstream h-BN exfoliation methods include: liquid-phase exfoliation (such as ultrasonication and ball milling), which relies on organic solvents (NMP, DMF) or surfactants, resulting in high costs and environmental pollution; low yields (<20%) and a tendency to introduce defects (significantly enhanced Raman D peak). Chemical vapor deposition (CVD) can prepare high-quality monolayer h-BN, but the equipment is complex and the yield is extremely low, failing to meet industrial needs. Intercalation exfoliation methods (such as molten salt and strong acid oxidation) require corrosive reagents (such as concentrated sulfuric acid and sodium hydroxide), leaving residual impurities that are difficult to remove; high-temperature molten salt methods (>800℃) are energy-intensive and easily damage the h-BN crystal structure. The thermal shock method, which has been successful in graphene preparation, cannot be directly applied due to its low efficiency (exfoliation rate <10%). Therefore, this invention proposes an optimized chemical intercalation-thermal shock exfoliation method that achieves ultrathin and large-size graphene while preserving the intrinsic high thermal conductivity of h-BN lattice as much as possible, and effectively eliminates surface parasitic debris. Summary of the Invention

[0005] Purpose of the invention: The purpose of this invention is to provide a highly efficient synergistic exfoliation method for hexagonal boron nitride based on urea intercalation-assisted thermal shock; for the large-scale preparation of few-layer / multi-layer hexagonal boron nitride.

[0006] Technical solution: The hexagonal boron nitride synergistic exfoliation method based on thermal shock described in this invention includes the following steps:

[0007] Step 1: Pre-intercalation processing;

[0008] Step 2: Wash multiple times;

[0009] Step 3: Secondary intercalation of urea;

[0010] Step 4: Wash several times until neutral;

[0011] Step 5: Thermal shock synergistic peeling;

[0012] Step 6: Gradient centrifugation sorting;

[0013] Step 7: Vacuum or freeze drying.

[0014] Further, step 1 includes mixing hexagonal boron nitride and sulfuric acid hydrogen peroxide at a ratio of 1g:15mL and pre-treating the mixture by reflux at 80°C in a reflux condenser.

[0015] Furthermore, in step 1, the material-to-liquid ratio is 1g hexagonal boron nitride corresponding to 10mL of 98% concentrated sulfuric acid plus 5mL of 30% hydrogen peroxide, and 3g hexagonal boron nitride corresponding to 30mL of 98% concentrated sulfuric acid plus 15mL of 30% hydrogen peroxide.

[0016] Furthermore, step 2 includes multiple centrifugal washings until neutral, i.e., pH≈7.

[0017] Further, step 3 includes mixing the washed hexagonal boron nitride with urea at a mass ratio of 1:10 and refluxing at 100°C for 6 hours.

[0018] Furthermore, step 4 includes washing repeatedly until neutral, vacuum drying, and sample preparation.

[0019] Furthermore, step 5 includes placing the obtained intercalation composite in a vacuum quartz tube, using a high-temperature flame gun generated by a hydrogen generator to perform instantaneous thermal shock, and using the high-pressure gas generated by the decomposition of the intercalating agent to achieve efficient stripping.

[0020] Furthermore, step 6 includes performing multi-stage differential centrifugation on the exfoliated product to collect large-sized, ultrathin, clean, ideal hexagonal boron nitride nanosheets obtained at 6000 rpm.

[0021] Furthermore, the multi-stage differential centrifugation sorting includes 3000 rpm / 10 min, 6000 rpm / 20 min, and 9000 rpm / 30 min.

[0022] Further, step 7 includes vacuum drying or freeze drying the sorted product to obtain a few-layer hexagonal boron nitride powder with a certain degree of rigidity without stacking.

[0023] Beneficial effects: Compared with the prior art, the present invention has the following significant advantages:

[0024] (1) Innovative intercalation-thermal shock synergistic delamination strategy: This invention combines "sulfuric acid / hydrogen peroxide pre-intercalation" with "urea secondary intercalation" for the first time to form a step intercalation mechanism, and synergizes with instantaneous high temperature thermal shock in a vacuum environment (hydrogen generator flame gun, 1500–2000℃). The gas generated by urea decomposition expands instantaneously between layers, generating huge pressure, which significantly weakens the lip-lip interaction of BN bonds between layers, thus achieving efficient and low-damage delamination of hexagonal boron nitride.

[0025] (2) Green and environmentally friendly, pollution-free: The intercalating agent used is urea, which is widely available, inexpensive, non-toxic and harmless. The decomposition products are NH3 and CO2, with no harmful residues. The entire preparation process avoids the use of strong acids, strong bases, organic solvents or surfactants, which meets the requirements of green chemistry. The post-processing is simple and environmentally friendly.

[0026] (3) High stripping efficiency and minimal structural damage: After treatment by the method of this invention, the interlayer spacing of hexagonal boron nitride is significantly increased. SEM further confirms that the product is mainly composed of 2-5 layers of few-layer hexagonal boron nitride, and the stripping yield can reach more than 40%, which is much higher than the traditional liquid phase stripping method (<20%).

[0027] (4) The number of layers is controllable and the reproducibility is good: By adjusting the urea ratio, hydrothermal temperature and thermal shock temperature, the controllable preparation of 2-5 layers of hexagonal boron nitride can be achieved.

[0028] (5) Simple process and easy to scale up: The process flow of this invention is clear, the equipment requirements are low, the hydrothermal and thermal shock steps are compatible for batch processing, the single processing volume can reach the gram level, it has the potential for industrial production, and it is suitable for large-scale two-dimensional material preparation.

[0029] (6) Excellent product performance and broad application prospects: The few-layer hexagonal boron nitride obtained has high thermal conductivity, good insulation and structural integrity, and is suitable for high-end application fields such as AI chip heat dissipation film, high thermal conductivity composite material, electronic packaging, and anti-oxidation coating. Attached Figure Description

[0030] Figure 1 This is a process flow diagram of the method of the present invention;

[0031] Figure 2 A comparison chart showing the results of multi-stage differential centrifugation of the stripped products;

[0032] Figure 3 A comparison chart of the original thickness of boron nitride and the thickness of boron nitride after intercalation and sorting.

[0033] Figure 4 The XRD patterns of boron nitride after intercalation and the original boron nitride are shown.

[0034] Figure 5 The thermal conductivity diagrams show the original boron nitride and the intercalated boron nitride.

[0035] Figure 6 To observe the thermal conductivity of raw boron nitride and processed boron nitride using an infrared camera. Detailed Implementation

[0036] The technical solution of the present invention will be further described below with reference to the accompanying drawings.

[0037] like Figure 1 As shown, this invention provides a hexagonal boron nitride synergistic exfoliation method based on thermal shock, comprising the following steps:

[0038] Step 1: Pre-intercalation treatment; Mix hexagonal boron nitride with sulfuric acid and hydrogen peroxide at a ratio of 1g:15mL, and pre-treat by reflux at 80℃ in a reflux condenser. The ratio is 1g hexagonal boron nitride corresponds to 10mL of 98% concentrated sulfuric acid and 5mL of 30% hydrogen peroxide, and 3g hexagonal boron nitride corresponds to 30mL of 98% concentrated sulfuric acid and 15mL of 30% hydrogen peroxide.

[0039] Step 2: Wash multiple times; centrifuge and wash multiple times until neutral, i.e., pH≈7.

[0040] Step 3: Secondary intercalation of urea; Washed hexagonal boron nitride and urea are mixed at a mass ratio of 1:10 and refluxed at 100°C for 6 h.

[0041] Step 4: Wash repeatedly until neutral; wash repeatedly until neutral, vacuum dry, and prepare the sample.

[0042] Step 5: Thermal shock synergistic exfoliation; The obtained intercalation complex is placed in a vacuum quartz tube and subjected to instantaneous thermal shock using a high-temperature flame gun generated by a hydrogen generator. High-pressure gas generated by the decomposition of the intercalating agent is used to achieve efficient exfoliation.

[0043] Step 6: Gradient centrifugation; The exfoliated product was subjected to multi-stage differential centrifugation to collect large, ultrathin, and clean ideal hexagonal boron nitride nanosheets obtained at 6000 rpm. The multi-stage differential centrifugation included 3000 rpm / 10 min, 6000 rpm / 20 min, and 9000 rpm / 30 min.

[0044] Step 7: Vacuum or freeze drying; The sorted product is vacuum dried or freeze dried to obtain a few-layer hexagonal boron nitride powder with a certain rigidity without stacking.

[0045] Example 1

[0046] 1. Pre-intercalation treatment: Take 3g of hexagonal boron nitride (h-BN) and add it to 45mL of sulfuric acid-hydrogen peroxide mixture (30mL of 98% concentrated sulfuric acid and 15mL of 30% hydrogen peroxide). Pre-treat the mixture by mechanical stirring at 80℃ in a reflux condenser for 2 hours to oxidize the edges of h-BN and initially expand the interlayer spacing.

[0047] 2. First washing: Centrifuge the above reaction solution, discard the supernatant, and repeatedly centrifuge and wash the precipitate with deionized water until pH=7 to obtain pretreated h-BN;

[0048] 3. Hydrothermal secondary intercalation of urea: The washed h-BN and urea were mixed at a mass ratio of 1:10 (i.e., 3g h-BN and 30g urea), dispersed in 30mL of deionized water, and reacted at 100℃ for 6 hours in a reflux condenser to achieve uniform intercalation depth.

[0049] 4. Secondary washing and drying: After the reaction is completed, the mixture is naturally cooled to room temperature, centrifuged and repeatedly washed with deionized water until neutral to remove residual urea and its decomposition products. Then, it is vacuum dried at 60°C for 12 hours to obtain the urea intercalated h-BN sample.

[0050] 5. Thermal shock synergistic exfoliation: The obtained intercalation complex is placed in a vacuum quartz tube and subjected to instantaneous thermal shock using a high-temperature flame gun generated by a hydrogen generator. High-pressure gas generated by the decomposition of the intercalating agent is used to achieve efficient exfoliation.

[0051] 6. Centrifugal separation: The quenched sample is dispersed in deionized water and subjected to multi-stage differential centrifugation, such as... Figure 2 As shown;

[0052] Low-speed pre-separation: Centrifuge at 3000 rpm for 10 minutes and discard the unseparated bulk particles at the bottom;

[0053] Intermediate layer collection: Centrifuge the supernatant at 6000 rpm for 20 minutes and collect the precipitate, which is 5–10 layers of h-BN (the main target product).

[0054] Upper layer collection: Centrifuge the supernatant at 9000 rpm for 35 minutes and collect the precipitate in the supernatant, which is the 2–5 layer h-BN.

[0055] 7. Vacuum drying: The collected 5–10 layers of h-BN product were vacuum dried at 60°C for 12 hours to obtain large-sized, ultrathin, and clean ideal h-BN nanosheets.

[0056] Effect verification

[0057] The few-layer h-BN prepared in Example 1 of this invention was characterized as follows:

[0058] (1) Peeling effect: Scanning electron microscopy (SEM) observation showed that, as Figure 3 As shown, the original h-BN is in the form of a thick block with tightly packed interlayers; the processed product is in the form of thin flakes with transparent edges and a significantly reduced thickness.

[0059] (2) Structural integrity: X-ray diffraction (XRD) patterns show, for example... Figure 4 As shown, the (002) diffraction peak of h-BN shifted to the left from 26.6° at the original position to about 26.0° after treatment. According to the Bragg formula, the interlayer spacing increased from 0.334 nm to 0.342 nm, proving that the intercalation and exfoliation were successful.

[0060] (3) Delamination yield: By weighing the ratio of the final dried product mass to the initial h-BN mass, the yield of 2–5 h-BN layers in this embodiment is approximately 42%, the yield of 5–10 h-BN layers is approximately 18%, and the overall delamination rate (<10 layers) reaches over 60%.

[0061] (4) Thermal conductivity: The in-plane thermal conductivity of the products was measured using the laser scintillation method. The results showed that the in-plane thermal conductivity of 2–5 layers of h-BN reached 1.3 W / m·K, which was significantly higher than that of the original bulk h-BN (approximately 0.6 W / m·K). Figure 5 As shown in the figure. Infrared thermal imaging tests further demonstrate that the treated h-BN film exhibits a more uniform surface temperature distribution, lower hotspot temperatures, and significantly enhanced thermal diffusion capacity under the same heat source. Figure 6 As shown.

[0062] (5) Stability and reproducibility: The experiment was repeated three times under the same process conditions. The coefficient of variation (CV) of the layer number distribution of the obtained product (statistically obtained by Raman 2D peak shift and SEM) was less than 12%, indicating that the method has good process reproducibility.

[0063] Comparative Example

[0064] A traditional ultrasonic liquid-phase exfoliation method was used: an equal amount of pristine h-BN was dispersed in N-methylpyrrolidone (NMP), and the mixture was ultrasonically treated with a cell disruptor for 6 hours, followed by centrifugation at 3000 rpm to remove unexfoliated particles. The resulting few-layer h-BN yield was less than 15%, and the Raman spectrum showed a significant enhancement of the D peak, indicating the formation of numerous structural defects. Furthermore, this comparative example used the toxic organic solvent NMP, which does not meet the requirements of green chemistry.

Claims

1. A hexagonal boron nitride synergistic exfoliation method based on thermal shock, characterized in that, The steps include the following: Step 1: Pre-intercalation processing; Step 2: Wash multiple times; Step 3: Secondary intercalation of urea; Step 4: Wash several times until neutral; Step 5: Thermal shock synergistic peeling; Step 6: Gradient centrifugation sorting; Step 7: Vacuum or freeze drying.

2. The hexagonal boron nitride synergistic exfoliation method based on thermal shock according to claim 1, characterized in that, Step 1 includes mixing hexagonal boron nitride and sulfuric acid hydrogen peroxide at a ratio of 1g:15mL and pre-treating the mixture by reflux at 80°C in a reflux condenser.

3. The hexagonal boron nitride synergistic exfoliation method based on thermal shock according to claim 2, characterized in that, In step 1, the material-to-liquid ratio is 1g hexagonal boron nitride corresponds to 10mL of 98% concentrated sulfuric acid plus 5mL of 30% hydrogen peroxide, and 3g hexagonal boron nitride corresponds to 30mL of 98% concentrated sulfuric acid plus 15mL of 30% hydrogen peroxide.

4. The hexagonal boron nitride synergistic exfoliation method based on thermal shock according to claim 1, characterized in that, Step 2 includes multiple centrifugal washings until neutral, i.e., pH≈7.

5. The hexagonal boron nitride synergistic exfoliation method based on thermal shock according to claim 1, characterized in that, Step 3 includes mixing the washed hexagonal boron nitride with urea at a mass ratio of 1:10 and refluxing at 100°C for 6 hours.

6. The hexagonal boron nitride synergistic exfoliation method based on thermal shock according to claim 1, characterized in that, Step 4 includes washing until neutral, vacuum drying, and sample preparation.

7. The hexagonal boron nitride synergistic exfoliation method based on thermal shock according to claim 1, characterized in that, Step 5 includes placing the obtained intercalation composite in a vacuum quartz tube, using a high-temperature flame gun generated by a hydrogen generator to perform instantaneous thermal shock, and using the high-pressure gas generated by the decomposition of the intercalating agent to achieve efficient stripping.

8. The hexagonal boron nitride synergistic exfoliation method based on thermal shock according to claim 1, characterized in that, Step 6 includes performing multi-stage differential centrifugation on the stripped product to collect large-sized, ultrathin, clean, ideal hexagonal boron nitride nanosheets obtained at 6000 rpm.

9. The hexagonal boron nitride synergistic exfoliation method based on thermal shock according to claim 8, characterized in that, The multi-stage differential centrifugation sorting includes 3000 rpm / 10 min, 6000 rpm / 20 min, and 9000 rpm / 30 min; Low-speed pre-separation: Centrifuge at 3000 rpm for 10 minutes and discard the unseparated bulk particles at the bottom; Middle layer collection: Centrifuge the supernatant at 6000 rpm for 20 minutes and collect the precipitate, which is 5-10 layers of hexagonal boron nitride; Upper layer collection: Centrifuge the supernatant at 9000 rpm for 35 minutes and collect the precipitate in the supernatant, which is 2–5 layers of hexagonal boron nitride.

10. The hexagonal boron nitride synergistic exfoliation method based on thermal shock according to claim 1, characterized in that, Step 7 includes vacuum drying or freeze drying the sorted product to obtain a few-layer hexagonal boron nitride powder with a certain rigidity without stacking.

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