Method for preparing hexagonal boron nitride by adding seed crystal

Abstract

The invention discloses a method for preparing hexagonal boron nitride by adding seed crystals, and relates to the field of boron nitride materials, the hexagonal boron nitride with two scales of nanoscale and micron scale is prepared at a relatively low temperature of 1100-1600 DEG C by adopting a molten salt method and adding the seed crystals, and the method has important significance for producing high-quality h-BN single crystals with different sizes. The operation difficulty is low, the energy consumption is low, the cost is saved, and large-scale production is easy.

Description

TECHNICAL FIELD

[0001] The application relates to the field of boron nitride materials, in particular to a method for preparing hexagonal boron nitride by adding crystal seeds. BACKGROUND

[0002] 5G is an important pillar of new generation information technology, and its leap-forward improvement is based on higher frequency electromagnetic waves and higher integration. However, the higher the integration, the more heat the electronic components generate, which leads to the decline of the stability and safety of the equipment. Epoxy resin is usually used for packaging electronic components, but the thermal conductivity of epoxy resin cannot meet the requirement of rapid heat conduction. Therefore, in order to improve the heat dissipation performance of the equipment, it is necessary to add a heat-conducting filler to the epoxy resin to improve its heat dissipation performance. As a two-dimensional material, hexagonal boron nitride (h-BN) has a layered structure similar to that of graphene, but unlike graphene, h-BN has a larger band gap of about 5.9 eV. At the same time, h-BN also has excellent dielectric properties, excellent insulation, high thermal conductivity, good thermal stability and low thermal expansion coefficient, and plays an irreplaceable role in semiconductor devices. With the development of science and technology, flaky h-BN particles are gradually filled into epoxy resin in the form of micron and nanometer particles (1-50 μm) to be applied to the heat-conducting and insulating field. Therefore, in order to meet the demand of h-BN of specific size in various fields, it is important to obtain high-quality h-BN single crystals of different sizes and shapes.

[0003] At present, the methods for preparing h-BN powder mainly include traditional high-temperature method, chemical vapor deposition method, self-propagating combustion method, solvent-thermal synthesis method and template method. These methods have advantages and disadvantages in terms of cost, equipment and operation, but they all have the problems of complex process conditions, high preparation cost, low product purity and low quality, which are difficult to meet the demand of semiconductor devices. Therefore, it is a prerequisite for the application of boron nitride to develop a h-BN preparation technology with low cost, high purity and size and morphology control.

[0004] Seed crystal induction technology refers to adding a crystal with the same structure and composition as the product as a seed crystal in the raw material, and crystallizing and epitaxially growing the target crystal on the surface of the seed crystal through a certain synthesis method. Based on the role of the seed crystal in the crystal growth process, by controlling the number and size of the seed crystal, the nucleation and growth of the crystal can be affected, and a crystal with high purity and crystallinity, specific morphology and size can be obtained. Seed crystal induction method is widely used in the preparation of high-purity and high-crystallinity ceramic materials such as alpha-Al2O3, SiC, alpha-Si3N4 and SnO2. At present, some scholars have studied the method for preparing h-BN by adding crystal seeds.

[0005] Patent CN115490523A discloses a preparation method of large-size hexagonal boron nitride and the obtained large-size hexagonal boron nitride. The technical scheme of the invention is as follows: first, prepare hexagonal boron nitride seeds, mix hexagonal boron nitride seeds, borides, carbon-nitrogen compounds, carbon powder and organic solvents in a certain proportion, then evaporate and dry, press to obtain a block-shaped composite blank, place it in a graphite crucible, and perform carbon thermal reduction reaction in flowing ammonia or nitrogen at 1500-1800℃ for 4-10h to obtain a crude product, then wash it to obtain large-size hexagonal boron nitride with D50 particle size of 0.1-20μm. Then, using the iterative growth method, the obtained large-size hexagonal boron nitride is used as the seed for secondary growth, and the same preparation method is used to make the hexagonal boron nitride grains continue to grow, and finally large-size hexagonal boron nitride with D50 particle size of 30-50μm is obtained. However, the method disclosed in the patent needs to be performed multiple times of high-temperature carbon thermal reduction, the reaction time is long and needs to be performed at a high temperature, resulting in a complex preparation process, a long cycle and being affected by factors such as uneven mixing of raw materials, different contact areas of materials, different heat transfer and heating degrees, etc., so that the prepared product is unstable in purity, crystallinity, morphology and particle size. At the same time, the method has a long cycle, a complex process and high cost, and is difficult to be applied in actual production. SUMMARY

[0006] In order to solve the problems in the prior art, the purpose of the present application is to overcome the shortcomings of the prior art, and to provide a method for preparing hexagonal boron nitride by adding seeds, which reduces the reaction temperature by adding seeds, and the prepared h-BN crystal has good development, high crystallinity and high purity, and the method is green and environmentally friendly, and the molten salt can be recycled.

[0007] To achieve the above-mentioned purpose, the specific technical scheme of the present application is as follows:

[0008] A method for preparing hexagonal boron nitride by adding seeds, comprising the following steps:

[0009] S1, grinding the source material, molten salt and seeds to mix uniformly to obtain a mixture;

[0010] S2, placing the mixture in a crucible and calcining under a flowing protective atmosphere, and cooling to room temperature under a flowing protective atmosphere after calcination to obtain a crude product;

[0011] S3, post-treating the crude product to obtain hexagonal boron nitride of different sizes.

[0012] Preferably, in step S2, the calcination temperature is 1100-1600℃,

[0013] Preferably, the method for preparing hexagonal boron nitride by adding seeds further comprises at least one of the following technical features:

[0014] A, the purity of the seed crystal in the step S1 is not less than 99.5%, the particle size is not more than 20 μm, and the mass ratio of the seed crystal to the source material is 1% to 25%.

[0015] B, the source material is selected from amorphous boron nitride, and the mass ratio of the molten salt to the source material is (0.5 to 4):1.

[0016] C, the molten salt is selected from any one or more of magnesium chloride, sodium chloride, potassium chloride, calcium carbonate, potassium carbonate or sodium carbonate.

[0017] Preferably, in the step S2, the protective atmosphere is nitrogen or argon, and the gas flow rate is 0.3 to 0.8 L / min.

[0018] Further preferably, in the step S2, the calcination time is 1 to 6 h, and the heating rate is 5 to 10 ℃ / min.

[0019] Preferably, the post-treatment comprises acid washing, soaking the crude product in a hydrochloric acid solution with w(HCl) = 8%, heating under a water bath at 20 to 100 ℃ and stirring using a magnetic stirrer for 10 to 60 min, and repeating 2 to 3 times.

[0020] Further preferably, the post-treatment further comprises water washing, soaking the product after the acid washing in deionized water, heating under a water bath at 20 to 100 ℃ and stirring using a magnetic stirrer for 20 to 60 min, and repeating 2 to 3 times.

[0021] Still further preferably, the post-treatment further comprises solid-liquid separation, which is performed by suction filtration, and the product after each acid washing and water washing is subjected to suction filtration using double-layer slow filter paper, and then subjected to the next acid washing or water washing.

[0022] Still further preferably, the post-treatment further comprises drying, and the drying temperature is 120 to 180 ℃.

[0023] The application also provides a hexagonal boron nitride prepared by the method for preparing hexagonal boron nitride by adding seed crystals.

[0024] Compared with the prior art, the application has the following advantages:

[0025] The method for preparing hexagonal boron nitride by adding seed crystals has the following advantages in the preparation process:

[0026] By using the molten salt method and adding seed crystals, micron and nanometer scale h-BN is prepared at 1100 to 1600 ℃, which is lower in energy consumption and saves cost compared with the traditional preparation method (the reaction temperature needs to reach 1800 to 1900 ℃).

[0027] The impurities are completely removed by washing and suction filtering this step, and the obtained hexagonal boron nitride has high purity.

[0028] The method for preparing hexagonal boron nitride by adding crystal seeds has the following advantages in the selection of reactants:

[0029] The source material is selected as a-BN and the crystal seeds are added, which is different from other reaction raw materials (such as borax, boric acid, melamine, etc.), and does not involve a chemical reaction stage, but only involves a crystal type conversion process. Therefore, the product conversion rate is high, the purity is high, and the environment is friendly;

[0030] Compared with the traditional solid-phase reaction, the molten salt at high temperature is used as a liquid-phase reaction environment, which reduces the loss of raw materials with gas, and improves the conversion rate of the product. At the same time, the liquid-phase environment accelerates the transmission of substances in the reactants, so that the h-BN crystal develops better and has higher crystallinity; and the molten salt can be recycled and used, achieving the effect of green and recycling;

[0031] By changing the types of crystal seeds, hexagonal boron nitride particles with different morphologies and sizes (micron and nanometer level) can be prepared, which is conducive to meeting the product demand under different use conditions.

[0032] In summary, the preparation method provided by the application has low operation difficulty, low energy consumption, cost saving and easy large-scale production. The final product h-BN is free of impurities and has high purity and high quality. At the same time, nanometer and micron level h-BN single crystal particles can be prepared, which is more suitable for application in the field of thermal insulation. BRIEF DESCRIPTION OF DRAWINGS

[0033] Figure 1 The morphology and particle size distribution of the three crystal seeds used in the application are shown in (a) and (d) for crystal seed A, (b) and (e) for crystal seed B, and (c) and (f) for crystal seed C;

[0034] Figure 2 The particle size distribution of the hexagonal boron nitride prepared in the application is shown in (a) for Example 1, (b) for Example 2, (c) for Example 3, and (d) for Comparative Example 1;

[0035] Figure 3 The XRD pattern of the hexagonal boron nitride prepared in Examples 1 to 3 of the application is shown in (a) for Example 1, (b) for Example 2, (c) for Example 3, and (d) for Comparative Example 1;

[0036] Figure 4 The XRD pattern of the hexagonal boron nitride prepared in Comparative Example 1 of the application is shown in (a) for Example 1, (b) for Example 2, (c) for Example 3, and (d) for Comparative Example 1;

[0037] Figure 5 The SEM pattern of the hexagonal boron nitride prepared in the application is shown in (a) for Example 3, (b) for Comparative Example 1, and (c) for Comparative Example 2. DETAILED DESCRIPTION

[0038] The technical solutions in the embodiments of the present application will be clearly and completely described below with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by a person of ordinary skill in the art without creative work fall within the protection scope of the present application.

[0039] In the detailed description, a method for preparing hexagonal boron nitride by adding crystal seeds is provided. Three crystal seeds are selected, which are named as crystal seeds A, B and C. The three crystal seeds are all hexagonal boron nitride crystal forms with a purity of more than 99.5%, but have different particle sizes. The corresponding SEM morphology diagram and particle size distribution diagram are shown in FIGS. 1-4. Figure 1

[0040] Embodiment 1

[0041] Reactant formula:

[0042] Amorphous boron nitride (a-BN): 6 g,

[0043] Magnesium chloride: 7.4 g,

[0044] Sodium chloride: 4.6 g,

[0045] Crystal seed A: 1.5 g.

[0046] Embodiment 2

[0047] Reactant formula:

[0048] Amorphous boron nitride (a-BN): 6 g,

[0049] Magnesium chloride: 7.4 g,

[0050] Sodium chloride: 4.6 g,

[0051] Crystal seed B: 0.6 g.

[0052] Embodiment 3

[0053] Reactant formula:

[0054] Amorphous boron nitride (a-BN): 6 g,

[0055] Magnesium chloride: 7.4 g,

[0056] Sodium chloride: 4.6 g,

[0057] Crystal seed C: 0.06 g.

[0058] The reactant formula of each of the above embodiments is used to prepare hexagonal boron nitride, and the preparation steps are as follows:​

[0059] S1, mixing and grinding various materials in the reactant formula to obtain a uniform mixed powder;

[0060] S2, pouring the mixed powder into a crucible and calcining it using a tube furnace, the calcining conditions being: heating to 1300℃ at a rate of 5℃ / min under a nitrogen flow rate of 0.5L / min for 4h, and then cooling to room temperature with the furnace under a nitrogen flow to obtain a crude product;

[0061] S3, treating the crude product by impurity removal, washing, drying, etc. to obtain hexagonal boron nitride with high purity and high crystallinity.

[0062] Further, in the S3 step, the prepared hexagonal boron nitride is extracted from the crude product, specifically:

[0063] Impurity removal: soaking the crude product in a hydrochloric acid solution with w(HCl) = 8%, stirring and washing the powder using a magnetic stirrer under a 70℃ water bath for 20min, and then suction filtering it using a Buchner funnel lined with double slow filter paper, repeating 2-3 times to obtain the impurity-removed product;

[0064] Washing: stirring and washing the impurity-removed product using deionized water and a magnetic stirrer under a 90℃ water bath for 20min, and then suction filtering it using a Buchner funnel lined with double slow filter paper, repeating 3 times to obtain the washed product;

[0065] Drying: drying the washed product in an oven at 180℃ for 6h to obtain the prepared hexagonal boron nitride;

[0066] Classification: classifying the dried hexagonal boron nitride by particle size using a classification device.

[0067] Comparative Example 1

[0068] Reactant formula:

[0069] Amorphous boron nitride (a-BN): 6g,

[0070] Magnesium chloride: 7.4g,

[0071] Sodium chloride: 4.6g.

[0072] The preparation steps are consistent with the preparation steps of the reactant formula for preparing hexagonal boron nitride in the above examples.

[0073] Comparative Example 2

[0074] The reactant formula is consistent with that in Comparative Example 1;

[0075] The preparation steps are substantially the same as those of the reactant formula of the above-mentioned examples for preparing hexagonal boron nitride, except that the calcination temperature is increased to 1500°C.

[0076] Figure 2 For the particle size distribution of the hexagonal boron nitride prepared in the three examples, it can be seen that by adjusting the size of the seed, nanoscale h-BN can also be obtained.

[0077] Figure 3 For the XRD spectrum of the hexagonal boron nitride prepared in the three examples, it can be seen that no other impurity phase is detected in the tested spectrum, indicating that the prepared h-BN particles have high purity. The sharp (002), (100), (101), (102), (004), (110), (112) h-BN characteristic peaks appear in the XRD spectrum without impurity peaks, indicating that the h-BN obtained by using the preparation method of the present application is high-purity and high-crystallinity. By using larger size seeds (Example 1), the h-BN diffraction peaks are more sharp and the peak intensity is higher, indicating that the crystal development is better.

[0078] Figure 4 For the XRD spectrum of the hexagonal boron nitride prepared in the three examples, it can be seen that no other impurity phase is detected in the tested spectrum, indicating that the prepared h-BN particles have high purity. The sharp (002), (100), (101), (102), (004), (110), (112) h-BN characteristic peaks appear in the XRD spectrum without impurity peaks, indicating that the h-BN obtained by using the preparation method of the present application is high-purity and high-crystallinity. By using larger size seeds (Example 1), the h-BN diffraction peaks are more sharp and the peak intensity is higher, indicating that the crystal development is better.

[0079] In order to further compare the effect of seed addition on the crystallinity of h-BN, the interplanar spacing d (002) of (002) crystal plane, the half-height width FWHM and the graphitization index G.I. of the XRD spectrum were calculated, as shown in Table 1. The lower these values, the better the h-BN crystal development and the higher the crystallinity. By comparison, it is found that the d (002) , FWHM and G.I. of Examples 1 to 3 and Comparative Example 1 show that the addition of seeds significantly improves the crystal development of h-BN and improves the crystalline quality. In particular, the crystallinity of Examples 1 and 2 at 1300°C is better than that of Comparative Example 2 at 1500°C, indicating that the addition of seeds can synthesize high-crystallinity h-BN at a lower temperature.

[0080] Table 1 XRD spectrum analysis of h-BN prepared in three examples

[0081]

[0082] Figure 5SEM morphology images of the hexagonal boron nitride prepared in Example 3, Comparative Example 1 and Comparative Example 2 at the same magnification, wherein (a) corresponds to Example 3, (b) corresponds to Comparative Example 1, and (c) corresponds to Comparative Example 2. Comparing (a) and (b), it can be seen that the size of the hexagonal boron nitride obviously increases and the morphology is obviously improved after adding the crystal seeds at the same temperature. Comparing (a) and (c), it can be seen that the size of the hexagonal boron nitride obtained by adding the crystal seeds at 1300 DEG C is close to that of the hexagonal boron nitride obtained by increasing the temperature to 1500 DEG C, and the morphology is also regular. It is indicated that the crystal seeds can be added to obtain the hexagonal boron nitride with good crystal development and large size at a lower temperature.

[0083] Further, the particle size distribution of the h-BN is counted by using the Image Pro Plus software, and it is found that the h-BN with different sizes and size distributions can be obtained by adjusting the size of the crystal seeds. Compared with Comparative Example 1 without adding the crystal seeds, the size of the product h-BN is obviously improved after adding the crystal seeds.

[0084] It should be noted that it is found through experiments that the preferred range of each process parameter in S2 in the above preparation steps is that the gas flow rate is 0.3-0.8 L / min, the high-temperature calcination temperature is 1100-1600 DEG C, the calcination time is 1-6 h, and the heating rate is 5-10 DEG C / min. The hexagonal boron nitride powder material can be prepared in this process parameter range.

[0085] In summary, the hexagonal boron nitride particles with high purity, high crystallinity and different sizes are prepared by the simple preparation process at a lower reaction temperature, which is suitable for scale production and is beneficial to meet the market demand of the high-thermal-conductivity low-dielectric-insulation boron nitride composite material, has important significance for the application and development of the boron nitride powder in the electronic industry field, and improves the development prospect of the boron nitride powder in the microelectronic device and integrated circuit field.

[0086] Although the embodiments of the present application have been shown and described, it can be understood by those skilled in the art that various changes, modifications, replacements and variations can be made to the embodiments without departing from the principles and spirits of the present application, and the scope of the present application is defined by the appended embodiments and their equivalents.

Claims

1. A method for preparing hexagonal boron nitride by adding seed crystals, characterized in that, Includes the following steps: S1. Grind the raw material, molten salt and seed crystals to mix them evenly to obtain a mixture; S2. Place the mixture in a crucible and calcine it under a flowing protective atmosphere. After calcineation, cool it to room temperature under a flowing protective atmosphere to obtain a crude product. S3. The crude product is post-processed to obtain hexagonal boron nitride of different sizes.

2. The method for preparing hexagonal boron nitride by adding seed crystals according to claim 1, characterized in that, In step S2, the calcination temperature is 1100–1600℃.

3. The method for preparing hexagonal boron nitride by adding seed crystals according to claim 1, characterized in that, The method for preparing hexagonal boron nitride by adding seed crystals further includes at least one of the following technical features: The seed crystals mentioned in steps A and S1 have a purity of not less than 99.5%, a particle size of not more than 20 μm, and a mass ratio of 1% to 25% with the source material. B. The source material is amorphous boron nitride, and the mass ratio of the molten salt to the source material is (0.5~4):

1. C. The molten salt is selected from one or more of magnesium chloride, sodium chloride, potassium chloride, calcium carbonate, potassium carbonate, or sodium carbonate.

4. The method for preparing hexagonal boron nitride by adding seed crystals according to claim 1, characterized in that, In step S2, the protective atmosphere is nitrogen or argon, and the gas flow rate is 0.3 to 0.8 L / min.

5. The method for preparing hexagonal boron nitride by adding seed crystals according to claim 2, characterized in that, In step S2, the calcination time is 1–6 h and the heating rate is 5–10 °C / min.

6. The method for preparing hexagonal boron nitride by adding seed crystals according to claim 1, characterized in that, The post-treatment includes acid washing, in which the crude product is soaked in a hydrochloric acid solution with w(HCl) = 8%, heated in a water bath at 20-100°C and stirred with a magnetic stirrer for 10-60 minutes, and repeated 2-3 times.

7. The method for preparing hexagonal boron nitride by adding seed crystals according to claim 6, characterized in that, The post-treatment also includes water washing, in which the acid-washed product is soaked in deionized water and stirred with a magnetic stirrer for 20 to 60 minutes under water bath heating conditions of 20 to 100°C, and repeated 2 to 3 times.

8. The method for preparing hexagonal boron nitride by adding seed crystals according to any one of claims 6 or 7, characterized in that, The post-processing also includes solid-liquid separation, which is carried out by vacuum filtration. Each product from the acid washing and water washing is filtered using double-layer slow filter paper before the next acid washing or water washing is performed.

9. The method for preparing hexagonal boron nitride by adding seed crystals according to claim 1, characterized in that, The post-processing also includes drying, with a drying temperature of 120–180°C.

10. A hexagonal boron nitride, characterized in that, The hexagonal boron nitride is prepared by the method of adding seed crystals as described in any one of claims 1 to 8.

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