A method for preparing boron nitride nanosheets with high aspect ratio by precursor conversion

Abstract

This invention discloses a method for preparing high aspect ratio boron nitride nanosheets through precursor conversion, belonging to the field of two-dimensional material preparation technology. The method includes the following steps: 1. Uniform blending of a boron nitride precursor and a foaming agent; 2. Programmed temperature foaming and high-temperature sintering of the mixed powder; 3. Liquid-phase fractionation and centrifugation of the sintered powder to prepare high aspect ratio boron nitride nanosheets. This invention achieves high-throughput preparation of boron nitride nanosheets with aspect ratios between 1000 and 3300 through in-situ foaming polymerization by adjusting the type and proportion of the foaming agent. This solves the problems of long processing time, low yield, complex post-processing, and small aspect ratio of the obtained boron nitride nanosheets in traditional boron nitride nanosheet preparation methods, laying the foundation for the industrial preparation of boron nitride nanosheets and their application in the field of advanced composite materials.

Description

Technical Field

[0001] This invention belongs to the field of two-dimensional material preparation technology, specifically relating to a method for preparing boron nitride nanosheets with high aspect ratio by precursor conversion. Background Technology

[0002] Hexagonal boron nitride has the same layered structure as graphite, with each layer containing a honeycomb structure composed of alternating boron and nitrogen atoms, hence its alternative name, "white graphite." Due to its excellent thermal conductivity, thermal stability, and mechanical properties, few-layer boron nitride nanosheets have broad application prospects in fields such as heat dissipation, catalysis, biomedicine, and water purification.

[0003] Currently, there are two main approaches to preparing boron nitride nanosheets: The first is the bottom-up synthesis method, which involves high-temperature deposition of boron-nitrogen precursors onto a metal substrate to synthesize boron nitride nanosheets. This method can produce few-layer, large-size boron nitride nanosheets, but the growth process is difficult to control, and problems such as transfer from the substrate exist, making it unsuitable for mass production. The second approach is the top-down exfoliation method, which weakens the interlayer forces of hexagonal boron nitride to exfoliate few layers of boron nitride from the bulk material, obtaining boron nitride nanosheets. This method has low raw material costs and is simple to operate, making it suitable for mass production of boron nitride nanosheets. However, due to the small crystal size of the boron nitride raw material and the difficulty in controlling the exfoliation energy, the boron nitride nanosheets prepared by this method have small lateral dimensions and a low overall aspect ratio (<200). For applications of boron nitride nanosheets, a low aspect ratio is detrimental to the expression of their excellent thermal and mechanical properties.

[0004] Existing preparation methods struggle to achieve high-yield, high-throughput preparation of boron nitride nanosheets with high aspect ratios (>1000), limiting their industrial application in composite materials, thermal management, and drug carriers. Therefore, exploring simple, high-yield, and high-throughput methods for preparing high aspect ratio boron nitride nanosheets is of significant importance for their industrial production and widespread application. Summary of the Invention

[0005] To address the problems of low yield, small aspect ratio, and long preparation time in existing boron nitride nanosheet preparation methods, this invention provides a method for preparing boron nitride nanosheets with high aspect ratio through precursor conversion. This invention achieves high-throughput preparation of boron nitride nanosheets with aspect ratios between 1000 and 3300 by adjusting the type and proportion of the foaming agent through in-situ foaming polymerization. This solves the problems of long processing time, low yield, complex post-processing, and small aspect ratio of the obtained boron nitride nanosheets in traditional boron nitride nanosheet preparation methods, laying the foundation for the industrial preparation of boron nitride nanosheets and their application in the field of advanced composite materials.

[0006] The technical solution of the present invention is as follows:

[0007] The present invention discloses a method for preparing high aspect ratio boron nitride nanosheets by precursor conversion, comprising the following steps: 1. uniformly mixing boron nitride precursor with a foaming agent; 2. foaming the mixed powder by programmed temperature rise and high-temperature sintering; 3. preparing high aspect ratio boron nitride nanosheets by liquid-phase fractionation and centrifugation of the sintered powder.

[0008] The objective of this invention is achieved through the following technical solution: A method for preparing high aspect ratio boron nitride nanosheets by precursor conversion includes the following steps: S1. The carbon-free polyboronazine precursor with high ceramic yield is mixed with the foaming agent at a mass ratio of (1-2):(1-5) and then ball-milled in a ball mill. The high ceramic yield carbon-free polycarbosilane precursor, wherein the ceramic yield is >70%, has the following structural formula: ; S2. The ball-milled mixed powder is foamed and sintered at high temperature to obtain boron nitride ceramic foam; S3. The boron nitride ceramic foam is ultrasonically treated in a solvent to obtain a suspension. After centrifugation of the suspension, the supernatant is filtered and the precipitate is vacuum dried to obtain boron nitride nanosheets with a high aspect ratio. The boron nitride nanosheets have a thickness of 1.5 nm-4 nm, a width of 3 μm-33 μm, and an aspect ratio of 1000-11000.

[0009] Furthermore, the foaming agent mentioned in S1 is one or more of ammonium chloride, ammonium carbonate, ammonium nitrate, melamine, and oxalic acid, and the mixture can be in any proportion.

[0010] In S1, the ratio of the high ceramic yield polyboron alkane precursor to the foaming agent is (1-2):(1-5). By adjusting the type and ratio of the foaming agent, the size of the bubbles generated during the foaming process can be controlled, and the aspect ratio of boron nitride nanosheets can be controlled.

[0011] The high-ceramic-yield carbon-free polycarbosilane precursor was prepared using the method described in patent number CN113716581A, and its structural formula is as follows:

[0012] According to its structural formula, this precursor does not contain carbon elements, but has a large number of B~H and N~H active bonds, which can serve as crosslinking sites during the sintering process, thus resulting in a high ceramic yield.

[0013] Furthermore, the foaming temperature described in S2 is 50℃~500℃, and the time is 5min~30min; the high-temperature sintering temperature is 1000℃~1800℃. The foaming temperature is precisely controlled according to the decomposition temperature of different foaming agents to achieve rapid and dense generation of bubbles.

[0014] Furthermore, in the foaming process described in S2, the heating rate before foaming is 5℃ / min to 20℃ / min, and the heating rate for high-temperature sintering is 2℃ / min to 10℃ / min. Adjusting the heating rate avoids the collapse of bubbles in the boron nitride foam, thereby achieving high-yield and mass production of boron nitride nanosheets with high aspect ratio.

[0015] The purpose of foaming in S2 is to cause the precursor to expand and bubble through the gas generated by the foaming agent during the cross-linking process. After high-temperature sintering, boron nitride foam with a fascia structure is formed. The membrane structure is separated by ultrasound to form boron nitride nanosheets with a high aspect ratio.

[0016] Furthermore, in S3, the solvent is one or more of water, N,N-dimethylformamide, ethanol, and isopropanol, and the mixture is in any proportion.

[0017] In S3, the ultrasonic power is 100 W to 500 W and the time is 1 min to 60 min. By controlling the ultrasonic power and time, the boron nitride nanosheets with high aspect ratio are peeled from the foam while ensuring that the boron nitride nanosheets do not break and maintaining the largest possible aspect ratio.

[0018] In S3, the centrifugation speed is 2000 rpm to 800 rpm and the time is 1 min to 60 min. By adjusting the centrifugation time and speed, bulk impurities in the suspension are separated, and high-purity extraction of boron nitride nanosheets with high aspect ratio is achieved.

[0019] The advantage of this invention over the prior art lies in: 1. The method for preparing high aspect ratio boron nitride nanosheets by precursor conversion described in this invention uses self-made high ceramic yield (>70%) carbon-free polycarbosilane as a precursor, which avoids the ammonia decarburization process in the traditional sintering ceramicization process, reduces the preparation cost, and simplifies the process flow.

[0020] 2. The method for preparing high aspect ratio boron nitride nanosheets by precursor conversion according to the present invention employs a foaming agent and a precursor with high ceramic yield for co-sintering, achieving high-yield preparation of high aspect ratio boron nitride nanosheets and avoiding the problems of long preparation time, low yield, and small aspect ratio of boron nitride nanosheets obtained by traditional mechanical exfoliation methods. Furthermore, the size of the boron nitride nanosheets can be controlled by selecting the appropriate foaming agent.

[0021] 3. The method for preparing high aspect ratio boron nitride nanosheets by precursor conversion described in this invention provides a boron nitride nanosheet preparation technology with advantages such as high stability, high efficiency, and low equipment requirements. It does not require a harmful gas heat treatment step and is suitable for the mass production of boron nitride nanosheets. Attached Figure Description

[0022] The accompanying drawings are provided to further illustrate the invention and form part of the specification. They are used together with the embodiments of the invention to explain the invention and do not constitute a limitation thereof.

[0023] Figure 1 The image shows the XRD pattern of boron nitride nanosheets obtained in the method for preparing high aspect ratio boron nitride nanosheets by precursor conversion as described in Example 1 of this invention. Figure 2 The image shows the AFM image of the boron nitride nanosheets obtained in the method for preparing high aspect ratio boron nitride nanosheets by precursor conversion as described in Example 1 of this invention. Figure 3 This is a size distribution diagram of the boron nitride nanosheets obtained in the method for preparing high aspect ratio boron nitride nanosheets by precursor conversion as described in Example 1 of the present invention. Detailed Implementation

[0024] 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 a part of the embodiments of the present invention, and not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0025] Furthermore, the technical solutions of the various embodiments of the present invention can be combined with each other, but only if they are based on the ability of those skilled in the art to implement them. When the combination of technical solutions is contradictory or cannot be implemented, it should be considered that such combination of technical solutions does not exist and is not within the scope of protection claimed by the present invention.

[0026] The realization of the objective, functional features and advantages of the present invention will be further explained in conjunction with the embodiments and with reference to the accompanying drawings.

[0027] Example 1: A method for preparing high aspect ratio boron nitride nanosheets by precursor conversion includes the following steps: S1. The carbon-free polyboronazine precursor with high ceramic yield was mixed with ammonium chloride at a mass ratio of 2:1 and placed in a ball mill. The ball milling was carried out at 500 rpm for 30 min at room temperature. S2. Place the ball-milled mixed powder into a boron nitride crucible, place it in a graphite furnace, heat it to 350℃ at 5℃ / min and hold it for 30min to foam it, then heat it to 1000℃ at 2℃ / min and hold it for 60min to sinter it at high temperature to obtain boron nitride ceramic foam. S3. Boron nitride ceramic foam was sonicated in ethanol at 100W for 1 min to obtain a suspension. The suspension was centrifuged at 2000rpm for 60 min, and the supernatant was filtered. The precipitate was dried under vacuum at 80℃ to obtain boron nitride nanosheets with a thickness of 3 nm, a width of 4 μm, and an aspect ratio of 1333.

[0028] Figure 1 The image shows the XRD pattern of the boron nitride nanosheets prepared in this embodiment. As can be seen from the image, the prepared nanosheets have a hexagonal boron nitride structure.

[0029] Figure 2 The image shows the AFM pattern of the boron nitride nanosheets prepared in this embodiment. As can be seen from the image, the thickness of the prepared nanosheets is 3 nm.

[0030] Figure 3 The figure shows the size distribution of the boron nitride nanosheets prepared in this embodiment. As can be seen from the figure, the average lateral size of the prepared nanosheets reaches 4 μm, and the aspect ratio exceeds 1000.

[0031] Example 2: A method for preparing high aspect ratio boron nitride nanosheets by precursor conversion includes the following steps: By changing the mass ratio of precursor to ammonium chloride in Example 1 to 1:5, while keeping other conditions unchanged, boron nitride nanosheets with a thickness of 2 nm, a width of 6 μm, and an aspect ratio of 3000 were obtained.

[0032] Example 3: A method for preparing high aspect ratio boron nitride nanosheets by precursor conversion includes the following steps: By changing the foaming temperature in Example 1 to 500℃, the heating rate to 20℃ / min, and the holding time to 1min, while keeping other conditions unchanged, boron nitride nanosheets with a thickness of 3nm, a width of 6μm, and an aspect ratio of 2000 were obtained.

[0033] Example 4: A method for preparing high aspect ratio boron nitride nanosheets by precursor conversion includes the following steps: By changing the sintering temperature in Example 1 to 1800 °C while keeping the other conditions unchanged, boron nitride nanosheets with a thickness of 4 nm, a width of 4.5 μm, and an aspect ratio of 1125 were obtained.

[0034] Example 5: A method for preparing high aspect ratio boron nitride nanosheets by precursor conversion includes the following steps: S1. The carbon-free polyboronazine precursor with high ceramic yield was mixed with oxalic acid at a mass ratio of 1:2 and placed in a ball mill. The mixture was then ball-milled at 500 rpm for 30 min at room temperature. S2. Place the ball-milled mixed powder into a boron nitride crucible, place it in a graphite furnace, heat it to 2200℃ at 7℃ / min and hold it for 30min to foam it, then heat it to 1600℃ at 8℃ / min and hold it for 30min to sinter it at high temperature to obtain boron nitride ceramic foam. S3. Boron nitride ceramic foam was sonicated in N,N-dimethylformamide at 100W for 2 minutes to obtain a suspension. The suspension was centrifuged at 3000rpm for 10 minutes, and the supernatant was filtered. The precipitate was dried under vacuum at 80℃ to obtain boron nitride nanosheets with a thickness of 2 nm, a width of 3 μm, and an aspect ratio of 1500.

[0035] Example 6: A method for preparing high aspect ratio boron nitride nanosheets by precursor conversion includes the following steps: S1. The carbon-free polyboron azela precursor with high ceramic yield was mixed with ammonium carbonate at a mass ratio of 2:3 and placed in a ball mill. The mixture was then ball-milled at 500 rpm for 30 min at room temperature. S2. Place the ball-milled mixed powder into a boron nitride crucible, place it in a graphite furnace, heat it to 500℃ at 10℃ / min and hold it for 10min to foam it, then heat it to 1600℃ at 5℃ / min and hold it for 60min to sinter it at high temperature to obtain boron nitride ceramic foam. S3. Boron nitride ceramic foam was sonicated in ethanol at 300W for 1 min to obtain a suspension. The suspension was centrifuged at 3000rpm for 20 min, and the supernatant was filtered. The precipitate was dried under vacuum at 80℃ to obtain boron nitride nanosheets with a thickness of 1.5 nm, a width of 5 μm, and an aspect ratio of 3333.

[0036] Example 7: A method for preparing high aspect ratio boron nitride nanosheets by precursor conversion includes the following steps: S1. The carbon-free polyboron azela precursor with high ceramic yield was mixed with ammonium nitrate at a mass ratio of 2:1 and placed in a ball mill. The mixture was then ball-milled at 500 rpm for 30 min at room temperature. S2. Place the ball-milled mixed powder into a boron nitride crucible, place it in a graphite furnace, heat it to 230℃ at 10℃ / min and hold it for 10min to foam it, then heat it to 1700℃ at 10℃ / min and hold it for 20min to sinter it at high temperature to obtain boron nitride ceramic foam. S3. Boron nitride ceramic foam was sonicated in ethanol at 300W for 1 min to obtain a suspension. The suspension was centrifuged at 4000rpm for 20 min, and the supernatant was filtered. The precipitate was dried under vacuum at 80℃ to obtain boron nitride nanosheets with a thickness of 3 nm, a width of 9 μm, and an aspect ratio of 3000.

[0037] Example 8: A method for preparing high aspect ratio boron nitride nanosheets by precursor conversion includes the following steps: S1. The carbon-free polyboron alkane precursor with high ceramic yield was mixed with melamine at a mass ratio of 1:4 and placed in a ball mill. The ball milling was carried out at 500 rpm for 30 min at room temperature. S2. Place the ball-milled mixed powder into a boron nitride crucible, place it in a graphite furnace, heat it to 300℃ at 3℃ / min and hold it for 10min to foam it, then heat it to 1800℃ at 5℃ / min and hold it for 60min to sinter it at high temperature to obtain boron nitride ceramic foam. S3. Boron nitride ceramic foam was sonicated in ethanol at 300W for 11 min to obtain a suspension. The suspension was centrifuged at 3000rpm for 30 min, and the supernatant was filtered. The precipitate was dried under vacuum at 80℃ to obtain boron nitride nanosheets with a thickness of 2 nm, a width of 2 μm, and an aspect ratio of 1000.

[0038] Comparative Example 1: The difference between Comparative Example 1 and Example 1 is that the precursor used is ammonia borane, and its ceramic yield is only 20%. Otherwise, it is the same as Example 1. The prepared boron nitride nanosheets have a thickness of 10 nm, a width of 1 μm, and an aspect ratio of 1000.

[0039] The results show that using precursors with high ceramic yields is beneficial for forming boron nitride nanosheets with a larger width-to-thickness ratio.

[0040] Comparative Example 2: The difference between Comparative Example 2 and Example 1 is that the foaming agent and the precursor with high ceramic yield were not used for co-sintering; otherwise, they are the same as Example 1. The prepared boron nitride nanosheets have a thickness of 10 nm, a width of 300 μm, and an aspect ratio of 30.

[0041] The results show that during the in-situ polymerization of the precursor, the gas molecules generated by the decomposition of the foaming agent can cause the polymer to expand and form a foam-like structure. The foam-like structure contains some boron nitride nanosheets with high aspect ratio. By fractional centrifugation of the foam, boron nitride nanosheets with high aspect ratio can be prepared.

[0042] The various embodiments of the present invention have been described above. These descriptions are exemplary and not exhaustive, nor are they limited to the disclosed embodiments. Many modifications and variations will be apparent to those skilled in the art without departing from the scope and spirit of the described embodiments. Therefore, the scope of protection of the present invention should be determined by the scope of the claims.

Claims

1. A method for preparing high aspect ratio boron nitride nanosheets by precursor conversion, characterized in that: Includes the following steps: S1. The carbon-free polyboronazine precursor with high ceramic yield is mixed with a foaming agent at a ratio of (1-2):(1-5) and then ball-milled thoroughly. The high ceramic yield carbon-free polycarbosilane precursor, wherein the ceramic yield is >70%, has the following structural formula: ； S2. The ball-milled mixed powder is foamed and sintered at high temperature to obtain boron nitride ceramic foam; S3. The boron nitride ceramic foam is ultrasonically treated in a solvent to obtain a suspension. After centrifugation of the suspension, the supernatant is filtered and the precipitate is vacuum dried to obtain boron nitride nanosheets with a high aspect ratio. The boron nitride nanosheets have a thickness of 1.5 nm-4 nm, a width of 3 μm-33 μm, and an aspect ratio of 1000-11000.

2. The method for preparing high aspect ratio boron nitride nanosheets by precursor conversion according to claim 1, characterized in that: The foaming agent mentioned in S1 is one or more of ammonium chloride, ammonium carbonate, ammonium nitrate, melamine, and oxalic acid, and can be mixed in any proportion.

3. The method for preparing high aspect ratio boron nitride nanosheets by precursor conversion according to claim 1, characterized in that: The foaming process described in S2 involves a foaming temperature of 50℃~500℃, a foaming time of 5min~30min, and a heating rate of 5℃ / min~20℃ / min before foaming; the high-temperature sintering process involves a temperature of 1000℃~1800℃, a foaming time of 1min~60min, and a high-temperature sintering heating rate of 2℃ / min~10℃ / min.

4. The method for preparing high aspect ratio boron nitride nanosheets by precursor conversion according to claim 1, characterized in that: In S3, the solvent is one or more of water, N,N-dimethylformamide, ethanol, and isopropanol, and the mixture can be in any proportion.

5. The method for preparing high aspect ratio boron nitride nanosheets by precursor conversion according to claim 1, characterized in that: In S3, the ultrasound power is 100 W to 500 W, and the duration is 1 min to 60 min.

6. The method for preparing high aspect ratio boron nitride nanosheets by precursor conversion according to claim 1, characterized in that: In S3, the centrifugation speed is 2000 rpm to 800 rpm, and the time is 1 min to 60 min.

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