Boron nitride nanosheet, preparation method and application

Abstract

The invention relates to the technical field of nano materials, in particular to a boron nitride nanosheet, a preparation method and application of the boron nitride nanosheet in a thermal interface material. According to the preparation method, the three-roller grinding machine is adopted to replace ball milling in the prior art, introduction of ball milling beads is avoided, and the functionalization degree of the boron nitride nanosheets is remarkably improved through double-mode treatment including a gap mode and a force mode of the three-roller grinding machine; the heat conductivity and the energy storage modulus are further improved when the boron nitride nanosheet serving as a filler is filled in the thermal interface material. In addition, the method does not need additional centrifugal operation, and the yield of the boron nitride nanosheet can be close to 100%.

Description

Technical Field

[0001] The present invention relates to the technical field of nanomaterials, and particularly to a boron nitride nanosheet, a preparation method thereof, and an application thereof in a thermal interface material. Background Art

[0002] With the rapid development of electronic devices, especially high-performance computing devices, 5G communication devices, and electric vehicles, etc., the demand for thermal management materials is increasing day by day. As a key bridge connecting heat-generating components and radiators, thermal interface materials (TIMs) play a crucial role in the heat dissipation of electronic devices. Boron nitride nanosheets (BNNSs), which have ultra-high intrinsic thermal conductivity, electrical insulation, good mechanical flexibility, more controllable orientation and structure, and protective barrier properties, are widely regarded as ideal filler candidates in TIM polymers. In addition, the high aspect ratio of two-dimensional fillers may reduce the thermal permeability of the composite material by more effectively constructing a network in the polymer. On the other hand, unexfoliated bulk hBN has also been used to manufacture epoxy resin composites and shows a certain enhancement effect on thermal conductivity at relatively high BN loadings.

[0003] The efficient large-scale production method of BNNSs is still in its infancy. The top-down method (usually exfoliating from bulk hBN) has become a viable route due to its better scalability and higher crystallinity, and thus is suitable for producing BNNSs for polymer TIM applications. The bottom-up method only allows the growth of very small amounts of high-quality BNNS thin sheets or films with extremely few defects on metal substrates by chemical vapor deposition (CVD). The prior art discloses ball-milling hexagonal boron nitride powder and urea solution under ultrasonic conditions, and centrifuging, washing, and drying the ball-milled liquid to obtain functionalized boron nitride nanosheets. However, when the boron nitride nanosheets prepared by the above method are used as fillers in thermal interface materials, the effect does not meet the expectations. Through research, it is found that on the one hand, during the ball-milling process, the ball-milling beads will collide with each other, and the wear and residue of the ball-milling beads may have an adverse impact on the performance of the final product. At the same time, the degree of functionalization of boron nitride nanosheets by the ball-milling method is limited, resulting in limited improvement in thermal conductivity and storage modulus when the boron nitride nanosheets are used as fillers in thermal interface materials. On the other hand, centrifugation is an indispensable step in the above method, but centrifugation will cause the loss of some boron nitride nanosheets, thereby reducing the yield of boron nitride nanosheets. Therefore, it is urgent to optimize the existing preparation method of boron nitride nanosheets to solve the above problems. Summary of the Invention

[0004] To solve the above problems, the present invention provides a new preparation method for preparing boron nitride nanosheets, which can avoid the introduction of ball-milling beads, effectively control the exfoliation degree and agglomeration phenomenon of BNNSs, and achieve 100% of the product.

[0005] The present invention provides a method for preparing boron nitride nanosheets, which comprises the following steps:

[0006] (1) Mix hexagonal boron nitride powder with an aqueous urea solution evenly to form a dispersion;

[0007] (2) Pass the dispersion through a three-roll grinder for cyclic grinding operations to strip the hexagonal boron nitride powder into a slurry of hydroxyl- and amino-functionalized boron nitride nanosheets;

[0008] (3) Dilute the slurry of boron nitride nanosheets and then perform dialysis treatment to remove urea;

[0009] (4) Perform drying treatment on the dialyzed slurry of boron nitride nanosheets to obtain boron nitride nanosheets.

[0010] Preferably, in the step (1), the concentration of the aqueous urea solution is 0.8 - 2 g / ml, and the mass ratio of hexagonal boron nitride to urea in the dispersion is 1:5 - 40.

[0011] Preferably, in the step (1), the mixing is selected from one or more of mechanical stirring mixing, ultrasonic mixing, or microfluidic mixing. The microfluidic mixing is to circulate a mixed slurry containing hexagonal boron nitride powder and an aqueous urea solution through a microfluidic channel with a width of 50 - 100 μm under a pressure of 100 - 300 MPa, and the number of circulation times is 10 - 80 times.

[0012] Preferably, the cyclic grinding operation of the three-roll grinder includes gap mode treatment and force mode treatment. The ratio of the number of circulation times of the two mode treatments is 1:0.5 - 2, and the total number of circulation times of the two modes is 20 - 80 times.

[0013] Preferably, in the gap mode treatment, the gap between the rolls of the three-roll grinder is gradually adjusted from 40 μm to 5 μm, and in the force mode treatment, the pressure is gradually adjusted from 2 N / mm to 26 N / mm.

[0014] Preferably, in the step (3), the dilution is to dilute the dialyzed slurry of boron nitride nanosheets to 30 - 50 mg / ml. The time of the dialysis treatment is 7 - 10 days, the dialysis solution for the dialysis treatment is water, and the dialysis solution is changed every 8 - 38 h during the dialysis treatment.

[0015] Preferably, in the step (4), the drying treatment is carried out at room temperature - 60 °C for 1 - 15 d.

[0016] The present invention also provides a boron nitride nanosheet prepared according to the above method. The boron nitride nanosheet is a urea-modified boron nitride nanosheet with an average size of 0.5 - 2 microns and an average thickness of less than 50 nanometers. The application of the boron nitride nanosheet prepared by the present invention in a thermal interface material is characterized in that the thermal interface material comprises a polymer and boron nitride nanosheets as fillers, and the loading amount of the boron nitride nanosheets in the thermal interface material is 2 - 45 wt%.

[0017] Preferably, the polymer is one or more of epoxy resin, silicone polymer, polyimide, fluororubber, and polyurethane.

[0018] Compared with the prior art, the present invention has the following advantages:

[0019] First of all, the present invention uses a three-roll grinder to replace the ball mill in the prior art, avoiding the introduction of ball mill beads. The dual-mode treatment of the three-roll grinder, including the gap mode and the force mode, significantly improves the functionalization degree of the hydroxyl and amino groups of the boron nitride nanosheets, and further improves the thermal conductivity and storage modulus when the boron nitride nanosheets are filled as fillers in the thermal interface material. Moreover, the above method does not require additional centrifugation operations and can achieve a yield of boron nitride nanosheets close to 100%.

[0020] Secondly, the present invention uses a microfluidic process as the mixing method. Its pre-shearing effect on the mixed slurry, combined with the treatment of the three-roll ball mill, further improves the functionalization degree of the boron nitride nanosheets. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 : SEM characterization diagrams of the boron nitride nanosheets prepared in Examples 1 and 2: (a) SEM photograph of hexagonal boron nitride powder; (b) SEM photograph of the boron nitride nanosheet prepared in Example 1; (c) SEM photograph of the boron nitride nanosheet prepared in Example 2;

[0022] Figure 2 : TEM characterization diagrams of the boron nitride nanosheets prepared in Examples 1 and 2: (a) and (c) TEM photographs of the boron nitride nanosheet prepared in Example 1; (b) and (d) TEM photographs of the boron nitride nanosheet prepared in Example 2;

[0023] Figure 3 : XRD characterization of the unpeeled and peeled boron nitride nanosheets in Example 1;

[0024] Figure 4: FTIR characterization of the boron nitride nanosheet prepared in Example 1;

[0025] Figure 5 : XPS characterization of the boron nitride nanosheet prepared in Example 1 and hexagonal boron nitride powder.

[0026] Figure 6 SEM images of the epoxy resin composites prepared in Example 3 and Example 4: (a) Example 3, (b) Example 4. Detailed implementation manners

[0027] Example 1

[0028] The boron nitride nanosheets in this example were prepared by the following steps:

[0029] (1) Urea was pre-dissolved in deionized water at 65 °C to reach a 1 g / ml solution; then the raw hexagonal boron nitride powder h-BN and the urea solution (the mass ratio of h-BN to urea was 1:10) were mixed by mechanical stirring to form a dispersion.

[0030] (2) The above dispersion was passed through a three-roll grinder for cyclic grinding treatment. Each complete cycle involved the material moving from the feed roll to the apron roll. The total number of cycles was 75 times. Among them, the intermittent mode treatment cycle was 40 times. During these 40 cycles, the gap between the rolls was gradually adjusted from 40 μm to 5 μm; the force mode treatment cycle was 35 times. In the first 20 cycles, the pressure was gradually adjusted from 2 N / mm to 26 N / mm and maintained at the above pressure in the subsequent 15 cycles.

[0031] (3) After grinding, the slurry was diluted to 40 mg / ml, and then dialyzed in deionized water for 7 - 10 days to remove urea.

[0032] (4) The above slurry was dried at room temperature for one week to obtain the final boron nitride nanosheets BNNS.

[0033] Example 2

[0034] The boron nitride nanosheets in this example were prepared by the following steps:

[0035] (1) Urea was pre-dissolved in deionized water at 65 °C to reach a 1 g / ml solution; then the raw hexagonal boron nitride powder h-BN and the urea solution (the mass ratio of h-BN to urea was 1:10) were mixed, and then the fluid circulated under a pressure drop of 207 Mpa through a microfluidic channel with a width of 87 microns for 16 times;

[0036] (2) The above dispersion was passed through a three-roll grinder for cyclic grinding treatment. Each complete cycle involved the material moving from the feed roll to the apron roll. The total number of cycles was 75 times. Among them, the intermittent mode treatment cycle was 40 times. During these 40 cycles, the gap between the rolls was gradually adjusted from 40 μm to 5 μm; the force mode treatment cycle was 35 times. In the first 20 cycles, the pressure was gradually adjusted from 2 N / mm to 26 N / mm and maintained at the above pressure in the subsequent 15 cycles.

[0037] (3) After the grinding is completed, dilute the slurry to 40 mg / ml, and then dialyze it in deionized water for 7 - 10 days to remove urea.

[0038] (4) Dry the above slurry at room temperature for one week to obtain the final boron nitride nanosheets BNNS.

[0039] Example 3

[0040] Using the boron nitride nanosheets BNNSs prepared in Example 1 as fillers, prepare an epoxy resin composite with a filler loading of 36 wt%, and the preparation method is as follows:

[0041] Mix the BNNSs with epoxy resin using a manual stirring rod for 5 minutes until the powder is wetted by the epoxy resin and no dry powder is visible; then, transfer the mixture to a high-speed mixer and mix it at 1000 rpm for 10 minutes; next, after the mixture and the high-speed mixer are cooled to room temperature, add a curing agent to the BNNSs / epoxy resin mixture according to the weight ratio of 38:100 of the curing agent to the epoxy resin, and mix it at 1000 rpm for another 10 minutes; finally, pour the mixture into a mold to form an epoxy resin composite with a BNNSs loading of 36 wt%.

[0042] Example 4

[0043] Using the boron nitride nanosheets BNNSs prepared in Example 2 as fillers, prepare an epoxy resin composite with a filler loading of 36 wt%, and the preparation method is the same as that in Example 3.

[0044] Comparative Example 1

[0045] Prepare boron nitride nanosheets by ball milling, and the specific method is as follows:

[0046] Pre-dissolve urea in deionized water at 65 °C to obtain a 1 g / ml solution; then mix the original hexagonal boron nitride powder h-BN with the urea solution (the mass ratio of h-BN to urea is 1:10) by mechanical stirring to form a dispersion. Then place the above dispersion together with zirconia milling beads in a ball mill jar and ball mill for 16 h, then take out the milling beads, centrifuge at 8000 rmp for 10 min, and repeat multiple times until the pH of the supernatant is 7.0. Finally, dry the precipitate to form boron nitride nanosheets.

[0047] Then use the same method as in Example 3 to prepare an epoxy resin composite with a filler loading of 36 wt%.

[0048] Characterization results: Characterize the boron nitride nanosheets prepared in the original Examples 1 and 2 by SEM and TEM, and the results are as Figure 1 and 2As shown, the particle sizes of the boron nitride nanosheets prepared in Examples 1 and 2 are significantly reduced compared to the original hexagonal boron nitride powder, showing an enlarged highly layered structure and good crystallization quality. Figure 3 XRD characterization of the boron nitride nanosheets prepared in Example 1 Figure 4 and Figure 5 Subsequently, FTI R characterization and XPS characterization were carried out on the boron nitride nanosheets prepared in Example 1. The characterization results show that both hydroxyl and amino groups were successfully grafted onto the BNNSs. TGA characterization was carried out on the boron nitride nanosheet samples of Example 1 and Example 2, and weight losses of 1.8 and 2.4 wt% were shown respectively. Therefore, it can be determined that the attachment of NH2 and -OH groups in the method of Example 2 is significantly increased compared to Example 1. And Figure 6 The epoxy resin composites prepared in Examples 3-4 are shown. As can be seen, at high filler loadings, the fillers in the composites can also be uniformly dispersed in the polymer without agglomeration.

[0049] Table 1 Performance comparison of epoxy resin composites prepared in different examples and comparative examples

[0050]

[0051]

[0052] The thermal conductivities and storage moduli of the epoxy resin composites prepared in Example 3, Example 4 and Comparative Example 1 were tested. As shown in the results of Table 1, the thermal conductivities and storage moduli of the epoxy resin composites prepared in Examples 3 and 4 are significantly increased compared to pure epoxy resin and Comparative Example 1.

[0053] The above content is a further detailed description of the present invention in combination with specific preferred embodiments, and it cannot be determined that the specific implementation of the present invention is only limited to these descriptions. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A preparation method of boron nitride nanosheets, characterized in that, It includes the following steps: (1) Mix hexagonal boron nitride powder with an aqueous urea solution evenly to form a dispersion; (2) Pass the dispersion through a three-roll grinder for cyclic grinding operations so that the hexagonal boron nitride powder is exfoliated into a slurry of hydroxyl- and amino-functionalized boron nitride nanosheets; (3) Dilute the boron nitride nanosheet slurry and then perform dialysis treatment to remove urea; (4) Dry the dialyzed boron nitride nanosheet slurry to obtain boron nitride nanosheets.

2. The preparation method according to claim 1, wherein In step (1), the concentration of the aqueous urea solution is 0.8 - 2 g / ml, and the mass ratio of hexagonal boron nitride to urea in the dispersion is 1:5 - 40.

3. The preparation method according to claim 1, wherein In step (1), the mixing is selected from one or more of mechanical stirring mixing, ultrasonic mixing, or microfluidic mixing. The microfluidic mixing is to circulate a mixed slurry containing hexagonal boron nitride powder and an aqueous urea solution through a microfluidic channel with a width of 50 - 100 μm under a pressure of 100 - 300 MPa, and the number of circulation times is 10 - 80 times.

4. The preparation method according to claim 3, wherein The cyclic grinding operation of the three-roll grinder includes gap mode treatment and force mode treatment. The ratio of the number of circulation times of the two mode treatments is 1:0.5 - 2, and the total number of circulation times of the two modes is 20 - 80 times.

5. The preparation method according to claim 1, wherein, In the gap mode treatment, the gap between the rolls of the three-roll grinder is gradually adjusted from 40 μm to 5 μm. In the force mode treatment, the pressure is gradually adjusted from 2 N / mm to 26 N / mm.

6. The preparation method according to claim 1, wherein In step (3), the dilution is to dilute the dialyzed boron nitride nanosheet slurry to 30 - 50 mg / ml; the time of the dialysis treatment is 7 - 10 days, the dialysis solution for the dialysis treatment is water, and the dialysis solution is replaced every 8 - 38 h during the dialysis treatment.

7. The preparation method according to claim 1, wherein In step (4), the drying treatment is carried out at room temperature - 60 °C for 1 - 15 d.

8. A boron nitride nanosheet prepared by the preparation method according to claim 1, characterized in that, The boron nitride nanosheets are urea-modified boron nitride nanosheets, with an average size of 0.5 - 2 microns and an average thickness of less than 50 nanometers.

9. The application of the boron nitride nanosheet according to claim 8 in a thermal interface material, characterized in that, The thermal interface material includes a polymer and boron nitride nanosheets as fillers, and the loading amount of the boron nitride nanosheets in the thermal interface material is 2 - 45 wt%.

10. The application according to claim 9, characterized in that The polymer is one or more of epoxy resin, silicone polymer, polyimide, fluororubber, and polyurethane.

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