CN112919431A - High-yield and high-crystallinity hexagonal boron nitride nanosheet and preparation method thereof - Google Patents
High-yield and high-crystallinity hexagonal boron nitride nanosheet and preparation method thereof Download PDFInfo
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Abstract
The invention provides a hexagonal boron nitride nanosheet with high yield and high crystallinity and a preparation method thereof, aiming at solving the technical problems that the nano boron nitride is low in yield and high in cost and large-scale preparation is difficult to realize. According to the invention, the submicron-grade boron nitride is prepared and used as a ball milling raw material, the submicron-grade boron nitride is effectively stripped by a mechanical ball milling method by utilizing the smaller particle size, the larger specific surface area and more active sites of the submicron-grade boron nitride, the stripping efficiency can be greatly increased, and the obtained boron nitride nanosheet is uniform in size, high in crystallinity, high in yield and high in production and application values.
Description
Technical Field
The invention belongs to the field of material preparation, and particularly relates to a high-yield and high-crystallinity hexagonal boron nitride nanosheet and a preparation method thereof.
Background
Hexagonal Boron Nitride Nanosheets (BNNSs), commonly known as "white graphene", have heat-conducting properties and mechanical properties similar to those of graphene, and also have cathodoluminescent effect, conductive insulation, thermal stability and high-temperature oxidation resistance, so that BNNSs have huge application market and prospect. However, hexagonal boron nitride (h-BN) is relatively more difficult to intercalate and exfoliate than graphite due to the strong van der waals forces between adjacent layers due to ionic bonding between the layers relative to graphene.
The current BNNSs preparation method mainly comprises a bottom-up vapor deposition method and a chemical stripping method, and a top-down mechanical stripping method and an ultrasonic-assisted solvent stripping method. The vapor deposition method requires high temperature which can reach 1800 ℃ at most, and has high equipment cost and difficult large-scale production. The chemical stripping method is to use chemical oxidation method for preparing graphene for effective stripping, namely, concentrated H is adopted2SO4、KMnO4And H2O2The chemical stripping is carried out on h-BN, the yield of BNNSs is low, and the raw material is not green and is not environment-friendly. The mechanical ball milling method is characterized in that a hard ball is used for stripping a block h-BN into a few-layer or single-layer boron nitride nanosheet under the action of high shearing force and impact force, and practice shows that if the ball milling time is short, the stripping efficiency is low, and if the ball milling time is too long, the h-BN structure can be damaged, so that the crystallinity of the h-BN structure is seriously reduced, and more crystal defects exist.
Chinese patent CN111137866A discloses a method for preparing boron nitride nanosheets by stripping h-BN, which utilizes sodium citrate surfactant to provide ions for intercalation, and strips the h-BN through high-temperature high-pressure reaction and repeated liquid-phase ultrasonic dispersion to prepare BNNS. The method combines a high-temperature high-pressure reaction and an ultrasonic-assisted solvent stripping method, and has the disadvantages of unsatisfactory stripping effect and low yield. Chinese patent CN111320149A discloses a preparation method of hexagonal boron nitride nanosheet dispersion, which adopts boric acid and industrial-grade h-BN as raw materials, and mechanical ball milling is carried out, and fractional gradient centrifugation treatment is carried out to obtain the hexagonal boron nitride nanosheet dispersionh-BN dispersion with the same size as the nano-sheet. The products obtained by the method have different sizes, and the BNNs with smaller sizes can be obtained only by carrying out centrifugal separation for multiple times in a grading way, so that the yield of the BNNs with smaller sizes is lower. Song Xiaoling et al (supercritical CO)2Preparation of h-BN nanosheet and study on catalytic performance of cobalt loaded by h-BN nanosheet [ J)]9,164-2Stripping processes for preparing BNNSs having less than 50 layers thick using supercritical CO2The equipment cost is high when stripping is carried out. Chen' an Hui et al (discussion of scale preparation process of boron nitride nanosheet, Zhang Pingqi, Master thesis of university of great design, 2016 (6) month) in combination with a ball milling method and an ultrasonic-assisted stripping method, urea is used as a dry powder ball milling reagent, and the ratio of boron nitride to urea is 1: and (3) performing ball milling for 10 hours, performing ultrasonic dispersion for 2 hours to prepare the boron nitride nanosheet, wherein the stripping efficiency is 43%, the concentration of the dispersion liquid is up to 2.7mg/mL, and the ball-milled product needs to be placed in a dialysis belt for dialysis treatment to remove urea. Although the BNNs can be obtained by the top-down stripping method, the stripping effect is not high, the BNNs yield is low, the sizes are not uniform, and the large-scale production is difficult to realize.
Disclosure of Invention
In order to solve the technical problems of low yield, high cost and difficulty in realizing scale preparation of the nano boron nitride, the invention utilizes the characteristics of small particle size, large specific surface area and more active sites of the submicron boron nitride to effectively strip the submicron boron nitride by a mechanical ball milling method, can greatly increase the stripping efficiency, and the obtained boron nitride (h-BN) nanosheet has uniform size, high crystallinity and high yield and has high production and application values.
One of the purposes of the invention is to provide a preparation method of a hexagonal boron nitride nanosheet with high yield and high crystallinity, which comprises the steps of preparing submicron boron nitride from a carbon nitride compound and a boron-containing compound by a solid phase method, and then carrying out ball milling on the obtained submicron boron nitride to obtain the hexagonal boron nitride nanosheet. The preparation method specifically comprises the following steps:
step one, uniformly mixing a carbon nitrogen compound and a boron-containing compound, and heating and preserving heat;
step two, after cooling to room temperature, heating for high-temperature roasting;
step three, washing and drying the product obtained after the high-temperature roasting in the step two to obtain submicron-grade boron nitride;
and step four, carrying out mechanical ball milling stripping on the submicron-grade boron nitride obtained in the step three to obtain the hexagonal boron nitride nanosheet.
In the first step of the above-mentioned preparation method,
the carbon nitrogen compound is selected from amino-containing small molecule carbon nitrogen compound, preferably at least one selected from urea and melamine;
the boron-containing compound is at least one of boric acid, metaboric acid and ammonium borate;
the mass ratio of the carbon nitrogen compound to the boron-containing compound is 1: (1.5-5); too small a proportion increases the amount of smoke during roasting and the subsequent treatment amount, and wastes boron-containing compound raw materials; too large proportion can lead to the failure of forming submicron boron nitride and can not meet the requirements of subsequent processes;
the heating rate of temperature rise is more than 600 ℃/h, the higher temperature rise speed is beneficial to the improvement of the yield of boron nitride, meanwhile, the rapid temperature rise is beneficial to the formation of a large amount of crystal seeds in the boron nitride in a short time, and the product with submicron particle size is easier to form in the subsequent reaction process;
the heating and heat preservation temperature is 700-900 ℃, the reaction speed is very slow when the temperature is lower than the temperature range, and boron nitride crystals are easy to grow into micron-sized particles with larger particle sizes and form irregular-shaped agglomerated products when the temperature is higher than the temperature range; the heating and heat preservation time is 2-4 h, the reaction is incomplete in a short time, and the boron nitride particles grow up in a longer heat preservation time.
In the first step, no reaction auxiliary agent such as alkali metal (NaCl, NaF, etc.) or alkaline earth metal halide (CaCl, etc.) is added2Etc.), thereby preventing the corrosion of the high-temperature reaction furnace by the inorganic salt added in the prior art.
In the second step of the preparation method, the heating rate is more than 600 ℃/h; the high-temperature roasting temperature is 1100-1300 ℃, the impurity oxygen element exists in the h-BN product when the high-temperature roasting temperature is lower than the temperature range, and the product particles are larger (>1 mu m) when the high-temperature roasting temperature is higher than the temperature range; the high-temperature roasting time is 2-6 h, the deoxidation is incomplete in a short time, and the boron nitride particles grow up in a longer roasting time. And step two, cooling the raw materials in the step one to room temperature after reaction, and carrying out annealing and cooling treatment, wherein the purpose is to avoid the situation that the temperature is continuously increased to 1100-1300 ℃ in the reaction process so as to cause the rapid and continuous growth of h-BN crystal particles.
In the third step of the preparation method, the washing is water washing and acid washing, wherein the water washing is carried out by using hot water at the temperature of 80-100 ℃, and the acid washing solution is a sulfuric acid solution with the mass percentage concentration of 0.5-1.0%; and step two, washing and acid-washing the product obtained after high-temperature roasting, filtering, washing with distilled water for multiple times, and drying at the drying temperature of 40-80 ℃, wherein the submicron-grade boron nitride is obtained after drying, and the particle size of the obtained submicron-grade boron nitride is 0.1-2 mu m, preferably 0.2-0.8 mu m.
In the fourth step of the preparation method, water is further added in the ball milling operation, the amount of the added water has no specific requirement, the mixture is preferably mixed without ball milling, and the preferable amount ratio of the submicron-order boron nitride to the water is 1: (5-20), more preferably 1: (10-15);
in the ball milling operation, the ball milling time is 3-10 h, preferably 4-6 h; the ball milling time is too short, the generated h-BN nanosheets are less, and the yield is low; the ball milling time is too long, so that the product BNNs generate a disordered layer phenomenon, in addition, the ball milling time is long, the abrasion degree of a ball mill and grinding balls is increased, and the energy consumption is increased;
the ball milling can be realized by common ball milling equipment and ball milling process, the ball milling mode can adopt planetary rotation and revolution rotation (Z-axis rotation), and positive and negative alternate operation, wherein the planetary rotation speed can be 250-400 rpm, preferably 300-380 rpm, and more preferably 320-370 rpm; the revolution speed can be 5-12 rpm, preferably 8-12 rpm, and more preferably 10-11 rpm. The planetary rotation speed is an important factor influencing the stripping effect, and the low rotation speed can cause low stripping efficiency and low yield; however, the loss of the ball mill and the grinding balls is increased due to the overlarge rotating speed, the amount of ammonia generated in the ball milling process is large, the crystallinity of the formed h-BN product BNNs is reduced, and the phenomenon of disordered layers is easily generated.
In the ball milling operation, the used grinding ball medium is selected from grinding balls with high hardness and strength, preferably at least one of stainless steel, ceramic and zirconia, and more preferably from zirconia; in the ball milling operation, the mass ratio of the submicron-grade boron nitride to the grinding balls is 1: (60 to 100), preferably 1: (75-85); the grinding balls comprise grinding ball particles with different sizes, the size range of the grinding balls is preferably 0.5-15 mm, more preferably 1-10 mm, particularly the size of the grinding balls is preferably 10mm, 5mm, 3mm and 1mm, the mass ratio of the grinding balls with corresponding sizes is 1 (2-5): 3-8), and the preferred size is 1 (2-4): 3-6;
washing, filtering, washing and drying the hexagonal boron nitride nanosheet obtained after ball milling to obtain the hexagonal boron nitride nanosheet powder with high yield and high crystallinity; wherein the drying temperature is 40-80 ℃, and the drying process is completed by adopting drying equipment commonly used in the field, such as vacuum drying equipment; the suction filtration is carried out by vacuum filtration through a microporous filter membrane, the aperture of the microporous filter membrane is 0.22-1.2 microns, preferably 0.22-0.45 microns, the microporous filter membrane is washed with distilled water for multiple times after suction filtration, then a filter cake is dried to obtain a well-dispersed BNNs product, or the hexagonal boron nitride nanosheets obtained by ball milling are washed with water, suction filtered and washed and then directly dispersed into a solvent to prepare a BNNs dispersion liquid with a certain concentration, wherein the solvent is selected from at least one of ethanol, water, acetone, chloroform, benzene, toluene and N, N-dimethylformamide.
The invention also aims to provide a hexagonal boron nitride nanosheet with high yield and high crystallinity, and the hexagonal boron nitride nanosheet is prepared by the preparation method.
The method firstly synthesizes submicron boron nitride (h-BN), and then strips the submicron boron nitride (h-BN) by a ball milling method to obtain the BNNs nano-sheet with uniform size. The h-BN nanosheet is synthesized by the two-step synthesis method, so that the problems that the h-BN nanosheet (or nanoparticle) prepared by the solid-phase synthesis method is low in crystallinity and difficult to synthesize nanosheets with fewer layers and the like are solved, the defects of low stripping efficiency, low BNNs yield, poor crystallinity and the like of a direct mechanical stripping method are overcome, the stripping time and the stripping strength are reduced, and the stripping efficiency is improved. The BNNs obtained by the method have uniform size, the centrifugal separation and washing processes which are carried out for multiple times in subsequent grading are avoided, the separation and washing efficiency is improved, no ball-milling auxiliary agent or intercalation agent is required to be added in the ball-milling process, and the product purity is improved.
Compared with the prior art, the invention has the following beneficial effects:
1. the method comprises the steps of preparing submicron h-BN through segmented high-temperature solid-phase reaction, and then performing ball-milling mechanical stripping on the submicron h-BN serving as a raw material to prepare hexagonal boron nitride nanosheets; the BNNs obtained by the method have uniform size and high yield, and compared with the prior art, the method avoids the step of carrying out centrifugal separation and washing for multiple times, thereby greatly improving the subsequent separation and washing efficiency;
2. in the preparation method provided by the invention, ball milling aids such as urea, boric acid, L-leucine and the like are not required to be added in the ball milling process, and the submicron h-BN slices are directly and effectively stripped by utilizing the lower layer number, the larger specific surface area and the higher activity of the submicron h-BN slices, so that the reduction of the purity and the crystallinity of the h-BN nanosheets caused by the surface reaction of the added aids and the h-BN in high shear force and high speed impact is avoided, the difficulty of subsequent impurity removal and washing is also avoided, and the production efficiency and the product purity are improved;
3. in the ball milling process, because the layer number of the submicron-grade h-BN slices is lower, the ball milling efficiency is increased, and the ball milling time (less than 10h) is greatly shortened; compared with the prior art, the defects of disordered layers, reduced crystallinity and the like caused by long-time ball milling are avoided, and the crystallinity of the BNNs is improved;
4. the preparation method provided by the invention is simple and easy to implement, is green and environment-friendly, and the obtained BNNs have higher crystallinity and yield, more uniform appearance and size, and wide application prospect.
Drawings
FIG. 1 is a Scanning Electron Microscope (SEM) photograph of submicron-sized boron nitride obtained from boric acid and melamine in example 1, and it can be seen from FIG. 1 that the submicron-sized boron nitride obtained in example 1 has a regular shape and a uniform size, and the size is 0.3-0.6 μm.
FIG. 2 is a Transmission Electron Microscope (TEM) photograph of BNNs obtained after ball milling in example 1.
Fig. 3 is an X-ray diffraction (XRD) pattern of BNNs prepared in example 1(a), comparative example 1(b) and comparative example 3(c), and the BNNs products (100), (101), (102) have diffraction angles of 41.5 °, 43.8 ° and 50.1 °, respectively.
FIG. 4 is a Transmission Electron Microscope (TEM) photograph of BNNs prepared in comparative example 1.
FIG. 5 is a photograph showing a comparison of the uniform dispersions obtained in example 1(a) and comparative example 1(b) after settling by standing for 15 days.
FIG. 6 is a Transmission Electron Microscope (TEM) photograph of BNNs prepared in comparative example 3.
Detailed Description
While the present invention will be described in detail with reference to the following examples, it should be understood that the following examples are illustrative of the present invention and are not to be construed as limiting the scope of the present invention.
The test instrument and test method used in the examples are as follows:
SEM: a scanning electron microscope;
XRD: an X-ray diffractometer;
the method for determining the concentration of the BNNs dispersion liquid comprises the steps of standing the boron nitride nanosheet dispersion liquid obtained through treatment for a period of time, taking out a certain volume of supernatant liquid by using a liquid transfer gun, and transferring the supernatant liquid into a clean small bottle with known mass. The vial was placed in a forced air oven, the solvent was evaporated and dried at 90 ℃ for 24 h. The total mass of the vial and the solid dry was weighed and the concentration of the nanoplatelets was calculated by the differential method as shown below.
In the formula, C-boron nitride nanosheet dispersion concentration (mg/mL), M-total mass of vial and solid dried material (mg), M-vial mass (mg), V-volume of supernatant taken out (mL).
The raw materials and sources used in the examples are as follows:
melamine (Shandong alloy Tai), boric acid (Russian BORMCC), ethanol (Zhongtian Fine)
Zirconia ball (Long Shamiqi instrument equipment Co., Ltd.)
Microfiltration membrane (Shanghai Xinya)
Dopamine (Shanghai Merlin biochemistry Co., Ltd.)
Example 1
(1) Weighing 200g of melamine and 600g of boric acid, uniformly mixing in a mixer, placing in a high-frequency induction furnace for roasting, rapidly heating to 900 ℃ at the speed of 600 ℃/h, and preserving heat for 4 h;
(2) annealing, cooling to room temperature, rapidly heating to 1200 deg.C at a heating rate of 600 deg.C/h, and continuously calcining for 4h to obtain submicron h-BN (shown in FIG. 1);
(3) collecting a product, crushing, sieving with a 40-mesh sieve, boiling in hot water for half an hour, filtering and separating to obtain a filter cake, carrying out acid washing with a sulfuric acid solution with the concentration of 0.8 percent, wherein the mass of the sulfuric acid solution is 10 times that of the wet filter cake, carrying out water washing to obtain a neutral product, and drying the filter cake obtained by filtering and separating at 150 ℃ to obtain submicron h-BN powder with the particle size of 0.3-0.6 mu m and the median particle size D50 of about 350 nm;
(4) weighing 20g of the submicron h-BN powder, placing the submicron h-BN powder into an all-directional planetary ball mill, adding 200ml of distilled water, adding 1500g of zirconia balls with different sizes and proportions, wherein the size and mass ratio of the zirconia balls is 10mm:5mm:3mm:1mm is 1:2.6:3.3:3.3, adjusting the planetary rotation speed to 350 revolutions per minute, the Z-axis rotation speed to 10 revolutions per minute, performing positive and negative alternate operation, performing ball milling for 5min of stop time every 30min, and the ball milling time is 4 h.
(5) After the ball milling is finished, the ball milling tank is opened, and gas with pungent smell can be smelled. Taking out the materials, washing the zirconia balls with distilled water, collecting washing liquor, performing vacuum filtration with a microporous filter membrane, washing the filter membrane with the aperture of 0.22 mu m for multiple times with the distilled water, and drying the filter cake in an oven at 50 ℃ for 24 hours to obtain BNNs powder.
And dispersing the filter cake obtained by suction filtration in ethanol or distilled water to obtain the BNNs dispersion liquid. The stripped BNNs can be well dispersed in the aqueous solution to form uniform dispersion liquid, and the whole body of the BNNs is milky white. A homogeneous stable dispersion was obtained with a concentration of up to 100mg/mL (0.1 g/L). The obtained BNNs nano-sheets are transparent and uniformly distributed, present a folded structure similar to the edge of graphene, have a small amount of agglomeration phenomenon, and have a thickness of about 4-10 nm and a BNNs layer number range of about 5-15 (as shown in FIG. 2).
In example 1, after ball milling was carried out using 350nm D50 h-BN as a raw material, the collected product solution was left to stand for 72 hours, and almost no sedimentation occurred, the suspension was in the form of thick white emulsion, and the lower layer was not precipitated and separated.
Example 2
200g of melamine and 300g of boric acid are used as raw materials, other preparation processes and conditions are the same as those of example 1, submicron h-BN powder is obtained in step (3), the median particle size D50 is about 600nm and is larger than that of example 1, and the size and thickness of BNNs nano-sheets obtained after ball milling are basically consistent with those of example 1.
Example 3
200g of melamine and 1000g of boric acid are used as raw materials, other preparation processes and conditions are the same as those of the example 1, submicron h-BN powder is obtained in the step (3), the median particle size D50 is about 350nm, and the size and the thickness of BNNs nano-sheets obtained after ball milling are basically consistent with those of the example 1.
Example 4
The preparation method is the same as that of the example 1, the ball milling time is adjusted to be 6h only in the step (4), other conditions are not changed, the product is not obviously changed from the example 1, and the ball milling time of 4h is proved to be enough to effectively strip the h-BN, so that the BNNs with good dispersity and uniform size are obtained.
Example 5
The preparation method is the same as that of the embodiment 1, only in the step (4), the Z axis is adjusted to rotate for 5 r/min, the planetary rotation speed is adjusted to 255 r/min, and other conditions are not changed. Washing the ball-milled materials with distilled water, collecting the materials in a beaker, standing the materials for 24 hours, wherein the materials have obvious layering phenomenon, the suspension liquid on the upper layer is light milk white, and the white precipitate is on the lower layer, which indicates that the formed h-BN particles are large and easy to settle, the yield of the BNNs is low, and the particle sizes are not uniform.
Comparative example 1
BNNs were prepared in comparative example 1 using commercially available h-BN (23. mu. mD50, large particles of h-BN were obtained by single calcination at high temperature) by the procedures of steps (4) to (5) in example 1. In the step (4), the commercial h-BN with the size of 23 mu m D50 is adopted as the raw material for ball milling, and in order to increase the impact force of large-particle h-BN and the milling balls, the added zirconia is 1600g in mass ratio of 10mm to 5mm to 3mm to 1:1.5:2.7:2.7, and other conditions are not changed. Washing the ball-milled materials with distilled water, collecting the materials in a beaker, standing the beaker for 12 hours, wherein the suspension liquid on the upper layer is light milky white, and the precipitation phenomenon on the lower layer is obvious, which indicates that the formed h-BN particles are large and easy to settle, the BNNs yield is low, the particle sizes are not uniform, and multi-stage multiple centrifugal separation and washing are required to obtain the h-BN with different sizes. The TEM picture of the product obtained in the comparative example 1 is shown in FIG. 4, and as can be seen from FIGS. 4a to c, the BNNs nano-sheet obtained in the comparative example 1 has uneven thickness, low transparency of the sheet-like structure h-BN, no obvious graphene-like fold structure, more edge defects, thickness of about 7 to 25nm and the number of BNNs layers ranging from about 10 to 25 (FIG. 4 d).
FIG. 5 is a graph showing a comparison between the homogeneous dispersions obtained in example 1(a) and comparative example 1(b) after 15 days of standing sedimentation. As shown in FIG. 5, the dispersion obtained by ball-milling exfoliation of large particles h-BN (23. mu. mD50) in comparative example 1 showed a significantly higher settling rate than that of example 1. Since the sedimentation speed of the particles in the suspension is in direct proportion to the particle size, the BNNs obtained from the large particles are uneven in thickness distribution and poor in stripping effect.
Comparative example 2
A500 ml three-necked flask was charged with 1.5g of hexagonal boron nitride (h-BN, 23. mu. mD50), 9g of KMnO4And 200mL 98% H2Adding the three-neck flask into a constant-temperature water bath kettle at 60 ℃ for magnetic stirring for 12 hours to obtain SO liquid, and adding 200g of ice and 40ml of H into a 1L beaker after the reaction is finished2O2The reaction product is slowly poured into a beaker, the obtained dispersion liquid is stood, and after standing for 1 hour, the effect is poor, the product particles are very large, a uniformly dispersed white sol-like liquid cannot be formed, the layering in the beaker is obvious, the bottom of the beaker has obvious white precipitate, and the upper layer is transparent colorless clear liquid.
Comparative example 3
The preparation method is the same as that of the embodiment 1, only in the step (4), a certain amount of hydrophilic and oleophilic amine molecule dopamine is added as a ball milling aid, the mass ratio of h-BN to dopamine is 10:1, and other conditions are unchanged. The product is grey white, other phenomena are similar to those in example 1, obvious sedimentation phenomenon hardly occurs within 72 hours, and the dispersion liquid is uniform and stable, so that the addition of a proper amount of the ball milling aid can contribute to effective stripping of h-BN to a certain extent, and dopamine molecules interact with the h-BN in the ball milling process to promote the stripping effect. As shown in FIG. 6, the obtained BNNs are transparent, the distribution of the BNNs is more uniform than that of example 1, the edge angles of the nanosheets are reduced, most of the nanosheets are round, as shown in FIG. 6b, the thickness of the BNNs is about 2-10 nm, the number of layers of the BNNs is about 3-15, and therefore the interaction between amine molecules and atoms of the h-BN edge structure promotes the effective stripping of the h-BN, but the addition of dopamine causes the product to carry impurities, and the separation and washing difficulty of subsequent products is increased.
Claims (10)
1. A preparation method of a hexagonal boron nitride nanosheet with high yield and high crystallinity comprises the steps of preparing submicron boron nitride from a carbon nitride compound and a boron-containing compound by a solid phase method, and then performing ball milling on the obtained submicron boron nitride to obtain the hexagonal boron nitride nanosheet.
2. The preparation method according to claim 1, wherein the preparation method specifically comprises the following steps:
step one, uniformly mixing a carbon nitrogen compound and a boron-containing compound, and heating and preserving heat;
step two, after cooling to room temperature, heating for high-temperature roasting;
step three, washing and drying the product obtained after roasting in the step two to obtain submicron-grade boron nitride;
and step four, carrying out mechanical ball milling stripping on the submicron-grade boron nitride obtained in the step three to obtain the hexagonal boron nitride nanosheet.
3. The method according to claim 2, wherein in the first step,
the carbon nitrogen compound is selected from amino-containing small molecule carbon nitrogen compound, preferably at least one selected from urea and melamine; and/or the presence of a gas in the gas,
the boron-containing compound is at least one of boric acid, metaboric acid and ammonium borate; and/or the presence of a gas in the gas,
the mass ratio of the carbon nitrogen compound to the boron-containing compound is 1: (1.5-5); and/or the presence of a gas in the gas,
the heating rate is more than 600 ℃/h; and/or the presence of a gas in the gas,
the heating and heat preservation temperature is 700-900 ℃, and the heating and heat preservation time is 2-4 h.
4. The production method according to claim 2, wherein in the second step,
the heating rate is more than 600 ℃/h; and/or the presence of a gas in the gas,
the high-temperature roasting temperature is 1100-1300 ℃, and the high-temperature roasting time is 2-6 h.
5. The method according to claim 2, wherein in the third step,
the washing adopts water washing and acid washing; and/or the presence of a gas in the gas,
the drying temperature is 40-80 ℃; and/or the presence of a gas in the gas,
the particle size of the submicron-grade boron nitride is 0.1-2 mu m.
6. The production method according to claim 5,
the particle size of the submicron-grade boron nitride is 0.2-0.8 mu m; and/or the presence of a gas in the gas,
the water washing is carried out by adopting hot water at the temperature of 80-100 ℃; and/or the presence of a gas in the gas,
the acid washing solution is a sulfuric acid solution with the mass percentage concentration of 0.5-1.0%.
7. The method according to claim 2, wherein in the fourth step,
water is also added in the ball milling operation; and/or the presence of a gas in the gas,
the ball milling time is 3-10 h, preferably 4-6 h; and/or the presence of a gas in the gas,
and washing, filtering, washing and drying the hexagonal boron nitride nanosheet obtained by ball milling to obtain the hexagonal boron nitride nanosheet powder with high yield and high crystallinity.
8. The production method according to claim 7,
in the ball milling operation, the dosage ratio of the submicron-grade boron nitride to the water is 1 (5-20), preferably 1 (10-15); and/or the presence of a gas in the gas,
the grinding ball used in the ball milling operation is selected from at least one of stainless steel, ceramic and zirconia, preferably from zirconia; and/or the presence of a gas in the gas,
the size range of the grinding ball adopted by the ball milling operation is 0.5-15 mm, and preferably 1-10 mm; and/or the presence of a gas in the gas,
in the ball milling operation, the mass ratio of the submicron-grade boron nitride to the grinding balls is 1 (60-100), preferably 1 (75-85); and/or the presence of a gas in the gas,
the suction filtration adopts a microporous filter membrane for vacuum filtration; and/or the presence of a gas in the gas,
the drying temperature is 40-80 ℃; and/or the presence of a gas in the gas,
and the hexagonal boron nitride nanosheet obtained by ball milling is dispersed in a solvent after being washed, filtered and washed, so that a hexagonal boron nitride nanosheet dispersion liquid is obtained.
9. The method according to claim 8,
the grinding balls have the sizes of 10mm, 5mm, 3mm and 1mm, and the mass ratio of the grinding balls with the corresponding sizes is 1 (2-5): 3-8, preferably 1 (2-4): 3-6; and/or the presence of a gas in the gas,
the aperture of the microporous filter membrane adopted for suction filtration is 0.22-1.2 μm, and preferably 0.22-0.45 μm; and/or the presence of a gas in the gas,
the solvent of the hexagonal boron nitride nanosheet dispersion is at least one selected from ethanol, water, acetone, chloroform, benzene, toluene and N, N-dimethylformamide.
10. A high-yield and high-crystallinity hexagonal boron nitride nanosheet prepared by the preparation method of any one of claims 1-9.
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