Small-size spherical hexagonal boron nitride nanopowder and preparation method thereof

Abstract

The present invention relates to a small-particle-size spherical hexagonal boron nitride nano powder and a preparation method thereof. The method comprises the following steps: mixing boric acid and a nitrogen source, then adding borax and mixing, and then treating the obtained mixture at 75-85°C or 250°C for 2-5h; ball milling and drying or freeze drying the obtained precursor, and then heating the obtained ball milled precursor to 900°C at a heating rate of 3-5°C / min in a high-pressure nitrogen atmosphere for 0.5-3h; placing the obtained product in 80-90°C water and stirring for 5-8h, and then post-treating to obtain a small-particle-size spherical hexagonal boron nitride nano powder. The spherical boron nitride powder prepared by the present invention has a small particle size, a high specific surface area and a high sintering activity, or the prepared spherical boron nitride powder has a small particle size and a high crystallinity, and the preparation method has low energy consumption, is green and efficient.

Description

Technical Field

[0001] The invention belongs to the technical field of inorganic non-metallic materials, and in particular relates to a small-diameter spherical hexagonal boron nitride nano powder and a preparation method thereof. Background Art

[0002] Hexagonal boron nitride (h-BN) has a layered structure similar to graphite. It is a highly potential non-oxide high-temperature structural functional material with excellent physical and chemical properties. It is widely used in high-temperature materials such as ceramics and metallurgical refractory materials. In recent years, it has shown broad application prospects in functional fields such as adsorption, catalysis, energy storage and carrier materials. As a representative zero-dimensional nanomaterial, hexagonal boron nitride nanospheres (h-BNNS) have the dual advantages of spherical morphology and h-BN intrinsic properties, and will have many novel optical, electrical and magnetic properties different from bulk materials, thereby expanding their applications in electronics, optoelectronics, energy conversion and storage, thermal electronics and other fields, and have therefore become the focus of researchers.

[0003] At present, the main methods for preparing hexagonal boron nitride nanospheres include chemical vapor deposition, spray drying spheroidization, template synthesis and solvothermal methods. Chemical vapor deposition is widely used in the synthesis of h-BN nanosheets. There are few results on h-BN nanospheres, and it also shows that this method has high requirements for raw materials, the precursor synthesis is relatively complex, and the cost is high; the spray drying spheroidization method is easy to control the morphology, but its equipment is relatively complex, occupies a large area, and the price of the powder recovery device is relatively high, resulting in high costs; the solvothermal method has the advantages of simple process and fine control, but has the disadvantages of low yield, low raw material selectivity and toxicity; the template synthesis method can direct the synthesis of nanomaterials with various morphologies and structures and has great application prospects, but the prepared materials mostly contain template impurities. Therefore, choosing an appropriate and easily removable template is the key to the template method. In addition, due to its own crystal characteristics, h-BN has a tendency to grow preferentially along the (002) crystal plane, which is easy to form a large-sized sheet structure, and it is difficult to synthesize nanospheres with uniform morphology and small particle size. Furthermore, the current methods for synthesizing high-crystallinity h-BN nanospheres often have problems such as high temperature, long time, uneven particle size distribution and large size. Therefore, it is urgent to develop a low-cost, green and efficient method for preparing small-particle h-BN nanospheres.

[0004] In addition, h-BN is a compound with extremely strong covalent bonds, high melting point, low solid-phase diffusion coefficient, and difficult to sinter; and because h-BN crystals have the characteristic of preferential growth along the (002) crystal plane, presenting a flaky microscopic morphology, the interior of the sintered body usually has a unique "card room" structure, which makes it very easy for h-BN to generate pores during the sintering process, and it is difficult to obtain a high-density sintered body, which limits the further improvement of the performance of h-BN ceramics. In order to improve the density of sintering of flaky boron nitride ceramics, it is mainly achieved by reducing the activation energy of ceramic sintering and increasing the sintering driving force. The specific measures mainly include increasing the sintering temperature, increasing the sintering pressure, adding sintering aids, and refining the powder particle size. However, too high a sintering temperature will cause abnormal growth of grains, which will not only consume excessive energy but also be unfavorable for performance improvement, while adding excessive low-melting-point sintering aids will reduce the high-temperature stability of BN ceramics and deteriorate product performance.

[0005] In contrast, refining the powder particle size and using spherical h-BN nanopowder can fundamentally solve the problem of h-BN sintering difficulties. This is because during the ceramic sintering process, the energy difference between surface energy and interface energy is one of the important driving forces for ceramic sintering. The higher the surface energy, the greater the sintering activity, and the higher the density of the prepared ceramic. Spherical h-BN nanopowder can not only reduce the anisotropy of the inherent flaky microstructure of flaky h-BN, but also has a higher filling degree, which can reduce the generation of pores. Compared with flaky h-BN, spherical h-BN powder can increase the packing density of the ceramic body and improve the sintering density of BN ceramics; at the same time, the powder particle size is limited to the nanometer level, the activity of the raw materials is increased, and the problem of difficult sintering of boron nitride ceramic products is alleviated; for example, Chinese patent application CN110386593A discloses a method for synthesizing spherical boron nitride (BN) nanopowder without precursor induction, but the lower particle size spherical boron nitride particles prepared by this method have serious adhesion between each other, and even bond into flaky form, and the uniformity and quality of the particle morphology still need to be further improved.

[0006] In summary, it is very necessary to provide a small-size spherical hexagonal boron nitride nanopowder and a preparation method thereof. Summary of the invention

[0007] In order to solve one or more technical problems existing in the prior art, the present invention provides a small-particle-size spherical hexagonal boron nitride nanopowder and a preparation method thereof. The preparation process of the present invention is simple to operate, green and efficient, low in cost, low in energy consumption, and can be prepared in large quantities. The prepared spherical boron nitride powder is nano-sized, with a particle size of only 10 to 20 nm, uniform particle size, stable structure, high specific surface area and high sintering activity, which can meet the demand for providing raw materials for the efficient preparation of dense boron nitride ceramics, or high in crystallinity and purity.

[0008] In a first aspect, the present invention provides a method for preparing a small-diameter spherical hexagonal boron nitride nanopowder, the method comprising the following steps:

[0009] (1) mixing boric acid and a nitrogen source uniformly, then adding borax and mixing uniformly to obtain a mixture, and then treating the mixture at 75 to 85° C. for 2 to 5 hours to obtain a precursor;

[0010] (2) ball-milling and drying the precursor to obtain a ball-milled precursor, and then heating the ball-milled precursor to 900° C. at a heating rate of 3 to 5° C. / min in a high-pressure nitrogen atmosphere and heat treating at 900° C. for 0.5 to 3 h to obtain a product;

[0011] (3) The product is placed in 80-90° C. water and stirred for 5-8 hours, followed by post-treatment to obtain small-size spherical hexagonal boron nitride nanopowder.

[0012] Preferably, the molar ratio of the boric acid to the nitrogen source is 1:(0.5-2); the mass amount of the borax is 8-15% of the sum of the mass amounts of the boric acid and the nitrogen source; and / or the nitrogen source is urea and / or ammonium chloride.

[0013] Preferably, the ball mill uses anhydrous ethanol as the ball milling medium; the rotation speed of the ball mill is 200-300 rpm; the ball milling time is 3-4 hours; and / or the high pressure pressure is 1-5 MPa.

[0014] Preferably, the post-treatment is: centrifuging the stirred product to obtain a centrifuged product, then placing the centrifuged product in 80-90°C water for ultrasonic treatment for 20-40 minutes, repeating the centrifugation and ultrasonic treatment for multiple times, and finally drying; preferably, the centrifugal speed is 6000-8000r / min, and the centrifugal time is 3-8min.

[0015] Preferably, the specific surface area of ​​the small-size spherical hexagonal boron nitride nanopowder is greater than 70 m 2 / g; and / or the particle size of the small-particle spherical hexagonal boron nitride nanopowder is 10 to 20 nm.

[0016] In a second aspect, the present invention provides a small-diameter spherical hexagonal boron nitride nanopowder prepared by the preparation method described in the first aspect of the present invention.

[0017] The present invention provides a method for preparing a small-diameter spherical hexagonal boron nitride nanopowder in a third aspect, the method comprising the following steps:

[0018] (a) mixing boric acid and a nitrogen source uniformly, then adding borax and mixing uniformly to obtain a mixture, and then treating the mixture at 250° C. for 2 to 5 hours to obtain a precursor;

[0019] (b) wet-ball-milling the precursor and freeze-drying it for 12 to 24 hours to obtain a ball-milled precursor, and then heating the ball-milled precursor to 900° C. at a heating rate of 3 to 5° C. / min in a high-pressure nitrogen atmosphere and heat-treating it at 900° C. for 0.5 to 3 hours to obtain a product;

[0020] (c) grinding the product and placing it in 80-90° C. water with stirring for 5-8 h, then ultrasonically treating it and freeze-drying it for 12-24 h to obtain small-size spherical hexagonal boron nitride nanopowder.

[0021] Preferably, the molar ratio of the boric acid to the nitrogen source is 1:(0.5-2); the mass amount of the borax is 8-15% of the sum of the mass amounts of the boric acid and the nitrogen source; the nitrogen source is urea and / or ammonium chloride; and / or the pressure of the high pressure is 1-5 MPa.

[0022] Preferably, the particle size of the small-size spherical hexagonal boron nitride nanopowder is 10 to 20 nm.

[0023] The fourth aspect of the present invention provides a small-diameter spherical hexagonal boron nitride nanopowder prepared by the preparation method described in the third aspect of the present invention.

[0024] Compared with the prior art, the present invention has at least the following beneficial effects:

[0025] (1) The present invention utilizes low-cost raw materials and can obtain spherical hexagonal boron nitride nanopowders with controllable size and morphology by adjusting the precursor processing conditions, raw material composition and ratio. The powder has uniform morphology and uniform size, and the particle size is between 10 and 20 nm. The controllable macroscopic preparation and batch production of small-size spherical BN nanopowders can be achieved. The process of the present invention is simple, green and low-cost. It provides a method for synthesizing spherical BN nanopowders at low cost and in large quantities, which is of great significance to boron nitride-based ceramics and is expected to improve the life of the side sealing plate in thin strip continuous casting.

[0026] (2) The present invention uses boric acid, borax and urea or ammonium chloride and nitrogen as raw materials to prepare spherical BN nanopowders, and the spherical BN has the characteristics of uniform morphology, small particle size and high purity; the spherical boron nitride powder prepared by the present invention has a high specific surface area and exhibits high sintering activity, and can be used as a raw material for BN ceramic powder with high sintering activity, promotes sintering densification, and provides a good raw material basis for boron nitride ceramics; the spherical boron nitride powder prepared by the present invention has a high specific surface area, and the specific surface area is greater than 70m 2 / g, while the specific surface area of ​​commercial hexagonal boron nitride nanopowder can only reach 26m 2 / g, the larger the specific surface area, the higher the specific surface energy, the greater the sintering activity, and the higher the density of the ceramic finally obtained.

[0027] (3) The present invention also provides a method for preparing hexagonal boron nitride nanospheres with small particle size and high crystallinity, wherein the precursor is synthesized by heat treatment, ball milling and freeze drying. This operation can not only refine the particle size and remove the influence of moisture on the subsequent process, but also freeze drying ensures the fluffiness of the precursor, so that it increases the reaction contact area in the subsequent closed high-pressure environment, thereby improving the yield; in addition, after hot water washing, the method of combining ultrasound with freeze drying helps to improve the purity and form a more uniform and fine spherical structure, while reducing the agglomeration phenomenon and improving the dispersibility of the nanospheres. The preparation method provided by the present invention is simple to operate, the sintering process temperature is low, the time is short, and the efficiency is high. At the same time, the synthesized h-BN nanospheres have a small particle size of 10-20nm and a high crystallinity. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 The XRD diagrams of the precursor and the small-size spherical hexagonal boron nitride nanopowder (product) in Example 1 of the present invention are shown in FIG.

[0029] Figure 2 This is a SEM image of the small-size spherical hexagonal boron nitride nanopowder prepared in Example 1 of the present invention;

[0030] Figure 3 This is a SEM image of commercial BN nanopowder (purity of 99.8%, particle size distribution in the range of 50-300nm);

[0031] Figure 4 This is a graph showing the specific surface area characterization results of the small-size spherical hexagonal boron nitride nanopowder (abbreviated as the prepared BN nanopowder) prepared in Example 1 of the present invention and the commercial BN nanopowder;

[0032] Figure 5 This is a macroscopic physical picture of the small-size spherical hexagonal boron nitride nanopowder prepared in Example 4 of the present invention;

[0033] Figure 6 is a SEM image of the small-size spherical hexagonal boron nitride nanopowder prepared in Example 4 of the present invention;

[0034] Figure 7 This is the XRD pattern of the small-size spherical hexagonal boron nitride nanopowder prepared in Example 4 of the present invention;

[0035] Figure 8 is a SEM image of a small-size spherical hexagonal boron nitride nanopowder prepared in Example 5 of the present invention;

[0036] Fig. 9 This is the XRD pattern of the small-size spherical hexagonal boron nitride nanopowder prepared in Example 5 of the present invention;

[0037] Fig.10 This is a SEM image of the small-size spherical hexagonal boron nitride nanopowder prepared in Example 6 of the present invention;

[0038] Fig.11 This is the XRD pattern of the small-size spherical hexagonal boron nitride nanopowder prepared in Example 6 of the present invention;

[0039] Fig.12 is a SEM image of the circular hexagonal boron nitride nanopowder prepared in Comparative Example 6 of the present invention;

[0040] Fig.13 This is the XRD diagram of the circular hexagonal boron nitride nanopowder prepared in Comparative Example 6 of the present invention. DETAILED DESCRIPTION

[0041] In order to make the purpose, technical solution and advantages of the present invention clearer, the technical solution of the present invention will be clearly and completely described below in combination with the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0042] In a first aspect, the present invention provides a method for preparing a small-particle spherical hexagonal boron nitride nanopowder (also referred to as a small-particle spherical hexagonal boron nitride nanopowder with a high specific surface area), the method comprising the following steps:

[0043] (1) Boric acid (H3BO3) and a nitrogen source are mixed evenly, and then borax (Na2B4O7·10H2O) is added and mixed evenly to obtain a mixture, and then the mixture is treated at 75 to 85°C (e.g., 75°C, 80°C or 85°C) for 2 to 5 hours (e.g., 2, 3, 4 or 5 hours) to obtain a precursor; in the present invention, when preparing the precursor, no solvent is added, and after the raw materials are mixed evenly, they are treated at a low temperature of 75 to 85°C. The present invention finds that the precursor obtained in this way is better than the precursor obtained by adding a solvent to the mixture and treating it at a relatively high temperature condition such as 200°C. The precursor is obtained by heat treatment at 500°C to 550°C. The method of the present invention adopts a method of treating a solvent-free mixture at low temperature to obtain a precursor, which can more effectively control the reaction process, is conducive to the subsequent formation of hexagonal boron nitride crystals, and is helpful to prepare spherical hexagonal boron nitride nanopowders with a larger specific surface area. The possible reason is that the treatment without adding a solvent can avoid the influence of solvent volatilization on the reaction system, and the decomposition of the raw materials can retain a part of the nitrogen source to help the growth of nanoparticles, which is helpful to control the grain growth rate of the nanoparticles and help to form nanoparticles with a spherical morphology with a large specific surface area;

[0044] (2) ball milling (wet ball milling) and drying the precursor to obtain a ball milled precursor, and then heating the ball milled precursor to 900°C at a heating rate of 3-5°C / min in a high-pressure nitrogen atmosphere and heat treating it at 900°C for 0.5-3h to obtain a product; the present invention finds that the high-pressure heat treatment at a relatively low temperature of 900°C can better control the growth process of the crystal, promote the formation of spherical hexagonal boron nitride nanopowders with uniform particle size, and ultimately facilitate the acquisition of spherical hexagonal boron nitride nanopowders with a larger specific surface area. The possible reason is that in the relatively low temperature range, it is helpful to control the morphology and size of the grains during the crystal growth process, promote the uniform growth of the grains, and help increase the reaction contact area to improve the yield, and help form small-diameter spherical particles with a high specific surface area; and the present invention finds that if the reaction is carried out at a relatively high temperature (e.g., not less than 1000°C) at normal pressure, it is easier to form hexagonal boron nitride nanopowders that are close to disc-shaped;

[0045] (3) placing the product in water at 80 to 90°C (e.g., 80°C, 85°C, or 90°C) and stirring for 5 to 8 hours (e.g., 5, 6, 7, or 8 hours), and then performing post-treatment to obtain a small-diameter spherical hexagonal boron nitride nanopowder with a high specific surface area; before the product is post-treated, the present invention places the product in water at 80 to 90°C and stirs for 5 to 8 hours. The present invention finds that stirring in high-temperature water can remove residual boric acid and intermediate products adhering to the surface of the particles, which helps to improve the purity of the product and make the particles form a more uniform and fine spherical structure, and is beneficial to regulating the dispersibility of the particles, thereby increasing the specific surface area of ​​the nanoparticles; in the present invention, the rotation speed of the stirring treatment can be, for example, 100 to 400 r / min; the operations of the present invention are all carried out in an air atmosphere unless otherwise specified.

[0046] The present invention utilizes low-cost raw materials and can obtain spherical boron nitride nanopowders with controllable size and morphology by adjusting precursor processing conditions, raw material composition and ratio. The powders have uniform morphology, uniform size and a particle size of 10 to 20 nm, and can realize controllable macroscopic preparation and batch production of small-size spherical BN nanopowders. The present invention has a simple process, is green and low-cost, and provides a method for synthesizing spherical BN nanopowders at a low cost and in a large amount. The method is of great significance to boron nitride-based ceramics and is expected to improve the life of side sealing plates in thin strip continuous casting. The present invention uses boric acid, borax and urea or ammonium chloride and nitrogen as raw materials to prepare spherical BN nanopowder, and the spherical BN has the characteristics of uniform morphology, small particle size and high purity; the spherical boron nitride powder prepared by the present invention has a high specific surface area and exhibits high sintering activity, can be used as a raw material for BN ceramic powder with high sintering activity, promotes sintering densification, and provides a good raw material basis for boron nitride ceramics; the spherical hexagonal boron nitride nanopowder prepared by the present invention has a high specific surface area, and the specific surface area is greater than 70m 2 / g, while the specific surface area of ​​commercial boron nitride nanopowder can only reach 26m 2 / g, the larger the specific surface area, the higher the specific surface energy, the greater the sintering activity, and the higher the density of the ceramic finally obtained.

[0047] According to some preferred embodiments, the molar ratio of the boric acid to the nitrogen source is 1:(0.5-2) (e.g., 1:0.5, 1:1, 1:1.5 or 1:2); and / or the mass dosage of the borax is 8-15% (e.g., 8%, 9%, 10%, 11%, 12%, 13%, 14% or 15%) of the sum of the mass dosages of the boric acid and the nitrogen source; in the present invention, controlling the mass dosage of borax within an appropriate range is conducive to obtaining spherical hexagonal boron nitride nanopowders with high specific surface area. The present invention finds that an appropriate amount of borax helps to control the growth rate of powder particles and avoid particles that are too large or incompletely generated. Too much borax may cause the particles to grow too large and tend to be plate-like, reducing the specific surface area; while too little borax may cause the particles to be incompletely generated, adhere to each other, and cause serious agglomeration. In addition, an appropriate amount of borax helps to form spherical hexagonal boron nitride nanopowders with uniform and stable morphology, while too much or too little borax is also likely to cause changes in the dispersion of particle morphology, and even secondary growth or incomplete growth may begin to appear on the surface.

[0048] According to some preferred embodiments, the nitrogen source is urea (CO(NH2)2) and / or ammonium chloride (NH4Cl).

[0049] According to some preferred embodiments, the ball mill uses anhydrous ethanol as the ball milling medium; the ball-to-material ratio of the ball mill is, for example, (5-10):1; the rotation speed of the ball mill is 200-300rpm (i.e., 200-300r / min); and / or the ball milling time is 3-4h; the present invention has no special requirements for the ball milling, which is a conventional technology in the field.

[0050] According to some preferred embodiments, the high pressure has a pressure of 1 to 5 MPa (eg, 1, 2, 3, 4 or 5 MPa), preferably 3 to 5 MPa.

[0051] According to some preferred embodiments, the post-treatment is: centrifuging the stirred product to obtain a centrifuged product, then placing the centrifuged product in 80-90°C water for ultrasonic treatment for 20-40 minutes, repeating the centrifugation and ultrasonic treatment multiple times, and finally drying.

[0052] According to some preferred embodiments, the centrifugal speed is 6000-8000 r / min, and the centrifugal time is 3-8 min.

[0053] According to some preferred embodiments, the specific surface area of ​​the small-size spherical hexagonal boron nitride nanopowder is greater than 70 m 2 / g.

[0054] According to some preferred embodiments, the particle size of the small-size spherical hexagonal boron nitride nanopowder is 10 to 20 nm.

[0055] According to some specific embodiments, the preparation of the small-size spherical hexagonal boron nitride nanopowder with high specific surface area comprises:

[0056] ① Prepare low-cost raw materials of boric acid H3BO3, urea CO(NH2)2 or urea NH4Cl in a molar ratio of 1:(0.5-2) and add 8-15% of nucleation template borax decahydrate (Na2B4O7·10H2O) and mix and grind for 0.5-2h, then treat the mixed raw materials in an oven at 80°C under air atmosphere for 2-5h to obtain a viscous, hard white crystal, i.e., a white precursor.

[0057] ② Place the precursor powder in a ball mill and add anhydrous ethanol, ball mill at a speed of 200-300 rpm for 3-4 hours to mix evenly, then dry at 80°C, and sieve to obtain a ball-milled precursor, that is, reduce the particle size through ball milling to obtain a more evenly mixed precursor.

[0058] ③ Nitriding synthesis: Spread the ball-milled precursor flat in a quartz boat, then put it into a tubular furnace, evacuate and introduce nitrogen, and in a high-pressure nitrogen atmosphere, heat up to 900°C at a heating rate of 5°C / min and keep it at high pressure for 0.5-3h. Then the reaction product is cooled to room temperature at a cooling rate of 3°C / min to obtain a harder white powder.

[0059] ④ The obtained white product is placed in 85°C deionized water and heated in a constant temperature magnetic stirrer for 6 hours, then centrifuged at a speed of 6000-8000r / min for 5 minutes, and after removing the supernatant, 85°C hot water is added for ultrasonic treatment for 30 minutes. The centrifugation and ultrasonic treatment process are repeated several times to remove soluble impurities, and finally dried in a drying oven at 80°C to obtain a white powder, which is the final product, a small-particle spherical hexagonal boron nitride nanopowder.

[0060] In a second aspect, the present invention provides a small-diameter spherical hexagonal boron nitride nanopowder having a high specific surface area and obtained by the preparation method described in the first aspect of the present invention.

[0061] In a third aspect, the present invention provides a method for preparing a small-particle spherical hexagonal boron nitride nanopowder (also referred to as a small-particle spherical hexagonal boron nitride nanopowder with high crystallinity), the method comprising the following steps:

[0062] (a) mixing boric acid and a nitrogen source uniformly, then adding borax and mixing uniformly to obtain a mixture, and then treating the mixture at 250° C. for 2 to 5 hours (e.g., 2, 3, 4 or 5 hours) to obtain a precursor;

[0063] (b) wet-milling the precursor and freeze-drying it for 12 to 24 hours (e.g., 12, 14, 16, 18, 20, 22 or 24 hours) to obtain a ball-milled precursor, and then heating the ball-milled precursor to 900°C at a heating rate of 3 to 5°C / min in a high-pressure nitrogen atmosphere and heat treating it at 900°C for 0.5 to 3 hours to obtain a product; in the present invention, the wet-milling, for example, uses anhydrous ethanol as a ball-milling medium; the ball-to-material ratio of the wet-milling, for example, is (5 to 10): 1; the rotation speed of the wet-milling is 200 to 300 rpm (i.e., 200 to 300 r / min); and / or the wet-milling time is 3 to 4 hours; the present invention has no effect on the wet-milling Special requirements are conventional technologies in the field; the present invention finds that the growth process of crystals can be better controlled by high pressure heat treatment at a relatively low temperature of 900°C, and the formation of spherical hexagonal boron nitride nanopowders with uniform particle size can be promoted. The possible reason is that in the relatively low temperature range, it is helpful to control the morphology and size of the grains during the crystal growth process, promote the uniform growth of the grains, and is conducive to increasing the reaction contact area to improve the yield, and is conducive to forming small-diameter spherical particles with high crystallinity, thereby improving the yield; and the present invention finds that if the reaction is carried out at a relatively high temperature (e.g., not less than 1000°C) at normal pressure, it is easier to form hexagonal boron nitride nanopowders that are close to a disc shape;

[0064] (c) grinding the product and placing it in 80-90°C water for stirring for 5-8h, then ultrasonically treating and freeze-drying for 12-24h to obtain a small-diameter spherical hexagonal boron nitride nanopowder; the present invention does not specifically limit freeze-drying, and conventional freeze-drying conditions can be used, for example, freeze-drying can be performed at a cold trap temperature of -45°C; the hexagonal boron nitride nanospheres prepared by the present invention have uniform particle size, high crystallinity and high purity, and the method used is low in energy consumption, green and efficient; before the product is ultrasonically treated and freeze-dried, the product is placed in 80-90°C water for stirring for 5-8h. The present invention finds that stirring in high-temperature water can remove residual boric acid and intermediate products adhering to the surface of the particles, which helps to improve the purity of the product and make the particles form a more uniform and fine spherical structure, and is conducive to regulating the dispersibility of the particles; in the present invention, the rotation speed of the stirring treatment can be, for example, 100-400r / min.

[0065] The preparation method of the small-particle-size high-crystallinity hexagonal boron nitride nanospheres provided by the present invention uses a heat treatment, ball milling combined with freeze drying method to synthesize a precursor, which can not only refine the particle size and remove the influence of moisture on the subsequent process, but also freeze drying ensures the fluffiness of the precursor, so that it increases the reaction contact area in the subsequent closed high-pressure environment to improve the yield; in addition, after hot water washing, the method of combining ultrasound with freeze drying helps to improve the purity and form a more uniform and fine spherical structure, while reducing the agglomeration phenomenon and improving the dispersibility of the nanospheres. The preparation method provided by the present invention is simple to operate, the sintering process temperature is low, the time is short, and the efficiency is high. At the same time, the synthesized h-BN nanospheres have a small particle size of 10-20nm and high crystallinity.

[0066] According to some preferred embodiments, the molar ratio of the boric acid to the nitrogen source is 1:(0.5-2); the mass amount of the borax is 8-15% of the sum of the mass amounts of the boric acid and the nitrogen source; in the present invention, controlling the mass amount of borax within an appropriate range is conducive to obtaining spherical hexagonal boron nitride nanopowder with a small particle size. The present invention finds that an appropriate amount of borax helps to control the growth rate of powder particles and avoid particles that are too large or incompletely generated. Too much borax may cause the particles to grow too large and tend to be plate-like, reducing the specific surface area; while too little borax may cause the particles to be incompletely generated, adhere to each other, and cause serious agglomeration. In addition, an appropriate amount of borax helps to form spherical hexagonal boron nitride nanopowders with uniform and stable morphology, while too much or too little borax is also prone to changes in the dispersion of particle morphology, and even secondary growth or incomplete growth begins on the surface.

[0067] According to some preferred embodiments, the nitrogen source is urea and / or ammonium chloride; and / or the pressure of the high pressure is 1 to 5 MPa (eg, 1, 2, 3, 4 or 5 MPa), preferably 3 to 5 MPa.

[0068] According to some preferred embodiments, the particle size of the small-size spherical hexagonal boron nitride nanopowder is 10 to 20 nm.

[0069] According to some specific embodiments, the small-size spherical hexagonal boron nitride nanopowder is prepared as follows:

[0070] ① Raw material treatment: Boric acid is used as the boron source, urea is used as the nitrogen source, borax is used as the boron source, nucleating agent and template, they are evenly mixed and treated at 250°C for 4 hours, the obtained precursor is wet ball-milled and freeze-dried for 12-24 hours to obtain a micron-sized ball-milled precursor;

[0071] ② Low-temperature sintering synthesis: The obtained micron-sized ball-milled precursor was placed in an OTF-1200X high-pressure furnace and evacuated. The temperature was raised to 900°C at a heating rate of 5°C / min under high pressure (1, 3 and 5MPa) in a nitrogen atmosphere and kept at high pressure for 0.5-3h, and then cooled to room temperature with the furnace to obtain an off-white product;

[0072] ③ Product purification: The obtained product is ground again and placed in 80-90°C water with stirring for 5-8h, then ultrasonically treated and freeze-dried for 12-24h to obtain spherical hexagonal boron nitride nanopowder with a particle size of 10-20nm.

[0073] The fourth aspect of the present invention provides a small-diameter spherical hexagonal boron nitride nanopowder prepared by the preparation method described in the third aspect of the present invention.

[0074] The present invention will be further described below by way of examples, but the protection scope of the present invention is not limited to these embodiments.

[0075] Example 1

[0076] ① Boric acid H3BO3 and urea CO(NH2)2 are mixed uniformly in a molar ratio of 1:1, and then borax decahydrate (Na2B4O7·10H2O) is added and mixed and ground for 1 hour to obtain a mixture, and then the mixture is treated in an oven at 80°C for 3 hours under an air atmosphere to obtain a precursor; wherein the mass amount of borax decahydrate is 10% of the sum of the mass amounts of boric acid and urea.

[0077] ②Put the precursor obtained in step ① into a ball mill with anhydrous ethanol as the ball milling medium, and ball mill at a speed of 250r / min for 3h under a ball-to-material ratio of 7:1 to mix evenly, then dry at 80°C, sieve, and obtain a ball-milled precursor with a particle size of <30μm.

[0078] ③ The ball-milled precursor obtained in step ② is spread flat in a quartz boat, then placed in a tubular furnace, evacuated and nitrogen is introduced, and in a high-pressure nitrogen atmosphere with a pressure of 3 MPa, the temperature is increased to 900°C at a heating rate of 5°C / min and kept at 900°C for 1 hour, and then the reaction product is cooled to room temperature at a cooling rate of 3°C / min to obtain the product.

[0079] ④ The product obtained in step ③ is placed in 85°C deionized water and heated in a constant temperature magnetic stirrer (stirring speed is 250r / min) for 6h, then centrifuged at a speed of 7000r / min for 5min, and after removing the supernatant, 85°C hot water is added for ultrasonic treatment for 30min. The centrifugation and ultrasonic treatment process are repeated 3 times, and finally dried at 80°C to obtain the final product, a small-particle spherical hexagonal boron nitride nanopowder (a small-particle spherical hexagonal boron nitride nanopowder with a high specific area).

[0080] The small-size spherical hexagonal boron nitride nanopowder prepared in this embodiment has a uniform morphology and is nano-sized, with a particle size distribution of 10 to 20 nm. Through nitrogen adsorption and desorption test, the specific surface area of ​​the spherical hexagonal boron nitride nanopowder prepared in this embodiment is 74.3 m 2 / g.

[0081] Example 2

[0082] Embodiment 2 is substantially the same as Embodiment 1, except that:

[0083] ① Boric acid H3BO3 and urea CO(NH2)2 are mixed uniformly in a molar ratio of 1:1, and then borax decahydrate (Na2B4O7·10H2O) is added and mixed and ground for 1 hour to obtain a mixture, and then the mixture is treated in an oven at 80°C for 3 hours under an air atmosphere to obtain a precursor; wherein the mass amount of borax decahydrate is 5% of the sum of the mass amounts of boric acid and urea.

[0084] Example 3

[0085] Embodiment 3 is substantially the same as Embodiment 1, except that:

[0086] ① Boric acid H3BO3 and urea CO(NH2)2 are mixed uniformly in a molar ratio of 1:1, and then borax decahydrate (Na2B4O7·10H2O) is added and mixed and ground for 1 hour to obtain a mixture, and then the mixture is treated in an oven at 80°C for 3 hours under an air atmosphere to obtain a precursor; wherein the mass amount of borax decahydrate is 20% of the sum of the mass amounts of boric acid and urea.

[0087] Comparative Example 1

[0088] Comparative Example 1 is substantially the same as Example 1, except that:

[0089] ① Boric acid H3BO3 and urea CO(NH2)2 (the molar ratio of boric acid to urea is 1:1) are uniformly mixed with an ethanol aqueous solution, and then stirred at 90°C until the ethanol aqueous solution volatilizes, and then borax decahydrate (Na2B4O7·10H2O) is added and mixed and ground for 1 hour to obtain a mixture, and then the mixture is treated in a muffle furnace at 200°C for 3 hours under an air atmosphere to obtain a precursor; wherein the mass amount of borax decahydrate is 10% of the sum of the mass amounts of boric acid and urea, the mass amount of the ethanol aqueous solution is 5 times the sum of the mass amounts of boric acid and urea, and the ethanol aqueous solution is prepared by mixing ethanol and water in a volume ratio of 3:7.

[0090] Comparative Example 2

[0091] Comparative Example 2 is substantially the same as Example 1, except that:

[0092] ③ Spread the ball-milled precursor obtained in step ② flatly in a quartz boat, then put it into a tubular furnace, evacuate and introduce nitrogen, and in a nitrogen atmosphere, heat it to 1100°C at a heating rate of 5°C / min and keep it at 1100°C at normal pressure for 2h, then cool the reaction product to room temperature at a cooling rate of 3°C / min to obtain the product.

[0093] Comparative Example 3

[0094] Comparative Example 3 is substantially the same as Example 1, except that:

[0095] ④ The product obtained in step ③ was added into 85°C deionized water, and then centrifuged at 7000r / min for 5min using a centrifuge. After removing the supernatant, 85°C hot water was added for ultrasonic treatment for 30min. The centrifugation and ultrasonic treatment process was repeated 3 times, and finally dried at 80°C to obtain the product hexagonal boron nitride nanopowder.

[0096] Comparative Example 4

[0097] Boric acid and urea are used as raw materials in a molar ratio of 2:3, and stirred at 90°C in an ethanol aqueous solution (ethanol accounts for 30 vol%) until the ethanol aqueous solution is completely evaporated. The obtained mixed raw material is kept at 200°C in a muffle furnace for 2h under an air atmosphere to obtain an amorphous precursor, the precursor is ground to less than 200 mesh, and the temperature is increased to 1000°C at 10°C / min in a nitrogen atmosphere for 240 minutes to obtain a reaction product; the reaction product is soaked in anhydrous ethanol, and then centrifuged at a speed of 7000r / min for 5min, the supernatant is removed, and the anhydrous ethanol soaking and centrifugation process is repeated 3 times, and finally dried at 80°C, and its morphology is nano-spherical.

[0098] Comparative Example 5

[0099] Comparative Example 5 is substantially the same as Example 1, except that:

[0100] ③ Spread the ball-milled precursor obtained in step ② flatly in a quartz boat, then put it into a tubular furnace, evacuate and introduce nitrogen, and in a nitrogen atmosphere, heat it to 900°C at a heating rate of 5°C / min and keep it at 900°C at normal pressure for 1h, then cool the reaction product to room temperature at a cooling rate of 3°C / min to obtain the product.

[0101] The boron nitride nanopowders finally prepared in Examples 1 to 3 and Comparative Examples 1 to 5 were tested for particle size and their specific surface areas were measured by nitrogen adsorption and desorption. The results are shown in Table 1.

[0102] Table 1

[0103] Example Particle size distribution range (nm) <![CDATA[Specific surface area (m 2 / g)]]> Example 1 10～20 74.3 Example 2 5～25 55.4 Example 3 10～50 47.1 Comparative Example 1 20～50 40.2 Comparative Example 2 30～60 33.2 Comparative Example 3 10～25 60.3 Comparative Example 4 5～30 34.1 Comparative Example 5 10～40 28.8 Commercial BN Nanopowder 50～300 26.2

[0104] Example 4

[0105] ① Boric acid is used as a boron source, urea is used as a nitrogen source, borax (borax decahydrate) is used as a boron source, a nucleating agent and a template, they are uniformly mixed and treated at 250°C for 4 hours, the obtained precursor is wet-milled and freeze-dried for 24 hours to obtain a micron-sized ball-milled precursor; wherein the molar ratio of boric acid to urea is 1:1, and the mass amount of borax decahydrate is 10% of the sum of the mass amounts of boric acid and urea; the wet ball milling uses anhydrous ethanol as a ball milling medium, and the wet ball milling is performed at a rotation speed of 250r / min for 3 hours under the condition of a ball-to-material ratio of 7:1.

[0106] ② The ball-milled precursor was placed in an OTF-1200X high-pressure furnace and evacuated. The temperature was increased to 900°C at a heating rate of 5°C / min under high pressure (3MPa) in a nitrogen atmosphere and kept at high pressure for 1 hour. The product was then cooled to room temperature with the furnace to obtain the product.

[0107] ③ The obtained product was ground again and placed in 85°C deionized water and heated and stirred in a constant temperature magnetic stirrer (stirring speed of 250r / min) for 6h, then placed in 85°C hot water for ultrasonic treatment for 30min and freeze-dried for 24h to obtain small-particle spherical hexagonal boron nitride nanopowder.

[0108] The small-size spherical hexagonal boron nitride nanopowder prepared in this embodiment has a uniform morphology and is nano-sized, with a particle size distribution of 10 to 20 nm. The SEM image of the small-size spherical hexagonal boron nitride nanopowder prepared in this embodiment is as follows: Figure 6 As shown; the XRD pattern of the small-size spherical hexagonal boron nitride nanopowder prepared in this embodiment is as follows Figure 7 shown; from Figure 7 It can be seen from the XRD half-peak width (FWHM) combined with the peak intensity that the small-size spherical hexagonal boron nitride nanopowder prepared in this example has high crystallinity.

[0109] Example 5

[0110] Embodiment 5 is substantially the same as Embodiment 4, except that:

[0111] ② The ball-milled precursor was placed in an OTF-1200X high-pressure furnace and evacuated. The temperature was increased to 900°C at a heating rate of 5°C / min under high pressure (5MPa) in a nitrogen atmosphere and kept at high pressure for 0.5h. The product was then cooled to room temperature with the furnace to obtain the product.

[0112] The small-size spherical hexagonal boron nitride nanopowder prepared in this embodiment has a uniform morphology and is nano-sized, with a particle size distribution of 10 to 20 nm. The SEM and XRD images of the small-size spherical hexagonal boron nitride nanopowder prepared in this embodiment are as follows: Figure 8 and Fig. 9shown; from Fig. 9 It can be seen from the XRD half-peak width (FWHM) combined with the peak intensity that the small-size spherical hexagonal boron nitride nanopowder prepared in this example has high crystallinity.

[0113] Example 6

[0114] Embodiment 6 is substantially the same as Embodiment 4, except that:

[0115] ② The ball-milled precursor was placed in an OTF-1200X high-pressure furnace and evacuated. The temperature was increased to 900°C at a heating rate of 5°C / min under high pressure (1 MPa) in a nitrogen atmosphere and kept at high pressure for 3 hours. The product was then cooled to room temperature in the furnace to obtain the product.

[0116] The small-size spherical hexagonal boron nitride nanopowder prepared in this embodiment has a uniform morphology and is nano-sized, with a particle size distribution of 10 to 20 nm. The SEM and XRD images of the small-size spherical hexagonal boron nitride nanopowder prepared in this embodiment are as follows: Fig.10 and Fig.11 shown; from Fig.11 It can be seen from the XRD half-peak width (FWHM) combined with the peak intensity that the small-size spherical hexagonal boron nitride nanopowder prepared in this example has high crystallinity.

[0117] Comparative Example 6

[0118] Comparative Example 6 is substantially the same as Example 4, except that:

[0119] ② The ball-milled precursor was placed in a tube furnace and evacuated, and the temperature was increased to 1100°C at a heating rate of 5°C / min under a nitrogen atmosphere and kept at normal pressure for 2 hours, and then cooled to room temperature with the furnace to obtain the product.

[0120] The particle size distribution of the disk hexagonal boron nitride nanopowder prepared in this comparative example is 25-60nm. The SEM and XRD diagrams of the disk hexagonal boron nitride nanopowder prepared in this comparative example are as follows: Fig.12 and Fig.13 shown; from Fig.13 It can be seen from the XRD half-peak width (FWHM) combined with the peak intensity that the circular hexagonal boron nitride nanopowder prepared in this comparative example has a low crystallinity.

[0121] Parts of the present invention that are not described in detail are well known to those skilled in the art.

[0122] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or make equivalent replacements for some of the technical features therein. However, these modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for preparing small-size spherical hexagonal boron nitride nanopowder, characterized in that: The method comprises the following steps: (1) mixing boric acid and a nitrogen source uniformly, then adding borax and mixing uniformly to obtain a mixture, and then treating the mixture at 75 to 85° C. for 2 to 5 hours to obtain a precursor; (2) ball-milling and drying the precursor to obtain a ball-milled precursor, and then heating the ball-milled precursor to 900° C. at a heating rate of 3 to 5° C. / min in a high-pressure nitrogen atmosphere and heat treating at 900° C. for 0.5 to 3 h to obtain a product; the high-pressure pressure is 1 to 5 MPa; (3) placing the product in 80-90° C. water and stirring for 5-8 hours, and then performing post-treatment to obtain a small-size spherical hexagonal boron nitride nanopowder; the particle size of the small-size spherical hexagonal boron nitride nanopowder is 10-20 nm, and the specific surface area of ​​the small-size spherical hexagonal boron nitride nanopowder is greater than 70 m 2 / g.

2. The preparation method according to claim 1, characterized in that: The molar ratio of the boric acid to the nitrogen source is 1:(0.5-2); The mass amount of the borax is 8-15% of the sum of the mass amounts of the boric acid and the nitrogen source; and / or The nitrogen source is urea and / or ammonium chloride.

3. The preparation method according to claim 1, characterized in that: The ball mill uses anhydrous ethanol as the ball milling medium; The rotation speed of the ball mill is 200-300 rpm; The ball milling time is 3 to 4 hours.

4. The preparation method according to claim 1, characterized in that: The post-treatment is as follows: centrifuging the stirred product to obtain a centrifuged product, then placing the centrifuged product in 80-90°C water for ultrasonic treatment for 20-40 minutes, repeating the centrifugation and ultrasonic treatment for multiple times, and finally drying.

5. The preparation method according to claim 4, characterized in that: The centrifugal speed is 6000-8000 r / min, and the centrifugal time is 3-8 min.

6. Small-diameter spherical hexagonal boron nitride nanopowder obtained by the preparation method described in any one of claims 1 to 5.

7. A method for preparing small-size spherical hexagonal boron nitride nanopowder, characterized in that: The method comprises the following steps: (a) mixing boric acid and a nitrogen source uniformly, then adding borax and mixing uniformly to obtain a mixture, and then treating the mixture at 250° C. for 2 to 5 hours to obtain a precursor; (b) wet-ball-milling the precursor and freeze-drying it for 12 to 24 hours to obtain a ball-milled precursor, and then heating the ball-milled precursor to 900° C. at a heating rate of 3 to 5° C. / min in a high-pressure nitrogen atmosphere and heat-treating it at 900° C. for 0.5 to 3 hours to obtain a product; the high-pressure pressure is 1 to 5 MPa; (c) grinding the product and placing it in 80-90° C. water with stirring for 5-8 hours, then ultrasonically treating it and freeze-drying it for 12-24 hours to obtain a small-sized spherical hexagonal boron nitride nanopowder; the particle size of the small-sized spherical hexagonal boron nitride nanopowder is 10-20 nm.

8. The preparation method according to claim 7, characterized in that: The molar ratio of the boric acid to the nitrogen source is 1:(0.5-2); The mass amount of the borax is 8-15% of the sum of the mass amounts of the boric acid and the nitrogen source; The nitrogen source is urea and / or ammonium chloride.

9. Small-diameter spherical hexagonal boron nitride nanopowder obtained by the preparation method described in any one of claims 7 to 8.

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