Preparation method of large-size high-length-diameter-ratio boron nitride fiber

Abstract

The invention belongs to the technical field of inorganic functional materials, and discloses a preparation method of large-size high-length-diameter-ratio boron nitride fibers. Comprising the following steps: (1) mixing a boron source and a nitrogen source according to a certain mass ratio, and uniformly mixing in a solvent to obtain a precursor solution; (2) adding the precursor solution into a reaction kettle, fully reacting, cooling and crystallizing to room temperature, filtering, washing and drying to obtain a white fibrous precursor; (3) calcining the fibrous precursor in a protective atmosphere to obtain a boron nitride fiber initial product; and (4) carrying out acid pickling on the boron nitride primary product, filtering, washing and drying to obtain the high-purity boron nitride fiber. The diameter of the prepared boron nitride fiber is 1-300 microns, the length is adjustable in a range of 500-4000 microns, the length-diameter ratio reaches up to 250-2000, and the purity reaches 99% or above. The whole technological process is simple and easy to control, low in cost, energy-saving and environment-friendly, and has obvious advantages in the aspects of manufacturing cost, product performance and the like.

Description

Technical Field

[0001] This invention belongs to the field of inorganic functional materials technology and relates to a method for preparing large-size boron nitride fibers with high aspect ratio. Background Technology

[0002] Boron nitride (BN) is the lightest III-V compound composed of boron and nitrogen. It mainly exists in hexagonal (h-BN), rhombohedral (r-BN), cubic (c-BN), and wurtzite (w-BN) phases, and has wide applications in aerospace, semiconductors, electronics, metallurgy, cosmetics, ceramics, and the nuclear industry. Flake-like boron nitride is currently the most widely used morphology in industry, but it has a wide particle size distribution and is prone to agglomeration. In contrast, fibrous boron nitride has better advantages in terms of mechanics, thermal properties, processing, and application adaptability. Its preparation process is currently a focus of attention in industry and science, and is still in the research and development stage. The difference in properties between the two different morphologies of boron nitride stems from the difference between two-dimensional layered structures and one-dimensional continuous structures. The difficulty in its preparation is attributed to its strong covalent bond structure, high melting point, and demanding conditions for phase transformation. The h-BN layered structure has a strong tendency for lateral growth, making it difficult to form axially continuous fibrous structures. Furthermore, due to the poor plasticity of boron nitride, it is also difficult to form using traditional metal / polymer melt spinning processes.

[0003] In recent years, some research techniques have involved melting and drawing boron oxide at high temperatures to prepare boron oxide fibers, and then nitriding these fibers in an ammonia atmosphere to prepare boron nitride fibers. This method has low raw material costs and controllable fiber diameter, but it easily leaves residual boron oxide and is prone to forming pore defects. Another technique involves preparing organic boron-containing polymer fibers through spinning, followed by high-temperature decomposition and nitriding to obtain fiber boron nitride. This method faces challenges such as demanding polymer precursor synthesis conditions, high costs, high fiber shrinkage, and a tendency to crack. Invention patent CN117587549 provides a high-conversion-rate boron nitride fiber and its continuous preparation method and system. The method involves spinning an organic boron nitride precursor (a mixture of aminoborane, aminocycloborane, and aniline) to obtain precursor fibers, coating the precursor fibers with a protective oil layer, and then subjecting them to curing treatment under an oil-controlled inert atmosphere and multi-temperature sintering treatment in an inert atmosphere to obtain boron nitride fibers. Invention patent CN104528671 uses spray freeze-drying technology to control the rapid nucleation of precursors, avoiding crystal growth, and obtains flocculent fibers through freeze-drying, followed by high-temperature heat treatment to prepare porous boron nitride fibers. Invention patent CN117164365 mixes and dissolves melamine, boric acid, and water in a certain proportion and pours it into a petri dish to obtain a supersaturated precursor. Then, a (111) oriented nickel-based single crystal high-temperature alloy is placed as a seed crystal at the bottom center of the petri dish, parallel to the electric field direction. The petri dish is then placed in an adjustable electric field to control the solvent evaporation rate and adjust the electric field strength to control the orientation, obtaining highly oriented precursor fibers, which are then calcined to obtain boron nitride fibers. Invention patent CN110629323B uses an organic solvent to cool the precursor solution to prepare boron nitride fibers with diameters ranging from 80-250 nm and lengths ranging from 100-300 μm. The aforementioned techniques each have their own characteristics, but they still suffer from varying degrees of problems such as low fiber purity, small size, low aspect ratio, complex preparation processes, and high energy consumption. Developing methods for preparing high aspect ratio boron nitride fibers is currently an urgent need. Summary of the Invention

[0004] The purpose of this invention is to provide a large-size boron nitride fiber with a high aspect ratio and a method for preparing the same, so as to achieve the controllable preparation of large-size, adjustable aspect ratio, and high-purity (>99%) boron nitride fibers, and meet the needs of high-end manufacturing fields such as aerospace, semiconductor, and nuclear power industries.

[0005] The technical solution of the present invention:

[0006] A method for preparing large-size boron nitride fibers with high aspect ratio, comprising the following steps:

[0007] Step 1: Mix the boron source, nitrogen source and auxiliaries in a certain mass ratio, add solvent, stir, adjust the pH and temperature of the solution, and react fully to obtain the precursor solution;

[0008] Step 2: Add the precursor solution obtained in Step 1 to the reaction vessel. Under stirring conditions, adjust the reaction pressure and reaction temperature. After the reaction is completed, slowly cool and crystallize to room temperature. After filtration and solvent washing, a white fibrous precursor is obtained.

[0009] Step 3: After vacuum drying or freeze drying of the fibrous precursor obtained in Step 2, heat it to 1100℃~2000℃ under a protective atmosphere for 2-20 hours, and then continue to cool it down to room temperature under a protective atmosphere to obtain the initial product of boron nitride fiber.

[0010] Step 4: Disperse the boron nitride fiber initial product obtained in Step 3 in an acidic solvent, soak, stir, filter, wash, and dry to obtain high-purity, large-size, high aspect ratio boron nitride fiber.

[0011] In step 1, the boron source is either boric acid or metaboric acid;

[0012] In step 1, the nitrogen source is either melamine or urea.

[0013] In step 1, the auxiliary agent is one of the following: polyvinylpyrrolidone, polyethylene glycol, sodium hydrogen phosphate, disodium hydrogen phosphate, disodium citrate, sodium hydroxide, and sodium dodecylbenzene sulfonate;

[0014] In step 1, the solvent is one of deionized water, ethanol, isopropanol, and tert-butanol;

[0015] In step 1, the mass ratio of nitrogen source, boron source, auxiliaries and solvent is 1:1~6:0.01~1:60~200;

[0016] In step 1, the pH value is 6~8. In addition to using the above-mentioned auxiliary agents, the pH value can also be adjusted by using 0.1mol / L hydrochloric acid solution or 0.1mol / L NaOH solution.

[0017] In step 1, the reaction temperature is 10~90℃ and the time is 0.5~5h;

[0018] In step 2, the reaction time is 5~24h and the reaction temperature is 100~180℃;

[0019] In step 2, nitrogen, air, and helium can be used to regulate the pressure inside the reactor, with a reaction pressure of 0.1~5 MPa;

[0020] In step 2, the cooling rate is a gradient cooling rate, which is 0.1~0.5℃ / min above 50℃ and 1~3℃ / min below 50℃;

[0021] In step 3, the vacuum drying temperature is 60℃~100℃; the freeze drying temperature is -40℃~-65℃.

[0022] In step 3, the protective atmosphere includes nitrogen, argon, helium, or ammonia;

[0023] In step 3, the heating rate is 2~15℃ / min, and the cooling rate is 3~10℃ / min;

[0024] In step 4, the acidic solvent is one of the following: 0.01~1 mol / L hydrogen chloride ethanol solution, 0.01~1 mol / L hydrochloric acid aqueous solution, 0.01~1 mol / L hydrochloric acid and hydrofluoric acid aqueous solution.

[0025] The beneficial effects of this invention are as follows: This invention achieves one-dimensional oriented growth through precursor molecular reactions and crystallization control. Auxiliary agents induce molecular nucleation, and the growth process regulates the fiber aspect ratio. The prepared highly crystalline boron nitride fibers possess a hexagonal crystal phase, with a diameter adjustable from 1-300 μm and a length adjustable from 500-4000 μm, achieving an aspect ratio as high as 250-2000. Residual boron oxide can be removed through crystallization and acid washing steps, resulting in boron nitride fiber purity exceeding 99%. The entire process is simple, easy to control, low-cost, energy-saving, and environmentally friendly, exhibiting significant advantages in manufacturing cost and product performance. The prepared boron nitride fibers can be used in special thermal conductive materials, nuclear power, high-performance coatings, water treatment, and catalysis. Attached Figure Description

[0026] Figure 1 This is a boron nitride fiber scanning electron microscope (SEM) image from Example 1;

[0027] Figure 2 This is a fiber scanning electron microscope (SEM) image of boron nitride in Comparative Example 3. Detailed Implementation

[0028] The specific embodiments of the present invention will be further described below with reference to the accompanying drawings and technical solutions.

[0029] Example 1

[0030] Weigh 8g of melamine, 35g of boric acid, and 4g of disodium citrate, add them to 500mL of deionized water and stir to mix. Adjust the reaction temperature to 60-65℃ and the pH of the solution to 6.5. After mixing for 4 hours, transfer the solution to a stirred autoclave, set the temperature to 120℃, the holding time to 720min, and the stirring speed to 400rpm. After the holding step, cool to room temperature in a gradient of 0.1℃ / min and 1℃ / min. Filter and wash with deionized water to obtain a white fibrous precursor. Freeze the precursor at -60℃ for 2h and then freeze-dry for 24h at a pressure of 30Pa to obtain a white fibrous solid precursor.

[0031] The obtained fiber precursor was then placed in a specially designed crucible and heated to 1500℃ at a rate of 4℃ / min under N2 protection, and held at that temperature for 2 hours. It was then cooled to room temperature at a rate of 5℃ / min under N2 protection, and obtained by soaking in 0.1mol / L hydrochloric acid solution, filtering, and washing with deionized water, with a yield of 20%. Boron nitride fibers were then obtained by soaking in 0.1mol / L hydrochloric acid solution, filtering, and washing, with diameters between 5-20 μm and lengths between 500-4000 μm. The boron nitride fiber content was above 99%, with a carbon content of 0.30% and a boron oxide content of 0.14%.

[0032] Example 2

[0033] 4g of melamine, 18g of boric acid, and 2.5g of disodium citrate were weighed and added to 300mL of deionized water. The reaction temperature was adjusted to 80℃, and the mixture was magnetically stirred at 400rpm for 0.5h until the pH of the solution was 6.2. The solution was then poured into a reaction vessel and reacted at 120℃ for 12h. The reaction vessel was then cooled to room temperature at a gradient cooling rate of 0.1℃ / min and 3℃ / min. The mixture was filtered and washed with deionized water to obtain a white fibrous precursor. The precursor was frozen at -60℃ for 2h and then freeze-dried for 12h at a pressure of 30Pa to obtain a white fibrous solid.

[0034] The obtained precursor was placed in a high-purity boron nitride ceramic crucible and heated using an electric furnace with a current of 50A. After 240 minutes, the furnace temperature was raised to 1800℃ and held for 2 hours, followed by natural cooling. The resulting product was then soaked in a 0.1 mol / L hydrochloric acid solution, filtered, and washed with deionized water to obtain boron nitride fibers with a yield of approximately 21.2%. The obtained boron nitride fibers had a diameter between 5 and 20 μm and a length between 500 and 4000 μm; the boron nitride fiber content was over 99%, with a carbon content of 0.21% and a boron oxide content of 0.12%.

[0035] Example 3

[0036] Weigh 0.95g melamine, 2.8g boric acid, and 0.5g polyvinylpyrrolidone, add them to 60mL deionized water, adjust the reaction temperature to 90℃, adjust the pH to 7.84 with 0.01mol / L NaOH solution, and magnetically stir at 400rpm for 0.5h. Then pour the solution into a reaction vessel and react at 120℃ for 12h. Afterward, cool the reaction vessel to room temperature at a gradient cooling rate of 0.1℃ / min and 3℃ / min. Filter and wash with deionized water to obtain a white fibrous precursor. Freeze the precursor at -60℃ for 2h, then freeze-dry for 12h at a pressure of 30Pa to obtain a white fibrous solid.

[0037] The prepared precursor was placed in a ceramic boat and heated to 1400℃ at a rate of 4℃ / min under N2 protection, and held at that temperature for 2 hours. Then, it was cooled to room temperature at a rate of 5℃ / min under N2 protection, with an N2 flow rate of 20ccm. After soaking in 0.1mol / L hydrochloric acid solution, filtering, and washing with deionized water, boron nitride fibers were obtained with a yield of 20.9%. The diameter of the boron nitride fibers was between 200-400μm, and the length was between 500-3000μm. The boron nitride fiber content was above 99%, of which the carbon content was 0.23% and the boron oxide content was 0.29%.

[0038] Comparative Example 1

[0039] 0.95 g of melamine and 2.8 g of boric acid were weighed and added to a mixture of 60 mL of ethanol and deionized water (volume ratio 1:1). The mixture was magnetically stirred at 400 rpm for 0.5 h. The suspension was then poured into a reaction vessel and reacted at 140 °C for 12 h. The reaction vessel was then cooled to room temperature at a gradient cooling rate of 0.1 °C / min and 3 °C / min. The mixture was filtered and washed with anhydrous ethanol to obtain a white fibrous precursor. The precursor was frozen at -60 °C for 2 h and then freeze-dried for 12 h at a pressure of 30 Pa to obtain a white fibrous solid.

[0040] The prepared precursor was placed in a ceramic boat and heated to 1500℃ at a rate of 4℃ / min under N2 protection, then held at that temperature for 2 hours. Afterward, it was cooled to room temperature at a rate of 5℃ / min under N2 protection, with an N2 flow rate of 20 ccm. Boron nitride fibers were obtained with a yield of 20.18%. The obtained boron nitride fibers had a diameter between 1 and 5 μm and a length between 200 and 500 μm; the boron nitride fiber content was greater than 98.5%, with a carbon content of 0.27% and a boron oxide content of 0.80%.

[0041] Comparative Example 2

[0042] 8g of melamine and 24g of boric acid were weighed and added to 500mL of deionized water. The mixture was mechanically stirred at 400rpm and magnetically stirred for 0.5h. The pH was adjusted to 7.34 with 0.01mol / L NaOH solution. The suspension was then poured into a reaction vessel and reacted at 120℃ for 12h. The reaction vessel was then cooled to room temperature at gradient cooling rates of 0.1℃ / min and 3℃ / min. After filtration and washing with deionized water, a white fibrous precursor was obtained. The precursor was frozen at -60℃ for 2h and then freeze-dried for 12h at a pressure of 30Pa to obtain a white fibrous solid.

[0043] The prepared precursor was placed in a ceramic boat and heated to 1500℃ at a rate of 4℃ / min under N2 protection, then held at that temperature for 2 hours. Afterward, it was cooled to room temperature at a rate of 5℃ / min under N2 protection, with an N2 flow rate of 20 ccm, yielding crude boron nitride fibers with a yield of 20.8%. These fibers were then soaked in 0.1 mol / L hydrochloric acid solution, filtered, and washed with deionized water to obtain boron nitride fibers. The obtained boron nitride fibers had a diameter between 1-20 μm and a length between 300-500 μm; the boron nitride fiber content was above 99%, with a carbon content of 0.30% and a boron oxide content of 0.10%.

[0044] Comparative Example 3

[0045] Weigh 8g of melamine and 35g of boric acid, add them to 500mL of deionized water and stir to react. Adjust the reaction temperature to 95℃. After the reaction is complete, cool to room temperature, filter and freeze dry for 24h to obtain a white solid precursor.

[0046] The precursor was placed in a ceramic boat and heated to 1500℃ at a rate of 4℃ / min under N2 protection, then held at that temperature for 2 hours. Afterward, it was cooled to room temperature at a rate of 5℃ / min under N2 protection, with an N2 flow rate of 20 ccm. The precursor was then soaked in 0.1 mol / L hydrochloric acid solution, filtered, and washed to obtain boron nitride whiskers with a diameter of 1-2 μm, a length of 10-20 μm, and porous surfaces. The boron nitride fiber content was over 98%, with a carbon content of 0.80% and a boron oxide content of 0.23%.

Claims

1. A method for preparing large-size boron nitride fibers with high aspect ratio, characterized in that, The steps are as follows: Step 1: Mix the boron source, nitrogen source and auxiliaries in a certain mass ratio, add solvent, stir, adjust the pH and temperature of the solution, and react fully to obtain the precursor solution; Step 2: Add the precursor solution obtained in Step 1 to the reaction vessel. Under stirring conditions, adjust the reaction pressure and reaction temperature. After the reaction is completed, slowly cool and crystallize to room temperature. After filtration and solvent washing, a white fibrous precursor is obtained. Step 3: After vacuum drying or freeze drying of the fibrous precursor obtained in Step 2, heat it to 1100℃~2000℃ under a protective atmosphere for 2-20 hours, and then continue to cool it down to room temperature under a protective atmosphere to obtain the initial product of boron nitride fiber. Step 4: Disperse the boron nitride fiber initial product obtained in Step 3 in an acidic solvent, soak, stir, filter, wash, and dry to obtain high-purity, large-size, high aspect ratio boron nitride fiber.

2. The method for preparing large-size, high aspect ratio boron nitride fibers according to claim 1, characterized in that, In step 1, The boron source is either boric acid or metaboric acid; The nitrogen source is either melamine or urea. The additive is one of the following: polyvinylpyrrolidone, polyethylene glycol, sodium hydrogen phosphate, disodium hydrogen phosphate, disodium citrate, sodium hydroxide, and sodium dodecylbenzene sulfonate; The solvent is one of deionized water, ethanol, isopropanol, or tert-butanol.

3. The method for preparing large-size, high aspect ratio boron nitride fibers according to claim 1, characterized in that, In step 1, The mass ratio of nitrogen source, boron source, additives and solvent is 1:1~6:0.01~1:60~200; The pH value is 6~8. In addition to using auxiliary agents, the pH value can also be adjusted by using 0.1mol / L hydrochloric acid solution or 0.1mol / L NaOH solution. The reaction temperature is 10~90℃ and the time is 0.5~5h.

4. The method for preparing large-size, high aspect ratio boron nitride fibers according to claim 1, characterized in that, In step 2, The reaction time is 5~24h, and the reaction temperature is 100~180℃; Nitrogen, air, and helium are used to regulate the pressure inside the reactor, with a reaction pressure of 0.1~5 MPa; The cooling rate is a gradient cooling rate, which is 0.1~0.5℃ / min above 50℃ and 1~3℃ / min below 50℃.

5. The method for preparing large-size, high aspect ratio boron nitride fibers according to claim 1, characterized in that, In step 3, Vacuum drying temperature is 60℃~100℃; freeze drying temperature is -40℃~-65℃; Protective atmospheres include nitrogen, argon, helium, or ammonia; The heating rate is 2~15℃ / min, and the cooling rate is 3~10℃ / min.

6. The method for preparing large-size, high aspect ratio boron nitride fibers according to claim 1, characterized in that, In step 4, The acidic solvent is one of the following: 0.01~1 mol / L hydrogen chloride ethanol solution, 0.01~1 mol / L hydrochloric acid aqueous solution, 0.01~1 mol / L hydrochloric acid and hydrofluoric acid aqueous solution.

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