Method for producing boron nitride nanotubes

Abstract

Provided is a method for producing boron nitride nanotubes, which reduces the proportion of by-products having a small reinforcing effect, such as boron nitride fullerene or boron nitride sheet, while increasing the yield, and does not require a thermal oxidation treatment. The present invention is a method for producing boron nitride nanotubes, comprising: a step of mixing a raw material containing boron nitride nanotubes, a nonionic polymer dispersant having an sp3-bonded CH group, and an organic solvent to obtain a suspension; and a step of centrifuging the obtained suspension to remove by-products contained in the raw material, thereby obtaining a dispersion liquid containing boron nitride nanotubes.

Description

Technical Field

[0001] This invention relates to a method for manufacturing boron nitride nanotubes. Background Technology

[0002] For example, as described in Patent Document 1, boron nitride nanotubes are obtained by reacting a mixture of magnesium oxide, iron(II) oxide (FeO), and boron powder with ammonia at 1100–1700 °C. The obtained boron nitride nanotubes are then treated with nitric acid to remove magnesium or iron used as catalysts. Using this method, uniform boron nitride nanotubes with a diameter of 20–50 nm can be manufactured. Regarding the obtained boron nitride nanotubes, it has been disclosed that a polymer, poly[(m-phenylenevinylene)-co-(2,5-dioctoxy-p-phenylenevinylene)], is dissolved in an organic solvent such as chloroform. Boron nitride nanotubes are then added to the resulting organic solvent solution, and the boron nitride nanotubes are coated with the polymer, i.e., polymer encapsulation is performed, thereby producing a uniform and transparent boron nitride nanotube dispersion. Furthermore, a purification method is disclosed: In this method, insoluble matter is removed by ultrasonic treatment and centrifugation at room temperature for 2 hours to produce a uniform and transparent dispersion. The organic solvent is then evaporated from this dispersion, and PmPV is removed by thermal decomposition to obtain boron nitride nanotubes with uniform diameter. In the following description, poly[(m-phenylacetylene)-co-(2,5-dioctyloxy-p-phenylacetylene)] is abbreviated as PmPV.

[0003] In recent years, as shown in Patent Document 2, it has been possible to produce fine (less than 10 nm in diameter) and moderately pure boron nitride nanotubes (BNNTs) very efficiently in high yields at or near atmospheric pressure without using a metal as a catalyst. Specifically, a method for producing boron nitride nanotubes (BNNTs) is disclosed, comprising: supplying one or more sources of boron, nitrogen, and hydrogen to a stable induced plasma at a plasma temperature in the range of 1,000–10,000 K in order to form a reaction mixture of boron, nitrogen, and hydrogen in a plasma at a pressure exceeding 0.6 atm and below 2 atm; and cooling the reaction mixture to form BNNTs, wherein the one or more boron sources include elemental boron, boron nitride, borane, ammoniaborane, cycloborazane, or mixtures thereof.

[0004] Using this method, Patent Document 3 discloses a boron nitride nanotube material, characterized by comprising boron nitride nanotubes containing boron nitride nanotubes and boron nitride fullerene hollow particles, wherein the boron nitride fullerene hollow particles are dispersed between the boron nitride nanotubes and are in contact with and located between the boron nitride nanotubes. The disclosed method involves, for example, converting boron to boron oxide (B₂O₃) in boron nitride nanotubes obtained in Patent Document 2, etc., through oxidative heat treatment, followed by washing with ethanol or methanol, water, etc., containing dissolved boron oxide, to remove it.

[0005] Existing technical documents

[0006] Patent documents

[0007] Patent Document 1: Japanese Patent Application Publication No. 2007-230830;

[0008] Patent Document 2: Japanese Patent Publication No. 2016-521240;

[0009] Patent document 3: International Publication No. 2020 / 031883. Summary of the Invention

[0010] Technical issues

[0011] The problem with the products synthesized using the manufacturing methods of Patent Documents 1 and 2 is the high proportion of byproducts such as boron nitride fullerenes or boron nitride flakes. These byproducts have a smaller aspect ratio than boron nitride nanotubes and offer less strengthening effect when combined with metals or ceramics. Boron nitride nanotubes have similar crystal structures to these byproducts, making them prone to formation during synthesis. Therefore, Patent Document 1 discloses a purification method to reduce the proportion of these byproducts, but this results in a decrease in the yield of boron nitride nanotubes. Furthermore, the adhesion of boron nitride nanotubes to byproducts caused by the thermal oxidation treatment described in Patent Documents 2 and 3, leading to reduced dispersibility of the boron nitride nanotubes, also presents a problem.

[0012] The purpose of this invention is to provide a method for manufacturing boron nitride nanotubes that reduces the proportion of byproducts with little reinforcing effect, such as boron nitride fullerenes or boron nitride flakes, while increasing the yield, and without requiring thermal oxidation treatment.

[0013] Solution to the problem

[0014] This invention relates to a method for manufacturing boron nitride nanotubes, characterized by comprising the following steps: mixing a raw material containing boron nitride nanotubes, a nonionic polymer dispersant having sp3-binding CH groups, and an organic solvent to obtain a suspension; and centrifuging the obtained suspension to remove byproducts contained in the raw material, thereby obtaining a dispersion containing boron nitride nanotubes.

[0015] The preferred polymer dispersant comprises a cellulose-based polymer or a vinyl-based polymer.

[0016] Invention Effects

[0017] The present invention provides a method for manufacturing boron nitride nanotubes that reduces the proportion of byproducts with little reinforcing effect, such as boron nitride fullerenes or boron nitride flakes, while increasing the yield, and without requiring thermal oxidation treatment. Attached Figure Description

[0018] Figure 1 This is a low-magnification TEM image of the recovered BNNT products.

[0019] Figure 2 This is a low-magnification TEM image of the sample after solvent removal from the BNNT dispersion of Example 1.

[0020] Figure 3 This is a high-resolution TEM image of the sample after solvent removal from the BNNT dispersion of Example 1.

[0021] Figure 4 This is a high-resolution TEM image of the sample after the BNNT dispersion of Example 1 was dried and then heated in the atmosphere at 500°C for 1 hour.

[0022] Figure 5 This is a low-magnification TEM image of the sample after solvent removal from the BNNT dispersion of Example 2.

[0023] Figure 6 This is a low-magnification TEM image of the sample after solvent removal from the BNNT dispersion of Comparative Example 1.

[0024] Figure 7 This is a low-magnification TEM image of the sample after solvent removal from the BNNT dispersion of Comparative Example 2. Detailed Implementation

[0025] Hereinafter, a method for manufacturing boron nitride nanotubes according to an embodiment of the present invention will be described with reference to the accompanying drawings. It should also be noted that in the following description, boron nitride nanotubes will sometimes be abbreviated as BNNT.

[0026] First, the process of mixing the synthesized product (containing boron nitride nanotubes, after water removal), a nonionic polymer dispersant with sp3-binding CH groups, and an organic solvent to obtain a suspension will be described. Specifically, to uniformly coat the boron nitride nanotubes with the polymer dispersant, it is preferable to dissolve the nonionic polymer dispersant with sp3-binding CH groups in the organic solvent beforehand to prepare a homogeneous solution. The synthesized product is added to this solution, and ultrasonic dispersion is performed using a homogenizer or similar device, thereby uniformly coating the boron nitride nanotubes with the polymer dispersant. To prevent the liquid temperature from rising during ultrasonic dispersion, it is preferable to perform the process while simultaneously cooling.

[0027] Furthermore, in this invention, "the synthesized product" includes not only the product immediately after synthesis, but also the product that has undergone other treatments after the synthesis of BNNT and before the steps of this invention. That is, the raw material containing boron nitride nanotubes is not limited to the synthesized original product, but also includes the boron nitride nanotube product after byproducts contained in the synthesized original product have been removed to some extent through other treatments. Therefore, in the following description, "the synthesized product" includes all boron nitride nanotube-containing raw materials used in the BNNT manufacturing steps of this invention described later.

[0028] As a nonionic polymer dispersant with sp3-binding CH groups, it is preferable to use: cellulose-based polymers such as ethyl cellulose, methyl cellulose, propyl cellulose, butyl cellulose, hydroxypropyl cellulose, and acetyl cellulose, which have a substituted glucose structure in which at least one of the substituted hydroxyl groups at positions 2, 3, and 6 is an alkyl ether, and are linked at positions 1 and 4; or vinyl-based polymers such as polyvinyl butyral, polyvinyl formal, polyvinyl acetate, ethylene-vinyl acetate polymers, polystyrene, polyvinyl alcohol, polyacrylonitrile, polyvinyl methyl ketone, and polymethyl methacrylate, which have at least one methylene group and at least one substituted methylene group in the repeating unit. This is because: due to the low symmetry of the π orbitals of boron nitride nanotubes, polymers that interact with boron nitride nanotubes via CH / π interactions are more likely to bind to boron nitride nanotubes than polymers that interact with boron nitride nanotubes via π / π interactions, as is the case with the polymers applicable in Patent Document 1. Furthermore, the backbone of the nonionic polymer with sp3-binding CH groups is more flexible than the backbone of the polymer with sp2-binding CH groups applicable in Patent Document 1, and therefore easily wraps around fine-diameter boron nitride nanotubes. Therefore, it is believed that, particularly for the fine-diameter boron nitride nanotubes obtained in Patent Document 2 or Patent Document 3, they can be easily coated using a nonionic polymer dispersant with sp3-binding CH groups.

[0029] Furthermore, carboxymethyl cellulose (CMC), widely used as a polymer dispersant for carbon nanotubes (CNTs), is an ionic polymer with sp3-binding CH groups and is used in aqueous solvents. When CMC is used as a dispersant for fine-diameter BNNTs, micelles form around the BNNTs, containing BNNTs within the hydrophobic spaces formed by the micelles, thus making the BNNTs soluble. However, since the size of the micelles follows the shape of the substance, solubility occurs regardless of the shape, and therefore byproducts other than BNNTs also become soluble. Therefore, it is considered difficult for CMC to selectively dissolve only fine-diameter BNNTs. On the other hand, in the case of nonionic polymers with sp3-binding CH groups, micelles do not form around the BNNTs due to the use of organic solvents. Therefore, due to the difference in shape or size between BNNTs and impurities (byproducts other than BNNTs), differences arise in their adsorption to their respective dispersants and their solubility, making it considered easier to separate BNNTs and impurities and selectively disperse BNNTs.

[0030] As organic solvents, the following can be used: benzyl alcohol, methanol, ethanol, isopropanol, butanol, acetone, butanone, diethyl ketone, methyl isobutyl ketone, ethyl acetate, butyl acetate, N-methylpyrrolidone, N-dimethylformamide, cyclohexanone, isophorone, tetrahydrofuran, 2-methyltetrahydrofuran, ethyl lactate, butyl lactate, ethylene glycol dimethyl ether, etc.

[0031] After mixing the raw materials, dispersant, and organic solvent, the mixture is stirred, for example, using an ultrasonic homogenizer. Following the specified stirring conditions, the separation process described later is performed. The SEM images of the supernatant or residue are checked, and the settings are determined based on the separability of byproducts and the effect of preventing BNNT degradation. For example, stirring is performed under multiple conditions, and the settings are determined based on the respective images after separation, aiming for a relatively small amount of byproducts in the supernatant and inconspicuous BNNT degradation / fracture, or an inconspicuous amount of broken BNNTs in the residue. For example, a frequency of 20 kHz, an ultrasonic amplitude of 40–80 μm, and a stirring time of approximately 20–40 minutes are preferred, as these conditions minimize the damage to boron nitride nanotubes and facilitate their dispersion. Through these operations, a suspension is obtained.

[0032] The composition of the suspension obtained by mixing the synthesized product (i.e., the raw material containing boron nitride nanotubes), a nonionic polymer dispersant with sp3-binding CH groups, and an organic solvent can be, for example, 1 part by mass of the raw material containing boron nitride nanotubes, 1 to 2000 parts by mass of the nonionic polymer dispersant with sp3-binding CH groups, and 200 to 100,000 parts by mass of the organic solvent. The lower limit of the amount of dispersant added is set, for example, based on the separability of the byproducts, after performing the separation process described later and confirming the SEM images of the supernatant or residue. For example, the amount of dispersant added can be varied under multiple conditions, and the setting can be based on the respective images after separation, under conditions where the amount of BNNT in the supernatant is relatively high and BNNTs do not cluster, or where the amount of BNNT in the residue is inconspicuous. Furthermore, regarding the upper limit of the amount of dispersant added, for example, after confirming the light absorption characteristics of the dispersion in the ultraviolet region after the separation process described later, it is set based on the relationship between the maximum absorption and the amount added, with the condition that the increase in light absorption is approximately saturated as the amount added increases. In this way, the range of dispersant and solvent is preferred because it is below the upper limit, thus avoiding waste and being economical; and it is preferred because it is above the lower limit, thus resulting in good dispersibility of BNNTs. In addition, regarding the determination of whether BNNTs are clustered, for example, in SEM images, if the dispersed BNNTs are coarser than the BNNTs in the raw material, it is determined that several to dozens of BNNTs have clustered during dispersion.

[0033] Next, the process of centrifuging the obtained suspension will be described. This operation removes byproducts. Here, byproducts refer to BN fullerenes or h-BN flakes with boron particles contained in the above solution as the core. The conditions for centrifugation to separate these byproducts are set based on the separability of the byproducts, such as by checking the SEM images of the supernatant or residue after the separation process. For example, by changing multiple conditions (time, centrifugal force), the conditions are set based on the respective images after separation, aiming for a relatively small amount of byproducts in the supernatant and a larger amount of dispersant polymer (a film-like substance that completely encapsulates the byproducts) in the residue. For example, the centrifugal acceleration can be set to 30,000 G or more, the processing time to 1 hour or more, and the liquid temperature to 25°C.

[0034] Finally, the process of removing byproducts from the raw material from the suspension obtained by centrifugation to obtain a dispersion containing boron nitride nanotubes will be described. Byproducts can be removed from the suspension using, for example, a high-speed cooling centrifuge. By removing byproducts from the raw material, a BNNT dispersion obtained by dispersing BNNTs coated with a nonionic polymer having sp3-binding CH groups in an organic solvent can be obtained.

[0035] The method for obtaining BNNT from the BNNT dispersion will be further described. Initially, an organic solvent is evaporated from the aforementioned BNNT dispersion. In this step, the BNNT becomes coated with a solid polymer dispersant. Next, the BNNT coated with the dispersant is heated in atmosphere to a temperature of 300°C or higher and 900°C or lower, thereby thermally decomposing and removing the dispersant. This purifies the BNNT contained in the dispersion to a high purity. Temperatures above 300°C are preferred because the dispersant readily and sufficiently decomposes thermally. On the other hand, temperatures below 900°C are preferred because the BNNT may remain without burning away. Temperatures below 650°C are preferred because they are below the temperature for the thermal oxidation treatment of boron particles, thus preventing the adhesion of boron nitride nanotubes and byproducts, and making the boron nitride nanotubes easily dispersed.

[0036] Example 1

[0037] Next, the embodiments will be described.

[0038] First, a boron nitride nanotube dispersion for evaluation was prepared according to the following method. First, using a small plasma apparatus (TEKNA Plasma Systems inc., TekNano-15), a BNNT product containing byproducts, i.e., a raw material containing boron nitride nanotubes, was synthesized according to the following procedure. Initially, the interior of the reaction vessel was purged with argon. Next, argon (flow rate: 10 L / min) was circulated in the central region, and a mixture of argon (30 L / min) and hydrogen (2.5 L / min) was circulated, thereby allowing the sheathing gas to flow around the outer periphery of the tubes encapsulating the plasma. Nitrogen flowed between the torch nozzle (10 L / min) and the porous wall surrounding the reaction vessel (47 L / min). Several minutes after plasma ignition, at the point when the temperature of the thermocouple located between the reaction vessel and the cyclone separator reached a constant, argon (2.5 L / min) was continuously supplied as a carrier gas from a feeder located at the top of the plasma torch to the h-BN powder (average particle size: 5 μm). The feed rate was set to 0.5 g / min, the running time to 2 hours, and the pressure in the reaction chamber to 1 atm. After synthesis, the apparatus was disassembled, and the products adhering to the plasma torch, reactor, cyclone separator, and filter section were recovered.

[0039] The recovered synthesized product was observed under a microscope. Figure 1These are low-magnification transmission electron microscope (TEM) images of the obtained product. The product contains BNNT101, BN fullerene 102, and h-BN flakes 103. BN fullerene 102 refers to a substance with a graphene structure consisting of alternating B and N atoms, and having a closed spherical or elongated spherical structure. h-BN flakes 103 refers to a sheet-like substance composed of crystalline h-BN. Furthermore, boron particles were incorporated into BN fullerene 102 (black contrast area). Other synthetic methods can also be used to synthesize the product.

[0040] Next, using the synthesized product as Example 1, various treatments were performed using the following methods. 25 mg of ethyl cellulose (EC) manufactured by Tokyo Chemical Industry Co., Ltd. was used as a dispersant, along with 20 cm... 3 After mixing with benzyl alcohol as an organic solvent, 15 mg of the synthesized product was added to the solution. That is, relative to 1 part by mass of the synthesized product (i.e., the raw material containing boron nitride nanotubes), the nonionic polymer dispersant with sp3-binding CH groups was set at 1.7 parts by mass, and the organic solvent at 1333 parts by mass. The mixture was dispersed at room temperature using an ultrasonic homogenizer for 20 minutes. Then, it was centrifuged at 30000G for 3 hours to remove byproducts from the raw material, yielding a BNNT dispersion.

[0041] Figure 2 These are low-magnification TEM images of the sample after solvent removal from the BNNT dispersion. This confirms... Figure 1 The BN fullerene or h-BN flakes contained in the synthesized product are removed, indicating that the proportion of byproducts with little reinforcing effect, such as boron nitride fullerene or h-BN flakes, has decreased.

[0042] Figure 3 yes Figure 2 High-resolution TEM images of the sample. The surface of BNNT301 is covered with an amorphous material 302, which is believed to be ethyl cellulose.

[0043] Next, in order to thermally decompose and remove the ethyl cellulose adhering to the surface of BNNT, the BNNT dispersion was dried and then heated in the atmosphere at 500°C for 1 hour. In Patent Document 2, an atmospheric oxidation step is required as a thermal oxidation treatment at a temperature in the range of 650°C to 850°C, but this step was not performed in Example 1.

[0044] Figure 4These are high-resolution TEM images of the sample, which was prepared by dropping a BNNT dispersion obtained by adding BNNT that had been heat-treated in the atmosphere to isopropanol and then ultrasonically treated onto a copper grid coated with a carbon film. The sidewalls of BNNT401 were clearly observed as the amorphous layer on the BNNT surface disappeared, confirming that the BNNT remained in a fully crystalline state.

[0045] Example 2

[0046] As Example 2, BNNT was obtained by operating in the same manner as in Example 1, except that the dispersant was set as polyvinyl butyral (PVB), a vinyl polymer. Figure 5 This is a low-magnification TEM image of the sample after solvent removal from the BNNT dispersion. Similar to Example 1, it is evident that the proportion of byproducts with minimal reinforcing effects, such as boron nitride fullerenes or h-BN flakes, decreases.

[0047] Comparative Example 1

[0048] As a comparative example 1, BNNT was obtained by operating in the same manner as in Example 1, except that the dispersant was set as a nonionic polymer with sp2-binding CH groups, namely poly[(m-phenylacetylene)-co-(2,5-dioctyloxy-p-phenylacetylene)](PmPV). Figure 6 These are low-magnification TEM images of the sample after solvent removal from the BNNT dispersion. It can be seen that the proportion of byproducts with minimal reinforcing effects, such as boron nitride fullerenes or h-BN flakes, has decreased.

[0049] Comparative Example 2

[0050] As a comparative example 2, BNNT was obtained by operating in the same manner as in Example 1, except that the dispersant was set as an ionic polymer with sp3-binding CH groups, namely CMC (carboxymethyl cellulose), and water was used as the solvent. Figure 7 This is a low-magnification TEM image of the sample after solvent removal from the BNNT dispersion. It shows that a large amount of byproducts with minimal reinforcing effect, such as boron nitride fullerenes or h-BN flakes, remain.

[0051] In Examples 1, 2, Comparative Example 1, and 2, based on the TEM images of the samples after solvent removal from the BNNT dispersion, the amount of residual byproducts (BN fullerenes or h-BN flakes) was approximately as follows: Example 2 had the least, followed by Examples 1 and Comparative Example 1, while Comparative Example 2 had the most byproducts. This is believed to be because: in Comparative Example 2, the dispersant was an ionic polymer and water was used as the solvent, thus micelles were formed around the BNNT as described above, and the larger byproducts (BN fullerenes or h-BN flakes) also became soluble along with the BNNT, resulting in poor selective dispersibility of the BNNT.

[0052] In addition, for Example 1, Example 2, Comparative Example 1, and Comparative Example 2, the yield of dispersed BNNTs was calculated using the following formula.

[0053] Yield (%) = {([mass of synthesized product] - [mass of residue after centrifugation]) / [mass of synthesized product]} × 100

[0054] As a result, the yield increased in the order of Example 1 (55%) > Example 2 (51%) > Comparative Example 1 (32%) > Comparative Example 2 (20%).

[0055] The dispersion of Comparative Example 1 has a backbone with sp2 binding properties, which is more rigid than EC and PVB with sp3 binding properties, and is particularly less prone to entanglement with fine-diameter BNNT, so it is considered to have poor dispersibility.

[0056] These results show that, compared with the dispersant used in the comparative examples, the amount of byproducts remaining (high purity of BNNT) is less and the yield of BNNT is increased when the dispersant of the present invention is used.

[0057] This invention is not limited to these embodiments, and various modifications can be made within the scope of the invention as set forth in the claims, which are of course also included within the scope of this invention. For example, the conditions for ultrasonic or centrifugal separation used to produce the dispersion can be appropriately selected according to the mixing ratio of the mass of BNNT, dispersant, and solvent.

[0058] Explanation of reference numerals in the attached figures

[0059] 101, 301, 401: BNNT;

[0060] 102: BN fullerene;

[0061] 103: h-BN thin film;

[0062] 302: Ethyl cellulose.

Claims

1. A method for manufacturing boron nitride nanotubes, characterized in that, It has the following processes: The process involves mixing a raw material containing boron nitride nanotubes, a nonionic polymer dispersant with sp3-binding CH groups, and an organic solvent, and dispersing them at room temperature to obtain a suspension. The process involves centrifuging the obtained suspension to remove byproducts contained in the raw materials, thereby obtaining a dispersion containing boron nitride nanotubes. as well as The removal process involves evaporating the organic solvent from the dispersion and heating the boron nitride nanotubes coated with the polymer dispersant to thermally decompose and remove the polymer dispersant.

2. The method for manufacturing boron nitride nanotubes according to claim 1, characterized in that, The polymer dispersant comprises cellulose-based polymers or vinyl-based polymers.

3. The method for manufacturing boron nitride nanotubes according to claim 1 or 2, characterized in that, The polymer dispersant is thermally decomposed by heating it in the atmosphere at a temperature above 300°C and below 900°C.

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