KR20170016539A - Method for preparing boron nitride nanotubes by thermal annealing of ball milled boron powder - Google Patents
Method for preparing boron nitride nanotubes by thermal annealing of ball milled boron powder Download PDFInfo
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- C01B21/00—Nitrogen; Compounds thereof
- C01B21/06—Binary compounds of nitrogen with metals, with silicon, or with boron, or with carbon, i.e. nitrides; Compounds of nitrogen with more than one metal, silicon or boron
- C01B21/064—Binary compounds of nitrogen with metals, with silicon, or with boron, or with carbon, i.e. nitrides; Compounds of nitrogen with more than one metal, silicon or boron with boron
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- C—CHEMISTRY; METALLURGY
- C01—INORGANIC CHEMISTRY
- C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
- C01B21/00—Nitrogen; Compounds thereof
- C01B21/06—Binary compounds of nitrogen with metals, with silicon, or with boron, or with carbon, i.e. nitrides; Compounds of nitrogen with more than one metal, silicon or boron
- C01B21/064—Binary compounds of nitrogen with metals, with silicon, or with boron, or with carbon, i.e. nitrides; Compounds of nitrogen with more than one metal, silicon or boron with boron
- C01B21/0648—After-treatment, e.g. grinding, purification
Abstract
The present invention relates to a method of manufacturing a boron nitride nanotube, and a manufacturing method according to an embodiment of the present invention is a method for manufacturing a boron nitride nanotube by precisely preparing a precursor for producing a nanoporous boron- catalyst precursor powder by ball milling a mixed powder obtained by mixing an amorphous boron powder and a catalyst powder A powder manufacturing step; And preparing a precursor-binder mixture by mixing the boron-catalyst precursor powder with a binder to produce a precursor-binder mixture; And forming the precursor-binder mixture into a film to form a precursor film.
Description
The present invention relates to a method for producing boron nitride nanotubes by heat treatment of ball milled boron powder, and more particularly, to a method for mass production of boron nitride nanotubes of high purity in a shorter production time.
Boron nitride nanotubes (BNNTs) have similar mechanical and thermal properties to carbon nanotubes (CNTs), which are commonly known. However, CNTs are electrically mixed with conductors and semiconductors, and have a characteristic of being oxidized at about 400 ° C. or more. However, BNNTs are electrically wide band gap materials and have insulating properties (band gap> ~ 5 eV) And has chemical stability even in a high temperature of about 1000 ° C or more in the air.
Due to the electrical insulation and high thermal conductivity of BNNT, BNNT-polymer composites are very useful as electrical insulation heat-dissipating materials indispensable for the IT industry. In the case of BNNT-ceramic composites, they have toughness at high temperatures. It can be used as a useful material for structures exposed to high temperature environment. In particular, BNNT is highly utilizable as a basic material for structural materials that shields radiation from the nuclear and space industries because of its excellent ability to absorb thermal neutrons.
In addition, BNNT is known to be useful in various fields of the IT industry such as medical treatment such as drug treatment for cancer treatment and treatment of boron-neutron capture cancer, energy related industry such as hydrogen storage and seawater desalination, ultraviolet ray emitting laser and piezoelectric sensor, It is a substance that application research is actively proceeding in related fields.
Conventional methods for synthesizing and growing BNNT have been developed, such as arc discharge, laser ablation, laser or thermal induction plasma, CVD and ball mill-heat treatment. Currently, laser or thermal induction plasma, CVD, It is mainly used. These BNNT production methods are currently being modified and developed in various aspects such as the type and atmosphere of the reaction gas, the target device, the characteristics of the synthesizer, and the like.
However, BNNT has not been developed in mass production technology because of difficulties such as synthesis at a high temperature of 1000 ° C or higher. In addition, since impurities or unreacted impurities are produced simultaneously with BNNTs by a precursor including a metal catalyst and a process gas, the purity is relatively low. Therefore, a high-cost purification step for removing impurities may be required.
At present, a small amount of commercial BNNT commercial products for research purposes are manufactured and marketed abroad, but it is not possible to mass-produce them due to the above-described process difficulties, and has a disadvantage that it contains a large amount of impurities. Thus, there is a need to develop a mass production method for high purity BNNTs for commercial use of BNNTs.
In the BNNT synthesis method, the ball milling-heat treatment method is expected to be able to further improve the productivity because the apparatus is simple, the process temperature is relatively low, and the simple and consistent process is applied. As a result, .
In one embodiment, the present invention provides a high yield BNNT production method suitable for large-scale production.
According to another embodiment of the present invention, there is provided a method for producing high purity BNNT.
However, the object of the present invention is not limited to these, and includes those known to those of ordinary skill in the art from the following description.
The present invention relates to a method for producing BNNT, wherein a production method according to an embodiment includes a step of preparing a precursor powder to produce a nanoporous boron-catalyst precursor powder by ball milling a mixed powder obtained by mixing amorphous boron powder and catalyst powder, - preparing a precursor-binder mixture to prepare a precursor-binder mixture by mixing the binder with a catalyst precursor powder; and forming a precursor film by molding the precursor-binder mixture into a film.
The catalyst powder may be at least one powder selected from the group consisting of Fe, Mg, Ni, Cr, Co, Zr, Mo, W and Ti.
The catalyst powder is preferably mixed in an amount of 5 to 20 parts by weight based on 100 parts by weight of the boron powder.
The ball milling may be performed by mixing 5 to 10 parts by weight of mixed powder obtained by mixing amorphous boron powder and catalyst powder with 100 parts by weight of milling balls.
In one embodiment of the present invention, a precursor powder preparation step is followed by a purification step, followed by a precursor-binder mixture preparation step, wherein the purification step comprises dispersing the boron-catalyst precursor powder in a solvent to form a dispersion, And precipitating the catalyst particles having a diameter exceeding 1000 nm in the dispersion liquid under the dispersion by using a magnet and collecting and drying the supernatant liquid to obtain a nanoporous boron-catalyst precursor powder, and the boron- Centrifuging the catalyst precursor powder at 500 to 2000 rpm to remove catalyst particles having a diameter greater than 1000 nm to obtain a nanoporous boron-catalyst precursor powder.
The binder may be at least one powder selected from the group consisting of sucrose, molasses, polypropylene carbonate (PPC), polyvinyl alcohol (PVA) or polyvinyl butyral (PVB) and ethyl cellulose (EC) , Wherein the precursor film can be prepared by applying the precursor-binder mixture onto a release film or substrate, followed by heating and pressing.
In addition, the binder may be an aqueous solution consisting of at least one selected from the group consisting of sucrose, molasses, corn starch and polyvinyl alcohol (PVA), wherein the precursor film is a precursor- Applying the mixture, and then heating and drying it.
In addition, the binder may be a solution in which at least one member selected from the group consisting of polypropylene carbonate (PPC), polyvinyl butyral (PVB) and ethyl cellulose (EC) is dissolved in a solvent. Specifically, the binder may be at least one of a solution in which polypropylene carbonate is dissolved in (ketone or ethyl acetate), a solution in which polyvinyl butyral is dissolved in (methanol or ethanol) and a solution in which ethyl cellulose is (terpineol) Lt; / RTI > In this case, the precursor film may be prepared by applying a precursor-binder mixture on a release film or a substrate, followed by heating and drying.
The precursor film may have a thickness of 100 to 1500 탆.
Furthermore, according to an embodiment of the present invention, the precursor film may further be disposed in a heat treatment reactor, and a nitrogen-containing reaction gas may be supplied into the heat treatment reactor to perform a heat treatment.
In addition, the precursor film can be disposed in the heat treatment reactor after removing the release film, and the precursor film can be disposed in the heat treatment reactor together with the substrate on which the precursor film is formed.
Wherein the substrate is a metal selected from the group consisting of stainless steel, tungsten, and titanium; Or a ceramic material selected from the group consisting of silicon carbide and alumina, and the precursor film may be formed on one side or both sides of the substrate.
It is preferable that the precursor films are vertically arranged in the heat treatment reactor and continuously arranged in a line in a spaced relation to each other. In addition, the precursor film may be arranged radially continuously with a plurality of precursor films spaced from each other with respect to the center of the circle.
The reaction gas may be nitrogen, ammonia, or a mixed gas thereof. Preferably, the reaction gas is supplied into the heat treatment reactor in parallel with the surface of the precursor film. At this time, the reaction gas is preferably supplied at a rate of 20 to 500 sccm.
Further, the heat treatment is preferably performed at a temperature of 1100 to 1300 캜 for 2 to 6 hours under a pressure of 2 atm or less.
As another aspect of the present invention, there is provided a boron nitride nanotube produced by the above production method.
According to an embodiment of the present invention, it is possible to manufacture a precursor powder for BNNT by milling amorphous boron, thereby solving the problem of excessive impurities being incorporated, and thus it is possible to manufacture a high purity BNNT, The time can be remarkably shortened and the BNNT manufacturing time can be shortened.
According to another embodiment of the present invention, the productivity of the BNNT can be greatly improved by preparing a precursor powder for the production of BNNT in the form of a film, arranging a plurality of the precursor powder in the heat treatment reactor at the same time,
Further, the heat treatment can be performed at atmospheric pressure instead of vacuum, so that a continuous process can be performed at a low cost, thereby enabling commercial production of BNNTs.
Although the effects of the present invention have been described above, the present invention is not limited thereto, and other effects not described may be clearly understood by those skilled in the art from the following description.
FIG. 1 is a view schematically showing a BNNT manufacturing method according to an embodiment of the present invention in accordance with a process order.
FIG. 2 schematically shows an example of a vertically mounted precursor film, in which (a) is an example in which precursor films are arranged side by side, (b) is a film in which a precursor film is radially arranged Is an example of a schematic cross section of one form.
FIG. 3 is an SEM image of each powder in the BNNT manufacturing process according to Example 1, wherein (a) amorphous boron base powder, (b) Fe metal catalyst base powder, (c) milled boron and Fe mixed powder, ) Refined boron and Fe mixed powder.
FIG. 4 is an image providing information on the components of the ball milled powder particles in Example 1, wherein (a) is an SEM image associated with the EDX spectrum, and (b) is a TEM image associated with the EDX spectrum.
5 is a SEM image (a) and a TEM image (b) of the BNNT powder obtained in Example 1. FIG.
6 is an XRD pattern of the specimen prepared in Example 1, wherein (a) shows an XRD pattern of the initial amorphous boron, ball milled amorphous boron and Fe mixed powder (supernatant), and heat treated BNNT, and (b) Denote the XRD patterns of the initial crystalline boron, the ball milled crystalline boron and the heat treated BNNT, respectively.
7 shows an SEM image of the BNNT produced by Example 2. Fig.
8 shows the XRD pattern of each of the three BNNT samples prepared after three runs under the same conditions as in Example 2. Fig.
The present invention relates to a method for producing BNNTs by a ball milling-heat treatment method, and is intended to provide an effective method for mass production at a higher yield than the conventional methods.
The method for producing BNNT according to one embodiment of the present invention includes a step of preparing a precursor powder to produce a boron-catalyst precursor powder by ball milling a mixed powder obtained by mixing boron powder and catalyst powder.
In one embodiment, the boron powder used in the present invention is not particularly limited as long as it can be generally used as boron for producing BNNTs. Such boron powder is nano-sized by the ball milling process during the process for producing BNNT, and the particle size of the powder is also not limited. However, the smaller the average particle size, the better the quality of the BNNT. It is more preferable to use the boron powder having a small particle size. For example, those having a particle size of 1 to 10 mu m can be used.
In the conventional general BNNT manufacturing process, precursor nanoparticles were prepared by ball milling with crystalline boron. Since crystalline hardness of boron is much higher than that of STS which is used as general milling balls and milling vessels, particles of catalytic metal particles are produced in nanometer size in milling balls and milling vessels by crystalline boron during ball milling, It has an advantage of being able to produce a precursor in the form of surface coating with boron, so that crystalline boron has been used as the parent powder. It is also recognized that the initial shape of the boron particles coated with the catalyst nanoparticles is important for determining the shape of the BNNT, such as cylinder or bamboo shape, during the growth of boron nitride (BN) through heat treatment. Thus, BNNTs were prepared using crystalline boron.
However, in the case of using such crystalline boron, it is necessary to perform ball milling for tens to hundreds of hours to prepare the boron nano powder. As a result, an excessive amount of time is required for ball milling in the production of BNNT, and the amount of milling at one time is limited to several grams due to the characteristics of a high energy ball milling apparatus for manufacturing nanoparticles, Respectively.
In addition, in the ball milling of crystalline boron, catalyst nanoparticles required as precursors are produced, but they are produced in excess of the required amount, and these excessive catalyst nanoparticles act as impurities in the finally produced BNNT, resulting in an increase of the impurity content Resulting in lowering the purity of BNNT. Further, an additional purification process for reducing the excessive amount of impurities is indispensably required. Since the object to be removed has a fine particle size, a complicated and precise process is required for its removal. This results in a complicated process and an increase in the price of the product.
In the present invention, the crystalline boron as described above may be used, but it is more preferable to use amorphous boron. In the case of using amorphous boron, it is possible to obtain a nano powder in which boron and catalyst powder are mixed by ball milling for a short time by milling amorphous boron mixed with a small amount of catalyst powder. Further, when the mixed nano powder of boron and catalyst powder thus obtained is used, BNNT can be synthesized at a high yield.
The ball milling may be performed in an ammonia or nitrogen gas atmosphere. Particularly, when a ball milling process is performed in an ammonia gas atmosphere, a nitriding reaction of boron powder may occur during the milling process, and the generated boron nitride may have a favorable influence on the formation of BNNTs.
On the other hand, the milling can be carried out by including the catalyst powder together with the amorphous boron powder. By including the catalyst powder, it is possible to produce precursor nanoparticles in which boron particles and catalyst powder are mixed during the milling process. These precursor nanoparticles serve as a seed for the preparation of BNNTs and contribute to the synthesis of boron nitride by reacting with nitrogen. The catalyst particles are not particularly limited and include, for example, Fe, Mg, Ni, Cr, Co, Zr, Mo, W and Ti.
The catalyst powder is preferably contained in an amount of 5 to 20 parts by weight based on 100 parts by weight of the boron powder. If the amount is less than 5 parts by weight, sufficient catalyst particles necessary for producing BNNT can not be obtained. If the amount is more than 20 parts by weight, it is not effective for complexing boron-catalyst particles by ball milling. An appropriate amount of boron-catalyst nanoparticles can be produced when the milling is carried out with the catalyst powder in the above range.
On the other hand, when amorphous boron is used, the nano-precursor powder can be prepared by performing the treatment for 2 hours to 6 hours. In case of milling within 2 hours, the nano-size of boron and catalyst powder is not enough, and when it exceeds 6 hours, the particle size of powder may be increased by re-agglomeration of nano powder.
On the other hand, it is preferable that the ball milling is performed at a temperature of room temperature (25 ° C) or less because unnecessary reaction of the powder due to the temperature rise of the milling vessel can be prevented. In order to prevent the temperature from rising during the milling process, The milling vessel can be water-cooled.
At this time, the ball used for the ball milling is not particularly limited, and any one of STS, WC, ZrO 2 , SiO 2 , Si 3 N 4 and Al 2 O 3 may be used. Those having an average diameter can be used.
Furthermore, in performing ball milling, the mixing ratio of the milling balls to the boron / catalyst powder is in the range of 5 to 10 parts by weight based on 100 parts by weight of the milling balls, and ball milling is most efficient in producing nano-boron powder . When the amount is less than 5 parts by weight based on the milling balls, the amount of the nanoized boron powder and catalyst powder obtained by performing the ball milling process once is advantageous, but if the amount is more than 10 parts by weight, There are difficulties in the nanotization of boron.
After the milling process as described above, nano-precursor powders in which boron and catalyst powder are mixed can be obtained, and they can be supplied to a heat treatment reactor for producing BNNTs.
However, the milled boron nanocrystals may include catalyst particles that are not nanoized during the ball milling process. The catalyst particles having such a large particle size act as impurities of the BNNT finally obtained and can lower the purity. It is preferable that the particles having a diameter exceeding 1000 nm are removed, and a purification process for removing catalyst particles having such a large particle size is included can do.
In the purification process, the ball milled nanopowder is dispersed in a solvent, and the catalyst powder having a large particle size in the dispersion can be separated by using a magnet. The supernatant of the dispersion is collected and dried to obtain a high purity A boron-based nano-precursor powder can be obtained.
On the other hand, the purification process can be performed by a centrifugal force other than the method using the magnet. For example, a centrifugal separator can be used to remove large particle catalyst powders at an energy of 500-2000 rpm.
The purification method of the present invention as described above can be performed at a low cost, and does not lead to an increase in the process cost. In the case of ball milling using conventional crystalline boron powder, it may contain a catalyst component such as nano-sized Fe derived from a ball or a milling container to be used, but when it contains an excessive amount, It is not easy to remove, which can cause high cost to be removed.
At this time, the solvent is not particularly limited, but it is preferable that the solvent does not act as an impurity in the production of BNNT. Therefore, it is preferable that the compound can be easily removed after the purification process, and preferable examples thereof include ethanol, water and the like. Among them, it is more preferable to use ethanol in consideration of the drying efficiency of the supernatant.
Generally, in manufacturing BNNT, BNNT can be manufactured by placing a boron nano powder in a heat resistant boat such as alumina, placing it directly in a horizontal heat treatment reactor (furnace), and then performing heat treatment while supplying a nitrogen containing reaction gas. However, in the case of performing the heat treatment by such a method, the space of the heat treatment reactor can not be fully utilized, so that a large amount of BNNT is prevented from being produced. Also, the precursor powder contained in the boat must react with nitrogen supplied to the reactor, And the exposed boron reacts mainly with nitrogen in the reaction, resulting in a remarkably low reaction yield.
Accordingly, in order to increase the reaction yield and mass production, the precursor powder is formed into a thin film and is introduced into the reactor. And it is possible to improve the productivity of BNNT by reacting with the reaction gas on both sides of the film.
As a method for producing the film, a film may be formed by mixing a binder which does not act as an impurity in the preparation of BNNT with a precursor powder and pressing or heating at a suitable temperature.
The precursor film can be produced according to the method of one embodiment of the present invention, and the obtained precursor film can be placed in a heat treatment reactor and heat-treated to produce BNNT. Therefore, the precursor film is sufficient to maintain its shape under the temperature and pressure in the heat treatment reactor, and does not require high bonding strength or stability of the form.
Examples of the binder include a vinyl type such as sucrose, molasses, phthalic acid, polypropylene carbonate (PPC), polyvinyl alcohol (PVA) or polyvinyl butyral (PVB), and a cellulose type Etc. may be used. These binders are all sublimated and removed in the gas phase in the high-temperature heat treatment step in which the precursor powder is calcined and nitrided, so that the binder remains in the BNNT and does not act as an impurity.
The binder may be used in an amount of 5 to 50 parts by weight based on 100 parts by weight of the precursor powder. If the binder content is less than 5 parts by weight, it may not be easy to form into a film, and it may be difficult to maintain the shape of the formed precursor film. On the other hand, when the content of the binder powder is more than 50 parts by weight, pores are formed in the film after the binder component is sublimated and removed, which may reduce the integrity of the precursor film due to excessive pores.
The precursor film can be formed on a removable film such as a release film. For example, a precursor film of a predetermined shape can be produced by inserting a release film into the mold, spreading the mixed powder of the precursor powder and the binder powder uniformly on the release film, and then performing pressure molding. Preferably, after removing the release film, the precursor film may be placed in a thermal treatment reactor.
At this time, the binder may be used in powder form or in liquid form.
Of the components exemplified as being suitable for use as the binder, the binder usable in powder form can be suitably used in the present invention as long as it has a solid phase at room temperature. Examples thereof include sucrose, molasses, polypropylene carbonate (PPC), polyvinyl alcohol (PVA) or polyvinyl butyral (PVB), and cellulose type such as ethyl cellulose (EC).
When the binder is used in powder form, the precursor film is prepared by mixing the precursor powder and the binder powder to prepare a mixed powder, uniformly spreading the mixed powder, and pressurizing the mixed powder at an appropriate temperature. Specifically, the mixed powder is uniformly spread in a mold capable of producing a film of a predetermined shape and then pressed with a hot press at a predetermined temperature, whereby the viscosity of the binder powder is increased, thereby inducing mutual adhesion of the precursor powder, Can be manufactured.
At this time, the temperature during the hot pressing is preferably in the range of 50-150 ° C. If the temperature is lower than 50 ° C, the adhesion of the binder powder by the viscosity can not be ensured. If it exceeds 150 ° C, the binder powder is not melted or sublimated and it is not easy to mold or form the film.
When the binder is in the form of a liquid, the precursor powder may be mixed with a binder in a liquid phase, spread evenly on the release film, and then heated and dried at a suitable temperature to easily form a film.
At this time, as the binder in the liquid phase, a binder such as sucrose, molasses, starch and polyvinyl alcohol (PVA) can be used as a binder by making water into a liquid.
Meanwhile, the binder such as polypropylene carbonate (PPC), polyvinyl butyral (PVB) and ethyl cellulose (EC) can be used as a binder in a liquid state using a solvent. For example, ketone or ethyl acetate may be used for polypropylene carbonate (PPC) and polyvinyl butyral (PVB) may be used for the solvent. Methanol or ethanol may be used, and for ethyl cellulose (EC), terpinol may be used.
As another embodiment, a precursor powder and a mixture of the binder may be dispersed on a predetermined substrate, and then the precursor film may be formed by pressing or heating the mixture, and the substrate on which the precursor film is formed may be placed in the reactor. At this time, the precursor film may be formed on both sides as well as one side of the substrate. When a film is formed on a substrate, the film forming method described above for forming the film on the release film can be applied as it is.
For example, the substrate may be made of stainless steel (STS), tungsten (W), and titanium (Ti). The substrate may be made of a material capable of withstanding heat treatment at a high temperature, ), Silicon carbide (SiC), and ceramics such as alumina.
The film is preferably as thin as possible considering the reaction efficiency with nitrogen in the heat treatment reactor, but it is preferable that the film is thicker in consideration of the morphological stability to maintain the shape of the film in the heat treatment reactor. Particularly, the binder contained in the preparation of the precursor film sublimates during the heat treatment process, thereby forming pores in the precursor film during the heat treatment. For example, if sugar is used as a binder, the pyrolysis process can be represented by the following formula.
C 12 H 22 O 11 (Surcrose) + heat → 3CO 2 + 5H 2 O + 6H 2
These pores can affect the morphological stability of the precursor film, which can lead to the collapse of the precursor film. Therefore, it is preferable to have a thickness of 100 mu m or more.
On the other hand, when the thickness of the precursor film is too thick, the reaction efficiency may decrease due to the thickness. However, as described above, the permeability of the reaction gas can be improved due to the pores formed by sublimation of the binder component, The yield of production can be improved. As a result, the problem of reducing the reaction efficiency due to the increase in the thickness can be canceled by pore formation. However, it is preferable not to exceed 1500 탆.
At this time, it is preferable that the precursor film is disposed vertically in the reactor, that is, perpendicular to the bottom surface of the reactor. By vertically arranging the precursor films in this manner, it is possible to arrange a plurality of precursor films in the reactor, and BNNT can be produced in a large amount by one heat treatment process, which is preferable. In addition, since the film is formed as a thin film, the precursor film can be contacted with the nitrogen-containing reaction gas on both sides of the precursor film, thereby widening the reaction region and improving the production yield of BNNT.
The manner of vertically disposing the precursor film in the reactor is not particularly limited as long as the reaction efficiency and efficiency of utilization of the space inside the reactor are taken into account in consideration of the internal shape of the reactor and the like. For example, as shown in Fig. 2 (a), the surfaces of the respective precursor films may be arranged so as to face each other so as to face each other. As shown in Fig. 1 (b) It can be radially arranged like a spoke.
The heat treatment reactor is not particularly limited as long as it is generally used for the synthesis of BNNT. However, it is preferable that the heat treatment reactor includes a facility for vertically standing the precursor film as described above. The equipment is not particularly limited as long as the film is disposed vertically.
In addition, the heat treatment reactor supplies a nitrogen-containing reaction gas to produce BNNT from a precursor film disposed therein. At this time, in order to supply the nitrogen-containing reaction gas, the heat-treatment reactor has a reaction gas disperser therein, and the reaction gas disperser preferably supplies the reaction gas in parallel to the precursor film. For example, as shown in Fig. 1 and Fig. 2 (b), since the precursor film is vertically erected, it can be supplied in the vertical direction from the top to the bottom. When the precursor films are arranged in a line at regular intervals, In addition to being supplied vertically as described above, it can be supplied in the side, that is, in the horizontal direction with respect to the surface of the precursor film as shown in Fig. 2 (a).
The nitrogen-containing reaction gas supplied to the heat treatment reactor is not particularly limited, and nitrogen (N 2 ) or ammonia (NH 3 ) can be used, and they can be mixed and supplied to the heat treatment reactor as a mixed gas.
The reaction gas is preferably supplied to the heat treatment reactor at a rate of 20 to 500 sccm. When the reaction gas is supplied at a rate of less than 20 sccm, the amount of the nitrogen element to be supplied is decreased and the nitriding efficiency of the boron is decreased. Therefore, it is necessary to perform the reaction for a long period of time. The boron powder is ablated and the yield of BNNT production is reduced.
The heat treatment can be performed at a temperature of 1100 to 1300 캜 under a pressure of 2 atm or less for 2 to 6 hours to obtain BNNT.
A schematic process of the BNNT production method according to one embodiment of the present invention is shown in FIG. As shown in Fig. 1, the boron powder is mixed with the catalyst powder to perform ball milling, and then the unreacted catalyst powder having a large particle size is purified and removed as necessary. The ball milled boron powder and the catalyst powder are mixed with a binder powder (or a liquid) and then compressed or heat-molded in a predetermined film form to form a precursor film. BNNT can be produced by vertically installing the precursor film in the heat treatment reactor and then heat-treating the precursor film while supplying the reaction gas from the upper part of the heat treatment reactor.
Example
Hereinafter, a method for producing BNNT will be described in more detail with reference to examples. However, the present invention is not limited to the embodiments.
Example One
To prepare a boron precursor for the synthesis of BNNT, amorphous boron powder having an average particle size of 1 mu m containing impurities of about 3.5 wt% of Mg and Fe powder having an average particle size of about 100 mu m as an additional metal catalyst were mixed, A milling ball with a diameter of 5 mm was placed in an STS milling vessel and then milled.
The milling vessel was filled with N 2 gas at 2 atm, and a mixed powder containing 4 g of amorphous boron and 0.4 g of Fe was filled at a weight ratio of 4.4: 100 with the milling balls. Thereafter, ball milling was performed at 600 rpm for 6 hours. The temperature of the milling vessel was cooled using flowing tap water to prevent unnecessary reaction of the milling vessel due to temperature rise of the milling vessel.
The ball milled powder was dispersed in ethanol, and a planar magnet for 20 minutes was placed under the ethanol beaker in which the powder was dispersed. Then, the supernatant liquid was decanted to separate the precipitated Fe powder having a large particle size, thereby separating and recovering the milled boron powder .
The boron precursor powder obtained by drying the supernatant was sprayed thinly on an alumina boat (4 cm x 5 cm area) to prepare a sample for producing BNNT. The alumina boat was installed in a vertical heat treatment reactor (furnace) in a plane below the disperser so that the reaction gas could be injected from top to bottom using a reaction gas disperser. Thereafter, a mixed gas of N 2 (90 vol%) and NH 3 (10 vol%) was heat-treated at a flow rate of 500 sccm at 1200 ° C for 6 hours to obtain BNNT.
SEM images were taken of the amorphous boron base powder containing a small amount of Mg used, the iron powder as a metal catalyst, the mixed powder of ball milled boron and iron, and the purified amorphous boron powder obtained by removing iron from the mixed powder This is shown in Fig. 3 (a) is an amorphous boron primary powder, (b) is an iron powder, (c) is a ball milled mixture of boron and iron, and (d) is a purified mixed powder.
As can be seen from FIG. 3, the amorphous boron particle size of the mother powder was in the range of submicrometer to several micrometers, and the Fe particles had a size of about 100 μm. However, after ball milling of boron and Fe powder The maximum particle size was found to be reduced to a submicron scale as shown in Figure 3 (c), with some large particles still visible as Fe.
As a result of application of a magnet to remove Fe particles having a large particle size, it was estimated that 60 to 70% of the particles were separated and purified as compared with the initial Fe powder. Most of the ethanol-dispersed particles, as shown in FIG. 3 (d) Sub-micron scale.
In order to analyze the components of the ball milled powder particles, an EDX spectrum was obtained in connection with an SEM image, and the photograph is shown in FIG. 4 (a).
As can be seen from Fig. 4 (a), the basic composition constituting an optional region in ball milled particles is about 84.5 wt% B, about 4.2 wt% Fe, about 2.3 wt% Mg, and 2.6 wt% there was. Mg is already present as an impurity in the amorphous boron powder and has the advantage of acting as a catalyst as well as Fe and elements such as O and Pt are expected to come from the oxidation and the lattice coating of the sample.
On the basis of this EDX analysis, the precursor nanoparticles produced by ball milling of the amorphous boron and Fe mixture are composed of nano-boron mixed with Fe-Mg. Here, Mg is contained in the existing amorphous boron and is used as a catalyst for producing BNNT similarly to Fe. The internal structure of the mixed B-Fe-Mg nanoparticles shows a form of boron homogeneously mixed with Fe and Mg from a TEM image associated with the EDX spectrum as shown in Fig. 4 (b) The weight ratio is similar to that obtained by SEM-EDX analysis.
The Fe nanoparticles are distinguished from B / Fe precursors which are Fe nanoparticles surrounded by amorphous boron which are generally obtained by ball milling of crystalline boron over 24 hours. The Fe nanoparticles are derived from the STS milling system as impurities , And amorphous boron due to the amorphization of crystalline boron.
However, by using amorphous boron mixed with a small amount of Fe, the precursor for synthesizing BNNT could be produced with a much shorter milling time of less than 6 hours, in which case a large amount of nano-sized It is easier to remove than Fe. As a result, the amount of Fe impurity can be reduced by using amorphous boron.
On the other hand, FIG. 5 shows an SEM photograph (a) and a TEM photograph (b) taken with respect to the BNNT powder obtained in Example 1. As can be seen from FIG. 5, it was confirmed that a crystalline structure of BNNT was produced.
Further, Fig. 6 shows the XRD pattern of the BNNT sample prepared using the precursor obtained by ball milling general crystalline boron with the XRD pattern (a) of the amorphous boron produced by Example 1. The XRD pattern shown in FIG. 6 (a) represents the initial amorphous boron, the ball milled amorphous boron and Fe mixture (the supernatant) and the heat-treated BNNT sample, respectively, and the XRD pattern shown in FIG. , Ball milled boron and a BNNT sample using the same.
As can be seen from Figure 6 (a), in the case of amorphous boron, there is a very small XRD peak, which can be derived from the incomplete amorphous structure of boron. On the other hand, most of the XRD peaks disappear in the case of the ball milled mixture (the supernatant) of amorphous boron and Fe, indicating that the BNNT precursor was made entirely of amorphous boron. Further, it is expected that the peaks of Fe and Mg are not observed because the intensity of the peaks is very small, and the Fe particles having a large particle size are purified by using magnets.
This very small amount of residual Fe and Mg is expected to be present in uniformly mixed B-Fe-Mg mixed nanoparticles as shown in FIG. 3 and can be used as a catalyst for synthesizing BNNTs. Through the BN (002) peak for the heat treated sample, it can be finally confirmed that the sample is BNNT along with the SEM and TEM image of FIG.
In the case of Fig. 6 (b), it can be seen that a large amount of Fe impurity is contained by ball milling of crystalline boron, and it is obvious that Fe impurity is also present as impurities in the BNNT sample. On the other hand, as can be seen from FIG. 6 (a), peaks for Fe and Mg were not observed in the BNNT sample prepared using the precursor obtained by milling amorphous boron as in the case of the precursor. This indicates that the B-Fe-Mg mixed precursor prepared by milling the Fe powder beforehand in the amorphous boron can maintain the BNNT sample purity and is more efficient than BNNT in the case of using crystalline boron.
Example 2
The amorphous boron mixed with about 3.5 wt% of Mg and Fe powder were mixed and ball-milled in the same manner as in Example 1, followed by purification using a magnet to prepare a B-Fe-Mg precursor powder.
Next, the precursor powder was heat-pressed to produce a film, and the resulting film was vertically placed in a heat treatment reactor and heat-treated to prepare BNNT.
20 g of B-Fe-Mg mixed precursor powder purified after ball milling and 4 g of finely pulverized sugar were uniformly mixed and then sprayed so as to be evenly distributed between a mold having a thickness of 300 탆 and a release film, Lt; RTI ID = 0.0 > um < / RTI >
The formed film was cut into 3.5 cm × 3.5 cm pieces, and then the three films were vertically mounted using a rectangular tungsten block measuring 0.5 cm × 0.5 cm × 4 cm, and then installed in a vertical heat treatment reactor.
Then, a mixed gas of N 2 (90 vol%) and NH 3 (10 vol%) was heat-treated at a flow rate of 200 sccm at 1200 ° C for 3 hours to obtain BNNT.
Result 2
FIG. 7 shows the SEM image of the BNNT specimen obtained after the heat treatment. It can be seen that the BNNT is grown in a somewhat different form from the BNNT obtained in Example 1. It can be seen that the BNNT is grown in a direction aligned in a certain direction and also grows into a slightly agglomerated form due to the binding of the precursor powder by compression .
FIG. 8 shows the XRD patterns of BNNT samples prepared three times under the same conditions. As can be seen from the pattern, it is shown that the BNNT was produced through the h-BN (200) peak, and the peak of Fe or Mg is high enough to be negligible as compared with the peak intensity of BNNT.
Claims (25)
A precursor-binder mixture preparation step of mixing the boron-catalyst precursor powder with a binder to prepare a precursor-binder mixture; And
Forming the precursor-binder mixture into a film to form a precursor film;
Wherein the boron nitride nanotubes have a thickness of about 10 nm to about 100 nm.
Dispersing the boron-catalyst precursor powder in a solvent to prepare a dispersion; And
Precipitating the catalyst particles in the dispersion liquid having a diameter exceeding 1000 nm by using a magnet under the dispersion, and collecting and drying the supernatant liquid to obtain a nanoporous boron-catalyst precursor powder
Wherein the boron nitride nanotubes are in contact with each other.
And centrifuging said boron-catalyst precursor powder at 500 to 2000 rpm to remove catalyst particles having a diameter of greater than 1000 nm to obtain a nanoporous boron-catalyst precursor powder.
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