RU2614012C1 - Method for boron nitride nanotubes production - Google Patents

Abstract

FIELD: chemistry.

SUBSTANCE: method comprises preparation of a reaction mixture of a boron oxide compound and a catalyst, reaction mixture heat treatment in ammonia at a temperature of 950°C-1200°C for 1 hour, nanotubes selection from the reaction products, washing and drying, at that, the reaction mixture consists of a boron oxide compound and a catalyst taken in a proportion providing cation ratio in the range of B/Me=7.1, where Me=Li, Mg , Ca, Sr, lithium, magnesium, calcium, or strontium hydroxide or carbonate, are used a catalyst, and boric acid or ammonium borate is used as a boron oxide compound, the reaction mixture is applied as a 0.1 to 1 mm thick layer to a closed strip of mild steel or a thin mesh which is passed through the furnace with ammonia atmosphere.

EFFECT: invention allows to obtain discrete cylindrical boron nitride nanotubes, 10-100 nm in diameter and 0,5-50 microns long, using cheap and low toxic reagents in a continuous manner.

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Description

The invention relates to the field of production of ceramic nanomaterials, namely, discrete boron nitride nanotubes having a diameter in the range of 10-100 nm and a length in the range of 0.5-50 microns.

Boron nitride nanotubes consist of sheets of hexagonal boron nitride twisted into a tube with a diameter of 10 to 100 nm, while the nanotubes have an internal channel with a diameter of 1 to 50 nm. The ratio of the length of the nanotube of boron nitride to its diameter is in the range from 20 to 1000.

Boron nitride nanotubes have high tensile strength and therefore can be used as a hardening phase for polymer and metal matrices. This requires discrete nanotubes.

US Pat. No. 1,077,712 describes a method for the production of hexagonal boron nitride based on the metallothermic reduction of reducible boron compounds, which consists in preparing the reaction mixture from a reducible boron compound, a reducing metal, and a decomposable nitrogen-containing compound. The reaction mixture is heated to a temperature of about 300 ° C, at which reactions in the reaction mixture are initiated. As a result of the heat generated in this process, the temperature of the mixture rises to a temperature of about 800 ° C, at which boron nitride synthesis occurs. After completion of the reactions in the reaction mixture, the resulting product is cooled, washed successively with dilute acid and alcohol, and the resulting boron nitride is dried under vacuum at a temperature of 60-70 ° C. Boric anhydride can be used as a reducible boron compound, magnesium in the form of a fine powder as a reducing metal, and ammonium chloride as a decomposable compound containing nitrogen.

The disadvantage of this method is that it does not allow to obtain boron nitride in the form of nanotubes.

US 2888325 describes a method for manufacturing hexagonal boron nitride, selected as an analogue, based on the interaction of a boroxide compound with ammonia, consisting in mixing a boroxide compound with an inert diluent compound, heating the mixture in an ammonia atmosphere to a temperature of from 700 ° C to 1100 ° C. , cooling and leaching boron nitride from the obtained product in dilute acid, water and alcohol to obtain high purity boron nitride. As a boroxide compound, boric acid can be used. The diluent compound is used to prevent the mixture from turning into a non-porous mass at nitriding temperatures. The diluent compound must be inert, non-melting and non-volatile at nitriding temperatures. Inorganic compounds of alkaline earth metals such as phosphates, for example, calcium triphosphate, or magnesium phosphate, and also oxides, for example, calcium or magnesium oxides, are used as a diluent compound.

The disadvantage of this method is that it does not allow to obtain boron nitride in the form of nanotubes.

A known method of producing nanotubes of boron nitride, which consists in creating an electric arc between the copper cathode and the anode, which is a tungsten tube with a diameter of 6.3 mm, into which a rod with a diameter of 3.17 mm from a pressed hexagonal boron nitride is inserted [N.G. Chopra, et. al. Science 269 (1995) 966-967]. The electrodes are in a helium atmosphere at a pressure of about 0.9 bar. A direct current of 50 to 140 A is created between the electrodes at a potential difference of 30 V. After some time, the current is turned off. After cooling the electrodes on a copper cathode, a dark gray powder is collected, in which there are single boron nitride nanotubes with a diameter of up to 8 nm and a length of more than 200 nm.

The disadvantage of this method is the low yield of nanotubes and a large number of impurity phases, in particular, tungsten.

There is a method of producing boron nitride nanotubes, consisting in the condensation of boron nitride vapors obtained by evaporation of a target from a compressed hexagonal boron nitride by laser radiation of a CO ₂ laser (λ = 10.6 μm, 70 W) [T. Laude, et. al. Appl. Phys. Lett. 76 (2000) 3239-3241]. The process is carried out in a nitrogen atmosphere at a pressure of 100 mbar. A depression forms on the surface of the target as a result of thermal dissociation of boron nitride. After cooling the plate, agglomerates of boron nitride nanotubes are found in the recess.

The disadvantage of this method is the low yield of nanotubes and a high degree of agglomeration, not allowing their use as a strengthening nanophase in polymer or metal materials.

A known method of producing nanotubes of boron nitride, which is a variant of chemical vapor deposition, consisting in the formation of volatile boron chloride as a result of chemical interaction in a reaction mixture consisting of amorphous boron and iron chloride [A. Pan, Y. Chen. Nanoscale Research Letters 9 (2014) 555-560]. The method includes: preparing a reaction powder mixture from amorphous boron and iron chloride, heating the mixture in an ammonia atmosphere to 1250 ° C, holding at this temperature for 5 hours, and cooling to room temperature. As a result, discrete boron nitride nanotubes with a large yield relative to the sample are formed on the surface of the reacted reaction mixture, however, the nanotubes are not cylindrical but bamboo-like, i.e. consist of small links interconnected by a thin sheet of boron nitride. Such nanotubes have a low tensile strength and cannot be used as a strengthening nanophase in polymer or metal materials.

A known method of producing boron nitride nanotubes, consisting in the condensation of boron nitride vapors obtained by evaporation of hexagonal boron nitride in high temperature argon-nitrogen-hydrogen plasma at a pressure below 1 atmosphere [K.S. Kim, et. al. ACS Nano, 8 (2014) 6211]. As a result of the evaporation of boron nitride in the plasma and its condensation in the cold zone of the reactor, boron nitride nanotubes are formed in high yield. The disadvantage of this method is that the nanotubes are highly agglomerated in the form of a cobweb-like material, which makes it impossible to use them as a strengthening nanophase in polymeric or metallic materials.

A known method of producing boron nitride nanotubes, consisting in the fact that in a high-temperature argon-nitrogen plasma serves boron-containing reagent at a total pressure in the reactor from 1 to 10 ATM [A. Fathalizadeh, et. al. Nano Lett. 14 (2014) 4881]. As a result, boron nitride nanotubes are formed with a large yield. The disadvantage of this method is that the nanotubes are highly agglomerated in the form of a cobweb-like material, which makes it impossible to use them as a strengthening nanophase in polymeric or metallic materials.

A known method of producing nanotubes of boron nitride, which is a variant of chemical vapor deposition, consisting in the preparation of a reaction powder mixture of FeO, MgO and B, heating the mixture in an induction furnace in ammonia to a temperature in the range of 1100-1250 ° C, holding at this temperature for several hours and subsequent cooling to room temperature [C. Zhi, et. al. Solid State Communications 135 (2005) 67-70]. Volatile boron oxide, which is formed as a result of chemical processes in the reaction mixture, is transferred by an argon stream to the colder part of the reactor, where, reacting with ammonia, it leads to the growth of boron nitride nanotubes. As a result, discrete cylindrical nanotubes of boron nitride are formed. At a temperature of the reaction mixture of 1100 ° C, nanotubes of high purity and high structural perfection are formed, however, with a small yield. With increasing temperature to 1250 ° C, the yield of nanotubes increases, however, their diameter increases sharply, which reduces their quality. At temperatures above 1250 ° C, only particles of hexagonal boron nitride are formed instead of nanotubes. The maximum yield of nanotubes of acceptable quality is about 100 mg per synthesis.

The disadvantage of this method is the frequency of the process associated with the need to load the reaction mixture of a limited volume, which leads to a small productivity of the process. This method does not allow to organize a continuous synthesis process, therefore, it cannot serve as a potential industrial method for the production of boron nitride nanotubes.

A known method of producing boron nitride nanotubes, which consists in the preparation of a porous precursor B ₃₁ Fe ₁₇ (MgO) ₂₇ by the method of self-propagating high-temperature synthesis of a mixture of B ₂ O ₃ , Fe ₂ O ₃ and FeB, taken in the appropriate proportion; in crushing the precursor and mixing its pieces with a thin iron mesh; in heating this composition in ammonia to a temperature in the range of 1000-1200 ° C, keeping it at this temperature for 6-12 hours and cooling to room temperature [J. Wang, et. al. Journal of Solid State Chemistry 184 (2011) 2478-2484]. After that, pieces of iron mesh were removed from the reaction composition. The remainder of the reaction composition was dissolved in a hydrochloric acid solution, washed and filtered. As a result, nanotubes of a wavelike or bamboo-like morphology with a purity of about 80% were obtained. The yield of such nanotubes was 95% or more. The pieces of iron mesh were sonicated in a hydrochloric acid solution, then filtered and washed. As a result, cylindrical boron nitride nanotubes with a purity of about 90% were obtained with a yield of no more than 5 weight relative to the total number of nanotubes. %

The disadvantage of this method is the small yield of cylindrical nanotubes, which can be used as a reinforcing additive in polymeric or metallic materials.

A known method of producing nanotubes of boron nitride, which consists in preparing a powder reaction mixture of Fe ₂ O ₃ and amorphous boron, taken in different ratios, applying the reaction mixture to a stainless steel substrate, heating the substrate with the reaction mixture in ammonia to 1300 ° C, holding at this temperature for 3 hours and cooling [J. Li, et. al. Chemical Engineering Journal 174 (2011) 687-692]. At a molar ratio of the components Fe ₂ O ₃ / B = 0.02, discrete nanotubes of boron nitride of high purity and structural perfection are formed, however, their yield is small. With an increase in the ratio of the components Fe ₂ O ₃ / B to 0.1 and higher, the number of nanotubes increases, however, their quality also decreases due to an increase in the content of O-Si-Mn impurities in their walls.

A known method of producing boron nitride nanotubes, selected as a prototype, based on the reaction of a boroxide compound with ammonia, consisting in preparing the reaction mixture from a boroxide compound and a catalyst, heating the resulting mixture in argon to 1300 ° C, replacing the argon atmosphere with ammonia at an ammonia pressure of 0 , 2 atm, holding under these conditions for 1 h, cooling to room temperature, and mechanically collecting nanotubes from the surface of the reacted reaction mixture [SD Yuan., Et. al. Journal of Crystal Growth 256 (2003) 67-72]. Barium metaborate is used as a boroxide compound, and gallium alkoxide is used as a catalyst. Barium metaborate is prepared by dissolving barium metal and boron triethylate in purified ethanol. Gallium alkoxide acts as a catalyst that promotes the growth of cylindrical boron nitride nanotubes; without the addition of gallium alkoxide, bamboo-like nanotubes are obtained.

The disadvantage of this method is the low quality of the resulting nanotubes of boron nitride containing a large number of surface defects. The disadvantages include the difficulty of preparing barium metabolite, and the need to use expensive reagents: barium metal, triethyl borate and gallium alkoxide. In addition, all reagents used require special precautions when working with them, as they are flammable and highly sensitive to moisture. When working with these reagents, it is necessary to use a box with a dry protective atmosphere. It is also necessary to use respiratory protection. These shortcomings make the method of producing nanotubes in accordance with the prototype low efficient.

The aim of the present invention is to increase the efficiency of the process for producing discrete cylindrical boron nitride nanotubes by using cheap reagents and creating a process that allows organizing the production of nanotubes as a continuous process.

The technical result of the invention is to increase the efficiency of the synthesis of discrete cylindrical nanotubes of boron nitride and reduce the environmental risks associated with their production through the use of cheap and low toxicity reagents.

The technical result is achieved by using instead of expensive and dangerous reagents: barium metal, triethyl borate and gallium alkoxide cheap, low-toxic and widely used in industry boric acid and hydroxides, or lithium, magnesium, calcium or strontium carbonates.

The specified technical result is achieved in the following way, including: preparation of the reaction mixture from a boroxide compound, which is used as boric acid or ammonium borate, and a catalyst, which is used as hydroxide or carbonate of lithium, magnesium, calcium, or strontium, taken in a proportion that provides cation ratio in the range:

B / Me = 1-7, where Me = Li, Mg, Ca, Sr,

heat treatment of the reaction mixture in ammonia at a temperature of from 950 ° C to 1200 ° C for 1 hour, the selection of nanotubes from the reaction products, washing and drying.

The use of boric acid or ammonium borate as a boroxide compound is preferred, but not required. Other boroxide compounds, for example, boron oxide, can be used instead of these compounds. The advantage of boric acid and ammonium borate in comparison with boron oxide is their lower hardness, which makes their grinding process easier and less labor intensive.

The use of a catalyst in the form of a hydroxide or carbonate is preferred, but not required. You can also use nitrates, or oxalates, or their other salts, however, the use of hydroxides and carbonates allows the production of boron nitride nanotubes with the greatest yield. The use of salts of other alkali or alkaline earth metals, other than those specified in this invention, gives a very small yield of nanotubes, which, apparently, is associated with their low catalytic activity.

When using a reaction mixture in which the boroxide compounds and the catalyst are taken in a proportion that provides the ratio of cations

B / Me <1, where Me = Li, Mg, Ca, Sr,

a sharp decrease in the yield of boron nitride nanotubes occurs, which is associated with the formation of low-reactivity meta- and tetraborates of lithium, magnesium, calcium, or strontium.

When using a reaction mixture in which the boroxide compounds and the catalyst are taken in a proportion that provides the ratio of cations

B / Me> 7, where Me = Li, Mg, Ca, Sr,

a significant decrease in the yield of boron nitride nanotubes is also observed, which is associated with a decrease in the amount of catalyst with respect to the boroxide compound.

Therefore, the optimal composition of the reaction mixture is a composition in which the boroxide compound and the catalyst are taken in a proportion that provides a ratio of cations in the range:

B / Me = 1-7, where Me = Li, Mg, Ca, Sr.

Heat treatment of the reaction mixture is carried out in ammonia at a temperature of from 950 ° C to 1200 ° C for 1 hour. When heat treated at temperatures below 950 ° C, the yield of nanotubes is significantly reduced, which is associated with a decrease in the reaction rate of the boroxide compound with ammonia. With an increase in the heat treatment temperature above 1200 ° С, the yield of nanotubes practically does not change; therefore, it is not advisable to increase the temperature for reasons of energy saving.

After heat treatment, nanotubes are formed on the surface and in the pores of the reacted reaction mixture. Nanotubes are collected from the surface of the reacted reaction mixture mechanically, or by dissolving the reaction products in a solution of hydrochloric acid, followed by filtration, washing and drying. For mechanical removal, you can use a brush, but it is better to use ultrasonic treatment of the reaction products, since it allows you to collect nanotubes with less loss. For this, the reaction products are ground in a mortar or by any other known method and dispersed in ethanol or isopropanol using an ultrasonic bath. After that, the suspension of nanotubes in alcohol is separated from the precipitate, washed several times with a water-alcohol solution by decantation or using a centrifuge and dried. A water-alcohol solution containing 5-10 vol. % ethanol or isopropanol is used for better washing of nanotubes, because the addition of alcohol contributes to their better wetting. During the mechanical separation of nanotubes from reaction products, their incomplete selection is possible, which leads to a certain decrease in the yield of nanotubes. When nanotubes are isolated from reaction products by the dissolution method, the reacted mixture is ground in a mortar or by any other known method, poured with a 10-15% hydrochloric acid solution and left in the solution for 5-6 hours with periodic stirring of the solution. The time of leaching of nanotubes depends on the degree of grinding of the reacted mixture. The use of ultrasonic treatment reduces the time of acid treatment by 2-3 times. After this, the suspension of the nanotubes is separated from the precipitate, washed several times with a water-alcohol solution by decantation or, using a centrifuge, and dried. This method of isolating nanotubes from reaction products leads to lower losses compared to the mechanical method of isolation, however, this increases the time of isolation and it is necessary to use acids.

The proposed method allows to organize the production of boron nitride nanotubes in the form of a continuous process. For this, the reaction mixture is applied in the form of a layer with a thickness of 0.1 to 1 mm on a closed tape of low-carbon steel foil (tin), or a thin mesh that passes through a tube furnace with an atmosphere of ammonia, at a speed that ensures the reaction mixture is at a temperature of 950 -1200 ° C for 1 hour. Mild steel is used because it weakly reacts with the reaction mixture, or with ammonia. After the ribbon leaves the oven, the reaction products are removed from the ribbon, crushed, and the tubes are extracted from them using one of the above methods.

Examples of carrying out the invention. In all examples, the mechanical method of isolating nanotubes from reaction products was used.

Example No. 1 (No. 1 in the table)

0.05 mol of B (OH) ₃ and 0.025 mol of Li ₂ CO _{3 were} thoroughly ground in an agate mortar with the addition of isopropanol. The resulting mixture was applied in a thin layer to a low carbon steel substrate, which was placed in a controlled atmosphere tube furnace. The furnace was evacuated to 10 ^-2 mbar, then ammonia was introduced and its flow was set at 50 ml / min, heated to 950 ° C at a speed of 300 ° / min, kept for 1 hour and cooled to room temperature. The sample was removed from the substrate, thoroughly ground in an agate mortar with the addition of isopropanol. After grinding, 100 ml isopropanol was added (until a diluted suspension was obtained) and placed in an ultrasonic bath for 30 minutes. After that, the suspension was allowed to settle for 5 minutes and it was drained, separating from the precipitate. With the precipitate, the grinding, dispersion in an ultrasonic bath and sedimentation procedures were repeated once more. The resulting second suspension was combined with the first suspension, placed in a tall narrow vessel (1 L volumetric vessel), distilled water was added to the full volume of the vessel, and allowed to stand for 15 minutes. After that, the suspension was separated from the precipitate, it was evaporated, and then the obtained material was first dried in air and then in a vacuum oven at 200 ° C for 1 hour. X-ray phase analysis showed that the obtained material has the structure of hexagonal boron nitride; the content of impurities in the form of boron oxide and lithium metabolite was less than 5 weight. % Scanning electron microscopy showed that the material consists almost entirely of discrete cylindrical tubes of boron nitride, having a diameter in the range of 30-100 nm and a length of 1-10 microns. Separate microparticles of boron oxide and lithium borate were observed. The output of nanotubes along boron (relative to the amount of boron in boric acid) was 11 weight. %

The table shows other examples of the use of the invention with a different composition of the reaction mixture (i.e., another boroxide compound or catalyst) and its heat treatment temperature; the procedures for heat treatment and isolation of nanotubes from the reacted mixture were the same. In these cases, the quality of boron nitride nanotubes (i.e., their cylindrical morphology, diameter, and length) did not change (see table).

Claims (1)

A method of producing boron nitride nanotubes, including preparing the reaction mixture from a boroxide compound and a catalyst, heat treating the reaction mixture in ammonia at a temperature of 950 ° C-1200 ° C for 1 hour, isolating the nanotubes from the reaction products, characterized in that the reaction mixture consists of boroxide compounds and catalyst, taken in a proportion that provides a cation ratio in the range B / Me = 1-7, where Me = Li, Mg, Ca, Sr, lithium, magnesium, calcium, or strontium hydroxide or carbonate are used as a catalyst, and inachestve boroksidnogo compound using boric acid or ammonium borate, the reaction mixture is applied in a layer thickness of from 0.1 to 1 mm on a closed strip of mild steel or a thin mesh which is passed through the furnace with an ammonia atmosphere, then washing and drying is performed.