Method for producing carbon nanotubes

Abstract

To provide a manufacturing method of a carbon nanotube capable of manufacturing a carbon nanotube having a high ratio of a two-layer structure at an excellent synthesis rate.SOLUTION: A manufacturing method of a carbon nanotube includes: a first raw material supply step of supplying a first raw material involving a first hydrocarbon compound, a catalyst source involving a metal element and a reaction accelerator to a reaction system; and a second raw material supply step of supplying a second raw material involving a gas-phase linear hydrocarbon compound as a second hydrocarbon compound which is different from the first hydrocarbon compound to the first raw material having been supplied to the reaction system; wherein the molar ratio of the linear hydrocarbon compound to the first hydrocarbon compound to is 70% or over and 400% or under.SELECTED DRAWING: None

Description

[Technical field]

[0001] The present invention relates to a method for producing carbon nanotubes, and more particularly to a method for producing carbon nanotubes that can produce carbon nanotubes with a high ratio of double-walled structure at an excellent synthesis rate. [Background technology]

[0002] Carbon nanotubes are a material with various properties and are expected to be applied in many fields. For example, carbon nanotubes are three-dimensional mesh structures consisting of a single layer of cylindrical bodies with a hexagonal lattice mesh structure, or multiple layers arranged approximately coaxially, and are lightweight and have excellent properties such as electrical conductivity, thermal conductivity, and mechanical strength.

[0003] Moreover, by controlling the number of walls, diameter, etc., of carbon nanotubes, it is possible to further improve various properties such as electrical conductivity, thermal conductivity, and mechanical strength.

[0004] On the other hand, electric wires consisting of a core wire made of one or more wires and an insulating coating covering the core wire are used as power lines and signal lines in various fields such as automobiles and industrial equipment. Copper or copper alloys are usually used as the material of the wire constituting the core wire from the viewpoint of electrical properties, but aluminum or aluminum alloys have been proposed in recent years from the viewpoint of weight reduction. However, due to the increase in the number of wirings associated with the high functionality of automobiles and industrial equipment, there is a demand for further weight reduction of electric wires. Therefore, it has been attempted to use carbon nanotube wires instead of metal wires as electric wire materials because of their excellent properties such as light weight, electrical conductivity, thermal conductivity, and mechanical strength.

[0005] When carbon nanotube wire is used as an electric wire material, it is particularly required to have excellent electrical conductivity. In order to impart excellent electrical conductivity to the carbon nanotube wire, it is preferable that the carbon nanotubes constituting the carbon nanotube wire have a two-layer structure. This is because the two-layer carbon nanotube has a high graphite structure and therefore has high electrical conductivity.

[0006] Therefore, it has been proposed to produce double-walled carbon nanotubes with high electrical conductivity by carrying out an oxidation reaction until the combustion peak on the high temperature side is at 700 to 850°C in thermogravimetric analysis and the weight loss on the low temperature side (TG(L)) and the weight loss on the high temperature side (TG(H)) is TG(H) / (TG(L)+TG(H))=0.75 or more (Patent Document 1).

[0007] However, in the manufacturing method of Patent Document 1, the control of the manufacturing process is complicated, and the synthesis rate of double-walled carbon nanotubes is not sufficiently high, leaving room for improvement in the productivity of carbon nanotubes. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] JP 2011-148674 A Summary of the Invention [Problem to be solved by the invention]

[0009] In view of the above circumstances, an object of the present invention is to provide a method for producing carbon nanotubes, which is capable of producing carbon nanotubes having a high proportion of double-walled structure at an excellent synthesis rate. [Means for solving the problem]

[0010] The gist of the configuration of the present invention is as follows. [1] a first raw material supply step of supplying a first raw material containing a first hydrocarbon compound, a catalyst source containing a metal element, and a reaction accelerator to a reaction system; A second raw material supply step of supplying a second raw material containing a gas-phase linear hydrocarbon compound as a second hydrocarbon compound different from the first hydrocarbon compound to a reaction system, A method for producing carbon nanotubes, wherein a molar ratio of the linear hydrocarbon compound to the first hydrocarbon compound is 70% or more and 400% or less. [2] The method for producing carbon nanotubes according to [1], wherein the first raw material supplying step includes a step of spraying the first raw material in a liquid phase into a reactor. [3] The method for producing carbon nanotubes according to [1], wherein the first raw material supply step includes a step of supplying the first raw material obtained by vaporizing the first raw material in a liquid phase to a reactor. [4] The method for producing carbon nanotubes according to any one of [1] to [3], wherein the first hydrocarbon compound is at least one hydrocarbon compound selected from the group consisting of aromatic hydrocarbon compounds, alicyclic hydrocarbon compounds, and chain hydrocarbon compounds having 5 or more carbon atoms. [5] The method for producing carbon nanotubes according to any one of [1] to [3], wherein the first hydrocarbon compound is at least one hydrocarbon compound selected from the group consisting of aromatic hydrocarbon compounds and alicyclic hydrocarbon compounds. [6] The method for producing carbon nanotubes according to any one of [1] to [5], wherein the linear hydrocarbon compound is a hydrocarbon compound having 1 to 4 carbon atoms. [7] The method for producing carbon nanotubes according to any one of [1] to [5], wherein the linear hydrocarbon compound is at least one hydrocarbon compound selected from the group consisting of methane, ethane, ethylene and acetylene.

[0011] In the above embodiment [1], a first raw material containing a first hydrocarbon compound and a second raw material containing a gaseous linear hydrocarbon compound as a second hydrocarbon compound are supplied to a reaction system for synthesizing carbon nanotubes. The first raw material contains the first hydrocarbon compound that is a carbon source for carbon nanotubes, a catalyst source containing a metal element, and a reaction promoter that lowers the melting point of the metal catalyst to promote catalytic action at a moderate temperature. The gaseous linear hydrocarbon compound contained in the second raw material is an additional carbon source for carbon nanotubes. Effect of the Invention

[0012] According to an embodiment of the method for producing carbon nanotubes of the present invention, the method includes a first raw material supplying step of supplying a first raw material containing a first hydrocarbon compound, a catalyst source containing a metal element, and a reaction accelerator to a reaction system, and a second raw material supplying step of supplying a gaseous linear hydrocarbon compound to the first raw material supplied to the reaction system, and the molar ratio of the linear hydrocarbon compound to the first hydrocarbon compound is 70% or more and 400% or less, so that carbon nanotubes having a high ratio of double-walled structure can be produced at an excellent synthesis rate. Furthermore, according to an embodiment of the method for producing carbon nanotubes of the present invention, the diameter of the carbon nanotubes can be made small (for example, an average diameter of about 2 nm) as in the conventional method while preventing the variation in the diameter of the carbon nanotubes.

[0013] According to an embodiment of the method for producing carbon nanotubes of the present invention, the first hydrocarbon compound is at least one type of hydrocarbon compound selected from the group consisting of aromatic hydrocarbon compounds, alicyclic hydrocarbon compounds, and chain hydrocarbon compounds having 5 or more carbon atoms, thereby reliably increasing the proportion of two-walled structures among the obtained carbon nanotubes.

[0014] According to an embodiment of the method for producing carbon nanotubes of the present invention, the first hydrocarbon compound is at least one type of hydrocarbon compound selected from the group consisting of aromatic hydrocarbon compounds and alicyclic hydrocarbon compounds, thereby further increasing the proportion of two-walled structures among the obtained carbon nanotubes.

[0015] According to an embodiment of the method for producing carbon nanotubes of the present invention, the linear hydrocarbon compound is at least one selected from the group consisting of methane, ethane, ethylene and acetylene, thereby further improving the ratio of double-walled structures and the synthesis rate of carbon nanotubes. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0016] First, the structure of carbon nanotubes will be described. Carbon nanotubes are cylindrical bodies having a single-layer structure or a multi-layer structure, and are called SWNT (single-walled nanotube) and MWNT (multi-walled nanotube), respectively. Of these, carbon nanotubes having a two-layer structure have a three-dimensional mesh structure in which two cylindrical bodies having a hexagonal lattice mesh structure are arranged approximately coaxially, and are called DWNT (double-walled nanotube). The hexagonal lattice, which is the structural unit, is a six-membered ring with carbon atoms at its vertices, and these are adjacent to other six-membered rings and are continuously bonded. Thus, carbon nanotubes are a six-membered ring structure having a three-dimensional mesh structure of a hexagonal lattice, and therefore have a large surface area and a large number of free electrons, making them a material with excellent conductivity.

[0017] For example, carbon nanotubes with a relatively small number of walls, such as a two-walled structure, are relatively more conductive than carbon nanotubes with a larger number of walls, and when doped, the doping effect is greatest in carbon nanotubes with a two-walled structure. Therefore, in order to improve the conductivity of carbon nanotube wires using carbon nanotubes, it is preferable to increase the proportion of carbon nanotubes with a two-walled structure. Specifically, the proportion of carbon nanotubes with a two-walled structure to the total number of carbon nanotubes is preferably 75% by number or more, and particularly preferably 80% by number or more. The proportion of carbon nanotubes with a two-walled structure can be calculated by observing and analyzing with a transmission electron microscope (TEM) and measuring the number of layers of each of 100 CNTs.

[0018] The carbon nanotube manufacturing method of the present invention will be described in detail below. The carbon nanotube manufacturing method of the present invention includes (1) a first raw material supplying step of supplying a first raw material containing a first hydrocarbon compound, a catalyst source containing a metal element, and a reaction accelerator to a reaction system, and (2) a second raw material supplying step of supplying a second raw material containing a gas-phase linear hydrocarbon compound as a second hydrocarbon compound different from the first hydrocarbon compound to the reaction system. In addition, in the carbon nanotube manufacturing method of the present invention, the molar ratio of the second hydrocarbon compound (i.e., linear hydrocarbon compound) supplied in the second raw material supplying step to the first hydrocarbon compound supplied in the first raw material supplying step is controlled to a range of 70% to 400%.

[0019] In the present invention, by controlling the above-mentioned production process and the molar ratio of the first hydrocarbon compound to the second hydrocarbon compound, carbon nanotubes having a high ratio of double-walled carbon nanotubes to the total carbon nanotubes can be produced at an excellent synthesis rate. Furthermore, in the present invention, carbon nanotubes having a small average diameter (for example, an average diameter of about 2 nm) can be obtained in the same manner as in the conventional method while preventing variation in the diameter of the carbon nanotubes.

[0020] (1) First raw material supply process The first raw material supply step is a step of supplying a first raw material containing a first hydrocarbon compound, which is a carbon source of carbon nanotubes, a catalyst source containing a metal element, and a reaction promoter to a carbon nanotube synthesis reaction system. The reaction promoter is a component that lowers the melting point of the metal catalyst and promotes catalytic action at a moderate temperature. The carbon nanotube synthesis reaction system includes a raw material introducer, which is a raw material introduction means, a reactor in which carbon nanotubes are synthesized, which is provided downstream of the raw material introducer, and a recovery device, which is provided downstream of the reactor, for recovering the synthesized carbon nanotubes. In the carbon nanotube synthesis reaction system, a carrier gas flows from the raw material introducer to the recovery device via the reactor. Examples of the carrier gas include inert gases such as nitrogen, argon, and helium, and hydrogen gas.

[0021] The reactor is provided with a heating furnace, and the first raw material flows from the raw material inlet to the recovery vessel through the reactor interior heated to a predetermined atmospheric temperature by the heating furnace. The atmospheric temperature inside the reactor is, for example, 1000°C or higher and 1500°C or lower.

[0022] The first raw material may be, for example, a mixture of a first hydrocarbon compound, a catalyst source containing a metal element, and a reaction promoter. The first raw material may be supplied to the carbon nanotube synthesis reaction system in a liquid phase state, or may be heated and vaporized, that is, supplied to the carbon nanotube synthesis reaction system in a gas phase state.

[0023] A method for supplying the liquid phase first raw material to the carbon nanotube synthesis reaction system includes, for example, a method for introducing the liquid phase first raw material into a raw material inlet provided in the carbon nanotube synthesis reaction system, and spraying the liquid phase first raw material from a sprayer provided in the raw material inlet toward the carrier gas flowing from the raw material inlet toward the reactor. Also, a method for supplying the gas phase first raw material to the carbon nanotube synthesis reaction system includes, for example, a method for heating and vaporizing the liquid phase first raw material in a vaporizer, and then supplying the gas phase first raw material from the raw material inlet to the carrier gas flowing from the raw material inlet toward the reactor.

[0024] The mixing ratio of the first hydrocarbon compound, the catalyst source containing a metal element, and the reaction promoter can be appropriately selected depending on the properties required of the carbon nanotubes. For example, the molar ratio of the first hydrocarbon compound:the catalyst source containing a metal element:the reaction promoter may be 100:1-10:1-10.

[0025] As the first hydrocarbon compound, for example, aromatic hydrocarbon compounds, alicyclic hydrocarbon compounds, and chain hydrocarbon compounds having 5 or more carbon atoms are preferable, since the ratio of double-walled carbon nanotubes among all the carbon nanotubes obtained is reliably increased. Examples of aromatic hydrocarbon compounds include toluene, benzene, xylene, naphthalene, and anthracene. Examples of alicyclic hydrocarbon compounds include decalin. Examples of chain hydrocarbon compounds having 5 or more carbon atoms include chain hydrocarbon compounds having 5 to 20 carbon atoms, such as hexane, heptane, octane, nonane, decane, undecane, dodecane, tridecane, tetradecane, pentadecane, hexadecane, and heptadecane.

[0026] Among these first hydrocarbon compounds, aromatic hydrocarbon compounds and alicyclic hydrocarbon compounds are preferred because they further increase the ratio of double-walled carbon nanotubes to the total carbon nanotubes obtained. These hydrocarbon compounds may be used alone or in combination of two or more.

[0027] The catalyst source containing a metal element is a metal element-containing compound, and examples of the catalyst source containing a metal element include catalyst sources containing Group VI transition metals such as chromium (Cr), molybdenum (Mo), and tungsten (W) (Group VI transition metal element-containing compounds), and catalyst sources containing Group VIII transition metals such as iron (Fe), cobalt (Co), nickel (Ni), ruthenium (Ru), rhodium (Rh), palladium (Pd), osmium (Os), iridium (Ir), and platinum (Pt) (Group VIII transition metal element-containing compounds). Among these, catalyst sources containing iron are preferred in terms of availability and the like. Examples of catalyst sources containing iron include iron element-containing compounds such as ferrocene, iron acetylacetonate, iron phthalocyanine, and iron porphyrin. These compounds may be used alone or in combination of two or more.

[0028] Examples of the reaction accelerator include sulfur and sulfur compounds. Examples of the sulfur compounds include carbon disulfide, thiophene, thiophene derivatives such as methylthiophene, ethylthiophene, hexylthiophene, benzothiophene, dibenzothiophene, and tetrahydrothiophene, phenylsulfide derivatives, phenyldisulfide derivatives, sulfolane, tetramethylene sulfoxide, alkylthiols, and thiophenols. These compounds may be used alone or in combination of two or more.

[0029] (2) Second raw material supply process The second raw material supplying step is a step of supplying a second raw material containing a gas-phase linear hydrocarbon compound as a second hydrocarbon compound different from the first hydrocarbon compound to the first raw material supplied to the carbon nanotube synthesis reaction system. That is, the second raw material supplying step is a step of supplying a second raw material containing a gas-phase linear hydrocarbon compound to the carbon nanotube synthesis reaction system. The gas-phase linear hydrocarbon compound is an additional carbon source for carbon nanotubes. Note that "gas-phase linear hydrocarbon compound" refers to a linear hydrocarbon compound that is in the gas phase at room temperature (25°C) and normal pressure (standard atmospheric pressure).

[0030] By supplying a first raw material and a second raw material containing a gas-phase linear hydrocarbon compound to a carbon nanotube synthesis reaction system, carbon nanotubes mainly having a two-wall structure are synthesized in the reactor at an excellent synthesis rate.

[0031] As a method for supplying the second raw material, which is a gas phase, to the carbon nanotube synthesis reaction system, for example, there is a method of supplying it from another raw material introducer other than the raw material introducer that introduces the first raw material into the reaction system. Specifically, for example, there is a method of supplying the second raw material from another raw material introducer provided upstream of the reactor to the carrier gas to which the first raw material flowing from the raw material introducer has been supplied. In addition, there is a method of first supplying the second raw material to the carrier gas to which the first raw material has not been supplied, using another raw material introducer provided upstream of the raw material introducer. In addition, there is a method of supplying the first raw material from the raw material introducer to the carrier gas and at the same time supplying the second raw material from the other raw material introducer to the carrier gas. In addition, there is a method of introducing the first raw material and the second raw material into the raw material introducer through separate introduction paths, and supplying the first raw material and the second raw material from the raw material introducer to the carrier gas. The process of synthesizing carbon nanotubes from the first raw material and the second raw material supplied to the synthesis reaction system as described above can be considered as follows. First, catalyst particles are synthesized from the catalyst source contained in the first raw material. Next, carbon nanotubes begin to grow on the catalyst from carbon derived mainly from the first hydrocarbon compound, which is the carbon source of the first raw material. Then, carbon nanotubes grow and become long, mainly from carbon derived from the second hydrocarbon compound contained in the second raw material.

[0032] The blending ratio of the first hydrocarbon compound contained in the first raw material and the second hydrocarbon compound contained in the second raw material is controlled such that the molar ratio of the second hydrocarbon compound to the first hydrocarbon compound is in the range of 70% to 400%. The molar ratio of the second hydrocarbon compound to the first hydrocarbon compound is not particularly limited as long as it is in the range of 70% to 400%, but the lower limit is preferably 80% and particularly preferably 90% from the viewpoint of further improving the synthesis rate of carbon nanotubes having a two-wall structure. On the other hand, the upper limit of the molar ratio of the second hydrocarbon compound to the first hydrocarbon compound is preferably 350% and particularly preferably 200% from the viewpoint of reliably improving the ratio of carbon nanotubes having a two-wall structure.

[0033] The linear hydrocarbon compound in the gas phase, which is the second hydrocarbon compound, may be a linear hydrocarbon compound having 1 to 4 carbon atoms. Examples of the linear hydrocarbon compound having 1 to 4 carbon atoms include methane, ethane, ethylene, acetylene, propane, propene, butane, butene, etc. Among these, methane, ethane, ethylene, and acetylene are preferred in terms of further improving the ratio of the two-wall structure and the synthesis rate of carbon nanotubes. These compounds may be used alone or in combination of two or more. EXAMPLES

[0034] Next, examples of the present invention will be described. However, the present invention is not limited to the following examples as long as they do not depart from the spirit of the present invention.

[0035] Carbon nanotube production Example 1 A liquid first raw material was prepared by mixing toluene as the first hydrocarbon compound, ferrocene as a catalyst source containing a metal element, and thiophene as a reaction accelerator. The molar ratio of toluene:ferrocene:thiophene in the first raw material was 100:2.5:3.0. The second raw material was prepared by using methane as the second hydrocarbon compound.

[0036] A ceramic tube (φ30 mm) was used as a carbon nanotube synthesis reaction system. Specifically, a raw material inlet for introducing a first raw material and another raw material inlet for introducing a second raw material were provided at one end of the ceramic tube. An electric furnace was installed at the center of the ceramic tube, and the center of the ceramic tube was used as a reactor. A cylindrical stainless steel mesh member was placed at the other end of the ceramic tube, and used as a carbon nanotube recovery vessel. A predetermined flow rate of hydrogen gas was flowed from one end to the other end of the ceramic tube as a carrier gas to be supplied to the carbon nanotube synthesis reaction system. The inside of the reactor was heated to about 1200° C. in an electric furnace, and the first raw material in liquid phase was sprayed from the sprayer of the raw material inlet at 150 μL / min, thereby supplying the first raw material to the carbon nanotube synthesis reaction system (first raw material supplying step). In addition, the second raw material in the gas phase was supplied to the hydrogen gas from another raw material inlet, thereby supplying the second raw material to the carbon nanotube synthesis reaction system (second raw material supply step). At this time, the total flow rate of the hydrogen gas and the second raw material in the gas phase was adjusted to be 10 L / min.

[0037] In Example 1, the first raw material and the second raw material were supplied to a carbon nanotube synthesis reaction system so that the molar ratio of methane to toluene was 100%. The carbon nanotubes synthesized in the reactor were attached to a mesh member serving as a collector, and the carbon nanotubes attached to the mesh member were collected to obtain a carbon nanotube sample of Example 1.

[0038] Example 2 A carbon nanotube sample of Example 2 was obtained in the same manner as in Example 1, except that the molar ratio of methane to toluene was 300%.

[0039] Example 3 A carbon nanotube sample of Example 3 was obtained in the same manner as in Example 1, except that ethylene was used instead of methane as the second hydrocarbon compound.

[0040] Example 4 A carbon nanotube sample of Example 4 was obtained in the same manner as in Example 1, except that ethane was used instead of methane as the second hydrocarbon compound.

[0041] Example 5 A carbon nanotube sample of Example 5 was obtained in the same manner as in Example 1, except that decaline was used instead of toluene as the first hydrocarbon compound.

[0042] Example 6 A carbon nanotube sample of Example 6 was obtained in the same manner as in Example 1, except that a mixture of toluene and decalin (the molar ratio of toluene to decalin was 1:1) was used instead of toluene as the first hydrocarbon compound.

[0043] Example 7 A carbon nanotube sample of Example 7 was obtained in the same manner as in Example 1, except that a mixture of methane and ethylene (methane to ethylene molar ratio of 1:1) was used instead of methane as the second hydrocarbon compound.

[0044] Comparative Example 1 A carbon nanotube sample of Comparative Example 1 was obtained in the same manner as in Example 1, except that the molar ratio of methane to toluene was 500%.

[0045] Comparative Example 2 A carbon nanotube sample of Comparative Example 2 was obtained in the same manner as in Example 1, except that the molar ratio of methane to toluene was 50%.

[0046] Evaluation items (1) Combined speed ratio Thirty minutes after the start of carbon nanotube synthesis, a sample of carbon nanotubes attached to the mesh member was collected, and the collected carbon nanotube sample was heated at 400°C for 30 minutes, immersed in 10% by mass hydrochloric acid for 30 minutes, washed with water, and dried at 100°C for 3 hours to remove iron and amorphous carbon, and the carbon nanotube sample was weighed. The weighed carbon nanotube sample was divided by the synthesis time to calculate the synthesis rate (mg / min) per unit time. The synthesis rate (mg / min) of the standard sample obtained by the same method as in Example 1 except that the second raw material was not supplied was set to 100%, and the synthesis rate ratio (%) of the example and the comparative example was calculated.

[0047] (2) The ratio of carbon nanotubes with a double-wall structure to the total number of carbon nanotubes (double-wall ratio) and the average diameter of the carbon nanotubes (nm) Approximately 0.1 mg of carbon nanotubes collected from the mesh member was added to 10 mL of ethanol and dispersed by ultrasonic treatment. The resulting dispersion was dropped onto a support film and observed under a high-resolution transmission electron microscope (JEM-F200 manufactured by JEOL, accelerating voltage 200 kV). Ten fields of view were observed, and the number of layers and the diameter of the outermost layer of approximately 100 random carbon nanotubes were measured from the images. From the obtained data, the ratio of carbon nanotubes with a two-layer structure to the total carbon nanotubes (two-layer ratio) and the average diameter (nm) of the carbon nanotubes were calculated.

[0048] The first hydrocarbon compound and the second hydrocarbon compound used in Examples 1 to 7 and Comparative Examples 1 and 2, their molar ratios, and the evaluation results are shown in Table 1 below.

[0049] [Table 1]

[0050] From the above Table 1, in the first raw material supply step, a first hydrocarbon compound, which is an aromatic hydrocarbon compound and / or an alicyclic hydrocarbon compound, and in the second raw material supply step, a linear hydrocarbon compound (second hydrocarbon compound) having a carbon number of 1 to 4 is supplied to the reaction system, and in the second raw material supply step, the molar ratio of the second hydrocarbon compound to the first hydrocarbon compound is within the range of 70% to 400%, in Examples 1 to 7, the ratio of carbon nanotubes having a double-walled structure to the total carbon nanotubes is 75% by number or more, and the synthesis rate ratio is 130% or more, and carbon nanotubes with a high ratio of the double-walled structure can be produced at an excellent synthesis rate. In addition, in Examples 1 to 7, while preventing the variation in the diameter of the carbon nanotubes, the average diameter of the carbon nanotubes can be about 2.0 nm, as in the conventional case.

[0051] In particular, in Examples 1, 3 to 7 in which the molar ratio of the second hydrocarbon compound to the first hydrocarbon compound was 100%, the ratio of carbon nanotubes having a two-wall structure to the total carbon nanotubes was 80% by number or more, and the ratio of the two-wall structure was further improved. In addition, in Examples 1, 3, and 4, when methane and ethylene were used as the second hydrocarbon compound, the ratio of the two-wall structure was further improved. In addition, in Examples 1, 5, and 6, when toluene and decalin were used in combination as the first hydrocarbon compound, the ratio of the two-wall structure was further improved. In addition, in Examples 1, 3 to 5, when toluene was used as the first hydrocarbon compound and methane and ethylene were used as the second hydrocarbon compound, the synthesis rate was further improved.

[0052] On the other hand, in Comparative Example 1, where the molar ratio of the second hydrocarbon compound to the first hydrocarbon compound was 500%, the ratio of carbon nanotubes having a double-walled structure to the total carbon nanotubes was 72% by number, and carbon nanotubes with a high ratio of double-walled structure could not be obtained. Also, in Comparative Example 2, where the molar ratio of the second hydrocarbon compound to the first hydrocarbon compound was 50%, the synthesis rate ratio was less than 130%, and an excellent synthesis rate was not obtained. [Industrial Applicability]

[0053] The carbon nanotube production method of the present invention can produce carbon nanotubes with a high proportion of double-walled structures at an excellent synthesis rate, and is therefore particularly useful in the field of electric wires such as power lines and signal lines, which require high conductivity.

Claims

1. a first raw material supply step of supplying a first raw material containing a first hydrocarbon compound, a catalyst source containing a metal element, and a reaction accelerator to a reaction system; A second raw material supply step of supplying a second raw material containing a gas-phase linear hydrocarbon compound as a second hydrocarbon compound different from the first hydrocarbon compound to a reaction system, a molar ratio of the linear hydrocarbon compound to the first hydrocarbon compound is 70% or more and 400% or less; A method for producing carbon nanotubes, comprising the steps of: obtaining carbon nanotubes in which the proportion of carbon nanotubes having a double-wall structure with respect to the total number of carbon nanotubes is 75% or more.

2. A method for producing carbon nanotubes as described in claim 1, which obtains carbon nanotubes having an average diameter of 1.8 nm or more and 2.0 nm or less.

3. A method for producing carbon nanotubes as described in claim 1 or 2, wherein the molar ratio of the linear hydrocarbon compound to the first hydrocarbon compound is 70% or more and 300% or less.

4. A method for producing carbon nanotubes as described in any one of claims 1 to 3, except for the case where benzene and toluene are used in combination as the first hydrocarbon compound and methane is used as the linear hydrocarbon compound.

5. 5. The method for producing carbon nanotubes according to claim 1, wherein the first raw material supplying step includes a step of spraying the first raw material in a liquid phase into a reactor.

6. 5. The method for producing carbon nanotubes according to claim 1, wherein the first raw material supplying step includes a step of supplying the first raw material obtained by vaporizing the first raw material in a liquid phase to a reactor.

7. 7. The method for producing carbon nanotubes according to claim 1, wherein the first hydrocarbon compound is at least one hydrocarbon compound selected from the group consisting of aromatic hydrocarbon compounds, alicyclic hydrocarbon compounds, and chain hydrocarbon compounds having 5 or more carbon atoms.

8. 7. The method for producing carbon nanotubes according to claim 1, wherein the first hydrocarbon compound is at least one type of hydrocarbon compound selected from the group consisting of aromatic hydrocarbon compounds and alicyclic hydrocarbon compounds.

9. 9. The method for producing carbon nanotubes according to claim 1, wherein the linear hydrocarbon compound is a hydrocarbon compound having 1 to 4 carbon atoms.

10. 9. The method for producing carbon nanotubes according to claim 1, wherein the linear hydrocarbon compound is at least one hydrocarbon compound selected from the group consisting of methane, ethane, ethylene and acetylene.

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