JP7645686B2 - Method for producing carbon nanotubes - Google Patents

Description

The present invention relates to a method for producing carbon nanotubes, and in particular to a method for producing carbon nanotubes with a high ratio of double-walled structures at an excellent synthesis rate.

Carbon nanotubes are a material with a variety of 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.

In addition, by controlling the number of layers, diameter, etc., of carbon nanotubes, it is possible to further improve various properties such as electrical conductivity, thermal conductivity, and mechanical strength.

On the other hand, electric wires consisting of a core wire made of one or more wire materials 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 for the wire material constituting the core wire from the viewpoint of electrical properties, but in recent years, aluminum or aluminum alloys have been proposed 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, attempts have been made 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.

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

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 between 700 and 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).

However, the manufacturing method described in Patent Document 1 required complex control of the manufacturing process, and the synthesis rate of double-walled carbon nanotubes was not sufficient, leaving room for improvement in the productivity of carbon nanotubes.

JP 2011-148674 A

In view of the above circumstances, the present invention aims to provide a method for producing carbon nanotubes that can produce carbon nanotubes with a high ratio of double-walled structures at an excellent synthesis rate.

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.

In the above aspect [1], a first raw material containing a first hydrocarbon compound and a second raw material containing a gas-phase linear hydrocarbon compound that is 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 that serves as the catalyst and promotes catalytic action at a moderate temperature. The gas-phase linear hydrocarbon compound contained in the second raw material is an additional carbon source for carbon nanotubes.

According to an aspect of the carbon nanotube manufacturing method 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 linear hydrocarbon compound in a gas phase to the first raw material supplied to the reaction system. Since the molar ratio of the linear hydrocarbon compound to the first hydrocarbon compound is 70% or more and 400% or less, carbon nanotubes having a high ratio of two-wall structure can be manufactured at an excellent synthesis rate. In addition, according to an aspect of the carbon nanotube manufacturing method 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 variation in the diameter of the carbon nanotubes.

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 improving the ratio of two-walled structures among the carbon nanotubes obtained.

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, which further increases the ratio of two-walled structures among the carbon nanotubes obtained.

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, which further improves the ratio of double-walled structures and the synthesis rate of carbon nanotubes.

First, the structure of carbon nanotubes will be described. Carbon nanotubes are cylindrical bodies with a single-layer structure or a multi-layer structure, and are called SWNTs (single-walled nanotubes) and MWNTs (multi-walled nanotubes), respectively. Of these, carbon nanotubes with a two-layer structure have a three-dimensional mesh structure in which two cylindrical bodies with a hexagonal lattice mesh structure are arranged approximately coaxially, and are called DWNTs (double-walled nanotubes). 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. In this way, carbon nanotubes are a six-membered ring structure with a three-dimensional mesh structure of a hexagonal lattice, so they have a large surface area and a large number of free electrons, making them a material with excellent conductivity.

For example, carbon nanotubes with a relatively small number of layers, such as a two-layer structure, are relatively more conductive than carbon nanotubes with a larger number of layers, and when doped, the doping effect is greatest with carbon nanotubes with a two-layer 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-layer structure. Specifically, the proportion of carbon nanotubes with a two-layer 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-layer 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.

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%.

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, it is possible to obtain carbon nanotubes having a small average diameter (for example, an average diameter of about 2 nm) as in the conventional method while preventing variation in the diameter of the carbon nanotubes.

(1) First raw material supply step 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 accelerator, to a carbon nanotube synthesis reaction system. The reaction accelerator 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 inlet, which is a raw material introduction means, a reactor in which carbon nanotubes are synthesized, which is provided downstream of the raw material inlet, and a collector, which is provided downstream of the reactor, and collects the synthesized carbon nanotubes. In the carbon nanotube synthesis reaction system, a carrier gas flows from the raw material inlet to the collector via the reactor. Examples of the carrier gas include inert gases such as nitrogen, argon, and helium, and hydrogen gas.

The reactor is equipped with a heating furnace, and the first raw material flows through the reactor interior, which is heated to a predetermined atmospheric temperature by the heating furnace, from the raw material inlet to the collector. The atmospheric temperature inside the reactor can be, for example, 1000°C or higher and 1500°C or lower.

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 accelerator. The first raw material may be supplied to the carbon nanotube synthesis reaction system in a liquid phase, or the liquid phase first raw material may be heated and vaporized, i.e., supplied to the carbon nanotube synthesis reaction system in a gas phase.

As a method for supplying the liquid phase first raw material to the carbon nanotube synthesis reaction system, for example, there is a method in which the liquid phase first raw material is introduced into a raw material inlet provided in the carbon nanotube synthesis reaction system, and the liquid phase first raw material is sprayed from a sprayer provided in the raw material inlet toward the carrier gas flowing from the raw material inlet toward the reactor. In addition, as a method for supplying the gas phase first raw material to the carbon nanotube synthesis reaction system, for example, there is a method in which the liquid phase first raw material is heated and vaporized in a vaporizer, and then the gas phase first raw material is supplied from the raw material inlet to the carrier gas flowing from the raw material inlet toward the reactor.

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

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, because they reliably increase the ratio of double-walled carbon nanotubes among the total carbon nanotubes obtained. 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.

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 among the total carbon nanotubes obtained. These hydrocarbon compounds may be used alone or in combination of two or more.

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 because of their ease of availability. 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.

Examples of reaction accelerators include sulfur and sulfur compounds. Examples of sulfur compounds include carbon disulfide; thiophene; thiophene derivatives such as methylthiophene, ethylthiophene, hexylthiophene, benzothiophene, dibenzothiophene, and tetrahydrothiophene; phenyl sulfide derivatives; phenyl disulfide derivatives; sulfolane; tetramethylene sulfoxide; alkylthiols; and thiophenol. These compounds may be used alone or in combination of two or more.

(2) Second raw material supplying step 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).

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 having a mainly two-walled structure are synthesized in the reactor at an excellent synthesis rate.

As a method for supplying the second raw material in the gas phase to the carbon nanotube synthesis reaction system, for example, 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 can be mentioned. Specifically, for example, 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 can be mentioned. Also, a method of first supplying the second raw material to the carrier gas to which the first raw material has not been supplied from another raw material introducer provided upstream of the raw material introducer can be mentioned. Also, a method of supplying the first raw material from the raw material introducer to the carrier gas and simultaneously supplying the second raw material from the other raw material introducer to the carrier gas can be mentioned. Also, 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 can be mentioned. 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.

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 so 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 more 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 more preferably 200%, from the viewpoint of reliably improving the ratio of carbon nanotubes having a two-wall structure.

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 linear hydrocarbon compounds 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-layer structure and the synthesis rate of carbon nanotubes. These compounds may be used alone or in combination of two or more.

Next, examples of the present invention will be described, but the present invention is not limited to the following examples as long as they do not deviate from the spirit of the present invention.

Production of Carbon Nanotubes Example 1
A liquid first raw material was prepared by mixing toluene as a 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 a second hydrocarbon compound.

A ceramic tube (φ30 mm) was used as a carbon nanotube synthesis reaction system. Specifically, a raw material inlet for introducing the first raw material and another raw material inlet for introducing the second raw material were provided at one end of the ceramic tube. An electric furnace was installed in 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 to serve as a carbon nanotube recovery vessel. A predetermined flow rate of hydrogen gas was flowed from one end of the ceramic tube to the other end 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 to supply the first raw material to the carbon nanotube synthesis reaction system (first raw material supply process). 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 process). 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.

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.

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%.

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.

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.

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.

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.

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.

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%.

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%.

Evaluation item (1) Synthesis rate ratio After 30 minutes from 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.

(2) The ratio of carbon nanotubes having a double-wall structure to the total carbon nanotubes (double-wall ratio) and the average diameter (nm) of the carbon nanotubes
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 with 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 having a two-layer structure to the total carbon nanotubes (two-layer ratio) and the average diameter (nm) of the carbon nanotubes were calculated.

The first and second hydrocarbon compounds 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.

As can be seen from Table 1 above, in Examples 1 to 7, in which a first hydrocarbon compound, which is an aromatic hydrocarbon compound and/or an alicyclic hydrocarbon compound, is supplied to the reaction system in the first raw material supply step, and a linear hydrocarbon compound (second hydrocarbon compound) having 1 to 4 carbon atoms is supplied to the reaction system in the second raw material supply step, and the molar ratio of the second hydrocarbon compound to the first hydrocarbon compound is within the range of 70% to 400%, 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 double-walled structure can be produced at an excellent synthesis rate. In Examples 1 to 7, the average diameter of the carbon nanotubes can be about 2.0 nm, as in the conventional method, while preventing variation in the diameter of the carbon nanotubes.

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

On the other hand, in Comparative Example 1, in which the molar ratio of the second hydrocarbon compound to the first hydrocarbon compound was 500%, the ratio of carbon nanotubes with a two-wall structure to the total carbon nanotubes was 72% by number, and carbon nanotubes with a high ratio of two-wall structure could not be obtained. In Comparative Example 2, in which 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.

The carbon nanotube manufacturing 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 (10)

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 . The method for producing carbon nanotubes according to claim 1 , wherein the carbon nanotubes have an average diameter of 1.8 nm or more and 2.0 nm or less. 3. The method for producing carbon nanotubes according to claim 1, wherein a molar ratio of the linear hydrocarbon compound to the first hydrocarbon compound is 70% or more and 300% or less. 4. The method for producing carbon nanotubes according to claim 1, except for a case in which benzene and toluene are used in combination as the first hydrocarbon compound and methane is used as the straight-chain hydrocarbon compound. 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. 5. The method for producing carbon nanotubes according to claim 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. 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. 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 and alicyclic hydrocarbon compounds. 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. 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.