JP7689785B1 - Carbon nanotube assembly - Google Patents

Abstract

The present invention provides a novel aggregate of carbon nanotubes.
[Solution] An aggregate of carbon nanotubes including a plurality of CNTs, wherein when 110 or more CNTs constituting the aggregate are observed with a transmission electron microscope, where the most frequently observed number of walls is n (n is an integer), the total percentage of CNTs having numbers of walls from n-1 to n+1 is 20-38%, and the total percentage of CNTs having numbers of walls from n-2 to n+2 is 35-80%, relative to a total of 100% of all observed CNTs; where the average value of the outer diameter of CNTs having number of walls n is X (nm), the average value of the outer diameter of CNTs having number of walls n-1 is X-2.8 (nm) to X+0.5 (nm), the average value of the outer diameter of CNTs having number of walls n+1 is X-0.5 (nm) to X+2.8 (nm), and the average value of the outer diameter of all observed CNTs is X-1.5 (nm) to X+1.5 (nm).
[Selection diagram] None

Description

This disclosure relates to an aggregate of carbon nanotubes.

Carbon nanotubes are used in electrical and electronic equipment and transportation machinery. For example, carbon nanotube films are sometimes used for heaters (see, for example, Patent Document 1).

JP 2010-257971 A

The present disclosure aims to provide a novel carbon nanotube aggregate.

One embodiment of the carbon nanotube (CNT) aggregate of the present disclosure includes a plurality of CNTs, and when 110 or more CNTs constituting the aggregate are observed with a transmission electron microscope (TEM), if the most frequently observed number of layers is n (n is an integer), the total percentage of CNTs with the number of layers being n-1 to n+1 is 20 to 38%, and the total percentage of CNTs with the number of layers being n-2 to n+2 is 35 to 80%, relative to a total of 100% of all observed CNTs. In addition, if the average value of the outer diameter of CNTs with n layers is X (nm), the average value of the outer diameter of CNTs with n-1 layers is X-2.8 (nm) to X+0.5 (nm), the average value of the outer diameter of CNTs with n+1 layers is X-0.5 (nm) to X+2.8 (nm), and the average value of the outer diameter of all observed CNTs is X-1.5 (nm) to X+1.5 (nm).

The present disclosure provides a novel carbon nanotube aggregate.

FIG. 1 is an example of a TEM image of the carbon nanotubes obtained in Example 1.

In this specification, the numerical range A to B means greater than or equal to A and less than or equal to B. In this specification, when the units of the numbers written before and after "~" indicating a numerical range are the same, the unit of the numbers written before "~" may be omitted.

In this specification, carbon nanotubes are also referred to as "CNTs", carbon nanotube forests are also referred to as "CNT forests", carbon nanotube fibers are also referred to as "CNT fibers", and carbon nanotube webs are also referred to as "CNT webs".

[Aggregation of carbon nanotubes]
The aggregate of carbon nanotubes (CNTs) of the present disclosure includes a plurality of CNTs. When 110 or more CNTs constituting the aggregate are observed with a transmission electron microscope (TEM), if the most frequently observed number of layers is n (n is an integer), the total percentage of CNTs having the number of layers n-1 to n+1 is 20 to 38%, and the total percentage of CNTs having the number of layers n-2 to n+2 is 35 to 80%, relative to a total of 100% of all observed CNTs. In addition, if the average value of the outer diameter of CNTs having the number of layers n is X (nm), the average value of the outer diameter of CNTs having the number of layers n-1 is X-2.8 (nm) to X+0.5 (nm), the average value of the outer diameter of CNTs having the number of layers n+1 is X-0.5 (nm) to X+2.8 (nm), and the average value of the outer diameter of all observed CNTs is X-1.5 (nm) to X+1.5 (nm).

The CNT aggregate of the present disclosure includes a plurality of CNTs.
CNTs can be produced using methods such as thermal chemical vapor deposition (thermal CVD), plasma CVD, laser ablation, arc discharge, or combustion.

The CNT aggregate of the present disclosure is, for example, a CNT forest provided on a substrate, or an aggregate obtained from a CNT forest. A CNT forest refers to an aggregate of multiple CNTs provided on a substrate and aligned in a direction perpendicular to the substrate surface. In a CNT forest, multiple CNTs stand tall on the substrate.
The aggregate obtained from the CNT forest is, for example, a powder-like aggregate of CNTs. Note that the powder-like aggregate of CNTs is also simply referred to as powder-like CNTs.

The number of layers, outer diameter, inner diameter, etc. of the CNTs calculated by observing the 110 or more CNTs that make up the CNT aggregate of the present disclosure with a transmission electron microscope (TEM) are described below. Observing CNTs using a transmission electron microscope (TEM) is also called "TEM observation," and an image obtained by TEM observation is also called a "TEM image."

The 110 or more CNTs to be observed with a TEM can be arbitrarily selected from the CNT aggregate. However, if a CNT with a changed number of walls is observed during TEM observation, a CNT with 110 or more CNTs is selected from the CNT aggregate so as not to include the CNT.
When the CNT assembly is a CNT forest disposed on a substrate, selecting 110 or more CNTs from the CNT assembly refers to harvesting 110 or more CNTs from the CNT forest.
Details of the observation method using a TEM will be described in the Examples section.

The CNT aggregate may include single-walled carbon nanotubes.

When 110 or more CNTs are observed using a TEM, if the most commonly observed number of layers is n (n is an integer), the total percentage of CNTs with layers between n-1 and n+1 is 20-38%, and the total percentage of CNTs with layers between n-2 and n+2 is 35-80%, out of a total of 100% of all observed CNTs.

The total proportion of CNTs with n-1 to n+1 walls refers to the proportion of the total number (number) of CNTs with n-1 to n+1 walls to the total number (number) of CNTs observed by TEM. The same applies to the total proportion of CNTs with n-2 to n+2 walls and the proportion of CNTs with n walls.
When 110 or more CNTs are observed under TEM, if the most commonly observed number of walls is two or more, the most commonly observed number of walls that is closest to the median of all the observed numbers of walls is defined as n. The median refers to the median in the distribution of the number of walls of CNTs, and more specifically, refers to the value (number of walls) at which the cumulative value in the cumulative distribution of the number of walls of CNTs is 50%.

When n is 1, the total proportion of CNTs having n-1 to n+1 walls means the total proportion of CNTs having 1 to 2 walls, and the total proportion of CNTs having n-2 to n+2 walls means the total proportion of CNTs having 1 to 3 walls.
When n is 2, the total proportion of CNTs having n-1 to n+1 walls means the total proportion of CNTs having 1 to 3 walls, and the total proportion of CNTs having n-2 to n+2 walls means the total proportion of CNTs having 1 to 4 walls.

n is preferably 3 or more, more preferably 3 to 20, even more preferably 4 to 15, still more preferably 5 to 15, and particularly preferably 7 to 13.
The total proportion of CNTs having wall numbers from n-1 to n+1 is preferably 22 to 36%, more preferably 24 to 35%, and even more preferably 26 to 34%.
The total proportion of CNTs having wall numbers from n-2 to n+2 is preferably 40 to 76%, more preferably 45 to 73%, and even more preferably 50 to 70%.
When the total proportion of CNTs with wall numbers n-1 to n+1 and n-2 to n+2 is within the above range, the CNT aggregate, particularly the CNT forest provided on the substrate, has excellent spinnability, and the CNT aggregate also has excellent dispersibility. In other words, the CNT aggregate of the present disclosure has an excellent balance between spinnability and dispersibility.

The proportion of CNTs having n walls is preferably 10 to 30%, more preferably 15 to 29%, and even more preferably 17 to 28%.
When the ratio of CNTs having n layers is within the above range, the CNT aggregate, particularly the CNT forest provided on the substrate, has excellent spinnability, and the CNT aggregate also has excellent dispersibility. In other words, the CNT aggregate of the present disclosure has an excellent balance between spinnability and dispersibility.

When the distribution of the number of layers of the CNTs that make up the CNT aggregate is wide, the total proportion of CNTs with layer numbers n, n-1 to n+1, and n-2 to n+2 becomes small.

The value of n and the total proportion of CNTs with layer numbers of n, n-1 to n+1, and n-2 to n+2 can be adjusted, for example, in the manufacturing method of CNT aggregates described below, by adjusting the type of substrate used for the catalyst substrate, the presence or absence of a buffer layer, the type and thickness of the buffer layer, the type and thickness of the catalyst layer, the pressure in the reaction chamber in the CVD method, and the flow rates of the source gas and carrier gas. These amounts may be adjusted while successively performing TEM observations.

When 110 or more CNTs were observed under a TEM, if the average outer diameter of CNTs with n layers is X (nm), the average outer diameter of CNTs with n-1 layers is X-2.8 (nm) to X+0.5 (nm), and the average outer diameter of CNTs with n+1 layers is X-0.5 (nm) to X+2.8 (nm). In addition, the average outer diameter of all CNTs observed under a TEM (used to calculate n) is X-1.5 (nm) to X+1.5 (nm).

The outer diameter of a CNT refers to the diameter of the outermost layer of the CNT measured using images obtained by TEM observation, and if amorphous material or the like is attached to the outside of the outermost layer, it refers to the diameter including the amorphous material.

From the viewpoint of providing a CNT aggregate, particularly a CNT forest provided on a substrate, with excellent spinnability and providing a CNT aggregate with excellent dispersibility, X is preferably 1 to 30 nm, more preferably 3 to 25 nm, even more preferably 5 to 20 nm, and particularly preferably 10.0 to 20.0 nm.
The average outer diameter of CNTs with n walls refers to the arithmetic average of the outer diameters of CNTs with n walls. The same applies to the average outer diameter of CNTs with n-1 walls, the average outer diameter of CNTs with n+1 walls, and the average outer diameter of all CNTs observed under TEM.

The average outer diameter of CNTs with n-1 walls is preferably X-2.6 (nm) to X+0.5 (nm), more preferably X-2.5 (nm) to X+0.5 (nm), and even more preferably X-2.4 (nm) to X+0.4 (nm), from the viewpoints that the CNT aggregate, particularly the CNT forest provided on the substrate, has excellent spinnability and that the CNT aggregate has excellent dispersibility.

The average outer diameter of CNTs with n+1 layers is preferably X-0.5 (nm) to X+2.6 (nm), more preferably X-0.5 (nm) to X+2.5 (nm), and even more preferably X-0.4 (nm) to X+2.4 (nm), from the viewpoints that the CNT aggregate, particularly the CNT forest provided on the substrate, has excellent spinnability and that the CNT aggregate has excellent dispersibility.

The average outer diameter of all CNTs observed under TEM is preferably X-1.3 (nm) to X+1.3 (nm), more preferably X-1.1 (nm) to X+1.1 (nm), and even more preferably X-0.9 (nm) to X+0.9 (nm), from the viewpoints that the CNT aggregate, particularly the CNT forest provided on the substrate, has excellent spinnability and that the CNT aggregate has excellent dispersibility.

The outer diameter of the CNTs can be adjusted, for example, in the manufacturing method of CNT aggregates described below, by adjusting the type of substrate used for the catalyst substrate, the presence or absence of a buffer layer, the type and thickness of the buffer layer, the type and thickness of the catalyst layer, the pressure in the reaction chamber in the CVD method, and the flow rates of the source gas and carrier gas. These amounts may be adjusted while successively performing TEM observations.

The average inner diameter of all CNTs observed under TEM (the number of layers was observed and used to calculate n) is preferably 1.0 to 6.0 nm, more preferably 3.5 to 6.0 nm, even more preferably 3.7 to 5.7 nm, and particularly preferably 4.0 to 5.5 nm, from the viewpoint that the CNT aggregate, particularly the CNT forest provided on the substrate, has excellent spinnability and the CNT aggregate has excellent dispersibility.

The inner diameter of a CNT refers to the diameter of the innermost layer of a CNT, which is measured using an image obtained by TEM observation.
The average value of the inner diameters of all CNTs observed under a TEM refers to the arithmetic average of the inner diameters of all CNTs observed under a TEM.

The inner diameter of the CNTs can be adjusted, for example, in the manufacturing method of CNT aggregates described below, by adjusting the type of substrate used for the catalyst substrate, the presence or absence of a buffer layer, the type and thickness of the buffer layer, the type and thickness of the catalyst layer, the pressure in the reaction chamber in the CVD method, and the flow rates of the source gas and carrier gas. These amounts may be adjusted while successively performing TEM observations.

The average length of the CNTs constituting the CNT aggregate is preferably 10 to 1000 μm, more preferably 30 to 800 μm, and even more preferably 50 to 500 μm. The average length of the CNTs can be adjusted, for example, by adjusting the time for which the CVD method is performed, i.e., the growth time of the CNTs.
The average length of CNT refers to the arithmetic average of the lengths of all CNTs observed using a scanning electron microscope (SEM). Specifically, the average length of CNT is measured by obtaining 10 images of CNTs using an SEM. For each of the 10 images, 10 length measurement points are arbitrarily selected and measured, measuring the length of a total of 100 points, and then arithmetically averaging the measured length values of the 100 points to obtain the average length of CNT.

The carbon purity of the CNTs constituting the CNT aggregate is preferably 95.0 to 99.999%. The lower limit of the carbon purity of the CNTs is preferably 96.0%, more preferably 97.0%, even more preferably 98.0%, even more preferably 99.0%, and particularly preferably 99.8%. The upper limit of the carbon purity of the CNTs may be, for example, 99.99% or 99.9%. The carbon purity of the CNTs can be determined, for example, by elemental analysis using fluorescent X-rays. In this disclosure, the percentage of carbon purity means mass%.

The crystallinity of the CNTs constituting the CNT aggregate can be evaluated, for example, by using Raman spectroscopy. In the evaluation of crystallinity by Raman spectroscopy, the value of the D/G ratio is used as an index. The D/G ratio is the ratio of the peak intensity of the D band appearing near 1360 cm ⁻¹ to the peak intensity of the G band appearing near 1580 cm ⁻¹ in the Raman spectrum measured by Raman spectroscopy. The smaller the value of the D/G ratio, the higher the crystallinity of the carbon nanotubes. The D/G ratio of the CNTs is preferably 0.5 to 1.0, more preferably 0.6 to 0.8.

The carbon purity and crystallinity of the CNTs can be adjusted, for example, in the manufacturing method of CNT aggregates described below, by adjusting the thickness of the buffer layer in the catalyst substrate, the type of material used for the buffer layer, the thickness of the catalyst layer, the type of catalyst, the type and flow rate of the source gas in the CVD method, and the temperature and pressure in the reaction chamber.

[Method for producing aggregates of carbon nanotubes]
The CNT aggregate of the present disclosure is, for example, a CNT forest provided on a substrate, or an aggregate obtained from a CNT forest. The CNT forest can be produced, for example, by the method described below. An aggregate obtained from a CNT forest (for example, a powder-like CNT aggregate) can be obtained, for example, by scraping off the CNTs from the substrate from the CNT forest using a scraper or the like.

CNT forests can be obtained, for example, by performing chemical vapor deposition (CVD) using a catalyst substrate that includes a substrate and a catalyst layer provided on the substrate. CVD is a method in which the catalyst substrate is placed in a reaction chamber, and then raw material gas is supplied into the reaction chamber to grow CNTs on the surface of the catalyst layer. As a CVD method, thermal CVD is preferred.

Examples of substrates include silicon substrates, alumina substrates, magnesium oxide substrates, glass substrates, sapphire substrates, titanium substrates, and stainless steel substrates.

From the viewpoint of ease of handling and cost of the substrate, the thickness of the substrate is preferably 0.03 to 2.0 mm, more preferably 0.05 to 1.8 mm, even more preferably 0.07 to 1.6 mm, and particularly preferably 0.09 to 1.4 mm.

The catalyst layer can be formed by depositing catalyst particles onto the substrate, for example, by sputtering.
Examples of the catalyst include metals, specifically, iron (Fe), nickel (Ni), cobalt (Co), molybdenum (Mo), gold (Au), and alloys containing at least one metal selected from the group consisting of these. Examples of the alloy include iron alloys, nickel alloys, and cobalt alloys. The catalyst may be a precursor of a metal, such as a metal oxide or a metal compound. Examples of the metal oxide include iron oxide, nickel oxide, and cobalt oxide. Examples of the metal compound include iron chloride. When a precursor is used, it is necessary to convert the precursor into a metal before carrying out the CVD method, such as by heating the precursor.
By changing the type of catalyst, the number of walls, outer diameter, and inner diameter of the CNTs can be changed.

The thickness of the catalyst layer is preferably 1 to 20 nm, more preferably 2 to 17 nm, even more preferably 3 to 15 nm, and particularly preferably 4 to 10 nm. The thicker the catalyst layer, the larger the number of layers and the outer diameter of the CNTs tend to be. The thinner the catalyst layer, the narrower the distribution of the number of layers of the CNTs that make up the CNT aggregates tends to be.

The catalyst substrate may further include a buffer layer between the substrate and the catalyst layer. Examples of materials used for the buffer layer include silica ( _SiO2 ), alumina ( _Al2O3 ), silicon nitride (SiN), zinc oxide ( _ZnO ), copper oxide ( _Cu2O ), and nickel oxide (NiO). The buffer layer can be formed by, for example, sputtering.
The outer and inner diameters of the CNTs can be changed by changing the type of material used for the buffer layer. For example, under one manufacturing condition, when the material used for the buffer layer is alumina ( _{Al2O3 )} , the outer and inner diameters of the CNTs tend to be small, and when the material is silica ( _SiO2 ), the outer and inner diameters of the CNTs tend to be large.

The thickness of the buffer layer may be, for example, 10 to 100 nm, 20 to 80 nm, or 30 to 60 nm.

The sputtering for forming the catalyst layer and the sputtering for forming the buffer layer can be performed using known equipment and conditions depending on the target of sputtering. The pressure conditions for sputtering are preferably about 0.01 to 10 Pa, and more preferably about 0.1 to 1 Pa.

As the raw material gas, a raw material gas containing carbon can be used, and examples thereof include hydrocarbons, sulfur-containing organic gases, phosphorus-containing organic gases, carbon monoxide, and alcohols. Examples of the hydrocarbons include alkane compounds such as methane and ethane, alkene compounds such as ethylene and butadiene, alkyne compounds such as acetylene, aryl hydrocarbon compounds such as benzene, toluene, and styrene, aromatic hydrocarbons having condensed rings such as indene, naphthalene, and phenanthrene, cycloalkane compounds such as cyclopropane and cyclohexane, cycloolefin compounds such as cyclopentene, and alicyclic hydrocarbon compounds having condensed rings such as steroids. Examples of the alcohols include methanol and ethanol. From the viewpoint of the carbon purity of the obtained CNTs, the raw material gas is preferably a hydrocarbon.

The flow rate of the source gas can be appropriately set depending on the size of the reaction chamber in the CVD method, the size of the substrate, etc. For example, when a CVD apparatus is used in which the volume of the quartz reaction tube is 2.0×10 ⁻³ m ³ and the heating zone is 60% of the quartz reaction tube, and the substrate size is 2 inches in diameter, the flow rate of the source gas may be 5 to 100 sccm, 7 to 80 sccm, 10 to 60 sccm, or 15 to 40 sccm.

A carrier gas, which is a gas that transports the raw material gas, may be supplied to the reaction chamber together with the raw material gas. Examples of carrier gases include helium, neon, argon, nitrogen, and hydrogen. Hydrogen is also called a reactive carrier gas, as it is believed to contribute to the productivity and quality of carbon nanotubes.

The flow rate of the carrier gas can be appropriately set depending on the size of the apparatus used in the CVD method, the size of the substrate, etc. For example, when a CVD apparatus is used in which the volume of the quartz reaction tube is 2.0×10 ⁻³ m ³ and the heating zone is 60% of the quartz reaction tube, and the substrate size is 2 inches in diameter, the flow rate of the carrier gas is preferably 50 to 2500 sccm, more preferably 200 to 2200 sccm, further preferably 300 to 2000 sccm, and particularly preferably 400 to 1900 sccm.

The temperature in the reaction chamber in the CVD method is preferably 600 to 850° C., more preferably 650 to 800° C., from the viewpoints of the growth rate of CNTs and the carbon purity of the obtained CNTs.
The pressure in the reaction chamber in the CVD method is preferably normal pressure from the viewpoint of the growth rate of CNTs and the carbon purity. The pressure in the reaction chamber may be reduced or increased from normal pressure depending on other conditions when the CVD method is performed. Under certain manufacturing conditions, the higher the pressure in the reaction chamber, the narrower the distribution of the number of layers of the CNTs constituting the CNT aggregate tends to be.
In the production of the CNT aggregate of the present disclosure, the pressure in the reaction chamber is preferably 450 to 800 torr, more preferably 480 torr to 750 torr, further preferably 510 torr to 700 torr, and particularly preferably 540 torr to 650 torr.

The average length of the CNTs in the CNT forest is, for example, similar to the average length of the CNTs described above.

[Uses of carbon nanotube aggregates]
The CNT aggregates of the present disclosure can be used, for example, for sports and leisure applications, such as shoes, fishing rods, golf shafts, and tennis rackets; for electrical and electronic equipment applications, such as secondary batteries, heat dissipation materials, electrode sheets, electromagnetic wave shields, electromagnetic wave absorbing sheets, antistatic sheets, battery parts, electronic parts, and housings for notebook computers, tablets, and smartphones; for construction applications, such as building materials; for transportation machinery applications, such as automobiles, motorcycles, bicycles, trains, drones, rockets, aircraft, and ships; for energy applications, such as hydroelectric generators and wind power generators; and for fashion applications, such as clothing and bags.

The CNT aggregates of the present disclosure, particularly the CNT forests provided on a substrate, are suitable for use in applications that use carbon nanotube fibers (CNT fibers) or carbon nanotube webs (CNT webs). Applications that use CNT fibers or CNT webs include, for example, applications that use CNT films, and specifically, in addition to the applications mentioned above, heaters are also included.

The CNT web can be produced, for example, by using an aggregate of CNTs in a CNT forest state provided on a substrate, and by drawing out a number of CNTs from the CNT forest, specifically by drawing out a number of CNTs in a sheet form. More specifically, the CNT web can be produced, for example, by using a pinching tool such as tweezers to pull out the CNTs located at the ends of the CNTs that make up the CNT forest, away from the CNT forest in a direction parallel to the surface of the substrate on which the CNT forest is provided. When the CNTs located at the ends of the CNT forest are pulled out, the CNTs adjacent to the pulled out CNTs are pulled out one by one due to van der Waals forces. The pulled out CNTs are oriented so that their longitudinal direction is aligned with the direction in which they are pulled out. Therefore, the multiple CNTs that make up the CNT fiber are oriented in one direction. The multiple CNTs that make up the CNT fiber are bonded to each other by van der Waals forces. This results in a CNT web that is composed of multiple CNT fibers that extend in the direction in which the CNTs are pulled out.

CNT films containing CNT fibers can be produced, for example, by producing multiple CNT webs obtained by extracting multiple CNTs from a CNT forest into sheets, and then laminating the CNT webs; or by producing a roll by wrapping a CNT web obtained by extracting multiple CNTs from a CNT forest around the periphery of a roller or the like multiple times, and then cutting the roll open along the direction of the roller's rotation axis.

The CNT aggregate of the present disclosure, particularly the CNT forest provided on a substrate, has excellent spinnability and excellent dispersibility due to the physical properties such as the number of CNT layers, outer diameter, inner diameter, and length being within the above ranges. If the CNT aggregate has excellent spinnability, there is less risk of the CNT fiber and CNT web being cut while the CNTs are being pulled out of the CNT forest when producing CNT fibers and CNT webs using the CNT forest. If the CNT aggregate has excellent dispersibility, it is suitable, for example, as a conductive auxiliary agent in secondary batteries such as lithium ion secondary batteries.

Examples of the dispersion medium for the CNT aggregates include water and organic solvents. Examples of the organic solvent include water-soluble organic solvents, specifically alcohol solvents, polyhydric alcohol ether solvents, amine solvents, amide solvents, heterocyclic solvents, sulfoxide solvents, sulfone solvents, lower ketone solvents, urea, and acetonitrile.

Examples of alcohol-based solvents include methanol, ethanol, propanol, isopropanol, butanol, isobutanol, ethylene glycol, diethylene glycol, polyethylene glycol, propylene glycol, dipropylene glycol, and polypropylene glycol.
Examples of polyhydric alcohol ether solvents include ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, ethylene glycol monobutyl ether, diethylene glycol monomethyl ether, diethylene glycol monoethyl ether, diethylene glycol monobutyl ether, propylene glycol monomethyl ether, and propylene glycol monoethyl ether.

Examples of the amine solvent include ethanolamine, diethanolamine, triethanolamine, N-methyldiethanolamine, N-ethyldiethanolamine, morpholine, N-ethylmorpholine, ethylenediamine, and diethylenediamine.
Examples of the amide solvent include N-methyl-2-pyrrolidone (NMP), N-ethyl-2-pyrrolidone (NEP), N,N-dimethylformamide, N,N-dimethylacetamide, N,N-diethylacetamide, and N-methylcaprolactam.

Heterocyclic solvents include, for example, tetrahydrofuran, cyclohexylpyrrolidone, 2-oxazolidone, and 1,3-dimethyl-2-imidazolidinone.
An example of the sulfoxide solvent is dimethyl sulfoxide.
Sulfone solvents include, for example, hexamethylphosphorotriamide and sulfolane.
Examples of lower ketone solvents include acetone and methyl ethyl ketone.

The present disclosure has, for example, the following aspects.
[1]
An aggregate of carbon nanotubes (CNTs) including a plurality of CNTs;
When the 110 or more CNTs constituting the aggregate were observed with a transmission electron microscope,
When the most frequently observed number of layers is n (n is an integer), the total number of layers is 100% of all the CNTs observed.
the total percentage of CNTs having a number of walls of n-1 to n+1 is 20 to 38%,
the total percentage of CNTs having a number of walls of n-2 to n+2 is 35 to 80%,
If the average outer diameter of a CNT with n walls is X (nm),
the average outer diameter of the CNTs having n-1 walls is X-2.8 (nm) to X+0.5 (nm);
the average outer diameter of the CNTs having n+1 walls is X-0.5 (nm) to X+2.8 (nm);
The average outer diameter of all the observed CNTs is X-1.5 (nm) to X+1.5 (nm);
An aggregate of carbon nanotubes.

[2]
The aggregate of carbon nanotubes according to [1], wherein the average inner diameter of all the observed CNTs is 1.0 to 6.0 nm.

[3]
The aggregate of carbon nanotubes according to [1] or [2], wherein n is 3 or more.

[4]
The carbon nanotube aggregate according to any one of [1] to [3], wherein the CNT aggregate is a CNT forest provided on a substrate.

[5]
The carbon nanotube aggregate according to any one of [1] to [3], wherein the CNT aggregate is an aggregate obtained from a CNT forest.

The CNT aggregates of the present disclosure will be described in more detail below with reference to examples, but the CNT aggregates of the present disclosure are not limited to these examples.

[Example 1]
First, vertically aligned CNTs were grown from a catalyst by the following steps (1) to (6), and a vertically aligned CNT forest aligned perpendicular to the wafer was produced.
(1) A 40 nm thick buffer layer of silica (SiO 2 ) was formed on a titanium metal foil substrate having a diameter of 2 inches and a thickness of 0.1 mm by reactive sputtering using silicon (Si) as a target while introducing 200 sccm of argon and 50 _sccm of oxygen to react with the silicon.
(2) On the silica buffer layer, a 5 nm-thick iron catalyst layer was uniformly formed by sputtering to obtain a catalyst substrate.
(3) The catalyst substrate was placed in the center of the heating zone in the CVD apparatus, and after evacuation, the temperature inside the furnace (reaction chamber) was raised to 730° C. to activate the catalyst particles (iron particles). The CVD apparatus used had a quartz reaction tube with a volume of 2.0×10 ⁻³ m ³ and a heating zone that was 60% of the quartz reaction tube.
(4) Nitrogen gas was introduced at 1,497 sccm, and the pressure inside the furnace was kept at 600 torr, creating a carrier gas (nitrogen gas) atmosphere inside the furnace.
(5) After the temperature inside the furnace was stabilized at 729° C., acetylene gas (C ₂ H ₂ ) was further introduced at 20 sccm and hydrogen gas at 100 sccm without changing the amount of nitrogen gas introduced, and CNTs were grown for 10 minutes.
(6) The furnace was then cooled, and the catalyst substrate and the CNT forest (a substrate with CNTs growing on top of it) were removed.

Next, the CNT forest formed on the catalyst substrate was scraped off the substrate using a scraper to obtain powdered CNTs.

The carbon purity of the CNTs making up the CNT forest was 99.8% or more, and the crystallinity (D/G ratio) was 0.6 to 0.8.
In addition, 114 CNTs were randomly selected from the obtained powder-like CNTs and subjected to TEM observation, which will be described later. The observation results are shown in Table 1.

A total of 114 CNTs were observed, and the most commonly observed number of walls was 10.
The total proportion of CNTs having 9 to 11 walls was 32.5%, and the total proportion of CNTs having 8 to 12 walls was 64.0%.

The average outer diameter of the CNTs with 10 walls was 10.1 nm, the average outer diameter of the CNTs with 9 walls was 10.4 nm, and the average outer diameter of the CNTs with 11 walls was 12.4 nm.
The average outer diameter of 114 CNTs observed under TEM was 10.3 nm, and the average inner diameter was 4.9 nm.
SEM observation revealed that the average length of each CNT making up the CNT forest was 250 μm.

Figure 1 shows an example of a TEM image of the CNT obtained in Example 1. The length D in Figure 1 is an example of the outer diameter of the CNT, and the length d is an example of the inner diameter.

[Comparative Example 1]
First, vertically aligned CNTs were grown from a catalyst by the following steps (1) to (6), and a vertically aligned CNT forest aligned perpendicular to the wafer was produced.
(1) On a titanium metal foil substrate having a diameter of 2 inches and a thickness of 0.1 mm, a buffer layer of alumina (Al ₂ O ₃ ) having a thickness of 40 nm was formed by reactive sputtering using aluminum (Al) as a target while introducing 98 sccm of argon and 21 sccm of oxygen to react with the aluminum.
(2) On the alumina buffer layer, a 5 nm-thick iron catalyst layer was uniformly formed by sputtering to obtain a catalyst substrate.
(3) The catalyst substrate was placed in the center of the heating zone in the CVD apparatus, and after evacuation, the temperature inside the furnace (reaction chamber) was raised to 730° C. to activate the catalyst particles (iron particles). The CVD apparatus used had a quartz reaction tube with a volume of 2.0×10 ⁻³ m ³ and a heating zone that was 60% of the quartz reaction tube.
(4) Nitrogen gas was introduced at 1,498 sccm, and the pressure inside the furnace was kept at 749 torr, creating a carrier gas (nitrogen gas) atmosphere inside the furnace.
(5) After the temperature inside the furnace was stabilized at 730° C., 21 sccm of acetylene gas (C ₂ H ₂ ) and 99 sccm of hydrogen gas were further introduced without changing the amount of nitrogen gas introduced, and CNTs were grown for 10 minutes.
(6) The furnace was then cooled, and the catalyst substrate and the CNT forest (a substrate with CNTs growing on top of it) were removed.

Next, the CNT forest formed on the catalyst substrate was scraped off the substrate using a scraper to obtain powdered CNTs.

The carbon purity of the CNTs making up the CNT forest was 99.8% or more, and the crystallinity (D/G ratio) was 0.6 to 0.8.
In addition, 161 CNTs were randomly selected from the obtained powder-like CNTs and were subjected to TEM observation, which will be described later. The observation results are shown in Table 2.

A total of 161 CNTs were observed, and the most commonly observed number of walls was six.
The total percentage of CNTs having 5 to 7 walls was 47.8%, and the total percentage of CNTs having 4 to 8 walls was 80.1%.

The average outer diameter of the CNTs with six walls was 7.6 nm, the average outer diameter of the CNTs with five walls was 7.4 nm, and the average outer diameter of the CNTs with seven walls was 7.5 nm.
The average outer diameter of 161 CNTs observed under TEM was 7.5 nm, and the average inner diameter was 4.1 nm.
SEM observation revealed that the average length of each CNT making up the CNT forest was 250 μm.

[Comparative Example 2]
First, vertically aligned CNTs were grown from a catalyst by the following steps (1) to (6), and a vertically aligned CNT forest aligned perpendicular to the wafer was produced.
(1) A 40 nm thick buffer layer of silica (SiO 2 ) was formed on a titanium metal foil substrate having a diameter of 2 inches and a thickness of _0.1 mm by reactive sputtering using silicon (Si) as a target and introducing 201 sccm of argon and 49 sccm of oxygen to react with the silicon.
(2) On the silica buffer layer, a 5 nm-thick iron catalyst layer was uniformly formed by sputtering to obtain a catalyst substrate.
(3) The catalyst substrate was placed in the center of the heating zone in the CVD apparatus, and after evacuation, the temperature inside the furnace (reaction chamber) was raised to 730° C. to activate the catalyst particles (iron particles). The CVD apparatus used had a quartz reaction tube with a volume of 2.0×10 ⁻³ m ³ and a heating zone that was 60% of the quartz reaction tube.
(4) Nitrogen gas was introduced at 1502 sccm, and the pressure inside the furnace was kept at 751 torr, creating a carrier gas (nitrogen gas) atmosphere inside the furnace.
(5) After the temperature inside the furnace was stabilized at 730° C., acetylene gas (C ₂ H ₂ ) was further introduced at 20 sccm and hydrogen gas at 98 sccm without changing the amount of nitrogen gas introduced, and CNTs were grown for 10 minutes.
(6) The furnace was then cooled, and the catalyst substrate and the CNT forest (a substrate with CNTs growing on top of it) were removed.

Next, the CNT forest formed on the catalyst substrate was scraped off the substrate using a scraper to obtain powdered CNTs.

The carbon purity of the CNTs making up the CNT forest was 99.8% or more, and the crystallinity (D/G ratio) was 0.6 to 0.8.
In addition, 166 CNTs were randomly selected from the obtained powder-like CNTs and subjected to TEM observation, which will be described later. The observation results are shown in Table 3.

A total of 166 CNTs were observed, and the most commonly observed number of walls was six.
The total percentage of CNTs having 5 to 7 walls was 42.2%, and the total percentage of CNTs having 4 to 8 walls was 65.1%.
The average outer diameter of the CNTs with six walls was 9.3 nm, the average outer diameter of the CNTs with five walls was 8.3 nm, and the average outer diameter of the CNTs with seven walls was 10.7 nm.
The average outer diameter of 166 CNTs observed under TEM was 11.5 nm, and the average inner diameter was 5.4 nm.
SEM observation revealed that the average length of each CNT making up the CNT forest was 246 μm.

[Comparative Example 3]
First, vertically aligned CNTs were grown from a catalyst by the following steps (1) to (5), and a vertically aligned CNT forest aligned perpendicular to the wafer was produced.
(1) A catalyst substrate was manufactured by forming a uniform iron catalyst layer having a thickness of 3 nm on a silicon wafer having a diameter of 2 inches and a thickness of 0.725 mm by a sputtering method using iron (Fe) as a target.
(2) The catalyst substrate was placed in the center of the heating zone in the CVD apparatus, and after evacuation, the temperature inside the furnace (reaction chamber) was raised to 730° C. to activate the catalyst particles (iron particles). The CVD apparatus used had a quartz reaction tube with a volume of 2.0×10 ⁻³ m ³ and a heating zone that was 60% of the quartz reaction tube.
(3) Nitrogen gas was introduced at 1502 sccm, and the pressure inside the furnace was kept at 751 torr, creating a carrier gas (nitrogen gas) atmosphere inside the furnace.
(4) After the temperature inside the furnace was stabilized at 732° C., acetylene gas (C ₂ H ₂ ) was further introduced at 20 sccm and hydrogen gas at 279 sccm without changing the amount of nitrogen gas introduced, and CNTs were grown for 10 minutes.
(5) The furnace was then cooled, and the catalyst substrate and the CNT forest (a substrate with CNTs growing on top of it) were removed.

Next, the CNT forest formed on the catalyst substrate was scraped off the substrate using a scraper to obtain powdered CNTs.

The carbon purity of the CNTs making up the CNT forest was 99.8% or more, and the crystallinity (D/G ratio) was 0.6 to 0.8.
In addition, 115 CNTs were randomly selected from the obtained powder-like CNTs and subjected to TEM observation, which will be described later. The observation results are shown in Table 4.

A total of 115 CNTs were observed, and the median number of walls observed was six, with six being the most commonly observed number of walls.
The total percentage of CNTs having 5 to 7 walls was 83.5%, and the total percentage of CNTs having 4 to 8 walls was 95.7%.

The average outer diameter of the CNTs with six walls was 9.2 nm, the average outer diameter of the CNTs with five walls was 8.1 nm, and the average outer diameter of the CNTs with seven walls was 10.6 nm.
The average outer diameter of 115 CNTs observed under TEM was 9.2 nm, and the average inner diameter was 3.9 nm.
SEM observation revealed that the average length of each CNT making up the CNT forest was 247 μm.

[TEM Observation]
The powdered CNTs produced in the Examples and Comparative Examples were subjected to TEM observation and measurements of the number of layers, outer diameter, and inner diameter of the CNTs by the following methods (1) to (3).
(1) The obtained powdered CNTs were dispersed in ethanol and dropped onto a microgrid.
(2) The dried sample was observed using an FE-TEM (JEM-2100F, manufactured by JEOL Ltd.) to obtain a TEM image. At this time, a predetermined number of CNTs were arbitrarily selected.
(3) For each TEM image, the number of layers, outer diameter, and inner diameter of the CNT were measured using ImageJ. The outer diameter and inner diameter of the CNT were measured at three points on the CNT shown in the TEM image, and the average value was calculated. When the outer diameter or inner diameter of the CNT shown in the TEM image changed visually, three points were selected as measurement points: a point with a thin outer diameter or inner diameter, a point with a thick outer diameter, and a point with a medium outer diameter or inner diameter.

[Spinnability evaluation]
Three evaluators with experience of pulling CNT webs from more than 100 CNT forests pulled CNT webs from the ends of CNT forests produced under the same conditions as in the examples and comparative examples, and evaluated the spinnability according to the following criteria.
Each evaluator performed the above operation on five CNT forests, and the average value was used as each evaluator's evaluation.
The average value of the evaluations by the three evaluators was taken as the spinnability of the CNT aggregate.

The spinnability of the CNT aggregate in Example 1 was 3.8, the spinnability of the CNT aggregate in Comparative Example 1 was 1.1, the spinnability of the CNT aggregate in Comparative Example 2 was 1.2, and the spinnability of the CNT aggregate in Comparative Example 3 was 4.4.

5: The CNT web can be pulled from one end of the CNT forest to the other in one go.
4: The CNT web can be pulled from one end of the CNT forest to the other in two passes.
3: The CNT web can be pulled from one end of the CNT forest to the other in three passes.
2: The CNT web can be pulled from one end of the CNT forest to the other in 4-6 passes.
1: The CNT web can be pulled from one end of the CNT forest to the other end 7 or more times, or it cannot be pulled from one end to the other end.

[Dispersibility Evaluation]
In the CNT forests obtained in the examples and comparative examples, the CNTs were scraped off the wafer using a scraper to obtain a powder-like aggregate of CNTs.
Three evaluators who had prepared CNT dispersions 100 or more times carried out the following operations and then evaluated the results according to the following criteria.

0.002 g of powdered CNT aggregates was weighed out and placed in a glass screw tube, 5 ml of ethanol was added, and the mixture was irradiated with ultrasonic waves of 40,000 Hz for 2 minutes.
The dispersion was visually observed and evaluated according to the following criteria.
Each evaluator evaluated each powder-like CNT aggregate three times, and the average value was used as the evaluation for each evaluator.
The average value of the evaluations by the three evaluators was regarded as the evaluation of the dispersibility of the CNT aggregates.

The dispersibility of the CNT aggregate of Example 1 was 2.8, the dispersibility of the CNT aggregate of Comparative Example 1 was 3.3, the dispersibility of the CNT aggregate of Comparative Example 2 was 3.2, and the dispersibility of the CNT aggregate of Comparative Example 3 was 1.4.
4: Uniform dispersion occurred within 2 minutes after ultrasonic irradiation.
3: Within 2 minutes after ultrasonic irradiation, slight lumps were observed, but the particles were dispersed to an extent that would not cause any practical problems.
2: Lumps were observed 2 minutes after ultrasonic irradiation, and additional dispersion for up to 3 minutes was required to disperse the mixture to a degree acceptable for practical use.
1: Lumps were observed 2 minutes after ultrasonic irradiation, and additional dispersion for more than 3 minutes was required to disperse the material to a degree that would not cause practical problems.

Claims (4)

An aggregate of carbon nanotubes (CNTs) including a plurality of CNTs;
When the 110 or more CNTs constituting the aggregate were observed with a transmission electron microscope,
When the most frequently observed number of layers is n (n is an integer from 3 to 15 ), the total number of layers is 100% of all the CNTs observed.
the total percentage of CNTs having a number of walls of n-1 to n+1 is 22 to 36%,
the total percentage of CNTs having a number of walls between n-2 and n+2 is between 50% and 70%;
If the average outer diameter of a CNT with n walls is X (nm),
the average outer diameter of the CNTs having n-1 walls is X-2.8 (nm) to X+0.5 (nm);
the average outer diameter of the CNTs having n+1 walls is X-0.5 (nm) to X+2.8 (nm);
The average outer diameter of all the observed CNTs is X-1.5 (nm) to X+1.5 (nm);
An aggregate of carbon nanotubes. The carbon nanotube aggregate according to claim 1, wherein the average inner diameter of all the observed CNTs is 1.0 to 6.0 nm. 2. The carbon nanotube aggregate according to claim 1, wherein the average length of the CNTs constituting the aggregate is 10 to 1000 μm. The carbon nanotube aggregate of claim 1 , wherein the CNT aggregate is a CNT forest disposed on a substrate.