JP7783102B2 - Method for manufacturing spun yarn made of carbon nanotubes and spun yarn made of carbon nanotubes - Google Patents

Description

The present invention relates to a method for producing spun yarn made from carbon nanotubes and to spun yarn made from carbon nanotubes.

Carbon nanotubes (hereinafter also referred to as "CNTs") have high mechanical strength, thermal conductivity, and electrical conductivity. Furthermore, multiple CNTs can be spun together to produce spun yarn made from CNTs (hereinafter also referred to as "CNT spun yarn"). Because CNT spun yarn also has high mechanical strength, thermal conductivity, and electrical conductivity, it is being developed as a material for a variety of products.

For example, Non-Patent Document 1 describes that by performing an electrical heating treatment under conditions of a temperature of 2450°C, a heating time (voltage pulse width) of 12 milliseconds, and a tension of 60 mN, the strength of the resulting CNT spun yarn increases to a breaking stress of 3.2 GPa and a Young's modulus of 123 GPa.

Patent Document 1 describes a method for manufacturing a structure spun from a spinning source member comprising a CNT forest, in which the entire open end of the CNT forest, which is a cylindrical opening substrate, is used as a spinnable portion, and a spinning process is performed in which the CNTs are pulled out from the spinnable portion and spun. The method describes a method for manufacturing a cylindrical web-like structure having an inner surface and an outer surface and comprising multiple entangled carbon nanotubes.

Patent No. 6667848

Y. Song et al., Nanoscale 11, 13909-13917 (2019)

As mentioned above, methods for producing CNT spun yarns have been developed. However, conventional methods have several issues. For example, CNT spun yarns obtained by spinning multiple CNTs extracted from a CNT forest typically have a density of approximately 0.7 g/ ^cm3 because they are spun using van der Waals forces between CNT bundles. CNT spun yarns with this density may have insufficient strength. Densification through tension application and structural transformation from amorphous carbon to graphene through heating are known to be effective in improving the strength of CNT spun yarns. However, when the temperature and applied tension of the electrical heating treatment are increased, the spun yarns with this density cannot withstand the applied tension at high temperatures and break. On the other hand, when electrical heating treatment is performed for an extremely short time to avoid breakage of the CNT spun yarns, although breakage of the CNT spun yarns can be avoided, the structural transformation from amorphous carbon to graphene is insufficient, resulting in insufficient strength improvement.

Therefore, the present invention aims to provide a means for producing high-strength CNT spun yarn.

The inventors have investigated various means for solving the above-mentioned problems. They have discovered that by applying a high tension to the CNT spun yarn before subjecting it to electrical heating treatment, within a range that does not cause the CNT spun yarn to break, to densify the CNT spun yarn, it is possible to increase the temperature and applied tension of the electrical heating treatment compared to conventional methods. The inventors have found that by producing CNT spun yarn using this procedure, the resulting CNT spun yarn has high crystallinity and high strength. Based on these findings, the inventors have completed the present invention.

That is, the present invention includes the following aspects and embodiments.
(1) A method for producing a spun yarn made of carbon nanotubes, comprising the following steps:
a step of producing a spun yarn precursor α, in which a plurality of carbon nanotubes are drawn out from the carbon nanotube forest, and the plurality of carbon nanotubes are spun while applying a tension of 6 mN or less per 1 cm width of the carbon nanotube forest to the plurality of carbon nanotubes, thereby producing a spun yarn precursor α made of carbon nanotubes;
a spun yarn precursor β production step in which a higher tension than that in the spun yarn precursor α production step is applied to the spun yarn precursor α to produce a spun yarn precursor β by densifying the spun yarn precursor α;
a spun yarn production step in which the spun yarn precursor β is electrically heated while applying tension to produce a spun yarn made of carbon nanotubes;
The method comprising:
(2) The method according to embodiment (1), wherein in the step of producing the spun yarn precursor β, tension is applied to the spun yarn precursor α while the spun yarn precursor α is brought into contact with a liquid selected from the group consisting of methanol, ethanol, acetone, water, paraffin, toluene, and mixtures thereof.
(3) The method according to the above embodiment (1) or (2), wherein tension is applied to the spun yarn precursor α in stages over multiple times in the spun yarn precursor β production step.
(4) The method according to any one of the above embodiments (1) to (3), wherein the spun yarn production step is performed by electrical heating at a temperature of 3200 K or less.
(5) The method according to any one of the above embodiments (1) to (4), wherein in the spun yarn production step, the spun yarn precursor β is electrically heated while being subjected to a tension of less than 480 MPa.
(6) The method according to any one of the above embodiments (1) to (5), further comprising a crosslinking step of impregnating the spun yarn made of carbon nanotubes obtained in the spun yarn preparation step with an aqueous solution containing a sulfide salt.
7. The method of claim 6, wherein the sulfide salt is sodium tetrasulfide or sodium disulfide.
(8) A spun yarn made of carbon nanotubes,
The ratio (IG/ID) of the G-band peak intensity (IG) to the D-band peak intensity (ID) in the spectrum obtained by Raman spectroscopy is 5 or more, and graphene is present in at least a part of the carbon nanotubes.
A spun yarn made of the carbon nanotubes.
(9) The spun yarn according to the embodiment (8), wherein the carbon nanotubes are cross-linked by disulfide bonds, trisulfide bonds, or tetrasulfide bonds.

This invention makes it possible to provide a means for producing high-strength CNT spun yarn.

Figure 1 shows the Young's modulus of the CNT spun yarns obtained in Test I-4 by carrying out the spun yarn precursor β production process while applying tension at different loads, and then carrying out the spun yarn production process without electrical heating. In the figure, the horizontal axis represents the load (mN) in the spun yarn precursor β production process, and the vertical axis represents Young's modulus (GPa). The values in the figure represent the average and standard deviation of the results of three tests. Figure 2 shows the breaking stress of CNT spun yarns obtained in Test I-4 by carrying out the spun yarn precursor β production process while applying tension at different loads, followed by the spun yarn production process without electrical heating. In the figure, the horizontal axis represents the load (mN) in the spun yarn precursor β production process, and the vertical axis represents the breaking stress (GPa). The values in the figure represent the average and standard deviation of the results of three tests. Figure 3 shows the density of the CNT spun yarn obtained in Test I-4 by carrying out the spun yarn precursor β production process while applying tension at different loads, and then carrying out the spun yarn production process without electrical heating. In the figure, the horizontal axis represents the load (mN) in the spun yarn precursor β production process, and the vertical axis represents the density (g/cm ³ ). The values in the figure represent the average and standard deviation of the test results obtained from cross-sectional area measurements at six locations. Figure 4 shows the density of CNT spun yarns obtained under various processing conditions in Test II-1. In the figure, the horizontal axis represents the processing conditions (tension and temperature of electrical heating) in the spun yarn production process, and the vertical axis represents density (g/ ^cm3 ). The open bars represent the results when the spun yarn precursor β production process was carried out under no load, and the filled bars represent the results when the spun yarn precursor β production process was carried out under load. The values in the figure represent the average and standard deviation of the test results obtained from cross-sectional area measurements at six locations. Figure 5 shows the Raman spectrum in the range of 1000 to 2000 cm ^-1 of the CNT spun yarn obtained by carrying out the spun yarn precursor β production process under load in Test II-2. In the figure, the horizontal axis represents the Raman shift (cm ^-1 ), and the vertical axis represents the intensity of Raman scattered light (au). Figure 6 shows the Raman spectrum in the range of 2200 to 3400 cm ^-1 of the CNT spun yarn obtained by carrying out the spun yarn precursor β production process under no load in Test II-2. In the figure, the horizontal axis represents the Raman shift (cm ^-1 ), and the vertical axis represents the intensity of Raman scattered light (au). Figure 7 shows the Raman spectra in the range of 2200 to 3400 cm ^-1 of the CNT spun yarn obtained by carrying out the spun yarn precursor β production process under load in Test II-2. In the figure, A is the Raman spectrum of the untreated CNT spun yarn and the CNT spun yarn obtained by electrical heating at 2500 K, and B is the Raman spectrum of the untreated CNT spun yarn and the CNT spun yarn obtained by electrical heating at 3000 K. In the figure, the horizontal axis is the Raman shift (cm ^-1 ), and the vertical axis is the intensity of Raman scattered light (au). FIG. 8 shows an image of the CNT spun yarn observed with a transmission electron microscope in Test II-3. In the figure, (A) is a transmission electron microscope image of an untreated CNT spun yarn; (B) is a transmission electron microscope image of a CNT spun yarn obtained by carrying out the spun yarn precursor β production process under unloaded conditions, followed by electrical heating at 2500 K while applying a tension of 100 mN (300 MPa for the spun yarn precursor β); (C) is a transmission electron microscope image of a CNT spun yarn obtained by carrying out the spun yarn precursor β production process under loaded conditions, followed by electrical heating at 2500 K while applying a tension of 100 mN (300 MPa for the spun yarn precursor β); and (D) is a transmission electron microscope image of a CNT spun yarn obtained by carrying out the spun yarn precursor β production process under loaded conditions, followed by electrical heating at 3000 K while applying a tension of 100 mN (300 MPa for the spun yarn precursor β). The scale bar in the figure is 10 nm. Figure 9 shows the relationship between the temperature of electrical heating in the spun yarn production process and the Young's modulus of the resulting CNT spun yarn in Test II-4. The horizontal axis of the figure represents the temperature (K) of electrical heating in the spun yarn production process, and the vertical axis represents the Young's modulus (GPa) of the CNT spun yarn. The dotted line in the figure also represents the Young's modulus of the untreated control CNT spun yarn, which was subjected to the spun yarn precursor β production process with or without load. The values in the figure represent the average and standard deviation of the results of three tests. Figure 10 shows the relationship between the temperature of electrical heating in the spun yarn production process and the breaking stress of the resulting CNT spun yarn in Test II-4. The horizontal axis of the figure represents the temperature (K) of electrical heating in the spun yarn production process, and the vertical axis represents the breaking stress (GPa) of the CNT spun yarn. The dotted line in the figure also represents the breaking stress of the untreated control CNT spun yarn, which was subjected to the spun yarn precursor β production process with or without load. The values in the figure represent the average and standard deviation of the results of three tests. Figure 11 shows scanning electron microscope images of the surface of CNT spun yarn obtained by the method including the crosslinking step in Test III-2. In the figure, A is a scanning electron microscope image of a CNT spun yarn obtained by a 2-hour impregnation treatment in the crosslinking step, B is a scanning electron microscope image of a CNT spun yarn obtained by a 24-hour impregnation treatment in the crosslinking step, and C is a scanning electron microscope image of a CNT spun yarn obtained by a 48-hour impregnation treatment in the crosslinking step. The scale bar in the figure is 10 μm. Figure 12 shows the diameter and density of the CNT spun yarn obtained in Test III-2 using a method including a crosslinking step. In the figure, graph A shows the diameter of the CNT spun yarn, and graph B shows the density of the CNT spun yarn. The horizontal axis represents the impregnation time in the crosslinking step, and the vertical axis represents the diameter (μm) or density (g/ ^cm3 ) of the CNT spun yarn. Figure 13 shows scanning electron microscope images of the cross-section of a CNT-spun yarn obtained using a method including a cross-linking step in Test III-3. In the figure, A is a scanning electron microscope image of a CNT-spun yarn obtained after a 2-hour impregnation treatment in the cross-linking step, B is a scanning electron microscope image of a CNT-spun yarn obtained after a 24-hour impregnation treatment in the cross-linking step, and C is a scanning electron microscope image of a CNT-spun yarn obtained after a 48-hour impregnation treatment in the cross-linking step. The dots in the images represent sulfur atoms present in the cross-section of the CNT-spun yarn, as mapped using an energy dispersive X-ray analyzer (EDS). Figure 14 shows the quantitative results of sulfur atoms present in the cross section of a CNT spun yarn obtained by a method including a cross-linking step, as determined using EDS in Test III-3. In the figure, graph A shows the percentage of the number of sulfur atoms relative to the total number of atoms present in the cross section of the CNT spun yarn, and graph B shows the percentage of sulfur atomic mass relative to the total mass of atoms present in the cross section of the CNT spun yarn. The horizontal axis represents the impregnation time in the cross-linking step, and the vertical axis represents the percentage (%) of the number of sulfur atoms or the percentage (%) of sulfur atomic mass. Figure 15 shows the stress-strain curve of the CNT spun yarn obtained in Test III-4 using a method including a crosslinking step. In the figure, the horizontal axis represents strain rate (%), and the vertical axis represents the breaking stress (GPa) of the CNT spun yarn. Figure 16 shows the stress-strain curve of the CNT spun yarn obtained in Test III-5 using a method including a cross-linking step. In the figure, the horizontal axis represents strain rate (%), and the vertical axis represents the breaking stress (GPa) of the CNT spun yarn. Figure 17 shows the stress-strain curve of the CNT spun yarn obtained in Test III-5 using a method including a cross-linking step. In the figure, the horizontal axis represents strain rate (%), and the vertical axis represents the breaking stress (GPa) of the CNT spun yarn.

A preferred embodiment of the present invention is described in detail below.

1. Method for producing spun yarn made from carbon nanotubes
The present inventors have discovered that by applying a high tension to a spun yarn made of CNTs (CNT spun yarn) before subjecting it to an electrical heating treatment, within a range that does not cause the CNT spun yarn to break, to densify the CNT spun yarn, it is possible to increase the temperature and applied tension of the electrical heating treatment compared to conventional methods. The present inventors have discovered that by producing CNT spun yarn using this procedure, the resulting CNT spun yarn has high crystallinity and high strength. Therefore, one aspect of the present invention relates to a method for producing a spun yarn made of CNTs.

In each aspect of the present invention, CNT refers to a carbon material in which a single layer of graphite has a tubular structure. CNTs with a single-layer tubular structure are typically classified as single-walled CNTs, and CNTs with two or more tubular layers are typically classified as multi-walled CNTs. The CNTs used in each aspect of the present invention may be either single-walled CNTs or multi-walled CNTs.

The method of this embodiment includes at least a spun yarn precursor α production process, a spun yarn precursor β production process, and a spun yarn production process. Each process is described in detail below.

[1-1. CNT forest preparation process]
The method of this embodiment may include a CNT forest preparation step of preparing a CNT forest.

In each aspect of the present invention, a CNT forest refers to an aggregate of CNTs having a structure in which multiple CNTs are oriented in a specific direction for at least a portion of their longitudinal axis.

The shape and dimensions of the CNT forest prepared in this process are not particularly limited and may be any shape and size. For example, the CNT forest is rectangular, with each side measuring 5 to 1000 mm, preferably 10 to 40 mm.

The CNT forest prepared in this process can be prepared by various methods known in the art. For example, a CNT forest can be formed by placing catalyst particles that serve as growth nuclei on the surface of a metal substrate and then contacting the surface of the substrate with a carbon-containing raw material gas in a reactor. In this process, the CNT forest can be prepared in-house by various methods known in the art, or it can be purchased.

[1-2. Spun yarn precursor α production process]
This process involves pulling out multiple CNTs from a CNT forest, spinning the multiple CNTs while applying tension to the multiple CNTs, and producing a spun yarn precursor α (hereinafter also referred to as "CNT spun yarn precursor α") made of CNTs.

In this process, various means commonly used in the art can be used to extract multiple CNTs from the CNT forest. For example, a combing blade with multiple blades can be placed on top of the CNT forest substrate, and a CNT web consisting of multiple CNTs can be extracted from between the blades. In this embodiment, it is preferable that the combing blade have three or more blades.

In this process, the formed CNT spun yarn precursor α is recovered by winding it around the circumferential surface of a rotating body. The rotating body is, for example, a bobbin or roller. By winding the CNT spun yarn precursor α around the circumferential surface of the rotating body, multiple CNTs can be pulled out from the CNT forest at the desired pulling speed and the desired tension can be applied to the multiple CNTs.

In this process, the pulling speed for pulling out multiple CNTs from the CNT forest is preferably in the range of 100 to 1000 mm/min, and more preferably in the range of 200 to 500 mm/min.

In this process, it is preferable to spin multiple CNTs by rotating the CNT forest at a predetermined rotational speed. In this embodiment, the rotational speed of the CNT forest is preferably less than 2000 rpm, more preferably in the range of 100 to 1000 rpm, and even more preferably in the range of 200 to 500 rpm. Furthermore, it is preferable that the ratio of the rotational speed of the CNT forest to the pulling speed of the multiple CNTs is in the range of more than 0.25 rev/mm and less than 25 rev/mm.

In this process, the tension applied to the multiple CNTs is typically 6 mN or less per cm of CNT forest width. The tension applied to the multiple CNTs is preferably in the range of 0.1 to 6 mN per cm of CNT forest width, more preferably in the range of 0.3 to 3 mN per cm of CNT forest width, and even more preferably in the range of 0.3 to 1 mN per cm of CNT forest width. For example, when the CNT forest has the shape and dimensions exemplified above, the tension applied to the multiple CNTs is typically 10 mN or less (30 MPa per CNT), preferably in the range of 0.15 to 10 mN, more preferably in the range of 0.5 to 5 mN (1.6 to 16 MPa per CNT), and even more preferably in the range of 0.5 to 2 mN (1.6 to 6.4 MPa per CNT). If the tension is less than the lower limit, the density of the resulting CNT spun yarn precursor α may be low. Furthermore, if the tension exceeds the upper limit, the CNTs may break and it may not be possible to obtain CNT spun yarn precursor α. Therefore, by carrying out this process while applying a tension within the above range to multiple CNTs, it is possible to obtain a high-density CNT spun yarn precursor α.

In this process, tension can typically be applied to multiple CNTs pulled from the CNT forest by applying a load using a tensioning rod, slider system, motor, or the like.

In this process, tension is preferably applied to the CNTs extracted from the CNT forest while they are in contact with a liquid. In this embodiment, the liquid is preferably selected from the group consisting of methanol, ethanol, acetone, water, paraffin, toluene, and mixtures thereof, and more preferably selected from the group consisting of methanol, ethanol, acetone, water, and mixtures thereof. The liquid may contain one or more additional components, such as metal salts (e.g., iron chloride or nickel chloride) or metal oxides (e.g., iron oxide). The liquid is particularly preferably methanol, ethanol, acetone, or water. In this embodiment, methods for contacting the CNTs with the liquid include, but are not limited to, impregnating the CNTs with the liquid, spraying the CNTs with the liquid, dripping the liquid onto the CNTs, and exposing the CNTs to liquid vapor. It is preferable to contact the CNTs with the liquid by impregnating them with the liquid. In this embodiment, it is preferable to dry the CNTs that have been contacted with the liquid at room temperature or higher. By bringing multiple CNTs into contact with a liquid using this method, the agglomeration force caused by vaporization of the liquid can be utilized to aggregate the multiple CNTs, improving the density of the resulting CNT spun yarn precursor α.

The twist angle of the CNT spun yarn precursor a formed in this step is usually about 4°. The density of the CNT spun yarn precursor a is usually less than 0.7 g/ ^cm3 . The Young's modulus of the CNT spun yarn precursor a is usually less than 100 GPa. The breaking stress of the CNT spun yarn precursor a is usually less than 1.4 GPa. By using the CNT spun yarn precursor a having the above properties in the following steps, a high-strength CNT spun yarn can be obtained in the method of this embodiment.

[1-3. Spun yarn precursor β production process]
This step involves applying a higher tension to the spun yarn precursor α than in the spun yarn precursor α production step, thereby producing a spun yarn precursor β by densifying the spun yarn precursor α. The purpose of this step is to densify the spun yarn precursor α by preliminarily applying a high tension within a range that does not cause the spun yarn precursor α to break, before subjecting the spun yarn precursor β to an electrical heating treatment in the spun yarn production step.

This process can be carried out, for example, by drawing the spun yarn precursor α obtained in the spun yarn precursor α production process from the circumferential surface of a rotor and applying tension. The formed spun yarn precursor β is recovered by winding it around the circumferential surface of a rotor. The rotor is, for example, a bobbin or roller. By winding the spun yarn precursor β around the circumferential surface of the rotor, the spun yarn precursor α can be drawn out at the desired drawing speed and the desired tension can be applied to the spun yarn precursor α.

In this process, the drawing speed for drawing out the spun yarn precursor α is preferably less than 4000 mm/min, and more preferably in the range of 1000 to 3000 mm/min.

In this process, the tension applied to the spun yarn precursor α is higher than that in the spun yarn precursor α production process. The tension applied to the spun yarn precursor α is, for example, less than 150 mN (480 MPa for the spun yarn precursor α), preferably in the range of 0.5 to 120 mN (1.6 to 380 MPa for the spun yarn precursor α), and more preferably in the range of 1 to 100 mN (3 to 300 MPa for the spun yarn precursor α). If the tension is below the lower limit, the density of the resulting CNT spun yarn precursor β may be low. If the tension exceeds the upper limit, the spun yarn precursor α may break, making it impossible to obtain a CNT spun yarn precursor β. Therefore, by performing this process while applying a tension within the above range to the spun yarn precursor α, the spun yarn precursor α can be densified, resulting in a high-density CNT spun yarn precursor β. This also results in a high-strength CNT spun yarn.

In this process, it is preferable to apply tension to the spun yarn precursor α in stages multiple times. In this embodiment, the tension is increased stepwise from substantially the same tension as in the spun yarn precursor α production process, and the tension is repeatedly applied stepwise to the spun yarn precursor α multiple times. In this case, the number of repeated tension applications is preferably two or more, more preferably two to five, and even more preferably three. Furthermore, the tension applied the first time is preferably in the range of 0.5 to 10 mN (1.6 to 30 MPa for the spun yarn precursor α), and more preferably 1 to 10 mN (3 to 30 MPa for the spun yarn precursor α). The tension applied in the final stage is preferably in the range of 10 to 120 mN (30 to 380 MPa for spun yarn precursor α), more preferably in the range of 20 to 100 mN (64 to 300 MPa for spun yarn precursor α), and even more preferably in the range of 50 to 100 mN (160 to 300 MPa for spun yarn precursor α). In a specific embodiment, it is preferable to apply tension to the spun yarn precursor α in a stepwise manner, increasing the tension in the order of 1 mN, 50 mN, or 100 mN (3 MPa, 160 MPa, or 300 MPa for spun yarn precursor α), three times. If the number of repetitions is less than the lower limit or if the tension applied in the first step exceeds the upper limit, the tension may increase rapidly, causing the spun yarn precursor α to break and preventing the production of CNT spun yarn precursor β. Furthermore, if the number of repetitions exceeds the upper limit, the processing time may become longer, potentially increasing the cost of the method of this embodiment. Therefore, by carrying out this process under the above conditions, the spun yarn precursor α can be densified, and a CNT spun yarn precursor β with a stable high density can be obtained. This also makes it possible to obtain a CNT spun yarn with high strength.

In this process, tension can typically be applied to the spun yarn precursor α pulled out from the circumferential surface of the rotor by applying a load using a tensioning rod, slider system, motor, or the like.

In this process, tension is preferably applied to the spun yarn precursor α while it is being brought into contact with a liquid. In this embodiment, the liquid is preferably selected from the group consisting of methanol, ethanol, acetone, water, paraffin, toluene, and mixtures thereof, and more preferably selected from the group consisting of methanol, ethanol, acetone, water, and mixtures thereof. The liquid may contain one or more additional components, such as metal salts (e.g., iron chloride or nickel chloride) or metal oxides (e.g., iron oxide). The liquid is particularly preferably methanol, ethanol, acetone, or water. In this embodiment, means for bringing the spun yarn precursor α into contact with the liquid include, but are not limited to, impregnating the spun yarn precursor α with the liquid, spraying the liquid onto the spun yarn precursor α, dripping the liquid onto the spun yarn precursor α, and exposing the spun yarn precursor α to the vapor of the liquid. It is preferable to bring the spun yarn precursor α into contact with the liquid by impregnating the spun yarn precursor α with the liquid. In this embodiment, it is preferable to dry the spun yarn precursor α that has been brought into contact with the liquid at room temperature or a higher temperature. By bringing the spun yarn precursor α into contact with the liquid using this method, the coagulation force caused by the vaporization of the liquid can be used to aggregate multiple CNTs, improving the density of the resulting CNT spun yarn precursor β.

By carrying out this step under the above conditions, the spun yarn precursor α can be densified, and a high-density CNT spun yarn precursor β can be stably obtained. The density of the spun yarn precursor β formed in this step is usually 1.0 g/ ^cm3 or higher, for example, in the range of 1.0 to 1.2 g/ ^cm3 . The Young's modulus of the CNT spun yarn precursor β is usually 100 GPa or higher, for example, in the range of 100 to 150 GPa. The breaking stress of the CNT spun yarn precursor β is usually 1.4 GPa or higher, for example, in the range of 1.4 to 2 GPa. By using the spun yarn precursor β having the above properties in the following steps, a high-strength CNT spun yarn can be obtained in the method of this embodiment.

[1-4. Spun yarn production process]
This step involves applying tension to the spun yarn precursor β while electrically heating it, to produce a spun yarn made of carbon nanotubes.

This process can be carried out, for example, by drawing the spun yarn precursor β obtained in the spun yarn precursor β production process from the circumferential surface of a rotor and subjecting it to an electrical heating treatment while applying tension. The formed CNT spun yarn is recovered by winding it around the circumferential surface of a rotor. The rotor is, for example, a bobbin or roller. By winding the CNT spun yarn around the circumferential surface of the rotor, the spun yarn precursor α can be drawn out at the desired drawing speed and the desired tension can be applied to the spun yarn precursor α.

In this process, the drawing speed for drawing out the spun yarn precursor α is preferably in the range of more than 60 and 6000 mm/min, more preferably in the range of 100 to 1000 mm/min, and even more preferably in the range of 100 to 600 mm/min.

In this step, the tension applied to the spun yarn precursor β may be the same as or greater than that applied in the spun yarn precursor β production step. The tension applied to the spun yarn precursor β is, for example, less than 150 mN (480 MPa for the spun yarn precursor β), preferably in the range of 0.5 to 120 mN (1.6 to 380 MPa for the spun yarn precursor β), and more preferably in the range of 1 to 100 mN (3 to 300 MPa for the spun yarn precursor β). If the tension is less than the lower limit, the density of the resulting CNT spun yarn may be low. If the tension exceeds the upper limit, the spun yarn precursor β may break, making it impossible to obtain a CNT spun yarn. Therefore, by performing this step while applying a tension within the above range to the spun yarn precursor β, the spun yarn precursor β can be densified, resulting in a CNT spun yarn with high density. This also results in a CNT spun yarn with high strength.

In this process, tension can typically be applied to the spun yarn precursor β pulled out from the circumferential surface of the rotor by applying a load using a tensioning rod, slider system, motor, or the like.

In this process, the means for electrically heating the spun yarn precursor β is not particularly limited, and various means commonly used in the relevant technical field can be applied. For example, this process may be carried out using a heater (e.g., a quartz heater) in an inert gas atmosphere (e.g., argon gas), or using an atmospheric furnace equipped with a tension application mechanism and capable of heating to ultra-high temperatures. In this embodiment, the distance between the electrodes of the heater is preferably in the range of 1 to 50 mm, and more preferably in the range of 1 to 10 mm.

In this process, the temperature of the electrical heating is preferably 3200 K or less, more preferably a temperature above room temperature and below 3200 K, even more preferably in the range of 1000 to 3200 K, and particularly preferably in the range of 2000 to 3000 K. If the temperature of the electrical heating is below the lower limit, the graphene conversion of the amorphous carbon in the CNTs contained in the spun yarn precursor β may not proceed sufficiently. Furthermore, if the temperature of the electrical heating exceeds the upper limit, the spun yarn precursor β may break, making it impossible to obtain a CNT spun yarn. Therefore, by performing this process under the above conditions, at least a portion of the amorphous carbon in the CNTs contained in the spun yarn precursor β can be graphene converted, resulting in a high-strength CNT spun yarn.

In this process, the duration of the electrical heating is preferably greater than 0.1 and less than 10 seconds, more preferably between 0.5 and 10 seconds, and even more preferably between 0.5 and 1 second. If the duration of the electrical heating is less than the lower limit, the amorphous carbon in the CNTs contained in the spun yarn precursor β may not be converted into graphene sufficiently. Furthermore, if the duration of the electrical heating exceeds the upper limit, the spun yarn precursor β may break, making it impossible to obtain a CNT spun yarn. Therefore, by carrying out this process under the above conditions, at least a portion of the amorphous carbon in the CNTs contained in the spun yarn precursor β can be converted into graphene, resulting in a high-strength CNT spun yarn.

In this process, tension is preferably applied to the spun yarn precursor β while it is being brought into contact with a liquid. In this embodiment, the liquid is preferably selected from the group consisting of methanol, ethanol, acetone, water, paraffin, toluene, and mixtures thereof, and more preferably selected from the group consisting of methanol, ethanol, acetone, water, paraffin, and mixtures thereof. The liquid may contain one or more additional components, such as metal salts (e.g., iron chloride or nickel chloride) or metal oxides (e.g., iron oxide). The liquid is particularly preferably methanol, ethanol, acetone, water, paraffin, or an ethanol solution of iron chloride. In this embodiment, means for bringing the spun yarn precursor β into contact with the liquid include, but are not limited to, impregnating the spun yarn precursor β with the liquid, spraying the liquid onto the spun yarn precursor β, dripping the liquid onto the spun yarn precursor β, and exposing the spun yarn precursor β to the vapor of the liquid. It is preferable to bring the spun yarn precursor β into contact with the liquid by impregnating the spun yarn precursor β with the liquid. In this embodiment, it is preferable to dry the spun yarn precursor β that has been brought into contact with the liquid at room temperature or a higher temperature. By bringing the spun yarn precursor β into contact with the liquid using this method, the coagulation force caused by the vaporization of the liquid can be utilized to aggregate multiple CNTs, improving the density of the resulting CNT spun yarn.

By carrying out this process under the above conditions, the spun yarn precursor β can be densified and/or at least a portion of the amorphous carbon contained in the spun yarn precursor β can be converted into graphene, resulting in a CNT spun yarn with high density and strength.

[1-5. Crosslinking process]
The method of this embodiment may further include a crosslinking step of impregnating the spun yarn made of carbon nanotubes obtained in the spun yarn preparation step with an aqueous solution containing a sulfide salt.

In each embodiment of the present invention, the sulfide salt used in the crosslinking step may be referred to as the "sulfurizing agent." Simulations predict that treating a CNT spun yarn with a sulfurizing agent crosslinks the CNTs via disulfide bonds, and that applying tensile force to this crosslinked CNT spun yarn improves tensile strength through repeated recombination of disulfide bonds (Composite Science and Technology, 2018, Vol. 166, pp. 3-9). In the method of this embodiment, treating the CNT spun yarn obtained in the spun yarn production step with an aqueous solution containing a sulfurizing agent crosslinks the CNTs constituting the CNT spun yarn via disulfide bonds, trisulfide bonds, or tetrasulfide bonds, reducing voids and increasing the density of the CNT spun yarn. Therefore, performing the crosslinking step can further improve the density and strength of the resulting CNT spun yarn.

In this process, the sulfide salt used as the sulfiding agent is preferably a salt of tetrasulfide, trisulfide, or disulfide, more preferably a salt of tetrasulfide or disulfide, even more preferably a salt of tetrasulfide or disulfide with sodium ions, potassium ions, calcium ions, or magnesium ions, and particularly preferably sodium tetrasulfide or sodium disulfide. When the sulfide salt is a salt of tetrasulfide with one of the counterions exemplified above, particularly sodium tetrasulfide, performing this process can further improve the strain resistance of the CNT spun yarn. Furthermore, when the sulfide salt is a salt of disulfide with one of the counterions exemplified above, particularly sodium disulfide, performing this process can further improve the breaking stress of the CNT spun yarn. Therefore, performing this process using one of the sulfide salts exemplified above can further improve the strength of the resulting CNT spun yarn.

In this process, the concentration of the sulfide salt in the aqueous sulfide salt solution is preferably 1 mmol/L or higher, more preferably in the range of 1 to 500 mmol/L, even more preferably in the range of 1 to 200 mmol/L, and particularly preferably in the range of 1 to 100 mmol/L. If the sulfide salt concentration is below the lower limit, disulfide bonds and the like will not be sufficiently introduced between the CNTs that make up the CNT spun yarn, which may result in insufficient improvement in the density and/or strength of the resulting CNT spun yarn. Therefore, by performing this process under the above conditions, the density and strength of the resulting CNT spun yarn can be further improved.

The aqueous sulfide salt solution used in this step may optionally contain one or more additional components, such as a water-miscible organic solvent. Examples of water-miscible organic solvents include methanol, ethanol, and acetone.

In this process, the impregnation time for impregnating the CNT spun yarn in the aqueous solution containing a sulfide salt is typically 1 hour or longer, preferably 2 hours or longer, more preferably 24 hours or longer, and even more preferably 48 hours or longer. There are no particular limitations on the upper limit of the impregnation time, but it is typically 72 hours or shorter, for example, 60 hours or shorter, and particularly 48 hours or shorter. If the impregnation time is shorter than the lower limit, the introduction of disulfide bonds between the CNTs that make up the CNT spun yarn may be insufficient, resulting in insufficient improvement in the density and/or strength of the resulting CNT spun yarn. Therefore, by performing this process under the above conditions, the density and strength of the resulting CNT spun yarn can be further improved.

In this process, the impregnation temperature at which the CNT spun yarn is impregnated with the aqueous solution containing a sulfide salt is typically room temperature or higher, preferably 10°C or higher, and more preferably in the range of 10 to 60°C. If the impregnation temperature is below the lower limit mentioned above, the introduction of disulfide bonds between the CNTs that make up the CNT spun yarn may be insufficient, resulting in insufficient improvement in the density and/or strength of the resulting CNT spun yarn. Therefore, by carrying out this process under the above conditions, the density and strength of the resulting CNT spun yarn can be further improved.

In this process, it is preferable to dry the CNT spun yarn impregnated with the aqueous solution containing the sulfide salt. In this embodiment, the drying temperature is typically above room temperature, preferably above 20°C, and more preferably in the range of 20 to 100°C. The drying time is typically at least several minutes, preferably at least 5 minutes and not more than several days, more preferably in the range of 5 minutes to 1 day, and even more preferably in the range of 5 minutes to 1 hour. In certain embodiments, it is preferable to air-dry the CNT spun yarn impregnated with the aqueous solution containing the sulfide salt at room temperature for 1 day, or vacuum-dry it at 80°C for 1 hour. If the impregnation temperature and/or impregnation time are below the above-mentioned lower limit, the CNT spun yarn may not be sufficiently dried. Therefore, by performing this process under the above-mentioned conditions, it is possible to obtain a CNT spun yarn with the desired properties.

In the method of this embodiment, the steps described above may be performed sequentially or continuously. It is preferable to perform the steps described above continuously in the method of this embodiment. By performing the steps of the method of this embodiment continuously, CNT spun yarn with high density and strength can be produced efficiently and at low cost.

2. Spun yarn made from carbon nanotubes
As explained above, a CNT spun yarn with high density and strength can be obtained by the manufacturing method of one embodiment of the present invention. Therefore, another embodiment of the present invention relates to a spun yarn (CNT spun yarn) that can be obtained by the manufacturing method of one embodiment of the present invention, and preferably made of carbon nanotubes obtained by said method.

In each embodiment of the present invention, the density of the CNT spun yarn precursor α, CNT spun yarn precursor β, and CNT spun yarn can be measured as bulk density, for example, by the following procedure, although this is not limited thereto. The weight per unit length of the CNT spun yarn precursor α, CNT spun yarn precursor β, or CNT spun yarn is measured using a microbalance. The cross-sectional area of the CNT spun yarn precursor α, CNT spun yarn precursor β, or CNT spun yarn is measured using a scanning electron microscope. The bulk density is then calculated from the weight per unit length and cross-sectional area obtained.

In each embodiment of the present invention, the strength of the CNT spun yarn precursor α, CNT spun yarn precursor β, and CNT spun yarn can be evaluated using, for example, Young's modulus and breaking stress as indicators. The Young's modulus and breaking stress of the CNT spun yarn precursor α, CNT spun yarn precursor β, and CNT spun yarn can be measured, for example, in accordance with ISO11566:1996 (JIS R 7606:2000), at a test speed of 1 mm/min and a grip distance of 25 mm.

The CNT spun yarn of this embodiment typically has a density of less than 1.6 g/cm ³ , for example, in the range of more than 1.0 g/cm ³ and less than 1.6 g/cm ³ , particularly in the range of 1.2 to 1.5 g/cm ^3. In contrast, CNT spun yarns obtained by conventional techniques that do not apply preliminary tension in the step of producing spun yarn precursor β typically have a density of 1.0 g/cm ³ or less. Therefore, the CNT spun yarn of this embodiment can have a denser structure than CNT spun yarns obtained by conventional techniques.

In the CNT spun yarn of this embodiment, the ratio of the G-band peak intensity (IG) to the D-band peak intensity (ID) in the spectrum obtained by Raman spectroscopy (IG/ID) is typically 5 or greater, for example, in the range of 5 to 35, and particularly in the range of 5 to 25. In contrast, in CNT spun yarns obtained by conventional methods that do not apply preliminary tension in the process of producing the spun yarn precursor β, IG/ID is typically less than 2, for example, less than 1.2. It is known in the art that in spectra obtained by Raman spectroscopy of CNTs and spun yarns made from CNTs (hereinafter also referred to as "Raman spectra"), the G-band peak is derived from a crystalline ^sp2 structure, and the D-band peak is derived from an ^sp3 structure indicating defects or amorphousness. Therefore, in the Raman spectra of CNTs and spun yarns made from CNTs, IG/ID serves as an indicator of the crystallinity of the CNTs. Furthermore, the structural change from amorphous carbon to graphene in the CNTs can be confirmed, for example, by the presence of a flake-like crystalline structure, indicating the presence of graphene, in an image obtained by observing the CNTs with a transmission electron microscope. The CNT spun yarn of this embodiment has a higher IG/ID than CNT spun yarns obtained by conventional methods, and therefore has higher CNT crystallinity than CNT spun yarns obtained by conventional methods. Furthermore, since the flake-like crystalline structure is confirmed in transmission electron microscope images of the CNT spun yarn of this embodiment, graphene may be present in at least a portion of the CNTs. Therefore, the CNT spun yarn of this embodiment has high crystallinity and a graphene structure that are not observed in CNT spun yarns obtained by conventional methods.

The CNT spun yarn of this embodiment has high strength due to its high crystallinity and graphene structure. The CNT spun yarn of this embodiment typically has a Young's modulus of 100 GPa or more, for example, in the range of 100 to 300 GPa, preferably in the range of 120 to 260 GPa, and particularly in the range of 190 to 260 GPa. The CNT spun yarn of this embodiment also typically has a breaking stress of 1.4 GPa or more, for example, in the range of 1.4 to 3.6 GPa, preferably in the range of 1.6 to 3.4 GPa, and particularly in the range of 2.2 to 3.2 GPa. In contrast, CNT spun yarns obtained using conventional methods that do not apply preliminary tension in the process of preparing the spun yarn precursor β typically have a Young's modulus of less than 100 GPa and a breaking stress of less than 1.4 GPa. Therefore, the CNT spun yarn of this embodiment can have a strength greater than that of CNT spun yarns obtained using conventional methods.

In a specific embodiment, the CNTs in the CNT spun yarn of this embodiment may be cross-linked by disulfide bonds, trisulfide bonds, or tetrasulfide bonds. The CNT spun yarn of this embodiment has one or more bonds selected from the group consisting of disulfide bonds, trisulfide bonds, and tetrasulfide bonds between the CNTs. The CNT spun yarn of this embodiment can be obtained by the production method of one embodiment of the present invention, which includes the cross-linking step described above. The CNT spun yarn of this embodiment has higher density and strength.

The CNT spun yarn of this embodiment has high strength in addition to thermal conductivity and electrical conductivity, and can therefore be used as a high-strength carbon material to replace carbon fiber reinforced plastic (CFRP). The CNT spun yarn of this embodiment can be used, for example, as a material for automobile parts (e.g., high-pressure tanks or bodies), wind turbine parts (e.g., blades), or aircraft parts (e.g., bodies).

The present invention will be explained in more detail below using examples. However, the technical scope of the present invention is not limited to these examples.

<I: Production of CNT spun yarn (1)>
[I-1: Spun yarn precursor α production process]
A combing blade with three blades was placed on top of an 18 x 18 mm CNT forest formed on the surface of a metal substrate. A CNT web consisting of multiple CNTs was drawn between the blades at a substrate rotation speed of 500 rpm and a drawing speed of 500 mm/min. The CNT web was spun while applying a tension of less than 1 mN (3 MPa for a CNT web consisting of multiple CNTs), resulting in a spun yarn precursor α consisting of CNTs with a twist angle of approximately 4°, wound around the circumferential surface of the rotating body. In this procedure, a load of less than 1 mN (3 MPa for a CNT web consisting of multiple CNTs) corresponds to a tension of 0.55 mN per cm of CNT forest width. This procedure produced spun yarn precursor α. The substrate rotation speed was varied within a range of less than 200 rpm, the ratio of substrate rotation speed to pulling speed was varied within a range of more than 0.25 rev/mm and less than 25 rev/mm, and the load was varied within a range of tension less than 10 mN (5.5 mN per cm width of the CNT forest, 30 MPa for a CNT web consisting of multiple CNTs). This process was also carried out by impregnating the CNT web pulled from the CNT forest with methanol, ethanol, acetone, or water while applying a load to the CNT web, and then winding it around the circumferential surface of a rotating body.

[I-2: Spun yarn precursor β production process]
Spun yarn precursor α was drawn from the circumferential surface of a rotor at a drawing speed of 2000 mm/min. After impregnating the spun yarn precursor α with ethanol while applying a higher tension than in the spun yarn precursor α production process, it was wound around the circumferential surface of a rotor. This procedure was repeated three times, applying a stepwise tension with increasing loads of 1 mN, 50 mN, or 100 mN (3 MPa, 160 MPa, or 300 MPa for spun yarn precursor α). This procedure densified the spun yarn precursor α to produce spun yarn precursor β. The drawing speed was varied within a range of less than 4000 mm/min, and the load was varied within a range of less than 150 mN (480 MPa for spun yarn precursor α). This process was also performed using a procedure in which the spun yarn precursor α was impregnated with methanol, acetone, or water instead of ethanol.

[I-3: Spun yarn production process]
Spun yarn precursor β was drawn from the circumferential surface of the rotor at a drawing speed of 600 mm/min. The spun yarn precursor β was subjected to electrical heating at a temperature of 1500 to 3000 K for 1 second with an electrode distance of 10 mm while applying a tension of 1 mN, 10 mN, or 100 mN (3 MPa, 30 MPa, or 300 MPa for spun yarn precursor β), and then wound around the circumferential surface of the rotor. This procedure produced CNT spun yarn. The drawing speed was varied between 60 and 6000 mm/min, the load was varied between 150 mN and 6000 mm/min, the electrical heating temperature was varied between room temperature and 3200 K or less, and the electrical heating time was varied between 0.1 and 10 seconds. This process was also carried out by impregnating the spun yarn precursor β with methanol, ethanol, acetone, water, paraffin, or an ethanol solution of iron chloride, and then electrically heating the impregnated yarn precursor β and winding it around the circumferential surface of a rotating body.

[I-4: Effect of load in the spun yarn precursor β production process]
In the spun yarn precursor β production step, spun yarn precursor β was produced by spinning while applying a tension of 1 mN, 50 mN, or 100 mN (3 MPa, 160 MPa, or 300 MPa for spun yarn precursor α). Next, in the spun yarn production step, a CNT spun yarn was produced without electrical heating. As a control, a CNT spun yarn was produced using the same procedure as above, except that spun yarn precursor β was produced by spinning without applying a load in the spun yarn precursor β production step. The Young's modulus and breaking stress of the resulting spun yarn were measured at a test speed of 1 mm/min and a grip distance of 25 mm according to ISO 11566:1996 (JIS R 7606:2000). The density of the resulting spun yarn was also measured using the following procedure. The weight per unit length of the spun yarn was measured using a microbalance. The cross-sectional area of the spun yarn (at six locations) was measured using a scanning electron microscope. The bulk density was then calculated from the weight per unit length and cross-sectional area. The Young's modulus, breaking stress, and density of the resulting spun yarn are shown in Figures 1, 2, and 3, respectively. The horizontal axis in each figure represents the load (mN) in the spun yarn precursor β production process. In Figures 1 and 2, the values represent the average and standard deviation of three test results. In Figure 3, the values represent the average and standard deviation of the test results obtained from the cross-sectional area measurements at six locations.

As shown in Figures 1 to 3, when spinning was performed while applying a load of 1 mN, 50 mN, or 100 mN (3 MPa, 160 MPa, or 300 MPa for spun yarn precursor α) during the process of preparing spun yarn precursor β, the Young's modulus, breaking stress, and density of the resulting CNT spun yarn were all significantly higher than those of the control spun yarn.

[I-5: Effect of electrical heating treatment in the spun yarn production process]
In the spun yarn precursor β production step, spun yarn precursor β was produced by spinning with no load or with a tension of 100 mN (300 MPa for spun yarn precursor α). Next, in the spun yarn production step, spun yarn precursor β was subjected to electrical heating at temperatures of 1500, 2000, 2500, 3000, or 3500 K for 1 second with an electrode distance of 10 mm while applying a tension of 1 mN, 10 mN, or 100 mN (3 MPa, 30 MPa, or 300 MPa for spun yarn precursor β), to produce a CNT spun yarn. Table 1 shows the relationship between the electrical heating temperature and the CNT spun yarn in the spun yarn precursor β production step when performed without a load, and Table 2 shows the relationship between the electrical heating temperature and the CNT spun yarn in the spun yarn precursor β production step when performed with a load. In the table, × indicates that the spun yarn precursor β was broken during the electrical heating treatment, and ◯ indicates that a CNT spun yarn was successfully produced.

As shown in Table 1, when the spun yarn precursor β production process was performed under unloaded conditions, the spun yarn precursor β broke during the electrical heating process when heated to a temperature of 3000 K or higher, making it impossible to obtain a CNT spun yarn. In contrast, when the spun yarn precursor β production process was performed under loaded conditions while applying tension to the spun yarn precursor α, the spun yarn precursor β did not break even when heated to 3000 K, and a satisfactory CNT spun yarn was obtained. As explained below, the higher the electrical heating temperature in the spun yarn production process, the stronger the resulting spun yarn. Therefore, by performing the spun yarn precursor β production process while applying tension to the spun yarn precursor α, the strength of the resulting spun yarn can be improved.

<II: Analysis of properties of spun yarn made from CNTs>
[II-1: Density of CNT spun yarn]
In the spun yarn precursor β production process, spun yarn precursor β was produced by spinning while applying a tension of either no load or 100 mN (300 MPa for spun yarn precursor α). Next, in the spun yarn production process, spun yarn precursor β was subjected to electrical heating at a temperature of 2500 or 3000 K for 1 second with an electrode distance of 10 mm while applying a tension of 1 mN, 10 mN, or 100 mN (3 MPa, 30 MPa, or 300 MPa for spun yarn precursor β), to produce a CNT spun yarn. As a control, a CNT spun yarn was produced without electrical heating. The densities of the CNT spun yarns obtained under various treatment conditions are shown in Figure 4. In the figure, the horizontal axis represents the treatment conditions (tension and electrical heating temperature) in the spun yarn production process, and the vertical axis represents density (g/cm ³ ). The open bars represent the results when the spun yarn precursor β production process was carried out under no load, and the solid bars represent the results when the spun yarn precursor β production process was carried out under load. The values in the figure represent the average and standard deviation of the test results obtained from the cross-sectional area measurements at six locations.

As shown in Figure 4, when the spun yarn precursor β production process was performed under unloaded conditions, the density of the CNT spun yarn obtained by electrical heating at 2500 K was substantially the same as that of the untreated CNT spun yarn. Furthermore, when electrical heating was performed at temperatures above 3000 K, the spun yarn precursor β broke during the electrical heating treatment, making it impossible to obtain a CNT spun yarn. In contrast, when the spun yarn precursor β production process was performed under loaded conditions while applying tension to the spun yarn precursor α, the density of the CNT spun yarn obtained by electrical heating at 2500 K while applying a tension of 100 mN (300 MPa for the spun yarn precursor β) was significantly higher than that of the untreated spun CNT yarn. Furthermore, the density of the CNT spun yarn obtained by electrical heating at a temperature of 3000 K was significantly higher than that of the untreated control CNT spun yarn, regardless of whether a tension load of 1 mN, 10 mN, or 100 mN (3 MPa, 30 MPa, or 300 MPa for spun yarn precursor β) was applied.

[II-2: Analysis of peaks observed in the Raman spectrum of CNT spun yarn]
The CNT spun yarn obtained in II-1 was subjected to Raman spectroscopic analysis. Figure 5 shows the Raman spectrum in the range of 1000 to 2000 cm ⁻¹ of the CNT spun yarn obtained by carrying out the spun yarn precursor β production process under load. Figure 6 shows the Raman spectrum in the range of 2200 to 3400 cm ⁻¹ of the CNT spun yarn obtained by carrying out the spun yarn precursor β production process under load. Figure 7 shows the Raman spectrum in the range of 2200 to 3400 cm ⁻¹ of the CNT spun yarn obtained by carrying out the spun yarn precursor β production process under load. In the figures, the horizontal axis represents the Raman shift (cm ⁻¹ ), and the vertical axis represents the intensity of Raman scattered light (au). In addition, in Figure 7, A is the Raman spectrum of the untreated CNT spun yarn and the CNT spun yarn obtained by electrical heating at a temperature of 2500 K, and B is the Raman spectrum of the untreated CNT spun yarn and the CNT spun yarn obtained by electrical heating at a temperature of 3000 K.

As shown in Figure 5, the Raman spectrum of the CNT spun yarn obtained by carrying out the spun yarn precursor β preparation process under load showed a G-band peak due to the crystalline ^sp2 structure and a D-band peak due to the ^sp3 structure, indicating defects or amorphousness. The ratio of the G-band peak intensity (IG) to the D-band peak intensity (ID), which is an indicator of CNT crystallinity, (IG/ID) was 1.2 for the untreated CNT spun yarn, 2.0 for the CNT spun yarn obtained by electrical heating at 2500 K while applying a tension of 100 mN (300 MPa for spun yarn precursor β), and 13 for the CNT spun yarn obtained by electrical heating at 3000 K while applying a tension of 100 mN (300 MPa for spun yarn precursor β). When experiments were performed multiple times under similar conditions, the IG/ID ratio was in the range of greater than 0.8 and less than 1.2 for the untreated CNT spun yarn, whereas it was in the range of greater than 1.2 and less than 35, particularly less than 25, for the CNT spun yarn obtained by applying tension and electrical heating.

The G ^* band peak in Raman spectra is known to originate from graphene (N. Ferralis, J. Mater. Sci. 45, 5135-5149 (2010), and M.S. Dresselhaus, et al., Nano Lett. 10 (3), 751-758 (2010)). As shown in Figures 6 and 7, the Raman spectra of the CNT-spun yarns obtained under all conditions showed a secondary D-band peak, a 2D-band peak, and a G ^* band peak originating from graphene. In particular, the G ^* band peak of the CNT-spun yarn obtained by electrical heating at 3000 K was more than twice as intense as ^that of the untreated CNT-spun yarn.

[II-3: Crystal structure analysis of CNT spun yarn]
Figure 8 shows an image of the CNT spun yarn obtained in II-1 observed with a transmission electron microscope. In the figure, (A) is a transmission electron microscope image of an untreated CNT spun yarn; (B) is a transmission electron microscope image of a CNT spun yarn obtained by carrying out the spun yarn precursor β production process under unloaded conditions, followed by electrical heating at 2500 K while applying a tension of 100 mN (300 MPa for the spun yarn precursor β); (C) is a transmission electron microscope image of a CNT spun yarn obtained by carrying out the spun yarn precursor β production process under loaded conditions, followed by electrical heating at 2500 K while applying a tension of 100 mN (300 MPa for the spun yarn precursor β); and (D) is a transmission electron microscope image of a CNT spun yarn obtained by carrying out the spun yarn precursor β production process under loaded conditions, followed by electrical heating at 3000 K while applying a tension of 100 mN (300 MPa for the spun yarn precursor β). The scale bar in the figure is 10 nm.

As shown in Figure 8A, amorphous carbon (arrows a-C in Figure 8A) was observed in the untreated control CNT spun yarn. In contrast, no amorphous carbon was observed in the CNT spun yarns obtained by carrying out the spun yarn precursor β production process under either load or load conditions, followed by electrical heating under tension (Figures 8B-D). Furthermore, in the case of the CNT spun yarn obtained by carrying out the spun yarn precursor β production process under load, flake-like crystalline structures indicating the presence of graphene were observed (arrows in Figures 8C and 8D). These results suggest that graphene may be present in at least some of the CNTs. Because flake-like crystalline structures were more prevalent in the CNT spun yarns obtained by electrical heating at 3000 K, it is inferred that electrical heating at 3000 K effectively promoted the structural transformation from amorphous carbon to graphene.

[II-4: Strength analysis of CNT spun yarn]
Figure 9 shows the relationship between the electrical heating temperature in the spun yarn production process and the Young's modulus of the resulting CNT spun yarn for the CNT spun yarn obtained in II-1, and Figure 10 shows the relationship between the electrical heating temperature in the spun yarn production process and the breaking stress of the resulting CNT spun yarn. In the figure, the horizontal axis represents the electrical heating temperature (K) in the spun yarn production process, and the vertical axis represents the Young's modulus (GPa) or breaking stress (GPa) of the CNT spun yarn. The dotted line in the figure also represents the Young's modulus or breaking stress of the untreated control CNT spun yarn, which was subjected to the spun yarn precursor β production process with or without load. The values in the figure represent the average and standard deviation of three test results.

As shown in Figure 9, the Young's modulus of the CNT spun yarn obtained by carrying out the spun yarn precursor β production process under unloaded conditions was substantially similar to that of the untreated CNT spun yarn. Furthermore, even when the electrical heating temperature in the spun yarn production process increased, no significant change was observed in the Young's modulus of the resulting CNT spun yarn. In contrast, the Young's modulus of the CNT spun yarn obtained by carrying out the spun yarn precursor β production process under loaded conditions significantly improved as the electrical heating temperature in the spun yarn production process increased. In particular, the Young's modulus of the CNT spun yarn obtained by electrical heating at a temperature of 3000 K reached a range of 190 to 220 GPa.

As shown in Figure 10, the breaking stress of the CNT spun yarn obtained by carrying out the spun yarn precursor β production process under unloaded conditions was substantially similar to the breaking stress of the untreated CNT spun yarn. Furthermore, even when the electrical heating temperature in the spun yarn production process increased, no significant change in the breaking stress of the resulting CNT spun yarn was observed. In contrast, the breaking stress of the CNT spun yarn obtained by carrying out the spun yarn precursor β production process under loaded conditions significantly improved as the electrical heating temperature in the spun yarn production process increased. In particular, the breaking stress of the CNT spun yarn obtained by electrical heating at a temperature of 3000 K reached a range of 2.0 to 2.8 GPa.

The results of this example demonstrate that a high-strength CNT spun yarn can be obtained by carrying out the spun yarn precursor β production process while applying high tension to spun yarn precursor α under load, and then performing electrical heating while applying tension. Furthermore, it was revealed that the obtained CNT spun yarn had an IG/ID ratio of 5 or greater in the Raman spectrum, and that graphene was present in at least some of the CNTs.

<III: Production of CNT spun yarn (2)>
[III-1: Crosslinking step]
In the spun yarn precursor α production process, multiple CNTs were spun while applying a tension of 1 mN per 1 cm of CNT forest width to produce spun yarn precursor α. Next, in the spun yarn precursor β production process, spun yarn precursor β was produced by spinning while applying a tension of 100 mN (300 MPa for spun yarn precursor α). Furthermore, in the spun yarn production process, spun yarn precursor β was subjected to electrical heating at a temperature of 3000 K for 1 second with an electrode distance of 10 mm while applying a tension of 1 mN (3 MPa for spun yarn precursor β) to remove residual oxygen, residual catalyst metal, and amorphous carbon, producing a CNT spun yarn.

Ultrapure water was produced by removing oxygen from the pure water through nitrogen bubbling for 30 minutes. _Na2S4 powder (sulfiding agent) was dissolved in ultrapure water to prepare _an aqueous solution containing 100 mmol/L (17.4 mg/mL) _Na2S4 . The CNT spun yarn produced using the above procedure was immersed in the aqueous solution containing _Na2S4 for a specified time at 20°C, and then air-dried at 20°C for one day to obtain a CNT spun yarn _{in which} the CNTs were crosslinked by disulfide bonds. An untreated control was used: a CNT spun yarn obtained by only the electrical heating treatment in the spun yarn production process, without the crosslinking step.

[III-2: Surface observation and density measurement of cross-linked CNT spun yarn]
The surface of the CNT spun yarn obtained by the above procedure was observed using a scanning electron microscope. The obtained images are shown in Figure 11. In the figure, A is a scanning electron microscope image of a CNT spun yarn obtained after 2 hours of impregnation treatment in the crosslinking process, B is a scanning electron microscope image of a CNT spun yarn obtained after 24 hours of impregnation treatment in the crosslinking process, and C is a scanning electron microscope image of a CNT spun yarn obtained after 48 hours of impregnation treatment in the crosslinking process. The scale bar in the figure is 10 μm. The diameter and weight of the CNT spun yarn were also measured to calculate the density (bulk density). The diameter and density of the CNT spun yarn obtained under various processing conditions are shown in Figure 12. In the figure, A is a graph showing the diameter of the CNT spun yarn, and B is a graph showing the density of the CNT spun yarn. The horizontal axis is the impregnation time in the crosslinking process, and the vertical axis is the diameter (μm) or density (g/ ^cm3 ) of the CNT spun yarn.

As shown in Figures 11 and 12, as the impregnation time in the crosslinking process increased, the diameter of the CNT spun yarn decreased significantly, and the density of the CNT spun yarn increased accordingly. For example, the diameter of the untreated CNT spun yarn was 18.4 μm, while the diameter of the CNT spun yarn obtained after 48 hours of impregnation treatment was 16.8 μm.

[III-3: Cross-sectional observation and constituent atomic analysis of cross-linked CNT yarn]
The CNT spun yarn obtained by the above procedure was cut using a focused ion beam (FIB). The sulfur atoms present in the cross section of the resulting CNT spun yarn were quantified using an energy dispersive X-ray analyzer (EDS). Scanning electron microscope (SEM) images of the cross section of the cut CNT spun yarn are shown in Figure 13. In the figure, A is a SEM image of a CNT spun yarn obtained after 2 hours of impregnation treatment in the cross-linking process, B is a SEM image of a CNT spun yarn obtained after 24 hours of impregnation treatment in the cross-linking process, and C is a SEM image of a CNT spun yarn obtained after 48 hours of impregnation treatment in the cross-linking process. The dots in the images represent sulfur atoms present in the cross section of the CNT spun yarn, as mapped using EDS. The quantitative results of the sulfur atoms present in the cross section of the cut CNT spun yarn, as quantified using EDS, are shown in Figure 14. In the figure, graph A shows the percentage of the number of sulfur atoms relative to the total number of atoms present in the cross section of the CNT spun yarn, and graph B shows the percentage of the mass of sulfur atoms relative to the total mass of atoms present in the cross section of the CNT spun yarn. The horizontal axis represents the impregnation time in the cross-linking step, and the vertical axis represents the percentage (%) of the number of sulfur atoms or the percentage (%) of the mass of sulfur atoms.

As shown in Figures 13 and 14, it was confirmed that sulfur atoms were introduced into the CNT spun yarn obtained using a method that included a crosslinking process. Furthermore, as the impregnation time in the crosslinking process increased, the number and mass of sulfur atoms introduced into the CNT spun yarn significantly increased.

[III-4: Strength analysis of cross-linked CNT spun yarn]
The breaking stress of the CNT spun yarn obtained by the above procedure was measured at a test speed of 1 mm/min and a grip distance of 10 mm according to ISO11566:1996 (JIS R 7606:2000). The stress-strain curves of the CNT spun yarn obtained under various processing conditions are shown in Figure 15. In the figure, the horizontal axis represents the strain rate (%), and the vertical axis represents the breaking stress (GPa) of the CNT spun yarn.

As shown in Figure 15, the CNT spun yarn obtained by the method including the crosslinking step had improved strain resistance and breaking stress compared to the control CNT spun yarn. In particular, when the impregnation time in the crosslinking step was 48 hours, both strain resistance and breaking stress were significantly improved.

[III-5: Strength analysis of CNT yarns cross-linked with different sulfurizing agents]
Crosslinked CNT _{spun yarns} were obtained using the same procedure as above, except that the sulfiding agent used in the crosslinking process was changed to _Na2S4 , _Na2S3 , or _Na2S2 , and the impregnation time was changed to 48 or ₂₄ hours. The breaking stress of the resulting CNT spun yarns was measured according to ISO11566:1996 (JIS R 7606:2000) at a test speed of 1 mm/min and a grip distance of 10 mm. Figures 16 and 17 show the stress-strain curves of the CNT spun yarns obtained under various processing conditions. In the figures, the horizontal axis represents strain (%), and the vertical axis represents the breaking stress (GPa) of the CNT spun yarns.

As shown in Figures 16 and 17, the strain tolerance and break stress of the CNT yarns obtained using either sulfiding agent were significantly _improved compared to the untreated control CNT yarn. The CNT yarn obtained by the crosslinking process using _Na2S4 exhibited higher strain tolerance than the CNT yarn obtained by the crosslinking process using _Na2S2 (Figure 16 ₎ . The CNT yarn obtained by _the crosslinking process using _Na2S2 also exhibited higher break stress than the CNT yarn obtained by the crosslinking process using _Na2S4 (Figure ₁₆ ). Furthermore, the CNT yarn obtained by the crosslinking process using _Na2S3 exhibited higher break stress and strain tolerance after 24 hours of impregnation treatment (Figure ₁₇ ). The CNT yarn obtained by the crosslinking process using _Na2S3 also exhibited higher strain tolerance than the CNT yarn obtained by the crosslinking process using _Na2S4 (Figures ₁₆ and ₁₇ ).

Simulations predict that when CNT spun yarn is treated with a sulfurizing agent, the CNTs are cross-linked via disulfide bonds and other mechanisms, and that when tensile force is applied to this cross-linked CNT spun yarn, the disulfide bonds repeatedly recombine, improving tensile strength (Composite Science and Technology, 2018, Vol. 166, pp. 3-9). The results of this example demonstrate that by performing the cross-linking process, the CNTs that make up the CNT spun yarn are cross-linked via disulfide bonds, trisulfide bonds, or tetrasulfide bonds, reducing voids and increasing the density of the CNT spun yarn. Furthermore, the resulting CNT spun yarn exhibited significantly improved strain resistance and breaking stress, demonstrating high strength, compared to yarns that did not undergo the cross-linking process.

The present invention is not limited to the above-described embodiments, but includes various modifications. For example, the above-described embodiments have been described in detail to clearly explain the present invention, and the present invention is not necessarily limited to those that include all of the described configurations. Furthermore, it is possible to add, delete, and/or replace part of the configuration of each embodiment with other configurations.

Claims (7)

A method for producing a spun yarn made of carbon nanotubes, comprising the following steps:
a step of producing a spun yarn precursor α, in which a plurality of carbon nanotubes are drawn out from the carbon nanotube forest, and the plurality of carbon nanotubes are spun while applying a tension of 6 mN or less per 1 cm width of the carbon nanotube forest to the plurality of carbon nanotubes, thereby producing a spun yarn precursor α made of carbon nanotubes;
a spun yarn precursor β production step in which a higher tension than that in the spun yarn precursor α production step is applied to the spun yarn precursor α to produce a spun yarn precursor β by densifying the spun yarn precursor α;
a spun yarn production step in which the spun yarn precursor β is electrically heated while applying tension to produce a spun yarn made of carbon nanotubes;
The method comprising: The method of claim 1, wherein in the step of preparing the spun yarn precursor β, tension is applied to the spun yarn precursor α while the spun yarn precursor α is brought into contact with a liquid selected from the group consisting of methanol, ethanol, acetone, water, paraffin, toluene, and mixtures thereof. The method according to claim 1 or 2, wherein tension is applied to the spun yarn precursor α in stages over multiple times during the spun yarn precursor β production process. The method according to any one of claims 1 to 3, wherein electrical heating is performed at a temperature of 3200 K or less in the spun yarn production process. The method according to any one of claims 1 to 4, wherein in the spun yarn production step, the spun yarn precursor β is electrically heated while being subjected to a tension of less than 480 MPa. The method according to any one of claims 1 to 5, further comprising a crosslinking step in which the spun yarn made of carbon nanotubes obtained in the spun yarn production step is impregnated with an aqueous solution containing a sulfide salt. The method of claim 6, wherein the sulfide salt is sodium tetrasulfide or sodium disulfide.