Carbon nanotube aggregate and method for producing same

Abstract

The purpose of the present invention is to provide carbon nanotube (CNT) aggregates having excellent dispersibility, and a method for producing the CNT aggregates.　The present invention pertains to carbon nanotube aggregates that satisfy at least one of conditions (1) to (3) below. (1) In an FT-IR spectrum of a CNT dispersion obtained by dispersing the CNT aggregates, a peak based on CNT plasmon resonance that is present in the range of a wave number exceeding 300 cm-1 but not more than 2,000 cm-1. (2) A maximum peak in differential pore volume distribution of the CNT aggregates falling in the range of a pore size exceeding 100 nm but less than 400 nm. (3) At least one peak in the two-dimensional spatial frequency spectrum of the CNT aggregates present in the range of 1 μm-1 to 100 μm-1, inclusive.

Description

[Technical Field]

[0001] The present invention relates to a carbon nanotube aggregate and a method for producing the same. [Background technology]

[0002] Carbon nanotubes (hereinafter sometimes referred to as "CNTs") have attracted attention as a material with various excellent properties due to their unique structure. For example, CNTs have excellent properties such as mechanical strength, optical properties, electrical properties, thermal properties, ion storage capacity, and ion adsorption capacity, and are expected to be used as functional materials for electronic devices, optical elements, conductive materials, and the like.

[0003] Due to their properties, CNTs tend to form aggregates easily. To improve the functionality of components that can be used in various applications, it has become necessary for CNTs to be uniformly dispersed within the components. Therefore, various methods for producing CNTs with excellent dispersibility have been investigated (see, for example, Patent Documents 1 to 3).

[0004] Specifically, for example, Patent Document 1 proposes a technology relating to a CNT dispersion liquid that can yield CNT spinning with good properties. Also, for example, Patent Document 2 proposes a technology relating to a CNT aggregate composed of CNTs that are easy to disentangle. Furthermore, for example, Patent Document 3 proposes a technology relating to a carbon nanotube aggregate that has high conductivity and high dispersibility. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2016-216863 [Patent Document 2] International Publication No. 2012 / 081601 [Patent Document 3] Japanese Patent Application Laid-Open No. 2008-201626 Summary of the Invention [Problem to be solved by the invention]

[0006] However, there is still room for further improvement in the dispersibility of CNTs that can be achieved by the above-mentioned conventional techniques.

[0007] Therefore, an object of the present invention is to provide an aggregate of carbon nanotubes having excellent dispersibility and a method for producing the same. [Means for solving the problem]

[0008] The present inventors have conducted extensive research to achieve the above object, and have newly discovered that an aggregate of carbon nanotubes that can satisfy at least one of three predetermined conditions can exhibit excellent dispersibility, thereby completing the present invention.

[0009] That is, an object of the present invention is to advantageously solve the above-mentioned problems, and the aggregate of carbon nanotubes of the present invention is characterized by satisfying at least one of the following conditions (1) to (3): (1) In a spectrum obtained by Fourier transform infrared spectrophotometric analysis of a carbon nanotube dispersion obtained by dispersing the carbon nanotube aggregate so that the bundle length is 10 μm or more, a peak due to plasmon resonance of the carbon nanotube dispersion is detected at a wave number of 300 cm -1 Super 2000cm -1 At least one of the following is present: (2) The maximum peak in the differential pore volume distribution measured for the aggregate of carbon nanotubes is in the pore diameter range of more than 100 nm and less than 400 nm. (3) The peak of the two-dimensional spatial frequency spectrum of the electron microscope image of the carbon nanotube aggregate is 1 μm -1 More than 100μm -1 At least one of the following ranges exists: An aggregate of carbon nanotubes that satisfies at least one of the above conditions (1) to (3) can exhibit excellent dispersibility.

[0010] Here, in the carbon nanotube aggregate of the present invention, at least one peak due to the plasmon resonance of the carbon nanotube dispersion has a wave number of 500 cm -1 It is preferable that the content is within the above range. The aggregate of carbon nanotubes that satisfies such conditions can exhibit even better dispersibility.

[0011] Furthermore, in the carbon nanotube aggregate of the present invention, the maximum peak in the differential pore volume distribution is 2 cm 3 / g or more. The aggregate of carbon nanotubes that satisfies such a condition can exhibit even better dispersibility.

[0012] The aggregate of carbon nanotubes of the present invention preferably satisfies at least two of the above conditions (1) to (3). The aggregate of carbon nanotubes that satisfies such conditions can exhibit even better dispersibility.

[0013] The aggregate of carbon nanotubes of the present invention preferably satisfies all of the above conditions (1) to (3). The aggregate of carbon nanotubes that satisfies such conditions can exhibit even more excellent dispersibility.

[0014] The present invention also aims to advantageously solve the above-mentioned problems, and the method for producing a carbon nanotube aggregate of the present invention is any of the methods for producing a carbon nanotube aggregate described above, characterized in that the conditions for growing the carbon nanotube aggregate satisfy all of the following (a) to (c): (a) The growth rate of the aggregate of carbon nanotubes is 5 μm / min or more. (b) The concentration of the catalyst activation material in the growth atmosphere of the aggregate of carbon nanotubes is 4% by volume or more. (c) When the carbon nanotube aggregate grows, an obstacle exists in the growth direction of the carbon nanotubes that constitute the carbon nanotube aggregate. According to the production method that satisfies these conditions, the aggregate of carbon nanotubes of the present invention can be produced efficiently.

[0015] Here, in the method for producing an aggregate of carbon nanotubes of the present invention, it is preferable that the growth atmosphere contains ethylene as a raw material gas, and that the thermal decomposition time of ethylene in the growth atmosphere is 0.5 seconds or more and 10 seconds or less. According to the production method satisfying such conditions, the aggregate of carbon nanotubes of the present invention can be produced more efficiently and with higher quality. [Effects of the Invention]

[0016] According to the present invention, it is possible to provide an aggregate of carbon nanotubes having excellent dispersibility. [Brief explanation of the drawings]

[0017] [Figure 1] 1 is a scanning electron microscope (SEM) image of a CNT aggregate according to an example of the present invention. [Figure 2] 1 is a spectrum obtained by Fourier transform infrared spectroscopic analysis of a CNT aggregate according to an example of the present invention. [Figure 3] 1 shows the differential pore volume distribution of each sample according to Examples 1 to 3, determined by the BJH method. [Figure 4A] This is one of ten electron microscope images obtained in Example 1. [Figure 4B] 4B is a two-dimensional frequency spectrum obtained for the electron microscope image shown in FIG. 4A. [Figure 5A] This is one of ten electron microscope images obtained in Example 2. [Figure 5B] 5B is a two-dimensional frequency spectrum obtained for the electron microscope image shown in FIG. 5A. [Figure 6A] FIG. 6A is one of the electron microscope images obtained in the comparative example. [Figure 6B] 6B is a two-dimensional frequency spectrum obtained for the electron microscope image shown in FIG. 6A. [Figure 7]1 shows the results of evaluating the dispersibility of the CNT aggregates obtained in Examples and Comparative Examples. [Figure 8] FIG. 1 is a diagram showing a schematic configuration of a CNT production apparatus used in Example 1. [Figure 9] FIG. 1 is a diagram showing a schematic configuration of a CNT production apparatus used in Example 2. DETAILED DESCRIPTION OF THE INVENTION

[0018] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described in detail below with reference to the accompanying drawings. The carbon nanotube aggregate (hereinafter sometimes abbreviated as "CNT aggregate") of the present invention can be efficiently produced by the production method of the present invention.

[0019] Here, the aggregate of carbon nanotubes of the present invention satisfies at least one of the conditions (1) to (3) and is excellent in dispersibility. (1) A carbon nanotube dispersion obtained by dispersing carbon nanotube aggregates so that the bundle length is 10 μm or more is subjected to Fourier transform infrared spectroscopy. In the spectrum obtained, a peak due to the plasmon resonance of the carbon nanotube dispersion is observed at a wave number of 300 cm. -1 Super 2000cm -1 At least one of the following is present: (2) The maximum peak in the differential pore volume distribution measured for the carbon nanotube aggregate is in the pore diameter range of more than 100 nm and less than 400 nm. (3) The peak of the two-dimensional spatial frequency spectrum of the electron microscope image of the carbon nanotube aggregate is 1 μm -1 More than 100μm -1 At least one of the following ranges exists:

[0020] The reason why a CNT aggregate satisfying at least one of the above conditions (1) to (3) has excellent dispersibility is unclear, but is presumed to be as follows. FIG. 1 shows a scanning electron microscope (SEM) image of an example of a CNT aggregate satisfying at least one of the above conditions (1) to (3). As shown in FIG. 1, the CNTs constituting a CNT aggregate satisfying at least one of the above conditions (1) to (3) have a wavy structure. It is presumed that this "wavy structure" can suppress interactions between the CNTs constituting the CNT aggregate. If interactions between the CNTs are suppressed, the CNTs contained in the CNT aggregate can be suppressed from tightly bundling and agglomerating. This can make it possible to easily disperse the CNT aggregate. Furthermore, if the CNT aggregate is easily dispersible, the CNT aggregate can be easily processed. This effect can result in an increase in the electrical capacity of the resulting secondary battery electrode, for example, when the CNT aggregate is formed into a sheet to produce a secondary battery electrode. Hereinafter, the above conditions (1) to (3) that can be satisfied by the CNT aggregate of the present invention will be described in detail.

[0021] -Condition (1) Condition (1) is "a carbon nanotube dispersion obtained by dispersing carbon nanotube aggregates so that the bundle length is 10 μm or more, and in the spectrum obtained by Fourier transform infrared spectroscopy, a peak due to the plasmon resonance of the carbon nanotube dispersion is at a wave number of 300 cm -1 Super 2000cm -1 At least one of the following ranges is present." Here, as an optical property of CNT, strong absorption characteristics in the far-infrared region have been widely known. Such strong absorption characteristics in the far-infrared region are thought to be due to the diameter and length of CNT. Note that the relationship between absorption characteristics in the far-infrared region, more specifically, the peak due to the plasmon resonance of CNT and the length of CNT, is described in a non-patent document (T. Morimoto et al., "Length-Dependent Plasmon Resonance in This has been discussed in detail in “Single-Walled Carbon Nanotubes,” pp. 9897-9904, Vol. 8, No. 10, ACS NANO, 2014.” Based on the investigations described in the above non-patent documents and their own findings, the present inventors speculated that the position at which a peak based on the plasmon resonance of a CNT is detected in a spectrum obtained by Fourier transform infrared spectroscopy may be affected in some way by the distance between defect points in the CNT, and conducted verification. The present inventors then found that the position at which a peak based on the plasmon resonance of a CNT is detected can serve as an index corresponding to the distance between bending points in a CNT having a wave-like structure, and set the above condition (1).

[0022] Under condition (1), the wave number is 300 cm -1 Super 2000cm -1 Within the following range, preferably within the wavenumber range of 500 cm -1 More than 2000cm -1 In the range below, preferably in the range of 700 cm -1 More than 2000cm -1 If a peak due to the plasmon resonance of a CNT exists in the following range, the CNT can exhibit good dispersibility.

[0023] As shown in Figure 2, the spectrum obtained by Fourier transform infrared spectroscopy shows a relatively gentle peak due to the plasmon resonance of the carbon nanotube dispersion, as well as a peak at a wavenumber of 840 cm -1 Near 1300cm -1 and around 1700cm -1 It can be seen that sharp peaks are observed around the wavelength of 840 cm. These sharp peaks do not correspond to "peaks based on plasmon resonance of the carbon nanotube dispersion," but each corresponds to infrared absorption due to functional groups. More specifically, -1 The sharp peak near 1300 cm is due to the CH out-of-plane bending vibration. -1 The sharp peak around 1700 cm is due to the epoxy three-membered ring stretching vibration; -1The sharp peak around 2000 cm is due to the C=O stretching vibration. -1 In the region above 2000, a peak similar to the S1 peak is detected in addition to the plasmon resonance, as mentioned in the non-patent document by T. Morimoto et al., and therefore the inventors set the upper limit for determining the presence or absence of a peak based on the plasmon resonance of the carbon nanotube dispersion under condition (1) to 2000. -1 The following is the result.

[0024] Here, in condition (1), when obtaining a spectrum by Fourier transform infrared spectroscopy, it is necessary to obtain a carbon nanotube dispersion by dispersing the carbon nanotube aggregate so that the bundle length is 10 μm or more. Here, for example, by blending the carbon nanotube aggregate, water, and a surfactant (e.g., sodium dodecylbenzenesulfonate) in an appropriate ratio and stirring the mixture for a predetermined time using ultrasound or the like, a dispersion liquid in which a carbon nanotube dispersion having a bundle length of 10 μm or more is dispersed in water can be obtained.

[0025] The bundle length of a carbon nanotube dispersion can be obtained by analyzing it with a wet image analysis particle size measuring device. Such a measuring device can calculate the area of ​​each dispersion from an image obtained by photographing the carbon nanotube dispersion, and obtain the diameter of a circle having the calculated area (hereinafter, also referred to as the ISO area diameter). In this specification, the bundle length of each dispersion is defined as the value of the ISO area diameter obtained in this manner.

[0026] -Condition(2) Condition (2) specifies that "the maximum peak in the differential pore volume distribution measured for the carbon nanotube aggregate is in the pore diameter range of more than 100 nm and less than 400 nm." The differential pore volume distribution of the carbon nanotube aggregate can be determined based on the BJH (Barrett-Joyner-Halenda) method from the adsorption isotherm of liquid nitrogen at 77 K. The BJH method is a measurement method that determines the pore diameter distribution assuming that the pores are cylindrical. The fact that the peak in the differential pore volume distribution measured for the carbon nanotube aggregate is in the range of more than 100 nm means that there are voids of a certain size between the CNTs in the carbon nanotube aggregate, and the CNTs are not in an excessively densely aggregated state. The upper limit of 400 nm is the measurement limit of the measurement device (BELSORP-mini II) used in the examples.

[0027] Here, from the viewpoint of further enhancing dispersibility, the differential pore volume value at the maximum peak in the differential pore volume distribution of the CNT aggregate is set to 2 cm 3 / g or more is preferable.

[0028] -Condition(3) Condition (3) is that the peak of the two-dimensional spatial frequency spectrum of the electron microscope image of the carbon nanotube aggregate is 1 μm -1 More than 100μm -1 At least one of the following conditions exists in the range below." Whether or not this condition is satisfied can be determined as follows. First, the CNT aggregate to be determined is observed at a magnification (for example, 10,000 times) using an electron microscope (for example, a field emission scanning electron microscope), and multiple electron microscope images (for example, 10 images) are obtained in a field of view of 1 cm square. A fast Fourier transform (FFT) is performed on the multiple electron microscope images obtained, and a two-dimensional spatial frequency spectrum is obtained. The two-dimensional spatial frequency spectrum obtained for each of the multiple electron microscope images is binarized to find the average value of the peak positions that appear on the highest frequency side. If the average value of the obtained peak positions is less than 1 μm -1 More than 100μm -1When it is within the following range, it was determined that condition (3) was satisfied. Here, as the "peak" used in the above determination, a distinct peak obtained by performing an outlier extraction process (i.e., the reverse operation of outlier removal) shall be used. Therefore, when the outlier extraction process is performed, if no distinct peak is obtained within the range of 1 μm -1 or more and 100 μm -1 or less, it is determined that condition (3) is not satisfied.

[0029] Here, from the perspective of further enhancing the dispersibility, it is preferable that the peak of the two-dimensional spatial frequency spectrum exists within the range of 2.6 μm -1 or more and 100 μm -1 or less. [[ID=​​​​​​​​​​​​​​​​​​​​​

[0032] Furthermore, the average height of the CNTs constituting the CNT aggregate is preferably 10 μm or more and 10 cm or less, and more preferably 100 μm or more and 2 cm or less. When the average height of the CNTs constituting the CNT aggregate is 10 μm or more, it prevents aggregation of adjacent CNT bundles and allows for easy dispersion. When the average height of the CNTs constituting the CNT aggregate is 100 μm or more, it facilitates the formation of a network between CNTs, making it suitable for applications requiring conductivity or mechanical strength, such as electrode formation. When the average height of the CNTs constituting the CNT aggregate is 10 cm or less, it can be produced in a short time, suppressing the adhesion of carbon-based impurities and improving the specific surface area. When the average height of the CNTs constituting the CNT aggregate is 2 cm or less, it allows for easier dispersion. The average height of the CNTs can be determined by measuring the height of 100 randomly selected CNTs using a scanning electron microscope (SEM).

[0033] The tapped bulk density of the CNT aggregate is 0.001 g / cm 3 More than 0.2g / cm 3 CNT aggregates in this density range have excellent dispersibility because the bonds between CNTs are not excessively strong, and can be molded into various shapes. 3 If the tap bulk density of the CNT aggregate is less than 0.001 g / cm, the bonds between the CNTs will be weak, making it easier to disperse the CNT aggregate homogeneously when stirred in a solvent or the like. 3 If this is the case, the integrity of the CNT aggregate will be improved and handling will be easier. Tapped bulk density is the apparent bulk density when powdered CNT aggregates are filled into a container and then tapped or vibrated to reduce the voids between the powder particles, resulting in a densely packed state.

[0034] Furthermore, the average outer diameter of the CNTs constituting the CNT aggregate is preferably 0.5 nm or more, more preferably 1.0 nm or more, and preferably 15.0 nm or less, more preferably 10.0 nm or less, and even more preferably 5.0 nm or less. If the average outer diameter of the CNTs is 1.0 nm or more, bundling of the CNTs can be reduced, allowing a high specific surface area to be maintained. If the average outer diameter of the CNTs is 5.0 nm or less, the proportion of multi-walled CNTs can be reduced, allowing a high specific surface area to be maintained. Here, the average outer diameter of the CNTs can be determined by measuring the diameter (outer diameter) of 100 randomly selected CNTs using a transmission electron microscope (TEM). The average diameter (Av) and standard deviation (σ) of the CNTs can be adjusted by changing the CNT manufacturing method and manufacturing conditions, or by combining multiple types of CNTs obtained by different manufacturing methods.

[0035] The G / D ratio of a CNT aggregate is preferably between 1 and 50. A CNT aggregate with a G / D ratio of less than 1 is thought to have low single-walled CNT crystallinity, a lot of contamination such as amorphous carbon, and a high content of multi-walled CNTs. Conversely, a CNT aggregate with a G / D ratio of more than 50 is highly linear, and the CNTs tend to form bundles with few gaps, which may reduce the specific surface area. The G / D ratio is an index commonly used to evaluate the quality of CNTs. The Raman spectrum of CNTs measured with a Raman spectrometer contains the G band (1600 cm -1 around 1350 cm -1 A vibration mode called the G band (near the G band) is observed. The G band is a vibration mode originating from the hexagonal lattice structure of graphite, which is the cylindrical surface of the CNT, and the D band is a vibration mode originating from the amorphous part. Therefore, the higher the peak intensity ratio of the G band to the D band (G / D ratio), the higher the CNT's crystallinity (linearity) can be evaluated.

[0036] To obtain a high specific surface area, it is desirable that the purity of the CNT aggregate be as high as possible. Purity here refers to carbon purity, a value that indicates what percentage of the mass of the CNT aggregate is made up of carbon. There is no upper limit to the purity required to obtain a high specific surface area, but in terms of manufacturing, it is difficult to obtain a CNT aggregate of 99.9999 mass% or more. If the purity is less than 95 mass%, it will be difficult to obtain a CNT aggregate of 1000m without being subjected to aperture treatment. 2 / g。 Furthermore, if the carbon purity is less than 95 mass % due to the inclusion of metal impurities, the metal impurities will react with oxygen during the opening treatment, preventing the opening of the CNTs, making it difficult to increase the specific surface area. From these points of view, it is preferable that the purity of the single-walled CNTs is 95 mass % or more. The purity of the CNT aggregate of the present invention is usually 98% by mass or more, preferably 99% by mass or more, and more preferably 99.9% by mass or more, even without purification treatment. The CNT aggregate of the present invention is almost free of impurities, and can fully exhibit the inherent properties of CNT. The carbon purity of the CNT aggregate can be obtained by elemental analysis using fluorescent X-rays, thermogravimetric analysis (TGA), or the like.

[0037] (Method for producing carbon nanotube aggregates) The method for producing an aggregate of carbon nanotubes of the present invention is characterized in that the conditions for growing the aggregate of carbon nanotubes satisfy all of the following (a) to (c): (a) The growth rate of the carbon nanotube aggregate is 5 μm / min or more. (b) The concentration of the catalyst activation material in the growth atmosphere of the carbon nanotube aggregate is 4% by volume or more. (c) During the growth of the carbon nanotube aggregate, an obstacle exists in the growth direction of the carbon nanotubes that constitute the carbon nanotube aggregate.

[0038] And, by the manufacturing method of the present invention, the CNT aggregate of the present invention described above can be efficiently manufactured. Further, in the manufacturing method of the present invention, as long as the above conditions (a) to (c) are satisfied during the growth of the CNT aggregate, without being particularly limited, a CNT synthesis step according to a known method such as a fluidized bed method, a moving bed method, and a fixed bed method can be employed. Here, the fluidized bed method means a synthesis method for synthesizing CNTs while fluidizing a granular carrier (hereinafter, also referred to as a granular catalyst carrier) supporting a catalyst for synthesizing CNTs. Also, the moving bed method and the fixed bed method mean synthesis methods for synthesizing CNTs without fluidizing a carrier (particulate carrier or plate-like carrier) supporting a catalyst.

[0039] In one example, the manufacturing method of the present invention includes a catalyst carrier forming step for forming a catalyst carrier, a CNT synthesis step for synthesizing CNTs using the catalyst carrier obtained in such catalyst carrier forming step, and a recovery step for recovering the CNTs synthesized in such CNT synthesis step. And, the catalyst carrier forming step can be carried out according to a known catalyst loading method, wet or dry. Also, the recovery step can be carried out using a known separation and recovery apparatus such as a classification apparatus.

[0040] <CNT synthesis step> In the CNT synthesis process, all of the above conditions (a) to (c) are satisfied during CNT growth. Specifically, the condition (a) that "the growth rate of the carbon nanotube aggregate is 5 μm / min or more" can be satisfied by appropriately adjusting the concentration and temperature of the raw material gas serving as the carbon source in the CNT growth atmosphere. The raw material gas serving as the carbon source is not particularly limited, and hydrocarbon gases such as methane, ethane, ethylene, propane, butane, pentane, hexane, heptane, propylene, and acetylene; lower alcohol gases such as methanol and ethanol; and mixtures thereof can also be used. The raw material gas may also be diluted with an inert gas. To further enhance the dispersibility of the resulting CNT aggregate, the growth rate of the CNT aggregate is preferably 10 μm / min or more. The temperature can be adjusted, for example, within a range of 400°C to 1100°C.

[0041] The raw material gas serving as the carbon source in the CNT growth atmosphere preferably contains ethylene. Heating ethylene within a specific temperature range (700°C to 900°C) promotes the decomposition reaction of ethylene, enabling rapid CNT growth when the decomposition gas comes into contact with the catalyst. However, if the pyrolysis time is too long, the ethylene decomposition reaction proceeds too quickly, causing catalyst deactivation and carbon impurities to adhere to the CNT aggregate. In the production of CNT aggregates of the present invention, a pyrolysis time of 0.5 seconds to 10 seconds is preferred for an ethylene concentration in the range of 0.1% to 40% by volume. If the pyrolysis time is less than 0.5 seconds, the ethylene pyrolysis is insufficient, making it difficult to rapidly grow a CNT aggregate with a high specific surface area. If the time is longer than 10 seconds, the ethylene decomposition proceeds too quickly, generating a large amount of carbon impurities, which can deactivate the catalyst and reduce the quality of the CNT aggregate. The pyrolysis time is calculated using the following formula: (Pyrolysis time) = (heating flow channel volume) / {(raw material gas flow rate) × (273.15 + T) / 273.15} Here, the heated flow channel volume is the volume of the flow channel heated to a predetermined temperature T°C through which the raw material gas passes before contacting the catalyst, and the raw material gas flow rate is the flow rate at 0°C and 1 atm.

[0042] Furthermore, by appropriately adjusting the supply rate of the catalytic activator supplied during CNT growth, the condition (b) that "the concentration of the catalytic activator in the growth atmosphere for the carbon nanotube aggregate is 4% by volume or more" can be satisfied. From the viewpoint of further improving the dispersibility of the resulting CNT aggregate, the concentration of the catalytic activator in the growth atmosphere for the CNT aggregate is preferably 5% by volume or more. The concentration of the catalytic activator in the growth atmosphere for the CNT aggregate is typically 40% by volume or less. Examples of catalytic activators include, but are not limited to, water, oxygen, ozone, acidic gases, nitrogen oxide, oxygen-containing compounds with low carbon numbers such as carbon monoxide and carbon dioxide; alcohols such as ethanol and methanol; ethers such as tetrahydrofuran; ketones such as acetone; aldehydes; esters; and mixtures thereof. Among these, carbon dioxide is preferred. Substances containing both carbon and oxygen, such as carbon monoxide and alcohols, may function as both a source gas and a catalytic activator. For example, carbon monoxide acts as a catalyst activator when combined with a more reactive raw material gas such as ethylene, and also acts as a raw material gas when combined with a catalyst activator such as water, which exhibits significant catalyst activation even in trace amounts.

[0043] Furthermore, by selecting a fluidized bed method in the CNT synthesis process, or by adjusting the spacing between catalyst carriers in a moving bed method or a fixed bed method, the condition (c) that "an obstacle exists in the growth direction of the carbon nanotubes that constitute the carbon nanotube aggregate during synthesis of the carbon nanotube aggregate" can be satisfied.

[0044] The catalyst carrier has a carrier and a catalyst supported on the surface of the carrier, and the carrier is a part that forms a matrix structure for supporting the catalyst on the surface of the carrier by adhering, fixing, forming a film, or forming. The carrier structure may consist of the carrier alone, or may be a carrier with an optional underlayer provided on the surface of the carrier for favorably supporting the catalyst. The carrier is preferably in the form of particles, and the particle diameter is preferably 1 mm or less, more preferably 0.7 mm or less, even more preferably 0.4 mm or less, and preferably 0.05 mm or more, in terms of volume average particle diameter. If the particle diameter is less than the above upper limit, the growing CNT bundles will be thinner, which is advantageous for forming a wave-like structure. The particle density is 3.8 g / cm in apparent density. 3 It is preferable that the concentration is 5.8 g / cm or more. 3 More preferably, it is 8 g / cm or more. 3 It is preferable that the particle density is equal to or less than the above lower limit. If the particle density is equal to or greater than the above lower limit, the force applied to the growing CNT bundles increases, which is advantageous for forming a wave-like structure. The support material is preferably a metal oxide containing at least one of Al and Zr. Among these, zirconia beads containing a large amount of Zr are particularly preferable.

[0045] In the CNT synthesis process, prior to the "growth process" performed so as to satisfy the above conditions (a) to (c), a "formation process" is performed to reduce the catalyst supported on the catalyst supporter. After the growth process is completed, a "cooling process" can be performed to cool the catalyst supporter on which the CNTs have grown. In the "formation process," for example, the atmosphere containing the catalyst supporter is a reducing gas atmosphere, and at least one of the reducing gas atmosphere or the catalyst supporter is heated to reduce and microparticulate the catalyst supported on the catalyst supporter. The temperature of the catalyst supporter or the reducing gas atmosphere in the formation process is preferably 400°C or higher and 1100°C or lower. The formation process can be performed for 3 minutes or longer and 120 minutes or shorter. Examples of the reducing gas include hydrogen gas, ammonia gas, water vapor, and mixtures thereof. The reducing gas may also be a mixture of these gases with an inert gas such as helium gas, argon gas, or nitrogen gas.

[0046] On the other hand, in the "cooling step," the catalyst support on which CNTs have been grown is cooled in an inert gas environment. Here, the inert gas may be the same as the inert gas that can be used in the growth step. In addition, in the cooling step, the temperature of the catalyst support on which CNTs have been grown is lowered preferably to 400°C or less, more preferably to 200°C or less. [Example]

[0047] The present invention will be specifically explained below with reference to examples, but the present invention is not limited to these examples. In the examples and comparative examples, various measurements and evaluations were carried out as follows.

[0048] (Fourier transform infrared spectroscopy (FT-IR)) To 10 mg of each CNT aggregate prepared in each Example and Comparative Example, 100 g of water containing sodium dodecylbenzenesulfonate as a surfactant at a concentration of 1 mass % was added, and the mixture was stirred at 45 Hz for 1 minute using an ultrasonic bath to obtain 100 ml of a dispersion of each CNT aggregate. Each dispersion prepared as described above was diluted two-fold with a solvent of the same composition, dropped onto a silicon substrate, and dried. Then, the plasmon effective length was measured using a Fourier transform infrared spectrophotometer based on the plasmon far-infrared (FIR) resonance peak. The plasmon effective length is shown in Table 1. The FIR spectrum obtained is shown in an FIR resonance chart in FIG. 2. The curves shown in FIG. 2 are, from top to bottom, for Examples 1 and 2 and the comparative example. As shown in the figure, the curves corresponding to Examples 1 and 2 are at 300 cm -1 On the other hand, the optical density peak in the curve corresponding to the comparative example was 214 cm -1 The plasmon peak top position was obtained from an approximate curve obtained by polynomial fitting using diagramming software.

[0049] (CNT bundle length measurement) For each dispersion prepared by FT-IR measurement, the ISO average circular diameter of the CNT particles present in the dispersion was measured using a flow particle image analyzer (a circulation type image analysis particle size distribution analyzer "CF-3000" manufactured by Jasco International Inc.), and the obtained value was taken as the CNT bundle length. The analysis conditions were as follows: <Analysis conditions> Injection volume: 50ml (sampling volume 1.2%) Flow cell spacer: 1000 μm Front lens magnification: 2x Telecentric lens magnification 0.75x Length per pixel: 2.3μm / pixel For each dispersion, measurements were taken four times under the same conditions while circulating, and the arithmetic mean value was calculated.

[0050] (Measurement of differential pore volume distribution) For 10 mg or more of each CNT aggregate from Examples 1 and 2 and Comparative Example, adsorption / desorption isotherms were measured using a BELSORP-mini II (Microtrack Bell) at 77 K with liquid nitrogen (adsorption equilibrium time was 500 seconds). As a pretreatment, the samples were vacuum degassed at 100°C for 12 hours. The differential pore volume distribution of each sample was calculated using the BJH method from the adsorption amount in the adsorption isotherm. The results are shown in Figure 3. As shown in Figure 3, Examples 1 and 2 showed the largest peak in differential pore volume in the pore diameter range of 100 nm or more, while Comparative Example 1 showed the largest peak in the pore diameter range of 100 nm or less.

[0051] (Two-dimensional spatial frequency spectrum analysis of electron microscope images) For each example and comparative example, 0.01 mg of the CNT aggregate was placed on carbon tape and blown with a blower to remove excess CNTs to prepare a sample. The sample was observed at 10,000x magnification using a field emission scanning electron microscope, and ten photographs were taken of randomly selected 1 cm square fields of view. Each of the ten electron microscope images was subjected to fast Fourier transform processing to obtain a two-dimensional spatial frequency spectrum. Each of the obtained two-dimensional spatial frequency spectra was binarized to determine the outermost (high-frequency) peak position and average value. Note that in the binarization process, values ​​greater than 0.75 obtained through fast Fourier transform processing were assigned a value of 1, and other values ​​were assigned a value of zero. Figure 4A shows one of the ten images obtained in Example 1, and Figure 4B shows the two-dimensional spatial frequency spectrum obtained for that image. In Figure 4B, components closer to the center represent low-frequency components, and components located further out from the center represent higher-frequency components. Arrows in the figure indicate the distances between 1 and 100 μm. -1 Among the clear peaks detected in the region, the peak position with the highest wavenumber (3 μm -1 ) is shown. FIG. 5A is one of the electron microscope images acquired in Example 2, and FIG. 5B is a two-dimensional spatial frequency spectrum acquired for this image. As a result of analysis of FIG. 5B, it was found that the -1 Among the clear peaks detected in the region, the peak position with the highest wave number was detected at 2.5 μm. -1It was confirmed that the above phenomenon occurred. Also, Fig. 6A is one of the electron microscope images acquired in the comparative example, and Fig. 6B is the two-dimensional spatial frequency spectrum acquired for this image. For all the images acquired in the comparative example, including the data shown in Figs. 6A and 6B, an isolated point extraction process was carried out to attempt to detect clear peaks, but only a few peaks were detected within the range of 1 to 100 µm. -1 No clear peak was detected in this region. Therefore, it was confirmed that the CNT aggregate according to the comparative example does not satisfy the above-mentioned condition (3).

[0052] (Dispersibility of CNT aggregates) 5 mg of each of the CNT aggregates obtained in each Example and Comparative Example was placed in separate vials, 25 mL of water was added, and the mixture was shaken 20 times to disperse. After leaving the mixture to stand for 1 hour, the state of the CNTs in the dispersion was observed. Sedimentation was confirmed in the CNT aggregate of the Comparative Example, but no sedimentation was confirmed in any of the CNT aggregates of the Examples. The results are shown in Figure 7. As is clear from Figure 7, Example 1 was uniformly dispersed without sedimentation or separation, while Example 2 did not sediment but did separate. The dispersibility of the CNT aggregates was evaluated according to the following criteria. A: Uniform dispersion confirmed after leaving it to stand for 1 hour B: After leaving it for 1 hour, check that there is no settling. C: Settling confirmed after leaving for 1 hour

[0053] Example 1 In this example, CNTs were synthesized using a fluidized bed method in the CNT synthesis process. The schematic configuration of the CNT production apparatus used is shown in Figure 8. The CNT production apparatus 100 shown in Figure 8 is composed of a heater 101, a reaction tube 102, a dispersion plate 103, a reducing gas / raw material gas inlet 104, an exhaust port 105, and a gas heating promotion section 106. The reaction tube 102 and dispersion plate 103 were made of synthetic quartz.

[0054] <Catalyst support forming step> The catalyst support formation process is described below. Zirconia (zirconium dioxide) beads (ZrO, volume average particle diameter D50: 350 μm, particle density: 6 g / cm) were used as the support. 3It was put into a rotary drum type coating apparatus, and while stirring the zirconia beads (20 rpm), an aluminum-containing solution was spray-sprayed by a spray gun (spray amount: 3 g / min, spray time: 940 seconds, spray air pressure: 10 MPa), and while supplying compressed air (300 L / min) into the rotary drum, it was dried to form an aluminum-containing coating film on the zirconia beads. Next, a firing treatment was performed at 480 °C for 45 minutes to produce primary catalyst particles having an aluminum oxide layer formed thereon. Further, while stirring the primary catalyst particles (20 rpm) put into another rotary drum type coating apparatus, an iron catalyst solution was spray-sprayed by a spray gun (spray amount: 2 g / min, spray time: 480 seconds, spray air pressure: 5 MPa), and while supplying compressed air (300 L / min) into the rotary drum, it was dried to form an iron-containing coating film on the primary catalyst particles. Next, a firing treatment was performed at 220 °C for 20 minutes to produce a catalyst support on which an iron oxide layer was further formed.

[0055] <CNT Synthesis Process> 300 g of the catalyst support thus produced was put into the reaction tube 102 of the CNT production apparatus 100, and while fluidizing the catalyst support 107 by flowing a gas, treatments were performed in the order of the formation process, the growth process, and the cooling process to produce a CNT aggregate.

[0056] The conditions of each process included in the CNT synthesis process were set as follows. - Formation Process · Set Temperature: 800 °C · Reduction Gas: Nitrogen 3 sLm, Hydrogen 22 sLm · Treatment Time: 25 minutes - Growth Process · Set Temperature: 800 °C · Source Gas: Nitrogen 15 sLm, Ethylene 5 sLm, Carbon Dioxide 2 sLm, Hydrogen 3 sLm · Treatment Time: 10 minutes · Source Gas Pyrolysis Time: 0.65 seconds - Cooling Process · Cooling Temperature: Room Temperature · Purge Gas: Nitrogen 25 sLm

[0057] The CNT aggregate synthesized on the catalyst support was separated and recovered using a forced vortex classifier (rotation speed 3500 rpm, air flow rate 3.5 Nm 3 / min). The recovery rate of the CNT aggregate was 99%.

[0058] The properties of the CNT aggregate produced by this example were tap bulk density: 0.01 g / cm 3 , CNT average height: 200 μm, BET specific surface area: 800 m 2 / g, average outer diameter: 4.0 nm, and carbon purity 99%. Also, various measurements and evaluations were carried out according to the above. The results are shown in Table 1.

[0059] (Example 2) In this example, in the CNT synthesis process, the fluidized bed method was carried out using a different apparatus from Example 1 to synthesize CNTs. The schematic configuration of the CNT production apparatus 200 used is shown in FIG. 9. The CNT production apparatus 200 is composed of a heater 201, a reaction tube 202, a dispersion plate 203, a reducing gas / raw material gas inlet 204, and an exhaust port 205. The reaction tube 202 is a double tube, and a double tube structure is adopted in which the reducing gas and the raw material gas flow between the outer tube and the inner tube to be efficiently heated to a predetermined temperature. The materials of the reaction tube 202 and the dispersion plate 203 are Inconel 601.

[0060] <CNT Synthesis Process> 1500 g of the catalyst support prepared in the same manner as in Example 1 was charged into the reaction tube 202 of the CNT production apparatus 200, and while fluidizing the catalyst support 206 by flowing gas, processing was carried out in the order of the formation process, the growth process, and the cooling process to produce a CNT aggregate.

[0061] The conditions for each process included in the CNT synthesis process were set as follows. [[ID=2​​​​​​​​​​​Feed gas: Nitrogen 30sLm, Ethylene 8sLm, Carbon dioxide 2sLm, Hydrogen 5sLm Processing time: 15 minutes - Raw gas pyrolysis time: 1.3 seconds -Cooling process ·Cooling temperature: room temperature Purge gas: Nitrogen 45sLm

[0062] The CNT aggregate synthesized on the catalyst support was separated and recovered in the same manner as in Example 1. The recovery rate of the CNT aggregate was 99%.

[0063] The properties of the CNT aggregate produced in this example are tapped bulk density: 0.01 g / cm 3 , CNT average height: 100 μm, BET specific surface area: 800 m 2 / g, average outer diameter: 4.0 nm, and carbon purity: 99%. Various measurements and evaluations were also carried out as described above. The results are shown in Table 1.

[0064] (Comparative Example) As a comparative example, a CNT aggregate (SG101) manufactured by Zeon Corporation was used. Various measurements and evaluations were carried out as described above. The results are shown in Table 1.

[0065] [Table 1]

[0066] From Table 1, it can be seen that the CNT aggregates of Examples 1 and 2, which satisfied the conditions (1) to (3), were excellent in dispersibility. [Industrial Applicability]

[0067] According to the present invention, it is possible to provide an aggregate of carbon nanotubes having excellent dispersibility. [Explanation of symbols]

[0068] 100,200 CNT production equipment 101,201 Heater 102,202 Reaction tube 103,203 Dispersion plate 104,204 Reducing gas / raw gas inlet 105,205 exhaust port 106 Gas heating promotion section 107 Catalyst support 206 Catalyst support

Claims

1. A method for producing an aggregate of carbon nanotubes, which satisfies at least one of the following conditions (1) to (3), wherein conditions during growth of the aggregate of carbon nanotubes satisfy all of the following conditions (a) to (c): (1) In a spectrum obtained by Fourier transform infrared spectrophotometric analysis of a carbon nanotube dispersion obtained by dispersing the carbon nanotube aggregate so that the bundle length is 10 μm or more, a peak due to plasmon resonance of the carbon nanotube dispersion is detected at a wave number of 300 cm －１ Super 2000cm －１ At least one of the following ranges is present: (2) The maximum peak in the differential pore volume distribution measured for the carbon nanotube aggregate is in the pore diameter range of more than 100 nm and less than 400 nm. (3) The peak of the two-dimensional spatial frequency spectrum of the electron microscope image of the carbon nanotube aggregate is 1 μm －１ 100 μm or more －１ At least one of the following ranges is present: (a) The growth rate of the aggregate of carbon nanotubes is 5 μm / min or more. (b) The concentration of the catalyst activation material in the growth atmosphere for the carbon nanotube aggregate is 4% by volume or more, and the growth atmosphere contains ethylene. (c) When the carbon nanotube aggregate grows, an obstacle exists in the growth direction of the carbon nanotubes that constitute the carbon nanotube aggregate.

2. At least one peak due to the plasmon resonance of the carbon nanotube dispersion has a wave number of 500 cm －１ The method for producing the carbon nanotube aggregate according to claim 1 , wherein the carbon nanotube content is within the above range.

3. The maximum peak in the differential pore volume distribution is 2 cm ３ The method for producing an aggregate of carbon nanotubes according to claim 1 or 2, wherein the surface roughness is 1 / g or more.

4. The method for producing an aggregate of carbon nanotubes according to any one of claims 1 to 3, wherein at least two of the conditions (1) to (3) are satisfied.

5. The method for producing an aggregate of carbon nanotubes according to any one of claims 1 to 3, which satisfies all of the conditions (1) to (3).

6. 6. The method according to claim 1, wherein the thermal decomposition time of ethylene in the growth atmosphere is 0.5 seconds or more and 10 seconds or less.

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