HK1150046A - Carbon nanotube assembly and process for producing the same - Google Patents

Description

Carbon nanotube assembly and method for producing same

Technical Field

The present invention relates to a carbon nanotube assembly and a method for producing the same.

Background

Carbon nanotubes with a small number of layers generally have a highly graphitic structure. Therefore, it is known that the single-walled carbon nanotube has high characteristics such as electrical conductivity and thermal conductivity. On the other hand, since the multi-walled carbon nanotubes have a low graphitization degree, it is known that the multi-walled carbon nanotubes generally have lower electrical and thermal conductivities than single-walled carbon nanotubes. Since the double-walled carbon nanotube has both the characteristics of a single-walled carbon nanotube and the characteristics of a multi-walled carbon nanotube, it has been spotlighted as a promising material for various applications.

In recent years, a technique for synthesizing a carbon nanotube aggregate having a high ratio of double-walled carbon nanotubes by using a chemical vapor deposition method (patent document 1), a plasma method (non-patent document 1), a pulsed arc method (patent document 2), or the like has been known.

Since impurities other than carbon nanotubes, such as catalytic metal, amorphous carbon, and granular carbon, are mixed in the produced carbon nanotube aggregate, an operation of removing the catalytic metal and the carbon impurities is required to sufficiently exhibit the original characteristics of the carbon nanotubes.

In order to remove carbon impurities, a method of heating in a gas phase is often used. To remove the catalyst metal, an acid is generally used. When a strong acid is used, the catalyst metal is easily removed, but when a strong acid is used, the carbon nanotubes are damaged and the properties are deteriorated, so that the acid used for removing the catalyst metal needs to have relatively smooth reactivity. Non-patent document 2 describes that when single-layer carbon nanotubes are treated in a nitric acid solution, defects such as functionalization and graphite structure occur. Non-patent document 3 discloses that if the heating of the multilayered carbon nanotube is continued, the carbon nanotube is functionalized, and the G/D ratio measured by raman spectroscopy, which is one of indexes showing the purity of the carbon nanotube, is lowered. Patent document 1 also reports that 20 or more layers (which is understood to mean that the distance between the carbon nanotubes is 0.34nm, calculated from the average diameter before and after treatment) are removed by using nitric acid.

However, in general, when an acid such as nitric acid, a mixed acid of nitric acid and sulfuric acid, or the like is used, the surface of the carbon nanotube may be functionalized, and therefore, hydrochloric acid is often used as a metal that can be removed by hydrochloric acid in practice for the above reasons. In particular, in the case of single-walled carbon nanotubes, since they are composed of only 1 graphite layer, they are significantly affected by functionalization.

In addition, in order to improve the conductivity as the carbon nanotube aggregate, a method of separating metallic carbon nanotubes having high conductivity from semiconducting carbon nanotubes by electrophoresis, a synthesis method in which metallic nanotubes are the main component in the synthesis stage, and the like are considered, but both techniques are difficult to be applied to carbon nanotubes having two or more layers, and at present, a double-layer carbon nanotube having high conductivity having advantages of both single layer and multi-layer is not obtained.

Patent document 1: japanese unexamined patent publication No. 2006-335604

Patent document 2: japanese laid-open patent publication No. 2004-168647

Patent document 3: japanese unexamined patent application publication No. 2005-154200

Non-patent document 1: journal of Physical Chemistry B, 107(2003), 8794-

Non-patent document 2: journal of American Chemical Society, 126(2004), 6095-

Non-patent document 3: carbon, 43(2005), 3124-

Disclosure of Invention

In view of the above problems, an object of the present invention is to provide a carbon nanotube assembly having high conductivity and a method for producing the carbon nanotube assembly simply and at high yield.

As a result of intensive studies, the present inventors have found that a carbon nanotube assembly containing double-walled carbon nanotubes having high conductivity can be obtained by treating a double-walled carbon nanotube assembly having a high degree of graphitization with a nitric acid solution.

That is, the present invention is a carbon nanotube assembly satisfying all of the following conditions (1) to (3):

(1) volume resistivity of 1X 10^-5～5×10^-3Ω·cm；

(2) When observed by using a transmission electron microscope, more than 50 of 100 carbon nanotubes are double-layer carbon nanotubes;

(3) when thermogravimetric analysis is performed at a temperature rise of 10 ℃/min, the weight loss at a temperature rise from 200 ℃ to 400 ℃ is 5-20%.

The present invention is also a method for producing the carbon nanotube aggregate by heating a double-walled carbon nanotube aggregate having a height ratio (G/D ratio) of a G band to a D band of 20 or more as measured by raman spectroscopy at a wavelength of 633nm in a nitric acid solution.

By using the carbon nanotube aggregate having excellent conductivity of the present invention, a film having excellent light transmittance and surface resistance can be obtained.

In addition, the carbon nanotube assembly having high conductivity can be obtained simply and with good yield by using the production method of the present invention.

Drawings

FIG. 1 is a schematic view of a fluidized-bed vertical reactor used for producing the carbon nanotube aggregate of example 1.

Fig. 2 shows the transparent conductivity evaluation results in example 1, example 2 and example 6.

Fig. 3 records results obtained by evaluating the outer diameter and the number-of-layers distribution of the carbon nanotube aggregate produced in example 1 using a transmission electron microscope, and the average value of the outer diameter and the standard deviation thereof at this time.

Fig. 4 shows binding energy (eV) of O1s as a result of XPS measurement of the carbon nanotube aggregate in example 1.

FIG. 5 is a schematic view of a fixed-bed vertical reactor used in the production of the carbon nanotube aggregate of example 5.

Description of the reference numerals

100 reactor

101 quartz sintered plate

102 sealed catalyst feeder

103 catalyst addition line

104 raw material gas supply line

105 waste gas pipeline

106 heater

107 detection port

108 catalyst

200 reactor

201 nonwoven fabric

204 raw material gas supply line

205 exhaust gas line

206 heater

207 detection port

208 catalyst

Detailed Description

In the present invention, the carbon nanotube aggregate refers to an aggregate in which a plurality of carbon nanotubes are present. The form of the carbon nanotubes is not particularly limited, and they may be present independently, may be present in the form of a bundle, a cross, or the like, or may be present in a mixture of these forms. In addition, carbon nanotubes of various numbers of layers and diameters may be contained. Further, when a plurality of carbon nanotubes are contained in a dispersion liquid, a composition in which other components are blended, or a composite body in which a plurality of carbon nanotubes are combined with other components, it is also understood that a carbon nanotube assembly is contained. The carbon nanotube aggregate may contain impurities (e.g., a catalyst) derived from the production method.

The volume resistivity of the carbon nanotube aggregate of the present invention is 1X 10^-5～5×10^-3Omega cm. Under the preferred production conditions, 1X 10 can be obtained^-51×10^-3Omega cm carbon nanotube assembly. The volume resistance value of the carbon nanotube assembly can be calculated by preparing a carbon nanotube film, measuring the surface resistance value of the film by a four-terminal method, and multiplying the surface resistance value by the film thickness of the carbon nanotube film. The surface resistance value can be measured by, for example, a low resistance meter EP MCP-T360 (manufactured by KAI' S ダィァィンスッルメンッ) using a four-terminal four-probe method defined in JISK 7149. For measuring the high resistance, the measurement was carried out by using a high resistance measuring instrument UP MCP-HT450 (manufactured by ダィァィンスッルメンッ, 10V, 10 seconds).

The measurement samples were prepared as follows. Mixing carbon nanotube 20mg and N-methylpyrrolidone 16mL, irradiating with ultrasonic wave at output power of 20W for 20 min by using ultrasonic homogenizer, mixing with ethanol 10mL, and using inner diameterThe filter of (2) to obtain a filtrate. The filtrate was dried at 60 ℃ for 2 hours together with the filter and the filter paper used in the filtration, thereby obtaining a carbon nanotube film for measurement. The carbon nanotube film thus produced is peeled off from the filter paper by using tweezers or the like, and then measured. When the carbon nanotube film cannot be peeled off from the filter paper, the filter paper and the carbon nanotube can be measuredThe thickness of the carbon nanotube film was calculated by subtracting the thickness of the filter paper alone from the total thickness of the tube film. As the filter paper used in filtration, for example, a membrane filter (microporous membrane filter, filter type: 1.0 μm JA,). The pore diameter of the filter paper may be 1.0 μm or less as long as the filtrate can pass through the filter paper. The material of the filter paper needs to be a material insoluble in NMP and ethanol, and a filter made of fluorine-based resin is preferably used.

Further, since the carbon nanotube aggregate of the present invention is particularly excellent in conductivity, when used for a conductive layer which requires transparency, such as a transparent electrode, for example, sufficient conductivity can be exhibited even when the amount of carbon nanotubes used is small, and the effect of improving transparency can be obtained by reducing the amount of carbon nanotubes used.

Carbon nanotubes have a shape in which 1 piece of graphite is rolled into a cylinder, and those rolled into 1 layer are called "single-walled carbon nanotubes", and those rolled into multiple layers are called "multi-walled carbon nanotubes". Among the multi-layered carbon nanotubes, those wound in 2 layers are particularly called "double-layered carbon nanotubes". The morphology of carbon nanotubes can be investigated using high resolution transmission electron microscopy. It is not preferable that the graphite layer is clear under a transmission electron microscope, but the graphite layer may be disordered.

When observed by using a transmission electron microscope, more than 50 carbon nanotubes among 100 carbon nanotubes in the carbon nanotube aggregate of the present invention are double-walled carbon nanotubes. The number of layers was evaluated by observing the carbon nanotubes 40 ten thousand times using a transmission electron microscope, and selecting 100 carbon nanotubes arbitrarily in a field of view in which 10% or more of the area of the field of view in a field of view having a radius of 75nm is a carbon nanotube. When 100 were not measured in one field, the measurement was taken from a plurality of fields until 100 were reached. In this case, 1 carbon nanotube means that a part of the carbon nanotube is 1 in the visual field, and it is not always necessary to see both ends. In addition, it is possible to count 2 items by considering 2 items in the visual field and 1 item by connecting them in the visual field.

In general, the smaller the number of carbon nanotube layers, the higher the graphitization degree, that is, the higher the conductivity, but the durability tends to be low. On the other hand, the larger the number of carbon nanotube layers, the lower the graphitization degree, that is, the lower the conductivity, but the higher the durability. The double-walled carbon nanotube has high durability and high conductivity because of its high graphitization degree. Therefore, the double-walled carbon nanotube has a high ratio. In the present invention, the ratio of the double-walled carbon nanotubes measured by the above method is preferably 50 or more out of 100, more preferably 70 or more out of 100, still more preferably 75 or more out of 100, and most preferably 80 or more out of 100.

The average value of the outer diameters of the carbon nanotubes is preferably in the range of 1.0 to 3.0 nm. The average value of the outer diameters is an arithmetic average value obtained by observing a sample and measuring the outer diameters of 100 carbon nanotubes by the same method as in the above evaluation of the number of layers.

In addition, the carbon nanotubes preferably have a narrow outer diameter distribution. Specifically, the standard deviation of the outer diameter is preferably 1.0nm or less, more preferably 0.8nm or less, and still more preferably 0.7nm or less. This standard deviation is a standard deviation calculated based on the obtained outer diameters of 100 pieces, evaluated by the same method as the above-described evaluation method of the outer diameters. When the distribution of the outer diameters of the carbon nanotubes is small, a carbon nanotube assembly having good conductivity can be easily obtained.

The carbon nanotube assembly of the present invention has a weight loss of 5 to 20% when heated from 200 ℃ to 400 ℃ in thermogravimetric analysis (Thermogravimetry) performed at a temperature rise of 10 ℃/min. The weight loss can be measured by thermal analysis of the carbon nanotube aggregate in an air atmosphere. About 1mg of the sample was placed in a differential thermal analyzer (for example, TGA-60 manufactured by Shimadzu corporation), and the temperature was raised from room temperature to 900 ℃ in air at a temperature raising rate of 10 ℃ per minute. The weight decrease from 200 ℃ to 400 ℃ means the proportion of the weight decrease from 200 ℃ to 400 ℃ to the weight decrease from 200 ℃ to 900 ℃.

Generally, since carbon impurities such as amorphous carbon other than carbon nanotubes are decomposed at 400 ℃ or lower, when the weight of heat of the carbon nanotube aggregate containing carbon impurities is measured, the weight decrease during the temperature increase from 200 ℃ to 400 ℃ is observed. The more carbon impurities, the more weight loss increases from 200 ℃ to 400 ℃. In general, the larger the amount of carbon impurities, the lower the conductivity as the carbon nanotube aggregate, and therefore, the conductivity of the carbon nanotube aggregate is generally improved by reducing the carbon impurities. In fact, a large number of methods for producing a carbon nanotube aggregate having a high rate of double-walled carbon nanotubes, in which the weight loss is less than 5% when the temperature is raised from 200 ℃ to 400 ℃, have been reported. However, the carbon nanotube assembly of the present invention has excellent conductivity when the weight loss is in the range of 5 to 20% when the temperature is raised from 200 ℃ to 400 ℃.

The reason why the carbon nanotube assembly of the present invention has high conductivity although the weight reduction from 200 ℃ to 400 ℃ is 5 to 20% is not clear. However, the carbon nanotube aggregate containing carbon impurities in an amount of 5% by weight or more does not exhibit the high conductivity as in the present invention unless it is subjected to a special treatment. It is presumed that the weight loss of 5 to 20% when the temperature is raised from 200 ℃ to 400 ℃ in the present invention means that not carbon impurities but a large amount of functional groups which are burned out in the above temperature range are contained, and as a result, the weight loss is in the above range.

It is considered that, in the case where a C ═ O group as an electron-withdrawing functional group is bonded to the surface of the carbon nanotube, the conductivity of the carbon nanotube becomes very high due to the effect of p-type doping to the carbon nanotube. However, since the graphite layer is defective due to the functionalization, the conductivity of the carbon nanotube itself is lowered if the functional group is excessive. On the other hand, if the functional group is too small, the doping effect and the defect effect cancel each other, and the effect of improving the conductivity cannot be exhibited. It is considered that the degree of functionalization in an appropriate amount is in a range of 5 to 20% by weight reduction in a range of 200 to 400 ℃. From the viewpoint of the conductivity of the carbon nanotubes, the weight reduction is more preferably 5 to 15%, and still more preferably 6 to 13%.

The C — O group and the C ═ O group in the functional group can be determined by X-ray photoelectron spectroscopy (XPS). For example, in exciting X-rays: monochromatic Al K_1，2Line, X-ray diameter: 1000 μm, photoelectron extraction angle: under the condition of 90 degrees (the detector is inclined relative to the surface of the sample), the peak of O1s can be determined by detecting the peaks of C-O group and C ═ O group near 532-533 (eV). In general, the binding energy (eV) in the case of carbon nanotubes is C — O > C ═ O. Preferably, it is appropriate to determine an element that does not peak at the same position in the result of analyzing the surface composition (atomic%) using X-ray photoelectron spectroscopy.

Further, as another index of the ratio of functionalization of the carbon nanotube, surface composition analysis by X-ray photoelectron spectroscopy (XPS) can be used. As a result of surface composition analysis by X-ray photoelectron spectroscopy (XPS) in the present invention, it is preferable that the ratio of oxygen atoms to carbon atoms is 4% (atomic%) or more, because a carbon nanotube assembly having excellent electrical conductivity can be obtained. As described above, when the proportion of the functionalization is too large, defects due to the functionalization increase, and as a result, the conductivity of the carbon nanotube itself decreases, so the proportion of the oxygen atom to the carbon atom is preferably 20% (atomic%) or less, more preferably 15% or less, and further preferably 11% or less.

In the carbon nanotube assembly of the present invention, when the differential thermal analysis is performed at a temperature rise of 10 ℃/min, the maximum peak of the DTA curve is preferably in the range of 650 to 750 ℃. This peak can be measured by performing differential thermal analysis on the carbon nanotube aggregate in an air atmosphere. About 1mg of the sample was placed in a differential thermal analyzer (for example, TGA-60 manufactured by Shimadzu corporation), and the temperature was raised from room temperature to 900 ℃ in air at a temperature raising rate of 10 ℃ per minute. The DTA curve (temperature (. degree. C.) on the x-axis and DTA signal (. mu.V/mg) on the y-axis) at this time was read. The maximum peak of the DTA curve is the temperature at which the value of the DTA signal becomes maximum, and is also referred to as the combustion peak temperature.

In general, the higher the graphitization degree of the carbon nanotube and the less the carbon impurities, the more the combustion peak temperature appears on the high temperature side, and the carbon nanotube having high durability is preferred because the combustion peak temperature is high. However, the carbon nanotube assembly of the present invention has both the degree of functionalization and purity of the carbon nanotubes, and therefore, preferably has a combustion peak temperature in the range of 650 to 750 ℃. More preferably 665 to 735 ℃.

Carbon nanotube aggregates formed of shorter carbon nanotubes are generally less conductive than carbon nanotube aggregates formed of longer carbon nanotubes. The carbon nanotube aggregate of the present invention has high conductivity even when the average length of the carbon nanotubes is 10 μm or less, preferably has high conductivity when the average length of the carbon nanotubes is 5 μm or less, and more preferably has high conductivity when the average length of the carbon nanotubes is 3 μm or less. If the lower limit of the average length of the carbon nanotubes is too short, the contact points between the carbon nanotubes become too large, which greatly affects the increase of the resistance value, and therefore, it is preferably 100nm or more. The average length is a value obtained by calculating an arithmetic average of lengths of all the carbon nanotubes observed when a length measurement sample is prepared and observed by AFM by the method described below.

The samples were prepared as follows. A carbon nanotube aggregate dispersion was prepared by charging 15mg of a carbon nanotube aggregate, 450. mu.L of an ammonium polystyrene sulfonate salt aqueous solution (30 wt.% manufactured by ァルドリッチ, having a weight average molecular weight of 20 ten thousand in terms of GPC measurement and polystyrene conversion) and 9.550mL of distilled water into a 20mL vessel, and carrying out a dispersion treatment with an ultrasonic homogenizer at an output of 20W under ice cooling for 20 minutes. The resulting dispersion was centrifuged at 20000G for 15 minutes using a high-speed centrifuge to obtain 9mL of a supernatant. The resulting supernatant was diluted 65-fold with distilled water, coated on mica using a bar coater (No.3), and then dried at 120 ℃ for 2 minutes, and the resulting film was used as a sample.

The carbon nanotube aggregate of the present invention can be used as a field emission material. For example, when the composition containing carbon nanotubes of the present invention is used for an electron source for field emission, since the conductivity is high, the applied voltage can be controlled to a low level. Further, it is assumed that the carbon nanotube aggregate of the present invention has good durability, and thus a good emission material can be obtained.

By using the carbon nanotube aggregate of the present invention, a carbon nanotube molded body having very high conductivity can be produced. Is suitable for producing a carbon nanotube molded body having extremely high conductivity and excellent strength. The carbon nanotube molded body refers to an object in all states shaped by molding or processing a carbon nanotube aggregate. The molding or processing means all operations that go through an operation or a step of changing the shape of the carbon nanotube aggregate. Examples of the carbon nanotube molded body include a system composed of an aggregate of carbon nanotubes, chips, particles, sheets, and blocks. These are combined and further molded or processed to obtain a product as a carbon nanotube molded body.

Examples of the molding method include a method of removing a liquid containing the carbon nanotube aggregate by filtration, evaporation, or the like to mold a film, or a sheet, and a method of injecting a liquid containing the carbon nanotube aggregate into a mold and then evaporating a dispersion medium. Further, a method of compressing the carbon nanotube aggregate by a press, a method of cutting or chipping the carbon nanotube aggregate by a cutter, or the like can be used. In addition, a method of aggregating carbon nanotubes in a liquid containing the carbon nanotube aggregate may be suitably used. The method of aggregating the carbon nanotubes in the liquid containing the carbon nanotube aggregates varies depending on the type of the dispersion medium, and for example, when the dispersion medium is water, the liquid containing the carbon nanotube aggregates is added to an organic solvent.

The carbon nanotube assembly of the present invention can be mixed with or dispersed in a substance other than carbon nanotubes, and used as a composition. The composition containing the carbon nanotube assembly of the present invention can be a composition having very high electrical conductivity, excellent strength, excellent thermal conductivity, high electrical conductivity and excellent strength. The term "substance other than carbon nanotubes" as used herein refers to, for example, resin, metal, glass, organic solvent, water, and the like. Further, the adhesive may be a binder, cement, gypsum, or ceramics. These may be used alone or in combination of 2 or more.

The term "dispersion" as used herein refers to a state in which carbon nanotubes are uniformly dispersed in the above-mentioned substance. The carbon nanotubes are dispersed as long as they are uniformly dispersed in the above-mentioned substance, regardless of whether the carbon nanotubes are in a state of being untwisted one by one, or in a state of being grouped into bundles, or in a state of being mixed with bundles of various thicknesses.

The term "mixed" as used herein means a state in which the carbon nanotube aggregate is not uniformly dispersed in the substance, or a state in which the carbon nanotube aggregate and the substance in a solid state are merely mixed together.

With respect to the content of the carbon nanotubes in the composition, there is no particular limitation in the case of mixing, and it may be mixed in a desired ratio. In the case of dispersing carbon nanotubes, the amount of the carbon nanotubes to be dispersed is preferably 0.01 to 20% by weight, more preferably 0.01 to 10% by weight, even more preferably 0.01 to 5% by weight, and particularly preferably 0.05 to 1% by weight, based on the composition. Although the carbon nanotube composition may be different depending on the purpose, if too many carbon nanotubes are added, the strength of the carbon nanotube composition may be reduced.

The resin in the substance other than the carbon nanotubes is not particularly limited as long as it can be mixed with or dispersed in the carbon nanotubes, and a natural resin or a synthetic resin may be used. Further, as the synthetic resin, both thermosetting resins and thermoplastic resins are suitably used. The thermoplastic resin is preferable because the resulting molded article has excellent impact strength and can be compression molded or injection molded with high molding efficiency.

The thermosetting resin is not particularly limited, and for example, unsaturated polyester resin, vinyl ester resin, epoxy resin, cyanate ester resin, benzoxazine resin, phenol (a-stage) resin, urea/melamine resin, thermosetting polyimide, etc., and copolymers, modified products thereof, and resins obtained by mixing 2 or more kinds thereof can be used. In addition, in order to improve the impact resistance, a resin obtained by adding a soft component such as an elastomer, a synthetic rubber, a natural rubber, or a silicone to the thermosetting resin may be used.

The thermoplastic resin is not particularly limited, and examples thereof include fluorine-based resins such as polyester, polyolefin, styrene-based resin, polyoxymethylene, polyamide, polycarbonate, poly (メチレン) methacrylate, polyvinyl chloride, polyphenylene sulfide, polyphenylene ether, polyimide, polyamideimide, polyetherimide, polysulfone, polyethersulfone, polyketone, polyetherketone, polyetheretherketone, polyetherketoneketone, polyarylate, polyethernitrile, phenol (novolak type, etc.) resin, phenoxy resin, polytetrafluoroethylene, and the like; thermoplastic elastomers such as polystyrenes, polyolefins, polyurethanes, polyesters, polyamides, polybutadienes, polyisoprenes and fluorines; copolymers and modified products thereof, and resins obtained by mixing 2 or more of these resins. In addition, in order to improve the impact resistance, the thermoplastic resin may be added with other soft components such as elastomer, synthetic rubber, natural rubber, and silicone.

Here, the styrene-based resin refers to a resin containing a unit derived from styrene and/or a derivative thereof (which may be collectively referred to as "aromatic vinyl monomer"). The aromatic vinyl monomer may be polymerized with 1 or 2 or more species, or may be copolymerized with 1 or 2 or more species of other monomers copolymerizable therewith. Further, a styrene-based resin reinforced with rubber is also suitably used. Examples of the rubber-reinforced styrene-based resin include 2 types, i.e., a structure in which a (co) polymer containing an aromatic vinyl monomer is partially grafted to a rubber polymer and a non-grafted structure. Specific examples of the styrene-based resin include PS (polystyrene), HIPS (high impact polystyrene), AS resin, AES resin, ABS resin, MBS (methyl methacrylate/butadiene/styrene copolymer) ("/" represents a copolymer) resin, ASA (acrylonitrile/styrene/acrylic rubber copolymer) resin, and the like.

The polycarbonate is not particularly limited, and examples thereof include an aromatic polycarbonate homopolymer or a polycarbonate copolymer having a viscosity average molecular weight of 10000 to 1000000, which is obtained by reacting an aromatic dihydric phenol compound with phosgene or a carbonic acid diester.

The polyamide is not particularly limited, and for example, nylon 6, nylon 66, nylon 46, nylon 610, nylon 612, nylon 9T (T is terephthalic acid), nylon 66/6, nylon 66/6T, nylon 66/6I (I is isophthalic acid), nylon 6/6T, nylon 6/6T, nylon 12/6T, nylon 6T/6I, nylon 66/6T/6I, nylon 66/6/6T, nylon 66/6/6I, nylon 6T/M5T (M is methylpentamethylenediamine), polymetaxylylene adipamide, a copolymer or a mixture thereof, and the like can be preferably used.

The polyester is not particularly limited, and examples thereof include a polycondensate of a dicarboxylic acid and a diol, a ring-opening polymer of a cyclic lactone, a polycondensate of a hydroxycarboxylic acid, and a polycondensate of a dibasic acid and a diol. Specific examples thereof include polyethylene terephthalate, polypropylene terephthalate, poly-1, 3-propylene terephthalate, polybutylene terephthalate, polyethylene naphthalate, polybutylene naphthalate, polycyclohexanedimethanol terephthalate, and poly-1, 2-bis (phenoxy) ethane-4, 4' -dicarboxylic acid ethylene ester, and copolymers and mixtures thereof.

When high flame retardancy or high moldability is to be imparted to the resin composition containing carbon nanotubes, a phenol resin or the like can be used as the resin. The phenol resin herein refers to a resin obtained by copolymerizing at least a component having a phenolic hydroxyl group alone or in combination, and examples thereof include various phenol resins (e.g., phenol novolac resin, cresol novolac resin, octyl phenol resin, phenyl phenol resin, naphthol novolac resin, phenol aralkyl resin, naphthol aralkyl resin, resol resin, etc.), modified phenol resins (e.g., alkylbenzene modification (particularly, xylene modification), cashew nut modification, terpene modification, etc.), and the like.

Further, there may be mentioned a polyol resin represented by polyvinyl alcohol, a polycarboxylic acid resin represented by polyvinyl acetate, an acrylic resin such as polyacrylate, and a resin such as polyacrylonitrile. Examples of the binder include binders and adhesives of vinyl type such as acrylic, silicone, vinyl acetate, and vinyl ether resins.

The metal is not particularly limited as long as it can be mixed with or dispersed in the carbon nanotubes, and aluminum, copper, silver, gold, iron, nickel, zinc, lead, tin, cobalt, chromium, titanium, tungsten, or the like can be used alone or in combination. The glass is not particularly limited as long as it can be mixed with or dispersed in carbon nanotubes, and examples thereof include soda lime glass, lead glass, and boric acid glass.

As a method for mixing or dispersing the above-mentioned substance with the carbon nanotube aggregate of the present invention, for example: a method of mixing the carbon nanotube aggregate while stirring the substance in a molten state, a method of melting the substance in a state where the powder of the substance and the powder of the carbon nanotube aggregate are mixed together, and then solidifying the melted substance, and the like.

The solvent is not particularly limited as long as the carbon nanotube aggregate can be mixed or dispersed, and various organic compounds such as alcohols, aromatic compounds, aliphatic compounds, diol compounds, amide compounds, ester compounds, and ether compounds can be used. These compounds may be used alone or in combination. Examples of the alcohol include methanol, ethanol, propanol, butanol, pentanol, hexanol, heptanol, octanol, nonanol, and decanol. Examples of the aromatic compound include benzene, toluene, xylene, chlorobenzene, dichlorobenzene, phenol, pyridine, thiophene, furan, and the like. Examples of the aliphatic compound include pentane, hexane, heptane, octane, nonane, decane, undecane, and dodecane. Examples of the diol compound include ethylene glycol, propylene glycol, and glycerin. Examples of the amide compound include dimethylformamide, ethylmethylformamide, and dimethylacetamide. Examples of the ester compound include a formate (e.g., methyl formate and ethyl formate), an acetate (e.g., ethyl acetate and methyl acetate), and a butyrate (e.g., methyl butyrate and ethyl butyrate). Examples of the ether compound include diethyl ether, ethyl methyl ether, and tetrahydrofuran. Isomers, derivatives, and the like of these compounds can be used. Further, chloroform, methylene chloride, dimethyl sulfoxide, supercritical carbon dioxide, carbon disulfide and the like can be cited.

The solid composition of the carbon nanotube composition may be shaped by molding or processing through compression, cutting, crushing, stretching, punching, or the like, or may be melted and changed into a solid state again in a specific shape to prepare a molded body.

In the present invention, a carbon nanotube composition in which a carbon nanotube aggregate is dispersed in a liquid dispersion medium such as an organic solvent or water is also preferable. Such a carbon nanotube composition is hereinafter sometimes referred to as a "carbon nanotube dispersion liquid" or a "dispersion liquid".

As a method for mixing or dispersing the above-mentioned substance with the carbon nanotube aggregate of the present invention, for example, in the case of mixing, it is preferable to simply mix and then stir using a screw, a rod, or the like, and it is also preferable to shake. In addition, in the case of dispersion, a ball mill, a bead mill, a roll mill, a pulverizer, an ultrasonic homogenizer are suitably used. Furthermore, it is also suitable to combine the above-mentioned methods.

In the case of dispersing the carbon nanotube aggregate in a liquid dispersion medium to prepare a carbon nanotube dispersion, it is preferable to contain additives such as a surfactant and various polymer materials. This is because the surfactant and some polymer materials can improve the dispersibility and dispersion stabilization ability of the carbon nanotubes.

The surfactant may be classified into an ionic surfactant and a nonionic surfactant, and any surfactant may be used in the present invention. The surfactants may be used alone or in combination of 2 or more.

The ionic surfactants are classified into cationic surfactants, amphoteric surfactants, and anionic surfactants. Examples of the cationic surfactant include alkylamine salts and quaternary ammonium salts. Examples of the amphoteric surfactant include alkyl betaine surfactants and amine oxide surfactants. Examples of the anionic surfactant include alkyl benzene sulfonate such as dodecylbenzenesulfonic acid, aromatic sulfonic acid surfactant such as dodecylphenyl ether sulfonate, monobasic acid soap anionic surfactant, ether sulfate surfactant, phosphate ester surfactant, and carboxylic acid surfactant, and among them, aromatic ionic surfactants containing an aromatic ring, that is, aromatic ionic surfactants are preferable because of their excellent dispersing ability, dispersion stability, and high concentration, and aromatic ionic surfactants such as alkyl benzene sulfonate and dodecylphenyl ether sulfonate are particularly preferable.

Examples of the nonionic surfactant include sugar ester surfactants such as sorbitan fatty acid esters and polyoxyethylene sorbitan fatty acid esters; fatty acid ester surfactants such as polyoxyethylene resin acid esters and polyoxyethylene fatty acid diethyl esters; ether surfactants such as polyoxyethylene alkyl ethers, polyoxyethylene alkylphenyl ethers, and polyoxyethylene/polypropylene glycols; aromatic nonionic surfactants such as polyoxyalkylene octyl phenyl ether, polyoxyalkylene nonyl phenyl ether, polyoxyalkyl dibutyl phenyl ether, polyoxyalkyl styryl phenyl ether, polyoxyalkyl benzyl phenyl ether, polyoxyalkyl diphenyl ether, and polyoxyalkyl cumyl phenyl ether. Among them, aromatic nonionic surfactants are preferable, and polyoxyethylene phenyl ether is particularly preferable, because of their excellent dispersibility, dispersion stability and high concentration performance.

Various polymer materials other than the surfactant may be added to the carbon nanotube dispersion liquid. For example, water-soluble polymers of polyvinyl alcohol, polyvinyl pyrrolidone, ammonium salts of polystyrene sulfonic acid, sodium salts of polystyrene sulfonic acid, and the like; and carbohydrate polymers such as sodium carboxymethylcellulose (Na-CMC), methylcellulose, hydroxyethylcellulose, amylose, cyclic amylose, and chitosan. In addition, conductive polymers such as polythiophene, polyethylenedioxythiophene, polyisothianaphthene, polyaniline, polypyrrole, and polyacetylene, and derivatives thereof can also be used. Among them, the conductive polymers and their derivatives are preferable because the conductive properties of the carbon nanotubes can be effectively exhibited by using the conductive polymers and their derivatives.

The method for producing the carbon nanotube dispersion liquid is not particularly limited, and for example, the carbon nanotube aggregate, the additive and the dispersion medium may be mixed together by using a mixing and dispersing machine (for example, a ball mill, a bead mill, a sand mill, a roll mill, a homogenizer, a mill, a mixer, a pigment disperser, etc.) which is generally used in coating production to produce the dispersion liquid.

In particular, in the case of applications requiring excellent conductivity and a conductive layer for a transparent electrode, it is preferable to size-classify the carbon nanotube dispersion by centrifugal separation or filter filtration before coating it. For example, the dispersion liquid is centrifuged to precipitate undispersed carbon nanotubes, an excessive amount of an additive, a metal catalyst that may be mixed in during the synthesis of carbon nanotubes, and the like. By collecting the supernatant liquid and removing impurities as precipitates or the like, re-aggregation of the carbon nanotubes can be prevented and the stability of the dispersion can be improved. Further, when centrifugal separation is performed by a strong centrifugal force, the carbon nanotubes can be classified into sizes according to the thickness and length, and the light transmittance of the film can be improved.

The centrifugal force during centrifugal separation may be 100G or more, but is preferably 1000G or more, and more preferably 10,000G or more. The upper limit is not particularly limited, but is preferably 200,000G or less in accordance with the performance of a general-purpose ultracentrifuge.

In addition, the filter used for the filter filtration can be selected appropriately from 0.05 to 5.0 μm. This makes it possible to remove relatively large substances such as undispersed carbon nanotubes and impurities that may be mixed during synthesis of carbon nanotubes.

In the case of performing size classification in this way, the carbon nanotube dispersion can be prepared so that the composition after size classification is within a desired range.

In the present invention, when a conductive layer is formed on a substrate using the carbon nanotube composition to form a composite, a composite having very good conductivity can be efficiently formed. In particular, when the substrate is a transparent substrate and a composite having both transparency and conductivity is required, the carbon nanotube assembly is particularly effective because it has conductivity and high transparency even when the amount of the carbon nanotube assembly used is reduced. Hereinafter, a composite using the carbon nanotube aggregate when the substrate is a film having transparency is sometimes referred to as a "transparent conductive film".

The method of forming the conductive layer using the carbon nanotube composition may be formed by coating the carbon nanotube dispersion liquid on a substrate. The method is not particularly limited, and a known coating method such as spray coating, dip coating, spin coating, blade coating, kiss coating, gravure coating, screen printing, inkjet printing, pad printing, other types of printing, roll coating, or the like can be used. The most preferred method of coating is roll coating. The coating may be performed a plurality of times, or 2 different types of coating methods may be used in combination. In the case where the dispersion medium of the dispersion liquid is volatile, the unnecessary dispersion medium can be removed by a method such as air drying, heating, or pressure reduction. Thereby forming a three-dimensional network structure of the carbon nanotubes and fixing the carbon nanotubes on the substrate. It is preferable to then remove the additives such as the surfactant, various polymer materials, and the like, which are components in the liquid, using an appropriate solvent. By this operation, electric charges are easily dispersed and the conductivity of the conductive layer is improved. The solvent used for removing the additives such as the surfactant and various polymer materials is not particularly limited as long as it can dissolve the additives such as the surfactant and various polymer materials, and may be an aqueous solvent or a nonaqueous solvent. Specifically, examples of the solvent include water and alcohols in the case of an aqueous solvent, and chloroform and acetonitrile in the case of a nonaqueous solvent.

In the case where the conductivity of the conductive layer is to be improved, the amount of carbon nanotubes in the carbon nanotube composition may be increased. In addition, in the case where the conductivity is to be further improved with a small amount of carbon nanotubes, it is preferable that the carbon nanotubes are uniformly dispersed in the carbon nanotube composition and the bundles are fine. More preferably, the bundle is disassembled, and the carbon nanotubes are dispersed one by one. The beam thickness can be adjusted by changing the dispersion time of the above dispersion method, the kind of surfactant added as an additive, various polymer materials, and the like.

The dispersion medium of the carbon nanotube aggregate for forming the conductive layer may be an aqueous solvent or an organic solvent. The organic solvent may be the above-mentioned organic solvent. Among them, as a dispersion medium for forming the conductive layer of the transparent electrode, a dispersion medium containing a solvent selected from water, alcohol, toluene, acetone, and ether, or a dispersion medium containing a solvent of a combination thereof is preferable. In the case where an aqueous solvent is required, and in the case where the binder is an inorganic polymer binder when the binder is used as described later, a polar solvent such as water, alcohols, and amines is used. When a binder that is liquid at ordinary temperature is used as the binder described later, the binder itself may be used as a dispersion medium.

The mixing ratio of each component in the dispersion liquid is as follows. The liquid of the carbon nanotube dispersion liquid preferably contains the carbon nanotube aggregate in an amount of 0.01 wt% or more, and more preferably 0.1 wt% or more. The upper limit of the concentration of the carbon nanotube aggregate is usually preferably 20 wt% or less, more preferably 5 wt% or less, and still more preferably 2 wt% or less.

The content of the surfactant and other additives is not particularly limited, but is preferably 0.1 to 50% by weight, more preferably 0.2 to 30% by weight. The mixing ratio of the additive to the carbon nanotubes (additive/carbon nanotubes) is preferably 0.1 to 20, more preferably 0.3 to 10, in terms of weight ratio.

The carbon nanotube dispersion may be diluted with a solvent to a desired concentration and used. In applications where electrical conductivity is not very desirable, the concentration of carbon nanotubes may be diluted for use, or may be initially made lower.

Further, the dispersion liquid of the present invention and a liquid to which a binder or the like is added can be used as a transparent coating liquid for coating all members to be coated, for example, a colored substrate and a fiber, not only as a transparent coating liquid on a transparent substrate. For example, if the coating is applied to a floor material or a wall material of a clean room or the like, the coating can be used as an antistatic floor wall material, and if the coating is applied to a fiber, the coating can be used as antistatic clothes, felt, curtain or the like.

After the carbon nanotube dispersion is applied to the substrate to form a composite as described above, the composite is preferably overcoated with a binder material capable of forming a transparent coating film. By the overcoating, the charge can be effectively further dispersed and moved.

Alternatively, a composite can be obtained by including a binder material capable of forming a transparent coating film in the carbon nanotube dispersion, applying the binder material to an appropriate substrate, and then heating the substrate as necessary to dry or burn (cure) the coating film. The heating conditions in this case are appropriately set according to the type of the adhesive. In the case where the adhesive is light-or radiation-curable, the coating film is cured by irradiating the coating film with light or radiation immediately after coating, rather than being cured by heating. As the radiation, ionizing radiation such as electron beam, ultraviolet ray, X-ray, and γ -ray can be used, and the amount of radiation is determined by the type of the binder.

The binder material is not particularly limited as long as it is used for the conductive coating material, and various organic and inorganic binders, that is, a transparent organic polymer or a precursor thereof (hereinafter, sometimes referred to as "organic polymer-based binder") or an inorganic polymer or a precursor thereof (hereinafter, sometimes referred to as "inorganic polymer-based binder") can be used. The organic polymer-based binder may be any of thermoplastic, thermosetting, or radiation-curable, such as ultraviolet rays and electron beams. Examples of suitable organic binders include polyolefins (polyethylene, polypropylene, etc.), polyamides (nylon 6, nylon 11, nylon 66, nylon 6, 10, etc.), polyesters (polyethylene terephthalate, polybutylene terephthalate, etc.), silicone polymers, vinyl resins (polyvinyl chloride, poly-1, 1-dichloroethylene, polyacrylonitrile, polyacrylate, polystyrene derivatives, polyvinyl acetate, polyvinyl alcohol, etc.), polyketones, polyimides, polycarbonates, polysulfones, polyacetals, fluorine resins, phenol resins, urea resins, melamine resins, epoxy resins, polyurethanes, cellulose polymers, proteins (gelatin, casein, etc.), chitin, polypeptides, polysaccharides, polynucleotides, and other organic polymers, and precursors of these polymers (monomers, casein, etc.), Oligomers). These may be cured by merely evaporating a solvent or by heat curing, irradiation with light or radiation to form an organic polymer-based transparent film or matrix (when incorporated in a liquid).

The organic polymer-based binder is preferably a compound having an unsaturated bond that can be cured by radical polymerization using radiation or light. Which is a monomer, oligomer or polymer having a vinyl or vinylidene group. Examples of such monomers include styrene derivatives (e.g., styrene and methylstyrene), acrylic acid, methacrylic acid, or derivatives thereof (e.g., alkyl acrylates or methacrylates, allyl acrylates or methacrylates), vinyl acetate, acrylonitrile, and itaconic acid. The oligomer or polymer is preferably a compound having a double bond in the main chain or a compound having an acryloyl group or a methacryloyl group at both ends of a linear chain. Such a radical polymerization curable adhesive can form a conductive film or a substrate (when blended in a liquid) having high hardness, excellent abrasion resistance, and high transparency.

Examples of the inorganic polymer-based binder include sols of metal oxides such as silica, tin oxide, alumina, and zirconia, and organic metal compounds such as hydrolyzable or thermally decomposable organic phosphorus compounds and organic boron compounds, organic silane compounds, organic titanium compounds, organic zirconium compounds, organic lead compounds, and organic alkaline earth metal compounds, which are precursors of inorganic polymers. Specific examples of the hydrolyzable or thermally decomposable organic metal compound include alkoxides or partial hydrolyzates thereof, lower carboxylates such as acetates, and metal complexes such as acetylacetone.

When these 1 or 2 or more types of inorganic polymer binders are fired, a vitreous inorganic polymer transparent film or matrix (when mixed in a liquid) composed of an oxide or a complex oxide can be formed. The inorganic polymer-based matrix is generally vitreous, and has high hardness, excellent abrasion resistance, and high transparency.

The amount of the adhesive used is an amount sufficient for overcoating, and an amount sufficient for obtaining a viscosity suitable for application when incorporated in a liquid. If too small, the coating cannot be smoothly performed, and if too large, the conductivity is impaired, which is not preferable.

The organic polymer-based binder curable by light or radiation may be a binder itself as a dispersion medium by selecting a binder that is liquid at room temperature. That is, a solvent-free composition can be prepared from a binder of 100% reaction system in the absence of a solvent or by diluting it with a non-reactive liquid resin component. This prevents the solvent from evaporating during the curing and drying of the coating film, thereby greatly shortening the curing time and eliminating the need for a solvent recovery operation.

The carbon nanotube dispersion liquid may contain additives such as a coupling agent, a crosslinking agent, a stabilizer, a precipitation preventing agent, a colorant, a charge control agent, and a lubricant, in addition to a dispersant such as carbon nanotubes and a surfactant, a solvent, and a binder.

The carbon nanotube dispersion liquid may contain other conductive organic materials, conductive inorganic materials, or a combination of these materials.

As the conductive organic material, Bucky-ball (Bucky-ball), carbon black, fullerene, a plurality of carbon nanotubes, and particles containing the same can be preferably cited.

Examples of the conductive inorganic material include aluminum, antimony, beryllium, cadmium, chromium, cobalt, copper, doped metal oxide, iron, gold, lead, manganese, magnesium, mercury, metal oxide, nickel, platinum, silver, steel, titanium, zinc, and particles containing these. Indium tin oxide, antimony tin oxide, and mixtures thereof are preferably listed.

The composite containing these conductive materials or the composite overcoated with these conductive materials is very advantageous in charge dispersion or transfer. Further, a layer containing these conductive materials other than the carbon nanotubes may be provided together with a laminated layer containing carbon nanotubes.

The transparent conductive film obtained using the carbon nanotube assembly of the present invention may be used as it is in a state of being bonded to a substrate, or may be used as a separate film by being peeled off from the substrate. The independent film may be produced by further applying an organic polymer adhesive to the transparent conductive film and then peeling off the substrate. The transparent conductive film may be used by transferring the transparent conductive film to another substrate by burning or melting the substrate at the time of production by thermal decomposition. In this case, the thermal decomposition temperature of the substrate at the time of production is preferably lower than the thermal decomposition temperature of the substrate to be transferred.

The thickness of the transparent conductive film may range from a moderate thickness to a very thin thickness. For example, the films of the present invention may take a thickness of between 0.5nm and 1000 μm. In a preferred embodiment, the film thickness is 0.005 to 1000. mu.m, preferably 0.05 to 500. mu.m, more preferably 1.0 to 200. mu.m, and still more preferably 1.0 to 50 μm.

The transparent conductive film of the present invention exhibits excellent transparency. In order to measure the light transmittance of the conductive film including the substrate, the following indices were used as the light transmittance. For example, when the film of the present invention is measured using a light source of 550nm, the light transmittance of the transparent conductive film/the light transmittance of the substrate is preferably at least 0.6, more preferably 0.8 or more, and further preferably 0.85 or more.

The conductivity of the transparent conductive film was evaluated by measuring the surface resistance value of the film. The surface resistance value can be measured by a four-terminal four-probe method defined in JISK7149 using, for example, a low resistance meter EPMCP-T360 (manufactured by KAI' S ダィァィンスッルメンッ Co., Ltd.). For measuring the high resistance, the measurement was carried out by using a high resistance measuring instrument UP MCP-HT450 (manufactured by ダィァィンスッルメンッ, 10V, 10 seconds). The surface resistance value of the transparent conductive film is preferably less than 10⁵Omega/□, more preferably less than 1X 10⁴Ω/□。

The transparent conductive film of the present invention can be used for various applications of transparent conductive coatings such as EMI/RFI (electromagnetic interference) shielding layers, low-visibility coatings, polymer electronic devices (for example, transparent conductive layers of OLED displays, EL lamps, and plastic chips), and the like. The surface resistance value of the transparent conductive film can be adjusted by controlling the film thickness of the conductive layer. For example, when the film thickness is increased, the sheet resistance value tends to be decreased, and when the film thickness is decreased, the sheet resistance value tends to be increased. For example, conductive coatings that typically require EMI/RFI shielding layers have surface resistance values less than 10⁴Omega/□. The surface resistance value of the conductive coating of the EMI/RFI shielding layer is preferably about 10¹～10³Omega/□. The surface resistance value of the transparent low-visibility coating is generally required to be less than 10³Omega/sq, preferably less than 10²Omega/□. In the case of polymer electronic devices and Inherently Conductive Polymers (ICP), the surface resistance value is generally less than 10⁴Omega/□, preferably having a surface resistance value of 10^-2～10⁰Omega/□. Therefore, in a preferred embodiment, the surface resistance of the transparent conductive film is less than 10⁴Ω/□.。

The carbon nanotube assembly of the present invention can be produced by heating a carbon nanotube assembly having a high height ratio (G/D ratio) of a G band and a D band measured by raman spectroscopy at a wavelength of 633nm in a nitric acid solution (hereinafter, sometimes referred to as "nitric acid treatment"). The layer structure of the carbon nanotube aggregate is not particularly limited as long as it contains double-walled carbon nanotubes. The carbon nanotube aggregate containing the double-walled carbon nanotubes will be hereinafter referred to as a "double-walled carbon nanotube aggregate". The ratio of the double-walled carbon nanotubes to 100 carbon nanotubes is preferably 50 or more, more preferably 70 or more, still more preferably 75 or more, and most preferably 80 or more. Since the existing ratio of the double-walled carbon nanotubes is generally reduced after the nitric acid treatment, the double-walled carbon nanotube assembly including a large number of double-walled carbon nanotubes is used in order to have a desired layer structure in consideration of the reduction. The larger the proportion of the double-walled carbon nanotubes in the double-walled carbon nanotube aggregate, the more conductive the carbon nanotube aggregate can be obtained after heating in a nitric acid solution.

In Raman spectroscopy, it will be at 1590cm^-1The Raman shift seen from the left and right is called the G-band from graphite and will be at 1350cm^-1The raman shift seen from the left and right is called D band from amorphous carbon, graphite defects. The higher the G/D ratio, the higher the graphitization degree and the better the quality. The double-walled carbon nanotube aggregate used herein preferably has a G/D ratio of 20 or more as measured by Raman spectroscopy at a wavelength of 633 nm. In order to obtain a carbon nanotube assembly having further improved conductivity by heating in a nitric acid solution, the G/D ratio is preferably 25 or more, more preferably 30 or more, and most preferably 40 or more. Although the higher the G/D ratio, the greater the conductivity-improving effect, it is difficult to obtain a carbon nanotube aggregate having a G/D ratio of more than 200, and therefore it is preferable to use a double-walled carbon nanotube aggregate having a G/D ratio of 200 or less.

Although the reason why the carbon nanotube aggregate having higher conductivity can be obtained after heating in a nitric acid solution as the G/D ratio in the double-walled carbon nanotube aggregate is higher is not known, the reason is considered as follows.

In general, a single-walled carbon nanotube easily produces a carbon nanotube aggregate having a high degree of graphitization, and since the degree of graphitization is high, the conductivity of the carbon nanotube itself is also very high. However, since the number of the graphite sheet having a conductive structure of the single-walled carbon nanotube is one, when the graphite sheet is heated in a nitric acid solution to cause a defect in the graphite layer, the conductive structure is often collapsed, and the conductivity is reduced by heating in the nitric acid solution.

When the carbon nanotube is a double-walled carbon nanotube having a high degree of graphitization, the inner layer is protected by the outer layer, and therefore, even if the outer layer is functionalized or a defect is generated by heating in a nitric acid solution, the inner layer having a high degree of graphitization maintains the conductive structure as it is with almost no defect generated in the graphite structure. Further, since the outer layer can receive the doping effect by the functionalization, both the inner layer having a high graphitization degree and the outer layer receiving the doping effect can be effectively utilized.

Here, the G band in the raman spectroscopy is derived from a graphite layer of the carbon nanotube, and the D band is derived from irregular carbon such as amorphous carbon other than the carbon nanotube, or a defect or an amorphous portion of the graphite layer of the carbon nanotube. In the present invention, raman spectroscopy can be used as an index for any of the D bands. This is because a low G/D ratio indicates that carbon impurities such as amorphous carbon in the carbon nanotube aggregate are more when the D band is irregular carbon derived from amorphous carbon other than carbon nanotubes. In this case, the heating time in the nitric acid solution requires a long time, and the carbon nanotubes themselves have many defects by the long-time nitric acid treatment, and even the inner layer has defects, and the conductivity of the carbon nanotubes themselves is deteriorated. On the other hand, when the D band is a defect derived from a graphite layer of a carbon nanotube, an amorphous portion, or the like, if heating is performed in a nitric acid solution, the outer layer is defective and the inner layer is exposed, and the degree of graphitization of the inner layer is low, so that the conductivity is not greatly improved. Or the inner layer is also defective due to nitric acid, and the carbon nanotube itself is lowered in conductivity due to nitric acid regardless of the amount of impurities. Therefore, regardless of the origin of the D band of the G/D ratio, it is preferable to use carbon nanotubes having a high G/D ratio.

The reason why heating in a nitric acid solution is preferable for producing the carbon nanotube aggregate of the present invention is not clear, but the reason is considered as follows.

Although there are various methods for removing carbon impurities in the liquid phase, when an acid or an oxidizing agent having a stronger oxidizing force than nitric acid is used, the outer layer of the multi-layered carbon nanotube is excessively damaged and defects occur in the inner layer in many cases while removing the carbon impurities. Further, when the treatment conditions are adjusted so that the inner layer does not suffer from defects, the reaction is stopped in a state where the outer layer is fragmented, and substances generated by the fragmentation of the outer layer become carbon impurities, resulting in a decrease in conductivity as an aggregate of carbon nanotubes. Examples of the acid having a stronger oxidizing force than nitric acid include a mixed acid composed of concentrated nitric acid and concentrated sulfuric acid, fuming sulfuric acid, and the like. It is considered that, compared with these acids having very strong oxidizing power, the nitric acid functionalizes the outer layer of the carbon nanotube appropriately by heating at a predetermined temperature, and decomposes and removes amorphous carbon and granular carbon impurities as carbon impurities, thereby obtaining a carbon nanotube assembly having improved conductivity.

The temperature at which the double-walled carbon nanotube assembly is heated in the nitric acid solution may be any temperature as long as the carbon nanotube assembly of the present invention can be obtained, but is preferably 70 ℃ or higher, more preferably 80 ℃ or higher, further preferably 90 ℃ or higher, and most preferably in the range from 100 ℃ to the temperature at which the nitric acid solution is in a reflux state. Generally, the higher the nitric acid concentration, the higher the reflux temperature of the nitric acid solution. In addition, the lower the G/D ratio of the double-walled carbon nanotube aggregate used, the lower the temperature to be set, and the higher the G/D ratio, the higher the temperature to be set, so that the degree of functionalization can be easily adjusted to an appropriate amount.

The method of heating the nitric acid solution is not particularly limited as long as the carbon nanotube assembly of the present invention can be obtained, and examples thereof include a method of immersing a container containing the nitric acid solution in an oil bath, a water bath, or a sand bath for heating, a method of winding the container with an electric wire for heating, and a method of direct firing. From the viewpoint of efficiency, the entire solution is preferably heated uniformly. The nitric acid solution may be heated while being stirred, or may be heated without being stirred, but from the viewpoint of efficiency, it is preferable to heat the solution in a stirred state. When the stirring is not carried out, the reaction time is preferably longer than the stirring time.

The concentration of nitric acid when the double-walled carbon nanotube assembly is heated in a nitric acid solution is preferably 10 wt% or more, more preferably 25 wt% or more, and still more preferably 30 wt% or more. More preferably, 55 wt% or more, most preferably 60 wt% or more concentrated nitric acid is used. The higher the concentration of nitric acid, the shorter the time required for the carbon impurities to be decomposed by nitric acid becomes, and the shorter the time required for the functionalization of the carbon nanotube surface becomes.

Here, the nitric acid is used to functionalize the carbon nanotubes, and therefore, the concentration of the nitric acid is preferably adjusted according to the G/D ratio of the carbon nanotube aggregate used. In general, a carbon nanotube aggregate having a high G/D ratio has a high graphitization degree, and thus is not easily functionalized. Therefore, the higher the G/D ratio, the higher the concentration of nitric acid becomes, and thus the carbon nanotube of the present invention can be easily obtained.

The time for heating the double-walled carbon nanotube assembly in the nitric acid solution is generally adjusted depending on the nitric acid concentration of the nitric acid solution and the heating temperature, and the lower the nitric acid concentration, the longer the time to be set. In addition, the time can be increased when the heating temperature is low, and the time can be shortened when the heating temperature is high. The conditions of temperature and time when the carbon nanotube assembly is heated in a nitric acid solution, and the concentration of nitric acid in the nitric acid solution are comprehensively adjusted so that the weight of the carbon nanotube assembly heated from 200 ℃ to 400 ℃ is reduced by 5 to 20% when the resulting carbon nanotube assembly is subjected to thermogravimetric analysis by raising the temperature at a rate of 10 ℃/min.

In addition, although a nitric acid solution is generally used in removing the catalyst or the catalyst support, the conditions are usually mild so as not to destroy the graphene sheet and not to generate a functional group, specifically, so as to reduce the nitric acid concentration unless a special purpose is provided. If only the catalyst and the carrier are removed, they can be sufficiently removed by heating in dilute nitric acid for a short time, and particularly, when a carbon nanotube aggregate having a high graphitization degree is used, functionalization is not easily caused under such conditions.

The method for producing the double-walled carbon nanotube aggregate is not limited, and the carbon nanotube aggregate having a high G/D ratio and a large proportion of double-walled carbon nanotubes can be produced in the following manner, for example, in view of the fact that the carbon nanotube aggregate can be produced easily.

The powdered catalyst with iron loaded on the carrier is contacted with carbon-containing compounds at 500-1200 ℃ in a reactor. The reactor may be any reactor as long as the carbon nanotube aggregate can be obtained, and various reactors can be used, but a vertical reactor is preferably used in order to obtain a homogeneous carbon nanotube aggregate. The vertical reactor has the following structure: there is a reactor arranged in a vertical direction (hereinafter sometimes referred to as "longitudinal direction"), through which a carbon-containing compound can flow from one end portion toward the other end portion thereof, the carbon-containing compound being allowed to flow in a manner to pass through a catalyst layer formed of a catalyst for carbon nanotube production. The reactor is preferably a reactor having a tubular shape, for example. The vertical direction also includes a direction inclined at a certain angle to the vertical direction (for example, 90 ° ± 15 °, preferably 90 ° ± 10 °). Most preferably in the vertical direction. Further, the supply portion and the discharge portion of the carbon-containing compound do not necessarily have to be at the end portions of the reactor as long as the carbon-containing compound can pass through the catalyst layer in this flow-through process.

In the vertical reactor, the catalyst is preferably present in the entire horizontal cross section of the reactor. Whereby the catalyst and the carbon-containing compound can be efficiently brought into contact. In the case of a horizontal reactor, it is necessary to hold the catalyst in the right and left directions in order to take such a state due to the gravity. However, in the reaction for producing carbon nanotubes, carbon nanotubes are produced on the catalyst as the reaction proceeds, and the volume of the catalyst increases, so that the method of sandwiching the catalyst from the left and right is not preferable. In the present invention, the reactor is made vertical, the gas-permeable table is provided in the reactor, and the catalyst is provided on the table, so that the catalyst can be uniformly present in the cross-sectional direction of the reactor without holding the catalyst from both sides. In the present invention, the state where the catalyst is present on the entire surface in the horizontal cross-sectional direction of the vertical reactor means a state where the catalyst is spread on the entire surface in the horizontal cross-sectional direction and the table surface at the bottom of the catalyst is not visible. The reactor is preferably heat-resistant, and preferably contains a heat-resistant material such as quartz or alumina.

The carbon-containing compound passes through the catalyst layer from the lower portion or the upper portion of the catalyst layer disposed in the reactor, and reacts in contact with the catalyst, thereby generating carbon nanotubes. The temperature of the catalyst contacting with the carbon-containing compound is 500-1200 ℃. More preferably, the temperature is 600 to 950 ℃, and still more preferably 700 to 900 ℃. When the temperature is too low, the yield of the carbon nanotubes becomes poor. In addition, when the temperature is too high, the material used for the reactor is limited, and the carbon nanotubes start to be bonded to each other, and it is difficult to control the shape of the carbon nanotubes. The reactor may be brought to the reaction temperature while the carbon-containing compound is brought into contact with the reaction solution, or the reactor may be brought to the reaction temperature after the pretreatment by heat is completed, and the supply of the carbon-containing compound may be restarted.

The iron-supporting support is preferably magnesium oxide. By supporting the catalyst iron on the carrier magnesium oxide, the particle size of the iron can be easily controlled, and in this case, even if the iron is present at a high density, sintering (metal aggregation) is less likely to occur at a high temperature. Therefore, high-quality carbon tubes can be synthesized efficiently in large quantities. Further, since magnesium oxide is soluble in an acidic aqueous solution, both magnesium oxide and iron can be removed by only treating with an acidic aqueous solution, and the purification process can be simplified.

The magnesium oxide may be commercially available or may be a synthetic one. A preferable method for producing magnesium oxide includes heating magnesium metal in air, heating magnesium hydroxide at 850 ℃ or higher, and heating magnesium carbonate hydroxide 3MgCO₃·Mg(OH)₂·3H₂O is above 950 DEG CHeating, and the like.

The iron supported on the catalyst is not limited to the 0-valent state. Although the state of the metal which becomes 0 valence in the reaction can be estimated, it may be a wide range of iron-containing compounds or iron species. For example, organic or inorganic salts such as iron formate, iron acetate, iron trifluoroacetate, ammonium iron citrate, iron nitrate, iron sulfate and iron halide, complex salts such as ethylenediamine tetraacetic acid complex and acetylacetone complex, and the like can be used. In addition, the iron is preferably fine particles. The particle diameter of the fine particles is preferably 0.5 to 10 nm. When iron is fine particles, carbon nanotubes having a small outer diameter are easily produced.

The method for supporting iron on magnesium oxide is not particularly limited. For example, a method (impregnation method) may be used in which magnesium oxide is immersed in a nonaqueous solution (e.g., ethanol solution) or an aqueous solution in which a salt of iron to be supported is dissolved, sufficiently dispersed and mixed by stirring, ultrasonic irradiation, or the like, and then dried. The magnesium oxide may be loaded with iron by heating at a high temperature (300 to 1000 ℃) in a gas selected from the group consisting of air, oxygen, nitrogen, hydrogen, an inert gas, and a mixture thereof, or in a vacuum.

The larger the amount of iron supported, the higher the yield of carbon nanotubes, but if too much, the larger the particle size of iron, and the coarsened carbon nanotubes produced. When the amount of iron supported is small, the particle size of the supported iron becomes small, and carbon nanotubes having a small outer diameter and a narrow outer diameter distribution can be obtained, but the yield tends to be low. The optimum amount of iron supported varies depending on the pore volume, the outer surface area, and the supporting method of the magnesium oxide, but it is preferable that 0.1 to 20 wt% of iron is supported on the magnesium oxide.

The catalyst may be pretreated by heat before the reaction for producing the carbon nanotubes. The time for the pretreatment by heat is not particularly limited, but if it is too long, the metal may aggregate on the magnesium oxide, and carbon nanotubes having a large outer diameter may be produced, and therefore, it is preferably within 120 minutes. The temperature of the pretreatment may be equal to or lower than the reaction temperature, or may be equal to or higher than the reaction temperature, as long as the catalyst activity is exhibited. The pretreatment by heat may bring the catalyst into a more active state. The pretreatment by heat and the reaction for producing carbon nanotubes are preferably carried out under reduced pressure or atmospheric pressure.

When the catalyst and the carbon-containing compound are contacted under reduced pressure, the reaction system may be depressurized using a vacuum pump or the like. In addition, when the pretreatment and the reaction are carried out under atmospheric pressure, the carbon-containing compound and the diluent gas may be mixed together and brought into contact with the catalyst as a mixed gas.

The diluent gas is not particularly limited, but a gas other than oxygen is preferably used. Oxygen is not normally used because of the potential for explosions, but can be used outside the explosive range. As the diluent gas, nitrogen, argon, hydrogen, helium, or the like is preferably used. These gases can effectively control the linear velocity and concentration of the carbon-containing compound, and can be used as carrier gases. Hydrogen gas is preferred because it has an effect of activating the catalyst metal. Since a gas having a relatively high molecular weight such as argon has a large annealing effect, a gas having a relatively high molecular weight such as argon is preferable for annealing. Particularly preferred are nitrogen and argon.

The carbon-containing compound to be used is not particularly limited as long as it can give a double-walled carbon nanotube aggregate having a high G/D ratio, but a hydrocarbon or an oxygen-containing carbon compound is preferably used. The hydrocarbons may be aromatic or non-aromatic. As the aromatic hydrocarbon, for example, benzene, toluene, xylene, cumene, ethylbenzene, diethylbenzene, trimethylbenzene, naphthalene, phenanthrene, anthracene, a mixture thereof or the like can be used. Further, as the non-aromatic hydrocarbon, for example, methane, ethane, propane, butane, pentane, hexane, heptane, ethylene, propylene, acetylene, a mixture thereof, or the like can be used. Examples of the oxygen-containing carbon compound include alcohols such as methanol, ethanol, propanol, and butanol; ketones such as acetone; aldehydes such as formaldehyde, acetaldehyde, etc.; ethers such as trioxane, dioxane, dimethyl ether, and diethyl ether; esters such as ethyl acetate; carbon monoxide or mixtures thereof. Among these, compounds selected from methane, ethane, ethylene, acetylene, propane and propylene are preferable carbon-containing compounds in terms of obtaining carbon nanotubes with high purity. In particular, the use of methane is preferable because a double-walled carbon nanotube having a high degree of graphitization can be obtained. Since these are gases at normal temperature and normal pressure, they can be easily supplied in a fixed amount as gases for use in the reaction. When other carbon-containing compounds are reacted at normal pressure, a step such as gasification is required.

When the carbon nanotube composition thus produced is subjected to an oxidation treatment in a gas phase, the single-walled carbon nanotubes and amorphous carbon are preferentially calcined and removed. Thereby, a double-walled carbon nanotube aggregate having a high G/D ratio can be obtained.

The method of oxidizing carbon nanotubes in a gas phase is a step of placing an aggregate of carbon nanotubes in an atmosphere in which an oxidizing gas is present. The oxidizing gas is not particularly limited as long as it is a gas that exhibits oxidation to the carbon nanotube assembly when the carbon nanotube assembly is exposed to the treatment temperature, and examples thereof include carbon monoxide, carbon dioxide, ozone, oxygen, and air. The composition of the gas may be a mixed gas of these gases, or may be mixed with another gas (inert gas) that does not exhibit oxidation properties with respect to the carbon nanotube aggregate.

When the oxidation treatment in the gas phase is a firing treatment at a temperature much lower than the combustion peak temperature of the carbon nanotubes, the single-walled carbon nanotubes are not fired and removed in many cases, and therefore, it is preferable to perform the oxidation treatment in the gas phase at a temperature of not less than the combustion peak temperature of the carbon nanotubes to 50 ℃ when the carbon nanotube composition is subjected to differential thermal analysis. When the temperature of the low-temperature side skirt corresponding to the combustion peak temperature of the carbon nanotube in the differential thermal analysis is a temperature of-50 ℃ or higher, it is preferable to perform the oxidation treatment at a temperature of higher than the temperature of the low-temperature side skirt corresponding to the combustion peak temperature of the carbon nanotube in the differential thermal analysis. In many cases, the carbon nanotubes are synthesized using a quartz tube as a reaction tube, and the temperature of the oxidation treatment is preferably 1200 ℃ or lower, and more preferably 1000 ℃ or lower. When the oxidation treatment is carried out at a temperature higher than 1200 deg.c, it is preferable to use a material of the apparatus capable of withstanding the temperature. In addition, when the oxidation treatment is performed at a temperature much higher than the combustion peak temperature of the carbon nanotubes, all the carbon nanotubes produced at this time are burned and disappeared. Therefore, the oxidation treatment is preferably performed at around the combustion peak temperature of the carbon nanotube, and more preferably at around ± 25 ℃.

The oxidation treatment may be performed in an electric furnace, or may be performed after the carbon nanotube is synthesized by reducing the atmosphere in the reactor or the concentration of air with an inert gas. When the oxidation treatment is performed in an electric furnace, the oxidation treatment is usually performed in an amount of about 10g, and if the oxidation treatment is not performed under the above conditions and is performed in a small amount, the oxidation treatment is performed in an amount that is feasible. The time for the oxidation treatment is not particularly limited, but is preferably 1 to 10 hours.

The number of layers of the carbon nanotubes after the oxidation treatment may be determined immediately after the synthesis of the carbon nanotubes, or may be determined after a purification treatment. For example, when iron/magnesium oxide is used as the catalyst, the catalyst may be further removed by purification treatment with an acid such as hydrochloric acid after the oxidation treatment, or the catalyst may be removed by purification treatment with an acid such as hydrochloric acid before the oxidation treatment.

In the present invention, the obtained carbon nanotubes are preferably treated with nitric acid by the above-described method.

Examples

The present invention is described in detail below with reference to examples, but the following examples are only for illustrative purposes and are not intended to limit the present invention.

< example 1>

(catalyst preparation)

2.459g of ferric ammonium citrate (green) (Wako pure chemical industries, Ltd.) was dissolved in 500mL of methanol (Kanto chemical Co., Ltd.). To this solution, 100g of light magnesium oxide (made by Seikagaku corporation, having an apparent density of 0.125g/mL) was added, the mixture was stirred at room temperature for 60 minutes, and the mixture was dried under reduced pressure at a temperature of 40 to 60 ℃ while stirring to remove methanol, thereby obtaining a catalyst in which a metal salt was supported on light magnesium oxide powder.

(production of carbon nanotube aggregate)

Carbon nanotubes were synthesized in a fluidized-bed vertical reaction apparatus shown in FIG. 1. The reactor 100 is a cylindrical quartz tube having an inner diameter of 32mm and a length of 1200 mm. The middle part has a quartz sintered plate 101, the lower part has an inert gas and raw material gas supply pipe 104, and the upper part has an exhaust gas pipe 105 and a catalyst addition pipe 103. Further, the reactor has a heater 106 surrounding the periphery of the reactor so that the reactor can maintain an arbitrary temperature. In order to determine the flow state in the apparatus, a detection port 107 is provided in the heater 106.

The catalyst 12g was taken out and fed from the closed catalyst feeder 102 through the catalyst feed pipe 103, and the prepared catalyst 108 was provided on the quartz sintered plate 101. Then, the supply of argon gas from the raw material gas supply line 104 was started at a rate of 1000 mL/min. After the inside of the reactor was changed to an argon atmosphere, the temperature was heated to 850 ℃ (heating time 30 minutes).

After reaching 850 ℃, the temperature was maintained, and the flow rate of argon gas in the raw material gas supply line 104 was increased to 2000 mL/min to start fluidization of the catalyst on the quartz sintered plate. After confirming that the catalyst had flowed from the detection port 107 of the heating furnace, the flow rate of methane was 95 mL/min (methane concentration: 4.5 vol%), and methane was mixed in argon and supplied to the reactor. After 90 minutes of supplying the mixed gas, the flow was switched to argon gas alone, and the synthesis was terminated. The heating was stopped, the mixture was left to stand until it became room temperature, and the carbon nanotube composition containing the catalyst and the carbon nanotubes was taken out of the reactor after it became room temperature.

About 10mg of the catalyst-containing carbon nanotube composition was placed in a differential thermal analyzer (TGA-60 manufactured by Shimadzu corporation), and the temperature was raised from room temperature to 900 ℃ in air at a temperature raising rate of 10 ℃ per minute. The weight change during this period was measured. The combustion peak temperature due to heat generation was read from the DTA curve at this time, and the result was 456 ℃.

Placing 23.4g of the carbon nanotube composition containing the catalyst on a ceramic dishThen, the mixture was put into an electric furnace (manufactured by ャマト scientific Co., Ltd., FP41) previously heated to 446 ℃ and heated at 446 ℃ for 2 hours in the air, and then taken out of the electric furnace. Next, to remove the catalyst, the carbon nanotube composition was added to a 6N aqueous hydrochloric acid solution and stirred at room temperature for 1 hour. The recovered product obtained by filtration was added to a 6N aqueous hydrochloric acid solution, and stirred at room temperature for 1 hour. This was filtered, washed with water several times, and then the filtrate was dried in an oven at 120 ℃ overnight, thereby obtaining 57.1mg of a carbon nanotube assembly from which the catalyst was removed. The above-described steps are repeated, and the following steps are further performed.

On the other hand, in order to examine the amount of carbon lost in the electric furnace, 5.2g of the catalyst-containing carbon nanotube composition which was not heated in the electric furnace was added to a 6N hydrochloric acid aqueous solution and stirred at room temperature for 1 hour. The recovered product obtained by filtration was added to a 6N aqueous hydrochloric acid solution, and stirred at room temperature for 1 hour. This was filtered, washed with water several times, and then the filtrate was dried in an oven at 120 ℃ overnight, thereby obtaining 107.2mg of an aggregate of carbon nanotubes.

When this amount was calculated as the original amount, the amount of carbon lost from the electric furnace was 88%. Further, when the carbon nanotube aggregate thus obtained was observed by a high-resolution transmission electron microscope, it was found that the carbon nanotubes were composed of clean graphite layers, and the carbon nanotubes having two layers were observed. In addition, of the 100 carbon nanotubes observed, the double-walled carbon nanotube accounted for 84. The double-walled carbon nanotube assembly was analyzed by Raman spectroscopy at a wavelength of 633nm, and the G/D ratio was 75.

Then, 80mg of the double-walled carbon nanotube assembly from which the catalyst was removed was added to 27mL of concentrated nitric acid (having an analytical content of 60 to 61% in 1-stage manufactured by Wako pure chemical industries, Ltd.), and the mixture was heated for 5 hours with stirring in an oil bath at 130 ℃. After the completion of the heating and stirring, the nitric acid solution containing the carbon nanotubes was filtered, washed with distilled water, and then dried at 120 ℃ overnight to obtain 57mg of carbon nanotube aggregates.

(measurement of volume resistivity)

The carbon nanotube assembly 20mg obtained above and 16mL of N-methylpyrrolidone were mixed together, and irradiated with 20W ultrasonic waves for 20 minutes using an ultrasonic homogenizer. The mixture was then mixed with 10mL of ethanol using the internal diameterThe filter of (4) was suction-filtered with a membrane filter. The filtrate was dried in a drier at 60 ℃ for 2 hours together with a filter and a membrane filter. The membrane filter with the carbon nanotube membrane attached thereto was removed from the filter, the thickness of the carbon nanotube membrane was measured together with the membrane filter, and the thickness of the membrane filter was subtracted, whereby the thickness of the carbon nanotube membrane was 55.7 μm. Membrane filter omnipore filter FILTERS, filtration type: 1.0 μm of JA, and,the surface resistance value of the obtained carbon nanotube film was measured by a four-terminal four-probe method defined in JISK7149 using a low resistance meter EP MCP-T360 (manufactured by Tokyo ダィァィンスッルメンッ), and was 0.134. omega./□. Therefore, the volume resistivity was 7.5X 10^-4Ω·cm。

(evaluation of the number of carbon nanotube layers Using Transmission Electron microscope)

0.5mg of the carbon nanotube assembly produced in the above carbon nanotube assembly production and 2mL of ethanol were put in a 2mL sample bottle, and ultrasonic waves were irradiated for 15 minutes using an ultrasonic bath (ultrasoniclean bench yamato 2510). The ethanol solution in which the carbon nanotubes were dispersed was dropped on a fine mesh (STEM 150Cu mesh, reinforced with carbon, mesh pitch 150 μm), and dried. The web coated with the sample was set on a transmission electron microscope (JEM-2100, manufactured by JE, Japan) to perform measurement. The measurement magnification was 40 ten thousand times. The acceleration voltage was 100 kV. The number of layers and the diameter of 100 carbon nanotubes were measured from the obtained measurement image. The results are shown in FIG. 3. The average diameter of 100 carbon nanotubes was 1.8nm, and the standard deviation of the diameters was 0.62 nm. 88 of the 100 carbon nanotubes are double-walled carbon nanotubes.

(weight loss measurement of temperature from 200 ℃ to 400 ℃ C.)

About 1mg of the carbon nanotube aggregate produced in the production of the carbon nanotube aggregate was set in a differential thermal analyzer (TGA-60, Shimadzu corporation), and the temperature was raised from room temperature to 900 ℃ in the air at a temperature raising rate of 10 ℃/min. The weight loss during the temperature increase from 200 ℃ to 400 ℃ was 9%. In addition, the maximum peak temperature of the DTA curve at this time was 695 ℃.

(evaluation of transparent conductivity)

20.0mg of the carbon nanotube assembly produced in the production of the carbon nanotube assembly and 200. mu.L of an aqueous solution of polystyrene sulfonic acid ammonium salt (30% by weight, weight average molecular weight 20 ten thousand, GPC measurement, polystyrene conversion, manufactured by ァルドリッチ) were put in a container, and 9.80mL of distilled water was added. The mixture was dispersed and treated with an ultrasonic homogenizer at an output of 20W under ice cooling for 20 minutes to prepare a carbon nanotube dispersion. The aggregates were not visually observed in the prepared liquid, and the carbon nanotube aggregates were sufficiently dispersed. The resulting liquid was centrifuged at 10000G for 15 minutes using a high-speed centrifuge to obtain 9mL of supernatant. 1mL of the residual solution was filtered through a filter having a pore size of 1 μm, washed, and the filtrate was dried in a dryer at 120 ℃. The weight of the filtrate was measured, and found to be 3.0 mg. From this, 17.0mg of carbon nanotubes were dispersed in 9mL of the supernatant. Thus, the carbon nanotube concentration of the supernatant was 0.18 wt%.

To 1mL of the carbon nanotube dispersion obtained was added distilled water so that the concentration of the carbon nanotubes was 0.09% by weight, and the mixture was applied to a polyethylene terephthalate (PET) film (ルミラ one (registered trademark) U46 ", manufactured by Tokyo レ K., light transmittance 90.6%, 15 cm. times.10 cm) by using a bar coater (Nos. 3 and 5), followed by air drying, rinsing with distilled water, and drying in a 120 ℃ dryer for 2 minutes. Thus, the carbon nanotube composition was fixed to the PET film to obtain a composite. The surface resistance of the composite was measured by the four-terminal four-probe method defined in JISK7149 using a low resistance measuring instrument EP MCP-T360 (manufactured by Tokyo ダィァィンスッルメンッ Co., Ltd.). The light transmittance was measured using a 550nm light from a U-2001 type two-beam spectrophotometer (Hitachi, Ltd.). The results are shown in FIG. 2.

(XPS measurement)

The carbon nanotube aggregate produced as described above was measured by XPS. The results of the surface composition (atomic%) analysis were: c: 94.4%, N: 0.2%, O: 5.1 percent. Therefore, the proportion of oxygen atoms to carbon atoms in the carbon nanotube is 5.4% (atomic%). The XPS measurement conditions were: excitation of X-rays: monochromatic Al K_1，2Line, X-ray path: 1000 μm, photoelectron extraction angle: 90 deg. (detector tilted relative to sample surface). The presence of C — O groups and C ═ O groups can be judged from the binding energy (eV) of O1 s. The results are shown in FIG. 4.

(measurement of the length of carbon nanotube)

The dispersion prepared in the above transparent conductivity evaluation was diluted 70 times with distilled water and coated on mica with a bar coater (No. 3). After drying at 120 ℃ for 2 minutes, the length of the carbon nanotubes was measured by AFM. As a result, the average length of 25 roots was 1.8. mu.m.

< example 2>

(catalyst preparation)

A catalyst was prepared in the same manner as in example 1.

(production of carbon nanotube aggregate)

A carbon nanotube assembly was produced in the same manner as in example 1. However, the mixture was heated in concentrated nitric acid and then fired at 400 ℃ for 1 hour in an electric furnace in air. The obtained carbon nanotube assembly was measured in the same manner as in example 1.

(measurement of volume resistivity)

Volume resistivity of 5.0X 10^-3Ω·cm。

(evaluation of the number of carbon nanotube layers Using Transmission Electron microscope)

The average diameter of 100 carbon nanotubes was 1.8nm, and the standard deviation of the diameters was 0.64 nm. 90 of the 100 carbon nanotubes are double-walled carbon nanotubes.

(weight loss measurement of temperature from 200 ℃ to 400 ℃ C.)

The weight loss during the temperature increase from 200 ℃ to 400 ℃ was 5.6%. The maximum peak of the DTA curve at this time was 773 ℃.

(evaluation of transparent conductivity)

The transparent conductivity of the composite was evaluated in the same manner as in example 1. The results are shown in FIG. 2.

< example 3>

(catalyst preparation)

A catalyst was prepared in the same manner as in example 1, except that the amount of ferric ammonium citrate (green) (manufactured by Wako pure chemical industries, Ltd.) used was 3.279g, and the amount of light magnesium oxide used was changed to Wako pure chemical industries, Ltd. (apparent density: 0.16 g/mL).

(production of carbon nanotube aggregate)

A carbon nanotube assembly was produced in the same manner as in example 1, except that the firing temperature in the electric furnace was set to 400 ℃. The obtained carbon nanotube assembly was measured in the same manner as in example 1.

(measurement of volume resistivity)

Volume resistivity of 2.7X 10^-3Ω·cm。

(evaluation of the number of carbon nanotube layers Using Transmission Electron microscope)

The average diameter of 100 carbon nanotubes was 1.8nm, and the standard deviation of the diameters was 0.79 nm. 85 of the 100 carbon nanotubes are double-walled carbon nanotubes.

(weight loss measurement of temperature from 200 ℃ to 400 ℃ C.)

The weight loss during the temperature increase from 200 ℃ to 400 ℃ was 12.0%. In addition, the maximum peak of the DTA curve at this time was 599 ℃.

(G/D ratio of double-walled carbon nanotube aggregate before heating in nitric acid solution)

The result of Raman spectroscopic analysis at a wavelength of 633nm was 20.01.

< example 4>

(catalyst preparation)

Example 1 the same procedure was used to prepare a catalyst.

(production of carbon nanotube aggregate)

A carbon nanotube assembly was produced in the same manner as in example 1, except that the firing time in the electric furnace was changed from 2 hours to 1 hour. The obtained carbon nanotube aggregate was measured in the same manner as in example 1.

(measurement of volume resistivity)

Volume resistivity of 1.5X 10^-3Ω·cm。

(evaluation of the number of carbon nanotube layers Using Transmission Electron microscope)

The average diameter of 100 carbon nanotubes was 2.0nm with a standard deviation of the diameter of 1.05 nm. 83 of the 100 carbon nanotubes are double-walled carbon nanotubes.

(weight loss measurement of temperature from 200 ℃ to 400 ℃ C.)

The weight loss during the temperature increase from 200 ℃ to 400 ℃ was 11.0%. In addition, the maximum peak of the DTA curve at this time was 624 ℃.

(G/D ratio of double-walled carbon nanotube aggregate before heating in nitric acid solution)

The result of Raman spectroscopic analysis at a wavelength of 633nm was 32.

< example 5>

(catalyst preparation)

A catalyst was prepared in the same manner as in example 1.

(production of carbon nanotube aggregate)

A carbon nanotube assembly was produced in the same manner as in example 1, except that the supply time of the mixed gas of methane and argon was changed to 30 minutes. The obtained carbon nanotube aggregate was measured in the same manner as in example 1.

(measurement of volume resistivity)

Volume resistivity of 2.4X 10^-3Ω·cm。

(evaluation of the number of carbon nanotube layers Using Transmission Electron microscope)

The average diameter of 100 carbon nanotubes was 1.8nm with a standard deviation of the diameter of 0.54 nm. 90 of the 100 carbon nanotubes are double-walled carbon nanotubes.

(weight loss measurement of temperature from 200 ℃ to 400 ℃ C.)

The weight loss during the temperature increase from 200 ℃ to 400 ℃ was 12.0%. In addition, the maximum peak of the DTA curve at this time was 611 ℃.

(G/D ratio of double-walled carbon nanotube aggregate before heating in nitric acid solution)

The result of Raman spectroscopic analysis at a wavelength of 633nm was 45.

< example 6>

An example of carbon nanotubes synthesized using a fixed-bed vertical reaction apparatus will be shown.

(catalyst preparation)

3.279g of ferric ammonium citrate (green) (Wako pure chemical industries, Ltd.) was dissolved in 500mL of methanol (Kanto chemical Co., Ltd.). To this solution, 100g of light magnesium oxide (apparent density 0.16g/mL, manufactured by Wako pure chemical industries, Ltd.) was added, and the mixture was stirred at room temperature for 60 minutes, and further stirred from 40 ℃ to 60 ℃ and dried under reduced pressure to remove methanol, thereby obtaining a catalyst in which a metal salt was supported on a light magnesium oxide powder.

(production of carbon nanotube aggregate)

Carbon nanotubes were synthesized using a fixed-bed vertical reactor shown in FIG. 5. The reactor 200 is a cylindrical quartz tube with an inner diameter of 250 mm. The middle part is provided with a non-woven fabric for placing catalyst, the lower part is provided with an inert gas and raw material gas supply pipeline 204, and the upper part is provided with an exhaust gas pipeline 205. The upper part of the quartz tube is made openable and closable so that the catalyst 208 can be introduced. The lower portion of the quartz tube is configured to be openable and closable so that the catalyst 208 can be removed. Further, the reactor has a heater 206 surrounding the periphery of the reactor so that the reactor can be maintained at an arbitrary temperature. The heater 206 is provided with a detection port 207 for checking the state of a catalyst 208 in the apparatus.

The reactor 200 was previously filled with argon gas before being filled with the catalyst. In addition, the heater 206 was heated to 850 ℃ prior to catalyst filling. The catalyst 10g prepared by the above catalyst was placed between the nonwoven fabrics 201 of the reactor shown in fig. 5, thereby being disposed in the reactor 200. Then, the apparatus was started to start the supply of argon gas from the raw material supply line 204 at a rate of 50000 mL/min. The reaction chamber was replaced with argon while heating the catalyst for 5 minutes, and then heated to 870 ℃ (30 minutes for temperature rise).

After the temperature reached 870 ℃, the flow rate of argon gas in the raw material gas supply line 204 was changed to 3000 mL/min, and methane was mixed with argon gas so that the flow rate of methane was 140 mL/min, and then the mixture was supplied to the reactor. After the mixed gas was supplied for 30 minutes, the flow was switched to argon gas alone, and the synthesis was terminated. The heating was stopped, the mixture was left to stand until it became room temperature, and the carbon nanotube composition containing the catalyst and the carbon nanotubes was taken out of the reactor after it became room temperature.

About 10mg of the catalyst-containing carbon nanotube composition was placed in a differential thermal analyzer (TGA-60 manufactured by Shimadzu corporation), and the temperature was raised from room temperature to 900 ℃ in air at a temperature raising rate of 10 ℃ per minute. The weight change during this period was measured. At this time, the combustion peak temperature due to heat generation was read from the DTA curve at that time, and the result was 515 ℃.

20.0g of the carbon nanotube composition containing the catalyst was placed on a ceramic dish (150)) Then, the mixture was put into an electric furnace (manufactured by ャマト scientific Co., Ltd., FP41) previously heated to 505 ℃ and heated at 505 ℃ for 2 hours in the air, and then taken out of the electric furnace. Next, to remove the catalyst, the carbon nanotube composition was added to a 6N aqueous hydrochloric acid solution and stirred at room temperature for 1 hour. The recovered product obtained by filtration was added to a 6N aqueous hydrochloric acid solution, and stirred at room temperature for 1 hour. This was filtered, washed with water several times, and then the filtrate was dried in an oven at 120 ℃ overnight, thereby obtaining 92.2mg of a carbon nanotube assembly from which the catalyst was removed.

The carbon nanotube aggregate thus obtained was observed by a high-resolution transmission electron microscope, and 90 double-walled carbon nanotubes were observed among 100 carbon nanotubes. In addition, in this case, the G/D ratio was 54 as a result of Raman spectroscopy at a wavelength of 633nm of the carbon nanotube assembly.

Heat treatment was performed in nitric acid in the same manner as in example 1, except that the above-described double-walled carbon nanotube assembly was used, and the heating time in concentrated nitric acid was changed to 12 hours. The obtained carbon nanotube aggregate was measured in the same manner as in example 1.

(measurement of volume resistivity)

The volume resistivity of the carbon nanotube aggregate was 2.4X 10^-3Ω·cm。

(evaluation of the number of carbon nanotube layers Using Transmission Electron microscope)

The average diameter of 100 carbon nanotubes was 1.9nm, and the standard deviation of the diameters was 0.71 nm. And 82 of the 100 carbon nanotubes are double-layer carbon nanotubes.

(weight loss measurement of temperature from 200 ℃ to 400 ℃ C.)

The weight loss during the temperature increase from 200 ℃ to 400 ℃ was 10.5%. In this case, the maximum peak of the DTA curve was 700 ℃.

(evaluation of transparent conductivity)

The transparent conductivity of the composite was measured in the same manner as in example 1. The results are shown in FIG. 2.

< comparative example 1>

The carbon nanotube assembly produced in example 1 was fired in an electric furnace in air at a temperature of 400 ℃ for 5 hours to obtain a carbon nanotube assembly in which the weight loss during the temperature increase from 200 ℃ to 400 ℃ was 3.6%.

(measurement of volume resistivity)

The volume resistivity of the carbon nanotube assembly was measured in the same manner as in example 1, and the result was 1.9X 10^-2Ω·cm。

(evaluation of the number of carbon nanotube layers Using Transmission Electron microscope)

74 out of 100 are double-walled carbon nanotubes. In addition, the average diameter of 100 carbon nanotubes was 1.9nm, and the standard deviation of the diameters was 0.60 nm.

(evaluation of transparent conductivity)

The transparent conductivity of the composite was measured in the same manner as in example 1. The results are shown in FIG. 2.

< comparative example 2>

A carbon nanotube aggregate manufactured by Tokyo ナノ carbon Co., Ltd was prepared in the same manner as in example 1 (the number of layers was evaluated in the same manner as in example 1, and the result was that 100 carbon atoms were usedOf the nanotubes, 30 were double-walled carbon nanotubes, and 67 were single-walled carbon nanotubes. ) After heating in a nitric acid solution, the volume resistivity was measured in the same manner as in example 1, and the result was 6.4X 10^-3Ω·cm。

(evaluation of transparent conductivity)

The transparent conductivity of the composite was measured in the same manner as in example 1. The results are shown in FIG. 2.

< comparative example 3>

ナノ テ ク ポ - ト mg of a double-walled carbon nanotube (59 out of 100 double-walled carbon nanotubes; weight loss during the temperature increase from 200 ℃ to 400 ℃ in this case was 9.5%), was added to 27mL of concentrated nitric acid (grade 1 manufactured by Wako pure chemical industries, Ltd., analytical content was 60 to 61%), and the mixture was heated in an oil bath at 130 ℃ for 5 hours while stirring, in the same manner as in example 1. The volume resistivity was measured in the same manner as in example 1, and was found to be 4.0X 10^-2Omega cm. In this case, 58 of the 100 carbon nanotubes are double-walled carbon nanotubes. The weight loss during the temperature increase from 200 ℃ to 400 ℃ was 10.4%.

Industrial applicability

By using the carbon nanotube aggregate having excellent conductivity of the present invention, a film having excellent light transmittance and surface resistance can be obtained.

In addition, the carbon nanotube assembly having high conductivity can be obtained simply and with high yield by using the production method of the present invention.

Claims (16)

1. A carbon nanotube assembly satisfying all of the following conditions (1) to (3):

(1) volume resistivity of 1X 10^-5～5×10^-3Ω·cm；

(2) When observed by using a transmission electron microscope, more than 50 of 100 carbon nanotubes are double-layer carbon nanotubes;

(3) when thermogravimetric analysis is performed at a temperature rise of 10 ℃/min, the weight loss at a temperature rise from 200 ℃ to 400 ℃ is 5-20%.

2. The carbon nanotube assembly according to claim 1, wherein the average value of the outer diameters of the carbon nanotubes is in the range of 1.0 to 3.0nm, and the standard deviation of the outer diameters is 1.0nm or less.

3. The carbon nanotube assembly according to claim 1 or 2, wherein a maximum peak of the DTA curve is in a range of 650 to 750 ℃ when the differential thermal analysis is performed at a temperature rise of 10 ℃/min.

4. The carbon nanotube assembly according to any one of claims 1 to 3, wherein the carbon nanotubes have a C-O group and a C ═ O group.

5. The carbon nanotube assembly according to any one of claims 1 to 4, wherein the ratio of oxygen atoms to carbon atoms in the carbon nanotubes is 4% or more.

6. A carbon nanotube molded body formed from the carbon nanotube assembly according to any one of claims 1 to 5.

7. A composition comprising the carbon nanotube assembly according to any one of claims 1 to 5.

8. The carbon nanotube composition of claim 7, the collection of carbon nanotubes dispersed in a liquid dispersion medium.

9. A molded body formed from the carbon nanotube composition of claim 7.

10. A composite comprising a conductive layer containing the carbon nanotube composition according to claim 7 or 8 formed on a substrate.

11. The composite according to claim 10, wherein the substrate is a film.

12. The composite body according to claim 11, satisfying the following conditions (1) and (2):

(1) surface resistance of less than 1 × 10⁴Ω/□；

(2) The light transmittance at a wavelength of 550nm satisfies the following conditions:

the light transmittance of the composite/that of the substrate was not less than 0.85.

13. A method for producing the carbon nanotube assembly according to any one of claims 1 to 5, wherein a carbon nanotube assembly containing double-walled carbon nanotubes, which has a G/D ratio of 20 or more as a height ratio of a G band to a D band measured by Raman spectroscopy at a wavelength of 633nm, is heated in a nitric acid solution.

14. The method for producing a carbon nanotube assembly of claim 13, wherein the temperature at the time of heating in the nitric acid solution is 70 ℃ or higher.

15. The method for producing a carbon nanotube assembly according to claim 13 or 14, wherein the nitric acid solution has a nitric acid concentration of 10% by weight or more.

16. The method for producing a carbon nanotube assembly according to claim 13, wherein the carbon nanotube assembly containing double-walled carbon nanotubes and having a G/D ratio of 20 or more is obtained by contacting a powdery catalyst in which iron is supported on magnesium oxide with methane at 500 to 1200 ℃ to produce a carbon nanotube composition, and then subjecting the carbon nanotube composition to an oxidation treatment in a gas phase.