CROSS REFERENCE

The present application claims the benefit of priority of Japanese Patent Application No. 2019-085046, filed on Apr. 26, 2019, the entire content of which is incorporated herein by reference. The entire contents of the patents, patent applications, and literatures cited in the present application are also incorporated herein by reference.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to a multi-walled carbon nanotube, a multi-walled carbon nanotube aggregate and a method for preparing a multi-walled carbon nanotube.

Description of the Related Art

A carbon nanotube (Carbon Nanotube: CNT) is a nano-scale material having a one or two-walled cylindrical structure obtained by rolling a graphene sheet. A single-walled carbon nanotube (Single-Walled Carbon Nanotube: SWCNT) having a single-walled cylindrical structure obtained by rolling a graphene sheet is excellent in various properties such as electrical properties, thermal properties, optical properties, hydrogen storage capacity, and metal catalyst support capacity. Further, SWCNT becomes metallic or semiconducting depending on the geometric structure determined by a chiral index (n, m). Accordingly, SWCNTs are receiving attention as materials for electronic devices, electrodes for power storage devices, MEMS members, fillers for functional composite materials and the like (See Japanese Patent Laid-Open No. 2014-166936).

On the other hand, a multi-walled carbon nanotube (Multi-Walled Carbon Nanotube: MWCNT) having two or more walls obtained by rolling a graphene sheet includes properties excellent in thermal stability and chemical stability. Even when the surface of MWCNT is modified, the internal cylindrical structure can be maintained and the properties as CNT can be exhibited. CNTs including MWCNT are materials difficult to mix into solvents such as water and organic solvents, or polymer solutions in general. However, by oxidizing the surface of MWCNT, a carboxy group is bonded to the surface, and further reacted with thionyl chloride to form a carbonyl group on the surface, and thereby it becomes easy to disperse MWCNT into the above described solvent or polymer while maintaining the properties as CNT (see Japanese Patent Laid-Open No. 2006-193380).

CNT can be prepared by, for example, an arc discharge method, a laser ablation method, or a chemical vapor deposition method (see Japanese Translation of PCT International Application Publication No. 2014-521589). Among these preparation methods, the chemical vapor deposition (Chemical Vapor Deposition: CVD) method enables cost reduction of CNT, because the CVD method can be used in preparing both SWCNT and MWCNT, and is more suitable for mass production as compared with the two former methods. The CVD method is broadly divided into a fluidized-bed type CVD method (also called a gas phase catalyst CVD method) that generates CNT on a catalyst surface while flowing the catalyst and reaction gas, and a fixed-bed type CVD method (also called a catalyst-supported CVD method) that positions a substrate supporting a catalyst in a reaction vessel and generate CNT on the substrate by supplying gas into the reaction vessel (see Japanese Patent Laid-Open No. 2019-014645). In general, the fluidized-bed type CVD method is more excellent in mass productivity as compared with the fixed-bed type CVD method.

The length of CNT that can be prepared by an arc discharge method, a laser ablation method, or a CVD method has been limited to 100 to 200 μm. One of the causes is that the life of the catalyst is short. As a CVD method that elongates the life of the catalyst like this, a method called a super growth method has been developed. The super growth method significantly increases the activity and life of the catalyst by adding a small amount of water to raw material gas, and enables production of a long SWCNT of several hundreds μm to 1 mm.

However, in the production of MWCNT, a method having a length on the millimeters has not yet been established. MWCNT with a small number of walls and a length on the order of millimeters (1 mm or more) has a low metal impurity concentration and high purity, so that the MWCNT has an excellent effect of being used as a conductive auxiliary agent for batteries, and can construct a high-strength molded body when the MWCNT is formed into the molded body because the MWCNT is excellent in network properties. When MWCNT is added into a resin or a rubber, MWCNT can be expected as a material that exhibits excellent electric conductivity performance or thermal conductivity in a small amount.

In order to respond to the above described expectation, the present invention has an object to provide a long multi-walled carbon nanotube with a small number of walls, and an aggregate of the multi-walled carbon nanotube.

SUMMARY OF THE INVENTION

(1) A multi-walled carbon nanotube according to one embodiment to achieve the above described object is a carbon nanotube having two or more tubes of a graphene sheet where carbon atoms are arranged in a hexagonal honeycomb form, coaxially, wherein a diameter of an outermost wall based on observation of an image by a transmission electron microscope is 3 nm or more and 15 nm or less, and a length based on observation of an image of a scanning electron microscope is 1.0 mm or more.
(2) In a multi-walled carbon nanotube according to another embodiment, a wave where an average period based on observation of an image by a scanning electron microscope is 0.5 μm or more and 2.0 μm or less is preferably formed in a lengthwise direction.
(3) A multi-walled carbon nanotube aggregate according to one embodiment to achieve the above describe object is an aggregate of multi-walled carbon nanotubes each having two or more tubes of a graphene sheet where carbon atoms are arranged in a hexagonal honeycomb form, coaxially, wherein an average diameter of outermost walls based on observation of an image by a transmission electron microscope is 6 nm or more and 12 nm or less, and an average length based on observation of an image of a scanning electron microscope is 1.4 mm or more.
(4) In a multi-walled carbon nanotube aggregate according to another embodiment, in a lengthwise direction of multi-walled carbon nanotubes in the multi-walled carbon nanotube aggregate, waves where average periods based on observation of an image by a scanning electron microscope are 0.5 μm or more and 2.0 μm or less are preferably formed.
(5) In a multi-walled carbon nanotube aggregate according to another embodiment, a metal content is preferably 250 ppm or less by weight.
(6) A method for preparing a multi-walled carbon nanotube according to one embodiment to achieve the above described object is a method for preparing a multi-walled carbon nanotube by a chemical vapor deposition method by holding a substrate supporting a metal catalyst on a surface, in a reaction vessel, and is a method including

forming a support film for supporting the metal catalyst, on a surface of a smooth plate,

supporting a metal catalyst formed from a manganese, iron, cobalt, nickel, molybdenum, palladium, indium, tin, or an alloy containing one or more of these metals, on the support film,

disposing the one or more substrates having a catalyst film formed of the metal catalyst formed on a surface of the support film, in the reaction vessel,

bringing an inside of the reaction vessel under an atmosphere of inert gas or oxidizing gas,

heating the inside of the reaction vessel to a synthesis temperature of the multi-walled carbon nanotube,

granulating the catalyst film by exposing the catalyst film to an atmosphere of gas having reducing activity, and

thermally decomposing the hydrocarbon to grow multi-walled carbon nanotubes on the surface of the catalyst film of the substrate, as introducing raw material gas containing at least hydrogen and hydrocarbon from one end of the substrate to another end to contact the raw material gas to a surface of the catalyst film while preheating the raw material gas to a temperature capable of promoting thermal decomposition of the hydrocarbon on the catalyst film and lower than the synthesis temperature, and exhausting the inside of the reaction vessel unidirectionally by a non-circulation method to generate a current in a direction from the one end of the substrate to the another end in the reaction vessel.

(7) A method for preparing a multi-walled carbon nanotube according to another embodiment is preferably a method that, by using an apparatus where the substrate is disposed in a space surrounded by a heater, and a pipe for introducing gas is disposed between the heater and the substrate, passes the gas containing at least hydrogen and hydrocarbon through the pipe and supplies the gas from the one end of the substrate while preheating the gas.
(8) A method for preparing a multi-walled carbon nanotube according to another embodiment is preferably a method, wherein the catalyst film is a film having a center line average roughness (Ra) after granulation of 0.80 nm or more.
(9) A method for preparing a multi-walled carbon nanotube according to another embodiment is preferably a method, wherein the support film is a film having a center line average roughness (Ra) of 0.37 nm or more.

According to the present invention, applicability to composite materials with high purity and high strength, and material requiring high conductivity or high thermal conductivity is increased.

