Carbon nanotubes (hereinafter referred to also as “CNTs”) are carbon structures each structured such that a carbon sheet composed of a planar hexagonal arrangement of carbon atoms is sealed in a cylindrical shape. The CNTs are classified into single-walled CNTs and multiwall CNTs, both of which are expected to develop into functional materials such as electronic device materials, optical element materials, and conducting materials because of their mechanical strength, optical properties, electrical properties, thermal properties, and molecular-adsorbing functions, etc.
Among the CNTs, the single-walled CNTs are excellent in various properties such as electrical properties (extremely high in current density), heat properties (comparable in specific thermal conductivity to diamonds), optical properties (emit light in a optical communication band of wavelengths), hydrogen storage capability, and metal catalyst supporting capability. Moreover, the single-walled CNTs exhibit the properties of both semiconductors and metals, and therefore have drawn attention as materials for nanoelectronics devices, nanooptical elements, and energy storage bodies.
In the case of making efficient use of CNTs for these purposes, it is desirable that a plurality of CNTs be aligned along a particular direction to form an aggregate in the form of a bundle, a film, or a mass, and that the CNT aggregate exhibit functionalities such as electric/electronic functionalities and optical functionalities. Further, it is preferable that the CNT aggregate be larger in height (length). It is predicted that creation of such an aligned CNT aggregate will lead to a dramatic expansion in the field of application of CNTs.
A known method for producing such CNTs is a chemical vapor deposition method (hereinafter referred to also as “CVD method”) (e.g., see Patent Literature 1). This method is characterized in bringing a carbon-containing gas (hereinafter referred to as “raw material gas”) into contact with a catalyst, i.e., fine metal particles in a hot atmosphere of approximately 500° C. to 1000° C., and as such, makes it possible to produce CNTs with variations in aspects such as the type and arrangement of the catalyst or the type and condition of reaction of the carbon compound, and therefore have drawn attention as being suitable to mass production of CNTs. Further, the CVD method has the advantages of: being capable of producing both single-walled carbon nanotubes (SWCNTs) and multiwall carbon nanotubes (MWCNTs); and being capable of, by using a substrate supporting a catalyst, producing a large number of CNTs aligned perpendicularly to a surface of the substrate.
The CVD method includes a CNT synthesis step. This CNT synthesis step may be divided into a formation step and a growth step, in which case a metal catalyst supported by a substrate is reduced by being exposed to a hot hydrogen gas (hereinafter referred to as “reducing gas”) in the formation step, and then in the growth step CNTs are synthesized by bringing the catalyst into contact with a raw material gas containing a catalyst activation material. The formation step and the growth step are executed in a single furnace to avoid exposure of the reduced catalyst to the outside air between the formation step and the growth step.
In the case of a normal CVD method, fine catalyst particles are covered with carbonaceous impurities generated in the process of synthesis of CNTs; therefore, the catalyst is easily deactivated, and the CNTs cannot grow efficiently. For this reason, it is common to synthesize CNTs in an atmosphere of low-carbon concentration with the volume fraction of a raw material gas during CVD reduced to approximately 0.1 to 1%. Since the amount of a raw material gas supplied is proportional to the production volume of CNTs, the synthesis of CNTs in an atmosphere of as high-carbon concentration as possible is directly linked to an improvement in production efficiency.
In recent years, there has been proposed a technique for the CVD method that remarkably increases the activity and longevity of a catalyst by bringing a catalyst activation material such as water, as well as a raw material gas, into contact with the catalyst (such a technique being hereinafter referred to as “super-growth technique”; see Non-Patent Literature 1). A catalyst activation material is believed to have an effect of cleansing the outer layer of a catalyst by removing carbonaceous impurities covering the fine catalyst particles, and such an effect is believed to remarkably increase the activity and longevity of the catalyst. Actually, there has been a case of success in remarkably improving efficiency in the production of CNTs by preventing deactivation of a catalyst even in such an environment of high-carbon concentration (approximately 2 to 20% of the volume fraction of a raw material gas during CVD) that the catalyst would normally be deactivated. CNTs that are synthesized by applying the super-growth technique to a substrate supporting a catalyst have the features of: being large in specific surface area, forming an aggregate of CNTs each aligned along a regular direction; and being low in bulk density (such an aggregate being hereinafter referred to as “aligned CNT aggregate”.
Conventionally, CNT aggregates are one-dimensional elongated flexible substances that are very high in aspect ratio, and because of their strong van der Waals' force, are likely to constitute random and non-aligned aggregates that are small in specific surface area. Because it is extremely difficult to restructure the orientation of an aggregate that is once random and non-aligned, it has been difficult to produce a CNT aggregate that is large in specific surface area with moldability and processability. However, the super-growth technique has made it possible to produce aligned CNT aggregates that are large in specific surface area, have orientation, and can be molded and processed into various forms and shapes, and such aligned CNT aggregates are believed to be applicable as substance/energy storage materials for various uses such as super-capacitor electrodes and directional heat-transfer/heat-dissipation materials.
Conventionally, there have been proposed various production apparatuses for realizing serial production of CNTs by the CVD method, a known example thereof being a technique for transferring a series of base materials into a synthesis furnace with use of transferring means such as a belt conveyor or a turntable (see Patent Literatures 2 to 4). However, it was found that in the case of serial production of aligned CNT aggregates with use of the super-growth technique, there are technical problems specific to high-carbon environment and/or a catalyst activation material, although there were no such problems with the conventional synthetic method.