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Process and apparatus for producing carbon nanotube

Process and apparatus for producing carbon nanotube description/claims

The present invention relates to process and apparatus for producing carbon nanotube by a combustion method.

Arc discharge, laser irradiation and chemical vapor deposition (CVD) have been conventionally known as methods for synthesis of carbon nanotubes. The arc discharge and laser irradiation methods require expensive and dangerous apparatuses such as an evacuator and a high-voltage large-current power source and produce carbon nanotubes in the low yield, which have been problems. The CVD method achieves production of carbon nanotubes generally under low pressure. This not only requires a vacuum apparatus and the like but also complicates control of process conditions such as temperature and pressure, which problematically increase the production cost.

A combustion method utilizing a flame has been noted as a promising industrial production method, because it can obtain both a raw material for carbon nanotubes and a formation energy of carbon nanotubes from a fuel gas. Fullerene, known as carbon nanomaterial other than carbon nanotube, can be produced at low prices by this combustion method.

In Non-patent Literature 1, synthesis of carbon nanotubes is achieved by the combustion method wherein a stainless steel mesh having a catalytic effect is exposed to a flame. However, this process is low in carbon nanotube productivity and also difficult to recover produced carbon nanotubes, which have been problems.

Patent Literature 1 proposes to hold a substrate having a catalyst metal perpendicular to a flame and physically shave off carbon nanotubes formed on the substrate. This case results in the improved yield of carbon nanotubes compared to the case where they are synthesized on the mesh substrate. However, a major portion of the flame that does not come into direct contact with the substrate is wasted because it is unserviceable as a raw material for the carbon nanotubes and only evacuated for diffusion, which raises a problem of low industrial production efficiency.

Patent Literature 2 discloses a method for producing carbon nanotubes by allowing a magnesium oxide substrate to support thereon a catalyst metal and a cocatalyst metal, forming carbon nanotubes by a CVD method and then contacting them with an acid or alkaline solution for dissolution of the substrate, catalyst metal and cocatalyst metal so that the carbon nanotubes are separated therefrom. However, although possible to readily achieve separation and recovery of carbon nanotubes, this method utilizes a CVD method in the production of carbon nanotubes, which requires a vacuum apparatus and the like and involves complex control of process conditions such as temperature and pressure. Accordingly, the problem of high production cost arises.

Patent Literature 2: Japanese Patent Laid-Open No. 2004-182548 Non-patent Literature 1: Randall L. VanderWal, Lee J. Proceedings of the combustion Institute, 29 (2002), PP. 1079-1085

It is an object of the present invention to provide a process for producing carbon nanotube by which carbon nanotubes can be produced more efficiently with a simpler apparatus and also provide an apparatus for producing carbon nanotube.

The production process of the present invention is a process which produces carbon nanotubes by a combustion method and is characterized as including the steps of preparing a catalyst-supporting powder comprising a base powder and a catalyst supported on a surface of the base powder, allowing a porous support having internal through-pores that permit passage of a flame to hold the catalyst-supporting powder non-adherently within the through-pores, placing the porous support holding the catalyst-supporting powder in a combustion oven having one open end, and generating a carbon-containing flame in a burner portion disposed at the other end of the combustion oven and feeding the flame into the through-pores of the porous support to form carbon nanotubes on the surface of the catalyst-supporting powder within the through-pores.

In the present invention, the catalyst-supporting powder comprising a base powder and a catalyst supported on a surface of the base powder is held non-adherently within the through-pores of the porous support and the carbon-containing flame is subsequently fed into the through-pores of the porous support so that carbon nanotubes are formed on the surface of the catalyst-supporting powder within the through-pores. Since the catalyst-supporting powder is held non-adherently within the through-pores, its entire surface can be utilized in the formation of carbon nanotubes thereon. This enables efficient production of carbon nanotubes.

Also, the catalyst-supporting powder having a surface on which carbon nanotubes have been formed is not affixed to the through-pores of the porous support. Accordingly, the catalyst-supporting powder subsequent to formation of carbon nanotubes can be easily extracted from the porous support.

Also in the present invention, the use of a combustion method in the production of carbon nanotubes eliminates the necessity of controlling complex production conditions such as temperature and pressure and thus permits production of carbon nanotubes with a simpler apparatus. Therefore, a large quantity of carbon nanotubes can be produced economically at low cost, which makes the present process more suitable for industrial production.

Also in the present invention, a shielding member is preferably provided at one end of the combustion oven for directing a part of a flow of the flame back to the porous support side. The provision of such shielding member allows a flow of the flame to partly return to the porous support side and enables the flame containing unreacted carbon to again contact with the catalyst-supporting powder within the through-pores of the porous support, leading to further formation of carbon nanotubes. Accordingly, carbon nanotubes can be produced further efficiently.

Also in the present invention, the base powder preferably comprises a material which is soluble in an acid or alkali. The use of such base powder eases recovery of carbon nanotubes by the fashion which follows. Subsequent to formation of carbon nanotubes on its surface, the catalyst-supporting powder is extracted from the porous support and then contacted to an acid or alkali that dissolves the base powder and the catalyst.

The base powder soluble in an acid or alkali can be illustrated by an oxide or salt of at least one metal selected from Mg, Si, Na, Li, Ca, Al and Zn. Examples of particularly preferred base powders include magnesium hydroxide, magnesium oxide, calcium hydroxide and calcium oxide.

The acid that dissolves the base powder can be illustrated by aqueous solutions of hydrochloric acid, nitric acid, sulfuric acid and organic acid. The alkali that dissolves the base powder can be illustrated by aqueous solutions of sodium hydroxide, potassium hydroxide and ammonia.

Selection of the acid or alkali may be suitably made depending on the type of the base powder used. A concentration of the acid or alkali may preferably be at least 0.01% by weight, more preferably 0.5-10% by weight, for example.

A temperature of an aqueous solution of the acid or alkali is not particularly specified, so long as it can maintain the state of aqueous solution, but generally preferably within the range of 30-80° C.

The catalyst-supporting powder for use in the present invention comprises the base powder and the catalyst supported on the surface of the base powder. A compound containing at least one metal selected from Co, Cr, Fe, Mo, Ni and V is preferably used as the catalyst to be supported.

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