JP2737736B2 - Method for producing carbon single-walled nanotube - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method for producing carbon single-walled nanotubes, and more particularly, to a method for producing carbon single-walled nanotubes capable of continuous production for a long time.

[0002]

2. Description of the Related Art Carbon nanotubes are formed by nesting one or several cylinders each having a rounded graphitic carbon atom surface having a thickness of several atomic layers, and are extremely minute substances having an outer diameter on the order of nm. .

A typical carbon nanotube in which several or more cylindrical graphite layers are formed concentrically has a large variation in the electrical and chemical properties of the carbon nanotube. Therefore, a carbon single-walled nanotube in which the shape of the tube is controlled to a single-walled type has been developed.

Conventionally, as a method for producing single-walled carbon nanotubes, arc discharge using a carbon rod as an electrode has been used. In this method, since carbon nanotubes are synthesized using the carbon material of the electrode as a raw material, it is inevitable that the electrode is consumed as the production time elapses, and the state of the arc discharge itself changes with time.

In order to synthesize carbon single-walled nanotubes with high efficiency, a metal catalyst such as iron or nickel is required in addition to carbon as a raw material. In the arc discharge method, these catalyst metals are often supplied in a form embedded in the carbon electrode. The ratio of the supply amount of the catalyst changes over time.

For the above reasons, in the arc discharge method, it is not possible to expect the continuity of the production condition exceeding several minutes, and it is difficult to synthesize the carbon single-walled nanotube for a long time.

[0007]

The carbon single-walled nanotube becomes a tough carbon fiber due to its good crystallinity.
In the conventional arc discharge device, the carbon rod electrode is consumed, and the ratio of the supply amount of carbon and the catalyst changes with time. It was difficult to obtain a yield.

[0008] Therefore, with the industrial development of carbon single-walled nanotubes, it has become an inevitable problem to establish a synthesis method capable of operating for a long time.

In addition, since amorphous carbon and graphite are also generated during the synthesis of carbon single-walled nanotubes, it has also been a problem to suppress the formation of these other phases and to obtain production conditions for obtaining high-quality carbon single-walled nanotubes. .

[0010]

In order to solve the above problems, it is necessary to generate high-temperature plasma without using a carbon electrode that directly contacts the plasma. I have.

According to this method, it is possible to prevent the plasma from contacting any wall surface by stabilizing the plasma shape by controlling the gas flow, and the use of the method for the synthesis of carbon single-walled nanotubes eliminates the limitation of the synthesis time. Was.

As the carbon raw material, it is preferable to use a gas having good controllability of the supply amount, and a hydrocarbon such as methane can be used.

If the metal catalyst is supplied to the plasma completely independently of the carbon source, the ratio of the supply amount of the carbon and the metal catalyst does not change with time.

Amorphous carbon and graphite are also generated when carbon single-walled nanotubes are formed. To suppress this, it is preferable to add hydrogen or oxygen which has an etching effect on carbon. Further, by supplying a gas to a part of the plasma from the outside, the plasma temperature can be rapidly lowered and the generated powder can be rapidly cooled, whereby the generation of by-products such as amorphous carbon and graphite can be suppressed.

Further, by cooling the substrate for collecting the carbon single-walled nanotubes by water cooling or the like, the production of multi-walled carbon nanotubes can be suppressed, and the single-walled carbon nanotubes can be selectively produced. .

[0016]

DESCRIPTION OF THE PREFERRED EMBODIMENTS One embodiment of the present invention will be described below.

Since the method for producing single-walled carbon nanotubes of the present invention uses electrodeless high-frequency plasma, there is in principle no problem of electrode consumption due to arc discharge, and long-term operation is possible.

In order to prevent the quality of the product from fluctuating with time during the production, a constant amount of the carbon raw material and the catalytic metal raw material may be constantly supplied into the plasma. The carrier gas is supplied in the form of powder in a steady flow of the carrier gas, and the carbon raw material is separately supplied in the form of a hydrocarbon gas whose flow rate is controlled.

In the embodiment of the present invention, usually several hundred Torr
Plasma in the arc region is generated at a degree of vacuum of r,
In this case, since the temperature of the atoms in the plasma reaches a high temperature of 10,000 ° C., the supplied catalytic metal powder having a particle size of several μm evaporates almost completely. Hydrocarbon gas, a carbon source, is more easily atomized in the plasma.

The carbon and the catalyst metal atomized in the plasma aggregate at low temperatures around the periphery of the plasma flame to form fine particles. Due to the catalytic action on these fine particles, the carbon atoms that have dissolved or fly away form carbon single-walled nanotubes.

However, at this time, amorphous carbon and graphite are also formed. However, since the growth rate of carbon single-walled nanotubes is higher, amorphous and graphite are more preferable if hydrogen or oxygen for etching carbon is added. Etching is preferentially performed, and a high-quality carbon single-walled nanotube is obtained.

Further, by supplying a gas from the outside to the peripheral portion of the plasma flame where the fine particles are formed and rapidly lowering the temperature of the plasma, amorphous carbon or graphite having a low growth rate is not sufficiently formed. The reaction can be stopped by quenching, and a high-quality carbon single-walled nanotube can be obtained.

Further, by cooling the substrate for collecting the carbon single-walled nanotubes by water cooling or the like, it is possible to suppress the formation of multi-walled carbon nanotubes and selectively produce single-walled carbon nanotubes.

[0024]

【Example】

Embodiment 1 An embodiment of the present invention will be described below with reference to FIG.

In the present embodiment, the high-frequency plasma generator can generate a high-temperature plasma 2 having a size of several cm at a degree of vacuum of about 300 Torr by supplying approximately 10 kW of power to the coil 3 at an oscillation frequency of 4 MHz. did it.

In order to stabilize the shape of the plasma, about 60 liters of argon gas is supplied per minute from above in the figure, and the powder generated in the plasma reaches the water-cooled substrate 4 by riding this gas flow. Collected.

In order to synthesize a carbon single-walled nanotube, a metal catalyst is required. Therefore, in this embodiment, the catalyst metal is supplied in the form of a powder having a particle size of about 3 to 5 μm by a flow type powder supply apparatus. At a rate of 3 liters per minute in a carrier gas stream of argon, and transported to the plasma via the nozzle 1.

When the flow rate of the carrier gas is less than 1 liter per minute, the powder is not supplied to the center of the plasma due to the reverse airflow inside the plasma, and the catalytic metal is not sufficiently evaporated. Therefore, in this embodiment, the carrier gas flow was set to 3 liters per minute.

The types of catalyst metals include iron, cobalt,
In each case of nickel, synthesis of carbon single-walled nanotubes was possible. Especially nickel 75%, cobalt 2
The one using 5% mixed powder had the highest yield of carbon single-walled nanotubes. The standard supply of metal powder was about 100 mg per minute.

As a carbon source, about 1 liter / minute of methane was supplied to the peripheral portion on the downstream side of the plasma flame. In addition, even if the upstream side of the plasma flame is selected as a place for supplying methane, if the supply rate is about 1 liter per minute, there is no practical problem since the stability of plasma is not impaired.

In order to suppress the formation of amorphous carbon and graphite formed together with the single-walled carbon nanotubes, 6 liters of hydrogen having an etching effect on carbon was supplied to the downstream side of the plasma. When the ratio of the supply amount of hydrogen to the supply amount of methane is 2 or 3, since a large amount of amorphous or graphite was generated around the catalyst metal fine particles, in this embodiment, the ratio of the supply amount of hydrogen was set to exceed 5. I chose.

By the above method, it was confirmed by an electron microscope that a large amount of fine particles were released in a soot form from the plasma flame, and that the fine particles contained many carbon single-walled nanotubes. However, when the fine particles are collected on a substrate which is exposed to a plasma flame and is at a high temperature of about 1000 ° C., multi-walled carbon nanotubes composed of two or three layers are more frequently observed than single-walled carbon nanotubes. As a result, it was confirmed that the carbon nanotubes grew in the diameter direction. Therefore, in order to obtain only single-walled nanotubes, it is preferable to collect them on a substrate cooled by water cooling or the like.

Example 2 In order to suppress the formation of amorphous carbon and graphite, 15 liters of argon gas per minute was supplied to the downstream of the plasma. The argon gas itself has no function of etching carbon, but by rapidly cooling the plasma accompanying the introduction of argon, preferentially leaving single-walled carbon nanotubes with a high growth rate and suppressing the formation of other amorphous carbon and graphite be able to.

The phenomenon of quenching could be confirmed visually because the radiation intensity of the downstream portion of the plasma was reduced when introducing argon gas. Compared with the case where argon was not introduced, the amount of amorphous carbon and graphite existing around the catalytic metal fine particles was reduced, and particularly the amount of graphite was significantly reduced. It is considered that gas quenching hindered the process of improving the crystallinity of amorphous carbon at high temperatures to become graphite.

On the other hand, the crystallinity of the carbon single-walled nanotubes was observed with an electron microscope, and it was confirmed that the gas cooling method did not adversely affect the crystallinity.

Example 3 Since commercially available metal powder has a large variation in particle diameter, when metal powder is used as a raw material for a catalytic metal, the metal powder having a large particle diameter is completely evaporated in plasma. In some cases, it is mixed with the product and collected.

To prevent this, an organic metal was used as a metal catalyst source. In this example, ferrocene, which is an organic metal of iron, was used. Since ferrocene had a low assembling temperature of 140 ° C., it was completely evaporated in the plasma, and undecomposed raw material powder as when metal powder was used was not mixed.

Using ferrocene as a catalyst raw material and methane as a carbon raw material, carbon single-walled nanotubes could be synthesized with high efficiency.

Since ferrocene itself also contains a carbon atom, carbon single-walled nanotubes could be synthesized by using only ferrocene without using methane, although the yield was low.

[0040]

According to the conventional method for producing single-walled carbon nanotubes by arc discharge, the carbon rod as the electrode is consumed during the discharge, and the catalytic metal embedded in the electrode is more rapidly consumed due to its high vapor pressure. You. The exhaustion of the carbon electrode causes a temporal change in the plasma state, and the selective evaporation of the catalyst metal causes a change in the supply amounts of the carbon and the catalyst metal to the plasma, so that it is difficult to synthesize carbon single-walled nanotubes for a long time. Was.

In order to solve this problem, the present invention uses an electrodeless plasma synthesis apparatus as shown in FIG. This is 4
A high-frequency plasma having a maximum temperature of about 10,000 ° C. was formed without contacting any wall surface by a high-frequency coil of MHz, so that it was possible to synthesize carbon single-walled nanotubes without any time limit.

The catalyst metal is supplied in the form of a powder having a particle size of several μm in a steady carrier gas flow, and the carbon raw material is supplied as methane gas to the plasma stably via a gas flow meter. If the metal catalyst and carbon are supplied to the plasma in different systems, the supply ratio of each component element is constant, and carbon single-walled nanotubes of uniform quality can be synthesized.

The carbon atomized in the plasma and the catalyst metal are aggregated in a low-temperature portion on the periphery of the plasma to form a generated powder. At this time, amorphous carbon and graphite are also formed. Therefore, in the present invention, utilizing the fact that the growth rate of carbon single-walled nanotubes is higher than that of other phases, amorphous and graphite are more preferentially etched by adding hydrogen or oxygen for etching carbon, thereby improving quality. Good single-walled carbon nanotubes were obtained.

Another method aiming at the same effect is to supply an argon gas from the outside to the periphery of the plasma and rapidly lower the temperature of the plasma, so that amorphous carbon or graphite having a low growth rate cannot be sufficiently formed. At this stage, the reaction was stopped by rapid cooling, and a high-quality carbon single-walled nanotube was obtained.

When a cooled collecting substrate is used, the production of a plurality of layers of carbon nanotubes can be suppressed and single-walled carbon nanotubes can be selectively obtained.

[Brief description of the drawings]

FIG. 1 is a conceptual diagram of an apparatus for producing single-walled carbon nanotubes using high-frequency plasma according to the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Powder supply port 2 Plasma 3 High frequency coil 4 Collection board

Claims (8)

(57) [Claims] 1. A method for producing single-walled carbon nanotubes, comprising generating plasma using an electrodeless high-frequency plasma and supplying a carbon material and a metal catalyst into the plasma. 2. A method for producing carbon single-walled nanotubes, wherein plasma is generated using an electrodeless high-frequency plasma, and a carbon material and a metal catalyst are separately supplied into the plasma. 3. The method according to claim 1, wherein a hydrocarbon gas is used as the carbon raw material. 4. The method according to claim 1, wherein a powdery metal catalyst is supplied into the plasma. 5. The method according to claim 1, wherein an organic metal is used as the carbon raw material and the metal catalyst. 6. The method for producing a carbon single-walled nanotube according to claim 1, wherein a gas for etching carbon is supplied into the plasma. Method. 7. The method of claim 1, wherein the temperature of the plasma region is locally reduced. 8. The substrate for collecting carbon nanotubes is cooled, wherein the substrate for cooling the carbon nanotubes is cooled. A method for producing the carbon nanotube according to the above.