JP2004018351A - Process for manufacturing carbon nanotube - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To improve manufacturing efficiency by synthesizing a large number of carbon nanotubes at a low temperature through arc discharge method, and to provide a process for easily manufacturing single- or double-layer carbon nanotubes having few defect. <P>SOLUTION: In the process, the carbon nanotubes are synthesized with a plasma generated through abnormal glow discharge between an anode part and a cathode part located inside a reaction chamber into which a reaction gas is introduced. Here, a catalyst is used for synthesizing the carbon nanotubes. <P>COPYRIGHT: (C)2004,JPO

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a method for producing carbon nanotubes, which generates plasma by abnormal glow discharge and synthesizes carbon nanotubes by using a catalyst.
[0002]
[Prior art]
In general, among fullerenes that are carbon substances having a hollow structure, carbon nanotubes that are tubular fullerenes form a hollow thin cylinder in which hexagonal carbon atoms are spirally connected, and have a diameter of 1 to several tens nm. It is a carbon fiber having a length of several μm. Further, carbon nanotubes have unique physical properties in electrochemical properties, mechanical properties, gas storage properties, and optical properties, and are expected to be widely applied in the future.
[0003]
Carbon nanotubes are classified into single-walled carbon nanotubes and multi-walled carbon nanotubes depending on the structure of the carbon layer.Single-walled carbon nanotubes are excellent in the various physical properties described above, but have difficulty in durability. However, various physical properties are not so high. The double-walled carbon nanotube in which only two carbon layers are stacked exhibits physical properties almost the same as those of the single-walled carbon nanotube, and has higher durability than the single-walled carbon nanotube.
[0004]
As a method for synthesizing carbon nanotubes, an arc discharge method, a laser evaporation method, and a chemical vapor deposition method (CVD method) are known. In the arc discharge method, high-quality carbon nanotubes with few defects can be obtained, but a method for mass synthesis has not been established. In addition, the laser evaporation method can obtain single-walled carbon nanotubes with relatively high purity, but is a synthesis method in which mass synthesis is difficult. Further, the CVD method is a synthesis method suitable for mass synthesis, although the obtained carbon nanotube has many defects. As described above, it is said that each synthesis method has advantages and disadvantages.
[0005]
A conventionally known method of synthesizing carbon nanotubes by an arc discharge method is to carry out arc discharge at a voltage of about 20 V and a current of about 50 A between carbon rods in an atmosphere of a reaction gas in a high temperature state, thereby forming a cathode electrode and an anode electrode. In this method, carbon nanotubes are synthesized in the space where the plasma is generated. The method of synthesizing carbon nanotubes by the CVD method is a method of synthesizing carbon nanotubes by bringing a carbon compound as a carbon source into contact with catalytic metal fine particles at 500 to 1000 ° C.
[0006]
[Problems to be solved by the invention]
By the way, the arc discharge method has a problem that the amount of carbon nanotubes synthesized is small because the plasma space generated by the discharge is narrow. In addition, there is a problem that the cooling operation during the synthesis is difficult because the reaction gas is in a high temperature state. Further, in the CVD method, since carbon nanotubes having many defects are synthesized, there is a problem that the characteristics of carbon nanotubes cannot be sufficiently exhibited. For example, the properties of a reinforcing material for a high-performance material, a hydrogen storage material for a fuel cell, and a high electrical conductivity material can be fully exhibited by using carbon nanotubes without defects. Further, there has been a demand for a manufacturing method capable of easily synthesizing a double-walled carbon nanotube having excellent properties and high durability.
[0007]
The present invention has been made under such a background, a method for producing carbon nanotubes in a large amount at a lower temperature than the arc discharge method to improve the production efficiency, and a method for producing carbon nanotubes with few defects, a single-layer method. It is another object of the present invention to provide a production method capable of easily synthesizing a double-walled carbon nanotube.
[0008]
[Means for Solving the Problems]
In order to solve the above problems, the present invention proposes the following means.
The invention according to claim 1 is a method for producing carbon nanotubes, in which carbon nanotubes are synthesized by plasma generated between an anode unit and a cathode unit disposed in a reaction chamber into which a reaction gas is introduced. And generating carbon plasma by using a catalyst.
[0009]
According to the method for producing carbon nanotubes according to the present invention, since plasma is generated by an abnormal glow discharge between the anode and the cathode, the plasma is generated in a space between the anode and the cathode which is wider than the arc discharge. Is generated. In other words, since the carbon nanotubes are synthesized in the plasma space, a wide area where the carbon nanotubes are synthesized means that the carbon nanotubes can be synthesized in a large amount. In addition, since the carbon nanotubes are synthesized in a reaction gas at a lower temperature than the arc discharge, the cooling operation during the synthesis is easily performed.
[0010]
In addition, the use of a catalyst speeds up the synthesis of carbon nanotubes, and facilitates the synthesis of single-walled and double-walled carbon nanotubes by selecting a catalyst. Further, by synthesizing using a discharge, a carbon nanotube having fewer defects can be synthesized than the synthesis using the CVD method. This makes it possible to produce a large number of carbon nanotubes having few defects in a low-temperature reaction gas.
[0011]
The invention according to claim 2 is a method for producing carbon nanotubes in which carbon nanotubes are synthesized by plasma generated between an anode unit and a cathode unit arranged in a reaction chamber into which a reaction gas is introduced, wherein a discharge current is reduced. The method is characterized in that plasma is generated by a discharge having a current value of 0.5 to 50 A and a voltage value of 350 V or more, and a carbon nanotube is synthesized by using a catalyst.
[0012]
According to the method for producing carbon nanotubes according to the present invention, the discharge current of the discharge for generating plasma is 0.5 to 50 A, more preferably 1 to 10 A, and the applied voltage is 350 V or more. High-density plasma is generated in a wide area between the cathode electrode and the anode electrode. In addition, the use of a catalyst can increase the synthesis rate of carbon nanotubes, and the selection of a catalyst facilitates the synthesis of single-wall and double-wall carbon nanotubes. Further, since the synthesis method is based on discharge, carbon nanotubes having fewer defects than the CVD method can be synthesized. As a result, a large number of carbon nanotubes with few defects can be manufactured.
[0013]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, embodiments of the present invention will be described with reference to the drawings.
First, abnormal glow discharge will be described with reference to FIG. The relationship between the current and the voltage when a DC voltage is applied between the anode and the cathode in the reaction chamber is conceptually shown as a curve in the figure. By controlling the current value shown on the horizontal axis, the voltage value shown on the vertical axis changes, and discharge is performed in a different state at each current value and voltage value. The area in the discharge state is divided by dashed lines D to H, and the area from dashed line F to dashed line H is an abnormal glow discharge region.
[0014]
In the abnormal glow discharge region, discharge is performed from the entire surface of the cathode portion, and the discharge voltage increases as the current density increases as the discharge current increases. For example, in the abnormal glow discharge region, the discharge voltage is 350 V or more, the discharge current is 0.5 to 50 A, more preferably 1 to 10 A, and the reaction gas temperature is 1000 to 10000C.
[0015]
The region from the broken line E to the broken line F is a normal glow discharge region, in which the discharge voltage is kept constant even when the discharge current is increased, which is a discharge state with a large discharge area and a low current density. For example, in the normal glow discharge region, the discharge voltage is 100 V to 350 V and the discharge current is 0.5 A or less.
[0016]
The area after the dashed line H is an arc discharge area, and a large amount of thermoelectrons is emitted by increasing the discharge current to increase the temperature of the cathode portion, thereby reducing the discharge voltage. Is a discharge state where the current density is narrow and the current density is high. For example, in the arc discharge region, the discharge voltage is 10 V to 100 V, the discharge current is 50 A or more, and the reaction gas temperature is 3000 ° C. to 20000 ° C.
As described above, in abnormal glow discharge, high-density plasma is generated in a wider area of the cathode than in arc discharge.
[0017]
Next, a carbon nanotube manufacturing apparatus using the carbon nanotube manufacturing method of the present embodiment will be described with reference to FIG.
The carbon nanotube manufacturing apparatus 1 includes a reaction chamber 2 in which an anode section 3, a cathode section 4, and a substrate base 9 are installed, a first power supply section 5a for applying a voltage between the anode section 3 and the cathode section 4, and And a second power supply unit 5b for applying a voltage between the anode unit 3 and the substrate base 9. The reaction chamber 2 is provided with a reaction gas introduction unit 6 for introducing a reaction gas and an exhaust unit 7 for exhausting gas inside. The inside of the reaction chamber 2 is made an atmosphere of a reaction gas by introducing a reaction gas such as hydrogen or methane from the reaction gas introduction unit 6. Further, an exhaust unit such as a vacuum pump (not shown) is connected to the exhaust unit 7 so that the inside of the reaction chamber 2 can be set to a reduced pressure atmosphere.
[0018]
The anode unit 3 is formed in a flat plate shape, and is connected to the positive electrodes of the first power supply unit 5a and the second power supply unit 5b. The cathode section 4 includes a terminal section 4a connected to the negative electrode side of the first power supply section 5a, and a rod-shaped discharge section 4b adjacent to the terminal section 4a. They are arranged so as to be substantially perpendicular to the electrode surface. Further, the discharge part 4b is formed of W (tungsten), Ta (tantalum), Mo (molybdenum) or the like which is a high melting point metal. The substrate base 9 is formed in a flat plate shape, is connected to the negative electrode side of the second power supply unit 5b, and is installed at a position close to the cathode unit 4. The substrate 8 on which the substrate on which the carbon nanotubes grow is applied is fixed to the substrate stand 9. Further, the substrate table 9 may be installed at a position in contact with a plasma space generated between the anode section 3 and the cathode section 4, and is preferably installed in a space where plasma is highly concentrated.
[0019]
The substrate 8 is formed by applying a powdery base containing a catalyst metal and a catalyst auxiliary metal to a plate-like silicon or the like. The substrate is obtained by immersing powdery high-purity alumina having an impurity of 0.05% or less, preferably 0.01% or less, in a solution in which the catalyst metal and the catalyst auxiliary metal are dissolved in the form of a soluble salt. And a high purity alumina containing a catalyst metal and a catalyst auxiliary metal. By forming the substrate in this manner, the catalyst metal and the catalyst auxiliary metal having a fine particle size are dispersed and contained in high-purity alumina.
[0020]
At least one of Pb, Cr, Co, Ni, and Cu is used as the catalyst metal, and Mo is used as the catalyst auxiliary metal. Further, any one of water, an organic solvent, or a mixture of water and a water-soluble organic solvent is used for the solution, and the concentration of the dissolved catalyst metal and catalyst auxiliary metal is 0.01 to 0.05%. ing. The temperature at the time of immersion is from room temperature to 80 ° C, preferably from 50 to 60 ° C. Further, the formed substrate contains 1 to 20%, preferably 5 to 10% of a catalyst metal, and 0.1 to 1.5%, preferably 0.3 to 0.8% of a catalyst auxiliary metal. It contains. The powdery substrate thus formed is coated on a plate-shaped Si or SiO ₂ at 100 to 250 cm ² / g, preferably 200 to 250 cm ² / g, and dried to form the substrate 8. Have been.
[0021]
A method of manufacturing carbon nanotubes using the above-described carbon nanotube manufacturing apparatus 1 will be described.
The inside of the reaction chamber 2 of the carbon nanotube production apparatus 1 is set to a reaction gas atmosphere, a pressure of 0.5 to 500 Torr, and a discharge current of 0.5 to 50 A, which is an abnormal glow discharge region, more preferably 1 to 10 A. . When the discharge is started under such conditions, first, heating of the discharge part 4b is started, and as the temperature of the cathode part 4 rises, thermoelectrons are emitted from almost the entire surface of the discharge part 4b. Then, stable plasma is formed by emitting thermions under the condition of the abnormal glow discharge, and carbon nanotubes are synthesized on the substrate 8 fixed to the substrate base 9 by using this plasma.
[0022]
Also, since high-density plasma is generated in a region between substantially the entire surface of the cathode portion 4 and substantially the entire surface of the anode portion 3, carbon nanotubes are synthesized in a wider region than the method of synthesizing carbon nanotubes by arc discharge. Further, since the carbon nanotubes can be synthesized using a reaction gas at a temperature lower than the arc discharge, cooling during the synthesis is easily performed. In addition, since the discharge is performed with a discharge current lower than the arc discharge, a smaller device configuration can be achieved as compared with the device configuration used for arc discharge. In addition, since carbon nanotubes are synthesized by electric discharge, carbon nanotubes having fewer defects and less contamination of impurities than the CVD method are synthesized.
[0023]
Further, by using the catalyst metal and the catalyst auxiliary metal, the synthesis rate is increased. Further, since the base is formed such that the particle diameters of the catalyst metal and the catalyst auxiliary metal are fine, single-walled and double-walled carbon nanotubes are easily synthesized. For example, when the catalyst metal is directly applied to a silicon plate or the like and a catalyst metal having a particle size larger than the above-mentioned catalyst metal is used, a multi-walled carbon nanotube according to the particle size is synthesized. That is, the layer structure of the carbon nanotube synthesized by selecting the particle size of the catalyst metal is selected.
[0024]
Thus, the production method of synthesizing carbon nanotubes using a catalyst while using an abnormal glow discharge can synthesize single-wall and double-wall carbon nanotubes in a large amount faster than the arc discharge method, and has more defects than the CVD method. A small number of carbon nanotubes can be synthesized. That is, high quality carbon nanotubes can be manufactured at low cost.
[0025]
【Example】
Next, the present invention will be described in more detail with reference to examples.
A mixed gas of hydrogen and methane was used as a reaction gas introduced into the reaction chamber 2 of the carbon nanotube manufacturing apparatus 1, and the supply amounts of the gas were 500 sccm for hydrogen and 10 sccm for methane. The pressure inside the reaction chamber 2 was set to 70 Torr. Further, the cathode part 4 includes a rod-shaped discharge part 4b made of Ta having a diameter of 12 mm and a length of 100 mm, and a terminal part 4a for connecting to a first power supply part 5a provided on the base end side of the discharge part 4b. It is configured.
[0026]
The substrate 8 fixed to the substrate stage 9 is prepared by immersing alumina powder in a solution in which a catalyst metal and a catalyst auxiliary metal are dissolved for 30 minutes, and further dispersing the solution by ultrasonication for 3 hours. It is obtained by applying to a plate material and drying in air at 120 ° C. for 1 hour. As the alumina powder, 1.0 g of high-purity γ-alumina powder having a sodium content of 0.01% or less and a purity of 99.95% or more is used. The solution in which the catalyst metal and the catalyst auxiliary metal are dissolved is prepared by dissolving 0.2 g of iron nitrate and 0.01 g of molybdenum acetylacetonate in 35 ml of methanol. By forming the substrate 8 in this manner, iron and molybdenum are carried on the alumina powder in a highly dispersed manner.
[0027]
The cathode section 4 and the substrate 8 are set in the reaction chamber 2 and the inside of the reaction chamber 2 is set to the atmosphere of the reaction gas, and then the anode section 3 and the cathode section are activated by the first power supply section 5a and the second power supply section 5b. 4 and between the anode part 3 and the substrate base 9 to control the discharge voltage so that the discharge current was constant at 2.5 A. For example, at the start of voltage application, the current value is set to 300 mA, and when a voltage is applied at 1000 V, thermoelectrons are emitted from the cathode section 4, and discharge is started between the cathode section 4 and the anode section 3. At this time, no thermoelectrons are emitted from the substrate table 9. When the temperature of the cathode portion 4 rises to 1800 ° C. or higher due to the discharge, the discharge current easily flows due to an increase in thermionic emission characteristics. At this time, since the constant current control is performed, the discharge voltage is reduced. When the temperature of the cathode portion 4 rises to 2000 ° C. due to the discharge, the discharge voltage drops to 400 V, and a normal glow discharge state occurs.
[0028]
When the discharge current is further increased, the discharge voltage increases as the density of the discharge current increases, the state changes from the normal glow discharge state to the abnormal glow discharge state, the temperature of the cathode part 4 is set to 2300 ° C., and the discharge current is reduced to 2.5 A. , And the discharge voltage was 700 V. At this time, the reaction gas temperature was set to 3000 ° C., and the entire surface of the substrate 8 was brought into contact with the plasma generated by the discharge, and the discharge was performed for 10 minutes in this state. By synthesizing the carbon nanotubes in this manner, single-wall and double-wall carbon nanotubes were synthesized on almost the entire surface of the substrate 8.
[0029]
3 and 4 show transmission electron microscope (TEM) images of the synthesized carbon nanotubes. In the figure, the scale shown at the lower right is 5 nm, and the carbon nanotubes A and B are projected in a substantially bar shape near the center of the figure. The carbon nanotube A shown in FIG. 3 is a single-walled carbon nanotube having a diameter of about 1 nm, and the carbon nanotube B shown in FIG. 4 is a double-walled carbon nanotube. No multi-walled carbon nanotube was observed.
[0030]
In addition, the diameters of the inner layer and the outer layer of the double-walled carbon nanotube were measured using RAMAN spectroscopy. RAMAN spectroscopy is a method of irradiating a sample of carbon nanotubes with excitation light and measuring the shift of RAMAN scattering due to the excitation light. It is known that a peak appears in RAMAN intensity in proportion to the reciprocal of the diameter of carbon nanotubes. I have. FIG. 5 shows the measurement results of the resonance Raman scattering of the synthesized double-walled carbon nanotube. In the figure, the horizontal axis indicates the RAMAN shift, and the vertical axis indicates the RAMAN intensity. It can be seen that the peak of the RAMAN intensity is observed at four points of the RAMAN shift. The diameters of the carbon nanotubes indicated by the respective peaks are 0.699 nm, 1.115 nm, 1.236 nm, and 1.527 nm. Further, it can be seen that the combination of the diameters of the inner and outer layers of the main double-walled carbon nanotube is a combination of an inner diameter of 0.699 nm and an outer diameter of 1.236 nm.
[0031]
In this embodiment, the shape of the alumina used for the substrate on which the carbon nanotubes grow is not limited to powder, but may be pellet or plate. Spray method may be used. The form of the metal contained in the alumina may be a form of a metal oxide or a metal hydroxide, as long as the form promotes the formation of carbon nanotubes.
[0032]
【The invention's effect】
As described above, according to the first aspect of the present invention, since plasma for synthesizing carbon nanotubes is generated by abnormal glow discharge, high-density plasma is generated over a wide area between the cathode electrode and the anode electrode. And a large amount of carbon nanotubes can be synthesized. In addition, since the carbon nanotubes are synthesized in the reaction gas at a lower temperature than the arc discharge, the cooling operation can be easily performed. In addition, the use of a catalyst can increase the synthesis rate of carbon nanotubes, and the selection of a catalyst facilitates the synthesis of single-wall and double-wall carbon nanotubes. In addition, since carbon nanotubes are synthesized by electric discharge, carbon nanotubes having fewer defects than the CVD method can be synthesized. Thereby, high-quality carbon nanotubes can be manufactured with high efficiency.
[0033]
According to the second aspect of the present invention, the discharge current of the discharge for generating plasma is a current value of 0.5 to 50 A and the applied voltage is a voltage value of 350 V or more. Obtainable.
[Brief description of the drawings]
FIG. 1 is a voltage-current relationship diagram illustrating an abnormal glow discharge region according to an embodiment of the present invention.
FIG. 2 is a schematic configuration diagram of a carbon nanotube manufacturing apparatus according to an embodiment of the present invention.
FIG. 3 is a TEM image of a carbon nanotube.
FIG. 4 is a TEM image of a carbon nanotube.
FIG. 5 is a measurement result of resonance Raman scattering of a double-walled carbon nanotube.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Carbon nanotube manufacturing apparatus 2 Reaction chamber 3 Anode part 4 Cathode part

Claims (2)

A method for producing carbon nanotubes, in which carbon nanotubes are synthesized by plasma generated between an anode unit and a cathode unit arranged in a reaction chamber where a reaction gas is introduced,
A method for producing carbon nanotubes, comprising generating plasma by abnormal glow discharge and synthesizing carbon nanotubes by using a catalyst. A method for producing carbon nanotubes, in which carbon nanotubes are synthesized by plasma generated between an anode unit and a cathode unit arranged in a reaction chamber where a reaction gas is introduced,
A method for producing carbon nanotubes, comprising generating plasma by discharging with a discharge current of 0.5 to 50 A and a voltage value of 350 V or more, and synthesizing carbon nanotubes by using a catalyst. Method.

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