JP3782041B2 - Method for producing carbon nanotube - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for producing carbon nanotubes, in which plasma is generated by abnormal glow discharge and carbon nanotubes are synthesized by using a catalyst.
[0002]
[Prior art]
In general, among the fullerenes that are carbon materials having a hollow structure, the carbon nanotubes that are tubular fullerenes form a hollow thin tube in which hexagonal carbon atoms are connected in a spiral, with a diameter of 1 to several tens of nm. It is a carbon fiber having a length of several μm. Carbon nanotubes have unique physical properties in terms of electrochemical properties, mechanical properties, gas storage properties, and optical properties, and are expected to be widely used in the future.
[0003]
Carbon nanotubes include single-walled carbon nanotubes and multi-walled carbon nanotubes depending on the structure of the carbon layer. Single-walled carbon nanotubes are excellent in various physical properties as described above, but they are difficult to durability. Multi-walled carbon nanotubes are excellent in durability. However, various physical properties are not so high. The double-walled carbon nanotube in which only two carbon layers are overlapped exhibits physical properties almost the same as those of the single-walled carbon nanotube, and is more durable than the single-walled carbon nanotube.
[0004]
As a carbon nanotube synthesis method, an arc discharge method, a laser evaporation method, and a chemical vapor deposition method (CVD method) are known. The arc discharge method is a synthesis method in which high-quality carbon nanotubes with few defects can be obtained, but a method for mass synthesis is not established. The laser evaporation method is a synthesis method in which single-walled carbon nanotubes with relatively high purity can be obtained, but mass synthesis is difficult. The CVD method is a synthesis method suitable for mass synthesis, although the obtained carbon nanotubes have many defects. Thus, it is said that each synthesis method has advantages and disadvantages.
[0005]
A conventionally known method for synthesizing carbon nanotubes by an arc discharge method is that a cathode electrode and an anode electrode are formed by performing 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. This is a method in which carbon nanotubes are synthesized in a space where plasma is generated between the two. Moreover, the carbon nanotube synthesis method by the CVD method is a method in which carbon nanotubes are synthesized by bringing a carbon compound serving as a carbon source into contact with catalytic metal fine particles at 500 to 1000 ° C.
[0006]
[Problems to be solved by the invention]
However, the arc discharge method has a problem that the amount of carbon nanotubes to be synthesized is small because the plasma space generated by the discharge is narrow. Further, since the reaction gas is in a high temperature state, there is a problem that it is difficult to perform a cooling operation during the synthesis. Further, the CVD method has a problem that carbon nanotubes with many defects are synthesized, so that the characteristics of the carbon nanotubes cannot be exhibited sufficiently. For example, a reinforcing material for a high-performance material, a hydrogen storage material for a fuel cell, a highly electrical conductive material, and the like can fully exhibit their characteristics by using carbon nanotubes having no defects. In addition, a production method capable of easily synthesizing double-walled carbon nanotubes having excellent characteristics and high durability has been demanded.
[0007]
The present invention has been made under such a background, and it is possible to improve the production efficiency by synthesizing a large amount of carbon nanotubes at a lower temperature than in the arc discharge method. And it aims at providing the manufacturing method which can synthesize | combine a double-walled carbon nanotube easily.
[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 carbon nanotube manufacturing method for synthesizing carbon nanotubes by plasma generated between an anode portion and a cathode portion arranged in a reaction chamber into which a reaction gas is introduced, and an abnormal glow discharge The carbon nanotubes are synthesized by generating plasma and using a catalyst.
[0009]
According to the carbon nanotube manufacturing method of the present invention, plasma is generated by an abnormal glow discharge between the anode part and the cathode part, so that plasma is generated in the space between the anode part and the cathode part wider than the arc discharge. Is generated. In other words, since carbon nanotubes are synthesized in the plasma space, the wide area in which carbon nanotubes are synthesized means that carbon nanotubes can be synthesized in large quantities. Further, since the carbon nanotubes are synthesized in a reaction gas having a temperature lower than that of arc discharge, the cooling operation during the synthesis is easily performed.
[0010]
In addition, the synthesis rate of carbon nanotubes can be increased by using a catalyst, and the synthesis of single-walled and double-walled carbon nanotubes can be facilitated by selecting the catalyst. In addition, carbon nanotubes with fewer defects than those synthesized by CVD can be synthesized by synthesizing using discharge. Thereby, a carbon nanotube with few defects can be manufactured in large quantities in the low temperature reaction gas.
[0011]
The invention according to claim 2 is a carbon nanotube manufacturing method in which carbon nanotubes are synthesized by plasma generated between an anode portion and a cathode portion arranged in a reaction chamber into which a reaction gas is introduced, and a discharge current is generated. Plasma is generated by discharge with a current value of 0.5 to 50 A and an applied voltage of 350 V or more, and carbon nanotubes are synthesized by using a catalyst.
[0012]
According to the method of manufacturing a carbon nanotube according to the present invention, the discharge current for generating the 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. Moreover, the synthesis rate of carbon nanotubes can be increased by using a catalyst, and the synthesis of single-walled and double-walled carbon nanotubes can be facilitated by selecting the catalyst. Further, since the synthesis method is based on discharge, it is possible to synthesize carbon nanotubes with fewer defects than the CVD method. Thereby, carbon nanotubes with few defects can be produced in large quantities.
[0013]
DETAILED DESCRIPTION OF THE INVENTION
Embodiments of the present invention will be described below with reference to the drawings.
First, the abnormal glow discharge will be described with reference to FIG. The relationship between current and voltage when a DC voltage is applied between the anode and 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 different states at each current value and voltage value. The region in the discharge state is divided by broken lines D to H, and the region from the broken line F to the broken 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 as the discharge current increases, the current density increases and the discharge voltage 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 10,000 ° C.
[0015]
The region from the broken line E to the broken line F is a normal glow discharge region, which is a region where the discharge voltage is kept constant even when the discharge current is increased, and 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]
Further, the area after the broken line H is an arc discharge area, and a large amount of thermoelectrons are emitted when the temperature of the cathode portion increases by increasing the discharge current, thereby reducing the discharge voltage. The discharge state 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 in the cathode portion than in arc discharge.
[0017]
Next, a carbon nanotube production apparatus using the carbon nanotube production method of the present embodiment will be described with reference to FIG.
The carbon nanotube production apparatus 1 includes a reaction chamber 2 in which an anode unit 3, a cathode unit 4, and a substrate base 9 are installed, and a first power supply unit 5 a that applies a voltage between the anode unit 3 and the cathode unit 4. The second power supply unit 5b for applying a voltage between the anode unit 3 and the substrate base 9 is provided. The reaction chamber 2 is provided with a reaction gas introduction part 6 for introducing a reaction gas and an exhaust part 7 for exhausting the internal gas. The reaction chamber 2 is filled with a reaction gas atmosphere by introducing a reaction gas such as hydrogen or methane from the reaction gas introduction unit 6. Further, exhaust means such as a vacuum pump (not shown) is connected to the exhaust part 7 so that the inside of the reaction chamber 2 can be in a reduced pressure atmosphere.
[0018]
The anode part 3 is configured in a flat plate shape, and is connected to the positive electrode side of the first power supply part 5a and the second power supply part 5b. The cathode part 4 includes a terminal part 4a connected to the negative electrode side of the first power supply part 5a, and a rod-shaped discharge part 4b adjacent to the terminal part 4a. They are arranged so as to be substantially perpendicular to the electrode surface. Moreover, the discharge part 4b is formed with W (tungsten), Ta (tantalum), Mo (molybdenum), etc. which are high melting point metals. The substrate base 9 is configured in a flat plate shape, and is connected to the negative electrode side of the second power supply unit 5 b and is installed at a position close to the cathode unit 4. A substrate 8 coated with a substrate on which carbon nanotubes grow is fixed to the substrate table 9. Further, the substrate table 9 only needs to be installed at a position in contact with the plasma space generated between the anode unit 3 and the cathode unit 4, and is preferably installed in a space where the plasma has a high concentration.
[0019]
The substrate 8 is formed by applying a powdery substrate containing a catalyst metal and a catalyst auxiliary metal to plate-like silicon or the like. The substrate is obtained by immersing powdery high-purity alumina having impurities 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. It is formed by adding a catalyst metal and a catalyst auxiliary metal to high-purity alumina. By forming the substrate in this manner, the catalyst metal and catalyst auxiliary metal having a fine particle size can be 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. In addition, 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 set to 0.01 to 0.05%. ing. Moreover, the temperature at the time of immersion shall be room temperature-80 degreeC, Preferably it is 50-60 degreeC. Further, the formed substrate contains 1 to 20%, preferably 5 to 10% of catalyst metal, and 0.1 to 1.5%, preferably 0.3 to 0.8% of catalyst auxiliary metal. Contains. The powdery substrate thus formed is applied to Si or SiO ₂ formed into a plate shape at 100 to 250 cm ² / g, preferably 200 to 250 cm ² / g, and dried to form a substrate 8. Has been.
[0021]
A method for producing carbon nanotubes using the carbon nanotube production apparatus 1 as described above 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, more preferably 1 to 10 A, which is an abnormal glow discharge region. . When the discharge is started under such conditions, heating of the discharge part 4b is started first, and thermoelectrons are emitted from almost the entire surface of the discharge part 4b as the temperature of the cathode part 4 rises. A stable plasma is formed by emitting thermoelectrons under abnormal glow discharge conditions, and carbon nanotubes are synthesized on the substrate 8 fixed to the substrate table 9 by using this plasma.
[0022]
In addition, since high-density plasma is generated in a region between almost the entire surface of the cathode portion 4 and almost 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. Moreover, since carbon nanotubes can be synthesized with a reaction gas at a temperature lower than that of arc discharge, cooling during synthesis is easily performed. In addition, since the discharge is performed with a discharge current lower than that of the arc discharge, the apparatus configuration can be made smaller than the apparatus configuration used for the arc discharge. In addition, since carbon nanotubes are synthesized by electric discharge, carbon nanotubes with fewer defects and fewer impurities are synthesized than with the CVD method.
[0023]
In addition, the synthesis rate is increased by using a catalyst metal and a catalyst auxiliary metal. Furthermore, since the substrate is formed so 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 a catalyst metal is applied directly to a silicon plate or the like and a catalyst metal having a particle size larger than that of the catalyst metal is used, multi-walled carbon nanotubes corresponding to the particle size are synthesized. That is, the layer structure of the carbon nanotube synthesized by selecting the particle size of the catalyst metal is selected.
[0024]
As a result, the production method that uses an abnormal glow discharge and synthesizes carbon nanotubes using a catalyst can synthesize single-walled and double-walled carbon nanotubes in large quantities faster than the arc discharge method. Fewer carbon nanotubes can be synthesized. That is, it is possible to produce high-quality carbon nanotubes at a 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 production apparatus 1, and the gas supply amount was 500 sccm for hydrogen and 10 sccm for methane. The pressure inside the reaction chamber 2 was 70 Torr. The cathode part 4 includes a rod-like discharge part 4b made of Ta having a diameter of 12 mm and a length of 100 mm, and a terminal part 4a for connection to the first power supply part 5a provided on the base end side of the discharge part 4b. Configured.
[0026]
Further, the substrate 8 fixed to the substrate base 9 is prepared by immersing alumina powder in a solution in which the catalyst metal and the catalyst auxiliary metal are dissolved for 30 minutes and dispersing the solution by ultrasonic treatment 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 oxide acetylacetonate in 35 ml of methanol. Thus, by forming the substrate 8, iron and molybdenum are supported on the alumina powder in a highly dispersed manner.
[0027]
After the cathode part 4 and the substrate 8 are installed in the reaction chamber 2 and the inside of the reaction chamber 2 is made the atmosphere of the reaction gas, the anode part 3 and the cathode part are formed by the first power supply part 5a and the second power supply part 5b. 4 and between the anode part 3 and the substrate base 9, a DC voltage was applied to control the discharge voltage so that the discharge current was constant at 2.5 A. For example, by setting the current value to 300 mA at the start of voltage application and applying a voltage of 1000 V, thermoelectrons are emitted from the cathode part 4 and discharge is started between the cathode part 4 and the anode part 3. At this time, thermoelectrons are not emitted from the substrate base 9. When the temperature of the cathode portion 4 rises to 1800 ° C. or higher due to the discharge, the thermoelectron emission characteristics are increased, so that a discharge current easily flows. At this time, since the constant current control is performed, the discharge voltage is lowered, and when the temperature of the cathode portion 4 rises to 2000 ° C. due to the discharge, the discharge voltage is lowered to 400 V and a normal glow discharge state is achieved.
[0028]
When the discharge current is further increased, the discharge voltage increases as the density of the discharge current increases, so that the normal glow discharge state is changed to the abnormal glow discharge state, the temperature of the cathode portion 4 is set to 2300 ° C., and the discharge current is 2.5 A. The discharge voltage was 700V. 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 in this state, discharge was performed for 10 minutes. By synthesizing the carbon nanotubes in this way, single-walled and double-walled carbon nanotubes were synthesized on almost the entire surface of the substrate 8.
[0029]
FIGS. 3 and 4 show images of the synthesized carbon nanotubes by a transmission electron microscope (TEM). In the figure, the scale shown in the lower right is 5 nm, and the carbon nanotubes A and B are projected in a substantially rod shape near the center of the figure. A carbon nanotube A shown in FIG. 3 is a single-walled carbon nanotube having a diameter of about 1 nm, and a carbon nanotube B shown in FIG. 4 is a double-walled carbon nanotube. Multi-walled carbon nanotubes were not observed.
[0030]
Moreover, the diameter of the inner layer and outer layer of a double-walled carbon nanotube was measured using RAMAN spectroscopy. RAMAN spectroscopic analysis is a method of irradiating a sample of carbon nanotubes with excitation light and measuring the shift of RAMAN scattering, and it is known that a peak appears in the RAMAN intensity in proportion to the inverse of the diameter of the carbon nanotubes. Yes. FIG. 5 shows the measurement result of resonance Raman scattering of the synthesized double-walled carbon nanotube. In the figure, the RAMAN shift is shown on the horizontal axis and the RAMAN intensity is shown on the vertical axis, and it can be seen that the peaks of the RAMAN intensity are observed at the 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. Moreover, it turns out that the combination of the diameter of the main double-walled carbon nanotube inner layer and outer layer 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 may be a pellet or a plate in addition to the powder, and the contact method between the alumina and the metal solution may be other than the dipping method. Alternatively, the spray method may be used. Moreover, the form of the metal contained in alumina may be a form of metal oxide or metal hydroxide as long as it promotes the generation of carbon nanotubes.
[0032]
【The invention's effect】
As described above, according to the first aspect of the present invention, the plasma for synthesizing the carbon nanotubes is generated by abnormal glow discharge, so that high-density plasma is generated over a wide region between the cathode electrode and the anode electrode. And carbon nanotubes can be synthesized in large quantities. In addition, since the carbon nanotubes are synthesized in the reaction gas at a temperature lower than that of the arc discharge, the cooling operation can be easily performed. Moreover, the synthesis rate of carbon nanotubes can be increased by using a catalyst, and single-walled and double-walled carbon nanotubes can be easily synthesized by selecting a catalyst. In addition, since carbon nanotubes are synthesized by discharge, carbon nanotubes with fewer defects than CVD methods can be synthesized. Thereby, a high quality carbon nanotube can be manufactured with high efficiency.
[0033]
According to the invention of claim 2, since 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, the same effect as in claim 1 is obtained. Obtainable.
[Brief description of the drawings]
FIG. 1 is a voltage-current relationship diagram illustrating an abnormal glow discharge region in an embodiment of the present invention.
FIG. 2 is a schematic configuration diagram of a carbon nanotube production apparatus according to an embodiment of the present invention.
FIG. 3 is a TEM image of carbon nanotubes.
FIG. 4 is a TEM image of carbon nanotubes.
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, comprising synthesizing carbon nanotubes by plasma generated between an anode part and a cathode part arranged in a reaction chamber into which a reaction gas is introduced,
A method for producing carbon nanotubes, characterized in that carbon nanotubes are synthesized by generating plasma with abnormal glow discharge and using a catalyst. A method for producing carbon nanotubes, comprising synthesizing carbon nanotubes by plasma generated between an anode part and a cathode part arranged in a reaction chamber into which a reaction gas is introduced,
Production of carbon nanotubes characterized in that plasma is generated by discharge with a discharge current of 0.5 to 50 A and an applied voltage of 350 V or more, and carbon nanotubes are synthesized by using a catalyst. Method.

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