JP3815421B2 - Method for producing carbon nanotube - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention is a carbon which can produce a product having a high synthesis ratio (purity) and yield of multi-walled or single-walled carbon nanotubes by an arc discharge method in an atmospheric pressure / atmosphere atmosphere, that is, in an open space, without using an airtight container. The present invention relates to a method for producing a nanotube.
[0002]
[Prior art]
A carbon nanotube (CNT) is obtained by performing an arc discharge between two carbon materials. A carbon nanotube (CNT) is a graphene sheet in which carbon atoms are regularly arranged in a hexagon and is rounded into a cylindrical shape. The single graphene sheet tube is a single-walled carbon nanotube (SWCNT), and its diameter is 1 to several nm. Further, multi-walled carbon nanotubes (MWCNTs) in which the graphene sheet cylinders are concentrically overlapped with each other have a diameter of several nanometers to several tens of nanometers. Single-walled carbon nanotubes are conventionally synthesized by using a carbon electrode containing a catalytic metal or embedding the catalytic metal in an anode electrode and performing arc discharge. The carbon material here is an amorphous or graphitic conductive material containing carbon as a main component (hereinafter the same).
[0003]
In any case, various techniques for synthesizing carbon nanotubes (CNT) by arc discharge between two carbon materials have been proposed. For example, a technique has been proposed in which carbon nanotubes are produced by filling a sealed container with helium or argon and performing a carbon direct current arc discharge with the pressure in the sealed container being 200 Torr or higher (see, for example, Patent Document 1).
[0004]
In addition, helium is filled in a sealed container, the inside of the sealed container is heated, the internal temperature is set to 1000 to 4000 ° C., and a DC arc discharge is performed between the discharge electrodes made of carbon rods while the temperature is controlled. Has proposed a technique for producing carbon nanotubes having a uniform length and diameter distribution (see, for example, Patent Document 2).
[0005]
In addition, arc discharge is performed between opposed electrodes arranged in a horizontal direction in a sealed container filled with an inert gas, and the electrodes are relatively or continuously or intermittently rotated or reciprocated. A technique for producing a nanotube has been proposed (see, for example, Patent Document 3).
[0006]
In addition, the surroundings of the opposed anode and cathode carbon electrodes placed in a sealed container are heated by a heater, and then DC arc discharge is performed between both electrodes, thereby increasing the purity and yield of the produced carbon nanotubes. The technique to make is proposed (for example, refer patent document 4).
[0007]
Also, a technology that enables efficient generation of carbon nanotubes of uniform size and quality by heating the tip of the anode consisting of carbon electrodes arranged in a sealed container with a heating means and then performing arc discharge. Has been proposed (see, for example, Patent Document 5).
[0008]
[Patent Document 1]
JP-A-6-280116
[Patent Document 2]
JP-A-6-157016
[Patent Document 3]
JP-A-7-216660
[Patent Document 4]
JP 2000-203820
[Patent Document 5]
JP 2000-344505 A
[0009]
[Problems to be solved by the invention]
By the way, carbon nanotubes are generated in a substance made of carbon atoms deposited on a carbon electrode on the cathode side of a portion where arc discharge is performed, or in a part of soot attached to the arc periphery. However, according to the conventional method for producing carbon nanotubes, it is inevitable that graphite other than carbon nanotubes, amorphous carbon, and the like are mixed in the product, and the ratio of carbon nanotubes itself is low.
[0010]
That is, in general arc discharge, the cathode spot is selectively generated at a location where the electron emission ability is high. However, if the cathode spot is generated for a while, the electron emission capability at that location is weakened, and the cathode spot moves to another location with higher electron emission capability. Thus, in general arc discharge, arc discharge is performed while the cathode spot moves violently and irregularly. Further, in some cases, the cathode spot is greatly deviated from the anode facing position, exceeding the load voltage capacity of the power source, and the arc may be extinguished. As described above, in arc discharge in which the cathode spot moves violently and irregularly, when one spot of the cathode is seen, the chemical factors such as the temperature and the carbon vapor density greatly vary with time. For this reason, the conditions are such that the carbon nanotubes are easily synthesized for a certain period, but the conditions are such that the carbon nanotubes are difficult to be synthesized in another period, or the carbon nanotubes are easily decomposed. It is synthesized at the entire cathode spot generation position. Here, the decomposition of carbon nanotubes means that the generation mechanism of carbon nanotubes itself is still unclear and cannot be determined. However, in a certain temperature range, carbon is in the form of graphite or amorphous carbon rather than carbon nanotube structure. When carbon nanotubes are more stable, the phenomenon that carbon nanotubes cause structural changes in graphite and amorphous carbon, and at a considerably high temperature, a group of carbon atoms that make up the generated carbon nanotubes is released, A phenomenon in which carbon nanotubes collapse. In addition, since the carbon nanotube production process itself is performed at a high temperature, it is considered that the cluster emission is also occurring in this production process. However, at the optimum temperature for the carbon nanotube production, the carbon nanotube production is performed. It is inferred that the rate exceeds the decay rate (cluster emission rate) and carbon nanotubes are synthesized.
[0011]
Therefore, conventionally, in order to increase the stability of the arc and the synthesis rate of the carbon nanotube, the arc discharge device is provided in the sealed container as described above, and the ambient gas type and pressure in the sealed container and the surroundings of the electrode in the sealed container The method of appropriately selecting and controlling the temperature of was taken.
[0012]
However, it is difficult to completely fix the cathode spot of the arc only by adjusting the atmospheric gas species and pressure in the sealed container and the temperature around the electrode in the sealed container, and it is still a mixture of many impurities and carbon nanotubes. It could only be recovered as cathode deposits or soot. Therefore, as a result, the yield of carbon nanotubes decreases, and complicated purification work must be performed to increase the purity of the carbon nanotubes, which increases the manufacturing cost of the carbon nanotubes. In addition, the apparatus becomes large and equipment costs increase, and it is difficult to synthesize a large amount of carbon nanotubes by arc discharge.
[0013]
The technical problem of the present invention is to enable the production of multi-layer or single-walled high-purity carbon nanotubes with high yield by an arc discharge method in an air atmosphere without using a sealed container or the like.
[0014]
[Means for Solving the Problems]
[0015]
The method for producing a carbon nanotube according to claim 1 of the present invention includes: When carbon nanotubes are synthesized by performing arc discharge between the anode electrode and the cathode electrode made of a carbon material, a hollow electrode is used as the anode electrode, and the atmosphere gas is directed from the hollow portion of the hollow electrode toward the cathode electrode. The arc generation path is constrained by blowing an inert gas with high ionization efficiency or a mixed gas containing this inert gas and forming an arc generation path along the flow of the sprayed inert gas or mixed gas. Do It is characterized by that.
[0017]
Claim 1 As in the invention, the anode electrode The hollow part of the hollow electrode If an arc discharge is performed while an inert gas such as argon gas or a mixed gas containing an inert gas is sprayed from the cathode to the cathode electrode, an ionization degree of the gas increases and an arc is generated in the gas ejection path. Easy-to-use conditions are formed. Further, it is considered that the anode electrode surface in contact with the gas containing the inert gas forms a stable anode point. For this reason, the arc generation path is restricted, and irregular movement of the cathode spot of the arc on the cathode electrode is prevented. As a result, carbon nanotubes can be preferentially synthesized at the position where the fixed cathode spot is generated (the center of the arc), and high purity is produced at the position where the fixed cathode spot is generated (the center of the arc). A composite of multi-walled carbon nanotubes can be produced.
[0018]
Claim 2 The carbon nanotube production method according to The inert gas or the mixed gas is Metal powder or metal compound powder used as catalyst Contains It is characterized by that.
[0020]
Claim 2 As in the invention, when an inert gas such as argon gas or a mixed gas containing an inert gas is sprayed from the anode electrode to the cathode electrode together with the metal powder or metal compound powder serving as a catalyst, the ionization degree of the gas is increased. Thus, a condition that an arc is likely to be generated in the gas ejection path is formed. In addition, the anode spot is stably formed on the surface of the anode electrode in contact with the inert gas or the mixed gas containing the inert gas. As a result, the arc generation path is constrained and irregular movement of the cathode spot of the arc on the cathode electrode is prevented. , The carbon nanotubes can be preferentially synthesized at the fixed cathode spot generation position (the center of the arc). Until here Claim 1 This is the same as the present invention. But, Claim 1 In the invention of the present invention, since it is an arc discharge only with an electrode, only multi-walled carbon nanotubes can be synthesized, Claim 2 In the invention, spraying from the anode electrode to the cathode electrode Said Inert gas or Said Gas mixture But, Metal powder or metal compound powder used as catalyst Contains For this reason, the catalyst is atomized by arc heat and becomes a nucleus, from which single-walled carbon nanotubes grow. In other words, it becomes possible to efficiently introduce a catalytic metal into the position where the fixed cathode spot is generated (the center of the arc), and a high-purity single-walled carbon nanotube composite is formed in the center of the arc or around the arc. Can be manufactured. In addition, it is desirable to make the metal powder particles as a catalyst as fine as possible.
[0022]
In order to cause arc discharge, it is necessary to ionize the space between the electrodes. There are various processes for ionization of atoms. In arc discharge, the ionization process by collision with electrons is dominant. In general, except for He and Ne having a small atomic number, inert gases such as Ar, Kr, and Xe have a high ionization efficiency due to collision with electrons and provide a space in which an arc is easily generated. Inert gases such as Ar, Kr, and Xe have higher ionization efficiency than oxygen and nitrogen. And big When arc discharge is performed while supplying an inert gas or a mixed gas containing an inert gas from the anode electrode to the cathode electrode in an air atmosphere, the arc is concentratedly generated along the gas flow path. be able to. That is, by using an inert gas or a mixed gas containing an inert gas supplied from the anode electrode toward the cathode electrode as the plasma gas, the arc can be concentrated and the cathode spot can be stabilized.
Further, in the atmosphere, carbon is oxidized and burned because oxygen is involved in the arc discharge part. At this time, the produced carbon nanotubes are also somewhat oxidized, but impurities such as amorphous carbon and polycrystalline graphite powder having a lower combustion temperature are preferentially oxidized and burned, resulting in the purity of the carbon nanotubes in the product. There is an effect to improve.
[0023]
Also, Claim 3 The carbon nanotube manufacturing method according to Hollow part Spray toward the cathode electrode Said Inert gas or Said Adjust the flow rate of the mixed gas to 1 mm of the cross-sectional area of the hole of the hollow electrode ² It is characterized by being 10 to 400 ml / min.
[0024]
Supply from the hole of the hollow electrode Said Inert gas or Said If the flow rate of the mixed gas is too small, it will not function sufficiently as a plasma gas. If the flow rate is too high, the concentration of the plasma gas will increase to the periphery of the electrode, causing arc discharge not only in the center but also in the periphery. It becomes an easy condition, and it becomes impossible to concentrate the arc. Therefore, Claim 3 Supplied from the hole of the hollow electrode as in the invention of Said Inert gas or Said Adjust the flow rate of the mixed gas to 1 mm of the cross-sectional area of the hole of the hollow electrode ² By setting it to 10 to 400 ml / min per unit, it is possible to create a condition in which only the central portion of the anode electrode is more likely to arc discharge than the peripheral portion while functioning as a plasma gas. As a result, the cathode spots can be concentrated, and high-purity carbon nanotubes can be produced with high yield.
[0025]
Also, Claim 4 The carbon nanotube production method according to Said Inert gas or Said As a mixed gas, argon or a mixed gas of argon and hydrogen gas is used.
[0026]
An inert gas such as Ar, Kr, or Xe having an atomic number equal to or higher than Ar has a high ionization efficiency due to collision with electrons, and provides a space where an arc is easily generated. In particular, since Ar is the cheapest and industrially easy to use gas, the production cost of carbon nanotubes can be reduced. As a mixed gas, H is added to Ar. ₂ By mixing several percent to several tens percent, the yield of carbon nanotubes can be increased without impairing the stability of the arc. This is H ₂ This is because the carbon sublimation on the anode electrode is prevented from growing as a cluster, and the carbon nanotubes are easily synthesized on the cathode electrode.
[0027]
Also, Claim 5 The carbon nanotube manufacturing method according to the present invention is characterized in that the cathode electrode is preheated to 500 to 2000 ° C. during arc discharge.
[0028]
When a general carbon electrode, that is, a carbon electrode having an electric resistivity (= specific resistance) of 500 to 2000 μΩ · cm is used, arc discharge after heating the cathode electrode to 500 to 2000 ° C. in advance, The temperature of the cathode spot can be made higher than when there is no preheating, and high purity carbon nanotubes can be synthesized. When the preheating temperature is 500 ° C. or less, the effect of preheating is not so great. When the preheating temperature exceeds 2000 ° C., the cathode carbon evaporates violently, and the yield of carbon nanotubes also decreases.
[0029]
Also, Claim 6 The carbon nanotube manufacturing method according to the present invention is characterized in that a carbon material having an electrical resistivity of 4000 μΩ · cm or more or a thermal conductivity of 40 W / m · K or less is used as the cathode electrode.
[0030]
In order to synthesize carbon nanotubes with high purity and yield, it is advantageous to raise the temperature of the arc cathode spot of the cathode material to some extent. The electric resistivity (= specific resistance) of the carbon electrode used as a normal electrode is in the range of about 500 to 2000 μΩ · cm as described above, but a carbon material having an electric resistivity of 4000 μΩ · cm or more is used as the cathode. When used as a material, a high current density is obtained in the vicinity of the cathode spot of the cathode material during arc discharge. Therefore, the vicinity of the cathode spot becomes a high temperature due to electric resistance heat generation. Therefore, the same effect as that obtained by preheating the cathode can be obtained, and carbon nanotubes with high yield and purity can be generated.
Moreover, the thermal conductivity of the carbon electrode normally used as an electrode is in the range of 50 to 200 W / m · K, and the electrical resistivity and the thermal conductivity in the carbon material have a substantially negative correlation. In other words, a material having a high electrical resistivity has a low thermal conductivity and is difficult to transfer heat, so that the temperature near the cathode spot becomes higher. The thermal conductivity of a carbon material having an electrical resistivity of 4000 μΩ · cm or more corresponds to approximately 40 W / m · K or less.
[0031]
DETAILED DESCRIPTION OF THE INVENTION
Embodiment 1. FIG.
Hereinafter, the manufacturing method of the carbon nanotube which concerns on the 1st Embodiment of this invention is demonstrated.
[0032]
FIG. 9 is a diagram schematically showing the state of arc discharge (general discharge) between carbon material electrodes in an argon gas atmosphere under atmospheric pressure. A rod-like carbon material is used for the anode 1 and a plate-like carbon is used for the cathode 2. Material is used. As shown in FIG. 9, in an argon gas atmosphere at atmospheric pressure, the position where the arc is generated moves greatly, and the position of the cathode spot also moves violently and irregularly on the cathode plate (flat carbon material 2) (in FIG. 9). Two arcs 3a and 3b having different times are shown in an overlapping manner). Reference numeral 4 denotes a cathode jet, which is a portion where carbon of the cathode is evaporated and some carbon atoms are ionized. Such intense and irregular movement of the arc is particularly remarkable in an argon gas atmosphere at atmospheric pressure, but the same movement is observed even in a helium gas or hydrogen gas atmosphere under a low pressure.
[0033]
FIG. 10 is a scanning electron microscope (SEM) photograph showing the result of observing the cathode spot when the arc is generated for a short time by the general discharge of FIG. 9, and (a) is the central part of the cathode spot and its periphery. (B) is an enlarged SEM photograph of the central part of the cathode spot, and (c) is an enlarged SEM photograph of the peripheral part of the cathode spot. As can be seen from these SEM photographs, the center of the cathode spot is formed by dense carbon nanotubes, whereas in the periphery of the cathode spot, a lump of amorphous carbon (amorphous carbon) is deposited. It is only doing. That is, it can be seen that the conditions for synthesizing the carbon nanotubes are prepared at the cathode spot of the arc, whereas the conditions for the carbon nanotubes not being synthesized in the peripheral portion thereof. From these results, in a general arc discharge mode in which the cathode spot moves violently and irregularly, the conditions for synthesizing carbon nanotubes on the cathode electrode and the conditions for not synthesizing carbon nanotubes are alternately repeated, so that amorphous It is considered that only the cathode deposit containing a large amount of impurities such as carbon can be recovered.
[0034]
Therefore, as shown in FIG. 1, a hollow electrode 11 having a hole 11a in the axial center is used as an anode made of a carbon material, and an arc 3 is formed from the hole 11a inside the hollow electrode 11 in an open space (under atmospheric pressure and in an atmospheric atmosphere). When a small amount of argon gas was fed to the arc, it was found that the arc 3 was generated along the gas flow path, and the cathode spot was always generated at a position opposite to the gas outlet. This is considered to be because the arc is generated along the argon gas flow path because the degree of ionization of the argon gas is increased and the conductivity is larger than that of the peripheral portion at a high temperature due to arc discharge. Further, it is considered that the inner surface of the hollow electrode is in contact with the inert gas, so that the anode point is easily formed stably. Further, as described above, an inert gas such as argon has a high ionization efficiency due to collision with electrons, and provides a space where an arc is easily generated. Therefore, if the arc 3 is generated after the supply of argon gas from the hole 11a in the hollow electrode 11 to the cathode 2 is started, the arc generation path can be constrained from the beginning of the arc generation, and the cathode can be restrained. Irregular movement of the cathode spot of the arc on 2 can be prevented. As a result, carbon nanotubes can be preferentially synthesized at the cathode spot generation position (arc center) fixed from the beginning of the arc, and at this fixed cathode spot generation position (arc center). A composite of multi-walled carbon nanotubes with high purity can be produced.
When the cathode deposit obtained by the static arc discharge with the hollow electrode 11 was observed with a scanning electron microscope (SEM), high-purity carbon nanotubes were synthesized even in a long-time arc at the cathode spot, which is the center of the cathode deposit. Turned out to be. In the static arc discharge by the hollow electrode 11, the cathode jet as described above with reference to FIG. 9 is not observed, and it is considered that the carbon vapor generated from the cathode 2 is ejected at a position overlapping the arc column. It was inferred that the carbon nanotube synthesis efficiency was also improved by increasing the concentration of carbon atoms at.
[0035]
The hollow anode electrode is not limited to a carbon material, and a non-consumable electrode such as a water-cooled copper electrode may be used.
[0036]
The inert gas or the mixed gas containing the inert gas flowing from the anode to the cathode uses, for example, a rod-shaped anode 111 as shown in FIG. A gas may flow toward the cathode electrode along the side surface. Even in this case, if the gas flow is sufficiently laminar, an arc is generated along the gas flow and the cathode spot is fixed. The same applies to other embodiments described later.
[0037]
In addition, even if the gas delivered from the hole 11a inside the hollow electrode 11 was pure argon or argon gas mixed with about 20% hydrogen gas or helium gas, there was no significant change in the arc form. In particular, when hydrogen gas was mixed with several percent to several tens of percent in argon, the yield of carbon nanotubes could be increased without impairing arc stability. This is considered to be because hydrogen gas has an effect of preventing the carbon sublimated on the anode electrode from growing as a cluster, and the carbon nanotubes are easily synthesized on the cathode electrode. The appropriate gas flow rate is affected by the cross-sectional area of the hole 11a of the hollow electrode 11, and the cross-sectional area of the hole 11a is 1 mm. ² 10 to 400 ml / min per unit.
[0038]
FIG. 5 is a graph of experimental results showing the relationship between the cross-sectional area of the hollow electrode hole, the gas flow rate flowing inside, and the arc generation mode. As is apparent from FIG. 5, the flow rate of pure argon supplied from the hole 11a of the hollow electrode 11 or argon gas mixed with about 20% hydrogen gas or helium gas has a cross-sectional area of 1 mm. ² If it is less than 10 ml / min, the plasma gas will not function sufficiently and the cross-sectional area of the hole 11a is 1 mm. ² If it is more than 400 ml / min, the plasma gas concentration increases up to the periphery of the electrode, and arc discharge is likely to occur not only in the center but also in the periphery, making it impossible to concentrate the arc.
As in this embodiment, the gas flow rate supplied from the hole 11a of the hollow electrode 11 is set to 1 mm in cross-sectional area of the hole 11a of the hollow electrode 11. ² By setting it to 10 to 400 ml / min per unit, it is possible to create a condition in which only the central portion of the anode electrode is more likely to arc discharge than the peripheral portion while functioning as a plasma gas. As a result, the cathode spots can be concentrated, and high-purity carbon nanotubes can be produced with high yield.
[0039]
Example
A hollow electrode having an outer diameter of 36 mm and an inner diameter of 10 mm is used as the anode electrode, and 10% of argon gas containing 3% hydrogen is directed from the hole in the hollow electrode toward the cathode electrode in an open space (under atmospheric pressure and in atmospheric air). Arc discharge was performed for 1 minute at a current of 500 A and a voltage of 35 V (arc length of about 5 mm) while feeding a flow rate of liters / minute.
[0040]
FIG. 3 is a scanning electron microscope (SEM) photograph of the central part of the cathode deposit obtained by a 1-minute static arc discharge with this hollow electrode. As is apparent from this SEM photograph, it can be seen that high-purity multi-walled carbon nanotubes are synthesized at the center of the cathode deposit. This static arc discharge for 1 minute yielded several tens mg of highly pure multi-walled carbon nanotubes.
[0041]
Embodiment 2. FIG.
FIG. 4 is an explanatory diagram of a carbon nanotube manufacturing method according to the second embodiment of the present invention. In the figure, the same parts as those in FIG. 1 of the first embodiment are denoted by the same reference numerals.
[0042]
The carbon nanotube manufacturing method according to the present embodiment uses a hollow electrode 11 having a hole 11a at the axial center similar to the anode of the first embodiment described above as an anode made of a carbon material, and a metal powder or catalyst used as a catalyst. The inside of the catalyst mixing container 22 containing the metal compound powder 21 and the hole 11a of the hollow electrode 11 are connected, and the inside of the hollow electrode 11 is passed through the catalyst mixing container 22 in an open space (under atmospheric pressure and in the atmospheric atmosphere). A small amount of an inert gas such as argon gas or a mixed gas containing an inert gas is sprayed from the hole 11a toward the cathode 2, and the catalyst metal powder or the metal compound powder 21 is injected on the gas flow. It is characterized in that the arc 3 is generated in the meantime.
[0043]
Also in this embodiment, pure argon or argon gas mixed with about 20% hydrogen gas or helium gas is used as the gas fed from the hole 11a inside the hollow electrode 11. In particular, when hydrogen gas was mixed with several percent to several tens of percent in argon, the yield of carbon nanotubes could be increased without impairing arc stability. This is considered to be because, as described above, there is an effect of preventing carbon sublimated on the anode electrode from growing as a cluster in hydrogen gas, and it becomes a condition that carbon nanotubes are easily synthesized on the cathode electrode.
[0044]
Also in the present embodiment, the appropriate gas flow rate is affected by the cross-sectional area of the hole 11a of the hollow electrode 11 as in the first embodiment, and the cross-sectional area of the hole 11a is 1 mm. ² By setting the gas flow rate to 10 to 400 ml / min, it is possible to create a condition in which only the central portion of the anode electrode is more likely to arc discharge than the peripheral portion while functioning as a plasma gas. As a result, the cathode spots can be concentrated, and high-purity carbon nanotubes can be produced with high yield.
[0045]
The metal powder or metal compound powder to be used may be of any type as long as it has a catalytic function, but here, a simple substance such as Fe, Ni, Co, or FeS or a mixture thereof was used.
[0046]
Also in this embodiment, since the inert gas or the mixed gas containing the inert gas is sprayed from the hole 11a inside the hollow electrode 11 toward the cathode 2, the inert gas or the inert gas at a high temperature by arc discharge. The ionization degree of the mixed gas containing increases, and the conductivity becomes larger than that of the peripheral portion. In addition, since the anode point is stably formed on the inner surface of the hollow electrode, a constrained arc is formed in which an arc is generated along the gas flow path.
[0047]
Furthermore, in this embodiment, since the catalyst metal powder or the metal compound powder 21 is injected in a gas flow, the catalyst is atomized by arc heat and becomes a nucleus, from which single-walled carbon nanotubes grow. I will do it. In other words, a high-purity single-walled carbon nanotube composite can be produced at the position where the fixed cathode spot is generated (at the center of the arc) and its periphery.
[0048]
Embodiment 3. FIG.
FIG. 6 and FIG. 7 are scanning electron microscope (SEM) photographs of the cathode spot portion showing the production state of carbon nanotubes by the cathode preheating temperature for explaining the carbon nanotube manufacturing method according to the third embodiment of the present invention. is there.
[0049]
In the synthesis of carbon nanotubes by arc discharge, carbon nanotubes and carbon ions generated mainly from the anode carbon electrode diffuse to the cathode side and condense on the cathode electrode surface at a temperature lower than that of the anode, thereby carbon nanotubes (particularly multi-walled carbon nanotubes). Is considered to be synthesized. Therefore, it is said that the lower the temperature of the cathode, the faster the growth rate of carbon nanotubes, and the cathode material need not be a carbon material if it is a heat-resistant conductive material.
[0050]
However, even if only the carbon vapor and carbon ions of the anode are increased, only a low carbon nanotube synthesis ratio can be generated, and high purity carbon can maintain the temperature of the cathode where the carbon nanotubes are generated within an appropriate temperature range. As a result of experiments by the present inventors, it has become clear that it is important in producing nanotubes. That is, under the same electrode configuration and conditions as in the first or second embodiment described above, when arc discharge is performed after the cathode electrode is heated to 500 to 2000 ° C. in advance, the temperature of the cathode spot is preheated. It was confirmed that high-purity carbon nanotubes could be synthesized at a higher temperature than in the case where there was not.
[0051]
That is, without preheating (FIG. 6 (a)), no carbon nanotubes were formed, and when the preheating temperature was 500 ° C. (FIG. 6 (b)), only a small amount of carbon nanotubes were formed, and the preheating effect was obtained. At 2000 ° C. (FIG. 7 (a)), a large amount of carbon nanotubes are formed, and at 2500 ° C. (FIG. 7 (b)), the cathode carbon evaporates violently and the yield of carbon nanotubes decreases. I found out that
[0052]
Embodiment 4 FIG.
FIG. 8 is an explanatory view showing the result of synthesizing carbon nanotubes with various carbon materials for explaining the carbon nanotube production method according to the fourth embodiment of the present invention.
[0053]
In order to synthesize carbon nanotubes with high purity and yield, it is advantageous to raise the temperature of the arc cathode spot of the cathode material to some extent as described in the third embodiment. The electrical resistivity (= specific resistance) of the carbon electrode that is normally used as an electrode is in the range of about 500 to 2000 μΩ · cm, but when a carbon material having an electrical resistivity of 4000 μΩ · cm or more is used as the cathode material, In the vicinity of the cathode spot of the cathode material, a high current density is obtained during arc discharge, and therefore the temperature near the cathode spot becomes high due to electric resistance heat generation. Therefore, the same effect as that obtained by preheating the cathode can be obtained, and carbon nanotubes with high yield and purity can be generated.
[0054]
Moreover, the thermal conductivity of the carbon electrode normally used as an electrode is in the range of 50 to 200 W / m · K, and the electrical resistivity and the thermal conductivity in the carbon material have a substantially negative correlation. In other words, a material having a high electrical resistivity has a low thermal conductivity and is difficult to transfer heat, so that the temperature near the cathode spot becomes higher. The thermal conductivity of a carbon material having an electrical resistivity of 4000 μΩ · cm or more corresponds to approximately 40 W / m · K or less.
[0055]
In FIG. 8, various carbon materials A to G made of carbonaceous, graphite, carbonaceous + graphitic, etc. are used, and arc discharge is performed under the same electrode configuration and conditions as in the first or second embodiment. The yield and purity of the obtained carbon nanotubes are evaluated in three stages (◯: good, Δ: normal, ×: bad). As is clear from FIG. 8, the carbon materials B, C, and D having normal electrodes, that is, electrical resistivity (= specific resistance) of about 500 to 2000 μΩ · cm and thermal conductivity of 40 W / m · K or more are all used. The purity of the carbon nanotube was poor, the yield was also poor for the carbon material B, and the carbon materials C and D were normal. In addition, even if the electrical resistivity (= specific resistance) exceeds 2000 μΩ · cm, the carbon material E having a thermal conductivity of 40 W / m · K or more is not less than 4000 μΩ · cm, The purity was normal.
[0056]
In contrast, the carbon materials A, F, and G having an electrical resistivity (= specific resistance) of 4000 μΩ · cm or more and a thermal conductivity of 40 W / m · K or less have good results in both the yield and purity of the carbon nanotubes. Obtained.
[0057]
In each of the above-described embodiments, the arc discharge is performed in an open space (under atmospheric pressure / air atmosphere) as an example. However, it is also possible to perform the discharge in a container. That is, the internal space of the container is opened to the outside through the relief valve, the internal space of the container is set to a constant pressure higher than the atmospheric pressure by the relief valve, and the atmosphere is forcedly introduced into the internal space of the container to form an atmospheric atmosphere. Of hollow electrodes placed inside Hollow part Alternatively, an arc discharge may be performed while spraying an inert gas such as argon gas or a mixed gas containing an inert gas toward the cathode electrode. Conversely, in a container that is lower than atmospheric pressure In space Arc discharge may be performed.
[0058]
【The invention's effect】
As described above, according to the present invention, a hollow electrode is used as the anode electrode, Hollow part From the cathode electrode Generate an arc by spraying an inert gas or a mixed gas containing this inert gas, which has a higher ionization efficiency than the atmospheric gas, and forming an arc generation path along the flow of the sprayed inert gas or mixed gas Constrain the path As a result, the arc generation path could be constrained and the cathode spot could be prevented from moving. As a result, carbon nanotubes can be preferentially synthesized at the position where the fixed cathode spot is generated (the center of the arc), and high purity is produced at the position where the fixed cathode spot is generated (the center of the arc). A composite of multi-walled carbon nanotubes could be produced.
[0059]
Also, Since the inert gas or the mixed gas contains a metal powder or metal compound powder serving as a catalyst, The catalyst is atomized by arc heat and becomes a nucleus, from which single-walled carbon nanotubes can be grown. High purity at the location of the fixed cathode spot (arc center) and its periphery Of single-walled carbon nanotubes could be produced.
[0061]
In addition, the flow rate of the inert gas or the mixed gas containing the inert gas sprayed from the inside of the hollow electrode toward the cathode electrode is set so that the cross-sectional area of the hole of the hollow electrode is 1 mm. ² Since it was 10 to 400 ml / min per minute, it was possible to create a condition in which only the central portion of the anode electrode was more likely to arc discharge than the peripheral portion while functioning as a plasma gas. As a result, the cathode spots could be concentrated, and high purity carbon nanotubes could be produced with good yield.
[0062]
In addition, as an inert gas or a mixed gas containing an inert gas, argon or a mixed gas of argon and hydrogen gas, which has a high ionization efficiency due to collision with electrons, is inexpensive, and can be used industrially, is used. The manufacturing cost of the nanotube could be reduced. Furthermore, by mixing hydrogen gas with argon as a mixed gas, it is possible to prevent carbon sublimated on the anode electrode from growing as a cluster, and to increase the yield of carbon nanotubes without impairing arc stability. I was able to.
[0063]
In addition, since the cathode electrode was previously heated to 500 to 2000 ° C. during arc discharge, carbon nanotubes with higher purity could be synthesized.
[0064]
In addition, since a carbon material having an electrical resistivity of 4000 μΩ · cm or more or a thermal conductivity of 40 W / m · K or less is used as the cathode electrode, the temperature near the cathode spot can be raised during arc discharge. The same effect as that obtained when the cathode was preheated was obtained, and carbon nanotubes with high yield and purity could be produced.
[Brief description of the drawings]
FIG. 1 is a diagram schematically showing an arc discharge state between carbon material electrodes by a carbon nanotube manufacturing method according to a first embodiment of the present invention.
FIG. 2 is a view schematically showing a modified example of the anode electrode by the carbon nanotube manufacturing method according to the first embodiment.
FIG. 3 is a scanning electron microscope (SEM) photograph of the central part of the cathode deposit obtained by the carbon nanotube manufacturing method according to the first embodiment.
FIG. 4 is a diagram schematically showing the state of arc discharge between carbon material electrodes by the carbon nanotube production method according to the second embodiment of the present invention.
FIG. 5 is a graph of experimental results showing the relationship between the cross-sectional area of the hollow electrode hole, the gas flow rate flowing inside, and the arc generation mode.
FIG. 6 is a scanning electron microscope (SEM) photograph of a cathode spot portion showing a state of carbon nanotube generation at a cathode preheating temperature for explaining a carbon nanotube production method according to a third embodiment of the present invention.
FIG. 7 is a scanning electron microscope (SEM) photograph of a cathode spot portion showing a state of carbon nanotube production at a cathode preheating temperature for explaining a carbon nanotube production method according to a third embodiment of the present invention.
FIG. 8 is an explanatory view showing a result of carbon nanotube synthesis using various carbon materials for explaining a carbon nanotube production method according to a fourth embodiment of the present invention.
FIG. 9 is a diagram schematically showing the state of arc discharge (general discharge) between carbon material electrodes in an argon gas atmosphere under atmospheric pressure.
FIG. 10 is a scanning electron microscope (SEM) photograph showing a result of observing a cathode spot when an arc is generated by a general discharge for a short time.
[Explanation of symbols]
2 Cathode electrode
3 Arc
11 Hollow electrode (anode electrode)
11a hole (inside of hollow electrode)
21 Catalyst metal powder or metal compound powder

Claims (6)

When a carbon nanotube is synthesized by performing arc discharge between an anode electrode and a cathode electrode made of a carbon material, a hollow electrode is used for the anode electrode, and the hollow portion of the hollow electrode is directed toward the cathode electrode. Generate an arc by spraying an inert gas or a mixed gas containing the inert gas, which has a higher ionization efficiency than the atmospheric gas, and forming an arc generation path along the flow of the sprayed inert gas or mixed gas A method for producing a carbon nanotube, characterized by constraining a path . The method for producing carbon nanotubes according to claim 1 , wherein the inert gas or the mixed gas contains a metal powder or a metal compound powder serving as a catalyst . Claim that the hollow portion of the hollow electrode the flow rate of the inert gas or the mixed gas blown toward the cathode electrode, characterized in that a ² per 10 to 400 / min cross-sectional area of the hollow electrode apertures 1 mm 1 Or the manufacturing method of the carbon nanotube of Claim 2 . The method for producing carbon nanotubes according to any one of claims 1 to 3 , wherein argon or a mixed gas of argon and hydrogen gas is used as the inert gas or the mixed gas. The method for producing carbon nanotubes according to any one of claims 1 to 4 , wherein the cathode electrode is preheated to 500 to 2000 ° C during arc discharge. The carbon nanotube production according to any one of claims 1 to 5 , wherein a carbon material having an electrical resistivity of 4000 µΩ · cm or more or a thermal conductivity of 40 W / m · K or less is used as the cathode electrode. Method.

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