JP2014166936A - Method of producing carbon nanotube aggregate - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method of producing a carbon nanotube aggregate in a high yield which increases the yield, without saturation of the yield even when the input of the raw gas is increased, in chemical vapor deposition by bringing a raw material gas and a catalyst activating substance into contact with a catalyst in a furnace at high temperatures to grow a CNT aggregate.SOLUTION: A production method is based on bringing a raw material gas and a catalyst activating substance into contact with a catalyst on a substrate to grow nanotube and comprises supplying a reducing gas to bring into contact with the catalyst on the substrate, heating at least one of the catalyst and the reducing gas to reduce and/or atomize the catalyst, bringing a raw material gas which contains a 2C or higher linear saturated hydrocarbon compound but no oxygen and a catalyst activating substance containing oxygen into contact with the catalyst, heating at least one of the catalyst, raw material gas and catalyst activating substance to grown carbon nanotube and increasing the carbon concentration of the raw material gas to increase the yield of CNT to 4 mg/cmor higher.

Description

The present invention relates to a method for producing carbon nanotubes. In particular, the present invention relates to a method for producing a carbon nanotube capable of growing a high yield carbon nanotube aggregate.

Recently, the development of carbon nanotubes (hereinafter also referred to as CNT) to functional new materials such as electronic device materials, optical element materials, conductive materials, and biological materials is expected. Studies on mass productivity are being conducted energetically.

As one of CNT manufacturing methods, a chemical vapor deposition method (hereinafter also referred to as a CVD method) is known (see Patent Document 1 and the like). This method is characterized in that a raw material gas such as a carbon compound is brought into contact with catalyst fine particles in a high temperature atmosphere of about 500 ° C. to 1000 ° C., and the type and arrangement of the catalyst, the type of the raw material gas, and the reducing gas It is possible to produce CNTs in various modes such as carrier gas, synthesis furnace, and reaction conditions, and it is attracting attention as being suitable for mass production of CNTs. In addition, this CVD method can produce both single-walled carbon nanotubes (SWCNT) and multi-walled carbon nanotubes (MWCNT), and uses a substrate carrying a catalyst, so that a large number of them are oriented perpendicular to the substrate surface. CNTs can be produced. A CNT aggregate having a high specific surface area and orientation is very suitable as a substance / energy storage material in various applications such as supercapacitor electrodes and heat transfer / heat radiation materials having directivity.

Among CNTs, single-walled CNTs have electrical properties (very high current density), thermal properties (thermal conductivity comparable to diamond), optical properties (light emission in the optical communication band wavelength region), hydrogen storage capacity, and Excellent in various properties such as metal catalyst support ability, as well as both semiconductor and metal properties, attracts attention as materials for electronic devices, electrodes for power storage devices, MEMS members, and fillers for functional composite materials Has been.

Furthermore, less metal impurities, CNT aggregate having a specific surface area in the range of 800m ² / g~2600m ² / g is effective as a support and energy and material storage material of the catalyst, applications such as supercapacitors, actuators It is suitable for.

If such a high specific surface area oriented CNT aggregate is created, the application field of CNTs is expected to expand dramatically, but in order to promote practical application, the high specific surface area oriented It is important to improve the mass productivity of the CNT aggregate.

However, in the conventional chemical vapor deposition method, carbon-based impurities generated in the CNT synthesis process coat the catalyst fine particles, the catalyst is easily deactivated, and CNT cannot be efficiently grown. The activity of the catalyst was usually on the order of a few percent and the lifetime was about 1 minute. For this reason, in the conventional single-walled CNT growth process, synthesis is usually performed in a low carbon concentration atmosphere. Here, the low carbon concentration atmosphere refers to a growth atmosphere in which the ratio of the source gas to the gas including the source gas and the atmosphere gas is about 0.1 to 1%. When the carbon concentration was increased by the conventional synthesis method, the catalyst was deactivated more easily, and the growth efficiency of CNT was further reduced.

As a result, in the conventional synthesis method, since the supply of the raw material gas to the catalyst is small, not only the CNT growth rate is slow, but also a single-walled CNT aggregate having a height of about several tens of μm can be produced.

The present inventors have found that the catalyst efficiency is dramatically improved by the presence of a very small amount of a catalyst activator such as moisture in the reaction atmosphere, and the efficiency is high, high purity, high specific surface area, highly oriented single unit. It was reported in Non-Patent Document 1 that it is possible to produce a layer CNT.

In this method, the catalyst activator added in the synthesis atmosphere of CNT removes the carbon-based impurities covering the catalyst fine particles and cleans the background of the catalyst film. As a result, the activity of the catalyst is remarkably improved and the life is extended. . As a result of the addition of the catalyst activator, the activity of the catalyst is enhanced and the life is extended. As a result, the growth of single-walled CNTs, which has been completed in about 2 minutes at the last, continues for several tens of minutes. It was improved from a few percent to 84%. As a result, a single-walled CNT aggregate having a height increased several hundred times from the conventional height of 4 μm was obtained. This is because, in the presence of a catalyst activator, the catalytic activity is remarkably improved. Therefore, even in a high carbon concentration environment, the catalyst does not lose its activity, and CNT can be grown for a long time, and the growth rate is remarkably high. It is for improving. Here, the high carbon concentration environment refers to a growth atmosphere in which the ratio of the source gas to the gas containing the source gas, the atmosphere gas, and the catalyst activator is about 2 to 20%.

JP 2003-171108 A

Kenji Hata et al, Water-Assisted Highly Efficient Synthesis of Impurity-Free Single-Walled Carbon Nanotubes, SCIENCE, 2004.11.19, vol.306, p.1362-1364

The synthesis of CNTs containing a catalyst activator has brought about significant improvements in growth efficiency, carbon efficiency, and growth rate compared to the prior art. However, when a CNT aggregate is produced using this method, a technical problem peculiar to a catalyst activation material that is not found in the conventional synthesis method occurs. When manufacturing CNTs in large quantities, the growth rate and growth efficiency such as yield are important factors for producing inexpensive CNTs in large quantities, so the growth efficiency is improved in an environment containing a catalyst activator. Development of synthetic methods is important.

In general, in order to improve the growth rate and the growth efficiency such as the yield, the amount (carbon concentration) of the raw material gas introduced into the synthesis furnace may be increased. Here, the carbon concentration is

Carbon concentration = (number of carbon atoms in source gas) x source gas flow rate / total flow rate

It is defined as However, in the case of using a raw material gas usually used in the synthesis of conventional CNTs such as ethylene, acetylene, methane, etc., if the input amount of the raw material gas is increased, the yield of the CNT is saturated, and more raw material gas However, there was a problem that the yield of CNTs decreased.

The present invention provides a chemical vapor deposition method in which a raw material gas and a catalyst activator are brought into contact with a catalyst in a furnace at a high temperature to grow a CNT aggregate. Even if the input amount of the raw material gas is increased, the yield is increased. The present invention provides a method for producing a high-yield carbon nanotube aggregate that increases without saturation.

In view of such problems of the prior art, the inventors have conducted intensive research and as a result of chemical vapor deposition in which a raw material gas and a catalyst activation material are brought into contact with a catalyst to grow a CNT aggregate in a furnace at a high temperature. Unlike conventional ethylene and acetylene, the raw material gas has a carbon number of 2 or more which increases without saturation even if the input amount of the raw material gas is increased within a carbon concentration range of 10% or more. The existence of gas was found, and by using the raw material gas, it was possible to grow a high yield of carbon nanotube aggregates.

According to one embodiment of the present invention, in a manufacturing method in which a raw material gas and a catalyst activator are brought into contact with a catalyst on a substrate to grow carbon nanotubes, a reducing gas is supplied to contact the catalyst on the substrate. And at least one of the catalyst and the reducing gas is heated to reduce and / or make fine particles of the catalyst, containing carbon that is a chain saturated hydrocarbon compound having 2 or more carbon atoms, and oxygen A source gas containing no oxygen and a catalyst activator containing oxygen are brought into contact with the catalyst, and at least one of the catalyst, the source gas and the catalyst activator is heated to grow carbon nanotubes. And a carbon nanotube aggregate manufacturing method for increasing the carbon concentration of the source gas so that the yield of CNT is at least 4 mg / cm ² or more. Provided.

In the carbon nanotube aggregate manufacturing method, the carbon concentration of the source gas may be 10% or more.

In the carbon nanotube aggregate manufacturing method, a carbon source flow rate of the chain saturated hydrocarbon compound may be 0.003 mol / min or more.

In the method for producing a carbon nanotube aggregate, the source gas may contain butane or propane.

In the method for producing an aggregate of carbon nanotubes, the carbon nanotube may be a single layer, and the aggregate may be oriented.

According to the present invention, in a chemical vapor deposition method in which a raw material gas and a catalyst activator are brought into contact with a catalyst in a furnace at a high temperature to grow a CNT aggregate, the yield can be increased even if the input amount of the raw material gas is increased. It is possible to provide a method for producing a high-yield carbon nanotube aggregate that increases without saturation.

It is a schematic diagram of the manufacturing apparatus 100 of the CNT aggregate which concerns on one Embodiment of this invention. (A) which concerns on one Embodiment of this invention is a schematic diagram which shows the growth of CNT using the source gas which concerns on this invention, (b) is a schematic diagram which shows the growth of CNT using the conventional source gas is there. (A) is a figure which shows the yield of a CNT aggregate with respect to the carbon concentration of a comparative example, (b)-(e) is optimal by one ethylene concentration corresponding to (b)-(e) of (a). It is a three-dimensional plot of the yield with respect to the amount of moisture and the growth temperature. (A) is a figure which shows the yield (mg / cm < ² >) per unit area for 10 minutes with respect to the carbon concentration of the raw material gas which concerns on one Example of this invention, (b) is a raw material from the butane which concerns on a present Example The growth curve used for gas is shown, (c) shows the growth curve which used ethylene of the comparative example for raw material gas. (A) shows the carbon efficiency with respect to the carbon concentration and the ratio of hydrogen to carbon for each type of molecule in the method for producing a CNT aggregate according to one embodiment of the present invention, and (b) shows the carbon concentration and initial growth rate. (C) is a figure which shows the relationship between carbon concentration and growth time. Water / carbon ratio for generating heat per carbon atom in the manufacturing method of the CNT aggregate according to one embodiment of the present invention (H 2 _{O /} C-ratio); FIG.

As a result of diligent studies to explain the above-mentioned problems, raw material gases such as ethylene, acetylene, and methane, which are usually used for the synthesis of conventional CNTs, are highly active carbon sources, while the input amount of the raw material gas is reduced. When increased, the yield of CNT was saturated, and when more raw material gas was introduced, the yield of CNT was decreased. On the contrary, in chemical vapor deposition, a low-activity carbon source was used. The inventors of the present invention have found for the first time that a high-yield CNT aggregate can be produced without increasing saturation even if the input amount of the raw material gas is increased by using a high-concentration gas.

As the manufacturing apparatus used for the method of manufacturing a carbon nanotube aggregate (CNT aggregate) according to the present invention, a known apparatus can be used as long as the apparatus can grow the CNT aggregate by a chemical vapor deposition (CVD) method. it can. As an example, an apparatus described in Japanese Patent Application No. 2010-279725 previously filed by the present inventors may be used. FIG. 1 is a schematic view of a CNT aggregate manufacturing apparatus 100 according to an embodiment of the present invention described in Japanese Patent Application No. 2010-279725.

The manufacturing apparatus 100 includes, for example, a synthesis furnace 110 in which a base material 101 (for example, a silicon wafer) on which a catalyst layer 103 is formed in advance is placed on a base material holder 105. In the synthesis furnace 110, a first gas channel 145 and a second gas channel 147 are disposed, and the substrate 101 is disposed so as to intersect the surface of the catalyst layer 103 generally perpendicularly. A source gas cylinder 161, an atmosphere gas cylinder 163, and a reducing gas cylinder 165 are connected to the first gas flow path 145 through a first gas supply pipe 141. In addition, a catalyst activation material cylinder 167 is connected to the second gas flow path 147 through a second gas supply pipe 143. The first gas supply pipe 141 is provided with a first carbon concentration adjusting means 171 for adjusting the carbon concentration of the first gas supplied from the source gas cylinder 161, the atmospheric gas cylinder 163, and the reducing gas cylinder 165. The second gas supply pipe 143 is provided with a second carbon concentration adjusting means 173 for adjusting the carbon concentration of the second gas supplied from the catalyst activation material cylinder 167.

The synthesis furnace 110 is provided with a gas discharge pipe 150 that discharges the gas supplied from the first gas flow path 145 and the second gas flow path 147 to the outside. The synthesis furnace 110 is provided with heating means 130. The first carbon concentration adjusting unit 171, the second carbon concentration adjusting unit 173, and the heating unit 130 can be controlled by a control unit (not shown).

[Raw material gas]
The source gas used in the method for producing a CNT aggregate according to the present invention as a carbon supply source is preferably an alkane which is a chain saturated hydrocarbon compound having 2 or more carbon atoms, 10 or less carbon atoms, and more preferably 5 or less carbon atoms. Ethane, propane, butane, pentane, hexane, heptane, octane, nonane and decane. In particular, butane and propane can be preferably used. Reference is now made to FIG. FIG. 2A is a schematic diagram showing the growth of CNTs using the source gas according to the present invention, and FIG. 2B is a schematic diagram showing the growth of CNTs using the conventional source gas. The conventionally used unsaturated chain hydrocarbon stream 9 such as ethylene or acetylene has been considered to be capable of producing CNT aggregates in high yield due to its high reactivity. When feed gas stream 9 is fed to the synthesis furnace of the production equipment, there is an optimum carbon concentration relative to the yield for a particular growth process (ie flow rate, temperature, catalyst synthesis, catalyst amount, etc.). Is an important parameter. In general, it is well known that the yield of CNT aggregates increases, reaches a peak, and then decreases as the concentration of carbon supplied to the synthesis furnace increases. This is because when the carbon concentration is low, the amount of carbon supplied becomes rate-limiting, and the yield of CNT aggregates increases as the carbon concentration increases. At the peak, the carbon concentration is balanced with the catalytic ability to convert the raw material gas 9 into CNT1.

However, in the CNT growth process, as shown in FIG. 2B, although part of the source gas 9 a contributes to the growth of the CNT 1, part of the source gas 9 b is carbon of impurities attached to the catalyst 103. 3 and part of the raw material gas 9c is discharged outside the system without being consumed. Therefore, when the carbon concentration of the raw material gas 9 exceeds the optimum level, the impurity carbon 3 adhering to the catalyst 103 forms a film, and the deactivation of the catalyst 103 occurs rapidly, which further increases the carbon concentration. Along with this, the yield of CNT1 decreases. As a result, the catalytic ability to convert carbon into CNT aggregates becomes rate limiting, and the lifetime of the catalyst is shortened. Acetylene can produce CNT aggregates in high yield only when the carbon concentration is low. However, when the catalyst activator 7 is added at a level of 100 ppm or less, the optimum carbon concentration is greatly increased, which is shown by CVD (also referred to as super-growth method) added by the present inventors so far. As described above, the yield of the CNT aggregate is dramatically improved. This is due to the suppression of the deactivation process of the catalyst 103, and the catalyst 103 can maintain its activity under high carbon concentration conditions. When water is not added, the lifetime of the catalyst 103 is about 1 minute at a carbon concentration as low as about 1%. However, when water 7 is added, the lifetime of the catalyst 103 can exceed 30 minutes. Thus, although the yield increases 1000 times by the addition of water 7, the peak of the yield still exists, the catalyst 103 is deactivated, and the growth of CNT1 is attenuated. Therefore, in the production of the conventional CNT aggregate, a carbon source having high activity has to be used at a low concentration.

On the other hand, the present invention is characterized in that a carbon source having low activity is used as a source gas at a high concentration. This is a completely opposite approach to the approach that has been reported so far, and the present invention can achieve a high yield of CNT aggregates that has not existed before. The source gas applied to the present invention is a chain saturated hydrocarbon compound. More specifically, the raw material gas contains carbon, which is a chain saturated hydrocarbon compound having 2 or more carbon atoms, and does not contain oxygen. As such a source gas, for example, butane and propane can be suitably used, but are not limited thereto. In the method for producing a CNT aggregate according to the present invention, for example, by using butane as a raw material gas, an optimum growth yield of 2.5 times that when using conventional ethylene can be achieved.

As shown in FIG. 2A, even in the raw material gas 5 according to the present invention, a part of the raw material gas 5a contributes to the growth of the CNT 1, but a part of the raw material gas 5b contains impurities adhering to the catalyst 103. Carbon 3 and a part of the raw material gas 5c are discharged without being consumed. However, unlike the conventional source gas 9, the source gas 5 according to the present invention is a carbon source with low activity, so that even if it is supplied into the synthesis furnace at a high concentration, many source gases 5c grow CNT1. Does not contribute. Further, a part of the source gas 5a contributes to the growth of the CNT 1 and a part of the source gas 5b becomes the impurity carbon 3 attached to the catalyst 103. The impurity carbon 3 generated here is the conventional source gas 9. Remarkably less than in the case of.

Therefore, when the source gas according to the present invention is used, the yield of the CNT aggregate is linearly improved with respect to the carbon concentration of the source gas. This is in contrast to the case where a yield peak occurs when the above-described conventional unsaturated hydrocarbon compound is used as a raw material gas. It means that the raw material gas according to the present invention does not saturate at least in the experimental range of Examples described later with respect to the flow rate supplied to the synthesis furnace.

When the carbon concentration is increased, the supply amount of the source gas per channel can be increased per unit time. A carbon concentration of 10% or more, more preferably 15% or more, and even more preferably 20% or more is preferable because a CNT aggregate can be produced with a high yield. If the carbon concentration is lower than this, the yield of the CNT aggregate decreases. On the other hand, even if the carbon concentration exceeds 50%, the yield does not increase, carbon impurities increase, and the specific surface area decreases.

[Catalyst activation material]
Also in this invention, it is preferable to supply a catalyst activation material to a synthesis furnace in the manufacturing process of a CNT aggregate. By adding a catalyst activator, the life of the catalyst can be extended and the activity can be increased. As a result, improvement in the production efficiency and high purity of CNT can be promoted. The catalyst activator used here is a substance having an oxidizing power such as oxygen or sulfur, and a substance that does not cause great damage to the CNT at the growth temperature. In the present invention, oxygen-containing catalyst activator is preferable. For example, water, oxygen, ozone, acidic gas, and low-carbon oxygen-containing compounds such as nitrogen oxide, carbon monoxide, carbon dioxide, or ethanol / methanol -Alcohols such as isopropanol, ethers such as tetrahydrofuran, ketones such as acetone, aldehydes, acids, salts, amides, esters, and mixtures thereof are effective. Among these, water, oxygen, carbon dioxide, carbon monoxide, ethers, and alcohols are preferable, but water that can be easily obtained is particularly preferable. When a material containing carbon is used as the catalyst activation material, carbon in the catalyst activation material can be a raw material for CNT.

[Atmosphere gas]
The atmospheric gas (carrier gas) for chemical vapor deposition may be any gas that is inert at the growth temperature of CNT and does not react with the growing CNT. As the atmospheric gas used in the practice of the present invention, As long as there is a track record in the manufacture of CNTs, an appropriate one can be used. In general, an inert gas is preferable, and examples thereof include helium, argon, hydrogen, nitrogen, neon, krypton, carbon dioxide, chlorine, and a mixed gas thereof. Particularly, nitrogen, helium, argon, hydrogen, and these A mixed gas is preferred.

[Reducing gas]
The reducing gas used in the catalyst formation step is a gas having at least one of the effects of reducing the catalyst, promoting atomization in a state suitable for the growth of the CNT of the catalyst, and improving the activity of the catalyst. As the reducing gas used in the practice of the present invention, any suitable gas can be used as long as it has a proven reductivity in the production of conventional CNTs. For example, hydrogen, ammonia, water, and mixtures thereof Gas can be applied. In the present invention, hydrogen can be suitably used as the reducing gas.

[Base material]
The base material (substrate) is a member capable of supporting a catalyst for growing CNTs on the surface thereof, and any suitable material can be used as long as the shape can be maintained even at a high temperature of 400 ° C. or higher. . As a form of the substrate, a planar form such as a flat plate is preferable for producing a large amount of CNTs using the effects of the present invention. However, it may be a base material that is a powder or an aggregate of linear bodies and has a planar shape. The use of a planar substrate is preferable because the raw material gas and the catalyst activator can be easily supplied to the catalyst uniformly.

The materials that have been used in the manufacture of CNTs so far include iron, nickel, chromium, molybdenum, tungsten, titanium, aluminum, manganese, cobalt, copper, silver, gold, platinum, niobium, tantalum, lead, zinc, gallium, Metals such as indium, germanium, arsenic, phosphorus, antimony, and alloys and oxides containing these metals, or silicon, quartz, magnesia, spinel, calcia, dolomite, chromia, zirconia, titania, mulay, glass, mica, graphite Non-metals such as alumina, magnesium oxide, potassium titanate, zirconium oxide, zeolite, silica, titanium oxide, diamond, etc., and ceramics and mixtures thereof. The metal is preferably less expensive than silicon or ceramic, and in particular, iron-chromium (Fe-Cr) alloy, iron-nickel (Fe-Ni) alloy, and iron-chromium-nickel (Fe-Cr-). Ni) alloys and the like are suitable for the practice of the present invention.

Specific examples of the powder or linear body include plate-like alumina, quartz flakes, quartz fibers, ceramic fibers, and fibrous titanium oxide.

[catalyst]
In the practice of the present invention, any catalyst that is supported on a substrate and forms a catalyst layer can be used as long as it has a proven record in the production of CNTs so far. Nickel / cobalt / molybdenum and their chlorides and alloys may be combined or layered with aluminium / alumina / titania / titanium nitride / silicon oxide. In the present invention, an alumina / iron (Al ₂ O ₃ —Fe) catalyst can be suitably used. An alumina / iron catalyst is preferable for producing a CNT aggregate with a high yield because catalyst fine particles can be placed at high density.

The amount of catalyst present in the practice of the present invention may be within a range that has been proven in CNT production so far. For example, when an alumina / iron metal thin film is used, the thickness of the alumina layer is 5 nm or more and 1000 nm or less. Is preferably 10 nm to 500 nm, and particularly preferably 15 nm to 300 nm. Further, the thickness of the iron layer is preferably 0.2 nm or more and 10 nm or less, more preferably 0.5 nm or more and 5 nm, and particularly preferably 0.8 nm or more and 2 nm or less. In the method for producing a CNT aggregate according to the present invention, a high specific surface area CNT aggregate can be produced with high yield by controlling the thickness of the catalyst. Moreover, since the CNT aggregate | assembly of single-walled CNT can be manufactured, it is preferable.

[Manufacturing process]
Referring to FIG. 1, a base material 101 (for example, a silicon wafer) on which a catalyst layer 103 (for example, an alumina-iron thin film) is formed in advance is placed on a base material holder 105. The atmospheric gas (for example, helium) supplied from the 1 gas supply pipe 141 via the first gas flow path 145 is filled. The base material 101 is disposed so that the surface of the catalyst layer 103 and the first gas flow path 145 and the second gas flow path 147 are generally perpendicular to each other so that the source gas is efficiently supplied to the catalyst.

Next, while supplying the reducing gas (for example, hydrogen) from the first gas supply pipe 141 to the synthesis furnace 110 via the first gas flow path 145, the interior of the synthesis furnace 110 is heated to a predetermined temperature (for example, 750 ° C.), This state is maintained for a desired time. In the process for producing a CNT aggregate according to the present invention, a reducing gas is supplied to be brought into contact with the catalyst on the substrate and at least one of the catalyst and the reducing gas is heated to reduce the catalyst and / or fine particles. Turn into.

By this reducing gas, the catalyst layer 103 is reduced into fine particles having various sizes, and is adjusted to a state suitable as a catalyst for CNT. In the formation step, a second gas containing a catalyst activation material may be added from the second gas supply pipe 143 through the second gas flow path 147 as necessary. By this step, at least one of the effects of reducing the catalyst, promoting atomization in a state suitable for the growth of the catalyst CNT, and improving the activity of the catalyst appears. For example, when the catalyst is an alumina-iron thin film, the iron catalyst layer is reduced into fine particles, and a large number of nanometer-sized catalyst fine particles are formed on the alumina layer.

Next, the supply of the reducing gas (hydrogen) and the atmospheric gas from the first gas flow path 145 is stopped or reduced according to the desired (reaction conditions), and each of the source gas and the catalyst activation material is disposed in the synthesis furnace 110. It supplies to the gas mixing area | region 180 of the vicinity of the catalyst layer 103 from the mutually different piping provided. That is, a raw material gas (for example, butane or propane) and a first gas containing an atmospheric gas are supplied from the first gas supply pipe 141 into the synthesis furnace 110 through the first gas flow path 145, and a catalyst activation material (for example, A second gas containing water is supplied from the second gas supply pipe 143 into the synthesis furnace 110 through the second gas flow path 147. These gases supplied from the first gas flow path 145 and the second gas flow path 147 form a gas flow directed in a direction substantially parallel to the surface of the catalyst layer 103 of the substrate 101, and then the catalyst layer 103. It mixes in the gas mixing area | region 180 of the vicinity, and is supplied to the catalyst layer 103 surface on the base material 101 by predetermined amount.

In the CNT growth process according to the present invention, the source gas contained in the first gas forms a source gas flow in a plurality of directions substantially parallel to the substrate plane, and then the source gas is applied to the substrate plane. And contact with at least one of the catalyst and the catalyst fine particles on the substrate from a substantially vertical direction. Moreover, it is preferable that the cross-sectional area of the first gas flow path 145 through which the raw material gas flows substantially matches the area of the surface where the first gas flow path 145 through which the raw material gas flows intersects the catalyst layer 103.

In the CNT growth process according to the present invention, a raw material gas containing carbon, which is a chain saturated hydrocarbon compound having 2 or more carbon atoms and not containing oxygen, and a catalyst activation material containing oxygen are used as a catalyst. At the same time, at least one of the catalyst, the source gas, and the catalyst activation material is heated to grow CNTs. At this time, the carbon concentration of the source gas is preferably 10% or more. In the CNT growth step according to the present invention, it is preferable that the H / C ratio is 2.0 or more and the H ₂ O / C ratio is 0.010 or less in the raw material gas and the catalyst activation material. Thus, by adjusting the gas supplied into the synthesis furnace 110, the CNT aggregate can be manufactured with high yield.

Here, the raw material gas contained in the first gas undergoes a decomposition reaction while passing through the first gas flow path 145, and is in a state suitable for the production of CNTs. Further, by supplying from the second gas channel 147, a predetermined amount of the catalyst activation material is supplied to the gas mixing region 180 without reacting with the source gas. Since the first gas and the second gas optimized in this way are mixed in the gas mixing region 180 and brought into contact with the catalyst layer 103, the catalytic activity, which is usually only a few percent, is 90% or more. Therefore, it is possible to synthesize CNTs from various catalysts having very high efficiency and different sizes. At this time, the initial growth rate of CNT is preferably 300 μm / min or more and 330 μm / min or less.

In general, since the size of the catalyst and the diameter of the CNT are correlated (matched), the present manufacturing method is composed of CNTs having peaks over a wide wavelength range in Raman spectroscopic analysis, that is, CNTs having a plurality of greatly different diameters. A CNT aggregate can be efficiently manufactured at high speed and high yield from the catalyst layer 103 deposited on the substrate 101.

In the production process of the CNT aggregate according to the present invention, carbon that is a chain saturated hydrocarbon compound having 2 or more carbon atoms and a source gas that does not contain oxygen, and a catalyst activation material that contains oxygen, The carbon concentration of the source gas is increased by bringing at least one of the catalyst, the source gas, and the catalyst activation material into contact with the catalyst to grow the CNT, thereby increasing the CNT yield to at least 4 mg / cm ² or more. Preferably 5 mg / cm ² or more, more preferably 6 mg / cm ² or more, further preferably 7 mg / cm ² or more, more preferably 8 mg / cm ² or more, and a CNT aggregate having a BET of 600 m ² / g or more. Can be manufactured.

After the CNT production is completed, the raw material gas contained in the first gas, the catalyst activation material contained in the second gas, the decomposition products thereof, or the carbon impurities existing in the synthesis furnace 110 remain in the synthesis furnace 110. In order to suppress adhesion to the CNT aggregate, only the atmospheric gas is allowed to flow from the first gas flow path 145.

Under a cooling gas environment, the CNT aggregate, the catalyst, and the substrate 101 are preferably cooled to 400 ° C. or lower, more preferably 200 ° C. or lower. As the cooling gas, an inert gas supplied from the second gas supply pipe 43 is preferable, and nitrogen is particularly preferable from the viewpoints of safety, economy, purgeability, and the like. Thus, CNT used for the CNT composite material of this invention can be manufactured.

[Characteristics of CNT]
The thus produced CNT aggregate according to the present invention has a specific surface area of CNTs of 600 m ² / g or more and a yield of 10 mg or more in a growth process of 4 mg / cm ² or more. In addition, the height of the CNT is 500 μm or more, the G / D ratio is 1 or more, the diameter is 10 nm or less, the density is 0.001 g / cm ³ or more and 0.1 g / cm ³ or less, and is a single-walled CNT, which is high Has orientation.

[Manufacture of single-walled CNT aggregates]
An Al ₂ O ₃ / Fe catalyst layer 103 (thickness 40 nm / 1.5 nm) was formed on the substrate 101. For the synthesis of single-walled CNT aggregates, a fully automated CVD apparatus with a 1-inch synthesis furnace was used that allowed a large number (˜2000) of required synthesis in a controlled and stable environment. In the production process, the single-walled CNT aggregate was synthesized with a growth time of 10 minutes using ethylene (25 sccm or less), water (50 to 500 ppm) as a catalyst activator, and He (total flow rate: 500 sccm) as a carrier gas. . In Examples and Comparative Examples, the same conditions (He / H ₂ ) = 1/9 and temperature of 750 ° C. were used for formation of catalyst nanoparticles. The growth temperature and the amount of added water were optimized by the fixed specific hydrocarbon flow rate (in the comparative example, the flow rate was low).

(Comparative example)
As a comparative example, for the sputtered Fe / Al ₂ O ₃ catalyst layer (1.5 / 40 nm), ethylene and He and H ₂ carrier gases are supplied to a 1-inch synthesis furnace with a total flow rate of 500 sccm. The CNT aggregate was synthesized at 750 ° C. The three input parameters of the CVD method for adding water are the moisture content, the growth temperature, and the carbon concentration of the source gas. FIG. 3A is a diagram showing the yield of CNT aggregates with respect to the carbon concentration of the comparative example. In order to optimize the yield, the ethylene concentration was fixed, and the growth temperature (700 ° C. to 900 ° C.) and water content were sequentially optimized. Up to 36 tests were performed for a specific ethylene concentration. This test was conducted in the range of 0.0010 to 0.0060 mol / min (5-55 sccm with respect to the total flow rate of 500 sccm, carbon concentration of 2% to 22%) for five different ethylene concentrations.

FIG. 3A shows cumulative optimization results when ethylene is used as a raw material gas, the maximum value at each carbon concentration is indicated by a large point, and the overall tendency is indicated by a solid line. FIGS. 3B to 3E are three-dimensional plots of the yield with respect to the moisture content and growth temperature optimized with one ethylene concentration (corresponding to (b) to (e) in FIG. 3A). . The yield jumped as the ethylene concentration increased, reaching a peak of 4 mg / cm ² when the ethylene carbon concentration reached 10%. When the ethylene concentration exceeded this value, the yield gradually decreased. This trend is a well-known behavior of carbon concentration and yield with and without water, but the addition of water makes the optimum ethylene concentration very high.

Cyclic saturated compounds, chain saturated compounds, cyclic unsaturated compounds, aromatic compounds, and chain unsaturated compounds were studied. Specifically, trans-decahydronaphthalene (t-DN), xylene, methane, propane, butane, n-hexane, dicyclopentadiene, 1,3-butadiene, ethylene and acetylene are included. The growth conditions (catalyst formation temperature and total gas flow rate) were fixed. Detailed structural features for the CNT aggregates grown with each source gas showed comparable diameter, number of layers and purity, but the growth kinetics were significantly different. In general, the yield trend for all feed gases is similar to ethylene with increasing, peaking and decreasing with increasing carbon concentration.

(Example)
As shown in FIG. 4A, butane and propane according to the examples of the present invention did not show a peak even at the highest carbon concentration, and exhibited an exceptional behavior. He and H ₂ carrier gas were supplied to a 1-inch synthesis furnace at a total flow rate of 500 sccm, and a CNT aggregate was synthesized at 750 ° C. Specifically, when butane is used as the source gas, the yield determined as the weight per substrate area in a certain growth time is from 1.3 mg / cm ² (height 300 μm) to carbon at a carbon concentration of 3%. It increased to 9.6 mg / cm ² (height 2400 μm) at a concentration of 32%. This is a yield that is more than twice that of the case of using another source gas containing acetylene and ethylene in the comparative example. It is clear that the butane peak, if present, will be present in a very high carbon concentration region.

Experimentally, it is difficult to handle such high carbon concentrations that the source gas decomposes and produces a thick smoke of gaseous tar. In fact, with increasing carbon concentration, the yield of CNTs using butane increased linearly up to the limits of the range handled in this example. CNT growth curves were measured with an in-situ telecentric height monitoring system. This is a real-time monitoring of the height of the CNT aggregate using the projected shadow of the CNT aggregate formed from parallel green light focused on the CCD from the telecentric optical system. The growth curve of the CNT aggregate using butane as the source gas indicated by a broken-line circle showed a sustained growth time exceeding 10 minutes so that the growth does not stop or saturate (FIG. 4A). 4 (b)). The linearity of the growth curve and the linearity of the yield suggest that there is no significant accumulation of carbon impurities on the catalyst. Unlike the unsaturated hydrocarbon of the comparative example (FIG. 4C), the utilization of the saturated hydrocarbon at a high carbon concentration according to this example can lead to a high yield of CNT aggregates. This means that during a certain growth time, butane can produce the highest yield of CNT aggregates, which is a significant feature of the synthesis of single-walled CNT aggregates.

FIG. 5A shows the carbon efficiency with respect to the carbon concentration and the ratio of hydrogen to carbon for each type of molecule. The carbon efficiency indicates the ratio of the supplied source gas converted into a single-walled CNT aggregate, and was estimated by the quotient (multiplied by 100) of the weight of the synthesized single-walled CNT aggregate and the supplied carbon. As an index of saturation, hydrocarbons were classified according to the ratio of hydrogen to carbon atoms. For example, this ratio for unsaturated hydrocarbons is low (acetylene: 1) and this ratio for fully saturated hydrocarbons is high (propane: 2.7). This plot is consistent with the conventional knowledge, and the raw material gas (for example, acetylene or ethylene) having a high degree of unsaturation in the comparative example showed high carbon efficiency (height of 51% or less). Such characteristics are considered to support the reason why acetylene and ethylene are frequently used in the study of the CVD method.

Furthermore, as for the unsaturated hydrocarbon such as acetylene of the comparative example, as the carbon concentration increased, the carbon efficiency increased sharply, reached a peak, and decreased sharply. Such behavior was observed in all groups of raw material gases except the highly saturated hydrocarbons (butane and propane) of this example. In contrast, the source gas with a high degree of saturation had a significantly different behavior depending on the carbon concentration, although the carbon efficiency was low (4.5 to 6.1%) even at the same carbon concentration. In contrast to the comparative example showing a sharp peak, the highly saturated source gas (for example, butane) of this example showed an exceptionally flat profile even when the carbon concentration was increased. In fact, the carbon efficiency of butane was only reduced by 23% over the entire range 2 of the test, in contrast to the acetylene and other saturated hydrocarbons of the comparative examples, which were reduced to approximately 100%. At a high carbon concentration of 11%, the carbon efficiency of the butane of this example was 4.6%, whereas that of the acetylene of the comparative example was 0%. Since the carbon efficiency of the butane of this example was significantly higher than that of the unsaturated hydrocarbon of the comparative example, it was suggested that the growth kinetics in the raw material gas of this example was different from that of the comparative example.

[In-situ telecentric height monitoring of single-walled CNT aggregates]
For the growth conditions described above, the growth curve of the single-walled CNT aggregate for each hydrocarbon was measured with an in-situ telecentric height monitoring system. In the wide dynamic range (maximum 1 cm) with high resolution (1 μm) of the monitoring system, the height of single-walled CNT aggregates from high precision and short (<100 μm) to long (> 2500 μm) was measured. For each measurement, the height of the single-walled CNT aggregate was continuously measured, and the growth curve shown in the inset of FIG. Due to the primary kinetics, the growth equation of the height (yield) of the single-walled CNT aggregate is expressed as follows.

Here, H represents the height of the single-walled CNT aggregate, and the two fitting parameters β and τ ₀ represent the initial growth rate (IGR) and lifetime, respectively. By fitting each growth curve to this growth equation, the IGR that characterizes the growth of single-walled CNT aggregates and the lifetime of the catalyst were plotted as a function of carbon concentration (FIGS. 5B and 5C). For each optimum condition (large point) in FIG. 4A, the corresponding IGR and lifetime values were plotted (FIGS. 5B and 5C).

As is already known in the comparative example ethylene, a general trend in which an inversely proportional relationship between the growth rate of the source gas and the lifetime with respect to the increase in carbon concentration was observed for almost all source gases. . Specifically, in the carbon concentration range of 0.00009 to 0.004 mol / min, the growth rate with most of the source gas increased to 500 μm / min. On the other hand, in this same range, the growth lifetime of the single-walled CNT aggregates was dramatically reduced from 50 minutes to 1 minute or less. This result explains why the growth yield peaks are observed in most of the source gases (FIG. 4a). Usually, the same tendency is shown also in the butane and propane which concern on a present Example. However, at high carbon concentrations, butane according to this example was less dependent on growth lifetime and showed a lifetime decrease of up to 40%. Therefore, when the butane according to this example was used, the growth lifetime was never zero, and the yield increased. This result explains the sustained increase in yield in butane (FIG. 4 (a)) and constant carbon efficiency (FIG. 5 (a)).

The optimized water / carbon ratio (H ₂ O / C-ratio) was plotted against the heat of formation for each source gas condition (FIG. 6). These values were derived from the growth conditions (optimum growth temperature, optimum moisture content and optimum carbon concentration) at the optimum yield. The optimum H ₂ O / C-ratio is an important value indicating the growth kinetics of a single-walled CNT aggregate to which water is added. Optimal H _{2 O} / C-ratio determines the amount of carbon and water molecules consumed before the catalyst deactivation. The H ₂ O / C-ratio associates the heat of formation of the source gas with the activation energy. In contrast to the H / C ratio, the heat of formation is an indicator of molecular stability (or reactivity) and was normalized by the number of carbon atoms in each source gas. Referring to FIG. 6, first, all data was almost directly proportional to the heat of formation. This result indicates that a source gas with high formation energy (ie, reactivity) required a high H ₂ O / C-ratio. This indicates the need for water to clean the catalyst surface in order to maintain activity. Furthermore, the acetylene of the comparative example showed that the H ₂ O / C-ratio was 10 times higher than the butane of this example.

Secondly, the generation energy of the raw material gas increased due to the hydrocarbon category, that is, the chain saturated compound, the cyclic saturated compound, the cyclic unsaturated compound, and the chain unsaturated compound. Third, the order of increase in generated heat showed a tendency opposite to the activation energy predicted for each raw material gas (FIG. 6, inset).

These results confirmed the mechanism of synthesis of CNT aggregates that explained the behavior observed with various source gases including butane as shown in FIG. For the growth model, the following scheme describing the process of the CVD method adding water is applied. In this model, two competitive reactions occur. 1) Generation of CNT aggregates and carbon impurities by exposure of the catalyst to heat and carbon, 2) Purification of the catalyst by exposure to water (ie, removal of carbon impurities). From the results described above, it can be interpreted that the rate-limiting step in the synthesis is different between the saturated hydrocarbon (for example, butane) according to the present invention and the unsaturated hydrocarbon (for example, acetylene) of the comparative example in the single-walled CNT aggregate. The raw material gas having high reactivity (low activation energy) such as the unsaturated hydrocarbon of the comparative example has a high H ₂ O / C-ratio, and the production rate of carbon impurities that cause catalyst deactivation is high. It is fast (FIG. 3). On the contrary, the low reactive material gas as shown in the chain saturated compound according to the present invention has a low optimum H ₂ O / C-ratio, suggesting that the generation of carbon impurities is relatively low.

Further, from the results of FIG. 5B and FIG. 5C, the unsaturated hydrocarbon of the comparative example has a long growth life only at a low carbon concentration at which the initial growth rate at which the optimum yield is limited is slow. Time has been achieved. In contrast, the saturated hydrocarbon according to the present invention showed a long growth lifetime when the initial growth rate was high and the carbon concentration was high. Therefore, as shown in FIG. 1B, in the case of an unsaturated hydrocarbon such as acetylene of the comparative example, the optimum H ₂ O / C-ratio increases as the carbon concentration increases, and the growth lifetime is increased. Since it becomes shorter, it is speculated that the purification of the catalyst surface becomes the rate-limiting step. This is consistent with the finding that, as previously reported by the present inventors, a decrease in the growth characteristics of the CNT aggregate is suppressed when the water content is increased in the catalyst purification step.

In contrast, in the butane of this example, as described above, a different chemical process was inferred that the carbon impurity generation rate was low. In this case, in the butane of this example, the rate-limiting step in the synthesis of the CNT aggregate is a conversion process from butane to the CNT aggregate (FIG. 1 (a)).
This mechanism was verified by an experimental result in which the carbon efficiency became substantially constant with respect to the raw material gas concentration (FIG. 5A). This explains the observed low optimal H ₂ O / C ratio and long growth lifetime at high carbon concentrations. Moreover, the difference in the rate-limiting step in the synthesis of the CNT aggregate between the saturated hydrocarbon of this example (butane, etc.) and the unsaturated hydrocarbon of the comparative example (acetylene, etc.) is that the peak of yield increases as the carbon concentration increases. Explain why not.

[Raman spectroscopy]
The Raman spectral characteristics were measured using a Raman spectrometer (Thermo Electron Co., Ltd.) for a sample having a diameter of 0.3 mm using an excitation wavelength of 532 nm.

1: CNT, 3: carbon, 5: raw material gas, 7: catalyst activation material, 9: raw material gas, 100: production apparatus, 101: base material, 103: catalyst layer, 105: base material holder, 110: synthesis furnace, 141: first gas supply pipe, 143: second gas supply pipe, 145: first gas flow path, 147: second gas flow path, 161: raw material gas cylinder, 163: atmospheric gas cylinder, 165: reducing gas cylinder, 167: catalyst Activating substance cylinder, 171: first carbon concentration adjusting means, 173: second carbon concentration adjusting means

Claims (5)

In the production method in which the raw material gas and the catalyst activator are brought into contact with the catalyst on the substrate to grow carbon nanotubes,
Supplying a reducing gas to contact the catalyst on the substrate and heating at least one of the catalyst and the reducing gas to reduce and / or atomize the catalyst;
A raw material gas containing carbon which is a chain saturated hydrocarbon compound having 2 or more carbon atoms and not containing oxygen and a catalyst activation material containing oxygen are brought into contact with the catalyst, and the catalyst and the raw material gas And heating at least one of the catalyst activation materials to grow carbon nanotubes,
A method for producing a carbon nanotube aggregate, wherein the carbon concentration of the raw material gas is increased so that the yield of CNT is at least 4 mg / cm ² or more. The method for producing an aggregate of carbon nanotubes according to claim 1, wherein the carbon concentration of the source gas is 10% or more. The method for producing a carbon nanotube aggregate according to claim 1 or 2, wherein a carbon source flow rate of the chain saturated hydrocarbon compound is 0.003 mol / min or more. The method for producing a carbon nanotube aggregate according to any one of claims 1 to 3, wherein the source gas contains butane or propane. The method for producing a carbon nanotube aggregate according to any one of claims 1 to 4, wherein the carbon nanotubes are single-walled and the aggregates are oriented.