JP5307195B2 - Carbon materials and identification marks with controlled isotope ratios - Google Patents

Description

  The present invention relates to carbon materials with controlled carbon isotope ratios and identification marks using these carbon materials. More specifically, the present invention relates to carbon isotope ratios obtained by CVD (chemical vapor deposition) or combustion flame methods. The present invention relates to a carbon material composed of single-walled carbon nanotubes, multi-walled carbon nanotubes, or carbon fibers with controlled carbon isotopes synthesized using a gas containing carbon that is controlled in a controlled manner, and an identification mark using these carbon materials.

Carbon nanotubes have been attracting attention since their discovery (1991), and various studies and studies have been conducted on their synthesis and utilization methods. Recently, the carbon isotope ratio has been changed to the natural abundance ratio ( ¹² C: 98.9). %, ¹³ C = 1.1%), and synthesis of carbon nanotubes has also been attempted. However, until now, there have been only a few reports of synthesis using the arc discharge method and laser irradiation method.

Robert Pallasser et al. And JM Jones et al. Reported synthesizing 0.002-0.034% ¹² C-rich carbon nanotubes by arc discharge (Journal American Chemical Society 115 11634-11635, 1993; Carbon 34 2,231-237, 1996). C. Goze Bac et al. Synthesized 10% ¹³ C-rich single-walled carbon nanotubes by the arc discharge method (Physical Review B 63, 100302, 2001).
Regarding the synthesis of carbon nanotubes by the laser irradiation method, X.-P.Tang et al. Synthesized single-walled carbon nanotubes rich in 10% ¹³ C by irradiating graphite with an Nd: YAG laser (Science 288,492). -494, 2000).

Journal American Chemical Society 115 11634-11635, 1993; Carbon 34,2,231-237, 1996 Physical Review B 63,100302, 2001 Science 288,492-494, 2000

  However, it is difficult to synthesize carbon nanotubes in which the carbon isotope ratio is controlled to an arbitrary isotope ratio by any of these arc discharge methods and laser irradiation methods. In the synthesis by the arc discharge method and the laser irradiation method, dangerous and expensive devices such as an exhaust device, a high voltage, a large current power source and the like are required. In addition, in these methods, there are various problems such as a small amount of production, and further, graphite, amorphous carbon, etc. are mixed in the product, and the obtained carbon nanotubes have large variations in diameter and length.

The inventors of the present invention have continued to experiment and research on carbon nanotube synthesis technology in which the carbon isotope ratio is controlled, and in particular, the carbon nanotube synthesis technology capable of arbitrarily controlling the carbon isotope ratio. The first successful synthesis of controlled single-walled carbon nanotubes, multi-walled carbon nanotubes and carbon fibers.
That is, the present invention relates to a carbon material composed of a single-walled carbon nanotube, a multi-walled carbon nanotube or a carbon fiber in which the carbon isotope ratio is controlled and the carbon isotope ratio is arbitrarily controlled, and an identification mark using the carbon material The purpose is to provide.

The present invention is (1) a carbon material comprising single-walled carbon nanotubes, multi-walled carbon nanotubes, or carbon fibers, the raw material being a gas containing carbon in which the isotope ratio of ¹² C and ¹³ C is artificially controlled. A carbon material comprising a single-walled carbon nanotube, a multi-walled carbon nanotube, or a carbon fiber with a controlled carbon isotope ratio is synthesized.

The present invention is (2) an identification mark using a carbon material comprising single-walled carbon nanotubes, multi-walled carbon nanotubes, or carbon fibers, and the carbon material artificially controls the isotope ratio of ¹² C and ¹³ C. There is provided an identification mark characterized by being a carbon material composed of single-walled carbon nanotubes, multi-walled carbon nanotubes, or carbon fibers with a controlled carbon isotope ratio synthesized by using a gas containing carbon as a raw material.

Carbon materials composed of single-walled carbon nanotubes, multi-walled carbon nanotubes or carbon fibers with controlled carbon isotope ratios, and carbon materials used for the identification marks described above are artificially controlled for ¹² C and ¹³ C isotope ratios. It is produced by synthesizing by a CVD method or a combustion flame method using a gas containing carbon as a raw material.

  According to the present invention, a carbon material composed of single-walled carbon nanotubes, multi-walled carbon nanotubes, or carbon fibers with a controlled isotope ratio is manufactured by a very simple CVD method or combustion flame method at a reaction temperature of 400 to 1200 ° C. can do. Since the carbon material of the present invention can synthesize single-walled carbon nanotubes, multi-walled carbon nanotubes, and carbon fibers particularly at a low reaction temperature of 400 to 800 ° C., it is very advantageous in terms of energy saving and apparatus cost.

  In addition, the carbon material comprising single-walled carbon nanotubes, multi-walled carbon nanotubes or carbon fibers with controlled isotope ratios according to the present invention has a Raman spectroscopic spectrum, infrared spectroscopic spectrum, mass spectrum, nuclear magnetic field depending on the difference in the carbon mass number. A resonance spectrum shift occurs. By utilizing this fact, these carbon materials can be used as identification marks for preventing counterfeiting of various articles such as authentication cards, banknotes, precious metals, and crafts. Although the carbon material itself is a material that is difficult to counterfeit, the carbon material is very useful in practice because of its simple manufacturing method.

The present invention (1) is, ¹² C and carbon isotope ratios obtained by combining the gas as raw material containing artificially controlled carbon isotope ratio of ¹³ C is controlled, i.e., the ¹² C and ¹³ C It is a carbon material composed of single-walled carbon nanotubes, multi-walled carbon nanotubes or carbon fibers in which the isotope ratio is different from the natural isotope ratio.

The present invention (2) includes a single-walled carbon nanotube, a multi-walled carbon nanotube having a controlled carbon isotope ratio obtained by synthesizing a gas containing carbon having an artificially controlled ¹² C and ¹³ C carbon isotope ratio as a raw material, It is an identification mark using a carbon material made of carbon fiber. This identification mark is given to various articles such as authentication cards, banknotes, precious metals, and crafts, and is used for preventing forgery of these articles.

The carbon material composed of single-walled carbon nanotubes, multi-walled carbon nanotubes or carbon fibers with controlled carbon isotope ratio of the invention (1), and the carbon material used for the identification mark of the invention (2) are ¹² C and ¹³ C It is possible to synthesize by a CVD method or a combustion flame method using as a raw material a gas containing carbon whose isotope ratio is artificially controlled. As the CVD method, a thermal CVD method and a plasma CVD method are preferably used. The following description will focus on the thermal CVD method, but the same applies to other methods.

As a gas source containing carbon in which the isotope ratio of ¹² C and ¹³ C is artificially controlled, that is, a gas source containing carbon in which the isotope ratio of ¹² C and ¹³ C is different from the natural isotope ratio, At least one gaseous raw material selected from carbon monoxide, carbon dioxide, acetylene, benzene and toluene is used. As an aspect in which the ratio is different from the natural isotope ratio, the isotope ratio may be larger or smaller than the natural abundance ratio.

Carbon atom number ratio [ ¹³ C / ( ¹³ C + ¹² C) × in the carbon isotope ratio of the carbon material comprising the single-walled carbon nanotube, multi-walled carbon nanotube, or carbon fiber of the present invention obtained using the gas source 100 (%)] can be arbitrarily controlled by artificially controlling the isotope ratio of ¹² C and ¹³ C of the gaseous raw material containing carbon. In the present invention, the carbon atom ratio is preferably 2.0% or more or 0.5% or less.

  The carbon material comprising single-walled carbon nanotubes, multi-walled carbon nanotubes or carbon fibers of the present invention is produced and produced on a substrate for growing the carbon material, that is, an active substrate. As the active substrate, a substrate made of alumina containing a catalyst metal and a catalyst auxiliary metal is used.

  As the catalytic metal, at least one kind of metal selected from single-walled carbon nanotubes, multi-walled carbon nanotubes or carbon fibers, that is, a metal selected from Fe, Co, Ni, Cr, Cu and Pd is used. When using two or more of them, they may be used as an alloy. Of these catalytic metals, Fe, Co, Ni, Cr and Pd are preferably used. And Mo is used as a catalyst auxiliary metal.

  As alumina, it is particularly important to use high-purity alumina with a low content of impurities such as alkali metals, Si, and S. Specifically, high-purity alumina having a total content of impurities composed of alkali metals such as Na and K, sulfur (S) and silica of 0.05 wt% or less, preferably 0.01 wt% or less. By suppressing the content of these impurities in alumina, the ability to adsorb source gases such as methane is increased, and production of single-walled carbon nanotubes, multi-walled carbon nanotubes or carbon fibers at a lower temperature such as 400 to 800 ° C. Can be made possible. The form of alumina can be various shapes such as powder, granules, tablets, pellets, and plates.

  The active substrate can be obtained by supporting a catalyst metal and its catalyst auxiliary metal on alumina, particularly high-purity alumina. When the above metal catalyst and its catalyst auxiliary metal are supported on alumina, the metal is preferably used in the form of a soluble salt, for example, a metal salt such as nitrate or acetate, and the soluble salt is dissolved in a solvent to obtain a metal salt solution. And As the solvent, water, lower alcohols such as methanol and ethanol, and other organic solvents, or a mixed solution of water and a water-soluble organic solvent are used, and water is preferably used.

  In the metal salt solution, that is, the solution containing the metal catalyst and the catalyst auxiliary metal, the concentration of the metal salt is not more than the saturated solution concentration, but is usually in the range of 0.01 to 0.05 wt%, preferably It is the range of 0.005-0.01 wt%. Then, the metal salt solution is impregnated with alumina and supported. As a contact method between alumina and the metal salt solution, an immersion method, a spray method, or the like can be used, but an immersion method is preferably used. The contact temperature is in the range of room temperature to 80 ° C, preferably in the range of 50-60 ° C. Due to the contact between the alumina and the metal solution, the metal catalyst solution is impregnated and supported on the alumina.

  In the alumina containing the metal catalyst and the catalyst auxiliary metal, the content of the catalyst metal is in the range of 1 to 20 wt%, preferably in the range of 5 to 10 wt% as the metal. Further, the content of Mo which is a catalyst auxiliary metal is in the range of 0.1 to 1.5 wt%, preferably in the range of 0.3 to 0.8 wt% as metal Mo.

  According to the results of various related experiments by the present inventors, when the metal catalyst is Fe, Co or Ni, there is a tendency that single-walled carbon nanotubes are likely to be formed, and when the metal catalyst is Fe or Co and at a high temperature (eg, 1200 ° C.). There is a tendency that multi-walled carbon nanotubes are likely to be generated, and when the metal catalyst is Pd, carbon fibers tend to be easily generated, and when the amount of catalytic metal is larger than that of alumina, carbon fibers are likely to be generated. Although the type varies depending on various conditions, according to the present invention, single-walled carbon nanotubes of about 0.5 to 4 nm (diameter), multi-walled carbon nanotubes or carbon fibers of about 2 to 100 nm (diameter) can be obtained.

  The form of these metals contained in alumina may be any form that can promote the formation of carbon nanotubes and carbon fibers, and may be in the form of metal oxide, metal hydroxide, in addition to metal form. In the case of a metal form, the metal salt-containing alumina obtained as described above may be reduced in an atmosphere such as hydrogen. In the case of the metal oxide form, the metal salt-containing alumina obtained as described above may be fired in an oxidizing atmosphere such as air.

According to the present invention, in producing a substrate containing a metal catalyst and a catalyst auxiliary metal, as a preferred method, first, powdered alumina containing the catalyst metal and the catalyst auxiliary metal is produced. Next, this powdery alumina is formed into a required shape, or this powdery alumina is applied to a substrate such as Si or SiO ₂ . For powdered alumina, the range of the specific surface area 100~250cm ² / g, preferably in the range of 200~250cm ² / g.

According to the present invention, in order to produce a carbon material composed of single-walled carbon nanotubes, multi-walled carbon nanotubes, or carbon fibers with a controlled carbon isotope ratio, a substrate made of alumina containing the catalyst metal and a catalyst auxiliary metal is used. In the presence, the carbon isotope ratio of ¹² C and ¹³ C is artificially controlled, that is, pyrolysis is performed while circulating a gas source in which carbon isotope separation is performed and ¹² C and ¹³ C are controlled to an arbitrary ratio. .

  The thermal decomposition temperature, that is, the reaction temperature is not particularly limited, but can be carried out in the range of 400 to 1200 ° C. According to the present invention, a carbon material composed of single-walled carbon nanotubes, multi-walled carbon nanotubes or carbon fibers can be produced even at a low temperature of 400 to 800 ° C., which is very advantageous in terms of energy saving and apparatus cost. is there.

As a gas source containing carbon in which the isotope ratio of ¹² C and ¹³ C is artificially controlled, a gas containing two kinds of carbons having different ¹³ C / ( ¹² C + ¹³ C) ratios is used in combination. be able to. That is, (A) ¹³ C / ( ¹² C + ¹³ C) × 100 gas having a large ratio, for example, methane gas having a ratio of 99.90% and (B) ¹³ C / ( ¹² C + ¹³ C) × 100 ratio is small. A gas, for example, methane gas having a ratio of 0.05% is prepared. Then, by mixing the carbon materials with a desired ratio necessary for producing a carbon material composed of single-walled carbon nanotubes with controlled carbon isotope ratios, multi-walled carbon nanotubes or carbon fibers, ¹² C and ¹³ C can be arbitrarily selected. It can be set as the gaseous raw material controlled to this ratio.

Among the gases (A) and (B), as a gas having a large (A) ¹³ C / ( ¹² C + ¹³ C) × 100 ratio, for example, methane gas having a ratio of 99.90% is used, and (B) ¹³ As a gas having a small C / ( ¹² C + ¹³ C) × 100 ratio, methane gas having a natural isotope ratio of 1.10% may be used. In this case, a gas with a natural isotope ratio is used as the gas (B), which is advantageous in terms of cost.

In this method, that is, a gas source containing carbon in which the isotope ratio of ¹² C and ¹³ C is artificially controlled, a gas containing two kinds of carbons having different ¹³ C / ( ¹² C + ¹³ C) ratios is mixed. According to the technique used, a gas containing two kinds of carbons having different ¹³ C / ( ¹² C + ¹³ C) ratios, for example, methane gas having a large ¹³ C ratio (that is, a small ¹² C ratio) and a ¹³ C ratio Since only a small amount of methane gas (that is, a large ¹² C ratio) is prepared, a gas material having an arbitrary desired ratio can be obtained, which is very advantageous in practice.

Velocity of the gas material of methane gas or the like, the range of 2000～200000H ^-1 gas hourly space velocity (GHSV), preferably in the range of 5000~100000h ^-1. When pyrolyzing methane gas, argon or hydrogen gas can be mixed in the gas as a carrier. In addition, a suitable amount of sulfur compounds such as hydrogen sulfide and mercaptans can be added to methane gas. Thereby, straight single-walled carbon nanotubes, multi-walled carbon nanotubes, or carbon fibers can be generated on the substrate.

  In the present invention, a carbon material composed of single-walled carbon nanotubes, multilayered carbon nanotubes, or carbon fibers in which the carbon isotope content ratio is artificially controlled is used as an identification mark for various articles. That is, these carbon materials are given to various articles as identification marks and used for authentication of various articles and forgery prevention. As the mode, for example, a solution (dispersion) in which these carbon materials are mixed with ink or the like is used, and this is applied to various articles such as authentication cards, banknotes, precious metals, and crafts. Since these carbon materials are fine materials of nanometer (nm) level, they can be easily applied to various articles such as being easily mixed into ink.

  The mass of single-walled carbon nanotubes, multi-walled carbon nanotubes, or carbon fibers with artificially controlled carbon isotope ratios should be identified by either Raman spectroscopy, infrared spectroscopy, mass spectra, or nuclear magnetic resonance spectra. Is possible. For example, in Raman spectroscopy, when the average mass of carbon increases, the spectrum shifts to the low wavenumber side, and when it decreases, the spectrum shifts to the high wavenumber side. This fact can be used to identify the carbon isotope ratio.

That is, the identification mark of the present invention has a carbon isotope content ratio different from the natural content ratio, so that a single-wall carbon nanotube, multi-wall carbon nanotube or carbon fiber having a natural content ratio has a vibration spectrum or NMR signal spectrum. It will be different. Infrared absorption / transmission spectra of carbon materials composed of single-walled carbon nanotubes, multi-walled carbon nanotubes, or carbon fibers in which the content ratio of ¹³ C is controlled to a value different from the natural isotope ratio are the carbon content of natural carbon isotopes. Shift with respect to the infrared absorption / transmission spectra of nanotubes and carbon fibers.

Since this difference differs individually depending on the content ratio of ¹² C and ¹³ C, the content ratio of ¹² C and ¹³ C can be detected by measuring the difference by infrared spectroscopy. Similarly, it can be detected by measuring a Raman spectrum, a mass spectrum, or a nuclear magnetic resonance spectrum. Thus, authentication using an identification mark and authenticity determination can be performed.

  EXAMPLES Hereinafter, although this invention is demonstrated in more detail based on an Example, it cannot be overemphasized that this invention is not limited to an Example. In the following examples, a tubular furnace (manufactured by Okura Riken Co., Ltd., electric furnace, maximum operating temperature = 1200 ° C.) having a reactor inner dimension: 112 mmφ (inner diameter) × 570 mm (length) was used as a reactor.

This reactor has three methane gas conduits and Ar conduits having a ratio of ¹³ C / ( ¹² C + ¹³ C) × 100 of 0.05%, 1.10%, and 99.95%. A control mechanism is provided for controlling the gas composition introduced into the furnace of the electric furnace by controlling the flow rate from the furnace. For this reason, a mass flow controller is provided for each of these conduits, and by controlling the flow rates of the three types of methane gases having different isotope ratios by the respective mass flow controllers, methane gas having a predetermined isotope ratio is obtained. It is introduced into the furnace. Here, among the above three types of methane gas, methane gas having a ¹³ C / ( ¹² C + ¹³ C) × 100 ratio of 0.05% and 99.95% was obtained by rectifying natural gas by a precision distillation method. Is.

<Example 1>
1.0 g of high-purity γ-alumina powder having a sodium content of 0.01 wt% or less and a purity of 99.95 wt% or more was converted to 0.2 g of iron nitrate [Fe (NO ₃ ) ₂ .9H ₂ O]. After being immersed in a solution obtained by dissolving 0.01 g of molybdenum oxide acetonate [CH ₃ COCHCOCH ₃ ) ₂ MoO _{3 in} 35 ml of methanol, it was dispersed by sonication for 3 hours. This dispersion was applied to a Si substrate and dried in air at 120 ° C. for 1 hour.

Next, the substrate is placed on an alumina boat and inserted into a furnace of an electric furnace, and the inside of the furnace is heated to 700 ° C. under an argon atmosphere, and then ¹³ C / ( ¹² C + ¹³ C is controlled by a control mechanism. ) X 100 ratios of six source gases of 0.05%, 0.50%, 1.10%, 2.00%, 49.97% and 99.90%, each of which in the furnace for 30 minutes Circulated. As a result, a tube-like product was observed on the substrate surface in each case by observation with an electron microscope (Hitachi, Ltd., S-5000).

When these products (samples) were observed with a transmission electron microscope (JEM-2000FX II manufactured by JEOL Ltd.), single-walled carbon nanotubes each having a thickness (diameter) of about 1 nm were bundled. It was observed that About each of these single-walled carbon nanotubes, the spectrum was further measured with a Raman spectroscope (manufactured by JASCO Corporation, FX-2100). Table 1 shows the ¹³ C / ( ¹² C + ¹³ C) × 100 ratio of each product sample and the spectral shift from the sample synthesized at ¹³ C / ( ¹² C + ¹³ C) × 100 = 0.05% [Δω ( cm ⁻¹ )].

As shown in Table 1, the ¹³ C / ( ¹² C + ¹³ C) × 100 ratio is 0.50%, 1.10%, 2.00%, 49.97%, 99.90% from any spectrum. A spectral shift [Δω (cm ⁻¹ )] from a sample of ¹³ C / ( ¹² C + ¹³ C) × 100 = 0.05 was observed. From this, it is understood that six types of single-walled carbon nanotubes can be distinguished from each other by measurement of Raman spectroscopy.

There is a relationship that m × ω ² is constant between the average mass number m of carbon and the wave number ω. From this, the following formula (1) is established. In the formula, Δm is an increase in mass number. From this relationship, it is possible to estimate the average mass number of the single-walled carbon nanotubes by measuring Δω (wave number shift or shift). If the average mass number of carbon is slightly different, the value of the wave number ω changes. By utilizing this fact, single-walled carbon nanotubes having a specific ratio of ¹³ C / ( ¹² C + ¹³ C) can be used as identification marks for preventing forgery of various articles.

<Example 2>
0.1 g of nickel nitrate [Ni (NO ₃ ) ₂ ] and 0.01 g of molybdenum oxide acetyl acetate were dissolved in methanol. In this solution, 1.0 g of high-purity γ-alumina powder having a sodium content of 0.01 wt% or less and a purity of 99.95 wt% or more was immersed for 30 minutes and dispersed by ultrasonic treatment for 3 hours. This dispersion was applied to a Si substrate. In the same manner as in Example 1, four kinds of source gases having ¹³ C / ( ¹² C + ¹³ C) × 100 ratios of 0.05%, 1.10%, 49.97%, and 99.90%, respectively, It was made to distribute | circulate for 30 minutes in a 1200 degreeC furnace.

When the product (sample) on the substrate surface was observed with a transmission electron microscope (JEM-2000FX II, manufactured by JEOL Ltd.), the thickness (diameter) was about 50 to 100 nm even when any of the above source gases was used. It was observed that multi-walled carbon nanotubes were formed. Moreover, about the product of each board | substrate surface, the spectrum measurement was carried out similarly to Example 1 with the Raman spectroscopic apparatus (The JASCO Corporation make, FX-2100). Table 2 shows the spectral shift [Δω (cm ⁻¹ )] from the sample synthesized at ¹³ C / ( ¹² C + ¹³ C) × 100 = 0.05% for each product sample.

As shown in Table 2, ¹³ C / ( ¹² C + ¹³ C) × 100 ratio is ¹³ C / ( ¹² C + ¹³ C from the spectrum of any sample of 1.10%, 49.97%, 99.90%. ) × 100 = 0.05% spectral shift [Δω (cm ⁻¹ )] from the sample was observed. From this, it is understood that four types of multi-walled carbon nanotubes can be distinguished from each other by measurement of Raman spectroscopy.

The above formula (1) holds between the average mass number m of carbon and the wave number ω because m × ω ² is constant. From this relationship, it is possible to estimate the average mass number of the multi-walled carbon nanotube by measuring Δω. If the average mass number of carbon is slightly different, the value of the wave number ω changes. Using this fact, multi-walled carbon nanotubes having a specific ratio of ¹³ C / ( ¹² C + ¹³ C) can be used as identification marks for preventing counterfeiting of various articles.

<Example 3>
1.2 g of high-purity γ-alumina powder having a sodium content of 0.01 wt% or less and a purity of 99.95 wt% or more, 1.2 g of palladium nitrate [Pd (NO ₃ ) ₂ ] and molybdenum oxide acetyl acetate It was immersed in a solution obtained by dissolving 0.01 g in methanol for 30 minutes and dispersed by ultrasonic treatment for 3 hours. This dispersion was applied to a Si substrate, and the ratio of ¹³ C / ( ¹² C + ¹³ C) × 100 was 0.05%, 1.10%, 49.97% and 99.90% in the same manner as in Example 1. Each of the four raw material gases was circulated in the furnace for 30 minutes. The temperature in the furnace, that is, the reaction temperature is 800 ° C.

When the products on the surface of each substrate were observed with a transmission electron microscope (JEM-2000FX II, manufactured by JEOL Ltd.), in each case, carbon fibers having a thickness (diameter) of about 20 to 30 nm were generated. It was observed. The spectrum of the product on the substrate surface was measured in the same manner as in Example 1. Table 3 shows the spectral shift [Δω (cm ⁻¹ )] from the sample synthesized at ¹³ C / ( ¹² C + ¹³ C) × 100 = 0.05% for each product sample.

As shown in Table 3, ¹³ C / ( ¹² C + ¹³ C) × 100 ratio is ¹³ C / ( ¹² C + ¹³ C from the spectrum of any sample of 1.10%, 49.97%, 99.90%. ) × 100 = 0.05% spectral shift [Δω (cm ⁻¹ )] from the sample was observed. From this, it can be seen that the four types of carbon fibers can be identified by Raman spectroscopy.

The above formula (1) holds between the average mass number m of carbon and the wave number ω because m × ω ² is constant. From this relationship, it is possible to estimate the average mass number of the carbon fiber by measuring Δω. If the average mass number of carbon is slightly different, the value of the wave number ω changes. By utilizing this, carbon fibers having a specific value of ¹³ C / ( ¹² C + ¹³ C) ratio can be used as identification marks for preventing forgery of various articles.

Claims (5)

A carbon material composed of single-walled carbon nanotubes, multi-walled carbon nanotubes, or carbon fibers, which is synthesized by using as a raw material a gas containing carbon in which the isotope ratio of ¹² C and ¹³ C is artificially controlled. A carbon material comprising a single-walled carbon nanotube, a multi-walled carbon nanotube, or a carbon fiber with a controlled carbon isotope ratio. The gas source containing carbon in which the isotope ratio of ¹² C and ¹³ C is artificially controlled is at least one gas source selected from methane, carbon monoxide, carbon dioxide, acetylene, benzene and toluene. The carbon material according to claim 1. An identification mark using a carbon material composed of single-walled carbon nanotubes, multi-walled carbon nanotubes, or carbon fibers, wherein the carbon material is a gas containing carbon whose isotope ratio of ¹² C and ¹³ C is artificially controlled An identification mark, characterized in that it is a carbon material composed of single-walled carbon nanotubes, multi-walled carbon nanotubes or carbon fibers with a controlled carbon isotope ratio.   The identification mark according to claim 3, wherein the identification mark is an identification mark used for preventing counterfeiting of an article such as an authentication card, banknote, precious metal, or craft. The gas source containing carbon in which the isotope ratio of ¹² C and ¹³ C is artificially controlled is at least one gas source selected from methane, carbon monoxide, carbon dioxide, acetylene, benzene and toluene. 5. The identification mark according to claim 3 or 4, wherein: