METHOD FOR PURIFYING CARBON NANOTUBES AND ULTRA-PURE CARBON NANOTUBES

Abstract

A method for cleaning carbon nanotubes, comprising: (1) Treating carbon nanotubes with an inert gas at a treatment temperature of 1600°C to 1800°C in a low vacuum with a reaction pressure of 13.3 Pascal (0.1 Torr) to 133 Pascal (1 Torr) in a reaction vessel, wherein the inert gas is used in an amount of 0.0025 to 0.25 times the volume of the reaction vessel per minute - , and (2) Obtaining ultrapure carbon nanotubes, wherein the ultrapure carbon nanotubes contain 50 ppm or less of each metal remaining therein.

Description

[0001] This application claims the benefit of Korean Patent Application No. 10-2016-0114409, filed on September 6, 2016, entitled “Method for purifying carbon nanotubes,” which is hereby incorporated by reference in its entirety into this application. BACKGROUND OF THE INVENTION 1. Technical field

[0002] The present invention relates to a method for purifying carbon nanotubes, particularly to a method for removing metal impurities remaining in the carbon nanotubes by treating carbon nanotubes with an inert gas at a high temperature in a low vacuum, and to ultrapure carbon nanotubes. 2. Description of the state of the art

[0003] Carbon nanotubes (CNTs) are a carbon allotrope composed of carbon in which one carbon atom is coupled with other carbon atoms in a hexagonal honeycomb shape to form a tube, with the diameter of the tube being very small, on the nanometer scale (nm = one billionth of a meter).

[0004] Carbon nanotubes, which possess highly desirable properties, exhibit a specific structure characterized by tube diameter, symmetry, layered structure, bundle structure, bond deformation, and the presence of impurities. They can be practically used in various fields of nanotechnology, electrical engineering, optical engineering, and materials engineering, among others. In particular, carbon nanotubes are known to be a new material exhibiting unusual field emission properties, properties as a high-performance hydrogen storage medium, and so on.

[0005] Nanotubes have a fullerene-based structure with a long, hollow tube structure. A membrane called graphene, consisting of a monolayer of carbon atoms, acts as the wall, hence the term carbon nanotubes. Depending on the angle at which the graphene is rolled up, different types of nanotubes can be formed. Depending on the rolling angle and diameter, they can take on the properties of a metal or semiconductor. The basic structure of nanotubes can be divided into single-walled nanotubes (SWNTs) and multi-walled nanotubes (MWNTs).

[0006] Carbon nanotube composites are excellent for conductive materials, high-strength lightweight structural materials, multifunctional composite materials, and the like. In the field of carbon nanotubes, extensive research is being conducted on the preparation of carbon nanotubes, their modification, various properties of carbon nanotube composites, and their application areas. Examples of application areas include electron emission sources for various devices, VFDs (vacuum fluorescent displays), white light sources, FEDs (field emission displays), lithium-ion battery electrodes, hydrogen storage fuel cells, nanowires, nanocapsules, nanotweezers, AFM / STM tips, single-electron devices, gas sensors, medical and engineering microparts, high-function composite materials, and so on.For industrial applications of carbon nanotubes, it is important to economically synthesize and purify carbon nanotubes with excellent properties through advanced synthesis methods.

[0007] Examples of methods for synthesizing carbon nanotubes typically include arc discharge, laser ablation, chemical vapor deposition (CVD), and the like. Arc discharge and laser ablation can improve the purity of carbon nanotubes, but they have the disadvantages of difficult mass production and expensive equipment. In CVD, a catalyzed CVD process allows for easy control of the diameter, length, density, structure, and crystallinity of carbon nanotubes, enabling the mass production of high-purity products. A metal catalyst mainly includes an alumina or silica catalyst supported with Fe, Co, Ni, or Mo.

[0008] Carbon nanotubes synthesized through a catalytic process can contain various nanocarbon materials, amorphous carbon, and transition metals that act as catalysts for their growth. Due to these impurities, carbon nanotubes are cleaved, thus reducing their length, or their surface is oxidized, thus deteriorating their electrical and mechanical properties, which is undesirable. Consequently, such impurities can impair the inherent properties of carbon nanotubes and thus limit their optimal performance in their application areas.

[0009] In particular, in the case of carbon nanotubes used in battery electrodes, the metal can rupture the battery separator and thus cause an explosion. Therefore, the metal content is limited to the order of ppm.

[0010] Various cleaning processes for removing transition metals and carbonaceous materials have been described, which can essentially be divided into chemical and physical processes.

[0011] Examples of chemical purification methods include vapor oxidation methods and liquid oxidation methods. Examples of the gas oxidant include air, Cl2, HCl, SF6, and H2S, and examples of the liquid oxidant include nitric acid, hydrogen peroxide, hydrochloric acid, sulfuric acid, and phosphoric acid. The use of nitric acid, sulfuric acid, or hydrochloric acid is economically efficient, but the metal catalyst encapsulated in the tube is difficult to remove, making it impossible to obtain ultrapure carbon nanotubes. US Pat. No. 8,628,748 B2 discloses a method for purifying carbon nanotubes by heat treatment using a halogen gas and / or a halogen compound, and US Pat. No. 6,752,977 B2 discloses a method for heat treating carbon nanotubes at 200 to 500°C using a halogen-containing gas.However, in this case, the halogen-containing compound used is problematic due to the cumbersome handling of the halogen gas, environmental risks and high investment costs.

[0012] Examples of physical purification methods include ultrasonic treatment, high-temperature annealing, supercritical CO2 fluid extraction, and the like. Among these, the high-temperature annealing process is known to induce graphitization, thus reducing the occurrence of chemical surface defects and effectively removing metal. Andrews et al. reported the purification of multi-walled carbon nanotubes (MWCNTs) by treating them at 3000 °C for 45 min in a nitrogen gas atmosphere at slightly superatmospheric pressure, so that the Fe content is limited to 100 ppm or less (Carbon 39 (2001) 1681-1687), and Chen et al. reported the purification of MWCNTs by treating them in an argon gas atmosphere at 2000 °C or more, so that the Fe content is limited to 100 ppm or less (Carbon 45 (2007) 274-280). Based on the research results of Huang et al. It was described that MWCNTs under conditions of 10 -3Pa to 10 Pa and 2000 °C or more for 5 h to remove Al2O3 and Fe-Mo (Carbon 41 (2003) 2585-2590). Thus, the reaction pressure required by Huang et al. must be very low. In CN 1 436 722 A, 86% MWCNTs are treated at 2300 °C for 5 h at a reduced pressure of 20 Pa to yield high-purity carbon nanotubes with a purity of 99.93% and a transition metal content of 0.05% or less. CN 1 436 722 A requires a very low reaction pressure and a very high reaction temperature. As described above, the high-temperature annealing processes described so far are mostly carried out under harsh conditions, including a high temperature of 2000 °C or more and a very deep vacuum, i.e., a high vacuum, which undesirably increases the manufacturing costs.

[0013] Furthermore, conventional methods may involve the use of halogen gas or oxygen gas to remove metal impurities. Although the halogen gas oxidation process is very effective for purifying carbon nanotubes, it has the disadvantages of long processing times, poor process stability due to the toxic gas, and the disposal of impurities and byproducts generated during gas treatment.

[0014] Furthermore, among metal impurities, Fe is known to be the most difficult to remove. Conventional methods for reducing the Fe content to 10 ppm or less are known to reduce the dispersibility of carbon nanotubes.

[0015] Carbon nanotubes are difficult to disperse due to their large length relative to their diameter and their strong intermolecular forces. Since the total surface area of ​​carbon nanotubes increases depending on their degree of dispersion, even when added in the same amount, their physical, electrical, and thermal properties can vary greatly. Thus, dispersibility is a very important factor in the application of carbon nanotubes. In particular, in battery applications, low dispersibility of carbon nanotubes can compromise their electrical conductivity.

[0016] In-depth research is being conducted on methods for removing impurities from carbon nanotubes, but methods suitable for the economical removal of metal impurities, especially Fe, to the order of a few ppm and ensuring the dispersibility of carbon nanotubes are still insufficient.

[0017] JP 2005 / 132701 A discloses purification processes for halogenated carbon nanomaterials. Antunes et al. (Applied Surface Science 257 (2011) 8038-8043) discloses an analysis method for carbon nanotubes prepared by camphor / ferrocene pyrolysis and purified by high-temperature annealing. CN 103 189308 A discloses manufacturing processes for modified carbon nanotubes. JP 2011 / 063458 A discloses carbon nanopowder formed by aggregation of oriented carbon nanotubes. SUMMARY OF THE INVENTION

[0018] Accordingly, one aspect of the present invention is to provide a method for purifying carbon nanotubes which, unlike conventional methods for purifying carbon nanotubes, is highly effective even without using harsh conditions of high temperature and high vacuum or using a halogen-containing gas to remove a metal catalyst or metal oxide.

[0019] Another aspect of the present invention is to provide ultrapure carbon nanotubes produced by the above-mentioned method for purifying carbon nanotubes.

[0020] Another aspect of the present invention is to provide ultrapure carbon nanotubes containing 50 ppm or less of each metal impurity.

[0021] Another aspect of the present invention is to provide ultrapure carbon nanotubes containing 50 ppm or less of Fe.

[0022] A first aspect of the present invention provides a method for purifying carbon nanotubes, comprising (1) treating carbon nanotubes with an inert gas at a treatment temperature of 1600°C to 1800°C or less in a low vacuum at a reaction pressure of 0.1 Torr to 1 Torr in a reaction vessel, wherein the inert gas is used in an amount of 0.0025 to 0.25 times the volume of the reaction vessel per minute, and (2) obtaining ultrapure carbon nanotubes, wherein the ultrapure carbon nanotubes contain 50 ppm or less of each metal remaining therein.

[0023] In an embodiment according to the first aspect, if the metal remaining in the ultrapure carbon nanotubes is Fe, Fe is contained in an amount of 10 ppm or less.

[0024] The low vacuum to a reaction pressure is set in the range of 0.1 Torr to 1 Torr.

[0025] The treatment temperature is in the range of 1600 °C to 1800 °C.

[0026] The inert gas is used in an amount of 0.0025 to 0.25 times the volume of the reaction vessel per minute.

[0027] In an embodiment according to the first aspect, the treatment of the carbon nanotubes is carried out for 15 min to 120 min.

[0028] In an embodiment according to the first aspect, the remaining metal comprises Fe, Co, Al2O3, Mg and a combination thereof.

[0029] Furthermore, carbon nanotubes produced by the above-mentioned method contain 50 ppm or less of each metal remaining therein.

[0030] If the metal remaining in the ultrapure carbon nanotubes is Fe, Fe is present in an amount of 10 ppm or less.

[0031] Furthermore, the carbon nanotubes have an electrical resistance of 1.0 × 10 2 up to 5.0 × 10 2 Ω / sq.

[0032] In the present invention, only carbon nanotubes are treated with an inert gas at a high temperature in a low vacuum, whereby metal impurities can be effectively removed therefrom, and the obtained ultrapure carbon nanotubes contain 50 ppm or less of each metal impurity and have high dispersibility. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 shows a CNT purification process; Fig. Figure 2 shows the purity (%) of CNTs depending on the treatment temperature; Fig. Figure 3 shows the metal content (ppm) remaining in CNTs depending on the treatment temperature; Fig. Figure 4 shows the purity (%) of CNTs depending on the treatment time; Fig. Figure 5 shows the metal content (ppm) remaining in CNTs depending on the treatment time; Fig. Figure 6 shows the purity (%) of CNTs depending on the flow rate of N2; Fig. Figure 7 shows the metal content (ppm) depending on the flow rate of N2; Fig. Figure 8 shows the electrical resistance of CNTs depending on the treatment temperature; Fig. 9 is a TEM image of CNTs treated at 1800 °C; and Fig. Figure 10 is a TEM image of CNTs treated at 2500 °C. DESCRIPTION OF SPECIFIC EMBODIMENTS

[0033] Embodiments of the present invention can be understood from the following description. The following description is intended to explain specific embodiments of the present invention, but the present invention is not necessarily limited thereto. The attached drawings are provided for illustration, and the present invention is not limited thereto. Details of the individual components can be clearly understood from the specific effects of the corresponding description given below.

[0034] The terms used herein may be defined as follows.

[0035] The term “ultrapure carbon nanotubes” may refer to carbon nanotubes designed such that the amount of any metal impurity remaining therein is 50 ppm or less.

[0036] The term “low vacuum” refers to a vacuum of 1 Torr or less.

[0037] The term “carbon nanotube residual metal” can refer to any impurity incorporated during the synthesis of carbon nanotubes, mainly a metal used as a catalyst.

[0038] The term “metal vapor pressure” can refer to a vapor pressure when a vapor is in dynamic equilibrium with the solid or liquid at a predetermined pressure and temperature.

[0039] One aspect of the present invention relates to a method for purifying carbon nanotubes, comprising treating carbon nanotubes with an inert gas under high temperature and low vacuum conditions in a reaction vessel to obtain ultrapure carbon nanotubes, wherein the ultrapure carbon nanotubes contain 50 ppm or less of each metal remaining therein.

[0040] In the present invention, the removal of metal impurities can be carried out by evaporating metal impurities, whereby the metal content can be limited to the order of a few ppm compared to other purification methods such as acid treatment, etc.

[0041] The removal of metal impurities by evaporation is conventionally performed by oxidizing the metal using a halogen gas and then evaporating it, or by a high-temperature annealing process involving evaporation at a high temperature in a high vacuum. Typically, a halogen gas of 900 to 1400 °C is used. However, the use of halogen gas can lead to problems with process stability, high manufacturing costs, and a long processing time. Furthermore, the high-temperature annealing process is performed by evaporation at a high temperature in a high vacuum under harsh conditions, which undesirably leads to high manufacturing costs.

[0042] In the present invention, metal impurities can be effectively removed even without the use of halogen gas and harsh conditions.

[0043] Fig.1 shows a CNT purification process according to the present invention.

[0044] Raw carbon nanotubes are loaded into a vacuum heating device equipped with a high-temperature furnace and a vacuum pump, and then treated at high temperature in a low vacuum with nitrogen flow, yielding ultrapure carbon nanotubes. The removed impurities are collected by a filter and scrubber.

[0045] In the present invention, the CNT purification process for removing metal impurities is carried out in a manner in which metal is evaporated into a gas, after which the evaporated metal impurities are removed.

[0046] It is known that metal evaporation requires high temperatures and a high vacuum. However, if the temperature is increased above 1800 °C, the resulting metal vapor pressure can increase, while the dispersibility of the purified carbon nanotubes can decrease due to graphitization. Furthermore, a high vacuum requires large facilities and is problematically uneconomical.

[0047] In the present invention, metal impurities can be evaporated even without the use of a reactive gas such as halogen or oxygen gas, and even without harsh conditions such as ultra-high temperature and ultra-high vacuum, whereby the metal impurities can be effectively removed using the inert gas.

[0048] According to the present invention, the reaction pressure can be 0.1 Torr to 1 Torr when the metal impurities in the carbon nanotubes are Fe, Co, Al2O3, and Mg. The case where the reaction pressure increases is undesirable because it impairs the evaporation of the metal to be removed.

[0049] The method according to the present invention comprises treating the carbon nanotubes with an inert gas in a reaction vessel at a high temperature under a low vacuum. The treatment temperature is 1600 to 1800°C. If the temperature is less than 1400°C, metal removal is difficult. On the other hand, if the temperature is higher than 1800°C, dispersibility may decrease and production costs may increase. The inert gas is not particularly limited, but can be nitrogen.

[0050] In one embodiment of the present invention, the amount of inert gas is closely related to the reaction pressure, so the amount of inert gas must be adjusted within a range that does not affect the low vacuum according to the present invention. In one embodiment, the vacuum level may drop if the inert gas is flowed in a large amount using the same pump. If the inert gas is flowed in a large amount, a high pump capacity is required to maintain a certain vacuum, thereby affecting the manufacturing cost.

[0051] In one embodiment, when the inert gas is introduced into a reaction vessel with a volume of 22 L at 2 L / min and 4 L / min, the amount of inert gas is 0.09 times and 0.18 times the volume of the reaction vessel per minute, respectively. The amount of inert gas is 0.0025 to 0.25 times the volume of the reaction vessel per minute. If its amount is less than 0.0025 times, metal cannot be removed effectively. On the other hand, if the amount exceeds 0.25 times, the pump capacity may need to be increased, and carbon nanotubes may be lost.

[0052] In one embodiment of the present invention, the treatment time with the inert gas may be 15 minutes or more. If the treatment time is less than 15 minutes, metal cannot be effectively removed.

[0053] In the treatment of metal contaminants according to the present invention, the low vacuum may be a pressure lower than the vapor pressure of the metal remaining in the carbon nanotubes, but is not limited thereto. Examples of the metal remaining in the carbon nanotubes may include Fe, Co, Al2O3, Mg, and combinations thereof, but the present invention is not limited thereto.

[0054] When the metal impurities in the carbon nanotubes are Fe, Co, Al2O3 and Mg, the reaction pressure can be 1 Torr or less, especially 0.1 to 1 Torr.

[0055] In the present invention, the purified carbon nanotubes may have a purity of 99% or more, and the amount of each metal impurity contained therein may be 50 ppm or less.

[0056] In the present invention, the purified ultrapure carbon nanotubes can exhibit high dispersibility and can be safely used when applied in batteries with a long cycle life.

[0057] In the present invention, the purified carbon nanotubes can have an electrical resistance of 1.0 × 10 2 up to 5.0 × 10 2Ω / sq and thus exhibit high dispersibility. The electrical resistance of carbon nanotubes is determined by measuring electrical resistance values ​​at five points (top, bottom, left, right, and center) on the carbon nanotubes using an electrical resistivity meter and then averaging them. The electrical resistance can vary depending on the extent of dispersion of the carbon nanotubes. If the carbon nanotubes are not well dispersed, the electrical resistance will be high or cannot be measured. ExamplesProcesses for cleaning carbon nanotubes

[0058] The carbon nanotubes used for testing were manufactured using a catalyzed CVD process. After cleaning, the amounts of metal components remaining in the carbon nanotubes were measured by ICP-OES (inductively coupled plasma optical emission spectroscopy, Agilent). The ICP pretreatment was performed by carbonizing the carbon nanotubes with sulfuric acid and then ashing them in a furnace at 800 °C.

[0059] The procedure for cleaning carbon nanotubes is as follows. (1) Carbon nanotubes to be purified are placed in a graphite crucible and loaded into a high-temperature vacuum furnace. (2) When a vacuum of 1 Torr or less is maintained, a predetermined amount of inert gas is allowed to flow. (3) The temperature is increased to the treatment temperature, after which the treatment is carried out for 15 to 120 minutes. (4) The temperature is lowered to room temperature, after which the vacuum is released and the sample is removed. [Example 1]

[0060] 23 g of carbon nanotubes (80 to 85% purity) were treated for 120 min at 1800 °C, 0.5 to 1 Torr, and 2 l / min of nitrogen. The reaction vessel volume was 22 l. The metal components of the purified carbon nanotubes were analyzed by ICP. The results of residual metal content and metal removal are shown in Table 1 below. [Comparison example 1]

[0061] The removal of metal impurities was carried out in the same manner as in Example 1, except that no nitrogen treatment was performed. The results of residual metal content and metal removal are shown in Table 1 below. [Comparison example 2]

[0062] The removal of metal impurities was carried out in the same manner as in Example 1, except that the reaction pressure was set to 2 Torr instead of 0.5 to 1 Torr. The results of residual metal content and metal removal are shown in Table 1 below. [Comparison example 3]

[0063] The removal of metal impurities was carried out in the same manner as in Example 1, except that the reaction pressure was set to 3 Torr instead of 0.5 to 1 Torr. The results of residual metal content and metal removal are shown in Table 1 below.

[0064] The results of metal impurity removal of Example 1 and Comparative Examples 1 to 3 are shown in Table 1 below. [Table 1] Purity(%) Metal content (ppm) Metal removal (%) Al2O3 Fe Co Mg Al2O3 Fe Co Mg Raw CNTs 84,3 81140,0 3170,0 1580,0 150,0 - - - - Example 1 99,9 0,0 6,5 0,0 0,0 100,0 99,8 100,0 100,0 Comparison example 1 99,7 0,0 86,0 14,0 0,0 100,0 97,3 99,1 100,0 Comparison example 2 99,7 150 5 0 0 99,8 99,8 100,0 100,0 Comparison example 3 99,6 90 10 0 0 99,9 99,7 100,0 100,0

[0065] As can be seen from the results in Table 1, Al2O3 and Mg were completely removed by nitrogen gas treatment and by nitrogen-free treatment, while Fe and Co were not as completely removed by nitrogen-free treatment as by nitrogen treatment.

[0066] In Comparative Examples 2 and 3, where the reaction pressure exceeded 1 Torr, the corresponding purities of the carbon nanotubes were 99.7% and 99.6%, respectively, indicating that the impurity removal effect was lower than at a reaction pressure of 1 Torr or less. In particular, at high reaction pressure, Al2O3 and Fe were not completely removed but remained. [Example 2]

[0067] Raw carbon nanotubes were treated for 90 min at various treatment temperatures under conditions of 2 l / min nitrogen and a reaction pressure of 0.5 to 1 Torr. The purity values ​​of raw carbon nanotubes were measured depending on the treatment temperature. The results are presented in Fig. 2. The amounts of metal impurities such as Al2O3, Fe, Co, and Mg were measured as a function of treatment temperature. The results are shown in Fig. 3 shown.

[0068] As in Fig. As shown in Figure 2, the purity of the carbon nanotubes increased sharply with increasing treatment temperature from 1300 °C to 1400 °C, while it increased less sharply in the temperature range higher than 1600 °C.

[0069] As in Fig. As shown in Figure 3, the amounts of metal impurities remaining in the carbon nanotubes decreased significantly up to 1400 °C, depending on the treatment temperature. Al2O3 was almost completely removed at 1600 °C, which is slightly higher than the temperature at which Fe, Co, and Mg were removed. [Example 3]

[0070] Raw carbon nanotubes were treated with different treatment times under conditions of 2 l / min nitrogen, a reaction pressure of 0.5 to 1 Torr, and a temperature of 1700 °C. The purity values ​​of raw carbon nanotubes were measured depending on the treatment temperature. The results are presented in Fig.4. The amounts of metal impurities such as Al2O3, Fe, Co, and Mg were measured depending on the treatment time. The results are shown in Fig. 5 shown.

[0071] As in Fig. 4, the purity of carbon nanotubes increased with increasing treatment time. As shown in Fig. As shown in Figure 5, the amounts of metal impurities remaining in the carbon nanotubes also decreased significantly depending on the treatment time. [Example 4]

[0072] Raw carbon nanotubes were treated with different nitrogen flow rates at 1700 °C for 15 min. The purity values ​​of raw carbon nanotubes were measured as a function of the nitrogen flow rate. The results are presented in Fig. 6. The amounts of metal impurities such as Al2O3, Fe, Co, and Mg were measured as a function of the treatment flow rate. The results are shown in Fig. 7 shown.

[0073] As in Fig. 6 and Fig. 7, which shows the purity of the carbon nanotubes depending on the nitrogen treatment flow rate, at a nitrogen flow rate of 2 l / min (reactor volume 22 l), the purity of the carbon nanotubes was 99.3% and the total metal content was 180 ppm, while at a nitrogen flow rate of 4 l / min (reactor volume 22 l), the purity of the carbon nanotubes was also 99.9% and the total metal content was reduced to 10 ppm or less. [Example 5]

[0074] Electrical resistance and dispersibility of carbon nanotubes Ten mg of the carbon nanotubes treated at each of the treatment temperatures of 1600 °C, 1700 °C, 1800 °C, and 2500 °C were dispersed in 10 g of a 2 wt% SDS (sodium dodecyl sulfate) aqueous solution using an ultrasonic device for 5 min, filtered with a 16 mm diameter filter paper, and dried at room temperature for 2 h. Afterward, the electrical resistance values ​​were measured and averaged at five points (top, bottom, left, right, and center) of the carbon nanotubes using an electrical resistivity meter. Fig. Figure 8 shows the electrical resistance values ​​of carbon nanotubes depending on the treatment temperature.

[0075] As in Fig.As shown in Figure 8, the carbon nanotubes treated at 1600 °C and 1700 °C showed similar electrical resistance values ​​and good dispersibility, but the carbon nanotubes treated at 2500 °C showed a greatly increased electrical resistance and were thus determined to be poorly dispersed. When the electrical resistance was in the range of 1.0 × 10 2 up to 5.0 × 10 2 Ω / sq, it is assumed that good dispersion has occurred.

[0076] Fig. 9 and Fig. Figure 10 shows TEM images of the carbon nanotubes treated at 1800 °C and 2500 °C, respectively.

[0077] It has been reported that carbon nanotubes actively undergo graphitization when annealed at 1800 to 2200 °C in an argon gas atmosphere at slightly above atmospheric pressure (Chen et al. 2007). Graphitization is known to reduce the occurrence of chemical surface defects on carbon nanotubes in conventional processes, but their dispersibility has not been investigated.

[0078] According to the TEM analysis of Fig. 9 and Fig. 10, the carbon nanotubes treated at 2500 °C exhibited numerous curved structures, while the structure of the carbon nanotubes treated at 1800 °C was comparatively smooth. In the present invention, graphitization occurred more actively during high-temperature treatment, but the growth layers of the carbon nanotubes were curved and intertwined, which impaired dispersibility.

[0079] The embodiments of the present invention have been disclosed for illustrative purposes, but it will be apparent to those skilled in the art that various modifications, additions and substitutions are possible without departing from the scope and spirit of the invention as disclosed in the accompanying claims.

Claims

[1] A method for cleaning carbon nanotubes, comprising: (1) Treating carbon nanotubes with an inert gas at a treatment temperature of 1600°C to 1800°C in a low vacuum with a reaction pressure of 13.3 Pascal (0.1 Torr) to 133 Pascal (1 Torr) in a reaction vessel, wherein the inert gas is used in an amount of 0.0025 to 0.25 times the volume of the reaction vessel per minute - , and (2) Obtaining ultrapure carbon nanotubes, wherein the ultrapure carbon nanotubes contain 50 ppm or less of each metal remaining therein. [2] The method according to claim 1, wherein Fe is present in an amount of 10 ppm or less when the metal remaining in the ultrapure carbon nanotubes is Fe. [3] The method according to claim 1, wherein the treatment of the carbon nanotubes is carried out for 15 min to 120 min. [4] The process according to claim 1, wherein the remaining metal comprises Fe, Co, Al2O3, Mg or a combination thereof. [5] Ultrapure carbon nanotubes produced by the method according to claim 1.

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