Method of purifying carbon nanotubes

Abstract

Disclosed is a method of purifying carbon nanotubes, including treating carbon nanotubes with an inert gas at a high temperature in a low vacuum in a reactor and obtaining ultrapure carbon nanotubes, wherein the ultrapure carbon nanotubes contain 50 ppm or less of each metal remaining therein.

Claims

1. A method of purifying carbon nanotubes, comprising: (1) treating carbon nanotubes with an inert gas at a treatment temperature of 1800 C. or less in a low vacuum at a reaction pressure of 0.1 Torr or more in a reactor; and (2) obtaining ultrapure carbon nanotubes, wherein the ultrapure carbon nanotubes contain 50 ppm or less of each metal remaining therein.

2. The method of 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 of claim 1, wherein the low vacuum is set at a reaction pressure ranging from 0.1 Torr to 1 Torr.

4. The method of claim 1, wherein the treatment temperature ranges from 1600 C. to 1800 C.

5. The method of claim 1, wherein the inert gas is used in an amount of 0.0025 to 0.25 times a volume of the reactor per minute.

6. The method of claim 1, wherein the treating the carbon nanotubes is performed for 15 min to 120 min.

7. The method of claim 1, wherein the remaining metal includes Fe, Co, Al.sub.2O.sub.3, Mg or a combination thereof.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows a CNT purification process;

(2) FIG. 2 shows the purity (%) of CNTs depending on the treatment temperature;

(3) FIG. 3 shows the metal content (ppm) remaining in CNTs depending on the treatment temperature;

(4) FIG. 4 shows the purity (%) of CNTs depending on the treatment time;

(5) FIG. 5 shows the metal content (ppm) remaining in CNTs depending on the treatment time;

(6) FIG. 6 shows the purity (%) of CNTs depending on the flow rate of N.sub.2;

(7) FIG. 7 shows the metal content (ppm) depending on the flow rate of N.sub.2;

(8) FIG. 8 shows the electrical resistance of CNTs depending on the treatment temperature;

(9) FIG. 9 is a TEM image of CNTs treated at 1800 C.; and

(10) FIG. 10 is a TEM image of CNTs treated at 2500 C.

DESCRIPTION OF SPECIFIC EMBODIMENTS

(11) Exemplary embodiments of the present invention may be understood through the following description. The following description should be understood to explain specific embodiments of the present invention, and the present invention is not necessarily limited thereto. Furthermore, the appended drawings are provided for clarity, and the present invention is not limited thereto, and details of the individual components thereof may be clearly understood through the specific effects of the relevant description, which will be described later.

(12) The terms used herein may be defined as follows.

(13) The term ultrapure carbon nanotubes may refer to carbon nanotubes configured such that the amount of each metal impurity remaining therein is 50 ppm or less.

(14) The term low vacuum refers to a vacuum of 1 Torr or less.

(15) The term metal remaining in carbon nanotubes may refer to any impurity incorporated during the synthesis of carbon nanotubes, and is mainly a metal used as the catalyst.

(16) The term metal vapor pressure may refer to a vapor pressure when a vapor is in dynamic equilibrium with the solid or liquid at predetermined pressure and temperature.

(17) An aspect of the present invention addresses a method of purifying carbon nanotubes, including treating carbon nanotubes with an inert gas under conditions of a high temperature and a low vacuum in a reactor and obtaining ultrapure carbon nanotubes, wherein the ultrapure carbon nanotubes contain 50 ppm or less of each metal remaining therein.

(18) In the present invention, the removal of metal impurities may be performed by vaporizing metal impurities, whereby the metal content may be controlled to the order of ones of ppm, compared to other purification methods such as acid treatment, etc.

(19) Removing the metal impurities through vaporization is conventionally performed in a manner in which metal is oxidized using halogen gas and then vaporized or through a high-temperature annealing process including vaporization at a high temperature in a high vacuum. Here, halogen gas is typically used at 900 to 1400 C. However, the use of halogen gas may cause problems related to processing stability, high manufacturing costs, and a long treatment time. Moreover, the high-temperature annealing process through vaporization at a high temperature in a high vacuum is conducted under harsh conditions, undesirably leading to high manufacturing costs.

(20) In the present invention, metal impurities may be effectively removed even without the use of halogen gas and harsh conditions.

(21) FIG. 1 shows a CNT purification process according to the present invention.

(22) Raw carbon nanotubes are loaded in a vacuum heating unit equipped with a high-temperature furnace and a vacuum pump and then treated at a high temperature in a low vacuum while allowing nitrogen to flow, after which ultrapure carbon nanotubes are obtained. The removed impurities are collected by a filter and a scrubber.

(23) In the present invention, the CNT purification process for removing metal impurities is carried out in a manner in which metal is vaporized into gas, after which the vaporized metal impurities are removed.

(24) Vaporizing the metal is known to require a high temperature and a high vacuum. However, when the temperature is increased higher than 1800 C., the generation of metal vapor pressure may increase, but the dispersibility of purified carbon nanotubes may decrease owing to graphitization. Furthermore, a high vacuum requires bulky equipment and is problematically uneconomical.

(25) In the present invention, metal impurities may be vaporized even without the use of reactive gas such as halogen or oxygen gas and even without harsh conditions such as an ultrahigh temperature and an ultrahigh vacuum, whereby the metal impurities may be effectively removed using the inert gas.

(26) According to the present invention, when metal impurities in the carbon nanotubes are Fe, Co, Al.sub.2O.sub.3 and Mg, the reaction pressure may be 1 Torr or less, and particularly 0.1 to 1 Torr. The case where the reaction pressure increases is undesirable because it affects the vaporization of metal to be removed.

(27) The method of the present invention includes treating the carbon nanotubes with the inert gas in a reactor at a high temperature in a low vacuum. The treatment temperature is 1400 C. or more, and particularly 1600 to 1800 C. If the temperature is less than 1400 C., it is difficult to remove metal. On the other hand, if the temperature is higher than 1800 C., dispersibility may decrease and manufacturing costs may increase. The inert gas is not particularly limited, but may be nitrogen.

(28) According to an embodiment of the present invention, the amount of the inert gas is closely associated with the reaction pressure, and thus the amount of the inert gas has to be set within a range that does not impair the low vacuum of the present invention. In an embodiment, when the inert gas is allowed to flow in a large amount using the same pump, the vacuum level may decrease. Also, when the inert gas is allowed to flow in a large amount, a large pump capacity is required in order to maintain a certain vacuum, which may affect manufacturing costs.

(29) In an exemplary embodiment, when the inert gas is allowed to flow at 2 L/min and 4 L/min into a reactor having a volume of 22 L, the amount of the inert gas is 0.09 times and 0.18 times the volume of the reactor per minute, respectively. Particularly, the amount of the inert gas may be 0.0025 to 0.25 times the volume of the reactor per minute. If the amount thereof is less than 0.0025 times, metal may not be efficiently removed. On the other hand, if the amount thereof exceeds 0.25 times, the capacity of the pump may need to be increased, and carbon nanotubes may be lost.

(30) In an embodiment of the present invention, the treatment time with the inert gas may be 15 min or more. If the treatment time is less than 15 min, metal may not be efficiently removed.

(31) In the treatment of metal impurities according to the present invention, the low vacuum may be, but is not limited to, a pressure lower than the vapor pressure of the metal remaining in the carbon nanotubes. Examples of the metal remaining in the carbon nanotubes may include Fe, Co, Al.sub.2O.sub.3, Mg and combinations thereof, but the present invention is not limited thereto.

(32) When the metal impurities in the carbon nanotubes are Fe, Co, Al.sub.2O.sub.3 and Mg, the reaction pressure may be 1 Torr or less, and particularly 0.1 to 1 Torr.

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

(34) According to the present invention, the purified ultrapure carbon nanotubes may exhibit high dispersibility, and may be safely used for a long lifetime when applied to batteries.

(35) According to the present invention, the purified carbon nanotubes may have an electrical resistance of 1.010.sup.2 to 5.010.sup.2 /sq and thus high dispersibility. The electrical resistance of carbon nanotubes is determined in a manner in which electrical resistance values at five points (top, bottom, left, right, and center) of the carbon nanotubes are measured using an electrical resistance meter and then averaged. The electrical resistance may vary depending on the extent of dispersion of carbon nanotubes. In the case where the carbon nanotubes are not dispersed well, electrical resistance is high or cannot be measured.

EXAMPLES

(36) Method of Purifying Carbon Nanotubes

(37) The carbon nanotubes used for testing were prepared using a catalyst CVD process. After purification treatment, the amounts of metal components remaining in the carbon nanotubes were analyzed using ICP-OES (Inductively Coupled Plasma-Optical Emission Spectroscopy, Agilent). ICP pretreatment was performed in a manner in which carbon nanotubes were carbonized with sulfuric acid and then ashed in a furnace at 800 C.

(38) The method of purifying carbon nanotubes is as follows.

(39) (1) Carbon nanotubes to be purified are placed in a graphite crucible and loaded in a high-temperature vacuum-firing furnace.

(40) (2) When a vacuum of 1 Torr or less is maintained, a predetermined amount of inert gas is allowed to flow.

(41) (3) The temperature is increased to the treatment temperature, after which treatment is performed for 15 to 120 min.

(42) (4) The temperature is cooled to room temperature, after which the vacuum is removed and the sample is unloaded.

Example 1

(43) 23 g of carbon nanotubes (purity of 80 to 85%) were treated under conditions of 1800 C., 0.5 to 1 Torr, and nitrogen of 2 L/min for 120 min, and the volume of the reactor was 22 L. The metal components of the purified carbon nanotubes were analyzed using ICP. The results of residual metal content and metal removal are shown in Table 1 below.

Comparative Example 1

(44) The removal of metal impurities was performed in the same manner as in Example 1, with the exception that nitrogen treatment was not conducted. The results of residual metal content and metal removal are shown in Table 1 below.

Comparative Example 2

(45) The removal of metal impurities was performed in the same manner as in Example 1, with the exception that the reaction pressure was set to 2 Torr in lieu of 0.5 to 1 Torr. The results of residual metal content and metal removal are shown in Table 1 below.

Comparative Example 3

(46) The removal of metal impurities was performed in the same manner as in Example 1, with the exception that the reaction pressure was set to 3 Torr in lieu of 0.5 to 1 Torr. The results of residual metal content and metal removal are shown in Table 1 below.

(47) The results of removal of metal impurities of Example 1 and Comparative Examples 1 to 3 are given in Table 1 below.

(48) TABLE-US-00001 TABLE 1 Metal content (ppm) Metal removal (%) Purity (%) Al.sub.2O.sub.3 Fe Co Mg Al.sub.2O.sub.3 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 Comparative 99.7 0.0 86.0 14.0 0.0 100.0 97.3 99.1 100.0 Example 1 Comparative 99.7 150 5 0 0 99.8 99.8 100.0 100.0 Example 2 Comparative 99.6 90 10 0 0 99.9 99.7 100.0 100.0 Example 3

(49) As is apparent from the results of Table 1, Al.sub.2O.sub.3 or Mg was completely removed upon nitrogen gas treatment or upon nitrogen-free treatment, but Fe or Co was not effectively removed upon nitrogen-free treatment, as in the nitrogen treatment.

(50) In Comparative Examples 2 and 3, in which the reaction pressure exceeded 1 Torr, the respective purities of carbon nanotubes were 99.7% and 99.6%, which means that the effect of removing impurities was low compared to when the reaction pressure was 1 Torr or less. In particular, when the reaction pressure was high, Al.sub.2O.sub.3 or Fe was not completely removed but was left behind.

Example 2

(51) Raw carbon nanotubes were treated for 90 min at different treatment temperatures under conditions of nitrogen of 2 L/min and a reaction pressure of 0.5 to 1 Torr. The purity values of raw carbon nanotubes depending on the treatment temperature were measured. The results are shown in FIG. 2. The amounts of metal impurities such as Al.sub.2O.sub.3, Fe, Co and Mg depending on the treatment temperature were measured. The results are shown in FIG. 3.

(52) As shown in FIG. 2, the purity of carbon nanotubes was increased with an increase in the treatment temperature but was drastically increased at 1300 C. to 1400 C. and then gently increased in a temperature range higher than 1600 C.

(53) As shown in FIG. 3, the amounts of metal impurities remaining in the carbon nanotubes depending on the treatment temperature were drastically decreased up to 1400 C. Al.sub.2O.sub.3 was almost completely removed at 1600 C., which is slightly higher than the temperature at which Fe, Co and Mg were removed.

Example 3

(54) Raw carbon nanotubes were treated for different treatment times under conditions of nitrogen of 2 L/min, a reaction pressure of 0.5 to 1 Torr and a temperature of 1700 C. The purity values of raw carbon nanotubes depending on the treatment time were measured. The results are shown in FIG. 4. The amounts of metal impurities such as Al.sub.2O.sub.3, Fe, Co and Mg depending on the treatment time were measured. The results are shown in FIG. 5.

(55) As shown in FIG. 4, the purity of carbon nanotubes was increased with an increase in the treatment time. Also, as shown in FIG. 5, the amounts of metal impurities remaining in the carbon nanotubes depending on the treatment time were drastically decreased.

Example 4

(56) Raw carbon nanotubes were treated at different nitrogen flow rates at 1700 C. for 15 min. The purity values of raw carbon nanotubes depending on the nitrogen flow rate were measured. The results are shown in FIG. 6. The amounts of metal impurities such as Al.sub.2O.sub.3, Fe, Co and Mg depending on the treatment flow rate were measured. The results are shown in FIG. 7.

(57) As is apparent from FIGS. 6 and 7, showing the purity of carbon nanotubes depending on the nitrogen treatment flow rate, when the nitrogen flow rate was 2 L/min (reactor volume of 22 L), the purity of carbon nanotubes was 99.3% and the total metal content was 180 ppm, and also, when the nitrogen flow rate was 4 L/min (reactor volume of 22 L), the purity of carbon nanotubes was 99.9% and the total metal content was decreased to 10 ppm or less.

Example 5

(58) Electrical Resistance and Dispersibility of Carbon Nanotubes

(59) 10 mg of the carbon nanotubes treated at each of treatment temperatures of 1600 C., 1700 C., 1800 C. and 2500 C. was dispersed in 10 g of a 2 wt % SDS (Sodium Dodecyl Sulfate) aqueous solution using a sonicator for 5 min, filtered with a filter paper having a diameter of 16 mm, and dried at room temperature for 2 hr, after which electrical resistance values at five points (top, bottom, left, right, center) of the carbon nanotubes were measured using an electrical resistance meter and averaged. The electrical resistance values of carbon nanotubes depending on the treatment temperature are shown in FIG. 8.

(60) As shown in FIG. 8, the carbon nanotubes treated at 1600 C. and 1700 C. exhibited similar electrical resistance values and good dispersibility, but the carbon nanotubes treated at 2500 C. were drastically increased in electrical resistance and were thus determined not to have dispersed well. When the electrical resistance falls in the range of 1.010.sup.2 to 5.010.sup.2 /sq, good dispersibility is assumed to have taken place.

(61) Also, the TEM images of the carbon nanotubes treated at 1800 C. and 2500 C. are shown in FIGS. 9 and 10, respectively.

(62) The carbon nanotubes are reported to actively cause graphitization thereof upon annealing at 1800 to 2200 C. in an argon gas atmosphere slightly higher than atmospheric pressure (Chen et al. 2007). In conventional techniques, graphitization is known to reduce the incidence of chemical surface defects of carbon nanotubes, but there has been no study on the dispersibility thereof.

(63) Based on the results of TEM analysis of FIGS. 9 and 10, the carbon nanotubes treated at 2500 C. had a large number of bent structures, and the structure of the carbon nanotubes treated at 1800 C. was relatively gentle. In the present invention, upon high-temperature treatment, graphitization occurred more actively, but the growth layers of the carbon nanotubes were bent and entangled, thus adversely affecting dispersibility.

(64) Although the embodiments of the present invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims.