Method for producing carbon nanotubes and carbon nanotube product produced thereby

Abstract

An aspect according to the present invention relates to a method for producing carbon nanotubes (CNTs), the method comprising the steps of: forming and growing CNTs by reacting a gaseous carbon compound in a catalytic chemical vapor deposition (CCVD) reaction; inhibiting the growth of CNT by applying low temperature conditions; and mechanically stirring the CNTs, wherein the mechanical stirring of the CNTs is performed while the growth of the CNTs is inhibited or is performed immediately after the growth of the CNTs is inhibited.

Description

Technical Field

[0001] The present invention relates to a method for producing carbon nanotubes (CNTs), in particular a method in which CNTs are formed and grown by a chemical vapor deposition (CVD) reaction, and a CNT product produced by the method. Background Art

[0002] Carbon nanotubes (CNT) bring great potential to many industries because of their abundant raw materials, their wide applications and the variation of structure and size. Their known general variants are single-walled carbon nanotubes (SWCNT), double-walled carbon nanotubes (DWCNT), few-walled carbon nanotubes (FWCNT) and multi-walled carbon nanotubes (MWCNT). Among these, SWCNT, FWCNT and DWCNT (collectively referred to as "small-diameter CNT") are more preferred than MWCNT due to their greater structural integrity and physical properties (such as conductivity and strength). And among small-diameter CNT, for the same reason, SWCNT is the most preferred. If the small-diameter CNT has a consistent length, they are more preferred because the consistency of the length imparts the consistency of the physical properties.

[0003] Because of this, it is desirable to control the production process so as to achieve a CNT product having a higher proportion of small diameter CNTs than MWCNTs, and it is even more desirable if those small diameter CNTs have a consistent length. Control is challenging, especially for methods of forming CNTs by chemical reactions that cause CNTs to "grow" on substrates. Examples of such reactions with particular relevance include chemical vapor deposition (CVD), which includes a preferred subset of catalytic chemical vapor deposition (CCVD), in which, as conventionally known, CNTs will grow randomly. Conventional CCVD reactions will result in CNT products with diameters distributed between 50nm and 150nm (which is considered to be a wider range), and are generally composed of less preferred MWCNTs. For large-scale CNT production by CCVD reactions, it is believed that these results are inevitable. In addition, there is currently no known method to effectively convert MWCNTs into small diameter CNTs.

[0004] An example of a past attempt to solve a similar problem is U.S. Pat. No. US 8,349,404 B2, which describes an example of such a CVD or CCVD method. In particular, the patent describes a method in which the purity of SWCNTs is controlled by forming a "shell" of randomly oriented nanotubes. The growth of SWCNTs on a flat substrate will occur below the shell and will therefore be limited by the shell. As a result, a SWCNT product with higher purity and more regular length can be achieved. However, this prior art method still relies on the development of a specific intermediate structure of the shell during the reaction, which must be precisely controlled, and can only guarantee a purity of at least 50% of the SWCNTs in the total CNT product.

[0005] Accordingly, there is a need to achieve CNT products having small-diameter CNTs with higher purity through simpler and more reliable methods. Summary of the invention

[0006] An object of the present invention is to provide a method for producing carbon nanotubes (CNTs) which is more effective in facilitating the production of small diameter CNTs and which is simpler and more reliable than conventionally known methods.

[0007] The inventors have found that the key concepts of the present invention are most effective when applied to a CNT formation method involving a catalytic chemical vapor deposition (CCVD) reaction. In particular, the key concepts involve inhibiting the growth of CNTs during the CCVD reaction by applying cryogenic conditions to the CNTs. Still substantially under cryogenic conditions, the CNTs are mechanically agitated to break and / or strip the MWCNTs into small diameter CNTs.

[0008] In an embodiment, the method according to the present invention is a method for producing carbon nanotubes (CNTs). The method comprises the steps of forming and growing CNTs by reacting gaseous carbon compounds in a catalytic chemical vapor deposition (CCVD) reaction; inhibiting the growth of CNTs by applying low temperature conditions; and mechanically stirring the CNTs. In this embodiment, mechanical stirring is performed while inhibiting the growth of the CNTs or just after inhibiting the growth of the CNTs.

[0009] In an embodiment, CNTs are continuously transferred from a reactor performing catalytic chemical vapor deposition (CCVD) to a cryogenic grinding unit that performs inhibition of CNT growth and mechanical agitation of the CNTs.

[0010] In an embodiment, inhibiting the growth of CNTs and mechanically agitating the CNTs are performed simultaneously in a batch unit operation.

[0011] The inventors have found that the method according to any of the foregoing embodiments can achieve a product that mainly includes CNTs with diameters distributed in the range of 10nm to 30nm, which includes all small diameter CNTs, but does not include MWCNTs. This range is also considered to be substantially narrower than the range of CNTs produced by the CCVD method in the prior art.

[0012] Embodiments may be configured as any of a batch process, a continuous process, or a semi-continuous process. In an exemplary embodiment, inhibition of CNT growth and mechanical agitation of CNTs are performed simultaneously in a batch unit operation, and the CCVD reaction may be performed in a batch reactor or a continuous reactor, the latter resulting in an overall semi-continuous process.

[0013] Preferably, cryogenic conditions involve temperatures ranging from -160°C to -40°C, and more preferably ranging from -160°C to -110°C.

[0014] As will be further shown in the detailed description below, the inventors have found that embodiments involving cryogenic conditions with temperatures ranging from -160°C to -40°C will produce particularly satisfactory results, with acceptable presence of CNTs with diameters of 30nm to 50nm, which is between the desired small diameter CNTs (10nm to 30nm diameter) and the relatively undesirable MWCNTs (50nm or larger diameter). The inventors have also found that embodiments involving cryogenic conditions with temperatures within a narrower range of -160°C to -110°C will produce excellent results, which embodiments generally include CNTs with diameters of 10nm to 30nm, with the desired small diameter CNTs falling therein.

[0015] Preferably, the mechanical agitation of the CNTs is grinding. Examples of ways in which grinding can be performed include ball mills and milling cutters. As will be shown in the detailed description below, the inventors' preferred grinding method is a ball mill, although it should be noted that other grinding methods are also applicable but are omitted from the detailed description for the sake of brevity without any intention to limit the scope of the invention.

[0016] The inventors prefer a ball mill because it is a relatively gentle alternative to crushing / stirring that damages and / or exfoliates the walls of the CNTs while controlling the risk of completely destroying them. Furthermore, the inventors believe that the mutual rolling of the ball mill on the walls of the grinding vessel causes specific bending stresses, which in turn overcome the van der Waals forces and thus make it a favorable mode for CNT cutting.

[0017] In embodiments where the mechanical agitation is grinding, the grinding is preferably carried out at a speed of at least 400 revolutions per minute, more preferably at a speed of at least 700 revolutions per minute, and most preferably at a speed in the range of 700 to 800 revolutions per minute.

[0018] The inventors have found that, although according to the present invention, a grinding speed of less than 400 revolutions per minute does produce some wall breaking and / or wall stripping effects, the effects occur very slowly and are not conducive to industrial-scale production. For grinding speeds of 400 to 4700 revolutions per minute, the wall breaking and / or wall stripping effects are satisfactory, producing small diameter CNTs with a purity of up to 70% to 80%. For grinding speeds of 700 to 800 revolutions per minute, the wall breaking and / or wall stripping effects are excellent, producing small diameter CNTs with a purity of 90% to 100%. For grinding speeds exceeding 800 revolutions per minute, the wall breaking and / or wall stripping effects are not significantly improved despite higher energy costs.

[0019] And, in the embodiment in which the mechanical agitation is crushing, the crushing is preferably performed for 15 minutes to 120 minutes, and more preferably for 60 minutes to 90 minutes.

[0020] The inventors have found that, although according to the present invention, a grinding time shorter than 15 minutes does produce some wall breaking and / or wall stripping effects, the effect will be very incomplete. For a grinding time of 15 to 60 minutes, a wall breaking and / or wall stripping effect will occur to a large extent, producing small-diameter CNTs with satisfactory purity. For a grinding time of 60 to 90 minutes, the wall breaking and / or wall stripping effect will be completely completed, producing small-diameter CNTs with minimal defects in general. For a grinding time of 90 to 120 minutes, the small-diameter CNT product contains obvious defects caused by excessive fracture. For a grinding time of more than 120 minutes, the small-diameter CNT product is substantially destroyed due to excessive fracture. The excessive fracture observable in a grinding time of 90 minutes or longer converts the structure of CNT from crystalline to amorphous.

[0021] The gaseous carbon compound used as a carbon source for the production of CNTs may be any organic or inorganic carbon species or mixtures thereof that are in a gaseous state under ambient conditions. In an embodiment, the gaseous carbon compound includes at least one of a C1 to C4 hydrocarbon. In the exemplary embodiment shown in the detailed description below, the gaseous carbon compound is natural gas, although it should be noted that other gaseous carbon compounds are omitted from the detailed description for the sake of brevity, but there is no intention to limit the scope of the present invention.

[0022] The present inventors have also discovered that preferred details of the catalytic chemical vapor deposition (CCVD) reaction are as follows.

[0023] Preferably, the CCVD reaction is performed at a reaction temperature of 800°C to 1200°C, and more preferably at a reaction temperature of 900°C to 1000°C.

[0024] The inventors have found that, although CCVD reaction temperatures of 800°C to 900°C do produce small diameter CNTs according to the concepts of the present invention, the CNT product also contains significant CNTs with less desirable large diameters (30nm to 100nm). Reaction temperatures of 900°C to 1000°C produce excellent CNT products that are substantially composed of small diameter CNTs. For reaction temperatures of 1000°C to 1200°C, the resulting CNT product shows decomposition into methane and / or soot. At reaction temperatures of 1200°C or higher, decomposition becomes significant.

[0025] Preferably, the CCVD reaction involves the use of an alumina-based catalyst comprising a metal loading content ranging from 1 wt % to 10 wt %. Exemplary embodiments involving alumina-based catalysts including Fe-Mo and Ni-Mo within a metal loading range are further described below.

[0026] Preferably, the CCVD reaction is conducted in a reactor tilted at an angle of 1 to 5 degrees, and more preferably at an angle of 4 to 5 degrees.

[0027] Preferably, the reactor rotates at a speed not exceeding 6 revolutions per minute, and more preferably rotates at a speed of 0.5 to 2 revolutions per minute to obtain small diameter CNTs of excellent purity, and most preferably rotates at a speed of 2 revolutions per minute to obtain higher yields of small diameter CNTs of excellent purity.

[0028] Preferably, the alumina-based catalyst is fed into the reactor at a rate of 0.1 g per minute to 0.5 g per minute.

[0029] Preferably, the gaseous carbon compound is fed into the reactor at a rate of 0.5 liters per minute to 1 liter per minute.

[0030] Preferably, the CCVD reaction involves the use of a carrier gas comprising hydrogen and nitrogen. More preferably, the carrier gas is a mixture comprising essentially hydrogen and nitrogen, most preferably mixed in a 1:1 H2:N2 ratio. Also more preferably, the carrier gas is fed into the reactor at a rate of 1 liter per minute to 2 liters per minute.

[0031] The inventors have determined that the presence of H2 in the CCVD reaction consistently improves the yield of CNT products. As the inventors observed, in the presence of H2, the reaction atmosphere becomes reducing, converting the oxide forms of the catalyst metals to their metallic forms, and thereby increasing the activity of the catalyst. The inventors also hypothesize that H2 can modify the surface of the catalyst by creating seeds for the nucleation of CNTs.

[0032] In an embodiment, the method further comprises the step of recovering small diameter CNTs (ie, CNTs having a diameter of 10 nanometers to 30 nanometers). BRIEF DESCRIPTION OF THE DRAWINGS

[0033] The principles of the present invention and its advantages will become apparent in the following description considering the accompanying drawings, in which:

[0034] Figure 1 A schematic flow chart representation of a method according to a preferred embodiment is shown.

[0035] Figure 2 A pilot production apparatus (not drawn to scale) for carrying out the examples is shown according to a preferred embodiment.

[0036] Figure 3 A scanning electron spectrum (SEM) of CNTs from Example 1 is shown.

[0037] Figure 4 A scanning electron spectrum (SEM) of CNTs from Example 2 is shown.

[0038] Figure 5 A scanning electron spectrum (SEM) of CNTs from Example 3 is shown.

[0039] Figure 6 A scanning electron spectrum (SEM) of CNTs from Example 4 is shown.

[0040] Figure 7 A scanning electron spectrum (SEM) of CNTs from Example 5 is shown.

[0041] Figure 8 A scanning electron spectrum (SEM) of CNTs from Example 6 is shown.

[0042] Fig. 9 A scanning electron spectrum (SEM) of CNTs from Example 7 is shown.

[0043] Fig.10 A scanning electron spectrum (SEM) of CNTs from Example 8 is shown.

[0044] Fig.11 A scanning electron spectrum (SEM) of CNTs from Example 9 is shown.

[0045] Fig.12 A scanning electron spectrum (SEM) of CNTs from Example 10 is shown.

[0046] Fig.13 A scanning electron spectrum (SEM) of CNTs from Example 11 is shown.

[0047] Fig.14 A scanning electron spectrum (SEM) of CNTs from Example 12 is shown.

[0048] Fig.15 A scanning electron spectrum (SEM) of CNTs from Example 13 is shown.

[0049] Fig.16 A scanning electron spectrum (SEM) of CNTs from Example 14 is shown.

[0050] Fig.17 A scanning electron spectrum (SEM) of CNTs from Example 15 is shown.

[0051] Fig.18 A scanning electron spectrum (SEM) of CNTs from Example 16 is shown.

[0052] Fig.19 Raman spectral peaks of CNTs from Example 5 are shown. DETAILED DESCRIPTION

[0053] It should be understood that the following detailed description will relate to embodiments provided as examples only for illustrating the inventive concept. The present invention is not actually limited to the specific embodiments described, and therefore the present invention can certainly be varied. It should also be understood that the terms used in this article are only used for the purpose of describing specific embodiments, and are not intended to be restrictive, because the scope of the present invention will be limited only by the appended claims.

[0054] For the convenience of readers only, the detailed description of the present invention is divided into sections, and the disclosure in any section can be combined with the disclosure in another section.

[0055] Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs.

[0056] It must be noted that as used herein and in the appended claims, the singular forms "a," "an," and "the" include plural referents unless the context clearly dictates otherwise.

[0057] When the term "about" is used before a numerical designation (e.g., size, time, amount, and the like, including ranges), it indicates an approximate value that may vary by +10% or -10%, +5% or -5%, or +1% or -1%, or any sub-range or sub-value therebetween.

[0058] "Comprising" or "comprises" is intended to mean that compositions and methods include the elements recited, but do not exclude other elements. When used to define compositions and methods, "consisting essentially of shall mean excluding other elements that have any significant effect on the combination for the purpose of the recitation. Thus, a device or method consisting essentially of the elements as defined herein will not exclude other materials or steps that do not materially affect the basic and novel characteristics of the claimed invention. "Consisting of shall mean excluding other ingredients and substantial method steps that exceed trace elements. Embodiments defined by each of these transition terms are within the scope of the present invention.

[0059] "Small diameter CNT" refers to a carbon nanotube having a diameter falling within the range of 10 nm to 30 nm. The diameter range includes and represents the diameter range of SWCNT, DWCNT and FWCNT, but does not include the diameter range of MWCNT.

[0060] Figure 1 A schematic flow chart representation of a method according to a preferred embodiment is shown. Here, a method 10 for producing carbon nanotubes (CNTs) includes the steps of forming and growing CNTs 12 in a catalytic chemical vapor deposition (CCVD) reaction; inhibiting the growth of the CNTs 14 by applying cryogenic conditions; mechanically agitating the CNTs 16; and recovering the CNTs 18. According to a preferred embodiment, the steps of inhibiting the growth of the CNTs 14 and mechanically agitating the CNTs 16 are performed simultaneously. As will be shown in more detail later, a preferred embodiment involves a batch unit operation in which the steps of inhibiting the growth of the CNTs 14 and mechanically agitating the CNTs 16 are performed simultaneously.

[0061] Figure 2 A pilot production apparatus for carrying out the examples (described further below) is shown according to a preferred embodiment. Figure 2 , a preferred embodiment is ensured by an apparatus 20 comprising: a reactor 210 for carrying out a desired catalytic chemical vapor deposition (CCVD) reaction; and a low temperature grinding unit 250 connected to the reactor 210 .

[0062] In particular, the reactor 210 in this embodiment is connected to an RLNG inlet 212, a nitrogen inlet 214, and a hydrogen inlet 216. The RLNG gas inlet 212 feeds regasified liquefied natural gas (RLNG) into the reactor 210, and the regasified liquefied natural gas (RLNG) mainly includes methane, and may also include any one of ethane, propane, and butane. The nitrogen inlet 214 and the hydrogen inlet 216 feed N2 and H2 gases into the reactor 210 at a ratio of about 1:1, respectively. These inlets 212, 214, 216 converge at a feed gas connector 218; therefore, the aforementioned gases are mixed at the feed gas connector 218. In this embodiment, the regasified liquefied natural gas is used as a gaseous carbon compound, and the N2 and H2 gases are used together as a carrier gas. The rate of feeding the regasified liquefied natural gas is 0.5 liters per minute to 1 liter per minute, and the combined rate of feeding the N2 and H2 gases (i.e., carrier gas) is 1 liter per minute to 2 liters per minute. In other embodiments, the feed gas connector 218 may be supplemented with another device, such as a mixing valve or the like, to promote uniformity of the resulting feed gas.

[0063] In particular, the feed side of the reactor 210 is also connected to a hopper 222, which in this embodiment is located between the feed gas connector 218 and the reactor 210. Here, the hopper 222 is an inlet for the catalyst for the desired CCVD reaction. In this embodiment, the catalyst is fed into the reactor 210 through the hopper 222 at a rate of 0.01 grams per minute to 0.5 grams per minute. In order to accommodate the desired CCVD reaction, the reactor 210 is configured to: Figure 2 ) to a temperature of 800° C. to 1200° C.; tilting its reaction chamber at an angle of 1 to 5 degrees; and rotating its reaction chamber at a speed of up to 6 revolutions per minute.

[0064] In this embodiment, the opposite side of the reactor 210 is connected to an outlet 224 through which the CNTs are continuously transferred from the reactor 210 to a cryogenic grinding unit 250. Within the cryogenic grinding unit 250, the desired inhibition of the growth of the CNTs and mechanical agitation of the CNTs are performed.

[0065] In particular, the cryogenic grinding unit 250 in this embodiment includes a ball mill 252, a cryogenic tank 254, and a powder separator 256 that are connected to each other.

[0066] More specifically, the ball mill 252 in this embodiment operates as a batch unit and is configured to provide mechanical agitation of the CNTs by means of grinding. The ball mill 252 is also configured to provide grinding at a speed of at least 400 revolutions per minute and at most 800 revolutions per minute.

[0067] More specifically, the cryogenic tank 254 in this embodiment contains liquid nitrogen that circulates around the ball mill 252, thereby providing a cooling chamber ( Figure 2 The low temperature conditions required for inhibiting the growth of CNTs are provided (not shown). Under the conditions, the CNTs are crushed. The low temperature conditions involve a temperature of -160°C to -60°C. With this configuration, inhibiting the growth of CNTs and mechanical stirring of CNTs are performed simultaneously in a batch unit operation.

[0068] Also more specifically, the powder separator 256 in this embodiment takes the form of a cyclone separator unit, in which the CNT product is recovered through the bottom outlet and the gas is discharged through the upper outlet.

[0069] Example

[0070] According to the above Figure 2 The pilot production equipment of was run several times to test the quantitative and qualitative effects of the embodiments. In all runs, a Fe-Mo alumina-based catalyst containing 5% iron (Fe) and 5% molybdenum (Mo) metal loading was fed into the reactor 210 at a rate of 0.05 grams per minute.

[0071] In the quantitative test, the distribution % of CNT diameter was measured for changes in the following factors: cryogenic temperature, grinding speed, grinding time, CCVD reaction temperature and reactor rotation speed. In those embodiments, cryogenic temperature, grinding speed and grinding time are applied to ball mill 252 (see Figure 2 ), and the CCVD reaction temperature and speed are applied to the reactor 210 (see Figure 2 ).

[0072] Next, in a quantitative test, the size and shape of the resulting CNTs were examined using a scanning electron microscope (SEM). The SEM image results for all examples are shown in Figures 3 to 18 middle.

[0073] Using Raman spectroscopy to determine from an example (as will be described later in Fig.19 The crystallinity and defects of CNTs obtained in ).

[0074] Effect of freezing temperature

[0075] In the following Examples 1 to 5, the following parameters were controlled: CCVD reaction temperature was 950° C.; reactor speed was 0.5 revolutions per minute (rpm); grinding speed was 700 revolutions per minute (rpm); and grinding time was 60 minutes (min).

[0076] On the other hand, the cryogenic temperature was changed: in Example 1, the CNTs were transferred from the reactor to a ball mill and then crushed without applying a cryogenic temperature; in Example 2, a cryogenic temperature of -40°C was applied; in Example 3, a cryogenic temperature of -80°C was applied; in Example 4, a cryogenic temperature of -110°C was applied; and in Example 5, a cryogenic temperature of -160°C was applied.

[0077] The resulting CNTs were then recovered from each example and purified by dissolving in a concentrated hydrogen fluoride solution (38 wt%) to remove alumina and metal catalysts. Next, at least 500 purified CNT particles were sampled from each example for SEM imaging, based on which the diameter size of the particles was observed. In this way, the distribution % of the diameter of the CNTs from each example was determined.

[0078] The results from the examples are shown in Table 1 below.

[0079] Table 1

[0080]

[0081] Effect of crushing speed

[0082] In the following Examples 6 to 8, the following parameters were controlled: the CCVD reaction temperature was 950°C; the reactor speed was 0.5 revolutions per minute (rpm); the cryogenic temperature was -110°C; and the grinding time was 60 minutes (min).

[0083] On the other hand, the grinding speed was changed: in Example 6, the grinding speed was lower than 400 revolutions per minute (rpm); in Example 7, the grinding speed was in the range of 400 revolutions per minute (rpm) to 700 revolutions per minute (rpm); and in Example 8, the grinding speed exceeded 700 revolutions per minute (rpm).

[0084] The resulting CNTs were then recovered, purified, the % distribution of diameters determined, and imaged by the methods previously described for Examples 1-5.

[0085] The results from these examples are shown in Table 2 below.

[0086] Table 2

[0087]

[0088] The impact of crushing time

[0089] In the following Examples 9 to 12, the following parameters were controlled: the CCVD reaction temperature was 950°C; the reactor speed was 0.5 revolutions per minute (rpm); the cryogenic temperature was -110°C; and the grinding speed was 700 revolutions per minute (rpm).

[0090] On the other hand, the crushing time is changed: in Example 9, the crushing time is less than 15 minutes (min); in Example 10, the crushing time is in the range of 15 minutes (min) to 60 minutes (min); in Example 11, the crushing time is in the range of 60 minutes (min) to 90 minutes (min); and in Example 12, the crushing time is greater than 90 minutes (min).

[0091] The resulting CNTs were then recovered, purified, the % distribution of diameters determined, and imaged by the methods previously described for Examples 1-5.

[0092] The results from these examples are shown in Table 3 below.

[0093] Table 3

[0094]

[0095] Effect of CCVD reaction temperature

[0096] In the following Examples 13 to 14, the following parameters were controlled: the reactor speed was 0.5 revolutions per minute (rpm); the cryogenic temperature was -110°C; the grinding speed was 700 revolutions per minute (rpm); and the grinding time was 60 minutes (min).

[0097] On the other hand, the CCVD reaction temperature was changed: in Example 13, the CCVD reaction temperature was in the range of 800°C to 900°C; and in Example 14, the CCVD reaction temperature was in the range of 900°C to 1000°C.

[0098] The resulting CNTs were then recovered, purified, the % distribution of diameters determined, and imaged by the methods previously described for Examples 1-5.

[0099] The results from these examples are shown in Table 4 below.

[0100] Table 4

[0101]

[0102] Effect of reactor speed

[0103] In the following Examples 15 to 16, the following parameters were controlled: the CCVD reaction temperature was 950° C.; the cryogenic temperature was −110° C.; the grinding speed was 700 revolutions per minute (rpm); and the grinding time was 60 minutes (min).

[0104] On the other hand, the reactor speed was changed: in Example 15, the reactor speed was in the range of 0.5 revolutions per minute (rpm) to 2 revolutions per minute (rpm); and in Example 16, the reactor speed was above 2 revolutions per minute (rpm).

[0105] The resulting CNTs were then recovered, purified, the % distribution of diameters determined, and imaged by the methods previously described for Examples 1-5.

[0106] The results from these examples are shown in Table 5 below.

[0107] Table 5

[0108]

[0109] Raman spectroscopy

[0110] The Raman spectrum of the CNT from Example 5 shows Fig.19 Among them, about 1344cm -1 and 1573cm -1 Two different Raman spectral peaks were observed at the Raman shift of about 1344cm -1 The Raman shift corresponds to the disordered bands of the graphene structure (D band, I D ); about 1573cm -1 The Raman shift corresponds to the high frequency E of the graphene structure. 2g The first vibration mode (G band, I G ). Correspondingly, relative to the G band (~1573cm -1 ), D belt (~1344cm -1 ) indicates the low density of defective structure and high crystallinity of CNTs.

[0111] Those skilled in the art may understand the remaining details of the embodiments and other possible optional embodiments based on the understanding of the aforementioned exemplary embodiments, and are therefore omitted for the sake of brevity without limiting the inventive concept.

[0112] Description of Reference Numerals

[0113] 10 Methods

[0114] 12 Steps to Form and Grow CNTs

[0115] 14 Steps to inhibit CNT growth

[0116] 16 Steps of mechanical stirring of CNT

[0117] 18 Steps for recycling CNT

[0118] 20 devices

[0119] 210 Reactor

[0120] 212RLNG Entrance

[0121] 214 Nitrogen inlet

[0122] 216 Hydrogen inlet

[0123] 218 Feed gas connector

[0124] 222 Hopper

[0125] Exit 224

[0126] 250 low temperature crushing unit

[0127] 252 ball mill

[0128] 254 cryogenic tank

[0129] 256 Powder Separator

Claims

1. A method for producing carbon nanotubes (CNTs), the method comprising the following steps: Forming and growing CNTs by reacting gaseous carbon compounds in a catalytic chemical vapor deposition (CCVD) reaction; Inhibiting the growth of the CNTs by applying low temperature conditions; as well as Mechanically stirring the CNTs, wherein the mechanical stirring of the CNTs is performed while suppressing the growth of the CNTs or the mechanical stirring of the CNTs is performed just after suppressing the growth of the CNTs.

2. The method according to claim 1, wherein the CNTs are continuously transferred from a reactor performing the catalytic chemical vapor deposition (CCVD) to a low temperature grinding unit performing the suppression of the growth of the CNTs and the mechanical agitation of the CNTs. 3 . The method of claim 1 , wherein inhibiting the growth of the CNTs and the mechanical agitation of the CNTs are performed simultaneously in a batch unit operation.

4. The method according to claim 1, wherein the cryogenic conditions involve a temperature ranging from -160°C to -40°C.

5. The method of claim 1, wherein the cryogenic conditions involve a temperature ranging from -160°C to -110°C. The method according to claim 1 , wherein the mechanical agitation of the CNTs is crushing.

7. The method of claim 6, wherein the crushing is performed at a speed of at least 400 revolutions per minute.

8. The method of claim 6, wherein the crushing is performed at a speed of at least 700 revolutions per minute.

9. The method of claim 6, wherein the crushing is performed at a speed ranging from 700 to 800 revolutions per minute.

10. The method of claim 1, wherein inhibiting the growth of the CNTs and the mechanical agitation of the CNTs are performed simultaneously in a batch unit operation; wherein the mechanical agitation of the CNTs is crushing; wherein the low temperature conditions involve a temperature ranging from -160°C to -40°C; and wherein the crushing is performed at a speed of at least 400 revolutions per minute. 11 . The method according to claim 10 , wherein the inhibiting the growth of the CNTs and the mechanical agitation of the CNTs are performed for 15 minutes to 120 minutes.

12. The method according to claim 11, wherein the cryogenic conditions involve a temperature ranging from -160°C to -110°C.

13. The method of claim 12, wherein the crushing is performed at a speed of at least 400 revolutions per minute. The method according to claim 13 , wherein the inhibiting the growth of the CNTs and the mechanical stirring of the CNTs are performed for 60 minutes to 90 minutes.

15. The method of claim 1, wherein the gaseous carbon compound comprises at least one of C1 to C4 hydrocarbons.

16. The method of claim 1, wherein the catalytic chemical vapor deposition (CCVD) reaction involves using a carrier gas comprising hydrogen and nitrogen.

17. The method of claim 1, wherein the catalytic chemical vapor deposition (CCVD) reaction is performed at a reaction temperature of 800°C to 1200°C.

18. The method of claim 1, wherein the catalytic chemical vapor deposition (CCVD) reaction is performed at a reaction temperature of 900°C to 1000°C.

19. The method of claim 17, wherein the catalytic chemical vapor deposition (CCVD) reaction involves the use of an alumina-based catalyst comprising a metal loading content in the range of 1 wt% to 10 wt%.

20. The method of claim 17, wherein the catalytic chemical vapor deposition (CCVD) reaction involves using an alumina-based catalyst comprising a Fe-Mo or Ni-Mo metal loading content ranging from 1 wt% to 10 wt%.

21. The method of claim 19, wherein the catalytic chemical vapor deposition (CCVD) reaction is performed in a reactor tilted at an angle of 1 to 5 degrees and rotating at a speed not exceeding 6 revolutions per minute.

22. The method of claim 19, wherein the catalytic chemical vapor deposition (CCVD) reaction is performed in a reactor tilted at an angle of 4 to 5 degrees and rotating at a speed of 0.5 to 2 revolutions per minute.

23. The method of claim 19, wherein the alumina-based catalyst is fed at a rate of 0.1 g / min to 0.5 g / min.

24. The method according to any one of the preceding claims, further comprising the step of recovering the CNTs having a diameter of 10 to 30 nanometers.

25. A carbon nanotube (CNT) product having a diameter of 10 nm to 30 nm, obtainable by the method according to any one of claims 1 to 23.

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