WO2023156821A1

WO2023156821A1 - A process for producing carbon nanotubes and a carbon nanotube product resulting thereform

Info

Publication number: WO2023156821A1
Application number: PCT/IB2022/051447
Authority: WO
Inventors: Trin JEDSUKONTORN
Original assignee: Ptt Lng Company Limited
Priority date: 2022-02-18
Filing date: 2022-02-18
Publication date: 2023-08-24

Abstract

Aspects in accordance with the present invention pertain to a process for producing carbon nanotubes (CNTs), said process comprising steps of: forming and growing the CNTs by reacting a gaseous carbon compound in a catalytic chemical vapor deposition (CCVD) reaction; inhibiting the growth of the CNTs by applying a cryogenic condition; and mechanically agitating the CNTs, wherein said mechanical agitation of the CNTs takes place simultaneously with or immediately after said inhibition of the growth of the CNTs.

Description

TITLE OF THE INVENTION

A PROCESS FOR PRODUCING CARBON NANOTUBES AND A CARBON NANOTUBE

PRODUCT RESULTING THEREFORM

FIELD OF THE INVENTION

The present invention relates to a process for producing carbon nanotubes (CNTs), particularly wherein the CNTs are formed and grown by a chemical vapor disposition (CVD) reaction, and the CNT product resulting from such process.

BACKGROUND OF THE INVENTION

Carbon nanotubes (CNTs) promise great potential for many industries for abundance of their raw materials, their broad applications, and structural and size variations. Their known general variants are single-walled carbon nanotubes (SWCNTs), double-walled carbon nanotubes (DWCNTs), few-walled carbon nanotubes (FWCNTs), and multi-walled carbon nanotubes (MWCNTs). Among these, SWCNTs, FWCNTs, and DWCNTs (collectively, “small-diameter CNTs”) are preferred over MWCNTs due to their greater structure integrity and thus physical properties (e.g. conductivity and strength). And among the small-diameter CNTs, SWCNTs are most preferred for the same reason. Said small-diameter CNTs are even more preferred if they have consistent lengths, for the consistency in length confers a consistency in physical properties.

As such, it is desirable to control the production process so as to achieve CNT products with high proportion of small-diameter CNTs over MWCNTs, and even more desirable if those smalldiameter CNTs are of consistent lengths. Said control is challenging, especially for a process which forms the CNTs by a chemical reaction which causes the CNTs to “grow” upon a substrate. Examples of such reaction with particular relevance include chemical vapor disposition (CVD), which includes a preferred subgroup of catalytic chemical vapor disposition (CCVD), in which, as conventionally known, CNTs would grow randomly. A conventional CCVD reaction would result in CNT products having the distribution of diameters within 50-150 nm, which is considered a wide range and consists essentially of less-preferred MWCNTs. These results are considered unavoidable for a large-scale CNT production by CCVD reaction. Furthermore, there is no known method to effectively convert said MWCNTs into small- diameter CNTs. An example of past attempts to address a similar problem is the U.S. Patent No. US 8,349,404 B2 which describes an example of such CVD or CCVD process. In particular, this document describes a process in which the purity of SWCNTs is controlled by forming a “crust” of randomly oriented nanotubes. The growth of SWCNTs on a flat substrate would take place under, and thus limited by, said crust. As a result, SWCNTs product of a higher purity and more regulated length may be achieved. However, this prior process still relies upon the development of a specific intermediate structure of a crust during the reaction, which must be precisely controlled and yet promises the SWCNTs purity of only at least 50 % of the total CNT product.

Accordingly, there is a need to achieve CNT products with a higher purity of small-diameter CNTs by a simpler and more reliable process.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a process for producing carbon nanotubes (CNTs) that is more effective in promoting the production of small-diameter CNTs, and is simpler and more reliable over the conventionally known processes.

The present inventors have found that the key concept of the present invention is most effective when applied to a CNT-forming process involving a catalytic chemical vapor disposition (CCVD) reaction. Particularly, the key concept involves inhibiting the growth of the CNTs during the CCVD reaction, said inhibition being carried out by applying a cryogenic condition to the CNTs. Still substantially under said cryogenic condition, the CNTs are mechanically agitated in order to break and/or peel the MWCNTs into small-diameter CNTs.

In an embodiment, a process in accordance with the present invention is a process for producing carbon nanotubes (CNTs). Said process comprises steps of: forming and growing the CNTs by reacting a gaseous carbon compound in a catalytic chemical vapor deposition (CCVD) reaction; inhibiting the growth of the CNTs by applying a cryogenic condition; and mechanically agitating the CNTs. In such embodiment, said mechanical agitation takes place simultaneously with or immediately after said inhibition of the growth of the CNTs.

In an embodiment, the CNTs are transferred continuously, from a reactor in which the catalytic vapor deposition (CCVD) is carried out, to a cryogenic crushing unit in which the inhibition of the growth of the CNTs and the mechanical agitation of the CNTs are carried out. In an embodiment, the inhibition of the growth of the CNTs and the mechanical agitation of the CNTs take place simultaneously in a batch unit operation.

The present inventor has found that a process according to any of the foregoing embodiments can achieve a product comprising mostly of CNTs having a diameter distribution within a range of 10-30 nm, which is inclusive of all small-diameter CNTs yet exclusive of MWCNTs. This range is also considered substantially narrower than those of CNTs produced from CCVD processes in the prior arts.

The embodiments may be configured to be any of a batch, continuous or semi-continuous process. In an exemplary embodiment where the inhibition of the growth of the CNTs and the mechanical agitation of the CNTs take place simultaneously in a batch unit operation, the CCVD reaction may take place in a batch or a continuous reactor, the latter alternative resulting in an overall semi-continuous process.

Preferably, the cryogenic condition involves a temperature within a range of -160 to -40 °C, and more preferably within a range of -160 to -110 °C.

As will be shown in the Detailed Description, further below, the present inventors have found that an embodiment of which the cryogenic condition involving a temperature within a range of -160 to -40 °C would yield particularly satisfactory results, with an acceptable presence of CNTs having 30-50 nm diameters, which borders between the desirable small-diameter CNTs (10-30 nm diameters) and relatively undesirable MWCNTs (50 nm or more diameters). The present inventors have also found that an embodiment of which the cryogenic condition involving a temperature within a narrower range of -160 to -110 °C would yield excellent results, comprising essentially of CNTs having 10-30 nm diameters, within which the desirable small-diameter CNTs fall.

Preferably, the mechanical agitation of the CNTs is crushing. Examples of means by which said crushing may be carried out include a milling ball and milling cutter. As will be shown in the detailed description below, the present inventors’ preferred means of crushing is the milling ball, though it should be noted that the other means of crushing are applicable yet omitted from the detailed description for brevity without any intention to limit the scope of the present invention.

The present inventors prefer the milling ball for its being a relatively mild alternative of crushing/agitating which breaks and/or peels the walls of CNTs while controlling the risks of destroying them entirely. Moreover, it is the inventors’ hypothesis that the interactive rolling of the milling balls against the walls of the milling container causes a particular manner of bending stress, which in turn overcomes the Van der Waal force and thus results in an advantageous mode of CNTs cleavage.

In an embodiment where the mechanical agitation is crushing, said crushing preferably takes place 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 within a range of 700-800 revolutions per minute.

The present inventors have found that though a crushing speed of lower than 400 revolutions per minute does produce some wall-breaking and/or wall-peeling effects in accordance with the present invention, said effects would occur very slowly and not conducive for an industrial-scale production. For a crushing speed of 400-700 revolutions per minute, the wall-breaking and/or wallpeeling effects are satisfactory, yielding up to 70-80 % purity of small- diameter CNTs. For a crushing speed of 700-800 revolutions per minute, the wall-breaking and/or wall-peeling effects are excellent, yielding 90-100 % purity of small-diameter CNTs. For a crushing speed of over 800 revolutions per minute, the wall-breaking and/or wall-peeling effects do not significantly improve despite a higher cost of energy.

Also, in an embodiment where the mechanical agitation is crushing, said crushing preferably takes place for 15-120 minutes, and more preferably for 60-90 minutes.

The present inventors have found that though a crushing time of shorter than 15 minutes does produce some wall-breaking and/or wall-peeling effects in accordance with the present invention, said effects would be very incomplete. For a crushing time of 15-60 minutes, the wallbreaking and/or wall-peeling effects would occur to a substantial extent, yielding small-diameter CNTs of satisfactory purity. For a crushing time of 60-90 minutes, the wall-breaking and/or wallpeeling effects would be complete, yielding essentially small- diameter CNTs with minimal defects. For a crushing time of 90-120 minutes, the small- diameter CNT products contain significant defects caused by excessive breakage. For a crushing time of over 120 minutes, the small-diameter CNT products are substantially destroyed by excessive breakage. Said excessive breakage, which is observable at a crushing time of 90 or more minutes, transforms the CNTs’ structure from crystalline into amorphous. Said gaseous carbon compound, which serves as a source of carbon for the production of CNTs, may be any organic or inorganic carbon species or a mixture thereof which is in the gas state at the ambient conditions. In an embodiment, the gaseous carbon compound comprises at least one of C1-C4 hydrocarbons. In an exemplary embodiment to be shown in the Detailed Description below, said gaseous carbon compound is a natural gas, though it should be noted that other gaseous carbon compounds are omitted from the detailed description for brevity without any intention to limit the scope of the present invention.

The present inventors have also found preferable particulars of the catalytic chemical vapor deposition (CCVD) reaction as follows.

Preferably, the CCVD reaction is carried out at a reaction temperature of 800-1200 °C, and more preferably at a reaction temperature of 900-1000 °C.

The present inventors have found that though a CCVD reaction temperature of 800-900 °C does produce the small-diameter CNTs in accordance with the concept of the invention, the said CNT product also contains a significant CNTs having large diameters (30-100 nm) which are less desirable. A reaction temperature of 900-1000 °C yields excellent CNT products, consisting essentially of small-diameter CNTs. For a reaction temperature of 1000-1200 °C, the resulting CNT products show decomposition into methane and/or soot. Said decomposition becomes substantial at a reaction temperature of 1200 °C or higher.

Preferably, the CCVD reaction involves the use of an alumina-based catalyst comprising a metal loading content within the range of 1-10 % by weight. Exemplary embodiments involving alumina-based catalysts comprising Fe-Mo and Ni-Mo within said range of metal loading will be further described below

Preferably, the CCVD reaction takes place in a reactor which is inclined at an angle of 1-5 degrees, and more preferably at an angle of 4-5 degrees.

Preferably, said reactor is rotated at a speed not exceeding 6 revolutions per minute, and more preferably at a speed of 0.5-2 revolutions per minute for an excellent purity of small-diameter CNTs, and most preferably at a speed of 2 revolutions per minute for an excellent purity of smalldiameter CNTs at a greater yield.

Preferably, the said alumina-based catalyst is fed into the reactor at a rate of 0.1-0.5 grams per minute. Preferably, the gaseous carbon compound is fed into the reactor at a rate of within 0.5-1 liter per minute.

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 of hydrogen and nitrogen, which are most preferably mixed at a H2:N2 ratio of 1: 1. Also more preferably, the said carrier gas is fed into the reactor at a rate of within 1-2 liters per minute.

The present inventors have determined that the presence of H2 in the CCVD reaction consistently improves the yield of CNT product. With presence of H2, as observed by the inventors, the reaction atmosphere becomes reductive, converting the catalyst metals’ oxide forms into their metal forms, and thereby increasing the catalyst’s activity. The inventors have also hypothesized that H2 may serve to modify the catalyst’s surface by creating the seeds for the nucleation of CNTs.

In an embodiment, the process further comprises a step of recovering the small-diameter CNTs, i.e. CNTs having diameters within 10-30 nanometers.

BRIEF DESCRIPTION OF DRAWINGS

The principle of the present invention and its advantages will become apparent in the following description, taking into consideration the accompanying drawings in which:

FIG. 1 shows a schematic flowchart representing a process in accordance with a preferred embodiment.

FIG. 2 shows a pilot production apparatus in accordance with a preferred embodiment which was used to carry out the Examples (not to scale).

FIG. 3 shows a scanning electron spectroscopy (SEM) of CNTs from Example 1.

FIG. 4 shows a scanning electron spectroscopy (SEM) of CNTs from Example 2.

FIG. 5 shows a scanning electron spectroscopy (SEM) of CNTs from Example 3.

FIG. 6 shows a scanning electron spectroscopy (SEM) of CNTs from Example 4.

FIG. 7 shows a scanning electron spectroscopy (SEM) of CNTs from Example 5.

FIG. 8 shows a scanning electron spectroscopy (SEM) of CNTs from Example 6.

FIG. 9 shows a scanning electron spectroscopy (SEM) of CNTs from Example 7.

FIG. 10 shows a scanning electron spectroscopy (SEM) of CNTs from Example 8.

FIG. 11 shows a scanning electron spectroscopy (SEM) of CNTs from Example 9. FIG. 12 shows a scanning electron spectroscopy (SEM) of CNTs from Example 10.

FIG. 13 shows a scanning electron spectroscopy (SEM) of CNTs from Example 11.

FIG. 14 shows a scanning electron spectroscopy (SEM) of CNTs from Example 12.

FIG. 15 shows a scanning electron spectroscopy (SEM) of CNTs from Example 13.

FIG. 16 shows a scanning electron spectroscopy (SEM) of CNTs from Example 14.

FIG. 17 shows a scanning electron spectroscopy (SEM) of CNTs from Example 15.

FIG. 18 shows a scanning electron spectroscopy (SEM) of CNTs from Example 16.

FIG. 19 shows a Raman spectrum peaks of CNTs from Example 5.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

It is to be understood that the following detailed description will be directed to embodiments, provided as examples for illustrating the concept of the present invention only. The present invention is in fact not limited to particular embodiments described, as such may, of course, vary. It is also to be understood that the terminology used herein is for the purpose of describing particular embodiments only, and is not intended to be limiting, since the scope of this invention will be limited only by the appended claims.

The detailed description of the invention is divided into various sections only for the reader’s convenience and disclosure found in any section may be combined with that in another section.

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

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.

The term “about” when used before a numerical designation, e.g., dimensions, time, amount, and such other, including a range, indicates approximations which may vary by ( + ) or ( - ) 10 %, 5 % or 1 %, or any sub-range or sub-value there between.

“Comprising” or “comprises” is intended to mean that the compositions and methods include the recited elements, but not excluding others. “Consisting essentially of’ when used to define compositions and methods, shall mean excluding other elements of any essential significance to the combination for the stated purpose. Thus, a device or method consisting essentially of the elements as defined herein would not exclude other materials or steps that do not materially affect the basic and novel characteristic(s) of the claimed invention. “Consisting of’ shall mean excluding more than trace elements of other ingredients and substantial method steps. Embodiments defined by each of these transition terms are within the scope of this invention.

“Small-diameter CNTs” refers to carbon nanotubes having diameters falling within a range of 10-30 nm. Said range of diameters includes and represents those of SWCNTs, DWCNTs, and FWCNTs, but not those of MWCNTs.

Fig. 1 shows a schematic flowchart representing a process in accordance with a preferred embodiment. Here, the process 10 for producing carbon nanotubes (CNTs), comprises steps of: forming and growing the CNTs 12 in a catalytic chemical vapor deposition (CCVD) reaction; inhibiting the growth of the CNTs 14 by applying a cryogenic condition; mechanically agitating the CNTs 16; and recovering the CNTs 18. In accordance with the preferred embodiment, the steps of inhibiting the growth of the CNTs 14 and mechanically agitating the CNTs 16 take place simultaneously. As will be shown later in more detail, the 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 take place simultaneously.

Fig. 2 shows a pilot production apparatus in accordance with a preferred embodiment which was used to carry out the Examples (to be described further below). According to Fig. 2, the preferred embodiment is enabled by an apparatus 20 comprising: a reactor 210, in which the intended catalytic vapor deposition (CCVD) reaction is carried out; and a cryogenic crushing unit 250 that is connected to the reactor 210.

Particularly, the reactor 210 in this embodiment is connected to an RLNG inlet 212, a nitrogen gas inlet 214, and a hydrogen gas inlet 216. The RLNG gas inlet 212 feeds regasified liquefied natural gas (RLNG), which comprises mostly methane and may also comprise any of ethane, propane, and butane, into the reactor 210. At a ratio of about 1:1, the nitrogen gas inlet 214 and the hydrogen gas inlet 216 feed pure N2 and H2 gases, respectively, into the reactor 210. These inlets 212, 214, 216 merge at the feed gas junction 218; thus the aforesaid gases are mixed at said feed gas junction 218. In this embodiment, the regasified liquefied natural gas serves as the gaseous carbon compound, whereas the N2 and H2 gases serve jointly as carrier gas. The rate at which the regasified liquefied natural gas is feed is within 0.5-1 liter per minute, whereas the combined rate at which the N2 and H2 gases (i.e. the carrier gas) is fed is within 1-2 liters per minute. In other embodiments, the feed gas junction 218 may be supplemented with another equipment, such as a mixing valve or like, in order to promote homogeneity of the resulting feed gas.

Particularly, the feeding side of the reactor 210 is also connected to a hopper 222, which in this embodiment is located between the feed gas junction 218 and the reactor 210. Here, the hopper 222 is an inlet for a catalyst for the intended CCVD reaction. In this embodiment, said catalyst is fed through said hopper 222 at a rate of 0.01-0.5 grams per minute into the reactor 210. In order to accommodate the intended CCVD reaction, the reactor 210 is configured to be capable of, among others: heating its internal reaction chamber (not shown in Fig. 2) to a temperature of 800-1200 °C; inclining its reaction chamber at an angle of 1-5 degrees; and rotating its reaction chamber at a speed of up to 6 revolutions per minute.

In this embodiment, the opposing side of the reactor 210 is connected to an outlet 224, through which the CNTs are transferred continuously from the reactor 210 to the cryogenic crushing unit 250. Within this cryogenic crushing unit 250, the intended inhibition of the growth of the CNTs and the mechanical agitation of the CNTs are carried out.

Particularly, said cryogenic crushing unit 250 in this embodiment comprises interconnecting mill ball 252, cryogenic tank 254, and powder separator 256.

More particularly, the mill ball 252 in this embodiment is a batch unit operation configured to provide a mechanical agitation to be performed upon the CNTs by way of crushing. Said mill ball 252 is also configured to provide said crushing at a speed of at least 400 revolutions per minute and up to 800 revolutions per minute.

More particularly, the cryogenic tank 254 in this embodiment contains liquid nitrogen which is circulated about the mill ball 252, thereby imparting the mill ball’s 252 milling chamber (not shown in Fig. 2) with cryogenic conditions required for inhibiting the growth of the CNTs. Under said conditions, the CNTs are to be crushed. Said cryogenic conditions involve a temperature of -160 to -60 °C. By this configuration, the inhibition of the growth of the CNTs and the mechanical agitation of the CNTs take place simultaneously in a batch unit operation.

Also more particularly, the powder separator 256 in this embodiment takes the form of a cyclone separator unit wherefrom the CNTs product is recovered through the bottom outlet and the gas is vented through the upper outlet. Examples

The pilot production apparatus as per the above Fig. 2 was run several times in order to test the quantitative and qualitative effects of embodiments. In all the said runs, a Fe-Mo alumina-based catalyst comprising 5 % iron (Fe) and 5 % molybdenum (Mo) metal loading was fed into the reactor 210 at a rate of 0.05 grams per minute.

In the quantitative tests, the % distributions of CNT diameters were measured over the variation of the following factors: cryogenic temperatures, crushing speeds, crushing times, CCVD reaction temperatures, and rotation speeds of the reactor. In those embodiments, the said cryogenic temperatures, crushing speeds, and crushing times were applied to the mill ball 252 (see Fig. 2), and the said CCVD reaction temperatures and rotation speed were applied to the reactor 210 (see Fig. 2).

Next, in the quantitative tests, scanning electron microscopy (SEM) was used to inspect the sizes and shapes of the resulting CNTs. The resulting SEM images for all Examples will be shown in Figs. 3-18.

Raman spectroscopy was used to determine the crystallinity and defects in CNTs obtained from one Example, as will be shown later in Fig. 19.

Effect of Cryogenic Temperatures

In the following Example Nos. 1-5, the following parameters were controlled: the CCVD reaction temperature, at 950 °C; the reactor’s rotation speed, at 0.5 revolutions per minute (rpm); the crushing speed, at 700 revolutions per minute (rpm); and the crushing time, at 60 minutes (min).

On the other hand, the cryogenic temperatures were varied: in Example No. 1, the CNTs were transferred from the reactor to the mill ball, in which the CNTs were then crushed without applying cryogenic temperature; in Example No. 2, the cryogenic temperature of - 40 °C was applied; in Example No. 3, the cryogenic temperature of - 80 °C was applied; in Example No. 4, the cryogenic temperature of- 110 °C was applied; and in Example No. 5, the cryogenic temperature of - 160 °C was applied.

From each Example, the resulting CNTs were then recovered and purified by dissolving in a concentrated hydrogen fluoride solution (38 wt. %) in order to eliminate the alumina and metal catalysts. Next, at least 500 particles of the purified CNTs from each Example were sampled for SEM imaging, based upon which the particles were observed for their diameter sizes. In this way, the CNTs from each Example were assayed for their % distribution of diameters.

The results from said Examples are shown below in Table 1 .

Table 1

Effect of Crushing Speeds

In the following Example Nos. 6-8, the following parameters were controlled: the CCVD reaction temperature, at 950 °C; the reactor’s rotation speed, at 0.5 revolutions per minute (rpm); the cryogenic temperature, at - 110 °C; and the crushing time, at 60 minutes (min).

On the other hand, the crushing speeds were varied: in Example No. 6, the crushing speed was below 400 revolutions per minute (rpm); in Example No. 7, the crushing speed was within a range of 400-700 revolutions per minute (rpm); and in Example No. 8, the crushing speed was above 700 revolutions per minute (rpm).

The resulting CNTs were then recovered, purified, assayed for the % distribution of diameters, and imaged by the methods as previously described with respect to Example Nos. 1-5.

The results from said Examples are shown below in Table 2.

Table 2

Effect of Crushing Times

In the following Example Nos. 9-12, the following parameters were controlled: the CCVD reaction temperature, at 950 °C; the reactor’s rotation speed, at 0.5 revolutions per minute (rpm); the cryogenic temperature, at - 110 °C; and the crushing speed, at 700 revolutions per minute (rpm). On the other hand, the crushing times were varied: in Example No. 9, the crushing time was less than 15 minutes (min); in Example No. 10, the crushing time was within a range of 15-60 minutes (min); in Example No. 11, the crushing time was within a range of 60-90 minutes (min); and in Example No. 12, the crushing time was more than 90 minutes (min).

The results from said Examples are shown below in Table 3.

Table 3

Effect of CCVD Reaction Temperatures

In the following Example Nos. 13-14, the following parameters were controlled: the reactor’s rotation speed, at 0.5 revolutions per minute (rpm); the cryogenic temperature, at - 110 °C; the crushing speed, at 700 revolutions per minute (rpm); and the crushing time, at 60 minutes (min).

On the other hand, the CCVD reaction temperatures were varied: in Example No. 13, the CCVD reaction temperature was within a range of 800-900 °C; and in Example No. 14, the CCVD reaction temperature was within a range of 900-1000 °C.

The results from said Examples are shown below in Table 4.

Table 4

Effect of the Reactor’s Rotation Speeds In the following Example Nos. 15-16, the following parameters were controlled: the CCVD reaction temperature, at 950 °C; the cryogenic temperature, at - 110 °C; the crushing speed, at 700 revolutions per minute (rpm); and the crushing time, at 60 minutes (min).

On the other hand, the reactor’s rotation speeds were varied: in Example No. 15, the reactor’s rotation speed was within a range of 0.5-2 revolutions per minute (rpm); and in Example No. 16, the reactor’s rotation speed was above 2 revolutions per minute (rpm).

The results from said Examples are shown below in Table 5.

Table 5

Raman Spectroscopy

Raman Spectroscopy of CNTs obtained from Example No. 5 is shown in Fig. 19, wherein two distinct Raman spectroscopy peaks were observed at the Raman shifts of about 1344 and 1573 cm’¹. The Raman shift of about 1344 cm’¹ corresponded to the disorder band (D-band, ID) of the graphite structure; whereas the Raman shift of about 1573 cm’¹ corresponded to the high-frequency E_2g first-order vibrational mode (G-band, IG) of the graphite structure. Accordingly, the low intensity of the D-band (-1344 cm’¹) relative to that of the G-band (-1573 cm’¹) indicated a low density of defective structures and high crystallinity of the CNTs.

The remaining detail of embodiments, as well as other possible alternative embodiments, may be appreciated by a skilled person upon the knowledge of the foregoing exemplary embodiments, and so is omitted for brevity without limiting the concept of the present invention. List of drawing references

10 process

12 step of forming and growing the CNTs

14 step of inhibiting the growth of the CNTs

16 step of mechanically agitating the CNTs

18 step of recovering the CNTs Apparatus

210 reactor

212 RLNG inlet

214 nitrogen gas inlet

216 hydrogen gas inlet

218 feed gas j unction

222 hopper

224 outlet

250 cryogenic crushing unit

252 mill ball

254 cryogenic tank

256 powder separator

Claims

1. A process for producing carbon nanotubes (CNTs), said process comprising steps of: forming and growing the CNTs by reacting a gaseous carbon compound in a catalytic chemical vapor deposition (CCVD) reaction; inhibiting the growth of the CNTs by applying a cryogenic condition; and mechanically agitating the CNTs, wherein said mechanical agitation of the CNTs takes place simultaneously with or immediately after said inhibition of the growth of the CNTs.

2. The process according to Claim 1 , wherein the CNTs are transferred continuously, from a reactor in which the catalytic vapor deposition (CCVD) is carried out, to a cryogenic crushing unit in which the inhibition of the growth of the CNTs and the mechanical agitation of the CNTs are carried out.

3. The process according to Claim 1, wherein the inhibition of the growth of the CNTs and the mechanical agitation of the CNTs take place simultaneously in a batch unit operation.

4. The process according to Claim 1, wherein the cryogenic condition involves a temperature within a range of -160 to -40 °C.

5. The process according to Claim 1, wherein the cryogenic condition involves a temperature within a range of -160 to -110 °C.

6. The process according to Claim 1, wherein the mechanical agitation of the CNTs is crushing.

7. The process according to Claim 6, wherein the crushing takes place at a speed of at least 400 revolutions per minute.

8. The process according to Claim 6, wherein the crushing takes place at a speed of at least 700 revolutions per minute. The process according to Claim 6, wherein the crushing takes place at a speed within a range of 700-800 revolutions per minute. The process according to Claim 1, wherein the inhibition of the growth of the CNTs and the mechanical agitation of the CNTs take place simultaneously in a batch unit operation; wherein said mechanical agitation of the CNTs is crushing; wherein the cryogenic condition involves a temperature within a range of -160 to -40 °C; and wherein said crushing takes place at a speed of at least 400 revolutions per minute. The process according to Claim 10, wherein the inhibition of the growth of the CNTs and the mechanical agitation of the CNTs take place for 15-120 minutes. The process according to Claim 11, wherein the cryogenic condition involves a temperature within a range of -160 to -110 °C. The process according to Claim 12, wherein the crushing take place at a speed of at least 400 revolutions per minute. The process according to Claim 13, wherein the inhibition of the growth of the CNTs and the mechanical agitation of the CNTs take place for 60-90 minutes The process according to Claim 1, wherein the gaseous carbon compound comprises at least one of C1-C4 hydrocarbons. The process according to Claim 1, wherein the catalytic chemical vapor deposition (CCVD) reaction involves the use of a carrier gas comprising hydrogen and nitrogen. The process according to Claim 1, wherein the catalytic chemical vapor deposition (CCVD) reaction is carried out at a reaction temperature of 800-1200 °C. The process according to Claim 1, wherein the catalytic chemical vapor deposition (CCVD) reaction is carried out at a reaction temperature of 900-1000 °C. The process according to Claim 17, wherein the catalytic chemical vapor deposition (CCVD) reaction involves the use of an alumina-based catalyst comprising a metal loading content within the range of 1-10 % by weight.

20. The process according to Claim 17, wherein the catalytic chemical vapor deposition (CCVD) reaction involves the use of an alumina-based catalyst comprising Fe-Mo or Ni-Mo metal loading content within the range of 1-10 % by weight.

21. The process according to Claim 19, wherein the catalytic chemical vapor deposition (CCVD) reaction takes place in a reactor which is inclined at an angle of 1-5 degrees and is rotated at a speed not exceeding 6 revolutions per minute.

22. The process according to Claim 19, wherein the catalytic chemical vapor deposition (CCVD) reaction takes place in a reactor which is inclined at an angle of 4-5 degrees and is rotated at a speed of 0.5-2 revolutions per minute. 23. The process according to Claim 19, wherein the alumina-based catalyst is fed at a rate of 0.1-

0.5 grams per minute.

24. The process according to any of the above Claims, further comprising a step of recovering the CNTs having diameters within 10-30 nanometers.

25. Carbon nanotubes (CNTs) product having diameters within 10-30 nanometers which can be obtained by a process according to any one of Claim 1-23.