WO2009040256A2

WO2009040256A2 - System and method for preparing single-wall carbon nanotubes

Info

Publication number: WO2009040256A2
Application number: PCT/EP2008/062101
Authority: WO
Inventors: Carlo Taliani; Petr Nozar; Viliam Vretenar
Original assignee: Carlo Taliani; Petr Nozar; Viliam Vretenar
Priority date: 2007-09-28
Filing date: 2008-09-11
Publication date: 2009-04-02
Also published as: WO2009040256A3; ITMI20071867A1

Abstract

A system for producing carbon nanotubes, comprising a body (2) with a cavity (3) which is connected at least partially to a device for loading a SWCNT growth catalyst (1) and to a SWCNT collection space (4), energy source means E for supplying energy in the cavity (3) and means for introducing a carbon source (C) in the cavity (3).

Description

SYSTEM AND METHOD FOR PREPARING SINGLE- WALL CARBON NANOTUBES

Technical Field

The present invention relates to a system and method for preparing carbon nanotubes, particularly single-wall carbon nanotubes (SWCNT). Background Art

Known methods for preparing carbon nanotubes currently can be divided into two main groups:

- physical methods, based on the treatment of carbon in the solid state,

- chemical methods, based on the treatment of carbon in the gaseous state (CVD method).

Methods based on the use of carbon in the solid state can be divided as follows: 1. Low- voltage high-current electric arc method

2. Process for graphite ablation by pulsed laser beam.

The electric arc method has led to the discovery of SWCNT. This is a low-voltage, high-current electric arc between two electrodes made of graphite doped with catalyst metals. The process occurs in a protective atmosphere (argon, helium, water, et cetera). This leads to a product with a relatively high concentration of nanotubes, both single-wall carbon nanotubes (SWCNT) and multi-wall carbon nanotubes (MWCNT) of different diameters, together with different types of fullerenes. However, the product is heavily contaminated by amorphous carbon and graphite (from 30 to 90%). Since this is a relatively inexpensive method, it is adopted by most manufacturers of unordered SWCNT, following it with processes for chemical-physical purification and separation.

The process for graphite ablation by means of a laser light beam can produce SWCNT of higher quality and with a lower level of contamination. However, the method is not suitable for industrial production, due to the high costs (the process requires the use of at least one power laser, a closed volume heated to a high temperature - 1000-1400⁰C - and an effective cooling system). All this for extremely limited quantities of produced material. The methods based on the use of carbon in the gaseous state

(chemical vapor phase deposition or CVD) can be divided into four groups:

1. Thermal CVD of methane/acetylene with pre-deposited solid-state catalyst

2. Plasma enhanced CVD of methane/acetylene, with pre-deposited solid-state catalyst

3. Thermal CVD of methane/acetylene, gaseous-state catalyst

4. Plasma enhanced CVD of methane/acetylene, gaseous-state catalyst

Methods for preparing SWCNT based on CVD utilize the low energy required for decomposition of gases containing carbon (methane, ethylene, et cetera). Decomposition of the working gas occurs by heating (internal - the gas flows around an electrically heated wire; external - the gas flows inside a reactor placed inside the furnace) or inside a plasma source. The plasma can be generated by oxidation of the working gas itself (plasma torch), by means of the microwave method or by using the electrical discharge. The catalyst of CNT growth can be pre-deposited on a suitable substrate with "classical" methods in the form of nanoparticles aggregated in a certain type of pattern suitable for the application. The substrate is heated appropriately during the process and the nanoparticles catalyze the growth of CNT. The predominant fraction of the nanotubes thus obtained is of the MWCNT (multi-wall carbon nanotubes) quality; moreover, the tubes are gradually covered with amorphous carbon during the process.

The other variation of the CVD process consists in adding catalyst in gaseous form (for example ferrocene, cobaltocene, nickelocene, et cetera) to the working gas (mixture of methane, ethylene, acetylene, et cetera, and argon, helium, nitrogen, et cetera). This mixture can be treated with both groups of methods cited above (thermal or plasma enhanced decomposition). The product of the process contains predominantly MWCNT, a small part of SWCNT, and is heavily contaminated with graphite and amorphous carbon.

A wide range of methods has been developed for preparing carbon nanotubes, but they all suffer a certain number of disadvantages that prompt the search for new methods.

In general, methods for preparing industrial quantities (with an electric arc) of single- wall carbon nanotubes have the disadvantage of having a product which is heavily contaminated with graphite and amorphous carbon. Accordingly, they require subsequent very complex and expensive physical and chemical methods for purifying the product.

The more controlled methods (CVD) have difficulty in growing SWCNT, often produce multi-wall nanotubes (MWCNT) and are not devoid of contamination with graphite and amorphous carbon. Disclosure of the Invention

The aim of the present invention is to eliminate the drawbacks noted above in known types of methods for producing carbon nanotubes, to provide a system and method for manufacturing carbon nanotubes with a high percentage of nanotubes in the single graphene layer (single-wall; SWCNT), which are free and scarcely contaminated by graphite and amorphous carbon.

This aim and other objects are achieved by the system and the method for producing carbon nanotubes defined in the claims. Brief Description of the Drawings

Figure 1 is a schematic view of the system for producing SWCNT according to the present invention.

Figure 2 is a schematic representation of an embodiment of the SWCNT production system according to the present invention. Figure 3 is a schematic view of another embodiment of the system for producing SWCNT according to the present invention.

Figure 4a, 4b is a transmission electron microscope (TEM) image of SWCNT attached to nanoparticles of catalyst. Figure 5 is the typical Raman spectrum of a SWCNT sample prepared by means of the method according to the present invention. Ways of carrying out the Invention

In one of its aspects, the present invention relates to a system for producing carbon nanotubes as shown in Figure 1 , which comprises a device for loading a SWCNT growth catalysts 1 , a body with a cavity 3 that is connected at least partially to the device for loading catalyst 1 , a space 4 for collecting SWCNT that is connected to the cavity 3, and energy source means E in order to provide energy in the cavity 3.

A source of carbon C can be introduced in the cavity 3. Examples of catalyst of the growth of SWCNT used in the present invention are powder or bar of nickel, cobalt, iron, et cetera, or their solid, liquid or volatile compounds.

The walls of the cavity 3 are constituted by a material 2 that is resistant to high temperatures and high pressures, for example Al₂O₃. The source of catalyst is suitable for the form of catalyst used (solid, liquid or gaseous). In the example described hereinafter, the catalyst has the form of the metallic cylinder of the selected composition.

The source of carbon C used in the present invention can be in solid form, for example powder or bar of graphite or diamond; in liquid form, for example alcohol or benzine; or in gaseous form, for example methane, ethane, acetylene, acetone, methanol and ethanol.

The source of energy used in the present invention can be for example a source of thermal or electromagnetic energy, a laser, electrical discharge or electron beam. An example of thermal energy source that can be used in the present invention is an electrical discharge heating system, which comprises a system with a cathode and an anode connected to a capacitor and capable of discharging an electrical discharge through a gas which is a source of carbon and is arranged inside the cavity 3.

The collection space 4 can be provided for example in a tube made of quartz or other materials that are resistant to high temperatures, such as alumina or stainless steel. The collection space 4 can be connected to the cavity 3 in the system according to the present invention for example by way of flow limiting means, such as flow limiting means that comprise a channel or duct. The collection space 4, or both the collection space 4 and the cavity 3, can be provided with means 9, 10 for introducing gas.

In another aspect, the present invention relates to a method for producing nanotubes, for example by using a system according to the present invention. The SWCNT growth process can be described generically by means of the following steps:

- At the beginning of each individual growth event (single plasma explosion), the reaction chamber is filled with a mixture of carrier gas (for example argon) and of carbon source gas (for example methane). - In the second step, the plasma is introduced, for example by means of a high- voltage, high energy content electrical discharge (high temperature - above 4000 K). The viscosity of the ionized plasma and neutral gas present in the reaction chamber is such as to not allow the exit of the gas from the chamber through a small-diameter channel provided in one of the electrodes.

- The plasma simultaneously ablates a small portion of alumina from the walls of the reaction chamber and melts part of the electrode, which also acts as a device for loading growth catalyst.

- The high temperature of the plasma ensures that the catalyst is evaporated in the form of nanometer particles. - The catalyst particles are deposited on the surface of the alumina particles previously (or simultaneously) created by reaction chamber walls.

- The carbon source gas is decomposed by the plasma into hydrogen ions and carbon ions; the carbon is absorbed (at least partially) by the catalyst nanoparticles, saturating them at high temperature.

- During the processes mentioned above, the plasma temperature decreases to a level that allows the mixture of plasma, gas and several particles to exit from the reaction chamber through the channel provided in one of the electrodes. The mixture is cooled further and the catalyst nanoparticles enter the carbon supersaturation state. The excess carbon is expelled from the inside of the nanoparticles in the form of nanotubes.

The process for preparing carbon nanotubes according to the present invention that uses the system according to the present invention comprises the stages of filling the cavity 3 with a mixture of a carrier gas and of a carbon source gas, applying inside the cavity 3 suitable energy to create inside the cavity 3 a plasma containing nanoparticles of catalyst and ionized carbon.

In one embodiment, initially the alumina of the walls of the cavity 3 is ablated in the form of small particles. The catalyst (which at that moment is in the gaseous phase) is deposited on these particles as nanoscopic clusters.

The clusters absorb the ionized carbon from the gas up to saturation at high temperature of the electrical discharge. After the exit of the particles from the cavity 3, the temperature decreases, the clusters become supersaturated with carbon and expel it in the form of nanotubes. The carbon source gas used in the present invention can be for example methane, acetylene or another volatile organic compound that contains predominantly carbon.

The carrier gas used in the present invention can be for example argon, nitrogen, helium or another inert gas. It is possible to add to the carrier gas a gas suitable for treating SWCNT, in order to prevent the deposition of minute quantities of amorphous carbon or to facilitate its removal during the growth and transport of SWCNT inside the apparatus.

The gas for treating the carbon tubes used in the present invention can be for example hydrogen or a small amount of oxygen.

The energy can be supplied continuously or in pulses. The possibility to supply pulsed energy to the system is an important advantage, because it allows the operator to adjust both the consumption of the energy and the average temperature of the system by adjusting the duty cycle.

The volume of the cavity 3 and the amount of energy supplied must be selected so that the amount of energy supplied brings the cavity to a high temperature, ensuring the melting and partial and gradual evaporation of the catalyst 1 and the decomposition of the source of carbon C. Simultaneously, other conditions of the growth of SWCNT from the mixture of catalyst 1 and carbon C must be met (for example relatively high pressure in the cavity 3). The spontaneous diffusion of the mixture of the catalyst 1 and of carbon C toward the space 4 allows the growth of individual SWCNT scarcely contaminated by graphite and amorphous carbon attached to the catalyst nanoparticles ("sea urchin of SWCNT").

In an embodiment of the system according to the present invention as shown in Figure 2, the energy source is constituted by an electrical discharge heating and ionization system. In this embodiment, the cavity 3 has a first end, which comprises the SWCNT growth catalyst 1, which is simultaneously the cathode of the electrical discharge heating system that constitutes the energy source of the system, and a second end, which comprises a conducting body 7, made for example of graphite, which also constitutes the anode of the heating system. The collection space 4 is connected to the cavity 3 through a channel or duct 8 in the body 7, which is preferably central, the hole 8 constituting a limiter of the flow of hot gas from the inside of the cavity 3.

Energy is supplied to the system by a capacitor 6 connected to the cathode 1 and to the anode 7.

In order to produce carbon nanotubes with the process according to the present invention by using this embodiment of the system according to the present invention, the system is filled with a mixture of carrier gas (argon) and of carbon source gas (methane, acetylene at 10-50% volume) with a total pressure from 100 to 800 mbar inside a quartz tube which creates in combination the space 4 for collecting SWCNT. At the beginning of the process, the cavity 3 is filled with a mixture of carrier gas and carbon source gas from the volume 4 by means of the hole in the channel or duct 8. The cathode 1 -anode 7 system is then connected to the capacitor 6, which is discharged by means of an electrical discharge between them through the ionized gas, creating the relatively hot plasma (3000-4500⁰C) containing nanoparticles of catalyst and ionized carbon released from gas. Such a high temperature is achieved by compressing the plasma in the relatively small volume and by reducing the duration of the discharge (typical discharge time τ = 1 μs). Simultaneously, the pressure increase required for the growth of SWCNT is achieved in the cavity 3 and contributes to the increase of the temperature of the plasma. Plasma at the temperature in the interval 3000-4500⁰C has such a viscosity that it prevents the immediate outflow of the ionized gas by means of the narrow channel or duct 8, assisting the system in increasing the temperature and extending the time of the reaction between the evaporated catalyst and the ionized carbon. The radiation of the heat from the cavity 3 through the material 2 leads to a decrease of the temperature of the plasma inside the cavity, which leads to a reduction of the viscosity of the ionized gas. The "cold" plasma exits from the cavity 3 through the channel or duct 8 toward the space 4, where it is cooled further. During this process, the carbon-saturated catalyst solidifies and SWCNT growth occurs. Expansion of the plasma outside the cavity 3 creates a void inside it. The cavity 3 is again filled with gas mixture from the space 4 by means of the channel or duct 8 and the cycle can be repeated. In another embodiment of the system of the present invention, the energy source comprises a laser beam or an electron beam, for example a laser beam or a pulsed electron beam, which enters the cavity 3, for example through the channel or duct 8.

In another embodiment, the supply of carbon source gas can be performed by means of a one-way valve 9 from a duct 1 1 directly into the cavity 3 and in the space 4 it is possible to supply, by means of a valve 10 from a duct 12, the carrier gas alone or in a mixture with the gas used for post-growth SWCNT treatment (for example hydrogen).

In the system according to Figure 3, the carbon source gas is supplied continuously by means of the duct 8 and the one-way valve 9. The pure carrier gas is supplied directly in the duct 8 for transporting nanotubes. The entire process of introducing plasma, creating nanoparticles of catalyst deposited on the surface of alumina particles, supersaturation of the carbon catalyst from carbon source gas, expulsion of particles from the reaction chamber and expulsion of carbon from the particles in the form of nanotubes occurs exactly in the same manner as in the preceding example. The two fundamental differences between the two systems are as follows:

- the direct supply of carbon source gas in the reaction chamber allows to increase substantially the concentration (and therefore supersaturation) of carbon, increasing SWCNT productivity significantly.

- the outflow of nanotubes from the reaction chamber directly inside the pure carrier gas prevents the process of carbon source gas decomposition (it is not present) at relatively low temperatures and therefore the creation of amorphous carbon and, in turn, contamination of the nanotubes.

Example An example of the embodiment of the system according to the present invention shown in Figure 2 is a system that comprises the catalyst 1, constituted by a cylindrical bar having a diameter D = 6 mm and a composition of 50% Co by weight + 50% Ni by weight, which forms an end wall of the cylindrical cavity 3, which has a diameter Dl = 5 mm and a length Ll = 8 mm, and simultaneously the cathode of the electrical discharge heating system. The body 2 of the cylindrical cavity 3 is made of a material with high thermal resistance and mechanical strength, such as alumina. The other end 5 of the cavity is constituted by a graphite cylinder, with a diameter D2 = 4 mm and a length L2 = 20 mm, which has a central channel or duct 8 with a diameter d = 1.5 mm. The channel or duct 8 in the end 5 constitutes a limiter of the flow of hot gas from the inside of the cavity 3. The graphite end 5 constitutes at the same time the anode of the heating system. Graphite was selected for its qualities of resistance to high temperatures, electrical conductivity and compatibility with the process (there is no danger of contamination with elements which are foreign to the process at high temperatures). The system is filled with a mixture of carrier gas (argon) and carbon source gas (methane, acetylene at 10-50% volume) with a total pressure from 100 to 800 mbar within a quartz tube which creates in combination the space 4 for collecting SWCNT. The pulsed energy is supplied to the system by a capacitor 6 with a capacitance C = 100 μF charged at the voltage U = 3000 V (energy 450 J) by means of the high- voltage power supply U_a = 3000 V and a maximum current I_a = 400 mA.

At the beginning of the process, the cavity 3 is filled with a mixture of carrier gas and of carbon source gas from the volume 4 by means of the channel or duct 8 in the end 7. The cathode 1 -anode 7 system is then connected to the capacitor 6, which is discharged by means of an electrical discharge between them through the ionized gas, creating the relatively hot plasma (3500-4100⁰C) containing nanoparticles of catalyst and ionized carbon released from gas. Such a high temperature is achieved by means of the compression of the plasma in the relatively small volume and by way of the reduction of the discharge duration (typical discharge time τ = 1 μs). Simultaneously, the increase in pressure that is necessary for the growth of SWCNT is achieved in the cavity 3 and in turn contributes to the increase of the temperature of the plasma. The plasma at the temperature of 4000⁰C has such a viscosity that it prevents the immediate outflow of the ionized gas by means of the narrow channel or duct in the end 5, assisting the system in increasing the temperature and extending the time of the reaction between the evaporated catalyst and the ionized carbon. The radiation of the heat from the cavity 3 through the material 2 leads to a drop in the temperature of the plasma inside the cavity, which leads to a reduction of the viscosity of the ionized gas. The "cold" plasma exits from the cavity 3 by means of the channel or duct in the end 5 toward the space 4, where it is cooled further. During this process, the carbon saturated catalyst solidifies and SWCNT growth occurs. Expansion of the plasma outside the cavity 3 creates a void inside it. The cavity 3 is again filled with a mixture of gas from the space 4 by means of the channel or duct in the end 5 and the cycle can be repeated. Results of the method

Figures 4a, 4b and 5 demonstrate the capability of the method according to the invention to generate single-wall carbon nanotubes (SWCNT, the scale can be seen in the bottom left corner of Figures 4a and 4b in comparison with tube diameter). Under certain growth conditions, the tubes grow separate (free) and scarcely contaminated by nanoparticles of amorphous carbon or graphite. The nanotubes grow on the surface of nanoparticles of the catalyst in all directions in space. This fact is difficult to verify from a single image, since the focusing field of TEM is very narrow. For this reason, TEM is unable to show different, positions on the surface of the nanoparticles. Moreover, the structural contrast of carbon is relatively low (low Z value) and in order to visualize the nanotubes it is necessary to shift the lens of the TEM slightly out of focus, and this leads to an overestimation of their diameter. Figures 4a, 4b also show that by varying the concentration of the gas (methane, acetylene) it is possible to control the growth of crossed (or joined) tubes. This possibility is fundamental for preparing single-molecule transistors. The typical Raman spectrum is shown in Figure 5. The important characteristics are the following:

- the band G of graphene (at Raman shift 1586 cm^'1) is relatively narrow but shows a broadening at the base, both factors pointing to the presence of SWCNT; - the band D of the defective mode (at Raman shift 131 1 cm^"1) demonstrates the very low intensity with respect to the band G ; this factor demonstrates the limited quantity of defects (both SWCNT defects and the presence of graphite and amorphous carbon);

- the band at Raman shift 196 cm^"1, i.e., SWCNT breathing mode, is irrefutable evidence of the presence of these nanotubes in the sample.

The processes mentioned above have demonstrated the importance of the presence of alumina micro- and nanoparticles in the volume where decomposition of the carbon carrier and saturation of the nanoparticles of the catalyst with the released carbon occur. The alumina particles act as a carrier (and separator) of the nanoparticles and clusters of the catalyst, effectively increasing its active surface and therefore its efficiency in the growth of carbon nanotubes and simultaneously store an important part of the carbon released by its carrier. This carbon is then transformed to carbon nanotubes by means of the process of surface diffusion on the surface of the particles and the process of absorption and expulsion of the carbon from the nanoparticles of the catalyst.

The importance of the alumina particles for the process of carbon nanotube growth has been verified by means of the following experiments based on the "traditional" method of preparing carbon nanotubes: the electric arc process, in which parameters were used in the following intervals:

Current in the arc I: 30 to 150 A.

Voltage in the arc V: 20 to 50 V.

Pressure in the CNT growth chamber p: 0.2 to 0.6 bars. The carrier gases in the deposition chamber are noble gases, particularly Ar, He, Ne, Xe.

The quantity of the catalyst is approximately 1 to 30% on the total weight of the electrode (in total among various elements, among them particularly Ni, Co, Fe and their mixtures). The quantity of alumina powder is approximately 1 to 60% on the total weight of the electrode.

The dimensions of the deposition chamber are not binding.

The experiment consisted of two successive steps: in the first step, growth of nanotubes was performed in the apparatus manufactured for this purpose (cylindrical vacuum chamber with a diameter of 150 mm and length of 400 mm, with two water-cooled axial electrode carriers). Two carbon electrodes containing 1% by weight of cobalt powder (in the form of nanoparticles) and 1% by weight of nickel powder (also in the form of nanoparticles) were introduced in the chamber. The electrodes were connected to a source of high electric current (up to I_raax = 80 A) and a voltage V = 40 V. The stream of argon with a pressure of 0.3 bar was introduced in the chamber. The electric arc was ignited by moving the electrodes mutually closer and then stabilized by manually controlling their mutual distance. The heat of the electric arc causes evaporation both of the carbon and of the catalyst. The mixture of vapors is cooled progressively by moving away from the region of the arc, the catalyst solidifies in the form of nanoparticles and clusters and expels the carbon from its interior in the form of carbon nanotubes.

The yield (proportion) of carbon nanotubes prepared by means of the process mentioned above is in the interval of 30-34% by weight with respect to the total weight of the product, which is composed of more than 2% by weight of catalyst, the remainder being amorphous carbon.

In the second step, the experiment was repeated in conditions which were strictly identical to the previous experiments, except for the composition of the electrodes. The electrodes were composed of a mixture of 10% by weight of catalyst (5% cobalt by weight and 5% nickel by weight, both in the form of nanoparticle powder), 40% by weight of alumina in the form of micrometer powder, and the remainder was ultrapure graphite. The product of this experiment is usually composed of 29-31% by weight of carbon nanotubes, 10% by weight of catalyst, 40% by weight of alumina and the remainder is amorphous carbon. This result means that the presence of the alumina particles in the region of the reaction causes the growth of the yield of carbon nanotubes from 82 to 94% by weight with respect to the experiment performed in the absence of alumina. The disclosures in Italian Patent Application No. MI2007A001867 from which this application claims priority are incorporated herein by reference.

Where technical features mentioned in any claim are followed by reference signs, those reference signs have been included for the sole purpose of increasing the intelligibility of the claims and accordingly, such reference signs do not have any limiting effect on the interpretation of each element identified by way of example by such reference signs.

Claims

1. A system for producing carbon nanotubes, comprising a body with a cavity which is connected at least partially to a device for loading a SWCNT growth catalyst and to a SWCNT collection space, energy source means E for supplying energy in the cavity and means for introducing a carbon source in said cavity.

2. The system according to claim 1, wherein said cavity has walls made of a material that is resistant to high temperatures and high pressures, in particular Al₂O₃ or tungsten.

3. The system according to claim 1 or 2, wherein said catalyst loading device is adapted to load into said cavity said catalyst in solid or liquid or gaseous form.

4. The system according to any one of the preceding claims, wherein said energy source E is selected from the group constituted by a source of heat energy, electromagnetic energy, laser, electrical discharge or electron beam.

5. The system according to any one of the preceding claims, wherein said space for collecting nanotubes is made of a material that is resistant to a high temperature, particularly a tube of quartz, alumina or stainless steel.

6. The system according to any one of the preceding claims, wherein said space for collecting nanotubes is connected to said cavity by flow limiting means, particularly by a channel or duct, particularly with a ratio between the transverse cross-section of the channel or duct and the transverse cross-section of the cavity from 0.05 to 0.25.

7. The system according to any one of the preceding claims, wherein said collection space, or both said collection space and said cavity, are provided with means for introducing gas.

8. The system according to any one of the preceding claims, wherein said SWCNT growth catalyst is selected from the group constituted by nickel, cobalt and iron, and their solid, liquid and volatile compounds, particularly ferrocene, cobaltocene or nickelocene.

9. The system according to any one of the preceding claims, wherein said carbon source C is in solid, liquid or gaseous form, particularly a bar of graphite or diamond, or comprises one or more gaseous organic compounds containing carbon, more particularly a material selected among methane, ethane, acetylene, acetone, methanol and ethanol.

10. The system according to any one of the preceding claims, wherein said energy source is an electrical discharge heating and ionization system which has a cathode and an anode.

11. The system according to claim 10, wherein said cathode is constituted by an end of the cavity that comprises said catalyst.

12. The system according to claim 10 or 1 1, wherein said anode is constituted by an end of said cavity that comprises a conducting body, particularly a graphite conducting body.

13. The system according to one of claims 10 to 12, wherein a capacitor is linked to said cathode and said anode.

14. The system according to one of claims 1 to 9, wherein said energy source is a laser beam or an electron beam that enters the cavity, particularly through a channel or duct for connection between said cavity and said collection space.

15. The system according to claim 14, wherein said laser beam or an electron beam is a laser beam or a pulsed electron beam.

16. The system according to claim 7, wherein said gas introduction means are constituted by one-way valves connected to means for supplying gas, particularly gas supply ducts.

17. A method for preparing carbon nanotubes using a system according to one or more of claims 1-16, comprising the steps of introducing in said cavity a mixture of a carrier gas and of a carbon source gas, applying inside the cavity an energy adapted to create inside the cavity a hot plasma that contains catalyst and ionized carbon, cooling said plasma with formation of carbon nanotubes and collection of said carbon nanotubes.

18. The method according to claim 17, wherein said carbon source gas is selected from the group constituted by methane, ethane, acetylene, acetone, methanol and ethanol.

19. The method according to one of claims 17 or 18, wherein said carrier gas is an inert gas, particularly a gas selected among argon, nitrogen and helium.

20. The method according to any one of claims 17 to 19, wherein said energy is supplied continuously or in a pulsed manner.

21. The method according to any one of claims 19 or 20, wherein said energy is applied in a quantity suitable to bring said cavity to a temperature at which said catalyst melts and evaporates and said carbon source gas decomposes.

22. The method according to any one of claims 17 to 21, wherein said energy is supplied by an electrical discharge heating and ionization source.

23. The process according to claim 21 using the system according to claim 13 or 14, comprising the step of feeding said mixture of carrier gas and carbon source gas, in said collection space, filling said cavity with said mixture through said connecting duct or channel, obtaining in said cavity a pressure preferably from 100 to 800 mbar, connecting said anode and said cathode to the capacitor, which is discharged by means of an electrical discharge between said anode and said cathode through said mixture of gas, creating a hot plasma which contains nanoparticles of catalyst and ionized carbon, cooling the hot plasma, preferably at a temperature from 3000 to 4500⁰C, and passing the cold plasma in said collection space through said channel or duct, further cooling the plasma of said collection space, growing and collecting carbon nanotubes in said collection space.