Abstract:
There is provided a method for producing single-wall carbon nanotubes comprising condensing a plasma comprising atoms or molecules of carbon and atoms of a metal suitable for catalyzing the formation of single-wall carbon nanotubes in an oven at a predetermined temperature so as to provide a temperature gradient, thereby forming single-wall carbon nanotubes; and collecting the so-formed single-wall carbon nanotubes by passing them through an electrostatic trap comprising a pair of electrodes generating an electrical current so as to deposit at least a portion of the single-wall carbon nanotubes on one of the electrodes.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to improvements in the field of carbon nanotube production. More particularly, the invention relates to an improved method and apparatus for producing single-wall carbon nanotubes. 
       BACKGROUND OF THE INVENTION 
       [0002]    Carbon nanotubes are available either as multi-wall or single-wall nanotubes. Multi-wall carbon nanotubes have exceptional properties such as excellent electrical and thermal conductivities. They have applications in numerous fields such as storage of hydrogen (C. Liu, Y. Y. Fan, M. Liu, H. T. Cong, H. M. Cheng, M. S. Dresselhaus, Science 286 (1999), 1127; M. S. Dresselhaus, K. A Williams, P. C. Eklund, M R S Bull. (1999), 45) or other gases, adsorption heat pumps, materials reinforcement or nanoelectronics (M. Menon, D. Srivastava, Phy. Rev. Lett. 79 (1997), 4453). Single-wall carbon nanotubes, on the other hand, possess properties that are significantly superior to those of multi-wall nanotubes. However, single-wall carbon nanotubes are available only in small quantities since known methods of production do not produce more than few grams per day of these nanotubes. For any industrial application such as storage or material reinforcement, the amount of single-wall carbon nanotubes produced must be at least a few kilograms per day. 
         [0003]    Nowadays, the most popular methods for producing single-wall carbon nanotubes are laser ablation, electric arc and chemical vapor deposition (CVD). The two first methods are based on the same principal: local evaporation of a graphite target enriched with a metal catalyst and subsequent condensation of the vapor to form nanotubes (A. A. Puretzky, D. B. Geohegan, S. J. Pennycook, Appl. Phys. A 70 (2000), 153). U.S. Pat. No. 6,183,714 discloses a method of making ropes of single-wall carbon nanotubes using a laser pulse to produce a vapor containing carbon and one or more Group VIII transition metals. U.S. Pat. No. 5,424,054 discloses a process for producing hollow carbon fibers having wall consisting essentially of a single layer of carbon atoms using an electric arc. The process involves contacting carbon vapor with cobalt vapor under specific conditions, and is thus limited to the use of cobalt vapor. 
         [0004]    Although the above methods are relatively efficient for the transformation of carbon into nanotubes, they have inherent drawbacks. The vaporisation of graphite is not energetically advantageous since 717 kJ are required to evaporate one mole of carbon. Therefore, the production of single-wall carbon nanotubes via laser ablation and electric arc consumes a lot of energy for small quantities of nanotubes produced. Moreover, these processes are non-continuous since they must be stopped for renewing the source of carbon once the graphite has been consumed. 
         [0005]    In the CVD method as well as in the other two methods described above, the metal catalyst plays a key role in the synthesis of the nanotubes. For example, in the CVD method, the carbon-containing gas is decomposed by the particles of metal catalyst on which the nanotubes form. The CVD method suffers from a major drawback since the encapsulation of the catalyst particles by carbon stops the growth of the nanotubes (R. E. Smalley et al. Chem. Phys. Lett. 296 (1998), 195). In addition, due to the non-selectivity of the method, nanotubes having two, three or multi-walls are obtained at the same time as the single-wall nanotubes. 
         [0006]    A promising method for the production of single-wall carbon nanotubes involves the use of a plasma torch for decomposing a mixture of carbon-containing substance and a metal catalyst and then condensing the mixture to obtain single-wall carbon nanotubes. This method has been recently described by O, Smiljanic, B. L. Stansfield, J.-P. Dodelet, A. Serventi, S. Ddsilets, in Chem. Phys. Lett. 356 (2002), 189 and showed encouraging results. Such a method, however, has an important drawback since a premature extinction of the plasma torch occurs due to a rapid formation of carbon deposit in the torch. This method is therefore non-continuous and requires removal of the carbon deposit. Thus, large quantities of single-wall carbon nanotubes cannot be produced. 
       SUMMARY OF THE INVENTION 
       [0007]    It is therefore an object of the present invention to overcome the above drawbacks and to provide a method and apparatus for the continuous production of single-wall carbon nanotubes in large quantities. 
         [0008]    According to a first aspect of the invention, there is provided a method for producing single-wall carbon nanotubes, comprising the steps of: 
         [0009]    a) providing a plasma torch having a plasma tube with a plasma-discharging end; 
         [0010]    b) feeding an inert gas through the plasma tube to form a primary plasma; 
         [0011]    c) contacting a carbon-containing substance and a metal catalyst with the primary plasma at the plasma-discharging end of the plasma tube, to form a secondary plasma containing atoms or molecules of carbon and atoms of metal catalyst; and 
         [0012]    d) condensing the atoms or molecules of carbon and the atoms of metal catalyst to form single-wall carbon nanotubes. 
         [0013]    According to a second aspect of the invention, there is provided a method for producing single-wall carbon nanotubes, comprising the steps of: 
         [0014]    a) providing a plasma torch having a plasma tube with a plasma-discharging end; 
         [0015]    b) feeding an inert gas and an inorganic metal catalyst through the plasma tube to form a primary plasma containing the atoms of metal catalyst; 
         [0016]    c) contacting a carbon-containing substance with the primary plasma at the plasma-discharging end of said plasma tube, to form a secondary plasma containing atoms or molecules of carbon and the atoms of metal catalyst; and 
         [0017]    d) condensing the atoms or molecules of carbon and the atoms of metal catalyst to form single-wall carbon nanotubes. 
         [0018]    According to a third aspect of the invention, there is provided an apparatus for producing single-wall carbon nanotubes, which comprises: 
         [0019]    a plasma torch having a plasma tube for receiving an inert gas so as to form a primary plasma, the plasma tube having a plasma-discharging end; 
         [0020]    a feeder for directing a carbon-containing substance and a metal catalyst towards the primary plasma so that the carbon-containing substance and the metal catalyst contact the primary plasma at the plasma-discharging end of the plasma tube, to thereby form a secondary plasma containing atoms or molecules of carbon and the atoms of the metal catalyst; and 
         [0021]    a condenser for condensing the atoms or molecules of carbon and the atoms of the metal catalyst to form single-wall carbon nanotubes. 
         [0022]    According to a fourth aspect of the invention, there is provided an apparatus for producing single-wall carbon nanotubes, which comprises: 
         [0023]    a plasma torch having a plasma tube for receiving an inert gas and an inorganic metal catalyst so as to form a primary plasma containing atoms of the metal catalyst, the plasma tube having a plasma-discharging end; 
         [0024]    a feeder for directing a carbon-containing substance towards the primary plasma so that the carbon-containing substance contacts the primary plasma at the plasma-discharging end of the plasma tube, to thereby form a secondary plasma containing atoms or molecules of carbon and the atoms of the metal catalyst; and 
         [0025]    a condenser for condensing the atoms or molecules of carbon and the atoms of the metal catalyst to form single-wall carbon nanotubes. 
         [0026]    Applicant has found quite surprisingly that by feeding the carbon-containing substance separately from the inert gas used to generate the primary plasma so that the carbon-containing substance contacts the primary plasma at the plasma-discharging end of the plasma tube to form the aforesaid secondary plasma, there is no undesirable formation of carbon deposit adjacent the plasma-discharging end of the plasma tube. Thus, no premature extinction of the plasma torch. 
         [0027]    The term “carbon-containing substance” as used herein refers to a substance which contains carbon atoms. Preferably, such a substance does not contain nitrogen atoms. The carbon-containing substance can be a solid, a liquid or a gas. 
         [0028]    The expression “organometallic complex” as used herein refers to a compound in which there is a bonding interaction (ionic or covalent, localized or delocalized) between one or more carbon atoms of an organic group or molecule with a main group, transition, lanthanide, or actinide metal atom or atoms. 
         [0029]    The expression “rapid condensation” as used herein refers to a condensation which occurs at a rate of at least 10 5  K/second. 
       DETAILED DESCRIPTION OF THE INVENTION 
       [0030]    In the method according to the first aspect of the invention, step (c) can be carried out by separately directing the carbon-containing substance and the metal catalyst towards the primary plasma. The carbon-containing substance can be in admixture with a carrier gas. Preferably, the carbon-containing substance is in liquid or gaseous phase and the carbon-containing substance in liquid or gaseous phase flows along a helical path prior to contacting the primary plasma. The carbon-containing substance in liquid or gaseous phase is preferably in admixture with a carrier gas. It is also possible to use a carbon-containing substance in solid phase, in admixture with a carrier gas; such a mixture preferably flows along a helical path prior to contacting the primary plasma. The metal catalyst can also be in admixture with a carrier gas. When use is made of a metal catalyst in liquid or gaseous phase, such a metal catalyst preferably flows along a helical path prior to contacting the primary plasma. The metal catalyst in liquid or gaseous phase is preferably in admixture with a carrier gas. It is also possible to use a metal catalyst in solid phase, in admixture with a carrier gas; such a mixture preferably flows along a helical path prior to contacting the primary plasma. 
         [0031]    Step (c) of the method according to the first aspect of the invention can also be carried out by directing a mixture of the carbon-containing substance and the metal catalyst towards the primary plasma. The latter mixture can be in admixture with a carrier gas. Preferably, the carbon-containing substance and the metal catalyst are in liquid or gaseous phase and the latter two flow along a helical path prior to contacting the primary plasma. The carbon-containing substance and the metal catalyst in liquid or gaseous phase are preferably in admixture with a carrier gas. It is also possible to use the carbon-containing substance and the metal catalyst in solid phase, in admixture with a carrier gas; such a mixture preferably flows along a helical path prior to contacting the primary plasma. 
         [0032]    The metal catalyst used in the method according to the first aspect of the invention is preferably an organometallic complex. It is also possible to use, as a metal catalyst, an inorganic metal complex or an inorganic metal catalyst comprising at least one metal in metallic form. Examples of suitable metal catalyst include those comprising at least one metal selected from the group consisting of Fe, Ru, Co, Rh, Ir, Ni, Pd, Pt, Y, La, Ce, Mn, Li, Pr, Nd, Tb, Dy, Ho, Er, Lu and Gd. The metal is preferably iron. 
         [0033]    The metal catalyst can also comprise cobalt and at least one metal selected from the group consisting of Ni, Fe, Y, Pt, Mo, Cu, Pb and Bi. Alternatively, the metal catalyst can comprise nickel and at least one metal selected from the group consisting of Fe, Y, Lu, Pt, B, Ce, Mg, Cu and Ti. 
         [0034]    Ferrocene, iron (II) phthalocyanine, iron in metallic form, iron pentacarbonyl and mixtures thereof can be used as suitable metal catalyst. Ferrocene is preferred. 
         [0035]    In the method according to the first aspect of the invention, it is possible to use the inert gas in admixture with an inorganic metal catalyst which may be the same or different than the one used in step (c). 
         [0036]    In the method according to the second aspect of the invention, step (c) can be carried out by directing the carbon-containing substance towards the primary plasma. The carbon-containing substance can be in admixture with a carrier gas. Preferably, the carbon-containing substance is in liquid or gaseous phase and the carbon-containing substance in liquid or gaseous phase flows along a helical path prior to contacting the primary plasma. The carbon-containing substance in liquid or gaseous phase is preferably in admixture with a carrier gas. It is also possible to use a carbon-containing substance in solid phase, in admixture with a carrier gas; such a mixture preferably flows along a helical path prior to contacting the primary plasma. 
         [0037]    The inorganic metal catalyst used in the method according to the second aspect of the invention can be an inorganic metal complex or at least one metal in metallic form. Preferably, the inorganic metal catalyst comprises at least one metal selected from the group consisting of Fe, Ru, Co, Ph, Ir, Ni, Pd, Pt, Y, La, Ce, Mn, Li, Pr, Nd, Tb, Dy, Ho, Er, Lu and Gd. The metal is preferably iron. The inorganic metal catalyst can also comprise cobalt and at least one metal selected from the group consisting of Ni, Fe, Y, Pt, Mo, Cu, Pb and Bi. Alternatively, the inorganic metal catalyst can comprise nickel and at least one metal selected from the group consisting of Fe, Y, Lu, Pt, B, Ce, Mg, Cu and Ti. 
         [0038]    The carbon-containing substance used in the method according to the first or the second aspect of the invention can be a carbon-containing gas, a carbon-containing liquid or a carbon-containing solid. It is also possible to use a mixture of a carbon-containing gas and a carbon-containing liquid, a mixture of a carbon-containing gas and a carbon-containing solid, a mixture of a carbon-containing liquid and a carbon-containing solid or a mixture of a carbon-containing gas, a carbon-containing liquid and a carbon-containing solid. Preferably, the carbon-containing gas is a C 1 -C 4  hydrocarbon such as methane, ethane, ethylene, acetylene, propane, propene, cyclopropane, allene, propyne, butane, 2-methylpropane, 1-butene, 2-butene, 2-methylpropene, cyclobutane, methylcyclopropane, 1-butyne, 2-butyne, cyclobutene, 1,2-butadiene, 1,3-butadiene or 1-buten-3-yne or a mixture thereof. When commercial acetylene is used, care should be taken to filter such a gas in order to remove impurities. The carbon-containing liquid is preferably a C 5 -C 10  hydrocarbon. Alternatively, the carbon-containing liquid can be selected from the group consisting of pentane, hexane, cyclohexane, heptane, benzene, toluene, xylene or styrene or mixtures thereof. The carbon-containing solid can be graphite, carbon black, norbornylene, naphthalene, anthracene, phenanthrene, polyethylene, polypropylene, or polystyrene or mixtures thereof. Graphite is preferred. More preferably, the graphite is in the form of a nano-powder. 
         [0039]    The inert gas used in the method according to the first or second aspect of the invention can be helium, argon or a mixture thereof. Argon is preferred. A further inert gas can be injected in the plasma torch and directed towards the primary and secondary plasmas. A cooling inert gas is preferably injected downstream of the secondary plasma; the cooling inert gas can be helium, argon or a mixture thereof. The cooling inert gas assists in providing a temperature gradient. The aforementioned carrier gas can be helium, argon, hydrogen or hydrogen sulfide or a mixture thereof. Argon is preferably used as carrier gas. 
         [0040]    According to a preferred embodiment, the metal catalyst and the carbon-containing substance are used in an atomic ratio metal atoms/carbon atoms of about 0.01 to about 0.06. More preferably, the atomic ratio metal atoms/carbon atoms is about 0.02. 
         [0041]    Step (d) of the method according to the first or second aspect of the invention is preferably carried out to provide a temperature gradient permitting rapid condensation of the atoms or molecules of carbon and the atoms of metal catalyst. Preferably, the temperature gradient is provided by directing the atoms or molecules of carbon and the atoms of metal catalyst through an oven disposed downstream of the plasma tube in spaced relation thereto, the oven being heated at a predetermined temperature. The predetermined temperature can be comprised between 500 and 1800° C. and preferably between 800 and 950° C. A temperature of about 900° C. is preferred. Such a temperature of about 900° C. is also particularly preferred when the metal catalyst comprises iron. The single-wall carbon nanotubes produced can be collected in a trap such as an electrostatic trap. 
         [0042]    In the apparatus according to the third aspect of the invention, the feeder preferably comprise a first conduit for directing the carbon-containing substance towards the primary plasma and a second conduit for directing the metal catalyst towards the primary plasma. Preferably, the first and second conduits each have a discharge end disposed adjacent the plasma-discharging end of the plasma tube. Alternatively, the feeder can comprise a single conduit for directing a mixture of the carbon-containing substance and the metal catalyst towards the primary plasma. In such a case, the single conduit preferably has a discharge end disposed adjacent the plasma-discharging end of the plasma tube. In a particularly preferred embodiment, the single conduit is disposed inside the plasma tube and extends substantially coaxially thereof. 
         [0043]    In the apparatus according to the fourth aspect of the invention, the feeder preferably comprises a single conduit for directing the carbon-containing substance towards the primary plasma. Preferably, the conduit has a discharge end disposed adjacent the plasma-discharging end of the plasma tube. In a particularly preferred embodiment, the conduit is disposed inside the plasma tube and extends substantially coaxially thereof. 
         [0044]    In the apparatus according to the third or fourth aspect of the invention, the condenser preferably comprise an oven disposed downstream of the plasma tube in spaced relation thereto, and a heat source for heating the oven to provide a temperature gradient permitting rapid condensation of the atoms or molecules of carbon and the atoms of metal catalyst. Preferably, a heat-resistant tubular member having a plasma-receiving end extends through the oven with the plasma-receiving end disposed upstream of the plasma-discharging end of the plasma tube. An injector is provided for injecting a cooling inert gas into the tubular member, downstream of the secondary plasma; the cooling inert gas assists in providing the temperature gradient. The heat-resistant tubular member can be made of quartz or boron nitride. The apparatus can be provided with a trap for collecting single-wall carbon nanotubes produced. Preferably, the trap is an electrostatic trap. The apparatus can also be provided with a cooling system disposed about the plasma tube and extends substantially coaxially thereof. Preferably, the apparatus comprises a Faraday shield made of a conductive material for preventing emission of electromagnetic radiations outside of the apparatus. 
         [0045]    Where the apparatus according to the third or fourth aspect of the invention has the aforementioned conduit disposed inside the plasma tube and extending substantially coaxially thereof, the apparatus preferably includes another heat-resistant tubular member disposed about the plasma tube and extending substantially coaxially thereof, and an injector for injecting a further inert gas between the plasma tube and the tubular member to prevent undesirable formation of carbon deposit adjacent the plasma-discharging end of the plasma tube. The latter heat-resistant tubular member can also be made of quartz or boron nitride. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0046]    Further features and advantages of the invention will become more readily apparent from the following description of preferred embodiments as illustrated by way of examples in the appended drawings wherein: 
           [0047]      FIG. 1  is a schematic, sectional elevation view of an apparatus for producing single-wall carbon nanotubes, according to a first preferred embodiment of the invention; 
           [0048]      FIG. 2  is a schematic, sectional elevation view of an apparatus for producing single-wall carbon nanotubes, according to a second preferred embodiment of the invention; 
           [0049]      FIG. 3  is a schematic, sectional elevation view of an apparatus for producing single-wall carbon nanotubes, according to a third preferred embodiment of the invention; 
           [0050]      FIG. 4  is a schematic, sectional elevation view of an injecting device according to a fourth preferred embodiment of the invention; 
           [0051]      FIG. 5  is a SEM (Scanning Electron Microscope) picture of a crude sample of single-wall carbon nanotubes; 
           [0052]      FIG. 6  is another SEM picture of the sample shown in  FIG. 5 ; 
           [0053]      FIG. 7  is a TEM (Transmission Electron Microscope) picture of the sample shown in  FIG. 5 ; 
           [0054]      FIG. 8  is another TEM picture of the sample shown in  FIG. 5 ; 
           [0055]      FIG. 9  is the graph of a Raman spectroscopy measurement performed on the sample shown in  FIG. 5  using a 514 nm laser; and 
           [0056]      FIG. 10  is the graph of another Raman spectroscopy measurement performed on the sample shown in  FIG. 5  using a 782 nm laser. 
       
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0057]    Referring first to  FIG. 1 , there is shown an apparatus  10  for producing single-wall carbon nanotubes, which comprises a plasma torch  12  having a plasma tube  14  with a plasma-discharging end  16 , and an oven  18  disposed downstream of the plasma tube  14  in spaced relation thereto. The plasma tube  14  is adapted to receive an inert gas for activation by electromagnetic radiation generated from a source (not shown) so as to form a primary plasma  20 . The electromagnetic radiations are propagated on the plasma tube  14  so as to maintain the primary plasma  20 . The primary plasma  20  comprises ionized atoms of the inert gas. A feed conduit  22  having a discharge end  24  is arranged inside the plasma tube  14  and extends substantially coaxially thereof. The discharge end  24  of the feed conduit  22  is disposed adjacent the plasma discharging end  16  of the plasma tube  14 . The feed conduit  22  serves to direct a carbon-containing substance, such as a carbon-containing gas, and a metal catalyst towards the primary plasma  20  so that the carbon-containing substance and the metal catalyst contact the primary plasma  20  at the plasma-discharging end  16  of the plasma tube  14 , whereby to form a secondary plasma  26  containing atoms or molecules of carbon and the atoms of metal catalyst. The carbon-containing gas is preferably ethylene or methane. 
         [0058]    The oven  18  serves to condense the atoms or molecules of carbon and atoms of metal catalyst to form single-wall carbon nanotubes  28 . A heat source  30  is provided for heating the oven  18  to generate a temperature gradient permitting rapid condensation of the atoms or molecules of carbon and the atoms of metal catalyst. A heat-resistant tubular member  32  having a plasma-receiving end  34  extends through the oven  18 , the plasma-receiving end  34  being disposed upstream of the plasma-discharging end  16  of the plasma tube  14 . An electrostatic trap  35  comprising a filter  36  and a rod  37  is extending downstream of oven  18 . The deposit of single-wall carbon nanotubes  28  occurs on the heat-resistant member  32  upstream and downstream of the oven  18 , as well as inside of the trap  35 . The filter  36  traps some of the fine particles (not shown) generated during the formation of single-wall carbon nanotubes  28  and reduces the emission of fine particles outside of the apparatus. The electrostatic trap  35  permits a more efficient recovery of the single-wall nanotubes produced by the apparatus  10 . The apparatus further includes a gas injector  38  for injecting a cooling inert gas into the tubular member  32 , downstream of the secondary plasma  26 . The cooling inert gas assists in providing the temperature gradient. Another heat-resistant tubular member  40  is disposed about the plasma tube  14  and extends substantially coaxially thereof, the tubular member  40  being fixed to the tubular member  32  and supporting same. Another gas injector  42  is provided for injecting a further inert gas between the plasma tube  14  and the tubular member  40  to prevent undesirable formation of carbon deposit adjacent the plasma-discharging end  16  of the plasma tube  14 . The plasma tube  14  is also provided with a cooling system (not shown), which preferably uses water. The apparatus  10  further comprises a Faraday shield (not shown) made of a conductive material, preferably aluminium. 
         [0059]    The inert gas flows through the plasma tube  14  along a helical path represented by the arrow  44 . Similarly, the carbon-containing gas and the metal catalyst, optionally in admixture with a carrier gas, flow through the feed conduit  22  along a helical path represented by the arrow  46 . The metal catalyst which is fed through the conduit  22  can be either an organometallic complex such as ferrocene, or an inorganic metal catalyst such as iron in metallic form. Instead of feeding the metal catalyst through the conduit  22 , it is possible to feed only the carbon-containing gas through the conduit  22  and to feed the metal catalyst in admixture with the inert gas through the plasma tube  14 . In such a case, the metal catalyst must be an inorganic metal catalyst to prevent undesirable formation of carbon deposit adjacent the plasma-discharging end  16  of the plasma tube  14 . It is also possible to feed the inert gas and an inorganic metal catalyst through the plasma tube  14  and to feed the carbon-containing gas in admixture with an organometallic complex or an inorganic metal catalyst through the conduit  22 . 
         [0060]      FIG. 2  illustrates another apparatus  48  for producing single-wall carbon nanotubes, which comprises a plasma torch  50  having a plasma tube  52  with a plasma-discharging end  54 , and an oven  56  disposed downstream of the plasma tube  52  in spaced relation thereto. The plasma tube  52  is adapted to receive an inert gas for activation by electromagnetic radiation generated from a source (not shown) so as to form a primary plasma  58 . A feed conduit  60  having a discharge end  62  disposed adjacent the plasma-discharging end  54  of the plasma tube  52  is provided for directing a carbon-containing substance, such as a carbon-containing gas, and a metal catalyst towards the primary plasma  58 . The carbon-containing substance and the metal catalyst discharged from the feed conduit  60  contact the primary plasma  58  at the plasma-discharging end  54  of the plasma tube  52 , thereby forming a secondary plasma  64  containing atoms or molecules of carbon and the atoms of metal catalyst. The carbon-containing gas is preferably ethylene or methane. Although only one feed conduit  60  is shown in  FIG. 2 , it is possible to have a plurality of such conduits disposed symmetrically about the plasma tube  52 . The plasma tube  52  is also provided with a cooling system (not shown), which preferably uses water. The apparatus  48  further comprises a Faraday shield (not shown) made of a conductive material, preferably aluminium. 
         [0061]    The oven  56  serves to condense the atoms or molecules of carbon and the atoms of metal catalyst to form single-wall carbon nanotubes  66 . A heat source  68  is provided for heating the oven  56  to generate a temperature gradient permitting rapid condensation of the atoms or molecules of carbon and the atoms of metal catalyst. A heat-resistant tubular member  70  having a plasma-receiving end  72  extends through the oven  56 , the plasma-receiving end  72  being disposed upstream of the plasma-discharging end  54  of the plasma tube  52 . The apparatus further includes a gas injector  74  for injecting a cooling inert gas into the tubular member  70 , downstream of the secondary plasma  64 . The cooling inert gas assists in providing the temperature gradient. The deposit of single-wall carbon nanotubes  66  occurs on the heat-resistant tubular member  70  upstream and downstream of the oven  56 . 
         [0062]    The inert gas flows through the plasma tube  52  along a helical path represented by the arrow  76 . Similarly, the carbon-containing gas and the metal catalyst, optionally in admixture with a carrier gas, flow through the conduit  60  along a helical path represented by the arrow  78 . The metal catalyst which is fed through the conduit  60  can be either an organometallic complex such as ferrocene, or an inorganic metal catalyst such as iron. Instead of feeding the metal catalyst through the conduit  60 , it is possible to feed only the carbon-containing gas through the conduit  60  and to feed the metal catalyst in admixture with the inert gas through the plasma tube  52 . In such a case, the metal catalyst must be an inorganic metal catalyst to prevent undesirable formation of carbon deposit adjacent the plasma-discharging end  54  of the plasma tube  52 . It is also possible to feed the inert gas and an inorganic metal catalyst through the plasma tube  52  and to feed the carbon-containing gas in admixture with an organometallic complex or an inorganic metal catalyst through the conduit  60 . Optionally, the apparatus  48  can be provided with the electrostatic trap  35  illustrated in  FIG. 1 . 
         [0063]    The apparatus  48 ′ illustrated in  FIG. 3  is similar to the apparatus  48  shown in  FIG. 2 , with the exception that an additional feed conduit  60 ′ is provided, the feed conduits  60  and  60 ′ being arranged on either side of the plasma tube  52 . The conduit  60 ′ has a discharge end  62 ′ disposed adjacent the plasma-discharging end  54  of the plasma tube  52  and serves the same purpose as the feed conduit  60 . The carbon-containing gas and the metal catalyst, optionally in admixture with a carrier gas, flow through the conduit  60 ′ along a helical path represented by the arrow  78 ′. Although two feed conduits  60  and  60 ′ are shown in  FIG. 3 , it is possible to have a plurality of such conduits disposed symmetrically about the plasma tube  52 . Instead of feeding the metal catalyst through the conduits  60  and  60 ′, it is possible to feed only the carbon-containing gas through the conduits  60  and  60 ′ and to feed the metal catalyst in admixture with the inert gas through the plasma tube  52 . In such a case, the metal catalyst must be an inorganic metal catalyst to prevent undesirable formation of carbon deposit adjacent the plasma-discharging end  54  of the plasma tube  52 . It is also possible to feed the inert gas and an inorganic metal catalyst through the plasma tube  52  and to feed the carbon-containing gas in admixture with an organometallic complex or an inorganic metal catalyst through the conduits  60  and  60 ′. The plasma tube  52  is also provided with a cooling system (not shown), which preferably uses water. The apparatus  48 ′ further comprises a Faraday shield (not shown) made of a conductive material, preferably aluminium. Optionally, the apparatus  48 ′ can be provided with the electrostatic trap  35  illustrated in  FIG. 1 . 
         [0064]      FIG. 4  illustrates an injecting device  80  comprising a reservoir  82  adapted to receive an oil  84 , and a reservoir  86  having filters  88 . The reservoir  86  is forming a chamber  89  for receiving a metal catalyst  90 , preferably ferrocene. The reservoir  86  has an inlet  92  and an outlet  94 , which are in fluid flow communication with conduits  96  having an inlet  98  and an outlet  100 . 
         [0065]    The chamber  89  of the reservoir  86  is provided with a metal catalyst  90  and the catalyst  90  is heated by the hot oil  84  so as to evaporate the metal catalyst  90 . A mixture of a carbon-containing gas and a carrier gas (not shown) or a carbon-containing gas is injected at the inlet  98  so as to flow into conduits  96  thereby passing through the reservoir  86  and carrying the evaporated metal catalyst  90  at the outlet  100 , which is connected to the apparatus  10 ,  48  or  48 ′. The filters  88  prevent solid particles of the metal catalyst  90  from being carried out into said conduits  96 . 
         [0066]    The following non-limiting example further illustrates the invention. 
       EXAMPLE 
       [0067]    The production or synthesis of single-wall carbon nanotubes has been performed by using a plasma torch as illustrated in  FIG. 1 . The following experiment has been carried out by the inventors by providing the plasma torch with a cooling system and a Faraday shield. The cooling system prevents the plasma torch from over-heating and being damaged. The Faraday shield comprising a conductive material, preferably aluminium, prevents the electromagnetic radiations from escaping from said apparatus, thereby protecting users of the plasma torch. All the parameters related to the plasma torch are controlled by a computer using the LABVIEW® software. The parameters can also be manually controlled. The inert gas used for generating the primary plasma was argon, the metal catalyst was ferrocene, the carbon-containing gas was ethylene and the cooling gas was helium. Helium was also injected toward the plasma discharging end so as to prevent carbon deposit. The injecting device illustrated in  FIG. 4  was used for injecting the ferrocene. Ferrocene was heated to 100° C. and the conduits were heated to 250° C. so as to prevent condensation of ferrocene in the conduit disposed downstream of the reservoir containing the latter metal catalyst. The argon flow varied from 1000 to 3000 sccm (standard cubic centimeters per minute). The helium flows were both stabilized at about 3250 sccm, and the ethylene flow varied between 50 and 100 sccm. The temperature of the oven was kept at 900° C. and measured with a pyrometer. The power of the source generating the electromagnetic radiations (microwaves) was 1000 W and the reflected power was about 200 W. The rod of the electrostatic trap was maintained at a tension of −1000 V. The heat-resistant tubular members were made of quartz. The plasma tube was made of brass. The feed conduit, on the other hand, was made of stainless steel. The metal catalyst (ferrocene) and the carbon-containing substance (ethylene) were used in an atomic ratio metal atoms/carbon atoms of 0.02. The software controlled the flow of the carrier gas, argon, so as to maintain the atomic ratio at such a value. The experiment was carried out at atmospheric pressure under inert conditions (helium and argon). 
         [0068]    The synthesis of single-wall carbon nanotubes was performed for a period of 20 minutes using the above-mentioned experimental conditions. During this period of time, 500 mg of the desired single-wall carbon nanotubes were produced. The purity of the nanotubes thus obtained was about 20%. 
         [0069]    The crude sample obtained in the above example was characterized by SEM; the results are illustrated in  FIGS. 5 and 6 . As it is apparent from  FIGS. 5 and 6 , single-wall carbon nanotubes were produced. The sample was also characterized by TEM; the results are illustrated in  FIGS. 7 and 8 . These two figures show that the growth of the single-wall nanotubes is initiated by metal catalyst particles of about 5 nm (indicated by the arrows). The rope-like structure shown in  FIGS. 7 and 8  is very common for single-wall nanotubes. The purity of the sample was estimated by comparing the surface occupied by the single-wall carbon nanotubes with the amorphous carbon residues in  FIGS. 7 and 8 . 
         [0070]    In order to determine the diameter of the single-wall nanotubes produced according to the above example, two Raman spectroscopy measurements were performed. In the first experiment, a 514 nm laser was used ( FIG. 9 ) whereas, in the second experiment, a 782 nm laser was used ( FIG. 10 ). In  FIG. 9 , the peaks at 149.10, 171.90, 184.22, 217.75 and 284.79 cm −1  correspond to single-wall carbon nanotubes having diameters of 1.50, 1.30, 1.22, 1.03 and 0.80 nm, respectively. 
         [0071]    In  FIG. 10 , the peaks at 127.91, 141.20, 147.59, 163.02, 181.64, 200.26, 211.96, 222.60, 230.05 and 263.57 cm −1  correspond to single-wall carbon nanotubes having diameters of 1.75, 1.60, 1.52, 1.37, 1.23, 1.12, 1.06, 1.00, 0.97 and 0.85 nm, respectively. 
         [0072]    The above data indicate that in the method according to the example, as opposed to the methods comprising vaporization of graphite, a plurality of single-wall nanotube chiralities was obtained. 
         [0073]    It should be noted that by using the method and apparatus of the invention, the production of single-wall carbon nanotubes can be performed for a period of several hours since the deposit of carbon at the plasma-discharging end, leading to the premature extinction of the plasma torch, is avoided. 
         [0074]    While the invention has been described with particular reference to the illustrated embodiment, it will be understood that numerous modifications thereto will appear to those skilled in the art. Accordingly, the above description and accompanying drawings should be taken as illustrative of the invention and not in a limiting sense.