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Growth of carbon nanotubes above the peritectic temperatureof tungsten metal catalyst

D. Harbec * , J.-L. MeunierDepartment of Chemical Engineering, McGill University, Montreal, QC, Canada

A R T I C L E I N F O

Article history:Received 10 September 2010Accepted 21 January 2011Available online 26 January 2011

A B S T R A C T

The mechanism of formation of carbon nanotubes (CNTs) produced from the dissociationof tetrachloroethylene (TCE) and the co-injection of tungsten vapour in the nozzle of a DCthermal plasma torch is discussed. Tungsten and the dissociation of TCE form nanoparti-cles of tungsten carbide (WC), which provide the catalyst activity for the formation of CNTs.A new route for the mechanisms of formation is discussed following thermodynamic equi-librium calculations of the plasma torch system, energy dispersive spectroscopy and nano-diffraction analyses of the WC nanoparticles formed. These analyses provide an under-standing of the dissociation pattern of TCE, evaluate the carbon concentration in the cat-alyst nanoparticles and determine the phase structure of these nanoparticles. Wepropose from these results a new mechanism of formation for CNT occurring above theperitectic temperature of the stoichiometric WC alloy.

Crown Copyright 2011 Published by Elsevier Ltd. All rights reserved.

1. Introduction

Carbon nanotubes (CNTs) are long hollow tubes made of car-bon uniquely and are characterized by their nanometricdiameter and their micron-size length, giving them a one-dimensional structure. They feature a high modulus of elas-ticity, a high tensile strain, superconductor and semiconduc-tor behaviours, a high thermal conductivity, the possibility forgas storage and applications linked to their very large specicarea. Novel promising applications are now being developedin advanced materials such as high strength materials [1,2],

in high conductivity nanouids [3,4], electronic componentssuch as transistors [5], electron emitters [6], fuel cell elec-trodes [7,8], and energy storage [9,10]. The availability andthe high cost of CNTs however still limit the developmentand the commercialization of many applications. The road to-wards industrial processes for the growth of CNTs involves anunderstanding of the mechanism of formation at the nano-scale and of the fundamental phenomena at the basis of thisformation.

In the late nineties, researchers noticed that the introduc-tion of a metal catalyst in CNT synthesis methods enhancethe yields [11]. For instance, doping the graphite anode withsome specic metal in the arc discharge method not only pro-duces CNT in the deposit on the cathode, but enables the con-densation of CNT on the walls of the arc chamber [1215].Metal catalysts enhance the yields of CNT by anchoring theedge of the CNT structure. These mainly inhibit the formationof pentagonal rings preventing a premature closure of theCNT structure into by-products, and allow an efcient elonga-tion and growth of the CNT. Researchers rst thought that the

metal catalysts in their elemental state acted as a transitionalelement permitting the addition of the carbon building blocksat the tip of the CNT structure [16,17]. Later, Zhang et al. [18]suggested that the metal catalyst in the form of nanoparticlesencapsulates the tip of the CNT structure allowing an efcientgrowth and preventing a premature closure of the tubestructure.

At the present time it is rather proposed that CNTs growrst from a diffusion process of elemental carbon into the

0008-6223/$ - see front matter Crown Copyright

2011 Published by Elsevier Ltd. All rights reserved.doi:10.1016/j.carbon.2011.01.050

* Corresponding author: Current address: LMCA/Department of Civil Engineering, Universite de Sherbrooke, Sherbrooke, QC, Canada.Fax: +1 819 821 7212.

E-mail address: david.harbec@usherbrooke.ca (D. Harbec).

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metal catalyst particles having a diameter in the nanometerscale. Authors [1926] indicated that the growth of CNTpasses rst through the nucleation of carbon clusters and me-tal nanoparticles at high temperature. The metal nanoparti-cles dissolve the carbon clusters forming a metalcarbonmixture. At some proper temperature for the phase separa-tion of the metalcarbon solid solution, the particle segre-gates a fraction of its carbon which graphitizes and formsCNTs at the surface of the particle. The nanometer size rangeof the particle is an essential condition for generating thetubular structures. In fact even for the growth of CNT directlyon at metal surfaces using CVD techniques, a prior surfacetreatment generating nano-scale defects of the metal surfaceis essential for the formation of this structure [27]. Onceextraction from the catalyst site is triggered, the growing CNT continuously uses the segregated carbon as building blocks to elongate their structure [1926]. Carbon in the gasphase may also be used in the growth of CNTs, this howeveris expected to occur through a surface diffusion process onthe tubular structure eventually feeding the metal solutionin carbon [19]. During the segregation process, the key param-eter for high yields of CNT is the control of the eutectic tem-perature for phase separation of the metalcarbon mixture.

Kataura et al. [20] proposed a growth mechanism com-posed of three steps, where the rst step occurs at high tem-perature, the second, at medium temperature and the third,at low temperature. In the rst step, the dissociation of boththe carbon raw material and metal catalyst at high tempera-ture leads to the nucleation of fullerene caps, carbon clustersand metal nanoparticles. The metal nanoparticles attract ful-lerene caps on their surface and dissolve some of the carbonclusters, forming a metalcarbon mixture. At the mediumtemperature state in step two, the metalcarbon mixturestops dissolving carbon clusters and attracting fullerene capson its surface. In step three, another quench occurs and thesolubility of carbon in the metal reduces signicantly. Themetalcarbon mixture segregates the excess of carbon andfullerene caps elongate their structure into CNTs using thesegregated carbon. Here, the diameter of the fullerene capsdetermines the diameter distribution of the CNTs. Theseauthors noticed that the temperature inuences the elonga-tion process of CNTin step three. Controlling the temperaturebelow the eutectic deactivates the elongation process andshortens the resulting CNT. A temperature near the eutecticstrongly activates the growth of CNT.

Nasibulin et al. [21] and Brukh and Mitra [22] worked onreaction pathways of the decomposition of the carbon sourceyielding the presence of free carbon. Their modeling calcula-tions and experimental data demonstrate a correlation be-tween high availability of free carbon and high yield of CNTs. They indicated that a high concentration of free carbonimproves the initial diffusion step in the metal catalystnanoparticles.

Larouche et al. [23] suggested that instead of the elonga-tion of fullerene caps as described above, their mechanismof formation rather passes through the extrusion of CNTcords from the metalcarbon mixture induced by the Marang-oni convective process in the liquid layer at the surface of theparticle. For achieving high yields of SWNT in their plasma-based gas phase synthesis, the quench of the iron nanoparti-

cle precursors must occur at a fast rate of at least 10 6 K/s inorder to generate the nanometer size structure. Following the metal nanoparticle generation stage, a lower temperaturein the range of the eutectic temperature of the particle ismaintained for the catalyst, leading to carbon extractionand growth of CNTs.

The importance of the eutectic transformation and of agood control over the specic eutectic temperature range forthe nanoparticle was further conrmed in many studies(see for example [28,29]). In situ and real time conrmationof this growth process within a transmission electron micro-scope was even obtained by Rodriguez-Manzo et al. [30], withthe added information of a gradual change of the Fe nanopar-ticle structure from the carbon-rich zone in the region of tubeformation, to the carbide phase forming away from this zonefollowing carbon depletion.

The importance of controlling a very specic temperaturefor the eutectic phase transformation, along with the need forthe nanoscale catalyst structure, is responsible of the maindifculties for the generation of CNT. In the present paper,we discuss a system which generates CNTs even though itdoes not have a eutectic transformation capable of extracting carbon. We show that another route for carbon extraction ispossible and that, contrary to the eutectic transformation,this transformation involves a continuous cooling of the li-quid metalcarbon system over a very wide temperature win-dow enabling easier control over the process. In a previouswork, we showed the catalytic production of CNTs using tet-rachloroethylene (C 2Cl4, TCE) as the carbon raw material in- jected in the nozzle of a DC thermal plasma torch. Theerosion of the DC torch electrode, made of tungsten, emitsmetallic vapours that nucleate in situ into catalytic nanoparti-cles within the plasma jet [3134]. The advantage of using thetechnology of thermal plasma torch is their scale-up capabil-ity, because of their high throughput and high power avail-ability. One should note that plasma torch systems arecurrently being used industrially at the megawatt powerscale. Moreover, such system enables decoupling of the cata-lyst nucleation zone from the CNT growth zone and an inde-pendent control of the process parameters: power, carbonsource and metal catalyst feeds. The present paper providesresults on the mechanism of formation of the CNTs withinthe torch nozzle using these particular tungsten/TCE precur-sors. This mechanism of formation evolves rst from thermo-dynamic equilibrium calculations for an understanding of theTCE dissociation in the plasma torch, from electron micros-copy observations and analyses for evaluating the carbonconcentration in the catalyst nanoparticles and determina-tion of the phase structure of these nanoparticles. The mech-anism of formation is observed here to occur above theperitectic temperature of the WC mixture.

2. Experimental

As previously outlined [3134], we inject TCE vapours at a rateof 0.050.29 mol/min using 20 slpm of argon carrying gas inthe water-cooled nozzle of a DC non-transferred thermalplasma torch, operated with 100 slpm of plasma-forming ar-gon and a torch power of 30 kW. Upstream of the torch nozzle,

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the arc plasma erodes the anode and the cathode both madeof tungsten and supplies a small amount of tungsten vapouras a source to nucleate the liquid catalyst nanoparticles in thelarge quench provided by the supersonic expansion in thetorch nozzle. The very high temperatures above 4000 K inthe nozzle zone downstream of the expansion are expectedto maintain the tungsten nanoparticles in the liquid state.The dissociation of TCE in the nozzle provides atomic carboncontamination to the nucleated tungsten particles, forming nanoparticles of tungsten with carbon in solution. TCE wasselected as the carbon precursor because of the complete dis-sociation of the CCl bonds and the generation of free C spe-cies in the temperature range of interest (see Section 3). Thetungstencarbon solution nanoparticles having suitable tem-perature and size represent the site for CNT growth. A largefraction of these nanoparticles are transported radially inthe torch nozzle by thermophoresis, and attach to the nozzlewall. These particles are characterized by having very long residence times in a given high temperature window, togetherwith a continuous supply of atomic carbon. Such a situationleads to the observed long CNTs, mainly MWCNT, showing average lengths exceeding 50 l m [31,34].

We characterize CNTs and the catalyst nanoparticles using High Resolution Transmission Electron Microscopy (HRTEM: Jeol JEM 2100F). The HRTEM is operated using 200 kV of accel-erating voltage. The CNT samples are deposited on a coppermesh observation grid coated with an amorphous carbonlm. The magnication during the soot observation variesfrom 100 to 1000 k times. To characterize the catalyst nano-particles, the elemental component are identied and quanti-ed using Energy Dispersion Spectroscopy (EDS) from OxfordInstrument, and their phase structure is determined by nano-diffraction techniques.

3. Thermodynamic analysis

We perform the thermodynamic equilibrium calculationsusing the operating conditions yielding high concentrationsof CNTs in an attempt to understand the dissociation patternof TCE at high temperatures. This analysis is based on themethod of the minimization of the Gibbs free energy. Thesecalculations are made using the equilibrium computationalprogram Ivtanthermo (CRC Press, 1993). There are however afew cautions in using such analysis. In thermal plasma chem-istry, the residence times of the different chemical species areshort, especially in the supersonic expansion within the torchnozzle. Equilibrium calculations are thus insufcient for pre-dicting the exact composition of the products. Furthermore,these calculations do not take into account any kinetic factorsthat may possibly inuence the formation of certain com-pounds. As a result, the calculations in this work serve onlyas a guide to understand the chemistry behind the dissocia-tion of TCE at high temperatures, rather than providing anaccurate prediction of the product concentration.

The following operating conditions are used in the equilib-rium evaluation: plasma-forming gas: 100 slpm of argon, TCEfeed rate: 0.15 mol/min, tungsten electrodes erosion rate:4.3 10 3 mg/s [31,34] and reactor pressure: 0.26 atm. Theamount of tungsten vapour is estimated from the electrode

erosion measurements for the plasma torch operating with100 slpm of plasma-forming argon at 30 kW. We base our cal-culations on one mole of chemical species. Fig. 1 representsthe calculated equilibrium molar fraction of the different pos-sible chemical species from the dissociation of TCE and theaddition of tungsten vapours for the 2739000 K temperaturerange. The upper temperature limit corresponds roughly tothe modeled temperature of the plasma jet entering the noz-zle region, which under the present conditions was evaluatedin the range between 9000 and 9500 K [35].

The thermodynamic analysis here predicts that the use of a chlorinated carbon precursor, TCE in this case, for the for-mation of CNTs leads to the formation of large amounts of graphite and of relatively low amounts of chlorinated carbonproducts in the range of temperatures of interest ( 8003000 K). In fact, we observe a complete break up of the CClbonds at 800 K. Around 2990 K, the eutectic and the peritecticpoints of WC [36], most of the carbon and chlorine presentare, respectively in the form of the graphitic structure andmono-atomic chlorine gas (Cl (g)). At high temperature (above3000 K), dissociation of TCE and the erosion of the tungstenelectrodes favour the formation of free gaseous C species:C(g), C2(g) and C3(g) . Species containing CCl bonds are not pre-dicted to be stable in this temperature range. Given the hightemperature of the plasma jet and the relatively long resi-dence time of the tungsten carbide particles deposited onthe nozzle wall [33,34], one may speculate that free carbonspecies produced at high temperature diffuses into the Wnanoparticles to form nanoparticles made of a WC solidsolution or some W x C y compound.

4. HRTEM and EDS analysis of the catalyst

nanoparticlesWe previously reported that the ow and the temperatureconditions within the torch nozzle enable a good dispersionof tungsten and the synthesis of large concentrations of

50 l m long MWCNT and carbon nano-onions [31,33,34].Using TEM, these MWCNT are typically of several l m long and their internal and external diameters are, respectively,23 and 1020 nm. TEM micrographs ( Fig. 2) show nanoparti-cles of tungsten carbide encapsulated at the tip of CNT and innano-onions. The typical size of these particles varies be-tween 20 and 30 nm.

In order to investigate if the growth of CNT is controlled bythe segregation of the excess of carbon contained in thenanoparticles of metal catalyst, we evaluated by EDS andHRTEM the concentrations of carbon and tungsten withinthe catalyst nanoparticles encapsulated at the tip of a CNT.For comparison purposes, we rst analyzed the nanoparticlesencapsulated in an early closed MWCNT made of concen-tric graphitic layers similar to nano-onions. We then com-pared this analysis with that of the catalyst particles locatedat the tip of a long MWCNT that extends as a full tubularstructure.

Fig. 3a and b, respectively represent the EDS analysis andthe HRTEM micrograph of the catalyst region of an encapsu-lated tungsten carbide particle and of a long MWCNT. Carewas taken to limit the EDS analysis to the metal nanoparticle

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on both observed structures with the surrounding layers of carbon remaining outside of the beam in order to isolate asmuch as possible the nanoparticle itself. The copper lines inthe spectra are attributed to the meshes of the HRTEM obser-vation grid.

The nano-onion structure around the encapsulated tung-

sten carbide particle in Fig. 3b consists of 14 layers of graph-itized carbon that encapsulate a nanoparticle of tungstencarbide of roughly 20 nm in diameter. In Fig. 3b, this structureis treated with an inverted Fast Fourier Transform to highlightthe graphitized layers from the superimposed MWCNT. Thisstructure is superposed in the gure over a long MWCNT alsohaving 14 layers. The presence of encapsulated tungsten car-bide particles in the nozzle deposit is presumably due to ashort residence time of the nanoparticle in the appropriatetemperature zone for CNT synthesis.

In Fig. 4, a long MWCNT is seen to have grown using a cat-alyst nanoparticle of tungsten carbide of similar diameter(20 nm). A different number of layers of graphitic carbon is

observed between the region directly around the nanoparticle

and that of the main tubular structure. The MWCNT pos-sesses nine layers around the catalyst nanoparticle, and 14concentric layers in the main tube structure. One can seethe additional layers on the main tube structure are locatedinside the tube and attach directly to the catalyst particle,these sites acting as carbon extraction zones. We can also ob-

serve that the tungsten particle region away from this carbonextraction zone shows aligned crystalline planes, while thezone of carbon extraction is depleted from any organizedcrystal plane arrangement. This behaviour is similar to thestructures observed on iron by Rodriguez-Manzo et al. [30].

Table 1 compares the relative concentrations of carbon inthe metallic nanoparticles for both cases. The calculationsof these concentrations are based on the intensity ratio of the tungsten and carbon lines on the EDS spectra. As statedearlier, the electron beam for the EDS analysis is focusedwithin an area of 20 nm in diameter bounded by the nano-particles of tungsten carbide. The carbon from the layeraround the nanoparticle is not taken into account in the

calculations. Besides the carbon contained within the nano-

0.00E+00

2.00E-02

4.00E-02

6.00E-02

8.00E-02

1.00E-01

1.20E-01

1.40E-01

0 1000 2000 3000 4000 5000 6000 7000 8000 9000Temperature (K)

M o l e

0.00E+00

5.00E-08

1.00E-07

1.50E-07

2.00E-07

2.50E-07

3.00E-07

3.50E-07

0 1000 2000 3000 4000 5000 6000 7000 8000 9000

Temperature (K)

M o l e

Cl (g)

Cl2 (g) Graphite

CCl 4 (g) C3 (g)

C2 (g)

C(g)

C+(g)

Cl+(g)

(a)

(b)WCl 4 (g)WCl 6 (g)

WCl 6 (s)

WCl 5 (g)

WCl 3 (g)

WCl (g)W (g)

W+(g)

Fig. 1 Computed mole fraction at equilibrium for the different chemical species resulting from the dissociation of 0.15 mol/ min of TCE and the addition of 4.3 10 3 mg/s of W vapours coming from the erosion of the torch cathode at 0.26 atm: (a)carbon and chlorine species; and (b) species related to tungsten.

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particle, the electron beam integrates the carbon observedabove and underneath the nanoparticles. In order to estimatethe concentration of carbon in the catalyst nanoparticles,some further calculations are needed to exclude these extralayers of carbon from the raw concentrations data. Thesecalculations use the following assumptions:

1. The contribution from the TEM carbon support in the con-centration of carbon is the same for both cases.

2. The extra layers are graphene sheets composed only of hexagonal rings and having a CC bond distance of 1.41 A .

3. The catalyst nanoparticles have a spherical shape.4. The analyzed structures are uniform in each direction

with regards to their amount of carbon layers.5. The density of the catalyst nanoparticles is the one of

tungsten carbide and it is independent of the concentra-tion of carbon.

5. Phase determination of the catalyst nanoparticles

The results in Table 1 indicate very high concentrations of car-bon in the catalyst nanoparticles in both cases of the carbonnanostructure. These may easily be biased towards high car-bon content by the measurement technique, however to ver-ify if such high concentrations of carbon are feasible and inagreement with the phase diagram of the WC system, exhib-ited in Fig. 5, we evaluated the phase structure of the catalystnanoparticles using electron diffraction and measured the in-

ter-layer spacing of the atomic planes [33,34]. As for the EDSanalyses in Section 4, the electron beam of the HRTEM is di-rectly projected on the catalyst nanoparticle encapsulated atthe tip of a long MWCNT (Fig. 4b), while leaving the externallayers of carbon outside of the beam.

The measured inter-planar distances evaluated from theelectron diffraction patterns are 1.30, 1.31, 1.46 and 2.5 A [33,34]. These four inter-planar values commonly representthe structure of the d phase in the WC system, which corre-sponds to the stoichiometric tungsten carbide (WC) phase[JCPDS File 51-0939]. The nano-diffraction pattern thus indi-cates that CNT growth generates nanoparticles of stoichiom-etric WC, while the zone showing an amorphous-likestructures in the TEM gures may possibly be a solid solutionsof tungsten with a high concentration of carbon. This type of structure agrees with high concentrations of carbon in theparticles. In the phase diagram of the WC system ( Fig. 5),stoichiometric WC ( d phase) is effectively generated uponcooling within the high carbon concentration range( 50 at.% C to

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torch nozzle deposit at high temperature [3134]. The EDSanalyses indicate concentrations of carbon in the nanoparti-cles that may be well above the stoichiometric CW phase.According to the CW phase diagram of Fig. 5, very high con-centrations of carbon are necessary in a high temperature li-quid tungsten system to enable the formation of thestoichiometric d phase of WC, and of the graphite phase thatcan be assimilated to the growth of CNTs. The temperatureexpected in the nozzle deposit, modeled by Guo [35] and citedin [33], takes into account the supersonic ow and the TCEreaction paths for evaluating the ow/energy elds. Resultsindicated temperatures well above 3000 K in this zone, downto temperatures of roughly 2000 K close to the nozzle wall.

This indicates the nucleated tungsten particles are effectivelyin the liquid state throughout most of their residence time onthe nozzle wall deposit, a period corresponding to the avail-ability of atomic carbon in the gas phase which is eventuallyloading the liquid droplets through diffusion. It is importantto note that the d phase nucleation observed here is not a eu-tectic transformation, but a peritectic transformation occur-ring above 2785 C of the L + graphite two-phase domain.Looking more closely at the phase evolution in the phase dia-gram for concentrations above 50 at.% C, we propose here anew and simple mechanism for the nucleation and thegrowth of CNTs from catalysts having a peritectic transforma-tion with carbon. This mechanism follows directly from the

Fig. 3 (a) EDS analysis of a nanoparticle of tungsten carbide encapsulated particle in a nano-onion. (b) TEM micrograph of theencapsulated tungsten carbide particle superposed over a long CNT. Scale bar: 20 nm.

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transformations occurring upon cooling within theliquid + graphite two-phase domain at temperatures abovethe peritectic temperature. The sketch of Fig. 6 anticipatesthis evolution within the phase diagram above 50 at.% C uponcooling down in the 27853500 C range. Note the temperature

values indicated here do not take into consideration thereduction in transition temperature imposed by the nanome-ter scale of the particles.

Upon the initial homogeneous nucleation of the W particlefrom the hot plasma in the supersonic shock zone, we expect

Fig. 4 (a) EDS analysis of a nanoparticle of tungsten carbide encapsulated at the tip of a long MWCNT. (b) TEM micrograph of the long MWCNT analyzed. Scale bar: 20 nm.

Table 1 Calculated concentrations of the different elements within the area bounded by the nanoparticle of metal catalyst.

Elements in the nanoparticle Nano-onion (at.%) Long MWCNT (at.%)

Carbon 99.19 91.91Tungsten 0.81 8.09

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this initial particle to be relatively pure of tungsten and to bein the state of a liquid droplet because of the high tempera-ture in the torch nozzle. The liquid W particle is then driventowards the nozzle wall within the layer of deposit observedexperimentally. During this transit, and even more during the period of residence in the nozzle wall deposit, its size isexpected to increase from coagulation processes. Its concen-tration in carbon during this period should also strongly in-crease. When cooling within the liquid + graphite domain at

some overall particle concentration Co in Fig. 5, the concen-tration of carbon in the liquid is decreasing along the liquidusline in favour of an increasing amount of solid graphite-likestructure being nucleated at a rate according to the lever ruleevolution upon cooling. The thermal evolution of a liquidWC system in this two-phase region effectively producesthe graphitic structure expected for the growth of CNTs.Contrary to the eutectic transformation, this process occursalong very wide ranges of both the temperature and the

Fig. 5 Phase diagram of tungsten carbide.

Fig. 6 Sketch of the evolution upon cooling within the phase diagram of tungsten carbide for carbon concentration above50 at.%, and resulting in the formation of the d phase of stoichiometric WC and CNT.

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carbon concentration in the catalyst particle. It stops uponcrossing the peritectic temperature value of the system. TheWC liquid particle then solidies into the stoichiometricsolid d phase of WC. Similarly to the eutectic transformation,the evolution of the graphitic structure from the particle isbased on the liquid phase of the particle and the change of concentration upon the solidication of this liquid phase.The main difference comes from the decreasing concentra-tion of carbon in the liquid phase upon cooling which forces,through the relative amounts of liquid and solid phasespresent, the growth of graphitic structures over the largetemperature range.

One can also note that an increase of the carbon concen-tration in the liquid particle will enhance this process. Suchincrease effectively leads to a greater proportion of graphiteat some temperature in the L + graphite domain and to a lar-ger temperature range for the growth of CNTs. This agreeswell with the increase of the CNT concentration with theTCE feed rate explained in [32,33]. The efciency of this mech-anism of formation of CNTs is expected to be quite high be-cause of the large temperature range (27853500 C) and thelarge carbon concentration within the catalyst particle (50%to near 100%). This seems to correlate well with the very long CNTs exceeding 50 l m produced in the torch nozzle [31,34].This mechanism is however possible only at temperatureshigher than the peritectic temperature. This explains whyno or very few CNTs are grown in the soot downstream of the torch nozzle and in the nozzle deposit during the rst3 min of operation seen in [33,34]. A nal note should be madeon the implications this peritectic route has on the processing aspects of CNT synthesis. The eutectic route involves main-taining a very narrow temperature window which is difcultto target and maintain, particularly in gas phase synthesisinvolving a priori the nucleation of metal catalyst nanoparti-cles using a strong quenching rate. The peritectic route de-scribed here on the contrary involves a slow cooling of thenanoparticles down a very large temperature domain thatshould be much easier to control downstream of a gas phaseow system.

7. Conclusions

In summary, experimental observations on CNTs generatedin a high temperature plasma jet using a tungsten catalystprecursor enables the proposition of a new mechanism forthe nucleation and the growth of CNTs from the catalystnanoparticles in this case. This mechanism follows fromthe cooling within the liquid + graphite two-phase domainof the WC phase diagram at temperatures above the peritec-tic temperature of 2785 C. When cooling within theliquid + graphite domain at high carbon concentrations, thethermal evolution of a liquid WC solution particle in thetwo-phase region produces the graphitic structure expectedfor the growth of CNTs. Contrary to the eutectic transforma-tion, this process occurs over a very wide temperature rangeof 700 C and a large carbon concentration range (50% tonear 100%) within the catalyst particle. This agrees with thehigh carbon concentrations observed in the tungsten catalystfollowing CNT growth, and with the very long CNTs exceeding

50 l m observed in the torch nozzle [31,34]. Based on thismechanism of formation, the yield of CNTs is anticipated toincrease following an increase of the carbon concentrationin the particle. This anticipation agrees with the increase of the yield of CNTs with the increase of TCE feed rate describedin [33,34]. Such mechanism of formation can possibly be ap-plied to more usual metal catalyst systems for the growth of CNT. In fact, the ironcarbon and other similar systems areexpected from the above analysis to show a similar growthbehaviour when loaded in their liquid state with concentra-tions of carbon that are well above their eutectic compositionvalue.

Acknowledgement

This work was supported by the Natural Sciences and Engi-neering Research Council (NSERC) of Canada.

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