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Synthesis of carbon nanotubes on Ni-alloy andSi-substrates using counterflow methane–air

diffusion flames

T.X. Li a,*, H.G. Zhang b, F.J. Wang a, Z. Chen b, K. Saito a

a Department of Mechanical Engineering, University of Kentucky, Lexington, KY 40506, USAb Department of Electrical and Computer Engineering and Center for Nanoscale Science and Engineering,

University of Kentucky, Lexington, KY 40506, USA

Abstract

We have conducted an experimental study to investigate the synthesis of multi-walled carbon nanotubes(CNTs) in counterflow methane–air diffusion flames, with emphasis on effects of catalyst, temperature, andthe air-side strain rate of the flow on CNTs growth. The counterflow flame was formed by fuel (CH4 orCH4 + N2) and air streams impinging on each other. Two types of substrates were used to deposit CNTs.Ni-alloy (60% Ni + 26% Cr + 14% Fe) wire substrates synthesized curved and entangled CNTs, whichhave both straight and bamboo-like structures; Si-substrates with porous anodic aluminum oxide(AAO) nanotemplates synthesized well-aligned, self-assembled CNTs. These CNTs grown inside nanop-ores had a uniform geometry with controllable length and diameter. The axial temperature profiles ofthe flow were measured by a 125 lm diameter Pt/10% Rh–Pt thermocouple with a 0.3 mm bead junction.It was found that temperature could affect not only the success of CNTs synthesis, but also the morphologyof synthesized CNTs. It was also found, against previous general belief, that there was a common temper-ature region (1023–1073 K) in chemical vapor deposition (CVD) and counterflow diffusion flames whereCNTs could be produced. CNTs synthesized in counterflow flames were significantly affected by air-sidestrain rate not through the residence time, but through carbon sources available for CNTs growth. Off-symmetric counterflow flames could synthesize high-quality CNTs because with this configuration carbonsources at the fuel side could easily diffuse across the stagnation surface to support CNTs growth. Theseresults show the feasibility of using counterflow flames to synthesize CNTs for particular applications suchas fabricating nanoscale electronic devices.� 2006 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

Keywords: Flame synthesis; Carbon nanotubes; Counterflow diffusion flame; Si-substrate; AAO-template

1. Introduction

Carbon nanotubes (CNTs) are honeycomb lat-tices rolled into a cylindrical structure. The length

of a CNT may be several micrometers and itsdiameter is of nanometers, which is much smallerthan conventional semiconductor devices.Because of CNTs small size and unique electricand mechanical properties, one application witha large potential impact is the fabrication of nano-scale sensors and electronic devices.

1540-7489/$ - see front matter � 2006 The Combustion Institute. Published by Elsevier Inc. All rights reserved.doi:10.1016/j.proci.2006.07.194

* Corresponding author. Fax: +1 859 257 3304.E-mail address: txli1@engr.uky.edu (T.X. Li).

Proceedings of the Combustion Institute 31 (2007) 1849–1856

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Since fullerenes were discovered in carbonevaporation [1] in 1985, later formed in low-pres-sure fuel-rich flame [2], and CNTs were discovered[3] both in 1991, synthesis of carbon nanomateri-als, especially, CNTs has been dominated by thechemical vapor deposition (CVD) method.Although CVD synthesis can produce largeamounts of high-quality CNTs, it requires com-plex and costly purification processes to removeother forms of carbon [4]. In recent years, greatefforts to synthesize CNTs have been devoted tousing flames. Flame synthesis, in comparison toCVD method, has advantage of being simpleand energy-efficient and able to produce high-pu-rity CNTs with no tedious purification process. In1986, Saito et al. [5] have studied soot producedby a co-flow methane–air diffusion flame deposit-ed on a quartz fiber and found brown materials ata certain flame region. Several years later, it wasfound by scanning electron microscopy (SEM)that these brown materials were polyhedral-shaped crystal-like particles [6]. Extended studieswere conducted and multi-walled carbon nano-tubes (MWCNTs) were successfully synthesizedin co-flow methane and ethylene diffusion flames[7,8]. The combustion science relevant to moderncarbon nanotechnology was reviewed [9]. Diffu-sion flames were also used to synthesize single-walled CNTs using metallocene-doped acetyleneas a fuel [10] and MWCNTs with metal catalystdispersed on TiO2 [11]. Vertically aligned, self-as-sembled CNTs were synthesized on two-dimen-sional (2-D) Si-substrates with porous anodicaluminum oxide (AAO) nanotemplates using eth-ylene co-flow diffusion flames [12] for the develop-ment of potential applications to fabricatenanoscale electronic devices.

Although efforts to synthesize well-aligned,self-assembled CNTs in co-flow flames have madeprogress, parameter influences have not been ade-quately studied due to smaller flame volume andstrong flame-substrate interactions. In addition,synthesis of CNTs on 2-D Si-substrates withAAO-nanotemplates for applications needs a rela-tively large surface area of the flame and growthof CNTs on 1-D AAO-array with nanopores hor-izontally aligned on sandwich-structured Si-sub-strates [13] particularly demands controllabletemperature and limited flame-substrate interac-tions. These requirements have stimulated us todevelop counterflow flames to provide betterexperimental conditions and sampling possibility.Recently, Kennedy’s group [14,15] has studiedsynthesis of CNTs and carbon nanostructures onNi-alloy substrates in opposed flow flames. Theflame was formed by impinging CH4 + 4% C2H2

and 50% O2 + 50% N2 streams. Different carbonmaterials were observed including MWCNTs,bundles, and coiled nanofibers. An electric fieldwas applied to control CNT alignment andmorphology.

In counterflow flame synthesis, a variety ofparameters influence CNTs growth, among whichcatalyst, temperature, and carbon sources are themajor factors. A three-step growth mechanismwas suggested [8] to explain CNTs growth in co-flow methane flames. Methane was first pyrolyzedin the preheat zone to produce hydrocarbon spe-cies as carbon sources, and catalyst particlesformed on the substrate surface. Under appropri-ate temperature, the catalyst particles absorbedthe carbon sources which formed a cylindricalstructure of CNTs. Whether the formed CNTswere single-walled or multi-walled depends onparticle sizes, while the diameter and length ofCNTs are determined by the geometry of thesubstrates.

In this paper, we present our experimentalresults obtained from synthesis of CNTs incounterflow methane–air diffusion flames on twodifferent substrates: Ni-alloy wire substrates andSi-substrates with AAO-nanotemplates. TheNi-alloy wire substrates were used for parameterstudies and the Si-substrates were developed tosynthesize well-aligned, self-assembled CNTs,which have a potential to fabricate nanoscale elec-tronic devices because these self-assembled CNTsare compatible with the Si planar processing tech-nology [13]. The synthesis region in the flamewhere CNTs could be harvested was determined,and the effects of temperature and air-side strainrate on CNTs growth were also experimentallystudied. It was found that there was a commontemperature region (1023–1073 K) in both CVDand counterflow diffusion flames where the CNTscould be synthesized.

2. Experimental method

In the present study, Ni-alloy wire substratesand Si-substrates with AAO-nanotemplates wereused to synthesize CNTs in counterflow methane–air diffusion flames. Figure 1 shows a schematic ofthe apparatus with a flame picture inserted. Thecounterflow diffusion flame apparatus providedair flow from the upper nozzle and the fuel (CH4

or N2-diluted CH4) flow through the lower nozzle.Diameter of each nozzle was 76.5 mm. A cooling-water tube was built in to keep nozzle temperatureconstant. The separation distance between upperand lower nozzles was adjustable. The flow ratesof fuel, air and nitrogen were separately controlledby each rotameter. The location of the diffusionflame in counterflow configuration depends onthe equivalence ratio of fuel and oxidizer; it usuallyremains on the oxidizer side of the stagnation sur-face, while it can shift to the fuel side if the fuelstream is highly diluted by inert gas. Around theupper air nozzle, coflow nitrogen was introducedto extinguish the flame near the edge of thenozzle and separated combustion flows from

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environments. The experiments were conducted atdifferent air-side strain rates, defined as a = 2u2/L + (2u1/L)(q1/q2)1/2 [16], where the subscript ‘‘1’’refers to the fuel stream and ‘‘2’’ to the air stream.The symbol u is the flow velocity at the nozzle exit,L the separation distance between two nozzles, andq the flow density.

To prepare a Ni-alloy wire substrate, a stain-less steel grid was used to hold a 0.4 mm diameterbare Ni-alloy wire (60% Ni + 26% Cr + 14% Fe),which acted as a catalyst to deposit sampled mate-rials. Previous experiences showed [7] that CNTsgrowth highly depends on catalyst particles (Niand Fe oxides). In the present study, the Ni-alloywires were pre-oxidized in a 70% nitric acid solu-tion for 10 s to form metal oxides.

Details of preparation of a Si-substrate withAAO-nanotemplate are given in Refs. [12,13]. Inbrief, a thin aluminum film was thermally coatedon the Si-substrate. The thickness of the coatedaluminum film was 0.5–2 lm. In order to fabricate2-D substrate, an anodic oxidation process wasapplied on the film during which vertical nanop-ores were self-generated into a highly-orderedhexagonal array as shown in Fig. 2a. In fabrica-tion of 1-D Si-substrate, a so-called sandwichstructure was built by depositing Si or SiO2/Si lay-ers on the top of the AAO-template so that theAAO-film was explored only at the side of thesandwich-structured substrate [13]. The nanop-ores were then formed at the side edge of the sub-strate by merging it in a 0.2 M oxalic solution at5–25 �C and 40 V for 3–7 min. Finally, the Si-sub-strate was dipped into a 5 wt% phosphoric solu-tion at 20 �C for 20 min for pore widening.Figure 2b is the SEM image of sandwich-struc-tured Si-substrate in side view, revealing a uni-form geometry of 1-D nanopore array in whichCNTs would be grown. The pore depth wasaround 500 nm according to the anodization rate

and voltage applied. Cobalt catalyst particlesmight be electrodeposited at the bottom of thepores depending on whether firm attachment ofCNTs was required.

In the experimentation, a substrate was mount-ed on a 34 mm long stainless steel probe andinserted in the fuel side of the yellow flame zoneto deposit carbon materials. For Si-substrates,the porous AAO-template was aimed face downtowards the flow direction. The sampling timewas varied from 5 to 20 min. The effect of temper-ature on CNTs growth was studied by samplingcarbon materials at different flame zones, as wellas by diluting CH4 with N2 and compared theresults with pure CH4. The effect of air-side strainrate on CNTs growth was studied by changing theratio of fuel and air flow rates and performingsymmetric and off-symmetry configurations. ASEM (Hitachi S900) with X-ray analyzer was usedto examine the materials deposited on the sub-strates. In order to explore CNTs morphologyby SEM, the samples were dipped in a chromicand phosphoric acid mixture, and usually experi-enced a sonication treatment to partially removeAAO-templates. A transmission electron micro-scope (TEM) (JEOL JEM-2000-FM) with anOxford INCA detector was used to analyze

Fig. 2. SEM images of Si-substrates with AAO-tem-plates, (a) top view of 2-D substrate, and (b) side view of1-D sandwich-structured substrate.

non-premixed flame

N2 airH2O

oxidizer

fuel stream

mixing

water

co-flow

fuel N2

X-Y positioner

Fig. 1. Schematic of the experimental apparatus.

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catalyst particles. For TEM analysis, the deposit-ed materials were dispersed in ethanol with a mildsonication, and then a few drops of the dispersedliquid were placed on copper TEM grids anddried. Energy dispersive X-ray (EDX) analysiswas preformed for elemental analysis. A high-res-olution TEM (JEOL 2010F) was also employed tostudy the microstructure of CNTs.

3. Results and discussion

3.1. Substrate effects

Typical SEM image of CNTs synthesized onNi-alloy wire substrates is shown in Fig. 3a,obtained at the fuel side of the counterflow meth-ane–air diffusion flame with 33% N2-diluted CH4

as a fuel at the air-side strain rate a = 53.3 s�1

for 10 min. They were curved and entangledMWCNTs with average diameter around 80 nm,similar to, but slightly larger than those (60 nm)obtained in co-flow flames [7,8]. The reason forslightly larger diameter is believed due to differenttype of flame used for synthesis. It was found inthe experiments that longer sampling time had alittle effect on CNT diameter and length, but pro-duced more CNTs. Two types of CNTs wereobserved to co-exist on the Ni-alloy substrate;TEM image of straight structure is shown inFig. 3b and bamboo-like structure with internalcompartment cap in Fig. 3c. The CNTs with bam-boo-like structure had several layers of graphi-tized sheets near the hollow core and werebridged at the internal cap. At this stage, nearlyidentical amounts of graphitized sheets were pro-duced at the outermost layer of CNTs. The CNTswith straight structure grew with spherical catalystparticles, while those with bamboo-like structureswere typically formed when the catalytic particleswere non-spherical [17,18].

The SEM image of well-aligned, self-assembledCNTs grown on 2-D Si-substrates with AAO-nanotemplates is shown in Fig. 4a with the localSEM image enlarged. It can be seen that uniformCNTs were grown inside nanopores and each poregrew one CNT. Growth of CNTs was stopped atthe surface of AAO film and the formed CNTshad the same diameter as the pores (35–40 nm).Figure 4b is the SEM image of CNTs obtainedafter the samples experienced a sonication treat-ment so that CNTs were pulled out of pores, dis-playing the same length for all CNTs. Thediameter and length of CNTs deposited on Si-sub-strates could be controlled in the processes of Si-substrate preparation.

It is expected that well-aligned CNTs grown on1-D sandwich-structured Si-substrates with AAO-nanopores is more compatible with the Siintegrated circuit processes than those grown on2-D Si-substrates [13]. Synthesis of 1-D CNTsarray requires a relatively large flame surface area,well-controlled temperature, and limited flame-substrate interaction. The co-flow flame may notbe most suitable for satisfying these requirementsbecause the interaction between substrate andflame is so strong that the AAO film at the sideedge cannot suffer high temperature. We havetried to synthesize 1-D CNTs array in co-flowflames with some difficulties. Fortunately, ourcounterflow methane–air diffusion flame satisfiesall these requirements. No literature has reportedsimilar synthesis. Figure 4c shows the SEM imageof 1-D CNTs array grown in the nanopores on thesandwich-structured Si-substrate shown inFig. 2b. CNTs shown in Fig. 4c had a uniformdiameter and length, parallel to each other andto the Si-substrate. The length of CNTs wasaround 3 lm and the mean diameter was approx-imately 50 nm. These preliminary studies substan-tiate that counterflow methane–air diffusionflames can be used to synthesize well-aligned, self

(b) (c)(a)

Fig. 3. CNTs synthesized on Ni-alloy wire substrates, (a) SEM image, (b) TEM image, showing a straight structure, and(c) TEM image, showing a bamboo-like structure.

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assembled CNTs for satisfying particularapplications.

3.2. Temperature effects

It was verified [6] that synthesis region in aflame where CNTs could be harvested was limit-ed. For example, in our previous study using62 mm height co-flow flame, the synthesis regionwas located inside the flame, between 7 and20 mm from nozzle exit. Limited synthesis regionis believed due to temperature effect. In the pres-ent study, a series of experiments were conductedto determine synthesis region in counterflow diffu-sion flames. It was observed that at the oxidizerside of the flame, no CNTs but the rectangularparticles were produced on the surface of Ni-alloysubstrate due to lack of carbon sources in thisregion. TEM analysis verified that they were metaloxides. In the high-temperature flame zone, a fewshort carbon fibers were deposited on the sub-strates. At the fuel side near the flame edge, a largeamount of curved and entangled CNTs were pro-duced (see Fig. 3a). The synthesis region for coun-terflow methane–air diffusion flames was at thefuel side somewhere near the flame edge.

Figure 5 is the measured temperature profilesplotted as a function of normalized distance (z/L) from fuel nozzle along the burner axis with dif-ferent fuel streams (CH4 and 33% N2-dilutedCH4), flow configurations, and air-side strainrates for comparison. The measurements showthat temperature increases along the burner axisuntil the flame zone, then decreases. Recall thatCVD synthesis usually operates at the tempera-ture range between 1023 and 1073 K, surprisingly,it was found that the temperature window appliedin CVD synthesis was well-correlated to that ofsynthesis region in the counterflow diffusionflame, as indicated in Fig. 5. This region was atthe fuel side, much closer to the fuel nozzle for

off-symmetric than symmetric configurations.N2-dilution to fuel stream lowered temperatureby approximately 20–50 K. Consequently, thesynthesis region for N2-diluted CH4 was slightlyshifted. For the symmetric configuration, the syn-thesis region was shifted more towards the oxidiz-er nozzle, but still at the fuel side.

Further study of temperature effect on CNTsgrowth was conducted on CH4 diluted with N2

at 33 and 21% mole fractions, and compared theresults with pure CH4. The SEM images of CNTsgrown on Ni-alloy substrates at different N2 addi-tions are shown in Fig. 6 for off-symmetric config-uration of the counterflow. It exhibited that CNTsgrown in pure CH4 (Fig. 6a) were much curvedand the diameter was various. CNTs synthesizedin N2-diluted CH4 at 21% (Fig. 6b) were stillcurved, but had a uniform size with outer diame-ter in the range of 60–70 nm. When CH4 was fur-ther diluted with N2 to 33%, CNTs were relatively

Fig. 4. SEM images of CNTs on 2-D Si-substrates, (a) top view with the local image enlarged, showing uniform CNTsgrown in each nanopore, (b) CNTs morphology after sonication treatment, and (c) 1-D CNTs array grown on sandwich-structured Si-substrate, showing CNTs parallel each other.

Relative distance from fuel nozzle, z/L

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

Tem

pera

ture

, K

200

400

600

800

1000

1200

1400

1600

CH4, off-symmetry, a= 46.6/s

CH4, symmetry,50.0/sCH4,+33%N2, off-symmetry, 53.3/s

temperature windowin CVD synthesis

stagnation planes

Fig. 5. Comparison of temperature profiles along theaxis of counterflow at a variety of conditions, showingthat temperature window in CVD synthesis is well-correlated to the synthesis region in the counterflowflame.

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straight (Fig. 6c), having better quality than thosein pure CH4 with the mean diameter around100 nm. These results indicate that temperatureis an essential parameter affected not only the suc-cess of synthesis, but also the morphology ofCNTs.

3.3. Strain rate effects

It is generally believed that the air-side strainrate of counterflow diffusion flames affects CNTssynthesis either through the residence time of theflow or carbon sources available for CNTsgrowth. A diffusion flame at high strain rate isstronger and produces more carbon sources thana weak flame with low strain rate, but the resi-dence time of carbon sources is short. Therefore,both factors should be experimentally examinedin the study of strain rate effect on CNTs growth.

In order to explore the effect of strain rate onCNTs growth, the experiments were performedin both symmetric and off-symmetric configura-tions. Experiments with symmetric counterflowflames were conducted at the air-side strain ratesof 50 and 39.8 s�1. Visual observation displayedthat the flames in symmetry located at the air sideaway from the stagnation surface. The flame atthe 50 s�1 strain rate was bright and appeared yel-lowish in color with a thin blue layer at the airside. The flame at the 39.8 s�1 appeared a thinneryellow and a blue layer, indicating less carbonsources in the flame. In both flames, only a fewcarbon fibers were sampled even in the appropri-ate temperature region shown in Fig. 5. The sam-pled carbon fibers were much curved and short,suggesting that the resident time might not beresponsible for poor synthesis, instead, it mightbe caused by less available carbon sources in theseflames to support CNTs growth, because in sym-metric flame configuration, the flame was far awayfrom the stagnation surface so that the carbonsources at the fuel side could not easily diffuse

across the stagnation surface to reach the flame.The flames at the strain rate lower than 39.8 s�1

were weak with only a blue layer, behaving likepremixed flames. We did not sample materials inthis flame.

In order to verify our suggestion, we conductedthe synthesis in off-symmetric flame configuration.Recall the studies conducted in Kennedy’s group[14,15], their experiments applied a lower strainrate at 20 s�1. The fuel stream was the mixtureof CH4 + 4% C2H2 to increase sooting character-istics. The oxidizer stream was O2-enhanced air(50% O2+ 50% N2) to enhance radicals availableto strip the carbon sources from the feedstock[14]. In the present study, off-symmetric configu-ration was achieved with higher air velocity thanfuel stream. Figure 7 shows the SEM images ofCNTs obtained in off-symmetric counterflow atdifferent air-side strain rates. At the high strainrate (46.6 s�1), synthesized CNTs (Fig. 7a) wererelatively straight, having a mean diameteraround 100 nm. At the moderate strain rate(42 s�1) produced by reducing air velocity, theCNTs were much more curved (Fig. 7b) with amean diameter of 50–60 nm. Further reducingair velocity (37.5 s�1, still higher than fuel veloci-ty) resulted in synthesized CNTs mixed with otherforms of carbons (Fig. 7c).

These results suggested that the effect of strainrate on CNTs growth was from the carbon sourc-es available to diffuse across the stagnation sur-face from fuel side to the flame, rather than theresidence time of carbon sources. In off-symmetricflame configuration, the flame with high air-sidestrain rate was strong and yellowish, and pushedtowards the fuel side and closer to the stagnationsurface so that the carbon sources at the fuel sidecould easily diffuse across the stagnation surfaceto support CNTs growth. When air velocitydecreased, the flame moved slightly away fromthe stagnation surface and carbon sources avail-able for CNTs growth were reduced, resulting in

Fig. 6. SEM images of CNTs synthesized in the off-symmetric counterflow diffusion flames at different CH4/N2 ratios,(a) pure CH4, (b) CH4 + 21% N2, and (c) CH4 + 33% N2 which produced relatively straight CNTs.

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decreased production of CNTs. However, the rea-son that CNTs synthesized at low strain rates weremuch more curved was unknown and a furtherstudy is necessary.

4. Conclusion

We have demonstrated in this study thatcounterflow methane–air diffusion flames couldsuccessfully synthesize curved and entangledCNTs on Ni-alloy wire substrates, well-aligned,self-assembled CNTs on 2-D Si-substrates withAAO-templates, and 1-D CNTs array on sand-wich-structured Si-substrates with 1-D AAO-nanotemplates. Ni-alloy substrates were usedfor parameter study, while Si-substrates wereused for development of potential applicationsto fabricate nanoscale electronic devices. Tem-perature profiles were measured by a thin ther-mocouple, and the effect of temperature onCNTs growth was studied by diluting CH4 withN2. It was found that there was a common tem-perature region (1023–1073 K) for both CVDand our counterflow diffusion flame under whichCNTs could be synthesized. For the symmetricconfiguration, the carbon sources at the fuel sidecould not easily diffuse across the stagnation sur-face to support CNTs growth, and the strain ratethrough the residence time of carbon sources hadalmost no effect on CNTs growth. Off-symmetriccounterflow with higher air velocity, however,pushed the flame closer to the stagnation surface,resulting in a significant amount of carbon sourc-es at the fuel side available to diffuse across thestagnation surface to support CNTs growth.Therefore, it was suggested that the effect ofair-side strain rate on CNTs growth was throughthe carbon sources at the fuel side available todiffuse across the stagnation surface to the flame,rather than the resident time of carbon sources.Relatively straight and long CNTs could be

produced on Ni-alloy substrates at the fuel sidenear the flame edge using CH4 + 33% N2 as afuel in the off-symmetric flame configuration atthe air-side strain rate of 53.3 s�1. Off-symmetricconfiguration with higher air velocity was morefavorable than symmetric configuration forhigh-quality CNTs production.

Acknowledgments

This study was supported in part by the Ken-tucky Science and Technology Corp. (KSEF-148-502-04097), in part by the National ScienceFoundation (ECS 0304129), and in part by theDepartment of Energy (DE-FG02-00ER4582and DE-FG26-04NT42171).

References

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Fig. 7. SEM images of CNTs synthesized in the off-symmetric counterflow diffusion flames at different air-side strainrates, (a) 46.6 s�1, (b) 42.0 s�1, and (c) 37.5 s�1.

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Comments

Lawrence Kennedy, University of Illinois at Chicago,

USA. Since your use of an aluminum foil to grow theCNT restricts the diameter of the CNT to the pore size,how flexible is the process to change the pore size in or-der to obtain small diameter CNTs?

In our earlier experiments [1,2], the aligned CNTgenerated also occurred in the 1000 �C range. Howev-er, growing carbon nanostructures at low temperatures(e.g., 700–800 �C) the morphology changed fromCNTs to ribbons and coil nanostructures. Did yourun any experiments in this low temperature region?If so did you see any of these other graphiticnanostructures?

References

[1] L.A. Kennedy, A.V. Saveliev, J.P. Bingue, A.A.Fridman, Proc. Combust. Inst. 29 (2002) 835–842.

[2] M. Silvestrini, W. Merchan-Merchan, H. Richter,A.V. Saveliev, L.A. Kennedy, Proc. Combust. Inst.

30 (2004) 2545–2552.

Reply. The nanopore diameter is mainly determinedby electrolyte species, species concentration and appliedvoltage, while the pore length depends on the anodiza-tion time. These parameters can be adjusted in the prep-aration process to control diameter, length of pores andtheir separation distance. There is an optimum solutionconcentration under which pores will grow regularly anduniformly. Since we fabricated AAO nanopore templateon Si-substrate with aluminum film deposition otherthan aluminum foil, it is possible to obtain small diam-eter nanopores.

We have run a series of experiments along the flowaxis to determine synthesis region in our counterflow dif-fusion flames. During the course of experiments, howev-er, we did not find these mentioned nanostructures,probably because of the differences in temperature rangeand in flow stream. Our flames (CH4 or N2-diluted CH4)are less sooty than your flames (CH4 + 4% C2H2). Thesampled results during these experiments were presentedin the text at Section 3.2.

d

Randy L. Vander Wal, NCSER at NASA-Glenn, USA.As you change the flame conditions for comparison of theCNTs produced, other factors may merit consideration.For example, at the different flame stoichiometries, thegas-phase carbon concentration is changing and this inturn affects the carbide induced breakup leading to the cat-alyst particles from the metal wire for subsequent CNTgrowth. Therein can you estimate to what extent wouldthe CNT density, size and uniformity reflect the catalystparticle formation versus the particular growth environ-ment at the different flame stoichiometries?

Reply. We can think of three effects by the nitrogendilution in counterflow methane–air diffusion flames: (1)reduction of flame temperature, (2) lowering the concen-tration of gas-phase carbon in the flame, and (3) dissocia-tion of nitrogen in the flame. In our paper, we thoroughlyinvestigated the first effect, leaving the other two effectsunknown. As you point out, the second effect may beimportant but we do not have the data. We appreciateyour comment and will consider for our future study.

1856 T.X. Li et al. / Proceedings of the Combustion Institute 31 (2007) 1849–1856