当前位置：文档之家› (2004)High-rate flame synthesis of vertically aligned carbon nanotubes using electric field control

(2004)High-rate flame synthesis of vertically aligned carbon nanotubes using electric field control

High-rate ?ame synthesis of vertically aligned carbon

nanotubes using electric ?eld control

Wilson Merchan-Merchan a ,Alexei V.Saveliev b ,Lawrence A.Kennedy

b,*

Department of Materials Sciences and Engineering,University of Illinois at Chicago,Chicago,IL 60607,USA

Department of Mechanical and Industrial Engineering,University of Illinois at Chicago,851South Morgan Street,Chicago,IL 60607,USA

Received 31July 2003;accepted 17December 2003

Abstract

The electric ?eld controlled synthesis of carbon nanomaterials on a Ni-based catalytic support positioned at the fuel side of the opposed ?ow oxy-?ame is studied experimentally.Carbon nanomaterials formed on the probe surface are comparatively analyzed for two characteristic operational modes:a grounded probe mode and a ?oating probe mode.In a grounded mode a number of various carbon nanostructures are formed depending on the probe location in ?ame.Observed nanoforms include multi-walled carbon nanotubes (MWNTs),MWNT bundles,helically coiled tubular nano?bers,and ribbon-like coiled nano?bers with rectan-gular cross-section.The presence of various carbon nanoforms is attributed to the space variation of ?ame parameters,namely ?ame temperature and concentration of chemical species.It is found that the presence of an electric potential (?oating mode operation)provides the ability to control the nanostructure morphology and synthesis rate.A thick layer (35–40l m)of vertically aligned carbon nanotubes (VACNTs)is found to be formed on the probe surface in the ?oating potential mode.This layer is characterized by high uniformity and narrow distribution of nanotube diameters.Overall,the electric ?eld control method demonstrates sta-bilization of the structure in a wide ?ame region while growth rate remains dependent on ?ame location.ó2004Elsevier Ltd.All rights reserved.

Keywords:A.Carbon nanotubes;B.Catalyst support;C.Scanning electron microscopy

1.Introduction

In recent years great e?ort has been devoted to the study of synthesis of carbon nanotubes in ?ames.Since the discovery of nanotubes in 1991by Iijima [1]con-ventional methods such as arc plasma discharges [2,3],pulsed laser vaporization [4],and chemical vapor deposition (CVD)[5]have dominated the ?eld.Re-cently,it has been demonstrated in a number of publi-cations that ?ames can be applied as a relatively inexpensive,robust,pyrolysing carbon source for growing tubular nanomaterials [6–14].Mainly studies in co-?ow di?usion ?ames with the introduction of a cat-alyst in the form of nano-aerosol or in the form of solid support have been reported.Yuan et al.[6]analyzed ?ame-based growth of nanotubes on a Ni/Cr support in laminar co-?ow methane–air di?usion ?ames.Forma-tion of entangled MWNTs with a diameter range of 20–

60nm was reported.In related work,Yuan and co-workers [7]used a catalytic support in the form of a stainless steel grid to produce MWNTs in an ethylene–air di?usion ?ame.The electroplating of the grid with cobalt resulted in synthesis of aligned MWNTs.The metal catalyst dispersed on TiO 2substrate was used by Vander Wal [8]to generate MWNTs in ethylene/air and acetylene/air di?usion co-?ow ?ames.It was demon-strated that the structure of the obtained nanotubes and nano?bers strongly depends on the catalytic particle shape and chemical composition.Recently,Vander Wal and co-workers [9,10]also reported that single-walled carbon nanotubes (SWNTs)can be grown in premixed co-?ow ?ames by seeding the fuel line with ferrocene and compositions of metal nitrates serving as catalyst precursors for the formation of nanotubes.It was demonstrated that ferrocene and Fe nanoparticles can yield bundles of self-assembled single-wall nanotubes with diameters as small as two nanometers.The rich premixed ?ame synthesis of carbon nanotubes using supported catalyst [11]was optimized by selection of optimal ?ame conditions including fuel composition and

Corresponding author.Tel.:+1-312-996-2400;fax:+1-312-996-8664.

doi:10.1016/j.carbon.2003.12.086

Carbon 42(2004)

599–608

fuel-to-air ratio.The studied fuels include methane, ethane,ethylene,acetylene,and propane.A recent work by Height and co-workers[12]used premixed co-?ow ?ames to grow SWNTs.A detailed characterization of SWNT growth was given with emphasis on?ame posi-tion and?ame air-to-fuel ratio.The non-catalytic for-mation of carbon nanotubes was reported in opposed ?ow oxygen enriched?ame studies[13].The strong potential of these?ames for carbon nanomaterial syn-thesis was recently proven employing a catalytic probe [14].

If compared with CVD and plasma methods,a typi-cal?ame is a reacting medium characterized by strong thermal and chemical non-uniformity.It is not surpris-ing,that a number of?ame studies show a high mor-phological and growth rate sensitivity of formed carbon nanomaterials to the?ame location.An e?cient control method is required to improve uniformity and produc-tivity of?ame based synthesis and utilization of elec-tromagnetic?eld control is one of the promising approaches.The electric?eld control is successfully tested in chemical vapor deposition(CVD)and plasma synthesis studies[15–21]as reported recently by several authors:Kuzuya et al.[15]applied an electric?eld for e?ective control of producing helically coiled carbon materials using CVD;Avigal et al.[16]grew short aligned nanotubes by positively biasing the substrate during the nanotube formation in CVD;Srivastava et al.

[17]showed that nanotubes that are grown under the in?uence of an electric?eld in plasma can permanently maintain their alignment when the electric?eld force is removed;Lee et al.[18]using CVD and magnetic?elds showed that it is possible to grow aligned carbon na-notubes at any angle with respect to the substrate sur-face;Ural et al.[19]used CVD to grow carbon nanotubes with and without the in?uence of electric ?elds to show the ability of electric?elds to grow aligned carbon nanotubes;Colbert et al.[20]used an arc plasma discharge to show that an electric?eld is essential for nanotube growth by stabilizing the open tip and pre-venting it from closure;Srivastava et al.[21]used plas-ma varying voltages to study the size,length,and alignment of carbon nanotubes.

To our knowledge,electric?elds have not been ap-plied to control growth of carbon nanotubes in?ames prior to this study.In this article the carbon nanoforms formed on a catalytic probe in opposed?ow oxy-?ame are comparatively analyzed for various probe potentials. It is shown that implementation of an electric?eld control allows to increase their structural uniformity and growth rate.

2.Experimental

Fig.1shows the schematic of the experimental setup employed in the present study.A counter-?ow burner forms two opposite streams of gases;the fuel(methane seeded with4%of acetylene)is supplied from the top nozzle and the oxidizer(50%O2+50%N2)is introduced from the bottom nozzle.The fuel and oxidizer?ows impinge against each other to form a stable stagnation plane,with a di?usion?ame established from the oxi-dizer side.The introduction of co-?owing nitrogen through a cylindrical annular duct around the outer edge of the oxidizer nozzle has the function of extin-guishing the?ame near the outer jacket and isolating it from the environment.A detailed description of the burner is given by Beltrame et al.[22].Technical purity methane(98%,AGA Gas Inc.)was seeded with atomic absorption purity acetylene(99.8%,AGA Gas Inc.).The experiments were conducted with constant fuel and oxidizer velocities and a strain rate equal to20sà1.

A40mm long catalytic probe was introduced through the?ame-protecting shield to the yellow soot-containing region of the?ame.The central part of

the

Fig.1.Schematic of the experimental setup.

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probe($25mm)was used to study the structure of deposited materials.The0.64mm diameter probe is an alloy with compositions of73%Ni+17%Cu+10%Fe. The axial position of the probeeZTwas controlled by the positioning system.Reported experiments were con-ducted with residence time of10min.

To generate radial electric?elds on the probe surface, the probe was supported on Te?onaisolators while the burner nozzles were kept at ground potential(Fig.1).In general this con?guration allows to generate a variety of electric?elds controlling the probe potential with the external electric source.In the present work,experi-ments were conducted at two characteristic probe potentials:grounded probe and probe at?oating po-tential.

When nodes one and two are connected no electric ?eld is generated;this mode is further referred as the grounding probe mode(GPM).On the other hand, when node one and two are disconnected,electric?eld is generated between the surface of the catalytic probe and the edges of the fuel and oxidizer nozzles;this con?gu-ration is further referred as the?oating potential mode (FPM).Due to the small size of the probe relative to size of the burner nozzles the electric?eld around the probe can be well approximated as radial.The source of the ?oating potential is the transport of ions and electrons formed in high-temperature oxygen?ame to the probe surface;a potential di?erence close to300mV was typically measured when the probe was in FPM.

The initial surface scans of the catalyst probe were performed by a scanning electron microscope(JEOL Inc.,Model JSM-6320F)with a cold?eld emission source.The specimens for TEM examinations were prepared by ultrasonic dispersion of carbon deposits collected from a speci?c probe location in acetone;a drop of the suspension was placed on the electron microscope grid.The detailed structural characteristics of the deposited material were obtained from high-res-olution electron microscopy studies performed in a JEOL JEM-3010electron microscope with the magni?-cation range from50to1,500,000times.Images were collected on a Gatan digital imaging system and pro-cessed by Digital Micrograph software.

3.Results and discussion

When the catalytic probe was inserted in GPM a variety of carbon nanostructures were observed depending on position of catalytic probe in the fuel zone of the opposed?ow?ame.Those carbon structures in-clude MWNTs and MWNT bundles,nano?bers with varying degree of crystallinity,helical regularly coiled tubular carbon nanotubes,ribbon-like coiled nano?bers with rectangular cross section,and,?nally,long($0.2 mm)uniform-diameter($100nm)tubular nano?bers with regular internal structure of carbon layers.Some of the observed structures,such as coiled tubular and rib-bon-like nano?bers,have been reported[23,24]to grow only under special conditions in CVD.

Fig.2(a)shows a typical regularly coiled carbon nano?ber formed in our?ames.The diameters of these coils ranged from20to100nm with lengths of several microns.These structures were typically observed in the zone close to Z?9mm,in the vicinity of the?ame front.Qualitatively,the formation of helical nanotubes can be explained by variation of deposition rates and, hence,extrusion velocities along the contact curve be-tween the active catalytic particle and the

already Fig.2.SEM images of carbon nanomaterials grown in various loca-tion of the opposed?ow?ame on a single catalytic support:(a)heli-cally coiled spiral carbon nanotube,(b)multi-walled carbon nanotubes,(c)coiled carbon nano?ber with rectangular cross-section.

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formed tube[23].The sharp gradients of temperature and chemical species in the vicinity of the?ame zone induce sensible variations of carbon deposition rates providing the condition for the growth of helical struc-tures.

Irregular carbon nanotubes are widely observed in the?ame region close to Z?8:5mm.They form a coating layer that typically contains entangled web of randomly directed tubes,carbon nano?bers and soot particulates,as shown in Fig.2(b).The tube diameters vary signi?cantly,typically from10to45nm.The internal tube structure is regular,although carbon layers often form an angle with the tube axis.As a result bamboo-like structures are widely observed.Irregularly oriented nanotubes frequently change direction of growth;kinks and bends are common for these struc-tures.

The other distinctive coiled carbon structure observed in our?ames possesses a nontubular form,Fig.2(c). These structures are typically found to be present in the low temperature zone Z?8mm.The nano?ber has a distinctive ribbon-like appearance with unwound ribbon rolls.The rectangular cross section has a height of300 nm and a thickness of100nm.It was reported previously [25],that growth of these rarely observed ribbon-like ?laments could be catalyzed by small iron-containing particles in CO containing atmosphere at700°C,which correspond to the?ame environment at the vicinity of Z?8mm.

The variation of structure and morphology of formed carbon nanomaterials is directly attributed to the strong variation of temperature and chemical composition in the studied?ame region.The distributions of several major hydrocarbons(CH4,C2H2,CO,and C6H6)that can contribute to the growth of carbon deposits are shown in Fig.3along with the temperature pro?le[22]. It is easy to conclude that all above components vary signi?cantly in the?ame region of interest.Thus,con-centration of CH4and C2H2diminishes below100ppm for Z>10:5mm;the concentration of CO grows with Z reaching its maximum at this point;C6H6is present in essential quantities only from8to10mm,maximizing at 9.5mm.

The essential variations of temperature and chemical composition are inevitable in a variety of?ame con?g-urations.The e?ective control method is necessary to stabilize growth of carbon nanomaterials with speci?ed structure and morphology.The electric?eld control method was implemented in this study electrically insulating the probe from the grounded support and operating it at a?oating potential.

The catalytic probe of the same metallic composition was inserted at the axial?ame position of Z?8:5mm in FPM(approximate probe potential)300mV).The produced carbon deposits were analyzed with SEM (Figs.4and5).Fig.4shows that controlled electric?eld growth generates a coat of VACNTs.Low magni?ca-tion image shows that VACNT layer uniformly coat the catalytic substrate.The generated nanotubes are char-

Fig.4.SEM image of orderly VACNT layer covering the probe sur-face;?oating potential mode,Z?8:5mm.The layer is partially re-moved revealing the bare catalytic surface.

Fig.5.High-resolution SEM of the wall edge of the layer shown in Fig.4displays nanotube purity and alignment.

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acterized by high purity and alignment,as shown in Fig.5.For this axial position,hundreds of nanotube diam-eters were measured.The results show a very narrow diameter distribution with a mean diameter close to 38nm.

Fig.2(b)and Figs.4,5depict the carbon nanomate-rials grown on identical probes in the same ?ame loca-tion respectively in GPM and FPM conditions.Analysis of SEM images on the synthesized carbon materials on both catalytic substrates suggests that nanotube growth is greatly enhanced when the electric ?eld is present (FPM)if compared with the case when the electric ?eld is absent (GPM).Other interesting aspects,displayed in Figs.4and 5,include not only the presence of VACNTs but also the absence of contaminants such as soot or other nontubular carbon structures that are often pres-ent in the ?ame synthesis of carbon nanotubes.It should be noted that the samples analyzed in this study were never puri?ed by any kind of chemical and/or physical treatment.

With electric ?eld stabilization,?ame position re-mains the important factor in controlling the thickness of the coat layer of nanotubes formed under FPM.Figs.6and 7show the variation of nanotube formation along the burner axis.Fig.6is a low resolution SEM image of the catalytic substrate inserted in the FPM at the axial distance of Z ?9:5mm.The well-de?ned layer formed here shows highly ordered carbon nanotube arrays,similar to those found in the previous position.The application of high-resolution imaging in Fig.6reveals a highly dense bundle of nanotubes attached to the tips of the undisturbed nanotube layer,as shown in Fig.7.High-resolution imaging shows that these nanotube bundles are free of contaminants as well.It is evident by comparing micrographs obtained at the axial position of Z ?8:5mm to those obtained at Z ?9:5mm that the thickness of the coating layer decreased when the sub-strate is inserted further down from the edge of the fuel nozzle.Several SEM images were collected at various

locations of the probe surface;an average layer thick-ness of 9l m was measured for this ?ame location.Another interesting aspect of these nanotube layers is the strong van der Waals body attraction force existing between the tubes in the self-formed macro bundles.This aspect is unique since it greatly simpli?es their harvesting.That is,micron size arrays of VACNTs can be easily removed from its roots.

A catalytic substrate inserted at the axial ?ame po-sition of Z ?10:5mm shows no apparent sign of arrays of VACNTs (Fig.8).But high-resolution SEM reveals that thinner and less uniform layers of VACNTs are still being grown as shown in Fig.9.The nanotube layer at this position is still aligned and de?ned but not as well as in the two previous positions.It can be speculated that at this ?ame position the conditions of CNT growth are far from optimal and the electric ?eld control has only limited e?ect.A thickness of VACNT arrays of 3l m is characteristic for this ?ame position.

Additionally,high and low resolution SEM analysis was performed on the surface of the support catalytic substrate at di?erent locations in order to con?rm the uniformity of the nanotube layer.In this particular experiment,the probe was inserted into the ?ame at

Fig.6.Low resolution SEM image shows the catalyst substrate coated with a layer of carbon nanotubes (FPM),Z ?9:5

mm.Fig.7.Higher resolution SEM of a micro area in Fig.

Fig.8.Low resolution SEM image on the catalyst substrate coated with a layer of carbon material (FPM),Z ?10:5mm.

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position where the most thick layer of nanotubes were observed,Z?8:5mm.After the exposure to the?ame, the probe was placed on a stainless steel stud for microscope studies.

Fig.10represents SEM images of arrays of orderly and high purity nanotubes scanned at top,center,and bottom of the probe surface,respectively.For all probe locations considered for microscopic analysis,arrays of nanotubes covered the probe with orientation perpen-dicular to the probe surface.By inspection of Fig.10it is evident that most of the formed material remains at-tached to the surface.Higher resolution SEM on the walls of this material showed that the bulk material is composed of arrays of nanotubes.The separated mate-rial always tends to remains packed like bundles of hay and always maintains its original length.In the upper right corner of Fig.10(b)a bundle of highly dense na-notubes is observed.This bundle has a cylindrical shape and a length of approximately40l m which coincide with the length of nanotubes in the attached VACNT layer.The material remains packed due to the strong van der Waals forces that exist between the tubes in the bundles.The same sample probe was then rotated180°for further SEM scanning and again,layers of nanotu-bes were present covering the catalytic substrate surface.

High-resolution TEM was employed to study the inner morphology of the aligned nanotubes.For TEM studies,nanotubes formed on the catalytic support were carefully peeled o?and transferred to acetone.After sonication,the drop of the suspension was placed on

a Fig.9.Higher resolution SEM of a micro area in Fig.

Fig.10.Low and high resolution(inserts)SEM images of scanned probe surface showing uniformity of VACNT coating layer.

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copper-substrate/carbon-?lm of microscope specimen grid and dried.TEM images of nanotubes synthesized at Z ?8:5mm are shown in Figs.11and 12.Very low resolution TEM imaging on the transferred material shows the presence of high-density closely packed nanotube bundles (Fig.11).It appears that even after sonication,closely packed bundles of nanotubes remain intact.Fig.11shows typical bundles of nanotubes ob-served by TEM in a sample grid,here the bundle pos-sesses a length and width of 31and 10l m,respectively.In a closer zoom on the nanotube bundle,no material other than tubular structures are present,Fig.12.From a number of micrographs the average diameter of the cylindrical multi-shells was measured,a monodisperse diameter distribution averaging 38nm was obtained.The application of high-resolution TEM imaging on these carbon nanotubes reveals a texture of well-aligned and highly graphitized concentric graphitic cylinders.The average interplanar distance of the concentric cylindrical graphene sheets was measured to be 0.34nm,as indicated in Fig.13.The layer planes appear to be perfectly parallel to the central tube axis.

The experimental results show the strong in?uence of the electric ?eld on alignment,size distribution,internal structure,and growth rate of carbon nanotubes.The mechanism of alignment is widely discussed in the lit-erature [26–28].As an example,employing a plasma discharge Merkulov and coworkers [27]successfully demonstrated,that the direction of the electric ?eld lines determines the orientation of the carbon nanotubes and nano?bers;electric ?eld alignment of single-walled na-notubes were considered by Zhang et al.[28].

Consider the alignment mechanism of a single CNT growing on a metal catalytic support at the presence of the electric ?eld E (Fig.14a).At the synthesis tempera-ture,both metallic and semiconducting CNT can be treated as good conductors.Consequently,the nanotube can be represented as a conductive cylinder of length L and radius R in contact with a conducting plane with a surface charge density r .For a cylinder of ?nite length,an electric ?eld generated by the plate in the normal direction will induce a bound charge at the nanotube end to screen the external ?eld over the nanotube vol-ume.The ?eld-screening area of the single nanotube can be estimated as L 2.Accordingly,the total charge of the nanotube is q %r L 2?e 0L 2E .The total electrostatic force acting on the nanotube is F %qE ?e 0L 2E 2.The nanotube inclined at an angle h with respect to the normal direction will have axial F a and tangential F t components of the force F inducing respectively axial stress and alignment force.For a small deviation of the nanotube from the normal direction d ,the alignment force F t ?F sin h ?e 0LE 2d is proportional to the nano-tube length.The more rigorous solution of the electro-static problem [29]gives an expression for the total electrostatic force acting on the cylinder as F ?pe 0E 2L 2=eln e2R =L Tà3=2T,showing that the acting force is a weak function of R .The solution also shows the pres-ence of ?nite surface charges over the lower part of the nanotube with the charge density distribution close to

linear.

Fig.11.Low resolution TEM image of material removed from the catalytic support.The material is produced using FPM at Z ?8:5

mm.

Fig.12.TEM image of metal catalyzed carbon nanotubes from a micro area shown in Fig.

11.

Fig.13.High-resolution TEM image of the wall of a tubule produced in ?oating potential mode.

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For CNTs growing on a conductive substrate as a vertically aligned array with a tube-to-tube separation distance of D,the external?eld is reduced by mutual screening(Fig.14b).The depth of the?eld penetration can be also estimated as D.Similar consideration for the charge induced on the nanotube tip provides estimation of the charge value q%e0D2E and the electrostatic force acting on the single tube F%qE?e0D2E2.Conse-quently,the aligning force F t?e0DE2d is linearly pro-portional to the nanotube separation D and the deviation from the vertical position d.The surface charges of the same polarity induced near the nanotube ends ensure their separation and alignment.These repulsive forces overcome van der Waals attraction forces.It is worth mentioning that the repulsive align-ment is quiet di?erent from the alignment induced by van der Waals interactions.The latter mechanism leads to the formation of nanotube bundles which in turn reduces the thermal randomization of the CNT growth. However,a direction of the bundle growth remains arbitrary.

Overall,the electric?elds near the tips of growing nanotubes can be extremely high.Even applied poten-tials as small as few tens of millivolts can develop an electric?eld exceeding1000V/cm at the characteristic nanotube diameter.The experimentally measured?eld enhancement factors are reaching800for multi-walled nanotubes[30]and3000for single-walled nanotubes [31].The enhancement of the electric?eld at the tip of closed conducting nanotube was calculated by Maiti et al.[32];the resulting force estimated from the axial stress is in a good agreement with Taylor solution[29].

The important aspect of the aligned growth is that the constant orientation of catalytic particles at the tips of the growing nanotubes is preserved by the electric?eld. The non-symmetric catalytic particle is polarized in the electric?eld,and the induced dipole moment tends to be aligned along the electric?eld lines.The formation of helical and spiral nanotubes requires variation of the particle orientation.As a result the constant orientation of the catalytic particle stabilizes the linear CNT struc-tures.

The high concentration of the electric charges near the nanotube end also induces axial stress between the growing nanotube body and conductive catalyst parti-cle.This can lead to the increase of the CNT growth rate.For clarity,we further consider a nanotube with a catalytic particle at the tip.The tip-growth proceeds via several important stages,including decomposition of ?ame hydrocarbons on catalytic surface,di?usion of carbon through and over metal particulate,formation of ordered CNT structure on the opposite side of the catalytic particle.The solution of electrostatic problem suggests strong concentration of charge and electric?eld near the tip of the growing nanotube,namely on the catalytic particle attached to the nanotube end.The stress introduced between the particle and the CNT body may be estimated as r s?F=p R2.While the force F has only weak dependence on R,the stress shows dra-matic increase with the reduction of contact area and can reach a critical value r?

corresponding to the pos-sible particle separation.This can discriminate the growth of nanotubes with R

Finally,the electric?eld can in?uence the transport of charged particles in?ames that include ions and charged soot particles.In this way,soot entrapment in the growing layer can be controlled by the electric

?eld.

The e?ect of the electric?eld control using various electric?eld amplitudes and geometries is an interesting subject for the future studies.The selected?ame con-?guration allows ease of application of internal(probe generated)and external(generated by external elec-trodes)electric?elds,as well as time-dependent elec-tromagnetic?elds.

4.Conclusions

The electric?eld controlled synthesis of carbon nanomaterials on a Ni-based catalytic support posi-tioned at the fuel side of the opposed?ow oxy-?ame is studied experimentally.The?ame environment is formed by fuel and oxidizer with compositions of CH4+4%C2H2and50%O2+50%N2,respectively.Car-bon nanomaterials formed on the probe surface are comparatively analyzed for two characteristic opera-tional modes:a grounded probe mode and a?oating probe mode.

In a grounded mode a number of various carbon nanostructures are formed depending on the probe location in?ame.Observed nanoforms include multi-walled carbon nanotubes(MWNTs),MWNT bundles, helically coiled tubular nano?bers,and ribbon-like coiled nano?bers with rectangular cross-section.The presence of various carbon nanoforms is attributed to the space variation of?ame parameters,namely?ame temperature and concentration of chemical species.

It is found that a presence of an electric potential (?oating mode operation)provides an ability to control nanostructure morphology and synthesis rate.A thick layer(35–40l m)of vertically aligned carbon nanotubes (VACNTs)is found to be formed on the probe surface in the?oating potential mode.This layer is character-ized by high uniformity and narrow distribution of nanotube diameters.Electric?eld control method demonstrates stabilization of the linear nanotube structure in a wide?ame region.The growth rate re-mains dependent on?ame location but the strong enhancement e?ect is observed with application of electric?eld.

Acknowledgements

This work was supported by the National Science Foundation grant CTS-0304528.This work was par-tially supported by the Air Liquide Corp.under an Unrestricted Laboratory Development Grant.The au-thors extend special thank to Dr.Alan Nicholls,Mr. John Roth,Ms.Linda J.Juarez,and Ms.Kristina Jarosius from the UIC Research Resource Center for day-to-day assistance in SEM and TEM studies, encouragement and helpful discussions.The authors would also like to thank Mr.Attilio Milanese for his input and helpful discussions.

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2018年同等学力申硕英语 2018年同等学力申硕英语?面对同等学力考试，相信每一位考生都做好了充足的准备，相信每一位考生的心中各不相同。但是提醒考生切勿兴奋过头，要保持一颗清醒的头脑，冷静答题。尤其是英语考试，考生要注意一下几方面：一、不要“抢”答，要做好答题前的准备按照英语考试的组织规程，考试两个阶段正式开始答题前几分钟，同学们就会拿到英语试卷。首先要快速地整体浏览一下试卷，大致判断一下对于自己来说试卷的难度。这里要特别注意留意两个部分：第一个是阅读理解，第二个是看一看书面表达题目二、不要“长”答，要合理分配答题时间在第一个阶段的英语考试中，控制好答题节奏，合理利用时间，这一点非常重要。不要在一道试题上耽误太多时间。阅读理解部分由于语篇多，词汇量和阅读量都大，因此比较耗时，但千万不能在这里“恋战”。三、调动语感做到“一答”准确英语考试考查语言的运用，因此，考试中有没有语感，就变得非常重要。答题时，不要一味地想语法，想考点，要把考试变成“调动语感”和进行“语言交流”。要自觉运用平时训练所形成的答题技巧。对应试卷各个题型，在回答问题时要注意：

(1)单项选择; (2)完型填空; (3)阅读理解; (4)翻译的重要一步就是一定要理解原文，然后在用自己的语言通顺的表达出来，英语重在意合，英语重在形合;(5)书面表达四、仔细填涂答题卡避免“非能力失分” 英语考试不仅检验平时训练是否“有素”，“功力”是否“完满”，也体现临场发挥的程度和水平。温馨提示：考试们在面对考试的时候不用有太多的担心，这2018年同等学力申硕英语其实英语考试除了考查学生英语知识，也是考查学生的应试能力，所以考生在考试中，一定要沉着冷静，保持一个稳定的心，希望同学们关注考试注意事项，注意答题的每一个细节，稳定心理，正常、甚至是超水平发挥，以期在考试中取得优异成绩。附：在职研究生热门招生院校推荐表

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15年高考理综全国卷1

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【解析】输入的溶液进入血液，随血液运输，同时从血浆通过毛细血管壁细胞，进入组织液。人体

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2015年高考全国一卷理综

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对于在职研究生英语单词是否“一见钟情”都是无所谓的，关键在于有更多不同类型的见面机会，因为一个单词能不能记住，取决于和它在不同场合见面的频率，不在于每次看着它的时间长短。一般想记住一个单词，每星期要和它在不同场合见三到四次面，具体问题一定要具体分析;另外，大家在背单词时，还要把握住最基础的部分，也就是所谓的词根，对付这些词根的最好方法，就是进行大量的，不间断的，简单的初级听力练习。因为阅读材料中，还有百分之二十其他词汇，所以光凭这个等级的词还看不懂那些阅读材料。附：在职研究生热门招生院校推荐表

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2015年全国卷1理综第Ⅰ卷(选择题共126分) 可能用到的相对原子质量：H 1 C 12 N 14 O 16 Cl 35.5 K 39 Cr 52 Fe 56 Cu 64 Br 80 Ag 108 I 127 一、选择题：本题共13小题，每小题6分。在每小题给出的四个选项中，只有一项是符合题目要求的。 1．下列叙述错误的是 A．DNA与ATP中所含元素的种类相同 B．一个tRNA分子中只有一个反密码子 C．T2噬菌体的核酸由脱氧核糖核苷酸组成 D．控制细菌性状的基因位于拟核和线粒体中的DNA上 2．下列关于植物生长素的叙述，错误的是 A．植物幼嫩叶片中的色氨酸可转变为生长素 B．成熟茎韧皮部中的生长素可以进行非极性运输 C．幼嫩细胞和成熟细胞对生长素的敏感程度相同 D．豌豆幼苗切段中乙烯的合成受生长素浓度的影响 3．某同学给健康实验兔静脉滴注0.9％的NaCl溶液(生理盐水)20 mL后，会出现的现象是A．输入的溶液会从血浆进入组织液 B．细胞内液和细胞外液分别增加10 mL C．细胞内液Na＋的增加远大于细胞外液Na＋的增加 D．输入的Na＋中50％进入细胞内液，50％分布在细胞外液 4．下列关于初生演替中草本阶段和灌木阶段的叙述，正确的是 A．草本阶段与灌木阶段群落的丰富度相同 B．草本阶段比灌木阶段的群落空间结构复杂 C．草本阶段比灌木阶段的群落自我调节能力强 D．草本阶段为灌木阶段的群落形成创造了适宜环境 5．人或动物PrP基因编码一种蛋白(PrP°)，该蛋白无致病性。PrP°的空间结构改变后成为PrP°°(朊粒)，就具有了致病性。PrP°°可以诱导更多的PrP°转变为PrP°°，实现朊粒的增殖，可以引起疯牛病。据此判断，下列叙述正确的是 A．朊粒侵入机体后可整合到宿主的基因组中 B．朊粒的增殖方式与肺炎双球菌的增殖方式相同 C．蛋白质空间结构的改变可以使其功能发生变化 D．PrP°转变为PrP°°的过程属于遗传信息的翻译过程 6．抗维生素D佝偻病为X染色体显性遗传病，短指为常染色体显性遗传病，红绿色盲为X 染色体隐性遗传病，白化病为常染色体隐性遗传病。下列关于这四种遗传病遗传特征的叙述，正确的是 A．短指的发病率男性高于女性

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