Gas and pressure effects on the synthesis of amorphous carbon nanotubes

ARTICLEShinese science bulletin 2004 Vol 49 No 24 2569"=2571in nitrogen at 900C. They explained the mechanism ofGas and pressure effects on the rameter. Liu et al. l ynthesized acnts by means of ardsynthesis of amorphousdischarge at controlled temperature using Co and Nipowders as catalyst in hydrogen gas at 600 C. The effectcarbon nanotubesof temperature on the formation of ACNTs was studiedThis research focuses on the effect of gas and presZHAO Tingkai, LIU Yongning ZHU Jiewusure on the formation of ACNTs using the temperaturecontrolled arc discharge furnace, aiming to developXian Jiaotong University, Xian 710049, ChinaCorrespondence should be addressed to Liu Yongning (e-mail: control method that can synthesize ACNTs in a largeyinliu@mail.xjtu,edu.cnAbstract The effects of gas, pressure and temperature onI Experimentalthe production of amorphous carbon nanotubes were invesAn arc discharging furnace with a vacuum chambertigated using an are discharging furnace at controlled tem- 300 mm. 400 mm in size was used in the experiment 8Jrature Co/Ni alloy powder was used as catalyst. The dis- The temperature of this furnace could be controlled duricharge current was 80 A and voltage was 32 V. The optimal the arcing processingde wheel on which 6 ceparameters were obtained: 600( temperature, hydrogen sumption anodes were mounted can be turned around. Thegas and 500 torr pressure. The productivity and purity ofamorphous carbon nanotubes are 6.5 gram per hour and Soots, which contained ACNTs, were collected on the wallof the furnace. Co/Ni alloy powder was used as catalyst80%, respectively. The diameter of the amorphous carbon and hydrogen or a mixture of hydrogen and helium, nitronanotubes is about 7--20 nmgen and argon were used as buffer gas. The temperatureeywords: amorphous carbon nanotubes, are discharge at con- was set at 600c and arcing current was 80a and maintained for 5 minutesDOI:10.136004we0132The microstructure of ACNTs is studied using tEmSEM, HRTEM, TGA and XRD. The purity of hydrogenSince the discovery of carbon nanotubes(CNTS)by helium, argon and nitrogen is 99.95%, 99.95%,99.95%lijima in 1991, there has been a world wide research on and 99.99%, respectivelynovel materials because of their unique physical, chemical 2 Results and discussionand mechanical properties. These properties are dependenton their structures determined by the preparation proce-Fig. I shows HRTEM image of ACNTs produced byduresthe arc furnace at 500 torr of hydrogen gas. The walls ofRecently, amorphous carbon nanotubes(ACNTs) the ACNTs are constructed by carbon clusters in shorthave attracted great attention. ACNTs with a diameter inthe range of 3---60 nm are different from normal carbonalls of the acnts are conmany carbon clusters with short-distance order orlong-distance disorder. The properties of the ACNTs aredifferent from single-walled and multi-walled carbonnanotubes due to their unique walls. Therefore, it is veryimportant to produce highly pure ACNTs in a large scaleWang et al. 2)prepared carbon nanotubes and found thathe aCNts was deposited on cathode using a traditionalarc discharge at a current of 100A and a pressure of 500torr in helium gas. With this method they only obtainedfew carbon nanotubes. Cal 13.4 also synthesizedACNTs with a diameter in the range of 10--60 nmthrough chemical vaporization deposition(CvD) withferrocene and thiophene serving as catalyst, hydrogen asbuffer gas and temperature kept at 1100--1200C. Theyreported that these ACNTs could absorb 3. 98% hydrogenYH中国煤化工CNMHGunder 10MPa after annealing at about 2200( Nishino etal.5.6) obtained 0.05 gram black powders by CVD usipoly(tetrafluoroethylene)and ferrous chloride as catalystFig 1. HRTEM image of the ACNTs at 600Chiese seience Bulletin Vol 49 No 24 December 20042569ARTICLESdistance order but long-distance disorder. The carbon of 85wt% is very close to the qualitative estimation withclusters which are arranged on the walls in turbo-like TEM and SEMshape are irregular and similar to bulk amorphous carboni) Effects of gases. Under our experimental conF1g.2 shows the XRD results of the as-grown ACNTs ditions, the effect of gases on the production and diametercorresponding to Fig. 1. For comparison, the diffraction of of acnts is studied and the results are shown in table 1Table 1 indicates thatkind of gases and theirpeak of (002)plane at-26.3 for crystal graphite disap- mixture had a great influence on the production of aCntpeared at 600G, indicating that the microstructure of the Compared with pure hydrogen gas, the production ofcarbon nanotubes was amorphousACNTs in other gases decreases obviously. Hydrogen hasthe maximum thermal conductivity (about 41.2.X10.w-m-K), the minimum density (0.08987 kg.m )anddynamic viscosity(0. 173 .>10 kgf-s-m )as comparedwith helium(0.394->10 kof-s-m). These factorsaffect the cooling rate of carbon clusters in the systemhence the production and purityi,)Effects of pressures. Under our experimentalconditions, the effect of different pressures on the production and diameters of acnts is studied and the results areACNTS(T=600℃)shown in table 2Table 2 shows the effect of pressures on the production and diameters of ACNTs. With increasing pressure,ACNTs(T=25℃)the productivity of ACNTs increased. 500 torr was a critial point. Beyond this prACNTs decreased. The possible reason might be thatlower pressure decreased the dynamic viscosity of hydro-gen gas and could not enhance the dynamics of carbonclusters on the formation of ACNTs, thus delaying the20()diffusion speed of carbon clusters. While the pressurereaches a moderate value or higher, the productivity ofFig. 2. XRD patterns of pure graphite and CNTS(T= 600.0ACNTs decreases. The main reason is that higher pressureincreases the density of gas, thus decreasing the speed ofFig. 3(a)and(b)shows the TEM and SEM photo- carbon clusters and increasing the Gibbs energy. Thereforegraphs of the ACNTs at 600. C. The main products are the optimal pressure is 500 torrmorphous carbon nanotubes with purity of about 80%0(ii) Formation mechanism of ACNTs. In order toig. 4 shows the TGa curve of the ACNT. With this minimize the free energy, carbon clusters cannot arrangecurve, the content of ACNT can be calculated. The purity themselves regularly along the axis to form crystal struc-NONE SHI 1S. 0kv中国煤化工CNMHGFig 3.(a) TEM image of as-grown ACNTs at 600.;(b) SEM image of ACNTs at 60025方数据Chinese Science Bulletin Vol 49 No 24 December 2004ARTICLESFurthermore, as is well known, the ACNTs are prone to1.0adsorb hydrogen. We think that the ACNTs can store hydrogen with a high capacity if the initial hydrogen can be0.8eleased25According to the above results and the formationprocess, the gas and pressure strongly affect the production of acnts, moderate conditions can increase theproduction of ACNTs3 ConclusionAmorphous carbon nanotubes with 7-20nm diame600ters, yield of 6.5 grams per hour and purity of 80wt % wereTemperature(℃)prepared by a hydrogen arc discharge using Co/Ni alloypowders as catalyst. The results indicate that the choice ofFig 4. TGA curve of the aCnts.of the gases has obvious effect on the productivity ofTable 1 Effect of gases on the production of ACNTsACNTs. The addition of helium, nitrogen and argon re-duces the production rate of ACNTs. The pressure alType of gas( volume ratio) Diameter/nm Production rate/g/haffects the production rate of ACNTs and there exists7-~20optimum pressure around 500 torr, beyond which, theH2.N2(I:1)10~410yield decreases a littleAcknowledgements This work was supported by the Industry Foundation of Shaanxi Province, China(Grant No. 2003k07-G12)H2.He.Ar(1:1:1)ReferencesH2.He.N2(l:1:1)1. lijima, S, Helical microtubules of graphitic carbon, Nature, 1991H2.+r.N2(1:1:1)7-~25354:56-58ang,W.L….Lu,WTable 2 Effect of hydrogen pressure on the production and diameters ofnanotubes: the growth intermediates of graphitic carbon nanotube?Pressure/torrDiameter/nmProduction rate/g-hElectrochemical Society Proceedings, 1997, 14: 814-824200amorphous carbon nanotubes prepared by the CVD method, Journal of Crystal Growth, 2001, 233: 823--8287004. Ci, L J, Zhu, H.W., Wei, B Q et al., Annealing amorphous ca10--60bon nanotubes for their application in hydrogen storage, Applied107-30Surface Science, 2003, 205: 39-45. Nishino, H, Yamaguchi, C, Haruyuki, N. et al., Carbon nanotubeure, because the regular array of the clusters into oneith amorphous carbon wall: a-CNT, Carbon, 2003, 41: 2165layer of graphitic sheet may make the surface area largerthan that of random stacking the clusters into irregularwalls in our experiments. Certainly, this may give rise to6. Nishino, H, Nishida, R, Matsui, T. et al, Growth of amorphousmany defects along the axis and around the circumferencebon nanotube from poly(tetrafluoroethylene) and ferrous chlo-of the tubes. Furthermore, the formed amorphous carbonride, Carbon,2003,41:2819-2823nanotubes are steady and the gibbs free energy of the7. Liu,Y. N, Song, X L, Zhao, T.K. et al., Amorphous carbonwhole system may be the lowest. In a short time, carbonnanotubes produced by a temperature controlled dC arc discharge,clusters can hardly arrange themselves in the structure ofCarbon,2004,42(8-9):1852--1855long-distance order. but form a short-distance order in8. Liu,Y N, Song, X L, An electric arc furnace for producing carstead. It indicates that the combination of hydrogen atomsand carbon clusters helps decrease the energy in systemTH中国煤化工hmCNMHGIed May 15, 2004)9数鸦 ence Bulletin Vo.49No.24 December20042571