Large scale solar production of fullerenes and carbon nanotubes

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Synthetic Metals 103 (1999) 2476-2477

Large

scale

solar

production

of fullerenes

and carbon

L. Alvareza, T. Guillardb , G. Olaldeb , B. Fiivoireb, J.F. Robertb, P. Berni&,

nanotubes

G, Flamantb, D. Laplazea

a-Groupe de Dynamique des Phases Condense’es, U.M.R.5581, lJniversit& Montpeliier II, F34095 Montpellier, France b-IMP-CNRS Odeilio,tJ.P.R. 8521, F66125 Font-Romeu, France Abstract: Direct vaporization of graphite,underinert gasatmosphere, usinga 2 kW solarfurnaceof the Odeillo institute,gives fullerenes with a yield which can reach 20 %. In the same way, vaporizing graphite&metal targets we can produce single-wall carbon nanotubes. The limiting factor is the target temperature which is around 3000 K. With a more powerful furnace (1000 kW) for which the vaporization temperature could reach 3400 K for larger targets we can increase significantly the fullerenes yield or the purity of carbon nanotubes. We report the first results obtained with this new set-up. Keywords:

High temperaturemethodsfor materialpreparation,Carbonnanotubes

1. introduction Various methods have been explored to produce fullerenes and carbon nanotubes but the most attractive is the vaporization of graphite under inert gas atmosphere[l] in which different possibilities are used to induce carbon vaporization like laser ablation, electric arc or inductive heating. Since 1993, it has been shown (both in USA and France) that we can produce Ceo, C7o and higher fullerenes by the solar route [1,2] and this approach could be a solution to the problem of large scale production of these material with high yield, selectivity or purity of carbon nanotubes. Direct vaporization of carbon, using the high intensity of solar radiation obtained with a 2 kW solar furnace of the Odeillo institute has been successfully used to produce fullerenes and carbon nanotubes. This small scale experimental set-up limits the target size and yields small amount of material due to the low power of the furnace and small size of the focus area. But we can easily observe the variation of yield and the structure evolution of the produced material in function of the synthesis parameters and have a better knowledge of their formation mechanisms. On the other hand, this process could be scaled up with the 1000 kW solar furnace of the Odeillo institute. 2. 2 kW solar

furnace

fast and disordered clustering which remains large at 3000 K despite the bleaching process occuring in the illuminated zone. For the production of carbon nanotubes we have used the same small scale set-up as for fullerenes, changing the target composition and adjusting conveniently the experimental conditions[3,4]. Generally we vaporized mixture of powdered graphite, nickel and cobalt (2 at % of Ni and 2 at % of Co) to study the effects of the synthesis parameters. When carbon nanotubes are produced large sheets (longer than 2 cm) of rubbery material can be collected in the condensing zone. These samples generally contain in addition catalyst nanoparticles embedded in amorphous carbon, empty carbon vesicles and amorphous carbon. The more important parameter is again the vaporization temperature TV. At low TV, the production of nanotubes is weak and the purity of material is poor. But when TV reaches 2950 K, we can obtain for each run 100 mg of material with the same purity as the electric-arc samples.from the whole set of expen’mentsdone with the same TV, SEM and TEM pictures clearly show that the structure of the nanotubes change with the pressure. At low pressure (120 mb) we do not find nanotubes but in some cases, large fibrils looking like carbon fiber precursors. When the pressure reaches 250 mb, the samples contains large amount of multiwalled carbon nanotubes,with bamboo like shape and we begin to find few numbers of small ropes of single wall nanotubes. For higher pressure (400-600 mb) we have only observed bundles of single wall nanotubes, some of them being formed by a large number of nanotubes.[Fig I]

With the small scale device the main difficulty encountered is to balance the very intense thermal losses due to the high thermal conductivity and high emissivity of graphite so that the target temperature reaches 3000 K. Last improvements consist in the use of powdered graphite set in a graphite crucible surrounded by a graphite pipe also heated by the sunlight at its top. This pipe works as a thermal screen and constitutes the annealing zone. From a lot of experiments, it becomes clear that the fullerenes yield is likely dependent on the vaporization temperature TV. A selection of results with the best experimental conditions (pressure and gas flow rate) for various lighting conditions shows that the efficiency varies between l-2% when the target temperature is around 2850 K to 20% when this temperature reaches 3000 K. It also appears that a pertinent parameter could be the relative density of carbon atoms in the vapor close to the crucible. This is consistent Fiaure 1: TEM pictures of bundles of SWCNT with the model proposed by R.E.Smalley [l] which assumes that carbon vapor must be expanded to prevent An increase of the pressure produces a dilution 0379-6779/99/$ - see front matter 0 1999 Elsevier Science S.A. All rights reserved. PII: SO379-6779(98)00709-7

of the

D. Laplaze

et al. I Synthetic

vaporized carbon atoms, favoring the growth of ordered structures. Temperature gradient is very large in the condensing area, but the structure of nanotubes remains the same whatever the place were they are collected. We can assume that carbon nanotubes growth fast in the high temperature zone close to the crucible. A reduction of the temperature gradient in this part could improve the process. Raman spectroscopy provides numerous informations about the structure of singlewall nanotubes. The observed Raman spectra of the as grown solar samples display the general features observed with single wall nanotubes [5]. Focusing our attentions on the low frequencies range 30-300 cm-t, where the breathing modes are strongly dependent of the tubes diameters [6], we observe a large dispersion of the nanotubes diameters. The great number of peaks at frequencies smaller than 180 cm-f show the presence of large diameter tubes and this result is in agreement with TEM observations.

Metals

103 (1999)

2477

2476-2477

at the rear part. However we can estimate the expected front temperature of the target with the maximum power of the furnace which is close to 3400 K.

,500 0

““I”“1”“1”“j”“/““I”‘/!“” 10 20 30 Distance

40 50 60 from front (nm)

Figure 3: Temp. along the graphite 3. 1000

kW

solar

,o

80

rod; power of 600 W/cm2

furnace

With the large furnace, a positive scaling effect is expected because of the reduction of the surface to volume ratio of the target which involves strong reductions of radiatives losses. The size of the focus area is larger than 30 cm in diameter and first numerical simulation have shown that the expected vaporization temperature TV could reach 3400 K. As a consequence, the temperature increase will result in both production and efficiency increase. For the first tests we have used the MEDIASE chamber, built in the Odeillo institute and which can operate under vacuum or various reduced atmospheres. This reactor is composed of a stainless steel chamber (465 mm in diameter, 310 mm high) close on the front by an hemispherical silica window (360 mm in diameter). The front face which supports the window is water cooled and reflects the excess of power.

The interest of this model which must be improved, is the determination of experimental parameters (front temperature, temperature gradient, mass transfer? and fluid dynamics) during runs with large vaporization rate where pyrometric measurements cannot be performed. The knowledge of these parameters is necessary to control and optimize the process. During the last run we have slightly increased the power to increase vaporization rate and verified the convenient drawing of the carbon vapor. The UV visible spectrum of the toluene soluble part of the soot collected during this experiments is typical of a mixture of Cao and C7c.

j ‘L---,_ --

0 300

wavelength(nm)

ml

Figure 4: Toluene soluble part of the soot produced with the 1000 kW solar furnace

4.

The target consist of a graphite rod (60 mm diameter, 80 mm length) surrounded by a graphite pipe insulated by graphite fiber. During experiments, the chamber is swept by argon gas (pressure 300 mb, flow rate 20l/mn). Our objectives were i) the test of the existing vessel with large carbon target; ii) the effect of the shape of the target on the drawn of the carbon vapor; iii) the validation of the used model. The main points are the prevention of the soot deposition on the silica window and on the front reflective face, and the determination of the vaporizing temperature. As we must perform temperature measurements with optical pyrometer during these tests we have reduced the usetu\ power of the furnace to keep the front temperature of the target lower than 3000 K and have a weak vaporization rate. Comparison between experimental and calculated temperatures along the graphite rod shows a good

agreement

for its top.

But some

discrepencies

appear

Conclusion

To produce fullerenes with high efficiency or high purity single wall nanotubes, with uniform structure it is necessary to increase the vaporization temperature and to keep under control the cooling rate of the vaporized carbon. This goal can be achieved with the 1000 kW solar furnace and the first experiments have shown that : i) our simulations yield temperature profiles along the graphite rod comparable to the measured ones, and we should get temperatures close to 3400 K; ii) the produced soot contains fullerenes; iii) it is possible to use large target to increase the production and get more easily higher fullerenes. w The authors would like to thank Prof. R.E. Smalley for the useful discussions they had for scaling up the process [I] [2] [3] [4] [5] [6]

Chibante and al, J.Phys.Chem., 1993, 97, 8696 Laplaze and al, J.Phys.6, 1996, 29, 4943 Thess and al, Science, 1996, 273,483 Laplaze and al, Carbon, published July, 1998 Rao and al, Science, 1997, 275, 187 Anglaret and al, ICSM 98, Montpellier, THC 45