D. Laplaze

Resonant Raman study of the structure and electronic properties of single-wall carbon nanotubes

We investigate the laser-energy dependence of the Raman profile of single-wall carbon nanotube (SWNT) samples with various distributions of diameters. We show that resonant Raman is an efficient tool for the study of the structure and electronic properties of SWNT. The tube diameter distribution is derived from the comparison between the experimental frequencies of the radial A1g breathing mode range (RBM) and the calculated RBM frequency of SWNT bundles. Metallic or semi-conducting tubes are identified in the light of calculations of allowed optical transitions. The assignments are confirmed by the observation (absence) of a Breit–Wigner–Fano-like lineshape for the tangential graphite-like modes of metallic (semiconducting) nanotubes.

Carbon nanotubes: The solar approach

Abstract Using the same experimental set-up as for the solar production of fullerenes, we can also produce carbon nanotubes by direct vaporization of a mixture of powdered carbon and catalyst (Co, Ni, Y). The structure of the nanotubes is strongly dependent on the experimental conditions (pressure and flow rate of Ar gas) and we can obtain either multi-walled nanotubes or ropes of single-walled nanotubes.

Carbon nanotubes: dynamics of synthesis processes

Abstract Heat and mass transport are obtained in a solar reactor using ‘in situ’ measurements linked to numerical simulation and allow the interpretation of the vaporization process as well as the determination of the cooling regime. Comparison with other processes (laser ablation or electric arc) point out some common behavior like the great influence of the cooling rate of vapors on the structure and yield of nanostructured carbon material. We also investigate the growth mechanisms of single wall carbon nanotubes (SWNTs) produced by the solar method as a function of the nature of catalysts and the temperature variation in the condensing area. The Raman spectra clearly show that the change of catalyst induces differences in the diameter of SWNT whereas TEM pictures enhance the change of both length and diameter of the bundles. All these results are explained considering that the key parameter is the temperature at which the SWNTs are formed. This temperature range can be related to the sublimation temperature of the target and to the eutectic temperature of the binary phase diagram. Finally we propose a new mechanism to explain the nucleation process and segregation rate which seems to depend on the capacity of catalyst to form carbide.

Fullerene production in a 3-phase AC plasma process

We present in this paper a new original process for fullerenes production consisting of treating carbon powders through a 3-phase thermal plasma. The main difference with the classical arc process is that the input carbon rate, not limited to the electrodes erosion, can be independently controlled. As expected, the experiments carried out using ten different industrial carbon grades have shown a high sensitivity depending on the carbon precursor. These results have confirmed that the new process is a very promising route for producing bulk quantities of fullerenes. Before optimization, 3.5% of extractable fullerenes were obtained at atmospheric pressure with acetylene black.

Growth mechanisms and diameter evolution of single wall carbon nanotubes

We investigate the growth mechanisms of single wall carbon nanotubes (SWNTs) produced by the solar method as a function of the nature of catalysts and the temperature variation in the condensing area. The Raman spectra clearly show differences in SWNT diameters induced by the change of catalyst whereas TEM pictures enhance the change of both length and diameter of the bundles. All the results are explained considering that the formation temperature range of SWNTs is the key parameter related to the target sublimation temperature and to the eutectic temperature of the binary phase diagram.

Large scale solar production of fullerenes and carbon nanotubes

Direct vaporization of graphite, under inert gas atmosphere, using a 2 kW solar furnace of the Odeillo institute, gives fullerenes with a yield which can reach 20 %. In the same way, vaporizing graphite/bimetal 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.

Scale up of a Solar Reactor for Fullerene and Nanotube Synthesis

Conventional methods for the synthesis of fullerenes and carbon nanotubes such as laser or electric arc ablation have failed when the process is scaled up. Our ultimate goal is to scale a solar process up from 2 to 250 kW; this paper shows that our method for achieving this scale-up is valid because we were able to predict process performance variables at the 50 kW level from preliminary experimental results from 2 kW experiments. The key parameters that characterize this process are the carbon soot mass flow rate and the desired product yield. The carbon soot production rate is a function of the target temperature and this can be predicted in a straightforward way from a heat transfer model of the larger system. The yield is a more complicated function of specific reactor variables such as patterns of fluid flow, residence times at various temperatures, and the reaction chemistry, but we have found that for fullerenes it depends primarily on the concentration of carbon vapor in the carrier gas, the target temperature and the temperature distribution in the cooling zone. Using these parameters, we scaled our process up to 50 kW and compared the predicted results to the measured performance, A graphite target 6 cm in diameter was vaporized in an argon atmosphere and a reduced pressure of 120-240 hPa with a solar flux density in the range 600-920 W/cm 2 . Vaporization rates as high as 50 g/h were measured with a fullerene production rate equal to about 2 g/h, i.e., the expected results.

Solar reactor scaling up

This paper aims at the optimum design of a solar reactor for carbon product processing by vaporization. The chosen reference case is the CNRS 1 MW solar furnace and graphite vaporization for fullerene synthesis. The method, which accounts for heat transfer and chemical reaction kinetics, is based on the combination of both experimental data obtained at laboratory scale and numerical simulation. For this very high temperature process (carbon vaporization is only significant at temperatures higher than 3200 K), the optimum diameter of the target is 22 cm and the soot production ranges between 80 and 150 g/h for an effective power of 325 kW (P=240 h Pa). The model is validated by experiments using a 6 cm o.d. graphite target.

Raman characterization of single wall carbon nanotubes prepared by the solar energy route

Abstract A Raman scattering characterization is reported that confirms the preparation of single wall carbon nanotubes (SWNT) by the solar energy route. The results are presented for samples synthesized with various catalysts—mixtures of Ni and Co (Y, La)—and compared to those obtained from electric arc discharge or laser ablation. In the light of the calculations of the vibrational spectra of SWNT by Eklund et al. (Carbon, 1995, 33, 959) it is shown that both the diameter and structure of the nanotubes depend strongly on the synthesis conditions. For the first time the presence of nanotubes with “zigzag” or “chiral” helical pitches for some of the samples are shown as well as a large distribution of tube diameters.

Solar-induced fluorescence (SIF) of C2 radical

Abstract C2 Swan band emission ( d 3 π g → a 3 π u ) near 517 nm is observed in a solar reactor for fullerene synthesis. On the basis of theoretical considerations and experimentals in the temperature range 3000–3400 K evidence is presented supporting the formation of excited C2 by absorption of solar photons. This phenomenon that we propose to name: solar-induced fluorescence (SIF) is described for the first time.