<I>In-Situ</I> Optical Analysis of the Gas Phase during the Formation of Carbon Nanotubes

Abstract

A reactor has been developed at ONERA to investigate the gas phase during carbon nanotube formation by laser-induced fluorescence (LIF), Laser-induced incandescence (LII), coherent anti-Stokes Raman Scattering (CARS), and emission spectroscopy. Continuous vaporization is achieved with a continuous wave CO2 laser. Optimized conditions are used for single-walled nanotube growth, that is, a graphite target doped with 2 atom % Ni and 2 atom % Co, helium as buffer gas at a flow rate of 50 ml/s, and a pressure of 300 hPa. Temperature profiles are measured by CARS on H2, and soot images are obtained by LII in the hot carbonaceous flow. LIF and spontaneous emission of the C2 radical and Ni and Co atoms are presented. Spectral investigations are conducted at 3100 and 3200 K to have an optimized pair of excitation/detection wavelengths. Spatial investigations of the relative concentrations in the hot carbonaceous flow are performed up to 3500 K. The concentrations are measured as a function of target temperature. Two regimes of vaporization are observed. Vaporization is slow up to 3350 K and becomes much faster above this temperature. The fast regime in the 3350-3500 K range corresponds to the observed spatial extent of the metal vapors region. At 3500 K, the C2 profiles obtained with and without catalysts are very different as a result of carbon coalescence as well as carbon dissolution into the metal nanoparticles when these are present in the gas phase. The shape of the C2 profile can be related to nanotube formation and growth at a target temperature of 3500 K.