In-situ optical analysis of the gas phase during the formation of carbon nanotubes

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 H-2, and soot images are obtained by LII in the hot carbonaceous flow. LIF and spontaneous emission of the C-2 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 C-2 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 C-2 profile can be related to nanotube formation and growth at a target temperature of 3500 K.