Diagnostics of carbon arc plasma under formation of carbon-encapsulated iron nanoparticles by optical emission and absorption spectroscopy

Absorption and emission spectra pertaining to the electronic transition a (3)Pi(u)-d (3)Pi(g) (0-0 band) of C-2 molecule as present in the carbon arc discharge zone are analysed to determine the temperature and C-2 radical content in function of carbon-iron anode composition. One of the objectives was to explore the use of emission and absorption, both approximate, methods for diagnostics of carbon arc plasma used for synthesis of carbon-encapsulated iron nanoparticles. The theory behind both methods was exploited to estimate the errors resulting from both approximate methods. The results obtained showed that the uncertainty in the determination of average temperatures and C-2 column densities should not exceed +/- 3% and 10%, respectively. Contrary to these predictions, the experimental values of temperature and C-2 column density determined from the plasma radiation significantly exceeded those resulting from the absorption measurements. It is concluded that some non-equilibrium state is between the ground (a (3)Pi(u)) and excited (d (3)Pi(g)) electronic states in carbon arc plasma under consideration. The column density distributions of non-excited C-2 radical were transformed into radial concentration distributions, and then to carbon vapour pressures assuming thermodynamic equilibrium between carbon species. Both quantities rise with Fe content in the anode. Additionally, clear relationship between the carbon vapour pressure and the yield of carbon encapsulated iron nanoparticles synthesis was found.