The nucleation of flame carbon by ions and the effect of electric fields

Applied electric fields have been shown to affect all the three phases in the process of carbon formation in diffusion flames: nucleation, growth in the pyrolysis zone, and deposition. It is possible to resolve these effects by using different burner and electric-field configurations and it is necessary to do so before conclusions regarding nucleation can be drawn. With a tubular burner as one electrode and a steam-cooled plate above it as the other it is found that the particles are positively charged, i.e., deposition occurs mainly on whichever is the negative electrode; also that the form of aggregate greatly increases in volume but not in mass with the current density. Using a counterflow diffusion flame system with the burner mouths as electrodes, it is possible to draw ions of either sign from the flame through the pyrolysis zone. The extent of growth of particles as determined by their size and mass is controlled entirely by their period of residence in the pyrolysis zone and therefore by the field strength, since they acquire their charge very early in their history. The mechanism of charging is discussed. Regarding nucleation, the number rate of generation of particles depends on both the sign and magnitude of the ion flux through the pyrolysis zone. It is concluded that positive ions, either flame ions or Cs + ions produced by the thermal ionization of CsCl added to the flame, act as nuclei for the formation of carbon particles. Negative charges do not appear to act as nuclei but tend to reduce carbon formation by neutralizing positive-ion nuclei.