70 4 Gas-Phase Synthesis of Nanoparticles
electrical fields. Figure 4.30 displays two experimental assemblies to study these
possibilities.
As mentioned above, at high temperature, one expects thermal emission of
electrons from the particles. However, as these electrons only have thermal energy
in the range of less than 1 eV, therefore, they can also attach at the surface of
the particles. Hence, one may expect positively and negatively charged particles.
Figure 4.31 displays a proof of this assumption. The figure displays a flame
Figure 4.30 Experimental arrangement to
influence flame synthesis with an external
electrical field. The two possibilities depicted
are an electrical field in-between two plate
electrodes (a) and in-between two needleshaped electrodes (b). In the latter case,
electron emission from the needles is
possible.
(a)
(b)
Figure 4.31 Flame in-between plate
electrodes with an electrical field of 2 kV cm
–1
.
According to the different signs of the
electrical charges, the flame is split up by the
electric field. The synthesized particles,
titania, were, according to the sign of their
electrical charges, attracted by the electrode
plates and deposited [17]. (Reproduced by
permission from Elsevier.)