4.5 Plasma Processes 55
4.5
Plasma Processes
4.5.1
Microwave Plasma Processes
Gas-phase synthesis processes, if unbiased, favor the coagulation of the larger
particles (see Box 4.4). Chemical and physical vapor synthesis processes and, to
some extent, the laser-ablation process are purely random processes. These processes allow manipulation of particle size and size distribution by altering the
concentration of active species in the gas, the residence time in the reaction zone,
temperature, and by quenching the carrier gas after leaving the reaction zone.
Fundamentally different is the agglomeration behavior in the case of particles
carrying electrical charges of equal sign (see Box 4.5). In this case, the probability
collision of large particles is reduced. These conditions may be reached in the case
of plasma processes, especially the microwave plasma process. Therefore, the
probability for coagulation and agglomeration is significantly reduced, as the collision parameter decreases with increasing particle size. The present author developed a microwave plasma process operating at reduced gas pressure for synthesis
of nanoparticles exploiting the benefits of charged particles, allowing high production rates of unagglomerated particles and narrow particle-size distribution, features that are contradictions with respect to the classical processes of gas-phase
synthesis. In a plasma, one finds free electrons, ions, dissociated gas and precursor
molecules in addition to neutral gas species. Interaction with an electrical field is
possible only with charged species. The oscillating electrical field of the microwaves transfers energy primarily to the electrons, because the energy transfer to
Figure 4.13 γ-Fe 2 O 3 powder synthesized by the laser-ablation process. The particle diameter
distribution of the product is nonsymmetric, typical of synthesis methods based on purely
random processes [7].
0
20
40
60
80
100
particle diameter [nm]
0
0.05
0.1
0.15
0.2
0.25
class
frequency
4.5
Plasma Processes
4.5.1
Microwave Plasma Processes
Gas-phase synthesis processes, if unbiased, favor the coagulation of the larger
particles (see Box 4.4). Chemical and physical vapor synthesis processes and, to
some extent, the laser-ablation process are purely random processes. These processes allow manipulation of particle size and size distribution by altering the
concentration of active species in the gas, the residence time in the reaction zone,
temperature, and by quenching the carrier gas after leaving the reaction zone.
Fundamentally different is the agglomeration behavior in the case of particles
carrying electrical charges of equal sign (see Box 4.5). In this case, the probability
collision of large particles is reduced. These conditions may be reached in the case
of plasma processes, especially the microwave plasma process. Therefore, the
probability for coagulation and agglomeration is significantly reduced, as the collision parameter decreases with increasing particle size. The present author developed a microwave plasma process operating at reduced gas pressure for synthesis
of nanoparticles exploiting the benefits of charged particles, allowing high production rates of unagglomerated particles and narrow particle-size distribution, features that are contradictions with respect to the classical processes of gas-phase
synthesis. In a plasma, one finds free electrons, ions, dissociated gas and precursor
molecules in addition to neutral gas species. Interaction with an electrical field is
possible only with charged species. The oscillating electrical field of the microwaves transfers energy primarily to the electrons, because the energy transfer to
Figure 4.13 γ-Fe 2 O 3 powder synthesized by the laser-ablation process. The particle diameter
distribution of the product is nonsymmetric, typical of synthesis methods based on purely
random processes [7].
0
20
40
60
80
100
particle diameter [nm]
0
0.05
0.1
0.15
0.2
0.25
class
frequency