be fitted with a log-normal distribution function. Taken together, Figures 4.18 and
4.19 demonstrate the potential of the laser ablation process for nonagglomerated
materials, notably because it has no special precursor requirements.
4.5
Radio- and Microwave Plasma Processes
The processes of chemical and physical vapor synthesis, as well as laser ablation, are
nonbiased random processes; hence, the only means by which particle size and size
distribution can be influenced are the concentrations of active species in the gas, the
temperature, and, most importantly, the rapid cooling (quenching) of the gas after
leaving the reaction zone. This situation is entirely different from that of the
microwave plasma process, where the particles originating in the plasma zone
carry electric charges. As a consequence, even when the process itself remains
random, the probability for coagulation and agglomeration is significantly altered, as
the collision parameter decreases with increasing particle size. These effects are
based on considerations leading to Eqs. (4.6)–(4.9) and Figure 4.4a and b. Vollath
et al. [3,13,14] developed a microwave plasma process for the synthesis of nanoparticles by exploiting the benefits of charged particles, as high production rates of
unagglomerated particles and narrow particle size distribution are in contradiction
to the classical processes of gas-phase synthesis.
In order to understand the special properties of the microwave plasma process, it
is first necessary to analyze the energy transfer in a microwave plasma. The energy U
transferred to a particle with the electric charge Q in an oscillating electrical field is
inversely proportional to the mass of the particle m and the squared frequency f of
the electrical field:
U /
Q
mf
2
ð4:18Þ
As the mass of the electrons is a few thousand times smaller than that of the ions,
a substantially larger amount of energy is transferred to the electrons, as compared
to the energy transferred to the ions. While Eq. (4.18) is valid for one charged particle
in an oscillating electrical field, in a plasma environment one finds free electrons,
ions, dissociated gas, and precursor molecules in addition to neutral gas species.
Therefore, collisions between charged and uncharged particles limit the mean free
path of the charged particles accelerated in the electric field, ruling the energy
transfer to the particles. Consequently, the collision frequency z of the gas species
must be considered [15]:
U /
Q
m
z
f
2 þ z 2
ð4:19Þ
Equation (4.19) does not alter the mass relationship of the energy transfer, but it
does show the reduction in energy transfer to the charged particles by collision with
other neutral species. Equation (4.19) introduces a dependency of the collision
64j 4 Gas-Phase Synthesis of Nanoparticles
4.19 demonstrate the potential of the laser ablation process for nonagglomerated
materials, notably because it has no special precursor requirements.
4.5
Radio- and Microwave Plasma Processes
The processes of chemical and physical vapor synthesis, as well as laser ablation, are
nonbiased random processes; hence, the only means by which particle size and size
distribution can be influenced are the concentrations of active species in the gas, the
temperature, and, most importantly, the rapid cooling (quenching) of the gas after
leaving the reaction zone. This situation is entirely different from that of the
microwave plasma process, where the particles originating in the plasma zone
carry electric charges. As a consequence, even when the process itself remains
random, the probability for coagulation and agglomeration is significantly altered, as
the collision parameter decreases with increasing particle size. These effects are
based on considerations leading to Eqs. (4.6)–(4.9) and Figure 4.4a and b. Vollath
et al. [3,13,14] developed a microwave plasma process for the synthesis of nanoparticles by exploiting the benefits of charged particles, as high production rates of
unagglomerated particles and narrow particle size distribution are in contradiction
to the classical processes of gas-phase synthesis.
In order to understand the special properties of the microwave plasma process, it
is first necessary to analyze the energy transfer in a microwave plasma. The energy U
transferred to a particle with the electric charge Q in an oscillating electrical field is
inversely proportional to the mass of the particle m and the squared frequency f of
the electrical field:
U /
Q
mf
2
ð4:18Þ
As the mass of the electrons is a few thousand times smaller than that of the ions,
a substantially larger amount of energy is transferred to the electrons, as compared
to the energy transferred to the ions. While Eq. (4.18) is valid for one charged particle
in an oscillating electrical field, in a plasma environment one finds free electrons,
ions, dissociated gas, and precursor molecules in addition to neutral gas species.
Therefore, collisions between charged and uncharged particles limit the mean free
path of the charged particles accelerated in the electric field, ruling the energy
transfer to the particles. Consequently, the collision frequency z of the gas species
must be considered [15]:
U /
Q
m
z
f
2 þ z 2
ð4:19Þ
Equation (4.19) does not alter the mass relationship of the energy transfer, but it
does show the reduction in energy transfer to the charged particles by collision with
other neutral species. Equation (4.19) introduces a dependency of the collision
64j 4 Gas-Phase Synthesis of Nanoparticles