4.5 Plasma Processes 59
To avoid parasitic absorption of the incoming microwaves the central reaction
tube is made of pure silica glass, which passes through a resonant microwave
cavity, connected via a waveguide to the microwave generator and the tuning
system [9]. Concentrically to the reaction tube, at each end of the cavity there is a
cut-off tube. This is a safety feature, as it attenuates microwaves and avoids leakage
into the environment. The plasma, the zone where the particles are synthesized,
burns at the intersection of the reaction tube and the microwave cavity. The evaporated precursor is introduced into the system via a stream consisting of carrier
and reaction gases. The gas flow is selected to keep the residence time of the
particles in the reaction as short as possible; in most cases, a residence time in
the range of less than 10 ms is selected. Depending on the intended product, the
temperature is selected in a range from 400 to 800 K. Such low reaction temperatures are possible, because in contrast to a chemical vapor synthesis in a tubular
furnace, the mechanism of the formation of the nanoparticles in the microwave
plasma is different, as the reactants are ionized and dissociated. Looking at the
price for the devices, in most cases it is advised to select the so-called “kitchen
frequency” of 2.45 GHz for the power supply. In special cases, primarily connected
to large-scale production, a frequency of 0.915 GHz is applied.
As proof that the above-discussed ideas can be realized in experimental reality,
Figure 4.17 displays the particle-size distribution of a ZrO 2 powder, synthesized
in a low-pressure microwave plasma system, with a mean particle size around
3 nm. This size distribution was determined by particle mass spectrometry.
Analyzing the particle-size distribution function depicted in Figure 4.17, one
realizes besides the extreme small mean particle size that the distribution is,
as expected, extremely narrow and asymmetric. Certainly, there are a few large
particles in the system, indicating that the particles grew by a process of random
collisions.
Figure 4.17 Particle-size distribution of
zirconia powder synthesized at low gas
pressure using the microwave plasma
process. This size distribution was measured
during synthesis, online, with a particle size
spectrometer. (Roth, P., University of
Duisburg, Germany) private communication.
0
1
2
3
4
5
6
particle size [nm]
0
0.1
0.2
0.3
class
frequency
To avoid parasitic absorption of the incoming microwaves the central reaction
tube is made of pure silica glass, which passes through a resonant microwave
cavity, connected via a waveguide to the microwave generator and the tuning
system [9]. Concentrically to the reaction tube, at each end of the cavity there is a
cut-off tube. This is a safety feature, as it attenuates microwaves and avoids leakage
into the environment. The plasma, the zone where the particles are synthesized,
burns at the intersection of the reaction tube and the microwave cavity. The evaporated precursor is introduced into the system via a stream consisting of carrier
and reaction gases. The gas flow is selected to keep the residence time of the
particles in the reaction as short as possible; in most cases, a residence time in
the range of less than 10 ms is selected. Depending on the intended product, the
temperature is selected in a range from 400 to 800 K. Such low reaction temperatures are possible, because in contrast to a chemical vapor synthesis in a tubular
furnace, the mechanism of the formation of the nanoparticles in the microwave
plasma is different, as the reactants are ionized and dissociated. Looking at the
price for the devices, in most cases it is advised to select the so-called “kitchen
frequency” of 2.45 GHz for the power supply. In special cases, primarily connected
to large-scale production, a frequency of 0.915 GHz is applied.
As proof that the above-discussed ideas can be realized in experimental reality,
Figure 4.17 displays the particle-size distribution of a ZrO 2 powder, synthesized
in a low-pressure microwave plasma system, with a mean particle size around
3 nm. This size distribution was determined by particle mass spectrometry.
Analyzing the particle-size distribution function depicted in Figure 4.17, one
realizes besides the extreme small mean particle size that the distribution is,
as expected, extremely narrow and asymmetric. Certainly, there are a few large
particles in the system, indicating that the particles grew by a process of random
collisions.
Figure 4.17 Particle-size distribution of
zirconia powder synthesized at low gas
pressure using the microwave plasma
process. This size distribution was measured
during synthesis, online, with a particle size
spectrometer. (Roth, P., University of
Duisburg, Germany) private communication.
0
1
2
3
4
5
6
particle size [nm]
0
0.1
0.2
0.3
class
frequency