54 4 Gas-Phase Synthesis of Nanoparticles
As the energy source, often frequency-converted Nd-YAG or excimer lasers with
pulse durations in the nanosecond range are applied. The interaction of these
nanosecond pulses leads, especially at the surface of good thermal conductors,
prior to evaporation to the formation of a pool of melted material. In the case of
targets with complex composition, this may lead to a composition of the powder
that differs from that in the precursor. The application of a picosecond laser avoids
this problem. During such a short high-power laser pulse, the material evaporates
instantaneously; therefore, melting at the surface is impossible.
A general problem of the laser-ablation process is the high concentration of
evaporated material in the plume. Insufficiently fast expansion leads to the formation of agglomerates, a phenomenon that is often observed. These agglomerates are, in most cases, fractal- or web-like. The formation of agglomerates is
not intended. Therefore, a huge effort went into the direction of synthesizing
individualized particles using this process. A typical example of a successful
approach was published by Wang et al. [6]. As a precursor for iron oxide, γ-Fe 2 O 3 ,
a wire of pure iron was used. These authors used, atypically, a YAG laser with
a wavelength of 1064 nm with a pulse width of 0.3–20 ms. The particle-size distribution of this product, as depicted in Figure 4.13, was in the range from 5 to
55 nm, only a minor amount of material was found in the size range from 50
to 90 nm.
The particle-size distribution displayed in Figure 4.13 is a nonsymmetric
distribution function, which is typical of a random process of particle formation.
This figure demonstrates the inherent high potential of the laser-ablation
process into the direction of nonagglomerated materials. This is of some importance, as the laser-ablation process has very few special requirements for the
precursor.
Figure 4.12 Production of nanoparticles using the laser-ablation process. The figure shows
the influence of the gas pressure in the reaction chamber on the mean particle size of the
reaction product, Co 3 O 4 [5].
10
1
10
2
10
3
10
4
10
5
pressure [ Pa]
0
1
2
3
mean
particle
diameter
[nm]
As the energy source, often frequency-converted Nd-YAG or excimer lasers with
pulse durations in the nanosecond range are applied. The interaction of these
nanosecond pulses leads, especially at the surface of good thermal conductors,
prior to evaporation to the formation of a pool of melted material. In the case of
targets with complex composition, this may lead to a composition of the powder
that differs from that in the precursor. The application of a picosecond laser avoids
this problem. During such a short high-power laser pulse, the material evaporates
instantaneously; therefore, melting at the surface is impossible.
A general problem of the laser-ablation process is the high concentration of
evaporated material in the plume. Insufficiently fast expansion leads to the formation of agglomerates, a phenomenon that is often observed. These agglomerates are, in most cases, fractal- or web-like. The formation of agglomerates is
not intended. Therefore, a huge effort went into the direction of synthesizing
individualized particles using this process. A typical example of a successful
approach was published by Wang et al. [6]. As a precursor for iron oxide, γ-Fe 2 O 3 ,
a wire of pure iron was used. These authors used, atypically, a YAG laser with
a wavelength of 1064 nm with a pulse width of 0.3–20 ms. The particle-size distribution of this product, as depicted in Figure 4.13, was in the range from 5 to
55 nm, only a minor amount of material was found in the size range from 50
to 90 nm.
The particle-size distribution displayed in Figure 4.13 is a nonsymmetric
distribution function, which is typical of a random process of particle formation.
This figure demonstrates the inherent high potential of the laser-ablation
process into the direction of nonagglomerated materials. This is of some importance, as the laser-ablation process has very few special requirements for the
precursor.
Figure 4.12 Production of nanoparticles using the laser-ablation process. The figure shows
the influence of the gas pressure in the reaction chamber on the mean particle size of the
reaction product, Co 3 O 4 [5].
10
1
10
2
10
3
10
4
10
5
pressure [ Pa]
0
1
2
3
mean
particle
diameter
[nm]