52 4 Gas-Phase Synthesis of Nanoparticles
4.4
Laser-Ablation Process
Instead of a tubular furnace or an electric arc, pulsed lasers are often applied as
the source of energy.
The “laser-ablation technique” has the advantage of allowing not only the application of metals, but also the use of oxides, even mixed targets are possible, for
the synthesis of nanoparticles. From the basic idea, this makes particle synthesis
extremely universal in application. Figure 4.11 displays a typical design of a production unit applying the laser-ablation process.
A laser-ablation system for powder production, generally, consists of the pulsed
high-power laser together with the optical focusing system and the feeding device
for the precursor. For the production of larger quantities, rotating targets and
automatic wire feeding systems were developed.
High-power laser pulses are focused onto the surface of the precursor target to
evaporate material. Depending on the thermal conductivity, at the focus point, the
target is heated to temperatures high enough to evaporate the material. Because
Box 4.7 Synthesis of Carbides and Nitrides
Because of their sensitivity against oxidation, it is more difficult to synthesize
nanoparticulate carbides or nitrides. A typical, often applied reaction for SiC
synthesis is
CH ClSi SiC CH HCl
3 3
4
(
)
⇒
+
+
2
.
(4.15)
Even though this reaction equation looks straightforward, this reaction is difficult to handle, as inappropriate selection of the reaction temperature leads to
the formation of elemental silicon particles and soot.
To synthesize nitrides, nitrogen is used as reaction and carrier gas. However,
chlorides are more stable than the nitrides, therefore, it is necessary to add
hydrogen to shift the equilibrium to the side of the formation of the nitride
and hydrochloric acid.
MeCl
N
H
MeN
HCl
2
2
n
m
m
n
n
+
+
⇒
+
2
2
.
(4.16)
Instead of hydrogen, one may also add ammonia to the carrier gas:
MeCl
NH
MeN
HCl
H
3
2
n
m
m
n
m n
n
+
⇒
+
+
−
2
3
.
(4.17)
Generally, the addition of ammonia instead of hydrogen reduces the reaction
temperature; however, there is the disadvantage of the formation of NH 4 Cl as
a byproduct.
4.4
Laser-Ablation Process
Instead of a tubular furnace or an electric arc, pulsed lasers are often applied as
the source of energy.
The “laser-ablation technique” has the advantage of allowing not only the application of metals, but also the use of oxides, even mixed targets are possible, for
the synthesis of nanoparticles. From the basic idea, this makes particle synthesis
extremely universal in application. Figure 4.11 displays a typical design of a production unit applying the laser-ablation process.
A laser-ablation system for powder production, generally, consists of the pulsed
high-power laser together with the optical focusing system and the feeding device
for the precursor. For the production of larger quantities, rotating targets and
automatic wire feeding systems were developed.
High-power laser pulses are focused onto the surface of the precursor target to
evaporate material. Depending on the thermal conductivity, at the focus point, the
target is heated to temperatures high enough to evaporate the material. Because
Box 4.7 Synthesis of Carbides and Nitrides
Because of their sensitivity against oxidation, it is more difficult to synthesize
nanoparticulate carbides or nitrides. A typical, often applied reaction for SiC
synthesis is
CH ClSi SiC CH HCl
3 3
4
(
)
⇒
+
+
2
.
(4.15)
Even though this reaction equation looks straightforward, this reaction is difficult to handle, as inappropriate selection of the reaction temperature leads to
the formation of elemental silicon particles and soot.
To synthesize nitrides, nitrogen is used as reaction and carrier gas. However,
chlorides are more stable than the nitrides, therefore, it is necessary to add
hydrogen to shift the equilibrium to the side of the formation of the nitride
and hydrochloric acid.
MeCl
N
H
MeN
HCl
2
2
n
m
m
n
n
+
+
⇒
+
2
2
.
(4.16)
Instead of hydrogen, one may also add ammonia to the carrier gas:
MeCl
NH
MeN
HCl
H
3
2
n
m
m
n
m n
n
+
⇒
+
+
−
2
3
.
(4.17)
Generally, the addition of ammonia instead of hydrogen reduces the reaction
temperature; however, there is the disadvantage of the formation of NH 4 Cl as
a byproduct.