form extended agglomerates (see Figure 4.14), whereas the primary particle size is
quite uniform, in the range of approximately 3 nm.
The other, most important groups of precursor compounds include metal alkoxides and acetyl acetonates. In particular, for the metals of groups II and IIa of the
periodic table, and for the rare earth elements, these are the most important
precursors.
In addition to oxides, chemical vapor synthesis also allows the synthesis of
carbides and nitrides, although in this case great care must be taken when selecting
the precursor as the free enthalpy of formation of carbides and nitrides is significantly smaller than for oxides. A typical example is the synthesis of silicon carbide
(SiC) by Erhart and Albe [7] via the pyrolysis of tetramethyl silane as precursor:
SiðCH 3 Þ 4 ) SiC þ 3CH 4
ð4:15Þ
As the free enthalpy of formation is quite low, many side-reactions, such as the
formation of Si 2 C and elemental silicon and carbon, are possible. An additional,
often-applied reaction for the synthesis of silicon carbide is:
ðCH 3 Þ 3 ClSi ) SiC þ 2CH 4 þ HCl
ð4:16Þ
However, even when the chemical reaction equation seems straightforward, this
reaction may be quite problematic, as an inappropriate selection of the reaction
temperature may lead to the formation of elemental silicon particles and carbon (as
soot). In order to synthesize nitrides, nitrogen is used as the reaction and carrier gas.
However, as chlorides are more stable than nitrides it is necessary to add hydrogen to
the system in order to shift the equilibrium towards nitride and hydrochloric acid
(HCl) formation:
MeCl n þ
m
2
N 2 þ
n
2
H 2 ) MeN m þ nHCl
ð4:17Þ
Apart from the problem of ammonium chloride (NH 4 Cl) being formed as a
byproduct, in most cases it is beneficial to use ammonia as the hydrogen carrier.
4.4
Laser Ablation Process
The situation is even more complex when applying a pulsed laser beam as a source
of energy. However, this “laser ablation technique” has the advantage of allowing not
only the use of metals, but also oxides as precursors, which makes this process
extremely versatile in its application. The general design of a production unit
applying the laser ablation process is shown in Figure 4.15.
A system for powder production using the laser ablation process generally
consists of two essential elements: the pulsed high-power laser, and the optical
focusing system and feeding device for the precursor. In order to produce larger
quantities with this process, rotating targets and automatic wire feeding systems
have been developed.
60j 4 Gas-Phase Synthesis of Nanoparticles
quite uniform, in the range of approximately 3 nm.
The other, most important groups of precursor compounds include metal alkoxides and acetyl acetonates. In particular, for the metals of groups II and IIa of the
periodic table, and for the rare earth elements, these are the most important
precursors.
In addition to oxides, chemical vapor synthesis also allows the synthesis of
carbides and nitrides, although in this case great care must be taken when selecting
the precursor as the free enthalpy of formation of carbides and nitrides is significantly smaller than for oxides. A typical example is the synthesis of silicon carbide
(SiC) by Erhart and Albe [7] via the pyrolysis of tetramethyl silane as precursor:
SiðCH 3 Þ 4 ) SiC þ 3CH 4
ð4:15Þ
As the free enthalpy of formation is quite low, many side-reactions, such as the
formation of Si 2 C and elemental silicon and carbon, are possible. An additional,
often-applied reaction for the synthesis of silicon carbide is:
ðCH 3 Þ 3 ClSi ) SiC þ 2CH 4 þ HCl
ð4:16Þ
However, even when the chemical reaction equation seems straightforward, this
reaction may be quite problematic, as an inappropriate selection of the reaction
temperature may lead to the formation of elemental silicon particles and carbon (as
soot). In order to synthesize nitrides, nitrogen is used as the reaction and carrier gas.
However, as chlorides are more stable than nitrides it is necessary to add hydrogen to
the system in order to shift the equilibrium towards nitride and hydrochloric acid
(HCl) formation:
MeCl n þ
m
2
N 2 þ
n
2
H 2 ) MeN m þ nHCl
ð4:17Þ
Apart from the problem of ammonium chloride (NH 4 Cl) being formed as a
byproduct, in most cases it is beneficial to use ammonia as the hydrogen carrier.
4.4
Laser Ablation Process
The situation is even more complex when applying a pulsed laser beam as a source
of energy. However, this “laser ablation technique” has the advantage of allowing not
only the use of metals, but also oxides as precursors, which makes this process
extremely versatile in its application. The general design of a production unit
applying the laser ablation process is shown in Figure 4.15.
A system for powder production using the laser ablation process generally
consists of two essential elements: the pulsed high-power laser, and the optical
focusing system and feeding device for the precursor. In order to produce larger
quantities with this process, rotating targets and automatic wire feeding systems
have been developed.
60j 4 Gas-Phase Synthesis of Nanoparticles