extremely high temperature of the arc may lead to high evaporation rates. Moreover,
a high concentration of the evaporated precursor in the carrier gas usually results in
large particles. In order to overcome these problems an extended knowledge of
industrial processing is required, although the next two steps – addition of the
reactive gas and quenching – are of similar dif ficulty.
The industrial product of Fe 2 O 3 (see Figure 4.13) typically shows a relatively broad
particle size distribution, characteristic of purely random processes. Furthermore, it
is interesting to see “twins ” in the electron micrograph, characterized by changes in
contrast (in this case, black lines) in many of these particles.
Although until now metal rods or powders have been used as precursors, this
approach is not in all cases either economic or ef ficient. In fact, it is often more
appropriate to use chemical compounds with a relatively high vapor pressure as the
precursor. This variant of the synthesis process, which is referred to as “chemical
vapor synthesis, ” utilizes a tubular furnace with temperatures up to 1500 K as a
source of heat. A carrier gas – in most cases argon or nitrogen – transports the
evaporated precursor through the heated reaction zone. As a precursor, chlorides,
carbonyls, or metal organic compounds are most often used – the ultimate
compound selection depending on properties, availability, and price. It must not
be forgotten that almost all precursors lead to typical reaction products in the off-gas
and may also leave some traces behind that might be dissolved in the matrix of the
particles or adsorbed at the particle’ s surface. This may cause severe disturbance; for
example, chlorine reacts readily with organic materials applied to functionalize the
surface of the particles. In order to obtain the metal oxide MeO y , a typical reaction
might be:
MeCl x þ
y
2
O 2 ) MeO y þ
x
2
Cl 2
ð4:12Þ
Figure 4.13 Fe 2 O 3 powder produced by the
physical vapor synthesis process. The broad
particle size distribution (here 5–50 nm) is
characteristic of this type of product.
(Reproduced with permission by Nanophase
Inc., Romeoville, IL, USA; www.nanophase.
com). The dark lines seen in larger particles
indicate twin boundaries.
58j 4 Gas-Phase Synthesis of Nanoparticles
a high concentration of the evaporated precursor in the carrier gas usually results in
large particles. In order to overcome these problems an extended knowledge of
industrial processing is required, although the next two steps – addition of the
reactive gas and quenching – are of similar dif ficulty.
The industrial product of Fe 2 O 3 (see Figure 4.13) typically shows a relatively broad
particle size distribution, characteristic of purely random processes. Furthermore, it
is interesting to see “twins ” in the electron micrograph, characterized by changes in
contrast (in this case, black lines) in many of these particles.
Although until now metal rods or powders have been used as precursors, this
approach is not in all cases either economic or ef ficient. In fact, it is often more
appropriate to use chemical compounds with a relatively high vapor pressure as the
precursor. This variant of the synthesis process, which is referred to as “chemical
vapor synthesis, ” utilizes a tubular furnace with temperatures up to 1500 K as a
source of heat. A carrier gas – in most cases argon or nitrogen – transports the
evaporated precursor through the heated reaction zone. As a precursor, chlorides,
carbonyls, or metal organic compounds are most often used – the ultimate
compound selection depending on properties, availability, and price. It must not
be forgotten that almost all precursors lead to typical reaction products in the off-gas
and may also leave some traces behind that might be dissolved in the matrix of the
particles or adsorbed at the particle’ s surface. This may cause severe disturbance; for
example, chlorine reacts readily with organic materials applied to functionalize the
surface of the particles. In order to obtain the metal oxide MeO y , a typical reaction
might be:
MeCl x þ
y
2
O 2 ) MeO y þ
x
2
Cl 2
ð4:12Þ
Figure 4.13 Fe 2 O 3 powder produced by the
physical vapor synthesis process. The broad
particle size distribution (here 5–50 nm) is
characteristic of this type of product.
(Reproduced with permission by Nanophase
Inc., Romeoville, IL, USA; www.nanophase.
com). The dark lines seen in larger particles
indicate twin boundaries.
58j 4 Gas-Phase Synthesis of Nanoparticles