4.6 Flame Processes 67
quenching influences the mean particle size and the width of the size distribution.
In this example, as a measure for the particle size, the specific surface area in
square meters per gram is given. Highly optimized industrial systems produce
nanopowders of good quality with particle sizes of less than 100 nm. Possible
products are metals, oxides, carbides, and nitrides. Except for oxides and precious
metals, powders of these products may be highly pyrophoric; therefore, it is
strongly recommended to handle these products in an inert gas atmosphere, if
possible within glove boxes.
4.6
Flame Processes
Since prehistoric times, in China, carbon black has been produced by flame processes, as pigments for inks. The flame aerosol process is the oldest of all processes
to produce nanoparticulate powders in our time. This technology, or one of its
many variants, is applied to produce thousands of metric tons of carbon black,
fumed silica, and titania, TiO 2 pigments. Lastly, it is the only process applicable
for mass production in the kiloton range. Even though this process is well established and widely applied in industry, the basis principles are not completely
understood. This is not least because these processes have been working for
decades; therefore, for a long time, there was no need for basic studies, which are
difficult and expensive, because particle formation takes place at extremely high
temperatures in very short times. With the trend to apply this process also for high
value added products, the situation has changed significantly.
The aerosol flame process has a long history and broad application; therefore, with respect to new materials, many highly specific variants, leading to
particles with different morphology, size, and crystallinity were developed. Important reviews on flame aerosol processes were published by Pratsinis [14] and
Wooldrige [15].
In the simplest case, a flame reactor is set up as shown in Figures 4.27a,b. Basically, a flame reactor consists of a primary flame that is fueled with hydrogen,
methane or another hydrocarbon fuel. As far as possible, the selection of a gaseous
precursor is advised. Such a precursor may be, for example, to synthesize silica,
SiO 2 , silane, SiH 4 or silicon tetrachloride, SiCl 4 . Generally, the gaseous fuel comes
premixed with oxygen or air in the burner. In general, the design of the burner
follows one of the two principles depicted in Figure 4.27: Figure 4.27a displays the
most conventional system, where the precursor, if necessary diluted with a carrier
gas, is blown from the side into the flame. The dilution of the precursor with a
carrier gas is also applied to adjust the particle size of the product.
The design displayed in Figure 4.27b is more advanced. Characteristic of this
design are the many small primary flames surrounding the secondary flame,
where the precursor reacts with the excess oxygen forming a secondary flame,
where the reaction for particle formation occurs.
quenching influences the mean particle size and the width of the size distribution.
In this example, as a measure for the particle size, the specific surface area in
square meters per gram is given. Highly optimized industrial systems produce
nanopowders of good quality with particle sizes of less than 100 nm. Possible
products are metals, oxides, carbides, and nitrides. Except for oxides and precious
metals, powders of these products may be highly pyrophoric; therefore, it is
strongly recommended to handle these products in an inert gas atmosphere, if
possible within glove boxes.
4.6
Flame Processes
Since prehistoric times, in China, carbon black has been produced by flame processes, as pigments for inks. The flame aerosol process is the oldest of all processes
to produce nanoparticulate powders in our time. This technology, or one of its
many variants, is applied to produce thousands of metric tons of carbon black,
fumed silica, and titania, TiO 2 pigments. Lastly, it is the only process applicable
for mass production in the kiloton range. Even though this process is well established and widely applied in industry, the basis principles are not completely
understood. This is not least because these processes have been working for
decades; therefore, for a long time, there was no need for basic studies, which are
difficult and expensive, because particle formation takes place at extremely high
temperatures in very short times. With the trend to apply this process also for high
value added products, the situation has changed significantly.
The aerosol flame process has a long history and broad application; therefore, with respect to new materials, many highly specific variants, leading to
particles with different morphology, size, and crystallinity were developed. Important reviews on flame aerosol processes were published by Pratsinis [14] and
Wooldrige [15].
In the simplest case, a flame reactor is set up as shown in Figures 4.27a,b. Basically, a flame reactor consists of a primary flame that is fueled with hydrogen,
methane or another hydrocarbon fuel. As far as possible, the selection of a gaseous
precursor is advised. Such a precursor may be, for example, to synthesize silica,
SiO 2 , silane, SiH 4 or silicon tetrachloride, SiCl 4 . Generally, the gaseous fuel comes
premixed with oxygen or air in the burner. In general, the design of the burner
follows one of the two principles depicted in Figure 4.27: Figure 4.27a displays the
most conventional system, where the precursor, if necessary diluted with a carrier
gas, is blown from the side into the flame. The dilution of the precursor with a
carrier gas is also applied to adjust the particle size of the product.
The design displayed in Figure 4.27b is more advanced. Characteristic of this
design are the many small primary flames surrounding the secondary flame,
where the precursor reacts with the excess oxygen forming a secondary flame,
where the reaction for particle formation occurs.