the precursor with excess oxygen forms the secondary flame, while the particle size is
adjusted by diluting the precursor with an inert gas such as argon or nitrogen. Instead
of a gaseous or vaporized precursor, it is also possible to use liquid precursors, which
may be sprayed via a two-phase nozzle into the primary flame. It is also possible to use a
premixed gas for the primary flame or as the spraying gas in a two-phase nozzle [21,22].
In order to obtain well reproducible results for this synthesis process, a stable
behavior of the flame is absolutely necessary. The appearance of a flame with
different additions of oxygen is shown in Figure 4.31; here, methane (1.4 l min
À1 )
was used as the fuel and hexamethyl disiloxane (HMDSO; 2.9 l min
À1 ) as the
precursor to produce 17 g of silica per hour. With regard to oxygen additions,
two regimes are observed: (i) at low flow rates, the flame is unstable and fluctuating,
and (ii) with increasing oxygen additions, the flame is more stable and hotter. The
transition between these two ranges is clearly pronounced, especially in terms of the
particle size produced [23].
As mentioned above, the transition from a fluctuating to a stable flame is reflected
significantly in the particle size obtained by the process. The data in Figure 4.32
show clearly that there is a critical flow rate for oxygen (in this case 5.7 l min
À1 ),
although interestingly, the maximum particle size is observed at the transition. Over
the range characterized by a fluctuating flame, it is difficult to obtain either stable or
reproducible conditions and consequently such a range is avoided.
The oxygen content in the flame not only influences the average particle size but
also has a major influence on the morphology of the powder. There are two reasons
for this:
Under otherwise constant conditions, with increasing the addition of oxygen to
the flame, the temperature of the flame increases.
Figure 4.31 Influence of different oxygen
additions on the appearance of a silicaproducing flame. For the synthesis of
17 g silica h
À1
, 2.9 l min
À1 HMDSO was used as
precursor and 1.4 l min
À1 methane as fuel [23]
(Pratsinis, ETH Zürich, private
communication). (a) O 2 flow 2.5 l min
À1
; the
flame was fluctuating; (b) O 2 flow 8.5 l min
À1
;
the flame was stable; (c) O 2 flow 24 l min
À1
; the
flame was the hottest. (Reproduced with
permission by Elsevier).
4.6 Flame Aerosol Process j73
adjusted by diluting the precursor with an inert gas such as argon or nitrogen. Instead
of a gaseous or vaporized precursor, it is also possible to use liquid precursors, which
may be sprayed via a two-phase nozzle into the primary flame. It is also possible to use a
premixed gas for the primary flame or as the spraying gas in a two-phase nozzle [21,22].
In order to obtain well reproducible results for this synthesis process, a stable
behavior of the flame is absolutely necessary. The appearance of a flame with
different additions of oxygen is shown in Figure 4.31; here, methane (1.4 l min
À1 )
was used as the fuel and hexamethyl disiloxane (HMDSO; 2.9 l min
À1 ) as the
precursor to produce 17 g of silica per hour. With regard to oxygen additions,
two regimes are observed: (i) at low flow rates, the flame is unstable and fluctuating,
and (ii) with increasing oxygen additions, the flame is more stable and hotter. The
transition between these two ranges is clearly pronounced, especially in terms of the
particle size produced [23].
As mentioned above, the transition from a fluctuating to a stable flame is reflected
significantly in the particle size obtained by the process. The data in Figure 4.32
show clearly that there is a critical flow rate for oxygen (in this case 5.7 l min
À1 ),
although interestingly, the maximum particle size is observed at the transition. Over
the range characterized by a fluctuating flame, it is difficult to obtain either stable or
reproducible conditions and consequently such a range is avoided.
The oxygen content in the flame not only influences the average particle size but
also has a major influence on the morphology of the powder. There are two reasons
for this:
Under otherwise constant conditions, with increasing the addition of oxygen to
the flame, the temperature of the flame increases.
Figure 4.31 Influence of different oxygen
additions on the appearance of a silicaproducing flame. For the synthesis of
17 g silica h
À1
, 2.9 l min
À1 HMDSO was used as
precursor and 1.4 l min
À1 methane as fuel [23]
(Pratsinis, ETH Zürich, private
communication). (a) O 2 flow 2.5 l min
À1
; the
flame was fluctuating; (b) O 2 flow 8.5 l min
À1
;
the flame was stable; (c) O 2 flow 24 l min
À1
; the
flame was the hottest. (Reproduced with
permission by Elsevier).
4.6 Flame Aerosol Process j73