60 4 Gas-Phase Synthesis of Nanoparticles
Box 4.9 Temperature in a Microwave Plasma
The definition of a temperature in a plasma environment is not straightforward, as in terms of thermodynamics, a plasma is a nonequilibrium system.
This is caused by the fact that electrical energy is transferred only to charged
species and also for charged species the energy transfer depends on the mass
of the charged particles (Eq. 4.17). The largest amount of energy is deposited
at electrons, the species with the lowest mass, as the energy transfer is indirectly proportional to the mass of the charged particles. To the neutral particles
in the electrical field no energy will be transferred. As temperature and energy
are just connected just by a constant factor (u = kT), these two quantities are
to some extent equivalent. Therefore, the electrons carry the highest temperature and the neutral particles (e.g., carrier gas atoms) the lowest. Temperature
measurements makes sense only directly after the plasma zone, leading to an
averaged temperature. The temperature in an AC or DC plasma is found to be
in the range up to 10
4 K. In low-pressure microwave plasma processes the
temperature can be adjusted in the range from 400 to 1200 K.
Box 4.10 Plasma Reaction in the Presence of Water
In general, in a microwave plasma, chemical reactions are, more or less, the
same as in thermal systems; however, because of dissociation and ionization
of the reactive components at lower temperature. Thermal activation of the
reactions is not necessary. In selecting chemical reactions, it is necessary to
avoid side reactions, which may shift particle formation into an unintended
direction. This problem should be clarified with the following example in connection with the synthesis of oxides using chloride precursors: To reduce the
reaction temperature, it is of advantage to add water to the reaction gas:
MeCl
H O
O
MeO
HCl
2
2
x
y
x
y x
x
+
+
−
⇒
+
2
2
2
.
(4.21)
In a plasma environment, the water will dissociate:
H O H
OH
+
2
⇒
+
−
(
) .
(4.22)
In general, the chemical reactions are the same as in a conventional furnace,
just at lower temperature. The lower temperatures are possible, because the reactive species are ionized or dissociated in the plasma. Therefore, there is no activation energy needed to initiate the chemical reactions. However, one must be
careful that the reaction or the reaction products are not neutralizing the electric
charges of the particles.
Box 4.9 Temperature in a Microwave Plasma
The definition of a temperature in a plasma environment is not straightforward, as in terms of thermodynamics, a plasma is a nonequilibrium system.
This is caused by the fact that electrical energy is transferred only to charged
species and also for charged species the energy transfer depends on the mass
of the charged particles (Eq. 4.17). The largest amount of energy is deposited
at electrons, the species with the lowest mass, as the energy transfer is indirectly proportional to the mass of the charged particles. To the neutral particles
in the electrical field no energy will be transferred. As temperature and energy
are just connected just by a constant factor (u = kT), these two quantities are
to some extent equivalent. Therefore, the electrons carry the highest temperature and the neutral particles (e.g., carrier gas atoms) the lowest. Temperature
measurements makes sense only directly after the plasma zone, leading to an
averaged temperature. The temperature in an AC or DC plasma is found to be
in the range up to 10
4 K. In low-pressure microwave plasma processes the
temperature can be adjusted in the range from 400 to 1200 K.
Box 4.10 Plasma Reaction in the Presence of Water
In general, in a microwave plasma, chemical reactions are, more or less, the
same as in thermal systems; however, because of dissociation and ionization
of the reactive components at lower temperature. Thermal activation of the
reactions is not necessary. In selecting chemical reactions, it is necessary to
avoid side reactions, which may shift particle formation into an unintended
direction. This problem should be clarified with the following example in connection with the synthesis of oxides using chloride precursors: To reduce the
reaction temperature, it is of advantage to add water to the reaction gas:
MeCl
H O
O
MeO
HCl
2
2
x
y
x
y x
x
+
+
−
⇒
+
2
2
2
.
(4.21)
In a plasma environment, the water will dissociate:
H O H
OH
+
2
⇒
+
−
(
) .
(4.22)
In general, the chemical reactions are the same as in a conventional furnace,
just at lower temperature. The lower temperatures are possible, because the reactive species are ionized or dissociated in the plasma. Therefore, there is no activation energy needed to initiate the chemical reactions. However, one must be
careful that the reaction or the reaction products are not neutralizing the electric
charges of the particles.