Chemistry Reference
In-Depth Information
•
For
P
0 the important collisions of neutral particles govern the afterglow,
which transfer the active plasma from complete decomposition to the final
state of CECD. That means for both CEEC and CECD a very “similar chem-
ical climate” exists. This includes: constant gas temperature and constant
wall conditions (in the case of heterogeneous reactions).
→
4.3.3 E
XPERIMENTAL
V
ERIFICATION AND
I
MPORTANCE
In the following we present selected experimental results on the occurrence of chemi-
cal quasi-equilibria in nonthermal plasma reactors. The experiments were performed
in a DC positive column, in an RF discharge, and a so-called spark discharge,
respectively.
Figure 4.5 shows the ozone synthesis in the positive column of a DC O
2
glow
discharge in a closed cylindrical glass tube at two different wall temperatures. The
ozoneconcentrationwasdetectedusingUVlightabsorptionat253.9 nm.Ineachcase,
the lower curves belong to the active phase AP (discharge operation). After discharge
interruption (passive phase PP) the O
3
concentration reaches a value which is nearly
independent of the ionization degree
x
e
, calculated from the current balance in the
positive column. The development of a finite ozone concentration is clearly shown
for
x
e
→
0, denoting the CEEC. This value is independent of the special type of the
discharge. Nearly the same value is reached using a weak spark discharge, produced
by a Tesla transformer (here named Tesla spark discharge). The almost horizontal O
3
curves manifest the typical case of identical quasi-equilibria CEEC
CECD for the
ozone synthesis as discussed in the last section. The strong dependencies of the level
of the quasi-equilibria on the wall temperature correspond to the sensitive temperature
dependence of the rate constants γ
2
and
k
5
for the synthesis and decomposition of O
3
(compare Table 4.1).
=
4
10
50
100
mA
0.05
i
0.001
T
= 195 K
x
O
3
300 K
0.001
x
e
10
-9
10
-8
10
-7
10
-6
FIGURE 4.5
Relative ozone concentration
x
O
3
in dependence on the ionization degree in a
closed reactor. Discharge: Positive column of a DC glow discharge. Tube:
L
=
47 cm,
r
=
1cm,
Al electrodes, input number density
n
0
=
5
·
10
17
cm
−
3
pure O
2
. Lower curves in each case:
Active phase AP
=
discharge operation. Upper curves in each case: Passive phase PP after
relaxation. Full symbols on left: Excitation for long time by a weak spark discharge.
