Chemistry Reference
In-Depth Information
pressure of oxygen. The origin of this model belongs to the field of the physics of
electrical breakdown in the long gaps (characteristic size in the 10
1
-10
2
cm range).
Under such conditions, any starting from the cathode avalanche may be expected
to reach a critical number of electrons in it necessary to cause a transition to the
streamer. For the ozonizers (gap width of a few millimeters), this mechanism has
never been observed experimentally and it seems to be hardly possible, excepting
probably only the case of the devices with pulse feeding voltage (rise time of about
1 kV/ns [18]).
8.1.1.2.2 Model with a Pulsed Electric Field
This model may be considered as a semiempirical one based on the results of the first
measurementsofthemicrodischargecurrentpulse[19].Inordertosimulatetheexper-
imentally observed current pulse within the frame of a simplified zero-dimensional
(homogeneous) kinetics, the authors [11,19] used a corresponding pulse variation of
the reduced field (Figure 8.2). Despite complying with the current measurements and
using an extremely detailed kinetic scheme, the model failed to provide a reasonable
estimate for the ozone yield [5].
8.1.1.2.3 Accumulation of Positive Space Charge Followed
by the Cathode-Directed Streamer
This model has been widely used in a number of computer simulations for MD
development since 1986 [12]. The first experimental evidence for the existence of
a cathode-directed streamer in a BD was obtained by means of the technique of
j
(A/cm
2
)
E
/
n
(Td)
300
600
250
500
Current
density
200
400
150
300
100
200
50
100
0
0
0
2
4
6
8
10
12
14
16
18
Time (ns)
FIGURE 8.2
Schematic illustration for the model of the
pulsed electric field
used in the
computer simulation of the MD in oxygen. Assumed variation of the reduced field (solid line),
and calculated current density (dashed line, right
y
-axis), is fitted to comply with the results of
experimental measurements of the MD current.
