Chemistry Reference
In-Depth Information
and chemistry. It is hard to carry out any experiment with a single MD, since high
temporal resolution (in a sub-nanosecond range) and spatial resolution (down to
10
−
1
-10
−
2
mm) is required. Moreover, in the case of a commonly used parallel-plane
electrode arrangement, it is usually impossible to predict the location of an MD
emergence. Thus, it is no wonder that a great deal of effort has been devoted to
computer simulation of MD development [9-15].
As regards experimental findings, the following important milestones should be
mentioned:
1972—Identification of separate MDs [7]
1980—Streak-photography of single MD [17,18]
1983—Accurate measurement of MD current pulses [19]
1995—Determination of the spectrally resolved spatiotemporal distribu-
tions of MD luminosity by means of cross-correlation spectroscopy (CCS)
[20,21]
It should be mentioned that the authors [20] also used their experimental data for
a qualitative characterization of the spatiotemporal structure of electric field within an
MD channel. A remarkable progress in experimental investigation of MD evolution
was achieved during the last few years [21-23]. The authors [22] reported their results
of an accomplished procedure of a BD plasma diagnostics by means of the spatially
resolved CCS, including the quantitative estimation of the electric field, relative
electron density, and ozone yield within an MD channel. These measurements and
calculations were performed for a BD with the symmetrical electrode arrangement,
discharge gap width of 1.2 mm, in air at atmospheric pressure, that is, under typical
conditions of ozone generation technologies. Experimental data [20-23] and their
sophisticated kinetic analysis provide, essentially, a deeper understanding of the MD
mechanism as well as a detailed quantitative description of the chemical activity of
plasma within the MD channel.
8.1.1.1 Chemistry of Ozone Formation in Gas Discharges
In order to understand in what way the physics of electrical breakdown determines
the chemical activity of a BD plasma, it seems reasonable to begin with the analysis
of the chemical mechanism of the formation of ozone, usually the dominant product
of plasma synthesis. This mechanism was established relatively long ago, and at
present the kinetic schemes for ozone generation in oxygen and air may be regarded
as sufficiently reliable ones [4,5]. Generally, under the typical conditions for an
ozonizer, there are only two dominant reaction channels as follows:
M
k
1
+
O
2
+
→
O
3
+
M
(
M
=
O
2
,N
2
)
,
(8.1)
Ozone formation: O
k
2
→
Ozone decomposition: O
+
O
3
O
2
+
O
2
.
(8.2)
Therefore, atomic oxygen can be regarded as a sole precursor of ozone, and the
conversion degree for the gross reaction O
+
O
2
→
O
3
is determined by the ratio of
the rate constants
k
1
/
k
2
as well as by the background (initial) concentration of O
3
.
