Chemistry Reference
In-Depth Information
breakdown event in an ozonizer (i.e., at the moment when an ozonizer is just switched
on), since only at this moment there are no surface charges on the dielectrics. Also,
in some laboratory experiments, the BD was operated in a special single-pulse mode
when the periodical slow voltage pulses were applied to the discharge cell. In order to
get rid of a surface charge, a rest time interval of about 30 min between the successive
single pulses was provided [18]. Obviously, for an ordinary (continuous) operation
mode of a BD, the model NSC should be used. Therefore, it is very important
to distinguish between two different modes of a BD operation: single pulse and
continuous. To provide a correct theoretical description of the MDs for an ordinary
continuous mode, a model should be used, which takes into account the existence of
initially nonuniformly charged dielectric surfaces.
In order to propose a qualitative description of the influence of the MD spa-
tiotemporal structure on the kinetics of ozone synthesis, the authors [22] used the
following semiempirical method. From the arrays of electric field strength
E
(
x
,
t
)
and
relative electron density
n
e
(
n
ma
e
, the spatially resolved kinetics of the formation
of atomic oxygen and triplet nitrogen
N
2
by the reactions (3,4) in a nanosecond
time scale were calculated. The rate constants
k
3
(
x
,
t
)/
were taken from
[14]. Since the relative values of electron density were used, the results of these
kinetic calculations also contain an unknown constant. However, this constant does
not depend on the space-time coordinates. Therefore, thus calculated spatiotemporal
distributions of the relative concentrations of considered species may be expected
to represent the real kinetics within the MD channel. The latter results lead to the
following important conclusions concerning chemical activity of plasma within the
MD channel.
There are two distinct regions with essentially different plasma properties of
plasma within the MD channel. Electric field near the cathode is higher than near the
anode; electron density, on the contrary, is considerably lower. Furthermore, different
physical properties of these regions result in a noticeable difference in chemical
kinetics. For example, as it was demonstrated in [22], the properties of the plasma in
the region near the anode favor dissociation of molecular oxygen by direct electron
impact. In the case of the excitation of triplet nitrogen states, the contributions of
both regions to this process appear to be comparable.
E
/
n
)
and
k
4
(
E
/
n
)
8.1.1.4 Concluding Remarks
Various physical models (that were explicitly or implicitly used in a number of
computer simulations of coming into being, development, and decay of a separate
MD) are distinguished, actually, by the choice of initial and boundary conditions
for the coupled Poisson and continuity equations. Assuming essentially different
mechanisms of electrical breakdown, they determine different contributions of the
terms in (8.6), corresponding to the conversion degree for ozone formation, fraction of
electron losses, and maximal yield of atomic oxygen, respectively. All these models
fail to provide a reasonable interpretation for the detailed spatiotemporal structure of
a real MD in air [6]. Use of the briefly described improved physical model of the
MD development under the conditions of initially charged dielectric surface [16] in
a computer simulation may be expected to avoid discrepancies between theoretical
