Environmental Engineering Reference
In-Depth Information
and the possibility for the compound
MX
k
to decay into metal clusters and halogen
molecules is determined roughly by the criterion [41]
ε
<
ε
M
,
X
where
ε
X
is the binding energy per halogen atom in molecule
MX
k
and
ε
M
is the
binding energy per atom for bulk metal.
From the chemical equilibrium for molecules
MX
k
it follows that this com-
pound decomposes into atoms at temperatures of the order
ε
X
ln(
N
0
/[
X
])
T
1
D
,
(6.88)
where [
X
] is the total number density of free and bound halogen atoms, and
N
0
is
of the order of a typical atomic value. In turn, metal clusters are transformed into
atomic vapor at temperatures of the order
ε
M
ln(
N
0
/[
M
])
T
2
D
,
(6.89)
where [
M
] is the total number density of free and bound metal atoms. Evidently,
clusters remain stable in the temperature range [132, 133]
T
1
<
T
<
T
2
.
(6.90)
Because of the halogen excess, [
X
]
[
M
], the possibility of the existence of clus-
ters and gas molecules in the system under consideration corresponds to the crite-
rion
ε
<
ε
<
k
ε
X
.
(6.91)
X
M
Onecanseethatthesecriteriaarecompatibleifthegaseouscompoundofaheat-
resistant metal contains several halogen atoms. The data related to this analysis are
given in Table 6.3. This table gives the values of
M
that are obtained on the
basis of the Gibbs thermodynamic potential for the compounds under considera-
tion, and the values that follow from the enthalpy values for these compounds are
given in parentheses. The temperatures
T
1
and
T
2
follow from (6.88) and (6.89),
and the temperature
T
3
coincides with
T
2
and results from the saturated vapor
pressure of the metal as a function of temperature. The data in Table 6.3 relate
to a total number density of halogen atoms of 1
ε
X
and
ε
10
16
cm
3
and a total number
density of metal atoms that is four or six times less depending on the type of metal-
containing molecules. If criteria (6.90) are fulfilled, clusters of this metal exist in
the temperature range
T
1
<
T
<
T
2
, and such cases are marked in the last column
in Table 6.3 by a plus sign.
We now analyze the character of evolution of a plasma flow with inserted metal-
containing molecules in the center of the flow. Then we take argon as a buffer
gas at a pressure of 1 atm, and metal-containing molecules
MX
k
are injected into
the central part of the flow of radius 1mm such that the concentration of metal
