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4.4.3 Ionic recombination
The neutralization of electrical carriers of opposite
sign in a gas is termed recombination. The positive carrier is a positive ion
while the negative carrier may be a negative ion or an electron depending on
the nature of the gas. This table deals only with ionic recombination, and only
with the situation in which the ions are randomly distributed throughout the
gas (see Loeb, 1955). Then dn+/dt =
dn−/dt = −αn−
.n+, where n+ and
n− are the ion densities and α is the recombination
coefficient. α
is commonly taken as a constant. Experimentally, however, measurements show
that α changes
with ion age, presumably due to a change in the nature of the ions owing to the
presence of small quantities of impurity. Further there is a small dependence
on ion density, α decreasing with
increasing ion density. α is dependent on
pressure and temperature and from 105 N m−2 to a
few hundred N m−2 is adequately described by the theory of
Thomson. For pressures above 106 N m−2 the Langevin
theory is employed. Three-body and mutual neutralization processes are by far
the most important for ionic recombination. For a general theoretical
discussion, see Loeb (1955).
All values given below are at 1.01 ×
105 N m−2 pressure unless otherwise stated.
|
Gas |
H2 |
N2 |
O2 |
Air |
Ar (impure) |
|
α/(10−12
m3 s−1) .
. . |
0.28 (22°C) |
1.06 (22°C) |
1.32 (2°C) |
1.65 (18°C) |
1.06 (22°C) |
| |
|
|
2.08 (25°C) |
|
|
| |
|
|
|
|
|
Dependence on ion density, pressure and
temperature
n = No. of ions/m3; p =
pressure; T = temperature
|
Air at 15°C .
. . . .
. . |
n/(1012
m−3) |
1.5 |
2.7 |
3.1 |
4.1 |
4.4 |
|
|
α/(10−12
m3 s−1) |
2.65 |
2.5 |
2.5 |
2.3 |
2.3 |
|
|
| |
|
p/103 N m−
2) . .
. . . |
5 |
10 |
20 |
30 |
40 |
50 |
60 |
70 |
80 |
90 |
100 |
110 |
|
Air 18°C α
/(10−12 m3 s−1) . |
0.12 |
0.21 |
0.37 |
0.55 |
0.7
|
0.88 |
1.05 |
1.22 |
1.4 |
1.55 |
1.72 |
|
|
O2 20°C
α
/(10−12 m3 s− 1) . |
|
0.7
|
1.3
|
1.6
|
1.75 |
1.88 |
1.96 |
2.01 |
2.05 |
2.08 |
2.08 |
2.08 |
| |
| |
|
p/(105 N m−2)
. . |
4.6 |
6.4 |
7.9 |
8.0 |
9.7 |
12.5 |
15.1 |
17.4 |
17.8 |
21.1 |
22.0 |
23.6 |
25.7 |
|
Air 18 °C
α
/(10−13 m3 s−1) |
10.58 |
8.44 |
|
6.98 |
5.94 |
4.80 |
4.07 |
|
3.44 |
2.90 |
|
2.59 |
2.37 |
|
CO2 18 °C
α
/(10−13 m3
s−1) |
5.76 |
|
3.58 |
|
|
2.24 |
|
1.56 |
|
|
1.24 |
|
|
|
|
| |
|
|
O2 T/K .
. . .
. |
200 |
250 |
300 |
350 |
400 |
|
|
α
/(10−12 m3 s−1)
. . |
3.7 |
2.7 |
1.9 |
1.5 |
1.2 |
|
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J.W. Leake
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