Environmental Engineering Reference
In-Depth Information
Ta b l e 1 . 5
Parameters in (1.70) for electron thermoemission from metals [59, 60],
A
R
is ex-
pressed in square meters per square centimeter per square Kelvin.
Material
A
R
W
,eV Material
A
R
W
,eV Material
A
R
W
,eV
Ba
60
2.11
Fe
α
26
4.5
Pt
33
5.32
Be
300
3.75
Fe
γ
1.5
4.21
Si
8
3.6
Cs
160
1.81
Mo
55
4.15
Ta
120
4.25
C
15
4.5
Ni
30
4.61
Th
70
3.38
Cr
120
3.90
Nb
120
4.19
Ti
60
3.86
Co
41
4.41
Os
1100
5.93
U
60
3.27
Cu
120
4.57
Pa
60
4.9
W
60
4.54
Hf
22
3.60
Pd
60
4.99
Y
100
3.27
Ir
120
5.27
Re
720
4.7
Zr
330
4.12
1.2.10
The Treanor Effect
A weakly ionized gas can be regarded as a system of weakly interacting atomic par-
ticles. This system can be divided into subsystems, and in the first approximation
each subsystem can be considered as an independent closed system. The next ap-
proximation, taking into account a weak interaction between subsystems, makes it
possible to establish connections among subsystem parameters. There are a variety
of ways in which this decomposition can be done, with the selection depending on
thenatureoftheproblem.Oftenitisconvenienttodivideanionizedgasintoatom-
ic and electronic subsystems. Energy exchange in electron-atom collisions is slight
owing to the large difference in their masses, so equilibrium within the atomic
and electronic subsystems is established separately. If a weakly ionized gas is locat-
ed in an external electric or electromagnetic field, which acts on electrons mostly,
the electron and atom temperatures may be different. This means that both atoms
and electrons can be characterized by Maxwell distributions for their translational
energies, but with different mean energies.
Another example of weakly interacting subsystems that we shall consider below
relates to a molecular gas in which exchanges of vibrational energy between col-
liding molecules have a resonant character. This is a more effective process than
collisions of molecules with transitions of the vibrational energy to excitations of
rotational and translational degrees of freedom. If vibrational and translational de-
grees of freedom are excited or are cooled in a different way, then different vibra-
tional and translational temperatures will exist in such a molecular gas. This situ-
ation occurs in gas discharge molecular lasers, where vibrational degrees of free-
dom are excited selectively, and also in gas dynamical lasers, where a rapid cooling
of translational degrees of freedom occurs as a result of gas expansion. The same
effect occurs in shock waves and as a result of gas expansion after a nozzle. There
is thus a wide variety of situations where a molecular gas is characterized by dif-
