Chemistry Reference
In-Depth Information
0.045
0.04
0.035
0.03
0.025
0.02
0.015
0.01
0.005
0
700
750
800
850
900
950
Temperature (K)
FIGURE 8.46
Chemical erosion yield as a function of temperature for incident H-flux of
10
22
m
−
2
s
−
1
.
of other hydrocarbons. At low temperature, the rate of thermal decomposition of
radicalic centers is small and the rather large cross section σ
H
provides effective
transfer of these centers back to
sp
3
; therefore one sees that at low temperatures
carbon is present mainly in
sp
3
hybridization state, whereas at higher temperatures,
the radical centers decompose readily and therefore
sp
2
carbon atoms dominate.
The hydrogen split-off reaction provides a pathway to destroy erosion relevant
groups. Only in an intermediate temperature range the balance between the supply
of
sp
x
centers through dehydrogenation, their thermal decomposition via hydro-
gen or CH
3
split-off, and hydrogenation to
sp
3
enables a noticeable chemical
erosion rate.
The particular field of research of plasma surface chemistry is definitively one
of the most complex problems to be understood for plasma applications. Up to
now only the beginning of an understanding exists and only for selected systems.
Further work is strongly needed to achieve a predictive quality of plasma chemistry
techniques.Thiswillrequireajointeffortofexperimentsandtheory,bringingtogether
different disciplines like surface science, quantum chemistry, solid state physics,
plasma physics, atomic and molecular physics, material science, and even, eventually,
biochemistry and biophysics (for medical applications of plasmas).
8.2.4.1.2 Plasma Polymerization
Plasma polymerization is the deposition of thin solid films (high-molecular-weight
molecules) on surfaces in more or less close contact with the plasma of electrical dis-
charges containing organic gases (low-molecular-weight molecules). The expression
polymerization describes the similar properties of the deposited films to classical
polymers, albeit structure and formation process being different from the known
polymers. The formation of solid depositions was observed in discharges in vapors
of hydrocarbons since the second half of the nineteenth century [301,302]. Plasma
