chemical physics of discharges - Argonne National Laboratory

214
states of molecular oqgen are also present.
It is instructive to compare these results with measurements of the activation
energy of graphite with atomic oxygen removed from the luminous discharge. In severearly
studies a vdue of zero was reported (2) (16). Hennig (8) observed no dependence
on temperature at a pressure of 1.0 torr and a value of 7 Kcal/mol at 10 torr. This
difference was attributed to ozone formation. In a recent detailed study of the
atomic oxygen-graphite reaction, Marsh et al. (U) reported an activation energy of
10 Kcal/mol between U and 2OO0C, which approached zero at 350°C. 'Although the rate
of oxidation in these studies was considerably lower than that obtained in the plasm,
their values are consistent with the belief that atomic oqygen is a major reactant
within the discharge.
Of a variety of specimens, only graphite exhibited an abrupt change in activation
energy near 3Oo0C. For example, the apparent activation energy in the decomposition
of sucrose is approximately 4 Xcal/mol over the entire measurement range. The independence
of oxidation rate for graphite at high temperatures is ascribed to the formation
of surface oxides. Numerous studies of graphite combustion have shown that such surface
compounds control rate at elevated temperatures in excess oxygen. As anticipated,
increasing radiofreqiiency power to the discharge does not measurablv increase the rate
of graphite oxidation at high sample temperature.
The exhaust gas of the ovgen-graphite reaction contains both carbon monoxide a d
carbon dioxide, with the latter predominating. Tn the lumi~ous discharge region these
gases react with graphite. The results of measuremepts in which carbon dioxide was
passed through the radiofrequency field and then exposed to graphite rods activated
are shown in figure 3. Between 150 and 4OOOC the Arhennius equation fits well, yielding
an apparent activation energy of 3.2 Kcal,/mol. Below 120°C less than one mg per
hour of carbon was removed. This small weight loss is attributed to ion and electron
bombardment.
W& Conditions: In a number of experiments, increasing power beyond an optimum
value led to the anomolous result of reducing ashing rate. Earlier it was noted that
placing a dry ice-acetone trap in the exhaust stream extended the length of the luminous
discharge (5). To study this effect, wall temperatures were varied while the
carbon rod was maintained at 200°C. The results of a series of measurements are shown
in figure 4.
Assuming that the decrease in oxidation rate at increased wall tempera-
ture ip due to recombination of active species at the wall, the overall activation
energy at low temperatures for this process is approximately 2 Kcal/ml.
The major sources leading to loss of active oxygen species are recombination of
atomic oxygen and ions at the'walls of the vessel. The recombination of discharged
oxygen on glass surfaces has been investigated. Linnett and Marsden (10) reported
that the recombination coefficient of atomic oxygen on clean borosilicate glass
(Pyrex) was independent of temperature. However, positive values were observed on
contaminated surfaces. In a later series of papers Greaves and Linnett (7) studied
a number of oxide surfaces, in all cases the recombination coefficient was temperature
'
dependent. For silica apparent activation energies were 1 to 13 Kcal/mol over the
temperature range 15 to 3OOOC. The exceptional nature of Pyrex was noted.
' The second major source of loss of active species results from electron-ion
recombination at the chamber walls. The electric field restricts the drift of ions
to the chamber walls. Rate of-recombination is controlled by ambipolar diffusion,
but is limited by the presence of negatively charged oqgen ions in the discharge (15).
Although diffusion is not dependent on wall temperature, a distortion of the
symmetric electrical field will enhance wall recombination, producing an inarease in
This effect is noted in t&le 1.
The presence of a 6-cm long metallic ring on the exterior wall of the
reduced oddation rate by more than ten per cent. Grounding the ring in comn with
. wall temperature and a reduction of oxidation rate.
<
/
I
1
I