chemical physics of discharges - Argonne National Laboratory

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the principal axis 43 cm from the gas inlet. A 0.6-cm tube, C, passed diametrically
through the cylinder and held the spechen on a central projection. Sample temperature
was regulated, in part, by circulating liquids between this tube and a constant
temperature bath. Radiofrequency excitation was supplied from a 250-watt, crystal
controlled 13.56-MHz generator, described previously (3). Except as otherwise noted,
an output of 115 watts was employed. Power was transferred to the gas by means of
impedance matching network terminating in a lo-turn coil of $-inch O.D. copper tubing
tapped 2 turns from ground.
The coil, B, which had an inside diameter of 4.5 cm and
was 12.5 cm long, was coaxial with the reaction tube and placed 10 hm from the gas
inlet.
After passage through a rotometer, molecular gas entered the reaction system
through a capillary orifice, A. Pressure was monitored by a McCloud gauge attached
to side ann E, located 50 cm beyond the gas inlet. Pressure was maintained at 1.2
torr; and flow rates at 150 cc per minute, S.T.P. U.S.P. grade oxygen and instrument
grade carbon dioxide were used.
The temperature of solid specimens in the plasma was determined by means of an
infrared radiation thermometer (Infrascope Model 3-lC00, Huggins Laboratories Inc.,
Sunnyvale, Calif.) attached to a chart recorder. This device employs a lead sulfide
detector and suitable filters to permit remote measurement of 1.2 to 2.5~ radiation
emitted by the specimen. To compensate for variations in emissitivity and inhomogeneity
in the optical field, empirical calibration curves were constructed.
Themcouples were employed to determine cylinder-wall temperatures. To eliminate
interaction with the radiofrequency field, the transmitter was inactivated during
the latter measurements.
High purity graphite rods (National Carbon Co., Grade AGKSP) were employed as
standard specimens. These rods were 0.61 cm in diameter and had a cavity machined
into their base to affix them to the cooling tube. Pellets of sucrose and carbon
containing small quantities of cupric acetate were also epployed. The latter were
produced by heating and then pressing a slurry of the salt solution and m-mesh
graphite powder.
Results and Discussion
Sample Temerature: The temperature variation of oxidation rate of graphite
exposed to the oxygen plasma is shown in figure 2. Over tpe range 120 to 300OC, the
Arrhenius equation, X = Ce'Ea/RT, fits the data well and yields a value of 6.5
Kcal/mol for the apparent activation energy, Ea. Between 300 and 450°C, the highest
temerature studied, oxidation rate is not dependent on temperature. The oxidation
of graphite by molecular oxygen at high tempgrature is strongly dependent on the purity
of the graphite. To determine if a similar effecb occurs in plasma oddation, pellets
composed of pure graphite powder and inorganic salts were utilized. The results of
the addition of 0.01 _M cupric acetate are shown in figure 2. At low temperature the
change in oxidation rate of pure and impure graphite with temperature is similar.
However, the rate of oxidation of the impure graphite becomes independent of taprature
at a lower temperature. Measurements performed with specimens contabhg 1-r ,
concentrations of cupric acetate led to results which fell between the illustrated
curves.
The gas in the law pressure electrodeless discharge is chemically similar to that k
in the positive column of a low pressure arc. Wlecule-molecule and ion-molecule
collisions are frequent. The ions and neutral species are, therefore, nearly in ther- 1
mal quildbrium.
Elastic collisions between these species and electrons are less
' frequent. At low pressure electron temperatures are quite high. In the oxygen die-
Charge atodc oxygen (%) is believed to be the most abundant active species. Higher
energy states of atomic oxygen, positive and negative ions, and electronically
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