chemical physics of discharges - Argonne National Laboratory

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appeared in the literature. O'Halloran, et al?', have directly sampled an argon
plasma jet using a specially designed entrance cone opening to a time-of-flight
mass spectrometer. Raisen, et a1.21J have made spectroscopic identifications of
species in an air plasma. Unlike the mass spectrometer measurements, however,
emission spectroscopy 1s difficult to quantify due to the wide variance in the
oscillator-strengths of the likely emitters.
I
Faced with the difficulty of determining plasma composition directly, one is
prone to apply equilibrium predictions as 8 guide to local plasma composition.
Referring both to Figs. 2 and 3, a highly dissociated composition can be expected
in most of the plasm region. Temperatures in excess of 10,000 K, expected over
most of the central region of the plasma, would dictate that the atomic species
H, C, and N, and their ions would predominate.
QUENCHING
At this juncture it is important to ask what Changes in composition would be
encountered on cooling the €I-C-N atom plasma. A slow, gradual cooling of the
labile intermediates resident in the plasma zone may well allow the system to
revert to the original reactants along an equilibrium path.
Kroepelin and Kippid2
found tNs effect in their study of a 10 amp, 40 volt d.c. arc burning in 8 hydro-
carbon-nitrogen atmosphere. The composition of the slowly cooled arc-heated gases
was found to be mainly w, &, and C1 and C2 hydrocarbons. No HCN was observed.
Under extremely rapid cooling, however, kinetic limitations can interpose along
the reaction path to yield different, uwre interesting or valuable products.
The Cold-Wall Tube. A small-diameter, water-cooled tube was selected from
the variaty of available high-cooling-rate devices, to provide rapid and convenient
quenching of the plasm species. Figure lb shows a diagram of the simple design
of the three-concentric tube arrangement of the quenching probe23. The overall
outside diameter of the probe was 318 in. The internal diameter of the quenching
tube which receives the hot plasma was 0.032 in. The surfaces of the inner tube
were stainless steel; other parts were fabricated from copper. The effect of a
variation in the composition of the cold surface waa not studied.
Cooling Rate. Due to the Ngh rate of heat transfer from plasma to adjacent
cold wall, rapid cooling occurs in the quenching tube. Freeman and ~ltrivan~~,~5
have measured an initial temperature decay rate of 5 x 107 Vsec in water-cooled
tubes. In their model of the heat-transfer process occurring in very smaU tubes,
Anmann and Timminsm have calculated cooling rates greater than lo9 Vsec. *obably
quenching rates in this range were associated with the quenching tube used
in. this study.
Reaction Path During Quench. Unfortunately, there is a sparsity of hightemperature
kinetic data for the H-C-N system. Hence one is unable to predict with
certainty the reaction path followed as the atomic species H, C, and N, are cooled
from 15,000 K to 500 K in 10 11tI~iBeCOnd6, for example. After an examination of