Oxygen recombination on various metal surfaces at low pressure

Harteck, P.

Loomis, K.

Reeves, R. R., Jr.

English

NASA Technical Reports Server

1__ t_ 5 35 8
9_002
Technical Status Report No. 3
A Study of the Effect of Surfaces on Oxygen Atom
Recombination at Low Pressures
t e
Rensselaer POl_Troy,N.Y. 88_1L _'
i May 1968 to 31 October 1968
Research Grant No. NGR-33-018-086
RPI Project 441.22
P. Harteck
R.R. Reeves, Jr.
K. Loomis
https://ntrs.nasa.gov/search.jsp?R=19690006026 2020-03-12T08:25:30+00:00Z
ABSTRACT
The study of O-atom recombination on various metal sur-
faces as applied to satellite mass spectrometer measurements
has continued. Results at this time are in the 10-6 - 10-5
torr pressure range and lower, mainly with gas mixtures of 02,
O-atoms, Ar and small amounts of NO. The results indicated that
no metal studied was totally inert to atom recombination. A
-4
recombination coefficient, y, of somewhat less than I0 was
obtained for the glass reaction vessel after conditioning the
surface. Particular attention has been given to gold and a re-
combination coefficient of 3 x 10-3 was found. Titanium was
found to be very catalytic (y>0.05); and aluminum relatively
passive, although the activity for each metal depended on a
number of variables.
INTRODUCTION
Three different types of experimental techniques were de-
vised as previously discussed in our interim report of 7 August
1968. Although similar results were obtained from each experi-
mental arrangement, only one method readily lead to expressing
the recombination coefficient as a number.
Hydrogen atoms and other gaseous impurities, temperature vari-
ation and surface preparation all appear to effect the interaction
of oxygen atoms with the surface.
No attempt was made to distinguish between "recombination"
of atoms on the surface to oxygen molecules and "combination" to
an oxide layer.
•EXPERIMENTAL
Oxygen atoms and NO are added to a reaction vessel in a
steady system at a low pressure using liquid helium for cryogenic
pumping. The electric discharge which produces the oxygen atom
supply is shut-off causing a decrease in the concentration of O-
atoms. The decrease in the NQ + 0 chemiluminescence observed is
first order and varies from 3 seconds in an empty system to very
fast decay rates with reactive samples from which the recombi-
nation coefficients,y, are calculated. It should be noted that
the decay rate is independent over a large pressure range• The
chemiluminescence was observed by means of a selected low noise
EMI photomultiplier (9502 SA) with a continuous output displayed
as a function of time.
RESULTS
Recombination efficiencies for oxygen atoms on various metal
surfaces are listed in Table i. The gold supplied to us by NASA
had a surface area of 5 cm 2 per side and its recombination effi-
ciency was twice that obtained for gold sheet. The gold sheet
sample showed a temperatur_ependence for efficiency with y=0.0033
at 20°C, 0.0086 at 160°C; and increased to 0.029 in the presence
of H-atoms (see interim report of 7 August 1968).
The test sample of titanium was an alloy #367 consisting of
6% AI, 4% V and the balance titanium. The results showed a very
rapid catalysis similar to silver. A series of reactions may be
postulated to explain these results:
Ti + 0 ÷ TiO (evaporates) (I)
TiO + 0 ÷ Ti + 02 (2)
TiO + 0 ÷ TiO 2 (3)
It is not certain at present if reaction 2 is possible ther-
modynamically. The TiO 2 is apparently inert, but the TiO may vol-
atilize and cause O-atom consumption for an extended period. When
the sample was removed and the O,atom pressure increased for a
period of time, reaction (3) oxidized all the invisible traces of
Ti (and TiO) completely to TiO 2 or (Ti02) n and the O-atoms then
survived for many collisions; i.e., the walls were "conditioned"
for low pressures again. In an analogous way vanadium may also go
through the same type reactions as 1-3.
The aluminum foil sample used had the same geometry as the
_old sheet. Initally, it gave a recombination efficiency similar
to the gold sheet and also exhibited temperature effects, but after
passing O-atoms at a higher pressure over its surface for a short
time an efficiency of 0.005 was obtained and show little change with
the addition of H-atoms.
SUMMARY AND FUTURE WORK
Results have been obtained on a variety of metals. Briefly
it can be said that:
Gold is frequently inert initially to O-atoms, cleaning-up
with time and actively combining O-atoms especially at higher tem-
oeratures or when hydrogen atoms are present. The gold sample,
similar to that used in the spectrometer and supplied by NASA-
GSFC, was reasonably rapid in recombination but initialy
•experiments on the one sample indicated greater stability with
increased temperature.
Titanium was actively recombining O-atoms over a series of
reactions discussed under results. Unless a satisfactory pro-
cedure can be found for accommodating the surface, which would
apparently consist of formation of a stable inert TiO 2 layer, the
use of titanium in probably inadvisable•
Aluminum when machined and polished appears relatively inert
and stable and is still one of the more promising metals for
satisfactory operation over a long period.
See also Table I for additional results.
Continued effort will be made along these lines with improve-
ment on the physical measurements and extension of the data. The
evaluation of non-metals will be pursued in future work.
An abstract of the technique and results has been submitted
for presentation at the Minneapolis meeting of the ACS, April 1969.
A copy of the abstract is appended.
TABLE I
TYPICAL RECOMBINATIONEFFICIENCIES FOR OXYGENATOMSON
VARIOUS SURFACESAT ROOMTEMPERATURE
Metals _7__
Gold plate 0.0060
Gold sheet 0.0033
AI 0.0032
AI accommodated 0.0056
Ta 0.0058
Stainless Steel 0.036
Ni 0.040
AI ii00 0.050
Ag >0.05
Ti >0.05
Remarks
Supplied by NASA
Increasing with
temperature
foil
Passed O-atoms at
high pressure over
surface
Very rough surface
CATALYTIC RECOMBINATIONOF OXYGENATOMSON
METAL SURFACESin the 10-4 to 10-6 torr
PRESSUREREGION
by
Keith E. Loomis
Robert R. Reeves, Jr.
Paul Harteck
Rensselaer Polytechnic Institute
Troy, N.Y.
ABSTRACT
The recombination efficiency of oxygen atoms impinging on
various surfaces has been studied. A pyrex reaction vessel was
used with O-atoms concentrations in the order of 10% and total
-4 -6pressures of i0 to i0 torr. The gas phase oxygen atom con-
centration was monitored by using the chemiluminescent reaction
of NO with O-atoms. A stream of oxygen atoms was supplied to a
stirred-flow reactor system which was cryogenicly pumped with
liquid helium. The afterglow intensity was proportional to the
oxygen atoms, and the decay rate of the oxygen atoms was a func-
tion of the pumping speed and the surface recombination. First
order decay of the intensity was obtained when the oxygen atom
discharge was turned off. They decay rate was then used to cal-
culate the recombination coefficient, y, for these operating con-
ditions. The recombination rates at these very low pressures are
found in general to be somewhat less than those reported in the
literature for much higher O-atom concentrations, possibly because
catalytic heating effects do not occur over the entire surface
(surface area ~ 10cm2) which would tend to increase the recombi-
nation rate. Localized heating and surface conditioning effects
will be discussed and recombination coefficients reported.


A study of the effect of surfaces on oxygen atom recombination at low pressures Technical status report no. 3, 1 May - 31 Oct. 1968

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