NATIONAL BUREAU OF STANDARDS REPORT

9802

SILVER OXIDATION IN ATOMIC OXYGEN

for

The Goddard Space Flight Center

of the

National Aeronautics and Space Administration

Contract S-998 142-G

<nb|>

U.S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
THE NATIONAL BUREAU OF STANDARDS

The National Bureau of Standards1 provides measurement and technical information services

essential to the efficiency and effectiveness of the work of the Nation’s scientists and engineers. The

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NATIONAL BUREAU OF STANDARDS REPORT

NBS PROJECT

3120444 March 25, 1968

NBS REPORT

9802

SILVER OXIDATION IN ATOMIC OXYGEN

Michael J. McBee

and

H . Thomas Yolken

for

The Goddard Space Flight Center

National Aeronautics and Space Administration

Contract S-998142-G

Final Report on NBS Project No. 3120444

by

of the

IMPORTANT NOTICE NATIONAL BUREAU OF S

for use within the Government, and review. For this reason, t whole or in part, is not authi Bureau of Standards, Washing

the Report has been speciticall

Approved for public release by the

Director of the National Institute of

Standards and Technology (NIST) on October 9, 2015

3ss accounting documents intended

> subjected to additional evaluation

3 listing of this Report, either in le Office of the Director, National by the Government agency for which

copies for its own use.

U.S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

The work reported here is concerned with problems which arose in

measuring the amount of atomic oxygen in the earth’s upper atmosphere

by the Goddard Space Flight Center of the National Aeronautics and

Space Administration. Preoxidized silver has been proposed as a surface coating on stainless steel for the gas entrance chambers of a satellite mass spectrometer for measurement of the earth's upper

atmosphere components. It was speculated that silver, which had been

preoxidized with atomic oxygen would behave as a catalytic surface

for the recombination of atomic oxygen from the environment. After

this recombination the oxygen would not undergo any further trapping

reactions before measurement in the spectrometer. The question that

this work answered was whether or not a stable surface can be formed

prior to use in the measurement process so that no further oxide

formation or film growth would introduce uncertainties into the measurement of the atomic oxygen in the upper atmosphere. Also of interest

was the stability of the surface formed under the low pressures

associated with the altitudes of measurement.

The specimen used for the study at the NBS was a 99.999% pure,

bulk, polycrystalline sample (see Table I for analysis). The specimen

was placed in a cell designed for incorporation into a high vacuum

system for the in situ ellipsometric study of metal surfaces exposed

o to atomic oxygen. Film growth to thicknesses of up to about 5000 A

could be measured with the ellipsometer for the particular film and

(1 2 ) substrate under investigation. The technique of ellipsometry v 9 '

determines thickness and optical constants of films on metal surfaces

by measuring the change in state of polarization of a light beam upon

reflection. The specimen was mechanically polished ending with a 6

micron diamond abrasive. An etching solution of about 50% reagent

HNO^ was used after each stage in the polishing. Electropolishing

was then accomplished in a cyanide bath containing the following

proportions of constituents: 35g AgCN, 37g KCN, 38g I^COg to 100 ml

H^O. Optimum conditions for polishing were established when the

voltage oscillated between about 0.8 to 1.7 volts. The specimen was

given a final rinse in spectro-grade methanol and dried in a stream of pure

oxygen. After positioning in the cell, the specimen was cleaned and

annealed at 2 x 10"^ torr by induction heating to red glow (about 600°C).

This was repeated until ellipsometer readings from the surface ceased

to change significantly upon subsequent heatings, denoting no further

oxide removal. After cooling to room temperature, atomic oxygen,

produced in a microwave discharge, was beamed to the center of the area

under observation across a pressure differential through a capillary.

-8

The pressure in the vacuum system during the study was about 10 torr,

although during the atomic oxygen exposure a pressure of about 10“^

torr on the high side of the capillary was required to maintain the

discharge.

2

We found that the silver oxide film continues to grow under

constant atomic oxygen bombardment, following a logarithmic growth

rate. This indicates that the oxidation rate controlling step is a

solid diffusion mechanism, and that more than enough oxygen is absorbed

to continue the process. The growth appears to be two stage with a slight increase in the logarithmic rate constant after a thickness of

o around 2200 A is attained (see Figure 1). In addition to ellipsometric

observations it was possible to follow visually changes in thickness by

means of interference colors as the thickness increased until the surface

o became blackened (greater than 5000 A). The microwave discharge of 0

^ is known to produce uncertain proportions of various species such as

0, 0", 0+

, 0^, and exited states. For this reason, further mechanistic

interpretation at the present time would be largely speculative.

o Approximate optical constants found for the 5461 A wavelength of

the Hg arc light source used in this experiment were for the "clean"

A A silver, n = 0.140 - 3 . 2 9 i , where n = n (1-ik); n = real refractive

index, k = absorption coefficient. Optical constants of the oxide were

determined by a method of best fit to computer calculated curves. The

ellipsometer computer program produces tables of polarizer and analyzer

values for a range of thicknesses corresponding to given values of

optical constants of the film and substrate. Constants of the substrate

being known, values for the oxide were found by matching the experimental

readings with those of the computer tables to find the best correspondence.

3
1 \
These values were found to be n = 2.00 - 0 . 0851

.

After the silver oxide film had been formed and the atomic oxygen

bombardment stopped, its thickness decreased a small amount as the gas

was pumped out of the system. Upon restarting the atomic oxygen beam,

the film was found to regrow at the same rate as before. Investigators

(3)

at NASA V ' found that an oxidized silver surface grown in a similar

manner consisted of approximately 30% AgO and 70% Ag^O which upon heating

to about 150°C in vacuum was converted to 100% Ag20. At low pressures

and room temperature, the stable oxide is probably nearly all Ag^O.

Since the dissociation of AgO at the pressures existing in the evacuated

system is favored thermodynamically , the instability in the oxide film

seen on removal of oxygen after oxidation, is explicable.

Since it was found that film growth never stops when silver is

exposed to atomic oxygen, it must be concluded that the pre-oxidized

silver surface is not suitable for making measurements of the concentration of atomic oxygen by using it for the catalytic recombination of

the oxygen atoms to be measured as molecular oxygen. These results

indicate that some of the atomic oxygen remains on the surface as part

of the oxide film. Additional silver oxidation investigations are

being continued and funded by the National Bureau of Standards.

4

USCOMM-NBS-DC
.

-
REFERENCES

1. McCrackin, F.L., Passaglia, E. , Stromberg, R.R., and Steinberg, H.L.,

"Measurement of the Thickness and Refractive Index of Very Thin

Films and the Optical Properties of Surfaces by Ell ipsometry"

,

J. Res., NBS, Vol . 67A, No. 4, 363 (1963).

2. Ell ipsometry in the Measurement of Surfaces and Thin Films,

Symposium Proceedings, National Bureau of Standards Mi sc. Pub. 256,

E. Passaglia, R.R. Stromberg and J. Kruger, editors, U.S. Gov't.

Printing Office, Washington, D.C. (1964).

3. Private communication.
, • l » | I Mje I - • ' • -
TABLE I

Spectrographic Analysis of 99.999 +

Element PPM

Mg 2

Si <1

Fe <1

Cu 2

Silver

OXIDATION

OF

SILVER

o

>0

o

in

o o

co o

CM

CD

Y‘SS3NH0IH±

TIME,

SEC.