2814
D. T.LARSONAND D. L. CASH
partly because of its unreactive and easily measured product. It is obvious, although not always recognized, that k(e NzO)/k(e S) cannot be worked out from a decrease in G(N2)if reaction 3 or 4 is affected by clustering proton transfer or any other similar reaction. The hydrogen and methane values agree reasonably well with the previously reached c o n ~ l u s i o n s . ~A~relatively small effect of additives and temperature on
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G(H2) does not present a significant difference. The increase of G(CH4) with temperature is probably due to enhanced fragmentation a t higher temperatures. No explanation is offered for the high value of G(CH,) in C3Hs NzO C H 8 1 a t room temperature.
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Acknowledgment. We are grateful to Professor J. J. Weiss for encouragement and Dr. G. R. A. Johnson for many stimulating discussions.
Ellipsometer Studies of Plutonium Oxidation1
by D. T. Larson and D. L. Cash T h e Dow Chemical Company, Rocky Flats Division, Golden, Colorado 80.401
(Eeceived November 1, 1068)
An ellipsometer-high vacuum system was used to investigate the kinetics of initial oxidation of plutonium stabilized in the A phase with 1.0 w t % gallium. “Clean” plutonium surfaces were prepared in a vacuum chamber using argon ion bombardment. The prepared surfaces were oxidbed thermally in 5.0 X 10-2 Torr of oxygen in a temperature range of 28 to 90”. Oxide thickness as a function of time was calculated from the ellipsometer measurements. These low-temperature oxidation data were fit to the parabolic equation d2 = kt C; where d is film thickness, t is oxidation time, and k is the rate constant. From values of k for the various oxidation temperatures and by the use of the Arrhenius equation, the activation energy was calculated to be 30 kcal/mol. The major oxide found under the experimental conditions used was identified by X-ray diffraction as PuOz with a high (111) orientation.
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Introduction Information available in the literature on the oxidation of plutonium does not include data on the parabolic rate constants at low temperatures.2-E This paper presents a system which is employed to investigate the low-temperature oxidation behavior of plutonium and discusses the kinetics observed. An ellipsometer is an effective instrument for in situ oxidation s t u d i e ~ . ~A, ~“clean” metal surface can be prepared, and the ellipsometer can measure a reproducible starting point before the beginning of the oxidation for the gas-metal reaction studies. After preparing the “clean” metal surface, it can be oxidized, and the thickness of the oxide is followed by ellipsometric measurements. The plutonium surfaces were oxidized thermally in 5.0 X low2Torr of oxygen in the temperature range 28 to 90”. Oxide thickness as a function of time was calculated from the ellipsometric measurements. These low-temperature oxidation data fit the parabolic equation d 2 = kt C. From values of k for the various oxidation temperatures and by use of the Arrhenius equation, the activation energy was calculated. The ellipsometer is an optical instrument that mea-
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T h e Journal of Physical Chemistry
sures the state of reflected polarized light. From these measurements one can determine the index of refraction of the “clean” metal substrate, the index of refraction of a thin film, and the thickness of the thin film.s-ll The optical arrangement of the ellipsometer used in (1) Work performed under U. S. Atomic Energy Commission Contract AT(29-1)-1106. (2) J. F. Sackman in ”Plutonium 1960,” E. Grison, W. B. H. Lord, and R. D. Fowler, Ed., Cleaver-Hume Press Ltd., London, 1961, p 222. (3) J. T. Waber and E. S. Wright in “The Metal Plutonium,” A. S. Coffinberry and W. N. Miner, Ed., The University of Chicago Press, Chicago, Ill., 1961, p 194. (4) J. T. Waber in ”Plutonium Handbook,” Vol. I, 0 . J. Wick, Ed., Gordon and Breach, New York, N. Y., 1967, p 145. (5) J. G. Schnizlein and D. F. Fischer, J . Electrochem. SOC.,114, 23 (1967). (6) M. A. Thompson in “Plutonium 1965,” A. E. Kay and M. B. Waldron, Ed., Chapman and Hall Ltd., London, 1967, p 592. (7) J. Kruger, Corrosion, 2 2 , 88 (1966). (8) J. V. Cathcart and G. F. Petersen in “Ellipsometry in the Measurement of Surfaces and Thin Films,” E. Passaglia, R. R. Stromberg, and J. Kruger, Ed., National Bureau of Standards Miscellaneous Publication No. 256, Washington, D. C., 1964, p 201. (9) A. C. Hall, J . O p t . Soc. Amer., 5 5 , 911 (1965). (10) F. L. McCrackin, E. Passaglia, R. R. Stromberg, and H. L. Steinberg, J . Res. Nat. Bur. Stand., 67A, 363 (1963). (11) R. J. Archer, J . Opt. SOC.Amer., 5 2 , 970 (1962).
28 15
ELLIPSOMETER STUDIES OF PLUTONIUM OXIDATION
rn
1 FAST DIRECTION OF COMPENSATOR
1k:2:;:Gm'
PLANE OF
*"",\"-
FOR EXTINCTION DIRECTION OF ANALYZER OF PLANE POLARIZED LIGHT
U
UNIVERSAL MOTION
DRY BOX BASE PLATE
---.La
'EED THROUGH
I
I I
Figure 1. Optical diagram of ellipsometer.
these investigations is shown in Figure 1. The quarterwave plate is set at k45" from the plane of incidence. Plane-polarized, monochromatic light is incident on the surface to be studied. After reflection from the metal surface, the light becomes elliptically polarized. This elliptically polarized light is extinguished by rotating the polarizer and analyzer. At extinction of light, the quarter-wave plate reproduces plane-polarized light which is extinguished by the analyzer. The angular orientations of the polarizer and analyzer determine psi, KP, and delta, A, as discussed by McCrackin, et aZ.'O Psi (KP) is defined as the arctangent of the amplitude ratio of the parallel and normal components to the plane of incidence, and delta (A) is the phase difference." The equations and computational techniques necessary for determining the index of refraction of a "clean" metal surface, index of refraction of a thin film, and thickness of the film from ellipsometric measurements of \k and A are discussed in the l i t e r a t ~ r e . ~ ~ - ' ~
Experimental Method Apparatus. A Rudolphle photoelectric ellipsometer (43303-2003) supported on a cast-aluminum tool and jig plate was used. The scales for the angle of incidence, polarizer, quarter-wave plate, and analyzer are measured in 0.01" increments. The angle of incidente used was 70.00", and the wavelength of light was 5461 A. Data were processed using an IBM-360 ~ o m p u t e r . ~ ' Because of the toxicity of plutonium, the samples were handled in a glove box. The sample chamber for ellipsometer studies was welded to the bottom of a glove box with the ellipsometer positioned underneath. Figure 2 shows the sample chamber extending from the glove box and the position of the viewing ports which are Pyrex (7056) windows located 140" apart for ellipsometer viewing. Figure 3 is a diagram of the gas injection system. Reagent grade gases were used in these experiments with the moisture content of the argon and oxygen listed as
