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The kinetics and oxygen pressure dependence of the high temperature oxidation of Pu-1wt.%Ga
Journal of the Less-Common
Metals, 136 (1988) 349 - 366
349
THE KINETICS AND OXYGEN PRESSURE DEPENDENCE OF THE HIGH TEMPERATURE OXIDATION OF Pu-lwt.%Ga JERRY L. STAKEBAKE and LLOYD A. LEWIS Rockwell
International,
(Received April 27,1987;
Golden,
CO 80402
(U.S.A.)
in revised form June 22,1987)
Oxidation of Pu-lwt.%Ga was measured between 150 and 500 “c in oxygen pressures of 0.004 - 500 Torr. Three stages of oxidation were identified beyond the initial oxide nucleation. The effect of temperature on oxidation rates was determined at an oxygen pressure of 500 Torr. A discontinuity was observed between 300 and 370 “c that resulted in a change in the activation energy for the Stage II and III processes. Oxygen pressure effects were measured at 200, 300 and 400 “c. Both the parabolic rates for Stage I and the linear rates for Stage II were independent of pressure below 60 Torr and directly proportional to pressure above 60 Torr. Stage III was an interface reaction created by cracking and spalling of the oxide. This reaction was controlled by oxygen adsorption and was directly proportional to P 1’4 or P ’ depending on the temperature at pressures above 37 Torr.
1. Introduction Studies involving actinide metal oxidation have recently been reviewed in depth [l]. One significant observation from this review is the lack of recent information on the oxidation of plutonium. Only a few studies related to plutonium oxidation have been reported in the last 10 years [ 2 - 41. This apparent lack of effort does not imply a complete understanding of plutonium oxidation. In fact, quite the opposite is true. Very little is actually known about the mechanism of oxidation and the role of the various oxides in the oxidation process. The majority of the early investigations focused on evaluating the effect of temperature, environment and alloying on oxidation rates [ 5,6]. Results from these investigations provided comparative information but did little to define the mechanism of oxidation. Recently two studies were completed on the oxidation of Pu-lwt.%Ga in air and in a mixture of nitrogen and 5.5% oxygen [7, 81. These studies concentrated on the effects of temperature on oxidation rates and the results generally support those previously reported. In addition, oxidation in the reduced oxygen environ0022-5088/88/$3.50
@ Elsevier Sequoia/Printed in The Netherlands
350
ment indicated a slightly lower oxidation rate; however, these results were not sufficient to determine a general pressure dependence. The present work was undertaken to determine the oxygen pressure dependence of the reaction between plutonium and oxygen and to evaluate plutonium oxidation kinetics in pure oxygen. Results from this investigation further define the mechanism of plutonium oxidation and the properties of the oxides formed. They also indicate some areas where additional work is required.
2. Experimental details Oxidation kinetics were measured gravimetrically using a Cahn vacuum microbalance. Sample temperatures were measured with a chromel-alumel thermocouple adjacent to the sample. A more complete description of the experimental apparatus as well as the characterization of the plutonium alloy used has been given in a previous paper [7]. This Pu-lwt.%Ga alloy was cast and then rolled into sheets approximately 1 mm thick. The total impurity content of the alloy was about 502 ppm. Samples with a surface area of 1.26 cm* were prepared by mechanically abrading with 400 grit silicon carbide paper to remove the oxide film. The resulting surface contained carbon and oxygen impurities as well as trace amounts of silicon carbide. Immediately after abrading, the samples were placed in a quartz bucket and suspended from the balance and the system was evacuated to lo-’ Torr for a period of 16 - 24 h. Following evacuation, the samples were heated to the desired reaction temperature and the system was backfilled with ultrapure oxygen. The total impurity content of the oxygen was less than 70 ppm. Initially, oxidation runs were made in a constant oxygen pressure of 500 Torr over the temperature interval of 150 - 470 “c. In order to evaluate the oxygen pressure dependence of the oxidation reaction, additional runs were made at 200, 300 and 400 “C with the oxygen pressure ranging from 0.004 to 500 Torr. Pressures were measured with an MKS capacitance manometer system. Constant pressures below 10 Torr were maintained with a GranvillePhillips automatic pressure controller. A computerized data acquisition system was used to monitor the weight gain of the sample, temperature and pressure. Approximately 450 data points were collected for each run.
3. Results 3.1. Oxidation products The exact composition of the oxide film is unknown. X-ray diffraction revealed only a single f.c.c. phase with a composition of Pu02 _-X. No X-ray evidence was found for any other phase. The fact that there is an abundance of metal at the metal-oxide interface and an excess of oxygen at the oxide surface suggests that various oxide phases might be present. A cross-section
(b) Fig. 1. (a) Metallographic cross-section of a Pu-lwt.%Ga sample oxidized at 400 “C in 1 Torr of oxygen (magnification 32~). (b) Metallographic cross-section revealing the porous nature of the oxide formed during the oxidation of a Pu-lwt.%Ga sample in 1 Torr of oxygen at 400 “C (magnification 365~).
through a sample coupon oxidized at 400 SCin 1 Torr of oxygen for 17 h is shown in Fig. 1. This represents the sample condition during the Stage II oxidation process. Figure l(a) shows the section of the entire coupon. Cracking is evident throughout the oxide layer. At a magnification of 365X (Fig. l(b)), the cracks appeared to contain what was thought to be unreacted metal. Electron microprobe analysis of these cracks identified only oxide and no metal inclusions were found. The electron microprobe was used in an attempt to determine the oxygen distribution across the cross-sectional surface of the sample shown in Fig. 1. There was a decrease in oxygen at the metal-oxide interface; however, oxidation of the plutonium after polishing prevented a sharp definition of oxygen distribution. Because of this behavior, the detection of two different oxides, or even an oxygen gradient in the oxide fihn, was not possible. The fate of the gallium alloy metal during oxidation was also investigated using the microprobe. Five random spots were analyzed in the metal and the average gallium concentration was 0.99 wt.%. Ten spots were analyzed in the oxide layer. The average gallium concentration in these spots was 0.28 wt.%. Actual locations of the spots analyzed were not recorded so it was not possible to obtain a gallium distribution throughout the oxide layer. It is not known whether the gallium in the oxide was oxidized. 3.2. Oxidation processes Oxidation of a plutonium coupon in 1 Torr of oxygen at 300 “c is illustrated in Fig. 2. The general shape of this curve is typical of all the runs made during this investigation and suggests the presence of three different processes; an initial parabolic rate followed by two linear reactions. This
352
0
500
1000
1500
21 IO
Elapsed Time (min)
Fig. 2. Three-stage oxidation of Pu-lwt.%Ga
in 1 Torr of oxygen at 300 “C.
type of behavior was also observed during the oxidation of plutonium in air [7] and in a mixture of nitrogen and 5% oxygen [8]. Processes contributing to this type of behavior were discussed in these earlier investigations. Oxide nucleation and the growth of the initial oxide layer or layers could not be observed by this technique. Larson and Motyl[9] have investigated the early stages of plutonium oxidation after cleaning the surface with an argon sputter etch. Auger electron spectroscopy was used to monitor the oxide film growth at 20 “c up to a thickness of about 40 - 50 A. Kinetics for this process appeared to follow a logarithmic rate law. Two different oxides were found in the initial oxide layer. The first oxide formed was PuO,_sZ and this oxide was then rapidly oxidized to PuO~-~. If this behavior is combined with the current three-stage process, the entire oxidation process would be described by equations for logarithmic, parabolic and linear rate laws. This is similar to the kinetic description of hafnium oxidation [lo]. The initial Stage I oxidation reaction obeys a parabolic, rate law that is generally indicative of a diffusion-controlled process. The actual thickness of the oxide fihn determines the oxidation rate. Minimum detectable film thicknesses measured with this microbalance were about 600 A. Based on the results of Larson and Motyl [9], the composition of a fihn of this thickness or greater is likely to be PuOZ_,. Calculations were made of the maximum thickness of the oxide films formed during the Stage I process. These thicknesses’ranged from 1 to 40 pm depending on temperature and oxygen pressure. The thickness of the rate-controlling layer increased with temperature up to about 375 “c and then decreased somewhat erratically at higher temperatures where spalling became a factor. Oxygen pressures below about 10 Torr produced thicker rate-controlling oxide layers. Above 50 Torr the oxide thickness was almost independent of pressure. Stage II of the oxidation process has linear kinetics that are attributed to changes in the dense oxide. These changes have not been identified but could range from cracking of the dense fihn to recrystallization or growth
353
of a new oxide. Evidence available on the nature of the oxide formed during the Stage I and II processes does not indicate the presence of two different oxides in quantities sufficient to cause the parabolic to linear transition. The more likely explanation appears to be formation of a porous outer oxide layer either by cracking or recrystallization. This is supported by metallographic analyses such as those shown in Fig. 1. From the information available, a two-layer oxide, one dense and the other porous, may exist. This could account for a linear diffusion-controlled rate [ 111. Stage III begins with the cracking of the oxide film and the resumption of an interface reaction controlled by the amount of metal exposed and the amount of oxygen available. The transition between Stage II and Stage III shows accelerating rates caused by the cracking and spalling of the oxide. A linear rate is observed when oxygen has free access through the oxide layer. This rate continues until the amount of metal available for reaction becomes rate limiting. Data are normalized for the initial surface area. However, the actual metal area available for oxidation in this stage is uncertain. 3.3. Oxidation kinetics Parabolic kinetics for the Stage I process were evaluated using the equation x=a+k,t”2
(1)
where x (mg cme2) is the amount of oxygen reacted, t (min) is the time and k, is the parabolic rate constant. The linear oxidation kinetics for Stages II and III were evaluated using the equation x = b + kit
(2)
where kl is the linear rate constant. All three stages discussed in this paper are treated independently in evaluating the kinetic parameters. Kinetics for Stages I and II are sometimes combined and referred to as paralinear kinetics. In this case the initial parabolic process is transformed into a linear reaction process. This type of behavior can also be explained if both linear and parabolic processes are occurring simultaneously. Applications of paralinear kinetics have been discussed by other workers [ll-131. 3.4. Effect of temperature on oxidation kinetics The effect of temperature on the oxidation kinetics at an oxygen pressure of 500 Torr was evaluated for the three stages using the Arrhenius equation: k = Ae-xJRr
(3)
The high heat of formation of plutonium dioxide has resulted in some concern about the accuracy of the sample temperature measurements used in calculating activation energies. Earlier measurements [7] indicated the temperatures measured with the present system were accurate to within 5 “C up
354 Temperature
I
1
I
500
I
400 350
“C
I
I
I
300
250
200
150
-3 -
-5 -
E, = 21 kcal/mole -9 -
-11 -
III
III
II 1.0
1.2
1.4
1.6
III 1.8
2.0
II
I
2.2
2.4
I
I 2.6
I
I 2.0
I 0
1000/T (K-‘)
Fig. 3. The effect of temperature on the parabolic oxidation rate obtained in 500 Torr of oxygen.
to about 400 “C. Therefore, no corrections were made in the sample temperature data prior to this analysis. Because of the form of the parabolic equation used in eqn. (l), a plot of 2 In 12us. l/T is required to satisfy the Arrhenius equation. Such a plot is shown in Fig. 3 for the Stage I oxidation. A discontinuity in the curve, indicated by the temperature-independent region at about 325 “C, prevents a single analysis of the data over the entire temperature range. This discontinuity was previously observed during oxidation in oxygen-nitrogen mixtures [7, 8 J, but is less pronounced in this atmosphere of pure oxygen. It appears that the process creating this discontinuity is influenced by the partial pressure of nitrogen in the oxidizing atmosphere. A least-squares fit of the data below 300 “C resulted in an activation energy of 21.4 kcal mol-‘. Above 350 “c the activation energy was determined to be 21.6 kcal mol-i . These values are the activation energies associated with the diffusion of oxygen through plutonium oxide. The agreement between these activation energies indicates that the phenomena causing the discontinuity does not affect the diffusion process. The actual cause of the discontinuity is presumed to be a change in the structure of the dense oxide film. Such a change might well be expected to be dependent on oxygen pressure. Figure 4 shows the temperature dependence of the Stage II oxidation process. The discontinuity observed for Stage I is also present in this stage beginning at about 350 “C. Below this temperature the activation energy was 18.8 kcal mol-‘, whereas above about 375 “c the activation energy increased to 32.5 kcal mall’ . The temperature dependence for the Stage III linear rate constants is shown in Fig. 5. Since the Stage III oxidation kinetics are determined to a large extent by the physical process of oxide cracking and spalling, it is not
355 Temperature 1
I
500
1
400
I
350
I
1
300
'C
I
I
200
250
150
E, = 33 kcal/mole
III1 1.0
II 1.4
1.2
I
II 1.6
1.6
I
I
I
I
2.0 2.2 lOOO/T(K-')
II
I
2.4
2.6
II
I
:
2.6
0
Fig. 4. The effect of temperature on the Stage II linear oxidation rate obtained in 500 Torr of oxygen.
"C
Temperature, 2
II 500
450
I 400
I 350
I
I
300
250
I 200
0 IE, = 46 kC8lhOle -1
-2
-3 I
Ee = 19 kC8l/IIIOl@
-6
1.0
I
I
I
I
I
I
I
I
I
I
I
I
1.1
1.2
1.3
1.4
1.5
1.6
1.7
1.6
1.9
2.0
2.1
2.2
1000/T
(K-')
Fig. 5. The effect of temperature Torr of oxygen.
on the Stage III linear oxidation rate obtained in 500
surprising that there is more scatter in the kinetic data. The discontinuity for this stage occurred at 362 “c and appeared quite sharp and not spread out over the 30 - 40 “C temperature range observed for Stages I and II. Below 362 “C the activation energy was 19.0 kcal moI_‘, which was nearly identical to the low temperature activation energy for Stage II. The activation energy for oxidation above 362 “C was 46.1 kcal mol-‘. Energetics for both the
356 TABLE 1 Evaluation of the effect of temperature on the oxidation kinetics of Pu-lwt.%Ga
ES
Reaction stage
Temperature range a
Rate eqz4ationb
A
I
Low High
Parabolic Parabolic
2.0
X lo6 (mg cme2 mir-1’2)
1.4
X
Low High
Linear Linear
8.2 2.1
Linear Linear
2.1 4.2
II III
LOW
High
(kcal mol-r)
X X X X
lo6 (mg cme2 min-1’2)
21.4 + 1.3 21.6 + 2.5
lo4 (mg cmW2 min-‘) lo9 (mg cmm2 min-‘)
18.8 f 1.5 32.5 + 5.4
10’ (mg cme2 mm-‘) 1014 (mg cmp2 min-‘)
19.0 f 0.8 46.1 + 7.5
aLow: T = 423 - 573 K. High: T = 633 - 723 K. bParabolic: kP2 = A exp(-EJRT). Linear: k = A exp(-E,/RT).
Stage II and Stage III processes are affected by the phenomena causing the discontinuity. Results for the temperature dependence of the kinetic rate parameters are summarized in Table 1. 3.5. Effect of oxygen pressure on oxidation kinetics The effects of oxygen pressure on oxidation rates was evaluated for each of the three stages of oxidation. These results are illustrated in Figs. 6 - 8. Pressure effects were evaluated using the equation k = @1/”
(41
where k is the parabolic or linear rate constant, P is the oxygen’ pressure, and a and l/n are constants. Oxidation rates were found to be pressure indePRESSURE, TORR 0.01 I
0
0.1 I
I
10 I
I -2
I 0
I 2
100 I
5 C E -1 k 2 m-2 E ; s -3 e 8 0 -4 5 ” j -5 -
--
e 5
-6 -
2 -10
I -6
I -6
I -4
I 4
I 6
I
6
10
V n (pressure, tow)
Fig. 6. The effect of oxygen pressure on the parabolic oxidation rate (Stage I) of Pulwt.%Ga at 200,300 and 400 “C.
357 PRESSURE, TOUR 0.1 1
0.01
;A
;ooOc;
3oooc
~‘~pl12
;
A
A
kpaP”3
A s 2 2 z g
-0 -
5 5
-9 -
-7 -
kyaP”5
-11 -10
I
I
I
-8
-6
-4
I
I
I
-2 0 2 Vn (pressure, torr)
I
I
4
6
I 8
10
Fig. 7. The effect of oxygen pressure on the linear Stage II oxidation rate of Pu-lwt.%Ga at 200,300 and 400 “C. PRESSURE (torr)
I -1
p E
-2
5
-3
P ,9
-4
a E
3d
I____ kCaPl/4
0
-6
1
23456
7
a
9 IC
P n (Pressure torr)
Fig. 8. The effect of oxygen pressure on the linear Stage III oxidation rate of Pu-lwt.%Ga at 300 and 400 “C.
pendent at low pressures and then abruptly change to a positive pressure dependence described by eqn. (4). Figure 6 shows this behavior for the parabolic oxidation observed for Stage I. The region of pressure independence varied with temperature. At 200 “C oxidation rates were pressure independent
358
below 13 Torr. As the temperature increased the transition pressure also increased until at 400 “C the transition pressure was 60 Ton. Above the transition pressure the parabolic rate was described by eqn. (4) where l/n varied from + to a again depending on temperature. Similar behavior was noted for Stage II (Fig. 7). The linear oxidation rates were independent of pressure below about 7 - 20 Torr. The transition pressure varied between 7 and 20 Torr and in this case did not appear to be influenced by temperature. Above 20 Torr, oxidation rates were proportional to P1’n where l/n varied from 5 to $ as the temperature increased from 200 to 400 “C. Data for Stage III are limited because some of the runs at low pressures were terminated before the reaction reached its final stage. Reaction rates for this stage were pressure independent up to 37 - 55 Torr. However, data were only available down to 10 - 15 Torr, which makes the behavior in the low pressure region somewhat questionable. Above the transition pressure, the pressure dependence of the linear oxidation rates was proportional to P”” where l/n varied between i and unity. Results for all three stages are summarized in Table 2. 4. Discussion Studies involving the oxidation of plutonium have most often focused on the temperature dependence of the oxidation kinetics [ 5, 61. These studies repeatedly found discontinuities in the Arrhenius plots that were attributed to properties of the oxide being formed. In general, this has not been verified by analytical evidence, but rather by a correlation with the Pu-0 phase diagram. Since the discontinuities occur near the phase boundary in the Pu-0 system, it appears reasonable to attribute them to the formation of the cubic PuO~.~~and the partially protective P~0i.s~ [ 141. The present investigation confirmed the presence of the discontinuities but did not provide any additional information on the role of the oxide. Results from this work did, however, identify two other areas in the oxidation process that are likely to be affected by the oxide being formed. These are firstly, the diffusion-controlled Stage I process, and secondly, the pressure dependence for all stages. Understanding the effect the oxide has on the oxidation process first requires a basic knowledge of oxide properties. During metal oxidation, the dense oxide fihn on the metal was generally found to be less than 50 pm thick. Although the primary interest is the characterization of thin oxide films, bulk properties of the oxide can provide a valuable source of information for application to thin oxide-metal systems. Properties such as electrical conduction, potential gradients and space charges, crystal orientation, and stresses within the oxide all affect the growth of the oxide film and the kinetics of the process. Information on these properties is quite limited for plutonium oxides as is information on the identity of the oxide formed at various stages of the oxidation process and on the effects of oxygen pressure.
359
TABLE2 Oxygen pressure dependence of the oxidation of Pu-lwt.%Ga Reaction stage
Temperature CC)
Pressure range (To=)
l/n
I
200
0.3 - 13 13 - 500
0
300
400
200
III
0.006 - 30 30 - 500 0.007 - 60 60 - 500
0.32-20 20-500
1 7
0 1 B
0 1 a
0 1 5
300
0.004 - 20 20 - 500
0 1 5
400
0.007 - 7 7 - 500
0 1 2
300
10 - 37 37 500
0 1 a
400
15-55
0
55 - 500
1
The Pu-0 phase diagram [15 - 171 provides a general indication of the most likely oxides to be formed during metal oxidation. The area of primary interest in the phase diagram is for 0:Pu ratios greater than 1.5. Specifically included is the formation of the Pu20s (hexagonal), Pu01.s2 (b.c.c.), PuO~.~~ (b.c.c.) and PuO~_~ (f.c.c.) oxides. One other compound, PuO,C, (often referred to as PuO in earlier literature), also needs to be considered in explaining the initial oxide formation. Electron spectroscopy [3, 41 and X-ray diffraction [18] analyses of oxides formed during plutonium oxidation have identified PuO~_~, Pu01.s2 and PuO,C,. In one case [18], the reported may actually have been PuO~.~~ since the measurements puo1.52 were made above 300 “C. Plutonium oxides are semiconductors [19] and as such exhibit electrical and semiconducting properties that are related to the type of defect structure. The electrical properties of the oxide control the migration of ionic or electronic species, and therefore, can also affect oxidation kinetics. A summary of the relationship between the semiconductor type, defect type and the pressure dependence of the defect structure is shown in Table 3. In
360 TABLE 3 Effects of oxygen pressure on defect concentrations Conductor
Defect structure
n-type
rvoX
(oxygen
vacancies) p-type (oxygen
interstitials) X = neutral,
Wo’l
1
tvo-’ 1 Eoixl [%‘I EOi”1
[20] Oxygen pressure dependence p-112 p-114 p-116 pll2
p1/4 pll6
* = positive charge, ’= negative charge.
the case of plutonium oxides, the predominant disorder consists of oxygen vacancies and interstitials. The role of plutonium interstitials and vacancies as the diffusing species can be ruled out because of the very small diffusion coefficients. However, these defects may play a role in recrystallization or other modifications of the oxide. The type of defect structure should be readily determined by evaluating the electrical properties of the oxide. Obtaining this information for plutonium is complicated, however, by the presence of multiple oxide phases. . . Stolchrometrrc PuOz.a,, is assumed to have Frenkel-type defects consisting of the defect pair (Oi, V,). Thermal e.m.f. measurements have shown that PuO~.~, as normally prepared, is a p-type semiconductor in the temperature range 200 - 1000 “C [21]. This is likely owing to the presence of divalent impurity ions that replace the Pu4+ ions in the fluorite structure. Substoichiometric PuOz --x is an oxygen deficient n-type semiconductor with oxygen vacancies as defects. The other principal oxide is Pu01_s2. The unit cell of this oxide consists of eight PuO, cells with one oxygen in four missing and a shift of the plutonium atoms. The large number of vacant sites allows excess oxygen to enter the lattice and results in p-type conductivity. The Pu-0 phase diagram shows that for 0:Pu ratios of 1.6 to 1.98 a two-phase oxide system consisting of p-type Pu01.s2 and n-type PuO~_~ exists below 300 “c. Between 300 and 650 ‘C (depending on the 0:Pu ratio) a second two-phase region exists and is composed of PuO,_, and Pu01.61. has the same structure as PuO~.~~, it may be presumed also to If puol.61 exhibit p-type conductivity. This has not, however, been verified. Compositions greater than 1.98 are single phase n-type PuOz _X at room temperature and above. Resistivity measurements show that there is a sensitive relationship between the conductivity of the two-phase oxides, oxide temperature and 0:Pu ratios [21]. This relationship suggests a phase transformation in the oxide resulting in p-type conductivity at low temperatures and n-type conductivity at higher temperatures. This behavior is summarized in Table 4.
361 TABLE 4 Transformation temperatures and activation energies for electronic phase plutonium oxides [21, 221 0:Pu
2.00 1.96 1.92 1.84 1.72
Resistiuity (n cm) 298 K
1250 K
10’4 1.0 x 1.5 x 2.0 x 8.0 x
102 2.5 0.5 0.2 0.055
T,, transformation
10’ 10’ 106 104
conduction
in two-
E, (below Tt ) (ev)
Transformation temperature (K)
-% (above Tt) (ev)
1.8 1.28 1.24 1.11 0.95
848 903 923 763
1.8 0.51 0.51 0.49 0.54
temperature.
Below the transformation temperature PuO~.~~(or possibly PuOi,, ) exists, resulting in p-type conductivity. Above the transformation temperature the reduced single-phase oxide PuO,_, exhibits n-type conductivity due to the excess metal. These measurements gave no indication that the phase transformation (Pu01.52-PuOz_x to PuO~.~~-PuO,__~) at 300 “C affects the electrical conductivity of the two-phase oxide. In the present investigation, the range of experimental conditions were such that a two-phase oxide product exists in contact with the plutonium metal phase. Oxidation was also measured under various oxygen pressures. Both of these factors, together with temperature, will affect the stoichiometry of the oxides and thus the defect structure of the oxide formed. Therefore, it is not surprising that the mechanism for the oxidation reaction changes with the experimental conditions. The parabolic behavior identified for the Stage I oxidation process results from the formation of a dense protective oxide. When this oxide layer forms, further reaction may proceed only after the reactants diffuse through the oxide. Therefore, oxygen diffusion becomes the rate-controlling process. Activation energies for the diffusion-controlled oxidation step were discussed earlier and found to be almost constant over the entire temperature range. This suggests the dense oxide is stable over a wide temperature range. Analytical evidence available at this time indicates that this oxide is PuO, __%.The magnitude of the activation energy (21 kcal mol-‘) was nearly identical to that determined for oxygen diffusion in PuOia9 [19]. Diffusion in the oxide layer or layers takes place via imperfections or defects in the oxide. These imperfections may be divided into two main categories which are point or lattice defects and surface defects. Point defects include vacancies, interstitial atoms and misplaced atoms that are responsible for lattice or bulk diffusion. Surface defects include grain boundaries, dislocations and inner and outer surfaces that contribute to grain boundary or surface diffusion. The type and magnitude of diffusion taking place is a function of temperature, partial pressure of oxygen, porosity and grain size. Because of the interrela-
tionship between oxidation and diffusion, any understanding of one process will aid in defining the other. Specific diffusion properties of plutonium oxides will be discussed in a future publication. The oxygen pressure dependence for the first stage of plutonium oxidation kinetics revealed a zero dependence at low pressures followed by a positive dependence at higher pressures. This type of behavior is not unique to plutonium. In the case of tantalum oxidation, similar behavior is observed for the parabolic oxidation of the metal to form TazO, [23]. The change in pressure dependence was correlated to a change in the semiconductor properties of the oxide. At low pressures Ta*Os is an n-type conductor, while at higher pressures it becomes a p-type conductor. Pressure independence for diffusion-controlled oxidation has also been observed for a number of other metals forming n-type semiconductor oxides [24]. In these cases, the number of defects at the gas-solid interface is small and the resulting oxidation rate should be nearly independent of oxygen pressure. This general process has been described by Wagner [25] for diffusion-controlled reactions. Oxygen diffusion in n-conducting oxides is proportional to P-1’n as shown in Table 3. However, if the dissociation pressure of this oxide is negligible, the oxidation rate then becomes independent of oxygen pressure [26]. In the case of p-conducting oxides involving oxygen interstitials, oxygen diffusion is proportional to P 1’n. If the dissociation pressure of this type of oxide remains negligible, the parabolic rate constant will remain proportional to P lh. Based on the observed pressure dependence for the Stage I oxidation of plutonium, oxidation in the pressure independent region (below 60 Torr depending on temperature) is controlled by oxygen vacancy diffusion in n-type PuOz p-X.This conclusion appears reasonable based on the identification of PuO, _-x by X-ray diffraction analysis of the product. Oxidation at pressures above 13 - 60 Torr remained diffusion controlled, but with a positive oxygen pressure dependence. In light of the previous discussion, oxidation in this region should be controlled by the diffusion of oxygen interstitials. Pressure dependence varied from P1” to P1’4 as the temperature increased from 200 to 400 “C. This suggests that the diffusion species is changing from doubly-charged to singly-charged oxygen interstitials. The change from pressure independence to a positive dependence requires that either the oxide film contain sufficient Pu01.s2 or PuO~.~~to provide p-type conductivity or that the defect structure of PuO~_~ be dependent on oxygen pressure and convert to p-type at higher pressures. The transition indicated by the pressure dependence is also affected by temperature as illustrated in Fig. 9. As pressures increased the temperature for the transition from n-type to p-type behavior also increased. Additional measurements of the electrical conductivity of the oxide as a function of temperature and oxygen pressure are required to confirm the observed n to p transition. Oxidation occurring during the linear Stage II process exhibited a pressure dependence at low pressures that was identical to the Stage I diffusion-
363
n-We)
1000/T
(K-l)
Fig. 9. Transition pressure for the n- to p-type transition behavior during plutonium oxidation as a function of temperature.
controlled reaction. Rates were independent of pressure up to a maximum of 55 Torr at 400 “c and proportional to P”” at higher pressures. Earlier it was stated that the linear Stage II was probably caused either by the cracking of the oxide layer allowing free access of oxygen to the metal or the formation of a two-layer oxide by cracking and recrystallization. The pressure behavior of this stage suggests the controlling step might be more closely related to the diffusion step seen in Stage I, which makes the two-layer oxide more probable. In this case the diffusion-controlling layer behaves like the n-type PuO, _-x and is subsequently recrystallized into a more porous form. Such a possibility has also been suggested by Greenholt and Weirick [27] for uranium and by Haycock [13] for several other metals. At this point the lack of knowledge with respect to the identity of the oxides formed and the effect of oxygen pressure on the electrical properties of the oxide, makes it very difficult to do more than propose possible mechanisms for the Stage II behavior. One alternative to the physical transformation of the dense oxide has been proposed by Loriers [ll] for cerium. In this case, a continuous protective layer of Cez03 forms on the metal surface and oxidizes at a constant rate to form CeOz. This makes diffusion through the CezO, layer the ratecontrolling step. If this same analogy were true for plutonium, then PuZOs (PuOi.& would be the rate-controlling diffusion barrier. However, the observed pressure dependence requires that this oxide behave like an n-type semiconductor at low pressures and a p-type semiconductor at high pressures. Since PuO 1.52is a p-type semiconductor, it appears to be eliminated from the rate-controlling role. Furthermore, the temperature range of the present investigation was primarily in the two-phase region where the n-type Pu02 _-x should be predominant along with the poorly defined PuO~.~~. Evaluation of the Stage III oxidation pressure dependence in the pressure range 10 - 55 Torr (Table 2) was based on the analysis of a limited
I -
Ignition
oLSd60 ELAPSED TIME (min.)
Fig. 10. Ignition of a Pu-lwt.%Ga 465 “C.
60
1010
coupon during oxidation
in 10 Torr of oxygen at
number of data points and cannot be considered reliable. At pressures above 55 Torr this interface-controlled process was proportional to P 1’4 at 300 “C and P’ at 400 “C. At 400 “c the oxidation rate is nearly catastrophic and may in fact approach ignition, as seen at 465 “C in Fig. 10. Control of the linear reaction rate rests on the availability of oxygen [28]. Oxygen adsorption follows the sequence 0, (gas) -+ Ki --f 02
(physically adsorbed)
(5)
2 sites + O2 (physically adsorbed) + & -+ 0 (chemisorbed ) + 0 (chemisorbed ) (6) 0 (chemisorbed) - k3 + 0 (reacted)
(7)
If equilibrium exists between chemisorbed and physically adsorbed oxygen, the linear oxidation rate becomes kl =
k3KP1” 1 + KP”’
where K = (K, K2)l”, S 1 and kl = k3
(8) At low temperatures and high oxygen pressures KP”2 (9)
Under these conditions the rate is independent of oxygen pressure and adsorption is not a factor. At high temperatures, such as those used in this investigation, and at low oxygen pressures (KP”’ 4 1) then kl = k3P1”
(W
365
This relationship suggests an adsorption equilibrium exists at the oxideoxygen interface and that a dissociative equilibrium (eqn. (6)) is rate controlling. The linear oxidation rate at 300 “C in Stage III was proportional to P1’4, which suggests an intermediate position between the conditions for eqn. (9) and eqn. (10). Oxidation at 400 “c was directly proportional to pressure. At high temperatures the lifetime of physically adsorbed oxygen is short so that the rate of chemisorption, 12*,depends directly on k 1 (the rate of physical adsorption) or more specifically, on the rate of impingement of oxygen molecules on the surface. According to kinetic gas theory the rate of impingement is given by SP
ww)
=
(2m~77)‘/2
(11)
where s is the sticking probability, m the mass of the oxygen molecule and k the Boltzmann constant. The direct pressure dependence dictated by this molecular adsorption mechanism satisfies the observed oxidation behavior at 400 “c.
5. Conclusion Oxidation kinetics were measured for the Pu-lwt.%Ga alloy over the temperature range 150 - 500 “C and with oxygen pressures of 0.004 to 500 Torr. Three processes were identified as follows: (i) the build-up of a dense protective oxide, (ii) a constant conversion of the protective layer to a porous oxide and (iii) an interface reaction resulting from severe cracking and spalling of the oxide layer. Results on the effects of temperature supported the results of earlier investigations in that a discontinuity existed between 300 and 370 “c. The cause of the discontinuity did not affect the 21.5 kcal mol-’ calculated for the activation energy of the diffusion-controlled Stage I process. Activation energies for the Stage II and III processes were, however, dependent on the temperature range. At present the cause of the discontinuity is unknown, although it is thought to be related to the properties of the oxide. Oxidation rates were independent of pressure up to about 60 Torr for Stages I and II. This behavior is attributed to a diffusion-controlled oxidation reaction where oxygen vacancy diffusion is rate controlling. Above 13 - 60 Ton oxidation was directly proportional to pressure, suggesting that the rate-controlling step is oxygen interstitial diffusion and that the oxide formed converts to a p-type semiconductor. Stage III of the oxidation process was an interface-controlled reaction that was dependent on the adsorption of oxygen in the 300 - 400 “c range. Above 400 “C ignition becomes an unpredictable factor in the Stage III oxidation process. Additional work is required to characterize oxide composition and properties as a function of oxygen pressure and temperature. This informa-
366
tion is needed to identify the cause of the discontinuity in the temperature behavior and to verify the explanation presented in this work for the oxygen pressure dependence.
Acknowledgment This work was performed under the auspices of the U.S. Department of Energy, Contract DE-AC04-76DP03533.
References 1 C. A. Colmenares, Prog. Solid State Chem., 15 (1984) 257. 2 J. L. Stakebake,J. Electrochem. Sot., 124 (1977) 460. 3 D. Courteix, J. Chayrouse, L. Heintz and R. Baptist, Solid State Commun., 39 (1981) 209. 4 D. T. Larson and J. M. Haschke, Znorg. Chem., 20 (1981) 1945. 5 C. A. Colmenares, in J. 0. McCaldin and G. Somorjai (eds.), Progress in Solid State Chemistry, Vol. 9, Pergamon, Oxford, 1975, p. 139. 6 0. J. Wick (ed.), Plutonium Handbook, Vol. 1, Gordon and Breach, New York, 1967. 7 J. L. Stakebake,J. Less-Common Met., 123 (1986) 185. 8 J. L. Stakebake,J. Nucl. Mater., 148 (1987) 115. 9 D. T. Larson and K. M. Motyl, personal communication. 10 W. W. Smeltzer and M. T. Simnad, Actu Metall, 5 (1957) 328. 11 J. Loriers, C.R. Acad. Sci., 231 (1950) 522. 12 P. Levesque and D. Cubicciotti, J. Am. Chem. Sot., 73 (1951) 2028. 13 E. W. Haycock, J. Electrochem. Sot., 106 (1959) 771. 14 J. G. Schnizlein and D. F. Fischer, J. Electrochem. Sot., 114 (1967) 23. 15 E. R. Gardner, T. L. Markin and R. S. Street, J. Znorg. Nucl. Chem., 27 (1965) 541. 16 T. D. ChikaIla, C. E. McNeilly and R. E. Skavdahl,J. NucL Mater., 12 (1964) 131. 17 M. H. Rand, in 0. Kubaschewski (ed.), Plutonium: Physico-Chemical Properties of its Compounds and Alloys, Special Issue No. 1, International Atomic Energy Agency, Vienna, 1966, p. 7. 18 K. Terada, R. L. Meisel and M. R. Dringman, J. Nucl. Mater., 30 (1969) 340. 19 C. Sari and G. Schumacher, NucL Tech., 28 (1976) 256. 20 P. Kofstad, High Temperature Oxidation of Metals, Wiley, New York, 1966, p. 51. 21 C. E. McNeiIly, J. Nucl. Mater., 11 (1964) 53. 22 R. E. Skavdahl and T. D. Chikalla, in 0. J. Wick (ed.), Plutonium Handbook, Vol. 1, Gordon and Breach, New York, 1967, p. 256. 23 P. Kofstad, J. Electrochem. Sot., 110 (1963) 491. 24 0. Kubaschewski and B. E. Hopkins, Oxidation of Metals and Alloys, Butterworths, London, 1962, p. 46. 25 C. Wagner, Atom Movements, American Society of Metals, Cleveland, OH, 1951, p. 153. 26 K. Hauffe, Oxidation of Metals, Plenum, New York, 1965, p. 202. 27 C. J. Greenholt and L. J. Weirick, J. Nucl. Mater., 144 (1987) 110. 28 P. Kofstad, High Temperature Oxidation of MetaZs, Wiley, New York, 1966, p. 245.
Metals, 136 (1988) 349 - 366
349
THE KINETICS AND OXYGEN PRESSURE DEPENDENCE OF THE HIGH TEMPERATURE OXIDATION OF Pu-lwt.%Ga JERRY L. STAKEBAKE and LLOYD A. LEWIS Rockwell
International,
(Received April 27,1987;
Golden,
CO 80402
(U.S.A.)
in revised form June 22,1987)
Oxidation of Pu-lwt.%Ga was measured between 150 and 500 “c in oxygen pressures of 0.004 - 500 Torr. Three stages of oxidation were identified beyond the initial oxide nucleation. The effect of temperature on oxidation rates was determined at an oxygen pressure of 500 Torr. A discontinuity was observed between 300 and 370 “c that resulted in a change in the activation energy for the Stage II and III processes. Oxygen pressure effects were measured at 200, 300 and 400 “c. Both the parabolic rates for Stage I and the linear rates for Stage II were independent of pressure below 60 Torr and directly proportional to pressure above 60 Torr. Stage III was an interface reaction created by cracking and spalling of the oxide. This reaction was controlled by oxygen adsorption and was directly proportional to P 1’4 or P ’ depending on the temperature at pressures above 37 Torr.
1. Introduction Studies involving actinide metal oxidation have recently been reviewed in depth [l]. One significant observation from this review is the lack of recent information on the oxidation of plutonium. Only a few studies related to plutonium oxidation have been reported in the last 10 years [ 2 - 41. This apparent lack of effort does not imply a complete understanding of plutonium oxidation. In fact, quite the opposite is true. Very little is actually known about the mechanism of oxidation and the role of the various oxides in the oxidation process. The majority of the early investigations focused on evaluating the effect of temperature, environment and alloying on oxidation rates [ 5,6]. Results from these investigations provided comparative information but did little to define the mechanism of oxidation. Recently two studies were completed on the oxidation of Pu-lwt.%Ga in air and in a mixture of nitrogen and 5.5% oxygen [7, 81. These studies concentrated on the effects of temperature on oxidation rates and the results generally support those previously reported. In addition, oxidation in the reduced oxygen environ0022-5088/88/$3.50
@ Elsevier Sequoia/Printed in The Netherlands
350
ment indicated a slightly lower oxidation rate; however, these results were not sufficient to determine a general pressure dependence. The present work was undertaken to determine the oxygen pressure dependence of the reaction between plutonium and oxygen and to evaluate plutonium oxidation kinetics in pure oxygen. Results from this investigation further define the mechanism of plutonium oxidation and the properties of the oxides formed. They also indicate some areas where additional work is required.
2. Experimental details Oxidation kinetics were measured gravimetrically using a Cahn vacuum microbalance. Sample temperatures were measured with a chromel-alumel thermocouple adjacent to the sample. A more complete description of the experimental apparatus as well as the characterization of the plutonium alloy used has been given in a previous paper [7]. This Pu-lwt.%Ga alloy was cast and then rolled into sheets approximately 1 mm thick. The total impurity content of the alloy was about 502 ppm. Samples with a surface area of 1.26 cm* were prepared by mechanically abrading with 400 grit silicon carbide paper to remove the oxide film. The resulting surface contained carbon and oxygen impurities as well as trace amounts of silicon carbide. Immediately after abrading, the samples were placed in a quartz bucket and suspended from the balance and the system was evacuated to lo-’ Torr for a period of 16 - 24 h. Following evacuation, the samples were heated to the desired reaction temperature and the system was backfilled with ultrapure oxygen. The total impurity content of the oxygen was less than 70 ppm. Initially, oxidation runs were made in a constant oxygen pressure of 500 Torr over the temperature interval of 150 - 470 “c. In order to evaluate the oxygen pressure dependence of the oxidation reaction, additional runs were made at 200, 300 and 400 “C with the oxygen pressure ranging from 0.004 to 500 Torr. Pressures were measured with an MKS capacitance manometer system. Constant pressures below 10 Torr were maintained with a GranvillePhillips automatic pressure controller. A computerized data acquisition system was used to monitor the weight gain of the sample, temperature and pressure. Approximately 450 data points were collected for each run.
3. Results 3.1. Oxidation products The exact composition of the oxide film is unknown. X-ray diffraction revealed only a single f.c.c. phase with a composition of Pu02 _-X. No X-ray evidence was found for any other phase. The fact that there is an abundance of metal at the metal-oxide interface and an excess of oxygen at the oxide surface suggests that various oxide phases might be present. A cross-section
(b) Fig. 1. (a) Metallographic cross-section of a Pu-lwt.%Ga sample oxidized at 400 “C in 1 Torr of oxygen (magnification 32~). (b) Metallographic cross-section revealing the porous nature of the oxide formed during the oxidation of a Pu-lwt.%Ga sample in 1 Torr of oxygen at 400 “C (magnification 365~).
through a sample coupon oxidized at 400 SCin 1 Torr of oxygen for 17 h is shown in Fig. 1. This represents the sample condition during the Stage II oxidation process. Figure l(a) shows the section of the entire coupon. Cracking is evident throughout the oxide layer. At a magnification of 365X (Fig. l(b)), the cracks appeared to contain what was thought to be unreacted metal. Electron microprobe analysis of these cracks identified only oxide and no metal inclusions were found. The electron microprobe was used in an attempt to determine the oxygen distribution across the cross-sectional surface of the sample shown in Fig. 1. There was a decrease in oxygen at the metal-oxide interface; however, oxidation of the plutonium after polishing prevented a sharp definition of oxygen distribution. Because of this behavior, the detection of two different oxides, or even an oxygen gradient in the oxide fihn, was not possible. The fate of the gallium alloy metal during oxidation was also investigated using the microprobe. Five random spots were analyzed in the metal and the average gallium concentration was 0.99 wt.%. Ten spots were analyzed in the oxide layer. The average gallium concentration in these spots was 0.28 wt.%. Actual locations of the spots analyzed were not recorded so it was not possible to obtain a gallium distribution throughout the oxide layer. It is not known whether the gallium in the oxide was oxidized. 3.2. Oxidation processes Oxidation of a plutonium coupon in 1 Torr of oxygen at 300 “c is illustrated in Fig. 2. The general shape of this curve is typical of all the runs made during this investigation and suggests the presence of three different processes; an initial parabolic rate followed by two linear reactions. This
352
0
500
1000
1500
21 IO
Elapsed Time (min)
Fig. 2. Three-stage oxidation of Pu-lwt.%Ga
in 1 Torr of oxygen at 300 “C.
type of behavior was also observed during the oxidation of plutonium in air [7] and in a mixture of nitrogen and 5% oxygen [8]. Processes contributing to this type of behavior were discussed in these earlier investigations. Oxide nucleation and the growth of the initial oxide layer or layers could not be observed by this technique. Larson and Motyl[9] have investigated the early stages of plutonium oxidation after cleaning the surface with an argon sputter etch. Auger electron spectroscopy was used to monitor the oxide film growth at 20 “c up to a thickness of about 40 - 50 A. Kinetics for this process appeared to follow a logarithmic rate law. Two different oxides were found in the initial oxide layer. The first oxide formed was PuO,_sZ and this oxide was then rapidly oxidized to PuO~-~. If this behavior is combined with the current three-stage process, the entire oxidation process would be described by equations for logarithmic, parabolic and linear rate laws. This is similar to the kinetic description of hafnium oxidation [lo]. The initial Stage I oxidation reaction obeys a parabolic, rate law that is generally indicative of a diffusion-controlled process. The actual thickness of the oxide fihn determines the oxidation rate. Minimum detectable film thicknesses measured with this microbalance were about 600 A. Based on the results of Larson and Motyl [9], the composition of a fihn of this thickness or greater is likely to be PuOZ_,. Calculations were made of the maximum thickness of the oxide films formed during the Stage I process. These thicknesses’ranged from 1 to 40 pm depending on temperature and oxygen pressure. The thickness of the rate-controlling layer increased with temperature up to about 375 “c and then decreased somewhat erratically at higher temperatures where spalling became a factor. Oxygen pressures below about 10 Torr produced thicker rate-controlling oxide layers. Above 50 Torr the oxide thickness was almost independent of pressure. Stage II of the oxidation process has linear kinetics that are attributed to changes in the dense oxide. These changes have not been identified but could range from cracking of the dense fihn to recrystallization or growth
353
of a new oxide. Evidence available on the nature of the oxide formed during the Stage I and II processes does not indicate the presence of two different oxides in quantities sufficient to cause the parabolic to linear transition. The more likely explanation appears to be formation of a porous outer oxide layer either by cracking or recrystallization. This is supported by metallographic analyses such as those shown in Fig. 1. From the information available, a two-layer oxide, one dense and the other porous, may exist. This could account for a linear diffusion-controlled rate [ 111. Stage III begins with the cracking of the oxide film and the resumption of an interface reaction controlled by the amount of metal exposed and the amount of oxygen available. The transition between Stage II and Stage III shows accelerating rates caused by the cracking and spalling of the oxide. A linear rate is observed when oxygen has free access through the oxide layer. This rate continues until the amount of metal available for reaction becomes rate limiting. Data are normalized for the initial surface area. However, the actual metal area available for oxidation in this stage is uncertain. 3.3. Oxidation kinetics Parabolic kinetics for the Stage I process were evaluated using the equation x=a+k,t”2
(1)
where x (mg cme2) is the amount of oxygen reacted, t (min) is the time and k, is the parabolic rate constant. The linear oxidation kinetics for Stages II and III were evaluated using the equation x = b + kit
(2)
where kl is the linear rate constant. All three stages discussed in this paper are treated independently in evaluating the kinetic parameters. Kinetics for Stages I and II are sometimes combined and referred to as paralinear kinetics. In this case the initial parabolic process is transformed into a linear reaction process. This type of behavior can also be explained if both linear and parabolic processes are occurring simultaneously. Applications of paralinear kinetics have been discussed by other workers [ll-131. 3.4. Effect of temperature on oxidation kinetics The effect of temperature on the oxidation kinetics at an oxygen pressure of 500 Torr was evaluated for the three stages using the Arrhenius equation: k = Ae-xJRr
(3)
The high heat of formation of plutonium dioxide has resulted in some concern about the accuracy of the sample temperature measurements used in calculating activation energies. Earlier measurements [7] indicated the temperatures measured with the present system were accurate to within 5 “C up
354 Temperature
I
1
I
500
I
400 350
“C
I
I
I
300
250
200
150
-3 -
-5 -
E, = 21 kcal/mole -9 -
-11 -
III
III
II 1.0
1.2
1.4
1.6
III 1.8
2.0
II
I
2.2
2.4
I
I 2.6
I
I 2.0
I 0
1000/T (K-‘)
Fig. 3. The effect of temperature on the parabolic oxidation rate obtained in 500 Torr of oxygen.
to about 400 “C. Therefore, no corrections were made in the sample temperature data prior to this analysis. Because of the form of the parabolic equation used in eqn. (l), a plot of 2 In 12us. l/T is required to satisfy the Arrhenius equation. Such a plot is shown in Fig. 3 for the Stage I oxidation. A discontinuity in the curve, indicated by the temperature-independent region at about 325 “C, prevents a single analysis of the data over the entire temperature range. This discontinuity was previously observed during oxidation in oxygen-nitrogen mixtures [7, 8 J, but is less pronounced in this atmosphere of pure oxygen. It appears that the process creating this discontinuity is influenced by the partial pressure of nitrogen in the oxidizing atmosphere. A least-squares fit of the data below 300 “C resulted in an activation energy of 21.4 kcal mol-‘. Above 350 “c the activation energy was determined to be 21.6 kcal mol-i . These values are the activation energies associated with the diffusion of oxygen through plutonium oxide. The agreement between these activation energies indicates that the phenomena causing the discontinuity does not affect the diffusion process. The actual cause of the discontinuity is presumed to be a change in the structure of the dense oxide film. Such a change might well be expected to be dependent on oxygen pressure. Figure 4 shows the temperature dependence of the Stage II oxidation process. The discontinuity observed for Stage I is also present in this stage beginning at about 350 “C. Below this temperature the activation energy was 18.8 kcal mol-‘, whereas above about 375 “c the activation energy increased to 32.5 kcal mall’ . The temperature dependence for the Stage III linear rate constants is shown in Fig. 5. Since the Stage III oxidation kinetics are determined to a large extent by the physical process of oxide cracking and spalling, it is not
355 Temperature 1
I
500
1
400
I
350
I
1
300
'C
I
I
200
250
150
E, = 33 kcal/mole
III1 1.0
II 1.4
1.2
I
II 1.6
1.6
I
I
I
I
2.0 2.2 lOOO/T(K-')
II
I
2.4
2.6
II
I
:
2.6
0
Fig. 4. The effect of temperature on the Stage II linear oxidation rate obtained in 500 Torr of oxygen.
"C
Temperature, 2
II 500
450
I 400
I 350
I
I
300
250
I 200
0 IE, = 46 kC8lhOle -1
-2
-3 I
Ee = 19 kC8l/IIIOl@
-6
1.0
I
I
I
I
I
I
I
I
I
I
I
I
1.1
1.2
1.3
1.4
1.5
1.6
1.7
1.6
1.9
2.0
2.1
2.2
1000/T
(K-')
Fig. 5. The effect of temperature Torr of oxygen.
on the Stage III linear oxidation rate obtained in 500
surprising that there is more scatter in the kinetic data. The discontinuity for this stage occurred at 362 “c and appeared quite sharp and not spread out over the 30 - 40 “C temperature range observed for Stages I and II. Below 362 “C the activation energy was 19.0 kcal moI_‘, which was nearly identical to the low temperature activation energy for Stage II. The activation energy for oxidation above 362 “C was 46.1 kcal mol-‘. Energetics for both the
356 TABLE 1 Evaluation of the effect of temperature on the oxidation kinetics of Pu-lwt.%Ga
ES
Reaction stage
Temperature range a
Rate eqz4ationb
A
I
Low High
Parabolic Parabolic
2.0
X lo6 (mg cme2 mir-1’2)
1.4
X
Low High
Linear Linear
8.2 2.1
Linear Linear
2.1 4.2
II III
LOW
High
(kcal mol-r)
X X X X
lo6 (mg cme2 min-1’2)
21.4 + 1.3 21.6 + 2.5
lo4 (mg cmW2 min-‘) lo9 (mg cmm2 min-‘)
18.8 f 1.5 32.5 + 5.4
10’ (mg cme2 mm-‘) 1014 (mg cmp2 min-‘)
19.0 f 0.8 46.1 + 7.5
aLow: T = 423 - 573 K. High: T = 633 - 723 K. bParabolic: kP2 = A exp(-EJRT). Linear: k = A exp(-E,/RT).
Stage II and Stage III processes are affected by the phenomena causing the discontinuity. Results for the temperature dependence of the kinetic rate parameters are summarized in Table 1. 3.5. Effect of oxygen pressure on oxidation kinetics The effects of oxygen pressure on oxidation rates was evaluated for each of the three stages of oxidation. These results are illustrated in Figs. 6 - 8. Pressure effects were evaluated using the equation k = @1/”
(41
where k is the parabolic or linear rate constant, P is the oxygen’ pressure, and a and l/n are constants. Oxidation rates were found to be pressure indePRESSURE, TORR 0.01 I
0
0.1 I
I
10 I
I -2
I 0
I 2
100 I
5 C E -1 k 2 m-2 E ; s -3 e 8 0 -4 5 ” j -5 -
--
e 5
-6 -
2 -10
I -6
I -6
I -4
I 4
I 6
I
6
10
V n (pressure, tow)
Fig. 6. The effect of oxygen pressure on the parabolic oxidation rate (Stage I) of Pulwt.%Ga at 200,300 and 400 “C.
357 PRESSURE, TOUR 0.1 1
0.01
;A
;ooOc;
3oooc
~‘~pl12
;
A
A
kpaP”3
A s 2 2 z g
-0 -
5 5
-9 -
-7 -
kyaP”5
-11 -10
I
I
I
-8
-6
-4
I
I
I
-2 0 2 Vn (pressure, torr)
I
I
4
6
I 8
10
Fig. 7. The effect of oxygen pressure on the linear Stage II oxidation rate of Pu-lwt.%Ga at 200,300 and 400 “C. PRESSURE (torr)
I -1
p E
-2
5
-3
P ,9
-4
a E
3d
I____ kCaPl/4
0
-6
1
23456
7
a
9 IC
P n (Pressure torr)
Fig. 8. The effect of oxygen pressure on the linear Stage III oxidation rate of Pu-lwt.%Ga at 300 and 400 “C.
pendent at low pressures and then abruptly change to a positive pressure dependence described by eqn. (4). Figure 6 shows this behavior for the parabolic oxidation observed for Stage I. The region of pressure independence varied with temperature. At 200 “C oxidation rates were pressure independent
358
below 13 Torr. As the temperature increased the transition pressure also increased until at 400 “C the transition pressure was 60 Ton. Above the transition pressure the parabolic rate was described by eqn. (4) where l/n varied from + to a again depending on temperature. Similar behavior was noted for Stage II (Fig. 7). The linear oxidation rates were independent of pressure below about 7 - 20 Torr. The transition pressure varied between 7 and 20 Torr and in this case did not appear to be influenced by temperature. Above 20 Torr, oxidation rates were proportional to P1’n where l/n varied from 5 to $ as the temperature increased from 200 to 400 “C. Data for Stage III are limited because some of the runs at low pressures were terminated before the reaction reached its final stage. Reaction rates for this stage were pressure independent up to 37 - 55 Torr. However, data were only available down to 10 - 15 Torr, which makes the behavior in the low pressure region somewhat questionable. Above the transition pressure, the pressure dependence of the linear oxidation rates was proportional to P”” where l/n varied between i and unity. Results for all three stages are summarized in Table 2. 4. Discussion Studies involving the oxidation of plutonium have most often focused on the temperature dependence of the oxidation kinetics [ 5, 61. These studies repeatedly found discontinuities in the Arrhenius plots that were attributed to properties of the oxide being formed. In general, this has not been verified by analytical evidence, but rather by a correlation with the Pu-0 phase diagram. Since the discontinuities occur near the phase boundary in the Pu-0 system, it appears reasonable to attribute them to the formation of the cubic PuO~.~~and the partially protective P~0i.s~ [ 141. The present investigation confirmed the presence of the discontinuities but did not provide any additional information on the role of the oxide. Results from this work did, however, identify two other areas in the oxidation process that are likely to be affected by the oxide being formed. These are firstly, the diffusion-controlled Stage I process, and secondly, the pressure dependence for all stages. Understanding the effect the oxide has on the oxidation process first requires a basic knowledge of oxide properties. During metal oxidation, the dense oxide fihn on the metal was generally found to be less than 50 pm thick. Although the primary interest is the characterization of thin oxide films, bulk properties of the oxide can provide a valuable source of information for application to thin oxide-metal systems. Properties such as electrical conduction, potential gradients and space charges, crystal orientation, and stresses within the oxide all affect the growth of the oxide film and the kinetics of the process. Information on these properties is quite limited for plutonium oxides as is information on the identity of the oxide formed at various stages of the oxidation process and on the effects of oxygen pressure.
359
TABLE2 Oxygen pressure dependence of the oxidation of Pu-lwt.%Ga Reaction stage
Temperature CC)
Pressure range (To=)
l/n
I
200
0.3 - 13 13 - 500
0
300
400
200
III
0.006 - 30 30 - 500 0.007 - 60 60 - 500
0.32-20 20-500
1 7
0 1 B
0 1 a
0 1 5
300
0.004 - 20 20 - 500
0 1 5
400
0.007 - 7 7 - 500
0 1 2
300
10 - 37 37 500
0 1 a
400
15-55
0
55 - 500
1
The Pu-0 phase diagram [15 - 171 provides a general indication of the most likely oxides to be formed during metal oxidation. The area of primary interest in the phase diagram is for 0:Pu ratios greater than 1.5. Specifically included is the formation of the Pu20s (hexagonal), Pu01.s2 (b.c.c.), PuO~.~~ (b.c.c.) and PuO~_~ (f.c.c.) oxides. One other compound, PuO,C, (often referred to as PuO in earlier literature), also needs to be considered in explaining the initial oxide formation. Electron spectroscopy [3, 41 and X-ray diffraction [18] analyses of oxides formed during plutonium oxidation have identified PuO~_~, Pu01.s2 and PuO,C,. In one case [18], the reported may actually have been PuO~.~~ since the measurements puo1.52 were made above 300 “C. Plutonium oxides are semiconductors [19] and as such exhibit electrical and semiconducting properties that are related to the type of defect structure. The electrical properties of the oxide control the migration of ionic or electronic species, and therefore, can also affect oxidation kinetics. A summary of the relationship between the semiconductor type, defect type and the pressure dependence of the defect structure is shown in Table 3. In
360 TABLE 3 Effects of oxygen pressure on defect concentrations Conductor
Defect structure
n-type
rvoX
(oxygen
vacancies) p-type (oxygen
interstitials) X = neutral,
Wo’l
1
tvo-’ 1 Eoixl [%‘I EOi”1
[20] Oxygen pressure dependence p-112 p-114 p-116 pll2
p1/4 pll6
* = positive charge, ’= negative charge.
the case of plutonium oxides, the predominant disorder consists of oxygen vacancies and interstitials. The role of plutonium interstitials and vacancies as the diffusing species can be ruled out because of the very small diffusion coefficients. However, these defects may play a role in recrystallization or other modifications of the oxide. The type of defect structure should be readily determined by evaluating the electrical properties of the oxide. Obtaining this information for plutonium is complicated, however, by the presence of multiple oxide phases. . . Stolchrometrrc PuOz.a,, is assumed to have Frenkel-type defects consisting of the defect pair (Oi, V,). Thermal e.m.f. measurements have shown that PuO~.~, as normally prepared, is a p-type semiconductor in the temperature range 200 - 1000 “C [21]. This is likely owing to the presence of divalent impurity ions that replace the Pu4+ ions in the fluorite structure. Substoichiometric PuOz --x is an oxygen deficient n-type semiconductor with oxygen vacancies as defects. The other principal oxide is Pu01_s2. The unit cell of this oxide consists of eight PuO, cells with one oxygen in four missing and a shift of the plutonium atoms. The large number of vacant sites allows excess oxygen to enter the lattice and results in p-type conductivity. The Pu-0 phase diagram shows that for 0:Pu ratios of 1.6 to 1.98 a two-phase oxide system consisting of p-type Pu01.s2 and n-type PuO~_~ exists below 300 “c. Between 300 and 650 ‘C (depending on the 0:Pu ratio) a second two-phase region exists and is composed of PuO,_, and Pu01.61. has the same structure as PuO~.~~, it may be presumed also to If puol.61 exhibit p-type conductivity. This has not, however, been verified. Compositions greater than 1.98 are single phase n-type PuOz _X at room temperature and above. Resistivity measurements show that there is a sensitive relationship between the conductivity of the two-phase oxides, oxide temperature and 0:Pu ratios [21]. This relationship suggests a phase transformation in the oxide resulting in p-type conductivity at low temperatures and n-type conductivity at higher temperatures. This behavior is summarized in Table 4.
361 TABLE 4 Transformation temperatures and activation energies for electronic phase plutonium oxides [21, 221 0:Pu
2.00 1.96 1.92 1.84 1.72
Resistiuity (n cm) 298 K
1250 K
10’4 1.0 x 1.5 x 2.0 x 8.0 x
102 2.5 0.5 0.2 0.055
T,, transformation
10’ 10’ 106 104
conduction
in two-
E, (below Tt ) (ev)
Transformation temperature (K)
-% (above Tt) (ev)
1.8 1.28 1.24 1.11 0.95
848 903 923 763
1.8 0.51 0.51 0.49 0.54
temperature.
Below the transformation temperature PuO~.~~(or possibly PuOi,, ) exists, resulting in p-type conductivity. Above the transformation temperature the reduced single-phase oxide PuO,_, exhibits n-type conductivity due to the excess metal. These measurements gave no indication that the phase transformation (Pu01.52-PuOz_x to PuO~.~~-PuO,__~) at 300 “C affects the electrical conductivity of the two-phase oxide. In the present investigation, the range of experimental conditions were such that a two-phase oxide product exists in contact with the plutonium metal phase. Oxidation was also measured under various oxygen pressures. Both of these factors, together with temperature, will affect the stoichiometry of the oxides and thus the defect structure of the oxide formed. Therefore, it is not surprising that the mechanism for the oxidation reaction changes with the experimental conditions. The parabolic behavior identified for the Stage I oxidation process results from the formation of a dense protective oxide. When this oxide layer forms, further reaction may proceed only after the reactants diffuse through the oxide. Therefore, oxygen diffusion becomes the rate-controlling process. Activation energies for the diffusion-controlled oxidation step were discussed earlier and found to be almost constant over the entire temperature range. This suggests the dense oxide is stable over a wide temperature range. Analytical evidence available at this time indicates that this oxide is PuO, __%.The magnitude of the activation energy (21 kcal mol-‘) was nearly identical to that determined for oxygen diffusion in PuOia9 [19]. Diffusion in the oxide layer or layers takes place via imperfections or defects in the oxide. These imperfections may be divided into two main categories which are point or lattice defects and surface defects. Point defects include vacancies, interstitial atoms and misplaced atoms that are responsible for lattice or bulk diffusion. Surface defects include grain boundaries, dislocations and inner and outer surfaces that contribute to grain boundary or surface diffusion. The type and magnitude of diffusion taking place is a function of temperature, partial pressure of oxygen, porosity and grain size. Because of the interrela-
tionship between oxidation and diffusion, any understanding of one process will aid in defining the other. Specific diffusion properties of plutonium oxides will be discussed in a future publication. The oxygen pressure dependence for the first stage of plutonium oxidation kinetics revealed a zero dependence at low pressures followed by a positive dependence at higher pressures. This type of behavior is not unique to plutonium. In the case of tantalum oxidation, similar behavior is observed for the parabolic oxidation of the metal to form TazO, [23]. The change in pressure dependence was correlated to a change in the semiconductor properties of the oxide. At low pressures Ta*Os is an n-type conductor, while at higher pressures it becomes a p-type conductor. Pressure independence for diffusion-controlled oxidation has also been observed for a number of other metals forming n-type semiconductor oxides [24]. In these cases, the number of defects at the gas-solid interface is small and the resulting oxidation rate should be nearly independent of oxygen pressure. This general process has been described by Wagner [25] for diffusion-controlled reactions. Oxygen diffusion in n-conducting oxides is proportional to P-1’n as shown in Table 3. However, if the dissociation pressure of this oxide is negligible, the oxidation rate then becomes independent of oxygen pressure [26]. In the case of p-conducting oxides involving oxygen interstitials, oxygen diffusion is proportional to P 1’n. If the dissociation pressure of this type of oxide remains negligible, the parabolic rate constant will remain proportional to P lh. Based on the observed pressure dependence for the Stage I oxidation of plutonium, oxidation in the pressure independent region (below 60 Torr depending on temperature) is controlled by oxygen vacancy diffusion in n-type PuOz p-X.This conclusion appears reasonable based on the identification of PuO, _-x by X-ray diffraction analysis of the product. Oxidation at pressures above 13 - 60 Torr remained diffusion controlled, but with a positive oxygen pressure dependence. In light of the previous discussion, oxidation in this region should be controlled by the diffusion of oxygen interstitials. Pressure dependence varied from P1” to P1’4 as the temperature increased from 200 to 400 “C. This suggests that the diffusion species is changing from doubly-charged to singly-charged oxygen interstitials. The change from pressure independence to a positive dependence requires that either the oxide film contain sufficient Pu01.s2 or PuO~.~~to provide p-type conductivity or that the defect structure of PuO~_~ be dependent on oxygen pressure and convert to p-type at higher pressures. The transition indicated by the pressure dependence is also affected by temperature as illustrated in Fig. 9. As pressures increased the temperature for the transition from n-type to p-type behavior also increased. Additional measurements of the electrical conductivity of the oxide as a function of temperature and oxygen pressure are required to confirm the observed n to p transition. Oxidation occurring during the linear Stage II process exhibited a pressure dependence at low pressures that was identical to the Stage I diffusion-
363
n-We)
1000/T
(K-l)
Fig. 9. Transition pressure for the n- to p-type transition behavior during plutonium oxidation as a function of temperature.
controlled reaction. Rates were independent of pressure up to a maximum of 55 Torr at 400 “c and proportional to P”” at higher pressures. Earlier it was stated that the linear Stage II was probably caused either by the cracking of the oxide layer allowing free access of oxygen to the metal or the formation of a two-layer oxide by cracking and recrystallization. The pressure behavior of this stage suggests the controlling step might be more closely related to the diffusion step seen in Stage I, which makes the two-layer oxide more probable. In this case the diffusion-controlling layer behaves like the n-type PuO, _-x and is subsequently recrystallized into a more porous form. Such a possibility has also been suggested by Greenholt and Weirick [27] for uranium and by Haycock [13] for several other metals. At this point the lack of knowledge with respect to the identity of the oxides formed and the effect of oxygen pressure on the electrical properties of the oxide, makes it very difficult to do more than propose possible mechanisms for the Stage II behavior. One alternative to the physical transformation of the dense oxide has been proposed by Loriers [ll] for cerium. In this case, a continuous protective layer of Cez03 forms on the metal surface and oxidizes at a constant rate to form CeOz. This makes diffusion through the CezO, layer the ratecontrolling step. If this same analogy were true for plutonium, then PuZOs (PuOi.& would be the rate-controlling diffusion barrier. However, the observed pressure dependence requires that this oxide behave like an n-type semiconductor at low pressures and a p-type semiconductor at high pressures. Since PuO 1.52is a p-type semiconductor, it appears to be eliminated from the rate-controlling role. Furthermore, the temperature range of the present investigation was primarily in the two-phase region where the n-type Pu02 _-x should be predominant along with the poorly defined PuO~.~~. Evaluation of the Stage III oxidation pressure dependence in the pressure range 10 - 55 Torr (Table 2) was based on the analysis of a limited
I -
Ignition
oLSd60 ELAPSED TIME (min.)
Fig. 10. Ignition of a Pu-lwt.%Ga 465 “C.
60
1010
coupon during oxidation
in 10 Torr of oxygen at
number of data points and cannot be considered reliable. At pressures above 55 Torr this interface-controlled process was proportional to P 1’4 at 300 “C and P’ at 400 “C. At 400 “c the oxidation rate is nearly catastrophic and may in fact approach ignition, as seen at 465 “C in Fig. 10. Control of the linear reaction rate rests on the availability of oxygen [28]. Oxygen adsorption follows the sequence 0, (gas) -+ Ki --f 02
(physically adsorbed)
(5)
2 sites + O2 (physically adsorbed) + & -+ 0 (chemisorbed ) + 0 (chemisorbed ) (6) 0 (chemisorbed) - k3 + 0 (reacted)
(7)
If equilibrium exists between chemisorbed and physically adsorbed oxygen, the linear oxidation rate becomes kl =
k3KP1” 1 + KP”’
where K = (K, K2)l”, S 1 and kl = k3
(8) At low temperatures and high oxygen pressures KP”2 (9)
Under these conditions the rate is independent of oxygen pressure and adsorption is not a factor. At high temperatures, such as those used in this investigation, and at low oxygen pressures (KP”’ 4 1) then kl = k3P1”
(W
365
This relationship suggests an adsorption equilibrium exists at the oxideoxygen interface and that a dissociative equilibrium (eqn. (6)) is rate controlling. The linear oxidation rate at 300 “C in Stage III was proportional to P1’4, which suggests an intermediate position between the conditions for eqn. (9) and eqn. (10). Oxidation at 400 “c was directly proportional to pressure. At high temperatures the lifetime of physically adsorbed oxygen is short so that the rate of chemisorption, 12*,depends directly on k 1 (the rate of physical adsorption) or more specifically, on the rate of impingement of oxygen molecules on the surface. According to kinetic gas theory the rate of impingement is given by SP
ww)
=
(2m~77)‘/2
(11)
where s is the sticking probability, m the mass of the oxygen molecule and k the Boltzmann constant. The direct pressure dependence dictated by this molecular adsorption mechanism satisfies the observed oxidation behavior at 400 “c.
5. Conclusion Oxidation kinetics were measured for the Pu-lwt.%Ga alloy over the temperature range 150 - 500 “C and with oxygen pressures of 0.004 to 500 Torr. Three processes were identified as follows: (i) the build-up of a dense protective oxide, (ii) a constant conversion of the protective layer to a porous oxide and (iii) an interface reaction resulting from severe cracking and spalling of the oxide layer. Results on the effects of temperature supported the results of earlier investigations in that a discontinuity existed between 300 and 370 “c. The cause of the discontinuity did not affect the 21.5 kcal mol-’ calculated for the activation energy of the diffusion-controlled Stage I process. Activation energies for the Stage II and III processes were, however, dependent on the temperature range. At present the cause of the discontinuity is unknown, although it is thought to be related to the properties of the oxide. Oxidation rates were independent of pressure up to about 60 Torr for Stages I and II. This behavior is attributed to a diffusion-controlled oxidation reaction where oxygen vacancy diffusion is rate controlling. Above 13 - 60 Ton oxidation was directly proportional to pressure, suggesting that the rate-controlling step is oxygen interstitial diffusion and that the oxide formed converts to a p-type semiconductor. Stage III of the oxidation process was an interface-controlled reaction that was dependent on the adsorption of oxygen in the 300 - 400 “c range. Above 400 “C ignition becomes an unpredictable factor in the Stage III oxidation process. Additional work is required to characterize oxide composition and properties as a function of oxygen pressure and temperature. This informa-
366
tion is needed to identify the cause of the discontinuity in the temperature behavior and to verify the explanation presented in this work for the oxygen pressure dependence.
Acknowledgment This work was performed under the auspices of the U.S. Department of Energy, Contract DE-AC04-76DP03533.
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