VAPORPRESSURE OF LIQUIDURANIUM the variation of the partial pressure of uranium over its range of composition. From the equilibrium reaction, eq 6, it is possible to calculate the partial pressure of U (g) in equilibrium with the stoichiometric solid and to compare it with the value measured a t the lower phase boundary. By means of eq 7 and 9, the equations for AG?(U, g) and AGP(U02, g) given in the text, and the observation of Ackermann, et al.,10 that a t 2000°K puo =0.03puo for nearly stoichiometric dioxide, UOl.g94(s),one calculates that a t this temperature the partial pressure of uranium is 3.8 X 10-la atm, which is to be compared with the value atm, obtained a t the lower phase boundary, 2.5 X from the equation in Table 11. Hence, this rather large change in the partial pressure relative t o the vapor
769 pressure of uranium over a relatively small change of composition indicates a positive deviation from Raoult's law. Acknowledgments. The authors are grateful t o Dr. Russell K. Edwards of the Chemical Engineering Division and to Dr. R. J. Thorn of the Chemistry Division for pertinent discussion throughout the course of the investigation, to Dr. Allan E. Martin of the Chemical Engineering Division for discussions dealing with liquid-solid phase equilibria, to John J. Hines and Michael A. Essling of the Chemistry Division for their assistance with the fission counting, and t o Reactor Operations for the use of the Juggernaut reactor facilities.
The Vapor Pressure of Liquid Uranium ; Effects of Dissolved Tantalum, Phosphorus, Sulfur, Carbon, and Oxygen1 by R. J. Aekermann and E. G. Rauh Chemistry Division, Argonne National Laboratory, Argonne, Illinois
60439
(Received May 9 , 1 9 6 8 )
The vaporization behavior of liquid uranium saturated with tantalum, phosphorus, sulfur, and carbon has been studied by mass effusion and mass spectrometric techniques in the temperature range 1800-2400°K. The results, combined with those previously reported for uranium saturated with oxygen, show that the activity coefficient of uranium decreases with increasing electronegativity of the dissolved component, varying from nearly unity in the case of tantalum and phosphorus to approximately 0.2 for oxygen. The vapor pressure of liquid uranium is given by the equation log p(atm) = (5.71 & 0.17) - (25,230 & 3 7 0 ) / T , and the heat of sublimation, AH0298, is 126.3 & 1.0 kcal/mol. At high temperatures uranium monophosphide and uranium monosulfide are converted to the two-phase systems UP(s) U(l) and US(s) U(1).
+
Introduction The vapor pressure and heat of sublimation of uranium appear thus far t o have escaped experimental verification by two or more laboratories. Several studies have yielded values of the heat of sublimation, A.Hoz9s, that lie within the range 117-129 kcal/mol and pressures that differ by an order of magnitude. More recently, the high limit of the heat of sublimation has been favored but values derived from experimental measurements and thermochemical cycles show systematic differences that lie outside the agreement that hopefully could be expected in high-temperature systems. Rauh and Thorn,2 using the effusion method and radiochemical analysis for uranium, measured the vapor pressure in the temperature range 1630-1970°K and derived a value of 116.6 kcal/mol for the heat of
+
sublimation a t 0°K. The effusion measurements of Alcock and Grievesona also support this value of the heat of sublimation. DeMaria, Burns, Drowart, and Inghram,4 using mass spectrometric analysis of the vapor effusing from an initially constituted system of uranium metal and aluminuin oxide, measured partial pressures of uranium that were approximately 0.1 those of Rauh and Thorn and reported a heat of sublimation, AH"298 = 126 =t 5 kcal/mol. (The change of reference temperature from 0 to 298'K is numerically unimpor(1) Based on work performed under the auspices of the U. S. Atomic Energy Commission. (2) E . G. Rauh and R . J. Thorn, J. Chem. Phys., 22, 1414 (1954). (3) 0.B. Alcock and P. Grieveson, J . Inst. M e t a l s , 90, 304 (1962). (4) G. DeMaria, R . P. Burns, J. Drowart, and M. G. Inghram, J . Chem. Phys., 32, 1373 (1960).
Volume '78,Number 4 A p r i l 1969
R. J. ACKERMANN AND E. G. RAUH
770 tant.) Ackermann, Rauh, and Thorn6 concluded that these two investigations need not be inconsistent if the activity of uranium is reduced to approximately 0.05 by the presence of dissolved oxygen. Drowart, Pattoret, and Smoes6have reported mass spectrometric studies of UOz(s) U(1) mixtures and pure liquid uranium which yield an average value of the heat of sublimation a t 298'K of 125.5 f 2.5 kcal/mol. That study indicated that the pressures measured by Rauh and Thorn2 and Alcock and Grieveson3 were too high as a result of oxygen contamination which vaporizes as UO(g) , Furthermore, the reduction of the vapor pressure of pure liquid uranium by dissolved oxygen was indicated to be less than a factor of 1.6. These authors' also reviewed the thermochemical data for the vaporization of uranium oxides and carbides and concluded that the use of the thermochemical data in the thermodynamic cycles was reasonably consistent with the latter value. Leitnaker and Godfreys recently analyzed the thermochemical data for the uranium-carbon system and calculated AH'zgs = 123.7 f 1.3 kcal/mol. Pattoret, Drowart, and Smoesghave recently reported new mass spectrometric data that yield an even higher value of = 128.5 f 2.0 kcal/mol. the heat of sublimation, The general intent of the present investigation is to directly measure by mass spectrometric and absolute effusion techniques the partial pressures of U(g) in equilibrium with liquid uranium saturated with tantalum, UP, US, and UC, to include those of V02(s) U(1) previously reported,'O and to demonstrate the significance of any observed trend of nonideality of the liquid solutions from the chemical and thermodynamic properties of the dissolved components.
+
+
Experimental Methods and Results
A . Mass Spectrometric Observations and Measurements. Mass spectrometric observations were made on the initially constituted systems UP (s) , UP (s) U(1) , US(s), US(s) U(l), UC(s), and UC(s) U(1).
+
+ +
The instrument used was a Bendix Model 12-101 timeof-flight mass spectrometer equipped with an electronbombarded effusion cell, Details of the cell assembly,'l the power regulator,lz and the methods, techniques, and measurement of temperature have been described elsewhere.l* The observations included identification of species, behavior of impurities (principally oxygen bearing species from oxide contamination of the starting materials), indications of changes in composition by a time dependence of the ion currents, and measurement of the temperature dependences of the ion currents. A typical behavior of the ion currents accompanying a change of composition has been demonstrated in the thorium-oxygen system.14 As the composition adjusts to the equilibrium value, the ion currents increase or decrease at varying rates and the rate of adjustment may become quite slow at the lower temperatures, for large temperature changes, and when the temperature The Journal of Phyeical Chemirtry
has been decreased. I n order to ensure steady-state conditions within reasonable times, large temperature changes were avoided and the ion currents were recorded a t temperatures sequentially decreased, then increased. The data for each series of measurements of the equilibrium ion currents as a function of temperature will be given by the least-squares equations of log IiT 21s. 1/T with the standard deviations. As shown previou~ly,'~ the data can reflect changes in partial pressures with both temperature and composition if the composition variable is important. An ionizing electron energy of 10 eV was used when there existed the possibility of U+ ions being formed from fragmentation of higher molecular weight species;lO when fragmentation was not possible 20 eV was used. Tungsten effusion cells were used throughout. The samples in all cases were in the form of dense cups of the monophosphide, sulfide, or carbide weighing about 1 g to which 50-200 mg of uranium metal was added after impurities had been removed by vaporization. I . The Systems U P ( s ) and U P ( s ) U(Z). Before the addition of uranium, oxide contamination in the UP was removed by vaporization as UO gas. At 2350OK the ions observed (20 eV) were Uf, UO+, P+, and a trace of Pz+. No UP+ or UOzf ions were observed. Initially the Uf and UO+ were of equal intensity but during a 7-hr period a t 2350'K, the U+ increased by about a factor of 2 and the UO+ and Pf decreased by factors of about 200 and 2, respectively, and thereafter both the U+and P+ remained constant. The temperature dependences of the partial pressures of U (g) and P (g) were then determined a t 20 eV over the temperature range 1890-2350'K. The leastsquares representation of the data and the measured heats of vaporization are given in Table I under Series UP-1. The errors quoted here and in the remainder of the paper are standard deviations. After an additional 4 hr a t 2350'K the temperature dependences were remeasured; the least-squares representation of these data is given in Table I under Series UP-2. Since the heats of the two series agree within the standard devia-
+
( 5 ) R. J. Ackermann, E . G. Rauh, and R. J. Thorn, J. Chem. P h y s . , 37, 2993 (1962). (6) J. Drowart,, A. Pattoret, and S. Smoes, ibid., 42, 2629 (1965). (7) J. Drowart, A. Pattoret, and S. Smoes, J. Nuclear JMat., 12, 319 (1964). (8) J . Iv1. Leitnaker and T. G. Godfrey, i b l d . , 21, 175 (1967). (9) A. Pattoret, J. Drowart, and 9. Srnoes, Paper Sm.98/49, published in the Proceedings of the Symposium on Thermodynamics of Nuclear Materials with Emphasis on Solution Systems, International Atomic Energy Agency, Vienna, Sept 4-8, 1967. (10) R . J. Ackermann, E. G. Rauh, and M. S. Chandrasekharaiah, J. P h y s . Chem.. 73, 762 (1969). (11) E. G. Rauh, R. 0. Sadler, and R. J. Thorn, Argonne National Laboratory Report, ANL-6536, April 1962. (12) H. H. Cremer, Z.Instr., 72, 209 (1964). (13) R. J. Ackermann and E. G. Rauh, J. Chem. P h y s . , 36, 448 (1962). (14) R. J. Ackermann, E. G. Rauh, R. J. Thorn, and A I . 0.Uannon, J. P h y s . Chem. 6 7 , 762 (1963).
VAPORPRESSURE OF LIQCID URANIUM
771
~~~
~~
Table I: Mass Spectrometric Determinations of Temperature Dependence of Partial Pressures of U and P in Equilibrium with the System UP(s)
+ U(1):
Linear Least-Squares Representation, Log I,T = A , B , / T . Measured Heats of Vaporization of U(g) and P(g)
+
Seriesa
AHw
Species
U
UP-1
P U P U P
UP-2 UP-U-1
A
-B
kcal /mol
15.391 f 0.086 17.447 f 0.164 15.628 i0.125 1 7 . 8 9 6 f 0.255 15.315 i0.086 17.706& 0.246
25,030 f 180 31,990 i 360 25,330 f 260 32,720 i 560 25,170 & 180 33,050 f 530
114.5 i0.8 146.4 & 1 . 6 115.9 i 1 . 2 149.7 i2 . 6 115.2 & 0 . 8 151.2 i 2.4
"UP-1: eight experimental points, temperature range 1890-2340°K. UP-2: ten experimental points, temperature range 1890-2400°K. UP-U-1: seven experimental points, temperature range 18852310°K.
The measured heats of evaporation of U agree within the standard deviations in all three series and there is agreement between the measured values of AH(P) of Series UP-2 and UP-U-1 although that of Series UP-1 is slightly low. The mass spectrometric evidence given here indicates that oxide contamination in UP (6) and UP (s) U (1) can be reduced to trace amounts by vaporization as UO but that during the time required, in this case 7 hr a t 2350°K, phosphorus is lost preferentially and the composition shifts into the two-phase region, UP(s) U(1). The observations are in agreement with those of Reishus, Gunderson, and Edwards.16 2. The Systems U S ( s )and U S ( s ) U ( 1 ) . During the high-temperature purification of the US the general vaporization behavior observed was essentially that described earlier.l6 Initially at 2110OK the ions observed were U+, UO+, US+, and GOz+ US+ (mass 270), and UOS+ (not U03 because of reducing conditions) in the approximate ratios 1:45 :15 : 1. During a period of 15 hr at 211OoK, the U+ increased by a factor of 5 and the UO+, US+ UOZ+,and UOS+ decreased by factors of 2, 3, and 8, respectively. During an additional 10-hr period at 2270°K, the U+ increased slightly, the US+ remained constant, the UO+ decreased by a factor of 103, and the UOS+ decreased beyond the limit of detection. The temperature dependences of the ion currents U+ and US+ were then determined over the temperature range 192O-227O0K. I n this range the ion currents showed no increase or decrease with time at each temperature indicating that the composition of the solid, stoichiometric or otherwise, did not change appreciably. However, when the temperature was raised to 2320°K, the V+ showed a small but steady increase while the US+ remained constant. After 1 hr a t 232OoK, the temperature dependences of U+ and US+ were remeasured over the temperature range 19202320'K. Again there appeared to be no change in composition at temperatures below 232OOK. The temperature was then raised to 2400OK for 1 hr during which time the U+ increased slightly, then became constant while the US+ remained unchanged. A third series of measurements of the temperature dependences was made in the range 1920-2320OK. The data for the three series of measurements are given in Table I1 under Series US-1, US-2, and US-3. An 85-mg sample of uranium metal was added to the US cup and the temperature raised to 2270OK for 5 hr during which time the U+ and US+ were essentially constant and a small ion current of UO+, initially about 3% of the total, decreased by a factor of 20. Three series of measurements of the temperature dependences
+
+
+
+
tions, it was apparent that equilibrium had been established and a 50-mg sample of anodically cleaned uranium was added. Again, at 235OoK, approximately equal intensities of U+ and LO+ were observed initially but after 3 hr the UO+ had decreased by a factor of about 250 and the U+and P+remained constant within a few per cent. A third measurement of the temperature dependences of the partial pressures of U(g) and P(g) resulted in the data given in Table I under Series UP-C-1 and shown in Figure 1. OK
2200
2300 1
'
1
2000
2100 1
1
I900
1
I
I
1
4.0
c
+H
3.0
0
-I
\/
AH=151.2t2.4 \
2.0
1
4.3
I
I
4.5
1
1
4.7
1
1
'
4.9
1
I
5. I
1
1
5.3
1O4/T Figure 1. Mass spectrometric measurements of the temperature dependence of U(g) and P(g) in equilibrium with UP(s) -I- U(1), Series UP-U-1, Table I; log liT = Ai Bi/T.
+
+
(15) J. W. Reishus, G. E. Gunderson, P. M. Danielson, and R. K. Edwards, Argonne National Laboratory Report, ANL-7514, Nov 1968. (16) E. D.
Cater. E. G. Rauh, and R . J. Thorn, J. Chem. Phys., 44, 3106 (1966);Erratum, i b i d . , 48, 538 (1968). Volume 79, Number 4 April 1.969
772
R. J. ACKERMANN AND E. G. RAUH ~
OK
Table I1 : Mass Spectrometric Determinations of Temperature Dependence of Partial Pressures of U and US in Equilibrium with US(s) and US(s) U(1):
2300
+
2200
+
H(U) = 131.6 t 0.8 KCALIMOLE
AHv,
us-1 us-2 US-3
us-u-1 us-u-2
us-u-3 a
Species
U US U US U US I; US U US U US
kcal /mol
-B
A
I900
2000
5.c
Linear Least-Squares Representation, Log IiT = A i B,/T. Measured Heats of Vaporization of U(g) and USrg)
Seriesu
2100
17.268 k 0.085 28,750 f 180 18.72OZt0.129 31,550 f 270 16.463 ic 0.077 26,850 f 160 18.172 It 0.180 31,270 k 380 16.221 f0.076 26,210 f 160 18.221Zt0.129 3 1 , 3 1 0 f 2 7 0 16.201 kO0.O63 26,240 k 130 1 8 . 1 0 4 f 0 . 0 8 8 31,170 i 180 16.209ZtO.030 26,340 k 60 18.065 i0.110 31,170 f 240 1 6 . 2 0 7 f 0 . 0 3 2 26,280 f 70 18.124 f0.090 31,240 f: 190
131.6 k 0.8 144.4f1.2 122.9 i0.7 143.1 i 1 . 7 119.9 f0.7 143.3f1.2 120.1 k 0 . 6 142.6i0.8 120.5k0.3 142.0 i 1.1 120.2i0.3 143.0 f 0.9
H(UI8 122.9 f 0.7 KCALIMOLE 4.c I-
L 0
s 3.c
0 1).
0
US-1 : ten experimental points, temperature range 1910-2310°K.
US-2: seven experimental points, temperature range 1930-2320°K. US-3: seven experimental points, temperature range 1930-2240°K. US-U-1: ten experimental points, temperature range 1930-2300°K. US-V-2: eight experimental points, temperature range 1945-2345°K. US-U-3: nine experimental points, temperature range 1925-2390°K.
-
'
0
USb) after IO hrs. a t 2270°K US(d after additional I hr. at 232Q'K US(d after additional I hr. ot 2400OK usk)-tU(L)
2.0
4:
I
1
4.4
I
I
4.6
I
I
4.8
l
I
5.0
I
=wJ I 5.2
1041~
+
of U+ and US+ from the system US(s) U(1) were carried out over the temperature range 1920-2320'K: US-U-1 after the original 5 hr a t 2270°K, US-IT-2 after an additional 5 hr at 2350'K, and US-U-3 after an additional 1 hr at 2400'K. The data are given in Table 11. Plotted in Figure 2 are the data for T;+ from Series US-1, US-2, US-3, and US-U-3 and the data for US+ from Series US-3 and US-U-3. The data for U+ from Series US-U-1 and PJS-U-2 reproduce those of US-3 and US-U-3 and all six sets of data for US+agree within the standard deviations, These mass spectrometric observations show that oxygen contamination in these condensed phases can be reduced to trace amounts by the evaporation of a small fraction of the total sample at 2270'K. The decrease in the measured heats of evaporation of U over TIS (US-1, US-2, US-3) as the sample was heated a t progressively higher temperatures suggests that the composition was becoming more substoichiometric. The agreement between the measured heats of Series GS-3 and US-U-1, VS-U-2, and US-U-3 indicates that in Series US-3 the composition had already been driven U(l), via the prefinto the two-phase region, US(s) erential vaporization of sulfur. The insensitivity of the measured heats of evaporation of US t o changes in composition suggests that the solubility of U in US is small and that the activity of the US remains near unity. 3. The Systems U C ( s ) and U C ( s ) U ( 1 ) . The UC cup was heated for a total of 30 hr a t 2475'K during which time only the ions U+ and CO+ were observed,
+
+
The Journal of Physical Chemislru
Figure 2. Mass spectrometric measurements of the temperature dependence of U(g) and US(g) in equilibrium with systems in various stages of decomposition of US(s) to US(s) U(1); log I$ = A i Bi/T for Series US-], 2, 3 and US-1:-3, Table 11.
+
+
initially in the ratio 1:1 (at 20 eV) and finally at about 20 :1. The ion in tensity Ufdid not change appreciably. The temperature dependence of U+ over the range 19602475'K was measured a t the end of the 4th, 9th, 14th, 20th, and 30th hours. The measured beats of evaporation, each with a standard deviation of i l kcal/mol, were successively, 141, 131, 126, 124, and 122 kcal/mol, while the partial pressure of uranium a t 2000'K increased by a factor of 2.5. This behavior indicates a progressive increase in the activity of uranium in the solid phase as the partial pressure of CO(g) decreases and possibly can be associated with the diminishing effect of stabilization of the carbide by oxygen and/or the preferential loss of carbon via CO(g) although this would require a ratio of partial pressure of U/CO less than the observed ratio of the ion currents. A sample of 175 mg of uranium metal was added to the UC and the temperature raised to 2385'K. Only Uf and CO+ were observed, in the ratio 1:3 initially and 3: 1 after 1.5 hr. A measurement of the temperature dependence of U+ over the range 1940-2385'K was made after 7 hr at 2385'K. An average slope of 116 f. 1.6 kcal/mol was measured but an examination of the residuals in the data indicated a departure from a linear relationship of log U+T us. 1/T. After 27 hr at 2385'K the temperature dependence data shown in Figure 3
773
VAPORPRESSURE OF LIQUID URANIUM OK
2400
r
1
1
2300 1
1
2200 1
I
2100
2000
I
I
1
I
i
C/U ( R e t 19)
0.50 0.45 0.40 0.35 0.30 5.0
0.25
0.20
-
I-
4.0
s
-
3.0
+
4.0
sensing only the vapor flux emitted directly from the evaporating surface of the sample. The utility of this method with sensitive radiochemical analysis is well established.2 More importantly, the spatial distribution of the vapor for a small solid angle located in the forward direction approaches closely the theoretical value for a cosine or random distribution whereas at solid angles away from the normal the intensity may be markedly less than the theoretical values. Ward, Mulford, and Kahn” have shown very striking departures from a cosine distribution for a variety of systems, samples, and cell geometries. In view of these findings an experiment was performed in which a direct comparison was made between the weight of uranium coIlected on platinum targets which intercepted a known solid angle, r2/(r2 d2) = 0,011 in eq 1, and the total weight lost from a tantalum effusion cell containing liquid uranium located in the bottom of a single crystal tantalum cup. At 2320OK this experimental arrangement yielded a ratio of the measured weight loss to that expected for a cosine distribution equal to 0.60. This value less than unity indicates a significant loss of particle intensity at larger angles from the normal, as do Ward’s results, and demonstrates the unreliability of total weight loss measurements for these systems. I n all cases except one, either the predominant or only vapor species was shown mass spectrometrically to be atomic uranium, and consequently, an apparent or “effective” vapor pressure of a particular system, p,, was calculated by assuming uranium to be the only uranium-containing species present in the vapor. Correction of this assumption will be effected when necessary in the subsequent thermodynamic analysis. The results are given in Table I11 and in Figure 4 and are compared with those of pertinent previous studies. Curve 1 represents the vapor pressure of liquid uranium reported by Rauh and Thorn2 and curve 2 represents the effective pressure which is predominantly that of UO(g) in equilibrium with mutually saturated UO,(s) U(I).’O Curve 3 corresponds to the system US(s) U(1) and an additional phase which was detected in the residue by metallographic sectioning of the sample. This additional phase was probably UOa since the X-ray diffraction pattern did not show the lines of a third phase such as UOS; the diffraction lines of UOz and US are virtually coincident. Mass spectrometric observations of the vapor arising from US contaminated with oxide indicated that UO(g) is the major component. At the present time, however, this system has not been quantitatively characterized and it is possible that the additional phase (if other than U02) may have escaped detection in the X-ray pattern as a result of inadequate sampling. It is important, however, to retain the
4.2
4.4
4.6 4.0 1041~
5.0
5.2
Figure 3. Mass spectrometric measurements of the temperature dependence of U(g) in equilibrium with UC(s) U(1). The limiting slopes have been included to show the departure from linearity.
+
indicated a definite curvature with a limiting slope of approximately 125 kcal/mol at 2000OK and 105 kcal/ mol a t 2400OK and could well reflect the effect of the increasing solubility of carbon in liquid uranium with temperature. B. Measurements of the Total Efusion Rate of Uranium. The effusion rates of the vapor species containing uranium in equilibrium with the heterogeneous systems cited in the preceding section were measured by the collection of known fractions of the effusate on platinum targets and the subsequent radiochemical analysis for uranium. Both tantalum and tungsten effusion cells of the same design were used interchangeably. The rate of collection is related to the vapor pressure inside the effusion cell by the equation
in which p i is the pressure of species i, ni and M i are the number of moles and atomic or molecular weight, respectively, of the uranium species collected on the target in time t, a is the area of the orifice, d is the orifice-to-collimator distance, r is the radius of the collimator, and R and T are the gas constant and temperature, respectively. The choice of the collection technique in which a relatively small fraction of the total effusate is collected in the forward direction, ;.e., normal to the plane of the orifice, was made for several reasons. Such a technique closely parallels the geometrical arrangement used in the mass spectrometer and possesses the advantage of
+
+
(17) J. W. Ward, R . N. R. Mulford, and M. Kahn, J. Chem. Phvs., 47, 1710 (1967); see also J. W. Ward, { b i d . , 47,4030 (1967), and J. W. Ward, Lo8 Alamos Scientiflc Report LA-3609, Jan 1966.
Volume Y9,Number .G April 1069
R. J. ACKERMANN AND E. G. RAUH
774 Table 111: Experimental Results of the Effusion Measurements Curve and designation
System
1 2 3
0 0 4 x
+
5 A 0 6
U(1)
0
+ UC(s)
Remarks
Least-squares equation
Temp range, OK
Ref 2 Ref 10 Normal uranium Normal uranium Normal uranium *36U enriched Normal uranium T J enriched
log p,(atm) = (5.702 =t 0.011) - (23,330 =t 21)/T logp,(atm) = (7.250 & 0.146) - (27,020 rt: 250)/T log p.(atm) = (7.450 f0.200) (28,050 rt: 420)/T
1630-1970 1580-2400 1984-2330
logpu(atm) = (5.493 f0.170) - (24,910 fi 370)/T
1980-2416
log pe(atm) = (10.340 f 1.783) - (4.388 f 0.751)104/T (0.1787 f 0.0787)10*/2’2 Iogpu(atm) = (4.880 f0.230) - (24,400 rt: 450)/T
1874-2383
2saUenriched
effusion data at least for the purpose of qualitative comparison. Curve 4 represents several sets of data which are statistically indistinguishable. The circle points correspond to the partial pressure of U(g) in equiljbrium with liquid uranium contained in a UP cup. The X points and points correspond to normal and 285Uenriched uranium contained in a cup made from single crystal tantalum. It is noteworthy to point out that single crystal tantalum as well as tungsten appear to be excellent container materials for liquid uranium a t temperatures up to at least 2400OK as the liquid metal
+
T
OK
-
+
1792-2138
was confined entirely within the cup. Containers fabricated from polycrystalline tantalum and tungsten are useful only up to approximately 2060 and 1900°K, respectively, above which the liquid metal rapidly disappears via creep into the grain-boundary network. The capability of heating liquid uranium to temperatures a t which the vapor pressure is relatively large ensures the complete loss of oxide contamination which vaporizes as UO (g) . It is interesting to note that the initially measured volatility of liquid uranium containing a surface film of oxide shown by points a and b in Figure 4 appears to be significantly higher than that corresponding to the system U02(s) U(l) given by curve 2. I n fact, the two points agree more closely with the pressures measured by Rauh and Thorns2 However, after 5 hr of heating at approximately 198OoI
