THE JOURNAL OF ! 1
i
J
PHYSICAL CHEMISTRY (Registered in U. S. Patent Office)
VOLUME 62
(0Copyright, 1958; by the American Chemical Society)
SEPTEMBER 3, 1958
NUMBER
8
OXYGEN DISSOCIATION PRESSURES OVER URANIUM OXIDES1 BY P. E. BLACKBURN Westinghouse Research Laboratories, Pittsburgh 36, Pennsylvania Received Aunust SO. 1967
Oxygen dissociation pressures over the uranium oxides, U02.0 to UO2.6, were measured between 950 and 1150' by the Knudsen effusion method. I n this temperature and composition range there are three stable uranium oxides: UOz in which the solubility of oxygen increases with temperature, U a 0 ~with a narrow homo eneity range and UbO13 in which oxygen dissolves. A phase diagram bf the uranium-oxygen system wa# constructed and t%ermodynadc values were derived.
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I. Introduction The purpose of this research is to determine the phases present between U02 and U30s at high temperatures, to find the oxygen pressure in equilibrium with these phases, and to derive the related thermodynamic properties. Biltz and Muller2 determined the static oxygen dissociation pressure of UOS, of V30s, and of the U3OSdecomposition products down t o a composition of U02.20,at1160'. These authors found that the dissociation pressure decreased with a diminishing O/U ratio from 2.644 t o between 2.613 and 2.607. From this latter composition to somewhere between u02.296 and UOz.zaothe pressure was constant. The data indicate that the uranium-rich limit' of the U3OSphase is U02.61. The information from these authors implies that there is a t least one boundary between U02.296 and UO2.20~a t 1160'; aside from this, they present insufficient data t o fix the phase limits. I n addition t o U308 and UOZ, two other oxides have been reported. Alberman and Anderson3 have recently noted the existence of a phase stable at high temperatures, U409. Another oxide, U6013, has been reported by Hoekstra, et aL4 The composition of the phases existing up t o 900" has been studied by Gr#nvold15who used a high temperature X-ray camera t o determine the (1) The work reported in this paper was supported by the U. S. Atomic Energy Commission under subcontract to Bettis Plant. (2) W. Biltz and H. Mliller, 2. anorg. allgem. Chem., 163, 257 (1927). (3) K. B. Alberman and J. S. Anderson, J . Chem. Soc., Suppl., 2 , 303 (1949). (4) H. R. Hoekstra, S. Siegel, L. H. Fuchs and J. J. Katr, THIS JOURNAL, 69, 136 (1955). (5) F. Gr@nvold,J. Inorp. and Nuclear Ckem., 1, 357 (1955).
oxide structure and composition. Above 300°, Grfinvold found three phases, UOZ,UlOg and u306. U02 was found to have an increasing oxygen solubility with temperature, while U40a had a very narrow homogeneity range. The lower limits of U3Os decreased from UOZ.62 at 25' to U 0 2 . 6 a~t 700". WagnerSa has pointed out that Coughlin's6b f ree energy of formation for U3OSand UOz, and Biltz and Muller's oxygen dissociation pressures are incoiisistent. The oxygen pressure measured by Biltz and Muller at 1160' over U409and U306is less than that over U40gand U02 calculated from a combination of Biltz and Muller's data and Coughlin's tables. Since this is thermodynamically impossible, one or more of the valhes must be in error. It is necessary, therefore, to find out which phases are present at high temperatures, and, a t the same time, to measure the dissociation pressure over these phases so that accurate thermodynamic values may be calculated. 11. Theory The Knudsen Method.-In the Knudsen method of measuring vapor pressures,' the procedure used here, a gas tight container with an orifice is heated in VUCZLO. The sample material inside the container is in quasi-equilibrium with the gaseous phase, and some of the gas effuses through the orifice. The gas pressure in the container may be ( 8 ) (a) C. Wagner, private communication; 03) J. P. Coughlin, Contribution t o the Data on Theoretical Metallurgy, XII. Heats and Free Energies of Formation of Inorganic Oxides," U. S. Bur. Mines (1954) Bull. 542 pp. 54-55. (7) A I . Knudsen, Ann. Physik, 28, 75 (1909); 28, 999 (1909); 29, 179 (1909).
897
P. E. BLACKBURN
898
calculated from the rate of effusion through the orifice using Langmuir’s equationS
where P is the pressure; m the weight of gas lost in time, t; B is the area of the orifice; Wg is Clausing’s factorg; R is the gas constant; and M is the molecular weight of the gaseous species. Motzfeldtll has derived an equation which expresses the equilibrium pressure as a function of the accommodation coefficient,12a P, = P,,
-A
(t + E 1
2)Pm
(2)
where Pmis the pressure calculated from equation 1, Pe, is the equilibrium pressure, A is the area of the sample material, and W Ais the Clawing factor for the Knudsen cell. For a cell in which the height is equal to the diameter, W Ahas a value very close to 0.5, reducing equation 2 thus P , = Pe,
- W B BP,
(3)
When Pmis plotted versus WBBP, for cells with varying orifice dimensions, the intercept, for B.= 0, is equal to P e q and the slope is 1/A a. 111. Apparatus and Samples A. Apparatus.-Briefly, the apparatus consists of a quartz beam mi~robalancel~ with a sensitivity of 0.5 pg. for a sample weighing 0.65 g. which is sealed into a “Pyrex” glass vacuum system containing metal valves so that grease and rubber joints are eliminated. The system may be baked out and evacuated to vacuums of about lod7mm. B. Knudsen Cells.-Knudsen cells were constructed from 0.005 inch platinum sheet by drawing a cylinder closed at one end. A lid was drawn so that it fitted tightly into the cylinder. The edge of the cylinder and of the lid were welded together making the cylinder gas tight, except for the orifice which was drilled through the center of the end opposite the lid. The finished cell (cell 1) was 0.64 cm. in diameter, 0.63 cm. high, and weighed 0.65314 g. The diameter of the orifice, measured with a traveling microscope, was 0.0658 f 0.0009 cm. and the edge of the hole was 0.025 f 0.001 cm. thick. A second cell with a hole 0.1498 f 0.0015 cm. in diameter and hole thickness 0.015 f 0.001 cm. was also prepared to determine the effects of hole size on the pressure measurements. I n calculating the rate of evaporation, the orifice dimensions are corrected for thermal expansion.l* C. Sample.-The sample material, UOZ powder prepared by Mallinckrodt Chemical Works and obtained from the Westinghouse Atomic Power Division, contained less than 0.1% total impurities as determined by spectroscopic analysis. The principal impurities were arsenic
