Sept., 1959
PREPARATION OF URANIUM MONOCARBIDE AND
I T S ~ E A TOF
FORMATION
1455
THE PREPARATION OF URANIUM MONOCARBIDE AND ITS HEAT OF FORMATION BY JOHND. FARR,ELMER J. HUBER,JR., EARLL. HEADAND CHARLES E. HOLLEY, JR. Contribution from the University of California, Los Alamos Scientijc Laboratory, Los Alamos, New Mexico Received February 81, 1.969
The preparation of uranium monocarbide, UC, by direct combination of the elements is described. Its heat of formation was found by combustion calorimetry to be AHOzssoK. = -21.0 f 1.0 kcal./mole. The combustion experiments were done with UC-UOz mixtures in order that the final uranium oxide formed would be stoichiometric u308.
are known, the heat of formation of UC was found. From preliminary experiments it was determined that when UC was burned by itself a fused oxide was formed which was not stoichiometric U&. But when UC was mixed with several times its weight of UOz and the mixture burned, the resulting oxide had not fused and was stoichiometric U308. This behavior is similar to that observed in the combusThe Preparation of Uranium Monocarbide.-High purity tion of uranium metal.3 The calorimeter and general procedure, which inuranium metal, depleted of U-235, was used. The surface oxide layer was removed with dilute nitric acid, and the volved the determination of the heat evolved from metal was then washed and dried and given a preliminary the simultaneous combustion of weighed amounts of arc-melting for outgassing and removal of any volatile impurities. Oxygen and nitrogen are soluble in the UC lattice uranium monocarbide and uranium dioxide in a and care had to be taken to exclude them from the reaction. bomb calorimeter a t a known initial pressure of oxyThe AUC graphite from the National Carbon Company gen, have been d e ~ c r i b e d . ~ The energy equivalent was outgassed at 2000’ in vacuo before use. 9988.0 f 3.3 joules/”C. as deof the calorimeter was The arc furnace was of water-cooled copper construction. It had a movable cathode with a tungsten tip. Pickup of termined by the combustion of standard benzoic either copper or tungsten by the sample was found to be neg- acid. The uncertainty interval for the calorimetric ligible. The furnace could be evacuated by an oil diffusion measurements is taken as twice the standard deviapump backed by a mechanical pump to approximately 10-8 tion of the mean. mm. The power for melting of samples came from a 34.5 The powdered carbide was mixed with powdered kw. welding generator of the rectifier type. A high fredioxide and burned in a thoria dish supported on a quency unit was used to initiate the arc. The UC was prepared by placing the uranium metal and platinum platform in the combustion bomb. Ignithe necessary graphite on the copper hearth and closing the tion was by means of a 0.010 in. diameter uranium furnace. After evacuation of the system, tank argon was admitted t.0 a pressure approximately 8-10 cm. below the wire as a fuse, which in turn was ignited electrically. The uranium monocarbide, after being ground to ambient atmospheric pressure. Next, a button of zirconium metal was melted in the arc furnace as a getter for the impuri- a powder in an “inert” atmosphere box contained ties in the argon. The uranium was then melted in contact with the graphite to form the carbide. This piece of carbide, 95.38% U, 4.52% C, 0.085’% 0, 0.005% H and after cooling, was turned over on the hearth by means of 0.015% Si. Apparently some oxidation took place the movable cathode without opening the furnace t o the during grinding. It was assumed that the carbon atmosphere. What was previously the bottom side of the was all present as UC, the oxygen as UO, the hydrosample could now be melted. This procedure was repeated gen as UH3, the silicon as elemental Si, and the exuntil the carbide had been melted on both sides a total of three times. The product was a shiny, brittle, metallic- cess uranium as U metal. The silicon was probably appearing button which analyzed 95.36%. U, 4.61% C, present as uranium silicide, but thermal data are 0.008% 0,0.005% H a n d 0.015% Si, other impurities being lacking on this compound and the amount of silicon negligible. This corresponds to 95.99% UC, 3.47% U metal, was so small that negligible error was introduced by 0.13% UO, 0.40% UH3 and 0.015% Si. All percentages given are weight percentages. That the excess uranium assuming it to be present as free silicon. The oxywas present as metal was shown metallographically and by gen was assumed to be present as UO because it is X-rays. If more graphite was used in an attempt to convert believed that UC stabilizes U O . 6 On these assumpall the uranium to UC, then a UC2 phase was found in the tions the actual composition of the powdered uraproduct as an impurity. The lattice constant of the UC nium monocarbide was thus 94.11% UC, 4.125% was found to be a = 4.9554 f 0.0003 A. U, 1.3570UO, 0 . 4 0 % UH3and 0.015% Si. The Heat of Formation of Uranium MonocarThe uranium dioxide had the formula Uoz.014. Its bide.-The heat of formation of uranium mono- heat of combustion under the conditions of the excarbide was determined indirectly from measure- periment was determined separately. ments of its heat of combustion in oxygen. From The solid conibustion product was n dark, slightly these measurements, AH was calculated for the re- sintered mass of uranium oxide which was easily action powdered. It was analyzed by heating to constant 3UC + 702 = Us08 + 3C01 weight in oxygen a t 750” and noting any change in (1) and, since the heats of formation of Ua08 and GOz weight. The exit gases were passed through ab-
Introduction The uranium carbides are of considerable interest in connection with the technology of nuclear reactors. Uranium monocarbide has been known for some time.2 This paper reports a method of preparing this compound by direct combination of the elements and a n experimental determination of its heat of formation.
(1) This work was done under the auspices of the Atomic Energy Commission. (2) See J. J . Kats and G . T . Seaborg, “The Chemistry of the hotinide Elements,” John Wiley and Sons, Inc., New York, N. Y.,1957, p. 148, for a recent review of the uranium-carbon system.
(3) E.J. Huber, Jr., C. E. Holley, Jr., and E. H. Meierkord, J . Am. Chsm. Sac., 74,3406 (1952). (4) C. E. Holley, Jr., and E. J. Huber, Jr.. ibid., 73, 5577 (1951). (5) R. E. Rundle, N. C. Baenziger, A. S. Wilson and R. A. &ICDonald, ibid., 70, 99 (1048).
J. D. FARR, E. J. HUBER, JR., E. L. HEADAND C. E. HOLLEY, JR.
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Vol. 63
TABLEI THEHEATOF COMBUSTION OF UC Mass UC, g.
Mass UOz, B.
Mass U, mg.
Energy e uiv., j./G
AT, ‘C.
1.2090 1.0601 1.2808 0.9844 0.9749 1.0241 1.1436 0.9578 1.0562 1.0182
19.5640 18,4404 17.4359 15.4431 14.4102 14,9250 15.3697 14.8057 14.7218 14.4235
73.3 64.3 72.9 74.5 76.2 73.7 76.0 66.3 72.6 67.6
10016.2 10015.8 10015.7 10015.0 10014.6 10014.8 10015,O 10014.8 10014.8 10014.7
1.4915 1.3578 1,4513 1.2064 1.1601 I.2097 1.2983 1.1633 1.2189 1.1845
sorption tubes to determine water and carbon dioxide. These smounted to about 5 and about 1mg., respectively, and are believed to be present because of adsorption. These amounts are negligible unless the carbon dioxide was due t o unburned UC in t,he combustion products. No correction was made for this possibility. The results confirmed the formula of the product oxide as U308. Ten runs were made. The results are presented in Table I. The average initial temperature of the runs was 25.05’. The initial volume of the bomb was 350 ml. The initial oxygen pressure was 25 atm. The heat of coinbustion of UC under the particu10.5 lar experimental conditions is thus 5876.9 j oules/g. I n order to find the heat of formation of UC, corrections must be made to a constant pressure process from a constant volume process. And, because an appreciable fraction of the heat from the actual process carried out came from U02, in addition to that from the UC, it was necessary to correct the total reaction in the bomb t o constant pressure and then subtract the contribution of t,he UOZ and the impurities to arrive a t AH298for the combustion reaction (eq. 1).6 After AH for reaction (1) was so (6) In the conversion t o a constant pressure process ( b A E / d P ) z u s for oxygen was taken as -6.51 joules/atm./mole IF. D. Rossini and M. Frandsen, J . Research Natl. Bur. Standards, 9, 733 (1932)J and for carbon dioxide as -28.5 joules/atm./mole (W. N. Hubbard, D. W. Scott and Guy Waddington, “Experimental Thermochemistry,” F. D. Rossini, Editor, Interscience Publishers, Inc., New York, N. Y., 1956, p. 101.) I n t h e correction for the impurities, AHf for UO was taken as half t h e value for UOz. This value is uncertain, but the fact t h a t UO has not
Firing energy,
Energy from u o z ,
1.
I.
Energy from UC, j ./g.
5.6 7487.5 4.8 7057.5 4.8 6673.0 4.9 5910.4 5515.1 5.6 5.0 5712.1 5.3 5882.3 4.9 5666.4 5.1 5634.3 4.9 5520.2 Av. 2 X standard dev.
5856.7 5864.3 5851.5 5887.2 5864.6 5888.5 5890.3 5897.3 5875.5 5893.0 5876.9
Dev.
20.2 12.6 25.4 10.3 12.3 11.6 13.4 20.4 1.4 16.1 14.4 10.5
found, this value was combined with the heats of formation of U308 arid COz t o give the heat of formation of UC, AHt = -87.9 f 4.2 k j o ~ l e s / m o l e . ~ ~ ~ In defined calories this is -21.0 f 1.0 kcal./mole. There are no previous experimental values for comparison. Estimates have been made by Brewer of -43 kcal./moleg and by Krikorian and Brewer of - 28 kcal./mole.lO Acknowledgments.-The authors wish to acknowledge the valuable assistance of D. Pavone, metallographic analysis, F. H. Ellinger, X-ray analysis, and &I. E. Smith, E. Van Kooten and 0. Kriege, chemical analysis. been prepared in bulk indicates t h a t i t is probably not appreciably more stable than UOP. For UHa, AHy was taken as -126.9 kcal./ mole and for Si02 a8 -205 kcal./mole. The heat of the reaction 3 U o z . o ~ 0.979 OP(1 atm.) = UaOs was calculated from separate measurements of the heat of combustion of this particular batch of uranium dioxide and found to be -313.05 i 0.31 kjoule/mole. (7) The heat of formation of UsOs was recalculated from the data in ref. 3 as -3567.B ==! 6.7 kjoules/niole and the heat of formation of COz was taken as -393.51 zt 0.04 kjoules/mole (NBS Circular 500). (8) The uncertainty includes the uncertainty in the calorimetric measurements on the UC-UOz mixtures and on the energy equivalent, both expressed as twice the standard deviation, the recalculated uncertainty in the heat of formation of UaOs. estimated values for the uncertainties in the heats of forination of COz, UO, UHS and SiOz, and estimated values for uncertainties in the amounts of oxygen. hydrogen and silicon inipntities in the UC, all combined according to standard methods for the propagation of errors. (9) Leo Brewer, el al., “The Thermodynamic Properties and Equilibria a t High Temperatures of Uranium Halides, Oxides, Nitrides and Carbides,” University of California Radiation Laboratory Report BC-82 (MDDC-1543) Sept. 1945. (10) 0. H. Krikorian, “High Temperatures Studies. Part 11. Thermodynamic Properties of the Carbides,” University of California Radiation Laboratory Report UCRL-2888, April 1955.
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